Effects of calcination on sol–gel synthesized floatable TiO2 microbubbles for azophloxine degradation

Anatase titanium dioxide was synthesized onto glass microbubbles via a sol–gel route to obtained low density TiO2 microbubbles. The TiO2 crystallite sizes were calculated to be 8.6, 9.1, 12.7 and 44.8 nm for the TiO2 microbubbles calcined at 350, 400, 650 and 800 °C, respectively. The bandgap energies were calculated to be 3.05, 3.23, 3.31 and 3.11 eV, respectively. The TiO2 microbubbles calcined at 400 °C had both the largest specific surface area (54.19 m2/g) and pore volume (0.1147 cm3/g). The productivity of hydroxyl radicals on the TiO2 microbubbles with the change of calcination temperature was in the sequence 400 °C > 650 °C > 350 °C > 800 °C. The TiO2 microbubbles prepared at 400 °C had the strongest activity on the degradation of azophloxine, and the degradation efficiency was 30.1% after 30 min of reaction. When the irradiation time was 160 min, a total of 96.9% of the dye were degraded on the TiO2 microbubbles.