Elucidating the role of surfactant structural parameters in Au nanoparticle morphology

The use of cationic quaternary ammonium halide as a surfactant is popular in aqueous seed-mediated growth methods of noble metal nanoparticles (NPs). Despite the effective shape control of NPs that can be achieved by modifying the surfactant chemistry, these structural parameters have been insufficiently probed. The number of cationic quaternary ammonium halide surfactants explored for the synthesis of Au NPs have been limited primarily to cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC). In this study, we investigate the effect of structural parameters, including the headgroup, chain length, and counter-anion of the quaternary ammonium halides, on the resultant Au NPs formed using seed-mediated synthesis. By employing pentatwinned Au NPs as the starting seeds, we observe that (i) increasing the bulkiness of the surfactant headgroup results in the formation of Au NPs with fewer stellations and more shape polydispersity, (ii) surfactant chain length affects the dimension of Au NPs, and (iii) the counter-anion associated with the surfactant affects the Au NPs' final morphology. These structural parameters provide a practical handle to tune the shape, size, and polydispersity of the final Au NPs.