DOI: 10.1021/la102510c 14901Langmuir 2010, 26(18), 14901–14908 Published on Web 08/24/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Silica Mesostructures: Control of Pore Size and Surface Area Using

a Surfactant-Templated Hydrothermal Process

Aparna Ganguly,†,‡ Tokeer Ahmad,‡ and Ashok K. Ganguli*,†

†Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India, and‡Department of Chemistry, Faculty of Natural Sciences, Jamia Millia Islamia, New Delhi 110025, India

Received June 21, 2010. Revised Manuscript Received August 3, 2010

The cooperative self-assembly of the silica precursor, tetraethyl ortho silicate (TEOS), with the surfactant moleculefollowed by the basic hydrolysis led to the formation of mesoporous silica with varying pore sizes. The pores are formedby the removal of the intermediate assemblies of the charged surfactant molecules. In the absence of formation of suchassemblies of surfactants (example in the case of nonionic surfactants), the resulting mesostructures have very smallpores, giving low surface area mesostructures. This study outlines the precise control of pore size in a wide sizedistribution (3.4-22 nm) by the systematic variation of the surfactant. The addition of polyethylene glycol (in situ) whilecarrying out the hydrolysis of TEOS results in the formation of large-sized cavities (∼40 nm). Uniform sphericalparticles with pores (different from the cavities) as large as 22 nm and surface areas of∼1100m2/g have been obtained bythe combined effect of the hydrothermal conditions on the cetyl trimethyl ammonium bromide-templated synthesis.

1. Introduction

Silica mesostructures have been gaining attention since theirdiscovery in the year 1992.1 Characterized by high surface areas(∼1200 m2/g) and large pore sizes (2-20 nm), these mesoporousstructures are of immense practical applications from catalysis tosensors and more recently in the area of drug delivery. A numberof synthetic approaches with proper surface functionalizationhave been designed, keeping in mind the desired properties. Thepresence of surface hydroxyl groups on silica particles allows easyfunctionalization, thereby extending its application. For example,polyamidoamine (PAMAM)-grafted silica particles have beenstudied for their use as water purifiers by selective removal of thedetergent and anionic dyes spilled in water.2 Porous silica with anuniform pore size distribution exhibits promising applications innanocasting,3 as adsorbents,4,5 in drug delivery,6 for controlledrelease of drugs, as nanoreactors,7,8 etc. The choice of pore sizedepends on the application. For example, a bimodal pore systemwhere both themesopores (2-50 nm) and themicropores (<2nm)are present has been effectively used as a catalyst andadsorbent forthe removal of volatile organic compounds (VOCs) in the lowpressure region especially below 2000ppm; a pore size of<2 nm isdesirable.9Because of its chemical inertness and high surface area,amorphous mesostructured silica nanoparticles are the mostpopular oxides for biomedical applications. Mesoporous silicananoparticleswith sizes of 100nmandpore sizes of 2 nmhavebeen

functionalized with multiple components like fluorescent moleculesfor easy tracking of anticancer drug-loaded particles or with photo-responsive azobenzene derivatives acting as nanoimpellars10 forsimultaneous imaging and delivery of biomolecules.11 On the otherhand, nanoparticles with large pores (∼20 nm) can be convenientlyloaded with high doses of plasmid DNA, which provides stabilityagainst enzymatic degradation.12 Not only the density of pores butalso the size of the pore are crucial for applications. Thus, controlover the size, porosity, and chemical homogeneity is an essentialrequirement for the consistency of techniques (biosensing or drugdelivery, etc.) based on such mesostructures.

Most of the reports onmesostructured silica discuss the use of thedouble surfactant system, that is, a triblock copolymer and afluorocarbon (FC)mixture acting in the SþX-Iþmechanismwherethe triblock copolymer acts as the supramolecular template and theFC surfactant controls the growth of the particle.12-14 Dependingon the synthesis pH, the cationic surfactant (Sþ), the anion (X-),and the inorganic silica (Iþ) self - assemble, leading to the formationof mesostructures.14,15 Organic swelling agents like trimethyl ben-zene (TMB) have also been used to adjust the pore size.13

Although a large number of studies emphasize the syntheticmethodology along with its application in the field of catalystsupports or removal of environmental hazardous materials, asystematic study on an increase in the pore size using differentsurfactants has not been carried out so far. From our earlierstudies (micromulsion-based synthesis), we observe that surfac-tants play a key role in guiding the morphology of the productsformed.16-18 A change in the morphology of silica mesoparticles*To whom correspondence should be addressed. Tel: 91-11-26591511. Fax:

91-11-26854715. E-mail: ashok@chemistry.iitd.ernet.in.(1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S.

Nature 1992, 359, 710.(2) Chu, C. C.; Uneo, N.; Imae, T. Chem. Mater. 2008, 20, 2669.(3) Lu, A. H.; Sch€uth, F. Adv. Mater. 2006, 18, 1793.(4) Lam, K. F.; Ho, K. Y.; Yeung, K. L.; McKay, G. Mater. Res. Soc. Symp.

Proc. 2003, 788 (Continuous Nanophase and Nanostructured Materials), 383.(5) Yan, Z.; Li, G.; Mu, L.; Tao, S J. Mater. Chem. 2006, 16, 1717.(6) Hadinoto, K.; Phanapavudhikul, P.; Kewu, Z.; Tan, R. B. H. Ind. Eng.

Chem. Res. 2006, 45, 3697.(7) Mal, N. K.; Fujiwara, M.; Tanaka, Y. Nature 2003, 421, 350.(8) Giri, S.; Trewyn, B. G.; Stellmaker, M. P.; Lin, V. S. Angew. Chem., Int. Ed.

2005, 44, 5038.(9) Kosuge, K.; Kubo, S.; Kikukawa, N.; Takemori, M. Langmuir 2007, 23,

3095.

(10) Lu, J.; Choi, E.; Tamanoi, F.; Zink, J. I. Small 2008, 4, 421.(11) Liong,M.; Angelos, S.; Choi, E.; Patel, K.; Stoddart, J. F.; Jink, J. J.Mater.

Chem. 2009, 19, 6251.(12) Gao, F.; Botella, P.; Corma, A.; Blesa, J.; Dong, L. J. Phys. Chem. B 2009,

1796.(13) Han, Y.; Ying, J. Y. Angew. Chem. Int. Ed. 2005, 44, 288.(14) Huo, Q.; Margolese, D. I.; Clesla, U.; Feng, P.; Gier, T. E; Sleger, P.; Leon,

R; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 1994, 368.(15) Ji, Y.; Wang, C.; Zou, Y.; Song, J.; Wang, J.; Li, F.; Xiao, F. S. J. Phys.

Chem. C 2008, 112, 19367.(16) Ranjan, R.; Vaidya, S.; Thaplyal, P.; Qamar,M.; Ahmed, J.; Ganguli, A.K.

Langmuir 2009, 25, 6469.

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(fibres to particles) was also observed onmoving from cationic tononionic surfactants.19 In this paper, we discuss the effect ofdifferent surfactants on the pore size of silica particles. We resortto a hydrothermal treatment followed by the basic hydrolysis ofthe silicate in the presence of a single surfactant system. A simpleroute has been developed for the fabrication of mesoporousparticles by manipulating the cooperative assembly of the surfac-tant and silicate species.Homogeneity and uniformity in size havebeen achieved to a high degree of precision using this method. Byvarying the type of surfactant, we can fine-tune the pore size of thesilica particle, that is, from microporous to mesoporous, so as tocater to different applications.

2. Experimental Section

2.1. Chemicals. All of the chemicals used were of analyticalgrade and were used as is without any further purification. TEOS(tetraethylortho silicate), PEG(polyethyleneglycol), andTergitolwere procured fromAldrich, while CTAB (cetyl trimethyl ammo-nium bromide) was bought from Spectrochem Laboratories.Sodium lauryl sulfate (SLS) was obtained from SRL (SiscoResearch Laboratories), and cetyl pyridinium bromide (CPB)was obtained fromBDHchemicals.Deionizedwater was used forall of the experiments.

2.2. Synthesis of Mesoporous Materials.Hydrothermallytreated silica was synthesized by mixing surfactant/TEOS/am-monia/ethanol/water in a weight ratio of 1:5:10:126:140 in allof the systems except for the dual surfactant system where theratio was changed to 2:5:10:126:140. Here, the surfactant wasused as the templating agent. Three different surfactants wereused for the synthesis of mesoporous silica particles, CTAB,Tergitol, SLS, and stabilizers like PEG, PEG-400. The surfac-tant was mixed with water followed by the addition of TEOSand ethanol. Ammonia was added dropwise to the mixturewhile stirring. It was then allowed to stir for 2 h. The pH ofthe reaction mixture was maintained between 10 and 11. Theslurry obtained was heated at 120 �C for 48 h in an autoclave atconstant pressure. The sample was centrifuged and washed withwater and ethanol and dried at room temperature. The powderobtained was then heated at 550 �C to remove the surfactanttrapped inside. For the dual surfactant template method, a 1:1molar ratio of CPB and CTAB was mixed together with othercomponents in a fixed ratio and treated hydrothermally asbefore.

2.3. Characterization of Material. The as-synthesizedand calcined products (silica mesoparticles) were characterizedby powder X-ray diffraction studies using a Bruker D-8 Ad-vance X-ray diffractometer with Ni filtered Cu KR radiation. IRspectra of the silica nanoparticles were recorded in the transmis-sion mode in the range of 200-4000 cm-1 on a Nicolet Prot�eg�e460 FTIR spectrometer using KBr discs. Transmission electronmicroscopy (TEM) was recorded on a Technai G2 20 (FEI)electron microscope operated at 200 kV. TEM specimens wereprepared by loading a drop of the ultrasonically dispersed samplein ethanol on a carbon-coated copper grid and dried in air. Theζ-potential was measured using a Malvern Zeta Sizer ZS 90fitted with a 633 nm laser. N2 adsorption and desorption studieswere carried out on a Belsorp max operated at 77 K using theBrunauer-Emmett-Teller (BET) method to determine the sur-face area. The as-prepared SiO2 samples were degassed at 150 �Cfor 4 h prior to the surface area measurements, whereas the poresize distribution was calculated according to the Barret JoynerHalenda (BJH) method.

3. Results and Discussion

Mesoporous silica has been synthesized by the dual action ofsurfactant and hydrothermal conditions. The PXRD patterns forall of the samples reveal that the silica particles are amorphous.Figure 1 shows thePXRDpattern for silica synthesizedusingCTAB.The formation of silica was confirmed using IR spectroscopy, whichshowed bands corresponding to stretching and bending modesof Si-O-Si linkages at 1101 and 800 cm-1. We have beensuccessful in tailoring the pore size and surface area of mesopor-ous silica particles using the surfactant template method. Theeffect of different type of surfactants like the cationic surfac-tant (CTAB), anionic surfactant (SLS), nonionic surfactant(Tergitol), and stabilizers like PEG-400, along with hydrothermaltreatment, on the surface area and pore volume has been studiedin this report. Spherical and uniform particles of size ∼400 nm(Figure 2) were obtained using the cationic surfactant, CTAB,along with the hydrothermal method. These mesoporous silicananoparticles have a high surface area of 442 m2/g. The samplewas heated at 550 �C to expel all of the surfactant from the core ofthe material, and the surface area increased to 1150 m2/g. Theshape and size of the particles remain unaffected on calciningthe sample at 550 �C as observed in the TEM image (Figure 3a).N2 adsorption-desorption isotherms exhibit typical H3 iso-therm characteristics of slit-shaped pores (Figure 3b). The averagepore diameter was calculated to be ∼22 nm from the adsorptionbranch of the isotherm using the BJHmethod (Figure 3c). To ourknowledge, this is the highest reported pore size so far obtainedbybasic hydrolysis in the presence of a cationic surfactant. However,

Figure 2. TEM image of silica microparticles synthesized usingCTAB as the surfactant.

Figure 1. PXRD pattern for the silica sample synthesized usingCTAB as the surfactant.

(17) Vaidya, S.; Rastogi, P.; Agarwal, S.; Gupta, S. K.; Ahmad, T.; Antonelli,A. M.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Phys. Chem. C2008, 112, 12610.(18) Ganguli, A. K.; Ganguly, A.; Vaidya, S. Chem. Soc. Rev. 2010, 39, 474.(19) Ganguli, A.; Ganguly, A. J. Cluster Sci. 2009, 20, 417.

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the pores obtained are not ordered. Besides this, we also observe alow pore volume for these samples. A sharp increase in nitrogenuptake at high relative pressures (P/Po > 0.9) corresponds to the

voids formed between the silica nanoparticles and is a measure ofthe textural porosity. A recent study on mesoporous silica using

Figure 3. (a) TEM image of silica synthesized usingCTAB and heated at 550 �C, (b) nitrogen adsorption-desorption isotherm, and (c) poresize distribution of particles calculated using the BJH method.

Figure 4. TEM image of silica particles synthesized using Tergitolas the templating surfactant.

Figure 5. Schematic diagram for the formation of porous silica.

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CTABmicroemulsion composed of different cosolvents like ethylether and ethoxyethanol reports a pore size of ∼3.8 nm.20 Thehydrothermal treatment enlarges the pore size to a greater extentthan a reaction carried out at ambient temperature and pressure,thereby acting as a better porogen. As the hydrolysis of the silaneproceeds, ethanol, a low-boiling solvent, is produced as a by-product. Ethanol gets trapped inside as the condensation ofTEOS occurs simultaneously with the help of the surfactant,CTAB, acting as the template. Under the high-temperature and -pressure conditions inside the hydrothermal vessel, ethanol getsevaporated at the reaction temperature (120 �C). This causesshort wormlike pores that can be seen throughout the surface ofthe particle in all of our samples, irrespective of the surfactantused. This is in accordance to the concept of dynamic templateas introduced by Yan in their study.20 On using a nonionicsurfactant, uniformly dispersed spherical particles of size 250-300 nm with wormlike pores were obtained with a low surfacearea of 10 m2/g (Figure 4). The isoelectric point of silica lies inbetween the pH range of 1.5-3; thus, at the synthesis pH (9.2), theinorganic species attain a negative charge. The self-assembly ofthe cationic surfactant, Sþ, and the anionic silicate ions, I-, takesplace in SþI- mechanism14 and is driven purely by electrostaticforces (Figure 5). On removing the surfactant, pores developinside the particle, leading to a very high surface area. Such anassembly is not possible with a neutral surfactant that have

uncharged head groups. The nonionic surfactant merely coatsthe surface of the particle (as seen in theTEM image,Figure 3).Asa result, the surface area is much lower where Tergitol is used asthe surfactant. Thus, in this synthesis, the cationic surfactantCTAB acts as the porogen, which on removal leaves behind theporous particle.

To confirm the role of CTAB,we have also studied the effect ofan anionic surfactant (SLS). We observe a large size distributionof silica particles varying from 20 to 150 nm (Figure 6a) in thereaction with SLS. This can be expected since both the negativelycharged silicate and the surfactant molecules repel each other,thereby preventing the formation of any kind of assembly as isseen in the case of CTAB. Despite the difference in the size of theparticles synthesized using different surfactants, all of the adsorp-tion isotherms show the type IV behavior, according to IUPACclassification, characteristic of mesoporousmaterials (Figure 6b).Typical H3 hysteresis plots are observed for these mesoporousmaterials. The pore size was centered at 3.4 nm along with a smallpeak at 12.3 nm, as calculated from the adsorption branch usingthe BJH method (Figure 6c).

Mesocellular foam type silica nanoparticles21 with disorderedpores are obtained when the hydrolysis is carried out in situ inpresence of PEG (used as a stabilizer). Large-sized cavities of sizesvarying from 20 to 40 nm were seen on the surface of the uni-formly sized silica particles (Figure 7a). These spherical particles

Figure 6. TEM image of the silica particles synthesized using SLS as the surfactant, (b) nitrogen adsorption-desorption isotherm, and(c) pore size distribution of particles calculated using the BJH method.

(20) Chen, H.; He, J.; Tang, H.; Yan, C. Chem. Mater. 2008, 20, 5894.(21) Lettow, J. S.; Han, Y. J.; Winkel, P. S.; Yang, P.; Zhao, D.; Stucky, G. D.;

Ying, J. Y. Langmuir 2000, 16, 8291.

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are surrounded by a nonporous shell of thickness around 25 nm.It is most likely that the hydroxyl groups on the surface of silicaallow the PEG to interact with its OH groups and prefer to stay atthe surface, which, however, is removed on heating at a hightemperature. The surface of the particle loses its uniformity alongwith the nonporous coating, suggestive of the presence of PEGonthe surface (Figure 7b). The surface area of the sample heated at550 �C was found to be 44 m2/g.

An increase in the density of cavities along with the size of thecavity was observed in the micrograph when the silica sampleobtained after the first hydrothermal treatment was subjected toanother hydrothermal treatment at 120 �C (Figure 8a). The size ofthe cavity increased from25 to 40 nmalongwith an increase in thesurface area from 16 to 55 m2/g. A strong mesoporous contribu-tion can be seen in the N2 adsorption-desorption plot for thissample in Figure 8b. A wide size distribution can be seen in theBJH plot (Figure 8c) with the major peak centered on 6 nmdiameter. However, the values obtained for the BET surface areaand the observed pore volume are very low for a mesocellularfoam type silicamaterial. Thus, nitrogen is not able to go throughthe silica wall of nanoparticles, and the reported textural char-acteristics of this material mainly correspond to the outer surface.

PEG had a profound role in the formation of these pores sincethe in situ hydrolysis of TEOS only leads to the formation ofcavities. To ascertain the importance of the in situ reaction, PEGwas added to the already synthesized silica nanoparticles andtreated hydrothermally under similar conditions. A closer look atthe high magnification image confirms that neither cavity forma-tion nor the additional nonporous shell is formed on the silicamesoparticles (Figure 9). The particles in this case are veryuniform. It is thus during the hydrolysis of TEOS that the PEGmolecule interacts and the cavities are formed at the raisedtemperature and pressure. Although the surface area of these

samples was much lower (1/3 when compared to SLS) than thesilica synthesized by surfactants, the cavities formed in theseparticles are large.The structure ofPEG is quite different fromtheionic or neutral surfactants; that is, the hydrophilic group is a longchain instead of the single polar head observed for the conven-tional surfactants.21 They are known to induce new effects in thebehavior of the template formed by polymer and the inorganicsilica material. Thus, the hydrophilic group plays an importantrole in determining the shape and volume occupied in thetemplate. The triblock copolymer of polyethylene oxide (PEO)and polypropylene oxide (PPO) has been used in the synthesis ofpolymer-templated mesoporous silica where the PEO block ishydrophilic and the PPO block provides a hydrophobic core.22

Along with swelling agents, these copolymers lead to the forma-tion of cylindrical pores in the size range of 6-12 nm. To see theeffect in the absence of PPO units and swelling agents, we haveused PEG, comprised of only ethylene oxide units, in our syn-thesis. PEOunits have a hydrophilic character interactingwith thesurface silanol groups, thereby forming shallow cavities only onthe surface of the particle.

All of the above synthesis has been carried out by using thesingle templatemethodwhere only one surfactant has beenused toform the dynamic template. In the dual template reaction,we haveused two cationic surfactants, CPB and CTAB, where one sur-factant is used as the template and the other is used to control thesize of the particle. On using the mixture of CPB (S1) and CTAB(S2) as the template, the increase in the porosity of the particle canbe clearly seen from the difference in the contrast at the edge andthe core of the particle (Figure 10). However, the reduction in thesize of the particles could not be observed, that is, the secondcationic surfactant (deliberately added) was not able to control thesize of the particle by interacting in the S1

þI-S2þ (surfactant 1:

inorganic silica:surfactant 2) mechanism as expected; instead, itforms a part of the template. As discussed above at the synthesispH (9.2), the inorganic species attains a negative charge andassembleswith the oppositely charged surfactant. On the additionof the second surfactant, CPB, the assembly formation takes placeby the SþI-Sþmechanism. Although both the surfactants carry apositive charge on their headgroup, they differ largely in theirstructures. Although both CTAB and CPB have a C-16 linearhydrocarbon chain, the size of the headgroup varies from anammonium ion in CTAB to the pyridinium ion in CPB therebyaffecting their surfactant packing parameter. Thus, when thesample was heated at 550 �C to expel the surfactant, a moreporous core was obtained compared to when only CTAB is usedas the template. Figure 11a shows the TEM image of the sample.Similar types of nitrogen adsorption-desorption curves wereobtained for the silica mesostructures obtained by single and dualtemplate surfactant synthesis (see Figures 3b and Figure 11b forcomparison). However, a change in the pore size for these meso-particles can be seen on using the double template method. Thisbehavior is explainable because the linear hydrophobic tail ofthe surfactant interacts and orients itself in accordance to thelinear silicate species. The pyridinium ion on the other hand getsfolded over relative to the hydrophobic tail to associate morestrongly with the silicate species, leading to a more rigid templateas discussed earlier by Ozin et al.23 Being large in size, it alsooccupies a comparatively larger volume in the dynamic templateformed under the hydrothermal conditions. When the template isremoved, pores are generated depending upon the size of the

Figure 7. TEM image of (a) silica particles synthesized using PEGas the template showing the nonporous coating and (b) sampleheated at 550 �C.

(22) Foster, B.; Cosgrove, T.; Hammouda, B. Langmuir 2009, 25, 6760.(23) Khushalani, D.; Kuperman, A.; Coombs, N; Ozin, G. A. Chem. Mater.

1996, 8, 2188.

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template formed by the surfactant molecule. The bulky pyridi-nium ion leaves behind large pores as seen in the micrographs.From the pore size distribution, we observe amajor peak centeredaround 22 nm along with a small peak at ∼32 nm, suggestive ofthe presence of larger pores (Figure 11c). The comparatively low

Figure 8. (a)TEMimageof the silica synthesizedusingPEGand twice treatedhydrothermally, (b) nitrogen adsorption-desorption isotherm(inset: enlarged part of the isotherm showing capillary condensation), and (c) pore size distribution of particles calculated using the BJHmethod.

Figure 9. TEM image of silica (post addition of PEG to the alreadysynthesized silica followed by hydrothermal treatment).

Figure 10. TEM image of silica particles synthesized using amixture of CPB and CTAB as the template.

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surface area of these mesostructures is also consistent with theavailable reports that with an increase in pore size a decrease insurface area is observed.12 Also, a 3-fold increase in the porevolume is observed in this dual template synthesiswhen comparedto the reactionusing onlyCTAB.Surface area, pore size, andporevolume data for all of the samples have been tabulated in Table 1.

We have also carried out the ζ-potential measurement of all ofthese samples in aqueous medium at neutral pH. Table 2 lists thevalues obtained for the silica samples synthesized using various

surfactants. For the nonionic surfactant and PEG, the maximumvalue of ∼50 mV was observed, which also explains our earlierobservation of the presence of the stabilizer (PEG or Tergitol) onthe surface of the particle. The hydroxyl group on the surfaceleads to the formation of a stable dispersion.

Thus, we observe a complete change in the formation mechan-ism of the mesoporous silica by varying the surfactant type. Acontrol over the pore size and pore volume can be achieved bychanging the type of surfactant, for example, anionic surfactant,SLS, generates a pore size of 3.4 nm, while the cationic surfactant(CTAB) can expand the pore up to 22 nm. Surface area and porevolume can also be tuned using a combination of surfactants asshown by our study using a double template (two surfactants)method.

Table 1. Pore Size, Pore Volume, and Surface Area of the

As-Synthesized and Calcined Materials

surfactant used

surfacearea(m2/g)

porevolume(cm3 g-1)

poresize(nm)

CTAB-assisted synthesis 442 0.21 21CTAB-assisted synthesis heated

at 550 �C1150 0.09 22

CPB and CTAB assisted 23 0.18 20CPB and CTAB assisted heated

at 550 �C110 0.28 22, 32

Tergitol 10 0.20PEG 16 0.08PEG-assisted synthesis after double

hydrothermal treatment55 0.09 6

SLS 35 0.29 3.4, 12.3

Table 2. ζ-Potential Studies of Silica Particles

sample (mesoporous silica) ζ (mV)

SLS assisted -22SLS assisted heated at 550 �C -27.5CTAB assisted -32.0PEG-assisted synthesis -50.5PEG assisted heated at 550 �C -28.7Tergitol-assisted synthesis -48.4CPB and CTAB -27CPB and CTAB heated at 550 �C -37

Figure 11. TEM image of the silica synthesized using a mixture of CPB and CTAB heated at 550 �C, (b) nitrogen adsorption-desorptionisotherm, and (c) pore size distribution of particles calculated using the BJH method.

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4. Conclusions

Acontrol over the pore size and pore volume has been achievedusing a surfactant-mediated hydrothermal method. Four differ-ent types of surfactants/stabilizers have been used to control thepore size from 3.4 to 22 nm, while a steady increase in the porevolume (0.09-0.28 cm3/g) was accomplished by employing eithera single surfactant or double surfactant system. The particles arequite stable despite the high temperature (even at 550 �C) andpressure. Surface cavities of sizes up to 40 nm were observed for

the in situ hydrolysis of TEOS in the presence of PEG. Mesopor-ous silica in high yield with varying pore size can be obtainedwithout the use of any organic additives or block copolymer. Theprecise control over the particle and pore size broadens the scopeof this material for future applications.

Acknowledgment.A.K.G. thanks CSIR for funding, andA.G.thanks CSIR for a fellowship. We thank Dr. Eswaramoorthy(JNCASR) for the surface area measurements.