NMR investigations of silane-coated nano-sized ZnO particles

29Si, 129Xe and 13C NMR is used to investigate a porous material consisting of nano-sized ZnO particles coated by silane molecules. 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 specific 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 concentration the BET specific surface area of the coated particles first increases with silane concentration, then reaches the maximum of 100 m2/g at a silane concentration 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 concentration till it levels off at 130 m2/g at 10 mol.% silane. Also the properties of the material have changed significantly upon UV irradiation, a wool-like structure is produced and by the UV irradiation the material changed from being hydrophobic to hydrophilic. From the spectroscopic results we conclude that the non-irradiated material consists of ZnO particles separated by a silicate network bonded to the ZnO surface. Above silane concentrations of 5 mol.% the organic parts of the silane form organic domains with sizes of several tens of nanometers. In that concentration range the material with hydrophobic properties contains nanometer-size ZnO particles and nanometer-size organic domains, separated and bound together by a silicate network. UV irradiation destroys the organic domains and the material becomes highly porous with a high BET surface and hydrophilic properties.