In-Situ Shape Transformation and Melting of Platinum Nanocrystals

Extract

The catalytic activity and selectivity of nanosize colloidal platinum (Pt) nanocrystals may depend strongly on their sizes and shapes. To determine the shape dependent catalytic behavior of Pt nanocrystals, two key steps are involved. First, shape controlled Pt nanocrystals must be synthesized at a high yield. Secondly, the passivation layer on the nanocrystal must be removed in order to measure the catalytic selectivity and activity while the particle shape is preserved. The former had been successfully demonstrated by us that platinum nanoparticles with a high percentage of cubic-, tetrahedral- and octahedral-like shapes, respectively, can be synthesized by a colloidal chemistry method [1,2]. The growth of the shape controlled Pt nanocrystal has been found to be the result of kinetic process [3]. To measure the shape dependent catalytic activity and selectivity of the Pt nanocrystals, the capping polymer must be removed. One method could be annealing the nanocrystals supported on a substrate for evaporating the capping polymer. Several questions, however, must be considered: (1) how high does the annealing temperature need to be to remove the capping polymer while the particle shape is still preserved? (2) to what temperature is the particle shape still stable? (3) is there a temperature induced shape transformation due to surface diffusion? (4) how high is the melting point of the Pt particles? and (5) what is the effect of substrate on nanoparticle melting? These questions have not only fundamental importance, but also practical impact on the catalysis applications of the particles because the chemical reaction may take place at a higher temperature.