Shibuya et al.
[26] reported the preparation of well-ordered WO3 nanotrees which are
composed of WO3 nanotrucks and WO3 nanobranches on tungsten
plate substrate. Similarly,Zhang et al. [27] synthesized hexagonal structured
WO3 films with tree-like morphology on tungsten foils. The film
thickness and the crystal structure of the film can be controlled by changing
the duration and annealing temperature, respectively.