The sol-gel procedure is the most important wet-chemical technique for fabricating ZnO nanoparticles. The procedure is simple, inexpensive, and produces materials of high homogeneity and purity. Some disadvantages include a long processing time, contraction during processing (may cause cracks), and residual (hydroxyl and/or carbon) groups, which is important because the morphology of the catalytic surface significantly affects the photosensitivity of sol-gel-derived ZnO thin films [67,68]. Therefore, research has been ongoing to fine-tune the microstructure (size and morphology) [69], homogeneity [70], and optical response efficiency of ZnO semiconductors through these solution-based syntheses protocols. A good example is the reported link between the shape and size of the ZnO nanoparticles and the concentration of surface hydroxide (OH^-) ions [71,72]. Hence, semiconductor shape controllers and capping agents were routinely added to the recipe [73,74,75]. However, it must be noted that a high photocatalytic performance has also been reported in the sol-gel synthesis of ZnO without a capping agent [69]. Similarly, high-temperature treatments are beneficial for the attainment of high crystallinity (minimum bulk defects), thereby favouring photocatalytic performance.

In the sol-gel method, metal alkoxide precursors undergo hydrolysis to form a colloidal suspension (sol). When the solvent is removed, a very porous and lightweight material called aerogel is produced (Equations (1)–(4)).

Kalisamy et al. [76] have described the fabrication of a ZnO-embedded S-doped graphitic carbon nitride (g-C₃N₄) heterojunction nanocarbon material with a good reduction of charge carrier recombination and high photocatalytic breakdown of dyes that was achieved via a mediator-free Z-scheme mechanism. Only 50 mg of the material was required to completely remove the 10 ppm initial concentration of dye pollutants [76]. Shemeena et al. [77] prepared similar ZnO/g-C₃N₄ material utilising the sol-gel technique. Here, the g-C₃N₄ was prepared using melamine, and the ZnO/g-C₃N₄ material was prepared by adding a few grams of the g-C₃N₄ to a ZnO sol and then the suspension was stirred, dried, and treated at 300 °C. This synthetic strategy resulted in a reduced band gap, offsetting the drawbacks of pristine ZnO. Thus, the ZnO/g-C₃N₄ catalyst achieved 100% degradation of Congo red (10 ppm) in 120 min.

In addition, Wu et al. [78] described the sol-gel preparation of a ternary ZnO/Fe₃O₄/g-C₃N₄ composite material that photodegraded three different dyes. In preparing the composite, a few grams of ZnO/Fe₂O₃ and melamine with a mass ratio of 1:1 were pulverised and then dispersed in deionised water under sonication. The dried materials were annealed at 550 °C. The ternary magnetic composite material was recyclable and the photocatalytic performance was attributed to a redshift in its absorption edge. Similarly, a heterojunction of AgIO₄/ZnO with efficiently separated photoinduced charges was prepared following the sol-gel approach. The materials were obtained by mixing known volumes of a AgNO₃ solution with a prepared ZnO suspension. Then, a known amount of a NaIO₃ solution was added in drops with strong stirring and the suspension was filtered and dried. The photoreactivity of the AgIO₄/ZnO in the removal of the dyes was three times better than the pristine ZnO with 100 mg of the catalyst removing 98% of indigo carmine and 81% of rhodamine B dyes [79].

In addition, the overall photocatalytic performance of the ZnO semiconductor was improved when some carbon-based material, such as reduced graphene oxide (rGO) [80], carbon nanotube [81], or graphene oxide (GO), was utilised [77]. In this regard, Zhu et al. [56] prepared an Fe-Cu-ZnO/GO composite that exhibited good adsorption and photodegradation of dyes. The authors used the sol-gel technique to combine Fe and Cu with ZnO in a heterostructured material in which C₁₈H₂₉NaO₃S was used as a shaping agent to control crystal growth. Graphene oxide was used to produce the gel that was dried and treated at 400 °C to produce the Fe-Cu-ZnO/GO composite. The composite removed 99% of dark green dyes from contaminated water in 90 min.

The method involves reacting precursor components in an appropriate solvent with a dopant added before precipitation occurs. Chemical methods involve precipitation from inorganic or organic solutions with an easy control of the nucleation, growth, and ageing of the solution. Its advantages include the easy control of particle size, composition, simplicity, and low-temperature processing, which may not involve an organic solvent. Some disadvantages of co-precipitation include:

• Impurities that may precipitate with the product;
• Time-consuming processes;
• Not applicable to uncharged species;
• Lack of batch-to-batch reproducibility;
• The difficulty encountered with handling components that precipitate at different rates.

Due to these shortcomings, the chemical precipitation method has been combined with other methods. Oppong et al. [75] have used the co-precipitation technique to prepare Vis active La-doped ZnO/GO nanostructured materials. In this synthetic approach, the La-doped ZnO was decorated on graphene oxide sheets to produce an advanced photocatalyst that overcame all the inherent deficiencies of unmodified ZnO. The material owes its efficient photocatalytic performance to doping with the lanthanum up to an optimally determined concentration, as higher amounts increased the recombination rate of the photoinduced charge carriers. Most importantly, the graphene oxide acted as an adsorbent for the eosin yellow dye, an enhancer of electron transfer, and an extender of visible light absorption. The study identified ^•OH and O₂^−• as the radicals responsible for the photodegradation of the dye. TOC analysis further confirmed the complete mineralisation of the dye. The degradation was 3.2 and 4.5 times slower in the presence of 2-propanol and benzoquinone (as radical scavengers), respectively. Within 210 min, the catalyst removed 100% of 20 ppm of the dye in water [75].

Praveen et al. [82] used the sol-gel method to synthesise ZnO nanoparticles decorated with Ag nanoparticles to remove toxic methylene blue and Cr(VI) from aqueous solutions. The ZnO-Ag composites were prepared by the addition of a few grams of ZnO to an N¹[3-(trimethoxysilyl)propyl]diethylenetriamine (TPDT) solution containing AgNO₃, with the resultant solution then reduced and stirred strongly at room temperature. All the materials with varying concentrations of the TPDT/ZnO-Ag showed a redshifted Vis absorption edge, implying that the interaction of the TPDT matrix and Ag nanoparticles with ZnO shifted the band gap of the ZnO into the visible light region. Hence, 20 mg of the catalyst completely removed 1.5 × 10⁻⁵ M aqueous solution of the dye pollutant in 60 min. In another work aimed at circumventing some of the problems inherent in the synthetic methods presented thus far, metal complexes were used to produce innovative hybrid materials via controlled morphology growth and band-gap expansion. There was no need for a stabiliser to direct growth. Hence, Sarmah et al. produced a zinc-sodium acetate complex (C₃₂H₄₈O₃₄Na₈Zn₄) from a combination of zinc (ZnC₄H₆O₄) and sodium (C₂H₃NaO₂) acetates in a 1:2 molar ratio in methanol at ambient temperatures. Hydrolysis reactions with hydrazine hydrate in methanol under hydrothermal conditions at 150 °C were done to produce metal hybrid materials (S-1, S-2, and S-3) as hybrids with Cu, Ag, and Au, respectively [83]. In addition to FT-IR spectral analysis, the AAS data confirmed the success of the metal exchange reactions, and the Zn^II, Cu^0, and Ag⁰ were confirmed in samples of S-1 and S-2 from X-ray photoelectron spectroscopic (XPS) analysis. Under Vis irradiation, 20 mg of the prepared photocatalysts completely degraded 100 ppm concentration of various organic pollutants through H₂O₂ generation from H₂O and O₂.

Researchers occasionally incorporate other techniques into chemical precipitation. Zarrabi et al. [84] prepared ZnO/GO by precipitation using sonication. When ultrasound is applied to a solution, it causes simultaneous compression and expansion, creating cavities in the liquid [85]. However, the calcination process (>400 °C) removes the cavities, affecting particle dispersion, homogeneity, and morphology. The group indicated that using ultrasound in the synthetic process improved the performance of the photocatalyst. Improvements include a higher surface area, larger pore volume, narrow and uniform particle size distribution, and better Vis sensitivity; all of which enhanced catalyst performance and the successful removal of dye pollutants by degradation [84].

A mixed metal oxide (ZnO/MgO, 0.5 M/0.5 M) nanomaterial prepared by co-precipitation achieved good photocatalysis under Vis irradiation with good antibacterial properties for the degradation of wastewater from textile dyeing. A hierarchical CuO/ZnO p-n heterojunction nanomaterial (p-CuO and n-ZnO) synthesised by Bharathi et al. [86] facilitated the efficient transfer of electrons from one semiconductor to the other despite the lattice mismatch and small chemical interaction. The study reported that the CuO/ZnO materials degraded methylene blue under Vis irradiation. Similarly, Santos et al. [87] used a combination of chemical and co-precipitation techniques to prepare heterostructured Ag/ZnO nanomaterials for the degradation of methylene blue and reactive black 5 textile dyes.

The hydrothermal procedure has become the method of choice in recent times. As a reaction procedure with a high control of the temperature and pressure conditions in an autoclave (pressurised steel vessel lined with/without Teflon), it allows for the careful control of the crystallite size and form. In addition, the nature of the thermal treatment and precursor solution used determines the strength of the interfacial adsorption, surface area, and morphology of the material [91,92].

Zhang et al. [93] fabricated a graphitic C₃N₄-ZnO@graphene aerogel (g-C₃N₄-ZnO@GA) nanomaterial by adding sonication to the hydrothermal method. Hence, a few grams of graphene oxide and zinc nitrate in deionised water was sonicated, and then a known amount of g-C₃N₄ was added to the suspension before hydrothermal treatment and final freeze-drying. The high catalytic performance was ascribed to its ability to suppress the recombination of photoinduced e⁻/h⁺ pairs due to their efficient transfer across the heterostructure. An amount of 5 mg of the hybrid catalyst with a high adsorption capacity and superior photocatalytic activity removed up to 83% of 20 ppm of organic pollutants in 150 min [93].

Similarly, Zhang et al. [94] successfully prepared a ‘pompon-like’ structured g-C₃N₄/ZnO composite photocatalyst using a two-step method. SEM analysis revealed that the heterojunction formed by the g-C₃N₄ nanosheets was wrapped (not 100%) around the spherical surface of ZnO. The prepared catalyst g-C₃N₄/ZnO successfully degraded rhodamine B dye.

The chemical composition of a material often determines its electronic properties. The coordination structure of ZnO may be altered by replacing (doping) some metallic Zn atoms with other atoms of choice in the structure. This changes its optical behaviour to improve its photocatalytic response and integrity. Gaurav et al. [95] prepared the Cu ion-doped ZnO material via the combination of zinc nitrate and copper acetate solutions in deionised water, with the pH adjusted before hydrothermal treatment. The study reported that the shift in the optical band gap energy (as low as 3.11 eV) to longer wavelengths is directly related to the concentration of Cu ions in the lattice of the Cu:Zn material.

Venugopal et al. [96] reported the fabrication of the ZnO-Ag₈S hybrid material using a three-step method that included spin coating, a hydrothermal method, and successive ionic layer adsorption reaction (SILAR). SILAR is a simple and cheap thin-film deposition method for the fabrication of composite materials. In this method, the substrate was spin-coated, subjected to hydrothermal treatment and, finally, SILAR. The substrate was then washed with water to remove any species that are loosely bound to the surface. The thin film efficiently degraded rhodamine B dye. The high efficiency resulted from the dual beneficial effects of the photocatalyst and the electric potential between the two semiconductors [96]. In addition, Karunakaran et al. [97] prepared nanodiscs with cubic ZnFe₂O₄ as the core and face-centred-cubic Ag-deposited hexagonal ZnO as the shell. The ZnFe₂O₄/Ag–ZnO composite exhibited surface plasmon resonance in the Vis region due to the silver ions. The nanodiscs were superparamagnetic, with the surface area, pore volume, and pore radius reported as 26.83 m² g⁻¹, 0.082 cm³ g⁻¹, and 4.88 nm, respectively. The combined solution of ZnFeO₄ and polyethylene glycol was sonicated, then zinc nitrate and silver nitrate solutions were added, and the pH was adjusted before microwave treatment. The catalyst exhibited excellent photocatalytic performance [97]. In another study, carbon-doped zinc oxide (C-ZnO) was found to alter the photoresponse of ZnO to the Vis region by altering its band structure. It also improved the photocatalytic performance of the catalyst by reducing the recombination speed of photogenerated charges and photocorrosion. BiVO₄ is a catalyst operating in the visible region with a small band gap energy of 2.4 eV, which suffers from a high recombination speed of e⁻/h⁺ charges. A composite C-ZnO/BiVO₄ was prepared to improve the catalytic properties for the efficient degradation of methylene blue under Vis irradiation. The ZnO and C-ZnO were prepared from zinc acetate, sucrose (carbon source), and CTAB in deionised water and subjected to hydrothermal treatment, respectively. Then, the obtained precipitate was calcined. Whereas the BiVO₄ was prepared by the hydrothermal method from a bismuth nitrate solution and ammonium metavanadate in HNO₃ and CTAB, the C-ZnO/BiVO₄ composite was synthesised by vigorously mixing the as-synthesised BiVO₄ and the C-ZnO in deionised water. The suspension was ultrasonicated, centrifuged, and dried [98]. As a photocatalyst, 75 mg of the material removed 100% of 10 ppm of the methylene blue dye in 50 min.

The microwave method applies high-frequency electromagnetic waves (900–2450 MHz). Microwave synthesis causes simultaneous nucleation and growth, provides even heat distribution, decreases overheating, and prevents an undesirable phase-contrast which is common with hydrothermal techniques. Microwave energy is used to synthesise numerous ZnO nanoparticles. High crystallinity (minimum bulk defects) results from high-temperature treatment in solid-state reactions. However, it can cause the agglomeration of smaller particles, and decrease the active surface area. In general, catalytic performance increases with an increasing active surface area and crystallinity; and enhancing these important features simultaneously is quite challenging [29]. Based on these and other parameters, Manjunatha et al. [102] applied the Taguchi strategic method for the analysis of variance (ANOVA) and Grey relation analysis to prepare ZnO nanoparticles using predicted best conditions. Under the various experimental conditions, the best photocatalyst with a maximum degradation efficiency of the pollutant was identified. The research reported that: (1) a trivalent metal ion (Al³⁺, Fe³⁺, Cr³⁺)/ZnO nanocomposite material synthesised via the microwave-assisted solution combustion technique yielded the best catalyst, (2) crystallite size is mainly affected by the treatment temperature, and (3) with a contributing factor of 85.77%, the type of dopant had a primary influence on the band gap. The group reported that the best catalyst achieved an 89% removal rate of the methylene blue dye in 160 min. The combination produced a photocatalyst with a crystallite size of 46.96 nm and a band gap of 3.05 eV [102].

In another example of the microwave synthesis of a ZnO nanostructure, Saad et al. [103] reported the synthesis of a chitosan/ZnO and a chitosan/Ce–ZnO nanocomposite material. In synthesising the composite, zinc nitrate and chitosan in deionised water were stirred vigorously, with the pH adjusted to form a suspension that was microwaved. The chitosan/Ce–ZnO (doped with 1% cerium) was prepared by following a similar procedure with the addition of Ce(NO₃)₃ at the first step. The composites had reduced band gap energies and exhibited good stability and reusability up to the fifth cycle with minimal reduction in photocatalytic performance.

Vignesh et al. [60] investigated the antibacterial and photocatalytic effectiveness of a polyvinylpyrrolidone (PVP)-capped Cd/Ag/ZnO nanocomposite fabricated by the use of microwave-aided precipitation. Silver nitrate, cadmium nitrate, zinc nitrate, and a few grams of PVP were dissolved in ethanol and deionised water, then precipitated with NaOH and microwaved. The obtained material was then sonicated, dried, and subjected to a high-temperature treatment. This synthetic approach prevented grain growth, but the lattice and cell parameters were slightly enhanced. When applied as a photocatalyst, the material completely degraded the methylene blue dye and showed good recyclability up to the fifth cycle with good stability.

Despite the wide variety of available nanoparticle synthesis methods, research has continued to evolve to produce catalysts with predefined properties through the combination and modification of existing ones and the development of novel routes. Other relevant methods for the fabrication of visible-light-driven ZnO photocatalysts are summarised in Table 5.

A highlight is the work by González-Casamachin et al. [104], who reported a ZnO on polypyrrole (PPy)-composite material synthesised by combining precipitation and polymerisation techniques. For the ZnO, solutions of ZnCl₂ and a surfactant in deionised water were mixed vigorously, precipitated using NaOH, and dried. While for the ZnO/Ppy, the ZnO nanoparticles, sodium dodecyl sulphate, and pyrrole in ammonium persulphate (for the polymerisation reaction) were thoroughly mixed. Methanol was then used to precipitate the ZnO/Ppy. The photocatalyst achieved a 66% TOC removal of the acid violet 7 pollutant dye in a continuous reactor system.