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.