Elsevier

Surface Science

Volume 546, Issues 2–3, 1 December 2003, Pages 117-126
Surface Science

Vanadium on TiO2(110): adsorption site and sub-surface migration

https://doi.org/10.1016/j.susc.2003.09.019Get rights and content

Abstract

The initial stages of the growth of vanadium overlayers on TiO2(1 1 0) at room temperature have been investigated with scanning tunneling microscopy. At very low coverages both individual vanadium adatoms and small vanadium clusters have been imaged with good resolution. The V adatoms adsorb preferentially on the so-called “upper threefold hollow” sites, as revealed by atomically resolved STM images: they are thus bonded to two bridging oxygen atoms and one threefold coordinated basal oxygen atom. At higher coverages the vanadium adlayers grow in form of poorly ordered three-dimensional islands. The number of V clusters at low coverages decreases by gentle annealing or with time even at room temperature. This kinetic effect has been interpreted in terms of sub-surface migration of V adatoms.

Introduction

In the past decade, both metallic vanadium and vanadia ultrathin films deposited on TiO2 single crystal surfaces have become a preferred field of activity of surface scientists concerned with filling the gap between well-defined––albeit simplified––model systems prepared under UHV conditions and real world catalysts and sensing devices. The enduring success of these systems in surface science is motivated on the one hand by the fact that titania-supported vanadia powders are extensively used as catalysts for the selective reduction of NO with NH3, although the atomic-scale mechanisms responsible for their extreme activity and selectivity are still a matter of debate. On the other hand, ultrathin vanadium and vanadia films can be easily prepared on rutile TiO2(1 1 0), a rather common substrate which is atomically well characterized and has a simple closed-shell electronic structure in its stoichiometric and well-ordered form [1], [2]. From the early work, which was mainly XPS-based [3], through structural studies relying upon photoelectron diffraction (XPD) [4], to a recent multi-technique approach [5], [6], which included XPS, NEXAFS and also STM as a local probe, a comprehensive understanding of the structure, electronic structure, morphology, and reactivity of V metal films and different vanadia phases on both (1 × 1) and (1 × 2)-reconstructed TiO2(1 1 0) has emerged. However, as a result of these investigations, new questions have arisen. As a matter of fact, almost all structural determinations to date have been performed with photoemission and electron diffraction techniques which have poor lateral resolution in their standard implementation. This means that the structural information obtained, both locally (e.g. XPD) and on the long range (e.g. LEED), is averaged over a large surface area. Local morphological information (STM) for V growth on TiO2(1 1 0) has been only reported by Biener et al. [5]. The determined growth mode is consistent with the previously published XPS/XPD data [3], [4], which demonstrated the occurrence of small V islands, partially oxidized at the expense of the substrate, with no long range coherence. However, some important features which have been found on the basis of XPD in the sub-monolayer coverage range [7], such as the adsorption site for V atoms in the initial stages of deposition, the sub-surface migration of V at room temperature (RT), and the concomitant occupation of sixfold-coordinated cation substitutional sites [8], have not been confirmed as yet by means of local probe experiments.

Taking advantage of atomically resolved STM images detailed links have been established in this work between previously reported laterally averaged structural information and the direct local atomic-scale view of the most relevant morphological and structural features of the V/TiO2(1 1 0) system. In particular, the reported images allow us to specify the V adsorption site, which has not been previously identified. Moreover, the time evolution of the system is addressed in this paper. The sub-surface migration of V atoms is a kinetic effect, showing the instability of the as-prepared system and the evolution towards a more stable atomic and electronic arrangement. Some hints of time dependent behaviour of the V/TiO2(1 1 0) system have already been reported previously [8]. Since the kinetics appears to be slow, it is possible to follow the time evolution of the system in a series of STM images in the time interval, where the most relevant structural changes occur.

Section snippets

Experimental

The experiments have been performed in a custom designed three-chamber UHV system with a base pressure <1 × 10−10 mbar, as described elsewhere [9]; it is equipped with low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and scanning tunneling microscopy (STM) facilities. The STM (micro-STM, Omicron) has been operated at room temperature in a constant current scanning mode, with typical sample bias between +1.2 and +2 V and a tunneling current between 0.5 nA and 1.5 nA.

In

Growth mode of vanadium at RT

Following the substrate cleaning procedure described above, atomically flat TiO2 surfaces have been obtained as shown in the STM image of Fig. 1(a). Chains of bright and dark rows separated by 6.5 Å, with a spacing of 2.9 Å along the rows (see insert), are clearly seen on the terraces, which demonstrate the presence of the TiO2 (1 × 1) structure, in agreement with the sharp (1 × 1) LEED pattern. There is a general consensus in interpreting the contrast in the STM TiO2(1 1 0) images as being dominated

Conclusions

The atomic structure of V overlayers on a TiO2(1 1 0) surface has been investigated by atomically resolved scanning tunneling microscopy during the early stages of overlayer growth. At very low coverages individual V adatoms and small clusters have been observed. The clusters are of the scale of 10–15 Å and are somewhat elongated along the [0 0 1] direction of the substrate. The clusters are between 1.5 and 3 Å high, corresponding to one or two V layers, and they tend to pair along the [1 1̄ 0]

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    This work has been supported by the Austrian Science Foundation.

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