In this paper, visible-light-active monoclinic-WO3 powders were synthesized by thermal oxidation of W powders at 200-1000 °C in air atmosphere. Morphology and crystal structure of annealed W powders were characterized by Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD), respectively. Based on the SEM and XRD results, a spherical orthorhombic-W3O8 obtained at 200 °C was transformed into a dendritic monoclinic WO2 + tetragonal WO3 + monoclinic WO3 structures at 400 °C accompanied by a color transition from grey into green powders. At 600 °C, yellow monoclinic WO3 + monoclinic WO2.96 powder was produced that ascribed to oxygen vacancies. Photocatalytic activity of the annealed W powders at 1000 °C demonstrated 70.7% Cr(VI) removal after 150 min attributed to the high photoactivity of monoclinic WO3. Nevertheless, the dendritic monoclinic WO2 + tetragonal WO3 + monoclinic WO3 obtained at 400 °C exhibited the lowest Cr(VI) photoreduction i.e. 45.2% suggesting poor photocatalytic activity of monoclinic WO2 and sluggish electron transport at WO2|WO3 interfaces.
