Along with the progress of nanoscience and nanotechnology, nanomaterials with attractive structural and functional properties have gained more attention than ever before, especially in the field of electronic sensors. In recent years, the gas sensing devices have made great achievement and also created wide application prospects, which leads to a new wave of research for designing advanced sensing materials. There is no doubt that the characteristics are highly governed by the sensitive layers. For this reason, important advances for the outstanding, novel sensing materials with different dimensional structures including 0D, 1D, 2D, and 3D are reported and summarized systematically. The sensing materials cover noble metals, metal oxide semiconductors, carbon nanomaterials, metal dichalcogenides, g-C3 N4 , MXenes, and complex composites. Discussion is also extended to the relation between sensing performances and their structure, electronic properties, and surface chemistry. In addition, some gas sensing related applications are also highlighted, including environment monitoring, breath analysis, food quality and safety, and flexible wearable electronics, from current situation and the facing challenges to the future research perspectives.

Recent Progress of Nanostructured Sensing Materials from 0D to 3D: Overview of Structure–Property-Application Relationship for Gas Sensors

Conventional gas-sensing materials for detecting trace NO2 gas are suffering from limitations such as low selectivity, high operation temperature and an agglomerated tendency during synthesis. In this work, uniform CuO nanoflakes modified with rGO nanosheets (CuO/rGO) were synthesized via an ultra-low-cost hydrothermal process with a thermal treatment. The gas sensor based on the as-synthesized CuO/rGO was fabricated by spin-coated method and applied to detect ppb-level NO2 gas at room temperature (23℃). Remarkably, the CuO/rGO-based sensor maintained an ultrahigh response behavior of approximately 400.8 % towards 5 ppm NO2 gas with ultrafast response time of 6.8 s at 23℃, and the limit of detection was down to 50 ppb (S = 20.6 %). In addition, the gas sensor had outstanding consistency, reliable repeatability, and long-term stability (30 days). The improved gas-sensing response was attributed to the synergetic effects of CuO and rGO. Specifically, the porous flake-like nanostructures of CuO and high surface areas of CuO/rGO may provide additional active adsorption sites for NO2 gas and accelerate the redox reaction on the surface of CuO/rGO, leading to the ultrahigh response. On the other hand, the rGO nanosheets with excellent transport capability may serve as highly conductive channels to accelerate carrier transfer, which dramatically reduced the response and recovery time. Furthermore, abundant surface defects of CuO/rGO were also conducive to surface reactions in the gas-sensing process. Undoubtedly, the scientific study of the present work will promote potential applications of ppb-level NO2 detection at room temperature.

A room-temperature NO2 gas sensor based on CuO nanoflakes modified with rGO nanosheets

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

Due to rapid advancement in technology, recently a significant amount of work has been carried out in the field of multi-mode sensor and multifunctional sensor. A single unit of multifunctional sensor provides multiple measurements, which eventually reduces the cost of multiple sensors and makes the system compact. Though the risk of sensor failure and reliability of the multifunctional sensor is always imminent, researchers are working out different methods to make the sensor fault tolerant. Multifunctional sensors are getting widespread acceptance in a variety of fields. This paper provides a review of recent advances in multifunctional sensor technology in the perspective of a multi-sensor system. Different aspects of design and implementation of multifunctional sensor technology have been addressed in this paper. This paper can serve as a basic reading material for students and researchers pursuing research on a multifunctional sensor.

Recent Advances in Multifunctional Sensing Technology on a Perspective of Multi-Sensor System: A Review

NO2 is an oxidizing gas, which is considered to be the most dangerous and most toxic gas that should be controlled. Because of the advantages of high sensitivity, selectivity, stability and fast response/recovery speed, the chemical resistance sensor is used as a NO2 gas sensor. This article reviews the NO2 gas sensor based on metal oxides. Firstly, the hazards of NO2 and the major disadvantages of chemiresistive sensors based on metal oxides were discussed. Then, different sensitive materials of chemical resistance sensors were discussed, including metal oxides (MO), conductive polymers and carbon-based materials. Finally, the NO2 gas sensors based on metal oxide (including ZnO, NiO, CuO, SnO2, WO3) were discussed from the aspects of morphology, preparation method, modification, doping and composite. 

Metal Oxide Gas Sensors For Detecting NO2 in Industrial Exhaust Gas: Recent Developments

pH sensor is very important in various fields, and has triggered many types of devices based on different materials and mechanisms. How to improve the performances of the sensors is one of major challenges now. In the present paper, we developed an idea to improve the sensitivity of pH sensors based on AlGaN/GaN high-electron-mobility transistors (HEMTs) by introducing multi-sensing segments, and experimentally fabricated prototype devicesand then investigated their responses to aqueous solutions under different pH values. The optimized sensor exhibits a very high linear sensitivity of 1.35 mA/pH at drain-source voltage of 1.5 V, which is much higher than those of conventional pH sensors with single open gate sensing area. This indicates that the multi-sensing segments is very effective in increasing the sensitivity of the pH sensor, rather than simply increasing the sensing area. The sensors degrade after measurement in solution for a long working time, but they can recover to their initial states in a certain duration after washing, typically ∼24 h. Our finding paves new strategy for the future design of high sensitive and stable pH sensors based on HEMTs.

AlGaN/GaN heterostructure pH sensor with multi-sensing segments

In this work, an ultra-low-cost and ecofriendly solvothermal strategy was employed to prepare gas-sensing materials for heterostructured p-Sb2O3/n-SnO2 nanospheres. The morphology, microstructure and other characteristics of the prepared products were comprehensively studied by several analyzers. The results show that the present SnO2 nanospheres modified with Sb2O3, exhibit exceptional mesoporous structures and high specific surface areas, as well as prominent thermal stability (only 0.02 % mass loss). For the first time, the cost-effective gas sensor based on Sb2O3/SnO2 nanospheres was fabricated, which maintained excellent gas-sensing behavior of 697 % towards 100 ppm H2 gas at operating temperature of 280 ℃, with a fast response/recovery time of 29.8/13.4 s, and the limit of detection was down to 100 ppb (S = 7.1 %). In addition, the gas sensor had quite consistency, reproducibility, long-term stability (over 40 days), and unique selectivity towards H2 gas. Investigations into a plausible gas-sensing mechanism imply that the p-n heterojunction, catalytic function of Sb2O3, mesoporous structures and high specific surface area synergistically induced the superior gas-sensing response. Undoubtedly, the present work may provide an outstanding strategy to improve H2 gas response in extremely environmental applications.

Hydrogen gas sensor based on SnO2 nanospheres modified with Sb2O3 prepared by one-step solvothermal route

In this work, novel mesoporous heterostructures composed of CdS, PbS and SnO2 (CdS/PbS/SnO2) were synthesized via a green and facile treatment. Interestingly, when the CdS/PbS/SnO2 composites were assembled into sensing layer for the fabrication of H2 gas sensor for the first time, the sensor based on CdS/PbS/SnO2 exhibited more prominent gas-sensing properties than those of the CdS/SnO2 and PbS/SnO2 sensors. The CdS/PbS/SnO2 sensor showed a fast response/recovery time of 10.6/36.9 s towards 100 ppm H2 gas at 200℃, with an ultralow limit of detection of 50 ppb (17.3 %), and the sensor had a largely enhanced response of 1125.2 %, which was approximately 16.8 and 7.4 times higher than those of the CdS/SnO2 (66.8 %) and PbS/SnO2 (151.4 %) sensors. In addition, the CdS/PbS/SnO2 sensor had an outstanding selectivity towards H2 gas against other gases, reliable reversibility and long-term stability for 40 days. Such enhanced properties was mainly attributed to the large surface-to-volume ratio, which can provide abundant active sites to gas adsorbtion and diffusion in surface redox reaction. Moreover, more numerous heterojunctions of the CdS/PbS/SnO2 composites may serve as highly conductive channels to accelerate carrier transfer, thus further leading to an improved performance of the sensors. Credibly, our present work will foresee a great potential application for ppb-level H2 gas monitoring in an extreme environment.

Yan Dong

Papers

A room-temperature NO2 gas sensor based on CuO nanoflakes modified with rGO nanosheets

AlGaN/GaN heterostructure pH sensor with multi-sensing segments

Hydrogen gas sensor based on SnO2 nanospheres modified with Sb2O3 prepared by one-step solvothermal route

Facile synthesis of mesoporous CdS/PbS/SnO2 composites for high-selectivity H2 gas sensor

Applications of AlGaN/GaN high electron mobility transistor-based sensors in water quality monitoring