Understanding and Controlling the Morphology of Silica Shells on Gold Nanorods

Subtle variations in the conditions for addition of a tetraethyl orthosilicate (TEOS)/methanol (MeOH) solution to gold nanorods (GNRs) stabilized by cetyltrimethylammonium bromide (CTAB) allow for morphological control of silica (SiO2) shells deposited onto the GNRs. The concentration of TEOS in the TEOS/MeOH mixture determines whether the SiO2 shell uniformly coats whole GNRs or forms lobes on the ends of the GNRs. Changes in the optical absorbance spectrum of SiO2-coated GNRs (SiO2-GNRs) after purification with MeOH suggest CTAB can be removed by dissolution through the porous SiO2 shells. The size of the SiO2 lobes can be controlled, but there is a minimum lobe size, below which full encapsulation is favored. The following mechanism of lobe formation is proposed: Initially, a SiO2 shell fully encapsulates the CTAB-stabilized GNR core. Under optimized reaction conditions, determined by the MeOH concentration, the SiO2 shells can reshape into lobes, which requires sufficient solubility of SiO2 and damage or modification of the CTAB coating on the GNRs. SiO2 lobes can be more thermodynamically stable than uniform shells because they reduce the surface energy. Mechanistic insights gained in this study may be applicable to related core materials and the deposition of other kinds of oxides.