| One of the basic restrictions in the development of human society is energy.Nowadays,the energy structure dominated by fossil fuels is facing the problem of resource scarcity and environmental pollution,which makes humans' craving for renewable and clean energy rapidly increasing.Among a number of possible solutions,photoelectrochemical-water-splitting-for-hydrogen-production has great potential,while the lack of efficient and stable photoanode materials is limiting its further development.?-Fe2O3,also known as hematite,is considered as a promising candidate for thecommercial photoelectrode with advantages of suitable band-gap,abundant reserves,high photoelectrochemical(PEC)stability and non-toxicity.Like many other photo-anode materials,hematite also faces the problem that the actual energy conversion efficiency is much lower than the theoretical value.Many methods have been proposed to improve its performance,such as element doping,high temperature treatment,nano engineering,construction of heterojunction,surface modification,etc.But there are still many problems worthy of further consideration and exploration.For example,the combination of high temperature treatment and elemental doping can further improve the water oxidation efficiency of ?-Fe2O3 photoanode under high bias condition while reducing the water oxidation efficiency under low bias condition.This phenomenon is confirmed but its mechanism has not been reported.Moreover,the current improvements of hematite's electronic structure mainly focus on element doping.But considering the adjustable electronic structures and excellent physicochemical properties of 2D materials,the potential PEC performance of 2D ?-Fe2O3 is worth looking forward to.Based on these two points,we have studies the effects of high temperature treatment and two-dimensionalization on the PEC performance of ?-Fe2O3 photoanodes.The main research contents are as follows:The influence of high temperature treatment on the onset potential of Ti-doped hematite photoanodes for solar water splitting.The Ti-doped ?-Fe2O3 photoanode was annealed at 750? for ten minutes,and the saturated photocurrent was doubled,while its onset potential had an unfavorable positive shift of 100mV.The basic characterizations showed that the morphology,phase,grain size and light absorption capacity of Ti:Fe2O3 photoanode did not change after high temperature treatment.XPS measurements showed that the content of Ti atoms on the surface increased,and the content of hydroxide group absorbted on the surface reduced after the high temperature treatment.Mott-Schottky measurements showed that the flat-band potential also had a positive shift of 100 mV,while the flat band potential of the photoanode can be understood as its the oretical turn-on potential.Therefore,the increase of onset potential can be initially attributed to the increase of flat band potential.After that,we employed the PEIS measurements to exclued the influence of the surface state on the flatband potential.Take the first-principles calculation results into consideration,we proposed the mechanism as follows:high-temperature treatment increases the Ti element content on the surface of Ti:Fe2O3 photoanode,thereby weakening the adsorption capacity of hydroxyl group on the surface.This would cause a change in the Hemholtz layer potential drop(VH),resulting in a positive shift of the flat-band potential and onset potential.This mechanism suggests that our subsequent studies can be carried out around surface modification,in order to eliminate the unfavourable effects of high temperature treatment on the onset potential of Ti:Fe2O3 photoanode.First-principles calculations on the electronic structure and water oxidation performance of 2D Fe2O3.The electronic structures of two-dimensional Fe2O3 and three-dimensional Fe2O3 were calculated and compared with a method of DFT+U theory.The results showed that two-dimensionalization will reduce the band gap of hematite from 2.17 eV to 1.62 eV,increasing the theoretical solar energy conversion efficiency of this material.In addition,two-dimensionalization will also cause the conduction band edge position to move up by 70mV,reducing the theoretical onset potential of this material.Furthermore,the calculation of atomic-projected energy band structure and(110)interplanar spacing indicated that the surface states and the reduction of atomic layer spacing may be the main reasons for the changes of electronic structure.DFT+U theoretical calculations were also employed to compare the water oxidation performance between two-dimensional and three-dimensional Fe2O3.The results showed that two-dimensionalization will reduce the water oxidation overpotential from 1.19V to 1.15V,which is favourable to the water oxidation reaction under low bias.Therefore,from the viewpoint of theoretical calculations,the 2D Fe2O3 will have a better PEC performance than the three-dimensional Fe2O3,indicating that further experimental investigations are worth trying. |
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