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Surface-Supported, Highly Ordered Macroporous Crystalline TiO2 Thin Films Robust up to 1000 °C
Scheme 1. Self Assembly of Primary Titania Sol Particle onto PEO-PS-PEO Micelles
Ya-Jun Cheng,† Linjie Zhi,† Werner Steffen,† and Jochen S. Gutmann*,†,‡ Max-Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany, and Institute of Physical Chemistry, UniVersity of Mainz, Welderweg 11, D-55099, Mainz, Germany ReceiVed December 23, 2007 ReVised Manuscript ReceiVed September 9, 2008
The synthesis of mesoporous and macroporous TiO2 thin films of anatase phase has attracted considerable interest because of their potential applications in photocatalysis, photovoltaics, gas sensing, lithium ion batteries, and photonic crystals.1-7 Mesoporous materials have a pore size in the range of 2-50 nm, whereas macroporous materials possess a pore size larger than 50 nm.8 Even though there has been a significant progress in synthesizing ordered mesoporous TiO2 materials by using an amphiphilic PEO-PPO-PEO triblock copolymer or Brij polymer as a templating agent, coupled with an evaporation induced self-assembly (EISA) process,8-15 the synthesis of macroporous films still remains very difficult with this strategy. Recently, macroporous TiO2 films have been reported when a sol-gel process is accompanied by a phase separation process; however, these results are not satisfactory because the pores are just randomly orientated and the pore size distribution is poorly controlled.16-18 In our opinion, the main difficulty in the * Corresponding author. E-mail:
[email protected]. Fax: (49) 6131-379-100. † Max-Planck Institute for Polymer Research. ‡ University of Mainz.
(1) Figueroa, O. L.; Lee, C.; Akbar, S. A.; Szabo, N. F.; Trimboli, J. A.; Dutta, P. K.; Sawaki, N.; Soliman, A. A.; Verweij, H. Sens. Actuators, B 2005, 107, 839–848. (2) Fujishima, A.; Zhang, X. T. Proc. Jpn. Acad., B 2005, 81, 33–42. (3) Gratzel, M. J. Photochem. Photobiol., A 2004, 164, 3–14. (4) Heller, A. Acc. Chem. Res. 1995, 28, 503–508. (5) Henderson, M. A. J. Phys. Chem. B 2005, 109, 12062–12070. (6) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T Chem. ReV. 1995, 95, 735– 758. (7) Stein, A.; Schroden, R. C. Curr. Opin. Solid State Mater. 2001, 5, 553–564. (8) Soler-illia, G. J. D.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093–4138. (9) Bartl, M. H.; Boettcher, S. W.; Frindell, K. L.; Stucky, G. D. Acc. Chem. Res. 2005, 38, 263–271. (10) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. AdV. Mater. 1999, 11, 579–n/a. (11) Choi, S. Y.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A. AdV. Funct. Mater. 2004, 14, 335–344. (12) Choi, S. Y.; Mamak, M.; Speakman, S.; Chopra, N.; Ozin, G. A. Small 2005, 1, 226–232. (13) Crepaldi, E. L.; Soler-Illia, G.; Grosso, D.; Cagnol, F.; Ribot, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125, 9770–9786. (14) Soler-Illia, G.; Crepaldi, E. L.; Grosso, D.; Sanchez, C. Curr. Opin. Colloid Interface Sci. 2003, 8, 109–126. (15) Yang, P. D.; Zhao, D. Y.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Nature 1998, 396, 152–155. (16) Fuertes, M. C.; Soler-Illia, G. Chem. Mater. 2006, 18, 2109–2117. (17) Angelome, P. C.; Fuertes, M. C.; Soler-Illia, G. AdV. Mater. 2006, 18, 2397–2402. (18) Nakanishi, K. Bull. Chem. Soc. Jpn. 2006, 79, 673–691.
synthesis of macroporous films is mainly due to restrictions in the accessible molecular weight of PEO-PPO-PEO block copolymers, which is often too low to form structures with a size of hundreds of nanometers thus limitng facile use of KLE block copolymer to smaller template structures. Furthermore, there is only one methyl group difference between the PEO and PPO structural repeating unit, leading to a small hydrophobicity/hydrophilicity difference. Besides pore size control, the retention of structural control gets progressively more difficult with increasing crystallization temperature. Even though some groups reported a so-called low temperature procedure to convert amorphous TiO2 into the crystalline anatase phase at temperatures lower than 450 °C, a complete crystallization of amorphous TiO2 is normally conducted at temperatures around or above 450 °C.11,19-22 However, porous structures tend to collapse at temperatures above 450 °C because the wall thickness is commonly too thin.20,22 Because of these restrictions, ordered macroporous TiO2 films are mainly achieved via an alternative strategy based on colloidal crystal templates.7,23,24 These strategies, however, are complicated multistep processes, in which organic polymer or SiO2 spheres need to be synthesized and self-assembled into colloidal crystal packing first, followed by incorporation of titania precursors and further removal of the templates. Especially, when SiO2 is used as a colloid template, HF, a hazardous substance, is needed to etch away SiO2 to produce macroporous TiO27,23 To simplify the colloidal templating process, we report herein a new strategy to achieve ordered crystalline macroporous TiO2 thin films through a convenient one-step approach. The resulting macroporous TiO2 structure exhibits an unusual thermal stability and retained its structural integrity during calcination at temperatures up to 1000 °C. Instead of synthesizing a separate polystyrene colloidal template, a symmetric triblock copolymer of PEO-PS-PEO was designed and synthesized.25 To achieve a structured hybrid sample, we coupled a good-poor solvent-pair-induced phase separation process with sol-gel chemistry to produce (19) Bosc, F.; Ayral, A.; Albouy, P. A.; Guizard, C. Chem. Mater. 2003, 15, 2463–2468. (20) Crepaldi, E. L.; Soler-Illia, G.; Grosso, D.; Sanchez, M. New J. Chem. 2003, 27, 9–13. (21) Kirsch, B. L.; Richman, E. K.; Riley, A. E.; Tolbert, S. H. J. Phys. Chem. B 2004, 108, 12698–12706. (22) Smarsly, B.; Grosso, D.; Brezesinski, T.; Pinna, N.; Boissiere, C.; Antonietti, M.; Sanchez, C. Chem. Mater. 2004, 16, 2948–2952. (23) Long, J. W.; Dunn, B.; Rolison, D. R.; White, H. S Chem. ReV. 2004, 104, 4463–4492. (24) Turner, M. E.; Trentler, T. J.; Colvin, V. L. AdV. Mater. 2001, 13, 180–183. (25) Floudas, G.; Tsitsilianis, C. Macromolecules 1997, 30, 4381–4390.
10.1021/cm703669d CCC: $40.75 2008 American Chemical Society Published on Web 10/17/2008
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Chem. Mater., Vol. 20, No. 21, 2008 6581
Figure 1. (a) Dynamic light scattering (DLS) profile and (b) corresponding size distribution of the aggreagtes.
polymer-titania composite nanostructures.26-28 The triblock copolymer undergoes a good-poor solvent-pair-induced phase separation in a solution mixture of DMF and concentrated HCl (Scheme 1), where DMF is a good solvent for both PEO and PS blocks; HCl is, however, a poor solvent for the PS block. The middle hydrophobic PS block is self-assembled into spherical polystyrene micelles decorated with hydrophilic PEO coronas because of the surface tension between the PS block and HCl solution. The PEO corona further incorporates titanium-tetraisopropoxide (TTIP), a titania precursor, via coordination bonds, where TTIP can be hydrolyzed and condensed into amorphous titania network with HCl as a catalyst.26-30 Consequently, these micelles are decorated at their surface with small primary titania sol-gel particles. Because of a significant hydrophobicity/hydrophilicity difference between the PS and PEO block, micelles with sizes between 200 and 400 nm are formed in solution, which is confirmed by dynamic light scattering as shown in Figure 1.31,32 The size distribution obtained from a CONTIN-based fit to the light scattern profile exhibits two pronounced peaks indicating particles in solution with very different sizes. The small particles (radius
