Study On The Fabrication Of Tungsten Trioxide Photoanode And Their Photoelectrochemical Properties

Seeking clean and renewable energy resources has been a research focus for the past decades as a series of environmental problems caused by the excessive consumption of traditional fossil fuels.Photoelectrochemical water splitting has widely been considered as a valid pathway to overcome energy dilemma and the key of this method is to find a suitable semiconductor as the photoelectrode.WO₃ has attracted great attention due to its suitable band gap,natural abundance and low-toxicity,and the light conversion efficiency can reach 6.3%theoretically.However,due to it’s insufficient utilization of light,poor charge transport properties,low oxygen evolution reaction kinetics and other inherent drawbacks,lower its efficiency to the calculated best value.In this paper,loading co-catalyst and tuning surface electronegativity were adopted to improve the PEC performances of WO₃.The details are as follows:（1）Ferrihydrite（Fh）as a cocatalyst was decorated on WO₃ photoanode through a hydrothermal method.By adjusting the hydrothermal time,the optimal reaction conditions were optimized.Importantly,the photocurrent density of the composite photoanode was boosted up to 0.61 mA/cm² at 1.23 V（vs.RHE）,which was 1.79 times than as-prepared WO₃ photoanode.The enhanced PEC performances were due to the deposition of Fh as co-catalyst,which functioned as a hole storage layer to capture and temporary store the generated holes,and thus inducing efficient charge-carrier separation on the electrode surface,improving the water oxidation reaction kinetics.（2）Zeolitic Imidazolate Framework-67（ZIF-67）as cocatalyst has been grown onto the WO₃ nanosheets with a facile surfactant-assisted controlled synthetic method to improve the PEC performances of WO₃ nanosheets.After the loading of ZIF-67,the carrier concentration increased,and the photocurrent density of the composite photoanode was boosted up to 0.63 mA/cm² at 1.23 V（vs.RHE）,which was 1.85 times higher than the as-prepared WO₃ photoanode.In addition,the charge separation efficiencies on the surface(η_surface)and in the bulk(η_bulk)were also improved.The enhanced PEC performance of the composite photoanode can be summarized as the following two reasons.Firstly,the Polyvinyl Pyrrolidone（PVP）assisted controllable synthesis method can provide good connection between WO₃ photoanode and ZIF-67.Secondly,the ZIF-67 can utilize efficiently the surface-reaching holes for PEC water oxidation and reduce surface charge recombination,and thus facilitating water oxidation kinetics（3）Fluorine anion,with the strongest electronegativity and similar ionic radius to O ions,could replace the surface O sites to form metal–fluorine bonds with a stronger iconicity and higher polarization.The surface fluorination was acompanished by immerging WO₃ in NH₄F solution,and the photocurrent density of the fluorinated photoanode had reached 0.68 mA/cm² at 1.23 V（vs.RHE）,and the oneset potential shifted negatively.The enhanced PEC performances might due to the surface enrichment of fluorine anions endows more hydrophilic surface character,which can provide more active site for the reaction.In addition,the formation of the metal–fluorine bonds with a stronger iconicity promote the separation of charge carriers,and thus improving the PEC performance.