Speciation of Gold Nanoparticles by Ex Situ Extended X‑ray Absorption Fine Structure and X‑ray 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^III samples. This allows monitoring the relative fraction of Au^III and Au⁰ 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 Debye–Waller 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>_<b>2</b><b>@Yne</b>) were synthesized by co-condensation of the difunctional organosilane [3-(2-propynylcarbamate)­propyl]­triethoxysilane (PPTEOS) with tetraethoxysilane (TEOS) in an alkaline medium. The alkynyl-carbamate functionalities present on silica are able to capture the gold precursor HAuCl₄, spontaneously reduce it, and stabilize the resulting supported Au_NPs, having an average size of ca. 11 nm. The prepared <b>Au</b>-<b>SiO</b>_<b>2</b><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 UV–vis spectroscopy. The catalytic activity of <b>Au</b>-<b>SiO</b>_<b>2</b><b>@Yne</b> was investigated for the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄, and kinetic constants <i>k</i> in the order of magnitude of about 10^–2 s^–1 were found

Straightforward Synthesis of Gold Nanoparticles Supported on Commercial Silica-Polyethyleneimine Beads

Stable silica-supported gold nanoparticles (Au_NPs) suitable for catalysis applications were conveniently obtained in a straightforward, one-step synthesis by simply adding an aqueous solution of HAuCl₄ to commercial polyethyleneimine-functionalized silica beads (SiO₂-PEI) as the only reactant without any external reducing agent and/or conventional stabilizing moieties. Six different types of 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 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>_{<b><i>x</i>–<i>y</i></b>}<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>_<b>1–</b>_{<b><i>x</i></b>}<b>h</b>, 40–70 nm 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 from 1 to 24 h the aggregation phenomena overcome the nucleation ones, promoting the formation of bigger aggregates at the expense of small Au_NPs. 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 NaBH₄ 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)