Straightforward Synthesis of Gold Nanoparticles Supported\non Commercial Silica-Polyethyleneimine Beads

Abstract

Stable silica-supported gold nanoparticles\n(Au_NPs) suitable\nfor catalysis applications were conveniently obtained in a straightforward,\none-step synthesis by simply adding an aqueous solution of HAuCl₄ to commercial polyethyleneimine-functionalized silica beads\n(SiO₂-PEI) as the only reactant without any external reducing\nagent and/or conventional stabilizing moieties. Six different types\nof Au_NPs/(SiO₂-PEI) beads termed <b>Au</b>_{<b><i>x</i>–<i>y</i></b>}<b>h</b>, where <i>x</i> is the initial HAuCl₄ concentration (1, 5, or 10 mM) and <i>y</i> is\nthe reaction time (1 or 24 h), were prepared and characterized by\nUV–vis diffuse reflectance spectroscopy, X-ray fluorescence,\nFE-SEM microscopy, and X-ray absorption spectroscopy. The SEM micrographs\nof <b>Au</b>_{<b><i>x</i>–<i>y</i></b>}<b>h</b> samples showed that the particle size\ndistribution decreases with the increase of the starting gold concentration,\ni.e., 70–100 nm for <b>Au</b>_<b>1–</b>_{<b><i>x</i></b>}<b>h</b>, 40–70\nnm for <b>Au</b>_{<b>5</b><b>–</b>}_{<b><i>x</i></b>}<b>h</b>, and <b>Au</b>_{<b>10</b><b>–</b>}_{<b><i>x</i></b>}<b>h</b>, whereas on passing\nfrom 1 to 24 h the aggregation phenomena overcome the nucleation ones,\npromoting the formation of bigger aggregates at the expense of small\nAu_NPs. The XAS analysis as a combination of XANES and EXAFS\nstudies provided detailed structural information regarding the coordination\ngeometry and oxidation state of the gold atoms present on the beads.\nMoreover, the catalytic activity of the modified silica beads in the\nreduction of 4-nitrophenol to 4-aminophenol by NaBH₄ was\ninvestigated and in one case the XAS analysis was repeated after recovery\nof the catalyst, demonstrating further reduction of the Au site to\nAu(0).