Abstract
In this study, we have utilized octahedral gold nanocrystals as the structure-directing cores to grow Ag shells in aqueous solution. Au–Ag core–shell heterostructures with different morphologies can be directly synthesized by using cetyltrimethylammonium chloride (CTAC) as capping agent, octahedral gold nanocrystals as seed, silver nitrate (AgNO3) as precursor of Ag shell, ascorbic acid (AA) as reducing agent and sodium hydroxide (NaOH) as controller of reducing rate. By simply varying the ratio of gold seed to AgNO3 and the concentration of reducing agent, shape evolution from cubes to cuboctahedra and octahedra can be achieved. The reaction was finished within 1 hour at 30 ºC. This is a time- and energy-saving method. These monodispersed nanocrystals can have sizes less than 50 nm and readily form self-assembled structures. By tuning the reduction rate and monitoring the solution color at different time points during synthesis, octahedra covered by {111} facets were found to grow at a faster rate. On the other hand, a slower reaction rate favors the generation of cubes enclosed by {100} facets. We can use the same gold cores to synthesize different sized Au‒Ag core‒shell cubes and octahedral with tunable shell thickness. UV–vis spectra were used to investigate their optical properties and suggested that their optical responses are closely related to silver shell thickness and gold core size. With very thin shell thicknesses, a spectral blue-shift was recorded. As particle size increases, red-shift appears. After absorbing light, these metal nanoparticles release the energy through heat generation. Upon laser illumination, rapid and significant solution temperature increase was recorded. In addition, the small polyhedral nanocrystals can form different shapes of supercrystals in a saturated moist atmosphere by using a simple drop-casting method at different temperatures.