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

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Tuning pore size and hydrophobicity of macroporous hybrid silica filmswith high optical transmittance by a non-template route†

Dongjiang Yang,a Yao Xu,*b Wujun Xu,b Dong Wu,b Yuhan Sunb and Huaiyong Zhu*a

Received 25th March 2008, Accepted 16th September 2008

First published as an Advance Article on the web 22nd October 2008

DOI: 10.1039/b804967j

Macroporous silica films with methyl groups grafted on their surface were prepared from

polymethylhydrosiloxane (PMHS) and tetraethylorthosilicate (TEOS) without templates. Compared

to the conventional template syntheses, this approach has several advantages. First, it avoids the

removal of templates, which is sometimes an environmentally unfriendly procedure. Second, it does not

require a post-synthesis grafting to form the hybrid structures with methyl groups on the silica surface.

Third, the pore size and hydrophobicity (expressed by the water contact angle) of the film can be tuned

readily. By adjusting the amount of PMHS used in the synthesis and introducing hexamethyldisilazane

(HMDS) we can tune the pore size in a wide range from 50 nm to 500 nm. The hydrophobicity increases

substantially as the pore size decreases. Evidently, the reaction mechanism of this synthesis is different

from the conventional template synthesis. A tentative mechanism is proposed: the hydrogen gas

released from PMHS results in the formation of the macropores in the films. With excellent

hydrophobicity, optical transmittance and thermal stability, the obtained silica films have

potential applications in the semiconductor industry.

Introduction

Recently, macroporous materials, having pore sizes larger than

50 nm, with well-defined and tunable pore structures have

attracted much attention because of their potential applications

(both structural and functional) in various areas. These materials

exhibit a full photonic band-gap effect, and have large specific

surface area and low thermal conductivity,1–7 and therefore can

be used as photonic crystals for optical communication devices,

supports for catalysts, separation and electronic industry mate-

rials for sensor devices and electrodes. Similar to the fabrication

of micro- and mesoporous materials using surfactants as

templates,8–11 the synthesis of macroporous materials relies

mostly on the use of a polymeric or inorganic template, usually in

the form of spheres packed in a periodic fashion.4,7 Several

methods for the synthesis of three-dimensional macroporous

inorganic materials have been reported, including sol–gel,12–15

salt preparation,16 nanocrystal infiltration 17,18 and polymeriza-

tion.19 Among the reported preparation approaches, the

template-based sol–gel process has gained popularity, because

materials in various configurations (films, monoliths, and

particles) can be readily prepared and utilized.4,7 In the sol–gel

process, metal alkoxides dissolved in alcohol are impregnated

into the voids of the colloidal sphere arrays, and they hydrolyze

aSchool of Physical and Chemical Sciences, Queensland University ofTechnology, Brisbane, QLD, 4001, Australia. E-mail: mailto:hy.zhu@qut.edu.au; Fax: + 61 73138 1804; Tel: + 61 73138 1581bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry,Chinese Academy of Sciences, Taiyuan, 030001, China. E-mail: xuyao@sxicc.ac.cn; Fax: + 86 351 4041153; Tel: + 86 351 4049859

† Electronic supplementary information (ESI) available: AFM 2D imageand arbitrary hole scanning plot of F3; photograph of H2 bubblesgenerated from the hydrolysis of PMHS. See DOI: 10.1039/b804967j

This journal is ª The Royal Society of Chemistry 2008

and condense on the sphere surfaces, forming an inorganic

framework. Subsequent removal of the template, by heat treat-

ment for polymeric spheres (e.g. polystyrene sphere) or etch

treatment for inorganic spheres (e.g. SiO2 sphere), results in an

ordered macroporous structure of metal oxide. Recently,

macroporous thin films and membranes have been successfully

prepared via the sol–gel method. Turner et al.20 reported the

synthesis of macroporous TiO2 films using silica spheres as

a template. The obtained thin films have highly ordered macro-

pores and intense optical diffraction properties, and they could

be used in photocatalysis and photonics. Khramov et al.21

reported macroporous silicate films prepared by condensing

a silica sol within a matrix of polystyrene latex spheres. These

silica films can selectively discriminate different probe

compounds based on their charge and exhibit potential for

chemical sensing applications. Furthermore, supported porous

silica films have low dielectric constant and good mechanical

strength and thermal stability.22,23 They are compatible with

silicon wafers and related materials used in integrated circuit (IC)

process technologies. Therefore, porous silica films are consid-

ered to be the most promising materials for the sub-100 nm

technology node and future IC process technologies.

In the present study, we synthesized hybrid macroporous silica

films using PMHS and TEOS as silica sources but employed no

template as pore formation directing agent. Therefore, our

method avoids an etch treatment or calcination operation to

remove the templates as in a conventional synthesis; the template

removal methods are usually environment-unfriendly proce-

dures. Moreover, we tune the pore size of the films in a wide

range, from 50 nm to 500 nm in diameter, simply by varying the

PMHS concentration or introducing another silica source,

HMDS. All obtained hybrid structures exhibit hydrophobicity,

high thermal stability and excellent optical transmittance.

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Experimental

Synthesis of samples

The hybrid macroporous silica films were prepared via the

following procedures. 0.47 mL (0.468 g) of poly-

methylhydrosiloxane (PMHS, �MW z 2300, Aldrich) was drip-

ped into 70 mL ethanol in the presence of strong base (NaOH,

pH ¼ 10) as catalyst and was kept at room temperature for 24 h.

During this period the hydrogen atoms of PMHS were replaced

by –OC2H5 groups. To this solution 5 mL (4.676 g) of tetrae-

thylorthosilicate (TEOS, 99%, Acros) and deionized H2O were

added. The TEOS:H2O molar ratio and the PMHS:TEOS mass

ratio of this system were 1:4 and 1:5, respectively. This mixture

was vigorously stirred for 3 h, and aged under ambient condi-

tions for 24 h. A sol with a light blue color formed during the

aging. The homogeneous sol was deposited onto clean single

crystal silicon wafers and glass substrates by spin-coating at 3000

rpm. Before spin-coating, the substrates were washed carefully

with pure water and acetone. The samples prepared with the

PMHS:TEOS mass ratios of 1:10, 1:5 and 1:2, were F1, F2 and

F3, respectively. Hexamethyldisilazane (HMDS, 99%, Acros)

was introduced into the sol with the PMHS:TEOS mass ratio of

1:5 to study its role in tuning the pore size. The films prepared

with the HMDS:TEOS molar ratios of 1:20, 1:10, 1:5 and 1:3

were F21, F22, F23 and F24, respectively. The obtained films

were moved to a vacuum oven and dried at 353 K for 8 h.

Fig. 1 3D AFM images (3 mm � 3 mm) of the hybrid silica films prepared

from the co-hydrolyzed PMHS and TEOS sols.

Characterization

The morphology of the macroporous films was investigated in

tapping mode by atomic force microscopy (AFM, Xe-100,

PASI). Scanning electron microscopy (SEM) micrographs were

acquired from Au coated sample powders with a Philips SEM

505 microscope with a LaB6 filament and operating at 4 keV. The

thickness of the films was measured by spectroscopic ellipsom-

etry (SE850, Stentch ellipsometer) using 632 nm wavelength,

and the measurement error was 1%. The transmission spectra of

films coated on glass substrates were recorded on an ultraviolet/

visible/near-infrared (UV/Vis/NIR) spectrometer (UV-3150,

Shimadzu). 29Si MAS NMR spectra were obtained on a UNITY

INOVA-500 Spectroscopy using a DOTY Scientific multinuclear

probe and 5mm zirconia rotors. 29Si resonance frequency was

99.745 M Hz; pulse width, 4ms; recycle delay time, 400 s; and

spinning speed, 8 kHz. Tetramethylsilane (TMS) was used to

calibrate 0 chemical shift. Thermogravimetric analysis (TGA)

was performed on a Perkin-Elmer Pyris Diamond TG instrument

at a heating rate of 10�C/min in N2. The infrared (IR) spectra

were measured with a FTIR spectrometer (FTS-25, Digilab). The

water contact angles of the films were measured with a FTA200

Dynamic Contact Angle Analyzer (First Ten Angstroms,

Portsmouth, VA).

Results and discussion

Pore size control

1. Pore size tuning from 200 nm to 500 nm. The atomic force

microscopy (AFM) images in Fig. 1 and field emission scanning

electron microscopy (FESEM) images in Fig. 2 show the

5558 | J. Mater. Chem., 2008, 18, 5557–5562

morphologies of some representative hybrid silica films. Some

craters perpendicular to the surface are randomly distributed

through the silica films (Fig. 1). The shape of the macropores is

similar to that of the silicate films prepared in the presence of

polystyrene microspheres as template.21 The diameters of the

pores varied with the amount of PMHS added into the sols. The

mean pore size of F3 with the PMHS:TEOS mass ratio of 1:2 is

approximately 500 nm, and the pore sizes in F2 and F1 decrease

to 300–400 nm and 200–300 nm, respectively. Using a sharp tip

we can determine that the holes go through the film from top to

bottom (Fig. 1S†). This observation is very different from the

template synthesis of macroporous silicas using polymeric or

inorganic templates, in which the pore size depends on the size of

the polymeric or inorganic spheres and is not related to the

template content.4,7 It is also found that the concentration of

PMHS has a strong influence on the pore size distribution. The

pore size of the sample synthesized with a small PMHS:TEOS

mass ratio, such as sample F1 with a ratio of 1:10, is more

uniform than that of sample F3 prepared with the large ratio of

1:2. It can be clearly observed that, besides the macropores

with diameters of about 500 nm, there are a few smaller pores

(200–300 nm) on the surface of sample F3. In sample F1 and F2,

This journal is ª The Royal Society of Chemistry 2008

Fig. 2 SEM images of the hybrid silica films obtained via the direct

hydrolysis and condensation of TEOS and PMHS.

Fig. 3 2D AFM images of hybrid silica macroporous films with tunable

pore size. Inset: 3D images of the hybrid macroporous films with Z axis

scales of 10 nm (F21, F22), 25 nm (F23) and 60 nm (F24), respectively.

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the pore sizes are more uniform. The SEM observations (Fig. 2)

are in good agreement with the AFM results. The detailed

structure information of the samples, such as thickness and

roughness, is listed in Table 1.

2. Pore size tuning from 50 nm to 300 nm. To further reduce

the pore size in the film we introduced HMDS into the sol

with the PMHS:TEOS mass ratio of 1:5. The role of HMDS in

tuning the pore size is illustrated by the F21, F22, F23 and F24

samples. The HMDS:TEOS molar ratios in these samples are

1:20, 1:10, 1:5 and 1:3, respectively. The morphologies of the

Table 1 The specific parameters of the obtained hybrid macroporousfilms

SamplePoresize/nm

Roughness/nm

Contactangle/�

Thickness/nm

Transmittance(%)

F1 200–300 6.1 82 71.7 93.12F2 300–400 3.5 92 120.9 93.42F3 400–500 10.1 99 224.2 93.10F21 200–300 1.2 102 115.1 92.96F22 150–200 1.3 104 122.2 92.88F23 80–100 2.1 107 133.7 92.68F24 — 1.9 112 135.9 92.58

This journal is ª The Royal Society of Chemistry 2008

HMDS doped films are shown in Fig. 3. Clearly, the size of the

pores in the film decreases with increasing HMDS content in the

sols. The size of the macropores of F2 is between 300 and 400 nm

(see Fig. 1 and 2), while the pore size in F21 is less than

approximately 300 nm. Increasing HMDS content reduced the

size of macropores further, and the maximum pore sizes in F22

and F23 samples are 200 nm and 100 nm, respectively. When the

HMDS:TEOS molar ratio increased to 1:3, the pores were too

small to be observed clearly in the AFM images. These results

show that the pore size of the macropores on the hybrid silica

coatings could be efficiently controlled by introducing HMDS.

Pore formation mechanism

Because no template, such as PS and SiO2 spheres, was intro-

duced in the synthesis, the formation mechanism of the macro-

porous hybrid films must be distinctly different from the

conventional template syntheses. TEOS is a widely used silicon

source that exhibits no structure directing function for forming

macrostructures. Therefore, PMHS is not only a silicon source,

but also plays a crucial role in the formation of macroporous

structures. With this knowledge, we tentatively propose the

formation process of macropores as illustrated in Fig. 4: in the

presence of NaOH as catalyst, ethoxy groups and hydroxys

can replace the active hydrogen atoms in PMHS, leading to the

formation of new polymethylhydroxysiloxane (PMHOS).24 The

replaced hydrogen atoms release as H2 (Fig. S2†), and the new

produced PMHOS molecules co-condense with Si–OH groups

from the product of TEOS hydrolyzation.24,25 Some of the Si–H

groups in PMHS remain unreacted, which will convert to Si–OH

groups in the subsequent spin-coating process. The H2 release in

this process results in formation of macropores perpendicular to

the film surface. Therefore, more PMHS is introduced into the

sols, more unreacted Si–H groups remain in the solution, more

H2 will be released during the spin-coating process, and the

pore size in the film will be larger.

J. Mater. Chem., 2008, 18, 5557–5562 | 5559

Fig. 4 Schematic formation process of macroporous hybrid films

prepared from PMHS and TEOS sol.

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Existence of methyl groups in the films

The PMHS molecules have methyl groups that are hydrophobic.

According to the reaction mechanism in Fig. 4, the methyl

groups will remain in the product films. As the PMHS content

increases, the product films contain more methyl groups. The

existence of methyl groups in the films is confirmed by both solid-

state 29Si MAS NMR spectra and FTIR spectra of the samples.

Fig. 5(a) shows the 29Si MAS NMR spectra of the samples F1,

F2, and F3. Noticeably, all spectra exhibit four distinct signals at

�110 ppm, �103 ppm, �64 ppm, and �56 ppm, corresponding

to Q4 (Si*(OSi)4), Q3 ((HO)Si*(OSi)3),26 T3 ((SiO)3Si*CH3) and

T2 ((HO)(SiO)2Si*CH3) 27 environments of silicon atoms,

respectively. The weak signal at 12 ppm is assigned to SiO-

Si*(CH3)3 at two ends of PMHS molecules 24,25 and labeled M1.

The observation of T3, T2 and M1 signals also means that the

Fig. 5 Solid-state 29Si MAS NMR spectra of the porous films (a and c)

and FTIR spectra of the film (b and d).

5560 | J. Mater. Chem., 2008, 18, 5557–5562

methyl groups (–CH3) in the product films are chemically bonded

to the silica framework. The –CH3 stretching band at 2900 cm�1

and the Si–C stretching peak at 1275 cm�1 are also observed in

the FTIR spectra of these samples (see Fig. 5b), which is

consistent with the results of 29Si MAS NMR spectra. For the

films prepared also using HMDS, the intensity of M1 peak at

12 ppm in 29Si MAS NMR spectra, which is due to silicon in

SiOSi*(CH3)3 environment, increases gradually (Fig. 5c) with

increasing HMDS content. This fact that more SiOSi*(CH3)3

exists in the films is also confirmed by the FTIR spectra of the

samples (Fig. 5d): the intensity of the stretching band of methyl

group at 2900 cm�1 and the peak of Si–C bond stretching at

1275 cm�1 is higher when the HMDS:TEOS molar ratio in the

sols is larger. Because the methyl groups are hydrophobic, we

may change the hydrophobicity of the film surface by varying

the content of methyl groups in the films.

Hydrophobicity

Given the hydrophobic nature of the methyl groups, it is antic-

ipated that macroporous films are hydrophobic and we could

tune the surface hydrophobicity by control of the methyl group

content. The wettability was studied by measuring the contact

angle (CA) of water on the surface of the macroporous hybrid

silica films. Fig. 6 shows the shape of a water droplet on the

surface of a single crystal silicon substrate and on the different

macroporous coatings. Clearly, the PMHS:TEOS mass ratio in

the synthesis of the porous films affects the CA of water on the

films remarkably. As shown in Fig. 6a, the single crystal silicon

substrate has a water CA of 32 � 2�, displaying a hydrophilic

trait. The macroporous hybrid silica films prepared with

PMHS:TEOS mass ratios of 1:10, 1:5, and 1:2 exhibit increasing

CAs of 82 � 2�, 92 � 2�, and 99 � 2�, respectively. The macro-

porous films are hydrophobic, and the CA increases with the

PMHS content. This is because the CA of water on the film is

affected by the content of methyl groups in the film and the

methyl groups were from PHMS. The CAs of the four HMDS

modified films are higher than 100�. As shown in Fig. 7, the CA

for pure water of F21 is 102 � 2� which is higher than that of the

unmodified film F2 (92 � 2�). Therefore, the HMDS addition

can not only tune the pore size of the macropores distributed on

Fig. 6 The profile of water droplets on (a) single crystal silicon substrate,

(b) F1, (c) F2, and (d) F3.

This journal is ª The Royal Society of Chemistry 2008

Fig. 7 The profile of a water droplet on the surface of sample F21.

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the surface but also tune hydrophobicity of the film surface

because more Si–(CH3)3 groups are introduced into the films.

Fig. 9 Thermogravimetric analysis (TGA) and derivative thermog-

ravimetry (DTG) curves of F2 prepared at the PHMS:TEOS mass ratio

of 1:5.

Optical transmittance

Another interesting change in properties of the macroporous

films is observed in optical transmittance. The transmission

spectra for coatings on glass are shown in Fig. 8. The maximum

obtainable transmission for the single-sided films prepared from

PMHS:TEOS hybrid sols is near to 94% (Fig. 8a), which corre-

sponds to �50% antireflective for a single side of glass. The value

is similar to that of the transparent antireflective silica nano-

particle coatings that are less reflective than their respective bare

glass substrates.30 The high transmittance indicates that the two

hydrolysis precursors, PMHS and TEOS, copolymerized well

and phase separation did not occur during the hydrolysis and

co-condensation process. For the HMDS films, the maximum

obtainable transmission is about 93% (Fig. 8b) which is slightly

lower than that of the films prepared with PMHS only. When the

HMDS:TEOS molar ratio was increased from 1:20 (sample F21)

Fig. 8 The transmission spectra of the PMHS (a) and HMDS (b) tuned

silica films coated on glass substrates.

This journal is ª The Royal Society of Chemistry 2008

to 1:3 (sample F24) a moderate decrease in the maximum

obtainable transmission from 93% to 92% is observed. The

reason for this phenomenon is unclear.

Thermal stability

In order to measure the thermal stability of the macroporous

films, they were peeled off the silica substrates and were analyzed

with the TGA technique. Fig. 9 presents the TG-DTG profile of

sample F2 that is obtained from the hybrid sol at the

PHMS:TEOS mass ratio of 1:5. In N2 atmosphere, the film

shows four obvious endothermic regions centered on 370, 500,

813 and 1025 K. Commonly, the process of mass loss in the

low-temperature region from 300 to 450 K is attributed to

desorption of physically adsorbed water.28 There are two water

release processes at higher temperature: the broad mass loss

region near 500 K is due to the condensation of vicinal silanol

groups,29 the loss centered at 1025 K is a result of condensation

of isolated and geminal silanols.28 According to Alekseev and

Zaitsev,28 the thermal decomposition of SiO2–Si(CH3)3 takes

place in the temperature region from 473 to 973 K. In the present

work, the mass loss peak at 813 K (Fig. 9) is due to the thermal

decomposition of –SiCH3 and –Si(CH3)3, which takes place at

temperatures between 700 and 900 K.

Conclusions

In summary, organic–inorganic hybrid macroporous silica films

can be achieved by a one-step synthesis approach in which

PMHS and TEOS were used as silica sources and no template

was introduced. In the products, methyl groups remain chemi-

cally bonded to the silica framework. Therefore, this approach

does not require additional grafting reaction, as is usually

necessary for creating hybrid macroporous structures. The

macropores were perpendicular to the surface and the pore size in

the product films can be tuned simply by varying the concen-

tration of PMHS or by introducing different amounts of HMDS.

The macroporous materials not only exhibited tunable hydro-

phobic nature, but also displayed super optical transmittance

properties and thermal stability. Given that the properties of the

hybrid films are desired for IC process, this study highlights new

opportunities for the preparation of porous hybrid functional

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materials that can be applied in advanced semiconductor

manufacture.

Acknowledgements

Financial support from the National Key Native Science

Foundation (20133040) and Australian Research Council

(DP0557945) is gratefully acknowledged. We acknowledge Dr E.

R. Waclawik for kindly providing the analysis for wettability.

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