Edge and facet atoms in graphene-supported nanoclusters

Nanometric atomic aggregates formed by tens to hundreds atoms, have properties remarkably different compared to their bulk crystalline counterparts. The unique size-dependent reactivity properties has long been recognized and used in heterogeneous catalysis. Many other domains in advanced technology have been attracted by the exotic physical and chemical properties of the nanocluster, the magnetism and energy storage being among them as well. The perspective of further developing cutting-edge materials for multiple applications has motivated a staggering interest in understanding and controlling the properties of atomic aggregates in the nanometer size range.

When the particle size is reduced to only few tens of Å, two main challenges are posed: the controllable and reproducible fabrication of large-scale arrays of clusters on a suitable substrate, and the determination of the atomic and electronic structure of the supported nanoclusters.
In order to meet the first challenge, we used epitaxial Graphene grown on Ir(111). Graphene has exceptional heat transport properties and an incomparably high room temperature electrical conductivity, and it is the strongest and thinnest material known: all these properties make it an excellent template for the growth and self-assembly of Rh nanoparticles.
The understanding of the relationship between the geometric and electronic structure of under-coordinated atoms in supported nanoclusters, and of their dependence on the cluster size, shape, and density, has been achieved by a multidisciplinary approach combining high-resolution core level spectroscopy, scanning tunneling microscopy and density functional theory simulations.
By carefully selecting the atomic Rh coverage and the annealing temperature, we have demonstrated the possibility to tune the morphology of small Rh cluster (see Figure 1a), and hence to determine the type and density of under-coordinated atoms, which are largely responsible for the nanocluster chemical activity and magnetic properties.
Notably, under particular growth condition, the graphene-supported nanoclusters exhibit a high degree of crystallinity. In fact, the Rh3d_5/2 core level spectra, measured at the SuperESCA beamline and obtained after low temperature Rh deposition and annealing to temperatures higher than 200 K (see Figure1b) show three distinct components. The higher binding energy peak (Rh_B) corresponds to the bulk component measured for Rh single crystals, and we therefore assign it to bulk-like atoms with a high coordination number C_N=12.
For high Rh coverages, the three components become much narrower, and a minimum becomes visible between the Rh_B and Rh_S photoemission peaks. The overall line-shape strongly resembles the one obtained for Rh single crystals. This suggests an evolution from clusters characterized by a wide range of locally non-equivalent atomic configurations, to a high structural order configuration, in which monodispersed particles expose well-defined nano-facets, mainly oriented along the {111} and {100} crystallographic planes.

Figure 1: (a) STM images (800×800 Å²) of Rh clusters grown on the Graphene/Ir(111). (b) Rh 3d_5/2 core level spectra measured at normal emission (T=90 K) corresponding to a Rh coverage of 0.37 ML annealed to increasing temperatures. Dark–blue, blue, and light–blue curves indicate Rh atoms with distinct local coordinations.

The results of the spectroscopic measurement turn out to be in very good agreement with the core level shift data (shown in Figure 2 as barcodes, superimposed on an experimental Rh 3d_5/2 spectrum measured at 0.37 ML Rh coverage) obtained from state of the art density functional theory calculations performed on clusters with distinct sizes and morphologies. The spectrum corresponding to a Rh(111) single crystal is also displayed for comparison. The structural models are reported in a color scale that reflects the binding energies of the Rh 3d_5/2 core levels.
The results clearly show that Rh atoms with a low coordination number typically have low BE values (Rh_Ecomponent). In particular, binding energies smaller than 306.4 eV are distinctive of edge atoms with C_N=7 or less.
Moreover the remarkably linear relationship between the core level shifts and the calculated d-band centre, which measures the reactivity of the nanoparticles, according to the Hammer-Nørskov model, highlights the efficiency of the synchrotron-based core level photoemission approach to probe the nanocluster electronic properties. The method we have used allows the detailed characterization of nanoscale fundamental properties, and paves the way to attractive technological applications of graphene-supported transition metals nanoclusters.

Figure 2: Rh 3d_5/2 spectra of Rh(111), 0.37 ML Rh on GR/Ir(111) annealed at 840 K. The barcodes in the bottom panel represent the calculated Rh 3d_5/2 binding energies for each of the 233 non–equivalent atomic configurations of the simulated Rh nanoclusters.

This research was conducted by the research teams of the Surface Science Laboratory and SuperESCA beamline of the Elettra Laboratory, in collaboration with researchers of the Physics Department, University of Trieste, the IOM and ISC of CNR and the University College London.

Alberto Cavallin, Alessandro Baraldi, Erik Vesselli, and Giovanni Comelli, Dipartimento di Fisica, Università di Trieste and IOM-CNR, Italy
Monica Pozzo, and Dario Alfè, Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, UK
Cristina Africh, and Carlo Dri, IOM-CNR Laboratorio TASC, Italy
Rosanna Larciprete, CNR-Institute for Complex Systems, Roma, Italy
Paolo Lacovig, and Silvano Lizzit, Sincrotrone Trieste, Italy

Reference

Alberto Cavallin, Monica Pozzo, Cristina Africh, Alessandro Baraldi, Erik Vesselli, Carlo Dri, Giovanni Comelli, Rosanna Larciprete, Paolo Lacovig, Silvano Lizzit, and Dario Alfè, Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template, ACSNANO 6, 4 3034 (2012), DOI: 10.1021/nn300651s

Last Updated on Thursday, 30 August 2012 15:33