Resistive gas sensors for the detection of NH3 gas based on 2D WS2, WSe2, MoS2, and MoSe2: A review.

Transition metal dichalcogenides (TMDs) with a two-dimensional (2D) structure and semiconducting features are highly favorable for the production of NH3 gas sensors. Among the TMD family, WS2, WSe2, MoS2, and MoSe2 exhibit high conductivity and a high surface area, along with high availability, reasons for which they are favored in gas-sensing studies. In this review, we have discussed the structure, synthesis, and NH3 sensing characteristics of pristine, decorated, doped, and composite-based WS2, WSe2, MoS2, and MoSe2 gas sensors. Both experimental and theoretical studies are considered. Furthermore, both room temperature and higher temperature gas sensors are discussed. We also emphasized the gas-sensing mechanism. Thus, this review provides a reference for researchers working in the field of 2D TMD gas sensors.

2D nanomaterials for realization of flexible and wearable gas sensors: A review.

Gas sensors are extensively employed for monitoring and detection of hazardous gases and vapors. Many of them are produced on rigid substrates, but flexible and wearable gas sensors are needed for intriguing usage including the internet of things (IoT) and medical devices. The materials with the greatest potential for the fabrication of flexible and wearable gas sensing devices are two-dimensional (2D) semiconducting nanomaterials, which consist of graphene and its substitutes, transition metal dichalcogenides, and MXenes. These types of materials have good mechanical flexibility, high charge carrier mobility, a large area of surface, an abundance of defects and dangling bonds, and, in certain instances adequate transparency and ease of synthesis. In this review, we have addressed the different 2D nonmaterial properties for gas sensing in the context of fabrication of flexible/wearable gas sensors. We have discussed the sensing performance of flexible/wearable gas sensors in various forms such as pristine, composite and noble metal decorated. We believe that content of this review paper is greatly useful for the researchers working in the research area of fabrication of flexible/wearable gas sensors.

Utilizing Data Mining for the Synthesis of Functionalized Tungsten Oxide with Enhanced Oxygen Vacancies for Highly Sensitive Detection of Triethylamine.

The optimal combination of metal ions and ligands for sensing materials was estimated by using a data-driven model developed in this research. This model utilized advanced computational algorithms and a data set of 100,000 literature pieces. The semiconductor metal oxide (SMO) that is most suitable for detecting triethylamine (TEA) with the highest probability was identified by using the Word2vec model, which employed the maximum likelihood method. The loss function of the probability distribution was minimized in this process. Based on the analysis, a novel hierarchical nanostructured tungsten-based coordination with 2,5-dihydroxyterephthalic acid (W-DHTA) was synthesized. This synthesis involved a postsynthetic hydrothermal treatment (psHT) and the self-assembly of tungsten oxide nanorods. The tungsten oxide nanorods had a significant number of oxygen vacancies. Various techniques were used to characterize the synthesized material, and its sensing performance toward volatile organic compound (VOC) gases was evaluated. The results showed that the functionalized tungsten oxide exhibited an exceptionally high sensitivity and selectivity toward TEA gas. Even in a highly disturbed environment, the detection limit for TEA gas was as low as 40 parts per billion (ppb). Furthermore, our findings suggest that the control of oxygen vacancies in sensing materials plays a crucial role in enhancing the sensitivity and selectivity of gas sensors. This approach was supported by the utilization of density functional theory (DFT) computation and machine learning algorithms to assess and analyze the performance of sensor devices, providing a highly efficient and universally applicable research methodology for the development and design of next-generation functional materials.

Functionalized Multiwalled Carbon Nanotubes for Highly Stable Room Temperature and Humidity-Tolerant Triethylamine Sensing.

Triethylamine (TEA) poses a significant threat to our health and is extremely difficult to detect at the parts-per-billion (ppb) level at room temperature. Carbon nanotubes (CNTs) are versatile materials used in chemiresistive vapor sensing. However, achieving high sensitivity and selectivity with a low detection limit remains a challenge for pristine CNTs, hindering their widespread commercial application. To address these issues, we propose functionalized multiwalled CNTs (MWCNTs) with carboxylic acid (COOH)-based sensing channels for ultrasensitive TEA detection under ambient conditions. Advanced structural analyses confirmed the necessary modification of MWCNTs after functionalization. The sensor exhibited excellent sensitivity to TEA in air, with a superior noise-free signal (10 ppb), an extremely low limit of detection (LOD ≈ 0.8 ppb), excellent repeatability, and long-term stability under ambient conditions. Moreover, the response values became more stable, demonstrating excellent humidity resistance (40-80% RH). Notably, the functionalized MWCNT sensor exhibited improved response and recovery kinetics (200 and 400 s) to 10 ppm of TEA compared to the pristine MWCNT sensor (400 and 1300 s), and the selectivity coefficient for TEA gas was improved by approximately three times against various interferants, including ammonia, formaldehyde, nitrogen dioxide, and carbon monoxide. The remarkable improvements in TEA detection were mainly associated with the large specific surface area, abundant active sites, adsorbed oxygen, and other defects. The sensing mechanism was thoroughly explained by using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and gas chromatography-mass spectrometry (GC-MS). This study provides a new platform for CNT-based chemiresistive sensors with high selectivity, low detection limits, and enhanced precision with universal potential for applications in food safety and environmental monitoring.

Room Temperature Chemiresistive Gas Sensors Based on 2D MXenes.

Owing to their large surface area, two-dimensional (2D) semiconducting nanomaterials have been extensively studied for gas-sensing applications in recent years. In particular, the possibility of operating at room temperature (RT) is desirable for 2D gas sensors because it significantly reduces the power consumption of the sensing device. Furthermore, RT gas sensors are among the first choices for the development of flexible and wearable devices. In this review, we focus on the 2D MXenes used for the realization of RT gas sensors. Hence, pristine, doped, decorated, and composites of MXenes with other semiconductors for gas sensing are discussed. Two-dimensional MXene nanomaterials are discussed, with greater emphasis on the sensing mechanism. MXenes with the ability to work at RT have great potential for practical applications such as flexible and/or wearable gas sensors.

Enhancing MgO efficiency in CO2 capture: engineered MgO/Mg(OH)2 composites with Cl-, SO42-, and PO43- additives.

The formation of a MgCO3 shell hampers CO2 capture efficiency in MgO. Our previous studies developed MgO/Mg(OH)2 composites to facilitate CO2 diffusion, improving capture efficiency. However, MgCO3 still formed along the interfaces. To tackle this issue, we engineered the MgO/Mg(OH)2 interfaces by incorporating Cl-, SO42-, and PO43- additives. Novel MgO-H2O-MgX (X = Cl-, SO42-, and PO43-) composites were synthesized to explore the role of additives in preventing MgCO3 formation. MgO-Mg(OH)2-MgCl2 nano-composites displayed enhanced CO2 adsorption and stability. This breakthrough paves the way for effective bio-inspired strategies in overcoming CO2 transport barriers in MgO-based adsorbents.

Metal Oxide Nanowires Grown by a Vapor-Liquid-Solid Growth Mechanism for Resistive Gas-Sensing Applications: An Overview.

Metal oxide nanowires (NWs) with a high surface area, ease of fabrication, and precise control over diameter and chemical composition are among the best candidates for the realization of resistive gas sensors. Among the different techniques used for the synthesis of materials with NW morphology, approaches based on the vapor-liquid-solid (VLS) mechanism are very popular due to the ease of synthesis, low price of starting materials, and possibility of branching. In this review article, we discuss the gas-sensing features of metal oxide NWs grown by the VLS mechanism, with emphasis on the growth conditions and sensing mechanism. The growth and sensing performance of SnO2, ZnO, In2O3, NiO, CuO, and WO3 materials with NW morphology are discussed. The effects of the catalyst type, growth temperature, and other variables on the morphology and gas-sensing performance of NWs are discussed.

Classification and concentration estimation of CO and NO2 mixtures under humidity using neural network-assisted pattern recognition analysis.

This study addresses the concerns regarding the cross-sensitivity of metal oxide sensors by building an array of sensors and subsequently utilizing machine earning techniques to analyze the data from the sensor arrays. Sensors were built using In2O3, Au-ZnO, Au-SnO2, and Pt-SnO2 and they were operated simultaneously in the presence of 25 different concentrations of nitrogen dioxide (NO2), carbon monoxide (CO), and their mixtures. To investigate the effects of humidity, experiments were conducted to detect 13 distinct CO and NO2 gas combinations in atmospheres with 40% and 90% relative humidity. Principal component analysis was performed for the normalized resistance variation collected for a particular gas atmosphere over a certain period, and the results were used to train deep neural network-based models. The dynamic curves produced by the sensor array were treated as pixelated images and a convolutional neural network was adopted for classification. An accuracy of 100% was achieved using both models during cross-validation and testing. The results indicate that this novel approach can eliminate the time-consuming feature extraction process.

Layer-dependent stability of 2D mica nanosheets.

We report on the layer-dependent stability of muscovite-type two-dimensional (2D) mica nanosheets (KAl3Si3O10(OH)2). First-principles calculations on mica nanosheets with different layer thicknesses (n = 1, 2, and 3) reveal their layer-dependent stability; odd-numbered 2D mica nanosheets are more stable than even-numbered ones, and the preferable stability of odd-numbered layers originates from electronic effects. A core-shielding model is proposed with a reasonable assumption, successfully proving the instability of the even-numbered mica nanosheets. Raman imaging supports that the population of odd-numbered mica nanosheets is predominant in exfoliated mica products. The alternating charge states with odd/even layers were evidenced by Kelvin probe force microscopy. We also demonstrate a unique photocatalytic degradation, opening new doors for environmental applications of mica nanosheets.

Tungsten oxide nanostructures peculiarity and photocatalytic activity for the efficient elimination of the organic pollutant.

The WO3 nanostructures were synthesized by a simple hydrothermal route in the presence of C14TAB and gemini-based twin-tail surfactant. The impact of using these special shape and size directing agents for the synthesis of nanostructures was observed in the form of different shapes and sizes. The WO3 web of chains type nanostructure was obtained using C14TAB in comparison to the cube-shaped nanoparticles through twin-tail surfactant. On contrary, the twin-tail surfactant provides sustainable and controlled growth of cube shape nanoparticles of size ~ 15 nm nearly half of the size ~ 35 nm obtained using conventional surfactant C14TAB, respectively. For the detailed structural features, the Williamson-Hall analysis method was implemented to find out the crystalline size and lattice strain of the prepared nanostructures. Owing to the strong quantum confinement effect, the WO3 cube-shaped nanoparticles with an optical band gap of 2.69 eV of the prepared nanoparticles showed excellent photocatalytic efficacy toward organic pollutant (fast green FCF) compared to the web of chain nanostructures with an optical band gap of 2.66 eV. The ability of the prepared systems to decompose the organic pollutant (fast green FCF) in water was tested under visible light irradiations. The percentage degradation was found to be 94% and 86% for WO3 cube-shaped nanoparticles and WO3 web of chains, respectively. The simplicity of the fabrication method and the high photocatalytic performance of the systems can be promising in environmental applications to treat water pollution.

Three-Dimensional van der Waals Heterostructure-Based Nanocages as Supersensitive 3-Hydroxy-2-butanone Gas Sensors at Room Temperature.

3-Hydroxy-2-butanone is one of the biomarkers of Listeria monocytogenes, which is quite important for the intelligent detection of 3H-2B. However, it is still a challenge to fabricate sensing materials obtaining excellent sensitivity and selectivity under the ppb-level detection limit. Herein, a plasma-assisted synthetic approach was proposed for the construction of hierarchical nanostructures and the simultaneous loading of TAPP-COFs, which could reduce interlayer interaction and convert the metallized sites on the surface of predesigned porphyrin rings into quantum nanoparticles. These multichannel pathways of Co-TAPP-COFs@SnO2@MWCNTs nanocages contributed to the gas adsorption and diffusion, thus enhancing the sensing behavior. The nanocages exhibited a highly specific sensing performance toward 3H-2B with the highest sensitivity (Ra/Rg = 100.9 to 0.5 ppm) in all reported sensing materials. The 3H-2B sensor presented outstanding long-term stability, and the detection limit was 100 ppb at room temperature. Furthermore, the synthesized materials were integrated into the sensing module connecting to an Internet of Things platform, providing rapid and real-time detection of 3H-2B. We also applied machine learning methods to analyze the nanocage-based sensors and found that the combined effects of modified sites on the heterointerfaces contributed to the improvement of the sensing performance.

Ethanol Electro-Oxidation on Catalysts with S-ZrO2-Decorated Graphene as Support in Fuel Cell Applications.

Direct ethanol fuel cells (DEFCs) are considered the most suitable direct alcohol fuel cell (DAFC) in terms of safety and current density. The obstacle to DEFC commercialization is the low reaction kinetics of ethanol (C2H5OH) oxidation because of the poor performance of the electrocatalyst. In this study, for the first time, graphene nanoplates (GNPs) were coated with sulfated zirconium dioxide (ZrO2) as adequate support for platinum (Pt) catalysts in DEFCs. A Pt/S-ZrO2-GNP electrocatalyst was prepared by a new process, polyol synthesis, using microwave heating. Field emission scanning electron microscope (FESEM) imaging revealed well-dispersed platinum nanoparticles supported on the S-ZrO2-GNP powder. Analysis of the Fourier transform infrared (FTIR) spectrometry confirmed that sulfate modified the surfaces of the sample. In X-ray diffraction (XRD), no effect of S-ZrO2 on the crystallinity net in Pt was found. Pt/S-ZrO2-GNP electrode outperformed those with unsulfated counterparts, primarily for the higher access with electron and proton, confirming sulfonating as a practical approach for increasing the performance, electrocatalytic activity, and carbon monoxide (CO) tolerance in an electrocatalyst. A considerable decrease in the voltage of the CO electrooxidation peak from 0.93 V for Pt/C to 0.76 V for the Pt/S-ZrO2-GNP electrode demonstrates that the new material increases activity for CO electrooxidation. Moreover, the as-prepared Pt/S-ZrO2-GNPs electrocatalyst exhibits high catalytic activity for the EOR in terms of electrochemical surface area with respect to Pt/ZrO2-GNPs and Pt/C (199.1 vs. 95 and 67.2 cm2.mg-1 Pt), which may be attributed to structural changes caused by the high specific surface area of graphene nanoplates catalyst support and sulfonating effect as mentioned above. Moreover, EIS results showed that the Pt/S-ZrO2-GNPs electrocatalyst has a lower charge transfer resistance than Pt/ ZrO2-GNPs and Pt/C in the presence of ethanol demonstrating an increased ethanol oxidation activity and reaction kinetics by Pt/S-ZrO2-GNPs.

Resistive-Based Gas Sensors Using Quantum Dots: A Review.

Quantum dots (QDs) are used progressively in sensing areas because of their special electrical properties due to their extremely small size. This paper discusses the gas sensing features of QD-based resistive sensors. Different types of pristine, doped, composite, and noble metal decorated QDs are discussed. In particular, the review focus primarily on the sensing mechanisms suggested for these gas sensors. QDs show a high sensing performance at generally low temperatures owing to their extremely small sizes, making them promising materials for the realization of reliable and high-output gas-sensing devices.

Counter-Intuitive Magneto-Water-Wetting Effect to CO2 Adsorption at Room Temperature Using MgO/Mg(OH)2 Nanocomposites.

MgO/Mg(OH)2-based materials have been intensively explored for CO2 adsorption due to their high theoretical but low practical CO2 capture efficiency. Our previous study on the effect of H2O wetting on CO2 adsorption in MgO/Mg(OH)2 nanostructures found that the presence of H2O molecules significantly increases (decreases) CO2 adsorption on the MgO (Mg(OH)2) surface. Furthermore, the magneto-water-wetting technique is used to improve the CO2 capture efficiency of various nanofluids by increasing the mass transfer efficiency of nanobeads. However, the influence of magneto-wetting to the CO2 adsorption at nanobead surfaces remains unknown. The effect of magneto-water-wetting on CO2 adsorption on MgO/Mg(OH)2 nanocomposites was investigated experimentally in this study. Contrary to popular belief, magneto-water-wetting does not always increase CO2 adsorption; in fact, if Mg(OH)2 dominates in the nanocomposite, it can actually decrease CO2 adsorption. As a result of our structural research, we hypothesized that the creation of a thin H2O layer between nanograins prevents CO2 from flowing through, hence slowing down CO2 adsorption during the carbon-hydration aging process. Finally, the magneto-water-wetting technique can be used to control the carbon-hydration process and uncover both novel insights and discoveries of CO2 capture from air at room temperature to guide the design and development of ferrofluid devices for biomedical and energy applications.

Gate-controlled gas sensor utilizing 1D-2D hybrid nanowires network.

Novel gas sensors that work at room temperature are attracting attention due to their low energy consumption and stability in the presence of toxic gases. However, the development of sensing characteristics at room temperature is still a primary challenge. Diverse reaction pathways and low adsorption energy for gas molecules are required to fabricate a gas sensor that works at room temperature with high sensitivity, selectivity, and efficiency. Therefore, we enhanced the gas sensing performance at room temperature by constructing hybridized nanostructure of 1D-2D hybrid of SnSe2 layers and SnO2 nanowire networks and by controlling the back-gate bias (Vg = 1.5 V). The response time was dramatically reduced by lowering the energy barrier for the adsorption on the reactive sites, which are controlled by the back gate. Consequently, we believe that this research could contribute to improving the performance of gas sensors that work at room temperature.

Effect of Ag Addition on the Gas-Sensing Properties of Nanostructured Resistive-Based Gas Sensors: An Overview.

Nanostructured semiconducting metal oxides (SMOs) are among the most popular sensing materials for integration into resistive-type gas sensors owing to their low costs and high sensing performances. SMOs can be decorated or doped with noble metals to further enhance their gas sensing properties. Ag is one of the cheapest noble metals, and it is extensively used in the decoration or doping of SMOs to boost the overall gas-sensing performances of SMOs. In this review, we discussed the impact of Ag addition on the gas-sensing properties of nanostructured resistive-based gas sensors. Ag-decorated or -doped SMOs often exhibit better responsivities/selectivities at low sensing temperatures and shorter response times than those of their pristine counterparts. Herein, the focus was on the detection mechanism of SMO-based gas sensors in the presence of Ag. This review can provide insights for research on SMO-based gas sensors.

Proton-beam engineered surface-point defects for highly sensitive and reliable NO2 sensing under humid environments.

Cross-interference with humidity is a major limiting factor for the accurate detection of target gases in semiconductor metal-oxide gas sensors. Under humid conditions, the surface-active sites of metal oxides for gas adsorption are easily deactivated by atmospheric water molecules. Thus, development of a new approach that can simultaneously improve the two inversely related features for realizing practical gas sensors is necessary. This paper presents a facile method to engineer surface-point defects based on proton-beam irradiation. The sensor irradiated with a proton beam shows not only an improved NO2 response but also considerable tolerance toward humidity. Based on surface analyses and DFT calculations, it is found that proton beams induce three types of point defects, which make NO2 molecules preferentially adsorb on the ZnO surfaces compared to H2O molecules, eventually enabling improved NO2 detection with less humidity interference.

Gas sensing materials roadmap.

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

Reduced Graphene Oxide (rGO)-Loaded Metal-Oxide Nanofiber Gas Sensors: An Overview.

Reduced graphene oxide (rGO) is a reduced form of graphene oxide used extensively in gas sensing applications. On the other hand, in its pristine form, graphene has shortages and is generally utilized in combination with other metal oxides to improve gas sensing capabilities. There are different ways of adding rGO to different metal oxides with various morphologies. This study focuses on rGO-loaded metal oxide nanofiber (NF) synthesized using an electrospinning method. Different amounts of rGO were added to the metal oxide precursors, and after electrospinning, the gas response is enhanced through different sensing mechanisms. This review paper discusses rGO-loaded metal oxide NFs gas sensors.

CsPbI3NC-Sensitized SnO2/Multiple-Walled Carbon Nanotube Self-Assembled Nanomaterials with Highly Selective and Sensitive NH3 Sensing Performance at Room Temperature.

It is an effective strategy to enhance the sensitivity of semiconductor metal oxides (SMOs) being sensitized with CsPbI3 nanocrystals (NCs) by adjusting the heterostructure between CsPbI3NC and SMO nanomaterials. In this work, for the first time, a porous 3D multiple-walled carbon nanotube (MWCNT) network uniformly coated with SnO2 quantum nanoparticles (QNPs) and CsPbI3 nanocrystals were prepared via a simple solvent vapor-induced self-assembly method. The fabricated CsPbI3NC-SnO2QNP/MWCNT nanocomposite with vapor-induced self-assembly exhibits superior stability against the moisture as well as an excellent sensing response. The results imply that the rational design of the metal halide perovskite NC/SMO heterostructure can not only improve the stability but also meet the requirements of sensing application. The self-assembled SnO2QNP/MWCNT can facilitate the dispersion of small-sized nanoparticles and efficaciously prevent the detachment of CsPbI3NC. Compared with pristine SnO2QNP and SnO2/MWCNT sensors, the CsPbI3NC-modified SnO2QNP/MWCNT nanostructure exhibited a remarkable sensitivity of 39.2 for 0.2 ppm NH3, rapid response/recovery time of 17/18 s, and excellent selectivity towards NH3. In particular, we applied machine learning methods, including principal component analysis (PCA) and support vector machines (SVMs), to analyze the sensing performance of the CsPbI3NC-SnO2QNP/MWCNT sensor and found that the combined effects of CsPbI3NC-SnO2QNP/MWCNT heterointerfaces contributed to the improvement of selectivity of sensors. The excellent NH3 for sub-ppm level concentration is ascribed to the high sensing activity of the CsPbI3 NC-based heterojunction. This work may not only enrich the family of high-performance breath detection materials but also provide a good example for designing reasonable composite materials with specific properties in the field of metal halide perovskite/SMO heterojunctions.