Surface And Interface Engineering Of Metal Oxide Photoanodes For Photoelectrochemical Water Oxidation

Due to the broad prospects in the efficient and economic solar fuel and chemical production,photoelectrochemical(PEC)water splitting and CO₂ reduction have recently attracted enormous interest from researchers in chemistry,physics,materials,and other fields.Photoanodes,as the central components in PEC water splitting cells,pose decisive effects on the device's solar-to-hydrogen conversion efficiency.Therefore,researchers have been committed to developing efficient,inexpensive,and stable photoanodes to meet the needs of water oxidation reactions.Among semiconductor photoanodes,metal oxides are particularly attractive because of their outstanding chemical stability,low cost,facile processing,favorable band edge positions and wide distribution of bandgaps.However,for a given metal oxide photoanode,insufficient charge separation and interfacial charge transfer capabilities are the two most critical factors that limit its PEC performance.In view of above issues,in this paper,we select typical?-Fe₂O₃,TiO₂ photoanodes as the research object.Through dual modification with electron transport layer and coatalyst,introduction of surface oxygen vacancies,surface polarization all of which are advanced surface and interface engineering strategies,mainly focusing on improvement of their charge separation and interfacial charge transfer capabilities.The roles they play and the effects they exert in improving the performance of photoanodes are disscussed in detail.The main research contents are as follows:1.Dual modification of?-Fe₂O₃ nanostructured film with electron transport layer and coatalyst:Poor carrier separation efficiency in the bulk and slow water oxidation kinetics on the surface are the key factors restricting the development of?-Fe₂O₃ photoanodes.We preparaed a WO₃ electron transport layer modified?-Fe₂O₃ nanostructured film by electrospray ionization method,and introduced an ultra-thin Co(OH)_x nanolayer cocatalyst on its surface via chemical bath deposition method.Due to the conformal deposition of?-Fe₂O₃ on the porous WO₃ framework,the close combination of the two phases and the matched band structure greatly enhance the charge separation efficiency of?-Fe₂O₃.We systematically studied the influence of factors such as the ratio of deposition amount of WO₃ to?-Fe₂O₃ and the substrate temperature on the film properties.Under optimal condition,the prepared WO₃/Fe₂O₃ photoanode shows a photocurrent of 0.32 m A cm^-2 under 1.23V_RHE,which is about 30 times that of the?-Fe₂O₃ film photoanode.The photocurrent was further increased to 0.62 m A cm^-2 after being modified with a Co(OH)_x nanolayer and an obvious cathodic onset potential shift of about 160 mV was observed.The detailed characterization results showed that the synergistic effect of WO₃ and Co(OH)_x simultaneously improved the bulk charge separation ability and surface water oxidation kinetics of the?-Fe₂O₃ film photoanode.2.?-Fe₂O₃ photoanode with enriched surface oxygen vacancies:Surface oxygen vacancies(V_O)hold great potential for efficient charge separation and transfer,but introducing V_O on the very surface of the photoanodes without affecting the bulk properties remains great challenge.We introduced V_O only on the surface of the?-Fe₂O₃ photoanode through a simple chemical reduction post-treatment method,which avoided the generation of bulk defects,thereby reducing bulk charge recombination.By tuning the processing time,we obtained?-Fe₂O₃ photoanodes with different surface V_O content and studied their effects on the performance of the photoanode.Experimental data proved that the surface V_Oincreases the majority carrier density,thereby shortening the width of the depletion layer and enhancing the band bending,which greatly improves the charge separation efficiency.After further loading the Co₃O₄ cocatalyst,the?-Fe₂O₃photoanode with enriched surface V_O obtained 1.41 m A cm^-2 photocurrent in 1.23V_RHE,which was twice that of the pristine?-Fe₂O₃ photoanode.3.Surface polarization of TiO₂ photoanode:As a direct bandgap semiconductor,rutile TiO₂ possesses insufficient charge separation ability caused by very fast recombination of photogenerated carriers,resulting in the poor PEC performance.Here,we prepared rutile TiO₂ nanorod arrays by hydrothermal method,and introduced abundant-OH groups on its surface via a simple post-treatment method.Detailed data show that these hydroxyls with electric dipoles polarize the surface,inducing substantial electric field in the surface region,and elevating band edges so as to form a homojunction within TiO₂.Such serves as the direct driving force for the photogenerated charge separation and migration.Thus,a fundamentally improved charge separation efficiency of TiO₂ photoanode is obtained,reaching 94%at 1.23 V_RHE.Moreover,these hydroxyls also facilitate water oxidation kinetics.Without tipycal doping or cocatalyst,the surface polarized TiO₂ photoanode achieved a significantly improved photocurrent of about 1.41 m A cm^-2 at 1.23 V_RHE(AM 1.5G)and a cathodic shift on onset potential over 200 mV.