Fabrication And Modification Of BiVO₄ Nano-film And Their Photoelectrocatalytic Performance Under Visible Light

In recent years,great advancement has been achieved in the research of photocatalytic technique.TiO₂ has been drawn much attention due to its good chemical stability, non-toxicity,low cost,easy fabrication and good photoelectrochemical performance.But TiO₂ can not utilize solar energy.Thus,the exploration of the catalysts with visible light response is necessary in order to use solar energy.BiVO₄ with monoclinic type has intense visible light response.The valence band(VB) of BiVO₄ is composed of hybridized Bi 6s and O 2p orbitals.This hybridization makes the VB largely dispersed,which favors the mobility of photogenerated holes in the VB and is beneficial to the oxidation reaction.In addition,the holes of the BiVO₄ have strong oxidation ability due to its more positive position of the VB.BiVO₄ nano-film was fabricated by modified metalorganic decomposition(MOD) technique and the photocatalytic(PC) activity of these photocatalysts was evaluated.To enhance the PC performance of the BiVO₄ film,the modification of BiVO₄ film was done.In this dissertation,the following several parts of work have been done:(1) BiVO₄ film coated on F-doped SnO₂(FTO) glass was successfully fabricated by modified MOD technique.The effects of the calcination temperature and coating time on the morphology,crystalline type and photoelectrochemical(PEC) performance of the BiVO₄ film were investigated.The obtained optimum fabrication technical conditions were 500℃of calcination temperature and four of coating time.Scanning electron microscope(SEM) and transmission electron microscopy(TEM) images of the prepared BiVO₄ film under the optimum conditions showed that the film was composed of grains with 200-300 nm in size and the grains were composed of particles,which were about 10 nm in size.Diffused reflectance spectroscopy(DRS) revealed that the absorption performance of the BiVO₄ film was intense in visible light region and the band gap was 2.43 eV.The analysis of X-ray diffractometry(XRD) and Raman spectra identified to the monoclinic type BiVO₄.The PEC measurement showed that the IPCE was 48.1%with 1.2 V of applied potential and 400 nm of the light weavelength and the highest photoconversion efficiency was 1.43%.The removal rate of phenol in PEC process by the BiVO₄ film electrode under visible light(＞400 nm) was 27.1 times that in PC process.The prominent enhancement was induced by the promoted separation of photogenerated electron-hole pairs.Furthermore,the BiVO₄ film electrode showed good stability in the PEC process. (2) Ag particles were deposited on BiVO₄ film by photoreduction technique.XRD analysis indicated that the chemical state of the Ag particles was metallic Ag.TEM observation confirmed that the sizes of the Ag particles were 10-20 nm.The investigation of the phenol degradation demonstrated that the PC degradation rate of the phenol on the Ag doped BiVO₄ film was enhanced by 1.61 times in PC process and by 42.7 times in PEC process compared with that of the BiVO₄ film.The transportation of the electrons from the BiVO₄ to Ag driven by the schottky barrier formed between Ag and BiVO₄ could increase the charge carrier separation,and consequently enhance the PC performance.The enhancement of the PC ability in PEC process could be attributed to the simultaneous movement of the photogenerated electrons to external circuit and the photogenerated holes to the Ag particles deposited on the BiVO₄ film.(3) The silicon-doped BiVO₄ film was fabricated by modified MOD method.The effects of Si doped on the chemical structure,surface hydrophilicity,morphology,crystalline size and photoabsorbance property were investigated.Doping Si into the BiVO₄ film did not change its crystalline type and the photoabsorbance property.However,the crystalline size of the BiVO₄ decreased and greater amount of oxygen vacancies in the BiVO₄ formed by the substitution V by Si in the crystalline lattice.In addition,the BiVO₄ film became more hydrophilic by doping Si.The phenol elimination rate on the Si-doped BiVO₄ film electrode in the PEC process was 1.84 times as great as that on the BiVO₄ film electrode.The enhanced PEC performance was attributed to the decease of the crystalline size,the increased amount of the oxygen vacancy and the enhancement of the hydrophilic performance.