PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Amphiphilic mesoporous silica composite nanosheets Shujiang Ding, Bing Liu, Chengliang Zhang, Ying Wu, Huifang Xu, Xiaozhong Qu,* Jiguang Liu and Zhenzhong Yang* Received 7th January 2009, Accepted 9th March 2009 First published as an Advance Article on the web 3rd April 2009 DOI: 10.1039/b900078j We have reported a new approach to fabricate mesoporous silica composite nanosheets by milling the corresponding hollow spheres. The mesoporous nanosheets are amphiphilic, and can be well dispersible both in water and oil, serving as particulate emulsifiers in o/w or w/o systems. The mesoporous silica can assist other functional materials, for example metal and carbon to be dispersible. An example is given to demonstrate the support of catalysts for a heterogeneous catalytic aerobic oxidation of benzyl alcohols by Pt/silica composite nanosheets.
Introduction Mesoporous materials have attracted much attention due to their broad potential applications.1 They are mainly synthesized based on co-assembly of surfactant and inorganic species, whose composition and pore microstructure can be controlled.1,2 Aiming at their practical applications, it is important to control macroscopic morphology, e.g. particle, thin film and fiber.3–5 Among the applications, mesoporous silica particles can be used as fillers to fabricate polymer nanocomposites with improved thermal and mechanical properties.6 Polymer chains can penetrate within the mesopores responsible for the improved properties. The particles are organically modified so as to enhance the compatibility.1c, 7 It is conjectured that if anisotropic mesoporous silica nanosheets instead of particulates are used as fillers for composites, polymer-inorganic multi-layered nanocomposites will be expected under shearing. Such materials can mimic natural hierarchically structured for example nacre, which is composed of alternative inorganic CaCO3 platelets and biomacromolecules.8 Although nacre mimetic composites have been extensively synthesized,9 the inorganic fillers lack of microstructure, thus interpenetration of polymer chains inside the fillers is impossible. How to improve dispersion of fillers in a matrix becomes another main concern. We have previously found that mesoporous silica thin film is amphiphilic, on which both water and oil can be spread completely and quickly due to a strong capillary effect from the mesopores.4f Therefore, we start to develop a method to synthesize mesoporous silica composite nanosheets and investigate their amphiphilic performance. Although it is reasonable to break a mesoporous thin film into small pieces, resulting in mesoporous nanosheets, the procedure can not be scaled up for massive synthesis. It is challenging to develop a method for massive production of the mesoporous nanosheets. While hollow spheres are template synthesized by removing cores from the core-shell structure, the osmosis pressure usually results in aperture even fragmentation of the shell into small pieces.10 By further exploring such fragmentation, especially under external sonication or milling, mesoporous State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. E-mail:
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nanosheets will be massively synthesized as Fig. 1. A monodisperse micron-sized polystyrene (PS) sphere was dispersed in a silica sol containing a porogen, for example F127, a very thin mesostructured silica was coated onto the PS core forming a core-shell structure by spraying the mixture at 90 C. For a complete sol–gel process, the core-shell structure was further treated at 100 C for 12 h. Both the PS core and porogen F127 could be removed by either calcination in air or dissolution with solvents, achieving a hollow sphere. By milling the hollow sphere into small pieces, those mesoporous silica nanosheets were produced.
Experimental Sample preparation Polystyrene sphere: styrene was distilled under reduced pressure after being washed with a 10 wt.-% sodium hydroxide solution. The other reagents were used as received, including azobisisobutylnitrile (AIBN), ethanol, poly N-vinylpyrrolidone (PVP, average weight molecular weight 30 000). The polymerization was carried out in a 250 mL four-necked flask equipped with a condenser, a stirrer, and a nitrogen inlet. The ethanol and water solution of PVP was introduced into the flask, and then the solution of styrene and AIBN were added slowly under stirring. After purging with nitrogen for 60 min, the flask was immersed in a water bath maintained at 73 C for 24 h for polymerization. The emulsion was centrifuged and washed with ethanol three times. In a typical experiment, monodisperse PS spheres of 2.5 mm in diameter were polymerized by introducing a solution
Fig. 1 Schematic synthesis of mesoporous nanosheets. Onto the PS core, a shell is achieved by sol–gel process forming a core-shell structure. During the process, a porogen is introduced for a mesostructured shell. After removal of both PS core and the porogen, a broken hollow sphere is derived with a mesoporous shell. After the hollow sphere is milled, the corresponding mesoporous nanosheets are synthesized.
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containing 20 g of styrene, 1.6 g of PVP, 0.6 g of AIBN, and 95 g of ethanol and 5 g of water. Mesoporous silica nanosheets: silica/PS core-shell particles were prepared by co-assembly of oligomer silicate from TEOS and triblock copolymer (EO106PO70EO106, Pluronic F127). In a typical preparation, 1.0 g of PS sphere, 0.25 g of F127 and 1.0 g of 0.2 M HCl were dispersed in 20 g of ethanol, and stirred at 40 C for 1 h. 0.43 g of TEOS was then added. After being stirred for 4 h, the mixture was sprayed forming core-shell particles. The core-shell particles were further thermally treated at 100 C for 12 h to allow a more complete polycondensation of the silica. After being treated with DMF or calcined at 450 C in air, hollow spheres were formed. The corresponding mesoporous silica nanosheets were created by ball milling the hollow spheres. Silica/carbon composite nanosheets: the resol precursor (Mw < 500) was prepared according to the literature method.11 In a typical procedure, 0.61 g of phenol was mixed with 0.13 g of 20 wt.-% NaOH aqueous solution at 40–42 C for 10 min under stirring. 1.05 g of formalin (37 wt.-% formaldehyde) was added dropwise below 50 C. Upon further stirring at 70–75 C for 1 h, the mixture was cooled to room temperature and the pH value was adjusted to neutral with aqueous HCl. After water was vacuum evaporated below 50 C, the final product was dissolved in ethanol obtaining a solid content of about 20 wt.-%. In a typical preparation of a silica/resol/PS core-shell particle, 1.0 g of PS sphere, 0.2 g of F127 and 1.0 g of 0.2 M HCl were dispersed in 20 g of ethanol at 40 C for 1 h under stirring. 0.25 g of TEOS and 0.625 g of 20 wt.-% resol were added. After being stirred for 4 h, the mixture was sprayed forming core-shell particles. After the core-shell particles were further thermally treated at 100 C, they were calcined at 800 C in nitrogen forming hollow spheres. By milling, mesoporous silica/carbon composite nanosheets were prepared. Pt/silica composite nanosheets were synthesized by additionally introducing K2PtCl6 in the precursor mixture for silica nanosheets followed by the similar procedure for mesoporous silica nanosheets at high temperature. The catalytic aerobic oxidation of benzyl alcohols in water: a mixture of Pt/silica nanosheets (0.002 mmol of effective platinum), 0.2 mmol of benzyl alcohol, 0.2 mmol of potassium carbonate and 2 mL of water was stirred at 60 C for 12 h under air at atmospheric pressure. After cooling, the mixture was washed with tert-butyl methyl ether and acidified with 5 wt.-% hydrochloric acid. The mixture was extracted with 1 mL of ethyl acetate for five times. The extract was dried over magnesium sulfate and concentrated in vacuo to get the benzyl acid.
voltage of 15 kV. The samples were ambient dried and vacuum sputtered with Pt. The dried samples were pressed into pellets with KBr for characterization by a BRUKER EQUINOX 55 FT-IR spectrophotometer. The silica content of core-shell composite microspheres and silica/carbon composite nanosheets were measured by a thermogravimetric analyzer (Perkin-Elmer TGA 7) in air at a heating rate of 20 C/min. Nitrogen adsorption was performed on a Micromeritics ASAP 2020M Surface Area and Porosity Analyzer. XRD patterns were recorded on a Rigaku, DMAX-2400 powder X-ray diffractometer by using CuKa radiation (60 kV, 200 mA).
Results and discussion Synthesis and characterization of mesoporous nanosheets Under acid conditions, both the co-assembly of F127/oligomer silicate and the condensation rates are rather slow in bulk. Alternatively, evaporation-induced self-assembly onto the PS core (2.5 mm in diameter, Fig. 2a) occurs very fast during spray drying the mixture. Aggregation among the core-shell spheres is avoided (Fig. 2b). After the core-shell spheres were dissolved with DMF or calcined in air, the PS cores and F127 were removed giving broken silica hollow spheres (Fig. 2c). The hollow spheres were subsequently milled into nanosheets (Fig. 2d), whose characteristic size is controlled by milling strength and time. The start pyrolysis temperature of triblock copolymer F127 and Linear PS are at about 200 and 400 C, respectively, and complete pyrolysis temperature is at 500 C in air (Fig. 3a, b). The silica weight content of core-shell composite spheres is about 9.1 wt.-% measured by TGA in air. (Fig. 3c) It is interesting that the organic content of core-shell composite shperes is pyrolyzed completely at 600 C, which is behind 100 C than F127 and PS. The possible reason is that silica matrix delays the reaction of air and organic component in silica nanocomposite.
Characterization Very dilute dispersions of the samples in ethanol were dropped onto carbon coated copper grids for transmission electron microscopy (TEM) characterization (JEOL 100CX operating at 100 kV). High resolution TEM (HR-TEM) characterization was performed on a HITACHI H-9000 NAR operating at 300 kV. The samples were embedded in organic glass resin (PMMA) and microtomed into slices about 30–50 nm thick using Leica ultracut UCT ultramicrotome at room temperature. Scanning electron microscopy (SEM) measurements were performed with a HITACHI S-4300 apparatus operated at an accelerating 3444 | J. Mater. Chem., 2009, 19, 3443–3448
Fig. 2 SEM image of: (a) the PS sphere; (b) PS/silica/F127 core-shell particles by spraying; (c) the broken silica hollow spheres derived from PS/silica/F127 composite spheres by calcination in air for 2 h; (d) the mesoporous silica nanosheets by milling the hollow spheres as shown in part (c).
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Fig. 3 TGA curves of: (a) the PS sphere; (b) F127; (c) PS/silica/F127 core-shell particles by spraying; (d) silica/carbon composite nanosheets.
Hexagonal mesopores are predominant with some wormlike mesopores coexistent, and the planar spacing is about 10 nm (Fig. 4a). Cross-section TEM image (Fig. 4b) indicates the nanosheet is about 50 nm thick, and composed of four layers of the mesopores, and the mesoporous structure is predominantly oriented parallel to the template sphere surface. The thickness can be tuned for example from 100 nm to 10 nm by changing the weight ratio of the silica sol to the PS template sphere (Fig. 4c, 4d). The pore structure of silica nanosheets was characterized by a surface area and porosity analyzer. There is an obvious hysteresis hoop at relative pressure 0.4–0.6 in nitrogen adsorption and desorption isotherm (Fig. 5a), which indicates the existence of hexagonal mesopores according to the H3 type pores.12 The mean pore size is 3.9 nm (Fig. 5b), and the BET surface area is 449 m2 g1. SAXD result (2q ¼ 0.9) reveals the corresponding inter-planar spacing d100 of about 10 nm (Fig. 6), consistent with the TEM result. No other evident peaks were found, indicating the mesopores are less ordered.
Fig. 5 (a) Nitrogen adsorption/desorption isotherm of the mesoporous silica nanosheets; and (b) the corresponding pore size distribution. The mesoporous silica nanosheets are synthesized by calcination in air for 2 h, followed by a further milling.
Fig. 6 SAXS pattern of the mesoporous silica nanosheets.
Amphiphilicity of mesoporous silica nanosheets
Fig. 4 TEM image of: (a) top view and (b) the cross-section (thickness is 50 nm) of the mesoporous silica nanosheets; mesoporous silica nanosheets with varied cross-section thickness of 100 nm (c) and 10 nm (d).
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Let us investigate dispersibility of the mesoporous silica nanosheets both in water and oil. Methylacrylate (MMA) was used as oil since it can be potentially polymerized. The mesoporous silica nanosheets are well dispersible both in water and in oil (Fig. 7a, 7b), indicating they are amphiphilic. The water and oil are J. Mater. Chem., 2009, 19, 3443–3448 | 3445
Fig. 7 Optical pictures of: (a, b) the mesoporous silica nanosheets dispersed in water (W) and MMA (O), respectively; (c) immiscible mixture of O (top) and W (bottom); (d) O/W mixture containing nanosheets without mesopores, the nanosheets are preferentially dispersed in W phase; (e, f) O/W and W/O emulsions with major W and O phases, respectively, stabilized with the same mesoporous silica nanosheets.
immiscible confirmed by the clear interface (Fig. 7c). A trace of methyl-orange was added into the water phase only for the guidance of eyes. In the presence of 1 wt.-% mesoporous nanosheets, a stable O/W emulsion is formed (Fig. 7e) when water is the major phase (90 vol.-%). The continuous phase is aqueous, confirmed by conductivity measurements. The suspension and emulsions can be stabilized for more than 1 h, and the picture was taken after 1 h. The droplet diameter is 10–20 mm (Fig. 8a). The diameter can be controlled by content of the nanosheets. For example, the diameter is 50–80 mm at 0.1 wt.-% nanosheets (Fig. 8a, inset image). After MMA was free radical polymerized to form a PMMA core, the emulsion is fixated. The PMMA core is enveloped with the silica nanosheets (Fig. 9a). In reverse, when
the oil is the major phase (90 vol.-%), a W/O emulsion is formed (Fig. 7f). The emulsion is insulated and the emulsion droplet diameter is 20–40 mm (Fig. 8b). In order to show the essential contribution of the mesopores to the amphiphilic performance, those silica nanosheets but without mesopores were added in the W/O mixture. The nanosheets are favorably dispersible in the bottom water phase, and the top oil phase is transparent (Fig. 7d). This verifies that the amphiphilic property is originated from the mesoporous silica. This phenomenon is different from Pickering emulsion based on slightly hydrophilic or hydrobobic particles.13 When the oil was changed from MMA to an alkane, for example paraffin, a stable O/W emulsion was also formed at high temperature. When the emulsion was cooled to room temperature, the melt paraffin core was solidified. The mesoporous silica nanosheets are coated onto the PS core (Fig. 9b). Pt/silica composite nanosheets and their heterogeneous catalysis performance Metallic hybrid mesoporous materials have been reported by many research groups.14 Since the mesoporous silica materials are amphiphilic, they can be used to support catalysts such metallic nanoparticles instead of traditional amphiphilic polymeric matrices.15 As an example, mesoporous Pt/silica composite nanosheets were accordingly prepared (Fig. 10a). Pt nanoparticles with a mean diameter 20 nm, are uniformly distributed in the silica matrix. The HR-TEM image reveals that the crystal lattice is 0.227 nm assigned to (111) (Fig. 10b), consistent with PDF 65-2868, 04-0802. Similarly, the heterogeneous catalysis performance of Pt/silica composite nanosheets are evaluated by catalyzing aerobic oxidation of benzyl alcohol in water at 60 C.15 Assisted by the amphiphilic mesopores, the less polar reagent benzyl alcohol is easily absorbed inside the mesopores and contacts Pt nanoparticles for being catalyzed. The product benzyl acid is water soluble, and escapes from the mesopores into a continuous aquoues phase. The yield of benzyl acid from benzyl alcohol is 80%. In our case, the amphiphilic mesoporous silica materials are more stable than the polymeric matrices at an elevating temperature. The heterogeneous catalysis at higher temperature is ongoing. Silica/carbon composite nanosheets
Fig. 8 Optical microscope images of two emulsions: (a) O/W (nanosheets content: 1wt.-%), inset image (nanosheets content: 1wt.-&) and (b) W/O (nanosheets content: 1wt.-%).
The amphiphilic performance of mesoporous silica can be used to assist other functional materials to be dispersible. Mesoporous silica/carbon composite nanosheets were synthesized to
Fig. 9 SEM images of two emulsions: (a) PMMA/W; (b) paraffin wax/W.
Fig. 10 (a) SEM and TEM image (inset) of Pt/silica nanosheets; (b) HRTEM image of Pt nanoparticle embedded in silica nanosheets.
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Fig. 11 (a, b) SEM and TEM images of the mesoporous silica/carbon composite nanosheets; (c, d) TEM images of the mesoporous carbon and silica nanosheets after selective removal of silica by dissolution with HF and carbon by calcination in air, respectively.
demonstrate the concept since the composite has many applications.16 Similar to the procedure for mesoporous silica nanosheets, soluble resol oligomer was added to the silica sol containing F127, the silica/carbon composite nanosheets (Fig. 11a) were thus prepared after the PS spheres and F127 were removed at 800 C under nitrogen followed by further milling. The composite nanosheets were conductive, implying carbon was continuous. The carbon was amorphous confirmed by XRD. The mesopores were also mainly hexagonal with some wormlike structure coexistent (Fig. 11b), confirmed by nitrogen adsorption and desorption isotherm (curve I, Fig. 12a).12 The mean pore size was 4.8 nm (curve I, Fig. 12b), and the surface area was 500 m2 g1, including a contribution of 164 m2 g1 from the micropores. Which component was continuous was determined by selective removal of one component from the silica/carbon composite. After the silica was selectively etched by HF, the corresponding carbon nanosheets were derived (Fig. 11c). From the adsorption and desorption isotherm (curve II, Fig. 12a), the slope of the curve at very low relative pressure near zero is higher; and slope of the curve between relative pressure 0.2 to 0.6 is lower. This indicates that the micropores are dominant. The total surface area is 350 m2 g1. The decreased area may be related with a partial collapse of the mesoporous carbon after removal of the support silica. On the other hand, after the carbon was calcined in air, the silica nanosheets are predominantly mesoporous (Fig. 11d), consistent of the H3 type isotherm (curve III, Fig. 12a). The mean size is 5.4 nm, and the surface area is 336 m2 g1. Therefore, both silica and carbon are continuous in the composite nanosheets. The silica content of silica/carbon nanosheets is about 48.6 wt.-% measured by TGA in air (Fig. 3d). The bi-continuity can be guaranteed in a broad silica/carbon ratio ranging 1/3 to 3/1.17 The mesoporous silica/ carbon composite nanosheets are well dispersible both in water and oil, respectively. Moreover, the composite nanosheets were used as an emulsifier to form stable O/W or W/O emulsion, similar to the mesoporous silica nanosheets (Fig. 13). This journal is ª The Royal Society of Chemistry 2009
Fig. 12 Nitrogen adsorption/desorption isotherms of three samples (a); and the corresponding pore size distribution (b). C (I): the mesoporous silica/carbon composite nanosheets; : (II): the mesoporous carbon nanosheets after removal of silica with HF; B (III) the mesoporous silica nanosheets after removal of carbon by calcination in air.
Fig. 13 Optical pictures of two emulsions: (a) O/W; and (b) W/O. Both emulsions are stabilized by the mesoporous silica/carbon composite nanosheets (nanosheets content: 1wt.-%), but with varied weight ratio of oil (MMA) to water.
Conclusions In conclusion, we have presented a simple approach towards massive synthesis of mesoporous silica composite nanosheets by milling the corresponding hollow spheres. The mesoporous silica nanosheets are amphiphilic, and dispersible in both water and oil. As a solid particulate emulsifer, stable emulsions are easily formed from immiscible liquid systems. Based on the amphiphilic performance, other functional materials can become dispersible J. Mater. Chem., 2009, 19, 3443–3448 | 3447
with the assistance of the mesoporous silica. As examples, mesoporous Pt/silica and carbon/silica composite nanosheets are also amphiphilic. Taking good advantage of such amphiphilic performance, the Pt/silica composite nanosheets show heterogeneous catalysis performance in aerobic oxidation of less polar benzyl alcohol to polar benzyl acid. It will be promising to explore such mesoporous nanosheets as multiple-functional additives for polymer nanocomposites.
Acknowledgements This work was supported by NSF of China (50573083, 50325313, 20128004 and 50521302), foundations from Chinese Academy of Sciences, and China Ministry of Science and Technology (200401-09, KJCX2-SW-H07 and 2003CB615600). We thank BASF company for their provision of F127. We thank Prof. Zhimin Liu for her valuable suggestion in heterogeneous catalysis.
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