Advancements in ZnO-Based Photocatalysts for Water Treatment: A Comprehensive Review

Abstract

Water contamination remains a pressing global concern, necessitating the development of effective and sustainable water treatment solutions. Zinc oxide (ZnO) has garnered significant attention for its potential applications in photocatalysis due to its unique properties and versatile nature. This review synthesizes recent research findings on the advancement in ZnO-based photocatalysts for water treatment, encompassing synthesis methods, structure modifications for photocatalytic efficiency enhancement, toxicity assessments, and applications in diverse water treatment processes. By critically analyzing the strategies to enhance the photocatalytic performance of ZnO and its role in addressing water pollution challenges, this review provides valuable insights into the evolving landscape of ZnO-based photocatalysts for achieving efficient and environmentally friendly water treatment systems. This review emphasizes the transformative potential of ZnO-based photocatalysts in revolutionizing water treatment methodologies and underscores the importance of continued research and innovation in harnessing ZnO’s capabilities for sustainable water purification.

1. Introduction

Water, the cradle of life, faces escalating challenges of global scarcity and pollution, primarily due to rapid population growth, unchecked industrial expansion, and extensive environmental degradation [1,2]. With over seven billion people—comprising more than 15% of the world’s population—experiencing acute freshwater shortages, the ability to maintain basic living standards and engage in productive activities is hindered [3,4]. Consequently, the purification of polluted water before its discharge into the environment is of paramount importance. Many traditional techniques like coagulation, filtration, adsorption, reverse osmosis, and treatment with ozone have been proposed to remove such organic pollutants in water. Although these methods are somewhat effective, they possess limitations such as high energy consumption and the potential for secondary pollution [5,6,7]. Hence, the development of more effective and energy-efficient techniques is imperative, with a focus on environmentally friendly, cost-effective approaches. Semiconductor-assisted photocatalysis has emerged as a green, low-cost technology that utilizes sunlight to dissociate organic pollutants into harmless byproducts, thereby contributing to a reduction in environmental pollution [8,9,10]. This approach has gained considerable attention due to its non-selective oxidative properties and its capability to mineralize organic pollutants into carbon dioxide and water. A crucial part of the ongoing research in the field of photocatalysis revolves around the exploration of various semiconductor materials to harness their potential in environmental purification and the conversion of light energy into chemical energy. Notably, semiconductors such as TiO₂ [11,12], ZnO [13,14], Fe₂O₃ [15,16], BiVO₄ [17,18], SnO₂ [19], MgO [20], SiO₂ [21], and others [22,23] have been utilized in photocatalytic applications, showing promise in addressing environmental challenges and energy conversion. Among these semiconductors, titanium dioxide (TiO₂) has been extensively studied for its exceptional photocatalytic characteristics, including high stability, potent oxidizing power, and long-term durability [24,25]. However, concerns about the potential nanoscale toxicity of TiO₂ have prompted a shift in focus toward zinc oxide (ZnO), which exhibits lower toxicity profiles. The exceptional properties of ZnO, including stability, ease of synthesis, low material cost, optical transparency, piezoelectric properties, and high biocompatibility, have positioned it as a promising alternative to TiO₂ [26,27,28]. Although ZnO possesses a wide band gap of 3.37 eV and a high exciton binding energy of 60 meV [29], it demonstrates superior photocatalytic performance compared to TiO₂ in the degradation of organic pollutants [30,31,32,33]. This is attributed to its higher electron mobility (200–300 cm²/V·s) than TiO₂ (0.1–4.0 cm²/V·s) [34,35], which accelerates electron transfer, contributing to high quantum efficiency. Additionally, the lower valence band (VB) position of ZnO compared to TiO₂ results in a higher oxidation potential of hydroxyl radicals, leading to the more effective degradation of pollutants [36].

However, the challenge lies in the limited light absorption of ZnO, which is largely confined to the UV spectrum (~5% of the solar spectrum) and lacks efficiency in harnessing the visible spectrum (~43% of the solar spectrum) [37]. To address this limitation, various methods have been employed to enhance ZnO’s performance and widen its light response range. Strategies such as doping with metals and non-metals [38,39,40,41], depositing noble metals [42,43,44], creating heterostructured photocatalysts based on ZnO [45,46,47,48,49,50,51,52,53,54,55,56], and incorporating carbon materials [57,58,59] have been instrumental in improving the efficacy of ZnO. Doping ZnO with metals and non-metals like Fe, Cu, Ag, Sn, C, N, F, P, and S introduces localized donor levels within the forbidden energy band, enabling enhanced light absorption and electron transfer. Additionally, depositing noble metals onto ZnO surfaces and developing heterostructured photocatalysts by combining ZnO with other semiconducting metal oxides with varying band gaps have shown promising results, augmenting ZnO’s photocatalytic capabilities.

Furthermore, the remarkable antibacterial activity of ZnO has positioned it as a valuable solution for addressing bacterial contamination in water sources. Various strategies, including the integration of antimicrobial agents and precise control of particle size and concentration, have been implemented to amplify the antibacterial performance of ZnO. The mechanisms underlying ZnO’s antibacterial action involve the generation of reactive oxygen species (ROS) through photocatalysis and the release of Zn²⁺, presenting a multifaceted approach to combat waterborne bacterial challenges efficiently [60,61,62,63]. Moreover, beyond its roles as a photocatalyst and antibacterial agent, ZnO plays a vital function as a transparent conductive oxide (TCO) in a diverse array of optoelectronic devices like solar cells, LEDs, and displays [64,65,66]. The exceptional properties of ZnO thin films, such as high transparency, excellent conductivity, and a wide bandgap, make them indispensable components in the realm of optoelectronic technologies. Furthermore, the compatibility of ZnO with flexible substrates further enhances its appeal for the burgeoning domains of flexible electronics and wearable technology [67], underscoring its versatility and significance beyond its traditional usage in environmental cleanup and water treatment applications.

In this comprehensive review, we strive to offer a detailed overview of zinc ZnO, focusing prominently on its photocatalytic and antibacterial properties, while integrating recent studies to enrich our analysis. Our primary objective was to present a thorough understanding of ZnO by exploring its fundamental properties and highlighting its diverse applications beyond traditional water treatment and disinfection methods. Unlike previous reviews that often segregate synthesis methods or application aspects, our approach integrates these facets to provide a cohesive narrative. We extensively cover various synthesis methods and their impacts on ZnO’s properties, emphasizing strategies to enhance its photocatalytic efficiency. Additionally, we underscore ZnO’s versatility in addressing environmental challenges and beyond, showcasing its potential in broader scientific and technological domains. This review endeavors to deliver a clear and comprehensive study of ZnO, consolidating existing knowledge while offering insights that contribute to its broader relevance in contemporary research and applications.

2. Generalities of ZnO

Zinc oxide emerges as a versatile and robust photocatalyst, offering a plethora of exceptional properties that underpin its efficacy in photocatalytic applications. This versatility is showcased by its extensive range of structures, spanning from one-dimensional (1D) nanorods, nanoneedles, nanohelixes, nanosprings, nanorings, nanoribbons, nanotubes, nanobelts, nanowires, and nanocombs to two-dimensional (2D) nanoplates/nanosheets and nanopellets and, finally, to intricate three-dimensional (3D) forms like flowers, dandelions, snowflakes, and coniferous urchin-like structures [68,69,70,71,72]. Examples of ZnO nanostructures are shown in Figure 1. The ability to exist in such diverse structural forms highlights the potential of ZnO for multifaceted applications across nanotechnology, making it a material of significant interest and research focus in various fields. Additionally, the outstanding characteristics of ZnO, including high photocatalytic efficiency, excellent physical and chemical stability, a high electrochemical coupling coefficient, a comprehensive radiation absorption range, notable photostability, cost-effectiveness, and robust environmental compatibility, render it highly suitable for environmental remediation and water treatment technologies [73,74]. At the structural level, ZnO encompasses various crystalline forms, with the hexagonal wurtzite structure emerging as the most energetically stable under ambient conditions. This crystalline arrangement endows ZnO with piezoelectric and spontaneous polarization properties, significantly influencing its defect formation and exceptional chemical reactivity. Moreover, the material’s exceptional luminescence properties stem from defect-related transitions and excitonic processes, characterizing robust UV emission bands, photoluminescence behavior, and distinct electrical attributes. ZnO’s lattice dynamics, including its interactions with optical phonons, markedly influence its optical properties and light–matter interactions, underpinning its pivotal role in optoelectronic and technological applications. Notably, ZnO also boasts remarkable electrical attributes, including enhanced electron mobility, high breakdown voltages, a substantial exciton binding energy of 60 meV at room temperature, and an exceptional breakdown field strength. These electrical properties further highlight ZnO’s versatility and utility, positioning it as a preferred material for diverse applications in high-powered electronic devices, electro-optical applications, and UV/blue light emitter applications. Additionally, the material’s mechanical properties, such as hardness and piezoelectric properties, position ZnO as a promising candidate for diverse device applications [75]. Furthermore, ZnO’s magnetic properties, particularly its classification as a magnetic semiconductor, diluted magnetic semiconductor (DMS), or nonmagnetic semiconductor, have garnered significant research interest, with clear implications for spintronic applications and innovative material design [76]. Finally, understanding the thermal properties of ZnO—including its thermal expansion coefficient, thermal conductivity, and specific heat—is crucial for leveraging its performance in high-power and demanding conditions, signifying its relevance in engineering and materials science applications [77]. Beyond these fundamental attributes, ZnO demonstrates remarkable antibacterial capabilities attributed to its diverse mechanisms of action. One key mechanism involves the induction of ROS generation, although the exact role of ROS remains debated due to factors like surface area, hydroxylation, interfacial traps, and particle size [78]. Furthermore, ZnO nanoparticles effectively disrupt biofilms—microorganisms’ defensive structures—through electrostatic interactions with extracellular polymeric matrices (EPMs) [62,63]. The positively charged ZnO nanoparticles can penetrate the negatively charged EPMs, impeding bacterial growth. Moreover, the release of Zn²⁺ ions by ZnO nanomaterials heightens their toxicity to bacteria, pathogens, and viruses, surpassing the efficacy of other elements like Cu⁺, Fe²⁺, and Al³⁺ [79]. Notably, ZnO demonstrates notably potent direct interactions with the surfaces of bacterial cells and pathogens, underlining its efficacy as a highly effective agent in combatting microbial threats [80]. Collectively, these antibacterial mechanisms aim to destabilize bacterial cell structures and disintegrate them into inert substances. Incorporating ZnO nanoparticles into water treatment systems can effectively control the growth of harmful bacteria, reducing the risk of waterborne diseases.

3. Synthesis Methods for ZnO Nanostructures for Photocatalysis

The unique properties of ZnO nanostructures, stemming from their nanoscale dimensions, surface effects, and quantum phenomena, differentiate them from their bulk counterparts. At the nanoscale, these materials demonstrate enhanced performance, particularly in photocatalytic applications, due to their substantial surface area and abundant active sites, resulting in significantly improved degradation rates [87,88,89]. A diverse array of ZnO nanostructures have been synthesized using various fabrication techniques, including solution processes, hydrothermal and solvothermal methods, sol–gel processes, precipitation, emulsions and microemulsions, vapor transport methods, thermal evaporation processes, chemical vapor deposition, physical vapor deposition, electrochemical methods, and environmentally friendly green synthesis approaches, documented in the literature [90,91]. In the forthcoming subsections, the focus will be on commonly employed methods such as hydrothermal and solvothermal techniques and chemical vapor deposition, among others, which aim to showcase the inherent advantages of each technique, such as cost-effectiveness, environmental considerations, scalability, and other pertinent factors. By elucidating the diverse synthesis techniques and their respective outcomes, this section aims to provide valuable insights into the selection and utilization of these methods for tailoring ZnO nanostructures to excel in advanced photocatalytic applications.

3.1. Hydrothermal Method

Hydrothermal synthesis emerges as a promising low-cost and low-temperature method yielding anhydrous materials with diverse sizes and morphologies directly from aqueous solutions, offering distinct advantages over traditional synthesis techniques. Notably, this approach enables one-step synthesis without necessitating high-temperature calcination or milling, leading to low aggregation levels and a narrow crystallite size distribution, making it a straightforward and environmentally friendly method. The technique excels in controlling particle morphology, resulting in enhanced purity exceeding that of starting materials as crystallites can expel impurities during growth. Operating at lower temperatures promotes unique defect structures, distinct from those obtained via conventional methods, potentially imparting exceptional properties for specific applications [92,93]. Hydrothermal synthesis conditions for ZnO powders and films span a wide range, encompassing temperatures from 20 to 400 °C, pressures from 0 to 4000 psi, diverse process times ranging from several hours to several days, various precursor types, reactant concentrations from 0.001 to 1.5 M, and pH values spanning 5–14, as well as employing additives, diverse precursor preparation methods, and autoclave types including both batch and continuous flow systems. Resulting ZnO powders exhibit a diverse array of sizes (ranging from 20 nm to tens of microns), morphologies (e.g., equiaxed, needles/whiskers, rods, fibers, sheets, flakes, plates, polyhedral, ellipsoid), and aggregation levels [94,95]. The synthesis of well-controlled ZnO nanostructured films via the hydrothermal process typically involves two key steps. First is the deposition of a ZnO buffer or seed layer on the substrate using methods like spin-coating, dip-coating, or sol–gel techniques; this seed layer plays a crucial role in nucleating and guiding the subsequent growth of ZnO nanostructures. Second, the growth of ZnO nanostructures on the seeded layer is governed by various synthesis parameters such as pH, temperature, and processing time, influencing the morphological and optical properties of the resulting nanostructures [96,97,98]. Various substrates, including glass, silicon, quartz, F-doped tin oxide (FTO), In-doped tin oxide (ITO), Al-doped zinc oxide (AZO), polyethylene terephthalate (PET), and other metals, are employed for ZnO hydrothermal deposition, each impacting nanostructure formation [98,99,100,101,102,103,104]. The surface type and condition of substrates play a crucial role in facilitating the formation of suitable ZnO layers, offering potential applications in a wide range of industrial and technological fields.

Khan et al. [105] conducted a study on the large-scale synthesis of well-crystalline ZnO NPs through a simple hydrothermal process using aqueous mixtures of (ZnCl) and ammonium hydroxide (NH₄OH). The resulting NPs exhibited a wurtzite hexagonal phase with an almost spherical shape and an average diameter of ~50 ± 10 nm. Their structural properties were extensively characterized, and the NPs demonstrated high efficacy as a photocatalyst for degrading acridine orange (AO) and as a chemical sensor for electrochemically sensing acetone in the liquid phase. Comparative analysis revealed that the ZnO NPs exhibited superior photocatalytic activity compared to commercially available TiO₂-UV100, achieving almost complete degradation of AO after 80 min of irradiation, as Figure 2 shows. Additionally, the ZnO NPs showed excellent performance as a chemical sensor for detecting acetone, with high sensitivity (~0.14065 μA/cm² mM) and a lower detection limit (0.068 ± 0.01 mM) within a short response time of 10 s.

Our research group has been dedicated to refining the hydrothermal method for synthesizing ZnO nanowire arrays (ZnO NWs). Through our previous endeavors, we successfully demonstrated the synthesis of well-aligned ZnO NWs using a silicon substrate coated with a pre-deposited ZnO seed layer. This involved depositing the seed layer on the Si substrate through spin-coating and then undergoing calcination to produce ZnO nanocrystallites as seeds. Subsequently, the growth of nanowires was achieved through hydrothermal treatment, enabling us to adjust growth conditions to fine-tune the morphology and defect concentration within the ZnO band gap. We highlighted the significant influence of key parameters of the hydrothermal method on the microstructure and morphology of the ZnO NWs. Factors such as the pH value of the aqueous solution, growth time, and solution temperature during ZnO NWs growth were found to affect the characteristics of the nanowires [98]. In our 2013 publication, we detailed the preparation of the growth solution and the optimal conditions for hydrothermal growth, emphasizing the importance of these parameters in achieving homogeneous ZnO NWs with higher aspect ratios. This research exemplified the optimization of hydrothermal synthesis conditions for ZnO NWs arrays, paving the way for their potential applications across various fields, particularly in water treatment [98].

In subsequent studies, we evaluated the photocatalytic performance of ZnO NWs in degrading three dyes (methylene blue (MB), methyl orange (MO), and acid red 14 (AR14)) under UV irradiation [106]. The results revealed substantial enhancements in degradation rates, with approximately 49% degradation for methyl orange, 86% degradation for methylene blue, and 93% degradation for AR14 after 180 min of UV exposure in the presence of ZnO nanowires (see Figure 3). Furthermore, our investigations into the effects of photocatalysis under static and dynamic modes highlighted the pivotal role of ZnO NWs in expediting degradation processes, particularly under dynamic conditions, where both MB and AR14 achieved total mineralization in less than 2 h (see Figure 4) [70].

Our contributions to the field include publications focusing on the synthesis of ZnO NWs through the hydrothermal method and exploring the impact of doping on their photocatalytic properties. If you require more detailed insights, we invite you to refer to the publication by Le Pivert et al. [107].

3.2. Solvothermal Method

The solvothermal method, a variation of the hydrothermal technique, uses organic solvents instead of aqueous ones (such as ethanol or ethylene glycol). This method allows for the precise control of temperature and pressure in an autoclave to synthesize materials under specific conditions. The higher temperatures that are possible with the solvothermal method, along with the use of organic solvents with high boiling points, provide better control over the size, shape, and crystallinity of ZnO NPs compared to hydrothermal methods. This method offers a versatile approach to tailor materials with specific characteristics based on the chosen solvent, expanding the range of achievable material properties and potential applications.

In a study by Wang et al. [108], the solvothermal method was used to produce uniform, sphere-like ZnO NPs ranging from 25 to 40 nm without the use of surfactants or agents. At a concentration of 1.5 g/L, these ZnO nanostructures exhibited exceptional effectiveness in degrading MO and p-Nitrophenol (PNP) at a concentration of 20 mg/L (Figure 5). The photocatalytic degradation rates, analyzed using pseudo-first-order kinetics, resulted in 92% degradation for MO and 56.2% for PNP after 180 min under UV light.

Additionally, Motelica et al. [109] conducted a study on ZnO nanoparticle synthesis through forced solvolysis using Zn(CH₃COO)₂·2H₂O in n-butanol, ethylene glycol, and glycerin. They found that the choice of alcohol influenced nanoparticle size and morphology, with alcohols containing a single –OH group resulting in smaller particles. ZnO nanoparticles produced in n-butanol were monocrystalline and polyhedral-shaped (ZnO_B), averaging 27 nm, while those from ethylene glycol were monocrystalline and round-shaped at approximately 44 nm. Glycerin synthesis led to larger, hexagonal, and polycrystalline particles around 120 nm. Their investigation into the photocatalytic performance with MB, rhodamine B (RhB), and MO revealed superior degradation efficiency in the order of MO > RhB > MB, increasing with nanoparticle size. The smallest nanoparticles exhibited exceptional degradation rates exceeding 99% and maintained high efficiency over multiple cycles (Figure 6). Furthermore, these NPs showed potent disinfection capabilities against various Gram-positive and Gram-negative bacterial strains.

In a different study, Xu et al. [110] made ZnO materials in various shapes using a solvothermal method with different solvents, and, for the first time, they used zinc acetylacetonate (Zn(C₅H₇O₂)₂) as the zinc source. They found that the ZnO took on shapes like cauliflower-like, truncated hexagonal conical, tubular and rod-like, hourglass-like, nanorod, and spherical forms, depending on the solvent used (THF, decane, water, toluene, ethanol, and acetone, respectively). They looked at how well these ZnO samples broke down phenol, and they found that ZnO made in THF broke it down the fastest at a rate of 0.1496 min⁻¹, nine times faster than commercial ZnO. Figure 7 illustrates the photodegradation results for phenol on the ZnO catalysts.

3.3. Chemical Vapor Deposition (CVD)

Chemical Vapor Deposition (CVD) is a deposition technique that involves introducing a substrate in contact with vapor-phase precursors and initiating reactions or decomposition, leading to the deposition of the desired molecules. The process removes unwanted by-products through gas flow circulating in the reactor. Variants of CVD include Atmospheric Pressure CVD (APCVD), conducted at atmospheric pressure; Plasma-enhanced CVD (PECVD), enhanced by plasma for room-temperature deposits; and Metalorganic CVD (MOCVD), utilizing metal–organic compounds as precursors. This method can yield a wide range of ZnO nanowire morphologies, sometimes within the same sample. For instance, Wang et al. [111] applied the one-step CVD method to produce distinctive propeller-shaped and flower-shaped ZnO nanostructures on Si substrates (Figure 8a,b). The propeller-shaped ZnO nanostructure consisted of axial nanorods (50 nm tip, 80 nm root, and 1 μm length) surrounded by radial-oriented nanoribbons (20–30 nm in thickness and 1.5 μm in length). Similarly, the flower-shaped ZnO nanostructure replicated the propeller-shaped ZnO morphology, with differences in the shape of the leaves. The nanorod leaves (30 nm in diameter and 1–1.5 μm in length) exhibited a radial alignment and pointed toward a common center. Additionally, the flower-shaped ZnO nanostructures demonstrated sharper and more intense UV emission at 378 nm compared to the propeller-shaped ZnO, indicating superior crystal quality and reduced structural defects. In contrast, the photodegradation of RhB under UV light illumination showed that the propeller-shaped ZnO nanostructures exhibited a higher photocatalytic efficiency relative to the flower-shaped ZnO nanostructures, as depicted in Figure 8c.

4. ZnO as a Photocatalyst

After discussing the generalities of ZnO and the various synthesis methods, we now turn our attention to the role of ZnO as a photocatalyst and explore the mechanism of photocatalysis along with the relationship between synthesis methods and properties, particularly in terms of photocatalytic efficiency. ZnO emerges as a highly efficient photocatalyst, leveraging its exceptional intrinsic qualities to confront modern environmental and energy challenges head-on. This photocatalysis process, driven by light energy to initiate chemical reactions, is pivotal in decomposing organic pollutants and harnessing clean energy resources. The ability of ZnO oxidation processes under standard environmental conditions enhances its practical utility, showcasing its significance in addressing both pollution issues and energy scarcities [112,113]. Upon absorbing photons, ZnO initiates the generation of electron–hole pairs, with electrons transitioning to the conduction band (CB) and holes remaining in the valence band (VB), depicted by Equation (1):

$$\mathrm{ZnO}+\mathrm{h}\mathsf{\nu }\text{→}{\mathrm{e}}_{\mathrm{CB}}^{-}+{\mathrm{h}}_{\mathrm{VB}}^{+}$$

(1)

These charge carriers migrate to the surface, reacting with adsorbed water and oxygen molecules to form highly reactive species such as hydroxyl radicals (^•OH) and superoxide anions (O₂^•−), as depicted in reactions (2) and (3):

$${\mathrm{h}}_{\mathrm{VB}}^{+}+{\mathrm{H}}_2\mathrm{O}\text{→}{\mathrm{H}}^{+}+{}^{\text{•}}\mathrm{OH}$$

(2)

$${\mathrm{e}}_{\mathrm{CB}}^{-}+{\mathrm{O}}_2\text{→}{\mathrm{H}}^{+}+{\mathrm{O}}_2^{\text{•}-}$$

(3)

These species play a crucial role due to their high reactivity, interacting with pollutant molecules to induce their degradation and discoloration. In addition, the generation of these radicals provides a mechanism to prevent the recombination of electrons and holes, thereby enhancing the photocatalytic efficiency of ZnO [114,115].

The photocatalytic efficiency of ZnO is significantly influenced by the band gap energy and the separation of charge carriers, which are intricately linked to the methods employed in synthesizing ZnO nanostructures. Various studies have demonstrated that different synthesis approaches can tailor the morphology, size, and specific surface area of ZnO nanoparticles, thereby impacting their photocatalytic efficiency [116,117,118,119,120]. The crystalline quality of ZnO is also crucial, as the presence of defect states can either enhance or inhibit photocatalytic activity. Specifically, defects such as oxygen vacancies (V_O) and zinc vacancies (V_Zn) act as traps that prolong the lifetime of excitons, allowing more of them to reach the surface and participate in the creation of ROS [121]. However, surface defects are beneficial only to a certain extent. While surface defects can trap excitons and facilitate the degradation of organic molecules, volume-related defects tend to act as recombination centers, preventing photogenerated electrons from reaching the surface and thus curtailing photocatalytic efficiency. Different strategies have been explored to enhance the photocatalytic performance of ZnO, such as increasing the specific surface area to reduce the prevalence of volume-related defects and doping ZnO with various elements to modify its band gap and improve its electrical and optical properties [122,123,124,125]. Additionally, the introduction of metal co-catalysts can serve as electron traps, discouraging radiative recombination and boosting photocatalytic efficiency [126]. In smaller nanoparticles with a high surface-to-volume ratio, surface defects dominate the material’s properties, whereas, in larger nanoparticles, both surface and volume defects are relevant. Surface defects, particularly V_Os, are advantageous as they can adsorb oxide radicals and other molecules, making them active centers for pollutant degradation. When excitons are captured by these vacancies, they are readily transferred to organic molecules adsorbed on the nanoparticle’s surface, enhancing dye degradation efficacy [118,119].

Understanding and optimizing these parameters and intrinsic properties is essential for maximizing the photocatalytic performance of ZnO, making it a promising material for environmental remediation and energy conversion applications.

4.1. Impurity Presence Effect

The organic contaminants in the synthesis process of ZnO can include residual chemicals or impurities from the precursor materials used during the synthesis, such as Polyvinylpyrrolidone (PVP), zinc nitrate hexahydrate, and sodium hydroxide (NaOH) [127]. These contaminants may also originate from the chemical reactions involved in the synthesis process, leading to the incorporation of organic residues on the surface of the ZnO nanoparticles [128]. The presence of these organic contaminants can detrimentally impact the photocatalytic performance of the catalyst by acting as hole traps on the semiconductor surface, which, in turn, leads to increased charge recombination and a subsequent decline in photocatalytic efficiency [30]. In their investigation, Umar et al. [129] explored the influence of annealing temperature on various properties and the photocatalytic efficiencies of ZnO NPs prepared through a facile solution combustion approach, employing zinc nitrate as the precursor and dextrose as the fuel and oxidizer. Notably, their findings revealed that annealing temperature significantly affects the particle size, morphology, bandgap, and photocatalytic performance of ZnO nanomaterials. The variations in key physical parameters for ZnO NPs with respect to annealing temperature are presented in

Table 1. Variations in physical properties for ZnO NPs with annealing temperature. Reprinted with permission from [129]. Copyright © 2015, Elsevier B.V.

Annealing Temp.	λmax. (nm)	Band Gap (eV)	Particle Size (nm) (from FESEM)	Crystallite Size (nm) (from XRD)
400	374.4	3.317	40–45	24.94
500	376.8	3.296	55–60	28.16
600	378.0	3.285	85–90	31.19
700	379.2	3.275	100–110	37.90
800	381.6	3.254	115–120	48.53

. Furthermore, the ZnO nanoparticles annealed at different temperatures were effectively employed in the photocatalytic degradation of a water-soluble direct red-23 (DR-23) dye. The outcomes demonstrated that specimens annealed at 400 °C, 500 °C, 700 °C, and 800 °C exhibited photodegradation percentages of 88.48%, 95.49%, 92.63%, and 86.40%, respectively, as depicted in Figure 9. Notably, ZnO nanoparticles annealed at 600 °C achieved nearly complete photodegradation of the DR-23 dye. The data in Table 1 indicate that particle size increased with annealing temperature, leading to a reduction in the bandgap. Additionally, a smaller particle size resulted in an increased surface area of the photocatalyst, both of which contributed to enhancing the photocatalytic activities of ZnO nanoparticles. The heightened crystallinity of ZnO nanoparticles at elevated annealing temperatures also accounted for the improved photocatalytic degradation. Furthermore, the introduction of native defects in the ZnO crystal in the form of neutral (V_O), singly charged ${(\mathrm{V}}_{\mathrm{O}}^{+}$), or doubly charged ${(\mathrm{V}}_{\mathrm{O}}^{++}$) oxygen vacancies at higher annealing temperatures up to 600 °C was found to play a pivotal role in enhancing the photocatalytic efficiencies of the synthesized ZnO nanoparticles. These defects in the ZnO mitigated electron–hole recombination, increased the quantum yield, and thereby resulted in the superior photocatalytic activities of the ZnO nanoparticles. Moreover, the greater prevalence of these oxygen vacancies on the ZnO surface at higher annealing temperatures also served as traps for electrons from the conduction band. The decline in the photocatalytic activities of the ZnO photocatalyst at even higher annealing temperatures of 700° C and 800 °C may be ascribed to the reduction in surface area due to the agglomeration of ZnO particles; however, it is noteworthy that the morphology of the ZnO NPs remained unchanged in the study.

In their study, Li et al. [130] synthesized ZnO NPs with varying average crystallite sizes (21.3 nm, 38.2 nm, 46.9 nm, 56.5 nm, and 89.8 nm) by calcining the prepared powders at different temperatures ranging from 200 °C to 1000 °C. Their investigation revealed that ZnO prismatic particles calcined at 800 °C exhibited the highest level of photocatalytic activity. Furthermore, Giraldi et al. [131] conducted research on the synthesis of ZnO nanoparticles through a chemical method, investigating the impact of annealing temperature on particle size, crystallinity, and photocatalytic performance in the degradation of RhB. They noted that as the annealing temperature increased from 100 °C to 500 °C, the particle sizes expanded from 13 nm to 28 nm, accompanied by enhanced crystallinity and improved photocatalytic degradation outcomes. Moreover, Franco et al. [132] highlighted the effectiveness of ZnO calcination in removing residual organic residues, emphasizing the crucial role played by the specific surface area in this process. Their findings indicated that an annealing temperature of 500 °C represented the optimal point, beyond which, particle size augmentation and a reduction in specific surface area occurred, subsequently leading to a detrimental impact on photocatalytic activity. This underscores the delicate balance between calcination temperature, particle size, specific surface area, and, ultimately, the photocatalytic performance of ZnO NPs.

4.2. Influence of Specific Surface Area

The surface area of a material decreases with an increase in the size of its crystallites, which depends on several factors such as calcination temperature and/or synthesis conditions, as shown in the preceding section. Most often, authors suggest that the photocatalytic activity of ZnO increases with an elevation in specific surface area due to a reduction in the recombination of photo-generated electron–hole pairs [117,122,133,134]. In their investigation, Hayat et al. [135] delved into the impact of calcination temperature on the photocatalytic activity of ZnO for phenol degradation. By varying the calcination temperature from 400 °C to 700 °C, the researchers explored its effect on the morphology and crystallite size of ZnO. The outcomes revealed that a reduction in particle size, leading to an increase in specific surface area, enhanced the photocatalytic efficiency by facilitating the adsorption of more phenol molecules. Employing the sol–gel method, they successfully synthesized ZnO NPs of different sizes by adjusting the calcination temperature. Interestingly, they observed that with increasing calcination temperature, the crystallites tended to agglomerate, resulting in a simultaneous loss of the catalyst’s activity. Notably, nano ZnO synthesized through sol–gel and calcined at 500 °C exhibited superior activity for the photocatalytic oxidation of phenol compared to the other photocatalysts considered in the study (Figure 10). Furthermore, the research highlighted the influence of various operating factors such as catalyst concentration, initial phenol concentration, solution pH, and irradiation intensity on the photocatalytic process. The obtained apparent kinetic rate constants followed the order nano ZnO-500 > nano ZnO-550 > nano ZnO-400 > nano ZnO-600 > nano ZnO-700, indicating the significant impact of these factors on photocatalytic efficiency. The smaller size, better dispersion, and homogeneity of nanoparticles were identified as pivotal factors contributing to the high photonic efficiency and accelerated degradation rate of phenol for nano ZnO-500.

Becker et al. [117] further investigated the impact of particle size on ZnO photocatalysts and highlighted that when ZnO NPs exhibit equivalent levels of crystallinity and morphology, the specific surface area emerges as the predominant factor influencing efficiency. In a similar vein, Flores et al. [122] emphasized the significance of a large specific surface area for augmenting photocatalytic efficiency. Nonetheless, they posited that surface area alone is not the sole crucial factor. Their research underscored the essential role of oxygen defects as a critical parameter for achieving heightened photocatalytic activity. Consistent with this, Wolski et al. [136] presented findings aligning with the notion that surface area does not stand alone as the singular critical factor. Their study indicated that a moderate surface area coupled with excellent crystallinity represents the optimal balance for maximizing performance. Contrarily, alternative studies [123,137] have suggested that a smaller surface area may yield greater effectiveness than a larger one, highlighting the significance of additional parameters such as morphology [123] and the exposure of polar planes [137]. This diverse body of research underscores the multifaceted nature of factors influencing the photocatalytic efficiency of ZnO NPs and the necessity of considering a range of parameters beyond specific surface area alone.

4.3. Influence of Morphology

The impact of morphology on the photocatalytic properties of ZnO is a subject of significant interest, with varied research findings shedding light on the complex relationship between morphology and effectiveness. Saravanan et al. [138] suggest that morphology plays a crucial role in determining photocatalytic efficiency, highlighting that ZnO spherical nanoparticles outperform nanorods despite the latter’s larger surface area. This raises intriguing questions about the genuine significance of morphology in dictating performance. On a different note, Daou et al. [139] found that ZnO with a spherical or hexagonal morphology exhibits higher efficacy in degrading MO compared to rod-shaped structures. In contrast, Li et al. [123] identified ZnO needles as the most effective, showcasing superior performance despite their relatively small surface area. Thakur et al. [125] advocate for nanospheres as optimal for enhanced photocatalytic activity. Similarly, Akir et al. [140] observed that a spherical morphology excels in degrading RhB when compared to nanosheets or hexagonal prismatic forms. Furthermore, spherical ZnO nanoparticles have shown remarkable activity in degrading MB when contrasted with nanorods, attributed to the presence of oxygen defects that mitigate electron–hole recombination and stimulate free radical generation [141]. Notably, Ahumada-Lazo et al. [142] established a significant correlation between the surface morphology of ZnO films and their photocatalytic activity, particularly in the decolorization of orange G dye solutions. Additionally, Jaafar et al. [143] demonstrated the exceptional degradation efficiency of irregular, nano-sized, sphere-like ZnO aggregates, achieving an impressive 87% degradation of phenol under visible light irradiation. Furthermore, Weerathunga et al. [144] underscored the catalytic prowess of pyramidal ZnO, achieving a remarkable 30% conversion rate for benzyl alcohol, surpassing the conversion rates of plate- and rod-shaped ZnO variants. Moreover, Shidpour et al. [145], in their comparison of different ZnO morphologies, including nanowires, nanorods, spheres, and welded nanoparticles, demonstrated the superior degradation capacity of nanowire-shaped particles in the breakdown of MB dye.

The literature emphasizes the importance of polar and non-polar facets and their relevance in photocatalysis [124,137,146,147,148]. However, this phenomenon is also linked to various other parameters such as morphology [147,149], surface [147], and oxygen vacancies [124,141]. Consequently, it is challenging to decisively determine the actual importance of morphology. The morphology and specific surface area of zinc oxide (ZnO) play pivotal roles in determining its photocatalytic performance [145]. Morphological variations, such as nanoparticles, nanorods, nanosheets, and porous structures, significantly influence the active surface area and light absorption properties of ZnO. These structures provide a high surface-to-volume ratio, enhanced directional electron transport, and exposure of reactive crystallographic facets, all of which are crucial for efficient photocatalysis. A higher specific surface area increases the availability of active sites and improves the separation of electron–hole pairs, thereby reducing recombination rates and enhancing photocatalytic efficiency. Optimized morphologies, often achieved through engineering hierarchical and porous structures, synergize these effects to maximize photocatalytic activity [150]. Consequently, ZnO with a tailored morphology and high specific surface area shows exceptional potential in environmental remediation, antibacterial applications, and solar energy conversion, underscoring the importance of precise material design in advancing photocatalytic technologies.

4.4. Impact of Structural Defects

In addition to focusing on the morphology and particle size of ZnO, structural defects have also been widely discussed. The presence of oxygen defects has drawn significant interest in current research concerning photocatalysis. Nandi et al. [151] prepared ZnO NPs and studied the impact of structural defects on photocatalytic efficiency for degrading the dye RhB under UV irradiation. They found that the catalyst with the most defects was the most effective, given that it had the largest specific surface area. Furthermore, ZnO exhibited better efficiency in degrading estrone, despite its smaller surface area compared to Aeroxide^® titanium dioxide P25 (TiO₂-P25) [152]. The authors attributed this efficacy to the ionic defects existing in the crystalline network of ZnO. They suggested that oxygen and zinc vacancies can trap charge carriers, thus preventing the recombination of electron–hole pairs and potentially enhancing photocatalytic performance.

Xu et al. [153] found that the photocatalytic degradation efficiency of ZnO with a high oxygen defect toward tetracycline was increased to more than 99.9%, about three times that of low-oxygen defect ZnO. Also, ultrathin ZnO/Al₂O₃ nanosheets with abundant oxygen defects could rapidly degrade tetracycline and RhB, affording 88.4% and 76.9% degradation in 150 min, respectively [154]. Similarly, oxygen defect-rich pencil-like ZnO nanorods possessed better photocatalytic performance for tetracycline degradation, which could reach 96% under optimal conditions [155]. The research of the Singh [156] and Pei [157] groups successively confirmed the conclusion of Li et al. [155] that more oxygen defects result in better photocatalytic performance.

On the other hand, while some publications consider Zn defects important to improve ZnO’s photocatalytic activity [136,152,158,159], a study by Danilenko et al. [160] demonstrated that zinc defects are deleterious to photocatalysis. The appearance of cationic defects in the structure decreased the photocatalytic activity in degrading phenol, while oxygen defects enhanced it, promoting charge separation, consistent with several other publications [161,162,163,164]. Additionally, He et al. [161] indicated that the photocatalytic efficiency of ZnO decreased with an increase in the calcination temperature, a phenomenon that could be attributed to the simultaneous reduction in oxygen defects. These results align with the work of Wang et al. [164], which showed that the concentration of oxygen defects is strongly dependent on the calcination temperature and that oxygen deficits render ZnO more effective. However, most publications highlighting the highest photocatalytic activity attributed to the catalyst with the highest level of oxygen defects do not account for the specific surface area, which increases in parallel with the increase in oxygen defects. Similarly, several other publications working on ZnO-based composite materials, such as Ag₃PO₄/ZnO [165] and C₃N₄/ZnO [166], also conclude that oxygen defects enhance photocatalytic efficiency and demonstrate that a higher concentration of defects corresponds to a larger surface. Conversely, contrary to the aforementioned suggestions, several other studies report an unfavorable impact of oxygen and zinc defects on ZnO’s photocatalytic activity, showing that they act as recombination centers, limiting its effectiveness [119,167,168].

5. Enhancing the Photocatalytic Efficiency of ZnO

While ZnO exhibits greater photocatalytic potential compared to its bulk counterparts, it is not without noticeable drawbacks. The primary limitations of bare ZnO include its susceptibility to dissolution at an acidic pH, photo corrosion in alkaline solutions under UV irradiation [169], rapid electron–hole recombination, a tendency toward agglomeration, and poor absorption of visible light [170]. Various strategies have been employed to address these drawbacks and enhance the structural stability of ZnO [171], such as doping with metals and non-metals, integrating with other semiconductors, employing surface deposition of conducting metals, and coupling with carbon materials, which will be further elaborated on in the subsequent subsections.

5.1. Doping

Doping, a method involving the introduction of elements into a semiconductor’s lattice, is a common approach to augment the photocatalytic performance of ZnO. This technique effectively reduces the band gap, modifies the energy level structure, enhances electrical conductivity, and broadens the absorption range to include visible light, thereby improving catalytic activity. However, excessive concentrations of dopants can generate physical defects, diminishing the photocatalytic efficiency of ZnO and altering its structure, thereby negating its advantages as an excellent photocatalyst [116].

5.1.1. Non-Metallic Doping

Non-metallic doping, particularly with nitrogen (N) [172,173,174,175], carbon (C) [176,177], fluorine (F) [178,179], and sulfur (S) [180,181], has gained prominence in the quest to enhance the activity of semiconductor photocatalysts. N-doping, notably, has revolutionized the field, following the pioneering work by Asahi’s team [158] demonstrating its efficacy in harnessing visible light utilization by TiO₂. N-doping achieves this by replacing oxygen vacancies with non-metallic ions or inducing defects, thereby reducing the energy band gap of ZnO and significantly expanding its optical response region. Furthermore, N-doping introduces unique characteristics to ZnO, enabling the synthesis of p-type ZnO and effectively narrowing the energy band, making it a notable non-metallic doping element.

Moreover, the incorporation of C into the ZnO lattice has shown remarkable effects, extending carrier lifetime, suppressing recombination rates, improving intrinsic defects, and significantly enhancing visible light capture efficiency. Research by Yu et al. [182] highlighted the effectiveness of C-doping in inhibiting the recombination of photo-generated electron–hole pairs in ZnO, thus providing new active sites and preventing h⁺ and e⁻ complexes. Additionally, the successful preparation of C–ZnO by Islam et al. [183] demonstrated a significant red shift in the UV absorption peak of ZnO post-C doping, enabling the efficient catalytic degradation of pollutants under visible light activation.

In a study by Pan et al. [184], a novel two-step calcination process was introduced, resulting in a porous ZnO structure with exceptional charge transfer and light absorption characteristics, showcasing notable photocatalytic activity. Furthermore, the close match in ionic radii between fluorine (F) and oxygen (O), measuring 1.33 Å and 1.4 Å for F⁻ and O²⁻ ions, respectively, enabled the effective substitution of F⁻ for O²⁻ in the crystal structure, facilitating the formation of n-type ZnO and the release of free electrons within the semiconductor matrix. Rueda-Salaya’s research team [185] investigated the doping of various F ratios into the ZnO lattice, demonstrating outcomes such as suppressed microcrystal growth, reduced oxygen vacancies, a narrowed band gap, and enhanced efficacy in degrading diclofenac (DCF) in aqueous environments, as depicted in Figure 11.

Moreover, boron (B) [186] and phosphorus (P) [187] have also been explored for ZnO doping, exhibiting remarkable performance in pollutant degradation, pathogen inactivation, and solar hydrogen production, among other applications.

5.1.2. Transition Metal Doping

Transition metal doping alters the ZnO lattice, enhancing its ability to separate charge carriers and boosting its photocatalytic performance [188]. By creating new electronic levels in the band gap, it can absorb visible light and facilitate charge transfer within the material [189].

Among the common dopant elements such as cobalt (Co), manganese (Mn), silver (Ag), iron (Fe), gold (Au), and copper (Cu), Co stands out due to its multiple electronic states and high solubility in the ZnO lattice, making it an ideal candidate for doping.

For instance, Co-doped ZnO quantum dots produced by Liu et al. [190] showed promising results in disinfecting Escherichia coli. Increasing Co doping improved light absorption and charge separation, leading to better photocatalytic activity. In another study, Lu et al. [40] synthesized Co-doped ZnO nanorods using a facile hydrothermal process, achieving a 93% degradation of alizarin red dye in 60 min. Similarly, Xiao et al. [191] demonstrated enhanced photocatalytic activity with Co-doped ZnO, effectively degrading MB dye in 300 min. These studies highlight the importance of defects in ZnO for improved photocatalytic performance.

In a different study, researchers developed Cu-doped ZnO through a solvothermal method, showcasing remarkable photocatalytic properties under visible light [192]. The UV–Vis absorption spectrum of Cu-doped ZnO displayed an effective optical performance throughout the visible light spectrum, with a broad tail band spanning approximately 400 nm to 750 nm. Shah’s work [193] on rod-shaped Cu-doped ZnO indicated that increased Cu content narrowed the optical band gap, subsequently enhancing the degradation rate. Cu-doped ZnO nanorods demonstrated efficient photocatalytic activity, with 20 mg showing a photodegradation efficiency of 57.5% for MO and up to 60% for MB. Pawar et al. [194] synthesized Cu-doped ZnO microstructures, with 7% Cu-doped ZnO requiring 50 min for the photocatalytic degradation of MB and RhB dyes (refer to Figure 12). The improved photocatalytic activity was attributed to the presence of (001) polar surfaces, oxygen vacancies, and the increased absorption of visible light. Additionally, Mohan et al. [195] conducted a study on Cu-doped ZnO nanorods synthesized through the vapor transport method, focusing on the UV light-driven photocatalytic degradation of resazurin dye, and attributed the enhancement in photocatalytic activity of Cu-doped ZnO nanorods to the presence of oxygen vacancies.

Furthermore, Li et al. [196] investigated the impact of Ag-doping on the structural, electronic, and optical properties of ZnO NWs using first-principles calculation. The study revealed that Ag dopants preferred substitution at the Zn sites, improving visible photocatalytic performance by introducing an additional absorption peak in the visible region, associated with the excitation of the Ag gap state. This led to a reduction in the gap of ZnO NWs, highlighting the potential application of AgO defects for visible light catalysis.

In their study, Chauhan et al. [197] reported on the photocatalytic dye degradation and antimicrobial activity of pure ZnO and Ag-doped ZnO synthesized using a green method involving the use of Cannabis sativa leaf extract as a reducing and stabilizing agent. The findings revealed that the synthesized Ag-ZnO and ZnO NPs exhibited antimicrobial activity against various human pathogenic bacteria (both Gram-positive and Gram-negative) and different fungal strains. Specifically, Ag-doped ZnO NPs demonstrated the maximum inhibition zone against all bacteria, including Escherichia coli, Klebsiella pneumonia, methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella typhi, and Staphylococcus aureus, while ZnO NPs exhibited the minimum inhibition zone. Furthermore, both ZnO NPs and Ag-doped ZnO NPs effectively inhibited the growth of two different plant pathogenic fungi, Fusarium spp. and Rosellinia necatrix. Additionally, under solar light, Ag-ZnO and ZnO NPs demonstrated the removal of 96% and 38% of Congo red and 94% and 35% of MO in 80 min, as illustrated in Figure 13.

In our previous work [198], we successfully synthesized the homogeneous transition metal-doped ZnO NWs on a Si substrate via hydrothermal method. The doped samples, including ZnO:Fe 3%, ZnO:Ag 2%, and ZnO:Co 2%, demonstrated enhanced efficiency in degrading MO compared to undoped ZnO. The doped samples achieved complete MO degradation within 180 min or less, while the undoped ZnO achieved only 95% degradation in the same time frame, as shown in Figure 14. This improved performance can be attributed to a reduction in bandgap energy, leading to increased photon absorption and more energy for the reaction, as well as an increased number of oxygen vacancies in the nanowire lattices, acting as electron traps. Additionally, the dopant ions on the nanowire surface helped reduce the recombination rate of electron–hole pairs.

Additionally, the doping of Mg [199], Mn [200], and Au [201] led to the optimization of ZnO’s photocatalytic performance.

5.1.3. Rare Earth Metal Doping

Recent research interest in using rare earth (RE) elements to modify ZnO has focused on their exceptional optical properties. RE ions like (La), europium (Eu), neodymium (Nd), samarium (Sm), dysprosium (Dy), praseodymium (Pr), ytterbium (Yb), and cerium (Ce) have been added to ZnO, sparking discussions regarding their impact on ZnO’s photocatalytic performance. Different views exist on the effects of RE doping. Some suggest it leads to lattice expansion, increasing oxygen vacancies but reducing photogenerated hole and electron formation. Others propose that rare earth ions broaden ZnO’s light absorption range, facilitating efficient photon energy transfer.

Studies on La-doped ZnO, including research by Nguyen et al. [202] and others [203,204,205], have shown excellent performance in gas-sensitive and photocatalytic activities. Research into various La-doping concentrations has offered valuable insights into reducing the band gap and enhancing pollutant degradation. For instance, Nguyen et al.’s study revealed a reduced band gap of ZnO upon La³⁺ ion doping, enabling efficient MO dye breakdown using La_0.1Zn_0.9O under visible light [202]. Similarly, Jia et al. [206] found significantly improved photocatalytic efficiency with solvothermally synthesized La³⁺-doped ZnO materials, identifying 2% La as the optimal concentration for the peak degradation of RhB. Additionally, Kumar et al. [207] reported notable effectiveness in degrading MO, RhB dyes, and picric acid (PA) with 1 mol% La-doped ZnO.

Continuing our discussion on RE element doping in ZnO, further research has shown promising outcomes. For instance, incorporating Eu into ZnO nanoparticles led to enhanced photocatalytic performance, with 1.62% Eu-doped nanoparticles achieving 99.3% degradation of methylene blue in just 60 min [208]. Dash et al. [209] found that the breakdown of MO increased with higher Eu levels, reaching 90% efficiency. Franco et al. [210] noted that Eu-doped ZnO, produced through a supercritical anti-solvent process, exhibited excellent mineralization and discoloration, almost entirely removing eriochrome black-T azo dye after 240 min of UV light exposure. In another investigation, Alam et al. [211] used a straightforward sol–gel approach to create ZnO NPs doped with RE metals (La, Nd, Sm, and Dy) and tested their photocatalytic activity in degrading MB and RhB under UV light. Among these, Nd-doped ZnO displayed the highest efficiency, achieving a degradation rate of 98%. Furthermore, investigations explored the photocatalytic efficacy of pure ZnO and ZnO doped with RE elements (La, Ce, Pr, Er, and Yb) for treating pollutants in water and wastewater. The results revealed that Er-, Ce-, Pr-, and, notably, Yb-doped ZnO exhibited higher photoactivity compared to unmodified ZnO and the established benchmark material TiO₂ P25 (refer to Figure 15) [212].

5.2. Co-Doping of ZnO

In addition to single-element doping, the investigation of the simultaneous doping of multiple elements into ZnO has revealed significant potential for enhancing photocatalytic activity. For instance, the simultaneous doping of sulfur (S) and chlorine (Cl) into ZnO resulted in the modification of ZnO with a high density of active sites and reduced charge complexation rate of electron–hole pairs, effectively reducing the optical band gap and substantially increasing the photocatalytic degradation rate compared to pure ZnO [213]. Furthermore, research efforts have highlighted the modification of ZnO’s physicochemical properties through co-doping with rare earth ions. For example, the co-doping of La³⁺ and Ce³⁺ led to the contraction of the ZnO band gap energy, shifting it from UV to visible light absorption, thereby enhancing luminescence and antibacterial properties [214]. Similarly, the co-doping of Ce and Yb induced changes in the crystal structure of ZnO, with variations observed in lattice constants based on the concentrations of the co-dopants [215]. Notably, the nature and concentrations of the doping elements in the composites significantly influenced the photocatalytic activity, especially with excessive co-dopant concentrations affecting carrier recombination before reaching the nano photocatalyst surface. These insights underscore the potential of co-doping strategies in tailoring the photocatalytic properties of ZnO for diverse applications [141].

5.3. Noble Metal Deposition on ZnO

The deposition of noble metal nanoparticles (e.g., Ag, Au, Pd) or their oxides onto ZnO represents a powerful strategy to improve ZnO’s photocatalytic activity by inhibiting the recombination of photogenerated h⁺ and e⁻ pairs. Unlike doping, noble metals deposited on ZnO surfaces have distinct Fermi levels, facilitating electron transfer from the semiconductor to the metal. This process creates a space charge layer that provides additional active sites, thereby prolonging redox reactions and generating more h⁺/e⁻ pairs, ultimately enhancing ZnO’s photocatalytic performance. Although effective, the high cost of precious metals necessitates exploration into economical deposition methods for noble metal-modified ZnO. Research has focused significantly on depositing Ag Nps on ZnO due to their affordability and stability. Despite the inherent difficulty in depositing Ag NPs on ZnO surfaces, various research endeavors have explored deposition methodologies at different temperatures and durations [42,216,217]. Moreover, Ag NPs exhibit efficient antibacterial properties against both Gram-negative and Gram-positive bacteria, although concerns about their cytotoxicity have been raised, prompting investigations into controlled release strategies to mitigate adverse effects [218]. Additionally, porous ZnO–Ag composites have demonstrated promising outcomes in ROS generation under visible light, showing potential in biomedical applications such as wound healing. Similarly, palladium (Pd) integration with ZnO has been explored extensively, driven by Pd’s catalytic prowess in converters and fuel cells. Methods like the one-pot synthesis by Bao et al. [43] have successfully yielded Pd-ZnO composites with exceptional catalytic and recyclability properties. Further innovations, such as microwave hydrothermal synthesis, reported by Veerakumar et al. [219], have led to stable Pd-ZnO nanostars capable of degrading hazardous pesticides under visible light, emphasizing their role in environmental sustainability. Chang et al. [220] also advanced Pd-ZnO integration through deposition and lattice doping, enhancing optical properties and catalytic efficiency for improved environmental remediation strategies. These developments highlight the versatile applications of noble metal-modified ZnO in addressing environmental challenges and promoting public health.

Similarly, the integration of noble metal Au NPs into ZnO has proven highly effective in enhancing its photocatalytic activity through various mechanisms. Au NPs act as effective charge carriers, facilitating interfacial charge transfer and extending light absorption into the visible spectrum, thereby optimizing solar energy utilization [44,221,222]. This has enabled the development of Au-ZnO nanocomposites with enhanced physical and chemical properties suitable for applications such as pollutant degradation (Figure 16), dye-sensitized solar cells, and biosensing, thereby advancing sustainable energy applications [44]. Udawatte et al. [44] demonstrated well-defined Au/ZnO Nps composites by modifying ZnO with preformed Au NPs shielded with bifunctional glutathione ligands. These composites exhibited significantly improved photocatalytic activity, eliminating the need for thermal activation processes in organic substrate breakdown via oxidative and reductive pathways. The photocatalytic efficiency was directly correlated with Au loading, where thiolate-protected Au NPs effectively extracted electrons from photoexcited ZnO, thereby enhancing charge separation and overall catalytic performance. Assessing the degradation of rhodamine 6G (R6G) dye in aqueous media, the absorption profiles during photocatalytic reactions with ZnO and the 2 wt% composite illustrated comparable degradation pathways for R6G (Figure 16a,b). However, the composite achieved complete dye bleaching within 30 min, contrasting with partial decomposition by ZnO alone. Furthermore, the time for 50% degradation of R6G decreased from approximately 8 min with ZnO to 6 min with a 0.4 wt% composite and further to about 2.5 min at 2 wt% Au NPs loading, demonstrating significant enhancement in catalytic activity with increasing Au NPs content. These findings underscore the pivotal role of Au NPs in augmenting the photocatalytic performance of ZnO-based composites, marking a substantial stride in enhancing their practical applicability.

Furthermore, the synergistic effect of bimetallic coupling, as demonstrated by Lee et al. [223] and Zhai et al. [224], showcased enhanced photocatalytic activity compared to monometallic deposition, making it a promising avenue for constructing efficient ZnO-based photocatalysts. These studies highlighted the generation of Schottky barriers at the interfaces of Au–ZnO and Pd–ZnO, resulting in improved photodegradation capabilities.

5.4. Semiconductor Composite

Coupling ZnO with other semiconductors has emerged as a highly promising approach to significantly enhancing its photocatalytic activity. This method involves creating nanocomposites with superior properties, particularly for photocatalysis, by boosting light absorption, suppressing photo-induced electron–hole pair recombination, and promoting increased charge separation. A study by Lin and Chiang showcased how the extended lifetime of charge carriers in nanocomposites, achieved through inter-particle electron transfer between the conduction bands, leads to enhanced photo-degradation reactions [225]. Additionally, the coupling of ZnO with semiconductors of varying band gaps, such as Cds, SnO₂, ZnS, TiO₂, CuO, α-Fe₂O₃, and Co₃O₄, has been extensively investigated, demonstrating a remarkable potential to improve the overall photocatalytic process [45,46,47,48,49,50,51,52,53,54,55,56,226,227,228,229].

For example, Zgura et al. [53] delved into the morphological, structural, optical, and photocatalytic aspects of ZnO and ZnO–CdS composites, uncovering that the introduction of CdS impacts the nucleation growth of ZnO. This was evidenced by the quasi-epitaxial growth of ZnO on the (111) crystal faces of CdS nanoparticles with a cubic crystal structure, resulting in flower-like structures of varying sizes. The intrinsic defect in ZnO–CdS acted as a h⁺/e⁻ trap, leading to a decreased complexation rate of electrons and holes, rendering it significantly more efficient in the degradation of RhB.

Additionally, a study addressing the photodegradation of MO highlighted the favorable performance of nanosized, coupled ZnO/SnO₂ photocatalysts, particularly when the Sn content was approximately 33.3 mol%, emphasizing the pivotal role of material composition in coupled semiconductor photocatalysts [229]. These instances collectively depict the valuable potential of coupling ZnO with other semiconductors to enhance its photocatalytic activity, positioning such systems as crucial in advancing environmental remediation technologies.

At present, a large number of studies have reported different semiconductor deposition methods to modify ZnO to improve its photocatalytic ability. For example, Ranjith et al. [47] reported vertically aligned core–shell ZnO–ZnS nanocomposites that exhibited high photocatalytic activity and excellent dye degradation stability under light irradiation. Upon visible light irradiation, these ZnO-based nanostructures exhibited varying levels of MB decomposition: 64.4%, 96.3%, 98.7%, 98.3%, 98.21%, 93.9%, and 87.4% for ZnO nanorod, ZnO-ZnS nanorod (4 h), ZnO-ZnS nanorod (8 h), ZnO-ZnS nanorod (12 h), ZnO-ZnS nanorod (16 h), and ZnO-ZnS nanorod (20 h) core–shell morphologies, respectively, within 135 min (Figure 17). This demonstrated a consistent trend in catalytic efficiency with varying shell wall thicknesses under both UV and natural sunlight irradiation. The UV–Vis absorption spectra revealed a blue shift at the absorption edge, attributed to the improved charge separation between ZnS and ZnO, a phenomenon further enhanced by the thickness of the shell wall.

In another work, it was reported that the photoluminescence spectra of CuO-coupled ZnO nanomaterials showed characteristic peaks in the visible range, and the composite material improved the absorption in the visible region of the solar spectrum [55]. The increase in the amount of ROS generated under visible light catalysis was effective in killing methicillin-resistant Staphylococcus aureus [56]. The α-Fe₂O₃ and ZnO composites also exhibited photocatalytic degradation and antibacterial properties, which provided a new strategy for the photodynamic antibacterial killing of multi-drug resistant bacteria [54]. The unpaired h⁺ and e⁻ within the surface of the Ag/Fe₂O₃/ZnO heterostructure fabricated by Rahmah et al. [48] using chemical precipitation could generate free radicals at both the CB energy level (superoxide ion) and VB energy level (hydroxyl group), possessing efficient photocatalytic and superhydrophobic properties. ZnO–Ag₃PO₄-WO₃ heterojunctions could improve the effective strategy of photocatalytic activity by improving chemical and structural properties as well as inducing inter-site charge transfer, promoting the utilization of visible light and showing excellent ammonia degradation activity in sunlight [230].

5.5. Coupling with Carbon-Based Materials

Carbon-based materials, including fullerene, carbon nanotubes, graphene materials, and graphitic carbon nitride, play essential roles across various technological applications, particularly in catalysis and water treatment, due to their diverse forms and exceptional properties [231,232]. Among these materials, graphene stands out for its unique hexagonal lattice structure of sp²-bonded carbon atoms, providing high electrical conductivity, mechanical strength, and a large specific surface area. When integrated with ZnO, graphene and its derivatives like graphene oxide (GO) and reduced graphene oxide (RGO) play a pivotal role in enhancing photocatalytic performance by facilitating efficient charge separation at the semiconductor interface [233].

The disparity in Fermi levels between reduced graphene oxide and the oxide semiconductor enables rapid electron transfer to the graphene surface, thereby promoting faster reduction reactions. This mechanism enhances photocatalytic efficiency by improving the separation efficiency of photogenerated electron–hole pairs [234]. Additionally, the π-π stacking interaction between the aromatic rings of graphene and incoming moieties significantly accelerates their adsorption on oxide semiconductors [235].

Recent advancements and research efforts have focused on utilizing various forms of ZnO composites incorporating graphene and GO for organic pollutant degradation. For instance, ZnO/graphene composites prepared via chemical precipitation demonstrated a notable 2.4 times increase in photocatalytic degradation efficiency concerning MB compared to bare ZnO [57]. However, a similar method followed by heat treatment resulted in the formation of multilayer graphene, leading to a reduced surface area and, subsequently, lower photocatalytic activity. On the other hand, graphene loaded onto ZnO nanofibers created via electrospinning with a 0.5% graphene content exhibited excellent photocatalytic efficiency in degrading MB under UV irradiation, attributed to better electron transport facilitated by graphene’s superior electrical conductivity [236]. Moreover, the enhanced photocatalytic activity of ZnO–graphene composites was associated with the robust interaction between ZnO and defect sites of graphene [57]. In another study, a graphene–ZnO composite with a quasi-shell–core structure was proposed for the photocatalytic degradation of RhB [58]. The establishment of an electric field between the ZnO core and the graphene shell layer reduced the charge transfer resistance for photogenerated electrons, enhancing the photocatalytic dye degradation capacity. The composite structure was further modified by coating ZnO nanospheres with reduced graphene oxide, achieved through the electrostatic attraction between positively charged ZnO nanospheres and negatively charged reduced graphene oxide [237]. Notably, approximately 10% reduced graphene oxide exhibited a fivefold enhancement in photoactivity compared to bare ZnO nanospheres, attributed to the decreased recombination of electron/hole pairs and the improved absorption capacity of ultraviolet light. The efficiency of photocatalytic degradation was closely linked to the fate and transfer of photogenerated electrons, with the highly enhanced photoactivity attributed to the substantial interfacial contact between ZnO nanospheres and RGO nanosheets, significantly improving the fate and transfer of photogenerated electron–hole pairs from ZnO.

Based on a study by Nguyen et al. [238], a comparison of the photoactivity of ZnO-rGO with different ZnO morphologies (nanospheres, nanodisks, and nanorods) was conducted for the photocatalytic degradation of dyes (MB and RhB) under UV irradiation. The results indicated that the smallest particle size (15–35 nm) was achieved with ZnO spheres, leading to the rapid transport of excited electrons compared to short rods and disks. Importantly, the ZnO sphere–RGO composites (sZG) exhibited higher adsorption and enhanced photocatalytic degradation of the dyes, as depicted in Figure 18.

In their study, Tayebi et al. [239] also reported how different shapes of ZnO, combined with rGO, affect photocatalytic activity, using MO degradation as a model. They compared ZnO nanoparticles (ZnO NPs), ZnO NP-RGO (GNP), ZnO nanorods (ZnO NRs), and ZnO NR-rGO (GNR) under UV and visible light in Figure 19. The results showed that the GNR composite achieved the highest photocatalytic activity, significantly reducing photo-corrosion to approximately 0.5%, compared to 10%, 35%, and 45% for ZnO/GNP, ZnO NR, and ZnO NP, respectively. This improvement in GNR was attributed to the better movement of photoexcited electron–hole pairs along the ZnO NRs, reducing electron–hole recombination compared to ZnO NPs.

Mohamed et al. [240] synthesized ZnO hollow microsphere–RGO nanocomposites and evaluated their efficacy in degrading 2,4-dichlorophenoxyacetic acid (2,4-D) under direct sunlight irradiation. Their study demonstrated that incorporating varying amounts of graphene oxide (GO) at 0% (ZGO0), 1% (ZGO1), 3% (ZGO2), and 5% (ZGO3) significantly enhanced photocatalytic activity, as depicted in Figure 20. This improvement was attributed to the reduction of electron–hole pair recombination and the modification of ZnO’s bandgap energy from 3.1 eV to 2.7 eV, enabling the absorption of visible light.

In another study, Nenavathu et al. [241] found that adding just 0.09 wt% functionalized graphene oxide nanosheets (FGS) to ZnO significantly boosted photocatalytic activity. They achieved around 94.5% degradation of safranin-T dye under visible light. Figure 21a shows how FGS-ZnO breaks down safranin-T. They tested different amounts of graphene oxide (0.05 wt%, 0.09 wt%, 0.10 wt%, 0.50 wt%, and 5.00 wt%) in FGS-ZnO compared to pristine ZnO nanoparticles for degrading safranin-T, as shown in Figure 21b. The improved performance of the 0.09 wt% FGS-ZnO was because it enhanced dye adsorption, separated charge carriers effectively, and combined FGS and ZnO components well.

Umukoro et al. [242] used Ag-Ag₂O-ZnO/GO nanocomposites as photocatalysts to degrade acid blue 74 dye in synthetic wastewater under visible light. They found that the nanocomposite removed 90% of the dye, compared to 85% with Ag-Ag₂O-ZnO and 75% with ZnO alone (Figure 22a). The nanocomposite’s superior performance was due to several factors: Ag’s ability to absorb light and enhance charge separation, reduced electron–hole pair recombination at the Ag₂O-ZnO interface, and graphene’s efficient electron mobility, facilitating the effective transfer of photogenerated electrons and holes from ZnO to graphene. Figure 22b illustrates the proposed mechanism of visible light photocatalytic activity with Ag-Ag₂O-ZnO/GO.

6. Role of ZnO in Water Treatment

Throughout the preceding sections, our exploration delved into the extraordinary photocatalytic properties of ZnO, meticulously assessing the diverse impacts of various synthesis methods on its catalytic effectiveness. We scrutinized the innovative strategies devised to overcome challenges and enhance ZnO’s photocatalytic prowess. Our in-depth analysis has highlighted the pivotal role of ZnO and ZnO-based photocatalysts in efficiently degrading organic pollutants, notably demonstrated through the impressive degradation of dyes. Moving beyond this foundational role, our focus shifts to illuminating the broader significance of ZnO in water treatment. This section encapsulates the multifaceted contributions of ZnO, from its adept removal of various organic pollutants including agricultural pesticides, pharmaceuticals, and antibiotics to its critical role in water disinfection. By untangling the complex web of ZnO’s involvement in water treatment, we set the stage for a compelling conclusion that magnifies its importance as a cornerstone in the pursuit of sustainable solutions for ensuring access to clean water.

6.1. Removal of Organic Pollutants

The utilization of ZnO in the removal of organic pollutants has emerged as a pivotal strategy in mitigating the severe environmental and health hazards posed by over 80,000 organic–synthetic compounds prevalent in various industrial sectors such as cosmetics, food processing, paper and pulp, textiles, petrochemicals, agrochemicals, and beyond [243]. These pollutants, due to their large-scale production, extensive use, and frequent discharge, significantly elevate the burden on the environment and pose acute and chronic risks to all living things through processes like biomagnification and accumulation [243]. ZnO’s exceptional photophysical properties make it an ideal catalyst for degrading these pollutants, involving a series of mechanisms including the diffusion of pollutants onto ZnO’s surface, their subsequent adsorption, redox reactions in the adsorbed phase, the desorption of the degradation products, and the final elimination of these products from the interface region.

In the context of dye pollutant degradation, ZnO NPs have exhibited exceptional efficacy, as demonstrated in the preceding section of this review. Building upon these foundational findings, Isa et al. [244] introduced an innovative approach by designing ZnO nanoparticles into a micro-flower architecture, resulting in a notable 99% removal rate of 10 mg/L MO within 300 min, utilizing 150 mg of a ZnO catalyst. Similarly, Inderyas et al. [245] reported a significant degradation of fast acid black dye, achieving an 85% removal under UV light in 75 min and a 65% removal under solar irradiation in 240 min.

For agricultural pesticides, ZnO has shown excellence in degrading compounds like phorate, as evidenced by Niranjani and Anchana [246], with degradation occurring within 120 min under solar light. Previous studies have consistently emphasized the significant influence of operational parameters, including pH, catalyst dosage, original pesticide concentration, and the type of synthetic pesticide, on degradation efficiency [247,248,249,250]. For example, Premalatha and Miranda [248] reported the highly efficient ZnO-Bi₂O₃ composite for the degradation of lambda-cyhalothrin (L-CHT), a harmful pyrethroid pesticide, under visible light irradiation. The proposed mechanism for the photocatalytic degradation of L-CHT using the ZnO-Bi₂O₃ composite is shown in Figure 23. Compared to bare ZnO, the ZnO-Bi₂O₃ composite achieved a degradation percentage of 85.7% within 120 min, using a catalyst dosage of 1.2 g/L in a 50 mg/L L-CHT solution. The intermediate products formed during the degradation process were identified as Phthalic acid, butyl 2-ethyl butyl ester, and Diethyl phthalate, with no other hazardous intermediates detected. The researchers investigated the influence of various parameters on the degradation efficiency, including pH, initial L-CHT concentration, and catalyst dosage. The optimum pH for efficient degradation was found to be 7, as higher pH values led to a decrease in degradation due to the repulsion between the negatively charged catalyst surface and the pesticide molecules. Regarding the initial L-CHT concentration, a degradation percentage of 89.91% was achieved with 30 mg/L, which decreased to 62.83% as the concentration increased to 100 mg/L. This was attributed to the increased adsorption of pesticide molecules on the catalyst surface, leading to reduced hydroxyl radical generation and the adsorption of intermediates on the catalyst, reducing the efficiency of active sites. The effect of catalyst dosage was also studied, and the degradation percentage increased linearly with an increase in the catalyst amount from 0.4 g/L to 1.2 g/L but then decreased with a further increase to 1.6 g/L. This was due to the enhanced absorption of photons and adsorption of L-CHT molecules at higher catalyst loadings, which was offset by the increased turbidity of the solution, reducing light penetration. Compared to dark adsorption, photolysis, and ZnO alone, the ZnO-Bi₂O₃ composite exhibited the highest degradation efficiency of 85.7% under the optimized conditions, demonstrating the synergistic effect of the heterojunction formation between ZnO and Bi₂O₃. The study highlighted the excellent performance of ZnO-based composites, such as ZnO-Bi₂O₃, in the photocatalytic degradation of recalcitrant pyrethroid pesticides like lambda-cyhalothrin.

Aulakh et al. [250] reported the synthesis of various ZnO nanostructures, including nanorods, cotton ball-like structures, and nanospheres, as well as their copper-loaded counterparts, for the effective photocatalytic degradation of the pesticide methyl parathion under UV light. The researchers demonstrated that ZnO nanorods proved to be a better catalyst compared to the other synthesized nanostructures, owing to their elongated morphology. The ZnO nanorods were nearly 1.4 times more efficient than the ZnO cotton ball-like structures. Furthermore, the photocatalytic activity was enhanced by the loading of Cu onto the ZnO nanostructures due to the reduced recombination rate of electron–hole pairs. The Cu-loaded ZNR were found to degrade the entire methyl parathion compound by up to 99% within just 80 min, whereas the bare ZnO nanorods took 3 h under similar conditions. This represents a nearly 6-fold increase in efficiency. A plausible mechanism for pesticide degradation proposed in this study is shown in Figure 24.

Furthermore, Khan et al. [247] evaluated the photodegradation activity of Fe-ZnO nanocomposites for the degradation of the organophosphate pesticide chlorpyrifos at different concentrations under UV irradiation. They reported up to 93.5% degradation of 10 ppm chlorpyrifos within 60 min, confirmed by reductions in TOC and Chemical Oxygen Demand (COD) values.

Apart from this, the surface properties of ZnO, such as surface defects and oxygen vacancies, can greatly determine its photocatalytic capability [251]. In a study by Korake et al. [251], the photocatalytic activity of 0.5 mol% La-doped ZnO was investigated for the degradation of the pesticide metasystox. The researchers found that the degradation efficiency of metasystox increased with up to 0.5 mol% La doping but then decreased at higher doping levels. Among the catalysts studied, the 0.5 mol% La-doped ZnO exhibited the highest photocatalytic activity for metasystox degradation. The maximum reduction in metasystox concentration was observed under static conditions at pH 8. Furthermore, a reduction in the COD of metasystox was observed after 150 min of treatment. Cytotoxicological studies on meristematic root tip cells of Allium cepa revealed that the photocatalytically degraded products of metasystox were less toxic compared to the original metasystox compound.

To further elevate the degradation rate, the addition of oxidants, mainly peroxide, can facilitate the dissociation process of pesticides to the safe end-product [252]. Regardless of the reputation of other semiconductors in agricultural pollutant degradation, ZnO has been proposed as the most promising catalyst, even more feasible than its commonly investigated counterpart, TiO₂ [253].

Moreover, ZnO’s versatility extends to the degradation of pharmaceuticals, particularly antibiotics and analgesics. The degradation efficiency is significantly enhanced through various modifications such as metal doping, heterostructure formation, or the incorporation of metal–organic frameworks (MOFs), enabling ZnO to effectively operate under solar light conditions [254]. Similar to other organic pollutants, the degradation rate of different antibiotics and analgesics is influenced by factors such as light sources, temperature, pH, and time [255]. Notably, studies have shown the successful degradation of antibiotics and analgesics including ampicillin, ciprofloxacin, diclofenac, ofloxacin, acetaminophen, paracetamol, and ibuprofen using ZnO photocatalysts in research by Mirzaei et al. [254], Eskandari et al. [256], Malakootian et al. [257], Kaur et al. [258], Thi and Lee [205], Choina et al. [259], and Ranjith Kumar et al. [260].

For example, Eskandari et al. [256] investigated the efficacy of utilizing UVC light and ZnO in the photocatalytic degradation of ciprofloxacin (CIP) in polluted water. They identified the optimal conditions for degrading CIP in a solution with 10 mg/L of the compound at a pH level of 5.0 and ZnO concentration of 0.15 g/L, with stirring at 600 rpm and an exposure time of 140 min. The obtained results maintained a relative standard deviation (RSD) below 7%. Through the kinetic analyses, Eskandari et al. determined pseudo-first-order reaction kinetics with a rate constant of 0.032 min⁻¹, underscoring the effectiveness of this method in CIP degradation within aqueous environments.

Additionally, Malakootian et al. [257] conducted a study on the photocatalytic ozonation degradation of CIP using ZnO nanoparticles immobilized on the surface of stones. The research aimed to explore the efficacy of photocatalytic ozonation for eliminating ciprofloxacin from aquatic environments and optimizing the key parameters of the process. The study revealed that the highest removal efficiency of CIP was achieved at specific optimal conditions: pH of 7, reaction time of 30 min, photocatalyst concentration of 3 g/L, and initial CIP concentration of 10 mg/L. The process yielded an impressive removal efficiency of 96% under these optimized conditions. Moreover, linear kinetic models demonstrated that the degradation process followed pseudo-first-order and Langmuir–Hinshelwood kinetics. The study concluded that this method exhibited a high removal efficiency and was well-suited for removing CIP from aquatic environments.

In their study, Kaur et al. [258] investigated the use of silver-modified ZnO nanoplates as an efficient photocatalyst for the degradation of the antibiotic drug ofloxacin under solar irradiation. Their comprehensive experiments demonstrated that silver-modified ZnO at a concentration of 0.25 g/L achieved approximately 98% photocatalytic degradation of ofloxacin (initially at 10 mg/L, pH 7) within 150 min under solar illumination. Notably, the prepared silver-modified ZnO exhibited significantly higher photocatalytic activity compared to bare ZnO. This enhancement in photocatalytic activity could be attributed to efficient charge separation as well as the role of silver nanoparticles as electron sinks on the ZnO surface. Furthermore, the photocatalytic degradation process of ofloxacin conformed well to a pseudo-first-order kinetic model.

Thi and Lee [205] detailed the effective photocatalytic degradation of paracetamol using La-doped ZnO photocatalyst under visible light irradiation. In their study, they applied La-doped ZnO photocatalysts to treat 100 mg/L paracetamol in an aqueous solution under visible light irradiation, achieving a remarkable degradation efficiency of 99% and total organic carbon (TOC) removal of 85% after 3 h. Specifically, the 1.0 wt% La-doped ZnO photocatalyst exhibited the highest photocatalytic activity for paracetamol degradation.

Choina et al. [259] reported the influence of the textural properties of ZnO nanoparticles on the adsorption and photocatalytic remediation of water from pharmaceuticals. Two types of zinc oxide nanoparticle photocatalysts were prepared and characterized. They were highly active in the photocatalytic degradation of tetracycline and ibuprofen. Smaller particles showed a somewhat better performance in the abatement of the drugs due to the increased specific surface area. Although the larger 100 nm sized ZnO nanoparticles had distinctly lower specific surface areas, their activity was relatively high compared to smaller ones. This finding was attributed to improved adsorption properties that facilitated the photocatalytic conversion. However, they deactivated faster at higher drug concentrations. Enhanced adsorption of tetracycline at the catalyst surface was observed at low substrate and catalyst concentrations compared to high concentrations, indicating different photocatalytic behaviors of high and low pollutant concentrations.

Ranjith Kumar et al. [260] delved into the impacts of N ion implantation in vertically aligned ZnO nanorod arrays and its effect on the photocatalytic degradation of acetaminophen. Their analysis of the ZnO nanorods’ X-ray diffraction revealed a wurtzite structure with a prominent (002) diffraction peak that exhibited a slight shift after N-ion implantation. Additionally, field emission scanning electron microscopy images of the NRAs displayed a length of approximately 4 μm and a diameter of roughly 150 nm. Furthermore, UV–visible spectroscopy unveiled that the band gap of pristine ZnO nanorod arrays decreased from 3.2 to 2.18 eV after N-ion implantation. Interestingly, under visible irradiation, the N-ion-implanted ZnO catalyst showcased a significant increase in the photocatalytic degradation of acetaminophen, elevating the degradation efficiency from 60.0% to 98.46% over a span of 120 min.

Additionally, ZnO has demonstrated its capability in degrading stimulants such as ketamine, amphetamine, morphine, and caffeine. For example, Lin et al. [261] reported a near-complete removal of methamphetamine, ketamine, and morphine found in the influent and effluent of domestic treatment plants, as well as in rivers, within 30 min using ZnO. Their study compared the degradation rates of different contaminants under varying light conditions and catalyst loadings, plotting the fitted pseudo-first order rate constants and calculating the half-lives for each target compound. The results presented in

Table 2. Fitted pseudo-first-order rate constants for contaminant degradation under various conditions. Reprinted with permission from [261]. Copyright © 2013, Springer Nature.

Compound and System	Catalyst Dosage (g/L)	Removal Efficiency (%) in 30 min	Reaction Rate, k (min−1)	t1/2 (min)
Ketamine, UV lamp/ZnO	0.01	98.5	0.14	5.90
	0.04	98.7	0.15	5.30
	0.05	99.9	0.23	3.83
	0.10	99.9	0.26	3.11
	0.40	99.9	0.43	2.25
	0.70	99.9	0.25	3.54
	1.00	99.9	0.27	2.72
Methamphetamine, UV lamp/ZnO	0.01	81.4	0.06	14.76
	0.04	98.8	0.15	7.46
	0.05	99.9	0.23	6.27
	0.10	99.9	0.32	4.70
	0.40	99.9	0.38	2.58
	0.70	99.9	0.14	5.44
	1.00	99.9	0.22	3.53
Morphine, UV lamp/ZnO	0.01	99.7	0.19	4.26
	0.04	99.9	0.21	3.61
	0.05	99.9	0.21	3.39
	0.10	99.9	0.37	2.38
	0.40	99.9	0.68	1.06
	0.70	99.9	0.48	1.52
	1.00	99.9	0.60	1.14

demonstrate the degradation efficiency of the three drugs under UV light at 254 nm with different ZnO catalyst dosages, revealing that morphine was most rapidly removed, while methamphetamine and ketamine were removed at roughly equal rates. Direct photolytic results further indicated that morphine exhibited the highest removal rate owing to its higher UV light absorption, facilitating more extensive degradation compared to ketamine and methamphetamine. Consequently, morphine showed the highest removal rate both by photocatalysis and photolysis, with ZnO proving to be a more effective catalyst for ketamine degradation than methamphetamine. Calculated times to achieve a 3-log reduction efficiency for each drug, detailed in

Table 3. Estimated time to achieve 3-log destruction efficiency through photocatalysis. Reprinted with permission from [261]. Copyright © 2013, Springer Nature.

System	Dosage (g/L)	Ketamine (min)	Methamphetamine (min)	Morphine (min)
UV lamp/ZnO	0.4	17	19	10
UVLED/ZnO	0.4	48	95	34

, clearly illustrate that morphine’s removal was the most rapid among the three drugs.

In their work, Elhalil et al. [262] presented a detailed study on Mg-ZnO-Al₂O₃ photocatalysts, achieving a remarkable 98.9% degradation rate of caffeine, one of the most consumed psychoactive substances, in just 70 min of irradiation. They meticulously explored various experimental parameters including doping amount, irradiation time, photocatalyst dosage, initial solution pH, caffeine concentration, and the photocatalyst’s reuse potential. The results indicated that Mg-ZnO-Al₂O₃ significantly outperformed undoped pure ZnO and commercial Degussa P-25 TiO₂ in terms of degradation efficiency. Crucially, the photocatalyst exhibited good stability and maintained its effectiveness after three regeneration cycles, showcasing its reusability and robustness.

6.2. Disinfection of Water

ZnO is one of the nanomaterials that has attracted significant research interest in recent years as an antibacterial agent, given the limitations of conventional disinfection methods such as chlorination, ultraviolet treatment, and ozonation. Chlorination proves ineffective for some highly resistant waterborne pathogens and also has a tendency to form carcinogenic disinfection by-product DBPs when chlorine is added to water [263]. Similarly, while ozonation forms fewer by-products, it is more costly than chemical disinfection and can form harmful bromate when ozone reacts with bromide ions in water. UV treatment also does not leave a residual in treated water, offering no protection against re-infection in the distribution network [264]. Along with other inorganic oxides such as Ag NPs and TiO₂, ZnO has attracted interest because it exhibits strong antibacterial activity even in small amounts [265]. ZnO is regarded as a good antibacterial agent because it is stable under harsh processing conditions and is considered safe for humans and animals. Compared to organic materials, inorganic materials like ZnO have greater durability, selectivity, and heat resistance [266].

Several mechanisms have been proposed for the antimicrobial activity of ZnO nanoparticles:

• Release of Reactive Oxygen Species (ROS): The antimicrobial activity of ZnO nanoparticles involves the release of oxygen species from the surface of ZnO, which cause fatal damage to microorganisms. ROS are known to cause oxidative stress by damaging DNA, cell membranes, and cellular proteins. The rupture of the cell wall is due to the surface activity of ZnO, which causes the decomposition of the cell wall and subsequently the cell membrane, the leakage of cell contents, and, eventually, cell death [266,267].
• Release of Zn²⁺ Ions: Another possible mechanism of ZnO antibacterial activity is the release of Zn²⁺ ions, which can damage the cell membrane and penetrate the intracellular contents. Studies have suggested that the toxicity of nano ZnO against Escherichia coli and Saccharomyces cerevisiae could result from the solubility of the Zn²⁺ ions in the microorganism-containing medium [268].
• Direct Contact of Nanoparticles with Cell Membrane: Cell damage does not necessarily result from the entry of the metal oxide particles into the cell. More importantly, the contact between the bacterial cell and the particle causes changes in the microenvironment within the contact area of the organism and particle. This can lead to increased membrane permeability and the subsequent cellular internalization of the nanoparticles [269,270].

Other research has also explored the role of photocatalytic activity in the antimicrobial properties of ZnO. By absorbing UV light, which activates its interaction with bacteria, ZnO nanoparticle suspensions can produce ROS such as H₂O₂, which have a phototoxic effect on bacteria [271]. However, studies have shown that inhibition could occur even in dark conditions, suggesting that mechanisms other than photocatalysis, such as oxidative stress and the destruction of membrane integrity, may also contribute to the antibacterial activity of ZnO [272]. The size and concentration of ZnO nanoparticles have also been found to play a crucial role in their disinfection efficiency [272,273,274]. Smaller nanoparticles with a higher surface area-to-volume ratio tend to exhibit enhanced antibacterial activity. Additionally, the photocatalytic efficiency of ZnO has been shown to increase with reduced particle size due to the quantum confinement effect [274].

In order to achieve a thorough comprehension of ZnO’s antimicrobial activity, conducting an extensive review of the literature is imperative. Additionally, it is important to note that antimicrobial activity refers to the process by which microbial growth is either inhibited or destroyed.

In our recent groundbreaking research led by Abou Zeid et al. [28], we presented a noteworthy contribution regarding the antibacterial efficacy of ZnO NWs against Pseudomonas putida, a significant Gram-negative bacterium. Our study meticulously investigated the antibacterial activity of ZnO NWs synthesized via hydrothermal methods in both dark and UV light conditions, with detailed insights illustrated in Figure 25a,b. Through comprehensive experimentation, we unequivocally highlighted that the unique morphology of ZnO NWs, achieved through hydrothermal growth, possesses the capability to disrupt bacterial cells, thereby effectively inhibiting bacterial growth. Furthermore, our research highlighted the release of Zn²⁺ ions from ZnO NWs as a key factor contributing to their antibacterial properties. These released ions demonstrate the potential to disturb the bacterial cell membrane, consequently impeding bacterial growth. Additionally, the generation of ROS under UV light further enhanced the antibacterial activity of the ZnO NWs. The proposed mechanism detailing these intricate interactions is clearly elucidated in Figure 25c, providing a comprehensive understanding of the multifaceted antibacterial mechanisms exhibited by ZnO NWs.

In their study, Baruah et al. [114] conducted a comprehensive examination focusing on the potential applications of photocatalysis as a promising approach for water disinfection. They detailed the growth of ZnO nanorods on a paper support crafted from softwood pulp. The study investigated the photocatalytic activity and antibacterial properties of a paper sheet embedded with ZnO nanorods within its porous matrix. The ZnO nanorods were securely anchored to cellulose fibers, enabling the reuse of the photocatalytic paper samples multiple times with only a nominal decrease in efficiency. The research findings revealed that the paper filled with ZnO nanorods exhibited significant photodegradation, with up to 93% degradation observed for methylene blue upon visible light irradiation at 963 W/m² for 120 min. Similarly, a photodegradation rate of approximately 35% was noted for MO under the same conditions. Antibacterial tests showcased the inhibitory effect of the photocatalytic paper on the growth of Escherichia coli under ambient lighting conditions. The zone of inhibition, indicating the absence of viable bacterial cells upon visible light exposure, demonstrated the immobilization effect facilitated by photocatalysis. The study observed that bacteria were immobilized on the surface of the photocatalytic paper, preventing their proliferation in the vicinity of the ZnO-treated paper. The results of the antibacterial experiments, depicted in Figure 26, highlighted the efficacy of the photocatalytic paper in inhibiting bacterial growth. The study concluded that photocatalytic paper presents an appealing functional membrane for potential applications in water purification purposes.

In a study by Navale et al. [271], the authors investigated the antimicrobial properties and mechanistic activities of synthesized ZnO NPs. The nanoparticles exhibited an average size of 20–25 nm and were subjected to concentrations ranging from 0 to 100 μg/mL. These concentrations were tested against four representative microorganisms: the bacterial pathogens Staphylococcus aureus (Gram-positive) and Salmonella typhimurium (Gram-negative), as well as the fungal plant pathogens Aspergillus flavus and Aspergillus fumigatus. The minimum inhibitory concentration (MIC), defined as the lowest concentration at which no visible growth of the cells was observed, was determined using a method recommended by the Clinical and Laboratory Standards Institute (formerly the National Committee for Clinical Laboratory Standards). Bacterial cells were incubated at 37 °C for 24 h with various concentrations of ZnO nanoparticles (20–100 μg/mL), and the MIC was found to be 40 μg/mL. The antifungal activity was assessed using the broth dilution method, where the fungal strains were incubated with varying ZnO concentrations for 24–72 h. It was reported that both the antibacterial and antifungal activities, as well as the viability of the microorganisms, increased with the rising ZnO concentration.

Furthermore, Narayanan et al. [275] conducted a study to evaluate the antimicrobial efficacy of ZnO against human pathogens, including Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Enterococcus faecalis, and Pseudomonas aeruginosa, using the well diffusion method. The study focused on determining the zone of inhibition (ZOI) to assess the antibacterial activity of ZnO nanoparticles at varying concentrations (20–100 μg/mL). The results of the study revealed that even at a lower concentration of 20 μg/mL, ZnO nanoparticles demonstrated robust antibacterial activity against all tested pathogens, including both Gram-negative and Gram-positive bacteria. In a study by Tayel et al. [276], the antimicrobial activity of ZnO was investigated on nine bacterial strains, encompassing both Gram-positive and Gram-negative types, with a focus on foodborne pathogenic strains. The research revealed that Gram-positive bacteria exhibited higher susceptibility to ZnO due to their comparatively simpler cell membrane structure. In contrast, the thicker cell membrane of Gram-negative bacteria was found to potentially impede ZnO penetration into the cells and hinder its interaction with internal bacterial components. Among the strains examined, Bacillus cereus displayed the highest sensitivity, while Pseudomonas spp. was identified as the most resistant. Notably, the exposure of Staphylococcus aureus to ZnO nanoparticles resulted in cell explosion or lysis after a 2 h exposure, with complete disappearance occurring after 4 h of exposure. These promising findings led the researchers to recommend ZnO as a preservative agent against foodborne pathogens, with a caveat to confirm its biosafety before implementation. In a study by Barnes et al. [277], the photocatalytic antibacterial activity of ZnO was evaluated on four bacteria, including two Gram-positive species (S. aureus and B. subtilis) and two Gram-negative species (E. coli and P. aeruginosa). The primary objective of the study was to establish a correlation between the antimicrobial action of ZnO and the generation of ROS. The findings revealed that two of the bacterial species exhibited susceptibility to ZnO nanoparticles even under dark conditions, indicating the potent antibacterial properties of ZnO. Furthermore, the study determined that there was no observed association between the type of bacteria cell wall and its inactivation by ZnO, suggesting a broad-spectrum antibacterial effect, irrespective of the bacterial cell wall type.

The antifungal activity of ZnO nanoparticles was studied by He et al. [278] on two pathogenic fungi, Botrytis cinerea and Penicillium expansum, using the agar dilution method at various ZnO concentrations (0, 3, 6, and 12 mM) by measuring fungal colonies. The results indicated that ZnO nanoparticles greater than 3 mM significantly inhibited the growth of the two fungi, and increased with increasing concentration. Compared to B. cinerea, P. expansum was more sensitive to ZnO nanoparticle treatment, which resulted from the difference in the growth morphologies of the two fungi. Further, the results suggested a different mechanism of the inhibitory effect of ZnO nanoparticles on fungi compared to that on bacteria.

Zhang et al. [279] conducted a study on the characteristics, dispersion, and stability of commercial metal oxide nanoparticles, including Fe₂O₃, ZnO, NiO, and SiO₂, in water, as well as their removal by water treatment processes. The investigation revealed that, with the exception of silica, these nanoparticles exhibited rapid aggregation in tap water, and they proved resistant to disaggregation via ultrasound or chemical dispersants. Furthermore, the study demonstrated that alum coagulation was only able to remove less than 80% of the total mass of nanoparticles. Additionally, the research findings indicated that the behavior of metal oxide nanoparticles in water is intricately linked to their physical properties and their interactions with other components present in the water.

In their investigation into the stability of three commercially available metal oxide nanoparticles (TiO₂, ZnO, and SiO₂) in aqueous solutions, Tso et al. [280] discovered that the nanoparticles rapidly aggregated and settled in pure water. Upon experimentation, ultrasonication emerged as the most effective method for dispersing the nanoparticles in water, thereby overcoming their tendency to aggregate. The study’s findings highlighted the varying stability of nanoparticles under different water conditions. Notably, in samples of lake water and wastewater, the aggregation of nanoparticles occurred at a slower rate compared to pure water. This phenomenon was attributed to the presence of organic colloids such as humic compounds and surfactants, which acted to hinder the rapid aggregation of the nanoparticles. The findings align with the research conducted by Li et al. [62], elucidating the influence of water chemistry on the toxicity of ZnO NPs in aqueous environments. Li and colleagues undertook a comprehensive investigation into the toxicity of ZnO NPs to E. coli, with a specific focus on understanding how water chemistry can modulate this toxicity. Their study aimed to uncover the mechanisms responsible for the toxic effects of ZnO nanoparticles on E. coli and investigate how the composition of the medium might influence these effects. The research findings from Li et al. shed significant light on the intricate and dynamic interactions between ZnO NPs and bacteria under diverse environmental conditions. Notably, the study highlighted the nuanced influence of various medium components on the toxicity of NPs in aqueous systems, providing valuable insights into the complex interplay between water chemistry and the impact of ZnO nanoparticles. Consequently, this work contributed substantially to enhancing the broader understanding of how environmental factors, particularly water chemistry, can dynamically modulate the behavior and impact of ZnO NPs, thereby emphasizing the critical importance of contextual environmental considerations in the assessment and management of nanoparticle toxicity [62].

7. Conclusions and Future Perspectives

In conclusion, this comprehensive review meticulously examined the progress in ZnO-based photocatalysts for water treatment, highlighting their diverse properties and burgeoning trends with the potential to reshape environmental restoration practices. The intricate interplay between ZnO’s inherent characteristics and the innovative enhancements discussed underscores their adaptability and efficacy in addressing the intricate challenges posed by water pollution. The extensive characterization of ZnO presented in this review, spanning structural, optical, electrical, magnetic, mechanical, thermal, photocatalytic, and antibacterial properties, as well as cost-effectiveness and various synthesis methods, collectively demonstrates the versatility of ZnO in enabling efficient and sustainable water treatment across diverse applications.

Moreover, this review delved into strategies that bolster the stability, affordability, and overall sustainability of ZnO-based photocatalysts, charting a course toward the implementation of large-scale solutions for clean water provision. By exploring methodologies to optimize water treatment efficiency, acknowledging the crucial balance between effectiveness and environmental responsibility, and addressing toxicity concerns related to modified ZnO-based materials, this review illuminates a roadmap for the evolution of advanced water treatment technologies. The insightful findings presented not only refine the present comprehension of ZnO-based photocatalysts but also unveil promising future research avenues and practical applications, paving the way for innovative, eco-friendly, and efficient water treatment modalities.

This extended review collects and comments on results reported in the literature, providing the scientific community with an up-to-date understanding of the topic. The collaborative endeavors and forward-thinking research directions articulated herein underscore the transformative potential of leveraging ZnO-based photocatalysts to establish widespread and effective water treatment solutions on a global scale. In essence, this review significantly contributes to the preservation of vital water resources for present and future generations, underscoring the pivotal role of advanced materials in upholding environmental integrity and public health.

Looking ahead, this review sets the stage for future research directions and practical applications in the realm of ZnO-based photocatalysts for water treatment. These perspectives encompass a wide array of opportunities, including innovative synthesis techniques and surface functionalization to tailor the properties of ZnO-based photocatalysts for specific water treatment applications. Additionally, the exploration of advanced composite materials and heterojunctions is crucial for enhancing the efficiency and performance of ZnO-based photocatalysts.

Environmental fate and impact studies are paramount to ensure the safe and sustainable application of ZnO-based photocatalysts, while techno-economic assessments will provide insights into the viability and scalability of these technologies. Collaboration between researchers, policymakers, and regulatory bodies is essential for the development of comprehensive frameworks for the safe implementation of ZnO-based photocatalysts. Engaging communities and stakeholders, integrating these technologies into industrial water treatment systems, and promoting continuous innovation and education are keys to advancing the field of ZnO-based photocatalysts for water treatment.

These future perspectives aim to foster a holistic approach to water treatment, leveraging the transformative potential of ZnO-based photocatalysts to address global water pollution challenges sustainably and effectively. The realization of these research directions holds promise for innovative, eco-friendly, and efficient water treatment solutions that contribute to the preservation of vital water resources and the enhancement of environmental and public health on a global scale.