MS-14-P-2011 In situ study of Pt-Pd nanoalloys in liquid

Pt-Pd nanoalloys were prepared in solution from organic precursors and self-assembled in a 2D array on SiO2 substrates, the particles are homogeneous in size and self- organized on the substrate. The size histogram gives an homogeneous average size of 3.2±0.8 nm.

Compared to pure Pt particles, Pt-Pd show a better homogeneity in size and less coalescence. The in situ growth of Pt-Pd particles does not follow the same mechanism as pure Pt particles.

The crystal growth in solution was in situ studied by TEM in a liquid cell.

In situ study by TEM of a solution with Pt and Pd organic compounds in OAm, initiates the nucleation by reduction of the precursors in the electron beam. As soon as the clusters are visible in the liquid, they start to grow without coalescence. The cluster size increases as a function of the time, as t ½, , during the first minute, then the growth rate decreases, which is consistent with a growth mechanism by monomer addition. At the end of the growth, the size of the nanoalloys is between 2 and 2.5 nm.

In the series (fig. 1a-c), the same cluster is imaged in liquid during 540 sec.

The shape starts to be isotropic and facetted after 60 sec, mostly limited by (001) and (111) faces. The particle orientation changes in the liquid, but it is clearly seen along the [110] direction in fig. 1 c.

The same experiment with pure Pt precursors in OAm leads to a fast coalescence of the clusters and to the formation of larger nanoparticles with sizes dispersed between 1 and 5 nm. The nucleation and growth of pure Pt is consistent with a growth mechanism by monomer addition and coalescence events.

The different growth mechanism is probably due to the concentration of Pd atoms on the surface which prevents from the coalescence during the growth.