Electrochemical Modification of Indium Tin Oxide Using
Di(4-nitrophenyl) Iodonium Tetrafluoroborate
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Abstract
Optoelectronic
applications often rely on indium tin oxide (ITO)
as a transparent electrode material. Improvements in the performance
of such devices as photovoltaics and light-emitting diodes often requires
robust, controllable modification of the ITO surface to enhance interfacial
charge transfer properties. In this work, modifier films were deposited
onto ITO by the electrochemical reduction of di(4-nitrophenyl) iodonium
tetrafluoroborate (DNP), allowing for control over surface functionalization.
The surface coverage could be tuned from submonolayer to multilayer
coverage by either varying the DNP concentration or the number of
cyclic voltammetry (CV) grafting scans. Modification of ITO with 0.8
mM DNP resulted in near-monolayer surface coverage (4.95 × 10<sup>14</sup> molecules/cm<sup>2</sup>). X-ray photoelectron spectroscopy
(XPS) analysis confirmed the presence of 4-nitrophenyl (NO<sub>2</sub>Ph) moieties on the ITO surface through the detection of a NO<sub>2</sub> nitrogen signal at 405.6 eV after grafting. Further XPS evidence
suggests that the NO<sub>2</sub>Ph radicals do not bond to the surface
indium or tin sites, consistent with modification occurring either
through bonding to surface hydroxyl groups or through strong physisorption
on ITO. CV in the presence of an electroactive probe and electrochemical
impedance spectroscopy (EIS) were used to investigate the electronic
effects that modification via DNP has on ITO. Even at submonolayer
coverage, the insulating organic films can reduce the current response
to ferrocene oxidation and reduction by more than 25% and increase
the charge transfer resistance by a factor of 10