Photocatalytic Activity of Metal- and Non-Metal-Anchored ZnO and TiO₂ Nanocatalysts for Advanced Photocatalysis: Comparative Study

Abstract

This review article provides useful information on TiO₂ and ZnO photocatalysts and their derivatives in removing organic contaminants such as dyes, hydrocarbons, pesticides, etc. Also, the reaction mechanisms of TiO₂ and ZnO photocatalysts and their derivatives were investigated. In addition, the impact of adding metallic (e.g., Ag, Co, Pt, Pd, Cu, Au, and Ni) and non-metallic (e.g., C, N, O, and S) cocatalysts to their structure on the photodegradation efficiency of organic compounds was thoroughly studied. Moreover, the advantages and disadvantages of various synthesis procedures of ZnO and TiO₂ nanocatalysts were discussed and compared. Furthermore, the impact of photocatalyst dosage, photocatalyst structure, contaminant concentration, pH, light intensity and wavelength, temperature, and reaction time on the photodegradation efficiency were studied. According to previous studies, adding metallic and non-metallic cocatalysts to the TiO₂ and ZnO structure led to a remarkable enhancement in their stability and reusability. In addition, metallic and non-metallic cocatalysts attached to TiO₂ and ZnO demonstrated remarkable photocatalytic efficiency in removing organic contaminants.

1. Introduction

Water scarcity has recently become a major problem owing to global warming, rapid industrial growth, depletion of water resources, environmental pollution, and uncontrolled development of groundwater [1,2]. The rapid industrial growth after the Industrial Revolution has remarkably increased the standard of living, but it has resulted in threats to human health [3]. Because of the development and growth of industries, industrial effluents are becoming more polluted and difficult to treat. The release of organic compounds and chemical fertilizers from various industries has polluted river water and has increasingly become a global pollution problem [4]. Moreover, it is difficult to completely remove non-degradable organic contaminants using biological treatment technologies [5]. Biological processes are safe, economical, and reliable, but the removal percentage of suspended solids is low, so they need better operation management [6]. The coagulation and precipitation techniques can suspend solid particles with the formation of flocs after adding polymeric coagulants and/or inorganic coagulants (such as Al, Fe, etc.). Coagulants bind solids together to form larger particles (flocs), then the flocs settle and separate from the effluent. This method has a high purification performance, but utilizing chemicals and generating biological sludge causes blockage of pipes and water deterioration, thus limiting the use of this process [7,8]. In the Fenton oxidation process, organic compounds are broken down using a strong oxidizer reagent. OH radicals can be produced by oxidizer reagents through a reaction between H₂O₂ and iron salts. This process is easy to apply, and additional equipment is not utilized excessively in comparison to other oxidation processes or photo-oxidation processes. However, this process has some drawbacks such as sludge production and high operating costs for secondary processes [9,10].

An advanced water treatment process can purify water at a high rate and effectively eliminate pollutants [11]. Advanced oxidation technology utilizes various techniques for the enhancement of oxidation power. For efficient water refinement, various contaminants must be removed economically [12]. Generally, various active species can play a key role in the photocatalytic reaction, including superoxide radicals (O₂^•−), holes (h⁺), and hydroxyl radicals (^•OH). These compounds play a crucial role in the efficient degradation of organic pollutants [13]. Advanced technology using TiO₂ and ZnO photocatalysts has attracted the most attention for producing OH^• using optical energy without additional chemicals. Also, the operating cost of this technique may be significantly decreased by solar energy. The low photocatalytic performance of ZnO and TiO₂ may limit the practical application of this advanced technology to treat wastewater [14]. Numerous studies have been performed to modify their surface by metal and non-metal cocatalysts for their possible practical utilization [15].

Various analyses are used to characterize the structure of ZnO and TiO₂ nanocatalysts, including XRD for determining crystal phases and crystal size, AFM for determining refractive index profiles, FTIR for specifying the functional groups, and EDX for determining the constituents. All these features are critical in the photocatalytic reaction. Also, SEM and TEM analyses are used to characterize the morphology and appearance structure of a catalyst [16,17]. Figure 1 demonstrates SEM and TEM photos of ZnO and TiO₂ nanocatalysts. As shown, ZnO and TiO₂ have various morphologies, shapes, and sizes.

TiO₂ and ZnO photocatalysts are extensively utilized for photodegradation of contaminants under UV light or sunlight. However, owing to their band gap range, they have limitations for the photodegradation of pollutants. For doing so, TiO₂ and ZnO photocatalysts can be combined with other compounds to synthesize efficient photocatalysts, some of which include La/TiO₂ [20], CuO/WO₃/TiO₂ [21], Fe₃O₄/TiO₂ [22], carbon-anchored TiO₂ [23], potassium titanate (K₂Ti₆O₁₃) [24], rGO/Fe₃O₄/ZnO [25], tungsten/silver/ZnO [26], Pb/ZnO, Cd/ZnO, Ag/ZnO [27], and Bi(12)ZnO(20) [28]. There are two common strategies for the synthesis of catalysts, including doping and heterojunction. Belousov and coworkers compared the photocatalytic activity of Bi₂WO₆ for water treatment. To prepare the catalyst, they used two methods including doping and heterojunction. Using the hydrothermal process, they synthesized Bi₂W_xMo_1–xO₆ through the doping strategy and Bi₂WO₆/g-C₃N₄ through the heterojunction method, and then employed them for the photodegradation of methylene blue. According to their results, the highest photodegradation efficiency of methylene blue was obtained using the catalyst prepared by the doping strategy. Overall, the attractiveness of the doping technique was seen in terms of photocatalytic efficiency, turnover number, and turnover frequency compared to the heterojunction method [29]. In this review article, the performance of the doping strategy for the development of ZnO and TiO₂ catalysts in the removal of organic pollutants was investigated.

This paper focuses on comparing the photocatalytic capabilities of TiO₂ and ZnO and their composites in the degradation of organic compounds in wastewater. Also, the advantages, disadvantages, and structural features of these photocatalysts were thoroughly investigated. Moreover, the photodegradation mechanism of contaminants using TiO₂ and ZnO as well as metal and non-metal cocatalysts anchored to TiO₂ and ZnO were completely discussed. Furthermore, the photodegradation efficiency of different contaminants using these photocatalysts and their derivatives (metallic and non-metallic cocatalysts in their structure) were compared. One of the main features of catalysts is their stability in the photocatalytic reaction, so the recyclability of TiO₂ and ZnO and their composites were fully studied. Therefore, this article makes a comprehensive comparison between these two catalysts and their composites in the photodegradation of organic pollutants, their photocatalytic efficiency, mechanisms, and reusability, which have not been scrutinized in previous review articles.

2. Organic Pollutants

Municipal sewage pollutants mainly include organic substances, a variety of pathogenic microorganisms, and so on, with complex structures such as drugs, antioxidants, polycyclic aromatic hydrocarbons, pharmaceutical intermediates, food additives, PPCPs, steroids, and phenolic compounds [30]. Also, pollutants in industrial wastewaters include gas condensates [5], petroleum products, non-chlorinated compounds, petroleum hydrocarbons, trinitrotoluene, and polycyclic aromatic hydrocarbon [31]. Hydrocarbon contaminants can enter wastewater through various industries such as oil and gas refineries, urban waste, dairy, the food industry, and slaughterhouses, and cause problems for human health and the environment. These pollutants, in addition to disrupting biodiversity, can directly or indirectly enter human food chains, reduce the quality of life, and cause many diseases. This type of wastewater contains aromatic hydrocarbons, dissolved gases, dispersed oil, inorganic salts and acids, suspended particles, ketones, and phenols. This type of wastewater is available in the form of oil/water emulsion [3,32]. The discharge of these wastewaters to sea must be performed under certain conditions. The highest permissible concentration of hydrocarbon compounds for discharge into the sea should be 40 ppm, and TDS less than 32,000 ppm [32].

Another kind of contaminant in wastewater is volatile organic compounds (VOCs), which have destructive effects on human health. Volatile organic pollutants are categorized into three groups, including semi-volatile organic compounds (SVOCs), volatile organic compounds (VOCs), and very volatile organic compounds (VVOCs). Propane and butane are some VVOCs which are poisonous compounds in very low concentrations. VOCs are also toxic compounds found in household products and the environment, including isopropyl alcohol, acetone, formaldehyde, toluene, vinyl chloride, and hexanal. The molecular weight and boiling point of SVOCs are higher than those of VOCs and can evaporate at room temperature. Pesticides such as chlordane and plasticizers such as phthalates are some of the SVOCs [33]. Pollution caused by pesticides disrupts the functioning of aquatic ecosystems, decreases biodiversity, reduces food sources, interferes with the concentration of dissolved oxygen, destroys the habitats of aquatic organisms, and increases the amount of biochemical oxygen demand (BOD) [34]. The utmost allowable limit of organic compounds for drainage into the river for utilization in agricultural irrigation should be less than 200 ppm [35].

The main source of VOCs includes incomplete burning of fossil fuels, solvents used in inks and paints (e.g., acetone, ethyl acetate, and glycol ether), utilizing biofuels (e.g., cooking oil and bioethanol), biomass combustion, especially from agricultural residuals, and VOCs released from metal working fluids [33]. The presence of these contaminants in sewages results in various problems for the environment. The important symptoms of exposure to these compounds include dizziness, headache, and in acute cases, it can lead to anesthesia. In more severe cases, respiratory and circulatory disorders can lead to death [33].

Also, tetracycline, sulfamethoxazole, cefixime, amoxicillin, gentamicin, erythromycin, and ciprofloxacin are kinds of antibiotics that are present in hospital effluents. The presence of antibiotics in effluents in low and high concentrations is dangerous and causes serious damage to human health. The presence of antibiotics in wastewater has detrimental impacts on human health and the aquatic environment owing to their toxicity, genotoxicity, and mutagenicity. According to reports, the concentration of antibiotics in surface and groundwater reaches 100 micrograms per liter. Also, the concentration of amoxicillin in raw effluent and urban wastewater treatment is 171 mg/L and 13 ng/L, respectively [36]. The acceptable standard of WHO for antibiotics in wastewater is 1 mg/L [37]. Therefore, their concentrations in wastewater should be attenuated.

Dyes are another kind of organic compound which are utilized in many industries like food, plastics, paper, cosmetics, and textiles. Among them, the most abundant dyes are related to the textile industry, and these dyes can enter the wastewater [38]. More than 7 × 10⁵ tons of dyes are produced annually, 10–15% of which enters the sewage during the dyeing process. Water pollution by dyes results in many environmental problems. The highest allowable limit of dyes in industrial discharge is in the range of 0.01–0.05 mg/L. Different groups of dyes are considered carcinogens or mutants. Dyes can prevent sunlight from entering the water and disrupt biological process in the water. Most dyes are poisonous to organisms and have adverse impacts on the aquatic life and the entire ecosystem. The presence of dye in water causes reproductive dysfunction, kidney damage, central nervous system damage, liver damage, brain damage, carcinogenesis, and mutagenicity. Basic dyes are cationic dyes and most of them are crystalline compounds, which can be derived from positively charged sulfur or nitrogen atoms. Under visible and UV light irradiation, dyes have high stability and insensitivity and do not degrade without photocatalysts [39,40,41]. Cationic, anionic, and non-ionic are three important categories of dyes, among which the toxicity of cationic dyes is more than the other two categories. There are various types of dyes in sewage, including crystal violet, Congo Red (CR), methyl orange (MO), reactive orange, reactive black, reactive red, methylene blue (MB), methyl violet, brilliant cresyl blue, and Safranin-T that may be generated by the plastic, leather, food dye, printing, paper, and textile industries [42,43]. Figure 2 illustrates different categories of organic compounds with some examples.

3. Various Processes for Wastewater Treatment

There are different processes for wastewater treatment, including the photocatalytic process, adsorption, coagulation, ion exchange, electrochemical, electrocoagulation, chemical deposition, and membrane technology [1,3]. Figure 3 demonstrates various processes to eliminate pollutants. Also, the advantages and disadvantages of each process can be seen in this figure [1,11]. As shown, the sorption process has many advantages such as low cost, high efficiency, simplicity, and wide adaptability [44,45]. However, this method has some drawbacks. For instance, the removal efficiency of microorganisms by the sorption process is low. Also, fine particles cannot be effectively removed from wastewater. Another method is ion exchange, which is not efficient for removing hydrocarbon and organic compounds. In addition, the advantage of membrane technology is to produce less waste. However, energy consumption in the membrane is high. Although the electrochemical process has high selectivity for most ions, this technique has high capital cost and high energy consumption. Among these approaches, the photocatalytic process is effective for removing different kinds of organic pollutants such as hydrocarbons, gas condensates, and oil from wastewater [1,11,46]. There are two important catalysts, namely TiO₂ and ZnO, which have been widely used by previous researchers. In the following sections, their advantages and disadvantages, reaction mechanisms, reusability, and performance in wastewater treatment are discussed.

4. TiO₂ Photocatalyst

TiO₂ is a non-toxic and insoluble substance in water. TiO₂ has high stability with a high catalytic performance. Other properties of TiO₂ include high specific surface area, high crystallinity, and high concentration of surface hydroxyls. The specific surface area is an important parameter in photocatalytic efficiency and usually a high value of the specific surface area is associated with a low crystallinity of the catalyst [47,48]. Another name for TiO₂ is Titania, which can be available in three forms such as anatase, brookite, and rutile [48]. The rutile (tetragonal) crystalline structure of TiO₂ is quite stable in particles greater than 35 nm [49]. TiO₂ in anatase and rutile structures have oxidation strengths of 3–3.2 eV, which are very strong oxidation strengths [14]. One of the most important applications of TiO₂ is its utilization in the photocatalytic process. To this end, UV light, halogen lamps, or solar irradiation can help accelerate the photocatalytic reaction in the presence of TiO₂ nanocatalysts [50]. The photocatalyst has two important properties. For instance, the photocatalyst should not be consumed or participate directly in the reaction. Also, the photocatalyst provides other mechanism pathways. The photocatalytic reaction is based on the absorption of solar energy in the semiconductor gap. Few semiconductors can be utilized as the catalyst in the photocatalysis process, of which, TiO₂ is the most extensively utilized [48]. Various morphologies of TiO₂ consist of zero-, one-, two-, and three-dimensional structures. By optimization of the shape and size, the photocatalytic activity of TiO₂ particles in the wastewater treatment process can be maximized [51].

TiO₂ can be synthesized easily by various physical, chemical, and thermal procedures such as chemical or physical vapor deposition, sol-gel, inverse micelle, solvothermal, hydrothermal, sonochemical, microwave, and electrodeposition processes [52]. Synthesizing metallic semiconductor oxides by conventional physical mixing or chemical deposition usually produces insoluble materials that are inherently difficult to control in terms of their morphology, size, and dispersing metal components. These methods need a long time and multi-stage processes. The sonochemistry procedure is an efficient process for preparing mesoporous materials. Ultrasound is useful to synthesize an extensive range of nanostructured materials, which include high specific surface area oxides, alloys, carbides, and transition metals [15]. Neppolian et al. synthesized TiO₂ nanophotocatalysts by a combination of ultrasonic and sol-gel processes. They investigated the impact of various factors on the synthesis method, including magnetic stirring, ultrasonic sources (e.g., bath and horn), ultrasonic time, power density, temperature, and reactor size [53].

Table 1. Advantages and disadvantages of various techniques for TiO₂ fabrication.

Process	Advantages and Disadvantages	Ref.
Hydrothermal	Advantages: good size distribution, crystal shape control, low defects, synthesizing large crystals with high quality, fine particle size Disadvantages: high equipment cost, high temperature and pressure needed, long synthesis time	[55]
Sol-gel	Advantages: high purity products, good size distribution, remarkable specific surface area, economical, uniform size of particles, fine particle size, ease of synthesis Disadvantages: agglomeration of particles, long processing time, using organic solvents which may be toxic	[55]
Flame pyrolysis	Advantages: rapid and mass production Disadvantages: requires high energy, ease of rutile formation	[52]
Solvothermal	Advantages: high crystallinity, suitability for materials, low defects, better control of features of TiO2 compared to hydrothermal process Disadvantages: requires organic solvents, unstable at high temperatures	[52,54]
Inverse micelle	Advantages: fine particle sizes, high crystallinity, low defects Disadvantages: high cost, high crystallization temperature	[52]
Sonochemical	Advantages: high specific surface area, simple control of particles and morphology, efficient for mesoporous materials, improved reaction rate, short time, no additives Disadvantages: low yield, inefficient energy	[15,55]
Microwave heating	Advantages: fast heating, short reaction time, high reaction rate and efficiency	[54]

indicates the advantages and disadvantages of various processes for TiO₂ synthesis. As shown, the production of high-quality crystals as well as easy control of crystals in TiO₂ can be achieved by the hydrothermal process. Also, the fabrication of TiO₂ with the sol-gel process has several benefits such as high purity products, good size distribution, remarkable specific surface area, economical, uniform size of particles, fine particle size, and simple synthesis. Among these processes, utilizing the microwave method has significant benefits such as a short reaction time, high reaction rate, and high efficiency [54].

4.1. Features and Reaction Mechanism of TiO₂ Photocatalyst

Utilization of TiO₂ as a photocatalyst for the elimination of contaminants from wastewater has several benefits, which include the following: (1) the process is conducted at room temperature and 1 atm, (2) complete decontamination without secondary contamination, (3) producing high surface area and high catalytic activity, and (4) using the photocatalyst in multiple cycles and decreasing the costs [56]. The decomposition process of contaminants is mainly an oxidative reaction, which depends on the features of the photocatalyst. TiO₂ is also low-cost, has good band gap energy, long-term stability against light, and is safe. Generally, the band distance and wavelength of TiO₂ are between 3 and 3.2 eV and 400 nm, respectively, indicating that UV light at a wavelength less than 400 nm leads to an inverse reaction. Also, UV irradiation at a wavelength less than 400 nm initiates a photoreaction. Unfavorable recombination of holes (h⁺) and electrons (e⁻), and low yield in the visible region under irradiation are two important disadvantages of TiO₂ [15]. The catalytic activity of the TiO₂ photocatalyst can be enhanced under UV irradiation because TiO₂ uses only 5% of the solar energy [57].

At UV light < 400 nm, the surface of TiO₂ may reach temperatures above 30,000 °C; this temperature can oxidize all substances. Hence, all organic compounds are completely decomposed into CO₂ and water. Figure 4a illustrates a schematic of the degradation of contaminants by forming photo-induced charge carrier electrons/holes (e⁻/h⁺) on the TiO₂ surface. Also, Figure 4b shows that the catalyst surface is surrounded by the contaminant molecules. On the TiO₂ surface, oxygen molecules react with electrons to produce oxygen radicals. Also, water molecules react with h⁺ to generate hydroxide radicals and H⁺ ions. The final products will be CO₂ and H₂O [14]. In other words, electrons introduced into the conduction band from the valence band can be easily transferred to the catalyst surface and are trapped by binding to noble metals, intermediate metals, and rare earth elements. Metallic cations increase the ability of radicals produced in the photocatalysis reaction and then decrease the recombination life of e⁻/h⁺ [58].

By irradiating UV light to the TiO₂ surface, the photo-induced electrons react with dissolved oxygen for producing O₂^•. The photo-induced h⁺ in the valence band penetrates the surface of the TiO₂ photocatalyst and reacts with the absorbed water molecules to form OH^•. OH• is a highly active species in the photocatalytic process. The reaction mechanism of TiO₂ and the electron–hole pair is defined as follows. As shown, TiO₂ reacts with hv to produce e⁻ and h⁺. Next, e⁻ and h⁺ can produce two important radicals such as OH^• and O₂^•, which have important roles in the photocatalytic reaction [14,58].

$${\mathrm{TiO}}_2+\mathit{hv}\to {\mathrm{TiO}}_2 (e_{CB}^{-}+h_{VB}^{+})$$

$$\mathrm{Oxidation}:e_{CB}^{-} + {\mathrm{H}}_2\mathrm{O} + \to {\mathrm{OH}}^{•}+{\mathrm{H}}^{+}$$

$$\mathrm{Reduction}:O_2 + e_{CB}^{-}\to {{\mathrm{O}}_2}^{•}$$

$$e_{CB}^{-}+{{HO}_2}^{•}\to {\mathrm{HO}}_2^{-}$$

$$h_{VB}^{+}+{\mathrm{HO}}_2^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

$$Ti{O}_2\left(h^{+}\right)+H_2{O}_{ads}\to Ti{O}_2+H{O}_{ads}^{.}+H^{+}$$

$$Ti{O}_2\left(h^{+}\right)+H{O}^{-}\to Ti{O}_2+H{O}_{ads}^{.}$$

$$Ti{O}_2\left(e^{-}\right)+O_2\to Ti{O}_2+O_2^{.}$$

$$O_2^{.-}+H_{2}O\to H{O}_2^{.}+H{O}^{-}$$

$$O_2^{•}+H^{+}\to H{O}_2^{•}$$

$$2H{O}_2^{.}\to H_2{O}_2+O_2$$

$$Ti{O}_2\left(e^{-}\right)+H_2{O}_2\to Ti{O}_2+H{O}^{-}+H{O}^{.}$$

$$Ti{O}_2\left(h^{+}\right)+R_{ads}\to Ti{O}_2+R_{ads}^{.+}$$

where, R is the adsorbed pollutant [59].

Also, TiO₂ is photochemically stable and non-toxic. Under irradiation, the charge-pair on TiO₂ reacts directly with solid lattice ions. Also, TiO₂ is resistant to various acidic and alkaline pHs. TiO₂ can be utilized in the photocatalytic treatment of the environment. Hence, TiO₂ can be used as a promising photocatalyst to degrade organic contaminants. Nevertheless, TiO₂ particles are inactive in visible light. To solve this problem, numerous studies have been performed on the synthesis of various composites of TiO₂ with metals and non-metals (e.g., ceramics, zeolites, carbon materials, fibers, and glasses) as supports in order to have photocatalytic activity in a wide range of visible light [14].

The recombination behavior of e⁻/h⁺ decreases with the decreasing size of TiO₂ NPs, which can be because of the increase in interfacial charge transfer at TiO₂ surfaces. Also, the photocatalytic activity of TiO₂ of less than a few nanometers decreases because of the predominant recombination of the electron/hole at the TiO₂ surface. According to the reports, TiO₂ nanotubes are more effective in degrading contaminants than TiO₂ particles, indicating the rapid mass transfer of contaminants on the surface of nanotubes [49].

4.2. Improved Photocatalytic Activity of TiO₂ Using Metallic and Non-Metallic Cocatalysts

As mentioned earlier, TiO₂ photocatalysts can decompose organic contaminants under UV irradiation. Researchers have tried to improve the optical sorption range of TiO₂ catalysts from ultraviolet irradiation to visible light to boost its photocatalytic activity. Therefore, for improving the photocatalytic activity of TiO₂ in visible light, its surface should be modified [60]. The catalytic activity of TiO₂ can be improved by placing suitable materials on the surface of TiO₂ for its application under visible irradiation [57,61,62]. Metallic particles such as Pd, Pt, Fe, Ru, Au, and Ag can be utilized on the TiO₂ surface to improve photocatalytic activity by suppressing recombination behaviors of e⁻/h⁺. The induced electrons migrate to the surface of the metal particles, stabilizing the photo-induced h⁺ on TiO₂ as the life time of the charge carrier increases. To this end, more superoxide (O₂^•) and OH^• radicals can be produced. As the crystallographic aspects of TiO₂ increase, the photocatalytic activity of TiO₂ also produces more OH•, which can contribute to the photodegradation of organic pollutants. Also, the element O in the lattice of TiO₂ may be replaced by various heteroatoms such as B, S, F, N, P, and binary elements B-C and N-S to perform photocatalytic reactions in the presence of visible light. In addition, metals and non-metals introduced to the TiO₂ structure can improve its catalytic activity for the decomposition of organic pollutants [14,63]. Also, modifying the surface of TiO₂ improves the uptake capacity of contaminants, which can be useful in advanced oxidation technology. The modified TiO₂ with nanotubes, foams, and mesoporous phases has shown better photocatalytic behavior compared to the unmodified TiO₂ [64].

The TiO₂ photocatalyst on a nanoscale has a great surface area/volume proportion, leading to increased charge separation and ion trapping at the TiO₂ surface. TiO₂ nanoparticles show enhanced oxidative power compared to TiO₂ microparticles. However, the TiO₂ nanocatalyst cannot be employed directly in wastewater treatment owing to the aggregation of particles during the degradation process as well as their physical and chemical features. To this end, the catalytic activity of TiO₂ NPs can be improved by incorporating TiO₂ onto carbon materials for composite synthesis such as graphene, carbon nanotubes, and activated carbon nanofibers. Carbon materials can be excellent supports for TiO₂ NPs owing to their unique features such as excellent mechanical, thermal, chemical, electrical, and optical properties, which lead to a rapid charge transfer on the surface of TiO₂/carbon nanocomposites [65]. TiO₂/carbon nanocomposites are suitable catalysts for the elimination of organic contaminants from effluents. Graphene NPs act as a bridge for excellent electron transfer and electron sinks. TiO₂/graphene nanocomposites have an extensive range of band gaps (2.66–3.18 eV), suggesting that long-wavelength lights in the visible region can be absorbed by the TiO₂/carbon nanocomposite. One of the greatest benefits of carbon nanomaterials is their high specific surface area, in which TiO₂ NPs may be distributed in their structure, leading to improved selectivity of organic contaminants. The TiO₂/carbon nanocomposite has a good impact on self-purification in comparison to UV irradiation and ozonation when used for wastewater treatment. These nanocomposites can be effective industrially for wastewater purification because carbon NPs are employed as a support to stabilize the composite structure [66].

The photocatalytic activity of TiO₂ can be enhanced by enhancing the interfacial charge transfer and reducing the recombination of e⁻/h⁺. To this end, various metals can be used to improve the photocatalytic activity of TiO₂, including Ag, boron (B), Si, Ni, and so on under UV light or sunlight irradiation. Generally, there are several ways to improve the photocatalytic activity of TiO₂, including enhancing the ratio of surface/volume, integrating TiO₂ with other semiconductors, optimizing its particle size, and anchoring metals or non-metals. The presence of metal cocatalysts in the TiO₂ structure remarkably affects its photo-reactivity because by shifting the catalyst band gap to the visible region, it changes the interfacial electron transfer rate and the charge-carrying recombination rate [67]. A cocatalyst ion can trap e⁻ or h⁺, leading to a longer lifespan of the produced charge carriers and increased photocatalytic activity. Figure 5 indicates various cocatalysts used in the TiO₂ structure. As shown, noble metals, transition metals, anionic compounds, metal oxides, and transition metal ceramics are the main cocatalysts for improving the TiO₂ surface. Au, Pt, Pd, Rh, and Ag are some noble metals. Also, Fe, Al, Zr, Co, Cu, and Ni are several types of transition metals. Moreover, Fe₂O₃, Cr₂O₃, and SiO₂ are the most important metal oxides. Furthermore, WO₃, SnO₂, and MoO₃ are the most important transition metal ceramics. Eventually, C, N, F, O, and S are non-metallic cocatalysts. The addition of transition metals to the TiO₂ surface can enhance its catalytic activity and reduce the recombination of photogenerated e⁻ and h⁺ [15,68]. Previous studies have shown that the addition of metals and metal oxides to the catalyst surface improves the photocatalytic activity significantly compared to semi-conductors [68]. The photodegradation mechanism of contaminants using metal- and non-metal-anchored TiO₂ is also illustrated in Figure 6 [69,70]. As shown, the presence of metals and non-metals on the TiO₂ surface can help improve its photocatalytic activity by producing more oxygen radicals. These oxygen radicals play a key role in the photodegradation of organic contaminants.

Numerous studies have been performed on the expansion of the solar absorption band by combining TiO₂ with new metals, intermediates, and anions, which maximizes the performance of the photocatalyst [49].

Table 2. Impact of various cocatalysts on TiO₂ photocatalytic activity.

Cocatalyst	Light Source/Pollutant	Conditions	PE for TiO2 (%)	PE for Anchored TiO2 (%)	Ref.
Pd	UV lamp/2,2′,4,4′-tetrabromodiphenylether	5% Pd, 300 WUV lamp	-	100	[71]
B	UV light/4-nitrophenol	5% B in TiO2, 1 g/L catalyst dose, 1 mg/L 4-nitrophenol	79	90	[72]
Au/UI	UV irradiation/2,4 dinitrophenol	20 mg/L contaminant concentration, 120 min, 1 g/L catalyst dose	60	37	[73]
Au/IWI	UV irradiation/2,4 dinitrophenol	20 mg/L contaminant concentration, 120 min, 1 g/L catalyst dose	60	50	[73]
Pd/IWI	UV irradiation/2,4 dinitrophenol	20 mg/L contaminant concentration, 120 min, 1 g/L catalyst dose	60	67	[73]
Pd/IWI	UV irradiation/Rhodamine 6G	20 mg/L contaminant concentration, 120 min, 1 g/L catalyst dose	88	96	[73]
Ag	UV-A illumination/2,4,6-trichlorophenol	0.5 wt.% Ag, 120 min	-	95	[74]
Ag	Halogen lamp/MB	2 Wt.% Ag, 120 min	-	82.3	[75]
Ni	Ultraviolet/Dipterex	pH 6, Dipterex concentration = 40 mg/L, 2 h	-	83.5	[76]
Ce	UV lamp, crystal violet	0.8 mol Ce in TiO2, 0.2 g/L catalyst, 30 ppm dye concentration, pH 6.5, intensity of 2000 W/cm2	70	92	[15]
Fe	UV lamp/crystal violet	1.2 mol Fe in TiO2, 0.2 g/L catalyst, 30 ppm dye concentration, pH 6.5, intensity of 2000 W/cm2	70	80	[15]
Au	UV lamp/total organic carbon	8.71 mg/L total organic carbon, 15W UV lamp	-	93	[77]
Si	UV lamp/MB	20 h for TiO2 and 2 h for Si/TiO2, 10 ppm MB	68	86.7	[65]

presents the influence of various metal cocatalysts on the photocatalytic activity of TiO₂. Accordingly, Wang et al. (2019) synthesized a 5% Pd-TiO₂ photocatalyst to degrade 2,2′,4,4′-tetrabromodiphenyl ether from water under a UV lamp and showed 100% photodegradation efficiency, which is a remarkable amount [71]. In another work, Yadav et al. (2020) utilized a B-TiO₂ nanophotocatalyst for 4-nitrophenol removal from water. The utmost photodegradation efficiencies using TiO₂ and B-TiO₂ were 79 and 90%, respectively, indicating that the photodegradation efficiency of TiO₂ was enhanced with its doping by boron [72]. Also, Sescu et al. (2020) synthesized a Au/TiO₂ photocatalyst using two different procedures, incipient wet impregnation (IWI) and ultrasound impregnation (UI). Then, Au/TiO₂ (IWI) and Au/TiO₂ (UI) were used for the degradation of 2,4 dinitrophenol under UV irradiation. According to their outcomes, the photodegradation efficiency of 2,4 dinitrophenol using Au/TiO₂ (IWI) and Au/TiO₂ (UI) were 50 and 37%, respectively, which were less than the photodegradation efficiency of pure TiO₂ (60%). Comparing these two methods displays that the IWI procedure is more efficient than the UI procedure. Therefore, they used the IWI procedure to synthesize Pd/TiO₂ for the degradation of 2,4 dinitrophenol and Rhodamine 6G contaminants under UV irradiation. The photodegradation efficiencies of 2,4 dinitrophenol and Rhodamine 6G using the Pd/TiO₂ photocatalyst were 67% and 96%, respectively, indicating that the Pd/TiO₂ photocatalyst can remove Rhodamine 6G dye with a significant efficiency [73]. In general, previous studies show that most photocatalytic processes are performed under UV light. Also, Ag and Pd cocatalysts showed greater potential for photodegradation of organic contaminants than other cocatalysts.

5. ZnO Nanophotocatalyst

The ZnO nanocatalyst is one of the most efficient catalysts in the degradation of organic contaminants due to its unique features like excellent oxidation ability, direct and extensive band gap in the spectral region close to UV, high photocatalytic activity, and high binding energy. UV light can be absorbed on ZnO at a wavelength lower than 385 nm [78]. The rock salt, wurtzite, and cubic structures are important crystalline structures of ZnO. Among the three structures, the wurtzite structure of ZnO is the most common structure and has the utmost stability. Also, ZnO in the form of rock salt is wholly rare. At room temperature and pressure, the crystalline structure of ZnO is a hexagonal wurtzite. Moreover, ZnO can crystallize in the structure of wurtzite. The ZnO wurtzite structure is the most common form of ZnO due to its stability in environmental conditions [79]. Different structures of ZnO are exhibited in Figure 7. In the ZnO wurtzite structure, each Zn atom is surrounded by four O atoms. ZnO has many benefits compared to TiO₂, including low cost, chemical stability, no toxicity, and abundance [80]. Other properties of ZnO include its insolubility in water, that it is odorless, and has a bitter taste. ZnO is used in catalysis processes, fertilizers, the rubber industry, the paint industry, and cosmetics. The development of nano-ZnO with precisely controllable properties has recently gained considerable scientific attention [81].

Different zinc salts can be utilized to fabricate ZnO, including Zn(C₂H₃O₂)₂.2H₂O, Zn(SO₄)₂.7H₂O, Zn(NO₃)₂·6H₂O, and ZnCl₂ [82]. ZnO nanoparticles and doped ZnO can be synthesized by various techniques, including pulsed-laser deposition, chemical coprecipitation, hydrothermal, thermal decomposition, sol-gel, liquid–solid solution, vapor condensation, microwave, and spray pyrolysis processes [78,83]. The hydrothermal procedure is a well-known and efficient process for synthesizing ZnO nanoparticles, which is performed at high pressure and temperature [83]. Chemical coprecipitation is one of the most successful processes to synthesize ZnO nanoparticles with a fine particle size distribution. Chemical coprecipitation can prevent complex steps like alkoxide reflux and therefore takes less time than other methods [78]. Although the chemical coprecipitation process has many benefits, this process will leave a large amount of solution, which causes waste and high costs for this method [84]. Compared to the coprecipitation process, the yield of the product obtained from the hydrothermal process is lower [85]. Also, the synthesis of ZnO nanoparticles by the sol-gel process has several important advantages such as a uniform distribution of particles and synthesis at low temperatures. However, microwave-based synthesis has attracted a lot of attention due to its advantages such as being simpler, faster, and more energy efficient. The precursor solution is irradiated by the microwave source. Energy transfer via relaxation and/or resonance can lead to a relatively fast heating process. Also, the heating process by the microwave source leads to uniform heating in a short time, which results in a uniform distribution of particles [86]. In addition, pulsed laser deposition processes can be easily understood, but their theoretical aspect is still not fully understood because of the complexities of the interaction between the laser beam and the target substance [87].

Table 3. Advantages and disadvantages of some important techniques for the synthesis of ZnO.

Process	Advantages and Disadvantages	Ref.
Hydrothermal	Advantages: high crystallinity Disadvantages: high temperature and pressure needed, long reaction time	[83,85]
Sol-gel process	Advantages: uniform distribution of particles, low temperature needed, simple synthesis, controlling the particle size and morphology, low cost Disadvantages: agglomeration of particles	[85,86,88]
Chemical coprecipitation	Advantages: short time for synthesis, excellent reproducibility, low temperature, technical simplicity Disadvantages: low crystallinity nanoparticles, produces high waste, expensive	[78,84]
Microwave	Advantages: simple, fast synthesis, more energy efficient, uniform distribution of particles Disadvantages: high cost of microwave reactors	[86,89]
Pyrolysis	Advantages: cost-effectiveness, easy way to stick to any element, easy control of the thickness of the films Disadvantages: operation at moderate temperature	[90]
Pulsed-laser deposition	Advantages: simple setup, high flexibility, good adaptability, high process speed, and generating high-quality transparent films Disadvantages: difficulty scaling up	[87,91]

indicates the advantages and disadvantages of various processes for the synthesis of ZnO. According to the characteristics of the different processes listed in Table 3, it seems that sol-gel and microwave processes can be suitable methods for ZnO synthesis compared to other techniques.

5.1. Characterization of ZnO Nanocatalyst

The ZnO nanocatalyst is a semiconductor with a broad band gap, as well as high binding energy in environmental conditions. ZnO NPs have better optical, electrical, and magnetic features than ZnO microparticles. ZnO NPs also have unique physical, thermal, and chemical properties like biocompatibility, low cost, and non-toxicity. ZnO is an environmentally friendly substance, which is well suited for a wide range of daily applications that pose no risk to human health. Also, ZnO NPs have a longer shelf life compared to other metal oxides like Fe₂O₃, SiO₂, WO₃, and TiO₂. Because of its exceptional features, ZnO nanocatalysts can be utilized as valuable catalysts to treat wastewater [82]. Considering that ZnO has the same band gap energy (3.2 eV) as TiO₂, their photocatalytic capabilities are similar. Also, ZnO nanocatalysts are relatively cheaper than TiO₂ nanocatalysts, and the use of ZnO is more economical in large-scale water purification processes. ZnO can adsorb a broad range of solar spectra and more optical quantum than many semiconductor metal oxides. However, the main disadvantage of ZnO is its wide band gap energy, therefore, the absorption of light by ZnO is limited to the visible light range. This leads to rapid recombination of light-generated charges, resulting in low photocatalytic performance [92]. The addition of cocatalysts affects the optical features of ZnO and possibly shifts the amplitude of optical absorption to the visible region. Therefore, the photocatalytic activity of ZnO can be enhanced by doping it with other materials, which is described in detail in Section 4.2.

5.2. Photocatalysis Mechanism of ZnO

In a desirable photocatalytic reaction in the presence of zinc oxide particles and active oxidizing species like oxygen, organic contaminants can be converted to H₂O, CO₂, and inorganic acids. The photocatalytic reaction begins when ZnO NPs absorb photons from light with energies greater than their band gap energy. Thus, the photo-induced electron rises from the valence band to the conduction band, creating h⁺ and e⁻ on the ZnO surface [93]. The presence of the O atom as an electron absorber lengthens the pair of recombinant electron cavities and the formation of superoxide radicals. The reaction between h⁺ and OH⁻ results in the production of hydroxyl radicals. Figure 8a indicates the photocatalytic mechanism of anionic- and cationic-anchored ZnO catalysts and the photodegradation mechanism of the contaminant on the ZnO surface under sunlight is illustrated in Figure 8b. The reaction mechanism of ZnO in the photocatalytic process has several steps, which are presented below [94,95]:

$$ZnO+hv\to e^{-}+h^{+}$$

$$\mathrm{Absorbed}\mathrm{oxygen}:O_2+e^{-}\to O_2^{.-}$$

$$\mathrm{Water}\mathrm{ionization}:H_{2}O\to O{H}^{-}+H^{+}$$

$$\begin{gathered} \mathrm{Superoxide}\mathrm{protonation}:O_2^{.-}+H^{+}\to HO{O}^{.} \\ HO{O}^{.}+e^{-}\to HO{O}^{-} \\ HO{O}^{-}+H^{+}\to H_2{O}_2 \\ {H}_2{O}_2+e^{-}\to O{H}^{-}+O{H}^{.} \\ {H}_{2}O+h^{+}\to H^{+}+OH \end{gathered}$$

5.3. Improving the Photodegradation Efficiency of ZnO

The recombination of photo-produced h⁺ and e⁻ is one of the main disadvantages of semiconductor photocatalysis. This recombination stage reduces quantum efficiency and wastes energy. Thus, the recombination process of e⁻/h⁺ must be inhibited to have an impressive photocatalysis process. Metallic cocatalysts can counteract the recombination problem by increasing the isolation of charge between h⁺ and e⁻. Also, cocatalysts can trap e⁻ and reduce the chance of electron/hole recombination, which inactivates the photocatalytic process. Moreover, the production of active oxygen species and hydroxyl radicals significantly increases the charge separation efficiency. In general, the cocatalyst concentration, operating conditions, and synthesis procedure have significant impacts on the metal-anchored semiconductor photocatalysis process [96].

Anion-anchored zinc oxide photocatalysts have shown higher photocatalytic degradation efficiency than pure zinc oxide. The presence of isolated N2p modes above the utmost ZnO valence band in the Ne ZnO sample increases its ability to absorb visible light. Under visible light irradiation, narrow-band gaps in N-ZnO require less energy to induce the charge carriers (e⁻ and h⁺). The increased photocatalytic activity of C–zinc oxide can have several reasons, including increased adsorption of contaminants on the surface of the catalyst, higher UV absorption than pure ZnO, and the creation of new energy levels below the conduction band of ZnO, where e⁻ photoexcited are eliminated by these new energy levels to prevent electron-hole recombination. All these factors increase the number of charge carriers and improve the photodegradation performance. Also, the number of oxygen vacancies in ZnO has a key role in influencing the photo-activity of ZnO attached to S. In the photocatalytic reaction, oxygen vacancies become centers for trapping e⁻. Therefore, the more oxygen vacancies, the greater the catalytic activity [97,98]. Figure 9 shows the mechanism of pollutant degradation by metal- and non-metal-doped ZnO [98]. As shown in Figure 9a, the metal anchored on the ZnO surface contributes to the generation of e⁻ and subsequently O₂⁻ radicals, resulting in better photodegradation of organic contaminants. The same role is played by non-metals attached to ZnO to generate O₂⁻ radicals followed by the photodegradation of pollutants (Figure 9b) [99].

Despite the high photodegradation efficiency of ZnO, studies show that the anchored ZnO has significant photocatalytic efficiency compared to the undoped ZnO. ZnO surface modification using cationic cocatalysts has attracted much attention. The chemical, electrical, magnetic, and structural features of ZnO can be adjusted by adding cationic cocatalysts such as Ni, Mn, Co, Al, and Sb. The anchored elements are usually iso-morphic to the Zn ion, like Cu(II), Co(II), Ni(II), and Mn(II). The doped ZnO photocatalyst shows a faster response to the degradation of organic contaminants than pure ZnO [100].

Table 4. Comparing different metallic cocatalysts in the ZnO structure on the photocatalytic activity.

Cocatalyst	Light Source/Pollutant	Operating Conditions	* PE (%) for ZnO	PE (%) for Anchored ZnO	Ref.
Co	Visible light irradiation/MO	10 wt.% Co, 130 min, 100 mg/L MO	46	93	[101]
Ag	Visible irradiation/MB	5 wt.% Ag, 120 min	2.7	45.1	[102]
Co	Visible light irradiation/MB	5 wt.% Co, 10 ppm dye concentration, 140 min	2.7	62.6	[103]
Cu	Visible light irradiation/MB	5 wt.% Cu, 10 ppm dye concentration, 140 min	2.7	42.5	[103]
Cr	UV-vis light illumination/MO	1 wt.% Cr, 100 min	-	99.8	[104]
Sn	Sunlight/brilliant green	120 min	72.6	96.52	[105]
Fe	Sunlight/MB	Time = 3 h	90	95	[106]
Ta	Visible light irradiation/MB	20 min, 1 g/L catalyst dosage, pH 8, 10 mg/L dye concentration	-	97.5	[107]

* PE: photodegradation efficiency.

reports the impact of different metallic cocatalysts on the photocatalytic efficiency of ZnO. As shown, Adeel and coworkers (2021) studied the degradation process of MO using Co/ZnO. The photodegradation efficiency of ZnO and Co/ZnO were 46 and 93%, showing that the photodegradation efficiency of ZnO doubled after its doping with Co [101]. In another work, Vallejo et al. (2020a) compared the photodegradation efficiency of MB using ZnO and Ag/ZnO. Their results displayed photodegradation efficiencies of 2.7 and 45.1% for ZnO and Ag/ZnO, respectively, indicating a remarkable increment in photodegradation efficiency after ZnO doping with Ag. These photodegradation efficiencies were attained after 120 min of visible irradiation [102]. Also, Vallejo et al. (2020b) synthesized Co/ZnO and Cu/ZnO photocatalysts and compared their photodegradation efficiencies with ZnO in MB removal under visible light after 140 min. Their findings demonstrated that the photodegradation efficiencies of ZnO, Co/ZnO, and Cu/ZnO were 2.7, 62.6, and 42.5%, respectively [103]. Moreover, Cr/ZnO was able to decompose MO dye with an excellent photodegradation efficiency of 99.8% under UV-vis light illumination after 100 min [104]. Among these works, Cr/ZnO showed the highest photodegradation efficiency (99.8%) for removing methyl orange after 100 min under UV light.

Table 5. Impact of various non-metallic cocatalysts on the ZnO photocatalytic activity.

Cocatalyst	Light Source/Pollutant	Conditions	PE (%) ZnO	PE (%) Anchored ZnO	Ref.
N	Visible light irradiation/Rhodamine 6G	60 min, 0.01 g N	76.2	81.6	[108]
C	Visible light/MB	60 min	54.3	98.1	[109]
N	Visible light irradiation/Rhodamine B	10 mg/L dye concentration, room temperature, 2 h	-	97	[110]
N	Visible light irradiation/MB	10 mg/L dye concentration, room temperature, 2 h	-	99	[110]
C	Visible light/MB	2.5% catalyst, 200 °C	26	80	[111]
N	UV light or visible light irradiation/MB	-	-	99.6	[112]
C	UV irradiation/Bisphenol A	24 h	-	100	[113]
C	Sunlight irradiation/Rhodamine B	2.5 h	54.6	92.9	[114]

PE: photodegradation efficiency.

also reports the impact of various non-metallic cocatalysts on the ZnO photocatalytic activity for some organic contaminants. As shown, Wu and coworkers (2014) studied the photodegradation performance of MB using N-anchored ZnO under visible light irradiation and compared its results with pure ZnO. Their outcomes indicated that the photodegradation efficiency of MN using pure ZnO and N-anchored ZnO photocatalysts are 76.2 and 81.6%, respectively [108]. Also, Li and coworkers (2012) used pure ZnO and C-anchored ZnO for photodegradation of MB under visible light. After 60 min, C-anchored ZnO was able to decompose MB with a photodegradation efficiency of 98.1%, which is a significant amount compared to pure ZnO (54.3%) [109]. Moreover, the photodegradation efficiency of Bisphenol A using C-anchored ZnO under UV irradiation was 100%, indicating high degradation efficiency [110]. Furthermore, Fu et al. (2012) studied the photocatalytic reaction of MB dye using C-anchored ZnO under visible light and compared the photodegradation efficiency of ZnO and C-anchored ZnO. The highest photodegradation efficiency of ZnO and C-anchored ZnO were 26 and 80% under optimal conditions such as 2.5% catalyst and 200 °C, which showed a significant increase in the photodegradation efficiency after ZnO doping with C [111]. In general, most studies have been performed by N- and C-anchored ZnO photocatalysts, and both cocatalysts showed significant photodegradation activity. Therefore, these studies demonstrate that utilizing C- and N-anchored ZnO photocatalysts is more attractive to researchers than other cocatalysts.

6. Recyclability of ZnO and TiO₂ Photocatalysts

The most important factor for the practical application of catalysts is their recyclability and stability. Catalyst recyclability shows how many times a catalyst can be utilized in the catalytic process. After repeated reuse, the degradation efficiency of the catalyst is reduced, which may be due to the saturation of active sites, deactivation of active sites, and chemical decomposition of the catalyst structure. If the degradation efficiency changes slightly after repeated use of the catalyst, it indicates that the catalyst is highly reusable. Thus, determining the reusability of a catalyst is critical [18,115]. Another important parameter in the stability of catalysts is the leaching of elements from the surface of the catalyst into water [116]. Usually, the leaching of cocatalysts takes place during the photocatalytic reaction, which reduces the reusability of catalysts [117]. The transition metal on the catalyst surface is intentionally eliminated by acid treatment, cyclic voltammetry, or thermal dealloying to form a core-shell structure. Because of differences in surface energy or lattice mismatch, transition metal atoms can be simply separated. Furthermore, when oxygen species, which are oxygen reduction reaction intermediates, are placed on the metal atom on the catalyst surface, the separation of metal atoms is accelerated. Choi et al. (2019) stated that the use of the gas phase reduction method increases the durability and activity of the catalyst and minimizes the leaching of cocatalysts from the catalyst surface [118]. Also, the generation of peroxides during the photocatalytic reaction can lead to deactivation and performance limitation, thus limiting its industrial applications [116].

Table 6. Stability and recyclability of various composites of ZnO and TiO₂.

Catalyst	Pollutant	PE * (%)	PE (%) After n Cycles	Ref.
ZnO nanorods	imidazole	83%	n = 4, 80%	[119]
Ag-anchored ZnO nanocomposite	MB	95%	n = 4, 89.5%	[120]
Fe3O4@S-ZnO	ofloxacin	Above 90%	n = 6, above 90%	[121]
ZnO NPs	MB	93.25%	n = 5, 86.63%	[22]
ZnO NPs	Rhodamine B	91.06%	n = 5, 83.61%	[22]
Mg-ZnO nanorods	MB and ciprofloxacin	82%	n = 4, 75%	[122]
2%Au-anchored TiO2 nanocatalyst	MO	100%	n = 11, 100%	[123]
Sm/N co-doped TiO2/diatomite	tetracycline	87.1%	n = 5, 83.2%	[124]
C-anchored TiO2/carbon nanofibrous	Rhodamine B	94.2%	n = 6, 92%	[125]
TiO2	MO	58.3%	n = 10, 36.1%	[126]
TiO2/polyaniline	MO	86%	n = 10, 46.2%	[126]
Fe3O4/AC/TiO2	MB	98%	n = 7, 93%	[127]
2-(methacryloyloxy) ethyltrimethylammonium chloride/TiO2	MB	99.66%	n = 20, 98.7%	[128]
TiO2/2NiO	MB	100%	n = 5, 72.6%	[115]

* PE: Photodegradation efficiency.

reports the reusability of various composites of ZnO and TiO₂ in different cycles.

Nikoofar et al. (2015) synthesized ZnO nanorods for imidazole removal. They studied the reusability of ZnO nanorods in four runs and their results showed that the elimination efficiencies of imidazole after the first, second, third, and fourth cycles were 83%, 83%, 81%, and 80%, respectively, which indicates that the elimination efficiency has declined slightly (less than 3% after four reuse cycles), showing remarkable reusability of ZnO nanorods [119]. In another study, the reusability of the Ag/ZnO nanocomposite was studied in eliminating MB, MO, and CR after five successive steps. A slight decrease in dye removal efficiency was observed after five cycles, indicating significant recyclability of Ag/ZnO for eliminating organic dyes [129]. Also, Shelar et al. (2020) investigated the stability and recyclability of the Ag-ZnO nanocatalyst for eliminating MB dye. The photodegradation efficiency of MB was 95%. Then, the nanocatalyst was eluted three times with deionized water and reused in the photocatalytic process. After four cycles, the photodegradation efficiency reduced to 89.5%, indicating a high stability of the Ag-ZnO nanocatalyst [120]. Moreover, Fe₃O₄@S-ZnO was utilized for degrading ofloxacin. The recyclability study illustrated that the photodegradation efficiency of ofloxacin was above 90% after six cycles, indicating remarkable stability and reusability of Fe₃O₄@S-ZnO [121]. Furthermore, Khan and coworkers (2021) studied the reusability of ZnO NPs for photodegradation of MB and Rhodamine B dyes after five consecutive cycles. The photodegradation efficiencies of MB and Rhodamine B dyes decreased from 93.25 to 86.63% and 91.06 to 83.61% after five cycles, respectively, indicating that the efficiency reduction percentage is less than 10%. ZnO indicates excellent photocatalytic activity and high stability after five cycles [18]. In addition, Ikram et al. (2021) studied the reusability of Mg-ZnO nanorods for eliminating a mixture of MB and ciprofloxacin. After four cycles, the photodegradation efficiency slightly reduced from 82 to 75% after 40 min [122].

Also, Padikkaparambil and coworkers (2013) studied the reusability of a 2%Au-TiO₂ nanocatalyst for MO photodegradation under UV irradiation in 11 reuse cycles. Under operating conditions (3 g/L catalyst, 10 mg/L MO, and 1 h irradiation time), the photocatalytic activity of Au/TiO₂ did not change (about 100% photodegradation efficiency) even after 10 consecutive tests, which indicates the high reusability of 2%Au-TiO₂ [123]. Also, Wu and Zhang (2019) studied the reusability and stability of a Samarium/Nitrogen co-TiO₂/diatomite for eliminating tetracycline under visible light. After five cycles, the photodegradation efficiency reduced from 87.1% to 83.2% (less than 5% reduction), indicating high stability and significant reusability of the Samarium/Nitrogen co-doped TiO₂/diatomite catalyst [124]. Moreover, Song et al. studied the reusability of C-TiO₂/carbon nanofibers in removing Rhodamine B from aqueous solutions. The nanocatalyst showed a photodegradation efficiency of 94.2% under UV light. After six reuse cycles, the photodegradation efficiency reached 92%, showing significant reusability and stability of C-TiO₂/carbon nanofibers [125]. In another work, Bahrudin and coworkers (2019) studied the reusability of TiO₂ and TiO₂/polyaniline for MO dye photodegradation. After ten reuse cycles, the photodegradation efficiency of MO using TiO₂ and TiO₂/polyaniline decreased from 58.3 to 36.1% and 86 to 46.2%, respectively [126]. Moreover, Moosavi et al. (2020) studied the recyclability and stability of Fe₃O₄/AC/TiO₂ for seven cycles of MB photodegradation. After seven reuse cycles, the photodegradation efficiency reduced from 98 to 93% (about 5% reduction). They attributed the decrease in the photodegradation efficiency after seven cycles to the following reasons: (i) Particle losses may take place in washing and drying steps, which result in lower doses in the next cycle, leading to the decrease in the surface catalytic activity as well as the efficiency. (ii) Characteristics of the catalyst like aggregation may change during different cycles because the aggregation of particles reduces the specific surface area and the number of active sites. (iii) The catalytic activity of the catalyst dwindles after each cycle due to the blockage of active sites and pores [127]. Furthermore, the reusability of the 2-(methacryloyloxy) ethyltrimethylammonium chloride/TiO₂ photocatalyst was studied for MB removal under UV irradiation. The photodegradation efficiency of the aforementioned photocatalyst after 270 min was 99.66% and after 20 reuse cycles, the photodegradation efficiency reached 98.7%, i.e., a reduction of less than 1% in the photodegradation efficiency, indicating remarkable stability of the photocatalyst [128]. In addition, a TiO₂/2NiO photocatalyst was used to decompose MB in five reuse cycles and the outcomes showed that the photodegradation efficiency of MB using TiO₂/2NiO reduced from 100% to 72.6% after five cycles, indicating its high stability [115].

In general, examining the reusability of ZnO and TiO₂ nanocatalysts demonstrates that they have high reusability and their utilization in industrial applications is cost-effective. Also, adding metallic and non-metallic cocatalysts to the structure of TiO₂ and ZnO catalysts increased their stability.

7. Literature Review

So far, many works have been conducted on the removal of organic compounds by catalysts. Most of these catalysts are based on TiO₂ or ZnO because these catalysts have shown high performance compared to other catalysts. These catalysts can degrade organic contaminants in the presence of light or UV. Table 7 reports the performance of TiO₂ and ZnO nanocatalysts in the photodegradation of organic contaminants.

According to Table 7, Mansoori and coworkers studied the photocatalytic efficiency of TiO₂ in the removal of toluene, phenol, nitrobenzene, and MO. According to their results, the photodegradation efficiency of phenol, nitrobenzene, and MO was 100%, which is a remarkable amount. In addition, the degradation efficiency of toluene by the TiO₂ photocatalyst was 71% [130]. Also, the Bi₁₂TiO₂₀ nanophotocatalyst could remove cefixime with a degradation efficiency of 94.93% after 3 h, while Bi₁₂ZnO₂₀ removed 80% of cefuroxime from water after 4 h [131]. In another study, Zhang and coworkers studied the performance of TiO₂ in the removal of parathion. Under optimal conditions (i.e., contaminant concentration of 50 ppm and catalyst dosage of 1 g/L), the maximum photocatalytic efficiency was 70% [132]. Also, Razip and coworkers (2019) synthesized an Fe₃O₄/TiO₂ nanocatalyst and used it in the removal of MO from wastewater. After 1h of UV irradiation, the photodegradation efficiency of MO dye was obtained at 90.3% [22]. In addition, Hou and coworkers synthesized a PVP/TiO₂/polydopamine nanocatalyst, and then used it to remove malachite green, MB, and MO dyes from wastewater. Based on their results, the photodegradation efficiency of malachite green, MB, and MO dyes using the nanocatalyst were 45, 25, and 24%, respectively, which are low values. Their results reveal that the PVP/TiO₂/polydopamine nanocatalyst is not suitable for the photocatalytic process [133]. Also, carbon-TiO₂ nanocatalysts can eliminate methyl ethyl ketone under UV light with a photodegradation efficiency of 94% [23]. Moreover, Akhlaghian and Najafi (2018) synthesized a CuO/WO₃/TiO₂ nanocatalyst to remove 4-chlorophen. According to their outcomes, the aforementioned nanocatalyst was able to remove the pollutant with a degradation efficiency of 94.8%, which was obtained at a catalyst dosage of 0.75 g/L and H₂O₂ amount of 563.16 mmol/L, after 3 h [21]. In another study, Nagaraju et al. (2020) investigated the photocatalytic degradation of chlorobenzene from water using several catalysts such as ZnO, Pb/ZnO, Cd/ZnO, and Ag/ZnO in the presence of LED light and tungsten light. Among these photocatalysts, Pb/ZnO could decompose chlorobenzene with 100% degradation efficiency in the presence of both lights. Also, pure ZnO showed the minimum degradation efficiency (71%) in the presence of LED light [28]. Furthermore, Elmolla and Chaudhuri (2010) utilized a ZnO nanophotocatalyst for removing amoxicillin, ampicillin, and cloxacillin and their outcomes demonstrated that ZnO could eliminate all contaminants with a significant photodegradation percentage of 100% under ultraviolet after 180 min [134]. Moreover, Boytsova and coworkers synthesized TiO₂ at different annealing temperatures (400–1200 °C), and then studied their photodegradation rates in the removal of crystal violet dye. According to their results, the highest photodegradation rate was obtained for the synthesized TiO₂ nanocatalyst at 800 °C [135]. In addition, Chiang and Lin studied the photocatalytic performance of ZnO/SnO₂ to remove methylene blue from aqueous media. Under optimal conditions, i.e., pH 12, a photocatalyst dosage of 0.5 g/L, and a reaction time of 60 min, the highest photodegradation efficiency was 96%, which is a considerable amount [136]. In another study, Ce/ZnO showed significant photocatalytic efficiency (99.5%) in the removal of direct red-23 dye after 70 min [137]. Also, Al-Namshah and coworkers compared the photocatalytic performance of ZnO, Ce/ZnO, and CeO₂/ZnO in the removal of methyl green dye. Based on their outcomes, the photodegradation efficiencies of methyl green using ZnO, Ce/ZnO, and CeO₂/ZnO were 68, 98, and 100%, which demonstrates that CeO₂/ZnO has a remarkable photocatalytic efficiency [138].

Overall, TiO₂ in the removal of nitrobenzene, phenol, and MO, Bi₁₂TiO₂₀ in the removal of cefixime, Fe₃O₄/TiO₂ (P25) in the removal of MO dye, CuO/WO₃/TiO₂ in the removal of 4-Chlorophen, carbon/TiO₂ in the removal of methyl ethyl ketone, as well as, ZnO in the removal of amoxicillin, ampicillin, and cloxacillin, Fe₃O₄/CuO/ZnO/graphene and ZnO/SnO₂ in the removal of MB, Ce/ZnO in the removal of direct red-23 dye, Pb/ZnO and Ag/ZnO in the removal of chlorobenzene, and Ce/ZnO and CeO₂/ZnO in the removal of methyl green, due to significant photodegradation efficiencies (more than 90%), are strongly suggested as powerful catalysts.

Table 7. The performance of TiO₂ and ZnO nanocatalysts and their derivatives in degrading different organic pollutants.

Nanocatalysts	Contaminants	Optimal Conditions	DE (%)	References
TiO2	Nitrobenzene	CD = 0.1 M, CC = 50 ppm	100	[130]
Bi12TiO20	Cefixime	3 h	94.93	[131]
TiO2	Parathion	CD = 1 g/L, CC = 50 ppm	70	[132]
TiO2	Toluene	CD = 5 g, CC = 45 ppm	71	[130]
TiO2	Phenol	1.8 g/L catalyst dose	100	[130]
TiO2	Benzene	CD = 5 g, CC = 45 ppm	72	[133]
TiO2	MO	CD = 3 g/L, CC = 30 ppm	100	[130]
Fe3O4/TiO2 (P25)	MO	1 h under UV light irradiation	90.3	[22]
Fe3O4/TiO2 (UV100)	MO	1 h under UV light irradiation	51.6	[22]
CuO/WO3/TiO2	4-Chlorophenol	CD = 0.75 g/L, H2O2 amount = 563.16 mmol/L, 3 h	94.8	[21]
CuO/WO3/TiO2	3-Phenyl-1-propanol	CD = 0.75 g/L, H2O2 amount = 563.16 mmol/L, 3 h	85.13	[21]
Carbon- TiO2	Methyl ethyl ketone	Under UV light	94	[23]
La/TiO2	Ramazol Brilliant blue	-	72	[22]
ZnO	Amoxicillin	Ultraviolet, pH = 11, catalyst dose = 0.5 g/L, time 180 min	100	[134]
ZnO	Ampicillin	Ultraviolet, pH = 11, catalyst dose = 0.5 g/L, time 180 min	100	[134]
ZnO	Cloxacillin	Ultraviolet, pH = 11, catalyst dose = 0.5 g/L, time 180 min	100	[134]
ZnO/SnO2	MB	pH 12, CD = 0.5 g/L, time 60 min	96	[136]
Ce/ZnO	Direct red-23	Reaction time = 70 min	99.5	[137]
ZnO	Methyl green	CC = 20 ppm, CD = 2 g/L, time = 60 min	68	[138]
Ce/ZnO	Methyl green	CC = 20 ppm, CD = 2 g/L, time = 60 min	98	[138]
CeO2/ZnO	Methyl green	CC = 20 ppm, CD = 2 g/L, time = 60 min	100	[138]
PVP/TiO2/polydopamine	Malachite green	60 min, 10 mg/L of dye	45	[139]
PVP/TiO2/polydopamine	MB	60 min, 10 mg/L of dye	25	[139]
PVP/TiO2/polydopamine	MO	60 min, 10 mg/L of dye	24	[139]
rGO/Fe3O4/ZnO	MV	120 min, CD = 0.04 g/L	83.5	[25]
Tungsten/silver/ZnO	Ponceau 4R	pH 5.64, CD = 0.08 g/L, 25 °C	78.8	[26]
Bi12ZnO20	Cefuroxime	4 h	80	[28]
ZnBi2O4	Cefixime	Solar light (98 mW/cm2), 30 min	89	[140]
ZnBi2O4	Cefixime	UV irradiation (20 mW/cm2), 2 h	88	[140]
Ag/TiO2	Phenol	pH 7, CD = 1.5 g/L, CC = 5 ppm, power light = 18 W	82.65	[141]
Ce/TiO2	Sulfur black	pH = 9.5, CC = 200 ppm	92	[142]
CeO2/ZnO/TiO2	Rhodamine B	pH 12, CC = 5 ppm, CD = 0.2 g/L, time = 180 min	80	[143]
Pure ZnO	Chlorobenzene	LED light	71	[27]
Pb/ZnO	Chlorobenzene	LED light	100	[27]
Ag/ZnO	Chlorobenzene	LED light	95	[27]
Cd/ZnO	Chlorobenzene	LED light	90	[27]
Pure ZnO	Chlorobenzene	Tungsten light	90	[27]
Pb/ZnO	Chlorobenzene	Tungsten light	100	[27]
Ag/ZnO	Chlorobenzene	Tungsten light	83	[27]
Cd/ZnO	Chlorobenzene	Tungsten light	73	[27]

CC = contaminant concentration; CD = catalyst dose.

Table 7. The performance of TiO₂ and ZnO nanocatalysts and their derivatives in degrading different organic pollutants.

Nanocatalysts	Contaminants	Optimal Conditions	DE (%)	References
TiO2	Nitrobenzene	CD = 0.1 M, CC = 50 ppm	100	[130]
Bi12TiO20	Cefixime	3 h	94.93	[131]
TiO2	Parathion	CD = 1 g/L, CC = 50 ppm	70	[132]
TiO2	Toluene	CD = 5 g, CC = 45 ppm	71	[130]
TiO2	Phenol	1.8 g/L catalyst dose	100	[130]
TiO2	Benzene	CD = 5 g, CC = 45 ppm	72	[133]
TiO2	MO	CD = 3 g/L, CC = 30 ppm	100	[130]
Fe3O4/TiO2 (P25)	MO	1 h under UV light irradiation	90.3	[22]
Fe3O4/TiO2 (UV100)	MO	1 h under UV light irradiation	51.6	[22]
CuO/WO3/TiO2	4-Chlorophenol	CD = 0.75 g/L, H2O2 amount = 563.16 mmol/L, 3 h	94.8	[21]
CuO/WO3/TiO2	3-Phenyl-1-propanol	CD = 0.75 g/L, H2O2 amount = 563.16 mmol/L, 3 h	85.13	[21]
Carbon- TiO2	Methyl ethyl ketone	Under UV light	94	[23]
La/TiO2	Ramazol Brilliant blue	-	72	[22]
ZnO	Amoxicillin	Ultraviolet, pH = 11, catalyst dose = 0.5 g/L, time 180 min	100	[134]
ZnO	Ampicillin	Ultraviolet, pH = 11, catalyst dose = 0.5 g/L, time 180 min	100	[134]
ZnO	Cloxacillin	Ultraviolet, pH = 11, catalyst dose = 0.5 g/L, time 180 min	100	[134]
ZnO/SnO2	MB	pH 12, CD = 0.5 g/L, time 60 min	96	[136]
Ce/ZnO	Direct red-23	Reaction time = 70 min	99.5	[137]
ZnO	Methyl green	CC = 20 ppm, CD = 2 g/L, time = 60 min	68	[138]
Ce/ZnO	Methyl green	CC = 20 ppm, CD = 2 g/L, time = 60 min	98	[138]
CeO2/ZnO	Methyl green	CC = 20 ppm, CD = 2 g/L, time = 60 min	100	[138]
PVP/TiO2/polydopamine	Malachite green	60 min, 10 mg/L of dye	45	[139]
PVP/TiO2/polydopamine	MB	60 min, 10 mg/L of dye	25	[139]
PVP/TiO2/polydopamine	MO	60 min, 10 mg/L of dye	24	[139]
rGO/Fe3O4/ZnO	MV	120 min, CD = 0.04 g/L	83.5	[25]
Tungsten/silver/ZnO	Ponceau 4R	pH 5.64, CD = 0.08 g/L, 25 °C	78.8	[26]
Bi12ZnO20	Cefuroxime	4 h	80	[28]
ZnBi2O4	Cefixime	Solar light (98 mW/cm2), 30 min	89	[140]
ZnBi2O4	Cefixime	UV irradiation (20 mW/cm2), 2 h	88	[140]
Ag/TiO2	Phenol	pH 7, CD = 1.5 g/L, CC = 5 ppm, power light = 18 W	82.65	[141]
Ce/TiO2	Sulfur black	pH = 9.5, CC = 200 ppm	92	[142]
CeO2/ZnO/TiO2	Rhodamine B	pH 12, CC = 5 ppm, CD = 0.2 g/L, time = 180 min	80	[143]
Pure ZnO	Chlorobenzene	LED light	71	[27]
Pb/ZnO	Chlorobenzene	LED light	100	[27]
Ag/ZnO	Chlorobenzene	LED light	95	[27]
Cd/ZnO	Chlorobenzene	LED light	90	[27]
Pure ZnO	Chlorobenzene	Tungsten light	90	[27]
Pb/ZnO	Chlorobenzene	Tungsten light	100	[27]
Ag/ZnO	Chlorobenzene	Tungsten light	83	[27]
Cd/ZnO	Chlorobenzene	Tungsten light	73	[27]

CC = contaminant concentration; CD = catalyst dose.

8. Factors Affecting Photodegradation Efficiency

8.1. Photocatalyst Dosage

Catalyst dosage or catalyst loading is a critical factor in the photocatalytic efficiency of organic contaminants. Increasing the catalyst dose leads to an enhancement in the specific surface area of the catalyst, the creation of more active sites, and ultimately the formation of more hydroxyl and superoxide radicals. Thus, the degradation of organic contaminants will be facilitated and a higher photodegradation efficiency will be attained. The photodegradation efficiency initially enhances with enhancing the photocatalyst dosage until it reaches an optimal value. At the photocatalyst concentration beyond the optimal concentration, the photodegradation efficiency decreases because of light scattering. Increasing the photocatalyst dosage beyond the optimal value leads to the agglomeration of catalyst particles and then decreases the specific surface area of the catalyst to absorb light, which ultimately leads to a reduction in the degradation efficiency. On the other hand, it prevents the penetration of light into the sewage. As a critical result, an optimal value of the catalyst must be determined to prevent overuse of the catalyst and to achieve maximum degradation efficiency [40].

In one study, Zhang and co-workers (2015) studied the photodegradation efficiency of MB using TiO₂ in the range of photocatalyst dosage from 1 to 10%. According to their outcomes, the maximum photodegradation efficiency of MB (93.78%) was attained at an optimal value of 7% [144]. Also, Notodarmojo et al. (2017) studied the degradation efficiency of RB-5 dye from water and found that the degradation efficiency increases with increasing the TiO₂ photocatalyst concentration from 0.5 to 2.5 g/L. Therefore, 2.5 g/L was considered the optimum photocatalyst dosage [145]. Moreover, Saiful Amran and co-workers (2019) investigated the maximum degradation efficiency of MB using carbon-TiO₂ in the photocatalyst dosage range of 1-3%. According to their outcomes, the highest photodegradation efficiency (82.67%) was obtained at 2 wt.% photocatalyst dosage [146]. Furthermore, Erdemoğlu et al. studied the influence of nano-TiO₂ dosage (0.1–1 wt.%) on photodegradation of CR under visible light. The nanocatalyst could remove 94% of CR at a catalyst dosage of 0.25 wt.%. By enhancing catalyst dosage from 0.25 to 1 wt.%, the photodegradation efficiency decreased [147].

8.2. Photocatalyst Structure

Photocatalytic efficiency is effectively influenced by the photocatalytic structure. The synthesis of nanostructured photocatalysts has recently received much attention because of their unique structures and features. There are various morphologies for nanostructured photocatalysts, including nanoflowers, nanosheets, nanowires, nanorods, nanodumbbells, nanobelts, and nanospiral disks. Some structures of these photocatalysts are illustrated in Figure 10. Nanostructured photocatalysts have a high specific active area with great catalytic strength. A high surface/volume proportion presents better physical and chemical features. Each of these structures has shown various photocatalytic features [148,149].

8.3. Contaminant Concentration on Photodegradation Efficiency

Contaminant concentration has an important influence on the photocatalytic process. Previous studies show that the concentration of organic pollutants in aqueous solution has an undesirable impact on photodegradation efficiency. The higher the concentration of contaminants in the effluent, the lower the photodegradation efficiency, which is because the concentration of the target contaminant becomes more and more. Organic contaminants can be adsorbed on the photocatalyst surface. The number of contaminant molecules increases, while the number of active sites remains constant [154]. Hence, the generation of hydroxyl radicals is not enough and there will be only a few active sites on the photocatalyst surface to absorb hydroxyl ions. Consumption of hydroxyl radicals (OH^.) by the produced intermediates decreases the photodegradation efficiency in solutions with high contaminant concentrations. Therefore, the lower the pollutant concentration, the less competition there is for consumption [155,156]. Parida and Parija (2006) investigated the impact of phenol concentration on the degradation efficiency using ZnO in various irradiation strengths. Under solar irradiation, the photodegradation yield decreased from 100% to 60% with an enhancement in the phenol dosage, while under ultraviolet light, the photodegradation efficiency reduced from 94% to 52% with an enhancement in the phenol concentration [157]. Also, Benhabiles et al. (2016) studied the influence of MB dye concentration (10–30 mg/L) on the photocatalytic process by commercial TiO₂. After 5 h, 70% of MB dye was degraded by TiO₂ at MB concentration of 10 mg/L, while at MB concentration of 30 mg/L, the catalyst could remove only 30% of MB [158]. Moreover, Shelar et al. (2020) surveyed the impact of MB dye concentration (10–40 mg/L) on photodegradation using Ag/ZnO. The photodegradation efficiency reduced from 95% to 65% with enhancing MB dosage from 10 to 40 mg/L, indicating that the highest photodegradation efficiency occurs at the lowest dosage of MB dye [120].

Generally, previous studies demonstrate that the highest photodegradation efficiency of contaminants occurs at the lowest contaminant concentration.

8.4. pH

Solution pH plays a vital role in the photocatalytic reaction of water purification. pH can change the surface charge of the photocatalyst [159]. Determination of optimal pH in the photodegradation process depends on the zero-point charge (pH_zpc) of the photocatalyst. pH and the photodegradation rate do not have a specific relation. The TiO₂ photocatalyst will be negatively charged if pH > pH_ZPC. In this case, the TiO⁻ anion will be formed. Also, if pH < pH_ZPC, the TiO₂ photocatalyst will be positively charged and form the TiOH⁺₂ cation. Depending on the catalyst used, the surface charge of the photocatalyst at 4.5 < pH_ZPC < 7 is neutral. When operating at pHzpc, the surface charge of the TiO₂ photocatalyst is positively charged and absorbs negatively charged molecules electrostatically over time. Industrial effluents may be discharged at different pHs, which complicates the photocatalytic reaction. Also, hydroxyl radicals are rapidly eliminated at high pHs, inhibiting their reaction with the contaminant [40,156]. There are many effective factors on pH in the photocatalytic reaction, including the electrostatic charge of catalyst particles, band structure, and crystal size. Each of these factors can alter the photocatalyst surface charge and generally alter the photodegradation efficiency [156]. Generally, the optimal pH in photodegradation depends on the contaminant and the photocatalyst. For example, Hameed et al. (2009) studied the impact of pH on the photodegradation efficiency of carbofuran using TiO₂/Ultraviolet and pH 7 was obtained as the optimal pH [160]. Also, Lopez-Alvarez et al. (2011) surveyed the impact of pH on the photodegradation efficiency of carbofuran using TiO₂/solar light and their outcomes showed that pH 7.6 is the optimal value [161]. Moreover, in the work conducted by Saien and Khezrianjoo (2008) for the degradation of carbendazim using TiO₂/ultraviolet, the optimal pH was 4 [162]. Furthermore, Elmolla and Chaudhuri (2010) found an optimal pH of 11 for the photodegradation of amoxicillin, ampicillin, and cloxacillin using ZnO/ultraviolet [135]. In another work, the highest photodegradation efficiency of TiO₂ for MB removal was attained at a pH of 11 [158]. Also, Shelar et al. studied the impact of pH on photodegrading MB dye in the range of 2–12 and found that the utmost photodegradation efficiency is obtained at pH 8 [120].

8.5. Light Intensity and Wavelength

Photodegradation efficiency depends on the absorption of light by a catalyst [163]. Light intensity specifies how much light can be adsorbed by a photocatalyst at a given wavelength. The higher the light intensity, the more radiation is deposited on the photocatalyst surface, leading to the production of more hydroxyl radicals and an enhancement of the reaction rate. At higher light intensities, the photodegradation reaction depends on the mass transfer between the reactants, because the photocatalyst surface is completely covered by saturated solids, limiting the mass transfer for sorption and desorption. Therefore, the photocatalytic reaction speed remains constant in spite of an increase in the intensity of light. Previous studies have shown that there is a relationship between the photodegradation efficiency of the contaminant and light intensity for different organic compounds. The generation of hydroxyl radicals increases with increasing light intensity, which leads to an improvement in the degradation rate [156,164]. Elaziouti et al. investigated the impact of light intensity in the range of 50–90 j/cm² on the photodegradation efficiency of CR and benzopurpurine 4B. The photodegradation efficiency of CR and benzopurpurine 4B increased with enhancing the light intensity from 50 to 80 j/cm² and 50 to 90 j/cm², respectively. Because more photons at higher light intensities will be available for excitation on the catalyst surface, more electron–hole pairs can be produced [165].

Based on the wavelength of UV irradiation, there are three electromagnetic spectra, including UV-A, UV-B, and UV-C. These are categorized based on the wavelength range. For example, the light wavelength ranges for UV-C, UV-B, and UV-A are between 100 and 280, 280 and 315, and 314 and 400 nm, respectively [166]. Because of the shorter penetration of higher energy photons, the photodegradation rate is greater at 254 nm, which increases the number of pairs of electron holes produced to decompose the target contaminant [167].

8.6. Temperature

It is better to perform the photodegradation reaction at 25 °C and 1 bar because of photonic activation, which is useful to purify water, by which the heating stage can be eliminated to save energy. However, the optimum temperature for photocatalytic reactions can occur at 20–80 °C. The results show that the photodegradation efficiency of organic pollutants increases with enhancing the reaction temperature; however, it may lead to a decline in the adsorption capacity of reactive species and dissolved oxygen, leading to lower photodegradation efficiency [168]. Chen and coworkers studied the impact of temperature (0–50 °C) on the photocatalytic activity of TiO₂ and Pd/TiO₂ photocatalysts for MB elimination under UV light, and their outcomes demonstrated that the photodegradation efficiency increases with increasing temperature [169]. Also, Hu and coworkers investigated the impact of temperature on the photodegradation of MO dye using TiO₂. According to their results, under UV-vis irradiation, the photocatalytic activity of TiO₂ was increased by enhancing temperature from 38 to 100 °C. Hence, the reaction was endothermic. Also, the reaction rate constant increased from 0.00031 to 0.00217 min⁻¹ with enhancing temperature from 34 to 100 °C [170]. Moreover, Barakat and coworkers studied the influence of temperature (5–55 °C) on the degradation of Rhodamine B using Ag-TiO₂. The highest photodegradation efficiency under irradiation occurred at 55 °C [171]. Generally, the photodegradation efficiency of organic contaminants increases with enhancing temperature and the highest removal performance occurs at high temperatures.

8.7. Reaction Time

Irradiation time or reaction time is a critical factor in the photodegradation process and is when the contaminant is exposed to light or photon energy. Many researchers have investigated the impact of reaction time on the photodegradation efficiency of organic contaminants using TiO₂ and ZnO photocatalysts. The longer the reaction time, the higher the photodegradation efficiency of the contaminant [40]. According to the work conducted by Shi et al. (2019), eliminating MO using TiO₂/chitosan increased with enhancing the reaction time and finally reached equilibrium. At the beginning of the photocatalytic reaction, there is a large number of active sites at the photocatalyst surface to attach with MO molecules, resulting in a high degradation efficiency. After the equilibrium reaction time, no change in the photodegradation efficiency was observed [172]. Elaziouti et al. studied the impact of reaction time (0–80 min) on the photodegradation efficiency of CR and benzopurpurine 4B. A total of 95% of CR dye and 97.2% of benzopurpurine 4B were degraded using 1 g/L ZnO after 60 and 80 min, respectively [165]. Also, Erdemoğlu et al. studied the influence of irradiation time on the photodegradation of CR dye using TiO₂. Under visible irradiation, CR was completely decomposed after 30 min [147].

9. Conclusions and Future Perspectives

TiO₂ and ZnO photocatalysts under either UV light or solar irradiation were studied for the degradation of organic compounds from wastewater. These catalysts have outstanding features like low cost, excellent degradation efficiency, high photocatalytic activity, etc., and are known as important catalysts in the water and wastewater industry. Both catalysts have been shown to be usable in the presence of sunlight and UV light, but the results generally indicate that using UV light is more efficient. Previous studies have shown that a mix of these catalysts with other materials can enhance the degradation efficiency of organic compounds. In this review study, the impact of various factors like temperature, time, photocatalyst loading, photocatalyst shape, pH, light wavelength, and light intensity was investigated on degrading organic compounds and the outcomes demonstrated that pH, photocatalyst dosage, temperature, and light intensity have significant impacts on the photodegradation efficiency. Also, the impact of these photocatalysts on the degradation rate of various organic compounds from wastewater was fully surveyed. Moreover, various cocatalysts can be utilized in the structure of TiO₂ and ZnO nanocatalysts, so the impact of adding metal and non-metal cocatalysts on their structures was fully studied. Studies show that the addition of some metal cocatalysts such as Ag, Pd, and Co and non-metal cocatalysts such as C and N in the structure of ZnO and TiO₂ significantly increases the photodegradation performance of organic contaminants. Furthermore, the reusability of ZnO and TiO₂ catalysts showed that they have high stability and significant reusability. Therefore, they can be used in several cycles without a significant decrease in their photocatalytic activity. The 2%Au-TiO₂ nanocatalyst with a photodegradation efficiency of 100% after 11 reuse cycles showed outstanding stability for the elimination of methyl orange dye. Also, the photodegradation efficiency of 2-(methacryloyloxy) ethyltrimethylammonium chloride/TiO₂ photocatalyst reduced from 99.66% to 98.7% after 20 reuse cycles for the elimination of MB dye, indicating remarkable stability.

Generally, previous studies have shown that they can remove more than 90% of most organic contaminants. Therefore, both metals and non-metals anchored to TiO₂ and ZnO are strongly recommended for the elimination of organic compounds from municipal and industrial wastewaters.