Transfer of Photoinduced Electrons in Anatase-Rutile TiO2 Determined by Time-Resolved Mid-Infrared Spectroscopy

It has been a long-standing debate about the photogenerated charge transfer in anatase-rutile mixed phase TiO2. In this work, we investigated this issue by studying anatase, rutile, and anatase-rutile mixed phase TiO2 with time-resolved mid-IR (MIR) spectroscopy (TR-MIR) in a vacuum or methanol vapor. Anatase TiO2 shows transient MIR absorption on microsecond time scale after 355 nm excitation, which is attributed to photoinduced electrons in the shallow states of anatase. Conversely, there is no transient MIR absorption detected for rutile TiO2. For anatase rutile mixed phase TiO2, the initial MIR absorption intensity decreases relative to that calculated from its phase composition, indicating that electron transfer takes place in the anatase-rutile phase junction of mixed phase TiO2. Our results suggest that the photogenerated electrons in the shallow states of anatase transfer to rutile across the anatase rutile interface, but this does not exclude the possibility that the net electron transfer from rutile to anatase due to the complicated energy levels and kinds of trapped states in mixed phase TiO2.