The Design Of BiVO₄ Photoanode And Its Application In Water Splitting

The conversion of solar energy into hydrogen by photoelectrochemical water splitting is an important for sustainable development.Water oxidation efficiency determines the efficiency of water splitting,and therefore the design of photoanode is the key for photoelectrochemical water splitting.BiVO4 owns stable chemical property and small band gap which is a kind of excellent photoanode material,has great potential in use of photoelectrochemical water splitting.However,the poor bulk electron transportation,interfacial charge transfer and surface water oxidation properties of BiVO4 limit its photoelectrochemical performance.Therefore,it is essential to overcome the above weaknesses for BiVO4 photoanode for its photoelectrochemical water splitting application.In this work,we study on the best design for BiVO4 as photoanode towords its morphology control,doping and surface modification.And assembling the modified BiVO4 photoanode with PbS solar cell to construct a device that can utilize near-infrared light for self-power water splitting.The details are as follow:(1)Combined hydrogenation treatment and surface modification of CoPi to synergistically enhance the photoelectrochemical performance of nanoporous BiVO4 photoanode.The nanoporous BiVO4 has the advantages of high specific surface area and short minor carrier diffusion length.However,its characteristics of small particles composition and inconsecutive surface leads to poor bulk electron transportation and heavily surface electron recombination.Hydrogenation treatment can induce oxygen vacancies in nanoporous BiVO4 photoanode and increase the semiconductor carrier density,promoting bulk electron transportation.While the deep defect states also can act as surface recombination centers.Further surface deposition of CoPi can efficient inhibit this surface recombination and accelerate surface water oxidation.The results showed that the photocurrent of the modified photoanode increased from 1.6 mA/cm2 to 4.8 mA/cm2 at the potential of 1.23 V vs.RHE,the highest incident-photoconversion efficiency achieved to 85%.A synergetic mechanism was proposed to interpret the enhancement of photoelectrochemical performance of the nanoporous BiVO4 photoanode.(2)Deposition of ultrathin amorphous TiO2 can improve the photoelectrochemical stability of hydrogenated nanoporous BiVO4.The oxygen vacancies induced by hydrogenation can be gradually healed with the increasing concentration of oxygen during water oxidation,leading to a decrease of carrier density and a worse photoelectrochemical performance.The amorphous TiO2 layer can prevent the oxygen vacancies form being healed by oxygen molecules and provides electron conductive pathway without adding excess overpotential for water oxidation.After 3 hours oxidation,the photoanode with amorphous layer only decreased 5% and 6% of photocurrent and carrier density while the photoelectrochemical performance of hydrogenated nanoporous BiVO4 already back to the pristine nanoporous BiVO4 photoanode.The “leaky” amorphous TiO2 layer could prevent the contact of oxygen molecules and oxygen vacancies while not impeding the interfacial charge transfer,and thereby improved the photoelectrochemical stability of hydrogenated BiVO4 electrode.(3)Fabricating BiVO4 photoanode with high photoelectrochemical performance by a facial spray pyrolysis method.The traditional spray pyrolysis method utilize ethanol as solvent obtains a BiVO4 film composed of small particles which owns large amount crystal defects that exhibits a poor photoelectrochemical performance.Using DMSO as the solvent can help the growth of bulk crystal and facilitate the bulk electron transportation.The obtained BiVO4 photoanode exhibited a high photoelectrochemical performance that the photocurrent density increased from 0.08 mA/cm2 to 1.2 mA/cm2 compared with ethanol prepared BiVO4 photoanode.The highest incident-photo-conversion efficiency achieved to 25%.The results show that DMSO with high boiling point helps the droplet spreading on the substrate and facilitates the growth of bulk crystal.This film with fewer crystal boundaries benefits the charge transportation and performs a better photoelectrochemical performance.(4)BiVO4 photoanode was in tandem with PbS quantum dots solar cells for self-power water splitting.PbS quantum dots can absorb the near-infrared light,complement with the absorption region of Bi VO4 that can expand the light absorption of water splitting device.Pb S quantum dot solar cells absorbs the light with wavelength that larger than the band-gap of BiVO4 and provides applied bias for BiVO4 photoanode for water splitting.The results showed that hydrogen bubble was continuously produced during water splitting,and the solarconversion-hydrogen efficiency was 0.7%,demonstrated that the device could use near-infrared light for self-power water splitting.