Image Simulation of Small Pt Particles and its Application to Lattice Spacing Measurements in Catalysts

Extract

The technique of high resolution imaging is important for characterizing the structure of small metal particle catalysts and nanophase materials. For bimetallic systems, it is possible to use local lattice parameter measurements to identify alloy compositions in ensembles of nanometer sized metal particles [1]. However, determining alloy composition is challenging because changes in lattice parameters of only a few percent must be reliably detected. We have performed measurements of the apparent d(111) fringe spacing on both simulated and experimental HREM images from Pt particles in the size range 15 - 35 Å. A series of initial image simulations of Pt cubeoctahedrons with 17 Å (201 atoms) and 34 Å (1289 atoms) in diameter have been studied in order to understand the effect of different parameters on the accuracy of lattice spacing. The clusters were built using the Rhodius program developed by Botana et al [2]. Starting with a bulk crystal we create cubeoctahedra by applying successive cuts along either the (111) or (100) direction. A supercell size of 50Å was selected and the particles were oriented at or close to the [110]. Images were calculated by the multi-slice techniques using both the CERIUS and EMS applications. The supercell was divided into 20 slices and the following parameters were used in the calculation E_o = 400 kV, C_s = 1mm, Δf= -320 Å, focal spread = 80 Å, convergence = 0.5 mrad and atomic vibration = 0.35Å.