Toxic gases are produced during the burning of fossil fuels. Room temperature (RT) fast detection of toxic gases is still challenging. Recently, MoS transition metal dichalcogenides have sparked great attention in the research community due to their performance in gas sensing applications. However, MoS based gas sensors still suffer from long response and recovery times, especially at RT. Considering this challenge, here, we report photoactivated highly reversible and fast detection of NO sensors at room temperature (RT) by using mixed in-plane and edge-enriched p-MoS flakes (mixed MoS). The sensor showed fast response with good sensitivity of ∼10.36% for 10 ppm of NO at RT without complete recovery. However, complete recovery was obtained with better sensor performance under UV light illumination at RT. The UV assisted NO sensing showed improved performance in terms of fast response and recovery kinetics with enhanced sensitivity to 10 ppm NO concentration. The sensor performance is also investigated under thermal energy, and a better sensor performance with reduced sensitivity and high selectivity toward NO was observed. A detailed gas sensing mechanism based on the density functional theory (DFT) calculations for favorable NO adsorption sites on in-plane and edge-enriched MoS flakes is proposed. This study revealed the role of favorable adsorption sites in MoS flakes for the enhanced interaction of target gases and developed a highly sensitive, reversible, and fast gas sensor for next-generation toxic gases at room temperature.

Nitrogen dioxide (NO2), a hazardous gas with acidic nature, is continuously being liberated in the atmosphere due to human activity. The NO2 sensors based on traditional materials have limitations of high-temperature requirements, slow recovery, and performance degradation under harsh environmental conditions. These limitations of traditional materials are forcing the scientific community to discover future alternative NO2 sensitive materials. Molybdenum disulfide (MoS2) has emerged as a potential candidate for developing next-generation NO2 gas sensors. MoS2 has a large surface area for NO2 molecules adsorption with controllable morphologies, facile integration with other materials and compatibility with internet of things (IoT) devices. The aim of this review is to provide a detailed overview of the fabrication of MoS2 chemiresistance sensors in terms of devices (resistor and transistor), layer thickness, morphology control, defect tailoring, heterostructure, metal nanoparticle doping, and through light illumination. Moreover, the experimental and theoretical aspects used in designing MoS2-based NO2 sensors are also discussed extensively. Finally, the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS2. Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO2 gas sensors for environmental monitoring.

The increased usage of hydrogen as a next generation clean fuel strongly demands the parallel development of room temperature and low power hydrogen sensors for their safety operation. In this work, we report strong evidence for preferential hydrogen adsorption at edge-sites in an edge oriented vertically aligned 3-D network of MoS 2 flakes at room temperature. The vertically aligned edge-oriented MoS 2 flakes were synthesised by a modified CVD process on a SiO 2 /Si substrate and confirmed by Scanning Electron Microscopy. Raman spectroscopy and PL spectroscopy reveal the signature of few-layer MoS 2 flakes in the sample. The sensor's performance was tested from room temperature to 150 C for 1% hydrogen concentration. The device shows a fast response of 14.3 s even at room temperature. The sensitivity of the device strongly depends on temperature and increases from $1% to $11% as temperature increases. A detail hydrogen sensing mechanism was proposed based on the preferential hydrogen adsorption at MoS 2 edge sites. The proposed gas sensing mechanism was verified by depositing $2-3 nm of ZnO on top of the MoS 2 flakes that partially passivated the edge sites. We found a decrease in the relative response of MoS 2-ZnO hybrid structures. This study provides a strong experimental evidence for the role of MoS 2 edge-sites in the fast hydrogen sensing and a step closer towards room temperature, low power (0.3 mW), hydrogen sensor development.

Controlled and tunable growth of chemically active edge sites over inert in-plane MoS2 flakes is the key requirement to realize their vast number of applications in catalytic activities. Thermodynamically, growth of inert in-plane MoS2 is preferred due to fewer active sites on its surface over the edge sites. Here, we demonstrate controlled and tunable growth from in-plane MoS2 flakes to dense and electrically connected edge-enriched three-dimensional (3D) network of MoS2 flakes by varying the gas flow rate using tube-in-tube chemical vapor deposition technique. Field emission scanning electron microscope results demonstrated that the density of edge-enriched MoS2 flakes increase with increase in the gas flow rate. Raman and transmission electron microscopy analyses clearly revealed that the as-synthesized in-plane and edge-enriched MoS2 flakes are few layers in nature. Atomic force microscopy measurement revealed that the growth of the edge-enriched MoS2 takes place from the in-plane MoS2 flakes. On the basis of the structural, morphological, and spectroscopic analysis, a detailed growth mechanism is proposed, where in-plane MoS2 was found to work as a seed layer for the initial growth of edge-enriched vertically aligned MoS2 flakes that finally leads to the growth of interconnected 3D network of edge-enriched MoS2 flakes. The surface energy of MoS2 flakes with different densities was evaluated by sessile contact angle measurement with deionized water (polar liquid) and diiodomethane (dispersive liquid). Both liquids show different nature with the increment in the density of the edge-enriched MoS2 flakes. The total surface free energy was observed to increase with increase in the density of edge-enriched MoS2 flakes. This work demonstrates the controlled growth of edge-enriched vertically aligned MoS2 flakes and their surface-energy studies, which may be used to enhance their catalytic activities for next-generation green fuel production.

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Two-dimensional layered materials have emerged prominently in the past decade, largely being investigated fundamentally and practically. Their unique layered structure and atomic-scale thickness make them attractive with exclusive electrical and optical properties compared to their bulk counterparts. Molybdenum disulfide (MoS2) is the most widely studied material in the family of transition metal dichalcogenides. The direct and variable bandgap, high carrier mobility, thermal and chemical stability makes it an attractive choice for next-generation photodetector applications. MoS2 heterojunction-based photodetectors offer ultrafast charge transfer and broadband photoresponse, adding more functionality beyond their individual counterparts. Enormous efforts have been devoted to adopting a new strategy that can improve photodetector performance in terms of responsivity and response time. This review briefly discusses the photo-induced current mechanism and performance parameters along with some important aspects to realize better device performance. Here, we critically review the current status and progress made towards MoS2-based photodetectors, followed by a discussion on open challenges and opportunities in their future application.

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