Preparation method of lead chromate photo-anode catalyst rich in oxygen vacancies

Abstract

The invention discloses a preparation method of a lead chromate photo-anode catalyst rich in oxygen vacancies. The method comprises the following steps: by taking Pb (NO3) 2 and Cr (NO3) 3 as raw materials, urea as a surfactant and 2-methoxyethanol as a solvent, carrying out ultrasonic treatment to uniformly mix the raw materials to obtain a precursor solution, and then repeatedly spin-coating and annealing on FTO (Fluorine-doped Tin Oxide) to prepare the lead chromate photo-anode catalyst rich in oxygen vacancies. The preparation method is low in cost, simple and controllable in process and good in repeatability, the obtained product is uniform and compact, the photocurrent density of the product is as high as 1.7 mA / cm < 2 >, and rich oxygen vacancies can effectively increase the carrier concentration and promote separation and transmission of photogenerated charges, so that the photoelectrochemical water decomposition performance is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalytic water splitting for hydrogen production materials, and relates to a preparation method of a lead chromate photoanode catalyst rich in oxygen vacancies. Background Art

[0002] Currently, the technology for converting solar energy into hydrogen energy is mainly photoelectrochemical (PEC) water splitting for hydrogen production. Among the existing photoanode semiconductor materials, lead chromate (PbCrO4) has received extensive attention due to its composition and electronic structure being closest to BiVO4. PbCrO4 is an n-type semiconductor with a band gap of 2.1 eV, having a good light absorption range, and the theoretical solar energy conversion efficiency can also reach 10.6%. However, the photocurrent value of the original PbCrO4 photoanode without modification is only a little over zero, less than one-tenth of the theoretical photocurrent value. This is mainly due to its own defects such as poor separation and transport performance of photo-generated carriers and relatively slow water oxidation kinetics, which limits its overall efficiency and makes the performance of the PbCrO4 photoanode unable to reach its theoretical potential.

[0003] A large number of studies have shown that the formation of an appropriate amount of oxygen vacancies can effectively increase the carrier concentration, thereby promoting the separation and transport of photo-generated charges. The existing preparation methods of lead chromate with high performance containing oxygen vacancies are complex, requiring annealing in an argon atmosphere, and the oxygen vacancies are not stable and will disappear after annealing in air again [Zhou H, et al. Modulating oxygen vacancies in lead chromate for photoelectrocatalytic water splitting. Advanced Materials (2023): 2300914.]. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of a lead chromate photoanode catalyst with low production cost, simple operation, and rich in oxygen vacancies. The present invention uses the sol-gel method to prepare lead chromate materials with uniform size and high density rich in oxygen vacancies, and the generation of oxygen vacancies mainly utilizes the consumption of oxygen atoms at the PbCrO4 interface during the heating decomposition of urea, and an appropriate amount of oxygen vacancies can be formed in PbCrO4 by regulating the concentration of urea.

[0005] The technical solution for achieving the purpose of the present invention is as follows:

[0006] A preparation method of a lead chromate photoanode catalyst rich in oxygen vacancies, comprising the following steps:

[0007] (1) Preparation of the precursor solution

[0008] Dissolve Pb(NO3)2 and Cr(NO3)3 in 2-methoxyethanol, and ultrasonically mix them thoroughly. Then add urea and continue ultrasonic mixing to obtain a precursor solution.

[0009] (2) Preparation of lead chromate photoanode catalyst rich in oxygen vacancies

[0010] Spin-coat the precursor solution on the surface of FTO glass, and then place the spin-coated FTO glass in a muffle furnace for precrystallization at 490 - 500 °C; repeat the above spin-coating and precrystallization steps to the required thickness. Finally, anneal the spin-coated FTO glass in a muffle furnace at 490 - 500 °C, and cool it to room temperature after annealing to obtain a lead chromate photoanode catalyst rich in oxygen vacancies.

[0011] Furthermore, in step (1), in the precursor solution, the molar ratio of Pb to Cr is 1:1.

[0012] Furthermore, in step (1), the concentrations of both Pb(NO3)2 and Cr(NO3)3 are 0.3 mol / L, and the concentration of urea is 0.2 - 0.3 mol / L.

[0013] Furthermore, in step (1), the total ultrasonic time ≥ 3 h.

[0014] Furthermore, in step (2), the spin-coating speed is 4000 - 5000 rpm, and the spin-coating time for each time is 30 s.

[0015] Furthermore, in step (2), the number of repetitions is 3 times.

[0016] Furthermore, in step (2), the thickness is 450 - 480 nm.

[0017] Furthermore, in step (2), the precrystallization time for each time is 10 - 15 min.

[0018] Furthermore, in step (2), the annealing time is 2 h.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] The present invention has low cost, simple and controllable process, good repeatability, and the obtained product is uniform and dense. The method of the present invention can generate lead chromate rich in oxygen vacancies in only one step, and the content of oxygen vacancies will not decrease rapidly with multiple annealings, having good repeatability. In addition, the lead chromate material rich in oxygen vacancies prepared by the present invention has a high carrier concentration, which can effectively promote the separation and transport of photo-generated carriers. Therefore, as a photoanode, it exhibits excellent photoelectrochemical performance in the photoelectrochemical water splitting reaction, and the photocurrent density is as high as 1.7 mA / cm 2 . Description of the Drawings

[0021] Figure 1 It is the X-ray diffraction (XRD) pattern of the lead chromate photocathode catalyst rich in oxygen vacancies prepared in Example 1 and Comparative Examples 1 and 3.

[0022] Figure 2 It is the scanning electron microscope (SEM) image of the lead chromate photocathode catalyst rich in oxygen vacancies prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 4.

[0023] Figure 3 It is the X-ray photoelectron spectroscopy (XPS) of the lead chromate photocathode catalyst rich in oxygen vacancies prepared in Example 1 and Comparative Example 1; among them, Figures a, b, and c are the XPS spectra of Pb 4f, Cr 2p, and O 1s, respectively; the pure lead chromate photocathode catalyst is the standard catalyst.

[0024] Figure 4 It is the linear sweep voltammetry (LSV) curves of the lead chromate photocathode catalyst rich in oxygen vacancies prepared in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 in KPi (pH = 7.0) solution; among them, Figure a is the photocurrent and Figure b is the dark current.

[0025] Figure 5 It is the analysis diagram of the carrier separation and transfer of the lead chromate photocathode catalyst rich in oxygen vacancies prepared in Example 1 and Comparative Example 1; among them, Figure a is the Mott-Schottky curve and Figure b is the electrochemical impedance spectrum. Detailed Description of the Invention

[0026] The present invention will be further described in detail below in conjunction with specific examples and drawings.

[0027] Example 1

[0028] First, 0.0024 mol of Pb(NO3)2 and 0.0024 mol of Cr(NO3)3 are dissolved in 8 mL of 2-methoxyethanol, and then placed in an ultrasonic machine for 1 h to mix them evenly; then, 0.0024 mol of urea is added to the solution, and the mixture is placed in the ultrasonic machine for another 2 h to mix evenly, thus obtaining a precursor solution with the concentrations of Pb(NO3)2, Cr(NO3)3, and urea all being 0.3 mol / L. Then, the washed and dried FTO is fixed on a spin coater, and 150 μL of the precursor solution is slowly dropped, and spin-coated at a speed of 4500 rpm for 30 s. The spin-coated FTO is placed in a muffle furnace and pre-crystallized at 500 °C; the above-mentioned spin-coating and pre-crystallization operations are repeated 3 times. The FTO after the last spin-coating is directly placed in the muffle furnace and annealed at 500 °C for 2 h, and then cooled to room temperature to obtain the lead chromate material rich in oxygen vacancies.

[0029] Example 2

[0030] First, dissolve 0.0024 mol of Pb(NO3)2 and 0.0024 mol of Cr(NO3)3 in 8 mL of 2-methoxyethanol, and place it in an ultrasonic machine for 1 h to mix them evenly; then add 0.0016 mol of urea to the solution, and continue to place it in the ultrasonic machine for 2 h to mix it evenly, and a precursor solution with the concentrations of Pb(NO3)2 and Cr(NO3)3 both being 0.3 mol / L and the urea concentration being 0.2 mol / L can be obtained. Then, place the washed and dried FTO on a spin coater and fix it, slowly drop 150 μL of the precursor solution, spin coat it at a speed of 4500 rpm for 30 s, put the spin-coated FTO into a muffle furnace, and perform pre-crystallization at 500 °C; repeat the above spin coating and pre-crystallization operations 3 times, directly put the FTO after the last spin coating into the muffle furnace, anneal it at 500 °C for 2 h, and cool it to room temperature to obtain a lead chromate material rich in oxygen vacancies.

[0031] Example 3

[0032] First, dissolve 0.0024 mol of Pb(NO3)2 and 0.0024 mol of Cr(NO3)3 in 8 mL of 2-methoxyethanol, and place it in an ultrasonic machine for 1 h to mix them evenly; then add 0.0024 mol of urea to the solution, and continue to place it in the ultrasonic machine for 2 h to mix it evenly, and a precursor solution with the concentrations of Pb(NO3)2, Cr(NO3)3 and urea all being 0.3 mol / L can be obtained. Then, place the washed and dried FTO on a spin coater and fix it, slowly drop 150 μL of the precursor solution, spin coat it at a speed of 4500 rpm for 30 s, put the spin-coated FTO into a muffle furnace, and perform pre-crystallization at 490 °C; repeat the above spin coating and pre-crystallization operations 3 times, directly put the FTO after the last spin coating into the muffle furnace, anneal it at 490 °C for 2 h, and cool it to room temperature to obtain a lead chromate material rich in oxygen vacancies.

[0033] Comparative Example 1

[0034] This comparative example is basically the same as Example 1, except that the urea concentration in the precursor solution is 0.1 mol / L.

[0035] Comparative Example 2

[0036] This comparative example is basically the same as Example 1, except that the urea concentration in the precursor solution is 0.4 mol / L.

[0037] Comparative Example 3

[0038] This comparative example is basically the same as Example 1, except that the pre-crystallization and annealing temperatures are 480 °C.

[0039] Comparative Example 4

[0040] This comparative example is basically the same as Example 1, except that the pre-crystallization and annealing temperatures are 510 °C.

[0041] Characterization Example

[0042] 1. Structure Characterization of PbCrO4 Photoanode Catalyst Rich in Oxygen Vacancies

[0043] Figure 1 XRD patterns of PbCrO4 photoanode catalysts rich in oxygen vacancies prepared for Example 1, Comparative Example 1 and Comparative Example 3. For the XRD pattern of PbCrO4 prepared in Example 1, compared with the standard card, it can be seen that at 2θ of 17.9°, 20.3°, 25.6°, 27.2°, 29.5°, 33.0°, 43.3°, they respectively correspond to the (011), (101), (200), (120), (012), (-202), (212) planes of PbCrO4 (PDF#08-0209). The diffraction peaks of the obtained samples have a good correspondence with the standard card, indicating that the phase structure of the obtained samples is uniform, with high purity and no impurity phase. The diffraction peaks of Comparative Example 1 and Comparative Example 3 also correspond one by one to the PbCrO4 standard card, indicating that the concentration of urea and the annealing temperature will not affect the phase structure of the obtained samples.

[0044] Figure 2 SEM images of PbCrO4 photoanode catalysts rich in oxygen vacancies prepared for Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 4. From Figure 2 it can be seen that the PbCrO4 material rich in oxygen vacancies prepared in Example 1 is composed of a large number of granular-structured grains evenly distributed on the FTO glass. The overall structure is clear, the morphology and size are uniform, the grain boundaries are distinct, and there are few impurities. For the PbCrO4 material prepared in Comparative Example 1, it can be clearly seen that the grains are distributed too closely, reducing the exposure of effective grain boundaries compared to Example 1 and decreasing the surface area when contacting the electrolyte. For the PbCrO4 material prepared in Comparative Example 4, it can be seen that the granular grains are damaged to a certain extent and there are a small number of impurity particles, indicating that the morphology of the sample will be damaged at high temperatures.

[0045] Figure 3 XPS spectra of PbCrO4 photoanode catalysts rich in oxygen vacancies prepared for Example 1 and Comparative Example 1. From Figure 3 it can be seen that for the O in the XPS spectrum of O1s in the PbCrO4 material vThe amount is significantly increased compared with the standard catalyst, and the characteristic peaks of Pb4f and Cr 2p are also shifted. Both of these phenomena indicate an increase in the oxygen vacancy content in the obtained sample.

[0046] 2. Photoelectrochemical performance of PbCrO4 photoanode catalyst rich in oxygen vacancies

[0047] Figure 4 are the LSV curves of the PbCrO4 photoanode catalysts rich in oxygen vacancies prepared in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4. As Figure 4 can be seen, at 1.23 V vs. RHE, the photocurrent density of the PbCrO4 photoanode catalyst rich in oxygen vacancies prepared in Example 1 is as high as 1.7 mA / cm 2 , which are all higher than those of Comparative Example 1 (0.58 mA / cm 2 ), Comparative Example 2 (1.14 mA / cm 2 ), Comparative Example 3 (1.03 mA / cm 2 ), and Comparative Example 4 (1.38 mA / cm 2 ). This indicates that both the amount of oxygen vacancy content and the annealing temperature will affect the photocurrent density of the sample. When the oxygen vacancy content is too low, the generated carrier concentration is insufficient and will increase with the increase of oxygen vacancies; however, an excessive oxygen vacancy content will cause crystal structure changes and an increase in surface defects, thus reducing the photoelectrochemical water splitting performance of the photoanode; similarly, too low an annealing temperature will result in insufficient growth of the material; while too high an annealing temperature will damage the crystal structure, thus generating impurity grains. At the same time, the dark current shows a similar pattern to the photocurrent. The PbCrO4 material prepared in Example 1 has the lowest overpotential, which indicates that the prepared PbCrO4 photoanode catalyst rich in oxygen vacancies has more excellent photoelectrochemical water splitting performance.

[0048] Figure 5 In a, it is the Mott - Schottky curve of Example 1 and Comparative Example 1. It can be seen from the figure that the carrier concentration of Example 1 is significantly higher than that of Comparative Example 1, which is attributed to the appropriate amount of oxygen vacancy content in Example 1, thereby increasing the carrier concentration. Figure 5 In b, it is the electrochemical impedance spectrum of Example 1 and Comparative Example 1. In the figure, Example 1 has a smaller arc radius, which indicates that the carriers in Example 1 have less resistance during transmission and are more conducive to the transmission of carriers. In summary, the prepared PbCrO4 photoanode catalyst rich in oxygen vacancies in the present invention significantly improves the carrier concentration and transmission performance in the material, effectively overcomes the problems of low carrier concentration and severe recombination in the prior art, and thus exhibits high photoelectrochemical water splitting performance.

Claims

1. Preparation method of lead chromate photoanode catalyst rich in oxygen vacancies, characterized in that, It includes the following steps: (1) Preparation of the precursor solution Dissolve Pb(NO3)2 and Cr(NO3)3 in 2-methoxyethanol, and ultrasonically mix them thoroughly. Then add urea and continue to ultrasonically mix to obtain the precursor solution; (2) Preparation of the lead chromate photoanode catalyst rich in oxygen vacancies Spin-coat the precursor solution on the surface of the FTO glass, and then place the spin-coated FTO glass in a muffle furnace for pre-crystallization at 490 - 500 °C; repeat the above spin-coating and pre-crystallization steps to the required thickness. Finally, anneal the spin-coated FTO glass in the muffle furnace at 490 - 500 °C, and cool it to room temperature after annealing to obtain the lead chromate photoanode catalyst rich in oxygen vacancies.

2. The preparation method according to claim 1, wherein In step (1), in the precursor solution, the molar ratio of Pb to Cr is 1:

1.

3. The preparation method according to claim 1, characterized in that, In step (1), the concentrations of both Pb(NO3)2 and Cr(NO3)3 are 0.3 mol / L, and the concentration of urea is 0.2 - 0.3 mol / L.

4. The preparation method according to claim 1, characterized in that, In step (1), the total ultrasonic time is ≥ 3 h.

5. The preparation method according to claim 1, characterized in that, In step (2), the spin-coating speed is 4000 - 5000 rpm, and the spin-coating time for each time is 30 s.

6. The preparation method according to claim 1, characterized in that, In step (2), the number of repetitions is 3 times.

7. The preparation method according to claim 1, characterized in that In step (2), the thickness is 450 - 480 nm.

8. The preparation method according to claim 1, characterized in that, In step (2), the pre-crystallization time for each time is 10 - 15 min.

9. The preparation method according to claim 1, characterized in that, In step (2), the annealing time is 2 h.

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