R. Memming

Tribological and electrical properties of metal‐containing hydrogenated carbon films

The formation of metal‐containing amorphous hydrogenated carbon films (a‐C:H) in a reactive sputtering process is reported. The layers were prepared at room temperature using various metals of different concentrations. According to tribological measurements the layers exhibit small friction values and an extremely low abrasive wear rate. The electrical conductivity depends on the metal concentration and could be varied over many orders of magnitude.

Mechanism of the Electrochemical Reduction of Persulfates and Hydrogen Peroxide

Electrochemical studies have shown that the reduction of persulfate and hydrogen peroxide is a two step mechanism, the first step occurs by electron transfer with the conduction band and the second step by hole injection with the valence band. It could be concluded from corresponding measurements performed with a semiconductor electrode that the electrochemical properties of these oxidizing agents have to be described by two instead of one redox (normal) potential. One normal potential is much lower and the other much larger than the theoretical value determined from thermodynamic data. These values are estimated as and .

Anodic dissolution of silicon in hydrofluoric acid solutions

Abstract The anodic dissolution rate of silicon is investigated as a function of the electrode potential, the doping of the crystal and of the hydrofluoric acid concentration. It is shown that at lower anodic electrode potentials two holes and at higher electrode potentials four holes are consumed. For the rate determining step which depends exponentially on the electrode potential in concentrated HF-solutions, however, only one hole is required. A dissolution mechanism is proposed partly based on the experimental results obtained by Turner.

Solar energy conversion by photoelectrochemical processes

Abstract The basic principles of conversion of solar energy by photoelectrochemical methods are reviewed. A distinction is made between systems used for conversion into electrical energy and those which are applicable for the production of chemical fuel. Besides photogalvanic cells, the main emphasis is put on semiconductor/electrolyte/metal-devices. The various mechanisms involved in these cells, the conversion efficiencies and the stability of the components are discussed in terms of energy schemes. The properties of these systems are compared with those of pure solid state devices. As far as the photolytic decomposition of water for the production of chemical fuel is concerned, the photoelectrochemical reactions in heterogeneous systems are briefly compared with purely photochemical processes in homogeneous solutions. Various conclusions are made concerning the applicability of the systems discussed in this paper.

The Role of Energy Levels in Semiconductor‐Electrolyte Solar Cells

The transfer of minority carriers across the semiconductor-electrolyte interface during illumination has been studied. In particular the parameters which determine the competition between pure redox processes and hydrogen evolution at p-type or anodic dissolution at n-type material have been investigated. In the first case the electron transfer between p-type electrodes and the redox system can simply be interpreted on the basis of an energy scheme. In the case of hole transfer from n-type electrodes across the interface the competition of a pure redox process and anodic dissolution is more complex. It is mainly determined by kinetic parameters. The results obtained with CdS and various III-V compounds are discussed in view of application of n- and p-type semiconductors in solar energy conversion systems.

Electrochemical properties of gallium phosphide in aqueous solutions

Abstract The anodic dissolution of GaP was investigated in alkaline and acid solutions. In acid solutions Ga and P are dissolved in the trivalent and pentavalent state, respectively, with the consumption of six positive holes. In alkaline solutions only three holes are consumed; P is dissolved only chemically. Furthermore, redox reactions at GaP electrodes were studied. It is shown that surface states are involved in certain redox reactions. The electroluminescence produced by applying a field between the electrode and a redox electrolyte is explained by assuming a radiative transition via surface states.

PHOTOCHEMICAL AND ELECTROCHEMICAL PROCESSES OF EXCITED DYES AT SEMICONDUCTOR AND METAL ELECTRODES

Abstract— Sensitization processes at semiconductor and metal electrodes were studied. With semiconductor electrodes, anodic as well as cathodic photocurrents were observed, depending on the semiconducting material. These photocurrents are caused by electron transfer between the electrode and the excited dye molecule adsorbed on the surface. With metals, photocurrents occur only in the presence of reducing or oxidizing agents, due to photochemical processes in the bulk of the dye solution. The different phenomena for semiconductors and metals are compared and the mechanism of supersensitization is discussed in detail.

Light-induced generation of hydrogen at CdS-monograin membranes

Abstract A new kind of membrane consisting of small particles of single-crystalline CdS embedded in a polymer film is described. In various photoelectrolysis experiments the properties of the membrane which separates a cell in two compartments were investigated. Relatively large quantities of H 2 were produced in the presence of an electron donor such as S 2− ions or EDTA and small quantities by direct photoelectrolysis of H 2 O. This technique gives information on the role of catalysts deposited on one or both faces of the membrane.

The Photoelectrochemical Behavior of Ferric Oxide in the Presence of Redox Reagents

The influence of dissolved sulfide ion and the ferri/ferrocyanide redox system on the photoelectrochemical behavior of iron oxide has been investigated. Particular attention was given to transient behavior and to dark current phenomena. An interpretation of the results is given in terms of a back‐reaction of photogenerated species with conduction band electrons via surface states.

Photocorrosion of cadmium sulfide: analysis by photoelectron spectroscopy

Abstract Photocorrosion of cadmium sulfide electrodes in aqueous electrolytes leads to sulfate formation in the presence and to sulfur formation in the absence of oxygen. All three sulfur species involved (S2−, S0 and SO2−4) can be detected on CdS electrodes after treatment in photoelectrochemical cells using AES or XPS. Both, the S2p/3p XPS and the S Auger peaks are broadened on photoelectrochemical oxidized CdS due to deposition of elemental sulfur. The XPS binding energy of the S electrons is shifted by about 1 eV per sulfur oxidation step from around 161.5 eV for S2− to around 163.5 eV for S0 and around 168 eV for SO2−4. The results obtained by photoelectron spectroscopy of electrodes transferred from the electrolyte directly into the UHV system without any cleaning and special precautions are in excellent agreement with the predictions based on photoelectrochemical experiments. For example, it could be proved by XPS that sulfur was transformed into sulfate in a reductive oxidation step in the presence of oxygen, leading to the CdS surface being cleaned of S0.