A mathematical model is developed to calculate the impedance response of a semiconductor electrode to a sinusoidal current perturbation. The model accounts explicitly for electron and hole transport as well as generation and recombination through band-to-band mechanisms and through bulk interband electron acceptors of specified energy. The resistive (real) component of the impedance is shown to be sensitive to the concentration, distribution, and energy level of the bulk sites. The capacitive (imaginary) component, while useful for determining the dopant level and the flat-band potential of the semiconductor, is relatively insensitive to low concentrations of bulk sites.

photocatalytic efficiency of different TiO2 powders used in photoassisted oxidation processes may have its origin in their different distribution of surface traps, where electrons are immobilized and from which electrons are transferred to oxygen. Materials with high densities of shallow (-0.1-0.3 eV) traps are likely to be most effective. However, catalysis of 02 reduction by group VIII metals should in all cases substantially increase the quantum efficiency of the photoassisted oxidation of organics by molecular oxygen.

The passive film on a zinc electrode is often assumed to be composed of ZnO. Since removal of the electrode from the solution could alter the film composition, an in situ measurement technique is required for a qualitative analysis. We have developed a model to simulate the kinetics of anodic dissolution and passivation on the zinc electrode in alkaline electrolyte. In our model ZnO is postulated to be responsible for the passivation of the electrode. To check this hypothesis we performed photocurrent measurements and we compared the results with those obtained on bulk material. Based on the differences in photospectra as a function of electrode potential, we infer that the surface composition changes with potential. Although photospectra from the passivated electrodes are not identical to those from bulk material, key characteristics closely correspond.We have developed a model of the anodic dissolution of the zinc electrode in alkaline electrolyte. The model is based on postulated elementary reactions coupled with a Langmuir treatment of the surface coverage by partially soluble species. Two parallel paths are considered in the prepassive region.In the initial dissolution region we proposed the following series of elementary reactions (1) Zn + OH-= ZnOH + e [1] ZnOH + 2 OH-= Zn(OH)~-+ e rds [2] Zn(OH)3-+ OH ~ Zn(OH)42 [3] where rds indicates the rate-determining step. Reaction [1] provides the ZnOH, which enters into the following parallel reaction path (2) ZnOH + OH-= Zn(OH)2 + e [4] Zn(OH)2 + OH-= Zn(OH)3 rds [5] Zn(OH)3-+ OH-= Zn(OH), 2-[6]In the passive region we suggest that a zinc oxide film forms and is responsible for passivation Zn + Zn(OH)2 + 2 OH = 2ZnO + 2H20 + 2e[7]Simulations of the current-potential behavior based on this model give quantitative agreement with the observed curves (3). From ellipsometric and chronocoulometric measurements we estimate that the passive film is of the order of 500A but varies with hydroxide concentration, potential, and hydrodynamic conditions (4). Pourbaix diagrams indicate that Zn(OH)2 is stable over a range of pH near 7, but no other solid species are thermodynamically favored over the range of potential and pH in our investigations. Ex situ x-ray diffraction measurements of anodic zinc films demonstrate the presence of ZnO (5-7), but in situ measurements to support the assumption that the film is composed of ZnO are less clear (8)(9)(10)(11)(12)(13)(14).ZnO is an n-type extrinsic semiconductor with a bandgap of 3.2 eV (388 nm) exhibiting direct electron transitions and nonstoiehiometry. Extrinsic conductivity is attributed to reactive interstitial zinc (Zni) (15) Zn, = Zni § + e[8]Upon illumination with super-bandgap energy photons, ZnO undergoes photoanodic dissolution (16) ZnO + 2 holes* = Zn 2 § + 1/2 O2 [9]Reviews of photoelectrochemical methods are given by Stimming (17), Chazalviel (18), and Peter (19). Photocurrent studies in the zinc system indicate the presence of a ZnO film on passive zinc in borate solutions (20), and data *Electrochemical Society Stud...

The impact of the assumptions inherent in using the Mott-Schottky theory to identify deep-level electronic states in semiconductors was assessed by comparison to the results of a less restrictive mathematical model. The model, developed in another paper, treated the transport and recombination reactions involving electrons, holes, and electronic states located within the bandgap. The capacitive component used in standard Mott-Schottky theory was found to be insensitive to bulk electronic states within the bandgap for concentrations significantly less than the doping level. The resistive component measured at low frequencies was much more sensitive to deep-level states and may be used to determine their distribution. For high concentrations of deep-level states, the model results were in agreement with the current practice of attributing changes in the slope of the Mott-Schottky curve to partial ionization of single-energy deep-level states with applied potential. In the absence of frequency dispersion, these changes in slope could be attributed instead to nonuniform dopant distribution.

Dilute-solution transport equations with constant activity coefficients are commonly used to mode] semiconductors. These equations are consistent with a Boltzmann distribution and are invalid in regions where the Species concentration is close to the respective site concentration. A more rigorous treatment of transport in a semiconductor requires activity coefficients which are functions of concentration. Expressions are presented for activity coefficients of electrons and holes in semiconductors for which conduction-and valence-band energy levels are given by the respective bandedge energy levels. These activity coefficients are functions of concentration and are thermodynamically consistent. The use of activity coefficients in macroscopic transport relationships allows a description of electron transport in a manner consistent with the Fermi-Dirac distribution.