Fig. 2

SEM and TEM images of porous transparent bulk TiO₂. a SEM image of the untreated porous transparent bulk TiO₂. b SEM image of porous transparent bulk TiO₂ heating at 500 °C. c SEM image of porous transparent bulk TiO₂ heating at 800 °C. d TEM image of porous transparent bulk TiO₂