Ti-F co-doped hematite photoelectrode and preparation method and application thereof

Abstract

The invention discloses a Ti-F co-doped hematite photoelectrode and a preparation method and application thereof, and belongs to the technical field of photoelectrocatalysis, the preparation method comprises the following steps: S1, dissolving ferric salt, titanium salt and villiaumite in deionized water, and stirring to obtain a precursor solution; s2, putting the precursor solution and FTO conductive glass into a hydrothermal reaction kettle for hydrothermal reaction to obtain a photoelectrode film precursor; and S3, annealing and calcining the photoelectrode film precursor in a tubular furnace to obtain the Ti-F: Fe2O3 photoelectrode. According to the Ti-F co-doped hematite photoelectrode and the preparation method and application thereof, the photoelectrocatalytic water oxidation activity is improved by reducing the surface state, forming a hydrogen bond structure to inhibit recombination of carriers and accelerating transfer of photo-generated holes, the preparation method is simple, operation is convenient, and experimental conditions are easy to control.

Description

Technical Field

[0001] The present invention relates to the technical field of photoelectrochemistry, and in particular to a Ti-F co-doped hematite photoelectrode and a preparation method and application thereof. Background Art

[0002] In recent decades, hematite (α-Fe2O3) has attracted extensive attention as a promising semiconductor photoanode for water splitting due to its good stability in aqueous solution, abundant reserves, suitable band gap, and non-toxic properties. Its band gap is 2.2 eV and the theoretical optical flux density is 12.6 mA cm -2 , which has good prospects for practical applications. However, the PEC water splitting efficiency of hematite is mainly limited by its electrical conductivity, surface defects and slow kinetics of oxygen evolution reaction (OER). To overcome these shortcomings, researchers have adopted many strategies, such as morphology design, metal and non-metal doping, acidic or alkaline surface treatment, use of co-catalysts or hole storage layer loading. Among metal doping, n-type doping (such as Ti 4+ 、Sn 4+ , Pt 4+ and Cr 4+ ) is the most common method to improve the conductivity of hematite by increasing the hopping probability of polarons with lattice strain and by 3+ The introduction of electrons near the site will Fe 3+ Reduction to Fe 2+ , which has also been shown to enhance donor concentration and charge transfer. However, the above metal doping creates another energy level between the conduction band (CB) and valence band (VB) of hematite, which easily generates recombination sites. Among them, titanium doping leads to more surface trapped states, which prevents the transfer of photogenerated holes from the photoanode to the electrolyte, thereby hindering efficient water oxidation.

[0003] Unlike metal ions, nonmetallic dopants (such as Si, P, B, and F) can resolve the recombination problem of hematite without hindering the charge transfer via the original polaron hopping mechanism in the hematite lattice, and the appropriate combination of these strategies is expected to achieve remarkable results. Summary of the Invention

[0004] The purpose of the present invention is to provide a Ti-F co-doped hematite photoelectrode and its preparation method and application, so as to improve the photoelectrocatalytic water oxidation activity by reducing surface states and forming hydrogen bond structures to inhibit carrier recombination and accelerate the transfer of photogenerated holes. The preparation method is simple, easy to operate, and the experimental conditions are easy to control. The co-doping strategy of metal and non-metal elements provides an idea and reference for designing an efficient photoelectrocatalytic water oxidation photoanode.

[0005] To achieve the above object, the present invention provides a method for preparing a Ti-F co-doped hematite photoelectrode, comprising the following steps:

[0006] S1. Dissolve iron salt, titanium salt and fluoride salt in deionized water and stir to obtain a precursor solution;

[0007] S2, placing the precursor solution and FTO conductive glass in a hydrothermal reactor for hydrothermal reaction to obtain a photoelectrode thin film precursor;

[0008] S3. Anneal and calcine the photoelectrode film precursor in a tube furnace to obtain a Ti-F:Fe2O3 photoelectrode.

[0009] Preferably, in S1, the iron salt is one of ferric chloride hexahydrate or ferric sulfate.

[0010] Preferably, in S1, the molar ratio of the iron salt, the titanium salt and the fluorine salt is (1100:1:19) to (1500:1:20).

[0011] Preferably, in S2, the hydrothermal reaction temperature is 90-120° C., and the hydrothermal reaction time is 3-5 h.

[0012] Preferably, in S3, the annealing calcination includes two stages, wherein the first stage is calcined at 400-600°C for 1-3 hours with a heating rate of 1-10°C / min; the second stage is calcined at 700-800°C for 10-30 minutes with a heating rate of 1-10°C / min.

[0013] The present invention provides a Ti-F co-doped hematite photoelectrode, which is prepared by adopting the above-mentioned preparation method of the Ti-F co-doped hematite photoelectrode.

[0014] The present invention provides an application of a Ti-F co-doped hematite photoelectrode, wherein the Ti-F co-doped hematite photoelectrode is applied to photoelectrocatalytic water decomposition.

[0015] Therefore, the present invention adopts the above-mentioned Ti-F co-doped hematite photoelectrode and its preparation method and application, which has the following beneficial effects:

[0016] (1) Based on Ti-doped hematite, the present invention introduces F to construct Ti-F co-doped hematite, thereby reducing the number of oxygen vacancies and thus reducing the surface state; at the same time, the formation of Ti-F bonds on the hematite surface helps to form a hydrogen bond network interface in the alkaline electrolyte, thereby accelerating hole transfer and thus improving the photoelectrocatalytic water oxidation activity;

[0017] (2) The Ti-F:Fe2O3 photoelectrode film provided by the present invention has a photoelectrocatalytic reaction activity under simulated sunlight that is about 5.3 times that of blank Fe2O3 and about 1.6 times that of Ti:Fe2O3, and has improved stability;

[0018] (3) The Ti-F:Fe2O3 photoelectrode film provided by the present invention is simple to synthesize, which provides a new idea for the synthesis of hematite-based photoelectrocatalytic anode materials for photoelectrocatalytic water decomposition and has good application prospects.

[0019] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is an XRD comparison chart of a Ti-F co-doped hematite photoelectrode and its preparation method and application example of the present invention, a blank Fe2O3, a Ti-F:Fe2O3 photoelectrode prepared in Example 1, and a Ti:Fe2O3 photoelectrode prepared in Comparative Example 1;

[0021] Figure 2 This is a SEM comparison of a Ti-F co-doped hematite photoelectrode and its preparation method and application example of the present invention, a blank Fe2O3, a Ti-F:Fe2O3 photoelectrode prepared in Example 1, and a Ti:Fe2O3 photoelectrode prepared in Comparative Example 1;

[0022] Figure 3 HRTEM and TEM comparison diagrams of a blank Fe2O3 of a Ti-F co-doped hematite photoelectrode and its preparation method and application example of the present invention, a Ti-F:Fe2O3 photoelectrode prepared in Example 1, and a Ti:Fe2O3 photoelectrode prepared in Comparative Example 1;

[0023] Figure 4 LSV curves of a Ti-F co-doped hematite photoelectrode and its preparation method and application example of the present invention, a Ti-F:Fe2O3 photoelectrode prepared in Example 1, and a Ti:Fe2O3 photoelectrode prepared in Comparative Example 1;

[0024] Figure 5 The charge separation efficiency (η) of the blank Fe2O3 of the Ti-F co-doped hematite photoelectrode and its preparation method and application example, the Ti-F:Fe2O3 photoelectrode prepared in Example 1, and the Ti:Fe2O3 photoelectrode prepared in Comparative Example 1 is shown in FIG. separation ) and charge injection efficiency diagram (η injection )picture;

[0025] Figure 6The impedance diagrams are of a Ti-F co-doped hematite photoelectrode and its preparation method and application example of the present invention, a blank Fe2O3, a Ti-F:Fe2O3 photoelectrode prepared in Example 1, and a Ti:Fe2O3 photoelectrode prepared in Comparative Example 1;

[0026] Figure 7 It is an It curve diagram of a Ti-F co-doped hematite photoelectrode and its preparation method and application example of the present invention, a blank Fe2O3, a Ti-F:Fe2O3 photoelectrode prepared in Example 1, and a Ti:Fe2O3 photoelectrode prepared in Comparative Example 1. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0028] Unless otherwise defined, technical or scientific terms used in the present invention shall have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs.

[0029] The raw materials used in the present invention are all purchased from the market.

[0030] Example 1

[0031] A Ti-F co-doped hematite photoelectrode, the preparation method of which comprises the following steps:

[0032] S1. Dissolve 0.8109 g of ferric chloride hexahydrate and 0.02 g of ammonium fluoride in 20 mL of deionized water, then add 2 μL of TiCl3 solution (18% wt), and stir to dissolve and mix uniformly to obtain a precursor solution;

[0033] S2. Place the precursor solution and FTO conductive glass in a 50 mL polytetrafluoroethylene-lined reactor and hydrothermally react at 100°C for 3 hours to grow the precursor Ti-F co-doped FeOOH on the FTO conductive glass. Remove the precursor after cooling. Scrape off the FeOOH adsorbed on the non-conductive surface and then dry in a drying oven at 60°C for 60 minutes to obtain the photoelectrode film precursor.

[0034] S3. Anneal and calcine the photoelectrode film precursor in a tube furnace. First, heat it to 600°C at 5°C / min and calcine it for 2h. Then heat it to 800°C at 10°C / min and calcine it for 30min. After cooling, a Ti-F:Fe2O3 photoelectrode is obtained.

[0035] Comparative Example 1

[0036] A Ti-doped hematite photoelectrode, the preparation method of which comprises the following steps:

[0037] S1. Dissolve 0.8109 g of ferric chloride hexahydrate in 20 mL of deionized water, add 2 μL of TiCl3 solution (18% wt), and stir to dissolve and mix well to obtain a precursor solution;

[0038] S2. Place the precursor solution and FTO conductive glass in a 50 mL polytetrafluoroethylene-lined reactor and hydrothermally react at 100°C for 3 hours to grow the Ti-doped FeOOH precursor on the FTO conductive glass. Remove the precursor after cooling. Scrape off the FeOOH adsorbed on the non-conductive surface and then dry in a drying oven at 60°C for 60 minutes to obtain the photoelectrode film precursor.

[0039] S3. Anneal and calcine the photoelectrode film precursor in a tube furnace. First, heat it to 600°C at 5°C / min and calcine for 2h. Then heat it to 800°C at 10°C / min and calcine for 30min. After cooling, a Ti:Fe2O3 photoelectrode is obtained.

[0040] Comparative Example 2

[0041] A hematite photoelectrode, the preparation method of which comprises the following steps:

[0042] S1. Place 150 mM FeCl3 solution and FTO conductive glass in a 50 mL polytetrafluoroethylene-lined reactor and hydrothermally react at 100 °C for 3 h to obtain β-FeOOH, which is then washed with deionized water and dried.

[0043] S2. Anneal and calcine the dried β-FeOOH in a tube furnace. First, heat it to 550°C at 5°C / min and calcine for 2h. Then heat it to 730°C at 10°C / min and calcine for 15min. After cooling, obtain α-Fe2O3, i.e. blank Fe2O3.

[0044] Experimental Test 1

[0045] Figure 1 The XRD comparison diagrams of blank Fe2O3, Ti-F:Fe2O3 photoelectrode prepared in Example 1, and Ti:Fe2O3 photoelectrode prepared in Comparative Example 1 show that the phases corresponding to the main diffraction peaks are hematite (JCPDS-33-0664, the dotted line in the figure is the phase corresponding to hematite) and SnO2 (JCPDS-41-1445, the peaks are not marked in the figure). Figure 1 It can be seen that both Ti-doped and Ti-F-co-doped hematite exhibit two crystal planes, namely (110) and (300). After Ti-F co-doping, the photoanode diffraction peak intensity and peak width do not change significantly, indicating that Ti-F co-doping has no obvious effect on the crystal form.

[0046] Figure 2SEM comparison of blank Fe2O3, Ti-F:Fe2O3 photoelectrode prepared in Example 1, and Ti:Fe2O3 photoelectrode prepared in Comparative Example 1. Figure 2 a in the figure is the SEM of blank Fe2O3. Figure 2 bc in the figure are SEM images of Ti:Fe2O3 photoelectrode at different scales. Figure 2 d in the figure is the cross-sectional SEM image of Ti-F:Fe2O3 photoelectrode. Figure 2 ef in the figure is the SEM image of Ti-F:Fe2O3 photoelectrode at different scales. Figure 2 The top view of the hematite-based photoelectrode shows a worm-like morphology, where the sample morphology becomes more dense and chaotic after single doping with Ti or co-doping with Ti-F, especially after co-doping with Ti-F. Figure 2 From the image d, we can see that Ti-F co-doped hematite is a nanorod structure with a length of about 450 nm.

[0047] Figure 3 This is a comparison of the high-resolution transmission electron microscopy (HRTEM) images of blank Fe2O3, the Ti-F:Fe2O3 photoelectrode prepared in Example 1, and the Ti:Fe2O3 photoelectrode prepared in Comparative Example 1. Figure 3 a and Figure 3 b in the figure are HRTEM images of Fe2O3 and Ti:Fe2O3, respectively, which correspond to the (110) crystal plane of hematite according to the lattice spacing. Figure 3 c and Figure 3 Figures d and d are transmission electron microscopy (TEM) and HRTEM images of the Ti-F:Fe2O3 photoelectrode, respectively. Ti-F:Fe2O3 exhibits a rod-like structure with a lattice spacing of d = 0.25 nm, which also corresponds to the (110) crystal plane of hematite. This is consistent with the crystal plane exposed by blank hematite and hematite doped with Ti alone, indicating that Ti-F co-doping does not change the crystal structure of hematite.

[0048] Experimental Test 2

[0049] Blank Fe2O3, Ti-F:Fe2O3 photoelectrode prepared in Example 1, and Ti:Fe2O3 photoelectrode prepared in Comparative Example 1 were all used as photoanodes to undergo photocurrent testing, photogenerated carrier separation efficiency and charge injection efficiency testing, electrochemical impedance spectroscopy (EIS) testing, and stability testing.

[0050] Photocurrent test: The light source is a 300W xenon lamp, under the conditions of simulating sunlight, simulating sunlight AM1.5G, 100mW / cm 2 The potential range is 0.6~1.6V vs RHE. The LSV curves of water oxidation under different bias voltages are as follows Figure 4As shown, the photocurrent density of different electrodes at the water oxidation potential (1.23 V vs RHE) is compared. Figure 4 It can be seen that the photocurrent density of all electrodes gradually increases with the increase of bias voltage. The photocurrent density of blank Fe2O3 and Ti:Fe2O3 photoanode at 1.23 V vs RHE is 0.29 and 1.27 mA cm, respectively. -2 When Ti-F is co-doped, the photocurrent density increases to 2.07 mA·cm -2 , which are 7.14 and 1.63 times of those of blank hematite and Ti-doped sample respectively.

[0051] Separation efficiency and charge injection efficiency of photogenerated carriers: In order to study the effect of Ti-F co-doping on carrier separation and transfer, 0.1 M Na2SO3 was added to 1 M KOH solution as a hole scavenger, and the charge separation efficiency (η separation ) and charge injection efficiency (η injection ), the result is as follows Figure 5 As shown, Figure 5 a in the figure is the bulk charge separation efficiency of the blank Fe2O3 photoanode, the Ti-F:Fe2O3 photoanode prepared in Example 1, and the Ti:Fe2O3 photoanode prepared in Comparative Example 1. Figure 5 b in the figure is the surface charge injection efficiency of the blank Fe2O3 photoanode, the Ti-F:Fe2O3 photoanode prepared in Example 1, and the Ti:Fe2O3 photoanode prepared in Comparative Example 1. As can be seen from the figure, the bulk charge separation efficiency and surface charge injection efficiency of the photogenerated carriers of the blank Fe2O3 are the lowest. The bulk separation efficiency and surface charge injection efficiency of the photogenerated carriers of the Ti-F:Fe2O3 photoanode are enhanced compared with those of Ti:Fe2O3, and this enhancement is mainly attributed to the doping of F. When F is introduced to form Ti-F co-doping, the F element enters the oxygen sites in the hematite instead of the iron sites, and more positive charges are formed at the iron sites. At the same time, charge compensation helps to reduce the recombination of photogenerated carriers, thereby promoting charge injection at the photoanode-electrolyte interface.

[0052] In order to further study the separation and transport mechanism of charge carriers, electrochemical impedance spectroscopy (EIS) tests were carried out on blank Fe2O3 photoanode, Ti-F:Fe2O3 photoanode prepared in Example 1, and Ti:Fe2O3 photoanode prepared in Comparative Example 1 under light irradiation ( Figure 6 ) and fitted the equivalent circuit ( Figure 6 The fitting results are shown in Table 1. Among them, R s represents the interfacial series resistance between the photoanode film and the FTO substrate, and R ctrepresents the charge transfer resistance of different photoanodes. For the Ti-F co-doped photoanode, the charge transfer resistance decreased from 2016.2Ω for the bare Fe2O3 and 263.1Ω for the Ti-Fe2O3 to 124.7Ω, respectively. Therefore, it can be inferred that Ti-F co-doping can significantly reduce the charge transfer resistance. In summary, the Ti-F co-doped photoelectrode is more likely to transfer photogenerated holes in the bulk phase.

[0053] Table 1 Resistance values ​​of blank Fe2O3, Ti:Fe2O3 and Ti-F:Fe2O3 photoanodes fitted using Zview software

[0054] sample <![CDATA[R s (Oh)]]> Error(%) <![CDATA[R ct (Oh)]]> Error(%) <![CDATA[Blank Fe2O3]]> 37.23 4.4 2016.2 6.0 <![CDATA[Ti:Fe2O3]]> 52.88 1.0 263.1 3.1 <![CDATA[Ti-F:Fe2O3]]> 45.38 0.5 124.7 3.0

[0055] The stability of photoelectrode materials is an important indicator for practical applications. Therefore, the stability of the Ti-F:Fe2O3 photoanode prepared in Example 1 and the Ti:Fe2O3 photoanode prepared in Comparative Example 1 were tested using the It curve. Figure 7 It can be seen that the stability of hematite is improved after Ti-F co-doping. Within 9000s, the decrease in photocurrent is reduced from 20.91% of Ti:Fe2O3 to 11.06%.

[0056] Therefore, the present invention adopts the above-mentioned Ti-F co-doped hematite photoelectrode and its preparation method and application to reduce the surface state and form hydrogen bond structure to inhibit the recombination of carriers and accelerate the transfer of photogenerated holes to enhance the photoelectrocatalytic water oxidation activity. The preparation method is simple, easy to operate, and the experimental conditions are easy to control. The co-doping strategy of metal and non-metal elements provides an idea and reference for the design of efficient photoelectrocatalytic water oxidation photoanode.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a Ti-F co-doped hematite photoelectrode, characterized in that: The following steps are involved: S1. Dissolve iron salt, titanium salt and fluoride salt in deionized water and stir to obtain a precursor solution; S2, placing the precursor solution and FTO conductive glass in a hydrothermal reactor for hydrothermal reaction to obtain a photoelectrode thin film precursor; S3. Anneal and calcine the photoelectrode film precursor in a tube furnace to obtain a Ti-F:Fe2O3 photoelectrode.

2. The method for preparing a Ti-F co-doped hematite photoelectrode according to claim 1, characterized in that: In S1, the iron salt is one of ferric chloride hexahydrate or ferric sulfate.

3. The method for preparing a Ti-F co-doped hematite photoelectrode according to claim 1, characterized in that: In S1, the molar ratio of the iron salt, the titanium salt, and the fluoride salt is (1100:1:19) to (1500:1:20).

4. The method for preparing a Ti-F co-doped hematite photoelectrode according to claim 1, characterized in that: In S2, the hydrothermal reaction temperature is 90-120° C., and the hydrothermal reaction time is 3-5 h.

5. The method for preparing a Ti-F co-doped hematite photoelectrode according to claim 1, characterized in that: In S3, annealing and calcination include two stages, wherein the first stage is calcination at 400-600°C for 1-3 hours with a heating rate of 1-10°C / min; the second stage is calcination at 700-800°C for 10-30 minutes with a heating rate of 1-10°C / min.

6. A Ti-F co-doped hematite photoelectrode, characterized in that: The photoelectrode is prepared by the method for preparing a Ti-F co-doped hematite photoelectrode according to any one of claims 1 to 5.

7. An application of a Ti-F co-doped hematite photoelectrode, characterized in that: The Ti-F co-doped hematite photoelectrode according to claim 6 is applied to photoelectrocatalytic water decomposition.

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