Study of the Thermal Annealing on Structural and Morphological Properties of High-Porosity A-WO(3) Films Synthesized by HFCVD

High-porosity nanostructured amorphous tungsten OXIDE (a-WO(3)) films were synthesized by a Hot Filament Chemical Vapor Deposition technique (HFCVD) and then transformed into a crystalline WO(3) by simple thermal annealing. The a-WO(3) films were annealed at 100, 300, and 500 °C for 10 min in an air environment. The films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), micro-Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and UV–vis spectroscopy. Results revealed that the a-WO(3) films were highly porous, composed of cauliflower-like structures made of nanoparticles with average sizes of 12 nm. It was shown that the effect of annealing on the morphology of the a-WO(3) films leads to a sintering process. However, the morphology is conserved. It was found that at annealing temperatures of 100 °C, the a-WO(3) films are of an amorphous nature, while at 300 °C, the films crystallize in the monoclinic phase of WO(3). The calculated bandgap for the a-WO(3) was 3.09 eV, and 2.53 eV for the film annealed at 500 °C. Finally, the results show that porous WO(3) films preserve the morphology and maintain the porosity, even after the annealing at 500 °C.