Microstructure evolution determined by the crystalline phases competition in self-assembled WO₃-BiVO₄ hetero nanostructures

A series of self-assembled WO₃-BiVO₄ nanostructured thin films were grown on the (001) yttria-stabilized zirconia (YSZ) substrate at the substrate temperatures of 400 °C, 500 °C, 550 °C, 600 °C, 650 °C and 700 °C by a pulsed laser deposition method. The microstructures including crystalline phases, epitaxial relationships, surface morphologies and interface structures were investigated by a combination of x-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. The sample grown at 400 °C was amorphous due to the low driving forces for nucleation and diffusion. For the samples made at 500 °C, 550 °C and 600 °C, the monoclinic BiVO₄ matrix epitaxially grew on YSZ, forming the matrix, where the WO₃ nanopillars were embedded in with a specific orientation relationship among BiVO₄, WO₃ and YSZ. However, in thin films deposited at 650 °C and 700 °C, 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-BiVO₄ composite thin film was proposed based on the growth dynamics determined by the competition between BiVO₄ and WO₃.