Formation Mechanisms of Pt and Pt3Gd Nanoparticles under Solvothermal Conditions: An in Situ Total X-ray Scattering Study

Platinum-based nanoparticles play a crucial role as catalysts, and solvothermal synthesis methods are attractive due to the excellent control of nanoparticle characteristics such as size, crystallinity, and morphology, which strongly affect the chemical and physical properties. Insight into the reaction mechanism leading to nanoparticle formation under solvothermal conditions generally remains elusive. This is mainly due to the experimental difficulties that lie in obtaining atomistic information on the nanoscale during the progression of the synthesis. Using in situ total X-ray scattering and pair distribution function (PDF) analysis with a time resolution of 1 s, we unravel the formation mechanisms of Pt and Pt3Gd nanoparticles under solvothermal conditions. We demonstrate that an octahedral Pt4+ platinic acid precursor complex is reduced in two steps. Both Pt and Pt3Gd nanocrystal formation proceeds via conversion of a square-planar Pt2+ complex to an unsaturated nanocluster (Pt-0), which subsequently grows by a combination of aggregation and ripening. In contrast to Rietveld analysis of powder X-ray diffraction data, the PDF method is able to uniquely establish that the structure of the Pt-Gd alloy is Pt3Gd rather than PtGd.