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Materials Chemistry and Physics 140 (2013) 148e153

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Tailored morphology and controlled structure of bimodal mesopores silicas viaadditive ammonia amount in the TEOSeCTABeH2O system

Huan Zhou, Jihong Sun*, Xia Wu, Bo Ren, Jinpeng WangCollege of Environmental & Energy Engineering, Beijing University of Technology, 100 PingLeYuan, Chaoyang District, Beijing 100124, PR China

h i g h l i g h t s

* Corresponding author. Tel.: þ86 10 67396118; faxE-mail address: jhsun@bjut.edu.cn (J. Sun).

0254-0584/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.03.013

g r a p h i c a l a b s t r a c t

� BMMs with uniform primary meso-pore around 2.7 nm weresynthesized.

� The secondary mesopores between15 and 30 nm were controlled viaammonia additive.

� The spherical particles and rod-likemorphology were obtained.

� The related cooperative mechanismwas proposed.

a r t i c l e i n f o

Article history:Received 4 July 2012Received in revised form1 February 2013Accepted 7 March 2013

Keywords:NanostructuresSolegel growthElectron microscopyMicrostructure

a b s t r a c t

Bimodal mesopores silicas (designated as BMMs) with tailored morphology and controlled structurehave been synthesized via varying additive ammonia amount in the TEOSeCTABeH2O system based onhydrolysisecondensation behaviors of silicate species (TEOS) and coefficient principles of cetyl-trimethylammonium bromide template (CTAB). The spherical BMMs with an average diameter of 25e150 nmwere prepared in a relative low ammonia concentration solution, while the rod-like BMMs withan average size of 2 mm in length were obtained under a high ammonia concentration aqueous solution.Meanwhile, their corresponding morphologies and textural properties were characterized by varioustechniques, such as X-ray diffraction, SEM, TEM, FT-IR, TG, and N2 sorption, and then, the relatedcooperative mechanism including solegel chemistry and self-assembly process was put forward.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Since the first discovery of the M41S family in 1992 [1,2], therehave beenmany research activities of synthesizing novel controlledmesoporous silicas, and exploring their various reaction mecha-nisms, as well as searching for versatile applications in the potentialnew fields [3e10]. In general, templated mesoporous materials canbe synthesized via the hydrolysisecondensation of organic pre-cursors and the self-assembled procedure of surfactant molecules[11]. Subsequently, the ordered controlled structure mesoporous

: þ86 10 67391983.

All rights reserved.

silicas with many excellent characteristics such as high specificsurface areas, large pore volume, regular and adjustable nanoporesizes and hydrophilic surface feature are usually synthesized byremoval of the encapsulated organic templates through calcination[12] or extraction [13].

The solegel procedures of the hydrolysis-polycondensation ofTEOS [14] have been widely used to prepare structure-controlledsilica sol and therefore obtain morphology-tailored MCM-41 typematerials frommicro- and, meso- to macroporous scales. However,the resultant structured silicas present very narrow monoporedistributions, which may result in more difficult accessibility of thelarger guest molecules to the active site due to the diffused limi-tation, thereby limiting their certain applications in many fields,especially for some shape/size selective catalysis and separations

H. Zhou et al. / Materials Chemistry and Physics 140 (2013) 148e153 149

[15]. Thus, innovations in the synthesis of inorganic structures, withnew approaches involving ways to improve reaction efficienciesand minimize blocking of channels during the sorption or diffusionbehaviors of the hosteguest molecules, are areas of intense inter-national scientific and technological research. So far, a great prog-ress has been made in designing hierarchically controllednanoporous silicas based on the template method via the solegeltechnique, and several new techniques are being developed toachieve even larger mesopores (diameter up to 50 nm) and mac-ropores (diameter >50 nm). Haskouri et al. [16] reported chemicalcontrol of the pore sizes via adjusting the critical micelle concen-tration (CMC) of the surfactant and the dielectric constant of thereaction medium. Zhang et al. [17] synthesized magnetic meso-porous silica nanoparticles with small particles (about 40e70 nm indiameter), tunable pores (3.8e6.1 nm), high surface areas (700e1100 m2 g�1), and large pore volumes (0.44e1.54 cm3 g�1). Sun andCoppens [18,19] reported a synthesis method for bimodal meso-pores materials (designated as BMMs), which possess the smallerpores with very uniform diameter of about 2.7 nm and the largerpores with a controlled diameter between 15 and 30 nm. Morerecently, our results demonstrated that BMMs either as by itself oras the starting material in the synthesis of multi-structured porousmaterials is very useful in potential applications for separations[20], drug controlled release delivery [21] and surface modificationof fluorescent molecules [22].

From this point of view, it is very interesting to study further theeffects of the synthesis parameters and to define the desired BMMsstructural characteristics and textural properties. Based on ourprevious works and combined the solegel process with the tem-plate method, the aim of this work is mainly to explore the in-fluences of ammonia additive on the bimodal mesoporousstructures and morphology. Meanwhile, the resultant series ofBMMs were well characterized by means of powder X-ray diffrac-tion (XRD), Fourier transform infrared (FT-IR), scanning electronmicroscopy (SEM), transmission electron microscopy (TEM), ni-trogen adsorption/desorption isotherms (N2-BET), and thermog-ravimetric analysis (TGA), meanwhile, the corresponding synthesismechanisms are proposed.

sample-3.0

(A

.U

.)

2. Experimental methods

2.1. Chemicals

The chemicals used in this work CTAB (A. R.) and TEOS (A. R.)were brought from Sinopharm Chemical Reagent Beijing Co.,Ltd. Ammonium hydroxide (A. R.) was obtained from Beijingchemical works. Besides, doubly distilled water was used in allexperiments.

2 4 6 8 10

sample-2.0

sample-1.6

sample-1.2

sample-0.8

sample-0.6

sample-0.4

In

te

ns

ity

2Thate (degree)

sample-0.2

Fig. 1. XRD patterns of BMMs series, sample-0.2, sample-0.4, sample-0.6, sample-0.8,sample-1.2, sample-1.6, sample-2.0, and sample-3.0.

2.2. Preparation of BMMs

BMMs was synthesized partially based on the literature method[23]. The detailed experimental procedure was as following: CTAB(1.31 g) was dissolved completely in doubly distilled water (52 mL)to get a transparent solution. Then TEOS (4 mL) was added to thesolution with stirring. Finally, a desired amount of ammoniumhydroxidewas added quickly. Themixturewas stirred continuouslyto get a white gel, and the molar composition of the starting so-lution was 1.0 SiO2:0.2 CTAB:165.9H2O:x NH3 (x ¼ 0.0, 0.2, 0.4, 0.6,0.8, 1.2, 1.6, 2.0, 3.0). The resultant precipitates were filtered,washed, and dried at 120 �C for 3 h. To remove surfactant, the solidwas calcined at 550 �C for 5 h, with a heating rate of 5 �C min�1

from room temperature to 550 �C, and finally, the resultant sampleswere marked as sample-x (x was on behalf of the molar ratio of

ammonia content with TEOS in the starting solution, for 0, 0.2, 0.4,0.6, 0.8, 1.2, 1.6, 2.0, and 3.0, respectively).

2.3. Characterization methods

XRD studies were performed on a Bruker D8 Advance XRD withCu Ka radiation (l ¼ 1.54 �A), operating at 35 kV and 35 mA, 0.02�

step size and 12.6 s step time over the range of 2q from 0.6� to9.991�. SEM images were recorded using a Hitachi S-4300, the ac-celeration voltage 15 kV and resolution around 9 nm. TEM micro-graphs were performed on a Tecnai F20 at 200 kV. Nitrogenadsorption/desorption isotherms at �196 �C were measured on aMicromeritics ASAP 3020 sorption analyzer. The sample was pre-treated at 100 �C overnight in vacuum. The isotherm data wereanalyzed by BET (BrunauereEmmetteTeller) and the plots of thecorresponding pore size distribution were obtained from desorp-tion branches of the isotherms by using BJH (BarretteJoynereHalenda) model. TGA were carried out between 25 �C and 800 �Cusing a Seiko TG/DTA 320 with an N2 flow rate 100 mL min�1 and aheating rate of 10 �Cmin�1. FT-IR spectrawere acquired on a Nicolet6700 spectrophotometer with KBr pellet. The pH value of thestarting synthesis solution was measured on Yiyuan Electronicdevice F-50A.

3. Result and discussion

3.1. XRD analysis

Fig. 1 presented the XRD patterns for BMMs series with differentamount of ammonia in the synthesis mixture system. As can beseen, the XRD pattern of BMMs series clearly exhibited twodiffraction peaks in the 2q range 2�e10�, indexed as (100) and (110)respectively, which indicated the existence of the mesoporousstructures in consistent with reported literature [23]. However, the(110) peak in samples-0.2, 0.4, 0.6, 3.0 were not obvious, and the

0.0 0.2 0.4 0.6 0.8 1.0

0

500

1000

1500

2000

sample-3.0

sample-2.0 sample-1.6

sample-1.2

sample-0.8

sample-0.6

sample-0.4

sample-0.2

Ab

so

rb

ed

V

olu

me (c

m3/g

, o

ffs

et 2

00

)

Relative Pressure (P/P0)

sample-0

A

100101

0

6

12

18

sample-3.0

sample-2.0

sample-1.6

sample-1.2

sample-0.8

sample-0.6

sample-0.4

sample-0.2

dV

/d

lo

gD

(o

ffs

et 2

)

Pore Diameter (nm)

sample-0

B

Fig. 2. N2 adsorptionedesorption isotherms (A) of BMMs series and correspondingpore size distribution plots (B), sample-0, sample-0.2, sample-0.4, sample-0.6, sample-0.8, sample-1.2, sample-1.6, sample-2.0, and sample-3.0. Isotherms are shifted verti-cally by 200 cm3 g�1 with respect to each other, and pore size distributions are shiftedvertically by 2 with respect to each other.

Table 1Texture properties of the BMMs series. The number after “sample-” represents themolar ratio of ammonia content with TEOS in solution.

Sample pHvalue

2q (o) d (nm) Surfacearea(m2 g�1)

Porevolume(cm3 g�1)

Primarypore size(nm)

Secondarypore size(nm)

Sample-0 7.50 151 0.37 20.0e100.0

Sample-0.2 8.96 1.86 4.73 1105 2.8 9.8Sample-0.4 9.37 1.89 4.68 1072 2.7 12.7Sample-0.6 9.60 1.94 4.55 1045 1.38 2.8 34.3Sample-0.8 9.82 2.09 4.22 1096 0.88 2.8 e

Sample-1.2 9.92 2.31 3.81 1237 0.77 2.6 e

Sample-1.6 10.01 2.39 3.68 1354 0.76 2.3 e

Sample-2.0 10.12 2.40 3.67 1334 0.72 2.5 e

Sample-3.0 10.29 2.43 3.64 843 0.46 2.3 e

H. Zhou et al. / Materials Chemistry and Physics 140 (2013) 148e153150

possible reason is that the peak (100) is too broad with a high in-tensity and so overlaps others in the XRD pattern. With anincreasing ammonia additive, the (100) peak intensity graduallyincreased until sample-0.8 showed the highest intensity and thendecreases. On the other hand, as the ammonia amount increased,the (100) peak position shifted from 1.86� (sample-0.2) to 1.89�

(sample-0.4), 1.94� (sample-0.6), 2.09� (sample-0.8), 2.31� (sample-1.2), 2.39� (sample-1.6), 2.40� (sample-2.0) and 2.43� (sample-3.0),respectively, and the corresponding d100 space value decreasedfrom 4.73 nm to 4.68 nm, 4.55 nm, 4.22 nm, 3.81 nm, 3.68 nm,3.67 nm and 3.64 nm. These observations indicate that effect ofammonia amount on the order degree of the BMMs’mesostructureis substantial, the main reason is due to the interactions betweensiloxane and the template of CTAB [24]. In addition, it is noted thatthe XRD pattern (not shown) of sample-0 without ammonia addi-tive revealed no any diffraction peaks in the 2q range 2�e10�,however, the broad diffraction peak with a low intensity from 15.3�

to 29.3� of the angle 2q can be ascribed to the amorphous charac-teristic [25e27].

3.2. N2 sorption analysis

The porosity of the BMMs series was characterized by using thesurface area, the total pore volume and pore size distribution fromthe N2 adsorptionedesorption isotherms. As can be seen in Fig. 2A,the isotherms of sample-0.2, 0.4, and 0.6 exhibited two inflections:thefirst hysteresis loop occurred at relative pressure 0.3< P/P0< 0.5and the second, much steeper one, at 0.75 < P/P0 < 1.0. In addition,with increasing ammonia additive in the synthesis system, the firstconspicuous condensation changed slightly, suggesting the forma-tion of amore uniform distribution of the mesopore size, consistentwith the result shown in Fig. 2B;meanwhile, the second one becameweak, showing a tendency to shift gradually toward the narrowrelative pressure range. However, as shown in Fig. 2A, it is clear thatfor sample-0.8, 1.2, 1.6, and 2.0, the relative pressures of the firsthysteresis loop around 0.2 < P/P0 < 0.4 were lower than that theabove-mentioned three samples (sample-0.2, 0.4, and0.6) in Fig. 2A,very similar to those of the reported MCM-41 [28e30], but theirsecond loops were much flatter, indicating the uniform monoporesize distribution. Particularly, sample-3.0 was very different fromthe other samples, and its capillary condensation was not obvious,implying an amorphous structure and broad pore distribution in thepresence of the BMMs integrity. Furthermore, the N2 adsorptionedesorption isotherms of sample-0 without ammonia additiveshowed type IV isotherms with an H1 type hysteretic loop. Mean-while, there was only a sharp increase in volume adsorbed at P/P0from 0.78 to 0.98, corresponding to a very broad pore distribution,which was different from that of BMMs series. These phenomenaclearly indicated the absence of primary mesopores for sample-0 ascompared to that of BMMs series [23].

Besides that, the textural properties of all of BMMs series weresummarized in Table 1. It is obvious that the mesopore size ofBMMs series is greatly influenced by the change of the ammoniaamount. The tendency can be seen clearly from Table 1 that theirsurface areas increased slightly from 1105 m2 g�1 (sampe-0.2) to1334 m2 g�1 (sample-2.0), except for sample-3.0 (843 m2 g�1), butpore volumes decreased sharply from 1.33 cm3 g�1 (sample-0.6) to0.46 cm3 g�1 (sample-3.0). Sample-3.0 showed a relatively lowsurface area (843 m2 g�1) and small pore volume (0.46 cm3 g�1),indicating that the mesopore structure may collapse after calcina-tion, which can be confirmed by XRD in Fig. 1. Moreover, it is easilyseen that with the increase of ammonia amount, the primarymesopores distribution was very narrow with a mean mesoporessize from around 2.7 nme2.3 nm, in accordance with N2 adsorp-tionedesorption isotherms, yet their secondary mesopores

gradually shifted to becoming large and even disappeared (inFig. 2B). Additionally, as shown in Table 1, the BET surface area ofaround 151 m2 g�1 and pore volume of about 0.37 cm3 g�1 ofsample-0 were much lower than that of BMMs series, obviously,due to its non-porous structures.

H. Zhou et al. / Materials Chemistry and Physics 140 (2013) 148e153 151

3.3. SEM and TEM analysis

The effects of the ammonia additive on the morphology andparticle size of BMMs series were investigated in the same syn-thesis condition. The representative SEM and TEM micrographs forBMMs series were shown in Fig. 3. As can be seen, the particle sizeof BMMs series increased with the increase of the ammonia con-centration of the synthesis mixture solution. On the other hand, it isfound that sample-0.2, 0.6, and 1.2 presented spherical particleswith average diameters of 25, 75 and 150 nm (shown in Fig. 3A, Cand E), respectively. In contrast, sample-3.0 exhibited an irregularshape with rod-like morphology in Fig. 3G. Moreover, TEM images

Fig. 3. Representative SEM/TEM images of (A, B) sample-0.2, (C

of the typical BMMs such as sample-0.2 showed the presence of abimodal mesoporous system, which revealed uniform intra-nanoparticle mesopores with a diameter of around 25 nm and alarge number of narrow small poreswith a size of 3 nm. As depictedin TEM micrographs (Fig. 3B, D, F, H), the mesopore channels ofBMMs showed awormlike characteristic with uniform structure forsample-0.2, sample-0.6, as well as sample-1.2. However, TEM image(not shown) of sample-0 with no ammonia additive indicateddense nanoparticle spheres with a size of around 20 nm.

In general, the formation process of the silica particle is acomplex competition, including hydrolysis of TEOS and nucleationbehaviors via poly-condensation, in which the hydrolysis process is

, D) sample-0.6, (E, F) sample-1.2, and (G, H) sample-3.0.

4000 3500 3000 2500 2000 1500 1000 500

Tra

ns

mis

sio

n (%

)

Wavenumber (cm-1)

800

956

Fig. 4. FT-IR spectra of the representative sample-0.2.

H. Zhou et al. / Materials Chemistry and Physics 140 (2013) 148e153152

the whole control step, leading to rapid nucleation in the earlystage. Matsoukas and Gulari [31] explained that the hydrolysis andpolycondensation rate of TEOS promoted simultaneously withincreasing ammonia concentrations, favors the resultant largeparticles. So, the particle sizes grow along with the ammoniacontent, and subsequently the edge shapes of the resultant parti-cles become very irregular and indistinct when excess ammonia isadded.

3.4. FT-IR and TG analysis

Fig. 4 showed the spectra (400e4000 cm�1) of typical BMMs,taking sample-0.2 as an example. As can be seen, the obviouscharacteristic peaks of mesoporous silica materials were exhibited.The strong wide bands in the range of 1000e1250 cm�1 can beascribed to the asymmetric SieOeSi stretching vibration [32]. Thepeaks at 800 cm�1 and 956 cm�1 can be attributed to the sym-metric SieO stretching vibration and the SieOH stretching vibra-tion [33] respectively. The peak at 1633 cm�1 was attributed to thesurface H2O, and the peak at 3450 cm�1 can be assigned to OeHstretching vibration [34].

Meanwhile, TGA profile of the sample-0.2, without calcination,was carried out under N2 atmosphere as depicted in Fig. 5. Itshowed three processes during the periods of weight loss according

0 100 200 300 400 500 600 700 800 900

40

60

80

100

Tempreture ( )

We

ig

ht (%

)

DT

G

TG DTG

Fig. 5. TG and DTG plots of the typical sample-0.2 without calcination.

to the DTG illustration. Firstly, a total weight loss of 2 wt% before180 �C, was caused by desorption of physisorbed and chemisorbedwater. Then, a sharp weight loss of 54 wt% around 180e350 �C wasassociated with the removal of the CTAB existing in the mesoporeschannels of the BMMs. Finally, a weight loss of around 3.16 wt%between 350 and 500 �C was ascribed to the dehydration of siliconalcohol [22]. These observations were in well agreement with theliterature [22,32].

The microstructure of templates stemming from CTAB is one ofkey of the texture properties of the resultant mesoporous materials[35]. Huo and co-workers [24] once demonstrated that the siloxaneanions and surfactant cations could be mediated by OH� ions in theTEOSeCTABeammonia aqueous system, and therefore proposedthat the variation of the alkali concentration in the synthesis sys-tem may affect the mesoporous structures by altering the chargematching pattern. In principle, varying the pH value can remarkablyadjust the electrostatic charge distribution of the solegel mixture,and strongly influence the interactions of hydrophilic head groupsof the CTAB, resulting in the ordered degree change of the templatestructure [36]. On the other hand, Brinker et al. also stated thatpoly-condensation reaction rate of the hydrolyzed silica species isslower than the hydrolysis reaction rate of TEOS under the lowammonia concentration [25,26], leading to formation of small andspherical particles. Based on the above-mentioned and combinedwith the characterization results, the mechanism for controlledmorphology and particle size of BMMs is suggested as illustrated inScheme 1.

At low ammonia concentration of the synthesis solegel, bothslow hydrolysis rate of TEOS and low electrostatic charge density ofsilica/surfactant are in favor of the generating of small sphericalparticles. Withmore ammonia content in the solution, the obtainedparticles of BMMs gradually become larger and uniform byaccompany with unstable and easily aggregation together. As theammonia concentration reaches a certain level, the hydrolysis rateof TEOS is faster and generates many more silica species. At thesame time, the strong alkaline environment might make negativesilicate ions interact strongly with the positive CTAþ template,leading to the rod-like morphology of the BMMs particles. Thepossible cooperative mechanism including the solegel processderived from hydrolysisepolycondensation of TEOS and self-assembly of CTAB was proposed as shown in Scheme 1.

Obviously, the effect of CTAB in an ammonia medium is mainlyuseful for the assembly of the primary mesopores, the other hand,the aggregates among the mesopores (primary) nanosilicas arebeneficial to the formation of the secondary mesopores. Accordingto the literature [24,35,36], as depicted in Fig. 3, the size of thenano-silicas gradually increases with increasing ammonia additivesduring the hydrolysis and polycondensation procedures of TEOS.While the secondary mesopores derived from intra-nanoparticlesare accompanied by an increase in size, eventually leading totheir disappearances, however, the primary mesopores are stillmaintained due to the presences of the ordered liquid crystaltemplate originating from micelles of CTAB [1,2], as can be seen inFig. 2 and Table 1.

Scheme 1. Schematic illustration of the possible mechanism of the morphology-tailored and structure-controlled BMMs.

H. Zhou et al. / Materials Chemistry and Physics 140 (2013) 148e153 153

4. Conclusions

Our present results showed that the morphology and structuralparameters of BMMs can be greatly influenced via the solegelprocess in different ammonia concentration solution. As the in-crease of the ammonia concentration in the synthesis system, theprimary mesopore diameter and the pore volume graduallydecrease slightly, the secondmesopores size grows gradually largeruntil they disappear, but the specific surface areas also increasedexcept in sample-3.0. In addition, the spherical particle size ofBMMs series shows an increasing tendency with an averagediameter from 25 to 150 nm as the increasing of ammonia content,except that sample-3.0 exhibited an irregular shape with rod-likemorphology. The unordered mesopore structures showed thewormlike channels with the uniform size. On the basis of the aboveresults, the proposed mechanism for bimodal mesoporous struc-tures may be available to understand, design and prepare the novelnano-silica material.

Acknowledgments

This project was supported by the National Natural ScienceFoundation of China (21076003), the National Basic Research Pro-gram of China (973 Program 2009CB930200), the Beijing MunicipalNatural Science Foundation (2093030) and the Funding Project forAcademic Human Resources Development in Institutions of HigherLearning under the jurisdiction of the Beijing Municipality (PHR200907105, PHR 201107104, and 005000543111517). The authorsalso thank Prof. Xu and associate Prof. Xiao at Beijing University ofTechnology for the TEM and XRD analysis, Dr, Bai Shi-Yang forfruitful suggestions.

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