Synthesis,characterization and antifungal property of Ti3C2Tx MXene nanosheets

Although the antibacterial properties of MXene nanosheets containing Ti₃C₂T_x are known, their antifungal properties have not been well studied. Herein, we present for the first time a report on the antifungal properties of Ti₃C₂T_x MXene. The Ti₃C₂T_x MXene was obtained by first exfoliating MAX phase of Ti₃AlC₂ with concentrated hydrofluoric acid, then the Ti₃C₂T_x was intercalated and deliminated by ethanol treatment and ultrasonication process. The delaminated Ti₃C₂T_x MXene nanosheets (d-Ti₃C₂Tx) were characterized using field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX), X-ray diffraction spectroscopy (XRD), and Raman spectroscopy. It was found that Ti₃C₂T_x MXene was characterized by lamellar structure alternating with layers of Ti, Al and C. The EDX results revealed that the delaminated Ti₃C₂T_x MXene nanosheets were composed of Ti, C, Si, O, F, and a trace amount of Al. The XRD and Raman spectra further indicated the elimination of Al and the formation of two-dimensional Ti₃C₂T_x MXene nanosheets. The antifungal activity of the delaminated Ti₃C₂T_x MXene was determined against Trichoderma reesei using the modified agar disc method. Observation using inverted phase contrastmicroscopy revealed inhibited fungus growth with the absence of hyphae around the discs treated wtih MXene. The surrounding of the control groups without an inclusion of MXene was found with large number of hyphae and spores. In addition, the spores of the fungi treated with the samples containing d-Ti₃C₂T_x MXene nanosheets did not germinate even after 11 days of culture. The results demonstrated disruption to the hemispheric structural formation of fungi colony, inhibition of hyphae growth and cell damage for fungi grown on the d-Ti₃C₂T_x MXene nanosheets. These new findings suggest that d-Ti₃C₂T_x MXene nanosheets developed in this work could be a promising anti-fungi material.     

Co-Catalyst Ti3C2TX MXene-Modified ZnO Nanorods Photoanode for Enhanced Photoelectrochemical Water Splitting

In order to obtain a high photoelectrochemical performance, co-catalysts loading is the most commonly used way, which can facilitate reactions and suppress the charge recombination. In this paper, a novel composite of ZnO/Ti₃C₂T_X photoanode was fabricated by a facile spin coating of precipitating Ti₃C₂T_X (MXene) flakes onto the surface of ZnO, as co-catalyst for enhanced photoelectrochemical (PEC) water splitting. Under simulated sunlight, the optimum composite of ZnO/Ti₃C₂T_X photoanode showed the photocurrent density as 1.2 mA cm^?2 at 1.23 V_RHE, which is 1.4 times higher than that of pristine ZnO without Ti₃C₂T_X co-catalyst (0.83 mA cm^?2 at 1.23 V_RHE). The ZnO/Ti₃C₂T_X photoanode showed a photoconversion efficiency of 0.32% and maintained a stable photocurrent over 2000s. The Ti₃C₂T_X (MXene) flakes as co-catalyst to promote the charge transfer and accelerates the reaction kinetics in ZnO/Ti₃C₂T_X photoanode. This work delivers a two-dimensional (2D) material Ti₃C₂T_X (MXene) as co-catalyst for enhanced ZnO photoanode PEC water splitting.

     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

The multiple synthesis of 2D layered Ti3C2Tx/Ag/MWCNTs/Ag composites with enhanced electrochemical properties

MXene, a novel two-dimensional material composed of alternating transition metal carbon/nitrogen, has the advantages of large specific surface area, many active sites, and short ion transport paths, and has shown great potential in the field of capacitors in the beginning. However, the unique two-dimensional structure is prone to collapse and accumulation, which inhibits the electron migration rate and ion penetration, resulting in poor energy storage capacity. In this paper, Ti₃C₂T_x/Ag/MWCNTs/Ag composites with good capacitive properties were successfully prepared by Ti₃C₂T_x which directly reduces silver nitrate solution and further introduces nanoscale MWCNTs. Ag NPs are introduced twice to grow and distribute uniformly on the surface and between the layers of Ti₃C₂T_x to play a supporting role. The introduction of MWCNTs in the intermediate process can enter the interlayer and act as spacers together with Ag NPs to prevent the collapse and stacking of Ti₃C₂T_x and improve its surface utilization. However, the preparation process unavoidably leads to partial oxidation of Ti₃C₂T_x, which deteriorates its electrochemical properties. Owing to the synergy between Ti₃C₂T_x, Ag NPs and MWCNTs, the Ti₃C₂T_x/Ag/MWCNTs/Ag composites show good electrical conductivity, low internal resistance (0.67 Ω) and extreme high capacitance contribution (97%) in electrochemical tests.     

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.     

Morphology and photocatalytic activity of TiO2/MXene composites by in-situ solvothermal method

In this work, TiO₂/MXene composites were successfully synthesized through an in-situ solvothermal method, where the morphology of TiO₂ nanoparticles (NPs) was modified by different concentrations of agents. Ti₃C₂T_x with electronic storage characteristics was employed as a co-catalyst to enhance the photocatalytic degradation activity by capturing photogenerated electrons. The experimental results reveal that, with the help of C₃H₈O agents, TiO₂ NPs are uniformly distributed on the surface of Ti₃C₂T_x. The TiO₂/Ti₃C₂T_x-C₃H₈O composite showed the highest photocatalytic activity of 90.5% after 75 min under mercury light irradiation, which is 57.9% higher than that of the pure TiO₂ photocatalyst. The photocatalytic activity is promoted due to the high photoelectron transmission performance of the TiO₂/Ti₃C₂T_x composites.     

A high-performance supercapacitor electrode based on freestanding N-doped Ti3C2Tx film

Two-dimensional transition metal carbide (MXene) is a promising electrode material for supercapacitors because of its excellent electrochemical properties. Here, we report a controllable and facile strategy to prepare a freestanding and flexible N-doped Ti₃C₂T_x (N–Ti₃C₂T_x) film electrode with a hydrothermal method using hydrazine hydrate (N₂H₄∙H₂O) as a nitrogen source. At a scan rate of 2 mV s⁻¹, the N–Ti₃C₂T_x film electrode exhibits a high specific capacitance of 340 F g⁻¹ and no capacitance degradation after 10,000 cycles in 1 M H₂SO₄ electrolyte. These results show that the N–Ti₃C₂T_x film could be used as an outstanding electrode material for high-performance supercapacitors. The operation of hydrazine treatment provides a more practical and convenient experimental method for N-doping.     

P-doped MoS2/Ni2P/Ti3C2Tx heterostructures for efficient hydrogen evolution reaction in alkaline media

By combining the advantages of doping to change the electronic structure of molybdenum disulfide (MoS₂), transition metal phosphides, and MXene, we proposed the idea of designing and preparing a new type of composite material, P-doped MoS₂/Ni₂P/Ti₃C₂T_x heterostructures (denoted as P@MNTC), to serve as the hydrogen evolution reaction (HER) catalyst of electrochemical water splitting. The as-prepared P@MNTC heterostructures show a significant HER activity with an overpotential of 120 mV at 10 mA cm^–2 in alkaline electrolyte, with decreasing 105 and 125 mV compared with those of MoS₂ and MXene, respectively. The density functional theory indicates that the P doping and synergy effect of Ti₃C₂T_x can enhance the activation of MoS₂ and thus promote dissociation and absorption of H₂O during HER process. This strategy provides a promising way to develop high-efficiency MoS₂- and Ti₃C₂T_x-based composite catalysts for alkaline HER.     

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.     

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.     

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.     

Enhanced mechanical properties of poly(lactic acid) composites with ultrathin nanosheets of MXene modified by stearic acid

MXene modified by stearic acid (Ti₃C₂T_x-g-SA) is incorporated into poly(lactic acid) (PLA) matrix to prepare Ti₃C₂T_x-g-SA/PLA composites. The effects of Ti₃C₂T_x-g-SA to pure PLA are investigated, including crystallization, mechanical, and thermal properties. Fourier transform infrared spectroscopy and X-ray diffraction analyses confirm that Ti₃C₂T_x interlayer is successfully intercalated by SA, and the interlayer spacing of Ti₃C₂T_x is increased. Differential scanning calorimetry illustrates that the cold crystallization enthalpy (ΔH_cc), melting enthalpy (ΔH_m), and crystallinity (X_c) of Ti₃C₂T_x-g-SA/PLA composites are improved by the plasticization and heterogeneous nucleation effect of Ti₃C₂T_x-g-SA. Specially, the Ti₃C₂T_x-g-SA/PLA composites exhibit excellent mechanical properties at an appropriate content of the Ti₃C₂T_x-g-SA. Compared with pure PLA, the elongation at break of the Ti₃C₂T_x-g-SA/PLA composite is increased 5.9-fold (up to 131.6%) when only containing 0.5 wt % Ti₃C₂T_x-g-SA. Besides, the Ti₃C₂T_x-g-SA/PLA composites exhibit good thermal stability in the low loading (lower than 1 wt %) of Ti₃C₂T_x-g-SA. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48621.     

Fabrication of urchin-like ZnO-MXene nanocomposites for high-performance electromagnetic absorption

The recently developed two-dimensional transition metal carbides (such as MXene) have shown amazing electrical properties. MXene and derivative two dimensional (2D) materials are widely used in electron devices, and have large potential application in electromagnetic (EM) absorber. Herein we describe a mild method to prepare an urchin-like ZnO-MXene Ti₃C₂T_x nanocomposite through a coprecipitation process. The nanocomposite delivers a substantially enhanced EM absorbing performance with an optimum reflection loss of −26.30 dB, which is significantly better than that of primitive Ti₃C₂T_x (−6.70 dB), owing to the construction of unique semiconductive networks and larger interfaces. The EM absorption performance can be effectively controlled in the range of 14.0–18.0 bands by changing the growth time of ZnO. Considering the large amount of members in MXene, this study demonstrates a new strategy applicable in maximizing their applications in EM absorbing materials.