Mesoporous Carbons via the

Apr 4, 2013 - ... e-mail [email protected] (M.Z.), [email protected] (J.C.). .... Functional mesoporous carbons from template methods for energy ...
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Fabrication of Hierarchical Macroporous/Mesoporous Carbons via the Dual-Template Method and the Restriction Effect of Hard Template on Shrinkage of Mesoporous Polymers Nianwu Li,†,§ Mingbo Zheng,*,†,‡,§ Shaoqing Feng,† Hongling Lu,† Bin Zhao,‡ Jiafei Zheng,† Songtao Zhang,† Guangbin Ji,† and Jieming Cao*,† †

Nanomaterials Research Institute, College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ‡ Nanjing National Laboratory of Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: A series of hierarchically ordered macro-/ mesoporous polymer resins and macro-/mesoporous carbon monoliths were synthesized using SiO2 opal as a hard template for the macropore, amphiphilic triblock copolymer PEO− PPO−PEO as a soft template for the mesopore, and phenolic resin as a precursor for the polymer or carbon. The obtained hierarchical macro-/mesoporous frameworks had highly periodic arrays of uniform macropores that were surrounded by walls containing the mesoporous structures. The mesoporous structure of the walls was adjusted using different precursors for the synthesis of FDU-14, FDU-15, and FDU-16. Results of the N2 adsorption−desorption analysis showed that the Brunauer−Emmett−Teller surface areas, the pore volumes, and the mesopore sizes of the macro-/mesoporous carbons were much larger than those of the FDU-14, FDU-15, and FDU-16 carbon materials. The mesopore size of the samples clearly increased with the increasing heat-treatment temperature when the temperature was below 700 °C. The results indicate that the SiO2 hard template successfully restricted the shrinkage of the framework during the thermosetting and carbonization process.

1. INTRODUCTION The synthesis of carbon materials with ordered mesoporous structures has attracted considerable interest because of their various applications in adsorbents, catalyst supports, sensors, electrode materials, etc.1−7 Ordered mesoporous carbons (OMC) with various morphologies have been synthesized using different types of ordered mesoporous SiO2 hard templates, such as MCM-48, SBA-15, HMS, MSU-H, and KIT-6.8−12 The generation of mesoporous carbon from SiO2 templates involves two main steps: (1) the assembly of surfactants and SiO2 precursors and the removal of the surfactants by calcination or solvent extraction; (2) the impregnation of mesoporous SiO2 templates with carbon precursors, followed by the carbonization and etching of the SiO2 framework with an HF or NaOH solution.13,14 The hard template procedure is rather complex, expensive, and timeconsuming. OMCs with various pore structures have also been synthesized via direct organic−organic assembly using amphiphilic block copolymers as the template and phenolic resin as the precursor.15−20 Zhao et al.19 successfully synthesized a family of highly ordered mesoporous polymers and carbon frameworks via the organic−organic assembly of triblock copolymers (Pluronic F127 or P123) and phenolic © 2013 American Chemical Society

resin precursors by the solvent evaporation induced selfassembly (EISA21) pathway. Diverse mesostructures, such as the two-dimensional p6̅m hexagonal (FDU-15), three-dimensional Ia3̅d bicontinuous (FDU-14), and three-dimensional Im3̅m body-centered cubic (FDU-16) structures, are obtained by simply varying the mass ratio of the polymer precursors and the amphiphilic surfactants. The evaporation of ethanol enriches the concentration of the copolymer and drives the organization of resol−copolymer composites into an ordered liquid-crystalline mesophase. A rigid covalently bonded construction from the simple thermopolymerization of phenolic resins at a relatively low temperature (100 °C) made the mesoporous structures rather stable at high temperature (900−1400 °C). These mesoporous carbon materials display highly ordered structures, high specific surface areas, large pore volumes, and uniform mesopore sizes. Compared with the two-step hard template procedure, this direct synthetic process is much easier and more cost-effective. Unfortunately, a significant structural shrinkage is generated Received: December 25, 2012 Revised: April 3, 2013 Published: April 4, 2013 8784

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during the thermosetting and carbonization process, such that the pore sizes of the mesoporous carbons are usually less than 4 nm. Thus, a feasible method needs to be developed to prevent the shrinkage of the mesostructure and to enlarge the mesopore sizes. Hierarchical porous carbon materials containing interconnected mesoporous and three-dimensionally ordered macroporous (3DOM) structures have been given much attention because they combine the efficient transport from macropores and the high surface area from mesopores.22,23 Colloidal crystals, including SiO2, poly(methyl methacrylate) (PMMA), or polystyrene (PS), can serve as desirable templates for 3DOM carbons.24−26 To date, hierarchically ordered carbons have been obtained by combining colloidal crystals and SiO2 nanoparticles or surfactants.27−29 Chai and co-workers22 synthesized 3DOM carbon with mesoporous walls templated by aggregates of PS spheres and silica particles. Kanamura et al.30 prepared 3DOM carbons that included large macropores and small spherical pores from a bimodal polymer−SiO2 colloidal crystal. However, these procedures are very complex and time-consuming. Stein’s group31 synthesized glassy carbon monoliths with ordered macropores and macroporous walls containing mesopores by nanocasting monolithic SiO2 with a hierarchical pore structure. The same group synthesized macroporous monoliths of the carbon/SiO2 composite with hierarchical porosity by combining triconstituent self-assembly with the use of a PMMA colloidal crystal template. The carbon/SiO2 monoliths can be converted into carbon monoliths with dual porosity after the extraction of SiO2 with HF.32 Zhao’s group33 prepared hierarchical macro-/mesoporous carbon (HMMC) monoliths using Pluronic F127 as a soft template, SiO2 opal as a hard template, and soluble resols as the carbon source. This method is much more direct and facile than the previous approaches. Similarly, our group synthesized HMMC monolith using Pluronic F127 and SiO2 opal as templates.34a Furthermore, the novel restriction effect of the hard template on the shrinkage of mesoporous polymers during the carbonization was proposed by our group.34b Most recently, Bein’s group also reported the restriction effect of anodic alumina membranes pores on the shrinkage of mesoporous polymers.35,36 In this work, we report a systematic study of the preparation of a series of hierarchical macro-/mesoporous polymer (HMMP) and HMMC monoliths with large mesopore sizes, high Brunauer−Emmett−Teller (BET) surface areas, and large pore volumes by combining organic−organic assembly with the use of SiO2 opal template. What’s more, the restriction effect of SiO2 opal on the shrinkage of mesoporous polymers was systematically investigated. The ordered mesopores are generated from the organic−organic self-assembly of phenolic resins and amphiphilic triblock copolymers (F127 or P123) in the voids of the SiO2 opal; the highly periodic arrays of uniform macropores are obtained from the accurate replication of SiO2 opal (Scheme 1). The mesoporous structures were controlled by different precursors for the synthesis of FDU-15, FDU-14, and FDU-16. The effects of the heat-treatment temperature on the mesopore sizes, BET surface areas, and pore volumes are systematically investigated.

Scheme 1. Schematic Illustration for the Preparation of HMMC

under basic conditions according to Stöber’s method.37 All of the microspheres were uniform in size with deviations of less than 5%. The resultant SiO2 microspheres were washed with ethanol six times and dispersed in an ethanol solution. SiO2 opal was obtained by self-assembling SiO2 microspheres using a vertical deposition technique.38 The SiO2 opal was sintered at 800 °C for 3 h to enhance its mechanical strength and to form small necks between neighboring SiO2 microspheres that provide tunnels to connect macropores in the carbon replica. 2.2. Preparation of Resol Precursor. Soluble lowmolecular-weight resol was synthesized from phenol and formaldehyde via a base-catalyzed method.19a A typical preparation procedure is as follows: Phenol (2.44 g) was melted at 41 °C in a round-bottom flask, and a 20 wt % NaOH (2.6 g) solution was slowly added with stirring. Formalin (37 wt %, 4.2 g) containing 1.55 g of formaldehyde was then added dropwise at 41 °C. After further stirring at 70 °C for 1 h, the reaction mixture was cooled to room temperature. The pH of the mixture was adjusted to 7.0 using 0.6 M HCl solution. Water was removed by vacuum evaporation at 43 °C. The final product was dissolved in 36.0 g of ethanol for further use. About 40.0 g of resol−ethanol solution was obtained. 2.3. Preparation of HMMP and HMMC. HMMP and HMMC were synthesized via a solvent EISA method using the SiO2 colloidal crystal as a hard template, the amphiphilic triblock copolymer F127 or P123 as a soft template, and soluble resol as a carbon source. 2.3.1. HMMP-1 and HMMC-1. The precursor solution was the same as the F127/resol ethanol solution for the synthesis of FDU-15.19a For a typical synthesis, 1.0 g of F127 was dissolved in 20.0 g of ethanol, and then 10.0 g of resol−ethanol solution containing 1.0 g of resol was added by stirring for 10 min to obtain a homogeneous solution. The solution was transferred to an evaporating dish and used to soak 1.0 g of SiO2 opal, which was composed of SiO2 microspheres that were 290 nm in diameter. After the evaporation of ethanol for 24 h at room temperature, the composites of F127/resol/SiO2 were carefully taken out and heated in an oven at 100 °C for 24 h. The obtained samples were calcined at different temperatures (350, 500, 700, and 900 °C) under a N2 atmosphere for 3 h with a heating rate of 1 °C/min. The SiO2 template was removed by using a HF solution (10 wt %), followed by washing with distilled water for six times and drying in air at 80 °C. The corresponding products were labeled as HMMP-1-350, HMMP-1-500, HMMC-1-700, and HMMC-1-900. Meanwhile,

2. EXPERIMENTAL SECTION 2.1. Preparation of SiO2 Opal. Monodisperse SiO2 microspheres were synthesized by tetraethyl orthosilicate hydrolysis and the subsequent seed growth polymerization 8785

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we prepared HMMC-1-700 using the SiO2 template, which was composed of microspheres that were 400 nm in diameter. 2.3.2. HMMP-2 and HMMC-2. The precursor solution was the same as the P123/resol ethanol solution for the synthesis of FDU-14.19b During the synthesis, 1.0 g of P123 was dissolved in 20.0 g of ethanol, and then 15.0 g of the resol−ethanol solution containing 1.5 g of resol was added by stirring for 10 min. After a homogeneous solution was obtained, the preparation was carried out using the same procedure as HMMC-1. The respective final products were labeled as HMMP-2-350, HMMC-2-700, and HMMC-2-900. 2.3.3. HMMP-3 and HMMC-3. The precursor solution was the same as the F127/resol ethanol solution of FDU-16.19a The synthesis was performed as follows: 1.0 g of F127 was dissolved in 20.0 g of ethanol, and then 20.0 g of resol−ethanol solution containing 2.0 g of resol was added by stirring for 10 min. After a homogeneous solution was obtained, the preparation was carried out using the same procedure as HMMC-1. The corresponding products were designated as HMMP-3-350, HMMC-3-700, and HMMC-3-900. 2.4. Characterization. The microscopic features of the samples were obtained by a field-emission scanning electron microscope (FE-SEM; LEO 1530) and a transmission electron microscope (TEM; JEOL JEM-2100). The samples were not gold-coated before SEM characterization. For the TEM measurements, the samples were ground to powder form and sonicated in ethanol for a few minutes. The N2 adsorption− desorption analysis was performed using a Micromeritics ASAP 2010 instrument. Specific surface areas were calculated by the BET method. The Barret−Joyner−Halenda (BJH) pore size distribution curves from the adsorption branches of the isotherms were employed to estimate the pore sizes and volumes.

Figure 1. SEM images of (a, b) SiO2 opal template and (c, d) HMMC1-700. TEM images of (e, f) HMMC-1-700.

which is the typical isotherm of polymer.39 The isotherms of the samples obtained at 500, 700, and 900 °C are similar to those of SiO2/HMMP-1-350. The mesopore sizes of the products clearly increased with the increasing temperature below 700 °C. The mesopore size distribution of the SiO2/ HMMC-1-900 is similar to that of SiO2/HMMC-1-700 (Figure 2b), suggesting the almost complete carbonization of the polymer frameworks at 700 °C. After the removal of SiO2 templates by HF etching, the resulting HMMP-1-350 also shows a representative type IV isotherm with clear capillary condensation steps and a type-H1 hysteresis loop (Figure 2c), indicating the mesoporous structure is very well maintained. The BJH average pore size of the product is ∼12.5 nm, which is the same as that of SiO2/HMMP-1-350 (Figure 2d). This finding suggests that no obvious shrinkage of the mesopores occurred after the removal of SiO2 template. For HMMP-1-500 and HMMC-1-700, similar isotherms can be observed. For HMMC-1-900, the hysteresis loop exhibited a small change after the removal of the SiO2 template, which is probably due to the slight collapse of the mesostructure. The wide pore size distribution for HMMC-1-900 (Figure 2d) confirmed the slight collapse of the mesostructure. The structural parameters of all the samples were investigated using the N2 adsorption method, and the results are summarized in Table 1. The mesopore size of the samples increases with increasing temperature until it reaches 700 °C. With the increasing heat-treatment temperature, the BET surface area and pore volume of the hierarchical macro-/mesoporous materials increase, and the values are much larger than those of FDU-15-350, FDU-15-500, FDU-15700, and FDU-15-900.19b This result indicates that the SiO2 template successfully prevents the shrinkage of the mesostructure during the thermosetting and carbonization process. This phenomenon was defined as the restriction effect of hard

3. RESULTS 3.1. HMMP-1 and HMMC-1. The HMMP-1 polymer and HMMC-1 carbon were prepared using the precursor of FDU15. The SiO2 opal, which was composed of SiO2 microspheres with diameters of about 290 nm, was used as the hard template (Figure 1a,b). After the carbonization of the phenolic resin/ F127/SiO2 composite in N2 at 700 °C and the further removal of the SiO2 template, a 3DOM network was observed (Figure 1c), thereby indicating that the SiO2 template was well replicated. The macropores are three-dimensionally interconnected by windows with sizes of ∼30 nm (Figure 1d), which are replicated from the coalescence necks between the SiO2 spheres. The TEM image of HMMC-1-700 shows a highly ordered macroporous structure (Figure 1e). The highmagnification TEM image (Figure 1f) reveals the formation of a quasi-2D hexagonal mesopore structure, with a pore sizes of ∼18 nm in the walls of the macropores, thereby indicating the bimodal porosity in the framework. The mesopore size of HMMC-1 is much larger than that of C-FDU-15 (3.1 nm).19b The N2 adsorption−desorption isotherms and BJH pore size distribution plots were determined from the adsorption branch for the SiO2/HMMP-1, SiO2/HMMC-1, HMMP-1, and HMMC-1 obtained at different temperatures (Figure 2a−d). SiO2/HMMP-1-350 exhibits the typical IV isotherm with a type-H1 hysteresis loop, and the isotherm shows a clear capillary condensation in the range of relative pressure of 0.80− 0.90 (Figure 2a), indicating the existence of uniform mesopores. The adsorption and desorption isotherm of SiO2/ HMMP-1-350 was not closed at the low relative pressures, 8786

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Figure 2. N2 adsorption−desorption isotherms and pore size distribution curves from the adsorption branch of (a, b) SiO2/HMMP-1 and SiO2/ HMMC-1, (c, d) HMMP-1 and HMMC-1. The isotherms of SiO2/HMMP-1-500, SiO2/HMMC-1-700, SiO2/HMMC-1-900, HMMP-1-500, HMMC-1-700, and HMMC-1-900 are vertically offset by 50, 120, 180, 600, 1300, and 2200 cm3 g−1, respectively.

Table 1. Textural Properties of SiO2/Polymer Resins, SiO2/Carbon Composites, and Hierarchical Meso-/Macroporous Materials

a

sample name

BET surf. area (m2/g)

micropore area (m2/g)

mesopore size (nm)

pore vol (cm3/g)

micropore vol (cm3/g)

SiO2/HMMP-1-350 SiO2/HMMP-1-500 SiO2/HMMC-1-700 SiO2/HMMC-1-900 SiO2/HMMC-1-700-400a HMMP-1-350 HMMP-1-500 HMMC-1-700 HMMC-1-900 HMMC-1-700-400a SiO2/HMMP-2-350 SiO2/HMMC-2-700 SiO2/HMMC-2-900 HMMP-2-350 HMMC-2-700 HMMC-2-900 SiO2/HMMP-3-350 SiO2/HMMC-3-700 SiO2/HMMC-3-900 HMMP-3-350 HMMC-3-700 HMMC-3-900

86 77 57 62 60 907 1133 1340 1667 1090 89 80 56 735 1134 1804 84 83 83 653 1067 1528

36 27 21 22 20 262 272 389 533 277 25 24 19 118 251 669 32 28 26 149 255 429

12.5 14.9 18.1 18.1 12.8 12.5 14.9 18.1 18.1 12.8 7.6/12.7 7.6/17.7 9.2/17.7 7.6 7.6/17.7 9.2 9.3 12.1 12.1 9.3 12.1 12.1

0.14 0.16 0.17 0.18 0.16 1.61 2.51 3.63 4.80 3.01 0.15 0.20 0.17 1.24 3.02 3.80 0.11 0.15 0.16 0.92 2.04 3.14

0.02 0.01 0.01 0.01 0.01 0.13 0.13 0.19 0.26 0.13 0.01 0.01 0.01 0.05 0.12 0.32 0.02 0.01 0.01 0.07 0.12 0.20

Used 400 nm SiO2 spheres as templates.

template in our previous work.34b For SiO2/mesoporous materials, the mass contents of SiO 2 are very high. Furthermore, the SiO2 sphere possesses a very small specific surface area because of its solid structure. Consequently, after

the removal of SiO2 template, the BET surface area, micropore area, and pore volume of the samples increase significantly. HMMC-700-400 was also obtained using SiO2 colloidal crystals with a larger diameter of 400 nm as the template (the 8787

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condensation steps at relative pressures of about 0.65−0.75 and 0.8−0.9, respectively, indicating the existence of two sizes of mesopores. The N2 adsorption−desorption isotherms of SiO2/ HMMC-2-700 and SiO2/HMMC-2-900 similarly have two capillary condensation steps. The BJH pore size distribution curves of SiO2/HMMP-2-350, SiO2/HMMC-2-700, and SiO2/ HMMC-2-900 show that these samples have a bimodal mesopore distribution (Figure 5b). During the carbonization process, the average size of the large mesopore obviously increased because of the restriction effect. However, the restriction effect does not obviously affect the size of the small mesopore. After the removal of SiO2 template by HF etching, the isotherms of HMMP-2-350 and HMMC-2-900 clearly change (Figure 5c), thereby indicating the transformation of the mesopore structure. The BJH pore size distributions of HMMP-2-350 and HMMC-2-900 reveal that only the small pore is retained (Figure 5d) because of the fragile mesoporous structure collapses after the removal of SiO2. However, the isotherms and BJH pore size distribution of HMMC-2-700 indicate that the pore structure is maintained after the removal of SiO2, which indicates that the sample obtained at 700 °C possessed a more stable structure. The BET surface area and pore volume of the samples increase with the increase of the heat-treatment temperature (Table 1). Moreover, the values of HMMP-2-350, HMMC-2-700, and HMMC2-900 (Table 1) are also much larger than those of FDU-14350, FDU-14-700, and FDU-14-900, respectively.19b 3.3. HMMP-3 and HMMC-3. The HMMP-3 polymer and HMMC-3 carbon were prepared using the precursor solution of FDU-16. The TEM images of HMMC-3-700 at different magnifications are presented in Figure 6a,b, where the ordered macroporous structure can be clearly observed. HMMC-3-700 possesses a quasi-Im3m body-centered cubic mesoporous structure. The average mesopore size is about 12 nm, which is much larger than that of C-FDU-16 (3.8 nm).19b Figure 7 shows the N2 adsorption−desorption isotherms and BJH pore size distributions from the adsorption branch for HMMP-3 polymer and HMMC-3 carbon obtained at different temperatures. The typical type IV curve with a representative H2-type hysteresis loop and a clear capillary condensation in the P/P0 range of 0.75−0.83 is observed for SiO2/HMMP-3350 (Figure 7a). A large H2-type hysteresis loop with delayed capillary evaporation at a relative pressure of about 0.5 is observed, thereby implying a caged mesopore with a window size smaller than 4.0 nm.41 Compared with SiO2/HMMP-3350, the steps of capillary condensation of the adsorption isotherms for SiO2/HMMC-3-700 and SiO2/HMMC-3-900 shift right, which indicates that the mesopore sizes of these two samples are larger than those of SiO 2/HMMP-1-350. Furthermore, the steps of desorption isotherms also shift right, which implies that the window size of caged mesopores similarly increases. The BJH pore size distribution curves (Figure 7b) from adsorption branches and desorption branches indicate that the size of caged mesopore and its respective window size increase with the increasing heat-treatment temperature. After the removal of SiO2 opal, the isotherms and BJH pore size distributions of HMMP-3-350 and HMMC3-700 barely change (Figure 7d), which indicates that the mesoporous structure of these two samples are stable. For SiO2/HMMC-3-900, the step of desorption isotherm obviously shifts right after the removal of SiO2 template (Figure 7c), which indicates the increased window sizes of some caged mesopores. As the heat-treatment temperature increases, the

sample was labeled as HMMC-700-400). The N2 adsorption− desorption isotherms and BJH pore size distributions from the adsorption branch for the SiO2/HMMC-700-400 are shown in Figure 3. The BJH pore size distribution curves indicate that

Figure 3. N2 adsorption−desorption isotherms and pore size distribution curves from the adsorption branch (inset) of SiO2/ HMMC-1-700-400.

the mesopore size of SiO2/HMMC-700-400 is smaller than that of SiO2/HMMC-1-700-290. The textural parameters of the samples are shown in Table 1. The results show that the BET surface area, pore volume, and mesopore size of the SiO2/ HMMC-1-700-400 are smaller than those of SiO2/HMMC-1700-290. The results indicate that the restriction effect weakens with the increasing size of the voids within the SiO2 template. 3.2. HMMP-2 and HMMC-2. The HMMP-2 polymer and HMMC-2 carbon frameworks were prepared by using the precursor of FDU-14.19b The TEM images of HMMC-2-700 are shown in Figure 4a,b. The ordered macropores were

Figure 4. TEM images of HMMC-2-700.

surrounded by walls with mesoporous structures. The twisted macropores were generated from the ultrasonic vibration during the preparation of homogeneous solution for TEM observation. The HMMC-2-700 exhibits a quasi-2D hexagonal structure. The top view of the structure is hexagonal, whereas the side view of the structure shows a tilted line pattern (marked by the arrow in the Figure 4b). This phenomenon can be confirmed by TEM tilting experiments.40 After 30° tilting along the [110]3DOM axis, the hexagonal arrays of pores (the red circle area in Figure S1a) were replaced by a tilted line pattern (the red circle area in Figure S1b). Furthermore, the mesopores in HMMC-1-700 and HMMC-2-700 are aligned parallel and perpendicular to the sphere surface, respectively. This difference is attributed to the different precursors. The N2 adsorption−desorption isotherm (Figure 5a) for SiO2/HMMP-2-350 shows a type IV curve with two capillary 8788

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Figure 5. N2 adsorption−desorption isotherms and pore size distribution curves from the adsorption branch of (a, b) SiO2/HMMP-2 and SiO2/ HMMC-2, (c, d) HMMP-2 and HMMC-2. The isotherms of SiO2/HMMC-2-700, SiO2/HMMC-2-900, HMMC-2-700, and HMMC-2-900 are vertically offset by 50, 150, 200, and 600 cm3 g−1, respectively.

the mesopore size of the obtained mesoporous carbon is very small (less than 4 nm).19b When the complex of the resin and surfactant filled in the voids of the SiO2 template, an interaction between the resin polymer and the surface of SiO2 spheres because both of them had a large number of hydroxyl groups.34b Moreover, the SiO2 template possessed a special 3D structure. The intense interaction and the 3D structure of the SiO2 template restricted the integral shrinkage of polymer during carbonization. Although the 3DOM polymer could not integrally shrink, shrinkage still occurred in the interior of the 3DOM polymer. The mesopore wall of 3DOM polymer continuously shrank with the increasing temperature. Thus, the thickness of the mesopore wall continuously decreased and the mesopore size increased with the increasing temperature. Finally, HMMC with a very large mesopore pore size was obtained. In addition, the micropore contents of the HMMC samples was much lower than those of the FDU-14, FDU-15, and FDU-16 samples because the continuous shrinkage of the mesopore wall reduced the formation of micropores. For the resin polymer, the framework shrinkage was very large when the temperature was below 600 °C because of its organic polymer properties.19a When the temperature was higher than 600 °C, the framework possesses amorphous carbon network structure. Therefore, the framework shrinkage above 600 °C is very slight. The above-mentioned reasons cause the increased mesopore sizes of HMMP and HMMC samples from 350 to 700 °C, with minimal change from 700 to 900 °C. In addition, after the removal of SiO2 template, the adsorption−desorption isotherms of HMMP-350 and HMMC700 barely change (except HMMP-2-350), which indicated that

Figure 6. TEM images of HMMC-3-700.

BET surface area and pore volume of the samples likewise increase (Table 1). The values of these materials (Table 1) are also much larger than those of FDU-16-350, FDU-16-700, and FDU-16-900.19b This phenomenon is due to the restriction effect of hard templates during heat treatment.

4. DISCUSSION The BJH pore size distribution curves indicate that the mesopore sizes of the HMMC samples are evidently larger than those of the corresponding samples of FDU-14, FDU-15, and FDU-16 that were obtained at the same heat-treatment temperature. Moreover, the mesopore size of the samples increases with the increasing heat-treatment temperature. These phenomena are attributed to the restriction effect of SiO2 hard temperature on polymer shrinkage during the heat treatment, as schematically described in Scheme 2. For general FDU-14, FDU-15, and FDU-16 mesoporous materials, the samples continuously shrink during the carbonization. Thus, 8789

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Figure 7. N2 adsorption−desorption isotherms and pore size distribution curves from the adsorption branch of (a, b) SiO2/HMMP-3 and SiO2/ HMMC-3, (c, d) HMMP-3 and HMMC-3. Insets: the corresponding pore size distribution curves from the desorption branches. The pore size distribution curves from the desorption branch of these the desorption branch of the isotherms of SiO2/HMMC-3-700, SiO2/HMMC-3-900, HMMC-3-700, and HMMC-3-900 are vertically offset by 50, 120, 200, and 700 cm3 g−1, respectively.

5. CONCLUSIONS A series of HMMP and HMMC monoliths have been prepared via a simple method that uses an amphiphilic triblock copolymer PEO−PPO−PEO as the soft template, SiO2 opal as the hard template, and phenolic resin as the carbon source. The mesoporous structures of HMMP and HMMC are controlled using different precursors for the synthesis of FDU-15, FDU-14, and FDU-16. The results of N2 adsorption− desorption analysis indicated that the heat-treatment temperature significantly affects the textural parameters of the samples. The BET surface area and pore volume of the hierarchically ordered macro-/mesoporous materials obviously increased with the increasing heat-treatment temperature. When the heattreatment temperature was lower than 700 °C, the mesopore sizes obviously increased with the increasing temperature. When the temperature exceeded 700 °C, the polymer framework transformed into carbon but the mesopore sizes were barely changed. The BET surface areas, pore volumes, and mesopore sizes of HMMC samples are much larger than those of the respective FDU-15, FDU-14, and FDU-16 due to the restriction effect of the SiO2 template. The SiO2 opal template successfully restricted the shrinkage of the polymer during the thermosetting and carbonization process.

Scheme 2. Variation of the Mesostructure during Carbonization

the porous structure of the samples obtained at 350 and 700 °C were stable. However, for the HMMC-900 samples, the isotherms obviously changed after the removal of SiO2. After heat treatment at 900 °C, the mesopore wall of the carbon framework is very thin and fragile. Thus, a few parts of the mesopore wall collapse after the removal of SiO2 template.

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ASSOCIATED CONTENT

S Supporting Information *

TEM tilting experiment result of HMMC-2-700 (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org. 8790

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AUTHOR INFORMATION

Corresponding Author

*Fax +86-25-84895289; Tel +86-25-84895289; e-mail [email protected] (M.Z.), [email protected] (J.C.). Author Contributions §

N.L. and M.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 51202106 and 51172109) and the Natural Science Foundation of Jiangsu Province of China (No. BK2010497).

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