Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

Single-atom catalysis has arguably become the most active new frontier in heterogeneous catalysis. Aided by recent advances in practical synthetic methodologies, characterization techniques and computational modelling, we now have a large number of single-atom catalysts (SACs) that exhibit distinctive performances for a wide variety of chemical reactions. This Perspective summarizes recent experimental and computational efforts aimed at understanding the bonding in SACs and how this relates to catalytic performance. The examples described here illustrate the utility of SACs in a broad scope of industrially important reactions and highlight the advantages these catalysts have over those presently used. SACs have well-defined active centres, such that unique opportunities exist for the rational design of new catalysts with high activities, selectivities and stabilities. Indeed, given a certain practical application, we can often design a suitable SAC; thus, the field has developed very rapidly and afforded promising catalyst leads. Moreover, the control we have over certain SAC structures paves the way for designing base metal catalysts with the activities of noble metal catalysts. It appears that we are entering a new era of heterogeneous catalysis in which we have control over well-dispersed single-atom active sites whose properties we can readily tune. Single-atom catalysts are heterogeneous materials featuring active metals sites atomically dispersed on a surface. This Review describes methods by which we prepare and characterize these materials, as well as how we can tune their catalytic performance in a variety of important reactions.

Heterogeneous single-atom catalysis

A new strategy for achieving stable Co single atoms (SAs) on nitrogen-doped porous carbon with high metal loading over 4 wt % is reported. The strategy is based on a pyrolysis process of predesigned bimetallic Zn/Co metal–organic frameworks, during which Co can be reduced by carbonization of the organic linker and Zn is selectively evaporated away at high temperatures above 800 °C. The spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurements both confirm the atomic dispersion of Co atoms stabilized by as-generated N-doped porous carbon. Surprisingly, the obtained Co-Nx single sites exhibit superior ORR performance with a half-wave potential (0.881 V) that is more positive than commercial Pt/C (0.811 V) and most reported non-precious metal catalysts. Durability tests revealed that the Co single atoms exhibit outstanding chemical stability during electrocatalysis and thermal stability that resists sintering at 900 °C. Our findings open up a new routine for general and practical synthesis of a variety of materials bearing single atoms, which could facilitate new discoveries at the atomic scale in condensed materials.

Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts

The growth, crystal and electronic structures, and photo-electrochemical properties of undoped and Ti doped hematite epitaxial films were studied. We evidence that Ti4+ substitutes Fe3+ in the hematite lattice inducing a slight modification of the oxygen octahedron. Ti doping is shown to induce a shift of the valence band toward higher binding energy due to a movement of the Fermi level toward the conduction band. The resulting modification of electrical conductivity appears as a possible origin of the improvement of photo-electrochemical properties in the doped sample.

Enhanced photoanode properties of epitaxial Ti doped α-Fe2O3 (0001) thin films

A widespread approach to modulate the performances of heterogeneous catalysts is the use of bimetallic nanoparticles (NPs) as the active phase. However, studying the relationship between the NPs structure and catalytic properties requires well-defined systems, having uniform composition, size and nanostructure, which cannot be achieved by traditional methods (e.g. impregnation). Here, we developed wet-chemistry synthetic routes to prepare Pd Pt NPs or Pt-core@Pd-shell NPs of small size and well-controlled composition and structure, protected by mercaptoundecanoic acid (MUA) moieties. The pristine NPs were tested for H2 production by NH3BH3 hydrolysis, in order to systematically investigate the effect of composition and of synthetic route on the activity of the systems. Depending on the preparation method, two distinct trends of activity were observed, rationalized in terms of the extent of surface functionalization by MUA. The MUA protective layer was found to effectively stabilize the NPs dispersion while maintaining high activity in certain cases (Pt-rich NPs), and was demonstrated to be essential for catalyst recycling. In order to further study structure-activity relationships of Pd Pt NPs after ligand removal, nanostructured Pd Pt@CeO2-based catalysts were prepared by self-assembly route. Regardless of the starting NPs structure (alloy or core-shell), similar water gas shift reaction performances were observed, due to the structural rearrangements occurring upon oxidation and reduction thermal treatments, which led to the formation of Pt-rich core@Pd Pt-shell under reducing conditions.

Nanostructured PdPt nanoparticles: evidences of structure/performance relations in catalytic H2 production reactions

The structural and magnetic properties of cobalt nanowires embedded in epitaxial CeO(2) layers are investigated. Combining electron microscopy with x-ray absorption spectroscopy, we show that polycrystalline metallic Co nanowires are formed during the growth. These nanowires are oriented in average along the growth axis with a misalignment distribution, have a predominant hcp structure and a narrow distribution of diameters. Their magnetic properties are overall dominated by shape effects with an easy axis oriented along the mean direction of the wires axis. The reversal mechanism is studied in details for samples with 3 and 5 nm mean diameters. The competition between shape and magnetocrystalline anisotropies is evidenced at 3 nm diameter. Upon reduction in the lateral dimensions, the nanowires tend to behave like aligned Stoner-Wohlfarth elongated particles with effective uniaxial anisotropy resulting from this competition.

Magnetic response of cobalt nanowires with diameter below 5 nm

Cerium oxide (IV) (CeO2) is extensively employed in heterogeneous catalysis, particularly as a promoter of noble metal action in three-way catalysts. For this reason there is a great scientific and economical interest in the development of any possible chemical or structural analysis technique that could provide information on these systems. EXAFS spectroscopy has revealed itself as a powerful technique for structural characterization of such catalysts. Unfortunately, good quality K-edge spectra of cerium are not yet easily obtainable because of the high photon energy required (>40 keV). On the other hand, at lower energies it is easy to collect very good spectra of the L3 edge (5.5 keV), but L3-edge spectra of cerium (IV) are characterized by the presence of two undesired additional phenomena that interfere with EXAFS analysis: final-state mixed-valence behaviour and intense multi-electron excitations. Here, a comparative analysis of the K, L3, L2 and L1 edges of Ce in CeO2 has been made and a procedure for obtaining structural parameters from L3-edge EXAFS, even in the presence of these features, has been developed. This procedure could allow further studies of catalytic compounds containing tetravalent cerium surrounded by oxygen ligands.

EXAFS analysis of the L3 edge of Ce in CeO2: effects of multi‐electron excitations and final‐state mixed valence

The ferromagnetic response from Co-doped CeO(2) epilayers is shown to be linked to the formation of Co metallic nanowires (diameters in the 3-7 nm range) within the oxide matrix. These results (i) illustrate the key role of the shape anisotropy of metallic inclusions in diluted magnetic oxides (DMO), (ii) stress the importance to monitor the out-of-plane response of DMO, and (iii) suggest that some dilute systems previously thought to be intrinsic ferromagnet may be reexamined. (C) 2009 American Institute of Physics. [doi:10.1063/1.3250173]

Emiliano Fonda

Papers

Enhanced photoanode properties of epitaxial Ti doped α-Fe2O3 (0001) thin films

Nanostructured PdPt nanoparticles: evidences of structure/performance relations in catalytic H2 production reactions

Magnetic response of cobalt nanowires with diameter below 5 nm

EXAFS analysis of the L3 edge of Ce in CeO2: effects of multi‐electron excitations and final‐state mixed valence

Nanowires formation and the origin of ferromagnetism in a diluted magnetic oxide