Letter pubs.acs.org/Langmuir
New Strategy to Prepare Hollow Silica Microspheres with Tunable Holes on the Shell Wall Hongri Wan,†,‡ Yue Long,‡ Hui Xu,† Kai Song,*,‡ Guoqiang Yang,‡ and Chen-Ho Tung‡ †
School of Chemistry and Materials Science, Ludong University, Yantai 264025, China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡
S Supporting Information *
ABSTRACT: Hollow silica microspheres with holes of tunable numbers and sizes on the shell wall were prepared in this study. Clusters with positively charged polystyrene (PS) microspheres as the central spheres (CSs) and negatively charged PS spheres as the “halo” spheres (HSs) were formed via electrostatic interactions and utilized as a template. In the subsequent silica coating process, only CS was selectively coated; hence, after calcination, porous hollow silica microspheres were obtained.
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INTRODUCTION Over the past few decades, hollow nanomaterials have stimulated great interest because of their broad potential applications ranging from drug delivery,1−3 ion exchange,4−6 sensors,7,8 electrooptics,9,10 and microreactors11−13 to building block of photonic crystals.14 Recently, this field has been advanced to the fabrication of hollow microspheres with holes on the shell wall, namely, porous hollow microspheres. Because of their high specific surface area, low density, adsorption capacity, and ability to encapsulate actives, such materials are very useful in catalysis,15,16 bioseparation,17,18 tissue engineering,19−21 solar cells,22 and reaction separation.23 Among the backbone materials, silica is widely used because of its facile controllability and surface functionality.24 However, previous studies of porous hollow silica spheres mainly focused on mesoporous ones in which the pores on the surface are less than 50 nm. In some cases, under certain demands of permeability, hollow silica microspheres with larger holes in the shell are also desired. A few groups succeeded in producing macroporous silica particles based on the encapsulation of colloidal particles in emulsion droplets and subsequent evaporation of the emulsion phase.25−27 However, precisely controlling the morphology of the inner cavity of hollow microspheres is difficult. Here, we now report a facile and novel approach to preparing hollow and porous silica microspheres, where holes on the shell wall can be tuned both in number and size. As presented in Scheme 1, clusters were formed via electrostatic attraction of the oppositely charged PS spheres (HSs and CS). When the concentration ratio of CSs to HSs was adjusted, clusters of one CS coordinated to different numbers of HSs were formed and used as the template for the subsequent silica coating (step a). Because of the different electrostatic nature of CS and HSs, only positively charged CS can be coated with silica. During this coating process, HSs acted as masks that protected the contact areas between CS and HSs from being coated with © 2014 American Chemical Society
Scheme 1. Strategy for the Preparation of Porous Hollow Silica Microspheresa
a
(a) Formation of clusters through electrostatic interaction. (b) Selective silica coating from the hydrolysis of TEOS. (c) Removal of PS microspheres by calcination.
silica (step b). After removing the PS spheres by calcination, hollow silica microspheres were left with a certain number of holes on their shell wall (step c). Furthermore, the pore size of the hollow microspheres can be tuned precisely by the substituent hydrolysis of tetraethyl orthosilicate (TEOS) from more than 100 nm to complete closure.
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EXPERIMENTAL SECTION
The PS spheres with functional groups of −COOH or −NH2 on the surface were prepared by a typical method of dispersion polymerization and emulsion polymerization.28 Charges on the PS spheres were measured with a zeta potential analyzer (Figure S1). Here positively charged PS spheres were used as the CS, and negatively charged PS spheres were used as HSs. In the experiment, a suspension of positively charged PS spheres in ethanol was added to an ethanol suspension of negatively charged PS spheres. Owing to the electrostatic interaction, oppositely charged particles attracted one another to form clusters in the suspension. To form the desired Received: December 1, 2013 Revised: January 9, 2014 Published: January 13, 2014 683
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structure shown in Scheme 1, the number ratio of HS to CS (NHS/ NCS) was controlled to ∼100:1 so that every CS is sufficiently surrounded by enough HSs. After ultrasonication for 30 min, the product was transferred to a polytetrafluoroethylene autoclave, followed by the addition of H2O, NH3·H2O, and TEOS. The resulting mixture was kept in oven at 80 °C for 3 h before being purified by three cycles of centrifugation−redispersion in ethanol. Finally, the dried product was calcinated at 500 °C for 1 h to remove the PS template.
Figure 1c,d, the smaller size ratio makes it geometrically possible for more HSs to be located on the surface of CS than in the case of Figure 1b. The subsequent silica coating was carried out on the basis of a modified Stöber method at a high temperature29 that was close to the glass-transition temperature Tg of PS spheres. The PS spheres began to transform from the glassy state to the highelastic state, so the HSs in the cluster were fixed to the surface of the CSs. Meanwhile, during the hydrolysis of TEOS, owing to the different surface charge of the PS spheres, silica can grow and fully cover only the positively charged CS spheres but will leave only a few unconnected silica globules on the surface of HS spheres. In this way, the contact areas of CS and HS were protected. This is consistent with the previous study.30 It was also found that at high reaction temperature the resulting silica shell is coarse, which is probably due to the faster rate of silica nucleation and growth. After purification, the as-prepared product was heated from room temperature to 500 °C at a rate of 1 °C/min and kept at 500 °C for 1 h to remove the PS template. Thus, porous hollow silica spheres were obtained, as presented in Figure 2a−c. However, a special case emerged for the silica spheres produced from γ = 0.14 (RHS = 150 nm, RCS = 1100 nm). It can be observed in Figure 2d that after calcination, instead of holes in the shell wall, there were uniformed buds sprinkled on the surface of the big silica shell, with the diameter similar to that of HS. As discussed above, HSs can be only partially coated with silica; hence after calcination the individual silica buds on the surface of HSs fell off. But for smaller HSs with diameters of 150 nm, the attached silica buds can almost completely cover the surface of PSs and consequently remain on the structure after calcination (Figure 2h). Figure 2e−g quantifies the abundance of the holes on the microspheres shell wall of Figure 2a−c. These results were deduced from statistical analysis based on SEM observation of the final porous hollow silica spheres. For each sample, the statistics were based on a manual counting of 50 hollow silica spheres. It is noteworthy that from the top-view images of the hollow spheres only part (less than half) of the holes on the shell wall can be observed and counted. Thus, the actual total number of holes on the shell wall of hollow silica spheres should be at least twice the counted result. In theory, the
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RESULTS AND DISCUSSION As clearly shown in Figure 1, the number of HSs around each CS can be modulated by their size ratio, γ = RHS/RCS. Because
Figure 1. SEM images of silica-coated clusters consisting of oppositely charged PS spheres with different coordination numbers. The sizes of HSs and CS are (a) 540 nm/280 nm, (b) 540 nm/1100 nm, (c) 260 nm/1100 nm, and (d) 150 nm/1100 nm.
the coordination number can be roughly calculated (Figure S2), the theoretical numbers of HSs attached to each CS for Figure 1b−d are 33, 99, and 252, respectively. Nevertheless, because of the electrostatic repulsion between negatively charged HSs, HSs cannot be close-packed around CS. Thus, the real coordination number was much less than the theoretical evaluation. In the case of γ = 1.93 (RHS = 540 nm, RCS = 280 nm), no more than three HSs can be found around one CS. As we expected, decreasing the size ratio γ will result in more coordinated HSs around each CS. The number of coordinated HSs increased as γ decreases from 0.49 to 0.14. As shown in
Figure 2. (a−d) SEM and TEM images of the porous hollow silica spheres after calcination from the templates with different size ratios: (a) 540 nm/280 nm, (b) 540 nm/1100 nm, (c) 260 nm/1100 nm, and (d) 150 nm/1100 nm. (e−g) Quantification of the abundance of the holes on the shell wall, corresponding to samples a−c; (h) magnification of image d. 684
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Figure 3. RCS/RHS = 1100 nm/540 nm; reaction temperature 100 °C. (a) SEM image of silica-coated clusters composed of oppositely charged PS spheres. (b) SEM and TEM images of the porous hollow silica spheres after calcination.
Figure 4. SEM images of produced porous hollow silica spheres after modification by further silica coating. The amounts of TEOS used are (a) 0, (b) 8, (c) 16, and (d) 64 μL. (e) Diagram of the size of holes due to increasing amounts of TEOS.
± 9 nm. After adding 64 μL of TEOS, the holes were completely closed, but the thickness was 164 ± 9 nm (Figure 5).
variation trends of the holes on the shell wall are the same as the coordination numbers in cluster templates. It was discovered that the reaction temperature can also affect the size of holes on the hollow spheres. For example, the holes in the products from hydrolysis at 80 °C have an average diameter of 125 ± 12 nm, and the products from 100 °C gave 190 ± 22 nm (Figure 3). This is because the higher reaction temperature results in deformation and consequently increases the contact areas between CS and HS, which give rise to larger holes after removing the PS templates. In addition, the size of the holes can be more widely and precisely tuned by a further silica coating process on the produced porous hollow silica spheres. Typically, 10 mg of the porous hollow silica spheres (540 nm/1100 nm) was added to ethanol (5 mL) and dispersed by ultrasonication. After adding the above suspension (50 μL) to the mixture of ethanol (3 mL), H2O (120 μL), and NH3·H2O (100 μL), various amounts of TEOS (8, 16, and 64 μL) were added to the reaction system with stirring. The reaction lasted for 8 h at room temperature before purification by three cycles of centrifugation−redispersion in ethanol. After this additional hydrolysis process, silica was uniformly coated onto the porous silica spheres. As shown in Figure 4, because of the uniformity of the subsequent silica coating, reasonably increasing the amount of TEOS resulted in an increased average thickness of the silica shells and a decreased average size of the holes. From the static measurement of SEM images (Figure 4e), the average size of each hole was chose as the diameter of the smallest circle that can include the hole inside (Figure S3). As the amount of TEOS increased from 0 to 16 μL, the average size of holes on the spheres decreased from 125 ± 12 to 65 ± 8 nm. Respectively, the average thicknesses of silica shells corresponding to different amounts of TEOS were 45 ± 7, 95 ± 6, and 128
Figure 5. TEM images of the produced porous hollow silica spheres after modification by further silica coating. The amounts of TEOS used are (a) 0, (b) 8, (c) 16, and (d) 64 μL.
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CONCLUSIONS We have developed a method to prepare porous hollow silica spheres. The primary advantage of this method is that the number and size of holes on the shell wall of hollow silica spheres can be well controlled. Clusters consisting of positively charged PS spheres as central spheres coordinated with negatively charged PS spheres were utilized as a template. By varying the size ratio of these two types of PS spheres, the coordination numbers of CS and HS can be adjusted. Because 685
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of the opposite surface charge, at high temperature the subsequent hydrolysis of TEOS resulted in a selective silica coating on the surface of positively charged CS. The hollow structure was then obtained by the following calcination process to remove the PS template. The size of the holes on the hollow spheres can be further reduced or even closed by an additional silica coating step. Also, the size of the inner cavity can be easily adjusted by choosing the desired size of central sphere. Thus, porous hollow silica spheres with a tunable number and size of holes on the shell wall were successfully prepared. We envision that these porous hollow spheres may be used as a novel microcontainer for catalysis or drug delivery.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details and calculated data are provided. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the national Basic Research Program of China (no. 2013CB834505).
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REFERENCES
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