Enhanced CO gas-sensing using zinc oxide decorated with mixed-valence manganese oxide

Abstract

Carbon monoxide (CO), a colorless and odorless gas commonly found in car exhausts and heating appliances, poses significant health and environmental risks, highlighting the critical importance of developing CO detection sensors. In this study, to enhance CO sensing performance, brush-like hierarchical zinc oxide (ZnO) nanostructures were prepared by vapor-phase growth and decorated with mixed-valence manganese oxide (MnO_x) nanoparticles synthesized via a solution process. MnO_x exhibits excellent catalytic properties for CO oxidation and effectively enlarges the depletion layer of ZnO by forming heterojunctions. The experiment involved adjusting the pH of the solution to evaluate the CO-sensing performance. Field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction were utilized to analyze the morphology, elemental composition, and crystal structure of the samples, respectively. The optical properties were analyzed using ultraviolet–visible spectroscopy, and the surface composition and oxidation states were analyzed using X-ray photoelectron spectroscopy. The analyses confirmed the brush-like morphology of ZnO decorated with MnO_x nanoparticles and revealed that higher pH values resulted in an increased proportion of Mn²⁺ and larger nanoparticle sizes. ZnO samples decorated with MnO_x demonstrated enhanced selectivity for CO gas over other gases, with the selectivity against NO₂ improving by 582 times at 300 °C and by 58 times at 350 °C compared to the pristine sample. The enhanced CO sensing performance can be attributed to the high surface-area-to-volume ratio of ZnO and the catalytic activity of MnO_x.