Measuring the Time-Dependent Monomer Concentration during the Hot-Injection Synthesis of Colloidal Nanocrystals

The shape of colloidal nanoparticles grown via hot-injection routes is largely determined by the reaction-limited rate of monomer nucleation. This offers an important synthetic benefit of tuning the morphology of colloidal nanocrystals simply by controlling the rate of monomer release during the thermal conversion of precursors. Unfortunately, the monomer concentration in colloidal reactions is difficult to track in situ, which obscures the actual effect of the temperature, monomer solubility, and ligand density on the probability of nanopartide nucleation. Here, we develop an experimental strategy for monitoring the time-dependent monomer concentration during the hot-injection synthesis of Ag nanocrystals. This approach employs Au nanoparticles as chemical probes of the Ag monomer build-up in the reaction flask. The precipitation of Ag on the surface of Au nanoparticles is diffusion-limited and results in a blue-shift of the plasmon resonance that is used to gauge the Ag monomer concentration, [Ag-0]. By measuring [Ag-0] immediately before the nucleation burst, we were able to elucidate the effect of several reaction parameters on the nucleation dynamics and the ultimate morphology of Ag nanocrystals. In particular, we show that the nucleation rate is independent of the reaction temperature but is highly sensitive to the concentration of free ligands in solution.