Dissociative gas sensing at metal oxide surfaces

The low- and high-temperature gas sensing behavior of hydrogenated diamond (HD) and metal oxide (MOx) materials is compared and contrasted. We present evidence that at room temperature and above both kinds of materials are coated with a thin surface electrolyte layer in which gas molecules can be adsorbed and in which adsorbed gases may undergo electrolytic dissociation. We show that both kinds of materials respond in a very similar way when exposed to acid and base vapors and that no gas response is observed otherwise. Heating beyond 200 degrees C removes the surface electrolyte layer from both kinds of materials. Whereas at MOx surfaces, the established combustive gas sensing effect sets in, the surface conductivity and the gas sensitivity of HD samples is lost due to the disappearance of the surface transfer doping effect.