Role of indium tin oxide electrode on the microstructure of self-assembled WO₃-BiVO₄ hetero nanostructures

Self-assembled WO₃-BiVO₄ nanostructured thin films were grown on a (001) yttrium stabilized zirconia (YSZ) substrate by the pulsed laser deposition method with and without the indium tin oxide (ITO) bottom electrode. Their microstructures including surface morphologies, crystalline phases, epitaxial relationships, interface structures, and composition distributions were investigated by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray energy dispersive spectroscopy. In both samples, WO₃ formed nanopillars embedded into the monoclinic BiVO₄ matrix with specific orientation relationships. In the sample with the 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 the ITO electrode. In addition, orthorhombic Bi₂WO₆ lamellar nanopillars were formed between WO₃ and BiVO₄ due to interdiffusion. Such a WO₃-Bi₂WO₆-BiVO₄ double heterojunction photoanode may promote the photo-generated charge separation and further improve the photoelectrochemical water splitting properties.