Synthesis And Coordinative Enhancement For Photocatalytic H₂ Evolution Of TiO₂ With Metals And Their Interfacial Active Sites

Recently,photocatalytic H₂ evolution from watersplitting by semiconductors has been considered as one of the most promising ways to solve energy problems.TiO₂ is one of the most promising photocatalyst due to its good stability,abundant resources and nontoxicity.Owing to its wide band gap（E_g=3.2 eV）,it can only respond to ultraviolet light（about 5%of sunlight）,rather than visible light.Moreover,TiO₂exhibits very low quantum yield and photocatalytic activity due to rapid recombination of photogenerated electron-hole pairs.Therefore,it is very important to improve the photocatalytic H₂-evolution performance by modification methods.Surface-cocatalyst modification has been regarded as one of the most ideal strategies for realizing efficient photocatalytic H₂-evolution performance.Noble metals（such as Pt,Au,Ag,Pd）as well-known cocatalysts have been extensively demonstrated to improve the photocatalytic H2-evolution performance of TiO2 and other photocatalysts.The enhanced activity can mainly be attributed to the fact that the noble metals can rapidly capture and transfer electrons from the semiconductor surface.In addition to noble metals,low-cost and earth-abundant non-noble metals（such as Cu,Co,Ni）as electronic transfer media in photocatalytic process have been reported as electronic cocatalyst.In the process of photocatalytic H₂ evolution,electron transfer and following interfacial H₂-evolution reactions both can affect the activity of photocatalytic H₂-evolution performance.However,almost all the metals（except for Pt）lack of interfacial active sites,which leads to a limited enhanced photocatalytic H₂-evolution performance.Therefore,loading catalytic active sites on the surface of metals can further enhance the H₂-evolution activity of photocatalyst.In this work,based on the synergistic effect of metals and their interfacial active sites for the enhanced H2-evolution performance of TiO2 photocatalyst,the synergistic effects of metals（Au and Ni）and interfacial active sites（thiurea and NiS_x）are mainly investigated.The results are as follows:First,the TiO₂ photocatalyst modified by noble-metal Au and thiourea（denoted as TiO₂/Au-S）was prepared via the photodeposition and impregnation method.The metallic Au was reduced on the surface of TiO₂ by photodeposition,and then the thiourea molecules were selectively adsorbed on the surface of Au nanoparticles by impregnation method,to obtain TiO₂/Au-S photocatalyst.The results of photocatalytic experiments showed that TiO₂/Au-S achieved the highest photocatalytic H₂-evolution rate(120.15μmol h^-1)when thiurea concentration was0.05 mmol L^-1,which was greater than those of TiO₂(10.10μmol h^-1),TiO₂/Au(75.35μmol h^-1),and TiO₂/S(13.23μmol h^-1)by factors of 11.9,1.6,and 9.1 times,respectively.After recycling four times,the TiO₂/Au-S sample still showed steady and highly efficient H₂-evolution performance.Based on the above experimental results,the enhanced H₂-evolution performance of TiO₂/Au-S can be attributed to the excellent synergistic effect of Au and thiourea,namely,the Au nanoparticles functions as an effective mediator to transfer electrons from the TiO₂ surface while amine groups（-NH₂）in thiourea serves as interfacial active sites to capture H⁺ions from solution and promote following interfacial H₂-evolution reactions.The synergistic effect of Au and thiourea can further enhance the H₂-evolution activity of TiO₂.Second,the TiO₂ photocatalyst modified by noble metal Ni and NiS_x（denoted as TiO₂/Ni-NiS_x）was prepared by photodeposition and ahydrothermal reaction in this study.Ni nanoparticles were loaded on the TiO₂ surface through a photodeposition process,and then the NiS_x was generated on the Ni surface by ahydrothermal reaction method,to obtain TiO₂/Ni-NiS_x photocatalysts.Photocatalytic H₂-evolution activity showed that the TiO2/Ni-NiSx sample achieved the highest photocatalytic activity(223.74μmol h^-1)when the amount of Ni converted to NiS_x was 30 wt%,which was greater than those of TiO₂(10.10μmol h^-1),TiO₂/Ni(27.91μmol h^-1),and TiO₂/NiS_x(99.75μmol h^-1)by factors of 22.2,8.0,and 2.2 times,respectively.After recycling four times,the TiO₂/Ni-NiS_x sample still kept steady and efficient H₂-evolution activity.The improved H2-evolution performance of TiO2/Ni-NiSx can be attributed to the excellent synergistic effect of Ni and NiS_x,where the Ni nanoparticles function as an effective mediator to transfer electrons from the TiO₂ surface and unsaturated S atoms in NiS_x generated on the surface of Ni nanoparticles serve as interfacial active sites to capture H⁺ions from solution for promoting following interfacial reactions.The synergistic effect of Ni and NiSx can further enhance the H2-evolution activity of TiO₂.