β‐Mercaptoethanol‐Enabled Long‐Term Stability and Work Function Tuning of MXene

The oxidation degradation by unsaturated metal atoms or dangling bonds at MXene edges and defects severely hinders the practical application of MXene. Herein, a passivation scheme for Ti3C2Tx MXene is demonstrated by utilizing a sulfhydryl‐containing molecule, β‐mercaptoethanol (BME), which can significantly suppress the Ti3C2Tx oxidation in various environments, including long‐term storage of Ti3C2Tx aqueous dispersions (2 m), single‐layer Ti3C2Tx‐based devices in humid air (2 m), and high‐temperature environment (12 h). Notably, the nonionic BME does not cause aggregation but maintains the 2D morphology of Ti3C2Tx. A comprehensive investigation of the protection mechanism through density functional theory (DFT) calculations and experimental characterizations reveals that BME is adsorbed especially at the edges and surface defects of MXene (binding energy −1.70 and −1.05 eV), where the degradation starts. Further, the electron‐donating effect of sulfhydryl groups tunes the work function of Ti3C2Tx from 4.70 to 4.39 eV, resulting in improved carrier‐transport performances in MoS2 field‐effect transistors owing to band alignment, where BME–Ti3C2Tx serves as the source electrode. The described methodology can largely contribute to the ultralong service life of 2D Ti3C2Tx without affecting its excellent properties, thereby promoting the practical application of this emerging material.

The oxidation degradation by unsaturated metal atoms or dangling bonds at MXene edges and defects severely hinders the practical application of MXene. Herein, a passivation scheme for Ti 3 C 2 T x MXene is demonstrated by utilizing a sulfhydryl-containing molecule, β-mercaptoethanol (BME), which can significantly suppress the Ti 3 C 2 T x oxidation in various environments, including longterm storage of Ti 3 C 2 T x aqueous dispersions (2 m), single-layer Ti 3 C 2 T x -based devices in humid air (2 m), and high-temperature environment (12 h). Notably, the nonionic BME does not cause aggregation but maintains the 2D morphology of Ti 3 C 2 T x . A comprehensive investigation of the protection mechanism through density functional theory (DFT) calculations and experimental characterizations reveals that BME is adsorbed especially at the edges and surface defects of MXene (binding energy À1.70 and À1.05 eV), where the degradation starts. Further, the electron-donating effect of sulfhydryl groups tunes the work function of Ti 3 C 2 T x from 4.70 to 4.39 eV, resulting in improved carrier-transport performances in MoS 2 field-effect transistors owing to band alignment, where BME-Ti 3 C 2 T x serves as the source electrode. The described methodology can largely contribute to the ultralong service life of 2D Ti 3 C 2 T x without affecting its excellent properties, thereby promoting the practical application of this emerging material.
Previous studies demonstrated effective strategies to delay the oxidation reaction of MXene, which can be roughly summarized into two categories: control of external factors that induce the oxidation reaction and stabilization of MXene by passivating the unsaturated Ti atoms at the defects and edges. For example, the oxidant of MXene, dissolved O 2 in water, can be removed by degassing using inert gases (Ar or N 2 ), [18] while another oxidant, H 2 O, can be replaced by dispersing MXene in organic solvents. [19] However, the dispersions of MXene in organic solvents without additional additives generally lead to issues of low concentration and low dispersion stability. [20] Another factor of the oxidation reaction is the reaction rate. A low temperature (as low as À80°C) [21] slows the oxidation reaction kinetics but has the disadvantages of high energy consumption and low efficiency. Overall, the control of external oxidation factors imposes harsh requirements on the application environment of MXene. In contrast, the method of stabilizing unsaturated Ti atoms has better adaptability because it improves the oxidation resistance of MXene. Mathis et al. [22] effectively reduced the defects in the Ti 3 AlC 2 MAX phase precursor by adding excessive Al during synthesis, thereby largely eliminating the unsaturated Ti atoms in the surface defective part of the MXene sheet, although the edge Ti is still unsaturated. Natu et al. [23] raised polyphosphates by capping the edges of MXene to mitigate oxidation. Polyphosphate protection caused MXene aggregation, as high concentrations of salt destroyed the hydration layer of MXene. Zhao et al. [24] introduced a sodium L-ascorbate additive to mitigate the oxidation while preserving the 2D dispersion state of MXene, but the susceptibility to mold limited its application in an unsterilized environment. Therefore, the development of a low-energyconsumption method and material that do not require additional repeatable washing processes, especially to simultaneously provide protection in both the storage of dispersion and the application of solid-state MXene, is still crucial for both fundamental research and practical applications of MXene.
In this study, we demonstrate that β-mercaptoethanol (BME) can significantly inhibit the degradation of Ti 3 C 2 T x MXene, in a colloidal dispersion, under humid air conditions, or even in a harsh 100°C water. Ti 3 C 2 T x was studied because it is the most representative and widely studied MXene, which can be attributed to the highest conductivity in the MXene family, rich surface groups for redox reactions and dispersibility in water, well-established methods for synthesis and material handling, etc.
[25] BME was selected because the sulfhydryl groups (─SH) have strong electron-donating effects and are expected to have strong interactions with the partially positively charged unsaturated Ti atoms at the edges and defects of MXene. This is based on the classic strong interaction between ─SH and gold, [26] thereby eliminating active sites in the oxidation reaction. ─SH has a reducing ability to eliminate oxygen free radicals in water. [27] Moreover, compared with the reported ionic protective materials, the nonionic BME seldom destroys the hydration layer of MXene, while the hydrophilic hydroxyl tail is conducive to the good water dispersibility of BME-Ti 3 C 2 T x MXene. Thus, 2D BME-Ti 3 C 2 T x flakes can be easily obtained by spin-coating without any further treatments. The oxidation-suppressing effect of BME and the related mechanism was confirmed by various characterizations such as ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectroscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), electron paramagnetic resonance (EPR), and X-ray diffraction (XRD). Using density functional theory (DFT) calculations and experimental characterizations, we propose that ─SH preferentially forms strong bonds (binding energy up to À1.70 eV) with unsaturated Ti atoms at the edges rather than at the surface of MXene, thereby suppressing the oxidation reaction. Furthermore, the electrical properties of BME-Ti 3 C 2 T x were investigated by fabricating MXene-based field-effect transistors (FETs). BME-Ti 3 C 2 T x exhibited better conductivity and considerably better humid air stability than those of pristine Ti 3 C 2 T x . Finally, the work function of Ti 3 C 2 T x was successfully tuned from 4.70 to 4.39 eV by the BME treatment because of the electron-donating effect of ─SH. BME-Ti 3 C 2 T x served as a considerably better source electrode material than pristine Ti 3 C 2 T x in MoS 2 FETs owing to the band alignment effect. The described methodology can largely contribute to the long-term storage of 2D MXene with a low energy consumption, which can likely improve the electronic properties of MXene, thereby promoting the practical application of MXene. In addition, the elucidation of the strong interaction of ─SH with MXene and improved stability of MXene at high temperatures is expected to inspire related studies and further expand the research field of MXene. Figure 1a illustrates the aqueous dispersed 2D Ti 3 C 2 T x nanosheets, the molecular structure of BME with highlighted lone pairs of electrons, and BME-Ti 3 C 2 T x composite structure to prevent MXene attack by O 2 or H 2 O. Aqueous Ti 3 C 2 T x colloids were produced by etching the Al layer from Ti 3 AlC 2 using HCl/LiF prior to removing extra acid and salts via centrifugation, as illustrated in Figure S2, Supporting Information (see Experimental Section). [14a] Regarding the mechanism of interaction between Ti 3 C 2 T x and BME molecules, the S atoms of BME with lone pairs of electrons are considered to preferentially bind with the unsaturated Ti of MXene. The excess BME can also form strong bonds with the defective and perfect MXene surface, which is discussed in detail later. To suppress the oxidation reaction of Ti 3 C 2 T x , the as-synthesized Ti 3 C 2 T x colloid was separated and diluted with three aliquots of an aqueous BME solution (volume ratio of 1:3), which yielded final BME concentrations in the 10 mL Ti 3 C 2 T x colloid of 0.005, 0.05, and 0.5 mol L À1 . An additional Ti 3 C 2 T x suspension diluted by deionized water without BME was regarded as a control. Figure 1b shows images of the as-prepared pristine Ti 3 C 2 T x colloid (0 d) and four Ti 3 C 2 T x dispersions containing different amounts of BME after storage at room temperature (25°C) for 28 d. The vials were shaken by hand and stood for around half an hour before taking photographs. Without BME, the Ti 3 C 2 T x dispersion aged for 28 d was almost transparent, which indicates that the concentration was significantly reduced. This reveals the natural oxidation of MXene at room temperature. With the protection of BME, even with the minimum BME amount of 0.005 M, the dispersion exhibited a pitch-black color, similar to the original Ti 3 C 2 T x . The concentration of Ti 3 C 2 T x in these dispersions was monitored using UV-vis-NIR absorption spectra, as shown in Figure S3, Supporting Information. The absorbances of the Ti 3 C 2 T x dispersions with 0.005 (0.005 BME-Ti 3 C 2 T x ), 0.05 (0.05 BME-Ti 3 C 2 T x ), and 0.5 M (0.5 BME-Ti 3 C 2 T x ) of BME were %1.92, 2.33, and 2.58 times that of Ti 3 C 2 T x in pure water, respectively. Only a slight concentration attenuation and peak red shift were found in aged 0.5 BME-Ti 3 C 2 T x compared to that in freshly synthesized pristine Ti 3 C 2 T x , which demonstrates that BME can effectively restrain the oxidation of Ti 3 C 2 T x . Figure 1c shows optical microscope (OM) images of a spin-coated Ti 3 C 2 T x (left panel) and 0.05 BME-Ti 3 C 2 T x (right panel) on Si/SiO 2 substrates after storage in a humid air atmosphere (25°C, relative humidity ¼ 50%) for 4 m. Without BME, Ti 3 C 2 T x sheets were oxidized and finally disintegrated in humid air, leaving shiny TiO 2 particles under OM light. By contrast, the spin-coated BME-Ti 3 C 2 T x maintained its 2D structure after aging, and AFM microscopy further proved that BME-Ti 3 C 2 T x retained its original morphology after 4 m ( Figure S4, Supporting Information). This reflects the long-term stability of BME-Ti 3 C 2 T x in humid air, and BME did not affect the dispersion of 2D Ti 3 C 2 T x . Future MXene chemistry may involve high temperatures. Thus, we further analyzed the stabilities of Ti 3 C 2 T x and BME-Ti 3 C 2 T x in H 2 O at 100°C. Figure 1d presents the relationship between the concentration of residual MXene and aging time, and the plots were well fitted by single-exponential decay (the dash lines). Based on the Lambert-Beer law, the concentration of residual MXene was estimated by the maximum absorption peak at %750 nm in the UV-vis-NIR spectrum regardless of the gradual change in the extinction coefficient ( Figure S5, Supporting Information). Without BME, Ti 3 C 2 T x degraded extremely fast at the beginning, then tended to slow down as the oxide layer formed, and was almost completely oxidized in 100°C H 2 O within 3 h. Under the protection of BME, 24.3%, 35.8%, and 60.5% of Ti 3 C 2 T x survived in the 0.005, 0.05, and 0.5 M BME solutions after 12 h, respectively. Without or with a low amount of BME, the oxidation rate of MXene slows down with time, which is due to the gradual thickening of the oxidation layer hindering the oxidation reaction. While the almost linear degradation kinetics of 0.05 BME-Ti 3 C 2 T x and the slow initial oxidation rate of 0.5 BME-Ti 3 C 2 T x suggest that BME participates in the oxidation reaction and is likely to be oxidized preferentially to Ti 3 C 2 T x . Additionally, the 0.5 BME-Ti 3 C 2 T x after 12 h reaction at high temperature was further stored at room temperature for 14 d, and its UV-vis-NIR spectrum showed only negligible changes ( Figure S6, Supporting Information), revealing that BME can still protect Ti 3 C 2 T x after high temperature. Therefore, BME is expected to provide high-temperature chemical reactions and high-temperature applications of MXene.

Results and Discussion
Further characterizations were performed to investigate the antioxidation effect of BME using the aforementioned Ti 3 C 2 T x and 0.05 BME-Ti 3 C 2 T x samples. AFM images of pristine Ti 3 C 2 T x , Ti 3 C 2 T x -28 (Ti 3 C 2 T x in water after 28 d), and BME-Ti 3 C 2 T x -28 (Ti 3 C 2 T x in the 0.05 M BME solution after 28 d) are presented in Figure 2a from left to right, respectively, with relevant height profiles. The 2D nature of the Ti 3 C 2 T x monolayer was confirmed by the AFM image with a height of 1.65 nm. [28] While the Ti 3 C 2 T x -28 lost the 2D feature, exhibiting many particles with sizes of dozens of nanometers. With the protection of BME, even after storage at room temperature for 28 d, the 2D feature of the Ti 3 C 2 T x nanosheet was completely retained. The Figure 1. a) Schematic of dispersed 2D Ti 3 C 2 T x nanosheets, molecular structure of β-mercaptoethanol (BME), and BME-Ti 3 C 2 T x composite structure to prevent Ti 3 C 2 T x attack by O 2 or H 2 O. b) Digital photographs of the freshly synthesized Ti 3 C 2 T x and 0, 0.005, 0.05, and 0.5 BME-Ti 3 C 2 T x samples after 28 d of storage at room temperature. c) Optical images of the Ti 3 C 2 T x and 0.05 BME-Ti 3 C 2 T x flakes on substrates in humid air after 4 m. d) Relationship between the normalized concentration of the 0, 0.005, 0.05, and 0.5 BME-Ti 3 C 2 T x aqueous solutions and aging time at 100°C. Scale bars: 5 μm.
www.advancedsciencenews.com www.small-science-journal.com height of BME-Ti 3 C 2 T x increased to 2.17 nm owing to excessive BME adsorption on the surface of Ti 3 C 2 T x . Further, the oxidation states of the Ti 3 C 2 T x nanosheets were studied using XPS. Figure 2b shows high-resolution XPS spectra of Ti 2p in pristine Ti 3 C 2 T x , Ti 3 C 2 T x -28, and BME-Ti 3 C 2 T x -28. The Ti 2p spectra were fitted to Ti─C, Ti 2þ , Ti 3þ , TiO 2 , and C─TiF x peaks. The peak at a% 459.00 eV can be attributed to TiO 2 , which indicates the oxidation degree of MXene. [29] For both pristine Ti 3 C 2 T x and BME-Ti 3 C 2 T x -28 samples, the small TiO 2 peaks reflect the low atomic percentage in the Ti 2p region of %10.04 and 12.45 atom%, respectively (Table S1, Supporting Information, presents the atomic ratio obtained by the Ti 2p peak fitting). Nevertheless, for the sample stored in pure water for 28 d, the high TiO 2 peak reflects a considerably higher atomic percentage of %58.32 atom % in the Ti 2p region, which reveals the severe oxidation of Ti 3 C 2 T x in water without BME ( Figure S7, Supporting Information, depicts the C 1s and O 1s core levels). To further analyze the effectiveness of BME in stabilizing the Ti 3 C 2 T x lattice, TEM measurements were conducted to characterize the morphologies of pristine Ti 3 C 2 T x , Ti 3 C 2 T x -28, and 0.005 and 0.05 BME-Ti 3 C 2 T x -28, as shown in Figure 2c and S8, Supporting Information. After oxidation in pure water for 28 d, a large number of aggregated large particles appeared on the surface of the survived Ti 3 C 2 T x , consistent with the AFM image, which are probably TiO 2 particles based on the XPS analysis. The Ti 3 C 2 T x nanosheet in the BME solution stored for 28 d still maintained its 2D structure, with an intact lattice like that of a pristine Ti 3 C 2 T x nanosheet according to the fast Fourier transform pattern (inset of Figure 2c). [30] In addition, Raman spectroscopy was used to monitor the oxidation state of Ti 3 C 2 T x , as shown in Figure S9, Supporting Information. The characterized A 1g (Ti, O, C) vibrational mode of Ti 3 C 2 T x was detected at %200 cm À1 in pristine Ti 3 C 2 T x , Ti 3 C 2 T x -28, and 0.05 BME-Ti 3 C 2 T x -28. Another obvious peak at %150 cm À1 appeared only in the Ti 3 C 2 T x -28 sample and was attributed to the E 1g mode of anatase TiO 2 formed by the oxidation reaction. [17a,31] Furthermore, the 0.05 BME-Ti 3 C 2 T x sample stored as long as 2 m (BME-Ti 3 C 2 T x -60) was also characterized by the AFM, XPS, TEM, and Raman in Figure S10, Supporting Information. The 60 d sample retains typical features of Ti 3 C 2 T x although the oxidation degree is increased compared to 0.05 BME-Ti 3 C 2 T x -28 (as discussed in the Supporting Information), indicating that Ti 3 C 2 T x can survive for 2 m under the protection of BME. To reveal the mechanism by which BME mitigates the Ti 3 C 2 T x degradation, DFT calculations [32] were performed to investigate  the interaction between ─SH groups of BME and Ti 3 C 2 T x , including the positively charged unpassivated titanium atoms at the edge, the typical single Ti atom defect site and functional groups on the surface of Ti 3 C 2 T x . Figure 3a presents the optimal structure of BME at the edge of the OH-terminated Ti 3 C 2 T x based on DFT calculations (refer to the DFT calculations in Experimental Section for details). The sulfur atom was aligned between two unsaturated titanium atoms with a Ti─S bond with a bond length of 2.50 Å and binding energy of À1.70 eV. The OH-terminated MXene plays a major role in the oxidation reaction because the stability of MXene follows OH < ─F < ¼O as the respective groups. [33] In addition, most ─OH functional groups disappeared in the O 1s core level of Ti 3 C 2 T x -28 ( Figure 2b). The three optimized BME adsorption conformations on the surface of OH-terminated Ti 3 C 2 T x are shown in Figure S11, Supporting Information. The ─SH groups of BME tend to interact with the Ti 3 C 2 (OH) 2 surface through the S···H hydrogen bond at a binding energy of À0.68 eV. In addition, as the number of ¼O groups in the proposed MXene was large according to the XPS O 1s peak, the interaction between the BME and Ti 3 C 2 O 2 has been presented and discussed in Figure S12, Supporting Information. BME is preferentially adsorbed to the edge of MXene rather than to the surface for either Ti 3 C 2 (OH) 2 or Ti 3 C 2 O 2 . Thus, it can effectively mitigate the oxidation of MXene because the oxidation reaction starts at the edge rather than at the perfect surface. The other oxidation reaction site is the defect on the surface of MXene. Therefore, we constructed a typical single Ti atom defect in Ti 3 C 2 O 2 and calculated the interaction between the BME and the defect site. [34] As shown in Figure S13, Supporting Information, both ─SH and ─OH of BME have a strong binding energy, exceeding 1 eV on the defect of MXene. It is stronger than that in the case of a perfect surface and BME can still strongly bind to the edge of the defective MXene. Furthermore, the Ti─S bond was experimentally proved. As shown in Figure 3b, the S 2p peaks in the XPS spectra of the aged 0.005 BME-Ti 3 C 2 T x sample provide further evidence of the interaction between ─SH and Ti 3 C 2 T x . The peak at 162.5 eV and its shoulder at 163.6 eV is a vital signal, which represents the bound S atom with the bare Ti atom (i.e., forming Ti─S bonds) at the edge or defect of Ti 3 C 2 T x . [35] The peaks at 163.4 eV (164.6 eV for S 2p 1/2 ) were assigned to the disulfide bonds originated by the oxidation of thiol groups and unbound thiol groups. [36] Figure S14a-c, Supporting Information, shows the binding energies of the S 2p core levels in the XPS spectra of the pristine Ti 3 C 2 T x , 0.05 and 0.5 BME-Ti 3 C 2 T x , respectively. The XRD patterns shown in Figure 3c reveal the (002) peaks of pristine Ti 3 C 2 T x and BME-Ti 3 C 2 T x in the range of 4°-30°, corresponding to the d spacing between the Ti 3 C 2 T x layers. [37] As the amount of BME increases, the interlayer spacing of Ti 3 C 2 T x increases from 1.27 nm for the pristine Ti 3 C 2 T x to 1.28, 1.46, and 1.48 nm for the 0.005, 0.05, and 0.5 BME-Ti 3 C 2 T x , respectively (Table S2, Supporting Information, summarizes the layer distances). Notably, the layer spacings of the pristine Ti 3 C 2 T x and 0.005 BME-Ti 3 C 2 T x are almost equal, because a small amount of BME is preferentially combined with the edge of Ti 3 C 2 T x . The 0.2 nm increase in layer spacing for 0.05 BME-Ti 3 C 2 T x compared to pristine Ti 3 C 2 T x may result from more BME adsorbed on the defective (preferentially) and perfect Figure 3. a) Configuration structure of the BME adsorbed to the Ti 3 C 2 (OH) 2 edge with the corresponding Ti─S bond length and binding energy (E be ). b) S 2p XPS spectra of 0.005 BME-Ti 3 C 2 T x . c) X-ray diffraction (XRD) patterns of pristine Ti 3 C 2 T x and BME-Ti 3 C 2 T x (0.005, 0.05, and 0.5) in a range of diffraction angles from 4°to 30°. d) Schematic of the pristine Ti 3 C 2 T x /BME-Ti 3 C 2 T x -based field-effect transistor (FET) device. e) Conductivity changes of Ti 3 C 2 T x and 0.005 and 0.05 BME-Ti 3 C 2 T x flakes on SiO 2 substrates in humid air over time. Ti 3 C 2 T x surface, but some BME molecules may be removed during the drying process of prepared XRD samples. The (002) peak of 0.5 BME-Ti 3 C 2 T x split into two peaks at 5.98°a nd 4.72°by the uneven intercalation of excessive BME. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted to analyze the thermal stability of the BME-Ti 3 C 2 T x and, in particular, the desorption process of BME. Figure S15a, Supporting Information, shows an additional weight loss of %3% occurred at 160-210°C in BME-Ti 3 C 2 T x as compared with pristine Ti 3 C 2 T x , which was related to the evaporation and desorption of BME. The DSC peaks ( Figure S15b, Supporting Information) of BME-Ti 3 C 2 T x at 102.7, 160.4, 176.7, and 209.5°C could be attributed to the evaporation of H 2 O and free BME, and to the desorption of BME on the surface and at the edge of Ti 3 C 2 T x , respectively. Electron paramagnetic resonance (EPR) was used to study the single electrons in pristine Ti 3 C 2 T x and BME-Ti 3 C 2 T x , as shown in Figure S16, Supporting Information. Consistent with the previous study, [38] both pristine Ti 3 C 2 T x and BME-Ti 3 C 2 T x showed negligible EPR signals, indicating that the strong binding energy between BME and Ti 3 C 2 T x edges originated from the electron donation of sulfhydryl groups rather than any possible radical reaction.
To quantitatively study the humid air stability and electrical properties of single-layer pristine Ti 3 C 2 T x and BME-Ti 3 C 2 T x , FET devices were fabricated using MXene as a channel. Two Au electrodes were thermally deposited to contact MXene on the Si/SiO 2 substrate, as shown in Figure 3d. The transfer and output characteristics of the pristine Ti 3 C 2 T x -and BME-Ti 3 C 2 T x -based FETs are presented in Figure S17a,b, Supporting Information, respectively. Consistent with the transfer curve, the output curve exhibits a slight n-type characteristic, and the ultra-linear I d -V d plot indicates Ohmic contact between the Au electrodes and (BMEÀ)Ti 3 C 2 T x ; thus, the conductivity can be directly calculated from the current. Figure 3e shows the changes in electrical conductivity of pristine Ti 3 C 2 T x and 0.005 and 0.05 BME-Ti 3 C 2 T x over time for 28 and 60 d, respectively. Previous studies [39] have extensively demonstrated the effect of molecular doping on the Ti 3 C 2 T x conductivity, and the current experiments indicate that incorporating BME with Ti 3 C 2 T x slightly improves the conductivity from 3534.1 to 4371.7 and 4514.9 S cm À1 for 0.005 and 0.05 BME-Ti 3 C 2 T x , respectively. In addition, Ti 3 C 2 T x degenerates into an insulator (0.015 S cm À1 ) after 7 d owing to oxidation, whereas 0.005 and 0.05 BME-Ti 3 C 2 T x retain excellent conductivities (96.5 and 1082.8 S cm À1 ) even after 28 d. Furthermore, 0.05 BME-Ti 3 C 2 T x retains a strong conductivity of 92.6 S cm À1 after 60 d, confirming the remarkable stability of BME-Ti 3 C 2 T x in humid air. The degradation of BME-Ti 3 C 2 T x in air also followed www.advancedsciencenews.com www.small-science-journal.com the exponential decay, and the accelerated degradation rate over time may be due to the gradual desorption of BME. Those results demonstrate that BME-Ti 3 C 2 T x can be easily fabricated onto 2D flakes on SiO 2 substrates, and the potential of BME to continuously protect Ti 3 C 2 T x in subsequent applications. Figure 4a presents work function maps of the pristine Ti 3 C 2 T x and 0.005 BME-Ti 3 C 2 T x sheets on the Si substrates obtained by Kelvin probe force microscopy (KPFM) measurements. [40] An evident color contrast is observed, from red (pristine Ti 3 C 2 T x ) to baby blue (BME-Ti 3 C 2 T x ), indicating the successful tuning of the work function. The bottom panel of Figure 4a shows work function distributions in the selected area of the pristine Ti 3 C 2 T x (red), Si substrate (green), and 0.005 BME-Ti 3 C 2 T x (blue), and their average work functions determined by Gaussian fitting are 4.70, 4.49, and 4.39 eV, respectively. The change of work function can be attributed to the electron-donating effect of BME, which changes the surface dipole moment of Ti 3 C 2 T x and the doping of the conductive layer. Figure S18a,b, Supporting Information, presents a work function map of 0.05 BME-Ti 3 C 2 T x and statistical work function (4.40 eV), which indicates that the doping of Ti 3 C 2 T x is saturated by 0.005 M BME. Owing to the 0.31 eV amplitude modulation, the work function of BME-Ti 3 C 2 T x is very close to the conduction band (CB) or lowest unoccupied molecular orbital of the n-type 2H-phase MoS 2 . Figure 4b presents a schematic band diagram of the pristine Ti 3 C 2 T x , BME-Ti 3 C 2 T x , and multi-layered MoS 2 . The work function of BME-Ti 3 C 2 T x matches the CB of MoS 2 (%4.4 eV), [41] which facilitates the barrier-free injection of electrons. In contrast, when electrons are injected from the pristine Ti 3 C 2 T x into MoS 2 , an energy barrier must be overcome. In the case of hole transport, BME-Ti 3 C 2 T x encounters larger obstacles than pristine Ti 3 C 2 T x to inject holes into the valence band (VB, %6.2 eV) of MoS 2 . [42] To verify this claim, MoS 2 FET devices were fabricated to compare the qualities of pristine Ti 3 C 2 T x and BME-Ti 3 C 2 T x as source electrodes, as shown in Figure 4c. The fabrication is described in Experimental Section. The bottom panel of Figure 4c shows an OM image of a typical MoS 2 FET device with a BME-Ti 3 C 2 T x source electrode. Figure 4d compares the transfer curves of the as-fabricated FETs using Ti 3 C 2 T x (black curve) and BME-Ti 3 C 2 T x (blue curve) electrodes. The BME-Ti 3 C 2 T x exhibits a high on/off ratio of 10 6 with an ON current 10 times higher than that of pristine Ti 3 C 2 T x . Figure S20, Supporting Information, depicts the output curves at various gate voltages. A nonlinear I d -V d for a low V d is observed in the pristine Ti 3 C 2 T x based FETs, indicating a typical Schottky contact, while a linear I d -V d supports Ohmic contact between BME-Ti 3 C 2 T x and MoS 2 . In addition, Figure S21, Supporting Information, shows that the performance of the FETs with the monolayer BME-Ti 3 C 2 T x electrodes is comparable to that of conventional thick Cr/Au (5/50 nm) electrodes, demonstrating the potential of BME-Ti 3 C 2 T x as a 2D electrode in future electronics.

Conclusion
We have demonstrated that the BME treatment could suppress the oxidation of Ti 3 C 2 T x MXene, in the colloidal dispersion (2 m), under humid air conditions (2 m), or even in harsh 100°C water (12 h). The absorption mechanism between BME and Ti 3 C 2 T x was investigated by comprehensive DFT calculations and experimental characterizations, revealing BME has strong interaction with both the edge (through Ti─S bond, binding energy À1.70 eV) and defect (by hydrogen bond, binding energy À1.05 eV) of Ti 3 C 2 T x , which significantly suppresses the degradation of Ti 3 C 2 T x . The work function of Ti 3 C 2 T x was successfully tuned from 4.70 to 4.39 eV by the BME treatment because of the electron-donating effect of sulfhydryl groups. BME-Ti 3 C 2 T x showed excellent electrical properties and much better humid air stability than Ti 3 C 2 T x in as-fabricated FET devices, and served as a better source electrode material than pristine Ti 3 C 2 T x in n-type MoS 2 FETs because of the band alignment effect. The described methodology can largely contribute to the long-term storage and service life of Ti 3 C 2 T x and simultaneously cover every aspect (in comparison to recently published antioxidant methods, see Table S3, Supporting Information), thus promoting the practical application of MXene. In addition, the elucidation of the strong interaction between sulfhydryl groups with MXene and the improved stability of MXene at high temperatures are expected to inspire related studies and further expand the research field of MXene.

Experimental Section
Synthesis of Ti 3 C 2 T x : To remove the Al layer in Ti 3 AlC 2 by chemical etching, 1 g of Ti 3 AlC 2 was slowly added to the etchant (precooled to below 5°C) containing 1.6 g of LiF (Sigma Aldrich), 5 mL of deionized H 2 O, and 15 mL of 12 M HCl (hydrochloric acid, Sigma Aldrich) in a 100 mL Teflon beaker. The mixture with a Teflon stirring bar was treated by N 2 blowing and then sealed prior to being placed in an ice bath and stirred at 500 rpm for 1 h. Subsequently, the reaction was continued at 25°C for 36 h. The product was then washed with centrifugation (5-6 cycles, at 3500 rpm) and manual shaking until the pH of the supernatant became neutral. The mixture was then centrifuged at 3500 rpm for 1 h, and the resulting colloid and swollen upper black precipitate were collected for follow-up experiments. The concentration of the as-prepared Ti 3 C 2 T x dispersion was determined to be approximately 13 mg mL À1 by the freezedrying technology. [43] Protection of Ti 3 C 2 T x by BME: Typically, in a 10 mL vial, 2.5 mL of Ti 3 C 2 T x obtained as described earlier was diluted with 7.5 mL of deionized water or BME solution, which yielded final BME concentrations in the 10 mL Ti 3 C 2 T x colloid of 0.005, 0.05, and 0.5 mol L À1 . The ratios of BME and Ti 3 C 2 T x were %0.12:1 (0.005 BME-Ti 3 C 2 T x ), 1.2:1 (0.05 BME-Ti 3 C 2 T x ), and 12:1 (0.5 BME-Ti 3 C 2 T x ), respectively. Thereafter, manual shaking was performed for %10 min to thoroughly mix Ti 3 C 2 T x and BME, followed by sonication in an ice water bath for 3 min to separate the oxidized Ti 3 C 2 T x maximally. The floating impurities on the dispersion surface were carefully removed and, finally, the vials were capped and stored in a dark place.
Device Fabrication: The highly doped n-type Si and thermally grown 285 nm thick SiO 2 served as the bottom gate and dielectric layer, respectively. The ten times diluted pristine Ti 3 C 2 T x or BME-Ti 3 C 2 T x dispersion was spin-coated on the O 2 plasma-treated substrate. An MXene flake with a regular shape was selected through OM and determined as a monolayer through AFM. For the fabrication of MXene-based FET devices, poly (methyl methacrylate) (PMMA) was coated onto the pristine Ti 3 C 2 T x and BME-Ti 3 C 2 T x samples, followed by electron-beam lithography (EBL) to pattern the source-drain electrodes. Subsequently, Cr (5 nm)/ Au (50 nm) was deposited through e-beam evaporation. For the fabrication of MoS 2 -MXene FETs, multilayer MoS 2 flakes were mechanically exfoliated from bulk MoS 2 crystals. High-quality MoS 2 flakes with suitable sizes (thicknesses % 20 nm) were attached to a polydimethylsiloxane (PDMS)