Alkylammonium Cation Intercalation into Ti3C2 (MXene): Effects on

Pressure-induced shear and interlayer expansion in Ti 3 C 2 MXene in the presence of water. Michael Ghidiu , Sankalp Kota , Vadym Drozd , Michel W...
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Alkylammonium Cation Intercalation into Ti3C2 (MXene): Effects on Properties and Ion-Exchange Capacity Estimation Michael Ghidiu,† Sankalp Kota,† Joseph Halim,‡ Alexander W. Sherwood,† Nils Nedfors,‡ Johanna Rosen,‡ Vadym N. Mochalin,*,§ and Michel W. Barsoum*,† †

Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States Thin Film Physics Division, Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden § Department of Chemistry and Department of Materials Science & Engineering, Missouri University of Science & Technology, Rolla, Missouri 65409, United States ‡

S Supporting Information *

ABSTRACT: Ti 3 C 2 T x MXene intercalated with Li + ions was produced and ion-exchanged with a series of trimethylalkylammonium (AA) cations of increasing alkyl chain length. A discontinuous expansion in the MXene layer spacing was observed, attributed to complete packing of the interlayer space at a critical chain length. The latter was used to estimate the number of cations per Ti3C2 formula unit, which was found to be in good agreement with a similar quantification obtained from X-ray photoelectron spectroscopy, energy-dispersive spectroscopy, and elemental analysis. The system was also modeled using density functional theory and molecular dynamics, arriving at cation concentrations in the same range. The intercalated AA cations led to tunable increases in resistivity of the normally highly electrically conductive MXene and were investigated as interlayer pillars in electrochemical capacitors.

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INTRODUCTION MXenes, now a fast-growing new family of two-dimensional (2D) materials since their discovery in 2011,1 have attracted interest due to their unique combinations of properties: high electrical conductivity,2,3 compositional variability,4,5 hydrophilicity,2 and their capability to host a broad range of intercalants.6−9 Because of these properties, they have potential uses in applications such as electrochemical energy storage,10,11 water purification,12 electromagnetic shielding and absorption,13,14 and polymer nanocomposites.8 Interlayer cations are of significant interest in MXenes, especially due to their involvement in many applications such as energy storage. A broad range of work has been undertaken to understand ion hosting between MXene layers.15−21 Early on, MXenes were dubbed “conductive clays”, a claim which we have recently expanded upon by investigating their ion exchange and humidity response, specifically with alkali and alkaline earth cations.9 With this work in mind, we were interested in whether much larger polyatomic cations such as alkylammonium (AA) cations could also be intercalated. Their © 2017 American Chemical Society

intercalation in other hosts (silicates, dichalcogenides, graphene oxide, and others) has been explored at length and has been shown to have drastic effects on structure, with the effects being tunable by changing the intercalant structure, for example, by increasing the length of alkyl chain of the cation.22−25 This is inherently important for four reasons: (i) Since most MXenes are highly electrically conductive, being able to fine-tune the interlayer space with semipermanent pillars (the cations can be exchanged for other cations, but cannot simply leave like a solvent molecule) could lead to fine-tunability of MXene conductivity. (ii) This could be a viable option for changing the inherent hydrophilic nature of MXene surfaces with the addition of nonpolar groups. (iii) If a large array of AA cations can be intercalated, this could prove a useful technique to introduce many different moieties into the MXene interlayer via grafting to the ammonium group.26 (iv) The presence of large, Received: October 4, 2016 Revised: January 10, 2017 Published: January 10, 2017 1099

DOI: 10.1021/acs.chemmater.6b04234 Chem. Mater. 2017, 29, 1099−1106

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Resistivity Measurements. The intercalated Ti3C2Tx powders were equilibrated at ∼50% RH for about 3 days, after which the powders were pressed at a load corresponding to 300 MPa into freestanding discs with an approximate diameter of 1 cm and thickness of 1 mm. The samples’ resistivities were then measured with a 4-point probe (ResTest v1, Jandel Engineering Ltd., Bedfordshire, UK). Electrochemical Characterization. To fabricate the electrodes for electrochemical testing, the intercalated Ti3C2Tx multilayered flakes, carbon black (Alfa Aesar, 100% compressed), and polytetrafluoroethylene binder (Aldrich) were combined in a weight ratio of 85:10:5, respectively, and homogeneously mixed via magnetic stirring in ethanol for ∼48 h until the ethanol was evaporated. The dried powder mixture was then kneaded with a metal spatula and rolled to a uniform thickness of 138−150 μm. The control sample of C0 used Ti3C2Tx powders etched in 10% HF without dissolved LiCl, as described above. Activated carbon (YP-40) counter electrodes were prepared in a similar manner, as previously reported.10 The rolled MXene electrodes were assembled in a three-electrode stainless steel Swagelok cell with an overcapacitive activated carbon counter electrode. The reference electrode was Hg/Hg2SO4 in saturated K2SO4, and the electrolyte was 1 M sulfuric acid, H2SO4. Gold and platinum disks were used as current collectors on the working electrode and counter electrode sides, respectively. Celgard 3501 polypropylene membranes were used as separators. Cyclic voltammetry was initially conducted 1,000 times at 10−20 mV/s as a precycling step to saturate the electrodes (see Figure S7) and subsequently conducted in the 2−200 mV/s range. Gravimetric capacitance values were calculated considering only the 85 wt % of intercalated or nonintercalated Ti3C2Tx as electrochemically active, similar to previous reports.6 Computational Modeling. Density functional theory (DFT) and classical molecular dynamics (MD) studies were performed using Accelrys Materials Studio. A Ti3C2(OH)2 cell was built and optimized using DFT (CASTEP module) with Perdew−Burke−Ernzerhof (PBE) generalized gradient approximation (GGA) exchange-correlation functional, energy cutoff 380 eV, 9 × 9 × 1 Monkhorst−Pack (MP) k-point set, and the following convergence criteria: energy, 5 × 10−6 eV/atom; maximal force, 0.01 eV/ Å; maximal stress, 0.02 GPa; and maximal displacement, 5 × 10−4 Å. The lattice parameters (a = 3.097, b = 3.097, and c = 20.192 Å) and angles (α = 90°, β = 90°, and γ = 120°) for the optimized cell are consistent with calculated30 and experimental7 literature data. Each atom in the optimized cell was assigned electric charge using a population analysis tool and Hirshfeld charging scheme. Each AA cation was built and placed in the center of a 18 × 18 × 18 Å periodic cubic cell. The system was assigned +1 electric charge. The geometry of the cations was then optimized by DFT PBE-GGA using an energy cutoff of 310 eV on the 2 × 2 × 2MP k-point set and convergence criteria listed above for Ti3C2(OH)2. Each atom in the optimized AA cation was assigned electric charge using a population analysis tool and Hirshfeld charging scheme. This charging scheme produced partial positive charges on N and H atoms and partial negative charges on C atoms in the AA cations as expected. For MD simulations, a 9 × 9 × 1 supercell of DFT optimized Ti3C2(OH)2 was built and intercalated with a predefined number of each of the DFT optimized AA cations using Amorphous Cell module, producing a number of AA cation-intercalated Ti3C2(OH)2 cells for subsequent studies. Classical MD simulations of intercalated MXenes are complicated since there is no general purpose force field that would include all atomic combinations present in MXenes and intercalants.31 Therefore, we used the Universal force field, which has been shown to give reasonably good results for MXenes intercalated with molecular species before.7 However, we kept the DFT calculated electric charges on all atoms in the system to achieve a better description of electrostatic interactions between the charged cations and Ti3C2(OH)2. To release the initial stress without distorting the cell, the geometry of each AA cation intercalated Ti3C2(OH)2 was optimized using the FORCITE module until 2.0 × 10−5 kcal/mol (energy), 1 × 10−3 kcal/mol/Å (force), and 1 × 10 −5 Å (displacement) convergence criteria were satisfied. In this step, no

pillaring cations in the interlayer space might ease electrolyte access in energy storage devices. In this work, we set out to find conditions suitable for intercalation of a large range of these cations and characterize the structural effects of the intercalations, as well as test the effects of these intercalated spacers on conductivity and in electrochemical capacitors.

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EXPERIMENTAL SECTION

MAX Phase Ti3AlC2. Powders of Ti3AlC2 were produced by pressureless sintering from Ti2AlC and TiC powders according to previous reports.1 In short, commercial Ti2AlC powders and TiC powders in a 1:1 molar ratio, accounting for ∼10 wt % Ti3AlC2 impurity present in the Ti2AlC powders, were ball milled with zirconia milling balls for 1 day. The mixture was placed in an alumina boat and heated at a rate of 5 °C/min under continuous Ar flow to 1350 °C. After 2 h at this temperature, the sample cooled passively in the furnace. The loosely sintered brick was milled into fine powders with a TiN-coated drill bit and passed through a 400 mesh sieve for etching later. Multilayer MXene Ti3C2Tx. One gram of Ti3AlC2 powder (−400 mesh/< 38 μm particle size) was immersed in a mixture of hydrofluoric acid (HF) and lithium chloride (10 mL of 5.8 M (∼10 wt %) HF with addition of 1.09 g of LiCl, for a LiCl/Ti3AlC2 molar ratio of 5:1). The reaction mixture was stirred at ∼25 °C for 24 h. Following etching, the mixture was centrifuged (3500 rpm or 2301 rcf for 1 min) to separate the sediment. The supernatant was discarded and replaced with hydrochloric acid (∼40 mL of 6 M HCl per 0.5 g of sediment) in each centrifuge tube. This HCl washing and centrifugation/decantation step was repeated 2 more times, followed by 2 washes with distilled water (∼40 mL/0.5 g sediment). The isolated wet sediments were then mixed immediately with solutions of the AA salts (40 mL of 0.5 M salts: tetramethylammonium chloride, and hexyl-, octyl-, decyl-, dodecyl-, and hexadecyltrimethylammonium bromide; TCI America). These solutions were allowed to equilibrate over 4 days, being agitated by hand daily. The final washing consisted of a similar centrifugation/decantation process (∼40 mL of distilled water/0.5 g of sediment, repeated 4 times) followed by suction filtration to isolate powders, with a final wash of 3 × 10 mL water on the filter. The samples were then air-dried. A control sample was also prepared by immersing 1 g of the Ti3AlC2 in 10 mL of 10 wt % HF for 24 h under continuous stirring. The powders were subjected to five cycles of washing with distilled water, centrifugation (3500 rpm or 2301 rcf for 2 min), and decanting the supernatant until the supernatant reached a pH of ∼6. This sample was collected by suction filtration, air-dried for several days, and used as the control sample for electrochemical studies. Characterization. X-ray Diffraction (XRD). A Cu KαI radiation source diffractometer (Rigaku SmartLab, Rigaku Corporation, Tokyo, Japan) was used to obtain all XRD patterns. Samples were scanned at a step size of 0.02° and dwell time of 0.5 or 1.0 s per step. Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS). SEM and EDS were performed on a Zeiss Supra 50VP (Carl Zeiss AG, Germany). X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed on multilayer Ti3C2Tx using a surface analysis system (Kratos AXIS UltraDLD, Manchester, U.K.) using monochromatic Al− Kα (1486.6 eV) radiation. The samples were mounted on double-sided tape and grounded to the sample stage with copper contacts. The Xray beam irradiated the surface of the sample at an angle of 45° with respect to the surface and provided an X-ray spot of 300 by 800 μm. Charge neutralization was performed using a coaxial, low energy (∼0.1 eV) electron flood source to avoid shifts in the recorded binding energy (BE). XPS spectra were recorded for Ti 2p, C 1s, N 1s, and Li 1s. The analyzer pass energy used for all regions was 20 eV with a step size of 0.1 eV. The BE scale of all XPS spectra was referenced to the Fermi-edge (EF), which was set to a BE of 0 eV. Peak fitting was carried out using CasaXPS version 2.3.16 RP 1.6 in the same manner as in refs 9, 27, 28, and 29, while the global elemental percentage was quantified as in ref 9. 1100

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Figure 1. Schematic of materials produced. First, Ti3AlC2 is etched with HF + LiCl to yield Li−Ti3C2Tx, which is then ion-exchanged with trimethylalkylammonium chlorides or bromides (that have cations of the form [(CH3)3NR]+, where R is an alkyl chain CnH2n+1 and n = 1, 6, 8, 10, 12, or 16) to form intercalated Ti3C2Tx. Note that these are sketches and not exact structural models. The structures in the interlayer of Cn-Ti3C2Tx represent alkylammonium cations.

Figure 2. (a) XRD patterns of intercalated Ti3C2Tx samples (after washing and drying of etched and ion-exchanged MXene powders); from top to bottom, n = 1, 6, 8, 10, 12, 16. Patterns recorded on powders, dried in air (∼40% humidity). The dashed line represents the peak position of Ti3C2Tx etched with HF alone (∼8.6°), and the dotted line represents the peak position of Li+-intercalated Ti3C2Tx (∼7.15°; with an H2O monolayer9). All peaks are from 002 reflections with the exception of C16, which shows reflections in this region for 004, 006, and 008 (weak). The values for Δd002 (d002 of Ti3C2 etched with HF alone subtracted from experimental d002) are shown on the right. (b) Nonintercalated structure, showing c, d002 (c/2), and layer separation s. From ref 32, c is 19.12 Å and s is 2.21 Å. (c) Expanded, intercalated structure of the same, showing, in addition, the interlayer space as Δd002 or Δd002 + s. cell optimization was allowed. The optimized cells were subjected to thermal MD equilibration at 298 K over the period of time of 100 ps (1 fs time step) in the NVT ensemble (again no changes in cell geometry were allowed at this stage) with a simple velocity scale thermostat. By the end of the equilibration MD run, the temperature and all components of energy were well stabilized. The equilibrated cells were subjected to 500 ps NPT MD at 298 K (using Nose thermostat) and 1 × 10−4 GPa isotropic pressure (maintained with Berendsen barostat) using final configurations of the cells from the equilibration NVT MD runs as starting configurations for the data collection NPT MD. In this final step, the cells were allowed to change geometry (expand) against external pressure applied by the barostat until a new equilibrium established. During this process, the c lattice parameter increased according to the number of the intercalated AA cations. Snapshots of the system were saved every 1000 MD steps (500 snapshots in total) of which only the last 300 snapshots were used for analysis by the tools provided in Materials Studio. During geometry optimization and the MD runs, the electrostatic interactions were summed up using the Ewald scheme, and the van der Waals interactions were summed up using the atom-based scheme.

Ti3C2Tx and wanted to now apply these to large polyatomic cations. For this, we chose AA cations of the form [(CH3)3NR]+, where R is an alkyl chain of variable length. In this study, R was chosen to be CH3, C6H13, C8H17, C10H21, C 12 H 25 , or C 16 H 33 (Figure 1); we designate samples intercalated with these cations as C1, C6, C8, C10, C12, and C16, respectively. The sample that was etched with 10% HF alone is designated C0. Following etching to produce the MXenes (Li−Ti3C2Tx), the wet powders were intercalated with cations of AA salts via ion exchange. For Ti3 C 2 Tx , the collapsed unit cell (u.c.) has an experimentally determined c lattice parameter (c-LP, or 2 x d002) of ∼19.12 Å (dashed line in Figure 2a and illustrated in Figure 2b).1,32 Since each u.c. contains two interlayer spaces, the expansion of one interlayer space, Δd002 (in Å), can be estimated by subtracting d002, collapsed from d002, expanded: Δd002 ≈ (d002,expanded − 9.56Å)

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(1)

where d002 of the collapsed structure comes from ref 32. From XRD, it was clear that there was an expansion of c-LP after equilibration with AA salts and that this expansion, of the order of 4.5−4.6 Å per interlayer space, was generally the same regardless of chain length up to R = C10H25 (Figure 2a). This expansion is attributed to intercalation of the AA cations in a configuration of the chains lying flat along the MXene sheets, as all the cations have roughly the same cross-sectional height.33 XRD of the C10 sample displays the onset of a lower-angle

RESULTS AND DISCUSSION As noted above, all Ti3C2Tx samples (with the exception of the control C0) were produced by reaction of Ti3AlC2 powders with a mixture of 10 wt % HF and LiCl. We have recently worked with this etchant mixture to intercalate exchangeable cations into MXene (when the material is etched with 10% HF alone, cations do not readily intercalate post-etching).9 In that work, we established procedures for cationic exchange in 1101

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Chemistry of Materials peak; sample C 12 completely shifts to this position, corresponding to a new c-LP of 37.8 Å or a Δd of +8.9 Å per interlayer. For sample C16, the c-LP increases to 75.9 Å or a Δd of +27.9 Å per interlayer space. We note that even at this large expansion, the material remains crystalline enough to give a sharp 002 peak (see XRD pattern labeled C16 in Figure 2a) and that despite the large layer spacing, the multilayer morphology was preserved in all cases (Figure S3). Drying the samples over P2O5 did not result in changes in the XRD patterns (examples of C1 and C16 are shown in Figure S1), confirming that the structural expansions were caused by the organic cations and not H2O. When the powder samples were pressed uniaxially into discs at a load corresponding to a stress of ≈200 MPa prior to measuring their electrical resistivity (discussed later), the resulting diffraction patterns (Figure S2) confirmed that no major structural changes took place. After establishing that the AA cations intercalate into the MXene sheets, we set out to understand the Δd002 values through simple modeling. By taking the density and molecular weight of various N,N-dimethylalkylamines (these are used as approximations for AA cations since the difference is one fewer CH3 group), a volume per cation in the bulk can be estimated. From densities and molar masses of commercially available N,N-dimethylalkylamines, the volume occupied per cation, in Å3, can be described through a regression (Figure S4) as Vcation ≈ 27.17 × nc + 123.99

(2)

where nc is the number of C atoms in the cation’s alkyl chain. The volume of one interlayer space of intercalated Ti3C2Tx (Cn) for the monolayer intercalated form can be calculated as the unit-cell basal area multiplied by height. It is important to note that each unit cell contains two Ti3C2 units and that we are describing half of a unit cell here. The area is related to the a unit cell parameter, which does not change appreciably with intercalation, as 2 × ((3)1/2/4) × a2; the height can either be taken as simply Δd002 (d002, expanded − d002, collapsed) or inclusive of separation in the van der Waals gap present in the collapsed structure, Δd002 + s (see Figure 2b). Then, Vinterlayer = h ×

3 × a2 2

Figure 3. Packing of cations of varied size, assuming that the number of cations per unit cell is constant: (a) the area of the cations is not enough to completely cover the available basal area; (b) the basal area is packed completely; (c) cation chains are made longer without more room available on the basal area, so they bulge, forming a bilayer structure. (d) Expected critical nc calculated as a function of ξ for two models, generated from eq 5. Red points denote where molecular dynamics results fall on this plot.

(3)

we presume ξ to be fixed while the cation chain length varies (assuming that the charge per Ti3C2 is relatively invariant). However, if ξ is allowed to vary, the same type of expansion can occur for a single selected cation chain length at a critical ξ as shown in Figure 3d. For our experimentally observed u.c. expansion, which occurs between C10 and C12 (nc = 10 to 12), Model 1 gives a value for ξ between 0.10 and 0.09, and Model 2 gives a value between 0.15 and 0.13. Both are in decent agreement with our recent XPS results in which we showed that ξ is of the order of 0.1−0.3 per Ti3C2 for select alkali metal cations.9 As further support of this range for ξ, XPS, EDS, and elemental analysis were performed on selected samples. XPS performed on C6, C10, and C12 (Table S1) revealed that intercalated N concentration per Ti3C2 was in the range of 0.14, 0.15, and 0.1, respectively (the N concentration was quantified with respect to Ti3C2 by taking the N signal originating from intercalated species). Further, XPS showed no signal from intercalated Li+, giving strong evidence that ion exchange was complete. Quantification of C/Ti ratios by EDS matches very closely the C/Ti ratios calculated by assuming a base ratio of 3Ti/2C, with additional C coming from the cations in the amount ξ × (nc + 3); e.g., intercalated C6 should have a cation

where h is given by Δd002 (from here on, Model 1) or as Δd002 + s (Model 2). Refer to Figure 2b,c for an explanation of these differences. A number, ξ, of cations occupy volume in the interlayer (Figure 3a), and the length, nc, of their alkyl chain is allowed to become larger, until a critical nc (Figure 3b) where they completely fill the volume available in the interlayer space, after which the c-LP must increase (Figure 3c) to accommodate them, or when Vinterlayer = Vcation × ξ

(4)

Solving for ξ, ξ=h×

3 × a 2 /(27.17 × nc + 123.99) 2

(5)

It should be noted that ξ will correspond to the charge per Ti3C2 if all negative charges on the MXene are being balanced by the cations; therefore, an upper limit to ξ is the negative charge per Ti3C2. A plot of the calculated values for critical nc as a function of ξ for both Models 1 and 2 is shown in Figure 3d. This gives a way to relate a value of critical nc observed experimentally to the number of cations. In our experiments, 1102

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At first, N is localized in the middle of the interlayer space (each MXene cell has 2 interlayer spaces), producing 2 sharp peaks in the concentration profile. In region iii, after the expansion occurs, the peaks become noticeably broader, and second peaks start to appear in each interlayer space of the MXene. This is evidence that the cations are no longer contained in one plane. MD snapshots of these regions are provided in Figure 4b. We find that ξ at which this happens is ∼0.12, in line with all other methods. Similar modeling and calculations were repeated with C1 and C6 (Figure S6). Extracting the ξ value at which expansion occurs, we find that the values fit within our simple model in Figure 3d (red points). To investigate some effects of intercalation on properties, we measured the DC conductivity of multilayered powders, in the form of compacted discs as reported in the literature.2 Figure 5

C contribution of 9 × ξ (Figure S5). Figure S5a shows experimental C/Ti ratios along with those calculated using ξ = 0.12; Figure S5b shows deviation from the experimental results with ξ values of 0.06 to 0.18. According to elemental analysis (Laboratory Testing, Inc., Pennsylvania, USA), the amount of N per Ti3C2 formula unit is ∼0.11 in both C6 and C12. Since each cation contains one N atom, this provides solid evidence that ξ obtained from various methods is in the correct range. Further, the C1−C10 expansions observed are in excellent agreement with Lagaly and Weiss,34 who model a basal expansion in silicates between 4.1 and 4.6 Å per interlayer for monolayer-intercalated AA cations. The larger expansion we observed for C12 of 8.9 Å is roughly double this, most probably representing a bilayer configuration of the chains after completely packing the monolayer. There are many other reports of similar expansions occurring in other materials, at varied nc, for example, MoS2 and montmorillonites.22,24 We note that this is an oversimplified model, but it provides a decent method for estimating ξ per Ti3C2. For the case of C16, the interlayer expansion (∼28 Å) is mostly in agreement with the length of [(CH3)3NC16H33]+ (roughly 25 Å),35 so it is likely that the intercalant chains are arranged so that they now stand up vertically between the layers. The reason for the transition to this possible arrangement is unclear at this time and requires more study that is beyond the scope of this work. To gain greater insight into the origins of the jump in c-LP at a critical nc, molecular dynamics simulations were carried out using the methodology outlined above. One u.c. of MXene was packed with C10 cations (the chain length at which our interlayer expansion was observed experimentally) at various values of ξ, and the c-LP was determined. Plotting c-LP against ξ, it is clear that there is a gradual slope at first (Figure 4a, region (i)), with a transition (region (ii)), followed by a much higher slope (region (iii)). The change in slope is caused by the interlayer being filled until it is forced to expand; this is evidenced by the evolution of the N concentration profile along the c axis over the course of the simulation (Figure 4c).

Figure 5. Effect of alkyl chain length on measured c-LP (red squares, left axis) and resistivity (blue squares, right axis). The latter was measured on discs cold pressed at 200 MPa. The lines joining the points are guides to the eyes.

(right axis) shows the effect of nc on disc resistivities, from which it is clear that a dramatic increase in resistivity is observed for nc > 10. This is most likely caused by the disruption of out-of-plane conductivity, which is already estimated to be lower than the in-plane conductivity,36 as the sheet-to-sheet separation increases. We suspect this is the case due to the fact that the trend in resistivity (blue squares, Figure 5) roughly follows the trend in c-LP expansion (red squares, Figure 5), with a few outlying points, in particular C1, for which we currently do not have an explanation. In the pressed discs, the in- vs out-of-plane conductivity difference may be a significant factor to the overall conductivity since the particles are not all aligned in the same direction, so that the path of a conducting electron through the disc may encounter a combination of in- and out-of-plane segments. It will be quite interesting to test the anisotropy of conductivity for single particles of intercalated MXene, but we reserve this for future studies. Since these are not fully dense discs, transport resistance due simply to porosity may also play a large role here. Delaminated Ti3C2Tx flakes have been known to reach high capacitances when used as electrochemical capacitor electrodes in sulfuric acid electrolyes.37 However, when composite Ti 3C 2Tx/carbon black electrodes, containing multilayer MXene particles, were tested in H2SO4, the capacitance was

Figure 4. Molecular dynamics simulation results for C10. (a) c-LP as a function of a number of cations per Ti3C2, ξ. (b) MD snapshots of the three regions highlighted in a, showing a cross-section view of the interlayers. (c) N concentration along the c axis for the three regions highlighted in a. There are two regions of peaks because each unit cell contains two interlayer spaces. 1103

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Figure 6. Electrochemical capacitance of AA-intercalated Ti3C2Tx: (a) cyclic voltammagrams of nonintercalated (C0) and intercalated C1, C6, and C10 samples; (b) gravimetric capacitance as a function of scan rate; (c) gravimetric capacitance at various scan rates as a function of alkylammonium chain length.

electrochemically inert in this potential range and given that roughly the same number of moles of each are preintercalated into each electrode, then the larger AA cations will inherently lower the specific capacitance since they would constitute a greater fraction of dead weight (and volume) in the electrode. While the AA interlayer spacers had a relatively small effect on the capacitance values associated with small protons, using ionic liquid electrolytes or organic electrolytes that contain much larger cations with sizes on the order of the interlayer space may be a more promising route for future studies. Interestingly, the pure Ti3C2Tx electrodes tested here reach higher capacitance values despite being about three times thicker than those reported by Lukatskaya et al. on Ti3C2 in H2SO4.37 Assuming the specific surface areas are roughly equal for 10% and 50% HF-etched Ti3C2Tx, the greater fraction of −O and −OH terminations32 in the former could contribute to higher pseudocapacitance. We note that it is not known at this time whether an alkylammonium/H+ ion exchange occurs during cycling that may change the structure. Despite the only modest effects on capacitance observed here, the addition of alkylammonium cations to the interlayer may have other interesting applications such as controlled hydrophobization of the surface for easier dispersion in solvents or insertion of various functional moieties into the interlayer.

considerably lower. It has been postulated that this is caused by potentially high activation energies required for cations to diffuse to electrochemically active sites deep inside the interlayers.38 To investigate whether the presence of large, pillaring interlayer AA cations can enhance charge storage, we performed cyclic voltammetry on similar composite Ti3C2Tx/ carbon black electrodes, with Ti3C2Tx etched with HF only (C0) and those which were intercalated with C1, C6, and C10. As shown in the cyclic voltammograms (CVs) obtained at 5 mV/s in Figure 6a, the area of the CV loop increased in going from the control sample C0 to C1 but decreased substantially moving to C10. For reasons not clear at this time, the stable potential window for C10 was 50 mV smaller than those of the control, C1, and C6 samples. In Figure 6b, the capacitance of the C1 sample is ≈12% higher than the control sample C0 at the lowest scan rate of 2 mV/s. On the other hand, the capacitance for the C10 sample decreases by up to 29% relative to C0. Figure 6c shows more clearly the dependence of the specific capacitance on nc at each scan rate. From this plot, it is clear that only the C1 sample shows a slightly enhanced capacitance at scan rates up to 20 mV/s, above which the measured capacitance is roughly independent of nc. The fact that the capacitances of all four electrodes converge at high scan rates indicates that charge is predominantly stored in the electrical double-layer on the particle surface or shallow sites in the interlayer at scan rates of 50 mV/s and above. These results reflect that the capacitance initially increases from smaller AA chains pillaring the interlayer enough to enhance electrolyte access. However, when considerably longer AA chains are present, the capacitance decreases, most probably due to higher resistivity impeding charge transfer and/or larger nonpolar C−C chains impeding the diffusion of the highly polar H2SO4. Furthermore, assuming that the AA cations are

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CONCLUSIONS Multilayered Ti3C2Tx was synthesized with intercalated Li+ cations. The Li+ were ion-exchanged with trimethylalkylammonium cations of varied alkyl chain lengths, resulting in expansion of the interlayer space. The degree of expansion did not monotonically increase with nc but remained independent of chain length up to nc ≈ 10, beyond which it increased significantly. This expansion can be simply modeled 1104

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Chemistry of Materials

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and is similar to many other classes of materials such as clays and dichalcogenides. As done with some of these systems, we have used the point of expansion to estimate a cation content per Ti3C2 unit (ξ) of the order of 0.09−0.15, which compares well with results we have previously obtained by other methods. As further evidence of the validity of this range, elemental analysis suggests a ξ value of 0.11, EDS analysis of C content suggests a ξ of ∼0.12, XPS analysis suggests a value of 0.1− 0.15, and molecular dynamics suggests a value of ∼0.12. From the convergence of these various techniques, we feel confident that we have determined the capacity for AA cations in Ti3C2Tx. On the basis of this work, we have demonstrated good control of the height of the Ti3C2Tx interlayer space in the range of ∼5−28 Å. Importantly, because a large range of AA cations can be intercalated into Ti3C2Tx, functionalized trimethylammonium intercalation seems to be a widely applicable technique that can greatly expand the intercalation chemistry of these materials. The conductivity of bulk pressed discs was found to decrease as the size of the intercalating AA cation increases. For reasons that we have not yet entirely elucidated, we find no marked enhancement in the capacitance as a result of incorporating AA cations; a mild improvement is seen for C1, with competing effects reducing the capacitance for larger cations such as C6 and C10.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04234. XRD, SEM, volume-packing analysis, EDS analysis, XPS quantification, molecular dynamics results, and electrochemistry (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(V.N.M.) E-mail: [email protected]. *(M.W.B.) E-mail: [email protected]. ORCID

Michael Ghidiu: 0000-0002-0599-9824 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation under Grant No. DMR-1310245. M.G. was supported by the National Science Foundation Graduate Research Fellowship under Grant 283036-3304. We thank the Drexel Core Facilities and staff for assistance with characterization involving XRD and SEM. J.R. acknowledges funding from the Knut and Alice Wallenberg (KAW) Foundation, and from the Swedish Foundation for Strategic Research (SSF) through the synergy grant FUNCASE.

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REFERENCES

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DOI: 10.1021/acs.chemmater.6b04234 Chem. Mater. 2017, 29, 1099−1106

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on January 27, 2017, with errors in eqs 3 and 5. The corrected version reposted January 30, 2017. 1106

DOI: 10.1021/acs.chemmater.6b04234 Chem. Mater. 2017, 29, 1099−1106