A surface-confined sulfur-doped metal oxide photoanode and its preparation method

Abstract

This invention relates to the field of photoelectrochemical water splitting materials technology, and particularly to a surface-confined sulfur-doped metal oxide photoanode and its preparation method. The method includes growing a metal oxide nanostructure on an FTO substrate using a hydrothermal method to obtain a metal oxide photoanode; repeatedly performing drop-coating and ignition treatments on the surface of the metal oxide photoanode to obtain an ignition-treated metal oxide photoanode; the drop-coating and ignition treatments include drop-coating sulfur-containing organic matter to form a liquid wet film, and igniting the liquid wet film in an air environment; cleaning and drying the ignition-treated metal oxide photoanode to obtain a surface-confined sulfur-doped metal oxide photoanode. This method utilizes the localized thermal field generated by liquid film combustion to drive sulfur atoms to diffuse to the surface and near-surface layer of the metal oxide, achieving precise surface-confined gradient doping.

Description

Technical Field

[0001] This invention relates to the field of photoelectrochemical water splitting materials technology, and in particular to a surface-confined sulfur-doped metal oxide photoanode and its preparation method. Background Technology

[0002] Photoelectrochemical (PEC) water splitting technology can directly utilize solar energy to decompose water into hydrogen and oxygen, and is one of the key pathways to achieve carbon neutrality. Among many photoanode materials, metal oxide semiconductors (such as TiO2, BiVO4, WO3, Fe2O3, etc.) are widely studied due to their good chemical stability, low cost, and mature preparation processes. However, these metal oxides generally face common problems such as poor intrinsic conductivity, low carrier mobility, and slow surface water oxidation kinetics. Severe recombination of photogenerated electron-hole pairs in the bulk phase and at the interface leads to actual photoelectric conversion efficiency far lower than the theoretical value.

[0003] To address the aforementioned issues, non-metallic element doping (especially sulfur doping) has proven to be an effective modification method. Sulfur doping can introduce intermediate energy levels, narrow the band gap, enhance visible light absorption, and optimize the surface electronic structure. However, for metal oxide photoanodes, controlling the doping site is crucial. Traditional "bulk homogeneous doping" strategies often introduce deep-level defects, which act as electron-hole recombination centers in the bulk phase, leading to a shortened carrier lifetime and thus offsetting the advantages of bandgap modulation.

[0004] Currently, methods for achieving surface-confined sulfur doping mainly include high-temperature annealing, ion implantation, and chemical vapor deposition (CVD). These methods generally suffer from the following problems: 1. Stringent process conditions: often requiring expensive vacuum equipment or a specific gas atmosphere; 2. Easily damages the substrate: Traditional flame treatment or high-temperature annealing, where the temperature is difficult to control precisely and usually exceeds 1000K, can easily damage the conductive layer of conductive glass (such as FTO), leading to a sharp increase in the series resistance of the electrodes. 3. Complex pretreatment: It usually involves pre-drying of the precursor, resulting in poor doping uniformity.

[0005] Therefore, developing a mild in-situ surface doping technique that is universally applicable, requires no special equipment, and can effectively protect the conductivity of the substrate is of great application value for developing high-performance metal oxide photoanodes. Summary of the Invention

[0006] To achieve the above objectives, this invention provides a method for preparing a surface-confined sulfur-doped metal oxide photoanode, and the technical solution provided by this invention is as follows: This invention provides a method for preparing a surface-confined sulfur-doped metal oxide photoanode, the method comprising: Metal oxide photoanodes were obtained by growing metal oxide nanostructures on FTO substrates using a hydrothermal method. The surface of the metal oxide photoanode is alternately subjected to drop-coating and ignition treatment multiple times to obtain an ignition-treated metal oxide photoanode. The drop-coating and ignition treatment includes drop-coating sulfur-containing organic matter to form a liquid wet film and igniting the liquid wet film in an air environment. The metal oxide photoanode after ignition treatment is cleaned and dried to obtain a surface-confined sulfur-doped metal oxide photoanode.

[0007] Furthermore, the metal oxide photoanode includes one of the following: TiO2 photoanode, BiVO4 photoanode, WO3 photoanode, or Fe2O3 photoanode.

[0008] Furthermore, metal oxide nanostructures include one of nanorod arrays, nanowire arrays, or nanosheet arrays.

[0009] Furthermore, the sulfur-containing organic compound includes one or more of dimethyl sulfoxide, dimethyl sulfide, diethyl sulfide, or thiophene.

[0010] Furthermore, the film-forming and ignition processes are alternately performed N times, where N is an integer not less than 1 and not more than 1000.

[0011] Furthermore, the film-forming and ignition processes are alternately performed N times, where N is an integer not less than 10 and not more than 150.

[0012] Furthermore, the drop volume is 0.05-0.20 mL / cm².

[0013] Furthermore, the size of the FTO substrate is greater than or equal to 1×1cm. 2 .

[0014] Furthermore, the size of the FTO substrate is greater than or equal to 5 × 5 cm. 2 .

[0015] The present invention also provides a surface-confined sulfur-doped metal oxide photoanode obtained by the preparation method described above, wherein the photocurrent density of the surface-confined sulfur-doped metal oxide photoanode is 1.60-4.35 mA cm⁻¹ under a bias voltage of 1.23 V vs. RHE. -2 .

[0016] The technical effects and advantages of this invention are as follows: 1. This invention achieves precise surface-confined gradient doping: This invention utilizes the in-situ self-sustaining combustion of liquid sulfur-containing organic compounds (such as DMSO) on the surface of metal oxides. The resulting localized thermal field (approximately 700-720 K) serves as the driving force, promoting the diffusion of sulfur atoms towards the lattice surface. This method successfully confines sulfur atoms to the surface and near-surface region of the photoanode, forming a concentration gradient distribution. This not only induces band bending to construct a built-in electric field and promotes charge separation but also creates surface catalytic active sites to accelerate reaction kinetics. It avoids excessive diffusion of sulfur atoms into the bulk phase, preventing the generation of deep-level defects (recombination centers). Thus, it achieves surface modification while preserving the integrity of the bulk lattice, resulting in a superior strategy for improving photoanode performance.

[0017] 2. This invention effectively modulates the band structure and surface reaction kinetics: the introduced surface sulfur doping significantly reduces the surface work function of the material (e.g., from 7.82 eV to 6.93 eV), causing the Fermi level to shift towards the conduction band, enhancing band bending and the built-in electric field. This not only broadens the visible light absorption range (redshift of the light absorption edge) but also significantly reduces the rate-determining energy barrier of the oxygen evolution reaction (OER) (theoretical overpotential decreases from 0.88 V to 0.43 V), thereby significantly improving the surface charge separation efficiency (from 22.76% to 71.69%).

[0018] 3. A self-limiting heat treatment mechanism was established: By controlling the liquid film thickness to regulate the combustion duration (several seconds) and the specific temperature field generated by combustion (approximately 720 K), a self-limiting heat treatment mechanism was formed. This mechanism ensures effective diffusion of sulfur atoms while concentrating heat only on the surface, preventing thermal damage to the bottom FTO conductive glass and maintaining its conductivity at 1.0 × 10⁻⁶. 5 High level above S / m.

[0019] 4. Simple process and low equipment cost: This invention completely eliminates the need for expensive vacuum equipment (such as ion implantation and CVD) or complex external heating equipment (such as muffle furnaces, tube furnaces, and gas spray guns) used in traditional doping methods. It only requires a simple "drop-coating-ignition" operation, which can be completed in an air environment at room temperature and pressure, greatly reducing the preparation cost and operation threshold, making it suitable for large-scale promotion.

[0020] 5. Significantly improved photoelectrochemical performance: Compared to the undoped sample, the sulfur-doped TiO2 photoanode prepared in this invention exhibits a photocurrent density increase of approximately 10.9 times at 1.23 V vs. RHE (reaching 4.35 mA cm⁻¹). 2The ABPE content was increased by 16 times, and it exhibited excellent photoelectrochemical stability (24-hour operating retention >75%). This performance is superior to most similar metal oxide photoanodes currently available.

[0021] 6. High versatility and applicability to various material systems: This invention utilizes the excellent wettability of organic solvents such as DMSO, and is not only applicable to TiO2, but has also been proven to be applicable to various metal oxide photoanode systems such as BiVO4, WO3, and Fe2O3. It can achieve effective surface sulfur doping and significantly improve their photoelectric performance, thus having broad applicability. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the preparation method of the surface-confined sulfur-doped metal oxide photoanode provided in the embodiments of this application; Figure 2 This is a graph showing the change trend of conductivity of the FTO substrate under different DMSO combustion cycles for the TiO2 photoanode provided in Example 1 of this application; Figure 3a The image shown is an FESEM image of the TiO2 sample from Example 1 after 70 cycles. Figure 3b HRTEM image of the TiO2 sample after 70 cycles in Example 1; Figure 3c SAED image of the TiO2 sample after 70 cycles in Example 1; Figure 3d The image shows the EDS spectrum of the TiO2 sample after 70 cycles in Example 1. Figure 4a This is a ring-shaped dark-field scanning transmission electron microscope (HAADF-STEM) image of a single nanorod from the TiO2-70 cycles cyclic sample in Example 1. Figure 4b This is a surface distribution diagram of sulfur (S) in a single nanorod of the TiO2-70 cycles sample from Example 1. Figure 4c This is a surface distribution diagram of oxygen (O) in a single nanorod of the TiO2-70 cycles sample from Example 1. Figure 4d The surface distribution of titanium (Ti) in a single nanorod of the TiO2-70 cycles sample in Example 1 is shown in Figure 1. Figure 5a ToF-SIMS 3D reconstruction of the spatial distribution of oxygen (O) in sulfur-doped TiO2; Figure 5b ToF-SIMS 3D reconstruction of the spatial distribution of titanium (Ti) elements in sulfur-doped TiO2; Figure 5c ToF-SIMS 3D reconstruction of the spatial distribution of sulfur (S) in sulfur-doped TiO2; Figure 5d ToF-SIMS depth profile of the spatial distribution of sulfur (S) in sulfur-doped TiO2; Figure 6a This is a comparison chart of linear sweep voltammetry (LSV) curves for samples with different DMSO cycle numbers. Figure 6b A comparison of the photoelectrochemical performance of the application deviation photon-current efficiency (ABPE) curves for samples with different DMSO cycle numbers; Figure 6c A comparative graph showing the photoelectrochemical performance of chronoamperometry (it) curves for samples with different DMSO cycle numbers; Figure 6d Comparison of electrochemical performance of samples with different DMSO cycle numbers using photochemical impedance spectroscopy (EIS) Nyquist plots; Figure 7 The 24-hour long-term stability test curve for TiO2-70 cycles photoanode (Note: the horizontal axis Time (h) represents time; the vertical axis Current density (mA cm⁻¹) represents the long-term stability test curve). -2 (Represents photocurrent density); Figure 8a The linear sweep voltammetry (LSV) curves of the sulfur-doped Fe2O3 photoelectrode material prepared in Example 2 of this invention before and after doping are shown. Figure 8b The figures show the chronocurrent (it) curves before and after doping on the sulfur-doped Fe2O3 photoelectrode material prepared in Example 2 of this invention. Figure 8c The Nyquist plots of the electrochemical impedance spectroscopy (EIS) before and after doping are shown for the sulfur-doped Fe2O3 photoelectrode material prepared in Example 2 of this invention. Figure 9a The linear sweep voltammetry (LSV) curves of the sulfur-doped WO3 photoelectrode material prepared in Example 3 of this invention before and after doping are shown. Figure 9b The figures show the chronocurrent (it) curves before and after doping on the sulfur-doped WO3 photoelectrode material prepared in Example 3 of this invention. Figure 9cThe Nyquist plots of the electrochemical impedance spectroscopy (EIS) before and after doping are shown for the sulfur-doped WO3 photoelectrode material prepared in Example 3 of this invention. Figure 10a The linear sweep voltammetry (LSV) curves of the sulfur-doped BiVO4 photoelectrode prepared in Example 4 of this invention before and after BiVO4 doping are shown. Figure 10b The chronocurrent (it) curves of the sulfur-doped BiVO4 photoelectrode prepared in Example 4 of this invention before and after BiVO4 doping are shown. Figure 10c The Nyquist plots of the electrochemical impedance spectroscopy (EIS) of the sulfur-doped BiVO4 photoelectrode prepared in Example 4 of this invention before and after BiVO4 doping are shown. Figure 11 The method of this invention is applicable to small areas (1×1cm) 2 ) and large area (5×5cm) 2 Comparison of LSV curves from magnified experiments on the substrate; Figure 12 This is a graph showing the change trend of conductivity of the pure TiO2 photoelectrode in the comparative example of the present invention after being treated with a traditional flame treatment method (continuous external open flame heat source) for different times. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] To address the shortcomings of existing technologies, this invention discloses a method for preparing surface-confined sulfur-doped metal oxide photoanodes, such as... Figure 1 As shown, the method includes, Step 1: A metal oxide nanostructure is grown on an FTO substrate using a hydrothermal method to obtain a metal oxide photoanode; wherein the metal oxide nanostructure includes one of a nanorod array, a nanowire array, or a nanosheet array. The metal oxide photoanode includes one of a TiO2 photoanode, a BiVO4 photoanode, a WO3 photoanode, or a Fe2O3 photoanode.

[0025] For example, the TiO2 photoanode is obtained by growing TiO2 nanostructures on an FTO substrate using a hydrothermal method, including: The titanium source is slowly added dropwise to the hydrochloric acid solution and stirred until homogeneous to form a precursor solution; the titanium source is tetrabutyl titanate, or may be selected from one or more of titanium isopropoxide, titanium ethoxide, titanium tetrachloride or titanium sulfate; The cleaned FTO was placed at an angle in a high-pressure reactor lined with polytetrafluoroethylene, the precursor solution was injected, and after hydrothermal reaction, it was naturally cooled to room temperature. The FTO after the hydrothermal reaction was removed, rinsed with deionized water, and dried to obtain the metal oxide photoanode precursor. The TiO2 photoanode precursor was kept at 450 °C for 2 hours to obtain the metal oxide photoelectrode.

[0026] Step 2: Repeatedly perform drop-coating and ignition treatment on the surface of the metal oxide photoanode to obtain an ignition-treated metal oxide photoanode. The drop-coating and ignition treatment includes drop-coating sulfur-containing organic matter to form a liquid wet film and igniting the liquid wet film in an air environment. The sulfur-containing organic matter includes one or more organic sulfur compounds such as DMSO, dimethyl sulfide, diethyl sulfide, and thiophene that have room temperature liquid and flammable properties.

[0027] Step 3: Clean and dry the ignition-treated metal oxide photoanode to obtain a surface-confined sulfur-doped metal oxide photoanode.

[0028] In one specific embodiment of the present invention, the surface of the metal oxide photoanode is subjected to drop-coating and ignition treatments alternately and repeatedly, including: Step 201: Drop coating to form a film: Drop sulfur-containing organic material (drop volume of 0.05-0.20 mL / cm²) onto the surface of the metal oxide photoelectrode to form a continuous liquid wet film on the electrode surface by utilizing the wettability of the sulfur-containing organic material.

[0029] Step 202: In-situ self-combustion of sulfur-containing organic matter: Without drying, the moist liquid film of sulfur-containing organic matter is directly ignited in air. The sulfur-containing organic matter itself serves as fuel for self-sustaining combustion. No external gas torch or continuous heat source is required during combustion. The generated localized thermal field (approximately 700-720 K) drives sulfur atoms to diffuse to the surface and near-surface of the metal oxide. The doping concentration and distribution gradient of sulfur atoms can be precisely controlled, resulting in good reproducibility.

[0030] Compared to existing technologies (such as the external flame method), this invention eliminates the "precursor drying" step and the "external flame equipment," directly igniting the moist sulfur-containing organic liquid film in the air, relying entirely on the self-combustion of the sulfur-containing organic matter for heating. This "self-limiting temperature" characteristic (surface temperature of approximately 720 K) and short combustion time (a few seconds) effectively avoid thermal damage to the FTO conductive substrate caused by strong external flames (>1000 °C), maintaining the high conductivity of the electrode.

[0031] Step 203: Alternately cycle through Step 201 (drop coating) and Step 202 (ignition treatment) N times, with the number of cycles ranging from 1 to 1000, to obtain metal oxide photoelectrodes with different sulfur doping levels. Preferably, N is an integer not less than 10 and not more than 150.

[0032] For example, among numerous semiconductor photoelectric anode materials, TiO2 has been widely studied due to its excellent chemical stability, environmental friendliness, and low cost. However, the inherent wide bandgap (approximately 3.2 eV) of TiO2 results in extremely low utilization of visible light, while its high recombination rate of photogenerated electron-hole pairs and slow surface water oxidation kinetics severely limit its practical application in photoelectric conversion efficiency. Sulfur doping can introduce intermediate energy levels into the TiO2 lattice, narrowing the bandgap and enhancing visible light absorption, while simultaneously optimizing the surface electronic structure and improving charge separation efficiency. Studies have shown that sulfur doping can also modulate surface catalytic active sites and accelerate water oxidation reaction kinetics.

[0033] For example, obtaining a TiO2 photoanode after ignition treatment includes the following steps: Step 201': DMSO liquid film coating: DMSO is dropped onto the TiO2 photoanode surface (drop volume is 0.05-0.20 mL / cm²), and a continuous liquid wet film is formed on the electrode surface by utilizing the wettability of DMSO.

[0034] Step 202': In-situ self-combustion of DMSO: Without drying, the moist DMSO liquid film is directly ignited in air. DMSO itself is used as fuel for self-sustaining combustion. No external gas torch or continuous heat source is required during combustion. The generated local heat field (about 700-720 K) drives sulfur atoms to diffuse to the TiO2 surface and near-surface layer.

[0035] Step 203': Cyclic operation: Repeat steps 201' and 202' several times, with the number of cycles ranging from 10 to 150, to obtain different doping levels.

[0036] This step involves forming a uniform liquid film of highly wettable DMSO on the electrode surface, which is then ignited in air. The localized high temperature (approximately 700-720 K) generated during combustion serves as the heat source, promoting the diffusion of sulfur atoms to the TiO2 surface and near-surface region, thereby constructing a gradient-distributed sulfur-doped layer. By controlling the number of drop-coating-combustion cycles (10-150 times), the degree of sulfur doping can be effectively adjusted, achieving precise control of the doping depth from the photoanode surface to the near-surface region, avoiding performance degradation caused by over-doping. The instantaneous characteristics of the combustion process ensure that the TiO2 crystal structure remains intact, and the substrate conductivity is maintained.

[0037] In one specific embodiment of the present invention, the size of the FTO substrate is greater than or equal to 1 × 1 cm. 2 Less than 5×5cm 2 Small-area substrates are also acceptable, as are FTO substrates with dimensions greater than or equal to 5×5cm. 2 A large-area base.

[0038] The present invention also provides a surface-confined sulfur-doped metal oxide photoanode, wherein the photocurrent density of the surface-confined sulfur-doped metal oxide photoanode is in the range of 1.60-4.35 mA cm⁻¹ under a bias voltage of 1.23 V vs. RHE. -2 .

[0039] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0040] Example 1: Preparation of a sulfur-doped TiO2 photoanode (1×1 cm²) Step 1: The FTO substrate area is 1×1 cm. 2 TiO2 nanorod arrays were prepared on FTO conductive glass using a hydrothermal method. The specific steps were as follows: 0.6 mL of tetrabutyl titanate was slowly added dropwise to 50 mL of 6 M hydrochloric acid solution, and the mixture was stirred continuously for 30 minutes to form a homogeneous precursor solution. The cleaned FTO substrate was placed tilted in a high-pressure reactor lined with polytetrafluoroethylene, and the aforementioned precursor solution was injected. The reaction was carried out at 170 °C for 7 hours. After the reaction, the sample was allowed to cool naturally to room temperature, removed, and repeatedly rinsed with deionized water, then air-dried. Finally, the sample was placed in a muffle furnace and heated to 450 °C at a heating rate of 5 °C / min, and held at this temperature for 2 hours to obtain a well-crystallized rutile phase TiO2 photoelectrode.

[0041] Step 2: Take the TiO2 photoelectrode (1×1 cm²) prepared in Step 1, and accurately measure 0.09 mL of DMSO solution using a micropipette, then uniformly drop-coat it onto the electrode surface. Utilizing the excellent wettability of DMSO, it spreads naturally on the electrode surface to form a continuous and uniform wet film. Without any drying or evaporation treatment, immediately ignite the wet film in air with an open flame. The film undergoes self-sustaining combustion, requiring no external continuous heat source (such as a gas torch) during combustion, until the fuel is exhausted and it extinguishes naturally, completing one combustion cycle. Repeat the above "drop-coating-wet film ignition" process 30, 50, 70, 90, and 110 times respectively.

[0042] Step 3: Finally, the samples are washed and dried sequentially to obtain the final sulfur-doped TiO2 photoanode.

[0043] like Figure 2The graph shows the conductivity variation trend of the FTO substrate under different DMSO combustion cycles for the TiO2 photoanode. Figure 2 The horizontal axis, Cycles, represents the number of combustion cycles; the vertical axis, Conductivity (10... 5 S / m represents conductivity, according to Figure 2 It was found that the performance was optimal when the number of loops was 70.

[0044] like Figures 3a-3d The structural characterization results of the TiO2-70 cycles (cycled) sample are shown. Figure 3a Scanning electron microscopy (FESEM) images show that the TiO2 sample maintained the nanorod array morphology after 70 combustion cycles, indicating that the combustion process did not damage the microstructure.

[0045] Figure 3b The high-resolution transmission electron microscope (HRTEM) image shown reveals clear lattice fringes with a crystal plane spacing of 0.27 nm, corresponding to the (101) crystal plane of rutile TiO2, indicating that the material maintains good crystallinity.

[0046] Figure 3c The selected area electron diffraction (SAED) pattern shown exhibits clear diffraction spots, confirming the single-crystal nature of the sample.

[0047] Figure 3d In the graph, the horizontal axis Energy (keV) represents energy; the vertical axis Intensity (au) represents signal strength; according to... Figure 3d The energy-dispersive X-ray spectroscopy (EDS) shown indicates the detection of Ti, O, and S elements, directly proving that sulfur was successfully doped into the TiO2 photoanode.

[0048] like Figures 4a-4d The elemental distribution diagram of a single nanorod in the TiO2-70 cycles shown indicates that sulfur atoms can be doped on the surface and near-surface region of TiO2 through a coating-ignition process.

[0049] like Figures 5a-5d The image shown is a characterization diagram of the spatial distribution of sulfur doping in TiO2 in TiO2 cyclic samples after 70 cycles. Figure 5dThe horizontal axis Sputtering time (s) represents the sputtering time, reflecting the depth; the vertical axis Intensity represents the signal intensity. It can be seen that by coating and ignition treatment, the doping concentration and distribution gradient of sulfur atoms can be precisely controlled in the TiO2 surface and near-surface region.

[0050] like Figures 6a-6d The graph shows a comparison of the photoelectrochemical performance of samples with different DMSO cycle numbers. Figure 6a Linear sweep volt-ampere (LSV) curves for samples with different DMSO combustion cycle numbers are shown, where the horizontal axis (Potential) represents electric potential (V) and the vertical axis (Current density) represents photocurrent density (mA cm⁻¹). -2 ).Depend on Figure 6a It can be seen that with the increase of DMSO combustion cycles, the photocurrent density of the sample first increases and then decreases. Under a bias voltage of 1.23V vs. RHE, the photocurrent density of the prepared surface-confined sulfur-doped metal oxide photoanode ranges from 1.60 to 4.35 mA cm⁻¹. -2 Within the specified range, the photocurrent density of the sample reached its maximum value of 4.35 mA cm⁻¹ when the number of combustion cycles was 70 (TiO₂-70 cycles). -2 .

[0051] Figure 6b The applied bias photon-current efficiency (ABPE) curves (where the vertical axis represents conversion efficiency and the horizontal axis represents potential) show that the TiO2-70 cycle sample reaches a maximum efficiency of 4.02% at 0.71 V, which is much higher than that of the undoped sample.

[0052] Figure 6c (The horizontal axis represents Time (s), and the vertical axis represents Current density (mA cm⁻¹) -2 The chronocurrent (it) curve shown (representing photocurrent density) demonstrates the rapid response of the photoanode under switched-on light conditions. S doping significantly improves the separation efficiency and stability of photogenerated carriers.

[0053] Figure 6d The electrochemical impedance spectroscopy (EIS) Nyquist plot shown (where the horizontal axis Z' (ohm) represents the real impedance and the vertical axis -Z'' (kOhm) represents the imaginary impedance) reveals that the radius of the arc of the S-doped sample is significantly reduced, indicating a substantial decrease in charge transfer resistance (Rct) and improved interfacial charge transport kinetics.

[0054] Figure 7 The 24-hour long-term stability test curves for TiO2-70 cycles photoanodes are shown. Figure 7 The horizontal axis, Time (h), represents time; the vertical axis, Current density (mA cm⁻¹), represents time. -2 () represents photocurrent density. According to... Figure 7 The results show the long-term stability curve of the TiO2-70 cycle photoanode after 24 hours under a bias of 1.23 V vs. RHE. The results indicate that the photocurrent density remained above 75.5% of the initial value after the test, demonstrating that the surface sulfur-doped photoanode prepared by this method exhibits excellent photoelectrochemical stability.

[0055] Example 2: Preparation of sulfur-doped Fe2O3 photoelectrode Step 1: Preparation of Fe2O3 (hematite) photoanode: Dissolve ferric chloride (FeCl3·6H2O) and sodium nitrate (NaNO3) in deionized water. Place the FTO (FeClO3) electrode with the conductive side facing down in the reactor and perform a hydrothermal reaction at 100 ℃ for 4-6 hours. After cleaning, anneal in air at 550 ℃ for 2 hours, and then briefly anneal at 800 ℃ (e.g., 20 minutes) to obtain the hematite phase.

[0056] Step 2: Take the Fe2O3 photoelectrode (1×1 cm²) prepared in Step 1, and accurately measure 0.09 mL of DMSO solution using a micropipette, uniformly drop-coating it onto the electrode surface. Utilizing the excellent wettability of DMSO, it naturally spreads on the electrode surface to form a continuous and uniform wet film. Without any drying or evaporation treatment, immediately ignite the wet film in air with an open flame. The film undergoes self-sustaining combustion, requiring no external continuous heat source (such as a gas torch) during combustion, until the fuel is exhausted and it extinguishes naturally, completing one combustion cycle. Repeat the above "drop-coating-wet film ignition" process for 30, 50, 70, 90, and 110 times respectively. Experiments showed that the performance was optimal when the number of cycles was 70.

[0057] Step 3: Finally, the samples are washed and dried sequentially to obtain the final sulfur-doped Fe2O3 photoanode.

[0058] Results analysis: Figure 8a This is a comparison of the linear sweep voltammetry (LSV) curves of the Fe2O3 photoanode before and after sulfur doping, where the horizontal axis represents potential (V) and the vertical axis represents current density (mA cm⁻¹). -2() represents photocurrent density. (From) Figure 8a As can be seen, compared with the undoped Fe2O3 sample, the sulfur-doped Fe2O3 photoanode treated by the method of the present invention exhibits a significantly enhanced photocurrent density and a more negative onset potential, proving that surface-confined sulfur doping effectively improves its photoelectrocatalytic activity.

[0059] Figure 8b This is a comparison of the chronocurrent (it) curves of the Fe2O3 photoanode before and after sulfur doping, where the horizontal axis represents Time (s) and the vertical axis represents Current density (mA cm⁻¹). -2 The figure represents the photocurrent density. As shown in the figure, the sulfur-doped sample exhibits rapid and stable photoresponse characteristics, indicating that the doped photoanode has good photoelectrochemical stability.

[0060] Figure 8c The figure shows the Nyquist plots of the electrochemical impedance spectroscopy (EIS) of the Fe2O3 photoanode before and after sulfur doping. The horizontal axis Z' (ohm) represents the real impedance, and the vertical axis -Z'' (kOhm) represents the imaginary impedance. As can be seen from the figure, the radius of the arc of the sulfur-doped sample is significantly smaller than that of the undoped sample, indicating that surface sulfur doping effectively reduces the charge transfer resistance at the electrode / electrolyte interface and promotes the separation and transport of photogenerated carriers.

[0061] Example 3: Preparation of sulfur-doped WO3 photoelectrode Step 1: Preparation of WO3 (tungsten trioxide) photoanode: Dissolve sodium tungstate (Na2WO4·2H2O) in deionized water, add hydrochloric acid (HCl) to adjust the pH value, and add an appropriate amount of oxalic acid or ammonium sulfate as a directing agent. Place FTO in the reactor and react at 120-180 ℃ for 2-12 hours. After cleaning, anneal at 500 ℃ for 2 hours.

[0062] Step 2: Take the WO3 photoelectrode (1×1 cm²) prepared in Step 1, and accurately measure 0.09 mL of DMSO solution using a micropipette, then uniformly drop it onto the electrode surface. Utilizing the excellent wettability of DMSO, it spreads naturally on the electrode surface to form a continuous and uniform wet film. Without any drying or evaporation treatment, immediately ignite the wet film in air with an open flame. The film undergoes self-sustaining combustion, requiring no external continuous heat source (such as a gas torch) during combustion, until the fuel is exhausted and it extinguishes naturally, completing one combustion cycle. Repeat the above "drop-coating-wet film ignition" process for 30, 50, 70, 90, and 110 times respectively. Experiments showed that the performance was optimal when the number of cycles was 70.

[0063] Step 3: Finally, the samples are washed and dried sequentially to obtain the final sulfur-doped WO3 photoanode.

[0064] Results analysis: Figure 9a This is a comparison of the linear sweep voltammetry (LSV) curves of the WO3 photoelectrode prepared in Example 3 of this invention before and after sulfur doping. The horizontal axis represents potential (V); the vertical axis represents current density (mA cm⁻¹). -2 The figure represents the photocurrent density. As shown in the figure, compared with the undoped pure WO3 sample, the sulfur-doped WO3 photoanode treated by the method of this invention exhibits a significantly improved photocurrent density throughout the entire test potential range, and the onset potential shifts negatively. This indicates that surface-confined sulfur doping effectively improves the photoelectrocatalytic activity of WO3 and reduces the overpotential of the water oxidation reaction.

[0065] Figure 9b This is a comparison of the chronocurrent (it) curves of the WO3 photoelectrode prepared in Example 3 of this invention before and after sulfur doping. The horizontal axis represents Time (s), and the vertical axis represents Current density (mA / cm²). -2 The figure represents the photocurrent density. As shown in the figure, the sulfur-doped WO3 photoanode exhibits rapid and stable photocurrent response during multiple switching cycles, with no significant decay in current density, demonstrating that the doped electrode prepared by this method possesses excellent photoelectrochemical stability.

[0066] Figure 9c The Nyquist plots of the electrochemical impedance spectroscopy (EIS) of the WO3 photoelectrode prepared in Example 3 of this invention before and after sulfur doping are shown. The horizontal axis Z' (ohm) represents the real impedance, and the vertical axis -Z'' (kOhm) represents the imaginary impedance. As can be seen from the figure, the radius of the impedance spectrum arc of the sulfur-doped sample is significantly smaller than that of the undoped sample. A smaller radius indicates a smaller charge transfer resistance (Rct), which means that surface sulfur doping significantly reduces the charge transport resistance at the interface between the WO3 electrode and the electrolyte, thereby accelerating the kinetics of the oxygen evolution reaction (OER).

[0067] Example 4: Fabrication of sulfur-doped BiVO4 photoelectrode Step 1: Preparation of BiVO4 (bismuth vanadate) photoanode: Typically, electrodeposition or water-drop coating followed by annealing is used. If a hydrothermal method is used: Bismuth nitrate (Bi(NO3)3) and ammonium metavanadate (NH4VO3) are dissolved separately, mixed, and the pH is adjusted. FTO is then added and reacted at approximately 160 °C. Finally, annealing is performed at 450-500 °C.

[0068] Step 2: Take the BiVO4 photoelectrode (1×1 cm²) prepared in Step 1, and accurately measure 0.09 mL of DMSO solution using a micropipette, uniformly drop-coating it onto the electrode surface. Utilizing the excellent wettability of DMSO, it naturally spreads on the electrode surface to form a continuous and uniform wet film. Without any drying or evaporation treatment, immediately ignite the wet film in air with an open flame. The film undergoes self-sustaining combustion, requiring no external continuous heat source (such as a gas torch) during combustion, until the fuel is exhausted and it extinguishes naturally, completing one combustion cycle. Repeat the above "drop-coating-wet film ignition" process for 30, 50, 70, 90, and 110 times respectively. Experiments showed that the performance was optimal when the number of cycles was 70.

[0069] Step 3: Finally, the samples are washed and dried sequentially to obtain the final sulfur-doped BiVO4 photoanode.

[0070] Results analysis: Figure 10a This is a comparison of the linear sweep voltammetry (LSV) curves of the BiVO4 photoelectrode prepared in Example 4 of this invention before and after sulfur doping. The horizontal axis represents potential (V); the vertical axis represents current density (mA cm⁻¹). -2 The figure represents the photocurrent density. As shown in the figure, compared with undoped pure BiVO4, the sulfur-doped BiVO4 photoanode treated by the method of this invention exhibits a higher photocurrent density throughout the entire scanning potential range, and the onset potential shifts negatively, proving that surface-confined sulfur doping effectively improves the photoelectric conversion efficiency of BiVO4.

[0071] Figure 10b This is a comparison of the chronocurrent (it) curves of the BiVO4 photoelectrode prepared in Example 4 of this invention before and after sulfur doping. The horizontal axis represents Time (s), and the vertical axis represents Current density (mA / cm²). -2 The figure represents the photocurrent density. As shown in the figure, the sulfur-doped BiVO4 photoanode exhibits rapid photoresponse characteristics under alternating light / dark conditions, and the photocurrent density remains stable during continuous testing, which is significantly better than that of the undoped sample.

[0072] Figure 10cThe figure shows the Nyquist plots of the electrochemical impedance spectroscopy (EIS) of the BiVO4 photoelectrode prepared in Example 4 of this invention before and after sulfur doping. The horizontal axis Z' (ohm) represents the real impedance, and the vertical axis -Z'' (kOhm) represents the imaginary impedance. As can be seen from the figure, the impedance radius of the sulfur-doped sample is significantly smaller than that of the undoped sample, indicating that sulfur doping effectively reduces the transfer resistance (Rct) of photogenerated charge at the electrode / electrolyte interface, thereby significantly accelerating the surface oxygen evolution reaction kinetics.

[0073] Specific test results are as follows: Figures 8a to 10c As shown: Figure 8a , 9a As shown in the LSV curves over 10 years, the photocurrent density of the three photoanodes, Fe2O3, WO3, and BiVO4, was significantly improved after doping; Figure 8c , 9c As shown in the EIS spectra at 10c, the interfacial charge transfer resistance of all three materials was significantly reduced after doping. These consistent experimental results demonstrate that the surface-confined sulfur doping method proposed in this invention is not only applicable to TiO2, but also to other metal oxide semiconductors, proving that the method has good universality.

[0074] Example 5: Fabrication of TiO2 electrode on large-area FTO substrate and sulfur doping The only difference from Example 1 is that the FTO substrate area is 5×5 cm. 2 The other steps and parameters are the same as in Example 1.

[0075] Figure 11 This method is applicable to small areas (1×1cm) 2 ) and large area (5×5cm) 2 A comparison of LSV curves from a scaled-up experiment on the substrate. The horizontal axis represents Potential (V); the vertical axis represents Current density (mA / cm²). -2 () represents photocurrent density.

[0076] As shown in Figure 8, the thermal simulation reveals a uniform temperature distribution on the surface of the large-area electrode, remaining within the range of 700-720 K. Photoelectric testing indicates that the photocurrent density of the large-area electrode reaches 3.74 mA·cm⁻¹. -2 For small-area electrodes (4.35 mA·cm²), -2 The result shows that the method has good scalability and is suitable for practical applications, with a yield of 86%.

[0077] Comparative Example 1 Step 1: The FTO substrate area is 1×1 cm. 2TiO2 nanorod arrays were prepared on FTO conductive glass using a hydrothermal method. The specific steps were as follows: 0.6 mL of tetrabutyl titanate was slowly added dropwise to 50 mL of 6 M hydrochloric acid solution, and the mixture was stirred continuously for 30 minutes to form a homogeneous precursor solution. The cleaned FTO substrate was placed tilted in a high-pressure reactor lined with polytetrafluoroethylene, and the aforementioned precursor solution was injected. The reaction was carried out at 170 °C for 7 hours. After the reaction, the sample was allowed to cool naturally to room temperature, removed, and repeatedly rinsed with deionized water, then air-dried. Finally, the sample was placed in a muffle furnace and heated to 450 °C at a heating rate of 5 °C / min, and held at this temperature for 2 hours to obtain a well-crystallized rutile phase TiO2 photoelectrode.

[0078] Step 2: Conventional Flame Treatment: Place the TiO2 photoelectrode prepared in Step 1 on a high-temperature resistant ceramic fiber plate. Use a handheld butane gas torch (or Bunsen burner) as a heat source, adjusting the gas flow rate to stabilize the flame, with the flame center temperature approximately 1300 K (>1000℃). Aim the torch flame at the back of the FTO substrate (or the photoanode surface, depending on the experimental conditions; usually, direct contact is used to simulate harsh environments), maintaining the nozzle approximately 3-5 cm from the electrode surface. Heat for 30 s, 50 s, 70 s, 90 s, and 110 s respectively without applying any organic reagents. After heating, allow it to cool naturally to room temperature. This process simulates conventional high-temperature annealing or uncontrolled external intense flame treatment.

[0079] The superiority of the "self-sustaining combustion" process of this invention over the traditional "external continuous heat source heating" process can be verified by comparing Example 1 and Example 1. Through comparative experiments, the same pure TiO2 nanorod array photoelectrode as in Example 1 was selected. Instead of DMSO being drip-coated, a traditional flame treatment method was used, placing it under an external open flame heat source (temperature > 1000 K) for continuous heating treatment (simulating traditional high-temperature annealing or flame treatment processes). The heating times were set to 0 s, 30 s, 50 s, 70 s, 90 s, and 110 s, respectively, and its conductivity was subsequently tested. The results are as follows: Figure 12 As shown in the figure, the horizontal axis Heating Time (s) represents the heating time; the vertical axis Conductivity (10) represents the heating time. 5 S / m represents conductivity. Figure 12 It can be known that: 1. Comparative Example 1: Severe substrate damage: The sample prepared using the DMSO self-ignition method of this invention ( Figure 1 Due to the self-limiting temperature characteristics of combustion, its conductivity remains at 1.0 × 10⁻⁶ after multiple cycles. 5The high level of S / m indicates that the conductivity of the FTO substrate has been well preserved.

[0080] 2. Poor electrical conductivity of external heat sources: In contrast, samples treated by traditional flame treatment methods ( Figure 12 The conductivity of the FTO substrate showed a significant decreasing trend. In the initial state (0 s), the FTO substrate remained intact, with a conductivity of approximately 1.95 × 10⁻⁶. 5 S / m; however, the conductivity rapidly decreases with prolonged processing time. After only 70 s of heating, the conductivity drops to 0.50 × 10⁻⁶. 5 Below S / m; after heating for 110 s, it drops even further to 0.12×10 5 The value is around S / m. This confirms that in traditional flame treatment methods, the heat accumulation effect generated by a continuous strong external heat source (>1000 K) can easily damage the structure of the FTO conductive glass, leading to a significant decrease in the overall conductivity of the electrode, which severely restricts the photoelectrocatalytic performance.

[0081] This result strongly demonstrates that the "liquid film self-ignition" technology proposed in this invention has the significant advantages of being gentle and not damaging to the substrate.

[0082] 3. Analysis of process advantages: Compared with existing external flame doping technology, the embodiments of the present invention have significant thermodynamic differences.

[0083] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a surface-confined sulfur-doped metal oxide photoanode, characterized in that, The method includes, Metal oxide photoanodes were obtained by growing metal oxide nanostructures on FTO substrates using a hydrothermal method. The surface of the metal oxide photoanode is alternately subjected to drop-coating and ignition treatment multiple times to obtain an ignition-treated metal oxide photoanode. The drop-coating and ignition treatment includes drop-coating sulfur-containing organic matter to form a liquid wet film and igniting the liquid wet film in an air environment. The metal oxide photoanode after ignition treatment is cleaned and dried to obtain a surface-confined sulfur-doped metal oxide photoanode.

2. The method for preparing a surface-confined sulfur-doped metal oxide photoanode according to claim 1, characterized in that, The metal oxide photoanode includes one of the following: TiO2 photoanode, BiVO4 photoanode, WO3 photoanode, or Fe2O3 photoanode.

3. The method for preparing a surface-confined sulfur-doped metal oxide photoanode according to claim 1, characterized in that, Metal oxide nanostructures include one of the following: nanorod arrays, nanowire arrays, or nanosheet arrays.

4. The method for preparing a surface-confined sulfur-doped metal oxide photoanode according to claim 1, characterized in that, The sulfur-containing organic compounds include one or more of dimethyl sulfoxide, dimethyl sulfide, diethyl sulfide, or thiophene.

5. The method for preparing a surface-confined sulfur-doped metal oxide photoanode according to claim 1, characterized in that, The film-forming and ignition processes are performed alternately N times, where N is an integer not less than 1 and not more than 1000.

6. The method for preparing a surface-confined sulfur-doped metal oxide photoanode according to claim 1, characterized in that, The film-forming and ignition processes are performed alternately N times, where N is an integer not less than 10 and not more than 150.

7. The method for preparing a surface-confined sulfur-doped metal oxide photoanode according to claim 1, characterized in that, The drop volume is 0.05-0.20 mL / cm².

8. The method for preparing a surface-confined sulfur-doped metal oxide photoanode according to claim 1, characterized in that, The size of the FTO substrate is greater than or equal to 1×1cm. 2 .

9. The method for preparing a surface-confined sulfur-doped metal oxide photoanode according to claim 1, characterized in that, The size of the FTO substrate is greater than or equal to 5 × 5 cm. 2 .

10. The surface-confined sulfur-doped metal oxide photoanode obtained by the preparation method according to any one of claims 1-9, characterized in that, The surface-confined sulfur-doped metal oxide photoanode exhibits a photocurrent density of 1.60–4.35 mA cm⁻¹ under a bias of 1.23 V vs. RHE. -2 .

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