Semiconductors electrodes, electrochemical preparation

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

The thin semiconductor particulate film prepared by immobilizing semiconductor nanoclusters on a conducting glass surface acts as a photosensitive electrode in an electrochemical cell. An externally applied anodic bias not only improves the efficiency of charge separation by driving the photogenerated electrons via the external circuit to the counter electrode compartment but also provides a means to carry out selective oxidation and reduction in two separate compartments. This technique has been shown to be veiy effective for the degradation of 4-chlorophenol [116,117], formic acid [149], and surfactants [150] and textile azo dyes [264,265]. 

Ottova et al. looked at two-compartment semiconductor-septum electrochemical photovoltaic cells with cadmium selenide and cadmium selenide telluride for water photolysis [126], They used cells consisting of two chambers separated by a CdSe or CdSe/CdTe bipolar electrode. The bipolar electrodes were prepared by painting a CdSe slurry on a metal substrate or by ultrasound-aided electrodeposition from CdSe solution in ZnCl2. The photoresponse (voltage and current output) and hydrogen yield from photo-induced electrolysis of H20 in the dark chamber of the cell were evaluated as a function of CdSe preparation method. The ultrasound-aided deposition technique gave excellent coatings of CdSe. 

Fransaer J., Roos J.R., Delaey L., Van Der Biest O., Arkens O., Celis J.R Sol-gel preparation of high-Tc Bi-Ca-Sr-Cu-0 and Y-Ba-Ca-0 superconductors. J. Appl. Phys. 1989 65 3277-3279 Fujishima A., Honda K. Electrochemical photolysis of water at semiconductor electrode. Nature 1972 238 37-38... 

The preparation of the electrode surface should be such as to permit reproducible experiments, and, hopefully, provide a well-defined interphasial system. Mercury does not require surface treatment, of course, but solid electrodes may require mechanical polishing and chemical and/or electrochemical polishing. The mounting of semiconductor electrodes may present even greater difficulties because many of the etchants used are extremely corrosive. This dictates the choice of encapsulating insulators. Bozhkov... 

Metal oxides. Noble metals are covered with a surface oxide film in a broad range of potentials. This is still more accentuated for common metals, and other materials of interest for electrode preparation, such as semiconductors and carbon. Since the electrochemical charge transfer reactions mostly occur at the surface oxide rather than at the pure surface, the study of electrical and electrochemical properties of oxides deserves special attention. 

One may expect that future work on the electrochemistry of diamond should take two paths, namely, an extensive investigation (search for new processes and applications of the carbon allotropes in the electrochemical science and engineering) and intensive one (elucidation of the reaction mechanisms, revealing the effects of crystal structure and semiconductor properties on the electrochemical behavior of diamond and related materials). It is expected that better insight into these effects will result in the development of standard procedures for thin-film-electrodes growth, their characterization, and surface preparation. 

The use of nanoparticles has extended throughout the field of biosensors in the electrochemical detection of DNA and immunoreactions (Murphy 2006). A wide range of nanoparticles including nanotubes and nanowires, prepared from metals, semiconductor, carbon or polymeric species, have been investigated. The enhanced electrochemistry is due to the ability of the small nanoparticles to reduce the distance between the redox site of a protein and the electrode, since the rate of electron transfer is inversely dependent on the exponential distance between them (Balasubramanian and Burghard 2006). CNT-modified electrodes have been most frequently used for the development of biosensors (Gooding 2005). 

After a short introduction to tunneling in Sec. 2, special attention is given in Sec. 3 to operating conditions on semiconductors because these are not as trivial as for metals and may raise experimental problems. Questions related to in-situ spectroscopic characterization are addressed in the following section. Section 5 reviews in-situ as well as ex-situ studies (in UHV or in air after treatment of the surface in solution) according to the materials and electrochemical reactions involved. Silicon electrodes are treated separately, mostly in relation to electrochemical etching and por-pous layer formation. The two final sections outline perspectives and draw general conclusions. Details related to instrumentation and tip preparation are not discussed here unless they are specific to semiconductors. They are reviewed in [9]. Experimental aspects of in-situ AFM are not presented either, because the immersion of the surface in an electrolyte raises no specific problem. The theory and other applications of AFM are discussed elsewhere [3, 4]. 

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