Adsorption in an ordered and non-interconnected mesoporous material: Single crystal porous silicon

Porous silicon is a network of parallel straight mesopores without lateral interconnection. The adsorption isotherm of nitrogen onto porous silicon exhibits a hysteresis loop of type H2 in the IUPAC classification, similar to those observed for highly interconnected mesoporous media. This shows that it is not possible to conclude from the observation of such a type of hysteresis whether a porous material is a network of interconnected or independent pores. Analysis of the adsorption and desorption branches according to the modified Kelvin equation does not account for the pore size distribution extracted from transmission electron micrographs. This suggests that the pore size dependence of the adsorbed layer thickness, prior to capillary condensation, needs to be considered. The steepness of the desorption branch cannot be explained in the framework of either the macroscopic Cohan model or the existing microscopic single pore models. It is believed that collective effects occur during the pore emptying process. Our experimental results bring strong arguments against the widely used application of the Barrett–Joyner–Halenda method to extract the pore size distribution from the desorption branch.