Size-control growth of thermally stable Au nanoparticles encapsulated within ordered mesoporous carbon framework

Abstract

Simultaneously controlling the size of Au nanoparticles and immobilizing their location to specific active sites while hindering migration and sintering at elevated temperatures is a current challenge within materials chemistry. Typical methods require the use of protecting agents to control the properties of Au nanoparticles and therefore it is difficult to decouple the influence of the protecting agent and the support material. By functionalizing the internal surface area of mesoporous carbon supports with thiol groups and implementing a simple acid extraction step, we are able to design the resulting materials with precise control over the Au nanoparticle size without the need for the presence of any protecting group, whilst simultaneously confining the nanoparticles to within the internal porous network. Monodispersed Au nanoparticles in the absence of protecting agents were encapsulated into ordered mesoporous carbon at various loading levels via a coordination-assisted self-assembly approach. The X-ray diffractograms and transmission electron microscopy micrographs show that the particles have controlled and well-defined diameters between 3 and 18 nm at concentrations between 1.1 and 9.0 wt%. The Au nanoparticles are intercalated into the pore matrix to different degrees depending on the synthesis conditions and are stable after high temperature treatment at 600 °C. N2 adsorption-desorption isotherms show that the Au functionalized mesoporous carbon catalysts possess high surface areas (1269–1743 m2/g), large pore volumes (0.78–1.38 cm3/g) and interpenetrated, uniform bimodal mesopores with the primary larger mesopore lying in the range of 3.4–5.7 nm and the smaller secondary mesopore having a diameter close to 2 nm. X-ray absorption near extended spectroscopy analysis reveals changes to the electronic properties of the Au nanoparticles as a function of reduced particle size. The predominant factors that significantly determine the end Au nanoparticle size is both the thiol group concentration and subjecting the as-made materials to an additional concentrated sulfuric acid extraction step.