Development of Highly Sensitive and Humidity Independent Room Temeprature NO2 Gas Sensor Using Two Dimensional Ti3C2Tx Nanosheets and One Dimensional WO3 Nanorods Nanocomposite

Novel metal oxides partially derived perovskite-structured hydroxides for room temperature trace NO2 gas sensors under UV irradiation

Perovskite-structured materials are used as gas-sensitive materials due to their wide bandgap and controllable morphology, but large initial resistance and low response limit their development. In this work, ZnSn(OH)6/ZnO composites derived from ZnO were synthesized by hydrothermal method. The gas-sensitive results show that all sensors show significantly improved response to NO2 under UV irradiation compared with without UV irradiation. Notably, under UV irradiation the operating temperatures of sensors are reduced from 110 °C to room temperature and have a low initial resistance. Compared with bare ZnO, the ZnSn(OH)6/ZnO-7 sensor shows a 4.4-fold improvement in response to 10 ppm NO2 under UV irradiation at room temperature and has response/recovery time (54.5/74 s). Meanwhile, the ZnSn(OH)6/ZnO-7 sensor has a practical detection limit of 50 ppb and a theoretical detection limit of 9.86 ppb, which enable efficient detection of trace NO2. The excellent gas-sensitive performance of the ZnSn(OH)6/ZnO sensors can be attributed to the highly efficient photogenerated carrier separation efficiency, the special morphology, high oxygen vacancy content and the construction of numerous heterostructures. Therefore, the ZnSn(OH)6/ZnO sensors provide insights into the realization of high-performance perovskite-structured NO2 sensors under UV irradiation at room temperature.

The MoS2/ZnO p-n heterostructure arrays for ultrasensitive ppb-level self-supporting NO2 gas sensors under UV irradiation

Light irradiation has emerged as a promising strategy to promote low operating temperatures of metal oxides semiconductors gas sensors. Traditional sensors have high operating temperatures, low electron-hole separation, and low gas response. Therefore, MoS2/ZnO heterostructure arrays were synthesized based on ITO conductive glass by hydrothermal and calcination methods as self-supporting sensors. Self-supporting sensors overcome limitations of traditional sensor fabrication. The successful preparation of self-supporting sensors is confirmed by a series of tests. The response of the gas sensor is determined as Rg/Ra or Ra/Rg (Ra and Rg indicate the resistance of the sensor in air and test gases). Regarding the gas-sensing performance, MoS2/ZnO-20 self-supporting sensor under UV irradiation exhibits ultrahigh response of 1088.43 to 10 ppm NO2 at 80 °C, which is 47 times higher than pure ZnO (23.21). Furthermore, operating temperature under UV irradiation is reduced by up to 60 °C. Additionally, MoS2/ZnO-20 self-supporting sensor demonstrates rapid response/recovery time (100/3 s), high selectivity, and ultralow theoretical detection limit (10.37 ppb). The p-n charge separation mechanism is employed to elucidate sensing mechanism of MoS2/ZnO self-supporting sensor for NO2 under UV irradiation. The efficient photogenerated carrier separation efficiency, large surface area, and the presence of multiple heterostructures are responsible for the high gas-sensing performance of MoS2/ZnO self-supporting sensor. Therefore, this study offers insights into the fabrication of ultrasensitive self-supporting sensors for low-temperature detection of NO2 under light irradiation.

Ultrasensitive Room-Temperature NO2 Gas Sensor Based on MXene-Cu2O Composites

The development of real-time trace-level NO2 quantification platforms that can be operated at room temperature constitutes a critical advancement for occupational safety and public health monitoring systems. This study demonstrates a room-temperature NO2 sensor using MXene-Cu2O composites prepared via a hydrothermal method. Systematic evaluation of MXene-introduced effects identified the 0.84 wt % MXene-Cu2O composite as optimal, exhibiting 4-fold enhanced sensitivity and shorter response (55 s)/recovery (35 s) time compared to pure Cu2O. Additionally, the sensor exhibits a low detection limit (10 ppb), high selectivity, great reversibility, and long-term stability. The enhanced sensing performance originates from precisely engineered interfacial architectures between MXene and Cu2O, which effectively adjust the charge-transfer behavior through the conduction tunnel in the sensing material. Furthermore, oxygen vacancy engineering creates defect-mediated adsorption centers that promote selective NO2 chemisorption through charge polarization effects. This research offers a novel strategy for designing optimized structures to enhance the sensitivity of MOS-based materials for NO2 gas detection.

Recent advances in 2D MXene-based heterostructures for gas sensing: mechanisms and applications in environmental and biomedical fields

MXenes, a unique class of 2D transition metal carbides, have gained attention for gas sensing applications due to their distinctive properties. Since the synthesis of Ti3C2Tx MXene in 2011, significant progress has been made in using MXenes as chemiresistive sensors. Their layered structure, abundant surface groups, hydrophilicity, tunable conductivity, and excellent thermal properties make MXenes ideal for low-power, flexible, room temperature gas sensors, fostering scalable and reproducible applications in portable devices. This review evaluates the latest advancements in MXene-based gas sensors, beginning with an overview of the elemental compositions, structures, and typical fabrication process of MXenes. We subsequently examine their applications in gas sensing domains, evaluating the proposed mechanisms for detecting common volatile organic compounds such as acetone, formaldehyde, ethanol, ammonia, and nitrogen oxides. To set this apart from similar reviews, our focus centered on the mechanistic interactions between MXene sensing materials and analytes (particularly for chemiresistive gas sensors), leveraging the distinct functionalities of MXene chemistries, which can be finely tuned for specific applications. Ultimately, we examine the current limitations and prospective research avenues concerning the utilization of MXenes in environmental and biomedical applications.

Microwave Sensor with Light-Assisted Enhancement Based on Ti3C2Tx MXene: Toward ppb-Level NO2 Detection Limits

Chemiresistive sensors are currently the most popular gas sensors, and metal semiconductor oxides are often used as sensitive materials (SMs). However, their high operating temperature means that more energy is required to maintain normal operation of the SM, resulting in an increase in power consumption of the entire sensing system. In order to solve this problem, a microwave gas sensor embedded with multilayer Ti3C2Tx MXene and split ring resonator (SRR) for nitrogen dioxide (NO2) detection was reported in this work. The sensor takes advantage of the weak coupling between the two SRRs to achieve a highly concentrated electric field and high Q-factor, in which the weak coupling region serves as the sensitive region to avoid damage to the resonator structure caused by the excessive conductivity of Ti3C2Tx. The sensor has good selectivity, a lower detection limit of 2 ppb, with an average detection sensitivity of 98.66 mdB ppm-1 in the range of 2-10,000 ppb at room temperature. Additionally, the effect of different lighting source to the sensor performance is investigated, and the sensor reached the best response (1.54 dB) under blue light. Finally, the mechanism of the enhanced sensitivity is discussed in detail based on the results of simulations and tests. The sensor circuit designed in this work provides a new approach for MGSs and for the first time introduces the photocatalytic pathway into microwave sensors, which will contribute to the optimization of microwave gas sensors in the future.

Polyoxometalate Cluster-Guided Dynamic Nucleation and Hierarchical Growth of Branched WO3 Nanofibers with Ultrafine Pt Nanoparticles for Advanced Gas Sensing

In the food industry, 2,3-butanedione is a significant volatile organic compound valued for its unique aroma and flavor. Real-time detection of its concentration during food preparation is crucial for ensuring optimal taste and food safety. However, accurately detecting low concentrations of 2,3-butanedione requires highly sensitive sensing materials. Herein, we present a novel synthesis of branched WO3 nanofibers decorated with ultrafine Pt nanoparticles (Pt NPs-WO3 NFs), templated by polyoxometalate (POM) clusters, through a combination of electrospinning and thermal oxidation strategies for advanced gas sensing applications. This Pt NPs-WO3 NFs-based sensor exhibits impressive sensitivity (Ra/Rg = 2.25 vs 500 ppb), a low detection limit of 10 ppb, high selectivity, excellent repeatability, and stable performance over a period of 25 days. Using POM clusters as templates offers significant advantages over the traditional WCl6 salt in synthesizing WO3 NFs with smooth surfaces. Specifically, the POM clusters guide the dynamic nucleation and hierarchical growth of branched NFs, enhancing the concentration of oxygen vacancies and increasing the number of active adsorption sites. Furthermore, the uniform dispersion of ultrafine Pt NPs (≈ 4 nm) within the WO3 NFs further enhances the catalytic activation of 2,3-butanedione, significantly improving the gas sensing performance. This study introduces an efficient method to synthesize Pt NPs-WO3 NFs with potential for manufacturing advanced nanostructured sensing materials using POM clusters as templates, paving the way for high-performance gas sensing technologies.

Novel Gas Sensing Approach: ReS2/Ti3C2Tx Heterostructures for NH3 Detection in Humid Environments

Continuous monitoring of ammonia (NH3) in humid environments poses a notable challenge for gas sensing applications because of its effect on sensor sensitivity. The present work investigates the detection of NH3 in a natural humid environment utilizing ReS2/Ti3C2Tx heterostructures as a sensing platform. ReS2 nanosheets were vertically grown on the surface of Ti3C2Tx sheets through a hydrothermal synthetic approach, resulting in the formation of ReS2/Ti3C2Tx heterostructures. The structural, morphological, and optical properties of ReS2/Ti3C2Tx were investigated using various state-of-the-art techniques, including scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, zeta potential, Brunauer-Emmett-Teller technique, and Raman spectroscopy. The heterostructures exhibited 1.3- and 8-fold increases in specific surface area compared with ReS2 and Ti3C2Tx, respectively, potentially enhancing the active gas adsorption sites. The electrical investigations of the ReS2/Ti3C2Tx-based sensor demonstrated enhanced selectivity and superior sensing response ranging from 7.8 to 12.4% toward 10 ppm of NH3 within a relative humidity range of 15-85% at room temperature. These findings highlight the synergistic effect of ReS2 and Ti3C2Tx, offering valuable insights for NH3 sensing in environments with high humidity, and are explained in the gas sensing mechanism.

A Comparative Review of Graphene and MXene-Based Composites towards Gas Sensing

Graphene and MXenes have emerged as promising materials for gas sensing applications due to their unique properties and superior performance. This review focuses on the fabrication techniques, applications, and sensing mechanisms of graphene and MXene-based composites in gas sensing. Gas sensors are crucial in various fields, including healthcare, environmental monitoring, and industrial safety, for detecting and monitoring gases such as hydrogen sulfide (H2S), nitrogen dioxide (NO2), and ammonia (NH3). Conventional metal oxides like tin oxide (SnO2) and zinc oxide (ZnO) have been widely used, but graphene and MXenes offer enhanced sensitivity, selectivity, and response times. Graphene-based sensors can detect low concentrations of gases like H2S and NH3, while functionalization can improve their gas-specific selectivity. MXenes, a new class of two-dimensional materials, exhibit high electrical conductivity and tunable surface chemistry, making them suitable for selective and sensitive detection of various gases, including VOCs and humidity. Other materials, such as metal-organic frameworks (MOFs) and conducting polymers, have also shown potential in gas sensing applications, which may be doped into graphene and MXene layers to improve the sensitivity of the sensors.

Advances in 2D Materials Based Gas Sensors for Industrial Machine Olfactory Applications

The escalating development and improvement of gas sensing ability in industrial equipment, or "machine olfactory", propels the evolution of gas sensors toward enhanced sensitivity, selectivity, stability, power efficiency, cost-effectiveness, and longevity. Two-dimensional (2D) materials, distinguished by their atomic-thin profile, expansive specific surface area, remarkable mechanical strength, and surface tunability, hold significant potential for addressing the intricate challenges in gas sensing. However, a comprehensive review of 2D materials-based gas sensors for specific industrial applications is absent. This review delves into the recent advances in this field and highlights the potential applications in industrial machine olfaction. The main content encompasses industrial scenario characteristics, fundamental classification, enhancement methods, underlying mechanisms, and diverse gas sensing applications. Additionally, the challenges associated with transitioning 2D material gas sensors from laboratory development to industrialization and commercialization are addressed, and future-looking viewpoints on the evolution of next-generation intelligent gas sensory systems in the industrial sector are prospected.

Synergy of Active Sites and Charge Transfer in Branched WO3/W18O49 Heterostructures for Enhanced NO2 Sensing

Achieving reliable detection of trace levels of NO2 gas is essential for environmental monitoring and protection of human health protection. Herein, a thin-film gas sensor based on branched WO3/W18O49 heterostructures was fabricated. The optimized WO3/W18O49 sensor exhibited outstanding NO2 sensing properties with an ultrahigh response value (1038) and low detection limit (10 ppb) at 50 °C. Such excellent sensing performance could be ascribed to the synergistic effect of accelerated charge transfer and increased active sites, which is confirmed by electrochemical impedance spectroscopy and temperature-programmed desorption characterization. The sensor exhibited an excellent detection ability to NO2 under different air quality conditions. This work provides an effective strategy for constructing WO3/W18O49 heterostructures for developing NO2 gas sensors with an excellent sensing performance.

Recent developments in 2D MXene-based materials for next generation room temperature NO2 gas sensors

MXenes with distinctive structures, good electrical conductivity and abundant functional groups have shown great potential in the fabrication of high performance gas sensors. Since the sensing mechanism of MXene-based gas sensors often involves a surface-dominant process, they can work at room temperature. In this regard, a significant amount of research has been carried out on MXene-based room temperature gas sensors and they can be viewed as one of the possible materials for NO2 sensing applications in the future. In this review, we focus on the most recent research and improvements in pure MXenes and their nanocomposites for NO2 gas sensing applications. First, we have explored the mechanisms involved in MXenes for NO2 gas sensing. Following that, other ways to tune the MXene sensing performance are investigated, including nanocomposite formation with metal oxides, polymers, and other 2D materials. A comparative analysis of the RT NO2 sensor performance based on MXenes and their hybrids is provided. We also discuss the major challenges of using MXene-related materials and the areas that can further advance in the future for the development of high-performance room temperature NO2 gas sensors.

Alkaloid Precipitant Reaction Inspired Controllable Synthesis of Mesoporous Tungsten Oxide Spheres for Biomarker Sensing

Highly porous sensitive materials with well-defined structures and morphologies are extremely desirable for developing high-performance chemiresistive gas sensors. Herein, inspired by the classical alkaloid precipitant reaction, a robust and reliable active mesoporous nitrogen polymer sphere-directed synthesis method was demonstrated for the controllable construction of heteroatom-doped mesoporous tungsten oxide spheres. In the typical synthesis, P-doped mesoporous WO3 monodisperse spheres with radially oriented channels (P-mWO3-R) were obtained with a diameter of ∼180 nm, high specific surface area, and crystalline skeleton. The in situ-introduced P atoms could effectively adjust the coordination environment of W atoms (Wδ+-Ov), giving rise to dramatically enhanced active surface-adsorbed oxygen species and unusual metastable ε-WO3 crystallites. The P-mWO3-R spheres were applied for the sensing of 3-hydroxy-2-butanone (3H2B), a biomarker of foodborne pathogenic bacteria Listeria monocytogenes (LM). The sensor exhibited high sensitivity (Ra/Rg = 29 to 3 ppm), fast response dynamics (26/7 s), outstanding selectivity, and good long-term stability. Furthermore, the device was integrated into a wireless sensing module to realize remote real-time and precise detection of LM in practical applications, making it possible to evaluate food quality using gas sensors conveniently.

Application of Titanium Carbide MXenes in Chemiresistive Gas Sensors

The titanium carbide MXenes currently attract an extreme amount of interest from the material science community due to their promising functional properties arising from the two-dimensionality of these layered structures. In particular, the interaction between MXene and gaseous molecules, even at the physisorption level, yields a substantial shift in electrical parameters, which makes it possible to design gas sensors working at RT as a prerequisite to low-powered detection units. Herein, we consider to review such sensors, primarily based on Ti3C2Tx and Ti2CTx crystals as the most studied ones to date, delivering a chemiresistive type of signal. We analyze the ways reported in the literature to modify these 2D nanomaterials for (i) detecting various analyte gases, (ii) improving stability and sensitivity, (iii) reducing response/recovery times, and (iv) advancing a sensitivity to atmospheric humidity. The most powerful approach based on designing hetero-layers of MXenes with other crystals is discussed with regard to employing semiconductor metal oxides and chalcogenides, noble metal nanoparticles, carbon materials (graphene and nanotubes), and polymeric components. The current concepts on the detection mechanisms of MXenes and their hetero-composites are considered, and the background reasons for improving gas-sensing functionality in the hetero-composite when compared with pristine MXenes are classified. We formulate state-of-the-art advances and challenges in the field while proposing some possible solutions, in particular via employing a multisensor array paradigm.