Stoichiometric surfaces, structures

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. 

RuHAP was synthesized from a stoichiometric HAP, Ca10(PO4)6(OH)2, with RuCl3nH20. Analysis by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX), IR and Ru K-edge X-ray absorption fine structure (XAFS) showed that a monomeric Ru phosphate species is created on the HAP surface. Figure 5.2a shows a proposed surface structure of RuHAP. 

Fig. 33. Perspective views from the top-layer side (Panel a) and from the bottom-layer side (Panel b) of the representative, nearly-stoichiometric surface cluster V360 98- The geometric structure of the V205 (010)- surface cluster (see Fig. 26) involves the SINDOl optimized intra-layer bond lengths [28] and the crystallographic values of the bond angles and the inter-layer V-O bond length. The surface layer views of alternative perpendicular adsorptions of toluene on the bridging oxygen 0(2) from the top layer side (c) and from the bottom layer side (d), and two alternative parallel adsorptions (Panels e, f) on the bottom layer. In Panel d the same adsorbate-substrate separation as in Figs. 7a, 9 and 25e has been adopted, while in the remaining panels this separation has been increased by 1 A relative to those shown in Fig. 25...

Fig. 8. Surface structures for SnO2(110). Large open circles are O small filled circles are Sn. (a) Stoichiometric 1x1 reconstruction with rows of bridging oxygen ions. All the surface Sn cations are in the +4 oxidation state, (b) Bare 1x1 reconstruction in which all the bridging oxygen ions are removed to leave both 4- and 5-coordmate surface Sn cations. To maintain electrostatic neutrality half the surface Sn cations are reduced to the +2 state, (c) One possible model for a (1x2) reconstruction with alternately missing rows of bridging oxygen.

Abstract The surfaces of model metal oxides offer many fundamental examples where the outcome of a specific chemical reaction might be linked to the surface structure and local electronic properties. In this work the reaction of simple molecules such as ammonia, alcohols, carboxylic and amino acids is studied on two metal oxide single crystals rutile TiO CllO) and (001) and fluorite UOj(l 11). Studies are conducted with XPS, TPD, and Plane Wave Density Functional Theory (DFT). The effect of surface structure is outlined by comparing the TiOj(llO) rutile surface to those of TiOjCOOl), while the effect of surface point defects is mainly discussed in the case of stoichiometric and substoichiometric UOjClll). 

Recently ALISS experiments and simple classical theory have been used to directly get the surface structure of TiO2(110) [41]. Figure 20 shows an unrelaxed stoichiometric Ti02 surface with bridging oxygen rows. 

As this review is intended to illustrate, the interplay between metal and oxygen leads to a richness of reactivity that is reflected in the surface structure of oxides. Much of this richness can be rationalised as varying proportions of ionic and covalent character in the metal-oxygen bonding, and is manifest in a variety of non-stoichiometric surfeces. We therefore focus on the prototypical transition metal oxide smface rutile Ti(>2 (1 1 0). This is contrasted with computational results for one of the most widely-studied p-block oxide surfaces - corundum Al2O3-(0 0 0 1) - and we refer also to computational surface studies on oxides of ruthenium, iron, vanadium, tin and silver, as well as ternary oxides. 

One possible explanation is that the surface models (both for the fitting of SXRD and for FP calculations) are too simple. The range of data for vacuum-annealed and (lx 3)-reconstructed TiOi (10 0) highlights the difficulties of fitting SXRD patterns to a limited set of over-simplified structural models [59]. However, an FP survey of over 60 stoichiometric surfaces of rutile (110) confirmed that the unreconstructed (1x1) bulk-termination is the most stable, with a computed surface energy of Ysuif=0-80 0.04 Jm [23]. 

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. 

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