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Langmuir 2009, 25, 3344-3348
Au-Coated 3-D Nanoporous Titania Layer Prepared Using Polystyrene-b-poly(2-vinylpyridine) Block Copolymer Nanoparticles Won-Jeong Shin, Fevzihan Basarir, Tae-Ho Yoon, and Jae-Suk Lee* Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro (Oryong-dong), Buk-gu, Gwangju 500-712, Korea ReceiVed December 12, 2008. ReVised Manuscript ReceiVed January 24, 2009 New nanoporous structures of Au-coated titania layers were prepared by using amphiphilic block copolymer nanoparticles as a template. A 3-D template composed of self-assembled quaternized polystyrene-b-poly(2-vinylpyridine) (Q-PS-b-P2VP) block copolymer nanoparticles below 100 nm was prepared. The core-shell-type nanoparticles were well ordered three-dimensionally using the vertical immersion method on the substrate. The polar solvents were added to the polymer solution to prevent particle merging at 40 °C when considering the interaction between polymer nanoparticles and solvents. Furthermore, Au-coated PS-b-P2VP nanoparticles were prepared using thiol-capped Au nanoparticles (3 nm). The 3-D arrays with Au-coated PS-b-P2VP nanoparticles as a template contributed to the preparation of the nanoporous Au-coated titania layer. Therefore, the nanoporous Au-coated titania layer was fabricated by removing PS-b-P2VP block copolymer nanoparticles by oxygen plasma etching.
Introduction In recent years, porous semiconducting materials have attracted much attention because of various applications in electronic, catalytic, and electrochemical devices such as solar cells,1 electrocatalysts,2 and sensors.3,4 A convenient, simple method for fabricating porous materials is by using templates such as 3-D latex arrays.5 By templating self-organized supramolecular assemblies of small molecules, surfactants, and block copolymers, it has been possible to prepare porous materials of various sizes. The porous materials generated by template removal have pore sizes in the range of 100 nm to 10 µm.6 In particular, templating against opalline arrays of colloidal spheres offers macroporous materials that exhibit precisely controlled pore sizes and highly ordered 3-D porous structures.7 There are several methods of fabricating 3-D arrays of particles such as sedimentation, evaporation, the Langmuir-Blodgett method, and others.8 All these fabrication methods are techniques for the formation of 3-D arrays from colloidal spheres or photonic crystals. Colloidal spheres have been assembled into 3-D arrays with relatively large domain sizes.7 Commercial polystyrene beads and silica spheres are the two most commonly used templates, and methods such as calcination, dissolution, and etching may be used to remove them.9 There have been many reports using 3-D arrays of microspheres as templates. Xia et al. and Pine et al. have reported the successful fabrication of a well-ordered 3-D particle layer and macroporous inorganic multilayer.6,7,9 They also described how metal nanoparticles coated with amorphous silica may be used to form spherical colloid * Corresponding author. Tel: +82-62-970-2306. Fax: +82-62-970-2304. E-mail:
[email protected]. (1) Hagfelf, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (2) Moriguchi, I.; Maeda, H.; Teroka, Y.; Kagawa, S. Chem. Mater. 1997, 9, 1050. (3) Hoyer, P.; Masuda, H. J. Mater. Sci. Lett. 1996, 16, 1228. (4) Matsushita, S. I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Langmuir 1998, 15, 6441. (5) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (6) Imhof, A.; Pine, D. J. AdV. Mater. 1998, 10, 697. (7) (a) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693. (b) Gates, B.; Yin, Y.; Xia, Y. Chem. Mater. 1999, 11, 2827. (8) (a) Zhou, Z.; Zhao, X. S. Langmuir 2004, 20, 1524. (b) Reculusa, S; Ravaine, S. Chem. Mater. 2003, 15, 598. (9) (a) Subramanian, G.; Manoharan, V. M.; Thorne, J. D.; Pine, D. J. AdV. Mater. 1999, 11, 1261. (b) Fo¨rster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195.
multilayers.10 Furthermore, Stein et al. demonstrated macroporous minerals from an ordered 3-D layer template using polystyrene spheres.11 There are various techniques used to control the size of particle used as templates. The most common technique for the preparation of colloidal particles is emulsion polymerization,12 which generates 100-1000 nm particles. Mini- and microemulsion polymerization are recently developed techniques that yield colloids in the ranges of 50-200 and 20-50 nm, respectively.13 Another method to control the size of polymeric particles is through self-assembly of amphiphilic block copolymers.14 This method can control the size of polymeric particles in the 10-100 nm range. In this work, we describe a 3-D template composed of selfassembled amphiphilic diblock copolymer nanoparticles below 100 nm. The core-shell nanoparticles were self-assembled and well-ordered on the substrate using a vertical immersion method. With solvent evaporation, the liquid surface moves down, and arrays of nanoparticles form on the substrate. We also produced an Au-coated nanoporous titania layer after removing a template from Au-coated diblock copolymer nanoparitcles and titania composite.
Experimental Section Materials. Polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer with a P2VP mole fraction (fP2VP) of 0.5 was synthesized by living anionic polymerization15 in our previous method.16,17 The molecular weight of PS-b-P2VP is 126 kg mol-1, (10) (a) Lu, Y.; Yin, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 183. (b) Lu, Y.; Yin, Y.; Li, Z.-Y.; Xia, Y. Nano Lett. 2002, 2, 785. (11) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (12) Durbin, D. P.; El-Aasser, M. S.; Poehlein, G. W.; Vanderhoff, J. W. J. Appl. Polym. Sci. 1979, 24, 703. (13) Zhang, G.; Niu, A.; Peng, S.; Jiang, M.; Tu, Y.; Li, M.; Wu, C. Acc. Chem. Res. 2001, 34, 249. (14) (a) Thurmond, K. B.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1997, 119, 6656. (b) Hadjichristidis, N.; Pispas, S. AdV. Polym. Sci. 2006, 200, 37. (c) Loos, K.; Bo¨ker, A; Zettl, H.; Zhang, M.; Krausch, G.; Mu¨ller, A. H. E. Macromolecules 2005, 38, 873. (d) Zhou, C.; Jao, T.-C.; Winnik, M. A.; Wu, C. J. Phys. Chem. B 2002, 106, 1889. (15) Ishizu, K. Prog. Polym. Sci. 1998, 23, 1483. (16) Cho, Y.-H.; Yang, J.-E.; Lee, J.-S. Mater. Sci. Eng., C 2004, 24, 293. (17) (a) Cho, Y.-H.; Cho, G.; Lee, J.-S. AdV. Mater. 2004, 17, 1815. (b) Goh, H.-D.; Kang, N.-G.; Lee, J.-S. Langmuir 2007, 23, 12817.
10.1021/la804101a CCC: $40.75 2009 American Chemical Society Published on Web 02/17/2009
Letters
Figure 1. Scheme of the preparation of 3-D arrays with Q-PS-b-P2VP nanoparticles by the vertical immersion method: (a) dispersion of the nanoparicles in toluene, (b) vertical immersion of the ITO glass substrate in the 40 °C solution for 5 days, and (c) formation of 3-D arrays.
and the polydispersity index (Mw/Mn) is 1.19. For the quaternization of the pyridine unit, PS-b-P2VP (0.5 g) was dissolved in 50 mL of methyl ethyl ketone (MEK) and stirred for 30 min at room temperature. A 5-fold excess of CH3I compared to the molar ratio of the P2VP block was added to the solution. After several hours, the color of the reacted solution changed from clear colorless to an opaque light-yellow solution, which is evidence of quaternization. After another 5 days, the reaction mixture was poured into a large amount of hexane, filtered, and dried under vacuum for 1 day.18 Quaternized PS-b-P2VP powder (Q-PS-b-P2VP, 0.0125 g) was dissolved in 10 mL of toluene under vigorous stirring and sonicated for 3 days. To remove dust and large aggregated particles, the
Langmuir, Vol. 25, No. 6, 2009 3345 nanoparticle solution was filtered with a 0.45 µm syringe membrane filter. Before using ITO glass, it was washed with acetone, methanol, and distilled water with sonication for 1 h each and then dried at 80 °C. The ITO glass was immersed vertically without stirring in polymer solution (nanoparticles in toluene) at 40 °C for 5 days. After 5 days, the multilayer of nanoparticles on the ITO glass was dried at room temperature for 1 day before measurement. Thiolcapped Au nanoparticles were synthesized with dodecanethiol as described elsewhere.20a The thiol-capped Au nanoparticles were dispersed in a 5 wt % Q-PS-b-P2VP nanoparticle solution for 12 h. Titanium(IV) isopropoxide (Aldrich, 97%) was mixed with ethanol (1:19 v/v) as a sol precursor solution for the preparation of the Au-coated nanoporous titania layer. The sol solution was used in quantities of 0.5s2 mL. Characterizations. Fourier-transform infrared spectroscopy (Perkin-Elmer System 2000) was used to confirm the quaternization. The 3-D polymer nanoparticle arrays were characterized with a fieldemission scanning electron microscope (FE-SEM, Hitachi S-4700) at 10 kV. A field-emission transmission electron microscope (FETEM, JEOL JEM-2100F) operating at 200 kV was used to observe the nanoparticles. The TEM sample was prepared by dropping a polymer solution onto a carbon-coated copper grid and drying at 60 °C. Before measurement, PS-b-P2VP was stained with I2 vapor for 10 h. Elemental analysis was performed by an energy-dispersive spectrometer (EDS) attached to the FE-SEM.
Figure 2. SEM images of 3-D arrays of Q-PS-b-P2VP nanoparticles: (a) 30° tilted cross-section when no polar solvent was added, (b) 30° tilted cross-section when a small amount of water was added, (c) cross-section of image b, (d) 30° tilted cross-section when a small amount of ethanol was added, (e) cross-section of image d, and (f) magnified 30° tilted surface of image d. (Inset) TEM images of Q-PS-b-P2VP nanoparticles for each case. The size of the nanoparticle is 80 ( 5 nm.
3346 Langmuir, Vol. 25, No. 6, 2009
Letters
Figure 3. Scheme of preparing Au-coated nanoporous titania layer using Q-PS-b-P2VP nanoparticles coated with thiol-capped Au nanoparticles: (a) dispersion of thiol-capped Au-coated Q-PS-b-P2VP in toluene, (b) infiltration titania precursors to the solution, (c) self-assembling of nanoparticles with titania precursors on the ITO glass at 40 °C for 5 days by vertical immersion method, (d) covering with the other glass plate to remove excess titania precursors, (e) drying the composites at room temperature for 12 h, and (f) formation of Au-coated nanoporous of titania after removing Q-PS-b-P2VP by oxygen plasma etching.
Figure 4. TEM images: (a) thiol-capped Au nanoparticles of
