Topological Transformation of Thioether-Bridged Organosilicas into

Article pubs.acs.org/cm

Topological Transformation of Thioether-Bridged Organosilicas into Nanostructured Functional Materials Jung Ho Kim,† Baizeng Fang,† Min Young Song,† and Jong-Sung Yu*,† †

Department of Advanced Materials Chemistry, Korea University, 208 Seochang, Jochiwon, ChungNam 339-700, Republic of Korea S Supporting Information *

ABSTRACT: The strong interest in nanostructured functional materials has motivated the scalable production of high quality mesoporous silicas and carbonaceous materials. Although many approaches have been explored for this goal, it is highly desired and still remains a challenge to develop a straightforward strategy for simple and cost-effective fabrication of nanostructured functional materials. Here we demonstrate a simple sol−gel preparation of bis[3-(triethoxysilyl)propyl]tetrasulfide-based organosilica nanostructured materials and their topological transformations, through which porous spherical silica or carbon and hollow silica or carbon capsule are synthesized. As a representative application, the hollow carbon capsule is employed as a catalyst support for dispersion of high loading of Pt, which exhibits much higher catalytic activity toward oxygen reduction reaction than other porous carbon materials prepared in this work due to its larger surface area and mesoporous volume, particularly the unique architecture composed of a hollow macroporous core and a mesoporous shell, facilitating not only small size and good dispersion of Pt nanoparticles but also fast mass transport. KEYWORDS: organosilica, mesoporous materials, porous carbon, TESPTS, catalyst support, oxygen reduction reaction

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INTRODUCTION An upsurge in mesoporous silica alchemy began after 1992 when a class of first periodic mesoporous silicas known as the M41S family was introduced by Mobil Oil Company.1 A wide range of synthesis routes to mesoporous materials with different mesophases and morphologies has been reported, including the use of cationic, neutral, and nonionic surfactants and even surfactant mixtures under basic, acidic, and neutral conditions.2−14 The great interest in developing inorganic−organic hybrid materials arises from the expectation that the assembly of organic and inorganic building blocks in a single material can combine their particular advantages. Compared with the mesoporous silicas or/and functional mesoporous silicas with terminally bonded organic groups, periodic mesoporous organosilicas (PMOs) exhibit combined advantages of periodic mesoporous materials and hybrid organic−inorganic materials.15−23 Thioether functionalities exhibit strong affinity for metal ions and allow ideal functional sites on mesoporous materials as an efficient adsorbent to remove heavy metal ions in wastewater. Moreover, thioether can be oxidized to sulfonic acid groups functioning as an acid catalyst, drawing much attention on the synthesis of ordered mesoporous materials with high loading of disulfide groups.24 Bis(3-triethoxysilylpropyl) tetrasulfide (TESPTS) is an organic silane with propyl, ethoxy, and tetrasulfide groups, and its interactions with silica are well documented.25,26 Despite many interesting properties, not much research has been performed on nanoporous materials possessing thioether moiety in the framework. © 2012 American Chemical Society

Recently, ordered mesoporous organosilicas with thioether moiety embedded in the framework have been synthesized via co-condensation of TESPTS and tetramethoxysilane (TMOS) by using cetyltrimethylammonium bromide (CTAB) as a surfactant in basic conditions.27 Lately, thioether-bridged mesoporous organosilicas with different mesostructures were synthesized by tuning the TESPTS/TMOS molar ratio.28 In this report, we present ongoing developments of new nanostructured functional materials with distinctive novelty. This work demonstrates the simple fabrication of uniform spherical thioether-bridged organosilica (TBOS) in alcoholic solution with ammonia and water by sol−gel process of TESPTS. In addition, the sol−gel synthesis route using TESPTS in combination with octadecyltrimethoxysilane (C18TMS) or CTAB gives rise to novel and differently functionalized monodisperse core−shell like TBOS spheres, and their topological transformations into novel nanostructured materials are successfully managed to create new functional nanoarchitectures. Particularly, the nanostructured TBOS composites with both organic and inorganic sources in the framework are demonstrated as suitable and versatile precursors for the direct structural control of various silica/carbon composites and hollow and nanoporous silica or carbon. Interesting hollow and nanostructured materials can be generated via different calcination conditions of the TBOS composites. Hollow and porous silicas are obtained by Received: November 24, 2011 Revised: May 6, 2012 Published: May 28, 2012 2256

dx.doi.org/10.1021/cm203520a | Chem. Mater. 2012, 24, 2256−2264

Chemistry of Materials

Article

silica composite particles, CTAB-TESPTS-N. The silica-free nanostructured spherical carbon particles, CTAB-TESPTS-NF, were obtained after selective removal of the silica using aqueous 48% HF. Synthesis of Three Different Carbon- (C18TMS-TESPTS-NF, CTAB-TESPTS-NF, and TBOS-NF) Supported Pt Catalysts and Their Catalytic Activity toward ORR. The C18TMS-TESPTS-NFsupported Pt (60 wt %) catalysts were synthesized by a widely used microwave assisted polyol process using ethylene glycol (EG) as a precursor of the reducing agent.40 For a fair comparison, CTABTESPTS-NF and TBOS-NF were also employed to support the same metal loading (i.e., 60 wt %) of Pt using the identical synthesis strategy. The supported Pt (60 wt %) catalysts were examined for their catalytic activity toward ORR. Details on the apparatus models and operation parameters for XRD, SEM, HR-SEM, TEM, nitrogen adsorption isotherms, Fourier transform infrared (FT-IR) spectrum, magic-angle spinning NMR spectra, and the electrochemical tests for the catalysts are provided in the Supporting Information.

calcination of the silica/carbon composite in air condition, while hollow and porous carbon are generated via carbonization of the organic moiety in N2 flow followed by silica etching from the composite. Fuel cells are being actively considered for automotive and stationary power applications as they offer high efficiency with little or no pollution. The enhancement of catalytic activity of electrodes represents one of the most important issues in fuel cell technology.29,30 Carbon is an ideal material for supporting nanosized metallic particles in the electrode for polymer electrolyte membrane fuel cells (PEMFCs).31 Consequently, rapid developments were made in synthesis approach of nanostructured carbon with controlled microporous and mesoporous structures. Presently, various porous carbon materials such as carbon black (i.e., Vulcan XC-72R, VC),32,33 carbon nanotubes,34,35 carbon nanofibers,36,37 carbon nanohorns,38 hollow core/mesoporous shell (HCMS) carbon, 39 and ordered hierarchical nanostructured carbon (OHNC),40 etc. are widely studied as catalyst supports for PEMFCs because of their large surface area, high electrical conductivity, and well-developed pore structure. In this study, as a representative application, hollow mesoporous carbon derived from the combination of C18TMS-TESPTS was selected as a catalyst support to disperse high loading (60 wt %) of Pt, which demonstrates superior catalytic activity toward oxygen reduction reaction (ORR) in PEMFC to its counterparts, CTAB-TESPTS-NF- and TBOS-NF-supported Pt catalysts.

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RESULTS AND DISCUSION Highly uniform and monodisperse TBOS particles were prepared using TESPTS as a source of both organic and inorganic components based on a modified Stöber method in ethanolic solution with water (Figure 1a). The ethoxysilane

EXPERIMENTAL SECTION

Preparation of Monodisperse Thioether-Bridged Organosilica (TBOS) Spheres. The monodisperse thioether-bridged organosilica spheres of ca. 350 nm in diameter were synthesized by the sol− gel process in alcoholic solution. In a typical synthesis, 30 mL of aqueous ammonia (28 wt.%) was added to a mixture solution containing 1000 mL of absolute ethanol and 100 mL of deionized (DI) water. After having been stirred for 30 min, 40 mL of TESPTS was added to the above-prepared mixture solution and then stirred at room temperature for 6 h. The solid product (TBOS) was retrieved by centrifugation, dried at 343 K overnight, and further calcined at 823 K for 6 h in air or at 1173 K for 6 h in N2 followed by HF etching to produce porous silica or carbon spheres, TBOS-A or TBOS-NF (A-air calcination, N-nitrogen calcination, NF-nitrogen calcination, and HF etching). Synthesis of TESPTS-Based Nanostructured Materials and Their Topological Transformations. For a combination of TESPTS and C18TMS, typically, a mixture solution containing 5 mL of TESPTS and 3 mL of C18TMS was added into a cosolvent solution including 300 mL of DI water, 100 mL of ethanol, and 6 mL of aqueous ammonia (28 wt.%) and stirred for 12 h. The resulting product (as-synthesized C18TMS-TESPTS) was retrieved by centrifugation and dried at 343 K overnight. Calcination at 823 K for 6 h in air produces porous spherical silica particles. Calcination at 423 K for 2 h and at 1073 K for 6 h in N2 followed by HF etching generates silica-free nanostructured hollow carbon spheres. The as-obtained materials were denoted as C18 TMS-TESPTS-A. -N or -NF, respectively. In the case of combination of TESPTS and CTAB, 5 mL of TESPTS and 1.2 g of CTAB were dissolved in a mixed solvent composed of 20 mL of DI water and 10 mL of ethanol, and the resulting solution was quickly added in the above cosolvent solution including 300 mL of DI water, 100 mL of ethanol, and 6 mL of aqueous ammonia (28 wt %) under vigorous stirring. After being stirred for 12 h, the solid product was retrieved by centrifugation, dried at 343 K overnight, and further calcined at 823 K for 6 h in air to produce the hollow silica spheres CTAB-TESPTS-A or calcined at 423 K for 2 h and at 1073 K for 6 h in N2 to produce spherical carbon/

Figure 1. Schematic illustration of TBOS synthesis procedure (a), SEM and TEM (insert) images (b), and 13C and 29Si MAS NMR spectra (c) of monodisperse spherical as-synthesized TBOS.

(−Si−OC2H5) can undergo hydrolysis by water to form silanol group (−Si−OH). This step can be accelerated by hydroxide ions in the system, which react faster than water molecules. The resulting silanol group further reacts with ethoxysilane or another silanol to form a siloxane linkage (−Si−O−Si−) with the release of ethanol or water. Figure 1b shows SEM and TEM micrographs of the asprepared TBOS spheres. The organosilica exhibits monodisperse spherical morphology with ca. 350 nm in diameter. Furthermore, low magnification TEM image (insert) shows uniform size distribution of the organosilica spheres. The 13C 2257



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TBOS sample was calcined in air, the resulting sample reveals surface characteristics typical of porous framework, exhibiting a high BET surface area of 210 m2/g as a consequence of removal of organic groups in the framework. It is white, indicative of silica framework. Nitrogen sorptions for the calcined TBOS indicate a type I characteristic of microporous materials. On the other hand, when the TBOS spheres were calcined in N2, the sample turned black indicative of formation of carbon. The organic moiety (−(CH2)3−S4−(CH2)3−) of TESPTS expected to act as a porogen in the synthesis of TBOS can be carbonized in the N2 flow to form a carbon framework. The resulting silica/carbon composite also showed a low surface area of ca. 23 m2/g similar to that of the parent as-synthesized TBOS, suggesting that the carbon chains were converted quantitatively to carbon framework. The N2-treated silica/carbon composite was further soaked in HF solution to remove the silica content in the framework, and the resulting silica-free carbon sample (TBOS-NF) exhibits an increased BET surface area of 360 m2/ g. The conversion of the organic moiety to carbon may be facilitated by means of the sulfide group in the framework, which could considerably increase the carbon yield via dehydration and sulfonation reactions. This facilitates the cross-linking process of alkyl groups and may help the aromatization process.41,42 Nitrogen sorptions for the TBOSNF also indicate a type I characteristic of microporous materials. The shape of the TBOS spheres remained almost identical before and after calcination in air and N2 conditions. The increase in surface area after calcination is attributed to the generation of micropores in the framework by removal of either organic or inorganic moiety in the tightly assembled organosilica structure. In another series of synthesis, new TESPTS-based materials were prepared through a base-catalyzed sol−gel process using either CTAB or C18TMS as a structure-mediator along with TESPTS mainly as a framework precursor as well as a structuredirecting agent as explained in the experimental. Interestingly, the new TESPTS-based nanostructured materials have been successfully generated for the both processes as shown in Figure 2. Spherical organosilica, high surface-area hollow silica or carbon capsules, and spherical porous silica or carbon nanoparticles (NPs) were successfully produced in response to further treatment conditions. Figures 2a (also Figure S3) and 2d (Figure S4) show representative TEM images of the assynthesized TESPTS-based composite organosilica materials, CTAB-TESPTS and C18TMS-TESPTS particles before heat treatments, respectively. The particles possess uniform spherical shape with a similar diameter of ca. 150 nm for both samples. The size of the spheres can be controlled mainly by concentration of reactants and reaction time. The same calcination treatments in either air or N2 atmosphere were applied to the TESPTS-based materials. Figures 2b-2c (Figure S5 and S6) and 2e-2f (Figure S7 and S8) show the evolution of the different nanostructured materials accompanied by different calcination treatments. Again, thermal treatment in air removes the organic moiety from framework, leaving silica, whereas the same treatment in nonoxidative N2 flow followed by HF etching leaves silica-free carbon framework. All the topological transformations after different treatments were studied through TEM analysis. The TEM images reveal unique hollow core/shell type structures for both the CTABTESPTS-A silica (Figure 2b) and C18TMS-TESPTS-NF carbon (Figure 2f) spheres. This is quite different from the TBOS

(CP-MAS) NMR of the TBOS spheres exhibits three signals corresponding to carbons at different environments at 12.5 (carbon of Si−*CH2−CH2−CH2−S−S−S−S−CH2−CH2− *CH2−Si), at 22.8 (carbon of Si−CH2−*CH2−CH2−S−S− S−S−CH2−*CH2−CH2−Si), and at 41.8 ppm (carbon of Si− CH2− CH2−*CH2−S−S−S−S−*CH2−CH2−CH2−Si) (Figure 1c). The 29Si NMR spectrum of the TBOS spheres reveals both Q sites and T sites as expected (Figure 1c). In the range between −90 and −150 ppm, a signal at −112.8 ppm and a shoulder at −102.6 ppm can be assigned to the resonances of Si(OSi)4 (Q4) and (OH)Si(OSi)3 (Q3), respectively, implying a high degree of cross-linking under the employed synthesis conditions. The signals originating from silicon bridged by organic group can be found in the range −50 to −90 ppm. The strong resonances at −57.0 and −65.0 ppm could be assigned to C−Si(OH)(OSi)2 (T2) and C−Si(OSi)3 (T3) sites, respectively, suggesting the organic moiety is stable at current synthesis conditions with successful incorporation of carbon chains into the TBOS spheres. The infrared (IR) spectrum also reveals the evidence of incorporation of organic moiety into the TBOS framework on the basis of the C−S and C−Si stretching bands at 698 and 1245 cm−1, respectively, which were not observed from the IR spectrum for “pure” silica, which was fabricated using tetraethyl orthosilicate (TEOS) instead of TESPTS based on a similar Stöber method to that used for the synthesis of the TBOS (Figure S1a). The S−S stretching modes in the organosilica sample were also detected weakly in wavenumber of 500−540 cm−1 probably due to strong device noise mainly caused by the detection limitation. Furthermore, the sharp vibration band at 790 cm−1 and broad stretching bands at 890 and 1020 cm−1 are also seen and assigned to Si−O bonds. The IR results confirm that the Si−O, C−Si, and C−S bonds are stable throughout the synthesis process of the TBOS. Although it is evident that sulfur functionalities have been incorporated into the asprepared TESPTS-derived TBOS from the FT-IR data shown in Figure S1b, it is very interesting to note that after being calcined in N2 at an elevated temperature (i.e., higher than 573 K) followed by silica etching, the TESPTS-derived TBOS samples do not reveal the C−S band at ca. 698 cm−1 and the S−S stretching modes in wavenumber of 500−540 cm−1. This can be easily understood according to their dissociation energies: among the various chemical bonds which may be involved in these samples, C−S and S−S have a dissociation energy of 259 and 266 KJ/mol, respectively, which are much smaller than that for C−C (i.e., 347 KJ/mol), Si−C (i.e., 360 KJ/mol), and Si−O (i.e., 452 KJ/mol). Therefore, the C−S and S−S bonds easily dissociate than other bonds and thus cannot be detected clearly from the FT-IR spectra if the calcining temperature is higher than 573 K because the equilibrium temperature of S−S bond breaking is near at 433 K.25 When the calcining temperature is higher than 433 K, the breaking of the −S−S−S−S− bridge starts in the molecule. In addition, from Figure S1b it is also found that after being calcined in N2, the CC band is clearly increased, suggesting TESPTS can work efficiently as carbon source. The as-synthesized TBOS sample with the organic −(CH2)3−S4−(CH2)3− group in it was investigated for surface properties and found to have a low surface area of SBET < 15 m2/g calculated from the nitrogen sorption isotherm shown in Figure S2. This proves that the as-synthesized TBOS particles possess nonporous framework with the organic moiety thoroughly incorporated into the TBOS spheres. When the 2258



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and 2.8 o, corresponding to a d spacing of 3.67, 3.42, and 3.42 nm, respectively on the basis of hexagonal symmetry. Especially, the CTAB-TESPTS-A sample shows a broad combined (110) and (200) reflection as well as the primary strong (100) diffraction peak, indicating a typical 2D hexagonal structure. Presented in Figure S10 are nitrogen adsorption−desorption isotherms of the TESPTS-based nanostructured materials shown in Figure 2 before and after various treatments. The treated samples show type IV isotherms according to the International Union of Pure and Applied Chemistry Nomenclature (IUPAC),43 corresponding to mesoporous framework. Mean mesopore sizes were calculated to be in the range of 3.57 to 4.65 nm. Some micropores are also expected from the isotherms of the calcined samples. N2 calcination resulted in porous silica/carbon composite in both CTABTESPTS and C18TMS-TESPTS, greatly increasing the surface area compared to those of the parent as-synthesized forms. BET surface areas of the CTAB-TESPTS-A hollow silica capsule and C18TMS-TESPTS-NF hollow carbon capsule were found to be 964 m2/g and 1070 m2/g along with high mesopore volumes, respectively. The structural properties of the TESPTS-based nanostructured materials before and after different treatments are summarized in Table 1. Interestingly, Table 1. Structural Parameters for TESPTS-Based Nanostructured Materials before and after Different Treatments Figure 2. TEM images of CTAB-TESPTS organosilica (a) in assynthesized form and subject to following subsequent topological transformation (b) after calcination in air (CTAB-TESPTS-A) and (c) after calcination in N2 followed by HF soaking (CTAB-TESPTS-NF). TEM images of and C18TMS-TESPTS organosilica (d) in assynthesized form and subject to following subsequent topological transformation (e) after calcination in air (C18TMS-TESPTS-A) and (f) after calcination in N2 followed by HF soaking (C18TMS-TESPTSNF). The scale bars represent 100 nm.

sample TBOS

CTABTESPTS

C18TMSTESPTS

samples prepared with only TESPTS as described above, where silica and carbon frameworks coexist closely throughout the structures, leaving only micropores in the framework upon removal of either carbon or silica. On the other hand, the resulting CTAB-TESPTS-NF carbon (Figure 2c) and C18TMSTESPTS-A silica (Figure 2e) show similar spherical particles. However, both the calcined spherical particles exhibit low material density compared to that of the corresponding parent as-synthesized particles, indicating the generation of porous structures in the framework due to the removal of silica or organic moiety in response to different treatments as clearly seen in the TEM images. To further investigate the morphology control of the resulting nanostructured materials, the samples produced under the various conditions were studied by X-ray diffraction (XRD), and their patterns are shown in Figure S9. The XRD patterns for the as-synthesized C18TMS-TESPTS, C18TMSTESPTS-A, and CTAB-TESPTS-NF samples show a single broad peak centered at 2θ = 2.0, 1.8, and 2.6 o, respectively, indicating amorphous nature of the framework. The absence of higher order reflections is indicative of lacking in long-range order. The XRD patterns for the CTAB-TESPTS, CTABTESPTS-A, and C18TMS-TESPTS-NF samples also exhibit a primary strong (100) diffraction peak centered at 2θ = 2.6, 2.8,

name As-syn silica (A) carbon (NF) As-syn silica (A) composite (N) carbon (NF) As-syn silica (A) composite (N) carbon (NF)

PSD

Vmicro (cm3/g)

Vmeso (cm3/g)

SBET (m2/g)a

0.01 0.16 0.20 0.01 0.35 0.17

0.04 0.11 0.10 0.04 0.61 0.11

14 210 360 22 964 230

1.1 0.8 1.0/3.6 1.0/3.2

0.20 0.01 0.07 0.23

0.17 0.04 0.13 0.20

347 18 228 520

1.1/3.6 1.7/4.7 0.9/3.2

0.40

0.80

1070

1.1/3.6

micro/meso b

(nm)

a

Multipoint Brunauer−Emmett−Teller (BET) surface area calculated over the relative pressure range P/Po = 0.1−0.2. bMean pore size calculated from the adsorption branch using the Horvath−Kawazoe (HK) and Barrett−Joyner−Halenda (BJH) methods.

much larger surface areas and mesoporous volumes were observed for the hollow silica or carbon capsules due to their unique hollow core/mesoporous shell structures compared with the simple porous spherical silica or carbon NPs. These excellent surface characteristics make the hollow mesostructured carbon an ideal candidate as a catalyst support for fuel cell application. On the basis of the observations for the TESPTS-based nanostructured materials evolved under various conditions, the formation mechanism of the TESPTS-based nanostructured organosilica is postulated and illustrated in Figure 3. Interesting structural evolutions accompanied by the generation of controlled solid, porous, or hollow architectures have been successfully regulated through organic−inorganic cooperative 2259



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extent. Electrostatic interaction of silanol groups of hydrolyzed TESPTS with ammonium ions of CTAB is faster and more favorable than the condensation between TESPTS molecules. Thus such micelle formation and electrostatic interaction between CTAB and TESPTS can be repeated many times to grow the size of spherical particle, giving exclusively hydrocarbon-rich structure with scattered silica distribution in the core. In addition, the intercalation of the TESPTS with hydrophobic moiety (i.e., −(CH2)3−S4−(CH2)3−) into the core of the surfactant micelles can possibly take place.28 The self-assembly between organosilica and CTAB around the CTAB-TESPTS micellar aggregates may take place rather quickly to form complex nanoarchitectures since the formed CTAB-TESPTS aggregates are stabilized by specific oil/water interface. Finally, when CTAB concentration decreases, relatively excess hydrolyzed TESPTS start condensing extensively over the hydrocarbon-rich core, producing silica-rich siloxane (−Si−O−Si−) shell structure. After all, such interactions and the intercalation of TESPTS with/into CTAB are proposed to lead to the formation of hydrocarbon-rich core and silica-rich shell like nanostructures. When calcined in air, the removal of CTAB micelles and the organic moiety (i.e., −(CH2)3−S4−(CH2)3−) of TESPTS creates pores in the core, leaving the silica-rich shell area with mesopores to generate hollow mesoporous silica since silica did not condense extensively in the hydrocarbon-rich core and thus cannot form extended silica framework structure in the core area. This is consistent with the observation from Figure 2b. On the other hand, N2 calcination and the subsequent silica etching process create new porous carbon spheres from organic −(CH2)3−S4−(CH2)3− and CH3−(CH2)15− moieties assembled mainly in the core as shown in Figure 2c. The conversion of the organic moiety to carbon is likely to be facilitated by means of the sulfide group of TESPTS, which could considerably increase the carbon yield via dehydration and sulfonation reactions, facilitating the cross-linking process of alkyl groups and helping the aromatization process as indicated in earlier work.41,42 Although C18TMS also contains trialkoxysilyl groups susceptible to hydrolysis, C18TMS is not expected to have a similar effect to that of TESPTS since the former has a simple hydrophobic long hydrocarbon chain (C18 carbon chain), while the latter contains a different hydrophobic electron rich tetrasulfide (i.e., −(CH2)3−S4−(CH2)3−) moiety. In fact, in our experiments, we did not observe regular core−shell assembly for the case of CTAB-C18TMS instead of CTABTESPTS. Instead, only some formation of hollow silica particles was observed with thin wall, but most of them were deformed and showed broken silica capsules after similar treatments as evident from Figure S11. In the conditions of N2 calcination followed by silica etching, the yield of carbon was very low, and only irregular carbon structures were seen with only some formation of broken capsule shapes. A similar mechanism can be considered, but without sulfur moiety, the formation of extended carbon structure from organic octadecyl bond and CTAB will be unfavorable. This may indicate the benefit/role of sulfur in the framework in promoting the formation of core− shell nanostructure. In order to further confirm the involvement of sulfur moieties in directing the structure of final materials, we performed another series of experiments using bis-triethoxysilyloctane and bis-triethoxysilylbenzene, a similar kind of materials just devoid of the tetra sulfur in TESPTS. The final products resulted in only highly malformed

Figure 3. Schematic illustration of TESPTS-based nanostructured organosilica materials from self-assembly between TESPTS and mediator (CTAB or C18TMS) and subsequent topological transformations.

self-assembly and subsequent thermal treatments. The following mechanistic considerations based on the results shown in Figure 2 also lead to interesting postulation on the relative distribution and self-assembly between TESPTS and CTAB or C18TMS for the formation process of TESPTS-based composite organosilica. In the CTAB-TESPTS system, CTAB is a cationic surfactant, which easily disperses in the studied aqueous solution to form single surfactant micelles. Although TESPTS also hydrolyzes in the aqueous solution, the dispersion of CTAB takes place much faster and easier than hydrolysis of TESPTS because the latter has the rigid ethoxy group and the bulk organic spacer which make the hydrolysis and condensation rate of TESPTS much slower than that of CTAB.28 This results in the formation of core mainly consisting of CTAB micelles, while TESPTS can anchor and assemble on the outer surface of the CTAB-based core due to the electrostatic interaction with ammonium ions of CTAB by silanol groups of hydrolyzed TESPTS. Then, it is likely that the hydrophobic moiety (i.e., −(CH2)3−S4− (CH2)3−) of each anchored TESTPS become a center for reaction with hydrophobic hydrocarbon tales of CTAB, which may group together to form another micelle maybe with a much larger size covering all over the hydrophobic −(CH2)3− S4−(CH2)3− moiety. Then, another TESPTS can assemble again through electrostatic interaction with ammonium ions of CTAB, which formed the micelle over the inner hydrophobic −(CH2)3−S4−(CH2)3− moiety. On the other hand, unreacted free silanol groups also might facilitate the micelle formation through the electrostatic interaction with ammonium ions of CTAB in reaction conditions. Of course, TESPTS also hydrolyzes in aqueous solution and condenses but not to a great extent since CTAB can much easily form micelles around TESPTS, prohibiting hydrolyzed TESPTS to condense to high 2260



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while air calcination produces porous silica spheres. This is in good agreement with the TEM images of Figure 2f and 2e. The formation of the hollow carbon capsules with mesoporous shell proves that the shell consists mainly of TESPTS assembled units. It is quite interesting to observe that CTAB and C18TMS as a structure mediator play different roles in the mediatorcontrolled self-assembly with TESPTS to generate different core−shell type organosilica composites with opposite distribution of silica-rich and carbon-rich areas as core and shell composition, which have been never observed before in other organosilica mesophases prepared with bridged organosilanes.11,23,27,28 The TESPTS-based organosilica composites contain both silica and carbon sources in the framework and thus provide a more direct route to the nanostructured silica and carbon with a reduced number of synthesis steps compared to a hard templating method, which usually involves several steps starting with the preparation of a mesoporous silica hard template, infiltration of the silica hard template with carbon precursor, carbonization of the carbon precursor, and finally silica etching for the fabrication of the mesoporous carbon.7,44,45 In particular, to the best of knowledge, this is the first example of direct scalable synthesis of uniform hollow silica and carbon capsules with high levels of mesoporosity in the shell, which have great potentials for many advanced applications. Thus, the current organosilica composite route offers a straightforward strategy for simple and cost-effective fabrication of novel nanostructured functional materials. The C18TMS-TESPTS-NF hollow carbon capsules are generated as individual discrete spherical particles and possess superior nanostructured characteristics such as larger specific surface area and mesoporous volume, particularly 3D interconnected unique hollow macroporous core/mesoporous shell nanoarchitecture, which make the hollow carbon capsules attractive as a catalyst support for active metal NPs. As a typical application, three different carbon materials prepared in this work, such as C18TMS-TESPTS-NF hollow carbon capsule, CTAB-TESPTS-NF mesoporous carbon spheres, and TBOSNF microporous carbon spheres were explored as catalyst supports to deposit high metal loading of Pt (60 wt %) for ORR in PEMFCs. The kinetics limitation of ORR on electrocatalysts is one of the most important issues in developing efficient PEMFCs.32,46−48 Catalyst support technology has been proved to be a very effective approach to improve the catalytic activity of Pt-based catalysts and reduce the Pt usage in catalysts as well. Various materials have been investgated as catalyst support to substitute the traditional carbon black VC for improved electrocatalytic property.49−53 Although so, ideal catalyst supports that can disperse high loading of Pt uniformly with small particle size are still highly desired. Figure S12 shows the representative HR-SEM images for Pt (60 wt %) deposited on the various catalyst supports. The HRSEM images reveal more uniform and smaller Pt NPs for the C18TMS-TESPTS-NF-supported Pt catalyst compared with the other carbon material-supported ones. The mean particle size of Pt NPs is estimated to be ca. 3.1 nm for the C18TMSTESPTS-NF-supported Pt catalyst, which is smaller than that (ca. 4.7 nm) of the CTAB-TESPTS-NF-supported catalyst and that (ca. 4.5 nm) of the TBOS-NF-supported catalyst. As shown in the HR-SEM images of Figure S12b and 12c, the large Pt aggregation and less uniform particles size were observed in CTAB-TESPTS-NF- and TBOS-NF-supported

and irregular structures, indicating the importance of sulfur moieties in directing the structures of final products. Through close examination of different end products obtained using TEPEST or bis-triethoxysilyloctane or bis-triethoxysilylbenzene as well as C18TMS, the difference in the material architecture makes it quite clear about the involvement of sulfur functionalities of TESPTS in formation of regular ordered core−shell structure. Interestingly, however, reverse phenomena are observed in combining bridging ((EtO)3Si-R-Si(OEt)3) and terminal (RSi(OMe)3) organosilane precursors. Unlike CTAB which easily hydrolyzes and forms surfactant micelles, when C18TMS is used as a surfactant to replace CTAB, there is no surfactant for micelle formation. The use of TESPTS and C18TMS in a basic medium resulted in materials with a spherical nanostructure containing tetrasulfide groups embedded in the structures. The relative hydrolysis/condensation rates of the two organosilane precursors should be different. It is likely that the TESPTSbridged precursor hydrolyzes/condenses slower than the C18TMS precursor. As the tetrasulfide bond is more electron rich compared to the octadecyl bond, the silicon atoms in C18TMS are more susceptible to nucleophilic attack from water causing hydrolysis rates to increase for C18TMS. For the formation of core−shell like organosilica in the C18TMSTESPTS system, the hydrophobic octadecyl chains of C18TMS first form micelle-like self-assembly structure with hydrophilic trihydroxysilyl groups as heads. A reactive core is then expanded by the base-catalyzed co-condensation of TESPTS or C18TMS over the C18TMS self-assembly structure in a mixture of ammonia, water, and ethanol. Under these conditions, the reactivity of monomer C18TMS is much larger than that of TESPTS because the latter has two bridging alkoxysilyl groups at both ends of the −(CH2)3−S4−(CH2)3− moiety. Thus, the core is formed mainly by the base-catalyzed co-condensation of hydrolyzed C18TMS and some by cocondensation between hydrolyzed C18TMS and TESPTS. Through the repeated condensation, the size of particles increases. On the other hand, organic octadecyl bonds and the −(CH2)3−S4−(CH2)3− moiety are separated from each other and isolated in the confined space left during high condensation of reactive hydroxysilyl groups. Thus, the resulting particles contain a silica-rich core with scattered −(CH2)3−S4−(CH2)3− groups. When the C18TMS concentration becomes lower, TESPTS is gradually more involved in condensation and can assemble on the outer surface of the silica-rich core. In this case, organic octadecyl bonds and −(CH2)3−S4−(CH2)3− moieties interact/self-assemble heavily in the outer surface. The resulting as-synthesized particle is inorganic−organic hybrid core−shell like structure with silica-rich aggregate derived from co-condensation of hydrophilic trihydroxysilyl groups of C18TMS and TESPTS as core composition, and excess TESPTS polymerizes/condenses with C18TMS over the silica-rich core structure to form shell composition. TESPTS can contribute as a carbon source as well as a silica source as seen from TBOS prepared with TESPTS alone in Figure 1. In this case, it is likely that −(CH2)3−S4−(CH2)3− of TESPTS interact/coassemble (during carbonization) with C18TMS over the silica-rich core structure to form shell framework. Thus, calcination in N2 and subsequent silica etching generate hollow carbon capsules with mesoporous shell since organic octadecyl bonds alone contribute very little to the formation of extended carbon structure, and organic octadecyl bonds and −(CH2)3− S4−(CH2)3− moiety cannot self-assemble heavily in the core, 2261



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catalysts. The smaller Pt particle size and more homogeneous particle distribution are mainly attributed to larger specific surface area and mesoporous volume of the C18TMS-TESPTSNF-support than the other carbons. Figure 4 shows the typical XRD patterns for the TBOS-NF-, CTAB-TESPTS-NF-, and C18TMS-TESPTS-NF-supported Pt

Figure 4. Typical XRD patterns for the C18TMS-TESPTS-NF-, CTAB-TESPTS-NF-, and TBOS-NF-supported Pt (60 wt %) catalysts.

(60 wt %) catalysts. All the Pt catalysts exhibit reflection lines characteristic of a Pt face-centered cubic structure. The average particle sizes calculated from the broadening of Pt (220) reflection are ca. 3.2 nm for Pt/C18TMS-TESPTS-NF, ca. 4.9 nm for Pt/CTAB-TESPTS-NF, and ca. 4.6 nm for Pt/TBOSNF using a Scherrer equation. The calculated particle sizes of the TBOS-NF, CTAB-TESPTS-NF, and C18TMS-TESPTSNF-supported Pt (60 wt %) NPs are well consistent with the direct measurements from the HR-SEM images. Larger surface area of the hollow carbon sphere and more uniform distribution of Pt NPs with smaller particle size are favorable for the better access of Nafion ions to the Pt NPs and the carbon support, and accordingly, a better three-phase interface can be developed which can be expected to contribute a lot to a high catalytic activity. The electrocatalytic property of Pt/C18TMS-TESPTS-NF was tested and compared with Pt/CTAB-TESPTS-NF and Pt/ TBOS-NF catalysts. Figure 5a shows the cyclic voltammograms (CVs) of these three catalysts dispersed on a glassy carbon electrode (GC, 3 mm in diameter, Pine Instruments), which were recorded in an O2-saturated 0.5 M H2SO4 solution at room temperature. The electrocatalytic activities of the various carbon-supported Pt catalysts toward the ORR were first evaluated from the CV plots recorded in O2-saturated 0.5 M H2SO4 as shown in Figure 5a. The onset potentials which correspond to a starting point of ORR for each catalyst were 728, 510, and 515 mV (vs Ag/ AgCl) for the Pt/C18TMS-TESPTS-NF, Pt/CTAB-TESPTSNF, and Pt/TBOS-NF catalysts, respectively. This indicates that C18TMS-TESPTS-NF hollow carbon capsule-supported Pt catalysts start the ORR earlier than the other carbon-supported Pt catalysts, which is attributable to the supporting effect of the catalyst supports, which favors the formation of highly dispersed Pt NPs with small particle size. Evidently, a C18TMS-TESPTS-NF carbon-supported Pt catalyst exhibits

Figure 5. (a) Cyclic voltammograms (CVs) of ORR recorded in O2saturated 0.5 M H2SO4 at a scan rate of 50 mV/s (vs Ag/AgCl) for various supported Pt (60 wt %) electrocatalysts, (b) polarization curves of Pt/C18TMS-TESPTS-NF recorded in O2-saturated 0.5 M H2SO4 solution with various rotation rates at a scan rate of 5 mV/s (vs dynamic hydrogen electrode (DHE)), and (c) the corresponding Koutecky−Levich plots (J−1 vs ω−1/2) at different electrode potentials of Pt/C18TMS-TESPTS-NF.

much higher catalytic activity than the other carbons-supported ones. The maximum peak currents are 1.272, 0.591, and 0.276 mA for the C18TMS-TESPTS-NF-, CTAB-TESPTS-NF-, and TBOS-NF-supported Pt catalysts, respectively, which corresponds to a mass activity (Am) of 285.8, 132.8, and 62.0 mA/ mgPt, respectively. Utilization efficiency is an essential and significant parameter reflecting the catalyst property, which can be calculated by dividing the electrochemical active surface area (ECSA) by the chemical surface area (CSA).32 ECSA can be estimated from the integrated charge (after subtraction of capacitance contribution) in the hydrogen absorption region of the steady state cyclic voltammogram in a supporting electrolyte (i.e., 0.5 M H2SO4), based on a monolayer hydrogen adsorption charge of 0.21 mC/cm2 on polycrystalline Pt. From Figure 5a, the ECSA of Pt was 2262



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determined to be 75 m2/g for the Pt/C18TMS-TESPTS-NF, which is much larger than that (i.e., 29 m2/g) of Pt/CTABTESPTS-NF and Pt/TBOS-NF (i.e., 18 m2/g), mainly resulting from the smaller Pt NP size and uniform particle dispersion of the former. CSA can be calculated using the following equation: CSA = 6000/ρd, where ρ represents Pt density (21.4 g cm−3) and d is the average diameter of Pt NPs obtained from XRD analysis. CSA was calculated to be ca. 87.6 m2/g for Pt/C18TMS-TESPTS-NF, 57.2 m2/g for the Pt/ CTAB-TESPTS, and 60.9 m2/g for the Pt/TBOS-NF. Thus, the corresponding utilization efficiency was ca. 85.6%, 50.7%, and 31.5% for the Pt/C18TMS-TESPTS-NF, Pt/CTABTESPTS-NF, and Pt/TBOS-NF, respectively. Evidently, much higher utilization efficiency of Pt/C18TMS-TESPTS-NF is mainly attributable to the smaller Pt NPs, the better particle dispersion and more efficient mass transport networks around the Pt NPs supported on the C18TMS-TESPTS-NF carbon than on the other carbon materials which enable more Pt NPs to work as the active sites for the desired reactions. To obtain further information into the kinetics of ORR, linear sweep voltammetry technique using rotating disk electrode (RDE) setup was employed to record the polarization curves that can generate significant amount of kinetic information on ORR, such as the catalyst activity measured by a current density at a given potential. Figure 5b shows the RDE behaviors of oxygen reduction recorded at the Pt/ C18TMS-TESPTS-NF electrode in an oxygen-saturated 0.5 M H2SO4 solution at different rotation rates with an increase in the rotation rate (from 400 to 2500 rpm). The onset potential of Pt/C18TMS-TESPTS-NF for ORR is at approximately 0.8 V (vs DHE). Figure 5c shows the Koutecky−Levich (K−L) plots of 1/J vs 1/ω1/2 at C18TMS-TESPTS-NF-supported Pt electrode potentials, and the data exhibit close to linearity and the slopes remain approximately constant over the potential range of 0.45 to 0.75 V, thus suggesting consistent electron transfer for oxygen reduction at different electrode potentials. The linearity and parallelism of the plots is an indication of first-order reaction kinetics with respect to the dissolved O2. The kinetic parameters can be analyzed using the Koutecky−Levich equation, 1/J = 1/JK + 1/Bω1/2, where J and JK are the measured and kinetic current densities, respectively.54 The value of constant B can be represented as 0.62nFD2/3υ−1/6C0. The number of electrons calculated is 3.78, based on the K−L equation, where JK is the limiting current density, n is the number of electrons transferred per oxygen molecule, F is the Faraday constant (96,485 C/mol), D is the O2 diffusion coefficient (1.4 × 10−5 cm2/s) in 0.5 M H2SO4, υ is the kinetic viscosity (0.01 cm2/s), and C0 is the concentration of oxygen (1.21 × 10−6 mol/cm3).54 The results suggest that the ORR catalyzed on the C18TMS-TESPTS-NFsupported Pt electrode is close to a favorable 4e− reduction process leading to the formation of H2O. Compared with the other carbon materials, larger specific surface area and mesoporous volume enable the C18TMSTESPTS-NF hollow carbon capsule to support high loading of Pt NPs with smaller particle size and more uniform dispersion, increasing Pt utilization efficiency and fuel cell performance. Furthermore, the fantastic hierarchical nanostructure consisting of hollow macroporous core and mesoporous shell along with 3D large interconnected interstitial volume facilitates fast mass transport. The well-combined multimodal porous structures with the mesopores in the shell open to outer surface and to the inner hollow macroporous core provide an open highway

network around the active catalyst for efficient diffusion of the reactants and products. In addition, the three-dimensionally interconnected large interstitial spaces between the packed spherical carbon particles are open to the mesoporous channels, providing main fast pathways for the transport of the reactants and products.39,55,56

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CONCLUSIONS This work demonstrated a simple preparation of interesting TESPTS-based organosilicas and their topological transformations, through which porous spherical silica or carbon and hollow silica or carbon capsule were synthesized. Sol−gel synthesis routes of TESPTS in combination with CTAB or C18TMS as a structure mediator gave rise to differently selfassembled core−shell type organosilica composites with opposite distribution of silica-rich and carbon-rich areas as core and shell composition, which have been never observed before in other organosilica mesophases. In particular, the organosilica composites with both silica and carbon sources in the framework were demonstrated as suitable and versatile precursors for direct structural control of a range of the new nanostructured materials, providing a direct scalable route to nanostructured silica and carbon and thus allowing a straightforward strategy for simple and cost-effective fabrication of novel hollow silica and carbon capsules with high levels of mesoporosity. In addition, the resulting hollow carbon capsule has demonstrated potential application as a promising catalyst support in PEMFC. Furthermore, some other applications can be expected from the hollow nanostructured materials such as in biology, controlled delivery of drug and materials, catalysis, chromatography, functional membranes for separation, hydrogen storage, double-layer capacitors, and electrodes and so on, and the energy-related applications may throw new insights. Mediator-controlled self-assembly of thioether-bridged organosilanes with other types of surfactants or co-condensing agents with or without addition of another silica source will lead to other series of new interesting organosilica composites and their topological transformations into new nanostructures full of wonder, which are currently under progress. Hence, the present work and further exploration in this trend may also shed new light on nanomaterial synthesis processes.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of surface characterization and electrochemical tests, FT-IR spectra, N2 adsorption−desorption isotherms, SEM images, TEM images, HR-SEM images, and XRD patterns. 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 This work was supported by a KRF grant funded by the Korean government (KRF 2010-0029245) and Human Resources Development Program (2009) of KETEP. The authors also would like to thank the Korean Basic Science Institute at 2263



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Jeonju, Chuncheon, and Daejeon for SEM, TEM, and XRD measurements.

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ABBREVIATIONS TESPTS, bis(3-triethoxysilylpropyl) tetrasulfide; CTAB, cetyltrimethylammonium bromide; TBOS, thioether-bridged organosilica; C18TMS, octadecyltrimethoxysilane; PEMFC, proton exchange membrane fuel cell; ORR, oxygen reduction reaction; CV, cyclic voltammogram; J. M, Johnson Matthey; VC, Vulcan XC-72R

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Topological Transformation of Thioether-Bridged Organosilicas into

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