Mechanism of optical transmittance change by NOx in CoO/SiO2 nanocomposites films

CoO-dispersed nanocomposite films, changing their optical transmittance by nitrogen oxide, have been developed using the co-sputtering method. Films sputtered from a SiO2 target with CoO pellets on under an Ar atmosphere showed reversible transmittance change by NO at 350°C in the wavelength range from 400 to 800 nm. Cobalt oxide dots 4–8 nm in diameter were observed in the CoO/SiO2 nanocomposite. The sensing properties can be considered to be originated from nanostructures composed of (a) functional CoO nanoparticles that change their optical transmittance, (b) nanopores that provide paths for gas molecules, and (c) a transparent SiO2 matrix that enables us to measure optical transmittance. A mechanism of optical transmittance change is inferred that the octahedral divalent cobalt ions confined in CoO nanoparticles effectively interact with NO, resulting in the consumption of electronic sites that are involved in the optical absorption process.