Calcination Temperature-Induced Morphology Transformation in WO3 Flower-like Thin Films for Photocatalytic Wastewater Treatment

WO3 nanoflowers were synthesized via anodization and subsequently calcined in air at different temperatures (200–700 °C) to evaluate their photocatalytic activity. The samples were characterized in terms of their morphological, crystallite, and optical properties. Anodization produced WO3 hydrate with a layer thickness of ~1.2 µm, which was transformed into WO3 after heating. All samples exhibited monoclinic phase, with Raman shift intensity increasing with the calcination temperature. Some residual WO3·H2O was detected at certain temperatures. The calculated bandgap energy ranged from 2.49 to 2.67 eV, with higher calcination temperatures leading to lower absorbance in the UV region. The photodegradation of phenol under UV-Vis radiation reached 35% in 60 min for WO3_700 °C, where the photocatalyst suffered a morphological transformation from a nanoflower to nanogranular structure, accompanied by increased crystallinity. Under visible light, the phenol abatement was limited, achieving 1–3% degradation. The WO3 surface is likely negatively charged at the solution’s pH (5.6), which may explain the low phenol adsorption (~1%).