Chemiresistive gas sensing properties of nanocrystalline Co_3O_4 thin films

摘要

The chemiresistive gas sensing characteristics of Co_3O_4 thin films - prepared by electron-beam deposition of Co films on (0001) Al_2O_3 substrate followed by oxygen annealing at three different temperatures (500℃, 650℃ and 800℃) - have been investigated for a host of gases (C_2H_5OH, CO, NO, Cl_2, NH_3 and H_2S). 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 H_2S gas in the concentration range 1-100ppm, films annealed at 650℃ 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 H_2S sensing mechanism has been explained on the basis of the depletion of chemisorbed oxygen at the film surface.