Communication pubs.acs.org/cm
Ultraporous Mesostructured Silica Nanoparticles Sam M. Egger,† Katie R. Hurley,† Ashish Datt,† Garrett Swindlehurst,‡ and Christy L. Haynes*,† †
Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue, Minneapolis, Minnesota 55455, United States
‡
S Supporting Information *
esoporous silica materials find use in many applications such as catalysis, separations, drug delivery, and gas adsorption wherein a large pore volume is desirable.1−4 High pore volumes can be achieved by swelling traditional surfactant templates,5−8 by employing larger templates such as block copolymers9,10 or sacrificial nanoparticles,11,12 or through interface-directed syntheses.13−20 For example, Stucky and coworkers demonstrated structural control over silica materials at two size scales by utilizing phase boundaries at the micelle level and at a bulk oil−water interface.13 Oil−water interfaces were also used in the biphasic stratification synthesis of dendritic mesoporous silicas.19 Further, solid−liquid interfaces between 3DOM carbon materials and water resulted in uniform silica nanoparticles with large pores.20 Despite novel structures and pore swelling strategies, pore volumes of mesoporous silica nanoparticles have seldom topped 2.0 cm3 g−1.21−23 In addition, swollen or hollow structures can suffer from other disadvantages. Some literature describes small micro- or mesopores on the shell portion of a particle,14,24 and many schemes result in thin silica walls,25,26 all of which are prone to breakage and aggregation. Herein, we report the development of a new mesoporous silica nanoparticle structure with extremely high pore volume and an open pore structure. These nanoparticles, which are formed from combinations of surfactant swelling strategies, stirring, and sonication, demonstrate pore volumes of up to 4.5 cm3 g−1 while maintaining the high surface areas of traditional mesoporous silica (>1000 m2 g−1). In addition, these structures demonstrate thermal stability and mechanical sturdiness. We propose that this new structure is formed through micellar aggregation during silica condensation. These ultraporous mesostructured nanoparticles (UMN) were synthesized with cetyltrimethylammonium bromide (CTAB) as a surfactant, dimethylhexadecylamine (DMHA) as a cosurfactant, and decane as an oil phase (exact combinations are shown in Table 1). Prior to silica condensation from tetraethylorthosilicate (TEOS), CTAB, DMHA, and decane mixtures were stirred and then sonicated for 90 min. The prepared suspension was equilibrated at 50 °C. After TEOS condensation, the synthesized material was dialyzed and centrifuged at 66 000 × g multiple times for purification. The textural properties of UMN are shown in Figure 1, Table 1, and Supporting Information Table S1 and Figures S1−S3. The difference in transmission electron microscopy (TEM) mass contrast between the center and edges of each particle suggests either a hollow or lacy interior silica network (Figure 1a). Image analysis of UMN-3, the structure with the highest pore volume, revealed average particle diameters of 71.3 ± 13.4
M
© 2015 American Chemical Society
Table 1. Synthetic Conditions and Properties of UMN
UMN-1 UMN-2 UMN-3 UMN-4 UMN-5
DMHA/ CTABa
decane/ CTABa
SAb [m2 g−1]
Vtc [cm3 g−1]
N/A 1.3 1.3 1.5 1.3
1.5 N/A 1.5 1.5 2.0
1161 1131 1235 1131 1221
1.8 3.3 4.5 4.2 4.1
a
Mole ratio. bSpecific surface area. cTotal pore volume calculated from nitrogen adsorption at P/P0 of 0.99.
Figure 1. Electron micrographs of UMN-3. (a−b) TEM shows texture and a relatively tight size distribution. (c) SEM displays surface structure following 10 Å platinum coating.
nm (see Supporting Information Figure S1 and Table S1 for other UMN characteristics). In contrast to other hollow or pore-expanded silica nanoparticles which often display either thin shells or small cavities, these materials demonstrate large interior voids/pores and web-like silica walls. These features work together to give UMN simultaneous high accessible surface area, high pore volume, and high stability (see below). The homogeneity of these particles can be seen in a wide-field image (Figure 1b, Supporting Information Figure S2). Scanning electron microscope (SEM) images show UMN-3 surface texture (Figure 1c). N2 physisorption isotherms measured from nanoparticles templated by CTAB/decane (UMN-1) or CTAB/DMHA (UMN-2) are compared to UMN-3 (CTAB/DMHA/decane) in Figure 2a to demonstrate the effects of a combined pore swelling strategy. When only decane is used, capillary condensation occurs at P/P0 = 0.3. This type IV hysteresis curve27 indicates pores of ∼4 nm diameter, typical of MCM-41type materials synthesized with oil phases.7,8 The UMN-1 plateau from P/P0 = 0.95 to 1.0 indicates that all spaces, Received: December 3, 2014 Revised: April 13, 2015 Published: April 16, 2015 3193
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and more porous. To this end, a geometrical model has been developed and is discussed in Supporting Information. Based on volume comparisons from interparticle spacings and interior particle cavities, it is shown that the major contributor to pore volume as mass decreases is from the cavity. This argument supports the high pore volumes reported in Table 1. A BJH pore diameter plot from the N2 physisorption data, shown in Figure 2b, reveals the complex porosity of UMN. BJH models were chosen over other techniques (e.g., DFT) because they are most commonly reported for mesoporous silica materials, and this thereby allows for fair comparison to literature.27,28 All samples, including those with CTAB/decane (UMN-1) or CTAB/DMHA (UMN-2), display a peak between 3 and 11 nm. This peak can be attributed to swollen mesopores throughout the particles. An additional broad peak is visible at higher pore diameters for each sample. In typical mesoporous silica nanoparticles, this broad peak represents the interparticle voids.28 For UMN-3, however, we argue that the peak represents both interparticle voids and intraparticle cavities (large meso- or macropores in the particles). The increase in pore volume for UMN-3 is due to increases in the meso- or macroporous region from 20 to 60 nm compared to UMN-1 or -2. DLS and TEM measurements in Supporting Information Table S1 and Figure S4 show the particle diameters do not change between conditions, so the increase in porosity for UMN-3 should not be attributed to larger interparticle voids caused by changes in packing in the powder form. The porous structure of UMN-type materials was confirmed using 2D (Supporting Information Figure S5 and Table S1) and 1D SAXS (small-angle X-ray scattering, Supporting Information Figure S6). 2D SAXS of UMN-3 showed single maxima at 2θ ∼ 1.16 (d = 7.29 nm). 2D SAXS displays only one broad peak with no higher order peaks, which indicates the pores are relatively disordered (see TEM in Figure 1). This dspacing and lack of higher order peaks is in excellent agreement with Sayari’s work (d-spacing of 7.6 nm) concerning pore swelling with DMHA of non-nanoparticle MCM-41 type films. As demonstrated by both Sayari and Hartono, pores with limited ordered domains (singly resolved peaks in SAXS) can be effectively measured via BJH pore diameter and Vt (P/P0 = 0.99) assessments despite the polydispersity in the pore diameters.25,32 Interestingly, Vt of Sayari’s and Hartono’s mesoporous silicas are 1.8 and 1.1 cm3 g−1 respectively, which suggests the major volume contributor of UMN is associated with the lacey cavities in the nanoparticles in addition to the large mesopores.25 Figure 2c shows the 1D SAXS patterns for UMN-3. There is a shoulder at a q value 0.8 nm−1, which is typical for large pore mesoporous silicas, such as SBA-15, with a q value ∼1 nm−1.33 Desmearing the profile reveals this shoulder is indeed a peak (Supporting Information Figure S6), as observed commonly in other mesoporous silicas.33 The higher order features at higher q values indicate the presence of short-range order. The pair distance distribution function (PDDF) of the SAXS profile (inset of Figure 2c), which represents the length density distribution,34 shows three different maxima. The first maximum at 2.5 nm corresponds to the wall thickness between mesopores and agrees with previous work.34 The second maximum, a shoulder around 12 nm, is attributed to the average mesopore size. Finally, the third maximum at 20 nm is a convolution of a variety of length scales including the lacey cavity, the periodicity of pores, polydispersity in the pore sizes, and interparticle spacings in the dried material.
Figure 2. N2 physisorption and SAXS measurements demonstrating the porosity of UMN. (a) N2 isotherms and (b) BJH pore distributions for UMN with CTAB/decane (red), CTAB/DMHA (blue), CTAB/DMHA/decane (black). Inset: primary pore peaks from 1 to 16 nm. (c) SAXS profile for UMN-3. Inset: pair distance distribution function.
including interparticle voids, have been filled with nitrogen and are smaller than 50 nm.28 Both the DMHA only (UMN-2) and UMN-3 show capillary condensation near P/P0 ∼ 0.6. This is typical of mesoporous silica materials with pore diameters > 6 nm.29 Once multilayer nitrogen adsorption begins, however, the difference between UMN-2 and UMN-3 becomes apparent. Although both samples display similar hysteresis,30 UMN-3 displays an unusually shaped isotherm; this large hysteresis area indicates a greater number density of pores, which results in large adsorption quantities (2931 cm3 g−1). The lack of isotherm plateauing from P/P0 = 0.95 to 1.0 in UMN-3 represents a combination of large mesopores, macropores (pores > 50 nm), and interparticle voids. Because of this convolution, an internal pore volume cannot be calculated separately from the total pore volume for multiple UMN samples. Because of these exceptionally high pore volumes, it is prudent to consider whether the results are artificially inflated by the decreasing mass of the particles as they become more 3194
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Figure 3. Proposed synthetic scheme. Aggregates of swollen micelles are formed during TEOS condensation to form a lacey structure reminiscent of dendritic-31 or raspberry-28 type structures.
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It has been suggested by Jaroniec and others that simultaneous high surface area and high pore volume would contribute to incredibly thin pore walls and result in low stability.25 To address this concern, the thermal robustness of UMN-3 was tested by subjecting a sample to 450 °C for 12 h. Subsequent nitrogen adsorption−desorption analysis demonstrates BET surfaces areas of 1229 m2 g−1 and total pore volumes of 4.0 cm3 g−1, suggesting that the textural properties of UMN-3 are not compromised at such temperatures (Supporting Information Figure S7). The structure of these materials is likely a result of micellar aggregation during silica condensation as depicted in Figure 3. Sonication of water suspensions containing CTAB, DMHA, and decane in the amounts detailed in Table 1 results in relatively stable, swollen micelles (Figure 3 steps 1 and 2). Assuming that the micellar aggregation number is n = 100 (typical of CTAB micelles) and that decane packs uniformly in the center of micelles with a density similar to the bulk, these swollen micelles should have a diameter between 6 and 12 nm, which corresponds well with the size of mesopores in the particles. When the sample is removed from the sonicator and TEOS is added slowly with stirring, silicate aggregates start to condense onto the spherical micelles, and individual micelles cluster together as is commonly reported (steps 3 and 4).16,28,35 Silica continues to condense at the interfaces between micelles, forming the walls or “wrinkles”31 seen in the final product (step 5). Vigorous and even stirring allows for micellar cluster rearrangement, resulting in particles with a small size distribution (step 6). Radially oriented mesopores in the final material (step 7) are possible due to the energetic preference of TEOS to condense on previously existing silica sites (i.e., growing silicas are more likely to deposit on existing silica walls or wrinkles as opposed to a fresh micellar surface at the edge of the growing particle). Other micellar rearrangement mechanisms, such as sonication gas cavitation,36 are considered unlikely, as the micelle template can be sustained over 24 h postsonication (Supporting Information Figure S8). The hypothesized process will be the subject of continued research. In summary, we report the synthesis of porous silica nanomaterials which provide ultrahigh pore volumes rivaling or surpassing the best performing porous materials. This simple combination of cosurfactants, sonication, and stirring prior to TEOS condensation resulted in a novel structure. Work to develop core/shell materials and to load molecular or biological cargo is ongoing. This material is a versatile platform that can be made appropriate for applications ranging from environmental remediation to biomedical sensing, retaining all the stability and surface-modification potential available in more traditional mesoporous silica nanomaterials syntheses.
ASSOCIATED CONTENT
S Supporting Information *
Synthetic details, a geometric model to understand the contributions to measure pore volume, and complete characterization of UMN including DLS, TEM, 1D and 2D SAXS, and N2 isotherms. This material is available free of charge via the Internet at http://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the DARPA IVN:Dx program. K.R.H. acknowledges support from NSF GRFP. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. The authors thank M. Brostrom, L. Sauer, J. Warner, J. Myers, N. Seaton, N. Klein, A. Stein, and N. Petkovich.
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REFERENCES
(1) Davis, M. E. Nature 2002, 417, 813−821. (2) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321−323. (3) Mamaeva, V.; Rosenholm, J. M.; Bate-Eya, L. T.; Bergman, L.; Peuhu, E.; Duchanoy, A.; Fortelius, L. E.; Landor, S.; Toivola, D. M.; Lindén, M.; Sahlgren, C. Mol. Ther. 2011, 19, 1538−1546. (4) Minakuchi, H.; Nakanishi, K.; Soga, N.; Ishizuka, N.; Tanaka, N. Anal. Chem. 1996, 68, 3498−3501. (5) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147−1160. (6) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710−712. (7) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 2759. (8) Blin, J. L.; Otjacques, C.; Herrier, G.; Su, B.-L. Langmuir 2000, 16, 4229−4236. (9) Khanal, A.; Inoue, Y.; Yada, M.; Nakashima, K. J. Am. Chem. Soc. 2007, 129, 1534−1535. (10) Du, B.; Cao, Z.; Li, Z.; Mei, A.; Zhang, X.; Nie, J.; Xu, J.; Fan, Z. Langmuir 2009, 25, 12367−12373. (11) Blas, H.; Save, M.; Pasetto, P.; Boissière, C.; Sanchez, C.; Charleux, B. Langmuir 2008, 24, 13132−13137. 3195
DOI: 10.1021/cm504448u Chem. Mater. 2015, 27, 3193−3196
Communication
Chemistry of Materials (12) Zhao, W.; Lang, M.; Li, Y.; Li, L.; Shi, J. J. Mater. Chem. 2009, 19, 2778−2783. (13) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schüth, F. Science 1996, 273, 768−771. (14) Huo, Q.; Feng, J.; Schüth, F.; Stucky, G. D. Chem. Mater. 1997, 9, 14−17. (15) Zhange, F.; Yongsheng, L.; Dechao, N.; Liang, L.; Wenru, Z.; Hangrong, C.; Lei, L.; Jianhua, G.; Meiling, R.; Jianlin, S. Chem. Commun. 2008, 2629−2631. (16) Wang, J.-G.; Li, F.; Zhou, H.-J.; Sun, P.-C.; Ding, D.-T.; Chen, T.-H. Chem. Mater. 2009, 21, 612−620. (17) Palani, A.; Wu, H.-Y.; Ting, C.-C.; Vetrivel, S.; Shanmugapriya, K.; Chiang, A. S. T.; Kao, H.-M. Microporous Mesoporous Mater. 2010, 131, 385−392. (18) Shanmugam, V.; Ching-Ting, C.; Hsien-Ming, K. New J. Chem. 2010, 34, 2109−2112. (19) Shen, D.; Yang, J.; Li, X.; Zhou, L.; Zhang, R.; Li, W.; Chen, L.; Wang, R.; Zhang, F.; Zhao, D. Nano Lett. 2014, 14, 923−932. (20) Wang, M.; Sun, Z.; Yue, Q.; Yang, J.; Wang, X.; Deng, Y.; Yu, C.; Zhao, D. J. Am. Chem. Soc. 2014, 136, 1884−1892. (21) Ikuno, T.; Nomura, A.; Iyoki, K.; Sugawara-Narutaki, A.; Okubo, T.; Shimojima, A. Chem. Lett. 2013, 42, 316−317. (22) Yang, Y.; Liu, J.; Bai, S.; Li, X.; Yang, Q. Chem. − Asian J. 2013, 8, 582−587. (23) Yang, G.; Gai, S.; Qu, F.; Yang, P. ACS Appl. Mater. Interfaces 2013, 5, 5788−5796. (24) Zhu, Y.; Shi, J.; Li, Y.; Chen, H.; Shen, W.; Dong, X. Microporous Mesoporous Mater. 2005, 85, 75−81. (25) Sayari, A.; Yang, Y.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B 1999, 103, 3651−3658. (26) Rong, Y.; Ku, Z.; Han, H.; Liu, G.; Xu, M.; Wang, H.; Liu, L.; Hu, M.; Yang, Y.; Li, X. J. Mater. Chem. A 2012, 1, 1475−1480. (27) Branton, P. J.; Hall, P. G.; Sing, K. S. W.; Reichert, H.; Schüth, F.; Unger, K. K. P. J. Chem. Soc., Faraday Trans. 1994, 90, 2965. (28) Zhang, K.; Xu, L.-L.; Jiang, J.-G.; Calin, N.; Lam, K.-F.; Zhang, S.-J.; Wu, H.-H.; Wu, G.-D.; Albela, B.; Bonneviot, L.; Wu, P. J. Am. Chem. Soc. 2013, 135, 2427−2430. (29) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712−10713. (30) de Miguel, Y.; Rohr, T.; Sherrington, D. C. In Polymeric Materials in Organic Synthesis and Catalysis; Buchmeiser, M. R., Ed.; Wiley-VCH: Weinheim, Germany, 2005; Ch. 1. (31) Du, X.; Qiao, S. Z. Small 2015, 11, 392−413. (32) Hartono, S. B.; Qiao, S. Z.; Liu, J.; Jack, K.; Ladewig, B. P.; Hao, Z.; Lu, G. Q. M. J. Phys. Chem. C 2010, 114, 8353−8362. (33) Chong, A. S. M.; Zhao, X. S. J. Phys. Chem. B 2003, 107, 12650− 12657. (34) Smarsly, B.; Groenewolt, M.; Antonietti, M. In Progress in Colloid and Polymer Science; Springer: Berlin, Heidelberg, 2005; pp 105−113. (35) Wu, S.-H.; Hung, Y.; Mou, C.-Y. Chem. Mater. 2013, 25, 352− 364. (36) Rana, R. K.; Mastai, Y.; Gedanken, A. Adv. Mater. 2002, 14, 1414−1418.
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