ARTICLE pubs.acs.org/Langmuir
Hydrothermal Synthesis of Hollow Silica Spheres under Acidic Conditions Qiyu Yu, Pengpeng Wang, Shi Hu, Junfeng Hui, Jing Zhuang, and Xun Wang* Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China
bS Supporting Information ABSTRACT: It is well-known that silica can be etched in alkaline media or in a unique hydrofluoric acid (HF) solution, which is widely used to prepare various kinds of hollow nanostructures (including silica hollow structures) via silicatemplating methods. In our experiments, we found that st€ober silica spheres could be etched in generic acidic media in a wellcontrolled way under hydrothermal conditions, forming welldefined hollow/rattle-type silica spheres. Furthermore, some salts such as NaCl and Na2SO4 were found to be favorable for the formation of hollow/rattle-type silica spheres.
’ INTRODUCTION Hollow silica spheres (HSSs) have attracted intense interest in a wide variety of applications, such as catalysis, sensors, and microvessels for drug delivery and nanoreactors.17 A conceptually straightforward strategy for preparation of HSSs should be the template method, which involves the formation of a core/silicashell structure and subsequent removal of the core material. Many studies have been directed to the fabrication of HSSs using hard/ soft templates.811 However, the heterogeneous deposition of a silica layer in many cases is difficult to control due to undesirable homogeneous nucleation of silica. To overcome this problem, researchers developed some homogeneous templating methods to produce HSSs. They used structurally different silica materials as the sacrificial templates and then selectively removed the templates by using appropriate etching agents, and eventually a hollow/rattletype structure formed.3a,1215 For example, Tang and co-workers13 elaborately synthesized organicinorganic hybrid solid silica spheres with a three-layer “sandwich” structure, and then selectively etched the middle layer by hydrofluoric acid (HF) to produce rattle-type hollow silica spheres. To fabricate mesoporous HSSs, Chen et al.14 first deposited mesoporous silica on solid silica spheres to make a solid silica core/mesoporous silica shell structure. Then by choosing the proper alkaline etching agent, the solid silica core was selectively dissolved based on the structural difference between the core and the shell. Furthermore, many efforts have been devoted to establishing template-free or self-templating methods to prepare HSSs, where no additional templates are needed. Ren et al.16 found that cationic poly(dimethyldiallylammonium chloride) (PDDA) could protect the silica surface from etching, and thus facilitate the formation of HSSs. Yin and co-workers15,1720 also r 2011 American Chemical Society
developed a surface-protecting etching method to fabricate hollow/rattle-type silica nanostructures using poly(vinyl pyrrolidone) (PVP) as the protecting ligand. The self-templating methods proved to be very simple and effective. Either in these structural difference-based etching methods or in the self-templated synthesis, HSSs were generally prepared in alkaline media. The used alkaline etching agents include NaOH,15,17 NH3 3 H2O,14,16 Na2CO3,14 NaBH4,18 and so on. Another effective etching agent is HF solution.13 The disadvantage of this etching agent is that it is extremely corrosive and toxic and thus the handling is not convenient. To our knowledge, the etching of silica materials to form HSSs in generic acidic media has not been reported. Herein, we show that st€ober silica spheres can also be hollowed out under acidic conditions. Moreover, our method of HSS fabrication was a self-driven process; no additional templates or protective surfactants were needed. Thus, as-prepared HSSs should have relatively clean surfaces, which are important in some application areas needing strict surface chemistry requirements, such as catalysis, electrochemistry, sensing, etc.
’ EXPERIMENTAL SECTION Materials. Tetraethyl orthosilicate (TEOS, A.R.), cetyltrimethyl ammonium bromide (CTAB, A.R.), ammonia hydroxide (28%), HCl (3638%), H2SO4 (98%), NaCl (A.R.), Na2SO4 (A.R.), NaI (A.R.), acetone (A.R.), and absolute ethanol were purchased from Beijing Chemical Regent Company. Deionized (DI) water was used for all syntheses. Hydrochloric acid and sulfuric acid were diluted before use. Received: February 23, 2011 Revised: April 29, 2011 Published: May 09, 2011 7185
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Langmuir € ber Silica Spheres. The preparation of Preparation of Sto different sized silica spheres involves the ammonia-catalyzed hydrolysis and condensation of TEOS in an aqueous ethanol solution via the classical st€ober method.21,22 Taking the synthesis of ∼190 nm silica spheres as an example, 15 mL of absolute ethanol, 5 mL of DI water, and 0.7 mL of 28% NH3 3 H2O were mixed and stirred. A total of 0.6 mL of TEOS was added into the mixture quickly. After a reaction time of about 10 h, the silica spheres were isolated by centrifugation. Then the white precipitate was washed with ethanol three times. Preparation of Silica/CTAB Composite Spheres. The mesoporous spheres of about 600 nm were prepared as follows: 0.1 g of CTAB was dissolved in an mixture of 15 mL of water and 10 mL of ethanol, and then 0.2 mL of NH3 3 H2O and 0.1 mL of TEOS were added under stirring. The reaction was carried out at about 35 °C for 10 h. As prepared silica/CTAB spheres were centrifuged and washed with deionized water and ethanol several times. Extraction of CTAB was carried out via the reported method.23 Preparation of Hollow Silica Spheres. Take the ∼190 nm spheres as an example. The newly prepared silica powder was dispersed into 30 mL of DI water by sonication. Next 5 mL of the silica dispersion and 15 mL of water were added into a beaker. Then 1 mL of 0.1 M HCl was added to tune the solution to be acidic (pH ∼3.0). A H2SO4 solution can also be used. The mixture was stirred at room temperature for about 24 h and then transferred into a sealed Teflon-lined autoclave and heated at 180 °C for a given time. For slightly acidic preparation, the acid treated dispersion was centrifuged, washed once by water, then dispersed into 20 mL of DI water (pH 56), and finally hydrothermally treated at 180 °C for a given time. For the other sized silica spheres, the procedure was similar. Except for scaled-up preparation, the amount of various silica spheres used in a single preparation was equivalent to 0.1 mL of TEOS, assuming that the precursor totally converted to silica spheres. Salt-Assisted Preparation of Hollow/Rattle-Type Silica Spheres. The synthesis procedure was just the same except that a salt was added along with the acid into the silica dispersion. For a typical synthesis, about 0.04 g of Na2SO4 (or 0.06 g of NaCl) was added. After incubation at room temperature for 24 h, the silica dispersion was also measured to have a pH value around 3.0. In the case of silica/CTAB composite spheres, 0.04 g of Na2SO4 and 1 mL of 0.1 M HCl were used. After synthesis, the products were dried at 100 °C and calcined at 550 °C for 6 h to remove CTAB. The Encapsulation and Release of Rhodamine B. The encapsulation process was as follows. 0.05 g of hollow spheres and 0.012 g of Rhodamine B were placed into 5 mL of water, and the mixture was agitated at room temperature for 48 h. Then the mixture was centrifuged and washed with water three times. The product was dried at 40 °C under a vacuum for 24 h. The release process was as follows. 0.01 g of the dried sample was dialyzed against 70 mL of phosphate buffer solution (0.025 M, pH 6.9) at 37 °C in a water bath. The UV absorbance of Rhodamine B in the outside buffer solution at different release times was obtained using a ultravioletvisible spectrophotometer. Release experiments were conducted until the concentration of the solution stopped changing significantly. Characterization. UVvis spectra were recorded on a Hitachi U-3010 spectrophotometer. The size and morphology of the silica particles were determined by a Hitachi H-7500 transmission electron microscope (TEM) operating at 80 kV and a Tecnai G2 F20 S-Twin high-resolution transmission electron microscope (HRTEM) operating at 200 KV. Scanning electron microscopy (SEM) images were obtained on a field emission FEI Sirion 200 microscope. Samples were prepared by dropping dilute ethanol solution of silica particles onto the surface of a carbon coated copper grid and silicon wafer for TEM and SEM. The X-ray diffraction patterns were collected by Rigaku D/max-2500/PC X-ray diffractometer using Cu KR radiation (λ = 1.5481 Å). Fourier
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Figure 1. Typical TEM images of the initial st€ober silica spheres (a) and the HSSs prepared in H2SO4 (b) and HCl (c, d) aqueus solution at 180 °C for 10 h. transform infrared (FTIR) spectra were performed on a Nicolet 560 spectrograph. Nitrogen adsorptiondesorption isotherms were measured on a Micromeritics Tristar II 3020 system. The silicon concentration in aqueous solution was analyzed by inductively coupled plasma (ICP) optical emission spectroscopy technique using a 2RIS Intrepid II XSP ICP-OES. To study the dissolution of the silica during hollow sphere preparation, a batch of preparation experiments was carried out simultaneously but stopped at different reaction times. The obtained reaction dispersions were first detected with UVvis spectra. Then they were centrifuged thoroughly. The supernatant clear solution was used to detect the silicon content with ICP method, while the sediment was analyzed by TEM.
’ RESULTS AND DISCUSSION Newly prepared st€ober silica spheres were used to fabricate HSSs. The st€ober silica spheres are slightly basic, due to the chemiadsorption of ammonia.24 We incubated the silica spheres in aqueous HCl (or H2SO4) solution for 24 h to obtain an acidic dispersion at pH ∼3.0. Another slightly acidic silica dispersion was made by redispersing the above incubated silica in water (pH 56). We found that too much acid or an alkaline condition was not favorable for the fabrication of HSSs via the present method (Supporting Information, Figures S1,S2). In the following part of this paper, we examined the hydrothermal synthesis of HSSs under these two acidic conditions. Then the effect of some salts such as NaCl and Na2SO4 to the acidic synthesis of HSSs was studied. Synthesis of HSSs at pH ∼3.0. Figure 1 shows the TEM images of the st€ober silica spheres around 190 nm and HSSs prepared at pH ∼3.0. The HSSs are evenly etched, nonaggregated and have narrow size distributions depending on the solid precursors. By using different hydrothermal reaction time in parallel experiments, we followed the solid-to-hollow transformation process. As illustrated in Figure 2a, the TEM pictures show that the silica spheres were gradually dissolved preferentially in the interior. The shell part also became less dense due to slight 7186
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Figure 2. Temporal evolution of the hydrothermally treated acidic silica dispersion. (a) TEM images of the gradually hollowed-out silica spheres. (b) UVvis absorption spectral evolution of the acidic silica reaction solution. (c) The variation of the concentration of the dissolved silicon species in the bulk solution. The theoretical value of the total silicon concentration is about 600 ug/mL.
Figure 3. (ac) TEM images of HSSs with different diameters: (a) 126 nm, (b) 160 nm, and (c) 340 nm respectively; (d) STEM image corresponding to (b), dark domains distributed on the spheres present small openings; (e) TEM image of HSSs prepared with a relatively short reaction time compared to (b); (f) TEM images of HSSs prepared from highly concentrated silica dispersion. The inset in b is a local HRTEM image.
dissolution, resulting in mesoscale pores, which were necessary for the outflow of the dissolved silicon species. The particle size exhibits little changes during the hollowing process. We measured the silicon content in the supernatant (obtained by careful centrifugation for several times) of the reaction solution after synthesis. As shown in Figure 2c, the silicon content in the bulk solution increased with reaction time, indicative of the gradual dissolution of silica spheres. After a reaction time of 10 h, the silica spheres lost about 63.6% of the silicon content theoretically. Accordingly, the reaction solution showed a decreasing UVvis absorption (Figure 2b), appeared as gradually increasing transparency. It is worth mentioning that the solution was also acidic
with almost unchanged room temperature pH value after hydrothermal treatment. Synthesis of Different Sized HSSs at pH 56. Hydrothermally heating of an acidic silica dispersion at pH 56 led to faster hollowing process. About 3 h was needed for the fabrication of ∼190 nm HSSs. Figure 3ac show as-prepared HSSs with different sizes. The hollowed-out silica spheres generally possess relatively larger pores due to faster etching, which can be obviously observed in Figure 3bd,f. The void space or the shell thickness can be tuned by changing the reaction time and the silica concentration. Figure 3, panels b and e ,are from two samples after 7 h and 1 h treatment respectively, which exhibit a 7187
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Figure 4. TEM images of the HSSs prepared in (a)HCl/Na2SO4 and (b)HCl/NaCl; (c) TEM images of silica spheres etched in HCl/Na2SO4 system at different reaction times.
Figure 5. TEM images of 180 nm (a) and 540 nm (b) rattle-type silica hollow spheres obtained from Na2SO4-assisted synthesis.
significant decrease in void space for the latter one. Increasing the silica concentration also leads to smaller void space. Figure 3f shows the HSSs prepared from a five times more dense dispersion. Two cycles of isolation/heating were adopted for this sample. Considering the effect of silica concentration, all samples elsewhere discussed in this paper were prepared in the same concentration with regard to silicon. Salt-Assisted Synthesis of HSSs. In the acidic reaction system, the presence of some salts was found to be favorable for the formation of HSSs. Figure 4a,b shows the HSSs after 5 h reaction in HCl/Na2SO4 and HCl/NaCl aqueous solution, respectively. The pH values were also kept around 3.0. Compared with the abovementioned systems, this salt-assisted synthesis seems to be even more efficient. Relatively shorter time was needed, and as-prepared HSSs usually have smoother and more well-defined shell structures. Amazingly, an absolutely different etching mode was observed in the case of Na2SO4. As illustrated in Figure 4c, an intermediate with rattle-type morphology was observed. For relatively small silica spheres, the rattle-type silica can evolve into totally hollow eventually. So, the rattle-type spheres can be obtained by controlling the reaction time (Figure 5a). However, for the large silica spheres of about 540 nm, the rattle-type silica exhibited little change with increasing reaction time (Figure 5b). A long time incubation of the silica spheres in the acidic media was found to be another critical issue for the formation of rattle-type silica spheres. If hydrothermal heating was carried out just after the mixing of the silica and HCl/Na2SO4 acidic solution, no hollow/rattle-type spheres were generated (Figure S3). The salt-assisted strategy is also applicable to the synthesis of HSSs from mesoporous silica/CTAB composite spheres (Figure 6a,b). In this case, we found that the existence of the surfactant template was critical for hollow structure formation. After CTAB molecules were removed by acetone extraction,23 no hollow structure 7188
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Figure 6. (a, b) TEM images of hollow silica spheres prepared from CTAB/silica composite. (c) TEM image of silica particles prepared from acetone extracted sample.
Figure 7. N2 adsorptiondesorption isotherms and pore size distributions of typical HSSs made from solid silica (a, b) and SiO2/CTAB (c, d) spheres.
formed (Figure 6c). We estimate that CTAB can delay the diffusing of the outer bulk solution into the spheres. After CTAB was extracted from the spheres, mesopores were left, and the outer solution can easily diffuse into the spheres through mesopores. This will make the etching occur evenly throughout the sphere, and thus no hollow structures can be generated. Possible Ethching Mechanism. In fact, the dissolution phenomenon of amorphous silica was already been studied tens of years ago.2527 Alexander et al.25investigated the solubility of amorphous silica in a wide pH range from 2 to 8. They found that amorphous silica in water could establish an equilibrium between solid phase and a monomeric form of silica, presumably Si(OH)4 SiO2 ðamorphousÞ þ H2 Oð1Þ h SiðOHÞ4 ðaqÞ However, the dissolution of amorphous silica at low temperature is quite slow. Keeping other conditions constant, we found that the silica spheres remained almost unchanged even after refluxing the reaction solution (pH ∼3.0) at about 100 °C for 1 day long. However, hollow spheres were observed in just a few
hours at 180 °C in our experiments. This can be explained by the significantly increased solubility and dissolution rate at high temperature.26 Kato et al.27 found that the dissolution rate can be accelerated by some coexisting salts, which was also consistent to our salt-assisted synthesis experiments. We noticed that Yin and co-workers20 recently reported the preparation of HSSs by surface-protected etching with water. They also attributed the etching to the dissolution of amorphous silica. We estimated that the formation process showed in Figure 2a proceeded via a dissolutiondiffusion process. Upon heating, the silica begins to dissolve slowly and diffuse outward, and the dissolved silicon species in the bulk solution gradually increases until a balance at the solidsolution interface is achieved. At higher silica sphere concentration, the balance can be achieved earlier, resulting in small void space. The salt effect in the formation of HSSs was quite elusive. We proposed a mechanism as follows: Naþ and Cl can be sorbed into the silica spheres during the long time incubation before hydrothermal treatment,28 and thus the dissolution rate was 7189
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Figure 8. Release of Rhodamine B from HSSs made from solid silica (a) and from SiO2/CTAB (b) spheres. The inset shows the HSS powders after loading of Rhodamine B. The last UV absorbance (21 h) of Rhodamine B was considered as complete release.
accelerated. When Na2SO4 was used instead, the sorbed Naþ ions can not reach the innermost of the spheres due to the electrostatic attraction of the SO42, which can not be sorbed into the silica spheres due to its large ion size. Therefore, only the dissolution between the core and the shell space was accelerated, leading to rattle-type hollow spheres at the beginning. However, further study is needed to get a full understanding of the salt effect in the formation of hollow/rattle-type silica spheres. Pore Structure Characterization and Dye Encapsulation Release. Figure 7 shows the BrunauerEmmettTeller (BET) N2 adsorption experiment results of typical HSSs made from solid silica and SiO2/CTAB composite spheres. The BET surface area and the total BarretJoynerHalenda (BJH) pore volume of the HSSs made from solid silica spheres were measured to be 103.9 and 0.17 cm3/g, respectively. The corresponding values of the SiO2/CTAB composite HSSs were 65.3 and 0.16 cm3/g. The pore diameter distributions of the two types of HSSs were relatively wide according to results from the adsorption branch of the isotherm. The HSSs with mesoporous structure may possess good performance in catalytic activity, drug delivery efficiency and so on. We preliminarily evaluated the in vitro drug encapsulation and release properties of the HSSs, using Rhodamine B as a model molecule. As shown in Figure 8, the dye was effectively loaded into the HSSs, and the release process lasted about 20 h. The release ratio for the HSSs made from solid silica reached above 95% in 89 h, whereas it took about 16 h for the other HSS sample to reach 95% release ratio.
’ CONCLUSION In conclusion, the present paper demonstrates that it is possible to fabricate various sized HSSs via a self-templating method in acidic aqueous media. The hollowed-out interior space was dependent on the reaction time and silica concentration. The pore structure in the shell of the HSSs can be controlled by tuning the acidity of the silica dispersion. Additional salts such as NaCl, and Na2SO4 are favorable for the process, leading to faster hollowing process and finally smoother, more well-defined shell structures. Especially, a rattle-type hollow silica spheres can be obtained with the help of Na2SO4. The acidic hollowing method was also extended to the fabrication hollow structure from mesoporous silica spheres. The acidic, nonsufactant syntetic condition may be desirable in the synthesis and application of some HSS-related nanomaterials.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional TEM images and other characterizations of the silica particles. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by NSFC (20725102, 20971078, 20921001), and the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011CB932402). ’ REFERENCES (1) Lou, X. W.; Archer, L. A.; Yang, Z. C. Hollow Micro-/Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987–4019. (2) (a) Ding, S. J.; Chen, J. S.; Qi, G.; Duan, X.; Wang, Z.; Giannelis, E. P.; Archer, L. A.; Lou, X. W. Formation of SnO2 Hollow Nanospheres inside Mesoporous Silica Nanoreactors. J. Am. Chem. Soc. 2011, 133, 21–23. (b) Tan, L. F.; Chen, D.; Liu, H. Y.; Tang, F. Q. A Silica Nanorattle with a Mesoporous Shell: An Ideal Nanoreactor for the Preparation of Tunable Gold Cores. Adv. Mater. 2010, 22, 4885–4889. (3) (a) Roca, M.; Haes, A. J. SilicaVoidGold Nanoparticles: Temporally Stable Surface-Enhanced Raman Scattering Substrates. J. Am. Chem. Soc. 2008, 130, 14273–14279. (b) Sanles-Sobrido, M.; Exner, W.; Rodríguez-Lorenzo, L.; Rodríguez-Gonzalez, B.; Correa-Duarte, M. A.; lvarez-Puebla, R. A.; Liz-Marzan, L. M. Design of SERS-Encoded, A Submicron, Hollow Particles Through Confined Growth of Encapsulated Metal Nanoparticles. J. Am. Chem. Soc. 2009, 131, 2699–2705. (4) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Dong, X. P.; Feng, J. W.; Ruan, M. L.; Li, Y. S. Stimuli-Responsive Controlled Drug Release from a Hollow Mesoporous Silica Sphere/Polyelectrolyte Multilayer Core Shell Structure. Angew. Chem., Int. Ed. 2005, 44, 5083–5087. (5) Li, L. L.; Tang, F. Q.; Liu, H. Y.; Liu, T. L.; Hao, N. J.; Chen, D.; Teng, X.; He, J. Q. In Vivo Delivery of Silica Nanorattle Encapsulated Docetaxel for Liver Cancer Therapy with Low Toxicity and High Efficacy. ACS Nano 2010, 4, 6874–6882. (6) Chai, G. S.; Yoon, S. B.; Kim, J. H.; Yu, J. S. Spherical Carbon Capsules with Hollow Macroporous Core and Mesoporous Shell structures as a Highly Efficient Catalyst Support in the Direct Methanol Fuel Cell. Chem. Commun. 2004, 23, 2766–2767. (7) Yamada, Y.; Mizutani, M.; Nakamura, T.; Yano, K. Mesoporous Microcapsules with Decorated Inner Surface: Fabrication and Photocatalytic Activity. Chem. Mater. 2010, 22, 1695–1703. 7190
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