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We reported the fabrication of a highly sensitive and fast switchable humidity sensor based on ZnO-TiO 2 core-shell nanorods that were synthesized using hydrothermal solution and atomic layer deposition. These nanorods were thermally treated under various physical conditions to improve their sensing performance.The structural investigation revealed that the crystal and microstructure changed with the thermal treatment. Notably, the amorphous TiO 2 shell layer transformed into various degrees of crystalline phase after annealing in air and a vacuum at 400 C. Furthermore, the responses of the sensors fabricated from the ZnO-TiO 2 nanorods, with and without thermal annealing, to relative humidity (RH) changes were proportional with the increase in humidity. Among various samples, ZnO-TiO 2 nanorods thermally treated in a vacuum exhibited the highest humidity selectivity in response to the cyclic changes in humidity from 11% RH to 33-95% RH at room temperature. Possible mechanisms for the enhancement of sensor performance have been discussed based on structural modifications caused by the thermal treatments.

AbstractsWe successfully prepared one-dimensional ZnO–ZnFe2O4 (ZFO) heterostructures for acetone gas-sensing and photoelectrochemical applications, by using sputter deposition of ZFO crystallites on ZnO nanostructure templates. The nanoscale ZFO crystallites were homogeneously coated on the surfaces of the ZnO nanostructures. Electron microscope images revealed that the ZnO–ZFO heterostructures exhibited a serrated surface morphology. Coating the ZnO nanostructures with a ZFO aggregated layer appreciably enhanced their acetone gas-sensing capability at 250 °C in comparison with pure ZnO nanostructures. The presence of many depleted nanoscale ZFO crystallites, the rugged surface of the heterostructures, and electron depletion at the ZnO/ZFO interface might contribute to the enhanced acetone gas-sensing response. Furthermore, the larger surface area and higher light absorption of ZnO–ZFO relative to the surface area and light absorption of ZnO were correlated with a substantial enhancement of the photocurrent value of ZnO–ZFO in photoelectrochemical tests produced by the simulated solar light irradiation.

We successfully prepared Au-nanoparticle-decorated ZnS (ZnS-Au) spheres by sputtering Au ultrathin films on surfaces of hydrothermally synthesized ZnS spheres and subsequently postannealed the samples in a high-vacuum atmosphere. The Au nanoparticles were distributed on ZnS surfaces without substantial aggregation. The Au nanoparticle diameter range was 5 to 10 nm. Structural information showed that the surface of the annealed ZnS-Au spheres became more irregular and rough. A humidity sensor constructed using the Au-nanoparticle-decorated ZnS spheres demonstrated a substantially improved response to the cyclic change in humidity from 11% relative humidity (RH) to 33% to 95% RH at room temperature. The improved response was associated with the enhanced efficiency of water molecule adsorption onto the surfaces of the ZnS because of the surface modification of the ZnS spheres through noble-metal nanoparticle decoration.