Assembly, Growth, and Catalytic Activity of Gold Nanoparticles in Hollow Carbon Nanofibers

Abstract

Graphitized carbon nanofibers (GNFs) act as efficient templates for the growth of gold nanoparticles (AuNPs) adsorbed on the interior (and exterior) of the tubular nanostructures. Encapsulated AuNPs are stabilized by interactions with the step-edges of the individual graphitic nanocones, of which GNFs are composed, and their size is limited to approximately 6 nm, while AuNPs adsorbed on the atomically flat graphitic surfaces of the GNF exterior continue their growth to 13 nm and beyond under the same heat treatment conditions. The corrugated structure of the GNF interior imposes a significant barrier for the migration of AuNPs, so that their growth mechanism is restricted to Ostwald ripening. Conversely, nanoparticles adsorbed on smooth GNF exterior surfaces are more likely to migrate and coalesce into larger nanoparticles, as revealed by in situ transmission electron microscopy imaging. The presence of alkyl thiol surfactant within the GNF channels changes the dynamics of the AuNP transformations, as surfactant molecules adsorbed on the surface of the AuNPs diminished the stabilization effect of the step-edges, thus allowing nanoparticles to grow until their diameters reach the internal diameter of the host nanofiber. Nanoparticles thermally evolved within the GNF channel exhibit alignment, perpendicular to the GNF axis due to interactions with the step-edges and parallel to the axis because of graphitic facets of the nanocones. Despite their small size, AuNPs in GNF possess high stability and remain unchanged at temperatures up to 300 °C in ambient atmosphere. Nanoparticles immobilized at the step-edges within GNF are shown to act as effective catalysts promoting the transformation of dimethylphenylsilane to bis(dimethylphenyl)disiloxane with a greater than 10-fold enhancement of selectivity as compared to free-standing or surface-adsorbed nanoparticles.

New Pathway for Heterogenization of Molecular Catalysts by Non-covalent Interactions with Carbon Nanoreactors

“…Indeed, it has been demonstrated previously for various metal nanoparticles that guest species are located predominantly on these step-edge sites. [17,18,19] Furthermore, the diameters of GNFs are sufficiently large to minimise diffusion resistance of the reactant and product molecules to and from the catalytic centres. by HR-TEM and although fullerene-free molecules were not resolved in the micrograph, the presence of the copper containing species was also confirmed by EDX.…”

Chemical reactions confined within carbon nanotubes

“…Raman spectroscopy of the TiO 2 NP@DWCNT shows a red shift of the G + band by 3.5-8.5 cm À1 , relative to empty DWCNT, which can be attributed to electron transfer from titanium dioxide to the CNT. 71 This observation can be attributed to concentration effects, whereby the increased local concentration of reactants within the GNF accelerates the reaction kinetics and hence, the build-up of the silanol intermediate accelerates its subsequent dimerisation. 28 Indeed, this work made a direct comparison between the catalytic activity of TiO 2 NP confined within narrow DWCNT (internal diameter 1.0-1.5 nm) and wider MWCNT (internal diameter 4-8 nm).…”

Photocatalytic Water Splitting Towards Hydrogen Production on Gold Nanoparticles (NPs) Entrapped in TiO2 Nanotubes

“…For example, Joongoo [14] has developed a nano-reactor system comprising gold core and silica hollow shell to catalyze the reduction of p-nitrophenol. Alessandro [15] has used hollow graphitic carbon nano-fiber as efficient template for the growth of Au NPs, and their size was limited to approximately 6 nm. Some mesoporous silica, such as SBA-15 and MCM-41 [16][17][18], were used as confined host to limit the growth of Au NPs.…”