NMR investigations of silane-coated nano-sized ZnO particles.
29Si, 129Xe and 13C NMR is used to study a porous material consisting of nano-sized ZnO particles coated by silane mols. These materials are synthesized by a novel sol-gel procedure where the nanoscale ZnO particles are formed in the presence of the coating material 3-methacryloxypropyl-trimethoxysilane (silane). The BET sp. surface of the uncoated particles, synthesized by the same process in the absence of silane, is 43 m2/g. As a function of the silane concn. the BET sp. surface area of the coated particles 1st increases with silane concn., then reaches the max. of 100 m2/g at a silane concn. of 1-2 mol.% and finally decreases to almost zero at 10 mol.% silane. This surface area behavior is very different when the same material is UV-irradiated directly after the synthesis. The BET value of the UV irradiated material continually increases with increasing silane concn. till it levels off at 130 m2/g at 10 mol.% silane. Also the properties of the material have changed significantly upon UV irradn., a wool-like structure is produced and by the UV irradn. the material changed from being hydrophobic to hydrophilic. From the spectroscopic results the nonirradiated material consists of ZnO particles sepd. by a silicate network bonded to the ZnO surface. Above silane concns. of 5 mol.% the org. parts of the silane form org. domains with sizes of several tens of nanometers. In that concn. range the material with hydrophobic properties contains nanometer-size ZnO particles and nanometer-size org. domains, sepd. and bound together by a silicate network. UV irradn. destroys the org. domains and the material becomes highly porous with a high BET surface and hydrophilic properties.
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