Mesoporous Co3O4 nanosheets with exposed Co2+-rich crystal facets for improved toluene detection

Crystal facet engineering strategy is an effective way to regulate the exposed facets, which affect the ability of the surface to adsorb and react with gas molecules, ultimately enhancing the gas-sensing properties. In this work, Co3O4 mesoporous nanosheets with dominant exposed {111} facets (Co3O4-60) or {112} facets (Co3O4-100) were synthesized by simply adjusting the condensation-reflux temperature. The sensing results toward toluene show that Co3O4-60 exhibited superior performance. According to the crystal structure analysis and work function calculations, the (111) facet composed of Co2+ would adsorb more oxygen molecules, which have been proved by experimental characterization, facilitating the sensing efficiency. The results of X-ray photoelectron spectroscopy indicated that Co3O4-60 possesses a higher Co2+ content and chemisorbed oxygen ratio (52.9%). Furthermore, Co3O4-60 has a high electrical conductivity and a minor apparent activation energy for toluene (38.75 kJ/mol). The gas sensor based on Co3O4-60 exhibited a response of 20.6 for 100 ppm toluene with rapid recovery and a detection limit of 1 ppm. Besides, other properties such as selectivity, repeatability, and humidity resistance were evaluated. This work evidences that crystal facet engineering is a practical approach to improving the gas-sensing properties of pure-phase Co3O4.