Hybrid, Tunable-Diameter, Metal Oxide Nanotubes for Trapping of

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Hybrid Tunable-Diameter Metal-Oxide Nanotubes for Organic Molecules Trapping Mohamed Salah Amara, Erwan Paineau, Stephan Rouzière, Béatrice Guiose, Marie-Eve Maryse Krapf, Olivier Taché, Pascale Launois, and Antoine Thill Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503428q • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Chemistry of Materials

Hybrid Tunable-Diameter Metal-Oxide Nanotubes for Organic Molecules Trapping Mohamed Salah Amara,a,b Erwan Paineau,*a,b Stéphan Rouzière,a Béatrice Guiose,b Marie-Eve M. Krapf,b Olivier Taché,b Pascale Launois,a and Antoine Thill*b a

Laboratoire de Physique des Solides, UMR CNRS 8502, Bât. 510, Université Paris Sud, 91405 Orsay CEDEX, France.

b

CEA Saclay, IRAMIS, UMR CEA/CNRS 3299 NIMBE, LIONS, F-91191 Gif-sur-Yvette, France.

ABSTRACT: New developments in nanosciences and nanotechnologies are strongly dependent of our ability to synthesize wellcontrolled nanobuilding units, with specific properties. We report in this article the first synthesis of hybrid single-walled imogolite nanotubes (OH)3Al2O3SixGe1-xCH3 with diameter-controlled hydrophobic nanopores varying from 1.8 to 2.4 nm. Methylation and nanotube dimensions are studied combining infrared spectroscopy, cryo-TEM observations and X-ray scattering measurements. We show that in solution, water density inside methylated nanotubes is decreased by a factor 3 compared to bulk value. Spontaneous confinement of bromopropanol molecules inside the nanotubes, when added to the solution, is demonstrated. These newlysynthesized nanotubes may open up possibilities for water filtration or water decontamination.

1. INTRODUCTION Since 1991 and the seminal paper of Iijima,1 considerable attention has been devoted to carbon nanotubes (CNT) due to their unique physical properties2 relevant for functional composite materials,3 or new electronic devices,4 to cite a few. These hollow structures also represent ultimate onedimensional (1D) nanochannels for molecular confinement and nanofluidic applications.5,6 Indeed, spontaneous filling7,8 and fast conduction of water9 through CNT have been recently reported, making 1-2nm diameter CNT good candidates for the next generation of water desalination membranes.10 However, carbon nanostructures are hardly miscible in aqueous suspensions for further manipulations without sophisticated functionalization11,12 and post-synthesis procedures are required to sort carbon nanotubes in diameter and chirality.13 Meanwhile, various inorganic metal-oxide nanotubes have been reported,14,15 opening the route towards a rich new field of platforms for nanofluidics.16 Imogolite metal-oxide nanotubes (INT) have emerged as one of the most promising competitor of CNT in several areas in nanosciences. Present naturally in volcanic soils,17 imogolites can also be easily synthetized by low-temperature solution phase protocols.18,19 Contrarily to CNT, these nanotubes benefit of a well-defined minimum of their strain energy which allows achieving monodisperse samples in diameter and chirality.20,21 INT consist of a curved gibbsite Al(OH)3 layer whose vacancies are bonded at the upright by isolated [(OH)SiO3] tetrahedral units on the internal surface (Figure 1), forming aluminosilicate Si-INT nanotubes with internal diameter typically equal to 1 nm.22 First, alumino-silicate nanotubes could only be synthesized using very low concentration of precursors,18 precluding their easy use in industrial processes. But recently, several teams have overcome this issue by substituting silicon by germanium, leading to the synthesis of single-walled (SW) aluminogermanate Ge-INT imogolite nanotubes, constructed from one curved gibbsite layer whose vacancies are bonded by isolated [(OH)GeO3] tetrahedra, and/or double-walled (DW) Ge-INT

imogolite nanotubes, with larger inner diameters of 1.4 to 2.8 nm, respectively.21,23-25

Figure 1. Atomic structure of non-modified aluminosilicate nanotube (Si-n-INT): (a) along the axial direction; (b) 3-dimensional view representing the octahedral gibbsite layer and isolated tetrahedral units.

The diameter of SW INT increases continuously with the substitution of Si by Ge.21,26 Synthesis of (OH)3Al2O3SixGe1x(OH) imogolite-like nanotubes thus offers a convenient and simple way to control INT diameter. The choice of initial precursors can also allow one to tune the diameter and length of INT.27,28 In addition to a tunable shape, the strength of INT lies in their peculiar structure composed of inner and outer hydroxyl groups, which makes such ‘normal’ imogolite nanotubes (n-INT) excellent candidates for surface functionalization. Dispersion of INT in polymer matrix was successfully achieved by modifying the aluminol external surface with hydrophilic or hydrophobic compounds.29 A wide range of applications for these modified INT is currently explored, for example, as nanofillers for improved tensile performances,30 for molecular sieving,31,32 or in biomaterials.33 Meanwhile, elaboration of INT with organic-modified inner surface needs either (i) a post-synthesis chemical modification; or (ii) the replacement of the initial alkoxide (prefiguring the tetrahedral layer) by a functionalized one. Post-functionalization can be obtained by different silane agents but requires previously a perfect dehydration of the hydrophilic INT samples.34 The use of a methyltriethoxysilane (MTES) agent during synthesis, first explored by Bottero et al, leads to the direct formation of methyl-functionalized aluminosilicate nanotubes (Si-mINT).35-37 Moreover, Kang et al reported very recently the

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direct synthesis of amino-modified aluminosilicate nanotubes via the design of a specific precursor.38 In both cases, an enhanced adsorption towards CO2 and methane was evidenced, which could find applications in molecular separation, gas storage or catalysis.35,37,38 While all these studies concern only silicon-based imogolite nanotubes, no data have been reported so far on germanium and Si/Ge modified nanotubes. We explore in this article the possibility to synthesize methylmodified imogolite nanotubes while tuning the Si/Ge substitution ratio by using a mixture of methyltriethoxysilane (MTES) and methyltriethoxygermane (MTEG). Like for n- INT, a dependence of the inner cavity diameter with the substitution of Si by Ge could be expected in m-INT. Modified m-INT (OH)3Al2O3SixGe1-xCH3 are characterized by combining infrared (IR) spectroscopy, cryo-Transmission Electron Microscopy (cryo-TEM) observations and X-ray Scattering (XRS) measurements. We demonstrate that methyl-modified m-INT can be obtained from the silicon to the germanium analogue end-members with a progressive increase of the nanotube diameter. On the basis of our XRS experiments and of their subsequent modeling, we are able to determine the inner and outer diameters of the nanotubes in suspension and to evidence unambiguously the hydrophobic character of the inner surface of m-INT. By using electronic contrast effect, we finally highlight for the first time, the possibility to confine organic molecules inside these nanotubes. The control of these hydrophobic nanochannels size and the possible capture of organic molecules confer to these tubular structures a wide range of potential applications, from filtration to depollution of water. 2. EXPERIMENTAL SECTION Methyl hybrid Si/Ge nanotubes (m-INT) synthesis. Synthesis of hybrid Si/Ge nanotubes with methylated inner cavity of nominal composition (OH)3Al2O3SixGe1-xCH3 was performed by adding a mixture of methyltriethoxysilane (MTES) and methyltriethoxygermane (MTEG) at different x ratios (x = [Si]/([Si]+[Ge]) = 0, 0.25, 0.5, 0.75, 1) to an aluminium perchlorate solution with an [Al]/([Si]+[Ge]) ratio equal to 2. The initial aluminum perchlorate concentration is C = 0.1 mol.L-1. Then, solutions were slowly hydrolyzed by the addition of a 0.1 mol.L-1 NaOH solution to reach a hydrolysis ratio ([OH]/[Al]) of 2. Solutions are stirred overnight at room temperature in Teflon beakers and then aged for 5 days into an oven at 90°C. Afterwards, all samples are dialyzed against ultrapure water, using 8 kDa membranes in order to withdraw residual salts and alcohol in excess. Dialyses were performed until the conductivity dropped below 5 µS.cm-1 except in the case of m-INT (x = 0.75) and n-INT (x = 0), for which dialysis was not as complete. Using this protocol, typical yield of ~ 15 g of nanotube sample can be obtained for 1 L of solution. Normal Si/Ge nanotubes (n-INT) synthesis. Hydroxylated aluminosilicate (x = 1) and aluminogermanate (x = 0) nanotubes of nominal composition (OH)3Al2O3SixGe1-xOH were also prepared as reference non-modified end-members. Syntheses were conducted in the same conditions of hydrolysis, heating and dialysis than m-INT by replacing only TEMS and TEMG by tetraethoxysilane (TEOS) and tetraethoxygermanium (TEOG), respectively. As shown previously,18 singlewalled (SW) Si-n-INT can be obtained only by using a dilute (C = 10-3 mol.L-1) aluminium perchlorate solution while for Ge-n-INT analogues, the synthesis was achieved using higher

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concentration C = 1.0 mol.L-1.25 Due to the low concentration of aluminium precursor, a concentration process was applied to Si-n-INT for further characterizations. Fourier Transform Infrared Spectroscopy. Infrared (IR) spectra were recorded using a Bruker Vertex 70 FTIR spectrometer at room temperature. About 1.5 mg of dry sample powder was mixed with 150 mg of potassium bromide powder and then pressed into a transparent disk. 200 scans at a resolution of 4 cm-1 were averaged to collect spectra in the range 4000-400 cm-1. Transmission Electron Microscopy. Cryo-TEM experiments were undertaken using a FEI Tecnai G2 Polara device operated at 300kV. The experiments were performed under low electron illumination conditions (low dose < 15 e-.Å-2). The images have been collected using a 4k x 4k Ultrascan Gatan camera with a calibrated magnification of 155000. A drop (4 µL) of the solution is deposited on a R2/2 Quantifoil grid made hydrophilic after glow discharging. A fully automated Vitrobot (FEI) device was used to blot the grid and rapidly plunge and freeze aqueous films of the nanotubes suspensions into liquid ethane cooled by liquid nitrogen to prevent the formation of ice crystals. X-ray Scattering (XRS). XRS experiments were carried out on a rotating anode generator (λMoKα = 0.711 Å) collimated by a multilayer optics and hybrid crystal slits, providing a monochromatic beam of 1×1 mm² at the sample position. A vacuum chamber behind the sample allows minimization of the X-ray scattering signal from air. A MAR research X-ray sensitive imaging plate detector with 100 µm pixel size is placed after the output window of the vacuum chamber at a distance of 650 mm from the sample. INT suspensions were introduced in kapton capillaries of about 0.24 cm in diameter. Scattered intensity I as a function of the wave-vector modulus Q is obtained by angular integration over the two-dimensional scattering pattern. Q is defined as Q = ||kd-ki|| (kd and ki are the wavevectors of the scattered and incident beams, respectively) and is equal to 4π/λ sin(θ), where λ is the incident wavelength and 2θ is the scattering angle. Data are corrected for water and kapton scattering as well as for electronic background. 3. RESULTS AND DISCUSSION Nanotubes with methylated inner cavity. We first investigated the replacement of hydroxyl groups by methyl groups in hybrid imogolite nanotubes by using infrared (IR) spectroscopy. Figure 2 presents the IR spectra obtained on hybrid mINTs (OH)3Al2O3SixGe1-xCH3 for x = 0, 0.25, 0.5, 0.75 and 1. For comparison, we also reported the IR curves for nonmodified nanotubes n-INT. Data were recorded at room temperature without any thermal treatment to remove water present within and around nanotubes after the synthesis. In addition to unresolved bands arising from multiple ν(OH) stretching modes of water, the OH stretching region (3800-2800 cm1 ) of n-INTs presents large absorption bands due to external Al-(OH)-Al groups (3800-3050 cm-1) and internal silanol or germanol functions (3050-2800 cm-1).35 The latter appears to be less intense in the case of m-INT and the presence of two new absorption peaks at 2978 and 2920 cm-1 point towards the inner cavity methylation of Si-m-INT. These bands, assigned to asymmetric (νas) and symmetric (νs) stretching vibrations of Si-CH3, respectively,35 are progressively shifted to 3000 and


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2925 cm-1 upon decreasing the substitution ratio x = [Si]/([Si]+[Ge]) down to 0 (inset ν(-CH3) in Figure 2).

Figure 2. IR absorbance spectra obtained for non-modified n-INT and methyl hybrid m-INT synthesized at different x = [Si]/([Si]+[Ge]) ratios. The insets display a zoom on the stretching ν(-CH3) and bending δ(-CH3) modes of methyl group. The star indicates characteristic vibrational bands of remaining perchlorate anions.

The bending mode of methyl groups δ(-CH3) is also affected by the Si/Ge substitution ratio, shifting from 1275 (x = 1) to 1252 cm-1 (x = 0) (inset δ(-CH3) in Figure 2). A possible explanation could be the modification of the nanotube diameter that will be confirmed in the following. The IR spectra of the samples also display absorption peaks at ~ 1100 cm-1, characteristic of the Cl-O vibrational bands from the perchlorate anions. These peaks were already observable in samples previously obtained by using the perchlorate synthesis route. 26,43,44 It is worth noting that samples m-INT (x = 0.75) and nINT (x = 0) containing the more ClO4- ions are those that have been on dialysis for the shortest time. The presence of residual perchlorate ions, however, does not modify the general conclusions of our work. Considering now the signatures of the imogolite structure at lower frequencies, all samples display absorption peaks close to 695, 550 and 420 cm-1, related to various Al-O stretching vibrations,39 while the band at 505 cm-1, related to Si-O-Al bending in Si imogolite,40 is shifted at 460 cm-1 for Ge analogues, as observed previously on n-INT Si/Ge mixtures.26,39 These features show that no noticeable modification of the outer surface occurs. Methylation of the INT structure can be considered as affecting mainly the inner cavity. Indeed, major changes are observed only in the bands arising from the tetrahedral layer. First, we evidenced a new absorption band in mINT, located at 780 cm-1 (x = 1) and assigned to Si-C stretching vibration in other methylated-clay systems,41,42 which shifts to 772 cm-1 in pure Ge-m-INT. Second, the two characteristic peaks of Si-n-INT at 995 and 940 cm-1, corresponding to Si-O-Al and Si-O stretching vibrations, are shifted at 960 and 910 cm-1 in Si-m-INT. A similar feature is evidenced for Ge analogues, with a shift of the 910 and 820 cm-1 bands in Ge-n-INT to 865 and 809 cm-1 in Ge-m-INT. In summary, our IR measurements reveal the successful synthesis of hybrid Si/Ge imogolite nanotubes with organic-modified inner cavity.

Inner and outer diameters of m-INT in aqueous suspension. To determine the diameters of the synthesized methyl hybrid nanotubes, we performed XRS measurements on INT samples. It was recently shown that single-walled n-INT (x = 0) are deformed and adopt a hexagonal base shape when arranged in bundles in powder samples while the cylindrical shape is preserved in suspension.43 Here, we chose to characterize our samples in suspension to avoid such effect. Figure 3 compares the scattering diagrams of n-INT and m-INT at x = 0 (Figure 3a) and x = 1 (Figure 3b). Scattering diagrams of mINT samples for x = 0, 0.25, 0.5 and 1 are presented in Figure 4.

Figure 3. XRS diagrams of non-modified n-INT and methylated m-INT imogolite suspensions for (a) Ge analogues, x = 0 and (b) Si analogues, x = 1. Simulations have been performed using eq. (2) for n-INT and eq. (4) for m-INT (see text). Curves are translated vertically for the sake of clarity.

XRS diagrams exhibit clear differences upon methylation of the nanotubes. While n-INT diagrams display regular oscillations characteristic of well-dispersed SW nanotubes,25,43 those obtained for m-INT present a strong deformation of the first oscillation. Furthermore, the positions of their minima are shifted at higher or lower Q-values for Ge and Si m-INT, respectively, except for the first minimum which is always located at lower scattering angles (Figure 3). For methylated tubes, the XRS diagrams shift progressively upon substitution of Si by Ge (Figure 4).

Figure 4. XRS diagrams of m-INT imogolite suspensions at different x = [Si]/([Si]+[Ge]) ratios. Curves are translated vertically for the sake of clarity.



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Table 1. Structural characteristics of Si(Ge)-m(n)-imogolite nanotubes.

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Material

Ri (Å)

Re (Å)

ρINT (e-.Å-3)

ρi (e-.Å-3)

N

L (Å)

Si-n-INT

7.4

13.4

0.71

0.334

12

5000

Si-m-INT

9.1

15.1

0.67

0.12

13

200

Ge-n-INT

13.8

20.3

0.74

0.334

20

450

Ge-m-INT

12.2

18.7

0.79

0.09

18

200

In addition, it was shown previously that below x = 0.2, a SW to DW transition can occur in n-INT26. While the synthesis conditions of Ge-m-INT are favourable in practice for the formation of the DW structure (CAl = 0.1 mol.L-1), we do not observe the characteristic XRS diagrams of the DW structure in the present study. The presence of -CH3 groups should thus hinder electrostatic interactions between external and internal walls. In the wave-vector range studied here (Q < 1 Å-1), XRS diagrams can be analyzed within the homogeneous approximation. Imogolite nanotubes are approximated by a homogeneous cylinder with an internal radius, Ri, a wall thickness e and an average electronic density ρINT (Figure 5).28,43 The external radius is Re = Ri + e. For an imogolite with an atomic ratio x = [Si]/([Si]+[Ge]), ρINT writes:

ρ INT =

(

2 NZ INT

)

π Re2 − Ri 2 T

(1)

where T ~ 8.5 Å is the repetition distance along the tube axis, N is the number of Si/Ge atoms in the tube circumference (within a period T, the nanotube contains 2N Si or Ge atoms) and ZINT = 86+14x+32(1-x) is the number of electrons associated to (OH)3Al2O3SixGe1-xOH or (OH)3Al2O3SixGe1-xCH3 units (note that hydroxyl and methyl groups have the same number of electrons).

Figure 6. Cryo-TEM images and diameter distribution histograms of methyl-modified imogolite nanotubes. (a,c) Si-mINT; (b,d) Ge-m-INT. The arrow points towards small bundles of nanotubes and the whitescale bar is 20 nm. 2 Re and σ refer to the average diameter and the relative standard deviation, respectively. Due to experimental resolution limitations,43 X-ray experiments are not sensitive to lengths exceeding ~ 100 Å. All samples studied here consisting of INT longer that 100 Å, we consider infinite length nanotubes to simplify calculations. The scattered X-ray intensity for individual nanotubes r I ind Q in well-dispersed suspensions writes:43

()

()

(

(

r 1 I ind Q ∝ ρ i FRi − ρ e FRe + ρ INT FRe − FRi Q Figure 5. (a) Schematic representations of a single-walled nanotube and (b) radial electronic density profiles along the nanotube diameter.

The average imogolite length was estimated from cryo-TEM images25,26 (Figure 6a,b) and is reported in Table 1.

))2

(2)

ρi and ρe are the electronic density of water inside and outside the nanotubes, respectively (Figure 5b). ρi is an adjustable parameter while ρe is taken equal to the bulk water density (ρw = 0.334 e-.Å-3).

In eq. (2), FR is the Fourier transform of the projection of a full cylinder along its axis:


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Chemistry of Materials r RJ (QR) FR Q ∝ 1 Q

()

(3)

where J1 is the cylindrical Bessel function of order 1. Using eq. (2), good agreement between simulated and experimental XRS diagrams of n-INT is obtained (Figure 3) and the fitting parameters are given in Table 1. Note that the number N of Si/Ge atoms on the nanotube circumference is related to the radius of the Ge layer, taken here as RGe = Ri + 0.3 e, by the T . Determination of radii values relation:45 RGe = 2 3 sin(π N ) enabled us to calculate N and the INT densities reported in Table 1. In the case of m-INT, the scattering model of individual nanotubes fails to reproduce the shape of the first oscillation (Figure S1 in S.I.). This modification of shape could be attributed to the appearance of a wide correlation peak at the first oscillation maximum, suggesting that m-INTs are not perfectly dispersed in suspensions. It is corroborated by our cryo-TEM observations which reveal the formation of small bundles in m-INT samples (Figure 6). While we have shown using IR spectroscopy that methylation mainly occurs on the inner surface of the nanotubes, very partial methylation of the outer tube surface could not be excluded, which might explain the formation of small bundles. To take into account this aggregation effect, we need to modify our scattering model, developed for individual nanotubes. The scattered intensity is now written: r r I bundle Q = I ind Q S bundle (Q )

()

()

(4)

The second term in eq. (4) is the structure factor  p Ntb  S bundle (Q) = J 0 QH ij  arising from the nano N tb  i , j  Ntb   tube arrangement in bundles, where the bundles can be formed with different numbers of tubes Ntb, the proportion of each type of bundle being given by pNtb . Hij is the modulus of the

∑

∑ (

)

vector joining the centers of tubes i and j in a bundle, in the plane perpendicular to its axis and J0 is the cylindrical Bessel function of order 0. Nanotubes are assumed to arrange on a hexagonal lattice within bundles. Figure S2 illustrates the contribution of each term in eq. (4) as well as the resulting calculated curve in the case of Ge-m-INT. Moreover, we show in Figure S3 that the lattice parameter a is equal to the nanotube diameter. Best agreement between experiments and simulations (Figure 3), for both Si-m-INT and Ge-m-INT, is obtained with p1 = 0.3, p 2 = 0.3, p3 = 0.3, p 4 = 0.1 and pNtb = 0 for Ntb > 4, that is for rather small bundles, and with a = 2Re, corresponding to close-packed nanotubes. Radii of the Si and Ge-m-INT, deduced from our best fits of XRS diagrams, are also reported in Table 1. These values are in good agreement with those obtained directly from measurements on cryoTEM pictures (Figure 6c,d). For silicon-based INT, m-INT cavity diameter is found to be increased by 3.4 Å in comparison with n-INT, as observed previously.35-37 But we evidence here for the first time a shrinkage effect of the nanotube diameter for Ge analogues. The replacement of –OH groups by – CH3 moieties reduces the number of Ge atoms in the nanotube circumference from 20 to 18, lowering the internal diameter

by 3.2 Å. A deeper understanding of this somehow surprising result, thanks to ab initio energetic simulations for instance, is beyond the scope of this article but would be highly informative. We have evidenced in our simulations that the positions of the minima of the scattered intensity for Q > 0.3 Å-1 are, at the first-order, controlled only by the tube radii Ri and Re, while the position of the first minimum, below 0.2 Å-1, depends also on the internal electronic density ρi (Figure S4 in S.I.). In the case of Ge-INT, minima above 0.3 Å-1 are shifted to larger Q values under methylation because nanotube radii increase but the first minimum shifts to lower Q values due to a decrease of the internal water density under methylation (Table 1). The large shift to lower Q values of the first minimum of the XRS diagram of Si-INT under methylation is explained by the diminution of the nanotube radii together with a strong diminution of its water content. The inner water density is reduced by a factor 3 compared to bulk water density, for both Si and Ge-m-INT, illustrating hydrophobic character of their inner cavity. It corroborates our conclusion, based on IR results, that the inner surface is covered by methyl groups. The continuous shift of scattering diagrams of m-INT from x = 0 to 1 (Figure 4) can now be easily interpreted. The water density inside methylated nanotubes can be assumed to be constant (equal to about 0.1 e-.Å-3) and the shift of the positions of all minima is related to the change in nanotubes radii, from Ri = 12.2 Å (Re = 18.7 Å) for x = 0 to Ri =9.1 Å (Re = 15.1 Å) for x = 1. The monodispersity of the nanotube diameter is confirmed by cryo-TEM measurements. The relative standard deviation   2 2 2 Re  is smaller than 5% for  σ = (2 Re ) − 2 Re   both Si and Ge-m-INT (Figure 6c,d).The tuning of Si/Ge substitution allows tailoring diameter-controlled m-INT over more than 6 Å, with hydrophobic properties inside. Our straightforward synthesis procedure offers therefore a precise control of m-INT diameter relevant to potential applications and to further research studies in the new field of nanofluidics,46 for instance. As mentioned in the Introduction section, hydrophobic CNT allow for ultra-fast conduction of water,9 with a diameter dependent friction coefficient.47 It would be most interesting to compare water transport properties through mINT imogolite nanotubes and CNT. In the following, we consider another interesting topic, with potential applications for water depollution: the confinement of organic molecules inside m-INTs in suspension. Confinement of organic molecules. We have shown in the previous section that XRS measurements are sensitive to the electronic density contrast inside and outside the nanotubes (Figure S4). In order to probe the affinity of the nano-cavity with organic compounds by X-ray scattering, we selected an organic molecule with a larger electronic density than water in the liquid state, whose insertion inside m-INT would thus enhance ρi. Experiments were performed using 3-bromo-1propanol (BRP, ρBRP = 0.453 e-.Å-3) with the two end-members of m-INT set (x = 0 and x = 1). Initial volume of m-INT solutions was set at 300 µL and different amounts of BRP (up to 10 µL) were added to the suspensions. The bulk (outer) density is increased by less than 1%, which is negligible. Thus, in our experiment, the modification (or not) of the position of the first minimum on the XRS diagram will be used to measure the electronic density of the fluid inside the nanotubes. Figure



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7 displays the impact of BRP addition on the XRS diagrams of m-INT suspensions. We observe that the position of the first minimum of I(Q) shifts significantly towards larger values up to an introduced volume of 5 µL for x = 0 and 3 µL for x = 1. As already shown the shift of the first minimum of XRS intensity towards higher wave-vector values is the signature of an increase of the internal density ρi. Simulations presented in Figure 7 (right) allowed us to determine the internal density, reported in Tables 2 and 3 together with the corresponding volume of BRP added in the suspension VBRP or equivalently, the initial concentration

CiBRP in BRP. In both Si- and Ge-m-INT, internal densities increase by a factor 3, up to the value of the bulk liquid density around the nanotubes.

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panol molecules are confined in m-INT either (i) with water molecules initially present Nw = cst or (ii) by expelling them out of the nanotubes Nw = 0. While the latter hypothesis allows BRP one to calculate the maximal value of ηtubes , the first one only gives an indicative value of the smaller amount inserted since it could happen that additional water molecules are confined together with BRP ones. In any case, the increase, by a factor of 3, of the inner electronic density ρi clearly proves the affinity of the methylated inner surface with BRP and evidences the capture of BRP molecules inside m-INT in solution. To the best of our knowledge, we report here the first evidence for organic molecules confinement in suspensions of methylmodified imogolite nanotubes. It opens the way towards water depollution for instance, as m-INT nanotubes may be used to take back organic waste rejected in water. The tunable diameters of the nanotubes could even allow one to sort out some of them, depending of their size. We underline that CNT cannot be used for such applications as they are hydrophobic on their external surface and do not form stable suspensions in water. BRP of Table 2. Internal electronic density and amount η tube inserted BRP molecules per gram of imogolite (mmol.g-1) deduced from XRS simulations for Ge-m-INT.

Figure 7. Evolution of the XRS diagram of m-INT suspensions upon addition of bromopropanol (BRP) solution: (a,b) Ge-m-INT and (c,d) Si-m-INT. Left panels: experimental diagrams. Right panels: corresponding simulated XRS diagrams using eq. (4). Initial m-INT volumes are set to 300 µL. The vertical dash line spots the position of the first minimum of I(Q) before adding BRP molecules. For the sake of clarity, experimental and calculated curves have been translated.

The internal electronic density per imogolite half period (4.25 Å) along the c axis is given by:

ρi =

10N w + 68N BRP 2

πRi T 2

VBRP (µL)

C iBRP (mol.L-1)

ρi (e .Å )

0

0

1

-

-3

BRP ηtubes (mmol.g-1)

Nw = cst

Nw = 0

0.09

0

-

0.037

0.10

0.07

0.67

2

0.073

0.18

0.6

1.2

4

0.146

0.27

1.2

1.8

5

0.181

0.32

1.53

2.13

10

0.357

0.32

1.53

2.13 BRP η tube

Table 3. Internal electronic density and amount of -1 inserted BRP molecules per gram of imogolite (mmol.g ) deduced from XRS simulations for Si-m-INT.

VBRP (µL)

C iBRP (mol.L-1)

ρi (e .Å ) -

-3

BRP ηtubes (mmol.g-1)

Nw = cst

Nw = 0

0

0

0.12

0

-

1

0.037

0.23

0.69

1.45

2

0.073

0.27

0.95

1.70

3

0.110

0.34

1.39

2.14

4

0.146

0.34

1.39

2.14

5

0.181

0.34

1.39

2.14

(5)

Nw and NBRP representing respectively the number of water and bromopropanol molecules with 10 and 68 electrons per molecule. Our X-ray experiments are extremely sensitive to any variation of the density of the confined fluid but they do not allow us to discriminate between water and BRP molecules inside the INT. The study of the possible co-confinement of water and BRP inside m-INT is beyond the scope of this article. However, to give some quantitative insights to the reader, we report in Tables 2 and 3 the amount of confined BRP molBRP ecules ηtubes per gram of imogolite, assuming that bromopro-

4. CONCLUSIONS


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We have shown experimentally that the diameter of methylmodified single-walled imogolite nanotubes can be tuned, with a control at the level of the angstrom, by partial substitution of -SiCH3 by -GeCH3 during the synthesis. The as-synthesized (OH)3Al2O3SixGe1-xCH3 m-INTs are monodisperse in diameter, contrarily to carbon nanotubes. Thanks to X-ray scattering, we determined precisely the inner and outer diameter values depending on the substitution ratio x, which will surely be at the origin of structural and energetically theoretical investigations in the future. We evidenced the actual methylation of the inner nanotubes surface by IR spectroscopy, as well as the hydrophobicity of m-INT through the decrease of the inner water density in suspension, probed using X-ray scattering. Finally, we demonstrated that m-INT nanotubes, in water, spontaneously catch inside them bromopropanol molecules added to the suspension, so that they could be used for water decontamination. The possible interest of these hydrophobic nanotubes for water transport, like carbon nanotubes, is also underlined and should be the subject of future work.

ASSOCIATED CONTENT Supporting Information. Supplementary X-ray simulation results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. Tel: +33 (0)1 69 15 60 51 * E-mail: [email protected]. Tel: +33 (0)1 69 08 99 82

ACKNOWLEDGMENT This work was funded by C’Nano Ile de France and by the French ANR agency (contract no. ANR-11-BS08-02, HIMO²). Use of the platforms of the Grenoble Instruct centre (ISBG; UMS 3518 CNRS-CEA-UJF-EMBL) with support from FRISBI (ANR-10INSB-05-02) and GRAL (ANR-10-LABX-49-01), within the Grenoble Partnership for Structural Biology (PSB), is acknowledged. We thank M. Bacia-Verloop for her help in cryo-TEM experiments, Philippe Joly for his work on the X-ray generator used and Luc Belloni for fruitful discussions.

REFERENCES (1) Iijima, S. Nature 1991, 354, 56-58. (2) Loiseau, A.; Launois, P.; Petit, P.; Roche, S.; Salvetat, J. P.; Eds. Understanding Carbon Nanotubes: from Basics to Applications; Series: Lecture Notes in Physics; Springer: New York, 2006. (3) Ajayan, P. M.; Tour, J. M. Nature 2007, 447, 1066-1068. (4) Anantram, M. P.; Léonard, F. Rep. Prog. Phys. 2006, 69, 507-561. (5) Monthioux, M.; Ed. Chapter 5. In Carbon MetaNanotube:Synthesis, Properties and Applications; Wiley: 2011. (6) Park, H. G.; Jung, Y. Chem. Soc. Rev. 2014, 43, 565-576. (7) Kyakuno, H.; Matsuda, K.; Yahiro, H.; Inami, Y.; Fukuoka, T.; Miyata, Y. Yanagi, K.; Maniwa, Y.; Kataura H.; et al. J. Chem. Phys. 2011, 134, 244501. (8) Paineau, E.; Albouy, P. A.; Rouzière, S.; Orecchini, A.; Rols, S.; Launois, P. Nano Lett. 2013, 13, 1751-1756. (9) Holt, J. K.; Park, H. G.; Wang, Y.; Stadermann, M.; Artyukhin, A. B.; Grigoropoulos, C. P.; Noy, A.; Bakajin, O. Science 2006, 312, 1034-1037. (10) Kar, S.; Bindal, R. C.; Tewari, N. K. Nano Today 2012, 7, 385389. (11) Ma, P. C.; Siddiqui, N. A.; Marom, G.; Kim, J. K. Composites Part A: Applied Sci. and Manuf. 2010, 41, 1345-1367.

(12) Karousis, N.; Tagmatarchis, N.; Tasis, D. Chem. Rev. 2010, 110, 5366-5397. (13) Fagan, J. A.; Khripin, C. Y.; Silvera Batista, C. A.; Simpson, J. R.; Hároz, E. H.; Hight Walker, A. R.; Zheng, M. Adv Mater. 2014, 26, 2800-2804. (14) Kijima, T. Inorganic and Metallic Nanotubular Materials: Recent Technologies and Applications. Springer-Verlag: Berlin, 2010. (15) Tenne, R. Nat. Nanotechnol. 2006, 1, 103-111. (16) Goldberger, J.; Fan, R.; Yang, P. Acc. Chem. Res. 2006, 39, 239248. (17) Yoshinaga, N.; Aomine, S. Soil Sci. Plant Nutr. 1962, 8, 22-29. (18) Farmer, V. C.; Fraser, A. R.; Tait, J. M. J. Chem. Soc. Chem. Comm. 1977, 13, 462-463. (19) Farmer, V.C.; Fraser, M.J.; Palmieri, F. Clay Miner. 1983, 18, 459-472. (20) Guimarães, L.; Enyashin, A. N.; Frenzel, J.; Heine, T.; Duarte, H. A.; Seifert, G. ACS Nano 2007, 1, 362-368. (21) Konduri, S.; Mukherjee, S.; Nair, S. ACS Nano 2007, 1, 393-402. (22) Cradwick, P. D. G.; Farmer, V. C.; Russell, J. D.; Masson, C. R.; Wada, K..; Yoshinaga, N. Nature (London) Phys. Sci. 1972, 240, 187189. (23) Ookawa, M.; Hirao, Y.; Watanabe, M.; Maekawa, T.; Inukai, K.; Miyamoto, S.; Yamaguchi, T. Clay Sci. 2006, 13, 69-73. (24) Maillet, P.; Levard, C.; Larquet, E.; Mariet, C.; Spalla, O.; Menguy, N.; Rose, J.; Masion, A.; Thill, A. J. Am. Chem. Soc. 2010, 132, 1208-1209. (25) Thill, A.; Maillet, P.; Guiose, B.; Spalla, O.; Belloni, L.; Chaurand, P.; Auffan, M.; Olivi, L.; Rose, J. J. Am. Chem. Soc. 2012, 134, 3780-3786. (26) Thill, A.; Guiose, B.; Bacia-Verloop, M.; Geertsen, V.; Belloni, L. J. Phys. Chem. C 2012, 116, 26841-26849. (27) Yucelen, G. I.; Kang, D. Y.; Guerrero-Ferreira, R. C.; Wright, E. R.; Beckham, H. W.; Nair, S. Nano Lett. 2012, 12, 827-832. (28) Amara, M. S.; Paineau, E.; Bacia-Verloop, M.; Krapf, M. E. M.; Davidson, P.; Belloni, L.; Levard, C.; Rose, J.; Launois, P.; Thill, A. Chem. Comm. 2013, 49, 11284-11286. (29) Ma, W.; Yah, W. O.; Otsuka, H.; Takahara, A. J. Mater. Chem. 2012, 22, 11887-11892. (30) Ma, W.; Otsuka, H.; Takahara, A. Polymer 2011, 52, 5543-5550. (31) Kang, D. Y.; Tong, H. M.; Zang, J.; Choudhury, R. P.; Sholl, D. S.; Beckham, H. W.; Jones, C. W.; Nair, S. ACS Appl. Mater. Interfaces 2012, 4, 965-976. (32) Barona, G.N.B.; Choi, M.; Jung, B. J. Colloid Interf. Sci. 2012, 386, 189-197. (33) Jiravanichanun, N.; Yamamoto, K.; Kato, K.; Kim, J.; Horiuchi, S.; Yah, W. O.; Otsuka, H., Takahara, A. Biomacromolecules 2012, 13, 276-281. (34) Kang, D. Y.; Zang, J.; Jones, C. W.; Nair, S. J. Phys. Chem. C 2011, 115, 7676-7685. (35) Bottero, I.; Bonelli, B.; Ashbrook, S. E.; Wright, P. A.. Zhou, W.; Tagliabue, M.; Armandi, M.; Garrone, E. Phys. Chem. Chem. Phys. 2011, 13, 744-750. (36) Zanzottera, C.; Vicente, A.; Celasco, E.; Fernandez, C.; Garrone, E.; Bonelli, B. J. Phys. Chem. C 2012, 116, 7499-7506. (37) Zanzottera, C.; Armandi, M.; Esposito, S.; Garrone, E.; Bonelli B. J. Phys. Chem. C 2012, 116, 20417-20425. (38) Kang, D. Y.; Brunelli, N. A.; Yucelen, G. I.; Venkatasubramanian, A.; Zang, J.; Leisen, J.; Hesketh, P.; Jones, C. W.; Nair, S. Nat. Comm. 2014, 5, 3342. (39) Wada, S. I.; Wada, K. Clay Miner. 1982, 30, 123-128. (40) Bishop, J. L.; Rampe, E. B.; Bish, D. L.; Abidin, Z.; Baker, L. L.; Matsue, N.; Henmi, T. Clays Clay Miner. 2013, 61, 57-74. (41) Mendelovici, E.; Carroz Portillo, D. Clays Clay Miner. 1976, 24, 177-182. (42) Frost, R. L.; Mendelovici, E. J. Colloid Interf. Sci. 2006, 294, 4752. (43) Amara, M. S.; Rouzière, S.; Paineau, E.; Bacia-Verloop, M.; Thill, A.; Launois, P. J. Phys. Chem. C 2014, 118, 9299-9306. (44) Farmer, V. C.; Fraser, A. R.; Tait, J. M. Geochim. Cosmochim. Acta 1979, 43, 1417-1420.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(45) Maillet, P.; Levard, C.; Spalla, O.; Masion, A.; Rose, J.; Thill, A. Phys. Chem. Chem. Phys. 2011, 13, 2682-2689. (46) Bocquet, L.; Charlaix, E. Chem. Soc. Rev. 2010, 39, 1073-1095. (47) Falk, K.; Sedlmeier, F.; Joly, L.; Netz, R. R.; Bocquet, L. Nano Lett. 2010, 10, 4067-4073.


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