Preparation And Characterization Of Metal Nanoclusters From Ionomer

As a novel cluster material, metal nanoclusters have superior optical, electrical, magnetic and catalytic properties owing to their small size as well as unique arrangement of atoms. However, because of the difficulties in preparation and storage of small-sized, uniformly distributed and stable metal nanoclusters under severe reaction conditions, an excellent preparation method, if any, would tremendously contribute to their practical applications. In this work, ionomer is used for the first time as a precursor to prepare and characterize metal nanoclusters under mild reaction conditions.Ionomers are ion-containing polymers having a small amount(up to 10–15 mol%) of pendant ionic groups that are covalently attached to the nonionic backbone chains and complexed partially or fully with counterions to form a salt. Although they reportedly were employed as precursors in successful syntheses of semiconductor quantum dots, no research has ever been conducted to prepare metal nanoclusters likewise, presumably due to the relative difficulties in synthesis and process control. Generally, metal-ion-rich aggregates can form inside an ionomer, which serve as unique nanoscale reaction chambers for the reduction to metal nanoclusters; further, the densely coating polymer-chain segments are able to effectively protect the metal nanoclusters from aggregation. Nevertheless, how to reduce less active metal ions under mild conditions remains an issue to be resolved. The investigations in this work are primarily the following:(1) In the dark and at room temperature, ionomers were used as precursor in solution to produce metal nanoclusters in the absence of a strong reducing agent, whose morphology, structure and optical properties were then characterized by means of transmission electron microscopy, X-ray diffraction, fluorescence spectrophotometry, UV–visible spectrophotometry, and infrared spectroscopy. The synthesized Ag0 nanoclusters of outstanding optical properties and Pd0 nanoclusters were observed to be ~4.5 and ~2.7 nm, respectively, in mean diameter, both with relatively uniform size distributions, which were both surface-coordinated via unsymmetrical bridging to the carboxyl oxygen atoms of a poly(methyl methacrylate-ran-methacrylic acid)(MMA–MAA), such that the MMA–MAA chain segments were effectively adsorbed chemically onto the surfaces of the nanoclusters to play a protective role.(2) The reduction mechanism has been discussed with the aid of several control experiments. It was found that the ionic aggregates in ionomer solution, primarily intramolecular, are activated and broken up upon intensive swelling of covalently attached polymer chains to give bare(i.e., uncoordinated), highly reducible metal ions, which subsequently are reduced by the active H atoms of a weak reducing agent, methanol, in the solvent to form metal nanoclusters. Meanwhile, the sizes of both the nanoclusters and the crystallites therein were observed to change little with reaction time across the entire reduction process, indicating that the metal nanoclusters do not grow overly nor agglomerate considerably.(3) The(unstable) ionic-aggregate structures of Ag- and Pd-salts ionomers solutions have been extrapolated to apply to other metal ions. It was found from our experiments that the ionic aggregates were generally intramolecular multiplets in(dilute) solution, which were activated towards reduction due to the swelling of connected polymer chains. According to the coordination structure and nature inside the ionic aggregates, it is speculated for the viability of preparation of metal nanoclusters other than Ag0 and Pd0 that this novel, facile approach can readily be extended to the solution reduction of ionomeric transition-metal counterions such as Au3+, Pt4+, and Rh3+, while is very difficult to apply to counterions of negative standard electrode potential such as Cd2+, Ca2+, and Zn2+.