posted on 2024-01-26, 11:06authored byAhmed M. El-Zohry, Lars Kloo, Lanlan He
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.