Synthesis of Porous Carbon Balls from Spherical Colloidal Crystal

Jul 6, 2012 - Copyright © 2012 American Chemical Society ... Accordingly, the meso-/macroporous porous carbon balls exhibited higher ..... It is impo...
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Synthesis of Porous Carbon Balls from Spherical Colloidal Crystal Templates Youngchan Kim,† Chang-Yeol Cho,† Ji-Hwan Kang,† Young-Sang Cho,‡ and Jun Hyuk Moon*,† †

Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea Department of Powder Materials, Korea Institute of Materials Science, Changwon 641-831, South Korea

‡

S Supporting Information *

ABSTRACT: Spherical inverse opal (IO) porous carbon was produced utilizing silica colloidal crystal spheres as templates. The spherical colloidal crystals were obtained through the selfassembly of monodisperse particles inside an emulsion droplet with confined geometry. The templates were inverted using a carbon precursor, phenol-formaldehyde (PF) resol. We demonstrated a two-step synthesis involving the subsequent infiltration of the PF resol precursor into the spherical colloidal crystal template and a one-step synthesis using a silica colloidal solution containing dissolved PF resol. In the former case, the sizes of the IO carbon balls were controlled by the size of the colloidal crystal templates, and diameters of a few micrometers up to 50 μm were obtained. The average diameter of the macropores created by the silica particles was 230 nm. Moreover, meso-/macroporous IO carbon balls were created using blockcopolymer templates in the PF resol. In the one-step synthesis, the concentration of PF resol in the colloidal solution controlled the diameter of the IO carbon balls. IO balls smaller than 3 μm were obtained from the direct addition of 5% PF resol. The onestep synthesis produced rather irregular porous structures reflecting the less ordered crystallization processes inside the spherical colloidal crystals. Nitrogen adsorption and cyclic voltammetry measurements were conducted to measure the specific area and electroactive surface area of the IO carbon balls. The specific area of the mesopores-incorporated IO carbon balls was 1.3 times higher than that of bare IO carbon balls. Accordingly, the meso-/macroporous porous carbon balls exhibited higher electrocatalytic properties than the macroporous carbon balls.

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INTRODUCTION Interest in the use of porous carbon for a variety of applications, including electrochemical energy storage, catalysis, and separation, has grown over the past several years.1−7 Porous carbon has a high surface area, large pore volumes, is chemically inert, and is mechanically and thermally stable. Control over the pore size is deterministic in these applications, and, as a result, synergistic benefits may be obtained from the coexistence of a variety of pore sizes ranging from the micropore to the macropore scale.1,5,8,9 Micropores are mainly used in gas or liquid molecular adsorption and separation applications due to their ability to induce strong van der Waals interactions with molecules.10 Wider pores, such as mesopores or macropores, permit efficient transport without losing significant surface area.1 The presence of macropores significantly enhances the mass transfer and diffusion properties, thereby aiding processes involving mass flow in many practical applications. Such pores are favorable for use in recent applications, including supercapacitors,3 fuel cell electrodes,11 lithium batteries,12 and biosensors.13 Ordered porous carbons have been prepared via templating methods using molecular templates (e.g., surfactant assemblies and block-copolymer assemblies) and particulate templates (e.g., colloidal crystals).14−17 The first use of templates, developed by Knox and Ross,18 employed highly © 2012 American Chemical Society

mesoporous silica gel, which was later captured by the liquid carbon precursor of the phenol-formaldehyde (PF) polymeric resin. This approach was further extended to produce ordered mesopores with structured silica gel directed by self-assembled surfactants.19 Typically, molecular templates provide a wide range of mesopores but have difficulty in providing macropores. Colloidal particle templates are the only templating method for forming macropores.20−23 Colloidal crystal templates provide several advantages, including a guaranteed highly monodisperse pore size and the facile control over pore size, from several tens of nanometers to several micrometers. Briefly, a typical procedure for forming macroporous carbon using colloidal crystal templates is as follows:20,21,24−27 (i) polymeric particles (e.g., polystyrenes or polymethylmethacrylate) or silica particles are used to prepare colloidal crystals via the evaporation-induced crystallization of colloidal particles during spin-coating, dip-coating, or drop-casting on a substrate; (ii) liquid-phase carbon precursors (e.g., resol, furfuryl alcohol, acetonitrile, and sucrose) or gas-phase precursors fill the pores in the colloidal crystals; and (iii) carbonization at high Received: February 19, 2012 Published: July 6, 2012 10543

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Scheme 1. Schematic Diagram Showing the Two-Step Preparation of IO Carbon Spheresa

a

(a,b) Colloidal crystallization of silica particles in an emulsion droplet. (b−d) Infiltration of phenol-formaldehyde resol into voids among the silica particles and subsequent washing of the residue of resol on the surface. (d,e) Carbonization of the resol and subsequent removal of the template.

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temperatures and the subsequent selective removal of particles and carbonization produces porous inverse opal (IO) carbon. Control over the macroscopic shapes of porous carbon materials in a desired morphology is crucial, in addition to the control of pore size and porosity, especially for practical applications of these materials. For example, in applications to electrochemical devices, such as supercapacitors or lithium batteries, a particulate form of a porous carbon material provided favorable results in coatings of porous carbon materials on substrates by solution-casting.28,29 The shapes of the porous carbon materials could only be controlled by controlling the shapes of the original templates. Molecular templating processes using surfactants or block copolymers mainly yielded micrometer-size mesoporous carbon, polydisperse pores, and irregular shapes.30−32 Thus, extensive studies were performed to obtain porous carbon in the form of films,33 spheres,34 rods,35,36 fibers,19,37 and platelets.38 Porous carbon templated from colloidal particles was prepared mainly as a film on a substrate.26,39 In some applications, the crushed particles of IO carbon films have been used to obtain dispersed solutions; however, in this case, the long-range order of the pores was often lost.21,40,41 The motivation of our work focused on shape control of IO carbon. We used spherical colloidal crystals to obtain freestanding IO carbon balls. Spherical colloidal crystals were obtained through the crystallization of monodisperse colloidal particles inside droplets.42,43 Previously, a variety of confined geometries were used to design colloidal crystal shapes in the form of rods,44 hollow cylinders,45 and fibers.46 We applied spherical colloidal crystals as a template and infiltrated a carbon precursor into the colloidal crystals (two-step process) or added a precursor to a colloidal droplet (one-step process). Carbonization and etching of the templates produced macroporous carbon balls. The sizes of the carbon balls and their macropore size could be controlled independently by changing the size of the spherical colloidal crystals in both the one- and the two-step processes or by changing the amount of carbon precursor added to the one-step process. We obtained ordered macroporous carbon balls with pore diameters of around 200 nm and particle diameters in the range of a few micrometers to 50 μm. The addition of block copolymers to the carbon precursors produced mesopores around 8 nm in diameter that were incorporated into the macroporous IO carbon. The carbon obtained by pyrolysis of the phenol-formaldehyde (PF) resol was glassy. The electrocatalytic characteristics of a dispersion of IO carbon balls were analyzed. Higher surface areas in the meso-macroporous structures yielded higher electrocatalytic reactions at the meso-macroporous IO carbon, that is, 1.8 times the reaction rate on the macroporous IO carbon.

EXPERIMENTAL SECTION

Materials. We used tetraethylorthosilicate (TEOS, Alfa Aesar, 99+ %), ammonia solution (NH4OH, JUNSEI, 28 wt %), hexadecane (Sigma-Aldrich, 99%), hexane (Sigma-Aldrich, ≥96.5%), hypermer 2296 (Croda), phenol (Sigma-Aldrich, 99%), formaldehyde (SigmaAldrich, 36.5−38%), sodium hydroxide (NaOH, Sigma-Aldrich, 20% w/v aq soln), hydro chloric acid (HCl, Sigma-Aldrich, 37%), Pluronic F-127 (Sigma-Aldrich), potassium hexacyanoferrate(II) trihydrate (Sigma-Aldrich, 99.99%), potassium chloride (Sigma-Aldrich, 99%), ethanol (DAEJUNG, 99.9%), and hydrofluoric acid (HF, SigmaAldrich, 50%). Preparation of Silica Colloidal Crystal Spheres. An aqueous suspension containing monodisperse silica particles 200−400 nm in diameter was prepared according to the Stöber Method.47 Colloidal silica particles (30 wt % in water) and hexadecane (6 mL), with 1 wt % Hypermer 2296, were mixed in a volume ratio of 1:10, and then emulsified using a vortex generator (with a constant angular velocity of 3000 rpm) for 10 min. Water evaporation and subsequent colloidal crystallization was achieved at a constant temperature of 60 °C without stirring. After 12 h, silica colloidal crystal spheres were formed. The silica colloidal crystal spheres were thoroughly washed with hexane to remove hexadecane and finally calcined at 700 °C for 6 h in air. Preparation of IO Carbon Spheres. Phenol-formaldehyde resol was used as a carbon precursor. The resol was prepared with an excess ratio of phenol in the presence of a base catalyst. Briefly, 6.50 mmol of phenol in a 20% NaOH solution and 13 mmol of a 37 wt % formalin solution were reacted at 70 °C for 1 h. After neutralization by adding 0.6 M HCl (required in a later step for cross-linking), the mixture was dried in a vacuum oven. Finally, the phenol-formaldehyde resol was obtained by dilution in ethanol to 50% (v/v). For the mesoporous carbon, the phenol-formaldehyde resin was mixed with 0.012 mol of block copolymer F127 in a 0.2 M HCl solution. The carbon precursor was injected into the silica colloidal crystals. The residue of the resin on the surface of the silica colloidal crystal spheres was washed with ethanol several times. The phenol-formaldehyde resin injected into the silica colloidal crystals was thermally cross-linked at 100−140 °C for 1 day. Subsequently, the samples were carbonized under a nitrogen atmosphere for 3 h at 900 °C. The silica colloidal crystal sphere templates were removed by incubating in a 25% HF solution for 6 h at 25 °C. One-Step Preparation of the IO Carbon Spheres. The phenolformaldehyde resol was added to the previously prepared aqueous silica colloid. The silica colloid was centrifuged to concentrate the solution to a 40−50 vol % solution. The volume ratio of the resol was controlled to be 10 v/v% in the silica colloidal solution. The resol− silica colloidal solution in a hexadecane emulsion was heated at 60 °C overnight to evaporate the water. The subsequent processes, including the washing of hexadecane, the cross-linking of resol, carbonization, and the removal of silica colloidal particles, were identical to the procedures described above. Characterization. Nitrogen adsorption isotherms were recorded on a Micromeritics ASAP 2405N. The specific surface area was calculated using the BET method applied to the nitrogen adsorption data. Cyclic voltammetry was measured with three electrodes: a Pt counter electrode, a glassy carbon electrode, and a reference electrode (Ag/AgCl (saturated KCl)). An electrolyte solution containing 0.057 10544

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Figure 1. (a) SEM images of spherical silica colloidal crystals. The inset shows a digital camera image of the spherical colloidal crystal powder. (b) Magnified SEM image of a single spherical colloidal crystal. The inset shows the reflective optical microscope image. (c) Magnified SEM image of the surface of the spherical colloidal crystal. mmol of potassium hexacyanoferrate (II) trihydrate in a 1 M KCl solution was also used. The IO carbon spheres were dispersed in a mixture of 25% IPA (v/v) and deionized water, then deposited on the carbon electrode. The scan range was −0.2 to 0.7 V, and the scan speed was 50 mV/s. Raman spectra were collected using a Horiba Jobin Yvon LabRAM HR equipped with an air-cooled Ar-ion laser operating at 541 nm. The surface morphologies were measured by SEM (Hitachi, Carl Zeiss) and TEM (Carl Zeiss).

spherical geometry led to the regular arrangement of the monodisperse particles.43 The inset image in Figure 1b shows a reflective opaline color from the balls under an optical microscope. This confirmed that the organizational arrangement of silica particles extended from the surface to the inside of the ball.43 Considering a silica particle concentration of approximately 30% (v/v), with the particles packed closely in a face-centered cubic (FCC) lattice in the spherical colloidal crystals, the sizes of the crystals were expected to be 4−40 μm; the estimated diameters of the spherical colloidal crystals were 20% smaller than the diameters measured in the SEM images. This discrepancy implied that the FCC packing shown in Figure 1c was achieved on the outer surface, whereas inside the ball, the colloidal particles were less ordered and loosely packed. Here, the formation of a loose packing structure inside the ball was likely due to colloidal crystallization inside the droplets. In a typical evaporation-induced colloidal assembly, colloidal crystallization occurred at a colloid particle concentration of 50−60 v/v% and resulted in FCC packing with a 76 v/v% packing fraction.51 The local crystallization of particles near the droplet surface during the colloidal assembly in a droplet may be achieved at concentrations below the 50−60 v/v% range due to a higher evaporation rate near the surface than within the particle.42,52 Thus, the subsequent growth of colloidal crystals resulted in a particle packing structure with higher void fractions inside the ball. Spherical silica colloidal crystals immersed in the resol solution were washed thoroughly to remove any residues on the surfaces of the spheres. Washing for at least 1 h with stirring (300 rpm) was required to remove the viscous PF resol residue covering the surface. It is important not to remove all of the resol because residual resol is crucial for revealing the porous morphology on the surface of the ball. The PF resol was further cross-linked upon heating. The resulting PF resol-coated silica spherical colloidal crystals are shown in Figure S1a and S1b. The SEM images clearly showed that the excess resol had been removed from the surface. SEM images of the balls after heat treatment at 900 °C are shown in Figure 2a and b. The complete carbon conversion of the resol was confirmed from the FT-IR spectrum, in which no peaks were observed after treatment at 900 °C under a nitrogen atmosphere (not shown). The center-to-center distance between the pores of the IO carbon balls was approximately 230 nm, similar to the distance in the silica colloidal crystals. The removal of silica particles in the HF solution left behind IO carbon balls, as shown in Figure 2c−f. The diameters of the IO carbon balls ranged from a few micrometers to 50 μm, similar to the diameters of the silica

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RESULTS AND DISCUSSION Two-Step Synthesis of IO Carbon Balls. The fabrication of IO carbon balls using spherical colloidal crystals as templates is described in Scheme 1. Spherical colloidal crystals were synthesized by the self-assembly of monodisperse colloidal particles inside an emulsion droplet (Scheme 1a). Previously, depending on the concentration of colloidal particles inside the droplet, colloidal clusters of a few particles to colloidal crystal assemblies have been obtained. 48−50 In this study, a monodisperse silica colloid in a hexadecane emulsion was generated by mechanical stirring. During the evaporation of water droplets at 60 °C, the silica particles self-assembled into colloidal crystals (Scheme 1b). A powder of the spherical silica colloidal crystals was obtained after washing (to remove hexadecane) and subsequent heat treatment (to cement the silica particles). The PF resol, which was poured over the powder, infiltrated the voids among the silica particles under capillary forces (Scheme 1c). To remove the resol residue on the surfaces of the spherical colloidal crystals, the spherical colloidal crystals were washed in ethanol with mild stirring (Scheme 1d). The resol was thermally cross-linked at 100−140 °C and subsequently pyrolyzed at 700−900 °C in a furnace for carbonization. The silica spheres were removed using a diluted HF solution, leaving behind the porous IO spheres (Scheme 1e). Emulsion droplets containing the monodisperse silica colloidal particles prepared here initially ranged in diameter from 10 to 50 μm (25−50 droplets were analyzed under an optical microscope). During drying, the emulsion droplets collapsed over 6−9 h, increasing the diameter from 10 to 100 μm. The diameters of the spherical silica colloidal crystals after drying ranged from a few micrometers to 50 μm, based on SEM image analysis (Figure 1a). In a single batch, around 0.1 g of the spherical silica colloidal crystal powder was produced (see the inset in Figure 1). A magnified image of the spherical silica colloidal crystals is presented in Figure 1b and c. The surface clearly showed a hexagonal packing of silica colloids, and the center-to-center distance between particles was approximately 250 nm. The size of a single domain, as shown in Figure 1c, was approximately 5 μm by 5 μm. The capillary force exerted by the 10545

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the surfaces of the balls, the shell partially collapsed during removal of the silica spheres. Moreover, in the image of the fractured IO carbon balls, as shown in Figure 2e and f, the IO structures of the ordered spherical carbon shell formed completely up to the inside edges of the balls. The pores were well-ordered on the surfaces and extended to several tens of layers, but the order deteriorated inside the balls. As the curvature in a sphere increased, the FCC packing of the colloidal crystals became more limited, thereby producing more defects. The images of the carbon-coated silica colloidal crystal powder and IO carbon ball powder are shown in the insets of Figure 2a and c. The carbon-coated spherical silica colloidal crystals produced a dark-green color and turned black after the removal of the silica colloidal crystals. A green reflective color (reflectance peak at around 540 nm) resulted from the photonic band gap properties of the colloidal crystals. The wavelength (λ) was estimated by Bragg’s diffraction law, λ = 2d111·navg, where d111 is the distance between the (111) planes in the colloidal crystals and navg is the averaged refractive index.26 Briefly, d111 was equal to 190 nm for the 230 nm silica sphere packing and navg = 0.5, with nsilica = 1.4 and ncarbon = 2.1.26 From the law, the wavelength, λ, was approximately 560 nm, consistent with the observed green color. The strong light absorptive properties of carbon in the IO carbon balls mainly yielded a black color. We incorporated mesopores into the IO carbon balls by applying a mixture of triblock copolymers (F127) and resol as a carbon precursor.14,53 SEM images of the meso- and macroporous IO carbon balls are shown in Figure 3a and b. The magnified images of the surfaces, shown in Figure 3b, revealed a hexagonal packing of pores in which the center-to-center distance between pores was measured to be 220 nm. The TEM image of the carbon shell shown in Figure 3c displayed a partial hexagonal ordering of mesopores with an average diameter of around 5 nm. One-Step Synthesis of IO Carbon Balls. One-step fabrication was achieved by the formation of a resol-added silica colloidal solution in a hexadecane emulsion, as shown in Scheme 2. Thus, the PF resol infiltrated the silica spherical colloidal crystals, and subsequent washing steps in the two-step synthesis were unnecessary. The PF resol remained trapped inside the particles after the water evaporation step (Scheme 2b). Carbonization and the removal of the silica particles produced IO carbon balls (Scheme 2c). It should be noted that because the resol was only present inside the balls, the inner regions of the silica colloidal crystals could be used as templates for producing smaller carbon balls. In this experiment, we

Figure 2. (a,b) SEM image of carbonized spherical colloidal crystals and their magnified surface image. (c,d) SEM image of IO carbon balls after the removal of the silica colloidal crystals and their magnified surface image. The inset shows the IO carbon ball powder taken by a digital camera. (e,f) SEM image of fractured IO carbon balls and their magnified image.

colloidal crystals. The removal of silica particles was confirmed by EDX, as shown in Table 1. The Si atom content almost disappeared after etching in a 25% HF aqueous solution overnight. Table 1. EDX Analysis of Spherical Silica Colloidal Crystals and IO Carbon Balls sample silica colloidal crystal sphere/ carbon composite IO carbon balls

C (wt %)

O (wt %)

Si (wt %)

9.34

56.71

33.95

84.66

14.1

1.24

The resol conformally coated the silica particles, as shown in Figure S1a. After removal of the silica particles, a carbon shell remained, as shown in the magnified image of Figure 2d. On

Figure 3. (a) SEM image a of meso-macroporous IO carbon ball. (b) Magnified SEM image of the surface of a meso-macroporous IO carbon ball. (c) TEM image of the carbon shell. 10546

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Scheme 2. Schematic Diagram of the One-Step Synthesis of IO Carbon Ballsa

a (a−c) Evaporation of water in an emulsion droplet of the silica colloidal solution mixed with PF resol produced spherical silica colloidal crystals and PF resol inside the ball. (b,c) Carbon balls were obtained after the carbonization resol and the removal of the silica colloidal crystals.

Figure 4. (a) SEM image of resol-containing silica spherical colloidal crystals. (b) SEM image of their magnified surface. (c) SEM image of IO carbon balls. (d) Magnified SEM image of a single IO carbon ball.

aqueous PF resol solution. The resol-containing water (72 mN/m for water) had a much higher surface tension than hexadecane (23 mN/m). Thus, the silica surface was not wetted by the water, and, consequently, the PF resol did not coat the silica particles. After the water was dried inside the droplet, the PF resol remained inside the ball. Here, we expect that an increase in the concentration of PF resol relative to the concentration of silica colloids in solution would yield larger IO carbon balls. Figure S2 shows SEM images of IO carbon balls with 20% (v/v) PF resol. The average diameter of the IO carbon balls was around 18 μm. The sizes of the IO carbon balls prepared here were estimated by considering the volume of resol in the silica colloidal solution. The volume of resol was 5% (v/v) relative to the volume of colloidal particles. The void fraction in a loose random packing of particles inside a ball could be assumed to be approximately 40%. In the case in which resol remained inside the ball, the volume of the sphere filled only with the resol could be approximated using the following relation: (the packing fraction of spherical colloidal crystals (60%))*(the volume of spherical colloidal crystals (VSCC))*(the volume ratio of the resol to the colloidal particles (5%)) = (the void fraction of spherical colloidal crystals (40%))*(the volume of

added the PF resol solution directly to the silica colloidal solution. The volume ratio of the PF resol solution to the silica colloidal solution was controlled to be 10% (v/v); thus, the volume ratio of the resol to the colloidal solution was around 5% (v/v). After the evaporation of water from the droplet, the sizes of the balls were measured and found to be 40−100 μm (Figure 4a). The magnified surface images in Figure 4b show that the center-to-center distance between particles was 230 nm. SEM images of the IO carbon balls after carbonization and the subsequent removal of silica particles in a one-step process are shown in Figure 4c and d. The sizes of the IO carbon balls, shown in Figure 4c and d, were 3 μm on average. The size of the ball shrunk by a large amount (3−6% of the size of the silica spherical colloidal crystals). This confirmed that the resol, which dissolved in the aqueous colloidal solution, remained present only in the centers of the silica spherical colloidal crystals, as described in Scheme 2b. This could be explained in terms of the wetting behavior of water at a silica− hexadecane interface during the water evaporation-induced colloidal crystallization. Colloidal crystallization started from the surface to the ball core. Simultaneously, hexadecane permeated the colloidal crystals and filled the volume where water was present while maintaining the interface with the 10547

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Figure 5. (a) Nitrogen adsorption/desorption isotherms and (b) the estimated pore size distributions (BJH method) of macroporous IO carbon balls and meso-/macroporous IO carbon balls.

spheres inside the spherical colloidal crystals that were filled with the resol (VSCC/R)). The calculation provides a ratio VSCC/R/VSCC of 8%. Thus, the diameters of the IO carbon balls could be estimated to be 3.2−8 μm under the assumption of no volume shrinkage during carbonization of the resol or removal of the colloidal crystals. The estimated diameters of the IO carbon balls agreed with the measured diameters under SEM images. The center-to-center diameters were found to be approximately 210 nm using an SEM image. The linear shrinkage was 9%, higher than the shrinkage observed in the one-step process. Moreover, the arrangement of pores was rather random, in contrast with the packing of the particles observed on the surfaces of the balls. A higher shrinkage and disordered pore structures implied that the packing of the silica particles inside the silica colloidal crystals was incomplete and random, thereby containing larger voids. Characterization of the IO Carbon Balls. The surface areas of the IO carbon balls were measured from Brunauer− Emmett−Teller (BET) nitrogen adsorption isotherms and compared the IO carbon balls prepared with or without mesoporous templates (F127), as shown in Figure 5. The isotherm indicated that the adsorption of the bare IO carbon balls did not increase. By contrast, IO carbon balls prepared using the resol with the block copolymers showed an increase in the adsorption for 0.6 > P/P0 > 0.4 and two hysteresis loops (Type IV isotherm), which clearly indicated the incorporation of mesopores into the IO carbon balls. The BET surface areas of the bare IO carbon balls and the mesoporous IO carbon balls were 564 and 741 m2 g−1. In Figure 5b, the pore size distribution was also determined by the Barrett−Joyner− Halenda (BJH) method using the desorption isotherm. The pore diameters of the mesoporous IO carbon balls were narrowly distributed around an average of 4 nm in diameter, which agreed with the results from TEM analysis. Raman spectra of the IO carbon balls were collected to characterize the carbon, as shown in Figure S3. The spectra showed that characteristic carbon peaks appeared at 1300 and 1600 cm−1. The two carbon peaks present at approximately 1360 and 590 cm−1 correspond to the D and G bands of carbon, respectively.35,54,55 D is characteristic of the A1g mode for in-plane disorder or defects. An increase in D represents an increase in the disordered amorphous carbon. G revealed the in-plane E2g vibrational modes of the graphite sheet. The G peak often overlaps with a peak at 1620 cm−1, which also represents disordered carbon. The spectra in Figure S3 show

that as the temperature increased, the D peak broadened, and the intensities relative to the G peak increased, whereas the G peak shifted only slightly toward lower frequencies. These results revealed that an increase in both graphitic carbon and disorder among the carbon atoms for increasing temperature led to the formation of glassy carbon above 700 °C. Glassy porous IO carbon balls are attractive candidates for use in electrochemical applications.6,56 For example, glassy porous carbon is suitable for use in high-power supercapacitors.3,17,29 Here, we evaluated the electrocatalytic properties of the porous IO carbon balls. Cyclic voltammetry was used to measure the electrocatalytic redox reaction of ferrocyanide electrolytes at the carbon surface. To use the IO carbon balls as a working electrode, the compounds were dispersed in ethanol and coated onto a glassy carbon electrode. IO carbon balls were prepared by a two-step procedure, either with or without mesopore incorporation into the carbon. The peaks in the voltammogram fell in the range 0.2−0.4 V, as shown in Figure 6, and revealed the presence of a redox reaction among ferrocyanide ions. The results indicated that the peak current was 12 μA for macroporous IO carbon balls and 25 μA for meso-/macroporous IO carbon balls. The electroactive area was linearly related to the peak current based on the Randles− Sevcik equation.57 The intensity of the mesoporous IO carbon balls was 1.8 times higher than that of the bare IO carbon balls.

Figure 6. Cyclic voltammetry measurements of macroporous IO carbon balls and meso-/macroporous IO carbon balls. 10548

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Thus, the electroactive surface area of the meso-/macroporous IO carbon displayed a larger electrocatalytic surface than the macroporous IO carbon. The mesoporous IO carbon balls showed a higher electroactive surface area (as determined by BET) than the IO carbon balls with or without mesopores. The mesopores may enhance ion transport, thereby increasing the electroactive area.

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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 This work was supported by grants from the National Research Foundation of Korea (2010-0028961, 2010-0011024). The Korea Basic Science Institute is also acknowledged for the SEM measurements.

CONCLUSIONS

We used spherical colloidal crystals as a template for macroporous and meso-/macroporous IO carbon balls. Spherical colloidal crystals were obtained through the selfassembly of monodisperse silica particles inside emulsion droplets. Specifically, we demonstrated a two-step and a onestep synthesis approach. The two-step synthesis involved infiltrating PF resol into the spherical colloidal crystals. Removal of the PF resol from the ball surface was crucial for obtaining a porous carbon surface. After carbonization of the PF resol and the subsequent removal of the colloidal crystal templates, macroporous IO balls were obtained. The carbonization of PF resol produced a glassy carbon phase, as determined by Raman analysis. The sizes of the porous balls, controlled by the size of the spherical colloidal crystals, measured 10−55 μm in this experiment. The diameters of the macropores, controlled by the sizes of the silica particles, measured approximately 230 nm. Meso-/macroporous IO carbon balls were created by the addition of block-copolymer templates to the PF resol solution. The one-step synthesis involved adding PF resol directly to the silica colloidal solution. Because colloidal crystallization involved water evaporation from the droplet surface to the core, PF resol dissolved in water remained inside the ball, which could be explained in terms of the surface tension of aqueous droplets in a hexadecane medium. Thus, only the core of the colloidal crystals provided a template for producing carbon balls. The sizes of the IO carbon balls depended on the weight fraction of the PF resol: a 5% (v/ v) PF resol solution resulted in IO balls with diameters of approximately 3 μm. Finally, we analyzed the material properties of carbon in the IO balls. BET measurements were collected to compare the surface areas of the macroporous and meso-/macroporous IO carbon balls. The results showed that the incorporation of mesopores increased the specific area by a factor of 1.3. The electrocatalytic properties of the porous IO carbon balls were evaluated by cyclic voltammetry on the redox reaction of the ferrocyanide electrolytes. The results showed that meso-/macroporous IO carbon possessed an electroactive capacity that was higher by a factor of 1.8 and retained a higher specific area than the macroporous IO carbon balls. We are currently investigating the use of IO carbon balls in a variety of energy storage devices, such as supercapacitors. The colloidal crystal templating process described here provides a facile approach to controlling the morphologies of porous carbon and achieving a wide range of pore sizes, which is critical for these applications.

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