Construction Of Transition Metal Loaded Black TiO₂ Nanosheet Arrays And Study On Photocatalytic Reduction CO₂ Performance

The gradual increase of CO₂ concentration in the atmosphere is one of the main factors leading to global warming.It has become a global problem to reduce CO₂ emissions from the source and realize its resource utilization.Photocatalytic CO₂ reduction can not only reduce the concentration of CO₂ in the atmosphere,but also convert it into valuable fuel,which is a more feasible and promising method to solve energy and environmental problems.However,the efficiency of photocatalytic CO₂ reduction is still very low at present.The design and synthesis of photocatalyst is the key to solve the efficiency problem.TiO₂ due to its non-toxic harmless,low cost,stable chemical properties and advantages to become one of the preferred material of photocatalytic CO₂ reduction.But its own existence big band gap(3.2 e V),low the visible light utilization rate,easily recombined photogenerated carriers,less surface active sites,this faults limit the catalyst widely to used in the field of photocatalytic CO₂ reduction.In order to solve the above problems,the photocatalytic CO₂ reduction of TiO₂ was improved by the method of oxygen vacancy doping and surface loaded co-catalyst.(1)First,TiO₂ nanosheet arrays with highly active{001}surface were grown on FTO conductive glass by a simple hydrothermal method,and black TiO₂ nanosheet arrays with oxygen vacancy doped were prepared by aluminothermic reduction method at 500?.Then,the effects of different aluminothermic reaction time on the carrier concentration,light absorption and photocatalytic CO₂ reduction of black TiO₂ were studied.The results show that,without damaging the structure of TiO₂ nanosheet,the aluminothermic reaction can capture oxygen atoms in the lattice of TiO₂ to form oxygen vacancies.The oxygen vacancies will form a Ti³⁺defect level below the conduction band of TiO₂,which narrates the band gap width of TiO₂,thus enlarging the light absorption range of black TiO₂ to the visible region(>400 nm).Meanwhile,when the aluminothermic reaction time is less than 4 h,the oxygen vacancy only forms on the highly active{001}surface of TiO₂,while when the aluminothermic reaction time is longer than 4 h,the oxygen vacancy forms on the{001}and{101}surfaces.When the oxygen vacancy is formed on the{001}facet,black TiO₂ has the highest carrier concentration and the highest photocatalytic reduction rate of CO₂ to CO,the carrier concentration is 1.178×10¹⁸cm^-3,and the CO generation rate is 128.5?mol·g^-1·h^-1.Further analysis shows that the Fermi level of{101}facet is higher than{001}facet when there are oxygen vacancies on{001}facet and{101}facet,which accelerates the recombination of electrons and holes on{001}facet,thus slowing down the rate of photocatalytic reduction of CO₂ to CO.(2)In order to improve the problems of less active site on the surface of black TiO₂nanosheets and the easy recombination of photogenerated carriers,a modification method of supporting cocatalyst on the surface of black TiO₂ was proposed.The effects of nickel-copper alloy co-catalyst on the photochemical properties of black TiO₂ and the photocatalytic CO₂reduction of products were studied by electrodeposition method.The results show that after electrodeposition,30-50 nm nickel-copper nanoparticles are uniformly distributed on the surface of black TiO₂,and nickel-copper act as electronic traps to promote the separation of photogenerated carriers on the sample surface.The photochemical test further showed that with the increase of copper content in the nickel-copper nanoparticles loaded on the surface of black TiO₂,the photocurrent density of the composite material gradually increased and the electrochemical impedance gradually decreased,indicating that copper would accelerate the electron transfer to the electrolyte solution.In the performance test of photocatalytic reduction of CO₂,it is found that nickel-copper alloy co-catalyst is favorable to photocatalytic reduction of CO₂ to methane,and when the content ratio of nickel-copper nanoparticles is 54:46,the rate of photocatalytic reduction of CO₂ to methane reaches 18.5?mol·g^-1·h^-1,and the methane selectivity of the product can reach 35%.