J. Phys. Chem. C, 118, 28227-28233 - Departamento de FÃsica

Article pubs.acs.org/JPCC

Model for Self-Rolling of an Aluminosilicate Sheet into a SingleWalled Imogolite Nanotube Rafael I. González,†,‡ Ricardo Ramírez,§,‡ José Rogan,†,‡ Juan Alejandro Valdivia,†,‡ Francisco Munoz,†,‡ Felipe Valencia,†,‡ Max Ramírez,†,‡ and Miguel Kiwi*,†,‡ †

Departamento de Física, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile 7800024 Facultad de Física, Universidad Católica de Chile, Casilla 306, Santiago, Chile 7820436 ‡ Centro para el Desarrollo de la Nanociencia y la Nanotecnología, CEDENNA, Avda. Ecuador 3493, Santiago, Chile 9170124 §

ABSTRACT: Imogolite is an attractive inorganic nanotube, formed from weathered volcanic ashes, that also can be synthesized in nearly monodisperse diameters. It has found a variety of uses, among them as an effective arsenic retention agent, as a catalyst support and as a constituent of nanowires. However, long after its successful synthesis, the details of the way it is achieved are not fully understood. Here we develop a model of the synthesis, which starts with a planar aluminosilicate sheet that is allowed to evolve freely, by means of classical molecular dynamics, until it achieves its minimum energy configuration. The minimal structures that the system thus adopts are tubular, scrolled, and more complex conformations as well, depending mainly on temperature. The minimal nanotubular configurations that we obtain are monodispersed in diameter and quite similar in diameter both to those of weathered natural volcanic ashes and to the ones that are synthesized in the laboratory. A tendency toward nanotube agglomeration is also observed, in agreement with experiment.

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INTRODUCTION Since the discovery by Iijima1 of carbon nanotubes (C-NTs), and their inorganic counterpart (inorganic concentric polyhedral and cylindrical structures of tungsten disulfide) by Tenne et al.,2 cylindrical structures have been intensely investigated by experimentalists and theorists. These hollow cylinders are very attractive because of their fascinating properties and their practical uses. C-NTs, and several inorganic ones, are fabricated by electric arc discharge, laser ablation, or chemical vapor deposition processes, while inorganic oxide nanotubes, like the imogolite aluminosilicate that is our present focus of attention, are created mainly by lowtemperature liquid phase chemical processes.3 Among the various NTs, imogolite is especially attractive for several reasons. In the C-NT synthesis, the graphitic sheet strain energy required to bend it into a NT decreases monotonically, as the nanotube diameter increases, thus precluding the possibility of tuning the NT diameter. With imogolite, on the contrary, highly monodisperse NT diameters are created. These NTs have a large arsenic retention capacity4 and, based on the experience acquired with 50 nm diameter halloysite clay nanotubes,5 imogolite may eventually be used as a vehicle for drug delivery. Imogolite also shows potential as additive of transparent polymers, such as molecular sieves,6−8 as support for insulating polymers,9 as a constituent of nanowires,10,11 as catalyst support,12−14 and as a base for organic− inorganic nanohybrides.15−21 Moreover, recently Kang et al.22,23 demonstrated their use as membranes and gas adsorbents. © XXXX American Chemical Society

Single-walled imogolite is a clay NT with chemical composition Al2SiO7H4. It is found in weathered volcanic ashes,24 whose conformation is illustrated in Figure 1. This structure, of basic relevance to the study of imogolite, was put forward in 1972 by Cradwick et al.25 It consists of a curved gibbsite cylinder with ortho-silicic acid, coordinated via oxygen with three aluminum atoms. At this point it is also worth

Figure 1. (a) Imogolite unit cell with N = 12 repetitions of the 28 atom circular sector, of angle 2π/N, marked by the black continuous line. (b) Lateral view of the 28 atom structure that is angularly repeated to form imogolite. Periodic repetitions of the unit cell are imposed in the axial direction. Lz is the axial length of the unit cell; H: light gray, O: red, Si: yellow, and Al: pink. Received: August 26, 2014 Revised: November 11, 2014

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dx.doi.org/10.1021/jp508637q | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 2. Illustration of the imogolite formation process that we simulate. (1) Planar aluminosilicate sheet; (2) Initial scrolling stages; (3) The NT is completed; (4) Axial and lateral view of the NT after relaxation; it is noticed that the defects at the NT seam, illustrated in (3), have disappeared. Lz is the axial length of the unit cell.

mentioning that Tamura and Kawamura,26 already in 2002, used molecular dynamics to model the structure of tubular gibbsite, finding that the total energy of the tubular gibbsite molecules, relative to that of flat gibbsite, tends to zero as the radius of the tube increases. In other words, the gibbsite energy does not have a minumum as a function of radius, which precludes diameter tuning. Shortly after the work of Cradwick et al.25 became known, Farmer et al.27 published a protocol for the synthesis of imogolite. Independently of the diverse synthesis procedures that have been reported afterward, the imogolite diameters turned out to be highly monodisperse.27−31 Consequently, it is of interest to explain this behavior on the basis of a microscopic model calculation,32 as the one we report here. The chemical formula for the Cradwick unit cell, ordered from the outside of the NT inward, is [(OH)3Al2O3SiOH]2N, where N is the number of repetitions of the circular sector, of angle 2π/N, that are required to build the NT cylinder illustrated in Figure 1b. Moreover, the values N ≈ 10 for natural and N ≈ 12 for synthetic imogolite have been reported.27,30,33,34 For the latter, the measured external diameter is 2.3 nm, with an average length of ≈100 nm. The imogolite diameter can be modified by the replacement of Si by Ge30,32,34−40 and, in this way, fine-tune the NT properties and capabilities, as reported by Mukherjee et al.,30 who obtained 3.3 nm diameter Ge NTs, of lengths of the order of 15 nm. Yucelen et al.41−44 contributed to elucidate the role of molecular precursors and nanoscale intermediaries, between 1− 3 nm in size, in the formation of single-walled aluminosilicate NTs. They also determined experimentally the relation between precursor structure and shape with the final imogolite NT curvature. Moreover, they, as well as Maillet et al.,45 formulated a generalized kinetic model for the formation and growth of single-walled metal oxide nanotubes. On the other hand, Levard et al.46,47 made significant contributions to the understanding of the role of the precursor shape and composition in the formation of imogolite, and Bac et al.48 synthesized single-walled hollow aluminogermanate nanospheres. In spite of the fact that the work by Cradwick et al.25 and Farmer et al.27 are 40 years old, and more than 30 years have passed since Wada et al.35 published the protocol for the synthesis of Ge-imogolite, the interest in the subject has not faded.39,41,42,44−46,49−54 Moreover, Maillet et al.45,55 reported in 2010 the synthesis of double-walled Ge-imogolite, and later on, Thill et al.37 showed that the formation of double walled Siimogolite NTs, as well as that of Ge-imogolite thicker than double-walled NTs, is quite unlikely. In 2012, Thill et al.38 reported that, in the synthesis process of imogolite-like NTs, a very small proportion of nanoscrolls formed when the Si

content in the mixture of Si and Ge is 10%. They may be envisioned as an intermediate stage between double- and triplewall NTs. All these results suggest that the self-rolling mechanism, as a path toward the formation of imogolite NTs, is a subject worth exploring from a theoretical point of view. What we report below are the results of classical molecular dynamics simulations. We start with an ideal planar aluminosilicate sheet and study the dynamics of the self-rolling that gives rise to imogolite NTs, and related curved structures, at various temperatures. We also compare these structures with the relaxed ones that are obtained, starting with tubular initial conditions (TIC) after their energy is minimized. As mentioned above, the synthesis of imogolite is mainly carried out by low-temperature liquid phase chemical processes.3 However, we choose to treat these problems separately since the simulation of the liquid sysnthesis implies a detailed study of the action of the solvent. In addition, it is significantly more intricate, as it implies systems an order of magnitude larger than the ones we report here, and we leave these issues for future work. This paper is organized as follows: after this Introduction we describe the method we used in Method. Next, the results obtained are given in Results, and the paper is closed with Summary and Conclusions.

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METHOD Our main objectives are to describe the dynamics of the rolling up process that starts with a flat aluminosilicate sheet and ends up as an imogolite NT or a related structure, and to find the minimum energy configurations, these structures do adopt, over a wide temperature range (10 K ≤ T ≤ 368 K). For the time being, we ignore the role played by precursors.41,42,53 Our present objective is illustrated in Figure 2, where the following are illustrated: (1) flat aluminosilicate sheet we denominate as planar initial conditions (PIC), (2) an intermediate stage of the bending process, (3) the closing of the structure into an N = 12 imogolite NT, which is allowed to relax in order to reach the perfect tubular conformation shown in (4), where both an axial and a side view are presented. These boundary conditions are adopted in order that the formation of a seamless tubular conformation is possible, in spite of the fact that the aluminum atoms at the edges are not fully coordinated. To achieve the above outlined objectives, the principal tool we use is molecular dynamics (MD) simulations. These MD simulations, as well as the structural relaxations, were carried out using the large-scale atomic/molecular massively parallel simulator (LAMMPS) code.56 For the atomic interactions, the CLAYFF potential57 is used, since it has been proven to be adequate to model aluminosilicate (imogolite) nanotubes.32,36,58−60 B



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The CLAYFF potential, developed by Cygan et al.,57 incorporates the charges of every single atom, the van der Waals interaction, and a harmonic potential for the O−H group stretching. Analytically it is given by E=

e2 4π ϵ0

∑ i≠j

In order to check our results, we also used tubular initial conditions (TIC). We chose to use the Fast Inertial Relaxation Engine (FIRE),63 in combination with conjugate gradient minimizations, to optimize the NT conformation and the length of the simulation cell, with encouraging results. The LAMMPS implementation of FIRE does not allow to vary the length of the simulation box; however, it does implement conjugate gradient. On the other hand, FIRE yields lower energy structures. Because of these reasons, we combined both methods, cell length and structural optimization, successively and repeatedly to achieve both better conformations and cell lengths. For the aluminosilicate structure, we adopted the model by Cradwick,25 with 9 ≤ N ≤ 24 values. Each one of these structures was relaxed using the FIRE algorithm,63−65 as implemented in the LAMMPS code, in order to have a representative set of structures to compare the rolling simulations with already reported results, varying the simulation parameters and carrying out convergence tests. We started our simulations with 30 slightly different tubular imogolite NTs; that is, they differed just by random displacements (of less than 0.1 Å) of the atoms from their “bulk” equilibrium positions. This process was carried out for each N value, the number of repetitions of the circular sector structure illustrated in Figure 1b.

⎡⎛

qiqj

+

rij

12 ⎛ R ⎞6 ⎤ R 0, ij ⎞ ⎟⎟ − 2⎜⎜ 0, ij ⎟⎟ ⎥ ⎥ r ⎝ rij ⎠ ⎦ ⎣⎝ ij ⎠

∑ εij⎢⎢⎜⎜ i≠j

+ kij(rij − r0)2

(1)

where the qi are the partial charges, obtained by means of quantum mechanics calculations, e is the electron charge, and ε0 is the dielectric vacuum permittivity. The second summation is the van der Waals contribution, where R0,ij and εij are empirical parameters derived by fitting to bulk structural and physical properties, and rij are the distances between atoms i and j. The last term in eq 1 describes the O−H bond stretch energy by means of a simple harmonic interaction, where the kij are the stretching constants, and r0 is the equilibrium O−H distance, given in Table 1. The work by Cygan et al.57 does not incorporate charge transfer and redistribution, and here we use the same approach in order to avoid unphysical results. Table 1. CLAYFF Parameters;57 “Bond Parameters” Parameterize the Bond Stretching

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RESULTS In Figure 3a we show the imogolite energy that we obtain for TIC as a function of N, and compare it with the ones that result keeping fixed the simulation cell length Lz = 8.40 Å, as done by Konduri et al.36,58 It is seen that the minimum corresponds to N = 10, in agreement with first-principles calculations,66 and with the configuration of natural imogolite. It is also apparent

nonbond parameters species hydroxyl H hydroxyl O bridging O octahedral Al tetrahedral Si

charge(e)

ε (eV)

0.425 0 −0.950 6.7 −1.050 6.7 1.575 5.7 2.100 8.0 bond parameters

× × × ×

10−3 10−3 10−8 10−8

R0(Å) 0 3.5532 3.5532 4.7943 3.7064

species i

species j

k (eV/Å2)

r0 (Å)

hydroxyl O

hydroxyl H

24.03

1.0

The Coulomb forces are long-ranged and, thus, converge slowly as a function of the size of the simulation box, requiring special techniques to handle them and to evaluate their contribution since the size of the simulation box is a crucial issue. Usually they are treated by means of an Ewald sum,61 but that requires computer times that are proportional to the square of the number of atoms. Here we handle them by means of the Particle−Particle−Particle Mesh (P3 M) method,62 which is a Fourier-based Ewald summation procedure, significantly more efficient, and that allows for parallelization. To ensure accuracy we took six repetitions of the unit cell along the symmetry axis to properly account for these long-range Coulomb interactions. Moreover, the use of periodic boundary conditions in the axis direction contributes to mantain the atomic coordination. The flat aluminosilicate is allowed to evolve freely in 2D, at a given temperature, to seek its minimum energy conformation. Once this putative minimal energy is reached, a 3D Gaussian velocity distribution, consistent with the given temperature, is applied to the system, which is allowed to evolve for 200 ps, and we use a time step of 1 fs in all our calculations. This procedure is carried out for a large number of randomly different seeds. In all these cases the aluminosilicate sheet rolled up, adopting a variety of shapes, but mainly forming nanotubes and nanoscrolls.

Figure 3. Energy E as a function of N. The blue × correspond to the case where the length Lz of the unit cell was kept fixed at 8.40 Å. The open black circles are the values of E/N obtained for the relaxed values of Lz. Only minor differences are observed. (b) Unit cell length Lz per angular repetition N, as a function of N. The completely relaxed structure corresponds to T = 0. The nonzero temperature values were obtained as a NPT average at 1 bar. C



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that the difference in energetics between keeping the cell size fixed and freeing it is not very significant at T = 0. There is general agreement in the literature on the structure and composition of the imogolite unit cell. However, the value of N varies between N = 10 (natural) and N = 11−15 (synthetic).42 In addition, Yucelen et al.41−43 showed that the NT diameter can be controlled by the replacement of the anions involved in the synthesis,42 and Lee et al.67 confirmed recently that the variation of the temperature also plays a major role. But, to the best of our knowledge, the length of the unit cell as a function of temperature and NT diameter has received little attention. The first experimental measurements25,35 reported a length Lz of 8.40 Å. However, in 2005 Mukherjee et al.30 reported a measured value of 8.51 Å, while ab initio calculations by Demichelis et al.66 used a length of 8.45 Å, and Li et al.68 adopted an even larger value of 8.70 Å. Since imogolite is stable at room temperature and only loses its tubular structure69 around 770 K, we investigated its thermal behavior as a function of N and of temperature T, up to 368 K, which is the temperature at which the synthesis is carried out experimentally. In Figure 3b the unit cell length Lz is plotted as a function of N for T = 0 (green), and for several final temperatures, Tf, where Tf = 10, 150, 300, and 368 K, and the pressure was kept constant at 1 atm by means of the Nosé− Hoover algorithm. Next we focus our attention on the N = 12 case, which is the one more frequently obtained in experiments. After structural relaxation, we obtain for N = 12 NT lengths of 8.39, 8.41, 8.44, and 8.50 Å at 10, 150, 300, and 368 K, respectively. In addition, it is worth remarking that the values we obtain coincide with those measured experimentally.30 Consequently, the different experimental results on Lz can be assigned to the temperature variation. Planar and Tubular Initial Conditions. Our main concern is the way in which an aluminosilicate sheet could roll up to form imogolite NTs. Therefore, we start with the planar (2D) counterpart of the unit cell illustrated in Figure 1 outstretched in the x and in the z-(axial) directions, while in the z direction periodic boundary conditions are adopted. In order to shed light on the rolling process we start with planar initial conditions (PIC) and allow the system to evolve. This is carried out in two steps: first, with periodic boundary conditions for the 2D unit cell along the z-axis, we apply a temperature ramp from 1 K up to a given final temperature Tf (Tf = 10, 150, 300, and 368 K), at a rate of 7.4 K/ps, restricting the system to remain planar. Once the system reaches Tf, and keeping the temperature fixed during 200 ps, random initial velocities are given to the atoms, and allowing them to move in 3D in contact with a thermostat. Shortly after the planar constraint is removed, that is within the next 50 ps, rolling up was observed. The system was then allowed to evolve for 200 ps. The average total energy of the configurations was calculated for 50 different initial velocity realization on the planar sheet, and the lowest energy configuration was kept. This process was briefly described above and illustrated in Figure 2. In Figure 4 we plot the total energy per circular sector structure (illustrated in Figure 1b) of the minimal energy conformation we obtain, for temperatures of 10, 150, 300, and 368 K for both planar (PIC) and tubular initial conditions (TIC). The latter correspond to the perfect imogolite NT of Figure 1a. The 10 K structure was investigated to eliminate thermal effects, and in this way we also avoid defects along the NT seam. In contrast, 368 K is the temperature at which the NT are grown in the laboratory.36,39 At T = 10 K the energies

Figure 4. Thermodynamic average of E/N for temperatures of 10, 150, 300, and 368 K. The black x correspond to tubular (TIC), and the red circles to planar (PIC), initial conditions. Snapshots of the scrolled conformations, obtained for the minimum energy, are provided for representative N values. The dashed horizontal line corresponds to the total energy of two interacting N = 11 NTs at the same temperature.

for the N = 12−18 NTs obtained with PIC and TIC show little difference, as can be seen in Figure 4a, since the NT generated with PIC become cylindrical and practically defect free. For N = 8−11 the seam of the PIC NTs are not defect free, which reflects in an energy increase. The defects consist in H atoms misplaced along the outer wall, and OH groups pointing inward. It is also relevant to point out that a small finite temperature is sufficient to shift the minimum energy from N = 10 to N = 11, as can be observed in Figure 3 for the TIC case. For N = 19, we observe an energy peak and thereafter, for N > 19, a transition to a double tube (“binocular”) structure, thereby reducing considerably its energy in relation to larger diameter single nanotubes. Experimentally, a NT agglomeration during growth has been observed.30 In this context, we calculated the energy of two parallel N = 11 NTs that are close to each other, which provides a lower energy bound. At higher temperatures (see Figure 4b−d), a tendency to the formation of nanoscrolls rather than nanotubes is apparent, suggesting that partial charges and their Coulomb interaction are key ingredients. These nanoscrolls have been reported38 for Ge-imogolite (with 0.1 Si/[Si + Ge]), but not for pure Si imogolite. Nevertheless, the energy of two coupled N = 11 imogolite NTs, indicated by the green dashed lines in Figure 4, is favorable when compared D



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to all conformations that form spontaneously. Hence, the NT packing is an issue that does deserve attention. At this point it is convenient to indicate that to calculate the energy of the two coupled N = 11 imogolite NTs we did as follows: (i) the optimized N = 11 NT was copied identically and laid 2.5 nm appart and parallel to the first NT, with periodic boundary conditions along the tube axis; (ii) next, the two tube system was optimized using the conjugate gradient method, followed by 200 ps of MD at the corresponding temperature. The value of N = 11 was chosen because it is intermediate between N = 10 (the imogolite found in volcanic ashes) and N = 12, the one synthesized experimentally. Moreover, the formation of Si and Ge double wall NTs was recently addressed by Guimaraes et al.70 using the selfconsistent charge density functional tight-binding method. They concluded that double wall NTs are energetically unfavorable and that single-wall NTs prefer to adopt zigzag chirality. We obtained similar results using classical molecular dynamics. In Figure 4b the T = 150 K results are presented, and we observe that for 8 ≤ N ≤ 14 almost perfect NTs form, which moreover, are very similar in energy per atom and very close to the N = 11 minimum, independently of the choice of PIC or TIC. In contrast with the 10 K results, kinetic effects help with the healing of the nanotube seam. A nanoscroll first forms for N = 15, followed by “binocular” configurations for larger N. However, for 23 ≤ N ≤ 27, a double tube structure is more favorable. For N = 22, the nanoscroll structure yields the minimal energy over the whole range of 8 ≤ N ≤ 30 values. However, the energy of two coupled NTs is slightly more favorable. The dynamics at 300 and 368 K are illustrated in Figure 4c,d. At 300 K, the local minimum, N = 9 for PIC, is present, and when its structure is inspected (see inset), a rather coarse match at the seams is clearly visible. However, the energies for N = 10 and N = 11 are very similar to this local minimum. For 300 K and N ≥ 12, only nanoscrolls form, which yields a decrease of the energy as a function of N up to N = 22, the global minimum for our calculations at T = 300 K, due to the fact that more OH groups interact. Much the same occurs at T = 368 K, but the minimum at N = 10 is much sharper, several local minima are present, and both scrolled and double NTs are generated, as displayed in the insets of Figure 4d. In the above-discussed simulations it is apparent that as the temperature increases the maximum value of E/N decreases. These maxima coincide with the N values above which the NTs fail to close as a single cylindrical tube. This behavior is illustrated in Figure 5, where we plot the N value for the maximum energy as a function of T for PIC (N > 10) and the global minimum for TIC. For PIC, a linear decrease of the largest E/N versus T is observed. The implication is that kinetic effects limit the NT diameter (maximum N). This is in agreement with the results reported by Lee et al.67 who observed that by varying the temperature the diameter of the NTs can be tuned. On the other hand, the minimum obtained using TIC sets a lower bound for the NT diameters, generating the rather monodisperse tubes observed experimentally. To stress the role played by temperature in Figure 6a,b, we sketch the structure of representative low lying energy NTs and indicate their respective energies at T = 150 and 300 K. At 150 K there is a predominance of closed NTs for N < 15. The same trend is observed at 300 K, but limited to N < 13. For T = 150, a global minimum is present for N = 22, which corresponds to a

Figure 5. Number of repetitions N of the local energy maxima for PIC, and energy minima for TIC as a function of temperature.

Figure 6. Families of conformations and of their energies, obtained using PIC; (a) at 150 K; (b) at 300 K. The red dashed line indicates the lowest energies for the various structures.

nanoscroll in a predominantly double tube scenario. This is in contrast to what occurs at 300 K, where the majority of the conformations for N > 13 are nanoscrolls and only a few double tubes are present, which underscores the importance of kinetic effects.

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SUMMARY AND CONCLUSIONS The main objective of this contribution is to shed light on the dynamics of the rolling process of an aluminosilicate sheet into imogolite-like nanostructures of various diameters and to determine the relevance temperature has on this procedure. To achieve this goal, we start with a planar aluminosilicate sheet and allow it to evolve spontaneously by means of classical molecular dynamics. We also construct tubular NTs to use as reference and control of our simulations. The first item we addressed is the relevance of keeping fixed the length of the unit cell that is repeated using periodic boundary conditions, and compare with results obtained when this length is allowed to relax in the axial direction. We found that the length of the unit cell along the imogolite symmetry axis does not have a significant relevance for the energy of the imogolite NTs, however, it explains the difference between E



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(10) Lee, Y.; Kim, B.; Yi, W.; Takahara, A.; Sohn, D. Conducting Properties of Polypyrrole Coated Imogolite. Bull. Korean Chem. Soc. 2006, 27, 1815−1818. (11) Kuc, A.; Heine, T. Shielding Nanowires and Nanotubes with Imogolite: A Route to Nanocables. Adv. Mater. 2009, 21, 4353−4356. (12) Marzan, L. L.; Philipse, A. Synthesis of Platinum Nanoparticles in Aqueous Host Dispersions of Inorganic (Imogolite) Rods. Colloids Surf., A 1994, 90, 95−109. (13) Imamura, S.; Hayashi, Y.; Kajiwara, K.; Hoshino, H.; Kaito, C. Imogolite: A Possible New Type of Shape-Selective Catalyst. Ind. Eng. Chem. Res. 1993, 32, 600−603. (14) Imamura, S.; Kokubu, T.; Yamashita, T.; Okamoto, Y.; Kajiwara, K.; Kanai, H. Shape-Selective Copper-Loaded Imogolite Catalyst. J. Catal. 1996, 160, 137−139. (15) Yamamoto, K.; Otsuka, H.; Takahara, A. Preparation of Novel Polymer Hybrids from Imogolite Nanofiber. Polym. J. 2007, 39, 1−15. (16) Yah, W.; Irie, A.; Jiravanichanun, N.; Otsuka, H.; Takahara, A. Molecular Aggregation State and Electrical Properties of Terthiophenes/Imogolite Nanohybrids. Bull. Chem. Soc. Jpn. 2011, 84, 893− 902. (17) Ma, W.; Yah, W.; Otsuka, H.; Takahara, A. Application of Imogolite Clay Nanotubes in Organic-Inorganic Nanohybrid Materials. J. Mater. Chem. 2012, 22, 11887−11892. (18) Geraldo, D.; Arancibia-Miranda, N.; Villagra, N.; Mora, G.; Arratia-Perez, R. Synthesis of CdTe QDs/Single-Walled Aluminosilicate Nanotubes Hybrid Compound and their Antimicrobial Activity on Bacteria. J. Nanopart. Res. 2012, 14, 1286−1293. (19) Thomas, B.; Coradin, T.; Laurent, G.; Valentin, R.; Mouloungui, Z.; Babonneau, F.; Baccile, N. Biosurfactant-Mediated One-Step Synthesis of Hydrophobic Functional Imogolite Nanotubes. RSC Adv. 2012, 2, 426−435. (20) Kang, D.; Zang, J.; Jones, C.; Nair, S. Single-Walled Aluminosilicate Nanotubes with Organic-Modified Interiors. J. Phys. Chem. C 2011, 115, 7676−7685. (21) Bottero, I.; Bonelli, B.; Ashbrook, S. E.; Wright, P. A.; Zhou, W.; Tagliabue, M.; Armandi, M.; Garrone, E. Synthesis and Characterization of Hybrid Organic/Inorganic Nanotubes of the Imogolite Type and their Behaviour Towards Methane Adsorption. Phys. Chem. Chem. Phys. 2011, 13, 744−750. (22) Kang, D.; Zang, J.; Jones, C.; Nair, S. Single-Walled Aluminosilicate Nanotube/Poly(vinyl alcohol) Nanocomposite Membranes. ACS Appl. Mater. Interfaces 2012, 4, 965−976. (23) Kang, D.-Y.; Brunelli, N. A.; Yucelen, G. I.; A. Venkatasubramanian, J. Z.; Leisen, J.; Hesketh, P. J.; Jones, C. W.; Nair, S. Direct Synthesis of Single-Walled Aminoaluminosilicate Nanotubes with Enhanced Molecular Adsorption Selectivity. Nat. Commun. 2014, 5, 3342−3351. (24) Yoshinaga, N.; Aomine, A. Imogolite in Some Ando Soils. Soil Sci. Plant Nutr. 1962, 8, 22−29. (25) Cradwick, P. D. G.; Farmer, V. C.; Russell, J. D.; Masson, C. R.; Wada, K.; Yoshinaga, N. Imogolite, a Hydrated Aluminium Silicate of Tubular Structure. Nat. Phys. Sci. 1972, 240, 187−189. (26) Tamura, K.; Kawamura, K. Molecular Dynamics Modeling of Tubular Aluminum Silicate: Imogolite. J. Phys. Chem. B 2002, 106, 271−278. (27) Farmer, V. C.; Fraser, A. R.; Tail, J. M. Synthesis of Imogolite: A Tubular Aluminium Silicate Polymer. J. Chem. Soc., Chem. Commun. 1977, 462−463. (28) Barret, S.; Budd, P.; Price, C. The Synthesis and Characterization of Imogolite. Eur. Polym. J. 1991, 27, 609−612. (29) Bursill, L. A.; Peng, J. L.; Bourgeois, L. N. Imogolite: An Aluminosilicate Nanotube Material. Philos. Mag. A 2000, 80, 105−117. (30) Mukherjee, S.; Bartlow, V. M.; Nair, S. Phenomenology of the Growth of Single-Walled Aluminosilicate and Aluminogermanate Nanotubes of Precise Dimensions. Chem. Mater. 2005, 17, 4900− 4909. (31) Levard, C.; Masion, A.; Rose, J.; Doelsch, E.; Borschneck, D.; Dominici, C.; Ziarelli, F.; Bottero, J. Y. Synthesis of Imogolite Fibers from Decimolar Concentration at Low Temperature and Ambient

several experimental results. In fact, the values we obtained for these lengths are in good agreement with experiment. When the simulations are started with planar initial conditions, we obtain several possible rolled conformations as the end result: nanotubes, nanoscrolls, and double nanotubes and nanoscrolls (“binocular” like configurations), depending on diameter and temperature. The minimal energy tubular structure at 150 K is obtained for N = 11, while the natural one corresponds to N ≈ 10 and the synthetic one, created at T ≈ 368 K, is of N ≈ 12. All in all, a rich assortment of configurations is obtained as the temperature and radius (N values) are varied. It is also worth remarking that kinetic factors and agglomeration are issues that play a significant role in the synthesis of imogolite. All these features do emerge naturally in our simulations. An additional feature that our simulations do yield is the diameter monodipersion that is observed in both natural and synthetic imogolites. This feature, which is only slightly affected by temperature variations in the range that experiments are carried out,67 can be traced to kinetic effects that limit imogolite NT diameters.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fondo Nacional de ́ Investigaciones Cientificas y Tecnológicas (FONDECYT, Chile) under Grants #11110510 (F.M.), #3140526 (R.I.G.), #1110630 (R.R.), #1110135 (J.A.V.), #1120399 and 1130272 (M.K. and J.R.), and Financiamiento Basal para Centros ́ Cientificos y Tecnológicos de Excelencia (R.R., J.R., J.A.V., F.M., F.V., M.R., and M.K.). F.V. was supported by CONICYT Doctoral Fellowship Grant #21140948.

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