Unraveling the Magnetic Properties of NiO Nanoparticles: From Synthesis to Nanostructure

Abstract

NiO nanoparticles have garnered significant interest due to their diverse applications and unique properties, which differ markedly from their bulk counterparts. NiO nanoparticles are p-type semiconductors with a wide bandgap, high discharge capacity, and high carrier density, making them ideal for use in batteries, sensors, and catalysts. Their ability to generate reactive oxygen species also imparts disinfectant and antibiotic properties. Additionally, the higher Néel temperature of NiO compared with other antiferromagnetic materials makes it suitable for high-temperature applications in spintronic devices and industrial settings. This review focuses on the critical role of structure and composition in determining the magnetic properties of NiO nanoparticles. It examines how finite-size surface effects, morphology, crystallinity, and nickel distribution influence these properties. Fundamental physical properties and characterization techniques are discussed first. Various synthesis methods and their impact on NiO nanoparticle properties are then explored. Their magnetic phenomenology is examined in detail, highlighting the effects of finite size, particle composition and surface, and crystal quality. The review concludes with a summary of key insights and future research directions for optimizing NiO nanoparticles in technological applications.

1. Introduction

Antiferromagnetic (AFM) materials represent a fascinating type of magnetic system, sharing some characteristics with their ferromagnetic (FM) counterparts [1,2,3,4,5,6]. For example, they both exhibit the formation of magnetic domains due to the cooperative magnetic behavior of the electronic spins within the crystal lattice. In AFM materials, spins align in two sublattices with opposite directions to minimize the system’s overall energy, allowing for complex interactions that influence magnetic and electronic properties.

Nickel oxide nanoparticles (NiO NPs) have gained significant interest in AFM materials research due to their size-dependent properties and diverse applications (see Figure 1) [7,8,9]. NiO NPs are cost-effective and easy to produce and isolate, while still possessing valuable chemical and physical characteristics [7,8,9]. For instance, they are p-type semiconductors with a wide bandgap (3.6–4 eV) [10,11], high discharge capacity (638 mAh/g) [12], and high carrier density (~7.35·10¹⁸ cm⁻³) [13]. These properties lead to superior electrochemical performance compared with traditional cathode materials, making them ideal for batteries [12,14,15,16,17]. Additionally, the porous nature of nanosized NiO NPs allows them to function as both sensors and catalysts [18]. They can detect relevant compounds, such as ethanol, and catalyze CO oxidation reactions [19,20,21,22]. Interestingly, NiO NPs also exhibit disinfectant and antibiotic properties by generating reactive oxygen species that damage microbial cell walls [23,24,25].

Furthermore, NiO NPs have the highest Néel temperature ($T_N$ = 525 K) among metal monoxides (e.g., FeO, CoO, MnO) [1], making them suitable for applications requiring magnetic stability at elevated temperatures. This is especially valuable in industrial settings where materials experience high heat exposure. As an AFM layer in spintronic devices, NiO maintains AFM order even at higher operational temperatures, enhancing reliability and performance. Moreover, NiO-based sensors can function effectively at higher temperatures compared with those made from materials with lower $T_N$ [26,27,28,29].

In bulk NiO, neighboring nickel spins align antiparallel, demonstrating AFM behavior [1,2,3,4,5,6,7,8]. However, when reduced to the nanoscale, surface and finite-size effects start to dominate, often resulting in a nonzero magnetic moment in NiO NPs. This exotic behavior has been studied for decades, beginning with the pioneering investigations by Néel [30], who suggested that the weak FM behavior in these systems could be due to incomplete compensation between the two AFM sublattices. In the 1990s, Kodama et al. proposed that the anomalous magnetic properties observed in these systems could not be fully explained by the two-sub-lattice AFM ordering observed in bulk NiO [31]. Instead, they suggested the emergence of more complex sublattice configurations within the NP due to finite-size effects associated with the size perturbation of the magnetic correlation in each of the two sublattices of the bulk AFM. Subsequent research has extensively examined the impact of finite-size and surface effects on the magnetic properties of NiO NPs, focusing on factors such as particle size, crystal domain boundaries, and Ni oxidation state.

The synthesis routes for synthesizing NiO NPs are crucial in determining their ultimate physical and chemical characteristics [7,8,9]. These methods range from traditional chemical precipitation to advanced techniques such as sol–gel [32,33,34], hydrothermal synthesis [35], and thermal decomposition [36,37], each with unique advantages and influencing factors on the NPs’ properties. The choice of a synthesis route directly impacts the size, shape, crystallinity, and surface characteristics of NiO NPs, which in turn, affects their electronic, magnetic, and catalytic behaviors. Understanding the synthesis of NiO NPs is not just about producing these particles but also about tailoring them for specific applications by manipulating their intrinsic properties through controlled fabrication processes. This critical link between the synthesis methods and the properties of NiO NPs underscores the importance of exploring these routes in depth, as it enables the development of NiO NPs with desired functionalities for various applications, from electronics and catalysis to biomedicine and environmental remediation.

This short review aims to shed light on the complex interplay between the crystal structure and the finite-size effects on the magnetic nature of NiO NPs, offering substantial knowledge beneficial to both scientific inquiry and technological advancement. This review is structured into several comprehensive sections, each focusing on a different aspect of NiO NPs. Thus, the second section focuses on the fundamental physical properties of NiO NPs and the primary physicochemical characterization techniques used to understand these properties. The third section explores the synthesis of NiO NPs, emphasizing the influence of crucial parameters such as reagent concentrations and solvents on the morphology and crystallinity of the particles. The fourth section summarizes the magnetic phenomenology of these systems detailing the impact of finite-size and surface effects, particle composition, and crystal quality on the magnetic properties of NiO NPs. Finally, this review concludes with a comprehensive summary and offers a critical perspective on the field, highlighting future research directions and potential applications of NiO NPs.

2. Structural and Magnetic Properties of NiO NPs: Analysis and Characterization Techniques

2.1. Structure vs. Magnetism in NiO NPs

NiO has a face-centered cubic (FCC) atomic arrangement, in which each Ni²⁺ atom is surrounded by six O²⁻ atoms in an octahedral geometry [7,8,9]. The space group is Fm3m, which renders a lattice parameter of 4.19 Å, and a density of 6.76 g/cm³ (see

Table 1. Key crystal and magnetic features of NiO, such as crystal structure, lattice parameters, unit cell volume, density, magnetic moment per atom, and $T_N$.

Property	Value
Crystal structure	Cubic (FCC)
Symbol	Fm3m
Length a = b = c (Å)	4.19
Unit cell volume (Å3)	73.37
Density (g·cm−3)	6.76
${\mu }_{NiO}$ (μB)	2
$T_N$ (K)	525

and Figure 2) [38,39]. Generally, stoichiometric NiO NPs exhibit a pale green color. However, minor changes in the Ni²⁺ oxidation state can result in samples that turn black. Such variations in the crystal structure can promote new energy levels in the band gap, modifying the optical and electronic properties of the material.

NiO is a type-II AFM with a collinear spin structure. The AFM superexchange interaction between Ni ions through oxygen (Ni-O-Ni) bonds along the 100 directions yields FM ordering within {111} planes. In these planes, spins in adjacent {111} planes align antiparallel to each other (see Figure 2) [39,40,41]. Below $T_N$, this AFM ordering causes magnetostriction, resulting in the formation of twinned crystal domains within the NiO crystal. This twinning produces four distinct magnetic twin domains (T domains), each characterized by different contractions along the {111} axes. Within each T domain, there are three possible spin easy axes oriented along the {112} directions, leading to a total of 12 different spin domains (S domains). This exchange striction leads to a small contraction of the distance between the ferromagnetically ordered planes below $T_N$. Despite this deviation from perfect cubic symmetry, the difference in plane spacing is only about 0.1% at 300 K.

Figure 2c depicts the overlap between the oxygen p-orbital and the nickel d-orbital. The fully occupied 2p orbital of oxygen, with its antiparallel electron configuration following the Pauli exclusion principle, induces opposite spin orientations in the nickel atoms’ 3d electrons through a superexchange interaction. This type of exchange via a nonmagnetic ligand is a well-known phenomenon in metal oxides, first reported by Kramers for MnO [42,43].

2.2. Magnetic and Structural Characterization Techniques

Understanding the inherent physical and chemical features of NiO NPs requires the application of several characterization techniques [44]. This section focuses on standard methods used to elucidate the main magnetic, structural, and compositional features of NiO NPs, while advanced techniques, although useful for single particle experiments, are not the primary focus of this review.

2.2.1. Magnetic Characterization of NiO NPs

They are dominated by pronounced surface effects arising from numerous uncompensated spins, which can also be present throughout the entire volume of the particle. Additionally, the odd number of sublattice planes and multiple sublattices due to finite-size effects can further contribute to the net magnetic moment observed [1,42]. These spins can exhibit superparamagnetic (SPM) behavior [45], for which thermal energy is sufficient to flip the magnetization within the particles spontaneously. Consequently, no magnetization is observed in the absence of a magnetic field. Interestingly, when a magnetic field is applied, there is a preferential orientation for the uncompensated spins, resulting in a net magnetic moment (see Figure 3). This alignment is analogous to that in a paramagnet but with higher magnetic moment, which can be up to 10² times larger than that of an atomic paramagnet [3]. Hysteresis loops (M–H) and zero-field cooled/field cooled (ZFC/FC) curves are the most commonly used measurements to examine the influence of an external magnetic field on the magnetic response of NiO NPs (see Figure 3) [1,2,4,44].

Measuring hysteresis loops at a constant temperature provides crucial insights into the magnetic properties of materials, specifically determining the maximum magnetization ($M_{max}$), zero-field saturation magnetization ($M_s$), remanent magnetization ($M_r$), and coercive field ($H_c$) [2,4]. The former is the highest net magnetization that a magnetic system can achieve by the action of a high enough external magnetic field. However, in AFM NiO and other AFM materials, full saturation is unreachable at any experimentally achievable magnetic field due to the intrinsic antiparallel alignment of magnetic moments and the effective large magnetic anisotropy. Therefore, the induced magnetization at any arbitrary value of the magnetic field is usually much smaller compared with that of a FM material. On the other hand, the weak FM behavior arising from either incomplete compensation between the two AFM sublattices or other more complex sublattice configurations within the NP due to finite-size effects can be measured by the so-called zero-field saturation magnetization, $M_s$. In the high magnetic field region, hysteresis loops at low temperature in the blocked state are typically linear and can be fitted to

$$M\left(H\right)=M_s+{\chi }_{d}H$$

(1)

where ${\chi }_d$ is the high-field differential susceptibility that accounts for the uncompensated spins. $M_s$ and ${\chi }_d$ are obtained from the average of the $H\to {\pm H}_{max }$ branches [2,4,46,47]. To recapitulate, $M_{max}$ is used when the hysteresis loop is dominated by the AFM component, while $M_s$ refers to the weak, superimposed FM component. Next, $M_r$ is the value of the residual magnetization when the applied magnetic field is removed ($H$ = 0). $H_c$ corresponds to the magnetic field that must be applied to cancel the sample magnetization. It is determined by $H_c={\displaystyle \raisebox{1ex}{$\left(\right|H_c^{+}|+|H_c^{-}\left|\right)$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$, where $H_c^{+}$ and $H_c^{-}$ are the intercepts of the hysteresis loop branches with the $H$-axis [2,4]. Interestingly, the interaction between FM and AFM phases can be evaluated by examining the shift in hysteresis loops, known as exchange bias phenomenon. This can be performed by recording hysteresis loops at low temperatures, after cooling the samples from room temperature to the desired low temperature under an applied magnetic field. The exchange bias field ($H_{eb})$ is then calculated as $H_{eb}= {(H}_c^{+}-H_c^{-})/2$ [2,4,46,47].

ZFC-FC curves are, on the other hand, commonly used for obtaining information about the distribution of magnetic volumes, verifying dipolar interactions, and estimating the average particle anisotropy in NP systems [4,48]. This protocol involves measuring particle magnetization as a function of temperature while subjecting the sample to a specific temperature/magnetic field profile. Typically, samples are cooled from room temperature to low temperatures (2–10 K) without an applied magnetic field. Then, a static magnetic field ($H>H_c$) is applied, and the zero-field-cooled (ZFC) magnetization ($M_{ZFC}$) is measured as the temperature increases [1]. Once room temperature is reached, the sample undergoes another cooling/heating cycle under the same magnetic field, and the field-cooled (FC) magnetization ($M_{FC}$) is collected during heating.

Initially, in a ZFC-FC protocol, as the sample is cooled in the absence of a magnetic field, the particle moments become blocked in random directions, resulting in very low magnetization. Then, as the sample is subsequently heated under the applied magnetic field, the particle moments progressively align with the external magnetic field, leading to a steady increase in net magnetization. This process continues until a peak is reached. Here most particles align with the magnetic field as they fluctuate between blocked and superparamagnetic states. Considering isolated SPM particles, we can define this temperature as the blocking temperature ($T_B$) [2,4,32], as follows:

$$T_B={\displaystyle \frac{K_{eff}V}{k_{B }\mathrm{l}\mathrm{n}\left(\raisebox{1ex}{${\tau }_m$}\!\left/ \!\raisebox{-1ex}{${\tau }_0$}\right.\right)}}$$

(2)

where $K_{eff}$ is the effective magnetic anisotropy constant, $V$ is the volume of the NP, ${\tau }_m$ is a characteristic measuring time usually around 100 s, and ${\tau }_0$ = 10⁻¹⁰ s is the characteristic relaxation time. On the contrary, $M_{FC}$ curve, obtained by cooling and heating the sample in the presence of a magnetic field, shows no peak because, during cooling, the particle moments progressively align with the magnetic field, increasing $M_{FC}$ as more particles become blocked. Upon heating, $M_{FC}$ curve shows a gradual decrease in magnetization due to thermal excitation. Finally, $T_N$ corresponds to the critical temperature at which the AFM order collapses. Below $T_N$, NiO NPs exhibit antiparallel spin alignments, characteristic of the AFM configuration. Above $T_N$, spins progressively disorder, as the remaining local AFM order vanishes, and paramagnetic behavior emerges.

2.2.2. Transmission Electron Microscopy (TEM)

This powerful technique allows a precise determination of NiO NPs’ size and shape, which can range from a few to several tens of nm [49,50,51]. Direct visualization through TEM enables measurement of particle size distribution and provides insights into phenomena like particle aggregation or sintering. Histograms of the size distributions of these systems can be determined by measuring some hundreds of particles using digital image analysis softwares and fitting the data commonly to a log-normal probability distribution [52], originating from the multiplicative processes involved in particle growth. The log-normal probability distribution can be described by Equation (3) [2,53,54].

$$P\left(D\right)={\displaystyle \frac{1}{S\sqrt{2\pi }D}}{e}^{-{\ln }^2\left({\displaystyle \frac{D}{D_0}}\right)/\left(2S^2\right)}$$

(3)

where D is the particle diameter, D₀ stands for the most probable value of D, and S is the standard deviation of the distribution of the logarithm of the reduced diameter $D/D_0$. D₀ and S are determined by fitting experimental histograms obtained from TEM data to Equation (3). The mean particle size D_TEM and the standard deviation σ of the diameter distribution are computed from Equations (4) and (5), respectively, as follows:

$$D_{TEM}=D_0{e}^{S^2/2}$$

(4)

$$\sigma =D_0{e}^{S^2/2\sqrt{e^{S^2}-1}}$$

(5)

Finally, the polydispersity index ${\mathsf{\sigma }}_{RSD}$ of the samples is estimated by dividing the standard deviation by the mean particle size ${\mathsf{\sigma }}_{RSD}$ = σ/D_TEM.

TEM also offers detailed observations of the surface roughness and porosity of NiO NPs. These surface characteristics are vital for applications that rely heavily on surface interactions, such as catalysis and sensors. Variations in surface texture, influenced by factors like synthesis methods and postsynthesis treatments, can affect the nanoparticles’ optical, electronic, and catalytic properties [7,8,9].

High-resolution TEM (HRTEM) takes this analysis a step further, enabling a highly detailed examination of NiO NPs’ crystallinity. HRTEM achieves this by directly imaging crystal planes and measuring the spacings between them. HRTEM also plays a crucial role in identifying defects within the crystal structure, such as dislocations or stacking faults, which have a significant impact on the physical and chemical properties of the NiO NPs, often influencing factors like electronic properties and catalytic activity. Additionally, structural and compositional analyses through techniques integrated with TEM, such as Selected Area Electron Diffraction (SAED) or Electron Energy Loss Spectroscopy (EELS), can confirm the chemical purity of NiO NPs [49,50,51].

SAED patterns of typical NiO NPs show ring-like patterns, indicating the random orientation of many small crystals. Analysis of these patterns is in general straightforward, as pure NiO presents a high-symmetry FCC crystal structure, so fewer rings are visible because many lattice planes are equivalent [38].

Analysis of EELS spectra helps detect both the presence of any additional phases and assess the structural crystallinity, which is essential for applications requiring consistent and reliable performance from the particles [55]. This technique is based on inelastic scattering phenomena, such as band transitions and phonon or plasmon excitations, and requires a more complex data processing and spectrum understanding. The area of interest for NiO comprises the O K-edge at (530–545 eV) and the Ni L-edges (853 eV L₃, 870 eV L₂), which form the so-called ‘core-loss spectrum’, where signals originate from excitations of inner-shell electrons to the conduction band; hence, the intensity of the signal can be directly related to the proportion between elements.

2.2.3. Scanning Electron Microscopy (SEM)

Similarly to TEM, SEM allows the study of NiO NP size, topography, and surface composition, trading resolution for cost-effective and simpler operating procedures [56]. SEM can complement TEM by better visualizing larger particles or agglomerated samples, where electron transparency is compromised, especially in metal oxides with particles larger than 30 nm. In addition to size and topographic information, SEM can provide compositional mappings using backscattered electrons. Many SEM systems include Energy-dispersive X-ray (EDX) add-ons, which analyze X-ray radiation emitted when sample electrons return to relaxed states. Depending on the synthesis method, a typical NiO spectrum contains O Kα and Ni Kα/Lα signals, along with traces of other elements like carbon. EDX can detect contamination and estimate the nickel-to-oxygen ratio. Area mappings can reveal compositional differences between the bulk and the surface of NPs, which is useful for identifying core–shell systems and detecting Ni/NiO coexistence.

2.2.4. X-ray Diffraction (XRD)

XRD plays a pivotal role in confirming the phase purity and crystallinity of NiO NPs [57]. XRD patterns provide a wealth of information, including the lattice parameters and symmetry of the crystal structure. This allows for the identification of the crystal structure [7,8,9]. Figure 4 shows the typical Bragg diffraction peaks observed in an XRD spectrum of these systems, which correspond to specific planes within the crystal lattice. This XRD spectrum typically exhibits peaks at 2θ values of 37.2°, 43.3°, 62.9°, 75.4°, and 79.4°. These correspond to the (111), (200), (220), (311), and (222) crystal planes of NiO, respectively. It is worth mentioning that the variation in intensities of these peaks could suggest a preferential growth along other crystal planes, while the presence of unexpected peaks could indicate potential impurities in the NiO NPs, such as metallic Ni or nickel (III) oxide (Ni₂O₃).

Accurate determination of these planes and their properties requires comparing the experimental XRD pattern with standard or computed patterns for NiO. The Scherrer–Debye Equation shown in Equation (6) is often used to estimate crystal size ($D)$ [58].

$$D={\displaystyle \raisebox{1ex}{$K\lambda $}\!\left/ \!\raisebox{-1ex}{$\beta \mathrm{c}\mathrm{o}\mathrm{s}\theta $}\right.}$$

(6)

where $K$ is a dimensionless shape factor, λ is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM), and θ is the Bragg angle. This equation helps estimate the size of crystallites in a material based on the broadening of XRD peaks, assuming that the broadening is due to small crystal size rather than strain or other factors.

Beyond structure identification, XRD offers valuable insights into the size of NiO NPs. By analyzing the broadening of diffraction peaks, techniques like Scherrer’s formula can be used to estimate the size of the crystallites. This information is crucial for understanding the catalytic and electronic properties of the NPs. Smaller particles generally possess a higher surface-area-to-volume ratio, which enhances their reactivity.

XRD can also detect strain within the crystal lattice, which can arise from distortions or defects. This strain can significantly impact the electronic properties and stability of the NPs. The technique’s ability to detect both microstrain and macrostrain provides a comprehensive picture of the internal stresses affecting the crystalline structure.

For a more detailed picture, XRD can be used in conjunction with other techniques like Rietveld refinement [59]. This process helps extract quantitative data on the crystal structure, such as the atomic positions and occupancy within the lattice. This information is essential for tailoring the properties of NiO NPs for specific applications. Rietveld refinement essentially achieves a more accurate fit between the observed and calculated diffraction patterns, leading to a deeper understanding of the structural parameters.

2.2.5. X-ray Photoelectron Spectroscopy (XPS)

XPS is a key analytical tool for evaluating the surface chemistry and electronic properties of NiO NPs [60]. It provides insights into the elemental composition, empirical formulas, and oxidation states by analyzing the kinetic energy and number of electrons emitted from the material’s surface after X-ray irradiation. In particular, XPS is crucial for understanding the electronic structure of Ni 2p core electrons in NiO NPs (see Figure 5) [61,62]. These Ni 2p orbitals, which are degenerated into two components, 2p_3/2 and 2p_1/2, due to spin-orbit coupling with relative intensities 2:1, are of great interest in XPS, as 2p orbitals of transition metals have high photoionization cross-section, yielding relatively high values of the electron ejection probability, and, hence, resulting in well-defined and intense peaks. Another advantage is that the binding energy of 2p orbitals is highly dependent on interactions with oxygen that can be attributed to the oxidation state of Ni. The main Ni 2p_3/2 peak, typically at about 854 eV, indicates nickel in the +2-oxidation state, characteristic of Ni(II) in the NiO lattice. It is often accompanied by a second 2p_3/2 component related to interactions with nonlocal oxygens, surface effects, and the presence of Ni³⁺. The Ni 2p_1/2 peak appears around 872 eV due to the spin-orbit splitting of the Ni 2p electrons. Satellite peaks, observed around 862 eV for Ni 2p_3/2 and approximately 879 eV for Ni 2p_1/2, arise from interactions between the photoelectron and unpaired electrons in the valence band, which are indicative of the material’s correlated electronic nature. The oxygen 1s peak in NiO, centered around 529.5 eV, signifies oxygen ions in the oxidized form and provides data on the oxygen environment. Shifts in this peak position can indicate variations in the electronic environment, potentially due to oxygen vacancies or interactions with other elements. Quantitative information is usually taken from consulting the signal ratio between Ni and O, since from the Ni 2p peaks analysis one can quantitatively differentiate Ni²⁺ from metallic nickel, yet it is not as straightforward with Ni³⁺. Note that in some cases the surface of the particles can present oxygen excess from the atmosphere, hence, it is recommended to perform Ar⁺ sputtering using the lowest possible ion beam energy to minimize reduction effects.

3. Chemical Approaches to Synthesized NiO NPs

A wide range of synthetic pathways has been developed to synthesize NiO NPs, allowing precise control over various aspects such as particle size, structural shape, crystallinity, and magnetic properties [7,8,9]. Two predominant approaches are commonly used for NiO NP preparation: the top-down and bottom-up methods. The former uses macroscopic initial structures, whose size is reduced in the nanoscale range by physical methods, such as, among others, laser ablation [63,64], mechanical milling [65], nanolithography [66], and sonochemical methods [67]. The major disadvantages of this approach are the high cost and notable difficulty in controlling the size of NPs, which specifically for NiO is a critical parameter as particles tend to agglomerate at larger sizes and adopt irregular pseudo-spherical shapes. On the other hand, bottom-up approaches focusing mainly on chemical and biological routes have become increasingly popular. This preference stems from several advantages, including reduced production costs, enhanced reaction yields, and superior control over the spatial characteristics of the resulting system. Nevertheless, a significant limitation is the need for annealing to convert the particles into their oxidized form. Given the predominant use of chemical routes in NiO NP synthesis for fundamental studies, this review is focused on these methods, detailing the main characteristics of the routes (see Table 2). Additionally, Figure 6 summarizes some typical morphologies synthesized through diverse chemical routes.

Figure 6. Distinct morphologies obtained from NiO NPs: (a) TEM image showing spherical NiO NPs with high crystallinity [68]. (b) HRTEM image illustrating the atomic arrangement and lattice fringes of NiO NPs [68]. (c) TEM image depicting rod-like NiO NPs [69]. (d) SEM image showing silk-flower NiO NPs [70]. (e) SEM image showing flower-like NiO nanostructures [71].

Figure 6. Distinct morphologies obtained from NiO NPs: (a) TEM image showing spherical NiO NPs with high crystallinity [68]. (b) HRTEM image illustrating the atomic arrangement and lattice fringes of NiO NPs [68]. (c) TEM image depicting rod-like NiO NPs [69]. (d) SEM image showing silk-flower NiO NPs [70]. (e) SEM image showing flower-like NiO nanostructures [71].

Table 2. Main chemical approaches for synthesizing NiO NPs detailing their pros and cons.

Synthesis Method	NP Range of Sizes (nm)	Morphologies	Pros	Cons
Co-precipitation [31,72,73]	~10–100 nm	spheres, cubes	Straightforward, low-cost, easy to scale up	Poor control over crystal quality and morphology compared with other methodologies
Hydrothermal [12,17,29,35,74]	~10 nm up to microns	spheres, rods, plates, flowers	Decent control over composition, relatively simple experimental setups	Lower crystallinity unless posterior annealing, need for specific instrumentation, lengthy reaction times, and difficulty in monitoring reaction
Sol-gel [32,33,34,39]	~10–100 nm	spheres, cubes, rods	Simple, cost-efficient process, high reaction yields, and scalability	Limited control over particle morphology, often involves a last-step calcination
Thermal decomposition [36,37,75,76,77,78,79]	~5–50 nm	spheres, cubes, hexagons, branches	Precise control over size, morphology, crystallinity, and composition	Requires many reagents, challenging to scale up and to reproduce
Microwave Assisted [25,28,80,81]	~5–50 nm	spheres, cubes	Rapid synthesis, high reaction yields, and scalability	High initial setup cost, limited size control

Table 2. Main chemical approaches for synthesizing NiO NPs detailing their pros and cons.

Synthesis Method	NP Range of Sizes (nm)	Morphologies	Pros	Cons
Co-precipitation [31,72,73]	~10–100 nm	spheres, cubes	Straightforward, low-cost, easy to scale up	Poor control over crystal quality and morphology compared with other methodologies
Hydrothermal [12,17,29,35,74]	~10 nm up to microns	spheres, rods, plates, flowers	Decent control over composition, relatively simple experimental setups	Lower crystallinity unless posterior annealing, need for specific instrumentation, lengthy reaction times, and difficulty in monitoring reaction
Sol-gel [32,33,34,39]	~10–100 nm	spheres, cubes, rods	Simple, cost-efficient process, high reaction yields, and scalability	Limited control over particle morphology, often involves a last-step calcination
Thermal decomposition [36,37,75,76,77,78,79]	~5–50 nm	spheres, cubes, hexagons, branches	Precise control over size, morphology, crystallinity, and composition	Requires many reagents, challenging to scale up and to reproduce
Microwave Assisted [25,28,80,81]	~5–50 nm	spheres, cubes	Rapid synthesis, high reaction yields, and scalability	High initial setup cost, limited size control

3.1. The Coprecipitation Method

In the realm of chemical bottom-up synthesis methodologies, coprecipitation stands out as one of the most utilized approaches for the obtention of NiO NPs. This technique is based on dissolving one or more soluble metal salts in aqueous media, followed by their precipitation induced by a basic pH [73]. The precipitated compounds, which typically form insoluble metal hydroxides or carbonates, require subsequent annealing to convert them into the NiO phase. Most common nickel salts employed include nickel(II) chloride hexahydrate (NiCl₂·6H₂O) or nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O) compounds. The precipitation is generally induced utilizing hydroxide or carbonate salts, such as sodium hydroxide (NaOH), ammonium hydroxide (NH₄OH), or hydrogen sodium carbonate (NaHCO₃).

The main strength of this route lies in its rapid and simple steps, as well as the low cost of the reagents and experimental setup required. These advantages make the coprecipitation method one of the most scalable chemical synthesis techniques available. However, the simplicity of the process is accompanied by low control over the crystal quality and morphology of the NPs. Consequently, a final annealing step is almost invariably required to improve the quality of the NiO NPs.

To illustrate the applicability of these systems, recent examples of NiO NPs are described in detail as follows. Rahal et al. presented a synthetic route to make 30 nm NIO NPs based on the capping of nickel(II) chloride with EDTA [72], followed by the dropwise addition of 4 M NaOH to adjust the pH to 12, inducing the precipitation of a green product. This product was then washed and air dried at 90 °C for 24 h. The dried powder was heated at 500 °C for 5 h to obtain black NiO NPs. The resulting particles were spherical, with sizes around 30 nm, displaying weak agglomeration but high polydispersity. Hysteresis loops at room temperature show typical trends of AFM NPs such as low $H_c, M_r,$and magnetization values probably due to the large crystal size of the particles.

The presence of metallic nickel is sometimes a significant issue that researchers must deal with. Factors such as type and amount of surfactant, annealing temperature and times are key aspects to control the Ni (II) reduction process. Sabouri et al. reported an interesting work focused on developing a green chemistry pathway for investigating the cytotoxicity of NiO NPs on cancerous cells [82]. The chosen nickel precursor and precipitation agent were Ni(NO₃)₂·6H₂O and NH₄OH, respectively, in a molar ratio of 1:4. The study’s main innovation was the addition of soluble starch to the mixture, which could stabilize the formation of small particles. Following a 1 h reaction at 60 °C and subsequent drying at 100 °C for 12 h, nickel hydroxide (Ni(OH)₂) particles were obtained. Calcination of these particles at 300–600 °C for 2 h yielded spherical NiO NPs with diameters below 40 nm. Magnetic measurements, specifically hysteresis loops, revealed a superimposed FM behavior with $M_s$ of approximately 15 emu/g, likely due to the presence of metallic nickel, as was corroborated by XRD spectra.

NiO NPs can also serve as templates for the addition of other metals to enhance or modify their magnetic properties. Gandhi et al. illustrated the magnetic properties and applications of NiO and Fe-doped NiO nanocomposites for magnetic memory effects [29]. In their study, Ni(NO₃)₂·6H₂O and Fe(NO₃)₃ were mixed in deionized water, followed by the addition of ammonium bicarbonate (NH₄HCO₃) to promote coprecipitation. The resulting light green precipitate was dried at 100 °C for 12 h and then annealed at 600 °C for 4 h in a muffle furnace. Magnetic studies revealed that Fe-doping enhances intraparticle interactions and gives rise to additional magnetic anisotropy barriers. For the undoped NiO NPs, they were found to be spherical with an average diameter of 63 nm, exhibiting complete AFM behavior, as expected for particles in this range of sizes.

3.2. Hydrothermal Synthesis of NiO NPs

The hydrothermal method is a chemical synthesis process involving two main stages. Initially, a metal hydroxide is formed by dissolving a selected metal precursor in water [83]. Subsequently, this hydroxide undergoes dehydration under medium to high pressures in autoclaves, leading to the formation of the NiO phase (see Figure 7). This method is particularly dependent on the solubility of the metal precursor in water, in addition to requiring high temperatures and pressures. A significant advantage of hydrothermal synthesis is its ability to produce crystalline phases that would be unstable under normal conditions. However, the method does not consistently produce materials with high crystallinity; some studies have reported the formation of amorphous precipitates. To address this, an annealing process is often employed post synthesis to enhance the crystallinity of the final product. This additional step involves heating the material at controlled temperatures to improve its structural properties.

Liu et al. reported the synthesis of two distinct flowerlike NiO structures combining a facial hydrothermal method coupled with subsequent annealing [71]. Their precursor of choice was Ni(NO₃)₂·6H₂O, which they dissolved in an aqueous solution in conjunction with mixtures of either ammonium fluoride (NH₄F) and urea to form ‘rose’ flowers or sodium acetate (NaAc) and ethylene glycol to obtain ‘silk’ flowers (see Figure 8). The hydrothermal reactions were conducted for 6 h in a Teflon-lined reactor with temperatures ranging from 120 °C to 180 °C based on the specific flower morphology. Finally, both structures were annealed at 350 °C for 2 h. The petal nanosheets had an average diameter of 0.7–1.0 μm for silk-flower architectures and 4–7 μm for rose-like, with a thickness of 11 nm and 25 nm, respectively. Magnetization was measured for both structures and found analogously to exhibit SPM loops with $M_s$ in the range of 8–10 emu/g. The authors attribute these values to synergistic effects of broken bonds and lattice distortion, which could be plausible as no metallic Ni traces were detected based on their obtained XRD patterns.

In addition to the traditional hydrothermal synthesis using an autoclave, more advanced systems can be employed. Cooper et al. conducted a study on core/shell magnetism in NiO NPs [35]. The hydrothermal synthesis was performed inside a confined jet mixer by reacting a supercritical water flow (450 °C, 24.1 MPa) with a second flow of 0.08 M Ni(NO₃)₂ and a third flow of 0.1 M KOH solution, both at 175 mL/min. The primary advantage of using this sophisticated experimental setup is the elimination of the final annealing step while maintaining precise control over system conditions. The resulting particles were spherical with a core/shell structure, featuring an AFM 5.2 nm core and a disordered 0.7 nm shell. The hysteresis loop exhibited a characteristic ‘S’ pattern, which did not reach saturation even at $H>60$ kOe probably due to the strong AFM-FM interactions in the core@shell NP structure.

In a more recent study, Zenkin et al. described the green hydrothermal synthesis of NiO and NiO@ZnO NPs for supercapacitor applications [74]. An amount of 7 mmol of Ni(NO₃)₂·6H₂O was dissolved in water and combined with 25 mL of S. officinalis plant extract, which is a natural reducing and capping agent used in traditional medicine. To regulate the pH, 0.1 M NaOH was added to the solution, and the final mixture was transferred to a 100 mL autoclave and left to react for 24 h at 180 °C. A final annealing at 500 °C for 2 h was then performed to obtain the NiO NPs. The achieved NiO NPs presented a spherical shape with an average size of 16 nm. Aggregation phenomena could though be observed as polydispersity was slightly above the expected (PI ≈ 25%). Hysteresis loops depicted a weak FM behavior for pure NiO NPs that was further reduced when integrated into a NiO@ZnO nanocomposite, foreseeable considering the diamagnetic ZnO contribution.

3.3. Thermal Decomposition

The synthesis of NiO NPs through this method involves the high-temperature decomposition of a nickel precursor with either organic solvents characterized by high boiling points or in solid media (see Figure 9) [4]. This multistep process requires higher temperatures, with reaction parameters playing a pivotal role in determining the outcome. By judiciously selecting the adjusting reaction time, controlling temperature, and choosing the appropriate organometallic precursor, researchers fine-tune NiO NP structural characteristics. Nickel oxalates, with their low melting points, are preferred precursors, yielding NPs with high surface area and small crystalline sizes. This method is often paired with TGA, granting a deeper understanding of the precursor’s decomposition profile to finely tune the reaction conditions.

Sharma et al. reported the synthesis of 6 nm NiO NPs via the thermal decomposition of nickel phosphine complexes in a high-boiling-point organic solvent [36]. The synthesis involved heating nickel phosphine complexes to 200 °C in the presence of trioctylphosphine oxide (TOPO) as a capping agent, resulting in the formation of NiO NPs with an average particle size of approximately 6 nm. These particles featured a crystalline core and a broad disordered surface shell, which accounted for about 75 ± 20% of the total particle diameter. Their findings indicated that magnetic behavior was primarily influenced by strong surface effects, especially at low temperatures.

Farhadi and Roostaei-Zaniyani demonstrated a low-temperature method for synthesizing NiO NPs, addressing the high energy consumption typically associated with such processes [75]. They prepared a Ni complex by combining Ni(NO₃)₂·6H₂O with excess ethylenediamine, precipitating the mixture with alcohol. The solidified crystals were then decomposed at temperatures between 200 and 400 °C for 60 min. The resulting spherical particles had an average size of 15.5 nm, although metallic nickel traces were found in samples annealed below 350 °C. Hysteresis loops of the NiO NPs prepared at 400 °C showed SPM behavior with a magnetization around 1 emu/g at 2 kOe.

Another interesting example was shown by Ibrahim et al. [76], who synthesized larger NiO NPs for fundamental studies using a Ni–Schiff base complex, followed by calcination. This Schiff base was prepared by condensing 2-hydroxy-1-naphthaldehyde (5 mmol, 0.86 g) with 2-aminopyridine (5 mmol, 0.47 g) in ethanol, refluxed for 1 h at 70 °C and purified. The Ni–Schiff complex was formed by reacting the Schiff base with a nickel nitrate precursor in ethanol at room temperature, followed by precipitation and heating to 500 °C in air for 1.5 h. This produced particles with an average diameter of 62 nm and noticeable polydispersity. Magnetic measurements indicated less significant SPM behavior compared with the 15.5 nm NPs, underscoring the size dependence of NiO magnetic properties. The hysteresis loop was measured up to 10 kOe, leaving the saturation behavior at higher fields undetermined.

An interesting advantage of thermal decomposition is its ability to be integrated as a final step in other synthetic pathways [84]. Gandhi and Lin studied $H_{eb}$ in Ni/NiO NPs using a two-step process involving initial precipitation followed by thermal decomposition. Ni(NO₃)₂·6H₂O was mixed with oxalic acid in deionized water, with the acid added dropwise to promote precipitate formation, which was then washed to remove reactants and byproducts [85]. The precursor was heated to 300 °C for 1 h under an Ar atmosphere to obtain the metallic Ni phase, followed by a second air annealing at 300 °C to form the NiO shell. The resulting core/shell particles had a Ni core of 30 nm and a NiO shell ranging from 8.6 to 13.6 nm. Magnetic measurements confirmed the exchange coupling in the Ni/NiO structures, showing consistent magnetization curves regardless of shell thickness.

3.4. Sol–Gel Synthesis Route

This process represents a bottom-up chemical synthesis approach, wherein the hydrolysis and subsequent condensation of a precursor material led to the formation of a colloidal solution, followed by its posterior gelation to obtain a gel (see Figure 10) [86]. To enhance the properties of the synthesized material, an annealing step is employed, facilitating particle size growth, byproduct removal, and the conversion of reaction intermediates into the desired final product. The incorporation of a surfactant is critical within this process, as it regulates the growth of NPs and prevents excessive agglomeration. This method’s versatility is underscored by its compatibility with both organic and inorganic nickel precursors, expanding its application range to a point where it is widely considered the most common method for NiO NPs synthesis. The sol–gel process is distinguished by its simplicity, involving straightforward setup requirements and the cost-effective nature of reactants and equipment. Despite this simplicity, it offers exceptional control over the particle size, morphology, and composition of the synthesized material. Thus, the ability to finely tune synthesis parameters without performing significant modifications to the synthetic route provides precise control over the final product unmatched by other alternatives.

Adiba et al. accomplished the synthesis of green NiO NPs by what is known as the citrate sol–gel method by mixing a solution of 0.01 mol of Ni(NO₃)₂·6H₂O with a 0.02 mol solution of citric acid [39]. The temperature was kept constant at 80 °C to perform the aging of the gel, following an annealing at 800 °C for 8 h. The obtained particles showed typical S-loops with small values of $M_s$ and $H_c$, probably due to surface effects.

More exhaustive research was conducted by Jafari et al., who not only studied the magnetic properties of NiO NPs but also the effect of the annealing temperature and ambient as well [34]. In their investigations, they selected a sol–gel approach, Ni(NO₃)₂·6H₂O being the precursor of choice and gelatin type B the surfactant, both acting as protection and gelation agent. The gel was formed by dilution of both components in an aqueous medium at 80 °C for 12 h. No dedicated aging step was performed, since the green gel was directly annealed inside a crucible. The authors controlled both the furnace atmosphere (air and O₂) and the annealing temperature (400–800 °C), no concise heating duration was provided in the article. The particles obtained presented a spherical shape with average diameters ranging from 8 to 76 nm, proportional to the increase in calcination temperature. The hysteresis loops followed similar trends to what is expected for finite-size and surface effects, as samples with $D>30$ nm present typical AFM behavior, while those under 15 nm show slight superparamagnetism. The latter is further highlighted in the case of NPs annealed under air atmosphere in comparison with O₂. A plausible explanation for these findings could be a minimal reduction of surface Ni²⁺ to metallic nickel when in the presence of atmospheric H_2.

3.5. Outlook on the Main Chemical Routes for Synthesizing NiO NPs

The synthesis approaches discussed above encompass the majority of the most common bottom-up chemical methods. They are briefly presented and described to provide a general overview of the advantages and drawbacks each method offers, which is crucial for selecting the most appropriate route based on specific needs.

For instance, coprecipitation is one of the most researched methodologies for the synthesis of NiO NPs and their integration into various applications, but it may not be the most suitable for fundamental studies. Similarly, hydrothermal synthesis is not ideally suited for magnetic studies because of the suboptimal crystallinity of the products. However, it is highly valuable for studies focused on obtaining exotic morphologies or phases.

Thermal decomposition and sol–gel methods are probably the most suitable for magnetic studies, especially the latter. The sol–gel method combines excellent control over the products by finely tuning the reaction parameters with cost-effectiveness. It can be seen as a more complex version of coprecipitation that trades simplicity for quality, with the gel acting as a template for particle growth, minimizing aggregation phenomena. Thermal decomposition, although not the simplest method, can be integrated as a final step for other synthetic techniques, making it quite versatile.

Other novel techniques, such as sono-chemical, microwave-assisted, pyrolysis, and nanocasting, are still in experimental stages. However, they have the potential to surpass existing mainstream methodologies shortly, as the synthesis of highly pure NiO particles remains a significant challenge. Another interesting methodology is the microemulsion method, which offers both advantages and drawbacks [87,88,89]. This technique involves creating a thermodynamically stable mixture of water droplets dispersed in an oil phase, stabilized by surfactants. Key steps include mixing an oil phase (e.g., cyclohexane), a surfactant (e.g., Triton X-100), and a cosurfactant (e.g., n-hexanol), followed by adding aqueous solutions containing the metal precursor and a precipitating agent. The resulting nanometer-sized droplets serve as reaction sites, allowing for precise control over particle size and morphology by adjusting the water-to-surfactant ratio and calcination temperature. While the microemulsion method provides excellent control over nanoparticle properties and requires simple apparatus, it also necessitates high annealing temperatures and is primarily limited to laboratory-scale synthesis, posing challenges for large-scale production. Despite these hurdles, researchers continue to explore its potential in nanotechnology and materials science.

The essential parameters that must be precisely regulated during the synthesis of NiO NPs to ensure uniform size and morphology depend on the synthesis method employed. In the coprecipitation method, the concentration of the reactants, the pH of the solution, the temperature at which the reaction is carried out, and the rate of addition of the precipitating agent are key parameters. Variations in these parameters can affect the nucleation and growth processes, resulting in differences in particle size, distribution, and morphology. Higher reactant concentrations and controlled pH can lead to more uniform and smaller NPs, while the temperature and rate of addition can influence the crystallinity and agglomeration of the particles.

In the hydrothermal method, temperature, pressure, type of solvent, and reaction time are critical factors. Higher temperatures and pressures generally produce better crystallinity and larger particles, while the choice of solvent can significantly influence the shape and size distribution of the NPs.

In the thermal decomposition method, the key parameters include temperature, heating rate, and the choice of precursor. Higher temperatures and controlled heating rates usually yield uniform particle sizes and higher purity. The type of precursor can determine the phase and morphology of the resulting NiO NPs, and faster heating rates often lead to smaller particles due to rapid nucleation.

For the sol–gel method, the concentration of precursors, pH of the sol, aging time, and drying conditions are crucial. Adjusting these parameters can lead to differences in particle size, morphology, and degree of agglomeration. Higher precursor concentrations or longer aging times typically result in larger particles and more defined crystalline structures.

In the microemulsion method, the water-to-surfactant ratio, type of surfactant, and reaction temperature are essential parameters. These factors control the size and shape of the NPs by influencing the size of the microemulsion droplets. Adjusting the water-to-surfactant ratio can fine-tune particle size, while different surfactants can stabilize droplets of various sizes and shapes.

Variations in these parameters influence the final product by altering nucleation and growth rates, particle size distribution, and surface characteristics, which in turn affect the physical and chemical properties of the NiO NPs. Postsynthesis processing techniques can markedly enhance the characteristics of NiO NPs. Annealing, a common technique across different synthesis methods, improves crystallinity, reduces defects, and enhances magnetic properties. Adjusting the annealing temperature and atmosphere (e.g., air, nitrogen, or reducing gases) allows one to tailor the NPs’ characteristics. Surface functionalization, which involves modifying the NP surface with organic molecules, polymers, or other inorganic materials, enhances stability, dispersibility, and compatibility with other materials. This technique is particularly useful for biomedical applications where biocompatibility and functionalization with targeting molecules are crucial.

4. The Impact of Structural and Compositional Variations on the Magnetic Behavior of NiO NPs

The fascinating magnetic properties of NiO NPs have garnered significant attention due to their potential applications in various fields, such as spintronics, catalysis, and biomedicine [7,8,9]. Unlike their bulk counterparts, NiO NPs exhibit unique magnetic behavior primarily influenced by their reduced structure and tunned composition. In this section, the key aspects governing the magnetic properties of these systems are reviewed, including finite-size and surface effects, NP morphology, crystal quality, and the impact of the parasitic Ni phase.

4.1. Finite-Size and Surface Effects

The magnetic properties of NiO NPs differ significantly from bulk materials due to their increased surface-to-volume ratio, the presence of uncompensated spins either on the surface or throughout the particle volume, and the effect of multiple sublattices at reduced sizes (see Figure 11) [1,2,3,24,26,32,42,90,91]. NiO NPs exhibit unique magnetic behaviors because of their smaller size and larger proportion of surface atoms. For instance, a 10 nm spherical NiO NP contains approximately 20% of its atoms on the surface, while smaller NPs, such as 5 nm spheres, have around 50% [4,80]. In these systems, the core typically exhibits AFM behavior, where the magnetic moments of nickel ions align AFM, resulting in a net zero magnetic moment. However, the surface of these systems can form an FM shell due to the presence of uncompensated spins (see Figure 11). This interaction between the FM shell and AFM core significantly alters the magnetic properties. Additionally, configurations of missing spins in NiO NPs can include randomly missing spins within the particle, on the surface, or due to an odd number of layers within a cubic structure. These phenomena lead to unique behaviors, such as reduced $T_B$ and $T_N$, larger magnetic moments, and increased coercivities [32,35,42,90,92,93].

In their seminal work, Kodama et al. delved into the finite-size effects exhibited by NiO NPs [31], shedding light on their anomalous magnetic properties. By investigating particles with sizes ranging from 6 to 80 nm, synthesized through chemical precipitation followed by annealing, the study revealed intriguing phenomena. The magnetic moments observed in these NiO NPs exceeded the predictions of a conventional 2-sublattice AFM model. Instead, a novel finite-size effect emerged: the reduced coordination of surface spins led to changes in the magnetic order throughout the particle. Remarkably, NiO NPs exhibited 8-, 6-, or 4-sublattice spin configurations, deviating from the expected 2-sublattice model. These multi-sub-lattice arrangements resulted in high net moments and diverse spin reversal paths, contributing to large coercivities and hysteresis loop shifts of up to 10 kOe. As particle size decreased, the impact of uncompensated surface spins became more pronounced. Numerical modeling corroborated the notion that this behavior stems from reduced coordination among surface spins. Interestingly, the average order parameter approached unity as particle size increased, while the number of sublattices decreased from 8 to 2 in larger particles.

Surface spins played a crucial role, particularly at lower temperatures, where stronger interactions contributed to the observed magnetic behavior [36,42,72,90,92,93,94]. For example, Sharma et al. reported a magnetic surface contribution in 6 nm NiO NPs synthesized by a thermal decomposition route [36], finding that the interaction between the FM shell and the AFM core enhanced the overall particle magnetization. The synthesized NiO NPs exhibited a crystalline core and a broad disordered surface shell, accounting for about 75 ± 20% of the total particle diameter. As a result, magnetic behavior was dominated by significant surface effects, particularly at low temperatures. Key magnetic properties included shifted hysteresis loops, enhanced coercivity, high field irreversibility, and a vertical shift in the magnetization curves after FC.

Another interesting example reported by Rinaldi et al. investigated the impact of the particle surface in ultra-small NiO NPs with sizes below 10 nm synthesized via the pyrolysis of Ni(NO₃)₂ [90,94]. Their findings revealed that 2 nm NiO NPs exhibit a completely disordered magnetic state, while particles larger than 4 nm exhibit a core/shell structure, featuring an uncompensated AFM NiO core and a spin glass (SG) shell, resulting in a net magnetic moment (see Figure 12). This core(AFM)/shell(SG) morphology induced $H_{eb }$ due to the exchange coupling between the SG shell spins and the uncompensated AFM core moments.

Particle size significantly impacts the magnetic properties, particularly $M_{ZFC}-M_{FC}$ curves and hysteresis loops in these systems. The former measurements reveal an enhancement of $T_B$ with the particle volume. Additionally, the peaks observed in the $M_{ZFC}$ curves at low temperatures suggest spin-glass behavior probably due to surface effects. On the other hand, hysteresis loops exhibit two key trends. Smaller NiO NPs show significantly higher magnetization due to uncompensated surface spins, whereas larger particles display behaviors more characteristic of AFM materials. The hysteresis loops for smaller particles reveal large $M_s$ at low temperatures, suggesting an FM-like behavior driven by surface effects. With rising temperatures, the coercivity diminishes, and the loops narrow, signaling a reduced impact of surface effects. These observations underscore the critical role of surface spin disorder and the effect of particle size reduction in enhancing the magnetic properties at lower temperatures in smaller NiO NPs. As the NiO NP size approaches the magnetic correlation length, surface spin disorder significantly influences the overall magnetic behavior. This disorder arises from the reduced coordination of surface atoms, bond relaxation, and increased static disorder, all of which weaken the exchange interactions necessary for maintaining AFM order. As a result, $T_N$ exhibits an intrinsic size dependence, progressively decreasing with smaller NP diameters [33,35,68]. One significant study that sheds light on this phenomenon was conducted by Rinaldi et al. [68], who investigated the size-dependent reduction of $T_N$ in small NiO NPs with average diameters ranging from 4 to 9 nm. Using neutron diffraction and X-ray absorption fine structure (EXAFS) measurements, they found that $T_N$ decreases with reducing NP size. Specifically, $T_N$ values decreased to 461 K for 4 nm particles, 497 K for 6 nm particles, 505 K for 7 nm particles, and 513 K for 9 nm particles. The observed variation was attributed to significant surface effects, including average undercoordination, bond relaxation, and static disorder, which collectively weaken the exchange interactions required for maintaining AFM order.

4.2. Impact of Structure and Coating on the Magnetic Behavior of NiO NPs

The magnetic properties of NiO NPs are significantly influenced by their morphology, as it directly affects surface area, surface energy, and the distribution of surface and bulk atoms—all of which play a role in determining magnetic behavior. Specific morphologies can either enhance or diminish magnetic interactions, resulting in diverse magnetic behaviors. Various shapes and structures, including nanorods [69], cuboctahedral [92], nanosheets [17], and branchlike forms [95] can lead to substantial variations in magnetic behavior. For example, Wesselinowa investigated the impact of particle size and anisotropy on the magnetic properties of cuboctahedral morphologies using a theoretical approach based on the Heisenberg model [92]. The study revealed that, as particle size decreases, magnetization increases due to enhanced surface spin contributions. $H_c$ exhibited notable dependence on particle size and surface anisotropy, indicating that the shape and size of NPs play crucial roles in determining their magnetic properties. Additionally, $T_N$ was found to decrease with decreasing particle size, linked to surface effects and reduced symmetry at the nanoscale.

Zhao, Yang, and Hua reported the synthesis and magnetic properties of 100 nm β-Ni(OH)₂ and NiO nanosheets [17]. The β-Ni(OH)₂ nanosheets, synthesized via a hydrothermal method, displayed FM behavior at low temperature around 5 K and paramagnetic behavior at 300 K. Upon thermal decomposition of β-Ni(OH)₂ at 450 °C, NiO nanosheets were formed, exhibiting $T_B$ greater than room temperature and nonzero values of $H_c$ even at 300 K.

Liu, Li, and Yang reported size-dependent magnetic properties of branchlike NiO NPs synthesized via the thermal decomposition of nickel(II) acetate in the presence of organic surfactants [95]. The resulting NiO nanocrystals ranged from 5 nm to 13 nm in size, demonstrating that both size and morphology significantly influence their magnetic properties. These branchlike structures exhibited higher $M_r$ and $H_c$ compared with spherical counterparts, attributed to their larger surface-to-volume ratio and an increased number of uncompensated surface spins. This unique two-dimensional nanosheet structure also affected magnetization relaxation processes, highlighting the critical role of the NP shape in determining magnetic properties. Additionally, they found that particles smaller than 13 nm exhibited weak FM behavior at room temperature due to uncompensated surface spins. Both average $T_B$ and irreversibility temperature increased with particle size, while $M_r$ and $H_c$ at 300 K decreased with increasing particle size, emphasizing the significant role of surface effects in smaller particles.

The particle coating plays a crucial role in modulating the magnetic properties of NiO NPs by mitigating interparticle interactions and stabilizing surface spins, influencing $T_B$ accordingly [65,93]. For example, Shim et al. investigated the size dependence of $T_B$ and electron magnetic resonance (EMR) spectra in NiO NPs with sizes ranging from 5 to 20 nm [93]. In the study, both uncoated and oleic acid-coated (OA) NiO NPs were synthesized using annealing methods. The research revealed that uncoated NiO NPs exhibited higher $T_B$ compared with OA-coated NPs, suggesting significant interparticle interactions in the uncoated samples. Key findings included the linear variation in $T_B$ with NP volume for OA-coated NPs, indicating negligible interparticle interactions. In contrast, uncoated NPs showed increased $T_B$ with decreasing size for NPs smaller than 8 nm, attributed to enhanced interparticle interactions. Figure 13 depicts the M_ZFC-M_FC curves of both uncoated and coated 7 nm NiO NPs, revealing a shift to lower $T_B$ from 170 to 55 K for uncoated and OA-coated NiO NPs, respectively. Besides,$K_{eff}$ increased with decreasing particle size, reflecting enhanced surface anisotropy contributions in smaller NPs. Finally, $H_{c }$and $H_{eb }$ measurements revealed that smaller particles exhibited higher $H_c$ probably due to greater surface anisotropy and uncompensated surface spins, with significant $H_{eb }$ values suggesting strong interactions between the FM-like surface spins and the AFM core.

4.3. Influence of Oxygen Stoichiometry and Metallic Nickel Presence on the Magnetic Properties of NiO NPs

The stoichiometry of NiO NPs significantly influences its magnetic and electronic properties. Dubey et al. investigated this effect on the magnetic and electronic properties of NiO NPs prepared through the thermal decomposition of Ni(NO₃)₂·6H₂O at various sintering temperatures ranging from 400 °C to 1100 °C [37]. The study revealed how varying levels of excess oxygen impact these properties. For example, the magnetic susceptibility and specific heat capacity measurements indicated a shift in $T_N$ with changes in stoichiometry. Besides, NiO samples sintered at 400 °C exhibited higher levels of excess oxygen and a corresponding lower $T_N$. In contrast, samples sintered at the highest temperatures of 1100 °C, had lower excess oxygen levels and higher $T_N$. Additionally, XPS revealed a higher concentration of Ni³⁺ ions in samples sintered at lower temperatures, indicating a higher degree of nonstoichiometry. This transition from Ni²⁺ to Ni³⁺ with increasing sintering temperature affects the magnetic properties due to modifications in the Ni-O bond length and Ni-O-Ni bond angle.

The presence and distribution of Ni within the NiO structure significantly influence the magnetic properties of NiO NPs. For example, Chaghouri et al. investigated the magnetic properties of Ni/NiO nanocrystals [96], focusing on the effects of exchange bias. In the study, Ni/NiO nanocrystals were synthesized by annealing Ni NPs coated with a thin amorphous NiO layer, resulting in nanocrystals with diameters ranging from 5.0 to 16.8 nm. Magnetic measurements showed that the smaller particles exhibited robust room temperature FM with strong exchange bias. M_ZFC-M_FC curves indicated a clear transition to FM order at 300 K, with $M_s=$ 15.8 emu/g and $H_c=$ 228 Oe. The occurrence of $H_{eb }$ was confirmed by a hysteresis loop shift of 62 Oe at room temperature, which increased significantly at 2 K with higher cooling fields. The annealing process, which induced crystallization of the NiO shell and increased oxidation of the Ni core, preserved and enhanced exchange bias features. $H_{eb }$ increased with particle size, from 34 Oe for 5.0 nm particles to 194 Oe for 16.8 nm particles, and $H_c$ values were higher than those previously reported for Ni/NiO systems.

Another interesting example was reported by Rostamnejadi et al. [10], who investigated the magnetic properties of Ni/NiO NPs, synthesized by the sol–gel method. The study found that Ni/NiO nanocomposites exhibit FM behavior at room temperature, which is unusual for AFM NiO. The observed FM moment was primarily attributed to uncompensated surface spins and FM clusters within the AFM NiO matrix. Magnetic hysteresis loops revealed that $M_s$ of the samples sintered at 450 °C (34 nm) and 550 °C (40 nm) were 3.5 emu/g and 0.2 emu/g, respectively. Interestingly, $H_c$ values were low for both samples, with 94 Oe for S1 and 95 Oe for S2, indicating the presence of weak FM phases.

4.4. Outlook on the Impact of Structural and Compositional Variations on the Magnetic Behavior of NiO NPs

To recapitulate, NiO NPs exhibit distinctive magnetic behaviors influenced by their structural and compositional characteristics. Key factors such as finite-size effects, surface effects, morphology, crystal quality, the presence of different phases of Ni, and impurities play crucial roles in determining the magnetic properties of these systems.

Finite-size effects in NiO NPs arise from the reduced dimensions of the particles, leading to unique magnetic properties compared with bulk materials. These effects are due to uncompensated spins within the particle volume, the presence of odd sublattices, and the formation of multiple sublattices that yield nonzero magnetization. Smaller particles, for instance, display larger magnetic moments and enhanced $H_c$ because of these finite-size effects. As particle size decreases, the influence of these internal spins becomes more prominent, resulting in significant deviations from the typical AFM behavior of bulk NiO.

Surface effects, on the other hand, are primarily due to the high surface-to-volume ratio in NiO NPs, where numerous uncompensated spins on the surface lead to a net magnetic moment. Surface defects, such as vacancies or interstitials, can create uncompensated spins, leading to FM behavior in otherwise AFM materials. The high proportion of surface atoms in NiO NPs results in FM order due to these spins. This FM shell interacts with the AFM core, changing the overall magnetic properties, especially in smaller particles, leading to increased $H_{c }$ and larger magnetic moments. The core–shell structures that form in smaller NPs exhibit AFM behavior in the core and FM signatures in the surface shell.

The magnetic properties are also highly dependent on the morphology of the NPs. Different shapes, such as nanorods, cuboctahedral, nanosheets, and branch-like structures, exhibit varying magnetic behaviors due to differences in surface area and anisotropy. Specific morphologies can enhance surface spin contributions, leading to increased magnetization and $H_c$. For instance, branch-like NiO structures demonstrate higher $M_r$ and $H_c$ compared with spherical counterparts due to their larger surface-to-volume ratios and uncompensated surface spins.

Coating the NPs, such as with oleic acid, can significantly modulate their magnetic properties by reducing interparticle interactions and stabilizing surface spins. Coated NPs generally exhibit lower $T_B$ compared with uncoated ones, indicating that coatings can effectively diminish magnetic interactions between particles. Additionally, coatings and functionalization of the NP surface can stabilize defects and further modify the magnetic properties.

The stoichiometry of NiO NPs, including the presence of excess oxygen and variations in the nickel oxidation state, plays a crucial role in their magnetic behavior. Changes in stoichiometry can alter the $T_N$ and the overall magnetic order, as seen in NPs sintered at different temperatures. The presence of metallic Ni within the NiO matrix can also induce FM behavior, further complicating the magnetic landscape of these systems.

Impurities also affect magnetic properties by introducing foreign atoms or ions that change magnetic interactions within the NiO structure. Introducing FM impurities like Ni or Fe can enhance the FM character of the particles, increasing both $M_s$ and $H_{eb}$. This can be particularly interesting in applications such as magnetic storage devices and spintronic components, where improved magnetic properties are crucial.

These defects and impurities can be introduced through various synthesis methods and postsynthesis treatments. By controlling synthesis parameters like temperature, precursor concentration, and annealing conditions, it is possible to adjust the degree and type of defects and impurities. For instance, annealing NiO NPs at different temperatures can vary oxygen vacancies, affecting the magnetic properties. Lower annealing temperatures generally result in higher defect levels, leading to increased FM behavior due to more uncompensated spins.

5. Conclusions and Perspectives

Research on NiO NPs is a promising frontier in material science, with significant implications for various advanced applications. Their unique magnetic properties, distinct from bulk materials, offer numerous opportunities and challenges for future research and technological innovations. Understanding surface effects is crucial, as uncompensated surface spins significantly influence the magnetic properties of NiO NPs. Future research should focus on elucidating the mechanisms governing surface spin interactions and their impact on overall magnetic behavior. Advanced characterization techniques, such as spin-polarized scanning tunneling microscopy, X-ray absorption spectroscopy, X-ray magnetic linear dichroism, and photoemission electron microscopy could provide detailed insights into these phenomena.

Tuning magnetic properties through size and morphology offers a pathway to tailor NiO NPs for specific applications. Systematic studies on anisotropic morphologies, such as nanostars, nanorods, and nanoflowers, could lead to NPs optimized for targeted uses. Additionally, exploring the impact of stoichiometry and dopants on the magnetic properties of NiO NPs could identify optimal doping strategies that enhance performance while maintaining stability.

The exchange bias effect observed in Ni/NiO core/shell structures opens new possibilities for spintronic devices. Investigating the stability and tunability of this effect at room temperature and under varying environmental conditions could pave the way for robust and efficient spintronic components. Controlling the core/shell ratio and interface quality will be crucial.

Another promising area is bio-inspired green synthesis, which utilizes natural materials and processes to create NiO NPs in an environmentally friendly manner. For instance, the use of plant extracts as reducing and capping agents can simplify the synthesis process and make it more sustainable [9,74,80,82]. Such green synthesis methods have already shown potential in producing NiO NPs with desirable properties for various applications.

In biomedicine, NiO NPs show significant promise due to their ability to generate reactive oxygen species, making them effective as antimicrobial and anticancer agents. Their high surface area and reactivity enable them to target and destroy pathogens and cancer cells, paving the way for innovative treatments and drug delivery systems [13,44,91].

Furthermore, the combination of NiO NPs with other magnetic materials can lead to the development of advanced FM and AFM composites. These composites are particularly valuable in spintronic devices, where the unique magnetic properties of NiO NPs can be utilized to enhance performance and functionality. Studies have demonstrated the potential of integrating NiO with ZnO, which affects magnetic behavior and opens new avenues for creating customized magnetic materials.

Overall, finely tuning the magnetic properties of NiO NPs through careful control of their size, shape, composition, and surface characteristics is key to unlocking their full potential in applications such as spintronics, catalysis, and biomedicine.