Role of vacancy sites and UV-ozone treatment on few layered MoS₂ nanoflakes for toxic gas detection

Various issues like global warming and environmental pollutions have led to the research of toxic gas detection worldwide. In this work, we have tried to develop a molybdenum disulfide (MoS₂) based gas sensor to detect toxic gases like ammonia and NO. MoS₂, an inorganic analog of graphene, has attracted lots of attention for many different applications recently. This paper reports the use of liquid exfoliated MoS₂ nanoflakes as the sensing layer in a handheld, resistive toxic gas sensor. The nanoflakes were exfoliated from MoS₂ bulk powder using a sonication based exfoliation technique at room temperature. The successful exfoliation of the nanoflakes was characterized using different techniques e.g., optical microscopy, atomic force microscopy, field emission scanning electron microscopy, high resolution transmission electron microscopy, x-ray diffraction, Raman spectroscopy, x-ray photoelectron spectroscopy and ultraviolet–visible spectrophotometry. The characterization results showed that few-layered nanoflakes have successfully been exfoliated. The MoS₂ nanoflakes showed reasonable sensing towards ammonia and NO. In order to explore the effect of particle size on ammonia sensing, the MoS₂ flakes were also exfoliated using different sonication times. We also observed that various factors like presence of vacancy sites, ambient oxygen, humidity, different contact electrodes have significant effect on the sensing characteristics. In fact, the response of the sensing layer against 400 ppm of ammonia increased from 54.1% to ∼80% when it was UV-ozone treated. This work holds promises to developing cost-effective, reliable and highly sensitive MoS₂ based ammonia sensors. (paper)