Adsorption of Ethylene on Neutral, Anionic and Cationic Gold Clusters

The adsorption of ethylene molecule on neutral, anionic and cationic gold clusters consisting of up to 10 atoms has been investigated using density-functional theory. It is demonstrated that C2H4 can be adsorbed on small gold clusters in two different configurations, corresponding to the pi- and di-sigma-bonded species. Adsorption in the pi-bonded mode dominates over the di-sigma mode over all considered cluster sizes n, with the exception of the neutral C2H4-Au5 system. A striking difference is found in the size-dependence of the adsorption energy of C2H4 bonded to the neutral gold clusters in the pi and di-sigma configurations. The important role of the electronic shell effects in the di-sigma mode of ethylene adsorption on neutral gold clusters is demonstrated. It is shown that the interaction of C2H4 with small gold clusters strongly depends on their charge. The typical shift in the vibrational frequencies of C2H4 adsorbed in the pi- and the di-sigma configurations gives a guidance to experimentally distinguish between the two modes of adsorption.Comment: 30 pages, 10 figure

Evolution of electronic and ionic structure of Mg-clusters with the growth cluster size

The optimized structure and electronic properties of neutral and singly charged magnesium clusters have been investigated using ab initio theoretical methods based on density-functional theory and systematic post-Hartree-Fock many-body perturbation theory accounting for all electrons in the system. We have systematically calculated the optimized geometries of neutral and singly charged magnesium clusters consisting of up to 21 atoms, electronic shell closures, binding energies per atom, ionization potentials and the gap between the highest occupied and the lowest unoccupied molecular orbitals. We have investigated the transition to the hcp structure and metallic evolution of the magnesium clusters, as well as the stability of linear chains and rings of magnesium atoms. The results obtained are compared with the available experimental data and the results of other theoretical works.Comment: 30 pages, 10 figures, 3 table

Impurity effects on the melting of Ni clusters

We demonstrate that the addition of a single carbon impurity leads to significant changes in the thermodynamic properties of Ni clusters consisting of more than a hundred atoms. The magnitude of the change induced is dependent upon the parameters of the Ni-C interaction. Hence, thermodynamic properties of Ni clusters can be effectively tuned by the addition of an impurity of a particular type. We also show that the presence of a carbon impurity considerably changes the mobility and diffusion of atoms in the Ni cluster at temperatures close to its melting point. The calculated diffusion coefficients of the carbon impurity in the Ni cluster can be used for a reliable estimate of the growth rate of carbon nanotubes.Comment: 27 pages, 13 figure

Structure and properties of small sodium clusters

We have investigated structure and properties of small metal clusters using all-electron ab initio theoretical methods based on the Hartree-Fock approximation and density functional theory, perturbation theory and compared results of our calculations with the available experimental data and the results of other theoretical works. We have systematically calculated the optimized geometries of neutral and singly charged sodium clusters having up to 20 atoms, their multipole moments (dipole and quadrupole), static polarizabilities, binding energies per atom, ionization potentials and frequencies of normal vibration modes. Our calculations demonstrate the great role of many-electron correlations in the formation of electronic and ionic structure of small metal clusters and form a good basis for further detailed study of their dynamic properties, as well as structure and properties of other atomic cluster systems.Comment: 47 pages, 16 figure

Lithiation of Silicon Anode based on Soft X-ray Emission Spectroscopy: A Theoretical Study

Due to its exceptional lithium storage capacity silicon is considered as a promising candidate for anode material in lithium-ion batteries (LIBs). In the present work we demonstrate that methods of the soft X-ray emission spectroscopy (SXES) can be used as a powerful tool for the comprehensive analysis of the electronic and structural properties of lithium silicides Li$_{x}$Si forming in LIB's anode upon Si lithiation. On the basis of density functional theory (DFT) and molecular dynamics (MD) simulations it is shown that coordination of Si atoms in Li$_{x}$Si decreases with increase in Li concentration both for the crystalline and amorphous phases. In amorphous a-Li$_{x}$Si alloys Si tends to cluster forming Si-Si covalent bonds even at the high lithium concentration. It is demonstrated that the Si-L$_{2,3}$ emission bands of the crystalline and amorphous Li$_{x}$Si alloys show different spectral dependencies reflecting the process of disintegration of Si-Si network into Si clusters and chains of the different sizes upon Si lithiation. The Si-L$_{2,3}$ emission band of Li$_{x}$Si alloys become narrower and shifts towards higher energies with an increase in Li concentration. The shape of the emission band depends on the relative contribution of the X-ray radiation from the Si atoms having different coordination. This feature of the Si-L$_{2,3}$ spectra of Li$_{x}$Si alloys can be used for the detailed analysis of the Si lithiation process and LIB's anode structure identification

Atomically Thin Hexagonal Boron Nitride Nanofilm for Cu Protection: The Importance of Film Perfection

Coatings are routinely applied to protect metallic surfaces, and polymer coatings have been conventionally used where the thickness is not a dramatic issue.[1] For the next generation of nanoelectronics, nanoscale coatings are needed to accommo-date the compact design. 2D materials that can be fabricated into atomically thin film as a coating over the substrate can be a great choice. Graphene has recently been considered for this purpose, since it is robust and flexible, and the hexagonal hon-eycomb structure can effectively block any species, including helium.[2] Mixed results, however, have been reported.[3-7] Good short-term anti-corrosion performance was observed,[3-5] but over time, accelerated Cu oxidation and corrosion in air were found in the presence of graphene compared to the bare Cu substrate.[8,9] This acceleration is likely due to the high con-ductivity that assists electron transfer in the two-component galvanic cell between Cu and graphene, facilitating oxygen reduction and Cu oxidation around the defects in the long run

Boron-induced transformation of ultrathin Au films into two-dimensional metallic nanostructures

The synthesis of large, freestanding, single-atom-thick two-dimensional (2D) metallic materials remains challenging due to the isotropic nature of metallic bonding. Here, we present a bottom-up approach for fabricating macroscopically large, nearly freestanding 2D gold (Au) monolayers, consisting of nanostructured patches. By forming Au monolayers on an Ir(111) substrate and embedding boron (B) atoms at the Au/Ir interface, we achieve suspended monoatomic Au sheets with hexagonal structures and triangular nanoscale patterns. Alternative patterns of periodic nanodots are observed in Au bilayers on the B/Ir(111) substrate. Using scanning tunneling microscopy, X-ray spectroscopies, and theoretical calculations, we reveal the role of buried B species in forming the nanostructured Au layers. Changes in the Au monolayer’s band structure upon substrate decoupling indicate a transition from 3D to 2D metal bonding. The resulting Au films exhibit remarkable thermal stability, making them practical for studying the catalytic activity of 2D gold

A computational investigation of H2 adsorption and dissociation on Au nanoparticles supported on TiO2 surface

The specific role played by small gold nanoparticles supported on the rutile TiO2(110) surface in the processes of adsorption and dissociation of H2 is discussed. It is demonstrated that the molecular and dissociative adsorption of H2 on Au_[n] clusters containing n = 1, 2, 8 and 20 atoms depends on cluster size, geometry structure, cluster flexibility and the interaction with the support material. Rutile TiO2(110) support energetically promotes H2 dissociation on gold clusters. It is demonstrated that the active sites towards H2 dissociation are located at corners and edges on the surface of the gold nanoparticle in the vicinity of the support. The low coordinated oxygen atoms on the TiO2(110) surface play a crucial role for H2 dissociation. Therefore the catalytic activity of a gold nanoparticle supported on the rutile TiO2(110) surface is proportional to the length of the perimeter interface between the nanoparticle and the support

Oxygen Reduction Reaction Catalyzed by Small Gold Cluster on h-BN/Au(111) Support

The catalytic activity for the oxygen reduction reaction (ORR) of a hexagonal boron nitride (h-BN) monolayer deposited on a Au(111) surface and decorated by a small planar Au-8 cluster has been studied theoretically using density-functional theory. It is shown that gold nanoparticles (Au-NP) deposited on the h-BN/Au(111) surface can provide catalytically active sites for effective ORR at the perimeter interface with the support. Stabilization of oxygen at the perimeter interface between Au-NP and h-BN/Au(111) support promotes OOH* dissociation opening effective 4-electron pathway of ORR with formation of H2O. It is suggested that increase in the perimeter interface area between the supported Au-NP and the surface would result in increase of the ORR activity. Such increase in the perimeter interface area can be achieved by decreasing the size of Au-NP. Our calculations demonstrate the principal ability to functionalize inert materials such as stand-alone h-BN monolayer or Au surface for the ORR and open new way to design effective Pt-free catalysts for fuel cell technology

Catalytic Activity of Au and Au2 on the h-BN Surface: Adsorption and Activation of O2

The structural, electronic, and catalytic properties of Au and Au2 supported on the pristine and defected hexagonal boron nitride (h-BN) surface have been studied theoretically using density functional theory. It is demonstrated that adsorption and catalytic activation of O2 on the h-BN supported Au and Au2 can be affected by the interaction with the support via electron pushing and donor/acceptor mechanisms. It is shown that even weak interaction of Au and Au2 with the defect-free "inert" h-BN surface can have an unusually strong influence on the binding and catalytic activation of the molecular oxygen. This effect occurs due to the mixing of the 5d orbitals of the supported Au and Au2 with the N-pz orbitals. Although the defect-free h-BN surface does not act as a good electron donor for the supported O2-Au, it promotes an electron transfer from the Au to O2, pushing electrons from the gold to the adsorbed oxygen. In the case of the defected h-BN surface, Au and Au2 can be trapped effectively by N or B vacancy and impurity point defects. Strong adsorption on the surface defects is accompanied by the large charge transfer to/from the adsorbate. The excess of the positive or negative charge on the supported Au and Au2 can considerably promote their catalytic activity. Therefore, the h-BN surface (pristine or defected) cannot be considered as an inert support for Au and Au2