Local electronic structure and density of edge and facet atoms at Rh nanoclusters self-assembled on a graphene template

Unearthing Atomic Dynamics in Nanocatalysts

Being able to access the rich atomic-scale phenomenology, which occurs during the reactions pathways, is a pressing need toward the pursued knowledge-based design of more efficient nanocatalysts, precisely tailored atom by atom for each reaction. However, to reach this goal of achieving maximum optimization, it is mandatory, first, to address how exposure to the experimental conditions, which will be needed to activate the processes, affects the internal configuration of the nanoparticles at the atomic level. In particular, the most critical experimental parameter is probably the temperature, which among other unwanted effects can induce nanocatalyst aggregation. This work highlights the high potential of experimental techniques such as the scanning probe microscopies, which are able to investigate matter in real space with atomic resolution, to reach the key challenge in heterogeneous catalysis of achieving access to the atomic-scale processes taking place in the nanocatalysts. Specifically, the phenomenology occurring in a nanoparticle system during annealing is studied with atomic precision by scanning tunneling microscopy. As a result, the existence of an internal atomic restructuring, occurring already at relatively low temperatures, within Ir nanoparticles grown over h-BN/Ru(0001) surfaces is demonstrated. Such restructuration, which reduces the undercoordination of the outer Ir atoms, is expected to have a significant effect on the reactivity of the nanoparticles. Going a step further, an internal restructuring of the nanoparticles during their involvement as catalysts has also been also identified.

Highly Ordered and Thermally Stable FeRh Cluster Superlattice on Graphene for Low-Temperature Catalytic CO Oxidation

We report on bimetallic FeRh clusters with a narrow size-distribution grown on graphene on Ir(111) as a carbon-supported model catalyst to promote low-temperature catalytic CO oxidation. By combining scanning tunneling microscopy with catalytic performance measurements, we reveal that Fe-Rh interfaces are active sites for oxygen activation and CO oxidation, especially at low temperatures. Rh core Fe shell clusters not only provide the active sites for the reaction, but also thermally stabilize surface Fe atoms towards coarsening compared with pure Fe clusters. Alternate isotope-labelled CO/O₂ pulse experiments show opposite trends on preferential oxidation (PROX) performance because of surface hydroxyl species formation and competitive adsorption between CO and O₂ . The present results introduce a general strategy to stabilize metallic clusters and to reveal the reaction mechanisms on bimetallic structures for low-temperature catalytic CO oxidation as well as preferential oxidation.

Single-Phase Formation of Rh2 O3 Nanoparticles on h-BN Support for Highly Controlled Methane Partial Oxidation to Syngas

Single-phase formation of active metal oxides on supports has been vigorously pursued in many catalytic applications to suppress undesired reactions and to determine direct structure-property relationships. However, this is difficult to achieve in nanoscale range because the effect of non-uniform metal-support interfaces becomes dominant in the overall catalyst growth, leading to the nucleation of various metastable oxides. Herein, we develop a supported single-phase corundum-Rh₂ O₃ (I) nanocatalyst by utilizing controlled interaction between metal oxide and h-BN support. Atomic-resolution electron microscopy and first-principle calculation reveal that single-phase formation occurs via uniform and preferential attachment of Rh₂ O₃ (I) (110) seed planes on well-defined h-BN surface after decomposition of rhodium precursor. By utilizing the Rh/h-BN catalyst in methane partial oxidation, syngas is successfully produced solely following the direct route with keeping a H₂ /CO ratio of 2, which makes it ideal for most downstream chemical processes.

Promoted activity of annealed Rh nanoclusters on thin films of Al2O3/NiAl(100) in the dehydrogenation of Methanol-d4

Annealed Rh nanoclusters on an ordered thin film of Al2O3/NiAl(100) were shown to exhibit a promoted reactivity toward the decomposition of methanol-d4, under both ultrahigh vacuum and near-ambient-pressure conditions. The Rh clusters were grown with vapor deposition onto the Al2O3/NiAl(100) surface at 300 K and annealed to 700 K. The decomposition of methanol-d4 proceeded only through dehydrogenation, with CO and deuterium as products, on Rh clusters both as prepared and annealed. Nevertheless, the catalytic reactivity of the annealed clusters, measured with the production of either CO or deuterium per surface Rh site from the reaction, became at least 2-3 times that of the as-prepared ones. The promoted reactivity results from an altered support effect associated with an annealing-induced mass transport at the surface. Our results demonstrate a possibility to practically prepare reactive Rh clusters, regardless of the cluster size, that can tolerate an elevated reaction temperature, with no decreased reactivity.

Atomic Undercoordination in Ag Islands on Ru(0001) Grown via Size-Selected Cluster Deposition: An Experimental and Theoretical High-Resolution Core-Level Photoemission Study

The possibility of depositing precisely mass-selected Ag clusters (Ag₁, Ag₃, and Ag₇) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d_5/2 core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag₇ deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level.

Tuning of 2D Nanographene Adlayers on Au(111) by Electrodeposition of Metal Halide Complexes

The electrodeposition of AuBr₄^- and PtBr₄^2- onto an adlayer of circobiphenyl-a structurally defined nanographene with low symmetry-on a Au(111) electrode was investigated via electrochemical scanning tunneling microscopy (EC-STM) to control and understand the formation of characteristic nanoclusters. By immersing a circobiphenyl-coated Au(111) substrate in a 0.1 mM aqueous AuBr₄^- solution, AuBr₄^- was spontaneously reduced, and a characteristic mixed adlayer consisting of circobiphenyl molecules and Br^- ions with monatomic Au islands was produced on the Au(111) surface. A similar electrodeposition process was performed in an aqueous solution of PtBr₄^2-, and an identical mixed adlayer was obtained with Pt nanoclusters. The electrodeposition of Au and Pt complexes was facilitated by the "negatively charged" reconstructed Au(111) surface, which is stabilized by the formation of a highly ordered circobiphenyl adlayer. EC-STM revealed the formation of characteristic dimers of Pt clusters ranging 2-4 nm in diameter on the circobiphenyl adlayer. Thus, Br^- metal complexes were found to play an important role in controlling the structure and size of a mixed adlayer containing Br^- and the shape of Pt clusters.

Translucency of Graphene to van der Waals Forces Applies to Atoms/Molecules with Different Polar Character

Graphene has been proposed to be either fully transparent to van der Waals interactions to the extent of allowing switching between hydrophobic and hydrophilic behavior, or partially transparent (translucent), yet there has been considerable debate on this topic, which is still ongoing. In a combined experimental and theoretical study we investigate the effects of different metal substrates on the adsorption energy of atomic (argon) and molecular (carbon monoxide) adsorbates on high-quality epitaxial graphene. We demonstrate that while the adsorption energy is certainly affected by the chemical composition of the supporting substrate and by the corrugation of the carbon lattice, the van der Waals interactions between adsorbates and the metal surfaces are partially screened by graphene. Our results indicate that the concept of graphene translucency, already introduced in the case of water droplets, is found to hold more generally also in the case of single polar molecules and atoms, which are apolar.

Recent Progress in Graphene-Based Noble-Metal Nanocomposites for Electrocatalytic Applications

The fast industrialization process has led to global challenges in the energy crisis and environmental pollution, which might be solved with clean and renewable energy. Highly efficient electrochemical systems for clean-energy collection require high-performance electrocatalysts, including Au, Pt, Pd, Ru, etc. Graphene, a single-layer 2D carbon nanosheet, possesses many intriguing properties, and has attracted tremendous research attention. Specifically, graphene and graphene derivatives have been utilized as templates for the synthesis of various noble-metal nanocomposites, showing excellent performance in electrocatalytic-energy-conversion applications, such as the hydrogen evolution reaction and CO₂ reduction. Herein, the recent progress in graphene-based noble-metal nanocomposites is summarized, focusing on their synthetic methods and electrocatalytic applications. Furthermore, some personal insights on the challenges and possible future work in this research field are proposed.

Tailoring Intercalant Assemblies at the Graphene-Metal Interface

The influence of graphene on the assembly of intercalated material is studied using low-temperature scanning tunneling microscopy. Intercalation of Pt under monolayer graphene on Pt(111) induces a substrate reconstruction that is qualitatively different from the lattice rearrangement induced by metal deposition on Pt(111) and, specifically, the homoepitaxy of Pt. Alkali metals Cs and Li are used as intercalants for monolayer and bilayer graphene on Ru(0001). Atomically resolved topographic data reveal that at elevated alkali metal coverage (2 × 2)Cs and (1 × 1)Li intercalant structures form with respect to the graphene lattice.

Exciting vibrons in both frontier orbitals of a single hydrocarbon molecule on graphene

Vibronic excitations in molecules are key to the fundamental understanding of the interaction between vibrational and electronic degrees of freedom. In order to probe the genuine vibronic properties of a molecule even after its adsorption on a surface appropriate buffer layers are of paramount importance. Here, vibrational progression in both molecular frontier orbitals is observed with submolecular resolution on a graphene-covered metal surface using scanning tunnelling spectroscopy. Accompanying calculations demonstrate that the vibrational modes that cause the orbital replica in the progression share the same symmetry as the electronic states they couple to. In addition, the vibrational progression is more pronounced for separated molecules than for molecules embedded in molecular assemblies. The entire vibronic spectra of these molecular species are moreover rigidly shifted with respect to each other. This work unravels intramolecular changes in the vibronic and electronic structure owing to the efficient reduction of the molecule-metal hybridization by graphene.

Pseudo-ordered distribution of Ir nanocrystals on h-BN

A 2D material consisting of a pseudo-ordered distribution of Ir nanocrystals supported on a h-BN/Rh(111) surface is presented here. The particular spatial distribution of the Ir nanoparticles is achieved thanks to the existence of a large variety of adsorption positions within the pores of the h-BN/Rh(111) nanomesh template with hexagonal symmetry. The resulting deviations of nanoparticle positions with respect to a perfect hexagonal lattice, which make this material of special interest in the field of optics, can be tuned by the temperature and the amount of Ir. Upon annealing, this material undergoes slight structural changes in the temperature range of 370-570 K and much more drastic ones, due to cluster coalescence, between 670 and 770 K. This relatively high onset of coalescence is encouraging for using this 2D material as a catalyst for reactions such as the oxidation of carbon monoxide or of nitrogen monoxide, which are especially relevant in the field of environmental science. Finally, metal nanostructures exhibiting regular geometries have been created from this material using a scanning tunneling microscope tip. Because of the insulating character of h-BN, these nanostructures could be very promising to use in the design of conductive nanotracks.

Preventing sintering of nanoclusters on graphene by radical adsorption

Metal nanoclusters, supported on inert substrates, exhibiting well-defined shapes and sizes in a broad range of temperatures are a major object of desire in nanotechnology. Here, a technique is presented that improves the thermal stability of monodisperse and crystalline transition metal nanoclusters grown in a regular array on metal-supported graphene. To stabilize the clusters after growth under ultrahigh vacuum the system composed of the aggregates and the graphene/metal interface is exposed to radicals resulting from the dissociation of diatomic gases. As a model system we have used Pt as the metal element for cluster growth and the template consisting of the moiré pattern resulting from the lattice mismatch between graphene and the Ir(111) surface. The study has been performed for deuterium and oxygen radicals, which interact very differently with graphene. Our results reveal that after radical exposure the thermally activated motion of Pt nanoclusters to adjacent moiré cells and the subsequent sintering of neighbor aggregates are avoided, most pronounced for the case of atomic O. For the case of D the limits of the improvement are given by radical desorption, whereas for the case of O they are defined by an interplay between coalescence and graphene etching followed by Pt intercalation, which can be controlled by the amount of exposure. Finally, we determined the mechanism of how radical adsorption improves the thermal stability of the aggregates.

Template Effect of the Graphene Moiré Lattice on Phthalocyanine Assembly

Superstructures of metal-free phthalocyanine (2H-Pc) molecules on graphene-covered Ir(111) have been explored by scanning tunnelling microscopy. Depending on the sub-monolayer coverage different molecular assemblies form at the surface. They reflect the transition from a graphene template effect on the 2H-Pc arrangement to molecular superstructures that are mainly governed by the intermolecular coupling.

Experimental and Theoretical Investigation of the Restructuring Process Induced by CO at Near Ambient Pressure: Pt Nanoclusters on Graphene/Ir(111)

The adsorption of CO on Pt nanoclusters grown in a regular array on a template provided by the graphene/Ir(111) Moiré was investigated by means of infrared-visible sum frequency generation vibronic spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy from ultrahigh vacuum to near-ambient pressure, and ab initio simulations. Both terminally and bridge bonded CO species populate nonequivalent sites of the clusters, spanning from first to second-layer terraces to borders and edges, depending on the particle size and morphology and on the adsorption conditions. By combining experimental information and the results of the simulations, we observe a significant restructuring of the clusters. Additionally, above room temperature and at 0.1 mbar, Pt clusters catalyze the spillover of CO to the underlying graphene/Ir(111) interface.

Dependence on size of supported Rh nanoclusters for CO adsorption

The adsorption and lateral interactions of CO molecules on Rh nanoclusters supported on an ordered thin film of Al₂O₃/NiAl(100) altered with the size of the Rh clusters.

Graphene growth and properties on metal substrates

Graphene-metal interface as one of the interesting graphene-based objects attracts much attention from both application and fundamental science points of view. This paper gives a timely review of the recent experimental works on the growth and the electronic properties of the graphene-metal interfaces. This work makes a link between huge amount of experimental and theoretical data allowing one to understand the influence of the metallic substrate on the electronic properties of a graphene overlayer and how its properties can be modified in a controllable way. The further directions of studies and applications of the graphene-metal interfaces are discussed.

Synergistic effects of a novel free-standing reduced graphene oxide film and surface coating fibronectin on morphology, adhesion and proliferation of mesenchymal stem cells

Fabrication of free-standing reduced graphene oxide (RGO) films by vacuum filtration of graphene oxide aqueous solution through a nanofiber membrane in combination with chemical reduction.