Facile Synthesis of Ordered Mesoporous Carbons with High Thermal

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Langmuir 2008, 24, 7500-7505

Facile Synthesis of Ordered Mesoporous Carbons with High Thermal Stability by Self-Assembly of Resorcinol-Formaldehyde and Block Copolymers under Highly Acidic Conditions Xiqing Wang, Chengdu Liang, and Sheng Dai* Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201 ReceiVed February 18, 2008. ReVised Manuscript ReceiVed April 2, 2008 The effect of phenols reactivity with formaldehyde on the formation of ordered mesoporous carbons has been investigated. A strategy to accelerate the polymerization of phenolic resins by using strongly acidic conditions is proposed. The self-assembly of resorcinol-formaldehyde and block copolymers (e.g., F127) under highly acidic concentrations (e.g., 1.5 M HCl) is probably driven by the I+X-S+ mechanism and hydrogen bonding and affords a highly reproducible approach for synthesis of ordered mesoporous carbons. The synthesis can be readily scaled up with no change in sample quality. The carbon material obtained (denoted as C-ORNL-1) exhibits highly ordered hexagonal mesostructure, with a typical BET surface area of ∼600 m2/g, pore size of 6.3 nm, and pore volume of ∼0.60 cm3/g. One of the unique structural features of C-ORNL-1 is its high thermal stability; it can be graphitized at 2600 °C while considerable mesoporosity is maintained.

1. Introduction Since the first report in 1999,1 ordered mesoporous carbons (OMCs) have received considerable attention because of their unique structural features and many promising applications.2,3 OMCs are usually prepared by the nanocasting method using mesoporous silica materials as hard templates. However, this multistep and costly process is difficult to employ for the largescale manufacturing of OMCs. This deficiency has stimulated the search for facile and reproducible routes for the preparation of OMCs.4–7 It has been shown that disordered porous carbons can be made by the carbonization of polymer blends with different thermal stabilities, in which a carbonizing polymer forms carbon networks while a pyrolyzing polymer decomposes to generate pores at high temperatures.8 By applying this concept, we have successfully prepared a highly ordered porous carbon film with a 2D hexagonal structure.9 Resorcinol was preorganized with a polystyrene-block-poly(4-vinylpyridine) scaffold via hydrogen bonding into a mesoscopically ordered film, followed by the formation of a nanostructured resorcinol-formaldehyde resin (RFR) through the in situ polymerization of resorcinol with formaldehyde vapor. A crack-free ordered porous carbon film was derived from the carbonization of this nanostructured RFR. According to the terminology for synthesis of mesoporous silicas, this self-assembly process can be classified as I0S0.10,11 We and several other groups have recently extended the above synthesis to the use of commercially available Pluronic block * Corresponding author. Tel: 1-876-576-7307. Fax: 1-865-576-5235. E-mail: [email protected]. (1) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (2) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. AdV. Mater. 2001, 13, 677. (3) Lee, J.; Kim, J.; Hyeon, T. AdV. Mater. 2006, 18, 2073. (4) Moriguchi, I.; Ozono, A.; Mikuriya, K.; Teraoka, Y.; Kagawa, S.; Kodama, M. Chem. Lett. 1999, 1171. (5) Liang, C. D.; Li, Z. J.; Dai,S. Angew. Chem., Int. Ed. 2008, 47, 3696. (6) Wan, Y.; Shi, Y. F.; Zhao, D. Y. Chem. Mater. 2008, 20, 932. (7) Li, Z. J.; Yan, W. F.; Dai, S. Carbon 2004, 42, 767. (8) Ozaki, J.; Endo, N.; Ohizumi, W.; Igarashi, K.; Nakahara, M.; Oya, A.; Yoshida, S.; Iizuka, T. Carbon 1997, 35, 1031. (9) Liang, C. D.; Hong, K. L.; Guiochon, G. A.; Mays, J. W.; Dai, S. Angew. Chem., Int. Ed. 2004, 43, 5785. (10) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gler, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 1994, 368, 317. (11) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024.

copolymers (e.g., F127, P123, and F108) as templates.12–18 Notably, we prepared hexagonal OMCs in the forms of monoliths, films, sheets, and fibers using phloroglucinol-formaldehyde as the carbon precursor and F127 as the template under mildly acidic conditions.12 Tanaka et al. synthesized a 2D hexagonal mesoporous carbon COU-1 using resorcinol-formaldehyde and triethyl orthoacetate (EOA) as the carbon coprecursors and F127 as the template.14 Zhao and his co-workers reported the systematic synthesis of a series of OMCs with various mesostructures by the organization of presynthesized phenol-formaldehyde resol with Pluronic block copolymers via the evaporation induced self-assembly (EISA) process under near neutral pH condition or by a dilute aqueous route via the self-assembly of phenol-formaldehyde and Pluronic block copolymers under weakly basic conditions.16,17 Song and co-workers prepared a cubic Im3jm OMC from resorcinol-formaldehyde and F108 under basic conditions via the EISA process.18 Although significant progress on the synthesis of OMCs via the soft-template method has been achieved recently,9,12–18 the understanding of the formation mechanism is still in the early stage. Compared to the hard template method, in which the mesostructure and structural ordering of carbon materials are determined by their silica parent templates, the soft-template method involves the cooperative self-assembly of surfactant (e.g., block copolymer) with polymeric carbon precursors (e.g., phenolic resin). In analogy to the synthesis of ordered mesoporous silicas, the self-assembly of surfactant templates directs the polymerization and organization of the carbon precursor, leading to the formation of mesostructured surfactant-polymer nanocomposites. The hydrogen bonding between block copolymer and (12) Liang, C. D.; Dai, S. J. Am. Chem. Soc. 2006, 128, 5316. (13) Steinhart, M.; Liang, C. D.; Lynn, G. W.; Gosele, U.; Dai, S. Chem. Mater. 2007, 19, 2383. (14) Tanaka, S.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Chem. Commun. 2005, 2125. (15) Tanaka, S.; Katayama, Y.; Tate, M. P.; Hillhouse, H. W.; Miyake, Y. J. Mater. Chem. 2007, 17, 3639. (16) Meng, Y.; Gu, D.; Zhang, F. Q.; Shi, Y. F.; Yang, H. F.; Li, Z.; Yu, C. Z.; Tu, B.; Zhao, D. Y. Angew. Chem., Int. Ed. 2005, 44, 7053. (17) Zhang, F. Q.; Meng, Y.; Gu, D.; Yan, Y.; Yu, C. Z.; Tu, B.; Zhao, D. Y. J. Am. Chem. Soc. 2005, 127, 13508. (18) Liu, C. Y.; Li, L. X.; Song, H. H.; Chen, X. H. Chem. Commun. 2007, 757.

10.1021/la800529v CCC: $40.75  2008 American Chemical Society Published on Web 06/12/2008

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Figure 1. TGA profile of the F127-phenolic resin nanocomposite and the evolving gases detected by mass spectrometry: water vapor, CO2, H2, and HCl. N2 was used as a carrier gas.

Therefore, the concentration of acid and the reactivity of phenols are two key factors in determining the polymerization rate of phenolic resins. In our previous study, the latter factor was investigated using different phenolic precursors. The different reactivities of these precursors had a profound effect on the polymerization rate of phenolic resins and ultimately the structures of the resulting carbon materials.12 Under a slightly acidic condition (e.g., 0.01 M HCl), the organization of phloroglucinolformaldehyde in the presence of F127 was able to produce highly ordered mesoporous carbon while the polymerization of resorcinol and phenol with formaldehyde took a long time and the resulting carbon materials exhibited poor structural order. The reactivity of phenols with formaldehyde follows a trend: phloroglucinol . resorcinol . phenol. Therefore, one strategy to enhance the polymerization rate, if phenols other than phloroglucinol are used, is to use stronger acidic conditions. Herein, we report a facile and highly reproducible synthesis of OMC using resorcinol-formaldehyde as the carbon precursor under highly acidic conditions. Another advantage of using highly acidic conditions is the possibility to assist the above self-assembly process through Coulombic interaction (i.e., I+X-S+ mechanism). The resulting OMC exhibits extremely high thermal stability and can undergo graphitization at 2600 °C to produce highly graphitic mesoporous carbon.

2. Experimental Section

Figure 2. Nitrogen sorption isotherm (A) and XRD pattern (B) of C-ORNL-1. The pore size distribution (PSD) plot (insert in A) was calculated from the adsorption branch based on the BJH method. Table 1. Structural Properties of C-ORNL-1-xa materials

a (nm)

pore size (nm)

surface area (m2/g)

pore volume (cm3/g)

850 1800 2200 2400 2600

12.24 12.20 -

6.3 6.2 6.3 6.4 6.6

607 390 371 288 230

0.58 0.46 0.47 0.37 0.30

a Unit cell parameter a ) 2/3d100, pore size refers to the maximum of the pore size distribution plot based on the BJH method, and wall thickness ) a - pore size.

phenolic resin has been acknowledged to act as a driving force for the formation of ordered mesostructures.9,12–19 However, the cooperative effect of degree of polymerization of phenolic resins on the resulting mesostructures has not received much attention. Under acidic conditions, the reaction and polymerization of phenols (e.g., phloroglucinol, resorcinol, and phenol) with formaldehyde involve the protonation of hydrated formaldehyde and electrophilic aromatic substitution reactions of phenols.20 (19) Hu, Q. Y.; Kou, R.; Pang, J. B.; Ward, T. L.; Cai, M.; Yang, Z. Z.; Lu, Y. F.; Tang, J. Chem. Commun. 2007, 601. (20) Gardziella, A.; Pilato, L. A.; Knop, A. Phenolic Resins; Springer: New York, 2000.

Mesoporous carbons with highly ordered structures can be made in a wide composition range of weight ratios of (1.1 resorcinol):(1.1 F127):(0.48 formaldehyde):(3.55-8.2 ethanol):(5.1-1.67 water): (0.16-0.66 HCl). In a typical synthesis, 1.1 g of resorcinol and 1.1 g of F127 were dissolved in 4.5 mL of EtOH and 4.5 mL of HCl aqueous solution (3.0 M). To this solution, 1.3 g of formaldehyde (37%) was then added. After stirring for about 11 min at room temperature, the clear mixture turned turbid, indicating the formation of RF-F127 nanocomposite and a phase separation. The polymerrich gel phase12 was obtained by centrifugation at 9500 rpm for 4 min after the mixture was stirred for 40 min. The gel was then loaded on a large Petri dish, dried at room temperature overnight, and subsequently cured at 80 and 120 °C for 24 h each. The TGA-MS (thermogravimetric analysis-mass spectrometry) profiles of assynthesized sample were recorded on a TGA 2850 thermogravimetric analyzer (TA Instruments, Inc.) equipped with a mass spectrometer (Pfeiffer-Vacuum, Inc.) using N2 (99.999%) as a carrier gas and a heating rate of 10 °C/min. Carbonization was carried out under N2 atmosphere at 400 °C for 2 h with a heating rate of 1 °C/min, which was followed by further treatment at 850 °C for 3 h with a heating rate of 5 °C/min. The resulting carbon material was denoted as C-ORNL1. N2 sorption analysis was performed on a Micromeritics Germini analyzer at -196 °C (77 K). Prior to measurement, the sample was degassed in a vacuum oven at 120 °C overnight. The specific surface area was calculated using the BET method from the nitrogen adsorption data in the relative range (P/P0) of 0.06-0.30. The total pore volume was determined from the amount of N2 uptake at P/P0 ) 0.95. The pore size distribution (PSD) plot was derived from the adsorption branch of the isotherm based on the BJH model. XRD patterns were recorded on a Siemens D5005 diffractometer operating at 40 kV and 40 mA. Both SEM and TEM images were taken on a Hitachi HD2000 STEM microscope operating at 200 kV under SE and TE modes, respectively. The heat treatment of C-ORNL-1 was carried out in a high-temperature furnace (Thermal Technology Inc.) under helium atmosphere at desired temperatures for 1 h with a heating rate of 20 °C/min.

3. Results and Discussion Figure 1 shows the TGA-MS profiles of the F127-phenolic resin nanocomposite under N2 atmosphere. The block copolymer template (F127) was gradually decomposed starting from 250 °C with concomitant evolution of water, CO2, and HCl. As seen

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Figure 3. High-resolution SEM image (A) and TEM images of C-ORNL-1 along the [001] (B) and [110] (C) directions.

Figure 4. Low-angle (A) and wide-angle (B) XRD patterns, N2 sorption isotherms (C), and pore size distribution plots (D) of C-ORNL-1-x. For clarity, the N2 sorption isotherm of C-ORNL-1-1800 was shifted up by 50 cm3 STP/g (C).

from Figure 1, the decomposition of the template was complete at 400 °C, giving rise to a mesoporous polymer.9,16,17 The carbonization of the corresponding mesoporous polymer was carried out at higher temperatures, which was accompanied by the evolution of H2 gas, finally resulting in a mesoporous carbon (C-ORNL-1) at 850 °C. C-ORNL-1 exhibits a type IV N2 sorption isotherm with a sharp capillary condensation step at relative pressure from 0.4 to 0.7 and a narrow pore size distribution, centered at 6.3 nm (Figure 2A). The BET surface area and pore volume are 607 m2/g and 0.58 cm3/g, respectively. As shown in Figure 2B, C-ORNL-1 displays three well-resolved XRD peaks, which can be indexed into 100, 110, and 200 reflections of the 2D hexagonal symmetry (p6mm), indicating a highly ordered mesostructure. The unit cell parameter a is calculated to be 12.24 nm and the wall thickness to be 5.94 nm. However, the carbon

framework wall of C-ORNL-1 is amorphous, as indicated by its wide-angle XRD pattern (Figure 4B). The highly ordered 2D hexagonal structure of C-ORNL-1 is further revealed by the high resolution SEM and TEM images. As shown in Figure 3, long-range hexagonal arrangement of porous structure is clearly visible along both the [001] and [110] directions. The cell unit parameter, pore size, and wall thickness of C-ORNL-1 estimated from the images are 12.2, 6.2, and 6.0 nm, respectively, which are in good agreement with the values determined from the N2 adsorption and XRD results. Unlike the OMCs made via hardtemplate method, whose wall thicknesses are determined by the pore sizes of silica templates,2,3 the thickness values of C-ORNL-1 (∼6 nm observed in our current study) are related to the domain sizes of phenolic resins structured by soft templates and the volume ratios of phenolic resins to F127 templates. One of the interesting features of C-ORNL-1 is its extremely high thermal stability. Parts A and B of Figure 4 show both the low-angle and wide-angle XRD patterns of C-ORNL-1-x (herein x refers to the temperature) after heat-treatment at different temperatures, ranging from 1800 to 2600 °C. Remarkably, C-ORNL-1 still exhibits a strong XRD peak at 2θ around 0.8° after being heated even up to 1800 °C. The low-angle XRD peak becomes less visible with an increase of heat-treatment temperature, suggesting a gradual loss of mesostructural order. However, the peak position does not shift to larger angle, indicating there is no structural shrinkage. While the wide-angle XRD patterns of C-ORNL-1-x clearly indicate the gradual development of graphitic character of carbon wall. The N2 sorption isotherms (Figure 4C) of C-ORNL-1-x show typical type IV curves, suggesting that the mesoporosity is preserved, even being heated at 2600 °C. However, the N2 uptake as well as the BET surface area (Table 1) of C-ORNL-1-x decreases with the increasing heat-treatment temperature. The pore-size distribution plots (Figure 4D) of C-ORNL-1-x show almost identical pore diameters for all samples, although the mesopores become broader as the heat-treatment temperature increases (see also in Table 1). Figure 5A-F shows the high-resolution SEM images and TEM images of C-ORNL-1-x. The obvious hexagonal arrangement of mesopores is still observed for C-ORNL-1-1800, suggesting that an ordered mesostructure is maintained, which is in good agreement with the results of XRD and N2 sorption analysis. Although the mesoporous carbon materials being heated at higher temperatures (2200-2600 °C) exhibit wormy structures (Figures 5C-F), all of the above data indicate that C-ORNL-1 can be graphitized at 2400–2600 °C to form a highly graphitic mesoporous carbon without considerable loss of mesoporosity and BET surface area. This observed high stability associated

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Figure 5. High-resolution SEM images (A, C, E, F) and TEM images (B, D) of C-ORNL-1-x after heat treatment at different temperatures.

with the mesostructure of C-ORNL-1 is in sharp contrast to the mesoporous carbons derived from other methodologies (e.g., soft-templating synthesis under neutral conditions).16,17 For example, the series of mesoporous carbons C-FDU-x prepared by Zhao and co-workers at neutral pH or weakly basic conditions underwent a structural shrinkage upon treatment at elevated temperatures, accompanying a decrease in pore size and an increase in BET surface area, which was due to the development of microporosity.16,17 The cubic mesoporous carbon synthesized under basic conditions by Song and co-workers displayed a structural loss even at 700 °C.18 The high thermal stability of C-ORNL-1 can be attributed to the highly cross-linked resorcinol-formaldehyde polymer and the resulting rigid carbon framework. The thick carbon wall may also be another reason for its high thermal stability. ThesuccessfulsynthesisofOMCusingresorcinol-formaldehyde as the carbon precursor can be attributed to the enhanced degree

of polymerization that resulted from the increased reaction rate. As a control experiment, catechol was selected to replace resorcinol, since catechol is expected to provide identical H-bonding with block copolymers as resorcinol. At 1.5 M HCl, it took about 3 weeks for the appearance of phase separation, as an indication of formation of block copolymer-catecholformaldehyde nanocomposite. However, the resulting carbon material exhibited very poor mesostructure (data not shown). When the concentration of HCl was increased to 2 M, the phase separation occurred after reaction for about 10 days and the carbon obtained thereafter displayed a well-ordered hexagonal mesostructure, as indicated by the steep type IV N2 sorption isotherm and SEM and TEM images (Figure 6). The fundamental guiding principles for synthesizing mesoporous metal-oxide materials via soft-template methodologies have been well developed.10 Different self-assembly schemes, such as I-S+, I+X-S+, and I0S0, have been utilized for the

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Figure 6. N2 sorption isotherm (A), SEM (B), and TEM (C) images of mesoporous carbon material made from catechol-formaldehyde-F127 (denoted as C-ORNL-1-c). The inset in part A is the PSD plot.

Figure 7. N2 sorption isotherm (A) and the PSD plot (B) of C-ORNL-1 prepared in a large batch. To increase the homogeneity of the sample, the polymer-rich phase obtained after centrifugation was dissolved in a mixture of 12 g of THF and 8 g of EtOH before being loaded on a substrate. For comparison, the curves of carbon made in the small batch are shown also.

synthesis of mesoporous oxide materials under different pH conditions.10 Currently, only self-assembly induced by hydrogen bonding (I0S0 scheme in oxide synthesis) has been successfully utilized for making mesoporous carbon materials. Although we7 and others4 explored the self-assembly synthesis of mesoporous carbon materials via Coulombic interaction (I-S+ scheme in oxide synthesis), no ordered and stable mesostructured carbon materials were derived from such a synthesis method. Two of the key issues are (a) uncontrollable fast coprecipitation of the strongly interacting polymer precursors and micellar templates and (b) difficulty in removal of cationic micellar templates without destruction of pore structures. The pKa values of most phenols (e.g., phloroglucinol, resorcinol, catechol, and phenol) are around 9.8.21 Under highly acidic conditions, phenols and cross-linked phenols are therefore protonated. The EO blocks of the Pluronic block copolymers are also protonated under these conditions.

Consequently, the highly acidic reaction conditions not only promote the polymerization rate of phenolic resins but also induce Coulombic interactions in the self-assembly of surfactant-polymer nanocomposites via the I+X-S+ mechanism.11 The presence of Cl- as a mediator in our synthesis of mesoporous carbons was experimentally validated by the observed evolution of HCl gas during the thermal treatment of the polymeric precursor (Figure 1). C-ORNL-1 with highly ordered hexagonal structure can be made in a wide range of acid concentrations (e.g., 0.5-2.0 M HCl in an ethanol-H2O mixture), while the attempt at low concentrations of HCl (e.g., 0.01 M) in our previous report12 gave only a porous carbon with mainly microporosity. The higher the acid concentrations that were used, the shorter the phase separation that was observed, as an indication of faster polymerization rates (see details in the Supporting Information). It should be noted that the synthesis recipe described here is suitable for large-scale synthesis. Figure 7 shows the N2 sorption isotherm of C-ORNL-1 prepared in a large batch (20 times the one shown in Figure 1 while the same mass ratios of precursors were maintained). The well-matched isotherms indicate no change in sample quality due to an increase of batch size and high reproducibility of the synthesis.

4. Conclusions In summary, we have investigated the effect of the phenols reactivity under different acid concentrations on the formation of mesoporous carbons and proposed a strategy to accelerate the polymerization of phenols with formaldehyde by using strongly acidic conditions. The self-assembly of the polymeric precursor under highly acidic concentrations is probably driven by the I+X-S+ mechanism along with I0S0. The better understanding (21) Olasz, A.; Mignon, P.; De Proft, F.; Veszpremi, T.; Geerlings, P. Chem. Phys. Lett. 2005, 407, 504.

Facile Synthesis of Ordered Mesoporous Carbons

of the interaction and self-assembly between surfactants and carbon precursor polymers will pave the way to the synthesis of OMCs with controllable structural features. The cooperative selfassembly of F127 and resorcinol-formaldehyde affords a simple and highly reproducible synthesis of a highly ordered mesoporous carbon, C-ORNL-1, with 2D hexagonal symmetry. The procedure described here allows a large-scale preparation of OMC. C-ORNL-1 exhibits extremely high thermal stability and can be graphitized at 2400-2600 °C. Highly graphitic wall structure coupled with uniform mesopore size and large BET surface area would promise C-ORNL-1 great potential for many applications, such as fuel-cell catalyst-supports.22 In addition, the carbon surface can be further modified with desired functional groups7,23–29 for some other applications. (22) Chang, H.; Joo, S. H.; Pak, C. J. Mater. Chem. 2007, 17, 3078. (23) Li, Z. J.; Del Cul, G. D.; Yan, W. F.; Liang, C. D.; Dai, S. J. Am. Chem. Soc. 2004, 126, 12782. (24) Li, Z. J.; Dai, S. Chem. Mater. 2005, 17, 1717. (25) Liang, C. D.; Huang, J. F.; Li, Z. J.; Luo, H. M.; Dai, S. Eur. J. Org. Chem. 2006, 586.

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Acknowledgment. This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. Supporting Information Available: The results of synthesis of mesoporous carbons at different HCl concentrations. This material is available free of charge via the Internet at http://pubs.acs.org. LA800529V

(26) Wang, X. Q.; Liu, R.; Waje, M. M.; Chen, Z. W.; Yan, Y. S.; Bozhilov, K. N.; Feng, P. Y. Chem. Mater. 2007, 19, 2395. (27) Shin, Y. S.; Fryxell, G. E.; Um, W.; Parker, K.; Mattigod, S. V.; Skaggs, R. AdV. Funct. Mater. 2007, 17, 2897. (28) Shin, Y. S.; Fryxell, G. E.; Johnson, I. C. A.; Haley, M. M. Chem. Mater. 2008, 20, 981. (29) Wan, Y.; Qian, X. F.; Jia, N. Q.; Wang, Z. Y.; Li, H. X.; Zhao, D. Y. Chem. Mater. 2008, 20, 1012.