Controllable Construction Of Bivo₄-based Photoanodes And Their Use For Photoelectrochemical Water Splitting

In recent years,the method of using semiconductor for photoelectrochemical water splitting to convert solar energy into hydrogen energy is considered to be one of the most ideal ways to solve the current energy problem,which has attracted widespread attention.Therefore,the development of stable and efficient semiconductor photoelectronic catalytic materials is the key to solar hydrogen production technology.Among many semiconductor materials,bismuth vanadate?BiVO₄?has attracted much attention due to its small band gap?2.4 eV?,suitable band edge position,and large theoretical photocurrent density.However,due to the severe electron-hole recombination and slow water oxidation kinetics during the photoelectrochemical water splitting process,the photon-to-current of BiVO₄ for water splitting efficiency is low,and the actual water splitting ability is much smaller than the theoretical value,which limits its wide application.In order to solve these problems,this Thesis is devoting to improving the photochemcial catalytic performance of BiVO₄ photoanode for water splitting through doping,cocatalyst modification,and surface ion regulation,which greatly improved its performance of photoelectrochemical water splitting.The main research achievements in this Thesis is presented as follows:In the first part of this Thesis,a one-step calcination method was carried out to modify BiVO₄ photoanode by partially replacing Bi ions with transition metal ions of Fe,Co,and Ni as dopant ions so as to achieve the goal of improving the electron-hole separation efficiency of BiVO₄ photoanode.At the same time,the in-situ formed oxyhydroxide layer on the surface of BiVO₄ photoanode during the synthetic process may also unprecedently assist for promoting the surface charge separation efficiency of BiVO₄ photoanode.The Bi sites in BiVO₄ photoanode replaced by transition metal ions and the oxyhydroxide layers on its surface synergistically enhance the electron-hole separation efficiency,thus greatly improve the photoelectrochemical?PEC?water oxidation performance of BiVO₄ photoanode.Among them,the Co:BiVO₄ photoanode shows the best PEC performance,and its photocurrent density could reach 4.2 mA cm-² under 1.23 V_RHE,which is twice than that of the undoped one.In the second part of this Thesis,a hydrothermal method was conducted to couple the cocatalyst of Co?CO₃?_xOH_y with BiVO₄ photoanode.In this way,a layer of highly active Co?CO₃?_xOH_y cocatalyst was built on the surface of BiVO₄ photoanode.Such modification may greatly improve the water oxidation performance of BiVO₄photoanode.The BiVO₄/Co?CO₃?_xOH_y photoanode exhibits a significantly enhanced photocurrent.The photocurrent density can reach 5.0 mA cm^-2 under 1.23 V_RHE,and its photocurrent far exceeds other Co-based cocatalysts modified BiVO₄ photoanode reported in the literature.More importantly,the Co?CO₃?_xOH_y as a cocatalyst exhibits a strange self-healing function in borate buffer electrolytes,in which the Co?CO₃?_xOH_y show a dissolving resistance ability under a long period of light.Moreover,the Co?CO₃?_xOH_y shows a phase transformation to form a new cobalt borate?Co-Bi?species that greatly improve the stability of the photoanode.In the third part of this Thesis,a hydrothermal method was employed to doped BiVO₄/Co?CO₃?_xOH_y photoanode with F ions to achieve a BiVO₄/F:Co?CO₃?_xOH_y coupling photoanode.The doping of F ions realizes the surface reconstruction of the catalytically active species to successfully activate the Co?CO₃?_xOH_y cocatalyst,and further improves the PEC water oxidation performance of the BiVO₄/F:Co?CO₃?_xOH_y photoanode.The photocurrent density of BiVO₄/F:Co?CO₃?_xOH_y photoanode can reach 5.9 mA cm^-2 under 1.23 V_RHE,and the stability is also improved to a certain extent.Meanwhile,the BiVO₄/F:Co?CO₃?_x OH_y photoanode shows a special self-healing function.The improvement of PEC performance of the BiVO₄/F:Co?CO₃?_xOH_y photoanode is mainly due to the in-situ formation of CoF₂ species during the stability test.The stronger ionic Co-F bond makes it easier for F ions to accumulate on the surface of the cocatalyst,participate in the surface reconstruction of the cocatalyst,thus playing an important role in regulating Co²⁺to greatly enhance the PEC water oxidation performance and stability of the photoanode.