Microjunction-Modulated Selective Ammonia Sensor with P-Type Oxides-Decorated WS₂ Microflakes

In this study, p-type oxides including NiO, Co₃O₄, and CuO had been heterostructured with WS₂ microflakes for chemiresistive-type gas sensors at room temperature. Microjunctions formed between p-type oxides and WS₂ microflakes effectively modulated the sensitivities of the sensors to ammonia. In comparison to Co₃O₄- or CuO-decorated WS₂-based sensors in which "deep energy puddles" were formed at the microjunctions between the oxides and WS₂, the fabricated NiO/WS₂ heterostructure-based sensor without the formed energy puddles exhibited a better sensing performance with improved sensitivity and a faster response to gaseous 1-10 ppm of NH₃. It also processes a good selectivity to some volatile organic compounds including HCHO, toluene, CH₃OH, C₂H₅OH, CH₃COCH₃, and trimethylamine (TMA). The underlying mechanisms for the enhanced responses were examined by employing in situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory computation. The oxidization of NH₃ on NiO/WS₂ was much more intensified compared to those occurred on Co₃O₄/WS₂ and CuO/WS₂. NiO/WS₂ has a stronger adsorption to NH₃ and gains more effective charges transferred from NH₃ which significantly contributes to the enhanced sensing properties.