Microstructure Evolution And Growth Mechanism Of WO₃-BiVO₄ Vertical Heteroepitaxial Nanocomposite Films

In the past decades,self-assembled vertical heteroepitaxial nanocomposite?VHN?films have attracted intense research interest because their high interface/volume ratio and strong interface coupling effects of the lattice,the charge,the spin and the orbital provide more degrees of freedom to design and develop intriguing physical properties.At the initial stage of the research,most of the VHNs were?pseudo?cubic-cubic composite films consisting of perovskite and spinel phase and so on,which were manily investigated for the prospective applications as electronic and magnetic device components.In recent years,VHN films composed of low-symmetry crystal phases like monoclinic and orthorhombic phases have achieved richer physical properties,namely,greatly improved photoelectrochemical activity.But the modulation mechanism of the properties also become more complicated.The microstructure investigations based on electron microscopy,which can be regarded as a bridge between materials growth and physical properties,is playing an increasingly important role in the development of material multifunctionalities.Here,we investigated the microstructures of WO₃-BiVO₄VHN films for photoelectrochemical water splitting,including crystalline phases,epitaxial relationship,interface structures and chemical composition distributions,by a combination of multiple electron microscopy techniques.In addition,we explored the in-situ crystallization and phase transformation of WO₃-BiVO₄ amorphous composite films.The microstructure evolution and growth mechanism of the composite film were summarized,which pave the way for design and device application of photoelectrochemical water splitting materials.?1?Microstructure evolution with composition ratio in self-assembled WO₃-BiVO₄ VHN films.A series of self-assembled WO₃-BiVO₄ VHN films with 17 mol%,25 mol%,50 mol%,67 mol%and 100 mol%WO₃ were grown on the?001?yttria-stabilized zirconia?YSZ?substrate by pulsed laser deposition?PLD?method.The monoclinic BiVO₄ formed the matrix,in which WO₃ nanopillars were embedded with specific epitaxial relationships.In BiVO₄-rich sample,orthorhombic Bi₂WO₆ was formed.However,metastable hexagonal WO₃ phase and orthorhombic WO₃ phase coexisted in other composite samples.The hydrostatic tensile strain due to thermal expansion mismatch and lattice mismatch between BiVO₄ and WO₃ as well as the diffusion of Bi into the WO₃ stabilized the metastable h-WO₃.The thin amorphous layer at the film/substrate interface indicated that the mismatch strain between films and substrate is released.A WO₃-BiVO₄ pseudo-binary phase diagram was proposed based on the magnitude of the mismatched strain and the distance of Bi diffusion,which can be applied to design the microstructures of WO₃-BiVO₄ heterojunctions and optimize their photoelectrochemical properties.?2?Microstructure evolution with deposition temperature in self-assembled WO₃-BiVO₄ VHN films.A series of self-assembled WO₃-BiVO₄ VHN films were grown on the?001?YSZ substrate at the substrate temperatures of 400?,500?,550?,600?,650?and 700?by PLD.The sample grown at 400?was amorphous due to the low driving forces for nucleation and diffusion.In the samples made at 500?,550?and 600?,the monoclinic BiVO₄ epitaxially grew on the YSZ,forming the matrix,where the WO₃ nanopillars were embedded in with specific orientation relationship among BiVO₄,WO₃ and YSZ.However,in the thin films deposited at 650?and 700?,the WO₃ grains randomly grew on the YSZ substrate,which dominated the microstructures of the resultant thin films.Quantitative analyses of the microstructures revealed that the lateral grain sizes of BiVO₄ and WO₃ increased and the volume fraction of BiVO₄ in the thin films decreased with the increase of the deposition temperature.A three-regime growth mechanism of the WO₃–BiVO₄composite thin film was proposed based on the growth dynamics determined by the competition between BiVO₄ and WO₃.?3?Role of indium tin oxide electrode on the microstructure of self-assembled WO₃-BiVO₄ VHN films.Self-assembled WO₃-BiVO₄ VHN films were grown on?001?YSZ substrate by PLD method with and without indium tin oxide?ITO?electrode.In both samples,WO₃ formed nanopillars embedded into the monoclinic BiVO₄ matrix.In the sample with ITO bottom electrode,an atomically sharp BiVO₄/ITO interface was formed and the orthorhombic WO₃ nanopillars were grown on a relaxed BiVO₄ buffer layer with a mixed orthorhombic and hexagonal WO₃ transition layer.In contrast,a thin amorphous layer appears at the interfaces between the thin film and the YSZ substrate in the sample without ITO electrode.In addition,orthorhombic Bi₂WO₆ lamellar nanopillars were formed between the WO₃ and Bi VO₄ due to the interdiffusion.Such a WO₃-Bi₂WO₆-BiVO₄ double heterojunction photoanode may promote the photo-generated charge separation and further improve the photoelectrochemical water splitting properties.?4?Crystallization and phase transformation investigation of amorphous WO₃-BiVO₄ composite films via in-situ annealing.The amorphous WO₃-BiVO₄ composite thin film was ex-situ and in-situ annealed,respectively.After ex-situ annealed in the air at 600 ^oC,BiVO₄ crystallized into orthorhombic phase and epitaxially grown on the substrate.Most of the WO₃ formed nanocrystaline grains embedded in the BiVO₄matrix,while other large WO₃ grains with stacking faults were distributed on the surface of the film.When the cross-sectional amorphous sample of thickness only⁵0 nm was in-situ annealed in the TEM column with high vacuum and anoxic environment,Bi elements easily evaporated.When heated to 550 ^oC,the main crystalline phase was cubic W phase with a small amount of orthorhombic WO_x?0<x?3?and tetragonal Bi_x VO_y?0<x?1,0<y?4?phases.Thus,the atmosphere during the annealing process and the heating mode greatly influenced the crystallization of the film.