Understanding Charge Recombination of TiO2 using Ultrafast mid-Infrared Spectroscopy: The Effect of Photogenerated Holes

We utilize mid-infrared probe to explore the mechanism for the dramatic charge recombination process of the photogenerated charges within the band gap of TiO2. Using the low-energy photons probes the free electrons in the conduction band of TiO2 and upon trapping to 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 or charge recombination. We compare the behavior of free electron dynamics within the bandgap of TiO2 and upon generating them across the interface with adsorbing organic dyes. This comparison shows that the main driving force of dramatic charge recombination of photogenerated charges is the presence of hole within the band-gap of TiO2 or close to the interface of TiO2. Once, the hole is far from the TiO2 surface, the electron trapping process is hindered and almost 100% of photogenerated charges can live up till nanoseconds. This paper allows for further understandings of the charge trapping and charge recombination processes in TiO2 and in other semiconductors.