Abstract
Photo-induced carrier generation and recombination have been regarded as
important steps in understanding the photocatalytic reactions on the surfaces of
semiconductors such as TiO<sub>2</sub>. During the investigation of a photocatalytic
water-splitting reaction on the surface of semiconductor TiO<sub>2</sub> (rutile)
single-crystal plate and sintered-nanoparticle (anatase) films coated on a
CaF<sub>2</sub> plate, a pulsed 355 nm laser was used for band-gap excitation
and a continuous-working, mid-infrared laser as the probe to trace the kinetics of
the photogenerated electrons. Fast oscillations with periods of 10-50 μs were
observed. We demonstrated experimentally that these oscillations are neither from
the nonlinear carrier recombination dynamics nor surface chemical reactions;
instead, they are caused by the thermal-induced flexural vibrations initiated by the
fast carrier recombination in the light-absorbing area. The observed oscillations
can be well accounted for by thermal stress-induced flexural vibrations of the
substrates due to rapid charge recombination, and the observed oscillation
frequencies match the flexural vibration frequencies predicted by the Kirchhoff-Love
thin-plate theory. Light-interference effect caused by the beam reflecting off the
surfaces of the substrate and windows, which can amplify the oscillation signals, is
also discussed. Several approaches are proposed to minimize or eliminate the
interference from the substrate vibration in the transient kinetic measurement when
a coherent laser is used as the probe beam.
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