Nanolaminar M_n+1X_nene (n=1–3) materials are new members of two-dimensional nanosheets, which were first discovered by Naguib et al. (2012). He dissolved Al element from MAX phases using high concentrated hydrofluoric acid (49 wt%) and got graphene-like carbides. These nanosheets exhibit excellent electrical conductivity and self-lubricity. To date, Ti₃C₂X, (V_0.5Cr_0.5)₃C₂X, Mo₂TiC₂X, Cr₂TiC₂X, Ti₂CX, Nb₂CX, V₂CX, Mo₂CX, Nb₄C₃X, Ta₄C₃X, Mo₂Ti₂C₃X, (Nb_0.8Ti_0.2)₄C₃X, and (Nb_0.8Zr_0.2)₄C₃X have been fabricated (Naguib et al., 2013; Ghidiu et al., 2014a; Sun et al., 2014; Anasori et al., 2015; Dall’Agnese et al., 2015; Meshkian et al., 2015; Yang et al., 2016). Here, X represents the functional groups of –OH, =O, and –F, which are formed during etching. Recently, Lukatskaya et al. systematically investigated the intercalation of ions into layered Ti₃C₂X in battery and electrochemical capacitor. She found that Na⁺, K⁺, NH₄⁺, Mg²⁺, and Al³⁺ ions could offer capacitance in excess of 300 F/cm³, much higher than that of porous carbons (Lukatskaya et al., 2013). This study provided the evidence of electrochemical energy storage application of two-dimensional MXene using single and multivalent ions. Additionally, Ghidiu et al. (2014b) improved the volumetric capacitance up to 900 F/cm³ with excellent cyclability and rate performance when using LiF/HCl solution etching Ti₃AlC₂ powder followed by rolling Ti₃C₂X clay as 5 μm thick film. This procedure provided the possibility of low cost production in industry. For other MXenes, the freestanding, flexible CNT/Nb₂CX paper electrodes were successfully prepared by Mashtalir et al. (2015) through filtering CNT/Nb₂CX suspension. More than 400 mAh/g of Li capacity at 0.5C and 325 F/cm³ of volumetric capacitance tested in an Li-ion capacitor were achieved. As the lubricant, it was determined that Ti₃C₂X nanosheets could effectively decrease the friction coefficient and enhance the wear resistance of countercouples of 440C stainless steel ball/45# steel disc as additive in 100 SN base oil (Zhang et al., 2015). The mechanism was attributed to sliding friction of nanosheets between the rubbing surfaces.

Recently, Ling et al. (2014) fabricated the Ti₃C₂X-PDDA/PVA polymer films and confirmed that as-obtained films showed high electrical conductivity of 2.24 × 10⁴ S/m and high tensile strength of 30 MPa when the polymer loading was 10 wt%. As another important reinforcing polymer, chitosan has been widely used by combining graphene to prepare the composites applied in the fields of food industry, drug delivery, sensor, lithium-ion battery, and supercapacitor (Schiffman et al., 2009; Ramkumar and Minakshi, 2015). Chitosan, the N-deacetylated derivative of chitin, is commercially 85% deacetylated. Therefore, it is interesting to fabricate MXene–chitosan films and evaluate their physical and mechanical properties. In the present work, Ti₃C₂X–chitosan films were first prepared by colloidal method followed by vacuum infiltration. In order to systematically evaluate the physical and mechanical properties of as-obtained films, the microstructure, tensile strength, electrical resistivity, and capacity of sodium battery of films were investigated.

Ti₃C₂X was prepared by immersing Ti₃AlC₂ powder into the solution of LiF/HCl for 40 h at 40°C followed by cleaning using deionized water until pH value of suspension reached 7. Then, the as-obtained suspension was ultrasonically stirred for 2 h at ambient temperature in a closed plastic bottle. After separating the Ti₃C₂X nanosheets through high-speed centrifugation (3,000 rpm) (ST16, Thermal Scientific, Inc., Odessa, TX, USA) for 10 min, the delaminated Ti₃C₂X could be well prepared by dispersing in the deionized water, showing a color of cyan. A total of 5 ml Ti₃C₂X suspension was filtered by vacuum filtration and dried in a vacuum chamber for 12 h. The concentration of suspension could be calculated according to the mass of Ti₃C₂X film, as 2.3 mg/ml. In order to prepare the Ti₃C₂X–chitosan films, first the chitosan from crab shells (molecular weight: 190,000–375,000 and DD: 83%) was weighed based on the designed content of 7, 10, and 14 wt%, and then mixed with 9 ml deionized water, 2 ml acetic acid (99.8 wt%), and 2 ml HCl (37 wt%). After oil bathing at 80°C for 5 min, the chitosan has been well dissolved in the solution. A total of 20 ml Ti₃C₂X suspension was poured into the chitosan solution and ultrasonically stirred for 10 min in order to get the homogeneous colloidal suspension. After vacuum filtering, as-prepared Ti₃C₂X–chitosan films were dried in the vacuum chamber for 12 h to remove the adsorbed water. For comparison, pure Ti₃C₂X film was also prepared.

The phase composition and microstructure of obtained films were characterized by an X-ray diffraction (XRD) diffractometer (SmartLab; Rigaku Corp., Tokyo, Japan) using CuKα radiation and a scanning electron microscope (Supra 50VP, Carl Zeiss AG, Germany) equipped with an energy dispersive spectroscope. The measurement of tensile strength was conducted by a KES G1 tensile tester (Kato Tech Co., Ltd., Kyoto, Japan) at a speed of 0.1 mm/s. For each composition, three to five specimens with a shape of dog bone were tested. The width of fracture region was 2 mm. The electrical resistivity of films was tested by a four-probe facility (JANDEL, Jandel Engineering Ltd., Linslade, UK). 2032-type coin cell was fabricated to test the sodium ion storage performance. The as-prepared Ti₃C₂X film was punched to 10 mm disk and directly used as anode electrode, pure sodium metal used as the counter electrode, Celgard 2400 as separator and 1.0 M NaPF₆ in (1:1 V/V) ethylene carbonate/diethyl carbonate served as electrolyte. Galvanostatic cycling was conducted with a Land CT2001A tester at room temperature with potential window from 0.05 to 2 V vs Na/Na⁺.

Figure 1 shows XRD patterns of as-prepared Ti₃C₂X–chitosan films. It is seen that the main diffraction peak refers to (002) plane, which presents the preferential texture microstructure of films. Owing to its two-dimensional crystal structure, Ti₃C₂X nanosheets were prone to stack layer by layer during filtering. The long chains of chitosan covered the surface of nanosheets and were easy to lap with each other in the dried films. Here, there was an interesting phenomenon that after infiltration the initial thickness of Ti₃C₂X–chitosan films with water was about 5 mm and after drying the thickness of final film was only 14–16 μm, reducing more than 300 times. This reason might be attributed to the saturated adsorption of water among the Ti₃C₂X nanolayers, which inspires one good idea to prepare the aerogel of Ti₃C₂X in the following works. Additionally, it was found that during the drying process if there existed one hole less than 1 mm diameter in the film, it could completely disappear after drying, which exhibited a self-healing behavior. Probably, this material could be useful in the biological research. It was determined that the displacements between two Ti₃C₂X layers were 24.254, 27.384, 28.822, and 28.132 Å, respectively, for 0, 7, 10, and 14 wt% chitosan reinforced films (Figures 1A–D). The introduction of chitosan could expand the displacement of Ti₃C₂X nanosheets. When the content of chitosan was above 10 wt%, effect of more chitosan on the displacement became weak.

Figure 2 shows the fracture surface of Ti₃C₂X–chitosan films after tensile testing. It is observed that in the fracture surface of pure Ti₃C₂X film nanosheets arranged orderly and were torn and pulled out (Figure 2A). Some vacancies existed among the layers. Owing to the weak Van der Waals force, nanosheets were easily delaminated. The tensile strength was only 8.20 ± 4.63 MPa, as shown in Figure 3. When 7, 10, and 14 wt% chitosan were used as additives, the tensile strength could be increased up to 9.79 ± 5.31, 30.69 ± 5.01, and 43.52 ± 1.61 MPa continuously. At the same loading of 10 wt%, the tensile strength of Ti₃C₂X–chitosan films has a close value compared to that of Ti₃C₂X–PVA film (30 MPa) (Ling et al., 2014). By observing the fracture surface of Ti₃C₂X–7 wt% chitosan film, it is seen that the layers stacked more tightly in comparison with pure Ti₃C₂X film (Figure 2B). In the magnified image, the nanosheets were torn to be fractured and spalling-off seldom appeared. Clearly, the chitosan has interacted with Ti₃C₂X nanosheets to form thin glue layers, acting as the intermedia to combine Ti₃C₂X nanosheets. However, some vacancies still existed in the fracture surface of film, which might be ascribed to the shortage of chitosan so that part surface of nanosheets could not be covered. Therefore, the tensile strength of film was only increased a few. When the chitosan content was designed above 10 wt%, especially as 14 wt%, nearly no vacancies could be observed in the fracture surface of film, as shown in Figure 2C. The morphology of film containing 10 wt% chitosan was similar to that of film with 14 wt% chitosan. So, the shell-like microstructure was successfully constructed and the as-obtained film with 14 wt% chitosan exhibited the highest tensile strength, about 5.3 times of that of pure Ti₃C₂X film. It is confirmed that 10 wt% chitosan is one critical value to strengthen the film. Whereas, in our works, more chitosan above 14 wt% could not be introduced into the film because the film was not possible to be prepared.

In addition, the electrical resistivity of as-prepared films was measured, as shown in Figure 4. With the increment of chitosan content, the film’s resistivity increased gradiently. For pure Ti₃C₂X film, the measured electrical resistivity was 0.39 mΩ cm, as 2.56 × 10⁵ S/m, which is close to the value determined by Ling et al. (2014) (2.4 × 10⁵ S/m). Under the loading of 7 and 10 wt% chitosan, those of films were 35.69 and 36.21 mΩ cm, respectively. Also, the Ti₃C₂X–14 wt% chitosan film exhibited electrical resistivity of 54.91 mΩ cm. So, it is reasonable to believe that the optimized chitosan content is 10–14 wt% to design Ti₃C₂X–chitosan films with excellent mechanical properties.

The sodium ion storage property of Ti₃C₂X–chitosan films was further performed. As shown in Figure 5, a revisable capacity above 100 mAh/g is obtained with Coulombic efficiency nearly to 100% during cycling. However, it was found that when the film contained chitosan, the sodium ion storage capacity dropped sharply. The reason might be that the introduction of chitosan might block electrolyte transport pathways within Ti₃C₂X flakes and decrease the electrical conductivity of electrodes as well.

Nanolaminar Ti₃C₂X–chitosan films were well prepared with the polymer loading of 0, 7, 10, and 14 wt%. It was found that these films exhibited the shell-like microstructure. With the increasing content of chitosan, the displacement between Ti₃C₂X nanosheets increased from 24.254 Å (0 wt%) to 28.822 Å (10 wt%) and then decreased to 28.132 Å (14 wt%). The tensile strength of films continuously increased from 8.20 to 43.52 MPa, enhancing 5.3 times. Additionally, the electrical resistivity of films increased from 0.39 (0 wt%) to 54.91 mΩ cm (14 wt%). Only pure Ti₃C₂X film could be used as electrode in the sodium battery.

XH and CH provided the idea and organized the manuscript. DZ, HZ, JX, and FS contributed to the experiments and characterization, and also discussed the results.

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 reviewer CW and handling editor declared their shared affiliation, and the handling editor states that the process nevertheless met the standards of a fair and objective review.