Semiconductive metal oxides, relation

In the case of dissolution of a number of semiconducting metal oxides in acid aqueous solutions, it can be calculated from the rate of solution, that a phase boundary reaction must be rate determining. Moreover, it can be shown, that this reaction is electrochemical in nature, for its rate depends on the potential of the dissolving oxide. As an example, Fig. 1 shows the influence of the electrode potential on the rate of dis -solution of FeQ g Oin sulfuric acid and hydrochloric acid. The rate of dissolution is expressed in terms of a dissolution current. A linear relation exists between the electrode potential... 

A promising application of this new functionalization method of metal oxides relates to the photosensitization of semiconducting oxides to convert light into electricity. Thus, using the cavity microelectrode technique, a significant photocurrent is produced under blue light illumination by Ic- and Id-modified, F-doped and undoped Sn02 nanoporous powders, in the presence of an aqueous electrolyte, the... 

Some metals form extensive series of oxides such as Ti 02n 1 or Mo 03 1 with structures related to simple oxides MO2 or MO3. Many transition-metal oxides show departures from stoichiometry leading to semiconductivity and others have interesting magnetic and electrical properties which have been much studied in recent years. We shall illustrate some of these features of oxides by dealing in some detail with selected metal-oxygen systems and by noting peculiarities of certain oxides. 

The nature of the barrier to charge transfer at the metal-oxide interface is open to speculation. Assuming semiconducting properties for the Pt-oxide layer, this additional barrier may simply represent the nonlinear resistance of a metal-semiconductor junction, i.e., resistance of a diode biased in the conduction direction. For high bias voltages, the current-voltage relation for such a junction may be expressed by an equation of the form of Eq. (31). 

The microscopic mechanisms for the MNM transition described in the previous section are quite general. They can be related to a wide variety of physical systems. These include not only expanded electronically conducting fluids, but liquid solutions such as the molten metal-salt solutions, metal-ammonia solutions, semiconducting liquid alloys, etc. The mechanisms are also relevant to the MNM transitions in various solids, including amorphous semiconductors, heavily doped crystalline semiconductors, and metal oxides. Our concern is with fluids and so we turn now to summarize briefly some of the theoretical investigations specifically focused on the MNM transition and its relation to the phase transition behavior of fluid metals. 

Schwab and co-workers (5-7) found a parallel between the electron concentration of different phases of certain alloys and the activation energies observed for the decomposition of formic acid into H2 and CO2, with these alloys as catalysts. Suhrmann and Sachtler (8,9,58) found a relation between the work function of gold and platinum and the energy of activation necessary for the decomposition of nitrous oxide on these metals. C. Wagner (10) found a relation between the electrical conductivity of semiconducting oxide catalysts and their activity in the decomposition of N2O. 

Metal sulfides and several important oxides display n-type or p-type semiconducting or nieialiic properties. As a result of their electronic conductivity, cermin minerals can participate in coupled charge tmasfer processes aanlogous to a metal corroding in an electrolye, and the kinetics of leaching can be related 1o the potential of the solid in contact with the aqueous electrolyte. 

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