Optical CO gas sensing using nanostructured NiO and NiO/SiO2 nanocomposites fabricated by PLD and sol–gel methods

Several kinds of NiO-based nanostructured films were prepared by pulsed-laser deposition (PLD) and sol–gel method, and CO sensing properties (1%, balanced by N2) of these films were studied. The sensitivity, defined as a difference of optical transmittance by gas atmospheric change (ΔT=T(CO)-T(air)), increased with increasing NiO content for the sol–gel prepared films, and increased with the film thickness for the laser deposited NiO films. Sol–gel films exhibited shorter response time than NiO films prepared by PLD under low Ar pressure of 6.7×10-2 Pa indicating a better gas permeability. A shorter response time was also obtained upon raising argon pressure from 6.7×10-2 Pa to 8.0 Pa during laser ablation due to the morphological change. Covering a NiO film even with a very thin (0.8 nm) layer of SiO2 by sputtering drastically reduced the CO sensitivity. The multilayered NiO/SiO2 films were substantially less sensitive to the CO gas than NiO films due to the same reason. Sensing mechanism of the NiO films is due to catalytic CO oxidation that reduces the concentration of adsorbed O2 species and results in optical transmittance increase upon change in the environment from air to CO.