Chemical Stability of Ti3C2 MXene with Al in the Temperature Range 500–700 °C

Ti3C2Tx MXene, a new 2D nanosheet material, is expected to be an attractive reinforcement of metal matrix composites because its surfaces are terminated with Ti and/or functional groups of –OH, –O, and –F which improve its wettability with metals. Thus, new Ti3C2Tx/Al composites with strong interfaces and novel properties are desired. To prepare such composites, the chemical stability of Ti3C2Tx with Al at high temperatures should be investigated. This work first reports on the chemical stability of Ti3C2Tx MXene with Al in the temperature range 500–700 °C. Ti3C2Tx is thermally stable with Al at temperatures below 700 °C, but it reacts with Al to form Al3Ti and TiC at temperatures above 700 °C. The chemical stability and microstructure of the Ti3C2Tx/Al samples were investigated by differential scanning calorimeter, X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy.


Introduction
MXenes, as a new family of two-dimensional (2D) materials, have attracted much attention because of their unusual combination of mechanical, physical, and chemical properties [1][2][3][4][5][6][7][8][9][10]. The MXenes are produced by the selective extraction of A-element from the layered MAX phases (where M is an early transition metal, A is an A group element, and X is C or N), such as Ti 3 AlC 2 , Ti 2 AlC, and Ti 3 SiC 2 by either HF solutions [1,2,[11][12][13], HCl/fluoride salt solutions [3,[14][15][16][17][18], or HF/oxidant solutions [19]. After removal of the A layers from MAX, the top and bottom surfaces of the bare MXene monolayer or single sheet are terminated by the redistribution of M atoms with metallic bonds. Hence, the MXenes exhibit metallic conductivity. However, the exposed M surfaces of MXenes are always attached by functional groups such as hydroxyl (-OH), oxide (-O), or fluorine (-F) after etching in acid solutions. Hence, the general formula M n+1 X n T x (where T x denotes the MXene terminal groups and x is number of terminal groups, n = 1, 2, or 3) is always used to represent the MXenes.
In the 2D MXene group, Ti 3 C 2 T x is the first reported and the most explored material because it is easily synthesized from the Ti 3 AlC 2 precursor using HF solution [1]. The 2D Ti 3 C 2 T x single nanosheet is micrometers in width but less than a nanometer in thickness. Hence, the Ti 3 C 2 T x MXene, with its attractive properties and high aspect ratios, has already been used as a reinforcement in composites. So far, the majority of the current work on composites containing Ti 3 C 2 T x has focused on polymer matrix composites [30][31][32][33][34], but less attention has been devoted to the use of Ti 3 C 2 T x to reinforce metal matrix composites.
In recent years, there has been intense research into Al matrix composites reinforced with 1D carbon nanotubes (CNTs) and 2D graphene to improve their mechanical properties [35][36][37][38]. However, there remains a challenge to overcome weak interfaces in the CNT/Al and graphene/Al composites, due to the poor wettability of CNTs and graphene with Al matrix.
Using Ti 3 C 2 T x to reinforce the Al metal matrix composites possibly endows the Ti 3 C 2 T x /Al composites with a strong interfacial strength because the surfaces of Ti 3 C 2 T x nanosheets are terminated with Ti or functional groups such as -OH, -O, and -F. It has been reported that the surface modification of CNTs by -O and -OH functional groups enhanced the interfacial bonding between metals and CNTs [39,40]. The functionalized 2D nanotubes can be employed as reinforcing fillers [41,42]. In addition, the -F and -O surface functional groups on the Ti 3 C 2 T x nanosheets can be eliminated at high temperatures [43,44]. The loss of surface functional groups possibly promotes the bonding of Ti surface atoms on bare Ti 3 C 2 with Al. However, work on the Ti 3 C 2 T x /Al composites has been much less focused so far. The ability to practically produce Ti 3 C 2 T x /Al composites remains to be demonstrated. Al matrix composites reinforced with 1D CNTs and 2D graphene have always been prepared in the temperature range 500-640 • C. To practically prepare Ti 3 C 2 T x /Al composites and to understand the chemical stability of Ti 3 C 2 T x with Al, the temperature range 500-700 • C was selected in the present study.
The purpose of this work was to assess the chemical stability of Ti 3 C 2 T x with Al in the temperature range from 500 • C to 700 • C by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and transmission electron microscopy methods. The interfaces between Ti 3 C 2 T x and Al were characterized.

Materials and Methods
Ti 3 C 2 T x MXene was prepared by acid etching of Ti 3 AlC 2 powder. The fabrication of Ti 3 AlC 2 has been described elsewhere [45]. Briefly, Ti (325-mesh, >99.2% purity), Al (particle size <5 µm, 99.5% purity), and C (graphite, <45 µm, >99.5% purity) powders with a molar ratio of Ti:Al:C = 3:1.1:2 were mixed for 10 h. The mixed mixture was cold-pressed to form compacts with a diameter of 50 mm and a height of approximately 5 mm. The compacts were pressurelessly sintered at 1450 • C for 1 h in an Ar atmosphere. The sintered samples were pulverized and then sifted with a 300-mesh sieve to make Ti 3 AlC 2 powder. A total of 2.5 g of Ti 3 AlC 2 powder was immersed in 60 mL of 40% HF solution in a polytetrafluoroethylene (PTFE) container. A heating magnetic stirrer was used to continuously stir the solution in the PTFE container at 50 • C for 0.5 h. The solution was centrifugally separated in a centrifuge (LD-4, Jinan Wohong Experimental instrument Co., Ltd., Jinan, China) with a rotation rate of 4000 rpm for 5 min and then washed with deionized water until a pH of about 7 was attained. The obtained sediment was vacuum dried at 80 • C for 24 h to make the desired Ti 3 C 2 T x powder. The prepared Ti 3 C 2 T x powder was sieved with a 200-mesh sieve.
Al and 10 wt % Ti 3 C 2 T x powders were mixed in a polypropylene container with agate balls for 10 h in a rotary drum type ball-miller with a speed of 150 rpm. The mixture was cold-pressed in a stainless steel mold with 100 MPa to form pellets with a diameter of 20 mm and a height of about 5 mm. The pellets were put into a graphite crucible coated with boron nitride and then sintered in the absence of additional pressure conditions in the temperature range 500-700 • C for 1 h in Ar.
The phase composition of the samples before and after sintering was identified by X-ray diffraction (XRD) analysis using a D/Max 2200 PC diffractometer (Rigaku Co. Ltd., Tokyo, Japan) applying monochromatic Cu Kα radiation. The operating voltage and current were 40 kV and 20 mA, respectively. The microstructures of the sintered samples were characterized with a ZEISS EVO 18 scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer system (EDS), and a JEM-2100F (JEOL Ltd., Tokyo, Japan) transmission electron microscope (TEM) with an operating voltage of 200 kV. TEM images were processed with the software RADIUS Desktop 2.0 (EMSIS GmbH, Muenster, Germany). Differential scanning calorimetry (DSC) analysis was used to measure the amount of energy absorbed or released by the Ti 3 C 2 T x /Al mixture heated in a NETZSCH STA 449F3 thermal analyzer (Netzsch, Germany) from room temperature to 800 • C with a heating rate of 15 • C/min in flowing Ar.

Results and Discussion
The DSC curve of the Ti 3 C 2 T x /Al mixture, together with that of pure Al for comparison, is presented in Figure 1. A broad endothermic peak appearing at around 110 • C on the DSC curve for Ti 3 C 2 T x /Al is ascribed to the evaporation of water absorbed in the interlayers of Ti 3 C 2 T x . The thermal gravity (TG) curve correspondingly exhibits a downward trend on mass loss. A sharp endothermic peak at 664 • C corresponds to the melting of Al (660 • C). Comparing the two DSC curves, it can be found that there is an exothermic peak at 717 • C on the DSC curve of Ti 3 C 2 T x /Al, which can be ascribed to the occurrence of a reaction between Ti 3 C 2 T x and Al. In the temperature range from 150 • C to 700 • C, the TG curve reveals a gradual mass loss, possibly due to the removal of surface groups such as -OH and -O. Zhou et al. [43] reported that there was a broad and weak exothermic peak in the DSC curve for the pure Ti 3 C 2 T x in the temperature range 200-800 • C in Ar due to the loss of surface groups. Shah et al. [46] reported that the bonding energies for -O, -OH, and -F groups in the Ti 3 C 2 T x MXene are 530.3 eV, 531.3 eV, and 685.1 eV, respectively. The bonding energies suggest that the removal sequence of the surface groups is -O > -OH > -F. Li and coworkers [44] reported that the disappearance of -OH and -O groups occurred at approximately 500 K and the removal of -F group was required at a high temperature of 1173 K in Ar. Sang and coworkers also confirmed that the functional group of -O was removed after annealing of Ti 3 C 2 T x MXene at 500 • C [47]. STA 449F3 thermal analyzer (Netzsch, Germany) from room temperature to 800 °C with a heating rate of 15 °C/min in flowing Ar.

Results and Discussion
The DSC curve of the Ti3C2Tx/Al mixture, together with that of pure Al for comparison, is presented in Figure 1. A broad endothermic peak appearing at around 110 °C on the DSC curve for Ti3C2Tx/Al is ascribed to the evaporation of water absorbed in the interlayers of Ti3C2Tx. The thermal gravity (TG) curve correspondingly exhibits a downward trend on mass loss. A sharp endothermic peak at 664 °C corresponds to the melting of Al (660 °C). Comparing the two DSC curves, it can be found that there is an exothermic peak at 717 °C on the DSC curve of Ti3C2Tx/Al, which can be ascribed to the occurrence of a reaction between Ti3C2Tx and Al. In the temperature range from 150 °C to 700 °C, the TG curve reveals a gradual mass loss, possibly due to the removal of surface groups such as -OH and -O. Zhou et al. [43] reported that there was a broad and weak exothermic peak in the DSC curve for the pure Ti3C2Tx in the temperature range 200-800 °C in Ar due to the loss of surface groups. Shah et al. [46] reported that the bonding energies for -O, -OH, and -F groups in the Ti3C2Tx MXene are 530.3 eV, 531.3 eV, and 685.1 eV, respectively. The bonding energies suggest that the removal sequence of the surface groups is -O > -OH > -F. Li and coworkers [44] reported that the disappearance of -OH and -O groups occurred at approximately 500 K and the removal of -F group was required at a high temperature of 1173 K in Ar. Sang and coworkers also confirmed that the functional group of -O was removed after annealing of Ti3C2Tx MXene at 500 °C [47]. To identify the reaction products and to characterize the microstructure, the samples sintered at temperatures from 500 °C to 700 °C were examined using XRD, SEM, and TEM. Figure 2 shows XRD patterns of 10 wt % Ti3C2Tx/Al samples before and after sintering at various temperatures. Before sintering, only Ti3C2Tx MXene and Al are detected, see Figure 2a. The broad peaks indicate the nanosheet structure of Ti3C2Tx. After sintering at 650 °C, some sharp diffraction peaks but with lower intensities appeared, in addition to the peaks corresponding to Al. These peaks are in good agreement with the simulated peaks for pure Ti3C2 and Ti3C2F2, see Figure 2b. It should be noted that the (002) peak at 8.7°, (004) peak at 17.98°, and (006) peak at 27.38° belonging to the Ti3C2Tx MXene disappeared in the XRD patterns of the sintered samples, see Figure 2b,c. The XRD results suggest that water molecules associated with the interlayer functional groups such as -O and -OH were gradually removed from 100 °C to 650 °C. The loss of water and functional groups causes a decrease in the basal space distance, or, alternatively, the newly formed surfaces possibly bond to form a new structure, which potentially induces the disappearance of basal peaks. The disappearance of basal plane peaks is similar to those for some layered compounds such as montmorillonite [48] and tetratitanate [49]. However, the diffractogram of the sample sintered at 700 °C shows the To identify the reaction products and to characterize the microstructure, the samples sintered at temperatures from 500 • C to 700 • C were examined using XRD, SEM, and TEM. Figure 2 shows XRD patterns of 10 wt % Ti 3 C 2 T x /Al samples before and after sintering at various temperatures. Before sintering, only Ti 3 C 2 T x MXene and Al are detected, see Figure 2a. The broad peaks indicate the nanosheet structure of Ti 3 C 2 T x . After sintering at 650 • C, some sharp diffraction peaks but with lower intensities appeared, in addition to the peaks corresponding to Al. These peaks are in good agreement with the simulated peaks for pure Ti 3 C 2 and Ti 3 C 2 F 2 , see Figure 2b. It should be noted that the (002) peak at 8.7 • , (004) peak at 17.98 • , and (006) peak at 27.38 • belonging to the Ti 3 C 2 T x MXene disappeared in the XRD patterns of the sintered samples, see Figure 2b,c. The XRD results suggest that water molecules associated with the interlayer functional groups such as -O and -OH were gradually removed from 100 • C to 650 • C. The loss of water and functional groups causes a decrease in the basal space distance, or, alternatively, the newly formed surfaces possibly bond to form a new structure, which potentially induces the disappearance of basal peaks. The disappearance of basal plane peaks is similar to those for some layered compounds such as montmorillonite [48] and tetratitanate [49]. However, the diffractogram of the sample sintered at 700 • C shows the appearance of new phases of Al 3 Ti and TiC, indicating the occurrence of a reaction between Ti 3 C 2 T x and Al at 700 • C. This result is in good agreement with the DSC analysis. Some weak peaks belonging to unreacted Ti 3 C 2 and Ti 3 C 2 F 2 can still be found. However, several peaks appearing in Figure 2c were not confirmed.     Figure 3 presents the morphologies of the raw Al and Ti 3 C 2 T x powders and microstructures of the sintered samples. The raw Al particles are spherical, and each Ti 3 C 2 T x particle has stacked multilayers, as shown in Figure 3a. After sintering at 650 • C, the polished surface shows that the Ti 3 C 2 T x particles mainly distribute at the Al grain boundaries, see Figure 3b. No other new phases were detected in the sample sintered at 650 • C. The fracture surface clearly shows the stacked multilayers of Ti 3 C 2 T x particles, see Figure 3c. appearance of new phases of Al3Ti and TiC, indicating the occurrence of a reaction between Ti3C2Tx and Al at 700 °C. This result is in good agreement with the DSC analysis. Some weak peaks belonging to unreacted Ti3C2 and Ti3C2F2 can still be found. However, several peaks appearing in Figure 2c were not confirmed.    Figure  4b, reveals the Al signal in the Ti3C2Tx flake, indicating the diffusion of Al atoms into the multilayers of Ti3C2Tx. The interlayer spacing areas of Ti3C2Tx provide active sites for Al nucleation and crystal growth. It should be noted that the F element was detected in the Ti3C2Tx multilayers, but O was mainly distributed in the areas between the Ti3C2Tx flake and Al grains. The presence of F in the multilayers suggests that the surface-layer termination of MXene is mostly F, i.e., Ti3C2F2. The -O group in Ti3C2Tx may react with Al to form Al2O3. Figure 5 presents the TEM micrographs of a Ti3C2Tx/Al sample sintered at 650 °C. These Ti3C2Tx flakes are from several nanometers to tens of nanometers in thickness, indicating the thinner flake is composed of at least two Ti3C2Tx layers, see Figure 5a. The interspaces in the Ti3C2Tx flakes are filled with Al. A high-resolution TEM (HRTEM) image shows that the interlayer spacing of Ti3C2Tx is about 0.855 nm, see Figure 5b. This value is smaller than the value of 1.17 to 1.28 nm for the initial Ti3C2Tx flakes due to the removal of functional groups after heat treatment at 650 °C. The HRTEM image of the Ti3C2/Al interface reveals that the lattices of the Ti3C2 and Al regions are in direct contact, as shown in Figure 5c. The interface is clean and continuous. Neither precipitates nor amorphous regions are observed at the interface. The above observation suggests that the Ti3C2/Al interface is chemically and structurally stable at 650 °C.  Figure 4a presents a TEM image of a multilayer Ti 3 C 2 T x flake. The EDS line analysis, see Figure 4b, reveals the Al signal in the Ti 3 C 2 T x flake, indicating the diffusion of Al atoms into the multilayers of Ti 3 C 2 T x . The interlayer spacing areas of Ti 3 C 2 T x provide active sites for Al nucleation and crystal growth. It should be noted that the F element was detected in the Ti 3 C 2 T x multilayers, but O was mainly distributed in the areas between the Ti 3 C 2 T x flake and Al grains. The presence of F in the multilayers suggests that the surface-layer termination of MXene is mostly F, i.e., Ti 3 C 2 F 2 . The -O group in Ti 3 C 2 T x may react with Al to form Al 2 O 3 .
(c)  Figure 4a presents a TEM image of a multilayer Ti3C2Tx flake. The EDS line analysis, see Figure  4b, reveals the Al signal in the Ti3C2Tx flake, indicating the diffusion of Al atoms into the multilayers of Ti3C2Tx. The interlayer spacing areas of Ti3C2Tx provide active sites for Al nucleation and crystal growth. It should be noted that the F element was detected in the Ti3C2Tx multilayers, but O was mainly distributed in the areas between the Ti3C2Tx flake and Al grains. The presence of F in the multilayers suggests that the surface-layer termination of MXene is mostly F, i.e., Ti3C2F2. The -O group in Ti3C2Tx may react with Al to form Al2O3. Figure 5 presents the TEM micrographs of a Ti3C2Tx/Al sample sintered at 650 °C. These Ti3C2Tx flakes are from several nanometers to tens of nanometers in thickness, indicating the thinner flake is composed of at least two Ti3C2Tx layers, see Figure 5a. The interspaces in the Ti3C2Tx flakes are filled with Al. A high-resolution TEM (HRTEM) image shows that the interlayer spacing of Ti3C2Tx is about 0.855 nm, see Figure 5b. This value is smaller than the value of 1.17 to 1.28 nm for the initial Ti3C2Tx flakes due to the removal of functional groups after heat treatment at 650 °C. The HRTEM image of the Ti3C2/Al interface reveals that the lattices of the Ti3C2 and Al regions are in direct contact, as shown in Figure 5c. The interface is clean and continuous. Neither precipitates nor amorphous regions are observed at the interface. The above observation suggests that the Ti3C2/Al interface is chemically and structurally stable at 650 °C.    Figure 5 presents the TEM micrographs of a Ti 3 C 2 T x /Al sample sintered at 650 • C. These Ti 3 C 2 T x flakes are from several nanometers to tens of nanometers in thickness, indicating the thinner flake is composed of at least two Ti 3 C 2 T x layers, see Figure 5a. The interspaces in the Ti 3 C 2 T x flakes are filled with Al. A high-resolution TEM (HRTEM) image shows that the interlayer spacing of Ti 3 C 2 T x is about 0.855 nm, see Figure 5b. This value is smaller than the value of 1.17 to 1.28 nm for the initial Ti 3 C 2 T x flakes due to the removal of functional groups after heat treatment at 650 • C. The HRTEM image of the Ti 3 C 2 /Al interface reveals that the lattices of the Ti 3 C 2 and Al regions are in direct contact, as shown in Figure 5c. The interface is clean and continuous. Neither precipitates nor amorphous regions are observed at the interface. The above observation suggests that the Ti 3 C 2 /Al interface is chemically and structurally stable at 650 • C.

Conclusions
The chemical stability of Ti3C2 with Al was investigated in the temperature range 500-700 °C in Ar for 1 h. Ti3C2Tx is thermally stable with Al at 650 °C, but it reacts with Al to form Al3Ti and TiC at 700 °C. For the Ti3C2Tx/Al sample sintered at 650 °C, the Ti3C2Tx/Al interface is clean and continuous, without precipitates and amorphous regions. The loss of water molecules and the removal of functional groups of -OH and -O induces a decrease in the interlayer spacing of Ti3C2Tx. The present work demonstrates the possibility to prepare Ti3C2Tx MXene reinforced Al or other metal matrix composites under certain processing conditions.

Conclusions
The chemical stability of Ti 3 C 2 with Al was investigated in the temperature range 500-700 • C in Ar for 1 h. Ti 3 C 2 T x is thermally stable with Al at 650 • C, but it reacts with Al to form Al 3 Ti and TiC at 700 • C. For the Ti 3 C 2 T x /Al sample sintered at 650 • C, the Ti 3 C 2 T x /Al interface is clean and continuous, without precipitates and amorphous regions. The loss of water molecules and the removal of functional groups of -OH and -O induces a decrease in the interlayer spacing of Ti 3 C 2 T x . The present work demonstrates the possibility to prepare Ti 3 C 2 Tx MXene reinforced Al or other metal matrix composites under certain processing conditions.