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 core–shell 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.16–19 Sometimes, special efforts

have to be made to maintain core–shell 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 pores

in 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 adsorption–desorption isotherms and pore sizedistribution curve of hollow carbon spheres, reflectance microscopephotographs of pure macroporous silicas and carbon–silica compo-sites, and simulations of photonic band structures of porous silicatemplate and carbon–silica composites. See http://www.rsc.org/supp-data/jm/b5/b503691g/*chezxs@nus.edu.sg

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

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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 obtain

a polymer–silica 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 before

subsequent infiltrations. But after the final infiltration step, the

sucrose-infiltrated silica was heated at 100 uC and 160 uC. In

addition 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 Adsorption–desorption isotherms were measured on a

Quantachrome NOVA 1200 system. Samples were degassed

at 200 uC for 2 h before measurement. Photographs of

the 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 be

seen 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 ordered

carbon 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.

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Upon carbonization at 800 uC, disassembly of the 3D

periodic 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 infiltration–heating 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 infiltration–heating 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 adsorption–desorption 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.{ The

surface 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 Dubinin–Astakhov

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, the

sucrose 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.

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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 and

160 uC, the precursor would not coat on the inner surface of

the 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 the

optical 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 Bragg’s 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=3ð ÞDp

(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).

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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 will

coalesce 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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