Three-dimensional p-n heterojunction photoelectrocatalytic material and preparation method and application thereof

Abstract

The application belongs to the technical field of photoelectrocatalytic materials, and particularly relates to a three-dimensional p-n heterojunction photoelectrocatalytic material and a preparation method and application thereof. The preparation method is as follows: a titanium mesh is used as a substrate, an ordered three-dimensional TiO2 nanotube array is prepared by an anodic oxidation method, and a narrow-band-gap p-type semiconductor FeS2 is loaded into and on the surface of a wide-band-gap n-type semiconductor TiO2 nanotube by an electrochemical deposition method, so as to prepare a composite double-layer three-dimensional p-n heterojunction FeS2 / TiO2 photoelectrocatalytic material. Compared with the TiO2 material, when the material is used as a photoelectrocatalytic material to reduce CO2 to generate ethanol, the absorption of TiO2 in the visible light range is increased, the utilization rate of visible light by TiO2 is promoted, the separation of photo-generated electron-hole pairs is more favorable, and the performance of TiO2 in photoelectrocatalytic reduction of CO2 to generate ethanol is greatly improved. The problem that the conversion efficiency and product selectivity of pure TiO2 in photoelectric conversion of CO2 to generate ethanol are difficult to control is solved.

Description

Technical Field

[0001] This invention relates to a composite photocatalytic material, and more particularly to a three-dimensional pn heterojunction photocatalytic material, its preparation method, and its application. Background Technology

[0002] With global economic development and a rapidly growing world population, the Earth's finite carbon resources, such as coal, oil, and natural gas, are being continuously depleted. In the near future, humanity will face the grim reality of depleting existing carbon resources and energy. On the other hand, the overexploitation of fossil fuels has led to massive CO2 emissions into the atmosphere, resulting in an increasingly severe greenhouse effect. Faced with this increasingly serious situation, countries around the world have implemented strict controls on CO2 emissions. However, from the perspective of sustainable development, simply relying on reducing CO2 emissions will inevitably affect the normal development of the social economy; therefore, the comprehensive utilization of greenhouse gases is an inevitable trend for economic and social development. Utilizing semiconductor materials and solar energy to convert CO2 and H2O into high-value-added organic chemical products such as CH4, CO, CH3OH, CH3O, CH3COOH, and CH3CH2OH can not only effectively utilize industrial waste and reduce CO2 emissions but also produce environmentally friendly new energy materials, possessing dual significance of energy conservation and environmental protection. Ethanol can be blended into gasoline as a vehicle fuel, improving fuel performance and quality while reducing emissions of major pollutants such as carbon monoxide and hydrocarbons. Ethanol itself can also be used to produce chemical raw materials such as acetaldehyde, diethyl ether, ethyl acetate, and ethylamine, and is also a raw material for the production of dyes, coatings, detergents, and other products. However, there are currently few methods for preparing ethanol, and industrially, ethanol is generally only produced through fermentation or synthesis.

[0003] To address the aforementioned issues, in the photoelectrocatalytic reduction of CO2 to ethanol, combining TiO2 with a semiconductor material that promotes ethanol yield can simultaneously improve the photoelectric conversion efficiency of the semiconductor and the selectivity of ethanol. In the photoelectrocatalytic reduction of CO2 reaction, photocatalysis and electrocatalysis interact; electrocatalysis promotes the directional transport of photocharge, while photocatalysis can compensate for the excessive energy consumption in the electrocatalytic process. Therefore, this technology has attracted considerable attention.

[0004] In recent years, a series of novel photoelectric materials for CO2 reduction have been reported by scientific researchers. Among them, the research on TiO2 photoelectrocatalytic materials for CO2 reduction is the most extensive and in-depth. TiO2 has a wide range of applications in photoelectrochemical degradation of organic matter, photoelectrochemical hydrogen production, and photoelectrochemical synthesis of chemical products due to its abundant reserves, low price, good photochemical stability, and non-toxicity. However, the traditional TiO2 prepared using titanium plates and sheets as substrates has a low light energy utilization rate in photoelectrocatalytic applications, and the photoelectrochemical conversion of CO2 to ethanol based on pure TiO2 has the following two major problems: (1) very low photoelectrochemical conversion efficiency, and (2) difficulty in controlling product selectivity. Therefore, improvements are still needed in the field of ethanol production based on TiO2 semiconductor photoelectrocatalysis. Summary of the Invention

[0005] One objective of this invention is to address the shortcomings of existing technologies in the photoelectric conversion of pure TiO2 to CO2 for ethanol production, namely low photoelectric conversion efficiency and difficulty in controlling product selectivity. This invention provides a method for preparing a three-dimensional pn heterojunction photocatalytic material. This method employs electrochemical deposition and thermal sulfidation to deposit FeS2 into the interior and surface of a TiO2 nanotube array, resulting in a high specific surface area, three-dimensional pn heterojunction FeS2 / TiO2 photocatalytic material with a titanium mesh substrate. This improves the performance of photocatalytic reduction of CO2 for the production of new energy materials, especially ethanol.

[0006] To achieve the above objectives, the present invention employs the following technical solution: a method for preparing a three-dimensional pn heterojunction photoelectrocatalytic material, comprising the following steps:

[0007] S1. Using deionized water as a solvent, glycerol and FeCl3 were added sequentially, and the mixture was stirred continuously to obtain a mixture of FeCl3 and glycerol.

[0008] S2. Using a three-electrode system, titanium dioxide nanotubes were added to the above mixture as the working electrode. A working voltage of -3 to 2V was applied, and electrodeposition was performed for 5 to 25 minutes to obtain Fe. 3+ / TiO2;

[0009] S3. Put Fe 3+ / TiO2 is placed in a muffle furnace and heat-treated at 300-500℃ for 1-4 hours and then cooled to obtain Fe2O3 / TiO2 precursor;

[0010] S4. The Fe2O3 / TiO2 precursor was placed in a vacuum tube furnace and heated to 500-600℃ under a sublimation sulfur atmosphere to obtain a three-dimensional pn heterojunction photoelectrocatalytic material.

[0011] Further improvements to the preparation method of three-dimensional pn heterojunction photoelectrocatalytic materials:

[0012] Preferably, the solubility of glycerol in the mixture of step S1 is 0.4-0.7 vol%, and the concentration of FeCl3 is 0.1-0.4 mol / L.

[0013] Preferably, the counter electrode of the three-electrode system is a platinum sheet, and the reference electrode is a saturated calomel electrode.

[0014] Preferably, in step S4, the Fe2O3 / TiO2 precursor is placed in a vacuum tube furnace and heat-treated in a sublimed sulfur atmosphere of 0.3-3g.

[0015] Preferably, the heat treatment time in step S4 is 10-20 hours.

[0016] Preferably, the TiO2 preparation method in step S2 is as follows: using a 100-mesh titanium mesh as a substrate, after being treated with a polishing solution, it is placed in an anodic oxidation electrolyte and oxidized at a voltage of 40-60V for 0.5-4h, and then heat-treated in a tube furnace at 500-600℃ for 1-4h.

[0017] Preferably, the polishing solution is an acidic diluted solution, which is composed of deionized water, nitric acid and hydrofluoric acid in a volume ratio of 5:4:1.

[0018] Preferably, the preparation method of the anodic oxidation electrolyte is as follows: 8.75-17.5 ml of deionized water and 1-4 g of ammonium fluoride are added to 1000 ml of ethylene glycol with stirring, and then stirring is continued for 1-5 hours.

[0019] The second objective of this invention is to provide a three-dimensional pn heterojunction photocatalytic material prepared by the preparation method of any one of the above-mentioned three-dimensional pn heterojunction photocatalytic materials.

[0020] The third objective of this invention is to provide an application of the above-mentioned three-dimensional pn heterojunction photoelectrocatalytic material in the photoelectrocatalytic reduction of CO2 to produce ethanol.

[0021] The advantages of this invention compared to the prior art are as follows:

[0022] 1) This invention provides a method for preparing a three-dimensional pn heterojunction FeS2 / TiO2 photoelectrocatalytic material. Using TiO2, which absorbs ultraviolet light, as the substrate, and FeS2, which absorbs visible light, as the loading, an electrochemical deposition method is employed to load the narrow-bandgap p-type semiconductor FeS2 onto the interior and surface of a wide-bandgap n-type semiconductor TiO2 nanotube. FeS2's absorption of visible light enhances the absorption of TiO2 in the visible light region, promotes the separation and transport of photogenerated carriers, and inhibits the recombination of photogenerated electrons and holes. This improves the solar light response and light utilization rate of the FeS2 / TiO2 photoelectrocatalytic material in the photoelectrocatalytic reduction of CO2 to prepare new energy materials, and enhances the performance of photoelectrocatalytic production of new energy materials, especially in the preparation of ethanol. The FeS2 loaded inside and on the surface of TiO2 constructs a pn-type heterojunction, in which TiO2 is an n-type semiconductor and FeS2 is a p-type semiconductor, enhancing the absorption of TiO2 in the visible light region, promoting the separation and transport of photogenerated carriers, and inhibiting the recombination of photogenerated electrons and holes.

[0023] This invention uses titanium mesh as the substrate for preparing TiO2 and prepares an ordered three-dimensional TiO2 nanotube array by anodizing. Since each titanium wire has a cylindrical three-dimensional structure, it has a higher specific surface area than TiO2 prepared by traditional titanium sheets and titanium plates, which can increase the specific surface area of ​​nanotubes and grow more nanotubes in a limited substrate space.

[0024] 2) The FeS2 / TiO2 photoelectrocatalytic material provided by this invention uses readily available, non-toxic, harmless, safe, and inexpensive raw materials. The preparation method is simple, providing a new photoelectrocatalytic material for the photoelectrocatalytic reduction of CO2 to ethanol, and has good application prospects. When preparing the three-dimensional pn heterojunction FeS2 / TiO2 photoelectrocatalytic material for the photoelectrocatalytic reduction of CO2 to ethanol, compared with TiO2, it increases the absorption and utilization rate of visible light in the visible light range of TiO2, which is more conducive to the separation of photogenerated electron and hole pairs, greatly improving the performance of TiO2 in the photoelectrocatalytic reduction of CO2 to ethanol, with a yield reaching 1032 μmol / L / cm. 2 . Attached Figure Description

[0025] Figure 1 The images are scanning electron microscope (SEM) images of the TiO2 photoelectrode prepared in Example 1 of the present invention at magnifications of 400X (a), 800X (b), 1200X (c) and 12000X (d).

[0026] Figure 2 The X-ray diffraction (XRD) patterns of the TiO2 photoelectrode and FeS2 / TiO2 photoelectrode prepared in Example 1 of this invention are shown.

[0027] Figure 3 The images show the Raman spectra of the TiO2 photoelectrode and the FeS2 / TiO2 photoelectrode prepared in Example 1 of this invention.

[0028] Figure 4 The images show scanning electron microscope (SEM) images of the TiO2 photoelectrode (a) and the FeS2 / TiO2 photoelectrode prepared at deposition times of 5 min (b), 10 min (c), 15 min (d), 20 min (e), and 25 min (f) in Example 1 of this invention.

[0029] Figure 5 These are the electrochemical impedance spectroscopy (EIS) spectra of the TiO2 photoelectrode and the FeS2 / TiO2 photoelectrode of Example 1 of this invention.

[0030] Figure 6 The transient photocurrent response diagram (it) of the TiO2 photoelectrode and FeS2 / TiO2 photoelectrode in Embodiment 1 of the present invention is shown.

[0031] Figure 7 The graph shows a comparison of the photoelectrocatalytic performance of the TiO2 photoelectrode of Example 1 of the present invention and the FeS2 / TiO2 photoelectrode prepared at deposition times of 5 min, 10 min, 15 min, 20 min and 25 min at -0.8 V.

[0032] Figure 8 The UV-Vis diffuse reflectance absorption spectra (UV-VisDRS, (a)) and Tauc function plots (b) of the TiO2 photoelectrode and FeS2 / TiO2 photoelectrode prepared in Example 2 are shown.

[0033] Figure 9 This is a transmission electron microscope (TEM) image of the FeS2 / TiO2 photoelectrode prepared in Example 2.

[0034] Figure 10 This is the X-ray photoelectron spectrum (XPS) of the FeS2 / TiO2 photoelectrode prepared in Example 2.

[0035] Figure 11 The image shows the Mott-Schottky curves (MS) of the FeS2 photoelectrode (a), TiO2 photoelectrode (b), and FeS2 / TiO2 photoelectrode (c) in Example 2.

[0036] Figure 12 The figures (a) and (b) show the comparison of the photoelectrocatalytic performance of the FeS2 / TiO2 photoelectrode prepared in Example 2 under different applied bias voltages. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0038] Example 1

[0039] This embodiment provides a method for preparing a high specific surface area composite bilayer three-dimensional pn heterojunction FeS2 / TiO2 photocatalytic material, which specifically includes the following steps:

[0040] S1. Dissolve 4g of ammonium fluoride and 17.5ml of deionized water in 1000ml of ethylene glycol solution and stir for 4h to obtain an anodic oxidation electrolyte; select 100-mesh titanium mesh as substrate, treat it with polishing solution and place it in the anodic oxidation electrolyte as anode, copper sheet as cathode, anodize at 60V for 2h, and then heat treat it in a tube furnace at 600℃ for 3h to obtain TiO2 nanotube material;

[0041] S2. Glycerol and FeCl3 were added sequentially to 50 ml of deionized water. The solubility of glycerol was 0.7 vol%, and the concentration of FeCl3 was 0.4 mol / L. The mixture was stirred continuously for 12 h to obtain a mixed solution of FeCl3 and glycerol.

[0042] S3. Add TiO2 nanotubes as the working electrode to the above mixture. Using a CHI760e electrochemical workstation, assemble a three-electrode deposition system with a platinum sheet as the counter electrode and a saturated calomel electrode as the reference electrode. Apply an external voltage of -2V for 15 minutes to deposit Fe. 3+ Loading Fe into the interior and surface of TiO2 nanotubes yields Fe 3+ / TiO2;

[0043] S4, Fe 3+ / TiO2 was placed in a muffle furnace and heat-treated at 500℃ for 2 hours, then cooled to obtain Fe2O3 / TiO2 precursor;

[0044] S5. The Fe2O3 / TiO2 precursor was placed in a vacuum tube furnace and heated to 500℃ for 20h under a 0.5g sublimed sulfur atmosphere to prepare FeS2 / TiO2 photoelectrocatalytic material.

[0045] As a control example, the deposition time of the three-electrode deposition system in step S3 was modified from 15 min to 5 min, 10 min, 20 min and 25 min respectively, and the other steps were the same as in Example 1; finally, FeS2 / TiO2 photoelectrocatalytic material was obtained.

[0046] The TiO2 nanotube material obtained in step S1 is referred to as the TiO2 photoelectrode when used as a photoelectrocatalytic anode electrode material to reduce CO2 and produce ethanol; the FeS2 / TiO2 photoelectrocatalytic material obtained in step S5 is referred to as the FeS2 / TiO2 photoelectrode when used as a photoelectrocatalytic anode electrode material to reduce CO2 and produce ethanol.

[0047] Figure 1 The images are scanning electron microscope (SEM) images of the TiO2 photoelectrode prepared in step S1 of Example 1 at magnifications of 400X (a), 800X (b), 1200X (c), and 12000X (d). Figure 1 As can be seen in (a), the TiO2 photoelectrode prepared with titanium mesh as the substrate has a 360° cylindrical annular high specific surface area morphology; from Figure 1 (b) and Figure 1 (c) shows that TiO2 grows vertically onto the titanium mesh substrate, with a smooth surface, close contact with the substrate, and good adhesion; from Figure 1 The nanotube structure can be clearly observed in (d), and the uniform and dense nanotube openings can be observed by combining the magnified image.

[0048] Figure 2 The X-ray diffraction (XRD) patterns of the TiO2 photoelectrode and FeS2 / TiO2 photoelectrode prepared in Example 1 of this invention are shown. The XRD data of TiO2 indicates that the TiO2 prepared in this invention is a mixed crystal phase of highly crystalline rutile and anatase phases. The diffraction peaks at 25.6°, 38.0°, 48.3°, 54.2° and 56.5° belong to anatase TiO2, and the diffraction peaks at 27.6 nm and 36.3 nm belong to rutile phase TiO2, proving that the TiO2 prepared by anodic oxidation is a mixed crystal phase. In the XRD image of FeS2 / TiO2, the diffraction peaks at 28.7°, 33.3°, 37.3°, 41.0°, 47.6° and 56.5° belong to the (111), (200), (210), (211), (220) and (311) crystal planes of FeS2. This result demonstrates the successful preparation of highly crystalline FeS2 / TiO2 photoelectrocatalyst materials. The XRD pattern shows that the FeS2 grown on the TiO2NT substrate exhibits high (200) and (311) orientations. This indicates the preferred growth of FeS2 in the (200) and (311) directions. This preferred growth of FeS2 in the (200) and (311) directions can lead to a decrease in the interfacial energy.

[0049] Figure 3 The images show the Raman spectra of the TiO2 photoelectrode and the FeS2 / TiO2 photoelectrode prepared in Example 1 of this invention. The images are shown at 339, 378, and 428.5 cm⁻¹. -1The Raman peaks of FeS2 and their corresponding vertices are 141.4, 197.8, 393.4, 514.6, and 637.9 cm. -1 The Raman peak of TiO2. Among them, 339cm -1 The Raman peak at 375 cm⁻¹ represents the vibrational peak (Eg) of FeS₂ in pyrite perpendicular to the SS axis. -1 The Raman peak at 425 cm⁻¹ represents the tensile vibration peak (Ag) of FeS₂ along the SS axis. -1 The Raman peak at the location indicates the coupled tensile vibration peak (Tg) of FeS2, further demonstrating that the prepared FeS2 is pure-phase pyrite.

[0050] Figure 4 The images show scanning electron microscope (SEM) images of the TiO2 material (a) and the FeS2 / TiO2 photoelectrodes prepared at deposition times of 5 min (b), 10 min (c), 15 min (d), 20 min (e), and 25 min (f) in Example 1 of this invention. Figure 4 It can be seen that the mixed-phase TiO2 material prepared by anodic oxidation has a smooth surface and grows vertically upwards from the cylindrical titanium wire. FeS2 deposited by electrochemical deposition mainly appears inside and on the surface of the TiO2 nanotube array. At a deposition time of 5 min, a small amount of FeS2 can be observed inside and on the surface of the TiO2 nanotube array in the FeS2 / TiO2-5 min electrode. With increasing deposition time, the amount of FeS2 deposited increases gradually. At a deposition time of 15 min, a large number of FeS2 particles are clearly visible uniformly distributed on the surface and sides of the nanotubes in the FeS2 / TiO2-15 min electrode. When the deposition time increases to 20 min, a large number of FeS2 particles aggregate on the sample surface, appearing as large blocky aggregates, which severely block light from reaching the nanotubes and have a negative effect on the photoelectrocatalytic performance of the sample. Increasing the deposition time to 25 minutes further revealed that the sample still exhibited blocky aggregates on its sides. Furthermore, due to the excessively long deposition time, the FeS2 loading was too high, and some of the initially loaded FeS2 had already detached, resulting in an overall uneven loading.

[0051] Figure 5 This is the electrochemical impedance spectroscopy (EIS) of the TiO2 material and FeS2 / TiO2 photoelectrode of Example 1 of this invention. By studying the electrode process kinetics and surface phenomena, it can be found that the composite photoelectrode FeS2 / TiO2 exhibits a smaller semicircular curve than the unmodified TiO2. This result indicates that the charge transfer resistance of the FeS2 / TiO2 photoelectrode composited with FeS2 in this invention is reduced, which promotes the transport of photogenerated charge carriers.

[0052] Figure 6The transient photocurrent response curves (it) of the TiO2 photoelectrode and FeS2 / TiO2 photoelectrode of Embodiment 1 of this invention are shown. This curve is an effective way to study the photogenerated carrier transfer behavior at the semiconductor / electrolyte interface. At the moment of illumination, the photocurrent of the photoanode increases rapidly, indicating that FeS2 / TiO2 has good photoresponse characteristics. The photocurrent response of the unmodified TiO2 photoelectrode shows a positive unidirectional current transient response, indicating that TiO2 is an n-type semiconductor; the photocurrent response of the FeS2 / TiO2 photoelectrode shows a bidirectional current transient response, with the photocurrent polarity reversed compared to TiO2, exhibiting a positive photocurrent under illumination and a negative photocurrent in darkness. Furthermore, both the TiO2 photoelectrode and the FeS2 / TiO2 photoelectrode show peaks of varying degrees in their transient photocurrent response curves. The peak appearing at the moment the light is turned on indicates the accumulation of electrons at the semiconductor / electrolyte interface, while the peak at the moment the light is turned off indicates the reverse recombination of holes. This indicates that the prepared photoelectrode exhibits a charge accumulation effect. This involves three processes: (1) At the moment of illumination, the holes that accumulate at the electrode / electrolyte interface and the instantaneously generated photogenerated carriers bring about a significant photocurrent; (2) The rapid recombination of photogenerated carriers causes a rapid decrease in photocurrent; (3) When the separation and recombination of photogenerated carriers reach a dynamic equilibrium, the photocurrent tends to stabilize.

[0053] Figure 7 This is a comparison of the photoelectrocatalytic performance of the TiO2 photoelectrode of Example 1 of this invention and the FeS2 / TiO2 photoelectrode prepared at deposition times of 5 min, 10 min, 15 min, 20 min, and 25 min at -0.8 V, labeled as FeS2 / TiO2-5 min, FeS2 / TiO2-10 min, FeS2 / TiO2-15 min, FeS2 / TiO2-20 min, and FeS2 / TiO2-25 min, respectively. The main photoelectrocatalytic product is ethanol. It can be seen that compared with unmodified TiO2, the composite photoelectrodes with deposition times of 5 min, 10 min, and 15 min all show significantly enhanced photoelectrocatalytic performance. Among them, the FeS2 / TiO2 with a deposition time of 15 min has the highest photoelectrocatalytic yield, with an ethanol yield of 833 μmol / L after 2 h of catalysis. The samples with deposition times of 20 min and 25 min exhibit poor photoelectrocatalytic performance, and no product formation can be detected by gas chromatography. Based on SEM and electrochemical characterization data, this situation is mainly due to excessive FeS2 loading, which hinders photoelectron transfer and thus negatively affects photoelectrocatalytic performance.

[0054] The photoelectrochemical performance of the prepared FeS2 / TiO2 photoelectrode was tested as follows: photoelectrochemical characterization was performed by transient photoresponse; the light absorption performance of the FeS2 / TiO2 photoelectrode was demonstrated by UV-Vis diffuse reflectance absorption spectroscopy; the photoelectric conversion kinetics of the FeS2 / TiO2 photoelectrode was demonstrated by impedance spectroscopy; the semiconductor characteristics of the FeS2 / TiO2 photoelectrode were demonstrated by Modulator-Schottky curve; and the performance of the FeS2 / TiO2 photoelectrode in the photoelectrocatalytic reduction of CO2 to ethanol was demonstrated by comparing it with the photoelectrochemical performance of TiO2. The transient photoresponse, impedance spectroscopy, and Modulator-Schottky curve were all tested in a 0.1 mol Na2SO4 aqueous solution using a three-electrode system under full-spectrum conditions.

[0055] Example 2

[0056] This embodiment provides a method for preparing a high specific surface area composite double-layer three-dimensional pn heterojunction FeS2 / TiO2 photoelectrode, which specifically includes the following steps:

[0057] S1. Dissolve 1g of ammonium fluoride and 8.75ml of deionized water in 1000ml of ethylene glycol solution and stir for 4h to obtain an anodic oxidation electrolyte; select 100-mesh titanium mesh as substrate, treat it with polishing solution and place it in the anodic oxidation electrolyte as anode and copper sheet as cathode, anodize at 60V for 2h, and then heat treat it in a tube furnace at 500℃ for 3h to obtain TiO2 photoelectrode;

[0058] S2. Glycerol and FeCl3 were added sequentially to 50 ml of deionized water. The solubility of glycerol was 0.4 vol%, and the concentration of FeCl3 was 0.1 mol / L. The mixture was stirred continuously for 12 h to obtain a mixed solution of FeCl3 and glycerol.

[0059] S3. Add TiO2 nanotubes as the working electrode to the above mixture. Using a CHI760e electrochemical workstation, assemble a three-electrode deposition system with a platinum sheet as the counter electrode and a saturated calomel electrode as the reference electrode. Apply an external voltage of 3V for 15 minutes to deposit Fe. 3+ Fe was loaded onto the interior and surface of TiO2 nanotubes to obtain Fe 3+ / TiO2;

[0060] S4, Fe 3+ / TiO2 was placed in a muffle furnace and heat-treated at 300℃ for 4 hours, then cooled to obtain Fe2O3 / TiO2 precursor;

[0061] S5. The Fe2O3 / TiO2 precursor was placed in a vacuum tube furnace and heated to 600℃ for 10h under a 1g sublimated sulfur atmosphere to prepare FeS2 / TiO2 photoelectrocatalytic material.

[0062] Figure 8 (a) shows the UV-Vis diffuse reflectance absorption spectra of the TiO2 photoelectrode and FeS2 / TiO2 photoelectrode prepared in Example 2. The TiO2 photoelectrode shows weak absorption in the visible region, but the absorption spectrum begins to rise sharply in the UV band below 380 nm, indicating the emergence of intrinsic light absorption by TiO2. This result confirms that TiO2 has strong absorption in the UV region but weak absorption in the visible region. The FeS2 / TiO2 photoelectrode, after being combined with FeS2, shows significant light absorption in the near-infrared region at 900 nm, indicating that the light absorption in the 380-900 nm band is mainly completed by FeS2. In the light absorption curve, the change in the linear absorption region reflects that the thin film possesses the intrinsic semiconductor light absorption characteristics of conduction band transitions of valence band electrons after absorbing a certain amount of photon energy. The band gap (…) can be obtained by calculating the linear absorption region using the Tauc function in this invention. Figure 8 (b) The calculated band gaps for TiO2 and FeS2 / TiO2 are 3.15 eV and 2.64 eV, respectively. It can be seen that after forming a composite semiconductor with FeS2, the band gap of the wide-bandgap TiO2 semiconductor is reduced, the light absorption performance is greatly improved, and the light absorption of the semiconductor in the visible light region is increased.

[0063] Figure 9 This is a TEM image of FeS2 / TiO2 prepared in Example 2 after 15 min. From... Figure 9 The three-dimensional nanotube structure can be clearly seen in (a). After high magnification, it can be seen that... Figure 9 (b) shows clear and continuous lattice fringes. Measurements revealed that the lattice fringes in the image are 0.352 nm, 0.325 nm, and 0.163 nm, corresponding to the (101) crystal plane of the anatase phase, the (110) crystal plane of the rutile phase, and the (311) crystal plane of pyrite FeS2, respectively. Mapping scans of the nanotube structure observed under TEM clearly showed FeS2 particles loaded on the surface and inside the TiO2 nanotubes. The elemental analysis revealed Ti, O, Fe, and S, with no other elements present. This further confirms that we have successfully prepared a pure FeS2 / TiO2 catalyst.

[0064] Figure 10 This is the X-ray photoelectron spectrum of FeS2 / TiO2-15min prepared in Example 2, revealing the surface composition of FeS2 / TiO2-15min nanotubes and the oxidation states of iron and sulfur in FeS2. All binding energies in the XPS analysis are referenced to the Cls binding energy of 284 eV. The XPS measured spectra can be seen at [insert image here]. Figure 10This confirmed the presence of Ti, O, Fe, and S elements in the FeS2 / TiO2 sample, which is completely consistent with the EDS results. Ti2p 3 / 2 and Ti2p 1 / 2 Located at 458.68 eV and 464.43 eV respectively, this indicates that the dominant state of Ti is Ti. 4+ The O1s peaks at 529.99 eV and 531.77 eV in the TiO2 sample can be attributed to the Ti-O peak and iron oxide, respectively. Comparison with the standard binding energy reveals a positive shift in the binding energies of both Ti and O after FeS2 deposition, indicating a significant electron transfer between TiO2 and FeS2. The XPS curve for Fe2p shows Fe2p at 706.50 eV. 3 / 2 Fe2p1 at 719.27 eV / 2 Simultaneously, Fe-surf-I, Fe-surf-II, and Fe-O peaks appeared, mainly because the sample is prone to oxidation when exposed to air. S2p 3 / 2 and S2p 1 / 2 The values ​​were located at 162.48 eV and 163.68 eV, respectively, and the binding energy peaks belonging to sulfur vacancies and sulfates were detected simultaneously.

[0065] Figure 11 The image shows the Mott-Schottky curves of the FeS2 (a), TiO2 (b), and FeS2 / TiO2 photoelectrodes (c) in Example 2. Figure 11 (a) The slope of the tangent line of the model Schottky curve of FeS2 is negative, indicating that FeS2 is a p-type semiconductor; Figure 11 (b) The slope of the tangent line of the model Schottky curve of TiO2 is positive, indicating that TiO2 is an n-type semiconductor, and it can be determined that the obtained TiO2 is an n-type semiconductor; Figure 11 (c) The slope of the tangent line of the model Schottky curve of the FeS2 / TiO2 photoelectrode shows both positive and negative values, indicating that the present invention has successfully synthesized a pn-type heterojunction FeS2 / TiO2 photoelectrocatalytic material with an inverted U-shaped Mott-Schottky curve.

[0066] The above test results demonstrate that the FeS2 / TiO2 photoelectrode is a pn-type heterojunction photoelectrocatalytic material with suitable valence and conduction band positions, and that it can absorb both visible and ultraviolet light. We then investigated the photoelectrocatalytic reduction performance of FeS2 / TiO2-15min at different voltages for 5h, with ethanol as the product. Figure 12 (a) shows that the reduction efficiency of CO2 is highest when the applied bias voltage is -0.7V, and the yield can reach 1137 μmol / L / cm after 3.5 h of reaction. 2 .

[0067] Based on the above experiments, we further investigated the mechanism of photoelectrocatalytic reduction of carbon dioxide, such as... Figure 12 As shown in (b), the valence band of FeS2 / TiO2 is located at -0.67 eV, which is more negative than the reduction potential of CO2 / ethanol (-0.30 V), and the conduction band is located at 1.97 eV, which is more positive than the oxidation potential of O2 / H2O (0.82 V). From a thermodynamic perspective, the composite semiconductor material FeS2 / TiO2 can be excited by visible light and can catalytically reduce carbon dioxide to ethanol. To explore the electron transport pathway in the composite semiconductor material, the band structure of FeS2 and TiO2 is also shown in the figure. Due to the energy level difference, the band edges of FeS2 and TiO2 are stepped. When FeS2 is photoexcited, photogenerated electrons jump to the conduction band and are rapidly injected into the conduction band of TiO2 and migrate to the electrode surface along the one-dimensional path of the nanotube. During this process, the photogenerated electrons are captured by hydrogen ions adsorbed on the electrode surface to generate active hydrogen atoms. The active hydrogen atoms then undergo an in-situ hydrogenation reduction reaction with carbon dioxide molecules near the electrode surface to generate ethanol. At the same time, a water oxidation decomposition reaction occurs on the surface of the FeS2 / TiO2 photoanode, releasing oxygen and generating a large amount of H2O. + Under the influence of applied voltage and photovoltage, hydrogen ions continuously migrate toward the photocathode and are adsorbed onto the electrode surface, where they continue to undergo in-situ reduction reactions with carbon dioxide.

[0068] Those skilled in the art should understand that the above descriptions are merely several specific embodiments of the present invention, and not all embodiments. It should be noted that many modifications and improvements can be made by those skilled in the art, and all modifications or improvements not exceeding the scope of the claims should be considered within the protection scope of the present invention.

Claims

1. The application of a three-dimensional pn heterojunction photoelectrocatalytic material in the photoelectrocatalytic reduction of CO2 to ethanol, characterized in that, The preparation method of the three-dimensional pn heterojunction photoelectrocatalytic material includes the following steps: S1. Using deionized water as a solvent, glycerol and FeCl3 were added sequentially, and the mixture was stirred continuously to obtain a mixture of FeCl3 and glycerol. S2. Using a three-electrode system, titanium dioxide nanotubes were added to the above mixture as the working electrode. A working voltage of -3 to 2V was applied, and electrodeposition was performed for 5 to 25 minutes to obtain Fe. 3+ The preparation method of / TiO2;titanium dioxide nanotubes is as follows: using a 100-mesh titanium mesh as a substrate, after being treated with polishing solution, it is placed in an anodic oxidation electrolyte and oxidized at a voltage of 40-60V for 0.5-4 h, and then placed in a tube furnace at 500-600 ℃ for heat treatment for 1-4 h. S3. Put Fe 3+ The Fe2O3 / TiO2 precursor was obtained by heat-treating the TiO2 in a muffle furnace at 300-500 ℃ for 1-4 h and then cooling it. S4. The Fe2O3 / TiO2 precursor was placed in a vacuum tube furnace and heated to 500-600 ℃ under a sublimation sulfur atmosphere to obtain a three-dimensional pn heterojunction photoelectrocatalytic material.

2. The application of the three-dimensional pn heterojunction photoelectrocatalytic material according to claim 1 in the photoelectrocatalytic reduction of CO2 to produce ethanol, characterized in that, The concentration of glycerol in the mixture of step S1 is 0.4-0.7 vol%, and the concentration of FeCl3 is 0.1-0.4 mol / L.

3. The application of the three-dimensional pn heterojunction photoelectrocatalytic material according to claim 1 in the photoelectrocatalytic reduction of CO2 to produce ethanol, characterized in that, The counter electrode of the three-electrode system is a platinum sheet, and the reference electrode is a saturated calomel electrode.

4. The application of the three-dimensional pn heterojunction photoelectrocatalytic material according to claim 3 in the photoelectrocatalytic reduction of CO2 to produce ethanol, characterized in that, In step S4, the Fe2O3 / TiO2 precursor is placed in a vacuum tube furnace and heat-treated in a sublimed sulfur atmosphere of 0.3-3 g.

5. The application of the three-dimensional pn heterojunction photoelectrocatalytic material according to claim 1 or 4 in the photoelectrocatalytic reduction of CO2 to produce ethanol, characterized in that, The heat treatment time in step S4 is 10-20 h.

6. The application of the three-dimensional pn heterojunction photoelectrocatalytic material according to claim 1 in the photoelectrocatalytic reduction of CO2 to produce ethanol, characterized in that, The polishing solution is an acidic diluted solution, which is made by mixing deionized water, nitric acid and hydrofluoric acid in a volume ratio of 5:4:

1.

7. The application of the three-dimensional pn heterojunction photoelectrocatalytic material according to claim 1 in the photoelectrocatalytic reduction of CO2 to produce ethanol, characterized in that, The preparation method of the anodic oxidation electrolyte is as follows: 8.75-17.5 ml of deionized water and 1-4 g of ammonium fluoride are added to 1000 ml of ethylene glycol and stirred for 1-5 h.

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