In this work, 2D/2D ZnO/g-C3N4 heterojunction composite was synthesized through an ultrasonic mixing and subsequent calcination process. The gas-sensing performance to NO2 was investigated at room temperature activated by visible/ultra-violet LED light sources. Noticeably, when ZnO/g-C3N4 composite is illuminated by 460 nm light, it exhibits the highest response of 44.8 to 7 ppm NO2, and the response and recovery time is 142 and 190 s, respectively. Furthermore, it possesses excellent repeatability and selectivity to NO2, and the limit of detection is 38 ppb. In addition, the effect of humidity on the sensing performance under visible light was also investigated. The excellent gas-sensing performance is attributed to the absorbance of g-C3N4 in the visible light region and the charge separation at the interface between ZnO and g-C3N4.

Visible light activated excellent NO2 sensing based on 2D/2D ZnO/g-C3N4 heterojunction composites

We report a highly sensitive and selective ammonia (NH3) gas sensor made from liquid exfoliated MoSe2 nanosheets. The powder obtained after exfoliation was used to make a two-terminal sensor on a q...

MoSe2 Crystalline Nanosheets for Room-Temperature Ammonia Sensing

In recent years, the utilization of gas sensors has increased tremendously in daily life and industry. Importantly, appropriate material selection should be made for gas sensors in order to achieve outstanding gas sensing performance, such as high sensitivity, good selectivity, a fast response/recovery time, and long-term stability. Numerous studies have shown that neither pure metal oxide semiconductor nor individual 2D nanomaterial (graphene, transition metal dichalcogenides, metal–organic frameworks, metal oxide nanosheets, MXenes, and phosphorene) based gas sensors are capable of showing excellent gas-sensing performance towards gas molecules. However, synergistic combinations of metal oxides and 2D nanomaterials have demonstrated enhanced gas-sensing performance in many studies. This review aims at providing a comprehensive summary of the current advancements in 2D/metal-oxide based heterostructures as gas sensors. Additionally, the underlying sensing mechanisms of various kinds of gas sensors are systematically described, and the device architectures and their corresponding sensing performances are summarized. Finally, the challenges and future prospects of 2D/metal-oxide nanocomposite-based gas sensors for sensing applications have been outlined.

Gas sensing performance of 2D nanomaterials/metal oxide nanocomposites: a review

Based on the density functional theory (DFT), the adsorption capabilities and electronic properties of Ag–MoS2 monolayer were discussed to fully discover the gas sensing performance to transformer oil decomposed gases (H2, CO, CH4, C2H6, C2H4 and C2H2). Specifically, the adsorption structure, adsorption energy, Mulliken charge analysis, deformation charge density, density of state (DOS), frontier molecular orbital analysis and band structure were explored. The analysis results illustrate that the adsorption to CO, C2H4 and C2H2 is strong, which is superior to the adsorption of H2, CH4 and C2H6. The narrower band gaps and increased electrical conductivity in all adsorption structures indicate that Ag–MoS2 monolayer is promising for the detection of studied gases. Therefore, the present study proposes an effective gas-sensing material to estimate the working state and insulation level of power transformers.

Gas sensing mechanism of dissolved gases in transformer oil on Ag–MoS2 monolayer: A DFT study

Recently, a new class of 2D materials called MXenes have attracted massive attention in a variety of applications. The abundant active sites, metallic conductivity, tunable surface chemistry and outstanding stability of MXenes make them desirable for gas sensing applications. In this context, an impactful amount of research has been performed on MXene based sensors and they can be considered one of the potential future materials for gas sensing. In a focused way, properties like high flexibility, convenient solution processability and easy functionalization of MXenes open the door to make their composites with other nanomaterials and provide a new avenue for advanced sensor research. Previously reported reviews on MXenes and their nanocomposites were not fully focused on the gas sensing application and covered a wide area of applications in energy storage, biomedicine, and photocatalysis. In this review, we focused on the latest research and advancement of pristine MXenes and their nanocomposites for the sensing of gases, volatile organic compounds (VOCs) and humidity. In this review, we first investigated the synthesis procedure of pure MXenes from the MAX phase by selective etching with the help of suitable etchants and delaminating agents. Subsequently, the synthesis procedure of MXene nanocomposites with other nanomaterials like metal oxides, polymers, 2D nanomaterials and other sensing materials has been reported. Later on, we briefly discuss the properties of MXenes involved in sensing performance and the modulation of properties after functionalization due to the synergistic effect of both materials. Finally, the detailed sensing performance and related sensing mechanism have been discussed for MXenes and their nanocomposite-based sensors for a variety of gases, VOCs and humidity.

MXene-based gas sensors

Detection and quantification of hydrogen sulfide (H2S) gas is important as it influences directly human health, our environment, and operations of several industries including food and beverages, oil, construction, and medicine. It also acts as biomarker in diagnosis of halitosis at early stage. We report herein liquid exfoliated MoSe2 nanoflakes based stable chemiresistive H2S gas sensor which operate at moderate temperature of 200℃. The response of p-type MoSe2 gas sensor device (when operated in ambient environment) was found to be varying between 15.87%–53.04% when the concentration of H2S was varied between 50 ppb – 5.45 ppm. The response of the device decreases when the measurements were done in synthetic air environment and it varies between 7.13%–19.87% for the concentration range of 500 ppb - 5.45 ppm. The response (%), recovery rate (%), hysteresis, experimental lowest detection limit etc. of the device suggest that the device performs better when operated in ambient than in synthetic air which suggest its real time device application. The response time and recovery time of the sensor are 15 s and 43 s respectively for 100 ppb of H2S. The sensor performance was found to be highly repeatable with sensitivity of 5.57%/ppm of H2S. The theoretical limit of detection and limit of quantization of the device were found to be 6.73 ppb and 22.44 ppb respectively. Based on chemical analysis, a plausible mechanism based on charge transfer phenomenon has been proposed for this sensor.

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Jostin Vinroy D’Costa

Papers

MoSe2 nanoflakes based chemiresistive sensors for ppb-level hydrogen sulfide gas detection