Gas sensing properties of nanocrystalline NiO and Co3O4 in porous silica sol-gel films

Thin SiO2-NiO and SiO2-Co3O4 nanocomposite films consisting of either NiO or Co3O4 nanocrystals in a porous SiO2 matrix have been prepared using sol-gel methods. The morphology, crystalline phase and chemical composition of the films have been characterized using X-ray diffraction, transmission electron microscopy and Fourier transform infrared techniques. The sensor response to H2 (20-850 ppm) and CO (10-500 ppm) in dry air and at different operating temperatures (50-300 °C) has been investigated using both conductometric and, for CO up to 1% in air, also with optical transmittance transduction methods. Both the NiO and Co3O4 doped films exhibit a conductometric p-type response, with a resistance increase upon exposure to the reducing gas. The nanocomposite films showed also a reversible change in the optical transmittance in the vis-NIR range when exposed to CO (10-10,000 ppm) in dry air. SiO2-NiO films have shown the highest response to H2 at 300 °C operating temperature and good selectivity to H2 if CO is the interfering gas. SiO2-Co3O4, which to the best of our knowledge may represent a new p-type material for gas sensing applications, shows similar behavior to the SiO2-NiO films. Detection limits of approximately 10 ppm CO and H2 are demonstrated.