The green fibers of PVA and HSiW were calcined in air at 600 °C in a muffle furnace. The furnace was heated within 1 h to the final temperature followed by 2 h dwell time. After heat treatment, the sample was left to cool down spontaneously to ambient temperature. The prepared -WO3/a-SiO2 nanofibrous material was analyzed by SEM and X-ray powder diffraction (XRD) (Supplementary Experimental Part 1).