Neutral Nanosheets that Gel: Exfoliated Layered Double Hydroxides

LETTER pubs.acs.org/JPCL

Neutral Nanosheets that Gel: Exfoliated Layered Double Hydroxides in Toluene Vikrant V. Naik, T. N. Ramesh,† and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

bS Supporting Information ABSTRACT: A simple strategy to exfoliate inorganic layered double hydroxide (LDH) solids to their ultimate constituent, intact single layers of nanometer thickness and micrometer size, is presented. The procedure involves intercalation of an ionic surfactant that forms a hydrophobic anchored surfactant bilayer in the galleries of the solid followed by simply stirring the intercalated solid in toluene. The method is rapid but at the same time gentle enough to produce exfoliated nanosheets of regular morphology that are electrically neutral and form stable gels at higher concentrations. In this Letter, we describe the phenomena and use molecular dynamics simulations to show that exfoliation of the LDH in toluene is a consequence of the modification of the cohesive dispersive interactions between surfactant chains anchored on opposing inorganic sheets by the toluene molecules. The toluene molecules function as a molecular glue, holding the surfactant-anchored LDH sheets together, leading to gel formation. SECTION: Nanoparticles and Nanostructures

D

elamination of inorganic layered solids by exfoliation resulting in the formation of highly dispersed monolayer colloidal phases has attracted considerable interest, particularly as building blocks for the construction of functional nanocomposites and nanostructures.1
3 It is, for example, an important step in the preparation of polymer
layered solid composites because homogeneity at the nanoscale is a prerequisite for enhanced properties.4
6 The spontaneous exfoliation of smectite clays in water to give stable colloidal dispersions is well-known.7,8 Similar delamination has been artificially achieved by controlling interlayer interactions for several classes of layered materials, layered oxides,9
14 dichalcogenides,15
18 metal phosphates,19,20 and, more recently, the anionic clays or layered double hydroxides (LDHs).21
25 The term nanosheets has been widely accepted to represent these inorganic layers of nanometer thickness with micrometer-sized lateral dimensions. Most existing methods have achieved exfoliation by use of a highly polar solvent that replaces the solvation shell of the interlamellar counterion, resulting in the swelling of the layers and consequent weakening of interlayer interactions.21
25 These methods result in the formation of either positively or negatively charged nanosheets, the polarity of the solvent ensuring that these inorganic macro-ions remain as stable dispersions. In situations where the layer charge is high, for example, the LDHs, delamination may be affected by using high dielectric solvents such as formamide.21,22 The Coulomb interaction between the sheets can be weakened by intercalation of long-chain ionic surfactants followed by solvation of the ionic head group by “large” polar solvent molecules, for exampe, hexanol or butanol.26 The swelling of oleate-intercalated LDH in n-alkanes, chloroform, and toluene has been reported.27 r 2011 American Chemical Society

In the work reported here, we have adopted a different strategy based on a recent molecular dynamics (MD) simulation that showed that in layered solids intercalated with long-chain surfactant molecules, dispersive or van der Waals’ interactions between chains anchored on opposing inorganic layers are responsible for holding the sheets together.28
30 This suggested that delamination of a surfactant-intercalated layered material can be affected by disrupting or weakening the van der Waals interactions between chains anchored on opposing layers. To do so, we take cognizance of the fact that intercalation of long-chain surfactants is known to convert the essentially hydrophilic interlamellar space of inorganic layered solids, such as the LDHs, into one that is hydrophobic and can solvate nonpolar molecules.31
33 The included nonpolar molecules are likely to modify the dispersive interactions between the chains anchored to opposing layers of the solid, thereby facilitating their separation, a premise shown by MD simulations, described in a subsequent section, to be true. We show here that by making the interlamellar region of the LDHs sufficiently hydrophobic by intercalation of dodecyl sulfate anions to form an anchored surfactant bilayer, delamination can be simply and rapidly realized by stirring the solid in a nonpolar solvent like toluene (Scheme 1). A unique feature of the delaminated nanosheets obtained by this procedure is that they are electrically neutral. In this Letter, we describe the phenomena and the use of MD

Received: April 7, 2011 Accepted: April 27, 2011 Published: April 29, 2011 1193

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Scheme 1. Delamination of a Surfactant-Intercalated LDH

Figure 2. Proton NMR spectra of surfactant-intercalated Mg
Al LDHDDS dispersed in deutrated toluene. For comparison, the spectrum of the “free” Na-DDS is shown (red line). The inset shows the expanded region of the DDS resonances.

Figure 1. (a) Photograph of a dispersion of Co
Al LDH. (b) A red laser beam directed through the dispersions to show the Tyndall effect. (c) Test tube inversion test demonstrating the formation of toluene gels for dispersions of (i)Mg
Al LDH, (ii) Co
Al LDH, (iii) Ni
Al LDH, and (iv) Zn
Al LDH. (d) The same dispersion as that in (a) after 7 days.

simulations to understand the underlying mechanism of the exfoliation process. The layered materials chosen for this study were the anionic clays, LDHs, that are particularly difficult to delaminate.34 These are derived from the brucite (Mg(OH)2) structure by the isomorphous substitution of M2þ ions by M3þ ions. The LDH layers are positively charged, and charge neutrality is preserved by the presence of interlamellar exchangeable anions. The LDH sheets studied in the present report are the positively charged Mg0.67Al0.33(OH)2 (Mg
Al LDH) and Co0.67Al0.33(OH)2 (Co
Al LDH). The method is easily extended to other LDHs. The Mg
Al LDH with NOh3 as the interlamellar anion and Co
Al LDH with Clh were prepared by standard procedures available in the literature (see Supporting Information).35,36 Procedures for the intercalation of dodecyl sulfate (DDS) surfactant anions in these materials as well as their characterization by powder X-ray diffraction (XRD) are well-established (see Supporting Information).30 Intercalation occurs with an increase in the interlayer layer spacing. The interlayer spacing of the DDS intercalated LDH, ∼2.7 nm, corresponds to an interlamellar arrangement wherein the surfactant chains are organized as bilayers with the ionic head group anchored to the inorganic sheet.30 The delamination of the LDH-DDS was carried out by stirring known weights of the solid in 3 mL of AR-grade toluene, followed by sonication for 5 min to obtain dispersions with differing volume fractions, Φv.37 Attempts to follow the

delamination process by diffraction measurements were unsuccessful as the process was too rapid. This situation is quite unlike that observed for the swelling clays in water, where the various stages — crystallite swelling by hydration of the interlamellar ions followed by osmotic swelling leading to gel formation and finally a free-flowing dispersion — are clearly delineated.7,38 At lower volume fractions (Φv e 0.005) of LDH, a clear transparent colloidal dispersion is obtained (Figure 1a). The color of the dispersion depends on the color of the LDH, for examepl, the Co
Al LDH dispersions are pink while the Ni
Al LDH are green. A clear Tyndall light scattering is observed from these solutions, indicating the presence of exfoliated nanosheets of the layered solid dispersed in the organic solvent (Figure 1b). Delamination of the LDH in nonpolar solvents like toluene occurs only when the interlamellar space of the solid is made sufficiently organophillic. The LDH-NO3
, for example, cannot be exfoliated in toluene. With increasing concentration of the dispersed layered solid in toluene, a gel-like state, as characterized by the test tube inversion test, is obtained (Figure 1c). The gels are stable even on long-term standing as long as care is taken to prevent solvent evaporation. It was also observed that the clear dilute dispersions on long-standing, typically a week, separated into a clear region (Figure 1d) and a gel-like phase that settled at the bottom of the container, suggesting that it is the gel that is the preferred state of the dispersion. Our results show that the delaminated nanosheets of the surfactant-intercalated LDH are effective gelators for toluene. Dispersions of the nanosheets of the LDH with the anchored surfactant chains were characterized by proton NMR spectroscopy. The 1H NMR spectra of dilute dispersions of the Mg
Al LDH nanosheets in deutrated (D8) toluene are shown in Figure 2. For comparison, the spectrum of the Na-DDS in D8toluene is also shown along with the assignment of the resonances. The NMR shows that DDS surfactant chains remain anchored to the Mg
Al LDH sheets in the dispersions. The resonances of the anchored surfactant chains are well-resolved; the R-CH2, the CH2-chain, as well as the terminal ω-CH3 resonances appear at the same position as that of the free NaDDS chains in toluene solution (Figure 2). The NMR spectra provide evidence that the schematic cartoon of the surfactantanchored delaminated LDH sheets as depicted in Scheme 1 is 1194

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Figure 3. Shape plots for four different concentrations of Mg
Al LDHDDS dispersed in toluene. The volume fraction of the solid in the dispersion is indicated. The plot shows the variation of the SAXS intensity, I(q), as a function of the scattering vector, q. The solid line shows the q
2 dependence.

essentially correct. These results also highlight one of the main differences of the present delamination strategy as compared to most methods available in the literature that yield either positively or negatively charged nanosheets, depending on the nature of the solid being delaminated. In the present approach, the dispersed nanosheets are electrically neutral as the ionic surfactant chains remain anchored to the layers even after exfoliation. The microstructures of the dispersions were characterized by small-angle X-ray scattering (SAXS). The shape plots, scattering intensity I(q) versus the scattering vector q, for four concentrations of the Mg
Al LDH-DDS dispersions in toluene are shown in Figure 3. At lower concentrations, the X-ray scattering intensity falls monotonically with increasing q, but for higher concentrations, in the gel-like phase, the scattering plots show a distinctive broad hump at q = 1.60 nm
1, the position of which does not change with further increase in concentration of the LDH-DDS in the dispersion. For all concentrations of the dispersion at low values of q (q < 0.7 nm
1), the scattering intensity I(q) roughly follows a q
2 power law. This variation is consistent with the form factor for randomly oriented thin disks.39 The broad hump at q = 1.60 nm
1 would then correspond to a loose stacking of the disks with an interdisk distance, d (=2π/q), of 3.92 nm. It may be noted that this distance is much larger than the interlayer distance of ∼2.76 nm in the solid Mg
Al LDH-DDS. The SAXS measurements indicate that the gel has a tactoidal microstructure with isolated LDH-DDS sheets that are loosely stacked with intervening toluene molecules. The morphology and size of the exfoliated nanosheets were examined by SEM, SAED, and tapping mode AFM. A typical SEM image of the delaminated nanosheets is shown in Figure 4a. The SEM image shows two-dimensional ultrathin sheets of micrometer size. A distinctive feature of the image is that the sheet is characterized by a regular well-defined hexagonal morphology. The selected area electron diffraction pattern of an individual sheet shows the characteristic hexagonal pattern confirming its single-crystal nature. The preservation of the lateral morphology of the nanosheets is a unique feature of the delamination process described here. In contrast, most of the exfoliation procedures described in the literature are violent,

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Figure 4. (a) Electron microscope images of Co
Al LDH DDS. The inset shows the selected area diffraction pattern. (b) Tapping mode AFM images of Co
Al LDH DDS nanosheets deposited on a mica wafer substrate. The height profiles of the particles along the white lines marked on the images are shown in the bottom panel.

leading to fractured sheets with irregular morphology. The tapping mode AFM image (Figure 4b) shows two-dimensional ultrathin sheets with lateral dimension up to 1 μm .The height profile reveals that the sheets have a fairly flat terrace with little wrinkling and an average thickness of 3 nm. The interlayer spacing of the anchored bilayer in the surfactant-intercalated Co
Al LDH-DDS and Mg
Al LDH-DDS as determined by XRD is 2.7 nm. The AFM images therefore correspond to a LDH
bilayer
LDH
bilayer
LDH stack. In none of the AFM images was an isolated anchored bilayer, that is, a surfactant bilayer sandwiched between two inorganic sheets, observed, indicating that the reassembly of the original layered materials from the dispersed nanosheets is facile. This is indeed borne out by the fact that when the solvent is allowed to evaporate from the bulk dispersion, the XRD pattern of the starting layered solid, with a high degree of orientational ordering along the interlayer normal, is recovered (see Supporting Information). MD simulations were performed to provide a molecular understanding of the delamination of the surfactant-intercalated LDHDDS in toluene and the associated gelling process. In a previous MD simulation, we had shown that when two single Mg
Al LDH sheets with anchored surfactant chains were brought closer from infinite separation in vacuum, it was the attractive dispersive interactions between chains on opposing layers that were responsible for holding the intercalated solid together.30 The simulations were able to correctly reproduce the X-ray-determined interlayer spacing of the solid, and it was these results that formed the basis of the delamination strategy presented here. In the present study, we have repeated these calculations but now in the presence of toluene and compared the results in the two situations. Details of the MD simulations are provided as part of the Supporting Information and also in refs 30 and 40. In these calculations, two single Mg
Al LDH sheets with anchored DDS chains were brought closer in incremental steps from infinite separation in toluene. At each value of the interlayer separation, a 2 ns isochoric, isothermal, NVT MD simulation was performed, and the relative contributions of the electrostatic and van der Waals interactions to the total energy of the equilibrium structure at that separation were evaluated. The total energy of Mg
Al LDH-DDS in toluene at each value of the interlayer separation may be written as ETotal ¼ Einter þ EToluene 1195

ð1Þ

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Figure 5. Plot of the interaction energy between two surfactantanchored sheets of Mg
Al LDH at different interlayer separations in vacuum and in toluene.

where ETotal is the energy of Mg
Al LDH-DDS in toluene, EToluene is the energy of toluene molecules, and Einter is the interaction energy between the toluene molecule and Mg
Al LDH-DDS sheet and between the two LDH sheets. Each of the energy terms in eq 1 has contributions from the Coulomb energy, van der Waals energy, internal energy, and kinetic energy. EToluene was calculated using the following approximation EToluene ¼ n  E1Toluene

ð2Þ

where n is the number of toluene molecules in the interlayer region in the simulation. E1Toluene is the energy of a single toluene molecule obtained from 2 ns NVT MD simulations at 298 K of 1065 molecules of pure toluene confined in a box of dimensions 5.728 nm  5.728 nm  5.728 nm. The density of the simulated system is identical to that of bulk toluene. The results of the simulation are shown in Figure 5 where the variation in interaction energy evaluated per unit surface area has been plotted as a function of interlayer separation. The energies at infinite separation are defined as 0, and the plots are the differences in Einter at a particular interlayer separation and Einter at infinite separation and therefore represent the interaction energy per unit surface area as a function of interlayer separation. It may be seen that the curve flattens out after a separation of 12 nm, justifying the assumption that a 13.0 nm separation may be considered as the infinite separation limit. The relative contribution of the van der Waals energy to the total energy at each interlayer spacing was evaluated and is shown in Figure 5. The electrostatic energy and the internal energy, which includes bond stretching, angle bending, and torsional terms, oscillate around 0 and hence are not shown (see Supporting Information). For comparison, the variation in the energy per unit surface as a function of the interlayer separation in vacuum is also shown in Figure 5. It may be seen that from a separation of 12.72
3.72 nm, the total interaction energy curve decreases with a rather sharp drop below a separation of 5.5 nm. The interaction energy curve shows a minima at 3.72 nm, while further compression causes the interaction energy to rise sharply. It is significant that the minimum occurs at an interlayer separation of 3.72 nm, comparable to the experimental SAXS-determined value of 3.92 nm for a concentrated dispersion of Mg
Al LDH-DDS in toluene. It is interesting to note that the van der Waals contribution accounts,

almost entirely, for the change in the total interaction energy as a function of intersheet separation. This scenario is identical to that when the sheets are brought closer together in vacuum, where too dispersive interactions are responsible for the change in the variation of the surface energy as a function of intersheet separation (Figure 5). It may also be seen from Figure 5 that for any value of the interlayer separation, the interaction energy per unit surface area in the presence of toluene is lower than the value in vacuum. The difference represents the strength of the interaction between DDS chains tethered to opposing sheets mediated via toluene molecules and indicates strong association between the toluene molecules and the DDS surfactant chains anchored to the Mg
Al LDH-DDS sheets. In fact, the interaction energy at an interlayer separation of 5 nm in toluene is lower than the value at the minimum, 2.72 nm, in vacuum. The simulations therefore predict that the surfactant-intercalated LDH-DDS would spontaneously delaminate in toluene and subsequentally associate with a tactoidal microstructure. This is of course what is observed experimentally. The structure at the minimum in Figure 5 corresponds to the simulation-determined equilibrium structure of Mg
Al LDHDDS in toluene. A snapshot of the simulation for this interlayer separation is shown in Figure 6a, and the projected density distribution of the methylene units and the toluene molecules along the c axis is shown in Figure 6b. For comparison, the equilibrium structure at the minimum in vacuum is shown in Figure 6c, and the corresponding projected density distribution is given in in Figure 6d. The density profiles in both toluene and vacuum show a pronounced layering behavior and are symmetrically displaced. In contrast to the situation in vacuum, the methylene units of the anchored DDS surfactant chains in toluene, especially toward the tail, are disordered. This may be inferred from a comparison of widths of the density profiles of the C11 and C12 carbons in the two situations. In vacuum, the DDS chains of the bilayer have a well-defined tilt angle;30 no such tilt angle coherence exists in the presence of toluene. The projected density distribution of the toluene molecules indicates that the they are located close to the tail (C10
C12) of the anchored DDS chains and do not penetrate the interior of the bilayer. The profile shows a bimodal distribution with toluene molecules filling up the empty space between DDS chains anchored on opposing sheets. The MD simulations indicate that the toluene solvent molecules function as a molecular “glue”, cementing the opposing surfactant-anchored LDH sheets. The simulations do validate the premise on which this work was initiated, that the hydrophobic solvent toluene molecules would penetrate the bilayer and modify the dispersive interactions between chains anchored on opposing LDH sheets, leading to delamination. Gel formation of the LDH dispersions with a tactoidal microstructure is a consequence of the attractive dispersive interactions of toluene molecules with the tails of DDS chains anchored to opposing LDH sheets. In conclusion, we have outlined a simple strategy to delaminate LDH solids to their ultimate constituent, intact single layers of nanometer thickness and micrometer size in nonpolar solvents like toluene. The procedure involves intercalation of an ionic surfactant to form a hydrophobic anchored surfactant bilayer in the interlamellar space of the solid. Delamination is affected by simply stirring the surfactant-intercalated layered solid in the solvent. The method is rapid but at the same time gentle enough to produce exfoliated nanosheets of regular morphology. A unique feature of the present procedure is that the delaminated nanosheets are 1196

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Figure 6. Snapshots of the equilibrium structure of the Mg
Al LDH DDS bilayer in (a) toluene at an interlayer separation of 3.72 nm and (b) the corresponding projected density distribution profiles along the 00 L axis for the methylene and methyl carbons of the DDS chain, obtained by averaging over all chains in the ensemble, and the center-of-mass of the toluene molecules. (c) A snapshot of the equilibrium structure of the Mg
Al LDH-DDS bilayer in vacuum at an interlayer separation of 2.72 nm and (d) the corresponding projected density distribution profile of the C atoms of the DDS chain along the 00 L axis. The numbering of the carbon atoms is shown.

electrically neutral because the ionic surfactants remain anchored to the sheets. The dispersions form stable gels that have a tactoidal microstructure. MD simulations showed that the cohesive dispersive interactions between surfactant chains anchored on opposing inorganic layers are modified by inclusion of nonpolar solvent molecules, leading to delamination and gel formation. Although the results presented here are restricted to the LDHs, we have been able to demonstrate that the procedure is universal. We have successfully delaminated a range of surfactant-intercalated layered solids, smectite clays, graphite oxide, and the divalent metal thiophosphates in toluene, to give neutral nanosheets that gel.

LDH-NO3 and (ii) Mg
Al LDH-DDS and (b) (i) Co
Al LDHCl and (ii) Co
Al LDH-DDS. (S5) Delaminated and restacked XRD patterns of (a) Co
Al LDH-DDS and (b) Mg
Al LDHDDS. (S6) Computational details. (S7) Contributions of the coulomb energy, internal energy, and van der Waals energy to the total interaction energy as a function of interlayer separation of Mg
Al LDH-DDS in toluene. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

’ ASSOCIATED CONTENT

*E-mail: [email protected]. Tel: þ91-80-2293-2661. Fax: þ91-80-2360-1552/0683.

bS

Present Addresses

Supporting Information. (S1) Synthesis of Mg
Al LDH-DDS. (S2) Synthesis of Co
Al LDH-DDS. (S3) Physical characterization. (S4) X-ray diffraction patterns of (a) (i) Mg
Al

†

University College of Science, Tumkur University, Tumkur 572102, India.

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’ ACKNOWLEDGMENT The authors would like to thank Mrs. J. Lakshmi and the Supercomputer Education and Research Center, Indian Institute of Science, for the computational resources. ’ REFERENCES (1) Osada, M.; Sasaki, T. Exfoliated Oxide Nanosheets: New Solutions to Nanoelectronics. J. Mater. Chem. 2009, 19, 2503–2511. (2) Li, L.; Ma, R.; Ebina, Y.; Fukuda, K.; Takada, K.; Sasaki, T. Layerby-Layer Assembly and Spontaneous Flocculation of Oppositely Charged Oxide and Hydroxide Nanosheets into Inorganic Sandwich Layered Materials. J. Am. Chem. Soc. 2007, 129, 8000–8007. (3) Akatsuka, K.; Haga, M.; Ebina, Y.; Osada, M.; Fukuda, K.; Sasaki, T. Construction of Highly Ordered Lamellar Nanostructures through Langmuir
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1 þ Φm)
1, where Φm is the partial mass of LDH-DDS in the dispersion and Ftoluene is the density of toluene. FLDH is the density of LDH-DDS and was assumed to be same as that of hydrotalcite (2 g/mL) as given by: Allman, R.; Jepsen, H. P. Die Struktur des Hrdrotalkits. Neues. Jahrb. Mineral. Monatsh. 1969, 544–551. (38) Hensen, E. J. M.; Smit, B. Why Clays Swell. J. Phys. Chem. B 2002, 106, 12664–12667. (39) Guinier, A.; Fournet, G. Small Angle Scattering of X-rays; Wiley: New York, 1955; pp 5
82. (40) Naik, V. V.; Vasudevan, S. Effect of Alkyl Chain Arrangement on Conformation and Dynamics in a Surfactant Intercalated Layered Double Hydroxide: Spectroscopic Measurements and MD Simulations. J. Phys. Chem. C 2011, 115, 8221–8232.

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