Semiconducting oxides, metal sites

The procedure is best illustrated by an example. Suppose that a nonstoichiometric phase of composition MA can have an existence range, which spans both sides of the stoichiometric composition, MX, oo. Assume that in this phase only vacancies are of importance, so that the stoichiometric composition will occur when the number of vacancies on the cation sublattice is exactly equal to the number of vacancies on the anion sublattice, which is, therefore, due to a population of Schottky defects. At other compositions, electrical neutrality is adjusted via mobile electrons or holes, leading to n-type or p-type semiconductivity. Thus there are four defects to consider, electrons, e, holes, h, vacancies on metal sites, Vm, and vacancies on anion sites, Vx. Finally, assume that the most important gaseous component is X2 as is the case in most oxides, halides, and sulphides. 

It has thus been elucidated that well-ordered micro- or mesopores of zeolites or mesoporous materials can accommodate transition metal oxides or ions in an isolated state as single-site photocatalysts to realize unique and selective photocatalytic reactions essentially different from those on semiconducting photocatalysts such as Ti02. It was observed that zeolite or mesoporous frameworks offer one of the most promising molecular reaction fields and approaches in the development of effective new photocatalytic systems that can contribute to the reduction of global air pollution and utilize solar energy as a clean, safe and abundant resource. 

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