Thermodynamic Control of Diameter-Modulated Aluminosilicate

Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea ... for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, J...
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Thermodynamic Control of Diameter-Modulated Aluminosilicate Nanotubes Hoik Lee,† Yangjun Jeon,† Youngil Lee,‡ Sang Uck Lee,*,‡,∥ Atsushi Takahara,§ and Daewon Sohn*,†,∥ †

Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, Seoul 133-791, Korea Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea § Department of Applied Molecular Chemistry, Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 819-0395, Japan ‡

ABSTRACT: The diameter of imogolite nanotubes was regulated by altering the synthesis temperature and was characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and 29Si crosspolarization magic angle spinning nuclear magnetic resonance (CP-MAS NMR). Imogolite diameter modulation via thermodynamic control was induced by curvature formation of the proto-imogolite cluster, which was dependent on the degree of silanol (SiOH) substitution. At lower temperature, 323 K, the amount of SiOH substitution in the protoimogolite clusters decreases and accordingly reduces the hydrogen bonding among SiOH substituents. In contrast, at higher temperature, 371 K, the large amount of SiOH substitution in the proto-imogolite clusters increases the hydrogen bonding among silanol groups, which also increases the degree of the curvature. The proto-imogolite clusters with a larger curvature can quickly create tubular structures by forming a circle with a smaller diameter.

1. INTRODUCTION Over the past 2 decades, a variety of non-carbon-based nanomaterials have been investigated for their unique properties in chemical reactivity, optical characteristics, high elastic modulus, thermal stability, and high specific surface area.1−6 Imogolites are one of many wonderful inorganic materials that can be obtained from nature. They were first identified as a fibrous acid-dispersible clay component of weathered pumice. Imogolites with the general formula of Al2O3·SiO2·2H2O form a hollow nanotube with an external diameter of ca. 2.5 nm, an internal diameter of less than 1 nm, and lengths ranging from several hundred nanometers to micrometers.7 Their basic structure consists of gibbsite sheets with orthosilicic acid groups. Imogolite was successfully synthesized by Farmer in 1977.8 The synthesis of imogolite nanotubes involves a simple hydrolysis step followed by a growth step at 95 °C.9 Recently, Nair et al. reported that the growth of imogolite nanotubes may be a thermodynamically driven self-assembly process.10 Additionally, both Rose et al.11 and Nair et al.12 elucidated a mechanism for the self-assembly of aluminogermanate nanotubes, which is a synthetic derivative of the aluminosilicate nanotube mineral. These mechanisms clearly suggest a possible method for the self-assembly of nanotube materials by controlling the temperature to form nanoparticle condensates, which sets the stage for the self-assembly process.12 Both of these reports reveal that the proto-imogolite clusters created at the initial stage are capable of assembling into a nanotube structure. In addition, they also investigated the effects of varying the temperature used during the condensation © 2014 American Chemical Society

stage on the growth of nanotube structures from protoimogolite clusters. As described in the above-mentioned papers, the protoimogolite clusters created in the initial stage form the building blocks of the nanotube structure. In our previous paper, we elucidated the driving force of tubular imogolite formation and the origin of the minimum strain energy using strain energy relaxation and hydrogen-bonding networks.13 The driving force for tubular imogolite formation is the strain energy relaxation caused by the Al−O and Si−O bond distance variation when the hydroxyl groups on the surface of gibbsite are substituted with SiOH groups to generate proto-imogolite clusters. Furthermore, the strain energy relaxation can be stimulated by the added stability of the hydrogen-bonding networks of the substituents (i.e., SiOH groups), which becomes the origin of the strain energy minimum. Meanwhile, the Nair group recently reported that the shape of proto-imogolite clusters can be controlled by anionic ligands and that the protoimogolite cluster shape is directly related to the diameter of the nanotubes.14 They accomplished this by rationally exploiting the relationship between the binding of different types of anionic ligands to the nanoscale proto-imogolite clusters and the alteration of proto-imogolite cluster curvature. These results imply that the control of the curvature of proto-imogolite clusters determines the shape of the aluminosilicate nanotubes. Received: November 29, 2013 Revised: March 25, 2014 Published: March 25, 2014 8148

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2.4. Solid-State NMR. Solid-state NMR spectra were obtained on a Bruker Avance 400WB NMR spectrometer operating at an 29Si resonance frequency of 79.5 MHz. 29Si CPMAS NMR spectra were recorded at a 7 kHz spinning rate using a commercial 4 mm Bruker double-bearing probe head. Typical acquisition parameters included a 3.5 μs 45° pulse, 14 s recycle delay, 10 scans, and an acquisition time of 40 ms. Chemical shifts were referenced to pure TMS solution 2.5. First-Principles Density Functional Theory Simulations. The Perdew−Burke−Ernzerhof (PBE) gradientcorrected density functional theory (DFT)17 and a double numerical plus d-functions (DND) basis set were employed to optimize all constructed models using the DMol3 program.18,19

In this article, we present the thermodynamically controlled synthesis of imogolite nanotubes with different diameters. Our results show the dependency of the curvature of protoimogolite clusters and the diameter of imogolite nanotubes on the degree of SiOH substitution, which is, in turn, influenced by the temperature used during synthesis. Imogolite nanotubes were synthesized by heating aluminum chloride (AlCl3·6H2O) with a tetraethoxysilane (Si(OEt)4) (TEOS) solution for periods of up to 5 days. Under these conditions, a mixture of gibbsite, imogolite, and halloysite was created and remained in equilibrium at 25 °C in solutions containing 20 μg cm−3 of SiO2. However, at temperatures above 25 °C, imogolite becomes more stable than gibbsite,15 which indicates that the formation of a gibbsite sheet and tubular imogolite or halloysite is most favorable at 25 °C, although it took a long period of time to prepare. In Wada’s experiment, it was shown that a 7 year long synthesis at 25 °C leads to synthetic imogolite having a diameter close to the natural ones.16 Conversely, SiOH substitution and the formation of tubular structures become more favorable under increased synthesis temperatures. Therefore, the temperature will govern the degree of SiOH substitution. Moreover, much experimental and theoretical research has proven that proto-imogolite clusters are created at the initial synthetic stage and that they are assembled into imogolite nanotubes.10−12,14 The shape of proto-imogolite clusters depends on the degree of SiOH substitution, which is controlled by the synthesis temperature used during processing.

3. RESULTS AND DISCUSSION 3.1. Formation Energy. To verify the dependency of SiOH substitution on the synthesis temperature, we investigated the formation energy of 2D planar gibbsite-like imogolite from a gibbsite sheet. The formation energy becomes endothermic, 43.45 kcal mol−1, as shown in Figure 1a. Therefore, SiOH

2. EXPERIMENTAL SECTION 2.1. Synthesis of Imogolite. An aqueous solution of aluminum chloride (AlCl3·6H2O) was mixed with an aqueous solution of tetraethoxysilane (Si(OEt)4). Tetraethoxysilane 99% (TEOS) was purchased from Alfa Aesar, and aluminum chloride hexahydrate 99% was purchased from Aldrich. The final concentration of the solution was 2.0 mmol L−1 with respect to Al and 1.0 mmol L−1 for Si. The solution was stirred for 1 h to hydrolyze the tetraethoxysilane. Aqueous sodium hydroxide 98% (0.1 mol L−1), which was purchased from Daejung, was slowly added by syringe pump at a rate of 0.5 mL min−1 until the aqueous solution reached pH 4. The solution was then refluxed with different temperature for 5 days. After being cooled to room temperature, the suspended material was gelated by supersaturated sodium chloride solution. NaCl 99.5% was purchased from Samchun. The suspended gel solution was filtered by 0.45 μm filter paper and rinsed more than five times by DI water to remove the remaining NaCl. The rinsed inorganic mineral gel in aqueous solution was dispersed by sonication. The cotton-like white solid was then obtained by freeze-drying the dispersed solution of aluminosilicate nanotubes. All specimens were dialyzed for 5 days before characterization to reduce the impurities. 2.2. X-ray Diffraction. Powder X-ray diffraction (XRD) was performed by D/MAX RINT 2000 with Cu Kα radiation (λ = 1.5406 Å). The diffraction pattern was recorded at scanning rate of 2° min−1. The d-spacing value of each sample was calculated by Bragg’s law from the XRD pattern. 2.3. Fourier Transform Infrared Spectroscopy. Each sample was confirmed by FT-IR using an ABB FTLA2000 spectrometer with the potassium bromide (KBr) dilution technique (1 wt % sample in KBr). The FT-IR spectra of each sample were collected after 64 scans at a resolution of 4 cm−1. The spectra were obtained in the region between 4000 and 500 cm−1.

Figure 1. (a) Formation energy of 2D planar gibbsite-like imogolite from a planar gibbsite sheet, 43.45 kcal mol−1, and (b) curvature of proto-imogolite clusters according to the degree of silanol group substitution. The curvature increases from 3.48 to 15.66° with an increase in the number of silanol groups.

substitution is enhanced by increasing the synthesis temperature. In addition, we have examined the change in the curvature in relation to the degree of SiOH substitution to determine the dependency of the curvature of proto-imogolite clusters on the degree of SiOH substitution. As shown in Figure 1b, the degree of curvature increases from 3.48 to 15.66° as the number of silanol groups increase. Consequently, the shape of the proto-imogolite clusters depends on the degree of SiOH substitution, which is influenced by the synthesis temperature. The formation of the tubular imogolite nanotubes originated from the hydrogen-bond (HB) network as well as the degree of Al−O and Si−O bond relaxation. The diameter of the imogolite nanotubes increases with the decreasing strength of the HB network, which is influenced by the aging time and the reaction temperature. The calculated formation energy shows that the degree of curvature could be controlled by temperature. 3.2. X-ray Diffraction Pattern. Additionally, we characterized our samples via XRD. Imogolite nanotubes synthesized at different temperatures (323, 343, and 371 K) were 8149

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Figure 2. (a) Imogolite nanotube X-ray diffraction patterns at different synthesis temperatures: black, red, and blue lines indicate 323, 343, and 371 K, respectively. (b) Monoclinic unit cell model of imogolite nanotubes with a characteristic angle of 78°. (c, d) Calculated energy diagram with different cell parameters of a or b: cell size and angle (γ) between a and b.

characterized by XRD, as shown in Figure 2a. The XRD patterns of imogolite nanotubes showed different diameter sizes at different synthesis temperatures. The XRD patterns also showed the characteristic features of imogolite nanotubes, exhibiting four broad bands at (100), (110), (001), and (211), each corresponding to an (hkl) reflection.20 It was found that imogolite nanotubes form monoclinic unit cells with a characteristic angle of 78°. To obtain the proper bundle structure of the imogolite nanotubes, we investigated the relative energies of the unit cell according to the cell parameters of a (or b): cell size and angle (γ) between a and b. The simulated results show that a unit cell of 19 × 19 Å2 with a 78° γ angle gives the most stable bundle structure of imogolite nanotubes, as shown in Figure 2c,d. Moreover, the simulated XRD pattern is well in agreement with experimental results. Nair et al.10 reported that imogolite nanotubes have typical dspacings of 2.11, 1.67, and 0.85 nm, which are in agreement with our results, as shown in Figure 2. These peaks reflect crystal structures that are refined and well-ordered along the a− b axes, as shown in Figure 2b. The XRD patterns exhibit an intense low-angle peak (3.8 to 4.1° (100)), an accompanying shoulder (9.2 to 10.0° (110)), and a series of other peaks ((001) and (211)), corresponding to the scattering from bundles of tubes. Detailed XRD studies show that the low-angle peak (3.8 to 4.1° (100)) is strongly correlated with the external diameter of the nanotube, but it has only a minor role in the spacing between the nanotubes.21 The low-angle peak (3.8 to 4.1° (100)) as well as other peaks ((110), (001), and (211)) shows systematic shifts as a function of the temperature, 323, 343, and 371 K, corresponding to the changes in the diameter of the nanotubes. Imogolite nanotubes synthesized at 323 K have the largest external diameter (2.32 ± 0.02 nm), whereas nanotubes synthesized at 371 K produce the smallest external diameter (2.12 ± 0.02 nm). The first peak corresponding to the (100) Miller plane shifts from 3.8 to 4.1° of 2θ, which indicates that the diameter changes from 2.32 to 2.12 nm by Bragg’s equation. The samples were freeze-dried and the water content

was carefully removed throughout all of the experiments; thus, we can assert that the shift of the (100) peak originates from diameter contraction. In Figure 2a, the (001) peak shows longitudinal contraction of the unit cell, and the peak intensity increases with increasing temperature. This can be explained by the formation of a well-ordered arrangement at higher temperature. However, we could not distinguish the nanotube length in the TEM images because it varies too much. The peaks at 27 and 40°, corresponding to the 2D unit cell of Al(OH)-Al, do not show dramatic shift that is dependent on temperature. The fact that the diameter of the imogolite nanotubes decreases with increasing temperature (2.32 ± 0.02 nm at 323 K, 2.26 ± 0.02 nm at 343 K, and 2.12 ± 0.02 nm at 371 K) clarifies the dependency of the proto-imogolite cluster curvature and the diameter of the imogolite nanotubes on the degree of SiOH substitution. 3.3. Fourier Transform Infrared Spectrum. To confirm the dependency of SiOH substitution on the size of the imogolite nanotube diameters, we quantitatively analyzed the Si content in the synthesized imogolite nanotubes using FT-IR and NMR. Figure 3 shows the FT-IR spectra of the products at different preparation temperatures. In all cases, the IR spectra have similar absorption bands resolved at 500, 600, and 680 cm−1. These bands indicate the presence of Al−OH bonds. Also, there is a broad absorption band between 2800 and 3800 cm−1 caused by various stretching vibrations of the O−H groups in imogolite, such as Al-(OH)-Al groups and silanol groups at the inner and outer surfaces of imogolite nanotubes, respectively. Lastly, the Si−O stretching mode between 940 and 995 cm−1 shows the important differences brought about by a change in the synthesis temperature. Although the length of the imogolite nanotubes is temperature- and aging timedependent, we could not see a difference in the length of our samples in the TEM images. The transmittance of the Si−O stretching mode between 940 and 995 cm−1 strongly depends on the number of Si−O bonds (i.e., the diameter of imogolite 8150

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SiOH substitution on the size of the imogolite nanotube diameter. It is worth mentioning that SiOH substitution affects the structural stability of the produced proto-imogolite clusters in two ways. The SiOH substitution imposes strain energy into the proto-imogolite clusters through bond-distance mismatching between the Si−O and Al−O bonds. However, substituted SiOH groups can generate hydrogen-bonding networks, which compensate for the strain energy and eventually stabilize the proto-imogolite clusters by setting off a curved structure.13 Therefore, the curvature of the proto-imogolite clusters becomes larger with the increasing degree of SiOH substitution as a result of the induced hydrogen-bonding networks. This means that the proto-imogolite clusters with a larger curvature can quickly create tubular structures by forming a circle with a small diameter. Because the degree of SiOH substitution depends on the synthesis temperature, we expect to obtain imogolite nanotubes of different sizes according to the temperature used during processing. This is shown graphically in Figure 5. At higher temperature, such as 371 K, high-

Figure 3. FT-IR spectra of products at different preparation temperatures: black, red, and blue lines indicate 323, 343, and 371 K, respectively.

nanotubes). Therefore, it can be seen that the transmission of imogolite nanotubes is higher at 371 K and lower at 323 K. These results are consistent with the XRD analysis, which showed that the imogolite nanotubes synthesized at 323 K have a larger diameter (2.32 ± 0.02 nm) than the imogolite nanotubes synthesized at 371 K (2.12 ± 0.02 nm). 3.4. Solid-State CP-MAS Analysis. Similar results were also seen in the CP-MAS NMR results. CP-MAS NMR offers an analytical method for quantitatively determining Si content. This is possible through the use of dipolar decoupling, magic angle spinning, and cross-polarization techniques, which enable line narrowing and considerable signal enhancement for 29Si nuclei in close proximity to protons.22 The solid-state NMR spectra of samples heated to various temperatures are shown in Figure 4. In all cases, the 29Si NMR spectra show a sharp

Figure 5. Shape control of imogolite nanotubes by temperature adjustment. The diameter of imogolite nanotubes decreases with increasing temperature (2.32 ± 0.02 nm at 323 K, 2.26 ± 0.02 nm at 343 K, and 2.12 ± 0.02 nm at 371 K).

curvature proto-imogolite clusters are generated by stimulated SiOH substitution, which quickly assemble into tubular structures with a small diameter. In contrast, at lower temperatures, such as 323 K, SiOH substitution is suppressed, and the curvature of proto-imogolite clusters decreases. Therefore, tubular structure formation requires a greater amount of proto-imogolite clusters to create imogolite nanotubes with larger diameters.

Figure 4. 29Si CP-MAS solid-state NMR spectra of products at different preparation temperatures: black, red, and blue lines indicate 323, 343, and 371 K, respectively.

4. CONCLUSIONS Our findings provide clear evidence of the remarkable role of temperature in the curvature of proto-imogolite clusters and the resulting imogolite nanotubes. We have conclusively demonstrated the dependency of the diameter of imogolite nanotubes on the degree of SiOH substitution according to the synthesis temperature, as shown Figure 5. We present a thermodynamically controlled imogolite nanotube synthesis for modulating proto-imogolite cluster curvature and imogolite nanotube diameter. These findings provide a rational and quantitative framework for angstrom-scale shaping and structuring of

resonance at −78.5 ppm from the SiOH groups on the inner surface of the imogolite nanotubes with a well-defined octahedral configuration. The peaks at −78.5 ppm indicate that imogolite nanotubes synthesized at 371 K have the lowest intensity, whereas imogolite nanotubes synthesized at 323 K have the highest intensity. This suggests that the imogolite synthesized at 371 K has a relatively low SiOH group content compared to imogolite nanotubes synthesized at 323 K. Thus, the CP-MAS NMR analyses also corroborate the XRD and FT-IR results. Our FTIR and CP-MAS NMR spectra clearly show the dependency of 8151

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(10) 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. (11) Levard, C.; Rose, J.; Thill, A.; Masion, A.; Doelsch, E.; Maillet, P.; Spalla, O.; Olivi, L.; Cognigni, A.; Ziarelli, F.; et al. Formation and Growth Mechanisms of Imogolite-Like Aluminogermanate Nanotubes. Chem. Mater. 2010, 22, 2466−2473. (12) Mukherjee, S.; Kim, K.; Nair, S. Short, Highly Ordered, SingleWalled Mixed-Oxide Nanotubes Assemble from Amorphous Nanoparticles. J. Am. Chem. Soc. 2007, 129, 6820−6826. (13) Lee, S. U.; Choi, Y. C.; Youm, S. G.; Sohn, D. Origin of the Strain Energy Minimum in Imogolite Nanotubes. J. Phys. Chem. C 2011, 115, 5226−5231. (14) Yucelen, G. I.; Kang, D.-Y.; Guerrero-Ferreira, R. C.; Wright, E. R.; Beckham, H. W.; Nair, S. Shaping Single-Walled Metal Oxide Nanotubes from Precursors of Controlled Curvature. Nano Lett. 2012, 12, 827−832. (15) Farmer, V. C.; Smith, B. F. L.; Tait, J. M. The Stability, Free Energy and Heat of Formation of Imogolite. Clay Miner. 1979, 14, 103−107. (16) Wada, S.-I. Imogolite Synthesis at 25°C. Clays Clay Miner. 1987, 35, 379−384. (17) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (18) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508−517. (19) Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756−7764. (20) Koenderink, G. H.; Kluijtmans, S. G.; Philipse, A. P. On the Synthesis of Colloidal Imogolite Fibers. J. Colloid Interface Sci. 1999, 216, 429−431. (21) Kang, D.-Y.; Zang, Ji; Wright, E. R.; McCanna, A. L.; Jones, C. W.; Nair, S. Dehydration, Dehydroxylation, and Rehydroxylation of Single-Walled Aluminosilicate Nanotubes. ACS Nano 2010, 4, 4897− 4907. (22) Barron, P. F.; Wilson, M. A.; Campbell, A. S.; Frost, R. L. Detection of Imogolite in Solid State 29Si NMR. Nature 1982, 299, 616−618.

nanotube objects in solution for a variety of possible applications by creating precursors of well-defined and controlled molecular structure and shape.

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AUTHOR INFORMATION

Corresponding Authors

*(S.U.L.) E-mail: [email protected]; Tel: 82-52-259-2339; Fax: 82-52-259-2348. *(D.S.) E-mail: [email protected]; Tel: 82-22-220-0933; Fax: 82-22-299-0762. Author Contributions ∥

S.U.L. and D.S. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF no. 2012M2A2A6035933) grant funded by the Korea government (MEST). We express our sincere thanks to the staff at the Center for Computational Materials Science of the Institute for Materials Research at Tohoku University for their continued support of the SR16000 supercomputing facilities.

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ABBREVIATIONS HB, hydrogen bonding; TEOS, tetraethoxysilane; XRD, X-ray diffraction; FT-IR, Fourier transform infrared; CP-MAS NMR, cross-polarization magic angle spinning nuclear magnetic resonance; PBE, Perdew−Burke−Ernzerhof; DFT, gradientcorrected density functional theory; DND, double numerical plus d-functions

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REFERENCES

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