Fe2o3 / n d composite photo-anode and preparation method thereof

Abstract

The application belongs to the technical field of photoelectric catalysis, and discloses a Fe2O3 / Nd composite photoanode and a preparation method thereof. First, FTO conductive glass is put into an aqueous solution prepared from FeCl3·6H2O and CH4N2O, and a FeOOH film grown on the FTO is obtained through a hydrothermal method; then, the Fe2O3 electrode is obtained by calcining at high temperature; and then, the prepared Fe2O3 electrode is placed in a negative pressure tubular furnace, and the Fe2O3 / Nd photoanode is obtained by replacement with NdCl3 as a replacement source. The application carries out surface layer metal cation replacement at high temperature to form a small amount of Nd element doping, does not damage the original iron oxide crystal structure, forms the Fe2O3 / Nd composite photoanode coexisting with Nd and Fe double metal elements, reduces the internal impedance of the semiconductor, improves the carrier concentration, and realizes effective improvement of the photo-generated current.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of photoelectrocatalysis, and particularly relates to a composite photoanode and a preparation method thereof. BACKGROUND

[0002] Modern industrialization has intensified resource development, and also caused irreversible damage to the ecology. Hydrogen energy, as a new type of energy, has the characteristics of green and no pollution, and has become one of the most potential clean energies in the 21st century. There are many ways to produce hydrogen energy, such as fossil fuel hydrogen production, biological hydrogen production, and water electrolysis hydrogen production. As a new method, photoelectrocatalytic (PEC) water splitting is gradually favored by people. In 1839, Edmond Becquerel immersed two same metal electrodes coated with silver halide particles into a dilute acid solution and found that a current was generated under light irradiation; in 1966, Gerischer studied the electrochemical and photoelectrochemical properties of the semiconductor / electrolyte contact interface; in 1972, Fujishima and Honda proved that the TiO2 photoanode could produce hydrogen under ultraviolet light, which led to a deep understanding of PEC water splitting. After that, many oxide semiconductors were widely studied and found to have photoelectrocatalytic activity, such as TiO2, SrTiO3, ZnO, and other wide band gap semiconductors, and BiVO4, WO3, Fe2O3, and other narrow band gap semiconductors. Fe2O3, as a transition metal oxide semiconductor, is abundant, non-toxic, has good photochemical stability, is thermodynamically stable, has a proper band gap (1.9-2.2 eV) and a theoretical solar-to-hydrogen (STH) efficiency of 15.3%, is cheap and easy to obtain, and has good chemical stability, etc. In recent decades, it has become a research hotspot in the field of photoelectric energy conversion. However, Fe2O3 has some undesirable physical behaviors, such as a fast hole recombination rate (~10 ps), a short hole diffusion length (2-4 nm), a high density of surface defects, and a low oxygen evolution reaction (OER) activity, etc. Therefore, it is very important to study reasonable and effective methods to reduce defects and improve the performance of photoelectrocatalytic water splitting. SUMMARY

[0003] The surface layer of the Fe2O3 photoanode is modified by an element doping method, that is, a composite photoanode is constructed by adding a replacement source in a negative pressure tube furnace to replace cations, thereby reducing the internal impedance of Fe2O3, increasing the carrier concentration, and improving the performance of photoelectrocatalytic water splitting.

[0004] In order to achieve the above-mentioned application purposes, the application is implemented by the following technical solutions:

[0005] The application provides a Fe2O3 / Nd composite photoanode, which is obtained according to the following preparation method:

[0006] S1: put the conductive substrate into the aqueous solution prepared by FeCl3·6H2O and CH4N2O, and obtain the FeOOH film grown on the conductive substrate by hydrothermal method; then calcine at high temperature to obtain the Fe2O3 electrode;

[0007] S2: put the Fe2O3 electrode prepared in S1 into the negative pressure tube furnace, and perform displacement with NdCl3 as the displacement source to obtain the Fe2O3 / Nd photoanode.

[0008] According to one aspect of the present application, a preparation method of Fe2O3 / Nd composite photoanode is provided, which is performed according to the following steps:

[0009] S1: put the conductive substrate into the aqueous solution prepared by FeCl3·6H2O and CH4N2O, and obtain the FeOOH film grown on the conductive substrate by hydrothermal method; then calcine at high temperature to obtain the Fe2O3 electrode;

[0010] S2: put the Fe2O3 electrode prepared in S1 into the negative pressure tube furnace, and perform displacement with NdCl3 as the displacement source to obtain the Fe2O3 / Nd photoanode.

[0011] In the above Fe2O3 / Nd composite photoanode and the preparation method thereof:

[0012] Further, in S1, the molar concentration ratio of FeCl3·6H2O to CH4N2O is 1:(1-1.5); CH4N2O can provide a suitable alkaline environment for the growth of FeOOH crystals; and the reasonable ratio of FeCl3·6H2O to CH4N2O can better ensure the uniformity of the morphology and the growth density.

[0013] Further, in S1, the temperature of the hydrothermal method is 90-110°C, and the time is 5-7h; a suitable growth temperature and sufficient growth time can be provided for the crystals, and better crystallinity can be obtained.

[0014] Further, in S1, the high-temperature calcination process is: first maintain at 500-600°C for 1-2h, and then maintain at 700-800°C for 5-30min; appropriate temperature can be selected to ensure better crystallinity.

[0015] Further, the temperature rising rate is 15-20°C / min from 500-600°C to 700-800°C. -1

[0016] Preferably, in S1, the morphology of Fe2O3 is nanorod.

[0017] ​The application is a method for improving the performance of a semiconductor photoanode by doping in the research of photoelectrocatalytic water splitting, which is a cation substitution with Nd. The method is to replace the position of Nd ions with Fe ions in a nitrogen atmosphere with high temperature and certain pressure in a tube furnace, without destroying the structure of the crystal itself, reducing the internal impedance of the semiconductor, increasing the carrier concentration, and further improving the photocurrent.

[0018] Further, in S2, the heating rate of the negative pressure tube furnace is 5-10℃ / min -1 ; The appropriate heating rate can ensure the accuracy of the temperature control of the negative pressure tube furnace.

[0019] Further, in S2, the holding temperature of the negative pressure tube furnace is 540-580℃, and the holding time is 5-10min; The reasonable holding temperature and holding time can ensure the reliability and accuracy of the results.

[0020] Further, in S2, the negative pressure of the negative pressure tube furnace is 200-500Pa.

[0021] The beneficial effects of the application are:

[0022] (1) The application can ensure the rapid crystallization of FeOOH to Fe2O3 by optimizing the preparation process parameters, and obtain Fe2O3 photoanode with good morphology;

[0023] (2) The application uses a negative pressure tube furnace for cation substitution to form a composite photoanode doped with Nd elements, which is different from the in-situ doping by forming a precursor, and the holding temperature and holding time of the negative pressure tube furnace are adjusted to improve the performance of the photoanode by a small amount of cation substitution;

[0024] (4) The preparation method of the application does not destroy the original crystal structure, maintains the stability of the photoanode, and only replaces the surface, which shows a one-fold performance improvement compared with pure Fe2O3 photoanode. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 (a), (b), (c) are scanning electron microscope (SEM) images of FeOOH, Fe2O3 and Fe2O3 / Nd prepared in Example 1;

[0026] Figure 2 (a), (b) are transmission electron microscope (TEM) images of Fe2O3 prepared in Example 1;

[0027] Figure 3 (a), (b) are transmission electron microscope (TEM) images of Fe2O3 / Nd prepared in Example 1;

[0028] Figure 4Fe2O3 / Nd single nanorod line scan map prepared in Example 1;

[0029] Figure 5 (a) and (b) are energy dispersive spectroscopy (EDS) maps of Fe2O3, Fe2O3 / Nd prepared in Example 1;

[0030] Figure 6 (a) is a total X-ray photoelectron spectroscopy (XPS) map of Fe2O3, Fe2O3 / Nd prepared in Example 1, and (b) is an X-ray photoelectron spectroscopy (XPS) map of Nd3d;

[0031] Figure 7 Different parameter current-voltage (J-V) curves of Comparative Example 1, Comparative Example 2;

[0032] Figure 8 Current-voltage (J-V) curves of Fe2O3, Fe2O3 / Nd;

[0033] Figure 9 Electrochemical impedance spectroscopy (EIS) maps of Fe2O3, Fe2O3 / Nd;

[0034] Figure 10 Mott-Schottky maps of Fe2O3, Fe2O3 / Nd. DETAILED DESCRIPTION

[0035] The specific implementation of the present application will be described in more detail below with reference to the accompanying drawings and examples, so that the scheme of the present application and the advantages of each aspect thereof can be better understood. It should be noted that the specific implementation and examples described below are for illustrative purposes only and are not limiting on the present application.

[0036] Example 1

[0037] The preparation method of Fe2O3 / Nd of the present example 1 comprises the following steps:

[0038] First step: 1.1g-1.3g FeCl3·6H2O, 0.2-0.3g CH4N2O were dissolved in 30ml deionized water to form a mixed solution with a concentration ratio of 1:1, then FTO (fluorine-doped tin dioxide) conductive glass was placed in the solution and put into a blast drying oven for hydrothermal incubation at 100℃ for 6h. After the reaction was completed, a light yellow FeOOH grown on FTO was obtained, and the sample FeOOH was washed with deionized water and dried;

[0039] Second step: the dried sample was placed in a muffle furnace and calcined at 550℃ for 2h, then the temperature was continuously increased to 800℃, incubated for 5min and naturally cooled to room temperature to obtain Fe2O3 photoanode;

[0040] Step 3: Put the Fe2O3 photoanode in a quartz boat and place it in the head region of the negative pressure tube furnace, and place 0.2 g of NdCl3 as the replacement source in the tail region of the negative pressure tube furnace, with a distance of 10 cm between them.

[0041] The internal pressure of the tube is extracted to about 8 Pa, and the temperature is increased by 10 ℃ / min from room temperature to 560 ℃ under a nitrogen atmosphere of 300 Pa -1 for 5 min, and then naturally cooled to room temperature to obtain the Fe2O3 / Nd composite photoanode.

[0042] Comparative Example 1

[0043] Comparative Example 1 prepared Fe2O3 / Nd photoanodes with different holding temperatures in the tube furnace, but the same holding time, and the preparation method comprises the following steps:

[0044] Put the Fe2O3 photoanode in a quartz boat and place it in the head region of the negative pressure tube furnace, and place the replacement source NdCl3 in the tail region of the negative pressure tube furnace, with a distance of 10 cm between them, and keep the same nitrogen gas pressure value, heating rate, and holding time as in Example 1, and change the highest temperature to 540 ℃ and 580 ℃. The subsequent experimental steps are the same as in Example 1 and will not be repeated here.

[0045] Comparative Example 2

[0046] Comparative Example 2 prepared Fe2O3 / Nd photoanodes with the same holding temperature in the negative pressure tube furnace, but different holding times, and the preparation method comprises the following steps:

[0047] Put the Fe2O3 photoanode in a quartz boat and place it in the head region of the negative pressure tube furnace, and place the replacement source NdCl3 in the tail region of the negative pressure tube furnace, with a distance of 10 cm between them, and keep the same nitrogen gas pressure value, heating rate, and highest temperature as in Example 1, and change the holding time to 1 min, 10 min, and 30 min.

[0048] Characterization and Testing

[0049] The FeOOH, Fe2O3, and Fe2O3 / Nd photoanodes prepared in Example 1 were characterized by scanning electron microscopy (SEM), as shown in Figure 1 (a), (b), and (c), respectively. As can be seen from Figure 1 (a), the FeOOH formed by hydrothermal treatment is in the form of a cuboid, with uniform growth, and the length and width are about 60 nm; as can be seen from Figure 1 (b), the Fe2O3 formed by high-temperature calcination is different from the cuboid shape of FeOOH, and becomes a rod shape, with uniform growth and larger gaps between the nanorods, which is due to the volatilization of crystalline H2O during high-temperature calcination, which is beneficial to the transfer of photo-generated carriers; as can be seen from Figure 1(c)It can be seen that the Fe2O3 / Nd photoanode formed after the high-temperature replacement in the tube furnace does not change the dispersion condition and does not form a collapse, which is due to the advantage of high-temperature sintering in the muffle furnace and good stability.

[0050] The Fe2O3 and Fe2O3 / Nd photoanodes prepared in Example 1 are characterized by transmission electron microscopy (TEM), and it can be seen that Figure 2 The lattice spacing of the (104) crystal plane of Fe2O3 in (a) and (b) is 0.27 nm. After cation replacement, as Figure 3 The lattice spacing of the (006) crystal plane of Fe2O3 / Nd in (a) and (b) is 0.23 nm, and a small amount of Nd doping does not affect the lattice structure of Fe2O3.

[0051] The Fe2O3 / Nd photoanode prepared in Example 1 is characterized by single nanorod line scanning, Figure 4 It can be seen that, in addition to Fe and O elements, there is a small amount of Nd element, which confirms the feasibility of the cation replacement experiment.

[0052] The Fe2O3 and Fe2O3 / Nd prepared in Example 1 are characterized by EDS, and the composition of the elements is analyzed, and it can be seen that Figure 5 (a) Fe and O elements exist, Figure 5 (b) Nd element also appears in addition to Fe and O elements, which indicates that Nd is successfully replaced into the Fe2O3 photoanode.

[0053] The Fe2O3 and Fe2O3 / Nd prepared in Example 1 are characterized by X-ray photoelectron spectroscopy (XPS), Figure 6 (a) is the XPS total graph. Figure 6 (b) can observe the peak value of Nd, which indicates that Nd is successfully replaced into Fe2O3.

[0054] The Fe2O3 and Fe2O3 / Nd photoanodes prepared in Example 1 are tested for photoelectric performance, a three-electrode system is used, the prepared photoanode is used as the working electrode, platinum sheet is used as the counter electrode, saturated (KCl) Ag / AgCl electrode is used as the reference electrode, and 1 mol / L NaOH aqueous solution is used as the electrolyte solution. A CEST electrochemical workstation is used, a PTFEL PLS-FX300HU high-uniformity integrated xenon lamp light source is used to simulate sunlight, and an AM 1.5G filter is used. The incident light power intensity on the surface of the working electrode is set to 100 mW cm -2 , and the battery detector is calibrated. The test method is linear sweep cyclic voltammetry, and the scan rate is 20 mV s -1 .

[0055] Figure 7The test of selecting experimental parameters for the above-mentioned Comparative Examples 1 and 2 is carried out by comparing different parameters for several times, and the optimal experimental parameters are selected, i.e., the temperature is 560 DEG C, and the holding time is 5 min.

[0056] Figure 8 For the J-V curves before and after the replacement, compared with Fe2O3, the photo-generated current of the Fe2O3 / Nd photoanode is increased from 0.4 mA·cm-2 to 0.9 mA·cm-2 at the potential of 1.23 V vs.RHE. -2 The performance is doubled. -2

[0057] Figure 9 For the electrochemical impedance (EIS) diagrams before and after the replacement, compared with Fe2O3, the semicircle of the Fe2O3 / Nd photoanode is smaller, which indicates that the successful replacement of Nd reduces the internal resistance of the semiconductor, and is beneficial to the faster separation and transport of the photo-generated carriers.

[0058] Figure 10 For the Mott-Schottky diagrams before and after the replacement, it can be seen that both Fe2O3 and Fe2O3 / Nd are n-type semiconductors, and the curve slope is reduced after the replacement, which indicates that the carrier concentration is increased, and also confirms the successful replacement of Nd.

[0059] It can be seen that the present application innovatively dopes on the basis of Fe2O3, i.e., placing the Fe2O3 electrode and the replacement source in a negative pressure tube furnace, and carrying out surface layer metal cation replacement at high temperature to form a small amount of Nd element doping, which does not damage the original iron oxide crystal structure, forms the Fe2O3 / Nd composite photoanode coexisting with Nd and Fe double metal elements, reduces the internal impedance of the semiconductor, increases the carrier concentration, and realizes the effective improvement of the photo-generated current.

[0060] Although the preferred embodiments of the present application are described above in combination with the drawings, the present application is not limited to the above-mentioned specific embodiments, and the above-mentioned specific embodiments are only illustrative, but not restrictive, and those skilled in the art can make many specific changes in form without departing from the purpose of the present application and the scope protected by the claims under the inspiration of the present application, which all belong to the protection scope of the present application.​

Claims

1. A method for preparing a Fe2O3 / Nd composite photoanode, characterized in that, Follow these steps: S1: A conductive substrate is placed in an aqueous solution prepared from FeCl3·6H2O and CH4N2O, and a FeOOH film is grown on the conductive substrate by hydrothermal method; then, Fe2O3 electrode is obtained by calcination at high temperature; wherein, the morphology of Fe2O3 is nanorods. S2: The Fe2O3 electrode prepared in S1 is placed in a negative pressure tube furnace and replaced with NdCl3 as the replacement source to obtain Fe2O3 / Nd photoanode; wherein, the holding temperature of the negative pressure tube furnace is 540 ~ 580 ℃, the holding time is 5 ~ 10 min, and the negative pressure of the negative pressure tube furnace is 200 ~ 500 Pa.

2. The method for preparing a Fe2O3 / Nd composite photoanode according to claim 1, characterized in that, In S1, the molar concentration ratio of FeCl3·6H2O to CH4N2O is 1:(1 ~ 1.5).

3. The method for preparing a Fe2O3 / Nd composite photoanode according to claim 1, characterized in that, In S1, the hydrothermal method is performed at a temperature of 90-110℃ for 5-7 hours.

4. The method for preparing a Fe2O3 / Nd composite photoanode according to claim 1, characterized in that, In S1, the high-temperature calcination process is as follows: first, maintain a temperature of 500 ~ 600℃ for 1 ~ 2 hours, and then maintain a temperature of 700 ~ 800℃ for 5 ~ 30 minutes.

5. The method for preparing a Fe2O3 / Nd composite photoanode according to claim 4, characterized in that, Maintain a temperature of 15-20°C for 15 min as the temperature increases from 500-600°C to 700-800°C. -1 The rate of temperature increase.

6. The method for preparing a Fe2O3 / Nd composite photoanode according to claim 1, characterized in that, In S2, the heating rate of the negative pressure tubular furnace is 5 ~ 10 ℃ min. -1 .

7. A Fe2O3 / Nd composite photoanode, characterized in that, It is obtained by the preparation method of Fe2O3 / Nd composite photoanode according to any one of claims 1-6.

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