Photophysical And Photocatalytic Properties Of BiVO₄ Epitaxial Films With Different Orientations

As a visible light responsive photocatalyst and photoanode material for water splitting,bismuth vanadate（BiVO₄）is one of the most promising materials.However,due to poor electron transport,slow water oxidation kinetics and low carrier mobility,the photocatalytic performance of pure BiVO₄ photocatalysts needs to be improved.The photocatalytic performance is affected by many factors,so the research on the photocatalytic mechanism will also become complicated.The kinetic and thermodynamic behavior of BiVO₄ on different exposed faces will be different,which also largely determine its oxidative and reductive ability.Single crystal thin films can advance the in-depth understanding of the intrinsic properties of materials,and it is more conducive to the study of charge transport and surface reactions at the molecular level.The crystalline anisotropy generally reveals distinct physical phenomena along different crystallographic orientations.Therefore,the study of BiVO₄ thin films exposed to different crystal planes is of great significance.In this paper,the epitaxial BiVO₄ thin films with different orientations were prepared on yttrium stabilized zirconia（YSZ）substrates with good lattice matching with BiVO₄ by pulsed laser deposition and magnetron sputtering technology,and the basic characteristics and photocatalytic performance of the BiVO₄ thin film were studied in detail.The specific contents are as follows:（1）BiVO₄ epitaxial films oriented along the b-axis were grown on YSZ（001）by pulsed laser deposition.And the BiVO₄ film was composed of nano-islands.BiVO₄ powder was also prepared by hydrothermal method,and it had a typical morphology of a truncated tetragonal bipyramid.Compared to nanocrystalline powder,the BiVO₄ epitaxial films with thinner thickness has a larger exposure ratio of the（010）facets,which can make it easier for electrons to migrate to the surface and it is also beneficial to the utilization of photogenerated electrons to promote the reaction of electrons with oxygen adsorbed on the surface.According to the results of rhodamine B（Rh B）degradation experiments under 440 nm monochromatic light irradiation,the apparent quantum yield of the BiVO₄ epitaxial films was 1.75 times higher than that of the powder.In addition,there are abundant oxygen vacancies on the surface of the BiVO₄ epitaxial films,which increases the number of adsorption sites on the surface,improves the utilization of electrons,and is conducive to the separation of photogenerated carriers.（2）BiVO₄ films were successfully grown on（001）-and（111）-oriented yttrium stabilized zirconia single crystal substrates（YSZ（001）and YSZ（111））by pulsed laser deposition.And the photocatalytic activity and photocorrosion mechanism of BiVO₄ films were systematically studied.The morphology of BiVO₄ films deposited on YSZ（001）was composed of square and flake nanoparticles,whereas that on YSZ（111）consisted of irregular hexagonal nanoparticles.The degradation rate of Rh B solution of BiVO₄/YSZ（001）film was 2.67 times that of BiVO₄/YSZ（111）film.What’s more,the photocatalytic activity of BiVO₄/YSZ（001）film was also much higher than that of polycrystalline BiVO₄film deposited on quartz substrate,indicating that the single crystal BiVO₄ film had a significantly higher photocatalytic activity than polycrystalline film.After the photocatalytic reaction,some smaller nanoparticles appeared on the edges and boundaries of the grains,and the V element was detected in the solution.And it found that BiVO₄/YSZ（001）film had stronger photostability.The electron paramagnetic resonance signal intensities of oxygen vacancy of the samples were reduced after the reaction.Furthermore,the amount of V dissolved in the solution decreased with the increase in the electron paramagnetic resonance signal intensity of oxygen vacancy,this showed that the existence of oxygen vacancy was conducive to inhibit the dissolution of V.（3）Tetragonal zircon BiVO₄ epitaxial films were prepared on YSZ（111）substrates by magnetron sputtering.By adjusting the substrate temperature,the growth conditions of pure tetragonal zircon BiVO₄ were mastered.In addition,the morphology evolution of the samples by controlling the deposition time was also studied.With the increase of deposition time,the morphology of the samples gradually changed from wirelike shape to banded shape,and finally to dense and continuous flakes.In situ growth of two-layer structure of tetragonal zircon BiVO₄ layer and V₂O₅ layer in the sample deposited for 120min can be vividly seen.The photocatalytic activity of the pure samples was evaluated by photodegradation of Rh B solution and H₂O₂ generation under full spectrum irradiation.And the photocatalytic degradation rate of Rh B solution of the sample deposited for 30 min was as high as 2.55-fold than that of the photolysis of Rh B.And the H₂O₂ generation rate of the sample deposited for 30 min(6.38×10^-3μmol/min)was higher than that of the samples deposited for 10 min and 5 min.These results stated clearly that tetragonal zircon BiVO₄ film had certain photocatalytic activity.