Controlled Synthesis of Self-Reduced CoO Decorated Co₃O₄

Nitrogen dioxide (NO₂) is a common toxic gas in automobile and locomotive emissions, industrial combustion, and power generation. Long-term exposure to nitrogen dioxide will not only cause photochemical smog and acid rain to pollute water, soil, and air but cause harm to human health, such as respiratory distress, heart failure, and pulmonary edema. Precise detection and emission control of nitrogen dioxide are thus critical.<br/>In this study, Co₃O₄ nano-assemblies with different morphologies, including sphere, pillow-shape, and star-shape, were successfully produced via the polyol method at different reaction temperatures. The appropriate operating temperature of these three nano-assemblies for having the highest gas responses were then evaluated under 1000 ppm NO₂, and were then found to be 130 °C. The observed highest response approaching 207% was found for the pillow-shaped nano-assembly. The spherical Co₃O₄ nano-assembly was selected for the subsequent self-reduction study due to its relatively regular morphology among these three nano-assemblies. The spherical Co₃O₄ nano-assembly was then reduced in a mixture of 95% nitrogen and 5% hydrogen with various controlled temperatures and time for having a nanocomposite comprising coexisted multiple cobalt oxides. The Rietveld, x-ray photoelectron spectroscopy (XPS), and energy-dispersive x- ray spectroscopy (EDX) were mainly performed to identify the phase fraction and chemical composition.<br/>It was found that after reduction at 250 °C for 120 minutes, a heterogeneous nanocomposite comprising CoO (Co²⁺) (43 wt%) and Co₃O₄ (Co²⁺/Co³⁺)(57 wt%) can be successfully obtained. The XPS analysis confirmed that the amount of the surface adsorbed oxygen (Oc) and the oxygen vacancy (Ov) associated with O₂-ions in oxygen-deficient regions within the matrix of Co₃O₄ increase with the increase of Co²⁺ ions newly formed via the reduction procedure from Co₃O₄ to CoO, which is considered to be beneficial for improving gas sensing performance. The sensing response at 400 ppm NO₂ at 130 °C increases from 61% of the unreduced Co₃O₄ nano-assembly to 91% of the self-reduced CoO / Co₃O₄ one. Compared with CH₄, C₂H6, C₃H8, C₂H₅OH, and CO, the self-reduced CoO / Co₃O₄ nano-assembly exhibits excellent selectivity to NO₂. The response of NO₂ is four times higher than that of other gases. Based on the dynamic recovery test, the self-reduced CoO / Co₃O₄ nano-assembly shows a reproducible response.<br/><br/>Keywords: gas sensor, nitrogen dioxide sensing, Co₃O₄, reduction, selectivity, nano-assembly.