Hydrothermal synthesis of mesoporous ZnO microspheres as NO_X gas sensor materials — Calcination effects on microstructure and sensing performance

This paper reports on the synthesis of mesoporous ZnO microspheres via hydrothermal production of zinc hydroxide carbonate (ZHC) and subsequent calcination of the ZHC precursor at 500 °C and their application to NO_X gas sensors. Materials characterization using various analytical techniques reveals that the ambient (air or vacuum) of the calcination process has a profound impact on the microstructure, morphology, and defect states of the resultant ZnO microspheres. The sample calcined under vacuum displays a microstructure of much smaller nanoparticles (<10 nm), a morphology of higher porosity and larger surface area, and a higher oxygen vacancy concentration compared to the air-calcined sample. The vacuum sample also has much better sensor performance (gas response: 19.8 to a low NO_X concentration of 0.1 ppm at 100 °C), with a high selectivity toward NO_X, than the air sample. We further discuss the sensing mechanism, suggesting that the differences in particle size, morphology, and oxygen vacancy concentration are responsible for the disparity in sensing performance.