Mesoporous silica framed by sphere-shaped silica nanoparticles

www.elsevier.com/locate/micromeso

Microporous and Mesoporous Materials 83 (2005) 262–268

Mesoporous silica framed by sphere-shaped silica nanoparticles

Dong-Wook Lee a,b, Son-Ki Ihm b, Kew-Ho Lee a,*

a Membrane and Separation Research Center, Korea Research Institute of Chemical Technology,

P.O. Box 107, Yuseong, Daejeon 305-606, South Koreab Department of Chemical and Biomolecular Engineering, National Research Laboratory for Environmental Catalysis,

Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea

Received 1 July 2004; received in revised form 23 December 2004; accepted 6 May 2005

Available online 16 June 2005

Abstract

Organic template-derived mesoporous silica with narrow pore size distribution was prepared through a colloidal silica–citric acid

system. Colloidal silica sol with 5 nm in particle diameter was firstly prepared under base-catalyzed condition via hydrolysis of tet-

raethyl orthosilicate (TEOS) and condensation reaction. Subsequently, colloidal silica–citric acid nanocomposite solution was syn-

thesized by addition of citric acid to the as-prepared colloidal silica sol. After the elimination of citric acid via thermal treatment at

500 �C, the mesoporous silica was successfully obtained, and its pore size was easily controlled in the range from 2 nm up to 15 nm

maintaining the narrow pore size distribution, the high specific surface area (800–900 m2/g) and the pore volume (0.6–1.4 cm3/g).

Comparing with a polymeric silica–citric acid system, the colloidal silica–citric acid system showed an increase in the controllable

pore size and the structural stability. The sphere-shaped silica nanoparticles as a framework with highly branched structure led to

the rigid structure of the framework and the thicker pore wall, suppressing the shrinkage of the silica structure in spite of the large

vacancy produced by the removal of citric acid.

� 2005 Elsevier Inc. All rights reserved.

Keywords: Mesoporous silica; Silica nanosphere; Citric acid; Nanocomposite

1. Introduction

Mesoporous silicates such as MCM-41 [1,2] with high

surface area and sharp pore size distribution have at-

tracted great technological and scientific interest since

1992 [3–7]. A sharp pore size distribution of MCM-41

in mesoscale, which is induced by a hexagonal array ofionic surfactants, is considered to provide molecular

recognition ability for large organic compounds. The

mesoporous materials are generally prepared with ionic

[7–9] or neutral [10–13] surfactants as a structure-direct-

ing agent, and the pore size of those materials is adjusted

by changing the structure and size of the surfactant

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2005.05.004

* Corresponding author.

E-mail address: [email protected] (K.-H. Lee).

molecules. Recently, Wei et al. and Takahashi et al. have

reported a novel preparation method of the mesoporous

silica via the non-surfactant templating route using

polymeric silica framework [14–19]. They synthesized

the amorphous silica framework from acid-catalyzed

sol–gel reaction of tetraethyl orthosilicate (TEOS), and

used non-surfactant organic compounds as a templatingagent such as D-glucose, D-maltose, dibenzoyl-L-tartaric

acid, citric acid, malic acid, lactic acid and tartaric acid.

After the elimination of the non-surfactant organic com-

pounds via solvent extraction [16] or thermal treatment

[19], the mesoporous silica was synthesized, and its pore

size was controlled in the range from 2 nm to 6 nm,

combined with high surface area and pore volume. In

addition, Takahashi et al. [19] proved the pore forma-tion mechanism of the mesoporous silica via the poly-

meric silica–citric acid system.

mailto:[email protected]

D.-W. Lee et al. / Microporous and Mesoporous Materials 83 (2005) 262–268 263

In this paper, we used colloidal silica nanoparticles

as a framework, synthesized from the base-catalyzed

sol–gel reaction of TEOS, to fabricate the mesoporous

silica with citric acid as a templating agent. Comparing

with the polymeric silica–citric acid system, the pore

properties of the mesoporous silica derived from colloi-dal silica–citric acid system were observed by investigat-

ing pore size distribution, specific surface area and pore

volume, obtained by nitrogen adsorption/desorption

isotherms. It is well-known that the colloidal silica clus-

ters show different shape and structure from the poly-

meric silica. We discussed the effect of such structure

of the colloidal silica nanoparticles on the pore structure

of the mesoporous silica.

2. Experimental

2.1. Synthesis of mesoporous silica via a polymeric

silica–citric acid system

Polymeric silica sol was synthesized under acid-cata-lyzed condition via hydrolysis of tetraethyl orthosilicate

(TEOS) and condensation reaction. A molar ratio of

TEOS, water and nitric acid was 1:5.8:0.083. A mixture

of TEOS, water and nitric acid was stirred at room tem-

perature for 20 min. The reaction mixture was diluted

with additional water to adjust the volume to 500 ml,

and refluxed for 8 h at 80 �C [20]. Polymeric silica–citric

acid nanocomposite solution was prepared by addition

Fig. 1. Preparation procedures of the mesoporous silica through (a) a polym

of citric acid to the as-prepared polymeric silica sol,

which is similar to the method previously reported by

other research groups [16,19]. The nanocomposite solu-

tion was stirred vigorously at room temperature for

10 min. Drying and calcination were carried out at

70 �C and 500 �C, respectively. Fig. 1a shows the proce-dure for synthesis of the mesoporous silica via the poly-

meric silica–citric acid system.

2.2. Synthesis of mesoporous silica via a colloidal

silica–citric acid system

Transparent colloidal silica sol was synthesized from

base-catalyzed condition at a TEOS:NH3:H2O:EtOHmolar ratio of 1:0.086:53.6:40.7. Prior to the addition

of the NH3/H2O mixture, the TEOS/ethanol mixture

was stirred vigorously at 50 �C. The addition of NH3/

H2O was carried out dropwise, followed by refluxing

the mixture for 3 h, resulting in the stable colloidal silica

sol including sphere-shaped silica nanoparticles with

5 nm in diameter. Subsequently, various concentrations

of citric acid were added to the as-prepared colloidal sil-ica sol, and the mixture was stirred vigorously at room

temperature for 10 min, followed by drying the solution

at 70 �C for 24 h. The citric acid surrounded by the silica

framework was removed by calcination at 500 �C for

2 h. The procedure for synthesis of the mesoporous sil-

ica via the colloidal silica–citric acid system is schemat-

ically shown in Fig. 1b.

eric silica–citric acid system and (b) a colloidal silica–citric acid system.

264 D.-W. Lee et al. / Microporous and Mesoporous Materials 83 (2005) 262–268

2.3. Characterization

Nitrogen adsorption/desorption isotherms of the

mesoporous silica were taken by a micromeritics ASAP

2400 instrument. Pore size distribution curves were

obtained from the desorption branch by using theBarrett–Joyner–Halenda (BJH) method. The XRD

patterns taken by a Rigaku D/MAX-2200V instrument

(operated at 1.6 kW) were utilized to investigate the

crystallization of citric acid in silica–citric acid nano-

composites [19]. For TEM analysis of the mesoporous

silica synthesized via the colloidal silica–citric acid nano-

composite, the samples were prepared by spin-coating

the nanocomposite solution on a Mo grid, followed bycalcination at 500 �C. TEM analysis was carried out

by using a Carl Zeiss EM912X instrument (operated at

120 kV). Cross polarization magic angle spinning (CP/

MAS) 29Si NMR spectra of the dried and calcined nano-

composites were taken by a Bruker DSX300 instrument.

Thermal gravimetry and differential thermal analysis

(TG–DTA) of the dried colloidal silica–citric acid nano-

composite was conducted at a heating rate of 10 �C/minunder a flow of air on a TA instrumental, SDT2960.

Fig. 2. A TEM image of sphere-shaped silica nanoparticles in the

transparent colloidal silica sol. Sphere-shaped silica nanoparticles

below 5 nm were obtained under base-catalyzed condition.

3. Results and discussion

3.1. Pore properties of mesoporous silica derived

from the colloidal silica–citric acid system

We report the synthesis method for the organic tem-

plate-derived mesoporous silica with narrow pore size

distribution and freely controlled pore size by using

sphere-shaped colloidal silica nanoparticles as a frame-

work. When we use the colloidal silica–citric acid sys-

tem, the pore size can be easily tailored from 2 to

15 nm maintaining narrow pore size distribution, high

surface area and pore volume. The easy control of poresize up to 15 nm is attributed to the highly branched

structure of the colloidal silica nanoparticles, which

gives rise to rigid structure and good thermal stability

of the silica framework, suppressing the shrinkage of sil-

ica framework in spite of the large vacancy produced by

the removal of citric acid as a template. The addition of

citric acid, which acts as a template and not an acid cat-

alyst for the sol–gel reaction of TEOS, to the silica solsleads to the formation of silica–citric acid nanocompos-

ites, and the space among the silica frameworks is filled

with the amorphous citric acid. However, at excess con-

centration of citric acid more than the saturated concen-

tration, the change in pore properties of the silica, such

as an increase in pore size with the concentration of cit-

ric acid, is not observed, because the citric acid not occu-

pying the volume among the silica frameworks iscrystallized [16,19]. In addition, when the organic tem-

plate is eliminated by thermal treatment, unexpected

shrinkage of the silica framework could partially or to-

tally occurs, which results in broad pore size distribution

or the restriction of the pore size control.

In this study, the limitation of pore size control in the

silica–citric acid system has been overcome by using col-

loidal silica nanoparticles as a framework due to thedenser structure of the primary particle of the colloidal

silica than that of the polymeric silica. Transparent col-

loidal silica sol with sphere-shaped silica nanoparticles

was synthesized under base-catalyzed condition via

hydrolysis of tetraethyl orthosilicate (TEOS) and con-

densation reaction. Fig. 2 shows a TEM image of the

transparent colloidal silica sol. Sphere-shaped silica par-

ticles with uniform particle size below 5 nm were ob-tained under base-catalyzed condition with low

concentration of NH3 as a catalyst. For comparison

with the colloidal silica system, polymeric silica sol

was also prepared under acid-catalyzed condition. Var-

ious concentrations of citric acid as a template were sim-

ply added to the as-prepared silica sols, and the mixture

was stirred vigorously at room temperature for 10 min.

The solution was dried at 70 �C, followed by the elimi-nation of citric acid via thermal treatment at 500 �C.In the case of the polymeric silica–citric acid system,

the pore size of the mesoporous silica was increased

up to 6 nm with an increase of the molar ratio of citric

acid to polymeric silica in the solution of the polymeric

silica–citric acid nanocomposite, designated as CA/PS

(see Fig. 3a). The specific surface area and total pore

volume (obtained by the nitrogen adsorption isothermfollowing the BJH method) was nearly constant over

the range of 1000–1200 m2/g and was increased from

0.6 cm3/g to 1.4 cm3/g, respectively. However, the in-

Fig. 3. Pore size distributions of the mesoporous silica prepared by

using (a) the polymeric silica–citric acid system and (b) the colloidal

silica–citric acid system. (c) N2 adsorption/desorption isotherms of the

mesoporous silica synthesized via the colloidal silica–citric acid system.

Fig. 4. The XRD patterns for (a) the dried polymeric silica–citric acid

nanocomposite at CA/PS = 0.97 and (b) the dried colloidal silica–citric

acid nanocomposite at CA/CS = 0.97.


crease in pore size and total pore volume was restricted

at CA/PS = 0.97. Maximum controllable pore size is

about 6 nm, and further increase of pore size is consid-

ered to be impossible due to the saturation of citric acid

(at CA/PS = 0.97) in the space surrounded by the silica

frameworks (see Figs. 3a and 4a). As shown in Fig.

4a, there are diffraction peaks corresponding to citric

acid crystals in the dried polymeric silica–citric acid

nanocomposite at CA/PS = 0.97. In the case of the col-

loidal silica–citric acid system, the pore size of the meso-

porous silica was surprisingly increased from 2 to

15 nm with an increase in the molar ratio of citric acid

to colloidal silica, designated as CA/CS. Although the

pore size of the mesoporous silica became larger up to15 nm, sharp pore size distribution was successfully

maintained (Fig. 3b). The specific surface area was

nearly constant over the range of 800–900 m2/g, and

total pore volume was increased from 0.6 cm3/g to

1.4 cm3/g with the increase in the CA/CS ratio. Due to

a little increase in wall thickness of the mesoporous silica

induced by larger diameter of the primary particles of the

colloidal silica, the specific surface area was somewhatsmaller than that of the mesoporous silica derived from

the polymeric silica–citric acid system, however the spe-

cific surface area is still high. Fig. 3c shows N2 adsorp-

tion/desorption isotherms of the mesoporous silica

synthesized from the colloidal silica–citric acid nanocom-

posites. All of the samples give typical type IV isotherms

with a H2 hysteresis loop as defined by IUPAC [21]. A

type H2 hysteresis loop is commonly associated withink-bottle pores or voids between close-packed spherical

particles. When the CA/CS ratio increased, an increase

in total pore volume was observed, and the sharp inflec-

tion of the hysteresis loop shifted toward higher P/Po

values, indicating that pore diameter of the mesoporous

silica increased. Fig. 4b shows that the saturation of cit-

ric acid in the colloidal silica–citric acid nanocomposite

occurred at CA/CS = 0.97. Comparing with the poly-meric silica, a ratio of Si interacting with CA to Si not

interacting with CA is smaller in the case of sphere-

shaped colloidal silica, because Si in the core of the silica

sphere substantially exists without interaction with CA.

Although the citric acid saturation for both polymeric

Fig. 6. The XRD patterns of the dried colloidal silica–citric acid

nanocomposites: (a) at CA/CS = 0.46 and (b) at CA/CS = 0.74. A

broad halo of amorphous silica indicates that the amorphous silica–

citric acid nanocomposite was formed.


silica and colloidal silica system occurred at CA/

Si = 0.97, considering a ratio of citric acid to the surface

silanol groups interacting with citric acid, the saturated

concentration of citric acid for the colloidal silica–citric

acid system is actually higher than that for the polymeric

silica–citric acid system. Therefore it is concluded thatthe increase in the saturated concentration of citric acid

contributed to the pore size tuning up to 15 nm via the

colloidal silica–citric acid system. Fig. 5 shows TEM

images of the mesoporous silica synthesized at different

CA/CS ratio. There are a number of regular mesopores

surrounded by the sphere-shaped silica nanoparticles

with highly branched structure.

3.2. Pore formation via the colloidal silica–citric acid

system

Under acid-catalyzed condition, the rate of hydroly-

sis is large compared to the rate of condensation. After

monomers are depleted, condensation between com-

pletely hydrolyzed species occurs by cluster–cluster

aggregation leading to weakly branched structures ofprimary silica particles with low fractal dimension. On

the contrary, under base-catalyzed condition, dissolu-

tion and redistribution reactions provide a continual

source of monomers which condense preferentially with

clusters rather than each other. Therefore growth occurs

primarily by monomer–cluster aggregation resulting in

highly branched primary particles with high fractal

dimension [22]. The highly branched structure of the col-loidal silica leads to rigidity of the silica framework,

making it possible to maintain the sharp pore size distri-

bution, high surface area and pore volume during calci-

nation of the colloidal silica–citric acid nanocomposite.

That is, even though the diameter of the space occupied

with citric acid was 15 nm large, sphere-shaped silica

nanoparticles with highly branched structure led to the

Fig. 5. TEM images of the calcined colloidal silica–citric acid nanocompos

prepared by spin-coating the nanocomposite solution on a Mo grid, followe

protection of the silica frameworks against the shrink-

age during calcination.

Below the saturated concentration of citric acid, the

addition of citric acid to the as-prepared colloidal silica

sol leads to the formation of an amorphous silica–citric

acid nanocomposite without crystallization of citric acid(Fig. 6). In the nanocomposite, a little attractive interac-

tion between citric acid and silica is induced by weak

hydrogen bonding of surface silanol groups with hydro-

xyl groups in citric acid. Because the strength of the

hydrogen bonding is not so strong to prevent additional

condensation of the silanol during calcination, the silica

gel can obtain high rigidity leading to the maintenance

of the bulky mesoporous structure without shrinkageof the framework [19]. In order to confirm the absence

ites: (a) at CA/CS = 0.16 and (b) at CA/CS = 0.46. The samples were

d by calcination at 500 �C.

Fig. 8. Thermal gravimetry and differential thermal analysis (TG–

DTA) profile of the dried colloidal silica–citric acid nanocomposite at

CA/CS = 0.46.


of any strong interaction between colloidal silica and cit-

ric acid which can inhibit condensation of the silanol

during calcination, cross polarization magic angle spin-

ning (CP/MAS) 29Si NMR was used (Fig. 7). The signals

around �90, �100 and �110 ppm are assigned to Si in

Q2 (Si(OSi)2(OH)2), Q3 (Si(OSi)3OH) and Q4 (Si(OSi)4),respectively [23]. There is relatively small difference be-

tween CP/MAS 29Si NMR spectra of the dried colloidal

silica (Fig. 7a) and those of the dried colloidal silica–cit-

ric acid nanocomposite (Fig. 7c), because any strong

interaction except for weak intermolecular hydrogen

bonding does not arise. After calcination at 500 �C for

2 h, both the colloidal silica and the colloidal silica–citric

acid nanocomposite show a decrease in Q2 and Q3 signaland an increase in Q4 signal due to additional condensa-

tion of the silanol groups during calcination. However,

comparing with CP/MAS 29Si NMR spectra of the cal-

cined colloidal silica (Fig. 7b), the smaller Q4 signal was

observed for the calcined colloidal silica–citric acid

nanocomposite (Fig. 7d). It means that part of the sila-

nol groups forming hydrogen bonding with the hydroxyl

group in citric acid does not participate in condensationduring calcination [19]. Thermal gravimetry and differ-

ential thermal analysis (TG–DTA) of the dried nano-

composite shows the endothermic peak at 190 �C and

Fig. 7. Cross polarization magic angle spinning (CP/MAS) 29Si NMR

spectra: (a) The dried colloidal silica. (b) The colloidal silica calcined at

500 �C for 2 h. (c) The dried colloidal silica–citric acid nanocomposite

at CA/CS = 0.59. (d) The colloidal silica–citric acid nanocomposite

(CA/CS = 0.59) calcined at 500 �C for 2 h.

the exothermic peak at 390 �C (Fig. 8). The endothermic

peak represents the elimination of the citric acid without

interaction with the colloidal silica, and the exothermic

peak corresponds to the decomposition of the citric acid

interacting with the colloidal silica. As a result, pore for-

mation mechanism of the mesoporous silica through the

colloidal silica–citric acid system is nearly consistentwith that via the polymeric silica–citric acid nanocom-

posites, which has been proved by Takahashi et al.

[19]. Consequently, although it is considered that pore

formation procedure of the colloidal silica–citric acid

system is nearly identical to that of the polymeric sil-

ica–citric acid system, extension of the range of control-

lable pore size via the silica–citric acid system was

successfully achieved through substitution of the colloi-dal silica nanoparticles as a framework for the polymeric

silica.

4. Conclusions

The pore size of the mesoporous silica was easily con-

trolled up to 15 nm maintaining narrow pore size distri-bution, high specific surface area and pore volume by

using the colloidal silica–citric acid system. For the poly-

meric silica–citric acid system, controllable pore size was

restricted to 6 nm, and further increase in pore size was

not observed due to the saturation of citric acid. The

range of controllable pore size for the citric acid tem-

plate-derived mesoporous silica was successfully ex-

tended through substitution of the sphere-shaped silicananoparticles as a framework for the polymeric silica,

because the saturated concentration of citric acid for

the colloidal silica–citric acid system is relatively higher

than that for the polymeric silica–citric acid system. In

addition, the highly branched structure of the colloidal

silica nanoparticles gave rise to rigidity of the silica

framework, resulting in maintaining the sharp pore size

distribution, high surface area and pore volume during


the pore size tuning up to 15 nm. This material is

potentially expected to be applied to selective catalysis,

chemical sensing, molecular engineering, biochemical

separation and membrane technology.

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