Understanding Charge Dynamics in TiO₂ Using Ultrafast Mid-infrared Spectroscopy: Trapping versus Recombination

We utilize herein ultrafast mid-infrared probe laser pulses to explore the mechanism for the charge recombination/trapping process of photogenerated charges within the band gap of TiO₂ and across interfaces. Low-energy photons solely probe the free electrons present in the conduction band of TiO₂ and those captured in shallow-trap states. We found that >70% of the photogenerated charges disappear from the conduction band in the first few nanoseconds due to electron trapping followed by charge recombination at longer time scales. Moreover, the behavior of the dynamics of the free electrons within the band gap of TiO₂ and electrons generated at the interface of adsorbed organic dyes was investigated and compared. This comparison shows that the main driving force for the efficient charge trapping of photogenerated charges within the picosecond time scale is the presence of photogenerated holes, within the band gap of TiO₂, or close to the interface of TiO₂. If the hole is far from the TiO₂ surface, the electron trapping process is hindered, and almost 100% of photogenerated charges can survive up to nanoseconds. This work offers a deeper understanding of the charge trapping and charge recombination processes, by knowing the spatial hole effect, in TiO₂ and similar semiconductors upon utilization in photonic devices and photocatalysis.