9.1 INTRODUCTION

Electrochemistry with semiconductor electrodes represents an interesting and highly interdisciplinary area of science especially in physical chemistry. It involves concepts of physics including band structure, zone theory, and charge transport as well as chemical concepts including electrodics, photochemistry, surface chemistry, catalysis, and Marcus theory. Semiconductor electrodes have been used in a variety of applications including liquid junction solar cells, photolytic splitting of water, pollutants and organic molecules, semiconductor processing, and sensor technology.

It is instructive to examine first the historical evolution of this field. The use of a semiconductor as an electrode in an electrochemical cell was first demonstrated by E. Becquerel in 1839. He observed a photovoltaic effect when AgCl electrode was illuminated with UV light. The "Becquerel effect" was not clearly understood until 1954, when researchers from Bell Laboratories studied photoelectrochemistry on Ge single crystals. They demonstrated that photochemical reactions occurring at Ge electrodes are affected substantially by the impurity levels in Ge. Based on these experiments and the zone theory that was developing at that time, the Becquerel effect was modeled as a photoinduced charge separation at the AgCl liquid interface. The effect was further tested with other semiconductor electrodes such as Si, GaAs, GaP, CdS, CdSe, ZnS, ZnSe, ZnTe, TiO ₂, SrTiO ₃, and Ta ₂O ₅. Electron transfer theories were also rapidly evolving during this period, starting from homogeneous systems at heterogeneous metal electrolyte interfaces leading, in turn, to...