Silica Nanotubes with Widely Adjustable Inner Diameter and Ordered

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Silica nanotubes with widely adjustable inner diameter and ordered silicas with ultra-large cylindrical mesopores templated by swollen micelles of mixed Pluronic triblock copolymers George Farid, and Michal Kruk Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 10, 2017

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Chemistry of Materials

Silica nanotubes with widely adjustable inner diameter and ordered silicas with ultra-large cylindrical mesopores templated by swollen micelles of mixed Pluronic triblock copolymers George Farida,b and Michal Kruka,b* a

Department of Chemistry, College of Staten Island, City University of New York, 2800 Victory Boulevard, Staten Island, NY 10314, USA; b Ph.D. Program in Chemistry, the Graduate Center of City University of New York, 365 Fifth Avenue, New York, NY 10016, USA ABSTRACT: Mixtures of commercially available Pluronic poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers combined with appropriate swelling agents are proposed as templates for silica nanotubes and ordered silicas with very large cylindrical mesopores. In particular, swollen micelles of Pluronic F127 (EO106PO70EO106) with long poly(ethylene oxide) (PEO) chains and low poly(propylene oxide) (PPO) content (30 wt.%) mixed with a Pluronic with a higher PPO content (60-70 wt.%) and short PEO chains were found at 11 °C to template 2dimensional hexagonal SBA-15 silicas with ultra-large (100) interplanar spacings up to ~30 nm. Moreover, when the ratio of the silica precursor to the triblock copolymer mixture was appropriately lowered, silica nanotubes were readily obtained. While the addition of the high-PPO-content Pluronic facilitated the uptake of the swelling agent (toluene), allowing one to achieve very large inner tube diameters (~35 nm), smaller diameters down to ~10 nm were systematically generated using lower relative quantities of the swelling agent. As-synthesized (surfactant-containing) nanotubes of very large diameter had a tendency to flatten into ribbons and twist or bend, resulting in materials that had rather featureless pore size distributions after the surfactant removal. However, the degradation of these nanotubes was suppressed by increasing the hydrothermal treatment temperature (from 100 to 110-130 °C). The nanotubes exhibited high surface areas (424-911 m2/g) and large total pore volumes (~2 cm3/g), in addition to the well-defined and widely tunable inner diameter (10-35 nm), even if they tended to cluster upon removal of the surfactant. It is envisioned that swollen mixed PEO-based surfactants will provide new templating opportunities for nanotubes of different compositions and for other nanoporous materials.

INTRODUCTION Over the last twenty five years, nanotubes have captivated the attention of scientific and engineering communities due to their fascinating structures and many useful properties.1-5 Among inorganic nanotubes, silica frameworks have been studied,4, 6-12 which paralleled a rapid development of hollow silica nanospheres13-18 and even more notably, surfactant-micelle-templated ordered mesoporous silicas.19-20 While the hollow silica nanospheres and ordered mesoporous silicas are available on gram scale (or larger) with readily adjustable pore size and wall structure, the syntheses of the silica nanotubes are typically small-scale and based on costly templates. The latter include noble metal nanowires,21 anodic aluminum oxides (AAOs),4, 22-23 and variety of molecules (often custom-made) that selfassemble into soft templating structures.10, 24-26 A gramscale synthesis of silica nanotubes templated by nickelhydrazine complex is an exception,27 while allowing for some degree of adjustment of the average inner tube diameter, but the diameters of individual tubes are appreciably scattered. Otherwise, the inner tube diameter adjustment opportunities are mostly documented for smallscale predictive syntheses (based on AAO templates22 or custom-made block-copolymer fibers26) and empirical ap-

proaches.28-29 While the surfactant-micelle-templating approach has been extremely successful for vast families of ordered mesoporous silicas with predictably adjustable structures and pore sizes,19-20, 30 and has also been highly suitable for silica nanospheres,14, 16-17 it has advanced the silica nanotube development to a much lower extent. This can be attributed to the fact that surfactants suitable as templates for tubular (cylindrical) pore voids usually afford silicas with periodic structures, while individual nanotubes are rarely observed and often have significant structural imperfections or low inner volume.9, 31-35 Recently, we proposed a predictive approach in which hollow nanospheres and nanotubes are templated by block copolymer surfactants with long poly(ethylene oxide), PEO, blocks that serve a dual role of providing the environment for the silica framework formation and protecting from aggregation into consolidated (periodic or disordered) structures.35-36 While this approach allows for the synthesis of uniform silica nanotubes under carefully selected conditions, the reported inner tube size range is quite narrow.35 Moreover, a limited choice of readily available surfactants with long PEO chains and their strong tendency to template spherical pores make further development challenging.

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Herein, it is shown that mixtures of commercially available Pluronic (poly(ethylene oxide)-poly(propylene oxide)poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer surfactants in the presence of a micelle swelling agent are convenient templates for silica nanotubes with widely tunable inner diameters. Earlier research showed that in the absence of a swelling agent, mixtures of surfactants with the same head group and different alkyl chain lengths afford a micelle templating behavior intermediate between that of the individual surfactants, thus allowing for a continuous adjustment of the pore size within limiting values for the two individual surfactants.37 In the case of surfactants with hydrophilic poly(ethylene oxide), PEO, moieties, and either hydrophobic alkyls or poly(propylene oxide), PPO, the mixing of surfactants with different PEO size and virtually the same hydrophobic moieties allowed one to change the ordered structure type, thus indicating a potential for a continuous adjustment of the effective surfactant packing parameter.38 Mixtures of Pluronic F127 (EO106PO70EO106) and P123 (EO20PO70EO20) with different PEO size and essentially the same PPO size38 were also used to enhance the pore volume and increase the pore size of SBA-16 silicas with body-centered cubic (Im3m symmetry) structures of spherical mesopores.39 Mixtures of surfactants of significantly different structures were also explored. For example, the addition of sodium dodecylsulfate, SDS, to Pluronic P123 promoted the formation of cubic Ia3d structure instead of 2-D hexagonal structure, suggesting the formation of Pluronic P123/SDS mixed micellar assemblies.40 Two vastly different surfactants may also form separate micelles that template pores of different sizes and shapes, as in the case of a combination of a block copolymer surfactant with an ionic liquid surfactant.41 The usefulness of swollen micelles of surfactant mixtures in templated syntheses of nanoporous materials has been nearly unexplored. For alkylammonium surfactants used to template disordered silicas of ill-defined pore geometry with large pore volumes and enlarged pore sizes (up to about 10 nm), a partial replacement of a longerchain surfactant with a shorter-chain one led to the narrowing of pore size distribution (PSD) or to a moderate pore diameter increase with broadening of PSD.42 The use of sodium dioctyl sulfosuccinate (AOT) as an additive to Pluronic F127 in the presence of 1,3,5-trimethylbenzene as a swelling agent allowed one to tune the structure type, including body-centered cubic (Im3m), face-centered cubic (Fm3m), gyroidal (cubic Ia3d), and 2-dimensional (2-D) hexagonal symmetry, while pore diameters were moderately large.43 Also, the addition of a small amount of a swelling agent moderately enlarged the pore size and volume in silicas with cubic Ia3d structure templated by Pluronic P123/SDS mixture.44 Herein, it is demonstrated that swollen Pluronic surfactant mixtures template highly ordered 2-D hexagonal materials with uncommonly large unit-cell sizes and pore diameters, and more importantly, provide access to silica nanotubes with inner diameters systematically adjustable in a wide range from ~10 to 35 nm by increasing the quantity of the swelling agent.

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RESULTS AND DISCUSSION The usefulness of swollen micelles of mixed Pluronic surfactants as templates was first explored in the case of ordered mesoporous silicas. A starting point was the synthesis of large-pore SBA-15 silica with 2-dimensional (2-D) hexagonal structure templated by Pluronic F127 (EO106PO70EO106; 70 wt.% PEO and 30 wt.% PPO) swollen by toluene at the initial synthesis temperature of 11 °C, which can be modified to obtain nanotubes, as well as materials with spherical mesopores, whether periodic or individual hollow nanospheres.35 In this case, one could expect that a low content of PPO domains (30 wt.%), which form the micelle core in water, may restrict the uptake of the swelling agent and thus the micelle size of Pluronic F127, thereby limiting the pore size of the final templated material. Therefore, an increase in the PPO content, while preserving the large size of PEO blocks, is expected to be beneficial, but the corresponding block copolymer surfactant is not commercially available. To overcome this limitation, we explored a templating behavior of a tolueneswollen mixture of Pluronic F127 with P104 (EO27PO61EO27) of higher PPO content (60 wt.% PPO; 40 wt.% PEO) at 80 : 20 wt. ratio (see Experimental section and Supporting Table S1 for other conditions). Ultra-largepore SBA-15 (ULP-SBA-15) silica was obtained with a well ordered 2-D hexagonal structure (as seen from SAXS, Figure 1) and appreciably increased (100) interplanar spacing, d100 (from 22.1 to 27.6 nm for as-synthesized and from 20.1 to 25.2 nm for calcined samples). It should be noted that d100 multiplied by 1.155 provides the unit-cell parameter for 2-D hexagonal structure, that is, the distance between the pore centers. A further increase in the proportion of Pluronic P104 in the surfactant mixture (F127 : P104 wt. ratio of 70 : 30 and 60 : 40) yielded d100 values of 33 nm for as-synthesized samples. The latter value is higher than the largest interplanar spacing values hitherto reported for SBA-15 silicas.45-46 The samples exhibited nitrogen adsorption isotherms with steep capillary condensation steps at high relative pressures and quite narrow pore size distributions (see Supporting Figure S1). The broad adsorption-desorption hysteresis loops of the isotherms indicated that the mesopores were accessible through openings appreciably narrower than the pore diameter, which is likely to be associated with the formation of a continuous silica envelope in the PEO corona of the cylindrical micelles, leading to caps at the ends of the cylindrical mesopores.47 However, a sample prepared with a hydrothermal treatment at 130 °C (instead of 100 °C) exhibited an isotherm with a narrow adsorption-desorption hysteresis loop, indicating virtually unrestricted access to its cylindrical mesopores (see Supporting Figure S2; the pore diameter of 25.8 nm was estimated using Eq. 1 in Supporting Information section). This sample exhibited an exceptionally well-resolved SAXS pattern with d100 of 26.0 nm after calcination (and 27.0 nm before calcination). TEM images of the materials indicated a high degree of structural ordering and large ordered domain size (Supporting Figure S3). Notably, a contamination with individual hollow nanospheres (or clusters of them), which was quite common on TEM images for highly ordered SBA-15 templated by F127/toluene pair,35 was largely diminished when Pluronic F127 was mixed with P104. Clearly, the


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addition of a surfactant with a higher fraction of the hydrophobic PPO component suppresses the formation of spherical-micelle-templated nanospheres, presumably through either the decrease in hydrophilic to hydrophobic domain volume ratio,38 or the decrease of crowding of PEO chains at the boundary between PEO and PPO domains. Both would promote a lower surface curvature, potentially making the formation of cylindrical pores more favorable. It is unusual to observe the unit-cell size increase with concomitant increase in the phase purity for micelletemplated ultra-large-pore materials, as it is shown here. 100 100 110 100

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An unexpected difference between the swollen micelles of single surfactants and those of mixed surfactants is that the latter produced unit-cell sizes that were quite independent of temperature in a sub-ambient range in which the former would exhibit a strong dependence.45, 48 While the strong dependence opens an avenue to the unit-cell size adjustment, it may limit reproducibility, especially if the temperature control is not sufficiently accurate. In the case of Pluronic F127 and P123 mixed at 80 : 20 wt. ratio and swollen by toluene, ultra-large-pore SBA-15 silicas formed in the temperature range from 11 to 21 °C with little variation in the unit-cell size (based on SAXS and TEM; see Supporting Figures S4 and S5) and ordering (based on TEM; although as temperature exceeded 19 °C, the content of an impurity phase significantly increased). While the unit-cell size was largely invariant (d100 on the order of 32 nm for as-synthesized samples, based on the position of the main SAXS peak), the pore diameter tended to decrease as the temperature increased, but was still large even for the sample synthesized at the initial temperature of 19 °C (Supporting Figure S6). Encouraged by the above results for ultra-large-pore SBA-15 with mesopores templated by cylindrical micelles, we attempted the synthesis of micelle-templated nanotubes by reducing the silica precursor (TEOS)/surfactant ratio, which had been a successful strategy in the case of Pluronic F127 combined with toluene.35 In cases of Pluronic F127/P123 mixture at 80 : 20 wt. ratio or Pluronic

F127/P104 mixture at 70 : 30 wt. ratio at 11 °C and the stirring rate of 150-200 rpm, the use of around 55 % of TEOS (in comparison to the amount used in the aforementioned SBA-15 synthesis) afforded surfactant-micelletemplated nanotubes based on TEM imaging of the synthesis mixture content. Less well defined nanotubes were also seen for Pluronic F127/P123 mixture and a lower amount of TEOS (50 %) and the stirring rate of 200 rpm. The inner diameter of the surfactant-containing nanotubes was on the order of 30 nm, which is very large as for micelletemplated mesopores. However, the nanotubes were found to be highly unstable. After the removal of the surfactants by calcination or solvent extraction, the nanotubes aggregated and distorted to such an extent that they were hardly discernible by TEM (except for small broken fragments in some cases). Moreover, their once uniform interiors were apparently so damaged or distorted that no uniform pores were seen on pore size distributions calculated from nitrogen adsorption isotherms. A closer inspection of TEM images for the nanotubes taken directly from the synthesis mixture revealed that some of these tubes were flattened to form ribbon-like structures that were often twisted (see Supporting Figure S7), thus resembling a behavior reported for some multi-wall carbon nanotubes.49 The bending of AAO-templated silica nanotubes at sharp angles without breaking was reported by others,22 but the flattening and twisting reported here suggest even greater flexibility, which may be due to the presence of PEO chains of the surfactant in the silica walls prior to the surfactant removal. In cases where the flattening was prevalent on TEM images of as-synthesized tubes, the surfactant removal typically rendered a clustered material in which the original tube shape was difficult to discern. To overcome the tube flattening issue and other degradation modes, we decreased the relative proportion of toluene used (in the synthesis involving 2.8 mL TEOS) from 3 mL (per 1 g of the mixture of Pluronic F127 and P104 at 70 : 30 wt. ratio) to 1.5, 1, 0.75, 0.5 and 0.25 mL, hoping that less swollen nanotubes would be more mechanically robust. To obtain individual nanotubes for low swelling agent volumes, the quantity of silica precursor, TEOS, was further decreased from 2.8 mL to 2.5, 2.3 and 1.8 mL for 0.75, 0.5 and 0.25 mL toluene, respectively (see Supporting Table S1). Moreover, in the case of the lowest volume of toluene (0.25 mL), the initial synthesis temperature was increased from 11 to 15 °C, because higher temperatures may promote the formation of single-micelletemplated hollow silica nanoparticles.17 We thus generated a series of surfactant-templated nanotubes with inner diameter tunable in a wide range (see TEM in Figure 2) by adjusting the relative quantity of the swelling agent. Notably, the nanotubes of different sizes were essentially free of branching, as the inspection of hundreds of TEM images (typically showing tens of tubes each) revealed only several apparent branching points. On the other hand, the branching was clearly apparent in the prior work on silica nanotubes templated by block-copolymer nanowires at the lower end of attainable inner tube diameters (13-17 nm).26 While the above procedure rendered well-defined assynthesized (surfactant-containing) silica nanotubes with a wide range of diameters, larger ones were increasingly



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prone to collapse in ways described above. The walls of large-diameter nanotubes were thin relative to the tube diameter, which appeared to make the nanotubes susceptible to distortions, and flattening in particular.

Figure 2. TEM image of as-synthesized silica nanotubes prepared with Pluronic F127/P104 mixture at 70 : 30 weight ratio and with different volumes (mL) of the swelling agent: (a) 0.25, (b) 0.5, (c) 0.75, (d) 1, (e) 1.5 and (f) 3.

We realized that the nanotube walls might have been too flexible. Therefore, the hydrothermal treatment temperature was increased above 100 °C to enhance the framework condensation, and indeed the treatment at 130 °C (or even 110-120 °C) greatly suppressed the tendency to flatten and allowed us to remove the surfactants by extraction or calcination at 300 °C while preserving a clear peak on the pore size distribution (see below). Even though the tube morphology was preserved very well during the surfactant removal when the hydrothermal treatment at temperatures above 100 °C was used (see Supporting Figure S8), the nanotubes still had a tendency to cluster into disorganized aggregates and/or break into shorter pieces, which has been documented for other silica nanotube syntheses.24, 35 To better understand the origin of the improved stability of the nanotubes prepared at higher hydrothermal treatment temperatures, solid-state 29Si MAS NMR measurements (see Supporting Figure S9) were carried out on freshly prepared as-synthesized large nanotubes synthesized with 3 mL toluene at different hydro-

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thermal treatment temperatures. The as-synthesized nanotubes hydrothermally treated for 1 d at 100 °C exhibited 53% of fully condensed Q4 silicon atoms (Si(OSi)4 sites) (peak at -111 ppm), 41% of Q3 silicons (Si(OSi)3(OH)sites) (peak at -102 ppm) and 6% of Q2 silicons (Si(OSi)2(OH)2 sites) (peak at ~-91 ppm), which corresponds to the formula SiO1.73(OH)0.54. The hydrothermal treatment for 1 d at 120 °C rendered 70% of fully condensed Q4 silicon atoms and 30% of Q3 silicons, while the intensity around -90 ppm was too low to meaningfully assign content of Q2 silicons. These data correspond to the formula SiO1.85(OH)0.30. Finally , the hydrothermal treatment for 1 d at 130 °C rendered 78% of fully condensed Q4 silicon atoms and 22% of Q3 silicons, while the intensity around -90 ppm was too low to meaningfully assign content of Q2 silicons. These data correspond to the formula SiO1.89(OH)0.22. Clearly, the degree of framework condensation (cross-linking) significantly increased as the hydrothermal treatment temperature increased, which provides a plausible explanation of the decreased tendency of the nanotubes to distort and flatten as the hydrothermal treatment temperature increases beyond 100 °C. It is notable that the tubes prepared at 100 °C, and thus flexible and prone to the distortions, already have an appreciable degree of framework condensation, so the nanotube wall is flexible even if the degree of its condensation is quite large. For these syntheses with hydrothermal treatments at 100 and 130 °C, the the yield was 0.74 g and 0.68 g silica, respectively (from 2.8 mL TEOS), which combined with compositions from NMR allowed us to estimate the percent yield as 91% and 87%, respectively. Shown in Figure 3 are nitrogen adsorption isotherms and pore size distributions (PSDs) of the resulting calcined nanotubes, while their TEM images are shown in Supporting Figure S8. Smaller tubes prepared with up to 1 mL of toluene exhibited nitrogen adsorption isotherms with capillary condensation steps whose midpoints shifted from a relative pressure of 0.78 to 0.92 as the volume of the micelle swelling agent increased, providing evidence of the inner tube diameter increase (see PSDs in Figure 3). These steps were followed by additional capillary condensation steps close to the saturation vapor pressure, which can be attributed to the capillary condensation between the aggregated nanotubes. In the case of the nanotubes prepared with the highest toluene volumes and the hydrothermal treatment at 130 °C (the latter being expected to be the most effective in preventing the tube shrinkage upon calcination), only one capillary condensation step was seen close to the saturation vapor pressure. Apparently, the voids between the tubes were too large to be filled by capillary condensation at the highest relative pressures reached in our nitrogen adsorption measurements (~0.989) and thus of the size on the order of 100 nm (or more). Alternatively, the pressures for capillary condensation between the tubes might have coincided with the capillary condensation inside the nanotubes, but this is less likely. The samples exhibited high BET specific surface areas of 730-911 m2/g (see Supporting Table S2), except for those hydrothermally treated at 130 °C, which had somewhat lower surface areas of 424-430 m2/g, due in part to their apparently negligible micropore volumes (00.01 cm3/g) in comparison to micropore volumes of other


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samples (0.04-0.06 cm3/g). The total pore volumes, which include volumes of inner tube voids, (micro)pores in the tube walls and inter-tube pores of diameter below ~100 nm, were 1.74-2.08 cm3/g, which are quite high. 2800 -1

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Figure 3. (right) Nitrogen adsorption isotherms and (left) pore size distributions of calcined silica nanotubes prepared with Pluronic F127/P104 mixture at 70 : 30 weight ratio and different volumes of toluene. Isotherms for samples prepared with 1.5, 1, 0.75, 0.5 and 0.25 mL toluene were offset vertically by 400, 800, 1200, 1600 and 1800 cm3 STP g-1.

The tubes prepared with 0.25 mL toluene per 1 g of Pluronic mixture had an inner diameter ~10 nm (see Figure 3). The use of 0.5 and 0.75 mL toluene rendered tubes of BJH-KJS nominal inner diameter 15 and 20 nm, the latter being analogous to the diameter of silica nanotubes templated by single Pluronic F127 surfactant.35 In the latter case, four times higher volume of the swelling agent was used, which demonstrates that the mixing of Pluronic F127 with a Pluronic with a higher PPO content significantly enhances the swelling agent uptake. When 1, 1.5 and 3 mL of toluene were used, BJH-KJS nominal pore diameters were 28, 43 and 60 nm. The BJH-KJS method/calibration used herein to calculate pore size distributions is known to overestimate the size of cylindrical mesopores of diameter above ~7 nm. The comparison of the BJH-KJS pore sizes with more reliable estimates for ordered silicas with cylindrical mesopores45 rendered pore diameters of 13, 17 and 21 nm for samples prepared with 0.5, 0.75 and 1 mL toluene. The BJH-KJS pore diameter of 43 nm for the sample prepared with 1.5 mL toluene is likely to correspond to the actual pore diameter of ~32 nm.46 While we do not have ordered mesoporous silica data corresponding to BJH-KJS pore diameter of 60 nm, TEM suggests the inner tube diameter of ~35 nm. We also evaluated the hydrothermal stability of calcined nanotubes in water at 100 °C, which is a common hydrothermal stability test. We first studied nanotubes hydrothermally treated at 130 °C and calcined at 300 °C. N2 adsorption (Supporting Figure S10) indicated the decrease in the adsorption capacity and widening of the hysteresis loop, both indicative of some pore structure change after water treatment for 2 h. The heating for 4 h led to a major adsorption capacity loss and significant broadening of the hysteresis loop (perhaps due to strong aggregation or local collapse of the tubes). Notably, some well-defined tubes were seen in TEM after both 2 hours (see Supporting Figure 10) and 4 hours. Similar conditions were found to

greatly degrade some MCM-41 silicas.50 To improve the stability, we calcined the nanotubes at 550 °C, which decreases the adsorption capacity, but does not lead to degradation of the tubes (Supporting Figure S11). The resulting material was largely unchanged after 2 hours of heating in water (Supporting Figure S12), but some broadening of the hysteresis loop was seen after 4 hours, while intact tubes were still seen in TEM. We also evaluated nanotubes hydrothermally treated at 120 °C and calcined at 550 °C and found that the porous structure was largely unchanged after both 2 and 4 hours of water treatment (Supporting Figure S13). TEM confirmed the retention of the nanotube structure. Clearly, the stability of our nanotubes calcined at 550 °C is comparable to that of many MCM-41 samples, while it is lower than that of SBA-15.20 The silica nanotubes discussed above were prepared at an initial synthesis temperature of 11 °C (except for the synthesis with 0.25 mL toluene at 15 °C). In the case of one of our synthesis mixture compositions (Pluronic F127/P123 at 80/20 wt. ratio), the increase in the initial synthesis temperature to 18 °C (Supporting Figure S14), or even to 21 °C, did not compromise the tube morphology, although the content of hollow nanosphere impurity increased. As we showed earlier, the nanospheres appear to form via fragmentation of nanotubes at their ends.35 It is hypothesized herein that several factors contribute to the exceptional performance of the considered Pluronic surfactant mixtures as micellar templates for silica nanotubes with widely tunable inner diameters. First, the mixtures consist of Pluronic F127 with long poly(ethylene oxide), PEO, blocks that facilitate the formation of quite thick silica walls around the PEO corona and still provide some dangling PEO ends to hinder aggregation of nanotubes into consolidated structures.35 Second, the addition of a Pluronic with a higher fraction of PPO to the one with a lower PPO fraction is known to moderately increase the micelle size,51 which is consistent with pore sizes observed for templated materials with spherical mesopores.39 Third, the addition of a Pluronic with a higher fraction of PPO to the one with a lower PPO fraction appears to significantly enhance the uptake of the swelling agent by the micelles (as noted above), thus allowing for the formation of micelle-templated nanotubes of unusually large diameters when a sufficient quantity of the swelling agent is added. This enhancement is consistent with the finding that the addition of a block copolymer with a high PPO content (Pluronic L121 with 90 wt.% PPO) to Pluronic F127 significantly increased the uptake of a hydrophobic compound in drug delivery feasibility studies.52 Finally, the mixing of Pluronic surfactants, one with long PEO chains and the other with quite short PEO chains, may help reduce crowding of PEO chains at the interface between PEO and (swollen) PPO domains, and thus to help achieve a lowercurvature cylindrical micelle morphology as opposed to a higher-curvature spherical one, which is typical38 for micelles of surfactants with large size of PEO relative to PPO. Perhaps also long PEO chains accompanied by much shorter PEO chains are prone to stretching more towards the periphery of the micelles (based on studies of polymer brushes53), which may contribute to the stabilization of the single-micelle-templated nanoparticles against aggrega-



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tion. Thus, the use of mixed PEO-based surfactants with a vast difference in PEO block sizes allows one to stabilize silica/micelle hybrids against aggregation (due to long PEO chains in the micelle corona) while achieving favorable volume ratio of the PEO corona to the swollen PPO domain for the formation of the cylindrical micelles (which is otherwise difficult to achieve for commercially available surfactants with long PEO blocks). In general terms, our results suggest that in complex soft-templating systems, the increase in the compositional complexity may enhance the structural homogeneity while vastly improving the robustness of the synthesis in terms of the range of applicable conditions and structural tunability.

Conclusions Swollen micelles of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer surfactant mixtures are remarkably robust in templating silica nanotubes with widely tunable inner diameters. The use of a surfactant with long PEO chains provides the stabilization of single-micelle-templated nanotubes against aggregation, as in single-surfactant templating systems, while an addition of a surfactant with shorter PEO blocks and a significant content of PPO promotes a lower micelle surface curvature and enhances the swelling agent uptake, both potentially contributing to the cylindrical shape. The resulting multi-component templating system affords products that approach or exceed homogeneity of single-surfactant templating systems, while offering tube diameter tunability (10-35 nm) through the adjustment of swelling-agent/surfactant ratio, which is unparalleled for surfactant-templated nanotubes and rarely observed for any surfactant-templated materials. Swollen mixed Pluronic surfactant micelles are also superior templates for ordered mesoporous silicas with large cylindrical mesopores, and thus are likely to provide new opportunities in the synthesis of ordered mesoporous materials, single-micelle-templated nanotubes and perhaps other nanoparticle morphologies of a variety of compositions. An exceptional flexibility of some single-micelle-templated silica nanotubes may lead to their significant degradation during the isolation and/or surfactant removal, but the degradation can be prevented by using a high-temperature hydrothermal treatment in the synthesis.

ASSOCIATED CONTENT Supporting Information. Tables with conditions and data; TEM images; SAXS and gas adsorption data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions The manuscript was written through contributions of both authors.

ACKNOWLEDGMENTS NSF (award DMR-1310260) is gratefully acknowledged for support. BASF is acknowledged for providing Pluronics. CSI

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imaging facility is acknowledged for access to TEM. Dr. Jianqin Zhuang (CSI) is gratefully acknowledged for help with NMR.

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Table of Contents artwork

silica nanotubes templated by swollen mixed micelles


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Silica Nanotubes with Widely Adjustable Inner Diameter and Ordered

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