Distinctive Behavior of Photogenerated Electrons and Holes in Anatase and Rutile TiO2 Powders

TiO2 powders have been widely used for photocatalysts; however, why anatase shows higher activity than ruffle has been a long-standing question. Here, we have elucidated the difference in the behavior of photogenerated electrons and holes by time-resolved visible to mid-IR absorption spectroscopy. In anatase TiO2, a considerable number of free electrons survive longer than 1 ms, but they are deeply trapped within a few picoseconds in the case of ruffle TiO2. The longer lifetime of free electrons is responsible for the higher activity for reduction processes on anatase TiO2. However, deep electron trapping in ruffle TiO2 elongates lifetime of holes and promotes multihole processes such as water oxidation. However, the low reactivity of deeply trapped electrons fails to increase the overall activity. These peculiar behaviors of electrons and holes are induced by defects on the powder particles and less sensitive to the physical properties such as particle size and specific surface area.