Structural relationship between V₂O₅ (0 0 1) surface and the bulk: cluster bulk termination models

Abstract

Ab initio periodic and cluster Hartree–Fock calculations have been performed to investigate the structural and electronic properties of both V₂O₅ bulk and ($\text{0}\text{0}\text{1}$) surface. A full bulk geometry optimization yields to good VO lengths which are found to be in agreement with experiment. The relationship between the bulk and surface structure is investigated by different cluster models with optimized bulk termination. The validity of this approach is discussed through the experimental and theoretical surface results.

Introduction

Vanadium oxides are of great scientific importance, they are widely used as prototype in chemical and electronic industries [1] for their interesting physical and chemical properties. These materials exist in many crystallographic forms with different stoichiometries, where vanadium exhibits various oxidation states from a structure to another. Vanadium pentoxide, V₂O₅, presents very interesting properties, it is used in catalysis [2], [3], as a n-type semiconductor [4], and as electrochemical material [5]. It has been a subject of many experimental studies [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], as well as theoretical one [18], [19], [20], [21], [22], [23], [24], [25], its catalytic properties depend essentially on their ability to provide surface oxygens as a reactant in oxidation of hydrocarbons.

V₂O₅ has an orthorhombic structure. The unit cell contains 14 atoms, 4 vanadium and 10 oxygen atoms (Fig. 1). The positions of the atoms in the unit cell and the crystallographic parameters are given in Table 1. This compound has a layered structure composed of corner and edge sharing VO₅ square pyramids, alternatively pointing upward and downward in the [0 0 1] direction [6], with only weak Van der Waals type interaction between the layers. In the full crystal, the building block is a deformed octahedron (VO₆). Within each layer, one can distinguish V chains (VO₅) in the [0 1 0] direction connected by oxygen bridges in the [0 1 0] and [1 0 0] directions. The shortest bond length corresponds to double vanadyl bond (1.57 Å) and the longest one to weak Van der Waals bond (2.791 Å) in the [0 0 1] direction. Hence, V₂O₅ presents an easy cleavage plane in this direction. Three different kinds of oxygen atoms exist: the vanadyl oxygen on the top of the vanadium with double bond coordinated with one metal, the oxygen chain coordinated to two vanadium atoms O₂ (the index corresponds to oxygen atoms coordination) in the chain parallel to the vanadium, the bridging oxygens coordinated to three vanadium atoms O₃ making corner between the pyramids in the neighboring chains [7].

Periodic Hartree–Fock (HF) calculations for the bulk [18] have shown that V₂O₅ is essentially ionic, and the vanadyl oxygens have a covalent character. The atomic charges (Q) on the different oxygen centers decrease in the order: Q(O₃)>Q(O₂)>Q(O₁). The covalent character of the vanadyl oxygen has been reported experimentally [8], [9]. Yin et al. [19] with a periodic density functional study of the bulk and the (0 0 1) surface have shown that the charge increases in the order: Q(O₃)>Q(O₂)>Q(O₁) and they have suggested a large reactivity for the oxygen with the highest coordination. They have also reported a large reactivity for the O₁ site from projected density of states.

Electronic spin resonance spectroscopy and the infrared spectroscopy studies have indicated that the vanadyl O₁ [10] or the triply coordinated centers O₃ [11], play the role of the reactive centers at the (0 0 1) surface. An adsorption model deduced from V₂O₅ ($\text{0}\text{0}\text{1}$) surface images [12], [13] have suggested the important role of the O₂ site. The various experimental and theoretical results are controversial and do not give a clear picture of surface reactivity of vanadium pentoxide.

In this paper, we are interested by the relationship between the bulk and surface structure in the lamellar compounds, which present an easy cleavage plane. V₂O₅ is a prototype of lamellar structure and it is well known that it cleaves easily along the (0 0 1) plane due to the weak electrostatic forces between the (0 0 1) layers and it is generally accepted that the remaining bonds are not modified [17]. So the (0 0 1) surface remains identical to the parallel bulk plane. In order to validate this assumption we have performed quantum chemical calculations of the (0 0 1) surface with optimized bulk termination in the cluster approach. The validity of our original approach is discussed through the experimental and theoretical surface results.