Crystallization and phase separation of tungsten oxidebismuth vanadate amorphous film by annealing in air

A WO3-BiVO4 amorphous film grown on the (001) yttria-stabilized zirconia (YSZ) substrate was annealed at 600 oC in air. The microstructures evolution was investigated by X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. After annealed, the amorphous composite film crystalized into monoclinic BiVO4 and orthorhombic WO3. BiVO4 epitaxially grew on the YSZ, forming the matrix, where part of small irregular WO3 grains were embedded in. Other large-sized sub-stoichiometric WO3-δ (0<δ<3) grains with stacking faults lay on the film surface, which may contribute to the compressed strain of BiVO4 matrix vertically. Compared to the typical vertical heteroepitaxial nanocomposite of WO3-BiVO4 film grown at 600 oC by pulsed laser deposition, in the annealed composite film, the vertical lattice mismatched strain at WO3/BiVO4 interface decreased and lead to the disappearance of metastable hexagonal WO3.