Polyol Synthesis of Platinum Nanoparticles:  Control of Morphology with Sodium Nitrate

Morphological control over platinum nanoparticles was realized by varying the amount of NaNO3 added to a polyol process, where H2PtCl6 was reduced by ethylene glycol to form PtCl42- and Pt0 at 160 °C. As the molar ratio between NaNO3 and H2PtCl6 was increased from 0 to 11, the morphology of Pt nanoparticles evolved from irregular spheroids with rounded profiles to tetrahedra and octahedra with well-defined facets. Absorption spectroscopy studies suggest that nitrate was reduced to nitrite by PtCl42- in the early stage of the synthesis, and the nitrite could then form stable complexes with both Pt(II) and Pt(IV) species. As a result, the reduction of Pt precursors by ethylene glycol was greatly slowed. This change in reaction kinetics altered the growth rates associated with different crystallographic directions of the Pt nanocrystals and ultimately led to the formation of different morphologies.