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Vanadium pentoxide is a layered compound in which VO monolayers are held together by van der Waals forces. It is therefore possible, in principle, to exfoliate the material and form two-dimensional monolayers. Density functional theory is used to calculate the structure and the energy of vacancy formation for hypothetical, two-dimensional VO systems and compare them to the same properties of VO slabs. We study a two-dimensional sheet (infinite in two directions) and two ribbons (infinite in one direction) whose edges are perpendicular to the [100] or [001] directions. These edges undergo a substantial reconstruction. When an oxygen vacancy is formed, the formal charge of two vanadium atoms is reduced from 5+ to 4+. The energy of oxygen vacancy formation is higher for the two-dimensional structures than for the corresponding slabs (i.e. it is more difficult to remove oxygen from the edge of a ribbon perpendicular to [001] than from the (001) surface of a slab). Therefore, the two-dimensional structures are less aggressive oxidants than vanadium pentoxide powders. [ABSTRACT FROM AUTHOR]

A two-dimensional ternary oxide layer, Cu-tungstate (CuWO 4 ) on Cu(110), has been tested as a substrate for the adsorption of (WO 3 ) 3 cluster molecules from the gas phase. Scanning tunneling microscopy/spectroscopy and X-ray photoelectron spectroscopy have been used to characterize the adsorbed species, while density functional theory has scanned the energy landscape for stable adsorption modes. The (WO 3 ) 3 clusters adsorb in preferential adsorption sites at low temperature (85 K) as stable, only slightly perturbed units, as established both experimentally and theoretically. The preferred sites are the Cu surface rows in the [100] substrate direction, which feature elastic flexibility as a result of ultrasoft phonon modes and are a characteristic of this two-dimensional ternary oxide structure. [ABSTRACT FROM AUTHOR]

• High-performance H 2 S sensor was fabricated based on 2D MoO 3 nanoflakes. • New sensing mechanism is discovered based on chemical reversible transition between sulfidation and oxidation. • Sulfur substitution in MoO 3 reduces band gap and promotes electron mobility. H 2 S is a flammable, explosive, and highly toxic gas; consequently, a high-performance H 2 S sensor based on metal oxides is required. In this study, two-dimensional MoO 3 nanoflakes were synthesized through facile liquid-phase exfoliation. The thickness of the prepared MoO 3 is approximately 4.2 nm. High-performance was obtained using a gas sensor based on two-dimensional MoO 3 nanoflakes at a working temperature of 300 °C. In addition, the lowest detection limit can be in the range of parts per billion, and the sensor exhibits high resistance to humidity. The sensing mechanism is discussed based on the results of ex situ characterizations of the sensing process and computation of the electronic structure. The results indicate that the chemically reversible sensing process is based on the transition between sulfidation and oxidation, where sulfidation promotes the mobility of electrons and increases the conductivity of MoO 3. This novel sensing mechanism provides a new perspective for the study of H 2 S sensors based on metal oxides. [ABSTRACT FROM AUTHOR]