Replicating novel carbon nanostructures with 3D macroporous silica template

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<ul><li><p>Replicating novel carbon nanostructures with 3D macroporous silicatemplate{</p><p>Zuocheng Zhou, Qingfeng Yan, Fabing Su and X. S. Zhao*</p><p>Received 14th March 2005, Accepted 27th April 2005</p><p>First published as an Advance Article on the web 16th May 2005</p><p>DOI: 10.1039/b503691g</p><p>Three-dimensional macroporous carbon, carbon capsules with and without opening windows,</p><p>solid carbon spheres and walnut-like carbon nanostructures were fabricated with macroporous</p><p>silica as a template. The relationship between the structures and experimental conditions was</p><p>studied. The carbon materials reported here may find application as photonic crystals, drug</p><p>delivery systems, catalyst supports, and adsorbents.</p><p>Introduction</p><p>Over the last few years, templating synthesis of porous solids</p><p>has received rapidly growing interest because it allows</p><p>one to sophisticatedly control pore size, length scale, and</p><p>morphology. A variety of templating strategies that afford</p><p>different porous architectures and nanostructures can be found</p><p>from a recent review paper.1</p><p>The colloidal-crystal-templating approach2,3 offers three-</p><p>dimensional (3D) macroporous materials of various composi-</p><p>tions with potential applications as photonic crystals,4</p><p>biosensors,5 catalysts and adsorbents.3 If a further step is</p><p>taken, colloidal-crystal-templated macroporous materials</p><p>can be used as exotemplates to guide the formation of other</p><p>functional structures that cannot be fabricated using other</p><p>different strategies.6 Johnson and co-workers6 were the first to</p><p>demonstrate the fabrication of silica nanoparticles by using</p><p>a colloidal-crystal-templated porous polymer exotemplate.</p><p>Jiang et al.7 described a lost-wax approach to the generation</p><p>of a wide variety of highly monodisperse inorganic, polymeric,</p><p>metallic solid and coreshell colloids, and hollow colloids with</p><p>controllable shell thickness by using macroporous polymers</p><p>as an exotemplate. Besides polymeric materials, other macro-</p><p>porous materials such as carbon8,9 and silica10 have also been</p><p>employed as exotemplates to grow spherical colloids and</p><p>periodic 3D structures.</p><p>Nanocarbons11 such as ordered nanoporous carbons,</p><p>carbon nanotubes, and hollow capsules have been shown to</p><p>be promising catalyst supports,12 adsorbents for gas separation</p><p>and purification,13 nanodevices,14 and energy storage mate-</p><p>rials,15 etc. So far, the methods for fabricating hollow carbon</p><p>spheres or capsules reported in the literature involve a multi-</p><p>step surface coating process.1619 Sometimes, special efforts</p><p>have to be made to maintain coreshell spheres separated from</p><p>each other.17 Recently, the exotemplating method was applied</p><p>to prepare porous carbons. Ryoo et al.20 reported the synthesis</p><p>of mesoporous carbon with silica molecular sieve as an</p><p>exotemplate.</p><p>In this paper, we describe a method for the fabrication of a</p><p>range of carbon structures including 3D macroporous carbons,</p><p>carbon capsules with and without opening windows, solid</p><p>carbon spheres and walnut-like carbon structures by using 3D</p><p>macroporous silica as a template and an appropriate carbon</p><p>precursor.</p><p>Experimental</p><p>Materials</p><p>The chemicals used in this study include styrene (99%,</p><p>Aldrich), potassium persulfate (KPS, 99%, Aldrich), sulfuric</p><p>acid (98%, Merck), fuming hydrochloric acid (HCl, 37%,</p><p>Merck), tetraethyl orthosilicate (TEOS, 98%, Fisher), absolute</p><p>ethanol (99.99%, Merck), ammonia solution (NH4OH, 28%,</p><p>Fisher), hydrofluoric acid (HF, 49%, J. B. Beaker), sucrose</p><p>(98%, Fluka). All the chemicals were used as received without</p><p>further purification except that styrene was washed with 10%</p><p>sodium hydroxide solution to remove the inhibitor and then</p><p>with deionized water to remove the sodium hydroxide.</p><p>Preparation of macroporous carbon and carbon capsules</p><p>The experimental procedure is schematically illustrated in the</p><p>Electronic Supplementary Information (ESI{). The prepara-tion of 3D macroporous carbon structures and hollow carbon</p><p>capsules is described as follows. First, PS spheres synthesized</p><p>according to Shim et al.21 were fabricated into a colloidal</p><p>crystal using a flow-controlled vertical deposition (FCVD)</p><p>method.22 The colloidal crystals were annealed at 105 or 110 uCfor 10 minutes to form necks of different sizes among the</p><p>spheres.23 The interstices in the colloidal crystal were then</p><p>infiltrated with a silica sol, which was prepared by mixing</p><p>tetraethyl orthosilicate (TEOS), ethanol and 0.1 M HCl</p><p>solution with a volume ratio of 1 : 3 : 0.1 and stirring for</p><p>4 h. A 3D macroporous silica was obtained after removal</p><p>of the PS spheres by calcination at 500 uC for 5 h. The poresin the macroporous silicas were connected with small channels</p><p>{ Electronic supplementary information (ESI) available: Calculationsof the stop band positions of macroporous silicas before and aftercarbon coating, N2 adsorptiondesorption isotherms and pore sizedistribution curve of hollow carbon spheres, reflectance microscopephotographs of pure macroporous silicas and carbonsilica compo-sites, and simulations of photonic band structures of porous silicatemplate and carbonsilica composites. See*</p><p>PAPER | Journal of Materials Chemistry</p><p>This journal is The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 25692574 | 2569</p><p>Publ</p><p>ishe</p><p>d on</p><p> 16 </p><p>May</p><p> 200</p><p>5. D</p><p>ownl</p><p>oade</p><p>d by</p><p> Uni</p><p>vers</p><p>ity o</p><p>f C</p><p>onne</p><p>ctic</p><p>ut o</p><p>n 29</p><p>/10/</p><p>2014</p><p> 11:</p><p>02:4</p><p>8. </p><p>View Article Online / Journal Homepage / Table of Contents for this issue</p><p></p></li><li><p>at the sites of the necks in the colloidal crystals. Second, the</p><p>porous silica was infiltrated with a 14.3, 20 or 33.3 wt% sucrose</p><p>solution containing 3.85 wt% H2SO4. Sucrose was used as</p><p>the carbon precursor because it is easy to handle and cost-</p><p>effective.24 The precursor-infiltrated template was heated at</p><p>100 uC for 2 h, followed by heating at 160 uC for 5 h to obtaina polymersilica composite. The thickness of the coated</p><p>polymer layer can be increased by repeating the above process.</p><p>Third, removal of the exterior silica shell from the composite</p><p>using a 49% HF solution followed by carbonization at 800 uCin N2 for 5 h resulted in either a 3D macroporous carbon or</p><p>hollow carbon capsules (HF etching is a hazardous process</p><p>and should be handled carefully).</p><p>Preparation of solid carbon spheres</p><p>The preparation of solid carbon spheres was similar to that of</p><p>the porous carbon structures. The only difference was that</p><p>after infiltration of sucrose, the sample was dried at room</p><p>temperature instead of heated at 100 uC and 160 uC beforesubsequent infiltrations. But after the final infiltration step, the</p><p>sucrose-infiltrated silica was heated at 100 uC and 160 uC. Inaddition to sucrose solution, furfural alcohol (FA) was also</p><p>used as a carbon precursor.</p><p>Characterization</p><p>A JEOL JSM-6700F scanning electron microscope (SEM)</p><p>and a JEOL 2010 transmission electron microscope (TEM)</p><p>were employed to observe the morphologies of the samples.</p><p>N2 Adsorptiondesorption isotherms were measured on a</p><p>Quantachrome NOVA 1200 system. Samples were degassed</p><p>at 200 uC for 2 h before measurement. Photographs ofthe samples were taken with an optical microscope (Leica,</p><p>DC 300F).</p><p>Results and discussion</p><p>Fig. 1a shows the SEM image of a sucrose-infiltrated (20 wt%)</p><p>macroporous silica after heated at 160 uC for 5 h. It can beseen that the composite material is a face-centered-cubic (fcc)</p><p>colloidal crystal embedded in a continuous phase. An</p><p>elemental analysis of this sample with the energy-dispersive</p><p>X-ray (EDX) technique showed the presence of carbon,</p><p>oxygen, and silicon with atomic percentages of 72.7%,</p><p>19.8%, and 7.47%, respectively, indicating that the sample</p><p>contains a substantial amount of carbon. From the inset</p><p>of Fig. 1a it can be seen that both the shape and the size</p><p>of the polymer were precisely replicated from the air holes</p><p>of the macroporous silica template, indicating a surface-</p><p>templating mechanism,10,25 rather than a volume-templating</p><p>mechanism,26 which are, in general, determined by the surface</p><p>properties of a porous template and the nature of an infiltrated</p><p>precursor. The surface of the macroporous silica used in this</p><p>study was hydrophilic in nature. Thus, the carbon precursor</p><p>solution could easily wet the silica surface. In addition,</p><p>dehydration of sucrose molecules and surface hydroxyl groups</p><p>of the silica during the heating steps further promoted surface</p><p>coating of the carbon species. After the first cycle of infiltra-</p><p>tion and heating, the surface would become less hydrophilic</p><p>because of the presence of a layer of polymerized sucrose</p><p>network. To enable subsequent coating of the precursor, a</p><p>small amount of ethanol was added to the carbon precursor</p><p>solution to decrease its surface tension, thus enhancing its</p><p>affinity to the earlier coated layer.</p><p>A 3D macroporous polymeric material was obtained after</p><p>leaching out the silica framework with HF solution. As shown</p><p>in Fig. 1b, the fcc periodic structure in Fig. 1a was well</p><p>preserved during the wet etching process. In the macroporous</p><p>silica template, the air pores are connected with small necks,2</p><p>which were also filled with the carbon precursor during the</p><p>infiltration process. As a result, after removal of the silica</p><p>framework, the polymeric hollow spheres were connected by</p><p>the carbon rods templated by the necks, which provide the</p><p>mechanical strength of the ordered 3D polymeric structure.</p><p>The hollow nature of the polymeric spheres is evident from a</p><p>broken sphere indicated by the arrow in Fig. 1b.</p><p>Sucrose precursor solutions with concentrations of 33.3 and</p><p>14.3 wt% were also used to prepare carbon structures using a</p><p>3D macroporous silica template. Fig. 2 shows the SEM images</p><p>of the carbon samples fabricated with these two sucrose</p><p>solutions. Although the same template was used, different</p><p>carbon structures were obtained. With the high-concentration</p><p>sucrose solution (33.3 wt.%) (see Fig. 2a), spheres with an</p><p>intact surface morphology was obtained. With the low-</p><p>concentration sucrose solution (14.3 wt%), spheres with open</p><p>windows were obtained. The positions of the windows (see</p><p>Fig. 2b) are exactly where the connections of two adjacent</p><p>polymeric spheres were. This observation indicates that a</p><p>higher concentration of the carbon precursor solution can lead</p><p>to thicker shells of the polymeric spheres. Further experiments</p><p>showed that further carbonization of the polymeric spheres</p><p>with or without windows at 800 uC did not alter the orderedcarbon structures in spite of slight framework shrinkage.</p><p>Fig. 1 (a) Low and high (inset) magnification SEM images of the</p><p>polymer coated macroporous silica and (b) 3D macroporous polymer</p><p>structure after removal of the silica shell. The arrow indicates a broken</p><p>hollow sphere.</p><p>2570 | J. Mater. Chem., 2005, 15, 25692574 This journal is The Royal Society of Chemistry 2005</p><p>Publ</p><p>ishe</p><p>d on</p><p> 16 </p><p>May</p><p> 200</p><p>5. D</p><p>ownl</p><p>oade</p><p>d by</p><p> Uni</p><p>vers</p><p>ity o</p><p>f C</p><p>onne</p><p>ctic</p><p>ut o</p><p>n 29</p><p>/10/</p><p>2014</p><p> 11:</p><p>02:4</p><p>8. </p><p>View Article Online</p><p></p></li><li><p>Upon carbonization at 800 uC, disassembly of the 3Dperiodic carbon structures by using sonication yielded</p><p>individual carbon capsules as can be seen from the SEM</p><p>images in Fig. 3a and b. Both sucrose and FA precursors</p><p>produced carbon capsules of uniform diameter. Shown in</p><p>Fig. 3c and d are the TEM images of the carbon spheres</p><p>fabricated with different infiltration cycles. It can be seen that</p><p>the thickness of the carbon shell, which is controllable with</p><p>repeating the infiltrationheating cycle, is relatively uniform.</p><p>The thickness of the shell after one infiltration, which is</p><p>about 21 nm, was increased to about 60 nm after four</p><p>infiltrations. However, it must be noted that the shell thickness</p><p>can not be increased infinitely with repeated infiltration</p><p>heating cycle. In other words, solid spheres can not be</p><p>synthesized using the infiltrationheating method because</p><p>repeated infiltration will block the pore necks of the 3D</p><p>macroporous silica template, preventing the subsequent</p><p>infiltration.8</p><p>The above experimental data demonstrate that the method</p><p>described here has switched complex chemical means to a</p><p>simple physical strategy for fabrication of novel carbon</p><p>structures,7 thus can be generalized to prepare various</p><p>carbon structures because the silica surface can be easily</p><p>wetted by many carbon precursors while no extra efforts to</p><p>avoid aggregation of spheres are needed.17</p><p>The nitrogen adsorptiondesorption isotherm of the hollow</p><p>carbon spheres with a hollow core size of about 190 nm in</p><p>diameter (sucrose as the precursor) is shown in the ESI.{ Thesurface area and total pore volume of the hollow carbon</p><p>spheres were calculated to be 630 m2 g21 and 0.63 mL g21,</p><p>respectively. The large volume of nitrogen adsorbed at the low</p><p>relative pressure region, together with a sharp increase in</p><p>the volume adsorbed at the relative pressure of about 0.95</p><p>due to capillary condensation indicate the existence of both</p><p>micropores and macropores in the material. The pore size</p><p>distribution data calculated using the DubininAstakhov</p><p>method showed that the micropores are uniform with a</p><p>narrow pore size distribution centered at about 1.4 nm. The</p><p>presence of the micropores is due to the carbonization of the</p><p>carbon precursor.27 As a result, the hollow carbon spheres</p><p>possess a microporous carbon shell.</p><p>In the present work, solid spheres were also obtained by</p><p>manipulating experimental parameters. As seen from the</p><p>broken edges of the spheres shown in Fig. 4, the spheres have</p><p>solid cores. It is believed that the heating process plays an</p><p>important part in forming this solid structure. As the</p><p>infiltrated sucrose solution was dried at room temperature,</p><p>no polymer formed during this process. Thus, the subsequent</p><p>infiltrations would allow the precursor solution to dissolve the</p><p>solidified sucrose of the previous infiltration. The highest</p><p>sucrose concentration used in this work was 33.3 wt%,</p><p>however, the solubility of sucrose is about 2.02 g sucrose per</p><p>1 g water at 20 uC, which is about 66.9 wt%. As a result, thesucrose concentration in the pores can be much higher than the</p><p>Fig. 2 SEM images of hollow polymer spheres synthesized with</p><p>different sucrose concentrations of (a) 33.3 wt% and (b) 14.3 wt%.</p><p>Fig. 3 SEM images of hollow carbon spheres synthesized with (a)</p><p>sucrose and (b) FA as the carbon precursors. TEM images of carbon</p><p>spheres fabricated with 20 wt% sucrose solution as the carbon</p><p>precursor after (c) one infiltration and (d) four infiltrations. Fig. 4 SEM image of solid polymer spheres.</p><p>This journal is The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 25692574 | 2571</p><p>Publ</p><p>ishe</p><p>d on</p><p> 16 </p><p>May</p><p> 200</p><p>5. D</p><p>ownl</p><p>oade</p><p>d by</p><p> Uni</p><p>vers</p><p>ity o</p><p>f C</p><p>onne</p><p>ctic</p><p>ut o</p><p>n 29</p><p>/10/</p><p>2014</p><p> 11:</p><p>02:4</p><p>8. </p><p>View Article Online</p><p></p></li><li><p>original precursor solution due to dissolving of the solidified</p><p>sucrose, which led to a high viscosity. In addition, the pore</p><p>necks which were blocked...</p></li></ul>