Study On Cocatalyst-Enhanced Water Oxidation Of Oxide Photoanode And Charge Kinetics

Energy shortage and environmental pollution make people realize the necessity and urgency of developing clean renewable energy to replace fossil energy.Semiconductor based photoelectricchemical for hydrogen production is considered as a sustainable clean energy conversion and storage technology.In this paper,bismuth vanadate and hemitate semiconductor were taken as examples to study the promoting effect of cocatalyst on photoanode water oxidation,and to explore the charge separation enhancement mechanism at the catalyst/semiconductor interface.Firstly,we used Fe-NiO as the oxygen evolution catalyst and Mo-BiVO₄ film as the photoanode.Ultrathin Fe-NiO was prepared by precipitation,ion exchange and thermal dehydration.The Mo-BiVO₄ semiconductor is synthesized by two steps.The catalyst was then loaded on the Mo-BiVO₄ thin film by spin coating.XRD and high resolution transmission microscope confirmed that the catalyst was layered structure Fe-NiO and bismuth vanadate was monoclinic phase.AFM showed that the catalyst had an ultrathin nanosheet structure of 2.1-4.8 nm thickness.Mo-BiVO₄ photoanode modified with Fe-NiO catalyst showed significant improvement in water oxidation:when the potential was 1.4 V vs.RHE,the current density increased from 0.88 mA/cm² to 1.65 mA/cm²,and the charge transfer efficiency increased to 99%.Transient surface photovoltage（TSPV）data show that bismuth vanadate thin films exhibit positive n-type semiconductor characteristic signals.The TSPV signal of Mo-BiVO₄modified with Fe-NiO was significantly enhanced.We use first-order serial reactions to simulate the separation and recombination of photo-induced charge in semiconductor.The effects of charge separation constant,recombination constant and concentration of photo-induced carriers on transient surface photovoltage curve are discussed.We found that the TSPV results of bismuth vanadate and Fe-NiO decorated bismuth vanadate films agree well with the kinetics simulation of serial reactions.Fe-NiO acts as a good charge reservoir and catalyst in Mo-BiVO₄.Electrochemical impedance data indicated a nearly fourfold increase in surface capacitance,consistent with cyclic voltammetry.In addition,we studied the catalytic effects of La-NiO and amorphous LaNiO_x on hematite.La-NiO is prepared by precipitation,ion exchange and thermal dehydration.Amorphous LaNiOx was prepared by ultraviolet ozone treatment with diethyl caproate as precursor.The performance of the water oxadation of the modified hematite photoanode was also significantly improved:when the applied potential was 1.6 V vs.RHE,the photocurrent density of the hemitate photocurrent modified by La-NiO increased from 0.73 mA/cm² to1.06 mA/cm².When the iron oxide modified by amorphous LaNiO_x was 1.3 V vs.RHE,the photocurrent increased from 0.19 mA/cm² to 0.61 mA/cm²,and the onset potential negatively shifted to 0.75 V vs.RHE@0.1 mA/cm².Lanthanum doped nickel oxide and amorphous LaNiO_x electrocatalysts may form an electrolyte permeable"adaptive junction"on the surface of hemitate,thus improving the photoanode performance.In conclusion,the ultrathin NiO nanosheet,as an oxygen evolution electrocatalyst,can be used to modify the bismuth vanadate and hematite photoanode,which can help enhance the charge storage of the surface charge of the semiconductor oxide photoanode,and promote the photo-induced carriers to participate in the oxidation of water on the surface.