Synthesis of Cobalt-Doped TiO<inf>2</inf>-RPNS as a Controlled Charge Transfer Photocatalyst: The Role of Scavengers and Oxygen as a Promoter**

Low band gap photocatalysts are desirable for efficient performance. We report synthesis of cobalt-doped TiO2 photocatalysts, inside radial pores of nano-spherical silica (RPNS) particles, having different weight percentages of dopant (0.1 to 4 wt.% Co-TiO2-RPNS). Doping decreases both optical band gap and electron-hole charge separation, with maximum decrease in charge transfer resistance, for 1 wt.% Co-TiO2-RPNS (0.74 kΩ.cm−2), compared to TiO2-RPNS (1.18 kΩ.cm−2). However, it shows only partial degradation of rhodamine B dye. Hence, the 1 wt.% Co-TiO2-RPNS catalyst was further investigated with respect to different e−/h+ recombination processes, in presence of any one of the scavengers - e− (K2S2O8) or OH* (butanol) or h+ (EDTA). An e− scavenger shows a higher dye degradation (rate constant 1.24 h−1), while h+ scavenger shows a lower rate (0.38 h−1), in contrast to no scavenger (0.91 h−1). Oxygen purging as a reaction promoter inhibited e−/h+ recombination, doubling overall rate (4.66 h−1). So, scavenger and oxygen purging together can further enhance a low band gap photocatalyst's activity.