Growth kinetics and metastability of monodisperse tetraoctylammonium bromide capped gold nanocrystals

Small-angle X-ray scattering (SAXS) was used to measure the time-dependent evolution of the size and size distribution of tetraoctylammonium-stabilized gold nanocrystals formed by arrested precipitation. The initial growth mechanism was varied by controlling the addition of reducing agent to the ionic gold precursor solution. Slow addition gives high Au-0 monomer supersaturation and provides discrete nucleation and growth events and a narrowing of the particle size distribution over short reaction times. Rapid addition of reducing agent leads to broad initial particle size distributions that begin to narrow only after the first few hours of the reaction. After 24 h, the nanocrystals grow to 52 Angstrom in diameter with a very narrow size distribution regardless of the initial growth kinetics. Analysis of the reaction and transport rates reveal that nanocrystal growth is limited by the surface reaction rate of monomer addition to the crystal surface. The final size appears to be thermodynamically metastable, with the system free energy, which includes the gold surface tension and the tetraoctylammonium binding energy, reaching a local minimum. Nanocrystal size is also found to vary inversely with the binding strength of the capping ligand present during growth, allowing the mean particle diameter to be tuned to between 1.7 and 5.2 nm.