Effect of zinc : cobalt composition in ZnCo2O4 spinels for highly selective liquefied petroleum gas sensing at low and high temperatures

Abstract

Nano ZnCo₂O₄ mixed phase materials were synthesized at varying zinc and cobalt ratios such as 1 : 1, 1 : 1.5, 1 : 2, 1 : 2.5 and 1 : 3. With a change in composition from 1 : 1 to 1 : 2.5, the gas sensing response characteristics increased two times (from 40 to 77.5 kΩ), while at higher zinc cobalt composition (1 : 3) a saturation point is shown (about 80 kΩ). This shows that optimal cobalt loading (1 : 2.5) leads to a two times enhancement in redox ability (from 174 to 346 mmol) and number of active sites, and this upshot significantly helps the sensing response reach a much lower 10 ppm and increases the saturation point to a higher 60 ppm LPG concentration. Furthermore, nano ZnCo₂O₄ (with a zinc and cobalt ratio of 1 : 2.5) material exhibited an excellent response time (∼80–90 s), rapid recovery time (∼65–75 s), excellent repeatability (fourth cycle), good selectivity (for LPG), higher gas response (∼77.5 kΩ), lower as well as higher operating temperature (from 30 to 250 °C). The results clearly reveal that by tuning cobalt composition in Zn_xCo_2−xO₄, we can achieve maximum sensing efficiency and repeatability.