A high-resolution electron microscopy study of photodeposited Pd particles on TiO 2 and their oxidation in air

Abstract

Small (1- to 20-nm) photodeposited Pd particles on thin polycrystalline TiO 2 films were examined by conventional and high-resolution electron microscopy (HREM). Lattice imaging in these particles and optical diffraction analysis showed that the particle structure was face centered cubic (fcc) and that the lattice parameters were expanded up to 10% with respect to the bulk value. When the samples were heated under high vacuum at 500 °C for 1 h, lattice expansions up to 5% were observed. In this case all Pd agglomerates (>4 nm), consisting of randomly oriented 2- to 4-nm Pd clusters which are fused together, recrystallized into spherical Pd monocrystallites. Heating in air at 500 °C for 1 h brought about an increase in the maximum lattice expansion up to 15% and in almost all particles a distortion of the fcc structure. Furthermore various other lattice defects such as lamellar twins, stacking faults, dislocations, and lattice bending were observed. Some observations also suggested an enhanced PdTiO 2 interaction. Prolonged oxidation up to 24 h revealed images that could be explained by a phenomenon which was recently described as pit formation in the center of Pd crystallites. In only a few cases did lattice imaging indicate the possible presence of stoichiometric PdO. It is suggested that the dissolution of oxygen in the bulk of the Pd particles is the main explanation of our observations.