The effect of stoichiometry on the stability of steps on TiO2(1 1 0)
Introduction
Although the surfaces of metals and semi-conductors have been studied extensively, the surfaces of oxides have received much less attention. In part this is because of their complexity [1], [2], but oxides also involve several practical experimental problems. First of all, most oxides are electrically insulating or very poorly conductive. Since many surface science techniques use an electric current to probe or prepare the surface, this can be a serious problem. A second practical problem concerns the surface composition. The sample preparation steps can leave the surface and near-surface region with a stoichiometry different from that of the bulk oxide.
In spite of the relatively modest number of investigations of oxide surfaces, oxides do have many important applications. Therefore oxide surfaces are now rapidly gaining interest. With the decreasing feature size in integrated circuits, the properties of the insulating oxide used, become more and more important. Also in catalysis oxides play an important role. They are used to support and disperse small catalytically active metal particles, and they can even act as the catalyst themselves [3], [4], [5].
One of the most studied oxide surfaces is TiO2(1 1 0). TiO2 was found to be an efficient catalyst for the dissociation of water [6], and it is currently used in a variety of applications, ranging from gas sensing to photocatalysis. Furthermore it is used as a support material in heterogeneous catalysis and it is one of the components of the TiO2–V2O5 catalyst that is being used for the reduction of NOx with NH3. TiO2 can have a relatively high electrical conductivity (see below), which makes it relatively easy to use standard techniques. Although this surface exhibits very complex behaviour, as will be explained in the remainder of this paper, TiO2(1 1 0) is considered to be one of the most easily prepared and stable oxide surfaces [1]. Therefore it is exploited as a model oxide surface, and it has been studied with several techniques, scanning tunnelling microscopy being one of the most prominent [1], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18].
The composition of titanium oxide is not constant but depends on the preparation conditions. The equilibrium stoichiometry depends both on temperature and on partial oxygen pressure. Typical surface science preparation procedures in ultra-high vacuum (UHV) usually result in crystals that are slightly oxygen deficient. Even though deviations from ‘perfect’ TiO2 stoichiometry can be very small (<10−4–10−3 [19], [20], [21], [22]) the consequences can be severe. The colour changes from transparent via blue to black with increasing oxygen depletion, and at the same time the electrical conductivity increases strongly.
In this paper we show that changes in composition affect the surface morphology. In particular the relative energies of steps in different directions depend on the average stoichiometry of the surface. When TiO2(1 1 0) is annealed and the oxide becomes increasingly depleted in oxygen, the equilibrium shapes of islands and vacancy islands change, and one specific step direction becomes strongly favoured over the others.
Section snippets
Experimental procedures
The TiO2 samples [23] were prepared in a high vacuum (HV) chamber with a base pressure of 2×10−8 mbar. The preparation chamber was equipped with a broad-beam ion source for sputtering the sample surface (Kaufmann 3 cm ion source model II). A neutralizer was used to supply electrons to the sputter beam, to prevent the samples from charging up. The ion source was mounted under an angle of 45° with respect to the surface normal. The samples ( mm) were mounted in a molybdenum holder. The
Results
In many studies on TiO2 it has been found that the surface morphology can depend critically on the preparation history, via the degree of bulk reduction (see for example Refs. [1], [10]). In order to maximize the reproducibility of the experiments, we started each series of experiments with a new, stoichiometric, fully oxidized, transparent sample. The result of a typical series of experiments is shown in Fig. 1, Fig. 2.
First, the stoichiometric crystal was sputtered in the HV preparation
Discussion
The changes in surface morphology observed at 1135 K (Fig. 2) are very different from what is usually found in annealing experiments on sputtered surfaces. The equilibrium shape of islands and holes is determined by the relative values for the step free energies along the different directions, via the two-dimensional analogue of the well-known Wulff construction [31], [32]. Annealing usually makes all shapes (e.g. islands) more compact and leads to a steady increase of the average feature size
Conclusions
Summarizing, we have used AFM to investigate the flattening of TiO2(1 1 0). At elevated temperatures the surface flattens, but at the same time the oxide is reduced. This has important consequences for the surface morphology. At 1135 K, first an elongated hexagonal equilibrium shape is formed, but as reduction continues the relative step free energies change and so do the shapes of the surface features. They become more and more elongated along the [0 0 1] direction. Eventually it even becomes
Acknowledgements
This work is part of the research program of the ‘Technologiestichting STW’ and the ‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’, and is financially supported by STW and the ‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)’.
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Present address: Philips Research Laboratories, Building WY 5-56, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands.