Article pubs.acs.org/Langmuir
Synthesis of Xylylene-Bridged Periodic Mesoporous Organosilicas and Related Hollow Spherical Nanoparticles Amanpreet S. Manchanda and Michal Kruk* Department of Chemistry, College of Staten Island, City University of New York, 2800 Victory Boulevard, Staten Island New York 10314, United States Ph.D. Program in Chemistry, The Graduate Center of City University of New York, 365 Fifth Avenue, New York, New York 10016, United States S Supporting Information *
ABSTRACT: A variety of organosilicas with p-xylylene bridging groups in the framework were synthesized using Pluronic F127 triblock copolymer as a micellar template under moderately acidic conditions in the presence of xylene as a micelle swelling agent. The resulting materials were characterized by using nitrogen adsorption, small-angle X-ray scattering, transmission electron microscopy, and 29Si and 13C cross-polarization magic angle spinning NMR. As the ratio of the organosilica precursor to Pluronic F127 was decreased, the structure evolved from highly ordered periodic mesoporous organosilica (PMO) to weakly ordered PMO, loosely aggregated hollow organosilica nanospheres, and finally to a significantly aggregated disordered structure. The highly ordered PMO with primarily face-centered cubic structure was effectively a closed-pore material. However, the weakly ordered variant exhibited large-diameter (∼15 nm) spherical mesopores, which were accessible after calcination under appropriate conditions or after extraction. The hollow nanospheres had readily accessible, uniform inner cavities whose size was readily tunable by adjusting the amount of the swelling agent used. It was also possible to convert the organosilica nanospheres into hollow silica nanospheres with inaccessible (closed) mesopores. The formation of distinct well-defined morphologies with spherical mesopores for an organosilica with large bridging groups in the framework shows that block−copolymer−surfactant templating is a powerful and versatile method for controlling the nanoscale structures of these remarkable materials.
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INTRODUCTION Ordered mesoporous silicas (OMSs)1,2 have been researched with keen interest because of their unique properties. The major advancement in this field was the development of periodic mesoporous organosilicas (PMOs), which have organic groups present as bridging groups between silicon atoms in the mesopore walls, thus replacing some oxygen-based siloxane (Si−O−Si) linkages.3−5 The presence of the organic groups in the framework gives rise to useful physical and chemical properties.6,7 Another major advancement was the use of large block copolymer surfactants, primarily Pluronics (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO triblock copolymers) as templates for the synthesis of OMSs.8−10 The use of such long-chain surfactants enabled the synthesis of OMSs8−10 and PMOs11−13 with mesopores of diameter above 7 nm. PMOs have been evaluated for many diverse applications, including catalysis,14−18 adsorption,19 coatings,20 light harvesting,17,21 guest molecule immobilization,16,22,23 and drug delivery.24,25 Block-copolymer-templated PMOs with 2D hexagonal structures of cylindrical mesopores and with short aliphatic or unsaturated bridges, such as methylene, ethylene, and ethenylene, as well as aryl bridging groups, such as phenylene, © XXXX American Chemical Society
thiophene, and 1,4-diethylbenzene, have been extensively studied.12,23,26−31 However, the successful block-copolymertemplated synthesis of PMOs with spherical mesopores, especially of large size, has been limited. Phenylene and thiophene appear to be the only aromatic bridges that have been successfully incorporated into standard PMO syntheses;32,33 moreover, the thiophene-bridged PMOs had a small mesopore size and volume.32 A confined environment of anodic alumina pores allowed one to obtain biphenylene-bridged PMO with body-centered cubic structure,34 but this approach could practically afford only small quantities of this interesting material. Although PMOs with spherical mesopores can be readily obtained with ethylene bridging groups,35−37 the reported syntheses involve only bis(trimethoxysilyl)ethane rather than a more benign and cheaper bis(triethoxysilyl)ethane framework precursor. PMOs with spherical mesopores and unsaturated ethenylene groups in the framework have been reported only recently,33,38,39 and well-ordered methylenebridged PMO is yet to be developed, even though a weakly ordered PMO of this kind has been reported.40 Received: November 21, 2015
A
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Langmuir Low temperature (0−7 °C) and moderate acid concentration (0.1 M HCl) were identified as particularly suitable for the block-copolymer-templated synthesis of PMOs with large spherical mesopores.33,41 However, these conditions have not been found to be suitable33 for the generation of single-micelletemplated hollow nanospheres,42−46 which can often be obtained simply through the reduction of the organosilicaprecursor/surfactant ratio under more strongly acidic conditions (2 M HCl).45 Also, there was no reported success in obtaining closed-pore PMOs with intact organic groups in the framework, although the mesopore closing involving the combustion of organic groups in the framework has been reported.37,40 As discussed above, only a small number of bridging groups, typically of relatively small size, have been successfully incorporated into Pluronic-templated PMOs with spherical mesopores. Herein, the incorporation of p-xylylene (−CH2− C6H4−CH2−) bridging group is demonstrated under moderately acidic conditions (0.1 M HCl), thus showing a possible way to overcome the current limitations. This interesting bridging group, which combines aromatic and aliphatic moieties and thus is less rigid than exclusively aromatic bridges, has been nearly unexplored in PMO synthesis. It appears that the only pertinent report was published by Ozin et al., who demonstrated the synthesis of p-xylylene-bridged organosilicas with the 2D hexagonal structure of cylindrical mesopores of diameter about 2.4 nm with Brij 56 oligomeric surfactant as a template.47 Moreover, the p-xylylene bridging group has not been incorporated into single-micelle-templated nanoparticles. Although the incorporation of a somewhat larger pdiethylbenzene bridge was attempted,42 the resulting nanospheres had a low pore volume and their TEM images suggested a mixed hollow-sphere/filled-sphere morphology. In the present study, Pluronic F127 triblock copolymer surfactant and p-bis(trimethoxysilylmethyl)benzene (pBTMSMB) organosilica precursor (Supporting Information Scheme S1) were used to generate p-xylylene-bridged organosilicas. PMO with a face-centered cubic structure of closed spherical mesopores was obtained, along with weakly ordered PMO with spherical mesopores as well as aggregated singlemicelle-templated hollow nanospheres and a largely disordered consolidated structure (Supporting Information Scheme S2). The transition between these structures was achieved by changing the pBTMSMB/Pluronic F127 ratio in the synthesis mixture. Because of the presence of large hydrophobic bridging groups in the framework, the resulting open-pore materials are expected to have interesting adsorption properties for hydrophobic molecules and may be surface-modified (via silanol chemistry or substitution on the benzene ring) to obtain catalytically useful materials.
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Alternatively, the sample (0.2 g) was extracted using 5 mL of 2 M HCl in 60 mL of ethanol at 60 °C for 6 h to remove the surfactant. Characterization. Small-angle X-ray scattering (SAXS) patterns were acquired on a Bruker Nanostar U SAXS/wide-angle X-ray scattering instrument with a Vantec-2000 2D detector and a rotating anode X-ray source. Samples were placed in the hole of a sample holder and secured from both sides with Kapton tape. Wide-angle XRD was recorded on a Rigaku MiniFlex II instrument with Cu Kα radiation. Transmission electron microscope (TEM) images were acquired on a FEI Technai Spirit TEM operated at 120 kV. Before being imaged, samples were dispersed in ethanol under sonication and deposited on a carbon-coated copper grid. Nitrogen adsorption measurements at −196 °C were performed on a Micromeritics ASAP 2020 volumetric adsorption analyzer. Before the measurements, samples were outgassed at 140 °C under vacuum in a port of the analyzer. 29Si and 13C cross-polarization (CP) magic-angle spinning (MAS) NMR experiments were carried out on a Varian INOVA 300 wide-bore spectrometer equipped with a superconducting magnet with a field of 7.1 T. The operating frequencies for 29Si measurements were 59.6 and 75.4 MHz for 13C measurements. The samples were packed into a 5 mm zirconia rotor, loaded into a 5 mm Doty XC-5 CP/MAS probe, and spun at 6−8 kHz. A total of 300−1100 scans were acquired depending on the sensitivity of a given sample. The recycle delay was 3 s, and the contact times were 3 ms for 13C and 2.3 ms for 29Si NMR measurements. The 90° pulse was 4 ms. The chemical shift reference was 4,4-dimethyl-4-silapentane-1-sulfonic acid (DSS). FTIR spectra were recorded on a Bruker Vertex 70 FTIR spectrophotometer operating at 4 cm−1 resolution. Calculations. The BET specific surface area (SBET) was determined from the nitrogen adsorption isotherm data at relative pressures from 0.04 to 0.20. The total pore volume (Vt) was calculated from the amount adsorbed at a relative pressure of 0.99. The micropore volume (Vmic) was determined using the αs plot method in the standard reduced adsorption, αs, range from 0.9 to 1.2 using LiChrospher Si1000 as a reference adsorbent.48 The pore size distribution was calculated from the adsorption branch of the nitrogen adsorption isotherm using the Barrett−Joyner−Halenda (BJH) algorithm with KJS correction for cylindrical mesopores49 and the statistical film thickness curve for a macroporous silica gel LiChrospher Si-1000.48 This pore size calculation method is known to underestimate the diameter of spherical mesopores of the considered size by about 2 nm.50
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RESULTS AND DISCUSSION As seen in Figure 1, p-xylylene-bridged organosilicas synthesized with two largest volumes of p-bis(trimethoxysilylmethyl)-
EXPERIMENTAL SECTION
Synthesis. Pluronic F127 (EO106PO70EO106, BASF, 0.5 g) was dissolved in 30 mL of 0.1 M HCl at 7 °C under stirring for 1 h. Then 1 g of the xylene isomer mixture (Aldrich) and 2.5 g of KCl were added. After 2 h of stirring, we added a selected volume (0.5, 0.9, 1.3, or 2.3 mL) of p-bis(trimethoxysilylmethyl)benzene (pBTMSMB, acquired from Gelest). The reaction mixture was stirred at 7 °C for 1 day, and then it was transferred to a polypropylene bottle or a Teflon-lined autoclave and heated for 1 day at 100 or 130 °C, respectively. The assynthesized material was filtered, washed with deionized water, and dried at ∼60 °C in a vacuum oven. The resulting sample was calcined at 350 °C under argon for 5 h (heating ramp 2 °C/min). If so noted, calcination was carried out under air at 250 °C for 5 h in some cases.
Figure 1. SAXS patterns for calcined organosilicas prepared with different volumes of pBTMSMB (per 0.5 g of F127). The samples were hydrothermally treated at 100 °C. B
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Table 1. Structural Properties of PMOs Synthesized with p-Bis(trimethoxysilylmethyl)benzene under Different Conditionsa amount of pBTMSMB (mL)
hydrothermal treatment temperature (°C)
2.30 1.30
100 100 130 100 130 100
0.90 0.50
d (nm) 18.3 18.2 19.4 14.9 14.4 14b
(20.3) (20.8) (20.5) (16.6) (15.9)
SBET (m2/g)
Vt (cm3/g)
Vmic (cm3/g)
wKJS (nm)
N/A 431 308 307 285 179
N/A 0.42 0.34 0.47 0.55 0.19
N/A 0.12 0.07 0.04 0.03 0.02
N/A 15.4 15.4 10.8 11.9 8.7
a
Notation: d, interplanar spacing for calcined sample (interplanar spacing for as-synthesized sample provided in parentheses); for samples prepared with 1.3 and 2.3 mL of the precursor, d is (111) interplanar spacing that provides the unit-cell parameter after multiplication by 31/2; SBET, BET specific surface area; Vt, total pore volume; Vmic, micropore volume; wKJS, pore diameter calculated using the BJH-KJS method. N/A, not available with acceptable accuracy because of the low porosity of the sample. bNot determined because of the absence of a peak on the SAXS pattern for the as-synthesized sample.
amorphous periodic mesoporous organosilica domains cannot be precluded for our samples with higher pBTMSMB/ surfactant ratios. TEM images for the sample prepared with the highest volume (2.3 mL) of pBTMSMB contained many dark regions, most likely related to large particle sizes and/or extensive particle aggregation, so most of the structure was not amenable to TEM characterization. However, edges of the dark regions and smaller particles revealed highly ordered structures with spherical mesopores (Figure 2(a), left) in addition to
benzene (pBTMSMB) (1.3 and 2.3 mL) exhibited SAXS patterns characteristic of a face-centered cubic structure (Fm3m symmetry), even though the pattern of the sample prepared with 1.3 mL of pBTMSMB was much less resolved. The unitcell parameters evaluated on the basis of (111) interplanar spacings (Table 1) were 31.5 and 31.7 nm for calcined samples prepared with 1.30 and 2.30 mL of pBTMSMB, respectively. It should be noted that these samples were hydrothermally treated at 100 °C. The unit-cell parameters of as-synthesized samples (Table 1 and SAXS patterns in Supporting Information Figure S1) were as large as 36.0 and 35.2 nm, respectively. These unit-cell dimensions are large as for PMOs with spherical mesopores.33,36,37,41 Only ethylene-bridged and ethenylenebridged PMOs prepared under certain conditions were reported to have larger unit-cell sizes.36,37,41,51 When the volume of the organosilica precursor in the synthesis mixture was decreased to 0.5−0.9 mL, SAXS peaks were much broader and appeared at higher angles, suggesting the absence of longrange mesoscale ordering and a decrease in the repeating distances in the materials. SAXS patterns for selected samples hydrothermally treated at 130 °C were similar to those for the samples treated at 100 °C (Supporting Information Figures S2 and S3 and Table 1). Wide-angle X-ray diffraction (XRD) patterns (illustrative pattern in Supporting Information Figure S4) featured broad peaks whose positions corresponded to interplanar spacings of ∼1.1 and ∼0.44 nm. As expected on the basis of the size of the bridging groups, the repeat distance of ∼1.1 nm observed for p-xylylene-bridged PMOs is between repeat distances observed for phenylene-bridged PMOs and biphenylene-bridged PMOs, which are 0.76 and 1.16 nm, respectively.52 The XRD peaks were strong for samples prepared with the two highest relative amounts of the precursor, but they were weak for the other two compositions. It is known that the presence of poly(ethylene oxide) chains of the PEO-PPO-PEO surfactant in the organosilica framework37 hinders the molecular-level crystallinity that could otherwise develop on the basis of the periodic arrangement of aromatic bridging groups.53 Therefore, it is not surprising that the framework crystallinity was moderate for the current PMOs and that, in general, the molecular-level ordering has been rarely reported for PMOs prepared using Pluronic block copolymer surfactants with long poly(ethylene oxide) (PEO) chains.32 The disruption of the crystallinity by PEO blocks is likely to be less significant for higher ratios of the framework precursor to the surfactant, thus providing a possible explanation of our observation mentioned above. However, the presence of bulk organosilica domains with atomic-scale ordering in addition to less atomic-scale-ordered or even
Figure 2. TEM images of calcined organosilica samples prepared with different volumes of pBTMSMB: (a) 2.30, (b) 1.30, (c) 0.90, and (d) 0.50 mL. The samples were hydrothermally treated at 100 °C.
disordered structures of spherical mesopores (Figure 2(a), right) and ordered structures of channel-like mesopores. Only the ordered structures of spherical mesopores are consistent with the well-resolved SAXS pattern for the sample, and thus the disordered and channel-like domains are expected to be a minor component of the sample. TEM images for the sample synthesized with a somewhat lower volume of pBTMSMB (1.3 mL) also showed large particles and/or aggregates of particles with their inner parts apparently too thick to be effectively imaged, but the edges were sufficiently transparent to the electron beam to reveal short-range-ordered as well as disordered structures with spherical mesopores (Figure 2(b)). C
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Langmuir As discussed above, the SAXS pattern suggested that mesopores were arranged in the face-centered cubic structure, and TEM images indicated that the size of ordered domains was small, which perhaps contributed to a less well resolved SAXS pattern in comparison to the sample prepared with 2.3 mL of pBTMSMB. TEM images for a sample synthesized with a smaller amount of pBTMSMB (0.90 mL) suggested the presence of hollow spherical nanoparticles that formed loose aggregates (Figure 2(c)). On the basis of their size, the nanospheres can be regarded as single-micelle-templated. It should be noted that in the case of phenylene-bridged organosilicas prepared at 7 °C in 0.1 M HCl solution, single-micelle-templated nanoparticles were not observed as the framework-precursor/surfactant ratio was decreased.33 Instead, quite small particles formed with multiple mesopores templated by multiple micelles. It is not fully clear why the current organosilica precursor interacted with the surfactant to form single-micelle-templated nanoparticles. In any case, the organosilicas discussed above are the first examples of block-copolymer-templated xylylene-bridged PMOs with an ordered Fm3m structure and of single-micelletemplated nanoparticles of this framework composition. It was found that the size of the hollow nanospheres can be adjusted over a wide range simply by changing the relative amount of the swelling agent while maintaining the pBTMSMB/Pluronic F127 ratio at 0.90 mL of the framework precursor per 0.5 g of the surfactant. TEM images for the considered products (Figure 3) showed the aggregated hollow sphere morphology. The images indicated an improvement in the uniformity of the product as the quantity of the swelling agent was increased from 0.5 to 1 g. Moreover, the samples prepared with higher quantities of the swelling agent (1.5 g or higher) were composed of large (outer diameter ∼30 nm) and very uniform hollow nanospheres. It is clear that the use of a large relative amount of the swelling agent did not lead to the loss of structural uniformity, which is remarkable. SAXS patterns of the considered samples (Supporting Information Figure S5) featured multiple broad peaks or shoulders whose position shifted to lower angles as the quantity of the swelling agent in the synthesis mixture increased (Supporting Information Figure S5 and Supporting Information Table S2), which is consistent with the increase in the outer diameter of the hollow nanospheres, even if it is difficult to extract more quantitative information from these patterns without their modeling.54 TEM images of a sample prepared with the lowest ratio of the framework precursor to the surfactant (0.5 mL pBTMSMB) were found to be much more aggregated than its counterpart discussed above (Figure 2(d)) and apparently weakly ordered in some small domains. This finding suggests that there might be a window of relative proportions of the framework precursor to the surfactant in which single-micelletemplated nanoparticles are formed and do not consolidate in compact aggregates. Figure 4(a) and Supporting Information Figures S5 and S6 show nitrogen adsorption isotherms for calcined and extracted xylylene-bridged PMOs (for the latter, the SAXS pattern is shown in Supporting Information Figure S7). Even though the SAXS patterns for samples synthesized with 1.30 and 2.30 mL of pBTMSMB were quite similar, the materials calcined under argon at 350 °C had very different adsorption properties. The sample synthesized with 1.30 mL of pBTMSMB exhibited a capillary condensation step at a relative pressure of ∼0.86 and a
Figure 3. TEM images of calcined organosilica samples prepared with 0.93 mL of pBTMSMB and different quantities of xylene: (a) 0.5, (b) 1, (c) 1.25, (d) 1.5, (e) 2, (f) 4, and (g) 6 g. The samples were hydrothermally treated at 100 °C.
steep capillary evaporation step at a lower limit of adsorption− desorption hysteresis55 (in this case, a relative pressure of ∼0.495). The steepness of the capillary condensation step indicated a uniform mesopore size, and the capillary evaporation at the lower limit of the hysteresis revealed that the mesopores were accessible through entrances that were less than 5 nm in diameter.55 The calcined sample exhibited a BET specific surface area of 430 m2/g, a total pore volume of 0.42 cm3/g, and a micropore volume of 0.12 cm3/g (Table 1). The pore size distribution (Figure 3(b)) was narrow and centered at 15.4 nm. The sample from which the surfactant was removed by extraction showed a lower uptake of nitrogen at −196 °C (Supporting Information Figure S6 and Table S1; see also SAXS pattern in Supporting Information Figure S7), possibly in part because of a lower efficiency of extraction in removing the D
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Figure 4. (a) Nitrogen adsorption isotherms and (b) pore size distributions for calcined samples prepared with different volumes of pBTMSMB and hydrothermally treated at 100 °C. For clarity, the isotherms for samples prepared using 0.90 and 0.50 mL of pBTMSMB were offset vertically by 150 and 400 cm3 STP g−1.
Figure 5. (a) Nitrogen adsorption isotherms and (b) pore size distributions for calcined samples prepared with 0.93 mL of pBTMSMB and different masses of xylene. The samples were hydrothermally treated at 100 °C. The isotherms for samples prepared using 1, 1.25, 1.5, 2, 4, and 6 g were offset vertically by 250, 350, 500, 700, 950, and 1150 cm3 STP g−1, respectively.
surfactant. However, pronounced low-pressure hysteresis was observed, suggesting that the mesopores were accessible to the surroundings through narrow passages of the diameter comparable to the size of a nitrogen molecule, thus resulting in a slow diffusion of nitrogen to the mesopores.56 Moreover, the sample calcined at 250 °C under air exhibited a very low uptake of nitrogen, thus indicating that the mesopores were not readily accessible to nitrogen. Likewise, the calcined (at 250 °C under air or at 350 °C under argon) sample synthesized with the largest volume (2.30 mL) of pBTMSMB exhibited low nitrogen uptake at −196 °C, even though its SAXS pattern indicated a well-ordered cubic Fm3m structure of mesopores. One can conclude that the ordered mesopores were present but were not accessible to nitrogen at −196 °C. A possible reason is that the large amount of organosilica precursor relative to the surfactant resulted in the formation of a thick and uniform organosilica framework around individual PEO blocks of the surfactant. As the surfactant was removed, the spaces once occupied by PEO chains shrank to the extent that nitrogen molecules were not able to pass through them to the mesopore interiors. It is also possible that some degree of flexibility of the
pBTMSMB bridge may facilitate the contraction of the spaces once occupied by PEO chains. Although the carbon backbone of the pBTMSMB bridge is expected to be rigid, the rotation about bonds that connect methylene carbons to the phenylene ring is not restricted and is likely to lead to higher flexibility than in the case of the phenylene bridge, which is directly bonded to silicon atoms. In fact, some phenylene-bridged PMOs prepared under similar conditions exhibited apparent diffusion problems in their nitrogen adsorption, but still an appreciable fraction of their mesopores was accessible to nitrogen at −196 °C.33 It is also possible that a residual surfactant contributed to the inaccessibility of the mesopores (see below). The sample prepared with 0.90 mL of pBTMSMB exhibited a significant uptake of nitrogen following the capillary condensation in its uniform mesopores (relative pressures above ∼0.81), which is in line with nitrogen adsorption results previously obtained in cases of single-micelle-templated nanoparticles.45 This adsorption behavior is consistent with TEM images of the sample, which showed a loose arrangement of the nanopartices. The sample was also hydrothermally treated at E
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Langmuir 130 °C (instead of 100 °C), which afforded a material with a similar isotherm but with even higher uptake close to the saturation vapor pressure (Supporting Information Figure S8). The considered sample and the other one prepared with smaller volumes of pBTMSMB (≤0.90 mL) had pore diameters of around 10 nm, which is much lower than the pore size of the sample with accessible mesopores prepared at a higher pBTMSMB/Pluronic ratio (that is, with 1.3 mL of pBTMSMB). The reason for this behavior is not clear, but it is possible that pBTMSMB precursor solubilizes to some extent in the micelles of the surfactant and/or promotes the solubilization of the swelling agent (xylene) used in the synthesis. Both of these factors could increase the aggregation number of surfactant micelles and/or could contribute to their size increase for higher pBTMSMB/surfactant proportions. Methanol formed from the hydrolysis of the framework precursor in the amount dependent on the volume of the precursor used also may have some influence on the process of the formation of the material. In the case of 0.90 mL of pBTMSMB, the evolution of the pore size with the increasing relative quantity of the swelling agent was followed. When the quantity of the swelling agent was moderate (0.5 g of xylene per 0.5 g of Pluronic F127), the pore diameter was small (∼7 nm) (Figure 5 and Supporting Information Table S2). The increase in the quantity of the swelling agent to 1, 1.25, and 1.5 g rendered hollow nanospheres with 11, 18, and 22 nm mesopores, respectively. A further increase in the proportion of the swelling agent to the surfactant (up to 6 g of xylene per 0.5 g of Pluronic F127) did not lead to any major pore size increase. It is notable that in the case of ethenylene-bridged organosilicas with face-centered cubic structures templated by the Pluronic F127/xylene pair,51 as in the present work, under otherwise analogous conditions (0.1 M HCl, 7 °C), the pore size increased as the quantity of xylene increased until it reached a plateau for 4 g of xylene (per 0.5 g of Pluronic F127). This indicates that the swelling agent/ surfactant ratio at which the pore size reaches a plateau depends on the composition of the synthesis mixture. However, it is remarkable that under the considered conditions (0.1 M HCl, 7 °C) the increase in the relative proportion of the swelling agent up to a large value and the resulting major pore size enlargement do not lead to the deterioration of the structural uniformity. It should be noted that a modest pore size increase (from ∼10 to ∼13 nm, that is, ∼30% enlargement) was reported when the amount of trimethylbenzene (TMB) swelling agent was increased in the Pluronic-F127-templated synthesis of ethylene-bridged organosilica nanospheres.44 In the present case, the pore diameter of the xylylene-bridged nanospheres increased by ∼200% and reached much larger values in comparison to the prior report.44 A comparably large pore diameter increase for single-micelle-templated nanospheres induced by the swelling agent was observed only for silicas templated by the Pluronic F108/toluene pair and was triggered by the lowering of the initial synthesis temperature.57 The samples prepared with a lower quantity of the swelling agent (0.5 and 1 g) had a rather small volume of uniform mesopores, as inferred from the small height of the capillary condensation step attributable to capillary condensation in uniform mesopores (at relative pressures of 0.6−0.75). However, the latter step greatly increased and shifted to a relative pressure of ∼0.85 as the quantity of xylene reached 1.25 g and continued to be prominent (at a relative pressure of ∼0.9) for higher relative proportions of the swelling agent. For
all of the samples, the capillary condensation step in uniform mesopores was followed by a further increase in the amount adsorbed as the pressure approached the saturation vapor pressure. This is consistent with the presence of uniform mesopores within the nanospheres in addition to large mesoporous/macroporous voids between the nanospheres, as seen from TEM (see above). The BET specific surface area of the nanospheres was in the range of 420 to 510 m2 g−1 and total pore volume was from 0.71 to 0.93 cm3/g, except for the sample prepared with 1 g xylene, for which these values were somewhat lower. The micropore volume was rather low (0.04− 0.09 cm3/g). The surfactant was removed from the xylylene-bridged organosilicas by calcination or extraction. Calcination was typically performed at 350 °C under argon, and extraction was carried out using the ethanol−HCl mixture, as described elsewhere.36 The 29Si cross-polarization (CP) magic-anglespinning (MAS) NMR spectrum of a sample prepared with 0.9 mL of pBTMSMB and calcined at 350 °C under argon indicates the preservation of the majority of Si−C bonds (based on T sites seen at −62 and −72 ppm), but Q sites were also seen at −100 to −110 ppm, suggesting some extent of Si−C bond cleavage in p-xylylene moieties during calcination at 350 °C under argon (Figure 6). A similar extent of preservation of
Figure 6. 29Si CP MAS NMR spectra of the calcined and extracted pxylylene bridged PMO prepared with 0.9 mL of pBTMSMB and hydrothermally treated at 100 °C.
Si−C linkages was inferred from the spectrum of the calcined sample prepared with 2.3 mL of pBTMSMB. The 29Si CP MAS NMR spectra of the extracted sample had peaks at about −62 and −72 ppm corresponding to T2 and T3 [Tn = RSi(OSi)n(OH)3−n] sites, respectively. In this case, there were no clear signals corresponding to Qn sites (Si(OSi)n(OH)4−n), implying no cleavage of Si−C bonds. Materials were also examined by 13C CP MAS NMR that showed peaks at around 128−133 ppm, which can be assigned to the carbon atoms in the phenyl group of the p-xylylene bridges. Peaks around ∼20 ppm (Supporting Information Figure S9) can be attributed to the methylene units.47 The absence or presence of a weak peak at ∼70 ppm suggests the effective removal of the surfactant for both the calcined and extracted samples. For comparison, the spectrum of the as-synthesized sample prepared with a higher amount of the framework precursor exhibited a strong peak at ∼70 ppm,58 which nearly disappeared after the calcination (Supporting Information Figure S10). Notably, the latter F
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Langmuir sample exhibited some features on the spectrum at about 50 ppm, which may indicate unhydrolyzed methoxy groups or even methanol trapped in the mesopores. Both could arise from limited mesopore accessibility as inferred from nitrogen adsorption. FTIR spectra of as-synthesized and calcined xylylene-bridged organosilicas are shown in Supporting Information Figure S11. As-synthesized xylylene-brided organosilica showed peaks at around 2930 and 1170 cm−1 corresponding to C−H and C− O−C asymmetric stretching vibrations of Pluronic F127 surfactant.59 These peaks are not distinctly visible for the calcined sample, yet there is a slight shoulder in place of the 1170 cm−1 peak. Also, a peak at 1460 cm−1, attributable to C− H bending,58 weakened appreciably. Although these features suggest a significant removal of the surfactant, consistent with 13 C NMR evidence, a surfactant residue may still be present. As discussed above, the synthesis strategy employed herein provided access to high-quality single-micelle-templated organosilica nanospheres with large bridging groups in the framework. The combination of the large size of the nanospheres (and thus the significant thickness of their walls) with an expected major shrinkage upon oxidative removal of the large organic bridging groups suggested that these organosilica nanospheres may be suitable as a precursor for closed-pore hollow silica nanospheres. Indeed, the calcination of the organosilica nanospheres at 500 °C under air rendered hollow nanospheres (as TEM images in Supporting Information Figure S12), whose inner voids were not accessible to nitrogen at 77 K. As shown in Supporting Information Figure S13, the sharp capillary condensation step so clearly seen on the isotherm of the hollow organosilica nanosphere sample calcined at 350 °C under argon was no longer observed, whereas a significant uptake of nitrogen close to the saturation vapor pressure was still seen. This is consistent with the retention of the nanosphere morphology (and thus large external surface area and interparticle porosity) but with no access to the hollow nanosphere interior. The consolidation of the nanosphere wall might be facilitated by KCl residue in the sample.60 The retention of the particle morphology was also supported by the SAXS pattern, which shifted to higher angles but was otherwise essentially unchanged after the calcination at 500 °C under air (Supporting Information Figure S14). The present finding extends the scope of the thermally induced pore-closing strategy for block-copolymer-templated ordered mesoporous (organo)silicas with closed (inaccessible) mesopores40 on closed hollow nanospheres. Our previous attempt to reach this goal using ethylene-bridged organosilicas did not render the desired closed-pore product.45
the organosilica-precursor/surfactant ratio afforded singlemicelle-templated hollow nanospheres, even though this strategy did not afford such nanoparticles in the cases of phenylene-bridged organosilicas under analogous conditions. The inner diameter of the nanospheres was adjusted in a wide range simply by changing the relative quantity of the micelle swelling agent used. Large-pore organosilica nanospheres were successfully converted to closed-pore silica nanospheres through calcination under air at 500 °C, which demonstrates that the thermally induced mesopore closing strategy is applicable to single-micelle-templated nanospheres. Further lowering of the organosilica-precursor/surfactant ratio afforded largely consolidated materials, showing that the distinct hollow nanosphere morphology may be accessible in a certain range of organosilica-precursor/surfactant ratios, whereas ratios that too high or too low may lead to consolidated structures. It is clear that the nature of the bridging group has a profound effect on the pore accessibility and propensity to form single-micelletemplated nanoparticles.
CONCLUSIONS Moderately acidic conditions (0.10 M HCl) and a low initial synthesis temperature (7 °C) were found suitable for the formation of a variety of p-xylylene-bridged organosilicas templated by micelles of Pluronic F127 block copolymer swollen by xylene. A well-ordered face-centered cubic structure formed, but its mesopores were not accessible after surfactant removal presumably because of some degree of flexibility of the p-xylylene-bridged framework with relatively thick walls. The lowering of the organosilica-precursor/surfactant ratio resulted in improved mesopore accessibility (presumably due to the lowering of the wall thickness) and a decrease in the structural ordering. The diameter of the mesopores of the resulting materials was remarkably large (∼15 nm). Further lowering of
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04284. Tables with structural parameters derived from nitrogen adsorption and SAXS. Figures with SAXS, WAXS patterns, adsorption isotherms, pore size distributions, 13 C CP MAS NMR spectra and IR spectra. TEM images. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS NSF is gratefully acknowledged for partial support of this research (award DMR-1310260) and for funding the acquisition of the SAXS/WAXS system through award CHE0723028. Partial support from PSC-CUNY award no. 66241-00 44 is acknowledged. Dr. Jianqin Zhuang (CSI) is acknowledged for assistance with NMR measurements. Dr. Manik Mandal (Lehigh University, Bethlehem, PA) is acknowledged for help in acquiring WAXS patterns at Lehigh University. The Imaging Facility at CSI is acknowledged for providing access to TEM. BASF is acknowledged for the donation of the Pluronic F127 block copolymer.
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REFERENCES
(1) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A new family of mesoporous molecular sieves prepared with liquid crystal templates. J. Am. Chem. Soc. 1992, 114, 10834−10843. (2) Inagaki, S.; Fukushima, Y.; Kuroda, K. Synthesis of highly ordered mesoporous materials from a layered polysilicate. J. Chem. Soc., Chem. Commun. 1993, 680−682. (3) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. Novel Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic Oxide in Their Frameworks. J. Am. Chem. Soc. 1999, 121, 9611−9614. G
DOI: 10.1021/acs.langmuir.5b04284 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir (4) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Mesoporous Sieves with Unified Hybrid Inorganic/Organic Frameworks. Chem. Mater. 1999, 11, 3302−3308. (5) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Periodic mesoporous organosilicas with organic groups inside the channel walls. Nature 1999, 402, 867−871. (6) Hunks, W. J.; Ozin, G. A. Challenges and advances in the chemistry of periodic mesoporous organosilicas (PMOs). J. Mater. Chem. 2005, 15, 3716−3724. (7) Van Der Voort, P.; Esquivel, D.; De Canck, E.; Goethals, F.; Van Driessche, I.; Romero-Salguero, F. J. Periodic Mesoporous Organosilicas: from simple to complex bridges; a comprehensive overview of functions, morphologies and applications. Chem. Soc. Rev. 2013, 42, 3913−3955. (8) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120, 6024− 6036. (9) Kleitz, F.; Choi, S. H.; Ryoo, R. Cubic Ia3d large mesoporous silica: synthesis and replication to platinum nanowires, carbon nanorods and carbon nanotubes. Chem. Commun. 2003, 2136−2137. (10) Fan, J.; Yu, C.; Lei, J.; Zhang, Q.; Li, T.; Tu, B.; Zhou, W.; Zhao, D. Low-Temperature Strategy to Synthesize Highly Ordered Mesoporous Silicas with Very Large Pores. J. Am. Chem. Soc. 2005, 127, 10794−10795. (11) Muth, O.; Schellbach, C.; Froeba, M. Triblock copolymer assisted synthesis of periodic mesoporous organosilicas (PMOs) with large pores. Chem. Commun. 2001, 2032−2033. (12) Zhu, H.; Jones, D. J.; Zajac, J.; Roziere, J.; Dutartre, R. Periodic large mesoporous organosilicas from lyotropic liquid crystal polymer templates. Chem. Commun. 2001, 2568−2569. (13) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Asefa, T.; Coombs, N.; Ozin, G. A.; Kamiyama, T.; Terasaki, O. Periodic Mesoporous Organosilica with Large Cagelike Pores. Chem. Mater. 2002, 14, 1903−1905. (14) Bhaumik, A.; Kapoor, M. P.; Inagaki, S. Ammoximation of ketones catalyzed by titanium-containing ethane bridged mesoporous silsesquioxane. Chem. Commun. 2003, 470−471. (15) Shylesh, S.; Singh, A. P. Vanadium-containing ethane-silica hybrid periodic mesoporous organosilicas: Synthesis, structural characterization and catalytic applications. Microporous Mesoporous Mater. 2006, 94, 127−138. (16) Fang, X.; Zhang, P.; Qiao, L.; Feng, X.; Zhang, X.; Girault, H. H.; Liu, B. Efficient Drug Metabolism Strategy Based on Microsome− Mesoporous Organosilica Nanoreactors. Anal. Chem. 2014, 86, 10870−10876. (17) Takeda, H.; Ohashi, M.; Goto, Y.; Ohsuna, T.; Tani, T.; Inagaki, S. Light-Harvesting Photocatalysis for Water Oxidation Using Mesoporous Organosilica. Chem. - Eur. J. 2014, 20, 9130−9136. (18) Waki, M.; Maegawa, Y.; Hara, K.; Goto, Y.; Shirai, S.; Yamada, Y.; Mizoshita, N.; Tani, T.; Chun, W.-J.; Muratsugu, S.; Tada, M.; Fukuoka, A.; Inagaki, S. A Solid Chelating Ligand: Periodic Mesoporous Organosilica Containing 2,2′-Bipyridine within the Pore Walls. J. Am. Chem. Soc. 2014, 136, 4003−4011. (19) López, M. I.; Esquivel, D.; Jiménez-Sanchidrián, C.; Van Der Voort, P.; Romero-Salguero, F. J. Vulcanized Ethene-PMO: A New Strategy to Create Ultrastable Support Materials and Adsorbents. J. Phys. Chem. C 2014, 118, 17862−17869. (20) Gehring, J.; Schleheck, D.; Trepka, B.; Polarz, S. Mesoporous Organosilica Nanoparticles Containing Superacid and Click Functionalities Leading to Cooperativity in Biocidal Coatings. ACS Appl. Mater. Interfaces 2015, 7, 1021−1029. (21) Takeda, H.; Goto, Y.; Maegawa, Y.; Ohsuna, T.; Tani, T.; Matsumoto, K.; Shimada, T.; Inagaki, S. Visible-light-harvesting periodic mesoporous organosilica. Chem. Commun. 2009, 6032−6034. (22) Hudson, S.; Cooney, J.; Hodnett, B. K.; Magner, E. Chloroperoxidase on Periodic Mesoporous Organosilanes: Immobilization and Reuse. Chem. Mater. 2007, 19, 2049−2055.
(23) Li, C.; Liu, J.; Shi, X.; Yang, J.; Yang, Q. Periodic Mesoporous Organosilicas with 1,4-Diethylenebenzene in the Mesoporous Wall: Synthesis, Characterization, and Bioadsorption Properties. J. Phys. Chem. C 2007, 111, 10948−10954. (24) Chen, Y.; Meng, Q.; Wu, M.; Wang, S.; Xu, P.; Chen, H.; Li, Y.; Zhang, L.; Wang, L.; Shi, J. Hollow Mesoporous Organosilica Nanoparticles: A Generic Intelligent Framework-Hybridization Approach for Biomedicine. J. Am. Chem. Soc. 2014, 136, 16326−16334. (25) Yang, Y.; Niu, Y.; Zhang, J.; Meka, A. K.; Zhang, H.; Xu, C.; Lin, C. X. C.; Yu, M.; Yu, C. Biphasic Synthesis of Large-Pore and WellDispersed Benzene Bridged Mesoporous Organosilica Nanoparticles for Intracellular Protein Delivery. Small 2015, 11, 2743−2749. (26) Goto, Y.; Inagaki, S. Synthesis of large-pore phenylene-bridged mesoporous organosilica using triblock copolymer surfactant. Chem. Commun. 2002, 2410−2411. (27) Guo, W.; Park, J.-Y.; Oh, M.-O.; Jeong, H.-W.; Cho, W.-J.; Kim, I.; Ha, C.-S. Triblock Copolymer Synthesis of Highly Ordered LargePore Periodic Mesoporous Organosilicas with the Aid of Inorganic Salts. Chem. Mater. 2003, 15, 2295−2298. (28) Vercaemst, C.; Ide, M.; Allaert, B.; Ledoux, N.; Verpoort, F.; Van Der Voort, P. Ultra-fast hydrothermal synthesis of diastereoselective pure ethenylene-bridged periodic mesoporous organosilicas. Chem. Commun. 2007, 2261−2263. (29) Zhang, W.-H.; Daly, B.; O’Callaghan, J.; Zhang, L.; Shi, J.-L.; Li, C.; Morris, M. A.; Holmes, J. D. Large Pore Methylene-Bridged Periodic Mesoporous Organosilicas: Synthesis, Bifunctionalization and Their Use as Nanotemplates. Chem. Mater. 2005, 17, 6407−6415. (30) Morell, J.; Guengerich, M.; Wolter, G.; Jiao, J.; Hunger, M.; Klar, P. J.; Froeba, M. Synthesis and characterization of highly ordered bifunctional aromatic periodic mesoporous organosilicas with different pore sizes. J. Mater. Chem. 2006, 16, 2809−2818. (31) Mandal, M.; Kruk, M. Versatile Approach to Synthesis of 2-D Hexagonal Ultra-Large-Pore Periodic Mesoporous Organosilicas. J. Mater. Chem. 2010, 20, 7506−7516. (32) Cho, E.-B.; Kim, D.; Gorka, J.; Jaroniec, M. Three-dimensional cubic (Im3m) periodic mesoporous organosilicas with benzene- and thiophene-bridging groups. J. Mater. Chem. 2009, 19, 2076−2081. (33) Mandal, M.; Manchanda, A. S.; Zhuang, J.; Kruk, M. Facecentered-cubic Large-pore Periodic Mesoporous Organosilicas with Unsaturated and Aromatic Bridging Groups. Langmuir 2012, 28, 8737−8745. (34) Li, Y.; Keilbach, A.; Kienle, M.; Goto, Y.; Inagaki, S.; Knochel, P.; Bein, T. Hierarchically structured biphenylene-bridged periodic mesoporous organosilica. J. Mater. Chem. 2011, 21, 17338−17344. (35) Guo, W.; Kim, I.; Ha, C.-S. Highly ordered three-dimensional large-pore periodic mesoporous organosilica with Im3m symmetry. Chem. Commun. 2003, 2692−2693. (36) Zhou, X.; Qiao, S.; Hao, N.; Wang, X.; Yu, C.; Wang, L.; Zhao, D.; Lu, G. Q. Synthesis of Ordered Cubic Periodic Mesoporous Organosilicas with Ultra-Large Pores. Chem. Mater. 2007, 19, 1870− 1876. (37) Mandal, M.; Kruk, M. Large-Pore Ethylene-Bridged Periodic Mesoporous Organosilicas with Face-Centered Cubic Structure. J. Phys. Chem. C 2010, 114, 20091−20099. (38) Vercaemst, C.; de Jongh, P. E.; Meeldijk, J. D.; Goderis, B.; Verpoort, F.; Van Der Voort, P. Ethynylene-bridged periodic mesoporous organosilicas with ultra-large mesopores. Chem. Commun. 2009, 4052−4054. (39) Vercaemst, C.; Friedrich, H.; de Jongh, P. E.; Neimark, A. V.; Goderis, B.; Verpoort, F.; Van Der Voort, P. Periodic Mesoporous Organosilicas Consisting of 3D Hexagonally Ordered Interconnected Globular Pores. J. Phys. Chem. C 2009, 113, 5556−5562. (40) Kruk, M.; Hui, C. M. Thermally Induced Transition between Open and Closed Spherical Pores in Ordered Mesoporous Silicas. J. Am. Chem. Soc. 2008, 130, 1528−1529. (41) Hu, Y.; Qian, K.; Yuan, P.; Wang, Y.; Yu, C. Synthesis of largepore periodic mesoporous organosilica. Mater. Lett. 2011, 65, 21−23. H
DOI: 10.1021/acs.langmuir.5b04284 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir (42) Liu, J.; Yang, Q.; Zhang, L.; Yang, H.; Gao, J.; Li, C. OrganicInorganic Hybrid Hollow Nanospheres with Microwindows on the Shell. Chem. Mater. 2008, 20, 4268−4275. (43) Liu, J.; Bai, S.; Zhong, H.; Li, C.; Yang, Q. Tunable Assembly of Organosilica Hollow Nanospheres. J. Phys. Chem. C 2010, 114, 953− 961. (44) Hao, N.; Wang, H.; Webley, P. A.; Zhao, D. Synthesis of uniform periodic mesoporous organosilica hollow spheres with largepore size and efficient encapsulation capacity for toluene and the large biomolecule bovine serum albumin. Microporous Mesoporous Mater. 2010, 132, 543−551. (45) Mandal, M.; Kruk, M. Family of Single-Micelle-Templated Organosilica Hollow Nanospheres and Nanotubes Synthesized through Adjustment of Organosilica/Surfactant Ratio. Chem. Mater. 2012, 24, 123−132. (46) Chi, F.; Guo, Y.-N.; Liu, J.; Liu, Y.; Huo, Q. Size-Tunable and Functional Core-Shell Structured Silica Nanoparticles for Drug Release. J. Phys. Chem. C 2010, 114, 2519−2523. (47) Hunks, W. J.; Ozin, G. A. Periodic Mesoporous Organosilicas with Phenylene Bridging Groups, 1,4-(CH2)nC6H4 (n = 0−2). Chem. Mater. 2004, 16, 5465−5472. (48) Jaroniec, M.; Kruk, M.; Olivier, J. P. Standard Nitrogen Adsorption Data for Characterization of Nanoporous Silicas. Langmuir 1999, 15, 5410−5413. (49) Kruk, M.; Jaroniec, M.; Sayari, A. Application of Large Pore MCM-41 Molecular Sieves To Improve Pore Size Analysis Using Nitrogen Adsorption Measurements. Langmuir 1997, 13, 6267−6273. (50) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Zhao, L.; Kamiyama, T.; Terasaki, O.; Pinnavaia, T. J.; Liu, Y. Ordered Mesoporous Silica with Large Cage-Like Pores: Structural Identification and Pore Connectivity Design by Controlling the Synthesis Temperature and Time. J. Am. Chem. Soc. 2003, 125, 821−829. (51) Manchanda, A. S.; Kruk, M. Synthesis of large-pore facecentered-cubic periodic mesoporous organosilicas with unsaturated bridging groups. Microporous Mesoporous Mater. 2016, 222, 153−159. (52) Mizoshita, N.; Tani, T.; Inagaki, S. Syntheses, properties and applications of periodic mesoporous organosilicas prepared from bridged organosilane precursors. Chem. Soc. Rev. 2011, 40, 789−800. (53) Hao, N.; Yang, Y.; Wang, H.; Webley, P. A.; Zhao, D. Synthesis of large-pore phenyl-bridged mesoporous organosilica with thick walls by evaporation-induced self-assembly for efficient benzene adsorption. J. Colloid Interface Sci. 2010, 346, 429−435. (54) Chen, Z. H.; Kim, C.; Zeng, X.-b.; Hwang, S. H.; Jang, J.; Ungar, G. Characterizing Size and Porosity of Hollow Nanoparticles: SAXS, SANS, TEM, DLS, and Adsorption Isotherms Compared. Langmuir 2012, 28, 15350−15361. (55) Kruk, M.; Jaroniec, M. Argon Adsorption at 77 K as a Useful Tool for the Elucidation of Pore Connectivity in Ordered Materials with Large Cagelike Mesopores. Chem. Mater. 2003, 15, 2942−2949. (56) Kruk, M.; Antochshuk, V.; Matos, J. R.; Mercuri, L. P.; Jaroniec, M. Determination and Tailoring the Pore Entrance Size in Ordered Silicas with Cage-like Mesoporous Structures. J. Am. Chem. Soc. 2002, 124, 768−769. (57) Li, Y.; Kruk, M. Single-micelle-templated Synthesis of Hollow Silica Nanospheres with Tunable Pore Structures. RSC Adv. 2015, 5, 69870−69877. (58) Kao, H.-M.; Chang, P.-C.; Wu, J.-D.; Chiang, A. S. T.; Lee, C.H. Direct synthesis, characterization and solid-state NMR spectroscopy of large-pore vinyl-functionalized cubic mesoporous silica FDU-12. Microporous Mesoporous Mater. 2006, 97, 9−20. (59) Matei Ghimbeu, C.; Vidal, L.; Delmotte, L.; Le Meins, J.-M.; Vix-Guterl, C. Catalyst-free soft-template synthesis of ordered mesoporous carbon tailored using phloroglucinol/glyoxylic acid environmentally friendly precursors. Green Chem. 2014, 16, 3079− 3088. (60) Huang, L.; Kruk, M. Versatile surfactant/swelling-agent template for synthesis of large-pore ordered mesoporous silicas and related hollow nanoparticles. Chem. Mater. 2015, 27, 679−689.
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