TiO₂-Based Nanoheterostructures for Promoting Gas Sensitivity Performance: Designs, Developments, and Prospects

Abstract

Gas sensors based on titanium dioxide (TiO₂) have attracted much public attention during the past decades due to their excellent potential for applications in environmental pollution remediation, transportation industries, personal safety, biology, and medicine. Numerous efforts have therefore been devoted to improving the sensing performance of TiO₂. In those effects, the construct of nanoheterostructures is a promising tactic in gas sensing modification, which shows superior sensing performance to that of the single component-based sensors. In this review, we briefly summarize and highlight the development of TiO₂-based heterostructure gas sensing materials with diverse models, including semiconductor/semiconductor nanoheterostructures, noble metal/semiconductor nanoheterostructures, carbon-group-materials/semiconductor nano- heterostructures, and organic/inorganic nanoheterostructures, which have been investigated for effective enhancement of gas sensing properties through the increase of sensitivity, selectivity, and stability, decrease of optimal work temperature and response/recovery time, and minimization of detectable levels.

1. Introduction

Since the 20th century, atmospheric pollution has been proved to be one of most urgent issues. For the sake of controlling the exhaust emissions, gas sensors for the quantitative detection of various toxic and harmful gases have been widely developed as a result of their high response, outstanding selectivity, excellent repeatability, and good stability [1,2,3]. So far a variety of gas sensors, such as metal oxide semiconductor-based gas sensors [4,5,6,7,8,9], solid electrolyte-based gas sensors [10], electrochemical gas sensors [11], carbon-based gas sensors [1,12,13,14], organic gas sensors [2,3], and so on, have been extensively investigated. Amongst these different types of gas sensors, resistance type metal oxide gas sensors offering low cost, simple manufacturing approaches, and excellent sensitivity to the great majority of gases, have attracted considerable attention during the past several years [15,16]. Since Seiyama [17] reported metal oxide-based gas sensors for the first time, a large amount of effort has been expended in exploring the sensing properties of metal oxide- based gas sensors [7,18,19].

As a representative semiconductor metal oxide, titanium dioxide (TiO₂) has attracted much attention since Fujishima et al. observed the photocatalytic splitting of water on a TiO₂ electrode under the irradiation of UV light in 1972 [20]. In the past decades, it has been discovered that TiO₂ could be employed in many promising fields including photovoltaics [21], photocatalysis [22,23], sensors, etc. [24,25,26]. In particular, because of its high stability, harsh environmental tolerance, and environmentally-friendly properties, TiO₂ has been widely investigated and is regarded as one of sensing materials for gas detection with the most potential [27,28,29,30].

Naturally, TiO₂ mainly exists in three polymorph forms: rutile phase (tetragonal, P42/mnm) [31], anatase phase (tetragonal, I41/amd), and brookite phase (orthorhombic, pbca) (Figure 1a–c), whose bandgaps are 3.02, 3.2, and 2.96 eV, respectively [32]. Besides the abovementioned three crystal phases, there exists another phase, TiO₂(B) (monoclinic, C2/m). As shown in Figure 1d, TiO₂(B) possesses a layer structure, thus the density is lower and the specific capacity is larger comparing with other phases [33,34,35]. Among these diverse crystal phases, the most stable bulk phase is rutile, whereas for nanomaterials, anatase and brookite are commonly recognized to be more stable phases because of their relatively lower surface energy than rutile, although this fact is still argued in previous reports [36,37]. In the practical applications, TiO₂ performance is commonly affected by the crystal phases, which can be obtained through controlling the experimental conditions, such as fabrication methods, pH, duration annealing, temperature, and so on. As for the sensing applications, rutile TiO₂ and anatase TiO₂ are the most studied polymorphs.

TiO₂ gas sensors are typical resistant-type sensors which can display a decrease or increase in resistance when probing a reductive gas (H₂, H₂S, NH₃, CO, VOCs) or oxidative gas (NO₂, O₂) [29,30,38], respectively. The sensing mechanism of TiO₂-based gas sensors can be described by the following two-step process: receptor process and transducer process, as shown in Figure 2 [39].

The receptor process occurs at the TiO₂ surface, and it involves physisorption and chemisorption processes [40]. Firstly, oxygen molecules can be physically absorbed on the surface when TiO₂ is exposed to an air environment at room temperature; the process is determined by Van der Waals and dipole interactions; secondly, oxygen molecules on the TiO₂ surface will capture electrons from the conductive band (CB) of TiO₂ to form chemisorbed oxygen species (O₂⁻) on the surface. The reactions taking place on the surface of TiO₂ are as follows:

$${\mathrm{O}}_2\left(g\right)\to {\mathrm{O}}_2\left(ads\right)$$

(1)

$${\mathrm{O}}_2\left(ads\right)+e\to {\mathrm{O}}_2^{-}$$

(2)

During the process, receptor capability is determined by the physisorption process and chemisorption process together, where the physisorption can be influenced by the temperature, whereas the rate of chemisorption process may be influenced by activation energy.

The transducer process includes the transportation of electrons in the TiO₂ and the transformation of electrons into the outward resistance signal. This can be influenced by the three typical electron transfer modes which are divided as surface-controlled mode, grain-controlled mode, and neck-controlled mode, respectively, [41,42] as shown in Figure 2. As for surface-controlled mode, compact layer structures determined by the thin film thickness of the materials are universally considered as the main pattern [39], where the gases can only affect the materials surfaces other than the internal body. On the contrary, in real polycrystalline materials, the TiO₂ grains connect to each other through grain boundaries or necks, in this way, the grain boundaries or necks will contribute significantly to the electroconductibility and gas sensing performance of the TiO₂. It has been reported that materials with large grain size would possess large neck cross sections, so accordingly the neck resistance is less significant than the grain-boundary resistance. However, for materials with much smaller grain size, the neck resistance is higher than the grain boundary resistance because of the much smaller neck cross section, in this case the neck resistance becomes more significant [41].

In these two typical processes presented above, surface-to-volume ratio, grain size, and the electron transport ability of TiO₂-based materials play important roles. Consequently, in recent years, substantial effort has been invested in increasing the specific surface area, decreasing the grain size, and enhancing the conductivity of TiO₂ through nanostructured materials, element doping, heterostructural materials and so on [27,30,43,44,45,46].

Nano-scale is a key factor in studying the gas sensing properties of metal oxide semiconductor-s based sensors. In fact, it is well known that the surface structure and specific surface area can play very important roles in sensing properties [47]. Nanocrystallization is an efficient way to improve the gas sensing performance because of their much larger specific surface area and rich surface chemical properties on the nanostructure surfaces, which may potentially lead to miniaturized sensors with outstanding performance. In this regard, many kinds of TiO₂ nanostructures with various morphologies, such as zero-dimensional (0D) nanocrystals [48], 1D nanofibers or nanowires [43,49,50], 2D nanoplates [29], and 3D hierarchical microstructures [27] have been developed.

Another important strategy to enhance the sensing performance of TiO₂ is the formation of heterostructures. The heterojunction theory dates back to as early as the 1930s [51]. Since then, more and more applications of nanoheterojunctions have been extensively investigated owning to the superinjection of charge carriers. Since 2005, a great many nanoheterostructural materials have been widely researched and applied in many fields, including solar cells [52], Li-ion batteries [53], photoelectrochemical cells [54], photocatalysis [55], and gas sensors [5,56]. As a matter of fact, several TiO₂-based nanoheterostructures, such as semiconductor/semiconductor, noble metal/ semiconductor, carbon-group-material/semiconductor and organic/inorganic nanoheterostructures, have attracted widespread interest in the preparation and investigation of their properties and applications. Various types of TiO₂-based nanoheterostructures including composite nanoparticles [57,58,59], quantum dots in nanowires/nanofibers [60,61,62,63], core/shell nanowires/nanofibers/ nanospheres [64,65], etc., have been studied intensively. Compared with the pure oxide, these nanoheterostructures achieve higher sensing performance. As a well-known strategy, the modification of TiO₂ nanostructures using noble metal nanoparticles, like platinum (Pt), palladium (Pd), silver (Ag), and gold (Au), forming noble metal/TiO₂ heterostructured sensor materials can further improve the sensing performance, including enhancement of sensitivity and selectivity and shortening of response/recovery times [66,67]. The enhanced properties can be generally ascribed to the catalytic activity of noble metal nanoparticles, increased active surface area, reduced electrical resistance, enhanced optical absorption, facilitated chemical adsorption of oxygen molecules and/or reaction of oxygen ions with probing gases, and improved gas diffusion inside the heterostructures [68]. In addition, the Schottky barrier formed at the interface of noble metal/TiO₂ heterojunction yielding efficient electron/hole separation is considered as another important factor in enhancing sensing performance. In general, the electronic and chemical properties of the metal/oxide interface, as well as the morphology of the noble metal nanoparticles and metal oxides matrix, play important roles in promoting the overall performance of the noble metal/TiO₂ nanoheterostructures [57,60,69,70]. Comprehensive coverage of metal/semiconductor nanoheterostructured sensors have been available elsewhere for many decades, so accordingly, in this review, we briefly trace the application of TiO₂-based semiconductor/semiconductor, carbon-group-material/semiconductor and organic/inorganic nanoheterostructures in the area of gas sensors, summarize their major design and preparation methods, describe some of the improvements and resulting achievements, and discuss the future challenges and perspectives.

2. Fabrication of TiO₂-Based Nanoheterostructures

In the past few decades, various fabrication methods, including chemical vapor deposition (CVD), atomic layer deposition (ALD), solid phase reaction, electrochemical deposition, chemical deposition, hydrothermal/solvothermal methods, sol-gel, electrospinning, etc., have rapidly developed and been successfully applied in preparing high quality TiO₂-based nanoheterostructures. Accordingly, in this section, most of the attention will be focused on the nanoheterostructure synthesis methods.

2.1. CVD and ALD Methods

The CVD route probably is one of most extensively explored approach in nanoheterostructure preparation, which can deposit various materials onto suitable substrates in order. Nano- heterostructures are generally synthesized using a two-step growth procedure. In the first step, the inner-core material is deposited on a suitable substrate through CVD or other synthetic routes. In the second step, the outer-layer shell material is subsequently grown on the core surface. This method has the capability to control the components, morphology, thickness, and length of the materials by controlling some technological parameters including temperature, pressure, carrier gases, gas-flow rates, substrates, and deposition time. For example, ZnO-TiO₂ nanocomposites were fabricated via an innovative CVD technique [46], where TiO₂ nanoparticles were grown on the initially deposited ZnO nanoplatelet host. The process was carried out at a relatively low temperature of 350–400 °C, this avoiding the effect of unsuitable thermal treatment and maintaining the chemical properties of the materials. Barreca et al. [71] have reported the fabrication of CuO-TiO₂ nanocomposites through a multistep vapor deposition process, where the first step was the synthesis of porous CuO nanomaterials on an Al₂O₃ substrate via a CVD approach, and the final step was the controllable growth of TiO₂ nanoparticles on the porous CuO matrices.

Although involving a similar chemical process as the CVD route, ALD has attracted much attention in the synthesis of heterostructures because of the accurate control of film thickness at atomic scale and the conformal growth of complex nanostructures. The excellent conformability between two materials obtained by ALD makes them possible to form heterojunctions at the semiconductor interfaces. Through growth control, Katoch et al. [72] have successfully prepared TiO₂/ZnO inner/outer double-layer hollow fibers (TiO₂/ZnO DLHFs), as shown in Figure 3. The TiO₂/ZnO DLHFs were synthesized using a three-step process. First, polyvinyl acetate (PVA) fibers were prepared by an electrospinning process; subsequently, TiO₂ and ZnO were sequentially grown on the PVA fibers through the ALD method and, finally, a thermal treatment was carried out for the removal of the PVA support and the crystallization of TiO₂ and ZnO.

2.2. Solid Phase Reactions

Solid phase reactions are a quite facile process to synthesize composite sensing materials. In a typical process, pure powders are mixed uniformly using a physical method to electrostatically self-assemble nanoheterostructures, which generally is a one-step procedure. For example, Zhou et al. have prepared Ag₂O/TiO₂ nanoheterostructures [73] through a solid phase reaction, where a certain amount of the pure Ag₂O and the corresponding amount of TiO₂ were uniformly mixed to get Ag₂O/TiO₂ nanoheterostructures. What’s more, some other nanoheterostructures, such as polyaniline-titanium (PANi-TiO₂) nanoheterostructures [74], reduced graphene oxide/titanium dioxide (rGO/TiO₂) layered nanofilm [75], ZnO–TiO₂ nanocomposites [59], and TiO₂/SnO₂ nanocomposites [76], have also been successfully synthesized using this facile method.

2.3. Electrochemical Deposition

Compared with the CVD route, electrochemical deposition can obtain large-scale nanostructures at relatively low temperature. The conventional technique is very propitious to the fabrication of ordered, uniform, and highly dense nanoheterostructures for widespread applications. Recent years, some types of TiO₂-based nanoheterostructures are prepared using a two-step electrochemical deposition process [60,77]. In the deposition process, the pre-grown nanomaterials on fluorine-doped tin oxide (FTO), indium-doped tin oxide (ITO), or other relevant substrates are treated as working electrode for the following deposition. For instance, Yang and co-workers [78] have reported the preparation of the Cu-Cu₂O/TiO₂ nanocomposites via the electrochemical deposition method. The fabrication process consisted of two main steps: in the first step, helical TiO₂ nanotube arrays (NTAs) were prepared through anodizing a Ti foil. The second step was the electrodeposition of Cu and Cu₂O nanoparticles on the TiO₂ NTAs surface.

Additionally, a great number of TiO₂-based nanoheterostructures were prepared using electrochemical deposition combined with other synthesis processes, where 1D TiO₂ arrays were commonly synthesized by electrochemical deposition, and then the as-prepared TiO₂ arrays would be used as templates for heterostructure fabrication in the second synthesis step by other methods, including chemical deposition, hydrothermal/solvothermal methods, sol-gel method, and so on. For example, coaxial Ni/NiTiO₃/TiO₂ NTAs (Figure 4a–c) were synthesized by hydrothermally treating the as-anodized TiO₂ NTAs [79]. Nano-coaxial p-Co₃O₄/n-TiO₂ heterojunctions were synthesized first using electrochemical deposition and secondly using hydrothermal reaction [80]. The SEM images are shown in Figure 4d–i, where obviously the NTs with open top and closed bottom display high order and directionality. When soaking the as-anodized TiO₂ NTs in Co-based solutions, new nanorods with smaller diameter can be formed in the center of the TiO₂ NTs, representing the formation of nano-coaxial shaped of nanoheterostructures. Similarly, CdS/TiO₂ nanoheterostructures were fabricated by electrochemical deposition and sequential chemical deposition [81]; Pd/TiO₂ nanoheterostructures were synthesized by electrochemical deposition and a UV irradiation chemical method [82]; polypyrrole (PPy)/TiO₂ heterojunctions were fabricated by chemical deposition and electronchemical deposition methods [78,83].

2.4. Chemical Deposition

Chemical deposition is a convenient and low-cost technique for fabricating nanoheterostructures. For this method, the nanomaterials are obtained from the solid precipitation in the solution, and the concentration of the precursor, pH value, deposition temperature, and deposition time play important roles in the process. Nowadays, chemical deposition is generally applied in preparing either pure nanomaterials or composite nanomaterials. For example, brookite-TiO₂/a-Fe₂O₃ nanoheterostructures have been synthesized via two-step facile chemical deposition without using any templates or surfactants [65], as shown in Figure 5.

Furthermore, chemical deposition combined with other methods can also be used to fabricate nanoheterostructures. Our group has recently reported the fabrication of Ag₂O/TiO₂/V₂O₅ nanoheterostructures (STV NHs) [84] through a two-step synthesis approach: the first step was the preparation of continuous TiO₂/V₂O₅ nanofibers (TV NFs) using an electrospinning method, and the second step was the deposition of Ag₂O nanoparticles on the TV NFs surfaces by the reaction of AgNO₃ solution and NaOH solution. Based on this facile method, some other nanoheterostructures could be easily obtained, such as TiO₂/CuO [85], TiO₂/LiCl [86], and TiO₂/FeOOH [87] nanoheterostructures.

It is worthy to note that room temperature chemical method is an efficient way to synthesize organic-functionalized TiO₂ nanoheterostructures, such as TiO₂/polypyrrole nanocomposites [88], TiO₂-diltiazem/tetrachlorobismuth core-shell nanospheres (TiO₂@DTMBi core-shell nanospheres) [89], polymer-functionalized TiO₂ nanorods [90], molecularly imprinted polymer on TiO₂ nanotubes (MIP@TiO₂ NTs) [91], polypyrrole/TiO₂ heterojunction (PPy/TiO₂ heterojunction) [78], and Prussian blue/TiO₂ nanowires (PB/TiO₂ NWs) [63]. Typically, Wang and his co-workers [92] have reported a simple, low cost, and effective chemical method to fabricate polythiophene/Pd/TiO₂ ternary composite nanoheterostructures, where TiO₂ spheres were synthesized in water/acetone solvent, and then the Pd species were loaded on the TiO₂ microspheres, finally the polythiophene was covered on the spheres to obtain polythiophene/Pd/TiO₂ ternary composites.

2.5. Hydrothermal/Solvothermal Technique

Hydrothermal/solvothermal methods are commonly applied in the synthesis of powdery nanostructures. In the typical process, reagents (such as amines) and precursors are firstly mixed with each other in an appropriate ratio and then injected into a solvent, which can not only speed up the precursor dissolution but also accelerate the reaction between reagent and precursor. Finally the solution is added into a special hydrothermal synthesis reactor for the reaction of reagent and precursor and the growth of nanomaterials at relatively high temperature and high pressure. For example, TiO₂/V₂O₅ nanoheterostructures could be prepared through solvothermally treating TiO₂ nanoparticles in vanadium chloromethoxide precursor solution to growing V₂O₅ on TiO₂ nanoparticle surfaces [93]. Similarly, branched 1D α-Fe₂O₃/TiO₂ nanoheterostructures [62] were synthesized by growing α-Fe₂O₃ nanorods on the TiO₂ nanofibers using hydrothermal treatment. Figure 6 displays the SEM images of the branched α-Fe₂O₃/TiO₂ nanoheterostructures, it can be observed that the sample is mainly formed by branch-like nanofibers with loose and rough surfaces. In addition, one-dimensional carbon nanotube (CNT)-TiO₂ heterostructures were prepared through a solvothermal route using multiwalled CNTs as templates [25]. As demonstrated by these examples, hydrothermal/solvothermal methods are suitable techniques to fabricate controlled nanoheterostructures.

2.6. Sol-Gel Method

The sol-gel method is a representative wet chemistry technique for synthesizing TiO₂-based nanoheterostructures which involves relatively low growth temperatures and the morphology of the products can thus be controlled. The general process is as follows: first, prepare the sol gel, then heat the solution at high temperature combined with vigorous stirring to make it hydrolyze, finally carry out a condensation reaction to obtain nanomaterials. For instance, Lee et al. have fabricated a quartz crystal microbalance (QCZ) gas sensor based on the polyacrilic acid (PAA)/TiO₂ nanofilm, where the (TiO₂/PAA)_n (n = 5, 10, and 20) nanofilms were deposited on gold-coated quartz crystal microbalance electrode using gas-phase surface sol-gel method [94].

2.7. Electrospinning

Since the electrospinning technique was first reported, more and more nanomaterials have been prepared by researchers via this simple method. During the typical process, a glutinous precursor solution is injected through a thin spinneret and then is stretched to form ultralong nanofibers; finally annealing the as-prepared samples at appropriate temperature can produce very highly crystalline nanofibers. In recent years, our group has successfully synthesized TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures using a simple one-step electrospinning method [95,96]. The synthesis process of TiO₂/Ag_0.35V₂O₅ nanoheterostructures and characterization of the nanoheterostructures are presented in Figure 7.

In the typical electrospinning process, the consistence of the composite, working voltage, inner diameter of the spinneret, distance between the spinneret tip and the collector substrate, and annealing temperature can all affect the morphology of the nanomaterials. Furthermore, Ag/TiO₂ and TiO₂/V₂O₅ nanoheterostructures have also been prepared using the electrospinning process by our group [97,98,99]. What’s more, various type of TiO₂-based nanoheterostructures fabricated by electrospinning process have been reported by other researchers, such as TiO₂/In₂O₃ [61], LiCl/TiO₂ [100], and Pt/TiO₂ [101] nanoheterostructures. Especially, Du’s group has synthesized TiO₂/ZnO core-sheath nanofibers using a coaxial electrospinning method [102].

3. Diverse Nanoheterostructural Gas Sensors

During the past decades, TiO₂ has attracted considerable attention because it was regarded as a promising candidate for waste gas detection. Unfortunately, the poor sensing activity of high resistance n-type TiO₂ seriously influences the development of TiO₂-based gas sensors. Recently, various heterostructured sensing materials have been reported. The coupling of different materials can result in improved sensing activity. In this section, we will focus on the sensing performance of multifarious TiO₂-based nanoheterostructure gas sensors, highlighting in particular, semiconductor/semiconductor nanoheterostructures.

3.1. Semiconductor/Semiconductor Nanoheterostructures

One of efficient ways to devise outstanding gas sensors is the modification of TiO₂ by coupling with other semiconductors to form nanoheterostructures, which could display enhanced sensitivity and selectivity, faster response/recovery times, and/or lower operational temperatures than pure TiO₂ [45,46,100,103,104].

Table 1. Summary of the gas sensing properties of TiO₂-based semiconductor/semiconductor nanoheterostructure sensors.

TiO2-Based Nanoheterostructures	Fabrication Method	Size	Detection Gas	Detection Range	Response				Ref.
					Operation Temperature (°C)	Sensitivity	Response/Recovery Time	Concentration
TiO2/Co3O4 acicular nanowires	hydrothermal + pulsed laser deposition	length: 1–3 μm; diameter: 200 nm	ethanol	10–500 ppm	160	Rg/Ra: 65		100 ppm	[106]
nano-coaxial p-Co3O4/n-TiO2 heterojunction	electronchemical anodization + hydrothermal process	nanotubes diameter: ~150 nm; core nanorods diameter: ~50 nm	ethanol		260	Ra/Rg: 40	1.4/7.2 s	100 ppm	[80]
Fe2O3/TiO2 tube-like nanostructures	hydrothermal + chemical deposition	diameter: 120 nm; length: 400 nm; outer wall thickness: 23.5 nm	ethanol	0.5–500 ppm	270	Ra/Rg: 19.4		500 ppm	[107]
SnO2-coated TiO2 nanobelts	hydrothermal	TiO2 nanobelts: length: over ten micrometers; width: 100–200 nm; thickness: 20–40 nm; SnO2 nanoplates: length: 20–200 nm; thickness: 20 nm	ethanol	10–500 ppm	43	Ra/Rg: 11.2	40/5 min	10 ppm	[108]
TiO2/SnO2 core shell nanocomposites	hydrothermal treatment + chemical deposition		ethanol	500–5000 ppm	200	Ra/Rg: 12.7	≤50/50 s	1000 ppm	[109]
SnO2 nanospheres functionalized TiO2 nanobelts	hydrothermal process	nanobelts diameter: 50–200 nm; length: several micrometers; nanospheres diameter: ~400 nm	ethanol	100–800 ppm	320	Rg/Ra: 27.5		400 ppm	[105]
Ag-TiO2/SnO2 nanocomposites	chemical deposition	diameter: ~100 nm	ethanol	1–500 ppm	275	Ra/Rg: 53	3.5/7 s	50 ppm	[104]
TiO2/V2O5 nanoheterostructures	electrospinning	nanobranches diameter: 15–20 nm; nanofibers diameter: 160 nm	ethanol	20–1000 ppm	350	Ra/Rg: 24.6	6/7 s	100 ppm	[97]
TiO2/Ag0.35V2O5 branched nanoheterostructures	electrospinning	nanobranches diameter: ~20 nm; nanofibers diameter: ~190 nm	ethanol	20–1000 ppm	350	Ra/Rg: 31.8	7/12 s	100 ppm	[95]
brush-like ZnO-TiO2 heterojunctions nanofibers	electrospinning + hydrothermal process	ZnO nanorods diameter: 100–300 nm; TiO2 nanofibers diameter: 100 nm	ethanol	20–500 ppm	320	Ra/Rg: 50.6	5/10 s	500 ppm	[110]
TiO2/ZnO core-shell nanorods	hydrothermal method + ALD	core width: 120 nm; shell width: 20 nm	ethanol	5–25 ppm	150	Ra/Rg: 2.37	100/70 s	10 ppm	[64]
ZnO surface functionalized TiO2	electrospinning + hydrothermal treatment	nanofibers diameter : 70–100 nm; ZnO nanosheets diameter: 500 nm	ethanol	10–200 ppm	280	Ra/Rg: 15.7	5/3 s	100 ppm	[111]
ZnO/TiO2 nanocomposites	CVD		ethanol		400	Ra/Rg: 5	~1/1 min	50 ppm	[46]
brookite TiO2 decorated a-Fe2O3 nanoheterostructures	chemical deposition	length: 50–100 nm; diameter: ~10 nm	butanol	10–500 ppm	370	Ra/Rg: 27.6	5/6 s	100 ppm	[65]
ZnO-TiO2 nanocomposites	CVD	diameter: 5–20 nm	acetone	20–100 ppm	350	Ra/Rg: 22.7	1/1 min	100 ppm	[46]
ZnO/TiO2 nanocomposites	CVD		acetone			Ra/Rg: 22		100 ppm	[46]
CuO-TiO2 heterostructure nanofibers	electrospinning + hydrothermal process	TiO2 nanofibers: length: several tens micrometers; diameter: 200 nm; CuO nanocubes: diameter: 40–80 nm	formaldehyde	5–100 ppm	200	Ra/Rg: 15.5		50 ppm	[112]
Cd/SnO2/TiO2 composites	sol-gel		formaldehyde	100–500 ppm	320	Ra/Rg: 32	25/17 s	200 ppm	[113]
α-Fe2O3/TiO2 branch-like hierarchical heterostructures	electrospinning + hydrothermal method	diameter: 600 nm; length: several micrometers	trimethylamine	10–200 ppm	250	Ra/Rg: 13.9	0.5/1.5 s	50 ppm	[62]
hierarchically assembled ZnO nanorods on TiO2 nanobelts	hydrothermal process	TiO2 nanobelts: width: 50–200 nm; length: several micrometers; ZnO nanorods: length: 500 nm	trimethylamine	5–500 ppm	200	Ra/Rg: 25		5 ppm	[114]
SnO2/TiO2 composites	sol-gel		methanol	50–400 ppm	360	Ra/Rg: 60	10–15/14–20 s	200 ppm	[115]
ZnO-TiO2 nanocomposites	physical mixture		humidity	5%–90% RH	RT (room temperature)	(ΔR)/(Δ%RH): 9.08 MΩ/%RH			[59]
LiCl/TiO2 electrospun nanofibers	electrospinning	diameter: 150–260 nm	humidity	11%–95% RH	RT	Ra/Rg: 103	<3/7 s	11%–95% RH	[100]
ZnSnO3 nanoneedles/TiO2 nanofibers heterojunction	electrospinning + hydrothermal treatment	TiO2 nanofibers diameters: 200–300 nm; ZnSnO3 nanorods: tip diameter: 0.5~1.5 nm; ratio of length to diameter: ~9.6	humidity	11%–95%	RT		2.5/3 s		[116]
ZnSnO3 nanoparticles/TiO2 nanofibers heterojunction	electrospinning + hydrothermal treatment	TiO2 nanofibers diameters: 200–300 nm; ZnSnO3 nanoparticles diameters: 30–50 nm	humidity	11%–95%	RT		3.5/29 s		[116]
Ce2O3/TiO2/SnO2 thin film	sol-gel		humidity	15%–95% RH	RT	Ra/Rg: 100		40%	[117]
polypyrrole-coated TiO2/ZnO nanofibers	electrospinning + chemical deposition	TiO2/ZnO core diameter: 100 nm; PPy shell thickness: 7 nm	NH3	0.5–450	RT	ΔR/Ra: 0.35		450 ppm	[118]
nanocrystalline TiO2/SnO2 composites	commercial powder		NH3	100–5000 ppm	400	ΔR/Ra: 0.5		1200 ppm	[119]
SnO2/TiO2nanoneedles	wet chemical method	diameter: 40–80 nm; length: 60–100 nm	NH3		150	(ΔR/Rg) × 100%: 300%	3/5 min	1000 ppm	[120]
TiO2/SnO2 thick film	sol		NH3	100–1000 ppm	250	Ra/Rg: 3		400 ppm	[121]
TiO2/ZnO inner/outer double-layer hollow fibers	electrospinning + ALD	inner diameter: ~320 nm; TiO2 layer thickness: ~30 nm; ZnO outer layer thickness: 20 nm	CO	0.1–10 ppm	375	Ra/Rg: 20.3		1 ppm	[72]
TiO2/Fe2O3 nanosized thin film	sputtering	diameter: 20–30 nm	CO			ΔR/Ra: 15	~50/- s	1000 ppm	[122]
TiO2/Al2O3/Pd composites	sol		H2S	200–1000 ppm	225	log(Ra/Rg): 0.9		1000 ppm	[48]
nanocrystalline CdO/ZnO/TiO2	pyrolyzation		H2S		225–250	ΔR/Ra: 0.8		10,000 ppm	[123]
TiO2 decorated CuO nanorods	thermal evaporative + sputtering	diameters: 50–100 nm; lengths: a few tens of micrometers	H2	0.1–5 ppm	300	Ra/Rg: 8.57		5 ppm	[124]
TiO2/SnO2 nanocomposites	physical mixture	diameter: 8–28 nm	H2	50–3000 ppm	375				[76]
TiO2 fibers supported β-FeOOH nanostructures	electrospinning + hydrothermal method	nanofiber diameter: ~500 nm	H2	100 500 ppm	RT	Ra/Rg: 52.5		500 ppm	[87]
mesoporous Nb2O5/TiO2	sol-gel	diameter: 4.1 nm	H2		450	Ra/Rg: ~5.5	~1/1 min	500 ppm	[125]
TiO2/NiO thin film	sputtering		H2	200 ppm–0.5%	300	Rg/Ra: 15	2/2.3 min	1000 ppm	[126]
PtO/Pt/TiO2 thin film	sol-gel		H2	1%–10%	180	ΔR/Ra × 100%: 40%	~10/10 min	2%	[103]
TiO2-In2O3 composite nanofibers	electrospinning	diameters: 250 nm; length: several micrometers	NO2	0.3–97 ppm	RT	ΔR/Ra: ~1.25	~ 9.5/- s	1 ppm	[61]
SnO2-core/V2O5-shell nanorods	thermal evaporation + sputtering	length: several tens micrometers; SnO2-core thicknesses: 100 nm; V2O5-shell thickness: 10 nm	NO2	10–80 ppm	300	Rg/Ra: 1.03%	≤4.5/4.5 min	10 ppm	[127]
Al2O3 decorated anatase TiO2 nanotubes	electrochemical anodization + thermal decomposition	nanotube outer diameter: ≤200 nm; length: several micrometers	NOx	0.97–97 ppm	RT	ΔR/Ra: 88.04%	8/- s	97 ppm	[45]
CuO-TiO2-Au nanosystems	CVD + sputtering		O3					300 ppb	[71]
V2O5/TiO2 thin film	sol-gel	3–5 nm	O2	1 ppm–20.9%	250	Rg/Ra: 3.5	5/30 min	120 ppm	[128]
CeO2/TiO2 thin film	sol-gel		O2	5–10,000 ppm	420	Rg/Ra: ~3	40–60/80	1000 ppm	[129]
SnO2/TiO2 thin film	sputtering	44–67 nm	O2	100–2000 ppm		Rg/Ra: 2		1000 ppm	[130]

summarizes the different TiO₂-based semiconductor/semiconductor nanoheterostructures and their performance in the gas sensing detection field. For example, Zeng et al. [105] have successfully fabricated a novel gas sensor based on the SnO₂-TiO₂ hybrid nanomaterials.

They demonstrated that the SnO₂ nanospheres-functionalized TiO₂ nanobelts-based sensor displayed very outstanding sensing properties, higher response and lower operating temperature, than pure TiO₂ nanobelts-based sensors, as shown in Figure 8.

Tomer and Duhan [104] reported a mesoporous Ag-(TiO₂/SnO₂) structure which exhibited high sensitivity, a low detection limit (1 ppm), and wide detection range (1 ppm to 500 ppm) to ethanol, as shown in Figure 9. Moreover, the mesoporous Ag-(TiO₂/SnO₂) nanohybrid sensor also displayed excellent stability and high selectivity for ethanol.

3.1.1. Sensing Mechanism

Although the mechanism of the heterostructures has not been investigated explicitly, it is clear that the enhanced sensing properties for semiconductor/semiconductor nanoheterostructures should be related to the heterojunction constracted at the interface between two semiconductors, where the changes of heterojunction energy barrier immerged into different gas atmospheres are benefit to the improvement of sensing properties.

One possible explanation for the enhancement of these heterostructure-based sensors is the heterojunctions formed between TiO₂ and other semiconductors. On the basis of band atructures and conductivity type of the semiconductors, two main types of semiconductor/semiconductor heteroatreuctures, n-n heterojunctions and p-n heterojunctions, can be considered. The work function (highest potential of valence band (VB)), electron affinity (lowest potential of CB), and bandgap of the coupled semiconductors determine the electron/hole dynamics in the heterojunctions. Since the work function of TiO₂ is different from that of coupling semiconductors, the electrons will transfer from one semiconductor to another, thus resulting in an additional depletion layer and an energy barrier at the interfaces of two semiconductors. Compared with the pure semiconductors, the conductivity of heterojunctions is mainly detrermined by the energy barrier, and the relationship between resistance and energy barrier of the heterojunctions can be presented by the following equation:

$$R\propto B exp\left(q\Phi /kT\right)$$

(3)

where B is a constant, k is the Boltzmann constant, T is the absolute temperature, and qΦ is the effective energy barrier at the heterojunction. When the heterostructures are in an air atmosphere, the electrons can be adsorbed by oxygen molecules to turn into various oxygen ions including O⁻, O²⁻, and O₂⁻, accordingly the height of the energy barrier in heterojunctions will increase, as shown in the first figure of Figure 10a. Similarly, the energy barrier height will further increase when put into oxidizing gases, whereas in reducing gases, the gases can react with the oxygen ions and result in the release of adsorbed electrons, therefore the energy barrier height will decrease, as shown in the second figure of Figure 10a. According to Equation (3), R_a/R_g is proportional to exp(ΔqΦ), thus the notable changes of energy barrier height can cause remarkable changes of the resistivity and superior enhancement of sensing properties for heterojunctions. For instance, Fe₂O₃/TiO₂ tube-like nanoheterostructures [107] and TiO₂/Ag_0.35V₂O₅ branched nanoheterostructures [95] based sensors exhibited improved ethanol sensing performance compared with pure matrix sensors.

Additionally, a synergetic effect between different nanomaterials is also regarded as one of important reasons for the enhanced sensing properties of nanoheterostructures. In fact, the synergetic effect is related only to the situation where both of the pure materials exhibit high sensitivity to the tested gases [61].

It is well known that some metal oxide semiconductors are effective catalysts which can help decompose organic gases. Therefore, the catalytic effect of TiO₂ and other materials should be taken into account in the sensing performance enhancement of nanoheterostructures. For example, according to the previous reports by our group [98,131], the TiO₂/V₂O₅ nanoheterostructures can act as much more effective catalysts than pure TiO₂ nanofibers, and should be capable of promoting the sensing reaction between volatile organic solvents (VOCs) and oxygen ions adsorbed at the surface, thus a TiO₂/V₂O₅ nanoheterostructures-based ethanol sensor displays much higher sensitivity than a pure TiO₂-based sensor [97].

In addition to the heterojunction effect, synergetic effect, and catalytic effect, there may be other mechanisms in play for enhancing the sensing performance of nanoheterostructure. For example, Chen et al. [108] considered that the electron transfer are facilitated because of the formation of SnO₂/TiO₂ heterostructures, thus the gas sensing response including sensitivity and selectivity is efficiently enhanced as a result of increased charge carrier concentration.

Actually, there is more than one reason for the enhanced mechanisms of nanoheterostructure sensors. Park and coworkers [64] demonstrated that the improved ethanol sensing performance of TiO₂-core/ZnO-shell nanorods compared with that of pure TiO₂ nanorods might be due to more efficient catalytic activity of ZnO and the potential barriers built in the heterojunctions. Wang and coworkers [61] found that the improved gas sensing activity of the porous single crystal In₂O₃ beads@TiO₂-In₂O₃ composite nanofibers (TINFs) could be ascribed to the Schottky junction formed between single crystal In₂O₃ beads and the Au electrode, the increased carrier density derived from the TiO₂ electron-donor, and the best gas absorption conditions provided by the surface-related defects, as shown in Figure 11.

3.1.2. The Influence of Morphology on Sensing Performance

It is reasonable that the influence of nanoheterostructures morphology on sensing performance should be taken into account, where the gas sensitivity and the response/recovery time affected by gas absorption and gas diffusion can be improved by the large specific surface area and the especial morphology of materials. On the one hand, like for resistance-type gas sensors, the fundamental sensing process is the reaction of the target gases and the electrons at the surface of the sensing materials. Larger surface area means more surface active sites can be provided for gas absorption and reactions, resulting in more noticeable changes of resistance in different gases, consequently the sensitivity of the sensors can be improved. On the other hand, the electron exchange process can only occur within a thin layer, the width of this surface layer is determined by the Debye length (L_D) of the materials, which is defined by following equation:

$$L_D={\left(kT\epsilon {\epsilon }_0/q^2{n}_c\right)}^{1/2}$$

(4)

where T is the absolute temperature in Kelvin, k is Boltzmann’s constant, ε₀ is the permittivity of vacuum, ε is the static relative dielectric constant, n_c is the carrier concentration, and q is the electrical charge of the carrier. When L_D is less or equivalent to the thickness of the nanostructures, the electrons in semiconductors can be totally depleted by the oxygen molecules adsorbed on the surface, thus will result in more evident resistance change after exposure in gases compared with that of not entirely depleted ones.

Hierarchical nanostructures are a particularly promising choice for further enhancing the sensing performance because of their extremely large specific surface area and/or thin thickness (less or equivalent to L_D). As a matter of fact, several hierarchical TiO₂-based nanoheterostructures, such as branch-like α-Fe₂O₃/TiO₂ hierarchical heterostructure [62], hierarchically assembled ZnO nanorods on TiO₂ nanobelts [114], SnO₂ nanospheres functionalized TiO₂ nanobelts [105], have been fabricated into gas sensors. In particular, Zhu and coworkers have reported that β-FeOOH/TiO₂ hierarchical heterostructures exhibited remarkably high sensitivity and reversibility [87]. Our group [95] demonstrated that the enhancement of the TiO₂/Ag_0.35V₂O₅ gas sensor could be attributed to the extraordinary branched-nanofiber structures with large surface area and thin branch diameter (the semidiameter of the branches was equivalent to the depletion layer of the Ag_0.35V₂O₅), where more gas molecules could be absorbed and electrons in the Ag_0.35V₂O₅ nanobranches could be totally depleted by the oxygen molecules adsorbed on the surface (Figure 10b). Furthermore, Deng et al. [110] have found that the enhanced performance of the ZnO-TiO₂-based gas sensors could be ascribed to the hierarchical structures, as shown in Figure 12, where the high specific surface area could result in a large number of gas molecules absorbed on the surface of ZnO-TiO₂ nanoheterostructures when compared with the pure TiO₂ nanofibers, additionally, well aligned structures of the nanoheterostructures would cause the unhindered gas diffusion to the whole surface of the sensor.

In addition to island type hierarchical heterostructured sensors, core/shell type and hollow type nanoheterostructures have also attracted much attention in gas sensor research. Recently, Zhu et al. [107] synthesized tube-like Fe₂O₃/TiO₂ core/shell nanoheterostructures (Figure 13). The special core/shell nanoheterostructures could act as a superior ethanol sensor material with respect to the pristine one, as shown in Figure 13. Katoch and coworkers [72] demonstrated that TiO₂/ZnO double layer hollow fibers exhibited superior CO sensing performance compared to the ZnO single layer hollow fibers, as shown in Figure 14a. The enhancement was ascribed to the fact that the electrons in ZnO outer layer could be easily absorbed to TiO₂ inner layer, thus ZnO would become more resistive owing to the noticeable loss of electrons, as shown in Figure 14b,c. When exposed in CO gas atmosphere, the resistance of ZnO outer layer would partially regain its original value, this could lead to more noticeable resistance change for the TiO₂/ZnO double layer hollow fibers in detecting CO gas.

3.2. Carbon-Group-Materials/Semiconductor Nanoheterostructures

The formation of carbon-group-materials/semiconductor heterostructures is also an important technique to improve the gas sensing performance of TiO₂. In recent years, carbon-group-materials have attracted much attention for applications in gas sensors [1,12,15,132]. The nanostructured carbon materials including carbon nanotubes (CNTs) and graphene possess outstanding physical, chemical and electrical properties, such as good flexibility, large surface area, high chemical stability, and high electrical conductivity [14,133]. These excellent features make them extremely suitable for use as candidates in enhancing semiconductors’ sensing properties [132,134,135,136]. It has been demonstrated that the absorptivity, conductivity, and/or electrochemical reaction of some small gas molecules of carbon/TiO₂ nanoheterostructures could be promoted, thus the nano- heterostructures can display remarkably improved sensing performance [75,137,138,139].

Table 2. Summary of the gas sensing properties of carbon-group-materials/TiO₂ nanoheterostructure sensors.

TiO2-Based Nanoheterostructures	Fabrication Method	Size	Detection Gas	Detection Range	Response				Ref.
					Operation Temperature (°C)	Sensitivity	Response/Recovery Time	Concentration
Pd/TiO2/reduced graphene oxide ternary composite	one-pot polyol		NH3	5–150 ppm	RT	(ΔR/Ra) × 100%: 39.9%		100 ppm	[142]
PPy/graphene nanoplatelets decorated TiO2 nanoparticles	sol-gel + chemical polymerization	TiO2 nanoparticles diameter: 10–30 nm	NH3	1–200 ppm	RT	(ΔR/Ra) × 100%: 102.2%	36/16 s	50 ppm	[146]
CNTs/TiO2 nanocomposites	screen-printing + dip-coating techniques		NH3		RT	ΔR/Ra: 93	9/2 min	1%	[148]
Pt/TiO2/MWCNTs nanocomposites	sol-gel		H2	5%–100%	50	(ΔR/Ra) × 100%: 30%		70%	[149]
CNTs/Pt-TiO2 NTs	anodization	diameter: 100 nm;length: 14 um	H2	0.5%–3%	100	(ΔR/Ra) × 100%: 2%		1%	[150]
Pt-TiO2/MWCNTs hybrid composites	wet chemical procedure		H2	0.5%–3%	150			0.5%	[140]
rGO/TiO2 thin film			formaldehyde	0.1–1 ppm	RT	(ΔR/Ra) × 100%: 0.64	70/126 s	1 ppm	[75]
MWCNTs/TiO2 nanocomposites	sol-gel	diameter: 20–40 nm	CO		350	Ra/Rg: 15.8	4/16 s	50 ppm	[151]
MWCNTs/TiO2 thin film	sol-gel		CO		400	Ra/Rg: 89.2	5.16/2.72 s	100 ppm	[152]
graphene-TiO2 nanocomposite	sol–gel	TiO2 nanoparticles: ~35 nm	CO2	500–15,000 ppm	200	Rg/Ra: 1.34		10,000 ppm	[144]
TiO2/carbon black	sol-gel		NO2	1–100 ppm	150	ΔR/Ra × 100%: 7%		100 ppm	[153]
single-walled carbon nanotube/TiO2 hybrid		length of carbon nanotubes: 20–50 nm	NO	50 ppb–1 ppm	RT	(ΔR/Ra) × 100%: 9%		50 ppb	[140]
CNT/TiO2 hybrid films	sol-gel		O2	10 ppm	350	ΔR/Ra: 6.5	8/- s	10 ppm in CO2	[154]
grapheme oxide/nano-anatase TiO2		~5 nm	humidity	35%–95%		power loss/ΔRH: ~0.47 dB/%RH	0.74/0.91 s		[141]

summarizes the carbon/TiO₂ nanoheterostructures and their performance in the detection of gases.

The mechanism of enhanced gas sensing performance of the carbon/TiO₂ nanoheterostructures is proposed to occur as follows: first, carbon-group-materials possess huge specific surface areas and nanoscale structures, thus a large number of surface sites are exposed for reacting with gases, therefore various gases can be easily detected at lower operating temperatures [140,141]. Second, the electric conductivity of carbon-group-materials is much higher in comparison with TiO₂, this can reduce the resistance and enhance the electrons transport capability of heterostructured sensors, thus making the nanoheterostructures based sensors work at lower operating temperature [142,143]. Third, since TiO₂ displays n-type semiconductor characteristic and carbon group- materials (graphene, CNTs) display p-type semiconductor characteristic, hence, a competitive mechanism may occur surrounding the carbon-group-materials/TiO₂ heterojunctions, which can lead to enhanced gas sensitivity owing to the decrease of the work function (barrier height) or increase of the conductivity of TiO₂ sensitive layer [144,145]. Furthermore, the catalytic activity of TiO₂ should also be considered as one of important reasons for the improved sensing performance of carbon-group-materials/TiO₂ nanoheterostructures [146,147].

As an emerging carbon material, graphene is regarded as a promising candidate for application in gas sensors due to its huge surface area, high electrical conductivity, inherently low electrical noise, environmental ultra-sensitivity, and ease microfabrication. In particular, the combination of graphene and TiO₂ can achieve efficiently improved sensing performance.

In Ye’s research [75], rGO/TiO₂ layered thin film were prepared on the interdigital electrode substrates via a spray method. Formaldehyde sensing tests demonstrated that the layered thin film exhibited high sensitivity (0.905 ppm⁻¹), and a reversible and linear response to 0.1–0.5 ppm formaldehyde at room temperature, and that this could be ascribed to the positive synergetic effect of the two materials, as shown in Figure 15. Xiang and coworkers [146] developed a room-temperature sensitive NH₃ gas sensor using TiO₂@PPy-graphene nanocomposites. The sensor displayed high sensitivity (102.2%), superior reproducibility, and excellent selectivity to 50 ppm NH₃. Additionally, Wang et al. [147] synthesized TiO₂/graphene composite film for O₂ sensing upon exposure to UV light. They found that the outstanding sensitivity of the composite film could be ascribed to the synergetic effect of the ultrasensitivity of single-layer graphene to the environment and photocatalytic activity of TiO₂.

Like graphene, CNTs have also been widely studied as preeminent gas sensing materials. Recently, multifarious effort has been devoted to the design and fabrication of CNTs/metal oxide nanocomposites [138,139,155,149]. The new composite materials will maintain the original properties of each component, or even show a synergistic effect, which is exceedingly significate to sensing performance. For example, Llobet et al. [154] proposed an effective O₂ sensor based on CNTs/TiO₂ hybrid films. The researchers compared the sensing properties of CNTs/TiO₂ hybrid film and Nb-doped TiO₂ films, and the results showed that the sensitivity of former was four times higher than that of latter, as shown in Figure 16. The outstanding sensing performance of CNTs/TiO₂ hybrid films arose from their very large surface area because of the central hollow cores and outside walls of CNTs. Luca and coworkers [149] have developed a room temperature sensor using Pt/TiO₂/MWCNTs composites. The results showed that increased resistance in response to H₂ possibly indicated a p-type conduction mechanism of the composite sensing material. Lee et al. [151] have demonstrated that the MWCNTs/TiO₂ xerogel composites film possessed better sensing performance. Compared with pure TiO₂ xerogel, the nanocomposites displayed improved sensitivity (15.8), and lower response/recovery times (4/16 s), as shown in Figure 17. The improved sensing properties could be attributed to the increased surface-to-volume area and enhanced conductivity which were benefit from the p-n heterojunction formed at the interface of MWCNTs and TiO₂. This result could also be confirmed by Kim et al. [152] who prepared a MWCNTs-modified direct-patternable TiO₂ thin film based CO gas sensor. It was found that the incorporation of MWCNTs could induce increase of surface morphology and roughness and thus resulted in promoted sensing performance.

3.3. Organic/Inorgnic Nanoheterostructures

Nowadays, more and more requirements for gas sensors applied in the safety control and environmental monitoring have been put forward. In these efforts, designing and manufacturing suitable and efficient sensing materials is an important factor in obtaining highly efficient sensors [156]. In recent years, conductive polymers were considered a promising sensing element because of the significant changes in electrical and optical properties of such materials exposed in different gas atmospheres. In particular, the simple preparation and high sensing performance at room temperature of these polymers make them more acceptable for applications in many fields [157].

Conducting polymer-based nanocomposites composed of metal oxides nanomaterials and conducting polymers have been well developed, and the corresponding sensors exhibit great potential in probing various hazardous gases [91] due to their enhanced sensing properties, such as fast and reversible responses to target gases at room temperature [92], and more noticeably, that defined organic material modified semiconductor sensors exhibit outstanding sensing selectivity to a single gas species because of their exclusive chemical and electronic conditions [6,158,159,160]. Particularly, organic-functionalized TiO₂ nanoheterostructures also exhibit significantly improved characteristics in gas sensing compared with pure TiO₂ nanomaterials [83,91,94,161,162] and thus have been investigated by several research groups, as shown in

Table 3. Summary of the gas sensing properties of TiO₂-based organic/inorganic nanoheterostructure sensors.

TiO2-Based Nanoheterostructures	Fabrication Method	Size	Detection Gas	Detection Range	Response				Ref.
					Operation Temperature (°C)	Sensitivity	Response/Recovery Time	Concentration
TiO2/PPy nanocomposites	in situ chemical polymerization	33–67 nm	NH3	20–140 ppm	RT	(ΔR/Ra) × 100%: 7.95%	19/85 s	141 ppm	[88]
PPy-coated TiO2/ZnO nanofibers	electrospinning + chemical deposition	TiO2/ZnO core diameter: 100 nm; PPy shell thickness: 7 nm	NH3	0.5–450 ppm	RT	ΔR/Ra: 0.35		450 ppm	[118]
PPy/TiO2 nanocomposites	layer by layer self-assembly technology.		NH3	10–1600 ppm	RT	frequency shift (ΔF): 50 Hz	~100/200 s	10 ppm	[165]
PPy/TiO2	in situ polymerization		NH3	20–500 ppm	RT	ΔR/Ra: 0.13		100 ppm	[169]
PANi/TiO2 nanofibers	electrospinning	diameter: 600 nm	NH3	>50 ppt	RT	ΔR/Ra: 0.018	<10/10 s	200 ppt	[49]
PANi/TiO2 thin film heterojunction	chemical polymerization + sol-gel		NH3	20–100 ppm	RT	(ΔR/Ra) × 100%: ~11%	41/- s	100 ppm	[74]
polyaniline/TiO2 nanorods heterostructure	hydrothermal method		NH3	5–100 ppm	RT	(Rg/Ra) × 100%: 610%	40/60 s	100 ppm	[167]
cellulose/TiO2/PANi composite nanofibers	electrospinning		NH3	10–250 ppm	RT	ΔR/Ra: 0.584		10 ppm	[168]
TiO2-PANi/PA6 nanofibers	electrospinning + sputtering		NH3	50–250 ppm	RT	ΔR/Ra: 18.3	<50/50 s	250 ppm	[170]
PANi/TiO2 nanocomposite thin film	in situ self-assembly technique	diameter: 90 nm	NH3	20–140 ppm	RT	ΔR/Ra: 0.3	2–3/~60 s	1 ppm	[88]
CSA/PANi/TiO2 thin film	sol-gel		NH3	20–100 ppm	RT	ΔR/Ra: 0.75	49/413 s	100 ppm	[171]
TiO2–PANi nanocomposite thin film	spin coating	~20 nm	CO2	53–1000 ppm	RT	Rg/Ra: 53	9.2/5.7 min	1000 ppm	[164]
PANi doped TiO2 nanocomposite thin film	spin coating	21 nm	LPG		RT	Rg/Ra: 2.37	2.6/2.4 min	2000 ppm	[164]
TiO2/PPy/poly 3-[(methacryoylamino) propyl trimethylammonium chloride] (PMAPTAC) nanocomposite thin film	in situ photopolymerization		humidity	13–90% RH	RT	log Z: ~6	30/45 s	60%	[172]
TiO2/PPy nanocomposite film	in situ photopolymerization		humidity	30–84% RH	RT	log Z: ~5.5	40/20 s	30%	[173]

.

Till now, a number of literatures proposed the sensing mechanism of polymer/TiO₂ nanoheterostructures. One of generally accepted factor is that the nanostructures of the heterostructures are beneficial to the high sensing properties, which mainly result from their large specific surface areas and much more active sites for the adsorption and interactions with the gases [163,164,165]. What is more, the formation of heterojunction at the interface between TiO₂ and polymers is usually regarded as a main factor for the ultrahigh sensitivity of the nanoheterostructures [83]. At the heterostructure interface, the electrons will transfer from the material with higher Fermi level to another with lower Fermi level, while the holes will transfer rightabout until the Fermi levels is equalized. In this process, electron depletion layer, an efficient current switch, will form at the interface of the heterostructures, thus leading to high gas sensitivity [166,167,168].

Polyacrylic acid (PAA) is one of the appropriate choices for probing NH₃ gas because the free carboxylic functional groups present on the PAA surface possess high sensitivity and selectivity to NH₃ molecules [174]. The incorporation of PAA and metal oxide nanomaterials may solve the long recovery time at higher NH₃ concentration (>1 ppm) and undesired sensitivity to humidity of the existing inorganic semiconductor sensors [175]. For instance, Lee and coworkers [94] have demonstrated that TiO₂/PAA-based amine gas sensors exhibited fast and stable response in a wide relative humidity range of 30%–70%, furthermore, a good linear response was also observed in the NH₃ concentration range between 0.3 ppm and 15 ppm, as shown in Figure 18. One of possible reasons for the decreased influence of humidity on gas sensitivity was that the presence of the TiO₂ would suppress the mobility of PAA, therefore the influence of water molecules on the sensing response would be reduced. As another very interesting conducting polymer, polypyrrole (PPy) has been widely investigated due to its relatively good environmental stability and easily controlled surface carrier properties adjusted by altering the dopant species in PPy during the synthetic process [169]. Accordingly, a number of researchers have attempted to enhance the gas sensing performance of TiO₂ by introducing PPy to form organic/inorgnic nanoheterostructures. For example, Bulakhe and coworkers [83] have prepared a PPy/TiO₂ heterojunction-based liquefied petroleum gas (LPG) sensor which could operate at room temperature. The maximum sensing sensitivity of 55% to 1040 ppm LPG was observed, as shown in Figure 19. Compared with other room temperature LPG sensors, the PPy/TiO₂ heterojunction-based sensor could work at low LPG concentrations and showed promoted response/recovery times (112/131 s), indicating the PPy/TiO₂ heterojunction was a promising choice for a room temperature LPG sensor. Furthermore, Tai et al. [88] have investigated the NH₃ sensing performance of TiO₂/PPy nanocomposite ultrathin films. The results revealed that the TiO₂/PPy ultrathin film presented outstanding sensing performance, such as shorter response/recovery time comparing with pure PPy thin film based sensor. Additionally, in the work of Wu et al. [169], the PPy/TiO₂ composite thin film based sensor displayed much lower detection limit of 2 ppm to NH₃ gas. The improvement of sensing performance of PPy/TiO₂ heterostructures is mainly attributed to the formation of p-n junctions at the interface between TiO₂ and organic PPy.

Moreover, polyaniline (PANi) has attracted much attention in commercial applications due to its excellent environment stability, novel photoelectrical and electrical characteristics, and easy fabrication [170]. It has been demonstrated that combining TiO₂ with PANi can enhance the sensitivity, selectivity, and stability of the resulting sensors [88]. For example, Gong et al. [49] have reported an ultrasensitive NH₃ gas sensor based on PANi/TiO₂ fibers p-n heterojunctions, it was found that the p-n heterojunctions could act as electronic transmission switches when NH₃ gas was absorbed by PANi, thus resulting in the enhancement of sensing properties. Similarly, using TiO₂ nanoparticles modified PANi fibers as sensor, a fast response of 2–3 s and a low detection limit of 1 ppm at room temperature were obtained by Tai et al. [88]. Additionally, Pawar et al. [74] have also investigated the sensing performance of nanostructured PANi/TiO₂ films synthesized via a spin-coating method on glass substrates, the results revealed that the nanocomposites film exhibited much enhanced sensing performance, such as higher sensitivity and lower respons/recover time toward NH₃ at room temperature, as shown in Figure 20.

4. Conclusions

In the last few decades, continuous breakthroughs in the fabrication, modification and application of TiO₂-based gas sensors have been reported. In this review, we describe the universal tactics and new progress in the preparation of TiO₂-based nanoheterostructures for exhaust gases detecting. We focus on then synthetic methods and sensing performances of TiO₂-based nanoheterostructures, including semiconductor/semiconductor nanoheterostructures, noble metal/semiconductor nanoheterostructures, carbon-group-materials/semiconductor nanoheterostructures, and organic/inorganic nanoheterostructures, which are summarized as follows:

(1)

Coupling TiO₂ by with other semiconducting materials to form heterostructures could result in enhanced sensitivity and selectivity, faster response/recovery times, and/or lower operational temperatures than pure TiO₂. The enhanced sensing properties could be related to the heterojunctions formed at the interface between two semiconductors, the synergetic effect and catalytic effect. Additionally, the influence of the nanoheterostructures’ morphology on sensing performance should also be taken into account, where the gas sensitivity and the response/recovery time affected by gas absorption and gas diffusion can be promoted by the specific surface area and the special morphology of structures. As compared with other types of nanoheterostructure-based sensors, the minimization of detectable levels of the semiconductor/semiconductor nanoheterostructure-based sensors might not bear comparison with the carbon-group-materials/semiconductor nanoheterostructures, and their operation temperatures might be higher than that of noble metal/semiconductor nanoheterostructures and organic/inorganic nanoheterostructures, however, their most excellent stability make the semiconductor/semiconductor nanoheterostructures more acceptable when applied in many fields.

(2)

Combining carbon-group-materials with TiO₂ is considered an efficient way to improve the gas sensing performance of TiO₂. It has been demonstrated that the absorptivity, conductivity, and/or electrochemical reactions of some small gas molecules with carbon/TiO₂ nano- heterostructures could be promoted, thus the nanoheterostructures can display remarkably improved sensing performance. Remarkably, carbon-group-material/semiconductor nano- heterostructure-based sensors display the most outstanding minimum detectable levels as compared with other nanoheterostructures, which could be attributed to their large surface area and the high electrical conductivity of the carbon-group-materials.

(3)

Conducting polymer-functionalized TiO₂ nanoheterostructures have also been demonstrated to be some of most promising materials for gas detection. These nanoheterostructures possess fast and reversible responses at room temperature due to the formation of organic/inorganic heterojunctions. It is noticeable that defined organic material modified semiconductor sensors exhibit outstanding sensing selectivity to a single gas species because of their exclusive chemical and electronic conditions. As a type of typical room temperature sensor, the controllable sensing selectivity of organic/inorganic nanoheterostructures is an outstanding characteristic that other types of nanoheterostructures do not possess.

In summary, much progress has been achieved in investigation of TiO₂-based nanoheterostructured sensors, and they are widely applicable in the detection of various gases such as H₂, CO, NH₃, H₂O, VOCs, etc. However, there are still some challenges in designing high-performance nanoheterostructured gas sensors. First, although the TiO₂-based nano- heterostructured gas sensors can respond to various gases based on the resulting resistance changes, a single sensor cannot discriminate different analytes, and the response is susceptible to other gases. Thus, most current studies are focused on the quantitative analysis of target gases using only simple matrices rather than complex matrices and actual samples. To meet the needs of practical applications, one possible solution is combing multiple sensors in a multimodal module sensor system, which can display distinctive response patterns across the array in probing different actual samples. Second, the understanding to the electrons transport and response on the interfaces of nanoheterostructures is not detailed enough, which is critical for the design and optimization of highly efficient sensors. Therefore it is necessary to further elucidate the dynamic behavior of electrons at the interface and surface of nanoheterostructures so that researchers can design TiO₂ based nanoheterostructures more rationally. Furthermore, although a variety of synthetic methods have been successfully used for preparing TiO₂-based nanoheterostructured sensors in the laboratory conditions, but these are far away from the large quantity industrial production. In addition, the chemical stability of TiO₂ based nanoheterostructures is still poor, which seriously hampers the performance and wide application of the sensors. Therefore, the in-depth understanding of the sensing mechanism and the establishing of general and reasonable design guidelines in TiO₂ based nanoheterostructured sensors are significant to achieve substantial breakthroughs in the practical application of the sensors.