Silicon Photoanode Modified By Core-shell Structured Ni@Ni?OH?₂ Particles For Water Oxidation

The sustainable development of human society is in urgent need the exploitation and utilization of renewable energy when faced with the growing shortage of fossil energy and serious problem of environmental pollution.The direct conversion of solar energy into clean and storable hydrogen fuel via photoelectrochemical(PEC)water splitting demonstrates an attractive approach to solve ever-growing shortage of fossil energy and environmental problem.According the study,during the reaction process of PEC,the separation and injection efficiency of photo-generated carriers on the surface of photoelectrode is the critical factor that restricts the performance improvement.Therefore,investigating the recombination mechanism of photo-generated carriers and enhancing the separation and injection efficiency is the key and difficult point in the field of PEC water splitting.In this dissertation,we aim to prepare high efficienct and stable Si-based PEC cell and put efforts into the investigation of too positive onset potential and poor separation and injection efficiency.By constructing metal-insulator-semiconductor-structured photoanode with core-shell nanoparticles,we successfully improve the photocurrent density,decrease the open potential of the photoelectrode and enhance the stability of the photoanode without influencing the absorption of light and reveal the critical factors that influence the PEC water splitting performance of Si-based photoanode.The main research contents include:(1)Ni modified n-Si photoanode with MIS structure is prepared by electrochemical deposition and the performance is optimized substantially.In order to solve the unstability of n-Si photoanode,a protection layer is needed for Si-based photoanode.In general,researchers use electron beam evaporation or megnetron sputtering to prepare a passivation layer like Al2O3?TiO2 or catalyst layer like Ni?Co?Ir on the surface of electrode,however this process is very strict with the equipment and air condition and the thickness of prepared catalyst layer must be contolled within several nanometers due to the influence of catalyst layer on light absorption.We take the electrochemical deposition method to deposit island-distributed Ni nanoparticles through reduction reaction and successfully prepare photoanode with n-Si/SiOx/Ni structure.There are three functions of the Ni nanoparticles:firstly,as a protection layer,Ni particles decrease the contact area between Si and electrolyte without affecting the light absorption obviously and increase the stability;secondly,as a high-work-function metal,Ni particles can construct MIS structure with n-Si/SiOx to form Schottky junction,which could improve the separation of photo-generated carriers;thirdly,as a catalyst,NiOOH formed after the oxidazion of Ni could accelerate the OER kinetics.(2)Core-shell structured Ni@Ni(OH)2 nanoparticles are deposited on the surface of n-Si/SiOx through two-step electrochemical deposition and the injection efficiency of photo-generated carriers is enhanced greatly.During the reaction of Ni modified n-Si photoanode,the outermost layer of Ni particle is going to be oxidized to form NiOx/NiOOH,while the eventual thickness of the oxide does not change with the time expending instead of maintaining at 2-4nm.To investigated the influence of oxide thickness on the PEC performance,prepared n-Si photoanodes with inherent SiOx passivation overlayers,which were partially coated with Ni@Ni(OH)2 core-shell nanoparticles.The photovoltage output of the photoanode increases with increasing thickness of the Ni(OH)2 shell,and is influenced by the interactions between Ni and Ni(OH)2,the electrolyte screening effect and the p-type nature of the Ni(OH)2 layer.Compared to n-Si/SiOx/Ni photoanode,Ni@Ni(OH)2 core-shell nanoparticles with appropriate shell thicknesses coupled to n-type Si photoanodes enhance the catalytic activity and promote the separation of photogenerated carriers and improve the charge-injection efficiency to nearly 100%.