A two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals

Abstract

The application relates to the technical field of semiconductor photocatalysis and functional nanomaterials, and discloses a two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, which comprises the following steps: preparation of a precursor solution, addition of a complexing agent, protonation reaction, hydrothermal treatment, product collection and washing, drying treatment and calcination treatment; the two-step hydrothermal method uses concentrated sulfuric acid as a protonation reaction medium, avoids the introduction of an organic carbon additive, realizes accurate control of crystal nucleation and growth kinetics in the hydrothermal reaction process, promotes the formation of an orthorhombic structure, and obtains WO3 crystals with a cubic morphology, pure crystal form and inhibited surface defects; meanwhile, the addition of citric acid as a complexing agent can stabilize tungsten species and ensure uniform distribution of the precursor, and improves the integrity and uniformity of the crystal structure.

Description

Technical Field

[0001] This invention relates to the field of semiconductor photocatalysis and functional nanomaterials technology, specifically a two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals. Background Technology

[0002] Tungsten trioxide (WO3), as an important n-type semiconductor material, has shown great application potential in the field of photocatalysis, especially in the photocatalytic reduction of carbon dioxide (CO2). Its catalytic performance depends on the crystal structure, morphological purity, and surface state of the material. Orthorhombic WO3 is considered an ideal crystal form for realizing photocatalytic reactions due to its suitable band structure and chemical stability. The regular nanocubic morphology helps to expose more active crystal faces and provide shorter carrier migration paths, thereby improving the separation and utilization efficiency of photoactive carriers.

[0003] Currently, in the synthesis of orthorhombic WO3 crystals, the traditional hydrothermal method generally uses hydrochloric acid (HCl) as a protonating agent and introduces glucose as an organic carbon source as a morphology modifier. This method makes it difficult to accurately control the nucleation and growth kinetics of the crystal during the reaction, which easily leads to problems such as impure crystal form, irregular cubic morphology, and numerous surface defects in the product, thus affecting the photogenerated carrier separation efficiency of the material. At the same time, the introduction of organic additives will leave carbon impurities after calcination. These residual carbons will cover the active sites and become recombination centers for electron-hole pairs, reducing the photocatalytic reduction activity of WO3 crystals under visible light irradiation of carbon dioxide (CO2), resulting in unstable and generally low yields of carbon monoxide (CO) and methane (CH4).

[0004] Therefore, a two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals is proposed to solve the above problems. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, which solves the problems of unstable and generally low yields of carbon monoxide (CO) and methane (CH4) mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, comprising the following steps: Step 1: Preparation of precursor solution: Dissolve sodium tungstate dihydrate in deionized water and stir to form a transparent and clear solution; Step 2: Adding a complexing agent: Add citric acid to the aforementioned solution and stir to mix, thus obtaining a tungstic acid precursor solution; Step 3: Protonation reaction. Concentrated sulfuric acid is added dropwise to the aforementioned tungstic acid precursor solution, and the mixture is stirred to carry out the protonation reaction. Step 4: Hydrothermal treatment, subjecting the solution after the protonation reaction to a hydrothermal reaction; Step 5: Product collection and washing. The product after hydrothermal reaction is subjected to solid-liquid separation and washed alternately with deionized water and ethanol until the pH of the washing solution is 6.5-7.5. Step 6: Drying treatment. The washed solid product is vacuum dried to obtain precursor powder. Step 7: Calcination treatment, the precursor powder is calcined to obtain the orthorhombic WO3 crystal; The method does not involve the addition of any organic carbon additives.

[0007] Preferably, the preparation of the precursor solution in step one specifically includes the following operations: The amount of sodium tungstate dihydrate Na2WO4·2H2O used is 0.33-0.40g, the amount of deionized water used is 40mL, and the mass-volume ratio of sodium tungstate dihydrate Na2WO4·2H2O to deionized water is 0.33-0.40g:40mL. The dissolution process is carried out at room temperature (20-25℃) with a stirring speed of 100-300 r / min and a stirring time of 10-30 minutes.

[0008] Preferably, the addition of the complexing agent in step two specifically includes the following operations: The amount of citric acid (C6H8O7) used is 0.28-0.30 g, and the molar ratio of citric acid to sodium tungstate dihydrate is 1.4-1.6:1. After adding citric acid, the stirring speed should be 100-300 rpm, and the stirring time should be 10-30 minutes.

[0009] Preferably, the protonation reaction in step three specifically includes the following operations: The amount of concentrated sulfuric acid (H₂SO₄) used is 2-4 mL, and the concentration of concentrated sulfuric acid is 95%-98%. The dropping rate is 0.5-1.0 mL / min, and the stirring speed is maintained at 100-300 r / min during the dropping process; After the addition is complete, continue stirring for 10-30 minutes.

[0010] Preferably, the hydrothermal treatment in step four specifically includes the following operations: The hydrothermal reaction is carried out in a high-pressure stainless steel reactor lined with polytetrafluoroethylene, with a reactor volume of 50-100 mL. The hydrothermal temperature is set to 100-150℃, the hydrothermal time is set to 20-28 hours, and the pressure naturally rises to 0.1-0.5MPa during the reaction.

[0011] Preferably, the product collection and washing in step five specifically includes the following operations: Solid-liquid separation is performed using a centrifuge at a speed of 3000-5000 r / min for 5-10 minutes. Washing is performed alternately with deionized water and ethanol. Each time, 10-20 mL of deionized water and 10-20 mL of ethanol are used. The washing is repeated 3-5 times until the pH value of the washing solution reaches 6.5-7.5.

[0012] Preferably, the drying process in step six specifically includes the following operations: Vacuum drying is performed in a vacuum drying oven at a vacuum degree of 1×10⁻⁶. -1 -1×10 -3 Pa, drying temperature is 50-70℃, drying time is 10-20 hours.

[0013] Preferably, the calcination treatment in step seven specifically includes the following operations: The calcination is carried out in a muffle furnace at a temperature of 450-550℃ for 1-3 hours, with a heating rate of 1-3℃ / min. The calcination atmosphere is static air with an air flow rate of 0.5-1.0 L / min.

[0014] Preferably, the equipment used in the method includes: The stirring operation in step one is completed by a magnetic stirrer, which is equipped with a stirring bar with a length of 2-3 cm. The centrifugal separation operation in step five is completed using a benchtop high-speed centrifuge. Steps six and seven are performed in a vacuum drying oven and a muffle furnace, respectively.

[0015] Preferably, the crystal includes the following characteristics: Structural and morphological characteristics: The crystal is a monodisperse cubic morphology with an average edge length of 100-110 nanometers and a standard deviation of edge length distribution of no more than 5 nanometers. Elemental and valence characteristics: The crystal is composed of tungsten and oxygen, with an atomic ratio of tungsten to oxygen of 1:2.9 to 1:3.1, and tungsten is predominantly W. 6+ Price states exist; Crystal structure characteristics: In the X-ray diffraction pattern of the crystal, the diffraction peaks corresponding to the (002), (020), and (200) crystal planes appear at 2θ of 23.1°±0.2°, 23.6°±0.2°, and 24.3°±0.2°, respectively, and the full width at half maximum (FWHM) of the (002) crystal plane diffraction peak is less than 0.15°.

[0016] Compared with the prior art, the present invention provides a two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, which has the following beneficial effects: 1. In this invention, the two-step hydrothermal method uses concentrated sulfuric acid as the protonation reaction medium, avoiding the introduction of organic carbon additives. It achieves accurate control of crystal nucleation and growth kinetics during the hydrothermal reaction, promotes the formation of orthorhombic crystal structure, and obtains WO3 crystals with cubic morphology, pure crystal form, and suppressed surface defects. At the same time, the addition of citric acid as a complexing agent can stabilize tungsten species and ensure uniform distribution of precursors, thereby improving the integrity and uniformity of the crystal structure.

[0017] 2. In the preparation stage of the precursor solution, by controlling the ratio and dissolution conditions of sodium tungstate dihydrate and deionized water, a uniform and stable solution basis is provided for the subsequent reaction. In the product collection and washing stage, deionized water and ethanol are used alternately for washing until the washing solution is neutral, which can thoroughly remove residual ions and impurities and reduce crystal defects caused by impurities. In the calcination process, by accurately controlling the heating rate and temperature, the orthorhombic crystal structure is consolidated, internal stress is eliminated, and a final product with crystal quality and stable physicochemical properties is obtained.

[0018] 3. In this invention, the method ensures the uniformity and controllability of the average edge length and edge size distribution of the orthorhombic WO3 crystal through the optimization and synergistic control of parameters throughout the entire process, including precursor solution preparation, complexing agent addition, protonation reaction, hydrothermal treatment, product collection and washing, drying, and calcination. This results in excellent charge separation efficiency and reaction stability in photocatalytic applications. The prepared crystal contains only tungsten and oxygen in its elemental composition, with tungsten predominating in W. 6+ The stable valence state exists, and the crystallographic parameters meet the characteristic requirements of the (002), (020), and (200) crystal planes in the X-ray diffraction pattern, which essentially ensures the high activity and durability of the material in the visible light-driven CO2 reduction reaction. Attached Figure Description

[0019] Figure 1 Transmission electron microscope image of WO3 crystals prepared by hydrothermal-calcination method; Figure 2 This is an elemental distribution diagram of WO3 crystals; Figure 3 The XRD pattern of WO3 crystal; Figure 4 A schematic diagram of photocatalytic CO2 reduction using cubic WO3 crystals. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Please see Figures 1-4 The specific implementation of a two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals is as follows: Example 1: A two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, comprising the following steps: Step 1: Preparation of precursor solution: Dissolve sodium tungstate dihydrate in deionized water and stir to form a transparent and clear solution; Step 2: Adding a complexing agent: Add citric acid to the aforementioned solution and stir to mix, thus obtaining a tungstic acid precursor solution; Step 3: Protonation reaction. Concentrated sulfuric acid is added dropwise to the aforementioned tungstic acid precursor solution, and the mixture is stirred to carry out the protonation reaction. Step 4: Hydrothermal treatment, subjecting the protonated solution to a hydrothermal reaction; Step 5: Product collection and washing. The product after hydrothermal reaction is subjected to solid-liquid separation and washed alternately with deionized water and ethanol until the pH of the washing solution is 6.5. Step 6: Drying treatment. The washed solid product is vacuum dried to obtain precursor powder. Step 7: Calcination treatment, the precursor powder is calcined to obtain orthorhombic WO3 crystals; No organic carbon additives are added in the method.

[0022] The preparation of the precursor solution in step one specifically includes the following operations: The amount of sodium tungstate dihydrate Na2WO4·2H2O used is 0.33g, the amount of deionized water used is 40mL, and the mass-volume ratio of sodium tungstate dihydrate Na2WO4·2H2O to deionized water is 0.33g:40mL. The dissolution process was carried out at room temperature (20-25℃) with a stirring speed of 100 r / min and a stirring time of 10 minutes.

[0023] Step two, the addition of the complexing agent, specifically includes the following operations: The amount of citric acid C6H8O7 used was 0.28g, and the molar ratio of citric acid to sodium tungstate dihydrate was 1.4:1; After adding citric acid, the stirring speed was 100 rpm and the stirring time was 10 minutes.

[0024] Step three, the protonation reaction, specifically includes the following operations: The amount of concentrated sulfuric acid (H₂SO₄) used is 2 mL, and the concentration of concentrated sulfuric acid is 95%. The dropping rate was 0.5 mL / min, and the stirring speed was maintained at 100 r / min during the dropping process; After the addition is complete, continue stirring for 10 minutes.

[0025] Step four, the hydrothermal treatment, specifically includes the following operations: The hydrothermal reaction was carried out in a stainless steel high-pressure reactor lined with polytetrafluoroethylene, with a reactor volume of 50 mL. The hydrothermal temperature was set to 100℃, the hydrothermal time was set to 20 hours, and the pressure naturally rose to 0.1MPa during the reaction.

[0026] Step five, product collection and washing, specifically includes the following operations: Solid-liquid separation was performed using a centrifuge at a speed of 3000 r / min for 5 minutes. Washing is performed using deionized water and ethanol alternately, with 10 mL of deionized water and 10 mL of ethanol used each time, for a total of 3 washes until the pH of the washing solution reaches 6.5.

[0027] Step six, the drying process, specifically includes the following operations: Vacuum drying is carried out in a vacuum drying oven at a vacuum degree of 1×10⁻⁶. -1 Pa, drying temperature is 50℃, drying time is 10 hours.

[0028] Step seven, the calcination process, specifically includes the following operations: The calcination was carried out in a muffle furnace at a temperature of 450°C for 1 hour, with a heating rate of 1°C / min. The calcination atmosphere is static air with an air flow rate of 0.5 L / min.

[0029] The equipment used in the method includes: The stirring operation in step one is completed using a magnetic stirrer, which is equipped with a 2cm long stirring bar; The centrifugal separation operation in step five is completed using a benchtop high-speed centrifuge; Steps six and seven are completed in a vacuum drying oven and a muffle furnace, respectively.

[0030] Crystals have the following characteristics: Structural and morphological characteristics: The crystals exhibit a monodisperse cubic morphology with an average edge length of 100 nm and a standard deviation of no more than 5 nm in edge length distribution. The obtained crystals have a uniform morphology and distinct cubic characteristics. Their transmission electron microscope images are shown below. Figure 1As shown; Elemental and valence characteristics: The crystal is composed of tungsten and oxygen, with an atomic ratio of tungsten to oxygen of 1:2.9, and tungsten is predominantly W. 6+ Valence states exist, and their elemental distribution is as follows: Figure 2 As shown, it is confirmed that it contains only tungsten and oxygen elements and that they are evenly distributed. Crystal structure characteristics: In the X-ray diffraction pattern of the crystal, the diffraction peaks corresponding to the (002), (020), and (200) crystal planes appear at 2θ of 23.1°, 23.6°, and 24.3°, respectively, and the full width at half maximum (FWHM) of the (002) crystal plane diffraction peak is less than 0.10°. The X-ray diffraction pattern is as follows: Figure 3 As shown, the diffraction peaks match well with the orthorhombic WO3 standard card, confirming high crystallinity and phase purity. A schematic diagram illustrating the photocatalytic CO2 reduction achieved by this crystal is shown below. Figure 4 As shown.

[0031] Example 2: A two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, comprising the following steps: Step 1: Preparation of precursor solution: Dissolve sodium tungstate dihydrate in deionized water and stir to form a transparent and clear solution; Step 2: Adding a complexing agent: Add citric acid to the aforementioned solution and stir to mix, thus obtaining a tungstic acid precursor solution; Step 3: Protonation reaction. Concentrated sulfuric acid is added dropwise to the aforementioned tungstic acid precursor solution, and the mixture is stirred to carry out the protonation reaction. Step 4: Hydrothermal treatment, subjecting the protonated solution to a hydrothermal reaction; Step 5: Product collection and washing. The product after hydrothermal reaction is subjected to solid-liquid separation and washed alternately with deionized water and ethanol until the pH of the washing solution is 6.8. Step 6: Drying treatment. The washed solid product is vacuum dried to obtain precursor powder. Step 7: Calcination treatment, the precursor powder is calcined to obtain orthorhombic WO3 crystals; No organic carbon additives are added in the method.

[0032] The preparation of the precursor solution in step one specifically includes the following operations: The amount of sodium tungstate dihydrate Na2WO4·2H2O used is 0.36g, the amount of deionized water used is 40mL, and the mass-volume ratio of sodium tungstate dihydrate Na2WO4·2H2O to deionized water is 0.38g:40mL. The dissolution process was carried out at room temperature (23°C) with a stirring speed of 200 r / min for 20 minutes.

[0033] Step two, the addition of the complexing agent, specifically includes the following operations: The amount of citric acid C6H8O7 used was 0.29g, and the molar ratio of citric acid to sodium tungstate dihydrate was 1.5:1; After adding citric acid, the stirring speed was 260 r / min and the stirring time was 20 minutes.

[0034] Step three, the protonation reaction, specifically includes the following operations: The amount of concentrated sulfuric acid (H₂SO₄) used is 3 mL, and the concentration of concentrated sulfuric acid is 96.5%. The dropping rate was 0.8 mL / min, and the stirring speed was maintained at 270 r / min during the dropping process; After the addition is complete, continue stirring for 25 minutes.

[0035] Step four, the hydrothermal treatment, specifically includes the following operations: The hydrothermal reaction was carried out in a high-pressure reactor lined with polytetrafluoroethylene and with a volume of 68 mL. The hydrothermal temperature was set to 130℃, the hydrothermal time was set to 25 hours, and the pressure naturally rose to 0.3MPa during the reaction.

[0036] Step five, product collection and washing, specifically includes the following operations: Solid-liquid separation was performed using a centrifuge at a speed of 4000 r / min for 8 minutes. Washing was performed using deionized water and ethanol alternately, with 15 mL of deionized water and 17 mL of ethanol used each time. The washing was repeated 4 times until the pH of the washing solution reached 7.0.

[0037] Step six, the drying process, specifically includes the following operations: Vacuum drying is carried out in a vacuum drying oven at a vacuum degree of 1×10⁻⁶. -2 Pa, drying temperature is 60℃, drying time is 15 hours.

[0038] Step seven, the calcination process, specifically includes the following operations: The calcination was carried out in a muffle furnace at a temperature of 500℃ for 2 hours, with a heating rate of 2.5℃ / min. The calcination atmosphere is static air with an air flow rate of 0.8 L / min.

[0039] The equipment used in the method includes: The stirring operation in step one is completed using a magnetic stirrer, which is equipped with a stirring bar with a length of 2.7cm. The centrifugal separation operation in step five is completed using a benchtop high-speed centrifuge; Steps six and seven are completed in a vacuum drying oven and a muffle furnace, respectively.

[0040] Crystals have the following characteristics: Structural and morphological characteristics: The crystals are monodisperse cubic in shape, with an average edge length of 106 nm and a standard deviation of edge length distribution of no more than 5 nm. The obtained crystal morphology is as follows: Figure 1 As shown, it exhibits uniform cubic characteristics; Elemental and valence characteristics: The crystal is composed of tungsten and oxygen, with an atomic ratio of tungsten to oxygen of 1:3.0, and tungsten is predominantly W. 6+ Valence states exist, and element distribution analysis is as follows: Figure 2 As shown, it is confirmed that it consists only of tungsten and oxygen elements and that they are evenly distributed. Crystal structure characteristics: In the X-ray diffraction pattern of the crystal, the diffraction peaks corresponding to the (002), (020), and (200) crystal planes appear at 2θ of 23.1°, 23.6°, and 24.3°, respectively, and the full width at half maximum (FWHM) of the (002) crystal plane diffraction peak is less than 0.12°. Its XRD pattern is as follows. Figure 3 As shown, the spectrum matches the standard spectrum of orthorhombic WO3, indicating high phase purity. This material can be applied to the photocatalytic CO2 reduction process as follows: Figure 4 As shown in the diagram.

[0041] Example 3: A two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, comprising the following steps: Step 1: Preparation of precursor solution: Dissolve sodium tungstate dihydrate in deionized water and stir to form a transparent and clear solution; Step 2: Adding a complexing agent: Add citric acid to the aforementioned solution and stir to mix, thus obtaining a tungstic acid precursor solution; Step 3: Protonation reaction. Concentrated sulfuric acid is added dropwise to the aforementioned tungstic acid precursor solution, and the mixture is stirred to carry out the protonation reaction. Step 4: Hydrothermal treatment, subjecting the protonated solution to a hydrothermal reaction; Step 5: Product collection and washing. The product after hydrothermal reaction is subjected to solid-liquid separation and washed alternately with deionized water and ethanol until the pH of the washing solution is 7.5. Step 6: Drying treatment. The washed solid product is vacuum dried to obtain precursor powder. Step 7: Calcination treatment, the precursor powder is calcined to obtain orthorhombic WO3 crystals; No organic carbon additives are added in the method.

[0042] The preparation of the precursor solution in step one specifically includes the following operations: The amount of sodium tungstate dihydrate Na2WO4·2H2O used is 0.40g, the amount of deionized water used is 40mL, and the mass-volume ratio of sodium tungstate dihydrate Na2WO4·2H2O to deionized water is 0.40g:40mL. The dissolution process was carried out at room temperature (25°C) with a stirring speed of 300 r / min for 30 minutes.

[0043] Step two, the addition of the complexing agent, specifically includes the following operations: The amount of citric acid C6H8O7 used is 0.30g, and the molar ratio of citric acid to sodium tungstate dihydrate is 1.6:1; After adding citric acid, the stirring speed was 300 rpm and the stirring time was 30 minutes.

[0044] Step three, the protonation reaction, specifically includes the following operations: The amount of concentrated sulfuric acid (H₂SO₄) used is 4 mL, and the concentration of concentrated sulfuric acid is 98%. The dropping rate was 1.0 mL / min, and the stirring speed was maintained at 300 r / min during the dropping process; After the addition is complete, continue stirring for 30 minutes.

[0045] Step four, the hydrothermal treatment, specifically includes the following operations: The hydrothermal reaction was carried out in a stainless steel high-pressure reactor lined with polytetrafluoroethylene, with a reactor volume of 100 mL. The hydrothermal temperature was set to 150℃, the hydrothermal time was set to 28 hours, and the pressure naturally rose to 0.5MPa during the reaction.

[0046] Step five, product collection and washing, specifically includes the following operations: Solid-liquid separation was performed using a centrifuge at a speed of 5000 r / min for 10 minutes. Washing is performed using deionized water and ethanol alternately, with 20 mL of deionized water and 20 mL of ethanol used each time. The washing is repeated 5 times until the pH of the washing solution reaches 7.5.

[0047] Step six, the drying process, specifically includes the following operations: Vacuum drying is carried out in a vacuum drying oven at a vacuum degree of 1×10⁻⁶. -3 Pa, drying temperature is 70℃, drying time is 20 hours.

[0048] Step seven, the calcination process, specifically includes the following operations: The calcination was carried out in a muffle furnace at a temperature of 550°C for 3 hours, with a heating rate of 3°C / min. The calcination atmosphere is static air with an air flow rate of 1.0 L / min.

[0049] The equipment used in the method includes: The stirring operation in step one is completed using a magnetic stirrer, which is equipped with a stirring bar with a length of 3cm. The centrifugal separation operation in step five is completed using a benchtop high-speed centrifuge; Steps six and seven are completed in a vacuum drying oven and a muffle furnace, respectively.

[0050] Crystals have the following characteristics: Structural and morphological characteristics: The crystals are monodisperse cubic in shape, with an average edge length of 110 nm and a standard deviation of edge length distribution of no more than 5 nm. Typical morphologies of the synthesized crystals are as follows: Figure 1 The transmission electron microscope image shows a uniform cube; Elemental and valence characteristics: The crystal is composed of tungsten and oxygen, with an atomic ratio of tungsten to oxygen of 1:3.1, and tungsten is predominantly W. 6+ Valence states exist, and their elemental distribution is as follows: Figure 2 As shown, tungsten and oxygen elements are evenly distributed; Crystal structure characteristics: In the X-ray diffraction pattern of the crystal, the diffraction peaks corresponding to the (002), (020), and (200) crystal planes appear at 2θ of 23.1°, 23.6°, and 24.3°, respectively, and the full width at half maximum (FWHM) of the (002) crystal plane diffraction peak is less than 0.14°. The X-ray diffraction analysis results are as follows: Figure 3 As shown, the product is confirmed to be high-purity orthorhombic WO3, and its application principle of photocatalytic CO2 reduction is illustrated in the diagram below. Figure 4 As shown.

[0051] Comparative Example 1 differs from Example 1 in that hydrochloric acid (HCl) is used instead of concentrated sulfuric acid (H2SO4) in the protonation reaction step.

[0052] Comparative Example 2 differs from Example 1 in that glucose was added as an organic carbon additive during the preparation of the precursor solution in this comparative example.

[0053] Comparative Example 3 differs from Example 1 in that citric acid (C6H8O7) was not added in the complexing agent addition step of this comparative example.

[0054] Comparative Example 4 differs from Example 1 in that: in the product collection and washing process, only deionized water was used for washing, and ethanol was not used for alternating washing.

[0055] The orthorhombic WO3 crystals prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to performance tests. The test items and test methods are as follows: X-ray diffraction analysis was performed using an X-ray diffractometer with a CuKα radiation source, a tube voltage of 40 kV, and a tube current of 40 mA. The scan rate was 2° / min and the range was 10° to 80°. The obtained spectra were compared with the orthorhombic WO3 standard card, and the full width at half maximum (FWHM) of the diffraction peaks on the crystal planes was calculated using software. Scanning electron microscopy morphology analysis: The sample was dispersed in ethanol and sonicated, then dropped onto the surface of a silicon wafer. After gold sputtering, the crystal morphology was observed using a field emission scanning electron microscope at an accelerating voltage of 5.0 kV. At least 100 crystals were randomly selected to measure their edge lengths and calculate the average value and standard deviation. The photocatalytic CO2 reduction performance was tested by uniformly dispersing 50 mg of sample in 10 mL of deionized water in a closed quartz reactor with a volume of 50 mL. After high-purity CO2 gas was introduced to purge the air and reach adsorption equilibrium, a 300 W xenon lamp equipped with a 420 nm cutoff filter was used as a visible light source for irradiation. After 1 hour of reaction, the amount of photocatalytic CO and CH4 generated in the gas at the top of the reactor was analyzed by gas chromatography equipped with TCD and FID detectors, and the yield was calculated. X-ray photoelectron spectroscopy analysis was performed using an X-ray photoelectron spectrometer with monochromatic AlKα rays as the excitation source, under a vacuum better than 5 × 10⁻⁶ ppm in the analysis chamber. -8 Full-spectrum and narrow-spectrum scans were performed under mBar conditions. The binding energy was calibrated using the C1s peak of surface contaminant carbon. The fine spectra of W4f and O1s orbitals were analyzed to determine the chemical valence state and relative abundance of elements.

[0056] The test data of the orthorhombic WO3 crystals prepared in Examples 1-3 and Comparative Examples 1-4 are recorded in the table below:

[0057] By comparing and analyzing the data in the table, it can be seen that the orthorhombic WO3 crystals prepared by the process in Examples 1-3 have significantly better performance than the WO3 crystals prepared by the process in Comparative Examples 1-4. This indicates that the two-step hydrothermal method uses concentrated sulfuric acid as the protonation reaction medium, avoiding the introduction of organic carbon additives. It achieves accurate control of crystal nucleation and growth kinetics during the hydrothermal reaction, promotes the formation of orthorhombic crystal structure, and obtains WO3 crystals with cubic morphology, pure crystal form, and suppressed surface defects. At the same time, the addition of citric acid as a complexing agent can stabilize tungsten species and ensure uniform distribution of precursors, thereby improving the integrity and uniformity of the crystal structure. In the precursor solution preparation stage, by controlling the ratio and dissolution conditions of sodium tungstate dihydrate to deionized water, a uniform and stable solution basis is provided for subsequent reactions. In the product collection and washing stage, deionized water and ethanol are used alternately for washing until the washing solution is neutral, which can thoroughly remove residual ions and impurities and reduce crystal defects caused by impurities. During the calcination process, the heating rate and temperature are accurately controlled to consolidate the orthorhombic crystal structure, eliminate internal stress, and obtain a final product with crystal quality and stable physicochemical properties. The method ensures the uniformity and controllability of the average edge length and edge size distribution of the orthorhombic WO3 crystal through the optimization and synergistic control of parameters throughout the entire process of precursor solution preparation, complexing agent addition, protonation reaction, hydrothermal treatment, product collection and washing, drying, and calcination, thus exhibiting excellent charge separation efficiency and reaction stability in photocatalytic applications. The prepared crystal contains only tungsten and oxygen elements in its elemental composition, with tungsten being the dominant element. 6+ The stable valence state exists, and the crystallographic parameters meet the characteristic requirements of the (002), (020), and (200) crystal planes in the X-ray diffraction pattern, which essentially ensures the high activity and durability of the material in the visible light-driven CO2 reduction reaction.

[0058] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0059] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, characterized in that: Includes the following steps: Step 1: Preparation of precursor solution: Dissolve sodium tungstate dihydrate in deionized water and stir to form a transparent and clear solution; Step 2: Adding a complexing agent: Add citric acid to the aforementioned solution and stir to mix, thus obtaining a tungstic acid precursor solution; Step 3: Protonation reaction. Concentrated sulfuric acid is added dropwise to the aforementioned tungstic acid precursor solution, and the mixture is stirred to carry out the protonation reaction. Step 4: Hydrothermal treatment, subjecting the solution after the protonation reaction to a hydrothermal reaction; Step 5: Product collection and washing. The product after hydrothermal reaction is subjected to solid-liquid separation and washed alternately with deionized water and ethanol until the pH of the washing solution is 6.5-7.

5. Step 6: Drying treatment. The washed solid product is vacuum dried to obtain precursor powder. Step 7: Calcination treatment, the precursor powder is calcined to obtain the orthorhombic WO3 crystal; The method does not involve the addition of any organic carbon additives.

2. The method for synthesizing high-purity orthorhombic WO3 crystals using a two-step hydrothermal method according to claim 1, characterized in that: The preparation of the precursor solution in step one specifically includes the following operations: The amount of sodium tungstate dihydrate Na2WO4·2H2O used is 0.33-0.40g, the amount of deionized water used is 40mL, and the mass-volume ratio of sodium tungstate dihydrate to deionized water is 0.33-0.40g:40mL. The dissolution process is carried out at room temperature (20-25℃) with a stirring speed of 100-300 r / min and a stirring time of 10-30 minutes.

3. The method for synthesizing high-purity orthorhombic WO3 crystals using a two-step hydrothermal method according to claim 1, characterized in that: The addition of the complexing agent in step two specifically includes the following operations: The amount of citric acid (C6H8O7) used is 0.28-0.30 g, and the molar ratio of citric acid to sodium tungstate dihydrate is 1.4-1.6:

1. After adding citric acid, the stirring speed should be 100-300 rpm and the stirring time should be 10-30 minutes.

4. The method for synthesizing high-purity orthorhombic WO3 crystals using a two-step hydrothermal method according to claim 1, characterized in that: The protonation reaction in step three specifically includes the following operations: The amount of concentrated sulfuric acid (H₂SO₄) used is 2-4 mL, and the concentration of concentrated sulfuric acid is 95%-98%. The dropping rate is 0.5-1.0 mL / min, and the stirring speed is maintained at 100-300 r / min during the dropping process; After the addition is complete, continue stirring for 10-30 minutes.

5. The method for synthesizing high-purity orthorhombic WO3 crystals using a two-step hydrothermal method according to claim 1, characterized in that: The hydrothermal treatment in step four specifically includes the following operations: The hydrothermal reaction is carried out in a high-pressure stainless steel reactor lined with polytetrafluoroethylene, with a reactor volume of 50-100 mL. The hydrothermal temperature is set to 100-150℃, the hydrothermal time is set to 20-28 hours, and the pressure naturally rises to 0.1-0.5MPa during the reaction.

6. The method for synthesizing high-purity orthorhombic WO3 crystals using a two-step hydrothermal method according to claim 1, characterized in that: Step five, product collection and washing, specifically includes the following operations: Solid-liquid separation is performed using a centrifuge at a speed of 3000-5000 r / min for 5-10 minutes. Washing is performed alternately with deionized water and ethanol. Each time, 10-20 mL of deionized water and 10-20 mL of ethanol are used. The washing is repeated 3-5 times until the pH value of the washing solution reaches 6.5-7.

5.

7. The method for synthesizing high-purity orthorhombic WO3 crystals using a two-step hydrothermal method according to claim 1, characterized in that: The drying process in step six specifically includes the following operations: Vacuum drying is performed in a vacuum drying oven at a vacuum degree of 1×10⁻⁶. -1 -1×10 -3 Pa, drying temperature is 50-70℃, drying time is 10-20 hours.

8. The method for synthesizing high-purity orthorhombic WO3 crystals using a two-step hydrothermal method according to claim 1, characterized in that: The calcination process in step seven specifically includes the following operations: The calcination is carried out in a muffle furnace at a temperature of 450-550℃ for 1-3 hours, with a heating rate of 1-3℃ / min. The calcination atmosphere is static air with an air flow rate of 0.5-1.0 L / min.

9. A method for synthesizing high-purity orthorhombic WO3 crystals using a two-step hydrothermal method according to any one of claims 1-8, characterized in that: The equipment used in the method includes: The stirring operation in step one is completed by a magnetic stirrer, which is equipped with a stirring bar with a length of 2-3 cm. The centrifugal separation operation in step five is completed using a benchtop high-speed centrifuge. Steps six and seven are performed in a vacuum drying oven and a muffle furnace, respectively.

10. A two-step hydrothermal method for synthesizing high-purity orthorhombic WO3 crystals, prepared by any one of the two-step hydrothermal methods for synthesizing high-purity orthorhombic WO3 crystals according to claims 1-9, characterized in that: The crystal has the following characteristics: Structural and morphological characteristics: The crystal is a monodisperse cubic morphology with an average edge length of 100-110 nanometers and a standard deviation of edge length distribution of no more than 5 nanometers. Elemental and valence characteristics: The crystal is composed of tungsten and oxygen, with an atomic ratio of tungsten to oxygen of 1:2.9 to 1:3.1, and tungsten is predominantly W. 6+ Price states exist; Crystal structure characteristics: In the X-ray diffraction pattern of the crystal, the diffraction peaks corresponding to the (002), (020), and (200) crystal planes appear at 2θ of 23.1°±0.2°, 23.6°±0.2°, and 24.3°±0.2°, respectively, and the full width at half maximum (FWHM) of the (002) crystal plane diffraction peak is less than 0.15°.

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