Semiconductors in Solution

When a conductive electrode (e.g., metallic or glassy carlxMi) is in contact with an electrolytic solution, the excess electronic charge is accumulated at the electrode surface and charge distribution occurs in the solution only. This is related to the fact that as the number of charged species increases, the space in which the redistribution of charges occurs shrinks. At a metallic electrode-solution interface, the charge redistribution in solution depends on the applied potential and is described by the Guy-Chapman-Stem theory. The characteristic thickness of the diffuse layer in nonadsorbing electrolytes varies from 0.3 nm in 1 M to 3 nm in 0.01 M aqueous electrolyte, while the thickness of the Helmholtz layer is much smaller [17]. 

In the case of semiconductor electrodes, the concentration of conductive species (electrons or holes) is much smaller than that in solution. This creates a redistribution of the space charge in the semiconductor electrode at distances much larger than that in solutions, 10-100 nm [462, 463]. This leads to much smaller capacitances of the semiconductor electrodes. Passive films formed on metallic surfaces behave as semiconductors, and their properties are important in studies of corrosion protection. 

Under depletion conditions there is a relation between 1/C f. and the potential, where Csc is the semiconductor electrode capacitance. For n-type semiconductors the following relation is found  

Boltzmann constant, Nj is the donor density, and kTle — 0.0257 V 25 25 °C. A similar expression exists for p-type semiconductors  

Capacitance measurements are usually carried out at one frequency, and the measured capacitance is determined as 

---

An electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) when a molecule in solution absorbs light. The excited electron in the LUMO may transfer to a neighboring molecule (oxidant) in solution, leading to the reduction of the oxidant, whereas the electronic hole (electron vacancy) in the HOMO may transfer to another neighboring molecule (reductant) in solution, resulting in the oxidation of the reductant. Quite similar photoinduced reduction-oxidation processes can occur at the semiconductor/solution (semiconductor/liquid) interface when a semiconductor in solution absorbs light. Fig. 4.1 schematically illustrates the... 

Surface states on a semiconductor in a vacuum can sometimes be explained by means of the spare bonds that dangle from atoms on surfaces, or defects associated with dislocations. Neither of these mechanisms works at the semicon-ductor/solution interface. The dangling bonds will be expunged by adsorbed water, etc. Experiment shows that the concentration of surface states on semiconductors in solution is strongly potential dependent, and that defects in the crystal structure would not be potential dependent, at least until anodic dissolution of the substrate itself began. 

Unfortunately, the to-electrode precipitation required for conventional (photo)electrochemical measurements on colloidal semiconductors necessarily perturbs the (assumed) spherical diffusion fields and surface adsorption equilibria that obtain at particles in the free solution state, phenomena which are instrumental in determining the dynamic and static charge transfer characteristics of the semiconductor. Consequently, there is a requirement for photoelectrochemical techniques capable of in situ, non-per-turbative investigations of the mechanistic details and catalytic properties of colloidal semiconductors in solution conditions typical of their intended ultimate application. Two such techniques are photoelectrophoresis and the Optical Rotating Disc Electrode (ORDE, developed by Albery et al.). As mentioned above, the former technique has already been reviewed by this author elsewhere [47]. Thus, the remainder of this review will concentrate on measurements that can be made with the latter... 

When photoelectrochemical solar cells became popular in the 1970s, many reports appeared concerning the stability, dissolution, and flat-band potential of semiconductors in solutions. These papers investigated parameters such as the energy level of the band edges, which is critical for the thermodynamic stability of the semiconductor and how to determine the potential for the onset of the (photo) electrochemical etching [38-40]. The criterion for thermodynamic stability of a semiconductor electrode in an electrolyte solution is determined by the position of the Fermi level with respect to the decomposition potential of the electrode with either the conduction band electrons or valence band holes E. Under illumination, the quasi-Fermi level replaces the Fermi level. The Fermi level is usually found within the band gap of the semiconductor and its position is not easily evaluated (especially the quasi-Fermi level of minority carriers). Therefore it was found more practical to use the conduction band minimum (Eq) and valence band maximum (Ey) as criteria for electrode corrosion. Thus, a semiconductor will be corroded in a certain electrolyte by the conduction band electrons if its... 

The combination of electrochemistry and photochemistry is a fonn of dual-activation process. Evidence for a photochemical effect in addition to an electrochemical one is nonnally seen m the fonn of photocurrent, which is extra current that flows in the presence of light [, 89 and 90]. In photoelectrochemistry, light is absorbed into the electrode (typically a semiconductor) and this can induce changes in the electrode s conduction properties, thus altering its electrochemical activity. Alternatively, the light is absorbed in solution by electroactive molecules or their reduced/oxidized products inducing photochemical reactions or modifications of the electrode reaction. In the latter case electrochemical cells (RDE or chaimel-flow cells) are constmcted to allow irradiation of the electrode area with UV/VIS light to excite species involved in electrochemical processes and thus promote fiirther reactions. 

Muller R et al 1996 Time-resolved identification of single molecules in solution with a pulsed semiconductor diode laser Chem. Phys. Lett. 262 716-22... 

An equally important challenge for nanocrystal assembly is the fonnation of specific nanocrystal arrangements in solution. By using complementary DNA strands as tethers, Mirkin et al [102, 103] fonned aggregates of gold nanocrystals with specific sizes Alivisatos et al also used DNA to stmcture semiconductor nanocrystal molecules, though in this case the molecules contained only a few nanocrystals placed controlled distances from each other [104, 105 and 106]. The potential applications of biomolecular teclmiques to this area of nanoscience are immense, and the opportunities have been reviewed in several recent publications [107, 108, 109 and 110]. 

Douglas T and Theopold K H 1991 Molecular precursors for indium phosphide and synthesis of small lll-V semiconductor clusters in solution inorg. Chem. 30 594... 

Primarily connected to corrosion concepts, Pourbaix diagrams may be used within the scope of prediction and understanding of the thermodynamic stability of materials under various conditions. Park and Barber [25] have shown this relevance in examining the thermodynamic stabilities of semiconductor binary compounds such as CdS, CdSe, CdTe, and GaP, in relation to their flat band potentials and under conditions related to photoelectrochemical cell performance with different redox couples in solution. 

Froment M, Lincot D (1995) Phase formation processes in solution at the atomic level Metal chalcogenide semiconductors. Electrochim Acta 40 1293-1303... 

Addition of hydrogen sulfide in solution was found to enhance the rate of this process albeit the efficiencies were generally low, partly due to concomitant precipitation of elemental sulfur during the photolytic experiments. The effects of reaction temperature, light intensity, and pH of the electrolyte were studied, and the photo-catalytic mechanism was discussed with reference to the theory of charge transfer at photoexcited metal sulfide semiconductors. 

---