pH-dependent evolution of five-star gold nanostructures: an experimental and computational study.

Dendritic structures, such as snowflakes, have been observed in nature in far-from-equilibrium growth conditions. Mimicking these structures at the nanometer scale can result in nanomaterials with interesting properties for applications, such as plasmonics and biosensors. However, reliable production and systematic fine-tuning morphologies of these nanostructures, with novel hierarchical or complex structures, along with theoretical understanding of these processes, are still major challenges in the field. Here, we report a new method of using pH to control HAuCl4 reduction by hydroxylamine for facile production of gold nanostructures with morphologies in various symmetries and hierarchies, both in solution and on solid surface. Of particular interest is the observation of five-star-like dendritic and hierarchical gold nanostructures under certain reaction conditions. Phase-field modeling was used to understand the growth and formation dynamics of the five-star and other gold complex nanostructures, and the results not only explained the experimental observations, but also predicted control of the nanostructural morphologies using both pH and hydroxylamine concentrations. In addition to revealing interesting growth dynamics in forming fascinating complex gold nanostructures, the present work provides a pH-directed morphology control method as a facile way to synthesize and fine-tune the morphology of hierarchical gold nanostructures.