A simulation of the NiO/Ag interface with point defects

The NiO/Ag interface has been modelled using established simulation techniques, which have been modified to include the image interactions between the oxide ions and the induced charge in the metal. The energies of point defects near the interface were calculated and it was found that the surface rumpling was such that defects with a negative net charge were favoured. This will result in a space charge layer with excess cation vacancies which will cancel the interfacial potential. A low energy interface was modelled in which the cation sub-lattice of the second oxide plane was saturated with vacancies and Ni3+. ions. Such a structure may be responsible for the observed excess of oxygen near the NiO/Ni interface, and also for the low wetting angles of metals on NiO, compared with MgO

Tomographic Heating Holder for In Situ TEM:Study of Pt/C and PtPd/Al2O3 Catalysts as a Function of Temperature

A tomographic heating holder for transmission electron microscopy that can be used to study supported catalysts at temperatures of up to ~1,500°C is described. The specimen is placed in direct thermal contact with a tungsten filament that is oriented perpendicular to the axis of the holder without using a support film, allowing tomographic image acquisition at high specimen tilt angles with minimum optical shadowing. We use the holder to illustrate the evolution of the active phases of Pt nanoparticles on carbon black and PtPd nanoparticles on ¿-alumina with temperature. Particle size distributions and changes in active surface area are quantified from tilt series of images acquired after subjecting the specimens to increasing temperatures. The porosity of the alumina support and the sintering mechanisms of the catalysts are shown to depend on distance from the heating filament.Peer Reviewe

Shape Changes of Pt Nanoparticles Induced by Deposition on Mesoporous Silica

Polyvinylpyrollidone (PVP)-capped platinum nanoparticles (NPs) are found to change shape from spherical to flat when deposited on mesoporous silica substrates (SBA-15). Transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and extended X-ray absorption fine structure (EXAFS) analyses are used in these studies. The SAXS results indicate that, after deposition, the 2 nm NPs have an average gyration radius 22% larger than in solution, while the EXAFS measurements indicate a decrease in first neighbor co-ordination number from 9.3 to 7.4. The deformation of these small capped NPs is attributed to interactions with the surface of the SBA-15 support, as evidenced by X-ray absorption near-edge structure (XANES).LNLS [D04B-XAFS1-7683]LNLSPIP (CONICET, Argentina)PIP (CONICET, Argentina) [112-200801-03079]ANPCYT [PICT-2008-00038]ANPCyTCNPq [PICT-2008-00038]CNPqOffice of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under the Department of Energy [DE-AC02-05CH11231]Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under the Department of Energ