Straightforward Synthesis of Gold Nanoparticles Supported
on Commercial Silica-Polyethyleneimine Beads
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Abstract
Stable silica-supported gold nanoparticles
(Au<sub>NPs</sub>) suitable
for catalysis applications were conveniently obtained in a straightforward,
one-step synthesis by simply adding an aqueous solution of HAuCl<sub>4</sub> to commercial polyethyleneimine-functionalized silica beads
(SiO<sub>2</sub>-PEI) as the only reactant without any external reducing
agent and/or conventional stabilizing moieties. Six different types
of Au<sub>NPs</sub>/(SiO<sub>2</sub>-PEI) beads termed <b>Au</b><sub><b><i>x</i>–<i>y</i></b></sub><b>h</b>, where <i>x</i> is the initial HAuCl<sub>4</sub> concentration (1, 5, or 10 mM) and <i>y</i> is
the reaction time (1 or 24 h), were prepared and characterized by
UV–vis diffuse reflectance spectroscopy, X-ray fluorescence,
FE-SEM microscopy, and X-ray absorption spectroscopy. The SEM micrographs
of <b>Au</b><sub><b><i>x</i>–<i>y</i></b></sub><b>h</b> samples showed that the particle size
distribution decreases with the increase of the starting gold concentration,
i.e., 70–100 nm for <b>Au</b><sub><b>1–</b></sub><sub><b><i>x</i></b></sub><b>h</b>, 40–70
nm for <b>Au</b><sub><b>5</b><b>–</b></sub><sub><b><i>x</i></b></sub><b>h</b>, and <b>Au</b><sub><b>10</b><b>–</b></sub><sub><b><i>x</i></b></sub><b>h</b>, whereas on passing
from 1 to 24 h the aggregation phenomena overcome the nucleation ones,
promoting the formation of bigger aggregates at the expense of small
Au<sub>NPs</sub>. The XAS analysis as a combination of XANES and EXAFS
studies provided detailed structural information regarding the coordination
geometry and oxidation state of the gold atoms present on the beads.
Moreover, the catalytic activity of the modified silica beads in the
reduction of 4-nitrophenol to 4-aminophenol by NaBH<sub>4</sub> was
investigated and in one case the XAS analysis was repeated after recovery
of the catalyst, demonstrating further reduction of the Au site to
Au(0)