One-Pot Preparation of Hollow Silica Spheres by Using Thermosensitive Poly(N-isopropylacrylamide) as a Reversible Template

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1 pubs.acs.org/langmuir 2009 American Chemical Society One-Pot Preparation of Hollow Silica Spheres by Using Thermosensitive Poly(N-isopropylacrylamide) as a Reversible Template Binyang Du,*, Zheng Cao, Zhenbing Li, Aixiong Mei, Xinghong Zhang, Jingjing Nie, Junting Xu, and Zhiqiang Fan Key Laboratory of Macromolecular Synthesis and Functionalization, Ministry of Education, Department of Polymer Science & Engineering and Department of Chemistry, Zhejiang University, Hangzhou , China Received May 12, Revised Manuscript Received September 7, 2009 Hollow silica nanospheres with mesoporous shells were successfully fabricated with a new one-pot strategy by using a thermosensitive polymer, poly(n-isopropylacrylamide) (PNIPAm), as a reversible template without the need of further calcination or chemical etching. By simply regulating the solution temperature with respect to the lower critical solution temperature (LCST) of PNIPAm, PNIPAm chains can reversibly form aggregates or dissolve in aqueous solution. The thermosensitive character makes PNIPAm chains behave as soft templates for the formation of core-shell silica nanospheres at elevated temperature (>LCST), and they will then diffuse out of the cores at lower temperature (<LCST), leading to the formation of hollow silica nanospheres. The TEM, SEM, XRD, and N 2 adsorption-desorption results indicate that the shells of such hollow silica nanospheres also contain large quantities of irregular mesopores. This new strategy was also tested with another thermosensitive polymer, poly(vinyl methyl ether) (PVME). However, only solid silica nanospheres with a broad size distribution were obtained when PVME was used. We speculated on the possible formation mechanism of hollow silica nanospheres with PNIPAm templates. The effects of the initial concentration of PNIPAm, the molecular weight of PNIPAm, and the pretreatment of silica precursor on the morphology and size of the resultant hollow silica nanospheres were also investigated. The PNIPAm soft templates were confirmed to be recyclable. Introduction Hollow silica spheres have attracted extensive attention because of their many potential applications in catalysis, drug delivery systems, and microcontainers. 1-4 The general strategy used to preparing hollow silica spheres is a template-based multistep method The template materials can be polymer latexes, 5-9 emulsion droplets, 10,11 inorganic nanoparticles, 12,13 aggregates or complexes of polymers, 14,15 and micelles of surfactants In the first step, the silica shells are fabricated *Corresponding author. duby@zju.edu.cn. (1) Song, X.; Gao, L. J. Phys. Chem. C 2007, 111, (2) Fujiwara, M.; Shiokawa, K.; Hayashi, K.; Morigaki, K.; Nakahara, Y. J. Biomed. Mater. Res. A 2007, 81A, (3) Arnal, P. M.; Comotti, M.; Schuth, F. Angew. Chem., Int. Ed. 2006, 45, (4) Zhu, Y.; Shi, J.; Shen, W.; Dong, X.; Feng, J.; Ruan, M.; Li, Y. Angew. Chem., Int. Ed. 2005, 44, (5) Zou, H.; Wu, S.; Ran, Q.; Shen, J. J. Phys. Chem. C. 2008, 112, (6) Blas, H.; Save, M.; Pasetto, P.; Boissiere, C.; Sanchez, C.; Charleux, B. Langmuir 2008, 24, (7) Deng, Z.; Chen, M.; Zhou, S.; You, B.; Wu, L. Langmuir 2006, 22, (8) Lu, Y.; McLellan, J.; Xia, Y. Langmuir 2004, 20, (9) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, (10) Singh, R. K.; Garg, A.; Bandyopadhyaya, R.; Mishra, B. K. Colloids Surf., A 2007, 310, (11) Fujiwara, M.; Shiokawa, K.; Tanaka, Y.; Nakahara, Y. Chem. Mater. 2004, 16, (12) Tsai, M. S.; Li, M. J.; Yen, F. H. J. Nanosci. Nanotech. 2008, 8, (13) Kim, M.; Yoon, S. B.; Sohn, K.; Kim, J. Y.; Shin, C. H.; Hyeon, T.; Yu, J. S. Microporous Mesoporous Mater. 2003, 63, 1 9. (14) Tsai, M. S.; Li, M. J. J. Non-Cryst. Solids 2006, 352, (15) Lynch, D. E.; Nawaz, Y.; Bostrom, T. Langmuir 2005, 21, (16) Feng, Z.; Li, Y.; Niu, D.; Li, L.; Zhao, W.; Chen, H.; Li, L.; Gao, J.; Ruan, M.; Shi, J. Chem. Commun. 2008, (17) Koh, K.; Ohno, K.; Tsujii, Y.; Fukuda, T. Angew. Chem., Int. Ed. 2003, 42, (18) Hubert, D. H. W.; Jung, M.; German, A. L. Adv. Mater. 2000, 12, via the fast hydrolysis and condensation of the silica precursor, typically tetraethyl orthosilicate (TEOS) under basic conditions, or via the layer-by-layer self-assembly technique onto the core templates to form core-shell-like particles. In the second step, the core templates are removed by either higher-temperature calcinations or chemical etching with acid, alkaline, or organic solvents to give hollow silica spheres These multistep processes are sometimes expensive and raise environmental and energy concerns. However, the removal of core templates may damage the silica shell and the compounds included in materials. 2,11 Recently, Wang et al. reported an interesting method for fabricating hollow silica nanospheres with air bubbles generated by ultrasound as soft templates, which can avoid the need to remove core templates. 22 Nevertheless, new strategies are still needed for the facile and nondestructive preparation of hollow silica spheres. Herein, we report a new one-pot method for the preparation of hollow silica nanosphere in water by utilizing thermosensitive polymer poly(n-isopropylacrylamide) (PNIPAm) as a recyclable template. PNIPAm is known to exhibit a lower critical solution temperature (LCST) at 32 C. 23,24 Below the LCST, PNIPAm is hydrophilic and soluble in aqueous solution. When raising the temperature above the LCST, the polymer becomes hydrophobic and insoluble and forms aggregates in aqueous solution. We found that the PNIPAm aggregates formed at elevated temperature (50 C) can serve as reversible templates for fabricating thin (19) Yang, X. L.; Yao, K.; Zhu, Y. H. J. Inorg. Mater. 2005, 20, (20) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, (21) Caruso, F.; Caruso, R. A.; M ohwald, H. Chem. Mater. 1999, 11, (22) Wang, J. G.; Li, F.; Zhou, H. J.; Sun, P. C.; Ding, D. T.; Chen, T. H. Chem. Mater. 2009, 21, (23) Wu, C.; Zhou, S. Macromolecules 1995, 28, (24) Schild, H. G. Prog. Polym. Sci. 1992, 17, Langmuir 2009, 25(20), Published on Web 09/18/2009 DOI: /la902531p 12367

2 Article Figure 1. Schematic illustration of the new strategy for the one-pot fabrication of hollow silica spheres with a mesoporous shell by using thermosensitive poly(n-isopropylacrylamide) as a recyclable template. silica shells, which are made up of small silica nanospheres formed via the prehydrolysis of TEOS in water at the same temperature. When cooling the aqueous solution below its LCST, the PNIPAm chains will diffuse out of the silica shell and redissolve in water. Hence, hollow silica nanospheres are obtained. The as-described one-pot method is shown schematically in Figure 1. This method exhibits the following advantages: Hollow silica nanospheres with mesoporous shells can be prepared in one step without the need for further calcinations or etching of the templates. By simply regulating the temperature of the aqueous solution, PNIPAm can serve as a soft template at elevated temperature (>LCST) and then diffuse out of the silica shell after cooling down to room temperature. Therefore, the polymer solution may be recycled after purification. This strategy was also tested with another thermosensitive polymer, namely, poly(vinyl methyl ether) (PVME). Note that PVME exhibits an LCST at around 35 C in aqueous solution. 25,26 PVME is a typical example of another series of thermosensitive polymers that contain ether polar groups and have an LCST in aqueous solution as well. However, the experimental results indicated that only solid silica spheres with a broad size distribution were obtained when PVME was applied. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), FTIR, X-ray diffraction (XRD), dynamic light scattering (DLS), and N 2 adsorption-desorption isotherms were used to characterize the resultant silica nanospheres. Experimental Section Chemical and Materials. N-Isopropylacrylamide (NIPAm 99%, Acros Organics) was purified by recrystallization from n- hexane. 1,4-Dioxane (Shanghai Chemical Reagents) was dried by refluxing in the presence of sodium flakes and distilled prior to use. The initiator, R,R 0 -azodiisobutyronitrile (AIBN), was recrystallized from methanol. Tetraethyl orthosilicate ([Si(OCH 2 - CH 3 ) 4 ], TEOS, 98%) and poly(vinyl methyl ether) (PVME, 30% in water, synthesized by Tokyo Kasei Kogro Co. LTD) were purchased from Acros Organics and used as received. All other reagents were of analytical grade and used as received. Synthesis of Poly(N-isopropylacrylamide). Poly(N-isopropylacrylamide) (PNIPAm) was synthesized by free radical polymerization of NIPAm in anhydrous dioxane at 80 C with AIBN as the initiator. 27,28 The weight-average molecular weight (25) Maeda, Y. Langmuir 2001, 17, (26) Schafer-Soenen, H.; Moerkerke, R.; Berghmans, H.; Koningsveld, R.; Dusek, K.; Solc, K. Macromolecules 1997, 30, (27) Cao, Z.; Du, B.; Chen, T.; Nie, J.; Xu, J.; Fan, Z. Langmuir 2008, 24, (28) Cao, Z.; Du, B.; Chen, T.; Nie, J.; Xu, J.; Fan, Z. Langmuir 2008, 24, (M w ) and polydispersity (PDI) of PNIPAm were determined by gel permeation chromatograph (GPC). Three linear polymers with M w = (PDI = 1.43), (PDI= 2.12), and (PDI= 1.94) were obtained. Preparation of Hollow Silica Spheres. Hollow silica spheres were prepared in aqueous solution in the presence of PNIPAm at elevated temperature. A typical procedure was given as follows: 250 mg of PNIPAm was first completely dissolved in 50 ml of distilled water to give a 0.5 wt % polymer solution 1 day before further use. The PNIPAm aqueous solution was then put into an oil bath at 50 C under magnetic stirring. The PNIPAm solution turned turbid at 50 C, indicating the formation of aggregates above its LCST. About 2.25 ml of TEOS was added to 18 ml of distilled water, and the solution was then also heated to 50 Cand stirred for 3 h. Afterwards, the hot PNIPAm solution was added to the hot prehydrolyzed TEOS solution. The mixed solution was continuously stirred for about 160 h at 50 C. The solution was then cooled to room temperature. A white suspension was obtained, indicating the formation of nanoparticles. The suspension was then purified by repeating the centrifugation and washing with distilled water and ethanol three times. The final products were separated into two parts. One part was redispersed in ethanol, and the other was dried in vacuum to give a white powder. Further characterization proved that hollow silica spheres were obtained under these conditions. (See below for details.) No strongly acidic or basic condition is used to catalyze the hydrolysis and condensation of TEOS. Only distilled water is used for the reactions. To test whether other thermosensitive polymers can serve as a reversible templates for the formation of hollow silica spheres, PVME was used. Note that the LCST of PVME is around 35 C, which is much lower than 50 C. 25,26 Similar experimental procedures described above were carried out by using PVME as a soft template. The effect of the initial polymer concentration on the final morphology of hollow silica nanospheres was investigated. Initial polymer concentrations of 0.3 and 0.2 wt % were tested. The effect of the pretreatment of TEOS on the final morphology of hollow silica nanospheres was also investigated. In the above experiments, TEOS was prehydrolyzed at 50 C for 3 h before the addition of hot polymer aqueous solutions. Here, the hot PNIPAm or PVME aqueous solutions were directly added to the as-prepared TEOS aqueous solution at 50 C without a prehydrolysis step, and the mixtures were then stirred for about 160 h. The resultant suspensions were purified with the same procedure described above. Silica spheres were also prepared by directly hydrolyzing TEOS aqueous solution at 50 C without using any template. Recycling of PNIPAm Templates. Three different methods were used to recycle PNIPAm templates after the first run of fabricating hollow silica nanospheres. PNIPAm with M w = (PDI = 1.94) was chosen as an example. First, the DOI: /la902531p Langmuir 2009, 25(20),

3 supernatant of the first run was collected after centrifugation at 2000 rpm and separated into three parts. One part of the supernatant was directly used as a template solution for second-run experiments. The ph of the second part was adjusted with HCl aqueous solution to be 1.5. After stirring for 3 h, the second part was further centrifuged at 2000 rpm and the supernatant was collected as a template solution to fabricate hollow silica nanospheres. For the third part, the ph value was adjusted with NH 3 3 H 2O to be ca. 11. Again, after stirring for 3 h, the third part was centrifuged at 2000 rpm again and the supernatant was collected as a template solution to fabricate hollow silica nanospheres. Adjusting the ph of the supernatant served to hydrolyze the possibly remaining TEOS completely, which may be incompletely hydrolyzed. The same procedure described above was used to purify the silica nanospheres obtained in the second run. Instruments and Characterization. The size and size distribution of the PNIPAm aggregates at 50 C weremeasured by dynamic light scattering (DLS) using a 90 Plus particle size analyzer (Brookhaven Instruments Corporation). Two PNIPAm s with M w = (PDI = 2.12) and (PDI = 1.94) were investigated. The polymers were dissolved in distilled water to give concentrations of 0.01, 0.03, 0.05, 0.1, 0.3, and 0.5 wt %, respectively. Dust was removed from the solutions by filtering them through Nylon membrane filters of 0.45 μm pore size prior to measurements. The PNIPAm solutions were kept at 50 C for 30 min to allow equilibration before DLS measurements. The morphologies of the resultant silica spheres were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The TEM measurements were performed on a JEOL JEM-1200 electron microscope operating at an acceleration voltage of 60 kv. TEM samples were prepared by dip coating with Formvar-coated copper grids in ethanol solutions of silica spheres. The solvent was gently absorbed by filter paper. The grids were then allowed to dry in air at room temperature before observation. The SEM measurements were carried out on a Hitachi S-4800 electron microscope. A droplet of the silica sphere solution was cast onto the aluminum foil at room temperature. After a few minutes, the excess solution was gently absorbed away by a filter paper. The aluminum foils were then allowed to dry in air. Before SEM observation, the samples were coated with gold vapor. FTIR spectra of dry silica spheres were recorded on a Vector 22 Bruker spectrometer. X-ray diffraction (XRD) measurements were performed on a Bruker AXS D8 X-ray diffractometer equipped with a Cu KR (λ= A ) X-ray source at room temperature. The hydrodynamic radius ÆR h æ of the hollow silica spheres in ethanol was measured at 25 CusingaBrookhaven laser light scattering system (BI-200SM). The N 2 adsorption-desorption isotherms of the dried silica spheres were measured at 77 K using a Micrometrics ASAP The specific surface area was determined with the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the adsorption isotherm using the Barrett-Joyner-Halenda (BJH) method. Results and Discussion Figure 2 shows the typical TEM and SEM images of the hollow silica spheres prepared with PNIPAm (M w = , initial concentration of 0.5 wt %) as a template at 50 C. The diameters of the hollow silica spheres shown in Figure 2A,B range from ca. 200 nm to ca. 410 nm with a mean diameter of 292 ( 48 nm. It can be clearly seen from the TEM images that the silica shells are asymmetric and nonuniform with a thickness range of nm The inner cavities of some hollow silica spheres in Figure 2A,B still preserve the shapes of the PNIPAm aggregates. This indicates that the PNIPAm aggregates can indeed serve as templates for the formation of silica shells. The higher-resolution images (Figure 2B,D) reveal that the shells are made up of many small Langmuir 2009, 25(20), Article silica nanoparticles condensed together, resulting in the rough surfaces. These condensed nanoparticles form a large number of interparticle voids. When cooling the reaction solution to room temperature, the PNIPAm molecules became hydrophilic again and can diffuse out of the core via the interparticle voids. Hollow silica spheres were thus successfully obtained. No PNIPAm can be detected in the dry powders of hollow silica spheres via the FTIR spectrum, as shown in Figure 3. The characteristic absorption bands at 462, 804, and 1098 cm -1 are attributed to the vibration of -Si-O-Si- groups in silica spheres. The absorption band at 945 cm -1 is assigned to the bending vibration of Si-OH groups. The broad absorption band ranging about from 3600 to 3000 cm -1 belongs to the stretching vibration of O-H. No characteristic absorption bands of amide groups can be observed, which suggests that there is no PNIPAm adsorbed onto the hollow silica spheres or if there is any the amount of PNIPAm is beyond the detection sensitivity of FTIR. For unpurified hollow silica spheres with PNIPAm templates, the FTIR spectrum clearly indicates the presence of PNIPAm templates (dashed line in Figure 3). Bands at 1642, 1547, and 1458 cm -1 are attributed to the -CO-NH- groups of PNIPAm. The ensemble behavior, hydrodynamic diameter, and size distribution of the hollow silica spheres in ethanol were investigated at 25 C by dynamic light scattering (DLS). Figure S1 shows the as-measured correlation functions of the hollow silica spheres at a scattering angle of θ = 90 (Supporting Information). The second-order cumulant expansion fits the decay curve well, indicating a monomodal distribution of particle size as shown in the inset. The peak hydrodynamic diameter D h of hollow silica spheres is ca nm with a polydispersity of at 90 and 25 C, which is in good agreement with those obtained by TEM and SEM. The decay rate Γ measured at four different scattering angles exhibits excellent linearity versus the square of wave vector q 2 (inset of Figure S1), indicating that the hollow silica spheres exhibit pure translational diffusive behavior in ethanol. Note that q =(4πn/λ)sin(θ/2) with λ and n being the wavelength of the laser light in vacuum (here λ = 636 nm) and the refractive index of the solvent, respectively. The N 2 adsorption-desorption isotherms of the hollow silica spheres are shown in Figure 4A. The typical type-iv hysteresis as usually observed for mesoporous or hollow silica spheres indicates the presence of large numbers of mesopores. The hysteresis at a relative pressure of >0.5 indicates the adsorption of N 2 molecules in the interparticle voids, 16 which is consistent with the results of TEM and SEM. The pore size distribution calculated by the Barrett-Joyner-Halenda (BJH) method is shown in Figure 4B. The Brunauer-Emmett-Teller (BET) specific surface area, mean pore diameter, and pore volume of the hollow silica spheres are about 245 m 2 g -1,9.79nm,and994mm 3 g -1.These results suggest that the hollow silica spheres have a macroporous core with a mesoporous shell structure. The XRD shows that the mesopores are irregularly distributed because no diffraction peak was observed in the 2θ range of 0.7 to 10 (Figure S2, Supporting Information). These mesopores of the silica shell provide channels for the outward diffusion of PNIPAm molecules from the core into the aqueous solution at lower temperature (<its LCST). The above experimental results show that the hollow silica nanospheres were successfully fabricated at 50 C by using thermosensitive PNIPAm as a template. No further chemical etching or calcination was needed. The N 2 adsorption-desorption isotherms suggest that the PNIPAm templates may diffuse out of the cores through the mesoporous silica shells when the PNIPAm molecules regain their solubility in aqueous solution at lower temperature (<its LCST), leading to the hollow inner DOI: /la902531p 12369

4 Article Figure 2. (A, B) TEM images and (C, D) SEM images of hollow silica nanospheres obtained with PNIPAm (M w = ,initial concentration 0.5 wt %) as a template at 50 C. Figure 3. FTIR spectra of hollow silica spheres obtained with PNIPAm (M w = , initial concentration of 0.5 wt %) as a template at 50 C(-) and solid silica spheres obtained with 0.5 wt % PVME as a template at 50 C ( 333 ). The FTIR spectrum of unpurified hollow silica spheres with PNIPAm templates (M w = ) was also included (---). cores. The FTIR spectrum confirms the absence of PNIPAm in the hollow silica spheres. It will be interesting to know whether such a novel strategy is also suitable for other thermosensitive polymers. In the present work, poly(vinyl methyl ether) (PVME) was tested. PVME is a thermosensitive polymer and exhibits an LCST at 35 C in aqueous solution. 25,26 In other words, both PNIPAm and PVME are thermosensitive and have similar LCSTs. Here, we are interesting in the experimental outcomes of using PNIPAm and PVME as soft templates at elevated temperature. We expect that PVME may have a capability similar to that of PNIPAm to be reversible and to function as a recyclable template for fabricating hollow silica spheres. However, the experimental outcomes proved that PVME does not work in such a way. Only solid silica spheres were obtained. This result indicates that the new strategy presented here is suitable only for PNIPAm and not for PVME. Figure 5 shows the typical TEM and SEM images of the solid silica spheres prepared with PVME (initial concentration of 0.5 wt %) as a template at 50 C. Solid silica spheres with a diameter of several hundreds of nanometers and a broad size distribution were observed. The broken silica sphere in Figure 5C clearly indicates that the silica sphere was solid. The surfaces of solid silica spheres are much smooth than those of hollow silica spheres shown in Figure 2. Such solid silica spheres are very easy to precipitate from the aqueous suspension. The FTIR spectrum confirms that the PVME molecules may be completely washed away by the purification procedure, and no PVME signal can be detected, as shown in Figure 3. The N 2 adsorption-desorption isotherms of the solid silica spheres is also included in Figure 4A. The adsorption volume of the solid silica spheres is much smaller than that of hollow silica spheres. No hysteresis was observed during the N 2 adsorption-desorption cycles, indicating that no DOI: /la902531p Langmuir 2009, 25(20),

5 Figure 4. (A) Nitrogen adsorption-desorption isotherms of the hollow silica spheres (O, b)obtainedwithpnipam(m w = , initial concentration of 0.5 wt %) as a template at 50 C and solid silica spheres (g) obtained with 0.5 wt % PVME as a template at 50 C. Open symbols present the adsorption branches. Solid symbols are desorption branches. (B) Pore size distribution calculated by the BJH method from the adsorption branches. Open circles represent the hollow silica spheres, and open stars represent the solid silica spheres. Scheme 1. Structures of PNIPAm and PVME mesopore exists in the solid silica spheres. The BJH pore size distribution of solid silica spheres is shown in Figure 4B as well. The BET specific surface area of the solid silica spheres is about 96.6 m 2 g -1, which is much less than that of hollow silica spheres. As mentioned above, hollow silica nanospheres were successfully prepared when using PNIPAm as a template at 50 C. However, by replacing PNIPAm with PVME, only solid silica nanospheres were obtained. This outcome is astonishing and unexpected. The structural difference between PNIPAm and PVME molecules consists of only the moieties of the side chain, as shown in Scheme 1. The side chain of PNIPAm is the N- isopropyl amide group, whereas it is the methyl ether group for PVME. Both N and O atoms can form hydrogen bonds with water molecules, which stabilize the PNIPAm and PVME molecules in aqueous solutions. Both the hydrophobic isopropyl and methyl groups tend to destabilize the PNIPAm and PVME molecules by altering the normal water structure and decreasing (29) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2000, 16, Langmuir 2009, 25(20), Article the entropy of water. 25,29 With increasing temperature of the aqueous solutions, the hydrophobic interactions of hydrophobic groups become dominant and the balance between the hydrogen bonding and hydrophobic interaction is broken, leading to phase separation and the existence of LCST for both PNIPAm and PVME. Both PNIPAm and PVME aqueous solutions were proven to exhibit a coil-to-globule transition followed by aggregation when heating their aqueous solutions above the LCST ,29-32 Tanev and Pinnavaia 33 systematically investigated the effects of ionic ammonium surfactants and neutral primary amine surfactants on the structure of mesoporous silica molecular sieves. The neutral hexagonal mesoporous silica molecular sieves were mainly attributed to the hydrogen bonding interactions between the precursor silanol hydrogens from TEOS and the lone electron pairs of the primary amine headgroups of the surfactants. 33 Although both amide and ether groups can hydrogen bond with silanol, it is speculated that the amount and extent of hydrogen bonding in the globule states may determine the structures of the resultant silica nanospheres. The remaining hydrogen bonding with water in the globule state of PNIPAm is much greater than that of PVME as a result of the existence of CdO groups of PNIPAm. 25,29 In a controlled experiment, pure silica nanospheres with a uniform size of around 30 nm were obtained after hydrolysis for 3 days at 50 C without adding any thermosensitive polymers (Figure S3, Supporting Information). When PNIPAm was used, TEM and SEM images indicate that the shells of hollow silica nanospheres were made up of many small silica nanospheres (Figure 2), the size of which is similar to that of pure silica nanospheres (Figure S3). However, with PVME as the template, the PVME globules behave as nucleation sites and the small silica nanospheres of prehydrolysis TEOS coarsened and grew into large solid silica spheres (Figure 5). The effects of the initial polymer concentration, molecular weight of the polymer, and pretreatment of TEOS on the morphology and size of the resultant silica spheres were also investigated (Figures S4-S10, Supporting Information). The experimental results indicate that the morphology and size of the hollow silica spheres were independent of the molecular weight of PNIPAm studied here. The diameters of hollow inner cores of the hollow silica spheres were in good agreement with the hydrodynamic diameters of PNIPAm aggregates at 50 C. The PNIPAm aggregates at 50 C exhibit a relative narrow size distribution with a mean hydrodynamic diameter of ca nm. The size distribution increases with increasing concentration of PNIPAm from 0.01 to 0.3 wt % (Figures S11-S12, Supporting Information). Note that the intensity of scattered light was out of the instrumental detection range for 0.5 wt % PNIPAm aqueous solutions at 50 C. However, the initial concentrations of PNI- PAm aqueous solutions do affect the morphology of the resultant hollow silica spheres. Low PNIPAm concentration (0.3 wt %) cannot lead to the successful formation of perfect hollow silica spheres. Solid silica spheres were always obtained regardless of the initial concentration of PVME and the prehydrolysis process. Efforts are currently underway to find a way to control the size and shell thickness of the hollow silica spheres. The use of monodispersive PNIPAm or mixing with amine-derivative surfactants will be considered. The formation mechanism of hollow silica nanospheres with PNIPAm as templates remains unanswered, and more systematic experiments are needed. (30) Spevacek, J.; Hanykova, L.; Starovoytova, L. Macromolecules 2004, 37, (31) Wu, C.; Wang, X. Phys. Rev. Lett. 1998, 80, (32) Wang, X.; Qiu, X.; Wu, C. Macromolecules 1998, 31, (33) Tanev, P. T.; Pinnavaia, T. J. Chem. Mater. 1996, 8, DOI: /la902531p 12371

6 Article Figure 5. (A) TEM image and (B and C) SEM images of solid silica nanospheres obtained with PVME (initial concentration of 0.5 wt %) as a template at 50 C. Figure 6. (A) TEM image and (B) SEM image of hollow silica nanospheres obtained from second-run experiments with recycled PNIPAm templates (M w = )at50 C. The supernatant of first-run experiments was directly used as a template solution for the second run of fabricating hollow silica nanospheres at 50 C. The PNIPAm soft templates after the first run of fabricating hollow silica nanospheres were confirmed to be recyclable. PNI- PAm with M w = (PDI= 1.94) was chosen as an example. The supernatant of the first run after centrifugation was directly used as a template solution for the second run of fabricating hollow silica nanospheres at 50 C. Figure 6 clearly shows that hollow silica nanospheres were successfully obtained from secondrun experiments, indicating the reusability of PNIPAm. Other methods were also tested for the reuse of PNIPAm templates. The ph value of the supernatant of the first-run experiment was first adjusted to be either ca. 1.5 or ca. 11 by using an HCl aqueous solution or NH 3 3 H 2O, respectively; the solution was then stirred for 3 h to hydrolyze the possibly remaining TEOS completely. Afterwards, centrifugation was performed again and the supernatant was used as a template solution for the second-run experiments. The experimental results indicate that the supernatant after treatment with HCl did not work (Figure S13), whereas welldispersed hollow silica nanospheres were successfully obtained for second-run experiments by using the supernatant at ph 11 as a template solution (Figure S14). Conclusions Thermosensitive PNIPAm can be used as reversible and recyclable soft templates for the fabrication of hollow silica DOI: /la902531p Langmuir 2009, 25(20),

7 nanospheres with a mesoporous shell in aqueous solution by simply regulating the reaction temperature with respect to its LCST. PNIPAm chains behave as soft templates for the formation of core-shell silica nanospheres at elevated temperature (>LCST) and will then diffuse out of the cores at lower temperature (<LCST), leading to the formation of hollow silica nanospheres. The morphology and size of the hollow silica spheres were found to be independent of the molecular weight of PNIPAm studied here. However, only solid silica nanospheres with a broad size distribution were obtained when another thermosensitive polymer, poly(vinyl methyl ether) (PVME), was used as a template. Such unique behavior of PNIPAm may be attributed to the hydrogen bonding interactions between the precursor silanol hydrogens from TEOS and the lone electron pairs of the amide groups of PNIPAm. Acknowledgment. We thank the National Natural Science Foundation of China ( , ), the Scientific Research Foundation for Returned Overseas Chinese Scholars (Ministry of Education), the Zhejiang Provincial Natural Science Article Foundation of China (Y406029), and the Zijin Program of Zhejiang University for financial support. We thank Prof. Qiang Zheng and Dr. Yonggang Shangguan for the use of the BI-200SM. Supporting Information Available: Correlation function of hollow silica spheres in ethanol measured by DLS. Smallangle XRD spectrum of hollow silica nanospheres obtained with PNIPAm. SEM image of silica nanospheres. TEM and SEM images of hollow silica nanospheres obtained with PNIPAm, another set of unsuccessful hollow silica nanospheres obtained with PNIPAm, solid silica nanospheres obtained with PVME, and hollow silica nanospheres obtained with PNIPAm. Photographs of PNIPAm aggregates. Size distribution of PNIPAm aggregates measured by DLS. SEM image of hollow silica nanospheres obtained in secondrun experiments with reused PNIPAm templates. Low-magnification TEM and SEM images of hollow silica nanospheres obtained in second-run experiments with reused PNIPAm templates. This material is available free of charge via the Internet at Langmuir 2009, 25(20), DOI: /la902531p 12373