Aerosol Synthesis of Self-Organized Nanostructured Hollow and

Sep 11, 2014 - Department of Mechanical and Nuclear Engineering, School of Engineering, Virginia Commonwealth University, 401 West Main. Street ...
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Aerosol Synthesis of Self-Organized Nanostructured Hollow and Porous Carbon Particles Using a Dual Polymer System Ratna Balgis,† Takashi Ogi,*,† Wei-Ning Wang,‡ Gopinathan M. Anilkumar,§ Sumihito Sago,†,§ and Kikuo Okuyama† †

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan ‡ Department of Mechanical and Nuclear Engineering, School of Engineering, Virginia Commonwealth University, 401 West Main Street, Richmond, Virginia 23284, United States § Research and Development Center, Noritake Co., Ltd., 300 Higashiyama, Miyoshi, Aichi 470-0293, Japan S Supporting Information *

ABSTRACT: A facile method for designing and synthesizing nanostructured carbon particles via ultrasonic spray pyrolysis of a self-organized dual polymer system comprising phenolic resin and charged polystyrene latex is reported. The method produces either hollow carbon particles, whose CO2 adsorption capacity is 3.0 mmol g−1, or porous carbon particles whose CO2 adsorption capacity is 4.8 mmol g−1, although the two particle types had similar diameters of about 360 nm. We investigate how the zeta potential of the polystyrene latex particles, and the resulting electrostatic interaction with the negatively charged phenolic resin, influences the particle morphology, pore structure, and CO2 adsorption capacity.

1. INTRODUCTION Carbon particles with hierarchical nanostructures are of great scientific interest because of their remarkable properties, such as high specific surface area and porosity, which enable their use in adsorbents, drug/gene carriers, supercapacitors, fuel cells, and lithium-ion batteries.1−4 A careful structuring strategy is highly desirable, yet very challenging, to obtain nanostructured carbon particles. Much effort has been expended to develop liquid- and gas-based processes to meet this challenge. In general, nanostructured carbon particles are prepared by using a template of inorganic materials, such as mesoporous silica particles, and an organic material as a carbon source that fills the voids in the template.5−7 However, removing the inorganic templates often requires corrosive chemicals, such as hydrofluoric acid, which greatly limits the possibilities for industrial scale-up.8,9 A promising alternative is to use organic templates, such as polystyrene latex (PSL), that readily decompose during carbonization.10−14 Aerosol synthesis of submicron-sized hollow and porous silica (SiO2) particles has been demonstrated by self-assembly of PSL, as the template, and colloidal SiO2 nanoparticles (NPs) as the host material.15−17 Submicron © 2014 American Chemical Society

SiO2 particles, containing ordered pores in a close-packed hexagonal arrangement, were obtained by adjusting the zeta potential and concentration of the PSL NPs. However, this strategy may not be easily applied to synthesize well-structured hollow or porous carbon particles because the hydrophobic nature of the host carbon NPs is not amenable to forming a homogeneous mixture with PSL NPs. Hence, it is highly required to find an alternative carbon source. Herein we develop a self-organized dual polymer system to address this issue. Phenolic resin was selected as the carbon source since it contains OH− groups that may allow control over the self-organization with PSL NPs through electrostatic interaction. The electrostatic interaction between phenolic resin and PSL can be tuned by adjusting the sign and magnitude of the zeta potential (ζ) of PSL. To the best of our knowledge, the present study is the first to report the use of electrostatic forces to promote the self-organization of polymers within a droplet. Received: March 31, 2014 Revised: August 29, 2014 Published: September 11, 2014 11257

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Scheme 1. Schematic of the Experimental Procedure

Figure 1. (a) Effect of AIBA concentration on zeta potential of PSL. SEM images of carbon particles prepared (b) without a PSL template and prepared using a PSL template with a zeta potential of (c) 53 mV, (d) 40 mV, (e) 22 mV, (f) 12 mV, and (g) −59 mV. TEM images of carbon particles prepared using a PSL template with a zeta potential of (h) 53 mV and (i) −59 mV. CO2 adsorption capacity was obtained by temperature-programmed desorption (TPD, BELCAT-A, BEL Japan, Inc., Osaka, Japan).

We investigate how the electrostatic forces between the two polymers influenced the morphology of the pores and particles.

3. RESULTS AND DISCUSSION To evaluate the electrostatic interaction between phenolic resin and PSL, the zeta potential of PSL is an important parameter to consider. Positively charged PSL were obtained by using the initiator 2,2′-azobis (isobutyramidine) dihydrochloride (AIBA), and their zeta potential was controlled by adjusting the quantity of initiator. Figure 1a indicates that the zeta potential is proportional to AIBA concentration and is saturated at 3 wt % and higher. To obtain negatively charged PSL, potassium persulfate was used as an initiator. The effect of PSL addition on particle morphology was observed by SEM and TEM. In this work, we have classified the morphology of particles into two types as follows; particles with open macropores on the surface are called porous particles, while particles having their pores covered by a complete shell are called hollow particles. Spray pyrolysis of the phenolic resin precursor resulted in spherical particles which are likely microporous, as shown in Figure 1b and the Supporting Information Figure SI 1a. Hollow carbon particles were obtained when highly positively charged PSL (ζ = 53 mV) was added to the precursor mixture, prior to spray pyrolysis (Figure 1c). The structure of the resulting carbon particles gradually changes from hollow to porous as the zeta potential of the PSL is decreased, because lowering the zeta potential

2. EXPERIMENTAL SECTION Spherical carbon nanostructures were prepared from an aqueous precursor solution containing phenolic resin (Sumitomo Bakelite Co., Ltd., Tokyo, Japan) and electrically charged PSL NPs (∼230 nm diameter), using spray pyrolysis, by a previously reported method.10 Droplets were generated by an ultrasonic nebulizer (1.7 MHz, NEU17, Omron Healthcare Co., Ltd., Kyoto, Japan) and sprayed through a tubular furnace with four stacked temperature zones set to 150, 300, 700, and 700 °C, as outlined in Scheme 1. N2 gas (0.8 L min−1) was used to carry the droplets through the furnace. Positively and negatively charged PSL NPs were used to control the self-assembly of phenolic resin. The mass ratio of phenolic resin to PSL was fixed at 0.625. Characterization. Particle morphology was observed using scanning electron microscopy (SEM, S-5000, 20 kV, Hitachi HighTech. Corp., Tokyo, Japan), transmission electron microscopy (TEM), and electron tomography (JEM-2010, 200 kV, JEOL Ltd., Tokyo, Japan). The pore size diameter was determined by gas sorption and estimated using nonlocalized density functional theory (NLDFT) and grand canonical Monte Carlo method (GCMC) from the computer simulation that have been developed in recent years as the evaluation method of pore size distribution of porous materials (BEL Japan, Inc., BELSORP-max, Osaka, Japan). The NLDFT/GCMC method well describes the adsorption of adsorptive to the porous materials and are useful for micropore and mesopore analysis. A profile of the particles’ 11258

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Figure 2. Schematic diagram of nanostructured particle formation during spray pyrolysis from the following precursors: (a) phenolic resin, (b) phenolic resin and positively charged PSL, and (c) phenolic resin and negatively charged PSL.

weakens the electrostatic interaction between the PSL and the phenolic resin (Figure 1d−f). Figure 1f shows that, instead of hollow particles, highly porous carbon particles were achieved when PSL with a zeta potential of 12 mV was used. Conversely, addition of negatively charged PSL produces a completely macroporous carbon, as depicted in Figure 1g. To investigate the inner structure of the carbon particles, we obtained TEM images of carbon particles prepared using highly positive charged PSL (ζ = 53 mV) and negatively charged PSL (ζ = −59 mV). The difference between the structures of the two particle types was clearly evident in their corresponding TEM images (parts h and i of Figure 1, respectively), although the two particle types had similar diameters of about 360 nm. The high-resolution (HR)-TEM image on the right-hand side of Figure 1h shows macropores covered by a thin carbon shell (∼5 nm), which confirms the hollow morphology. In Figure 1i, the left-hand image shows the particle surface occupied by macropores, and the right-hand HR-TEM image shows a triangle-like carbon shell, indicating that no coating was formed; the phenol filled only the voids between PSL particles. Micro- and mesopore formation, in both the hollow and porous carbon samples, can be attributed to the release of gas during PSL decomposition and carbonization of the phenolic resin.18 Figure 2 shows a proposed model for the formation of nanostructured carbon from phenolic resin. During spray pyrolysis, the temperature of the droplets is usually lower than the set-point temperature of the tubular furnace. This is because the rapid flow of the carrier gas propels each droplet

quickly through each stack of the furnace before heat has been completely transferred from the furnace to the droplet. The precise difference in the temperature of the droplet and furnace cannot be analyzed easily because of the paucity of quantitative data. As a consequence, cross-linking and curing processes were not individually examined in this work. Figure 2a illustrates the self-assembly of the phenolic resin precursor. In the lowtemperature zone, a droplet containing oligomers of the phenolic resin is compacted into a spherical cross-linked phenol as water evaporates.6 A three-dimensional interconnected network of methylene bridges forms among the phenol oligomer units to yield spherical carbon particles. The release of gas during carbonization of the phenolic resin generates microor even mesopores in the particles (Figure SI 1 in the Supporting Information).18 Figure 2b shows the self-assembly of phenolic resin with positively charged PSL. Phenolic resin in oligomeric form contains OH− groups and has a zeta potential of −40 mV. Strong electrostatic attraction between the highly positively charged PSL (ζ = 53 mV) and the OH− groups of the phenol oligomers causes the phenolic resin to associate closely with the PSL NPs, packing the individual PSL NPs tightly into the center of each droplet. The phenol oligomers then crosslink and cure as the water evaporates, leaving phenolic-resincoated PSL. Complete polymerization of these phenol oligomers occurs at lower temperatures than required for PSL decomposition, as indicated previously by thermogravimetric analysis (Figure SI 1b in the Supporting Information).10 Hollow particles are formed when the number of highly 11259

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Figure 3. SEM images of (a) PSL (ζ = 53 mV) and (b) PSL (ζ = −59 mV). The insets show the obtained carbon particles. (c) Model of PSL arrangement in a droplet.

positively charged PSL NPs in a droplet is small enough to be covered by phenolic resin, so that the excess phenolic resin serves as a shell. Porous particles may form if not enough phenolic resin is available to form an outer shell. When the electrostatic attraction is weaker (ζ ≤ 40 mV) most of the phenolic resin fills the voids between PSL NPs, which become randomly distributed inside the droplet. In this case, a larger number of porous particles are formed after PSL decomposition because the greater uniformity in the distribution of phenolic resin throughout the droplet tends not to favor a shell structure. This is also very similar to the result of incorporating negatively charged PSL NPs, where the PSL NPs are evenly distributed inside the droplet, as shown in Figure 2c. SEM and TEM analysis support the proposed formation pathways. PSL NPs naturally form a hexagonal arrangement because of their high monodispersity, as shown in Figure 3a,b. Highly positively charged PSL NPs become coated by phenolic resin due to electrostatic attraction and resulting hollow carbon particles which possess compact, regular polyhedral structures, as depicted in the inset of Figure 3a. In contrast, negatively charged PSL NPs are repelled from the phenolic resin until the water completely evaporates from the droplet and polymerization occurs. The resulting porous carbon particles have spherical structures, with hexagonally arranged pores, as shown in the inset of Figure 3b. The morphology of the carbon particles also depends on the number of PSL contained in a droplet. The schematic in Figure 3c shows that each sprayed droplet may contain a different number of PSL, which may influence the final particle morphology. The trends in particle morphology are similar to those reported previously.17,19 The number of PSL particles also influences the size of the final carbon particles. The 2D correlation between carbon particle radius, R, and PSL radius, r, as derived from trigonometry rules, is presented here in eqs 1.a−1.f. R=r

R = 2r

R = 2.7r

R = 3r

(1.b)

for n = 2 for n = 3

2 )r

for n = 4

for n = 5

for n = 6

(1.d) (1.e) (1.f)

To analyze the formation of the carbon shell more accurately, samples were investigated by electron tomography (Figure SI 2 in the Supporting Information). This method was consistent with the SEM observations, confirming that the hollow particles consist of pores surrounded by a thin carbon shell and that the porous particles comprise an interconnected system of open macropores. The CO2 adsorption characteristics of prepared hollow and porous carbon particles were investigated by temperatureprogrammed desorption (TPD) in the temperature range 50− 500 °C and at a pressure of 1 bar. The samples were heated at 500 °C under He atmosphere prior to TPD measurements, to eliminate the possibility of CO2 generation from oxygenated groups of the samples during TPD measurements. In order to perform the CO2 adsorption on sample, after cooling, the sample chamber was filled with CO2 gas, which adsorbed on the samples. The sample chamber was then filled again with He to remove excess adsorbed gas. As the temperature was gradually increased to 500 °C, the amount of desorbed gas was measured to obtain the TPD curve. Because TPD is also known as thermal desorption spectroscopy, the data/graph collected represents the mass of desorbed CO2. Two major desorption peaks were observed for both types of particles at 120 and 280 °C, respectively (Figure SI 3 in the Supporting Information). The CO2 adsorption capacity was 3.04 and 4.78 mmol g−1 for hollow and porous carbon particles, respectively, which is much higher than that of other materials summarized in Table SI 1 in the Supporting Information.20,21 The surface functional groups on the prepared porous carbon particles were characterized via FT-IR to determine the performance limit of the prepared carbon particles (Figure SI 4 in the Supporting Information). Because both the porous and hollow carbon particles were prepared from the same source, we believe that they should contain similar functional groups, based on previously reported IR spectra of mesoporous carbon particles synthesized from phenolic resin.18 The C−O stretching vibration (1320 cm−1)

(1.a)

for n = 1

⎛ ⎞ 2 R = ⎜1 + 3 ⎟r ⎝ ⎠ 3

R = (1 +

(1.c) 11260

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and Porous Carbon Nanospheres Using Spray Pyrolysis. ACS Nano 2013, 7, 11156−11165. (2) Qiao, Z.-A.; Guo, B.; Binder, A. J.; Chen, J.; Veith, G. M.; Dai, S. Controlled Synthesis of Mesoporous Carbon Nanostructures via a “Silica-Assisted” Strategy. Nano Lett. 2013, 13, 207−212. (3) Yang, C.-M.; Noguchi, H.; Murata, K.; Yudasaka, M.; Hashimoto, A.; Iijima, S.; Kaneko, K. Highly Ultramicroporous Single-Walled Carbon Nanohorn Assemblies. Adv. Mater. 2005, 17, 866−870. (4) Liu, J.; Yang, T.; Wang, D.-W.; Lu, G. Q. (Max); Zhao, D.; Qiao, S. Z. A Facile Soft-Template Synthesis of Mesoporous Polymeric and Carbonaceous Nanospheres. Nat. Commun. 2013, 4, 1−7. (5) Boissiere, C.; Grosso, D.; Chaumonnot, A.; Nicole, L.; Sanchez, C. Aerosol Route to Functional Nanostructured Inorganic and Hybrid Porous Materials. Adv. Mater. 2011, 23, 599−623. (6) Yu, X.; Ding, S.; Meng, Z.; Liu, J.; Qu, X.; Lu, Y.; Yang, Z. Aerosol Assisted Synthesis of Silica/Phenolic Resin Composite Mesoporous Hollow Spheres. Colloid Polym. Sci. 2008, 286, 1361− 1368. (7) Schuster, J.; He, G.; Mandlmeier, B.; Yim, T.; Lee, K. T.; Bein, T.; Nazar, L. F. Spherical Ordered Mesoporous Carbon Nanoparticles with High Porosity for Lithium−Sulfur Batteries. Angew. Chem., Int. Ed. 2012, 51, 3591−3595. (8) Ryoo, R.; Joo, S. H.; Jun, S. Synthesis of Highly Ordered Carbon Molecular Sieves via Template-Mediated Structural Transformation. J. Phys. Chem. B 1999, 103, 7743−7746. (9) Muylaert, I.; Verberckmoes, A.; De Decker, J.; Van Der Voort, P. Ordered Mesoporous Phenolic Resins: Highly Versatile and Ultra Stable Support Materials. Adv. Colloid Interface Sci. 2012, 175, 39−51. (10) Balgis, R.; Sago, S.; Anilkumar, G. M.; Ogi, T.; Okuyama, K. Self-Organized Macroporous Carbon Structure Derived from Phenolic Rein via Spray Pyrolysis for High-Performance Electrocatalyst. ACS Appl. Mater. Interfaces 2013, 5, 11944−11950. (11) Ko, Y. N.; Park, S. B.; Jung, K. Y.; Kang, Y. C. One-Pot Facile Synthesis of Ant-Cave-Structured Metal Oxide-Carbon Microballs by Continuous Process for Use as Anode Materials in Li-Ion Batteries. Nano Lett. 2013, 13, 5462−5466. (12) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Synthesis of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120, 6024− 6036. (13) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a LiquidCrystal Template Mechanism. Nature 1992, 359, 710−712. (14) Thomas, A. Functional Materials: from Hard to Soft Porous Frameworks. Angew. Chem., Int. Ed. 2010, 49, 8328−8344. (15) Iskandar, F.; Mikrajuddin; Okuyama, K. Controllability of Pore Size and Porosity on Self-Organized Porous Silica Particles. Nano Lett. 2002, 2, 389−392. (16) Balgis, R. B.; Anilkumar, G. M.; Sago, S.; Ogi, T.; Okuyama, K. Ultrahigh Oxygen Reduction Activity of Pt/Nitrogen-Doped Porous Carbon Microspheres Prepared via Spray-Drying. J. Power Sources 2013, 229, 58−64. (17) Lee, S. Y.; Gradon, L.; Janeczko, S.; Iskandar, F.; Okuyama, K. Formation of Highly Ordered Nanostructured by Drying Micrometer Colloidal Droplets. ACS Nano 2010, 4, 4717−4724. (18) Kim, Y. J.; Kim, M. I.; Yun, C. H.; Chang, J. Y.; Park, C. R.; Inagaki, M. Comparative Study of Carbon Dioxide and Nitrogen Atmospheric Effects on The Chemical Structure Changes During Pyrolysis of Phenol-Formaldehyde Spheres. J. Colloid Interface Sci. 2004, 274, 555−562. (19) Talapin, D. V.; Schevchenko, E. V.; Bodnarchuk, M. I.; Ye, X.; Chen, J.; Murray, C. B. Quasicrystalline Order in Self-Assembled Binary Nanoparticle Superlattices. Nature 2009, 461, 964−967. (20) Zhao, Y.; Zhao, L.; Yao, K. X.; Yang, Y.; Zhang, Q.; Han, Y. Novel Porous Carbon Materials with Ultrahigh Nitrogen Contents for Selective CO2 Capture. J. Mater. Chem. 2012, 22, 19726−19731.

observed in the spectrum suggests that a higher temperature of spray pyrolysis may be needed to obtain oxygen-free carbon particles, which may have better CO2 adsorption capacity. The porous carbon particles show higher adsorption capacity than the hollow carbon particles, likely because the open pores on their surface cause their surface area to be larger than that of the hollow particles. The results of the present study suggest that carbon particles derived from phenolic resin have promising applications in CO2 adsorption and storage. Further research is needed to confirm how the functional groups and particle morphology influence the adsorption capacity, to optimize the experimental conditions of the spray pyrolysis system, and to understand the physical and chemical properties of the particles. Nevertheless, achieving nanostructured hollow and porous carbon particles, through electrostatic self-organization of dual polymer systems, offers exciting possibilities for using carbon particles as a high-performance gas adsorbent.

4. OUTLOOK Nanostructured hollow and porous carbon particles were prepared by spray pyrolysis of a dual polymer system. The morphology of the prepared particles can be tailored by tuning the attractive or repulsive forces between the precursor components. Strong electrostatic attraction between the phenolic resin and highly positively charged PSL formed hollow carbon particles. Weaker attractive force, as in the case of particles with a small positive charge, or repulsions due to negatively charged PSL resulted in porous carbon particles. The as-prepared particles readily adsorb CO2 because they contain various aromatic groups derived from the phenolic resin and large active surface area. This study opens new opportunities to develop advanced adsorbent material for future needs. Our group continues to study the optimal conditions for the spray pyrolysis system and the properties of the carbon particles, which will be reported in the near future.

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ASSOCIATED CONTENT

S Supporting Information *

Details about experimental procedure; TGA of phenolic resin, PSL, and phenolic resin-PSL composites; electron tomography, CO2 TPD profile, and FT-IR of the prepared carbon particles. 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]. Phone: +81-82-424-7850. Fax: +81-82-424-7850. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.B. gratefully acknowledges The Japan Society for the Promotion of Science (JSPS) for providing a postdoctoral fellowship. The authors would like to thank Dr. Eishi Tanabe for TEM analysis, Miss. Keiko Ohta from the R & D center of Noritake Co. Ltd. for TPD analysis and discussions, and Mr. Takahiro Mori for experimental assistance.

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REFERENCES

(1) Wang, C.; Wang, Y.; Graser, J.; Zhao, R.; Gao, F.; O’Connell, M. J. Solution-Based Carbohydrate Synthesis of Individual Solid, Hollow, 11261

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(21) Wang, S.; Zhao, L.; Wang, W.; Zhao, Y.; Zhang, G.; Ma, X.; Gong, J. Morphology Control of Ceria Nanocrystals for Catalytic Conversion with Methanol. Nanoscale 2013, 5, 5582−5588.

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