Mn-doped TiO2 photocatalysts: Role, chemical identity, and local structure of dopant

The response of TiO2 is extended to visible light by doping with Mn through a hydrothermal route. The role, chemical identity, and local structure of the Mn dopants are comprehensively elucidated to find the key factors behind the improved photocatalytic activity. X-ray absorption fine structure and X-ray near-edge structure spectroscopy with synchrotron light reveal that the Mn dopants partially replace Ti atoms in the bulk of the TiO2 host to form a MnO2-TiO2 solid solution. The Mn dopants extend the light absorption of the host to the visible region and restrict electron-hole recombination, as probed by diffuse reflectance spectroscopy and light-induced infrared absorption spectroscopy, respectively. The extension of light absorption is not monotonically correlated with the improvement in photocatalytic activity. At an excessive Mn concentration, the conduction band edge bottom becomes too low to enable the reduction of the adsorbed oxygen by the photoexcited electrons. The unconsumed electrons eventually recombine with holes to accelerate the recombination, resulting in a low electron population. There is a trade-off between the extended light absorption and the recombination. This is optimized at a Mn concentration of 4 wt%, to yield the highest electron population, and thus, the highest photocatalytic activity.