Multiphase multistage WO3 homojunction photoanode and preparation method and application thereof

Abstract

The invention belongs to the technical field of photoelectric materials, and particularly relates to a multiphase multistage WO3 homojunction photoanode and a preparation method and application thereof. The preparation method of the photo-anode comprises the following steps: preparing a precursor solution by taking sodium tungstate as a raw material, hydrochloric acid as an acid source and ammonium oxalate as a dispersing agent; placing the pretreated conductive glass in a reaction kettle, adding the precursor solution, and carrying out a hydrothermal reaction to obtain conductive glass deposited with yellow tungstic acid; and cleaning, drying and annealing the yellow tungstic acid deposited conductive glass to obtain the yellow tungstic acid deposited conductive glass. The multiphase multistage WO3 homojunction photoanode prepared by the invention can promote the separation of photon-generated carriers, so that the photoelectrocatalysis efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of photoelectric materials, and in particular relates to a multi-phase multi-level WO3 homojunction photoanode and a preparation method and application thereof. Background Art

[0002] Due to the global wave of energy structural reform and the increasing demand for energy, hydrogen energy, as a clean, efficient, green and pollution-free energy source, is expected to become the mainstream of energy market consumption in the future. Compared with replacement hydrogen production, reforming hydrogen production and other methods, photoelectrocatalytic water / seawater decomposition to produce hydrogen is based on the coordinated use of energy and is a more energy-saving and efficient way of producing hydrogen. The entire catalytic process consists of two parts, namely, the cathode reduces water to hydrogen, namely the hydrogen evolution reaction (HER) and the anode oxidizes water to oxygen, namely the oxygen evolution reaction (OER). After years of research by predecessors, the cathode HER reaction has made great progress, reaching 10mA·cm -2 Only a 12mV overpotential is required for the catalytic water / seawater splitting reaction. Therefore, the sluggish anode kinetics limit the hydrogen production efficiency. Regarding the anode OER reaction, its catalytic performance can be further improved through the optimization of semiconductor catalyst design.

[0003] A variety of oxide materials have been successfully applied to photoelectrocatalytic water / seawater decomposition. However, according to the solar radiation spectrum and its energy distribution, metal oxide semiconductors with a large bandgap (Eg) absorb wavelengths in the near-ultraviolet region, which cannot achieve reasonable utilization of visible light energy. Considering the influence of solution pH on the OER reaction kinetics, metal oxide semiconductors with moderate Eg and a certain degree of acid and alkali resistance are required. WO3, as a metal oxide material with a moderate bandgap and a certain degree of acid and alkali tolerance, can fully realize the effective utilization of solar radiation energy and will not invalidate the utilization of absorbed photons due to its own excessive non-radiative recombination.

[0004] WO3 materials mainly have monoclinic, orthorhombic, and hexagonal crystal forms. Different WO3 crystal forms have different photoelectrocatalytic activities. Among the many crystal forms, the (002), (020), and (200) crystal planes exposed in the monoclinic phase WO3 have the highest electrochemical activity. However, the carrier transport in its bulk phase is relatively delayed, resulting in the retention and recombination of photogenerated carriers. The hexagonal phase WO3, due to its porous structure, can adsorb some hydrated ions, thereby enhancing its bulk conductivity. However, the existing technology does not have a solution for preparing a multi-phase, multi-level homojunction photoanode using monoclinic WO3 as the main catalytic site and hexagonal WO3 as the current collecting site. Moreover, the existing photoanode is less effective in improving its photoelectric conversion efficiency. Summary of the Invention

[0005] One of the purposes of the present invention is to provide a method for preparing a multi-phase multi-level WO3 homojunction photoanode, which is easy to operate.

[0006] A second object of the present invention is to provide a multi-phase multi-level WO3 homojunction photoanode that can improve the photoelectrocatalytic performance.

[0007] The third object of the present invention is to provide an application of a multi-phase multi-level WO3 homojunction photoanode, which has broad application prospects in photoelectrocatalytic water decomposition.

[0008] One of the purposes of the present invention is achieved by the following technical solution:

[0009] A method for preparing a multi-phase multi-level WO3 homojunction photoanode comprises the following steps:

[0010] (1) Using sodium tungstate as raw material, hydrochloric acid as acid source, and ammonium oxalate as dispersant, a precursor solution is prepared;

[0011] (2) placing the pretreated conductive glass in a reactor, adding the precursor solution to carry out a hydrothermal reaction, and obtaining a conductive glass with deposited multi-phase and multi-level tungstic acid;

[0012] (3) The conductive glass deposited with the multi-phase and multi-level tungstic acid is cleaned, dried and annealed to obtain the conductive glass.

[0013] Among them, the present invention uses ammonium oxalate as a dispersant. On the one hand, oxalate ions and tungstate ions are complexed to form a six-component complex to obtain a preliminary hexagonal phase WO3. Subsequently, oxalate acts as a surfactant to form vesicles that wrap H2WO4 molecules and deposit them on a large scale. A monoclinic phase grows on the basis of the original hexagonal phase, thereby obtaining a multiphase structure.

[0014] Furthermore, in step (2), the conductive surface of the conductive glass faces the precursor solution; the temperature of the hydrothermal reaction is 140-180° C., and the time is 10-14 hours.

[0015] Furthermore, the preparation method of the precursor solution in step (1) is: dissolving sodium tungstate in deionized water A to obtain a sodium tungstate solution; adding hydrochloric acid dropwise to the stirred sodium tungstate solution, and continuing to stir for time A after the addition of hydrochloric acid is completed; then adding ammonium oxalate and deionized water B, and continuing to stir for time B, to obtain the precursor solution.

[0016] Furthermore, the sodium tungstate is sodium tungstate dihydrate; the mass ratio of the sodium tungstate dihydrate to ammonium oxalate is 1:(1.5-2.0); the volume ratio of the mass of the sodium tungstate dihydrate to deionized water A is 1 g:45-91 mL; the volume ratio of the mass of the sodium tungstate dihydrate to hydrochloric acid is 1 g:18-25 mL; the concentration of the hydrochloric acid is 2-5 mol / L; the volume ratio of the ammonium oxalate to deionized water B is 1 g:30-60 mL; the stirring time A is 30-40 min; and the stirring time B is 30-50 min.

[0017] Furthermore, the mass ratio of the sodium tungstate dihydrate to ammonium oxalate is 1:(1.5-1.6).

[0018] The present invention involves adding hydrochloric acid to a sodium tungstate solution. After stirring, free tungstate ions in the solution combine with hydrogen ions to form tungstic acid, which then gradually aggregates to form a turbid solution. After stopping stirring, the turbid solution rapidly separates into layers, forming a colorless, transparent upper layer and a pale yellow flocculent precipitate in the lower layer. Without any subsequent operation, the pale yellow flocculent precipitate remains undissolved. Subsequently, ammonium oxalate is added, and the solution transforms into a transparent gel. This is because ammonium oxalate is an ionic compound that hydrolyzes upon addition to an aqueous solution, forming free oxalate and ammonium ions and affecting the solution pH. Due to the pH change, tungstic acid complexes with free oxalate to form complex macromolecules (coordination compounds), which are then hydrolyzed and dissolved in the solution. The aggregated tungstic acid gradually hydrolyzes during the gradual coordination process with the oxalate, and the aggregation phenomenon gradually disappears, forming a colorless, transparent gel solution.

[0019] The present invention first prepares a tungstic acid solution as a WO3 precursor solution through a hydrolysis-coordination-rehydrolysis method, uses an acetone / deionized water mixed solution to clean and immerses the conductive glass surface in a piranha solution to etch it to increase its hydrophilicity, then places the conductive glass in a symmetrical geometric manner, injects the WO3 precursor solution, and finally synthesizes multi-phase and multi-level WO3 through hydrothermal treatment and annealing.

[0020] Furthermore, in step (3), the specific operation of the annealing is: heating to 400-600°C at a rate of 1-5°C / min, and keeping the temperature for 2-5 hours after reaching the target temperature.

[0021] Furthermore, in step (2), the pretreatment includes: placing the conductive glass in an acetone aqueous solution for ultrasonic treatment, then immersing it in a piranha solution for etching, and after cleaning and drying, obtaining the pretreated conductive glass.

[0022] Furthermore, in step (2), the volume ratio of water to acetone in the acetone aqueous solution is 1:1-2; and the piranha solution is prepared by mixing a 30 wt% hydrogen peroxide aqueous solution and concentrated sulfuric acid in a volume ratio of 1:0.5-1.5.

[0023] Furthermore, the conductive glass substrate is fluorine-doped tin oxide conductive glass (FTO) or indium-doped tin oxide conductive glass (ITO).

[0024] The second object of the present invention is achieved by adopting the following technical solution:

[0025] A multi-phase multi-level WO3 homojunction photoanode is prepared by using the above preparation method.

[0026] Furthermore, the multiphase structure of the multiphase multilevel WO3 homojunction photoanode includes a monoclinic phase and a hexagonal phase; the mass ratio of the monoclinic phase to the hexagonal phase in the multiphase structure is 19:1; the multilevel structure of the multiphase multilevel WO3 homojunction photoanode is a hexagonal snowflake structure; in the hexagonal snowflake structure, the hexagonal phase is located at the central axis of the hexagonal prism, presenting a primary structure; the monoclinic phase is located at the hexagonal snowflake, presenting a secondary structure.

[0027] The third object of the present invention is achieved by adopting the following technical solution:

[0028] For example, the application of multiphase and multilevel WO3 homojunction photoanode in photoelectrocatalytic water splitting.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. The present invention provides a method for preparing a multi-phase and multi-level WO3 homojunction photoanode. A WO3 precursor solution is prepared by hydrolysis-coordination-rehydrolysis, injected into the conductive surface of conductive glass placed according to geometric symmetry requirements, and a multi-phase and multi-level WO3 homojunction photoanode is formed by hydrothermal reaction. This preparation method is simple, has obvious improvement effects, and has prospects for large-scale commercial production.

[0031] 2. The present invention provides a multi-phase multi-level WO3 homojunction photoanode, which combines the advantages of monoclinic WO3 and hexagonal WO3 to form a multi-phase multi-level structure, which can effectively improve its photoelectrocatalytic efficiency.

[0032] (1) The multi-phase multi-level WO3 homojunction photoanode of the present invention organically combines two different crystal forms (monoclinic and hexagonal) of WO3 and forms a type II homojunction at the interface of the two crystal phases, thereby accelerating the transport of photogenerated carriers. Monoclinic WO3 has extremely high electrochemical activity and a suitable band structure, but the adjacent unit cells in the monoclinic WO3 are twisted at a certain angle, the electron cloud overlap area is small, and the hole structure formed is small, which cannot effectively adsorb charged particles, resulting in poor conductivity and serious photogenerated carrier recombination. However, the hexagonal hole structure formed between the six unit cells of the hexagonal WO3 can effectively adsorb hydrated ions. When filled with additional ions, the conductivity of the hexagonal WO3 can be greatly improved.

[0033] (2) The multi-phase multi-level WO3 homojunction photoanode of the present invention has a hexagonal snowflake-like multi-level structure. Since the snowflake structure is thin and symmetrically distributed, it is beneficial to the transmission of photogenerated carriers, reduces the retention and recombination during the transmission process, and thus improves the photoelectrocatalytic efficiency.

[0034] 3. The multiphase and multilevel WO3 homojunction photoanode of the present invention generates a photocurrent of approximately 1.5-3 times that of a pure WO3 photoanode at a voltage of 1.23V. The electrochemical reaction resistance Rct is significantly reduced by approximately 150Ω. It can operate for a long time, and the overall quality of the device is stable. It has potential prospects for large-scale production and provides new ideas for the construction of photoanodes.

[0035] 4. The present invention also provides the use of the above-mentioned multi-phase multi-level WO3 homojunction photoanode in photoelectrocatalytic water decomposition. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a reaction mechanism diagram of the multi-phase multi-level WO3 homojunction photoanode of the present invention;

[0037] Figure 2 These are SEM images of the multi-phase multi-level WO3 homojunction photoanode of Example 1 of the present invention at different magnifications;

[0038] Figure 3 Schematic diagram of the multi-phase and multi-level structure of the multi-phase and multi-level WO3 homojunction photoanode of Example 1;

[0039] in, Figure 3 a is a schematic diagram of the multi-stage structure of the multi-phase multi-stage yellow tungstic acid in Example 1, Figure 3 b is Figure 3 a Enlarged image within the blue dotted box;

[0040] Figure 4 The SEM images of the monoclinic WO3 photoanode of Comparative Example 1 of the present invention at different magnifications are shown;

[0041] Figure 5The XRD patterns of the multi-phase multi-level WO3 homojunction photoanode of Example 1 of the present invention and comparative examples 2-4 are shown;

[0042] Figure 6 Schematic diagram of the energy band structure of the multi-phase multi-level WO3 homojunction photoanode according to Example 1 of the present invention;

[0043] Figure 7 This is a JV curve diagram of the multi-phase multi-level WO3 homojunction photoanode according to Example 1 of the present invention;

[0044] Figure 8 This is the Nyquist diagram of the multi-phase multi-level WO3 homojunction photoanode of Example 1 of the present invention;

[0045] Figure 9 This is a stability test diagram of the multi-phase multi-level WO3 homojunction photoanode according to Example 1 of the present invention;

[0046] Figure 10 These are the XRD patterns before and after the stability test of the multi-phase multi-level WO3 homojunction photoanode according to Example 1 of the present invention. DETAILED DESCRIPTION

[0047] The following describes the embodiments of the present invention through specific examples. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented or applied through various other specific embodiments, and the details in this specification may be modified or altered based on different viewpoints and applications without departing from the spirit of the present invention.

[0048] Before further describing the specific embodiments of the present invention, it should be understood that the scope of protection of the present invention is not limited to the specific embodiments described below; it should also be understood that the terms used in the examples of the present invention are for describing specific embodiments rather than for limiting the scope of protection of the present invention.

[0049] When numerical ranges are given in the examples, it should be understood that, unless otherwise specified herein, both endpoints of each numerical range and any value between the two endpoints may be selected. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs.

[0050] The relevant parameters of the FTO conductive glass of the present invention are as follows:

[0051] The length, width and height dimensions are 20mm*20mm*2.2mm respectively, the square resistance is 7-14Ω, the transmittance is higher than 80%, and it can withstand high temperatures of 650℃.

[0052] The relevant parameters of the ITO conductive glass of the present invention are as follows:

[0053] The length, width and height dimensions are 20mm*20mm*2.2mm respectively, the square resistance is 7~18Ω, the transmittance is about 90%, and it can withstand high temperatures of 600℃.

[0054] (1) Embodiment

[0055] Example 1

[0056] This embodiment provides a method for preparing a multi-phase multi-level WO3 homojunction photoanode. The specific preparation process is as follows:

[0057] (1) Prepare a tungstic acid mixed solution by hydrolysis-coordination-rehydrolysis: weigh 0.332g of sodium tungstate dihydrate and place it in a beaker, add 30mL of deionized water, and stir at 500rpm until it is completely dissolved; use a pipette to measure 8mL of 3mol / L concentrated hydrochloric acid solution and add it dropwise to the stirred sodium tungstate solution until the color of the solution gradually changes from colorless and transparent to light yellow turbid liquid, stir the above turbid liquid for 40min, weigh 0.502g of ammonium oxalate and add it to the above solution, and add 30mL of deionized water, and stir at 500rpm for 40min until the solution is in a transparent gel state, thus obtaining a precursor solution;

[0058] (2) The FTO conductive glass was completely immersed in an acetone solution (the acetone solution was prepared by mixing deionized water and acetone in a volume ratio of 1:1), and ultrasonically treated for 20 minutes. The ultrasonically treated FTO conductive glass was then immersed in a piranha solution for 10 minutes. The piranha solution was prepared by mixing a 30wt% hydrogen peroxide solution and concentrated sulfuric acid in a volume ratio of 1:1. The FTO conductive glass was then repeatedly rinsed with an ethanol solution (prepared by mixing ethanol and deionized water in a volume ratio of 1:1). The glass was then blown dry with a hair dryer to obtain a pretreated FTO conductive glass.

[0059] The conductive surface of the pretreated FTO conductive glass was placed in a 50 mL polytetrafluoroethylene liner in an equilateral triangle with the conductive surface facing the solution. 30 mL of the precursor solution was measured with a pipette and injected into the liner. The glass was hydrothermally reacted at 160°C for 12 hours. After the hydrothermal reaction, the glass was naturally cooled to room temperature to form a conductive glass with tungstic acid deposited on the conductive surface.

[0060] (3) Remove the conductive glass with tungstic acid deposited on the conductive surface, repeatedly wash it with ethanol and deionized water, and blow dry it with a hair dryer. At this time, the conductive glass has shown multi-phase and multi-level structural characteristics. Remove the conductive glass and place it in an alumina porcelain boat and place it in a muffle furnace. Heat it to 450℃ at a rate of 1℃ / min and hold it for 3 hours. After natural cooling, remove it and obtain a multi-phase and multi-level WO3 homojunction photoanode.

[0061] This embodiment also provides a multi-phase multi-level WO3 homojunction photoanode, which is prepared by the above preparation method.

[0062] Example 2

[0063] This embodiment provides a method for preparing a multi-phase multi-level WO3 homojunction photoanode. The specific preparation process is as follows:

[0064] (1) Prepare a tungstic acid mixed solution by hydrolysis-coordination-rehydrolysis: weigh 0.532g of sodium tungstate dihydrate and place it in a beaker, add 30mL of deionized water, and stir at 500rpm until it is completely dissolved; use a pipette to measure 10mL of 3mol / L concentrated hydrochloric acid solution and add it dropwise to the stirred sodium tungstate solution until the solution color gradually changes from colorless and transparent to light yellow turbid liquid, stir the above turbid liquid for 30min, weigh 0.817g of ammonium oxalate and add it to the above solution, add 30mL of deionized water, and stir at 700rpm for 30min until the solution is in a transparent gel state, thus obtaining a precursor solution;

[0065] (2) The ITO conductive glass was completely immersed in an acetone solution (the acetone solution was prepared by mixing deionized water and acetone in a volume ratio of 1:1.5), and ultrasonically treated for 30 minutes. The ultrasonically treated ITO conductive glass was then immersed in a piranha solution for 10 minutes. The piranha solution was prepared by mixing a 30wt% hydrogen peroxide solution and concentrated sulfuric acid in a volume ratio of 1:1.5. The FTO conductive glass was then repeatedly rinsed with an ethanol solution (prepared by mixing ethanol and deionized water in a volume ratio of 1:1). The glass was then blown dry with a hair dryer to obtain a pretreated ITO conductive glass.

[0066] The conductive surface of the pretreated ITO conductive glass was placed in a 50 mL polytetrafluoroethylene liner in an equilateral triangle with the conductive surface facing the solution. 35 mL of the precursor solution was measured with a pipette and injected into the liner. The glass was hydrothermally reacted at 180°C for 14 hours. After the hydrothermal reaction, the glass was naturally cooled to room temperature to form a conductive glass with tungstic acid deposited on the conductive surface.

[0067] (3) Remove the conductive glass with tungstic acid deposited on the conductive surface, repeatedly wash it with ethanol and deionized water, and blow dry it with a hair dryer. At this time, the conductive glass has shown multi-phase and multi-level structural characteristics. Remove the conductive glass and place it in an alumina porcelain boat and place it in a muffle furnace. Heat it to 450℃ at a rate of 2℃ / min and hold it for 2h. After natural cooling, remove it and obtain a multi-phase and multi-level WO3 homojunction photoanode.

[0068] This embodiment also provides a multi-phase multi-level WO3 homojunction photoanode, which is prepared by the above preparation method.

[0069] Example 3

[0070] This embodiment provides a method for preparing a multi-phase multi-level WO3 homojunction photoanode. The specific preparation process is as follows:

[0071] (1) Prepare a tungstic acid mixed solution by hydrolysis-coordination-rehydrolysis: weigh 0.429g of sodium tungstate dihydrate and place it in a beaker and add 20mL of deionized water, stirring at 600rpm until it is completely dissolved; use a pipette to measure 8mL of concentrated hydrochloric acid solution with a concentration of 3mol / L and add it dropwise to the stirred sodium tungstate solution until the solution color gradually changes from colorless and transparent to light yellow turbid liquid, stir the above turbid liquid for 30min, weigh 0.657g of ammonium oxalate and add it to the above solution and add 20mL of deionized water, stir at 700rpm for 40min until the solution is in a transparent gel state, and thus obtain a precursor solution;

[0072] (2) The FTO conductive glass was completely immersed in an acetone solution (the acetone solution was prepared by mixing deionized water and acetone in a volume ratio of 1:2), and ultrasonically treated for 20 minutes. The ultrasonically treated FTO conductive glass was then immersed in a piranha solution for 15 minutes. The piranha solution was prepared by mixing a 30wt% hydrogen peroxide solution and concentrated sulfuric acid in a volume ratio of 2:1. The FTO conductive glass was then repeatedly rinsed with an ethanol solution (prepared by mixing ethanol and deionized water in a volume ratio of 1:1). The glass was then blown dry with a hair dryer to obtain a pretreated FTO conductive glass.

[0073] The conductive surface of the pretreated FTO conductive glass was placed in a 50 mL polytetrafluoroethylene liner in an equilateral triangle with the conductive surface facing the solution. 30 mL of the precursor solution was measured with a pipette and injected into the liner. The glass was hydrothermally reacted at 140°C for 10 hours. After the hydrothermal reaction, the glass was naturally cooled to room temperature to form a conductive glass with tungstic acid deposited on the conductive surface.

[0074] (3) Remove the conductive glass with tungstic acid deposited on the conductive surface, repeatedly wash it with ethanol and deionized water, and blow dry it with a hair dryer. At this time, the conductive glass has shown multi-phase and multi-level structural characteristics. Remove the conductive glass and place it in an alumina porcelain boat and place it in a muffle furnace. Raise the temperature to 450℃ at a rate of 5℃ / min and hold for 5h. After natural cooling, remove it and obtain the multi-phase and multi-level WO3 homojunction photoanode.

[0075] This embodiment also provides a multi-phase multi-level WO3 homojunction photoanode, which is prepared by the above preparation method.

[0076] (2) Comparative Example

[0077] Comparative Example 1

[0078] Comparative Example 1 provides a monoclinic WO3 photoanode, which is different from Example 1 in that the dispersant added in step (1) is citric acid, and the rest is the same as Example 1.

[0079] Comparative Example 2

[0080] The difference between Comparative Example 2 and Example 1 is that the time of the hydrothermal reaction in step (2) is 9 hours, and the rest is the same as Example 1.

[0081] Comparative Example 3

[0082] The difference between Comparative Example 3 and Example 1 is that the time of the hydrothermal reaction in step (2) is 6 hours, and the rest is the same as Example 1.

[0083] Comparative Example 4

[0084] The difference between Comparative Example 4 and Example 1 is that the time of the hydrothermal reaction in step (2) is 3 hours, and the rest is the same as Example 1.

[0085] (3) Product structure characterization and performance testing

[0086] The structure and application of the multi-phase multi-level homojunction photoanode provided by the embodiments of the present invention are described in detail below.

[0087] 1. The preparation process of the photoanode provided by the present invention is explained, and the reaction mechanism diagram is as follows Figure 1 shown.

[0088] Coordination stage: When hydrochloric acid is added to the sodium tungstate solution, yellow tungstic acid molecules are formed. Under stirring conditions, the solution gradually becomes turbid. When ammonium oxalate is added, the alkalinity of the solution increases, and the originally formed yellow tungstic acid is hydrolyzed and complexed with oxalate ions to form Figure 1 The molecular combination structure in the coordination stage, the solution gradually becomes clear;

[0089] Deposition stage: The complex macromolecules formed above are continuously stirred and aged and dehydrated. The yellow tungstic acid gradually dehydrates and grows using the complex macromolecules as templates, bonding with tungstate ions to form the molecular structure of the deposition stage.

[0090] Hydration stage: Due to the high temperature and high pressure in the hydrothermal reaction, the bonds formed between oxalate and tungstate are gradually subjected to repeated impacts of water molecules and internal thermal motion, resulting in the breaking of the bonds, and the tungstates reduce their energy by forming bonds.

[0091] Annealing stage: After the above process, tungstate is gradually deposited on FTO. In the subsequent annealing process, the crystal water will be released, that is, the crystal water formed with tungstate will be gradually decomposed through annealing and finally form WO3.

[0092] 2. Microstructural characterization of multiphase and multilevel WO3 homojunction photoanode

[0093] The top view of the microstructure of the multi-phase multi-level WO3 homojunction photoanode in Example 1 was observed by scanning electron microscope SEM. Figure 2 ,in, Figure 2 a and Figure 2 b are SEM images of the multi-phase multi-level WO3 homojunction photoanode of Example 1 at different observation angles at a 200nm scale, Figure 2 c and Figure 2 d are SEM images of the multiphase multilevel WO3 homojunction photoanode of Example 1 at 500nm and 1μm scales, respectively.

[0094] Figure 2 This is the SEM image of the multi-phase multi-level WO3 homojunction photoanode of Example 1. Figure 2 It can be seen that its main structure is a hexagonal snowflake structure, and its composition is uniform in the macroscopic range. Its snowflake structure is thinner in the range around the central hexagonal prism, thicker at the distal end of the central axis, and distributed around the central axis of the hexagonal prism in a certain C6 symmetrical structure. The additional scattered parts in other areas have not fully formed a hexagonal snowflake structure, but it can be seen that it has grown along the main central axis.

[0095] Figure 3 This is a schematic diagram of the multi-phase multi-level structure of the multi-phase multi-level WO3 homojunction photoanode in Example 1. Figure 3 a and Figure 3 b, where the dotted line is the central axis of the hexagonal prism of the hexagonal snowflake structure, corresponding to the hexagonal phase WO3, and Figure 3 The position marked by the blue solid line on the periphery of b is the monoclinic WO3. The overall structure forms a multi-phase and multi-level structure with the monoclinic WO3 as the main catalytic site and the hexagonal WO3 as the main current collector site.

[0096] Figure 4 The SEM image of the material obtained in Comparative Example 1 is Figure 4 It can be seen that the structure is quite different from that of Example 1, and no multi-phase and multi-level structure appears.

[0097] 3. Characterization of the crystal structure of WO3 in multiphase and multilevel WO3 homojunction photoanode

[0098] The results of observing the WO3 in the multi-phase multi-level WO3 homojunction photoanode of Example 1 by X-ray diffractometer are shown in FIG. Figure 5 .

[0099] Figure 5The XRD patterns of the homojunction photoanodes of Example 1 and Comparative Examples 2-4 show the crystal growth process for different hydrothermal times (Example 1 and Comparative Examples 2-4). XRD testing of samples with different hydrothermal times shows that the growth process is that the hexagonal WO3 phase first grows, and then the monoclinic WO3 phase grows on this basis. The color mapping based on the diffraction intensity in the figure shows that as the hydrothermal reaction time increases, the growth order of the different crystal phases differs, and the growth of the two crystal phases becomes more dense. Among them, 2θ=14.2°, 23.0°, and 28.3° correspond to the (100), (001), and (200) crystal planes in the hexagonal WO3, respectively, which have been marked with heart-shaped symbols in the figure; 2θ in the range of 23.3-24.5° corresponds to the (002), (020), and (200) crystal planes of the monoclinic WO3, 2θ=28.9° and 36.8° correspond to the (111) and (201) crystal planes of the monoclinic WO3, respectively, and 2θ=26.7°, 37.9°, and 51.7° correspond to the (110), (200), and (211) crystal planes of SnO2 in the FTO conductive glass, respectively, indicating that the components of the photoanode material obtained in Example 1 are monoclinic WO3 and hexagonal WO3, indicating that the homojunction photoanode material was successfully synthesized. And according to the content calculated based on its peak intensity, it can be seen that the mass ratio of monoclinic phase to hexagonal phase in its multi-phase and multi-level WO3 is 19:1.

[0100] 4. Band structure characterization of multiphase and multilevel WO3 homojunction photoanode

[0101] Figure 6 The band structure of the multi-phase multi-level WO3 homojunction photoanode of Example 1 (in the figure, Monoclinic is monoclinic phase and Hexagonal is hexagonal phase) is observed. Figure 6 It can be seen that there are slight differences in the valence and conduction band positions between monoclinic WO3 and hexagonal WO3, and their band structures conform to the type II heterojunction structure after contact with each other. Since both are composed of WO3, a homojunction is established between the two.

[0102] 5. Photoelectrocatalytic performance of multiphase and multilevel WO3 homojunction photoanode

[0103] The photoelectric performance of the multi-phase multi-level WO3 homojunction photoanode of Example 1 of the present invention and the WO3 photoanode of Comparative Example 1 was tested as follows:

[0104] Photoelectrochemical tests were performed using a Metrohm PGSTAT302N electrochemical workstation using a classic three-electrode two-loop system. The multiphase multi-level WO3 homojunction photoanode of Example 1 and the WO3 photoanode of Comparative Example 1 were used as working electrodes, respectively, to form a polarization loop with a Pt counter electrode, and a measurement loop with an Ag / AgCl reference electrode.-1 Na2SO4 solution and 6.0 mol·L -1 of NaCl and 0.1 mol·L -1 The mixed solution of Na2SO4 was used as the electrolyte. The multi-phase multi-level WO3 homojunction photoanode of Example 1 and the WO3 photoanode of Comparative Example 1 were processed into a test area of ​​1×1 cm 2 The electrode sheet was fixed to the electrode holder, and the counter electrode and reference electrode were fixed and their contact area with the electrolyte was adjusted. Photoelectrochemical measurements were performed using a Microsolar 300 xenon lamp with a built-in 300W xenon lamp from Perfectlight (Beijing).

[0105] (1) Linear sweep voltammetry (LSV) was used to detect the photocurrent of the two groups of experiments. The scan rate was maintained at 0.01 V / s. The test results are shown in Figure 7 (In the figure, MH WO 3 corresponds to the multi-phase multi-level WO 3 homojunction photoanode obtained in Example 1; MWO 3 corresponds to the WO 3 homojunction photoanode obtained in Example 1).

[0106] Figure 7 The JV curves of the WO3 homojunction photoanode obtained in Example 1 and Comparative Example 1 are shown. Figure 7 It can be seen that at a voltage of 1.23 V (vs. reversible hydrogen electrode (vs RHE)), the photocurrent of the multi-phase multi-level WO3 homojunction photoanode of Example 1 is as high as 0.52 mA·cm -2 , which is 2.16 times that of the WO3 photoanode in comparative example 1. This shows that the multi-phase multi-level WO3 homojunction photoanode of the present invention can significantly improve the photoelectrocatalytic water splitting performance of the WO3 photoanode, proving the feasibility of introducing its multi-phase multi-level structure.

[0107] (2) Select the electrochemical impedance spectroscopy (EIS) method to analyze the electrochemical reaction impedance (R ct ) and other parameters were measured, with the bias voltage set to 1.23V (vs RHE), the frequency range from 1MHz to 0.1Hz, and the disturbance amplitude set to 10mV. Figure 8 (In the figure, MHWO3 corresponds to the multiphase multilevel WO3 homojunction photoanode obtained in Example 1; MWO3 corresponds to the WO3 homojunction photoanode obtained in Example 1);

[0108] Figure 8 The amplitude-phase frequency characteristic curve (Nyquist curve) of the WO3 homojunction photoanode obtained in Example 1 and Comparative Example 1 is shown in FIG. Figure 8 It can be seen that compared with the M WO 3 of Comparative Example 1, the Rct of the synthesized multiphase multilevel WO 3 homojunction photoanode of Example 1 is about 690Ω, which is significantly lower than the monoclinic WO 3 photoanode of Comparative Example 1.

[0109] (3) The chronoamperometry (It) was used to test the stability of the multi-phase multi-level WO3 homojunction photoanode under seawater conditions at 1.23 V (vs RHE). The results are shown in Figure 9 .

[0110] observe Figure 9 The results show that the catalytic activity of the photocatalyst can be maintained for 12 hours in a 0.5M H2SO4 solution at a bias voltage of 1.23V, and after 12 hours of reaction, it still maintains more than 50% of its original activity. These results indicate that the introduction of a multi-phase, multi-level structure can significantly accelerate the bulk charge transport process, thereby promoting the separation of photogenerated carriers and accelerating the catalytic reaction process.

[0111] (4) The XRD results of the photoanode materials before and after the stability test of Example 1 were compared. Figure 10 In the figure, After It is the result of the photoanode material obtained in Example 1 after the chronoamperometry test; Before is the result of the photoanode material obtained in Example 1 before the amperometry test; h WO3 is the standard spectrum of hexagonal phase WO3; m WO3 is the standard spectrum of monoclinic phase WO3.

[0112] The XRD results of the stability test of the multi-phase multi-level WO3 homojunction photoanode in Example 1 of the present invention are as follows: Figure 10 It can be seen that after the chronoamperometry test (After It), compared with the standard spectra of hWO3 and mWO3, the material itself is relatively stable after a long period of stability testing and can still maintain its crystal structure.

[0113] The above is only a preferred embodiment of the present invention, and only specifically describes the technical principles of the present invention. It is only for the purpose of explaining and clarifying the underlying principles of the present invention and should not be construed as limiting the scope of protection of the present invention. Based on the explanations herein, any modifications, substitutions, or improvements derived from the spirit and principles of the present invention, as well as other specific embodiments of the present invention that practitioners in related fields can conceive of without creative effort, should in principle be included in the scope of protection of the present invention.

Claims

1. A method for preparing a multi-phase multi-level WO3 homojunction photoanode, characterized in that: The following steps are involved: (1) Using sodium tungstate as raw material, hydrochloric acid as acid source, and ammonium oxalate as dispersant, a precursor solution is prepared; (2) placing the pretreated conductive glass in a reactor, adding the precursor solution to carry out a hydrothermal reaction, and obtaining a conductive glass with deposited multi-phase and multi-level tungstic acid; (3) The conductive glass deposited with the multi-phase and multi-level tungstic acid is cleaned, dried and annealed to obtain the conductive glass.

2. The method for preparing the multi-phase multi-level WO3 homojunction photoanode according to claim 1, characterized in that: In step (2), the conductive surface of the conductive glass faces the precursor solution; the temperature of the hydrothermal reaction is 140-180° C., and the time is 10-14 hours.

3. The method for preparing the multi-phase multi-level WO3 homojunction photoanode according to claim 1, characterized in that: The preparation method of the precursor solution in step (1) is as follows: dissolving sodium tungstate in deionized water A to obtain a sodium tungstate solution; adding hydrochloric acid dropwise to the stirred sodium tungstate solution, and continuing to stir for time A after the addition of hydrochloric acid is completed; then adding ammonium oxalate and deionized water B, and continuing to stir for time B to obtain the precursor solution.

4. The method for preparing a multi-phase multi-level WO3 homojunction photoanode according to claim 3, characterized in that: The sodium tungstate is sodium tungstate dihydrate; the mass ratio of the sodium tungstate dihydrate to ammonium oxalate is 1:(1.5-2.0); the volume ratio of the mass of the sodium tungstate dihydrate to deionized water A is 1 g:45-91 mL; the volume ratio of the mass of the sodium tungstate dihydrate to hydrochloric acid is 1 g:18-25 mL; the concentration of the hydrochloric acid is 2-5 mol / L; the volume ratio of the ammonium oxalate to deionized water B is 1 g:30-60 mL; the stirring time A is 30-40 min; and the stirring time B is 30-50 min.

5. The method for preparing the multi-phase multi-level WO3 homojunction photoanode according to claim 1, characterized in that: In step (3), the specific operation of annealing is: heating to 400-600°C at a rate of 1-5°C / min, and keeping the temperature for 2-5 hours after reaching the target temperature.

6. The method for preparing the multi-phase multi-level WO3 homojunction photoanode according to claim 1, characterized in that: In step (2), the pretreatment includes: placing the conductive glass in an acetone aqueous solution for ultrasonic treatment, then immersing it in a piranha solution for etching, and washing and drying to obtain the pretreated conductive glass.

7. The method for preparing a multi-phase multi-level WO3 homojunction photoanode according to claim 6, characterized in that: In step (2), the volume ratio of water to acetone in the acetone aqueous solution is 1:1-2; and the piranha solution is prepared by mixing a 30wt% hydrogen peroxide aqueous solution and concentrated sulfuric acid in a volume ratio of 1:0.5-1.

5.

8. A multi-phase multi-level WO3 homojunction photoanode, characterized in that: The preparation method is described in any one of claims 1 to 7.

9. The multi-phase multi-level WO3 homojunction photoanode according to claim 8, characterized in that: The multiphase structure of the multiphase multilevel WO3 homojunction photoanode includes a monoclinic phase and a hexagonal phase; the mass ratio of the monoclinic phase to the hexagonal phase in the multiphase structure is 19:1; the multilevel structure of the multiphase multilevel WO3 homojunction photoanode is a hexagonal snowflake structure; in the hexagonal snowflake structure, the hexagonal phase is located at the central axis of the hexagonal prism, forming a primary structure; the monoclinic phase is located at the hexagonal snowflake, forming a secondary structure.

10. Use of the multi-phase multi-level WO3 homojunction photoanode as claimed in claim 8 or 9 in photoelectrocatalytic water splitting.

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