Article pubs.acs.org/cm
Dispersions of Two-Dimensional Titanium Carbide MXene in Organic Solvents Kathleen Maleski,† Vadym N. Mochalin,*,‡ and Yury Gogotsi*,† †
A. J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States ‡ Department of Chemistry and Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409, United States S Supporting Information *
ABSTRACT: Two-dimensional titanium carbide (Ti3C2Tx) MXene has attracted a great deal of attention in the research community and has already showed promise in numerous applications, but only its dispersions in aqueous solutions have previously been available. Here we show that Ti3C2Tx can be dispersed in many polar organic solvents, but the best dispersions were achieved in N,Ndimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, propylene carbonate, and ethanol. The dispersions were examined by measuring the concentration and absorbance spectra of MXene in organic solvents as well as correlating the concentration to solvent physical properties, such as surface tension, boiling point, and polarity index. Hildebrand and Hansen solubility parameters were additionally used to provide an initial understanding of how Ti3C2Tx MXene behaves in organic media and potentially develop quantitative correlations to select solvents and their combinations that can disperse Ti3C2Tx and other MXenes. Using this analysis, we have outlined a range of organic solvents, which can disperse Ti3C2Tx, expanding the opportunities for processing techniques, such as mixing MXenes with other nanomaterials or polymers to form composites, preparing inks for printing, and deposition requiring solution processable materials, allowing the use of Ti3C2Tx in a multitude of applications.
T
A relatively new but already very widely studied class of twodimensional materials, transition metal carbides, and/or nitrides, MXenes, are formed upon the selective extraction of monatomic layers of aluminum (Al) or other group III and IV elements from their bulk precursors, layered ternary or quaternary transition metal carbides, and nitrides, MAX phases (Figure 1a,b). Most commonly, MXene’s exfoliated structure originates from etching the “A” element, with either aqueous hydrofluoric acid16 or a mixture of lithium fluoride and hydrochloric acid,17 from a MAX phase that typically has a formula of MnAXn−1, where M commonly stands for a transition metal (Ti, Mo, V, etc.), A is Al, Si, Ga, etc., X is carbon and/or nitrogen, and n is an integer from 1 to 3.18 Synthesized from the Ti3AlC2 precursor, titanium carbide (Ti3C2Tx) was the first reported16 and is the most studied MXene that has already shown great promise for applications in energy storage,17,19,20 electromagnetic interference shielding,21 transparent conducting coatings,22−24 and many other applications,25 but ∼20 other MXenes have already been synthesized. Conventionally, Tx is used to denote the groups that terminate the MXene surface such as O, −OH, and −F,
wo-dimensional (2D) materials have attracted attention recently because of their advantageous properties (mechanical, optical, electronic, etc.), which are revealed when the 2D material is isolated from its bulk precursor.1 Separation from the three-dimensional (3D) bulk structure into 2D sheets often occurs by exfoliation, a process accomplished by mechanical cleavage or by chemical means, increasing the available surface area and drastically changing the material’s properties. The process of exfoliation, which occurs in the liquid phase, is closely associated with the dispersion of 2D materials into a colloidal state; therefore, the dispersion behavior and stability of the 2D material in the liquid colloidal state become important to understand. Two-dimensional materials such as graphene,2−6 transition metal dichalcogenides (TMDs),7−9 hexagonal boron nitride (h-BN),10−12 clays,13 and layered double hydroxides (LDH)14 have been exfoliated and stabilized in colloidal solutions with a surfactant, via surface charge, by adjusting surface chemistry, or with polymer assistance. Dispersing nanomaterials in solution provides necessary means of developing new techniques for deposition and/or allows materials to be utilized in the ways they could not be used before, in processes such as spray coating, casting, spin coating, 2D and 3D printing, sol−gel techniques, etc.,15 allowing many possible routes to fabricating multifunctional devices and exploring new applications. © XXXX American Chemical Society
Received: November 11, 2016 Revised: January 12, 2017
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DOI: 10.1021/acs.chemmater.6b04830 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
previous studies of other 2D materials,3,8,37 Hildebrand and Hansen solubility parameters were also investigated to screen the solvents and provide more information regarding Ti3C2Tx dispersion behavior.
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EXPERIMENTAL DETAILS
Synthesis of Ti3C2Tx and Ti3C2Tx Clay. In this study, two methods previously reported in the literature were used to synthesize titanium carbide: Ti3C2Tx produced by etching Ti3AlC2 with aqueous hydrofluoric acid (HF) and Ti3C2Tx clay produced by etching in a mixture of lithium fluoride and hydrochloric acid (LiF/HCl).16,17 One gram of Ti3AlC2 MAX powder (20). When the polarity of the solvent matches the polarity of the material, the latter is expected to be more stable in the dispersion because “like dissolves like”. By plotting the concentration versus polarity index for each solvent (Figure 4b), we can linearly fit it to reveal that solvents with a higher polarity index are, indeed, best for dispersing Ti3C2Tx. Upon comparison of correlations between MXene solution concentrations and surface tension, boiling point, polarity, etc., it is evident that although none of the solvent properties alone
δT =
ΔHv − RT Vm
(1)
where ΔH, R, T, and Vm represent the heat of vaporization, the gas constant, the temperature, and the molar volume of the condensed phase, respectively.41 The three Hansen solubility parameters correspond to three contributions to the cohesive energy density due to dispersion, dipole, and hydrogen bonding interactions. These contributions are called the dispersion Hansen parameter (δD), the polarity Hansen parameter (δP), and the hydrogen bonding Hansen parameter (δH), respectively. Because no other contributions to cohesive energy density are postulated in the Hansen model and cohesive energy is an additive parameter, the sum of squares of these three Hansen parameters should be equal to a square of the Hildebrand solubility parameter, which represents the total cohesive energy density (eq 2). δ T 2 = δ D2 + δ P 2 + δ H 2 E
(2) DOI: 10.1021/acs.chemmater.6b04830 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
high dispersion interaction strength. Additionally, the relationship between surface tension and Hansen solubility parameters (polar/dispersive) has been used for other 2D materials in the literature to investigate the parameters in parallel.38 More information about this investigation with Ti3C2Tx can be found in the Supporting Information. The long-term stability of the dispersions was also examined by leaving the samples undisturbed for 40 days in a lab drawer away from sunlight. When observed after 40 days, DMF, NMP, PC, and ethanol each exhibited only a small amount of precipitation (Figure S4). Importantly, the control solution of Ti3C2Tx in water showed some oxidation (visible by the white sediment of TiO2), while DMF, NMP, PC, and ethanol all remained as dark supernatants. This leads to an important conclusion about the chemical behavior of MXene. Previously, TiO2 formation in aqueous MXene suspensions over time was ascribed to slow reaction of Ti3C2Tx with oxygen dissolved in water. However, the solubility of oxygen in ethanol and other organic solvents shown in Figure S4 is at least an order of magnitude higher than its solubility in water.45 Still, TiO2 is observed in a noticeably larger amount in water than in other solvents, in which no TiO2 formation over time is visible at all. This observation leads us to conclude that Ti3C2Tx MXene slowly reacts with H2O itself or a H2O/O2 mix, not just with dissolved oxygen. Other 2D materials, such as black phosphorus,46 also demonstrate a low oxidation stability, which can be circumvented by using different strategies. Hanlon et al. hypothesized that the solvation shell of the organic solvent molecules protects the nanosheets from reaction with water and/or oxygen.46 However, the first necessary step to suppressing the undesired reactions of MXenes and other 2D materials with the environment is to understand these reactions. If water reacts with MXenes, then switching to organic solvents provides additional benefits. Uncovering solvents that can extend the shelf life of delaminated Ti3C2Tx nanosheets will become important for long-term storage and large-scale production of this promising nanomaterial. Although the majority of results were obtained with Ti3C2Tx MXene produced by MAX phase etching in HF, as a proof of concept and an extension of this work, delaminated Ti3C2Tx clay MXene produced by MAX phase etching in a LiF/HCl solution (Figure S3) and exfoliation of multilayered, LiF/HCletched, Ti3C2Tx (Figure S6) were also investigated in organic solvents. This delaminated material proved to be more challenging to disperse once it was in film form, as the Li+ ions that are intercalated during etching more strongly held together the negatively charged MXene nanosheets. When the Ti3C2Tx clay was sonicated in organic medium for 1 h, large clumps of material remained at the bottom of the container. After being sonicated for 3 h, the dispersions were free of large particles, allowing stability studies. After dispersions were formed, trends similar to those of HF-etched Ti3C2Tx were achieved. In this case, methanol was still a poor solvent, as well as the nonpolar solvents, as noticed directly after sonication. However, following 96 h, stable solutions were exhibited in ethanol, acetonitrile, DMSO, DMF, NMP, and PC. With these results in hand, it can be suggested that Ti3C2Tx clay differs only slightly from HF-etched Ti3C2Tx, even though the surface termination is substantially different, with a lower content of fluorine on the surface.26 Differing from Ti3C2Tx etched by HF, Ti3C2Tx clay may be even more polar, because acetonitrile with
The solubility parameters have been used frequently to predict the solubility or dispersibility of 2D materials,38 such as graphene,3,42 reduced graphene oxide, and graphene oxide,37 TMDs,8 and others, as well as one-dimensional materials such as single-walled carbon nanotubes, 43 etc. The Hansen parameters of the solvents used in this study are depicted in a radar plot in Figure 5c. Hansen solubility parameters are advantageous because solvent−solute interactions can be investigated component by component. By measurement of the absorbance spectrum of each Ti3C2Tx/solvent solution (Figure 5a), the absorbance per path length (A/l) at a specific wavelength (800 nm for HFetched Ti3C2Tx) can be extracted. Then, plotting A/l versus the Hildebrand parameter (Figure 5b) and fitting to a Gaussian, we find a peak appears around ∼27 MPa1/2, suggesting Ti3C2Tx MXene may disperse better in solvents with values close to this cohesion energy density. In comparison, graphene shows good dispersion in solvents with a Hildebrand solubility parameter of ∼23 MPa1/2.42 Hansen solubility parameters δD, δP, and δH are analyzed individually (Figure 5d−f, respectively). As follows from the figure, for good MXene solvents, δD is moderate to high (15.5− 20.0 MPa1/2), δP is high (8.8−18 MPa1/2), while δH is less important and may vary over a broad range from 4.1 to 42.3 MPa1/2. Thus, a good solvent for MXene must have (1) high polarity while (2) providing a high strength of nonspecific dispersion interactions, whereas its ability to form hydrogen bonds is not necessarily important. (Poly)aromatic solvents provide high-strength of dispersion interactions but are not polar enough, which suggests they may not work for MXenes, although they work for fullerenes, CNTs, graphene, and other sp2 carbon nanoparticles.44 This explains why DCB and toluene are poor solvents for MXene and why slightly more polar DCB is still better than toluene (Figure 2). Acetonitrile is an example of a poor solvent that is on the opposite side: it is very polar but has a low dispersion interaction strength. Still, acetonitrile works better than acetone, which has a similar small dispersion interaction contribution but in addition is not polar enough to disperse Ti3C2Tx. Five of six “good” solvents (H2O, NMP, PC, DMF, and DMSO) have a high strength of both polar and dispersion interactions, and in addition, H2O has an unusually high hydrogen bonding strength. The case of alcohols remains unclear. Ethanol polarity is low, and its dispersive parameter is moderate; thus, we may assume that its strong hydrogen bonding ability makes it a good MXene solvent. However, if this logic is correct, then methanol with a higher polarity, a comparable dispersion interactions contribution, and a larger hydrogen bonding term should be a better solvent than ethanol, which is not the case. Modeling showed that methoxy groups may form on the Ti3C2Tx surface in contact with methanol.40 Potentially, this may result in the improved solubility of methoxy-terminated MXenes in alcohols, but this hypothesis needs more theoretical and experimental support. Aliphatic solvents with low polarity and low to moderate dispersive energy contributions cannot be good solvents for MXenes. The analysis of the solubility parameters allows for a better understanding of the factors governing MXene colloidal stability and formulates a set of properties that are essential for being a “good” solvent. These considerations may be extended beyond simple screening of the individual solvents. For example, on the basis of this analysis, it should be possible to formulate a good solvent for MXene by mixing highly polar acetonitrile with a (poly)aromatic solvent, which will provide a F
DOI: 10.1021/acs.chemmater.6b04830 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials a high polarity and small dispersion energy contribution also becomes a “good” solvent. Exfoliation of multilayered, LiF/HCl-etched Ti3C2Tx showed promise in select “good” solvents DMF, DMSO, and PC, similar to the water control. The A/l was obtained for each of the dispersions, as a method for probing the quantity of nanomaterial retained in solution (Figure S6). The high surface tension of these solvents could provide a route to successful exfoliation; however, more investigation of the mechanisms and efficiency of exfoliation of multilayered Ti3C2Tx in organic solvents is necessary. We assume that Ti2CTx MXene, having a similar surface chemistry,40,47 will show comparable behavior in terms of dispersion stability in organic solvents. MXenes with other transition metals may also behave similarly because of oxide/hydroxide-like surface terminations, but this hypothesis needs to be confirmed experimentally.
of dispersion stability of MXene in organic solvents could lead to advances in many functional applications, including a wider range of Ti3C2Tx−polymer composites, inks for additive manufacturing, and more effective methods for producing films and 3D structures from MXene colloidal solutions.
CONCLUSIONS The dispersions of Ti3C2Tx and Ti3C2Tx clay nanosheets in organic solvents were directly investigated for the first time. The results indicate that polar solvents better disperse Ti3C2Tx MXene, being somewhat similar to polar termination groups, which are present on the surface of MXene sheets (Table 1).
Corresponding Authors
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04830. Characterization methods, physical solvent literature values, Hansen and Hildebrand literature values, and additional information about Ti3C2Tx dispersions (PDF)
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solvent deionized water ethanol methanol acetone ACN DMSO DMF NMP PC DCB toluene hexane
solvent group polar protic polar protic polar protic polar aprotic polar aprotic polar aprotic polar aprotic polar aprotic polar aprotic nonpolar nonpolar nonpolar
for MXene produced by the LiF/HCl method
for multilayered Ti3C2Tx (LiF/ HCl) exfoliated
good
good
good
good
good
X
fair
X
n/a
fair
fair
n/a
fair
good
n/a
good
good
good
good
good
good
good
good
X
good
good
good
X X X
X X X
n/a n/a n/a
AUTHOR INFORMATION
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Kathleen Maleski: 0000-0003-4032-7385 Yury Gogotsi: 0000-0001-9423-4032 Notes
Table 1. Dispersibility Based on Solvent Group and Type of MXene Investigateda for MXene produced by the 50% HF method
ASSOCIATED CONTENT
S Supporting Information *
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Korean National Research Foundation via the NNFC-KAIST-Drexel Nano2 Co-op Center (NRF-2015K1A4A3047100). The authors thank Dr. Babak Anasori for providing MAX phase materials, Mohamed Alhabeb for helpful discussions about synthesis, and Katherine Van Aken for helpful comments on the manuscript (all from Drexel University).
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ABBREVIATIONS EtOH, ethanol; MeOH, methanol; ACN, acetonitrile; ACE, acetone; DMF, N,N-dimethylformamide; DMSO, dimethyl sulfoxide; NMP, N-methyl-2-pyrrolidone; PC, propylene carbonate; HEX, hexane; TOL, toluene; DCB, 1,2-dichlorobenzene
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REFERENCES
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a
Legend: good, disperses in solvent and is stable over at least 96 h; fair, originally disperses but falls out of solution over 96 h; X, disperses very little or not at all; n/a, no test performed
We have identified several organic solvents (DMF, NMP, PC, and ethanol) in which Ti3C2Tx can form dispersions with longterm stability. By using solvent properties found in the literature, as well as Hildebrand and Hansen solubility parameters, we identified a library of organic solvents that disperse Ti3C2Tx. Moreover, when organic solvents are compared to Ti3C2Tx dispersions in water, results show that organic solvents can mitigate or slow MXene degradation, extending the shelf life of delaminated Ti3C2Tx. This first study G
DOI: 10.1021/acs.chemmater.6b04830 Chem. Mater. XXXX, XXX, XXX−XXX
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