Chemiresistive gas sensing properties of nanocrystalline Co3O4 thin films

The chemiresistive gas sensing characteristics of Co3O4 thin films - prepared by electron-beam deposition of Co films on (0 0 0 1) Al2O3 substrate followed by oxygen annealing at three different temperatures (500 °C, 650 °C and 800 °C) - have been investigated for a host of gases (C2H5OH, CO, NO, Cl2, NH3 and H2S). The results of atomic force microscopy and X-ray diffraction revealed that the grains of these films are composed of nano-crystallites (size: 24-56 nm). X-ray photoelectron spectroscopy data revealed that the content of adsorbed oxygen decreases with increasing annealing temperature. For H2S gas in the concentration range 1-100 ppm, films annealed at 650 °C exhibited high selectivity, low base line drift and highest response, which are attributed to small crystallite size, improved crystallinity, better grain-to-grain connectivity and higher adsorbed oxygen content. The H2S sensing mechanism has been explained on the basis of the depletion of chemisorbed oxygen at the film surface.