The Study Of Spatial Field Effect On Improving Semiconductor Photocatalytic Hydrogen Evolution

Solar-to-hydrogen(STH)conversion is considered to be an ideal approach to alleviate the energy crisis and environmental pollution problems.Semiconductor-based photocatalytic water splitting to produce hydrogen is one of attractive routes for solar energy conversion.Although great progresses have been made in resent years,the conversion efficiency of solar energy to hydrogen over semiconductor photocatalysts has been a key factor for restricting the development of photocatalytic water splitting.The crucial issues to enhance the photocatalytic efficiency is improving the number of photoexcited charges for surface reaction,reducing photoexcited charges recombination possibility.Spatial field effect has the intrinsic potential to improve photocatalytic performance of the semiconductor.Rational configuration of internal electric field in the semiconductor can not only drive the separation of photogenerated charges and reducing the recombination possibility,but also regulate density distribution of charges for improving the dynamic process.The present thesis was demonstrated four strategies for regulating inner spatial field in the semiconductor photocatalysts through designing these structures as following: spatial bandgap engineering,regulation and utilization of surface/interface defects,electromagnetic field effect of plasmonic metal.Theoretical calculation and experimental characterization are applied to meticulously explore the promotion effect of inner electric field to photocatalytic hydrogen evolution activity.The main contents and results are as follows:(1)A synergistic strategy of spatial bandgap engineering and surface defect was introduced into the development of photocatalysts.Tuning the composition of semiconductor particle from inside to outside can realize gradient distribution of the bandgap width.A two-step approach,including a sol-gel method and an anion exchange route,was adopted to prepared ZnxCd1-xS/SiO2 photocatalyst.The composition of tunability of ZnxCd1-xS particles is thermodynamically driven by the different solubility product constant between Zn S and CdS.ZnxCd1-xS nanoparticles are well dispersed on the surface of silica.XPS results by Ar+ bombardment show that the molar ratio of Cd:Zn in ZnxCd1-xS particles is present as a decreasing trend of gradient,which is gradually tuned from 0.305:1 to 0.490:1.DFT theoretical analyses and UV-vis spectra show that the spatial bandgap engineering formed along the inside to outside of the ZnxCd1-xS particles.At least a 0.17 e V potential difference is present between the inside and outside.This bulit spatial electric field from potential difference could effectively drive the initiative transportation of photogenerated electron-hole pairs to the surface.Without adding any cocatalysts,the photocatalytic H2 evolution performance shows that the quantum efficiency of ZnxCd1-xS/SiO2 at 420 nm can reach up to 45.6%.PL spectra and XPS analyses show that the surface defects originated from surface Cd cations with a low oxidation state.According to the theoretical result,this in situ formed Cd plays the role of a cocatalyst for hydrogen evolution.(2)Au nanoparticle with surface plasmon resonance(SPR)effect was introduced into the construction of semiconductor photocatalysts.By controlling the distribution and arrangement of Au nanoparticles,three kinds of plasmonic building blocks with Au and ZnxCd1-xS semiconductor were designed and prepared,including isolated distribution of Au nanoparticles embedded into ZnxCd1-xS,Au nanochain enbedded into ZnxCd1-xS,and Au nanoparticles loaded on the surface of ZnxCd1-xS.The plasmonic building block that embedding Au-nanochain into ZnxCd1-xS semiconductor(Au@ZnxCd1-xS-C)can boost the highest photocatalytic hydrogen evolution activity among three Au@ZnxCd1-xS photocatalysts.The optimal quantum efficiency at 420 nm can reach up to 54.6%.Both FDTD theoretical simulation and experimental characterizations show that Au-nanochain could give rise to much higher electromagnetic field by resonance coupling effect between Au particles when excited by the incident light.Embedded build block facilitates the nearby ZnxCd1-xS semiconductor capturing the highest field strength and boosts the formation rate and lifetime of electron-hole pairs in the nearby ZnxCd1-xS semiconductor.Ultimately,the photocatalytic hydrogen evolution activity of semiconductor photocatalyst was improved.(3)In this section,we have meticulously studied the positive role of surface defect of the semiconductor in photocatalytic hydrogen evolution.CdS is chosen as a parent semiconductor to build the surface defect.Firstly,theoretical simulation was adopted to investigate the formation process of surface defects,bandgap structure variation,and charge density change in the photocatalytic reaction process.The result shows that the configuration of S vacancy in the second layer(2L-Sv)could give rise to the higher electron cloud density around the nearest neighboring Cd atoms.And the introduction of defect state level in the surface layer of CdS coud thermodynamically drive directional transfer of photogenerated electrons to the surface,which provides the theoretical basis for promoting the photocatalytic reaction process.In experiment,CdS nanoparticles with abundant surface defects were well dispersed on the surface of TiO2 semiconductor by impregnation and anion exchange method,which is denoted as CdS/TiO2.The photocatalytic hydrogen evolution rate of TiO2 increases significantly with the introduction of CdS nanoparticles.When the weight amount of CdS was 1.0%,the H2 evolution rate can be enhanced to 152.4 mmol/h/g CdS under full spectrum irradiation.The experimental characterization shows that S vacancy of defect configuration is formed on the surface of CdS nanoparticle,which induces the higher electron cloud density around the neighboring Cd atoms and the formaction of Cd cations with a low oxidation state.The photogenerated electrons transfer to the surface of CdS nanoparticle and join in the reduction of surface Cd cations to Cd.Hence,an induction period was present at the beginning of the reaction.These defect states build inner electric field for not only driving the transformation of photogenerated electrons to the surface,but also providing the active site for surface reduction reaction.Photogenerated electrons of both CdS and TiO2 are effectively utilized due to the defect configuration on the surface of CdS,which results in the improvement of photocatalytic hydrogen evolution performance.(4)Learning from natural photosynthesis,a freestanding cocatalyst was designed to spatially separate the reaction sites of semiconductor-based photocatalysts.Silica-sphere-supported Pt nanoparticles(SSP)was prepared with the impregnation method and used as a hydrogen evolution center of spatial separation from two typical semiconductors TiO2 and CdS in the photocatalytic H2 evolution reactions.The experiments show that SSP could be used as a freestanding hydrogen evolution cocatalyst for semiconductor photocatalysts to increase photocatalytic H2 evolution rate,in which some rates are higher than the photocatalyst with semiconductor directly supported Pt nanoparticles.In situ photoluminescence characterization shows that SSP could effectively trap electrons from the photoexcited semiconductors in the process of particle collision,and then complete the hydrogen evolution reaction over the Pt nanoparticles.The reduction active sites were spatially separated from semiconductor photocatalysts and the recombination possibility of electron-hole pairs in semiconductors were well supressed due to the presence of SSP.In summary,four strategies were applied to regulate and control spatial electric field and surface active site of the semiconductor.The photocatalytic hydrogen evolution performance of the semiconductor is enhanced.Combined with theoretical calculation and experimental characterization,the impact of the spatial field effect on the structure of photocatalysts,bandgap variation,and photogenerated charge behavior was studied systematically.This work might provide some reference and guidance for further designing highly efficient photocatalysts for water splitting.