Synthesis of Silica Nanotubes with Orientation Controlled Mesopores

Article pubs.acs.org/cm

Synthesis of Silica Nanotubes with Orientation Controlled Mesopores in Porous Membranes via Interfacial Growth Anfeng Zhang, Keke Hou, Lin Gu, Chengyi Dai, Min Liu, Chunshan Song, and Xinwen Guo* State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R. China ABSTRACT: Silica nanotubes with mesopores perforating the wall were synthesized on the channel surface of porous anodic alumina (PAA) or polycarbonate membranes via interfacial growth. Transmission and scanning electron microscopy characterizations indicated that the mesopore orientation can be regulated from perpendicular to the wall to circling the axis when the thickness of the silica nanotube wall increases from about 15 to 40−60 nm, without any modification of the channel surface or external forces, just by optimizing the amount of the tetraethoxysilane confined in the channels of PAA. Mesopores in the silica nanotube wall arrange hexagonally when the diameter of the silica nanotubes is larger than 200 nm.

KEYWORDS: silica nanotube, mesoporous film, mesopore orientation, perpendicular, interfacial growth solid−liquid interface40,41 or by electrochemical strategy have been reported,42−44 the preparation of mesoporous thin films with one-dimensional alignment of the surfactant-templated silica mesochannels oriented perpendicular to the film surface has proved to be very difficult. Correspondingly, to our knowledge, silica nanotubes with hexagonally arranged mesopores perforating the wall have not been synthesized in PAA or PC membranes. Zhao et al. synthesized mesh-like vesicles of polyisoprene[1,4-addition]-b-polystyrene-b-poly(2vinyl pyridine) triblock copolymer in PAA membrane, but they only synthesized a polymer without silica.45 Recently, we reported that hollow silica spheres with mesopores perforating the shell can be synthesized at the benzene−water interface,46 which opened up a new approach for the synthesis of mesoporous film with perpendicular mesochannels on the chemically non-neutral surface. Inspired by this, we demonstrate here that silica nanotubes with mesopores perforating the wall can be synthesized in the channel of PAA or PC membranes without any modification. This means that perpendicular mesoporous membranes can be synthesized on the channel surface of porous membranes. We used triblock surfactant P123 as the template of mesopores, which has HO(C2H4O)20(C3H6O)70(C2H4O)20OH as the average composition. Instead of dipping conventional synthesis solutions,28−39 our strategy is to slowly add TEOS and a P123 acid solution into the channel of PAA one after the other. According to the laminar flow principle,47 this procedure will result in a TEOS−P123 solution interface where TEOS hydrolyzes and self-assembles with P123, leaving a thin layer of mesoporous silica on the channel surface of PAA to form silica

1. INTRODUCTION Recently, the control of morphology and mesopore orientation of mesoporous silica has become a hot topic. Mesoporous silica spheres,1,2 films,3,4 fibers,5 and tubes6 have been prepared by many research groups. Among the morphologies, silica nanotubes are particularly important for fundamental science and potential applications such as adsorption,7−9 separation,10 catalysts,11 drug delivery, and controlled release.12,13 Silica nanotubes have been prepared by means of soft templates, such as surfactants,14−21 acids,22,23 block copolymer,24 and gel systems.25−27 Among them, mesoporous silica nanotubes are particularly important because of their hierarchical structure. Up to now, various orientations of mesopores in the silica wall have been realized by soft templates; that is, silica nanotubes with mesochannels either run circularly around the tubular axis20 or parallel to the tubular axis,19 even with radially orientated mesopores.21 However, it remains a challenge to control the diameter and aspect ratio of the mesostructured silica nanotubes. Well-distributed tubular porous media such as porous anodic alumina (PAA) and/or polycarbonate (PC) membranes have been extensively used as template to fabricate uniform mesostructured silica nanorods5,28−35 and nanotubes36−39 with regulated diameter and aspect ratio. However, in the mesoporous silica nanotubes, the mesochannels of mesopores usually run circularly around the tubular axis.37,39 Wu et al. have synthesized silica nanotubes with chiral mesopores, such as single- and double-helical geometries, inside PAA channels, which is equivalent to rolling up a two-dimensional, hexagonal SBA-15 mesostructured thin film on flat substrates with the long axes of the mesochannels oriented parallel to the substrate surface.38 Although some recent strategies to control the orientation of mesopores through chemical modification of the © 2012 American Chemical Society

Received: September 9, 2011 Published: February 24, 2012 1005

dx.doi.org/10.1021/cm300242n | Chem. Mater. 2012, 24, 1005−1010

Chemistry of Materials

Article

Scheme 1. Schematic Diagram for the Preparation of Silica Nanotubes

intervals from 0.04−0.3. Pore size distribution curves were calculated by the density functional theory (DFT) from the adsorption branch. The total pore volume was estimated from the amount adsorbed at the relative pressure of 0.99. Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max 2400 diffractometer, which employed Cu Kα radiation.

nanotubes (Scheme 1). It will be shown that silica nanotubes with mesopores perforating the wall can be synthesized under the right conditions, and the mesochannel orientation can be regulated by adjusting the synthesis conditions.

2. EXPERIMENTAL SECTION Chemicals. All materials were of analytical grade and were used as received without any further purification. Triblock copolymer (EO)20(PO)70(EO)20 (P123) was purchased from Sigma-Aldrich. Tetraethoxysilane (TEOS), hydrochloric acid, and other reagents were obtained from Shanghai Chemical Reagent Inc. of the Chinese Medicine Group. The PAA membranes were purchased from Whatman (membrane diameter ca. 4.3 cm, channel diameter ca. 200 nm, thickness ca. 60 μm), and PC membranes were purchased from Millipore (membrane diameter ca. 4.3 cm, channel diameter ca. 50, 100, or 200 nm, thickness ca. 10 μm). The membranes were used without any modifications. Synthesis of Silica Nanotubes. The P123 solution was prepared according to the literature with some modification.48 At first, P123 (1.0 g) was dissolved in a mixture of HCl (0.26 g, 12 M aqueous solution) and water (8.0 g) and stirred at 38 °C for about 1 h. The processing details for fabricating silica nanotubes are presented in Scheme 1. The alumina membrane became transparent after TEOS was added for 5 min, which means that the channels were completely filled. Then a certain amount of P123 solution (2.0, 1.0, or 0.5 mL) was spread on the top surface of PAA (or PC) in uniform thickness. Because of the pressure produced from the P123 solution layer, P123 solution begins to flow into the channels of PAA. Four minutes later, part of the TEOS in the channels of PAA (or PC) was expelled by the P123 solution, which determined the amount of TEOS attached on the PAA (or PC) channels. Then a small amount of liquid paraffin wax (to a thickness of 1 mm) was put on one side of the surfaces of the PAA (or PC), and the other side was dipped into a P123 acid solution at 38 °C for 8 h. After the sol−gel process, a mesoporous silica film on the channel surface of the membrane was formed. Then the surfactant and alumina were removed by calcination and leaching with hydrochloric acid, respectively, and silica nanotubes with uniform morphologies were obtained. Characterization. Transmission electron microscopy (TEM) images were taken on a Tecnai G2 20 S-twin instrument (FEI Company) with an acceleration voltage of 200 kV. The samples for TEM analysis were prepared by dipping the carbon-coated copper grids into the ethanol solutions of silica and drying at ambient condition. Scanning electron microscopy (SEM) images were recorded using a Hitachi S-4800 instrument. Nitrogen sorption isotherms were measured on a Autosorb iQ sorption analyzer at liquid nitrogen temperature. Prior to the measurements, the samples were degassed at a temperature of 250 °C overnight. Their specific surface areas were determined by the Brunauer−Emmett−Teller (BET) method, which were calculated from the adsorption data in the relative pressure

3. RESULTS AND DISCUSSION 3.1. Silica Nanotubes Synthesized in the Channels of PAA Membranes. As typically shown in SEM images (Figure 1), the length and diameter of the columnar mesoporous silica

Figure 1. SEM micrographs of the silica nanotubes. (a) Overview; (b) separated nanotubes.

are estimated as ca. 45 μm (Figure 1a) and ca. 200 nm (Figure 1b), respectively. The interior cavity is clearly discerned in the open ends of the silica nanotubes. These values and morphologies of the silica nanotubes depend on the channel structure of PAA membrane. The local structure of the silica nanotubes was examined by TEM and high magnification SEM measurements. Three types of mesostructures were observed for the silica nanotubes synthesized in PAA membranes depending on the volume of P123 solution (2.0, 1.0, or 0.5 mL) spread on the surface to replace TEOS in the channel. Those structures are mesopores perforating the wall of the silica nanotubes (Figure 2), mesochannels circling the axis of the silica nanotube (Figure 3b,c), and a mixture of the perpendicular, circling, and/or parallel alignment of the mesopores in the wall (Figure 3a). When 2.0 mL P123 solution was spread on the PAA, silica nanotubes with mesopores perforating the wall were obtained. The TEM image of the open end (Figure 2a) clearly shows that the mesopores perforate the wall and arrange hexagonally; the diameter of the mesopores is about 10 nm (Figure 2b). To further prove the alignment of the mesopores in the wall, high magnification SEM images were obtained. Figure 2c shows that 1006



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the channels of the mesopores are perpendicular to the wall and that the mesopores in the silica nanotubes of larger diameter arrange more orderly. The SEM image of the open end of the silica nanotube (Figure 2d) also confirms that the mesopores perforate the wall. The thickness of the wall is about 15 nm, which is apparently less than two repeat distances of the mesopores in the wall (Figure 2a,d). Although sometimes the mesopores observed from the outer surface of the silica nanotube are not clear, under the outer surface they still arrange hexagonally in the nanotubes, which have a diameter larger than 200 nm (Figure 2e). It means that the alumina attached to the silica nanotubes sometimes was not dissolved completely. Interestingly, the mesopores in the enclosed part of some silica nanotubes are still perpendicular to the wall (Figure 2f). The results indicate that a whole perpendicular mesoporous membrane was created on the channel surface of PAA via interface growth to form silica nanotubes. The porous nature of the silica nanotubes with mesopores perforating the wall is revealed by nitrogen sorption measurement. As seen in Figure 4, the N2 sorption isotherms of the

Figure 2. TEM and SEM micrographs of the silica nanotubes with mesopores perforating the wall. (a) TEM, broken part of the silica nanotube. (b) TEM, hexagonally arranged mesopores in the wall. (c) SEM, hexagonally arranged mesopores perpendicular to the wall. (d) SEM, cylindrical mesopores perforating the wall in the open end. (e) SEM, hexagonally arranged mesopores under the outer surface. (f) Mesopores in the enclosed end of the silica nanotube.

Figure 4. N2 adsorption/desorption isotherm of the silica nanotubes with mesopores perforating the wall. The inset shows the adsorption pore-size distribution determined by the DFT method.

silica nanotubes is of type IV and displays a small hysteresis loop due to the short channels of the mesopores in the silica nanotubes. The average length/diameter ratio of the mesopores is about 15 nm/10 nm, which makes the mesochannels seem to be spherical cavities. Most of the mesopores have diameters of about 8.0 nm, and some have a diameter of about 12 nm (Figure 4, inset). These values are much larger than those of SBA-15 synthesized under the same conditions.48 During the leaching of alumina in hydrochloric acid solution at 85 °C,the diameter of the mesopores may be expanded by the hydrothernal treatment.46 Because the thickness of the alumina attached on silica nanotubes is different, the expansion effect on the mesopores is different. As a consequence, the pore size distribution curve calculated from the adsorption branch is not uniform (Figure 4, inset). Furthermore, the specific surface area and pore volume are 294 m2 g−1 and 0.52 cm3 g−1, respectively. The surface area is much smaller than that of SBA-15 synthesized under the same conditions,48 partly because the alumina attached on the silica nanotubes was not dissolved completely. The diameters of the silica nanotubes are not uniform; some are smaller than 200 nm. The arrangement of the mesopores in

Figure 3. TEM images. (a) Silica nanotube with mesostructure at transition state. (b) Silica nanotube with mesochannel circling the axis. (c) Enclosed part of some silica nanotubes. 1007



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area and pore volume were 408 m2 g−1 and 0.748 cm3 g−1, respectively. The hexagonal arrangement of the mesopores with orientation circling the axis were not obviously changed when the diameter of the silica nanotube was larger than 100 nm. Also, the influence of the hydrothermal treatment on the arrangement of the mesopores is little, which was confirmed by the XRD analysis, that is, one peak was shown in the XRD pattern of the samples (Figure 6), which can be indexed as the (100) diffractions associated with a 2D hexagonal symmetry (p6mm).

the silica nanotubes with diameter smaller than 200 nm is not hexagonal (Figure 2c,f). Also because of the hydrothermal treatment at 85 °C during the leaching of alumina, the arrangement of the mesopores becomes irregular (Figure 4, inset), which can be confirmed by the XRD analysis, that is, no well-resolved peaks in the XRD pattern (not shown here) of the samples. Decreasing the amount of P123 solution spread on PAA to 1.0 mL, which means that the TEOS attached on the channel surface of PAA increased, silica nanotubes with a mixture of perpendicular, circling, and/or parallel alignment of the mesopores in the wall were obtained. Figure 3a shows that three types of mesostructure coexist in a single nanotube, which means that the alignment of the mesopores in the wall is at a transition state under the synthesis conditions. The diameter of a single silica nanotube changed to some extent, which may be affected by the phase transition of the mesostructure of the wall. Further decreasing the amount of P123 solution spread on PAA to 0.5 mL, which means that the TEOS attached on the channel surface of PAA further increased, silica nanotubes with mesopores perforating the wall were no longer observed. Instead, mesopores with orientation circling around the axis were in the silica nanotube wall. As seen in Figure 3b, the wall of the silica nanotube became thicker, about 40−60 nm, or 4−6 times the repeat distance of the mesopores in the wall. Interestingly, mesochannels also circle the axis at the enclosed end of some silica nanotubes (Figure 4c), which means that parallel-oriented mesoporous membranes were synthesized on the channel surface of PAA.38 The N2 sorption isotherm of the silica nanotubes with mesopore orientation circling the axis are also of type IV and display an obvious hysteresis loop (Figure 5), indicating the

Figure 6. Small-angle X-ray diffraction pattern of the silica nanotubes with mesochannels circling the axis.

The above experiments indicate that the mesostructure of the silica nanotubes changes with the thickness of the wall. As is well-known, during the synthesis of SBA-15, flocs of spherical micelles are first held together by the polymerizing silica. Then the structure of these flocs evolves from spherical to cylindrical hexagonally packed micelles.50 However, during the synthesis of silica nanotubes in PAA by this method, flocs hydrolyzed from TEOS were limited by the confinement of the PAA channels. When the amount of TEOS attached on the channel surface was small, there were not enough flocs to form long cylindrical channels. Then the flocs which were attached on the channel surface arranged to perpendicular films with a thickness of about 15 nm. With increasing amount of TEOS attached on the channel surface, the flocs confined in the channels of PAA increased and began to form cylindrical channels parallel to the channel surface of PAA. 3.2. Silica Nanotubes Synthesized in the Channels of PC Membranes. By controlling the amount of TEOS attached on the channel surface of PC membranes, silica nanotubes with mesopores perforating the wall vertically can be synthesized in the same way. As shown in the SEM images (Figure 7), the length of the columnar mesoporous silica is estimated as

Figure 5. N2 adsorption/desorption isotherm of the silica nanotubes with mesochannels circling the axis. The inset shows the adsorption pore-size distribution determined by the DFT method.

presence of mesopores.49 During the leaching of alumina in hydrochloric acid solution at 85 °C, the diameters of the mesopores were also expanded. Because the mesochannels circle the axis and only the outer layer of mesopores is attached on the alumina, the expansion effect on the mesopores is almost the same. As a consequence, the pore size distribution calculated from the adsorption branch is more uniform (Figure 5, inset) than that of the silica nanotubes with mesopores perforating the wall (Figure 4, inset). Most of the mesopores are about 7.0 nm, still larger than that of SBA-15 synthesized under the same conditions.48 Furthermore, the specific surface

Figure 7. SEM micrographs of the silica nanotubes synthesized in polycarbonate membranes. 1008



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approximately 10 μm (Figure 7a). The interior cavity is also clearly discerned in the open ends of the silica nanotubes (Figure 7b). Figure 8 shows a series of TEM images of the silica nanotubes formed inside the PC channels of varying diameters.

PC membranes, the hexagonally arranged mesopores in the silica nanotube wall become disordered. In summary, we confirmed that silica nanotubes with mesopores perforating the wall can be synthesized on the channel surface of PAA or PC membranes via interfacial growth. When the diameter of PAA or PC channels is larger than 200 nm, mesopores in the silica nanotube wall arrange hexagonally. In addition, the mesopore orientation of the silica nanotubes can be regulated without any modification of the channel surface or external forces, just by optimizing the amount of TEOS confined in the channels of PAA. When the thickness of the silica nanotube walls increases from about 15 to 40−60 nm, the mesochannel orientation shifts from perpendicular to circling the axis. The knowledge obtained in this work provides an opportunity to control the mesopore orientation of functional materials, such as perpendicular mesoporous membranes on various substrates, which are carried out in our group.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 8. TEM micrographs of the silica nanotubes synthesized in polycarbonate membranes with different diameter channels. (a) 200 nm; (b) 100 nm; (c) 50 nm.

With the diameter of PC channels decreasing from 300 to 50 nm, the mesostructure of the silica nanotubes changes from hexagonally arranged mesopores (Figure 8a) to disordered mesopores (Figure 8c). At the open ends of the silica nanotubes of 100 nm, the mesostructure is at a transition state (Figure 8b). Scheme 2 summarizes the mesostructure transition of the mesopores as a function of the cylinder channel confinement. Scheme 2. Schematic Structural Correlation between Mesostructure Formed on a Flat Substrate and within a Cylindrical Confinement

Viewed in this way, the 2D hexagonally arranged mesostructure, which forms vertically on a planar substrate, is the limiting case that would be observed when the diameter of the confining cylinder channel is infinite. All of the silica nanotubes with various diameters can be geometrically constructed by coiling up perpendicular membranes in different diameters. This paradigm provides a simple rationale of how the boundary symmetry (flat plane versus cylinder) changes with decreasing cylinder diameter. To counteract the curvature impact of the cylinder, with the decrease of the channel diameter of PAA or 1009



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Synthesis of Silica Nanotubes with Orientation Controlled Mesopores

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