Replicating novel carbon nanostructures with 3D macroporous silica template

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  • Replicating novel carbon nanostructures with 3D macroporous silicatemplate{

    Zuocheng Zhou, Qingfeng Yan, Fabing Su and X. S. Zhao*

    Received 14th March 2005, Accepted 27th April 2005

    First published as an Advance Article on the web 16th May 2005

    DOI: 10.1039/b503691g

    Three-dimensional macroporous carbon, carbon capsules with and without opening windows,

    solid carbon spheres and walnut-like carbon nanostructures were fabricated with macroporous

    silica as a template. The relationship between the structures and experimental conditions was

    studied. The carbon materials reported here may find application as photonic crystals, drug

    delivery systems, catalyst supports, and adsorbents.

    Introduction

    Over the last few years, templating synthesis of porous solids

    has received rapidly growing interest because it allows

    one to sophisticatedly control pore size, length scale, and

    morphology. A variety of templating strategies that afford

    different porous architectures and nanostructures can be found

    from a recent review paper.1

    The colloidal-crystal-templating approach2,3 offers three-

    dimensional (3D) macroporous materials of various composi-

    tions with potential applications as photonic crystals,4

    biosensors,5 catalysts and adsorbents.3 If a further step is

    taken, colloidal-crystal-templated macroporous materials

    can be used as exotemplates to guide the formation of other

    functional structures that cannot be fabricated using other

    different strategies.6 Johnson and co-workers6 were the first to

    demonstrate the fabrication of silica nanoparticles by using

    a colloidal-crystal-templated porous polymer exotemplate.

    Jiang et al.7 described a lost-wax approach to the generation

    of a wide variety of highly monodisperse inorganic, polymeric,

    metallic solid and coreshell colloids, and hollow colloids with

    controllable shell thickness by using macroporous polymers

    as an exotemplate. Besides polymeric materials, other macro-

    porous materials such as carbon8,9 and silica10 have also been

    employed as exotemplates to grow spherical colloids and

    periodic 3D structures.

    Nanocarbons11 such as ordered nanoporous carbons,

    carbon nanotubes, and hollow capsules have been shown to

    be promising catalyst supports,12 adsorbents for gas separation

    and purification,13 nanodevices,14 and energy storage mate-

    rials,15 etc. So far, the methods for fabricating hollow carbon

    spheres or capsules reported in the literature involve a multi-

    step surface coating process.1619 Sometimes, special efforts

    have to be made to maintain coreshell spheres separated from

    each other.17 Recently, the exotemplating method was applied

    to prepare porous carbons. Ryoo et al.20 reported the synthesis

    of mesoporous carbon with silica molecular sieve as an

    exotemplate.

    In this paper, we describe a method for the fabrication of a

    range of carbon structures including 3D macroporous carbons,

    carbon capsules with and without opening windows, solid

    carbon spheres and walnut-like carbon structures by using 3D

    macroporous silica as a template and an appropriate carbon

    precursor.

    Experimental

    Materials

    The chemicals used in this study include styrene (99%,

    Aldrich), potassium persulfate (KPS, 99%, Aldrich), sulfuric

    acid (98%, Merck), fuming hydrochloric acid (HCl, 37%,

    Merck), tetraethyl orthosilicate (TEOS, 98%, Fisher), absolute

    ethanol (99.99%, Merck), ammonia solution (NH4OH, 28%,

    Fisher), hydrofluoric acid (HF, 49%, J. B. Beaker), sucrose

    (98%, Fluka). All the chemicals were used as received without

    further purification except that styrene was washed with 10%

    sodium hydroxide solution to remove the inhibitor and then

    with deionized water to remove the sodium hydroxide.

    Preparation of macroporous carbon and carbon capsules

    The experimental procedure is schematically illustrated in the

    Electronic Supplementary Information (ESI{). The prepara-tion of 3D macroporous carbon structures and hollow carbon

    capsules is described as follows. First, PS spheres synthesized

    according to Shim et al.21 were fabricated into a colloidal

    crystal using a flow-controlled vertical deposition (FCVD)

    method.22 The colloidal crystals were annealed at 105 or 110 uCfor 10 minutes to form necks of different sizes among the

    spheres.23 The interstices in the colloidal crystal were then

    infiltrated with a silica sol, which was prepared by mixing

    tetraethyl orthosilicate (TEOS), ethanol and 0.1 M HCl

    solution with a volume ratio of 1 : 3 : 0.1 and stirring for

    4 h. A 3D macroporous silica was obtained after removal

    of the PS spheres by calcination at 500 uC for 5 h. The poresin the macroporous silicas were connected with small channels

    { 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 http://www.rsc.org/supp-data/jm/b5/b503691g/*chezxs@nus.edu.sg

    PAPER www.rsc.org/materials | Journal of Materials Chemistry

    This journal is The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 25692574 | 2569

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    View Article Online / Journal Homepage / Table of Contents for this issue

    http://dx.doi.org/10.1039/b503691ghttp://pubs.rsc.org/en/journals/journal/JMhttp://pubs.rsc.org/en/journals/journal/JM?issueid=JM015026

  • at the sites of the necks in the colloidal crystals. Second, the

    porous silica was infiltrated with a 14.3, 20 or 33.3 wt% sucrose

    solution containing 3.85 wt% H2SO4. Sucrose was used as

    the carbon precursor because it is easy to handle and cost-

    effective.24 The precursor-infiltrated template was heated at

    100 uC for 2 h, followed by heating at 160 uC for 5 h to obtaina polymersilica composite. The thickness of the coated

    polymer layer can be increased by repeating the above process.

    Third, removal of the exterior silica shell from the composite

    using a 49% HF solution followed by carbonization at 800 uCin N2 for 5 h resulted in either a 3D macroporous carbon or

    hollow carbon capsules (HF etching is a hazardous process

    and should be handled carefully).

    Preparation of solid carbon spheres

    The preparation of solid carbon spheres was similar to that of

    the porous carbon structures. The only difference was that

    after infiltration of sucrose, the sample was dried at room

    temperature instead of heated at 100 uC and 160 uC beforesubsequent infiltrations. But after the final infiltration step, the

    sucrose-infiltrated silica was heated at 100 uC and 160 uC. Inaddition to sucrose solution, furfural alcohol (FA) was also

    used as a carbon precursor.

    Characterization

    A JEOL JSM-6700F scanning electron microscope (SEM)

    and a JEOL 2010 transmission electron microscope (TEM)

    were employed to observe the morphologies of the samples.

    N2 Adsorptiondesorption isotherms were measured on a

    Quantachrome NOVA 1200 system. Samples were degassed

    at 200 uC for 2 h before measurement. Photographs ofthe samples were taken with an optical microscope (Leica,

    DC 300F).

    Results and discussion

    Fig. 1a shows the SEM image of a sucrose-infiltrated (20 wt%)

    macroporous silica after heated at 160 uC for 5 h. It can beseen that the composite material is a face-centered-cubic (fcc)

    colloidal crystal embedded in a continuous phase. An

    elemental analysis of this sample with the energy-dispersive

    X-ray (EDX) technique showed the presence of carbon,

    oxygen, and silicon with atomic percentages of 72.7%,

    19.8%, and 7.47%, respectively, indicating that the sample

    contains a substantial amount of carbon. From the inset

    of Fig. 1a it can be seen that both the shape and the size

    of the polymer were precisely replicated from the air holes

    of the macroporous silica template, indicating a surface-

    templating mechanism,10,25 rather than a volume-templating

    mechanism,26 which are, in general, determined by the surface

    properties of a porous template and the nature of an infiltrated

    precursor. The surface of the macroporous silica used in this

    study was hydrophilic in nature. Thus, the carbon precursor

    solution could easily wet the silica surface. In addition,

    dehydration of sucrose molecules and surface hydroxyl groups

    of the silica during the heating steps further promoted surface

    coating of the carbon species. After the first cycle of infiltra-

    tion and heating, the surface would become less hydrophilic

    because of the presence of a layer of polymerized sucrose

    network. To enable subsequent coating of the precursor, a

    small amount of ethanol was added to the carbon precursor

    solution to decrease its surface tension, thus enhancing its

    affinity to the earlier coated layer.

    A 3D macroporous polymeric material was obtained after

    leaching out the silica framework with HF solution. As shown

    in Fig. 1b, the fcc periodic structure in Fig. 1a was well

    preserved during the wet etching process. In the macroporous

    silica template, the air pores are connected with small necks,2

    which were also filled with the carbon precursor during the

    infiltration process. As a result, after removal of the silica

    framework, the polymeric hollow spheres were connected by

    the carbon rods templated by the necks, which provide the

    mechanical strength of the ordered 3D polymeric structure.

    The hollow nature of the polymeric spheres is evident from a

    broken sphere indicated by the arrow in Fig. 1b.

    Sucrose precursor solutions with concentrations of 33.3 and

    14.3 wt% were also used to prepare carbon structures using a

    3D macroporous silica template. Fig. 2 shows the SEM images

    of the carbon samples fabricated with these two sucrose

    solutions. Although the same template was used, different

    carbon structures were obtained. With the high-concentration

    sucrose solution (33.3 wt.%) (see Fig. 2a), spheres with an

    intact surface morphology was obtained. With the low-

    concentration sucrose solution (14.3 wt%), spheres with open

    windows were obtained. The positions of the windows (see

    Fig. 2b) are exactly where the connections of two adjacent

    polymeric spheres were. This observation indicates that a

    higher concentration of the carbon precursor solution can lead

    to thicker shells of the polymeric spheres. Further experiments

    showed that further carbonization of the polymeric spheres

    with or without windows at 800 uC did not alter the orderedcarbon structures in spite of slight framework shrinkage.

    Fig. 1 (a) Low and high (inset) magnification SEM images of the

    polymer coated macroporous silica and (b) 3D macroporous polymer

    structure after removal of the silica shell. The arrow indicates a broken

    hollow sphere.

    2570 | J. Mater. Chem., 2005, 15, 25692574 This journal is The Royal Society of Chemistry 2005

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  • Upon carbonization at 800 uC, disassembly of the 3Dperiodic carbon structures by using sonication yielded

    individual carbon capsules as can be seen from the SEM

    images in Fig. 3a and b. Both sucrose and FA precursors

    produced carbon capsules of uniform diameter. Shown in

    Fig. 3c and d are the TEM images of the carbon spheres

    fabricated with different infiltration cycles. It can be seen that

    the thickness of the carbon shell, which is controllable with

    repeating the infiltrationheating cycle, is relatively uniform.

    The thickness of the shell after one infiltration, which is

    about 21 nm, was increased to about 60 nm after four

    infiltrations. However, it must be noted that the shell thickness

    can not be increased infinitely with repeated infiltration

    heating cycle. In other words, solid spheres can not be

    synthesized using the infiltrationheating method because

    repeated infiltration will block the pore necks of the 3D

    macroporous silica template, preventing the subsequent

    infiltration.8

    The above experimental data demonstrate that the method

    described here has switched complex chemical means to a

    simple physical strategy for fabrication of novel carbon

    structures,7 thus can be generalized to prepare various

    carbon structures because the silica surface can be easily

    wetted by many carbon precursors while no extra efforts to

    avoid aggregation of spheres are needed.17

    The nitrogen adsorptiondesorption isotherm of the hollow

    carbon spheres with a hollow core size of about 190 nm in

    diameter (sucrose as the precursor) is shown in the ESI.{ Thesurface area and total pore volume of the hollow carbon

    spheres were calculated to be 630 m2 g21 and 0.63 mL g21,

    respectively. The large volume of nitrogen adsorbed at the low

    relative pressure region, together with a sharp increase in

    the volume adsorbed at the relative pressure of about 0.95

    due to capillary condensation indicate the existence of both

    micropores and macropores in the material. The pore size

    distribution data calculated using the DubininAstakhov

    method showed that the micropores are uniform with a

    narrow pore size distribution centered at about 1.4 nm. The

    presence of the micropores is due to the carbonization of the

    carbon precursor.27 As a result, the hollow carbon spheres

    possess a microporous carbon shell.

    In the present work, solid spheres were also obtained by

    manipulating experimental parameters. As seen from the

    broken edges of the spheres shown in Fig. 4, the spheres have

    solid cores. It is believed that the heating process plays an

    important part in forming this solid structure. As the

    infiltrated sucrose solution was dried at room temperature,

    no polymer formed during this process. Thus, the subsequent

    infiltrations would allow the precursor solution to dissolve the

    solidified sucrose of the previous infiltration. The highest

    sucrose concentration used in this work was 33.3 wt%,

    however, the solubility of sucrose is about 2.02 g sucrose per

    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

    Fig. 2 SEM images of hollow polymer spheres synthesized with

    different sucrose concentrations of (a) 33.3 wt% and (b) 14.3 wt%.

    Fig. 3 SEM images of hollow carbon spheres synthesized with (a)

    sucrose and (b) FA as the carbon precursors. TEM images of carbon

    spheres fabricated with 20 wt% sucrose solution as the carbon

    precursor after (c) one infiltration and (d) four infiltrations. Fig. 4 SEM image of solid polymer spheres.

    This journal is The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 25692574 | 2571

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  • original precursor solution due to dissolving of the solidified

    sucrose, which led to a high viscosity. In addition, the pore

    necks which were blocked by the solidified sucrose can be

    re-opened by such a dissolving process, facilitating the sub-

    sequent infiltrations. Thus after several infiltration cycles, the

    pores were fully filled with sucrose solution of high concentra-

    tion and viscosity. During the final heating steps at 100 uC and160 uC, the precursor would not coat on the inner surface ofthe silica macropores due to the high viscosity of the sucrose

    solution. Instead, the precursor would crosslink to form

    solid spheres.

    Ordered 3D macroporous silica has been observed to display

    a photonic bandgap.2 Upon coating with carbon, its optical

    properties are believed to change. Shown in the ESI{ are theoptical photographs of porous silicas with different pore sizes

    (220 and 300 nm) before and after coating of one layer of

    carbon. After coating, the colors became green and red,

    respectively, indicating a red shift of the reflective wavelength.

    Because of the presence of different faces of the fcc structure,

    multi-colors are seen on the iridescent surfaces. The color

    changes can be explained with Braggs equation:28

    l~2dhkl

    ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

    n2eff{ sin2 h

    q

    (1)

    where l is the wavelength of the stop band maximum, dhkl is

    the inter-planar spacing, neff is the effective refractive index of

    the structure, and h is the angle of incident light to the normal

    surface. To demonstrate, here we use the [111] direction as the

    dominant light-incident direction, the inter-planar spacing can

    be calculated with the following equation:

    dhkl~d111~ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

    2=3 Dp

    (2)

    where D is the pore diameter of the porous silica. The effective

    refractive index of the structure is related to the volume

    fractions of carbon, silica and air that can be obtained from

    the following equation:28

    neff 5 nairfair + ncarbonfcarbon + nsilicafsilica (3)

    where f is the volume fraction of the compositional materials.

    The refractive indexes of carbon, silica and air are taken as

    2.1,29 1.4 and 1.

    The calculated results (see ESI{ for details of calculation)showed that before infiltration, the wavelengths of the stop

    bands of the macroporous silicas with pore diameters of 220

    and 300 nm are 397 and 541 nm, respectively. Upon one-time

    infiltration, the wavelengths will increase to 534 and 730 nm,

    respectively. The calculated wavelengths of the reflectance

    waves are consistent with the dominating colors of the sample

    surfaces shown in the ESI.{Because carbon has a higher refractive index than air, when

    the partial volume of the silica voids is occupied by a layer of

    carbon, the effective refractive index of the carbon-coated

    silica will increase. Thus, the refractive index of the carbon-

    coated 3D silica structures can be finely tuned for photonic

    applications. With a plane wave expansion (PWE) method,30

    the photonic band structures of a macroporous silica before

    and after coating of carbon of different thicknesses were

    simulated and are shown in ESI.{ It can be seen that by

    manipulating the thickness of the carbon shell, the photonic

    band structures can be varied (e.g. the mid-gap frequency and

    the gap-width of the pseudo-gap at the L point changes

    with the variation of the thickness of coated carbon layer).

    Thus, the strategy described in this work offers a route to

    finely tuning the optical properties of 3D photonic crystals.

    The size of the necks between the air holes of the

    macroporous silica template and the thickness of the coated

    carbon shell are two important parameters determining the

    structure of the resultant carbons. As schematically illustrated

    in Fig. 5a, if the size of the necks of the macroporous silica

    template is relatively large, a carbon precursor will coat on the

    surfaces of both the interior pores and the necks. The resultant

    carbon will be an integrated macroporous carbon structure

    instead of individual hollow carbon spheres. This has been

    confirmed by the SEM image of a macroporous carbon

    templated by a porous silica with a neck size of 100 nm

    (Fig. 5b). On the other hand, if the size of the necks of the

    macroporous silica is relatively small, the necks will be

    blocked by the carbon precursor. Thus, when the macroporous

    carbon is disassembled, individual hollow carbon spheres can

    be obtained, which has been confirmed by the SEM image of

    a carbon structure templated by a porous silica with a neck

    size of 20 nm (Fig. 5c).

    The thickness of the coated carbon is also important to the

    structure of the resultant carbon. Upon disassembly by

    sonication, the carbon spheres from which the 3D carbon

    structure was made up of are separated because of the strong

    Fig. 5 (a) Schematic illustration of coating mechanisms in porous

    silica templates with different neck sizes. SEM images of hollow

    spheres with (b) and without open windows (c).

    2572 | J. Mater. Chem., 2005, 15, 25692574 This journal is The Royal Society of Chemistry 2005

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  • mechanical force applied during sonication. Normally, frac-

    ture occurs across inter-sphere walls because the stress there is

    stronger than that of other places.25 Thus, if the carbon shell is

    thin, the wall between the two spheres will belong to only one

    of them when breaking up takes place. It should be noted that

    in a fcc structure each sphere connects with twelve neighbors.

    So it is not easy to obtain hollow carbon spheres without

    opened windows. However, if the carbon shell is thick enough,

    the shared carbon wall will break into two pieces and keep

    both spheres intact at the broken site. Here this breaking is not

    even as can be seen from the SEM image shown in Fig. 5c. The

    concave surfaces of the spheres are due to loss of carbon

    during disassembly. From the above discussion it can be

    concluded that by precisely controlling the thickness of the

    coated carbon shell and the size of the necks of the porous

    silica template, either 3D macroporous carbon structures or

    individual hollow carbon spheres can be prepared.

    In this work a walnut-like carbon structure was also

    obtained as shown in Fig. 6a. According to the experiment,

    the structure was only obtained when the concentration of the

    sucrose solution is high (33.3 wt%) and the necks among

    the template pores are relative large (larger than 50 nm).

    Apparently, it resembles the morphology of the carbon

    structure shown in Fig. 5b. However, a closer look revealed

    that the hollow body is separated into several cells with

    partition carbon walls. The scheme shown in Fig. 6b can be

    used to help understand the formation of this carbon structure.

    Heating of the sucrose solution infiltrated in the 3D macro-

    porous silica at 100 uC produces small bubbles, which willcoalesce to form large bubbles in the regions of the necks. With

    further evaporation of the solvent, larger bubbles will continue

    to grow at the expense of smaller ones. In the meantime, the

    viscosity of the sucrose solution increases, resulting in

    solidification of the solution. When the solidified precursor is

    rigid enough, the bubbles separated by the solidified precursor

    are no longer growing. Eventually, hollow spheres with

    partition walls can be obtained.

    Conclusion

    In summary, template synthesis of novel carbon structures

    including 3D macroporous carbon, micron-scale hollow

    carbon spheres with and without partition carbon walls,

    and carbon solid spheres has been demonstrated. Instead of

    involving complex chemical coating steps, the method

    described here is simple and versatile, allowing one to fabricate

    various carbon structures simply by controlling the neck size of

    a 3D macroporous silica template and fabrication parameters

    such as concentration and infiltration time of carbon

    precursor, neck size of the porous silica template.

    Acknowledgements

    This work was supported by Agency for Science, Technology

    and Research (A*STAR) of Singapore.

    Zuocheng Zhou, Qingfeng Yan, Fabing Su and X. S. Zhao*Department of Chemical and Biomolecular Engineering, NationalUniversity of Singapore, 10 Kent Ridge Crescent, Singapore 119260.E-mail: chezxs@nus.edu.sg; Fax: +65-67791936; Tel: +65-67794727

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    http://dx.doi.org/10.1039/b503691g

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