Surface And Interface Design Of Noble Cocatalyst On TiO₂ Nanosheets And Related Photocatalytic Mechanism Study

The combination of metal cocatalyst with semiconductor is a promising route to improve the solar-to-chemical conversion efficiency of photocatalysts.On one hand,the noble metal cocatalyst can trap the photo-generated electron from semiconductor,thus prevent the electron-hole separation.On the other hand,the metal cocatalyst act as high active sites for photocatalytic reduction reaction.This article mainly underlines the surface and interface design of cocatalyst in realizing high photocatalytic performance in H2 evolution from water spliting and reducing carbon dioxide(CO2)to hydrocarbons.Suitable shape of metal cocatalyst was chosen in realizing the surface and interface design of semiconductor-metal hybrid photocatalysts,and the advantages of 2D metal nanostructure was used to realize high photocatalytic performance.Metal hydrides was used as ideal cocatalyst to realize high photocatalytic performance in the reduction of CO2 to hydrocarbons.The photocatalytic performance in CO2 reduction can be promoted by grain boundaries(GBs)on metal cocatalysts.The specific contents of the research can be summarized as follows:1.Ultrathin palladium(Pd)nanosheets are integrated with semiconductor titanium dioxide(TiO2)nanosheets for photocatalytic hydrogen evolution,which acts as cocatalyst and plasmonic agent in ultraviolet and visible-near-infrared spectral regions,respectively.Owing to the unique two-dimensional(2D)nanostructure,the Pd nanosheet cocatalyst realizes the large TiO2-Pd interfacial area for electron transfer as well as large Pd exposed area for reduction reaction,while the plasmonic Pd nanosheets offer strong vis-NIR light absorption for "hot" electron production as well as large interfacial area for "hot" electron injection.As a result,the Pd nanosheets achieve improved photocatalytic activity in comparison with three-dimensional Pd nanotetrahedrons under both light irradiations.2.The hydriding treatment of palladium(Pd)cocatalysts into ?-phase palladium hydride(PdH0.43)can greatly promote the photocatalytic reduction of CO2 into methane(CH4).Pd nanocrystals in cubic and tetrahedral shapes were firstly in situ grown on TiO2 nanosheets,which were then transformed to PdH0.43 nanocubes and nanotetrahedrons through a facile solvothermal treatment,respectively.It was found that the hydrogen doping could change the electronic structure and enhance the photo-induced electron trapping ability of Pd.On the other hand,PdH0.43 cocatalyst with lattice expansion could adsorb the produced H2 during the photocatalytic reactions,while the hydrogen atoms on the surface of PdH0.43 cocatalyst could accelerate the transformation of CO2 into CH4 through a hydrogenation process.As a result,in comparison with Pd,the corresponding PdH0.43 cocatalysts not only facilitate the electron-hole separation and prolong the carrier lifetime,but also enhance the CH4 yield while decrease the hydrogen(H2)and carbon monoxide(CO)production.3.The photocatalytic performance in CO2 reduction can be promoted by grain boundaries(GBs)on metal cocatalysts.In this work,metal(Pd and Rh)nanowires with high density of grain boundary terminations on the surface were loaded on TiO2 nanosheets,which as cocatalyst effectively prevent the H2 evolution reaction and greatly enhance the photocatalytic performance in CO2 reduction as compared with the corresponding metal nanoparticles without GB terminations.Two effects were believed to contribute to this enhancement:(1)nanowire structure facilitates the interfacial electron transfer from TiO2 to metal cocatalysts;(2)the surface GB terminations of metal cocatalysts are highly active sites for CO2 reduction reaction.This work highlights the rational architectural design of cocatalyst for enhanced photocatalytic performance.