Synthesis of 1 nm Pd Nanoparticles in a Microfluidic Reactor: Insights from in Situ X-ray Absorption Fine Structure Spectroscopy and Small-Angle X-ray Scattering

A novel microfluidic reactor for in situ small-angle X-ray scattering (SAXS) and X-ray absorption fine structure spectroscopy (XAFS) of Pd colloidal nanoparticles synthesis is reported. The microreactor allows time resolution ranging from milliseconds to several minutes with a residence time of over an hour. The synthesis of colloidal Pd nanoparticles is investigated in the presence of oleylamine and trioctylphosphine ligands. For both ligands, SAXS results show the synthesis proceeds through slow, continuous nucleation as evidenced by a continuous increase in the number of particles. Growth is autocatalytic and fast at the early times; then, despite the availability of a large percentage of unreacted Pd precursor, growth slows dramatically and the Pd nanoparticle diameter reaches a plateau while more nanoparticles continue to form. In situ XAFS reveals an increase in Pd-P coordination coinciding with the time of slowed growth. The combined SAXS and XAFS results strongly suggest that the capping ligands play an important role in slowing growth by binding to the nanoparticle surface leading to a self-limiting nanoparticle size. Despite the slow continuous nucleation and overlapped fast autocatalytic growth, 1 nm Pd nanoparticles with narrow size distribution (+/- 20%) can be synthesized using strong capping ligands (e.g., trioctylphosphine).