Synthesis and structural evolution of mesoporous silica-silver nanocomposites

Mesoporous silica materials were prepared by a sol - gel procedure using Si(OCH2CH3)(4) (TEOS) as the silica source and the non-ionic alkyl-poly( ethyleneoxide) oligomer Brij76 as the structure-directing agent. Pure inorganic hexagonal-like mesostructured SiO2 powders were obtained after annealing, performed in air between 400 and 600 degrees C. Upon calcination, the organic template was removed and silica networks with a different amount of Si - OH groups as well as porous features were prepared. Ag-SiO2 nanocomposites were obtained by metallation of the mesoporous silica powders with aqueous solutions of silver acetate. Ag+ ions were chemically grafted on the silica pores by taking advantage of the basic character of acetate anions and the acidic properties of Si - OH groups. Different silver concentrations were achieved on the mesostructured silica powders, depending on their former annealing treatment. For all samples the formation of silver nanoclusters occurred spontaneously at room temperature. Monodispersed, spherical Ag crystallites, with an average diameter of a few nanometres, were obtained starting from the 400 degrees C-treated silica, whereas clusters with different sizes (3 - 20 nm) and irregular shapes were grown for metallation of the 600 degrees C-heated SiO2 matrix. The sample's chemical composition was studied by x-ray photoelectron spectroscopy (XPS) and x-ray excited Auger electron spectroscopy (XE-AES), whereas the porous features were investigated by N-2 BET adsorption. Information concerning structure and microstructure was obtained by x-ray diffraction (XRD) and transmission electron microscopy (TEM).