Ternary Oxide Nanostructures: New Synthesis Routes and Nanoscale Properties

Oxide structures with nanometric dimensions on well-defined metal surfaces have novel physical and chemical properties, whose exploitation will require controlled fabrication. The focus of this project is on the preparation of low-dimensional ternary oxide MWOx (M denotes a transition metal: Cu, Ni, Fe, Mn), or so-called tungstate nanostructures, and the subsequent characterization of their properties. Particular interest will be in the effects of the dimension, scale and chemical composition of the MWOx nanostructures on their structural, electronic and vibrational properties. For the preparation of the ternary oxide nanolayers a new fabrication approach will be employed, which involves a two-dimensional solid state reaction process of (WO3 ) 3 cluster molecules, deposited from the gas phase, onto a well-ordered surface oxide MOx phase in ultrahigh vacuum. A major focus of the work will be to elucidate and control the physically relevant thermodynamic and kinetic parameters governing the solid state reaction process. The characterization of the structural, electronic and vibrational properties of the tungstate nanostructures will be performed in situ and with atomic precision using the full palette of modern surface science techniques. In the first phase of the project Cu-and Ni-tungstate nanolayers will be grown onto oxygen- reconstructed O- Cu(110) and O-Ni(110) surfaces, respectively. The FeO(111) bilayer on a Pt(111) surface will be then considered as a template for the construction of ordered FeWOx two-dimensional structures. To explore the possibility to create (quasi-)one-dimensional Mn-tungstate structures, the reaction of (WO3 ) 3 clusters with MnO2 nanowires and Mn 3 O4 nanostripes, supported on stepped Pd(100) surfaces will be investigated. The morphology and geometric structure of the various surfaces will be studied by combination of STM and LEED. The chemical nature of the tungstate overlayers will be determined by high-resolution XPS and NEXAFS measurements, whereas their electronic and vibrational structure will be derived from ARPES and HREELS data. To rationalize the atomic structure of the successfully prepared MWOx nanostructures density functional theory calculations will be performed in the theory group of Prof. Alessandro Fortunelli in Pisa. Particular structures will then be selected for detailed investigations of their formation mechanism and chemical reactivity.

The research project has been focussed on the controlled fabrication by physical vapour deposition of low-dimensional ternary oxide nanostructures on single crystal metal surfaces and the understanding of their novel physical and chemical properties at the atomic level using a combination of state-of-the-art surface science techniques and ab-initio density functional theory (DFT). The ternary oxides investigated belong to the family of metal tungstates (MWO4), which are functional materials with a high application potential in the modern nanotechnologies. A new preparation approach has been developed, utilizing the on-surface solid-state chemical reaction between two-dimensional (2D) binary metal (M) oxide and tungsten oxide phases on metal substrates to create various structurally well-ordered 2D metal tungstate layers. The latter have been found to support completely different building architectures, with little correspondence to the known bulk MWO4 structures, where both the M and W cations adopt an octahedral O coordination. Depending on the chemical potential of oxygen three different types of 2D MWOx nanolayers form, where W atoms exhibit threefold (e.g. FeWO3 on Pt(111)), fourfold (e.g. CuWO4 on Cu(110) and mixed four/sixfold (e.g. MnWOx on Pd(100)) coordination to O. Due to the thickness confinement at the atomic scale, the 2D MWOx layers exhibit physical and chemical properties not shared with their bulk counterparts, for example, a ferromagnetic ground state in honeycomb 2D FeWO3 layers on Pt(111), a high activity for methanol decomposition in 2D CuWO4 on Cu(110), or a faceting transition in NiWO4(100) layers on Ni(110), as a polarity compensation mechanism. Our model studies on 2D metal tungstates may open up exciting perspectives in the field of ternary oxides and could be transformative in creating new areas of research in more complex oxide systems.