Single-Crystalline Co3O4 Nanocubes for Highly Sensitive Acetone Gas Sensors
Authors: Ma Ahyeon , Kim Ha Young , Nam Ki Min 
Published: December 2020

Semiconductor metal oxide (SMO) based gas sensors have been considered a promising candidate for gas detection over the past few years due to their simplicity of use, low cost, and flexibility in production. Semiconductor metal oxide gas sensors have various application field such as non-invasive diagnosis of diseases as well as monitoring flammable and toxic gases in domestic and industrial environments. Nowadays, with the increase in interest in health care, gas sensor for early diagnosis of disease and monitoring health status are becoming the next generation of medical technology. However, it is still difficult to selective and accurate analyze only a small amount of target gas in a respiratory gas with a humidity of 95% or more. According to many studies, important detection parameters including sensitivity and selectivity of resistive SMO sensors can be improved through the synthesis of high-efficiency nanostructures.
Various metal oxides are suitable for detecting oxidizing or reducing gases by measuring conductivity. Most p-type oxide semiconductors such as NiO, CuO, Cr2O3 and Co3O4 have been widely studied as excellent catalysts for promoting the selective oxidation of various volatile organic compounds (VOCs). The inherent oxygen adsorption of p-type oxide semiconductors can be used to design high-performance gas sensors with low humidity dependence and high recovery rates. Moreover, by introducing a p-type oxide semiconductor as a complex to the n-type oxide semiconductor, it can be used to change the electrical properties near the hetero interface and to change the gas sensing properties.
According to the gas sensing mechanism, the operating characteristics of solid-state gas sensors are determined by the receptor and transducer functions. Therefore, it is very important to synthesize metal oxides with optimal morphology and crystal structure. The sensitivity of the sensor can be improved by using nanoparticles with a size of 1 – 100 nm as a sensing material, which is in good agreement with previously reported studies. However, since it is difficult to sufficiently increase the gas sensitivity by simply adjusting the size, it is required to form high sensitivity and selectivity to the target gas by using various types of nanostructures.
Among the various p-type SMO, Co3O4 has been studied as a potential material in many applications, including catalysts, magnetism, supercapacitors, batteries, and gas sensors, with indirect band gaps in the region of 1.6-2.2 eV. In most cases, Co3O4 play an important role in strong catalytic activity for specific reactions based on their surface catalytic properties. In particular, the microstructure of Co3O4 in gas sensors is known to be an important factor in determining sensing characteristics. The shape and size of the sensing material greatly influences the sensing performance due to the exposed surface. The selectivity also can be improved by the inherent surface characteristics of p-type oxide semiconductors, and thus the synthesis and characterization of shape-related structure with controlled crystal facet exposure is very interesting research area. Recently, a lot of research has been studied on the gas sensing properties according to the crystal plane of sensing materials.For example, Zhou et al reported that the {111} plane of Co3O4 has a significant effect on ethanol gas detection performance and reacts more sensitively. Liu et al demonstrated that both p-type and n-type sensing responses are facet dependent and sensitivity increases as the exposed {001} facet of TiO2 increases.Therefore, exposing a crystal surface rich in unsaturated metal cations has been identified as the most promising approach to achieving excellent gas sensing performance.
In this paper, we report facile preparation of Co3O4 nanostructures for highly active gas-sensing properties for acetone at sub-ppm levels controlling size and shape of nanoparticles. Co3O4 nanocubes shows high reactivity for 1 ppm acetone compared to Co3O4 nanospheres. These results show the potential of Co3O4 as a promising gas sensing material having a different gas response, selectivity, and response kinetics.