A Review on Cu2O-Based Composites in Photocatalysis: Synthesis, Modification, and Applications

Photocatalysis technology has the advantages of being green, clean, and environmentally friendly, and has been widely used in CO2 reduction, hydrolytic hydrogen production, and the degradation of pollutants in water. Cu2O has the advantages of abundant reserves, a low cost, and environmental friendliness. Based on the narrow bandgap and strong visible light absorption ability of Cu2O, Cu2O-based composite materials show infinite development potential in photocatalysis. However, in practical large-scale applications, Cu2O-based composites still pose some urgent problems that need to be solved, such as the high composite rate of photogenerated carriers, and poor photocatalytic activity. This paper introduces a series of Cu2O-based composites, based on recent reports, including pure Cu2O and Cu2O hybrid materials. The modification strategies of photocatalysts, critical physical and chemical parameters of photocatalytic reactions, and the mechanism for the synergistic improvement of photocatalytic performance are investigated and explored. In addition, the application and photocatalytic performance of Cu2O-based photocatalysts in CO2 photoreduction, hydrogen production, and water pollution treatment are discussed and evaluated. Finally, the current challenges and development prospects are pointed out, to provide guidance in applying Cu2O-based catalysts in renewable energy utilization and environmental protection.


Introduction
With the development of industrialization, the use of fossil fuels in industry has caused many problems, such as carbon dioxide emissions causing global warming, water pollution, and the destruction of surrounding biological habitats. Energy shortages and environmental pollution pose a serious threat to the development of industry and agriculture, and have become hot topics that need to be addressed [1,2]. Photocatalytic technology utilizes semiconductor materials to achieve the photoreduction of CO 2 , the photocatalytic decomposition of water, and the degradation of pollutants, and has the advantages of a low cost, simple operation, and no secondary pollution [3][4][5]. Photocatalysis is a green technology that fully utilizes solar energy, and is considered one of the most feasible and promising methods to solve environmental and energy problems.
Since Fukushima et al. [6] discovered in 1967 that TiO 2 can decompose water to produce hydrogen under light, tremendous progress has been made in photocatalytic technology. Due to its stable structure, high efficiency, low cost, nontoxicity, and high optical stability, TiO 2 has become widely studied in the past few decades [7,8]. However, TiO 2 can only absorb 3-5% of total ultraviolet light, so its utilization of sunlight is not high, significantly limiting its practical application under sunlight [9,10]. To effectively utilize the maximum proportion of visible light covering the solar spectrum (λ > 400 nm), in addition to modifying TiO 2 , researchers have studied a series of novel photocatalysts with a visible light response, such as simple oxides (ZnO [11] and Cu 2 O [12]), sulfides (CdS [13] and MoS 2 [14]), Bi-based materials (Bi 2 WO 6 [15] and BiVO 4 [16]), and nitrides (C 3 N 4 [17]).

Synthetic Methods
The low utilization of visible light by single CuO materials, and the easy complexation of electron-hole pairs generated by CuO under photoexcitation limit the application of CuO materials in photocatalysis. There is currently more research on doping Cu2O with high Li or Na metal concentrations. The bonding network of the off-domain two-and three-electron centers is disrupted, effectively localizing the electrons in the limited space.

Preparation Methods
Different preparation methods to prepare effective Cu2O materials for photocatalytic experiments have been reported in the literature. The thermal oxidation of metals is a widely used method for synthesizing high-quality oxides. The final desired thickness of

Synthetic Methods
The low utilization of visible light by single CuO materials, and the easy complexation of electron-hole pairs generated by CuO under photoexcitation limit the application of CuO materials in photocatalysis. There is currently more research on doping Cu 2 O with high Li or Na metal concentrations. The bonding network of the off-domain two-and three-electron centers is disrupted, effectively localizing the electrons in the limited space.

Preparation Methods
Different preparation methods to prepare effective Cu 2 O materials for photocatalytic experiments have been reported in the literature. The thermal oxidation of metals is a widely used method for synthesizing high-quality oxides. The final desired thickness of the Cu 2 O layer is prepared based on the oxidation of the high-purity copper foil. The temperature range is between 1000 and 1500 • C under a pure oxygen atmosphere or mixed gas atmosphere (e.g., Ar + O 2 ). The obtained Cu 2 O is polycrystalline, with different grain structures depending on the chosen experimental conditions. In general, a mixture of CuO and Cu 2 O appears in copper foil at the end of oxidation. The Cu 2 O appears first, at the beginning of the oxidation process, while the CuO takes a long time to appear in the oxidation process. Electrodeposition is one of the methods for the production of high-quality Cu 2 O. The advantages of this method are that it is cheap, can efficiently work on different substrates, and allows for adjustments in the material properties and morphology according to the following parameters: the applied potential, current, and temperature, and the pH of the tank solution. The first electrochemical synthesis of Cu 2 O was by Stareck [27]. Subsequently, many other scholars developed different synthesis methods, using copper precursors, electrolytes, and electrochemistry. Two types of Cu 2 O nanoparticles were successfully synthesized by adjusting the pH of the electrolyte [28]. Firstly, the pH of the electrolyte was adjusted to 10, and a pyramid-shaped p-type Cu 2 O crystal was grown on the FTO substrate. Subsequently, the pH was adjusted to 4.9, and an ultra-thin layer of n-Cu 2 O deposition product was obtained ( Figure 3). The p-Cu 2 O nanoparticles and the n-Cu 2 O protective layer on the surface formed a p/n heterojunction. The modified p/n-Cu 2 O had a bandgap of 2-2.2 eV, and could be excited by visible light with a wavelength less than 600 nm. Its photocurrent response was significantly improved, increasing the charge transfer rate and the stability of the catalyst. Thus, the modified p/n-Cu 2 O catalyst exhibited much higher activity than the original p-Cu 2 O. electrolyte was adjusted to 10, and a pyramid-shaped p-type Cu2O crystal was grown on the FTO substrate. Subsequently, the pH was adjusted to 4.9, and an ultra-thin layer of n Cu2O deposition product was obtained ( Figure 3). The p-Cu2O nanoparticles and the n Cu2O protective layer on the surface formed a p/n heterojunction. The modified p/n-Cu2O had a bandgap of 2-2.2 eV, and could be excited by visible light with a wavelength less than 600 nm. Its photocurrent response was significantly improved, increasing the charge transfer rate and the stability of the catalyst. Thus, the modified p/n-Cu2O catalyst exhib ited much higher activity than the original p-Cu2O. Magnetron sputtering is a process that uses high-energy particles to bombard a solid target, so that atoms or molecules sputtered from the surface of the target form a thin film in a specific region. The CuO films prepared using this method exhibit nanometer-sized columnar structures, and the crystallinity, grain size, and film thickness of the Cu2O film can be controlled by varying the sputtering parameters (e.g., the sputtering power, oxygen content, oxygen concentration, sputter deposition time, and annealing temperature) Cu2O-CuO films with an excellent photocatalytic performance have been deposited on glass substrates using RF magnetron sputtering ( Figure 4) [29]. It has been observed tha with the prolongation of the sputtering deposition time, the size of the Cu2O-CuO nano particles has increased from 7 nm to 13 nm, and the thickness of the thin films from 7 nm to 50 nm, resulting in a rougher surface, reduced bandgap, and decreased PL strength The results indicate that the structure, morphology, and optical and photocatalytic prop erties of prepared Cu2O-CuO films are strongly dependent on the deposition thickness Under sunlight exposure, Cu2O-CuO films can completely degrade pollutants (methylene blue and methyl orange) from water within only 60 min. Magnetron sputtering is a process that uses high-energy particles to bombard a solid target, so that atoms or molecules sputtered from the surface of the target form a thin film in a specific region. The CuO films prepared using this method exhibit nanometersized columnar structures, and the crystallinity, grain size, and film thickness of the Cu 2 O films can be controlled by varying the sputtering parameters (e.g., the sputtering power, oxygen content, oxygen concentration, sputter deposition time, and annealing temperature). Cu 2 O-CuO films with an excellent photocatalytic performance have been deposited on glass substrates using RF magnetron sputtering ( Figure 4) [29]. It has been observed that with the prolongation of the sputtering deposition time, the size of the Cu 2 O-CuO nanoparticles has increased from 7 nm to 13 nm, and the thickness of the thin films from 7 nm to 50 nm, resulting in a rougher surface, reduced bandgap, and decreased PL strength. The results indicate that the structure, morphology, and optical and photocatalytic properties of prepared Cu 2 O-CuO films are strongly dependent on the deposition thickness. Under sunlight exposure, Cu 2 O-CuO films can completely degrade pollutants (methylene blue and methyl orange) from water within only 60 min.

Other Methods
In addition to the above methods, different surfactants [30][31][32][33][34][35] and micelles [36] have been used, mainly to control the morphology of the prepared Cu-based catalyst particles. Cu 2 O nanocrystals and nanoarray with cubic [37][38][39], octahedral [40], and multipod structures [41] have been prepared using these methods. Yang et al. [42] have proposed a metal-induced thermal reduction (MITR) method for the in-situ growth of Cu 2 O crystals on a copper substrate. The corresponding scheme is shown in Figure 5, and the operation is divided into two steps: (a) under alkaline conditions, the Cu(OH) 2 nanorod array is in-situ grown by impregnating copper foil with a mixed solution of (NH 4 ) 2 S 2 O 8 and ammonia; and (b) the Cu(OH) 2 on copper foil is directly thermally reduced to Cu 2 O nanorod array films in a N 2 atmosphere at 500 • C. The average diameter of a nanorod was 400 ± 100 nm, with a length of several micrometers. The method is simple and efficient, and the preparation process has a low energy consumption and is controllable. In addition, the introduction of the substrate metal Cu can significantly reduce the reduction temperature, by changing the Gibbs free energy of the reaction. Surfactant-free synthesis has also been developed to reduce the interference of these surfactants [43][44][45]. Solvothermal [46,47] and sol-gel [48] methods have also been tested. The wet chemistry route [49,50], thermal evaporation [51,52], chemical vapor deposition [53,54], and hydrothermal route [55][56][57][58] are also common methods for synthesizing such semiconductors. In addition, the corresponding properties of Cu 2 O-based materials synthesized by different methods are detailed in the Section 4, including their morphology, structure, band gap, and photocatalytic applications.
In addition to the above methods, different surfactants [30][31][32][33][34][35] and micelles [36] been used, mainly to control the morphology of the prepared Cu-based catalyst part Cu2O nanocrystals and nanoarray with cubic [37][38][39], octahedral [40], and multipod s tures [41] have been prepared using these methods. Yang et al. [42] have proposed a m induced thermal reduction (MITR) method for the in-situ growth of Cu2O crystals copper substrate. The corresponding scheme is shown in Figure 5, and the operati divided into two steps: (a) under alkaline conditions, the Cu(OH)2 nanorod array situ grown by impregnating copper foil with a mixed solution of (NH4)2S2O8 and am nia; and (b) the Cu(OH)2 on copper foil is directly thermally reduced to Cu2O nan array films in a N2 atmosphere at 500 °C. The average diameter of a nanorod was 100 nm, with a length of several micrometers. The method is simple and efficient, an preparation process has a low energy consumption and is controllable. In addition introduction of the substrate metal Cu can significantly reduce the reduction tempera by changing the Gibbs free energy of the reaction. Surfactant-free synthesis has also developed to reduce the interference of these surfactants [43][44][45]. Solvothermal [46,47 sol-gel [48] methods have also been tested. The wet chemistry route [49,50], thermal e oration [51,52], chemical vapor deposition [53,54], and hydrothermal route [55][56][57][58] are common methods for synthesizing such semiconductors. In addition, the correspon properties of Cu2O-based materials synthesized by different methods are detailed i Section 4, including their morphology, structure, band gap, and photocatalytic app tions.

Modification Strategies
Although noble metals have been used in photocatalytic organic waste degradation and CO 2 reduction, their efficiency is still high. However, the cost is also high (e.g., Pt and Au), making them unsuitable for future industrial development. In contrast, Cu 2 O is inexpensive to use. It also has excellent CO 2 capture ability and photochemical and structural properties, and shows unlimited development potential in CO 2 reduction. However, the high electron-hole complexation rate and the low optical quantum efficiency limit the application of Cu 2 O in photocatalysis. To improve the photocatalytic efficiency of Cu 2 O, the structure of Cu 2 O needs to be modified. The modified structures are mainly divided into binary and ternary Cu 2 O heterostructure structures, and the addition of co-catalysts, in this section.

Cu 2 O/Noble Metal Heterojunction
The Fermi energy level of the noble metal material is relatively low compared to that of the catalyst in the photocatalytic reduction of CO 2 , which has a higher work function than that of the catalyst. The mutual contact between the two will form a Schottky barrier at the metal-semiconductor interface, which can effectively inhibit the complexation of photoexcited electron-hole pairs, thus promoting the catalytic process, and improving the catalytic efficiency of the catalyst. The currently synthesized Cu 2 O/noble metal composites are Cu 2 O/Ag [59], Cu 2 O/Au [60], and Cu 2 O/Pt [61]. These materials show more than 90% photocatalytic efficiency for modified Cu 2 O. Cu 2 O/Au nanostructures have been extensively investigated in recent years. Kuo et al. [62] reported the synthesis of Au@Cu 2 O core-shell nanocrystals using a chemical reduction method. The nanocrystals exhibited high activity in degrading methyl orange. Ag is relatively inexpensive, and has a higher electron transfer efficiency than metallic Au. Therefore, Ag/Cu 2 O catalysts have been more widely studied. Yang et al. [63] prepared Cu 2 O/Ag spherical microstructures by depositing silver nanoparticles on the surface of Cu 2 O through the thermal decomposition of silver acetate.

Cu 2 O/Graphene (GO) Heterojunction
From amorphous carbon black to crystalline structured natural layered graphite, and from zero-dimensional nanostructured fullerenes to two-dimensional structured graphene, carbon materials have been the most widely used and endlessly promising materials on earth. In recent decades, carbon nanomaterials have attracted much attention. The discovery of graphene self-assembled hydrogels with three-dimensional mesh structures has dramatically enriched the carbon material family, and provided a new growth point for new materials. Due to their unique nanostructure and properties, they have also shown significant scientific significance and experimental results. Thus, they provide a new target and direction when it comes to researching carbon-based materials. Graphene has been compounded with semiconductor photocatalysts, using its regular two-dimensional planar structure as a photocatalyst carrier. On the one hand, this could improve the dispersion of the catalyst. On the other hand, it could accelerate the photogenerated charge migration rate, and improve the photocatalytic activity of the composites.
Huang et al. [64] used the hydrothermal method to add graphene with the mass fractions of 0.1, 0.5, and 1 to Cu 2 O, which were noted as Cu 2 O/GO-0.1, Cu 2 O/GO-0.5, and Cu 2 O/GO-1, respectively ( Figure 6). The experimental results showed that the highest hydrogen yield of Cu 2 O modified with graphene (118.3 mmol) was more than twice that of pure Cu 2 O (44.6 mmol). During the formation of Cu 2 O/GO composites, many negatively charged functional groups in graphene can recombine with positively charged copper ions by electrostatic adsorption, thus forming Cu 2 O/graphene composite structures directly during the reduction process. This principle has been used to synthesize cubic and octahedral Cu 2 O/GO composites. This structure could improve the efficiency of electron-hole separation. It could also improve the stability of the prepared catalysts. The experimental results showed that the cubic and octahedral Cu 2 O/GO composites degraded methyl orange with more than 90% efficiency. After six replicate tests, the efficiency remained above 70%, indicating that the prepared catalysts had excellent stability [65].
Graphene has properties such as the half-integer Hall effect, a unique quantum tunneling effect, and the bipolar electric field effect. In particular, its excellent electrical conductivity and huge specific surface area provide a feasible way to solve the bottleneck problem in the photocatalytic reaction of Cu 2 O-based composites. hole separation. It could also improve the stability of the prepared catalysts. The exper mental results showed that the cubic and octahedral Cu2O/GO composites degraded me thyl orange with more than 90% efficiency. After six replicate tests, the efficiency remaine above 70%, indicating that the prepared catalysts had excellent stability [65]. Graphene has properties such as the half-integer Hall effect, a unique quantum tun neling effect, and the bipolar electric field effect. In particular, its excellent electrical con ductivity and huge specific surface area provide a feasible way to solve the bottlenec problem in the photocatalytic reaction of Cu2O-based composites.

Ternary Cu2O-Based Heterojunctions
In recent years, binary photocatalytic composites of Cu2O have achieved hig achievements in the treatment of organic matter form wastewater and CO2 reduction However, it will be a long time before binary photocatalytic composites can be used i society and daily life. Therefore, the development of ternary photocatalytic composite has become inevitable.
Yang et al. [66] used ternary Ag-CuO/GO as a photocatalytic material in the photo catalytic degradation of methyl orange, and the degradation efficiency of Ag-CuO/GO o the methyl orange was 90% after 60 min of visible light irradiation. Fu et al. [67] prepare TiO2-Ag-Cu2O composite catalysts for enhanced photocatalytic hydrogen production. Th experimental results showed that the synergistic effect of Ag and Cu2O improved the pho tocatalytic efficiency of the reaction. In addition, the prepared composite catalysts had double Z-scheme charge transfer pathway, which reduced the electron-hole complexatio probability. The weak oxidation holes and weak reduction electrons in the charge transfe

Ternary Cu 2 O-Based Heterojunctions
In recent years, binary photocatalytic composites of Cu 2 O have achieved high achievements in the treatment of organic matter form wastewater and CO 2 reduction. However, it will be a long time before binary photocatalytic composites can be used in society and daily life. Therefore, the development of ternary photocatalytic composites has become inevitable.
Yang et al. [66] used ternary Ag-CuO/GO as a photocatalytic material in the photocatalytic degradation of methyl orange, and the degradation efficiency of Ag-CuO/GO on the methyl orange was 90% after 60 min of visible light irradiation. Fu et al. [67] prepared TiO 2 -Ag-Cu 2 O composite catalysts for enhanced photocatalytic hydrogen production. The experimental results showed that the synergistic effect of Ag and Cu 2 O improved the photocatalytic efficiency of the reaction. In addition, the prepared composite catalysts had a double Z-scheme charge transfer pathway, which reduced the electron-hole complexation probability. The weak oxidation holes and weak reduction electrons in the charge transfer process were directly quenched, and the photogenerated carrier separation efficiency and catalyst reduction capacity were significantly enhanced.

Co-Catalyst Addition
In addition to constructing heterojunction structures, the photocatalytic efficiency can be improved by adding co-catalysts. Suitable co-catalysts are often present on the photocatalyst surface as active centers for oxidation or reduction, which can reduce the oxidation or reduction overpotential, and thus contribute to the photocatalytic reaction. In general, co-catalysts have three primary roles: (1) promoting the separation of the photoexcited electron-hole pairs; (2) inhibiting side reactions; and (3) improving the selectivity of the target products. Yu et al. [68] reported that adding the co-catalyst Cl to Cu 2 O nanorods led to a strong CO 2 reduction ability. The experimental results showed that the addition of co-catalyst Cl mainly reduced its direct energy band, and also achieved an increase in the carrier density and conductivity. Zhang et al. [69] doped Zn in Cu 2 O microcubes, and the hydrogen production rate of Cu 2 O was six times higher than that of pure Cu 2 O when the Zn content was 0.1 wt.%. Kalubowila et al. [70] proposed a new method for introducing cocatalysts. They used ascorbic acid (AA) to reduce the prepared Cu 2 O/GO, where Cu 2 O was partially converted to Cu, and GO was fully converted to rGO. Cu nanoparticles with tens of nanometers have acted as co-catalysts in Cu 2 O/Cu/rGO composites, providing centers for effective charge transfer, and enhancing the performance of photocatalytic degradation.

Photocatalytic Applications
Semiconductor photocatalytic reactions are based on the solid energy band theory. Under the light, the available photogenerated electrons (e − ) and holes (h + ) in the conduction band (CB) and valence band (VB) of the semiconductor migrate to the surface, to participate in the redox reaction. Therefore, the appropriate match between the CB/VB position of the photocatalyst and the redox potential determines whether the reaction can occur. In general, the CB position of the photocatalyst should be more negative to the reduction potential of the reaction, to promote the transfer of e − from CB to the reactant; at the same time, the VB position should be corrected to the oxidation potential of the reaction, to ensure that holes can be transported from VB to the reactant. The bandgap of Cu 2 O-based materials is shown in Figure 7 [71]. It has been proven that they can be used as photocatalysts to achieve CO 2 reduction (CO 2 RR), hydrogen production from water, pollutant degradation, and the reduction reaction of Cr. This section summarizes and discusses the latest progress in applications of Cu 2 O-based photocatalysts. rods led to a strong CO2 reduction ability. The experimental results showe tion of co-catalyst Cl mainly reduced its direct energy band, and also achie in the carrier density and conductivity. Zhang et al. [69] doped Zn in Cu and the hydrogen production rate of Cu2O was six times higher than th when the Zn content was 0.1 wt.%. Kalubowila et al. [70] proposed a n introducing cocatalysts. They used ascorbic acid (AA) to reduce the prep where Cu2O was partially converted to Cu, and GO was fully converted to particles with tens of nanometers have acted as co-catalysts in Cu2O/Cu/r providing centers for effective charge transfer, and enhancing the perform catalytic degradation.

Photocatalytic Applications
Semiconductor photocatalytic reactions are based on the solid ener Under the light, the available photogenerated electrons (e − ) and holes (h + tion band (CB) and valence band (VB) of the semiconductor migrate to the ticipate in the redox reaction. Therefore, the appropriate match between tion of the photocatalyst and the redox potential determines whether the cur. In general, the CB position of the photocatalyst should be more negat tion potential of the reaction, to promote the transfer of e − from CB to the same time, the VB position should be corrected to the oxidation potential to ensure that holes can be transported from VB to the reactant. The ba based materials is shown in Figure 7 [71]. It has been proven that they photocatalysts to achieve CO2 reduction (CO2RR), hydrogen production f lutant degradation, and the reduction reaction of Cr. This section summ cusses the latest progress in applications of Cu2O-based photocatalysts.

Photocatalytic CO 2 Reduction
Photocatalytic technology can convert CO 2 into CO and hydrocarbon fuels, achieving carbon recycling, and reducing greenhouse gas emissions. The application of Cu 2 O has been hampered largely by its inherent photocorrosion, ultra-fast charge recombination rate, and slow charge transport dynamics. In recent years, researchers have conducted and developed a series of novel Cu 2 O-based photocatalysts, making significant progress.
As is well known, semiconductors with different morphologies often expose different crystal faces, and exhibit varying photocatalytic activity. Celaya et al. [72] calculated by density-functional theory (DFT) that the (110) and (111) crystal faces of Cu 2 O have the potential of photocatalytic reduction of CO 2 to produce hydrocarbon derivatives. To further determine the catalytic mechanism and active site, Wu and his colleagues [73] successfully prepared Cu 2 O nanocrystals with (110) and (100) crystal faces through colloidal synthesis, and carried out photocatalytic reactions using CO 2 and H 2 O. gas chromatography-mass spectrometry (GC-MS), confirming that methanol was the only product of photoreduction, and the internal quantum yield was approximately 72%. In photocatalytic reactions, the (110) surface of a single Cu 2 O particle showed photocatalytic activity, while the (100) surface was inert. The electronic density of the Cu active site on the (110) surface moved from Cu (i) to Cu (ii), and the oxidation state of the Cu changed from Cu (ii) to Cu (i) after CO 2 conversion under light. In 2022, Sahu et al. [74] synthesized and characterized Cu 2 O photocatalysts with cubic and truncated cubic structures. Their correspondingly exposed crystal faces were different ( Figure 8). Due to the selective accumulation of e − and h + on different crystal planes, the photocatalytic activity in selectively reducing CO 2 to methanol on cubic Cu 2 O with anisotropic {100} and {110} crystal planes was nearly 5.5 times higher than that on cubic Cu 2 O with only {100} crystal planes. been hampered largely by its inherent photocorrosion, ultra-fast charge r rate, and slow charge transport dynamics. In recent years, researchers ha and developed a series of novel Cu2O-based photocatalysts, making signific As is well known, semiconductors with different morphologies often ex crystal faces, and exhibit varying photocatalytic activity. Celaya et al. [72] density-functional theory (DFT) that the (110) and (111) crystal faces of C potential of photocatalytic reduction of CO2 to produce hydrocarbon deriva ther determine the catalytic mechanism and active site, Wu and his colleag cessfully prepared Cu2O nanocrystals with (110) and (100) crystal faces thro synthesis, and carried out photocatalytic reactions using CO2 and H2O. ga raphy-mass spectrometry (GC-MS), confirming that methanol was the on photoreduction, and the internal quantum yield was approximately 72%. lytic reactions, the (110) surface of a single Cu2O particle showed photocata while the (100) surface was inert. The electronic density of the Cu active sit surface moved from Cu (i) to Cu (ii), and the oxidation state of the Cu chan (ii) to Cu (i) after CO2 conversion under light. In 2022, Sahu et al. [74] syn characterized Cu2O photocatalysts with cubic and truncated cubic structure spondingly exposed crystal faces were different (Figure 8). Due to the selec lation of e − and h + on different crystal planes, the photocatalytic activity in ducing CO2 to methanol on cubic Cu2O with anisotropic {100} and {110} cryst nearly 5.5 times higher than that on cubic Cu2O with only {100} crystal plan Meanwhile, researchers have adopted various modification methods to structure and performance of the photocatalyst. Element doping is a com Meanwhile, researchers have adopted various modification methods to optimize the structure and performance of the photocatalyst. Element doping is a commonly used method to effectively change the physical properties of semiconductors, to improve their catalytic activity. Cl doping has been shown to optimize the catalytic activity of Cu 2 O [75]. At 400 nm, the apparent quantum yield (AQE) of Cl-doped Cu 2 O photocatalytic reduction of CO 2 to CO and CH 4 increased, with 1.13% and 1.07% for CO and CH 4 , respectively. The reason behind the enhanced performance of CO 2 RR was not only that the Cl doping optimized the energy band structure and conductivity of Cu 2 O, and improved the adsorption capacity of CO 2 and the separation efficiency of the photogenerated carriers, but also that the Cl-doped Cu 2 O was conducive to the conversion of CO 2 into the intermediates of *COOH, *CO, and *CH 3 O, thus improving the yield and selectivity of CO and CH 4 .
Constructing heterojunction structures is also an effective method for band reconstruction. In heterostructures, the internal electric field is formed at the contact interface of two or more semiconductors with the movement of the Fermi level, which drives the directional migration and separation of photogenerated electrons and holes. Common heterojunctions include the traditional (Type-I, II, and III), p-n, Z-scheme, and S-scheme. The p-n heterojunction of Cu 2 O and n-type semiconductors can effectively delay the recombination of photogenerated carriers, and promote electron transfer [76]. The yield of the photocatalytic reduction of CO 2 to CH 3 OH from the Cu 2 O/TiO 2 heterojunction after 6 h of UV-Vis irradiation has been 21.0-70.6 µmol/g cat . At the p-n heterojunction, the photogenerated electrons and holes are separated and transferred to the CB/VB with lower potential energy, respectively, resulting in a redox ability closer to the lower of the two semiconductors. The Z-scheme heterostructure solves this problem perfectly. Electrons and holes in the CB/VB with lower energy recombine, and cancel each other out in the Z-scheme heterojunction, thus retaining the higher conduction and valence band values in the two semiconductors, and enhancing the redox ability of the photocatalyst. For example, the Ag-Cu 2 O/ZnO nanorods (NRs) reported by Zhang and his team showed an enhanced photocatalytic CO 2 reduction performance [77]. Under UV-vis light, the yield of CO significantly increased, which was seven times higher that of pure ZnO or Cu 2 O NRs. The results showed that the deposited Cu 2 O can enhance the chemical adsorption of CO 2 on the catalyst surface, and the Z-scheme charge transfer pathway formed between the ZnO and Cu 2 O can promote effective charge separation, thereby improving the photocatalysis performance.
Due to the small bandgap energy and high conduction band value of Cu 2 O-based materials, the products of photocatalytic CO 2 RR are complex, mainly including CO and various organic compounds (CH 4 , CH 3 OH, HCOOH). According to the different reaction products, the application of Cu 2 O-based materials in photocatalytic CO 2 reduction is summarized in Table 1.

Photocatalytic H 2 Production
Hydrogen energy is abundant and renewable, which can effectively avoid energy exhaustion, and the products of hydrogen energy combustion will not cause pollution. Photocatalytic hydrogen production has the advantages of high efficiency, low cost, and environmental friendliness, and has great potential in high-efficiency hydrogen evolution. Common semiconductor photocatalysts (such as TiO 2 , ZnO, and g-C 3 N 4 .) have the disadvantage of a low utilization of sunlight, and the photocatalytic hydrogen evolution efficiency is not ideal. In recent years, Cu 2 O has become a research hotspot in photocatalytic hydrogen evolution because of its excellent photoresponsiveness. However, the poor charge separation ability of pure Cu 2 O lowers its hydrogen evolution performance. It is essential to modify and adjust Cu 2 O-based catalysts to meet the practical need to increase the hydrogen production yield.
Hybridizing Cu 2 O with other semiconductor materials to construct heterojunctions can achieve the effective separation of photo-induced charge carriers, which is an effective method to enhance photocatalytic activity, and has been validated in numerous studies on photocatalytic hydrogen production. NiFe 2 O 4 /Cu 2 O with different mass percentages has been synthesized by impregnation and thermal annealing methods to construct p-n heterojunctions [94]. The photocatalytic hydrogen production rate of all heterojunctions was significantly higher than that of the original material. The 50/50 mass ratio was the most effective, and the hydrogen production rate within 24 h was 102.4 mmol·g −1 , while NiFe 2 O 4 and Cu 2 O only obtained 1.35 and 0.85 mmol·g −1 , respectively. The increase in activity came from the enhanced charge separation at the heterojunction, which increased the concentration of charge carriers (Figure 9). Cu 2 O/CaTiO 3 series samples were synthesized using the hydrothermal method and NaBH 4 reduction treatment [95]. The photocatalytic hydrogen production effect of the 50Ca10Cu sample was the best (8.268 mmol·g −1 ·h −1 ), about 344.5 times that of the CaTiO 3 sample. It also exhibited perfect stability after multiple cyclic tests. exhaustion, and the products of hydrogen energy combustion will not ca Photocatalytic hydrogen production has the advantages of high efficiency, environmental friendliness, and has great potential in high-efficiency hydro Common semiconductor photocatalysts (such as TiO2, ZnO, and g-C3N4.) h vantage of a low utilization of sunlight, and the photocatalytic hydrogen ciency is not ideal. In recent years, Cu2O has become a research hotspot in hydrogen evolution because of its excellent photoresponsiveness. Howe charge separation ability of pure Cu2O lowers its hydrogen evolution perf essential to modify and adjust Cu2O-based catalysts to meet the practical ne the hydrogen production yield.
Hybridizing Cu2O with other semiconductor materials to construct h can achieve the effective separation of photo-induced charge carriers, which method to enhance photocatalytic activity, and has been validated in num on photocatalytic hydrogen production. NiFe2O4/Cu2O with different mas has been synthesized by impregnation and thermal annealing methods to heterojunctions [94]. The photocatalytic hydrogen production rate of all h was significantly higher than that of the original material. The 50/50 mass most effective, and the hydrogen production rate within 24 h was 102.4 m NiFe2O4 and Cu2O only obtained 1.35 and 0.85 mmol•g −1 , respectively. The tivity came from the enhanced charge separation at the heterojunction, wh the concentration of charge carriers (Figure 9). Cu2O/CaTiO3 series samples sized using the hydrothermal method and NaBH4 reduction treatment [95 catalytic hydrogen production effect of the 50Ca10Cu sample was th mmol•g −1 •h −1 ), about 344.5 times that of the CaTiO3 sample. It also exhibited ity after multiple cyclic tests. The above p-n heterojunctions are typical type-Ⅱ heterojunctions, whic the redox capacity of photogenerated electrons and holes. Researchers hav signed and constructed Z-scheme and step-scheme (S-scheme) heterojuncti catalytic hydrogen production. For example, dendritic branched Cu2O wa hydrothermally, and Cu2O/TiO2 composites were prepared via surface charg [96]. The hydrogen production rate of the optimized CT-70 (Cu2O coupled TiO2) photocatalyst reached 14.020 mmol −1 within six hours, which was 264 higher than that of pure Cu2O and TiO2, respectively. The electron transfer the Z-scheme was proposed and verified via DFT calculation and EPR an The above p-n heterojunctions are typical type-II heterojunctions, which often impair the redox capacity of photogenerated electrons and holes. Researchers have recently designed and constructed Z-scheme and step-scheme (S-scheme) heterojunctions for photocatalytic hydrogen production. For example, dendritic branched Cu 2 O was synthesized hydrothermally, and Cu 2 O/TiO 2 composites were prepared via surface charge modulation [96]. The hydrogen production rate of the optimized CT-70 (Cu 2 O coupled with 70 wt.% TiO 2 ) photocatalyst reached 14.020 mmol −1 within six hours, which was 264 and 44 times higher than that of pure Cu 2 O and TiO 2 , respectively. The electron transfer mechanism of the Z-scheme was proposed and verified via DFT calculation and EPR analysis. Under simulated sunlight, photoexcited electrons migrate from the CB of TiO 2 to the VB of Cu 2 O, and then recombine with photogenerated holes in the VB of Cu 2 O, thereby retaining highly reducing electrons and highly oxidizing holes ( Figure 10). Therefore, under the conditions of sensitive photosensitivity and the effective separation of photogenerated electrons and holes, the performance of photocatalysts in hydrogen evolution under visible light is significantly improved. The S-scheme heterojunction photocatalyst has a similar efficient carrier separation performance and enhanced redox capacity. Cu 2 O/g-C 3 N 4 composites were successfully synthesized using a simple wet chemical method, and applied in the field of photocatalytic energy production. Cu 2 O/g-C 3 N 4 series samples showed high catalytic activity. In particular, 1-Cu 2 O/g-C 3 N 4 showed the highest hydrogen evolution rate of 480.6 µmol·g −1 ·h −1 under visible light irradiation, 12.0 times that of the original Cu 2 O sample. Based on the analysis of the experimental and simulation results, the ideal catalytic performance of the Cu 2 O/g-C 3 N 4 photocatalyst was derived from the efficient interfacial charge separation and transfer of the S-scheme heterostructure [97].

FOR PEER REVIEW 14 of 25
and then recombine with photogenerated holes in the VB of Cu2O, thereby retaining highly reducing electrons and highly oxidizing holes ( Figure 10). Therefore, under the conditions of sensitive photosensitivity and the effective separation of photogenerated electrons and holes, the performance of photocatalysts in hydrogen evolution under visible light is significantly improved. The S-scheme heterojunction photocatalyst has a similar efficient carrier separation performance and enhanced redox capacity. Cu2O/g-C3N4 composites were successfully synthesized using a simple wet chemical method, and applied in the field of photocatalytic energy production. Cu2O/g-C3N4 series samples showed high catalytic activity. In particular, 1-Cu2O/g-C3N4 showed the highest hydrogen evolution rate of 480.6 μmol•g −1 •h −1 under visible light irradiation, 12.0 times that of the original Cu2O sample. Based on the analysis of the experimental and simulation results, the ideal catalytic performance of the Cu2O/g-C3N4 photocatalyst was derived from the efficient interfacial charge separation and transfer of the S-scheme heterostructure [97]. Furthermore, photocorrosion is currently an urgent problem for Cu2O photocatalysts, and finding effective strategies to suppress photocorrosion in photocatalysts is still an enormous challenge. To overcome this challenge, Liu et al. [98] proposed a core-shell model: the Cu2O/PyTTA-TPA COF nanocube photocatalyst was constructed using an energy level matching the Cu2O and 2D PyTTA-TPA COF. It exhibited an excellent photocatalytic hydrogen evolution rate of 12.5 mmol•g −1 •h −1 , approximately 8.0 and 20.0 times higher than the PyTTA TPA COF and Cu2O, respectively. Most importantly, under the protection of the stable PyTTATPA-COF shell, the Cu2O nanocube core was protected from photocorrosion, and did not show noticeable morphological or crystal structure changes after 1000 light excitations, thus significantly improving the photocorrosion resistance stability of the catalyst. Table 2 shows the recently reported Cu2O-based materials for photocatalytic hydrogen production. Furthermore, photocorrosion is currently an urgent problem for Cu 2 O photocatalysts, and finding effective strategies to suppress photocorrosion in photocatalysts is still an enormous challenge. To overcome this challenge, Liu et al. [98] proposed a core-shell model: the Cu 2 O/PyTTA-TPA COF nanocube photocatalyst was constructed using an energy level matching the Cu 2 O and 2D PyTTA-TPA COF. It exhibited an excellent photocatalytic hydrogen evolution rate of 12.5 mmol·g −1 ·h −1 , approximately 8.0 and 20.0 times higher than the PyTTA TPA COF and Cu 2 O, respectively. Most importantly, under the protection of the stable PyTTATPA-COF shell, the Cu 2 O nanocube core was protected from photocorrosion, and did not show noticeable morphological or crystal structure changes after 1000 light excitations, thus significantly improving the photocorrosion resistance stability of the catalyst. Table 2 shows the recently reported Cu 2 O-based materials for photocatalytic hydrogen production.

Photocatalytic Degradation of Pollutants
With the rapid development of the global economy, industrial and agricultural waste is produced in large quantities, and continues to enter the environment. Many organic pollutants also enter the environment, and some show persistent pollution, which is difficult to remove through microbial action and hydrolysis. The long-term existence and accumulation of refractory pollutants leads to environmental pollution and ecological imbalance, and even threatens human survival and development. Research and development around pollutant degradation technology are critical. Photocatalytic technology has shown promising prospects for treating refractory pollutants, such as the photocatalytic processes that mineralize organic pollutants into water and CO 2 , and which essentially eliminate secondary pollution, rather than concentrating these pollutants and their by-products into the waste stream. In the past few decades, extensive research has been conducted on Cu 2 O-based photocatalysts to purify the environment. Table 3 summarizes the recent reports of Cu 2 O-based photocatalysts in pollutant degradation.
Among all the types of pollutants, organic dyes have become an important source of water pollution. As refractory organic pollutants, dyes cause severe damage to human health and the ecological balance. Traditional adsorption methods only transfer toxic organic molecules to the solid surface, without eliminating them, and still run the risk of desorption. MBC@Cu 2 O composites have been prepared by loading porous spherical Cu 2 O onto wood biochar carriers, with a liquid-phase synthesis strategy, at room temperature [109]. As a bi-functional adsorption-based photocatalytic composite, MBC@Cu 2 O showed great potential in removing anionic dye methyl orange (MO) from water. Under visible light irradiation, the photocatalytic degradation efficiency of MO reached 94.5%, and remained above 80% after five cycles. In another work, Sehrawat and his team prepared MoS 2 /Cu 2 O composites with different weight ratios via precipitation, using MoS 2 nanosheets and Cu 2 O nanospheres [110]. The photocatalytic degradation of indigo carmine (IC) dye was carried out under simulated visible light. Compared to the original MoS 2 , the optimized MC-3 sample showed the best degradation performance, with a degradation rate of 99.59% for IC within 90 min, and no significant change in performance after five cycles. Experiments regarding the capture of active species showed that the photocatalytic reaction relied on the production of the superoxide radical (•O 2 − ), and further verified the Z-scheme mechanism of the MoS 2 /Cu 2 O photocatalyst. In the same year, Li et al. synthesized the core-shell WO 3 -Cu 2 O Z-scheme heterojunction via hydrothermal and electrochemical deposition methods for the photocatalytic degradation of methylene blue (MB) under visible light [111]. The Cu 2 O nanoparticles deposited on the surface of WO 3 enhanced the visible light absorption ability. The Z-scheme heterojunction achieved the effective spatial separation of the charges, and retained the strong redox ability of the photogenerated electrons and holes. The WO 3 -Cu 2 O-120s photocatalyst showed the highest reaction rate, almost twice that of the original WO 3 .
As a typical persistent organic pollutant, antibiotics are difficult to degrade and remove, due to their low biodegradability, which has become a thorny problem in water pollution control. Research has shown that the defect states and vacancies caused by element doping significantly impact the catalytic performance of semiconductor materials. Doping semiconductor functional materials with specific elements provides a feasible way to overcome the obstacles in applications for photocatalytic degradation. Nie et al. synthesized Cl-doped Cu 2 O microcrystals using a simple hydrothermal method, and used them to treat levofloxacin contaminants (LVX) under mild reaction conditions [112]. Compared with other reaction systems, the synthesis of Cl-doped Cu 2 O has a higher degradation efficiency for levofloxacin. After 240 min of photocatalytic reaction, the maximum degradation rate of LVX was 85.8% and 80.3% after eight cycles, indicating the stability and reusability of the photocatalyst. Based on the theoretical calculation and test results, it can be concluded that introducing hybrid orbitals and oxygen vacancy defects into Cu 2 O crystal cells by doping Cl reduces the band gap of Cu 2 O, resulting in a red shift in the absorption edge. Compared with pure Cu 2 O microcrystals, the prepared Cl-doped Cu 2 O single crystals with oxygen vacancy had a narrower band gap, and higher photogenerated electron-hole separation and transport efficiency. Considering the close relationship between the morphology and electronic structure, surface energy, and chemical reactivity of nanocrystals, it is of great significance to explore the influence of the morphology/exposed crystal surface of Cu 2 O on the synthesis process and the photocatalytic performance. Wu et al. developed a series of Cu 2 O@HKUST-1 core-shell structures via self-constrained strategies, using Cu 2 O nanocrystals with different morphologies as templates [113]. The characterization results indicated that the (111) surface of Cu 2 O was more favorable for the growth of HKUST-1 than the (100) surface. Comparing the photocatalytic degradation performance of tetracycline hydrochloride (TC-HCl), it was found that Cu 2 O@HKUST-1 had the best photocatalytic performance among the three types of composite material, with a degradation efficiency of 95.35% for TC-HCl. It was attributed to the excellent photoresponse, and the most effective interfacial charge transfer and separation in the Cu 2 O@HKUST-1 cubes.
In addition to organic dyes and antibiotics, solar-powered Cu 2 O-based photocatalysts can degrade heavy metal pollutants in wastewater, mainly toxic hexavalent chromium (Cr (VI)). Xiong et al. [114] constructed a Cu 2 O/LDH photocatalyst by grafting Cu 2 O-NP, and embedding it into the LDH host layer through an in-situ reduction strategy. CuZnTi LDH is valuable in two aspects: (a) as a source of Cu 2 O, and (b) as a support bracket to avoid the self-oxidation of Cu 2 O-NPs. The optimized photocatalyst showed a high degradation efficiency for difficult-to-degrade pollutants under visible light conditions, with a reduction rate of 95.5% for Cr (VI) by Cu 2 O/LDH0.10, and a degradation rate of 71.6% for TC. The excellent photocatalytic efficiency was attributed to the charge transfer mechanism of the Cu 2 O/ZnTiLDH p-n heterojunction, effectively promoting the separation and migration of the photogenerated electron-hole. Recently, Zhu et al. [115] used the Si and Cu of waste serpentine tailings and WPCB to prepare low-cost waste-based Cu-Cu 2 O/SiO 2 photocatalysts. Due to the dispersion of Cu-Cu 2 O 3 on the surface of the SiO 2 carrier, the composite material obtained a higher specific surface area. The photocatalytic reduction of Cr (VI) using waste-based catalysts was the best at a loading rate of 9% Cu and 7g·L −1 SiO 2 , and the photocatalytic activity decreased by only 4.93% after five cycles. The mechanism of Cr (VI) reduction by the waste Cu-Cu 2 O/SiO 2 photocatalyst is to excite the waste Cu 2 O to produce photoelectron-hole pairs. The electrons in the waste group Cu 2 O CB reduce Cr (VI) adsorbed on the surface to Cr (III), and the surface Cu drives the electrons to the surface of the Cu metal, without returning the waste group Cu 2 O.
Moreover, the accumulation in soil and water of herbicides, insecticides used in the agriculture and food industries, and phenolic compounds emitted from industry, such as petrochemicals and pharmaceuticals, can have significant harmful effects on humans and aquaculture systems. The use of metal oxide photocatalysts has been proven to be an effective, low-cost, and green method for treating such wastewater. In 2021, Alp [116] successfully synthesized hybrid Cu 2 O-Cu cubes by reducing D(+)-glucose in an alkaline solution using a one-step aqueous solution synthesis method, without any toxic reagents or surfactants. The Cu 2 O-Cu exhibited excellent photocatalytic properties for dyes and herbicides, due to the effective separation of photogenerated electron-holes and the enhanced charge transfer mechanism at heterojunctions. In particular, when dealing with 2,4-Dichlorophenoxyacetic acid (2,4-D), one of the widely used herbicides in agriculture and urban landscaping, the degradation effect of the Cu 2 O-Cu heterojunction was outstanding. It photodegraded all of the 2,4-D in the medium within 40 min, while the original Cu 2 O cube photodegraded 85% within 60 min. In the same year, Mkhalid et al. [117] prepared a Cu 2 O photocatalyst loaded with Cu nanoparticles via sol-gel and photo-assisted deposition technology. The structure and optical and photoelectric properties of the prepared photocatalyst were improved by adjusting the Cu content. The results showed that the band gap of the Cu 2 O loaded with 15% Cu was reduced to 1.95 eV, significantly enhancing the visible light absorption ability. The optimized Cu@Cu 2 O photocatalyst completely photodecomposed atrazine (AZ, a commonly used triazine herbicide) within 30 min, and demonstrated excellent durability. In recent years, effectively solving the problem of phenolic pollutants in livable environments has also been a major challenge faced by humanity, and has received a high level of attention from many researchers. A lowcost but highly efficient phosphate-doped carbon/Cu 2 O composite (HKUST-1-P-300) was reported by Dubai et al. [118]. The catalyst was derived from the modification of HKUST-1 with triphenylphosphine and conditioned calcination. Under visible light irradiation, the degradation efficiency of HKUST-1-P-300 for phenol was 99.8%, the hydrogen evolution rate was 1208 µmol, and the external quantum efficiency was 48.6% (at 425 nm) within 90 min, and the high performance could still be maintained after four cycles. Mechanism studies showed that the excellent photocatalytic activity of HKUST-1-P-300 came from multiple synergistic effects: an enhanced visible light absorption efficiency, a larger surface area, the effective separation of photogenerated carriers, a reduced aggregation of Cu 2 O, and the P-doped carbon/Cu 2 O structure. These novel Cu 2 O-based materials, as highly efficient photocatalysts, have potential applications in removing environmental pollutants, and generating clean energy, to promote sustainable environmental construction. Lv et al. [133] analyzed the electronic structure and photocatalytic properties of Cu 2 O doped with different contents of Mn, using first-principle calculations. The simulation results showed that the visible light absorption intensity and photocatalytic efficiency were enhanced with the increase in doping concentration, and varied with the doping configuration, compared to pure Cu 2 O. The enhanced light absorption was mainly attributed to the in-band leaps of the electrons in the three-dimensional state of Mn. The enhancement of light absorption was mainly due to the in-band leaps of electrons in the three-dimensional state of Mn, which gave the semiconductor material certain metallic properties, and increased the absorbance of the visible light. Therefore, Cu 2 O applied to the future industrialization of photocatalysis could be doped with a small amount of Mn in the semiconductor, to improve the photocatalytic efficiency.

Conclusions
In recent years, the practical photocatalytic applications of Cu 2 O-based materials in scientific fields such as solar energy conversion and environmental remediation have attracted great interest. As a transition metal oxide, Cu 2 O has the advantages of a narrow band gap, strong visible light response, suitable conduction band position, low cost, and great potential as a photocatalyst. This paper introduces the basic properties, synthesis methods, and modification strategies of Cu 2 O-based materials. Recently reported Cu 2 O-based photocatalysts and their recent advances in photocatalysis, such as photocatalytic CO 2 reduction, photocatalytic hydrogen production, and pollutant degradation, are reviewed. However, the research on Cu 2 O-based materials is still in its early stages, and there is room for improvement in their photocatalytic performance.

1.
Currently, most Cu 2 O-based composites and sacrificial agents are synthesized from noble metal materials, which have high costs and significantly limit their large-scale applications. The development of non-precious metal catalysts, such as graphene, is vital to future development. More importantly, the catalytic efficiency of most Cu 2 Obased composites is very low, and the catalytic performance needs to be improved to meet the requirements of practical applications.

2.
Although many experimental studies on the photocatalysis of Cu 2 O-based composites are introduced in this paper, these works are still in their infancy. In addition, the large-scale production of high-quality Cu 2 O-based photocatalysts faces numerous difficulties, considering the secondary hazards of nanomaterials. Therefore, it is urgent that we further study the photocatalytic mechanism of Cu 2 O-based composites from the above perspectives, and promote the industrial application process of Cu 2 O-based composite catalysts. 3.
The photocorrosion of Cu 2 O still deserves attention. Although the current method of constructing heterojunctions to suppress photocorrosion has achieved certain results, the photocorrosion phenomenon of Cu 2 O still exists, and affects its long-term use. Establishing a core-shell structure is a good governance measure but, when synthesizing photocatalysts, it is necessary to carefully handle the thickness of the shell layer, to ensure sufficient absorption of light by the Cu 2 O. 4.
The structure of the catalyst determines the catalytic activity, while the catalytic microstructure and mechanism of Cu 2 O-based composites is still unclear. Theoretical calculations should be introduced when designing a Cu 2 O-based photocatalyst, and studying the mechanism of improving photocatalytic activity at the molecular level. In future research, DFT simulations and experiments are needed, to reveal the relationship between the establishment of the microstructure and the catalytic activity of photocatalysts.
Author Contributions: Q.S.: conceptualization, methodology, software, investigation, writing-original draft. C.Z.: methodology, validation, formal analysis, and visualization. M.L. and X.T.: funding, acquisition, and supervision. All authors have read and agreed to the published version of the manuscript.
Funding: Financial support in carrying out this work was provided by the Doctoral Research Foundation of Weifang University (2022BS13).

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.