Construction Of ?-Fe₂O₃-Based Nanorods Array Photoanode For Photoelectrochemical Water Oxidation

With the continuous consumption of fossil energy and excessive emission of greenhouse gases,photoelectrochemical?PEC?water splitting to generate hydrogen has attracted wide attention in recent years.The bottleneck of achieving high-efficiency solar conversion hugely depends on the development of efficient photoanode materials.Due to its excellent optical absorption characteristics,earth abundance and chemical stability,?-Fe₂O₃ has become alternative photoanode for PEC water oxidation.However,the research achievements of hematite-based photoanodes are still far below the theoretical value,which is caused by its poor conductivity,inferior hole-electron pairs separation efficiency and low valence band energy.To overcome the above problems,the dual modification strategy via heteroatom doping combined with further surface treatment or loading of the co-catalyst was adopted to enable simultaneously facilitating charge separation from the bulk to the surface.The reasons for the enhancement in the water oxidation activity are revealed through physical characterization and PEC analysis.The details are as follows:1.Activating Hematite Nanorod Photoanode via Fluorine-Doping and Further Surface Fluorination for Enhanced Oxygen Evolution Reaction:The target photoanode of FeF_x/F-Fe₂O₃ was constructed by fluorine doping and further surface fluorination,enabling simultaneously promoting the charge separation from the bulk to the interface.The fluorine anion was incorporated into?-Fe₂O₃ by a simple hydrothermal method,which significantly improved the conductivity and charge transfer capacity of?-Fe₂O₃.Furthermore,fluorine anion serving as donor dopant not only increases the charge carriers'concentration,but also partially removes the surface charge trapping states and improves the holes'injection efficiency.Then the fluorine-doped?-Fe₂O₃ was fluorinated on the surface to further eliminate the surface charge trapping states.Meanwhile,the enriched F-Fe bonds on the surface effectively reduced the internal energy barrier of water oxidation by forming hydrogen bonds with water molecules,thus remarkably improving the water oxidation efficiency.In the absence of sacrificial reagent,FeF_x/F-Fe₂O₃ showed the photocurrent response of 2.41 mA cm^-2 at1.23 V vs.RHE,3.2 times increased than that of pristine?-Fe₂O₃(0.75 mA cm^-2).The charge separation efficiencies in bulk and on the surface were enhanced to 32.4%and80.3%,respectively,which significantly improved compared with the pristine?-Fe₂O₃?11.5%and 45.6%?.2.Conformally Coupling CoAl-Layered Double Hydroxides on Fluorine-Doped Hematite:Surface and Bulk Co-Modification for Enhanced Photoelectrochemical Water Oxidation:The composite photoanode CoAl-LDH/F-Fe₂O₃ was constructed by conformally coating the amorphous co-catalyst CoAl-LDH on the fluorine-doped?-Fe₂O₃ nanorods array.At the potential of 1.23 V vs.RHE,the photocurrent density and incident photon-to-current efficiency?IPCE?increased to 2.46 mA cm^-2 and 47.66%,respectively.The source of the PEC water oxidation activity was revealed through systematic electrochemical analysis.Fluorine-doping endows?-Fe₂O₃ an excellent conductivity to boost charge transfer and separation.The redox pair of cobalt ions changes the transmission path of holes in the process of water oxidation,thus improving the water oxidation efficiency at the interface.Moreover,the cocatalyst of CoAl-LDH acting as hole extraction layer partially passivates the charge-trapping state,which effectively delays the recombination of electron-hole pairs on the surface.Therefore,he dual modification strategy via heteroatom doping combined with further surface treatment or loading of the co-catalyst is a powerful means to improve the PEC water oxidation performance of?-Fe₂O₃.