Elucidating the Role of Reduction Kinetics in the Phase-Controlled Growth on Preformed Nanocrystal Seeds: A Case Study of Ru.

This study demonstrates the crucial role of reduction kinetics in phase-controlled synthesis of noble-metal nanocrystals using Ru nanocrystals as a case study. We found that the reduction kinetics played a more important role than the templating effect from the preformed seed in dictating the crystal structure of the deposited overlayers despite their intertwined effects on successful epitaxial growth. By employing two different polyols, a series of Ru nanocrystals with tunable sizes of 3-7 nm and distinct patterns of crystal phase were synthesized by incorporating different types of Ru seeds. Notably, the use of ethylene glycol and triethylene glycol consistently resulted in the formation of Ru shell in natural hexagonal close-packed (hcp) and metastable face-centered cubic (fcc) phases, respectively, regardless of the size and phase of the seed. Quantitative measurements and theoretical calculations suggested that this trend was a manifestation of the different reduction kinetics associated with the precursor and the chosen polyol, which, in turn, affected the reduction pathway (solution versus surface) and packing sequence of the deposited Ru atoms. This work not only underscores the essential role of reduction kinetics in controlling the packing of atoms and thus the phase taken by Ru nanocrystals but also suggests a potential extension to other noble-metal systems.

Facile Synthesis and Characterization of Uniform Au Nanospheres Capped by Citrate for Biomedical Applications.

We report a simple and versatile method for effectively replacing the toxic ligands, such as cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC), on the surface of Au nanospheres with different sizes by citrate. The method involves the deposition of an ultrathin shell of fresh Au in the presence of sodium citrate at an adequate concentration. After the ligand exchange process, multiple techniques are used to confirm that the surface of the resultant Au nanospheres is covered by citrate while there is no sign of aggregation. We also demonstrate the mitigation of cell toxicity after exchanging the surface-bound CTAB/CTAC with citrate, opening the door to a range of biomedical applications.