Intercalation of well-dispersed gold nanoparticles into layered oxide nanosheets through intercalation of a polyamine.

Gold nanoparticles have attracted widespread interest in both materials chemistry and biomedical science because of their special electronic, magnetic, catalytic, and optical properties1,2 For these applications, it is important to be able to control the size, shape, and dispersion of the nanoparticles on a chemically diverse range of supports. Oxides are among the most interesting kinds of support materials for nanogold, particularly for plasmonic and catalytic applications.3,4 However, in most of these studies, the nanoparticles are synthesized in situ, and it is difficult to control their size distribution and dispersion on the oxide support. The layer-by-layer (LBL) assembly of oppositely charged polyelectrolytes and nanoparticles has been widely studied as a route to functional composite materials.5,6 Structural control in the LBL method derives from the fact that the surface charge inverts in each sequential adsorption step. By adapting this principle to intercalation compounds, we recently showed that one could overcompensate the charge of anionic polysilicate nanosheets by intercalating coiled polycationic chains. This makes the layered solid an anion exchanger, which can intercalate large molecular anions into the cationic interlayer galleries.7 Because gold and many other kinds of nanoparticles are negatively charged at neutral and alkaline pH, we sought to investigate their intercalation into these cationic hosts. We report here a general method for intercalating gold nanoparticles (NPs) into the galleries of layered materials. This method allows us to disperse gold NPs within the interlayers of exfoliated fluoromica (Na-TSM) as well as a Dion-Jacobson phase layered perovskite (HCa2Nb3O10) with little aggregation. With amine polymer-intercalated hosts, the driving force for the reaction is covalent bonding between NPs and the free amine groups of the polymer. With quaternary ammonium polymer modified hosts, intercalation occurs through anion exchange. In both cases, a polymer loading in excess of that needed to compensate the layer charge is required for NP intercalation. Colloidal Au NPs with diameter of 1-3 nm (Au(S)) or 2-6 nm (Au(L)) were prepared by the method of Pham et al.9 HCa2Nb3O10 was made by ion-exchange of KCa2Nb3O10 in aqueous HNO3. Poly(allylamine), PAA, was intercalated at pH 12.0-12.5 to minimize protonation of the primary amine groups of the polymer. In the preparation of Au/PAA/Na-TSM (n), where n denotes the adsorbed amount (in millimoles) of Au per gram of host solid, a PAA/Na-TSM aqueous suspension was added to the appropriate amount of Au NP suspension and stirred at pH 12.0-12.5. After centrifugation, the resulting product was washed thoroughly with water, followed by drying at 333 K. In cases where the reaction did not go to completion, the amount of Au absorbed was calculated by comparing the absorption spectrum of a 0.0625 wt % suspension with that of an intercalation compound of known composition. For Au/PAA/HCa2Nb3O10, the same procedure was followed except that the solid was exfoliated using tetra(n-butyl)ammonium hydroxide before intercalation of PAA. Figure 1 shows typical XRD patterns of PAA/Na-TSM and PAA/ HCa2Nb3O10 before and after Au intercalation. The basal plane spacings of Na-TSM and HCa2Nb3O10 prior to intercalation were 9.6 and 14.7 Å, respectively. After intercalation of PAA into NaTSM or HCa2Nb3O10 (Figure 1a and 1d), the spacings increased to 16.5 and 30.9 Å, respectively, indicating intercalation of coiled PAA chains. The coiled chains contain ionizable amine groups in excess of that needed to compensate the anionic charges of the sheets.7 The larger change in spacing in the case of HCa2Nb3O10 reflects the higher charge density of the sheets relative to Na-TSM. After intercalation of Au(S) into PAA/Na-TSM (Figure 1b), a broad shoulder was observed at d ) 20-35 Å. For Au(L)/PAA/Na-TSM (Figure 1c), a more prominent diffraction peak was observed at a d-spacing of 35-45 Å, indicating that the larger Au(L) NPs were also accommodated by the interlayer galleries. The XRD patterns of Au/PAA/Na-TSM both show an additional phase with shorter gallery height (15-16 Å) than PAA/Na-TSM. In this phase the PAA chains must lie flat, and they do not accommodate intercalated NPs. Similar behavior was observed in the intercalation of large molecular anions into polycation/Na-TSM composites.7 The same pattern of reactivity was found with HCa2Nb3O10, PAA, and Au NPs. A broad shoulder was observed at d ) 32-38 Å, indicating intercalation of Au NPs. As in the case of the Na-TSM host, a broad peak was observed on the high-angle side of the original diffraction peak, indicating the formation of a second phase with a single flat layer of intercalated polymer. TEM images of Au(S) and (L)/PAA/Na-TSM and Au(S)/PAA/ HCa2Nb3O10 (Figure 2a, b, and c), show that the Au NPs, regardless of particle size and host material, are well-dispersed within the interlayer. From TEM image analysis,11 the average NP diameters for Au(S)/PAA/Na-TSM (0.79 and 0.20) were 1.7 ( 0.6 and 1.3 † The Pennsylvania State University. ‡ Tokyo Institute of Technology. Figure 1. XRD patterns of PAA/Na-TSM (a), Au(S)/PAA/Na-TSM (0.79) (b), Au(L)/PAA/Na-TSM (0.79) (c), PAA/HCa2Nb3O10 (d), and Au(S)/PAA/ HCa2Nb3O10 (<0.90) (e). The patterns were recorded on Philips X’Pert MPD diffractometer (monochromatized Cu KR1). Published on Web 02/28/2007