Abstract
With the continuous advancement of smart greenhouse technologies, digital and information-based environmental monitoring has emerged as a focal point of research. The development of high-performance gas sensors is central to achieving this objective. In recent years, MXene materials have been widely applied in the field of gas sensors due to their excellent ion mobility, favorable hydrophilicity, outstanding electronic conductivity, and unique physicochemical properties. Various MXene heterojunction structures have been synthesized for gas detection. This review aims to summarize the current state of research on Ti3C2Tx-based gas sensors, explore methods for synthesizing different morphologies of Ti3C2Tx heterojunction structures, and evaluate the sensing behaviors of these configurations to fully harness their potential for gas monitoring in greenhouse environments. Additionally, an in-depth analysis of the sensing mechanisms associated with Ti3C2Tx heterojunction structures will be provided, offering theoretical support for future investigations. The findings indicate that Ti3C2Tx-based nanomaterials demonstrate considerable promise as high-performance sensors for gas detection in greenhouse settings. This innovative research not only provides new insights into the development of gas sensor technologies but also serves as an important foundation for the digitization of environmental monitoring.
1 Introduction
In recent years, greenhouses have become essential facilities for agricultural production; however, their relatively enclosed environments lead to the accumulation of toxic and harmful gases released from improper fertilizer application and insufficient ventilation (; ). This phenomenon has exacerbated air quality issues within greenhouses. For instance, when ammonia (NH3) concentration exceeds 5 mg/m3 (approximately 6.579 ppm), crop leaves may exhibit water-soaked spots, ultimately resulting in wilting; at concentrations above 30 mg/m³ (approximately 39.474 ppm), NH3 binds with hemoglobin, causing symptoms such as chest tightness, cough, and respiratory difficulties, alongside conjunctival congestion and headaches (; ; ). Similarly, when nitrogen dioxide (NO2) concentration surpasses 5 mg/m3 (approximately 2.433 ppm), water-soaked spots appear on the edges of crop leaves, which quickly fade to a bleached appearance; if concentrations exceed 20 mg/m3 (approximately 9.73 ppm), NO2 can inhibit enzyme activity in humans, affecting the nervous system and leading to chest tightness and respiratory distress, with long-term exposure potentially resulting in neurosis and chronic respiratory inflammation (; ; ). Furthermore, elevated levels of ethylene (C2H4), exceeding 2 mg/m3 (approximately 1.598 ppm), significantly impact the edges of leaves and young shoots, manifesting as chloroplast degradation that leads to yellowing or whitening of the leaves. Although ethylene exhibits low toxicity to humans, prolonged exposure may induce symptoms such as dizziness and lack of concentration; high concentrations can even cause paralysis (; ; ). Therefore, the development of high-response and low-detection-limit sensors for real-time monitoring of toxic gases within greenhouse environments at room temperature is of critical significance. Such advancements would not only ensure healthy crop growth but also protect the health of agricultural workers, thereby supporting the progression of smart agriculture. Furthermore, it is essential to recognize that humidity levels within greenhouse settings play a pivotal role in determining sensor performance. Fluctuations in moisture can markedly influence both the sensitivity and stability of gas detection mechanisms, necessitating the design of sensors that can effectively operate under varying humidity conditions. This adaptability is crucial for ensuring accurate and reliable monitoring, as excessive moisture may lead to false readings or degradation of sensor materials. Addressing these challenges will enhance the efficacy of gas sensors in dynamic agricultural environments, ultimately contributing to improved environmental control and optimal crop management practices (; ; ).
MXenes are a class of two-dimensional (2D) materials that have emerged as popular alternatives to graphene in recent years. Composed of transition metals (Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta), X (C or N), and surface functional groups T (-O, -OH, and -F), MXenes are represented by the chemical formula Mn+1XnTx, where n ranges from 1 to 3 (; ; ). Due to their exceptional ion mobility, favorable hydrophilicity, and outstanding electronic conductivity, MXenes have found extensive applications in sensors, catalysts, lithium-ion batteries, electromagnetic shielding, and supercapacitors (; ; ; ). Zhang et al. successfully synthesized accordion-like V2CTx MXene materials and combined them with SnS2 nanosheets, resulting in the development of a highly efficient MXene heterojunction gas sensor (). Gas sensing results indicate that the V2CTx material composite with SnS2 nanosheets exhibits an exceptional sensitivity towards NO2, being 581 times greater than that of intrinsic V2CTx materials. Furthermore, at a relative humidity of 51.9%, rapid response and recovery times of 4.8 s and 4.7 s, respectively, are observed, alongside commendable selectivity. Wang et al. successfully incorporated SnS2 onto Nb4C3Tx materials through an in situ growth approach (). This composite demonstrates a high response of 1046.6% to 50 ppm triethylamine at room temperature, with a response time of 11 s. Lu et al. designed a MXene/Na2Ti3O7@polyaniline composite, which exhibits a response of 283% to NH3 at room temperature with 90% humidity and achieves a detection limit of 186% (). Mo2CTx nanospheres were synthesized using ultrasonic technology by Wang et al. (). Gas sensing tests revealed that this material exhibits a favorable response to 5 ppm of NO2 at room temperature. Additionally, a sensor composed of Nb2CTx nanosheets combined with polyaniline was fabricated, demonstrating high selectivity and sensitivity towards low-concentration NH3 at room temperature and 87.1% relative humidity (). Collectively, the synthesis of MXene heterojunction structures holds promise for achieving outstanding gas sensing capabilities for toxic gases in greenhouse environments at room temperature, thereby providing robust technical support for the advancement of smart agriculture.
Ti3C2Tx is among the most stable and cost-effective MXene materials, with a well-established preparation methodology. Its exceptional thermal stability, high electrical conductivity, and outstanding chemical and physical properties position it as a potential candidate for climate monitoring applications (; ; ). A two-dimensional α-Fe2O3/Ti3C2Tx MXene composite was designed by Zhang et al. (), and gas sensing tests demonstrated that this material exhibits a response of 0.27–100 ppm H2S at room temperature, with an exceptionally low detection limit. A unique sacrificial technique was employed by Qiu et al. to successfully fabricate Pd/Cu-modified Ti3C2Tx materials, which demonstrate a response time of 4 s to 0.5% H2 at room temperature, with a detection limit of 0.1% H2 (). Li et al. successfully fabricated Fe2O3/TiO2/Ti3C2Tx MXene composite sensors, which respond to 100 ppm NH3 at room temperature with response and recovery times of 62 s and 74 s, respectively (). Additionally, a Google Scholar investigation into the keyword “nano + Ti3C2Tx + gas sensor” revealed a steady increase in the publication count related to this topic from 2020 to 2023, with records showing 9, 22, 59, and 64 publications annually. Although this data may not be entirely precise, an increasing body of literature indicates that gas sensors based on Ti3C2Tx are garnering heightened attention. Therefore, summarizing the latest advancements in Ti3C2Tx-based gas sensors is crucial for a deeper understanding of their advantages in monitoring toxic gases within greenhouse environments. This will not only provide a foundation for future research but also establish robust theoretical support for the development of smart agriculture.
In this review, several highly cited published studies have been selected to provide a concise overview of typical Ti3C2Tx MXene gas sensors. The objective of this research is to summarize and compare the exceptional performance characteristics of these gas sensors. Furthermore, various methods for synthesizing Ti3C2Tx MXene-based materials are presented to offer valuable references for related investigations. Finally, a brief exploration of the sensing mechanisms associated with Ti3C2Tx MXene gas sensors is conducted. This review not only establishes a foundation for understanding the applications of Ti3C2Tx MXenes in the realm of gas sensing but also elucidates potential directions for future research.
2 Recent advances in the gas sensing performance of Ti3C2Tx
2.1 Metal atom modification of Ti3C2Tx MXene nanomaterials
Nam et al. successfully developed gas sensors based on Au/Pt-modified Ti3C2Tx MXene (). The Ti3C2Tx MXene material was synthesized through the HF etching of Ti3AlC2. The Ti3C2Tx MXene materials modified with varying ratios of Au/Pt particles are illustrated in Figures 1A–F. These sensors demonstrated the capability to operate in self-heating mode at room temperature, exhibiting good selectivity towards ammonia when containing 1.92 at% Au or 0.83 at% Pt NPs. Additionally, the Au-Ti3C2Tx MXene and Pt-Ti3C2Tx MXene sensors demonstrate adaptability to voltages of 5 V and 3 V, respectively, and both have successfully passed rigorous strength tests. These findings indicate that noble metal-modified Ti3C2Tx MXene exhibits excellent compatibility for gas detection at room temperature. This research provides novel insights into the development of high-performance gas sensors.
FIGURE 1
A method for the preparation of Pt-modified Ti3C2Tx/TiO2 heterojunction sensors using a simple hydrothermal approach was proposed by Zhang et al. (). Experimental comparative results indicate that the response direction of the Pt-modified Ti3C2Tx/TiO2 system changes significantly, enhancing its NH3 detection performance at room temperature compared to intrinsic Ti3C2Tx materials and Ti3C2Tx/TiO2 heterojunctions. As shown in Figure 1G, the response of the Pt-modified Ti3C2Tx/TiO2 heterojunction sensor to 100 ppm NH3 at room temperature is 45.5%, which is 13.8 times and 10.8 times greater than that of the intrinsic Ti3C2Tx material and the Ti3C2Tx/TiO2 heterojunction, respectively. Furthermore, the Pt-modified Ti3C2Tx/TiO2 heterojunction sensor offers advantages such as low detection limits, high response, rapid response and recovery times, and selectivity, as illustrated in Figures 1H, I. In terms of long-term stability and durability under greenhouse conditions, the Pt-Ti3C2Tx/TiO2 sensor exhibited a slight decrease in response to 100 ppm NH3 after a duration of 22 days. This research provides new insights into the development of efficient gas sensors, particularly for monitoring harmful gases under ambient conditions.
Qin et al. reported an environmentally friendly and efficient one-step pulsed laser ablation method for synthesizing iron cluster-loaded Ti3C2Tx MXene (L-Fe-Ti3C2Tx) gas sensing materials (). Subsequently, the research team developed sensors based on L-Fe-Ti3C2Tx nanosheets to enable real-time detection of NH3 at room temperature. As illustrated in Figures 1J–L, the L-Fe-Ti3C2Tx-232 sensor exhibited a response of 64.03% under 10 ppm NH3 conditions and demonstrated significant reproducibility even in high-humidity environments, making it a strong candidate for specific applications such as agricultural monitoring and human health assessment. This exceptional ammonia sensing performance is primarily attributed to the robust adsorption capability conferred by the abundant defect sites on the Ti3C2Tx nanosheets, along with the catalytic effects provided by the supported iron clusters. The L-Fe-Ti3C2Tx-232 sensor demonstrated commendable long-term stability and durability under greenhouse conditions, maintaining consistent responsiveness to 10 ppm NH3 over a continuous monitoring period of 44 days. This study introduces a method for modifying Ti3C2Tx materials with non-noble metal clusters, thereby further enhancing the sensing performance of Ti3C2Tx MXene materials compared to previous research. Such advancements are poised to not only advance gas sensing technology but also open new avenues for practical applications in related fields.
2.2 Metal oxide composite Ti3C2Tx MXene nanomaterials
Kale et al. developed a Ti3C2Tx material with optimized surface terminating groups (-O and -OH), utilizing 30% HF as an etching agent to enhance the interaction with ammonia molecules, designated as Ti3C2Tx-30 (). This material exhibits the advantage of a low work function (3.78 eV) and is embedded in a MoO3 matrix to achieve both high sensitivity and room temperature operation. The MoO3 matrix provides stability to Ti3C2Tx-30, effectively reducing its risk of oxidation, while Ti3C2Tx-30 introduces additional free electrons into the MoO3 matrix, enabling efficient detection of NH3 at room temperature. When exposed to 100 ppm ammonia, the sensor response reached 93%, with a response time of approximately 10 s, representing a tenfold improvement over bare MoO3. The surplus electrons on the Ti3C2Tx-30 surface facilitate the generation of species, further enhancing the stability of the MXene surface. In the presence of adsorbed MoO3, these species actively react with ammonia molecules, leading to significant changes in system resistance. Consequently, by incorporating an optimally proportioned Ti3C2Tx, a significant breakthrough has been achieved for metal oxides in realizing high gas sensitivity at room temperature.
Yang et al. introduced a novel sensor by integrating SnO2 quantum dot (QD) nanoparticles with Ti3C2Tx MXene, achieving a composite material that exhibits enhanced sensing capabilities (). The successful fabrication of this composite was thoroughly characterized through transmission electron microscopy (TEM) imaging and elemental mapping analysis, as demonstrated in Figures 1M–O. This innovative approach not only highlights the synergistic effects of combining these two materials but also paves the way for advanced gas sensing applications with improved sensitivity and selectivity. The abundant nucleation sites on the MXene surface facilitated the dense deposition of SnO2 QD nanoparticles with diameters ranging from 2 to 3 nm. Compared to traditional methods, SnO2-composited Ti3C2Tx MXene materials markedly improve the detection performance for NH3 at room temperature, exhibiting a response that is 100 times greater than that of intrinsic Ti3C2Tx MXene materials. These findings provide new insights into the development of gas sensors and signify the potential applications of MXene-based materials in environmental monitoring.
Peng et al. successfully synthesized transition metal oxide-derived copper oxide (CuO) nanoparticles (NPs) through a hydrothermal method combined with stirring aging techniques (). The resultant CuO NPs exhibited octahedral growth on the surface of Ti3C2Tx MXene nanosheets. The successful fabrication of this composite material was substantiated by X-ray diffraction (XRD) analysis, which revealed distinct crystallographic peaks corresponding to the synthesized phases, as illustrated in Figure 1P. This innovative approach signifies a notable advancement in the integration of metal oxides with MXene substrates, paving the way for enhanced functionalities in various applications. The resulting binary heterostructure, CuO NPs/Ti3C2Tx MXene, retains the unique structure of the metal-organic framework (MOF), featuring abundant heterojunction interfaces. Ti3C2Tx MXene materials compounded with CuO nanoparticles exhibit the unique structure of metal-organic frameworks (MOFs), and the characteristics associated with MOFs result in a larger specific surface area, abundant active sites, and a more intricate porous structure, thereby significantly enhancing their adsorption performance for NO2. Under room temperature conditions, the sensor exhibits a sensitive response time of 38.54 s–100 ppm NO2, with a total response time of 2.84 s. Furthermore, the detection limit for NO2 is as low as 30 ppb, maintaining good stability over a testing period of 10 weeks. This research positions the composite sensor as a high-sensitivity tool for NO2 monitoring, showcasing significant potential for applications at room temperature.
Xiong et al. developed a flexible ammonia sensor utilizing a composite material comprising polyaniline (PANI), Ti3C2Tx, and TiO2 through ultrasonic spray pyrolysis and in situ polymerization techniques (). The successful synthesis of this composite was characterized by X-ray diffraction (XRD) and Raman spectroscopy, as shown in Figures 1Q, R. This innovative methodology not only enhances the sensor’s performance but also exemplifies the potential for integrating advanced materials to facilitate heightened sensitivity and responsiveness in gas detection applications. The synthesized TiO2 nanoparticles are uniformly adhered to the surface of Ti3C2Tx, effectively preventing aggregation in the PMT. Leveraging this characteristic, the enhanced PMT film sensor demonstrates high response (2.30), low detection limit (20 ppb), and high selectivity for 10 ppm NH3 at room temperature. Currently, considerable attention is being devoted to the incorporation of metal oxides into Ti3C2Tx MXene materials to form heterojunction structures. In addition to the aforementioned typical metal oxide-Ti3C2Tx heterojunctions, we provide an overview of representative studies on metal oxide-Ti3C2Tx heterojunctions for greenhouse gas detection, focusing primarily on NO2 and NH3 detection, which are summarized in Table 1.
TABLE 1
| Gas | Sensing material | Synthesis method | Concentration (ppm) | Temperature (°C) | Response | Selectivity | References |
|---|---|---|---|---|---|---|---|
| NH3 | Pt-Ti3C2Tx/TiO2 | hydrothermal | 100 | room temperature | 45.5% | NO2, H2S, C2H6O, C4H10O, C3H8O, C3H6O, CH4O, CH2O | |
| SnO2 QDs/Ti3C2Tx | water-dispersible | 100 | room temperature | 10.4 | C3H6O, C2H6O, Toluene, CH4O | ||
| Ti3C2Tx/CeO2/TiO2 yarns | hydrothermal | 100 | room temperature | 42.9% | C3H7NO, CH3COOH, H2S, HCHO, C2H5OH | ||
| BaTiO3/Ti3C2Tx | hydrothermal | 20 | room temperature | 10.43 | C2H6O, CO, SO2, H2S, C6H6 | ||
| Ti3C2Tx/SnO2 | hydrothermal | 500 | room temperature | 75% | C2H6OH, H2, CH4, H2S, C3H6O | ||
| NO2 | Ti3C2Tx/ZnO nanorod hybrids | hydrothermal | 0.2 | room temperature | 346% | SO2, NH3, H2S, CO | |
| In2O3/Ti3C2Tx | hydrothermal | 0.25 | room temperature | 193.45% | NO, CO2, NH3, CH4, H2 | ||
| SnO2 nanoparticles@Ti3C2Tx | one-step solvothermal | 300 | room temperature | 78.2% | CO, H2, NO2, H2S, NH3 | ||
| few-layered Ti3C2Tx/WO3 | electrostatic self-assembly | 20 | 200 | 89.46 | NH3, CO, C6H6, C3H6O, Xylene, Toluene, CH4O, CH2O, C2H6O | ||
| Ti3C2Tx/ZnO | electrostatic self-assembly, hydrothermal | 50 | 80 | 190% | NH3, C3H6O, C7H8, CH3OH, CH2O, C3H8O, C2H6O, CO | ||
| C3H6O | alpha-Fe2O3/TiO2/Ti3C2Tx | hydrothermal, annealing treatments | 100 | 220 | 34.66 | C2H6O, CH4O, CH2O, C7H8 | |
| α-/γ-Fe2O3/TiO2/Ti3C2Tx | hydrothermal | 100 | 240 | 105.5 | C8H10, C2H6O, CH4O, CH2O, H2S, NO2, NH3, C2O | ||
| TiO2/Fe2O3-Ti3C2Tx | hydrothermal | 100 | 268 | 446.8 | C8H10, C2H6O, CH4O, CH2O, H2S, NO2, NH3, CO | ||
| Ti3C2Tx/rGO/CuO | hydrothermal | 100 | Room Temperature | 52.9% | C6H6, N(CH3)3, CH4O, C7H8, (C2H5)3N | ||
| (C2H5)3N | In2O3 nanofibers/Ti3C2Tx | electrospinning | 50 | 120 | 12.44 | CH2O, C2H6O, C3H6O, CH4O, NH3, C6H6 | |
| Cr2O3/TiO2/Ti3C2Tx | hydrothermal | 5 | Room Temperature | 28% | NH3, C2H6O, C3H8O, C3H6O, C7H8, N(CH3)3, CH4O, CO, NO2 | ||
| CH2O | Ti3C2Tx/SnO2 | hydrothermal, freeze-dry | 10 | room temperature | 29.16% | C2H6O, C3H6O, N(CH3)3, CH4O, NH3 | |
| ZnSnO3 nanocubes/Ti3C2Tx | hydrothermal | 5 | room temperature | 62.4% | NH3, C2H6O, C6H6, C3H6O, N(CH3)3 | ||
| C2H6O | ZnO/Ti3C2Tx | hydrothermal | 100 | 300 | 79 | C3H6O, CH4O, C3H8O, NH3 | |
| MoO2/MoO3/Ti3C2Tx | hydrothermal | 200 | room temperature | 19.77 | N(CH3)3, C3H6O, CH4O, CH2O, (C2H5)3N |
Summary of recent researches on Metal oxide composite Ti3C2Tx MXene sensors for gas detection.
2.3 Alternative materials composite Ti3C2Tx MXene nanomaterials
Kim et al. successfully prepared three-dimensional MoS2/MXene van der Waals heterostructure aerogels through the physical mixing of two-dimensional MXene and MoS2, followed by a freeze-drying method (). This low-temperature synthesis approach effectively mitigated the significant oxidation of Ti3C2Tx MXene while forming a layered, independent three-dimensional heterostructure composed of high-quality MoS2 and MXene nanosheets. The catalytic layer of MoS2 significantly enhanced the functionalization of MXene, improving its sensitivity and long-term stability towards NO2 gas. Additionally, a resistive sensor based on WS2/Ti3C2Tx MXene multilayer composites was designed by Sardana et al. (). The optimal concentration ratio of a chemical resistive sensor was explored by combining different concentrations of WS2 with MXene. Notably, the composite Ti3C2Tx MXene sensor containing 25 wt% WS2 exhibited responses of 29% to 5 ppm NH3 and 44% to 100 ppb NO2 at room temperature, along with strong selectivity.
To achieve low detection limits and high sensitivity for NH3 detection at room temperature, a gas sensor based on a polypyrrole (PPy) coated MXene/MoS2 nanocomposite has been developed by Lu et al. (). This loosely assembled heterogeneous structure of MXene-loaded MoS2 was synthesized through a one-step hydrothermal technique based on electrostatic self-assembly. The MoS2 nanosheets uniformly wrapped around the etched MXene surface formed a highly synergistic metal-semiconductor contact, effectively generating a resistive layer capable of capturing electrons produced during NH3 sensing. PPy was subsequently coated onto the MXene/MoS2 composite surface via an in situ polymerization process, resulting in a self-supporting structure. The Ti3C2Tx MXene/MoS2/PPy composite material exhibits a high response to 100 ppm NH3 that is five times greater than that of intrinsic Ti3C2Tx MXene materials, while maintaining a response of 2.08 for 10 ppm NH3 at room temperature. The MXene/MoS2/PPy nanocomposite sensor also displayed excellent long-term stability, high response and recovery rates, outstanding humidity resistance, and notable selectivity towards NH3.
A shell-type Ti3C2Tx@PDAC (croconaine) composite material was designed by Zhou et al. through a simple in situ polymerization reaction for the detection of NH3 at room temperature (). Compared to the original Ti3C2Tx, sensors constructed from the Ti3C2Tx-PDAC composite exhibited remarkable sensitivity, achieving a response of 2.8% ppm-1 with an estimated detection limit of 50 ppb. Under room temperature conditions, the Ti3C2Tx-PDAC sensor demonstrated commendable stability and durability, maintaining effective responsiveness to both 10 ppm and 500 ppb NH3 over a period of 45 days. Concurrently, Quan et al. devised a fully flexible paper-based gas sensor that integrated non-metallic Ti3C2Tx MXene electrodes with a Ti3C2Tx/WS2 sensing film, forming ohmic contacts and Schottky heterojunctions within a single gas sensing channel (). This Ti3C2Tx/WS2 composite demonstrated high conductivity, effective charge transfer capabilities, and abundant gas-sensing active sites. Under room temperature conditions, this gas sensor achieved a response of 15.2% to 1 ppm NO2, which is 3.2 times greater than that of an Au fingertip electrode integrated with a Ti3C2Tx/WS2 sensor (4.8%) and 76.0 times greater than that of a MXene electrode integrated with a Ti3C2Tx sensor (0.2%). Furthermore, the design operated effectively at a detection limit of 11.0 ppb for NO2 gas while exhibiting outstanding stability in high-humidity environments.
A film-type sensor based on a hybrid polyaniline (PANI:PSS) was developed by Wen et al. using a unique in situ polymerization technique (). This sensor exhibits high sensitivity for NH3 detection, yielding a favorable response to 1 ppm NH3. In terms of long-term stability and durability under greenhouse conditions, the PANI:PSS/Ti3C2Tx sensor exhibited a mere 23% decrease in responsiveness to 1 ppm NH3 after 30 consecutive hours of operation at room temperature. This result underscores the sensor’s commendable stability, highlighting its potential for reliable application in extended environmental monitoring scenarios. Furthermore, it demonstrates notable selectivity, stability, and mechanical properties at room temperature. Additionally, a novel MoS2/Ti3C2Tx heterostructure sensor was designed by Ta et al. using a hydrothermal method (). Compared to traditional heterojunction sensors, the incorporation of two-dimensional metal sulfide MoS2 with Ti3C2Tx MXene significantly enhances the sensing performance, while the compatibility between MoS2 and Ti3C2Tx ensures stable sensing capabilities for NO2 at room temperature. Building upon the work of Ta’s team, Tian et al. incorporated a TiO2 system into MoS2/Ti3C2Tx materials to enhance the capacity for electron supply in the sensing material (). Gas sensing tests revealed that, compared to intrinsic Ti3C2Tx MXene and MoS2 materials, the sensing performance of this composite material improved by 1.79 times and 2.75 times, respectively, for 100 ppm NH3 at room temperature, while also exhibiting a lower detection limit. The Ti3C2Tx MXene@TiO2/MoS2 sensor demonstrated exceptional responsiveness to 100 ppm NH3 at room temperature over a continuous monitoring period of 60 days, with a slight decrease in response of only 9.1%. The response value stabilized at approximately 163.3%, indicating the sensor’s robust performance and potential for long-term applications in gas detection. These findings provide new perspectives for the development of gas sensors and illustrate the broad application potential of MXenes and their composites in environmental monitoring.
3 Sensing mechanisms of Ti3C2Tx MXene heterojunction structures
Understanding the gas sensing mechanisms is crucial for the development of high-performance sensors based on Ti3C2Tx MXene. The adsorbed gases can be primarily classified into oxidative and reductive gases. As shown in previous analyses, Ti3C2Tx MXene exhibits exceptional responsiveness to toxic gases such as NO2 and NH3 commonly found in greenhouse environments (; ; ). In this study, NO2 is selected as a representative oxidative gas and NH3 as a reductive gas for in-depth investigation. Research indicates that Ti3C2Tx MXene, due to its high conductivity, large specific surface area, favorable hydrophilicity, and abundant surface functional groups, possesses excellent adsorption capabilities toward NO2 (). Furthermore, existing studies have demonstrated that Ti3C2Tx MXene materials can also respond favorably to the reductive gas NH3 (). Consequently, the traditional charge transfer theories associated with metal oxides may not be applicable to the Ti3C2Tx MXene materials.
3.1 Sensing mechanisms of Ti3C2Tx MXene heterojunction structures for oxidative gases
During the composite process of Ti3C2Tx MXene with In2O3, it has been observed that when NO2 is adsorbed at room temperature, oxygen is initially adsorbed on the surface of In2O3, where it captures electrons to form superoxide anions , as shown in Equations 1, 2 (). Due to the greater electron affinity of NO2 compared to that of O2, electrons on the surface of the sensing material are preferentially attracted to NO2, resulting in the formation of . The specific process is illustrated in Equations 3–5. As electrons are continually captured from In2O3 and flow towards Ti3C2Tx, the Schottky barrier layer at the interface between Ti3C2Tx and In2O3 gradually expands. Upon expulsion of NO2 gas from the chamber, the generated reacts with , reverting back to NO2, as illustrated in Equation 6. In parallel, Guo et al. investigated the behavior of Ti3C2Tx/CuO sensors when exposed to air (). Research conducted by the team reveals that the sensing mechanism of the Ti3C2Tx/CuO sensor involves the initial interaction of oxygen molecules in the air with the sensing layer, leading to the formation of adsorbed . Subsequently, the adsorbed oxygen combines with captured electrons to form . As illustrated in Figure 2A, oxygen vacancies within the Ti3C2Tx/CuO nanocomposite also facilitate the conversion of O2 (ads) into . The depletion of electrons results in a continuous increase in hole concentration within CuO. Due to the work function difference between Ti3C2Tx (3.9 eV) and CuO (4.7 eV), holes in CuO migrate towards Ti3C2Tx while electrons from Ti3C2Tx flow into CuO until the Fermi levels reach equilibrium. Throughout this process, the valence band thickens and the hole accumulation layer widens, resulting in an initial observation of reduced resistance in the Ti3C2Tx/CuO sensor.
FIGURE 2
The MoS2/Ti3C2Tx sensor developed by Ta captures O2 molecules from the air at room temperature, resulting in the formation of superoxide anions as electrons are captured on the sensor surface (). Moreover, the presence of TiO2 facilitates the acquisition of additional oxygen ions by the sensor. As shown in Equation 7, due to its strong electron affinity and oxidative nature, NO2 is capable of extracting electrons from the surrounding medium, resulting in electron depletion. The higher electronegativity of NO2 compared to oxygen facilitates the reaction between NO2 molecules and oxygen ions, leading to the formation of nitrate ions (). Concurrently, the accumulation of holes promotes a reduction in resistance, inducing p-type semiconductor behavior. Notably, a continuous heterojunction interface forms between vertically oriented MoS2 and layer-stacked Ti3C2Tx MXene, facilitating the accelerated migration of additional electrons. In this context, edges terminated with sulfur (S) and those terminated with molybdenum (Mo) exhibit high d-orbital electron densities, enhancing electronic interactions with NO2 gas. Furthermore, hydrogen bonding between NO2 molecules and water molecules under ambient humidity conditions also contributes to the adsorption of NO2. In their investigation, Quan et al. similarly observed that when Ti3C2Tx/WS2 materials are exposed to ambient air at room temperature, oxygen molecules can capture electrons from the material, resulting in the formation of chemically adsorbed oxide ions () on the surface (). This interaction may lead to the establishment of a hole accumulation layer. Upon the introduction of NO2, which possesses a higher electron affinity (2.30 eV) compared to that of oxygen (0.44 eV), electrons migrate toward the NO2, facilitating the formation of nitrate ions (). This process not only promotes electron extraction but also contributes to an increase in the hole accumulation layer, ultimately resulting in a reduction in electrical resistance. The detailed reaction mechanism is illustrated in Figures 2B–E.
3.2 Sensing mechanisms of Ti3C2Tx MXene heterojunction structures for reductive gases
It has been noted that in TiO2/Ti3C2Tx MXene composites, TiO2, as an n-type semiconductor with high electronegativity, may contain intrinsic oxygen vacancies and tends to adsorb oxygen from the atmosphere on its surface (). The adsorbed oxygen molecules capture electrons at the oxide surface, transforming into superoxide anions (), as represented in Equations 8, 9. It is important to acknowledge that competition for oxygen adsorption may occur between the semiconductor oxide and Ti3C2Tx within the composite. In any case, the adsorbed oxygen will convert to . As an n-type semiconductor, TiO2 has a work function of 5.1 eV, while the typical range of the work function for Ti3C2Tx lies between 3.9 eV and 4.8 eV, depending on its surface terminating groups. Assuming a work function of 3.9 eV for MXene, electrons will flow from MXene to TiO2, leading to the formation of a depletion layer at the TiO2/Ti3C2Tx interface. When NH3 interacts with the sensing surface, it can contribute electrons through two different mechanisms, as illustrated in Equations 10, 11. On one hand, NH3 molecules may interact with the adsorbed ions, releasing electrons in the process; on the other hand, gas molecules might also interact with ions, similarly resulting in electron release. The released electrons further compensate for the charge in the depletion layer, causing an increase in the sensor resistance.
Zhu et al. discovered that at room temperature (below 100 °C), oxygen adsorbed on the surface of SnO2/Ti3C2Tx nanocomposites can ionize into superoxide anions (), as shown in Equation 9 (). The entire reaction process can be described by Equations 12, 13. Yao et al. further investigated the behavior of Ti3C2Tx/ZnO when exposed to NH3(). Upon adsorption of NH3 molecules on the Ti3C2Tx/ZnO surface, a reaction occurs with water molecules to form ammonium ions (). Subsequently, the interactions between the adsorbed NH3 and can also be articulated using Equations 12, 13. The electrons released from these reactions flow back to the Ti3C2Tx/ZnO material, where they recombine with holes in Ti3C2Tx MXene, resulting in a reduction of hole carriers in Ti3C2Tx/ZnO and an increase in its resistance. Subsequently, Yao et al. delved into the adsorption mechanism of NH3 on Ti3C2Tx/SnO sensors and discovered that this mechanism aligns with Equations 12, 13 (). The presence of p-n junctions enables the composite material to function as an electron acceptor, receiving electrons from the external environment. This interaction leads to a reduction in resistance during the NH3 adsorption process for Ti3C2Tx MXene/SnO, as depicted in detail in Figures 2F–H. This insightful analysis underscores how the existence of p-n junctions facilitates effective electron transfer within the compound, resulting in enhanced sensitivity to NH3.
Ti3C2Tx MXene exhibits high conductivity, a large specific surface area, favorable hydrophilicity, and abundant surface terminal groups, enabling effective gas adsorption. However, research on the formation of heterojunctions between Ti3C2Tx MXene and transition metal sulfides for the detection of oxidative and reductive gases remains insufficient. Given that Ti3C2Tx MXene demonstrates good adsorption capabilities for both types of gases, directly applying traditional p-type semiconductor theory to Ti3C2Tx MXene materials may not be entirely appropriate (; ). Therefore, it is essential to consider additional factors in the analysis of resistance changes. On one hand, Ti3C2Tx MXene materials do not conform to the conventional definition of semiconductors; their metallic sensing layers may continually hinder the effective transport of charge carriers (). On the other hand, the interlayer spacing of Ti3C2Tx MXene could influence its tolerance, representing an intriguing direction for further exploration. Additionally, the surface functional groups of Ti3C2Tx MXene materials play a crucial role in the gas sensing mechanism. Hydrophilic groups such as -OH and = O can interact with n-type or p-type semiconductors to enhance the adsorption of gas molecules onto the sensing material. For instance, MXene terminated with oxygen is an ideal candidate for NH3 sensing due to its semiconductor-like electronic properties (). Conversely, research by Hu et al. indicates that MXene terminated with sulfur is optimal for nitrogen oxide gas sensors (). This research direction provides a new perspective on the applications of Ti3C2Tx MXene in gas sensing and lays the groundwork for the optimization and design of related materials.
4 Conclusion
This review provides a concise overview of the current research landscape surrounding gas sensors based on Ti3C2Tx MXene heterojunction structures. Analytical results indicate that Ti3C2Tx MXene nanostructures exhibit exceptional gas-sensing performance in greenhouse environments, attributed to their high electrical conductivity, extensive specific surface area, excellent hydrophilicity, and abundant surface terminal groups. The formation of heterojunction structures through composite materials has emerged as an effective strategy for enhancing the performance of Ti3C2Tx MXene sensors. The incorporation of metal atom doping, metal oxides, and transition metal sulfides into Ti3C2Tx MXene significantly improves the gas-sensing characteristics. Consequently, the development of Ti3C2Tx MXene heterojunction-based gas sensors presents immense potential for achieving room-temperature operation, low detection limits, and high responsiveness, particularly for gas monitoring in agricultural greenhouses. However, existing studies predominantly focus on the long-term stability and durability of these sensors under actual greenhouse conditions, mainly concerning ammonia, while investigations regarding common nitrogen oxides and other agricultural waste products remain limited. Moreover, the duration of performance stability testing is often short; thus, future research should extend these analyses to ensure prolonged usability of the sensors within intelligent greenhouse environments. In summary, this review underscores the capabilities of Ti3C2Tx MXene heterojunction gas sensors for monitoring toxic gases in greenhouses, highlighting their primary advantage of enabling high-sensitivity detection and rapid recovery of hazardous gases at room temperature. Future endeavors could integrate the methodologies discussed herein with reported approaches facilitating room-temperature, low-power, and highly sensitive sensing techniques, paving the way for the development of online monitoring sensor arrays for toxic gases in intelligent greenhouse systems. Additionally, establishing a quantitative detection model for multi-component mixed gases using artificial intelligence algorithms may enable direct detection of various toxic gas components within agricultural greenhouse settings.
Statements
Author contributions
HZ: Conceptualization, Investigation, Writing–original draft. HX: Data curation, Writing–original draft. WZ: Writing–review and editing. ZW: Writing–review and editing. QZ: Writing–review and editing.
Funding
The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This work has been supported in part by the National Natural Science Foundation of China (Nos 52077177 and 51507144) and the Fundamental Research Funds for the Central Universities (No. XDJK 2019B021).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.
Generative AI statement
The author(s) declare that no Generative AI was used in the creation of this manuscript.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
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Summary
Keywords
Ti3C2Tx, heterojunction, gas sensor, greenhouse, sensing mechanism
Citation
Zhang H, Xu H, Zeng W, Wang Z and Zhou Q (2024) Gas sensing performance of Ti3C2Tx MXene heterojunction structures in greenhouse environments: a mini review. Front. Chem. 12:1509732. doi: 10.3389/fchem.2024.1509732
Received
11 October 2024
Accepted
18 November 2024
Published
27 November 2024
Volume
12 - 2024
Edited by
Tianying Sun, Sun Yat-sen University, China
Updates
Copyright
© 2024 Zhang, Xu, Zeng, Wang and Zhou.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Qu Zhou, zhouqu@swu.edu.cn
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.