Colloidal Amphiphile-Templated Growth of Highly Crystalline

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Colloidal Amphiphile-Templated Growth of Highly Crystalline Mesoporous Nonsiliceous Oxides Ben Liu,† Zhu Luo,‡ Anthony Federico,† Wenqiao Song,† Steven L. Suib,*,†,‡ and Jie He*,†,‡ †

Department of Chemistry and ‡Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States S Supporting Information *

T

carbon-based molecular amphiphiles. The polymer tethers of CAMs that are essentially “soft templates” can interact with inorganic precursors and direct the self-assembly to yield the mesostructures, while the nanoparticle cores of CAMs can serve as “hard templates” to support the frameworks at high temperatures.16 Our synthetic approach is based on the use of CAMs containing silica-like nanoparticle cores and poly(ethylene oxide) (PEO) hydrophilic tethers as a structuredirecting agent (Figure S1). The hybrid CAMs as structuredirecting agents can overcome the shortcomings of other soft templates (e.g., carbon-based molecular or polymeric surfactants) with poor thermal stability and mechanical strength. The as-made oxides can be directly crystallized by calcination at 1000 °C under air atmosphere without disrupting well-ordered mesostructures. The CAM-templating approach, thus, combines merits of both the (i) soft-templating method where PEOtethered CAMs as structural-directing agents can directly selfassemble into periodically ordered mesostructures with tunable pore sizes and (ii) hard-templating method where silica-like micelles have an ultrahigh thermal stability to fabricate highly crystalline frameworks. Due to its excellent thermal stability, our method also offers a straightforward pathway to more precisely control the growth and transition of crystalline phases, as well as the interface and composition of two crystalline phases that are beyond the capabilities of current synthetic methods. Our synthetic route is schematically illustrated in Scheme 1. As a proof-of-concept, the amphiphilic BCP of poly(ethylene oxide)-block-poly(3-(trimethoxysilyl)propyl methacrylate)

he design and synthesis of mesoporous nonsiliceous oxides, especially transition-metal oxides, are of great importance for the fields of photovoltaics, energy storage, sensing, and catalysis.1−5 However, the amorphous mesoporous oxides having a low thermal/mechanical stability and poor optoelectronic performance have difficulty meeting the demand for these applications.6,7 The conversion of solid mesostructured materials from amorphous to crystalline that often needs thermal treatment at higher temperatures becomes critical. In particular, the crystallization of mesoporous oxides often results in the collapse of mesoporous frameworks because their crystallization is accompanied by a large volume shrinkage.2,4 It remains a grand challenge to convert as-made, amorphous mesoporous oxides to highly crystalline ones while simultaneously retaining welldefined mesostructures.8,9 Much effort has been dedicated to two methods currently. On the one hand, nanocasting, also known as the hard-templating method, was proposed as a key method to prepare crystalline mesoporous materials.10,11 To do so, mesoporous silica12,13 and carbon14 are first fabricated and then used as hard templates. The mesostructures and pore sizes of obtained oxides are totally dependent on the hard templates (e.g., SBA series12 and KIT-613), and therefore the produced pores are generally very small (800 °C under air. All traditional carbon-based soft templates, e.g., surfactants or polymers, cannot be used under these harsh conditions. As such, a simple and general method using thermally stable and mechanically strong “soft” templates to prepare mesoporous transition-metal oxides with highly crystalline frameworks is much desired. In this contribution, we report utilizing a new type of colloidal amphiphiles (CAMs) toward the templated growth of mesoporous nonsiliceous oxides with highly crystalline frameworks. CAMs generally composed of nanoparticle cores tethered by well-defined polymer ligands are engaged with many of the right characteristics of molecular amphiphiles and colloids. The incorporation of nanoparticle cores will endow CAMs with excellent thermal stability and mechanical strength that surpass © XXXX American Chemical Society

Scheme 1. Schematic Illustration of Highly Crystalline Transition-Metal Oxides Based on Colloidal AmphiphileTemplating Method

Received: June 15, 2015 Revised: August 11, 2015

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DOI: 10.1021/acs.chemmater.5b02248 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials (PEO114-b-PTMSPMA298 with Mn = 73.9 kg/mol and PDI = 1.28) was synthesized using atom transfer radical polymerization with a macroinitiator of PEO114−Br (see Supporting Information for synthetic details). The hybrid CAMs were prepared via the self-assembly of PEO114-b-PTMSPMA298 in a mixed solvent of ethanol and water, followed by the hydrolysis/polycondensation of silane moieties in PTMSPMA blocks (Scheme 1a).17,18 The obtained hybrid CAMs, similar to polymer micelles, composed of polysilsesquioxane cores and PEO tethers have an average diameter of 25 ± 2 nm (see TEM in Scheme 1). After the addition of inorganic sources, the coordination interaction between PEO and transition-metal sources (e.g., Ti4+ and Zr4+) thus triggers the self-assembly to form periodically ordered polymer-oxide hybrids by sol−gel chemistries via an evaporationinduced self-assembly (EISA) process (Scheme 1b). The asmade amorphous samples were sequentially calcined at elevated temperatures under air directly to form highly crystalline frameworks. PEO blocks were directly decomposed upon heating at >400 °C,1 while polysilsesquioxane cores of CAMs converted into amorphous silica-like nanoparticles to support the mesostructures and prevent the structural reconstruction of frameworks during the thermal treatment at high temperature (Scheme 1c). After the removal of CAM templates, periodically ordered and highly crystalline mesoporous oxides could be yielded (Scheme 1d). The nanostructures and crystallinity of the obtained mesoporous oxides were carefully investigated by smallangle X-ray scattering (SAXS), wide-angle X-ray diffraction (WXRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), and nitrogen adsorption−desorption surface area analysis. Using group-IV transition-metal oxide TiO2 as an example, the mesoporous TiO2 was prepared by calcining at 900 °C under air (denoted as mTiO2-900) (Figure 1). Note that the calcination temperature was increased manually at a rate of 100 °C per 2 h until reaching 900 °C and all calcination cycles were performed under air directly. The periodically ordered mesoporous structure of mTiO2-900 was confirmed by both SEM (Figure 1a,b) and TEM (Figure 1d,e) images. The average pore size is ∼24 nm close to the size of CAM cores (see Figure S1), indicating that the formation of mTiO2 mesostructures was templated by CAMs. The wall thickness is in the range of 8−10 nm. It should be pointed out that the pores of mTiO2 look less organized under TEM because of the high crystallinity of walls (see SAXS patterns below). The high-resolution TEM image (Figure 1c) showed the anatase crystalline structure of mTiO2. The measured lattice fringe of 0.36 nm corresponds to (101) planes of anatase TiO2. The SAED (Figure 1f) and WXRD patterns (Figure 1h) further confirmed the formation of a crystalline TiO2 framework composed of mixed crystalline phases of anatase (79%) and rutile (21%).19 The average grain size (AGS) of mTiO2-900 was estimated to be 16 nm using the Scherrer formula.20,21 mTiO2-900 has a similar composition of the commercial Degussa TiO2 P25 (∼80% of anatase and ∼20% of rutile).22 The SAXS pattern of mTiO2-900 (Figure 1g) presents well-resolved (111), (311), and (420) reflections, assigned to an ordered face-centered cubic ( fcc) mesostructure with the space group of Fm3̅m. The d-spacing of mesostructures for (111) reflection is calculated to be 30.5 nm in good consistency with SEM and TEM observations. The resulting unit cell parameter (a) for mTiO2-900 is estimated to be 52.4 nm (Table S1). The Brunauer−Emmett−Teller (BET) specific surface area from nitrogen absorption−desorption analyses of

Figure 1. Structural characterization of mTiO2-900. (a, b) SEM and (d, e) TEM images of mTiO2-900 indicate the ordered mesostructures with uniform large pores. (c) High-resolution TEM image showing the highly crystalline structure. (f) SAED pattern of mTiO2-900. (g) SAXS pattern indicates ordered fcc mesoporous structures. (h) WXRD patterns of commercial Degussa P25 TiO2 (black, bottom) and mTiO2-900 (red, top). The crystal planes of rutile TiO2 are marked with “*”.

mTiO2-900 was estimated to be 54.5 m2 g−1 with a pore size of 21 nm (Figure S3). The structural and crystalline evolutions of mTiO2 have been systemically investigated by controlling calcination temperatures. The mesostructures of mTiO2 were completely retained even after being calcined at 400−1000 °C (Figure 2a−d, also see the low magnification TEM images in Figures S2−S8), while the growth of TiO2 crystalline domains was seen at higher calcination temperatures. The slight decrease of pore sizes to ∼15 nm at 1000 °C could be found as a result of the framework shrinkage during the formation of large rutile domains. The corresponding SAXS patterns also suggest that d-spacing of mTiO2 (111) reflection gradually decreases from 32.5 nm (mTiO2-400) to 31 nm (mTiO2-800) and eventually to 28.8 nm (mTiO2-1000) (Figure 2e). When further increasing the calcination temperature to 1100 °C, the mesostructure of mTiO2 was completely disrupted (Figure S9), possibly due to the phase segregation between silica and TiO2. The AGS of rutile domains in TiO21100 increased to 50 nm, much larger than the wall thickness (Table S2), confirming the segregation of TiO2 from the templates as well. The evolution of mTiO2 crystal phases was characterized by WXRD as shown in Figure 2f. Two conclusions can be made from WXRD results. First, below 800 °C, the poorly crystalline anatase phase of TiO2 was present as evidenced by the broadening and low intensity of TiO2 (101). The elevation of the calcination temperature can significantly improve the crystallinity of TiO2. This was confirmed by the growth of AGSs of mTiO2 from 3.8 nm (mTiO2-400) to 7.8 nm (mTiO2700). Second, new diffraction peaks at 27.5°, 36.1°, and 47.4° appeared, and their intensities gradually increased with the calcination temperature, indicating the formation of rutile TiO2 phase. A clear phase boundary between anatase and rutile phases has also been observed from high-resolution TEM (see the inset B

DOI: 10.1021/acs.chemmater.5b02248 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

shell structures.21 The small AGS with high crystallinity is known to be essential to improve the photocatalytic activity of mTiO2.21 On the contrary, using traditional amphiphilic BCPs as soft templates, e.g, poly(ethylene oxide)-block-polystyrene, the mesoporous framework of TiO2 totally collapsed during the crystallization at 600 °C in our control experiment (Figure S12). The key to our CAM-templating approach is the improvement of the thermal stability of templates that can preserve the mesoporous framework during the calcination at elevated temperatures. Our synthetic methodology can be readily applied to prepare other mesoporous transition-metal oxides, e.g., group-V zirconium dioxide (mZrO2) and group-IV/group-V mixed transition-metal oxide, Zr0.33Ti0.67O2 (mSrilankite). Among these, the crystallization of mSrilankite normally needs thermal treatment at >700 °C.23 As shown in Figure S13, both mZrO2 and mSrilankite calcined at 800 °C present a long-range mesoporous structure with uniform large pores. The WXRD patterns further suggest that both of them have highly crystalline walls (Figure S13e). In particular, the crystalline mSrilankite with well-defined pores has not been obtained using previous methods. This powerful method is expected to offer us a library of crystalline mesoporous (mixed) transition-metal oxides. The enhancement of crystallinity and the controllable interface of anatase/rutile phases of TiO2 are known to improve its photocatalytic activity. Using the standardized photodecomposition of organic dye of Rhodamine B (RhB), the photocatalytic activity of mTiO2 was further evaluated under the irradiation of UV light (320−390 nm). The decomposition of RhB was followed by measuring the change in UV−vis absorption of RhB as given in Figure 3a. The standard TiO2

Figure 2. (a−c) TEM and (d) SEM images of mTiO2 calcined at various temperatures: 400 °C (a), 600 °C (b), 800 °C (c), and 1000 °C (d), respectively. The insets in (b−d) are the corresponding high-resolution TEM images. The dashed line in the inset of (d) indicates the boundary between anatase and rutile phases. (e) SAXS and (f) WXRD patterns of mTiO2 calcined at different temperatures.

in Figure 2d) for mTiO2-1000, indicating the high content of rutile TiO2 in the resulting materials. The detailed analysis of the crystalline phases and AGSs of mTiO2 is summarized in Table S2. The proportion of anatase and rutile phases seems to be controllable by calcination temperature. For mTiO2-1000, ∼39% of rutile phase and ∼61% of anatase phase were present, compared to that of mTiO2-600 with nearly 100% anatase. This result is particularly important for the synthesis of TiO2-based photocatalysts where the anatase/rutile interface is believed to be crucial for the photocatalytic performance.20 We further evaluated the influence of calcination time on the thermal stability of mesostructures of mTiO2-800 (Figures S10 and S11). The corresponding SEM images of mTiO2-800 clearly showed that all materials preserved the periodically ordered mesoporous structures even after calcination at 800 °C for 48 h under air, indicating the extraordinary thermal stability of these mesomaterials. The anatase-rich mTiO2 with improved crystallinity was obtained after thermal treatment for a longer time. Interestingly, the ratio of anatase/rutile phases did not change over the calcination time as shown in WXRD patterns. The AGS of mTiO2-800 was 6.6 nm with the calcination time of 0.5 h (mTiO2-800-0.5h) and grew to 17 nm after calcined for 24 h (mTiO2-800-24h). The AGS, however, did not further increase for a longer thermal treatment time (15 nm for mTiO2-800-48h) (see Table S3). This was likely because of the nanoconfinement of colloidal templates, i.e., silica-like nanoparticles formed by CAMs, preventing the further growth of TiO2 nanocrystals in between templates. The similar phenomenon was reported in the confined crystallization of TiO2 using SiO2@TiO2@SiO2 core−

Figure 3. Photocatalytic activity of mTiO2. (a) Photocatalytic degradation of RhB under the irradiation of UV light and (b) the apparent reaction rate vs the reaction time with mTiO2 catalysts: blank control (■), P25 (●), mTiO2-600 (▲), mTiO2-800 (▼), mTiO2-800− 48h (◆), mTiO2-900 (◀), and mTiO2-1000 (▶).

photocatalyst of P25 nanoparticles and the photobleaching of RhB without any photocatalyst were also tested as a comparison. The photocatalytic results were fitted with a first-order reaction kinetics by plotting ln(C/C0) as a function of reaction time under irradiation (see Supporting Information for details). The photobleaching of dyes without catalysts is very slow with a rate constant (k) of 9 × 10−5 s−1. Both mTiO2-800 and mTiO2900 have an enhanced photocatalytic activity with k = 5.1 × 10−3 s−1 and 4.4 × 10−3 s−1, comparable to the state-of-art activity of P25. The mTiO2-800-48h catalyst gives a highest activity with a k of 8.65 × 10−3 s−1, 1.7-fold higher than that of P25. The general tendency of photocatalytic activity is in good consistency with the crystallinity and the controllable interface of anatase/rutile C

DOI: 10.1021/acs.chemmater.5b02248 Chem. Mater. XXXX, XXX, XXX−XXX

Communication

Chemistry of Materials

(3) Li, W.; Wu, Z.; Wang, J.; Elzatahry, A. A.; Zhao, D. A perspective on mesoporous TiO2 materials. Chem. Mater. 2014, 26, 287−298. (4) Gu, D.; Schüth, F. Synthesis of non-siliceous mesoporous oxides. Chem. Soc. Rev. 2014, 43, 313−344. (5) Liu, B.; Kuo, C. H.; Chen, J.; Luo, Z.; Thanneeru, S.; Li, W.; Song, W.; Biswas, S.; Suib, S. L.; He, J. Ligand-Assisted Co-Assembly Approach toward Mesoporous Hybrid Catalysts of Transition-Metal Oxides and Noble Metals: Photochemical Water Splitting. Angew. Chem., Int. Ed. 2015, 54, 9061−9065. (6) Luo, H.; Wang, C.; Yan, Y. Synthesis of mesostructured titania with controlled crystalline framework. Chem. Mater. 2003, 15, 3841−3846. (7) Lee, J.; Orilall, M. C.; Warren, S. C.; Kamperman, M.; DiSalvo, F. J.; Wiesner, U. Direct access to thermally stable and highly crystalline mesoporous transition-metal oxides with uniform pores. Nat. Mater. 2008, 7, 222−228. (8) Tsung, C. K.; Fan, J.; Zheng, N.; Shi, Q.; Forman, A. J.; Wang, J.; Stucky, G. D. A general route to diverse mesoporous metal oxide submicrospheres with highly crystalline frameworks. Angew. Chem., Int. Ed. 2008, 47, 8682−8686. (9) Brezesinski, T.; Wang, J.; Tolbert, S. H.; Dunn, B. Ordered mesoporous [alpha]-MoO3 with iso-oriented nanocrystalline walls for thin-film pseudocapacitors. Nat. Mater. 2010, 9, 146−151. (10) Lu, A. H.; Schüth, F. Nanocasting: a versatile strategy for creating nanostructured porous materials. Adv. Mater. 2006, 18, 1793−1805. (11) Ren, Y.; Ma, Z.; Bruce, P. G. Ordered mesoporous metal oxides: synthesis and applications. Chem. Soc. Rev. 2012, 41, 4909−4927. (12) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 1998, 279, 548−552. (13) Kleitz, F.; Choi, S. H.; Ryoo, R. Cubic Ia 3 d large mesoporous silica: synthesis and replication to platinum nanowires, carbon nanorods and carbon nanotubes. Chem. Commun. 2003, 2136−2137. (14) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Ordered nanoporous arrays of carbon supporting high dispersions of platinum nanoparticles. Nature 2001, 412, 169−172. (15) Zhou, W.; Li, W.; Wang, J.-Q.; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K.; Wang, L.; Fu, H.; Zhao, D. Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst. J. Am. Chem. Soc. 2014, 136, 9280−9283. (16) Bastakoti, B. P.; Li, Y.; Imura, M.; Miyamoto, N.; Nakato, T.; Sasaki, T.; Yamauchi, Y. Polymeric Micelle Assembly with Inorganic Nanosheets for Construction of Mesoporous Architectures with Crystallized Walls. Angew. Chem., Int. Ed. 2015, 54, 4222−4225. (17) Du, J.; Chen, Y. Hairy nanospheres by gelation of reactive block copolymer micelles. Macromol. Rapid Commun. 2005, 26, 491−494. (18) Li, W.; Kuo, C.-H.; Kanyo, I.; Thanneeru, S.; He, J. Synthesis and Self-Assembly of Amphiphilic Hybrid Nano Building Blocks via SelfCollapse of Polymer Single Chains. Macromolecules 2014, 47, 5932− 5941. (19) Spurr, R. A.; Myers, H. Quantitative analysis of anatase-rutile mixtures with an X-ray diffractometer. Anal. Chem. 1957, 29, 760−762. (20) Zhong, J.; Chen, F.; Zhang, J. Carbon-deposited TiO2: synthesis, characterization, and visible photocatalytic performance. J. Phys. Chem. C 2010, 114, 933−939. (21) Joo, J. B.; Dahl, M.; Li, N.; Zaera, F.; Yin, Y. Tailored synthesis of mesoporous TiO 2 hollow nanostructures for catalytic applications. Energy Environ. Sci. 2013, 6, 2082−2092. (22) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR. J. Phys. Chem. B 2003, 107, 4545−4549. (23) Kondo, J. N.; Yamashita, T.; Nakajima, K.; Lu, D.; Hara, M.; Domen, K. Preparation and crystallization characteristics of mesoporous TiO 2 and mixed oxides. J. Mater. Chem. 2005, 15, 2035−2040. (24) Luo, Z.; Poyraz, A. S.; Kuo, C.-H.; Miao, R.; Meng, Y.; Chen, S.-Y.; Jiang, T.; Wenos, C.; Suib, S. L. Crystalline Mixed Phase (Anatase/ Rutile) Mesoporous Titanium Dioxides for Visible Light Photocatalytic Activity. Chem. Mater. 2015, 27, 6−17.

phases where (i) the high crystallinity reduces the defects and traps in mTiO2 where the recombination of excitons normally occurs21 and (ii) the presence of anatase/rutile interface separates excitons to diminish the recombination within anatase phase.24 In summary, we demonstrated a straightforward approach to fabricating thermally stable, periodically ordered, and highly crystalline mesoporous (mixed) transition-metal oxides via a CAM-templating approach. We highlighted that the utilization of colloidal silica-like nanoparticles with hydrophilic PEO tethers prepared from silane-containing hybrid BCP largely enhanced the thermal/mechanical stability of templates to prevent the structural reconstruction and pore collapse during calcination at higher temperature. The application of CAMs as soft templates could open up a new realm of possibilities to directly manipulate the crystallinity and AGSs of mesoporous oxides, as well as the controllability of crystalline phase transition. We are currently expanding the scopes of hybrid polymer micelles to control the porosity and inorganic sources to grow other types of mesoporous oxides. The synthetic strategy may stand out as a new methodology to build up highly crystalline mesostructured materials with potential applications for photovoltaics, photocatalysis, and energy storage.

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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.chemmater.5b02248. Synthetic procedures of polymers and mesoporous materials and characterization results for nanostructures and crystal phases of TiO2, ZrO2, and Srilankite (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.L.S.). *E-mail: [email protected] (J.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H. thanks the financial support of startup funds from the University of Connecticut. S.L.S. acknowledges support of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical, Biological and Geological Sciences, under Grant DE-FG02-86ER13622.A000. The authors thank Dr. Haoquan Zheng (Stockholm University) for his helpful discussion on SAXS results and Dr. Lichuan Zhang for his assistance on TEM characterizations. The used P25 was a gift kindly provided by Evonik Industry and Dr. Rick Muisener. This work was also partially supported by Research Excellence Program Awards of UConn and the Green Emulsions Micelles and Surfactants (GEMS) Center.

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REFERENCES

(1) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Generalized syntheses of large-pore mesoporous metal oxides with semicrystalline frameworks. Nature 1998, 396, 152−155. (2) Wang, D.; Choi, D.; Yang, Z.; Viswanathan, V. V.; Nie, Z.; Wang, C.; Song, Y.; Zhang, J.-G.; Liu, J. Synthesis and Li-ion insertion properties of highly crystalline mesoporous rutile TiO2. Chem. Mater. 2008, 20, 3435−3442. D

DOI: 10.1021/acs.chemmater.5b02248 Chem. Mater. XXXX, XXX, XXX−XXX