Microporous and Mesoporous Materials 139 (2011) 1–7

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Microporous and Mesoporous Materials

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Synthesis of hollow mesoporous silica microspheres through surface sol–gel processon polystyrene-co-poly(4-vinylpyridine) core–shell microspheres

Shengnan Wang, Minchao Zhang, Da Wang, Wangqing Zhang ⇑, Shuangxi Liu ⇑Key Laboratory of Functional Polymer Materials of Ministry of Education, Institute of Polymer Chemistry, Chemistry College, Nankai University, Tianjin 300071, China

a r t i c l e i n f o

Article history:Received 22 July 2010Received in revised form 30 September2010Accepted 6 October 2010Available online 30 October 2010

Keywords:Core–shell microspheresHollow mesoporous silica microspheresSol-gel processTemplate

1387-1811/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.micromeso.2010.10.002

⇑ Corresponding authors. Tel.: +86 22 23509794; fax:E-mail addresses: wqzhang@nankai.edu.cn (W. Zhang

a b s t r a c t

Synthesis of hollow mesoporous silica microspheres (HMSM) through directed surface sol–gel process oftetraethylorthosilicate (TEOS) on the template of core–shell microspheres of polystyrene-co-poly(4-vinylpyridine) (PS-co-P4VP) in the presence of the CTAB surfactant in neutral aqueous solution at roomtemperature is discussed. Ascribed to the inherently pendent catalyst of the Lewis alkaline P4VP segmenton the template surface of the PS-co-P4VP core–shell microspheres, the sol–gel process of TEOS is direc-ted exclusively onto the template, and thus well-defined HMSM are fabricated. The thickness of the mes-oporous silica shell of HMSM can be tuned by changing the weight ratio of the template of the PS-co-P4VPcore–shell microspheres to the coating material of TEOS. Following this method of directed surface sol–gel process, well-defined HMSM with shell thickness ranging from 13 to 39 nm are synthesized. Trans-mission electron microscopy (TEM), thermogravimetric analysis (TGA), powder X-ray diffraction (XRD)and nitrogen adsorption–desorption analysis are applied to characterize the synthesized HMSM.

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1. Introduction

Well-defined hollow microspheres have received considerableattention recently because of their unique properties such aslow density, large surface area, excellent loading capacity, highpermeability, and potential applications in catalysis, chromatog-raphy, fillers, microreactors, waste removal, drug/gene storage,and controlled release [1–9]. The hollow interior makes it possi-ble to encapsulate a variety of guest molecules within the hollowmicrospheres. However, when the shell of the hollow micro-spheres is dense, encapsulation of guest molecules within thehollow microspheres and release of the encapsulated guest mole-cules off the hollow microspheres are impossible or time consum-ing, limiting use of such hollow microspheres. Thus, controlledsynthesis of hollow micropsheres with porous shell especiallythe hollow mesoporous silica microspheres (HMSM) is desired,since these HMSM microspheres have been demonstrated to benontoxic, highly biocompatible, and mechanically stable, andhave many practical applications mentioned above [10–21]. Inthis regard, various methodologies including template-assistedsynthesis and template-free routes are proposed to fabricateHMSM [22–35]. In comparison, the template-assisted method ismore controllable, although there has been an increase in thenumber of template-free routes [36–39]. In a typical template-

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+86 22 23503510 (W. Zhang).), sxliu@nankai.edu.cn (S. Liu).

assisted synthesis, mesoporous silica shell is formed initiallythrough sol–gel process of a silica precursor such as tetraethylor-thosilicate (TEOS) catalyzed by acid or base on a suitabletemplate in the presence of surfactant such as hexadecyltrime-thylammonium bromide (CTAB) or Pluronic� followed by removalof the template through calcination at elevated temperature orselective etching in an appropriate solvent [29–35].

Generally, it is deemed that the sol–gel process of TEOS on asuitable template involves (1) fast hydrolysis catalyzed by HCl orNH3 aqueous solution, (2) condensation of Si(OH)4 to form silicacoated template microspheres, (3) condensation of Si(OH)4 to formfree silica nanoclusters, (4) capture of free silica nanocluster ontothe template, and (5) aggregation of the free silica nanoclustersinto unwanted and irregular aggregates of silica, which are shownin Scheme 1 [40]. Although many success examples of template-assisted synthesis of HMSM through sol–gel process are reported[22–35], whereas great care should be taken to avoid formationof unwanted and irregular aggregates, since it is sometimes veryserious due to the fast hydrolysis of TEOS catalyzed by a free andsoluble catalyst of HCl or NH3 aqueous solution [40,41]. For exam-ple, Hotta and coworkers reported that a lot of TEOS was convertedto unwanted and irregular aggregates of silica during the sol–gelprocess of TEOS on negatively charged polystyrene microsphereswhen HCl aqueous solution was employed as catalyst [40]. There-fore, the key to controlled synthesis of HMSM is to make the sol–gel process of TEOS take place exclusively on the template to avoidformation of unwanted and irregular aggregates.

TEOS(1)

Si(OH)4

SiO2nanoclusters template

SiO2irregular aggregates

(2)(3)

(4)

(5)

TEOS(1)

Si(OH)4

SiO2nanoclusters templatetemplate

SiO2irregular aggregates

(2)(3)

(4)

(5)

silica

silica

TEOS(1)

Si(OH)4

SiO2nanoclusters template

SiO2irregular aggregates

(2)(3)

(4)

(5)

TEOS(1)

Si(OH)4

SiO2nanoclusters templatetemplate

SiO2irregular aggregates

(2)(3)

(4)

(5)

silica

silica

Scheme 1. Sol–gel process of TEOS on a template employing a free catalyst.

2 S. Wang et al. / Microporous and Mesoporous Materials 139 (2011) 1–7

To avoid formation of unwanted and irregular aggregates of sil-ica, Shinkai and coworkers proposed a surface sol–gel mechanism,following which the component to catalyze the formation andgrowth of silica is appended on the template surface and thereforethe sol–gel process of TEOS is directed exclusively onto the tem-plate since no free catalyst exists in solution [42]. This surface di-rected sol–gel process leads to a unitary product, which is veryimportant when clean and well-defined materials are needed [42].

Herein, we report a controllable synthesis of HMSM throughsurface sol–gel process of TEOS on the template of core–shellmicrospheres of polystyrene-co-poly(4-vinylpyridine) (PS-co-P4VP) in the presence of the CTAB surfactant in neutral aqueoussolution at room temperature. In our procedures, the dual tem-plates of the CTAB surfactant and the PS-co-P4VP core–shell micro-spheres are employed to construct mesopores in the silica shellsand hollow interiors of HMSM, respectively. The selection of thePS-co-P4VP core–shell microspheres as template is ascribed totwo concerns. First, the PS-co-P4VP core–shell microspheres,which have a polystyrene core and a P4VP shell, contain the pen-dent catalyst of the Lewis alkaline P4VP segment on the surfaceof the template to initiate sol–gel process of TEOS [43,44]. Second,the PS-co-P4VP core–shell microspheres can be easily synthesizedby one-stage soap-free emulsion polymerization as described else-where [45,46], which provides great convenience and economicalalternation for the present methodology. The synthesis demon-strates that the sol–gel process of TEOS in neutral aqueous solutionat room temperature is directed onto the template of thePS-co-P4VP core–shell microspheres, formation of unwanted andirregular aggregates is avoided, and therefore well-defined HMSMwith controllable shell thickness are produced.

Scheme 2. Schematic synthesis of HMSM through directed surface sol–gel p

2. Experimental section

2.1. Materials

The monomers of styrene (St, >98%, Tianjin Chemical Company)and 4-vinylpyridine (4VP, >95%, Aldrich) were distilled under vac-uum before being used. Tetraethylorthosilicate (TEOS, >99%, AlfaAesar), cetyltrimethylammonium bromide (CTAB, >99%, TianjinGuangfu Fine Chemical Research Institute), K2S2O8 (>99.5%, TianjinChemical Company) and other analytical reagents were used as re-ceived. Deionized water was employed in the present study.

2.2. Synthesis of the PS-co-P4VP core–shell microspheres

The PS-co-P4VP core–shell microspheres were synthesized byone-stage soap-free emulsion polymerization as described else-where [45,46]. The typical synthesis was introduced as follows.To a flask, 80.0 mmol of 4VP (8.41 g) and 360.0 mL of water wereadded at room temperature. Then 160.0 mmol of styrene(16.66 g) was added to the homogeneous aqueous solution of4VP. The mixture was vigorously stirred at about 300 rpm forabout 30 min at room temperature, and then 4.0 mmol of K2S2O8

(1.08 g) was added into the mixture. Finally, the mixture was de-gassed under nitrogen purge, and then polymerization was per-formed with vigorously stirring at 80 �C for 24 h under nitrogenatmosphere. After completion of the polymerization, the resultantPS-co-P4VP core–shell microspheres were collected by centrifuga-tion, washed thrice with water (3 � 20 mL), and finally dispersedin 300 mL of water for next use. The concentration of the PS-co-P4VP core–shell microspheres in the aqueous dispersion is0.080 g/mL.

2.3. Synthesis of hollow mesoporous silica microspheres (HMSM)

A given amount of TEOS was added into 90.0 mL of water con-taining the cationic surfactant CTAB in which the molar ratio ofTEOS/CTAB was set at 2/1. The mixture was initially dispersed bymeans of an ultrasonic bath set at 20 �C for about 30 min(KQ-200KDE, 40 kHz, 200 W, Zhoushan, China), and then 10.0 mLof the aqueous dispersion of the PS-co-P4VP core–shell micro-spheres was added. The sol–gel process of TEOS was performedat room temperature for 40 h with vigorous stirring. The productof the silica-coated PS-co-P4VP microspheres (PS-co-P4VP@SiO2)was collected by centrifugation, washed thrice with water(3 � 10 mL), and dried under vacuum at 50 �C for 12 h. After

rocess of TEOS on the template of PS-co-P4VP core–shell microspheres.

S. Wang et al. / Microporous and Mesoporous Materials 139 (2011) 1–7 3

calcination at a given temperature of 500–600 �C in air for 4 h,HMSM were obtained.

2.4. Characterization

Transmission electron microscopy (TEM) measurement wasconducted by using a Philips T20ST electron microscope at anacceleration voltage of 200 kV, whereby a small drop of the sam-ple was deposited onto a carbon-coating copper grid and dried atroom temperature under atmospheric pressure. Scanning electronmicroscopy (SEM) measurement was conducted by using a HIT-ACHI S-3500N electron microscope. The thermogravimetric anal-ysis (TGA) was performed on a thermogravimetric analyzer (TG209, NETZSCH) in air with a heating rate of 10 �C /min from roomtemperature to 800 �C. The powder X-ray diffraction (XRD)measurement was performed on a Rigaku D/max 2500 X-ray

Fig. 1. The TEM images of the PS-co-P4VP microspheres (A and B), silica-coated PS-comicrospheres of HMSM-26-550 (E and F).

diffractometer. The nitrogen adsorption–desorption analysis wascarried out at 77 K on a Micromeritics TriStar 3000 apparatus.The analytical data were processed by the Brunauer–Emmett–Teller (BET) equation for surface areas and by the Barret–Joyner–Halenda (BJH) model for pore size distribution.

3. Results and discussion

Scheme 2 shows the synthesis of HMSM. First, the template ofthe PS-co-P4VP core–shell microspheres, which contain a polysty-rene (PS) core and a poly(4-vinylpyridine) (P4VP) shell, is synthe-sized by one-stage soap-free emulsion polymerization asdescribed elsewhere [45,46]. The template of the PS-co-P4VPcore–shell microspheres has a noticeable character that the shell-forming P4VP segment is a catalyst of a typical Lewis alkali toinitiate sol–gel process of TEOS in water [43,44]. The inherently

-P4VP microspheres of PS-co-P4VP@SiO2 (C and D), and hollow mesoporous silica

4 S. Wang et al. / Microporous and Mesoporous Materials 139 (2011) 1–7

pendent catalyst sites of the P4VP segment afford a directedsol–gel process on the template of the PS-co-P4VP core–shellmicrospheres. Second, the silica-coated PS-co-P4VP microspheresof PS-co-P4VP@SiO2 are produced through sol–gel process of TEOSon the template of the core–shell microspheres in the presence ofthe CTAB surfactant. Third, the PS-co-P4VP@SiO2 microsphereswere collected by centrifugation, and washed with water toremove the CTAB surfactant. Lastly, calcination to remove thepolymeric template affords HMSM.

Fig. 1A and B show the transmission electron microscope (TEM)images of the PS-co-P4VP core–shell microspheres used in thepresent study, indicating an average diameter of 325 nm of themonodispersed template microspheres. After sol–gel process ofTEOS on the template of the PS-co-P4VP core–shell microspheres,

Fig. 2. The SEM images of the PS-co-P4VP microspheres (A), silica-coated PS-co-P4VP microspheres of PS-co-P4VP@SiO2 (B), and hollow mesoporous silica micro-spheres of HMSM-26-550 (C).

the product is collected and washed with water. TEM observationshows the unitary product of the PS-co-P4VP@SiO2 microspheres(Fig. 1C), confirming the directed sol–gel process exclusively onthe template due to the inherently pendent catalyst sites of theP4VP segment. After the silica coating, the size of the PS-co-P4VP@SiO2 microspheres increases from 325 to 375 nm.Meanwhile, the mesoporous structure of the coated silica on theperiphery of the PS-co-P4VP@SiO2 microspheres can also be clearlydiscerned from the TEM images shown in Fig. 1C and D, confirmingthe sol–gel process exclusively on the template of the PS-co-P4VPcore–shell microspheres. After calcination at 550 0C for 4 h, thepolymeric template is removed, 26 nm of mesoporous silica shellremains, and the hollow mesoporous silica microspheres ofHMSM-26–550 (herein and in the next text, the first number inHMSM-26–550 represents shell thickness of the mesoporous silicaand the second number indicates the calcination temperature) arefabricated, as clearly indicated by the high contrast of the shell andthe core demonstrated in the TEM images (Fig. 1E and F). Further-more, no evidence of coalescence, extensive cracking, or collapsedstructures among HMSM is found during the TEM observation. Be-sides, the synthesis of HMSM is also tracked by SEM. As shown inFig. 2, after silica coating on the PS-co-P4VP core–shell micro-spheres (Fig. 2A), the sole product of uniform PS-co-P4VP@SiO2

microspheres (Fig. 2B) is produced. Furthermore, as shown inFig. 2C the resultant HMSM keep perfect morphology after calcina-tion and only very few of broken ones, which are indicated by cy-cles, are observed.

The formation of mesopores in the HMSM is further discussed.It has been demonstrated that the polymer chains of the poly(eth-ylene oxide) (PEO) block are enveloped within the resultant silicaduring the synthesis of mesoporous silica materials employingthe typical structure directing agent of Pluronic [47–49]. In thepresent synthesis of HMSM, the outer layer of the P4VP shell ofthe PS-co-P4VP core–shell microspheres are swollen in water dueto the somewhat hydrophilic character of the P4VP segment. Thus,it is expected that both the mesoglobules of the P4VP chains andthe CTAB micelles are operative in the synthesis of HMSM asshown in Scheme 2. When washing with water, the P4VP mesoglo-bules transfer to coils, and release from the silica layer, and there-fore mesopores in the silica layer of the PS-co-P4VP@SiO2

microspheres are produced as indicated in Fig. 1C and D. Further-more, the CTAB micelles are also removed to contribute the forma-tion of mesopores in the silica layer in this washing procedure,which is confirmed by thermogravimetric analysis (TGA) as dis-cussed subsequently.

Fig. 3. Weight ratio of PS-co-P4VP/TEOS dependent shell thickness of HMSM.

Fig. 5. X-ray diffraction spectra of the typical hollow mesoporous silica micro-spheres of HMSM-26-550.

100 200 300 400 500 600 700 800

0

20

40

60

80

100

Weig

ht (w

t %)

Temperature (°C)

B

A

Fig. 6. TGA curves of the PS-co-P4VP core–shell microspheres (A) and the silica-coated PS-co-P4VP microspheres of PS-co-P4VP@SiO2 (B).

S. Wang et al. / Microporous and Mesoporous Materials 139 (2011) 1–7 5

As similar as the general coating methodology [50–52], thethickness of the mesoporous silica shell of HMSM can be tunedby the weight ratio of PS-co-P4VP/TEOS. It is found that the shellthicknesses of HMSM linearly decreases from 39 to 13 nm whenthe weight ratio of PS-co-P4VP/TEOS increases from 0.025 to 0.25as shown in Fig. 3, which provides great convenience to designHMSM with controllable shell of mesoporous silica. From the in-sets of TEM images in Fig. 3, it is clearly observed that almost allthe HMSM samples including HMSM-39-550, HMSM-26-550 andeven the HMSM-13-550 with the thinnest shell of 13 nm keep per-fect morphology after calcination at 550 �C, indicating their ther-mal stability.

To compare the present surface sol–gel process with the generalsol–gel coating, a reference sol–gel process employing a free cata-lyst of 1.0 mL of 25 wt.% NH3 aqueous solution under other similarconditions was checked. After 24 h of sol–gel process of TEOS, theseparated product was characterized by TEM observation. The TEMimage shown in Fig. 4A clearly indicates a mixture of irregular sil-ica sheets and microspheres. Further from a high-magnificationTEM image of a single microsphere (Fig. 4B), it is discerned thatonly slight amount of silica (indicated by an arrow) is coated onthe template of the PS-co-P4VP core–shell microspheres. These re-sults confirm the advantage of the present synthesis of HMSMthrough surface sol–gel process.

Next, we tend to characterize the synthesized HMSM. The typ-ical XRD spectra of HMSM-26–550 are shown in Fig. 5. Clearly, nocharacteristic Bragg diffraction peaks at 2h below 10� but a broadpeak with 2h centered at 21� is observed, possibly indicating a dis-ordered mesostructure of the present HMSM [53].

The weight density of the mesoporous silica shell of the typicalhollow mesoporous silica microspheres of HMSM-26-550 isfurther calculated. Fig. 6 shows the TGA curves of the PS-co-P4VP@SiO2 microspheres and the reference sample of the PS-co-P4VP core–shell microspheres. As indicated, three weight-lossstages below 270 �C, at 270–400 �C and 400–650 �C are observed.These weight-losses correspond to the evaporation of physicallyadsorbed water and residual solvent, the decomposition of thepolymeric template of the PS-co-P4VP core–shell microspheres,and the decomposition of the silica-bonded groups such as –OHand/or unhydrolyzed –OR, respectively [54]. Clearly, no signalcorresponding to the decomposition of CTAB at about 220 �C isobserved [23], confirming that CTAB is removed from the PS-co-P4VP@SiO2 microspheres by washing with water. The mass frac-tion of 35% corresponds to the resultant HMSM after calcination.From the shell thickness of HMSM-26-550 (26 nm by TEM imageas shown in Fig. 1) and assuming that the weight density of the

Fig. 4. TEM images of silica materials synthesized in the presence of PS-co-P4VP microsphsilica-coated PS-co-P4VP microspheres (B).

polymeric template of the PS-co-P4VP core–shell microspheres is1.0 g/cm3, the weight density of the mesoporous silica shell ofHMSM-26-550 is calculated to be 1.7 g/cm3. Clearly, the weight

eres and the free catalyst of aqueous ammonia at room temperature (A) and a single

Fig. 7. N2 adsorption–desorption isotherms of HMSM-26-500 (A), HMSM-26-550(B) and HMSM-26-600 (C). Insets: mesopore diameter distribution and TEM imageof HMSM.

Table 1BET surface area and total mesopore volume of HMSM obtained under different ratiosof PS-co-P4VP/TEOS.

Sample Shellthickness(nm)

BET surfacearea (m2/g)

Volume ofmesopore (cm3/g)

Averagemesoporesize (nm)

HMSM-13-550 12 546 1.41 5.4HMSM-26-550 26 347 0.44 2.8HMSM-39-550 39 72 0.16 2.7

6 S. Wang et al. / Microporous and Mesoporous Materials 139 (2011) 1–7

density of the present mesoporous silica shell of HMSM-26-550 ismuch lower than those of amorphous silica (2.2 g/cm3), confirmingmesoporous structure of the present HMSM.

Fig. 7 shows the nitrogen adsorption–desorption isotherms ofthree HMSM samples of HMSM-26-500, HMSM-26-550 andHMSM-26-600 prepared from calcination of the same PS-co-P4VP@SiO2 microspheres at different temperatures of 500 �C,550 �C, and 600 �C for 4 h, respectively. For HMSM-26-500 andHMSM-26-550, the N2 adsorption–desorption isotherms exhibittypical type-IV hysteresis with a sharp increase in nitrogen uptakeat high relative pressure P/P0 �0.9 and a wide hysteresis loop at P/P0 > 0.2 (Fig. 7A and B). In addition, the mesopore size distributioncurves of HMSM-26-500 and HMSM-26-550 are determined fromthe adsorption branch of the isotherms (seeing the top-left insets

in Fig. 7A and B). The mesopore size distributions exhibit a sharppeak centered around 2.9 and 2.8 nm, respectively, implying a uni-form mesopore size. The BET surface area and total pore volumeare 365 m2/g and 0.44 cm3/g for HMSM-26-500 and 347 m2/gand 0.44 cm3/g for HMSM-26-550, respectively. In comparison,the nitrogen adsorption–desorption isotherms of HMSM-26-600is slightly different (Fig. 7C). It is found that the BET surface areaof 138 m2/g and total mesopore volume of 0.25 cm3/g are muchlower than those of HMSM-26-500 and HMSM-26-550. These re-sults illustrate that when calcination temperature increases from500 to 550 �C the HMSM keeps thermal stability and when temper-ature further increases to 600 �C, part of the mesopores in HMSMcollapsed and therefore BET surface area and total mesopore vol-ume decrease. The collapse of the HMSM samples are further con-firmed by the top-right insets of TEM images in Fig. 7, from whichit can be seen that mesopores in HMSM-26-500 and HMSM-26-550are clearly more abundant than those in HMSM-26-600 and theshell thickness of HMSM-26-600 is thinner than those of HMSM-26-500 and HMSM-26-550 (21 nm vs 26 nm).

Lastly, the porous parameters including specific surface area,mesopore volume, and mesopore size of HMSM-39-550, HMSM-26-550 and HMSM-13-550 are compared. These three HMSM havedifferent shell thickness as indicated by the insets of the TEMimages shown in Fig. 3, but they are prepared from calcination atthe same temperature of 550 �C. The results shown in Table 1 dem-onstrate that the specific surface area and mesopore volume ofHMSM decrease with the increases in the thickness of the meso-porous silica shell. Compared with the general mesoporous silica[55], the present HMSM especially the HMSM-39-550 have a lowerspecific surface area. The reason is possibly ascribed to the thicksilica layer in HMSM-39-550. As discussed above 2, formation ofmesopores in the synthesis of HMSM is ascribed to both theP4VP mesoglobules and the CTAB micelles. However, the P4VPmesoglobules are operative just near the surface of the templateas shown in Scheme 2, since the P4VP shell of the PS-co-P4VPcore–shell microspheres is just swollen but not soluble in water.With the increase in the thickness of the silica shell, only the CTABmicelles are operative in the formation of mesopores, which resultsin lower specific surface area for HMSM with thicker silica shell.

4. Conclusions

Synthesis of hollow mesoporous silica microspheres (HMSM)through surface sol–gel process on the template of core–shellmicrospheres of polystyrene-co-poly(4-vinylpyridine) (PS-co-P4VP) in the presences of the CTAB surfactant in aqueous solutionat room temperature is discussed. Ascribed to the inherently pen-dent catalyst of the Lewis alkaline P4VP segment on surface of thetemplate of the PS-co-P4VP core–shell microspheres, the sol–gelprocess of TEOS is directed exclusively onto the template, andthus well-defined HMSM are fabricated. The thickness of themesoporous silica shell of HMSM can be tuned by changing theweight ratio of the template of the PS-co-P4VP microspheres tothe coating material of TEOS. Following this method, well-definedHMSM with 3–5 nm mesopores in the shell and with shell

S. Wang et al. / Microporous and Mesoporous Materials 139 (2011) 1–7 7

thickness ranging from 13 to 39 nm are synthesized. XRD analysisindicates the disordered mesostructure of the present HMSM. Theweight density of the mesoporous silica shell of HMSM is esti-mated to be 1.7 g/cm3. The porous parameters including BET sur-face area, mesopore volume, and mesopore size of HMSM arecharacterized and the results show that part of mesopores col-lapsed during calcination, the BET surface area decreases from365 to 138 m2/g and mesopore volume decreases from 0.44 to0.25 cm3/g when calcination temperature increase from 500 to600 �C. Furthermore, when the mesoporous silica shell thicknessincreases, the BET surface area and mesopore volume of HMSMdecrease.

Acknowledgments

The financial support by National Science Foundation of China(No. 20974051) and Tianjin Natural Science Foundation (No.09JCYBJC02800) is gratefully acknowledged.

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