Role of Hydrazine and Size-Tuning Parameter in GoldNanoparticle Synthesis by Water-in-Oil Microemulsion: Experiment andSimulation

By tuning the drop size, self-assembled microemulsion drops are used to control the size of nanoparticles synthesized in it. However, nanoparticle size control is challenging, specifically when particles outgrow the initial drop diameter. This necessitates the search for a robust operating parameter to control the size of the nanoparticles in the microemulsion route. In this pursuit, gold nanoparticle (GNP) size is controlled here, via the molar ratio (<i>P</i>) of concentrations of reducing agent (hydrazine) to precursor (aurochloric acid). A kinetic Monte Carlo (kMC) simulation scheme was devised to investigate the underlying mechanism behind decreasing particle diameter on increasing <i>P</i>. Binary pairs of GNPs seen to be partially fused from TEM images confirmed the involvement of particle–particle coagulation as a key step. Coagulation was regulated in the presence of hydrazine, the latter stabilizing GNPs by chemisorbing on its surface. Addition of NaCl caused the Cl^– ion to compress the diffuse double layer around hydrazine and form agglomerated GNPs. Finally, we found that at a fixed value of <i>P</i> = 12, even a 4-fold increase in precursor concentration does not affect the final diameter of GNPs, signifying <i>P</i> as a very important robust parameter. Therefore, the current proposed single parameter <i>P</i> reduces the experimental parametric space by eliminating the need to track individual concentrations of all reagents. It can be used to tune or predict the nanoparticle size even in the regime where the final particle diameter is bigger than the initial drop diameter.