Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal

Abstract

In situ band gap mapping of the V 2 O 5 001 crystal surface revealed a reversible metal-to-insulator transition at 350-400 K, which occurs inhomogeneously across the surface and expands preferentially in the direction of the vanadyl (V O) double rows. Supported by density functional theory and Monte Carlo simulations, the results are rationalized on the basis of the anisotropic growth of vanadyloxygen vacancies and a concomitant oxygen loss driven metal-to-insulator transition at the surface. At elevated temperatures irreversible surface reduction proceeds sequentially as V 2 O 5 001 ! V 6 O 13 001 ! V 2 O 3 0001. DOI: 10.1103/PhysRevLett.99.226103 PACS numbers: 68.47.Gh, 68.35.ÿp, 73.20.ÿr Vanadium oxides represent an important class of materials with high potential in many technological applications based on their diverse temperature-dependent electronic, magnetic, and catalytic properties (e.g., [1][2][3][4] and references therein). There is a large variety of the vanadium oxide phases such as VO (V 2 , rocksalt structure),Depending on the ambient conditions and temperature, phase transformations between these oxides can occur. They may involve the formation of mixed valence phases. In addition, several vanadium oxides undergo metal-to-insulator transitions (MIT), e.g., V 2 O 3 at 150 K, VO 2 at 340 K. These complex structural and electronic transformations may play a crucial role in the behavior of vanadia-based systems.Among the surface structures of vanadium oxides [4], the V 2 O 5 001 surface seems to be the most studied [3,[5][6][7][8][9][10][11][12]. This surface exposes vanadyl (V O) double rows along the [010] direction [ Fig. 1(a)], which were observed also with scanning tunneling microscopy (STM) [10,11] and atomic force microscopy [12]. Surprisingly, the V O termination has been found also for the most stable, (0001) surface of V 2 O 3 [4,13,14], whereas the VO 2 110 surface appears to follow the respective bulk termination [1,4,15,16].Since the thermally induced transformations of vanadium oxides may be initiated at the surface and even be restricted to the surface layers, it is important to study their surface structures in the early stages of transitions when several structures may coexist. In this respect, STM combined with scanning tunneling spectroscopy (STS) can provide direct information on both geometrical and electronic structures of oxide surfaces (see, e.g., [17] ). We here report thermally induced reconstructions observed by STM/STS on a V 2 O 5 001 single crystal surface. Data show that V 2 O 5 , which is not known for the MIT in the bulk, exhibits, however, the MIT at the surface which proceeds through the formation and anisotropic growth of vanadyl-oxygen vacancies and is followed by an irreversible surface reduction. The experimental findings are supported by density functional theory (DFT) and

Selectivity in Methanol Oxidation as Studied on Model Systems Involving Vanadium Oxides

“…[21]. Figure 1a shows a scanning tunneling microscopy image of a V 2 O 5 (001) single crystal surface revealing the double rows of vanadyl-oxygen atoms protruding out of the surface of the V 2 O 5 (001) sheets which are held together by van der Waals forces along the (001) direction as expected from the bulk structure schematically represented in Fig.…”

“…The setups are described in the literature. The V 2 O 5 (001) single crystal surfaces were prepared by cleavage in UHV using adhesive tape [21].…”

“…Specifically, we will report changes of the surface structure of a V 2 O 5 (001) single crystal surface upon heat treatment. The experimental studies are paralleled by theoretical investigations using electronic structure calculations and Monte Carlo simulations [21].…”

Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

“…[21,23,27,28,38,39,50]), we aim to understand the role of ceria as a support in the ODH of methanol.…”

“…To elucidate the ODH reaction mechanism over vanadia on a molecular level, different model systems have been invoked that range from single crystals [22,23] and thin films prepared on TiO 2 [24][25][26], W(110) [27] , Au(111) [27][28][29][30][31] , Rh(111) [32][33][34] , and Cu 3 Au(100) [35][36][37] to vanadia particles deposited onto oxide substrates [21,38,39] including ceria [40][41][42]. In addition to experimental investigations, density functional theory h as been employed to explore the structural characteristics and reactivity towards methanol of model vanadia systems [43][44][45][46][47][48].…”