Application of Degenerately Doped Metal Oxides in
the Study of Photoinduced Interfacial Electron Transfer
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Abstract
Degenerately doped In<sub>2</sub>O<sub>3</sub>:Sn semiconductor
nanoparticles (<i>nano</i>ITO) have been used to study the
photoinduced interfacial electron-transfer reactivity of surface-bound
[Ru<sup>II</sup>(bpy)<sub>2</sub>(4,4′-(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>-bpy)]<sup>2+</sup> (RuP<sup>2+</sup>) molecules
as a function of driving force over a range of 1.8 eV. The metallic
properties of the ITO nanoparticles, present within an interconnected
mesoporous film, allowed for the driving force to be tuned by controlling
their Fermi level with an external bias while their optical transparency
allowed for transient absorption spectroscopy to be used to monitor
electron-transfer kinetics. Photoinduced electron transfer from excited-state
-RuP<sup>2+*</sup> molecules to <i>nano</i>ITO was found
to be dependent on applied bias and competitive with nonradiative
energy transfer to <i>nano</i>ITO. Back electron transfer
from <i>nano</i>ITO to oxidized -RuP<sup>3+</sup> was also
dependent on the applied bias but without complication from inter-
or intraparticle electron diffusion in the oxide nanoparticles. Analysis
of the electron injection kinetics as a function of driving force
using Marcus–Gerischer theory resulted in an experimental estimate
of the reorganization energy for the excited-state -RuP<sup>3+/2+*</sup> redox couple of λ* = 0.83 eV and an electronic coupling matrix
element, arising from electronic wave function overlap between the
donor orbital in the molecule and the acceptor orbital(s) in the <i>nano</i>ITO electrode, of <i>H</i><sub>ab</sub> =
20–45 cm<sup>–1</sup>. Similar analysis of the back
electron-transfer kinetics yielded λ = 0.56 eV for the ground-state
-RuP<sup>3+/2+</sup> redox couple and <i>H</i><sub>ab</sub> = 2–4 cm<sup>–1</sup>. The use of these wide band
gap, degenerately doped materials provides a unique experimental approach
for investigating single-site electron transfer at the surface of
oxide nanoparticles