A CdS / TiO2 / CdS semi-inverted quantum well nanosheet, its preparation method and application

Abstract

This invention belongs to the field of photocatalysis technology, and particularly relates to a CdS / TiO2 / CdS semi-inverted quantum well nanosheet, its preparation method, and its applications. This invention uses the ultrathin edge layer of TiO2 nanosheets as the quantum well bottom, cleverly utilizing the redox properties of TiO2 crystal heterojunctions, and employs a simple photoreduction and ion exchange method to grow CdS quantum dots on the TiO2 {101} surface as a quantum well barrier, thus constructing a CdS / TiO2 / CdS semi-inverted quantum well nanosheet. TiO2 is a wide-bandgap semiconductor with a bandgap of 3.2 eV, absorbing only about 4% of the ultraviolet light in sunlight. CdS is an n-type semiconductor with a bandgap of 2.4 eV, capable of absorbing visible light. Combining TiO2 and CdS extends the photoresponse range to the visible light region, improving the utilization efficiency of sunlight; the photocatalytic performance for the reduction of water and carbon dioxide is significantly enhanced.

Description

Technical Field

[0001] This invention relates to the field of photocatalysis technology, and in particular to a CdS / TiO2 / CdS semi-inverted quantum well nanosheet, its preparation method, and its application. Background Technology

[0002] In recent years, rapid industrial development and a continuously rising global population have led to severe energy shortages and environmental pollution. Semiconductor-based photocatalysis technology can directly utilize solar energy to produce valuable chemical fuels, such as hydrogen and hydrocarbon fuels. Therefore, photocatalysis is considered a green technology with significant energy and environmental application prospects. In photocatalytic reactions, the separation efficiency of photogenerated carriers is one of the key factors limiting photocatalytic activity. To address this issue, researchers have proposed various strategies to effectively transfer photogenerated electron-hole pairs in semiconductor photocatalysts, such as doping and vacancy manipulation, constructing semiconductor heterojunctions, and loading noble metals. However, in the field of semiconductor physical optics devices, there is also an effective method to promote photogenerated carrier transfer: constructing quantum well structures.

[0003] Traditional semiconductor quantum well structures are "sandwich" structures formed by sandwiching a narrow-bandgap semiconductor B between two wide-bandgap semiconductor A layers, creating potential wells for electrons or holes with significant quantum confinement effects. Because semiconductor quantum well structures can rapidly transfer and confine photogenerated electrons and holes from the two outer semiconductor A layers to the conduction and valence bands of the middle semiconductor B, electrons and holes readily recombine and emit photons. Therefore, semiconductor quantum well structures disclosed in existing technologies have found numerous applications in the field of optical devices. However, their application in photocatalysis is relatively limited. This is because the effective separation of photogenerated electron-hole pairs is a crucial pathway to enhancing photocatalytic activity, while traditional semiconductor quantum well structures can only achieve rapid recombination of photogenerated electrons and holes, not effective separation. Therefore, how to modify existing semiconductor quantum well structures to enable their application in photocatalysis has become a major research hotspot. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a CdS / TiO2 / CdS semi-inverted quantum well nanosheet, its preparation method, and its application. The CdS / TiO2 / CdS semi-inverted quantum well nanosheet provided by this invention exhibits excellent photocatalytic performance.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] This invention provides a CdS / TiO2 / CdS semi-inverted quantum well nanosheet, which is composed of TiO2 nanosheets and CdS nanoparticles;

[0007] The TiO2 nanosheets are anatase heterojunction structures with both {101} and {001} crystal planes exposed.

[0008] The CdS nanoparticles are assembled on the {101} crystal plane of the TiO2 nanosheets and exhibit a quantum dot structure.

[0009] Preferably, the length of the {001} crystal plane of the TiO2 nanosheet is 10-80 nm; the width of the {101} crystal plane of the TiO2 nanosheet is 2-20 nm.

[0010] Preferably, the particle size of the CdS nanoparticles is 1–10 nm.

[0011] This invention also provides a method for preparing CdS / TiO2 / CdS semi-inverted quantum well nanosheets as described above, comprising the following steps:

[0012] Titanium source, hydrofluoric acid and ethanol were mixed and subjected to a solvothermal reaction to obtain TiO2 nanosheets;

[0013] The TiO2 nanosheets were dispersed in water and then activated by ultraviolet light to obtain photoactivated TiO2 nanosheets.

[0014] The photoactivated TiO2 nanosheets were mixed with silver ammonia solution and subjected to a reduction reaction under simulated sunlight to obtain Ag / TiO2 / Ag nanosheets.

[0015] The Ag / TiO2 / Ag nanosheets, sulfur source and water were mixed and subjected to a complexation reaction to obtain Ag2S / TiO2 / Ag2S nanosheets;

[0016] The Ag2S / TiO2 / Ag2S nanosheets, a cadmium source, and water were mixed and cadmium ion exchange was performed to obtain the CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

[0017] Preferably, the titanium source is one or more of titanate, titanium tetrachloride, and titanium sulfate; the temperature of the solvothermal reaction is 160-200°C, and the holding time is 10-14 h.

[0018] Preferably, the activation time of the ultraviolet lamp is 2 to 5 hours.

[0019] Preferably, the molar ratio of titanium in the photoactivated TiO2 nanosheets to silver in the silver ammonia solution is 1:(0.2-0.4); and the reduction reaction time is 1-3 min.

[0020] Preferably, the sulfur source is one or more of thioacetamide, sodium sulfide, or thiourea; the complexation reaction is carried out at a temperature of 35–60°C for 2–5 hours.

[0021] Preferably, the cadmium source is one or more of Cd(AC)2·2H2O, CdCl2, and Cd(NO3)2; the cadmium ion exchange temperature is 40–70°C, and the time is 2–5 h.

[0022] The present invention also provides the application of CdS / TiO2 / CdS semi-inverted quantum well nanosheets described in the above technical solution or CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared by the preparation method described in the above technical solution in photocatalytic water splitting for hydrogen production and carbon dioxide reduction.

[0023] This invention provides a CdS / TiO2 / CdS semi-inverted quantum well nanosheet, which is composed of TiO2 nanosheets and CdS nanoparticles; the TiO2 nanosheets are anatase heterojunction structures with both {101} and {001} crystal planes exposed; the CdS nanoparticles are assembled on the {101} crystal plane of the TiO2 nanosheets and have a quantum dot structure.

[0024] The present invention has the following advantages:

[0025] (1) Traditional semiconductor quantum well structures, due to their confinement effect on photogenerated carriers, make it easy for electrons and holes to recombine and emit photons, which has led to many applications in the field of optical devices. This invention is the first to modify the traditional semiconductor quantum well, confine and collect a large number of photogenerated electrons, promote the efficient separation of photogenerated carriers, and inject new inspiration into the application of quantum well structures in the field of photocatalysis.

[0026] (2) The ultrathin edge layer of TiO2 nanosheets is selected as the bottom of the quantum well. The redox properties of TiO2 crystal plane heterojunction are cleverly utilized. CdS quantum dots are grown on the TiO2{101} surface as quantum well barriers by simple photoreduction and ion exchange methods to construct CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

[0027] (3) TiO2 is a wide bandgap semiconductor with a bandgap of 3.2 eV, which can only absorb about 4% of the ultraviolet light in sunlight. CdS is an n-type semiconductor with a bandgap of 2.4 eV, which can absorb visible light. Combining TiO2 and CdS can extend the photoresponse range to the visible light region and improve the utilization efficiency of sunlight.

[0028] (4) Compared with the classic CdS / TiO2 "II" type heterojunction, the CdS / TiO2 / CdS semi-inverted quantum well nanosheet synthesized in this invention has significantly improved photocatalytic performance for the reduction of water and carbon dioxide. In addition, the selectivity of its CH4- products is also increased. Attached Figure Description

[0029] Figure 1 Transmission electron microscope images and selected area electron diffraction patterns of CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1;

[0030] Figure 2 Transmission electron microscope image of the CdS / TiO2 “Type II” heterojunction nanosheets obtained in Comparative Example 1;

[0031] Figure 3 The X-ray diffraction pattern of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1;

[0032] Figure 4 The graph shows the performance of photocatalytic water splitting for hydrogen production from the CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1.

[0033] Figure 5 The graph shows the performance of photocatalytic water splitting for hydrogen production from the CdS / TiO2 “Type II” heterojunction nanosheets obtained in Comparative Example 1.

[0034] Figure 6 The graph shows the photocatalytic CO2 reduction performance of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1.

[0035] Figure 7 The graph shows the photocatalytic CO2 reduction performance of the CdS / TiO2 "Type II" heterojunction nanosheets obtained in Comparative Example 1.

[0036] Figure 8 The image shows a comparison of the CH4 selectivity of CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1 and CdS / TiO2 "Type II" heterojunction nanosheets obtained in Comparative Example 1 for photocatalytic CO2 reduction. Detailed Implementation

[0037] This invention provides a CdS / TiO2 / CdS semi-inverted quantum well nanosheet, which is composed of TiO2 nanosheets and CdS nanoparticles;

[0038] The TiO2 nanosheets are anatase heterojunction structures with both {101} and {001} crystal planes exposed.

[0039] The CdS nanoparticles are assembled on the {101} crystal plane of the TiO2 nanosheets and exhibit a quantum dot structure.

[0040] The CdS / TiO2 / CdS semi-inverted quantum well nanosheets provided by this invention include TiO2 nanosheets. In this invention, the TiO2 nanosheets are anatase heterojunction structures with both {101} and {001} crystal planes exposed. In this invention, the length of the {001} crystal plane of the TiO2 nanosheet is preferably 10–80 nm, more preferably 20–50 nm. In this invention, the width of the {101} crystal plane of the TiO2 nanosheet is preferably 2–20 nm, more preferably 4–10 nm.

[0041] The CdS / TiO2 / CdS semi-inverted quantum well nanosheets provided by this invention include CdS nanoparticles, wherein the particle size of the CdS nanoparticles is preferably 1–10 nm, more preferably 1–4 nm. In this invention, the CdS nanoparticles are assembled on the {101} crystal plane of the TiO2 nanosheets and exhibit a quantum dot structure.

[0042] This invention also provides a method for preparing CdS / TiO2 / CdS semi-inverted quantum well nanosheets as described above, comprising the following steps:

[0043] Titanium source, hydrofluoric acid and ethanol were mixed and subjected to a solvothermal reaction to obtain TiO2 nanosheets;

[0044] The TiO2 nanosheets were dispersed in water and then activated by ultraviolet light to obtain photoactivated TiO2 nanosheets.

[0045] The photoactivated TiO2 nanosheets were mixed with silver ammonia solution and subjected to a reduction reaction under simulated sunlight to obtain Ag / TiO2 / Ag nanosheets.

[0046] The Ag / TiO2 / Ag nanosheets, sulfur source and water were mixed and subjected to a complexation reaction to obtain Ag2S / TiO2 / Ag2S nanosheets;

[0047] The Ag2S / TiO2 / Ag2S nanosheets, a cadmium source, and water were mixed and cadmium ion exchange was performed to obtain the CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

[0048] Unless otherwise specified, all raw materials used in this invention are preferably commercially available products.

[0049] This invention involves mixing a titanium source, hydrofluoric acid, and ethanol, and then performing a solvothermal reaction to obtain TiO2 nanosheets.

[0050] In this invention, the titanium source is preferably one or more of titanate, titanium tetrachloride, and titanium sulfate, more preferably titanate or titanium tetrachloride. In this invention, the mass concentration of the hydrofluoric acid is preferably 40%. In this invention, the ethanol is preferably anhydrous ethanol. In this invention, the volume ratio of the titanium source, hydrofluoric acid, and ethanol is preferably (1-3):0.8:8. In this invention, the mixing of the titanium source, hydrofluoric acid, and ethanol preferably comprises: dissolving the titanium source in ethanol to obtain a titanium source solution; and mixing the titanium source solution with hydrofluoric acid.

[0051] In this invention, the temperature of the solvothermal reaction is preferably 160-200°C, more preferably 170-190°C, and even more preferably 180°C; the holding time is preferably 10-14 hours, and even more preferably 12 hours.

[0052] Following the solvothermal reaction, the present invention preferably further includes: naturally cooling the obtained solvothermal reaction solution to room temperature, then centrifuging to collect the white powder, and washing and drying the white powder. In this invention, the washing preferably includes sequential ethanol washing and water washing; the water washing reagent is preferably deionized water; the present invention does not specifically limit the amount and number of ethanol and water washing reagents used, as long as impurities are removed completely. In this invention, the drying temperature is preferably 60°C.

[0053] After obtaining TiO2 nanosheets, the present invention disperses the TiO2 nanosheets in water and then activates them with ultraviolet light to obtain photoactivated TiO2 nanosheets.

[0054] In this invention, the activation time of the ultraviolet lamp is preferably 2-5 hours, more preferably 3-4 hours. In this invention, during ultraviolet lamp activation, the obtained TiO2 nanosheet aqueous dispersion is preferably stirred.

[0055] After activation by the ultraviolet lamp, the present invention preferably further includes: centrifuging the obtained ultraviolet lamp activation solution to collect powder, and washing and drying the powder. In this invention, the washing preferably includes sequential ethanol washing and water washing; the water washing reagent is preferably deionized water; the present invention does not specifically limit the amount and number of ethanol and water washing reagents, as long as impurities are removed completely. In this invention, the drying temperature is preferably 60°C.

[0056] After obtaining photoactivated TiO2 nanosheets, the present invention mixes the photoactivated TiO2 nanosheets with silver ammonia solution and carries out a reduction reaction under simulated sunlight to obtain Ag / TiO2 / Ag nanosheets.

[0057] In this invention, the preferred method for preparing the silver ammonia solution is as follows: dissolving silver nitrate in water to obtain a silver nitrate solution; mixing the silver nitrate solution with ammonia water to perform a complexation reaction to obtain the silver ammonia solution. In this invention, the mass concentration of the ammonia water is preferably 25-28%. In this invention, the preferred ratio of the amounts of silver nitrate, water, and ammonia water is (17-26) mg: 30 mL: (2.34-3.58) g. In this invention, the preferred temperature for the complexation reaction is 30-50°C, more preferably 40°C; the preferred time is 30-60 min, more preferably 40-50 min; the complexation reaction is preferably carried out under stirring conditions.

[0058] In this invention, the molar ratio of titanium in the photoactivated TiO2 nanosheets to silver in the silver ammonia solution is preferably 1:(0.2-0.4), more preferably 1:(0.25-0.35), and even more preferably 1:0.3; the reduction reaction time is preferably 1-3 min, more preferably 2 min. In this invention, the simulated sunlight is preferably AM 1.5.

[0059] Following the reduction reaction, the present invention preferably further includes: centrifuging the obtained reduction reaction solution to collect the powder, and washing and drying the powder. In this invention, the washing preferably includes sequential ethanol washing and water washing; the water washing reagent is preferably deionized water; the present invention does not specifically limit the amount or number of ethanol and water washing reagents used, as long as impurities are completely removed. In this invention, the drying temperature is preferably 60°C.

[0060] After obtaining Ag / TiO2 / Ag nanosheets, the present invention mixes the Ag / TiO2 / Ag nanosheets, sulfur source and water to carry out a complexation reaction to obtain Ag2S / TiO2 / Ag2S nanosheets.

[0061] In this invention, the sulfur source is preferably one or more of thioacetamide, sodium sulfide, or thiourea, and more preferably thioacetamide or thiourea. In this invention, the preferred ratio of Ag / TiO2 / Ag nanosheets, sulfur source, and water is 30 mg:(15-30) mg:40 mL.

[0062] In this invention, the temperature of the complexation reaction is preferably 35-60°C, more preferably 40-55°C, and even more preferably 45-50°C; the time is preferably 2-5 hours, and even more preferably 3-4 hours; the complexation reaction is preferably carried out under the conditions of water bath and stirring.

[0063] Following the complexation reaction, the present invention preferably further includes: centrifuging the obtained complexation reaction solution to collect the powder, and washing and drying the powder. In this invention, the washing preferably includes sequential ethanol washing and water washing; the water washing reagent is preferably deionized water; the present invention does not specifically limit the amount and number of ethanol and water washing reagents used, as long as impurities are completely removed. In this invention, the drying temperature is preferably 60°C.

[0064] After obtaining Ag2S / TiO2 / Ag2S nanosheets, the present invention mixes the Ag2S / TiO2 / Ag2S nanosheets, a cadmium source, and water to perform cadmium ion exchange, thereby obtaining the CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

[0065] In this invention, the cadmium source is preferably one or more of Cd(AC)₂·2H₂O, CdCl₂, and Cd(NO₃)₂, more preferably Cd(AC)₂·2H₂O or Cd(NO₃)₂. In this invention, the preferred ratio of Ag₂S / TiO₂ / Ag₂S nanosheets, the cadmium source, and water is 30 mg:(53-65) mg:40 mL.

[0066] In this invention, the temperature of the cadmium ion exchange is preferably 40-70°C, more preferably 50-60°C; the time is preferably 2-5 hours, more preferably 3-4 hours; the cadmium ion exchange is preferably carried out under the conditions of water bath and stirring.

[0067] Following the cadmium ion exchange, the present invention preferably further includes: centrifuging the obtained cadmium ion exchange solution to collect the powder, and washing and drying the powder. In this invention, the washing preferably includes sequential ethanol washing and water washing; the water washing reagent is preferably deionized water; the present invention does not specifically limit the amount or number of ethanol and water washing reagents used, as long as impurities are removed completely. In this invention, the drying temperature is preferably 60°C.

[0068] The present invention also provides the application of CdS / TiO2 / CdS semi-inverted quantum well nanosheets described in the above technical solution or CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared by the preparation method described in the above technical solution in photocatalytic water splitting for hydrogen production and carbon dioxide reduction.

[0069] The present invention does not impose specific limitations on the application of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets; the application can be set according to the actual situation.

[0070] The following detailed description, in conjunction with embodiments, illustrates the CdS / TiO2 / CdS semi-inverted quantum well nanosheets provided by this invention, their preparation methods, and their applications. However, these descriptions should not be construed as limiting the scope of protection of this invention.

[0071] Example 1

[0072] ① Weigh 3 mL of tetrabutyl titanate and dissolve it in 8 mL of ethanol. Stir magnetically until a clear solution is obtained to obtain a titanium source solution. Add 0.8 mL of hydrofluoric acid (mass concentration of 40%) to the titanium source solution and stir magnetically to form a homogeneous solution.

[0073] ② The homogeneous solution obtained in ① was placed into a 20 mL reaction vessel and reacted at 180 °C for 12 h using a solvothermal method. After naturally cooling to room temperature, the white powder was collected by centrifugation, washed several times with ethanol and deionized water, and dried at 60 °C to obtain TiO2 nanosheets.

[0074] ③ The TiO2 nanosheets synthesized in ② were dispersed in water, stirred and irradiated under ultraviolet light for 2 hours, the powder was collected, washed several times with ethanol and deionized water, and dried at 60℃ to obtain photoactivated TiO2 nanosheets.

[0075] ④ Weigh 17 mg of silver nitrate and dissolve it in 30 mL of water. Add 2.34 g of ammonia (mass concentration of 25-28%) and stir magnetically at 40 °C for 40 min to obtain a silver ammonia solution.

[0076] ⑤ Weigh 30 mg of the photoactivated TiO2 nanosheets obtained in ③ and add them to the silver ammonia solution obtained in ④. Irradiate the solution under simulated sunlight (AM 1.5) for 2 min, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60℃ to obtain gray powder Ag / TiO2 / Ag nanosheets.

[0077] ⑥ Weigh 30 mg of the gray powder Ag / TiO2 / Ag nanosheets obtained in ⑤ and 15 mg of thioacetamide, disperse them in 40 mL of water, stir in a water bath at 40 °C for 2 h, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60 °C to obtain gray-black powder Ag2S / TiO2 / Ag2S nanosheets.

[0078] ⑦ Weigh 30 mg of the gray-black powder Ag2S / TiO2 / Ag2S nanosheets obtained in ⑥ and 53 mg of Cd(AC)2·2H2O, disperse them in 40 mL of water, stir and ion exchange in a water bath at 40 °C for 2 h, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60 °C to obtain orange-yellow powder CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

[0079] Example 2

[0080] ① Weigh 1 mL of titanium tetrachloride and dissolve it in 8 mL of ethanol. Stir magnetically until a clear solution is obtained to obtain a titanium source solution. Add 0.8 mL of hydrofluoric acid (mass concentration of 40%) to the titanium source solution and stir magnetically to form a homogeneous solution.

[0081] ② The homogeneous solution obtained in ① was placed into a 20 mL reaction vessel and reacted at 160 °C for 12 h using a solvothermal method. After naturally cooling to room temperature, the white powder was collected by centrifugation, washed several times with ethanol and deionized water, and dried at 60 °C to obtain TiO2 nanosheets.

[0082] ③ The white powder TiO2 nanosheets synthesized in ② were dispersed in water, stirred and irradiated under ultraviolet light for 2 hours, the powder was collected, washed several times with ethanol and deionized water, and dried at 60℃ to obtain photoactivated TiO2 nanosheets.

[0083] ④ Weigh 26 mg of silver nitrate and dissolve it in 30 mL of water. Add 3.58 g of ammonia (mass concentration of 25-28%) and stir magnetically at 40 °C for 40 min to obtain a silver ammonia solution.

[0084] ⑤ Weigh 30 mg of the photoactivated TiO2 nanosheets from ③ and add them to the silver ammonia solution obtained in ④. Irradiate the solution under simulated sunlight (AM 1.5) for 4 min, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60℃ to obtain gray powder Ag / TiO2 / Ag nanosheets.

[0085] ⑥ Weigh 30 mg of the gray powder Ag / TiO2 / Ag nanosheets obtained in ⑤ and 30 mg of thioacetamide, disperse them in 40 mL of water, stir in a water bath at 40 °C for 2 h, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60 °C to obtain gray-black powder Ag2S / TiO2 / Ag2S nanosheets.

[0086] ⑦ Weigh 30 mg of the gray-black powder Ag2S / TiO2 / Ag2S nanosheets obtained in ⑥ and 65 mg of Cd(AC)2·2H2O, disperse them in 40 mL of water, stir and ion exchange in a water bath at 40 °C for 2 h, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60 °C to obtain orange-yellow powder CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

[0087] Example 3

[0088] ① Weigh 3 mL of tetrabutyl titanate and dissolve it in 8 mL of ethanol. Stir magnetically until a clear solution is obtained to obtain a titanium source solution. Add 0.8 mL of hydrofluoric acid (mass concentration of 40%) to the titanium source solution and stir magnetically to form a homogeneous solution.

[0089] ② The homogeneous solution obtained in ① was placed into a 20 mL reaction vessel and reacted at 160 °C for 14 h using a solvothermal method. After naturally cooling to room temperature, the white powder was collected by centrifugation, washed several times with ethanol and deionized water, and dried at 60 °C to obtain TiO2 nanosheets.

[0090] ③ The white powder TiO2 nanosheets synthesized in ② were dispersed in water, stirred and irradiated under ultraviolet light for 2 hours, the powder was collected, washed several times with ethanol and deionized water, and dried at 60℃ to obtain photoactivated TiO2 nanosheets.

[0091] ④ Weigh 25 mg of silver nitrate and dissolve it in 30 mL of water. Add 3.44 g of ammonia (mass concentration of 25-28%) and stir magnetically at 40 °C for 40 min to obtain a silver ammonia solution.

[0092] ⑤ Weigh 30 mg of the photoactivated TiO2 nanosheets obtained in ③ and add them to the silver ammonia solution obtained in ④. Irradiate the solution under simulated sunlight (AM 1.5) for 5 min, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60℃ to obtain gray powder Ag / TiO2 / Ag nanosheets.

[0093] ⑥ Weigh 30 mg of the gray powder Ag / TiO2 / Ag nanosheets obtained in ⑤ and 15 mg of thiourea, disperse them in 40 mL of water, stir in a water bath at 40 °C for 2 h, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60 °C to obtain gray-black powder Ag2S / TiO2 / Ag2S nanosheets.

[0094] ⑦ Weigh 30 mg of the gray-black powder Ag2S / TiO2 / Ag2S nanosheets obtained in ⑥ and 53 mg of Cd(NO3)2, disperse them in 40 mL of water, stir and ion exchange in a water bath at 40 °C for 2 h, collect the powder, wash it several times with ethanol and deionized water, and dry it at 60 °C to obtain orange-yellow powder CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

[0095] Comparative Example 1

[0096] ① Weigh 3 mL of tetrabutyl titanate and dissolve it in 8 mL of ethanol. Stir magnetically until a clear solution is obtained to obtain a titanium source solution. Add 0.8 mL of hydrofluoric acid to the titanium source solution and stir magnetically to form a homogeneous solution.

[0097] ② The homogeneous solution obtained in ① was placed into a 20 mL reaction vessel and reacted at 180 °C for 12 h using a solvothermal method. After naturally cooling to room temperature, the white powder was collected by centrifugation, washed several times with ethanol and deionized water, and dried at 60 °C to obtain TiO2 nanosheets.

[0098] ③ Weigh 30 mg of the white powder TiO2 nanosheets obtained in ② and ultrasonically disperse them in 20 mL of water.

[0099] ④ Weigh 53 mg Cd(AC)2·2H2O and 15 mg thioacetamide, and dissolve them in 20 mL of water by magnetic stirring.

[0100] ⑤ Mix the liquid obtained in ③ and the liquid obtained in ④ together, stir in a water bath at 40℃ for 1 hour, and evaporate the water at 100℃ until dry to obtain CdS / TiO2 "Type II" heterojunction nanosheets.

[0101] Transmission electron microscopy (TEM) characterization

[0102] The samples synthesized in Example 1 and Comparative Example 1 were subjected to morphological and structural analysis. The instrument used was a JEOL JEM-2100 transmission electron microscope with an operating voltage of 300 kV. The results are as follows: Figure 1 and Figure 2 As shown. Figure 1 The images show transmission electron microscope (TEM) images and selected area electron diffraction (SAD) patterns of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1. The left image is the TEM image, and the right image is the SAD pattern. Figure 1 As shown in the left figure, CdS quantum dots are uniformly distributed at the edges of TiO2 nanosheets, i.e., the TiO2 {101} planes. The size of the TiO2 nanosheets is 20–35 nm, and the size of the CdS quantum dots is 1–2 nm. Figure 1 As shown in the right figure: the diffraction rings correspond to the TiO2 (101), CdS (002) and TiO2 (200) crystal planes respectively, proving that CdS was successfully grown on the TiO2 surface.

[0103] Figure 2 The image shows a transmission electron microscope (TEM) image of the CdS / TiO2 "Type II" heterostructure nanosheets obtained in Comparative Example 1. Figure 2 As shown, CdS quantum dots are uniformly distributed on all surfaces of TiO2 nanosheets. The size of the TiO2 nanosheets is 20–35 nm, and the size of the CdS quantum dots is 1–2 nm.

[0104] X-ray diffraction (XRD) characterization

[0105] The sample synthesized in Example 1 of this invention was characterized by XRD using a Shimadzu XRD-6000 X-ray powder diffractometer. The XRD test results are as follows: Figure 3 As shown, Figure 3 The X-ray diffraction pattern of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1; from Figure 3 It can be seen that the diffraction peaks are TiO2 anatase phase with high crystallinity. No diffraction peaks of CdS were found in the sample, which may be due to the small particle size of CdS and its high dispersion in TiO2.

[0106] Photocatalytic activity test

[0107] 1. Photocatalytic hydrogen production

[0108] Application Example 1

[0109] The photocatalytic hydrogen production experiment was conducted in a 35 mL quartz reactor. 10 mL of aqueous solution [containing 15 vol.% triethanolamine (sacrificial agent: to eliminate photogenerated holes) and 0.1 mol / L Na₂S (to inhibit sulfide photocorrosion), and Pt co-catalyst (H₂PtCl₆, 0.05 mg / mL)] was added to the reactor. 5 mg of the CdS / TiO₂ / CdS semi-inverted quantum well nanosheets prepared in Examples 1-3 were added as photocatalysts. The reactor was sealed with a rubber stopper, and residual air was then purged using argon as the carrier gas. A 300 W xenon lamp with an AM 1.5 filter was then turned on to perform the photocatalytic water splitting reaction.

[0110] The changes in product gas with illumination time were measured using a gas chromatograph equipped with a TCD detector. The results are shown in [Figure number missing]. Figure 4 , Figure 4 The graph shows the performance of photocatalytic water splitting for hydrogen production from the CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1; the corresponding specific data are shown in Table 1.

[0111] Table 1 Gas yield at different times during photocatalytic water splitting reaction

[0112] Time / min 30 60 90 120 Hydrogen production in Example 1 (μmol) 24.5 49 73.5 98 Hydrogen production in Example 2 (μmol) 23.2 46.4 69.6 92.8 Hydrogen production in Example 3 (μmol) 23.8 47.6 71.4 95.2

[0113] from Figure 4 As shown in Table 1, the CdS / TiO2 / CdS semi-inverted quantum well nanosheet catalyst obtained in this invention has a water splitting rate of ~49 μmol·h. -1 .

[0114] Comparative Application Example 1

[0115] The CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared in Example 1 of Application Example 1 were replaced with the CdS / TiO2 / CdS nanosheets prepared in Comparative Example 1, and the rest were the same as in Application Example 1.

[0116] The changes in product gas with illumination time were measured using a gas chromatograph equipped with a TCD detector. The results are shown in [Figure number missing]. Figure 5 And Table 2. Figure 5 The graph shows the performance of photocatalytic water splitting for hydrogen production from the CdS / TiO2 “Type II” heterojunction nanosheets obtained in Comparative Example 1.

[0117] Table 2 Gas yield at different times during photocatalytic water splitting reaction

[0118] Time / min 30 60 90 120 Hydrogen production / μmol 3.97 7.94 11.91 15.88

[0119] Depend on Figure 5 As shown in Table 2, the CdS / TiO2 "II" type heterojunction nanosheets, under photocatalysis, achieve a water splitting rate of ~7.94 μmol·h⁻¹. -1 In other words, the hydrogen production rate of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared in this invention is ~6 times that of the CdS / TiO2 "II" heterojunction nanosheets.

[0120] Photocatalytic CO2 reduction

[0121] Application Example 2

[0122] 5 mg of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared in Examples 1-3 were weighed and dispersed in 100 μL of deionized water as a photocatalyst, and a Pt co-catalyst (H2PtCl6, 0.05 mg / mL) was added to form a suspension. The suspension was then uniformly dropped onto a 2 cm × 2 cm glass substrate. As the aqueous solvent evaporated, a thin film composed of the original synthesized sample was formed on the glass substrate. Subsequently, the glass substrate containing the synthesized sample was placed in a quartz reactor, and CO2 was used as the carrier gas to purge any residual air from the sealed reactor. Simultaneously, 100 μL of deionized water was injected into the reactor to provide water vapor. Finally, a 300 W xenon lamp with an AM 1.5 filter was turned on to conduct the photocatalytic CO2 reduction reaction.

[0123] The changes in product gas with illumination time were measured using a gas chromatograph equipped with a TCD detector. The results are shown in [Figure number missing]. Figure 6 , Figure 6 This is a graph showing the photocatalytic CO2 reduction performance of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1. Figure 6 The specific data is shown in Table 3.

[0124] Table 3 Gas yield at different times during photocatalytic CO2 reduction reaction

[0125]

[0126]

[0127] from Figure 6 As shown in Table 3, the CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared in this invention exhibit photocatalytic CO2 reduction to CO and CH4. The CO yield is ~17.8 μmol·g. -1 ·h -1 The yield of CH4 was ~31.4 μmol·g. -1 ·h -1 .

[0128] Comparative Application Example 2

[0129] The CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared in Example 1 of Application Example 2 were replaced with the CdS / TiO2 / CdS nanosheets prepared in Comparative Example 1, and the rest were the same as in Application Example 2.

[0130] The changes in product gas with illumination time were measured using a gas chromatograph equipped with a TCD detector. The results are shown in [Figure number missing]. Figure 7 And Table 4. Figure 7 The graph shows the photocatalytic CO2 reduction performance of the CdS / TiO2 “Type II” heterojunction nanosheets obtained in Comparative Example 1.

[0131] Table 4. Gas yield at different times during the photocatalytic CO2 reduction reaction.

[0132] Time / h 1 2 3 CO production / μmol / g 3.1 6.2 9.3 <![CDATA[CH4 production / μmol / g]]> 1.72 3.44 5.16

[0133] Depend on Figure 7 As shown in Table 4, CdS / TiO2 "Type II" heterojunction nanosheets reduce CO2 to CO and CH4 under photocatalysis. The yield of CO is ~3.1 μmol·g⁻¹. -1 ·h -1 The yield of CH4 was ~1.72 μmol·g. -1 ·h -1 In other words, the photocatalytic activity of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared in this invention for CO2 reduction is ~6 times (TCEN) that of the CdS / TiO2 "Type II" heterojunction nanosheets.

[0134] The overall activity of the corresponding CO / CH4 generation can be evaluated using the total number of electrons consumed (TCEN), which can be calculated using Equation 1:

[0135]

[0136] In Formula 1, TCEN is the total number of electrons consumed in the generation of CO / CH4, and C... CO and C CH4 V refers to the concentrations of CO and CH4 generated, respectively. reactor It is the reactor volume, m cat. The mass of the catalyst is expressed in t. irr. It refers to the irradiation time.

[0137] The selectivity for CH4 is calculated using Equation 2:

[0138]

[0139] In formula 2, and n co These represent the number of moles of CH4 and CO generated in 1 hour, respectively.

[0140] Figure 8 This is a comparison of the CH4 selectivity for photocatalytic CO2 reduction between the CdS / TiO2 / CdS semi-inverted quantum well nanosheets obtained in Example 1 and the CdS / TiO2 "Type II" heterojunction nanosheets obtained in Comparative Example 1. Figure 8 As shown, the CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared in this invention have a CH4 selectivity of 63.82%, which is 1.3 times that of the CdS / TiO2 "Type II" heterojunction nanosheets (CH4 selectivity of 49.22%).

[0141] This invention addresses the shortcomings of semiconductor photocatalysts, such as easy recombination of photogenerated electron-hole pairs and low utilization of sunlight. It leverages the advantages of semiconductor quantum well structures, which can rapidly transfer and confine photogenerated carriers, by modifying the quantum well for the first time: using an ultrathin TiO2 {101} plane as the quantum well bottom and CdS quantum dots grown on both sides of the TiO2 {101} plane as quantum well barriers, a CdS / TiO2 / CdS semi-inverted quantum well nanosheet is constructed. This confines and collects a large number of photogenerated electrons, promoting efficient separation of photogenerated carriers. The strong absorption of ultraviolet and visible light by TiO2 and CdS, respectively, enhances the utilization efficiency of sunlight. Photocatalytic performance tests show that compared with the classic CdS / TiO2 "Type II" heterojunction, the CdS / TiO2 / CdS semi-inverted quantum well nanosheet exhibits ~6 times (TCEN) enhanced photocatalytic activity in the reduction of water and carbon dioxide. Correspondingly, the selectivity of its CH4- products also increases by ~130 times.

[0142] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A CdS / TiO2 / CdS semi-inverted quantum well nanosheet, composed of TiO2 nanosheets and CdS nanoparticles; The TiO2 nanosheets are anatase heterojunction structures with both {101} and {001} crystal planes exposed. The CdS nanoparticles are assembled on the {101} crystal plane of the TiO2 nanosheets and exhibit a quantum dot structure. The preparation method of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets includes the following steps: Titanium source, hydrofluoric acid and ethanol were mixed and subjected to a solvothermal reaction to obtain TiO2 nanosheets; The TiO2 nanosheets were dispersed in water and then activated by ultraviolet light to obtain photoactivated TiO2 nanosheets. The photoactivated TiO2 nanosheets were mixed with silver ammonia solution and subjected to a reduction reaction under simulated sunlight to obtain Ag / TiO2 / Ag nanosheets. The Ag / TiO2 / Ag nanosheets, sulfur source and water were mixed and subjected to a complexation reaction to obtain Ag2S / TiO2 / Ag2S nanosheets; The Ag2S / TiO2 / Ag2S nanosheets, a cadmium source, and water were mixed and cadmium ion exchange was performed to obtain the CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

2. The CdS / TiO2 / CdS semi-inverted quantum well nanosheet according to claim 1, characterized in that, The length of the {001} crystal plane of the TiO2 nanosheet is 10-80 nm; the width of the {101} crystal plane of the TiO2 nanosheet is 2-20 nm.

3. The CdS / TiO2 / CdS semi-inverted quantum well nanosheet according to claim 1, characterized in that, The CdS nanoparticles have a particle size of 1–10 nm.

4. The method for preparing CdS / TiO2 / CdS semi-inverted quantum well nanosheets according to any one of claims 1 to 3, characterized in that, Includes the following steps: Titanium source, hydrofluoric acid and ethanol were mixed and subjected to a solvothermal reaction to obtain TiO2 nanosheets; The TiO2 nanosheets were dispersed in water and then activated by ultraviolet light to obtain photoactivated TiO2 nanosheets. The photoactivated TiO2 nanosheets were mixed with silver ammonia solution and subjected to a reduction reaction under simulated sunlight to obtain Ag / TiO2 / Ag nanosheets. The Ag / TiO2 / Ag nanosheets, sulfur source and water were mixed and subjected to a complexation reaction to obtain Ag2S / TiO2 / Ag2S nanosheets; The Ag2S / TiO2 / Ag2S nanosheets, a cadmium source, and water were mixed and cadmium ion exchange was performed to obtain the CdS / TiO2 / CdS semi-inverted quantum well nanosheets.

5. The preparation method according to claim 4, characterized in that, The titanium source is one or more of titanate, titanium tetrachloride, and titanium sulfate; the temperature of the solvothermal reaction is 160-200℃, and the holding time is 10-14h.

6. The preparation method according to claim 4, characterized in that, The activation time of the ultraviolet lamp is 2 to 5 hours.

7. The preparation method according to claim 4, characterized in that, The molar ratio of titanium in the photoactivated TiO2 nanosheets to silver in the silver ammonia solution is 1:(0.2-0.4); the reduction reaction time is 1-3 min.

8. The preparation method according to claim 4, characterized in that, The sulfur source is one or more of thioacetamide, sodium sulfide, or thiourea; the complexation reaction is carried out at a temperature of 35–60°C for 2–5 hours.

9. The preparation method according to claim 4, characterized in that, The cadmium source is one or more of Cd(AC)2·2H2O, CdCl2, and Cd(NO3)2; the cadmium ion exchange temperature is 40–70℃ and the time is 2–5 h.

10. The application of the CdS / TiO2 / CdS semi-inverted quantum well nanosheets according to any one of claims 1 to 3 or the CdS / TiO2 / CdS semi-inverted quantum well nanosheets prepared by the preparation method according to any one of claims 4 to 9 in photocatalytic water splitting for hydrogen production and carbon dioxide reduction.

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