We report a facile and robust room-temperature NO₂ sensor fabricated using bi- and multi-layered 2H variant of tungsten di-selenide (2H-WSe₂) nanosheets, exhibiting high sensing characteristics. A simple liquid-assisted exfoliation of 2H-WSe₂, prepared using ambient pressure chemical vapor deposition, allows smooth integration of these nanosheets on transducers. Three sensor batches are fabricated by modulating the total number of layers (L) obtained from the total number of droplets from a homogeneous 2H-WSe₂ dispersion, such as ∼2L, ∼5–6L, and ∼13–17L, respectively. The gas-sensing attributes of 2H-WSe₂ nanosheets are investigated thoroughly. Room temperature (RT) experiments show that these devices are specifically tailored for NO₂ detection. 2L WSe₂ nanosheets deliver the best rapid response compared to ∼5–6L or ∼13–17L. The response of 2L WSe₂ at RT is 250, 328, and 361% to 2, 4, and 6 ppm NO₂, respectively. The sensor showed nearly the same response toward low NO₂ concentration even after 9 months of testing, confirming its remarkable long-term stability. A selectivity study, performed at three working temperatures (RT, 100, and 150 °C), shows high selectivity at 150 and 100 °C. Full selectivity toward NO₂ at RT confirms that 2H-WSe₂ nanosheet-based sensors are ideal candidates for NO₂ gas detection.

Among the most reliable techniques for exfoliation of two-dimensional (2D) layered materials, sonication-assisted liquid-phase exfoliation (LPE) is considered as a cost-effective and straightforward method for preparing graphene and its 2D inorganic counterparts at reasonable sizes and acceptable levels of defects. Although there were rapid advances in this field, the effect and outcome of the sonication frequency are poorly understood and often ignored, resulting in a low exfoliation efficiency. Here, we demonstrate that simple mild bath sonication at a higher frequency and low power positively contributes to the thickness, size, and quality of the final exfoliated products. We show that monolayer graphene flakes can be directly exfoliated from graphite using ethanol as a solvent by increasing the frequency of the bath sonication from 37 to 80 kHz. The statistical analysis shows that ∼77% of the measured graphene flakes have a thickness below three layers with an average lateral size of 13 μm. We demonstrate that this approach works for digenite (Cu₉S₅) and silver sulfide (Ag₂S), thus indicating that this exfoliation technique can be applied to other inorganic 2D materials to obtain high-quality few-layered flakes. This simple and effective method facilitates the formation of monolayer/few layers of graphene and transition metal chalcogenides for a wide range of applications.

We report a facile two-furnace APCVD synthesis of 2H-WSe₂. A systematic study of the process parameters is performed to show the formation of the phase-pure material. Extensive characterization of the bulk and exfoliated material confirm that 2H-WSe₂ is layered (i.e., 2D). X-ray diffraction (XRD) confirms the phase, while high-resolution scanning electron microscopy (HRSEM), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM) clarify the morphology of the material. Focused ion beam scanning electron microscopy (FIB-SEM) estimates the depth of the 2H-WSe₂ formed on W foil to be around 5–8 μm, and Raman/UV–vis measurements prove the quality of the exfoliated 2H-WSe₂. Studies on the redox processes of lithium-ion batteries (LiBs) show an increase in capacity up to 500 cycles. On prolonged cycling, the discharge capacity up to the 50th cycle at 250 mA/g of the material shows a stable value of 550 mAh/g. These observations indicate that exfoliated 2H-WSe₂ has promising applications as an LiB electrode material.

2D layered transition metal selenides grown in ambient pressure CVD for promising energy storage applications.