Mechanism of hierarchical porosity formation in silica thin films using cellulose nitrate

Ordered mesoporous silica thin films have potential applications as, e.g., sorbents, catalysts, and analytical instruments. In addition to high internal surface areas, these applications also require high permeability, which can be obtained by engineering hierarchical porosity into the film. This paper presents a novel approach using cellulose nitrate to generate a system of larger pores (15-30 nm diameter) connected by the smaller mesopores (2-10 nm diameter) generated using traditional organic surfactants. The unimolecular reaction for cellulose nitrate deflagration avoids the usual diffusion limitations of traditional combustion synthesis. A family of hierarchically porous films was produced using a range of surfactant mixtures, and exhibited a range of surface areas along with relatively nonporous surface phases ('skins') on the film free boundaries. These skins were formed by an extrusion mechanism due to pore expansion coupled with the lateral (in-plane) symmetry constraints unique to thin films. The skin thickness depends primarily on the ratio of the large pore spacing (l0) over its diameter (d). The specific surface area (SA, m2/g) of the film increases monotonically as l0/d decreases, reaching a maximum at about l0/d ∼ 0.67. Precipitous reductions in SA for smaller values of l0/d are caused by pore intersections and the associated excluded surface areas. At high pore intersection (low l0/d), the film structure evolves from 'a solid with pores' to 'pores surrounded by a cage-like silica matrix.' The cage-like film structure is less susceptable to thin film cracking behavior, but is probably in a more metastable state that is susceptable to thermal or hydrothermal exposure.