DETERMINATION OF METAL-PARTICLE SIZES FROM EXAFS

Simulations of molecular dynamics carried out for Cu particles with sizes up to about 17 000 atoms, i.e. up to 70 Angstrom in diameter, have shown that the surface atoms move in a much more anharmonic potential than the bulk atoms. Since the fraction of surface atoms increases with decreasing particle size, the pair distribution functions become more and more asymmetric when going to smaller particles. This anharmonicity may introduce significant errors in the apparent bond distance and in the coordination number when determined by use of the standard EXAFS analysis formalism which assumes that the thermal vibrations are harmonic. Such errors in the coordination numbers will result in large errors in the calculated particle sizes. Consequently, a method was developed to correct these errors. In the present work we have extended the simulation of molecular dynamics to other metals, such as Pt, in an attempt to make the above model generally applicable to all transition metals. Since the motion of the atoms in Pt particles is found to be more harmonic than the atomic motion in Cu particles, the corrections for Pt are smaller than for Cu.