Simultaneous synthesis and assembly of noble metal nanoclusters with variable micellar templates.

Abstract

Simultaneous synthesis and assembly of Au, Pt, and Pd nanoclusters (NCs; with sizes ≤3 nm) into mesoscale structures with defined boundaries are achieved using their metal halides, cetyltrimethylammonium bromide (CTAB), and thiourea (Tu). Geometric shape, hierarchical organization, and packing density of resultant assemblages vary depending on metal precursors and CTAB concentration. For example, rod- or tube-like assemblages are formed from Au NCs, giant vesicles and/or dandelion-like assemblages from Pt NCs, and rhombic/hexagonal platelet assemblages from PdS NCs and Pd NCs. These assemblages inherit pristine shapes from their respective variable micelles of CTA(+)-metal halide complexes. Owing to dynamical nature, the assembled NCs demonstrate various structural reforming behaviors. The metal halides, which serve as counterions of positively charged surfactant heads, screen the electrostatic repulsion among the surfactant molecules as well as the micelles, providing the driving force for the formation of soft templates. Meanwhile, the formation of NCs can be addressed from the perspective of nucleation and growth kinetics. The unique protecting role of surface sulfur, controlled release of S(2-) from Tu, and formation of NCs of metal sulfides as intermediates together lead to a relatively low rate-ratio of growth to nucleation and thus limit the size of product NCs. Our preliminary study also indicates that the assembled noble metal NCs have high catalytic activity and recyclability. In this regard, the present approach not only provides a facile means to construct NC-based metal catalysts but serves also as a simple way to visualize interaction and evolution of micelles of CTA(+)-metal halide complexes.