Activity of a Semiconductor

As we have seen, the participation of the free electrons and holes of the catalyst in chemisorption bonds results in the chemisorbed particle spending a part of its lifetime on the surface in a radical state. Thus, the very fact that a molecule goes over from the gaseous phase to the chemisorbed state leads to an increase in its reactivity. 

We shall consider different types of heterogeneous reactions and their possible radical mechanisms 10, 27). Consider a reaction between two molecules AB and CD, where A, B, C, D denote the separate atoms or atomic groups. 

Let A and B, and also C and D, be connected by single bonds. Consider the exchange reaction 

Suppose now that one of the reacting molecules, CD, for example, contains a double bond, whereas A and B in molecule AB are connected by a single bond. Consider the reaction. 

Other examples of reactions of this type are the reactions of addition of hydrogen-halides to olefins, for example, 

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A number of works have been devoted to the effect of preadsorbed foreign gases on the catalytic activity of a semiconductor in relation to the hydrogen-deuterium exchange reaction. 

VIII. Factors Affecting the Adsorptivity and Catalytic Activity of a Semiconductor 241... 

The fifth consequence of the theory is that the adsorptivity and catalytic activity of a semiconductor are affected by illumination. When a crystal absorbs light waves of photoelectrically active frequencies (i.e., frequencies exciting the internal photoeffect), this leads, generally speaking, to a change... 

In conclusion we stress once more that the above-considered mechanism of the effect of illumination on the adsorptivity and catalytic activity of a semiconductor holds in the case when the absorption of light increases the number of free electrons or holes (or both) in the crystal. This, however, does not always take place. The absorption of light by the crystal may proceed by an exciton mechanism. This seems to be the case in the region of intrinsic absorption, which is as a rule photoelectrically inactive. 

This effect predicted by the theory (effect of an external electric field on the adsorptivity and catalytic activity of a semiconductor) has not been... 

Thus, sensor effect deals with the change of various electrophysical characteristics of semiconductor adsorbent when detected particles occur on its surface irrespective of the mechanism of their creation. This happens because the surface chemical compounds obtained as a result of chemisorption are substantially stable and capable on numerous occasions of exchanging charge with the volume bands of adsorbent or directly interact with electrically active defects of a semiconductor, which leads to direct change in concentration of free carriers and, in several cases, the charge state of the surface. 

Thus, the rigorous solution of kinetic equation describing the change in electric conductivity of a semiconductor during adsorption of radicals enables one to deduce that information on concentration of radicals in ambient volume can be obtained measuring both the stationary values of electric conductivity attained over a certain period of time after activation of the radical source and from the measurements of initial rates in change of electric conductivity during desactivation or activation of the radical flux incident on the surface of adsorbent, i.e. 

Up to now we have considered the relationship between the concentration of active particles in systems like gas (vapour) - solid body (semiconductor) and variation of conductivity of a semiconductor. In connection to these systems we mentioned numerous relationships which may be used for quantitative assessment of the content of gaseous media on the basis of data provided by semiconductor sensors when analyzing various active components. 

NH2 radicals, hydrogen atoms adsorbed on the surface of a semiconductor sensor more actively affect the electric conductivity of the sensor. 

We found that the increase of film conductivity observed without magnetic field disappeared immediately after the field tum-on and did not appear again until the filed was turned off again. This result seems to confirm the earlier supposition of electric activity being present only if the surface of a semiconductor film is covered with silver atoms. We used the method of physical development of an oxide film alter sufficiently long exposition of the film to the beam of silver particles, with the magnetic field turned on, in order to be sure that particles of silver... 

From the positive results of these experiments we concluded that the behaviour of atomic and molecular particles of silver with respect to their influence on electrophysical properties of oxide films is similar to that of atoms and molecules of nonmetals, with the only difference that metal-atom interstitials behave similar to hydrogen-like donors of electrons, independent of the kind of a metal. As to metal molecules, at low temperatures of a semiconductor film, when their surface dissociation does not occur, they do not reveal considerable activity with respect to electrophysical properties of the film. 

The passivation of deep level defects and shallow impurities in semiconductors by hydrogen has been studied extensively in recent years (Pearton et al., 1987, 1989 Haller, 1989). For Si in most cases, complexing with hydrogen eliminates the electrical activity of a defect.Once passivated, the... 

In classical kinetic theory the activity of a catalyst is explained by the reduction in the energy barrier of the intermediate, formed on the surface of the catalyst. The rate constant of the formation of that complex is written as k = k0 cxp(-AG/RT). Photocatalysts can also be used in order to selectively promote one of many possible parallel reactions. One example of photocatalysis is the photochemical synthesis in which a semiconductor surface mediates the photoinduced electron transfer. The surface of the semiconductor is restored to the initial state, provided it resists decomposition. Nanoparticles have been successfully used as photocatalysts, and the selectivity of these reactions can be further influenced by the applied electrical potential. Absorption chemistry and the current flow play an important role as well. The kinetics of photocatalysis are dominated by the Langmuir-Hinshelwood adsorption curve [4], where the surface coverage PHY = KC/( 1 + PC) (K is the adsorption coefficient and C the initial reactant concentration). Diffusion and mass transfer to and from the photocatalyst are important and are influenced by the substrate surface preparation. 

Upon excitation of a semiconductor, the electrons in the conduction band and the hole in the valence band are active species that can initiate redox processes at the semiconductor-electrolyte interface, including photocorrosion of the semiconductor, a change in its surface properties (photoinduced superhydrophilicity [13]), and various spontaneous and non-spontaneous reactions [14-19]. These phenomena are basically surface-mediated redox reactions. The processes are depicted in Fig. 16.1. Owing to the slow spontaneous kinetic of the reactions between the... 

A. Relation between Catalytic Activity and Electrical Conductivity of a Semiconductor... 

The third important consequence of the theory is the relation between the catalytic activity and the electrical conductivity of a semiconductor. 

The existence of a correlation between the catalytic activity and the electrical conductivity which follows from the theory was indicated by us back in 1950 (37, 6S), when there were as yet no measurements available that could either corroborate or refute this theoretical prediction. To date we have already a whole series of experimental work in which such a correlation has been observed (e.g., 36, 56, 66-70). A number of authors have measured the electrical conductivity and the catalytic activity of various samples of a semiconductor which differed in the method of preparation and have discovered that these two properties of the semiconductor vary in the same or in opposite directions from one sample to another. The results of some of these experiments are presented in Table II. 

It should be observed that in several cases the relation between the electrical conductivity and the activity may break down. This will occur in those intervals of variation of ,+, in which the reaction rate is independent of e,+, e.g., for the reaction of dehydrogenation of alcohols in the region of sufficiently high values, and for dehydration in the region of sufficiently low values of e,+ (Sec. V,B and Fig. 19). It may also occur in the case of a semiconductor with a quasi-isolated surface, when e,+ is independent of e,+ (Sec. VI,B) if the dimensions of the crystal are not too small (Sec. VI,C). 

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 ... 

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