DOI: 10.1021/la101225j 12267Langmuir 2010, 26(14), 12267–12272 Published on Web 06/24/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Interfacially Controlled Synthesis of HollowMesoporous Silica Spheres withRadially Oriented Pore Structures

Juan Li,†,‡ Jun Liu,*,† Donghai Wang,† Ruisong Guo,*,‡ Xiaolin Li,† and Wen Qi†

†Pacific Northwest National Laboratory, Richland, Washington 99352, and ‡School of Material Science andEngineering, Tianjin University, Tianjin 300072, P. R. China

Received March 28, 2010. Revised Manuscript Received May 13, 2010

This paper reports an alternative process to prepare hollow mesoporous silica spheres (HMS) using a single cationicsurfactant with a tunable wall thickness and radially oriented pore structures. Using N,N-dimethylformide (DMF) asthe intermediate solvent bridging the organic and aqueous phase, hollow mesoporous silica spheres were synthesizedwith interfacial hydrolysis reactions at the surface of liquid droplets. These spheres have an ordered pore structurealigned along the radial direction, and the wall thickness and sphere sizes can be tuned by adjusting the experimentalconditions. Transmission electron microscopy and nitrogen absorption techniques were used to characterize HMS andits formation procedure. A hypothetic formation mechanism was proposed on the basis of a morphology transforma-tion with the correct amount of DMF and a careful observation of the early hydrolysis stages. Au and magnetic Fe3O4

nanoparticles have been encapsulated in the HMS hollow core for potential applications.

1. Introduction

Since the pioneering work by the scientists at the MobilCorporation in 1992, great effort has beendevoted to synthesizingmesoporous silica nanoparticles with controlled particle sizes,morphologies, and tunable pore structures.1 In these materials,the surface area, pore sizes, and pore structures can be preciselycontrolled with different surfactants/polymer molecules or bycarefully tuning the synthetic conditions.Recently, hollowporoussilica particles and spheres attracted wide attention for drugdelivery, optical and magnetic imaging, absorption, encapsula-tion, and catalysis applications because the large cavities in theseparticles or spheres can be used as high-capacity reservoirs tostore chemicals, biological agents, or other nanoparticles. Further-more, the mesoporosity provides easy pathways for moleculardiffusion.2 Hollow mesoporous particles with three-dimensionalpore channels3-6 and core-shell micellar silica nanoparticles7

have been investigated for high-capacity loading and controlledrelease of biomolecules and drugs. High drug loading andsustained, or even switchable, release7 have been reported. Dye-loaded hollow silica particles can be used for direct optical imag-ing of cancer cells.7 Hollow mesoporous silica spheres (HMS)loaded with magnetic particles have the potential for magneticimaging andmagnetic separation.8 HMShave also been prepared

with mixed magnetic and semiconductor nanoparticles.9 It isinteresting that enzymes trapped in multilayered silica vesiclescould catalyze the hydrolysis of tributyrin with a high activity,10

showing the potential of such materials for biomolecule encapsu-lation and catalytic applications.

There are several general approaches to prepare hollowmesoporous materials. The core templated approach is basedon surface deposition on inorganic or organic materials orpolymers.10-13 This method is simple to use, but it is difficult toscale up. The templates need to be removed after the synthesis,and it is difficult to load the materials in the cavities. The secondapproach uses water/oil emulsion as the template and formshollow mesoporous structures at the water-oil interfaces.4,14

The most widely studied method is to use surfactant vesicles asthe template. The vesicle template approach was first studied byPinnavaia et al.15,16 using neutral bolaamphiphile surfactant thatcontains two polar head groups linked by a hydrophobic alkylchain. Silicate precursors were condensed in interlayered regionsof multilamellar vesicles, and they formed micro- and mesopor-ous multilayer lamella silica with a high surface area and a highthermal stability. Also, many other symmetric and asymmetricdiamine surfactants as well as different silicate precursors havebeen used to prepare vesicle silicate particles with a wide range ofparticle sizes,morphologies, and pore structures.17-19Vesicle-like

*Corresponding authors. E-mail: jun.liu@pnl.gov (J.L.), rsguo@tju.edu.cn(R.G.).(1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S.

Nature 1992, 359, 710.(2) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Chen, H. R.; Dong, X. P.; Ruan, M. L.

Nanotechnology 2005, 16, 2633.(3) Zhu, Y. F.; Shi, J. L.; Shen,W.H.; Dong, X. P.; Feng, J.W.; Ruan,M. L.; Li,

Y. S. Angew. Chem., Int. Ed. 2005, 44, 5083.(4) Li, Y. S.; Shi, J. L.; Hua, Z. L.; Chen, H. R.; Ruan, M. L.; Yan, D. S. Nano

Lett. 2003, 3, 609.(5) Li, Y. S.; Shi, J. L.; Chen, H. R.; Hua, Z.; Zhang, L. X.; Ruan,M. L.; Yan, J.;

Yan, D. S. Microporous Mesoporous Mater. 2003, 60, 51.(6) Zhu, Y. F.; Shi, J. L.; Li, Y. S.; Chen, H. R.; Shen, W. H.; Dong, X. P.

J. Mater. Res. 2005, 20, 54.(7) Huo, Q. S.; Liu, J.; Wang, L. Q.; Jiang, Y. B.; Lambert, T. N.; Fang, E.

J. Am. Chem. Soc. 2006, 128, 6447.(8) Zhang, L.; Qiao, S. Z.; Jin, Y. G.; Chen, Z. G.; Gu, H. C.; Lu, G. Q. Adv.

Mater. 2008, 20, 805.

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(10) Zhu, Y. F.; Shi, J. L.; Chen, H. R.; Shen, W. H.; Dong, X. P. MicroporousMesoporous Mater. 2005, 84, 218.

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44, 489.(18) Karkamkar, A. J.; Kim, S. S.; Mahanti, S. D.; Pinnavaia, T. J. Adv. Funct.

Mater. 2004, 14, 507.(19) Hossain, K. Z.; Sayari, A.Microporous Mesoporous Mater. 2008, 114, 387.

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silica particles can be obtained with nonionic surfactants,20-22

cationic surfactants,23,24 and fluorocarbon surfactants.25Anotherwidely studied method is to use mixed systems of cationic-anionic surfactants26 and fluorocarbon-hydrocarbon surfac-tants.27 In the cationic and anionic system, the positive andnegative charges are neutralized, and the two surfactants stronglybind to one another, favoring the formation of the lamella sheetsand vesicles that function as the template for mesoporous silicastructures. In the fluorocarbon surfactant system, the fluorocar-bon surfactant forms the template for the vesicle template, and thehydrocarbon surfactant forms the template for the silicate mesos-tructures. Both cationic and nonionic surfactant28,29 can be usedwith the fluorocarbon surfactants, and the morphologies andstructures can be controlled by adjusting the surfactant ratio andexperimental conditions.30 Periodic mesoporous organosilicahollow spheres with tunable wall thickness have also been pre-pared with fluorocarbon and cationic double templates.31

Many scientific and practical issues still need to be addressed toallow an efficient synthesis of HMS. For example, many widelystudied approaches involve expensive or special surfactants. Inthe vesicle template synthesis, the hollow silica spheres are basedon surfactant bilayers and lamella structures. The silica particleshave onionlike structures and mono- or multilayered shell struc-tures. The wall or the shell structures range from microporous,mesoporous, spongelike, and, in a few cases, ordered three-dimen-sional mesoporosity. For many applications, such as controlledrelease and catalysis, ideally the pore channels should be orientedperpendicularly with respect to the walls. Although orientedmesoporous particles have been reported with a hard templateapproach,11 there is a need to develop a simple, one-step methodthat not only produces hollow mesoporous particles and sphereswith controlled pore orientation using commonly available com-mercial surfactant but also could integrate the encapsulation offunctional materials inside the HMS during the synthesis.

In this paper, we report a simple and one-step route tosynthesize HMS particles with radially oriented mesoporous wallstructures and tunable wall thickness at room temperature usingone commonly available cationic surfactant. These mesoporoussilica spheres are formed through an interfacial reaction at thesurface of liquid droplets and are different from the surfactantbilayer vesicle structures. These materials can be used to loadmultiple gold and Fe3O4 nanoparticles.

2. Experimental Methods

Materials. All chemicals were used directly without fur-ther purification, including N,N-dimethylformamide (DMF,

anhydrous, 99.8%, Sigma-Aldrich), cetyltrimethylammoniumchloride solution (CTAC, 25 wt % solution in water, Aldrich),tetraethyl orthosilicate (TEOS, Fluka), triethanolamine (TEA,minimum 98%, Sigma), ethanol (anhydrous, Sigma-Aldrich),and deionized (DI) H2O. Hydrochloric acid (HCl, 37%, ACSReagent, Sigma-Aldrich) and ethanol were used for templateextraction.

Synthesis of HMS Spheres. Our methods for making HMSspheres were derived from a method used to prepare regularmesoporous nanoparticles.32-34 Typically, 6.4 mL of DI H2O,1.05 mL of ethanol, and 1.04 mL of 25 wt % CTAC were mixedand stirred at room temperature for 30 min. A total of 1.85 g ofTEA was subsequently added and further stirred for 30 min untildissolved to control the pH. The mixtures were stirred at roomtemperature, to which the mixture of 0.692 mL of TEOS and anappropriate amount of DMF was added under stirring. Thewhole mixtures were further stirred for an appropriate time atroom temperature.

After the reaction, we stopped the stirring and added 5 mL ofethanol into the translucent, colloidal aqueous suspension toprecipitate the mesoporous materials. The mixture was centri-fuged to collect the aggregate HMS at the bottom. The resultingprecipitates were washed by hydrochloric acid in ethanol solvent(3mLof concrntratedHCl in 240mLof ethanol) with sonication.This procedurewas repeated three times. Subsequentwashingwasperformed by centrifugation at 9000 rpm for 6 min and redis-persed in DI H2O under sonication. This washing procedure wasrepeated three times. All products were stocked in DI H2O.

To study the effect of stirring and sonication, similar proce-dures were followed, except that themixed solutionwas sonicatedin a water bath (Branson 1510) during part or the entire period ofreaction. The same procedure was used to extract the surfactant.

Preparation ofAuColloids.ColloidalAunanoparticleswereprepared according to the citrate method reported in theliterature.35 A total of 500 mg of HAuCl4 was dissolved in 250 gof DI H2O to form 2 mg/mL of HAuCl4 aqueous solution. Then4.31mLof this HAuCl4 aqueous solutionwas added into 200 g ofDIH2O.This solutionwas loosely coveredwithAl foil and heatedto boil. Then 20 mL of 1% sodium citrate aqueous solution wasadded.Themixturewas boiled for 5min to form thewine red goldcolloidal solution with gold nanoparticles about 15 nm indiameter.35 This colloidal solution was used as the seed solutionto grow the particles to a larger size. A total of 100 mL of the as-prepared colloidal solution was heated to boil. Then 25 mL ofHAuCl4 (2 mg/mL) aqueous solution was added dropwise duringboiling. After boiling for 5 min, the mixture was cooled down toroom temperature. The resultant solutionwas centrifuged at 3500rpm for 10 min to collect the large gold particles at the bottomsection of the centrifuge tube.

Preparation ofMagnetic Fe3O4 Nanoparticles. Fe3O4 nano-particles were synthesized according to a method reported in theliterature.36A total of 1 g ofKOHand0.5 gofKNO3was dissolvedin 75 mL of DI H2O, and the solution was heated to boil. Sub-sequently, 2.25 g of FeSO4 dissolved in 15 mL of DI H2O wasadded. After boiling for about 5min, the granular black precipitatewas separated by a magnet. Then 200 mL of hot DI H2O wasadded. After dispersion, a magnet was brought into contact withthe beaker for a short period of time (5min), and the large particlesattracted to the magnet were removed. The small Fe3O4 particlesleft in the solution were washed by hot DI H2O three times anddispersed in 5 mL of DI H2O. The crystalline phase of Fe3O4

(magnetite) was confirmed by X-ray diffractometry analysis.

Synthesis of Mesoporous Silica Spheres with Au andFe3O4 Nanoparticles. Mesoporous silica nanoparticles with

(20) Wang, H. N.; Wang, Y. H.; Zhou, X.; Zhou, L.; Tang, J.; Lei, J.; Yu, C.Adv. Funct. Mater. 2007, 17, 613.(21) Zhang, Y.; Yu, M. H.; Zhu, L.; Zhou, X. F.; Zhao, Q. F.; Li, H. X.; Yu,

C. Z. Chem. Mater. 2008, 20, 6238.(22) Yu, M. H.; Wang, H. N.; Zhou, X. F.; Yuan, P.; Yu, C. Z. J. Am. Chem.

Soc. 2007, 129, 14576.(23) Hentze, H. P.; Raghavan, S. R.; McKelvey, C. A.; Kaler, E. W. Langmuir

2003, 19, 1069.(24) Zhang, L. X.; Li, P. C.; Liu, X. H.; Du, L. W.; Wang, E. Adv. Mater. 2007,

19, 4279.(25) Tan, B.; Lehmler, H. J.; Vyas, S. M.; Knutson, B. L.; Rankin, S. E. Adv.

Mater. 2005, 17, 2368.(26) Yeh, Y. Q.; Chen, B. C.; Lin, H. P.; Tang, C. Y. Langmuir 2006, 22, 6.(27) Lin, C. X.; Yuan, P.; Yu, C. Z.; Qiao, S. Z.; Lu, G. Q. Microporous

Mesoporous Mater. 2009, 126, 253.(28) Yuan, P.; Yang, S.; Wang, H. N.; Yu, M. H.; Zhou, X. F.; Lu, G. Q.; Zou,

J.; Yu, C. Z. Langmuir 2008, 24, 5038.(29) Gu, X.; Li, C. L.; Liu, X. H.; Ren, J. W.;Wang, Y. Q.; Guo, Y. L.; Guo, Y.;

Lu, G. Z. J. Phys. Chem. C 2009, 113, 6472.(30) Yang, S.; Zhou, X. F.; Yuan, P.; Yu, M. H.; Xie, S. G.; Zou, J.; Lu, G. Q.;

Yu, C. Z. Angew. Chem., Int. Ed. 2007, 46, 8579.(31) Djojoputro, H.; Zhou, X. F.; Qiao, S. Z.; Wang, L. Z.; Yu, C. Z.; Lu, G. Q.

J. Am. Chem. Soc. 2006, 128, 6320.

(32) Moller, K.; Kobler, J.; Bein, T. Adv. Funct. Mater. 2007, 17, 605.(33) Kobler, J.; Moller, K.; Bein, T. ACS Nano 2008, 2, 791.(34) Moller, K.; Kobler, J.; Bein, T. J. Mater. Chem. 2007, 17, 624.(35) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 55.(36) David, I.; Welch, A. J. E. Trans. Faraday Soc. 1956, 52, 1642.

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Au cores were prepared with the following method: 6.4 mL of DIH2O, 1.05 mL of ethanol, and 1.04 mL of 25 wt % CTAC werestirred at room temperature for 30min and then put under a bathsonication while the rest of the reactants were added. A total of1.85 g of TEA was subsequently added into the mixture. Afterthat, 0.2 mL of colloidal Au solution was added. Then a mixtureof 0.692 mL of TEOS and 2 mL of DMF was added. The entiremixtures were further sonicated for 5 min followed by stirring atroom temperature for 1 h. After the reaction, all of the productswere washed by hydrochloric acid in ethanol solvent (3 mLof concentrated HCl in 240 mL of ethanol) with sonication. Thisprocedure was repeated twice. Subsequent washing was per-formed by repeated centrifugation at 9000 rpm for 6 min andredispersed in DI H2O under sonication. All products werestocked in DI H2O.

Mesoporous sphereswithFe3O4 nanoparticles were prepared asfollows: 6.4 mL of DI H2O, 1.05 mL of ethanol, and 1.04 mL of25 wt % CTAC were stirred at room temperature for 30 min andthenput under abath sonicationwhile the rest of the reactantswereadded. A total amount of 1.85 g of TEA was subsequently addedinto the mixture. After that, 0.1 mL of Fe3O4 aqueous suspensionwas added.Amixtureof 0.692mLofTEOSand2mLofDMFwasthen added. The mixture was further sonicated for 1 h.

Characterization. Transmission electron microscopy (TEM)was performed on a JEOL JSM-2010 TEMmicroscope operatedat 200 kV. Scanning electron microscopy (SEM) was carried outon an FEI Helios Nanolab dual-beam focused ion beam (FIB)/SEM. The surface area was determined using QuantachromeAutosorb Automated Gas Sorption Systems. The result of theBrunauer-Emmett-Teller (BET) surface areawas obtainedwiththe single-point adsorption method.

3. Results and Discussion

DMF was found to be important in morphology control. Weobtained very small mesoporous nanoparticles to HMSby tuningthe amount of DMF. As shown in the TEM images of Figure 1,withoutDMF,nanoparticles of about 30-40nm indiameterwithdisorderedmesopore structureswere produced (Figure 1a).Whilea small amount ofDMF (200 μL) was added, similarmesoporousnanoparticles were obtained (Figure 1b). However, when theamount ofDMFwas increased to 0.976mL, some hollow sphereswith radially oriented mesopore features were observed in addi-tion to the small nanoparticles (Figure 1c). These hollow spheresare about 200 nm in diameter, and the mesopore features areabout 3 nm wide, consistent with what is expected from thecationic synthesis approach.23,24 The wall thickness of the hollowspheres is about 60 nm. Further addition of DMF (more than2 mL of DMF) caused the formation of mostly HMS spheres(Figure 1d-f). The radially oriented mesopore can be clearlyobserved at a high magnification from Figure 1d. Figure 1e is alower magnification TEM image with 4 mL of DMF. A fewregular, “dense” mesoporous nanoparticles are stilled observed,but the exact amount is difficult to quantify from the TEMimages. Figure 1f shows one single mesoporous particle with avery small cavity in the center. The hollow structure of the HMSobtained with 2 mL of DMF was further confirmed by SEMimages (Figure 1g,h), fromwhich spheres with broken shells wereclearly observed.

To prove the existence of the mesoporosity, the materialsmadeof hollow silica particles were precipitated with ethanol, dried,and calcined at 500 �C for BET measurement. The nitrogenabsorption isothermof the sample synthesizedwith 4mLofDMFwas obtained and is shown in Figure 2. The material has a surfacearea of 800 m2/g (Figure 2a). The Barrett-Joyner-Halenda(BJH) method gives an average pore size of 3 nm (Figure 2b)from the adsorption branch.

A series of experiments were carried out to study the growth ofthe hollow spheres. It is critical to understand how the particlesformed. Therefore, we collected the samples synthesized with2 mL of DMF of different reaction times. Figure 3 shows theparticle morphologies at different reaction times. Before 30 min,only disordered, amorphous precipitateswere observed.However,at 30 min, hollow spheres already formed with well-defined andradially oriented features (Figure 3a). The wall thickness was lessthan 40 nm. After 1 h, similar morphologies were observed, andthe wall thickness increased slightly (Figure 3b). After 3 h, the wallthickness further increased to more than 60 nm, and the wallstructure seemed to becomedenser (Figure 3c). This series ofTEMresults suggests that the particles were produced in the early stagesof the reaction, and further reaction resulted in increasing the wallthickness of the particles. Another noticeable feature is that the

Figure 1. TEM and SEM images of particle morphologies withdifferent amounts of DMF. (a) No DMF. (b) 0.2 mL of DMF.(c) 0.976 mL of DMF. (d) 2 mL of DMF. (e) 4 mL of DMF. (f) Asingle sphere with almost no cavity obtained by using 4 mL ofDMF. (g, h) SEM images of hollow spheres synthesized with 2mLof DMF. Broken spheres are marked out with arrows.

Figure 2. Brunauer-Emmett-Teller (BET) result of the hollowmesoporous spheres synthesized using 4mLofDMF. (a)Nitrogenabsorption-desorption isotherm curves. (b) Pore size distributionfrom the absorption curve using the Barrett-Joyner-Halenda(BJH) method.

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formation of the hollow particles may be kinetically controlled. Ifthemixed solution containing all the ingredients is sonicated at thebeginning of the reaction, the large hollow particles will be brokendown into smaller hollowparticles, andmostly fillednanoparticleswill be formed, as shown by Figure 3d for a sample that wassonicated for 1 h at the beginning of the reaction.

Figure 4 shows high-magnification TEM images of somehollow spheres with 4 mL of DMF and further reveals how theinterfacial reactions take place. Figure 4a is a sphere with a smallcavity at the center. The mesostructure near the outer surface ofthe walls seems to bemore ordered than the structures close to thecenter of the sphere. Figure 4b also shows that the inner walls ofthe spheres remained disordered for these thick-walled particles.These results suggest that the self-assembly of the mesoporestructure most likely started at the outer surface of the spheres.Furthermore, when compared with the morphologies of thespheres formed at an early stage of reaction (inset in Figure 4b),the mesostructure also became denser and more uniform.

Sometimes, fused hollow mesoporous particles/spheres areobserved connecting two or more spheres together. The grainboundary structures of these fusedparticles are interesting. Figure5 shows the grain boundaries of the fused spheres with differentdegrees of overlapping. In Figure 5a, the boundary in the interiorof the particles is totally fused together and becomes quitedisordered because of the high degree of overlap, but on the edge,the pore structures on both spheres remain intact until they meetat the grain boundary. There is a sharp grain boundary and not agradual change of themesostructure in the gain boundary region.Similarly, in Figure 5b, the mesostructures in both spheres arewell maintained, and there is again a sharp grain boundarydividing the two spheres. This observation again suggests thatthe mesostructures are formed in the very early stage of reactionand are locked in. There is almost no change in the porousstructure even if the particles are fused together.

On the basis of these TEM results and our observation of thehydrolysis process, it is believed that the formation of the hollowmesoporous spheres may be explained by an interfacial reactionmechanism (Figure 6). When pure TEOS is mixed with thesurfactant solution, initially the solubility of TEOS is very pooruntil it is hydrolyzed by TEA.Without stirring, the TEOS rapidlyphase separates from the aqueous surfactant phase and forms itsown liquid phase above the aqueous phase because of the densitydifference, and no reaction will be observed for many days.Reactions take place through slow hydrolysis of TEOS that favorthe formation of regular mesoporous nanoparticles (Figure 1a).DMF is soluble in both the TEOS and the aqueous surfactantsolution. DMF added in the solution probably plays two roles.First, as a bridging solvent, it helps the dispersion of TEOS in theaqueous solution to form small droplets of TEOS and DMF(Figure 6a). The inside of the droplet is silicate-rich, and theoutside aqueous phase is surfactant-rich. Another effect forDMFis that it speeds up the hydrolysis at the interface. When thesurfactant and the TEOS meet at the water-TEOS interface,hydrolysis and condensation of the silicate occur and formordered mesostructures. The surfactant and water diffuse towardthe centers of the spheres, and the silicate species and DMFdiffuse toward the aqueous phase, forming the radially alignedmesoporouswalls (Figure 6b). Since the formation of the dropletsis not an equilibrium process, the sizes of the droplets depend onthe stirring and rate ofmixing. Sonicating the solution in the earlystages of the reaction can have a large effect and breaks up thedroplets, and thus, the sizes of the spheres are reduced. Figure 6cshows a schematic of the grain boundary structure.

The above interfacial reactionmechanism is supported by con-trolled experiments. First, when TEOS is added to the aqueous

Figure 3. Hollow silica spheres formed at different reaction timeswith the addition of 2 mL of DMF. (a) After 30 min of reaction.(b) After 60 min of reaction. (c) After 3 h of reaction. (d) Smallparticles and spheres with 1 h of sonication after the ingredients aremixed.

Figure 4. High-magnification TEM images of the wall structuresin HMS spheres with 4 mL of DMF. (a) The oriented porestructures on the outer surfaces. (b) The disordered structures inthe inner walls. White arrows marked the ordered pore structures.Inset is a zoom-in image showing the ordered porous wall of theHMS.

Figure 5. Grain boundary structures in fused silica spheres with 2mL of DMF. (a) Fused particles with disordered boundary in theinterior and ordered pore structure on the edge of the particles.(b) Two overlapped HMS spheres with ordered boundary andsharp edge.

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surfactant solution without stirring, the TEOS phase separatesfrom the surfactant solutions and forms a stable interface withoutany reaction for many days (Figure 6d). However, when theTEOS-DMF is slowly added to the surfactant solution withoutstirring, the TEOS-DMF phase separates to form the interface,

but an interfacial reaction zone that grows thicker with time isimmediately observed, which supports the interfacial reactionmechanism (Figure 6e). Figure 6f,g compares the solutions withand without DMF after 5 min of stirring. The solution withoutDMF remains clear withoutmuch reaction, and the solutionwith

Figure 6. Schematics of the formation of hollow mesoporous spheres and the wall structures. (a) TEOS droplets in aqueous solution.(b) Formation of orientedmesostructures at the interface. (c)Grain boundary structures. (d) Immiscible interface formedbetweenTEOSandthe aqueous phase without stirring. (e) Interface formed between the aqueous phase and the TEOS-DMF solutionwithout stirring, showingthe reacted interfacial region. (f) An almost clear solution ofmixed TEOS and aqueous solution after 5min of stirring, without addingDMF.(g) Amixed solution of TEOS and aqueous solution withDMF, after 5min of stirring, showing the reaction products in the solution. Imageswere taken right after mixing of the solutions.

Figure 7. Au and Fe3O4 nanoparticles trapped in the hollow mesoporous spheres. Usually several particles are encapsulated inside of oneHMS. (a-c) Typical TEM images of multiple gold nanoparticles in hollow silica spheres. (d-f) TEM images of multiple magneticnanoparticles in hollow silica spheres. The images were taken with the materials obtained in one representative synthesis of Au@HMS andFe3O4@HMS, respectively.

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DMF becomes translucent due to particle formation. This ob-servation confirms that DMF indeed accelerates the hydrolysisreactions.

For controlled release, catalysis, and sensing, it is important todemonstrate that the cavities in the spheres can be filledwith othermaterials or chemicals. Figure 7a-c shows multiple gold nano-particles trapped inside the mesoporous spheres. The gold nano-particles were prepared according to the well-known citratereduction methods, and the particle sizes were increased throughmultiple growth. The nanoparticles were then dispersed in thesurfactant solutions. Since the gold nanoparticles were negativelycharged in the solution at the pH conditions (pH 10), most likely,the cationic surfactants were attracted to the particle surfacesthrough the positively charged headgroup through ion pairing,leaving the tails exposed.37 When mixed with the TEOS andDMF, the particle core with the surfactant tails on the surfacewaseasily displaced in the more hydrophobic TEOS droplets. Similarresults were reported in other systems that showed single nano-particles trapped inside radially aligned micellar structures.38

Similarly, magnetic Fe3O4 particles were prepared according toa method reported in the literature, and we have shown thatmultiple magnetic particles can be loaded in the hollowmesopor-ous spheres (Figure 7d-f). The Fe3O4 loaded silica spheres aremagnetic and can be easily separated from the solution with amagnet. In Figure 7, the mesoporous walls in the particles are notas well ordered. This may be attributed to the effect of other

impurities that coexist with the nanoparticles from the prepara-tionmethod, such as citrate ions in the gold nanoparticle solution.

4. Summary and Conclusions

In summary, we have developed a simple and fast method tosynthesize HMS spheres through interfacial hydrolysis reactionsat the surface of liquid droplets of TEOS-DMF. By tuning theamount ofDMF and experimental conditions, we can control thepore structure well aligned along the radial direction, the wallthickness, and sphere sizes.We proposed a hypotheticmechanismto explain the formation process of HMS spheres on the basis ofmorphology transformation with the amount of DMF and thecareful observation of the hydrolysis. Our as-synthesized HMSspheres have been used to encapsulate multiple Au and magne-tic Fe3O4 nanoparticles and can be extended to encapsulateother materials like drugs and dyes as potential stably dispersedcatalysts at high temperatures, magnetic imaging agents, drugcargos, and optical imaging agents.

Acknowledgment. This research is supported by the U.S.Department of Energy, Office of Basic Energy Sciences, Divisionof Materials Sciences and Engineering, under Award KC020105-FWP12152. Pacific Northwest National Laboratory (PNNL) is amultiprogram national laboratory operated for DOE by Battelleunder Contract DE-AC05-76RL01830. Juan Li thanks the par-tially financial support from the China Scholarship Council. JunLiu thanksWayneCosby at PNNL for his help in the preparationof this manuscript.

(37) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Son:New York, 1978; p 34.(38) Gorelikov, I.; Matsuura, N. Nano Lett. 2008, 8, 369.