Semiconductor-Electrolyte Systems

Fig. 6.3 Schematic picture of the electrochemical potential ( > as a function of distance x in an oxide semiconductor electrolyte system a) bulk semiconductor potential b) solid/solution interface potential c) space charge potential d) flat band potential e) potential in the double layer (White, 1990, with permission.

R. de Gryse, W. P. Gomes, F. Garden, and J. Veimik, On the interpretation of Mott-Schottky plots determined at semiconductor/electrolyte systems, J. Electrochem. Soc. 122, 712, 1975. 

Microstructural factors also play important roles in determining the electrochemical and physical properties of semiconductor-electrolyte systems. For example, semiconductor electronic properties are usually interpreted in terms of ideal band models for perfect crystals—i.e., for systems that exhibit absolute long-range order. For many systems, however, this is a gross oversimplification and, in the extreme of the amorphous state, it may be appropriate to abandon band models altogether... 

Semiconductor-Electrolyte Systems.—Titanium Dioxide. Interest continues in the cell devised by Fujishima and Honda30 for the electrochemical photolysis of water ... 

Semiconductor/Electrolyte System in Dark and upon Illumination... 

Semiconductor/electrolyte system in dark and upon illumination... 

Kolbasov G.Ya., Gorodyski A.B. The processes of photoexited charge transfer in semiconductor-electrolyte system. Kiev, Naukova dumka , 1993, 192 p., in Russian. 

The charge distribution for the semiconductor-electrolyte system is given by the excess minority carrier profile Ap(x) (2.40a) and the corresponding neutralizing majority carrier profile. The microwave signal due to the minority carriers, AR, is given by the excess carrier distribution throughout the sample ... 

Modem first principles computational methodologies, such as those based on Density Functional Theory (DFT) and its Time Dependent extension (TDDFT), provide the theoretical/computational framework to describe most of the desired properties of the individual dye/semiconductor/electrolyte systems and of their relevant interfaces. The information extracted from these calculations constitutes the basis for the explicit simulation of photo-induced electron transfer by means of quantum or non-adiabatic dynamics. The dynamics introduces a further degree of complexity in the simulation, due to the simultaneous description of the coupled nuclear/electronic problem. Various combinations of electronic stmcture/ excited states and nuclear dynamics descriptions have been applied to dye-sensitized interfaces [54—57]. In most cases these approaches rely either on semi-empirical Hamiltonians [58, 59] or on the time-dependent propagation of single particle DFT orbitals [60, 61], with the nuclear dynamics being described within mixed quantum-classical [54, 55, 59, 60] or fuUy quantum mechanical approaches [61]. Real time propagation of the TDDFT excited states [62] has... 

Information aboutthe mechanism of dissolution of semiconductors may be obtained from investigations of current-potential characteristics of a semiconductor-electrolyte system (16)( 18-23), and from photoluminescence and electroluminescence spectra of semiconductor-electrolyte interface (26-28). For details of surface reaction mechanisms, the reader is referred to the above cited literature. 

Photoelectrochemistry is still an emerging discipline in that it is not yet possible to write this chapter in the form of a tutorial in which one can give a series of steps and techniques that, once applied to a given system, results in a complete understanding of the system in terms of structure, mechanism and kinetics. There is no single semiconductor/electrolyte system of which our understanding is sufficient for its predictive power to provide any degree of satisfaction. Afew systems such as Si and InP come close. 

Figure 6 shows a typical Gss/w and Bss spectra of n-lnSe electrode in iodide solution (51), from which two surface state elements were assigned, with relaxation times of 1 ms. and 10 ms., respectively. An example of the dependence of the surface state capacitances on the electrode potential, is shown in Fig 7. The data were fitted to a Gaussian distribution of surface states (52). For the fast surface state, the distribution is centered around -0.23 volt (vs. Pt) with area density of 1.05x10 /cm and width of 0.11 eV. For the slow surface state, the distribution is centered around -0.21 volt (vs. Pt) with area density of 3.12x 10 /cm and width of 0.13 eV. If we take 10 /cm as typical density for a monolayer, both states occupy a small fraction of a monolayer. Similar analysis of the impedance data was done on other semiconductor/electrolyte systems that include TiO, CdSe, CuInjSej etc. 

The first consideration is the choice of model system, and this may involve an aqueous electrolyte or nonaqueous media such as an organic electrolyte or molten salt. The selection of electrolyte and scavenger will depend on the energetics of the electrolyte reduction (good stability to reduction is needed) and upon the acceptor reactivity. Apparatus and electrochemical cells for metal/ electrolyte and semiconductor/electrolyte systems are similar generally. Some guidelines are presented below to assist with experimental practice (see also Chapter 3 in Ref. 10). Theses, also, are a good source of practical information, e.g., Refs. 20, 21, and 81. 

The intention of this monograph has been to assimilate key practical and theoretical aspects of those spectroelectrochemical techniques likely to become routine aids to electrochemical research and analysis. Many new methods for interphasial studies have been and are being developed. Accordingly, this book is restricted in scope primarily to in situ methods for studying metal/electrolyte or semiconductor/electrolyte systems moreover, it is far from inclusive of the spectroelectrochemical techniques that have been devised. However, it is hoped that the practical descriptions provided are sufficiently explicit to encourage and enable the newcomer to establish the experimental facilities needed for a particular problem. 

---