Elementary processes at semiconductor/electrolyte interfaces: perspectives and limits of electron spectroscopy (original) (raw)
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The semiconductor-electrolyte interface 11.1 Electrochemistry at semiconductors
Many naturally occurring substances, in particular the oxide films that form spontaneously on some metals, are semiconductors. Also, electrochemical reactions are used in the production of semiconductor chips, and recently semiconductors have been used in the construction of electrochemical photocells. So there are good technological reasons to study the interface between a semiconductor and an electrolyte. Our main interest, however, lies in more fundamental questions: How does the electronic structure of the electrode influence the properties of the electrochemical interface, and how does it affect electro-chemical reactions? What new processes can occur at semiconductors that are not known from metals? 11.2 Potential profile and band bending When a semiconducting electrode is brought into contact with an electrolyte solution, a potential difference is established at the interface. The conductivity even of doped semiconductors is usually well below that of an electrolyte solution; so practically all of the potential drop occurs in the boundary layer of the electrode, and very little on the solution side of the interface (see Fig. 11.1). The situation is opposite to that on metal electrodes, but very similar to that at the interface between a semiconductor and a metal. The variation of the electrostatic potential φ(x) in the surface region entails a bending of the bands, since the potential contributes a term −e 0 φ(x) to the electronic energy. Consider the case of an n-type semiconductor. We set φ = 0 in the bulk of the semiconductor. If the value φ s of the potential at the surface is positive, the bands band downwards, and the concentration of electrons in the conduction band is enhanced (see Fig. 11.2). This is called an enrichment layer. If φ s < 0, the bands bend upward, and the concentration of electrons at the surface is reduced; we speak of a depletion layer. On the other
The Journal of Physical Chemistry B, 2006
Perspectives of a new approach for the synchrotron photoemission spectroscopic analysis of chemical processes at solid/liquid interfaces under UHV conditions have been explored. A thin layer of HCl-2-propanol solution was frozen-in on the semiconductor GaAs(100) wafer surface by cooling the substrate to liquid nitrogen temperature after etching off the native oxide layer under N 2 atmosphere. Chemical reactions induced in situ by exposure to synchrotron radiation (SR) and by stepwise heating have been monitored. Right after etching and freezing, the surface is covered by gallium chlorides with 1, 2, 3, and 4 Cl ions attached and lattice back-bonded to As atoms, as well as by elemental arsenic As 0 and 2-propanol. Exposure to SR at low temperature produces surface As chlorides at the expense of As 0 . The GaCl 3 and GaCl 2 emissions diminish while GaCl is enhanced. On the other hand, heating the sample to approximately 130 K just above H 2 O desorption causes the thermodynamically expected reaction of AsCl 3 with the substrate GaAs to form Ga chloride species and As 0 . Heating the sample to room temperature leaves only As 0 on the surface and for gallium the content of all surface chlorides is drastically reduced. By further heating to 400 K elemental arsenic starts to desorb and the Ga chloride surface content is reduced. Using different excitation energies the depth composition of the reaction products has been monitored indicating a tendency of decreasing chlorination numbers and increasing Ga vs As chloride content toward the pristine substrate at each stage of the reaction.
Solar Energy Materials and Solar Cells, 2004
The application of surface science techniques to investigate elementary processes in photoelectrochemistry is shown based on a number of experiments performed during the last years. For these experiments, layered chalcogenide WSe 2 (0 0 0 1) van der Waals surfaces are used as semiconductor substrates as they provide ideal surface properties for fundamental studies. After a short introduction to the experimental techniques, we will present results of adsorption and coadsorption experiments performed to investigate different aspects of semiconductor/electrolyte contacts. For H 2 O and Br 2 adsorbed as single species and coadsorbed with each other, the electronic states involved in contact formation can be identified and different mechanisms of interface interaction have been deduced. Na and H 2 O coadsorption experiments give information about solute solvent interaction as, e.g. solvation energies. Finally, a complex electrolyte phase containing all relevant species has been prepared by coadsorbing Br 2 , Na and H 2 O. r
High-resolution synchrotron photoemission spec-troscopy has been applied to detail the electrochemical and photoelectrochemical corrosion reactions at the liquid junction n‑GaAs(100)/1 M aqueous HCl solution. Under anodic polarization of 1.8 eV, the main process initiated by the presence of holes in the Ga−As bonding states of the valence band is the formation of soluble gallium chloride complexes and insoluble elemental arsenic on the surface. In addition, arsenic hydroxide forms, which reacts further to soluble HAsO 2. In toto, the As/Ga atomic ratio increases, which is accompanied by an increase of the work function. The anodic decomposition reaction is enhanced by illumination as more holes reach the n-semiconductor/electrolyte junction. Under cathodic polarization of 1.5 eV, only minor changes are observed in Ga and As core-level spectra, giving no indication of corrosion, but specific adsorption of hydrated HCl molecules and/or Cl − ions considerably modifies valence band spectra.
Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1991
Through band edge shift (BES) measurements the paper puts forward new aspects concerning the study of reactions that take place at the semiconductor/electrolyte interface. In particular it focuses on the problem raised by reaction coupling, that is the interdependence between the redox and the photocorrosion reactions at photoanodes for instance. BES determination coupled with stabilization characterization was performed with n-GaAs photoanodes in contact with various redox solutions. With respect to surface charging, it is shown that reactions do not always act in parallel; surface effects (adsorption of redox species for example) may strongly affect the reaction scheme. For the simplest case (reaction in parallel) we propose a way of determining the electrode stability from BES measurements.
Analytical and Bioanalytical Chemistry, 2002
Electrochemically-induced oxidation and reduction reactions of UHV-cleaved GaAs(110) surfaces have been studied after emersion under potential control using high resolution synchrotron-induced photoelectron spectroscopy. High quality spectra of the As and Ga core 3d lines and the valence band region have been obtained. The spectra of the anodic oxide show strong emission of bulklike Ga 2 O 3 and some As 2 O 3 with the admixture of suboxides and hydroxides. Ga 2 O 3 and As 2 O 3 are cathodically reduced leaving the GaAs surface covered mostly with elemental As, some As-H and remnants of Ga-suboxides and -hydroxides.
Possibilities of chemical sensing at the semiconductor/electrolyte interface
Sensors and Actuators B-chemical, 1994
A review of the knowledge base on semiconductor/electro&te interfaces (including many references to the relevant results in the former SU) reveals the existence of many experimental data which can be useful for chemical sensor construction. The phenomena discussed include selective adsorption of ions at the interface and kinetic effects. In some cases very low detection limits (about 10e9 M) can be. achieved.