3 research outputs found
Speciation of Gold Nanoparticles by Ex Situ Extended Xâray Absorption Fine Structure and Xâray Absorption Near Edge Structure
A combined
X-ray absorption near edge structure (XANES) and extended
X-ray absorption fine structure (EXAFS) methodology is here presented
on a series of partially and fully reduced Au<sup>III</sup> samples.
This allows monitoring the relative fraction of Au<sup>III</sup> and
Au<sup>0</sup> in the studied samples, displaying a consistent and
independent outcome. The strategy followed is based, for the first
time, on two structural models that can be fitted simultaneously,
and it evaluates the correlation among strongly correlated parameters
such as coordination number and the DebyeâWaller factor. The
results of the present EXAFS and XANES approach can be extended to
studies based on X-ray absorption spectroscopy experiments for the
in situ monitoring of the formation of gold nanoclusters
Novel Synthesis of Gold Nanoparticles Supported on Alkyne-Functionalized Nanosilica
A novel,
convenient method for the preparation of gold nanoparticles
supported on alkyne-functionalized nanosilica is presented. Silica
nanoparticles functionalized with alkynyl carbamate moieties (<b>SiO</b><sub><b>2</b></sub><b>@Yne</b>) were synthesized
by co-condensation of the difunctional organosilane [3-(2-propynylcarbamate)Âpropyl]Âtriethoxysilane
(PPTEOS) with tetraethoxysilane (TEOS) in an alkaline medium. The
alkynyl-carbamate functionalities present on silica are able to capture
the gold precursor HAuCl<sub>4</sub>, spontaneously reduce it, and
stabilize the resulting supported Au<sub>NPs</sub>, having an average
size of ca. 11 nm. The prepared <b>Au</b>-<b>SiO</b><sub><b>2</b></sub><b>@Yne</b> was thoroughly analyzed by
X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis
(TGA), transmission electron microscopy (TEM), atomic absorption spectroscopy
(AAS), FT-IR, and UVâvis spectroscopy. The catalytic activity
of <b>Au</b>-<b>SiO</b><sub><b>2</b></sub><b>@Yne</b> was investigated for the reduction of 4-nitrophenol to
4-aminophenol by NaBH<sub>4</sub>, and kinetic constants <i>k</i> in the order of magnitude of about 10<sup>â2</sup> s<sup>â1</sup> were found
Straightforward Synthesis of Gold Nanoparticles Supported on Commercial Silica-Polyethyleneimine Beads
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>â<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
UVâvis 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>â<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., 70â100 nm for <b>Au</b><sub><b>1â</b></sub><sub><b><i>x</i></b></sub><b>h</b>, 40â70
nm for <b>Au</b><sub><b>5</b><b>â</b></sub><sub><b><i>x</i></b></sub><b>h</b>, and <b>Au</b><sub><b>10</b><b>â</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)