Sensitivities of a 6:4 (by molar ratio) ZnO/WO₃ composite nanoparticle sensor to reducing and oxidizing gases

For n–n composite nanoparticle gas sensors the dependence of the sensitivities of composite nanoparticle sensors to reducing and oxidizing gases on the molar ratio of the two n-type semiconducting materials is not established yet. In this study, the following hypothesis has been made for the ZnO/WO₃ composite NP chemiresistive sensor: If the molar ratio of ZnO (smaller work function) to WO₃ (larger work function) is larger than 1, the reducing gas-sensing properties of the ZnO NWs will be more significantly enhanced than the oxidizing gas-sensing properties. In contrast, if the molar ratio of ZnO to WO₃ is smaller than 1, the oxidizing gas-sensing properties of the ZnO NWs will be more significantly enhanced than the reducing gas-sensing properties. ZnO and WO₃ nanoparticles (NPs) were synthesized using sol-gel and hydrothermal methods, respectively. A 6:4 ZnO/WO₃ composite NP chemiresistive sensor was prepared by adjusting the molar ratio of the ZnO and WO₃ precursor solutions. The sensing test results for the sensor verified the feasibility of the hypothesis. The response of the 6:4 ZnO/WO₃ NP composite sensor to 200-ppm CO was 39.7, which is the 2nd highest value reported to date. In addition, the undying sensing mechanism is discussed in detail.