Design Of ZnO-based Composite Photoanodes And Studies On Photoelectrochemical Water Splitting

ZnO nanometrials,a kind of n-type,direct,wide-band-gap,?-? metal oxide semiconducting material,possess a variety of outstanding optical and electrical properties,which have being widely applied in the solar cells,photoelectrochemical?PEC?cells and other energy conversion devices.Based on such materials,surface modification materials?gold nanoparticles,ZnS and Al₂O₃ etc.?,doping?vanadium?and structure optimization?nanosheet-nanorod mixed dimensional hierarchical structure?were sequentially taken into combination to form ZnO-based composite nanostructures,in which each component worked synergistically to compensate each other.Then the effects of surface plasmon resonance,surface passivation,poling and structure optimization on the photogenerated carriers' generation,separation and recombination were systematically explored for improved PEC performance of water splitting.The ZnO/ZnS/Au nanorods array was successfully sythesized as photoanode for PEC water splitting.The surface plasmon resonance of Au NPs extended the absorption spectrum to the visible light region,and the ZnS interlayer favored the movement of hot electrons into the conduction band of ZnO,resulting in an improved IPCE both in the visible regions.The solar-to-hydrogen efficiency of the ZnO/ZnS/Au photoanode was 2 times and 3.5 times that of ZnO/Au and pristine ZnO photoanodes,respectively.Au NPs and Al₂O₃ overlayer were sequentially deposited on the surface of ZnO NRs for PEC water splitting.The Al₂O₃ passivation layer by ALD decreased the surface defect states and photoexcited hole concerntration for the suppress of surface charge recombination,which finally enhanced the PEC performance.The optimal STH effiency reached 0.67%,1.7 times and 6.7 times that of ZnO/Au and pristine ZnO photoanodes,respectively.However,excessive deposition of the insulation overlayer would possibly hinder the transmission of electron resulting in photocurrent decay.Ferroelectric V-doped ZnO nanorods array was synthesized by hydrothermal process.Compared to pure ZnO NRAs,the V-doped ZnO NRs array showed low charge recombination,high charge carrier density and a negative shift of flat band,resulting in efficient charge separation.Thus,the maximum ABPE reached 0.83%,which was about 120%improvement compared to pure ZnO NRAs photoanode.After negative poling,the maximum ABPE increased to 1.04%,attributed to the enlarged Schottky barrier height at the photoanode/electrolyte interface by the dipole alignment.The ZnO nanosheet-nanorod mixed dimensional hierarchical structure was synthsized to improve the heterostructure interface area.The reuslts of FDTD simulation indicated that the 2D/1D hierarchical structure largely improved the effective heterostructure interface area of ZnO/CdS heterojunction,resulting in effective generation and separation of photogenerated electron-hole pairs.The introduction of Ni2+/Ni3+/Ni4+ red ox procedures largely accelerated holes consumption,and finally resulted in significantly improved photoconversion efficiency,as well as the stability.The solar-to-hydrogen efficiency of H-ZnO/CdS/Ni?OH?2 reached 4.12%,20.6 times that of ZnO nanosheets photoanode.In this thesis,the PEC performance of ZnO based composite nanostructures as well as their corresponding enhancement mechanism were in-depth studied.These works enriched the application of ZnO nanomaterials in PEC field and provided theoretical guidance and experimental basis for the application of ZnO nanomatrials in energy conversion deivces.