Superior Room-Temperature Ammonia Sensing Using a Hydrothermally Synthesized MoS₂/SnO₂ Composite

Abstract

Layered two-dimensional transition metal dichalcogenides, due to their semiconducting nature and large surface-to-volume ratio, have created their own niche in the field of gas sensing. Their large recovery time and accompanied incomplete recovery result in inferior sensing properties. Here, we report a composite-based strategy to overcome these issues. In this study, we report a facile double-step synthesis of a MoS 2 /SnO 2 composite and its successful use as a superior room-temperature ammonia sensor. Contrary to the pristine nanosheet-based sensors, the devices made using the composite display superior gas sensing characteristics with faster response. Specifically, at room temperature (30° C), the composite-based sensor exhibited excellent sensitivity (10%) at an ammonia concentration down to 0.4 ppm along with the response and recovery times of 2 and 10 s, respectively. Moreover, the device also exhibited long-term durability, reproducibility, and selectivity toward ammonia against hydrogen sulfide, methanol, ethanol, benzene, acetone, and formaldehyde. Sensor devices made on quartz and alumina substrates with different roughnesses have yielded almost an identical response, except for slight variations in response and recovery transients. Further, to shed light on the underlying adsorption energetics and selectivity, density functional theory simulations were employed. The improved response and enhanced selectivity of the composite were explicitly discussed in terms of adsorption energy. Lowdin charge analysis was performed to understand the charge transfer mechanism between NH 3 , H 2 S, CH 3 OH, HCHO, and the underlying MoS 2 /SnO 2 composite surface. The long-term durability of the sensor was evident from the stable response curves even after 2 months. These results indicate that hydrothermally synthesized MoS 2 /SnO 2 composite-based gas sensors can be used as a promising sensing material for monitoring ammonia gas in real fields.

Temperature-Based Selective Detection of Hydrogen Sulfide and Ethanol with MoS2/WO3 Composite

“…The spectrum in panel a consists of peaks at 228.6 and 231.5 eV, corresponding to Mo 4+ 3d 5/2 and Mo 4+ 3d 3/2 , respectively. These belong to +4 oxidation state of Mo and govern the highest signal in the spectra, while the other low-intensity peak at 232.1 and another one at 235.6 eV corresponds to Mo 6+ , implying the presence of MoO 3 or MoO 4 2– , which may be formed during the synthesis of the materials. ,, The Mo 4+ peaks are at 228.6 and 231.5 eV, and the S 2p region matches the 2H-phase of MoS 2 . , In panel b, the two major peaks of S 2p centered at 161.4 and 163.0 eV due to S 2– 2p 3/2 and S 2– 2p 1/2 can be seen. , The low-intensity high-energy component at 168.5 eV can be assigned to S 6+ species in sulfate groups (SO 4 2– ) . Panel c displays the prominent peaks for the W 6+ oxidation state, which correspond to W 4f 7/2 and W 4f 5/2 at 38.7 and 36.0 eV, respectively. , In addition to these two peaks of W 6+ , one more low-intensity peak at the binding energy of 32.4 is detected, confirming the presence of the W 5+ oxidation state in the hybrid materials .…”

“…Chemical field-effect transistor or chemiresistive and electrochemical sensors based on metal-oxides, polymers, and two-dimensional materials are among the most commonly used sensors for the detection of various toxic gases present in the environment and for biomedical purposes. − Among these, metal-oxide-based chemiresistive sensors are ideal candidates because of their low cost, simple structure, facile integration with electronics technology, and excellent sensitivity. − Their variable morphologies have governed enormous applications in the area of sensing. ,, One of the common problems related to metal-oxide semiconductor (MOS)-based sensors is their higher operating temperature and inferior selectivity. Among various MOS, WO 3 , an n-type semiconductor, possesses a larger bandgap, high thermal and chemical stability, and better sensing features, − except for its poor selectivity and low sensing performance at higher humidity levels. , Various other strategies, such as doping, grain size reduction, or composite formation with semiconducting materials, have been adopted to address some of these issues. − For instance, MoS 2 , a semiconducting transition metal dichalcogenide (TMDC), has been successfully employed in various sensing applications. , Inferior sensitivity, slow response and recovery to gas molecules, and the negative impact of humidity on sensing performance hinder its practical applications . It is to be noted that MOS and layered TMDCs possess complementary properties.…”

Selective N,N-Dimethylformamide Vapor Sensing Using MoSe2/Multiwalled Carbon Nanotube Composites at Room Temperature

“…However, increasing the amount of MWCNTs considerably increases the effective conductivity of the composite, masking the gas-induced change in conductivity and decreasing the relative response . Plausible reasons for the increased sensitivity of composite-based devices could be enhanced specific-surface area, the presence of defects, oxygen vacancies, and synergistic effects that come into play after a composite is formed. ,,, Hence, all sensing measurements were carried out using only the MoSe 2 /MWCNTs-10 mg sample. A good chemiresistive sensor should ideally possess a greater response at low analyte concentrations and room temperature.…”

“…In contrast, two-dimensional (2D) layered materials, particularly transition-metal dichalcogenides (TMDCs) have shown a useful and tunable electronic band gap and optical and electrochemical properties, leading to applications in various fields including optoelectronic devices, catalysis, solar cells, supercapacitors, and gas sensing. − Layered TMDCs, for instance, MoS 2 and its composites with other materials such as SnO 2 , , ZnO, and ZnS, have shown excellent performance toward the sensing of gases such as NH 3 and NO 2 . Due to the dangling-bond-free surface, charge transport capabilities, and biocompatibility, they are also considered as excellent options for flexible electronics. − Among the 2D TMDCs, molybdenum diselenide (MoSe 2 ) is a layered nanomaterial with a large surface to volume ratio, exceptional adsorption–desorption properties, and a direct energy band gap of 1.55 eV, indicating higher electrical conductivity. , It also possesses a higher adsorption energy for chemical compounds in comparison to graphene, black phosphorus, and molybdenum sulfide and has the capability of selective detection at room temperature. , …”

“…17,18 In contrast, two-dimensional (2D) layered materials, particularly transition-metal dichalcogenides (TMDCs) have shown a useful and tunable electronic band gap and optical and electrochemical properties, leading to applications in various fields including optoelectronic devices, catalysis, solar cells, supercapacitors, and gas sensing. 19−23 Layered TMDCs, for instance, MoS 2 and its composites with other materials such as SnO 2 , 24,25 ZnO, 19 and ZnS, 23 have shown excellent performance toward the sensing of gases such as NH 3 and NO 2 . Due to the dangling-bond-free surface, charge transport capabilities, and biocompatibility, they are also considered as excellent options for flexible electronics.…”