Microemulsion synthesized silica/ZnO stable core/shell sensors highly selective to ethanol with minimum sensitivity to humidity

Silica/ZnO core/shell and composite nanostructured sensors containing 15-90 wt% zinc oxide were prepared by a two- and one-step microemulsion method, respectively. The samples were characterized by XRD, EDX, FESEM, TEM and BET surface area measurement techniques and used for detection of 300 ppm of C2H5OH, CO, C2HCl3, C3H8, C2H4O, C3H6O or C6HSCH3 and 1% CH4 in air at 200-450 degrees C. For core/shell samples, containing less than 40 wt% ZnO, 7.5-20.5 nm porous layers of ZnO nanoparticles is formed on the silica spherical particles. At higher ZnO loadings, separate bulk ZnO nanoparticle aggregates are also formed. Heterogeneous nucleation on the silica surface is preferred at the low ZnO concentrations, while homogeneous nucleation occurs at the higher ones too. On the other hand, mixtures of ZnO nanostructures including plate and particle morphologies as well as silica nanoparticles are observed in the composite samples. Core/shell sensors are sensitive and selective to ethanol with responses at least 10 times higher than those to interfering gases. However, the composite sensors with less than 70 wt% ZnO have no response to any gas. The core/shell sensor containing 40 wt% ZnO shows a very slow drift in the response to ethanol in 14 days and only 3-14% lower response in 56% relative humidity. (C) 2016 Elsevier B.V. All rights reserved.