Oxidation Stability of Colloidal Two-Dimensional Titanium Carbides

Article pubs.acs.org/cm

Oxidation Stability of Colloidal Two-Dimensional Titanium Carbides (MXenes) Chuanfang John Zhang,†,‡,# Sergio Pinilla,§,∥,# Niall McEvoy,†,‡ Conor P. Cullen,†,‡ Babak Anasori,⊥ Edmund Long,†,§ Sang-Hoon Park,†,‡ Andrés Seral-Ascaso,†,‡ Aleksey Shmeliov,†,‡ Dileep Krishnan,†,‡ Carmen Morant,∥ Xinhua Liu,†,‡ Georg S. Duesberg,†,‡,△ Yury Gogotsi,*,⊥ and Valeria Nicolosi*,†,‡,§ †

CRANN and AMBER Research Centers, Trinity College Dublin, Dublin 2, Ireland School of Chemistry, Trinity College Dublin, Dublin 2, Ireland § School of Physics, Trinity College Dublin, Dublin 2, Ireland ∥ Applied Physics Department, Universidad Autónoma de Madrid, 28049 Madrid, Spain ⊥ Department of Materials Science and Engineering, and A. J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, United States ‡

S Supporting Information *

ABSTRACT: Two-dimensional (2D) transition metal carbides and nitrides (MXenes) have shown outstanding performances in electrochemical energy storage and many other applications. Delamination of MXene flakes in water produces colloidal solutions that are used to manufacture all kinds of products (thin films, coatings, and electrodes, etc.). However, the stability of MXene colloidal solutions, which is of critical importance to their application, remains largely unexplored. Here we report on the degradation of delaminated-Ti3C2Tx colloidal solutions (T represents the surface functionalities) and outline protocols to improve their stability. Ti3C2Tx MXene solutions in open vials degraded by 42%, 85%, and 100% after 5, 10, and 15 days, respectively, leading to the formation of cloudy-white colloidal solutionss containing primarily anatase (TiO2). On the other hand, the solution could be well-preserved when Ti3C2Tx MXene colloidal solutionss were stored in hermetic Ar-filled bottles at 5 °C, because dissolved oxygen, the main oxidant of the MXene flakes, was eliminated. Under such a recipe, the time constant of the solution was dramatically increased. We have found that the degradation starts at the edges and its kinetics follows the singleexponential decay quite well. Moreover, we performed size selection of the MXene solution via a cascade technique and showed that the degradation process is also size-dependent, with the small flakes being the least stable. Furthermore, a dependence between the degradation time constants and the flake size allows us to determine the size of the nanosheets in situ from UV−vis spectra and vice versa. Finally, the proposed method of storing the MXene colloidal solution in Ar-filled vials was applied to Ti2CTx to improve its stability and time constant, demonstrating the validity of this protocol in improving the lifetime of different MXene solutions.

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INTRODUCTION In the past decade, two-dimensional (2D) materials, including graphene,1 hexagonal boron nitride (h-BN),2 transition metal dichalcogenides (TMDs),3 oxides (TMOs),4−6 and layered double hydroxides (LDHs),7 have been the focus of intensive research with synthesis routes ranging from mechanical exfoliation8,9 to liquid-phase exfoliation10,11 and chemical vapor deposition (CVD).12 These 2D nanomaterials have shown excellent performance in various applications, such as electrochemical energy storage (EES),13−24 gas sensing,25 composites,26 and field-effect transistors (FETs).27 A large family of 2D materials, the so-called MXenes, are produced by a wet hydrofluoric chemical treatment of the Mn+1AXn phases (MAX, M stands for early transition metal such as Ti, V, Nb, © 2017 American Chemical Society

and Mo, etc., A is an A-group element such as Al, Si, and Ga, etc., X is carbon and/or nitrogen, and n = 1, 2, 3).28−31 Due to their wet etching synthesis method, MXenes are terminated by -O, -OH, and/or -F with the general formula Mn+1XnTx (where T represents surface termination and x is the number of the surface groups per formula unit).31,32 MXenes are composed of stacked 2D sheets held together by weak van der Waals and/or hydrogen bonding between these terminating functional groups,31 opening up the possibility to delaminate them into a monolayer-enriched MXene colloidal solution. To date, Received: February 24, 2017 Revised: May 9, 2017 Published: May 9, 2017 4848

DOI: 10.1021/acs.chemmater.7b00745 Chem. Mater. 2017, 29, 4848−4856

Article

Chemistry of Materials

Figure 1. TEM images of (a) MXene flakes from fresh d-Ti3C2Tx solution and aged solutions in Air@RT for (b) 7 days and (c) 30 days, respectively. (d−f) High-resolution TEM images in panels a−c, respectively. Inset in panel d is the corresponding SAED pattern, and those in panels e and f are the corresponding FFT patterns. (g) Raman spectra of fresh as well as aged MXene films for different durations. XPS results of (h) fresh d-Ti3C2Tx film and (i) film that was aged in Air@RT for 25 days.

be revealed. Moreover, since the preferred flake size of the 2D materials differs depending on the application, with applications such as hydrogen evolution48 requiring the MXene flakes to be small, while larger flakes are preferred for conductive films39 or mechanical reinforcements,46 it would be beneficial if the MXene flakes could be well-separated by lateral dimensions, and the intrinsic relationship between the degradation kinetics and flake size could be established. However, no reports on either the lifetime of MXene aqueous colloidal solutions or the size effect are available. Herein, we report on the stability of MXene aqueous colloidal solutions monitored by UV−vis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy (TEM). We begin with the most widely studied MXene, Ti3C2Tx, as a model, to reveal the degradation mechanisms of the delaminated solution. A protocol to improve the MXene stability is proposed, and degradation kinetics of the solution are analyzed. In addition, we establish a correlation between the degradation kinetics and the MXene flake size, which allows us to obtain the flake size from the time constant in situ and vice versa. Finally, we apply the proposed protocols to Ti2CTx, another type of MXene which has demonstrated promising capacitances and Li+ storage performances in supercapacitors and Li-ion batteries, respectively.49−51 Both the stability and time constant of the solution have been remarkably improved compared to the sample that was stored following the standard conditions, demonstrating the generality of the proposed protocol for improving the lifetime of the delaminated-MXene solutions.

multilayered Ti3C2Tx, Ti2CTx, Ti3CNTx, Nb2CTx, V2CTx, Mo2CTx, Mo2TiC2Tx, and many other MXenes, as well as their corresponding colloidal solutions, have been reported,33−38 with Ti3C2Tx being the most widely investigated one. For instance, Ti3C2Tx has shown excellent volumetric capacitance (>900 F/cm3) in supercapacitors,30 excellent metallic conductivity (as high as 6500 S cm−1) in thin films,39 and high capacity and rate capability in Li-ion batteries and Li-ion capacitors.34,40 Beside the EES applications, Ti3C2Tx has also shown promising results in FETs,41 selective molecular sieve,42 and electromagnetic shielding43 fields, to name just a few. Similar to many other 2D materials,14,44,45 multilayered MXene particles can be intercalated with organic molecules (such as hydrazine, urea, or dimethyl sulfoxide).35 Sonication35 or manual shaking in water41 results in the formation of MXene aqueous colloidal solutions, which are used for the processing of MXene thin films, manufacturing of thick membranes, or reinforcement of polymer-based composites.33,46 Previous studies found that both the multilayered Ti3C2Tx MXene and monolayer flakes degrade gradually either in humid air41 or water.47 Investigating the stability of aqueous solutions of delaminated-MXene is very important, as many practical applications rely on solution instead of powder processing. However, the lifetime of the MXene colloidal solutions and the degradation mechanisms still remain largely unexplored, which limits our understanding of the properties of delaminatedMXenes. Protocols for protecting the delaminated-MXene solutions from rapid decay should be established and insights from the characterization, such as the effect of MXene concentration or flake size on the oxidation process, should 4849


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RESULTS In general, the as-synthesized Ti3C2Tx MXene aqueous colloidal solutions are usually stored in open vials at room temperature (23 °C, Air@RT).33,40 Therefore, we began the MXene stability studies under these conditions. The asobtained fresh colloidal solution exhibited a dark green color and showed a Tyndall effect (Supporting Information Figure S1a,b). Figure 1a is a typical TEM image of a pristine Ti3C2Tx flake. A clean surface and edges were observed. Lattice fringes were observed in a high-resolution TEM image (HRTEM, Figure 1d), suggesting the nanosheets are single crystalline. A selected area electron diffraction (SAED) pattern (inset of Figure 1d) shows a hexagonal atomic arrangement, in agreement with previous reports on Ti3C2Tx MXenes.28,35 The surface of these “pristine flakes” was uniform with a mean lateral size of 600 nm, as seen in the SEM image and the corresponding histogram (Figure S2a,b). After aging in Air@ RT for 1 week, some “branches” formed at the edge sites and nanoparticles appeared on the flakes’ basal planes (Figure 1b,e). The nanoparticles on the basal planes are 2−3 nm in size, much smaller than the “branches” on the edges (up to 100 nm). The latter were crystalline, with the fast Fourier transform (FFT) matching well to anatase (inset of Figure 1e). The SEM images in Figure S2c also showed contrast differences between edge and basal plane sites, with flakes much brighter on the edge. Further aging of the colloidal solutions in Air@RT for 30 days led to a complete decomposition of the material into anatase debris and disordered carbon, as shown in Figure 1c,f and Figure S2d. The color of the colloidal solutions also evolved from green to cloudy-white during the aging process (compare Figure S2a,c,d). The electron microscopy analysis suggests the edge sites are more vulnerable than the basal planes. During the course of aging, the branches grow from the edge sites to the basal plane in a manner consistent with the so-called “scissor effect”, which eventually shreds the nanosheets into small particles (debris). To further confirm the composition of MXene solution after different periods of degradation, Raman analysis was performed. The fresh sample showed the expected vibrational modes for Ti3C2Tx MXene (Figure 1g).6 After aging the MXene solution in Air@RT for 14 days, anatase and disordered carbon peaks were detected (Figure 1g and Figure S3). Further exposure to Air@RT for 60 days resulted in the formation of more anatase, as peaks at 156, 399, 528, and 630 cm−1 were well-indexed to Eg(1), B1g(1), A1g and B1g(2), and Eg(3) vibrational modes of anatase, respectively.6 The chemical composition of the surface of the Ti3C2Tx flakes was further monitored by XPS over time. Figure 1h shows the high-resolution XPS of the fresh MXene film in the Ti 2p region, revealing a pair of asymmetric peaks corresponding to a Ti−C bond (formation of Ti = Carbide) at a binding energy of 455 eV, two sets of peaks attributed to Ti(II) suboxide and/or hydroxide (∼456 eV), and Ti(III) suboxide and/or hydroxide (∼458 eV), and Ti(IV) oxide (TiO2; ∼459 eV), a typical XPS spectrum for the Ti3C2Tx MXene, where the oxide peaks can be indexed.52 After aging the solution for 10 days in Air@RT, the Ti(IV) oxide peak became well-resolved while a substantial decrease of the carbide peak intensity was observed (Figure S4). After the MXene colloidal solution was aged in Air@RT for 25 days, no carbide signal was detected and the remaining signal was attributed to 5% Ti(III) suboxide and 95% TiO2 (Figure 1i). In addition,

substantial amounts of Ti(II) and -(III) suboxides and/or hydroxides were found in the initial 10 days (Figure S4) which then decreased substantially after 25 days, suggesting Ti(II,III) hydroxides and/or oxides are intermediate phases before the MXene solution fully converts to an anatase (TiO2) which sediments on the bottom of the vial. This oxidation phenomenon is also found in monolayered or multilayered MXene exposed to air45 or water.47 The evolvement of the morphology of the single flake in Air@RT could also be seen in the scanning transmission electron microscopy (STEM) images (Figure S5a−c). Electron energy loss spectroscopy (EELS) was employed to further understand the degradation mechanisms of the flakes (Figure S5d). While all the studied samples showed an 0 K signal, an indication of degradation, the pre-edge peaks appeared in the Air@RT sample after 2 weeks, suggesting the presence of a Ti−O bond. Upon aging for 30 days, two preedge peaks appeared in the 0 K EELS, which could be indexed as t2g and eg states of the Ti4+, in agreement with XPS results. To quantify the degradation phenomenon, UV−vis measurements of the solution were performed. Extinction spectra and the normalized spectra (to the local minimum) are shown in Figure S6. The intensity at 785 nm was chosen as the metric for the concentration of the MXene nanosheets, and the normalized intensity plotted as a function of time allows us to track the degradation of the colloidal solution. As shown in Figure 2a, a continuous decrease in the concentration of MXene nanosheets over time in Air@RT was

Figure 2. (a) Stability of colloidal d-Ti3C2Tx in different environments. The dotted lines are the fitting results according to the empirical equation A = Aunre + Aree−t/τ. (b) Time constants of colloidal d-Ti3C2Tx in different environments. (c) Low- and (d) highmagnification TEM images of aged d-Ti3C2Tx flakes in Ar@LT for 30 days. Inset in panel d is the corresponding FFT pattern.

observed. We fitted these points to an empirical function,53 A = Aunre + Aree−t/τ, where Aunre and Are represent the stable/ unreactive and reactive/unstable MXene nanosheets, respectively, and τ is the time constant (days). Quite interestingly, the MXene nanosheets followed the exponential decay with a time constant of only 4.8 days. Moreover, the Are/(Aunre + Are) ≈ 99.92, indicates that the entirety of the MXene nanosheets are vulnerable to degradation. The exponential decay is similar to the degradation behavior of aqueous solutions of black 4850


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Chemistry of Materials phosphorus which showed a comparable 7.9 days.53 The stability data were acquired from extinction spectroscopy, which is complex for 2D materials due to the presence of nonnegligible light scattering. However, it is reasonable to use the extinction spectra, rather than absorbance spectra, for these studies since similar decay behavior and time constants were previously found from both extinction and absorbance spectroscopy of aqueous solutions of black phosphorus.53 The MXene sample decreased in intensity by 42%, 85%, and 100% after 5, 10, and 15 days, respectively. Based on these results, it is clear that d-Ti3C2Tx MXene flakes are unstable when the aqueous solution is stored in Air@ RT. Within 2 weeks, almost all carbides had been oxidized to anatase titania. To ensure the performance of the MXenes in various applications, methods for improving the lifetime of the aqueous solution need to be developed. We realized this by injecting the very fresh d-Ti3C2Tx aqueous solution into Arfilled bottles, which were preassembled inside the glovebox. As a result, the pressure inside the hermetically sealed bottles was as high as 2.3 mbar. The compressed Ar could effectively prevent the air from entering the water medium and forming dissolved oxygen. Moreover, the multilayered Ti3C2Tx colloidal solution was degassed prior to and during bath sonication to remove the dissolved oxygen from the fresh d-Ti3C2Tx aqueous solution. Consequently, the solution stored in Ar-filled bottles was well-protected from the dissolved oxygen. Additionally, lowering the temperature suppresses the rate of the oxidation process by the Arrhenius equation.54 It is thus anticipated that Ar at low temperature (Ar@LT at 5 °C) could possibly provide the best environment for MXene solution storage. The stability of MXene aqueous solutions in different environments, namely, Air@LT, Ar@RT, and Ar@LT, are compared with Air@RT in Figure 2a. The normalized UV−vis spectra (Figure S7) show that the stability was remarkably improved by introducing a lower temperature and/or Ar environment. After 25 days, 57% of the initial intensity (concentration) was retained in the Air@ LT sample, in sharp contrast to the 86% and 95% in Ar@RT and Ar@LT, respectively (Figure 2a). The improvement in stability of Ar-stored samples compared to the Air-stored ones suggests that expelling the dissolved oxygen from the colloidal solution is more effective in suppressing the MXene degradation than merely lowering the temperature of the solution. This was further supported by fitting the degradation results with the exponential decay model. As shown in Figure 2a,b and Table 1, the time constant

solutions. This point is further supported by the Raman spectra in Figure S8. The Ti 2p core-level XPS spectra of d-Ti3C2Tx MXene solutions stored in Ar@LT and Air@LT for 14 days are compared in Figure S9, which clearly shows that the Ar@LT environment has effectively suppressed the growth of nanoscale TiO2, as the TiO2 intensities at 459.30 eV (Ti 2p3/2) and 465.00 eV (Ti 2p1/2) in Air@LT are much higher than in the Ar@LT sample. The “Ti-carbide” signals in Ar@LT at 455.08 eV (C−Ti−Tx 2p3/2) and 461.10 eV (C−Ti−Tx 2p1/2) correspond to the interaction between the titanium atoms with the core carbon atoms and the surface functionalities, respectively. The positions of Ti 2p region are summarized in Table S1. Figure 2c shows a TEM image of the MXene flakes stored in Ar@LT for 30 days. Despite some sub-micrometer branches being visible at some edge sites, most of the nanosheets have a relatively clean and even surface, indicating similar thickness in the nanosheets. A higher magnification TEM image in Figure 2d shows that no nanoscale titania was observed, in sharp contrast to Figure 1e,f. The FFT in the inset also indicates the crystallinity has been preserved in the Ar@LT sample. The faster degradation speed on the edge sites detected by the TEM indicates that flake size plays a critical role in the degradation rate of the MXene solution, as the size of the flake determines the ratio of perimeter to surface area. To quantify this observation, investigations focusing on the degradation kinetics of colloidal solutions with different flake sizes need to be conducted. Standard d-Ti3C2Tx solution, right after 1 h of centrifugation at 3500 rpm, has a quite broad size distribution55 which makes it less appropriate for studying the effect of flake size on their stability. We performed size selection on the colloidal solution via a cascade method by elevating the centrifugation speed. The sediments that were trapped between the centrifugation speeds could be readily re-dispersed via simple manual shaking, forming various colloidal solutions with different mean flake sizes.56,57 We note this size selection process is quite important, as it allows us to choose the flakes with optimal lateral dimensions for specific applications. Panels a−c of Figure 3 and panels a and b of Figure S10 show the SEM images of size-selected flakes, indicating smaller flakes were obtained as the centrifugation speed was increased. Statistical SEM image analysis was conducted for all samples, and the histograms are displayed in Figure 3d−f and Figure S10c,d, demonstrating mean lateral dimensions from 110 nm in the small sample to 800 nm in the large one. We then chose three representatives, namely, large (⟨L⟩ = 800 nm), medium (⟨L⟩ = 300 nm), and small (⟨L⟩ = 110 nm) flake samples and studied their UV−vis optical responses and degradation behaviors in different environments. The normalized extinction spectra of fresh samples can be seen in Figure 4a. Similar to other 2D materials such as black phosphorus,53 molybdenum disulfide57 and gallium sulfide,58 etc., both the shape and the normalized extinction intensities of the MXene spectra are size-dependent, which could be attributed to the different electronic structure associated with edge sites compared to the basal plane.53,59 The stability of colloidal solutions with various sizes, shown in Figure 4b−e, suggested that large MXene flakes are the most stable, followed by the medium and small ones. The dependence of degradation rate on the nanosheet size is also indicative of edge-driven reactions. Introducing Ar at LT greatly improves the lifetime of various colloidal solutions. For

Table 1. Time Constant and Coefficients from the Fitting of the Standard Sample in Different Environments ambient

time constant (days)

Are

Aunre

A re A re + A unre

Air@RT Air@LT Ar@RT Ar@LT

4.8 42 198 740

1.28 0.98 0.98 0.98

0.001 0.04 0.02 0.02

0.999 0.961 0.980 0.980

was improved by almost an order of magnitude when storing the MXene solution at LT (from 4.8 days in Air@RT to 42 days in Air@LT) and was boosted to 190 and 740 days (about 2 years) by storing the solution in Ar@RT and Ar@LT, respectively, the latter showing that a combination of Ar and LT provided the best environment for storing MXene colloidal 4851


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Figure 3. (a−c) SEM images of size-selected samples ranging from the largest size on the left to the smallest size on the right. (d−f) Corresponding lateral flake size histograms of panels a−c.

Figure 4. (a) Normalized extinction spectra of fresh d-Ti3C2Tx solutions with different flake sizes. (b−e) Stability results of small, medium, and large d-Ti3C2Tx flakes in Air@RT, Air@LT, Ar@RT, and Ar@LT, respectively. The dotted lines are the fitting results according to the empirical equation, A = Aunre + Aree−t/τ. The time constant extracted from the fitting of the graphs is plotted as a function of flake length, as shown in panel f. The dotted lines in panel f are the fitted results according to the empirical equation, τ = 10bLa.

Table 2. Time Constants and the Fitting Coefficients of Different Sizes in Different Environments A re A re + A unre

time constant size

Air@RT

Air@LT

Ar@RT

Ar@LT

Air@RT

Air@LT

Ar@RT

Ar@LT

large medium small

9 5.1 6.2

75 43 30

2100 205 138

2400 555 255

0.999 0.999 0.991

0.900 0.989 0.900

0.950 0.950 0.950

0.960 0.930 0.930

instance, after 25 days, the large MXene flakes only degraded by 1.2% in Ar@LT and 1.4% in Ar@RT, while degrading by 29.1% and 98.5% in Air@LT and Air@RT, respectively (Figure S11a). The reduction in degradation speed in Ar@LT environments again suggests that dissolved oxygen is the major oxidant while the aqueous medium is a mild one. The degradation rate of the small-flake sample in all environments can be found in Figure S11b, showing a shorter lifetime compared to larger flakes. The lower stability of smaller flakes could also be confirmed by

TEM as shown in Figure S12a,b. The XPS of the fresh and aged samples with different flake sizes could be seen in Figure S13, showing more TiO2 on the small flakes when the solutions were aged for 14 days in Air@RT. We further estimated the time constants (τ) of colloidal solutions with varying flake sizes by fitting the points in Figure 4b−e. Once again, the exponential decay model explains the degradation behavior of the colloidal solutions quite well, regardless of flake size. The results are summarized in Table 2. 4852


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Chemistry of Materials The τ of the small-flake sample was 6.2 days in the Air@RT sample and was improved to 255 days in Ar@LT. For the large-flake sample, τ reaches 2400 days in Ar@LT, suggesting the large-flakes-enriched colloidal solution could be sufficiently stable for composites processing or other applications. τ of the standard d-Ti3C2Tx sample in Ar@LT was 740 days, a value between that of the small-flake sample and the large-flake one, indicating that the instability of smaller flakes is the main cause for the short lifetime of the standard MXene colloidal solution. To gain more insights into the size effect, we first plotted τ as a function of flake size in Figure 4f and then fitted the points using the following formula: τ = a × 10bL,53 where a and b are coefficients and L is the mean flake size (nm). It is worth mentioning that this model is valid for and achieved under a specific, constant concentration of the colloidal solution. In this case, various colloidal solutions were diluted to 0.007 mg/mL before subjecting them to UV−vis measurements. Quite interestingly, we observed a well-defined correlation. Table 3

simple manual shaking, it quickly re-dispersed and formed a colloidal solution (Movie S1 and Figure S15g), demonstrating excellent reversibility. We note this is important, as it allows us to keep the quality of MXene high, even for prolonged periods of time in Air@RT. The degradation behavior of Ti3C2Tx MXene colloidal solutions can be summarized as follows: (1) when dissolved oxygen and water are present, the former would preferably react with the most active flake edges, leading to the formation of anatase (TiO2). Reducing the lateral size, concentration and/or storing the colloidal solutions at RT accelerates degradation of the flakes. This process is schematically illustrated in Figure 5a.

Table 3. Fitting the Time Constant vs Flake Length To Provide the Metrics sample Air@RT Air@LT Ar@RT Ar@LT

metrics τ τ τ τ

= = = =

5.7 × 100.00026L 25 × 100.0006L 80 × 100.00168L 180 × 100.0014L

lists the coefficients and the corresponding correlations. For example, the relationship between τ and flake size when the MXene solution was stored in Ar@LT could be described as τ = 180 × 100.0014L. We note this correlation is extremely important and useful, as it allows us to quickly know the mean size of the MXene flakes in situ from the UV−vis spectra without requiring electron microscopy or other size measurements and vice versa. The effect of concentration on the colloidal solutions’ stability can be found in Figure S14, showing a more rapid decay when the solution is diluted. Based on the above discussion, the ideal condition for maximizing the lifetime of MXene colloidal solutions is to prepare large-flake-concentrated colloidal solutions and store them in an Ar@LT environment. In this case, the solution is stable enough to be processed into composites for various applications. Nevertheless, the flakes still degraded by 1.2% over 25 days (Figure S11), indicating water medium is a mild oxidant for MXenes. The reaction led by water could be written as

Figure 5. Schematics of degradation of the colloidal d-Ti3C2Tx MXene aqueous solution in (a) Air@RT and (b) Ar@RT. In panel a, the high solubility of oxygen in water provides a continuous source for MXene degradation. In panel b, in the absence of dissolved oxygen, water (a mild oxidant) slowly degrades the MXene flakes in the Ar-based environment.

(2) Through elimination of the dissolved oxygen from water via compressed Ar gas, the degradation phenomenon could be suppressed substantially, but the flakes still degrade slowly over time (Figure 5b). (3) Separating flakes from water via vacuumassisted filtration results in films whose quality is independent of time and environment. To see if the above proposed scheme is general and universal, we employed another popular MXene, Ti2CTx, which has shown good electrochemical performance as a battery and supercapacitor electrode.50,51 The as-delaminated (d-) Ti2CTx MXene colloidal solution, a bright-brown color (Figure 6a), consists of ultrathin, electron-transparent nanosheets (Figure S17a). These Ti2CTx nanosheets have a mean lateral dimension of ⟨L⟩ = 400 nm, as shown in the representative SEM image and associated histogram (Figure S17b,c). To examine its stability, we first stored the d-Ti2CTx colloidal solutions in Air@RT, as described in the standard recipe.49 The normalized UV−vis spectra can be seen in Figure S18a. Surprisingly, it degraded in hours. A 50% amount of d-Ti2CTx concentration was decayed after aging in Air@RT for 8 h, and it was completely gone after 24 h, as shown in Figure 6c. The degradation of d-Ti2CTx flakes follows a single-exponential decay quite well. From the fitted line we obtain the time constant (τ) of 0.4 day (10 h), which is quite believable, as the solution became cloudy-white after aging the solution in Air@ RT for 12 h (Figure S19a). The TEM images reveal that numerous branches/nanoparticles were formed, which were

Ti3C2O2 + 4H 2O = 3TiO2 + 2C + 4H 2

To completely avoid the degradation, we separated the flakes from water by vacuum filtration of the colloidal solution. The morphologies of the as-obtained and the aged (Air@RT for 30 days) Ti3C2Tx MXene films are shown in Figure S15. The Raman spectra of films that were aged in Air@RT and Ar@RT for 30 days are compared in Figure S16a, showing identical spectra and suggesting the flakes are well-preserved when water is absent. The uniform distribution of Ti, C, and O signals in EDX mapping of the aged film (Figure S16b) also suggests that no apparent oxidation happened to the flakes. This is understandable, as the compact morphology prevents the inner nanosheets from interacting with humid air. Once the filtrated film was magnetically stirred in water or subjected to a 4853


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the lifetime of MXene solutions by either storing the colloidal solutions in hermetic Ar-filled bottles at low temperatures (5 °C) or by filtrating the colloidal solutions and storing the resulting films and re-dispersing MXene in solutions when needed. We have confirmed that the degradation starts at MXene nanosheet edges and showed that it follows a singleexponential decay behavior. Moreover, the separation of the dTi3C2Tx solution by flake size showed the degradation process to be size-dependent, with flakes of smaller sizes being less stable. A correlation between the time constants and flake size allows us to determine the size of the nanosheets in situ from the UV−vis spectra and vice versa. Finally, we extended the study to another type of MXene, d-Ti2CTx, and found that both the stability and time constant were improved substantially by refrigeration and storage in deaerated environment, demonstrating that the proposed protocols for extending the lifetime of MXene colloidal solutions are generally applicable. We note that the stability of MXene colloidal solutions is of importance as 2D MXenes are attracting ever-increasing attention in various applications and many MXene-based materials and devices are manufactured using solutions of single- or few-layer nanosheets. Most important is the size selection of MXene flakes in the colloidal solutions. The separation of larger MXene flakes from smaller ones would not only increase the shelf life but also improve performance of MXenes in various applications.

Figure 6. (a) Optical image of d-Ti2CTx MXene solution. (b) TEM image of d-Ti2CTx stored in Ar@LT for 12 h. (c) Stability of d-Ti2CTx in Air@RT and Ar@LT. The dotted lines are the fitting results according to the empirical equation, A = Aunre + Aree−t/τ.

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confirmed to be anatase (Figure S19b,c and inset). The poor lifetime would negatively limit the applications of d-Ti2CTx. To improve the stability of d-Ti2CTx solutions, we applied the proposed protocol, as developed for d-Ti3C2Tx, by storing the solution in Ar@LT. After being stored in Ar@LT for 12 h, the d-Ti2CTx flakes were relatively clean with well-defined edges and shapes (Figure 6b). The stability was improved significantly (Figure S18b and Figure 6c). The time constants (τ) increased to 10 days in Ar@LT, 24 times higher than that of the Air@RT sample. After 8 days, the colloidal solutions had degraded by 54%. Nevertheless, by isolating the flakes from water/dissolved oxygen via vacuum filtration, d-Ti2CTx flakes could be wellprotected from degradation over time and good-quality solutions could be recovered via a simple re-dispersing process. It is also worth pointing out that the much shorter lifetime of dTi2CTx solution compared to d-Ti3C2Tx agrees with previous reports28 and could be attributed to the fact that Mn+1Xn becomes more stable as n increases from 1 to 3.31 The Raman (Figure S20) and XPS spectra (Figure S21) also suggest that by storing the d-Ti2CTx solution in Ar@LT, the lifetime of colloidal Ti2CTx was greatly improved, demonstrating that the proposed method for improving the stability of MXene aqueous solution is effective and universal. Despite that, the much smaller time constant in d-Ti2CTx compared to dTi3C2Tx MXene in Ar@LT indicates the reaction speed between d-Ti2CTx and water is quite fast. Said otherwise, water is not a suitable solvent for the storage of this colloidal solution. Future work should focus on the identification of an ideal solvent that can simultaneously disperse and protect the Ti2CTx nanosheets.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00745. (Figures S1−S20) Additional images, SEM, TEM, Raman, XPS, and UV−vis extinction spectra, and histograms of studied compounds, (Table S1) XPS spectra fitted positions of MXene, and experimental details (PDF) (Movie S1) Film being re-dispersed forming a colloidal solution (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*(Y.G.) E-mail: [email protected]. Tel.: +1 215 8956446. *(V.N.) E-mail: [email protected]. Tel.: +353 (0) 1 8964408. ORCID

Chuanfang John Zhang: 0000-0001-8663-3674 Yury Gogotsi: 0000-0001-9423-4032 Present Address △

(G.S.D.) Universität der Bundeswehr München, 85577 Neubiberg, Germany.

Author Contributions

# The manuscript was written by C.J.Z with the contributions from all coauthors. C.J.Z. and S.P. contributed equally.

Notes

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The authors declare no competing financial interest.

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CONCLUSIONS In summary, we have studied the degradation of MXene aqueous colloidal solutions in detail using SEM, TEM, UV−vis, Raman spectroscopy, and XPS. Dissolved oxygen, a water medium, and elevated temperatures greatly speed up the decay process. Therefore, we have proposed methods of improving

ACKNOWLEDGMENTS We acknowledge the following funding support: SFI AMBER, SFI PIYRA, ERC StG, 2DNanoCaps, ERC PoC 2DUSD, ERC PoC 2DInk, FP7MC ITN MoWSeS, Horizon2020 NMP CoPilot, and MINECO research Project ENE2014-57977-C2-1-R, 4854


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etc. We thank the Advanced Microscopy Laboratory (AML) in CRANN, Trinity College. Mohamed Alhabeb and Kathleen Maleski (Drexel University) are especially acknowledged for the helpful suggestions on the material synthesis and data discussion. Prof. Duesberg acknowledges the support from SFI under Contract No. 12/RC/2278 (AMBER) and PI 15/IA/ 3131.

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Oxidation Stability of Colloidal Two-Dimensional Titanium Carbides

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