Characterization of microporous Si by flow calorimetry: Comparison with a hydrophobic SiO2 molecular sieve

Abstract

Flow microcalorimetry has been used to study microporous silicon produced by electrochemical corrosion of bulk p‐type silicon wafers in highly concentrated (50 wt %) aqueous hydrofluoric acid. Calorimetry data on pore size and hydrophobicity of freshly etched crystalline silicon structures are compared with similar measurements on silicalite, a well‐studied microporous form of crystalline silicon dioxide. Silicalite has a tetrahedral SiO₂ framework with interconnected ‘‘ultramicropores’’ that only readily admit molecules of less than 6 Å diameter. Its measured heat of immersion in n‐heptane (kinetic diameter 4.3 Å) consequently far exceeds that in iso‐octane (kinetic diameter 6.2 Å) and it preferentially adsorbs the normal alkane from the branched alkane. In direct contrast the microporous Si layers studied exhibited comparable heats of immersion for n‐heptane and iso‐octane, and did not show any preferential adsorption of the narrower molecule. In addition, the microporous Si layers studied exhibited appreciable heats of immersion in 1, 3, 5 tri‐isopropylbenzene (kinetic diameter 8.5 Å). The majority of their pore volume is thus constrained to the ‘‘supermicropore’’ size regime of 10–20 Å width. Both silicalite and freshly etched microporous Si are shown, however, to be highly hydrophobic and organophilic materials. Their exothermic heats of immersion in n‐heptane far exceed those in water and both materials preferentially interact with the polar alcohol (n‐butanol) more strongly from water than from n‐heptane.