Designing a material with novel sensing properties under extreme working conditions has remained a challenging task. Here, we report a facile two-step approach to develop a MoS 2 /MoO 3 composite with enhanced surface properties. When used as a gas sensor at 25 °C, it displayed superior sensing properties, selectivity, and a stable response toward ammonia against various reducing and oxidizing gases under highly humid conditions (relative humidity ≈ 95%). The composite exhibited a relative response of ≈55% (15% for 1 ppm) toward 50 ppm of NH 3 with smaller response τ res. and recovery τ rev. times of 45 and 53 s, respectively. It also displayed complete recovery without any external optical or thermal stimulus. The enhanced sensing properties of the composite are attributed to the synergistic effect arising from heterostructure formation between two base materials. The sensor displayed a decrease in resistance when exposed to NH 3 , a reducing gas, thus indicating its n-type character, which was further confirmed by performing Mott−Schottky (MS) measurements on MoS 2 and the MoS 2 /MoO 3 composite, both displaying n-type behavior with increased electron densities of the composite. Further, to understand the adsorption process and the resulting sensing properties, density functional theory simulations were performed using a pristine and a defect-enriched MoS 2 /MoO 3 surface. Large negative adsorption energies (for NH 3 ) of −344 and −519 meV, respectively, reflect that the adsorption process is feasible, and mechanism change from physisorption to chemisorption is predicted. Bader scheme was employed to evaluate the charge transfer between the NH 3 molecule and the pristine (defect-enriched) MoS 2 /MoO 3 surface and gave an amount of 0.073e (0.010e). Therefore, these results collectively justify the use of the MoS 2 /MoO 3 composite as a selective NH 3 sensor that can operate in humid air and environmental monitoring applications where such conditions exist.

This article demonstrates the use of a p-MoS2/n-WO3 heterojunctions based ultra sensitive and selective chemiresistive ammonia sensor that operates at 200◦ C. Surprisingly, the composite based sensor exhibited significant enhancement in ammonia sensing as compared to MoS2 (p-type) and WO3 (n-type) counterparts. The device also displayed excellent response-recovery features over a wider range of ammonia concentration together with superior selective nature toward ammonia as compared acetone, ethanol, methanol, isopropanol, formaldehyde, benzene, and hydrogen sulfide. Empowered by better signal-to-noise ratio, ammonia detection down to 1 ppm has become possible and can be further improved with the use of serpentine type electrodes. The device has shown a relative response of 207% for 200 ppm of ammonia with response and recovery times of 80 and 70 s, respectively. Moreover, these experimental results were further supplemented by density functional theory (DFT) simulation that were used to understand the adsorption kinetics and the sensing mechanism. A significant amount of charge transfer (0.082 e) between the adsorbed ammonia molecule and the MoS2/WO3 surface has been predicted by Bader analysis. Analysis also revealed a large negative adsorption energy ≈3.86 eV (373 kJ/mol) per ammonia molecule, implying the adsorption process to be chemisorption in nature. The band structure analysis further confirmed that ammonia adsorption on MoS2/WO3 is accompanied by an increase in band gap (by ≈ 96 meV). The present work illustrates the potential use of composite based heterostructures for monitoring ammonia gas in real fields.

We report a highly sensitive and selective ammonia (NH 3 ) gas sensor made from liquid exfoliated MoSe 2 nanosheets. The powder obtained after exfoliation was used to make a two-terminal sensor on a quartz substrate with predeposited silver contacts. The device so obtained, exhibited excellent sensitivity (5.5%) at an ammonia concentration down to 1 ppm, a fast response and recovery time of 15 and 135 s, respectively, better reproducibility, and impressive selectivity against various gases at room temperature. Moreover, density functional theory (DFT) simulations were used to understand the adsorption kinetic and electronic structure and therefore to shed light on the fundamentals of the sensing mechanism. Bader analysis was performed to understand the charge transfer process between the adsorbed ammonia gas molecule and underlying MoSe 2 surface. The resulting analysis confirmed that the electrons transfer from NH 3 molecules to MoSe 2 . The slight shift of the valence band toward the Fermi level which is clear from band structure analysis, along with the experimental fact that after exposure to ammonia the sensor displays an increase in resistance, indicates p-type behavior of the processed MoSe 2 crystalline nanosheets. These results imply the potential use of scaled nanosheets of MoSe 2 as a promising sensing material for enhanced and selective NH 3 gas monitoring at room-temperature.

Here we proposed a novel two-dimensional (2D) carbon allotrope namely pentagraphyne (PG-yne) which is energetically favourable than other graphyne members, including experimentally synthesized graphyne and graphdiyne monolayer. State-of-the-art theoretical calculations...