1.1.1. Pure ZnO Nanoparticles

Metallic oxides are compounds composed from metallic elements and oxygen, exhibiting a wide range of properties that make them highly valuable in various applications [51]. They serve as semiconductors and can form composite with other semiconductor materials and act as catalysts. Additionally, metallic oxides are used as pigments in paints and coatings, providing color and stability [52], while their antimicrobial properties make them effective against bacteria and fungi in medical and environmental applications. This versatility enables metallic oxides to play crucial roles across multiple fields, from technology to healthcare. Among these oxides, ZnO is one of the predominant n-type semiconductors [54]-. Different synthesis methods such as sol–gel [27], precipitation [37], green method [57], combustion [58], hydrothermal [59], and spray pyrolysis [50] have been reported to synthesize zinc oxide nanoparticles for photocatalytic application.

Nguyen and Nguyen [60] used green synthesis method for zinc oxide preparation. Typically, zinc acetate decomposed thermally without adding any chemical agents (different thermal decomposition temperatures; Figure 1). The authors aimed to optimize the reaction conditions and minimize the formation of undesired compounds. These authors also used X-ray diffraction (XRD) and FE-SEM to analyze the synthesized photocatalyst (Figure 1, left side). A hexagonal wurtzite structure was observed at all temperatures as XRD results showed [29]. The average crystallite size of ZnO decreased from 42 to 33 nm as the temperature increased from 450 to 750^oC.

The authors also used FE-SEM to examine the morphology of ZnO nanoparticles. The FE-SEM images revealed spherical and rod-like structures for ZnO nanoparticles at higher and lower temperatures, respectively. This temperature-dependent variation in morphology suggests that the growth conditions during the synthesis process play a crucial role in determining the structure of ZnO nanoparticles. Controlling the morphology of ZnO nanoparticles is significant. Because the calcination temperatures can affect the shape and the surface area of ZnO nanoparticles, as the calcination temperature increased, the rod-like shape changed to a spherical shape. Based on the authors’ conclusions, the calcination temperatures were also found to influence the shape and surface area of ZnO nanoparticles. Specifically, as the calcination temperature increased, the rod-shaped nanoparticles transformed into spherical shapes. Changes in shape and surface area can have a significant impact on the properties of nanoparticles, such as their catalytic activity, optical properties, and surface reactivity. Therefore, optimizing the calcination temperature can be a critical parameter to tailor the properties of ZnO nanoparticles for specific applications (Figure 1, right side).

The authors [60] also carried out photocatalytic activities of these materials toward the photodegradation of methylene blue under ultraviolet (UV) light. The results show that the efficiency depends on the size and morphology of the ZnO nanomaterials. ZnO nanoparticles prepared at 750°C exhibited the largest crystallite size and nanorod shape. This suggests that the change in shape and surface area can have a significant impact on the nanoparticle’s properties, such as catalytic activity, optical properties, and surface reactivity. Therefore, optimizing the calcination temperature can be a critical parameter for tailoring the properties of ZnO nanoparticles for specific applications.

Uribe-L´opez et al. [61] synthesized ZnO nanoparticles through precipitation (ZnO-PP) and sol–gel (ZnO-SG) methods using zinc acetylacetonate hydrate as a precursor, ethanol and deionized water for washing and dissolving, and ammonium hydroxide for precipitation formation, respectively. The authors used 70^oC and 500^oC for drying and annealing the gel, respectively, to obtain the ZnO powders. XRD finding shows high purity and crystallinity. In addition to phase analysis, the authors characterized the synthesized ZnO nanoparticles using FTIR to identify the presence of functional groups in materials. The results show the presence of a characteristic broad band between 3500–3250 cm⁻¹. The peak at 3500 cm⁻¹ indicates the stretched vibration OH groups in both samples with an insignificant variety of intensity. For ZnO-PP, a small band around 3704 cm⁻¹ was observed and an intense signal appeared around 408 cm⁻¹ followed by a shoulder centered around 506 cm⁻¹ for the ZnO-SG nanomaterial. The authors also confirmed the presence of metal-oxygen bonds stretching 408 and 506 cm^−1.

Uribe-L´opez et al. [61] examined the structure and grain size of the sample using FE-SEM and HR-TEM. They observed spherical and rod-like structures for the ZnO-PP and ZnO-SG samples, respectively. They also detected the presence of hexagonal tubes in both samples. When they increased magnification, they noticed minor variations in atomic arrangement; the ZnO crystals produced by the precipitation method exhibited O₂ effects. The authors confirmed that the grain sizes and crystallite sizes of ZnO NPs were found to be varied together. This correlation implies that changes in the particle size of the nanoparticles were accompanied by changes in their crystallite size.

The authors [61] also evaluated the band gap energies of ZnO nanomaterials. The estimated band gap energies of 3.28 and 3.26 eV for ZnO were confirmed when the sol–gel and precipitation method were used. The authors noted that both ZnO samples displayed a strong absorbance band with a cutoff wavelength below 380 nm. This indicates that ZnO nanoparticles exhibit intrinsic band gap absorption, where electrons transition from the valence band (VB) to the conduction band (CB) upon absorption of light. The cutoff wavelength below 380 nm suggests that ZnO nanoparticles can absorb light in the UV region of the electromagnetic spectrum. In the study conducted by the authors, they observed slightly higher band gap energy for the ZnO nanoparticles synthesized through the precipitation method, which could be attributed to the larger particle size. Similar results have been reported in [36].

The ZnO nanoparticles synthesized by the precipitation method exhibited a photodegradation efficiency higher than that of the one synthesized by the sol–gel method. The precipitation-synthesized ZnO nanoparticles achieved a maximum degradation, whereas the sol–gel–synthesized nanoparticles only achieved 80% degradation [61]. This may be due to its crystal structure, defect density, surface charge, and adsorption capacity that play a major role in the photocatalytic performance.

Kumaresan et al. [62] synthesized ZnO nanoparticles using the hydrothermal method. They observed different XRD patterns and crystallite sizes of ZnO nanoparticles at different pH values. At pH 7, 9, and 11, the authors observed hexagonal disk XRD patterns of ZnO. However, at pH 13, the authors observed nanoflower structures of ZnO. The band gap decreased slightly as the pH value increased from 7 to 9, but remained stable afterward. -Kumaresan et al. [62] reported a similar idea for ZnO nanoparticles. The average band gap energy for ZnO nanoparticles was almost similar. The paper by Qin et al. [63] explored the effects of pH on the morphology of ZnO nanoparticles. They used a simple hydrothermal method to synthesize ZnO nanoparticles with different shapes and sizes. They found that the pH values influenced the growth rate and direction of ZnO crystals, resulting in different shapes such as hexagonal disks at pH 7, prismoids at pH 8, prisms at pH 9, and pyramids at pH 10. They also measured the structure, crystallite size, and surface area of the ZnO nanoparticles and found that they varied with pH values.

Kumaresan et al. [62] presented the photodegradation activity for zinc oxide nanoparticles over rhodamine B (RhB) dye under different pH conditions (7, 9, 11, and 13). They found maximum degraded efficiency (94%) at pH 9 value. They also showed that the zinc oxide catalyst at pH 9 performed better than the ZnO catalyst at other pH values. They explored the reusability of ZnO and they found almost the same efficiency before and after reuse. The authors obtained a degradation efficiency of 94%, 93%, 90%, 88%, and 82% in the first, second, third, fourth, and fifth cycles, respectively. Because of this variation, the results suggest that the material is unstable. This might be due to the problem of choosing the right calcination temperature. Zhu et al. [64] reported on the photocatalytic effect of zinc oxide nanomaterials on RhB using 30 mg of zinc oxide nanoparticles in 50 mL of the RhB solution. Almost RhB completely degraded after 150 min. Other researchers like Kumaresan et al. [62] evaluated the photocatalytic ability of zinc oxide nanosized with RhB dye-. In general, although the observed variations in structural and morphological properties may imply some instability in the material, the consistent efficiency before and after reuse suggests that the ZnO nanoparticles maintained their photocatalytic activity.

Balcha et al. [65] also reported the synthesis of pure zinc oxide particles through precipitation and sol–gel methods. In this typical work, zinc oxide particles were prepared using zinc nitrate hexahydrate, ammonia hydroxide, ethanol, polyethylene glycol, starch, and ammonium carbonate. The authors [65] also investigated the photocatalytic activity of zinc oxide using various photocatalyst loads and initial dye concentrations for the photodecomposition of MB under UV light radiation. The degradation of the dye decreased as its concentration increased. Nevertheless, efficiency increased as the catalyst load increased to an optimum load and then decreased beyond optimum concentration. Based on the results obtained, the authors concluded that a higher dye concentration and catalyst load after the optimum amount prevent the photons of light from covering the surface of zinc oxide particles during photodegradation processes; optimum adsorbate and adsorbents are paramount.

Saif and colleagues [34] prepared ZnO nanoparticles using green and chemical methods. Optical absorption was observed at 370 nm for green-synthesized ZnO nanoparticles and at 360 nm for chemically synthesized ZnO nanoparticles. This indicates that the biological method is less efficient in enhancing the absorption of ZnO nanoparticles in the visible region. The estimated band gaps of biologically and chemically synthesized are 3.35 and 3.44 eV, respectively. The results show that the energy band gap decreased when synthesized with the biological method and increased when the reaction mixture was chemically synthesized. In other words, biologically synthesized ZnO particles are more reliable for photocatalytic activity.

The study also suggested that green-synthesized ZnO nanoparticles exhibited photocatalytic activity higher than that of chemically prepared nanoparticles. Green synthesized ZnO nanoparticles were able to degrade up to 90% of MB, while chemically prepared nanoparticles achieved a degradation of 78%. These findings were consistent with UV-vis spectroscopy analysis, which likely provided additional evidence supporting the superior photocatalytic performance of the green-synthesized ZnO nanoparticles. General mechanism for photodegradation of organic dyes by employing ZnO nanoparticles is indicated in Scheme 2. When ZnO photocatalyst interacts with UV light, the electron and holes are generated at CB and VB, respectively. Electrons react with oxygen to form superoxide radicals, whereas holes react with water to generate hydroxyl radicals. These reactive species can easily break down dyes into CO₂, water, and other substances. The production of reactive species is influenced by the oxidation-reduction potentials or VB and CB positions. Photocatalysts have high redox potentials when their VB and CB positions become more positive and more negative potentials than the standard oxidation-reduction potentials (Scheme 2).

In the conclusion of this section, many researchers explored different synthesis methods to tailor the properties of ZnO for different uses [65]. They investigated characterization techniques to identify the effect of various factors such as particle size, morphology, crystalline structure, and surface modifications on photocatalytic performance [66]. The study also estimated the effectiveness of ZnO in dye degradation, measured the degradation kinetics, and explored the potential mechanisms involved. Optimization of photocatalyst loading, initial pollutant content, pH values, temperature, and others focused on improving the efficiency of ZnO photocatalysis [8, 60]. However, ZnO-NPs have drawbacks (large band gap, recombination, and surface defects) that limit their visible light performance. To overcome these confines, metal doping of ZnO-NPs is a promising strategy that can enhance their optical, electronic, biological, and catalytic properties [67]. Metal-doped ZnO-NPs can exhibit improved photocatalytic activity, stability, and selectivity for various environmental and energy applications [68]. Therefore, the relevance of this review lies in addressing environmental pollution challenges and developing sustainable and efficient treatment methods.

For a general understanding of the use of ZnO for the degradation of organic dyes, here we recommended some potential to: (1) Optimize the particle size and morphology of ZnO, as it can have a significant impact on its photocatalytic activity. Researchers should explore different synthesis methods or modifications to effectively control the particle size and morphology of ZnO, to improve its ability to degrade dyes. (2) Extend the light absorption range of ZnO by doping it or coupling it with other materials or modifying its surface to enhance visible light absorption. (3) Improve the surface defects of ZnO. Surface defects play a crucial role in the degradation efficiency of ZnO. (4) Optimize the operating conditions such as pH, temperature, and initial dye concentration which can affect the efficiency of ZnO-mediated dye degradation. Researchers should carefully consider and adjust these parameters to achieve the highest possible degradation efficiency. (5) Consider catalyst recovery and reusability to reduce the environmental impact and cost associated with the use of ZnO.

1.1.2. Single Metal–Doped ZnO Nanoparticles

Numerous photocatalysts such as MnO₂ [32], CuO [33], ZnO [34], TiO₂ [35], and Cu₂O [36] have been reported for the photodegradation of organic dyes. ZnO is an appropriate candidate material to substitute other metal oxides, among others [19]. However, ZnO has a wide band gap, recombination of charge carriers, and surface defects [30]. Many researchers have formulated different strategies to enhance the photocatalytic performance of single ZnO, such as doping with metal or nonmetal. These approaches aim to improve charge separation and migration within the material, enhance light absorption, and prolong the catalyst’s lifetime. For instance, Wang et al. [69] reported Mn, Ni, Fe, Co, and Cr-doped ZnO to modify its efficiency in terms of surface area, particle size, band gap energy, mechanical, and some other properties. Doping ZnO with rare earth elements such as lanthanum, europium, cerium, and samarium has also been reported by other groups [70–72]. Several other investigators, such as Shahpal et al. [73], Nguyen and Nguyen [60], Alshamsi -Hussein and Hassan [68], and Gaurav et al. [74], reported single metals such as Mn, Ag, La, and Cu-doped ZnO nanoparticles. In most cases, single-metal doping improves surface area and light absorption and inhibits the recombination of electron-hole pairs of single ZnO [60, 68, 74].

Metal-doped ZnO nanomaterials can be synthesized by various methods such as sol–gel, hydrothermal, spray pyrolysis, and coprecipitated approaches [60, 71, 72, 74]. These methods can be solution-based and vapor-phase methods; nevertheless, solution-based routes have attracted more attention due to their low energy cost and eco-friendliness. The structural properties of the nanoparticles were also studied using various techniques to determine significant structures, size, crystallinity, functional groups, elemental composition, phase change, particle distribution, and thermal stability [52–55].

Achouri et al. [75] synthesized single and Mn-doped zinc oxide using the solvothermal method for enhanced visible-light photocatalysis. The authors [75] observed the wurtzite crystal structure of zinc oxide particles. The XRD peaks were found to be very sharp in intensity, and this is due to their purity and high crystallinity. The authors also characterized the internal composition of Mn-doped ZnO nanoparticles using TEM and HR-TEM. The ZnO nanomaterial is typically made up of sphere-shaped structures. Hexagonal images of certain particles can also be perceived. A reduction in the average size was observed after 3% of Mn loading on ZnO. This reduction may be attributed to the inhibition of the growth of crystals by the presence of Mn ions. The size reduction of Mn-doped ZnO nanocomposites would be advantageous for enhanced photocatalytic efficiency. The selected area electron diffraction pattern of a sphere-like structure shows the formation of the hexagonal structure of ZnO particles.

Jothibas and colleagues [76] synthesized Ag-doped ZnO nanoparticles using the sol–gel method. The synthesis process included the use of ZnCl₂ and various contents of AgNO₃ (0.01, 0.03, and 0.05), along with 25% dissolved NH₃. After the sol–gel reaction, a precipitate formed, which was subsequently washed with ethanol and distilled water, dried at 100°C for 6 h, and annealed at 400°C for 2 h. The authors also examined the structural purity, morphology, particle size, elemental composition, and optical properties of these synthesized undoped and doped ZnO materials to confirm surface enhancement. The authors investigated XRD patterns of doped ZnO using various amounts of silver (0.01, 0.03, and 0.05) doped ZnO-NPs. In all cases, the hexagonal wurtzite structure of zinc oxide was observed (Figure 2(a)). Almost many of the ZnO peaks and three additional peaks appeared in the XRD patterns. The additional peaks show the integration of Ag in the ZnO crystal sites. As a result of silver doping, the crystallinity and peak position of ZnO are little altered; the pure silver peak in the zinc oxide photocatalyst predicts the place of Ag to the void place.

The authors also utilized TEM to examine the size and shape of the prepared Ag-doped ZnO nanoparticles. In particular, the TEM results of the 3% Ag-doped ZnO nanoparticles were captured to analyze their shape, size, and consistency. The images revealed hexagonal-shaped nanoparticles with a grain size distribution (Figures 2(b) and 2(c)). Based on the size distribution, the average particle size of the single and Ag-doped ZnO nanoparticles was determined to be 25 and 11 nm, respectively (Figure 2(d)). These outcomes are consistent with the analysis obtained from XRD.

The photodegradation performance of a single ZnO and a doped one with various contents of Ag (0.01–0.05) and dye without any catalyst was examined. The highest % of degradation is 94 (Figures 2(e), 2(f), and 2(g)). However, the 0.03 content of Ag-doped ZnO photocatalysts degraded higher than other Ag-doped ZnO and a single one. This reveals that the photocatalytic performance increases with increasing Ag doping. This is probably because the number of active surface sites increases as the grain size decreases, and hence it enhances the photocatalytic performance. The smaller grain size of the doped photocatalysts contributes to their modified photocatalytic performance in the degradation of organic dyes. This correlation between photocatalytic performance and crystalline size highlights the potential to control ZnO size to enhance its photocatalytic performance. By reducing the grain size, the redox potential of the VB holes and CB electrons is enhanced, facilitating the rapid transfer of electrons generated from a single ZnO to the photocatalyst’s surface. This, in turn, leads to enhanced photocatalytic performance under sunlight irradiation. Based on this information, it is recommended to explore and further investigate the effects of grain size reduction on ZnO photocatalysts.

The created vacancies engage in direct reactions with MB dye or interact with surface-bound H₂O to generate highly reactive hydroxyl radical species, which act as a potent oxidizing agent for the complete breakdown of MB. Furthermore, it is proposed that the generated electrons can undergo reactions with oxygen molecules, resulting in the formation of superoxide free radicals. Subsequently, these superoxide free radicals can react with the dye, leading to the production of carbon dioxide, water, and other substances (Figure 2(h)).

Alshamsi and Hussein [68] synthesized Ag-doped ZnO particles using three different methods, namely, solvothermal, coprecipitation, and deposition precipitation methods, using zinc acetate, silver acetate, sodium hydroxide, and ethanol (Figure 3). The authors compared the photocatalytic activities of ZnO and Ag-doped ZnO particles prepared by those three methods. The experimental results show that the homogeneity of metallic Ag in samples decreased from Ag-doped ZnO prepared using the deposition-precipitation method to the Ag-doped ZnO solvothermal method; the concentration of the Ag-ZnO interface decreased from Ag-doped ZnO prepared via the deposition-precipitation method to Ag-doped ZnO by the solvothermal method. The photocatalytic efficiency of silver-doped ZnO synthesized by the coprecipitation method is much lower than that synthesized via the solvothermal method because of its lower dispersity of Ag-ZnO photocatalyst in the dye. This indicates that synthesis methods can affect the efficiency of metal-doped zinc oxide nanocomposites [61]. Based on these, the choice of synthesis method for metal-doped zinc oxide nanocomposites is important to consider when aiming to maximize their photocatalytic performance. Researchers should carefully select the appropriate synthesis method based on the desired properties and requirements of the photocatalyst, such as dispersity, particle size, and morphology, to achieve the desired photocatalytic efficiency in a specific application.

Gaurav et al. [74] synthesized copper-doped zinc oxide nanomaterials and estimated their energy band gap (3.11 eV). The decrease in the band gap was mainly attributed to the amendment of the electronic structure, resulting in nearly complete or 100% photodegradation of the MO by copper-doped zinc oxide catalyst. They also studied the photodegradation of various organic dyes using copper-doped and single zinc oxide nanoparticles by a polyol reduction process with a capping agent. The capping agent aims to prevent uncontrollable particle growth during synthesis. The authors observed enhanced photodegradation performance at high concentrations of copper. Therefore, ZnO nanostructures are efficient for photodegradation of dyes.

Alshamsi and Hussein [68] examined the photodegradation of Ag-doped ZnO particles by comparing them with pure ZnO under solar radiation. The authors reported the effect of catalyst load and initial dye concentration. The authors studied the activities of the photocatalyst loaded with its various contents in a series of 50–150 mg/L through an optimized cibacron brilliant yellow (CB) dye content (Figure 4(a)). In this investigation, 100 mg of Ag-ZnO nanocomposite degraded 65% of the CB dye. The experimental findings showed a lower degradation efficiency of the CB dye solution when 100 mg of Ag-ZnO was used; a lower efficiency of the CB dye was observed after 150 mg/L of Ag-ZnO was used. Furthermore, the degradation efficiency for 25 mg of Ag-ZnO is 40%. The results reveal that the degradation efficiencies become high at higher values of the catalyst load and lower at lower values of the photocatalyst load. By keeping the catalyst load, the consequence of the initial concentration of CB dye was also carried out in the range of 25–125 mg/L. The highest percent of the photodegradation performance of CB was documented at 50 mg/L of dye; however, reduced efficiency was observed at 125 mg/L (Figure 4(b)). The result obtained reveals that the degradation ability of the catalyst load decreases after 50 mg/L of initial dye content. This is probably due to its lower number of reactive species on the surface of the catalysts. It is important to note the exact mechanism and factors influencing the decrement in efficiency at higher dye concentrations.