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

TiO2/ZnO heterostructure nanowire based NO2 sensor

In this report, we present improved NO2 gas sensing properties based on CdTe-functionalized ZnO filled porous Si (PSi) hybrid hierarchical nanostructured thin films (CdTe/ZnO@PSi). The CdTe and ZnO nanostructured thin films were grown on the PSi substrate using a scalable magnetron sputtering technique. The PSi substrate was prepared using the electrochemical anodization method. The proposed CdTe/ZnO@PSi sensor was characterized in terms of structural, morphological, and compositional properties. The sensing performance and corresponding mechanism of NO2 gas sensor based on CdTe/ZnO@PSi were described comprehensively. In order to compare, the sensing characteristics of both CdTe/ZnO@PSi and pristine ZnO@PSi thin film sensors were tested in different temperature range varied from 30 °C to 180 °C for 1 ppm concentration of NO2 gas. We found that CdTe/ZnO@PSi sensor displays enhanced NO2 sensing characteristics (∼3.5-fold greater sensor response ∼19.82 %, fast response/recovery time ∼13 s/54 s) at the relatively low working temperature of 90 °C towards 1 ppm NO2 gas in comparison to the pristine ZnO@PSi sensor (sensor response ∼5.72 % and response/recovery time ∼41 s/124 s at an operating temperature of 150 °C). Further, the fabricated NO2 gas sensor demonstrated excellent reversibility and good stability, and enabling CdTe/ZnO@PSi a promising candidate for its practical use in IoT sensing devices.

Low-temperature highly selective and sensitive NO2 gas sensors using CdTe-functionalized ZnO filled porous Si hybrid hierarchical nanostructured thin films

2D materials and their heterojunctions have been explored for gas sensing applications due to their tremendous surface-to-volume ratio, active edges with atomic thickness, and tunable electrical properties. Heterostructures of 2D materials exhibit absolutely novel physics and versatility with accelerated device performance by integrating the atomic scale properties of individual materials. Traditional gas sensors use homogeneous materials as the sensing interface in which the surface adsorbed oxygen ion species play an important role in its performance. But the performance of the sensors suffers greatly due to their selectivity and high working temperature leading to poor stability and short-term uses; the van der Waals 2D p-n heterojunction-based gas sensors hold several advantages since both the materials and the depletion layer formed at the junction can actively tune the sensing performance. By choosing 2D materials with different band structures, charge polarity, carrier concentration, and work function, band alignment at the interface can be precisely engineered to achieve the selective gas sensing performance with low operating temperature. Herein, we have reviewed the working principles, recent developments, and future perspectives of p-n heterojunctions of 2D materials for gas sensing applications.

A review on mechanisms and recent developments in p-n heterojunctions of 2D materials for gas sensing applications

Role of sensitizers in imparting the selective response of SnO2/RGO based nanohybrids towards H2S, NO2 and H2

Here, we demonstrate improved NO2 gas sensing properties based on reduced graphene oxide (rGO) decorated V2O5 thin film. Excluding the DC sputtering grown V2O5 thin film, rGO was spread over V2O5 thin film by the drop cast method. The formation of several p-n heterojunctions was greatly affected by the current-voltage relation of the rGO-decorated V2O5 thin film due to the p-type and n-type nature of rGO and V2O5, respectively. Initially with rGO decoration on V2O5 thin film, current decreased in comparison to the pristine V2O5 thin film, whereas depositing rGO film on a glass substrate drastically increased current. Among all sensors, only the rGO-decorated V2O5 sensor revealed a maximum NO2 gas sensing response for 100 ppm at 150 °C, and it achieved an approximately 61% higher response than the V2O5 sensor. The elaborate mechanism for an extremely high sensing response is attributed to the formation and modulation of p-n heterojunctions at the interface of rGO and V2O5. In addition, the presence of active sites like oxygenous functional groups on the rGO surface enhanced the sensing response. On that account, sensors based on rGO-decorated V2O5 thin film are highly suitable for the purpose of NO2 gas sensing. They enable the timely detection of the gas, further protecting the ecosystem from its harmful effects.

https://iopscience.iop.org/article/10.1088/1361-6528/ab0321/pdf

NO2 gas sensing performance enhancement based on reduced graphene oxide decorated V2O5 thin films.

We report synthesis, characterization and IR photoresponse properties of 150 ± 10 nm thick high quality VO2(M1) thin films synthesized by DC reactive sputtering. Phase formation was confirmed by X-ray diffraction and Raman spectroscopic measurements. Morphology and microstructure were analysed by atomic force microscope, scanning electron microscope and transmission electron microscope which revealed polycrystalline nature of nanosized films with root mean square (rms) roughness value of 8 ± 0.7 nm. Electrical measurements revealed 1st order transition of thin films with a change in resistance of more than two orders of magnitude and temperature coefficient of resistance, TCR of − 1.24% K−1 at 30 °C. The fabricated VO2(M1) IR photodetector exhibited excellent reproducible photoresponse properties when subjected to a 1064 nm laser under 250 mW cm−2 power density with a bias voltage of 5 V at the ambient conditions of temperature and pressure. The sensitivity, responsivity, external quantum efficiency and specific detectivity were observed to be 1775%, 40.09 mA W−1, 4.67% and 7.07 × 1011 Jones, respectively.

Highly photoresponsive VO2(M1) thin films synthesized by DC reactive sputtering

Kalanchoe pinnata and Ficus racemosa are naturally occurring plant products that are gaining importance because of their excellent medicinal properties. Determining the toxicity of a plant extract is crucial, based on which the therapeutic dose of extracts can be adjusted. The present study evaluates phytochemical screening and toxicity studies (acute and sub-acute) of the Kalanchoe pinnata and Ficus racemosa plant products. Phytochemical screening showed the presence of alkaloids, tannins, flavonoids, steroids, phenols, and glycosides. The acute toxicity study of both the plant extracts was performed in Wistar albino rats at a single dose of 2000mg/kg body weight in both genders. Sub-acute toxicity study was performed in Wistar albino rats (both male and female) for 28 days at various doses of 500, 1000, and 2000 mg/kg body weight. The evaluation results of the acute toxicity study did not show any signs of toxicity, behavioral changes, mortality, or differences in gross histopathology appearance. The results of the sub-acute toxicity study also showed no changes in body weight, toxic signs, mortality, or behavioral changes. All biochemical and hematological parameters were in normal ranges comparable to the control group's. At the end of the treatment period, all the rats were sacrificed, and the liver and kidneys were sent for histopathological examination. No abnormalities were observed in the treatment groups. The present study concludes that both plant extracts are safe, and the LD50 of both plant extracts was above 2000mg/kg bw.

https://doi.org/10.22376/ijlpr.2023.13.4.p52-p60

D. Sheela

Papers

NO2 gas sensing performance enhancement based on reduced graphene oxide decorated V2O5 thin films.

Highly photoresponsive VO2(M1) thin films synthesized by DC reactive sputtering

Toxicity Studies and Phytochemical Screening of Ficus Racemosa and Kalanchoe Pinnata