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Determining the surface and interface structure of nanomaterials

Description: This paper informally speculates on the challenges of determining the atomic-scale surface and interface structure of nanomaterials. The relative capabilities of different techniques are compared. This includes discussion of theoretical methods needed to interpret experimental techniques.
Date: June 14, 2004
Creator: Van Hove, Michel A.
Partner: UNT Libraries Government Documents Department

Quantitative Prediction of Surface Segregation in Bimetallic Pt-MAlloy Nanoparticles (M=Ni, Re, Mo)

Description: This review addresses the issue of surface segregation inbimetallic alloy nanoparticles, which are relevant to heterogeneouscatalysis, in particular for electro-catalysts of fuel cells. We describeand discuss a theoretical approach to predicting surface segregation insuch nanoparticles by using the Modified Embedded Atom Method and MonteCarlo simulations. In this manner it is possible to systematicallyexplore the behavior of such nanoparticles as a function of componentmetals, composition, and particle size, among other variables. We choseto compare Pt75Ni25, Pt75Re25, and Pt80Mo20 alloys as example systems forthis discussion, due to the importance of Pt in catalytic processes andits high-cost. It is assumed that the equilibrium nanoparticles of thesealloys have a cubo-octahedral shape, the face-centered cubic lattice, andsizes ranging from 2.5 nm to 5.0 nm. By investigating the segregation ofPt atoms to the surfaces of the nanoparticles, we draw the followingconclusions from our simulations at T= 600 K. (1) Pt75Ni25 nanoparticlesform a surface-sandwich structure in which the Pt atoms are stronglyenriched in the outermost and third layers while the Ni atoms areenriched in the second layer. In particular, a nearly pure Pt outermostsurface layer can be achieved in those nanoparticles. (2) EquilibriumPt75Re25 nanoparticles adopt a core-shell structure: a nearly pure Ptshell surrounding a more uniform Pt-Re core. (3) In Pt80Mo20nanoparticles, the facets are fully occupied by Pt atoms, the Mo atomsonly appear at the edges and vertices, and the Pt and Mo atoms arrangethemselves in an alternating sequence along the edges and vertices. Oursimulations quantitatively agree with previous experimental andtheoretical results for the extended surfaces of Pt-Ni, Pt-Re, and Pt-Moalloys. We further discuss the reasons for the different types of surfacesegregation found in the different alloys, and some of theirimplications.
Date: June 20, 2005
Creator: Wang, Guofeng; Van Hove, Michel A.; Ross, Phil N. & Baskes,Michael I.
Partner: UNT Libraries Government Documents Department

Non-free-electron momentum- and thickness-dependent evolution ofquantum well states in the Cu/Co/Cu(001) system

Description: We present systematic k{sub {parallel}}-dependent measurements of the Fermi surface and underlying band structure of quantum well states in Cu/Co/Cu(001). Compared to bands from normal emission, we find a complicated evolution of ''split'' QW states as a function of the thicknesses of both the copper overlayer and the cobalt barrier layer. Self-consistent calculations show that the penetration of the quantum well states into the cobalt barrier layer is significant and leads to the observed very non-free-electron behavior of these states.
Date: May 21, 2005
Creator: Rotenberg, Eli; Wu, Y.Z.; An, Joonhee M.; Van Hove, Michel A.; Canning, Andrew; Wang, Lin-Wang et al.
Partner: UNT Libraries Government Documents Department

Monte Carlo Simulations of Segregation in Pt-Ni Catalyst Nanoparticles

Description: We have investigated the segregation of Pt atoms in the surfaces of Pt-Ni nanoparticles, using Modified Embedded Atom Model potentials and the Monte Carlo method. The nanoparticles are assumed to have disordered fcc configurations at two fixed overall concentrations (50 at. percent Pt and 75 at. percent Pt). We use four kinds of nanoparticle shapes [cube, tetrahedron, octahedron, and cubo-octahedron] terminated by {l_brace}111{r_brace} and {l_brace}100{r_brace} facets to examine the extent of the Pt segregation to the nanoparticle surfaces and determine the equilibrium structures of Pt-Ni nanoparticles at T=600 K. The model particles contain between 560 and 4631 atoms (particle size ranging from 2.5 to 5 nm). Our results imply that a complete (100)-facet reconstruction could make the cubo-octahendral Pt-Ni nanoparticles most energetically favorable, consistent with experimental observations. We predict that at 600 K due to segregation the equilibrium Pt50Ni50 nanoparticles with fewer than 2000 atoms and Pt75Ni25 nano particles with fewer than 4000 atoms would achieve a surface-sandwich structure, in which the Pt atoms are enriched in the outermost and third atomic shells while the Ni atoms are enriched in the second atomic shell. We also find that due to an order-disorder transition the Pt50Ni50 cubo-octahedral nanoparticles containing more than 2000 atoms would form a core-shell structure with a Pt-enriched surface and a Pt-deficient homogeneous core.
Date: April 1, 2004
Creator: Wang, Guofeng; Van Hove, Michel A.; Ross, Philip N. & Baskes, M.I.
Partner: UNT Libraries Government Documents Department