Cuboctahedral Nanoparticles Research Articles

Carbon-supported shape-controlled Pt nanoparticle electrocatalysts for direct alcohol fuel cells

The demand for power sources alternative to fossil fuels makes urgent the development of more efficient electrocatalysts for fuel cells applications and the maximization of the performances of the existent ones. This work reports, for the first time, the use of carbon-supported shape-controlled Pt nanoparticles as anode catalysts in direct ethanol fuel cells. By using cubic Pt nanoparticles, on which (100) surface sites are predominant, the performance of the fuel cell can be increased from 14 to 24 mW per mg of Pt when compared with cuboctahedral nanoparticles. Moreover, the open circuit potential shifts about 50 mV toward more positive potentials. In comparison with commercially available Pt catalysts, the performance for the (100) preferentially oriented nanoparticles is about three times higher. The reported results evidence that, from an applied point of view, the effect of the surface structure/shape of the electrocatalysts can be also considered to improve the performance of real fuel cell systems.

Controlled synthesis of concave cuboctahedral nitrogen-rich metal–organic framework nanoparticles showing enhanced catalytic activation of epoxides with carbon dioxide

Nitrogen-rich metal–organic framework (N-MOF) nanoparticles with an unusual concave cuboctahedral morphology were prepared through a facile wet-chemical route using benzimidazole as the modulator. Benzimidazole plays an essential role in controlling the concave cuboctahedral morphology of the nanoparticles. By changing the benzimidazole dosage, the resultant morphology can be tuned from a concave octahedron to a concave cuboctahedron. The concave cuboctahedral nanoparticles exhibit high catalytic efficiency for the activation of epoxides with carbon dioxide.

Facile Preparation of Cuboctahedral Copper Nanoparticles and Their Surface Enhanced Raman Scattering Activity

Facile Preparation of Cuboctahedral Copper Nanoparticles and Their Surface Enhanced Raman Scattering Activity

Influence of surface segregation on magnetic properties of FePt nanoparticles

Surface segregation leads to chemical disordering in magnetic alloy nanostructures and thus could have profound impact upon the magnetic properties of these nanostructures. In this study, we used the first-principles density functional theory calculation method to determine how Pt surface segregation (exchanging interior Pt with surface Fe atoms) would affect the magnetic properties of L10 ordered FePt nanoparticles. For both cuboid and cuboctahedral FePt nanoparticles, we predicted that the Pt surface segregation process could cause a decrease in total magnetic moments, a change in (easy and/or hard) magnetization axes, and a reduction in magnetic anisotropy.

Increase of catalyst utilization in polymer electrolyte membrane fuel cells by shape-selected Pt nanoparticles

In the present work, we succeeded in supporting predominantly cuboctahedral Pt nanoparticles onto high surface area carbons while maintaining their shape. These novel catalysts were applied in a realistic fuel cell set-up for the first time and showed remarkable fuel cell performance. A 95% fraction of cuboctahedral Pt nanoparticles was synthesized using tetradecyltrimethylammonium bromide (TTAB) as a stabilizer. Transmission electron micrographs of the synthesized samples demonstrated the presence of monodispersed cuboctahedral particles of 12 nm in size. Cyclic voltammetry (CV) studies of the unsupported cuboctahedral nanoparticles revealed the presence of Pt (110) and (100) facets. The shape-selected Pt nanoparticles were let to absorb onto Vulcan carbon by a simple dispersing procedure to obtain supported shape-selected Pt nanoparticles. Only by this gentle adsorption step of the surfactant-stabilized nanoparticles on the carbonaceous support material, the nanoparticles retained their shape. Finally an MEA was fabricated using the supported shape-selected nanoparticles and tested in a realistic H2-PEM fuel cell environment. In terms of Pt utilization, shape-selected Pt particles were found to be more effective by a factor of four in weight compared to the commercial catalyst.

Thermally induced solid-solid structural transition of copper nanoparticles through direct geometrical conversion

Molecular dynamics simulations of small Cu nanoparticles using three different interatomic potentials at rising temperature indicate that small nanoparticles can undergo solid-solid structural transitions through a direct geometrical conversion route. The direct geometrical conversion can happen for cuboctahedral nanoparticles, which turn into an icosahedra shape: one diagonal of the square faces contracts, and the faces are folded along the diagonal to give rise to two equilateral triangles. The transition is a kinetic process that cannot be fully explained through an energetic point of view. It has low activation energy and fast reaction time in the simulations. The transition mechanism is via the transmission of shear waves initiated from the particle surface and does not involve dislocation activity.

Relative stability of icosahedral and cuboctahedral metallic nanoparticles

The size and form of metallic nanoparticles (NPs) significantly affects their adsorptive, chemical, and catalytic activity. One of the most interesting nanoscale size effects is the transition from icosahedral to octahedral forms with growth in the NP size. We compared the stability of icosahedral, decahedral and cuboctahedral NPs made from eight metals Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au using the local optimization of total energy, which was computed from the tight-binding second moment approximation and quantum Sutton–Chen potentials. The obtained results predicted that the icosahedral form would be most stable for Ni, and least stable for Au. For Rh, and especially for Ir, a strong dependency of the stability of the different forms on the NP size was revealed.

Electrochemical ammonia production on molybdenum nitride nanoclusters

Theoretical investigations of electrochemical production of ammonia at ambient temperature and pressure on nitrogen covered molybdenum nanoparticles are presented. Density functional theory calculations are used in combination with the computational hydrogen electrode approach to calculate the free energy profile for electrochemical protonation of N2 and N adatoms on cuboctahedral Mo13 nanoparticles. Pathways for electrochemical ammonia production via direct protonation of N adatoms and N2 admolecules with an onset potential as low as -0.5 V and generally lower than -0.8 V on both a nitrogen covered or a clean Mo nanoparticle. Calculations presented here show that nitrogen dissociation at either nitrogen vacancies on a nitrogen covered molybdenum particle or at a clean molybdenum particle is unlikely to occur under ambient conditions due to very high activation barriers of 1.8 eV. The calculations suggest that the nitrogen will be favored at the surface compared to hydrogen even at potentials of -0.8 V and the Faradaic losses due to HER should be low.

Electronic and magnetic properties of bimetallicL10cuboctahedral clusters by means of fully relativistic density-functional-based calculations

By means of density functional theory (DFT) and the generalized gradient approximation (GGA) we present a structural, electronic and magnetic study of FePt, CoPt, FeAu and FePd based L1$_0$ ordered cuboctahedral nanoparticles, with total numbers of atoms, N$_{tot}$ = 13, 55, 147. After a conjugate gradient relaxation, the nanoparticles retain their L1$_0$ symmetry, but the small displacements of the atomic positions tune the electronic and magnetic properties. The value of the total magnetic moment stabilizes as the size increases. We also show that the Magnetic Anisotropy Energy (MAE) depends on the size as well as the position of the Fe-atomic planes in the clusters. We address the influence on the MAE of the surface shape, finding a small in-plane MAE for (Fe,Co)$_{24}$Pt$_{31}$ nanoparticles.

Modelling electrified interfaces in quantum chemistry: constant charge vs. constant potential

The proper description of electrified metal/solution interfaces, as they occur in electrochemical systems, is a key component for simulating the unique features of electrocatalytic reactions using electronic structure calculations. While in standard solid state (plane wave, periodic boundary conditions) density functional theory (DFT) calculations several models for describing electrochemical environments exist, for cluster models in a quantum chemistry approach (atomic orbital basis, finite system) this is not straightforward. In this work, two different approaches for the theoretical description of electrified interfaces of nanoparticles, the constant charge and the constant potential model, are discussed. Different schemes for describing electrochemical reactions including solvation models are tested for a consistent description of the electrochemical potential and the local chemical behavior for finite structures. The different schemes and models are investigated for the oxygen reduction reaction (ORR) on a hemispherical cuboctahedral platinum nanoparticle.

Ab initiodetermination of structure-property relationships in alloy nanoparticles

I present a computational study on atomic order in 2-nm cuboctahedral Au-Pd nanoparticles. Equilibrium atomic structures, energies, and electronic surface $d$-band centers have been calculated across the entire range of compositions at different temperatures using a Bayesian approach to cluster expansions. The estimated prediction error in formation energies calculated by the cluster expansion, relative to density functional theory, is approximately 1 meV/atom. This prediction error would be low for a cluster expansion on a bulk material, and it is exceptionally low for a study of nanoparticles of this size. This result was accomplished by extending the Bayesian approach for cluster expansions to account for nonlocal, composition-dependent effects that might otherwise not be captured. For this system the Bayesian approach is estimated to be approximately five times as efficient as more common cluster selection techniques.

A Comprehensive Search for Stable Pt–Pd Nanoalloy Configurations and Their Use as Tunable Catalysts

Using density-functional theory, we predict stable alloy configurations (ground states) for a 1 nm Pt-Pd cuboctahedral nanoparticle across the entire composition range and demonstrate their use as tunable alloy catalysts via hydrogen-adsorption studies. Unlike previous works, we use simulated annealing with a cluster expansion Hamiltonian to perform a rapid and comprehensive search that encompasses both high and low-symmetry configurations. The ground states show Pt(core)-Pd(shell) type configurations across all compositions but with specific Pd patterns. For catalysis studies at room temperatures, the ground states are more realistic structural models than the commonly assumed random alloy configurations. Using the ground states, we reveal that the hydrogen adsorption energy increases (decreases) monotonically with at. % Pt for the {111} hollow ({100} bridge) adsorption site. Such trends are useful for designing tunable Pd-Pt nanocatalysts for the hydrogen evolution reaction.

Unknown Aspects of Self-Assembly of PbS Microscale Superstructures

A lot of interesting and sophisticated examples of nanoparticle (NP) self-assembly (SA) are known. From both fundamental and technological standpoints, this field requires advancements in three principle directions: (a) understanding the mechanism and driving forces of three-dimensional (3D) SA with both nano- and microlevels of organization; (b) understanding disassembly/deconstruction processes; and (c) finding synthetic methods of assembly into continuous superstructures without insulating barriers. From this perspective, we investigated the formation of well-known star-like PbS superstructures and found a number of previously unknown or overlooked aspects that can advance the knowledge of NP self-assembly in these three directions. The primary one is that the formation of large seemingly monocrystalline PbS superstructures with multiple levels of octahedral symmetry can be explained only by SA of small octahedral NPs. We found five distinct periods in the formation PbS hyperbranched stars: (1) nucleation of early PbS NPs with an average diameter of 31 nm; (2) assembly into 100-500 nm octahedral mesocrystals; (3) assembly into 1000-2500 nm hyperbranched stars; (4) assembly and ionic recrystallization into six-arm rods accompanied by disappearance of fine nanoscale structure; (5) deconstruction into rods and cuboctahedral NPs. The switches in assembly patterns between the periods occur due to variable dominance of pattern-determining forces that include van der Waals and electrostatic (charge-charge, dipole-dipole, and polarization) interactions. The superstructure deconstruction is triggered by chemical changes in the deep eutectic solvent (DES) used as the media. PbS superstructures can be excellent models for fundamental studies of nanoscale organization and SA manufacturing of (opto)electronics and energy-harvesting devices which require organization of PbS components at multiple scales.

Cuboctahedral Pd Nanoparticles on Tungsten Carbide for Improved Electrocatalytic Activity in DMFCs

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Influence of the Shape of Nanostructured Metal Surfaces on Adsorption of Single Peptide Molecules in Aqueous Solution

Self-assembly and function of biologically modified metal nanostructures depend on surface-selective adsorption; however, the influence of the shape of metal surfaces on peptide adsorption mechanisms has been poorly understood. The adsorption of single peptide molecules in aqueous solution (Tyr(12) , Ser(12) , A3, Flg-Na(3) ) is investigated on even {111} surfaces, stepped surfaces, and a 2 nm cuboctahedral nanoparticle of gold using molecular dynamics simulation with the CHARMM-METAL force field. Strong and selective adsorption is found on even surfaces and the inner edges of stepped surfaces (-20 to -60 kcal/mol peptide) in contrast to weaker and less selective adsorption on small nanoparticles (-15 to -25 kcal/mol peptide). Binding and selectivity appear to be controlled by the size of surface features and the extent of co-ordination of epitaxial sites by polarizable atoms (N, O, C) along the peptide chain. The adsorption energy of a single peptide equals a fraction of the sum of the adsorption energies of individual amino acids that is characteristic of surface shape, epitaxial pattern, and conformation constraints (often β-strand and random coil). The proposed adsorption mechanism is supported and critically evaluated by earlier sequence data from phage display, dissociation constants of small proteins as a function of nanoparticle size, and observed shapes of peptide-stabilized nanoparticles. Understanding the interaction of single peptides with shaped metal surfaces is a key step towards control over self-organization of multiple peptides on shaped metal surfaces and the assembly of superstructures from nanostructures.

Electro-oxidation of COchem on Pt Nanosurfaces: Solution of the Peak Multiplicity Puzzle

An understanding of the oxidation of chemisorbed CO (CO(chem)) on Pt nanoparticle surfaces is of major importance to fuel cell technology. Here, we report on the relation between Pt nanoparticle surface structure and CO(chem) oxidative stripping behavior. Oxidative stripping voltammograms are obtained for CO(chem) preadsorbed on cubic, octahedral, and cuboctahedral Pt nanoparticles that possess preferentially oriented and atomically flat domains. They are compared to those obtained for etched and thermally treated Pt(poly) electrodes that possess atomically flat, ordered surface domains separated by grain boundaries as well as those obtained for spherical Pt nanoparticles. A detailed analysis of the results reveals for the first time the presence of up to four voltammetric features in CO(chem) oxidative stripping transients, a prepeak and three peaks, that are assigned to the presence of surface domains that are either preferentially oriented or disordered. The interpretation reported in this article allows one to explain all features within the voltammograms for CO(chem) oxidative stripping unambiguously.

Oxygen Reduction Reaction Activity of Shape Controlled Pt Catalysts

In order to clarify the effect of surface atomic structure on oxygen reduction reaction (ORR) activity for Pt/C catalysts, cubic and cuboctahedral Pt nanoparticles (NPs) colloidal solutions were prepared from K2PtCl4 solutions using sodium polyacrylate (PAA) as a capping reagent, and furthermore the shape-controlled Pt NPs were deposited on a Ketjen Black support. The cubic and cuboctahedral Pt/C catalysts (Ptcubic/C and Ptcuboctahedral/C) gave characteristic H2 absorption/desorption peaks in the CV curves attributed to their crystal facets. The Ptcuboctahedral/C having the (100) and (111) faces showed higher ORR activity than a commercially available polycrystalline Pt/C and the Ptcubic/C composed of mainly (100) facet.

Concentration effects on segregation behavior of Pt-Rh nanoparticles

Based on the first-principles calculation combined with the cluster expansion technique and the Monte Carlo statistical simulation, concentration dependence of segregation behavior for Pt-Rh cuboctahedral nanoparticles is quantitatively examined. Several ground-state atomic arrangements for the nanoparticles at $T=0\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ are predicted where Pt atoms prefer surface sites, particularly, at vertex sites with the lowest coordination number. We find strong Pt segregation at the surfaces of nanoparticles. Concentration dependence of segregation behavior for the nanoparticle is essentially different from that of the bulk Pt-Rh surface. Temperature dependence of segregation in the nanoparticles only appears for Pt composition at around 30%--70% due to the finite number of constituent atoms and to substantial Pt segregation at the surface. Warren-Cowley short-range-order parameters for vertex-edge and vertex-(100) pairs change their signs with respect to the Pt composition. This can be attributed to the significant Pt segregation at the vertex site, which should disrupt the tendency for the preference of unlike-atom pairs found at low Pt compositions. We find that the magnitude relationship of the Pt composition for surface sites is significantly affected by Pt composition in the nanoparticle.

Stability and Electronic Structures of Pt-Rh Icosahedral Nanoparticles

We investigate the energetic stability and electronic structures of Pt-Rh icosahedral nanoparticles based on first-principles calculations. We find that Pt atom energetically prefers vertex and edge sites at the surface rather than subsurface and core sites, which is a tendency similar to Pt-Rh cuboctahedral nanoparticles. This can be attributed to lower surface energy of Pt compared with Rh. Edges of nanoparticles with second-lowest coordination number are the most favorable sites for Pt atom. This could be attributed to the enhanced interatomic distance at the edge site. 1st order moment of d-state electronic contribution for Pt atom exhibits almost negative dependence in terms of Pt coordination number. This can be qualitatively interpreted by positive dependence of 2nd order moment of the density of states for the Pt atom on the coordination number. Pt surface segregation is expected mainly due to contribution from Pt on-site segregation energy compared with weak ordering tendency of Pt-Rh unlike-atom pairs.

Shape-Controlled Platinum Nanoparticles of Different Sizes and Their Electrochemical Properties

Platinum (Pt) nanoparticles with controlled shapes and sizes were prepared from a solution of K2PtCl4 in the presence of polyacrylic acid (PAA) as a capping reagent. The effect of temperature, pH, and PAA concentration on the shape, size, and stability of the nanoparticles were investigated. Cubic Pt nanoparticles of different sizes in the range of 4.4–14 nm and cuboctahedral nanoparticles of an average size of 14 nm were obtained. The cubic and cuboctahedral Pt nanoparticles showed strong features of Pt {1 0 0} faces and Pt {1 0 0} and Pt {1 1 1} faces, respectively, on their cyclic voltammograms under argon atmosphere, respectively. Rotating ring disk electrode measurements in O2-saturated H2SO4 solution revealed that the catalytic activity for oxygen reduction reaction (ORR) of cubic Pt nanoparticles (average size 10 nm) was higher than that of cuboctahedral Pt nanoparticles (average size 14 nm). The activity for ORR of both cubic and cuboctahedral Pt nanoparticles decreased slightly after polycrystallization by repeated potential cycling in the range of 0.05 to 1.4 V.