The Design of Nano/Micro Structure of the Composite Metal Oxides and the Gas Sensing Performances
Authors: Zhou Tingting , Zhang Tong 
Published: May 2020

The gas sensor based on metal oxide semiconductors (MOS), which is known as one of the most widely popular gas sensors, plays an important role in gas detection in all walks of life. The sensing principle of MOS-based gas sensors results from changes in resistance caused by the adsorption–desorption reaction between target gases and chemisorbed oxygen ions on the surface of the sensing layers [1-3]. To generate adjustable oxygen vacancies on the surfaces of MOS is an effective strategy to improve the gas sensing performances [4, 5]. This work reported the synthesis of spinel type cobalt-based MOS (MCo2O4, M=Zn, Ni and Mn) (Fig. 1(a)) and the performance optimization by controlling the stoichiometric proportion of sensing materials. The results show that sensing properties can be significantly enhanced by substituting Co with other transition metals in the lattices. ZnCo2O4 with typical normal spinel structure can be regarded as a representative sensing material among the studied cobaltates, which exhibits the best sensing performances to acetone. The sensitivity of the ZnCo2O4 sensor is 24 and 106 times higher than those of the NiCo2O4-sensor and MnCo2O4-sensor at 190 °C, respectively. Zn substitution in tetrahedrally coordinated positions can provide more adsorbed oxygen molecules and oxygen vacancies and enable a better gas-sensing capability (Fig. 1(b-d)). In addition, the transition metal content can also be adjusted to further increase oxygen vacancy. The Zn content was controlled in ZnxCo3-xO4 bimetallic oxides during the growth process. When tested as an acetone sensing layer for gas sensors, the prepared ZnxCo3-xO4 materials (Zn/Co=0.37) composed of nano-sized building blocks exhibited a response of 35.6 to 200 ppm acetone within 60 s (Fig. 1(e)). In addition, the sensor device also demonstrates a low detection limit of 0.5 ppm to acetone, even in a high humidity environment. The better acetone sensing performance is attributed to the adsorbed oxygen and abundant oxygen vacancies of ZnCo-0.37 samples. More adsorbed oxygen molecules that take part in the sensing reaction are beneficial for the release of a mass of electrons to increase the extent of the change in resistance. Oxygen vacancies, serving as electron donors, endow ZnCo-n nanocages with unpaired electrons to provide free electrons and active sites to enhance the gas sensing properties. This approach sheds light on the rational design of bimetallic oxides with abundant oxygen vacancy and the development of superior sensors targeted for ultratrace acetone gas.

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Fig. 1 (a) Schematic diagrams of normal spinel structure and adsorbed acetone molecules. Polar graphs and sensitivity (ΔR/R0) of (b) ZnCo2O4, (c) NiCo2O4, and (d) MnCo2O4 to 50 ppm of different gases at 160-210 °C. (e) The resistance transient of the sensors based on ZnCo-0.27, ZnCo-0.37 and ZnCo-0.48 nanocages towards 200 ppm acetone gas.

Figure 1