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Density functional calculations are carried out for the (0 0 0 1) surface of V2O3 in the corundum structure. In thermal equilibrium with the bulk, the dominant surface termination is characterised by vanadyl (VO) groups adsorbed on the (0 0 0 1) surface. Under increased oxygen pressure, the calculations predict that a pure oxygen termination is stable, whereas under very oxygen poor conditions the removal of the oxygen from the VO group can result in a stoichiometric metal termination. The electronic states in the valence band regime, the oxygen and vanadium core-level binding energies, and the vibrational spectra are calculated for the bulk and the surface by density functional theory and compared to recent experimental studies. [Copyright &y& Elsevier]

Well-ordered epitaxial V2O3(0 0 0 1) films (thickness > 80 A˚) have been prepared on the clean Rh(1 1 1) surface and investigated with scanning tunnelling microscopy, low-energy electron diffraction, high-resolution electron energy loss spectroscopy and high-resolution X-ray photoelectron spectroscopy with synchrotron radiation. Atomically flat V2O3(0 0 0 1) surfaces, terminated by vanadyl (V&z.dbnd6;O) groups, have been obtained after the reactive oxide deposition and subsequent annealing in vacuum to 600 °C. Annealing the V&z.dbnd6;O termination in oxygen atmosphere (500 °C, 5 × 10−6 mbar) results in the partial removal of the vanadyl groups and in the formation of an oxygen-richer surface termination, exhibiting a (√3×√3)R30° structure. Structure models for this oxygen-rich termination are proposed, which are characterised by (O&z.dbnd6;V)0.66–O3–V–V⋯ and (O&z.dbnd6;V)0.33–O3–V–V⋯ stacking sequences. Conversely, deposition of submonolayer V coverages onto the V&z.dbnd6;O surface leads to the formation of a metal-rich V2O3(0 0 0 1) surface, terminated by a close-packed V3 layer on top of the bulk-type O3 plane. Such a VO(1 1 1)-layer is only metastable and oxidises back readily upon annealing in vacuum to the vanadyl-terminated V2O3(0 0 0 1) surface. [Copyright &y& Elsevier]

Abstract: The growth, morphology and epitaxial relationship of Ag nanostructures deposited onto the c(4×2)-NiO x /Pd(100) surface have been investigated by photoemission (both core and valence levels), scanning tunneling microscopy and angle-scanned photoelectron diffraction. Small Ag nanoparticles are obtained on the terraces, whereas the tendency of Ag to decorate the step edges can lead to the formation of extended nanowire-like features. The Ag nanoparticles adopt a fcc structure epitaxially related to the substrate according to the following relationship: Ag(100)[001]//Pd(100)[001], while by means of STM we have found that the shape of the islands is typically rectangular with the edges running along the [011] directions. According to the reported data, the NiO x ML is locally disrupted in the vicinity of the Ag nanoparticles so that the c(4×2)-NiO x /Pd(100) surface cannot be considered as an efficient template to stabilize metal nanoparticles against Ostwald ripening phenomena. [Copyright &y& Elsevier]

The growth morphology of ultrathin (up to 5 ML) vanadium oxide films on TiO2(1 1 0) has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). It has been found that the reactive evaporation technique produces more uniform and better-ordered vanadia layers than the post-oxidation method. At low coverages V-oxide clusters adsorb on top of the fivefold-coordinated Ti rows of the substrate. With increasing coverage the clusters agglomerate and form strands, which are oriented along the [0 0 1] titania direction. For oxide coverage >2 ML the strands cover uniformly the titania substrate, forming a texture along the [0 0 1] direction, and give rise to a (1 × 1) LEED pattern. The latter is consistent with the growth of an epitaxial rutile-type VO2 phase. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

X-ray photoelectron diffraction (XPD) has been applied to the study of the surface-(s)-V2O3 (2 × 2) layer on Pd(1 1 1), which is a novel interface-stabilised vanadium oxide phase with no bulk oxide counterpart. It has been detected by scanning tunnelling microscopy (STM) during the growth of ultrathin films of vanadium oxide on Pd(1 1 1). XPD confirms the general features of the model for s-V2O3/Pd(1 1 1), which has been proposed previously on the basis of STM measurements and ab initio density-functional-theory (DFT) calculations. In addition, quantitative agreement is found between the DFT model and the XPD experiment in the estimate of the average V–O interlayer spacing: the experimental result is 0.72±0.07 A˚, while the DFT-derived value is 0.723 A˚. [Copyright &y& Elsevier]

W- and Mo-oxides form an interesting class of materials, featuring structural complexities, stoichiometric flexibility, and versatile physical and chemical properties that render them attractive for many applications in diverse fields of nanotechnologies. In nanostructured form, novel properties and functionalities emerge as a result of quantum size and confinement effects. In this topical review, W- and Mo-oxide nanosystems are examined with particular emphasis on two-dimensional (2D) layers and small molecular-type clusters. We focus on the epitaxial growth of 2D layers on metal single crystal surfaces and investigate their novel geometries and structures by a surface science approach. The coupling between the oxide overlayer and the metal substrate surface is a decisive element in the formation of the oxide structures and interfacial strain and charge transfer are shown to determine the lowest energy structures. Atomic structure models as determined by density functional theory (DFT) simulations are reported and discussed for various interface situations, with strong and weak coupling. Free-standing (quasi-)2D oxide layers, so-called oxide nanosheets, are attracting a growing interest recently in the applied research community because of their easy synthesis via wet-chemical routes. Although they consist typically of several atomic layers thick-not always homogeneous-platelet systems, their quasi-2D character induces a number of features that make them attractive for optoelectronic, sensor or biotechnological device applications. A brief account of recently published preparation procedures of W- and Mo-oxide nanosheets and some prototypical examples of proof of concept applications are reported here. (MO 3 ) 3 (M = W, Mo) clusters can be generated in the gas phase in nearly monodisperse form by a simple vacuum sublimation technique. These clusters, interesting molecular-type structures by their own account, can be deposited on a solid surface in a controlled way and be condensed into 2D W- and Mo-oxide layers; solid-state chemical reactions with pre-deposited surface oxide layers to form 2D ternary oxide compounds (tungstates, molybdates) have also been reported. The clusters have been proposed as model systems for molecular studies of reactive centres in catalytic reactions. Studies of the catalysis of (MO 3 ) 3 clusters in unsupported and supported forms, using the conversion of alcohols as model reactions, are discussed. Finally, we close with a brief outlook of future perspectives., (Creative Commons Attribution license.)

Mixed CuO(2 × 1)-CuWO 4 layers on a Cu(110) surface have been prepared by the on-surface reaction of the CuO(2 × 1) surface oxide with adsorbed (WO 3 ) 3 clusters. The adsorption and decomposition of methanol on these well-defined CuO-CuWO 4 surfaces has been followed by high-resolution X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and temperature-programmed desorption (TPD) to assess the molecular surface species and their concentration, while the state of the surface oxide phases before and after methanol decomposition has been characterized by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and XPS. Surface methoxy species form the primary methanol decomposition products, which desorb partly by recombination as methanol at 200-300 K or decompose into CH x and possibly CO. The most reactive surfaces are mixed CuO-CuWO 4 phase, with CuWO 4 coverages 0.5-0.8 monolayer, thus pointing at the importance of oxide phase boundary sites. In a minority reaction channel, a small amount of formaldehyde is detected on the CuWO 4 surface. The CuWO 4 oxide phase becomes modified as a result of reduction and a morphology transition triggered by the methanol decomposition, but the pristine surface state can be recovered by a postoxidation treatment with oxygen.

The structure and properties of ternary oxide materials at the nanoscale are poorly explored both on experimental and theoretical levels. With this work we demonstrate the successful on-surface synthesis of two-dimensional (2D) ternary oxide, MnWO x and FeWO x , nanolayers on a Pd(1 0 0) surface and the understanding of their new structure and phase behaviour with the help of state-of-art surface structure and spectroscopy techniques. We find that the 2D MnWO x and FeWO x phases, prepared under identical thermodynamic conditions, exhibit similar structural properties, reflecting the similarity of the bulk MnWO 4 and FeWO 4 phases with the wolframite structure. Structure models of prototypical 2D ternary oxide phases are proposed and discussed in the light of new structure architecture concepts which have no analogues in the bulk.

An ultrathin two-dimensional CeO2 (ceria) phase on a Cu(110) surface has been fabricated and fully characterized by high-resolution scanning tunneling microscopy, photoelectron spectroscopy, and density functional theory. The atomic lattice structure of the ceria/Cu(110) system is revealed as a hexagonal CeO2(111)-type monolayer separated from the Cu(110) surface by a partly disordered Cu-O intercalated buffer layer. The epitaxial coupling of the two-dimensional ceria overlayer to the Cu(110)-O surface leads to a nanoscopic stripe pattern, which creates defect regions of quasi-periodic lattice distortions. The symmetry and lattice mismatch at the interface is clarified to be responsible for the topographic stripe geometry and the related anisotropic strain defect regions at the ceria surface. This ceria monolayer is in a fully oxidized and thermodynamically stable state.

The growth morphology and structure of ceria nano-islands on a stepped Au(788) surface has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Within the concept of physical vapor deposition, different kinetic routes have been employed to design ceria-Au inverse model catalysts with different ceria nanoparticle shapes and arrangements. A two-dimensional superlattice of ceria nano-islands with a relatively narrow size distribution (5 ± 2 nm²) has been generated on the Au(788) surface by the postoxidation method. This reflects the periodic anisotropy of the template surface and has been ascribed to the pinning of ceria clusters and thus nucleation on the fcc domains of the herringbone reconstruction on the Au terraces. In contrast, the reactive evaporation method yields ceria islands elongated in [01-1] direction, i.e. , parallel to the step edges, with high aspect ratios (~6). Diffusion along the Au step edges of ceria clusters and their limited step crossing in conjunction with a growth front perpendicular to the step edges is tentatively proposed to control the ceria growth under reactive evaporation conditions. Both deposition recipes generate two-dimensional islands of CeO₂(111)-type O-Ce-O single and double trilayer structures for submonolayer coverages.

Manipulation of chemistry and film growth via external electric fields is a longstanding goal in surface science. Numerous systems have been predicted to show such effects but experimental evidence is sparse. Here we demonstrate in a custom-designed UHV apparatus that the application of spatially extended, homogeneous, very high (>1 V nm(-1)) DC-fields not only changes the system energetics but triggers dynamic processes which become important much before static contributions appreciably modify the potential energy landscape. We take a well characterized ultrathin NiO film on a Ag(100) support as a proof-of-principle test case, and show how it gets reduced to supported Ni clusters under fields exceeding the threshold of +0.9 V nm(-1). Using an effective model, we trace the observed interfacial redox process down to a dissociative electron attachment resonant mechanism. The proposed approach can be easily implemented and generally applied to a wide range of interfacial systems, thus opening new opportunities for the manipulation of film growth and reaction processes at solid surfaces under strong external fields.

Metal tungstates (with general formula MWO4) are functional materials with a high potential for a diverse set of applications ranging from low-dimensional magnetism to chemical sensing and photoelectrocatalytic water oxidation. For high level applications, nanoscale control of film growth is necessary, as well as a deeper understanding and characterization of materials properties at reduced dimensionality. We succeeded in fabricating and characterizing a two-dimensional (2-D) copper tungstate (CuWO4). For the first time, the atomic structure of an ultrathin ternary oxide is fully unveiled. It corresponds to a CuWO4 monolayer arranged in three sublayers with stacking O-W-O/Cu from the interface. The resulting bidimensional structure forms a robust framework with localized regions of anisotropic flexibility. Electronically it displays a reduced band gap and increased density of states close to the Fermi level with respect to the bulk compound. These unique features open a way for new applications in the field of photo- and electrocatalysis, while the proposed synthesis method represents a radically new and general approach toward the fabrication of 2-D ternary oxides.

A bottom-up approach to produce a long-range ordered superlattice of monodisperse and isomorphic metal-oxide nanoparticles (NP) supported onto an oxide substrate is demonstrated. The synthetic strategy consists of self-assembling metallic NP on an ultrathin nanopatterned aluminum oxide template followed by a morphology-conserving oxidation process, and is exemplified in the case of Ni, but is generally applicable to a wide range of metallic systems. Both fully oxidized and core-shell metal-metal-oxide particles are synthesized, up to 3-4 nm in diameter, and characterized via spectroscopic and theoretical tools. This opens up a new avenue for probing unit and ensemble effects on the properties of oxide materials in the nanoscale regime.

Distinct one-dimensional (1D) oxide nanowires decorating the step edges of a stepped Pd(1 1 9) surface are formed by partial and complete oxidation of a 1D Mn-Pd alloy. Under full postoxidation treatment at 470-570 K, 1D MnO(2) nanowires coupled pseudomorphically to the Pd steps are obtained. Oxidized nanowires, which maintain the basic structural pattern of the 1D Mn-Pd alloy, are instead prepared by exposure of the Mn-Pd alloy to O(2) at 90 K and subsequent short heating to 400 K. A relatively weak Mn-O bonding characterizes these oxidized alloy wires, which are readily reduced by reaction with CO at moderate temperature (350 K). The here reported system emphasizes the influence of kinetic constraints in the formation of oxide nanostructures.