Semiconductor Electrodes

Irradiated suspensions of sem~ single crystal and polycrystalline conductor powders, e.g., Ti02, have semiconductor materials currently been employed in such heterogeneous being investigated. Measurements photo processes as the oxidation of involving such slurry electrodes cyanide, the decomposition of carare of use not only for the charboxylic acids, and the deposition acterization of the powders but of metals. Details about these and may also have practical applicaother reactions are given in recent reviews (1-3). The mechanism proposed for these processes is based on the formation of an electronhole (e-h +) pair at the particle surface upon absorption of light of energy greater than the semiconductor band-gap. The particle then behaves as a short-circuited cell with both oxidation and reduction taking place at the particle sur m face. Recent measurements of the electrophoretic mobility of TiO 2 powder have demonstrated that in solution, the surface charge on the tions in photoelectrochemical cells. In a typical experiment, a suspension of n-TiO 2 particles in 0.02 M HCI was deaerated for 24 hours prior to measurements. APt foil (area ~ 3 cm2), polished with AI203 and cleaned in boiling HN03, was used as the collector electrode. Electrochemical measurements were conducted in a Pyrex Hcell, with the compartments separated by a fine-porosity glass frit. One compartment contained the deaerated TiO 2 suspension particle under illumination is more while the other compartment connegative than that measured in the tained a Pt counter electrode and dark (4). Moreover, this negative 0.02 M HCI with a low oxygen concharge persists for at least 10-15 centration to support a current minutes after the irradiation stop~ flow. A 2500 W xenon lamp, operaThese results suggest that when the ted at 1600 W, was used for illuholes are scavenged by an oxidation mination. A Pyrex water bath approcess, electron density builds up on the particle surface. We report here that this excess charge can be collected on an inert electrode to produce an anodic current flow. Thus, semiconductor powder suspensions (e.g., TiO 2, WO 3, CdS) can behave as electrodes ("slurry electrodes") in the same manner as the