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

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.