Salt-Induced Phase Separation to Synthesize Ordered Mesoporous

Self-assemly of block copolymers (BCPs) and phenolic resin (PR) is an important method to prepare ordered mesoporous polymers (OMPs) and carbon ...
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Salt-Induced Phase Separation to Synthesize Ordered Mesoporous Carbon by pH-Controlled Self-Assembly Youxin Duan,†,‡ Fuping Pan,† Qiao Liu,§ Yan Zhou,† Aimin Liang,*,† and Junyan Zhang*,† †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § School of Materials Science and Engineering, Ningbo University of Technology, Ningbo 315016, China S Supporting Information *

ABSTRACT: Self-assemly of block copolymers (BCPs) and phenolic resin (PR) is an important method to prepare ordered mesoporous polymers (OMPs) and carbon materials (OMCs). In the process, phase separation of the BCP−PR composite is a critical step which is, however, time-consuming in aqueous solution. Here we report, for the first time, a new salt-induced phase separation strategy to achieve this goal. Triblock copolymer F127 and phenol-formaldehyde resin (PF) are used as the template and precursor, respectively, and sodium chloride (NaCl) is applied to induce the coagulation and phase separation of the F127−PF composite which is transformed to be OMC at high temperature. It is found that the maintenance of the ordered mesostructure is highly dependent on the pH of the F127−PF solution under NaCl interference. A hypothetical mechanism is proposed to explain the role of pH in the formation of ordered mesostructure when salt is introduced into the self-assembly system. The effects of pH, salt concentration, and varied salts on the structures and properties of the as-prepared OMCs are investigated in detail. The new salt-induced phase separation strategy can synthesize OMC facilely and can provide a new insight into understanding the process of preparing ordered mesoporous materials by self-assembly more deeply.

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INTRODUCTION Ordered mesoporous carbon materials (OMCs) have been widely investigated in different scientific fields such as catalysis,1,2 adsorption,3 and electrochemical energy storage/ conversion systems4,5 due to their unique structures such as regular pore arrangement and large specific surface areas. Simple, rapid, and massive preparation of OMCs is of significant importance for their practical applications. Selfassembly of block copolymers (BCPs) and phenolic resin (PR) (also called soft template method) has been a very important method to prepare ordered mesoporous polymers (OMPs) and carbon materials.6−9 Compared with the hard template method, the procedure of BCP−PR self-assembly is much more simple. There are mainly three different approaches in the BCP−PR self-assembly method to prepare OMPs/OMCs: (1) evaporation-induced self-assembly (EISA);10 (2) dilute aqueous selfassembly;11 and (3) the hydrothermal method.12,13 The EISA method is conducted in volatile organic solvents such as ethanol which evaporate slowly to induce the formation and phase separation of the BCP−PR ordered mesostructured composite.10 The evaporation of a large amount of organic solvents is obviously unfeasible in practical production. Its other disadvantage is that the preparation of PR ethanol solution is complex. The hydrothermal method is carried out in © XXXX American Chemical Society

aqueous solution at high temperature and pressure. Although it is facile and rapid, the high pressure in the process and the limited volume of the autoclave hinder its large-scale application. In contrast to these two methods, self-assembly in aqueous solution is more convenient. It is conducted in dilute aqueous solution under mild temperature (normally lower than 373 K) and ambient pressure without the evaporation of solvent (water). A critical step in the whole process is the phase separation (precipitation) of the BCP−PR composite from solution. In the EISA method, it is accomplished by the evaporation of the volatile organic solvent, while in aqueous solution, the phase separation is achieved by the polymerization of phenolic resin because its solubility decreases with the polymerization degree increase. For example, phenol-formaldehyde resin and triblock copolymers P123 and F127 have been used to prepare OMC in basic aqueous solution successfully; however, the process is very time-consuming (taking 5−7 days) mainly due to the slow polymerization rate of phenol-formaldehyde resin.8,11 If phenol was replaced by resorcinol or phloroglucinol and the selfReceived: November 3, 2016 Revised: December 24, 2016 Published: January 4, 2017 A

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The Journal of Physical Chemistry C Scheme 1. Process of Synthesizing Ordered Mesoporous Carbon by Self-Assembly and Salt-Induced Phase Separation

centrifuged and heated at 100 °C in air for 24 h. Similarly, 3.0, 3.5, 4.0, and 4.5 mol L−1 NaCl solution and 3.5 mol L−1 KCl solution were also used, respectively, to induce precipitation from F127−PF-a solution. The obtained precipitations were then pyrolized at 500 or 800 °C in nitrogen atmosphere to transform the polymers to carbons. Materials Characterization. Small-angle X-ray diffraction (XRD, X’PERT PRO, 40 kV/20 mA, Cu Kα radiation, λ = 1.5406 nm) was used to identify the regular mesopore arrangement in a 2θ range of 0.6−5°. Nitrogen sorption isotherms were collected on a Micromeritics (ASAP 2010) porosimetry analyzer. Transmission electron microscopy (TEM) images were taken on a Tecnai-G2-F30 (300 keV) field-emission TEM (FE-TEM).

assembly process was conducted in acid solution, the phase separation of the BCP−PR composite can be relatively faster than that in the case of phenol in basic conditions.14 It is obvious that an efficient phase separation strategy is of significant importance for the synthesis of ordered mesoporous polymer and carbon materials. In the current work, we for the first time develop a new saltinduced deposition strategy to achieve the goal of phase separation and prepare ordered mesoporous carbon. The new method does not rely on the precipitation of phenolic resin naturally but induces the BCP−PR composite to precipitate from solution by adding inorganic salt into the solution. It is based on a well-known phenomenon that electrolyte can destroy the stability of a colloidal solution and induce the coagulation of the colloidal particles. The effect of pH, salt concentration, and salt kind on the structures of obtained OMCs is explored in detail, and a possible mechanism is proposed to explain the results. By the new method, we can prepare OMCs more facilely and rapidly. Moreover, the results can provide us new insight into understanding the self-assembly process more deeply.

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RESULTS AND DISCUSSION Synthesis of OMC by pH-Controlled Self-Assembly and Salt-Induced Phase Separation. The whole process of synthesizing OMC by the self-assembly and salt-induced phase separation strategy can be illustrated in Scheme 1. Triblock colopymer Pluronic F127 was used as a soft template which can form ordered mesophases in solution depending on the concentration and temperature. Phenol-formaldehyde resin (PF) was used as a carbon precursor in that it can combine with F127 through hydrogen bonding to form an ordered mesostructured composite.8 The reaction was first conducted in weakly basic NaOH solution to catalyze the polymerization of PF resin. If pH was too high, the PF resin would transform to be anions, and then the hydrogen bonding between F127 and PF would be disrupted and the F127−PF composite destroyed.8 If the reaction was conducted in acid aqueous solution, the F127 and PF could not be compatible with each other and led to a turbid solution, which would be harmful to the self-assembly of F127 and PF at the molecule scale. So ethanol was always used as cosolvent to improve the compatibility of the triblock copolymer and phenolic resin in acid solution. In previous reports, the reaction always lasted for several days in order to accomplish the phase separation of the F127−PF composite.8,11 Here, we skipped the time-consuming process and realized the phase separation of the F127−PF composite by salt. The F127−PF system first reacted in weakly basic aqueous solution for a relatively short time (8 h) in order to form F127−PF composite and make the polymerization degree of the PF polymer to a certain extent. The resulting F127−PF solution was red, and its pH was about 9.7 (denoted as F127−PF-b solution). After the reaction was stopped and

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EXPERIMENTAL SECTION Synthesis of Ordered Mesoporous Carbons. Pluronic F127 was used as a soft template, and soluble phenolformaldehyde resin (PF) was used as a carbon precursor. In a typical synthesis, 1.0 g of phenol was dissolved in 25 mL of 0.1 mol L−1 NaOH solution at 70 °C, and 3.5 g of 37% formaldehyde solution was added into the phenol solution. The new solution was stirred and heated at 70 °C for 30 min (denoted as PF solution). Then 2.2 g of F127 dissolved in 25 mL of water was added into the PF solution and heated at 70 °C for 8 h during which the color of the solution changed from colorless to red. After the solution was cooled to room temperature, its pH was about 9.7 (denoted as F127−PF-b solution; b means the solution was basic). In 20 mL of F127− PF-b solution, 20 mL of 5.0 mol L−1 NaCl solution was added slowly, and red precipitation appeared gradually. In another 20 mL of F127−PF-b solution, 37% concentrated hydrochloric (HCl) acid was added to adjust its pH to be less than 1 (the solution was denoted as F12−PF-a; a means the solution was acid). During the pH-adjusting process, the color of the solution changed from red to yellow. Here the pH was 0.6−0.7. In 20 mL of the F127−PF-a solution, 20 mL of 5.0 mol L−1 NaCl solution was added slowly, and yellow precipitation appeared gradually. The yellow and red precipitations were B

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structure in the range of mesopores. The mesoporous structure of OMCs can be characterized by nitrogen sorption isotherms. The type-IV curve with H2-type hysterisis loop in the P/P0 range of 0.4−0.6 (Figure 1b) indicated the existence of mesopores16 in the OMC-5.0NaCl-500 sample which had a BET surface area of 385.11 m2 g−1 and average pore size of 3.65 nm (Figure 1b, inset). The isotherm was not close which was different from previously reported ordered mesoporous carbons. This phenomenon was reported previously in ordered mesoporous polymers which was probably due to the existence of micropores.16 By contrast, the AC-5.0NaCl-500 sample only had a BET surface area of 6.8 m2 g−1 and pore volume of 0.0091 cm3 g−1 (Figure S1b and S1c) indicating that it had little mesopores. The TEM images showed the difference of their nanostructures clearly. The OMC-5.0NaCl-500 sample possessed 2D hexagonally packed regular channels belonging to the p6mm space group (Figure 2), while the AC-5.0NaCl-500

cooled to room temperature, concentrated (37%) hydrochloric acid (HCl) was added slowly into the red F127−PF-b solution to adjust its pH to be less than 1.0. During adjusting pH, the color of the solution changed from red to yellow when the pH was ∼7. We fixed the pH to be 0.6−0.7 and denoted the yellow solution as F127−PF-a solution. Then sodium chloride (NaCl) was added into the F127−PF-a solution to induce the precipitation of the F127−PF composite from solution. This can be explained from the view of colloidal chemistry. The F127−PF solution can be viewed as a colloidal system. It is well-known that electrolytes can disrupt the stability of a colloidal system and induce the coagulation or precipitation of the colloidal particles. For example, ammonium sulfate [(NH 4 ) 2 SO 4 ] is always applied to purify protein in biochemistry because it can induce the coagulation of protein and cause little harm to the activity of protein. Here, NaCl played a similar role in the phase separation of the F127−PF composite. So the addition of NaCl could induce yellow precipitation of the F127−PF composite from F127−PF-a solution. NaCl could also induce red precipitation from F127− PF-b solution, but its mass was just 70% that of yellow precipitation induced from F127−PF-a solution when 20 mL of 5 mol L−1 NaCl solution was added into 20 mL of F127−PF-a and F127−PF-b solution, respectively. After the yellow and red precipitation were pyrolyzed at 500 °C in N2 atmosphere, the resulting two carbon materials possessed very different nanostructures. The sample resulted from F127−PF-a solution was a typical ordered mesoporous carbon (denoted as OMC-5.0NaCl-500), while that resulting from the F127−PF-b solution was just an amorphous carbon (denoted as AC-5.0NaCl-500) without any characteristics of ordered mesoporous carbon materials. The structures of the OMC-5.0NaCl-500 and AC-5.0NaCl-500 materials were investigated by small-angle XRD, nitrogen sorption isotherm, and transmission electron microscope (TEM). Due to their regular pore arrangement, the XRD pattern of ordered mesoporous materials shows diffraction peaks usually in the 2θ range of 0−5°. The small-angle XRD pattern of the OMC5.0NaCl-500 showed an apparent peak at 2θ value of 0.87° (Figure 1a) associated with (100) reflection of the 2D p6mm space group,15 while that of AC-5.0NaCl-500 did not show any peaks (Figure S1a). This result indicated that the pores of the OMC-5.0NaCl-500 were really arranged in a regular pattern and that the AC-5.0NaCl-500 sample did not have any ordered

sample was just amorphous and did not have any mesopores (Figure S2). When the yellow precipitation induced from F127−PF-a solution by 5.0 mol L−1 NaCl was pyrolyzed at 800 °C in N2 atmosphere (denoted as OMC-5.0NaCl-800), it also remained a well-ordered mesoporous structure (Figure 3 and Figure 4) indicating that the as-prepared OMC sample possessed good thermal stability. Effect of pH on the Structures of the As-Prepared Carbon. Given the above results, we can see that NaCl can induce the phase separation of the F127−PF composite from solution effectively and thus can accelerate the process of synthesizing ordered mesoporous carbon materials. However, although NaCl can induce phase separation in both F127−PF-a

Figure 1. Small-angle XRD pattern (a), nitrogen sorption isotherm (b), and pore size distribution (b inset) of the OMC-5.0NaCl-500 sample.

Figure 3. Small-angle XRD pattern (a), nitrogen sorption isotherm (b), and pore size distribution (b inset) of the OMC-5.0NaCl-800 sample.

Figure 2. TEM images of the OMC-5.0NaCl-500 sample.

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bonding is the dominant force (Scheme 1 and Scheme 2a). If pH is too high, PF resin may become resin anions, and thus the hydrogen bonding will be disrupted.8 This is why in basic conditiond the pH is limited in a narrow range. In strongly acid solution, both F127 and PF resin may be protonated to be positively charged, and the dominant force between F127 and PF resin is the electrostatic interaction bridged by chloride ions (Cl−) in solution17 (Scheme 1 and Scheme 2c). The strength of hydrogen bonding is much weaker than that of electrostatic interaction. According to the biochemistry theory, hydrogen bonding in protein can be destroyed by certain external forces such as heat,18 alcohol,19 salt,20 etc. So we can propose that when NaCl is added into F127−PF-b solution it can disrupt the hydrogen bonding between F127 and PF, and then PF will not arrange in an ordered pattern around the corona of the F127 micelles and disperse randomly in solution (Scheme 2b). The F127 and PF polymer chains twist and tangle up to form a disordered composite, so although NaCl can induce the coagulation of the F127−PF composite it can just result in amorphous carbon without mesopores. In the case of F127− PF-a solution, the dominant force between F127 and PF is an electrostatic interaction which is much stronger than hydrogen bonding and thus cannot be destroyed by NaCl (Scheme 2d). So the F127−PF composite maintains its original ordered mesostructure, and ordered mesoporous carbon is obtained. This can also explain why the same amount of NaCl can induce more precipitation in the F127−PF-a solution than that in the F127−PF-b solution: because separated F127 and PF in F127− PF-b solution need more salt to precipitate them completely than the combined F127−PF composite in F127−PF-a solution. Effect of Salt Concentration. In addition to the pH effect, the concentration of salt also had an important influence on the phase separation of the F127−PF composite. When 20 mL of NaCl solutions with varied concentrations (3.0, 3.5, 4.0, 4.5, and 5.0 mol L−1) were added into 20 mL of F127−PF-a solution, respectively, different amounts of precipitations were induced. As shown in Figure 5, the mass of the precipitations after heating at 100 °C for 24 h and the concentration of NaCl solutions showed a good linear relationship suggesting that the

Figure 4. TEM images of the OMC-5.0NaCl-800 sample.

and F127−PF-b solution, structures of the resulting carbon materials are entirely different. The only difference of the two solutions is their pH values. So we can conclude that pH plays a critical role in the formation of the ordered mesostructured F127−PF composite. But why can the difference of pH lead to completely different nanostructures? We do not have a clear understanding of the reason at present. However, based on previous reports and some fundamental theory, we can propose a possible hypothesis to explain the experimental results, and the process is illustrated in Scheme 2. According to previous reports, the forces that combine F127 surfactant and PF resin together are believed to be hydrogen bonding8,14 or electrostatic interaction.17 In basic aqueous solution, hydrogen Scheme 2. Hypothetical Mechanism Explaining the Role of pH in the Salt-Induced Phase Separation Process

Figure 5. Relation between the mass of salt-induced precipitations and the concentration of salt solution. The precipitations were induced by adding 20 mL of NaCl solution with varied concentrations (3.0, 3.5, 4.0, 4.5, and 5.0 mol L−1) into 20 mL of F127−PF-a solution and then centrifuged at 10 000 rpm for 5 min. The mass of the precipitations was recorded after heating at 100 °C for 24 h. D

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Different salt concentrations lead to OMC with slightly different properties, and there exists a minimum concentration below which there will be no precipitation. Varied salts such as NaCl and KCl can induce the phase separation effectively, and the properties of the resulting OMCs are not completely identical.

amount of precipitation was proportional to the amount of salt. According to the linear equation, we can conclude that when the concentration of NaCl is below 2.3 mol L−1 there will be no precipitation induced by the NaCl solution. Experiment showed that 2 mol L−1 NaCl could not induce any precipitation but only resulted in yellow and milk-like liquid. The liquid was very stable, and there was no precipitation even centrifuged at 10 000 rpm or after standing for a month. The structure of the samples prepared from F127−PF-a solution by 3.0 and 4.0 mol L−1 NaCl and pyrolyzed at 500 °C (denoted as OMC-3.0NaCl500 and OMC-4.0NaCl-500, respectively) was investigated by XRD, nitrogen sorption isotherm, and TEM. It can be seen that the two samples possessed well-ordered mesoporous structures. The OMC-3.0NaCl-500 showed an obvious diffraction peak at 2θ value of 0.84° (Figure S3a), and the OMC-4.0NaCl-500 showed a weak and unconspicuous peak at 2θ value of 0.92° (Figure S5a). Both their nitrogen sorption isotherms were typeIV curves with H2-hysteresis loop which was a typical characteristic of mesoporous materials (Figures S3b and S5b). The BET suface areas of the two samples were 323.42 and 465.79 m2 g−1, respectively, and their average pore diameters were 3.88 and 3.66 nm, respectively (see Table 1). Their TEM images (Figure S4 and Figure S6) showed that both of their ordered mesostructures belonged to the 2D p6mm space group.

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CONCLUSION In summary, we developed a new salt-induced phase separation strategy to prepare ordered mesoporous carbon materials by pH-controlled self-assembly in aqueous solution. The pH plays a key role in controlling the formation of ordered mesostructure when salt is added into the system. The concentration of salt also has an important effect on the structures of OMCs. The selection of salt is not limited to NaCl, and other salts can also induce the phase separation. The new strategy not only provides a facile method to prepare ordered mesoporous carbons but also provides a new insight into the principle of preparing ordered mesoporous materials by self-assembly. We believe this work can be helpful to the further study and research of the ordered mesoporous materials and self-assembly.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11056. The small-angle XRD, nitrogen sorption isotherm, pore size distribution and TEM images of the as-prepared AC5.0NaCl-500, OMC-3.0NaCl-500, OMC-4.0NaCl-500, and OMC-3.5KCl-500 samples (PDF)

Table 1. Lattice Size, Pore Size, BET Surface Area, and Pore Volume of Carbon Samples Prepared under Different pH, Salt Concentration, and Varied Salt d100

lattice sizea

pore size

BET surface area

pore volume

sample

nm

nm

nm

m2 g−1

cm3 g−1

OMC-3.0NaCl-500 OMC-4.0NaCl-500 OMC-5.0NaCl-500 OMC-3.5KCl-500 OMC-5.0NaCl-800 AC-5.0NaCl-500

10.46 9.56 10.11 10.42 9.04 -

12.09 11.04 11.67 12.03 10.44 -

3.88 3.66 3.65 4.22 3.31 -

323.42 465.79 385.11 390.41 384.08 8.6

0.13 0.20 0.20 0.24 0.091 0.0091

ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Authors

*Aimin Liang. E-mail: [email protected]. *Junyan Zhang. E-mail: [email protected]. ORCID

a

Youxin Duan: 0000-0002-5384-8596

Lattice size was calculated by the formula a = 2d100/√3; d100 was calculated by the peak of the XRD pattern.

Notes

The authors declare no competing financial interest.

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Effect of Other Salt. NaCl is not the only salt that can induce the phase separation. Many other salts can also have a similar effect. Here we took potassium chloride (KCl) as an example. We used 3.5 mol L−1 of KCl to induce the phase separation, and the resulting sample was denoted as OMC3.5KCl-500. The small-angle XRD (Figure S7a), nitrogen sorption isotherm (Figure S7b), and TEM (Figure S8) showed that it had well-ordered mesoporous structure. The salt effect on the stability of the colloidal syetem has been studied for a long time, and the Hofmeister series was summarized to show the effect of cations and anions on the stability of colloidal systems.21 From this view, we can expect that different salts will result in OMCs with different structures and properties, and we can control the structures and properties of OMC by different salts to some extent. A more detailed study is still in progress. Table 1 shows the lattice size, pore size, BET surface area, and pore volume of the as-prepared five OMC samples and the amorphous carbon sample. From all these results, we can conclude that pH of the F127−PF solution plays a critical role in determining whether the ordered mesostructure can be obtained when salt is introduced into the self-assembly system. The as-prepared OMCs possess good thermal stability.

ACKNOWLEDGMENTS This work was supported by the National Key Basic Research and Development (973) Program of China (Grant No. 2013CB632300) and the National Natural Science Foundation of China (Grant No. 51275508).

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