Two-dimensional (2D) nanostructures are highly attractive for fabricating nanodevices due to their high surface-to-volume ratio and good compatibility with device design. In recent years 2D nanostructures of various materials including metal oxides, graphene, metal dichalcogenides, phosphorene, BN and MXenes, have demonstrated significant potential for gas sensors. This review aims to provide the most recent advancements in utilization of various 2D nanomaterials for gas sensing. The common methods for the preparation of 2D nanostructures are briefly summarized first. The focus is then placed on the sensing performances provided by devices integrating 2D nanostructures. Strategies for optimizing the sensing features are also discussed. By combining both the experimental results and the theoretical studies available, structure-properties correlations are discussed. The conclusion gives some perspectives on the open challenges and future prospects for engineering advanced 2D nanostructures for high-performance gas sensors devices.

Two-Dimensional Nanostructured Materials for Gas Sensing

Two-dimensional (2D) nanomaterials have demonstrated great potential in the field of gas sensing due to their layered structures. Especially for 2D transition metal dichalcogenides (TMDs), inherent high surface areas and their unique semiconducting properties with tunable band gaps make them compelling for sensing applications. In combination with the general benefits of 2D nanomaterials, the incorporation of metal oxides into 2D TMDs is a recent approach for improving the gas sensing performance of these materials by the synergistic effects of the hybridization. This Review aims to comprehend the sensing mechanisms and the synergistic effects of various hybridizations of 2D TMDs and metal oxides. The Review begins with the gas sensing mechanisms and synthesis methods of 2D TMDs. Achievements in recent research on 2D TMDs and their metal oxide hybrids for sensor applications are then comprehensively compiled. To clearly understand the collective benefits of TMDs and metal oxide hybrids, the hybridization effects are discussed in three aspects: geometrical, electronic, and chemical effects.

Two-Dimensional Transition Metal Dichalcogenides and Metal Oxide Hybrids for Gas Sensing.

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS2, MoSe2, MoTe2, WS2 and WSe2, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1–2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Developing a facile, cost-saving, and environment-friendly method for fabricating a multifunctional humidity sensor is of great significance to expand its practical applications. However, most humidity sensors involve a complex fabrication process, resulting in their high cost and narrow application fields. Herein, a multifunctional paper-based humidity sensor with many advantages is proposed. This humidity sensor is fabricated using conventional printing paper and flexible conductive adhesive tape by a facile pasting method, in which the paper is used as both the humidity-sensing material and the substrate of the sensor. Owing to the moderate hydrophilicity of the paper and the rational structure design of the paper-based humidity sensor, the sensor exhibits an excellent humidity-sensing response of more than 103 as well as good linearity ( R2 = 0.9549) within the humidity range from 41.1 to 91.5% relative humidity. Furthermore, the paper-based humidity sensor has good flexibility and compatibility, endowing it with multifunctional applications for breath rate, baby diaper wetting, noncontact switch, skin humidity, and spatial localization monitoring. Although the resistance of the paper-based humidity sensor is relatively large, the humidity-sensing response signals of the sensor can be conveniently processed by the designed signal processing system. The readily available starting materials and facile fabrication technique provide useful strategies for the development of multifunctional humidity sensors.

Facile, Flexible, Cost-Saving, and Environment-Friendly Paper-Based Humidity Sensor for Multifunctional Applications.

Skin-mountable chemical sensors using flexible chemically sensitive nanomaterials are of great interest for electronic skin (e-skin) application. To build these sensors, the emerging atomically thin two-dimensional (2D) layered semiconductors could be a good material candidate. Herein, we show that a large-area WS2 film synthesized by sulfurization of a tungsten film exhibits high humidity sensing performance both in natural flat and high mechanical flexible states (bending curvature down to 5 mm). The conductivity of as-synthesized WS2 increases sensitively over a wide relative humidity range (up to 90%) with fast response and recovery times in a few seconds. By using graphene as electrodes and thin polydimethylsiloxane (PDMS) as substrate, a transparent, flexible, and stretchable humidity sensor was fabricated. This senor can be well laminated onto skin and shows stable water moisture sensing behaviors in the undeformed relaxed state as well as under compressive and tensile loadings. Furthermore, its high sensing performance enables real-time monitoring of human breath, indicating a potential mask-free breath monitoring for healthcare application. We believe that such a skin-activity compatible WS2 humidity sensor may shed light on developing low power consumption wearable chemical sensors based on 2D semiconductors.

Transparent, flexible, and stretchable WS2 based humidity sensors for electronic skin.

WS2 nanosheets have been synthesized by ultrasonication in a binary mixture of acetone and 2-propanol, with a volume ratio of 80:20. Hansen solubility parameters were taken into consideration as part of the process. These nanosheets have been characterized by electron microscopy, atomic force microscopy, and x-ray diffraction, along with spectroscopy such as ultraviolet-visible spectroscopy, Raman spectroscopy, and x-ray photoelectron spectroscopy. The nanosheets were further used as a sensing material to fabricate a humidity sensor on interdigitated aluminum electrodes, realized over Si/SiO2 substrate using a conventional photolithography technique. The response for our sensor varied from 11.9 for 40% RH to as high as 37.5 for 80% RH. Response and recovery time were found to be 13 ± 2 s and 17 ± 2 s respectively. The suspended nanosheets were also treated with UV light in a nitrogen environment. The response for UV treated nanosheets shows better linearity, however its response decreases in the presence of humidity. This is due to a decrease in oxygen content of the UV treated sample. Furthermore, the effect of sonication time has been investigated, and it was found that samples with 10 h sonication are better than others due to their high surface-to-volume ratio. The repeatability and stability of the sensor have been investigated and found to be excellent. The hysteresis in the sensors was also explored. The mechanism of humidity sensing has been discussed in detail.

https://iopscience.iop.org/article/10.1088/0957-4484/27/47/475503/pdf

Liquid exfoliated pristine WS2 nanosheets for ultrasensitive and highly stable chemiresistive humidity 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.

/pdf/mose2-nanoflakes-based-chemiresistive-sensors-for-ppb-level-3qn1n2ubih.pdf

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

Polyaniline (PANI)/inorganic-nanomaterial based nanocomposites are attractive for gas sensing applications due to the synergistic behavior arising at their interfaces. In this work, HCl doped-PANI-nanofibers/WS2 nanosheet based nanocomposites were prepared by the template-free in situ polymerization of aniline on WS2 nanosheets, which were previously obtained by direct liquid exfoliation. The optimal performance was shown by the PANI/WS2 (w/w 10%) sample and it was characterized extensively using electron microscopy (SEM and TEM), several spectroscopic techniques (UV-Vis, Raman and FTIR), and X-ray diffraction. The nanomaterial was deposited on gold interdigitated electrodes which were then used as chemiresistive transducers for ammonia sensing at room temperature. The device was tested for ammonia concentrations of 50–200 ppm. The resistance of the device was found to increase with the concentration of ammonia vapours and a maximum response of up to 81% was recorded for 200 ppm of ammonia. The long-term stability, selectivity, sensitivity, and repeatability of the device were investigated. Apart from these, the device was also found to perform well under humid conditions. A mechanism based on the synergistic behavior due to the p–n heterogeneous characteristics at the interface of PANI and WS2 is proposed.

Ammonia vapour sensing properties of in situ polymerized conducting PANI-nanofiber/WS2 nanosheet composites

Tungsten sulfide is a promising, though less explored layered transition metal dichalcogenides material which can be exfoliated into mono and fewlayers with enhanced electronic, optoelectronics, sensing and catalytic properties. Liquid-phase exfoliation in high-boiling-point solvents (i.e., N-methyl-2-pyrrolidone), surfactant-assisted exfoliation and polymer-associated exfoliation are some of the widely used techniques for mass exfoliation of layered materials. Due to the high boiling point of these well-established solvents, surfactants and co-solvents, it is difficult to get pristine nanosheets, which are of highest importance in applications like gas sensing. The common solvents like acetone and water have been employed in this work along with low power ultrasonication waves to exfoliate bulk WS2 which makes this process low cost and convenient. The effect of freezing on exfoliation has been effectively utilized to tailor Hansen solubility parameters of solvents. Monolayers and few layers with lateral dimension of few hundred nanometers have been synthesized with yield of 2.7% and characterized extensively using electron microscopy, atomic force microscopy and optical spectroscopies. The nanosheets have been found to be stable with no precipitation formation for months. This method uses low-boiling-point solvents, hence guaranties the pristine two-dimensional surfaces.

An effective liquid-phase exfoliation approach to fabricate tungsten disulfide into ultrathin two-dimensional semiconducting nanosheets

This paper reports humidity sensing performance of WS2/GO hybrid nanostructures WS2, being a layered material, can be exfoliated into ultrathin nanosheets These nanosheets when combined with other materials with lamellar structures easily form hybrid structures even when processed in liquid Hybrid structures with materials like graphene oxide (GO) have several advantages for humidity sensing like increase in defect sites and oxygen rich groups, which ultimately enhance the performance of humidity sensor Interdigitated electrodes (made of aluminum), fabricated using photolithography technique on Si/SiO2 substrate, was used as sensor device platform The WS2/GO (in the wt% ratio of 1:3) sensing layer showed excellent optimal response when exposed to different humidity levels The response achieved was 658 times at 40% relative humidity (RH) and 590 times at 80% RH The response time and recovery time was also found to be good, eg, only 25 and 29 s, respectively, at 60% RH The device showed repeatable results when exposed to the same RH% The stability of device was tested for over a month and the response at each RH% have found to be the same with its day-zero performance A fast response combined with excellent repeatability due to low hysteresis loss and high stability (in the ambient) of the device makes it an excellent sensing material Overall, the performance of WS2/GO nanohybrid-based humidity sensors seems to be comparable to other established sensing nanomaterials used in solid state humidity sensors

Ravindra Kumar Jha

Papers

Liquid exfoliated pristine WS2 nanosheets for ultrasensitive and highly stable chemiresistive humidity sensors

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

Ammonia vapour sensing properties of in situ polymerized conducting PANI-nanofiber/WS2 nanosheet composites

An effective liquid-phase exfoliation approach to fabricate tungsten disulfide into ultrathin two-dimensional semiconducting nanosheets

WS 2 /GO Nanohybrids for Enhanced Relative Humidity Sensing at Room Temperature