A defect-layer modified TiO2 nanowire array photoanode and its preparation method

Abstract

The application discloses a defect layer modified TiO2 nanowire array photoanode and a preparation method thereof, and belongs to the technical field of photoelectrochemistry. 2 The Ti film is deposited on the surface of the NWAs by a magnetron sputtering method, and then annealing treatment is performed under a N2 atmosphere; during the annealing, Ti can capture O atoms in the TiO2 lattice to form a TiO2 defect layer containing oxygen vacancies on the surface, which helps to obtain a lower Fermi level; when a homojunction is formed between the defect layer and the main TiO2 nanowire, the Fermi levels tend to be balanced, thereby generating a built-in electric field, promoting the migration of photo-generated carriers and enhancing the utilization of light; the defect state concentration on the surface can be regulated by further regulating the Ti film deposition time and the annealing temperature and the like, so that the prepared photoanode has better PEC performance; the application provides an effective means for regulating the photoelectrocatalytic performance of TiO2 and also provides a theoretical basis for related applications.

Description

Technical Field

[0001] This invention belongs to the field of photoelectrochemical technology, specifically relating to a defect-layer modified TiO2 nanowire array photoanode and its preparation method. Background Technology

[0002] Photoelectrochemical water splitting systems are a clean energy production technology that converts solar energy into hydrogen. With abundant sources, environmental friendliness, and low cost, it is considered one of the most promising ways to achieve carbon neutrality. One of the key factors affecting the operating efficiency of this system is the selection of the photoanode material.

[0003] Titanium dioxide (TiO2) has been widely used in photocatalysts, chemical sensors, and luminescent materials due to its strong photoelectrochemical oxidation (PEC) capability, non-toxicity, and high stability. For example, TiO2 crystals, under sunlight irradiation, can absorb solar energy for solar power generation, photocatalytic water splitting for hydrogen production, and photocatalytic degradation of pollutants through the photoelectric conversion effect. Based on these properties, TiO2 has become a key photoanode material under research.

[0004] However, due to the relatively wide band gap of TiO2 (~3.2 eV), it can only utilize the shorter wavelengths of solar radiation, limiting its utilization of solar energy. Furthermore, in TiO2 photoanode materials, photogenerated electron-hole pairs rapidly recombine, which is detrimental to redox reactions and leads to low photoelectric conversion efficiency.

[0005] Modifying the photoelectric properties of materials is the most common method to improve their photoelectric conversion efficiency. Common methods for modifying oxide semiconductors include nanostructuring, doping with foreign elements, introducing defects, constructing heterojunctions, and supporting co-catalysts. Among these, defects, as a ubiquitous structural feature in materials, can be effectively improved by introducing and properly controlling defects, thus enhancing the physicochemical properties and photoelectrocatalytic performance of TiO2. Vacancies are the most common point defects in metal oxide semiconductors, formed at specific temperatures by minimizing the Helmholtz free energy required to establish thermodynamic equilibrium. Unlike element doping, vacancies can effectively alter the charge distribution and electronic energy levels near the defect sites, affecting the electronic structure and surface properties of the material. Furthermore, vacancy defects can also change the optical absorption and emission properties of the crystal, affecting the carrier concentration of the semiconductor and thus regulating the Fermi level.

[0006] In semiconductor oxides, oxygen vacancies (Vo) have the lowest formation energy among various donor defects and are the most common vacancy defects. They play a crucial role in controlling and altering various physical and chemical properties, influencing every step of the photoelectrochemical water splitting reaction in metal oxide semiconductors, including light absorption, photogenerated carrier separation and transport, and semiconductor-electrolyte interface reactions. Vo is generally considered to promote the PEC water splitting process by inducing changes in the TiO2 energy levels and facilitating the separation of electron-hole pairs. Furthermore, Vo introduces localized states into the energy band, altering the optical absorption of the crystal and extending the effective light range of nano-TiO2 to visible light, thus improving the utilization rate of sunlight. Therefore, defect engineering is a feasible method to address the low solar energy utilization rate and high photogenerated carrier recombination rate of TiO2 photoanodes.

[0007] The main methods for introducing oxygen vacancies in TiO2 photoanodes include heat treatment, doping, and chemical reduction. Heat treatment involves heating TiO2 in a reducing atmosphere (such as hydrogen) to induce oxygen vacancy formation. This method does not introduce impurities, but it is energy-intensive and carries certain risks. Using the reducing agent NaBH4 can generate Vo on the TiO2 surface at room temperature, but it often only generates surface Vo, resulting in poor repeatability and stability. Chemical reduction directly introduces oxygen vacancies into the TiO2 nanostructure through wet chemical methods or solid-state redox methods. This method is simple, but the introduced chemical substances may introduce impurities. Doping refers to incorporating an element during or after the synthesis of TiO2, introducing oxygen vacancies by substituting titanium or breaking Ti-O bonds. Common doping methods for introducing oxygen vacancies include metal doping, non-metal doping, and co-doping. For example, reducing metal powders (Li, Mg, Zn, Al) can introduce Vo by grinding to extract O atoms from the surface of TiO2 powder. While grinding and doping is a straightforward and easy method, it is difficult to introduce Vo into TiO2 nanowires generated on fluorine-doped tin oxide (FTO) substrates. This is because FTO nanowires typically exhibit high structural stability, and their surface atoms are likely more firmly bonded to the substrate compared to powdered TiO2, making it difficult to extract O atoms through a simple grinding process. Furthermore, the arrangement and spatial constraints of FTO nanowires prevent the reducing metal powder from making sufficient contact with the nanowire surface, thus limiting the reaction. Additionally, the structural limitations and longer diffusion paths between reactants in FTO nanowires can significantly reduce the reaction rate. In summary, grinding is ineffective in introducing Vo into FTO nanowires. FTO, as a highly conductive and transparent photoelectrocatalytic substrate material, possesses strong binding capabilities to photoanodes or photocathodes, effectively transferring electrons, reducing electron-hole recombination, and improving photoelectric efficiency, making it highly valuable for applications. Therefore, improving the preparation process to successfully introduce Vo into the TiO2 nanowire photoanode based on FTO substrate to improve the photoelectric performance of the corresponding photoanode material still has very good industrial application prospects. This requires further research on how to successfully modify the Vo defect layer on the corresponding material. Summary of the Invention

[0008] To address the aforementioned problems, this invention aims to provide a defect-modified TiO2 nanowire array photoanode and discloses its specific preparation method. By depositing a metallic Ti thin film on the surface of TiO2NWAs on an FTO substrate and annealing it in a nitrogen atmosphere, a TiO2 defect layer can be formed on the surface of TiO2NWAs. The oxygen vacancy defect layer can increase the light utilization rate of the photoanode and fully improve the PEC performance of the photoanode.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for preparing a defect-layer modified TiO2 nanowire array photoanode, comprising the following steps:

[0010] 1) Add the titanium source to the acid solution and mix to obtain the precursor solution;

[0011] 2) Pretreatment of the substrate;

[0012] 3) Place the substrate with the conductive side down at an angle in the liner of the reactor, add the precursor solution, seal the reactor, and heat the reaction. After the reaction is complete, clean the sample and dry it to grow TiO2NWAs on the substrate surface.

[0013] 4) A Ti thin film was deposited on the TiO2NWAs surface using magnetron sputtering, and the resulting sample was denoted as TiO2@Ti;

[0014] 5) Anneal the TiO2@Ti material to obtain the final product.

[0015] Furthermore, the substrate material is any one of fluorine-doped tin oxide (FTO), indium tin oxide (ITO), aluminum zinc oxide (AZO), and gallium zinc oxide (GZO).

[0016] Furthermore, in step 3), the heating reaction temperature is 130~200℃ and the heating reaction time is 10~24 h.

[0017] Furthermore, in step 3), after drying the sample, it is annealed in a muffle furnace to improve the crystallinity of the nanowires on the substrate surface. The annealing temperature is 450~550℃ and the annealing time is 0.5~3 h.

[0018] Furthermore, in step 4), DC magnetron sputtering is used. During the deposition process, the substrate rotation speed is 6~10 r / min, the deposition power is 220~240 W, and the deposition time is 1~6 min.

[0019] Furthermore, in step 5), the annealing is carried out in a nitrogen atmosphere, with an annealing temperature of 250~450℃ and an annealing time of 0.5~3 h.

[0020] The defect-modified TiO2 nanowire array photoanode prepared using the above method comprises a substrate, a TiO2NWAs layer, and a TiO2 defect layer on the surface, wherein the defect layer contains oxygen vacancies. This photoanode material can be efficiently applied to the photoelectrocatalytic splitting of water to produce hydrogen and oxygen, and the photoelectrocatalytic process can be achieved in neutral, acidic, and alkaline solutions.

[0021] The beneficial effects of this invention are as follows:

[0022] 1. In this application, a metal Ti thin film was successfully deposited on the surface of TiO2NWAs grown on FTO glass substrate by magnetron sputtering and the sample was annealed. The annealing of the Ti thin film in N2 can capture O atoms in the TiO2 lattice and form a TiO2 defect layer on the surface. The presence of the oxygen vacancy defect layer significantly improves the photoelectrocatalytic performance of TiO2.

[0023] 2. The photoanode material prepared by constructing an oxygen-containing vacancy defect layer on the surface of TiO2NWAs on an FTO glass substrate has a lower Fermi level compared to the TiO2 photoanode without a defect layer. When a homojunction is formed with the host TiO2 nanowire, the Fermi level tends to be in equilibrium, thereby generating a built-in electric field. Under the action of the electric field, the energy band of the defect layer shifts to a more negative potential. Under light excitation, photogenerated electrons migrate from the conduction band of the defect layer to the conduction band of the TiO2 nanowire, while holes migrate in the opposite direction. This process promotes the migration of photogenerated carriers and greatly increases the utilization rate of light.

[0024] 3. By controlling the magnetron deposition time, the thickness of the Ti film on the TiO2NWAs surface can be adjusted, thereby improving the PEC performance of the defect-modified TiO2NWAs photoanode.

[0025] 4. The TiO2@Ti-3-N350 photoanode material obtained by adjusting the deposition time and annealing conditions after Ti film deposition exhibits good photoelectrocatalytic activity. Defect layer modification increased the hydrogen generation rate of TiO2 by 2.9 times, and the constructed defect layer still maintains good structure and photoelectrocatalytic stability during the catalytic water splitting process.

[0026] 5. This application modifies TiO2NWAs photoanodes through an oxygen vacancy defect layer. This process provides an effective means to regulate the photoelectrocatalytic performance of TiO2, and can be achieved in neutral, acidic, and alkaline solutions, providing possibilities for large-scale applications in the future and a theoretical basis for related applications. Attached Figure Description

[0027] Figure 1 The images are scanning electron microscope (SEM) images of the photoanodes. In the image a, the smaller image is a SEM planar view of the TiO2 photoanode with TiO2NWAs grown on the FTO substrate, and the smaller image b is a SEM planar view of the TiO2@Ti-3 photoanode obtained after depositing a Ti film on the top of the TiO2 nanowire.

[0028] Figure 2The smaller image (a) shows the Raman spectra of TiO2, TiO2@Ti-3, TiO2@Ti-3-N350, and TiO2@Ti-3-A350; the smaller image (b) shows the full width at half maximum (FWHM) of the main peak in the Raman spectra of TiO2, TiO2@Ti-3, TiO2@Ti-3-N350, and TiO2@Ti-3-A350.

[0029] Figure 3 The photocurrent density data for different photoanode materials under a 1.23 V vs. RHE bias are shown below. In plot a, the photocurrent density data for photoanode materials TiO2, TiO2@Ti-1-N350, TiO2@Ti-3-N350, TiO2@Ti-6-N350, and TiO2@Ti-9-N350 under a 1.23 V vs. RHE bias is shown below. In plot b, the photocurrent density data for photoanode material TiO2@Ti-1-A350 under a 1.23 V vs. RHE bias is shown below. In plot c, the photocurrent density data for photoanode materials TiO2@Ti-3, TiO2@Ti-3-N250, TiO2@Ti-3-N350, TiO2@Ti-3-N450, and TiO2@Ti-3-N550 under a 1.23 V vs. RHE bias is shown below.

[0030] Figure 4 The small figure (a) shows the total number of electrons and the evolution of gas over time obtained from the photoelectrocatalysis test of TiO2@Ti-3-N350; the small figure (b) is a comparison of the hydrogen generation rate of TiO2 photoanode and defect-layer modified TiO2@Ti-3-N350 photoanode; the small figure (c) shows the water decomposition stability test results of TiO2@Ti-3-N350 photoanode under illumination conditions; and the small figure (d) shows the photocurrent stability test results of TiO2@Ti-3-N350 photoanode under illumination conditions. Detailed Implementation

[0031] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0032] Unless otherwise specified, all reagents and raw materials used in the following examples are commercially available. Example

[0033] This embodiment discloses a TiO2 nanowire array photoanode with defect layer modification. First, a Ti film of a certain thickness is deposited on the surface of TiO2 nanowires grown on an FTO substrate by magnetron sputtering and then annealed. Ti atoms are used to capture O atoms in the TiO2 lattice to form a TiO2 defect layer on the nanowire surface. The specific preparation process is as follows:

[0034] 1) Mix 4.5 mL of hydrochloric acid with 4.5 mL of deionized water, add 0.15 mL of tetrabutyl titanate, and stir until clear to obtain a precursor solution.

[0035] 2) Pretreatment of FTO glass: Cut FTO glass into small pieces of 1 cm × 3 cm × 1.6 mm. First, sonicate in acetone for 30 min to remove most of the organic contaminants on the FTO surface. Then, sonicate in ethanol for 30 min to remove the remaining organic matter and the acetone residue from the previous cleaning step. Finally, sonicate with deionized water for 30 min to remove the last residue. Dry at 70℃.

[0036] 3) The precursor solution prepared in step 1) was transferred into a polytetrafluoroethylene (PTFE) liner. The FTO glass was placed in the PTFE liner with the conductive side facing down, and the edge was at a 45° angle to the liner wall. The hydrothermal reactor was sealed and placed in an oven. It was heated at 150°C for 15 h. After the reaction was completed, the reactor was allowed to cool naturally. The liner was removed, and the FTO glass was taken out of the liner. It was washed with anhydrous ethanol and deionized water, dried in an oven at 70°C, and then annealed in a muffle furnace at 500°C for 1 h to improve crystallinity. TiO2 nanowire arrays (TiO2NWAs) were successfully grown on the FTO substrate surface.

[0037] The SEM image of the TiO2 nanowire photoanode (denoted as TiO2) based on the FTO substrate obtained in this step is shown below. Figure 1 As shown in the small figure in the middle, its microstructure is a nanorod structure. TiO2 nanowires are grown vertically and closely on the FTO substrate, with a height of about 3.5 μm.

[0038] Ti thin films were deposited on the surface of TiO2NWAs using DC magnetron sputtering: the deposition chamber was evacuated to 5.0 × 10⁻⁶ using a mechanical pump and a molecular pump. -3 High-purity Ar (99.99%) was introduced into the cavity at a flow rate of 45 sccm below Pa, causing the cavity pressure to reach 3.0 × 10⁻⁶. -1 Pa; Before magnetron deposition, the target was pre-sputtered with Ar plasma for 10 min. During the deposition process, the substrate rotation speed was 8 r / min, the deposition power was 220 W, and the deposition times were 1 min, 3 min, 6 min, and 9 min to deposit Ti films of different thicknesses on the TiO2NWAs surface. The samples obtained based on different deposition times were denoted as TiO2@Ti-1, TiO2@Ti-3, TiO2@Ti-6, and TiO2@Ti-9.

[0039] Observe the SEM planar image of the TiO2@Ti-3 photoanode ( Figure 1(See small image b) It was found that after depositing a Ti film on the tip of the TiO2 nanowire, the Ti film wrapped around the tip of the TiO2, making the tip of the nanowire smoother. Furthermore, observation of the microstructure of the TiO2@Ti-6 and TiO2@Ti-9 photoanodes revealed that with increasing deposition time, the tip of the nanowire was completely wrapped, and the thickness of the shell layer increased significantly.

[0040] 6) The four different samples obtained in step 5) were annealed at 350℃ for 1 h in a nitrogen atmosphere. The resulting photoanode materials were denoted as TiO2@Ti-1-N350, TiO2@Ti-3-N350, TiO2@Ti-6-N350, and TiO2@Ti-9-N350.

[0041] 7) PEC performance was measured using a typical three-electrode setup on a CHI660E electrochemical workstation (Shanghai Chenhua). The photoanode material, Ag / AgCl (saturated potassium chloride), and Pt wire were used as the working electrode, reference electrode, and counter electrode, respectively. A 300W Xe lamp (CHF-XM300) was used to simulate full-spectrum illumination. The photoelectrochemical performance of the samples was analyzed, and the light intensity was stabilized at 100 mW·cm⁻¹. -2 The test area is 1.5 cm². 2 PEC was determined using 0.5 M Na2SO4 solution (pH=7) as the electrolyte.

[0042] See the obtained photocurrent density data. Figure 3 The smaller figure (a) shows that under full-spectrum illumination (100 mW cm⁻¹), -2 Under the conditions of 0.5 M Na2SO4 (pH=7) electrolyte, the photocurrent density of the TiO2 photoanode is 1.09 mA·cm⁻¹. −2 The photocurrent density of the TiO2@Ti-3-N350 photoanode reached 3.26 mA·cm⁻¹. −2 This demonstrates that modifying TiO2NWAs photoanodes with oxygen vacancy defects significantly improves the PEC water-decomposition performance and stability under neutral reaction conditions. Furthermore, it was verified that they also exhibit excellent PEC water-decomposition performance and stability in both acidic and alkaline solutions.

[0043] Photoelectrocatalytic testing using TiO2 and TiO2@Ti-3-N350 photoanodes revealed that the TiO2@Ti-3-N350 photoanode exhibited excellent photoelectrocatalytic activity, with oxygen and hydrogen production rates of 18.3 μmol·cm⁻¹. -2 ·h -1 and 37.6 μmol·cm -2 ·h -1 This conforms to the stoichiometric ratio of 1:2 for water decomposition. Figure 4 (Small figure a); The initial hydrogen generation rate of the TiO2 photoanode was 9.6 μmol·cm⁻¹. -2 ·h -1 Defect layer modification increased the hydrogen generation rate of TiO2 by 2.9 times. Figure 4 (Small image in b). The stability of water decomposition and photocurrent of the TiO2@Ti-3-N350 photoanode under illumination conditions was tested. Figure 4 (See small figures c and d). It can be seen that the hydrogen generation and photocurrent density of the photoelectrode remain stable over time, indicating that the constructed defect layer maintains good structure and photoelectric stability during the catalytic water splitting process. Example

[0044] The difference between this embodiment and Example 1 is that, after preparing TiO2@Ti-3 using the same method as in Example 1, the materials were annealed at 250℃, 350℃, 450℃, and 550℃ for 1 h under a nitrogen atmosphere. The resulting photoanode materials were designated as TiO2@Ti-3-N250, TiO2@Ti-3-N350, TiO2@Ti-3-N450, and TiO2@Ti-3-N550, respectively. The PEC performance of the materials was tested using the same testing method as in step 7) of Example 1. Photocurrent density data can be found [link to relevant documentation]. Figure 3 The smaller image in the middle (c) shows the effect under full-spectrum illumination (100 mW cm⁻¹). -2 Under the conditions of 0.5 M Na2SO4 (pH=7) electrolyte, the photocurrent densities of TiO2@Ti-3-N250, TiO2@Ti-3-N350, TiO2@Ti-3-N450, and TiO2@Ti-3-N550 at a bias voltage of 1.23 V vs. RHE were 2.46, 3.26, 2.93, and 1.46 mA·cm⁻¹, respectively. −2 .

[0045] Comparative Example 1

[0046] The difference between this comparative example and Example 1 is that, after preparing TiO2@Ti-3 in the same way as in Example 1, the sample was annealed in air instead of nitrogen: the sample was annealed in air at 350°C for 1 hour, and the resulting photoanode material was denoted as TiO2@Ti-3-A350.

[0047] The PEC measurement method is the same as in Example 1. The photocurrent density of the photoanode material TiO2@Ti-3-A350 obtained in this comparative example is as follows: Figure 3 As shown in the small figure (b), under full-spectrum illumination (100 mW cm⁻¹), -2Under the conditions of 0.5 M Na2SO4 (pH=7) electrolyte, the photocurrent density of the TiO2@Ti-3-A350 photoanode is 1.77 mA·cm⁻¹. -2 .

[0048] Figure 2 The smaller figure (a) shows the Raman spectra of TiO2, TiO2@Ti-3, TiO2@Ti-3-N350, and TiO2@Ti-3-A350. As can be seen from the figure, compared to TiO2, TiO2@Ti-3 has a higher E... 1g The redshift of the vibrational mode characteristic peaks indicates that Ti deposition disrupted the symmetry of the O-Ti-O bonds. Further annealing caused the redshift to disappear, indicating that the Ti layer was oxidized. 1g The full width at half maximum (FWHM) of a characteristic peak often reflects the disorder of the sample surface.

[0049] like Figure 2 As shown in the small figure (b), Ti deposition renders the TiO2 surface disordered. After annealing in N2, the full width at half maximum (FWHM) of the characteristic peak decreases, but remains greater than that of the initial TiO2, indicating that the formed shell surface is a defect layer containing Vo. In contrast, the FWHM of the sample annealed in air is consistent with that of TiO2, indicating that its shell has a good lattice structure. This suggests that in the air-annealed sample, Ti is oxidized by oxygen in the air, thus passivating the surface.

[0050] Comparative Example 2

[0051] The difference between this comparative example and Example 1 is that, after preparing TiO2@Ti-3 according to the method in Example 1, this comparative example did not perform annealing treatment, and directly tested the PEC performance of the TiO2@Ti-3 photoanode according to the same test method as in Example 1.

[0052] The PEC measurement method is the same as in Example 1. The photocurrent density of the photoanode material TiO2@Ti-3 obtained in this comparative example is as follows: Figure 3 As shown in the small figure in middle c, under full-spectrum illumination (100 mW cm⁻¹), -2 Under the conditions of 0.5 M Na2SO4 (pH=7) electrolyte, the photocurrent density of the TiO2@Ti-3 photoanode is 1.31 mA·cm⁻¹. -2 .

[0053] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. However, the above description is merely a specific embodiment of the present invention, and the technical features of the present invention are not limited thereto. Any other embodiments derived by those skilled in the art without departing from the technical solution of the present invention should be covered within the scope of the present invention.

Claims

1. A method for preparing a TiO2 nanowire array photoanode modified with a defect layer, characterized in that, Includes the following steps: 1) Add the titanium source to the acid solution and mix to obtain the precursor solution; 2) Pretreatment of the substrate; 3) Place the substrate with the conductive side down at an angle in the liner of the reactor, add the precursor solution, seal the reactor, and heat the reaction. After the reaction is complete, clean the sample and dry it to grow TiO2NWAs on the substrate surface. 4) A Ti thin film was deposited on the surface of TiO2 NWAs by magnetron sputtering, and the resulting sample was denoted as TiO2@Ti; 5) Anneal the TiO2@Ti material to obtain the final product; In step 3), after drying the sample, it is annealed in a muffle furnace to improve the crystallinity of the nanowires on the substrate surface. The annealing temperature is 450~550℃ and the annealing time is 0.5~3 h. In step 5), annealing is carried out in a nitrogen atmosphere at a temperature of 250-450°C for 0.5-3 hours.

2. The method for preparing the defect-layer modified TiO2 nanowire array photoanode as described in claim 1, characterized in that, The substrate material is any one of fluorine-doped tin oxide, indium tin oxide, aluminum zinc oxide, and gallium zinc oxide.

3. The method for preparing the defect-layer modified TiO2 nanowire array photoanode as described in claim 1, characterized in that, In step 3), the heating reaction temperature is 130~200℃ and the heating reaction time is 10~24 h.

4. The method for preparing the defect-layer modified TiO2 nanowire array photoanode as described in claim 1, characterized in that, In step 4), DC magnetron sputtering is used. During the deposition process, the substrate rotation speed is 6~10 r / min, the deposition power is 220~240 W, and the deposition time is 1~6 min.

5. A TiO2 nanowire array photoanode modified with a defect layer, characterized in that, It is prepared based on the preparation method of any one of claims 1-4, comprising a substrate, a TiO2 NWAs layer and a TiO2 defect layer on the surface arranged sequentially, wherein the defect layer contains oxygen vacancies.

6. The application of the defect-layer modified TiO2 nanowire array photoanode as described in claim 5 in the photoelectrocatalytic water splitting reaction to produce hydrogen and oxygen.

7. The application as described in claim 6, characterized in that, Photocatalytic processes can be applied to neutral, acidic, or alkaline solutions.

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