Influence of exposure time and bias voltage on the response kinetics of toluene vapour on Ho³⁺-doped NiO chemiresistive sensors: Yellow-emitter nanophosphors

The incorporation of rare earths (REs) into semiconducting metal oxides is advantageous for creating surface defects and manipulating electronic structures. Therefore, the hydrothermal method fabricated p-type NiO nanostructures with various holmium (Ho³⁺) ion concentrations of 0, 1, 2, 3, and 4 wt% are investigated. The gas sensing features of the Ho³⁺-doped NiO sensors were conducted at different bias voltages and exposure times. At the bias voltage of 1 V with the exposure time of 5 min, the Ho³⁺-doped NiO sensors displayed a relatively high response of 36.35%, detection limit and sensitivity of 449 ppb and 0.749 ppm⁻¹, respectively towards 60 ppm toluene at 100 °C. Upon increasing the exposure time to 1 h, at the applied bias voltage of 0.5 V, the response improved by almost 6 times (R_g/R_a ∼ 201%) towards 60 ppm of toluene, showing the outstanding toluene selectivity against benzene, xylene, ethyl-benzene and acetone vapours. The gas sensing findings proved that an appropriate concentration of Ho³⁺ doping, exposure time and applied bias voltage have a significant influence on the sensing performances. Further tuning of these features could result in an improved sensing response. The increased sensing mechanism is assigned to the creation of oxygen vacancies.