Yx. Weng et al., Back electron transfer from TiO2 nanoparticles to Fe-III(CN)(6)(3-): Origin of non-single-exponential and particle size independent dynamics, J PHYS CH B, 104(1), 2000, pp. 93-104
Back-electron-transfer (ET) dynamics in Fe-II(CN)(6)(4-)-sensitized colloid
al TiO2 nanoparticles are studied using ultrafast pump probe spectroscopy.
Excitation of the adsorbate-to-nanoparticle charge-transfer band at 400 nm
leads to direct injection of electrons from Fe-II(CN)(6)(4-) to TiO2. The k
inetics of back electron transfer from TiO2 to the Fe-III(CN)(6)(3-) are me
asured by monitoring the bleach recovery of the charge-transfer band in the
430-600 nm region. The measured back-ET kinetics are non-single-exponentia
l, and a multiexponential fit requires at least four components on the <1 n
s time Scale. The kinetics are independent of pump power, indicating a gemi
nate recombination process. Recombination kinetics are very similar in two
samples of 5 and 11 nm (A-type) particles prepared from dried-nanoparticle
powder, but they are noticeably different from those in samples of 3 and 9
nm (B-type) nanoparticles prepared directly from colloids without drying. T
his result indicates that the back-ET kinetics in this system are more infl
uenced by the surface properties of the nanoparticles than their sizes. Two
models with different distributions of trapped electrons are used to descr
ibe the back-ET kinetics. Model I assumes a homogeneous distribution of ele
ctrons on the surface of the entire particle. This model predicts a large p
article size dependence and cannot fit the observed kinetics. Model II assu
mes a more localized distribution of injected electrons and takes account o
f relaxation from shallow to deep trap states during the recombination proc
ess. This model can fit the back-ET kinetics with three fitting parameters.
According to this model, the injected electrons are trapped near the adsor
bate, which accounts for the size independent back-ET kinetics. This model
also predicts that trapped electrons at longer distance and/or larger trap
energy recombine slower. A distribution of distance and trap energy as well
as relaxation between trap states give rise to multiexponential back-ET ki
netics.