Method for carrying out photoelectrocatalytic modification on ferric oxide photo-anode by constructing bottom layer and top layer structures

Abstract

The invention discloses a method for carrying out photoelectrocatalytic modification on a ferric oxide photoanode by constructing a bottom layer and top layer structure, a large number of dense holes are formed in a nanorod through simple annealing in a manner of spin-coating ZrOx on the surface of FeOOH for several times, the specific surface area is greatly increased, the hole transfer distance is shortened, and the photoelectrocatalytic modification of the ferric oxide photoanode is realized. In addition, the problem of electron reinjection of the contact interface of the iron oxide and the FTO glass is solved by regulating and controlling the thickness of the Nb2O5 at the bottom layer, meanwhile, exogenous niobium ion doping of the iron oxide is realized, the carrier concentration and conductivity in the iron oxide are effectively improved, and the conductivity of the FTO glass is improved. And meanwhile, the existence of the bottom layer can effectively inhibit large-ion-radius Sn < 4 + > in the SnO2 conducting layer from entering iron oxide crystal lattices to generate internal defects.

Description

Technical Field

[0001] The invention belongs to the technical field of photoelectrochemical catalysis, and specifically relates to a method for photoelectrocatalytic modification of an iron oxide photoanode by constructing a bottom layer and a top layer structure. Background Art

[0002] As environmental pollution caused by the burning of fossil fuels becomes increasingly serious, the demand for clean energy is rising. Photoelectrochemical (PEC) water splitting technology, which uses semiconductor electrodes and solar energy to decompose water into hydrogen and oxygen, has become an important direction for clean energy production. Among them, semiconductor metal oxides such as TiO2, BiVO4, WO3, etc., have attracted much attention in the field of PEC water splitting. In particular, iron oxide (α-Fe2O3) is regarded as an ideal material for PEC water splitting due to its suitable band gap, low cost and abundant resources, as well as a theoretical solar-to-hydrogen (STH) conversion efficiency of up to 15.5%.

[0003] However, iron oxide photoanodes face many challenges in practical applications, such as limited hole diffusion length, high electron-hole recombination rate, and low conductivity. In addition, the problem of recombination centers at the semiconductor / electrolyte and semiconductor / semiconductor interfaces further restricts the improvement of PEC performance. In order to overcome these limitations, researchers have proposed a variety of modification strategies. For example, the team of Professor Su Jinzhan of Xi'an Jiaotong University used TiO2 lower and upper layer deposition on hematite nanorod films, followed by rapid annealing. The TiO2 lower and upper layers not only serve as doping sources for increased carrier density, but also reduce charge recombination at the fluorine-doped tin oxide (FTO) / hematite interface and accelerate charge transfer at the hematite / electrolyte interface. This synergistic doping and interface modification effect enhances the photoelectrochemical water oxidation performance of hematite nanorod arrays. The carrier concentration and photocurrent density of Fe2O3 treated by this method are improved, and the internal reaction kinetics are improved to a certain extent. The final photoanode showed a photocurrent density of 1.49mA cm-2; however, this method still has the following defects: ① The experimental annealing was carried out using a traditional muffle furnace, and the pure Fe2O3 itself showed a very low photocurrent density of 0.005mA / cm2. ② Although the electron-hole separation effect has been improved by simultaneously constructing the bottom and top layers, it has not effectively solved the problem of short hole diffusion distance and difficulty in quickly transferring to the surface. ③ The improvement in photocurrent caused by the overall modification process is still very limited.

[0004] Therefore, the present invention provides a new method for photoelectrocatalytic modification of iron oxide photoanode by constructing bottom and top structures to solve the above problems. Summary of the invention

[0005] In order to solve the above technical problems, the present invention designs a method for photoelectrocatalytic modification of iron oxide photoanode by constructing a bottom layer and a top layer structure, using Nb2O5 as the bottom layer, and realizing high-valent ion doping to solve the problem of poor conductivity of iron oxide while blocking electron backflow. The top layer of ZrOx further promotes the negative shift of the starting potential by protecting the surface state; in addition, the present invention can effectively solve the problem of short hole diffusion distance by forming a large number of holes in the nanorods. A large number of holes bring a larger specific surface area, which brings more reaction sites for the oxygen evolution reaction.

[0006] In order to achieve the above technical effects, the present invention designs a method for photoelectrocatalytic modification of iron oxide photoanode by constructing a bottom layer and a top layer structure, which specifically includes the following steps:

[0007] S1: Ultrasonic cleaning of F:SnO2 substrate glass (FTO) with water, ethanol and acetone for 30 min respectively;

[0008] S2: Spin coating 0.03M NbCl5 solution on FTO, and then heat it at 400℃ for 2 hours to form Nb2O5 bottom film;

[0009] S3: Synthesize Nb2O5 / FeOOH nanorods by hydrothermal method; after the FTO is cooled to room temperature, immerse the FTO in 50 mL aqueous solution containing ferric chloride hexahydrate (FeCl3·6H2O), sodium nitrate (NaNO3), and nitric acid (HNO3) in a ratio of 0.5-1.5:2.0-2.5:0.2-0.5, and hydrotherm at 100°C for two hours;

[0010] S4: Spin coat 0.1 M ZrCl4 solution on the surface of Nb2O5 / FeOOH by spin coating and heat at 100°C for 5 minutes.

[0011] S5: Place graphite powder (60-80 mL) in a beaker (100 mL), press the prepared sample onto the graphite powder, and microwave it at full power for 2-3 min to convert the yellow sample into a red Nb2O5 / Fe2O3@ZrOx photoanode.

[0012] Furthermore, the cleaning time in S1 is 25min-40min;

[0013] Furthermore, in S2 and S4, the spin coating is performed in batches, first at 500 rpm for 10 s, and then at 3000 rpm for 20 s;

[0014] Furthermore, the frequency of the microwave in S5 is 2.45 GHz and the power is 1000 W.

[0015] The beneficial effects of the present invention are:

[0016] The present invention utilizes the method of spin coating ZrOx on the surface of FeOOH in batches, and realizes the formation of a large number of dense holes on the nanorods through simple annealing, greatly improving the specific surface area, shortening the distance of hole transfer, and better realizing electron-hole separation. In addition, the present invention avoids the loss of spin coating solution by spin coating in batches, realizes the uniform dispersion of spin coating solution, and ensures that the film formed by spin coating has a uniform thickness.

[0017] In addition, the present invention solves the problem of electron back-injection at the contact interface between iron oxide and FTO glass by adjusting the thickness of the bottom Nb2O5 layer, and at the same time realizes exogenous niobium ion doping of iron oxide, effectively improving the internal carrier concentration and conductivity of iron oxide. At the same time, the presence of the bottom layer can effectively inhibit the SnO2 conductive layer with large ion radius. 4+ Entering the iron oxide lattice creates internal defects.

[0018] Finally, the present invention adopts hybrid microwave annealing technology: graphite powder is used as a heating carrier, and a microwave oven is used as a microwave energy source. The equipment and related equipment used are easy to obtain and cheap. The iron oxide nanorod film after hybrid microwave annealing has high crystallinity, and its surface state is significantly reduced compared with traditional thermal annealing treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solution of the embodiment of the present invention, the drawings required for describing the embodiment are briefly introduced below.

[0020] Figure 1 This is a schematic diagram of the synthesis of the Nb2O5 / Fe2O3@ZrOx porous nanorod photoanode of the present invention;

[0021] Figure 2 This is a test diagram of the photoelectrochemical performance of the Nb2O5 / Fe2O3@ZrOx porous nanorod photoanode, in which a is the photocurrent curve; b is the starting potential; c is the transient photoelectric; d is the unbiased photoelectron conversion efficiency; e is the Mott-Schottky curve; f is the impedance spectrum; g is the surface charge separation efficiency; h is the bulk charge separation efficiency; and i is the photoelectric conversion efficiency. DETAILED DESCRIPTION

[0022] Example 1

[0023] In this embodiment, the present invention discloses a method for photoelectrocatalytic modification of an iron oxide photoanode by constructing a bottom layer and a top layer structure, comprising the following steps:

[0024] S1: Ultrasonic cleaning of F:SnO2 substrate glass (FTO) with water, ethanol and acetone for 30 min respectively;

[0025] S2: Spin coating 0.03M NbCl5 solution on FTO, and then heat it at 400℃ for 2 hours to form Nb2O5 bottom film;

[0026] S3: Synthesize Nb2O5 / FeOOH nanorods by hydrothermal method; after the FTO is cooled to room temperature, immerse the FTO in 50 mL aqueous solution containing ferric chloride hexahydrate (FeCl3·6H2O), sodium nitrate (NaNO3), and nitric acid (HNO3) in a ratio of 0.5-1.5:2.0-2.5:0.2-0.5, and hydrotherm at 100°C for two hours;

[0027] S4: Spin coat 0.1 M ZrCl4 solution on the surface of Nb2O5 / FeOOH by spin coating and heat at 100°C for 5 minutes.

[0028] S5: Place graphite powder (60-80 mL) in a beaker (100 mL), place the prepared sample on the compacted graphite powder, and microwave it at full power for 2-3 min to convert the yellow sample into a red Nb2O5 / Fe2O3@ZrOx photoanode.

[0029] The specific purpose of the multiple spin coating is to avoid the loss of the spin coating solution and to achieve uniform dispersion of the spin coating solution. The first step of 10s 500rpm is to disperse the spin coating solution on the FTO glass. Too fast a rotation speed will cause all the solution to be thrown away. After the solution is spread, a higher rotation speed of 20s 3000rpm is used to allow the solution to be evenly dispersed on the FTO, ensuring that the film formed by spin coating has a uniform thickness.

[0030] In addition, in this embodiment, the present invention uses a hybrid microwave annealing method for annealing; hybrid microwave annealing is a heating method that converts microwave energy into thermal energy compared to traditional muffle furnace annealing, and can be heated as a whole. The heating body absorbs microwave energy and heats itself as a whole to a certain temperature, which can effectively form high crystallinity and purity nanorods, and the damage to the conductivity of FTO can be ignored, as well as the unconventional solid interface reaction of the new material. Graphite powder is used as a heat carrier because incomplete combustion of carbon will occur during the heating process, producing CO reducing gas, and the reducing atmosphere formed can induce the formation of more oxygen vacancies in iron oxide, thereby improving its photoelectrocatalytic performance.

[0031] Example 2

[0032] In this embodiment, a method for photoelectrocatalytic modification of an iron oxide photoanode by constructing a bottom layer and a top layer structure comprises the following steps:

[0033] S1: Ultrasonic cleaning of F:SnO2 substrate glass (FTO) with water, ethanol and acetone for 30 min respectively;

[0034] S2: Spin coating 0.03M NbCl5 solution on FTO, and then heat it at 400℃ for 2 hours to form Nb2O5 bottom film;

[0035] S3: Synthesize Nb2O5 / FeOOH nanorods by hydrothermal method; after the FTO is cooled to room temperature, immerse the FTO in 50 mL aqueous solution containing ferric chloride hexahydrate (FeCl3·6H2O), sodium nitrate (NaNO3), and nitric acid (HNO3) in a ratio of 1:2.1:0.1, and hydrothermally heat at 100°C for two hours;

[0036] S4: Spin coat 0.1 M ZrCl4 solution on the surface of Nb2O5 / FeOOH by spin coating and heat at 100°C for 5 minutes.

[0037] S5: Place graphite powder (60-80 mL) in a beaker (100 mL), press the prepared sample onto the graphite powder, and microwave it at full power for 2-3 min to convert the yellow sample into a red Nb2O5 / Fe2O3@ZrOx photoanode.

[0038] Example 3

[0039] Immerse FTO in a 50mL aqueous solution containing ferric chloride hexahydrate (FeCl3·6H2O), sodium nitrate (NaNO3), and nitric acid (HNO3) in a ratio of 1:2.1:0.3, and heat at 100℃ for two hours. (The rest of the process is the same)

[0040] Example 4

[0041] The FTO was immersed in 50 mL of an aqueous solution containing ferric chloride hexahydrate (FeCl3·6H2O), sodium nitrate (NaNO3), and nitric acid (HNO3) in a ratio of 1:2.1:0.5, and hydrothermaled at 100°C for two hours.

[0042] Example 5

[0043] The FTO was immersed in 50 mL of an aqueous solution containing ferric chloride hexahydrate (FeCl3·6H2O), sodium nitrate (NaNO3), and nitric acid (HNO3) in a ratio of 1:2.1:0.7, and hydrothermaled at 100°C for two hours.

[0044] The performance of the Nb2O5 / Fe2O3@ZrOx photoanode prepared by the above-mentioned Examples 2-5 is verified by the conversion change of the current, because in this field, the photoanode is a key component, which is responsible for absorbing light energy and converting it into electrical energy or chemical energy. When the photoanode is illuminated, it can effectively convert light energy into current. Therefore, the magnitude of the current is an important indicator for measuring the performance of the photoanode. Now, we further compare and measure it by the magnitude of the current, as shown in the following table:

[0045] Table 1 Current conditions under different ratios

[0046]

[0047] It can be seen from the above table that, among the different proportions of ferric chloride, sodium nitrate and nitric acid, the current is best when the ratio is 1:2.1:0.3. By spin-coating ZrOx on the surface of FeOOH in batches, a large number of dense holes are formed on the nanorods through simple annealing, which greatly increases the specific surface area and shortens the distance of hole transfer, thus better realizing electron-hole separation. Therefore, at this optimal ratio, the photoanode can more effectively convert light energy into current, thereby showing better performance.

[0048] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to only the specific implementation methods described.

Claims

1. A method for photoelectrocatalytic modification of an iron oxide photoanode by constructing a bottom layer and a top layer structure, characterized in that: The specific steps include: S1: Ultrasonic cleaning of F:SnO2 substrate glass (FTO) with water, ethanol and acetone for 30 min respectively; S2: Spin coating 0.03M NbCl5 solution on FTO, and then heat it at 400℃ for 2 hours to form Nb2O5 bottom film; S3: Synthesize Nb2O5 / FeOOH nanorods by hydrothermal method; after the FTO is cooled to room temperature, immerse the FTO in 50 mL aqueous solution containing FeCl3·6H2O, NaNO3, and HNO3 in a ratio of 0.5-1.5:2.0-2.5:0.2-0.5, and hydrothermally heat at 100°C for two hours; S4: Spin coat 0.1 M ZrCl4 solution on the surface of Nb2O5 / FeOOH by spin coating and heat at 100°C for 5 minutes. S5: Place graphite powder in a beaker, press the prepared sample onto the graphite powder, and microwave at full power for 2-3 min to convert the yellow sample into a red Nb2O5 / Fe2O3@ZrOx photoanode.

2. According to claim 1, a method for photoelectrocatalytic modification of an iron oxide photoanode by constructing a bottom layer and a top layer structure, characterized in that: The cleaning time in S1 is 25 min-40 min.

3. According to claim 1, a method for photoelectrocatalytic modification of an iron oxide photoanode by constructing a bottom layer and a top layer structure, characterized in that: In S2 and S4, the spin coating is performed in batches, first at 500 rpm for 10 s, and then at 3000 rpm for 20 s.

4. A method for photoelectrocatalytic modification of an iron oxide photoanode by constructing a bottom layer and a top layer structure according to claim 1, characterized in that: The frequency of the microwave in S5 is 2.45 GHz, and the power is 1000 W.

5. A method for photoelectrocatalytic modification of an iron oxide photoanode by constructing a bottom layer and a top layer structure according to claim 1, characterized in that: The ratio of FeCl3·6H2O, NaNO3 and HNO3 in the S3 is 1:2.1:0.3.

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