Facet Effect Of Palladium Nanoparticle Cocatalyst On Photocatalytic Performance Of Semiconductor

With the rapid development of the world economy,the demand of energy is increasing year by year.However,the resulting problems of environmental pollution and energy shortage have restricted the development of human societies.Therefore,the exploration of clean and renewable energy has become one of the hot topics concerned by the scientists all over the world.In recent decades,semiconductor materials,as photocatalysts,using solar energy to drive photocatalytic hydrogen production or carbon dioxide reduction reactions have attracted much attention.However,the quantum yield of a traditional semiconductor photocatalyst is very low due to the poor solar utilization and high recombination rate of photo-generated electron-hole pairs,leading to a weak photocatalytic efficiency.Pd nanoparticles are usually used as co-catalysts to improve the photocatalytic activity of semiconductors,because Pd can promote the separation of photogenerated electron-hole pairs to extend the lifetime of photogenerated carriers,meanwhile reducing the over-potential of the reactions.In addition,Pd shows a shape-dependent selectivity when used as the co-catalyst due to the different surface structures of different shape of Pd nanoparticles.Therefore,the study of shape-dependent performance of Pd co-catalysts is of great significance for the design and optimization of metal/semiconductor composite photocatalysts.In this thesis,we investigate the photocatalytic carbon dioxide reduction or hydrogen production activities of Pd co-catalysts with differnet exposed facets coupled with different semiconductor photocatalysts,to further demonstrate the facet effect of Pd nanoparticle cocatalyst on the photocatalyt performance of semiconductors.The main research contents are as follows:First,nanotetrahedral and nanocube Pd nanoparticles are synthesized by solution-based morphology control method and are loaded on the surface of g-C₃N₄substrate by electrostatic adsorption-deposition method for photocatalytic carbon dioxide reduction performance test.The two types of Pd nanoparticles exhibit well-defined shapes and have a similar average size?11 nm?but with different exposed facets.The main products of photocatalytic carbon dioxide reduction in this study are methane and methanol.The results show that the photocatalytic activity of Pd nanoparticles supported on g-C₃N₄ photocatalyst is affected by the shape of Pd co-catalyst.The experimental results and theoretical calculations show that the tetrahedral Pd nanoparticles with exposed{111}facets are more efficient to be co-catalysts than cubic Pd nanoparticles with exposed{100}facets,which is due to the more preferable electron-sink effect,CO₂ adsorption and CH₃OH desorption capability.Secondly,nanotetrahedral and nanocube Pd/cubic-TiO₂ composites are used for photocatalytic hydrogen production to study the facet effect of Pd nanoparticles.First,cubic-TiO₂ is prepared via a hydrothermal method at 180 ^oC for 24 h.Then the tetrahedral and cubic Pd nanoparticles are in-situ deposited onto the surface of cubic-TiO₂,respectively.The size of the prepared cubic-TiO₂ is about 100 nm,and the sizes of cubic and tetrahedral Pd nanoparticles are both about 14±2 nm.Experiment results show that the Pd nanoparticles used as co-catalysts can significantly improve the hydrogen production efficiency of cubic-TiO₂,because Pd can promote the transfer of photo-generated electrons from the conduction band of TiO₂ to the surface of Pd nanoparticles,surpressing the recombination of the photogenerated charge carriers.Moreover,the photocatalytic hydrogen production efficiencies of tetrahedral Pd/TiO₂ composite are all higher than that of the cubic Pd/TiO₂ composite in the presence of different sacrificial agents,indicating that the photocatalytic activity of Pd/cubic-TiO₂ composite photocatalysts is greatly affected by the shape of Pd nanoparticles but shows no selectivity towards the type of sacrificial agent.The experimental results and theoretical calculations indicate that the tetrahedral Pd nanoparticles with exposed{111}facets possess better electron capture ability and H₂O molecular adsorption capability than that of the cubic Pd nanoparticles with exposed {100} facets.