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Investigation on Relaxational Behavior of Alkylammonium Ions Intercalated in Graphite Oxide Xiaoqian Ai,† Ji Yu,† Min Gu,*,† Tong B. Tang,‡,§ and Tao Zhang∥ †

National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Department of Physics, Hong Kong Baptist University, Kowloon, Hong Kong SAR, People’s Republic of China ∥ College of Engineering and Applied Science, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: Graphite oxide (GO) nanocomposites have been synthesized to contain various concentrations of intercalated alkylammonium ions and characterized with X-ray diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and differential scanning calorimetry. Depending on their concentration, the alkyl chains may lie parallel to the GO plane one or two layers thick or they form two columns inclined at an angle ∼37° to the plane. Dielectric spectroscopy reveals a relaxation process far below room temperature, attributed to small-angle wobbling around the long molecular axis. The activation energy of this relaxation increases as the intercalate changes from one to two layers, and to dual columns, with increasing interactions among the intercalated molecules. An additional phase transition occurs in composites with high concentrations of intercalate between a rotator-type solid phase to a disordered phase for the confined alkyl chains.

1. INTRODUCTION Amphiphiles are long-chain compounds containing both hydrophobic groups (at their tails) and hydrophilic groups (at their heads) that can function as surfactants, which may then be classified by reference to their polar hydrophilic group.1 A nonionic surfactant has no charge groups in its head, whereas the head of an ionic surfactant carries a net charge. If the charge is negative, the ionic surfactant is more specifically called anionic; if the charge is positive, it is cationic. Cationic surfactants, such as alkylammonium compounds, are often used to modify the surface properties of hydrophilic substances like layered double hydroxides,2,3 layered metal phosphate4 and two-dimensional layered materials.5 The thermodynamics of surfactants occupies great practical importance and theoretical interest, due to their configurational complexity, and indeed they may become ordered or disordered when their concentrations or chemical environments change. In particular, for alkylammonium ions confined inside layered materials, their dynamical behaviors have been probed with computational and experimental techniques such as molecular simulation, Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and solid state nuclear magnetic resonance (NMR). Generally, confined alkylammonium ions self-assemble to assume an all-trans conformation at low temperature but undertake a transition to gauche conformation upon heating, followed by an order− disorder phase transition at elevated temperature.6−12 Specifically, confined hexadecyltrimethylammonium ions (CTA+) have recently been revealed by NMR measurement to undergo small-angle wobbling around their long molecular axes, and © 2015 American Chemical Society

then, with increasing temperature, fast and unrestricted rotations about symmetry axes.13 Graphite oxide (GO) as a nonstoichiometric layer compound has variable layer spacings, because it contains additional functional groups, such as hydroxyls, epoxides, and carboxyls.14 Its epoxide and hydroxyl groups can form hydrogen bonds with water15 or amine16 or form ionic bonds with ammonium ions.17 Currently, dynamic of intercalates trihexylmethyl (d3) ammonium within graphene or GO galleries were investigated, and the result of 2H NMR spectra demonstrated that the intercalates have restricted overall rotation.18 With GO to provide confinement, the cationic may be expected to exhibit complex behaviors, thus, the motivation of the present work. We looked into conformational dynamics and phase transition of CTA+ intercalated into GO, using impedance spectroscopy, and discovered a dielectric relaxation far below room temperature that arose from small-angle wobbling around the long molecular axes of the chains. Dielectric loss further reveals, in surfactant-intercalated graphite oxide (SIGO) with the highest concentration of hexadecyltrimethylammonium chloride (CTAC), a cusp reflecting the order-to-disorder transition of confined alkyl chains.

2. EXPERIMENTAL DETAILS 2.1. Preparation of GO. GO was synthesized by completely oxidizing natural graphite powder via the modified Hummers method.19 Graphite powder (6 g), NaNO3 (6 g), and Received: May 11, 2015 Revised: June 18, 2015 Published: July 2, 2015 17438

DOI: 10.1021/acs.jpcc.5b04493 J. Phys. Chem. C 2015, 119, 17438−17443

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The Journal of Physical Chemistry C 98% H2SO4 (132 mL) were added into a beaker in an ice bath and stirred for 30 min, followed by the gradual addition of KMnO4 powder (24 g). After another 30 min, distilled water (276 mL) was dropped slowly into the constantly stirred mixture of reactants, with its temperature kept below 35 °C. Then the beaker was transferred to an oil bath and stirred at 95 °C for 1 h. Lastly, distilled water and 30% H2O2 were used to remove unreacted KMnO4. GO powder was obtained after the residue was washed, centrifuged, dialyzed, filtered, and dried. 2.2. Preparation of Nanocomposites. GO powder (50 mg) was dissolved in 0.05 M NaOH solution (12 mL) and exfoliated for 30 min inside a GA92-IID ultrasonicator (Wuxi, China), followed by the addition of 100 mL aqueous solutions of the chloride of CTA+ or CTAC. Three concentrations were adopted, the respective amounts of the salt being 24, 72, and 800 mg. The GO changed from hydrophilicity to hydrophobicity and readily precipitated. Filtration and drying in vacuum at 50 °C for 12 h produced SIGO, labeled as SIGO-1 (with GO/CTAC being 50:24 mg), SIGO-2 (50:72), and SIGO-3 (50:800). 2.3. Experimental Techniques. Small-angle X-ray diffraction (SAXD) patterns were recorded from all four samples in a Brüker D8 Advance diffractometer operating at 40 kV and 40 mA and using nickel filtered Cu Kα radiation with a wavelength λ = 0.15406 nm over the range 2θ = 0.5−30°. Additionally, wide-angle X-ray diffraction (WAXD) was performed with the help of a Shimadzu 600 powder diffractometer (40 kV and 30 mA, Cu Kα and 5−50°). X-ray photoelectron spectroscopy (XPS) proceeded in a Thermo Scientific K-Alpha that provided Al Kα radiation as the photon source. Survey spectra were measured with a pass energy of 200.0 eV in 1.0 eV steps, followed by high-resolution scans of the C 1s signals in 0.10 eV steps, with a pass energy of 50.0 eV. After subtraction of linear baseline, the scan curves were fitted by a mixed Gaussian-Lorentzian product function. FTIR absorption was measured in a Nicolet Nexus 870 spectrometer on pressed KBr pellets under ambient conditions, over the range 4000 to 400 cm−1. DSC experiments proceeded under a N2 atmosphere between 223 and 343 K at 10 K/min, in a PerkinElmer Pyris-1 calorimeter. Two heating and one cooling runs were completed in a continuous cycle for each sample. After these characterizations, dielectric spectroscopy was conducted on pellets made into 8 mm diameter and 0.4 mm thickness by uniaxial compaction at 6 MPa. The sample under investigation sat on a gold-plated copper block inside a steel chamber, to be heated at the constant rate of 1 K/min, with sample temperature monitored by a closely placed Cu−CuNi thermocouple calibrated to the accuracy of 0.1 K. An Agilent E4980A meter applied a working voltage of 1.0 V in threeterminal configuration at 12 chosen frequencies within the range of 11.1 to 1000 kHz. Preliminary runs on pellets previously dried in situ under dynamic vacuum of 30 Pa, at 50 °C for 12, 18, or 24 h, confirmed the reproducibility of impedance spectra and indicating the effective elimination of moisture after 24 h, henceforth all samples were dried for a day.

Figure 1. SAXD patterns of (a) GO, (b) SIGO-1, (c) SIGO-2, and (d) SIGO-3. Inset: WAXD patterns of GO and SIGOs, in same sequence. Symbols g and r indicate reflections relative to the order of GO planes and long alkyl chains of SI, respectively.

the six-membered ring structure within each sheet.20 With increasing CTAC content, (001) moved to a smaller angle, consistent with the dilation of interlayer spacing due to the intercalation of CTAC. Spacings of 0.75, 1.02, 1.47, and 3.41 nm are thus calculated, respectively, for GO, SIGO-1, SIGO-2, and SIGO-3. After heating at 70 °C under dynamic vacuum for 24 h to remove water, dehydrated GO had a spacing of about 6.3 Å. Outstandingly, SIGO-3 showed six higher-order diffraction peaks of (001) and a 100r reflection at 21.4°, proving a high crystalline order of its alkyl chains intercalated in layered GO, as M. Mauro et al. have also reported.21 As the alkyl chains in CTAC has a length of 2.17 nm,22 for it to fit inside the spacing of d(001) = 3.41 nm of SIGO-3 implies a bilayer that is inclined to the GO plane at an angle of 37°.21,23,24 From the XPS data in Figure 2 we can tell that GO and the SIGOs contain the chemical elements of carbon, nitrogen, and

Figure 2. XPS survey scans of (a) GO, (b) SIGO-1, (c) SIGO-2, (d) SIGO-3, and (e) CTAC.

oxygen, as expected. The absence in SIGOs of chlorine, originally at the heads of CTAC molecular chains, can be explained by the replacement of Cl− by C−O− on the skeleton of GO. In Figure 3 the C 1s peak of GO is well-resolved into contributions from the following kinds of carbons: CC, CO, OC−OH, C−OH, and C−O−C, with respective binding energy at 284.5, 287.8, 289.2, 286.2, and 287.3 eV,25,26

3. RESULTS Figure 1 depicts representative X-ray diffraction results, revealing two prominent peaks that can be indexed as (001) and 100g. The (001) peak correlates with the spacing between adjacent layers, while the 100g peak enlarged in the inset shows 17439


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The Journal of Physical Chemistry C

Figure 3. High-resolution XPS C 1s spectra of (a) GO, (b) SIGO-1, (c) SIGO-2, and (d) SIGO-3.

Table 1. Area Percentages of Various Bonded Carbons Derived from Figure 3 sample

CC

C−C

C−N

C−OH

C−O−C

CO

OC−OH

GO SIGO-1 SIGO-2 SIGO-3

44 35 30 23

0 18 28 43

0 4.9 7.9 12

22 18 16 12

27 19 16 9.0

3.8 2.5 0 0

3.2 2.6 2.1 1.0

based on the 13C MAS NMR spectroscopy of GO in Supporting Information. The percentage concentrations of these various bonded carbons are estimated from areas and listed in the top row of Table 1. When CTAC rises in content (as we go down in rows) those carbons in oxygen-containing functional groups consistently become more scarce, but carbons with C−C and C−N bonds, as shown in the 13C MAS NMR spectroscopy of SIGO-3 in Supporting Information, at 285.1 and 285.9 eV,27 increase their concentrations. These trends demonstrate the reaction of GO with CTAC and the detachment of oxygen-containing functional groups from GO. In the IR spectrum of GO (Figure 4), the broad band above 3000 cm−1 is ascribable to the O−H stretching motion, whereas the shoulder at 3600 cm−1 derives from C−OH groups in GO.14 Typical peaks appearing at 1725, 1620, and 1225 cm−1 relate to stretching vibrations of CO at the edges and CC in carbon skeleton and phenolic groups in the bulk of the carbon network, respectively. The peak at 1060 cm−1 is associated with C−O−C stretching.15,28,29 In SIGO with rising CTAC content, the features at 3600 and 1225 cm−1 all weakened. These changes further indicate that the surfactant was intercalated into GO layers via electrostatic binding between CTA+ and C−O− groups on GO. At the same time, new peaks emerged that derived from CTAC, as Figure 5 more clearly reveals. The peaks arising from −CH2 stretching, at 2920 and 2850 cm−1, shifted to higher frequencies with decreasing CTAC content, indicating the

Figure 4. FTIR absorption in (a) GO, (b) SIGO-1, (c) SIGO-2, (d) SIGO-3, and (e) CTAC.

strengthening of the gauche conformer in alkyl chains.27,30 The bands at 1530−1430 and 770−670 cm−1, assigned to methylene scissoring modes and methylene rocking modes, respectively, are sensitive to interchain interactions. In these regions, doublets were found in the spectrum for pure crystalline CTAC, the splitting owing to strong intermolecular interaction between adjacent hydrocarbon chains.7,31 The doublets were replaced by singlets at 1468 and 721 cm−1 in SIGO, implying that there the alkyl chains can freely rotate around their long axes.31 Band broadening and decreasing intensity of 17440


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Figure 5. Comparison of FTIR spectra of SIGO-1 (red), SIGO-2 (blue), SIGO-3 (magenta), and CTAC (black) in the regions 2980−2820, 1530− 1430, and 770−670 cm−1.

Figure 6. DSC scans of SIGO-3 nanocomposite in the sequence (black) first heating, then (red) cooling, and last (blue) second heating, all at 10 K/min. Figure 7. Temperature dependence of permittivity and dielectric loss at five designated frequencies in a SIGO-3 nanocomposite pellet. Inset: cusps magnified.

the singlets indicate lessening interchain interaction and growing chain motion.32 Figure 6 presents the calorimetry of a SIGO-3 nanocomposite. On initial heating it exhibited endothermicity with a peak temperature at 317 K, during subsequent cooling an exothermic peak at 312 K, and on repeated heating an endothermic peak at 316 K. This is evidence for a reversible order−disorder phase transition of the rotator-type alkyl chain molecules confined in GO, which has been recognized for many layered materials intercalated with long hydrocarbon molecules.13,21 Finally, we come to the results of impedance spectroscopy. The illustrative case in Figure 7 shows a loss peak that moved to higher temperature with increasing frequency, an attribute of a relaxation process. Additionally, a cusp in the dielectric loss curve appeared at 316 K, irrespective of the frequency, so it was not associated with relaxation. Obviously it has the same origin as the endothermic peak just discussed; therefore, it derives from the transition of confined alkyl chains from an ordered phase to a disordered phase.13 The relaxation was seen in SIGO-1, SIGO-2, and CTAC, too (Figure 8). Figure 9 confirms the Arrhenius fits to the relaxation times τ τ(T ) = τ0 exp(Ea /kT )

where k denotes the Boltzmann’s constant, all lead to straight lines, from which plots Ea, the activation energy, and τ0, the preexponential factor can be calculated, to be listed in Table 2. It is interesting to note that the activation energy of the relaxation apparently increases linearly with the interlayer spacing.

4. DISCUSSIONS Characterization measurements show that CTAC, the surfactant, has been successfully inserted into the interlayers of GO through electrostatic attraction between CTA+ and C−O− functional groups. The picture they help to construct is that each of our three kinds of SIGO has different arrangements of the alkyl chains. SIGO-1 has an interlayer spacing of 1.02 nm, and its intercalated molecules form a single layer parallel to the GO sheets. In SIGO-2 the spacing widens to 1.47 nm, and the stack of confined alkyl chains increases to double layers. SIGO-3 has the highest spacing of 3.41 nm, and to accommodate its greatest fraction of surfactant, it holds bilayers of hydrocarbon chains that stand up, not quite normal to the GO sheets, but inclined at ∼37°, displaying hexagonal symmetry if projected on the confining plane.21 In reacting with GO the surfactant CTAC lost its head ion Cl− and turns into CTA+, which, bonded with C−O− group on

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f = ω/2π ∼ 104−106 Hz and τη ∼ 10−11s at a temperature of 316 K,13,35 we got ωτη ≪ 1. That is to say, the probe frequency ω ≪ 1/τη, under which small value of dielectric loss ε′ can be anticipated. That is the reason why the dielectric loss peak of the ordered-disordered phase transition is rather indistinct. At probe frequency f = 104−106 Hz and at lower temperature, a relaxational process was clearly observed in all the SIGO and in CTAC. This relaxational process is attributed to restricted small-angle reorientations of the chains.13 Its motional correlation time τ is in the 10−3−10−4 s range, which coincided with our probe frequencies and resulted in ωτ ∼ 1. As seen in Figure 10, the activation energy of the relaxation increases

Figure 8. Temperature dependence of dielectric loss in (a) SIGO-1, (b) SIGO-2, (c) SIGO-3, and (d) CTAC measured at 31.6 kHz. Inset: (d) magnified.

Figure 10. Activation energy as a function of interlayer spacing d.

linearly with interlayer spacing. This is understandable because more room means a greater number of neighboring molecules.

5. CONCLUSIONS The dynamics of CTA+ confined in GO has been been investigated and a hitherto unknown relaxation identified, that arises from electrostatic coupling between polar headgroup of CTA+ and C−O− group of GO and small-angle wobbling of the alkyl chains around their long axes. Additionally, in GO with high concentration of intercalated CTA+, we have observed a phase transition of the surfactant from an ordered phase to a disordered phase. These properties can all be explained by the geometrical arrangements of the intercalate in confinement.

Figure 9. Temperature dependence of relaxation time for the dielectric loss peaks fitted with an Arrhenius relationship.

Table 2. Arrhenius Parameters Derived from Figure 9 sample SIGO-1 SIGO-2 SIGO-3 CTAC

Ea* (meV) 354 371 408 433

± ± ± ±

4 5 5 8

log(τ0[s]) −15.02 −15.21 −16.56 −16.53

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ASSOCIATED CONTENT

S Supporting Information *

GO, possess a permanent electric dipole. At room temperature in SIGO-3, the alkyl chains form a crystalline structure with hexagonal symmetry,21 but rotate along their axes, with angle and rotational rate increasing with temperature.13,33 At Tc = 316 K, as indicated by both DSC and dielectric experiments, this rotator phase passes into a disordered phase, where the conformations of most confined chains have transformed from all-trans to gauche,7,13,33 that is, chain melting. Near Tc using a relaxational behavior of the order parameter fluctuation δη(r,t) = η(r,t) − η0, we may describe the dielectric response at a probe frequency ω as ε(ω,T) ∝ Δε′/(1 + iωτη), where Δε′ stands for the relaxation strength, corresponding to the relative magnitudes of the relaxational features in Figure 7, and τη is the characteristic time for the response of the order parameter to the external electric field.34 The dielectric response in SIGO-3 results from the rotation of chains, accompanying a reorientation of its dipole moment. In our case, because of the applied frequency

13

C NMR spectrum for the samples GO and SIGO-3 under the magic angle spinning (MAS) condition with spinning rate 10 kHz using an aring-pulse on a Bruker Avance-300 spectrometer operating at a 7 T. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b04493.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 25 83593508. Fax: +86 25 83595535. E-mail: mgu@ nju.edu.cn. Present Address §

Asia Power Development (Group), 2502 Win Plaza, San Po Kong, Hong Kong SAR, People’s Republic of China (T.B.T.). Notes

The authors declare no competing financial interest. 17442


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ACKNOWLEDGMENTS This work was supported by the National Basic Research Programme of China (under Grants Nos. 2011CB933400 and 2012CB934000) and the National NSF of China (10674060).

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