Surfaces, non-stoichiometric

To summarize, on insulating oxide polar surfaces, a reduction of the ionic charge density a = aR/ R + R ) is required on the outer layers to cancel the macroscopic electric field, and stabilize the system. It may be induced by non-stoichiomety in the surface layers, by faceting or by charged impurities introduced during surface preparation. Systematic chemical analysis and measurements of the surface stoichiometry should be able to point out which mechanism takes place in a given case. 

In ternary or more complex partly covalent oxides, some surfaces have a weak polarity, although they are considered to be non-polar according to Tasker s classification. However, the charge compensation on these weakly polar surfaces is more easily achieved than on polar surfaces, since it only involves a bond-breaking mechanism. 

While several calculations have proved that a charge density a = a jl may appear at a surface when the electronic degrees of freedom are taken into account, it should be noted that no one has found a dipole density a R with a = ajA. It is likely that, in actual systems with a discrete atomic structure, the oscillating macroscopic potential (G = 0) (Fig. 3.7b) is strongly damped by microscopic non-uniform contributions (Gy f= 0 terms). 

The interest in surface defects on oxides also relies on considerations of reactivity, since steps or vacancies are known to be active centres for chemical reactions. For example, in photo-electrochemical cells, anodes made of reduced Ti02 or SrTiOs present a larger activity in the decomposition of water, than their stoichiometric counterparts, which was assigned to the presence of Ti ions (Lo et al, 1978 Lo and Somoijai, 1978). The reduction of ReOs surfaces (Tsuda and Fujimori, 1981) and of SnO2(110) surfaces (Gercher and Cox, 1995) induces similar effects. Many structural and electronic studies have thus been devoted to the analysis of defects, and more particularly of oxygen vacancies. 

The sub-stoichiometric (001) face of V2O5 presents a gap state and a charge transfer towards the vanadiums (Zhang and Henrich, 1994). 

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The energetics of non-stoichiometric surfaces can be characterized in two different ways. One consists in defining the vacancy formation energy Evf. It is the energy required to extract one neutral oxygen atom from the... 

Because the formation of non-stoichiometric surfaces in this scheme is governed by the balanced chemical equation between metal, oxygen and oxide, the allowed range of chemical potential (po -Mo ) is given by the formation energy of the oxide (AGf). For example, since... 

The link between structure and reactivity is again demonstrated by the complicated succession of vanadium oxide surface phases predicted by FP [77]. At certain O2 partial pressures, the metal substrate is computed to stabilise thin film phases that are not known in equivalent bulk form. The impliaation is that STM studies of thin insulator fihm on conducting substrates may have to contend with the complex, and sometimes novel, chemistry of thin films [2]. A phase diagram of non-stoichiometric surfaces is also generated by FP in Ref. [53], this time for silver oxidation. The aim is to bri(%e the pressure gap between ultra-high-vacuum research and the industrial reality of high-pressure reactors. 

Non-Stoichiometric Surface Phases Hydroxides and Reactivity with Water... 

Nano-composite materials with fine semiconductor particles dispersed in the matrix have attracted considerable interest because the properties of the particles are much different from their bulks when the diameters are l s than the Bohr exciton radius. Such particles, which are generally named as nano-particles, are characterized by non-stoichiometric surface structure and quantum size effect 2). These properties would lead to new phenomena, new theoretical insights, and new materials and devices. 

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