Small-angle X-ray scattering and electron microscopy investigation of silica and carbon replicas with ordered porosity

Ordered nanoporous carbons can be prepared by a replica technique starting from an organized silica template. The silica template used, SBA-15, displays hexagonal arrangements of mesopores interconnected by micropores. Two routes are possible for introducing carbon into the pores of the silica: liquid impregnation by a solution of sucrose followed by carbonization or chemical vapor infiltration (CVI). After dissolution of the silica template by hydrofluoric acid treatment, a carbon material is obtained. Small-angle X-ray scattering (SAYS) measurements were performed over a broad range of wave vector q in order to investigate the multiscale structure of the carbon replica as a function of the method of infiltration (liquid or gas route) and the amount of infiltrated carbon. Because of the close match in electron density between silica and carbon, it is possible to investigate the silica mesopore filling. It appears that at least 50% of the pore volume must be filled in order to obtain an organized carbon replica after dissolution of the silica template. It is also shown that the gas route (CVI) prevents the spatially proportional (i.e., affine) shrinkage observed for replicas prepared by liquid impregnation. TEM confirms that the organized carbon is a nearly perfect negative replica of the silica porous structure.