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

Abstract

Stable silica-supported gold nanoparticles (AuNPs) suitable for catalysis applications were conveniently obtained in \na straightforward, one-step synthesis by simply adding an aqueous solution of HAuCl4 to commercial polyethyleneiminefunctionalized \nsilica beads (SiO2-PEI) as the only reactant without any external reducing agent and/or conventional stabilizing \nmoieties. Six different types of AuNPs/(SiO2-PEI) beads termed Aux−yh, where x is the initial HAuCl4 concentration (1, 5, or 10 \nmM) and y is the reaction time (1 or 24 h), were prepared and characterized by UV−vis diffuse reflectance spectroscopy, X-ray \nfluorescence, FE-SEM microscopy, and X-ray absorption spectroscopy. The SEM micrographs of Aux−yh samples showed that \nthe particle size distribution decreases with the increase of the starting gold concentration, i.e., 70−100 nm for Au1−xh, 40−70 \nnm for Au5−xh, and Au10−xh, whereas on passing from 1 to 24 h the aggregation phenomena overcome the nucleation ones, \npromoting the formation of bigger aggregates at the expense of small AuNPs. The XAS analysis as a combination of XANES and \nEXAFS studies provided detailed structural information regarding the coordination geometry and oxidation state of the gold \natoms present on the beads. Moreover, the catalytic activity of the modified silica beads in the reduction of 4-nitrophenol to 4- \naminophenol by NaBH4 was investigated and in one case the XAS analysis was repeated after recovery of the catalyst, \ndemonstrating further reduction of the Au site to Au(0). \n1. INTRODUCTION