A photoanode with urea oxidation activity and a preparation method thereof

Abstract

The application discloses a photoanode with urea oxidation activity and a preparation method thereof. x Ti 100‑x The alloy sheet is polished and ground to remove a surface oxide layer and is single-side insulated and packaged to serve as a conductive base; then, the alloy sheet is anodized to generate amorphous TiO2 nanotubes decorated with NiO, and heat treatment is performed to make the amorphous TiO2 phase change into a rutile crystal phase; then, surface states are generated by electrochemical protonation under a negative voltage to serve as a photo-generated electron transmission path; finally, NiO is activated into Ni(OH)2 by electrochemical cyclic scanning in an alkaline electrolyte, and the photoanode with urea oxidation activity is obtained. The photoanode obtained by the application has high charge separation efficiency and urea oxidation selectivity, and the preparation process is easy to be industrialized, and has great application potential in the fields of energy and environment.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of photoelectrochemistry, and relates to a urea oxidation active photoanode and a preparation method thereof, which is applied to photoelectrochemical urea oxidation for hydrogen production and photoelectrochemical urea wastewater purification. BACKGROUND

[0002] With the rapid development of society, environmental pollution and energy crisis problems are increasingly prominent. Industrial and agricultural, animal husbandry and daily life will produce a large amount of urea wastewater. In order to prevent water pollution, the current electrolysis method consumes a large amount of electric energy, and the biological hydrolysis method will produce a large amount of ammonia [1] . The photoelectrochemical urea oxidation method uses the photoanode to oxidize urea into harmless nitrogen and carbon dioxide, and the cathode reduces water into hydrogen, which is a one-two method for the environment and energy [2] . The theoretical decomposition voltage of urea is 0.37V vs.RHE (Reversible hydrogen electrode), which is much lower than the theoretical decomposition voltage of water 1.23V vs.RHE, so replacing the anode reaction of electrolysis of water with urea oxidation is a thermodynamically optimized reaction path [3] .

[0003] At present, the urea oxidation photoelectrode has the problems of low charge separation efficiency and insufficient urea oxidation kinetics [4-8] . In view of these problems, the application provides a high-efficiency photoanode with urea oxidation activity and a preparation technology thereof. Taking TiO2 photoelectrode as a model material, a surface state is formed by electrochemical surface protonation to serve as a photo-generated electron separation path, and a high work function Ni(OH)2 layer is further modified in situ to improve the TiO2 solid-liquid junction surface band bending, so as to inhibit the photo-generated charge recombination caused by back electron transfer. Meanwhile, the Ni(OH)2 serves as an electrocatalytic layer to improve the urea oxidation selectivity and catalytic efficiency. The application of the method is conducive to the design of a high-efficiency photoanode applied to the energy and environmental fields related to urea oxidation reaction.

[0004] REFERENCES

[0005] [1] Zhu B J, Liang Z B, Zou R Q. Designing Advanced Catalysts for Energy Conversion Based on Urea Oxidation Reaction. Small, 2020, 16, 1906133.

[0006] [2]Sun W B,Zhang M,Li J N,Peng C.Solar-Driven Catalytic UreaOxidation for Environmental Remediation and Energy Recovery.ChemSusChem,2022,e202201263.

[0007] [3]Singh R K,Rajacelu K,Montag M,Schechter A.Advances in CatalyticElectrooxidation of Urea:A Review.Energy Technol,2021,2100017.

[0008] [4]Zhao R,Schumacher G,Leahy S,Radich E J.Ni(OH)2as Hole Mediator forVisible Light-Induced Urea Splitting,J.Phys.Chem.C 2018,122,13995-14003.

[0009] [5]Wang G M,Ling Y C,Lu X H,Wang H Y,Qian F,Tong Y X,Li Y.Solardriven hydrogen releasing from urea and human urine,Energy Environ.Sci.,2012,5,8215–8219.

[0010] [6]Xu D D,Fu Z W,Wang D J,Lin Y H,Sun Y J,Meng D D,Xie T F.ANi(OH)2-modified Ti-doped a-Fe2O3 photoanode for improved photoelectrochemicaloxidation of urea:the role of Ni(OH)2as a cocatalyst.Phys.Chem.Chem.Phys.,2015,17,23924—23930.

[0011] [7] Zhu H, Zhao MM, Zhou JK, Li WC, Wang HY, Xu Z, Lu L, Pei L, Shi Z, YanS C, Li ZS, Zou Z G. Surface states as electron transfer pathway enhanced charge separation in TiO2 nanotube water splitting photoanodes.Appl.Catal.B, 2018, 234, 100–108.

[0012] [8] Yang QM, Zhu H, Hou YH, Liu DD, Tang H, Liu DP, Zhang WN, Yan SC, Zou Z G. Surface polaron states on single-crystal rutile TiO2 nanorod arraysenhancing charge separation and transfer. Dalton Trans., 2020, 49, 15054–15060. Summary of the Invention

[0013] The technical problem of this invention is that the oxidation and decomposition of urea using a photoanode is a thermodynamically optimized reaction pathway that is beneficial to both the environment and energy. However, urea oxidation photoelectrodes suffer from low charge separation efficiency and insufficient urea oxidation kinetics. The purpose of this invention is to design and develop a photoanode with urea oxidation activity and its preparation method, and to further improve the charge separation efficiency of the photoelectrode by applying an electrochemical surface protonation process.

[0014] The technical solution of this invention is: a urea oxidation-active photoanode, the photoanode being composed of a conductive substrate, a photoactive semiconductor, and a urea oxidation-active layer, wherein the conductive substrate is Ni. x Ti 100-x The alloy sheet has a value of 0 < x ≤ 50. The photoactive semiconductor is a rutile phase TiO2 nanotube array grown on the alloy sheet, and the urea oxidation active layer is Ni(OH)2 generated in situ by electrochemical activation on the nanotube array.

[0015] Furthermore, Ni x Ti 100-x The Ni content in the alloy sheet affects the urea oxidation activity; the higher the Ni content, the better the urea oxidation kinetics.

[0016] Furthermore, the rutile TiO2 nanotube array has a length of 800-1000 nm, an outer diameter of 60-80 nm, and a wall thickness of 15-20 nm. The uniformly covered nanotube array has a high electrolyte contact interface, which is beneficial to increasing the number of chemical reaction sites per unit projected area.

[0017] This invention also proposes a method for preparing a photoanode with urea oxidation activity. After polishing and encapsulation, a NiTi alloy sheet is anodized in an electrolyte of ethylene glycol, water, and ammonium fluoride to generate NiO-modified amorphous TiO2 nanotubes. Then, heat treatment in air transforms the amorphous TiO2 phase into a rutile phase, denoted as Ni. x Ti 100-x / TiO2@NiO; then, through electrochemical surface protonation, doping generates surface states that serve as photogenerated electron transport pathways, denoted as Ni. x Ti 100-x / p-TiO2@NiO, electrochemically activating NiO to Ni(OH)2, generating a urea oxidation active layer, thus preparing the above-mentioned urea oxidation active photoanode, denoted as Ni. x Ti 100-x / p-TiO2@Ni(OH)2; wherein the electrochemical surface protonation is performed by electrochemical treatment with a negative voltage applied in a neutral or alkaline solution, the urea oxidation active layer is generated in situ by electrochemical cyclic scanning in an alkaline solution, and the urea oxidation activity is demonstrated by detection in alkaline aqueous solutions of urea at different concentrations.

[0018] Furthermore, the preparation of amorphous TiO2 nanotubes is specifically as follows:

[0019] (1) Alloy sheet pretreatment: Select Ni according to Ni content x Ti 100-x For alloy sheets with 0 < x ≤ 50, the target oxide surface is sequentially polished vertically with metallographic sandpaper of different grits until a mirror-like smooth surface is achieved. The polished alloy sheets are then ultrasonically cleaned for 10 minutes each in acetone, anhydrous ethanol, and deionized water, and dried with compressed air to complete the pretreatment. Finally, they are adhered to the pretreated Ni alloy sheet with insulating tape. x Ti 100-x The alloy sheet completes the single-sided encapsulation;

[0020] (2) Anodizing of alloy sheets: using packaged Ni x Ti 100-x Anodizing was performed using an alloy sheet as the anode and a platinum sheet as the cathode in an ethylene glycol electrolyte containing 0.5-1 vol% deionized water and 0.2-0.35 wt% ammonium fluoride. The anode was then rinsed with ethanol and distilled water before use. The oxidation voltage was 30-60 V, the electrolyte temperature was 20-30 °C, and the oxidation time was 15-60 min. x Ti100-x NiO-modified amorphous TiO2 nanotubes are grown on an alloy substrate.

[0021] Furthermore, the heat treatment to obtain the rutile phase TiO2 nanotube array specifically involves: after removing the insulating encapsulation, heat treatment in a muffle furnace transforming the amorphous TiO2 phase into rutile phase TiO2. The heat treatment heating rate is 10℃ / min, the holding temperature is 550℃-650℃, and the holding time is 2 hours. The array is then naturally cooled to room temperature with the furnace to obtain NiO-modified rutile phase TiO2 nanotube electrodes, i.e., NiO-modified rutile phase TiO2 nanotube electrodes. x Ti 100-x / TiO2@NiO.

[0022] Furthermore, the specific steps for preparing the urea oxidation active layer to obtain a urea oxidation active photoanode are as follows:

[0023] Electrochemical surface protonation doping of rutile TiO2 nanotube arrays: A three-electrode electrochemical cell was used with Ni x Ti 1-x Using / TiO2@NiO as the working electrode, and a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, chronoamperometry was performed in neutral or alkaline electrolytes with a negative voltage negative to the flat band potential of TiO2. By adjusting the magnitude and duration of the applied voltage, the surface state density was controlled, resulting in a NiO-modified electrochemically protonated rutile TiO2 nanotube electrode, denoted as Ni. x Ti 100-x / p-TiO2@NiO, wherein the applied voltage is -1.5 to -1.7 V relative to the Ag / AgCl reference electrode vs. Ag / AgCl, and the voltage holding time is 30 to 60 seconds;

[0024] Then, electrochemical activation is performed to obtain a photoanode with urea oxidation activity: a three-electrode electrochemical cell is used, with Ni x Ti 1-x Using p-TiO2@NiO as the working electrode, and a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, an electrochemical cyclic scan was performed at a rate of 100 mV / s in an alkaline NaOH electrolyte with pH = 14, within a voltage range of -1.0 to 0.6 V vs. Ag / AgCl. The activated NiO surface was converted to Ni(OH)2, resulting in a photoanode with urea oxidation activity, i.e., Ni... x Ti 100-x / p-TiO2@Ni(OH)2;

[0025] Finally, rinse the obtained photoanode with distilled water and dry it with compressed air before use.

[0026] The photoanode conductive substrate of this invention is Nix Ti 100-x The Ni content affects the urea oxidation activity. Under the anodizing process, Ni 2+ When immersed in the electrolyte, the nanotubes made from alloy sheets with a high proportion of Ni have more NiO particles on their surface, resulting in more urea oxidation active sites after electrochemical activation and faster urea oxidation kinetics; at the same time, it is also beneficial to increase the surface bending to reduce back electron transport.

[0027] The photoanode of this invention also has high photoelectrochemical efficiency: Ni(OH)2, as a high work function layer, increases the surface band bending of the semiconductor, effectively suppressing the back transport of photogenerated electrons, that is, suppressing the capture of photogenerated electrons by reducing species in the electrolyte, and improving charge separation efficiency. At the same time, Ni(OH)2 can interact strongly with urea molecules through Ni-N bonds, effectively promoting the consumption of photogenerated holes. The faster chemical reaction kinetics effectively reduce photogenerated charge recombination.

[0028] In this invention, surface states are generated through electrochemical surface protonation during the fabrication of the photoanode. Under a negative voltage, the protonation of oxidants follows two protonation mechanisms. At lower negative voltages, the reduction of lattice oxygen atoms to generate oxygen vacancies is insufficient, at which point the H+ in the electrolyte... + Under the influence of an electric field, protons undergo a proton insertion reaction with O atoms on the TiO2 surface to generate Ti-OH. If the negative voltage is sufficiently high, the lattice oxygen in TiO2 will be reduced to generate highly reactive oxygen vacancies, which then react with water to form Ti-OH. The resulting surface states enhance photoelectric properties. Ti-OH, as a surface state, can be converted into Ti... 3+ / 4+ -OH ions capture or transport electrons through valence changes, becoming a pathway for photogenerated electrons. For N-type conductive TiO2 semiconductors, the surface state energy levels are distributed near the conduction band. Due to the energy level difference between the conduction band and the surface energy levels, photogenerated electrons can be captured by the surface states. By changing the applied voltage, the capture and extraction of photogenerated electrons from the surface states can be effectively controlled. When the applied voltage causes the semiconductor conduction band energy level to be lower than the surface state energy level, the electrons captured by the surface states will be extracted to the conduction band and transported to the external circuit under the influence of the external electric field. Therefore, under the influence of the anodic voltage, Ti... 3+ -OH tends to be oxidized to Ti 4+ -OH, while photogenerated electrons tend to reduce Ti 4+ -OH represents Ti 3+ These two mechanisms—-OH—enable the continuous capture of photogenerated electrons by surface states and extraction by external voltage, achieving effective separation of photogenerated charges. This solves the problem of low charge separation efficiency in existing urea oxidation photoelectrodes.

[0029] The photoanode urea oxidation mechanism of this invention is as follows: the photoactivity originates from the photoelectric conversion characteristics of the TiO2 semiconductor, and the urea oxidation activity originates from the catalytic performance of the Ni(OH)2 active species on the nanotube surface. Photogenerated holes in TiO2 oxidize Ni(OH)2 to NiOOH, and NiOOH oxidizes CO(NH2)2 to N2, while CO2 is simultaneously reduced to Ni(OH)2. This process continues under light irradiation, thus achieving urea oxidation. To address the problem of insufficient urea oxidation kinetics, the urea oxidation activity of the photoanode of this invention can be demonstrated by testing in alkaline aqueous solutions of urea at different concentrations, using a three-electrode system with Ni... x Ti 100-x Using the p-TiO2@Ni(OH)2 photoanode as the working electrode, and a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, and a 300W xenon lamp as the light source, linear sweep voltammetry was performed in 1M KOH electrolytes containing 0.033M, 0.1M, 0.33M, and 1M urea, as well as in a 1M KOH alkaline solution without urea. Figure 5 The photocurrent of the NixTi100-x / p-TiO2@Ni(OH)2 photoanode changes with urea concentration, demonstrating the urea oxidation activity of the photoanode of the present invention.

[0030] The beneficial effects of this invention are:

[0031] (1) The photoanode of the present invention has high charge separation efficiency and good urea oxidation activity, and can be applied to photoelectric treatment of urea wastewater, urea fuel cells, photoelectric water splitting to produce hydrogen, etc., generating good environmental and energy benefits.

[0032] (2) The photoanode preparation method of the present invention is relatively simple and easy to control, and is suitable for large-scale industrial production;

[0033] (3) The novel photoanode of the present invention has the dual functions of water purification and hydrogen production, which is conducive to achieving low-cost treatment of wastewater;

[0034] (4) This invention proposes a technical route for improving the photoelectrochemical performance of photoanodes by one-step electrochemical oxidation of alloys and combining it with electrochemical protonation to generate surface states. This route can be applied to improve the photoelectric conversion efficiency of various oxidation photoelectrodes and is beneficial for designing high-efficiency electrochemical devices. Attached Figure Description

[0035] Figure 1 Ni in Example 1 50 Ti 50 Alloy sheet, made of Ni 50 Ti 50 Ni prepared by anodizing alloy sheets 50 Ti 50 / TiO2@Ni(OH)2 and protonated Ni 50 Ti 50 XRD pattern of / p-TiO2@Ni(OH)2.

[0036] Figure 2 Ni prepared in Example 1 50 Ti 50 SEM image of / TiO2@Ni(OH)2.

[0037] Figure 3 Ni as described in Example 1 50 Ti 50 / TiO2@Ni(OH)2、Ni 50 Ti 50 Comparison of photoelectrocatalytic performance of / p-TiO2@Ni(OH)2 in 1M potassium hydroxide solution containing 0.33M urea and 1M potassium hydroxide solution without urea.

[0038] Figure 4 Ni as described in Example 1 50 Ti 50 Comparison of photoelectrocatalytic performance of / TiO2@Ni(OH)2 in 1M potassium hydroxide solution containing 0.33M urea and 1M potassium hydroxide solution without urea by chopper test.

[0039] Figure 5 Ni as described in Example 1 50 Ti 50 Photoelectrocatalytic performance test results of / p-TiO2@Ni(OH)2 in 1M KOH electrolyte containing 0.033M, 0.1M, 0.33M, and 1M urea, as well as in 1M KOH alkaline solution without urea.

[0040] Figure 6 Ni as described in Comparative Example 2 25 Ti 75 Photoelectrocatalytic performance test results of / p-TiO2@Ni(OH)2 in 1M KOH electrolyte containing 0.033M, 0.1M, 0.33M, and 1M urea, as well as in 1M KOH alkaline solution without urea. Detailed Implementation

[0041] This invention proposes a photoanode with urea oxidation activity, which consists of a conductive substrate and a photoactive semiconductor, wherein the conductive substrate is Ni. x Ti 100-x (0<x≤50) alloy sheet, the photoactive semiconductor is a rutile phase TiO2 nanotube array, and the urea oxidation active layer is an in-situ activated Ni(OH)2.

[0042] This invention also proposes a method for preparing the aforementioned photoanode: NiTi alloy sheets are polished and encapsulated, then NiO-modified TiO2 nanotubes are anodized in an electrolyte of ethylene glycol, water, and ammonium fluoride. The nanotubes are then heat-treated in air to transform into a rutile phase. Electrochemical surface protonation doping generates surface states that serve as photogenerated electron transport pathways. Electrochemical activation of NiO transforms it into Ni(OH)2, forming a urea active layer to prepare a highly active photoanode. The electrochemical protonation is performed by applying a negative voltage to an alkaline solution. The urea oxidation active layer is generated in situ through electrochemical cyclic scanning in an alkaline solution. The urea oxidation activity is fully demonstrated by exposure to alkaline urea solutions of different concentrations.

[0043] First, the alloy sheet undergoes pretreatment: Ni x Ti 100-x (0<x≤50) The alloy sheet thickness is 0.2-1mm. Cut it into 1cm x 2cm pieces with aviation scissors. Use metallographic sandpaper of 400 grit, 600 grit, 1000 grit and 2000 grit to grind the target oxide surface vertically until it is mirror smooth. The ground alloy sheet is ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 10 minutes each. After drying with compressed air, it is ready for use.

[0044] Then, use insulating tape to adhere it to the pretreated Ni. x Ti 100-x (0<x≤50) The alloy sheet is encapsulated on one side to obtain a conductive substrate.

[0045] Next, anodizing of the alloy sheet is performed: using the packaged NiTi alloy sheet as the anode and a platinum sheet as the cathode, anodizing is carried out in an ethylene glycol electrolyte containing 0.5-1 vol% deionized water and 0.2-0.35 wt% ammonium fluoride. After rinsing with ethanol and distilled water, it is ready for use. The oxidation voltage is 30-60V, the electrolyte temperature is 20-30℃, and the oxidation time is 15-60min. This achieves the growth of NiO-modified TiO2 amorphous nanotubes on the NiTi alloy substrate. The grown TiO2 amorphous nanotubes have a length of 800-1000nm, an outer diameter of 60-80nm, and a wall thickness of 15-20nm.

[0046] Amorphous TiO2 nanotubes were heat-treated in a muffle furnace after the encapsulating insulating tape was removed, transforming the amorphous TiO2 phase into rutile TiO2. The heating rate was 10℃ / min, the holding temperature was 550℃-650℃, and the holding time was 2 hours. The nanotubes were then naturally cooled to room temperature with the furnace to obtain NiO-modified rutile TiO2 nanotubes, denoted as NiO. x Ti 100-x / TiO2@NiO.

[0047] The growth mechanism of TiO2 nanotube arrays is as follows:

[0048] (1) Electrochemical oxide layer formation of NiTi alloy sheet. The pretreated alloy sheet is exposed to the metal substrate. An anodic voltage is applied to generate a directional electric field, which causes an electrochemical reaction between the alloy and active oxygen ions and corrosive hydroxide ions in the electrolyte, thereby forming a thin metal oxide layer between the alloy substrate and the solution.

[0049] (2) The surface of the electrochemical oxide layer of the NiTi alloy sheet is eroded and pitted, and then directionally dissolved into nanotubes. The oxide on the surface of the NiTi alloy sheet is then coated with F. - Ion etching leads to chemical dissolution, forming small pits and pores. Under anodic voltage, the more stable Ti-O bonds form TiO2, while the less stable Ni-O bonds tend to be dissolved by F. - Ion etching dissolves the TiO2 nanotubes into the electrolyte. As the electrochemical oxidation and ion etching processes continue, the length of the TiO2 nanotubes increases. When the TiO2 nanotubes reach a certain length, the F at the growth interface... - When both the ion concentration and the electrochemical partial pressure decrease to a level insufficient to continue the electrochemical oxidation and chemical etching processes, the nanotubes cease growth.

[0050] The nanotubes prepared by the anodizing process of this invention are amorphous and transform into the rutile phase at 550℃. Simultaneously, the Ni adsorbed on the nanotube surface oxidizes and aggregates to grow into NiO particles, achieving NiO modification and obtaining Ni... x Ti 100-x / TiO2@NiO electrode.

[0051] Preferably, the rutile nanotube array has a length of 800-1000 nm, an outer diameter of 60-80 nm, and a wall thickness of 15-20 nm. The nanotube array prepared by this invention has good uniformity, with active species uniformly covering the substrate, which is beneficial for increasing the number of chemical reaction sites per unit projected area.

[0052] This invention utilizes electrochemical surface protonation doping to generate surface states as a photogenerated electron transport pathway, thus broadening the application field of electrochemical surface protonation. Specifically:

[0053] (1)Ni x Ti 100-x / TiO2@NiO electrochemical surface protonation doping: A three-electrode electrochemical cell was used with Ni x Ti 100-xUsing a TiO2@NiO electrode as the working electrode, and a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, chronoamperometry was performed in an alkaline electrolyte with a negative voltage (-0.14V vs. RHE, pH=14) at the TiO2 flat band potential. The degree of surface protonation was controlled by adjusting the applied voltage magnitude and duration, thereby controlling the surface state density and obtaining NiO2@NiO2. x Ti 100-x / p-TiO2@NiO. The applied voltage relative to the Ag / AgCl reference electrode is -1.5 to -1.7 V vs. Ag / AgCl, and the application time is 30 to 60 seconds;

[0054] (2) Rinse the product obtained by electrochemical surface protonation treatment with distilled water, and dry it with compressed air for later use.

[0055] Then, electrochemical activation is performed to obtain a photoanode with urea oxidation activity: a three-electrode electrochemical cell is used, with Ni x Ti 1-x Using p-TiO2@NiO as the working electrode, and a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, an electrochemical cyclic scan was performed at a rate of 100 mV / s in an alkaline NaOH electrolyte with pH = 14, within a voltage range of -1.0 to 0.6 V vs. Ag / AgCl. The activated NiO surface was converted to Ni(OH)2, resulting in a photoanode with urea oxidation activity, i.e., Ni... x Ti 100-x / p-TiO2@Ni(OH)2; Finally, rinse the obtained photoanode with distilled water, dry it with compressed air, and set it aside for later use.

[0056] The technical solutions in the embodiments of the present invention will be described in further detail below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0057] The photoelectrocatalytic performance testing conditions used in this embodiment of the invention are as follows: a 300W xenon lamp with a simulated sunlight (AM1.5G) filter is used as the light source; a three-electrode testing system is employed, with a platinum sheet as the counter electrode and Ag / AgCl as the reference electrode; and the test is conducted using a Shanghai Chenhua CHI 660e electrochemical workstation. All test voltage values ​​are converted to voltage values ​​relative to the reversible hydrogen electrode.

[0058] Example 1

[0059] (1) Using Ni 50 Ti 50The alloy sheet, 0.2 mm thick, was cut to 1 cm × 2 cm using aviation scissors. This size is for testing convenience and is designed to correspond to the size of the electrolytic cell used. The thickness of the alloy sheet has no effect on the photoanode performance of this invention; the performance is determined by the growth of the surface semiconductor. The target oxide surface was vertically polished sequentially with 400-grit, 600-grit, 1000-grit, and 2000-grit metallographic sandpaper until it was mirror-smooth. The polished alloy sheet was then ultrasonically cleaned for 10 minutes each in acetone, anhydrous ethanol, and deionized water, and dried with compressed air before use. Insulating tape was used to attach the pretreated sample for single-sided encapsulation.

[0060] (2) Using the packaged sample as the anode, a titanium sheet of the same size as the cathode, and an ethylene glycol solution of 0.5 Vol% deionized water and 0.35 Wt% ammonium fluoride at 30°C as the electrolyte, anodize for 60 min at 60 V.

[0061] (3) Remove the insulating tape used for sample packaging, and heat the sample in a muffle furnace at a rate of 10℃ / min from room temperature to 550℃, hold for 2 hours, and then allow it to cool naturally to room temperature to obtain NiO-modified rutile TiO2 nanotubes (denoted as Ni). 50 Ti 50 / TiO2@NiO).

[0062] (4) In the three-electrode system, Ni 50 Ti 50 Using / TiO2@NiO as the working electrode, a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, and a 1M potassium hydroxide solution as the electrolyte, electrochemical surface protonation was performed for 60 seconds at a potential of -1.6V vs. Ag / AgCl to obtain a photoanode with abundant surface states, denoted as Ni. 50 Ti 50 The XRD pattern for / p-TiO2@NiO is shown below. Figure 1 As shown, the SEM image is as follows: Figure 2 As shown in the figure, the prepared photoanode is a rutile phase TiO2 nanotube array, and the electrochemical surface protonation treatment does not change the phase structure.

[0063] (5) Three-electrode electrochemical cell, with Ni 50 Ti 50 / TiO2@NiO、Ni 50 Ti 50Using p-TiO2@NiO as the working electrode, and a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, electrochemical cycling was performed at a rate of 100 mV / s in an alkaline NaOH electrolyte with pH = 14, within a voltage range of -1.0 to 0.6 V vs. Ag / AgCl. The activated NiO surface was then converted to Ni(OH)2, resulting in NiO with high urea oxidation activity. 50 Ti 50 / TiO2@Ni(OH)2 and Ni 50 Ti 50 / p-TiO2@Ni(OH)2 photoanode. Both Ni50Ti50 / TiO2@NiO and Ni50Ti50 / p-TiO2@NiO before and after protonation can be activated by cyclic voltammetry to obtain a surface Ni(OH)2 active layer. However, the photoanodes with and without protonation surface state treatment have different performance, which was compared and verified in step (6).

[0064] (6) 1M KOH solution containing 0.033M, 0.1M, 0.33M, and 1M urea and 1M KOH alkaline solution without urea were used as electrolytes for photoelectrochemical testing. The test was conducted using a three-electrode system with a platinum sheet as the counter electrode and Ag / AgCl as the reference electrode to test the photoelectrochemical urea oxidation activity.

[0065] Take Ni from step (5) of Example 1 50 Ti 50 / TiO2@Ni(OH)2 and Ni 50 Ti 50 The photoelectrocatalytic performance of two electrodes, p-TiO2@Ni(OH)2, was tested in a 1M potassium hydroxide solution containing 0.33M urea and a 1M potassium hydroxide solution without urea. The results are as follows: Figure 3 As shown, Ni 50 Ti 50 The urea oxidation saturation photocurrent of / p-TiO2@Ni(OH)2 is significantly higher than that of Ni 50 Ti 50 The / TiO2@Ni(OH)2 method demonstrates that electrochemical surface protonation significantly improves the separation efficiency of photogenerated charges. At the same time, the turn-on potential is significantly lower in electrolytes with high urea concentrations, indicating the existence of a competitive reaction process between urea molecule oxidation and water oxidation. This also proves that the invented photoanode has dual activity of water oxidation and urea oxidation, and can be applied to the treatment of water pollution with different urea concentrations.

[0066] Take Ni from step (5) of Example 1 50 Ti 50The photoelectrocatalytic performance of / TiO2@Ni(OH)2 was tested by chopper assay in 1M potassium hydroxide solution and 1M potassium hydroxide solution containing 0.33M urea, respectively. The results are as follows: Figure 4 As shown. It can be seen that Ni 50 Ti 50 / TiO2@Ni(OH)2 exhibited smaller transient recombination in a 1M potassium hydroxide solution containing 0.33M urea, indicating that Ni… 50 Ti 50 The / TiO2@Ni(OH)2 photoanode exhibits higher urea oxidation kinetics. Here, Ni without protonation treatment is used. 50 Ti 50 The study of / TiO2@Ni(OH)2 further verified that the Ni(OH)2 active species on the surface of the nanotube designed in this invention have good urea oxidation activity. The photoanode designed in this invention to achieve urea oxidation activity with Ni(OH)2 can have smaller transient recombination and faster urea oxidation kinetics.

[0067] Take Ni from step (5) of Example 1 50 Ti 50 / p-TiO2@Ni(OH)2, perform the test in step (6), the results are as follows Figure 5 As shown, the results indicate that the photocurrent gradually increases with the increase of urea concentration, confirming that the prepared photoanode has strong urea oxidation activity.

[0068] Example 2

[0069] (1) Using Ni 25 Ti 75 The alloy sheet, 0.2 mm thick, was cut into 1 cm x 2 cm pieces using aviation scissors. The target oxide surface was then vertically polished sequentially with 400, 600, 1000, and 2000 grit metallographic sandpaper until a mirror-smooth finish was achieved. The polished alloy sheet was then ultrasonically cleaned for 10 minutes each in acetone, anhydrous ethanol, and deionized water, and dried with compressed air before use. The pretreated sample was then sealed on one side with insulating tape.

[0070] (2) Using the packaged sample as the anode and a titanium sheet of the same size as the cathode, anodize in 0.5 vol% deionized water and 0.35 wt.% ammonium fluoride in ethylene glycol solution. The oxidation voltage is 60 V, the electrolyte temperature is 30 °C, and the oxidation time is 60 min. The obtained electrode is rinsed with ethanol and distilled water and then dried for later use.

[0071] (3) Remove the insulating tape used for sample packaging, and heat the sample in a muffle furnace at a rate of 10℃ / min from room temperature to 600℃, hold for 2 hours, and then allow it to cool naturally to room temperature to obtain NiO-modified rutile TiO2 nanotubes (denoted as Ni). 25 Ti 75 / TiO2@NiO).

[0072] (4) In the three-electrode system, Ni 25 Ti 75 Using the / TiO2@NiO photoanode as the working electrode, and a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, and a 1M potassium hydroxide solution as the electrolyte, electrochemical doping was performed for 60 seconds at a potential of -1.6V vs. Ag / AgCl to obtain an electrochemically protonated photoanode (denoted as NiO2@NiO). 25 Ti 75 / p-TiO2@NiO).

[0073] (5) Three-electrode electrochemical cell, with Ni 25 Ti 75 Using p-TiO2@NiO as the working electrode, and a platinum sheet electrode and an Ag / AgCl electrode as the counter and reference electrodes, respectively, an electrochemical cyclic sweep was performed in an alkaline NaOH electrolyte at pH 14, within a voltage range of -1.0 to 0.6 V vs. Ag / AgCl, at a rate of 100 mV / s to activate the surface NiO to Ni(OH)2, thus obtaining Ni 25 Ti 75 / p-TiO2@Ni(OH)2 electrode.

[0074] (6) A 1.0M potassium hydroxide solution without urea and a 1M potassium hydroxide solution containing 0.033M, 0.1M, 0.33M and 1M urea, respectively, were used as electrolytes for photoelectrochemical testing. The test was conducted using a three-electrode system with a platinum sheet as the counter electrode and Ag / AgCl as the reference electrode to test the photoelectrochemical urea oxidation activity.

[0075] Take Ni from step (5) of Example 2 25 Ti 75 The / p-TiO2@Ni(OH)2 electrode was used for step (6) testing, and the results are as follows: Figure 6 As shown. Comparison Figure 5 and Figure 6 It is known that reducing the Ni content in the alloy reduces the photoanodic performance. This is mainly because a lower Ni content results in a lower number of Ni species adsorbed on the electrode surface. Low Ni content modification is not conducive to improving surface bending to reduce back electron transport and improve urea oxidation kinetics.

[0076] The photoanodes with urea oxidation activity prepared in the above embodiments can be applied to fields such as photoelectrocatalytic urea wastewater treatment, urea fuel cells, photocatalytic water splitting for hydrogen production, and organic dye degradation. In particular, the photoanodes have higher photoelectric conversion efficiency after electrochemical surface protonation, which proves that electrochemical surface protonation is an effective technology to improve the separation efficiency of photogenerated charges. The design scheme of the present invention expands the application of electrochemical surface protonation.

[0077] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A photoanode of urea oxidation activity, characterized by The photoanode consists of a conductive substrate, a photoactive semiconductor, and a urea oxidation active layer, wherein the conductive substrate is Ni x Ti 100-x alloy sheet, 0 < x < 50, the photoactive semiconductor is an array of rutile phase TiO2 nanotubes grown on the alloy sheet, and the urea oxidation active layer is Ni(OH)2 generated in situ electrochemically activated on the nanotube array.

2. The photoanode of urea oxidation activity according to claim 1, characterized in that The rutile TiO2 nanotube array has a length of 800-1000 nm, an outer diameter of 60-80 nm and a wall thickness of 15-20 nm.

3. A method for producing a photoanode having urea oxidation activity, characterized by NiTi alloy sheet is polished and packaged, and then anodized in an electrolyte of ethylene glycol, water and ammonium fluoride to form NiO modified amorphous TiO2 nanotube, and then heat treated in air atmosphere to make amorphous TiO2 phase change into rutile phase, recorded as Ni x Ti 100-x / TiO2@NiO; and then electrochemically surface protonated and doped to form surface state as a photo-generated electron transport path, recorded as Ni x Ti 100-x / p-TiO2@NiO, electrochemically activated NiO into Ni(OH)2, to form a urea oxidation active layer, and to prepare the urea oxidation active photoanode of claim 1 or 2, recorded as Ni x Ti 100-x / p-TiO2@Ni(OH)2; wherein the electrochemical surface protonation is performed by electrochemical treatment with negative voltage in alkaline solution, and the urea oxidation active layer is generated in situ by electrochemical cyclic scanning in alkaline solution.

4. The method for preparing the photoanode with urea oxidation activity according to claim 3, characterized in that the preparation of the amorphous TiO2 nanotube is specifically as follows: (1) Alloy sheet pretreatment: according to the Ni content, select Ni x Ti 100-x Alloy sheet, 0 < x ≤ 50, is polished vertically on the target oxidation surface with different mesh numbers of metallographic sandpaper until it is mirror smooth; the polished alloy sheet is sequentially ultrasonically cleaned in acetone, anhydrous ethanol and deionized water for 10 min, dried by compressed air, and pretreated, and is stuck on the pretreated Ni x Ti 100-x Alloy sheet is completed single-sided packaging; (2) Alloy sheet anodization: The packaged Ni x Ti 100-x Alloy sheet as anode, platinum sheet as cathode, anodization in ethylene glycol electrolyte containing 0.5-1 vol% deionized water, 0.2-0.35 wt% ammonium fluoride, rinsed with ethanol and distilled water, and then stored for use; wherein the oxidation voltage is 30-60 V, the electrolyte temperature is 20-30℃, the oxidation time is 15-60 min, and the Ni x Ti 100-x NiO modified amorphous TiO2 nanotubes are grown on the substrate of alloy sheet.

5. The method of claim 3, wherein the method is characterized by The heat treatment obtains the rutile phase TiO2 nanotube array, specifically: after removing the insulating package, heat treatment in a muffle furnace makes the amorphous TiO2 phase change into the rutile phase TiO2, wherein the heating rate of the heat treatment is 10 ℃ / min, the holding temperature is 550-650 ℃, the holding time is 2 h, and then the furnace is naturally cooled to room temperature to obtain the NiO modified rutile phase TiO2 nanotube electrode, namely Ni x Ti 100-x / TiO2@NiO.

6. The method of claim 3, wherein the method is characterized by The urea oxidation activity layer is prepared to obtain the photoanode with urea oxidation activity. Electrochemical surface protonation doping of rutile TiO2 nanotube arrays: a three-electrode electrochemical cell was used with Ni x Ti 1-x / TiO2@NiO as the working electrode, platinum sheet electrode and Ag / AgCl electrode as the counter electrode and reference electrode respectively, chronoamperometry was carried out in neutral or alkaline electrolyte with negative voltage lower than the flat band potential of TiO2, the surface state density was controlled by adjusting the applied voltage size and applied voltage time, and the NiO modified electrochemical surface protonated rutile TiO2 nanotube electrode was obtained, denoted as Ni x Ti 100-x / p-TiO2@NiO, wherein the applied voltage was-1.5 to-1.7 V relative to the Ag / AgCl reference electrode, and the voltage holding time was 30 to 60 seconds. Then electrochemically activate to obtain photoanode with urea oxidation activity: adopt three-electrode electrochemical cell, with Ni x Ti 1-x / p-TiO2@NiO as working electrode, platinum sheet electrode and Ag / AgCl electrode as counter electrode and reference electrode respectively, in pH=14 NaOH alkaline electrolyte, electrochemically cyclic scan surface NiO to Ni(OH)2 in-1.0-0.6V vs.Ag / AgCl voltage range at 100mV / s rate, to obtain photoanode with urea oxidation activity, namely Ni x Ti 100-x / p-TiO2@Ni(OH)2; Finally, the obtained photoanode is washed clean with distilled water, dried by compressed air and stored for use.

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