A Review of Microwave Synthesis of Zinc Oxide Nanomaterials: Reactants, Process Parameters and Morphologies

Abstract

Zinc oxide (ZnO) is a multifunctional material due to its exceptional physicochemical properties and broad usefulness. The special properties resulting from the reduction of the material size from the macro scale to the nano scale has made the application of ZnO nanomaterials (ZnO NMs) more popular in numerous consumer products. In recent years, particular attention has been drawn to the development of various methods of ZnO NMs synthesis, which above all meet the requirements of the green chemistry approach. The application of the microwave heating technology when obtaining ZnO NMs enables the development of new methods of syntheses, which are characterised by, among others, the possibility to control the properties, repeatability, reproducibility, short synthesis duration, low price, purity, and fulfilment of the eco-friendly approach criterion. The dynamic development of materials engineering is the reason why it is necessary to obtain ZnO NMs with strictly defined properties. The present review aims to discuss the state of the art regarding the microwave synthesis of undoped and doped ZnO NMs. The first part of the review presents the properties of ZnO and new applications of ZnO NMs. Subsequently, the properties of microwave heating are discussed and compared with conventional heating and areas of application are presented. The final part of the paper presents reactants, parameters of processes, and the morphology of products, with a division of the microwave synthesis of ZnO NMs into three primary groups, namely hydrothermal, solvothermal, and hybrid methods.

1. Introduction

1.1. Nanotechnology

Nanotechnology is one of the most rapidly developing disciplines of science and technology. It was the reason for the global industrial revolution of the 21st century [1,2,3,4,5,6,7,8,9,10]. At present, this is a leading technology within various research areas, such as: physics, biology, chemistry, biochemistry, biotechnology, medicine, materials and biomedical engineering, electronics, optoelectronics, mechatronics, spintronics, energy generation, food and agriculture, environmental protection, and interdisciplinary fields [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. This technology enables testing, controlling, regulating, modifying, processing, producing, and using structures in which at least one of the dimensions does not exceed 100 nanometres (1 nm = 10⁻⁹ m) [39]. Materials in the nano-scale are characterised by new specific properties and phenomena, which sometimes considerably deviate from the characteristic properties of the same materials appearing in the micro-scale [40,41,42,43,44,45,46,47,48,49,50]. The observed changes to the physicochemical properties are caused primarily by two factors, namely the quantum confinement of electrons and increased share of surface atoms/ions in relation to atoms/ions present inside the particle. The big development of the specific surface area of the nanomaterial leads to an increase in the number of unsaturated coordination centres, defects and stresses in the crystal lattice, and in the chemical reactivity of particles [51,52].

Nanotechnology enables the application of nanomaterials to create innovative products, devices, and complex systems that make use of material properties in the nano-scale [11]. Products developed and created thanks to the use of nanotechnology are present in all aspects related to the human activity on Earth and in the outer space [2,3,4,5,6,7,8,9,10,11,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67]. The current research concerning the applications of nanotechnology in the broadly understood catalysis, optoelectronics, microelectronics, biomedicine and pharmacy concentrate on the control of the physicochemical properties of nanostructures (NSs) [68,69], including of the nano zinc oxide (ZnO) [70]. Numerous tests related to the use of nano ZnO for application purposes are in progress and attempts are made to produce them on an industrial scale. The dynamic development of methods of repeatable production of nano ZnO, characterised by high purity, controlled properties, high efficiency, and shortest synthesis duration possible, is desirable [71].

1.2. Bulk ZnO: Properties and Application

Owing to its exceptional physical (

Table 1. Physical properties of ZnO with the wurtzite structure. Data were derived from literature [98,99,100,101,102,103,104,105,106,107].

Properties	Value
Molecular formula	ZnO
State (colour, form)	white powder
CAS Reg No.	314-13-2
Molar mass	81.39 g/mol
Density at room temperature	5.606 g/cm3 (crystal theoretical density 5.61 g/cm3)
Solubility in water (25 °C)	1.6 mg/L
Melting point	1975 °C
Boiling point	2360 °C
Stable phase at room temperature	wurtzite
Structure	Hexagonal, where a0 = b0 ≠ c0
Space group symmetries	C4 6v (P63mc)
Bulk effective piezoelectric constant	9.9 pm/V
Hardness	5.0 ± 0.1 GPa
Lattice parameters at 300 K
a0	3.2495 Å
c0	5.2069 Å
c0/a0	1.602 (ideal hexagonal structure shows 1.633)
U	0.345
Thermal conductivity	0.6, 1–1.2 W·cm−1·K−1
Specific heat	0.125 cal/gm·°C
Linear expansion coefficient	a0: 6.5 cm 3 × 10−6 K
	c0: 3.0 cm 3 × 10−6 K
Static dielectric constant	8.656 ε(0), ε(∞)
Thermoelectric constant at 573 K	1200 mV/K
Refractive index	2.008–2.029
Band gap	at RT: 3.370 eV
	at 4 K: 3.437 eV
Exciton binding energy	60 meV
Intrinsic carrier concentration	<106 cm3
Electron effective mass	0.24 m0
Hole effective mass	0.59 m0
Electron Hall mobility at 300 K	200 cm2/V·s
Hole Hall mobility at 300 K	5–50 cm2/V·s
Ionicity	62%

) and chemical properties, ZnO counts as a multifunctional material [72,73,74,75]. Its properties include high electrochemical coupling coefficient, broad range of radiation absorption, high photostability, low toxicity, biocompatibility, and biodegradability [76]. It occurs naturally in Earth’s crust in the form of a mineral called zincite (Zn_1−xM_xO). Zincite rarely occurs in a pure form; most often it includes dopants of other bivalent metals (among others: Mn(II) and Fe(II)), which is manifested in various colours of the mineral, among others, red, red and yellow, orange, and brown [77]. ZnO is insoluble in water and its powdered form is white. Due to its amphoteric properties it reacts both with acids and bases. ZnO can crystallise in three primary crystal systems: hexagonal wurtzite, cubic zinc blende, and cubic rocksalt (Figure 1). In normal conditions, the thermodynamically stable crystal structure of ZnO is wurtzite (Figure 2) [78]. Hexagonal ZnO is an II–VI semiconductor characterised by a wide band gap. A wide band gap (3.37 eV) and a high bond energy (60 meV) make ZnO a subject of enormous interest as regards applications in optoelectronics and electronics (among others, voltage dependent resistors) [79,80]. Owing to its antibacterial (disinfecting) properties, ZnO is widely applied as an ingredient of medications for various purposes [81,82]. The earliest information about the use of ZnO as an ingredient of therapeutic ointments for treating skin boils and ulcers comes already from 2000 BC [83,84,85,86]. It is common knowledge that ZnO was widely produced and used in the 13th century in Persia, where it was applied in treatment of eye inflammation and production of brass [86]. ZnO accelerates wound healing, which is why it is used as an ingredient of pastes (e.g., zinc paste) for treating skin inflammations (among others, acme, skin eruptions) and against itching. It is also commonly used in dentistry, e.g., as temporary tooth dressing [87] and root canal filling materials for deciduous teeth [88]. Moreover, ZnO is used as an additive in feed for pigs, where it helps protect piglets from diarrhoea by inhibiting the growth of pathogenic flora [89,90]. In the past, ZnO was also an ingredient of a medication for treating diarrhoea in humans, but at present, it is used in the oral form, most often as an ingredient of food products and dietary supplements, where it acts as a source of zinc (Zn²⁺). Zinc is the second most important trace element in the human body after iron, as it fulfils the catalytic, structural and regulatory function. The average zinc (Zn²⁺) content in the organism of an adult is 2–3 g [91], while the recommended daily average dose for a healthy adult is 8–11 mg [92]. ZnO is used as an ultraviolet (UV) radiation filter in sunscreen cosmetics (creams, balms, lotions). Nowadays ZnO is commonly used as an additive in such products as pigments (zinc white, zinc green), flame retardants, cements, ceramics, porcelain, glass, plastics, sealants, adhesives, lubricating oils and paints [70,76,89,90,93,94]. The presence of ZnO in paint, apart from changing the colour, contributes also to providing the paint coat with anti-corrosion and antifouling properties. The most important area of ZnO application is the rubber industry, where it is widely used as an activator of the conventional process of sulphur vulcanisation of natural rubber [95]. At present, as much as ca. 50% of the produced ZnO is used for the vulcanisation of rubber, which is a material for manufacturing tyres, sports equipment, garden and industrial hoses, shoe soles, belts, and other rubber products [96]. The presence of ZnO in rubber enhances its strength, hardness, dynamic stability, capacity to absorb heat, and provides important pigmenting properties.

1.3. Nano ZnO: Properties and Application

Due to its unique properties ZnO enjoys unfading popularity among research units worldwide (Figure 3). Increasingly often, researchers focus on ZnO in the nano-form, which is proven by the search results in the ScienceDirect scientific paper search engine (Figure 3). The number of hits was as follows: “zinc oxide”—264,763 hits, and “nano zinc oxide”—38,999 hits. The overall share of publications concerning “nano zinc oxide” among all the hitherto published papers about ZnO was ≈14.7%. The number of publications concerning nano ZnO have been growing dynamically year by year (Figure 4), while the annual share of publications concerning nano ZnO in 2020 was as high as 32.6% (Figure 4). Over the past 10 years, the number of scientific publications related to nano ZnO increased by ca. 400% (Figure 3). The growing popularity of nano ZnO results among others from the technological development of production of ZnO nanostructures, which are characterised by novel physical properties allowing hitherto unknown applications and possibilities [71,108,109,110].

The main limitation related to the synthesis and application of various types of nanopowders, obviously including nano ZnO, is the problem of repeatability of parameters of nanopowders. Most “wet methods” [73,76,86,111] used for producing metal oxide nanoparticles lead to obtaining powders with a broad particle size distribution, insufficient crystallinity degree, variable morphology and insufficient purity. This causes a range of problems with respect to their application in the broadly understood industry, where properties of raw materials are the crucial and decisive parameter for their choice and guarantee the performance characteristics of the product being manufactured. Research on ZnO nanomaterials (ZnO NMs) can be intensified in line with the development of new methods of production and control. The evolution of nanoparticle characterisation techniques [112] and the standardisation of nanotechnology [113] currently permit determining whether changes to the ZnO NMs properties are, e.g., caused by unrepeatability of methods of obtaining, actual changes to NMs features (among others, material ageing, etc.) or unrepeatability of research procedures. An important role in research on ZnO NMs is played by a network of Accredited Research Laboratories, where increasingly more accurate and appropriately validated measurement techniques are introduced. The large number of published scientific papers devoted to the issues of nano ZnO prove that the topic of synthesis, properties, and application of such powders is researched intensively. This indicates an enormous development potential of nano ZnO, which was noticed and appreciated by numerous research groups worldwide.

Nano ZnO is regarded as a safe material [114], which excellently fulfils the function of an ultraviolet (UV) radiation filter, because it permits creation of protective layers which are invisible to the human eye. Other advantages of the use of ZnO nanoparticles (NPs) as a UV filter in the area of personal hygiene and sun protection are, among others, long-term protection and broadband protection (UV-A (315–380 nm) and UV-B (280–315 nm)) [115]. The impact of contact between ZnO NPs and human skin is still being monitored and tested [116,117,118,119,120,121]. ZnO NPs are also applied in deodorants, medical and sanitary materials, glass, ceramic, as well as self-cleaning materials [115,122]. In optoelectronics, various ZnO nanostructures are used for producing e.g., refractive index sensors [123], surface-enhanced Raman scattering (SERS) sensors [124], lasers, UV detectors, and UV diodes [74,125,126,127,128]. ZnO nanostructures may serve for producing sensors of gases [129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152] such as steam (humidity, H₂O) [133], ammonia (NH₃) [129,130,131,143], nitrogen (N₂) [131], nitrogen monoxide (NO) [129,130,139], nitrogen dioxide (NO₂) [129,131,134,135,136,151,152]_, hydrogen (H₂) [129,130,131,132,149]_, ozone (O₃) [131,137]_, hydrogen sulfide (H₂S) [129,130,131,138,139], carbon monoxide (CO) [129,130,131,135,152], carbon dioxide (CO₂) [140], methane (CH₄) [141], acetylene (C₂H₂) [131], ethylene (C₂H₄) [147], ethane (C₂H₆) [129], 1,2-dichloroethane (C₂H₄Cl₂) [129], p-xylene (C₈H₁₀) [129], phenol (C₆H₅OH) [130], chlorobenzene (C₆H₅Cl) [130], methanol (CH₃OH) [131], ethanol (C₂H₅OH) [129,130,131,148,150], formaldehyde (HCHO) [129,130,131,151], acetaldehyde (CH₃CHO) [131], acetone ((CH₃)₂CO) [129,130,131,144,145,146], mixture of propane (C₃H₈) and butane (C₄H₁₀) [130,142], and triethylamine (C₆H₁₅N) [129]. The application of ZnO nanostructures in the commercial development of new gas sensors is quite strongly limited for the time being, which is a consequence above all of the ageing effect [153]. Thin films of nano ZnO are most often used as inorganic conductors in flexible and transparent devices [154,155], e.g., transparent electrodes, transparent windows, flat panel displays, and components of devices with the surface acoustic wave [156,157,158,159,160,161].

The antibacterial and antifungal properties of ZnO NPs have become the focus of interest of pharmacy, biomedicine and dentistry due to their low toxicity to the human organism [162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181]. It has been proved that ZnO NPs applied in textiles effectively fight bacteria, e.g., Staphylococcus aureus or Klebsiella pneumoniae [182]. In medicine, research is in progress on the application of ZnO NMs as a potential contrast agent (imaging), drug carrier, iron delivery, gene carrier, biosensor, a potential anti-cancer agent, in a photodynamic therapy, for prophylactic and therapeutic vaccines, support for antifungal treatments, in photocatalytic antibiotics, inhibition of influenza virus infection, diagnostic-therapeutic functions, wound dressing and tissue engineering [70,166,168,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213]. However, before implementing ZnO NMs in biomedical applications on a commercial scale, the toxicity of ZnO NMs must be carefully learnt and their toxicity mechanisms must be explained [121,162,171,174,178,179,214,215,216,217,218,219]. Once doped, they display new properties, e.g., electrical conductivity, magnetic, magneto-optical, photocatalytic, antibacterial and optical [74,161,220,221,222,223,224,225,226,227,228,229,230,231,232]. At present, various ZnO nanostructures are being used in attempts to produce a new generation of light-emitting diodes [233,234], lasers [235], field emission devices [236,237], memory carriers [238,239], solar cells [240,241,242,243,244,245,246], liquid crystals [247], polymer nanocomposites [248,249,250,251], food packaging materials [252,253,254,255,256,257,258], transparent ultraviolet light absorbers in unplasticised polymers [259], catalysts [260], photoluminescent NPs [261], photocatalysts [262,263,264,265,266,267,268,269,270,271,272,273], hybrid materials [272,273,274], chemical sensors [275,276], optical biosensors [277], nanofibrous materials [278,279], piezoelectric nanogenerators [280,281], nano-piezotronics [282], supercapacitors [155], flexible and transparent thin-film transistors [154,155], batteries [283,284], photoelectrochemical applications [285], water treatment [286,287,288,289,290,291], water filters [292], solar water splitting [266,293,294,295], functional coatings [296,297,298,299,300,301,302,303,304,305], and in crop cultivation [306,307]. ZnO nanostructures are investigated intensively for the purpose of their application production of chemical and biological sensors [190,308,309].

In agriculture, research is in progress on the application of the antifungal properties of ZnO NPs to protect crops [307,310]. ZnO NPs are used in a broad range of commercial applications, thus it is important to understand their fate and behaviour in soil, water, their manner of absorption and spreading/accumulation in plants, animals and microorganisms, but also interactions with impurities [311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334]. It must be borne in mind that the dynamically increasing the commercial application of ZnO NPs without appropriate supervision and tests may contribute to their adverse impact on land and water plants and animals [335,336].

The properties of nanostructures depend strongly on their size and shape [40,41,42,43,44,45,46,47,48,49,50]. The size-dependent effect of ZnO NPs has been observed on the: photocatalytic activity [337,338,339,340,341,342], catalyst activity [343], dielectric properties [344], piezoelectric property [345,346], breakdown voltage varistors [347], visible emission property of quantum dots displays [348], equilibrium constant of chemical reactions [349], gas sensing properties [350,351,352], thermal diffusivity of water nanofluid [353], photoluminescence [354], UV absorption [355,356], biomedical potential [357], toxicity [312,328,357,358,359,360,361,362], bioavailability [363], and interactions with biomatrices [364]. A nano ZnO material may be characterised by piezoelectric parameters of even a few times higher values than a bulk ZnO material [365]. The photoluminescence, band gap width, conductivity, or magnetic properties, in turn, may be controlled by doping ZnO NPs with transition metal ions (dopant quantity and type): e.g., Co, Mn, Cr, Ni, Fe, V) [74,223,224,225,226,227,228,229,230,231,232]. It was found that the UV sensing performance of doped ZnO improved in term of sensitivity and photoresponse properties compared to non-doped ZnO [366]. One of the goals of research concerning doped ZnO (diluted magnetic semiconductor—DMS) is to enable a combination of communication, memory, and data processing in a single device. They will serve for creating a new generation of smaller, faster and less energy-consuming devices than the present ones, acting based on a combination of conventional micro-electronics and spin-dependent effects [367,368].

In order that ZnO could be more willingly used in products on the industrial scale, the development of ZnO NPs synthesis methods is necessary to enable simultaneous precise control of the average particle size, obtaining narrow particle size distributions and their doping with transition ions. For the purpose of achieving the required performance characteristics, e.g., in pharmaceutical applications, it is also necessary that the material is homogeneous, fully crystalline, and characterised by high purity.

1.4. ZnO Market

It is estimated that the production of NPs of metal oxides in 2020 will be ca. 660 thousand tons, with the estimated production of ZnO NPs alone being between 45 and 56 thousand tons, i.e., ca. 7–8% of the market [115,122]. At present, ZnO NPs are used mainly for producing the following (Figure 5):

-

pharmaceuticals,

-

cosmetics,

-

paints,

-

various coatings,

-

antibacterial products,

-

electronics,

-

and in scientific research.

1.5. Obtaining ZnO Nanomaterials

The synthesis of ZnO NMs with repeatable properties is a quite difficult topic because some of the ZnO nanopowders available on the market are characterised by an insufficient degree of crystallinity, variable morphology, wide particle size distribution, presence of impurities and unrepeatability of properties. In addition, available ZnO nanopowders are mostly composed of a mixture of NPs agglomerates and aggregates, which can be confirmed by a test of average particle size for a dry powder using the laser diffraction method (ISO 13320:2020), while for suspension samples - using the dynamic light scattering method (ISO 22412:2017) [369].

The literature provides numerous chemical, physical and biological methods of producing ZnO nanostructures [73,76,86,154,169,170,171,172,173,174,270,307,370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390]. The following are most frequently enumerated methods of obtaining ZnO NMs [76]: co-precipitation with subsequent calcination, sol-gel method with subsequent calcination, mechanical synthesis combined with high-energy milling, hydrothermal synthesis and solvothermal synthesis. The unrepeatability of ZnO NMs properties arises above all from the limitations of the synthesis methods, unrepeatability of the reactants used and complexity of chemical reactions. The main obstacle in obtaining NMs with intended physicochemical properties is the lack of technologies that ensure the simultaneous control of, among others, average particle size, particle size distribution, shape, phase purity, dopant content, and particle agglomeration and aggregation. The synthesis of nanomaterials is currently at a new stage of development [391,392], at which the main criterion of the approach to obtaining ZnO NMs is understanding the synthesis mechanisms [393,394,395,396,397,398,399,400,401,402]. The familiarity with mechanisms of ZnO NMs synthesis allows obtaining repeatable and reproducible ZnO nanostructures.

Nowadays, when the humans are fully aware of their harmful environmental impact [403,404] and of the climate change on the planet [405], particular attention is drawn to technologies of obtaining such nanomaterials that are environment friendly. One of the most interesting dynamically developing “green technologies” of ZnO nanostructure syntheses is the “microwave assisted synthesis”, which is proved, e.g., by the constant trend of the growing number of publications concerning microwave synthesis of ZnO (Figure 6). At present, the number of scientific papers describing the microwave synthesis of ZnO accounts for ca. 10.9% of the number of all hitherto published papers describing the synthesis of ZnO, while the annual percentage share in 2020 was as high as ca. 16.0%. It must be noted that microwave synthesis is also commonly and willingly employed for obtaining different nanomaterials, e.g., gold (Au) [406,407], silver (Ag) [406,407,408], rhodium (Rh) [406,407], copper (Cu) [406,407], hydroxyapatite (HAp) [409], zirconium dioxide (ZrO₂) [410,411,412], titanium dioxide (TiO₂) [413], silicon oxide (SiO₂) [414], cerium dioxide (CeO₂) [415], tin oxides (SnO and SnO₂) [416], and nanocomposites (i.a. ZrO₂-AlO(OH) [417,418], MoS₂- polydopamine-Ag [419]).

2. Microwave Heating

Microwaves constitute the part of the electromagnetic spectrum with the wavelength (λ) between 1 mm and 1 m (Figure 7), which corresponds to the frequency range between 300 MHz (λ = 1 m) and 300 GHz (λ = 1 mm) [420,421]. Generally, microwaves are commonly used for commercial and military purposes, namely in cellular, Internet, and satellite communications, in radar technologies (radiolocation and radio navigation) and in the food industry (heating and drying). The International Telecommunication Union (ITU) [422] introduced the official division of the microwave range by frequency for the first time in 1953 [423,424]. ITU periodically organises World Radiocommunication Conferences (WRC) [425], during which legal radio regulations, i.e., international agreements controlling the use of the radio frequency spectra, are reviewed and corrected as necessary. In accordance with international agreements, some frequencies were assigned for industrial, scientific and medical purposes to avoid interferences in telecommunication. In each country, a dedicated governmental department deals with the assignment of frequencies and the frequency range as well as the terms and conditions of their use; in Poland, this role is fulfilled by the Ministry of Infrastructure [426]. The division of frequencies for industrial and scientific purposes is presented, e.g., by Torgovnikov [427]. Given the fact of the frequent use of microwave radiation in telecommunication, its individual frequencies were assigned to specific fields, e.g., microwave ovens and equipment employed at a laboratory were assigned the frequency of 2.45 GHz, which corresponds to the wavelength of ca. 12.25 cm.

The interaction of the microwave radiation with a substance may be divided into three types (Figure 8): absorption, transmission and reflection. Polar substances (e.g., solvents: H₂O, ethylene glycol) absorb microwave radiation, as a result of which they are heated. Non-polar substances may display low interactions with microwave radiation, for which they are a microwave transparent medium. The third type includes substances (electrical conductors, e.g., metals) which reflect the microwave radiation from their surface.

The heating of substances by microwave radiation has already been thoroughly described in the literature by numerous scholars [428,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457]. Microwave heating (MH) is based on the transformation of electromagnetic field energy stemming from microwave radiation into kinetic energy (heat) by interacting with the polar particles of the material. MH arising from the electric component of electromagnetic radiation may occur in four ways (Figure 9 and Figure 10): by rotation of dipoles (dipolar polarisation), ionic conduction (ionic polarisation), electronic polarisation (atomic polarisation), and interfacial polarisation (surface polarisation).

The first mechanism of MH consists of the rotation of dipoles having a dipole moment, which try to position themselves in line with the direction and sense of the variable electromagnetic field, which imposes their motion. During the rotation, the microwave radiation energy is converted into kinetic energy, which is transferred between the particles that collide and rub against each other. The result is a uniform spreading of heat in the heated material and a rapid increase in temperature. The second mechanism of MH based on ionic conduction concerns systems (solutions, suspensions) containing ions. Ions present in the microwave field move in line with the direction of the variable electric field. The collision of migrating ions with those moving in the opposite direction causes a heating effect, which is the stronger the higher the concentration and mobility of ions are. The third mechanism of MH constitutes the induction of a dipole moment by shifting the centre of the electron charge in relation to the nucleus. The fourth mechanism of MH is based on the polarisation of a material in the microwave field by accumulation of charges on the interface surface (particle boundaries, phase boundaries), i.e., partial charging of the surface (induced surface charges) through the action of the microwave radiation.

The capability of a substance (solvent) in the conditions of being radiated with microwaves at a given frequency and temperature of converting the microwave radiation energy is determined by the so-called “loss factor” or “loss angle” or “loss tangent”, tanδ. Loss factor is expressed as $\tan \mathsf{\delta }=\frac{{\epsilon }^{″}}{{\epsilon }^{\prime }}$, where ${\epsilon }^{″}$ is the dielectric loss (F·m⁻¹), which is indicative of the efficiency with which electromagnetic radiation is converted into heat and $\epsilon \prime$ is the dielectric constant (F·m⁻¹), which is indicative of the polarisability of molecules in the electric field [428]. The higher the tanδ value, the more efficient the heating of a given substance by microwave radiation will be. The tanδ and $\epsilon$ values depend on the electromagnetic radiation frequency and temperature. The tanδ values of popular pure solvents are presented in

Table 2. tanδ values of different solvents at 2.45 GHz and 20 °C [428,458,459].

High (>0.5)		Medium (0.1–0.5)		Low (<1)
Solvent	tanδ	Solvent	tanδ	Solvent	tanδ
Ethylene glycol	1.350	2-Butanol	0.447	Chloroform	0.091
Ethanol	0.941	Dichlorobenzene	0.280	Acetonitrile	0.062
DMSO	0.825	NMP	0.275	Ethyl acetate	0.059
2-Propanol	0.799	Acetic acid	0.174	Acetone	0.054
Formic acid	0.722	DMF	0.161	THF	0.047
Methanol	0.659	Dichloroethane	0.127	Dichloromethane	0.042
Nitrobenzene	0.589	Water	0.123	Toluene	0.040
1-Butanol	0.571	Chlorobenzene	0.101	Hexane	0.020

. The heating capability (tanδ) of solvents also depends on the substances it contains. The following classification of solvents was adopted depending on tanδ value:

-

low microwave absorbing, where tanδ value <0.1

-

medium microwave absorbing, where tanδ value ranges from 0.1 to 0.5

-

high microwave absorbing, where tanδ value is higher than 0.5.

Generally, microwaves are capable of heating only such solvents with particles being dipoles, e.g., water (H₂O), methanol (CH₃OH), ethanol (C₂H₅OH), ethylene glycol (C₂H₄(OH)₂), diethylene glycol ((C₂H₄OH)₂O), dimethylformamide ((CH₃)₂NCHO), dimethylsulfoxide ((CH₃)₂SO), ethyl acetate (CH₃COOC₂H₅), chloroform (CHCl₃), methyl chloride (CH₂Cl₂), acetic acid (CH₃COOH), acetonitrile (CH₃CN), and acetone ((CH₃)₂CO). Liquid (and solid) substances which do not have particles with the dipole moment and electric charge carriers do not absorb microwave radiation and do not heat. Therefore, such solvents as hexane (C₆H₁₄), benzene (C₆H₆), toluene (C₆H₅CH₃), diethyl ether (C₂H₅)₂O, tetrachloromethane (CCl₄), tetrahydrofuran (C₄H₈O), or polytetrafluoroethylene (PTFE) oil virtually do not heat under the influence of microwave radiation or heat insufficiently. Our experience shows that even a small quantity of an impurity that has a dipole moment and is present in a non-polar solvent may contribute to a considerable improvement of the solvent heating under the influence of microwave radiation.

It must be remembered that during the interaction of microwave radiation with unearthed metals, electric charge carriers (electrons) move under the influence of the electric component of radiation, which leads to metal polarisation. Therefore, unearthed metal subject to microwave radiation will heat intensively and the charges generated on its surface will cause electric discharges and sparking.

2.1. Comparison of Conventional Heating with Microwave Heating

Conventional methods of obtaining nanomaterials employ direct resistance heating and indirect resistance heating. In direct resistance heating, current flows directly through the feedstock itself, causing its heating. This type of heating is not popular because it mostly requires a high current to flow through the feedstock, which flows through the contacts connecting the power source with the feedstock but this causes a range of complications. Indirect resistance heating uses a heat source, i.e., a heating element (e.g., an electric heater). There are two possibilities of applying the heating element: at the direct contact with the feedstock (e.g., electric kettle) or at the indirect contact, where the heating element is in contact with the vessel walls and with the feedstock (e.g., heating mantle or heating jacket). The application of a heating element directly in the feedstock is not popular, which results mainly from heavy contamination caused by corrosion of heater walls and from accumulation of solid synthesis products on the heater surface. Heating jackets are used in the majority of conventional chemical reactors. It must be borne in mind that during conventional heating the heating jacket first heats the walls of a reaction chamber/vessel and subsequently the reaction feedstock, which is shown in Figure 11.

Conventional heating of the reactor feedstock involves the following disadvantages:

(a)

long heating time, which depends on the thermal conduction of the material of which the reaction chamber walls are made;

(b)

temperature maximums occur on the reaction vessel/chamber wall surface, which is one of the direct causes of the heterogeneity of the obtained products (so-called wall effect);

(c)

limited reaction control caused by a high thermal inertia of the system, which results from the heating of the heating jacket and the reaction chamber walls;

(d)

difficulties involved in the speed of the feedstock cooling process;

(e)

high heat losses.

MH is characterised by shorter heating times in relation to the conventional method. With optimum parameters (MH power, resonance structure), feedstock may be heated rapidly and uniformly due to the direct molecular heating by the energy of microwaves. MH is defined as endogenous or volumetric heating [461], which means that heat may be generated within the whole precursor mass (sample inside) rather than transferred from an external heat source (Figure 11). However, it must be borne in mind that microwave radiation does not always have to heat the whole sample volume. When electromagnetic radiation is cast on the surface of a given substance, a part of the radiation may reflect from the surface while the remaining part may penetrate inside the (volume of the) substance. Electromagnetic radiation which penetrated inside interacts with substance molecules and ions. Depending on the properties of a given substance, electromagnetic radiation may penetrate a given substance at various depths, which is illustrated in Figure 12. Penetration depth is the key parameter of each substance in the process of microwave radiation heating. The penetration depth of a field is defined as the distance from the surface to a certain internal point where the magnitude of field strength decreases to 1/e (=36.8%) of the original magnitude at the surface [462]. This parameter is described by the following formula [437]:

$$d_p=\frac{{\lambda }_0}{2\pi }{\left(\frac{1}{2\mu {\epsilon }_0{\epsilon }^{\prime }}\right)}^{1/2}[{(1+{(\tan \mathsf{\delta })}^2{)}^{\frac{1}{2}}-1]}^{-1/2}$$

where $d_p$—penetration depth (cm), ${\lambda }_0$—wavelength at vacuum conditions (cm), ${\mu }_0$—magnetic permeability of free space (H·m⁻¹), ${\mu }^{\prime }$—magnetic relative permeability (H·m⁻¹), ${\epsilon }_0$—permittivity in free space, (F·m⁻¹), ${\epsilon }^{\prime }$—relative permeability (F·m⁻¹), and $\tan \mathsf{\delta }$—loss angle. If considering, e.g., water, the penetration depth of the electromagnetic radiation with the frequency of 2.45 GHz at the temperature of 25 °C for deionised water is 2.88 cm, while for water with an addition of NaCl (0.5 M)—merely 0.45 cm [456] (

Table 3. Penetration depths of the 2.45 GHz microwaves for common solvents and materials.

Material	Temperature (°C)	Penetration Depth (cm)	Ref.
Water (distilled)	20	1.6	[463]
Water (distilled)	25	2.88	[456]
Water (distilled)	100	80	[463]
0.125 M NaCl solution of salt water	25	0.88	[464]
0.5 M NaCl solution of salt water	25	0.45	[456]
2 M NaCl solution of salt water	25	0.14	[464]
Water (ice)	−12	1100	[465]
Ethylene glycol	25	0.46	[464]
Methanol	25	0.68	[464]
Ethanol	25	0.93	[464]
1-propanol	25	1.39	[464]
Acetone	25	7.07	[464]
Ethyl acetate	25	11.05	[464]
Xylene	25	28.32	[464]
Rubber, styrene-butadiene (SBR), vulc.	-	19	[463]
Nitrile rubber, natural	-	65	[463]
Aluminium oxide (Al2O3) ceram, for MW use	-	3000	[463]
Polyethylene	25	4000	[463]
Polyethylene	-	5907.1	[456]
Polystyrene	-	7619.3	[456]
PTFE (Teflon®)	-	9000	[463]
Quartz, pure	-	20,000	[463]
Silver	-	0.33 × 10−4	[463]
Zinc, pure (Zn)	-	1.24·× 10−4	[463]
Copper (Cu)	-	1.3 × 10−4	[456]
Aluminium 100% (Al)	-	0.86 × 10−4	[463]
Aluminium (Al)	-	1.7 × 10−4	[456]
Nickel (Ni)	-	2.7 × 10−4	[456]
Iron (Fe)	-	3.2 × 10−4	[456]
Titanium, pure (Ti)	-	3.3 × 10-4	[463]
Stainless steel (304)	-	4.3 × 10−4	[463]

). When the size of the heated substance is greater than the microwave penetration depth, only a part of the volume of this substance is heated, which is illustrated in Figure 12.

Reaction vessels of microwave reactors are mostly made of polytetrafluoroethylene (PTFE, one of the trade names is Teflon®), which is characterised by a low thermal conductivity (0.25 W/(m·K)) and acts as a thermal insulator, thanks to which a low temperature gradient is maintained in the reaction chamber [461,466,467]. Compared with conventional heating, MH has the following advantages:

(a)

No direct contact of heat source with heated material (contactless method).

(b)

Minimisation of the “wall effect” because the wall of the vessel (reaction chamber) is not heated directly.

(c)

Volumetric heating of the feedstock.

(d)

Instantaneous and precise electronic control. Quick heating switching on and off, e.g., heating process can be controlled with the accuracy of 1 s, namely after switching off the magnetron power unit, the heat source supply is interrupted immediately.

(e)

Rapid heating with preservation of low thermal gradients (rapid energy transfer) [468].

(f)

Heating uniformity (which is translated into the quality of obtained products, e.g., homogeneity and narrow size distribution of obtained nanomaterials [437,438,439,440,441,469,470]).

(g)

Shorter duration of syntheses/processes. Microwave synthesis, depending on type of organic compound obtaining, may be 5–2500 times quicker than in the case of the same syntheses carried out by conventional thermal methods [429,433,471].

(h)

Fewer side reactions [472,473].

(i)

Selectivity and purity of generated products [432,474].

(j)

Easy to conduct under solvent-free conditions [451].

(k)

Very high power densities developed in the processing zone [452].

(l)

Superior moisture levelling [452].

(m)

Energy saving [467].

(n)

Higher production efficiency (faster throughputs) [452].

(o)

Lower apparatus size (compact equipment) [452].

(p)

Shorter time of apparatus start-up.

(q)

A green chemistry approach [475,476,477,478,479,480].

2.2. Application of Microwave Heating, Chemical Microwave Apparatus

Microwave heating is used for various applications, among others:

(a)

food processing (np. home cooking, pasteurisation, drying) [445,446,447];

(b)

industrial application (np. drying, wood curing, rubber curing and vulcanisation, disinfection, coal pre-treatment and processing, ceramic processing, polymer processing, polymeric composites, ceramic composites, melting of glasses, melting of metallic materials, roasting of tea/coffee beans, plant extraction processes) [442,447,448,449,450,451,481,482,483],

(c)

waste treatment (np. medical waste, garbage, sludge) [447];

(d)

medical applications (np. sterilisation, drying, diagnosis, novel methods for treating inoperable tumours) [447,484];

(e)

analytical chemistry (laboratory sample processing, ashing, digestion, extraction, moisture analysis) [432,485];

(f)

chemical reactions: organic and inorganic synthesis (microwave assisted synthesis of e.g., medications, polymers and nanomaterials) [406,407,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,447,455,475,476,477,478,479,480,486,487,488,489,490,491,492,493,494,495].

Recent years have seen a noticeable constant trend of growing popularity of microwave technologies, which is proved e.g., by the increase in the number of publications that were published in the period 2011–2020 (Figure 13). According to the scientific search engine ScienceDirect, 304,138 scientific publications have been released so far which contain a reference to the keyword “microwave” (Figure 13).

The main reason for the increased interest in microwave technologies, which is visible particularly in chemical laboratories, is the dynamic development of microwave apparatus, in particular chemical reactors [428,461,496,497,498,499,500,501], and the decrease in apparatus prices. The use of microwave reactors involves a range of advantages, which are already discussed in the previous Section 2.1 describing the advantages of microwave heating. Control and measurement technologies have reached a very high development level over the past 25 years. Very difficult to control at the beginning of the 21st century, microwave devices are currently equipped with advanced control systems and their operation parameter repeatability matches older and well developed technologies. Microwave chemical reactors of a new generation are characterised by a high repeatability of processes thanks to, among others, the optimisation of reactor structure (batch or flow type) in terms of applied materials, shape and size of the reaction chamber/vessel, recess structure (single synthesis or parallel synthesis), classes of microwave applicators (monomode or multimode) and automation. The capability of monitoring of the real course of reaction (temperature (T), pressure (P)) in microwave chemical reactors is crucial for meeting the requirements of chemical product generation, e.g., in the pharmaceutical industry. New reactor designs enable the rapid cooling of products, e.g., through adiabatic opening of the reactor chamber, which enables freezing the course of reaction over just a few seconds after the finish of the heating process [461,502]. Microwave reactors can always experience a failure, just like other ones [503]. It must be emphasised that microwave ovens, which are often used for conducting chemical reactions, are not professional equipment. Processes performed in microwave ovens are characterised by unrepeatability, which is often related to:

-

random setting of the reaction vessel,

-

random geometry of the reaction vessel (shape and size),

-

impossibility to monitor the course of the process (temperature (T), pressure (P)).

One of the main reasons why microwave chemical reactors are popular is their heating speed. Figure 14 shows the kinetics of distilled water heating in an experimental reactor, MSS3, which was designed and constructed at the Laboratory of Nanostructures, Institute of High Pressure Physics, Polish Academy of Sciences (IHPP PAS). The MSS3 is a reactor intended exclusively for research applications with a reaction vessel made of quartz, which is cooled continuously. The selection of the appropriate microwave power (Figure 14) enables the precise control of kinetics of feedstock heating, which, as was proven later, permits the application of unique features to the synthesised nanomaterials (ZnO NPs) [502]. Differences between the dynamics of heating and cooling are perfectly visible in the chart (Figure 14). It is easier to heat a material because energy is supplied by the power of the microwave field, while cooling takes place through thermal conduction and this process is limited by the thermal conduction of the applied materials.

Table 4. Summary of durations of microwave heating of a 70 mL distilled water sample. Source: Experimental data achieved in the MSS3 reactor (IHPP PAS).

Microwave Power	Heating Time after Which the Temperature Was Reached (s)
	100 °C	140 °C
100 W	443	-
200 W	139	353
300 W	106	171
400 W	61	112
500 W	43	141
600 W	37	61
700 W	31	52
800 W	27	45
900 W	22	41
1000 W	22	39

presents times after which the water sample reached the temperature of 100 °C and 140 °C depending on the applied microwave power.

3. Microwave Hydrothermal Synthesis of ZnO

The hydrothermal synthesis was defined in the literature [504] as a process in a closed system, in which chemical reactions take place exclusively in a water solvent at increased temperatures, at a pressure that is higher than atmospheric pressure (P > 101,325 Pa). This definition is often modified by scholars, e.g., Byrappa and Yoshimura [505] proposed a definition of the “hydrothermal reaction” as any heterogeneous chemical reaction in the presence of a solvent (whether aqueous or non-aqueous) above the room temperature and at a pressure greater than 1 atm in a closed system. For the purposes of this review, we assume that the “hydrothermal synthesis” is a process occurring in an aqueous environment (where mH₂O > 50%), with the pressure equal to or higher than atmospheric pressure. In hydrothermal processes, reaction products are mainly oxides and salts due to the properties of water as a solvent. The results of the literature review concerning microwave hydrothermal synthesis of ZnO are divided into the following subgroups:

(1)

Microwave hydrothermal synthesis of ZnO nanostructures without any additional heat treatment, where the literature review results [506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639] are summarised in

Table 5. Summary of microwave hydrothermal synthesis of ZnO without any additional heat treatment. SSA: specific surface area.

Substrates	Conditions during Preparation	Properties	Ref.
Zn(NO3)2·6H2O (0.1, 0.5 and 2 M), NaOH, H2O	pH: 8–12; T: 100–190 °C; P: 1–13 bar, duration: 2 min–2 h; microwave reactor	SSA: 4.7–18.1 m2/g; particles, submicrometre grains and star-like morphology	[506]
Zn(NO3)2·6H2O (0.1 M), NaOH (2 M), H2O	P: 9–39 bar, duration: 3–7 min; power: 70–100%; microwave reactor (750 W)	heterogeneous nano- and microstructures; particles size: 10–300 nm	[507]
ZnCl2·2H2O (0.1 M), KOH or urea, H2O	pH: 12; P: 10–40 bar; duration: 3–15 min; microwave reactor	SSA: 8.6–102.2 m2/g; particles size: 37–114 nm, flower-like morphology	[508]
Zn(NO3)2·6H2O (0.005 M), hexamethylenetetramine (C6H12N4) (0.005 M), H2O	T: 90 °C; duration: 2 min; microwave reactor	ZnO rods (e.g., bipods, tripods, tetrapods and multipods); diameter: 160–220 nm; length: 1.25–1.3 µm	[509]
Zn(NO3)2·6H2O (0.13 M), NaOH (1.3 M), 1-n-butyl-3-methyl imidazolium tetrafluoroborate, H2O	T: 90–125 °C; duration: 2–10 min; microwave reactor	morphology: flower-like + needle-like, from 60 to 450 nm and lengths up to several micrometres	[510]
Zn(CH3COO)2·2H2O, N,N dimethylformamide, H2O	duration: 23 min; power: 50%; microwave oven	spherical particles ~160 nm and nanoplatelets and nanorods ~2 nm in diameter and ~80 nm in length	[511]
ZnO was dissolved in NH4(OH) and Zn2+ 0.08 M, H2O, NH4(OH) (0.5, 1, 5, 10 and 14.8 M) was obtained	T: 90–150 °C; P: 0.7–4.8 bar; power: 1000 W; microwave reactor	flower-like agglomeration	[512]
Zn(NO3)2·6H2O (0.005 M), hexamethylenetetramine (C6H12N4) (0.010 M), NaOH (3 M)	pH: 9 and 13; T: 96 °C; duration: 60 min;	flower-like ZnO microstructures (2–3 µm) of hexagonal prisms (length: 1–2 μm, diameter: 50–130 nm) with planar and hexagonal pyramid tips (length: 1.5 μm, diameter: 300 nm)	[513]
Zn(NO3)2·6H2O (0.43 M), NaOH (0.43 M), H2O, NaCl, wet mechanical mixtures obtained	T: 75–135 °C; power: 650 W; microwave oven	SSA: 9–13 m2/g; microtubes	[514]
Zn(NO3)2·6H2O (0.05 M), urea (0.05 M), H2O	duration: 40 min; power: 180 W; microwave oven	nanotubes have regular polyhedral shapes, hollow cores with diameters of 100–200 nm, lengths of 1–3 mm and wall thicknesses of 10–40 nm.	[515]
Zn(NO3)2·6H2O (0.43 M), NaOH (0.43 M), H2O	duration: 15 min; T: 75–170 °C; power: 40–450 W; Teflon cell in microwave oven; pulsed mode	SSA: 9–19 m2/g, crystallite size: 30–45 nm	[516]
Zn(CH3COO)2·2H2O and hydrazine (N2H4·H2O) in a molar ratio of 1:4 in H2O	duration: 10 min; power: 150 W; microwave oven	nanorods; diameter about 25–75 nm and length in the range of 500–1500 nm	[517]
Zn(NO3)2·6H2O, NaOH, H2O	duration: 20 min; T: 100–180 °C; power: 0–1000 W; microwave reactor	nanorods; nanowires; nanothruster vanes; nanodandelions; nanospindles	[518]
Zn(NO3)2·6H2O (1.6 M), NaOH (3.2 M), H2O	pH: 8.3; duration: 1–5 min; microwave oven	nanorods (diameter: 100–200 nm) and flower structures	[519]
Zn(NO3)2·6H2O, NaOH (different concentrations), H2O	duration: 1 h; T: 110 °C; microwave oven	submicron starshaped structures, chrysanthemum flower structures, nanoflakes	[520]
Zn(NO3)2·6H2O, pyridine (C5H5N), aniline (C6H5NH2) and triethanolamine (TEA, C6H15NO3) (different concentrations), H2O	duration: 10 min; T: 90 °C; microwave reactor	various morphologies: linear linked needles, regularly hexagonal cross section of a needle, hollow structures, hexagonal nanorings, hexagonal columns, nanosheets	[521]
Zn(CH3COO)2·2H2O, NaOH, 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imid, H2O	duration: 2–7 min; T: power: 255 W; microwave oven (850 W)	nanoparticles, length less than 100 nm	[522]
Zn(NO3)2·6H2O (0.025 M); hexamethylenetetramine (C6H12N4) (0.025 M), H2O	duration: 1–30 min; T: 100–180 °C; power: 120–700 W; microwave oven	nanowires	[523]
Zn(NO3)2·6H2O, NaOH (5 M) nasturtium officinale leaf extract, H2O	pH: 10; duration: 10 min; microwave oven (1000 W)	heterogeneous aggregates of NPs	[524]
Zn(NO3)2·6H2O (0.025 M), Zn(CH3COO)2·2H2O, NH4(OH) (0.16 M), H2O, ethylenediamine (EDA, C2H8N2), hexamethylenetetramine (C6H12N4), triethyl citrate (C12H20O7), tripotassium citrate monohydrate (C₆H₅K₃O₇·H₂O)	duration: 15 min; T: 90 °C; microwave reactor	nanorods, nanoneedles, nanocandles, nanodisks and nanonuts	[525]
Zn(NO3)2·6H2O, hexamethylenetetramine (HMTA), polyvinylpyrrolidone (PVP), H2O	duration: 60 min; T: 100 °C; power: 300 W; microwave reactor	nanostars, average size: ≈625 nm, crystallite size ≈550 nm, SSA = 20.6 m2/g	[526]
Zn(CH3COO)2·2H2O (different concentrations), NH4(OH), H2O	duration: 85 s; T: 90 °C; power: 800 W; microwave oven	flower-like shapes with diameter of 3 to 5 µm, flowers with rod-like nanostructures, spherical particles in 2–4 µm diameter; 2–3 µm structured balls with occasional large 10 µm lumps	[527]
Zn(CH3COO)2·2H2O, NH4(OH) (different concentrations), H2O	pH: 7.0–11.1; duration: 2 h; T: 150 °C; microwave reactor	hexagonally shaped prismatic (width ≈ 1 μm, length ≈ 5 μm); flower-like structures formed by a micron sized crystals; heterogeneous particles (size from ~50 nm to 300 nm)	[528]
Zn(CH3COO)2·2H2O, NaOH, (o- and m- and p)-nitrobenzoic acid, H2O	duration: 10 min; microwave reactor	flower-like products consist of sword-like ZnO nanorods, which were 60–100 nm in width and several micrometres in length.	[529]
ZnCl2·2H2O, NaOH, H2O, bis(dodecyldimethyl ammonium bromide) (C26H56BrN)	duration: 5–10 h; T: 100–140 °C; power: 300–400 W; microwave reactor	flower-like with mean r 0.5–1.5 µm, sphere-like with mean diameter of 0.5 µm	[530]
ZnCl2·2H2O, NaOH, H2O, cetyltrimethylammonium bromide (CTAB, C19H42BrN), Pluronic F127	duration: 5 min; power: 130 W; microwave oven	SSA: 15.5–24.8 m2/g, diameter: 58–93 nm, heterogeneous shape	[531]
Zn(CH3COO)2·2H2O, Na(OH) (different concentrations), H2O	duration: 5 min; power: 450 W; microwave oven	nanoplates flowers	[532]
Zn(CH3COO)2·2H2O, NaOH, H2O, polyethylene glycol, ethanol	duration: 30 min; T: 140 °C; power: 700 W; microwave reactor (multimode)	nanorods, flowers	[533]
Zn(CH3COO)2·2H2O (0.5 M), KOH (2 M), H2O	pH: 8; T: 120 and 140 °C; duration: 8 min; microwave oven (800 W)	multiwires with a flower-like shape of 50–400 nm in width and length	[534]
Zn(CH3COO)2·2H2O (0.1 M), Zn(NO3)2·6H2O (0.1 M), NH4OH, polyvinilpirrolidone, hydrazine hydrate solution (N2H4 in H2O), H2O	pH: 7.5–8; duration: 5–10 min; microwave oven (1000 W)	spherical nanoparticles, stars, flowers	[535]
Zn(NO3)2·6H2O, NaOH, gum arabic (stabilising agent), NaOH, H2O	pH: 10; duration: 5 min; power: 450 W	stars (diameter: 1020 nm), spherical particles (diameter: 240 nm)	[536]
Zn(NO3)2·6H2O, NaOH, gum arabic (stabilising agent), NaOH, H2O	pH: 10; duration: 2–10 min; power: 350 W	aggregates of NPs (20–40 nm), size of aggregates: 150–200 nm	[537]
Zn(NO3)2·6H2O (0.1 M), NH4(OH), hydrazine hydrate (N2H4·H2O), H2O	pH: 8; duration: 10 min; microwave oven	flower morphology consists of sharp nanorods which look like petals	[538]
Zn(NO3)2·6H2O (0.005 M), KOH (4 M), H2O	pH: 12; T: 120 °C; duration: 4 h; microwave oven	nanowires with diameter of 80 nm and lengths of up to 10 µm.	[539]
Zn(NO3)2·6H2O (0.01 M), urea (0.1 M), H2O	T: 120 °C; duration: 10–24 min; power: 150 W; microwave oven	javelins, length: 14–17 µm, width: 0.9–1.4 µm	[540]
Zn(CH3COO)2·2H2O, KOH	duration: 20 min; power: 180 W; microwave oven	flowerlike structures composed of hexagonal ZnO spear-shaped nanorods with diameters and lengths of 50 nm and 2–4 μm,	[541]
ZnCl2·2H2O (different concentrations), NH4(OH), H2O	duration: 10–40 min; power: 10–50%; microwave oven (800 W)	nanorods	[542]
ZnCl2·2H2O, arginine (C6H14N4O2), H2O	T: 120–180 °C; duration: 3–10 min; microwave reactor	rods and flowers	[543]
Zn(CH3COO)2·2H2O (different concentrations), NaOH, H2O	pH: 12; T: 120–140 °C; duration: 15–60 min; power: 0–100%; microwave oven (900 W)	nanosheets	[544]
Zn(CH3COO)2·2H2O, NH4(OH), H2O	pH: 9; duration: 90 sec; microwave oven (900 W)	narcissus-like nanostructures with crystallite sizes of 10–15 nm and average diameter of 1–2.5 µm	[545]
Zn(CH3COO)2·2H2O, NaHCO3, H2O	duration: 15 min; power: 200 W; microwave oven	amorphous material	[546]
Zn(CH3COO)2·2H2O (0.0026 M), NaOH (1 M), H2O, cetyltrimethylammonium bromide (CTAB, C19H42BrN)	T: 130 °C; duration: 30–180 min; Teflon autoclave in microwave oven (800 W)	nanowires and microwires of about 50–400 nm in width and several micrometres in length	[547]
Zn(NO3)2·6H2O (different concentrations), hexamethylenetetramine (C6H12N4), H2O	duration: 2–3 min; power: 600–900 W; Teflon bottle in microwave oven (1000 W)	nanorods diameters from 117 to 156 nm	[548,549]
Zn(NO3)2·6H2O (different concentrations), NaOH, H2O	pH: 7–13.1; duration: 20; power: 180 W; microwave oven	nanoparticles in clusters, nanoplates in flower-like clusters, and spear-shaped particles in flower-like clusters	[550]
ZnCl2·2H2O (0.066 M), NaOH (1.75 M), H2O	pH: 13.75; duration: 5 min; power: 150–1000 W; microwave oven (1000 W)	nanoparticles, nanoneedles, nanosheets (leaf-like)	[551]
Zn(CH3COO)2·2H2O (0.1 M), NaOH (4 M), CH3(CH2)11OSO3Na (0.1 M), C12H25C6H4SO3Na (0.025 M), H2O	T: 75–130 °C; duration: 1–5 h; power: 400 and 700 W; microwave reactor	SSA: 33.1–419.7 m2/g; square shaped sheets	[552]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4)	duration: 30–45 sec; power: 700 W, microwave oven	multiple linked rods such as bipods, tripods (T- shaped), tetrapods (+ and X-shaped), tassel brush and flower shaped and individual rods	[553]
Zn(CH3COO)2·2H2O (different concentrations), triethanolamine (TEA, C6H15NO3), H2O	T: 80–100 °C; duration: 10–30 min; microwave reactor (1000 W)	pompon-like spheres, peach nut-like spheres, misshapen spheres	[554]
Zn(CH3COO)2·2H2O, NaOH, bis(triaminomethyl) carbonate (C3H12N6O3), H2O	pH: 12; duration: 2 min; power: 600 W; microwave oven	flower-like structure (2 µm) composed of petals with average size of about 600–700 nm in length, 300–400 nm in width, and 50–70 nm in tip	[555]
Zn(NO3)2·6H2O, NaOH, H2O	duration: 15–50 min; power: 120–420 W; microwave reactor (700 W)	nanostructures consisted of flower-like, sword-like, needle-like and rods-like structures	[556]
Zn(CH3COO)2·2H2O (different concentrations), NH4(OH), H2O	pH: 10.2; duration: 50–70 s; microwave oven (800 W)	rod-arrays film on glass	[557]
Zn(NO3)2·6H2O, polyvinyl pyrrolidone, NH4(OH), H2O	pH: 10.2; duration: 10 min; microwave oven (1000 W)	star-shaped nanostructures	[558]
Zn(NO3)2·6H2O (0.06 M), NaOH (0.06 M), polyethylene glycols (PEG)-2000, H2O	T: 180 °C; duration: 10, 20, 30, 60 min; microwave reactor	hierarchical structured nanorods	[559]
ZnSO4·7H2O (0.1 M), NaOH (0.4 M)	duration: 2 min; microwave oven	nanoparticles (10–15 nm)	[560]
Zn(CH3COO)2·2H2O, NaOH, 1-butyl-3-ethyl imidazolium tetrafluoroborate (C8H15BF4N2), H2O	T: 120–140 °C; duration: 5 min; microwave reactor (800 W)	calthrop-like framework	[561]
Zn(NO3)2·6H2O (0.03 M), NaOH (0.06 M), H2O, polyethylene glycol (PEG) 2000, H2O	T: 180 °C; duration: 30 min; microwave reactor	rods with the diameter of 300 nm and length of 1 µm	[562]
Zn(NO3)2·6H2O, Zn(C5H7O2)2·xH2O, urea, C2H4(OH)2 (different concentrations), H2O	T: 150 °C; duration: 1–30 min; microwave reactor	microrods with width of 200–300 nm and length of up to 4 µm	[563]
Zn(NO3)2·6H2O, dodecylamine (C12H27N), H2O	T: 80–130 °C; duration: 1–50 min; power: 150 W; microwave oven	hexagonal quasi-hourglasses (tripods, tetrapods, pentapods, multipods)	[564]
Zn(CH3COO)2·2H2O, triethanolamine (TEA, C6H15NO3), NH4(OH), H2O	pH: 6-12; T: 80–160 °C; duration: 10–60 min; power: 150 W; microwave reactor	spherical nanoparticles 60–90 nm, rugby-like nanostructures with diameter of 450 nm and length of about 700 nm	[565]
Zn(CH3COO)2·2H2O, triethanolamine (TEA, C6H15NO3, (different concentrations), NaOH (different concentrations), H2O	pH: 9.0–12; duration: 90 s; power: 900 W; microwave oven	nanospheres with the crystallite size of 57 nm; raspberry-like nanostructures with the crystallite size of 62 nm; hollow nanospheres with the crystallite size of 78 nm; nanoparticles with the crystallite size of 24 nm	[566]
Zn(NO3)2·6H2O (0.1 M), C6H12N4 (0.1 M)	T: 90 °C; duration: 2 h; microwave oven	nanorods on the surface of GaN 80–170 nm, nanorods on the surface of glass 40–100 nm	[567,568,569]
Zn(NO3)2·6H2O, C6H12N4	duration: 5 h; microwave oven	nanorods on the surface of glass, average width of nanorod: 20–1000 nm, average length of nanorod: 150–5000 nm	[570]
Zn(CH3COO)2·2H2O, KOH,	T: 90 °C; duration: 10–30 min; microwave oven	micro-tube structure 200–400 nm in diameter, flower-like structure composed of spear-shaped nanorods with diameters and lengths of 70 nm and 1–5 μm,	[571]
Zn(CH3COO)2·2H2O (different concentrations), NaOH, C2H5OH	pH: 10; T: 100 °C duration: 45–60 min; power: 800 W; microwave reactor	plates (SSA: 10.7 m2/g), rounded plates (SSA: 9.18 m2/g), brush-like (SSA: 9.5 m2/g), flower-like (SSA: 8.5 m2/g)	[572]
Zn(CH3COO)2·2H2O, NH4(OH), H2O	pH: 8; duration: 180 s; microwave oven (900 W)	SSA: 22.9 m2/g; uniform flower-like nanostructures composed of petals attached in the centre with lengths in the range of 700–950 nm and a width in the range of 130–230 nm; each single petal is composed of nanoparticles with lengths of 45–95 nm	[573]
Zn(CH3COO)2·2H2O, NH4(OH), C25N3H30Cl, Polyethylene glycol (PEG) 400, C19H42BrN, H2O	duration: 10 min; microwave reactor	rod-like nanostructures, star-like nanostructures	[574]
ZnSO4·7H2O, NaOH, H2O	pH: 9; duration: 5–25 min; microwave reactor	sheet nanostructures	[575]
Zn(NO3)2·6H2O (0.1 M), NH4(OH), H2O, cetyltrimethylammonium bromide (CTAB, C19H42BrN)	pH: 7; T: 150 °C; duration: 1 h; microwave reactor	nanorods, length: 1–2 μm and width: 100–150 nm	[576]
Zn(CH3COO)2·2H2O, NH4(OH), H2O	duration: 8 min; power: 900 W; microwave oven	nanoparticles (15 nm) which were self-assembled to form a sheet-like structure	[577]
Zn(NO3)2·6H2O, NaOH, polyvinyl alcohol, H2O	duration: 10 min; power: 700 W; microwave oven	nanoparticles (40 nm)	[578]
Zn(NO3)2·6H2O (0.005 M), Zn(CH3COO)2·2H2O (0.005 M), ZnSO4·7H2O (0.005 M), KOH (2 M), H2O	pH: 12; T: 130 °C; duration: 1 h; microwave reactor (800 W)	flower-like structures, plates	[579]
ZnCl2·2H2O (0.5 M), urea, H2O	duration: 5 min; power: 800 W microwave reactor	sponge-like nanostructure	[580]
Zn(CH3COO)2·2H2O, Zn(NO3)2·6H2O, ZnCl2·2H2O, NaOH, KOH, NH4(OH), sodium di-2-ethylhexyl-sulfosuccinate (C20H36Na2O7S), H2O	T: 80–140 °C; duration: 5–20 min; power: 300–1200 W; microwave reactor	hexagonal rods (3–4 μm long and 1 μm wide), hexagonal prismatic, bihexagonal rod-like structure (6 μm long and 2 μm wide), hexagonal prismatic particles (60–80 nm in diameter and length between 90 and 110 nm)	[581]
ZnCl2·2H2O, NH4(OH), H2O	T: 80–140 °C; duration: 20 min; power: 240 W; microwave oven	flower-shaped ZnO microcrystals (about 5 μm)	[582]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), H2O	T: 170 °C; duration: 2–20 min; microwave reactor	irregular sheet-like structures and rods, tripods	[583]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), H2O	T: 90 °C; duration: 3 h; microwave reactor	nanorods on paper	[584]
Zn(NO3)2·6H2O (0.1 M), NH4(OH), albumen, H2O	pH: 8; duration: 5 min; microwave oven	sheet-like and spherical-like nanostructures (13–50 nm), nanowhiskers and nanorods (10–57 nm)	[585]
Zn(NO3)2·6H2O (0.03 M), hexamethylenetetramine (C6H12N4), H2O	duration: 2–30 min; power: 10–100%; microwave oven (1100 W)	nanostructured-films (networked-nanoflakes morphology)	[586,587]
Zn(CH3COO)2·2H2O, NaOH, glutamic tetrofluoroborate (different concentrations), H2O	T: 80 °C; duration: 10 min; power: 1000 W; microwave reactor	clew-like hierarchical nanosheet spheres, nanoneedle-like structures	[588]
Zn(NO3)2·6H2O (different concentrations), hexamethylenetetramine (C6H12N4), H2O	duration: 5–20 s; power: 180-1100 W; microwave oven	growth of nanorods, diameters: 50–80 nm	[589,590,591]
Zn(NO3)2·6H2O (0.03 M), hexamethylenetetramine (C6H12N4), H2O	duration: 30 min; power: 110 W; modified microwave oven (1100 W)	porous nanostructures grown on Al-Si substrate (Al layer thickness form 0 to 150 nm); Al-free Si substrate: nanorods were formed (length: 350 nm, diameter: 50 nm); Al on Si substrate: nanoflake (height ~380 nm ) with pores sizes ranging from 50 nm to several hundreds of nanometres.	[592]
Zn(NO3)2·6H2O (different concentrations), hexamethylenetetramine (C6H12N4, different concentrations), H2O	T: 80 °C; duration: 10 min; power: 30–50%; microwave oven	ZnO nanoarray (rod-like structures) on glass, size control achieved by regulating the parameters	[473]
Zn(NO3)2·6H2O (different concentrations), NH4(OH), H2O	duration: 8 min; power: 800 W; microwave oven	flower-like and rod-like structures	[593]
Zn(CH3COO)2·2H2O (0.45 M), NaOH (8 M), Triton X-100	duration: 1–6 min; power: 100–600 W; microwave oven	rods (400–800 nm), flower structures	[594]
Zn(CH3COO)2·2H2O, KOH, H2O	duration: 3 min; power: 800 W; microwave oven	hexagonal nanorods (length from ~1.5 μm to 3 μm and in diameter from ~30 nm to 80 nm)	[595]
Zn-dust, HNO3, NaOH, polyethylene glycol (PEG, MW 2000), H2O	duration: 10–20 min; microwave oven	SSA: 14.4-21.8 m2/g; particles with irregular shape (plate and rod-like particles), crystallite size: 34–42 nm	[596]
Zn(CH3COO)2·2H2O, NaOH, cetyltrimethylammonium bromide (CTAB, C19H42BrN, different concentrations), H2O	T: 130°; duration: 15–60 min; microwave oven	wire-like architecture with a width in the range of 60–80 nm, flower-like microstructures composed of nanorods, rod has a width of 300–400 nm and a length of 3–4 µm	[597]
Zn(CH3COO)2·2H2O, Zn(NO3)2·6H2O, NaOH, NH4(OH), di-2-ethylhexyl sodium sulfosuccinate (C20H36Na2O7S), H2O	T: 80–140 °C; duration: 5–20 min; power: 300–1200 W; microwave reactor	cauliflower-like structures, hexagonal prismatic type particles (200–300 nm)	[598]
Zn(CH3COO)2·2H2O, Zn(NO3)2·6H2O, NH4(OH), hydrazine hydrate (N2H4·H2O), H2O	pH: 11.5; duration: 10–25 min; power: 510–680 W; microwave oven	nanoparticles, nanorods, flowers	[599]
Zn(CH3COO)2·2H2O, NaOH, H2O	T: 140 °C; duration: 45 min; power: 400 W; microwave reactor (1600 W)	nanorods, diameter ranging from 60 to 80 nm with average length of about 250 nm	[600]
Zn(CH3COO)2·2H2O (0.005 M), NaOH (0.025 M), H2O	duration: 6 min; power: 400–600 W; microwave oven	mixture of nanorods and nanoplates	[601]
Zn(CH3COO)2·2H2O (0.18 M), NaOH (different concentrations), H2O	pH: 7–9.5; T: 50 °C; duration: 3 min; microwave reactor (600 W)	nanorods (width: 80–300 nm, height: 150–1000 nm) grown on Si, GaAs and GaN substrate	[602]
Zn(CH3COO)2·2H2O (0.18 M), NaOH (different concentrations), H2O	pH: 6.75–7.75; T: 50 °C; duration: 2 min; microwave reactor (600 W)	rods (width: 0.5–2.5, height: 1.5–2.2 µm) grown on GaN substrate	[603]
Zn(CH3COO)2·2H2O, NH4(OH), H2O	pH: 10.1–10.9; T: 90 °C; duration: 20 min; power: 100 W; microwave oven	nanorods grown on Si substrate	[604]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4 ), H2O	T: 90 °C; duration: 2 h (switched on and off automatically); microwave oven	nanorods grown on Si substrate (thickness: ~1 µm)	[605]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), H2O	duration: 10–30 min; power: 120 W; microwave oven	rods, bipods (length: 0.46–1 µm, width: 0.1–0.13 µm)	[606]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), H2O, oxalic acid dihydrate (C2H2O4·2H2O)	P: 20.68 bar; duration: 15 min; microwave reactor (300 W)	nanorods or flower grown on paper	[607]
Zn(NO3)2·6H2O, hexamethylene tetramine (C6H12N4), H2O	T: 80 °C; duration: 5–20 min; power: 100–1600 W; microwave reactor (1600 W)	nanorods grown on glass substrate	[608]
Zn(NO3)2·6H2O, hexamethylene tetramine (C6H12N4, different concentrations), H2O	duration: 10 min; power: 240 W; microwave oven	nanorods (diameter: 89–216 nm) grown on glass substrate	[609]
Zn(NO3)2·6H2O, NH4(OH), H2O	pH: 10.0–12.0; T: 90–120 °C; duration: 1 h min; microwave reactor	nanorods grown on glass substrate, control of size of diameter of rods within the size range between circa 125 and 770 nm	[610]
Zn(CH3COO)2·2H2O (different concentrations), hexamethylenetetramine (C6H12N4), H2O	T: 60–110 °C; duration: 5–40 min; microwave reactor	nanorods (diameter from ~30 to ~300 nm, length from ~60 nm to ~520 nm) grown on Si substrate	[611]
Zn(CH3COO)2·2H2O, NH4(OH), NaOH, CH3COOH, H2O	pH: 9.8 or 10.8; duration: five steps (each step included 30 s of irradiation and 10 s off); microwave oven	dandelion-like nanostructures (needles: 50–200 nm, height ~2 µm) or a flower-like microstructures grown on activated carbon cloth	[612]
Zn(CH3COO)2·2H2O, NH4(OH), palmitic acid (CH3(CH2)14COOH), H2O	pH: 4–5; duration: 10–30 min; microwave oven	rod shaped structures	[613]
ZnSO4·7H2O, NH4(OH), H2O	pH: 10; duration: 2 min; power: 600 W; microwave oven	nanoparticles (~50 nm)	[614]
Zn(NO3)2·6H2O, potassium sodium citrate, NaOH, H2O	T: 90 °C; duration: 2 min; power: 600 W; microwave oven (650 W) with a refluxing apparatus	sphere-like particles (~2.32 μm)	[615]
tris(ethylenediamine)zinc nitrate ([Zn(en)3](NO3)2), NaOH, H2O	pH: 7–12; T: 180 °C; duration: 20 min; power: 400 W; microwave reactor (1600 W)	nanorods, diameter: from 40 nm (pH 12) to 600 nm (pH 7)	[616]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), H2O	T: 100 °C; duration: 60 min; power: 100 W; microwave reactor	nanorods (180–350 nm) grown on glass	[617]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), polyethylenimine, NH4OH, H2O	T: power on and off in order to control the solution temperature; duration: 6–10 × (30–60 s “on” and 5 min “off”); microwave oven (350 W)	nanorods (180–350 nm) grown on glass	[618]
Zn(CH3COO)2·2H2O, NH4(OH), carbon fibre, H2O	duration: 3 × (30 s irradiation and 30 s stop); microwave oven (1120 W)	rods (diameter: 0.3–0.5 µm, length: 1.0–1.5 µm) grown on carbon fibre	[619]
Zn(NO3)2·6H2O, NaOH, NH4(OH), polyethylene glycol (PEG, MW 400), H2O	T: 100 °C, duration: 5 min; microwave oven (800 W)	quasi-spherical shape and dimensions of less than 5 μm, flower-like structures (>5 μm),	[620]
Zn(NO3)2·6H2O, C6H12N4, polyethylenimine, NH4(OH), H2O	duration: 5–15 min; microwave oven (800 W)	nanoflowers and nanowalls grown on P–Si	[621]
Zn(CH3COO)2·2H2O, NaOH, 1-hexyl-2-ethyl-3-methylimidazoliumtetrafluoroborate (C6H11BF4N2), H2O	duration: 2–9 min; power: 30%; microwave oven	flakes-shaped particles, flower-like shaped particles	[622]
Zn(NO3)2·6H2O, NaOH, plant extract, H2O	pH: 10; duration: 15 min; microwave oven	nanoparticles	[623]
Zn(NO3)2·6H2O, (C6H12N4)2, H2O	T: 70–130 °C; duration: 10 min; microwave reactor	nanorods on paper, length 120–480 nm, thickness 55–75 nm	[624]
Zn(NO3)2·6H2O, triethanolamine (TEA, C6H15NO3), H2O	duration: 10 min; power: 640 W; microwave oven	nanoparticles	[625]
Zn(CH3COO)2·2H2O, KOH, triethanolamine (TEA, C6H15NO3), 1,2,4,5-benzenetetracarboxylic acid, H2O	pH: 8–12; T: 150 °C; duration: 30 min; power: 800 W; microwave reactor	dumbbell-like structures, football-like structures, hexagonal bi-pyramidal structures, SSA: 7–24 m2/g; size 50 nm–10 μm	[626]
Zn(CH3COO)2·2H2O, NaOH, H2O	pH: 8–10; duration: 6 min; microwave oven (700 W)	sheet-like structures and uniform microstructures	[627]
Zn(CH3COO)2·2H2O, Tris (hydroxymethyl) aminomethane (C4H11NO3), H2O	duration: 3 min; microwave oven (300 W)	spherical nanoparticles	[628]
Zn(CH3COO)2·2H2O, KOH, benzene-1,2-dicarboxylic acid, benzene-1,3-dicarboxylic acid, benzene-1,4-dicarboxylic acid, H2O	pH: 7–12; T: 150 °C; duration: 30 min; power: 800 W; microwave reactor	rod-like structures, needle-like structures, platelet-like structures, hexagonal columnar shape of the particles, rice-grain shape structures	[629]
Zn(CH3COO)2·2H2O (0.1 M), NaOH (0.1 M), {4-[(E)-2-(furan-2-yl)ethenyl]pyridin-1-ium-1-yl} acetate (1 wt% and 3 wt%), CH3OH, H2O	duration: 20 min; microwave oven	heterogeneous shape of nanoparticles, size: 200–800 nm, average crystallite size: 21–23 nm	[630]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), NH4(OH), H2O	pH, 6.8–13, duration: 10–15 min; pulsed microwave heating in microwave oven (850 W)	rod, flower, star, tetrapod	[631]
Zn(NO3)2·6H2O (different concentrations), hexamethylenetetramine (C6H12N4), H2O	T: 105 °C; duration: 10–30 min; microwave oven (850 W)	growth of nanorods on P-type silicon wafer, diameters: from 26–32 nm to 35–40 nm, lengths: from 330 nm–607 nm	[632]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), H2O	duration: 20 min; power: 750 W; microwave oven	growth of nanorods on silicon substrate, diameters ~80 nm, lengths ~500 nm	[633]
Zn(NO3)2·6H2O, coffee powder extract, H2O	duration: 5 min; power: 540 W; microwave oven	spherical nanoparticles (80–120 nm)	[634]
Zn(NO3)2·6H2O, tomato extract, H2O	duration: 5 min; power: 180–540 W; microwave oven	spherical nanoparticles (40–100 nm)	[635]
Zn(NO3)2·6H2O, tea leaf extract, H2O	duration: 7 min; power: 540 W; microwave oven	spherical nanoparticles (26 nm)	[636]
Zn(CH3COO)2·2H2O, Longan fruits extract, H2O	duration: 1 min on & 1 min off irradiation cycle (1–30 cycles); power: 450–800 W; microwave oven	SSA: 35 m2/g, diameter: 10–100 nm, heterogeneous shape	[637]
Zn(CH3COO)2·2H2O (0.22 M), carbinol, H2O	duration: 5–15 min; power: 900 W; microwave oven	spherical nanoparticles, diameters: 30 nm–50 nm; hexagonal facetted nanostructures, average size: 400–450 nm	[638]
Zn(NO3)2·6H2O, NH4(OH) (28%), H2O	pH: 12; duration: 5–25 min; power: 180–540 W; microwave oven (1200 W)	spherical and flower-like particles on paper; non-uniform size	[639]
ZnO powder, hydrogen peroxide (H2O2, 30%)	P: 30 bar; duration: 15 min; power: 1200 W; microwave oven (1200 W)	rod-like nanostructures, average size: 36 nm	[640]

.

(2)

Microwave hydrothermal synthesis of ZnO nanostructures with additional heat treatment, where the literature review results [585,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673] are summarised in

Table 6. Summary of microwave hydrothermal synthesis of ZnO nanostructures with additional heat treatment.

Substrates	Conditions during Preparation	Parameters of Additional Heat Treatment	Properties	Ref.
Zn(CH3COO)2·2H2O, NH4OH, H2O	duration: 5–10 min; power: 240–400 W; microwave oven	500 °C in air for 1 h	dumbbell-shaped structures built of particles sized ~100 nm	[641]
Zn(CH3COO)2·2H2O, KOH, H2O	duration: 15 min (irradiation 12 s, stop 10 s); power: 180 W; microwave oven	400 °C in air for 1 h	nanorods assembled in flower shaped, rods: diameter 150–190 nm (tip diameter ~15 nm), length 2 μm, with an aspect ratio of 20–22	[642,643]
Zn(CH3COO)2·2H2O, NaOH, guanidinium carbonate, acetyl acetone (ACAC), H2O	pH: 8–12; duration: 2 min; power level: 75%; microwave oven	without and 600 °C in air for 2 h	petals: length 600–700 nm, width 300–400 nm, tip 50–70 nm; rod-like nanostructures diameters 60–90 nm maximal length 1.5 µm; spherical-like nanostructures: diameter 50 nm	[644]
Zn(CH3COO)2·2H2O (0.2 M), NaOH (0.4 M), H2O, triethanolamine (TEA, C6H15NO3)	duration: 20 min; power: 20%; microwave oven	900 °C in air for 1 h	spherical particles ~50 nm	[645]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), H2O	duration: 2 h; microwave oven	250–550 °C for 1 h in oxygen flow (5 cm3/min)	nanorods (diameter from 50–300 nm) grown on surface of silicon substrates	[568]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), polyethylenimine, NH4OH, H2O	duration: 4–80 min; power: 180–850 W; microwave oven	350 °C for 20 min in air	nanowire grown on an ITO-coated glass substrate	[646]
Zn(CH3COO)2·2H2O, NaOH, H2O	duration: 10 min; power: 200 W; microwave oven	400–800 °C for 1 h in air	circular- and hexagonal-shaped particles	[647]
Zn(CH3COO)2·2H2O, sodium dodecyl, NH4OH, (CH3)2CHOH (2-propanol) H2O	duration: 2 h; power: 100–800 W; microwave oven	500 °C for 3 h in air	flakes-like structures, spherical-like, crystallite size: 31–39 nm (morphology dependent on the microwave power)	[648]
Zn(CH3COO)2·2H2O, ZnCl2·2H2O, Zn(NO3)2·6H2O, pyridine (C5H5N), H2O	pH: 13.75; duration: 2–5 min; power: 1000 W; microwave oven	300–500 °C for 2 h in air	nanoparticles, nanoflowers, nanorods	[649]
Zn(CH3COO)2·2H2O, urea, H2O	T: 220 °C; duration: 15 min; microwave oven	400 °C for 90 min in air	nanosheets	[650]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), NaOH, H2O	pH: 13; T: 90–220 °C; duration: 15 min; power: 110–710 W; microwave reactor (1400 W)	200 °C for 2 h in air	nano-platelets	[651]
Zn(CH3COO)2·2H2O, trisodium citrate dihydrate (Na3C6H5O7·2H2O) (different amounts), NH4OH, H2O	T: 90 °C; duration: 10–45 min; power: 300 W; microwave reactor	400 °C for 4 h in air	hollow microspheres average dimensions ~4 μm; thickness 400–600 nm.	[652]
Zn(NO3)2·6H2O, NH4OH, albumen, H2O	pH: 8; duration: 5 min; microwave oven (1000 W)	130 °C for 5 h in air	whisker-like and rod-like nanostructures: thickness 10–57 nm	[585]
Zn(NO3)2·6H2O, KOH, H2O	pH: 9–13; duration: 30 min; power: 180 W microwave oven	200 °C for 2 h in vacuum	nanorods	[653]
Zn(NO3)2·6H2O, urea (different concentrations), H2O	T: 90 °C; duration: 40 min; power: 400 W microwave oven	400–600 °C for 3 h in different atmospheres (O2, N2, H2 and air),	various flower-like nano and microstructures, urchin-like structures	[654]
Zn(CH3COO)2·2H2O, NH4OH, hexamethylenetetramine ((CH2)6N4), hexadecyl trimethyl ammonium bromide, polyethylene glycol (PEG400), H2O	duration: 10 min; power: 300 W; microwave reactor	500 °C for 2 h in air	needle-assembled structures with flower-like morphology; bundle-like microstructures assembled by nanorods; flower-like hierarchical structures composed of some tight aggregations	[655]
Zn(CH3COO)2·2H2O, trisodium citrate dihydrate (Na3C6H5O7·2H2O), urea, H2O	T: 140 °C; duration: 20 min; microwave reactor	500 °C for 2 h in air	porous core–shell microstructures	[656]
Zn(NO3)2·6H2O, NaOH, H2O,	pH: 13; duration: 1–5 min with 30 s on–off cycling mode; power: 900 W; microwave oven	400 °C for 1 h in air	microspheres	[657]
Zn(CH3COO)2·2H2O, NaOH, H2O,	pH: 12.66–13; duration: 10–30 min; power: 300 W; microwave reactor	500 °C for 2 h in air	nanorods, nanoplates	[658]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), H2O	duration: 45 min; power: 1800 W; microwave oven	350 °C for 1 h in air	nanorods, length: ~4.3 ± 0.2 µm, diameter: 100 ± 10 nm	[659]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4) (different concentrations), H2O	duration: 10 min; power: 750 W; microwave reactor	400 °C for 1 h in air	shape from random spherical to highly conserved hexagonal shaped rods, size: from ~25 nm to µm/sub µm	[660]
Zn(NO3)2·6H2O, hexamethylenetetramine (C6H12N4), hydrazine hydrate (N2H4), H2O	pH: 10; duration: 10–30 min; microwave reactor (1000 W)	500 °C for 2 h in air	nanorods, length: 579–909 nm, diameter: 116–240 nm	[661]
Zn(CH3COO)2·2H2O, Tuber (Amorphophallus konjac) extract, H2O	duration: 5 min; microwave oven	400 °C for 1 h in air	rice shaped nanoparticles, length: 237 nm, diameter: 76 nm	[662]
ZnCl2, NaOH, H2O	pH: 6.1–13.7; T: 80 °C; duration: 5–20 min; microwave reactor (900 W)	150 °C for 3 h in air	hexagonal flake, velvet flower-like, rough globular, needle bunch-like, cauliflower-like, clew-like, nanorods, and rhombic microstructures	[663]
Zn(CH3COO)2·2H2O, ZnCl2·3H2O, Zn(NO3)2·6H2O, ZnSO4·7H2O, NH4OH, H2O	duration: 10 min; microwave reactor	600 °C for 2 h in air	nanoflakes, nanorods, hexagonal tubular, pseudo-spherical	[664]
Zn(NO3)2·6H2O, NaOH, H2O (solvent) and C2H5OH (solvent)	P: 20–40 bars; duration: 15 min; microwave reactor	750 °C for 1 h in air	particles, irregular shape	[665]
ZnCl2·3H2O, H2O, microcrystalline cellulose	T: 100 °C; duration: 10–60 min; microwave oven	600 °C for 3 h in air	heterogeneous nanostructures and microstructures	[666]
Zn(CH3COO)2·2H2O, ZnCl2·3H2O, ZnSO4·7H2O, ZnCO3, Psidium guajava Linn. Extract, H2O	T: 100 °C; duration: varying cycles of 3 min-on and 1 min-off min; power: 720 W; microwave oven	900 °C for 1.5 h in air	diameter: 60–180 nm	[667]
Zn(NO3)2·6H2O, pelargonium leaf extract, H2O	duration: 3 min; power: 800 W; microwave oven	400 °C for 2 h in air	heterogeneous particles	[668]
ZnSO4·7H2O (different concentrations), banana corm extract, H2O	duration: 15 min; power: 540 W; microwave oven	400 °C for 3 h in air	microparticles	[669]
Zn(CH3COO)2·2H2O, KOH, H2O	duration: 0.5–2 min; microwave oven (1000 W)	400 °C for 1 h in air	nanorods, tetrapods (length: 255 nm), flowers (petal length: 387 nm)	[670]
Zn5(OH)8(NO3)2·2H2O, 1,2,3- trimethyl-imidazole tetrafluoroborate, H2O	duration: 2 h; power: 180 W; microwave oven (900 W)	400 °C for 2 h in air	nanobelts, width: 500–800 nm; length: several micrometres, thickness: 100 nm	[671]
Zn(CH3COO)2·2H2O, NH4OH, C2H5OH, H2O	T: 120 °C; duration: 10 min; power: 180 W; microwave reactor	527 °C in air	heterogeneous hollow NPs, SSA: 17.1 m2/g	[672]
Zn(NO3)2·6H2O, NaOH, polyethylene glycols (MW= 1500 and MW= 4000), sugar, cassava starch, H2O	duration: 10 min (with 5 s/15 s on/off step); power: 320 and 480 W; microwave oven	450 °C for 1 h in air	rod-like structures and needle-like structures, length: 300–3000 nm	[673]

.

(3)

Microwave hydrothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures without any additional heat treatment, where the literature review results [674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723] are summarised in

Table 7. Summary of the microwave hydrothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures without any additional heat treatment.

Type of Composite	Substrates	Conditions during Preparation	Properties	Ref.
Mn (5%) doped ZnO	Zn(CH3COO)2·2H2O, Mn(CH3COO)2·4H2O, NaOH, polyvinylpyrrolidone	T: 60 °C, power: 700 W; microwave oven	nanoparticles sized 10–59 nm	[674]
Mn (5–70% wt%) doped ZnO	Zn(NO3)2·6H2O, Mn(NO3)2·4H2O, KOH, H2O	P: 38 bars; duration: 15 min; microwave reactor	nanoparticles, irregular spherical shape, ZnO and ZnMn2O4 phases, size: 33–99 nm	[675]
ZnO/ZnMn2O	Zn(NO3)2·6H2O, Mn(NO3)2·4H2O, KOH, H2O	P: 38 bars; duration: 15 min; microwave reactor	ZnO (30 wt%)-MnO (70 wt%); nearly spherical in shape and agglomerated to the form of irregular clusters; crystallite size of ZnO and ZnMn2O4 was 99 and 27 nm, respectively; SSA = 25 m2/g	[676,677]
Fe (0.18, 1.70 and 3.05 at%) doped ZnO	ZnSO4·7H2O, FeSO4·7H2O, NaOH, H2O	power: 140 W; microwave oven	nanorods length ~1 µm and diameter in the range of ~50 nm	[678]
Fe doped ZnO	Zn(CH3COO)2·2H2O, Fe(NO3)3⋅9H2O, NH4(OH), H2O	T: 80 °C; power: 400 W; duration: 40 min; microwave oven	star-like structure (~433 nm)	[679]
ZnO/ZnFe2O4, Fe (5–95%) doped ZnO	Zn(NO3)2·6H2O, Fe(NO3)2·4H2O, KOH, H2O	P: 39 bars; duration: 15 min; microwave reactor	particles, irregular spherical shape, diameter 4–13 nm	[680,681,682,683,684,685]
Ce (0–0.15 mol) doped ZnO	Zn(CH3COO)2·2H2O, Ce(SO4)2, NaOH, H2O	pH: 13; duration: 10 min; power: 55%; microwave oven (1000 W)	nanosheets	[686]
Ce doped ZnO	Zn(CH3COO)2·2H2O, Ce(CH3COO)3·xH2O, NH3·H2O, H2O	pH: 9.5–11; duration: 5–15 min (10/20 s power on/off); power: 200 W; microwave oven	heterogeneous particles (2% of dopant)	[687]
CoO doped ZnO	Zn(NO3)2·6H2O, Co(NO3)2·4H2O, KOH, H2O	duration: 15 min; pressure: 38 bar; microwave reactor	CoO content from 5 to 50%, particles >100 nm	[688,689,690,691]
K (0–5 mol%) doped ZnO	Zn(NO3)2·6H2O, KNO3, H2O	T: 160 °C; duration: 30 min; microwave reactor	lamellar-like and granule-like structures (100–300 nm)	[692]
Ce (5 wt%) doped ZnO, Ce (5 wt%) doped carbon nanotube/ZnO	Zn(CH3COO)2·2H2O, Ce(SO4)2, NaOH, H2O, commercial multi-walled carbon nanotubes (length 5–9 μm, diameter 110–170 nm)	pH: 11; duration: 5 min; power: 450 W; microwave oven	nanorods on carbon nanotubes	[693]
Eu doped ZnO	Zn(NO3)2·5H2O, Eu(NO3)3·5H2O, NH4(OH), H2O	pH: 10; pressure: 1–100 bar; duration: 20 min; microwave reactor	nanoplate-like structures, elongated hexagonal prisms	[694]
Cu doped ZnO	Zn(CH3COO)2·2H2O, Cu(CH3COO)2·2H2O, NaOH, H2O	T: 100 °C; duration: 20 min; microwave reactor	Zn1−xCuxO (x = 0.00, 0.01, 0.02, 0.03, and 0.04), particles	[695]
Ga doped ZnO	Zn(CH3COO)2·2H2O, Ga(NO3)3·xH2O, NH4(OH), H2O	pH: 10; duration: 15–20 min; microwave oven (850 W)	Zn(1−x)GaxO (x = 1, 2, and 5 mol%), nanorods are grown on p-Si substrates	[696]
Ga doped ZnO	Zn(CH3COO)2·2H2O, Ga(NO3)3·xH2O, KOH, H2O	T: 180 °C; duration: 15 min; power 250 W; microwave reactor (1900 W)	Zn(1-x)GaxO (x=0, 0.05, 1.00, 3.00 mol%), micron-sized rods (slightly above 1 mm in length)	[697]
Cd doped ZnO/ carbon nanotube	commercial carbon nanotube diameter 10–20 nm and length 30 μm (modified HNO3), Zn(CH3COO)2·2H2O, Cd(NO3)2·5H2O, NH4(OH), H2O	pH: 10; duration: 6 min; power: 490 W; microwave reactor (1000 W)	growing Cd doped ZnO nanoparticles over the surface of carbon nanotube	[698]
Sr doped ZnO	Zn(NO3)2·6H2O, Sr(NO3)2·6H2O (different concentrations), NaOH, H2O	T: 160 °C, duration: 30 min; microwave reactor	Zn1−xSrxO (x = 0.00, 0.001, 0.002, and 0.003), ZnO - lamellar structures, size: 200–300 nm; Zn1−xSrxO - heterogeneous granule nano and microstructures	[699]
ZnO/ZnS	ZnCl2·2H2O, NH4(OH), thioacetamide, H2O	duration: 30 min; power: 400 W; microwave refluxing system	core–shell nanorods	[700]
Au-decorated ZnO	Zn(NO3)2·6H2O, hexamethylenetetramine, NaOH, HAuCl4·3H2O, sodium citrate dihydrate, H2O	duration: 10 min; power: 475 W; microwave oven	growing Au NPs (~20 nm) over the surface of ZnO nanorods (diameter of 162 nm and an average length of 1.27 μm)	[701]
Au-decorated ZnO	Zn(CH3COO)2·2H2O, Ag(NO3)·6H2O, C6H12N4, H2O	duration: 5 min; microwave oven	ZnO particles sized up to 2 μm and spherical particles are sized up to 200 nm	[702]
Ag-decorated ZnO	ZnO, Ag(NO3), cetyltrimethylammonium bromide (CTAB, C19H42BrN), H2O	duration: 20 min; power: 900 W; microwave oven	spike-like nanostructures, length: few microns, diameter: 50–100 nm	[638]
Ag-ZnO	Zn(NO3)2·6H2O, Ag(NO3), C6H12N4, H2O	T: 120 °C, duration: 30 min; power: 400 W; microwave reactor	star-like structures (up to 2 μm), rod-like structures (~0.5 μm)	[703]
Ag-ZnO	Zn(CH3COO)2·2H2O, AgNO3, C2H4(OH)2 (ethylene glycol, EG), Na2O2, H2O	duration: 5 min; power: 400 W, microwave oven	flower-like structures Ag-ZnO with a molar ratio of 0:100; 2:98; 4:96; 6:94; 8:92; 10:90	[704]
Ag-ZnO	ZnO (nanorods 10–20 nm), AgNO3, glucose, H2O	pH: 7; duration: 10 min; microwave oven	Ag/ZnO nanoparticles (ZnO nanorods 10–20 nm; Ag nanoparticles ~60 nm)	[705]
Ag-ZnO-clay composite, ZnO-clay composite	chemically activated bentonite clay, AgNO3, ZnO NPs, H2O	T: 80 °C, duration: 20 min; power: 500 W; microwave reactor	nanoparticles in clay, ZnO NPs size 15–70 nm, Ag NPs size 9–30 nm, SSA(ZnO-clay) = 33 m2/g, SSA(Ag-ZnO-clay) = 25 m2/g	[706]
ZnO-reduced graphene oxide	graphite (modified Hummers method), ZnSO4·7H2O, NaOH, H2O	pH: 8; T: 150 °C, duration: 10 min; microwave reactor	graphene nanosheets are decorated densely by ZnO nanosheets (20–30 nm),	[707]
ZnO-reduced graphene oxide	graphite (modified Hummers method), ZnSO4·7H2O, NaOH, H2O	pH: 9; T: 150 °C, duration: 30 min; microwave reactor	graphene sheets packed by nanosized and irregularly shaped ZnO nanoparticles	[708]
ZnO-reduced graphene oxide	graphite (modified Hummers method), Zn(NO3)2·6H2O, NaOH, H2O	pH: 9–11; T: 100 °C, duration: 30 min; microwave reactor	reduced graphene oxide sheets with wrinkles and folds are decorated densely by the ZnO nanorods	[709]
ZnO-reduced graphene oxide	graphite (modified Hummers method), Zn(CH3COO)2·2H2O, HN(CH2CH2NH2)2, NaOH, H2O	pH: 13; duration: 30 min; microwave oven	reduced graphene oxide sheets with ZnO rod-like (diameter ~100 nm and length ~1 μm)	[710]
ZnO-reduced graphene oxide	graphite (modified Hummers method), ZnCl2·2H2O, NaOH, H2O	duration: 5 min; power: 450 W; microwave oven	nanowires of ZnO were decorated/anchored on the surface of graphene oxide	[711]
ZnO-reduced graphene oxide	graphite (modified Hummers method), Zn(CH3COO)2·2H2O, NaOH, H2O	duration: 20 min; power: 450 W; microwave oven	reduced graphene oxide sheets with ZnO nanoparticles (10–20 nm)	[712]
ZnO-reduced graphene oxide	graphite (modified Hummers method), Zn(CH3COO)2·2H2O, KOH, H2O	T: 100 °C; duration: 8 min; power: 450 W; microwave reactor	reduced graphene oxide sheets with ZnO nanoparticles (irregular elongated shapes and agglomerates with particle size of 122 nm)	[713]
ZnO, ZnO-reduced graphene oxide	graphite (modified Hummers method), Zn(NO3)2·6H2O, hexamethylenetetramine (HMTA), polyvinylpyrrolidone (PVP), H2O	T: 100 °C; duration: 8 min; power: 450 W; microwave reactor	reduced graphene oxide nanosheets with ZnO nanostars, SSA = 34.3 m2/g, size of ZnO nanostars: ~625 nm	[526]
ZnO-TiO2-reduced graphene oxide	graphite (modified Hummers method), commercial TiO2, ZnSO4·7H2O, NaOH, H2O	pH: 9; T: 150 °C, duration: 10 min; power: 150 W; microwave reactor	reduced graphene oxide nanosheets are decorated by ZnO nanosheets and TiO2 nanoparticles	[714]
ZnO-MOF-reduced graphene oxide	zeolitic imidazolate framework-8, graphite (modified Hummers method), ZnO, H2O2, H2O	T: 150 °C, duration: 10 min; power: 150 W; microwave reactor	ZnO-MOF-reduced graphene oxide with 0, 0.5, 1.0, 1.5 and 2 wt% reduced graphene oxide	[715]
ZrO2-ZnO	ZnSO4·5H2O (1 M), Na2CO3 (2 M), ZrOCl2·8H2O (1 M), H2O	T: 180 °C, duration: 10 min; microwave reactor	needle-shaped micro- and nanoparticles, mass concentrations of ZrO2: 1%, 5%, 10%, 20%	[716]
ZrO2 coated ZnO	ZnO NPs (diameter: 12–25 nm, 38.4 m2/g), (Zr(SO4)2·4H2O), NH4OH	T: 70 °C, duration: 5 min; microwave reactor (500 W)	core-shell nanocomposites	[717]
ZnO chitosan/ZnO	Zn(NO3)2·2H2O, NaOH, chitosan, H2O	duration: 4–8 min; power: 400–800 W; microwave oven	nanoparticles, diameter: 32–82 nm	[718]
Fe3O4/ZnO/AgBr	Zn(NO3)2·2H2O, FeCl3, AgNO3, NaBr, NH4OH, NaOH, C2H5OH, malt extract agar (MEA), H2O	duration: 10 min; power: 550 W (55%); microwave oven (1000 W)	composition (weight ratio): Fe3O4/ZnO (1:8), Fe3O4/ZnO/AgBr (1:2; 1:4; 1:6; 1:8; 1:10); in homogeneous oval particles	[719]
MoS2/ZnO, ZnO	ZnCl2·2H2O, Na2MoO4·2H2O, C2H5NS, H2O	T: 60 °C; duration: 20 min; power: 140 W	heterogeneous nano- and microstructures	[720]
ZnO doped CeO2	ZnCl2·2H2O, Ce(NO3)3·6H2O; NH4OH, H2O	pH: 8; T: 200 °C, duration: 30 min; microwave reactor (1500 W)	homogeneous spherical particles (diameter ≈ 5–40 nm)	[721]
mesoporous Si@ZnO	Zn(NO3)2·6H2O, 3-aminopropyl trimethoxy silane (APTMS, (CH3O)3-Si-(CH2)3NH2), LiOH, H2O	T: 80 °C; duration: 30 min; power: 300 W; microwave reactor (1000 W)	APTMS/ZnO molar ratios: 0.15, 0.2, 0.3 and 0.4.; agglomerate size ≈ 200 nm	[722]
ZnO/zinc aluminium hydroxide	Zn(CH3COO)2·2H2O, AlCl3, NH4OH, tripotassium citrate monohydrate (HOC(COOK)(CH2COOK)2·H2O, H2O	T: 95 °C; duration: 20 min; microwave reactor	ZnO nanorod on zinc aluminium hydroxide heterostructures; sunflower-like ZnO nanorods on zinc aluminium hydroxide heterostructures; ZnO nanotubes on zinc aluminium hydroxide heterostructures; ZnO film on zinc aluminium hydroxide heterostructures	[723]

.

(4)

Microwave hydrothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures with additional heat treatment, where the literature review results [724,725,726,727,728,729,730,731,732,733,734,735,736,737,738] are summarised in

Table 8. Summary of the microwave hydrothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures with additional heat treatment.

Type of Composite	Substrates	Conditions during Preparation	Parameters of Additional Heat Treatment	Properties	Ref.
ZnO/TiO2	Zn(NO3)2·6H2O, C4K2O9Ti·2H2O, NH4(OH), H2O	pH: 10; duration: 40 min; power: 180 W; microwave oven	without and T: 500–700 °C; duration: 3 h	without calcination: nanoparticles <10 nm; after calcination: nanoparticles 25 nm and 80–100 nm	[724]
ZnO/TiO2	Zn(CH3COO)2·2H2O, titanium isopropoxide, NaOH, H2O	T: 180 °C; duration: 5 min; microwave reactor	T: 500–800 °C	rods: diameters: 150–250 nm, length: 1000–2000 nm	[725]
ZnO/CuO	Zn(CH3COO)2·2H2O, Cu(CH3COO)2·2H2O NaOH, H2O	duration: 15 min; power: 450 W; microwave oven	T: 800 °C; duration: 7 h	nanorod like structures	[726]
Cr doped ZnO	Zn(CH3COO)2·2H2O, (Cr(CH3COO)2)2·2H2O, NaOH, H2O	duration: 5 min; power: 400 W; microwave oven (1350 W)	T: 700 °C; duration: 2 h	particles (~100 nm)	[727]
Cr doped ZnO	Zn(NO3)2·6H2O, Cr(NO3)3·9H2O, citric acid (C6H8O7), H2O	microwave oven (650 W)	250 °C for 1 h in air	doped concentration (5%, 10%, and 15%), porous structures	[728]
Co doped ZnO	Zn(NO3)2·6H2O, Co(NO3)2·6H2O, citric acid (C6H8O7), H2O	duration: until reaction was completed; microwave oven (650 W)	250 °C for 1 h in air	doped concentration (5%, 10%, and 15%), porous structures	[728]
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, polyethylene glycol, NaOH, H2O	pH: 9; duration: 30 min; power: 300 W; microwave oven	T: 400 °C; duration: 1 h	Zn1−xCoxO (x = 0.00, 0.01, 0.03 and 0.05), needle shaped microstructures, nanospheres	[729]
CdO-ZnO	ZnCl2·2H2O, CdCl2·2H2O, NH4OH	pH: 8; duration: 10–20 min; microwave oven (1000 W)	500 °C for 4 h in air	heterogeneous nanostructures and microstructures	[730]
Eu doped ZnO	Zn(NO3)2·5H2O, Eu(NO3)3·5H2O, NaOH, H2O	pressure: 60 bar; duration: 15 min; microwave reactor (600 W)	750 °C for 1 h in air	10 mol% of Eu, heterogeneous size and shape of NPs	[731]
In2O3-ZnO	Zn(CH3COO)2·2H2O, In(NO3)3·4.5H2O, CO(NH2)2, H2O	T: 120 °C; duration: 30 min; power: 800 W; microwave reactor	550 °C for 2 h in air	n(In):n(Zn) (0.03:1, 0.05:1, 0.07:1), rods (long: 200–300 nm; wide 75 nm) and flowers	[732]
Sn doped ZnO	Zn(NO3)2·6H2O, SnCl4, H2O	duration: 5 min; microwave oven	400 °C for 3 h in air	0, 5, 10 and 15 wt% Sn doped ZnO; needle-like structures for the pure ZnO (30–70 nm); agglomerated spherical crystallites in all Sn doped ZnO samples	[733]
ZnO-reduced graphene oxide	graphite (modified Hummers method), ZnCl2·2H2O, NaOH, H2O	duration: 2 min; power: 450 W; microwave oven	stainless steel (SS-316) Teflon lined autoclaves were kept at 150 °C in hot air oven for 24 h	SSA: 140–182 m2/g; flower-like ZnO nanoparticles well decked on graphene/graphene oxide sheet	[734]
Ag-ZnO	Zn(NO3)2·6H2O, AgNO3, citric acid (C6H8O7)	duration: 15–20 cycles (cycling mode: on for 30 s and off for 30 s); power: 800 W; microwave oven	500 °C for 2 h in air	nanoparticles; SSA: 61 m2/g; size: ~17 nm	[735]
Ag/Ag2SO4/ZnO	Zn(CH3COO)2·2H2O, AgNO3 (different concentrations), urea, thiourea (different concentrations),	duration: 30 min; power: 400 W; microwave reactor	500 °C for 4 h in air	plate-like aggregates, diameter: 1.5–2 µm, thickness: 100–200 nm	[736]
Ag-N co-doped ZnO	Zn(NO3)2·6H2O, hexamethylenetetramine (HMT; C6H12N4), CH3COONH4, AgNO3	duration: 40 min; power: 500 W; microwave oven	800 °C for 1 h in oxygen	Ag-N co-doped ZnO nanorods (diameter: 50–200 nm) were vertically grown on n-type Si substrate	[737]
Au-ZnO	Zn(NO3)2·6H2O, hexamethylenetetramine (HMT; C6H12N4), hydrazine hydrate (N2H4), HAuCl4, H2O	pH: 10; microwave oven (1000 W)	500 °C in oxygen	Au NPs on the surface of ZnO nanorods (width: 140 nm, length: 626 nm) Au size of NPs: 11–36 nm	[738]

.

3.1. Reactants

The most popular reactants of the “Zn²⁺” zinc cation precursor used for ZnO synthesis according to the data derived from the literature review [506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738] (Table 5, Table 6, Table 7 and Table 8) are Zn(NO₃)₂·6H₂O, Zn(CH₃COO)₂·2H₂O, ZnCl₂, ZnSO₄·7H₂O and ZnO (Figure 15), respectively. The popularity of these reactants results from their low price and easy availability. Reactants being a source of “Zn²⁺” ions are commonly produced at a large scale mostly by several producers in each developed country. It should be emphasised that ZnCl₂ is one of the problematic reactants, which arises from the possibility that during the ZnO synthesis a stable by-product (impurity), called simonkolleite (Zn₅(OH)₈Cl₂·H₂O), is formed. If Zn(NO₃)₂·6H₂O is used as the reactant and if the temperature of the ZnO synthesis is too low (below 100 °C), one of the synthesis by-products (impurity) may be the salt Zn₅(OH)₈(NO₃)₂·2H₂O [515]. It must be borne in mind that ZnO obtained by the hydrothermal method, depending on the parameters employed (T, P, pH), may contain impurities being hydroxides, oxide hydroxides or basic salts. Depending on the quantity and size of crystallites of “impurities”, they are often invisible in the results of the X-ray powder diffraction analysis (XRD) because they are below the method’s detection limit. When discussing the results, it must be remembered that the XRD analysis is capable of indicating only crystalline impurities of ZnO.

The most popular reactants being hydroxide anion precursor chemicals “OH⁻” used for the ZnO synthesis according to the data derived from the literature review [506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738] (Table 5, Table 6, Table 7 and Table 8) (Figure 16) are sodium hydroxide (NaOH), ammonia water (NH₃·H₂O, NH₄(OH)), hexamethylenetetramine (urotropin, HMTA, C₆H₁₂N₄), potassium hydroxide (KOH), urea (CH₄N₂O, CO(NH₂)₂), hydrazine (N₂H₄·H₂O) as well as other amides and amines, e.g., 1-n-butyl-3-methyl imidazolium tetrafluoroborate (C₈H₁₅BF₄N₂), N,N dimethylformamide (C₃H₇NO), pyridine (C₅H₅N), aniline (C₆H₅NH₂), arginine (C₆H14N₄O₂), tris (hydroxymethyl) aminomethane (C₄H₁₁NO₃), bis(triaminomethyl) carbonate (C₃H₁₂N₆O₃), butyl-3-ethyl imidazolium tetrafluoroborate (C₈H₁₅BF₄N₂), dodecylamine (C₁₂H₂₇N), and 1-hexyl-2-ethyl-3-methylimidazoliumtetrafluoroborate (C₆H₁₁BF₄N₂).

The chemical properties of amines are similar to those of ammonia. These are compounds with alkaline properties, which form salts with acids and produce an alkaline reaction in aqueous solutions. The alkalinity of amines depends on substituents by the nitrogen atom. Aliphatic amines are, as a rule, more alkaline than ammonia while the alkaline properties of aromatic amines are weaker than those of ammonia. Amides undergo a slow hydrolysis, which leads to the formation of OH⁻ and the formation of an intermediate, Zn(OH)₂.

3.2. Surfactants

Surfactants are commonly used in nanotechnology for passivation of surface, control of shape, and giving of photophysical properties [740]. Similar impact on properties of nanomaterials, apart from various types of surfactants (anionic, nonionic, cationic and amphoteric), is exerted also by surface active polymers, amines, amino acids, proteins, and carbohydrates. One of the primary purposes of the use of surfactants in obtaining nanoparticles is their strong contribution to obtaining dispersoids (emulsions, suspensions, colloids), in which they counteract the agglomeration of NPs [741,742,743]. Surfactants enable achieving a dispersion of NPs in non-polar solvents, e.g., in lubricating oils [744]. Surfactants are also used for controlling the shape of nanomaterials [740,745]. The shape control mechanism is related above all to the adsorption of the surfactant particles on various planes of crystalline nucleation centres of crystallites; these particles selectively promote the growth of these centres or to the contrary: inhibit their growth. The following surfactants were used in microwave hydrothermal syntheses of ZnO NMs for the purpose of controlling the morphology:

-

Ethylenediamine (EDA, C₂H₈N₂) for obtaining nanoneedles [525].

-

Hexamethylenetetramine (HMT, (C₆H₁₂N₄)) for obtaining nanorods [525].

-

Triethyl citrate (C₁₂H₂₀O₇) for obtaining hexagonal disks [525].

-

Triethanolamine (TEA, C₆H₁₅NO₃) for obtaining nanosheets [521], pompon-like spheres [554], peach nut-like spheres [554], misshapen spheres [554], rugby-like nanostructures [565], raspberry-like nanostructures [566], hollow nanospheres [566], dumbbell-like [626], football-like shape [626], and spherical nanoparticles [565,566,625,645].

-

1,2,4,5-benzenetetracarboxylic acid (BTCA, C₆H₂(CO₂H)₄) for obtaining nanosheet [554] and elongated triangles [554].

-

Cetyltrimethylammonium bromide (CTAB, (C₁₉H₄₂BrN)) for obtaining heterogeneous shapes [531], wires [547], nanorods [576], wire-like architecture [597], flower-like microstructures composed of nanorods [597], and spike-like nanostructures [638].

-

Sodium di-2-ethylhexyl-sulfosuccinate (C₂₀H₃₆Na₂O₇S) for obtaining hexagonal rods [581], hexagonal prismatic [581], bihexagonal rod-like structures [581], wire-like architecture [597], and flower-like microstructures composed of nanorods [597] and rods [597].

-

Pluronic F127 (polyoxypropylene polyoxyethylene block copolymer) for obtaining heterogeneous shapes [531].

-

Polyethylene glycol 400 (PEG400, C_2nH_4n+2O_n+1) for obtaining nanorods [533], flowers [533], rod-like nanostructures [574], star-like nanostructures [574], particles with an irregular shape (plate and rod-like particles) [596], quasi-spherical shapes [620], flower-like structures [620], flower-like hierarchical structures [655], rod-like structures [673], and needle-like structures [673].

-

Acetyl acetate (ACAC, (CH₃CO)₂O) for obtaining rod-like structures [644].

-

Guanidinium carbonate ((CH₅N₃)₂·H₂CO₃) for obtaining spherical nanoparticles [644] and flower-like ZnO [644].

-

Polyvinyl alcohol 2000 (PVA2000, (C₂H₄O)_n) for obtaining spherical nanoparticles [578].

-

Polyvinyl Pyrrolidine (PVP, (C₆H₉NO)_n) for obtaining star-like nanostructures [526,558].

-

Triethyl citrate (C₁₂H₂₀O₇) for obtaining disk- and nut-like structures [525].

-

Tripotassium citrate for obtaining UFOs and balls-like structures [525].

-

Arginine (C₆H₁₄N₄O₂) for obtaining rods and flowers [543].

-

Albumen for obtaining whisker-like and rod-like nanostructures [585].

-

Triton X-100 (C₁₄H₂₂O(C₂H₄O)_n (n = 9–10)) for obtaining rods (400–800 nm) and flower structures [594].

3.3. Morphology

The morphologies of ZnO NMs reported in the literature are presented in Table 5, Table 6, Table 7 and Table 8 [506,507,508,509,510,511,512,513,514,515,516,517,518,519,520,521,522,523,524,525,526,527,528,529,530,531,532,533,534,535,536,537,538,539,540,541,542,543,544,545,546,547,548,549,550,551,552,553,554,555,556,557,558,559,560,561,562,563,564,565,566,567,568,569,570,571,572,573,574,575,576,577,578,579,580,581,582,583,584,585,586,587,588,589,590,591,592,593,594,595,596,597,598,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,614,615,616,617,618,619,620,621,622,623,624,625,626,627,628,629,630,631,632,633,634,635,636,637,638,639,640,641,642,643,644,645,646,647,648,649,650,651,652,653,654,655,656,657,658,659,660,661,662,663,664,665,666,667,668,669,670,671,672,673,674,675,676,677,678,679,680,681,682,683,684,685,686,687,688,689,690,691,692,693,694,695,696,697,698,699,700,701,702,703,704,705,706,707,708,709,710,711,712,713,714,715,716,717,718,719,720,721,722,723,724,725,726,727,728,729,730,731,732,733,734,735,736,737,738] along with a description of their properties. ZnO NMs synthesised with the use of microwave hydrothermal synthesis were characterised by the following shapes: belts-like [671], bundle-like [655], brush-like [572], calthrop-like framework [561], candles [525], dandelion-like [518,612], disks [525], dumbbell-like [626,641], football-like structure [626], hexagonal column [521,629], hexagonal nanoring [521], hexagonal bi-pyramidal [626], hexagonal prisms [513,694] hexagonally shaped prismatic [528], hexagonal tubular [664], hollow structures [521], javelins [540], lamellar-like [692,699], misshapen spheres [554], peach nut-like spheres [554], petals [644], nuts-like [525], pompon-like spheres [554], raspberry-like nanostructures [566], rhombic [663], rugby-like nanostructures [565], spindles [518], sponge-like [580], spike-like [638], thruster vanes [518], tetrapod-like [631,670], tubes [514,515,571], whiskers [585], wires [518,523,534,539,547,646,711], flakes [520,586,587,592,622,648,663,664], plates [511,532,572,579,596,629,651,658,694,736], star-like [506,526,535,536,558,574,631,679,703], heterogeneous structures [507,524,528,531,630,637,665,666,668,672,720,730,731], flower-like [508,510,512,513,519,520,527,528,529,530,532,533,535,538,541,543,545,550,553,555,556,557,558,559,560,561,562,563,564,565,566,571,572,579,582,593,594,597,598,599,620,621,622,631,639,649,654,655,663,670,704,734,737,738], needle-like [510,521,525,551,556,588,588,629,655,663,673,716,729], sheets [521,544,551,552,575,577,583,585,588,627,650,686,707,714], spherical particles [511,535,536,551,560,565,566,578,585,599,614,615,623,625,628,634,635,636,638,639,645,648,649,674,680,681,682,683,684,685,702,706,712,718,721,727,735], and rods [473,509,511,517,518,519,525,527,529,533,538,541,542,543,548,549,553,556,557,559,562,563,567,568,570,573,574,576,581,583,584,585,589,590,591,592,593,594,595,596,597,599,600,601,602,603,604,605,606,607,608,609,610,611,612,613,616,617,618,619,624,629,631,632,633,640,642,643,644,649,653,655,658,659,661,663,664,670,673,678,693,696,697,700,701,705,709,710,723,725,726,732].

Examples of various morphologies of ZnO are presented in Figure 17, Figure 18, Figure 19 and Figure 20. The modification of morphology of ZnO NMs with a 3D architecture is extremely important e.g., for producing sensors with high sensing capabilities [739]. The 3D architecture of ZnO NMs, characterised by a small particle size, a high specific surface area and porosity, enables the improvement of the sensing response of gas sensors [152].

3.4. Microwave Hydrothermal Synthesis of ZnO without Any Additional Heat Treatment

The microwave hydrothermal synthesis of ZnO NMs owes its widespread use and popularity to water (H₂O). This solvent is cheap, non-toxic and easy to dispose of. The main advantage of this solvent is that it can be easily separated from the product by e.g., centrifuging, drying with the use of a filter, in a vacuum dryer, or freeze-drying. Another advantage of water is that the majority of Zn²⁺ salts are soluble in it (among others: Zn(NO₃)₂·6H₂O, Zn(CH₃COO)₂·xH₂O, ZnCl₂·xH₂O, ZnSO₄·7H₂O).

Considering dispersoids of the reaction mixture in the microwave hydrothermal synthesis of ZnO NMs, it is possible to perform the synthesis in a solution or in a suspension. It must be underlined that currently the synthesis of ZnO NMs has reached a new stage of development, where the common requirement is to control of properties of the obtained ZnO NMs, their repeatability and reproducibility. One of examples of papers which report, e.g., control of the ZnO shape is the publication by Wang et al. [510]. They demonstrate an example of obtaining ZnO with different morphologies by using an imidazolium salt 1-n-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM]BF₄) (an ionic liquid). The aqueous mixture of Zn(NO₃)₂·6H₂O, NaOH and [BMIM]BF₄ was used as the precursor solution. They obtained a strongly alkaline pH of the solution, where Zn²⁺ existed mainly as Zn(OH)₄²⁻. The mechanism of the ZnO synthesis reaction proposed by Wang et al. [510] is as follows in Equations (1)–(4):

$$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{NO}}_3{}^{-}$$

(1)

$$\mathrm{NaOH}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}$$

(2)

$${\mathrm{Zn}}^{2+}+4{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }\mathrm{Zn}{\left(\mathrm{OH}\right)}_4{}^{2-}$$

(3)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_4{}^{2-}\stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}+2{\mathrm{OH}}^{-}$$

(4)

Wang et al. [510] controlled the change in the morphology from flower-like, to flower-like + needle-like and to needle-like by changing such parameters as concentration of reactants, temperature and synthesis duration.

Pulit-Prociak et al. [507] report the effect of process parameters on the size and shape of ZnO in their paper. They used Zn(NO₃)₂·6H₂O and NaOH for the synthesis. The mechanism of the ZnO synthesis reaction proposed by Pulit-Prociak et al. [507] was as follows in Equations (5)–(8):

$$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{NO}}_3{}^{-}$$

(5)

$$\mathrm{NaOH}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}$$

(6)

$${\mathrm{Zn}}^{2+}+2{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\to }\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\downarrow$$

(7)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}$$

(8)

Pulit-Prociak et al. [507] contributed to a change in the shape and size of aggregates of ZnO particles by changing the microwave power, synthesis duration and pressure.

Hu et al. [509] report obtaining ZnO rods using a water suspension composed of Zn(NO₃)₂·6H₂O and (CH₂)₆N₄ (hexamethylenetetramine, HMT) in their paper. The mechanism of the ZnO synthesis reaction proposed by Hu et al. [509] was as follows in Equations (9)–(12):

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{N}}_4+6{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }4{\mathrm{NH}}_3+4\mathrm{HCHO}$$

(9)

$${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{NH}}_4{}^{+}+{\mathrm{OH}}^{-}$$

(10)

$${\mathrm{Zn}}^{2+}+2{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\to }\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\downarrow$$

(11)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\downarrow \stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}$$

(12)

Hu et al. [509] changed the morphology from flower-like, to flower-like + needle-like, to needle-like by changing such parameters as concentration of reactants, temperature and synthesis duration.

Phuruangrat et al. [513] report the control of morphologies and a growth mechanism of hexagonal prisms with planar and pyramid tips of ZnO microflowers. They used Zn(NO₃)₂·6H₂O, NaOH, and hexamethylenetetramine (HMT) for the synthesis. Figure 21 presents the as-synthesised products which were pure hexagonal wurtzite ZnO microstructured flowers of hexagonal prisms with planar tips at pH 9 and of hexagonal prisms with hexagonal pyramid tips at pH 13.

Phuruangrat et al. [513] explained the formation of ZnO microflowers composed of petals in the shapes of hexagonal prisms with planar and pyramid as follows: the size and morphology of particles is controlled by the chemical state of Zn²⁺ ions in the solution. Thermodynamic and kinetic factors affect the process of precipitation of compounds containing Zn²⁺ in various ways. The chemical state of Zn²⁺ ions (dissociated form, dissociated complex form, sediment) strongly depends on the pH of the medium and may be strongly controlled by the pH of the solution and anionic type. By adding the NaOH solution gradually to the Zn(II) salt solution with the pH of 13, the Zn(OH)₄²⁻ complex is formed, which can be denoted by in Equations (13)–(16):

$$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{NO}}_3{}^{-}$$

(13)

$$\mathrm{NaOH}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}$$

(14)

$${\mathrm{Zn}}^{2+}+4{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }\mathrm{Zn}{\left(\mathrm{OH}\right)}_4{}^{2-}$$

(15)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_4{}^{2-}\stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}+2{\mathrm{OH}}^{-}$$

(16)

Due to a large number of growth units, [Zn(OH)₄]²⁻ complexes are able to easily adsorb on different sites of ZnO nanorods. The positively charged Zn-(0001) surfaces are the most reactive. Thus OH⁻ ions may stabilise the positive charge of the Zn-(0001) surfaces to some extent, allowing rapid growth along the [1] direction zone axis of the hexagonal phase, leading to the formation of ZnO hexagonal prisms with rod-like crystal. ZnO microflowers of hexagonal pyramid tips were obtained at a higher concentration of NaOH, where the pH of the solution was 13. It must be emphasised that higher concentrations of OH⁻ result in an increase in the solubility of Zn(OH)₂ and ZnO at the room temperature, which is related to the formation of an ionic complex, above all Zn(OH)₄²⁻. The solubility of Zn²⁺ complexes decreases in line with the increase in temperature during the microwave synthesis, which results in the precipitation of solid ZnO. The growth mechanism of ZnO microflowers with the petals of hexagonal pyramids may be explained as follows: when the pH value of the solution is high, a strong electrostatic interaction emerges between ions and polar surfaces, which leads to an increase in surface energies of polar surfaces (0001) and (1011) in comparison with the energy of the other surfaces of the hexagonal crystal. The growth rate of these polar surfaces decelerates and the polar surfaces appear as external surfaces with sharp edges. When the pH value is high, a strong electrostatic interaction emerges between ions and polar surfaces, which leads to an increase in surface energies of polar surfaces (0001) and (1011) in comparison with the energy of the other crystalline surfaces. The growth rate of these polar surfaces decelerates and the polar surfaces appear as external surfaces with sharp tips of the hexagonal prisms with pyramid tips.

Klofac et al. [574] report obtaining ZnO rods using a water suspension composed of Zn(CH₃COO)₂·2H₂O, aqueous ammonia, polyethylene glycol (PEG; Mr = 400), and cetyltrimethylammonium bromide (CTAB, C₁₉H₄₂BrN) in their paper. The mechanism of the ZnO synthesis reaction proposed by Klofac et al. [574] was as follows in Equations (17)–(21):

$${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{NH}}_4{}^{+}+{\mathrm{OH}}^{-}$$

(17)

$${\mathrm{Zn}}^{2+}+4{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{[\mathrm{Zn}{\left(\mathrm{OH}\right)}_4]}^{2-}$$

(18)

$${\mathrm{Zn}}^{2+}+4{\mathrm{NH}}_3\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4]}^{2+}$$

(19)

$${[\mathrm{Zn}{\left(\mathrm{OH}\right)}_4]}^{2-} \stackrel{{\mathrm{H}}_2\mathrm{O} }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}+2{\mathrm{OH}}^{-}$$

(20)

$${[\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4]}^{2+}+2{\mathrm{OH}}^{-}\stackrel{\mathrm{M}\mathrm{H} }{\to }\mathrm{ZnO}\downarrow +4{\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}$$

(21)

Klofac et al. [574] obtained star- or flower-like ZnO particles aggregated from rod-like components united at a common centre depending on the surfactant used.

Based on these papers, a general conclusion may be drawn that depending on pH and NH₃ derivatives the key role in reactions is played by stages related to the formation of complex compounds of zinc ions (Zn²⁺).

Details of other research papers concerning the microwave hydrothermal synthesis of ZnO without any additional heat treatment are presented in Table 5.

3.5. Microwave Hydrothermal Synthesis of ZnO Nanostructures with Additional Heat Treatment

The additional heat treatment process at the temperature from 150 °C to 900 °C (Table 6) of the microwave hydrothermal synthesis products is applied in order to remove organic impurities and to cause thermal decomposition of unreacted substrates and intermediates, e.g., Zn(OH)₂, Zn₅(OH)₈(NO₃)₂·2H₂O, Zn₅(OH)₈Cl₂·H₂O. The additional heat treatment process is applied mainly when reaction products are obtained in an open vessel in a microwave oven, which results in reaching a low reaction temperature (max. ca. 100 °C) (Table 6). The main disadvantage of the additional heat treatment in terms of structure of the obtained ZnO NMs is the sintering of particles and the increase in their size (recrystallisation process), which results in formation of large agglomerates/aggregates/conglomerates. Thus obtained ZnO NMs, despite the use of efficient homogenisation methods (e.g., ultrasounds), are characterised by a large average particle/agglomerate/aggregate size and high polydispersity in water suspensions.

When discussing aspects of application of ZnO NMs in a dry form, the additional heat treatment of samples improves the stability of nanostructures and gives unique properties by soaking ZnO in various gaseous atmospheres, which may result in the formation of oxygen vacancies in their structure. The controlled introduction of oxygen vacancies into ZnO NMs can manipulate their optical, electronic and surface properties [746]. An example that confirms the advantages of heat treatment is the paper by Gu et al. [654], where the authors used the obtained ZnO NMs for detecting gases. They used the aqueous solution of Zn(NO₃)₂ with varied urea contents for the synthesis. At the first stage, precursors were heated by microwaves for 40 min at the temperature of 90 °C, while at the second stage, the samples, after rinsing and drying, were soaked in various gaseous atmospheres (O₂, N₂, H₂ and air) for 3 h at the temperature of 400, 500, and 600 °C. Gu et al. [654] obtained ZnO in the form of 3D nanostructures. Figure 22 shows the impact of the change in the urea content on the shape of the obtained 3D structures. With the lowest urea content (0.3 g) urchin-like structures were obtained (Figure 22a), while as a result of adding a higher quantity of urea (3.0 g and 6.0 g) flower-like structures were obtained (Figure 22b,c). The change in the soaking temperature from 400 °C to 500 °C caused a change in the shape of the flower structure and a change in the morphology of the nanoflakes themselves (Figure 23a,b). Structures soaked at 600 °C were characterised by a ragged and porous structure of the nanoflakes (Figure 23c).

The paper by Li et al. [652] is another example of application of 3D structures of ZnO obtained by the microwave hydrothermal synthesis with an additional soaking process in applications related to gas detection. For the synthesis of ZnO hollow microspheres, Li et al. [652] used a solution created by mixing zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), trisodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O) with aqueous ammonia (NH₃·H₂O). At the first stage, precursors were heated by microwaves for 40 min at the temperature of 90 °C, while at the second stage the samples, after rinsing and drying, were soaked in an air environment for 4 h at the temperature of 400 °C. The mechanism of the ZnO synthesis reaction proposed by Li et al. [652] was identical with the mechanism proposed by Klofac et al. [574] per Equations (12)–(16). Figure 24 reveals how the authors controlled the morphology of the change to the microsphere filling by changing the synthesis duration. Figure 25 presents changes to the morphology caused by the change in the amounts of trisodium citrate dihydrate in the precursor solution.

Salah et al. [660] report developing a method of controlling the ZnO nanostructure size. They used an aqueous mixture of zinc nitrate dehydrate (Zn(NO₃)₂·6H₂O) with hexamethylenetetramine (C₆H₁₂N₄) as the reaction precursor. The first stage of microwave heating at the temperature of 120 °C lasted 10 min, and subsequently after rinsing and drying the synthesis products were soaked at 400 °C for 1 h. By applying various proportions of zinc nitrate dehydrate and hexamethylenetetramine in the precursor mixture, the authors [660] controlled the size of the structures within a range from several dozen nm to several µm (Figure 26). The change in the size of the ZnO structures was accompanied by the change in their morphology from spherical nanoparticles (Figure 26a) in micro sized hexagonal nanorods.

Details of other research papers concerning the microwave hydrothermal synthesis of ZnO with additional heat treatment are presented in Table 6.

3.6. Types of ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Hydrothermal Synthesis

The microwave hydrothermal synthesis enables obtaining ZnO doped with the following ions: Cd²⁺ [698], Ce⁴⁺ [686,687,693], Co²⁺ [688,689,690,691,728,729], Cu²⁺ [695], Cr³⁺ [727,728], Eu³⁺ [694,731], Fe²⁺ [678,679,680,681,682,683,684,685], Ga³⁺ [696,697], K⁺ [692], Mn²⁺ [674,675], Sn⁴⁺ [733], and Sr²⁺ [699].

The microwave hydrothermal synthesis enables obtaining the following composite and hybrid materials: ZnO/ZnMn₂O [676,677], ZnO/ZnFe₂O₄ [680,681,682,683,684,685], ZnO/ZnS [700], Au-decorated ZnO [701,702,738], Ag-decorated ZnO [638,703,704,705,706,735], reduced graphene oxide [707,708,709,710,711,712,713], ZrO₂ coated ZnO [717], Fe₃O₄/ZnO/AgBr [719], MoS₂/ZnO [720], ZnO doped CeO₂ [721], mesoporous Si@ZnO [722], ZnO/zinc aluminium hydroxide [723], ZnO/TiO₂ [724,725], ZnO/CuO [726], CdO-ZnO [730], and In₂O₃-ZnO [732].

3.7. ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Hydrothermal Synthesis without Any Additional Heat Treatment

The potential of the microwave hydrothermal synthesis in obtaining ZnO hybrid nanostructures is proved by the results achieved by Cho et al. [723]. The paper by Cho et al. [723] is one of the more interesting publications reporting a synthesis of hierarchical hexagonal zinc oxide/zinc aluminium hydroxide heterostructures through epitaxial growth using microwave irradiation. For obtaining a substrate being zinc aluminium layered double hydroxide heterostructures (ZnAl:LDH), the authors used the solution of zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), aluminium chloride (AlCl₃) and ammonia water (NH₃·H₂O). The microwave hydrothermal synthesis of ZnAl:LDH lasted 20 min at the temperature of 95 °C. The product, ZnAl:LDH substrate, was thoroughly rinsed and dried after the synthesis. Depending on the selected synthesis parameters and modifications of the precursor composition, the authors obtained various structures that grew on the surface of the ZnAl:LDH substrate, namely: hexagonal ZnO nanorods (Figure 27 and Figure 28), sunflower-like ZnO nanorods (Figure 29), ZnO nanotubes (Figure 30) and ZnO film (Figure 31). The process of ZnO nanorod growth on the substrate surface was as follows: the substrate (ZnAl:LDH) was introduced to the solution of the mixture of Zn(CH₃COO)₂·2H₂O and NH₄OH and was subjected to microwave heating (20 min, 95 °C), and subsequently the product was rinsed with deionised water and dried. On the surface of some of the substrates, the growing ZnO nanorods formed repeatable patterns, which can be seen in Figure 28. The formation mechanism of these regular patterns was not tested by the authors [723], but the occurring regular patterns indicate that the nucleation of ZnO on the surface of the ZnAl:LDH strongly depends on the atomic arrangements of the ZnAl:LDH surface. The process of sunflower-like ZnO nanorod growth on the substrate surface (Figure 29) was virtually identical to the growth of ZnO nanorods apart from the applied Zn(CH₃COO)₂·2H₂O concentration, which was increased twice relative to the original solution used for the growth of ZnO nanorods. The growth of ZnO nanotubes (Figure 30) on the ZnAl:LDH surface, in turn, consisted in introducing the substrate to the solution of the mixture of Zn(CH₃COO)₂·2H₂O with NH₄OH and subjected to microwave heating (20 min, 95 °C), and subsequently the suspension was aged at the room temperature for 10 h, after which the product was rinsed with deionised water and dried. For the synthesis of a 2D ZnO film on the surface of ZnAl:LDH heterostructures, the composition of the precursor solution was modified by adding tripotassium citrate monohydrate (HOC(COOK)(CH₂COOK)₂·H₂O). As a result of microwave heating, sandwich-like heterostructures were obtained (Figure 31). In our opinion, the results of that paper indicate that the microwaves could have potentially acted as a stimulator of a stereochemical reaction (i.e., spatial orientation of the structure). The reaction temperature was not high, merely 95 °C, where an identical temperature could have been reached by pre-soaking of substrates on a heating plate, but the clear spatial organisation of the structure (Figure 27, Figure 28, Figure 29, Figure 30 and Figure 31) suggests the participation of additional mechanisms, e.g., an electromagnetic field.

Details of other research papers concerning the microwave hydrothermal synthesis of ZnO with additional heat treatment are presented in Table 7.

3.8. ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Hydrothermal Synthesis with Additional Heat Treatment

As already mentioned, the additional soaking process of the microwave hydrothermal synthesis product aims, above all, to complete the conversion of the substrates or intermediates. In the case of a synthesis of doped ZnO, additional heat treatment mostly leads to the precipitation of foreign phases, e.g., Cr₂O₃ [727,728], Eu₂O₃ [731], and SnO₂ [733]. Depending on the type of gaseous atmosphere, dopants may also undergo a partial oxidation or reduction process. Synthesis products subjected to an additional soaking process may be also characterised by a compact structure, where the particles are sintered or strongly agglomerated, which is indicated, e.g., by the results of the research by Bhattia et al. [728] in Figure 32.

One of the purposes of applying an additional process of synthesis product soaking is to give unique features both to the doped ZnO materials and composite ZnO materials, which results in the possibility to apply them e.g., as photocatalysts [724,732,734], gas sensors [725], dilute magnetic semiconductors (DMS) for spintronics applications [728]. A good example of obtaining a hybrid material with the use of an additional soaking process is the paper by Cao et al. [736], which describes obtaining Ag/Ag₂SO₄/ZnO nanostructures. The authors [736] used the aqueous mixture of Zn(CH₃COO)₂·2H₂O, CO(NH₂), CS(NH₂)₂ and AgNO₃ as the precursor. The composition of the mixture was modified by changing the molar ratio of thiourea and Ag⁺, which was kept at 1:2, 1:1, 2:1, respectively. The samples were heated by microwaves for 30 min at the temperature of 170 °C, subsequently rinsed (water, ethanol) and were subjected to calcination for 4 h at the temperature of 500 °C in the air atmosphere. Figure 33 contains XRD results of an example as-synthesised sample (only after the microwave synthesis) and the XRD results of samples after an additional soaking process.

The as-synthesised sample reveals a considerable quantity of foreign inclusions, above all Zn₅(CO₃)₂(OH)₆ product (intermediate, by-product). The authors [736] proposed a mechanism of the formation of (Zn₅(CO₃)₂(OH)₆) by-product and main products (ZnO, Ag₂S), which is described by the following Equations (22)–(30):

$$\mathrm{CO}{({\mathrm{NH}}_3)}_2+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{T} > 60 °\mathrm{C} }{\to }2{\mathrm{NH}}_3{}^{+}+{\mathrm{CO}}_2$$

(22)

$$\mathrm{CS}{({\mathrm{NH}}_3)}_2+2{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{T} > 60 °\mathrm{C} }{\to }2{\mathrm{NH}}_3{}^{+}+{\mathrm{H}}_2\mathrm{S}+{\mathrm{CO}}_2$$

(23)

$${\mathrm{NH}}_3+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{NH}}_4{}^{+}+{\mathrm{OH}}^{-}$$

(24)

$${\mathrm{CO}}_2+2{\mathrm{OH}}^{-}\stackrel{{\mathrm{H}}_2\mathrm{O} }{\leftrightarrow }{\mathrm{CO}}_3{}^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(25)

$$5{\mathrm{Zn}}^{2+}+2{\mathrm{CO}}_3{}^{2-}+6{\mathrm{OH}}^{-}\stackrel{\mathrm{MH} }{\to }{\mathrm{Zn}}_5{\left({\mathrm{CO}}_3\right)}_2{\left(\mathrm{OH}\right)}_6\downarrow$$

(26)

$${\mathrm{Zn}}^{2+}+2{\mathrm{OH}}^{-}\stackrel{}{\to }\mathrm{Zn}{(\mathrm{OH})}_2\downarrow$$

(27)

$$\mathrm{Zn}{(\mathrm{OH})}_2\stackrel{ \mathrm{MH}}{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}$$

(28)

$${\mathrm{Zn}}_5{\left({\mathrm{CO}}_3\right)}_2{\left(\mathrm{OH}\right)}_6 \downarrow \stackrel{\mathrm{MH} }{\to }5\mathrm{ZnO}\downarrow +2{\mathrm{CO}}_2+3{\mathrm{H}}_2\mathrm{O}$$

(29)

$${\mathrm{H}}_2\mathrm{S}+2{\mathrm{Ag}}^{+}\stackrel{ \mathrm{MH}}{\to }{\mathrm{Ag}}_2\mathrm{S}\downarrow +2{\mathrm{H}}^{+}$$

(30)

It must be underlined that ZnO, Ag₂S and Zn₅(CO₃)₂(OH)₆ compounds were present in samples obtained only as a result of the microwave synthesis. However, after the calcination (Figure 34), the presence of such compounds as Ag₂SO₄ and metallic Ag was observed in the samples, which was related to the soaking of samples in an oxidising atmosphere (air). During the soaking, Ag₂S and Zn₅(CO₃)₂(OH)₆ decomposed. In the sample with the highest addition of thiourea also ZnS formed. The course of the oxidation process was explained by the following Equations (31)–(32):

$${\mathrm{Ag}}_2\mathrm{S}+{\mathrm{O}}_2\stackrel{500 °\mathrm{C},4 \mathrm{h}, \mathrm{air} }{\to }2\mathrm{Ag}\downarrow +{\mathrm{SO}}_2$$

(31)

$${\mathrm{Ag}}_2\mathrm{S}+2{\mathrm{O}}_2\stackrel{500 °\mathrm{C},4 \mathrm{h}, \mathrm{air} }{\to }2{\mathrm{AgSO}}_4\downarrow$$

(32)

An example morphology of the Ag/Ag₂SO₄/ZnO composite sample is shown in Figure 34. The obtained plates structure was composed of multiple nanoparticles and had rough and irregularly porous surfaces. Obtaining a plates structure was explained by Cao et al. [736] as follows: due to the hydrolysis reactions of urea and thiourea, Ag₂S/Zn₅(CO₃)₂(OH)₆/ZnO nanoparticles are obtained at the first stage of microwave heating. At the second stage, the existing nanoparticles rapidly formed into plates aggregates for the purpose of decreasing their surface energy. The plates shown in Figure 34 had the diameter of ca. 1.5–2 µm and the thickness of ca. 100–200 nm. The TEM image in Figure 34b indicates monodispersity of Ag/Ag₂SO₄ NPs with the diameters of ca. 4–7 nm, which were uniformly anchored on the surface of ZnO plates. Figure 34c presents two different kinds of the lattice fringes with 0.24 nm and 0.26 nm, corresponding to the distance of Ag (1 1 1) and ZnO (0 0 2) planes, respectively. Figure 34d confirms the qualitative composition of the Ag₂S/Ag₂SO₄/ZnO composite obtained by Cao et al. [736].

Details of other research papers concerning the microwave synthesis of ZnO nanocomposites or ZnO hybrid nanostructures with additional heat treatment are presented in Table 8.

4. Microwave Solvothermal Synthesis of ZnO

According to the literature [747], a solvothermal synthesis is a process in a closed system, in which chemical reactions take place exclusively in non-aqueous solvents at increased temperatures, at a pressure that is higher than atmospheric pressure (P > 101,325 Pa). The literature [748,749] provides a different definition of the solvothermal synthesis, e.g., it is a chemical reaction based on non-aqueous solvents or their mixtures. The application of water mixtures and organic solvents in syntheses of metal oxides made the definitions of the hydrothermal synthesis and the solvothermal synthesis often used interchangeably. However, this is not reasonable and there is no need to use these definitions interchangeably. The main reason for developing the solvothermal synthesis was attempts to obtain non-oxide materials (in particular metal particles) [747,748,749,750,751,752]. In syntheses with the use of organic solvents, the critical parameter is the temperature of the onset of initial decomposition, because after exceeding this temperature solvents begin undergoing the thermal decomposition, polymerisation or oxidation process. Solvothermal synthesis is much more expensive than hydrothermal synthesis, which results from the costs of application of organic solvents, their disposal and the need to adapt the laboratory (ventilation, laboratory fume hood, etc.). However, the costs related to the maintenance and safe use of reactors operating in milder conditions of solvothermal synthesis are considerably lower than in the case of high temperatures and pressures dedicated to hydrothermal syntheses. As a result, the solvothermal technology is becoming cheaper, safer, and more popular year by year.

For the purposes of this review, we assume that the “solvothermal synthesis” is a process occurring in an inorganic solvent environment (where m_s > 50%), with the pressure equal to or higher than atmospheric pressure. The results of the literature review concerning microwave solvothermal synthesis of ZnO are divided into the following subgroups:

(1)

Microwave solvothermal synthesis of ZnO nanostructures without any additional heat treatment, where the literature review results [402,573,758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797] are summarised in Table 10.

(2)

Microwave solvothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures without any additional heat treatment, where the literature review results [798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822] are summarised in Table 11.

(3)

Microwave solvothermal synthesis of undoped ZnO, ZnO nanocomposites or ZnO hybrid nanostructures with additional heat treatment, where the literature review results [823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838] are summarised in Table 12.

4.1. Reactants

The most popular organic solvents used for the microwave solvothermal synthesis of ZnO according to the data derived from the literature review [758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838] (Tables 10–12) are ethylene glycol (EG, C₂H₄(OH)₂), ethanol (C₂H₅OH), diethylene glycol (DEG, (HOCH₂CH₂)₂O), propanol (C₃H₇OH), methanol (CH₃OH), benzyl alcohol (C₆H₅CH₂OH) and other solvents (Figure 35). The following were used sporadically as a solvent in the microwave solvothermal synthesis: acetone (C₃H₆O) [778], acetonitrile (C₂H₃N, [789]), alkoxyethanol [772], butanediol (C₄H₈(OH)₂) [769], butoxyethanol (CH₃C₃H₆OC₂H₄OH) [772], ethoxyethanol (C₂H₅OC₂H₄OH) [772], glycerol ((CH₂OH)₂CHOH) [822], hexanol (C₆H₁₃OH) [766], methoxyethanol (CH₃OC₂H₄OH) [772], N,N-dimethylacetamide (CH₃CON(CH₃)₂) [792], mixture of oleic acid (C₈H₁₇CH=CH(CH₂)₇COOH) with oleylamine [807], propanediol (C₃H₆(OH)₂) [776], tetraethylene glycol (TTEG, C₈H₁₈O₅) [402], triethylene glycol (TEG, C₆H₁₄O₄) [402], tetrahydrofuran (C₄H₈O, THF) [795], and polyethylene glycol (PEG400) [768].

The popularity of solvents in microwave solvothermal syntheses is affected above all by such factors as availability, price, toxicity and boiling point (

Table 9. Summary of boiling points of several selected organic solvents for the pressure of 1013.25 hPa.

Solvent	Boiling Point (°C)
Acetone	56.2
Methanol	64.6
Tetrahydrofuran	65–66
Ethanol	78.5
2-Propanol	82.4
1-Propanol	97.0
Water	100.0
1-Butanol	117.6
1-Hexanol	156.5
Ethylene glycol	197.6
1,3-Butanediol	207.0
Benzyl alcohol	203–205
1,4-Butanediol	228.0
Diethylene glycol	244.9
Triethylene glycol	288.0
Glycerine	290.0
Tetraethylene glycol	~327

). Ethylene glycol and diethylene glycol are relatively cheap solvents, which are produced on an industrial scale and can be applied above all as components of coolants and as components for production of polyester resins. Ethanol and methanol are two most popular alcohols, which are applied on a global scale as primary solvents in organic syntheses. The application of organic solvents enables a considerable decrease in the pressure, temperature, and duration of the process in relation to a synthesis of the same product in water alone [839]. For example, the reaction of ZnO synthesis at the temperature of 230 °C in water is characterised by the equilibrium pressure of ca. ~28 bar [839], while in ethylene glycol for the same temperature the pressure is merely ~4 bar [502]. In a chemical reaction in the liquid state, only temperature is important, while pressure is only a derivative of the process conditions.

The most popular reactants of the “Zn²⁺” zinc cation precursor used for ZnO synthesis according to the data derived from the literature review [758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838] (

Table 10. Summary of the microwave solvothermal synthesis of ZnO without any additional heat treatment.

Substrates	Conditions during Preparation	Properties	Ref.
Zn(CH3COO)2·2H2O, C2H4(OH)2 (solvent), H2O (different concentrations)	T: 190–220 °C, duration: 25 min; power: 100%; microwave reactor (600 W)	control of size of particles within the size range between circa 15 and 120 nm	[402,758]
Zn(CH3COO)2·2H2O, diethylene glycol (solvent), triethylene glycol (solvent), tetraethylene glycol (solvent), H2O (constant concentration)	T: 190 °C, duration: 25 min; power: 100%; microwave reactor (600 W)	spherical nanoparticles and rod-like shape nanoparticles; diameters: 32–47 nm	[402]
Zn(CH3COO)2·2H2O, C2H4(OH)2 (solvent), H2O (constant concentration)	pressure: 4 bar; duration: 12 min; power: 1000–3000 W; microwave reactor (3000 W)	nanoparticles (27 nm) aggregates precise size-control ranging from about 60 to 120 nm	[502]
Zn(CH3COO)2·2H2O (different concentrations), diethylene glycol (solvent)	T: 180 °C, duration: 5 min; microwave reactor (300 W)	nanoparticles (6–10 nm) clusters precise size-control ranging from about 57 to 274 nm	[759]
Zn(CH3COO)2·2H2O (different concentrations), oleic acid, diethylene glycol (solvent)	T: 250 °C, duration: 15 min; power: 100%; microwave reactor	nanoparticles (4–14 nm)	[760]
Zn(CH3COO)2·2H2O, C2H4(OH)2 (ethylene glycol - solvent), H2O	different irradiation cycling modes, duration: 3–60 min; power: 200–600 W; microwave oven (750 W)	straw-bundle-like, wide chrysanthemum-like, nanorod-based microspheres, microspheres (irregular), nanorod-based microspheres, mixture of straw-bundle-like, wide chrysanthemum-like oat-arista-like	[761]
Zn(CH3COO)2·2H2O, C6H5CH2OH (anhydrous benzyl alcohol, solvent)	T: 120–180 °C duration: 30 s–35 min; power: 300 W; microwave reactor	nanoparticles (4–8 nm)	[762]
Zn(CH3COO)2, Zn(C5H7O2)2, C6H5CH2OH (benzyl alcohol, solvent)	T: 120–180 °C duration: 30 s–35 min; power: 300 W; microwave reactor	nanoparticles (20 nm for Zn(C5H7O2)2; 25–30 nm for Zn(CH3COO)2)	[763]
Zn(CH3COO)2·2H2O, C2H5OH (solvent); C3H7OH (solvent), H2O (solvent), NaOH	duration: 5 min; power: 150 W; microwave oven	water (solvent): ZnO nanoparticles had elliptical shape and large size with size of larger axis of about 100 nm and size of the other axis of about 40 nm; ethanol (solvent): rod nanostructures form with length of ~45 nm and radius of ~20 nm; isopropanol (solvent): spherical particles with radius of 10–12 nm.	[764]
Zn(CH3COO)2·2H2O, CH3OH (solvent), N(CH2CH2OH)3, NaOH, H2O	pH: 9.5; duration: 150 s; microwave oven (900 W)	sword-like wires with diameters of about 80–250 nm and the length of ∼1–4 μm	[765]
Zn(C5H7O2)2·xH2O, CH3OH+C6H13OH (1-hexanol) (solvent)	T: 160 °C; duration: 1 h; microwave reactor	spherical aggregates of NPs, SSA: 52 m2/g	[766]
Zn(CH3COO)2·2H2O, C2H4(OH)2 (solvent), H2O (solvent), various volume ratio H2O:C2H4(OH)2	T: 200 °C; duration; 30 min; power: 1000 W; microwave reactor	rods, hexagonal prisms, peanut-like, butterfly-like, spheres	[767]
Zn(CH3COO)2·2H2O, Na2CO3, polyethylene glycol (PEG400) (solvent)	duration: 10 min; microwave oven (700 W)	nanorods, diameters: 10–25 nm, length: 60–200 nm	[768]
Zn(CH3COO)2·2H2O, C4H8(OH)2 (1,4- butanediol, solvent)	duration: 2 min with on–off mode with a duration interval of 30 sec; power: 200 W; microwave oven (800 W)	nanoparticles 59 ± 16 nm	[769]
Zn(C5H7O2)2 (zinc acetylacetonate), C2H5OH (solvent), polyvinylpyrrolidone (various concentrations)	duration: 15 min; power: 800 W; microwave oven	irregularly shaped nanoparticles, nanorods and nanotubes	[770]
Zn(C5H7O2)2 (zinc acetylacetonate), C2H5OH + H2O (solvent), cetyltrimethylammonium bromide (CTAB)	duration: 15 min; power: 100 W; microwave reactor	nanorods coatings deposited on Cr/Si, Al/Si, Au/Si, ITO/glass; nanorod (length > 1 μm, width ~140–180 nm)	[771]
Zn(C5H7O2)2·H2O (zinc acetylacetonate monohydrate), alkoxyethanol (solvent), methoxyethanol (solvent), ethoxyethanol (solvent), n-butoxyethanol (solvent),	duration: 4 min; power: 800 W; microwave oven	SSA: 10–70 m2/g, nanoparticles 30–200 nm	[772]
Zn(C5H7O2)2·xH2O (zinc acetylacetonate hydrate, various concentrations), C4H9OH (1-butanol, solvent)	T: 120 °C; duration: 30 min; power: 250 W; pulsed microwave irradiation in microwave reactor	spherical-shaped (diameter 10–60 nm) and rod-shaped structures (length 100 nm)	[773]
Zn(CH3COO)2·2H2O, NaOH, (CH3)2CHOH (isopropanol, solvent)	duration: 5 min; power: 150 W; microwave reactor	spherical nanoparticles (3–5 nm)	[774]
commercial ZnO was dissolved in 5 M NaOH, C2H8N2, C2H5OH (solvent)	T: 150 °C; duration: 30 min; microwave reactor	flower-like microstructures, rod-like microstructures (length: 5 μm, diameter: 1 μm)	[775]
Zn(CH3COO)2·2H2O, C3H6(OH)2 (1,3-propanediol, solvent)	duration: 3 min with on–off mode having duration interval of 30 s; power: 360 W; microwave oven (800 W)	nanoparticles, diameter: 10–50 nm	[776]
Zn(CH3COO)2·2H2O, N(CH2CH2OH)3), CH3OH (solvent), H2O (solvent), NaOH	pH: 10.6–11.0; duration: 120–180 s; microwave oven (900 W)	uniform flower-like ZnO nanostructures (water), lengths: 700–950 nm, width: 130–230 nm; spheres (methanol): 250–400 nm	[573]
Zn(CH3COO)2·2H2O, N(CH2CH2OH)3, NaOH, C4H9OH (butanol, solvent)	pH: 8–10; T: 110 °C; duration: 40–60 s; microwave oven (900 W)	aggregated particles, average diameters: 550 nm; semi-spherical particles along with rice-like particles, average diameters ~600 nm; nanospheres average diameter 250 nm	[777]
Zn(NO3)2·6H2O, CH3(CH2)15NH2 (hexadecylamine), NaOH, C2H5OH (solvent), C3H6O (acetone, solvent), H2O (solvent)	T: 120 °C; duration: 15 min; microwave reactor	ethanol: spherical particles (20–60 nm) and rods with aspect ratio of 8–60; acetone: spherical particles (45–100 nm) and rods with aspect ratio which ranges from 37 to 94. water: star-shaped nanoparticles	[778]
Zn(C5H7O2)2·xH2O (zinc acetylacetonate hydrate), ethylenediamine (solvent), water (solvent), polyvinyl alcohol (PVA), polyethylene glycol 400 (PEG-400), polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB)	T: 100–110 °C; duration: 10 min, microwave oven	ethylenediamine: flower with rod-like petals, nanorod, nanoflake, flower with rod-like petals, flower with rod-like petals; water: flower cluster with spindle-like petals	[779]
Zn(CH3COO)2·2H2O, C2H5OH (anhydrous solvent), C6H5CH2OH (benzyl alcohol, solvent), H2O (solvent)	T: 200 °C; duration: 5–30 min; microwave reactor	SSA: 32–34 m2/g; ethanol: nanoparticles (~20 nm), hollow spheres consisting of nanoparticles (20–30 nm) ranging from 200 to 700 nm in diameter; benzyl alcohol: nanoparticles (10–80 nm) water: nanorods, length: 1–2 µm	[780]
Zn(CH3COO)2·2H2O, NaOH, C2H5OH (solvent)	T: 60 °C; duration: 5–6 min; microwave oven	spherical nanoparticles (4–8 nm)	[781,782]
Zn(CH3COO)2·2H2O, (CH3)2CHOH (isopropanol, solvent)	duration: 180 s; power: 140 W; microwave oven	nanowires	[783]
Zn(CH3COO)2·2H2O, (CH3)2CHOH (isopropanol) + H2O (solvent, various concentrations)	duration: 1–3 min; microwave oven (1450 W)	spheroidal nanostructures	[784]
Zn(NO3)2·6H2O, NaOH (different concentrations), sodium dodecyl sulfate (SDS), C2H5OH + H2O (solvent)	T: 80–180 °C duration: 5–60 min; microwave reactor	rod-like, sheet-like, needle-like and flower-like nanostructures	[785]
Zn(NO3)2·6H2O, NaOH, C2H5OH + H2O (solvent), hexamethylene tetramine (C6H12N4), triethanolamine ((TEA, C6H15NO3, various concentrations)	T: 180 °C duration: 15 min; microwave reactor (600 W)	mulberry-like structures (~150 nm) was constructed by many nanoparticles (~5 nm); flower-like structures; hexagonal structure	[786]
Zn(CH3COO)2·2H2O, Zn(NO3)2·6H2O, zinc metal powder, NaOH, NH4(OH), C2H5OH (solvent), C2H4(OH)2 (solvent)	pH: 10–14; duration: 5–10 min: power: 300–900 W: reactor microwave	marigold-flower like, multipod jasmine- flower like, urchin-rod-flower like, calendula-flower like and rice-grain-shape like	[787]
Zn(CH3COO)2·2H2O, NaOH, H2O (solvent), C2H4(OH)2 (solvent), 2-ethoxyethanol (solvent), Triton X-100	duration: 3 min; power: 300 W; reactor microwave	nanorods with a pencil-like tip, nanorods with hexagonal flat tops, flower-like nanostructures	[788]
bis(acetylacetonato)zinc monohydrate, bis(methylacetato)zinc, bis(dimethylmalonato)zinc, C2H3N (acetonitrile, solvent)	T: 160–220 °C; duration: 30 min; power: 300 W; microwave reactor	spherical nanoparticles (3–16 nm)	[789]
Zn(CH3COO)2·2H2O, NaOH, C2H5OH (solvent), H2O	T: 90 °C; duration: 20 min; microwave reactor	spherical nanoparticles (20–25 nm)	[790]
Zn(CH3COO)2·2H2O, (CH3)2CHOH (isopropanol, solvent), (CH2CH2OH)2NH (diethanolamine)	T: 150–200 °C; microwave reactor	nanospheres	[791]
Zn(CH3COO)2·2H2O, C4H9NO (N,N-dimethylacetamide, DMAc, solvent), H2O	duration: 1.5–6 min; microwave oven (800 W)	particles (300–510 nm) and nanowires	[792]
Zn(CH3COO)2·2H2O, oleic acid, diethylene glycol (solvent)	T: 220–230 °C; duration: 10–15 min; microwave reactor	nanoparticles; diameters: 5–9 nm	[793]
Zn(NO3)2·6H2O, CO(NH2)2, H2O, C2H4(OH)2 (ethylene glycol, solvent)	T: 150 °C; duration: 15 min; microwave reactor (850 W)	flowers	[794]
ZnCl2·2H2O, sodium oleate, tetrabutylammonium hydroxide, tetrahydrofuran (C4H8O, THF, solvent)	T: 125–200 °C; duration: 5 min; microwave reactor	spherical nanoparticles, size: 2.6–3.8 nm	[795]
Zn(CH3COO)2·2H2O, Na2CO3, polyvinyl alcohol (PVA), polyethylene glycol (PEG), diethylene glycol (DEG, solvent),	duration: 1.5–5 min; microwave oven (1150 W)	nanoparticles, SSA: 35–86 m2/g	[796]
Zn(NO3)2·6H2O, NaOH, polyethylene glycol, C2H5OH (solvent)	T: 140 °C; duration: 10 min; microwave oven	rods, diameter: 0.167 ± 0.05 µm, length: 1.63 ± 0.33 µm	[797]
Zn(NO3)2·6H2O, NaOH, C2H5OH (solvent)	duration: 10–15 min; power 33%; microwave reactor (800 W)	nanorods, diameters: 10–20 nm	[705]

,

Table 11. Summary of the microwave solvothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures without any additional heat treatment.

Type of Composite	Substrates	Conditions during Preparation	Properties	Ref.
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, C2H4(OH)2 (solvent)	T: 220 °C, duration: 25 min; power: 100%; microwave reactor (600 W)	Zn1−xCoxO (x = 0, 0.01, 0.05, 0.10 and 0.15); spherical nanoparticles: SSA: 37–39 m2/g, diameter: 30–32 nm, paramagnetic behaviour	[798,799]
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, C2H4(OH)2 (solvent), H2O (different concentrations)	T: 190 °C, duration: 25 min; power: 100%; microwave reactor (600 W)	Zn0.90Co0.10O, control of size of particles within the size range between circa 20 and 53 nm, SSA: 43–21 m2/g	[800]
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, oleic acid, (HOC2H4)2O (diethylene glycol, DEG, solvent)	T: 250 °C, duration: 15 min; power: 100%; microwave reactor	Zn1−xCoxO (x = 0, 0.01, 0.05, 0.10), spherical nanoparticles: diameter: 5–40 nm	[801]
Mn doped ZnO	Zn(CH3COO)2·2H2O, Mn(CH3COO)2·4H2O, C2H4(OH)2 (solvent)	T: 200 °C, duration: 25 min; power: 100%; microwave reactor (600 W)	Zn1−xMnxO (x = 0, 0.01, 0.05, 0.10 0.15, 0.20, 0.25); spherical nanoparticles, diameter: 19–30 nm, SSA: 40–63 m2/g	[802]
Co-Mn co-doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·4H2O Mn(CH3COO)2·4H2O, C2H4(OH)2 (solvent)	T: 190 °C, duration: 25 min; power: 100%; microwave reactor (600 W)	Zn(1−x−y)MnxCoyO NPs was x = y = 0.00, 0.01, 0.05, 0.10, 0.15 (the amount of both ions was equal), spherical nanoparticles, diameter: 19–30 nm, SSA: 40–56 m2/g, paramagnetic and ferromagnetic behaviour	[803]
M doped ZnO (M = Co, Cr, Fe, Mn, Ni)	Zn(NO3)2·6H2O, Co(NO3)2·6H2O, Cr(NO3)3·9H2O, Cr(NO3)3·9H2O, Mn(NO3)2·4H2O, Ni(NO3)2·6H2O, NaOH, C2H5(OH) (solvent), polyethylene glycol MW ≈ 2000	T: 280 °C; pressure: 20 bar; microwave reactor (300 W)	nanoparticles, paramagnetic behaviour	[232]
Mn doped ZnO	Zn(CH3COO)2·2H2O, Mn(CH3COO)2·4H2O, C2H4(OH)2 (solvent)	duration: 30 s cycles (on for 10 s, off for 20 s) for 10 min; power: 33%; microwave reactor (650 W)	Zn1−xMnxO (x = 0, 0.05, 0.10, 0.20), nanoparticles	[804]
Ag-ZnO	Zn(CH3COO)2·2H2O, Ag(CH3COO), C2H4(OH)2 (solvent)	duration: 12 min; power: 1 kW (33%); microwave reactor (3 kW)	ZnO spherical nanoparticles (30–35 nm); Ag spherical nanoparticles (35–39 nm)	[181]
Ag-ZnO	Zn(CH3COO)2·2H2O, AgNO3, C2H4(OH)2 (solvent), H2O	duration: 30 s cycles (on for 21 s, off for 9 s) for 10 min; power: 900 W; microwave oven	spherical nanoparticles (13–30 nm); hexagonal disks (14–165 nm); nanorods diameter: 104 nm, aspect ratio of 2.8; 3.5 nm Ag particles were inserted into the pores of ZnO; SSA: 25–51 m2/g	[805]
Ag-ZnO	Zn(CH3COO)2·2H2O, AgNO3, C2H4(OH)2 (solvent), H2O	T: 170 °C; duration: 30 min; microwave oven	hierarchical architectures constructed by nanoparticles (50 nm)	[806]
Au-ZnO	Zn(CH3COO)2, HAuCl4, oleic acid + oleylamine (solvent)	duration: 10 s–15 min; power: 100–1000 W; microwave oven	nanopyramids, height: 100–130 nm, diameter of the hexagonal basal plane of the final Au-ZnO nanopyramid is about 50–60 nm	[807]
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·2H2O, C2H4(OH)2 (solvent)	T: 280 °C; pressure: 20 bar; duration 20 min; microwave reactor (300 W)	Zn1−xCoxO (x = 0, 0.001, 0.01; 0.05, 0.10, 0.15), nanoparticles	[808]
M doped ZnO (M = Mn, Ni, Co, Cr)	Zn(CH3COO)2·2H2O, Mn(CH3COO)2·4H2O, Ni(CH3COO)2·4H2O, Co(CH3COO)2·2H2O, Cr(CH3COO)3, C2H4(OH)2 (solvent)	T: 280 °C; pressure: 20 bar; duration 40 min; microwave reactor (300 W)	Zn1−xMxO (x = 0, 0.05, 0.10, 0.15), nanoparticles (20–30 nm)	[809]
M doped ZnO (M = V, Co, Fe, Ni, Mn)	Zn(CH3COO)2, Co(CH3COO)2, Fe(CH3COO)2, Ni(CH3COO)2·4H2O, Mn(CH3COO)2, C6H5CH2OH (benzyl alcohol, solvent)	T: 160 °C; duration: 3 min; microwave reactor	Zn1−xMxO (x = 0–0.3), nanoparticles (10–20 nm), Fe doped samples showed room temperature ferromagnetism	[810]
In doped ZnO, Al doped ZnO	Zn(CH3COO)2·2H2O, InCl3·4H2O, AlCl3·6H2O, diethylene glycol (solvent), H2O	T: 200 °C; duration 30 min; laboratory microwave oven (1200 W), 1 h at 400 °C in H2/N2 = 10/90%	nanoparticles (10–15 nm)	[811]
Co doped ZnO, Mn doped ZnO	Zn(NO3)2, Co(NO3)2, Mn(NO3)2, NaOH, C2H5(OH) (solvent), H2O	pH: 12; duration 5 min; power: 150 W; microwave oven	concentration of the dopant was 5%; spherical nanoparticles (10–15 nm), paramagnetic	[812]
Ni doped ZnO	Zn(CH3COO)2·2H2O, Ni(CH3COO)2·2H2O, NaOH, polyvinylpyrrolidone, (CH3)2CHOH (isopropanol, solvent)	duration 5 min; power: 150 W; microwave oven	ZnO:Ni nanorods with diameter: 8–10 nm and length: 35-45 nm	[813]
Al doped ZnO	Zn(CH3COO)2·2H2O, Al(NO3)3, NaOH, C2H5(OH) (solvent)	T: 80 °C; duration 60 min; power: 400 W; microwave oven	Al doping levels: 0, 1.0, 2.0, 3.0, 4.0 at%; spherical-like structures, crystallite size: 11–15 nm	[814]
Al. doped ZnO, Ga doped ZnO, Al, Ga co-doped ZnO	Zn(CH3COO)2·2H2O, Al(NO3)3·9H2O, Ga(NO3)3·xH2O, diethylene glycol (solvent), H2O	T: 200 °C; duration 30 min; microwave reactor (1500 W)	Al and Ga dopant levels were from 0.5 to 2.5 at%; doped and co-doped powders exhibited a broad size distribution with particles around 100–200 nm	[815]
Al2O3 coated ZnO	ZnO NPs (diameter: 12–25 nm, 38.4 m2/g), aluminium triisopropoxide (Al(O-i-Pr)3), NH4OH, C2H5(OH) (solvent)	pH: 12; T: 70 °C; duration 5 min; microwave reactor (500 W)	core-shell structures	[816]
Mg doped ZnO	Zn(NO3)2·6H2O, Mg(NO3)2·6H2O, CO(NH2)2, urea, C2H4(OH)2 (solvent), H2O	T: 130 °C; duration 4 h; power: 150–200 W; microwave reactor	Zn1−xMgxO (x = 0, 0.2, 0.4, 0.6, 0.8), nano- and sub-micron particle size	[817]
Fe3O4@SiO2/ZnO	Fe3O4@SiO2 (different quantities), Zn(CH3COO)2·2H2O, diethylene glycol (DEG, solvent)	T: 160 °C; duration 15–60 h; mechanical stirring	Fe3O4 content: 10–30 wt%; spherical shape, size 250–850 nm	[818]
coaxial ZnO/C/CdS nanocables	ZnO/C core-shell nanocables (80 nm in diameter and a range of 0.5–2 mm in length), CdCl2·2H2O, C2H5(OH) (solvent), thioacetamide	duration 10 min; power: 280 W; microwave refluxing system	CdS nanoparticles (5.5 nm) uniformly deposited on the surface of nanocables	[819]
ZnO/reduced graphene oxide	graphite (modified Hummers method and Fan’s method), Zn(CH3COO)2·2H2O (various concentrations), diethylene glycol (solvent)	duration 10 min; power: 300 W; microwave refluxing system	ZnO nanocrystals (10–14 nm) anchored onto reduced graphene oxide sheets	[820]
ZnO/reduced graphene oxide, Ag/ZnO/reduced graphene oxide	graphite, Zn(CH3COO)2·2H2O (various concentrations), AgNO3, NaOH, C2H4(OH)2	duration: 4 cycles (heated 1 min, stirred for 3 min); microwave oven	ZnO NPs and Ag NPs anchored onto reduced graphene oxide sheets	[821]
C/ZnO	ZnO nanorods grafted by glucose, glycerol	T: 100 °C; duration 30 min; microwave reactor	carbon-coated ZnO nanorods	[822]

and

Table 12. Summary of the microwave solvothermal synthesis of ZnO nanocomposites or ZnO hybrid nanostructures with additional heat treatment.

Type of Composite	Substrates	Conditions during Preparation	Calcination Parameters	Properties	Ref.
undoped ZnO	Zn(CH3COO)2·2H2O, NaOH, isopropanol (solvent)	duration 5 min; microwave oven	600 °C in air for 1 h	spherical nanoparticles (30 nm)	[823]
undoped ZnO	Zn(CH3COO)2·2H2O, NaOH, 1-butyl-3-methylimidazolium chloride (solvent)	duration 1 min; power: 400–1000 W; microwave oven	500 °C in air for 3 h	nanoparticles; size from 15–25 to 50–70 nm	[824]
undoped ZnO	Zn(C5H7O2)2·xH2O, C2H5(OH) (solvent), CH3OH (solvent), polyvinylpyrrolidone (PVP), cetyltrimethylammonium bromide (CTAB), and Triton X100, H2O	duration: 20 s–5 min; power: 160–800 W; microwave oven (800 W) with a water-cooled condenser (reflux system)	500 °C in air for 2–5 min	films on Si, Ge, Cr/Si, glass, indium tin oxide coated glass and polymer substrates, (spherical nanoparticles (diameter: ∼15) nm or nanorods (length: 1–3 μm))	[825,826,827,828,829,830]
undoped ZnO	Zn(CH3COO)2·2H2O, NaOH, dimethylformamide (DMF)	duration 60 min; power: 300 W; microwave oven	500 °C in air for 3 h	spherical nanoparticles (as-synthesised 24–26 nm, annealed 33–34 nm)	[831]
undoped ZnO	Zn(NO3)2·6H2O, 2-methylimidazole, HCOONa, CH3OH (solvent)	T: 120 °C; duration: 2 h; microwave reactor	550 °C in air for 2 h	uniform particle approximately rhombic dodecahedron facets, size: ~97 nm	[832]
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·2H2O, urea, C2H4(OH)2 (solvent)	duration 30 min; power: 500 W; microwave oven	400 °C in air	Zn1−xCoxO (x = 0.1, 0.2, 0.3, 0.4), nanoparticles (~24 nm), paramagnetic behaviour	[833]
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, C2H4(OH)2 (solvent)	T: 220 °C, duration: 25 min; power: 100%; microwave reactor (600 W)	800 °C in nitrogen for 0.5 h, 800 °C in synthetic air for 0.5 h	Zn1−xCoxO (x = 0, 0.01, 0.05, 0.10, 0.15); spherical nanoparticles: SSA: 3 m2/g, diameter: 300–400 nm, paramagnetic behaviour	[798]
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·2H2O, NaOH, HCl, C2H4(OH)2 (solvent)	pH: 6–12; duration: until microwave heating solvents were evaporated; power: 500 W; microwave oven	400 °C in air	Zn0.94Co0.06O; nanoparticles (7–23 nm), paramagnetic behaviour	[834]
Co doped ZnO	Zn(CH3COO)2·2H2O, Co(CH3COO)2·2H2O, urea, C2H4(OH)2 (solvent)	until microwave heating solvents were evaporated; microwave oven	300 °C in air for 1 h	Zn1−xCoxO (x = 0.001-0.004), average crystallite size 18–28 nm; super paramagnetic nature and ferromagnetic behaviour	[835]
Fe doped ZnO	Zn(CH3COO)2·2H2O, Fe(NO3)2·9H2O, NaOH, polyvinylpyrrolidone (PVP), C2H5OH (solvent)	pH: 11; duration 2 min; power: 140 W; microwave oven	200 °C in air for 1 h	nanoparticles (11–17 nm), Fe content: 0–20%	[836]
Mn doped ZnO	Zn(CH3COO)2·2H2O, Mn(CH3COO)2·4H2O, C2H4(OH)2 (solvent)	duration: 30 s cycles for 20 min in total; power: 650 W; microwave oven	400 °C in air	Zn1−xMnxO (x = 0.1–0.4), spherical nanoparticles	[837]
Ag-ZnO	Zn(CH3COO)2·2H2O, AgNO3, Na2O2, isopropanol (solvent), cetyltrimethylammonium bromide (CTAB)	T: 200 °C; duration 2–6 h; microwave reactor	300 °C in air for 2 h	chrysanthemum-like prismatic nanorods	[838]

) are zinc acetate (Zn(CH₃COO)₂·xH₂O), zinc nitrate (Zn(NO₃)₂·6H₂O), zinc acetylacetonate (Zn(C₅H₇O₂)₂), zinc chloride (ZnCl₂), and other (Figure 36). The following were used sporadically as the “Zn²⁺” zinc cation precursor in the microwave solvothermal synthesis: ZnO [775,816,819,822], bis(acetylacetonato)zinc [789], bis(methylacetato)zinc [789], bis(dimethylmalonato)zinc [789].

The popularity of zinc acetate (68.7%) results mainly from its availability and solubility in most organic solvents.

The most popular reactants being hydroxide anion precursor chemicals “OH⁻” used for the ZnO synthesis according to the data collected from the literature review [758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838] (Table 10, Table 11 and Table 12) (Figure 37) are sodium hydroxide (NaOH), ammonia water (NH₃·H₂O, NH₄(OH)), as well as amides and amines, e.g., C₂H₈N₂ [775], hexamethylene tetramine (C₆H₁₂N₄) [786], triethanolamine (TEA, C₆H₁₅NO₃) [786], diethanolamine ((CH₂CH₂OH)₂NH) [791], thioacetamide (C₂H₅NS) [819]. In over a half of publications, no hydroxide anion precursor chemical “OH⁻” was used, because the “OH⁻” anion was formed during the synthesis as a result of a reaction of a Zn²⁺ salt with a solvent, forming basic salts [402].

4.2. Surfactants

As mentioned before, surfactants are commonly used in nanotechnology for passivation of surface, control of shape, and giving of photophysical properties [740]. The following surfactants were used in microwave solvothermal syntheses of ZnO NMs for the purpose of controlling the morphology:

-

Polyvinyl alcohol (PVA) for obtaining nanoflakes [779].

-

Polyethylene glycol 400 (PEG-400) for obtaining nanorods [779].

-

Polyvinylpyrrolidone (PVP) for obtaining flowers with rod-like petals [779] and nanorods [825,827,828,829,830].

-

Cetyltrimethylammonium-bromide (CTAB) for obtaining flowers with rod-like petals [779], nanorods [827,830] and chrysanthemum-like prismatic nanorods [838].

-

Polyethylene glycol for obtaining NPs [232].

-

Methylimidazole for obtaining uniform particles with approximately rhombic dodecahedron facets [832].

-

Triton X100 for obtaining nanorods [827,830].

4.3. Morphology

Morphologies of ZnO reported in the literature are presented in the description of properties in Table 10, Table 11 and Table 12 [758,759,760,761,762,763,764,765,766,767,768,769,770,771,772,773,774,775,776,777,778,779,780,781,782,783,784,785,786,787,788,789,790,791,792,793,794,795,796,797,798,799,800,801,802,803,804,805,806,807,808,809,810,811,812,813,814,815,816,817,818,819,820,821,822,823,824,825,826,827,828,829,830,831,832,833,834,835,836,837,838]. ZnO nanomaterials and micromaterials synthesised with the use of the microwave solvothermal synthesis had the following shapes: butterfly-like [765], cables [819], core-shell structures [816], hexagonal prisms [765], hierarchical architectures constructed by nanoparticles [806], mulberry-like structures [786], needle-like [785], peanut-like structures [765], pyramids [807], rice-like particles [777,787], spheroidal nanostructures [784], sword-like wires [765], tubes [770], wires [783,792], sheet-like structures [785], spheres [765,791], flower-like structures [573,775,779,785,786,787,794,838], rod-like shape structures [402,705,764,767,770,771,778,780,785,788,797,805,813,822,825,826,827,828,829,830], and spherical-like shape structures [181,402,758,759,760,761,762,763,764,766,769,772,773,774,776,778,780,789,790,793,795,796,798,799,800,801,802,803,804,808,809,810,811,812,814,818,823,824,825,826,827,828,829,830,831,833,834,835,836,837,864].

Examples of various morphologies of ZnO obtained by the microwave solvothermal method are presented in Figure 38, Figure 39, Figure 40, Figure 41 and Figure 42. Changes in the colours of suspensions of doped ZnO NMs caused by the type and quantity of dopant are reflected in Figure 43, Figure 44, Figure 45 and Figure 46, while Figure 47 shows a change in the colour of ZnO NPs colloid caused by the use of various sizes of NPs.

4.4. Microwave Solvothermal Synthesis of ZnO without Any Additional Heat Treatment

One of the main advantages of the solvothermal technology is the possibility to use mixtures of precursors in the form of solutions rather than sedimenting suspensions of hydroxides, as is the case in the majority of hydrothermal syntheses of ZnO in water. Thanks to that, numerous problems are avoided, above all with heterogeneity and unrepeatability of the precursor (suspension sedimentation) and the lack of stirring of the precursor suspension in the reactor, which critically affects the quality and homogeneity of the obtained products. The application of non-aqueous solutions or those with a limited water content eliminates the presence of competitive mechanisms during the ZnO NPs synthesis, which lead to formation of heterogeneous products, with varying morphology and chemical composition. Among various synthesis routes, the solvothermal method is noted as attractive for its simplicity and the possibility to have a better control of the reaction mechanism and the better rate of particle growth [402,753,754,755,756,757].

The microwave solvothermal synthesis enables also a ZnO NMs synthesis from a suspension, which consists in obtaining a suspension of a Zn²⁺ salt in an organic solvent with an addition of a substance precipitating the hydroxide anion precursor chemical “OH⁻”, e.g., NaOH.

4.5. Microwave Solvothermal Synthesis of ZnO from a Solution

During our research on the microwave solvothermal synthesis we noticed that by changing the water content in the solution obtained by dissolving zinc acetate in ethylene glycol it is possible to control the size of ZnO NPs precisely within the range from ca. 15 nm to 120 nm [758]. Figure 48 presents an example size distribution of ZnO NPs crystallites obtained from precursor solutions with varying H₂O contents. All microwave solvothermal syntheses were carried out with the same parameters (25 min, 230 °C). Due to the lack of access to research enabling us to analyse by-products, we proposed a general equation of the reaction (33) of ZnO NPs in ethylene glycol:

$$\mathrm{Zn}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·2{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2,{ \mathrm{H}}_2\mathrm{O}, \mathrm{MH} \left(\mathrm{T}, \mathrm{P}\right) }{\to }\mathrm{ZnO}\downarrow +\mathrm{other} \mathrm{products} \left(\mathrm{liquid} \mathrm{or} \mathrm{gas}\right)$$

(33)

We proved that by determining an experimental correlation between the average size of ZnO NPs and the water content in the precursor, a calibration curve of the ZnO NPs size is obtained, based on which particle size can be controlled. Of course, each calibration curve of ZnO NPs size refers to a specific lot of reactants for which it was determined. Each time, when changing the lot of reactants, the calibration curve of ZnO NPs size must be determined again to preserve the precision of particle size control.

Thorough research on the microwave solvothermal synthesis of ZnO NPs obtained from a precursor solution, zinc acetate dissolved in ethylene glycol with an addition of heavy water, allowed us to explain and verify the synthesis mechanism [402]. It was crucial for our experiment to track and learn the fate of water at various stages of the synthesis. Owing to the results of XRD tests of ZnO NPs synthesis products for different durations (Figure 49) and the elemental analysis, we managed to determine that the solid intermediate of the solvothermal synthesis of ZnO was lamellar hydroxy-zinc acetate (LHZA, Zn₅(OH)₈(CH₃COO)₂·2H₂O) with the lamellar structure (Figure 50a). Based on the results of the gas chromatography with mass spectrometry and Fourier transform infrared spectroscopy we proved that the liquid by-products were esters, mainly ethanediol monoacetate (HOC₂H₄OOCH₃), and small quantities of ethanediol diacetate (C₂H₄(OOCH₃)₂). Thanks to the water content analysis by the Karl Fischer method, we determined that also water was one of the liquid by-products.

Learning all the primary products of the solvothermal synthesis of ZnO allowed us to write a general equation of the reaction (34):

$$\mathrm{Zn}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2,{ \mathrm{H}}_2\mathrm{O}, \mathrm{MH} \left(\mathrm{T}, \mathrm{P}\right) }{\to }\mathrm{ZnO}\downarrow +{\mathrm{H}}_2\mathrm{O}+\left(2-\mathrm{m}\right){\mathrm{HOC}}_2{\mathrm{H}}_4{\mathrm{OOCH}}_3+{\mathrm{mC}}_2{\mathrm{H}}_4{\left({\mathrm{OOCH}}_3\right)}_2$$

(34)

The above equation may be expressed in a more general way (35):

$$\mathrm{Zn}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2+\mathrm{alcohol}\stackrel{ \left(\mathrm{T}, \mathrm{P}\right) }{\to }\mathrm{ZnO}\downarrow +\mathrm{esters}+{\mathrm{H}}_2\mathrm{O}$$

(35)

An identical general equation of the reaction (35) was proposed by Du te al. [394], Tonto et al. [396], Šarić et al. [398], Zhao et al. [780], Yiamsawas et al. [840], and Yuan et al. [841], while the results achieved in the papers by Bilecka et al. [762], Bhatte et al. [769,776], and Demir [842] permit deriving a general equation independently (35). The results in the paper by Gotic et al. [843], which describes the solvothermal synthesis of Fe₃O₄ (hematite), and the results in the paper by Bilecka et al. [763], which describes the solvothermal synthesis of CoO, ZnO, Fe₃O₄, MnO and Mn₃O₄, permit deriving a general equation of the reaction (36) of synthesis of metal oxide (M_xO_z) NPs from a salt of a metal acetate (M(CH₃COO)_x) dissolved in an alcohol.

$$\mathrm{M}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_x+\mathrm{alcohol}\stackrel{ \left(\mathrm{T}, \mathrm{P}\right) }{\to }{\mathrm{M}}_y{\mathrm{O}}_z+\mathrm{esters}+{\mathrm{H}}_2\mathrm{O}$$

(36)

Learning the fate of heavy water allowed us to explain the mechanism of ZnO NPs size control in the microwave solvothermal synthesis, which we described in detail in our paper [402]. The mechanism of size control of ZnO NPs obtained by the microwave solvothermal synthesis may be divided into 4 stages (Republished with permission of ©IOP Publishing from [402], Copyright (2018), permission conveyed through Copyright Clearance Center, INC. All rights reserved.):

(1)

Dissolution of zinc acetate in ethylene glycol (37,38), preparation of the precursor with a specified H₂O content (39)–(41):

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{solid}\right)}\stackrel{70°\mathrm{C}, \mathrm{EG} }{\to }{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{dissolved}\right)}$$

(37)

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{solid}\right)}+ 2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{70°\mathrm{C}, \mathrm{EG} }{\to }{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2{}_{\left(\mathrm{dissolved}\right)}+2{\mathrm{H}}_2\mathrm{O}$$

(38)

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}\stackrel{{ \mathrm{H}}_2\mathrm{O}, \mathrm{EG}}{\leftrightarrow }\left({\mathrm{CH}}_3\mathrm{COO}\right){ \mathrm{Zn}}^{+}+{ \mathrm{CH}}_3{\mathrm{COO}}^{-}$$

(39)

$${\mathrm{CH}}_3{\mathrm{COO}}^{-}+{ \mathrm{H}}_2\mathrm{O}\stackrel{{ \mathrm{H}}_2\mathrm{O}, \mathrm{EG}}{\leftrightarrow }{\mathrm{CH}}_3\mathrm{COOH} +{ \mathrm{OH}}^{-}$$

(40)

$$\left({\mathrm{CH}}_3\mathrm{COO}\right){\mathrm{Zn}}^{+}+{\mathrm{OH}}^{-}\stackrel{{ \mathrm{H}}_2\mathrm{O}, \mathrm{EG}}{\leftrightarrow }\left({\mathrm{CH}}_3\mathrm{COO}\right)\left(\mathrm{OH}\right)\mathrm{Zn}$$

(41)

(2)

Formation (42)–(45) and growth of the intermediate (46):

$$5{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+8{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+x{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{T}, \mathrm{P} }{\to }{\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{precipitation}\right)}+8{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(42)

or possibly e.g.,

$$5{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+x{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{T}, \mathrm{P} }{\to }{\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{precipitation}\right)}+8{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2$$

(43)

$$5\left({\mathrm{CH}}_3\mathrm{COO}\right)\left(\mathrm{OH}\right)\mathrm{Zn}+3{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+x{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{T}, \mathrm{P} }{\to }{\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{precipitation}\right)}+3{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(44)

$$2\left({\mathrm{CH}}_3\mathrm{COO}\right)\left(\mathrm{OH}\right)\mathrm{Zn} + 3{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn} + x{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{T}, \mathrm{P} }{\to }{\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{precipitation}\right)}+6{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(45)

$${\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2\mathrm{O}+ 5{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn} + 8{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+x{\mathrm{H}}_2\mathrm{O} \stackrel{\mathrm{T}, \mathrm{P} }{\to }\left(1+1\right){\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left(\mathrm{CH}3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{growth}\right)}+8{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(46)

nH₂O comes from the simultaneous esterification reaction (47) or (48)

$${\mathrm{C}}_2{\mathrm{H}}_4\left(\mathrm{OH}\right){ }_2+{\mathrm{CH}}_3\mathrm{COOH}\leftrightarrow {\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}$$

(47)

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+{\mathrm{CH}}_3\mathrm{COOH}\leftrightarrow {\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}·{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2·{\mathrm{H}}_2\mathrm{O}+{ \mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(48)

(3)

Achievement of equilibrium constant of the ester hydrolysis reaction for Equation (49) and at the same time of equilibrium constant of the esterification reaction (47) and decomposition of the intermediate caused by temperature (50):

$${\mathrm{HOC}}_2{\mathrm{H}}_4{\mathrm{OOCH}}_3+{ \mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2+{\mathrm{CH}}_3\mathrm{COOH}$$

(49)

$${\mathrm{Zn}}_5{\left(\mathrm{OH}\right)}_8{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2·x{\mathrm{H}}_2\mathrm{O}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2 \stackrel{\mathrm{T}, \mathrm{P} }{\to } 5\mathrm{ZnO}+2{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}+5{\mathrm{H}}_2\mathrm{O}+x{\mathrm{H}}_2\mathrm{O}$$

(50)

(4)

Growth of existing ZnO NPs (51,52), which is confirmed by the results in Figure 51:

$$\mathrm{ZnO}+{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{\mathrm{T}, \mathrm{P} }{\to }\left(1+1\right){\mathrm{ZnO}}_{\left(\mathrm{particle} \mathrm{growth}\right)}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(51)

$$\mathrm{ZnO}+\left({\mathrm{CH}}_3\mathrm{COO}\right)\left(\mathrm{OH}\right)\mathrm{Zn}+{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{\mathrm{T}, \mathrm{P} }{\to }\left(1+1\right){\mathrm{ZnO}}_{\left(\mathrm{particle} \mathrm{growth}\right)}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(52)

The general equation of the microwave solvothermal synthesis reaction of ZnO NPs in ethylene glycol that takes into account obtaining only HOC₂H₄OOCH₃ ester is as follows:

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+2{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2\stackrel{{\mathrm{C}}_2{\mathrm{H}}_4{\left(\mathrm{OH}\right)}_2,{ \mathrm{H}}_2\mathrm{O}, \mathrm{T}, \mathrm{P} }{\to }\mathrm{ZnO}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{CH}}_3{\mathrm{COOC}}_2{\mathrm{H}}_4\mathrm{OH}$$

(53)

The detailed description of the above mechanism of the ZnO NPs synthesis, which is also presented in Figure 52, is included in the paper [402]. For the purposes of this review, we present a general description below:

• As a result of hydrolysis of zinc acetate, water leads to the formation of acetic acid, which participates in an esterification reaction with ethylene glycol during the microwave solvothermal synthesis.
• The products of the esterification reaction are esters and water. However, the course of the reaction of obtaining and growth of the intermediate, Zn₅(OH)₈(CH₃COO)₂·xH₂O, is possible only through the co-existence of the esterification reaction. Only water forming in the esterification reaction participates in reactions of obtaining/growth of the intermediate, Zn₅(OH)₈(CH₃COO)₂·xH₂O. Once the equilibrium constant of the esterification reaction is reached, the intermediate rapidly decomposes into ZnO NPs, H₂O and esters.
• The control of particle size arising from a change in the water content in the precursor is a consequence of the change in the quantity of formed crystalline nuclei of ZnO (NPs) relative to the remaining unconverted quantity of substrate (zinc acetate). After the decomposition of the intermediate into homogeneous nuclei of ZnO (NPs), no subsequent nuclei of ZnO (NPs) are formed as a result of further reactions. The only process that might occur is the growth of the existing nuclei of ZnO (NPs) until the still unreacted substrates are used up.
• Water fulfils the function of a catalyst in the described ZnO NPs solvothermal synthesis reaction. Water participates in the reaction with substrates and forms an unstable intermediate, Zn₅(OH)₈(CH₃COO)₂·xH₂O, which at the same time is a catalyst of the esterification reaction.

We believe that generally each ZnO solvothermal synthesis based on a precursor obtained as a result of dissolution of zinc acetate in an alcohol or a polyol will proceed according to the mechanism described by us above [402]. In this solvothermal synthesis, the alcohol/polyol fulfils a double function: the solvent of zinc acetate, and the reactant. Particular attention should be drawn to the fact that the lack of control over the H₂O content in organic solvents will be manifested in the lack of repeatability of properties of the obtained ZnO NMs. It must be borne in mind that precursors obtained based on organic solvents must be always closed tightly because they absorb moisture from air/environment.

Other authors considered a mechanism of ZnO synthesis from zinc acetate dissolved in an alcohol but without analysing the solid intermediate, e.g., Bhatte et al. [769,776] suggest that Zn(OH)₂ is the intermediate of the discussed solvothermal reaction of ZnO synthesis.

4.6. Microwave Solvothermal Synthesis of ZnO from a Suspension

The microwave solvothermal synthesis of ZnO NMs has one major advantage: once the reactants (Zn²⁺) have undergone a complete reaction, the further growth of ZnO NMs crystals through the Ostwald ripening process is considerably limited by the presence of the organic solvent. ZnO solubility in organic solvents is so low that it prevents recrystallisation. Of course, theoretically, the Ostwald ripening process may not take place where:

-

content of, ${\mathrm{m}}_{{\mathrm{H}}_2\mathrm{O}}=0 \mathrm{wt}\%$,

-

water being formed is collected physically or bound chemically,

-

other substances which may digest/dissolve ZnO are not formed.

A limited Ostwald ripening process in the solvothermal synthesis enables obtaining ZnO NPs with the size below 10 nm (quantum dots), which is extremely rare in the case of hydrothermal syntheses. ZnO quantum dots are NPs with the size ranging from 2 to 10 nm.

The paper by Kuo et al. [774] describes obtaining ZnO NPs from a precursor formed by precipitation of Zn(CH₃COO)₂ dissolved in isopropanol, in an aqueous solution of NaOH. The authors [774] proposed the following general equation of the reaction (54):

$${\left({\mathrm{CH}}_3\mathrm{COO}\right)}_2\mathrm{Zn}+\mathrm{NaOH}\stackrel{}{\to }\mathrm{ZnO}+{\mathrm{H}}_2\mathrm{O}+2\mathrm{Na}\left({\mathrm{CH}}_3\mathrm{COO}\right)$$

(54)

The product of their synthesis were spherical ZnO NPs with the average size of 3.5 nm.

The paper by Saloga et al. [795] describes obtaining ZnO NPs through a multi-stage synthesis. First, their prepared zinc oleate by mixing ZnCl₂ with sodium oleate, which they rinsed with water and dried. Next, dry zinc oleate was dissolved in tetrahydrofuran and then tetrabutylammonium hydroxide was added to the solution. Thus obtained suspension was subjected to a microwave synthesis lasting 5 min at the temperatures of 125 °C, 150 °C, 175 °C and 200 °C. Saloga et al. [795] describe obtaining ZnO NPs with the average size of 2.6 nm (125 °C), 2.7 nm (150 °C), 3.1 nm (175 °C), and 3.8 nm (200 °C).

Saoud et al. [705] obtained nanorods from a suspension of a precursor obtained by dissolving Zn(NO₃)₂ in ethanol, subsequently adding such a quantity of the NaOH solution until the pH of the obtained solution reached 10. The suspension was exposed to microwave irradiation for approximately 10–15 min and was removed upon onset of boiling. The synthesis product was nanorods with diameters ranging from 10 to 20 nm (Figure 53).

Details of other research papers concerning the microwave solvothermal synthesis of ZnO without any additional heat treatment are presented in Table 10.

4.7. Types of ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Solvothermal Synthesis

The microwave solvothermal synthesis enables obtaining:

-

ZnO doped with the following ions: Al³⁺ [811,814,815], Co²⁺ [232,798,799,800,801,808,809,810,812,833,834,835], Cr³⁺ [232,809], Fe²⁺ [232,810,836], Ga³⁺ [815], In³⁺ [811], Mg²⁺ [817], Mn²⁺ [232,802,804,809,810,812,837], Ni²⁺ [232,809,810,813] and V⁵⁺ [810].

-

ZnO co-doped with the following ions: Co²⁺–Mn²⁺ [803] and Al³⁺–Ga³⁺ [815].

-

ZnO composites or ZnO hybrid structures: Ag–ZnO [181,805,806,838], Au–ZnO [807], Fe₃O₄@SiO₂/ZnO [818], coaxial ZnO/C/CdS nanocables [819], ZnO/reduced graphene oxide [819,821], Ag/ZnO/reduced graphene oxide [821], and carbon-coated ZnO nanorods [822].

4.8. ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Solvothermal Synthesis without Any Additional Heat Treatment

One of the main advantages of the solvothermal synthesis of doped ZnO is the multifunctionality of the organic solvent, which may fulfil the function of a reactant, a dopant stabilising agent, limit the uncontrolled growth of particles, and enables a precise control of the particle size.

Our research [798,799,801] proves that the solvothermal synthesis, through selection of appropriate reactants (Zn(CH₃COO)₂·2H₂O, Co(CH₃COO)₂·4H₂O, C₂H₄(OH)₂), enables obtaining Co doped ZnO with the nominal content of Co²⁺ ions being at least 15 mol% in the form of a single-phase material (without foreign phase inclusions). We used ethylene glycol, which has slightly reducing properties, which enabled stabilisation of the oxidation state of the Co²⁺ dopant during the synthesis. Co doped ZnO obtained by us displayed paramagnetic properties with a contribution of antiferromagnetic coupling of Co–Co pairs [798]. We also confirmed that the method, which we discovered, of ZnO particle size control in the microwave solvothermal synthesis [402,758] permitted also control of Co doped ZnO particle size within the size range of 20–53 nm.

Bhattacharyya et al. [805] and Wang et al. [806] described obtaining an Ag–ZnO composite with the use of a mixture containing identical components: (Zn(CH₃COO)₂·2H₂O, AgNO₃, C₂H₄(OH)₂ (solvent), H₂O and C₂H₄(OH)₂ (solvent)). Bhattacharyya et al. [805] carried out 15-min syntheses in a domestic microwave oven modified with a refluxing system, while Wang et al. [806] carried out 30-min syntheses at the temperature of 170 °C in a microwave reactor. Bhattacharyya et al. [805] obtained an Ag–ZnO composite depending on the Ag content (Ag:ZnO molar ratio of 0.02, 0.05, 0.15, and 0.22) with the nanodisk and nanorod shape (Figure 54), where Ag NPs sized 3.5 nm were inserted into the pores of ZnO. Wang et al. [806], in turn, obtained an Ag–ZnO composite composed of microspheres (Figure 55), where the increase in the content of Ag NPs with the diameter below 5 nm from 0.5 mol% to 3 mol% resulted in a decrease in the size of irregular semi-circular microspheres.

One of the most interesting papers is the one by Liu et al. [820], which describes obtaining reduced graphene oxide sheets covered with ZnO NPs. As the synthesis precursor, zinc acetate dissolved in diethylene glycol with the addition of reduced graphene oxide sheets was used. Synthesis reactions lasting 10 min were carried out in a microwave refluxing system. Products prepared with different amounts of zinc acetate 0.0023, 0.0046, 0.0069, 0.0092, and 0.0115 M. By changing the zinc acetate concentration, the authors controlled the degree of coverage of the reduced graphene oxide sheets surface, which is presented in Figure 56 and Figure 57. The authors [820] successfully obtained the ZnO NPs (10–14 nm) content in ZnO/reduced graphene oxide samples from 11.55 wt% to 26.89 wt%.

Details of other research papers concerning the ZnO nanocomposites or ZnO hybrid nanostructures obtained by the solvothermal synthesis without any additional heat treatment are presented in Table 11.

4.9. ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Solvothermal Synthesis with Additional Heat Treatment

One of the main purposes of additional heat treatment of microwave solvothermal synthesis products, among others, undoped ZnO NMs in the solvothermal synthesis according to the publication [823,824,825,826,827,828,829,830,831], was to remove unconverted reactants. A perfect example here is the cycle of papers by Brahma et al. [825,826,827,828,829,830], which describe the growth of ZnO nanorods on an amorphous or disordered surface (silica, glass and polymer substrates). Brahma et al. [825] used a mixture obtained by mixing zinc acetylacetonate dissolved in ethanol with polyvinylpyrrolidone dissolved in water for the synthesis. The obtained mixture together with the silica substrate immersed therein was introduced for a duration between 20 s and 5 min (800 W) in a domestic-type microwave oven equipped with a water-cooled condenser (reflux system) placed outside the microwave oven. After cooling the suspension, the substrates were removed, rinsed and dried. For the purposes of removing the surfactant residue, the covered substrates were soaked (2–5 min) at 500 °C in the air atmosphere. An example of the obtained morphology of the ZnO film grown on Si(100) is presented in Figure 58.

In the case of a microwave solvothermal synthesis of doped ZnO, additional heat treatment of samples may lead to an increase in the ZnO particle size (recrystallisation process), oxidation or reduction of dopant ions and formation of foreign inclusions. The results of our research [798] show the effect of soaking ZnO doped with Co²⁺ ions. We obtained Co doped ZnO from a solution obtained by dissolving a mixture of zinc acetate with cobalt acetate (II) in ethylene glycol. We performed syntheses lasting 25 min at the temperature of 220 °C in a microwave reactor. Figure 59 shows a homogeneous product of Zn_1−xCo_xO NPs synthesis (x = 0, 0.01, 0.05, 0.10, 0.15) in the form of spherical particles with the size of 30–40 nm without foreign phase inclusions. The obtained samples were subjected to additional soaking for 0.5 h at 800 °C in an oxidising (synthetic air, Figure 60) and a reducing (nitrogen, Figure 61) atmosphere. XRD tests proved the presence of a Co₃O₄ phase in the Zn_0.85Co_0.15O sample soaked in air, while the presence of metallic Co in the Zn_0.95Co_0.05O, Zn_0.90Co_0.10O and Zn_0.85Co_0.15O samples soaked in nitrogen. Co²⁺ ions underwent oxidation to Co³⁺ ions during the soaking in synthetic air, which is presented by the equation of the reaction (55).

$$4{\mathrm{Co}}^{2+}+{\mathrm{O}}_2\to 4{\mathrm{Co}}^{3+}+2{\mathrm{O}}^{2-}$$

(55)

Co²⁺ ions underwent reduction to metallic Co⁰ in nitrogen, which is presented by the equation of the reaction (56).

$$2{\mathrm{Co}}^{2+}+{\mathrm{N}}_2\to 2{\mathrm{Co}}^0+2{\mathrm{N}}^{2+}$$

(56)

The soaking of the Zn_1−xCo_xO NPs samples resulted in:

-

a change in their specific surface area from 37–39 m²/g to merely 3 m²/g,

-

an increase in the particle size from the range of 30–40 nm to the range of 50–2000 nm depending on the amount of Co,

-

a change in the morphology from homogeneous spherical NPs (Figure 59) to heterogeneous hexagonal or cubic NPs (Figure 60 and Figure 61).

Details of other research papers concerning the ZnO nanocomposites or ZnO hybrid nanostructures obtained by the solvothermal synthesis with additional heat treatment are presented in Table 12.

5. Microwave Hybrid Synthesis of ZnO

Microwave hybrid syntheses of ZnO NMs are divided into two following groups:

(1)

Microwave hybrid synthesis method of pure ZnO nano and microstructures, where the literature review results [844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886] are summarised in

Table 13. Summary of synthesis of pure ZnO by the microwave hybrid method.

Type of Method	Substrates	Conditions during Preparation	Calcination Parameters	Properties	Ref.
Ultrasonic microwave synthesis	Zn(CH3COO)2·2H2O, ZnCl2·2H2O, 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) (different concentrations), H2O	pH: 5.4–9.4; T: 110 °C; duration: 17 min (discontinuous ultrasonic irradiation (1 s sonication and 2 s interruption); power: 500 W; microwave and ultrasonic wave combined reactor	-	half-backed grenade-like ZnO microstructures, uniform spindle-like ZnO microstructures; spindle-like to double-prism-like structures	[844]
Ultraviolet assisted - ultrasonic microwave synthesis	Zn(CH3COO)2·2H2O, C16H33(CH3)3NBr (cetyltrimethylammonium bromide, CTAB), (CH2)6N4 (hexamethylenetetramine, HMT), H2O	T: 98 °C; duration: 10–25 min; power: 150 W; UV- microwave and ultrasonic wave combined reactor	-	hourglass-like ZnO microstructures (diameter: 1 µm, length: 2 µm)	[845]
Microwave induced combustion synthesis	ZnSO4, oxalic acid, polyvinyl alcohol (fuel)	duration: 0–30 min; power level: 0–90%; microwave oven	-	nanoparticles: crystallite size: ~33 nm	[846]
Microwave induced combustion process	Zn(NO3)2·6H2O, urea (fuel), H2O	power: 170–680 W; microwave oven	-	flower-like microstructures (2–5 μm) and irregular block-shaped particles (100–300 nm)	[847]
Microwave induced combustion process	Zn(NO3)2·6H2O, urea (fuel), H2O	duration: 10 min; microwave oven (700 W)		spherical nanoparticles (10–100 nm)	[848]
Microwave induced combustion process	Zn(NO3)2·6H2O, urea (fuel), H2O	duration: 20 min; microwave oven (800 W)		nanoplatelets	[849]
Microwave induced combustion process	Zn(NO3)2·6H2O, C2H4(OH)2 (fuel)	microwave oven (800 W)	-	foamy and porous structures	[850]
Microwave induced combustion process	Zn(NO3)2·6H2O, glycine (fuel)	duration: 2 min; power: 80%; microwave oven	-	nanoparticles (20–25 nm)	[851]
Microwave induced combustion process	Zn(NO3)2·6H2O, using citric acid (C6H8O7) as a fuel, NH4OH, H2O	pH: 4; duration: 2 min; power: 80%; microwave oven	800 °C for 2 h in air	rods	[852]
Microwave induced combustion process	Zn(NO3)2·6H2O, glycine (fuel), H2O	duration: 10 min; microwave oven (750 W)	-	nanoflakes	[853]
Microwave induced combustion process	Zn(CH3COO)2·2H2O, H2O	duration: 6 min; power: 800 W; microwave oven	-	powder	[854]
Microwave induced combustion process	Zn(NO3)2·6H2O, Indian bael (Aegle marmelos) juice - different volumes (fuel)	duration: 10 min; microwave oven	500 °C for 1 h in air	heterogeneous particles, sheet like structures,	[855]
Microwave assisted annealing	Zn(OH)2, NH4(OH)	T: 140–320 °C, duration: 30 min; microwave reactor	-	layers on SiO2	[856]
Sol-gel -microwave assisted annealing	Zn(CH3COO)2·2H2, 2-methoxyethanol (ME), monoethanolamine (MEA),	T: 140–320 °C, duration: 3–12 min; power: 65 W; microwave oven (750 W)	-	layers (40 nm) on indium-tin oxide cathodes	[857]
Microwave assisted annealing	Zn(NO3)2·6H2O, H2O	T: 250 °C, duration: 3 min; microwave oven (3 kW)	-	layers on SnO2	[858]
Microwave assisted annealing	Zn(NO3)2·6H2O, extract (fruit, seed or pulp), H2O	duration: 8 min; power: 340 W; microwave oven	-	flower-like, hexagonal, and block-shaped nanostructures, size: 27–85	[859]
Microwave assisted annealing	Zn(NO3)2·6H2O, cetyltrimethylammonium bromide (CTAB), hexamethylenetetramine – (HMTA) (different concentrations), H2O	duration: 30, 60 min; microwave oven	-	nanopowders	[860]
Microwave assisted annealing	Zn(NO3)2·6H2O, PEG400 (polyethylene glycol, M = 400), NaOH, H2O, C2H5(OH)	duration: 10 × 10 min and 4 × 10 min with on–off mode with a duration interval of 1 min; power: 136–800 W; microwave oven (800 W)	500 °C in air for 0.5 h	sphere particles (10–50 nm) and rods (lengths: 50–200 nm, diameters: 15–50 nm)	[861]
Sol-gel – microwave assisted annealing	Zn(NO3)2·6H2O, HNO3 (65%), H2O	duration: 30–40 min; microwave oven	-	SSA: 0.12–0.2 m2/g, corn-like microstructures	[862]
Sol-gel - microwave assisted annealing	Zn(CH3COO)2·2H2O, C2H4(OH)2 (solvent)	pH: 6; duration: 10 min; power: 600 W; microwave oven	450 °C in air for 2 h	nanoparticles (average crystallite size 24 nm)	[863]
Sol-gel - microwave assisted annealing	Zn(CH3COO)2·2H2O, LiOH·H2O, C2H5OH (solvent)	T: 30–60 °C; duration: 10–40 min; microwave reactor (600 W)	80 °C in air for 2 h	quantum-sized particles 4–7 nm	[864]
Microwave assisted sintering	ZnO, organic compounds	duration: 10–30 min; microwave oven (900 W)	-	thick films on alumina substrate, heterogeneous microstructures	[865]
Microwave assisted sintering	ZnO powder, carbon charcoal powder	duration: 20–60 min; microwave oven (1000 W)	-	ZnO nanoclusters, size from 8 μm to 10 μm	[866]
Microwave sintering	ZnO powder, graphite, O2	duration: 3 min; power: 100–1200 W; microwave oven; oxygen flow	-	nanowires, diameters: 2–70 nm, length: 5–15 µm	[867]
Microwave sintering	ZnO nano and micropowders, acetone	T: 1100–1350 °C; duration: 40 min; power 5 kW;	-	hexagonal tubes, diameter: 10 µm, length: 100 µm, wall thickness: 0.5–1 µm	[868]
Microwave vapour deposition	ZnO nanoparticles (20–30 nm; 48.9 m2/g) )	T: 1350–1400 °C; duration: 15 min; power: 3850 W; traveling-wave mode microwave system (5000 W)	-	microtubes (hexagonal hollow), average diameter: 60 µm, length: 250 µm; wall thickness: 3–5 µm	[869,870]
Microwave vapour deposition	Zn powder, steel-wool	duration: 30 s; microwave oven (1000 W)	-	nanofiber: diameters 50 nm, lengths 0.5–1 μm	[871]
Microwave vapour deposition	Zn powder, steel-wool, O2/N2 = 20/80, 40/60, 60/40, 80/20	microwave oven (1000 W)	-	nanoparticles with controlled morphologies	[872]
Microwave vapour deposition	Zn(CH3COO)2·2H2O, C2H5OH, vacuum of 5 kPa, O2	power: 400–1200 W; microwave based plasma deposition unit (2000 W)	-	films (NPs diameter: 10–18 nm) deposited on glass substrates	[873]
Microwave vapour deposition	metallic Zn flakes (2–3 mm)	duration: <5 min; microwave oven (800 W)	-	nanowires: diameter 70–80 nm	[874]
Microwave vapour deposition	ZnO microtubes (wall thickness less than 0.5–1)	T: 800–1450 °C; microwave system (3000 W)	-	single-crystal microtubes rods (above 1300 °C); crystal rods (below 1150 °C, diameters from 50 nm to a few μm)	[875]
Microwave vapour deposition	ZnO, Zn, graphite, N2 (carrier gas, 25 cm3)	T: 1100 °C; duration: 3–5 min microwave system (3000 W)	-	nanowires, nanobelts and microrods on different substrates materials (sapphire, silicon carbide, polycrystalline alumina); micro/nanotubes up 150 mm, wall thickness 0.3–1 mm, length up to 2–4 mm	[876]
Microwave vapour deposition	ZnO, graphite	duration: 2 min; microwave oven (800 W)	-	nanosheets: widths several to tens of µm thickness 20–80 nm	[877]
Microwave plasma assisted chemical vapour deposition	ZnO powder, graphite powder, He (carrier gas)	duration: 5 min.; microwave oven (1000 W)	-	nanonails: size of caps: 25–150 nm, necks: 30–200 nm, shank: 250–450 nm	[878]
Microwave vapour deposition	Zn(CH3COO)2·2H2O, C2H5(OH)	duration: 70 s; carrier gas: air and O2; microwave plasma system (700 W)	-	film like, worm-like, flower like and dot-like structures on glass, silicon wafer and Al2O3/Si wafer substrates	[879]
Microwave vapour deposition	Zn, air	duration: 4 s; carrier gas: air; microwave plasma system	-	nanowires deposited on aluminium foil substrate, glass, paper, microfibre, polycarbonate film, paraffin wax	[880]
Microwave vapour deposition	Zn, H2O	P: 20 kPa; power: 135 W; microwave plasma system	-	flower-like structures composed of nanorods (diameter 50 nm, lengths 150–200 nm)	[881]
Microwave vapour deposition	Zn(CH3COO)2·2H2O, Zn(NO3)2·6H2O, ZnCl2·2H2O, ethanol	duration: 10–120 s; protective gas and carrier gas: pure Ar (300 cm3/g); microwave plasma system	-	glass quadratic slides were coated by ZnO and/or Zn particles, whose sizes ranged from a few micrometres to ∼20 nm	[882]
Microwave plasma	Zn powder (10 µm)	protective gas and carrier gas: air, O2, O2/N2 (20/80 vol%); flow rate 10 l/m; microwave plasma system	-	wires (diameter: 111 nm, length: 5835 nm), tetrapods (diameter: 30 nm, length: 257 nm), rods (diameter: 82–627 nm, length: 309–853 nm), tetrapods (diameter: 30 nm, length: 257 nm)	[883]
Microwave vapour deposition	Zn(CH3COO)2·2H2O, NaOH, H2O, atmospheric pressure	pH: 11; power: 800 W; home-made microwave induced plasma in liquid system (1.5 kW)	-	NPs; diameter: 23 nm	[884]
Microwave assisted ball milling	Zn(CH3COO)2·2H2O	duration: 4–20 h; power: 800 W; microwave reactor	-	nanoparticles, diameters: 15 nm	[885]
-	1 cm × 1 cm × 125 μm Zn sheet, gas mixture of O2 (20 sccm) and Ar (80 sccm)	duration: 3 s; microwave oven (1000 W)	-	growth of nanoneedles on the Zn sheet; length ~500 nm, tip ~40, pillar: ~100	[886]

.

(2)

Microwave hybrid synthesis method of ZnO composites or ZnO hybrid structures, where the literature review results [887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927] are summarised in

Table 14. Summary of the microwave hybrid synthesis method of ZnO nanocomposites or ZnO hybrid nanostructures.

Type of Product	Type of Method	Substrates	Conditions during Preparation	Calcination Parameters	Properties	Ref.
Ag/ZnO, Au/ZnO, ZnO	ultrasonic microwave synthesis - UV irradiation	Zn(CH3COO)2·2H2O, NaOH, H2O, HAuCl4 - C2H5(OH) (solvent) AgNO3 - C2H5(OH) (solvent)	duration: 30 min (combined discontinuous ultrasonic irradiation (1 s sonication and 2 s interruption); power: 500 W; microwave and ultrasonic wave combined reactor	-	flower-like ZnO nanostructures (~800 nm)	[887]
Ag/ZnO, ZnO	ultrasonic microwave synthesis and deposition-precipitation	Zn(CH3COO)2·2H2O (0.5 M), urea (0.001 M), cetyltrimethylammonium bromide (CTAB_ (0.00035 M), C2H4(OH)2 (solvent) AgNO3 (10 M), NaOH (1 M)	duration: 20 min (combined discontinuous ultrasonic irradiation (1 s sonication and 2 s interruption); power: 500 W; microwave and ultrasonic wave combined reactor	-	spindle-like micro-and nanostructures	[888]
Ag/ZnO, Ag/ZnO/graphene, ZnO	ultrasonic microwave synthesis	graphite oxide (modified Hummers method), Zn(NO3)2·6H2O, AgNO3, hexamethylenetetramine [HMT, (CH2)6N4], H2O	T: 90 °C, duration: 2 h, ultrasonic microwave reaction system (700 W)	-	ZnO: rods, Ag/ZnO: rods with nanoparticles Ag/ZnO/graphene: rods with nanoparticles and sheets	[889]
Al doped ZnO	microwave induced combustion process	Zn(NO3)2·6H2O, Al(NO3)3·9H2O, C2H4(OH)2 (fuel)	duration: 10 min with on/off cycles of 30/10 s; power: 300 W; microwave oven	-	0 to 7% atomic weight percentage Al doped ZnO, hexagonal shaped, diameter: 20–53 nm	[890]
Al doped ZnO/Sn doped In2O3	microwave plasma assisted chemical vapour deposition	In2O3:SnO2 = 90:10 wt%, ZnO:Al2O3 = 98:2 wt%, gas pressure: 25 torr, hydrogen flow rate: 100 sccm	duration: 5 min; power: 400–800 W; microwave plasma system	-	films on glass substrates	[891]
Au/Fe2O3-ZnO	microwave assisted annealing	ZnCl2·5H2O, HAuCl4, FeSO3∙7H2O, NaOH, H2O	power: 700 W	400 °C for 4 h in air	nanostructures	[892]
Ba doped ZnO	microwave assisted annealing	Zn(NO3)2·6H2O, BaCl2, NaOH, H2O	T: 500 °C, duration: 30 min; microwave oven	-	doping range of 0 to 2 at%; nanoparticles (crystallite sizes: 20–22 nm)	[893]
Ce–Cu co-doped ZnO	microwave induced combustion process	Zn(NO3)2·6H2O, Ce(NO3)2·6H2O, Cu(NO3)2·6H2O, H2O, using urea as a fuel	duration: 20 min; power: 800 W; microwave oven	600 °C in air for 2 h	Zn1−2xCexCuxO (x = 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05), crystallite size: 35–46 nm, heterogeneous shape	[894]
Co doped ZnO	microwave induced combustion process	Zn(NO3)2·6H2O, Co(NO3)2·6H2O, using citric acid (C6H8O7) as a fuel (solution obtained was heated up to 80-90 °C until the excess water was removed and a highly viscous precursor gel was gained)	T: 900 and 1000 °C; duration: 50 s: power level: 100%; microwave oven (900 W)	-	Zn0.90Co0.10O, micron particle size	[895]
Co doped ZnO	microwave sintered	Co doped ZnO from (Zn(CH3COO)2·2H2O, Co(CH3COO)2·4H2O, C2H5OH)	T: 900–1075 °C duration: 15 min microwave furnace (850 W)	-	doping range of 0 to 7 mol%, powders, crystallite size: 36–210 nm	[896]
Cr doped ZnO	microwave induced combustion process	Zn(NO3)2·6H2O, Cr2(SO4)3·6H2O (various concentrations), C2H4(OH)2 (fuel)	duration: 8 min; power 210 W; microwave oven (420 W)	500 °C in air	Cr doped ZnO (0.00≤ x ≤0.15), heterogeneous nano/microstructures	[897]
ZnO/BiOBr ZnO	ultrasonic microwave synthesis	Bi(NO3)3·5H2O, KBr, Zn(CH3COO)2·2H2O, NH4OH, C2H5OH	pH: 8–9; T: 85 °C; duration: 35 min (discontinuous ultrasonic irradiation (2 s sonication and 1 s interruption); power: 500 W; microwave and ultrasonic wave combined reactor	-	ZnO/BiOBr grown on cotton fabric	[898]
Zn–ZnO, ZnO	microwave plasma	Zn powder and bulk ZnO, H2, Ar, O2	duration: 15–30 min; protective gas and carrier gas: pure H2; microwave plasma system (400 W)	-	nanowire-nanocable-nanotube route has been designed to fabricate ZnO nanotubes with desired dimensions	[899]
Zn–ZnO ZnO	microwave plasma	Zn(CH3COO)2·2H2O, H2O, air (0.3 L/min)	gases: Ar (flow rate 10 L/m) N2 (flow rate 5 L/m); power: 650 W; microwave plasma system	400 °C for 1 h in air	ZnO/Zn films on glass substrate	[900]
ZnO–ZrO2, ZnO	microwave induced combustion process	Zn(NO3)2·6H2O, ZrO(NO3)2·xH2O, urea (fuel), H2O	duration: 7 min; power: 850 W; microwave oven	-	heterogeneous particles, ZnO-ZrO2 M ratio: 1-1, 2-1, 1-2	[901]
ZnAl2O4/ZnO, ZnO	microwave induced combustion process	Zn(NO3)2·6H2O, Al(NO3)3·9H2O, urea (fuel), H2O	duration: 1–2 min; power: 900 W; microwave oven	1000 °C for 1 h in air	ZnAl2O4/ZnO nanocomposites with different ZnO (20, 30, and 40 mol%); flakes and plates structures of fine particles (ZnAl2O4 diameter 55 nm; ZnO diameter 38 nm)	[902,903]
Cu doped ZnO	microwave induced combustion process	Zn(NO3)2·6H2O, Cu(CH3COO)2·2H2O, C2H4(OH)2	duration: 10 min; power: 320 W; microwave oven (800 W)	450–750 °C	ZnO micro/nanostructures, wt% of Cu: 0, 0.8, 1.6, 2.5 and 5%	[904]
Cu–ZnO–Al2O3	microwave induced combustion process	Zn(NO3)2·6H2O, Cu(NO3)2·3H2O, Al(NO3)3·9H2O, H2O, using C2H4(OH)2 as a fuel(Cu, Zn and Al nitrates with 3/6/1 molar ratio)	microwave oven	-	nanoparticles	[905]
Cu–ZnO, ZnO	microwave assisted annealing	ZnSO4·7H2O (0.1 M), CuSO4·5H2O (0.1 M) NaOH (0.2 M), ascorbic acid, H2O	microwave oven	-	heterogeneous structures	[906]
Eu doped ZnO	microwave assisted annealing	Zn(CH3COO)2·2H2O, Eu(NO3)3·5H2O, NaOH, polyvinyl alcohol, H2O	pH: 10; duration: 5 min with on-off cycle (20 s on - 40 s off); microwave oven (600 W)	200 °C for 3 h in air	doping range of 0 to 0.5 mol%, nanorods	[907]
Eu doped ZnO	microwave induced gel combustion process	Zn(NO3)2·6H2O, Eu2O3, HNO3, using citric acid (C6H8O7) and glycine (NH2CH2COOH) as a fuel (different fuel mixtures); (solution obtained was heated up to 80 °C for 1 h until the excess water was removed and a highly viscous precursor gel was gained)	duration: 50 s, microwave oven	900 °C for 1 h in air	Zn0.99Eu0.01O nanoparticles; diameter: 30 nm	[908]
Fe2O3/ZnO	microwave sintering (solid state reaction)	ZnO powders (1 µm), γ-Fe2O3 (20–40 nm)	T: 1000–1400 °C; duration: different; power: 3 kW and 4 kW; microwave system (2.45 GHz and 28 GHz)	-	Fe2O3(ZnO)x (x = 6, 8, 34); homogeneous micro- and nanostructures	[909]
Ga doped ZnO	ultrasonic microwave synthesis	Zn(NO3)2·6H2O, Ga(NO3)3·xH2O, NaOH	T: 140 °C, duration: 30 min, ultrasonic microwave reaction system (150 W)	-	nanoparticles, Ga mol% content: 1, 2, 3, 5 and 10%)	[910]
Ga doped ZnO	microwave assisted annealing	Zn(CH3COO)2·2H2O, Ga(NO3)3·xH2O, NaOH, H2O	T: 150 °C, duration: 5 min, microwave reactor (4 × 800 W)	-	Zn1−xGaxO (x = 0, 0.01, 0.02, 0.03), heterogeneous nanoparticles	[911]
In2O3-Ga2O3-ZnO	microwave assisted annealing	Zn(CH3COO)2·2H2O, Ga(NO3)3·xH2O, In(NO3)3·xH2O, mono-ethanolamine (C2H7NO), methoxyethanol (solvent) (various reactant concentrations)	duration: 2 min; power: 150–1400 W; microwave annealing system	600 °C for 0.5 h in air	thin films	[912]
In2O3-Ga2O3-ZnO	post treatment process microwave assisted annealing	In2O3-Ga2O3-ZnO (layer is deposited by atmospheric pressure plasma enhanced chemical vapour deposition (AP-PECVD))	duration: 50/100 s; power: 150/300 W; microwave oven	-	thin films on silicon wafer	[913]
M and Co co-doped ZnO	microwave induced combustion process	Mn(NO3)2·4H2O, Co(NO3)2·6H2O, Zn(NO3)2·6H2O, Fe(NO3)3·9H2O, Ni(NO3)2·6H2O, water, using urea as a fuel	duration: 20 min microwave oven (1000 W)		M0.1Co0.1Zn0.8O (M = Cu, Fe, Mn, Ni); nanoparticles (25–34 nm)	[914]
Mg doped ZnO	microwave induced combustion process	Zn(NO3)2·6H2O, Mg(NO3)2·6H2O (various concentrations), vera plant extract (fuel), H2O	duration: 10 min; microwave oven (800 W)	-	0 and 1.5 wt% Mg doped ZnO	[915]
MgO doped ZnO	microwave sintered	ZnO, MgO powders used with the same purity and particle size of (99.9%, 20–30 nm)	T: 1400–1470 °C; duration: 20 min; power: 1300–1500 W; microwave reactor (5000 W)	-	microtubes, lengths of up to several micrometres in the ranges between 150 and 200 µm and an average diameter of 70 µm	[916]
Mn and Co co-doped ZnO	microwave induced combustion process	Mn(NO3)2·4H2O, Co(NO3)2·6H2O, Zn(NO3)2·6H2O, water, using urea as a fuel	duration: 15 min microwave oven (800 W)	-	MnxCo0.1Zn0.9−xO (x = 0.0, 0.05, 0.1, 0.15, and 0.2); nanoparticles (24–33 nm)	[917]
Sb2O3-MnO-CoO-Cr2O3-ZnO	microwave sintering	ZnO nanopowder (35 nm), Sb2O3, MnO, CoO, Cr2O3	T: 1000–1150 °C; duration: 1–60 min; microwave oven (2000 W)	-	average particle size: 2.1 µm	[918]
Sm doped ZnO	sol-gel-ultrasonic microwave sintering	Zn(NO3)2·6H2O, Sm(NO3)3·6H2O, polyvinylpyrrolidone (PVP, MW ≈ 40,000), diethylene glycol (DEG), triethylenetetramine (TETA), NaOH, H2O	pH: 9; duration: 7 min; power: 700 W; microwave reactor	400 °C for 2 h in air	nanorods, lengths: 200–400 nm, widths: 50–90 nm	[919]
TixOy–ZnO	microwave sintering(solid state reaction)	ZnO powder (1 µm), Ti powder (53 µm)	T: 550–1100 °C; duration: 1 min; microwave sintering furnace	-	rods (lengths: 1.5–3 µm, diameters: 0.2–0.5 µm) and coarser and irregularly shaped ZnO whiskers	[920]
ZnO/ZnFe2O4	microwave induced combustion process	Zn(NO3)2·6H2O, Fe(NO3)2·9H2O, NaCH3COO (different concentrations) using polyethylene glycol as a fuel	duration: 5 and 10 min; power: 120 and 700 W; microwave oven	-	nanoparticles, size in range from 23–54 nm (SSA = 94.5 m2/g) to 102–209 nm (SSA = 59.5 m2/g)	[921]
ZnO/multi-walled carbon nanotube	microwave assisted annealing	Zn(CH3COO)2·2H2O, multi-walled carbon nanotubes, (diameter about 20–50 nm), H2O	power: 300–700 W; microwave oven	-	nanocomposites	[922]
ZnO–exfoliated graphene	microwave assisted annealing	graphite (modified Staudenmaiers method), Zn(NO3)2·6H2O, NH4OH, H2O	duration: 5 min; power: 700 W; microwave oven	-	wrinkles and a fluff-like structure	[923]
ZnO–reduced graphene oxide	microwave assisted annealing	graphite (modified Hummers method), Zn(CH3COO)2·2H2O, H2O	duration: 5 min; power: 1000 W; microwave reactor	-	reduced graphene oxide sheets with ZnO nanoparticles (10–20 nm), SSA = 109.5 m2/g; ZnO NPs diameter range of 50 to 100 nm	[924]
ZnO–graphene oxide	microwave assisted annealing	graphite (modified Staudenmaiers method), Zn(NO3)2·6H2O, NaOH, C2H5OH, H2O	duration: 3–7.5 min; power: 800 W; microwave oven	-	ZnO microcubes-graphene oxide; ZnO nanoflakes-graphene oxide; ZnO nanoneedles-graphene oxide	[925]
ZnO–Pr2O3–CoO–Cr2O3–K2O	microwave induced combustion process	ZnO (96.7 mol%), Pr2O3 (2 mol%), CoO (0.5 mol%), Cr2O3 (0.5 mol%), K2O (0.3 mol%); agate mortar in presence of n-hexane as a fuel	duration: 8 min; power: 600 W; microwave oven	1200, 1250 and 1350 °C for 2 h in air	Agglomerated particles, size 4–6 µm	[926]
expandable graphite–ZnO	microwave sintered	exfoliated graphite, ZnO nanopowder	duration: 5 min; power: 1000 W; microwave oven	-	graphite-ZnO nanocomposite powders	[927]

.

The following were used for obtaining ZnO nano- and microstructures in the literature:

(1)

Ultrasonic microwave synthesis, which consists in the use of a new generation of microwave reactors, which permit the presence of an ultrasonic homogeniser’s sonotrode in the precursor mixture during the microwave heating. The ultrasonic homogeniser during its operation converts electrical energy into mechanical energy by moving the tip of the titanium sonotrode immersed in the fluid with a high frequency (19.5–40 kHz). Due to its inertia, the fluid no longer catches up with the rapid motion of the sonotrode, which results in cavitation, i.e., formation of gas bubbles that rapidly collapse, which is accompanied by sudden pressure changes, and as a consequence creates an impact wave.

(2)

Microwave assisted combustion synthesis, which consists in an exothermic reaction of combustion of one of the reactants of the reaction mixture in an oxygen atmosphere. Generally, a mixture composed among others of a Zn²⁺ salt and an organic component (fuel) is thoroughly mixed. There are several possibilities of the final state of the reaction mixture, among others, powder, pressed pastilles, gel, emulsion. The ready reaction mixture is introduced to a microwave reactor or oven, subjected to microwave radiation, which leads to a rapid increase in the sample temperature and ignition of the fuel, resulting in the formation of a ZnO powder.

(3)

Microwave assisted annealing, which consists in decomposition of the reaction mixture to ZnO only under the influence of its heating as a result of microwave radiation.

(4)

Microwave assisted sintering, which consists in microwave soaking of the earlier obtained ZnO.

(5)

Microwave vapour deposition, which consists in ZnO deposition from a gaseous phase, mostly at the atmospheric pressure, on the wafer (substrate) surface. For example, powdered ZnO, Zn or a Zn²⁺ salt is introduced to a ceramic crucible made of Al₂O₃, which is closed with a cover to which the substrate is attached on its inside part. Under the influence of microwave heating, a plasma arc appears in the crucible, enabling the evaporation of the Zn²⁺ substrate, which is deposited at the same time in the form of thin films on the whole surface of the ceramic container in the form of ZnO. Of course, there are professional microwave based plasma deposition units, which enable the application of inert carrier gases (e.g., argon, helium) or such gases (e.g., O₂) that can participate in chemical reactions leading to the formation of ZnO layers.

5.1. Reactants

The most popular reactants of the “Zn²⁺” zinc cation precursor used for the hybrid ZnO synthesis according to the data derived from the literature review [844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927] (Table 13 and Table 14) are Zn(NO₃)₂·6H₂O, Zn(CH₃COO)₂·2H₂O, ZnO, metallic Zn, and others (Figure 62). Other reactants are ZnCl₂, ZnSO₄, and Zn(OH)₂.

5.2. Morphology

Morphologies of ZnO NMs reported in the literature are presented in Table 13 and Table 14 [844,845,846,847,848,849,850,851,852,853,854,855,856,857,858,859,860,861,862,863,864,865,866,867,868,869,870,871,872,873,874,875,876,877,878,879,880,881,882,883,884,885,886,887,888,889,890,891,892,893,894,895,896,897,898,899,900,901,902,903,904,905,906,907,908,909,910,911,912,913,914,915,916,917,918,919,920,921,922,923,924,925,926,927] along with a description of their properties. ZnO NMs synthesised with the use of the microwave hybrid method were characterised by the following shapes: block-shaped structures [859], cables [899], corn-like microstructures [862], cubes [925], fibres [871], fluff-like structures [923], foamy and porous structures [850], half-backed grenade-like structures [844], hourglass-like structures [845], nails [878], wrinkle structures [923], spindle-like structures [844,888], spindle-like to double-prism-like structures [844], tetrapods [883], platelets [849], needles [886,925], plates [902,903], sheet-like structures [855,877,889], flakes [853,902,903,925], flower-like structures [847,859,881,887], tubes [868,869,870,899,916], wires [867,874,880,883,899], layers (films) [856,857,858,865,873,891,900,912,913], rods [852,861,881,883,889,907,919,920], and spherical particles [846,848,851,861,863,864,884,885,889,893,905,908,910,921].

Examples of various morphologies of ZnO obtained by the microwave hybrid synthesis are presented in Figure 63, Figure 64, Figure 65, Figure 66, Figure 67, Figure 68 and Figure 69.

5.3. Synthesis of Pure ZnO by the Microwave Hybrid Method

An example of an interesting publication reporting results of the microwave and ultrasonic wave combined synthesis of ZnO micro-/nanostructures is the paper by Li et al. [844]. As the synthesis precursor, a solution obtained by mixing solutions of Zn²⁺ salts (zinc acetate dehydrate, zinc acetylacetonate hydrate, zinc chloride) with 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES) was used. Later, the mixture was continuously sonicated for 5 min with the power of 1000 W. Subsequently, the mixed solutions were heated to 110 °C within 3 min and kept at this temperature for 17 min under microwave heating combined with discontinuous ultrasonic irradiation (1 s of sonication and 2 s of interruption) with the power of 500 W. After finishing the synthesis the products were centrifuged, thoroughly rinsed (water, ethanol) and subsequently dried. Li et al. [844] obtained various morphologies of ZnO products, including grenade-like, column-like, spindle-like, rod-like, shuttle-like and flower-like micro-/nanostructures. The size and shape of the ZnO nanostructures was controlled by the authors by changing the Zn/HEPES molar ratio, pH value, and Zn precursor. Figure 70 demonstrates the control of the ZnO morphology by changing the Zn/HEPES molar ratio. When the Zn/HEPES molar ratio was adjusted to 3:8, half-backed grenade-like ZnO microstructures were formed (Figure 70a,b). When the Zn/HEPES molar ratio was adjusted to 1:4, spindle-like ZnO microstructures were formed (Figure 70c,d). When the Zn/HEPES molar ratio was adjusted to 1:8, spindle-like to double-prism-like ZnO structures were formed (Figure 70e,f). When the Zn/HEPES molar ratio was adjusted to 1:20, short spindle-like ZnO nanostructures were formed (Figure 70g,h). When the Zn/HEPES molar ratio was adjusted to 1:40, irregular ZnO agglomerates were formed (Figure 70g,h). Figure 71 presents a change in the ZnO morphology caused by changes in the pH (7.4, 8.4, and 9.4) in a precursor with the constant Zn/HEPES molar ratio of 1:5. Figure 72, in turn, shows a change in the ZnO morphology caused by a change in the pH (7.4, 8.4, and 9.4) in a precursor with the constant Zn/HEPES molar ratio of 1:2.

An example of a publication showing results of obtaining ZnO with the use of the microwave induced combustion process is the paper by Cao et al. [847]. In this paper, an aqueous solution of zinc nitrate with an addition of urea was used as a reaction precursor. The samples were placed in an ordinary microwave oven and subjected to microwaves. After boiling, evaporating, and concentrating, the precursor rapidly foamed up, deflagrated, and released heat and gases. Synthesis products were heterogeneous ZnO microstructures, the morphology of which changed depending on the urea/Zn²⁺ molar ratio (1:1, 5:3, and 3:1), which is presented in Figure 73. The effect of the high temperature of the microwave induced combustion process was that the vast majority of the obtained synthesis products were ZnO microstructures.

Hu et al. [864] report obtaining ZnO NPs by sol-gel—microwave assisted annealing. As the reaction precursor, a suspension obtained by mixing a solution of zinc acetate in anhydrous ethanol with a solution of LiOH·H₂O in anhydrous ethanol was used. Hu et al. [864] performed a range of syntheses to examine the impact of temperature and duration on a change in ZnO NPs properties, namely:

-

for the same duration (20 min) at various reaction temperatures (30, 40, 50 and 60 °C),

-

for the same reaction temperature (50 °C) with various durations (10, 20, 30 and 40 min).

The product in the form of a transparent ZnO sol was removed from the microwave reactor, and subsequently introduced to hexanol and cooled to 4 °C. Thus prepared suspensions were centrifuged and the obtained white powders were dried in the air at the temperature of 80 °C. According to the Scherrer’s formula, the particle size of ZnO prepared at 30 °C for 20 min, 40 °C for 20 min, 50 °C for 10 min, 50 °C for 20 min, 50 °C for 30 min, 60 °C for 20 min, and 50 °C for 40 min are 3.86, 4.09, 4.33, 4.78, 4.96, 5.14, and 6.68 nm, respectively [864]. Zhou et al. [868] report the results of obtaining ZnO hexagonal tubes by the microwave sintering method. For the synthesis, a mixture of nanometric and micrometric ZnO was used, which was obtained by triturating for 1 h in an agate mortar and then pressed into pills with a hollow core, which itself acted as a form of a growth chamber. Thus prepared pastilles were placed in a microwave chamber on a quartz plate surface. The pastilles were subjected to microwave heating at the temperature of 1100–1350 °C for 40 min. The product was tubes (Figure 74) with the diameter exceeding 10 μm, length exceeding 100 μm, while wall thickness ranging from 0.5 to 1 μm. The hexagonal tubes growth process was explained by the authors [868] by proposing a vapour-solid mechanism.

Takahashi [871] reports obtaining ZnO nano-fibres by means of a domestic microwave oven. As the reaction precursor, he used a mixture of Zn powder and steel-wool, which was introduced to an Al₂O₃ crucible. The crucible was covered by a glass substrate, on which the crucible cover was placed (Figure 75a). The sample was subjected to microwave heating with the power of 1000 W for 30 s. As a result of microwave heating, ZnO nano-fibres (Figure 75b) with the diameter of ca. 50 nm and the length of 0.5–1 μm (Figure 75c) grew on the glass substrate.

Details of other research papers concerning the pure ZnO obtained by the microwave hybrid method are presented in Table 13.

5.4. Types of ZnO Nanocomposites or ZnO Hybrid Nanostructures Obtained by the Microwave Hybrid Synthesis Method

The microwave hybrid synthesis enables obtaining:

-

ZnO doped with the following ions: Al³⁺ [890], Ba²⁺ [893], Co²⁺ [895,896], Cu²⁺ [904,914], Cr³⁺ [897], Eu³⁺ [907,908], Ga³⁺ [910,911], Mg²⁺ [915] and Sm³⁺ [919];

-

ZnO co-doped with the following ions: Ce²⁺-Cu²⁺ [894] and Mn²⁺-Co²⁺ [917], where M = Mn²⁺, Ni²⁺, Fe³⁺ and Cu²⁺ [914];

-

composite and hybrid materials: Ag-ZnO [887,888,889], Au-ZnO [887], Ag–ZnO–graphene [889], Al³⁺ doped ZnO/Sn doped In₂O₃ [891], Au/Fe₂O₃–ZnO [892], ZnO/BiOBr [898], Zn–ZnO [899,900], ZnO–ZrO₂ [901], ZnAl₂O₄/ZnO [902,903], Cu–ZnO–Al₂O₃ [905], Cu–ZnO [906], Fe₂O₃/ZnO [909], In₂O₃–Ga₂O₃–ZnO [912,913], MgO–ZnO [916], Sb₂O₃–MnO–CoO–Cr₂O₃–ZnO [918], Ti_xO_y–ZnO [920], ZnO/ZnFe₂O₄ [921], ZnO/multi-walled carbon nanotube [922], ZnO—exfoliated graphene [923], ZnO–expandable graphite [927], and ZnO–reduced graphene oxide [924,925].

5.5. Synthesis of ZnO Nanocomposites or ZnO Hybrid Nanostructures by the Microwave Hybrid Method

Dou et al. [889] report obtaining ZnO NPs, Ag/ZnO nanocomposites and Ag/ZnO/graphene nanocomposites by the microwave and ultrasonic wave combined method. In order to obtain products, the aqueous mixture of:

-

zinc nitrate with hexamethylenetetramine was used to obtain ZnO NPs;

-

zinc nitrate, silver nitrate with hexamethylenetetramine was used to obtain Ag/ZnO nanocomposites;

-

zinc nitrate, silver nitrate with hexamethylenetetramine and an addition of graphene was used to obtain Ag/ZnO/graphene nanocomposites.

Identical synthesis parameters were applied for each sample, namely after stirring the suspensions for 10 min, the reaction vessel with the sample was put into an ultrasonic microwave reaction system and kept at 90 °C for 2 h and sonicated for 30 min. Subsequently, the samples were centrifuged, rinsed with water and then dried in a vacuum dryer. Dou et al. [889] provide only the results of the morphology of Ag/ZnO/graphene nanocomposites (Figure 76).

Mary et al. [894] report obtaining ZnO co-doped with Ce²⁺ and Cu²⁺ ions by the microwave induced combustion process. Reaction precursors were obtained by dissolving zinc nitrate, cerium nitrate, copper nitrate in deionised water with an addition of urea as a fuel. The crucible with a precursor was placed in a microwave oven, where the precursor solution underwent various reactions for 8 min of heating. Namely, the solution first boiled, then underwent dehydration, and subsequently decomposition with emission of large quantities of vapours in the form of smoke. When the solution reached the spontaneous ignition point, the combustion heat released and transformed the sample into a solid powder. ZnO powders were prepared with the addition of copper (Cu) and cerium (Ce) of different molar ratios (Zn_1−2xCe_xCu_x)O with x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05 to ZnO. The general equation of the synthesis reaction is described by Equation (57).

$$\mathrm{Zn}{({\mathrm{NO}}_3)}_2+\mathrm{Cu}{({\mathrm{NO}}_3)}_2+\mathrm{Ce}{({\mathrm{NO}}_3)}_2\stackrel{\mathrm{urea}, \mathrm{water} \mathrm{MH} \left(\mathrm{T}\right)}{\to }{\mathrm{Zn}}_{1-2x}{\mathrm{Ce}}_x{\mathrm{Cu}}_x\mathrm{O}+\mathrm{gaseous} \mathrm{products}$$

(57)

The size of Zn_1−2xCe_xCu_xO crystallites was 35 nm, 38 nm, 40 nm, 44 nm, 46 nm and 46 nm depending on the dopant content of 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05, respectively. The SEM results (Figure 77) indicated that the size of Zn_1−2xCe_xCu_xO particles decreased in line with the increase in the dopant content. The structure of the obtained pure ZnO was polycrystalline, where the ZnO particle size reached several hundred nm, while for the Zn_0.90e_0.05Cu_0.05O sample the largest particle size reached up to several dozen nm.

Konou et al. [893] described Ba doped ZnO by the microwave assisted annealing process. Figure 78 shows a scheme according to which samples were obtained by Konou et al. [893]. First, NaOH (a molar ratio of OH⁻ and Zn²⁺ kept to 3) was dropped into solutions of zinc nitrate and barium chloride, and subsequently the obtained suspension was filtered, rinsed and dried. The obtained powders were soaked in a microwave furnace at the temperature of 500 °C for 30 min. The barium doping concentration of ZnO was 0, 1, and 2 at%. Figure 79 presents the morphology of ZnO, Ba (1 at%) doped ZnO and Ba (2 at%) doped ZnO. The obtained products were characterised by a compact (sintered) structure, which resulted from the application of a high soaking temperature (500 °C). All three samples were characterised by a heterogeneous shape and by a large range of particle/aggregate sizes although the crystallite size was the same in all samples (20–22 nm).

Kim et al. [927] described a microwave sintering synthesis of ZnO NPs/graphene nanocomposites, where individual steps of obtaining ZnO NPs/graphene nanocomposites are presented in Figure 80. Commercially available ZnO NPs and expandable graphite were used for the synthesis. The expandable graphite sample was introduced to a crucible (Al₂O₃) and subjected to microwave heating (1 kW, 1 min), after which the product was dispersed in ethanol and sonicated for 10 min to exfoliate the graphite. Subsequently, ZnO NPs together with exfoliated graphite (0.01 wt%) were mixed in ethanol through sonication. The mixture was filtered off on a filter paper and subsequently the powders were placed in a crucible (Al₂O₃) and heated in a microwave oven (1 kW, 5 min). Thus obtained ZnO-graphene nanocomposite powders were dispersed in ethanol and were spray-coated on a heated SiO₂ substrate using a spray gun (140–160 °C). Figure 81a presents the morphology of commercial ZnO NPs, while Figure 81b,c show SEM images of ZnO/graphene nanocomposites without the microwave sintering process and after the microwave sintering process, respectively. The main difference between the morphology of samples in Figure 81b,c was the emergence of secondary ZnO NPs with smaller sizes in the sample after the microwave sintering process, which is perfectly visible in TEM images (Figure 82). The results achieved by Kim et al. [927] are very interesting, because generally the sintering process of NPs leads to their size growth, but in this particular case, a secondary ZnO NPs phase with smaller sizes was obtained.

Zhang et al. report [899] obtaining Zn-ZnO nanocables and ZnO nanotubes by microwave vapour deposition with the use of a microwave plasma system. The scheme of their experiment is shown in Figure 83. Zinc powder present in the quartz crucible was placed inside a horizontal quartz tube. Pure hydrogen (H₂) was used as the protective gas and the carrier gas, which was introduced to the reaction chamber with the flow rate of 50–90 sccm. The pressure in the reaction chamber was maintained at ca. 4 Torr. Microwaves (400 W) were introduced with the use of a wave guide in the central part of the quartz tube for generating stable plasma. The temperature of the microwave plasma was estimated at about 1000 °C. As an additional heat source, a movable tubular furnace was used, which generated the temperature of 700 °C at a stretch of 20 cm, while the estimated temperature in the quartz tube along the same stretch was ca. 500 °C. After switching the heating off for 30 min, the H₂ flow was stopped and a black product covering the internal surface of the quartz tube was observed. For the purpose of oxidising the formed product, O₂/Ar gas (volume ratio 1:50) was introduced to the microwave plasma system for 15–30 min. Then, the tubular furnace was moved back-and-forth along the quartz tube until only white powder was left. The morphology of the obtained products could be controlled through the oxidation duration (15–30 min) and the H₂ flow rate (50–90 sccm). Figure 65f,g shows the morphology of the final products. The nanotubes had a uniform inner and outer diameter of about 10 and 40 nm, respectively, while the average length of the nanotubes was ca. 1 µm. Figure 65f,g reveal that the outer diameter of ZnO nanotubes is virtually identical to the outer diameter of Zn–ZnO nanocables.

Details of other research papers concerning the of ZnO nanocomposites or ZnO hybrid nanostructures obtained by the microwave hybrid method are presented in Table 14.

6. Conclusions

ZnO is a multifunctional material, among others thanks to its semi-conductor, optical, biological, antibacterial, piezoelectric properties. Probably, to the surprise of many, the popularity of ZnO contyinues to grow, which is mainly caused by the development of new methods of obtaining ZnO NMs. These methods permit a synthesis of various ZnO NMs, e.g., owing to the possibility to control the shape and size, and to modify the chemical composition (dopants, composites etc.), which defines their properties and enables their use in new applications. ZnO NMs are also promising unique components for producing various innovative devices. However, for the potential of all ZnO NMs to be fully tapped in consumer products, numerous comprehensive tests are still needed. Nowadays, the microwave heating technology is already a classic method for fabrication of distinct chemical compounds and materials, including ZnO NMs. This has become possible thanks to the availability of new professional microwave apparatus on the market, which is continually improved by the manufacturers to meet the users’ requirements. A microwave apparatus can be encountered increasingly often in laboratories, the industry, and virtually all households. Microwave heating is environment-friendly and has been classified as a green chemistry approach. The main advantages of microwave heating include speed, homogeneity, and purity (contactless method).

This comprehensive review describes the microwave synthesis of ZnO NMs (pure, doped, composites). We concentrated in particular on reactants, process parameters and morphologies of the obtained products. The microwave synthesis of ZnO NMs was divided into three primary groups: hydrothermal, solvothermal and hybrid methods. Hybrid methods include such methods as: ultrasonic microwave synthesis, microwave assisted combustion synthesis, microwave assisted annealing, microwave assisted sintering, and microwave vapour deposition. Statistics of the use of reactants were presented. The overall results point that the microwave synthesis of ZnO NMs has an enormous potential that enables obtaining a wide range of product morphologies, beginning with quantum dots, to core-shell structures and hierarchical structures, and ending with films on substrates (surface modification). The discovery and explanation of some of the mechanisms of microwave syntheses have made it possible to obtain ZnO NMs with controlled properties (among others size, shape) and has eliminated the unrepeatability of syntheses. The present review indicates that the microwave synthesis of ZnO NMs is an extremely vast research topic, with many phenomena yet to be explained. Moreover, new issues (effects, mechanisms) related to the microwave synthesis of ZnO NMs certainly remain to be discovered.

Unfortunately, the cited literature does not provide examples of the achieved daily capacity of the microwave synthesis of ZnO NMs. Nevertheless, based on our many years of experience with the microwave synthesis of nanomaterials, we have become convinced in practice that, thanks to microwave solvothermal synthesis, ca. 100 g of ZnO NPs can be achieved daily.