Nonstoichiometric Co-rich ZnCo2O4 Hollow Nanospheres for High Performance Formaldehyde Detection at ppb Levels

Since metal oxide semiconductors were investigated as chemiresistors, rapid advances have been reported in this field. However, better performance metrics are still required, such as higher sensitivity and selectivity levels for practical applications. To improve the sensing performance, we discuss an optimal composition of the active sensing material, nonstoichiometric Co-rich ZnCo2O4, prepared by the partial substitution of Co(2+) into Zn(2+) in Co3O4 without altering a hollow sphere morphology. Remarkably, this Co-rich ZnCo2O4 phase achieved detection limits for formaldehyde as low as 13 ppb in experimental measurements and 2 ppb in theory, which were the lowest values ever reported from actual measurements at a working temperature of 225 °C. It was also unprecedented that the selectivity for formaldehyde was greatly enhanced with respect to the selectivity levels against other volatile organic compounds (VOCs). These excellent sensing performances are due to the optimal composition of the Co-rich ZnCo2O4 material with a proper hole concentration and well-organized conductive network.