Preparation Of Metal Sulfide Composites And Their Photoelectrocatalytic Reduction Of CO₂

As non-renewable and the most widely used energy sources,the over-consumption of fossil fuels will eventually result in the depletion of energy.And the excessive emission of CO₂ by burning fossil fuels will consequently cause a series of disasters in climate change and sea level rise.In nature,the photosynthesis of plants is the process of converting CO₂ and H₂O into organics under the sunlight irradiation.Inspired by this,scientists simulate the photosynthesis of plants through photoelectrocatalytic reduction of CO₂,hoping to solve the two major challenges of energy shortage and environmental pollution.The key to photoelectrocatalytic reduction of CO₂ is the design of catalysts.Among the common catalysts,metal sulfides attract the attention of many researchers due to the properties of unique electronic states and excellent absorption of visible light.However,the rapid recombination of photogenerated carriers and intrinsic photo-corrosion of metal sulfides inhibit the applications in large-scale.In this dissertation,a series of strategies,including morphology control,introduction of defects and heterojunction construction,have been used in this paper to further improve the performance of stability,activity and selectivity for metal sulfides in photoelectrocatalytic reduction of CO₂.（1）A hollow heterojunction of CdS/V_S-MoS₂ rich in S-vacancies was designed,synthesized and applied to the photoelectrocatalytic reduction of CO₂ in the mixe solution of KHCO₃ and Na₂SO₃.The morphology,chemical composition and defects of Cd S/V_S-Mo S₂ were characterized and analyzed by the technologies of SEM,TEM,XRD,Raman,XANES and EPR.In addition,the quick separation and migration of photo-generated carriers was confirmed by the XPS,femtosecond transient absorption spectra,PL spectra and charge separation efficiency.The different selectivity for products between inner and external surfaces of hollow catalysts was carefully investigated.The external surface generated HCOOH with the formation rate of 210μMh^-1 cm^-2,while the inner surface mainly produced C₂ chemicals（CH₃CH₂OH and CH₂OHCH₂OH）due to the increased collision probability of the intermediates in the confined cavity.A new mechanism of SH-assisted CO₂ reduction was proven by DFT calculation and the important active species such as^*OCHO,SH were further verified by the operando IR spectra.（2）Since the morphology and structure of the catalysts play a crucial role in the selectivity of C₂ products in CO₂ reduction,In₂S₃ catalyst with a honeycomb structure was grown on carbon paper and modified Cd S nanoparticles on the top and inside of the honeycomb In₂S₃.Experimental results indicated that the In₂S₃/Cd S heterojunction could phototlectrocatalytic reduction of CO₂ to HCOOH,CH₃COOH and CH₃CH₂OH products with a C₂ selectivity as high as 88.8%.Reasons for the excellent catalytic performance and selectivity of C₂ product were as follows:the construction of the heterojunctions that inhibited the recombination of photo-generated carriers;the unique photothermal properties of the carbon substrate for providing additional energy in the reaction and the honeycomb structure that increased the collision probability of intermediates and facilitated the C-C coupling.FDTD simulations showed that the larger aperture of the honeycomb structure,the slower decays of the electric field strength along the radial direction.And the electric field strength around Cd S nanoparticles was also significantly enhanced.（3）Nitrogen-doped carbon materials have merits of unique electronic states,large specific surface areas,and high electron mobility.Thus the hollow N-doped carbon spheres were prepared by a template method,followed by growing Cd In₂S₄nanosheets on the surfaces of N-C.XPS and elemental analysis indicated that the amount of the doping N,especially pyridine N and pyrrolic N,could be tuned by changing the annealing temperature.In addition,infrared thermal images and pyroelectric results confirmed the excellent photothermal and thermoelectric properties of carbon-based composite materials.Based on this understanding,the Cd In₂S₄-N/C-800 photoanode could generate the C₁ and C₂ products with the selectivity of C₂ products reaching up to 67%.Considering the electrolyte effect,K⁺ion was more conducive to the CO₂reduction than Na⁺and（NH₄）⁺ions.DFT calculations revealed that pyridine N could effectively adsorb K⁺ions,thereby lowering the energy barrier for CO₂ reduction.The key intermediates（^*=C=O and^*OC-COH）related to the formation of ethanol were observed in operando IR spectra.On the one hand,the above researches provide inspiration for the design of metal sulfide semiconductors,such as improving the catalytic activity and stability of catalysts through morphology regulation,modification of the chemical composition on surface and construction of heterojunctions.On the other hand,in-depth exploration is conducted on the mechanism of photoelectrocatalytic CO₂ reduction through the techniques of operando IR spectra,DFT calculation and finite element simulation,which provides theoretical guidance for the design of new photoelectrocatalytic systems in the future.