Rutile surfaces

A packing density of 6.6 Ti + ions per 100 A was estimated on a theoretical basis by Hollabaugh and Chessick (301). From the values of irreversible and reversible water vapor adsorption, a surface density of 3.7 OH/100 A was calculated for the substance activated at 450° and of 11.4 OH/100 A for a fully hydroxylated rutile surface. 

An estimation of the average electrostatic field of the rutile surface is possible, since the net heat of adsorption, or the heat of wetting directly... 

The slope of the line yields the average electrostatic field strength F of 2.7 X 10 e.s.u./cm. (at the operative distance from the rutile surface to the center of the dipole) this value is in excellent agreement with that calculated for rutile by de Boer (44) from the results for argon adsorption by Morrison, Los, and Drain (4 ). 

Munuera and Stone (142) successfully applied the model of a (110) plane to their adsorption studies of water, isopropanol, and acetone. The dry (110) face exposes 5.1 five-coordinate Ti4+ ions. Half of these were able to chemisorb water dissociatively, leading to the formation of two types of surface OH groups. The activation energy for desorption of this water was 107 kJ mole-1. At this state of hydroxylation the rutile surface consists of isolated five-coordinated Ti4+ ions, which on further addition of water coordinate water molecu-larly, of isolated O2- ions, and of pairs of OH groups, one being monodentate and the other bidentate with respect to the cations. 

Fig. 28. Alternatives sites of a molecular adsorption of water on the (110)-rutile surface cluster (146 atoms) on the central, five fold O-coordinated Ti site (the singly coordinated water molecule) and on the bridging oxygen vacancy (the doubly coordinated water molecule). The corresponding reaction coordinates of the dissociative adsorption that produce two OH species involve the bending of water towards the bridging oxygen of the singly coordinated adsorbate and rotation of the doubly coordinated water...

Fig. 29. Representative external MEC diagrams for a model (147 atom) cluster of the (llO)-rutile surface. The numbers below the diagrams report rk values (a.u.). The primarily displaced atom k is identified by the largest open circle. Only displacements around the central five-fold coordinated Ti site of the surface are included in the figure...

Somewhat later, the surface and colloidal properties of rutile were studied in considerable detail (Day, 1973 Wiseman, 1976 Parfitt, 1981 Rochester, 1986). In the early 1970s, extensive use was made of infrared spectroscopy for characterizing the rutile surface and its interaction with water and other molecules. An improved understanding of the mechanisms involved in coating the rutile surface was also provided by studies of the energetics of immersion and electrokinetic behaviour together with the application of electron microscopy. 

Fig. 8.17. Structure of a Ti40,6 cluster as a model for the ideal (110) rutile surface. The shaded plane shows the (110) surface (after Tsukada et al., 1983 reproduced with the publisher s permission).

This defect level located near the conduction band is very important because of its high chemical activity. Experimentally, it has been established that the photocatalytic activity of the rutile surface increases greatly when reduced (oxygen deficient) compared to the clean surface (Mavroides et al., 1975). Defect sites on all oxide surfaces are usually very active for adsorption, whereas the nearly perfect surfaces of oxides where the cation is in its highest oxidation state are generally inert (Flen-rich, 1987). 

Contrary to the (110) face, the clean (100) rutile surface is unstable, and facets upon annealing to form (110) and other low index faces [76, 77]. Annealing in UHV the sputter-cleaned (100) surface at 873 K results in the formation of a (1x3) reconstruction. 

There is only one way to cut a rutile crystal in (001) direction (Fig. 16.) Although this creates a non-polar, autocompensated surface, it does not represent a low-energy configuration. This becomes clear immediately when reviewing the coordination of the surface atoms. All the Ti atoms are 4-fold coordinated, and all the O atoms 2-fold coordinated. Hence the number of broken bonds on this surface is higher than on the other low-index rutile surfaces discussed so far. Consequently, the (001) surface has a high surface... 

The expanding data base has made rutile the model system for metal oxides. Nevertheless, there are still many open questions concerning the crystal structure of rutile surfaces as pointed out throughout this Chapter. One interesting aspect is the advent of surface studies on anatase. From the recent... 

Fig. 14. AFM images of polycrystalline rutile surfaces before (a,c,e) and after (b,d,f) the photochemical deposition of silver (white contrast).

In the present work, a quantum chemical study of interaction of rutile surface (110) with dimer Agz was carried out. Calculations were performed within two adsorption models a cluster model and a periodic one. 

The DFT study of adsorption of silver dimer on rutile (110) surface within the cluster and periodic models shows that the interaction occurs both with chain oxygen atoms of the surface and with atoms located between the chains of 0(2c) atoms. Positive binding energy of Ag2 with rutile surface during adsorption between the oxygen chains was obtained only for the periodic model. The latter is concluded to be the preferable for theoretical study of Ag /Ti02 systems. 

Some observations, relevant to the effect of the UV illumination upon energetics of water adsorption on the (100) plane of rutile Ti02, have been made by Lo et al.. The band-gap (hv > 3 eV) irradiation of the prereduced, Ti -rich rutile surface, containing adsorbed water, resulted in a distinct increase of the work function, indicating a possible change of the nature of adsorbed species. 

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). 

The proposed mechanism also explains why the slopes of the H2 chemisorption versus In t curve for Rh on anatase were smaller than those for Rh on rutile. For the Ti + cations at the rutile surface are predicted to be more easily reduced than those at the anatase surface (30). 

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