Possible sources of deviations of semiconductor/metal and semiconductor/electrolyte Schottky barriers from ideality are discussed. The conditions under which the flat-band potential of the barrier can be determined by capacitance measurements are deduced. Results on the flat-band potential and the position of the band edges for several semiconductor/electrolyte barriers are given.

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The (photo)electrochemical behavior of n‐GaN[0001] in various aqueous solutions was studied using rotating‐disk voltammetry, cyclic voltammetry, and electrical impedance measurements. It was found that the bandedges of the semiconductor shift over 60 mV/pH unit, indicating acid‐base equilibria at the interface. In H2SO4 and KOH solutions, the photocurrent under anodic bias is associated with the oxidation of the semiconductor according to a three‐equivalent reaction, leading to dissolution and roughening of the surface. In 1.2 M HCl solutions, n‐GaN is stabilized for anodic decomposition due to the competing oxidation of Cl− ions. In the presence of oxalic acid and citric acid, anodic photocurrent multiplication was observed. Under cathodic polarization in the dark, Fe3+ , Ce4+ , HIO3 , in acid medium and normalFefalse(CN)63− in alkaline medium are electrochemically reduced at a diffusion‐limited rate. In 1 M KOH a high reactivity for O2/H2O reduction is observed, explaining why n‐GaN can be photoetched under open‐circuit conditions in this solution. © 2000 The Electrochemical Society. All rights reserved.

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The impedance of several nearly ideally polarizable semiconductor electrodes (CdSe, CdS, TiO,) has been studied as a function of applied voltage and of frequency. The differential capacitance and resistance appear to obey to simple frequency laws. A mathematical model accounting for these laws is presented, in which a distribution of time constants is assumed, associated with dielectric relaxation phenomena in the double layer at the semiconductor/electrolyte interface. The experimental results indicate, that the source of the frequency-dependence has to be sought in structural irregularities of the surface region of the electrode. The proposed model appears to be also applicable to results, mentioned in the literature, concerning other semiconductor/electrolyte systems. The possibility of determining the flat-band potential from frequency-dependent capacitance data is discussed. Die Impedanz verschiedener fast ideal polarisierbaren Halbleiterelektroden (CdSe, CdS, TiO,) wurde in Abhangigkeit yon der angelegten Spannung und der Frequenz untersucht. Die Differentialkapazitat und der Differentialwiderstand zeigen einfache Frequenzabhangigkeiten. Diese Gesetze werden aufgrund eines mathematischen Modells gedeutet, in dem eine Verteilung von Zeitkonstanten angenommen wird, die mit dielektrischen Relaxationserscheinungen in der Doppelschicht an der Halbleiter/Elektrolyt-Phasengrenze in Zusammenhang stehen. Die Ergebnisse deuten darauf hin, Strukturdefekte im Oberflachengebiet der Elektrode seien fur die Frequenzabhangigkeit verantwortlich. Es zeigt sich, daO das vorgeschlagene Modell auch auf Literaturergebnisse betrefis anderer Halbleiter/Elektrolyt-Systeme anwendbar ist. Die Moglichkeit, das Flachbandpotential aus frequenzabhangigen Kapazitatsergebnissen zu bestimmen, wird besprochen.

The electrochemical behaviour of semiconducting TiO, electrodes has been investigated by means of impedance, current and potential measurements in darkness, and as well in indifferent electrolyte as in redox electrolyte solutions. The results allow the determination of the flat-hand potential and of the position of the energy band edges at the TiO, surface. The data obtained on electrochemical reactivity, both anodically and cathodically, appear to he qualitatively interpretable on the basis of the accepted theories for direct electron transfer at the semiconductor/electrolyte interface. Some particular features inherent to the TiO,/electrolyte system are discussed. Das elektrochemische Verhalten halbleitender Ti0,-Elektroden wurde im Dunkeln durch Impedanz-, Strom-und Spannungsmessungen, sowohl in indifferenten Elektrolyt-wie in Redoxelektrolyt-Losungen untersucht. Die Ergebnisse lassen die Bestimmung des Flachbandpotentials und der energetischen Lage der Bandkanten an der TiO,-Oherflache zu. Es zeigt sich, daR die anodischen und kathodischen Reaktivitatsergebnisse aufgrund der allgemein angenommenen Theorien fur direkten Elektronenubergang an der Halbleiter/Elektrolyt-Phasengrenze qualitativ interpretierbar sind. Einige dem TiO,/Elektrolyt-System inharente Eigentiimlichkeiten werden besprochen.

The anodic oxidation of various reactants dissolved in water was studied on a zinc oxide single‐crystal electrode by electrical techniques. It was found that only a few substances inject electrons into the conduction band. However, many reactants are oxidized by holes, which are created in the crystal by illumination: A class of two‐or multi‐equivalent reducing agents, called current‐doubling agents, undergoes oxidation by a two‐step mechanism in which reaction with a hole leads to the formation of a radical‐type intermediate, which then injects an electron in the conduction band. Relative hole reactivities for different reagents were determined by making current measurements during competitive oxidation of current‐doubling and noncurrent‐doubling reactants. The reactivity of different radical‐type intermediates toward oxidizing agents was investigated. Experimental support is given for the idea that a common intermediate, the hydrogen atom, is formed during the oxidation of many organic current‐doubling species. Differences in behavior depending on the crystal face are discussed.