Nickel tungstate derived from WO₃ and NiO for room temperature CO₂ sensing

Abstract

The room temperature sensing of carbon dioxide (CO$_2$) has potential applications in diverse areas such as food industry, health care, and environmental monitoring. However, CO$_2$ is chemically inert and room temperature detection of this gas is rare, especially using undoped binary oxides such as tungsten oxide (WO$_3$) and nickel oxide (NiO). This study reports on the preparation and investigation of a nickel tungstate (NiWO$_4$) pellet-based sensor for room temperature detection of CO$_2$. The mixed oxide is prepared by first physically mixing NiO and WO$_3$ and then compacting them with polyvinyl alcohol (PVA) binder using a hydraulic press. These pellets are then sintered in the solid state at 900 $^{\circ }\hbox {C}$ for 2 h and the resultant product is structurally characterized by X-ray diffraction and scanning electron microscopy. The optical band gap is measured using UV–Vis spectroscopy in reflectance mode. The gas sensing behavior of the fabricated sensor is investigated at different concentrations of carbon dioxide (CO$_2$) at room temperature and measuring the change in electrical resistance. The obtained results showed that the response of the NiWO$_4$ sensor is greatly improved when compared to its binary constituents. This can be attributed to the hetero-contacts formed in the sintered material and also the increase in porosity of the mixed oxide when compared with pure WO$_3$. Further, the proposed mechanism that explains CO$_2$ sensing is discussed for NiWO$_4$ based on the formation of p–p junction between the constituents. This work provides a feasible route for the efficient fabrication of NiWO$_4$ for room temperature CO$_2$ sensing.