MXene Ti3C2Tx-Derived Nitrogen-Functionalized Heterophase TiO2 Homojunctions for Room-Temperature Trace Ammonia Gas Sensing

Unveiling superior NH3 sensing performance: ultrafast response and enhanced recovery kinetics in Ti3C2Tx/ZnO nano-hybrid sensors with UV-induced Schottky junctions

Achieving high sensitivity and rapid response/recovery times at ambient temperatures remains a significant challenge in gas sensing. Ti3C2Tx MXenes have gained attention for their gas-sensing potential due to their high conductivity and active surface functional groups, but challenges such as limited sensitivity and slow response/recovery persist. In this study, we present an ultrafast, reversible Ti3C2Tx/ZnO hybrid composite sensor for NH3 detection at room temperature. We evaluated the sensor's performance under both ambient and UV illumination conditions. Under ambient conditions, the Ti3C2Tx/ZnO sensor exhibited a 50-fold enhancement in sensitivity compared to pristine ZnO, with response and recovery times of 49 s and 39 s, respectively, at 10 ppm NH3. Under UV illumination, the optimized Ti3C2Tx/ZnO configuration achieved a sensor response of 88 at 50 ppm NH3, with ultrafast response and recovery times of 10 s and 13 s, respectively, at 10 ppm NH3, and a limit of detection (LOD) of 0.1 ppm. These improvements are attributed to charge perturbation at the sensor surface facilitated by Ti3C2Tx/ZnO interactions and the formation of a Schottky barrier at their interface, accelerating adsorption-desorption kinetics. The sensor also demonstrated excellent selectivity for NH3 and high long-term stability and repeatability, making it highly suitable for environmental monitoring, industrial safety, and medical diagnostics.

Optical Analysis System Based on Binary Differential Absorption Intensity Reconstruction Combined with Discretization Variability Analysis: Parallel Temperature and Concentration Detection of NH3, NO Mixtures

In industrial combustion, parallel measurements of ammonia (NH3), nitric oxide (NO), and temperature are essential for analyzing and regulating the selective catalytic reduction (SCR) process. Although ultraviolet differential optical absorption spectroscopy (UV-DOAS) is regarded as an ideal measurement method, the simultaneous detection of NH3, NO, and temperature has not been achieved due to the analytical complexities arising from spectral overlap. Therefore, an optical analysis system is proposed based on binary differential absorption intensity reconstruction combined with discretization variability analysis. First, considering the complexity of the mixed spectra of NH3 and NO, we establish a spectral mapping transformation from the wavelength domain to the reconstructed domain, using NH3 and NO as standard spectra, respectively. With the known mutual interference between NH3 and NO, the absorption features of the two compounds are separated in the reconstructed domain by constructing specific Boolean screening matrices. Second, the law of temperature-induced discretization of the reconstructed spectra of NH3 and NO is clarified, and the relationship between temperature and discretization is established within the reconstruction domain. Finally, we construct three-dimensional field maps of temperature, concentration, and reconstructed optical parameter (ROP) to measure NH3, NO, and temperature simultaneously. The results indicate that the analysis system shows good parallel measurement accuracy, with Mean Relative Errors (MRE) of 3.9%, 3.5%, and 2.8% for NH3 (0-50.0 ppm), NO (0-50.0 ppm), and temperature (290.15-773.15 K), respectively. To the best of our knowledge, the study is the first report on simultaneous measurements of NH3, NO, and temperature using UV-DOAS.

Nb2CTx/MoSe2 composites for a highly sensitive NH3 gas sensor at room temperature

The detection of ammonia (NH3)gas holds significant importance in both daily life and industrial production. In this study, the Nb2CTx/MoSe2 sensor was synthesized using a one-step hydrothermal method and applied for NH3 detection. The morphology and elemental composition of the composites were analyzed through a series of characterization techniques including XRD, TEM, SEM, and XPS, confirming the successful synthesis of Nb2CTx/MoSe2 composite with the optimal mass ratio. The sensing performance of the sensor for NH3 (0.1-100 ppm) was tested at room temperature (∼25 °C). The results showed that, compared to pure Nb2CTx, the sensor based on Nb2CTx/MoSe2 composite exhibited more stable baseline resistance, a 3.5-fold increase in response to 50 ppm NH3, and a reduction in response/recovery time by 56.4 s/32.1 s. Additionally, the sensor's response to NH3 (1 ppm, 50 ppm, 100 ppm) varied by less than 10 % over 90 days, demonstrating excellent stability. The sensing mechanism of NH3 by Nb2CTx/MoSe2 composite is attributed to the formation of a p-n heterojunction and surface charge transfer at the interface between p-type Nb2CTx and n-type MoSe2. Finally, the superior selectivity mechanism of the composite for NH3 was investigated using first-principles calculations. This work opens a new avenue for exploring the application potential of Nb2CTx MXene-based nanocomposites in NH3 detection.

Room-Temperature Wearable Chemiresistor Based on a Flexible Inorganic Photoactive Anatase-Rutile TiO2/Yttria-Stabilized Zirconia Nanofiber Network

Wearable gas sensors offer remarkable advantages in terms of portability and real-time monitoring, rendering them highly promising for various applications such as environmental detection, health monitoring, and early disease diagnosis. However, the most widely used oxide semiconductor gas sensors encounter substantial challenges in achieving mechanical flexibility and room-temperature gas detection due to their inherent rigidity, brittleness, and reliance on high operating temperatures. Herein, an all-inorganic wearable oxide semiconductor gas sensor is fabricated by depositing the anatase/rutile TiO2 (TiO2-A/R) homojunction on a flexible yttria-stabilized zirconia (YSZ) nanofiber substrate using atomic layer deposition technology. The combination of the YSZ nanofiber and the ultrathin TiO2 sensing layer (∼13 nm) endows the wearable sensor with tiny linear strains (0.55%) when subjected to a radius of curvature of 25 μm. As a result, the wearable inorganic YSZ/TiO2-A/R sensor can be folded multiple times without fracturing and maintain a stable electrical connectivity during cyclic bending. Furthermore, the utilization of photoactive TiO2 homojunctions allows the sensor to be activated by UV light and operated at room temperature. The efficient separation efficiency of photogenerated carriers, which stems from the interfacial electric field of TiO2 homojunctions, significantly enhances the sensor's response, leading to a low detection limit of 0.15 ppm for acetone. In addition, the wearable sensor was anchored on a mask and successfully utilized for the detection of a simulated breathing gas of diabetics; the real-time and stable response signals demonstrate its potential for noninvasive diabetes diagnosis. This study provides a valuable reference for the advancement of wearable room-temperature inorganic semiconductor gas sensors, offering valuable insights into their potential applications in disease diagnosis.

MXene-based materials for enhanced water quality: Advances in remediation strategies

Two-dimensional MXenes are promising candidates for water treatment because of their large surface area (e.g., exceeding 1000 m²/g for certain structures), high electrical conductivity (e.g., >1000 S/m), hydrophilicity, and chemical stability. Their strong sorption selectivity and effective reduction capacity, exemplified by heavy metal adsorption efficiencies exceeding 95 % in several studies, coupled with facile surface modification, make them suitable for removing diverse contaminants. Applications include the removal of heavy metals (e.g., achieving >90 % removal of Pb(II)), dye removal (e.g., demonstrating >80 % removal of methylene blue), and radioactive waste elimination. Furthermore, 3D MXene architecture exhibit enhanced performance in antibacterial activities (e.g., against bacteria), desalination rejection percentage, and photocatalytic degradation of organic contaminants. However, several challenges have remained, which necessitate further investigation into toxicity (e.g., assessing effects on aquatic organisms), scalability, and cost-effectiveness of large-scale production. This review summarizes recent advancements in 3D MXene-based functional materials for wastewater treatment and water remediation, critically analyzing their both potential and limitations.

Humidity-Activated Ammonia Sensor Based on Carboxylic Functionalized Cross-Linked Hydrogel

Owing to its extensive use and intrinsic toxicity, NH3 detection is very crucial. Moisture can cause significant interference in the performance of sensors, and detecting NH3 in high humidity is still a challenge. In this work, a humidity-activated NH3 sensor was prepared by urocanic acid (URA) modifying poly (ethylene glycol) diacrylate (PEGDA) via a thiol-ene click cross-linking reaction. The optimized sensor achieved a response of 70% to 50 ppm NH3 at 80% RH, with a response time of 105.6 s and a recovery time of 346.8 s. The sensor was improved for response and recovery speed. In addition, the prepared sensor showed excellent selectivity to NH3 in high-humidity environments, making it suitable for use in some areas with high humidity all the year round or in high-humidity areas such as the detection of respiratory gas. A detailed investigation of the humidity-activated NH3-sensing mechanism was conducted using complex impedance plot (CIP) measurements.

Room temperature gas sensors for NH3 detection based on the heterojunction of 2D Ti3C2Tx MXenes and Bi2S3

Bi2S3/Ti3C2Tx nanomaterials were successfully prepared through a simple hydrothermal method. Various methods were used for their characterization, including XRD, XPS, SEM, EDS, and BET, along with testing their gas-sensing properties. The results showed that the response value to 100 ppm ammonia at room temperature reached 107%, which was 14.1 times higher than that of pure few-layer MXene. After undergoing anti-humidity interference testing, we observed that Bi2S3/Ti3C2Tx exhibited a higher response value in real-time monitoring of ammonia as humidity increased. Specifically, under 90% humidity conditions, its response value reached 1.32 times that of normal humidity conditions. This exceptional moisture resistance ensures that the sensor can maintain stability, and even exhibit superior performance, in harsh environments. Therefore, it possesses excellent selectivity, high-moisture-resistance, and long-term stability, making it significant in the field of medical and health monitoring.

Biofunctionalized magnetic nanoparticles incorporated MoS2 nanocomposite for enhanced n-butanol sensing at room temperature

N-butanol is well known to be a flammable and harmful liquid that is a potential threat to human health and property. Therefore, it is important to monitor the concentration of n-butanol in the surroundings. The need for highly efficient toxic gas detection is urgent and has been driving the research on gas sensors for practical applications. Molybdenum disulfide (MoS2) has been attracting significant interest for gas detection at room temperature. Herein, we report biofunctionalized magnetic nanoparticles incorporated MoS2 for sensing n-butanol. The biosynthesized magnetite nanoparticles (CT-Fe3O4) were synthesized by the addition of Cinnamomum Tamala (CT) leaf extract, and subsequently, the nanocomposite was synthesized using a hydrothermal method. Highly sensitive sensors based on MoS2-CT-Fe3O4 were fabricated and tested for sensing different concentrations of n-butanol. The nanocomposites showed a good sensing performance ( Δ R/Rair %) of 72% towards 20 ppm of n-butanol, indicating the potential use of MoS2-CT-Fe3O4 nanocomposite for sensing n-butanol at room temperature.

Bioinspired multi-scale interface design for wet gas sensing based on rational water management

Natural organisms have evolved multi-scale wet gas sensing interfaces with optimized mass transport pathways in biological fluid environments, which sheds light on developing artificial counterparts with improved wet gas sensing abilities and practical applications. Herein, we highlighted current advances in wet gas sensing taking advantage of optimized mass transport pathways endowed by multi-scale interface design. Common moisture resistance (e.g., employing moisture resistant sensing materials, post-modifying moisture resistant coatings, physical heating for moisture resistance, and self-removing hydroxyl groups) and moisture absorption (e.g., employing moisture absorption sensing materials and post-modifying moisture absorption coatings) strategies for wet gas sensing were discussed. Then, the design principles of bioinspired multi-scale wet gas sensing interfaces were provided, including macro-level condensation mediation, micro/nano-level transport pathway adjustment and molecular level moisture-proof design. Finally, perspectives on constructing bioinspired multi-scale wet gas sensing interfaces were presented, which will not only deepen our understanding of the underlying principles, but also promote practical applications.

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.

Multifunctional MnFe2O4/TiO2/Ti3C2Tx composites based on in-situ grown TiO2 for efficient microwave absorption, high hydrophobicity, and heat dissipation properties

Despite the fact that the 2D structure Ti3C2Tx with abundant defects and functional groups contributes to the high microwave absorption (MA) performance, it is difficulty to improve the strength and bandwidth by pursuing higher conductivity or loading more groups due to the limitation of intrinsic properties. Therefore, it is important to ingeniously design efficient Ti3C2Tx based MA composites assembling the features of abundant surface groups, good dispersibility, multiple composition, and precise structure. Inspired by the fact that Ti3C2Tx contains thermodynamically metastable marginal Ti atoms, TiO2 nanoparticles can be grown in-situ on Ti3C2Tx nanosheets uniformly and increase the spacing of Ti3C2Tx layers, and then MnFe2O4 nanoparticles are introduced into the layers of Ti3C2Tx by electrostatic self-assembly method for optimized impedance matching. This designed hierarchical MnFe2O4/TiO2/Ti3C2Tx composites shows excellent MA performance, and the minimum reflection loss (RLmin) reaches -46.91 dB with a thickness of 2.5 mm at frequency of 10.4 GHz. The high MA performance mainly comes from the enhanced interfacial polarization induced by edges location and interface region among TiO2, MnFe2O4, and Ti3C2Tx. In addition, the conduction loss existed in the interior untreated Ti3C2Tx, the dielectric loss generated by multiple composition, the multiple scattering from improved large surface specific area all contribute to the excellent MA performance. Meanwhile, the simple preparation process and good stability storage at room temperature under air atmosphere of the MnFe2O4/TiO2/Ti3C2Tx composites promote its exploration on practical use, and the lab-gown cloth coated with MnFe2O4/TiO2/Ti3C2Tx composites shows better electromagnetic shielding properties, hydrophobicity, and heat transfer ability than pure fabric, showing the potential for practical application.

Manganese doped two-dimensional zinc ferrite thin films as chemiresistive trimethylamine gas sensors

Trimethylamine (TMA) is highly toxic and can have lethal effects on living organisms. Detecting the presence of TMA in air is very important because, if the TMA level exceeds the OSHA (Occupational Safety and Health Administration) limit, it may harm the environment and endanger human life. Doping is an appropriate flexible way to change the electrical structures of metal oxide semiconductors (MOSs) and improve their ability to detect toxic gases. In this work, Mn-doped zinc ferrite thin film nanorods with agglomerated morphology were fabricated by a spray pyrolysis technique. For the first time, a comprehensive investigation was done on the gas sensing capabilities of Mn-doped ZnFe2O4 thin films. The findings showed that ZFM1 had the best gas sensing characteristics, with high sensitivity (S = 6.24), good selectivity, and quick recovery, towards 10 ppm TMA at ambient temperature. The alternate Mn-ZF sites are responsible for the rapid recovery because they can significantly increase the concentration of oxygen vacancies in the ZF crystal. 0.1 Mn doped ZnFe2O4 (ZFM1) thin film exhibits greatly enhanced gas sensing properties towards TMA, because of its high surface-to-volume ratio and rough surface with a small nanorod structure. The sensor's response to 10 ppm TMA was measured 13 weeks later for stability testing. The stability test results show that the coated ZFM1 film works well as a TMA gas sensor. This work shows that ZF thin films are effective in detecting TMA in the atmosphere.

Room-temperature chemiresistive ammonia sensors based on 2D MXenes and their hybrids: recent developments and future prospects

Detection of ammonia (NH3) gas at room temperature is essential in a variety of sectors, including pollution monitoring, commercial safety and medical services, etc. Two-dimensional (2D) materials have emerged as fascinating candidates for gas-sensing applications due to their distinct properties. MXenes, a type of 2D transition metal carbides/nitrides/carbonotrides, have drawn the interest of researchers due to their high conductivity, large surface area, and changing surface chemistry. The review begins by describing the NH3 gas-detecting methods of 2D materials and then concentrates on MXene-based sensors, emphasising the benefits that MXenes provide in this context. The study also explains the prime factors involved in evaluating sensor performance, which include sensor response, sensitivity, selectivity, stability, charge transfer values, adsorption energy and response/recovery times. Subsequently, the review covers two main categories: pristine/intercalated MXenes and MXene-based hybrid materials. The review investigates the approaches for improving the sensing characteristics of pristine and intercalated MXenes by introducing MXene hybrids like MXene-metal oxide hybrids, MXene-transition metal dichalcogenides hybrid, MXene-other 2D materials hybrid, MXene-polymers and other hybrids and other MXene-derived materials. In summary, this review offers a thorough overview of current advancements and potential applications for room-temperature ammonia sensors based on 2D MXenes and their hybrids. In order to pave the way for future improvements in MXene-based gas-sensing technology for room temperature ammonia detection, the study concludes by outlining potential future scope and conclusions.

Low Temperature Chemoresistive Oxygen Sensors Based on Titanium-Containing Ti2CTx and Ti3C2Tx MXenes

The chemoresistive properties of multilayer titanium-containing Ti₂CT_x and Ti₃C₂T_x MXenes, synthesized by etching the corresponding MAX phases with NaF solution in hydrochloric acid, and the composites based on them, obtained by partial oxidation directly in a sensor cell in an air flow at 150 °C, were studied. Significant differences were observed for the initial MXenes, both in microstructure and in the composition of surface functional groups, as well as in gas sensitivity. For single Ti₂CT_x and Ti₃C₂T_x MXenes, significant responses to oxygen and ammonia were observed. For their partial oxidation at a moderate temperature of 150 °C, a high humidity sensitivity (T, RH = 55%) is observed for Ti₂CT_x and a high and selective response to oxygen for Ti₃C₂T_x at 125 °C (RH = 0%). Overall, these titanium-containing MXenes and composites based on them are considered promising as receptor materials for low temperature oxygen sensors.

High-performance selective NO2 gas sensor based on In2O3-graphene-Cu nanocomposites

The control of atmosphere content and concentration of specific gases are important tasks in many industrial processes, agriculture, environmental and medical applications. Thus there is a high demand to develop new advanced materials with enhanced gas sensing characteristics including high gas selectivity. Herein we report the result of a study on the synthesis, characterization, and investigation of gas sensing properties of In₂O₃-graphene-Cu composite nanomaterials for sensing elements of single-electrode semiconductor gas sensors. The nanocomposite has a closely interconnected and highly defective structure, which is characterized by high sensitivity to various oxidizing and reducing gases and selectivity to NO₂. The In₂O₃-based materials were obtained by sol-gel method, by adding 0-6 wt% of pre-synthesized graphene-Cu powder into In-containing gel before xerogel formation. The graphene-Cu flakes played the role of centers for In₂O₃ nucleation and then crystal growth terminators. This led to the formation of structural defects, influencing the surface energy state and concentration of free electrons. The concentration of defects increases with the increase of graphene-Cu content from 1 to 4 wt%, which also affects the gas-sensing properties of the nanocomposites. The sensors show a high sensing response to both oxidizing (NO₂) and reducing (acetone, ethanol, methane) gases at an optimal working heating current of 91-161 mA (280-510 °C). The sensor with nanocomposite with 4 wt% of graphene-Cu additive showed the highest sensitivity to NO₂ (46 ppm) in comparison with other tested gases with an absolute value of sensing response of (- ) 225 mV at a heating current of 131 mA (430 °C) and linear dependence of sensing response to NO₂ concentration.

Accelerating the Gas-Solid Interactions for Conductometric Gas Sensors: Impacting Factors and Improvement Strategies

Metal oxide-based conductometric gas sensors (CGS) have showcased a vast application potential in the fields of environmental protection and medical diagnosis due to their unique advantages of high cost-effectiveness, expedient miniaturization, and noninvasive and convenient operation. Of multiple parameters to assess the sensor performance, the reaction speeds, including response and recovery times during the gas-solid interactions, are directly correlated to a timely recognition of the target molecule prior to scheduling the relevant processing solutions and an instant restoration aimed for subsequent repeated exposure tests. In this review, we first take metal oxide semiconductors (MOSs) as the case study and conclude the impact of the semiconducting type as well as the grain size and morphology of MOSs on the reaction speeds of related gas sensors. Second, various improvement strategies, primarily including external stimulus (heat and photons), morphological and structural regulation, element doping, and composite engineering, are successively introduced in detail. Finally, challenges and perspectives are proposed so as to provide the design references for future high-performance CGS featuring swift detection and regeneration.

Room-temperature ammonia gas sensing via Au nanoparticle-decorated TiO2 nanosheets

A high-performance gas sensor operating at room temperature is always favourable since it simplifies the device fabrication and lowers the operating power by eliminating a heater. Herein, we fabricated the ammonia (NH₃) gas sensor by using Au nanoparticle-decorated TiO₂ nanosheets, which were synthesized via two distinct processes: (1) preparation of monolayer TiO₂ nanosheets through flux growth and a subsequent chemical exfoliation and (2) decoration of Au nanoparticles on the TiO₂ nanosheets via hydrothermal method. Based on the morphological, compositional, crystallographic, and surface characteristics of this low-dimensional nano-heterostructured material, its temperature- and concentration-dependent NH₃ gas-sensing properties were investigated. A high response of ~ 2.8 was obtained at room temperature under 20 ppm NH₃ gas concentration by decorating Au nanoparticles onto the surface of TiO₂ nanosheets, which generated oxygen defects and induced spillover effect as well.

Two-dimensional black phosphorus/tin oxide heterojunctions for high-performance chemiresistive H2S sensing

Hydrogen sulfide (H₂S) emission from industrial fields and bacteria decomposing of sulfur-containing organic matter poses a significant impact on human health and atmospheric environment, thus necessitating the development of a H₂S sensor with high sensitivity and exclusive selectivity especially at a very low dose. Chemiresistive sensors based on traditional metal oxides were readily limited by the elevated operating temperature and severe cross-sensitivity. To overcome these obstacles, we prepared two dimensional (2D) tin oxide (SnO₂) nanosheets decorated with thin black phosphorus (BP) as the sensing layer of MEMS H₂S sensors. Compared with pure SnO₂ counterparts, BP-SnO₂ sensors demonstrated lower optimal working temperature (130 °C vs. 160 °C), higher response (8.1 vs. 4.6) and faster response/recovery speeds (39.8 s/47.4 s vs. 79 s/140 s) toward 5 ppm H₂S as well as larger sensitivity (1.3/ppm vs. 0.342/ppm). In addition, favorable repeatability, long-term stability, selectivity and humidity tolerance were exhibited. Thin BP not only served as an excellent conductivity platform within the composites, but enriched the adsorption sites by constructing p-n heterojunctions and introducing more oxygen vacancy, thus separately accelerating and strengthening the gas-solid interaction. This study showcased the application superiorities of BP nanosheets in the field of gas sensing, simultaneously providing a new strategy for trace H₂S sensing via the 2D heterojunctions.

Enhanced ammonia sensing response based on Pt-decorated Ti3C2Tx/TiO2composite at room temperature

Herein, we report a Pt-decorated Ti₃C₂T_x/TiO₂gas sensor for the enhanced NH₃sensing response at room temperature. Firstly, the TiO₂nanosheets (NSs) arein situgrown onto the two-dimensional (2D) Ti₃C₂T_xby hydrothermal treatment. Similar to Ti₃C₂T_xsensor, the Ti₃C₂T_x/TiO₂sensor has a positive resistance variation upon exposure to NH₃, but with slight enhancement in response. However, after the loading of Pt nanoparticles (NPs), the Pt-Ti₃C₂T_x/TiO₂sensor shows a negative response with significantly improved NH₃sensing performance. The shift in response direction indicates that the dominant sensing mechanism has changed under the sensitization effect of Pt NPs. At room temperature, the response of Pt-Ti₃C₂T_x/TiO₂gas sensor to 100 ppm NH₃is about 45.5%, which is 13.8- and 10.8- times higher than those of Ti₃C₂T_xand Ti₃C₂T_x/TiO₂gas sensors, respectively. The experimental detection limit of the Pt-Ti₃C₂T_x/TiO₂gas sensor to detect NH₃is 10 ppm, and the corresponding response is 10.0%. In addition, the Pt-Ti₃C₂T_x/TiO₂gas sensor shows the fast response/recovery speed (23/34 s to 100 ppm NH₃), high selectivity and good stability. Considering both the response value and the response direction, the corresponding gas-sensing mechanism is also deeply discussed. This work is expected to shed a new light on the development of noble metals decorated MXene-metal oxide gas sensors.

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.

Recent Progress on Anti-Humidity Strategies of Chemiresistive Gas Sensors

In recent decades, chemiresistive gas sensors (CGS) have been widely studied due to their unique advantages of expedient miniaturization, simple fabrication, easy operation, and low cost. As one ubiquitous interference factor, humidity dramatically affects the performance of CGS, which has been neglected for a long time. With the rapid development of technologies based on gas sensors, including the internet of things (IoT), healthcare, environment monitoring, and food quality assessing, the humidity interference on gas sensors has been attracting increasing attention. Inspiringly, various anti-humidity strategies have been proposed to alleviate the humidity interference in this field; however, comprehensive summaries of these strategies are rarely reported. Therefore, this review aims to summarize the latest research advances on humidity-independent CGS. First, we discussed the humidity interference mechanism on gas sensors. Then, the anti-humidity strategies mainly including surface engineering, physical isolation, working parameters modulation, humidity compensation, and developing novel gas-sensing materials were successively introduced in detail. Finally, challenges and perspectives of improving the humidity tolerance of gas sensors were proposed for future research.

Humidity-Tolerant Room-Temperature Selective Dual Sensing and Discrimination of NH3 and NO Using a WS2/MWCNT Composite

Continuous detection of toxic and hazardous gases like nitric oxide (NO) and ammonia (NH₃) is needed for environmental management and noninvasive diagnosis of various diseases. However, to the best of our knowledge, dual detection of these two gases has not been previously reported. To address the challenge, we demonstrate the design and fabrication of low-cost NH₃ and NO dual gas sensors using tungsten disulfide/multiwall carbon nanotube (WS₂/MWCNT) nanocomposites as sensing channels which maintained their performance in a humid environment. The composite-based device has shown successful dual detection at temperatures down to 18 °C and relative humidity of 90%. For 0.1 ppm ammonia, it exhibited a p-type conduction with response and recovery times of 102 and 261 s, respectively; on the other hand, with NO (10 ppb, n-type), these times were 285 and 198 s, respectively. The device with 5 mg MWCNTs possesses a superior selectivity along with a relative response of ≈7% (5 ppb) and ≈5% (0.1 ppm) for NO and NH₃, respectively, at 18 °C. The response is less affected by relative humidity, and this is attributed to the presence of MWCNTs that are hydrophobic in nature. Upon simultaneous exposure to NO (5-10 ppb) and NH₃ (0.1-5 ppm), the response was dominated by NO, implying clear discrimination to the simultaneous presence of these two gases. We propose a sensing mechanism based on adsorption/desportion and accompanied charge transfer between the adsorbed gas molecules and sensing surface. The results suggest that an optimized weight ratio of WS₂ and MWCNTs could govern favorable sensing conditions for a particular gas molecule.

A review on MXene and its nanocomposites for the detection of toxic inorganic gases

In the search of the viable candidate for the sensing of pollutant gases, two-dimensional (2D) material transition metal carbides (MXenes) have attracted immense attention due to their outstanding physical and chemical properties for sensing purposes. The formation of unique 2D layered structure with high conductivity, large mechanical strength, and high adsorption properties furnish their strong interactions with gaseous molecules, which holds a promising place for developing ideal gas sensing devices. This review looks at recent achievements in diversified MXenes, with a focus gaining on in-depth understanding of MXene-based materials in room temperature inorganic gas sensors through both theoretical and experimental studies. In the first part of the review, the properties and advantages of sensing material (MXene) in comparison with other 2D materials are discussed. In the second part, the unique advantages of chemiresistive based sensors and the demerits of other detection methods are summarized in detail. This section is followed by the unique structural design of MXene bases materials for improving the sensing performance towards detection of inorganic gases. The interaction between MXene and the adsorbed gases on its surface is discussed, with a possible sensing mechanism. Finally, an overview of the current progress and opportunities for the demand of MXene is emphasized and perspectives for future improvement of the design of MXene in gas sensors are highlighted. Therefore, this review highlights the opportunities and the advancement in 2D material-based gas sensors which could provide a new avenue for rapid detection of toxic gases in the environment.

Synergistic Effect of Surface Acidity and PtOx Catalyst on the Sensitivity of Nanosized Metal-Oxide Semiconductors to Benzene

Benzene is a potentially carcinogenic volatile organic compound (VOC) and its vapor must be strictly monitored in air. Metal-oxide semiconductors (MOS) functionalized by catalytic noble metals are promising materials for sensing VOC, but basic understanding of the relationships of materials composition and sensors behavior should be improved. In this work, the sensitivity to benzene was comparatively studied for nanocrystalline n-type MOS (ZnO, In₂O₃, SnO₂, TiO₂, and WO₃) in pristine form and modified by catalytic PtO_x nanoparticles. Active sites of materials were analyzed by X-ray photoelectron spectroscopy (XPS) and temperature-programmed techniques using probe molecules. The sensing mechanism was studied by in situ diffuse-reflectance infrared (DRIFT) spectroscopy. Distinct trends were observed in the sensitivity to benzene for pristine MOS and nanocomposites MOS/PtO_x. The higher sensitivity of pristine SnO₂, TiO₂, and WO₃ was observed. This was attributed to higher total concentrations of oxidation sites and acid sites favoring target molecules' adsorption and redox conversion at the surface of MOS. The sensitivity of PtO_x-modified sensors increased with the surface acidity of MOS and were superior for WO₃/PtO_x. It was deduced that this was due to stabilization of reduced Pt sites which catalyze deep oxidation of benzene molecules to carbonyl species.

Mesoporous cellulose nanofibers-interlaced PEDOT:PSS hybrids for chemiresistive ammonia detection

Chemiresistive ammonia (NH₃) detection at room temperature is highly desired due to the unique merits of easy miniaturization, low cost, and minor energy consumption especially for portable and wearable electronics. In this regard, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) has sparked considerable attention due to the benign room-temperature conductivity and environmental stability, but it is undesirably impeded by limited sensitivity and sluggish reaction kinetics. To overcome these, we incorporated cellulose nanofibers (CNF) into PEDOT:PSS via a facile blending. The constituent-optimized composite sensor displayed sensitive (sensitivity of ∼7.46%/ppm in the range of 0.2-3 ppm), selective, and stable NH₃ sensing at 25 °C at 55% RH, with higher response and less baseline drift than pure PEDOT:PSS counterparts. Additionally, the response/recovery times (4.9 s/5.2 s toward 1 ppm NH₃) ranked the best cases of conducting polymers based NH₃ sensors. The humidity involved more than twofold response enhancement indicated a huge potential in exhaled breath monitoring. Furthermore, we observed an excellent flexible NH₃-sensing performance with bending-tolerant features. This work provides an alternative strategy for trace NH₃ sensing with low power consumption, superfast reaction, and high sensitivity.

Plastic Waste Precursor-Derived Fluorescent Carbon and Construction of Ternary FCs@CuO@TiO2 Hybrid Photocatalyst for Hydrogen Production and Sensing Application

A sustainable nexus between renewable energy production and plastic abatement is imperative for overall sustainable development. In this regard, this study aims to develop a cheaper and environmentally friendly nexus between plastic waste management, wastewater treatment, and renewable hydrogen production. Fluorescent carbon (FCs) were synthesized from commonly used LDPE (low-density polyethylene) by a facile hydrothermal approach. Optical absorption study revealed an absorption edge around 300 nm and two emission bands at 430 and 470 nm. The morphological analysis showed two different patterns of FCs, a thin sheet with 2D morphology and elongated particles. The sheet-shaped particles are 0.5 μm in size, while as for elongated structures, the size varies from 0.5 to 1 μm. The as-synthesized FCs were used for the detection of metal ions (reference as Cu2+ ions) in water. The fluorescence intensity of FCs versus Cu2+ ions depicts its upright analytical ability with a limit of detection (LOD) reaching 86.5 nM, which is considerably lesser than earlier reported fluorescence probes derived from waste. After the sensing of Cu2+, the as-obtained FCs@Cu2+ was mixed with TiO2 to form a ternary FCs@CuO@TiO2 composite. This ternary composite was utilized for photocatalytic hydrogen production from water under 1.5 AM solar light irradiation. The H2 evolution rate was found to be ~1800 μmolg−1, which is many folds compared to the bare FCs. Moreover, the optimized FCs@CuO@TiO2 ternary composite showed a photocurrent density of ~2.40 mA/cm2 at 1 V vs. Ag/AgCl, in 1 M Na2SO4 solution under the illumination of simulated solar light. The achieved photocurrent density corresponds to the solar-to-hydrogen (STH) efficiency of ~0.95%. The efficiency is due to the fluorescence nature of FCs and the synergistic effect of CuO embedded in TiO2, which enhances the optical absorption of the composite by reaching the bandgap of 2.44 eV, apparently reducing the recombination rate, which was confirmed by optoelectronic, structural, and spectroscopic characterizations.

High-Sensitivity Metal Oxide Sensors Duplex for On-the-Field Detection of Acetic Acid Arising from the Degradation of Cellulose Acetate-Based Cinematographic and Photographic Films

In this work, a system consisting of two resistive sensors working in tandem to detect and quantify the acetic acid released during the degradation of cellulose acetate-based ancient cinematographic and photographic films is presented. Acetic acid must be constantly monitored to prevent reaching concentrations at which autocatalytic degradation processes begin. The sensors are constituted by a thin layer of metal oxide (tungsten oxide and tin oxide) deposited over an interdigitated electrode capable of being heated, chosen to maximize the array response towards acetic acid vapors. The signals obtained from the sensor array are mathematically processed to reduce the background signal due to interferent gases produced during degradation of ancient cinematographic films. The sensor array reported a LOD of 30 ppb for acetic acid, with a linearity range up to 30 ppm. Finally, the sensor array was tested with different cinematographic and photographic film samples made of cellulose acetate, whose degradation state and acetic acid production was validated using the conventional technique (A-D strips). The presented array is suitable for remote monitoring large number of films in collections since, compared to the official technique, it has a lower detection limit (30 ppb vs. 500 ppb) and is much quicker in providing accurate acetic acid concentration in the film boxes (15 min vs. 24 h).