Tuning the Formation and Growth of Platinum Nanoparticles Using Surfactant: In Situ SAXS Study of the Aggregative Growth Mechanism

The synthesis of platinum nanoparticles (Pt NP) via chemical reduction with ascorbic acid (AA) and kinetic stabilization with the cationic surfactant tetradecyltrimethylammonium bromide (TTAB) was investigated, with emphasis on the influence of the TTAB/Pt2+ ratio on particle size and growth behavior. Based on small-angle X-ray scattering (SAXS), ultraviolet-visible (UV-vis), and transmission electron microscopy (TEM) analyses, a four-stage mechanism was proposed for Pt NP formation, starting from nucleation and initial growth of primary nanoparticles (NPp), followed by a hierarchical aggregation process governed by the interplay between attractive and repulsive forces. While the ascorbic acid governs the reduction pathway and remains central to defining the morphology of Pt NP, the addition of TTAB was found to significantly modulate aggregation kinetics and structural organization, even though it does not act as a direct shape-directing agent. The higher the TTAB concentrations, the smaller and more monodisperse the primary NP, the enhanced the electrosteric stabilization, and the denser the aggregates with lower porosity. These changes were closely correlated with a decrease in the aggregation rate and an increase in the activation barrier for aggregation. This work advances the understanding of how cationic surfactants, even when not acting as shape-directing agents, can critically influence the assembly and final architecture of Pt nanostructures, providing valuable insights into the rational design of nanoparticle-based materials.