Semiconductors electrodes

The general scheme for a semiconductor electrode takes into account a two-step charge transfer process. One step corresponds to the transfer of electrons and ions through the Helmholtz layer. Let be the corresponding impedance that 

Zgc- ACs c plotting vs the potential V gives the Mott-Schottky plot. From this one can determine the flat-band potential of a semiconducting electrode [17]. 

Silver-overlayer SERRS also has been used to study in situ the redox reactions of methyl viologen (MV) adsorbed on to a p-type InP SC electrode [42]. These experiments are related to earlier time-resolved resonance Raman spectroscopy (TR3S) work on electron transfer reactions at the surfaces of photoexcited semiconductor colloids (TiOa and CdS) involving 

MV2+ acceptors and SCN electron donors in solution [43], Colloidal semiconductor particles, typically of ca. 10-100 nm diameter, in aqueous sols may be treated as isolated microelectrode systems. Steady-state RRS experiments with c.w. lasers can be used to study phototransients produced at the surfaces of such colloidal semiconductors in flow systems [44], but pulsed laser systems coupled with multichannel detectors are far more versatile. Indeed, a recent TR3S study of methyl viologen reduction on the surface of photoex-cited colloidal CdS crystallites has shown important differences in mechanism between reactions occurring on the nanosecond time scale and those observed with picosecond Raman lasers [45]. Thus, it is apparent that Raman spectroscopy may now be used to study very fast interface kinetics as well as providing sensitive information on chemical structure and bonding in molecular species at electrode surfaces. 

My thanks go to Dr. Hari Virdee for help with the literature survey which preceded this review, to the SERC Rutherford Laboratory for help with pulsed laser experiments, and to the SERC for financial support. 

Pettinger and H. Wetzel, in R.K. Chang and T.E. Furtak (Eds.), Surface Enhanced Raman Scattering, Plenum Press, New York, 1982, p. 293. 

The general scheme for a senticonductor electrode takes into account a two-step charge-transfer process [17]. One step corresponds to the transfer of electrons and ions through the Helmholtz layer. Let Zh be the corresponding impedance that is in parallel to Ch, the capacity of the Helmholtz layer and let Z be the impedance of this parallel combination. The other step exists due to the localization of charges in surface states or intermediates. Here the corresponding impedance, Zsc, is in 

Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India 

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Sato N 1998 Electrochemistry at Metal and Semiconductor Electrodes (Amsterdam Elsevier)... 

R. K. Pandey, S.N. Sahu, S. Chandra, Handbook of Semiconductor Electrode-position", Marcel Dekker, Inc., 1996, ISBN 0-8247-9701-9. 

After starting his own laboratory in 1982, the author built microwave measurement facilities with his collaborators and resumed research on microwave electrochemical phenomena. While the potential of combining photoelectrochemistry with microwave conductivity techniques became evident very soon,6,7 it was some time before microwave experiments could be performed at semiconductor electrodes under better-defined microwave technical conditions.8... 

Stationary potential-dependent measurements are not the only measurements that can be performed with microwaves. Figure 6 shows a scheme indicating the different techniques that can be used for microwave characterization of semiconductor electrodes. 

Other situations may also occur that allow a simple determination of the sensitivity factor. When, for example, a sufficiently negative electrode potential forces all minority carriers to drift into the interior of the semiconductor electrode, where they recombine subject to the bulk lifetime Tfr we will see a limiting PMC signal (given a sufficiently thick electrode). Knowing the photonflux /0 (corrected for reflection), we may expect the following formula to hold ... 

Otherwise, the effect of electrode potential and kinetic parameters as contained in the relevant expression for the PMC signal (21), which controls the lifetime of PMC transients (40), may lead to an erroneous interpretation of kinetic mechanisms. The fact that lifetime measurements of PMC transients largely match the pattern of PMC-potential curves, showing peaks in accumulation and depletion of the semiconductor electrode and a minimum at the flatband potential [Figs. 13, 16-18, 34, and 36(b)], demonstrates that kinetic constants are accessible via PMC transient measurements, as indicated by the simplified relation (40) derived for the depletion layer of an n-type electrode. 

On the basis of our theoretical considerations and preliminary experimental work, it is hoped that fast processes of charge carriers will become directly measurable in functioning photoelectrochemical cells, Typical semiconductor electrodes are not the only systems accessible to potential-dependent microwave transient measurements. This technique may also be applied to the interfacial processes of semimetals (metals with energy gaps) or thin oxide or sulfide layers on ordinary metal electrodes. 

In this chapter we have attempted to summarize and evaluate scientific information available in the relatively young field of microwave photoelectrochemistry. This discipline combines photoelectrochemical techniques with potential-dependent microwave conductivity measurements and succeeds in better characterizing the behavior ofphotoinduced charge carrier reactions in photoelectrochemical mechanisms. By combining photoelectrochemical measurements with microwave conductivity measurements, it is possible to obtain direct access to the measurement of interfacial rate constants. This is new for photoelectrochemistry and promises better insight into the mechanisms of photogenerated charge carriers in semiconductor electrodes. 

The schemes in Figs. 44 and 45 may serve to summarize the main results on photoinduced microwave conductivity in a semiconductor electrode (an n-type material is used as an example). Before a limiting photocurrent at positive potentials is reached, minority carriers tend to accumulate in the space charge layer [Fig. 44(a)], producing a PMC peak [Fig. 45(a)], the shape and height of which are controlled by interfacial rate constants. Near the flatband potential, where surface recombination... 

At present, the microwave electrochemical technique is still in its infancy and only exploits a portion of the experimental research possibilities that are provided by microwave technology. Much experience still has to be gained with the improvement of experimental cells for microwave studies and in the adjustment of the parameters that determine the sensitivity and reliability of microwave measurements. Many research possibilities are still unexplored, especially in the field of transient PMC measurements at semiconductor electrodes and in the application of phase-sensitive microwave conductivity measurements, which may be successfully combined with electrochemical impedance measurements for a more detailed exploration of surface states and representative electrical circuits of semiconductor liquid junctions. 

Self-doped polymers, 334 Semicircles, Albery and Mount interpretation of, 584 Semiconductor electrodes with polymer layers, 499 diffusion length in, 492 Semiconductors, lifetime for carriers and, 495... 

N. Sato, Electrochemistry at metal and semiconductor electrodes, Elsevier, Amsterdam (1998). 

Park SM, Barber ME (1979) Thermodynamic stabilities of semiconductor electrodes. Electroanal Chem 99 67-75... 

A thorough insight into the comparative photoelectrochemical-photocorrosion behavior of CdX crystals has been motivated by the study of an unusual phenomenon consisting of oscillation of photocurrent with a period of about 1 Hz, which was observed at an n-type CdTe semiconductor electrode in a cesium sulfide solution [83], The oscillating behavior lasted for about 2 h and could be explained by the existence of a Te layer of variable width. The dependence of the oscillation features on potential, temperature, and light intensity was reported. Most striking was the non-linear behavior of the system as a function of light intensity. A comparison of CdTe to other related systems (CdS, CdSe) and solution compositions was performed. 

Junctions involving a bilayer (two-semiconductor) electrode and a liquid have been investigated as to their photoelectrochemical properties primarily in works which... 

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

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