Facile Fabrication of Carbon Spheres with Tunable Morphologies from

Sep 30, 2014 - This study presents a facile and general method for fabrication of carbon spheres with tunable morphologies based on the sol–gel reac...
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Facile Fabrication of Carbon Spheres with Tunable Morphologies from Novel Polymeric Carbon Precursors Wei Sun, Min Chen, Shuxue Zhou, and Limin Wu* Department of Material Science and State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: This study presents a facile and general method for fabrication of carbon spheres with tunable morphologies based on the sol−gel reaction of a novel polymeric carbon precursor. The carbon precursor was fabricated by the synthesis of resole, a low-molecular weight polymer of phenol and formaldehyde, and then the modification with poly(ethylene glycol) monomethyl ether (PEG). By turning the modification degree of resole with different amounts of PEG and the hydrolysis and condensation reactions of this precursor, carbon spheres with various morphologies, including regular spheres, hollow spheres of different pore sizes, and raspberry- and peanut-like spheres, were produced easily. This should be attributed to the condensation, self-assembly, and phase separation of the new polymeric carbon precursors during the sol−gel process.

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INTRODUCTION Carbon has been recognized as one of the most promising materials in numerous fields, such as gas capture, sensing, catalyst supports, separation and purification, supercapacitors, fuel cells, and lithium-ion batteries, etc.,1−6 due to its specific properties, such as high surface-to-volume ratio, high thermal conductivity, good electrical conductivity and mechanical property, biocompatibility, and quite chemically stable under nonoxidizing conditions.7,8 These properties and performances are highly dependent upon not only the inner composition and structure, but also the shape and size of the materials. Various elegant strategies have been developed to synthesize carbon nanostructures in diverse forms, such as nanospheres, hollow spheres, nanosheets, nanotubes, and graphene in recent years.9−15 One of the most widely used and effective approaches toward the synthesis of carbon materials is based on the sol−gel strategy, by which the as-formed cross-linked polymeric materials are converted into carbon after carbonization. The low-cost and easy large-scale preparation procedure makes this method especially suitable for industrial production.16 For example, Liang et al. described the poly(ethylene glycol)-induced self-assembly of phenol-formaldehyde (PF) resole with the triblock copolymer Pluronic P123 by improving the interaction between the PF resol and P123 to form a two-dimensional hexagonal ordered mesostructure. The resultant polymer with rigid network frameworks can be directly transformed after a carbonization process into ordered mesoporous carbons.17 Among these reported morphologies and textures, spherical structures are particularly advantageous to handle in a closed packed or slurry bed because of their strong packing ability, low fluid resistance, and low particle release characteristics.18 Recently, a modified sol− gel method was successfully extended for the preparation of © 2014 American Chemical Society

carbon spheres by ammonia-catalyzed polymerization of resorcinol and formaldehyde followed by carbonization.19−21 However, due to the very limited amount of carbon precursors and the hard-to-control sol−gel reactions of these precursors, little research has involved the synthesis of carbon spheres with tunable morphologies using the sol−gel strategy. In this study, we report a facile method for the fabrication of carbon spheres with tunable morphologies based on the sol− gel reaction of a novel polymeric carbon precursor. This new precursor was obtained by the synthesis of resole, a lowmolecular weight polymer of phenol and formaldehyde, and then the modification with poly(ethylene glycol) monomethyl ether (PEG). As compared to the previous carbon precursors such as glucoses, cross-linking polymers, and pure resins, this new resole-PEG precursor has the following properties and merits: (i) It can form three-connected covalently bonding zeolite-like frameworks by thermopolymerization and can be directly transformed into ultrastable carbon frameworks by carbonization process with high carbon yield;22,23 (ii) it may have various substitution degrees of PEG due to the abundant terminal methoxy groups of resole; and (iii) the grafted PEG macromolecular segments can be used to adjust the selfassembly behavior of precursors during the sol−gel process, producing tunable morphologies for nanospheres. Besides, as an additional benefit, the intermediate product, phenolformaldehyde resin polymer sphere (PF sphere), itself also exhibits many interesting properties and may find potential applications in various fields, especially for biological Received: May 2, 2014 Revised: August 15, 2014 Published: September 30, 2014 12011

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on a Philips XL 30 field emission microscope at an accelerating voltage of 10 kV. Nitrogen adsorption−desorption isotherms were determined at 77 K using an ASAP 2010 analyzer. Surface area was calculated using the Brunauer−Emmett−Teller (BET) method, and the pore volumes and pore size distributions were obtained using the Barrett− Joyner−Halenda (BJH) model. Proton nuclear magnetic resonance (1H NMR) measurements were carried out on a Bruker (500 MHz) NMR instrument. GPC analyses were performed at Waters calibrated by narrow polystyrene standards with a DAWN HELEOS (Wyatt multiangle laser light scattering detector, He−Ne 658.0 nm) and THF as the eluent at a flow rate of 1.0 mL/min at 35 °C. Dynamic light scattering (DLS) measurements were carried out on the diluted reaction solutions using a Nano-ZS90 (Malvern).

applications, such as cellular delivery vehicles, cell targeting, imaging, and so on.19

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EXPERIMENTAL SECTION

Materials. Phenol, formaldehyde (37 wt % aqueous solution), tetraethylorthosilicate (TEOS), hydrogen peroxide (H2O2, 30 wt % aqueous solution), and ammonia (NH3·H2O, 25 wt % aqueous solution) were purchased from Aladdin Chemical Reagent Corp. Potassium chloride (KCl), hydrochloric acid (HCl, 36−38 wt % aqueous solution), sodium hydrate (NaOH), ethanol, tetrahydrofuran (THF), and magnesium sulfate anhydrous (MgSO4) were purchased from Sinopharm Chemical Reagent Corp. Mercaptopropyltrimethoxysilane (MPTMS), Pluronic 123 (P123), and poly(ethylene glycol) monomethyl ether (PEG, Mn: 500 g/mol) were purchased from Sigma-Aldrich. Synthesis of Resole. Resole was synthesized according to the literature as follows:22 Phenol (8.0 g, 85 mmol) was molten at 45 °C and then dropwise added by 2.04 g of NaOH aqueous solution (0.34 g of NaOH dissolved in 1.7 g of water) under stirring, followed by addition of 14.5 g of formalin aqueous solution (5.36 g, 177 mmol). This mixture was stirred at 70 °C for 45 min, and then cooled to room temperature, and the pH was adjusted to neutral using 1 M HCl. After water was distilled out by vacuum distillation, the product was dissolved in THF and dried using magnesium sulfate anhydrous. GPC characterization: Mn = 320, Mw/Mn = 1.50. Synthesis of Propyl-sulfonic Acid-Functionalized Mesostructured Silica (SiO2−SO3H). P123 (4 g, 0.69 mmol) and KCl (6 g, 80 mmol) were dissolved in 120 g of 2.0 M HCl aqueous solution and heated to 40 °C, followed by addition of TEOS (7.69 g, 36.9 mmol) and stirring for another 1 h. This mixture was then added by MPTMS (0.805g, 4.1 mmol) and H2O2 (4.17 g aqueous solution, 36.9 mmol) and stirred at 40 °C for an additional 24 h and aged at 105 °C for another 24 h. The resultant solid was collected by centrifugate and washed with water and ethanol, respectively, each three times. Finally, the solid was refluxed in ethanol for 24 h to remove the template. The H+ exchange capacity of SiO2−SO3H was determined by chemical titration as follows: 0.1 g of acid solid was suspended in 10 mL of aqueous solution of KCl (0.5 M) and stirred for 1 h. Titration was carried out using a solution of 0.01 M NaOH with phenolphthalein as the indicator. Chemical Modification of Resole with PEG. Resole was reacted with PEG under the catalysis of solid acid SiO2−SO3H. Typically, resole (1.68 g, corresponding to 10.7 mmol of phenol and 22.4 mmol of formalin), PEG (0.82 g, 1.65 mmol), and SiO2−SO3H (0.1 g, containing 0.11 mmol of exchangeable H+) were charged into a flask and heated to 50 °C under stirring. The water was continuously fractionated off until no bubble arose under vacuum. The mixture then was stirred at 50 °C for another 12 h. This process of distillation under vacuum/stir under constant pressure was repeated for another cycle. After that the mixture was cooled, and 4 mL of THF was added. The product was finally obtained by removal of the solid acid by centrifugate and vacuum distillation by THF. GPC characterization: Mn = 1040, Mw/Mn = 3.22. Fabrication of PF Spheres and Carbon Spheres. Typically, 0.1 g of resole-PEG was dispersed in a 5 mL mixture of ethanol and water (ethanol:water = 2:1 by volume ratio) and heated to 65 °C, followed by the addition of 100 μL of ammonia aqueous solution (25%). This mixture was stirred at 65 °C for 24 h. During this reaction, the color of this solution turned from colorless transparent to green. After that, the mixture was cooled and diluted with the same solvent to 25 mL. The mixture was then held in a Teflon-lined autoclave at 130 °C for 24 h. The brown solid product was collected by centrifugation and washed with ethanol three times and dried at 40 °C. The PF spheres were carbonized under Ar atmosphere at 700 °C for 3 h, by a heating rate of 1 °C/min and held at 180 and 350 °C for 3 h, respectively, before reaching the final temperature. Characterizations. Transmission electron microscopy (TEM) images were taken on a Philips CM200FEG field emission microscope. Scanning electron microscopy (SEM) was conducted for microanalysis

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RESULTS AND DISCUSSION Synthesis of Resole-PEG. Resole was modified by the etherification reaction with PEG to yield resole-PEG, as shown in Scheme 1. The key factor for successful modification was to

Scheme 1. Chemical Modification of Resole with Poly(ethylene glycol) Monomethyl Ether

use the propyl-sulfonic acid-functionalized mesostructured silica (SiO2−SO3H) as the catalyst, which has been proved to be an efficient catalyst for etherification reaction between the hydroxyl groups of benzyl alcohol and alkyl alcohol with very high selectivity.24,25 SiO2−SO3H was synthesized using a modified sol−gel method. As shown in Scheme 2, the initially Scheme 2. Schematic Mechanism for the Synthesis of Propyl-sulfonic Acid-Functionalized Mesostructured Silica (SiO2−SO3H)

formed SiO2 underwent sulfhydrylation and in situ oxidation reactions to form SiO2−SO3H. The N2 adsorption−desorption isotherm of the as-obtained SiO2−SO3H, as shown in Supporting Information Figure S1, reveals a type IV curve, typical for mesoporous materials according to the IUPAC nomenclature. The BET surface area was 742 m2/g, and the mean mesopore size was 8.3 nm calculated by BJH model. The successful grafting of MPTMS could be proved by elemental analysis of SiO2−SO3H, which contained 4.1% of sulfur atom. The H+ exchange capacity of SiO2−SO3H was determined to be 1.09 mmol/g by chemical titration using NaOH solution as the reagent. This etherification reaction produced a large number of bubbles, and shifted toward the formation of resole-PEG as the water was distilled out continuously by vacuum distillation. The structure of resole-1PEG was characterized by 1H NMR. As shown in Figure 1, after the reaction, the peak at 2.3 ppm for hydroxyl groups of PEG disappeared, indicating a successful grafting of PEG onto resole. Because of the uncompleted 12012

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used, the as-obtained PF spheres aggregated due to the too low content of resole within the precursor. After carbonization, these PF spheres were directly transformed into carbon spheres, as shown in Figure 2. Thanks to

Figure 1. 1H NMR spectra of PEG (black line), resole (red line), and resole-1PEG (blue line) in CDCl3.

reaction between formaldehyde and phenol during resole synthesis, the integral ratio of aromatic (6.8−7.3 ppm) and benzylic (4.4−4.8 ppm) protons decreased from 1:1.8 to 1:2.7 due to the further coupling of residual formaldehyde with aromatic protons. The degree of modification was defined by the theoretical mole ratio of PEG to the total phenol and formaldehyde and determined by monitoring the peak height of the initial and residue PEG before and after reaction from GPC curves (Supporting Information Figure S2). As summarized in Table 1, 0, 1, 2.5, 5, 7.5, and 10 mol % of PEG modifications were prepared and designated as resole-0PEG, resole-1PEG, resole-2.5PEG, resole-5PEG, resole-7.5PEG, and resole-10PEG, respectively, and the degree of modification is between 80% and 90% depending on the addition amount of PEG. Fabrication of PF Spheres and Carbon Spheres with Tunable Morphologies. The as-obtained resole-PEG precursors were then thermopolymerized in a mixture solvent of ethanol and water at 65 °C and then aged at 130 °C to produce the intermediate PF spheres with various morphologies, depending upon the degree of substitution, as shown in Supporting Information Figure S3. For pure resole (resole0PEG), only regular spherical particles with smooth surface and a mean diameter of 620 nm were obtained (Supporting Information Figure S3a). When resole-1PEG was used as the precursor, irregular spheres with smooth surface were obtained (Supporting Information Figure S3b). Further increasing the substitution degree of PEG led to the formation of hollow structures within the spheres. As PEG substitution increased, the volume ratio of void to the whole sphere increased while the average diameters of PF spheres decreased. Meanwhile, the shape of PF sphere became much more irregular and the surface turned to be much rougher. When resole-10PEG was

Figure 2. TEM images of the carbon spheres obtained from resolePEG precursors with different PEG substitutions: (a) resole-0PEG, (b) resole-1PEG, (c) resole-2.5PEG, (d) resole-5PEG, (e) resole-7.5PEG, and (f) resole-10PEG. Inset images: Corresponding SEM images.

the covalently bonding zeolite-like frameworks of PF spheres, the carbon spheres almost duplicated the morphologies of PF spheres, except about 20−30% of contraction in the diameter. Elemental analysis results showed that these carbon spheres contained about 87 wt % of carbon after carbonization at 700 °C. By carefully adjusting the sol−gel reaction condition of resole-PEG precursors, we also successsfully synthesized the PF and corresponding carbon spheres with other morphologies. For example, when various ammonia amounts such as 200 and 50 μL were used as the catalysts, the resole-1PEG precursor produced raspberry- and peanut-like spheres, as shown in Figure 3. The N2 adsorption−desorption isotherms of the as-obtained carbon spheres, as shown in Supporting Information Figure S4a, all reveal a combination of type I and II curves, indicating there were few pores within the carbon spheres. The type II curve may be due to the external surface area of carbon spheres. Besides, the typical BJH pore size distribution curves of hollow spheres suggest that this structure has irregular pore

Table 1. Characterizations of Resole Modified with Different Amounts of PEG samples

phenol (mmol)/formalin (mmol)

resole-1PEG resole-2.5PEG resole-5PEG resole-7.5PEG resole-10PEG

10.7/22.4 10.7/22.4 10.7/22.4 10.7/22.4 10.7/22.4

PEG (mmol) 0.33 0.82 1.65 2.46 3.30

(0.17 (0.41 (0.82 (1.23 (1.65

g) g) g) g) g)

Mn (g/mol)

Mw/Mn

degree of modification, theoretical (%)

degree of modification from GPC (%)

860 980 1040 1100 1300

3.24 3.18 3.22 3.03 2.65

1 2.5 5 7.5 10

0.9 2.2 4.3 6.2 8.0

12013

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the surfaces of the PF spheres but little on the morphology of the inner structure, acting as the surfactant in these reactions. Accordingly, only simple spherical morphology could be obtained. In the same way, only one type of morphology could be obtained by adding appropriate amphiphilic copolymers during the sol−gel process of resole using a physical mixing method.19,23 As compared to noncovalent bonds (such as hydrogen bond and hydrophobic interactions), the chemical bond is much more stable, especially during the sol−gel process, which thus can cause much stronger controllability over the morphologies of the final products.26,27 As a result, after the PEG segments were chemically grafted to the resole to form new polymeric carbon precursors, they should have different hydrolysis, condensation, and assembling behaviors from pure resole during the sol−gel process. Formation Mechanism of PF and Carbon Spheres with Tunable Morphologies. On the basis of the above results and discussion, a possible mechanism by which the PF and corresponding carbon spheres formed is proposed on the basis of the self-assembly of the new polymeric carbon precursor, as shown in Scheme 3a. The resole-PEG was

Figure 3. TEM and SEM images of (a) raspberry- and (b) peanut-like shaped carbon spheres.

distribution with the mean size of 4.1 nm. Similar results were obtained for carbon spheres with other morphologies. As summarized in Table 2, the BET surface area of these carbon Table 2. Physical Properties of the As-Obtained Carbon Spheres carbon spheres regular sphere hollow spheres-1a hollow spheres-2b raspberry-like sphere peanut-like spheres a

surface area (m2/g)

pore size (nm)

pore volume (cm3/g)

425 469 466 456

3.4 4.1 4.1 3.1

0.03 0.04 0.04 0.02

3.8

0.03

437

Scheme 3. (a) Schematic Mechanism for the Formation of Carbon Spheres with Tunable Morphologies; and (b) Typical Condensation Polymerization between Resole-PEG Precursors

b

Same sample as shown in Figure 2c. Same sample as in Figure 2f.

spheres is around 450 m2/g, and the pore volume is between 0.03−0.04 cm3/g, indicating a low porosity of these carbon structures without addition of mesopore structure directing agent. Effect of Chemical Modification of Resole with PEG. An important innovation in this study is to use the PEG chemically modified resole as the precursor. To verify the effect of chemical modification of resole with PEG, we further ran some controlled experiments by thermopolymerization of the physical mixtures of resole with the same PEG amounts as the resole-5PEG and resole-10PEG and other parameters equal. As shown in Figure 4, only regular spherical particles with smooth

Figure 4. TEM images of the carbon spheres from the carbonization of PF spheres obtained from thermopolymerization of the physical mixtures of resole with PEG. The mole ratios of PEG to phenol and formaldehyde: (a) 5% and (b) 10%.

initially soluble in the reaction medium. During the sol−gel reaction, the dehydration reaction between the hydroxyl groups of benzyl alcohol and ortho or para hydrogen of phenol occurred under the catalysis of ammonia (Scheme 3b), forming three-dimensional dendritic architecture and thus decreasing the hydrophilicity of polymer segments.22,23 Thus, the wettability of these copolymers from the condensation of resole-PEG precursors gradually turned from hydrophilic to amphiphilic, and the hydrophobic part of the copolymers became larger and larger as the reaction time extended. When the hydrophobic part of copolymer reached a certain ratio, these amphiphilic dendritic copolymers could not dissolve in

surfaces were observed in both cases, just like the case of resole0PEG as the precursor. Also, the more PEG was used, the smaller the PF spheres were. It was reported that hydrophilic PEG has good compatibility with hydrophilic PF resole due to a strong hydrogen-bonding interaction between them.17 However, this hydrogen-bonding interaction was not stable in this system, and would become weaker and even vanish along with the sol−gel reaction process because of the thermopolymerization of resole. As a result, the influence of PEG was limited to 12014

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model for the assembly behaviors of amphiphilic dendritic polymers.28,29 At the very beginning, these amphiphilic dendritic copolymers condensed from the resole-PEG precursors, and formed unimolecular micelles with a hydrophobic resole core and a hydrophilic linear PEG shell. These unimolecular micelles with large hydrophobic segments would further self-assemble into MMAs intermicellar due to the hydrophobic interactions. Because the molecular structures of the amphiphilic dendritic copolymers were not uniform at the initial stage, the reaction degree of each micelle was not synchronized either. Thus, two size distributions were observed (Figure 5a), which were corresponding to the unimolecular micelles and MMAs, respectively.28 What is different from the previously reported dendritic polymers with stable molecular structures is that these dendritic copolymers in this study could undergo further condensation reaction in virtue of many residue active groups on hydrophobic segments, and thus the hydrophobic part of micelles could be further enlarged as the reaction time went on. As a result, the ratio of unimolecular micelles decreased (Figure 5b and c), and ultimately disappeared completely because of more unimolecular micelles aggregating into MMAs (Figure 5d), which became larger, denser, and more hydrophobic to finally produce PF spheres precipitating from the medium as a result of the gelation transformation. In the previously reported systems, when resole was used as the precursor, its condensation reaction only caused gelation transformation and formation of regular PF spheres due to the uniform distribution of a single component in resole.20,21 However, in this study, when resole-PEG was used as the precursor, the condensation reaction would increase the crosslinking degree of PF molecular segments, which caused the microphase separation between PF and linear PEG segments. To minimize the surface free energy, the hydrophilic linear PEG segments within the MMAs preferred to migrate toward the surfaces during the phase separation process.30 The structures of the resultant products could be controlled by cross-linking degree and the content of cross-linking components.31,32 When the PEG content used for modification is low (e.g., resole-1PEG), the high density of PF framework made the PF spheres highly stable. Accordingly, the microphase separation and the migration of PEG only resulted in the deformation of PF spheres to form irregular spherical particles without any cavities. As the PEG content in resole-PEG increased, more PEG but less PF segments were contained in the MMAs. Also, the speed and possibility of condensation reaction between PF segments decreased, causing incompact and unstable PF framework. Driven by the minimization of surface energy, these hydrophilic PEG and even these loose PF segments migrated toward the shells, forming hollow structures. When more ammonia was used, the condensation reaction between PF segments was accelerated, which availed the formation of more compact structure for PF framework, which restrained the deformation of PF spheres. When resole-1PEG was used as the carbon precursor at this condition, the phase separation and the migration of PEG only induced the formation of humps on the surfaces of PF spheres with raspberry-like structure (Figure 3a). On the contrary, when less ammonia was used, the decreasing condensation reaction caused much looser structure for PF framework, which favored the deformation of PF spheres, forming peanut-like spheres (Figure 3b).

the solvent completely. Instead, they began to self-assemble into micelles. The hydrodynamic diameter (Dh) of these micelles during the sol−gel reaction as a function of the reaction time was in situ monitored by dynamic light scattering (DLS), as shown in Figure 5. After 5 h, a bimodal size distribution with two median Dh values of 3.0 and 88 nm was observed. This phenomenon could be explained by the multimicelle aggregates (MMAs)

Figure 5. Typical change of hydrodynamic diameter of system as a function of reaction time. Reaction conditions: 0.1 g of resole-2.5PEG, 0.1 mL of ammonia (25%), 5 mL of ethanol and water (volume ratio, ethanol:water = 2:1), at 65 °C. 12015

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(7) Ma, T.; Liu, L.; Yuan, Z. Direct synthesis of ordered mesoporous carbons. Chem. Soc. Rev. 2013, 42, 3977−4003. (8) Rondeau-Gagné, S.; Morin, J.-F. Preparation of carbon nanomaterials from molecular precursors. Chem. Soc. Rev. 2014, 43, 85−98. (9) Wickramaratne, N.; Perera, V.; Ralph, J.; Huang, S.; Jaroniec, M. Cysteine-assisted tailoring of adsorption properties and particle size of polymer and carbon spheres. Langmuir 2013, 29, 4032−4038. (10) Lu, A.; Li, W.; Hao, G.; Spliethoff, B.; Bongard, H.-J.; Schaack, B. B.; Schüth, F. Easy synthesis of hollow polymer, carbon, and graphitized microspheres. Angew. Chem., Int. Ed. 2010, 49, 1615−1618. (11) Lu, A.; Sun, T.; Li, W.; Sun, Q.; Han, F.; Liu, D.; Guo, Y. Synthesis of discrete and dispersible hollow carbon nanospheres with high uniformity by using confined nanospace pyrolysis. Angew. Chem., Int. Ed. 2011, 50, 11765−11768. (12) Fang, Y.; Lv, Y.; Che, R.; Wu, H.; Zhang, X.; Gu, D.; Zheng, G.; Zhao, D. Two-dimensional mesoporous carbon nanosheets and their derived graphene nanosheets: synthesis and efficient lithium ion storage. J. Am. Chem. Soc. 2013, 135, 1524−1530. (13) Böttger-Hiller, F.; Kempe, P.; Cox, G.; Panchenko, A.; Janssen, N.; Petzold, A.; Thurn-Albrecht, T.; Borchardt, L.; Rose, M.; Kaskel, S.; Georgi, C.; Lang, H.; Spange, S. Twin polymerization at spherical hard templates, an approach to size-adjustable carbon hollow spheres with micro- or mesoporous shells. Angew. Chem., Int. Ed. 2013, 52, 6088−6091. (14) Jiang, H. Chemical preparation of graphene-based nanomaterials and their applications in chemical and biological sensors. Small 2011, 7, 2413−2427. (15) Geim, A. K. Graphene: status and prospects. Science 2009, 324, 1530−1534. (16) Hartmann, S.; Brandhuber, D.; Husing, N. Novel possibilities for the synthesis of hierarchically organized (hybrid) porous materials. Acc. Chem. Res. 2007, 40, 885−894. (17) Liang, Y.; Lu, S.; Wu, D.; Sun, B.; Xu, F.; Fu, R. Polyethylene glycol-induced self-assembly to synthesize an ordered mesoporous polymer with a two-dimensional hexagonal structure. J. Mater. Chem. A 2013, 1, 3061−3067. (18) Long, D.; Lu, F.; Zhang, R.; Qiao, W.; Zhan, L.; Liang, X.; Ling, L. Suspension assisted synthesis of triblock copolymer-templated ordered mesoporous carbon spheres with controlled particle size. Chem. Commun. 2008, 2647−2649. (19) Liu, J.; Qiao, S. Z.; Liu, H.; Chen, J.; Orpe, A.; Zhao, D.; Lu, G. Q. Extension of the stöber method to the preparation of monodisperse resorcinol−formaldehyde resin polymer and carbon spheres. Angew. Chem., Int. Ed. 2011, 50, 5947−5951. (20) Lu, A.; Hao, G.; Sun, Q. Can carbon spheres be created through the stöber method. Angew. Chem., Int. Ed. 2011, 50, 9023−9025. (21) Choma, J.; Jamio, D.; Augustynek, K.; Marszewski, M.; Gao, M.; Jaroniec, M. New opportunities in Stöber synthesis, preparation of microporous and mesoporous carbon spheres. J. Mater. Chem. 2012, 22, 12636−12642. (22) Meng, Y.; Gu, D.; Zhang, F.; Shi, Y.; Yang, H.; Li, Z.; Yu, C.; Tu, B.; Zhao, D. Ordered mesoporous polymers and homologous carbon frameworks- amphiphilic surfactant templating and direct transformation. Angew. Chem. 2005, 117, 7215−7221. (23) Zhang, F.; Meng, Y.; Gu, D.; Yan, Y.; Yu, C.; Tu, B.; Zhao, D. A facile aqueous route to synthesize highly ordered mesoporous polymers and carbon frameworks. J. Am. Chem. Soc. 2005, 127, 13508−13509. (24) Gu, Y.; Azzouzi, A.; Pouilloux, Y.; Jérôme, F.; Barrault, J. Heterogeneously catalyzed etherification of glycerol: new pathways for transformation of glycerol to more valuable chemicals. Green Chem. 2008, 10, 164−167. (25) Grieken, R.; Melero, J. A.; Morales, G. Etherification of benzyl alcohols with 1-hexanol over organosulfonic acid mesostructured materials. J. Mol. Catal. A: Chem. 2006, 256, 29−36. (26) Wang, H.; Agrawal, G.; Tsarkova, L.; Zhu, X.; Möller, M. Selftemplating amphiphilic polymer precursors for fabricating mesostructured silica particles. Adv. Mater. 2013, 25, 1017−1021.

CONCLUSIONS Carbon spheres with tunable morphologies were successfully fabricated by a facile sol−gel strategy using PEG-modified resole (resole-PEG) as the new polymeric carbon precursor. The condensation, self-assembly, and phase separation of resole-PEG could be well controlled by the modification degree of PEG and ammonia amounts, producing carbon nanospheres with different morphologies, including regular spheres, hollow spheres of different pore sizes, and raspberry- and peanut-like spheres. This study is the first proto and provides an intriguing method to synthesize carbon spheres with tunable morphologies for their uses in gas capture, sensing, catalyst supports, separation and purification, supercapacitors, lithium-ion batteries, and so on. The present strategy from the perspective of the new precursors can be easily extended for fabricating other inorganic colloid particles with interesting and specific morphologies.

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ASSOCIATED CONTENT

* Supporting Information S

Typical nitrogen adsorption/desorption isotherm of the as prepared SiO2−SO3H. GPC profiles of PEG and the asprepared resole-7.5PEG after reaction. TEM images of the PF spheres obtained from resole-PEG precursors with different degrees of substitution. Typical nitrogen adsorption/desorption isotherm of the as-prepared carbon spheres. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was received from the National Natural Science Foundation of China (Grants 51133001 and 21374018), National “863” Foundation, Science and Technology Foundation of Ministry of Education of China (20110071130002), and Science and Technology Foundation of Shanghai (12nm0503600, 13JC1407800).

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REFERENCES

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dx.doi.org/10.1021/la5026476 | Langmuir 2014, 30, 12011−12017