Porous coral-like NiCo<inf>2</inf>O<inf>4</inf> nanospheres with promising xylene gas sensing properties

Xylene is a volatile organic compound, widely used in paints, antiseptics, adhesives, varnishes etc. It is a well-known irritant and health hazard. Hence low cost and effective sensing materials that are both sensitive and selective to xylene are increasingly important for examining health of environments (primarily indoor). Here we show that coral-like NiCo2O4 nanospheres, developed through a template-free hydrothermal method results in a material with specific surface area as high as ∼83.8 m2/g (and pore volume ∼0.6 m3/g), demonstrating sensitivity as low as 16 ppm for xylene (much below the values currently deemed dangerous to health or life). The sensitivity is ∼4 times better that solid (hence non-porous) NiCo2O4 nanospheres, and the response varies almost linearly with xylene concentration (and saturates at ∼100 ppm). The reported material has good selectivity towards xylene over other gases at 260 °C. The optimum operating temperatures appear to be ∼260 and 300 °C with a response/recovery time of ∼30 s. However the sensor based on solid NiCo2O4 nanospheres exhibits poor selectivity to ethanol as well as acetone. The observed gas sensing performance suggests that the reported coral-like NiCo2O4 nanospheres are a plausible and promising sensing material for xylene detection.