Pure oxygen terminated MXenes prepared by NaOH/NaI eutectic molten salt

Abstract

MXenes, a promising class of two-dimensional materials derived from transition metal carbides and nitrides, are attracting considerable attention for their remarkable properties and potential uses in energy storage, EMI shielding, electronics, etc. Modifying surface terminations is essential for tuning the electron structure of MXenes, significantly affecting their properties. However, existing methods for altering the surface chemistry, particularly regarding oxygen termination, encounter significant challenges, such as low substitution efficiency and self-oxidation during treatment, resulting in the absence of pure O-terminated MXenes in the literature. Here, we present a general NaOH/NaI molten salt modification strategy that successfully prepares pure O-terminated MXenes. The effectiveness of this reaction stems from the naked OH ions in the molten salt, along with elevated temperatures that facilitate substitution, all while remaining below the temperatures that cause structural degradation and self-oxidation of MXenes. Consequently, the intrinsic properties of pure O-terminated MXenes undergo significant changes, including continuous tunability of the work function (3.67 to 4.17 eV) and distinct temperature-dependent resistance behavior. In addition, pure O-terminated Ti₃C₂ demonstrates impressive performance for Li⁺ storage, achieving a high specific capacity of 258 mAh g⁻¹ at 0.1 A g⁻¹, which is more than two times that of pristine Ti₃C₂T_x. In addition, after 5000 cycles at a current density of 2 A g⁻¹, the capacity retention of pure O-terminated Ti₃C₂ is 97%. Our results not only demonstrate the possibility of obtaining pure O-terminated MXenes but also validate the importance of pure O terminations on tuning the properties and boosting the performance of MXenes in various fields.

Graphical abstract

1. Introduction

MXenes have attracted significant attention for applications in EMI shielding [[1], [2], [3]], energy storage [[4], [5], [6], [7]], catalysis [8,9], optoelectronics [10,11], etc., due to their superior hydrophilicity, metallic conductivity, and ease of processing [[12], [13], [14]]. MXenes have a general formula of M_n+1X_nT_x (where n = 1–4), with M representing early transition metals (such as Ti, Nb, Mo, V, and W), X as carbon and/or nitrogen, and T_x denoting surface terminations [[15], [16], [17], [18]]. These terminations critically influence intrinsic properties such as biaxial lattice strain [16,17,19], density of states at the Fermi level [20], work function [21], band gap [21], electron-phonon coupling strength of MXenes, etc. Among various terminations (including but not limited to O, F, OH, Cl, Br, I, NH, S, and Se) [19], O-terminated MXenes present significant advantages across various fields due to the high electronegativity of oxygen to form a strong M (transition metal)-O bond [22]. This strong bond usually leads to the transfer of electronic states in the MXene structure, which results in an increase in valence band energy due to the presence of O 2p states [23]. Besides, in energy storage, O-terminated MXenes are predicted to have a low lithium-ion barrier that can significantly enhance capacity and cycling stability in lithium-ion batteries [24]. Their exceptional electronic properties further enable them to serve as efficient electrode materials, capable of accommodating rapid charge/discharge cycles. In the realm of catalysis, O-terminated MXenes demonstrate superior catalytic activity due to their favorable surface chemistry and electronic structures, which are able to optimize interactions for specific key reactions such as hydrogen evolution and CO₂ reduction, thereby contributing to sustainable energy solutions [25]. In addition, the tunable work function of MXenes (by adjusting the percentage of O-terminations) allows for tailored charge injection in electronic devices [26], remarkably improving the performance of perovskite solar cells [26], sensors [27], etc. More significantly, theoretical studies have predicted that some metallic Mxenes, such as Ti₂C, are capable of converting into a semiconductor if the surface is capped by pure O terminations, showing enormous modulation potential in optoelectronic applications [28,29]. These multifaceted features underscore the significance and versatility of O-terminated MXenes, making them integral across diverse applications. Unfortunately, the current attempts to realize pure O-terminated MXenes remain in the early stage, severely hindering their applications both in fundamental research and practical applications.

We would like to note that the ideal O-terminated MXenes should meet two criteria: first, a very high percentage of O termination with the absence of other terminations; second, an extremely low percentage of oxides. Currently, the prevalent approaches to preparing O-terminated MXenes can be primarily classified into two strategies. The first one involves alkaline solution modification (such as LiOH, NaOH, or KOH), where OH ions replace surface F terminations and subsequently convert to O terminations (Fig. 1a). However, this method faces challenges with low substitution efficiency, typically around 50%, as the first step of F termination substitution by OH often occurs at room temperature. If the reaction temperature is raised above 60 °C, the combination of water and NaOH can quickly lead to the oxidation of MXene [25,30]. The second method is tube furnace heating treatment, which removes a large amount of F terminations at a temperature above 700 °C (Fig. 1a) [31]. However, this process leads to an increase in MXene surface defects when the temperature exceeds 500 °C [32]. Worse still, the interlayer water and oxygen-containing termination of MXene will react with metal atoms on the surface of MXene at high temperature (above 700 °C) to form metal oxides [33]. In this context, it is a huge challenge to obtain pure O-terminated MXenes while preserving their structural integrity in the absence of oxides in the traditional methods. Accordingly, pursuing a new suitable reaction system is highly demanded to control the terminations of MXenes for precise tuning of the properties in fundamental research and performance boosting in practical applications.

Fig. 1.

Preparation and characterization of pure O-terminated Ti₃C₂. (a) Schematic of the traditional O termination modification method and (b) our novel method based on NaOH/NaI molten salt. High resolution XPS spectra of (c) O 1s and (d) Ti 2p of pure O-terminated Ti₃C₂, respectively. (e) Ratio of O termination/TiO₂ (Ti 2p) of Ti₃C₂T_x prepared by NaOH/NaI molten salt, NaOH aqueous solution and tube furnace heating, respectively. (f) XRD patterns of Ti₃C₂T_x and pure O-terminated Ti₃C₂ (inset is a magnified view of the 002 peaks. (g) Atomic-resolution high-angle annular dark-field (HAADF) TEM image of pure O-terminated Ti₃C₂. (h) TEM image of pure O-terminated Ti₃C₂. The inset is the diffraction pattern.

Here, we present a general NaOH/NaI molten salt modification strategy to prepare pure O-terminated MXenes. In the NaOH/NaI molten salt system, the naked OH ions undergo a substitution reaction with the F termination on the MXene surface, which is subsequently converted to the O termination at 380 °C. A key advantage of this reaction is that the molten salt environment prevents the presence of water, while also enabling higher temperatures that facilitate the substitution process. Importantly, all the reacting processes are taking place well within a mild temperature range, which is much lower than the temperature that causes structural degradation and self-oxidation of MXene [31]. As a result, the intrinsic properties of the resulting pure O-terminated MXenes exhibit significant changes. We achieved continuous tunability of the work function for Ti₃C₂ through modification with different percentages of O termination, and the temperature-dependent resistance behavior of O-terminated Nb₄C₃ differs markedly from that of the pristine one. Additionally, the pure O-terminated Ti₃C₂ is demonstrated as the electrode material for Li⁺ storage, which shows a high specific capacity of 258 mAh g⁻¹ at 0.1 A g⁻¹ (more than two times higher than pristine Ti₃C₂T_x), accompanied by high-rate capability. Notably, this method is broadly applicable to various MXenes, including both mono- and double-metal, as well as 21, 32, and 43 phase MXenes.

2. Materials and methods

2.1. Synthesis of multilayer MXenes

Multilayer MXenes were prepared by selectively etching the A element from MAX (the method for the synthesis of MAX is in Supporting Information). Ti₃C₂T_x: 1 g of Ti₃AlC₂ powder was added to 10 mL HF (30 wt.%, Sigma-Aldrich), followed by stirring for 24 h at room temperature (RT) at 400 rpm. Nb₄C₃T_x: 1 g of Nb₄AlC₃ powder was added to 10 mL HF and stirred at RT for 100 h Mo₂Ti₂C₃T_x: 1 g of Mo₂Ti₂AlC₃ powder was added to 10 mL HF and stirred at 55 °C for 96 h Mo₂CT_x: 1 g of Mo₂Ga₂C powder was added to 20 mL HF and stirred at 55 °C for 100 h The various multilayer MXenes powders were operated by washing with deionized (DI) water for 5–6 times, until the pH of the supernatant reached ∼6. All multilayer MXenes powders were vacuum dried at 60 °C for 8 h for further use in the molten salt modification reaction.

2.2. Synthesis of pure O-terminated MXenes

All experiments were carried out in an argon atmosphere. Pure O-terminated Ti₃C₂: 200 mg of Ti₃C₂T_x powder and molten salt (about 3 g) with a composition of NaOH (98%, Macklin): NaI (99.9%, Macklin) = 63.6: 36.4 mol % were mixed and ground for 10 min. The mixture was heated at 380 °C for 1 h in a graphite crucible under argon with a heating rate of 5 °C min⁻¹. After cooling, pure O-terminated MXenes were washed with deionized water until the pH of the supernatant was about 8. For pure O-terminated Nb₄C₃, Mo₂Ti₂C₃ and Mo₂C, the reaction temperature of the tube furnace was 220 °C (NaOH: NaI = 75.6: 24.4 mol %) for 30 min. It should be noted here that by adjusting the molar ratio between NaOH and NaI, the melting point of the molten salt system can vary from about 220 to 650 °C, which is more conducive to selecting the temperature to treat different MXenes to avoid oxidation. And after washing, the obtained Nb₄C₃, Mo₂Ti₂C₃ and Mo₂C MXenes powders were heated at 380 °C for 1 h in order to convert OH to O terminations.

2.3. Synthesis of O-terminated Ti₃C₂ by NaOH aqueous solution and tube furnace heating methods

For the NaOH solution method, 200 mg of Ti₃C₂T_x was added to 1 M NaOH aqueous solution and n-decyl alcohol solution, respectively, and stirred for 12 h at room temperature. MXenes were washed with deionized water until the pH of the supernatant was about 8. And after washing, the obtained Ti₃C₂ powder was heated at 380 °C for 1 h in order to convert OH to O terminations (named Ti₃C_2—NaOH-AS-380 °C). For the tube furnace heating method, 200 mg of Ti₃C₂T_x was heated in a tube furnace at 700 °C for 4 h under argon with a heating rate of 5 °C min⁻¹. After cooling, the obtained MXene is named Ti₃C₂–700 °C–4 h.

2.4. Material characterization

The crystal structure was analyzed using X-ray diffraction (XRD) patterns (D/max 2600). To examine the sample morphology, scanning electron microscopy (SEM, JSM-7600F) coupled with energy dispersive spectroscopy (EDS) was employed. Transmission electron microscopy (TEM) images were captured using a Tecnai G2 F20 S-TWIN (FEI) operating at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed with a PHI VersaProbe 5000 instrument (Physical Electronics), using a 100 µm spot size and a 25 W monochromatic Al-Kα (1486.6 eV) X-ray source. The work function of the MXenes was determined via ultraviolet photoelectron spectroscopy (Thermo ESCALAB 250XI). Temperature-dependent resistance measurements of the MXenes were conducted using a standard four-probe method in a Physical Property Measurement System (PPMS, Quantum Design). Thermogravimetric analysis with differential scanning calorimetry (TG-DSC) curves was obtained under an oxygen atmosphere using TGA/DSC 3+ (Mettler Toledo).

2.5. Electrochemical measurements

The electrodes were prepared by mixing anode material, acetylene black, and a 5% PVDF solution in N-methyl-2-pyrrolidone (NMP) at a mass ratio of 8:1:1. The resulting paste was evenly coated onto copper foil using a squeegee. The copper foil was then dried in a vacuum at 60 °C for 12 h. After drying, the copper foil was cut into 13 mm diameter discs to be used as electrodes. Lithium sheets served as the counter electrodes. The active material loading was approximately 0.7 mg cm⁻². The electrolyte consisted of a 1 M LiPF6 solution in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl carbonate (EMC) in a 1:1:1 vol ratio. The cells were assembled in an argon-filled glove box with oxygen and water levels maintained below 0.01 ppm. The electrodes were incorporated into CR2016 coin-type cells, which were then tested for electrical performance using an automatic battery tester (LAN-2100) at room temperature. Cyclic voltammetry (CV) profiles were recorded on an electrochemical workstation (BioLogic, France), and electrochemical impedance spectroscopy (EIS) was conducted with a frequency range of 100 kHz to 10 mHz and an AC perturbation amplitude of 5 mV. The specific capacitance and capacity from the CV profiles are calculated using the following formulas:

$$C=\frac{{\int }_0^{\mathrm{V}/\mathrm{s}}\left|i\right|d\mathrm{t}}{\mathrm{m}}$$ (1)

$$Q=\text{CV}$$ (2)

$${\mathrm{Q}}_{\mathrm{m}}=\frac{{\mathrm{Q}}_{\mathrm{c}}}{3.6}$$ (3)

where C represents the specific capacitance (F g⁻¹), i (A) represents the current changed by time t (s), m is the active material mass (g), V is the voltage window (V), s is the scan rate (V s⁻¹), and m is the active material mass (g). Additionally, Q_c (C g⁻¹) and Q_m (mAh g⁻¹) are the specific capacities.

3. Results and discussion

3.1. Preparation and characterization of pure O-terminated Ti₃C₂

Ti₃C₂T_x was first synthesized by selectively etching Ti₃AlC₂ MAX phase following the reported methods [1]. A NaOH/NaI eutectic molten salt system is utilized to modify the as-synthesized Ti₃C₂T_x. By virtue of high-concentration OH ions (15 M) in the molten salt and the suitable melting point (about 380 °C, which is high enough to promote the termination substitution while much lower than Ti₃C₂T_x oxidation temperature), OH ions first react with F termination, and the substituted F ions are combined with Na ions to form NaF. At this intermediate stage, the surface of Ti₃C₂T_x is covered with a mixture of OH and O terminations (Fig. 1b). As the heating time increases, the OH terminations gradually convert into O terminations at 380 °C [29,34,35]. According to X-ray photoelectron spectrum (XPS), after treatment with NaOH/NaI, the peak intensity of the F element on Ti₃C₂T_x nearly disappears, while the peak intensity of the O element increases significantly (Fig. S1). Only 1.57 at % of F element remains, which is attributed to AlF₃ residue and/or TiO_2-xF_x rather than F termination (F 1s spectrum, Fig. S2). Based on high-resolution XPS spectra of O 1s (Fig. 1c), we find that the high percentage of O (28.48 at %) is attributed to the O terminations on the surface of Ti₃C₂T_x. The Ti-O(I) and Ti-O(II) bonds correspond to O terminations located at different sites on the Ti₃C₂T_x surface. As shown in Fig. S3, the Ti-O(I) bond stands for O termination located directly above the central Ti atoms (site A, indicated by a circle), whereas the Ti-O(II) bond features O termination positioned above the central carbon atoms (site B, indicated by a square) [31].

Importantly, compared with the O 1s spectrum, the Ti 2p spectrum is more suitable for the analysis of the surface terminations because the metal (M) component is highly sensitive to oxidation states [30]. Therefore, we further use the high-resolution XPS of Ti 2p to calculate the TiO₂ percentage (Fig. 1d). It should be noted that there are still 2.1% of TiO₂ (Ti 2p) present in our sample, however, such amount is almost equivalent to that of the pristine Ti₃C₂T_x (Table S2, and the XPS spectrum of Ti 2p of Ti₃C₂T_x was shown in Fig. S4, see Table S3 for fitting details), indicating that the NaOH/NaI molten salt modification method doesn’t lead to further oxidation of the Ti₃C₂T_x (the original TiO₂ on the Ti₃C₂T_x surface arises from the loss of electrons from Ti atoms during the etching process). As mentioned in the introduction, two criteria should be considered to evaluate the purity of O termination on Ti₃C₂T_x: 1) a very high percentage of O termination with the absence of other terminations, and 2) an extremely low percentage of oxides. Accordingly, we further propose to use the content ratio of O termination to TiO₂ (Ti 2p) to quantitatively evaluate the purity of O-terminations. It is found that the O termination/TiO₂ (Ti 2p) ratio of the NaOH/NaI molten salt modification method is 8.4, which is more than an order of magnitude higher than that of the NaOH aqueous solution modification method (0.59, Fig. 1e). Such a large difference highlights the advantages of the NaOH/NaI molten salt modification strategy. In addition, the substitution of the O termination along with the insertion of the Na cations leads to an increase in the interlayer spacing of the pure O-terminated Ti₃C₂, from 9.77 to 12.22 Å (X-ray diffraction pattern, Fig. 1f). Meanwhile, a highly open structured accordion-like morphology could be observed for multilayered pure O-terminated Ti₃C₂ according to scanning electron microscopy (SEM, Fig. S5a). Such an increased interlayer spacing facilitates the exfoliation of multilayered pure O-terminated Ti₃C₂, as we successfully obtain the mono/few-layer pure O-terminated Ti₃C₂ using a straightforward ultrasonic method (Fig. S5b). Furthermore, scanning transmission electron microscopy (STEM) analysis reveals that in O-terminated MXene, the three brightest middle layers correspond to Ti atoms, while the atoms on either side represent the surface termination sites (Fig. 1g). Notably, due to the low atomic number of oxygen element, the O atoms exhibit negligible brightness in the HAADF (high-angle annular dark-field) image. Nevertheless, the surface terminations can be seen neatly arranged on both sides of the middle three layers of Ti atoms. The structure of the pure O-terminated Ti₃C₂ remains intact after the treatment with NaOH/NaI molten salt, based on the analysis of transmission electron microscopy (TEM) and selected area diffraction pattern (SAED) (Fig. 1h).

3.2. Design principle and mechanism of NaOH/NaI molten salt modification strategy

To clarify the design criteria of precise O termination synthesis and provide guidance for subsequent preparation, we deeply analyze the traditional O termination modification methods, including MXene treatment with NaOH solution and heating in a tube furnace. In the NaOH solution method, OH ions first replace the F terminations on the surface of Ti₃C₂T_x, resulting in a mixture of OH and O terminations. Then the OH termination is subsequently converted to O termination upon heating at 380 °C [29,34]. However, we find that, after modification, there is still a substantial amount of F termination remaining, along with a sharp rise in the percentage of TiO₂ (Ti 2p) according to the XPS analysis (increase by 7.1%, Fig. 2a). This is also confirmed by the observation of small particles on the surface of Ti₃C₂T_x (Fig. S6b). As a result, the chemical formula of Ti₃C₂T_x modified by NaOH aqueous solution method is Ti₃C_1.8O_1.48F_0.45 (the pristine Ti₃C₂T_x is Ti₃C_2.2(OH)_0.48O_0.69F_0.8, Fig. 2e). This result indicates that: 1) the termination substitution of F to OH process is incomplete, where the substitution efficiency is calculated to only 44% (see details in Methods); 2) the presence of water causes the oxidation of MXenes. It is a natural idea to prevent oxidation by changing the solvent to a nonaqueous one (such as n-decyl alcohol). However, although the TiO₂ percentage (Ti 2p) of Ti₃C₂T_x in the NaOH alcohol system is significantly lower than that in the NaOH aqueous solution (Fig. 2a), the O termination substitution efficiency is still not high (28%), which is apparently due to the low substitution (F to OH step) temperature. Notably, one cannot obtain pure O-terminated Ti₃C₂ by simply increasing the treatment temperature of NaOH/n-decyl alcohol solution because the boiling point of n-decyl alcohol is 233 °C (the complete conversion of OH to O terminations requires a temperature between 350–400 °C, and the solubility of NaOH in alcohols is also limited to <1M).

Fig. 2.

Precise design and mechanism of NaOH/NaI molten salt treatment. (a) High-resolution XPS spectra of Ti 2p of Ti₃C₂T_x prepared by NaOH aqueous solution and alcohol solution. (b) High-resolution XPS spectrum of Ti 2p of Ti₃C₂T_x prepared by tube furnace heating. (c) Raman spectrum of Ti₃C₂T_x, intermediate state of reaction (Ti₃C₂T_x-5 min) and pure O-terminated Ti₃C₂. (d) High-resolution XPS spectra of Ti 2p of pure O-terminated Ti₃C₂ with different treatment time. (e) Moles of T_x per M_n+1X_nT_x formula unit and the increased TiO₂ (Ti 2p) percentage of Ti₃C₂T_x and Ti₃C₂T_x prepared by NaOH aqueous solution and tube furnace heating, and NaOH/NaI molten salt, respectively. It is worth noting that because the surface Ti atoms lose electrons during etching, TiO₂ is found in the pristine Ti₃C₂T_x, which is an inevitable phenomenon. Therefore, we used the increased percentage of TiO₂ to evaluate the level of oxidation during the modification of O-terminated Ti₃C₂.

Another method for O termination modification involves heating Ti₃C₂T_x in a tube furnace, where a large amount of F termination is removed at temperatures exceeding 700 °C, allowing O to occupy the resulting vacancies [31]. In fact, after annealing Ti₃C₂T_x at 700 °C for 4 h (denoted as Ti₃C₂-700 °C-4 h) under argon atmosphere, although more O termination percentage is achieved (the substitution efficiency is 64%) compared to pristine Ti₃C₂T_x, the percentage of TiO₂ (Ti 2p) (4.6%) in the high-resolution spectrum of Ti 2p (Fig. 2b, chemical formula is Ti₃C₂O_1.2F_0.29) of Ti₃C₂–700 °C–4 h is more than double of pristine Ti₃C₂T_x (2.1%) (Fig. 2e). Interestingly, Ti₃C₂T_x with lower amount of TiO₂ (tube furnace method) possesses the lower conductivity compared with the Ti₃C₂ modified by NaOH aqueous solution method (Table S4, the value of Ti₃C₂-700 °C–4 h and Ti₃C₂-NaOH-380 ℃ is 330 S cm^-1 and 403 S cm^-1, respectively), implying that high-temperature method may lead to structural damage in Ti₃C₂T_x which is consistent with SEM morphology and the previous report (Fig. S6, the related XPS spectra of Ti₃C₂T_x modified by tube furnace and NaOH aqueous solution are shown in Fig. S7 and Fig. S8, see Table S5 for fitting details) [32].

Based on the above analysis, we believe that the ideal O-termination modification strategy should meet the following requirements: 1) providing the high concentrated OH ion environment for F to OH substitution, higher temperature is required for efficiency enhancement; 2) absence of water or other oxidants to prevent the oxidation of MXenes; 3) suitable treatment temperature above 350 °C for complete conversion from OH to O while much lower than 700 °C in case of MXene self-oxidation. In this context, we focus on the molten salt system, which offers a wide operating temperature range, unmatched anion and cation concentrations, and an oxygen-free environment. The high concentration of naked OH ions (15 M) in the NaOH/NaI molten salt system greatly enhances the F to OH substitution step, as calculated by the Arrhenius equation [7]. This is further promoted by the melting temperature of 380 °C, resulting in the complete conversion of OH to O terminations. Structural damage is effectively prevented (Fig. 1g and 1h) and the formation of TiO₂ on Ti₃C₂T_x is avoided by maintaining an oxygen-free environment.

A Raman spectrum is conducted to further analyze the process of our O-termination modification method. As shown in Fig. 2c, the range between 230 and 470 cm⁻¹ corresponds to the in-plane (E_g) vibrations of the surface terminations associated with Ti atoms [36,37]. Meanwhile, the region between 580 and 730 cm⁻¹ is commonly utilized for identifying surface terminations in electrochemical studies [38]. Based on the literature [[36], [37], [38]], we assigned the Raman peaks, with the blue and red regions corresponding to O and OH terminations, respectively. It shows that O and OH terminations co-exist on the surface of pristine Ti₃C₂T_x obtained by HF etching. After 5 min of reaction in a NaOH/NaI molten salt, the peaks for O and OH terminations increase significantly. This suggests that the OH ions in the molten salt have replaced the F termination, with a portion being converted into O termination. As the reaction time increases, the transformation of OH into O termination continues until we obtain Ti₃C₂T_x that consists entirely of O termination, a finding consistent with our XPS results. In addition, when Ti₃C₂T_x is treated in the NaOH/NaI molten salt system for varying durations (10 min, 30 min, 1 h, 2 h, 6 h), analysis of the XPS Ti 2p orbital peak splitting reveals that the intensity of the peak corresponding to TiO₂ at ∼459 eV remained unchanged (Fig. 2d) [39]. This indicates that the NaOH/NaI molten salt system does not oxidize the Ti₃C₂T_x during the reaction, regardless of treatment duration.

3.3. Universality of NaOH/NaI molten salt treatment for various MXenes

The universality of the NaOH/NaI molten salt surface modification strategy is explored for different MXenes beyond Ti₃C₂T_x. When Nb₄C₃T_x is treated under the same conditions as Ti₃C₂T_x (380 °C), however, partial oxidation is noted. The surface morphology displays a significant increase in particles compared to pristine Nb₄C₃T_x, as observed in the SEM image (Fig. S9). This is corroborated by a dramatic rise in Nb₂O₅ percentage in the XPS analysis (Fig. S10–12, see Table S6 for fitting details) [40], highlighting the different oxidation resistance among various types of MXenes. By adjusting the ratio of NaOH to NaI (the specific proportions are in Methods), the melting point of the molten salt system is reduced from 360 to 220 °C. At this milder temperature (220 °C), the SEM morphology exhibits a smoother accordion structure without surface particles (Fig. 3a). The substitution of the O termination and the insertion of the Na cations lead to a forward shift of the (002) peak of the XRD pattern (Fig. 3b). Moreover, the percentage of Nb₂O₅ is nearly equivalent to that of pristine Nb₄C₃, calculated from the high-resolution XPS spectrum of Nb 3d (Fig. S11), demonstrating that tuning the molten salt ratio and reaction temperature is crucial for controlling oxidation during surface modification. The high-resolution XPS spectrum of O 1s provided evidence for pure O terminations in Nb₄C₃ (Fig. 3c), as indicated by the presence of Nb-O(I) and Nb-O(II) peaks, along with the absence of peaks associated with OH and F terminations (F 1s spectrum, Fig. S12c). The chemical formula of Nb₄C₃T_x modified by NaOH/NaI molten salt is Nb₄C_3.1O_1.87 (Fig. 3d). For the double metal MXenes, such as Mo₂Ti₂C₃T_x, after the NaOH/NaI molten salt modification, an obviously enlarged layered structure of multilayer pure O-terminated Mo₂Ti₂C₃ is observed (Fig. 3e and Fig. S13). The intensity of the (002) peak in the XRD pattern is significantly stronger compared to the pristine Mo₂Ti₂C₃T_x (Fig. 3f), indicating that pure O-terminated Mo₂Ti₂C₃ sheets became more orderly stacked. XPS analysis showed that the F terminations are completely removed, leaving only O terminations on the surface (Fig. S14–16 and Fig. 3g, see Table S7 for fitting details), and the chemical formula of pure O-terminated Mo₂Ti₂C₃ is Mo₂Ti_2.4C_3.3O_1.75 (Fig. 3h). We also successfully modified the O terminations in the phase 21 MXenes, Mo₂CT_x, with results consistent with those in Ti₃C₂, Nb₄C₃, and Mo₂Ti₂C₃ MXenes (Fig. S17–20, see Table S8 for fitting details). These results highlight the universality and high efficiency of such NaOH/NaI molten salt treatment strategy for modifying O terminations across 21, 32, 43 phases, and double metal MXenes.

Fig. 3.

Universality of NaOH/NaI molten salt treatment. (a, e) SEM images of pure O-terminated Nb₄C₃ and Mo₂Ti₂C₃ MXenes. (b, f) XRD patterns of Nb₄C₃ and Mo₂Ti₂C₃ MXenes before and after NaOH/NaI molten salt treatment. (c, g) High-resolution XPS spectra of O 1s and (d, h) Moles of T_x per M_n+1X_nT_x formula unit for pristine MXenes and MXenes prepared by NaOH/NaI molten salt, respectively.

3.4. Properties and performance of pure O-terminated MXenes

To verify the importance of pure O terminations for tuning intrinsic properties and improving the performance in various fields, we tested the Li-ion battery performance, working function and resistance characteristics. Theoretical calculations have suggested that O terminations can enhance the lithium-ion storage capacity due to a lower energy barrier for Li⁺ migration [41]. Therefore, we compared the pristine Ti₃C₂T_x and pure O-terminated Ti₃C₂ materials for Li⁺ storage in a 1 M LiPF₆+/EC/DEC electrolyte. Pure O-terminated Ti₃C₂ exhibited an irreversible reduction peak at ∼0.56 V (Fig. S21), correlated with the formation of a solid electrolyte interface (SEI) on the electrode surface [42]. The area under the CV curve for pure O-terminated Ti₃C₂ (Fig. 4a) is significantly larger than that for Ti₃C₂T_x at a scan rate of 5 mV s⁻¹, indicating a much higher Li⁺ storage capacity for pure O-terminated Ti₃C₂. The lithium-ion storage capacity of O-terminated Ti₃C₂ is more than two times that of the pristine Ti₃C₂T_x with the improved rate performance at various scan rates (Fig. 4c and Fig. S22). Specifically, pure O-terminated Ti₃C₂ exhibits a maximum capacity of 258 mAh g⁻¹ at a current density of 0.1 A g⁻¹ (Fig. 4b). Compared to pristine Ti₃C₂T_x, the charge transfer resistance (R_ct) of pure O-terminated Ti₃C₂ also has a significant decrease from 217 to 86 Ω (Fig. 4d), indicating that pure O-terminated Ti₃C₂ facilitates faster Li ion transport. After 5000 cycles, the specific capacity of pure O-terminated Ti₃C₂ was still 185 mAh g^-1 at a current density of 2 A g^-1 with the capacity retention rate exceeded 97% (Fig. 4e). Moreover, by simply controlling the percentage of O terminations on Ti₃C₂ (varying OH to O terminations ratios), a tunable work function range from 3.67 to 4.17 eV is achieved (Fig. 4f). This tunability enables enhanced performance across a range of applications, including electronic devices, and optoelectronics [26,43]. For instance, in electronic devices and sensors, MXenes with different work functions can optimize electron injection or extraction, enhancing device switching performance and response speed. In optoelectronics, tuning the work function of MXenes can improve photoelectric conversion efficiency, offering potential for optimization in applications such as photovoltaic cells and photodetectors [26]. We also investigated the temperature-dependent resistance of Nb₄C₃T_x and pure O-terminated Nb₄C₃ using a four-probe method (Fig. 4g). Both samples exhibit a decrease in resistance with falling temperatures down to 80 K, indicative of metallic conductivity. Significantly, as the temperature dropped further to around 8 K, Nb₄C₃T_x displayed a sharp resistance decline similar to superconducting behavior, dramatically differing from the pure O-terminated Nb₄C₃ that has a slight upward trend, likely due to a higher tunneling barrier between the sheets [11]. Therefore, we can rule out O termination as a factor in the superconducting behavior, which prompts the question of whether pure F or pure OH terminations could exhibit superconductivity, highlighting the need to investigate other single surface terminations. This disparity in the conductivity behaviour of MXenes with different single terminations demonstrates the changes in their electronic structures and their potential in developing novel electronics. We investigated the stability of pure O-terminated Ti₃C₂ from room temperature to 1000 °C under an atmosphere of oxygen, and found that the initial oxidation temperature of MXene increased from 302 °C to 360 °C, as well as the complete oxidation temperature increased from 432 °C to 574 °C (Fig. 4h and Fig. S24), which benefited from the stable Ti-O bond formation in MXene.

Fig. 4.

Properties of pure O-terminated MXene. (a) CV profiles of Ti₃C₂T_x and pure O-terminated Ti₃C₂ MXenes at a scan rate of 5 mV s^-1. (b) Voltage profiles of pure O-terminated Ti₃C₂ at various specific currents. (c) Comparison of the capacity of Ti₃C₂T_x and pure O-terminated Ti₃C₂ MXenes at various scan rates. (d) Electrochemical impedance measurements of Ti₃C₂T_x and pure O-terminated Ti₃C₂ electrodes. (e) Cycling stability of Ti₃C₂T_x and pure O-terminated Ti₃C₂ at 2 A g⁻¹. (f) Continuously tuning work function of Ti₃C₂T_x depends on different ratios of OH and O terminations. (g) Temperature-dependent resistance of Nb₄C₃T_x and pure O-terminated Nb₄C₃ MXenes. (h) TG and DSC curves of pure O-terminated Ti₃C₂ powder from room temperature to 1000 °C in an oxygen atmosphere.

4. Conclusion

In conclusion, we present a general strategy for the precise preparation of O-terminated MXenes using NaOH/NaI molten salt. By virtue of the absence of water, high OH^- concentration, and suitable melting point being far below the temperature that causes structural degradation and self-oxidation of MXenes, our NaOH/NaI molten salt method meets the ideal criteria of O-termination modification, resulting in the extremely pure O-terminated MXenes with no addition of metal oxides or other termination species. Notably, this approach is applicable to a variety of MXenes, including 21/32/43 phases and mono/double metal MXenes. The properties of pure O-terminated MXenes differ markedly from pristine MXenes, with continuous tunability of the work function (3.67 to 4.17 eV) by controlling the proportion of O and OH terminations, more than two times the storage capacity in a Li⁺ battery, and distinct temperature-dependent resistance behavior. We anticipate that pure O-terminated MXenes will significantly contribute to advancements in energy storage, catalysis, electronic technologies, etc. The implications of this study extend beyond MXenes, offering valuable insights and methodologies for the surface modification of two-dimensional materials.

CRediT authorship contribution statement

Tianze Zhang: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Pei-Yu Cao: Methodology, Data curation. Qiuxiang Wang: Methodology, Data curation. Bin Yang: Methodology, Data curation. Hujie Wan: Data curation. Libo Chang: Conceptualization. Guozheng Zhang: Data curation. Qi Jin: Methodology, Data curation. Yi Deng: Data curation. Chao Yang: Data curation. Yang Wang: Data curation. Ye-Chuang Han: Supervision. Tianpeng Ding: Supervision. Xu Xiao: Visualization, Validation, Supervision, Funding acquisition, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no conflicts of interest in this work.

Biographies

Tianze Zhang is a Ph.D. student in the School of Physics at University of Electronic Science and Technology of China under the supervision of Prof. Xu Xiao. His research directions mainly focus on the surface and interface regulation of 2D transition metal nitrides and carbides (MXenes), and the application of MXenes in terahertz technology.

Ye-Chuang Han (BRID: 01161.00.38728) received his Ph.D. from Xiamen University (2022) under the supervision of Prof. Zhong-Qun Tian, and then worked as a postdoctoral research associate in Prof. Zhong-Qun Tian's group from 2022 to 2024. Now Dr. Han is an associate research fellow at the Fujian Science & Technology Innovation Laboratory for Energy Materials of China (IKKEM). Dr. Han has published >10 papers in high-profile journals like PNAS, Nat. Commun., Natl. Sci. Rev., Adv. Funct. Mater., and Mater. Horiz.. Dr. Han's group now mainly focuses on synthetic methodology under non-equilibrium high-temperature conditions, particularly in solid catalysts and multi-field coupled heterogeneous catalysis.

Tianpeng Ding (BRID: 08672.00.02135) received his Ph.D from Huazhong University of Science and Technology under the supervision of Prof. Jun Zhou in 2017 and worked as a postdoctoral fellow at National University of Singapore from 2017 to 2021. Subsequently, Dr. Ding became a full professor at University of Electronic Science and Technology of China. Currently, Dr. Ding's research interests include flexible thermoelectric, thermal energy conversion, THz-matter interaction and MXenes.

Xu Xiao (BRID: 07973.00.01750) received his BS degree in Physics and Ph.D. degree in Physical Electronics from Huazhong University of Science and Technology under the guidance of Prof. Jun Zhou in 2011 and 2016, respectively. He then joined Department of Materials Science and Engineering at Drexel University as a postdoctoral researcher under the guidance of Prof. Yury Gogotsi in 2016 2020. Currently, he is a professor in the School of Physics at University of Electronic Science and Technology of China. His research group focuses on synthesis and modulation of two-dimensional transition metal nitrides and carbides for applications in terahertz technology.