Controlled synthesis of ultrathin MoS 2 nanoflowers for highly enhanced NO 2 sensing at room temperature.

Fabrication of a high-performance room-temperature (RT) gas sensor is important for the future integration of sensors into smart, portable and Internet-of-Things (IoT)-based devices. Herein, we developed a NO 2 gas sensor based on ultrathin MoS 2 nanoflowers with high sensitivity at RT. The MoS 2 flower-like nanostructures were synthesised via a simple hydrothermal method with different growth times of 24, 36, 48, and 60 h. The synthesised MoS 2 nanoflowers were subsequently characterised by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy. The petal-like nanosheets in pure MoS 2 agglomerated to form a flower-like structure with Raman vibrational modes at 378 and 403 cm -1 and crystallisation in the hexagonal phase. The specific surface areas of the MoS 2 grown at different times were measured by using the Brunauer-Emmett-Teller method. The largest specific surface area of 56.57 m 2 g -1 was obtained for the MoS 2 nanoflowers grown for 48 h. This sample also possessed the smallest activation energy of 0.08 eV. The gas-sensing characteristics of sensors based on the synthesised MoS 2 nanostructures were investigated using oxidising and reducing gases, such as NO 2 , SO 2 , H 2 , CH 4 , CO and NH 3 , at different concentrations and at working temperatures ranging from RT to 150 °C. The sensor based on the MoS 2 nanoflowers grown for 48 h showed a high gas response of 67.4% and high selectivity to 10 ppm NO 2 at RT. This finding can be ascribed to the synergistic effects of largest specific surface area, smallest crystallite size and lowest activation energy of the MoS 2 -48 h sample among the samples. The sensors also exhibited a relative humidity-independent sensing characteristic at RT and a low detection limit of 84 ppb, thereby allowing their practical application to portable IoT-based devices.