RELATION WITH SEMICONDUCTOR PROPERTIES

After some general remarks on the relations between semiconductor properties and their use as photocatalysts, this text will first deal with oxidations of organic compounds.The interactions of illuminated semiconductors with gaseous C>2 (and, for comparison, with gaseous NO) will be then presented, whereas the last part will consider metal/semiconductor photocatalysts and the organic reactions they allow. In this presentation, the results of this laboratory will be highlighted. 

Above reasoning can be confirmed by a number of experimental results which showed that although with some peculiarities irrelevant to the properties of semiconductor sensors the correlation between the amount of the atoms in the flux incident on the target, or their surface concentration, and the variation (increase, if we are dealing with semiconductor of n-type) of the target conductivity takes place [28]. Based on the relations cited in Chapter 1, one can estimate concentrations (i. e., flow intensities) of these particles in vacuum or in gaseous medium if these values are quite small, using the values of conductivity variation of the semiconductor film. 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

The link between chemisorption and semiconductivity, as illustrated by this example, was first clearly perceived by Wagner and Hauffe (2) in 1938. Whereas the production of a semiconductor by chemisorption presents relatively little interest for our purpose, the reverse problem is currently receiving a great deal of attention. How is a given semiconductor going to behave in chemisorption Is it possible to relate semiconductor characteristics with catalytic properties and, if so, what are the properties of the semiconductor that have to be changed in order to modify and control catalytic activity ... 

Chapter 1 of the present volume provides the basic concepts related to the properties and characterization of the centres known as shallow dopants, the paradigm of the H-like centres. This is followed by a short history of semiconductors, which is intimately connected with these centres, and by a section outlining their electrical and spectroscopic activities. Because of the diversity in the notations, I have included in this chapter a short section on the different notations used to denote the centres and their optical transitions. An overview of the origin of the presence of H-related centres in crystals and guidelines on their structural properties is given in Chap. 2. To define the conditions under which the spectroscopic properties of impurities can be studied, Chap. 3 presents a summary of the bulk optical properties of semiconductors crystals. Chapter 4 describes the spectroscopic techniques and methods used to study the optical absorption of impurity and defect centres and the methods used to produce controlled perturbations of this absorption, which provide information on the structure of the impurity centres, and eventually on some properties of the host crystal. Chapter 5 is a presentation of the effective-mass theory of impurity centres, which is the basis for a quantitative interpretation... 

Solvent Effects, Crystal Fields. - This report is concerned with molecular properties and full coverage of intermolecular effects and solid state susceptibilities is not attempted. The papers reviewed in this section have been selected because they contain material closely related to the calculated properties of individual molecules. For example, calculations based on the electronic band structures of semiconductors etc. are excluded, but a few papers relating molecular crystal susceptibilities to the molecular hyperpolarizabilities are included. 

Can the present-day electronics based on sihcon and related inorganic semiconductor materials, in particular micro- to nanoelectronics, be extended in the not-too-distant future to include electronics with molecules which makes use of molecular functional units and applies their specific molecular properties. Can molecular devices be integrated into circuits based on silicon. The fascinating aspect of such ideas is a conceivable further miniaturisation beyond that possible with inorganic semiconductors, combined with the enormous variability which is offered by organic chemistry, and perhaps also the greater ease of development and fabrication of devices based on such materials. 

Consequently, DFT is restricted to ground-state properties. For example, band gaps of semiconductors are notoriously underestimated [142] because they are related to the properties of excited states. Nonetheless, DFT-inspired techniques which also deal with excited states have been developed. These either go by the name of time-dependent density-functional theory (TD-DFT), often for molecular properties [147], or are performed in the context of many-body perturbation theory for solids such as Hedin s GW approximation [148]. 

Yanagida S, Ishimaru Y, Miyake Y, Shiragami T, Pac CJ, Hashimoto K, SakataT Semiconductor photocatalysis. 7. ZnS-catalyzed photoreduction of aldehydes and related derivatives 2-Electron-transfer reduction and relationship with spectroscopic properties. J Phys Chem 1989, 93(6) 2576—2582. 

---