Evidence for Bilayer Assembly of Cationic Surfactants on the Surface of Gold Nanorods

The surface structure of gold nanorods (NRs) capped with cationic surfactants in water was studied by FTIR, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). For gold nanorods, the FTIR results show the formation of new bands, which indicate binding of the surfactant headgroup to the surface of the NR. These bands are stable at temperatures as high as 350 °C. For a surfactant mixture (used as capping material), TGA shows a weak weight loss peak at 235 °C and a strong peak at 298 °C assigned to the surfactant molecules in monomer and aggregated forms, respectively. For gold nanorods, three weight loss peaks at about 230, 273, and 344 °C are observed. For gold nanospheres (NSs), TGA shows a strong mass loss at 225 °C and two weak mass loss peaks at 255 and 288 °C. The released material after combustion in the TGA process was analyzed by FTIR spectroscopy and found to be CO2. Our results suggest the following for both NRs and NSs: (1) There are two different binding modes for the surfactant molecules capping these nanoparticles. (2) Surfactant molecules form a bilayer structure around the gold nanoparticles in which the inner layer is bound to the gold surface via the surfactant headgroups. (3) With increase of the temperature, the outer layer desorbs at lower temperature and consequently the inner layer leaves the surface at higher temperature. (4) The higher desorption temperature of the bilayer in the NRs compared to NSs is explained in terms of the difference in packing of the surfactant molecules and their adsorption energy to the different facets present in these nanoparticles. (5) TEM results suggest that the shape transformation of NRs to NSs occurs as the inner layer is released from the surface. (6) The CH2 rocking mode at 720 cm-1 suggests that the methylene chains have free rotation and surfactants are packed in a hexagonal structure.