Semiconducting properties transition metal oxides

E3.41 Low oxidation number d-metal oxides can lose electrons through a process equivalent to the oxidation of the metal atoms, with the result that holes appear in the predominately metal band. The positive charge carriers result in their p-type semiconductor classification. NiO is an example of this p-type semiconduction. Early transitional metal oxides with low oxidation number such as TiO and VO have metallic properties owing to the extended overlap of the d orbitals of the cations. See Section 24.6b for more details. 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

Reactions involving the creation, destruction, and elimination of defects can appear mysterious. In such cases it is useful to break the reaction down into hypothetical steps that can be represented by partial equations, rather akin to the half-reactions used to simplify redox reactions in chemistry. The complete defect formation equation is found by adding the partial equations together. The mles described above can be interpreted more flexibly in these partial equations but must be rigorously obeyed in the final equation. Finally, it is necessary to mention that a defect formation equation can often be written in terms of just structural (i.e., ionic) defects such as interstitials and vacancies or in terms of just electronic defects, electrons, and holes. Which of these alternatives is preferred will depend upon the physical properties of the solid. An insulator such as MgO is likely to utilize structural defects to compensate for the changes taking place, whereas a semiconducting transition-metal oxide with several easily accessible valence states is likely to prefer electronic compensation. 

Since pure mesoporous silica phases does not show any catalytic activity many successful attempts have been made to vary the inorganic composition towards transition metal oxides or metal chalcogenides [5-12], In particular the semiconducting properties of the latter offer a great range of possible applications in materials chemistry. 

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. 

Some ceramics are semiconductors. Most of these are transition metal oxides, such as zinc oxide. Ceramicists are most interested in the electrical properties that show grain boundary effects. Semiconducting ceramics are also employed as gas sensors. When various gases are passed over a poly crystalline ceramic, its electrical resistance changes. With tuning to the possible gas mixtures, very inexpensive devices can be produced. 

One of the early triumphs of quantum mechanics in the area of solid state materials was the ability to explain why certain materials are metallic conductors while others are insulating or semiconducting. The energy band structure, which can be calculated by ab initio or semiempirical methods, provides access to these electrical properties. As will be shown below, DFT methods are well suited to treat metallic systems whereas there are problems in the accurate prediction of energy band gaps in semiconductors and insulators. Furthermore, certain transition metal oxides and solids containing rare-earth and actinide elements present serious theoretical challenges which have not been completely resolved yet,... 

The semiconductive properties and tunnel structure of sulfide and transition-metal oxides led to the use of these materials in lithium power sources (Table 2.5). Several lithium-based chemistries were successfully applied to replace the prior system Zn/AgO and later the lithium-iodine batteries in implantable medical devices [59-61]. For example, Li//CuO, Li//V205, Li//CF and more recently Li// Ag2V40ii couples have been adopted to power cardiac pacemakers requiring less that 200 pW [62,63]. The lithium/carbon monofluoride (Li//CFJ primary cells are very attractive in several applications because of the double energy density with respect to the state-of-the-art LiZ/MnOa primary batteries (theoretically 2203 against 847 Wh kg ). 

The bridged transition metal complexes described here are of interest for technical applications due to their comparatively high thermal stability and their good semiconducting properties without external oxidative doping. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

Interesting results have been obtained in studies of the catalytic activity for oxidation by phthalocyanine polymers, containing different metal ions in the same molecule 87-90>. If Fe was mixed with a series of other transition metal ions, differences in activity were found to be dependent on the metal ion, and correlations between the catalytic activity and the thermal activation energy of semiconductivity were found. With copper as the second metal ion, maximum activities were found at a ratio Fe/Cu = 1. Many other chelate polymers have been tested for their oxidation activity, and a dependence of the catalytic activity on the donor properties of the ligand was found 91>92). 

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