Band Structure Engineering And Defect Control Of Tantalum Nitride Photoanode Material

The constantly increasing global energy demand and related environmental issues urgently motivate the pursuit of renewable and eco-friendly energy sources.Artificial photosynthesis that converts solar energy into storable chemical fuels is an attractive way to produce green and sustainable energy,as well as to address the intermittency of solar energy.Solar-driven photoelectrochemical(PEC)water splitting and carbon dioxide(CO₂)reduction are two principal artificial photosynthesis processes to produce sustainable hydrogen or carbon-based fuels.Regardless of the fuel product,water-oxidizing photoanodes are required for the construction of such PEC water splitting or CO₂reduction systems.Tantalum nitride(Ta₃N₅)is a promising photoanode material for PEC water splitting.However,due to the fact that many intrinsic defects are easily formed during the preparation of the material,the further improvement of its PEC performance is limited.In this dissertation,the effects of cationic doping on the intrinsic defects and PEC performance of Ta₃N₅ are studied.This study focuses on the effect of different dopants(Mg,La),doping concentrations,dopant distributions on the defect properties and energetic band structure of Ta₃N₅,and uses these knowledges as guidlines to construct high-efficiency PEC water splitting photoanodes.The main research contents and conclusions are as follows:(1)The controlled doping of cations in Ta₃N₅ thin films is achieved by nitriding oxide precursor films deposited using dual-source electron-beam co-evaporation.Compared with the traditional solid-state reaction,the doped cations and Ta ions are uniformly dispersed in the precursor films using the dual-electron beam co-evaporation method,which greatly reduces the thermodynamic barrier for ion diffusion and rearrangement in the nitridation process.This is beneficial to form effective and controllable doping in Ta₃N₅ thin films,and the prepared film structure can be directly used as an electrode for PEC water splitting.(2)Mechanism studies reveal that the deep-level defect density in Ta₃N₅ films can be effectively reduced by Mg doping,which suppresses the defect-related charge recombination,and significantly improve the separation and collection efficiencies of photogenerated carriers during PEC water splitting.After modifying with a borate-intercalated nickel cobalt iron oxyhydroxide(Ni Co Fe-B_i)oxygen evolution reaction co-catalyst,relatively high PEC water splitting performance is achieved with the Mg-doped Ta₃N₅ photoanode,and both the fill factor and the onset potential have been greatly improved.With a Mg doping concentration of about 13%(Mg/Ta),the Ni Co Fe-B_i/Mg:Ta₃N₅ photoanode yilelds an applied bias photon-to-current efficiency(ABPE)of2.87%,which surpasses the previously reported highest value of 2.72%for tantalum(oxy)nitride-based photoanode.In addition,Mg doping also changes the band structure of Ta₃N₅ films:the conduction band and valence band position gradually shift to the lower energy level with the increasing Mg content.(3)Based on the above findings,we propose to use gradient Mg doping for band structure engineering and defect control of Ta₃N₅.The gradient Mg doping profile in Ta₃N₅ induces a gradient of the band edge energetics,which greatly enhances the charge separation efficiency.Furthermore,defect-related recombination is significantly suppressed due to the passivation effect of Mg dopants on deep-level defects and,more importantly,the matching of the gradient Mg doping profile with the distribution of defects within Ta₃N₅.When combined with a Ni Co Fe-B_i oxygen evolution reaction co-catalyst,the gradient Mg-doped Ta₃N₅ photoanode achieves a low onset potential of?0.4V versus RHE,a high ABPE of 3.31%,which compares favourably with the previous literature on Ta₃N₅-based photoanodes,also exceeds that of photoanodes based on Fe₂O₃,Bi VO₄.Moreover,the photoanode also exhibited significantly improved stability,with no obvious decay at a high photocurrent density of over 8 m A cm² for 5 h.These results demonstrate that the use of this gradient doping strategy to suppress gradient-distributed defects and create internal band bending can be used as a guideline for improving the solar-to-hydrogen energy conversion efficiency of other semiconductor thin film light absorbers.(4)Studies have shown that La doping can inhibit the formation of deep level defect in Ta₃N₅ films,and increase the content of donor impurity oxygen instead of nitrogen defects,so as to significantly increase the concentration of carrier in Ta₃N₅ films,and then significantly improve the photocurrent.However,due to the large difference between the radius of La³⁺and that of Ta⁵⁺,the crystallinity of Ta₃N₅ is deteriorated by La doping,resulting in serious recombination of photogenerated carriers at the grain boundaries of the films.Therefore,La can only be doped with a lower concentration in the Ta₃N₅ films or with thinner thickness to prepare photoanode.Based on the fact that the onset potential and filling factor of Ta₃N₅ photoanode are significantly improved by gradient Mg doping,and a considerable photocurrent can be obtained with a thin layer of La-doped Ta₃N₅photoanode,a thin layer of La-doped Ta₃N₅ is deposited on top of the gradient Mg-doped Ta₃N₅ film to prepare a La-Mg co-doped photoanode.After the surface modification of Ni Co Fe-B_i co-catalyst,a photocurrent density of about 10 m A cm^-2 is obtained at the water oxidation potential(1.23 V versus RHE),which is very close to the theoretical maximum photocurrent(12.9 m A cm^-2)of Ta₃N₅ photoanode.Finally,the La-Mg co-doped Ta₃N₅ photoanode obtained a maximum ABPE of 4.07%,which further improved the solar-to-hydrogen energy conversion efficiency of Ta₃N₅-based photoanode.