Digestive ripening, heating a colloidal suspension at or near the solvent boiling point in the presence of a surface-active ligand, was applied to polydisperse colloidal gold in toluene using a series of alkylthiols, viz., octyl-, decyl-, dodecyl-, and hexadecylthiols. In all the instances, digestive ripening significantly reduced the average particle size and polydispersity. All the colloids remain suspended in solution above 80 °C, but at room temperature the tendency to form 3D superlattices and precipitate increased with declining alkyl chain length. For example, using octanethiol as the ligand makes the colloids aggregate into big 3D superlattices and precipitate; decane-and dodecanethiol also produce precipitated 3D superlattices along with separate particles, while hexadecanethiol-coated particles remain well separated from each other. The optical spectra at room temperature reveal, apart from the gold plasmon band at 530 nm, a large tail above 700 nm for Au-octanethiol and Au-decanethiol cases and a shoulder at 630 nm for Au-dodecanethiol attributed to the superlattices. Au-hexadecanethiol, on the other hand, shows only the gold plasmon band as expected from separate particles. However, at higher temperatures only the gold plasmon band is observed for all the colloids indicating the dissolution of the superlattices. The aggregation of the particles into 3D superlattices or their stability as a colloidal suspension is qualitatively explained on the basis of decreasing van der Waals attraction between the gold nanoparticles as the separation between them is increased through the alkyl chain length of the capping ligand from octyl to hexadecyl.
Several ligands, such as alkylthiols, -amines, -silanes, -phosphines, -halides, and simple alkanes, were employed for digestive ripening, a process in which a colloidal suspension in a solvent is refluxed at the solvent boiling temperature in the presence of a surface-active ligand to convert a highly polydisperse colloid into a nearly monodisperse one. Apart from thiols, which are the only established digestive-ripening ligands, amines, silanes, and phosphines were found to be similarly efficient for this purpose. The important steps involved in the digestive ripening were identified to be (1) breaking the polydisperse colloid into smaller size particles upon addition of the ligand, (2) isolating this colloid from the reaction side products, and finally (3) heating this isolated colloid in the presence of the ligand to form a nearly monodisperse colloid. The successful ligands could be differentiated from the others based on their effectiveness to perform the different tasks in each step. Namely, they broke the bigger nanoparticles into smaller ones in the first step, formed a stable redispersable colloid in toluene after the second step, and at the end of the third step lead to a nearly monodisperse colloid. The ability of the different ligands to break the bigger, prismatic as-prepared particles in the first step varied as RSH ≈ RNH2 ≈ R3P ≈ RSiH3 > RI > ROH ≈ RBr and simple alkanes completely failed to induce any changes in the size and shape of the as-prepared colloid. Ligands such as RI, RBr, and ROH failed in the second step, possibly because of the poor ligand−gold interaction. The ligand−gold interaction trends observed here could be rationalized semiqualitatively by invoking the hard and soft acid and base theory, which suggests that a soft acid-like gold likes to interact with softer bases such as RSH and R3P rather than hard bases such as ROH. After the third step, the sizes of the nearly monodisperse particles depended on the ligand used for digestive ripening and correlated well with the ligand−gold interaction trends.
Dodecanethiol-stabilized gold nanoparticles with similar average size organize into different superlattice structures depending upon the method of preparation of the nanocrystals. Particles synthesized by the inverse micelle technique preferentially assemble into face-centered cubic (fcc) structures with long-range translational and orientational ordering. Gold nanoparticles obtained by the solvated metal atom dispersion (SMAD) method behave like "hard" spheres and predominantly organize into hexagonal close-packed (hcp) nanocrystal superlattices with long-range translational ordering. Different packing behavior results from differences in nanoparticle core morphologies induced by the synthetic method; fcc ordering is preferred by single crystalline nanoparticles, while hcp is preferred by polycrystalline nanoparticles. A combination of optical microscopy, transmission electron microscopy (TEM and HRTEM), selected area electron diffraction (SAED), atomic force microscopy (AFM), and X-ray diffraction (XRD) were used to characterize both the dispersed nanoparticles and the nanocrystal superlattices.
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