Photo-generated Carriers' Efficient Separation And Photoelectrochemical Water Splitting Performance Of Nitride Photoanodes

Renewable and clean new energy that will replaced the use of fossil fuels is imminently needed for the sustainable development of human society.A photoelectrochemical cell(PEC)is a potential way to convert solar energy into clean energy by water splitting.The key of an efficient PEC solar water splitting cell is developing a perfect photoelectrochemical electrode material.The most important quality of a photoelectrochemical material is a suitable band gap,which is suitable to make the best use of the visible light.Compared with oxide semiconductors,nitride semiconductors have narrower band gaps,which are beneficial to visible light utilization.Moreover,the carriers' recombination on the surface of a photoelectrode is the limiting step for the photoelectrochemical performance.Therefore,finding a simple and efficient method to eliminate the surface recombination centers is the emphasis and difficulty in a PEC water splitting cell.In this paper,we intend to increase the solar conversion efficiency of a PEC cell.The InxGa1-xN and Ta3N5,which can use the visible light,have been prepared as the photoanode in a PEC water splitting cell.Chemical and physical methods for removal of surface recombination centers have been developed for different semiconductors.The photoelectrochemical performance of InxGa1-xN and Ta3N5 has been greatly increased after the removal of surface recombination centers.Moreover,the charge transfer mechanism of Ta3N5 films prepared by different methods has been studied to further increase the solar energy conversion efficiency.The main conclusions are as follows:The photoelectrochemical performance of InxGa1-xN has been greatly improved by electrochemical etching of surface recombination centers.InxGa1-xN semiconductors(band gaps ranging from 0.7 to 3.4 eV by varying indium composition)are promising photoelectrode materials for solar hydrogen production because their absorption can match the solar spectrum very well.However,indium segregation and phase separation happen during the growth of InxGa1-xN with higher In content or thicker film.The In-rich segregation layer on the surface of InxGa1-xN,which acts as recombination centers of photo-generated carriers,will decrease the photoelectrochemical performance.The photocurrent and IPCE of InxGa1-xN were greatly increased after the removal of indium segregation by the electrochemical etching method.Ta3N5 photoanode achieved record solar photocurrent after being modified by surface recombination center removal and Co(OH),loading.Compared with InxGa1-xN,Ta3N5 is cheaper,more simple to prepare,has a narrower band gap and a more suitable position of conduction and valence band for water splitting.Ta3N5 film has been prepared by oxidation and nitridation of Ta foil.However,the photoelectrochemical performance of Ta3N5 film is poor because there are carriers'recombination centers on surface of the films.The carrier transfer efficiency and performance has been greatly improved after the surface layer removal by mechanical exfoliation,automatic thermal exfoliation or HF etching.Moreover,the photoelectrochemical performance and photostability of Ta3N5 is further increased by Co(OH)x loading.After recombination center removal and Co(OH),loading,the solar photocurrent of Ta3N5 @ 1.23 VRHE is about 5.5 mA/cm2,which is the record photocurrent for the photoanode.The restrictive factors for the solar energy conversion efficiency have been explored by investigating the charge transport mechanism of photo-generated carriers in Ta3N5 films.The photo-generated carriers' transport mechanism on the surface and in the bulk of the Ta3N5 films prepared by different methods has been studied.The results show that the Ta3N5 film composed of close packed small particles was beneficial to the transport of photo-generated electrons in the bulk of the film,which could decrease the photo-generated carriers' recombination and improve the phtotoelectrochemical performance of Ta3N5.In this paper,the photoelectrochemical performance of photoanodes has highly enhanced after the removal of surface recombination centers by inspired methods.These surface recombination center removal methods may also favor improving the performances of other optoelectronic devices,such as solar cells and photodetectors.After recombination center removal and Co(OH)x loading,the solar photocurrent of Ta3N5 @ 1.23 VRHE is about 5.5 mA/cm2,which is the record photocurrent for the photoanode.