ZnO and Au/ZnO thin films: Room-temperature chemoresistive properties for gas sensing applications

Zinc Oxide has been widely investigated for its photocatalytic properties, which enhance the bulk/surface charge transfer at room temperature. At the same time, the doping of semiconductor materials with metals allows the modification of their physical and chemical properties, and hence their performance as gas sensors. The aim of this work was to investigate the difference between thin-film sensors based on pure ZnO nanoparticles and on gold-cluster decorated ZnO in photo-activation mode. The nanopowders were synthesized through simple sol-gel methods and chemically, morphologically and structurally characterized. Thin films were deposited by spin coating onto alumina substrates. The sensing layers were tested with several gases in photo-activation mode, illuminated with radiation wavelengths of 525, 468, 400 and 385 nm. Gold-decorated ZnO thin film showed some interesting features, such as better chemoresistive properties than pure ZnO under photo-activation mode, when illuminated with a wavelength of 385 nm. A selective response of the sensing film to NO2 under green light irradiation was observed, in particular in dry air condition. Furthermore, the humidity effect on the sensing responses was investigated, highlighting a possible application of Au/ZnO for the detection of few ppm of SO2 with a high percentage of relative humidity.