Characterization and photocatalytic properties of hexagonal and monoclinic WO3 prepared via microwave-assisted hydrothermal synthesis

WO3 nanoparticles with hexagonal and monoclinic structures were synthesized via a microwave-assisted hydrothermal process without the use of any additives in which the microwave hydrothermal reaction time was 30 or 60 min and was followed by different thermal treatments. The formation at different temperatures of hexagonal (h-WO3) and monoclinic (m-WO3) crystal structures of WO3 was confirmed by X-ray powder diffraction (XRD). The characterization of WO3 samples was complemented with scanning electron microscopy (SEM) images, which revealed a natural tendency of the oxides to form particles with ovoid shapes. The particle size of the prepared WO3 samples was between 20 and 100 nm depending on the heat-treatment temperatures. The surface areas of the WO3 samples, as determined by the BET method, ranged from 2 to 113 m2 g−1 depending on their temperature of synthesis. The highest obtained specific surface area was nearly 37 times greater than that observed for commercial WO3. The photocatalytic activity of WO3 was evaluated with respect to the degradation reactions of rhodamine B (rhB), indigo carmine (IC) and tetracycline hydrochloride (TC) under UV–vis irradiation. The highest activity was observed in the sample obtained by thermal treatment at 700 °C and with a microwave hydrothermal reaction time of 30 min. The degree of mineralization of the organic dyes by WO3 was determined by the total organic carbon analysis (TOC) and was found to reach percentages of 95% for rhB, 65% for IC and 65% for TC after 96 h of irradiation.