A sub-ppm level formaldehyde gas sensor based on Zn-doped NiO prepared by a co-precipitation route

Abstract

Doping is an important and effective way to improve the gas sensing properties of sensors based on metal oxide semiconductors. Undoped NiO and Zn-doped NiO (with 2%, 3% and 4% of Zn) were successfully synthesized by a thermal treatment of the corresponding nickel zinc malonate previously prepared by a controlled coprecipitation in aqueous solution. The phase identification, the texture, the morphologies and the chemical composition (including the chemical state of Zn and Ni) for all the samples were investigated by a set of techniques such as XRD, BET, SEM and XPS respectively. The comparison of the gas-sensing properties towards formaldehyde of the undoped NiO and the Zn-doped NiO was also carried out. Among all the as synthesized materials, the 3% Zn-doped NiO exhibited significantly enhanced formaldehyde sensing properties, including lower operating temperature (200 °C), higher sensitivity, better selectivity, low detection limit and good reproducibility. The response (defined by S = (Rgas/Rair -1) for reducing gases) of the as prepared 3% Zn-doped NiO to 1.4 ppm at 200 °C is 5 times higher than that of undoped NiO. The experimental detection limit is 74 ppb which is lower than the limit set by WHO (80 ppb). The possible gas-sensing mechanism is discussed. The enhancement of sensor properties for 3% Zn-doped NiO can be explained by the effect of Zn on both, the quantity of adsorbed oxygen on the surface and the conductivity of the material. The catalytic activity of ZnO also plays a key role on the sensor performance.