Photoinduced electron transfer involving localized electronic states

The Role of Oxides in Reduction Reactions at the Metal-Water Interface

“…Thus, the electrons involved in ET to RX could originate from the conduction band or localized states in the oxide. If a space charge barrier exists (which we believe to be the case for passivated iron in the presence of aqueous CCI4), conduction band electrons will have to tunnel through the barrier either directly or via localized states (55,71,74,76,80). There is evidence that ET to dissolved ferrous cyanide can occur via tunneling from the conduction band through the space charge region of a passivated iron electrode (81,82).…”

“…Passive films on Fe° are typically only a few monolayers thick, so longrange order normal to the surface cannot develop. Thicker oxide layers tend to be amorphous because a more coherent film would be strained and break up (55,(71)(72)(73)(74). While thick oxide layers will have bands equivalent to the conduction and valence bands found in single crystal semiconductors (shown in Figure lc), additional bands, which result from a large density of impurities or ions in a different oxidation state, may also be present (55,75).…”

“…In addition, there will be a much greater density of localized states (energy levels that exist in the bandgap) than is found in single crystal semiconductors. These localized states result from the lack of identical lattice sites throughout the oxide (55,71,(73)(74)(75). Impurity bands and localized states are important because they can serve as electron donors in ET (68,76), and because they are likely to be abundant in the oxide layers that develop during remediation applications of Fe°.…”

“…Direct tunneling of electrons from the underlying Fe° through the oxide layer is only possible for thin films of oxide (-10 Â). Resonance tunneling, in which the electrons tunnel or "hop" between localized states in the oxide, is more probable for the oxide layer that forms on grains of Fe° in remediation applications (55,73,74,78,79).…”

Photoeffects on the Reduction of Carbon Tetrachloride by Zero-Valent Iron

“… Electron transfer from Fe 0 to the aqueous CCl 4 may occur directly from the metal or indirectly through the oxide film. Direct electron transfer between the metal and the organic could occur by tunneling (direct or resonance) − or through defects such as pits or pinholes that allow the solution to directly contact the iron. ,, For indirect electron transfer through the oxide film to be significant, there must be a facile mechanism for transporting electrons across the oxide layer, presumably through the conduction band. Exploring the role of conduction-band electrons in reduction of organic adsorbants is the primary objective of this study.…”