In Situ Synthesis Of (Oxy) Nitride Films And Their Applications In Photoelectrochemical Water Splitting

Considering the forthcoming fossil energy exhaustion and environmental pollution,developing and utilizing clean and renewable energy is of great strategic significance for the sustainable development of environment and society.As a recognized clean and renewable energy,hydrogen has high energy density and is abundant in natural resources.Solar energy is abundant and widespread in nature,it's far and away sufficient to satisfy human energy demands.Photoelectrochemical water splitting is a promising way to convert solar energy into storable and clean hydrogen fuel,and solve the current energy exhaustion and environmental pollution,therefore attracting wide attention.However,the relatively low quantum efficiency and poor photoelectrochemical stability of photoelectrodes,especially photoanodes,limit the practical application.To achieve practical application,the solar-to-hydrogen efficiency of a photoelectrochemical cell must be higher than 10%.However,the earlier studied oxide photocatalysts,like TiO2 and NaTaO3,suffer from large bandgap,which cannot absorb most of the visible light.So the theoretical solar-to-hydrogen efficiencies of these oxide photoelectrodes are less than 10%.(Oxy)nitrides usually have smaller bandgaps than oxides,emerge as an important class of materials for photoelectrochemical water splitting.Among them,Ta3N5 has been reported as a promising photoanode with a bandgap of 2.1 eV and a suitable band structure straddled the redox potentials of water.Perovskite oxynitrides,like SrTaO2N,also have suitable band gap and band structure for photoelectrochemical water splitting.In this article,a facile carbonate-assisted one-step nitridation method has been proposed to synthesize Ta3N5 films directly on Ta foils,which enables the direct synthesis of high-oxidation-state Ta3N5 films from metal Ta under ammonia flow.By introducing vaporized molten salt as precursors,a serial of oxynitride films have been prepared by reactive inorganic vapor deposition method.Transparent Ta3N5 photoanode films have been prepared on transparent conductive substrate by carbonate-assisted one-step nitridation method.The main research contents and conclusions are as follows:A facile carbonate-assisted one-step nitridation method has been proposed to synthesize high-oxidation-state metal nitride films directly from metal precursors under ammonia flow.Usually,to prepare Ta3N5,metal Ta has to be oxidized to high-oxidation-state Ta precursors,like Ta2O5 and NaTaO3,before perfoming nitridation process,which is a time-consuming and laborious process and may introducing undesirable impurities.Here,we proposed a facile carbonate-assisted one-step nitridation method,which enables the direct synthesis of high-oxidation-state Ta3N5 films from metal Ta under ammonia flow,by introducing CO2 released from carbonates to oxide Ta to Ta2O5.After surface modified with NiFe layered double hydroxide,the as-prepared Ta3N5 photoanode exhibits a photocurrent of ca.6 mA cm-2 at 1.23 V vs.RHE and maintains approximately 82%of the initial photocurrent after 60 min irradiation,under AM 1.5G simulated sunlight.Transparent Ta3N5 photoanodes have been synthesized on GaN,TiN and TaC conductive substrate by carbonate-assisted one-step nitridation method.Ta3N5 photoanode has a maximum photocurrent density of 12.5 mA cm-2,when it's coupled with an appropriate photocathode to form a photoelectrochemical cell,the solar-to-hydrogen efficiency can be over 15%.But the lack of transparent conductive substrates hinders the synthesis of transparent Ta3N5 photoanodes.As the carbonate-assisted one-step nitridation method avoids the preoxidation process of metal Ta,a series of conductive metal nitrides and metal carbides can be considered as conductive substrates.We prepared transparent Ta3N5 photoanodes on GaN,TiN and TaC conductive substrates.The Al2O3/TiN/Ta3N5 photoanode and Al2O3/TaC/Ta3N5 photoanode exhibits a plateau photocurrent of 0.7 mA cm-2 and 1.2 mA cm-2 respectively,with good photoelectrochemical stability.A reactive inorganic vapor deposition method has been proposed to prepare a serial of perovskite oxynitride films.Perovskite oxynitrides have the advantages of both oxides and nitrides,and are predicted to be promising photoanode candidates for photoelectrochemical water splitting.However,their synthesis is usually accomplished by high-temperature ammonolysis of oxides and carbonates precursors,at temperatures over 800 ? under ammonia flow.In order to prepare oxynitride films,the oxynitride powders have to be synthesized firstly,and subsequently transfering them to conductive substrates by electrophoretic deposition or particle transfer methods.These particle-assembled oxynitride films often suffer from poor charge carrier transport among film particles,and also between film particles and the conductive substrate.Here,taking SrTaO2N as an example,we prepared SrTaO2N films on Ta foils under NH3 flow at 950? for 2 hours,by utilizing vaporized SrCl2/SrCO3 eutectic salt to provide Sr sorce,and using CO2 to oxide metal Ta.The as-prepared SrTaO2N film,after surface modified with Co/CoOOH layer,exhibits a photocurrent of ca.1.9 mA cm-2 at 1.23 V vs.RHE,which is ca.240%higher comparing with the particle-assembled SrTaO2N film.Meanwhile the SrTaO2N/Co/CoOOH photoanode maintains approximately 62%of the initial photocurrent after 240 min irradiation,under AM 1.5G simulated sunlight.This strategy also allows the preparation of a serial of perovskite oxynitride films including CaTaO2N,BaTaO2N,SrNbO2N and BabO2N.