Analysis of the growth process of gold nanorods with time-resolved observation

Gold nanorods are generated spontaneously in a surfactant solution. We developed an experimental setup where the growth of gold nanorods can be completely stopped at any instant. With this method, a time series of the growth process of gold nanorods was determined by the direct observation of nanorods with transmission electron microscopy. We estimated the growth rate of nanorods from the change in the average long-axis length over time. To understand the experimental results, we developed a mathematical model for the growth of nanorods. The present results should help to clarify the mechanism of the growth of gold nanorods.

Figure 1

Time changes in the adsorbed peak intensities at 550 nm just after adding seed. Without adding dodecanethiol-ethanol solution (thick line), the intensity increases gradually and saturates. With adding a dodecanethiol-ethanol solution (dotted line), the intensity is fixed. It means that the growth of nanoparticles completely stopped.

Figure 2

(Color online) Experimental observations for the time series of reaction-stopped suspensions. (a) TEM images of the time series of the growth process of gold nanorods. The numbers in each image indicate the time interval (min) when the chemical reactions for nanorod growth were stopped by addition of the dodecanethiol-ethanol solution. (b) Images of the time series of reaction-stopped suspensions. (c) Time series of the average long-axis length, short-axis length, and aspect ratio of gold nanorods with standard deviations measured from the TEM images shown in (a). (d) TEM image of seed particles. The typical diameter is 5 nm.

Figure 3

Numerical results. Changes in the (a) long-axis length, $L$, (b) short-axis length, $D$, and (c) aspect ratio over time.

Figure 4

(Color online) Dependence of the change in the long-axis length with time on the amount of seed particles. (a) Numerical results with changes in the parameter $N_0$. $N_0$ is 0.2 (top; red), 1 (middle; green), and 5 (bottom; blue). (b) Experimental results with various amounts of seed particles. 3.4 (top; red), 17 (middle; green), and 85 (bottom; blue) $\mu \text{l}$ of seed suspension were added to the growth solution, respectively. (c) Images of the time series of reaction-stopped suspensions corresponding to the experimental results in (b). 3.4 (top), 17 (middle), and 85 (bottom) $\mu \text{l}$ of seed suspension were added to the growth solution, respectively.