Structural Evolution Mechanism Of Electron-induced BiVO₄ Nanomaterials

Bismuth vanadate（BiVO₄）is considered as the most promising semiconductor optoelectronic material due to its narrow band gap（～2.4 e V）.However,BiVO₄is prone to photocorrosion because of its poor stability,which seriously restricts its wider application.Therefore,it is particularly important to study the structural stability and evolution mechanism of BiVO₄under the external field.Due to the poor stability of BiVO₄,the relevant evolution mechanism is often limited to macroscopic physical and chemical properties so that it is difficult to intuitively reveal its corrosion and evolution mechanism in essence.The state-of-art transmission electron microanalysis technology provides the potential to solve this problem.In particular,in-situ transmission electron microscopy（TEM）can directly reveal the structural evolution mechanism of materials by introducing an external field into TEM.The structure evolution of materials can be directly induced by the electron beam,which can control the fine structure of materials at nanometer or even atomic scale.Meanwhile,the structure evolution mechanism of materials under the effect of external field can be studied directly.In this paper,the structure evolution of BiVO₄photoanode material induced by electron irradiation was analyzed by in-situ transmission electron microscopy.The reduction process and related products of BiVO₄photoanode materials under electron beam irradiation were observed.The structure transformation and sublimation process were also studied with in-situ heating holder.In conclusion,the microstructure evolution mechanism of BiVO₄photoanode materials induced by electron beam is revealed,which provides important guidance for the design and construction of BiVO₄photoanode materials with high efficiency and stability.The main contents include:（1）the evolution mechanism of BiVO₄nanomaterials induced by in-situ irradiation field is studied.The transformation process of BiVO₄nanomaterial under electron induction was discussed by in-situ transmission electron microscopy.It was observed that BiVO₄nanomaterial would decompose after selective electron implantation,resulting in metallic Bi nanocrystals and amorphous VO₂.During the experiment,we found that the precipitated Bi nanoparticles would nucleate and grow.Therefore,in order to study the microscopic growth mechanism of Bi nanoparticles,the growth kinetics of metal Bi nanoparticles was studied at atomic scale in situ with BiVO₄nanosheets as the substrate.We use low dose rate(4.0×10³e??^-2?s^-1)electron beam to irradiate Bi nanoparticles directly.Experimental results show that compared with the classical nucleation theory,there are two non-classical growth paths for Bi nanoparticles,and the stability plane of growth induction is{012}crystal plane.In GPA,it is further demonstrated that the growth mechanism of Bi nanoparticles is layer by layer.（2）the evolution mechanism of BiVO₄nanomaterials induced by in-situ irradiation and thermal field is studied.Under the direct irradiation of electron beam,a heating holder（DENSsolution）was further used to control the temperature.It was found that the metal Bi nanocrystals sublimated,and the remaining V atoms existed in the form of BiVO₄(V⁵⁺),VO₂(V⁴⁺)and V₂O₃(V³⁺),respectively.Bi nanoparticles with good crystallinity and porous V₂O₃crystals were synthesized.Based on the above theoretical research,after the surface of BiVO₄nanosheet is coated with an electron transport layer,the results show that the structural damage of BiVO₄nanomaterial can be greatly inhibited.In addition,we observed the sublimation kinetics of Bi nanoparticles at the atomic scale for sublimation phenomenon of Bi nanocrystals.When high dose rate electron(2.5×10⁴e??^-2?s^-1)beam irradiates Bi nanoparticles,metal Bi nanoparticles with low melting point are prone to sublimation.According to the symmetry of crystal structure,it can be divided into uniform sublimation of symmetric crystal and non-uniform sublimation of asymmetric crystal.The stability plane induced by sublimation of Bi nanoparticles is also{012}crystal plane.Further studies showed that particle size and surface energy also affected the sublimation pathway.In addition,we found that the sublimation rate of Bi nanoparticles changed from linear to nonlinear during uniform and non-uniform sublimation processes.