pubs.acs.org/Langmuir © 2010 American Chemical Society
Preparation of Patterned Zinc Oxide Films by Breath Figure Templating Kenichi Kon,† Chris Norman Brauer,§ Kosuke Hidaka,‡ Hans-Gerd L€ohmannsr€oben,§ and Olaf Karthaus*,†,‡ †
Graduate School of Photonic Science, and ‡Department of Bio- and Material Photonics, Chitose Institute of Science and Technology, and §Potsdam University, Institute of Chemistry & Innovation Center innoFSPEC Potsdam Received December 29, 2009. Revised Manuscript Received May 31, 2010
A large variety of microporous polymer films can be prepared by the breath figure technique. Here, we report on its use for the formation of microporous zinc oxide films. Zinc acetylacetonate, a zinc oxide precursor, is either dissolved in a polymer solution that is cast at high humidity to form microporous films or is vacuum evaporated onto a preformed microporous polymer film. Annealing leads to the pyrolysis of the organic material and the formation of zinc oxide films, which show increased photocatalytic activity as compared to unstructured films.
Introduction The self-assembly of polymeric materials by the so-called ‘breath figure’ technique1 is a promising approach to creating regular pore arrays with submicrometer to micrometer dimensions in thin films. Such microstructured films have received increased interest in recent years because they can be used for many applications, ranging from catalysis2 to cell proliferation.3 Notably in the field of photonics, films with structures in the range of several tens of nanometer to several micrometers show interesting properties, such as enhanced light outcoupling for LEDs,4 surface plasmon effects,5 light scattering and structural colors,6 reduced reflectivity,7 and enhanced photocatalytic activity. Franc-ois et al. reported in 1994 that stable polymeric microporous honeycomb films can be easily prepared by casting a polymer solution at high humidity.8 Water droplets condense on the evaporating polymer solution that act as templates for microstructure formation. Franc-ois’ report led to the opening of a new research platform to produce many different honeycomb films with various functionalities for a wide variety of applica*Corresponding author. Tel/Fax: þ81-123-27-6102. E-mail: kart@ photon.chitose.ac.jp. (1) Lord Rayleigh Nature 1912, 90, 436–438. (2) Li, L; Yang, H.; Yu, J.; Chen, Y.; Ma, J.; Zhang, J.; Song, Y.; Gao, F. J. Cryst. Growth 2009, 311, 4199–4206. (3) Tsuruma, A.; Tanaka, M.; Fukushima, N.; Shimomura, M. e-J. Surf. Sci. Nanotechnol. 2005, 3, 159–164. (4) Schnitzer, I.; Yablonovitch, E.; Caneau, C; Gmitter, T. J.; Scherer, A. Appl. Phys. Lett. 1993, 63, 2174–2176. (5) Sawai, Y.; Takimoto, B.; Hideki Nabika, H.; Ajito, K.; Murakoshi, K. J. Am. Chem. Soc. 2007, 129, 1658–1662. (6) Structural Colors in Biological Systems: Principles and Applications; Kinoshita, S., Yoshioka, S., Eds.; Osaka University Press: Osaka, Japan, 2005. (7) Clapham, P. B.; Hutley, M. C. Nature 1973, 244, 281–282. (8) Widawski, G.; Rawiso, M.; Franc- ois, B. Nature 1994, 369, 387–389. (9) Ishii, D.; Yabu, H.; Shimomura, M. Chem. Mater. 2009, 21, 1799–1801. (10) Karthaus, O.; Maruyama, N.; Cieren, X.; Shimomura, M.; Hasegawa, H.; Hashimoto, T. Langmuir 2000, 16, 6071–6076. (11) Cheng, C. X.; Tian, Y.; Shi, Y. Q.; Tang, Stenzel-Rosenbaum, M. H.; Davis, T. P.; Fane, A. G.; Chen, V. Angew. Chem., Int. Ed. 2001, 40, 3428–3432. (12) Franc-ois, B.; Ederle, Y.; Mathis, C. Synth. Met. 1999, 103, 2362–2363. (13) Zhao, B.; Li, C.; Lu, Y.; Wang, X.; Liu, Z.; Zhang, J. Polymer 2005, 46, 9508–9513. (14) Hernandez-Guerrero, M.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. Eur. Polym. J. 2005, 41, 2264–2277. (15) Nygard, A.; Davis, T. P.; Barner-Kowollik, C.; Stenzel, M. H. Aust. J. Chem. 2005, 58, 595–599. (16) Franc-ois, B.; Pitois, O.; Franc- ois, J. Adv. Mater. 1995, 7, 1041–1044.
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tions.8-21 The void diameter and spacing of the honeycomb film can be controlled by the casting volume, polymer concentration, humidity, and other factors and typically range between 100 nm and 10 μm. The crucial step in honeycomb formation is the prevention of the coalescence of water droplets that form during solvent evaporation, either by using gelling or amphiphilic polymers. By using a small amount of an amphiphilic additive, many nonamphiphilic polymers can also form honeycomb films,22 and thus honeycomb fabrication does not depend so much on the molecular structure of the polymer that is used. Such highly structured films with periodicities similar to the wavelength of visible light strongly scatter light, and one obvious application of such microporous films is in the field of photonics, for example, as a photocatalyst. The most well-known material for photocatalysis is TiO2, which can also be used for the lightinduced splitting of water.24 We have already reported on the photocatalytic activity of microstructured honeycomb TiO2 films prepared by coating a honeycomb polymer template with a nanocrystalline TiO2 suspension. The resulting TiO2 films, after pyrolysis at 400 °C to remove the polymer, show a hierarchical structure with nanometer- and micrometer-sized pores.25 This type of structure is produced because the cross-linked polymer template is initially stable enough to retain its 3D structure at the temperature necessary for pyrolysis.22,23 Another important environmentally friendly and chemically stable metal oxide semiconductor is zinc oxide, which has a band gap of 3.37 eV. It also has a high charge-carrier mobility26 and is (17) Srinivasarao, M.; Collings, D.; Philips, A.; Patel, S. Science 2001, 292, 79–82. (18) Bolognesi, A.; Mercogliano, C.; Yunus, S.; Civardi, M.; Comoretto, D.; Turturro, A. Langmuir 2005, 21, 3480–3485. (19) Cui, L.; Xuan, Y.; Li, X.; Ding, Y.; Li, B.; Han, Y. Langmuir 2005, 21, 11696–11703. (20) Englert, B. C.; Scholz, S.; Leech, P. J.; Srinivasarao, M.; Bunz, U. H. F. Chem.;Eur. J. 2005, 11, 995–1000. (21) Yabu, H.; Shimomura, M. Langmuir 2005, 21, 1709–1711. (22) Karthaus, O.; Y. Hashimoto, Y.; Kon, K.; Tsuriga, Y. Macromol. Rapid Commun. 2007, 28, 962-965. (23) Kabuto, T.; Hashimoto, Y.; Karthaus, O. Adv. Funct. Mater. 2007, 17, 3569–3572. (24) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (25) Kon, K.; Nakajima, K.; Karthaus, O. e-J. Surf. Sci. Nanotechnol. 2008, 6, 161–163. (26) Li, L.; Yang, H.; Yu, J.; Chen, Y.; Ma, J.; Zhang, J.; Song, Y.; Gao, F. J. Cryst. Growth 2009, 311, 4199–4206.
Published on Web 06/17/2010
DOI: 10.1021/la904897m
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used as a transparent electrode material in liquid-crystal displays,27 and dye-sensitized solar cells,28 transistors, and lightemitting diodes and as a field-effect emitter.29 It can be expected that a hierarchical microstructure also has a beneficial effect on catalytic activity, as already shown in the case of TiO2. It has been reported that hierarchically structured ZnO fiber arrays can be produced by the hydrothermal reaction of metallic zinc foil.30 In addition, zinc oxide can be deposited on pincushion films that were derived from honeycomb films by the adsorption of zinc salts, followed by in situ formation of the oxide by the electroless generation of hydroxide anions.31 Here we report on the formation of microporous ZnO films by two different processes: the casting of a mixed Zn(acac)2/polyion complex (PIC) solution at high humidity and vacuum evaporation of zinc acetylacetonate (Zn(acac)2) onto a polymer honeycomb. The experimental parameters used to produce regular films were determined, and after pyrolysis, the films were characterized by fluorescence microscopy, X-ray diffraction, and scanning electron microscopy. Furthermore, the photocatalytic activity was evaluated for films prepared by the first method.
Figure 1. Absorption spectrum of the dye solution. The inset shows the chemical structure. Scheme 1. Chemical Structure of the Used Polymers: Polyion Complex (Top) and Poly(styrene-co-maleic anhydride) (Bottom) That Can Be Cross-Linked with Diamino Octane
Methods and Materials Method 1: Incorporation of Zinc into the Honeycomb Film. The amphiphilic polyion complex was prepared by mixing equimolar amounts of an aqueous solution of poly(styrene sulfonate) (Aldrich Inc.) and a vesicular emulsion of bishexadecyldimethyl ammonium bromide (TCI, Japan). The precipitate was centrifuged, washed several times with water, and vacuum dried. Zinc acetylacetonate hydrate (Aldrich Inc.) was dehydrated in vacuum, after which it was completely soluble in chloroform (Wako Corp.). Solutions were prepared by mixing the polyion complex and zinc complex in various ratios and amount. Honeycomb films were prepared as described in the literature by placing 10 μL of the solution onto a 25 50 mm2 glass cover slide (Matsunami Neo) and letting the solvent evaporate at a humidity of 70-80% at room temperature.10 The humidity was achieved by pumping air through a column of distilled water. The humidified air was led through a tube that was connected to the lower part of a glass funnel with 10 cm diameter, which was placed over the sample. The flow rate was 2-4 L/min, depending on the relative humidity that needed to be adjusted.
Method 2: Vacuum Evaporation of Zn(acac)2 onto a Polymer Honeycomb Film. Honeycomb films of poly(styrene-comaleic anhydride) were prepared as described above and subsequently cross-linked with diamino octane.23 A 5 5 mm2 glass slide covered with a honeycomb film was attached to the coldfinger of a microsublimation apparatus (Aldrich Inc.) filled with Zn(acac)2 hexahydrate. The evacuated sublimation apparatus was placed in an oil bath at 60 °C for 5 to 40 min. The films prepared by either method were pyrolyzed at 400 °C in a muffle oven (As One Corp.). For imaging, a scanning electron microscope (Hitachi 5300) was used. Film thicknesses were estimated from SEM pictures taken at an oblique angle. X-ray diffraction (XRD) patterns were taken with an RINT2000 (Rigaku Corp.) diffractometer. The photocatalytic activity was determined by irradiation of a 1 μM aqueous solution of a sulfonated cyanine dye, sodium (2-[5-chloro-1,3-benzothiazol2(3H)-ylidenmethyl]-5-chloro-1,3-benzothiazole-N,N0 -dipropane1-sulfonate) (Aldrich Inc.) with a 5 mW black light (As One (27) Wagner, P.; Helbig, R. J. Phys. Chem. Solids 1974, 35, 327–335. (28) Oh, B.-Y.; Jeong, M.-C.; Moon, T.-H.; Lee, W.; Myoung, J.-M.; Hwang, J.-Y.; Seo, D.-S. J. Appl. Phys. 2006, 99, 124505–124508. (29) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455–459. (30) Li, Y. B.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2004, 84, 3603–3605. (31) Hirai, Y.; Yabu, H.; Shimomura, M. Colloids Surf., A 2008, 313-314, 312– 315.
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Corp.) for several hours. The dye was chosen because it is highly water-soluble and anionic and thus is not likely to physisorb onto the photocatalyst. As can be seen in Figure 1, the dye has an absorption minimum of around 335 nm in deionized and distilled water and thus does not significantly absorb actinic light but allows it to pass through the liquid layer to reach the photocatalyst. The absorption between 400 and 450 nm shows two peaks. At a concentration of 1 μM, the dye exists in monomer/ dimer equilibrium. The monomer state has an absorption maximum at 428 nm, and the dimer has an absorption maximum at 407 nm. It should be noted that the dye concentration is low enough that no J aggregates, which have a maximum at 464 nm, are present. The change in absorption during the photoreaction was measured at given intervals.
Results and Discussion The morphology of a honeycomb film (pore pitch and diameter, rim width, and total film thickness) depends on several parameters, one of them being the solute concentration. The mechanism of honeycomb formation has been described by several authors8,10,17,20 and will be only briefly outlined here. When the water-immiscible organic solvent starts to evaporate, its surface cools. Water from the atmosphere starts to condense. Because the condensation takes place on an unstructured liquid surface, the water droplets have a narrow size distribution. With ongoing evaporation of the organic solvent, these water droplets can grow until the solution becomes too viscous. A lower concentration of the solute leads to a lower total amount of material in the film and thus thinner rims and pillars (structures between three adjacent water droplets that connect the substrate with the upper honeycomb layer). Langmuir 2010, 26(14), 12173–12176
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Figure 2. Scanning electron micrographs of films before (left) and after (right) annealing at 400 °C. The ratio between Zn(acac)2 and PIC is 5:1 with total concentrations of (A) 5 and (C) 10 mg/mL. The scale bar is 10 μm.
Method 1. Three mixing ratios of Zn(acac)2 and PIC were investigated: 1:1, 5:1, and 10:1 (w/w). When casting solutions that contained a total amount of 5 mg/mL, we found that the 10:1 mixture had a concentration of PIC that was too low to prevent the crystallization of Zn(acac)2. Thus, we observed the formation of heterogeneous samples of crystal fibers with a broad size distribution. However, the 1:1 mixture leads to well-developed honeycomb films, but the structure is destroyed upon pyrolysis because the amount of inorganic material is not enough to retain a continuous structure. As can be seen in Figure 2A,C, the 5:1 mixing ratio produced honeycomb structures but a difference in film morphology could be observed after pyrolysis. At a low concentration of the casting solution, the pillars that connect the upper honeycomb layer with the substrate were not thick enough and collapsed during pyrolysis. The upper honeycomb layer was thick enough to retain its structure, leading to a 2D honeycomb structure (Figure 2B). By increasing the concentration 2-fold, the honeycomb film had enough strength to prevent the collapse and a stable 3D honeycomb film was formed (Figure 2D). Obviously, pyrolysis leads to a shrinkage of the film, but the integrity of the in-planne structure of the honeycomb film is well preserved, as can be seen in Figure 2B,D. The organic material in the rim decomposes, and thus the rim gets thinner but the pore-pore distance, measured as the center-to-center distance, is not affected. Fluorescent images of such films under violet excitation show weak blue fluorescence, which points to a crystalline ZnO film with few defects (Supporting Information).26 Method 2. A second method to produce micropatterend inorganic structures is the use of a polymer honeycomb as a template for the vacuum deposition of a volatile metal oxide precursor. The honeycomb needs to be stable at the temperatures reached during vacuum evaporation, and we chose poly(styreneco-maleic anhydride), which can be chemically cross-linked to produce a thermally stable honeycomb film, as already published.10,23 As the metal oxide precursor, we used zinc acetylacetonate hexahydrate. Upon heating in vacuum, the complex evaporates and loses its water of hydration. The anhydrous Zn(acac)2 then adsorbs onto the substrate. Figure 3 shows the scanning electron micrographs of samples after vacuum evaporation. Figure 3A is a sample after 5 min of evaporation. The border between the polymer honeycomb film and the evaporated metal complex can be seen at the rim edges, which is indicated by the black arrow. With increasing evaporation time, the metal complex layer gets thicker and the hole diameter shrinks. This is due to the geometry of the evaporation apparatus. The honeycomb Langmuir 2010, 26(14), 12173–12176
Figure 3. Scanning electron micrographs of films prepared by method 2: (A) after 5 min of evaporation; (C) after 10 min of evaporation; and (E) after 30 min of evaporation. The corresponding pictures after pyrolysis are B, D, and F, respectively. The center-to-center distance of the pores is between 2.1 and 2.4 μm. Table 1. Size Parameters of the Honeycomb Films Prepared by Method 2 evaporation time (min) 0 5 (Figure 3A) 10 (Figure 3C)
pitch hole diameter rim width Zn(acac)2 P (μm)a D (μm) W (μm)b height (μm) D/W 2.12 2.30 2.40
1.75 1.85 1.47
0.37 0.45 0.93
n.a. 0.3 0.7 1.5
4.7 4.1 1.6 0.2 30 (Figure 3E) 2.10 0.43 1.67 6 40 n.a. 0 n.a. 1.6 0 a Center-to-center distance of two neighboring holes. b Measured along the hole center-hole center line. n.a.: not applicable.
substrate is separated from the source container, which has a diameter of 12 mm, by ca. 15 mm. Thus, the evaporating metal complex hits the sample not only at a normal angle but also at shallow angles. Still, the preferential growth is perpendicular to the honeycomb film normal, as can be seen from the data in Table 1. Four different samples were analyzed, and because of the locally slightly different environments during honeycomb formation, the hole diameter of the honeycomb template shows a slight variation and is in the range of 2.1 to 2.4 μm. Notwithstanding the slight differences in pore diameter, all samples show similar behavior after Zn(acac)2 evaporation. When the metal complex film becomes thicker (Figure 3C,E), the hole diameter decreases. At 60 °C, it takes around 40 min for the holes to be completely covered. By that time, the inorganic film has a thickness of around 1.6 μm. Table 1 gives a summary of the size parameters for the different honeycomb films obtained by the SEM measurements. As the Zn(acac)2 thickness increases, the hole diameter becomes smaller and the rim thickness increases. After an initial induction period of 5 min, during which the film mainly growth in the z direction (Figure 3B), the ratio of hole diameter D to rim width W decreases exponentially, as would be expected for isotropic growth in the x-y plane of the honeycomb film. After pyrolysis, the polymer is burned away and the metal complex is decomposed into ZnO. XRD patterns of such a sample indicate a highly crystalline Wurtzite structure (Supporting Information).25 This crystallization and the loss of the organic ligands caused the metal oxide film to collapse onto the substrate DOI: 10.1021/la904897m
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and to laterally shrink, which led to partial rupture of the 2D honeycomb network, especially in the case of a thin film, which forms domains of 5-10 honeycomb pores, as can be seen in Figure 3B. Thicker films, prepared with 10 and 30 min of evaporation, are more resistant to lateral stress, and only minor cracks can be seen in the films (Figure 3D,F). Still, all three films are very porous and effectively scatter visible light and thus, to the unaided eye, are white and nontransparent. Photocatalytic Activity. We have already reported on the formation of titanium dioxide-containing honeycomb films that can be used as photocatalysts.25 The honeycomb structure has the following advantages. First, an increase in surface area, which is also hierarchically structured, facilitates the diffusion of photogenerated reactive species from the photocatalyst surface and the diffusion of reactants into the porous photocatalyst structure. The second advantage is that the microporous structure is strongly light scattering. This leads to multiple passes of photons through the film, thus increasing the efficiency. Zinc oxide is a versatile metal oxide that has many interesting characteristics that make it suitable as a gas sensor32 or photocatalyst33,34 and sometimes has a higher efficiency than TiO2.35-40 Here, we demonstrate the increase in photocatalytic activity by fabricating a ZnO film with honeycomb surface morphology. By using method 1, equal amounts of Zn(acac)2 solution were cast onto glass slides, one at 30% humidity (normal laboratory atmosphere) resulting in an unstructured flat film and one at 80% humidity (by blowing humidified air over the solution) to form 3D honeycomb films. Each film was used to determine the photocatalytic activity. The decrease in absorbance during irradiation with 365 nm light shown in Figure 4 indicates that the ZnO films are all catalytically active. A control sample that did not contain any ZnO film showed only an insignificant drop in dye absorption. On the other hand, solutions that contain both the flat as well as the honeycomb film show a steady decrease in absorption, indicating the photodegradation of the dye. The strongest decrease occurs with the honeycomb film, showing the anticipated enhancing effect of the free-standing 3D honeycomb structure. For the longtime stability of these films in solution, the attachment of ZnO to the substrate is crucial. We found that using a cover glass as a substrate limits the annealing temperature because of the softening of the substrate at temperatures above 450 °C. All of our (32) Eriksson, J.; Khranovskyy, V.; S€oderlind, F.; K€all, P. O.; Yakimova, R.; Spetz, A. L. Sens. Actuators, B 2009, 137, 94–102. (33) Moribe, S.; Yamamoto, Y.; Ikoma, T.; Akiyama, K.; Zhang, Q.; Saito, F.; Tero-Kubota, S. Chem. Phys. Lett. 2007, 436, 373–377. (34) Daneshvar, N.; Salari, D.; Khataee, A. R. J. Photochem. Photobiol., A 2004, 162, 317–322. (35) Dindar, B.; Icli, S. J. Photochem. Photobiol., A 2001, 140, 263–268. (36) Pirkanniemi, K.; Sillanpaa, M. Chemosphere 2002, 48, 1047–1060. (37) Yeber, M. C.; Rodriguez, J; Freer, J.; Baeza, J.; Duran, N.; Mansilla, H. D. Chemosphere 1999, 39, 1679–1688. (38) Khodja, A. A.; Sehili, T.; Pihichowski, J. F.; Boule, P. J. Photochem. Photobiol., A 2001, 141, 231–239. (39) Serpone, N.; Maruthamuthu, P.; Pichat, P.; Pelizzetti, E.; Hidaka, H. J. Photochem. Photobiol., A 1995, 85, 247–255. (40) Marci, G.; Augugliaro, V.; Munoz, M. J. L.; Martin, C.; Palmisano, L.; Rives, V.; Schiavello, M.; Tilley, R. J. D.; Venezia, A. M. J. Phys. Chem. B 2001, 105, 1033–1040.
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Figure 4. Relative absorbance change during irradiation with 365 nm UV light.
samples were annealed at 400 °C and were stable against peeling for up to 2 weeks.
Conclusions Honeycomb films of inorganic metal complexes have been produced in the past by using either chainlike metal complexes41,42 or amphiphilic metal complexes,43,44 and our new results extend the types of honeycomb-forming compounds to mixtures of low-molar-mass metal complexes. These complexes can either be incorporated into the honeycomb film by mixing with a polyion complex solution or vacuum evaporated onto a polymer honeycomb film. With this extension, it is possible to increase the amount of the inorganic component because the polyion complex need not be present in an equimolar amount, which was the case for the previously reported examples. We could demonstrate that, after pyrolysis of the organic matter, the resulting inorganic honeycomb-patterned film has a higher photocatalytic activity than an unstructured film. Furthermore, the present route of metal oxide film formation can also be used to prepare mixed metal oxide films either by casting a mixture or by layer-by-layer evaporation. Acknowledgment. This research was partially supported by the Collaborative Development of Innovative Seeds of Japan Science and Technology Agency (JST, SEEDS). C.N.B. acknowledges support from Photonic World Consortium, Chitose, and from InnoFSPEC, Potsdam. Supporting Information Available: Fluorescence micrograph of a sample prepared by method 1. XRD pattern of a film prepared by method 2. This material is available free of charge via the Internet at http://pubs.acs.org. (41) Lee, C.-S.; Kimizuka, N. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4922–4926. (42) Fraxedas, J.; Vergaguer, A.; Sanz, F.; Baudron, S.; Batail, P. Surf. Sci. 2005, 588, 41–48. (43) Maruyama, N.; Koito, T.; Nishida, J.; Cieren, X.; Ijiro, K; Karthaus, O.; M. Shimomura, M. Thin Solid Films 1998, 327-329, 854–856. (44) Karthaus, O.; Cieren, X.; Maruyama, N.; Shimomura, M. Mater. Sci. Eng., C 1999, 10, 103–106.
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