Catalytic Modification of Indium Tin Oxide Electrode Surfaces

Abstract

Conductive and transparent oxide thin films such as Sb-doped tin oxide and Sn-doped indium oxide (ITO) generally display sluggish electrochemical behavior towards multi-electron redox systems. As a critical step towards the construction of a photoelectrolysis device based on Si/ITO heterojunctions, we describe herein our success with the design and optimization of catalytic charge-storage centers on the ITO electrode surface. The key feature of our approach is the control of catalyst morphology such that the excellent optical transparency of the ITO layer is not compromised to any significant extent. This is accomplished by a two-stage spray pyrolysis method which will be described in detail elsewhere (I). Aside from solar energy storage possibilities, the results described here may have relevance to other applications which require a combination of optical transparency and electrocatalytic property for the electrode material.The important finding of this study is that in the case of Ru, our catalytic modification approach results in the formation of a ring-like island morphology on the parent ITO surface. Representative photomicrographs are contained in Fig. I. This figure also illustrates the gradual overlap of the catalyst rings with increased loading (spray duration). Auger electron maps of the surface revealed the peripheral portions of the rings to be predominantly Ru with the center regions comprising mainly ITO. Consistent with the expectation that these catalyst islands are mainly localized on the surface, Auger depth profiles reveal a precipitous decline in the Ru levels a few X into the ITO layers. Detailed surface analyses of these catalytically-modified ITO layers will be presented in a forthcoming paper (i). Good optical transparency was maintained for these layers even at relatively high Ru loading (cf. Fig. Id). For example, transmittance measurements yielded values of 72% and 64% at 500 nm for samples shown in Fig. Ib and d, respectively. These values should be compared with the reference level of 80% recorded for bare ITO, cf. Fig. la. *Electrochemical Society Student Member, **Electrochemical Society Active Member.Platinization of the ITO surface results in a different island morphology relative to~e Ru case described above. Contrasting the ring-like structure noted with Ru, clusters form in this case, the distribution of which again is dependent on the spray-duration. Electrodeposited Pt yielded a similar morphology. Similar to the Ru case, the optical transparency of these platinized ITO layers was excellent.Representative polarization curves for Ru-and Pt-modified ITO electrodes are contained in Figs. 2-4. Curves for bare ITO are also shown for comparison. Details on ITO electrode fabrication have been given previously (2). The electrochemical data in Figs. 2-4 demonstrate the efficacy of catalytically-modified ITO layers for driving the 02 evolution, bromide oxidation, and proton reduction reactions. The modified electrodes show no deterioration in performance over many cycles of operation....

Electrocatalytic Modification of Indium Tin Oxide Electrode Surfaces: Surface Analyses and Electrochemistry

“…This paper describes our success with the design and optimization of catalytic charge-storage centers on the ITO surface (10). We show that, by proper control of the catalyst morphology, it is possible to significantly improve the catalytic activity of the ITO without compromising its excellent optical transparency to any significant extent.…”