Behavior of Tin Oxide Semiconducting Electrodes Under Conditions of Linear Potential Scan

Abstract

Linear sweep voltammetric studies of thin layer semiconducting tin oxide layers on glass substrates revealed three regions of electrochemical interest. In the anodic region, oxygen or chlorine evolution occurs without disruption of the surface. In the intermediate region of potentials, cathodic-anodic proc-esses attributable to charging and discharging of positive surface states oc-cur. The charging-discharging process is most pronounced in the presence of high concentrations of hydrogen ions and chloride ions and is closely re-lated to the specific adsorption of chloride ion. In the cathodic region, hydro-gen ion discharge is accompanied by a disruption of the tin oxide layer, which is reduced to metallic tin. This investigation describes the charging and dis-charging of the double layer on the semiconductor side of the semiconductor-solution interface under conditions of linear potential scan. Also included is a brief study of the cathodic decomposition of the tin oxide electrode to metallic tin. While pure polycrystalline tin oxide is an insulator, the addition of certain electron donating species can render the material into a broad-band n-type semi-conductor. These doped or unstoichiometric tin ox-ides with free carrier densities of approximately 10 TM cm-3 are ideally suited for use as indicator electrodes in electrochemical investigations. Thin semiconducting tin oxide films on glass or quartz substrates have been used as indicator elec-trodes for potentiometry (1), coulometry, and am-perometry (2, 3). Kuwana and co-workers (4, 5) have taken advantage of the transparent nature of the tin oxide film electrodes in the visible region of the spectrum to directly observe electroactive spe-cies at the electrode-solution interface using internal reflection spectroscopy. The literature also contains several papers des-cribing the electrical and optical properties of stannic oxide-based semiconductors (6-13). The physical prop-erties of this material have been described in detail, and several mechanisms of electron conduction have been proposed.