The Study Of A New-type Tantalum Nitride Semiconductor Thin Films Photoelectrode

With the depletion of fossil fuels and the environmental problems caused by the annual increase in greenhouse gas emiss ions emitted by increasing use of fossil fuels,so,development and utilization of renewable energy are imminent.In the long run,solar energy is the only renewable energy source that can replace fossil fuels in totally-consumed amount.Therefore,it is of great significance to develop and use solar energy in relation to Chinese energy security and ecological civilization construction.Photoelectrochemical production of solar fuel,that is,fuels?e.g.methanol,ethanol,methane,etc.?which is produced by solar energy driving reaction between carbon dioxide and water,can not only store solar energy in the form of chemical energy,but also can be used to turn greenhouse gas CO₂ into useful chemicals,so it has received extensive attention and research.While utilization of solar fuel is still a forward-looking science and technology,it is considered to be one of the best way to solve both environmental and energy problems in the future.In order to convert solar energy into solar energy effectively,one of the primary tasks of the technology is to find semiconductor photoelectrode materials with high photoelectrical conversion efficiency.In the past decades,metal oxide semiconductors have been mainly studied,but the band gap of metal oxidation is relatively larger,and the absorption of visible light is so poor that the photoelectrical conversion efficiency is lower.In recent years,metal nitride,especially Ta₃N₅ semiconductor material,has attracted wide attention due to its suitable band gap?about 2.1 e V?and good absorption in the visible region,so,it is an urgent problem to be solved that how to prepare Ta₃N₅ thin film photoelectrode with uniform film formation and good photoelectric al conversion by simple and easy method and to deeply understand the relationship between the microscopic structure and its properties.In this paper,Ta ₃N₅ semiconductor is studied as the photoelectrode,and the research works have been carried out on the preparation,characterization and density functional theory calc ulation of this materials.The main contents are as follows:?1?Preparation and characterization of Ta₃N₅ thin film photoelectrode.The Ta₃N₅ film photoelectrode is prepared by a two-step method of anodic oxidation and high temperature nitridation.The surface morphology and crystal structure of the film layer of the Ta₃N₅ photoelectrode are characterized by means of FESEM and XRD.The result shows that the prepared Ta₃N₅ photoelectrode is a porous lamellar structure constructed by nano particles,and its crystal type is monoclinic.Furthermore,the XPS test is performed to analyse the element on the surface of photoelectrode.The result confirmed that Ta₃N₅ thin-film electrode is successfully obtained through high-temperature nitridation of Ta₂O₅ thin-film electrode that prepared by anodic oxidation in ammonia gas.?2?Characterization of band structure and charge transfer impedance of Ta₃N₅ thin film photoelectrode.The band gap,and the potentials of flat band,valence band and conduction band of Ta₃N₅ thin film photoelectrode are measured by UV-vis absorption spectroscopy,Mott-Schottky electrochemical impedance and linear sweep voltammetry under dark light,respectively.The results show that band gap of the Ta₃N₅ thin film photoelectrode is 2.08 e V,and the top of valence band and the bottom of conduction band and the flat band potential are located at 1.79 V,-0.29 V and-0.18 V?vs.RHE?,respectively.The electrochemical impedance method is used to determine the charge transfer impedance of Ta₃N₅ film photoelectrode under the light power density of 100m W/cm² and the bais of 1.6 V?vs.RHE?,and the result shows that the impedance value is 284.2?.?3?Determination of photoelectric conversion performance of Ta₃N₅ thin film photoelectrode.The result of volt-ampere cure of the photocurrent density with the applied potential shows that the Ta₃N₅ film photoelectrode exhibits excellent photoelectrical conversion performance,and the photocurrent density can reach 640?A/cm² at 1.23 V?vs.RHE?,indicating that the prepared Ta₃N₅thin film photoelectrode can be used as a good photoanode in the photoelectrochemical reaction system.?4?The first-principles calculation based on density functional theory of density of states and band structure of Ta₃N₅.The density of states and energy band structure of Ta₃N₅ are theoretically calculated by density functional theory,and the results show that the valence band of Ta₃N₅ is mainly composed of the 2p orbital of N,while the conduction band is mainly composed of the 5d orbital of Ta.Since the lowest point of the conduction band and the highest point of the valence band of Ta₃N₅ are not at the same k point,this indicates that Ta₃N₅ is an indirect semiconductor.In the three exchange-correlation functionals of the adopted PBE,GGA+U and HSE06,the band gap of Ta₃N₅ obtained from the hybrid functional HSE06 is 2.12 e V,which is closest to the experimental value of2.08 e V.