A black phosphorus / bismuth tungstate two-dimensional S-type heterojunction, its preparation method and application

Abstract

The present invention belongs to the field of photocatalysis technology, and specifically relates to a black phosphorus / bismuth tungstate two-dimensional S-type heterojunction and its preparation method and application. To solve the problems of low selectivity and yield of photocatalytic ethanol production and large energy barrier of the oxidation half-reaction in the prior art, the present invention provides a black phosphorus / bismuth tungstate two-dimensional S-type heterojunction and its preparation method and application. The preparation method of the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction includes the following steps: obtaining two-dimensional bismuth tungstate and black phosphorus nanosheets through a hydrothermal method and a liquid-phase exfoliation method, mixing the two under the condition of an inert atmosphere of argon and ultrasonically stirring, and electrostatic self-assembly to obtain the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction. The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction can effectively promote the separation of photo-generated carriers, accelerate the multiple PCET process, use black phosphorus as the active site, couple with the benzylamine oxidation with a low energy barrier, and show more excellent photocatalytic activity and selectivity in the process of CO2 conversion to ethanol.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalysis, and particularly relates to a black phosphorus / bismuth tungstate two-dimensional S-type heterojunction, a preparation method thereof and an application thereof. Background Art

[0002] Disclosing the information of this background art section is only intended to enhance the overall understanding of the present invention, and is not necessarily regarded as an admission or an implication in any form that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] The catalytic conversion of carbon dioxide into value-added fuels can alleviate the exhaustion of fossil resources and global warming. The photocatalytic reduction of carbon dioxide driven by solar energy to high-value hydrocarbons is considered a promising strategy for solving energy and environmental problems. Compared with common carbon dioxide reduction products such as carbon monoxide, methane, methanol, etc., higher-value C 2+ products such as ethanol are more ideal fuels because they have high energy density and are easy to store and transport. However, due to reasons such as low efficiency of the multiple proton-coupled electron transfer (PCET) reduction half-reaction process and slow C-C coupling process, it is extremely challenging to photocatalytically produce ethanol with high activity and selectivity simultaneously. In addition, the oxidative half-reaction of generating oxygen in the water solvent system has a high reaction barrier, which limits the reaction of carbon dioxide reduction to produce ethanol. To reduce the reaction barrier of the oxidative half-reaction, sacrificial agents are usually added to the photocatalytic system, and the sacrificial agents are used to provide protons and replace the oxidative half-reaction of oxygen generation, thereby promoting the overall progress of the photocatalytic reaction. The addition of sacrificial agents leads to an increase in the cost of the photocatalytic reaction, and the oxidation products of oxygen and sacrificial agents often do not have high economic value, resulting in a weakened overall economic benefit of the photocatalytic carbon dioxide reduction reaction. Summary of the Invention

[0004] To solve the deficiencies of the prior art, the purpose of the present invention is to provide a black phosphorus / bismuth tungstate two-dimensional S-type heterojunction, a preparation method thereof and an application thereof. The two-dimensional S-type heterojunction can effectively promote the separation of photo-generated carriers. Through the Bi-O-P carrier transport channel as a bridge for transporting electrons, the multiple PCET process is accelerated. At the same time, electron-rich black phosphorus (BP) serves as an active site and plays a crucial role in the C-C coupling process. In addition, the oxidation of benzylamine to replace water oxidation can reduce the thermodynamic barrier of the oxidative half-reaction, further enhancing the photocatalytic performance of CO2 reduction to ethanol, while obtaining a higher-value oxidation product N-benzylidene benzylamine. The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction exhibits more excellent photocatalytic activity and selectivity in the process of CO2 conversion to ethanol.

[0005] To achieve the above purpose, the present invention is realized through the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing a black phosphorus / bismuth tungstate two-dimensional S-type heterojunction, comprising the following steps:

[0007] S1. Dissolve bismuth nitrate pentahydrate in an aqueous nitric acid solution, stir to form solution A; dissolve sodium tungstate dihydrate and cetyltrimethylammonium bromide in water to form solution B; drop solution B into A, stir and then carry out a hydrothermal reaction, wash and dry the product to obtain bismuth tungstate;

[0008] S2. Immerse black phosphorus in N-methylpyrrolidone, introduce an inert gas to exhaust air, seal and ultrasonicate in an oxygen-free environment, centrifuge after ultrasonication, collect the supernatant and centrifuge again to obtain black phosphorus nanosheets;

[0009] S3. Mix bismuth tungstate and black phosphorus nanosheets, introduce an inert gas to exhaust air, seal and ultrasonicate and then stir to obtain a mixture, wash and dry the mixture to obtain the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction.

[0010] In a second aspect, the present invention provides a black phosphorus / bismuth tungstate two-dimensional S-type heterojunction obtained by the preparation method as described in the first aspect.

[0011] In a third aspect, the present invention provides an application of the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction as described in the second aspect in photocatalytic carbon dioxide reduction coupled with benzylamine oxidation.

[0012] In a fourth aspect, the present invention provides a method for photocatalytic carbon dioxide reduction coupled with benzylamine oxidation, characterized by comprising the following steps:

[0013] Disperse the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction as described in the second aspect in a reaction solvent containing benzylamine, ultrasonically disperse, introduce CO2 until saturation, and irradiate with full light for photocatalytic reaction;

[0014] Preferably, the reaction solvent is a mixture of acetonitrile, water and benzylamine, and the volume ratio of acetonitrile, water and benzylamine is 490-510:1:1;

[0015] Preferably, the ratio of the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction to benzylamine is 1 mg:3-5 μL.

[0016] The beneficial effects obtained by one or more of the above technical solutions of the present invention are as follows:

[0017] The synthesis method of the present invention is simple. Two-dimensional nanosheets are obtained by simple hydrothermal and liquid-phase exfoliation methods respectively, and then electrostatically assembled into a heterojunction photocatalyst material. This work provides a new possibility for the effective photocatalytic reduction of CO2 to high-value-added compounds.

[0018] The reaction conditions of the present invention are different from the traditional carbon dioxide reduction reaction. The oxidation of benzylamine is coupled with the carbon dioxide reduction reaction, which not only promotes the reduction process but also oxidizes to produce imine products with high added value. This work provides a reference for developing a new reaction system.

[0019] The two-dimensional S-type heterojunction of black phosphorus / bismuth tungstate prepared by the present invention has good photocatalytic activity and product selectivity. Among them, for 6% BP / BWO under 300W xenon lamp illumination, the rate of CO2 reduction to produce C2H5OH is 61.3 μmol g -1 h -1 −1 h−1, the selectivity reaches 91%, and the rate of producing N-benzylidene benzylamine is 413.3 μmoL / g / h. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The specification drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0021] Figure 1 TEM images (a), AFM images (b), thickness curves obtained from AFM (c) of BWO prepared in Comparative Example 1, TEM images (d), AFM images (e), thickness curves obtained from AFM (f) of BP prepared in Comparative Example 2, and XRD patterns of BWO in Comparative Example 1, BP in Comparative Example 2, and BWO / BP in Examples 1-4;

[0022] Figure 2 Morphology and element distribution maps of 6% BP / BWO in Example 3, where a is the TEM image of 6% BP / BWO, b is the HRTEM image, c is the SEM image of the selected area, and d-g are the element distribution maps of Bi (d), W (e), O (f), and P (g) in the selected area;

[0023] Figure 3 UV-visible diffuse reflectance spectra and band gap analysis of BWO in Comparative Example 1 and BWO / BP in Examples 1-4, where (a) is the diffuse reflectance comparison diagram and (b) is the Tauc curve calculated from (a);

[0024] Figure 4 XPS analysis of BWO in Comparative Example 1, BP in Comparative Example 2, and 6% BWO / BP in Example 3, where (a) is the Bi 4f diagram, (b) is the W 4f diagram, (c) is the O1s diagram, and (d) is the P 2p diagram;

[0025] Figure 5Performance comparison diagrams of BWO of Comparative Example 1, BP of Comparative Example 2, and BWO / BP of Examples 1-4, where a is the CO2 reduction performance diagram, b is the benzylamine oxidation performance diagram, c is the reduction performance control diagram of 6% BWO / BP under different reaction conditions, d is the oxidation performance control diagram of 6% BWO / BP under different reaction conditions, e is the cyclic stability diagram, and f is the selectivity comparison diagram of different materials;

[0026] Figure 6 In-situ XPS diagram of 6% BP / BWO prepared in Example 3 and reaction mechanism diagram of BP / BWO. Among them, a is the change in the binding energy of W 4f in 6% BP / BWO before and after light irradiation, b is the change in the binding energy of P 2p in 6% BP / BWO before and after light irradiation, and c is the mechanism diagram of the heterojunction of BP / BWO in the photocatalytic reaction before and after recombination and after the reaction;

[0027] Figure 7 Steady-state fluorescence spectra (a), time-resolved photoluminescence spectra (b), photocurrent response diagrams (c), and Nyquist impedance diagrams (d) of BWO of Comparative Example 1 and 6% BP / BWO of Example 3;

[0028] Figure 8 Adsorption energy comparison diagram (a) and activation free energy change diagram (b) of BWO and BP / BWO for the oxidation of benzylamine and benzylcarbamic acid;

[0029] Figure 9 Theoretical calculation configuration diagram during the adsorption, activation, and conversion of CO2 molecules on the surface of BP / BWO;

[0030] Figure 10 In-situ infrared, theoretical calculation, and catalytic reaction mechanism diagrams of BWO and BP / BWO. Among them, a is the change in reaction intermediates shown by in-situ infrared under light irradiation, b is the Gibbs free energy change diagram of the formation of CO, CH3OH, and C2H5OH during the CO2 reduction process, and c is the oxidation reaction path diagram of the photocatalytic reaction and the total mechanism process diagram of the coupling reaction;

[0031] Figure 11 Schematic diagram of the structure of BP / BWO material and the reaction mechanism of photocatalytic reduction of carbon dioxide and coupling oxidation of benzylamine (BA) to N-benzylidene benzylamine (BDA). Detailed implementation mode

[0032] The first typical implementation mode of the present invention, a preparation method of a black phosphorus / bismuth tungstate two-dimensional S-type heterojunction, includes the following steps:

[0033] S1. Dissolve bismuth nitrate pentahydrate in an aqueous nitric acid solution, and stir to form solution A; dissolve sodium tungstate dihydrate and cetyltrimethylammonium bromide in water to form solution B; drop solution B into A, stir, and then carry out a hydrothermal reaction. Wash and dry the product to obtain bismuth tungstate.

[0034] S2. Immerse black phosphorus in N-methylpyrrolidone, purge with an inert gas, seal, and ultrasonicate in an anaerobic environment. After ultrasonication, centrifuge, collect the supernatant, and centrifuge again to obtain black phosphorus nanosheets.

[0035] S3. Mix bismuth tungstate and black phosphorus nanosheets, purge with an inert gas, seal, ultrasonicate, and then stir to obtain a mixture. Wash and dry the mixture to obtain the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction.

[0036] An inert atmosphere is selected during the reaction because it can ensure an anaerobic environment and avoid the oxidation of black phosphorus. If carried out in air, the photocatalytic activity will deteriorate.

[0037] N-methylpyrrolidone, as the reaction solvent during the liquid-phase exfoliation of bulk black phosphorus, can break the van der Waals bonds between the black phosphorus layers and promote the exfoliation of bulk black phosphorus into single or few layers.

[0038] Nitric acid, as an important solvent for dissolving bismuth nitrate pentahydrate when synthesizing bismuth tungstate, can, on the one hand, promote the dissolution of bismuth nitrate pentahydrate, form a homogeneous solution to participate in the reaction, and on the other hand, avoid its hydrolysis and generate unnecessary other bismuth-based compounds during the hydrothermal process.

[0039] Cetyltrimethylammonium bromide (CATB), as a surfactant, is adsorbed on the surface of bismuth tungstate when bismuth nitrate pentahydrate and sodium tungstate dihydrate hydrothermally generate two-dimensional bismuth tungstate nanosheets, inhibiting stacking and controlling the growth thickness of the nanosheets. + Adsorb on the surface of bismuth tungstate, inhibit stacking, and control the growth thickness of the nanosheets.

[0040] All three play a key role in the synthesis of two-dimensional bismuth tungstate and black phosphorus nanosheets respectively.

[0041] In one or more embodiments of this implementation manner, in step S1: the molar ratio of bismuth nitrate pentahydrate to nitric acid is 1:19 - 21, the molar ratio of sodium tungstate dihydrate to cetyltrimethylammonium bromide is 1:1.3 - 1.4, and the molar ratio of bismuth nitrate pentahydrate to sodium tungstate dihydrate is 1.9 - 2.1:1.

[0042] In one or more embodiments of this implementation manner, in step S1: the stirring time is 0.5 - 1 h; the temperature of the hydrothermal reaction is 190 - 210 °C, and the time of the hydrothermal reaction is 5 - 7 h.

[0043] In one or more embodiments of this embodiment, in step S2: the ratio of black phosphorus to N-methylpyrrolidone is 1 mg: 0.9-1.1 mL; the inert gas is argon.

[0044] In one or more embodiments of this embodiment, in step S2: the ultrasonic time is 24-48 h; the rotation speed of centrifugation after ultrasonic treatment is 1900-2100 rpm, the time is 15-25 min, and it is carried out 2-3 times; the rotation speed of centrifugation of the supernatant again is 10500-11500 rpm, and the time is 15-25 min.

[0045] In one or more embodiments of this embodiment, in step S3: the mass ratio of black phosphorus nanosheets to the total mass of black phosphorus nanosheets and bismuth tungstate is 0.01-0.09:1, and the inert gas is argon.

[0046] In one or more embodiments of this embodiment, in step S3: the time of sealed ultrasonic treatment is 0.5-2 h, and the time of stirring is 6-12 h.

[0047] The second typical embodiment of the present invention is a black phosphorus / bismuth tungstate two-dimensional S-type heterojunction obtained by the preparation method described in the first typical embodiment.

[0048] The synergistic effect of the formed S-type heterojunction and the Bi-O-P carrier transport channel after compounding can promote the separation of carriers, provide sufficient electrons and holes for oxidation and reduction reactions respectively, and improve the photocatalytic efficiency.

[0049] The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction has abundant reduction active sites BP, which can enhance the adsorption and activation of CO2. The S-type heterojunction provides sufficient electrons for the PCET process and accelerates the kinetic process of the reduction reaction. At the same time, bismuth tungstate, as the active site for oxidizing benzylamine, reduces the thermodynamic energy barrier of the oxidation reaction. The mutual promotion of the coupling reaction makes it show excellent catalytic activity, stability and product selectivity in the process of carbon dioxide reduction.

[0050] The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction has obvious performance advantages compared with pure BWO and BP photocatalysts. It has better CO2 physical and chemical adsorption capabilities compared with the BWO photocatalyst, can better promote the activation process of carbon dioxide, and also has better light response performance and carrier transfer efficiency, can significantly inhibit the recombination of photogenerated electron-hole pairs, and promote the kinetic process of the reduction reaction. In addition, in the process of benzylamine oxidation, it also has better ability to adsorb reactants and reduces the reaction activation energy.

[0051] The third typical embodiment of the present invention is the application of the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction described in the second typical embodiment in photocatalytic carbon dioxide reduction coupled with benzylamine oxidation.

[0052] The fourth typical embodiment of the present invention is a method for photocatalytic carbon dioxide reduction coupled with benzylamine oxidation, which includes the following steps:

[0053] Disperse the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction described in the second typical embodiment in a reaction solvent containing benzylamine, ultrasonically disperse it, introduce CO2 until saturated, and irradiate with full light for photocatalytic reaction;

[0054] Preferably, the reaction solvent is a mixture of acetonitrile, water and benzylamine, and the volume ratio of acetonitrile, water and benzylamine is 490 - 510:1:1;

[0055] Preferably, the ratio of the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction to benzylamine is 1mg:3 - 5μL.

[0056] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the following will specifically describe the technical solution of the present invention in detail with reference to specific examples and comparative examples.

[0057] Example 1

[0058] (1) Dissolve 0.9701g of bismuth nitrate pentahydrate in 40mL of nitric acid aqueous solution (molar concentration is 1M / L), stir for 0.5h to form solution A; dissolve 0.3297g of sodium tungstate dihydrate and 0.05g of cetyltrimethylammonium bromide (CTAB) in 40mL of water to form solution B, slowly drop B into A, stir for 1h, then transfer it to a 100mL autoclave, react at 200°C for 6h, wash and dry the obtained sample, which is marked as BWO.

[0059] (2) Immerse 20mg of bulk black phosphorus (BP) in 20mL of N-methylpyrrolidone (NMP), introduce argon to exhaust for 30min, seal it and ultrasonically treat it for 24h in an oxygen-free environment, centrifuge at a speed of 2000r for 20min to remove bulk BP twice, centrifuge the supernatant again at 11000r for 20min to obtain BP precipitate nanosheets, and then disperse them in 10mL of NMP to obtain a BP suspension with a concentration of 0.5mg / mL.

[0060] (3) After mixing 0.04g of BWO with 1mL of BP suspension, exhaust for 30min under an argon atmosphere, then seal and ultrasonically treat it for 1h, and then stir for 12h to obtain a uniform mixture. Wash and vacuum dry to finally obtain light gray black phosphorus / bismuth tungstate two-dimensional S-type heterojunction (BP / BWO) powder, denoted as 1% BP / BWO.

[0061] Example 2

[0062] Different from Example 1, 0.04 g of BWO was mixed with 2.5 mL of BP suspension, and the obtained black phosphorus / bismuth tungstate two-dimensional S-type heterojunction (BP / BWO) was denoted as 3% BP / BWO.

[0063] Example 3

[0064] Different from Example 1, 0.04 g of BWO was mixed with 5 mL of BP suspension, and the obtained black phosphorus / bismuth tungstate two-dimensional S-type heterojunction (BP / BWO) was denoted as 6% BP / BWO.

[0065] Example 4

[0066] Different from Example 1, 0.04 g of BWO was mixed with 8 mL of BP suspension, and the obtained black phosphorus / bismuth tungstate two-dimensional S-type heterojunction (BP / BWO) was denoted as 9% BP / BWO.

[0067] Comparative Example 1

[0068] 0.9701 g of bismuth nitrate pentahydrate was dissolved in 40 mL of nitric acid aqueous solution (molar concentration is 1 M / L), and stirred for 0.5 h to form solution A; 0.3297 g of sodium tungstate dihydrate and 0.05 g of cetyltrimethylammonium bromide (CTAB) were dissolved in 40 mL of water to form solution B. Solution B was slowly added dropwise to A, stirred for 1 h, and then transferred to a 100 mL autoclave, reacted at 200 °C for 6 h, and the washed and dried sample was marked as BWO.

[0069] Comparative Example 2

[0070] 20 mg of bulk black phosphorus (BP) was immersed in 20 mL of N-methylpyrrolidone (NMP), purged with argon for 30 min, sealed and then ultrasonically treated for 24 h in an oxygen-free environment. The centrifugation speed was 2000 r, and the time was 20 min. The bulk BP was removed twice, and the supernatant was centrifuged again at 11000 r for 20 min to obtain BP nanosheets.

[0071] The TEM image of BWO obtained in Comparative Example 1 is as shown in Figure 1 a in, and the AFM image is as shown in Figure 1 b in, and the thickness curve obtained by AFM is as shown in Figure 1 c in. The TEM image of BP obtained in Comparative Example 2 is as shown in Figure 1 d in, and the AFM image is as shown in Figure 1 e in, and the thickness curve obtained by AFM is as shown in Figure 1as shown in f in. TEM shows that both BWO and BP are thin transparent flake structures, and the thicknesses obtained by AFM are 3.04 and 2.3 nm respectively, both belonging to two-dimensional layered structures. The XRD of BWO obtained in Comparative Example 1, BP obtained in Comparative Example 2, and BP / BWO obtained in Examples 1-4 is as Figure 1 shown in g in. It can be seen that BP / BWO all maintain a crystal structure similar to that of BWO, and diffraction peaks of BP appear when the addition amount of BP increases.

[0072] The morphology and element distribution of the 6% BP / BWO photocatalyst prepared in Example 3 are as Figure 2 shown. TEM ( Figure 2 a) in and HRTEM ( Figure 2 c) in can see that it is still nanosheet-like after compounding, and the change of lattice fringes in close contact at the interface can be observed, indicating the successful synthesis of the heterojunction. The Mapping diagrams ( Figure 2 c-g) in show that the elements Bi, W, O, and P are evenly distributed.

[0073] The DRS and band gap analysis of BWO obtained in Comparative Example 1, BP obtained in Comparative Example 2, and BP / BWO obtained in Examples 1-4 are as Figure 3 shown. It can be seen from the DRS test that after the addition of BP, the absorbance of BP / BWO in the visible light range increases, and the band gap of BWO obtained by Tauc calculation is 2.6 eV.

[0074] The XPS analysis of BWO obtained in Comparative Example 1 and BP / BWO obtained in Examples 1-4 is as Figure 4 shown. Compared with BWO, the Bi 4f and W 4f orbitals in 6% BP / BWO shift to lower binding energies, while the P 2p orbital shifts in the opposite direction, indicating that electron transfer occurs between bismuth tungstate and black phosphorus, and it is towards the BP end.

[0075] Performance test experiment:

[0076] For the transient photocurrent experiment, the photoelectrochemical test of the catalyst adopts the standard three-electrode mode. A 0.5 M Na2SO4 (pH = 6.8) solution is used as the electrolyte, the FTO substrate coated with the catalyst is used as the working electrode, Ag / AgCl is used as the reference electrode, and a Pt sheet is used as the counter electrode. A 300 W xenon lamp equipped with a 420 nm cut-off filter (λ≥420 nm) is used as the light source. The working electrode is prepared by the spin-coating method as follows: 50 mg of the catalyst is dispersed in a mixed solution of water, ethanol, isopropanol, and Nafion and ultrasonically dispersed for 30 minutes, and the obtained suspension is spin-coated on a clean fluorine-doped tin oxide (FTO) glass.

[0077] Photocatalytic CO2 reduction test:

[0078] The photocatalytic activity of the prepared samples was carried out in a sealed Pyrex container with a 300 W xenon lamp as the light source. The specific process was as follows: 5 mg of the catalyst was evenly dispersed in the reaction solvent, namely 10 mL of acetonitrile, 20 μL of water, and 20 μL of benzylamine, and ultrasonic dispersion was carried out evenly. Before irradiation, high-purity CO2 was continuously injected into the reactor for 30 min, and the temperature was maintained at 25 °C throughout the test process. The reduction gas and liquid products were analyzed by a GC9790II gas chromatograph equipped with FID and TCD detectors. The oxidation products were analyzed by QP2020plus GC-MS. The recycling test was as follows: The used catalyst was washed with water multiple times and dried in a vacuum oven for 12 hours. The subsequent tests were carried out similarly to the above photocatalytic reaction steps.

[0079] Test results:

[0080] The photocatalytic performance of BWO obtained in Comparative Example 1 and BP / BWO obtained in Examples 1-4 is as Figure 5 shown. As Figure 5 shown in a of Figure 5 , with the increase of the addition amount of BP, the yields of CO, CH3OH, and C2H5OH first increased and then decreased, and the highest yields reached 12.4, 8.6, and 61.3 μmol / g / h, respectively. As Figure 5 shown in b of Figure 5 , with the increase of the addition amount of BP, the oxidation products also showed a trend of first increasing and then decreasing, and the highest yield was 413.3 μmol / g / h. Figure 5 shown in c of Figure 5 , when no light, catalyst, and CO2 were added, no reduction products were generated, indicating that this reaction was a photocatalytic carbon dioxide reduction reaction.

[0081] Figure 6 are the in-situ XPS diagram of the 6% BP / BWO photocatalyst and the reaction mechanism diagram of the S-type heterojunction. As Figure 6 shown in a and b of Figure 6c in it summarizes the mechanism of this heterojunction in the photocatalytic reaction. Before the combination of BWO and BP, BWO has a larger work function. After the combination, electrons spontaneously shift from BP to BWO until the Fermi levels of the two reach the same position, and band bending and built-in electric fields are generated. When the photocatalytic reaction is carried out, the electrons of BWO and BP are excited from the valence band to the conduction band. Under the action of the bent band and the built-in electric field, the electrons on the conduction band of BWO recombine with the holes on the valence band of BP, promoting the separation of photo-generated carriers and enabling them to maintain good redox capabilities respectively. The carbon dioxide reduction and benzylamine oxidation reactions occur on BP and BWO respectively.

[0082] The PL spectrum TRPL diagram, photocurrent diagram and impedance of BWO and 6% BP / BWO photocatalysts are as Figure 7 shown. 6% BP / BWO has better carrier separation ability than BWO. Figure 7 a in it shows that compared with BWO, the PL spectrum shows more obvious PL quenching of 6% BP / BWO, indicating more effective carrier transfer and separation in 6% BP / BWO. Figure 7 b in it, the TRPL spectrum shows that the average emission lifetime of 6% BP / BWO (0.35 ns) is shorter than that of BWO (0.88 ns), indicating that rapid carrier transfer has occurred in the reaction. Obvious PL quenching and lifetime reduction indicate more effective carrier transfer and separation in 6% BP / BWO. In addition, photoelectrochemical tests were also carried out to further examine the charge separation and transfer ability. As Figure 6 c in it shows, compared with BWO, 6% BP / BWO exhibits the highest photocurrent, which means that the transfer and separation of photo-generated carriers in 6% BP / BWO are more effective. In addition, Figure 6 d in it also observes that the interfacial charge transfer resistance of 6% BP / BWO is lower than that of BWO. It is proved that the photo-excited carriers of 6% BP / BWO are more easily captured by the reaction substrate, so the carbon dioxide reduction reaction can be triggered faster.

[0083] Figure 8 It is the comparison diagram of the adsorption energy and the activation free energy change diagram for the oxidation of benzylamine and benzyl carbamate by the BWO and BP / BWO photocatalysts prepared in Example 1. Figure 8 a in it is the calculation of the adsorption ability of BWO and BP / BWO for benzylamine and benzyl carbamate molecules respectively. Compared with BWO, BP / BWO has a more negative adsorption energy, indicating that this material is beneficial to the adsorption of the two reactants, thus facilitating the subsequent reaction. And compared with benzylamine, both materials are more inclined to adsorb benzyl carbamate molecules, indicating that in the oxidation reaction, the oxidation of benzyl carbamate is the reaction that occurs preferentially. Figure 8 b in it and Figure 8In this case, c is the calculation of the activation free energy of benzylamine and benzylcarbamic acid molecules for the two materials. Similar to the adsorption energy situation, BP / BWO requires the lowest free energy barrier to overcome when activating benzylcarbamic acid, so it occurs preferentially.

[0084] Figure 9 The figure shows the theoretical calculation configuration diagrams during the adsorption, activation, and conversion processes of CO2 molecules on the surface of the BP / BWO photocatalyst. The reaction steps are as follows: a. CO2(g) → *CO2; b. *CO2 + H + + e - → *COOH; c. *COOH + H + + e - → *CO + H2O; d. *CO + H + + e - → *CHO; e. *CHO + H + + e - → *CH2O; f. *CH2O + H + + e - → *CH3O; g. *CH3O + H + + e - → *CH3OH; h. *CH3O + 2H + + 2e - → *CH3 + H2O; i. *CH3 + *CO → *CH3CO; j. *CH3CO + H + + e - → *CH3CHO; k. *CH3CHO + H + + e - → *CH3CH2O; l. *CH3CH2O + H + + e - → *CH3CH2OH.

[0085] Figure 10 It is the in-situ infrared, theoretical calculation, and catalytic reaction mechanism diagrams of the BWO and BP / BWO photocatalysts. Figure 10 In a, it shows the changes of intermediates during the CO2 reduction process after illumination. Key intermediates such as *COOH, *CO, *CH3, *CHO, and the intermediate *C2H5O after C-C coupling can all be detected, and with the prolongation of the illumination time, the signal peaks become stronger. Figure 10 In b, it is the free energy change diagram comparing BWO and BP / BWO. BWO has a lower energy barrier in generating CO, so it tends to generate CO, while BP / BWO has a lower barrier in generating CH3CH2OH than in generating CO and CH3OH, so it preferentially generates ethanol products. Figure 10Among them, c is the path diagram of benzylamine oxidation and the overall photocatalytic reaction mechanism diagram. Path 1 is the main oxidation route, that is, benzylamine reacts with carbon dioxide to form benzylcarbamic acid, which is then oxidized by holes. Path 2 is the spontaneous oxidative coupling process of benzylamine. Under light illumination conditions, after the S-scheme heterojunction effectively separates photogenerated carriers, reduction and oxidation reactions occur at the BP and BWO ends respectively.

[0086] In summary, the material structure of BP / BWO and the schematic diagram of the reaction mechanism for photocatalytic reduction of carbon dioxide and oxidative coupling of benzylamine (BA) to N-benzylidene benzylamine (BDA) are as Figure 11 shown.

[0087] The above description is only the preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A black phosphorus / bismuth tungstate two-dimensional S-type heterojunction, characterized in that, The preparation method of the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction includes the following steps: S1. Dissolve bismuth nitrate pentahydrate in an aqueous nitric acid solution, stir to generate solution A; dissolve sodium tungstate dihydrate and cetyltrimethylammonium bromide in water to generate solution B; add solution B dropwise to A, stir and then carry out a hydrothermal reaction, wash and dry the product to obtain bismuth tungstate; S2. Immerse black phosphorus in N-methylpyrrolidone, introduce an inert gas for exhaust, seal and ultrasonicate in an oxygen-free environment, centrifuge after ultrasonication, collect the supernatant and centrifuge again to obtain black phosphorus nanosheets; S3. Mix bismuth tungstate and black phosphorus nanosheets, introduce an inert gas for exhaust, seal and ultrasonicate and then stir to obtain a mixture, wash and dry the mixture to obtain the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction.

2. The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction according to claim 1, wherein In step S1: The molar ratio of bismuth nitrate pentahydrate to nitric acid is 1:19-21, the molar ratio of sodium tungstate dihydrate to cetyltrimethylammonium bromide is 1:1.3-1.4, and the molar ratio of bismuth nitrate pentahydrate to sodium tungstate dihydrate is 1.9-2.1:

1.

3. The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction according to claim 1, characterized in that, In step S1: The stirring time is 0.5-1 h; the temperature of the hydrothermal reaction is 190-210 °C, and the time of the hydrothermal reaction is 5-7 h.

4. The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction according to claim 1, wherein In step S2: The ratio of black phosphorus to N-methylpyrrolidone is 1 mg: 0.9-1.1 mL; the inert gas is argon.

5. The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction according to claim 1, characterized in that In step S2: The ultrasonication time is 24-48 h; the rotation speed of centrifugation after ultrasonication is 1900-2100 rpm, the time is 15-25 min, and it is carried out 2-3 times; the rotation speed of centrifugation of the supernatant again is 10500-11500 rpm, and the time is 15-25 min.

6. The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction according to claim 1, wherein In step S3: The mass ratio of black phosphorus nanosheets to the total mass of black phosphorus nanosheets and bismuth tungstate is 0.01-0.09:1, and the inert gas is argon.

7. The black phosphorus / bismuth tungstate two-dimensional S-type heterojunction according to claim 1, characterized in that, In step S3: The time of sealed ultrasonication is 0.5-2 h, and the stirring time is 6-12 h.

8. The application of the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction according to claim 1 in photocatalytic carbon dioxide reduction coupled with benzylamine oxidation.

9. A method for photocatalytic carbon dioxide reduction coupled with benzylamine oxidation, characterized in that, It includes the following steps: Disperse the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction according to claim 1 in a reaction solvent containing benzylamine, ultrasonically disperse, introduce CO2 until saturated, and irradiate with full light for photocatalytic reaction.

10. The method according to claim 9, characterized in that, The reaction solvent is a mixture of acetonitrile, water and benzylamine, and the volume ratio of acetonitrile, water and benzylamine is 490-510:1:

1.

11. The method according to claim 9, characterized in that, The ratio of the black phosphorus / bismuth tungstate two-dimensional S-type heterojunction to benzylamine is 1 mg:3-5 μL.

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