Carlo Dri

Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template

The chemical and physical properties of nanoclusters largely depend on their sizes and shapes. This is partly due to finite size effects influencing the local electronic structure of the nanocluster atoms which are located on the nanofacets and on their edges. Here we present a thorough study on graphene-supported Rh nanocluster assemblies and their geometry-dependent electronic structure obtained by combining high-energy resolution core level photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. We demonstrate the possibility to finely control the morphology and the degree of structural order of Rh clusters grown in register with the template surface of graphene/Ir(111). By comparing measured and calculated core electron binding energies, we identify edge, facet, and bulk atoms of the nanoclusters. We describe how small interatomic distance changes occur while varying the nanocluster size, substantially modifying the properties of surface atoms. The properties of under-coordinated Rh atoms are discussed in view of their importance in heterogeneous catalysis and magnetism.

Reverse Water–Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure

The interaction of CO, CO2, CO + H2, CO2 + H2, and CO + CO2 + H2 with the nickel (110) single crystal termination has been investigated at 10(-1) mbar in situ as a function of the surface temperature in the 300-525 K range by means of infrared-visible sum frequency generation (IR-vis SFG) vibrational spectroscopy and by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Several stable surface species have been observed and identified. Besides atomic carbon and precursors for graphenic C phases, five nonequivalent CO species have been distinguished, evidencing the role of coadsorption effects with H and C atoms, of H-induced activation of CO, and of surface reconstruction. At low temperature, carbonate species produced by the interaction of CO2 with atomic oxygen, which stems from the dissociation of CO2 into CO + O, are found on the surface. A metastable activated CO2(-) species is also detected, being at the same time a precursor state toward dissociation into CO and O in the reverse water-gas shift mechanism and a reactive species that undergoes direct conversion in the Sabatier methanation process. Finally, the stability of ethylidyne is deduced on the basis of our spectroscopic observations.

Temperature-Driven Changes of the Graphene Edge Structure on Ni(111): Substrate vs Hydrogen Passivation

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 °C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges.

Reactivity of Carbon Dioxide on Nickel: Role of CO in the Competing Interplay between Oxygen and Graphene

The catalytic conversion of carbon dioxide to synthetic fuels and other valuable chemicals is an issue of global environmental and economic impact. In this report we show by means of X-ray photoelectron spectroscopy in the millibar range that, on a Ni surface, the reduction of carbon dioxide is indirectly governed by the CO chemistry. While the growth of graphene and the carbide-graphene conversion can be controlled by selecting the reaction temperature, oxygen is mainly removed by CO, since oxygen reduction by hydrogen is a slow process on Ni. Even though there is still a consistent pressure gap with respect to industrial reaction conditions, the observed phenomena provide a plausible interpretation of the behavior of Ni/Cu based catalysts for CO2 conversion and account for a possible role of CO in the methanol synthesis process.

Trapping of Charged Gold Adatoms by Dimethyl Sulfoxide on a Gold Surface.

We report the formation of dimethyl sulfoxide (DMSO) molecular complexes on Au(111) enabled by native gold adatoms unusually linking the molecules via a bonding of ionic nature, yielding a mutual stabilization between molecules and adatom(s). DMSO is a widely used polar, aprotic solvent whose interaction with metal surfaces is not fully understood. By combining X-ray photoelectron spectroscopy, low temperature scanning tunneling microscopy, and density functional theory (DFT) calculations, we show that DMSO molecules form complexes made by up to four molecules arranged with adjacent oxygen terminations. DFT calculations reveal that most of the observed structures are accurately reproduced if, and only if, the negatively charged oxygen terminations are linked by one or two positively charged Au adatoms. A similar behavior was previously observed only in nonstoichiometric organic salt layers, fabricated using linkage alkali atoms and strongly electronegative molecules. These findings suggest a motif for anchoring organic adlayers of polar molecules on metal substrates and also provide nanoscale insight into the interaction of DMSO with gold.

Effects of lattice expansion on the reactivity of a one-dimensional oxide.

By means of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we characterize at the single-atom level the mechanism of the water formation reaction on the (10 x 2)-O/Rh(110) surface, a prototype of a one-dimensional (1D) oxide where the lattice expansion and the segmentation of the surface play a fundamental role. When the reaction is imaged in the 238-263 K temperature range (35 s/image acquisition time), a peculiar comblike propagation mechanism for the reaction front is found. Fast STM measurements (33 ms/image) prove that this mechanism holds also at room temperature, being therefore an intrinsic characteristic of the reaction on the 1D oxide. DFT calculations explain the observed behavior as due to the interplay between the lattice expansion in the initial surface and its relaxation during the reaction that leads to varying configurations for the reactants. At low temperatures, the reaction produces, in its final stages, a low-coverage, ordered patterning of the surface with residual oxygen. The pattern formation is related to the segmentation of the oxide phase.

Tailoring Bimetallic Alloy Surface Properties by Kinetic Control of Self-Diffusion Processes at the Nanoscale

Achieving control of the nanoscale structure of binary alloys is of paramount importance for the design of novel materials with specific properties, leading to, for example, improved reaction rates and selectivity in catalysis, tailored magnetic behavior in electronics, and controlled growth of nanostructured materials such as graphene. By means of a combined experimental and theoretical approach, we show that the complex self-diffusion mechanisms determining these key properties can be mostly defined by kinetic rather than energetic effects. We explain how in the Ni-Cu system nanoscale control of self-diffusion and segregation processes close to the surface can be achieved by finely tuning the relative concentration of the alloy constituents. This allows tailoring the material functionality and provides a clear explanation of previously observed effects involved, for example, in the growth of graphene films and in the catalytic reduction of carbon dioxide.

Real-time imaging of adatom-promoted graphene growth on nickel

Watching graphene grow The growth of graphene on metal surfaces can be catalyzed by mobile surface metal atoms. Patera et al. used a high-speed scanning tunneling microscope to image the growth of graphene islands on a nickel surface. High temperatures caused carbon to diffuse to the surface, where mobile nickel atoms catalyzed graphene growth on the edges of islands. Molecular dynamics and density functional theory calculations provide mechanistic insights into the reaction steps. Science, this issue p. 1243 Individual Ni atoms catalyzing the growth of a graphene flake along its edges were imaged on the millisecond time scale. Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

Multi-orbital charge transfer at highly oriented organic/metal interfaces

The molecule–substrate interaction plays a key role in charge injection organic-based devices. Charge transfer at molecule–metal interfaces strongly affects the overall physical and magnetic properties of the system, and ultimately the device performance. Here, we report theoretical and experimental evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules adsorbed on Cu(100). The exceptional charge transfer leads to filling of the higher unoccupied orbitals up to LUMO+3. As a consequence of this strong interaction with the substrate, the porphyrin’s macrocycle sits very close to the surface, forcing the phenyl ligands to bend upwards. Due to this adsorption configuration, scanning tunneling microscopy cannot reliably probe the states related to the macrocycle. We demonstrate that photoemission tomography can instead access the Ni-TPP macrocycle electronic states and determine the reordering and filling of the LUMOs upon adsorption, thereby confirming the remarkable charge transfer predicted by density functional theory calculations.Charge transfer at molecule-metal interfaces affects the overall physical and magnetic properties of organic-based devices, and ultimately their performance. Here, the authors report evidence of a pronounced charge transfer involving nickel tetraphenyl porphyrin molecules adsorbed on copper.

Electronic properties of the boroxine–gold interface: evidence of ultra-fast charge delocalization

We performed a combined experimental and theoretical study of the assembly of phenylboronic acid on the Au(111) surface, which is found to lead to the formation of triphenylboroxines by spontaneous condensation of trimers of molecules. The interface between the boroxine group and the gold surface has been characterized in terms of its electronic properties, revealing the existence of an ultra-fast charge delocalization channel in the proximity of the oxygen atoms of the heterocyclic group. More specifically, the DFT calculations show the presence of an unoccupied electronic state localized on both the oxygen atoms of the adsorbed triphenylboroxine and the gold atoms of the topmost layer. By means of resonant Auger electron spectroscopy, we demonstrate that this interface state represents an ultra-fast charge delocalization channel. Boroxine groups are among the most widely adopted building blocks in the synthesis of covalent organic frameworks on surfaces. Our findings indicate that such systems, typically employed as templates for the growth of organic films, can also act as active interlayers that provide an efficient electronic transport channel bridging the inorganic substrate and organic overlayer.