Ligand-dominated temperature dependence of agglomeration kinetics and morphology in alkyl-thiol-coated gold nanoparticles

The stability of nanoparticle suspensions and the details of their agglomeration depend on the interactions between particles. We study this relationship in gold nanoparticles stabilized with different alkyl thiols in heptane. Temperature-dependent interactions were inferred from small-angle x-ray scattering, agglomeration kinetics from dynamic light scattering, and agglomerate morphologies from transmission electron microscopy. We find that the particles precipitate at temperatures below the melting temperatures of the dry ligands. Agglomerates grow with rates that depend on the temperature: Around precipitation temperature, globular agglomerates form slowly, while at lower temperatures, fibrilar agglomerates form rapidly. All agglomerates contain random dense packings rather than crystalline superlattices. We conclude that ligand-ligand and ligand-solvent interactions of the individual particles dominate suspension stability and agglomeration kinetics. The microscopic packing is dominated by interactions between the ligands of different nanoparticles.

Figure 1

Characterization of the particles. (a) The scattered intensity $I\left(q\right)$ in SAXS spectra of dilute particle suspensions stabilized with C12, C16, and C18 at 35 ${}^{\circ }$C. The solid lines represent fits of a polydisperse particle form factor. (b) The measured intensity autocorrelation $g_2$ from DLS experiments of the same suspensions. The solid lines represent the results of cumulant fits. No traces of agglomerates are visible. The curves are offset by the indicated factors or addends for clarity. The inset gives a representative TEM image of a C12 sample.

Figure 2

Temperature-dependent behavior of the particles. (a) The hydrodynamic radius measured in C12, C16, and C18 suspensions as function of temperature. The arrows indicate the cooling/heating cycles; the continuous lines are meant to guide the eye. The increase of the hydrodynamic radius with cooling indicates the onset of agglomeration, the decay with heating the disassembling of agglomerates. The temperature of agglomeration onset grows markedly with the ligand chain length. (b) DSC measurements on C12, C16, and C18 particles. The curves are offset for clarity. An endothermal reaction in the C16 and the C18 sample is detected by an energy consumption $\Delta E$ of the samples, indicating a melting of the ligand shell.

Figure 3

Evolution of (a) the structure factor $S\left(q\right)$ of a C16 suspension and (b) the depth of the interparticle potential $V_{\min }$ upon cooling. (a) The structure factor indicates a uniform distribution of particles at elevated temperatures. Upon cooling the particles agglomerate, leading to oscillations in the structure factor. (b) The interparticle potential as determined from $S\left(q\right)$ decays below $-3/2$ kT upon temperature decrease, leading to the observed structuring in the suspension. A distinct drop in the potential occurs below 15 ${}^{\circ }$C. Lines are meant to guide the eye.

Figure 4

Initial agglomeration stage of C16 samples. (a) The increase of hydrodynamic radii measured after quenching the sample to the target temperatures. The growth rate increases with decreasing temperatures. (b) The stability ratio calculated from the growth rates and the theoretical Smoluchowski rate constant. Lines indicate the two regimes: Agglomeration below 15 ${}^{\circ }$C is less temperature sensitive than above.

Figure 5

Further growth of quenched C16 samples. (a) Two temperature-dependent regimes of the agglomeration kinetics are visible: At 16 ${}^{\circ }$C, the agglomerates grow exponentially; at 14 ${}^{\circ }$C, their growth follows a power law (solid lines show fits). The inset shows a double-logarithmic plot of the data at 14 ${}^{\circ }$C to illustrate the power-law behavior. (b) The final growth kinetics of C16 samples above 15 ${}^{\circ }$C. The inset shows double-logarithmic plots of the final growth regime, emphasizing that the growth follows a power law.

Figure 6

TEM micrographs of agglomerates of C16 particles. (a) and (b) Agglomerates grown for 5 min at 10 ${}^{\circ }$C have filamentous structure. (b) Agglomerates grown for 5 min at 16 ${}^{\circ }$C have globular structures after reaction-limited growth. (d) Agglomerates grown for 30 min at 16 ${}^{\circ }$C show coalescence of agglomerates.

Figure 7

Scattered intensities $I\left(q\right)$ from SAXS measurements of the dilute suspension of C16 particles at 35 ${}^{\circ }$C, and suspensions that rested for 1 h at the denoted temperatures. The samples at lower temperatures exhibit altered profiles due to scattering from agglomerates. The solid lines represent fits to the profiles using the form factor alone for the dilute sample and the form factor and a cluster term for the samples at the lower temperatures. The curves are offset by the indicated factors for clarity.