2D/2D SnO2 nanosheets/Ti3C2Tx MXene nanocomposites for detection of triethylamine at low temperature

Highly active two-dimensional (2D) nanocomposites have been widely concerned in the field of gas sensors because of their unique advantages and synergistic effects. 2D/2D SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites were synthesized by using layered Ti₃C₂T_x MXene and uniform SnO₂ nanosheets by hydrothermal method. Characterization results show that the SnO₂ nanosheets are well dispersed and vertically anchored on the layered Ti₃C₂T_x MXene surface, forming heterogeneous interfaces. Based on the gas-adsorption capabilities and synergistic effects of electronic properties, SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites show high triethylamine (TEA) gas-sensing performance at low temperature (140 °C). The sensor responses of the nanocomposites and pure SnO₂ nanosheets to 50 ppm of TEA are 33.9 and 3.4, respectively. An enhancement mechanism for SnO₂ nanosheets/Ti₃C₂T_x MXene nanocomposites is proposed for highly sensitive and selective detection of TEA at low temperature. The combination strategy of two-dimensional metal oxide semiconductor and multilayer MXene provides a new way for the development of cryogenic gas sensors in the future.     

Broadband electromagnetic absorption of Ti3C2Tx MXene/WS2 composite via constructing two-dimensional heterostructure

Ti₃C₂T_x MXene, an emerging two-dimensional (2D) ceramic material, has rich interfaces and strong conductive networks. Herein, we have successfully built a heterostructure between Ti₃C₂T_x MXene and WS₂ to improve electromagnetic absorption performance. X-ray diffraction and X-ray photoelectron spectroscopy were used to determine the successful synthesis of Ti₃C₂T_x/WS₂ composite. Field emission scanning electron microscopy and transmission electron microscopy images show that WS₂ nanosheets are evenly dispersed on the accordion-like Ti₃C₂T_x MXene. Importantly, Ti₃C₂T_x MXene/WS₂ composite has sufficiently high dielectric loss and impedance matching due to self-adjusting conductivity and 2D heterostructure interfaces. As a result, the Ti₃C₂T_x/WS₂ composite has a minimum reflection loss (RL_min) of −61.06 dB at 13.28 GHz. Besides, it has a broad effective absorption bandwidth (EAB) of 6.5 GHz, with EAB >5.0 GHz covering a wide range of thickness. Such impressive results may provide experience for the application of Ti₃C₂T_x ceramics and 2D materials.     

Tuning the photoluminescence of large Ti3C2Tx MXene flakes

Ti₃C₂T_x MXene has been reported to be a metallic two-dimensional (2D) material with high conductivity, whereas its photoluminescence (PL) mechanism is still under debate. Herein, we demonstrate that large Ti₃C₂T_x MXene flakes exhibit tunable PL under ambient conditions. The as-prepared Ti₃C₂T_x MXene flakes emit blue, yellow-green and red light under different excitation wavelengths. Their PL emission wavelengths redshift as the excitation wavelength changes from violet to red light. Surface modification of the MXenes can further tune the PL peak wavelength into the near infrared region. Using density function theory (DFT) calculations, this excitation wavelength-dependent PL can be correlated to TiO₂ defects that exist on the surface of Ti₃C₂T_x. Our study expounds on the optical properties of Ti₃C₂T_x MXene and is helpful for comprehensively understanding this novel material.     

Ti3C2Tx aerogel with 1D unidirectional channels for high mass loading supercapacitor electrodes

As one of the typical MXenes materials, 2D Ti₃C₂T_x has attracted extensive attention in the field of energy storage. However, due to the restacking problem of Ti₃C₂T_x nanosheets, the electrochemical performance of Ti₃C₂T_x is unsatisfactory. In this paper, a scheme is proposed to obtain 3D aerogel with 1D channels by directional freeze drying of Ti₃C₂T_x. With the help of the unidirectional channels, the 3D Ti₃C₂T_x/Sodium alginate (SA) aerogel can effectively solve the stacking problem of Ti₃C₂T_x nanosheets, and it also accelerates the diffusion of ions. The Ti₃C₂T_x/SA-5 electrode can still reach the mass capacitance of 284.5 F g^?1 and the areal capacitance of 4030.4 mF cm^?2 at 2 mV s^?1 when the loading is 14.2 mg cm^?2 in 1 M H₂SO₄ electrolyte. In addition, the electrode showed good cycling performance without capacitor degradation after 20,000 cycles at 50 mV s^?1. These results suggest that by using the strategy of building special 3D structure of 2D MXene with 1D unidirectional channels, high performance supercapacitor electrodes with high mass loading can be realized.     

Electrochemical storage mechanism of interstratification-assembled Ti3C2Tx MXene/NiCo-LDHs electrode in alkaline,acid and neutral electrolytes

Different kinds of two-dimensional hybrid electrodes have high theoretical capacitance and energy density. However, the origin of the electrochemical storage mechanism still remains elusive in alkaline, acid and neutral electrolytes. Herein, the interstratification-assembled Ti₃C₂T_x MXene/NiCo-LDHs electrodes were successfully prepared and studied in different electrolytes by in-situ Raman spectroscopy. The results show that H₂O molecules in neutral electrolyte combine with –OH at the end of Ti₃C₂T_x MXene during charging, and debonding occurs during discharge. Similarly, this reaction also occurs in the discharge process with NiCo-LDHs and provides smaller pseudocapacitance characteristics. Although this pseudocapacitance reaction also occurs in acidic and alkaline electrolytes, however, the difference is that the hydrogen ions will promote the electrochemical performance of Ti₃C₂T_x MXene and has a certain corrosion consumption effect on NiCo-LDHs, but generally improve the electrochemical performance of Ti₃C₂T_x MXene/NiCo-LDHs. Interestingly, the OH^? in alkaline electrolyte can promote the electrochemical performance of NiCo-LDHs, and produce a new electrochemical reaction with –F between the layers of Ti₃C₂T_x MXene, which greatly improves the overall electrochemical performance of this hybrid electrodes. As a result, Ti₃C₂T_x MXene/NiCo-LDHs electrodes have the best electrochemical performance in alkaline electrolyte with capacitance of 283 F g^?1, energy density of 14.2 Wh kg^?1 and power density of 3007.1 W kg^?1. This work lays a foundation for the preparation of high-performance two-dimensional hybrid electrochemical energy storage devices.     

Improvement of gas sensing property for two-dimensional Ti3C2Tx treated with oxygen plasma by microwave energy excitation

Two-dimensional layered Ti₃C₂T_x MXene was prepared through hydrothermal etching method with LiF and hydrochloric (HCl) acid. Ti₃C₂T_x was further treated with oxygen plasma activated by microwave energy to obtain the activated Ti₃C₂T_x at different temperatures ranging from 350 °C to 550 °C. The gas-sensing properties of raw Ti₃C₂T_x and Ti₃C₂T_x activated with oxygen microwave plasma were tested toward different volatile organic compounds gases. The results indicated that Ti₃C₂T_x activated at 500 °C exhibited excellent gas-sensing properties at room temperature (25 °C) to 100 ppm ethanol with a value of 22.47, which is attributed to the enhancement of the amount of oxygen functional groups and defects on the MXene Ti₃C₂T_x film, and in turn to lead to more oxygen molecules adsorption and desorption reaction in the active defect sites. The enhancement of ethanol-sensing performance demonstrated that the activated Ti₃C₂T_x possess great potential in gas sensing.     

Preparation and characterization of polymer‐Ti3C2Tx (MXene) composite nanofibers produced via electrospinning

MXene, a recently‐discovered family of two‐dimensional (2D) transition metal carbides and/or nitrides, have attracted much interest because of their unique electrical, thermal, and mechanical properties. In this study, poly(acrylic acid), poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), and alginate/PEO were electrospun with delaminated Ti₃C₂ (MXene) flakes. The effect of small additions of delaminated Ti₃C₂ (1% w/w) on the structure and properties of the nanofibers were investigated and compared with those of the neat polymer nanofibers using scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, and Fourier transform infrared spectroscopy. Ti₃C₂ had an effect on the solution properties of the polymer and a greater effect on the average fiber diameter. The Ti₃C₂T_x/PEO solution exhibited the largest change in viscosity and conductivity with an 11% and 73.6% increase over the base polymer, respectively. X‐ray diffractograms demonstrated a high degree of crystallization for Ti₃C₂/PEO and a slight decrease in crystallinity for Ti₃C₂/PVA. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45295.     

Fabrication of PANI@Ti3C2Tx/PVA hydrogel composite as flexible supercapacitor electrode with good electrochemical performance

Developing a new strategy to effectively prevent the restacking of MXene nanosheets will have significant impacts on designing flexible supercapacitor electrodes. Herein, a novel Ti₃C₂T_x/polyvinyl alcohol (PVA) porous sponge with 3D interconnected structures is prepared by sol-gel and freeze-dried methods. This Ti₃C₂T_x/PVA porous sponge is used as the template of in-situ polyaniline (PANI) polymerization, and the fabricated PANI@Ti₃C₂T_x/PVA hydrogel composite is applied as flexible supercapacitors electrodes. 1D conductive polymer chains PVA could increase the interlayer spacing of Ti₃C₂T_x nanosheets, which is beneficial to expose more electrochemical active sites. The supercapacitor based on PANI@Ti₃C₂T_x/PVA hydrogel composite exhibits the coexistence of double-layer capacitance and pseudocapacitance behavior. This supercapacitor shows a maximum areal specific capacitance of 103.8 mF cm^?2 at 2 A m^?2, and it also exhibits a maximum energy density of 9.2 μWh·cm^?2 and an optimum power density of 800 μW cm^?2. The capacitance of this supercapacitor is almost not change under different bending angles. Moreover, 99% capacitance retention is achieved after 10 000 charge/discharge cycles of the supercapacitor. The synergistic effect between PANI and Ti₃C₂T_x/PVA composite may improve the number of reactive sites and provide efficient channels for ion diffusion/electron transport.     

Fabrication strategies and application fields of novel 2D Ti3C2Tx (MXene) composite hydrogels: A mini-review

Ti₃C₂T_x (MXene), a new kind of 2D ceramic nanosheets, is receiving more and more attention in the fields of medicine, biology, energy, electronics, etc. However, the preparation and application of MXene in hydrogel is still in its infancy period. Here, we review the latest progress (after 2018) related to MXene hydrogels in time. Aiming at the key issue of the dispersion stability of MXene in hydrogel systems, the preparation strategy, mechanism, advantages and disadvantages of MXene hydrogels are sorted out in detail, and the potential application prospects of MXene composite hydrogel are introduced. Finally, future viewpoints are put forward for the dispersion stability challenges that need solving in the design of MXene hydrogel.     

Scavenging activity and reaction mechanism of Ti3C2Tx MXene as a novel free radical scavenger

A novel free radical scavenger, multi-layered Ti₃C₂T_x MXene (ML-Ti₃C₂T_x), has been studied by evaluating its scavenging activity against 2,2-diphenyl-1-picrylhydrazyl (DPPH?). It exhibits high scavenging efficiency (95% in 10 min) at low dosage (0.06 mg/mL). Based on the analysis of structure and surface states of ML-Ti₃C₂T_x before and after reaction with DPPH? and a series of comparative experiments, including few-layered Ti₃C₂T_x MXene (FL-Ti₃C₂T_x), original Ti₃AlC₂, and soluble Ti species derived from ML-Ti₃C₂T_x, the observed high scavenging activity is attributed to the intrinsic reducing property of ML-Ti₃C₂T_x rather than the hydrogen donation ability from surface functional groups. A model is proposed to explain the scavenging mechanism.     

Synthesis of few-layered Ti3C2Tx/ WO3 nanorods foam composite material for NO2 gas sensing at low temperature

The few-layered Ti₃C₂T_x/WO₃ nanorods foam composite material was synthesized by electrostatic self-assembly and bidirectional freeze-drying technologies. The phase structure and microstructure of synthesized samples was characterized by XRD, FESEM, TEM and their gas sensing properties estimated via a self-designed equipment with four test channels. The results demonstrate WO₃ nanorods were successfully anchored on the surface and between layers of few-layered Ti₃C₂T_x MXene by electrostatic self-assembly strategy and the composite material simultaneously has a low-density foam morphology by means of bidirectional freeze-drying processes. There exists a typical heterostructure at the interfaces owing to the inseparable contact between the few-layered Ti₃C₂T_x MXene and WO₃ nanorods. Compared with the original WO₃ nanorods, the few-layered Ti₃C₂T_x/WO₃ nanorods foam composite material displays excellent gas sensing properties for NO₂ detection at low temperature, in particular the optimal value of gas sensing response (R_g/R_a) reaches to 89.46 toward 20 ppm NO₂ at 200 °C. The gas sensing mechanism was also discussed. The increase of gas sensitivity is attributed to a fact that during the reaction process of gas sensing, the excellent conductivity of the few-layered Ti₃C₂T_x MXene provided faster transport channels of free carriers, and the heterojunctions formed by few-layered Ti₃C₂T_x MXene and WO₃ nanorods enhanced the carriers separation efficiency. Meanwhile, the low-density layered structure of few-layered Ti₃C₂T_x/WO₃ nanorods foam composite material provides convenient diffusion paths for gas molecules to the surface of WO₃ nanorods.     

In2O3 nanocubes/Ti3C2Tx MXene composites for enhanced methanol gas sensing properties at room temperature

Highly active two-dimensional (2D) nanocomposites, integrating the unique merits of individual components and synergistic effects of composites, have been recently receiving attention for gas sensing. In this work, In₂O₃ nanocubes/Ti₃C₂T_x MXene nanocomposites were synthesized using In₂O₃ nanocubes and layered Ti₃C₂T_x MXene via a facile hydrothermal self-assembly method. Characterization results indicated that the In₂O₃ nanocubes with sizes approximately 20–130 nm in width were well dispersed on the surface of layered Ti₃C₂T_x MXene to form numerous heterostructure interfaces. Based on the synergistic effects of electronic properties and gas-adsorption capabilities, In₂O₃ nanocubes/Ti₃C₂Tx MXene nanocomposites exhibited high response (29.6%–5 ppm) and prominent selectivity to methanol at room temperature. Meanwhile, the low detection concentration could be reduced to ppm-level, the response/recovery times are shortened to 6.5/3.5 s, excellent linearity and outstanding repeatability. The strategy of compositing layered MXene with metal oxide semiconductor provides a novel pathway for the future development of room temperature gas sensors.     

Multilayer Ti3C2Tx: From microwave absorption to electromagnetic interference shielding

Ti₃C₂T_x MXene has attracted extensive attention in the field of electromagnetic (EM) protection over recent years. Multilayer Ti₃C₂T_x (M-Ti₃C₂T_x), as an intermediate product of MXene ultra-thin structure, has potential advantages in the field of EM protection. Herein, the M-Ti₃C₂T_x was obtained by HCl/LiF etching Ti₃AlC₂. The microwave absorption (MA) and electromagnetic interference (EMI) shielding performance of Ti₃AlC₂ and M-Ti₃C₂T_x were compared. The mechanism research of MA and EMI shielding indicates that the construction of local conductive network plays a leading role in the EM wave attenuation. The sample with 30% M-Ti₃C₂T_x display RL_min of ?50.26 dB, and corresponding bandwidth of 4.64 GHz at the thickness of 1.7 mm. Especially, the metastructure based on the EM parameters of M-Ti₃C₂T_x/wax exhibits ultra-wide bandwidth (15.54 GHz). Our research will provide a basis for the design of MXene-based EM protection performance.     

Enhanced dielectric and mechanical properties of CaCu3Ti4O12/Ti3C2Tx MXene/silicone rubber ternary composites

Dielectric polymer composites with conducting fillers would have great potential for diverse applications if their severe leakage loss could be addressed. In this regard, ternary composites using both ceramic and conducting materials as fillers might be an enabler for high dielectric constant and low dielectric loss. Herein, ternary composites with both Ti₃C₂T_x MXene conducting nanosheets and CaCu₃Ti₄O₁₂ (CCTO) dielectric particles embedded in silicone rubber were studied. It was found that a ternary composite with 1.2 wt% (0.40 vol%) Ti₃C₂T_x MXene and 12 wt% (2.58 vol%) CCTO could provide an overall superior performance that include a high dielectric constant of 8.8, low dielectric loss of less than 0.0015, good thermal stability up to 450 °C, and excellent mechanical properties with tensile strength of 569 kPa, elastic module of 523 kPa and elongation at break of 333%. The outstanding performance is attributed to the improved uniform dispersion and good interfacial compatibility of mixed fillers in the polymer matrix, suggesting ternary composites might be a better option over their binary counterparts in preparing high performance dielectric composites.     

Synthesis of Ti3C2Fx MXene with controllable fluorination by electrochemical etching for lithium-ion batteries applications

Ti₃C₂T_x MXene has attracted remarkable attention due to its promising applications in energy storage and sensors. However, traditional MXene preparation methods used HF as etchant, which was highly toxic and harmful to human and environment. Moreover, the aqueous etchants will also result in the combination of OH, O and F groups on the surfaces, making it difficult to control the varieties and contents of the surface terminations. In this paper, a green and mild electrochemical exfoliation method was proposed to synthesize Ti₃C₂F_x and synchronously control its fluorination degree on the surface. A non-aqueous ionic liquid, [BMIM][PF₆]-based solution was used as electrolyte. The as-prepared Ti₃C₂F_x was fluorinated with the CF and TiF₃ groups, which were electrochemically active and contributed to the excellent cycling stability of the MXene anode-based Li-ion batteries. These findings provided a facile strategy to prepare MXene materials and dope MXene with tailored property for MXene-based energy devices applications.     

Bifunctional Ti3C2Tx–CNT/PANI composite with excellent electromagnetic shielding and supercapacitive performance

Ti₃C₂T_x exhibits excellent electromagnetic (EM) shielding and electrochemical properties. However, the inherent re-stacking tendency and easy oxidation of Ti₃C₂T_x limit its further application. In this study, a multi-walled carbon nanotube/polyaniline composite (CNT/PANI, denoted as C–P) was introduced into Ti₃C₂T_x nanosheets to obtain a Ti₃C₂T_x–CNT/PANI composite (T@CP). Owing to the integrated effects of Ti₃C₂T_x and C–P, the contribution of absorption was significantly improved, which finally enhanced the EM shielding performance of T@CP. The highest total EM shielding effectiveness (SE_T) was close to 50 dB (49.8 dB), which was substantially higher than that of pure Ti₃C₂T_x (45.3 dB). Moreover, T@CP demonstrated outstanding supercapacitive performance. The specific capacitance of T@CP (2134.5 mF/cm² at 2 mV/s) was considerably higher than that of pure Ti₃C₂T_x (414.3 mF/cm² at 2 mV/s). These findings provide a new route for the development of high-efficiency Ti₃C₂T_x-based bifunctional EM shielding and electrochemical materials.     

Novel Ti3C2Tx MXene/epoxy intumescent fire-retardant coatings for ancient wooden architectures

Most of the ancient buildings are made of inflammable wooden structures, which have serious potential safety hazards. Applying fire-retardant coating is one of the simplest and most effective means of fire prevention in ancient wooden buildings. In this work, we have demonstrated that the Ti₃C₂T_x transition metal carbide/carbonitride (MXene) was applied as the synergetic agent, waterborne epoxy resin as the film-forming agent, ammonium polyphosphate, dipentaerythritol, and melamine (P-C-N system) as the intumescent fire-retardant system to prepare Ti₃C₂T_x/epoxy intumescent fire-retardant coating (TEIFC). The results showed that MXene has significantly improved the fire-retardant performance of the coating. By incorporating 3 wt% Ti₃C₂T_x (TEIFC-3, with 62 wt% P-C-N system), the coating displayed UL-94 V-0 rating with the limiting oxygen index value of 38%. In addition, the combination of Ti₃C₂T_x and P-C-N system enhanced the Shore hardness of the coating to 95 SHD (TEIFC-3). Furthermore, TEIFC-3 presented high thermal stability with the T_HRI of 177.0°C and T_dmax of 380.5°C. This work provides a novel strategy for the design and preparation of intumescent fire-retardant coating, which will greatly broaden the industrial applications of MXene-based polymer composites in the field of fire prevention of ancient buildings.     

Manganese dioxide nanosheets decorated on MXene (Ti3C2Tx) with enhanced performance for asymmetric supercapacitors

The incorporation of nanosized pseudocapacitive materials and structure design are general strategies to enhance the electrochemical performance of MXene-based materials. Herein, the decoration of manganese dioxide (MnO₂) nanosheets on MXene (Ti₃C₂T_x) surfaces was prepared by a facile liquid phase coprecipitation method. Ti₃C₂T_x is initially modified by polydopamine (PDA) coating to ensure the homogeneous distribution of MnO₂ nanosheets and tight and close connections between MnO₂ and the Ti₃C₂T_x backbone. Due to the obtained three-dimensional (3D) nanostructure, facilitating electron transport within the electrode and promoting electrolyte ion accessibility, the δ-MnO₂@Ti₃C₂T_x-0.06 electrode yields superior electrochemical performances, such as a rather large areal capacity of 1233.1 mF cm^?2 and high specific capacitance of 337.6 F g^?1 at 2 mV s^?1, as well as high cyclic stability for 10000 cycles. Furthermore, δ-MnO₂@Ti₃C₂T_x-0.06 composites are employed as positive electrodes, and activated carbon (AC) materials act as negative electrodes with an aqueous electrolyte of 1 M Na₂SO₄ to assemble asymmetric supercapacitors. The prototype device is reversible at cell voltages from 0 to 1.8 V, and manifests a maximum energy density of 31.4 Wh kg^?1 and a maximum power density of 2700 W kg^?1. These encouraging results show enormous possibilities for energy storage applications.     

In-situ growth of bamboo-shaped carbon nanotubes and helical carbon nanofibers on Ti3C2Tx MXene at ultra-low temperature for enhanced electromagnetic wave absorption properties

The potential of two-dimensional layered MXenes in electromagnetic wave (EMW) absorption needs further development. Herein, we carried out the in situ growth of carbon nanotubes (CNTs) on the surface of Ti₃C₂T_x MXene at ultra-low temperature via chemical vapor deposition. The obtained CNTs exhibited a bamboo-like structure and were accompanied by helical carbon nanofibers. The ultra-low temperature solved the problem that the high temperature required in the traditional CNT growth process would destroy the structural integrity of MXene. The lush CNT forest cross-linked the MXene layers, transforming the two-dimensional layered structure into a three-dimensional conductive network, providing abundant conductive channels for carriers, optimizing the impedance matching of the CNT/MXene hybrid, and resulting in a significant dielectric loss. The as-prepared CNT/MXene hybrid exhibited a minimal reflection loss of ?52.56 dB (99.9994% EMW absorption) in the X-band. This work proposes a new idea to enhance the EMW absorption properties of Ti₃C₂T_x MXene and fabricate high-performance MXene-based EMW absorbers.     

MXene supported rhodium nanocrystals for efficient electrocatalysts towards methanol oxidation

Since conventional Pt/carbon catalysts usually suffer from CO poisoning as well as carbon corrosion issues during the methanol oxidation reaction, it is essential to explore high-efficiency Pt-alternative electrocatalysts supported by a robust matrix in the direct methanol fuel cells. Herein, we report a convenient low-temperature approach to the controllable fabrication of well-dispersive Rh nanocrystals in situ grown on Ti₃C₂T_x MXene nanosheets. The ultrathin lamellar MXene structure reveals unique superiorities on the construction of advanced Rh-based hybrid catalysts, which can not only provide a large number of efficient anchoring sites for immobilizing small-sized Rh nanocrystals with abundant exposed catalytic crystal planes, but also enable direct electronic interaction with Rh for strong synergistic effects and facilitate the fast charge transportation during the catalytic process. As a consequence, the resulting Rh/Ti₃C₂T_x hybrid exhibits prominent electrocatalytic properties towards methanol oxidation reaction, such as a large electrochemical active surface area of 71.6 m² g^?1, a high mass activity of 600.2 mA mg^?1, and good long-term stability, all of which are much better than those of conventional carbon-supported Rh as well as Pt/C and Pd/C catalysts.