Interaction of Na with 2D WO₃ and MoO₃ Layers on Pd(100): From Doping to 2D Bronze Formation

Abstract

Doping of tungsten trioxide (WO 3 ) and molybdenum trioxide (MoO 3 ) materials with alkali atoms, leading to the formation of the so-called sodium bronzes, is a viable approach to achieve a precise control of their electronic, optical, and magnetic properties via electron band structure engineering. Driven by the ongoing trend for thickness reduction and the resulting new functionalities at the nanoscale, using a combination of state-of-the-art experimental and computational techniques, we investigate here the interaction of two isostructural two-dimensional (2D) WO 3 and MoO 3 layers, grown epitaxially onto a Pd(100) surface, with Na dopants. We identify two interaction regimes as a function of the Na coverage: a low-coverage regime up to 0.3 ML, which we describe in terms of doping interactions, and a reaction regime, where at higher Na coverages, the 2D WO 3 /MoO 3 lattices become destroyed and several ordered 2D bronze-type phases form upon thermal activation. In the doping regime, Na initially decorates the oxide domain boundaries and later adsorbs in a (2 × 2) superstructure, filling the regular adsorption sites within the oxide domains. Further Na accommodation in the 2D oxide lattice is unfavorable due to the poor lateral electrostatic screening and elastic strain increase. In the reaction regime, the most prominent and energetically stable phase is the hexagonal 2D bronze-like layer, whose atomic details are resolved in a density functional theory (DFT) analysis and compared with the structure of the bulk counterpart.

Tungsten and molybdenum oxide nanostructures: two-dimensional layers and nanoclusters

“…A hand waving argument of the higher 3d binding energy of these latter Mo atoms may be that these atoms at the boundaries are coordinatively unsaturated, and electron density is withdrawn yielding a higher binding energy. DFT calculations have established a cubic 2D MoO 2 + O sheet model, similar to the one for WO 3 /Pd (100), as the stable structure for this c(2 × 2) MoO 3 phase [145]. It is noted parenthetically that the MoO 2 + O sheet model may also be derived by suitable cuts from an idealized cubic α-MoO 3 structure.…”

“…The investigation of structural phase transitions in 2D W(Mo)O 3 layers, e.g. from a quasi-cubic to a hexagonal structure as a function of some thermodynamic variables, which has been suggested on the basis of recent DFT calculations [145], may be another area of experimental activity. Experimental studies of the charge transfer to/from W(Mo) oxide clusters adsorbed on 2D oxide layers, which has been predicted theoretically [197], is an interesting topic, since both structure and chemical reactivity of the clusters depend strongly on their charge state.…”

“…The effects of the dimensionality reduction from 3D to 2D on the physical and chemical properties are of importance in view of the technological trend towards thickness reduction of active oxide layers and further miniaturization in green energy nanodevices. For example, the doping behaviour of the oxides in the 2D limit is of interest, and first studies of doping 2D W-oxides with alkali metals are currently underway [145].…”

Structure and Stoichiometry Self-Organization in a Mixed Vanadium–Iron Oxide Honeycomb Film on Ru(0001)

“…While Co and Fe are immiscible in the bulk, oxide films with variable Co/Fe ratios and unexpected hydroxylation properties can be prepared on an Au(111) support . The structure and stoichiometry of 2D tungsten bronzes (CuWO 3 , NaWO 3 , and FeWO 3 ) were found to deviate strongly from their 3D analogues. , While Mo and Ca crystallize in powellite (CaMoO 4 ) and perovskite structures (CaMoO 3 ) in the bulk, a number of specific 2D phases could be stabilized on Mo(001) . Ultrathin Fe silicate films on Ru(0001) undergo a spontaneous phase separation into Fe- and Si-rich layers .…”

Evolution of 2D MoO3 layer structures on Pd(100): interplay of interfacial interactions and polarity

“…At the 2D limit, most of the atoms are at the surface and therefore the material properties are more sensitive to surface chemistry and the underlying substrate, and strongly influenced by point and extended defects. Among the many well-defined 2D oxide layers grown on metallic substrates in the last decades, ultrathin films of MoO 3 have been epitaxially grown on different single crystal metal substrates, including Au(111) [9][10][11][12] and Pd(100) [13]. The MoO 3 monolayer (ML) exhibits well-ordered commensurate structures (c(4 × 2) on Au(111), and c(2 × 2) on Pd(100)), which significantly differ from their bulk α-MoO 3 counterpart, and were interpreted within the framework of a MoO 2 + O model [10,12].…”