Band gap, semiconductor electrodes

In a similar manner, during the process of the existing metal particles growth and the deposition of new species using cathodically biased electrode in a solution of metal ions, the growing metal phase will be also localized at the sites of the surface exposure of the continuous donor centers. The reason for this is that namely these sites possess substantially enhanced electrocatalytic activity in comparison with the stoichiometric oxide surface and exhibit the properties of current channels non-restricted by the Schottky barrier at the interface with the electrolyte. Actually, a peculiar decoration of the sites of donor centers accumulation and donor clusters localization by the metal nanoparticles takes place in the dark processes of metal particle deposition onto the surface of the chemically inert wide-band-gap oxides. The increased electrocatalytic activity of the wide-band-gap semiconductor electrodes resulted from the deposition of metal nanoparticles on their surface may be also regarded as a kind of such decoration . 

Cathodic photocurrents have been observed near the flat-band potential or at more negative potentials in the presence of dioxygen or other electron acceptors with sensitized and unsensitized wide band-gap semiconductor electrodes [84-88]. Al-... 

R. Malpas, F. R. Mayers, and A. G. Osborne, The chemical modification of small band gap semiconductor electrodes using chromium-carboxylic acid complexes of redox couples, J. Electroanal. Chem. 153, 97, 1983. 

Organic dyes, aside from their role as sensitization agents for wide band gap semiconductors have been employed also for stabilization of narrow band gap semiconductors. The majority of such studies have considered metal or metal-free phthalocyanine films for both sensitization and electrode protection purposes [35]. 

Such an interfacial degeneracy of electron energy levels (quasi-metallization) at semiconductor electrodes also takes place when the Fermi level at the interface is polarized into either the conduction band or the valence band as shown in Fig. 5-42 (Refer to Sec. 2.7.3.) namely, quasi-metallization of the electrode interface results when semiconductor electrodes are polarized to a great extent in either the anodic or the cathodic direction. This quasi-metallization of electrode interfaces is important in dealing with semiconductor electrode kinetics, as is discussed in Chap. 8. It is worth noting that the interfacial quasi-metallization requires the electron transfer to be in the state of equilibrimn between the interface and the interior of semiconductors this may not be realized with wide band gap semiconductors. 

Figure 8-37 shows a shift in the redox electron level of a reductant from the hydrated state, which can not anodically iiyect electrons into the conduction band because its electron levels are located within the band gap, to the adsorbed state, which then can iiyect electrons into the conduction band of semiconductor electrodes... 

Although several single-crystal, wide-band gap semiconductors provide electrochemical and optical responses close to those expected from the ideal semiconductor-electrolyte model, most semiconducting electrodes do not behave in this manner. The principal and by far overriding deviation from the behavior described in the previous section is photodecomposition of the electrode. This occurs when the semiconductor thermodynamics are such that thermal or photogenerated valence band holes are sufficiently oxidizing to oxidize the semiconductor lattice [8,9]. In this case, kinetics routinely favor semiconductor oxidation over the oxidation of dissolved redox species. For example, irradiation of n-CdX (X = S, Se, or Te) in an aqueous electrolyte gives rise exclusively to semiconductor decomposition products as indicated by... 

Such unfavorable phenomena must be alleviated in order for small band-gap semiconductors to be used in PECs. Much effort has been devoted to the problem of photocorrosion, particularly in the decade following the first report concerning PECs. Beginning with independent reports in 1976 by groups at Bell Labs,20 MIT22) and the Weizmann Institute20 that the aqueous S2 /S22 couple could be used to stabilize CdS and CdSe, it has become well-established that stabilization of these electrodes can be achieved through the addition of suitable redox species to the electrolyte. 

The use of porphyrins as sensitizers for the nano-crystalline electrode in DSSC is particularly attractive given their important role in photosynthesis and the relative ease modification on molecular structure. Not only the porphyrin monomers, but also the large porphyirn arrays have been tested extensively as sensitizers of wide-band-gap semiconductors like NiO, ZnO and Ti02 in the past several decades. The focus of this section is the various molecular structure modification of porphyrins for the purpose of applying as sensitizer in DSSCs. 

Germanium — (Ge, atomic number 32) is a lustrous, hard, silver-white metalloid (m.p. 938 °C), chemically similar to tin. Ge is a low-band-gap - semiconductor that, in its pure state, is crystalline (with the same crystal structure as diamond), brittle, and retains its luster in air at room temperature. Anodic dissolution of the material occurs at potentials more positive than ca. -0.2 V vs SCE. Peaks in the voltammograms of germanium in acidic electrolyte are ascribed to a back-and-forth change between hydrogenated and hydroxy-lated surfaces [i]. Studies are often conducted at p-doped and n-doped Ge electrodes [ii] or at Ge alloys (e.g., GeSe) where photoelectrochemical properties have been of considerable interest [iii]. 

Silver halide microcrystals are wide band gap semiconductors which exhibit weak photoconductivity. Early experiments demonstrated that dyes that sensitized silver halide photographic action also sensitized silver halide photoconductivity [6c]. Since the observation of photoconductivity necessitates the movement of free charge within the crystals, dye sensitization processes must inject charge into the silver halide lattice in some way. Initial theories of sensitization were based on the semiconductor view of silver halides, especially as espoused by Gurney and Mott [10]. Current ideas are based on thorough studies of the absorption spectroscopy and luminescence of silver halide emulsions and of adsorbed, sensitizing dyes, and the oxidation-reduction properties of the dyes at silver/silver halide electrodes [11]. 

The utility of visible- or UV-sensitive semiconductors as initiators for redox chemistry is limited by the instability of the surface under irradiation while in contact with an oxidizable substrate or an electrolyte. This decreased activity is caused by the chemical reactivity of the semiconductor itself, so that an insulating (blocking) layer is formed or the electrode quickly corrodes. This lack of stability is particularly troublesome with small-band-gap semiconductors that adsorb strongly in the visible region. 

Mark and Gora [24], commenting on the results, considered a model in which initiation is associated with a critical interface field at the Schottky-barrier contact between the metal electrode and the azide. Interface fields depend on properties of the sample and on the work function of the electrode, and are larger than the applied voltage divided by sample thickness. The model predicted an effect for uniform samples which was qualitatively consistent with experiment, but whose magnitude was too small to observe. However, the experimental samples were pressed pellets composed of individual grains which are likely to be separated by potential barriers [25,26]. Taking this into account, the model was consistent with experiment if initiation occurs at a critical interface field of about 2 X 10 V/m. This is a plausible value, in that fields in excess of 10 -10 V/m applied to surfaces of wide band-gap semiconductors commonly result in destructive breakdown due to carrier emission into the bulk from interface states [27-29]. 

PC-doubling Reactions PC doubling refers to a type of charge-transfer reaction in which both bands of the semiconductor are involved, thus emphasizing the distinctive features of semiconductor electrochemistry. The first examples of such reactions relate to the photoan-odic oxidation of species such as formate and tartrate at wide band gap -type electrodes [74, 75]. A photon generates an electron-hole pair in the semiconductor. The electron and hole are separated by the electric field of the depletion layer. The electron is detected as photocurrent in the external circuit. The hole oxidizes a species from solution producing an intermediate... 

WO3 is an example of another class of electroactive material, metal oxides, which has been used to construct microelectrochemical devices. WO3 is a wide-band-gap semiconductor, with high resistance in its neutral state.Upon reduction, WO3 intercalates cations such as H" ", Li" ", and Na and becomes conducting. W03 based transistors, showing sensitivity to pH and to Li" concentration have been demonstrated in solution electrolytes. A schematic of a MEEP/WO3 device is shown in Figure 3. WO3 is confined to the required electrodes, using standard photolithographic techniques. 

In the previous section discussion has centered on electrode reactions stimulated by light absorption in the semiconductor electrode. In the present section, electron transfer processes between electrodes and excited molecules will be treated. In order to avoid any light absorption by the electrode itself, only large band gap semiconductors can be used. 

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