Optical pump/x-ray probe studies of lattice parameter evolution in Pt nanoparticles due to sintering and grain growth*

An \textit{in-situ }optical pump/x-ray probe technique has been developed to study the evolution in the lattice parameter of nanometer-sized Pt particles as a function of the particle size during sintering and grain growth. In particular, the lattice parameter of the as-prepared nanoparticles was observed to be about 0.4{\%} smaller than the corresponding bulk value in good agreement with the value expected for isolated spherical particles subject only to a simple surface stress. As the as-prepared nanoparticles sinter and grow as a result of the photo-thermal irradiation, however, the evolution in the lattice parameter reflects the effects of both an increasing grain size and the evolving particle-particle interface. As a result, the lattice parameter does not evolve monotonically with increasing grain size.