COMMUNICATION pubs.acs.org/cm
Silica Nanotubes by Templated Thermolysis of Silicon Tetraacetate Xin Chen,*,† Andreas Berger,‡ Meiying Ge,† Sigrid Hopfe,‡ Ning Dai,† Ulrich G€osele,‡ Sabine Schlecht,*,§ and Martin Steinhart*,^ †
National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China ‡ Max-Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle/Saale, Germany § Justus-Liebig-Universit€at Giessen, Institut f€ur Anorganische und Analytische Chemie, Heinrich-Buff-Ring 58, 35392 Giessen, Germany ^ Institute for Chemistry, Universit€at Osnabr€uck, Barbarastrasse 7, 49069 Osnabr€uck, Germany KEYWORDS: nanotubes, silica, thermolysis, block copolymers, template synthesis, nanoporous alumina
ilica nanotubes (SNTs) prepared by sol gel chemistry1 or atomic layer deposition (ALD)2 inside nanoporous, shapedefining hard templates have been explored for a broad range of applications. Functionalized membrane configurations consisting of SNTs located in the pores of the hard templates may mimic ligand-gated ion channels1 or can be used as microreactor arrays.3 Released SNTs, which can bear different functional groups at their inner and outer surfaces, were applied in bioseparations and biocatalysis,4,5 whereas SNTs containing magnetic nanoparticles were also used for magnetic-field-assisted immunobinding and drug delivery.6 ALD requires special equipment as the surfaces to be coated are successively exposed to different gaseous precursors, and the uniformity of the coatings deteriorates in high-aspect ratio pores far away from the pore mouths. Sol gel approaches under acidic conditions, in which commonly tetraethyl orthosilicate (TEOS) is employed as a silica precursor, involve hydrolysis steps and require careful adjustment of the pH value,1 aging times of preaged gels, dipping times, and temperatures.7,8 In alternative approaches, toxic chemicals or chemicals difficult to handle are processed, such as solutions of SiCl4 in tetrachloromethane.9 We used silicon tetraacetate (SiAc4), which decomposes into silica and acetic anhydride between 160 and 170 C,10 as the silicon precursor in the synthesis of SNTs inside the pores of selfordered nanoporous anodic aluminum oxide (AAO) hard templates prepared by two-step anodization.11,12 While hydrolytic cross-linking of silica precursors is a delicate process sensitively depending on the conditions applied, thermolysis leads to complete conversion of the precursor within a well-defined temperature range. Thin silica films on smooth substrates obtained by thermolysis of Si(Ac)4 showed, as compared to silica films obtained by hydrolytic sol gel chemistry, significantly enhanced dynamic hardness.13 As hydrolysis of SiAc4 yields silica too, it is not necessary to work under dry conditions. As in previous synthetic approaches to mesoporous silica nanorods based on hydrolytic sol gel chemistry under acidic conditions,14,15 an amphiphilic block copolymer (BCP), polystyrene-block-poly (ethylene oxide) (PS-b-PEO; Mn (PS) = 32.0 kg/mol, Mn (PEO) = 11.0 kg/mol; polydispersity index =1.06) served as a structure-directing soft template. Mixtures of SiAc4 and PS-b-PEO were dissolved in THF or a mixture of THF and toluene (at a weight ratio of 3:1) (weight ratios for SiAc4:PS-b-PEO:THF/toluene = 4:1:16). Self-ordered
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AAO with a mean pore diameter of 60 nm11 and a pore depth of 10 μm was dipped into the thus-obtained homogeneous solutions. After drying in air, the infiltrated AAO membranes were put in furnaces, heated to 200 C at a rate of 1.0 K/min, and kept at this temperature in air for 24 h to decompose the SiAc4. Transmission electron microscopy (TEM)16 on silica/PS-b-PEO nanorods released by etching the AAO with H3PO4 (10 wt %) for 8 h at room temperature followed by five centrifugation/washing cycles with deionized water confirmed their nontubular, solid nature (Figure 1a). To elucidate the spatial distribution of silica and PS-b-PEO across the silica/PS-b-PEO nanorods, electron energy loss spectra (EELS) were recorded along a path perpendicular to the long axis of a silica/PS-b-PEO nanorod. Panel (b) of Figure 1 displays the relative elemental composition, i.e., the proportions of Si, C, and O, derived from the EEL spectra as a function of the position. The highest Si content was detected at the edges of the nanorod, whereas the proportion of carbon is highest in the nanorod core confined by the silica-rich regions. The regions with maximum oxygen content coincide with the regions with maximum silicon content, while in the carbon-rich innermost nanorod core little oxygen is present. Clearly, the outermost shell of the silica/PS-bPEO nanorod predominantly consists of silica. The energy-loss near-edge structure (ELNES) of the Si-L2,3 peak of a representative EEL-spectrum taken at the edge of a silica/PS-b-PEO nanorod (Figure 1c) shows the typical ELNES features of silicon oxygen bonds with peaks at 108 and 115 eV. As obvious from the distribution of oxygen, the PEO is located next to the outermost silica shell, whereas the innermost core of the silica/PS-b-PEO nanorod predominantly consists of PS. We interpret these findings as follows. Like TEOS,17 SiAc4 selectively segregates to the polar PEO domains, and the PEO/ SiAc4 mixture in turn segregates to the likewise polar AAO pore walls. Within the PEO/SiAc4 domains the SiAc4 is enriched at the AAO pore walls because the acetate groups of SiAc4 should have a higher capability for forming hydrogen bonds with the AAO surface hydroxyl groups than PEO. Moreover, the conformational space available to polymers at hard confining Received: March 10, 2011 Revised: June 7, 2011 Published: June 20, 2011 3129
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Figure 1. Released silica/PS-b-PEO nanorods. (a) TEM image; (b) EELS-derived relative compositions across a silica/PS-b-PEO nanorod; (c) ELNES of the Si-L2,3 EELS peak taken at the edge of a silica/PS-b-PEO nanorod.
Figure 2. Cross-sectional specimen of AAO with a pore diameter of 60 nm containing SNTs (specimen plane perpendicular to long axes of AAO pores and SNTs, view along direction of long axes of AAO pores and SNTs). (a) TEM image; (b) EDX intensity map of the KR-peak of Si.
interfaces such as the AAO pore walls is significantly reduced as compared to that available to unconfined polymers. Therefore, the low molecular mass compound SiAc4 enriches at the AAO pore walls also for entropic reasons. When the SiAc4 is thermolized, the SiO2 layer formed at the pore walls does not decrease the conformational entropy of the PEO blocks, as is the case if silica particles are formed inside the PEO domains. This aspect appears to be important as it was predicted that formation of nanoparticles within BCPs confined to cylindrical pores can even enforce morphological transitions.18 To remove any organic material, AAO containing silica/PS-bPEO nanorods was heated to 550 C at a rate of 1 K/min and kept at this temperature for 6 h. Then, the heating was switched off, and the samples were removed from the furnace after they had cooled to room temperature. Panel (a) of Figure 2 shows a TEM image of a section across AAO containing SNTs perpendicular to the pore axes, and panel (b) of Figure 2 shows the corresponding spatial distribution of Si obtained by energydispersive X-ray (EDX) spectroscopy mapping of the intensity of the Si KR peak at ∼1.47 keV.16 SNTs with ∼7.5 nm thick walls appearing brighter than the surrounding AAO and uniform wall thickness are seen inside the AAO pores with a diameter of ∼60 nm. The SNTs are free of defects and are tightly connected to the AAO pore walls. Figure 3 shows TEM images of SNTs that were released as described above. Their openings initially located at the AAO pore mouths are shown in panel (a) of Figure 3, and their hemispherical tips, which are replicas of the likewise hemispherical AAO pore bottoms are shown in panel (b) of in Figure 3. The length of the SNTs, which amounts to ∼9.5 μm (Figure 3c), is in reasonable agreement with the depth of the pores in the AAO hard template. This outcome indicates that the SNTs have good
mechanical properties as they did not break in the centrifugation steps carried out to remove the aqueous H3PO4 solution used to etch the AAO. Moreover, the apparent wall thickness of ∼6.0 nm of the released SNTs is in reasonable agreement with the value determined from the cross-sectional specimen shown in panel (a) of Figure 2. Therefore, the released SNTs were perfect replicas of the AAO pores and uniform along their entire length. Remarkably, after dipping AAO containing SNTs into a ∼0.05 wt % aqueous suspension of gold nanoparticles ∼20 nm in diameter and subsequent release of the SNTs, gold nanoparticles were still located at the tips of the SNTs (Figure 3b) and occasionally near their pore mouths (Figure 3a). It should be noted that thermolysis of Si(Ac)4 inside AAO in the absence of a structure-directing BCP soft template yields solid silica nanorods with grainy structure. For example, the silica nanorods shown in panel (d) of Figure 3 were obtained in exactly the same way as the SNTs shown in panels (a c) of Figure 3 except that no BCP was added to the solution infiltrated into the AAO hard templates. BCPs may develop complex nanoscopic morphologies in the two-dimensional confinement of AAO nanopores characterized, for example, by internal helices. Replication by classical hydrolytic sol gel chemistry yielded silica nanorods having helical pores.19 Tentative experiments revealed that complex internal domain structures of the PS-b-PEO soft template can also be replicated by thermolysis of SiAc4. For example, the released SNT shown in Figure 4 was prepared exactly in the way described above; the only difference being that AAO with a pore diameter of 180 nm12 was used as a hard template. It contains a low-pitch helix with a pitch of ∼70 nm spanning across the entire SNT as well as an inner highpitch double helix in the center with a pitch of ∼140 nm and a helix diameter of ∼40 nm. The strand diameter of all helices is ∼15 nm. The space between the outer SNT shell and the helices is empty, and the helices themselves appear to be hollow. Assuming that PEO/SiAc4 forms a matrix surrounding helical PS domains, we conclude that formation of silica is strongly preferred at already existing interfaces, that is, at the AAO pore walls and at the internal boundary between the PEO/SiAc4 and PS domains. We speculate that reduction in conformational entropy of the PEO chains associated with the formation of silica nanoparticles within the PEO domains is avoided in this way.20 In conclusion, thermolysis of silicon tetraacetate in selfordered AAO hard templates assisted by amphiphilic BCP soft templates yielded silica nanotubes as well as complex onedimensional silica nanostructures. Silica forms preferentially at the AAO pore walls and at internal BCP domain boundaries to avoid the entropic penalty associated with the formation of silica entities within the polar BCP domains. We anticipate that the 3130
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Figure 3. TEM images of (a c) SNTs and (d) silica nanorods released from AAO with a pore diameter of 60 nm and a pore depth of 10 μm. (a) Openings of SNTs initially located at the AAO pore mouths; (b) tips of SNTs, which are replicas of the hemispherical pore bottoms of the AAO hard template. The AAO containing the SNTs was immersed in a gold nanoparticle suspension before etching the AAO. (c) Large-field view of released SNTs. (d) Silica nanorods prepared by thermolysis of SiAc4 inside an AAO hard template in the absence of a structure-directing BCP soft template.
Figure 4. TEM image of a SNT released from an AAO hard template with a pore diameter of 180 nm and a pore depth of 10 μm.
synthetic approach reported here might enable the encapsulation of pharmaceuticals into silica nanocontainers. Moreover, we expect that it can be extended to prepare three-dimensional silica specimens with ordered nanoscopic fine structure by thermolyis of SiAc4 templated by bulk BCPs.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (X.C.); Sabine.Schlecht@ anorg.chemie.uni-giessen.de (S.S.);
[email protected] (M.S.).
’ ACKNOWLEDGMENT Financial support by the German Research Foundation (Priority Program 1165) as well as additional funding by the National Basic Research Program of China (2010CB933700 and 2011CBA00900), NSFC (20704042) and 863 (2009AA03Z412) is gratefully acknowledged. The authors thank R. Scholz for technical support and S. Sklarek for preparation of AAO. ’ REFERENCES (1) Steinle, E. D.; Mitchell, D. T.; Wirtz, M.; Lee, S. B.; Young, V. Y.; Martin, C. R. Anal. Chem. 2002, 74, 2416–2422. (2) Knez, M.; Nielsch, K.; Niinist€o, L. Adv. Mater. 2007, 19, 3425–3438. (3) Choi, K. Y.; Han, J. J.; He, B.; Lee, S. B. J. Am. Chem. Soc. 2008, 130, 3920–3926. (4) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; S€oderlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864–11865.
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