An investigation into the factors governing the oxidation of two-dimensional Ti₃C₂ MXene

Abstract

Two-dimensional (2D) transition metal carbides (MXenes) exhibit outstanding performances in many applications, such as energy storage, optoelectronics, and electrocatalysts. However, colloidal solutions of Ti₃C₂T_x MXene flakes deteriorate rapidly under ambient conditions due to the conversion of the titanium carbide to titanium dioxide. Here, we discuss the dominant factors influencing the rate of oxidation of Ti₃C₂T_x MXene flakes, and present guidelines for their storage with the aim of maintaining the intrinsic properties of the as-prepared material. The oxidation stability of the Ti₃C₂T_x flakes is dramatically improved in a system where water molecules and temperature were well-controlled. It was found that aqueous solutions of Ti₃C₂T_x MXene can be chemically stable for more than 39 weeks when the storage temperature (−80 °C) is sufficiently low to cease the oxidation processes. It was also found that if the Ti₃C₂T_x flakes are dispersed in ethanol, the degradation process can be significantly delayed even at 5 °C. Moreover, the oxidation stability of the Ti₃C₂T_x flakes is dramatically improved in both cases, even in the presence of oxygen-containing atmosphere. We demonstrate practical applications of our approach by employing Ti₃C₂T_x in a gas sensor showing that when oxidation is inhibited, the device can retain the original electrical properties after 5 weeks of storage.

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Introduction

In the past decade, two-dimensional (2D) materials have received increased attention because of their outstanding performances in electronic devices such as gas sensors and^1,2 energy storage^3–5 and as transparent conductive electrodes.^6,7 MXenes are a new family of 2D materials that consist of a few atomic layers of transition metal carbides or carbonitrides and are selectively etched from the bulk MAX phase in an acid etchant solution; MAX (M_n+1AX_n) consists of M, which is an early transition metal, A, which is a group of 13 or 14 elements, and X, which is carbon and/or nitrogen.^8–12 To obtain Ti₃C₂T_x from the Ti₃AlC₂ MAX phase, the Al atoms are removed by wet chemical etching with HF or LiF/HCl or with an etchant containing hydroxides.^3,13–15 The surfaces of MXenes are terminated by –OH, –F, or –O groups, which is denoted as T_x in the formula M_n+1X_nT_x. In general, MXenes are stored in deionized (DI) water because exfoliated individual flakes are hydrophilic and easily form an aqueous colloidal suspension. However, it has been experimentally confirmed that when Ti₃C₂T_x is stored in water for long periods, oxygen dissolved in water and/or in air acts as an oxidizing agent that induces the conversion of titanium carbide to its oxidized form (titanium dioxide, TiO₂).^16–18 The chemical stability of Ti₃C₂T_x MXene limits its potential in applications where longevity of materials is required. Although it has been demonstrated that serious problems arise due to the oxidation of electronic materials, including MXenes, investigations into the mechanism of oxidation and methods to prevent Ti₃C₂T_x MXene oxidation are necessary to understand and control the degradation. Previous reports have shown that storing Ti₃C₂T_x MXene solutions in organic solvents, Ar-filled bottles, and polymer matrices, and at lower temperatures (≤5 °C or freeze dried) slows the oxidation process, hypothesized to be because the solutions exposed to less dissolved oxygen were stable longer.^16–19 Recently, it has been shown that the stability of Ti₂CT_x and Ti₃C₂T_x was improved when materials were dispersed in isopropyl alcohol (IPA), displaying that dissolved oxygen (O₂) may not be as detrimental to oxidation as reactions with water, revealing that the mechanism is more complex than that was originally proposed.¹⁶

In this study, we investigated the dominant factors governing the oxidation of two-dimensional Ti₃C₂T_x MXene. We present an effective storage system where the water and temperature environment were controlled, and the oxidation rate was significantly slowed. Due to the previous reports showing that the oxidation reaction rate is slower at low temperatures,²⁰ we followed these protocols by storing the Ti₃C₂T_x MXene solutions at low temperatures. In addition, we developed two methods for the storage of MXene that do not require degassing or purging and that also minimize the influence of water. First, we kept air-filled bottles of Ti₃C₂T_x aqueous solution in a refrigerator and at controlled temperatures of 5 °C, −18 °C, and −80 °C (samples named D@5, D@-18, and D@-80; D meaning DI water). The water molecules were quickly frozen and converted to ice at the cryogenic temperature (5 °C is a normal cold storage temperature, −18 °C is a normal freezing temperature, and −80 °C is a typical cryogenic temperature). In another method, the MXenes were stored at 5 °C in anhydrous ethanol (sample named E@5, E meaning ethanol). Furthermore, we examined the effects of humidity and temperature on the solidified Ti₃C₂T_x prepared from these two stored solutions by fabricating films through vacuum-assisted filtration. We also assessed the stability of the Ti₃C₂T_x colloids and films by performing scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive spectrometry (EDS), and dynamic light scattering (DLS). To probe the longevity of an electronic device, we evaluated Ti₃C₂T_x MXene gas sensor device performance tests over the course of 5 weeks.

Results and discussion

Fig. 1a shows an optical image of the as-synthesized Ti₃C₂T_x solutions that display a dark colour. The Ti₃C₂T_x solutions were stored in air at 5 °C, −18 °C, and −80 °C. Fig. 1b shows an optical image of aged Ti₃C₂T_x solutions stored at 5 °C, −18 °C, and −80 °C for 5 weeks; the colour of solutions stored at −18 °C, and −80 °C retained their dark colour, suggesting that oxidation was minimized. On the other hand, the colour of the solution stored at 5 °C starts to change, suggesting that oxidation has started and the concentration of Ti₃C₂T_x has started to decrease (Fig. S1†). Furthermore, Fig. 1c shows an optical image of aged Ti₃C₂T_x solutions stored at 5 °C, −18 °C, and −80 °C for 10 weeks; the colour of the solution stored at 5 °C has become almost transparent. Scanning electron microscopy (SEM) of vacuum filtered films was utilized to compare the extent of oxidation under each experimental condition (Fig. 1d–f, inset shows films). Fig. 1d displays the expected morphology of a vacuum filtered Ti₃C₂T_x film. As shown in Fig. 1e, the solution stored at 5 °C has undergone oxidation, i.e. the filtered film has a light green film morphology (inset Fig. 1e) and SEM reveals that the flakes have white edges (TiO₂ particles forming). As shown in Fig. 1f, the solution D@5 for 10 weeks was heavily oxidized and the SEM exposes a rough film surface. From these observations, it can be seen that the oxidation of D@5 solution proceeds slowly from the as-synthesized solution, degrading over the course of 10 weeks; however, the materials stored at D@-18 and D@-80 appear different.

Fig. 1 (a–c) Optical images of (a) the as-synthesized Ti₃C₂T_x aqueous solutions stored at D@-80, D@-18, and D@5 and (b) solutions aged for 5 weeks and (c) solutions aged for 10 weeks at D@-80, D@-18, and D@5. Concentration of the as-synthesized Ti₃C₂T_x solutions is 0.2 mg ml⁻¹. (d–f) SEM images of films prepared with vacuum filtration. (d) An as-synthesized Ti₃C₂T_x solution. (e) A D@5 solution for 5 weeks. (f) A D@5 solution for 10 weeks. The insets show optical images of the vacuum-filtered films.

Fig. 2a displays the integrated Raman spectra of the as-synthesized solution and solutions stored for 5 weeks at −80 °C, −18 °C, and 5 °C. The Ti₃C₂T_x monolayer modes at 228 cm⁻¹ and 599 cm⁻¹ correspond to the out-of-plane stretching vibrations of Ti₂C and C, respectively.²¹ Please note that the measurement was based on the 5 weeks of storage when the oxidation began to appear.

Fig. 2 Characterization of the Ti₃C₂T_x vacuum filtered film. The solutions are stored as-synthesized and at −80 °C, −18 °C, and 5 °C. Raman spectra: (a) the as-synthesized and after 5 weeks aging in DI water at −80 °C, −18 °C, and 5 °C. (b) Size distribution of flakes obtained via dynamic light scattering (DLS). The energy dispersive X-ray (EDX) maps in (c), (d), and (e) show flakes from the Ti₃C₂T_x solutions and the corresponding titanium, carbon, and oxygen elemental mapping analyses of the Ti₃C₂T_x layers (in the horizontal direction, Ti is red, C is blue, and O is green). Images show flakes of (c) the as-synthesized Ti₃C₂T_x and flakes stored at (d) −80 °C and (f) 5 °C for 5 weeks.

In particular, the storage for 5 weeks of the solution at 5 °C results in the formation of anatase, as is evident from the peak at approximately 156 cm⁻¹, and the three other peaks at 399 cm⁻¹, 528 cm⁻¹, and 630 cm⁻¹, which are assigned to the E_g(1), B_1g(1), and A_1g and E_g(3) vibrational modes of TiO₂, respectively.¹⁹ Thus, the Raman analysis contains peaks due to TiO₂ and disordered carbon.²²Fig. 2b shows the hydrodynamic sizes of the flakes of Ti₃C₂T_x in the solutions after storage for 5 weeks; these results confirm that the flake size decreases from approximately 1700 nm to 300 nm when the Ti₃C₂T_x flakes are oxidized. TiO₂ particles form on the edges of the flakes, and then the particles gradually spread over the surface as oxidation proceeds. The oxidized components gradually detach from the original flakes and the flakes become smaller. Therefore, our measurements of the flake size demonstrate that the Ti₃C₂T_x flakes stored at 5 °C are oxidized and that the flakes stored for 5 weeks at −80 °C undergo oxidation more slowly. The as-prepared Ti₃C₂T_x flakes typically have an intact morphology and uniform distributions of Ti, C, and O elements, as shown in Fig. 2c. After storing Ti₃C₂T_x for 5 weeks at −80 °C, the flakes were examined with EDS to identify any changes in the chemical composition. The TEM image in Fig. 2d shows that the surface of this Ti₃C₂T_x sample is clean and that the elements Ti, C, and O are still evenly distributed.¹⁹ However, Fig. 2e shows that the Ti₃C₂T_x stored at 5 °C for 5 weeks has undergone severe oxidation and that agglomerated particles have formed on its surface. The titanium and oxygen elements are preserved during this process, but the carbon is slowly removed through the conversion of Ti₃C₂T_x to TiO₂.²³

We investigated how the water and oxygen dissolved in solution influence the surface morphologies and components of the films. The Ti₃C₂T_x solutions stored for 5 weeks were examined by XPS to assess changes in the surface chemical bonding during aging. The Ti 2p components at 454.7, 455.8, 457.2, and 458.6 eV shown in Fig. 3a can be assigned to Ti–C, Ti–X, Ti_xO_y, and TiO₂, respectively.^24–26 These XPS data show that the freshly prepared Ti₃C₂T_x barely shows the TiO₂ peak but the Ti₃C₂T_x solution stored at 5 °C produces a remarkable TiO₂ peak. That is, Ti–C bonding of 42.6% was not detected after 5 weeks and TiO₂ is quantitatively confirmed to increase from 3.0% to 94.3%. The C 1s components at 281.8, 284.1, 286.3, and 288.8 eV shown in Fig. 3b can be assigned to C–Ti, C–C, C–O, and O–C=O, respectively.^26,27 The MXenes stored at −80 °C, 5 °C in ethanol and the as-synthesized MXene show C–Ti bonding of 45.7% approximately but MXene stored at 5 °C did not show the C–Ti bonding.

Fig. 3 X-ray photoelectron spectra samples (as-synthesized, aged for 5 weeks at D@5, aged for 5 weeks at D@-80, and aged for 5 weeks at E@5) with deconvoluted components at two core levels for the films: (a) Ti 2p and (b) C 1s.

Also, the amount of C–C bonding represents 84.0% in MXene stored at 5 °C. As the oxidation of the Ti₃C₂T_x sample stored in water at 5 °C progresses, the MXene flakes are converted to TiO₂.²⁸ According to the Arrhenius equation,²⁰ the amounts of water and oxygen that come into contact with the Ti₃C₂T_x flakes per unit time depend on the storage temperature. At −80 °C, the amount of liquid water in the solution with access to the Ti₃C₂T_x per unit time is small and the diffusion rate of oxygen is low.²⁹ Whereas, Ti₃C₂T_x flakes of D@5 could more frequently contact with water or dissolved oxygen than those stored at −80 °C, resulting in the oxidation of the flakes. Therefore, the storage of Ti₃C₂T_x solutions at extremely low temperatures can ensure reproducibility for maintaining the characteristics of the flakes. In addition, deconvoluted O 1s XPS data and integrated XPS data according to temperature conditions are also shown as insets in Fig. S2 and S3.†

To probe the longevity of materials over time, D@-80, D@-18 and D@5 samples were fabricated into a film to measure the sheet resistances and perform gas sensor tests. Our experimental results for the sheet resistances are shown in Fig. 4a and Fig. S4.† As mentioned above, the sheet resistance of the film prepared from a D@5 solution for 10 weeks increased to approximately 2.5 × 10⁶ Ω sq⁻¹; the resistance ratio (R/R₀) reached 3 × 10⁵ after 10 weeks. Typically, Ti₃C₂T_x rapidly oxidizes under an atmosphere with abundant moisture and oxygen. As MXenes are typically well-dispersed in water, the functional groups on the surface of Ti₃C₂T_x react with water to form TiO₂ products, and the water molecules trapped between the Ti₃C₂T_x thin films can accelerate the oxidation of the MXene.

Fig. 4 (a) Sheet resistances of Ti₃C₂T_x films during storage for 10 weeks in D@-80, D@-18, D@5 and E@5. In addition, we measured the sheet resistance of a Ti₃C₂T_x sample stored at −80 °C for 39 weeks. The inset shows optical images of Ti₃C₂T_x films obtained from MXene solutions with vacuum filtration after 5 weeks of storage in D@-18, D@5 and E@5. (b) Resistance variation in a gas-sensing chamber for Ti₃C₂T_x flakes stored at D@5 for 5 weeks. (c and d) Gas sensor responses toward 5 ppm of NO₂ of (c) the as-synthesized Ti₃C₂T_x, and (d) Ti₃C₂T_x flakes stored at D@-80 for 5 weeks.

On the other hand, the sheet resistance of Ti₃C₂T_x solutions stored at −80 °C was almost constant even after storage for up to 39 weeks, displaying the importance of storing the materials in a controlled environment. Thus, the oxidation rate of a Ti₃C₂T_x solution over time can be reduced, even though the solution is stored in air, by using a cryogenic storage method, which eliminates the effect of the liquid water. Interestingly, at a temperature of −18 °C, the sheet resistance increased from 2.9 Ω sq⁻¹ to approximately 225 Ω sq⁻¹ after the storage for 10 weeks, while the sheet resistance is kept constant for 39 weeks at −80 °C. The reason for the dramatic difference in the storage at −80 °C and −18 °C can be interpreted that the lower the temperature, the slower the oxidation reaction occurs (lower diffusion rates of oxidants).²⁹ Besides, the lower the environmental temperature, the faster the freezing rate, and generally, the more the crystal disorder becomes. Therefore, we believe that it is hard for the oxidants to penetrate into the Ti₃C₂T_x MXene in the disordered crystals (−80 °C).^30–34

To further investigate the influence of water, additional experiments were conducted on dispersions of Ti₃C₂T_x in an anhydrous organic solvent. The reason why Ti₃C₂T_x stored in EtOH can maintain sheet resistance for 10 weeks is because the amount of water in anhydrous organic solvent is negligible and the oxidation factor does not affect MXene (Fig. 4a, Fig. S5 and S6†). Another reason for the constant sheet resistance after the storage at 5 °C is the reduction in the contact area exposed to the environment, as particles were aggregated in ethanol. To extend our insights into the influence of water on the Ti₃C₂T_x flakes, the sheet resistances of the MXene thin films were measured during storage in various environments, as shown in Fig. S7 and S8.†

The MXene thin films were stored in four different storage environments, with different levels of oxygen and moisture, and the variations with time in the sheet resistance were measured. In this process, some samples were stored in an 80 °C oven to activate the air and others were placed in a wet environment to determine the effects of moisture. The RH values of the humid environment and the dry environment were approximately 95% and 40%, respectively. The humidity (r.h.) was measured by using a commercial sensor with a nominal detection range of 20–99% r.h. and an uncertainty of 0.1% r.h. The Ti₃C₂ MXene films are typically kept at room temperature (RT) and a humidity of 40% (h40%). However, the sheet resistance of Ti₃C₂T_x film D, stored for 5 weeks at RT–h40%, is approximately 10 times higher than the original resistance. Thus, these storage conditions are not appropriate for the preservation of electrical performance. In contrast, film B was stored at the same temperature as film D, but the water content was very high (h95%), so less than 5 days were required for the sheet resistance to increase tenfold. The sheet resistance of film B increased up to 8 × 10⁴ in approximately 4 weeks. It is obvious that the sheet resistance undergoes a significant change at the high temperature from the comparison of the results for films A and C. The resistance of film A, which was stored at 80 °C-h95%, the highest temperature and humidity tested in this study, reached 2 × 10⁵ in one day, which is a dramatic increase. Film C was stored at a high temperature but very low humidity, which results in a relatively slow increase in resistance. The films stored in a glovebox are completely protected from oxygen and moisture, so oxidation is unlikely.

To further examine the stability of Ti₃C₂T_x MXene samples stored under various conditions, we measured their gas sensing responses. To measure the resistance signal of each synthesized Ti₃C₂T_x sensor, a constant bias was applied to the two-probe-resistor-type sensor, and the changes in the electrical resistance of the sensor upon exposure to analytes were monitored and recorded as sensing signals. The sensors were simultaneously loaded into a home-made gas sensing chamber, and the sensing signals of each channel were measured with a multi-channel sensing system.³⁵ The gas delivery system, which was fabricated in-house, is shown in detail in Fig. S9.† The gas sensing response was defined as the relative change in the channel resistance compared to the baseline resistance (100% × (R − R_b)/R_b). Sensing devices were fabricated by using the as-prepared D@-80, D@5 and E@5 Ti₃C₂T_x aged for 5 weeks. 5 ppm of NO₂ gas was introduced into a N₂-based environment; NO₂ was used for comparison as it is easily detected by most gas sensing channel materials, including MXene.

D@5 samples aged for 5 weeks did not exhibit a meaningful gas response behavior since the sample resistance was higher than the sensing range (>120 M Ohm) (Fig. 4b). As shown in Fig. 4c, the gas response of the as-synthesized MXene toward 5 ppm NO₂ was approximately 4.5%. The gas responses of the MXenes stored for 5 weeks in water vary significantly with the storage temperature. The D@-80 sample for 5 weeks was found to exhibit a gas response of approximately 4.5%, which is similar to that of the as-synthesized Ti₃C₂T_x MXene, as shown in Fig. 4d. The response speeds and recovery behaviors of these two samples are also similar. Since surface functionality and electrical conductivity both influence the gas response of the films, such results imply that both properties should be well-preserved when MXene is stored at −80 °C. These results show that a low storage temperature is crucial to the preservation of the original surface/electrical properties of MXene. For Ti₃C₂T_x dispersed in an anhydrous organic solvent (ethanol), a valid gas response toward NO₂ was observed, but with a lower value (0.63%) than that of the as-prepared Ti₃C₂T_x MXene (Fig. S10†). We assume that this lower value is due to the adsorption of the organic content onto the Ti₃C₂T_x flake surface, which inhibits gas adsorption, or due to the inhibition of the diffusion of gas molecules by the intercalated organic molecules. Compared to dispersions in water, titanium carbides MXenes do not disperse in ethanol also, and the gas response may be affected by the aggregation of Ti₃C₂T_x MXene in ethanol.

Conclusions

In summary, although Ti₃C₂T_x MXene has outstanding electrical properties, the use of Ti₃C₂T_x is difficult because of its oxidation rate. In the attempt to solve these problems, we have identified the temperature and exposure to water molecules as the most important elements of oxidation. We have studied the dominant factors influencing the oxidation of Ti₃C₂T_x MXene flakes, and have presented guidelines for their storage to maintain the intrinsic properties of the as-prepared MXene. The oxidation stability of the Ti₃C₂T_x flakes is dramatically improved in the systems where the influence of water molecules and the temperature was well-controlled, even in the presence of oxygen-containing air (up to 39 weeks). With their superior performance and long lifetimes, our Ti₃C₂T_x MXenes and the storage system will have a broad range of applications in research into MXenes.

Experimental sections

List of materials

Ti powders (Yee Young Cerachem, 99.5%, 1–2 μm), TiC powders (Yee Young Cerachem, 99.5%, <45 μm), Al powders (Yee Young Cerachem, 99%, 35–45 μm), LiF (Aldrich, 300 mesh), hydrochloric acid (Daejung, 35%), ethyl alcohol, pure (Aldrich, ≥99.5%), Anodisc inorganic filter membrane (AAO, pore diameter 0.2 μm, Whatman), and polycarbonate membrane (PC, 0.1 μm pore size, Whatman) were used.

Preparation of Ti₃C₂T_x

Ti₃AlC₂ was prepared by ball-milling the TiC, Ti, and Al powders in a molar ratio of 2 : 1 : 1 for 18 h with zirconia balls. The mixture was prepared under a pressure of 7 MPa for 30 s to create a 2 g pellet (see Fig. S11†). The pellets were heated to 1450 °C for 3 h under argon. The compact pellet was then crushed by using a mortar and pestle. Roughly, 2 g of each powder was sieved with a 74 μm mesh to prepare a fine powder. 2 g of lithium fluoride (LiF) was dissolved in 20 mL of 9 M LiF/HCl, then 2 g of the fine powder (≤74 μm) was slowly poured into the solution over a few minutes. This solution was stirred continuously (350 rpm) for 24 h at room temperature. The resulting Ti₃C₂T_x product was shaken by using a shaker (FINEPCR, Multi shaker 3D-300) for 1 h and centrifuged at 5000 rpm for 10 min. Ti₃C₂T_x solution was washed using deionized water until the pH ≈ 7. To obtain multilayered Ti₃C₂T_x, two grams of MXene were added to 100 mL of deionized water, then the solution was shaken for 1 h with a shaker again. The resulting Ti₃C₂T_x product was centrifuged at 5000 rpm for 5 min and this process was once again carried out with the obtained suspension.

Fabrication of the Ti₃C₂T_x MXene film

To confirm the sheet resistance of a Ti₃C₂T_x film, the films were fabricated via the vacuum filtration process of 1.5 mL of Ti₃C₂T_x solutions (0.2 mg ml⁻¹) on a PC (polycarbonate, 0.1 μm pore size) filter membrane. Then, the sheet resistances of films prepared from the solutions stored at each temperature were measured every 7 days.

Ti₃C₂T_x film fabrication and transfer onto a sensor device

To fabricate sensor devices using the Ti₃C₂T_x samples, 70 nm thick Au electrodes with a pre-deposited 5 nm thick adhesion layer of Cr, a spacing of 100 μm, and a width of 100 μm were deposited with e-beam evaporation by using a customized mask on SiO₂/Si substrates. Thin films were prepared from the Ti₃C₂T_x solutions by performing vacuum filtrations through porous Anodisc inorganic filter membranes (AAO). The filtered Ti₃C₂T_x films on AAO membranes were placed in a vacuum oven to dry at 60 °C for 1 hour, and then floated on 2 M NaOH solution. The Ti₃C₂T_x membrane floats free on the NaOH solution because the AAO film gradually dissolves in the NaOH solution. We transferred the prepared Ti₃C₂T_x membranes onto SiO₂ wafers, after washing with DI water. The MXenes on the SiO₂ wafers were dried in a vacuum oven at 60 °C for 1 h.¹

Gas sensor device fabrication and measurements

The sensing devices were mounted in a sensing chamber designed to measure resistance signals by using a data acquisition module (Agilent 34970A). NO₂ gas was passed through the sensing chamber. A gas delivery system designed in-house was used to control the gas flow into the sensing chamber to measure the sensors’ responses to the analytes. The serial dilution system used to vary the analyte concentration in the range of 0.125 to 1000 ppm consisted of a mass flow controller (MFC, Brooks 5850E), Teflon tubing (PFA, 1/8′′), Lok-type fittings, and a system of valves. N₂ was used as the reference gas, and the total flow rate for the reference gas and tested analytes was maintained at 400 sccm. The injection times of the analytes and N₂ were 5 min and 15 min, respectively. The dimensions of the sealed gas sensing chamber were approximately 10 cm (width), 5 cm (length), and 8 mm (height).

Characterization of the products

A scanning electron microscope (SEM, FEI, Sirion) and a transmission electron microscope (TEM, JEOL Ltd, JEM-3011HR) were used to obtain high-magnification images of the surfaces of the as-synthesized and oxidized Ti₃C₂T_x. Raman spectroscopy (NOST, FEX) was used to confirm the production of Ti₃C₂T_x. Energy-dispersive X-ray spectroscopy (EDS, JEM-ARM200F) images were obtained to assess changes in the surfaces. Dynamic light scattering (DLS, Malvern, Zetasizer Nano ZS) was used to determine the sizes of the flakes. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-Alpha) was used to characterize the chemical compositions of the flakes. A four-point probe station (AIT, CMT-100MP) was used to determine the sheet resistances of the flakes.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank Natalia Noriega (Drexel University) for helpful comments on the manuscript. This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (NRF-2015K1A4A3047100, NRF-2015M3A7B6027973, and NRF-2015M3A7B7046618). This work was supported by the Open Innovation Program funded by NNFC (PCOI1801M004).

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Footnotes

† Electronic supplementary information (ESI) available: (1) Optical images of MAX powder and pellets, (2) SEM images of Ti₃C₂ MXene films dispersed in ethanol and DI water, (3) sheet resistances of Ti₃C₂ MXenes dispersed in ethanol stored under various conditions, (4) normalized sheet resistances of Ti₃C₂ MXenes stored under various conditions, (5) (a) normalized resistances of various Ti₃C₂T_x MXene films obtained with vacuum filtration after storage for up to 8 weeks at various temperatures and humidity. (b) Sequential SEM images of film B, which was stored at room temperature (25 °C) and a humidity of 95%. (6) Schematic diagram of the gas delivery system. (7) Gas sensor responses of Ti₃C₂T_x flakes stored at 5 °C in an anhydrous organic solvent. See DOI: 10.1039/c9nr00084d

‡ Present address: Materials Architecturing Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, South Korea.

§ Present address: Department of Nano-structured Materials Research, National Nano Fab Center (NNFC), Daejeon 34141, Korea.

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