Surface charge driven size evolution during the formation of self-assembled nanostructures from discrete hydrophilic silver nanoparticles.

We investigate the spontaneous evolution of mercaptosuccinic acid-capped silver nanoparticles (MSA-Ag) of size 2.5 ± 1.0 nm during the formation of assembled nanostructures at the air/water interface. In the phase transfer approach induced by the cationic surfactant cetyltrimethylammonium bromide (CTAB), the fusion among MSA-Ag nanoparticles is notably accelerated owing to the formation of a CTA-MSA bilayer on the particle surface. Provided that the size-selective separation is carried out in advance for these polydisperse MSA-Ag nanoparticles, either a unitary or a binary orderly superlattice composed of evolved nanoparticles is obtained at the air/water interface. For self-corrective equilibrium growth by adjusting the pH value of the bulk solution through the diffusion of HCl vapor, a competition is found between the superlattice formation and the size growth. The possible size evolution mechanism of each method is proposed. This work also shows that controllable evolution is a useful approach to prepare nanostructures with structural diversity.