H. Gerischer

Underpotential deposition of metals and work function differences

Summary The electrolytic deposition of metal atoms onto foreign metal substrates at underpotentials has been studied in aqueous and non-aqueous solutions. It is shown, that the potential difference between monolayer and bulk deposition is closely related to the difference in the work functions of substrate and deposit, causing a partial charge of the adatoms. The ionic contribution to the chemical bond, arising from this partial electron transfer, can account for the favourable deposition of the first monolayer. A formula is given which allows the prediction of the amount of underpotential shift from differences in work function. This is explained in analogy to Paulings treatment of the chemical bond between atoms of different electronegativity.

Electrochemical photo and solar cells principles and some experiments

Summary It is shown how the well-known photoeffects observed at semiconductor electrodes can be used to construct an electrochemical photocell. The photovoltaic effects in such cells are based on the formation of a Schottky barrier at the interface between a semiconductor and a suitable redox electrolyte. A cell design is developed and some results with CdS, CdSe or GaP electrodes are presented. The characteristics of such photocells and the problems involved in their application as solar cells are discussed.

On the stability of semiconductor electrodes against photodecomposition

Abstract The thermodynamics of cathodic reduction of semiconductors by electrons and of anodic oxidation by holes are discussed and related to the free energy of electrons and holes in illuminated semiconductor electrodes. Typical correlations between the energetic positions of band edges and decomposition potentials are derived which can be used as an indicator of stability or instability. Decomposition can be prevented by redox reactions with suitable position of the redox potential and high enough reaction rate. A few typical examples are discussed which demonstrate the importance of kinetics for the stability behaviour.

The impact of semiconductors on the concepts of electrochemistry

Abstract The study and kinetic interpretation of electrode reactions was, from the early years of electrochemistry, dominated by processes occurring at electrodes with metallic conductivity. Only since the late 1950s and early 1960s has it been realized that semiconductor electrodes behave differently in many respects and offer new insights into the role played by the electronic properties of a solid in its electrochemical reactivity. The investigation of semiconductor electrodes has intensified the link between electrochemistry and solid state physics, has created a close connection to photochemistry and has profoundly improved the understanding of interfacial reactions. Selected examples are presented in this lecture.

ELECTROCHEMICAL TECHNIQUES FOR THE STUDY OF PHOTOSENSITIZATION

Abstract— Spectral photosensitization is discussed from an electrochemical point of view, as caused by electron transfer from or to excited molecules. General principles for a description of electron transfer processes at semiconductor or insulator electrodes are reviewed and applied to reactions which involve excited electronic states. The conditions for suitable experimental arrangements are characterized. Illustrations for the application of these techniques are given. The examples show spectral sensitization of photocurrents due to electron or hole injection into inorganic or organic solids.

Etching of silicon in NaOH solutions. I: In-situ STM investigation of n-Si(111)

The etching of n-type silicon (111) has been investigated by means of in situ scanning tunneling microscopy (STM) observations performed over a wide range of bias of the sample. A special procedure has been used to observe topography changes at potentials close and positive of the rest potential. Irrespective of the bias, images show that the surface consists in atomically smooth terraces separated by 3.1 A high steps. At cathodic bias, the etching occurs principally at terrace edges and (111) terraces are most probably H terminated, which prevents their reconstruction, as could be seen in atomically resolved pictures taken in situ. Triangular etch pits nucleate when the potential approaches the rest potential

Photocatalytic Oxidation of Organic Molecules at TiO2 Particles by Sunlight in Aerated Water

This paper reports that a key issue in the photoassisted oxidation of organic materials, e.g., oil slicks on sea water, on semiconducting particles is the charging of the particles by electrons as a result of the hole transfer (e.g., to adsorbed organics or to water) being fast, and the two-electron transfer to dioxygen being potentially slower. Such charging leads to recombination, i.e., loss of quantum efficiency. Here we show that at an {approximately}1 sun irradiance of n-TiO{sub 2} particles in water, the rate of depolarization of the particles by molecular oxygen will not be limited by O{sub 2} diffusion. For large ({gt}1 {mu}m) TiO{sub 2} particles, but less probably for small ({lt}0.1 {mu}m) ones, the quantum efficiency may be limited by the rate of electron transfer to adsorbed O{sub 2}. The latter rate depends on the density of shallow (0.1-0.3 eV) surface and near surface electron traps. If the density of these traps is too low, group VIII metal O{sub 2} reduction catalysts should increase the rate of electron transfer to O{sub 2} and thereby raise the quantum yield.

Etching of Silicon in NaOH Solutions II . Electrochemical Studies of n‐Si(111) and (100) and Mechanism of the Dissolution

In Part I of this work, the bias dependence of the etching of silicon (111) has been investigated by means of in situscanning tunneling microscopy observations. In this second part, current‐voltage curves and etch rate results derived from the loss of material and performed with n‐type Si samples of various orientations, show that electrochemical and chemical reactions coexist in the oxidation of Si. A model is presented for the oxidation of a Si atom in a kink site in different situations of polarization. The key feature of the description is the understanding of the persistent hydrogen termination of the surface in spite of the continuous oxidative removal of Si atoms from the surface. The model includes the hydrolytic splitting of Si—H and Si—Si bonds as the important chemical contributions to the etching process. At the rest potential, the chemical component is dominant. The sequence of reactions leaves the surface in the terminated state. The anodic current is due to the injection of electrons which are produced during the substitution of Si—H by Si—OH bonds. This results above a critical electrode potential in passivation. In this respect, (111) and (100) faces present quite different behaviors. At cathodic bias where the hydrogen evolution becomes fast, due to the accumulation of electrons at the surface, not only the anodic component of the etching reaction vanishes but also the chemical component decreases in rate and is eventually stopped.

Untersuchungen Zur anodischen Oxidation von Ammoniak an Platin-Elektroden*

Zusammenfassung Die elektrochemische Oxidation des Ammoniak in stark alkalischer Losung wurde an platinierten Platin-Elektroden untersucht. Aufgrund der gefundenen Messergebnisse und der Analyse der Reaktionsprodukte wird ein Reaktionsmechanismus vorgeschlagen, bei dem die Oxidation des NH3-Molekuls im adsorbierten Zustand durch gleichzeitig adsorbierte OH-Radikale erfolgt. Aus den als Zwischenprodukten entstehenden NHx-Radikalen (x=2 oder 1) enstehen durch Rekombination N2Hy-Molekule (y=4 oder 2) bzw. Radikale (y=3), welche sehr rasch zu N2 oxidiert werden. In einer Nebenreaktion entstehende N-Atome bleiben dagegen auf der Oberflache chemisorbiert und bewirken die beobachtete irreversible Inaktivierung der Elektrode.

The mechanisms of the decomposition of semiconductors by electrochemical oxidation and reduction

The several possible ways in which semiconductor electrodes can be electrochemically oxidized and reduced are discussed, giving main emphasis to the role of holes and electrons in the mechanism. It is shown that the disruption of a chemical bond normally results in the formation of a radical intermediate and that the polarity of the chemical bond is very important for the reaction mechanism. The possibility of non-electrochemical oxidation in certain cases, eg GaAs by Br2 in solution, is considered.