L‐Cysteine‐Encapsulated MXene Nanosheet Possessing Ultra‐Antioxidation in Aqueous Suspension for Electrophoretic Deposition Assisted Carbon‐Fiber‐Surface Modification

With recent progress in 2D materials, Ti3C2Tx MXene featured high metallic electrical conductivity, high electromagnetic interference shielding effectiveness, and super in‐plane stiffness, exhibits unique advantages in many fields, but is rarely applied as a sizing agent in long‐time continuous processing of carbon fiber sizing because of its poor ambient stability in aqueous suspension. Herein, a new strategy to chemically encapsulate the reactive sites of MXene nanosheet with l‐cysteine for restricting the attacking of water and/or oxygen in aqueous suspension is proposed. Based on the ultra‐antioxidation, polarity, and electrical properties of l‐cysteine‐encapsulated Ti3C2Tx MXene (LC‐MX) nanosheet, the LC‐MX, even if aging for weeks in aqueous suspension, can be deposited on the surface of high‐modulus carbon fiber (HMCF) uniformly via the electrophoretic deposition assisted sizing. Benefiting from the enhanced surface energy, wettability, and roughness of LC‐MX‐sized HMCF (HMCF‐LCMX) relative to that of unsized one (U‐HMCF), the interfacial property of HMCF‐LCMX/epoxy (EP) composites is also improved, for which the interlayer shear strength (ILSS) of the composite reached 88.5 MPa, 52.8% higher than that of U‐HMCF/EP composite (57.9 MPa). This work makes an essential step toward the application of ultra‐stable MXene nanosheet suspension in large‐scale continuous carbon fiber sizing.


Introduction
[3] However, HMCF without surface treatment and resin usually exhibit chemical inertness and poor wettability, leading to insufficient interfacial bonding strength, for which the composites always suffer fiber debonding or fiber pull-out under loading. [4,5]8] In recent decades, the emerging 2D material composed of transition metal carbides, carbonitrides, and nitrides, namely MXene with the general chemical formula of M n+1 X n T x (M, X, and T x , represent the early transition metal, carbon and/or nitrogen, and surface functional groups, respectively, the value of n ranges from 1 to 4), [9,10] especially Ti 3 C 2 T x , have been widely explored in a variety of fields, such as supercapacitors, [11] batteries, [12] catalysts, [13] sensors, [14,15] electromagnetic interference (EMI) shielding, [16] and sizing agent. [17]Owing to the unique acidic etching process for preparing MXene, there are abundant hydrophilic and electronegative groups (e.g., ─O, ─OH, ─F) on the surface of MXene, [18,19] endowing good aqueous dispersibility, tremendous interlayer bonding force, and convenient modifiability, which makes it a fascinating sizing candidate for optimizing the surface energy, wettability, and roughness of carbon fiber (CF) surface, then enhancing the interfacial and mechanical properties of the CF/resin composites. [20,21]Different from traditional sizing agents, MXene is easier to prepare and applicable to most sizing processes.Moreover, the structure and chemical composition of MXene are adjustable, which makes it easy to carry out the structural design at the interface and improve the strength of the interface with additional functionality. [17,22]Up to now, a variety of methods involving socking with MXene supernatant, [23] chemical grafting with structure-modified MXene, [21,24] and electrophoretic deposition (EPD) MXene suspension, [25] have been developed to introduce Ti 3 C 2 T x MXene onto CFs for surface modification.However, the development of large-scale continuous production of MXene-modified CFs has seriously lagged behind, ascribing to the heavy chemical consumption for generating supernatant and tedious chemical processing for grafting. [17,20,26][29] This is mainly due to the concern that the degradation of MXene, deriving from its oxidation in air and/or water, [30] will diminish its excellent properties and hinder its engineering applications.Thus, developing an effective method for anticipating the oxidation of MXene in suspension is the key to enabling large-scale production of MXene-modified CFs via EPD, which is fascinating but challenging.
According to previous studies, the oxidation of MXene starts at its edges and defects, [31,32] where the oxidation rates are mainly affected by the temperature, [33] surrounding media, [34,35] available oxidant concentration, [36,37] and the existing states, for instance, the MXene nanosheets in aqueous dispersions are more unstable relative to the stacked MXenes in clays or bucky paper films. [38]o fight against the oxidation of MXene, tremendous efforts, such as freezing the crystals in a disordered state, [39] keeping them in a dark place, [40] inhibiting the infiltration of oxidants, [35] and the passage of Ar in a closed container, [33] have been developed to delay the oxidation process of MXene colloidal solutions and prolong the storage time. [36,38]However, effective techniques to eliminate or restrict the oxidation of MXene nanosheets still need to be discovered, particularly in the dispersed aqueous state for the continuous production of CF modification via EPD.Fortunately, other methods based on reactive species capture or chemical modification open new ways of resisting MXene oxidation.Encapsulating MXene nanosheets by chemical re-action is an effective way to moderate the rapid oxidation of MXene in aqueous dispersions.For example, reactive oxygen species quenchers such as imidazolium inion liquids (1-aminoethyl-3methyl-imidazolium chloride) have been demonstrated to protect Ti 3 C 2 T x MXene from oxidation and degradation. [41]Lee et al. [42] introduced mussel-inspired dopamine for highly ordered MXene film, which noticeably enhanced the oxidation resistance of Ti 3 C 2 T x MXene due to the prevention of oxygen and humidity inside the densely packed MXene that edge occupied by polydopamine.Besides, sodium l-ascorbate was also demonstrated to be an efficient antioxidant for stabilizing Ti 3 C 2 T x . [43]According to these reports, bonding the edges and defects with antioxidants containing reactive groups, which can restrict the attacking of water and/or oxygen from the reactive sites, may be efficient for forbidding the oxidation of MXene in water suspension.
49] To date, only a handful of studies have been reported on encapsulated nanomaterials by l-cysteine, but it looks to be a notable study based on the results of the available research.For example, Abdelhalim et al. [50] synthesized l-cysteine reduced graphene oxide (LC-GO) by combining l-cysteine on GO via covalent bonding, achieving the functionality.Yang et al. [51] confirmed that the Ti─S and Ti─N bonds were formed by the chemical reaction between Ti on MXene and the ─SH and ─NH 2 functional groups on l-cysteine.Besides, the efficient removal of numerous oxygen function groups on the MXene surface by l-ascorbate and l-cysteine achieved the ultralong lifetime of the Ti 3 C 2 T x MXene hybridized hydrogels.
Herein, we demonstrate an efficient strategy to fight against the degradation of Ti 3 C 2 T x MXene nanosheet in aqueous suspension by chemically encapsulating the nanosheet with l-cysteine.It prolongs the shelf life of MXene significantly owing to the occupation of reactive sites with l-cysteine that restrict the attacking of water and/or oxygen, thus guaranteeing their application for HMCF surface treatment in large-scale continuous production via EPD.The mechanism of the antioxidative l-cysteine for MXene is discussed and studied systematically.With the treatment of L-cysteine-modified MXene (LC-MX) via EPD, even if LC-MX suspension for EPD is aging for weeks, the surface energy, wettability, and roughness of HMCF were significantly optimized.Therefore, the interfacial property of LC-MX modified HMCF/epoxy (EP) resin composite is largely enhanced, for which the interlayer shear strength (ILSS) of the composite is as high as 88.5 MPa, with an enhancement of 52.8% relative to that without LC-MX modification.This work unveils the considerable potential of ultra-stable LC-MX nanosheet suspension in large-scale continuous carbon fiber sizing.

Results and Discussion
The MXene nanosheets with surface modification of l-cysteine (LC-MX) were prepared by adding the l-cysteine deionized water solution to the as-prepared Ti 3 C 2 T x MXene dispersion, then stirring for 24 h.Owing to the existence of ─NH 2 and ─SH groups in l-cysteine, the MXene nanosheets can be chemically encapsulated by l-cysteine via bonding with the Ti on the surface of the nanosheets (Figure 1a).As is known, the reactive sites and defects on MXene occupied by l-cysteine will significantly forbid the attacking of oxygen and/or water, which can enhance the resistance to oxidation of LC-MX nanosheets and prolong their shelf-life. [52]Subsequently, the scanning electron microscopy (SEM) images and corresponding energy dispersive spectroscopy (EDS) mapping images of C, O, Ti elements for MXene (Figure S1, Supporting Information) and C, O, Ti, N, S elements for LC-MX are carried out.Because of the treatment of l-cysteine, the LC-MX nanosheets possess rougher surfaces than that of pure MXene ones, and the homogenous distribution of the characteristic elements, such as Ti, N, S, for LC-MX nanosheets suggests the successful introduction of lcysteine on MXene surface (Figure 1b; Figure S2, Supporting Information). [53]Further, the chemical structure composition of pure MXene and LC-MX is deeply investigated by the Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra.In comparison with FTIR of pure MXene, except for the peaks at 558, 1631, and 3433 cm −1 respectively corresponding to the stretching vibration of Ti─O, C─O, and O─H bonds, there are another two intense absorption peaks located at 1449 and 1688 cm −1 assigned to the stretching vibration of ─COO─ and bending vibration of N─H bonds in the FTIR spectrum of LC-MX (Figure S3, Supporting Information). [51]In addition, the peak at 3433 cm −1 is significantly enhanced with the appearance of N─H stretching vibration. [31]These results indicate that l-cysteine has been successfully introduced to the surface of MXene.Then, the direct evidence for the chemical composition of the LC-MX nanosheet is provided by the chemical analysis of XPS spectra.As shown in Figure S4 (Supporting Information), except for the characteristic peaks of F 1s, Ti 2s/2p/3s/3p, O 1s, and C 1s in the wide-scan XPS spectra of pure MXene nanosheets, it reveals the additional N 1s and S 2p peaks at 399.00 eV and 165.00 eV in that of LC-MX, confirming the presence of l-cysteine on the MXene surface. [54]Then, the bonding information was also analyzed via XPS C 1s and O 1s spectra of MX and LC-MX nanosheets and displayed in Figure S5 (Supporting Information).Despite typical binding energies of MXene, such as the binding energies for C═O/C─F, C─O, C─C, C─Ti─O, and C─Ti peaks in C 1s (Figure S5a,c, Supporting Information), C─Ti─(OH)x, C─Ti─O, and C─Ti─Ox peaks in O 1s (Figure S5b,d, Supporting Information), LC-MX presents two extra peaks with the binding energy of 285.47 and 286.24 eV (Figure S5a,c, Supporting Information), assigned to the C─S and C─N bonds in l-cysteine, respectively, [53] suggesting that the LC-MX nanosheets are composed of l-cysteine and MXene.As previous reports, the sulfhydryl groups (─SH) and amine groups (─NH 2 ) in l-cysteine are highly reactive, which tend to nucleophilically react with oxygen-containing groups on the surface of MXene, such as hydroxyl group (─OH), and graft to MXene via chemical bonding. [50]To confirm the chemical bonding information between l-cysteine and MXene, the binding energy is also distinguished with the Ti 2p, N 1s, and S 2p spectra of LC-MX. Figure 1c shows the high-resolution Ti 2p spectra of LC-MX.The appearance of Ti─N (44.84%) and Ti─S (21.68%) bonds which are located at 456.52/462.52 and 455.63/461.63 eV [55,56] confirmed the introduction of l-cysteine (66.52%) via bonding with Ti on MXene.As shown in Figure 1d, the 400.33 and 402.37 eV peaks are assigned to the binding energy of ─NH 2 and ─NH─ in lcysteine, respectively, [54] and the peak at 397.44 eV is believed to be the binding energy of N─Ti formed by amine bonding at the unterminated Ti sites. [57]S 2p XPS spectra of LC-MX further demonstrate the existence of the C─S─Ti with binding energies of 163.63 and 162.09 eV, which is derived from the reaction between sulfhydryl group in l-cysteine and Ti sites on MXene (Figure 1e). [52]The C─SH in l-cysteine with binding energies of 165.20 and 164.61 eV are also distinguished, confirming the existence of l-cysteine on MXene.Thus, for LC-MX, the l-cysteine is connected to MXene via S─Ti and N─Ti bonding derived from the interaction of ─SH and ─NH 2 groups with the reactive sites on the surface of MXene (Figure 1a).As a result, the reactive sites of LC-MX vulnerable to water molecule attack are occupied, suggesting the antioxidant possibility of LC-MX in water.
Motivated by this, the antioxidation of LC-MX is assessed.The suspensions with the mixture of MXene (0.5 mg mL −1 ) nanosheets and l-cysteine (LC) at different concentrations (0, 0.5, and 2.5 mg mL −1 ) were obtained (Figure S6.Supporting Information).The freshly pure MXene aqueous suspension and LC-MX suspension present a black color.After being stored in a sealed glass bottle at room temperature for four weeks, the aqueous suspension of MXene becomes cloudy white while that of LC-MX remains black.When aging for 3 months, the pure MXene aqueous suspension and LC-MX suspension containing 0.5 mg mL −1 LC change to entirely white and cloudy-white, respectively, but the LC-MX suspension containing 2.5 mg mL −1 LC still presents a black color.It is indicated that the concentration of LC obvi-ouslyb affects the degradation of Ti 3 C 2 T x , and the MXene treated by higher concentrated LC offers more excellent antioxidant ability.So, we select the LC-MX nanosheet suspension containing 2.5 mg mL −1 LC for further investigation.Then high-resolution transmission electron microscopy (HRTEM) was also carried out.As shown in Figure 2a, the surface of fresh MXene nanosheet is clean and flat, and the corresponding selected area electron diffraction (SAED) pattern image (inset of Figure 2a) demonstrates its hexagonal atomic arrangement of single crystalline.After the age of four weeks, the typical 2D nanosheet morphology of pure MXene disappears (Figure 2b), which transfers into amorphous TiO 2 and disordered carbon structure according to the previous studies. [58]While, the 2D nanosheet morphology and single crystal phase of LC-MX keep well after storage for four weeks (Figure 2c), suggesting that the LC can postpone the degradation of Ti 3 C 2 Tx MXene nanosheets in aqueous suspension effectively.
Further, the zeta potentials of pure MXene and LC-MX suspensions are measured to assess their colloidal stability.As shown in Figure 2d, the terminal groups of MXene involving ─OH, ─O, ─Cl, and ─F result in its negatively charged surface with a zeta potential value of −31.6 mV, which is nearly in agreement with the previous study. [41]As they age for 4 weeks, the zeta potential of pure MXene nanosheets increases to −14.1 mV because of their oxidation and degradation.However, for LC-MX suspension, a lower zeta potential value of −37.0 mV is obtained, and the zeta potential value remains −30.6 mV after 4 weeks of storage (Figure 2d), confirming that the introduction of l-cysteine significantly contributes to slowing down the decrease of terminal groups and improving the stability of LC-MX aqueous suspension.Figure 2e,f represents the X-ray diffraction (XRD) patterns of MXene and LC-MX nanosheet film.For pure MXene, the (002) peak at 2 = 6.25°and (004) peak at 2 = 18.23°disappeared completely after aging four weeks.At the same time, some typical peaks for rutile, TiO 2 , appeared, indicating that the MXene nanosheets are entirely oxidized to TiO 2 (Figure 2e,f). [58]In contrast, for LC-MX, the characteristic peaks of (002) and (004) are still clearly visible after aging for four weeks, and the peaks corresponding to TiO 2 are absent, indicating that LC could effectively prevent the oxidation and retain the crystal structure of MXene nanosheets in aqueous suspension.
Moreover, the antioxidation ability of LC-MX was also confirmed by the evolution of surface chemical components of LC-MX with different aging times in aqueous suspension.As the storage time increased, the typical peak of O 1s at 531.00 eV enhanced for pure MXene (Figure 2g), suggesting that much oxygen entered the surface of MXene nanosheets.While for LC-MX, the intensity of peak O 1s is nearly unchanged owing to the presence of LC antioxidants.Then, to probe the state of oxidation on nanosheets, the XPS Ti 2p and C 1s spectra of MXene and LC-MX nanosheets with different aging times are plotted in Figure 2h,i.After aging for 4 weeks, the peaks at 455.69 and 461.65 eV, corresponding to Ti 2p 3/2 and Ti 2p 1/2 , disappeared.Meanwhile, the peaks at 459.08 and 464.77eV, assigned to TiO 2 , are observed, confirming the severe oxidation of pure MXene nanosheets (Figure 2h). [36]However, only a very weak TiO 2 peak near 458.69 eV is observed in the spectrum of 4week aging LC-MX nanosheets spectrum, indicating that the presence of l-cysteine could retain the lamellar structure of MXene nanosheets, besides a slight oxidation of MXene (Figure 2h).Coincidentally, the changes for peaks in C 1s spectra with different aging times also demonstrate the antioxidation ability of LC-MX.As shown in Figure 2i, the intensity of the C─Ti─Tx peak of LC-MX located near 281.95 eV changes slightly after aging four weeks.In contrast, after shelving pure MXene for four weeks, MXene nanosheets suffered serious oxidation and the C─Ti─Tx peak disappeared, resulting in TiO 2 and amorphous C. To sum up, owing to the reaction between the active groups (─SH, ─NH 2 ) of l-cysteine and oxygen-containing groups (─OH, etc.) on the MXene surface (Figure 1d,e; Figure S3.Supporting Information), the reactive sites on the surface of LC-MX nanosheets, that are vulnerable to water molecule attack, are occupied.Therefore, the LC-MX nanosheets in aqueous suspension exhibit excellent antioxidation ability due to the lack of reactive sites for water attack.The amazing resistance to oxidation of LC-MX nanosheets in an aqueous suspension favors the stability of their unique characteristics, such as electrical conductivity, mechanical strength, and hydrophilicity, which is beneficial for practical application.
Motivated by this, the LC-MX nanosheets are used to optimize the surface of HMCF by electrophoretic deposition (EPD) method continuously.This is more suitable for actual production than our previous work, [59] where the MX nanosheets aqueous suspension was placed directly in an open container at room temperature, leading to their oxidization and degradation over time.As described in Figure 3a, for the EPD process of LC-MX onto HMCF under a DC power supply, the carbon fiber acting as anode is wound parallel to the spindle, and the metal plate in the LC-MX electrophoresis solution is used as the cathode, where the vertical distance between the metal plate and the fibers is 1.2 cm.When an external electric field is applied, the negatively charged LC-MX nanosheets move toward the positive electrode, resulting in LC-MX deposition onto the HMCF followed by the drying process in an N 2 atmosphere at 100 °C.As shown in Figure 3b and Then, the SEM and the atomic force microscope (AFM) are carried out to investigate the surface morphology of HMCF treated with the LC-MX or MX aqueous suspensions exposed for different periods via EPD.The surfaces are rough for both freshly LC-MX-and MX-modified HMCF (Figure 4a) because of the deposition of LC-MX and MX.While the fiber surface modified with the MX aging in aqueous suspension for two weeks (HMCF-MX-2wks), became smooth and flat, indicating the near absence of MX on the surface of HMCF.According to the surface chemical structural composition analysis by XPS, the peak of Ti 2p at 474 eV for HMCF-MX-2 wks decreased relative to that of freshly HMCF-MX (Figure S8a and Table S1.Supporting Information), confirming that the smooth surface of HMCF-MX-2 wk is suffered from the reducing content of MX nanosheet on the surface of HMCF (Figure 4a ii).As time aged, the MX nanosheets oxidatively decompose into TiO 2 and amorphous carbon (Figure 2h,i; Figure S8b.Supporting Information), leading to the reduction of negative charge (Figure 2d) that is detrimental to the deposition of MX nanosheets by EPD.As a result, few MX nanosheets are deposited to the surface of HMCF when the MX nanosheets suspension aging for two weeks is used for EPD.At the same time, less oxygen-containing groups (─OH, ─O, etc.) are introduced on the surface of HMCF-MX-2wk (Figure S8a and Table S1.Supporting Information) compared to that of HMCF-MX, which is unfavored for enhancing the wettability of HMCF surface.From the AFM image of HMCF-MX (insets of Figure 4a i,ii), it is found that the nanosheets on its surface are looser.The surface roughness (Ra) of HMCF-MX-2 wk decreased to 38.2 nm (Table S2.Supporting Information) due to the bare MX nanosheets deposited on the surface of HMCF-MX-2 wk.In sharp contrast, obviously, large LC-MX nanosheets could be observed on the surface of fiber modified with the LC-MX aging in aqueous suspension for 2 weeks (HMCF-LCMX-2wks) (Figure 4a iii,iv).The chemical composition of HMCF-LCMX-2 wks changes little relative to that of HMCF-LCMX (Figure S8a,c, and Table S1.Supporting Information), indicating the ultra-stability of LC-MX suspension for EPD even aging for two weeks owning to the antioxidation of l-cysteine.Moreover, by analyzing the AFM images, the LC-MX nanosheets on the HMCF-LCMX surface are denser and the Ra of HMCF-LCMX surface is higher compared with that of HMCF-MX.Especially, the Ra of HMCF-LCMX-2 wk surface is enhanced to 64.7 nm, even higher than that of HMCF modified by freshly LC-MX (56.3 nm) (Table S2.Supporting Information).It can be ascribed to the localized aggregation of LC-MX nanosheets resulting from the formation of adequate bonding, such as covalent and hydrogen bondings, between l-cysteine and MX nanosheets as exposed time extended. [60,61]Besides, the peak at 282.3 eV assigned to C-Ti change little for HMCF-LCMX-2wks relative to HMCF-LCMX due to the antioxidative ability of l-cysteine (Figure S8c.Supporting Information).In sharp contrast, the C─Ti peak in the C 1s spectrum of HMCF-MX-2wks disappears, indicating the absent MX on the surface of HMCF-MX-2wk (Figure S8b.Supporting Information).Thus, the introduction of l-cysteine not only improves the stability of MX nanosheets but also favors the local aggregation of nanosheets for enhancing the fiber surface roughness when deposited on the surface of HMCF, which would contribute to improving the interfacial binding strength of the composites.
Further, the resin wettability of the HMCF surface with different modifications is investigated.As shown in Figure 4b, both the contact angles of HMCF-MX and HMCF-LCMX in deionized water and diiodomethane decrease in comparison with that of HMCF without any modification (U-HMCF), which is positively related to the surface roughness of HMCF with different modification (Table S2.Supporting Information).Especially, the contact angles of HMCF-LCMX are as low as 42.7°for water and 65°for diiodomethane, suggesting excellent wettability of HMCF with resin, so as to the HMCF-LCMX-2wks.Then, the polar component p, the dispersion component d, and the surface energy  of HMCF with different modifications are calculated according to the measured contact angles via the OWRK method.As is known, the polar groups, for instance, the ─OH and ─O on the surface of MX, contribute to the enhancement of the polar part p.Meanwhile, the elevated surface roughness favors the increase of the dispersion part d. [62]Therefore, the p and d for HMCF-LCMX increase to 21.03 and 33.8 mN m −1 (Figure 4c) because of the existence of nanosheets that contribute to the introduction of polar groups and enhancing the roughness of the HMCF surface.The surface energy  of HMCF-LCMX-2wk is also significantly improved to 53.78 mN m −1 , nearly the same as that of HMCF-LCMX (54.83 mN m −1 ), because of the existence of enough LC-MX on the surface of HMCF.However, the  of HMCF-LCMX-2wks decreases relative to that of HMCF-LCMX due to the fewer MX nanosheets deposited on the HMCF surface mentioned above (Figure 4a).Thus, the introduction of LC-MX enhances the wettability and surface energy of HMCF, which is beneficial for optimizing the interfacial properties of carbon fiber-reinforced resin composite.
To investigate the effects of different HMCF surface modification on the interfacial properties of HMCF reinforced resin composite, the HMCF/EP composites is prepared, and then the interlaminar shear strength (ILSS) is tested to evaluate the composite interfacial properties macroscopically.As shown in Figure 5a, the ILSS of MX and LC-MX modified HMCF/EP composites are significantly improved compared to that of U-HMCF/EP composites.The HMCF-MX/EP and HMCF-LCMX/EP present the maximum ILSS of 86.8 and 88.5 MPa, respectively, which are 49.9% and 52.8% higher than that of the U-HMCF/EP composite (57.9 MPa).Owing to the excellent antioxidation properties of LC-MX in suspension, even LC-MX nanosheets aging in aque-ous suspension for two weeks are deposited on the surface of HMCF fiber enough, thus the ILSS of HMCF-LCMX-2 wk/EP (85.2 MPa) do not show a substantial decrease.However, the ILSS of HMCF-MX-2wk/EP decrease by 20.3%, indicating that the oxidation of MX in suspension is detrimental to optimizing the interfacial properties of fibers and the resin matrix.It is in accordance with our previous discussion that the oxidation of MX aging for two weeks in suspension will fail to be deposited on the surface of HMCF, leading to the lack of polar groups and roughness for optimizing the  of HMCF-MX-2wks.As a result, HMCF-MX-2wk/EP composites possess poor interfacial properties and a decreased ILSS.The shear strength test of composites prepared with different fibers provides the load-displacement curves (Figure 5b), where the area of the curve represents the energy absorption during the failure process of composites.It is evident that the maximum load and the corresponding displacement of HMCF-MX/EP or HMCF-LCMX/EP composites are larger, suggesting that more energy is absorbed by these composites during failure.Similar to the variation of ILSS, HMCF-MX, HMCF-LCMX, and HMCF-LCMX-2wk reinforced composites could maintain similarly high mechanical properties, while that of HMCF-MX-2wk is lower.It is confirmed that the antioxidative properties of LC-MX guarantee its chemical and physical properties, which is vital for optimizing the mechanical properties of HMCF-reinforced resin composites.
To understand the mechanical mechanism deeply, the failure interface of the HMCF-LCMX/EP composite is carefully studied by the SEM images.There are apparent adhesive failures between HMCF-MX-2 wk and resin matrix in axial and cross-section (Figure 5c i,iv).Large microcracks are observed in the cross-section (Figure 5c ii) where the EP matrix shows a broken irregular morphology, indicating that the cracks in the interface transfer from the fibers to the resin matrix for dispersion. [63]This phenomenon agrees with the energy absorption during the failure process of composites displayed in the load-displacement curve.From the axial section of HMCF-LCMX, there are some LC-MX, and matrix resins are residual on the surface of HMCF (Figure 5c v), which result from the effectively enhanced interfacial strength of the composite due to the dense arrangement of LC-MX on the surface of HMCF mentioned above (Figure 4a).By comparing the SEM images of HMCF-MX-2 wk/EP with those of HMCF-LCMX-2 wk /EP, it is apparent to feel the effect brought by l-cysteine.In the crosssection of HMCF-MX-2 wk/EP (Figure 5c i), we can see obvious gaps caused by the debonding of the fibers from the resin, indicating that the TiO 2 particles in the axial cross-sectional image (Figure 5c iv) are not sufficient to substitute the strengthening effect of MX.Unlike the substantial failure of HMCF-MX-2 wk/EP composites, the LC-MX nanosheet aggregates are found on the cross-section of HMCF-LCMX-2 wk /EP (Figure 5c iii,vi).Some residual LC-MX and matrix resin could be found on its axial section of the HMCF surface, which suggests that HMCF-LCMX and resin matrix could still firmly bond during the composite failure.Above all, the interfacial strengthening mechanism of LC-MX modified HMCF/EP composites is summarized, and the failure diagram of composites is shown in Figure 5d.Due to the modification of l-cysteine, more polar functional groups involving ─NH 2 , ─SH, ─COOH, ─OH, and other oxygen-containing groups are introduced to LC-MX.Thus LC-MX nanosheets on the surface of HMCF possess a tighter arrangement to forbid the interfacial layer sliding, which is confirmed by the more residual LC-MX nanosheets and resin matrix on the surface of HMCF (Figure 5c ii,v).Besides, the high surface roughness combined with the rich polar groups of HMCF-LCMX surface (Figure 4a iii; Table S1 and S2.Supporting Information), are beneficial for improving the fiber wettability and the chemical bonding with resin and achieving interfacial bonding effect.Thus, the LC-MX nanosheets formed a modulus transition layer at the interface between HMCF and EP, allowing the stress to be uniformly transferred to the resin and create an excellent composite interface. [64]Notably, the LC-MX-2wk has larger lateral dimensions than MX-2wk nanosheets, which means HMCF-LCMX-2wk /EP presents higher yield strength at the interface when receiving pure shear forces, as LC-MXene-2wk has a higher potential to resist crack growth. [65]The surface of HMCF-LCMX-2wk is similar to HMCF-LCMX.However, there is a localized aggregation of nanosheets, which leads to a decrease in wettability and affects the mechanical properties of the composite.As to HMCF-MX-2 wk/EP, MX-2wk has decomposed into TiO 2 particles, amorphous C, and a few small-sized MX nanosheets, which results in the disappearance of high roughness and high functional group content surface of HMCF-MX, and significantly reduced the mechanical properties of the composites.

Conclusion
The l-cysteine chemically treated MXene nanosheets, LC-MX, with excellent antioxidation in aqueous suspensions, have been developed for HMCF surface modification via the EPD method.Through chemical bonding and physical interaction, l-cysteine occupied the reactive sites of MXene nanosheets that inhibit water and/or oxygen attacks.Therefore, the 2D layered structure and crystalline structure of MXene can be maintained in an aqueous solution, even after aging for weeks.The surface roughness, wettability, and surface energy of HMCF-LCMX are significantly improved, resulting in significantly enhanced ILSS of HMCF-LCMX/EP composite, which could be enhanced up to 52.8% compared to U-HMCF/EP composites.Besides, the interfacial properties of composites with the HMCF treated by LC-MX suspension aging for weeks.Thus, l-cysteine functionalized modification could effectively extend the service life of Ti 3 C 2 T x MXene nanosheets and perfectly meet the requirements of continuous processing as a carbon fiber sizing agent in aqueous suspension.This study also contributes to broader application scenarios of MXene nanosheets in the future.

Experimental Section
Preparation of LC-MX Nanosheet: Ti 3 C 2 T x Mxene was prepared by etching Ti 3 AlC 2 (purchased from Jilin Yiyi Technology Co., Ltd.China.) with LiF/HCl mixed solution (HCl was provided by Beijing Tongguang Fine Chemical Co., Ltd. and LiF was supplied by Aladdin Biochemical Technology Co., Ltd.).l-Cysteine powder (purchased from Shanghai Haohong Biomedical Technology Co., Ltd.China.) was dissolved by deionized water (0.5 mg mL −1 ), then the MXene dispersion (2.5 mg mL −1 ) was added and stirred for 24 h to obtain the LC-MX aqueous suspension.
Deposition of MXene Nanosheets via EPD Method: HMCF (BHM2) was produced by the Beijing University of Chemical Technology, China.The HMCF fiber bundles and the metal plate were immersed in the electrophoretic solution containing MXene or LC-MX nanosheets, and deposited at a voltage of 15 V for 2 min.
HMCF/EP Composites Preparation: HMCF/EP composites were prepared by the molding method.Epoxy resin (E44, purchased from Nantong Xingchen Synthetic Materials Co., Ltd.China.) and curing agent (TETA, mass fraction > 65%, purchased from Macleans Biochemical Technology Co., Ltd.China.) were homogeneously mixed at a mass ratio of 10:1, then uniformly applied to the surface of HMCF and placed into a mold.Followed by curing at 80 °C for 2 h.The volume fraction of HMCF in the composites was about 60%.
Characterization: The morphology and microstructure of samples were obtained from scanning electron microscope (SEM, TESCAN, MAIA3), atomic force microscopy (AFM, Bruker, Dimension Icon), and transmission electron microscope (TEM, JEM-2100Plus).The FTIR spectra were obtained by Fourier-transform infrared spectroscopy (FTIR, Thermo Fisher, Nicolet iS10).The surface chemical composition was obtained from X-ray photoelectron spectroscopy (XPS, Thermo Fisher, K-Alpha).To measure the degree of crystallization, X-ray diffraction (XRD, Rigaku, ULTIMA IV) was acquired at a scan rate of 5°min −1 .The zeta potential was observed from the zeta potential analyzer (Malvern, Nano-ZS ZEN3600).The contact angle was obtained from a dynamic contact angle analyzer (Dataphysics, DCAT21), and the surface energy was calculated by the WORK method.The mechanical properties of HMCF/EP composites were tested according to EN ISO 14 130, for each composite, at least ten samples were tested using an electronic universal testing machine (INSTRON, 3345).

Figure 1 .
Figure 1.a) Schematic illustration of the fabrication of l-cysteine modified MXene (LC-MX) nanosheets from pure MXene nanosheets (MX), b) SEM image and SEM elemental mapping of Ti, N, S of LC-MX, c) Ti 2p, d) N 1s and e) S 2p high-resolution XPS spectra of fresh LC-MX.

Figure 2 .
Figure 2. TEM images and the corresponding SAED pattern of MX and LC-MX: a) fresh MX, b) MX and c) LC-MX aging in aqueous suspension for four weeks, marked as MX-4wks and LC-MX-4wks respectively; various characterizations of fresh MX, fresh LC-MX, MX-4wks, and LC-MX-4wks: d) zeta potentials, e) XRD pattern, f) enlarged version of XRD pattern, g) Wide-scan XPS spectra, h) Ti 2p and i) C 1s high-resolution XPS spectra.

Figure 3 .
Figure 3. a) Schematic of EPD of LC-MX on HMCF, b) SEM image and SEM elemental mapping of C, O, Ti, N, S of HMCF surface modified by LC-MX (HMCF-LCMX).
Figure S7 (Supporting Information), the S and N elements on LC-MX modified HMCF surface, namely HMCF-LCMX, are monitored besides the typical elements of C, O, and Ti for both pure MX modified HMCF (HMCF-MX) and HMCF-LCMX, suggesting the successful deposition of LC-MX on HMCF surface via EPD.What is more, the XPS spectrum is also carried out.In comparison with HMCF-MX, two extra peaks for HMCF-LCMX at 399 and 165 eV appear, which could be assigned to N 1s and S 2p respectively (Figure S8a.Supporting Information).Accompanied by the binding energies distinguished as C─S (285.5 eV), C─N (286.2 eV), and C─Ti (282.3 eV) (Figure S8c.Supporting Information), the existence of LC-MX on the surface of HMCF-LCMX is confirmed again.