- Email: [email protected]
Filling behavior of ZnO nanoparticles into opal template via electrophoretic deposition and the fabrication of inverse opal
Electrochimica Acta 54 (2009) 3677–3682
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Discussion
Filling behavior of ZnO nanoparticles into opal template via electrophoretic deposition and the fabrication of inverse opal Yi-Wen Chung a , Ing-Chi Leu b,∗ , Jian-Hong Lee c , Min-Hsiung Hon c a
Flexible Electronics Equipment Dep., Laser Application Technology Center, South Industrial Technology Research Institute, No. 8. Gongyan Rd., Liujia Shiang, Tainan County, 734, Taiwan, ROC b Department of Material Science, National University of Tainan, 33, Sec. 2, Shu-Lin St., Tainan, 360, Taiwan, ROC c Department of Materials Science and Engineering, National Cheng Kung University, 1, Ta-Hsueh Rd., Tainan, 701, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 6 September 2008 Received in revised form 3 December 2008 Accepted 1 January 2009 Available online 15 January 2009 Keywords: Electrophoretic deposition Colloidal template-mediated process Inverse opal ZnO photonic crystal ZnO nanoparticles
a b s t r a c t Combining colloidal crystal template (artificial opal) and electrophoretic deposition (EPD) process, wellordered ZnO inverse opal can be formed by finding the optimum driving potential of EPD. Through providing the various driving potentials from −25 V, −10 V, −5 V to −2.5 V, the different mechanism of electrophoretically depositing ZnO nanoparticles into the colloidal crystal template was determined by the SEM observation of the filled templates. Because the nano-channels of colloidal crystal template are the network type, the results of surface jam, incomplete filling and perfect filling are found under specific applied voltages. The high-quality ZnO inverse opal can be only fabricated under the perfect nano-channel-filling condition. The filling behavior can be monitored dynamically by tracing the current transients, and the optimum conditions for filling the interstitial spaces of templates constructed from colloidal particles with 180 nm and 300 nm diameter can be obtained by applying a voltage of −5 V and −15 V, respectively. After the complete filling of ZnO nanoparticles into the colloidal crystal template consisting of 300 nm colloids, high-quality ZnO photonic crystal possessing an absorptive peak at the wavelength of 560 nm can be fabricated by removing the template. It is expected that the EPD can find extensive applications for preparing photonic crystals of various oxides only if their nanoparticles are available. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The atomic clusters [1], nanoparticles [2] and nanostructures [3] of materials exhibit size-dependent properties that are profoundly different from their corresponding bulk material counterparts. Even, there has been extensive interest in the creation of nano-porous materials with three-dimensional periodicity due to potential applications, such as catalysts [4], chemical [5] and gas [6] sensors, photonic [7] and magnetic devices [8]. When ZnO macroprous structure was fabricated, it is useful for the applications in sensors and semiconducting electrodes in virtue of the large internal surface area and improved mass transportation originated from size-controlled macropores [9]. The template-mediated process is a conventional and important technique in forming nanostructure, including 1D nano-fiber [10] and 2D [11] macroporous monolayer or 3D well-ordered macroporous structures (the so-called inverse opal) [12]. The entire template-mediated process contains three steps. Firstly, appropri-
∗ Corresponding author. E-mail address: [email protected] (I.-C. Leu). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.013
ate templates must be fabricated in order to act as a host. Secondly, the material of interest can be filled into the interstitial space of template via induced filling of precursors into nano-channel. In general, nano-channel-filling technologies include chemical vapor deposition [13], dipping process [14], electrophoresis [10] and electrodeposition [15]. Finally, the replica can be obtained by removing template. Among these filling methods, electrophoresis is a powerful tool for fabrication of nanostructures. As compared to other methods, the advantages of electrophoresis deposition process (EPD) are simplicity, efficient, material general, low cost and highly reproducibility. Besides, it has also been widely applied for coatings and laminated materials in the micrometer size region. However, the main disadvantage is the possibility of jamming of the nanochannels leading to the poor filling ratio during the material filling process [16]. An illustration of electrophoresis process is shown in Fig. 1. In this technique, an electric field is used to perform the filling of voids of the colloidal crystal template with nanoparticles. The entire process includes two steps. First, nanoparticles in a suspension are forced to move toward working electrode having an opposite charge by applying an electric field to the suspension. Second, the particles are collected at the electrode and fill the interstitial spaces of template, until complete infiltration is achieved.
3678
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
Fig. 2. High-quality colloidal crystal with 300 nm colloidal diameter fabricated via a capillary-enhanced process.
Fig. 1. A schematic diagram of electrophoresis process.
In this manuscript we report the preparation of well-ordered ZnO macroporous structure via a template-mediated electrophoresis process. Under the different potentials of electrophoresis, different behaviors for filling interstitial spaces will be described. Because the colloidal crystal template appears as face-centered cubic (FCC) structure as prepared via the capillary-enhanced evaporation process, a ZnO replica constructed by removing the template is expected to have the well-ordered macroporous structure (inverse opal), and also can be used as photonic crystals. 2. Experimental section 2.1. Preparation of opal template Monodispersed Poly(stylene-co-methacrylic acid) microshperes were synthesized according to an emulsifier-free-emulsion copolymerization process [17], where styrene (St) and methacrylic acid served as reactants. The process, capillary-enhanced evaporation method, for preparing high-quality colloidal crystals (the so-called artificial opal) is characterized by the presence of multi-capillary forces [18]. First, the substrate was placed horizontally at the bottom of a container, which was filled with an aqueous dispersion of the P(St-co-MAA) microspheres obtained by ultrasonication. Then, the self-assembly of microspheres and the formation of colloidal crystals were conducted at 45◦ C while maintaining high humidity (90%). After 24 h, evaporation was terminated and the substrate was covered with colloidal crystal films. 2.2. Electrophoresis process of ZnO nanoparticles The fabrication of ZnO nanoparticle suspensions followed the method of Bahenmann et al. [19]. The ZnO colloidal suspensions were synthesized from zinc acetate (ACS reagent grade) in 2propanol (ACS reagent grade) with NaOH (ACS reagent grade) added to adjust the pH. Then, a transparent suspension of ZnO nanoparticles was obtained and the particle diameter was determined by transmission electron microscopy to be about 5 nm.
The schematic illustration of electrophoresis process and equipment is shown in Fig. 1. The conductive ITO-glass substrate with deposited colloidal crystal and a platinum plate were used as the working electrode and the counter electrode, respectively. The distance between the two electrodes was 2 cm. The electrophoresis was performed at different voltages. After electrophoresis, calcinations at 330 ◦ C for 8 h were required to both remove the P(St-co-MAA) microspheres and sinter the oxide nanoparticles. Finally, 3D macroporous structure of ZnO can be formed on the substrate. 3. Results and discussion 3.1. Fabrication of colloidal crystal template Colloidal crystal template (also named artificial opal) for synthesizing macroporous structure studied in this work was prepared using an efficient technology described previously [18], with an entire process time in only 24 h. In general, the gathering behavior of colloidal particles generally depends on an appropriate driving force. Subsequently, the self-assembly of colloidal particles can then form a crystal-like structure, named colloidal crystal. The capillary-enhanced method relies on multiple capillary driving forces, including primary and secondary capillary forces by controlling the ambient humidity, to organize colloids. Finally, high-quality colloidal crystal will be fabricated. SEM image of the (1 1 1) surface of the synthetic crystal reveals a close-packed arrangement, as shown in Fig. 2. Each colloidal particle is surrounded by other six colloids. Only few vacancies and dislocations are found in this domain. As shown in the inset figure, the square pattern in (1 0 0) facet can be observed clearly. In general, this geometric arrangement of colloidal particles indicates the stack of face-centered cubic structure [20]. 3.2. The filling characteristics of ZnO nanoparticles via electrophoresis process Preparation of nanoparticles with a small diameter is an important step for template-mediated electrophoresis process. This is because smaller nanoparticles will migrate easier in the interstitial space of colloidal crystal template. Zeta-potential measurements show that the surface charge of the ZnO nanoparticles is positive. Thus, a negative voltage is applied to the working electrode in order
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
3679
Fig. 3. The relationship between current and time for ZnO electrophoresis in the interstitial space of colloidal crystal template with different diameter and voltage. (a) 180 nm, −25 V, (b) 180 nm, −10 V, (c) 180 nm, −5 V, (d) 180 nm, −2.5 V and (e) 300 nm, −15 V.
to infiltrate the colloidal crystal template with nanoparticles, as shown in Fig. 1. The main disadvantage of EPD is the jamming of nano-channels near the surface. Hence, the effect of varied voltages on the possibility of jamming, including −25 V, −10 V, −5 V and −2.5 V, are studied for the template constructing from 180 nm colloidal particles. The recorded current transient curves for the first 60 s are shown in Fig. 3. The trend of curve (a) is different from the other curves. When a larger voltage (−25 V) is employed in the process of filling interstitial space, a large nanoparticle flux will be induced near the template surface. This will cause jamming of the mouth of the interstitial space near the template surface with ZnO nanoparticles. After jamming, ZnO nanoparticles agglomerate rapidly on the surface and form a continuous layer. This phenomenon will cause an increase of the electrophoretic current values quickly for the first 7 s of deposition. P(St-co-MAA) microsphere is a polar macromolecule and possess a negative zeta-potential in the suspension. It will adsorb the ZnO nanparticles with positive zeta-potential before the interstitial spaces of colloidal template are filled by EPD process. Even though the surface of colloidal crystal template is jammed by the ZnO nanoparticles, electrons can still be transported from ITO-glass substrate through the path constituted by ZnO coatings possessing a certain degree of electrical conductivity, and finally reach the surface. Hence, the surface jamming cannot screen the electric field. The ZnO nanoparticles continuously migrate toward the working electrode under the application of an electrical field. Then, the thickness of coating will increase gradually with electrophoresis time, which in turn results in an increase of electric resistance. Hence, the trend of current values appears to decrease accordingly for a long time. As shown in Fig. 4(a), there is a coating consisting of ZnO nanoparticles on the template surface for a deposition time of 1 min. In brief, the entire process exhibits the deposition characteristics similar to films deposited on planar surface by electrophoresis [21]. As observed in the inset figure, the surface of P(St-co-MAA) colloidal particles had adsorbed many ZnO nanoparticles, which is due to the electrostatics-induced adsorption effect between the positively charged ZnO and negatively charged PS microsphere. These ZnO nanoparticles adsorbed onto the PS particle surface also act as the medium for conducting electrons to working electrode. The recorded current curves (b–d), as compared with curve (a), are shown to exhibit a peak in the initial stage of filling
Fig. 4. ZnO nanoparticles filled into interstitial spaces of colloidal crystal template by electrophoresis with different voltages of (a) −25 V, (b) −10 V, (c) −5 V and (d) −2.5 V.
3680
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
interstitial spaces via an electrophoresis process. This is the phenomenon of nano-channel-filling resulting from the presence of time-dependent diffusion regime [22]. When a driving potential is applied at the working electrode, current value will first increase rapidly due to a large number of ZnO nanoparticles moving to working electrode. Subsequently, the decrease of current indicates the consumption of electroactive species (i.e., salt ions and complexes) [23] and ZnO nanoparticles near the electrode and the formation of a diffusion layer near the electrode [24]. Hence, the migration characteristics of ZnO nanoparticles in the interstitial spaces will influence the current value. The curve (b) of Fig. 3 performed at −10 V possesses a larger current value and a weaker peak. When the large nanoparticle flux is caused by applying a potential (−10 V), both the filling of interstitial spaces and jamming of template surface occur in 5 min of deposition. As shown in Fig. 4(b), the inset figure shows a thick coating covering the surface. Moreover, the boundary between filled and unfilled interstitial spaces can be observed clearly, as indicated by a white arrow in Fig. 4(b). The large current values in the initial stage result from the large driving potential (−10 V). If the effect of time-dependent diffusion regime [22] can be more pronounced, the recorded current curve (b) will possess a strong peak in the initial stage of EPD. However, the jamming of template surface will occur under this condition. The turning point which takes place at 13 s in curve (b) means the onset of the jamming of the template surface. The ZnO nanoparticles in the solution not only deposit into the interstitial spaces, but also accumulate onto the template surface. Thereafter, recorded current values caused by the effect of ZnO nanoparticle accumulation on template surface are larger, which is similar to the curve (a) in Fig. 3. Hence, this phenomenon exhibits a weaker peak in the initial stage of EPD. Finally, the interstitial spaces of colloidal crystal template are filled incompletely with ZnO nanoparticles, and the surface of colloidal crystal template is then covered completely. In order to infiltrate the nanoparticles into the interstitial spaces, optimization of parameter had been tried. The jamming of surface nano-channels can be avoided by decreasing voltages in the EPD system. Fig. 4(c) shows the SEM micrograph of colloidal crystal that is filled with ZnO nanoparticles. The electrophoresis is performed at constant voltage of −5 V and for a deposition time of 5 min. Firstly, ZnO nanoparticles in the interstitial spaces of colloidal crystal template will move towards and accumulate onto the substrate surface. Subsequently, ZnO nanoparticles in the solution will also be continuously attracted and move into the interstitial spaces. Finally, the interstitial space among the P(St-co-MAA) microspheres can be filled completely from bottom to top, as depicted in insets of Fig. 4(c). The jamming of template surface observed at high electrophoretic voltage had disappeared. The curve (c) in Fig. 3 possesses a sharp peak, which is because the effect of time-dependent diffusion is more significant for the present case without surface jamming. The current values are smaller due to the only source for charge transport from the ZnO nanoparticle infiltrating into the interstitial space. This result demonstrates that the ZnO nanoparticles can move under a voltage condition of −5 V, and fill the interstitial spaces of colloidal crystal template completely. The deposit thickness fabricated at −10 V in 5 min via an electrophoresis process is about 14 m, as shown in Fig. 4(b). This value is lower than that (20 m) fabricated at −5 V in 5 min. This result is not anticipated, because a larger driving potential is expected to obtain a thicker film. The main reason is the jamming of template surface. The surface jamming decreases the efficiency of infiltration into the interstitial spaces, so a thinner coating (14 m) can be obtained at −10 V in 5 min via an electrophoretic process. Hence, according to the result of the filling thickness of the interstitial spaces, it is also demonstrated that the jamming of template surface and the infiltration of interstitial space occur in the same time.
Fig. 5. ZnO nanoparticles filled into interstitial spaces of colloidal crystal template by electrophoresis with a voltage of −2.5 V in a suspension with high nanoparticle concentration.
If a potential of −2.5 V is applied, a smaller nanoparticle flux can be caused. Because of lower driving potential, the curve (d) of Fig. 3 possesses a smaller current value with weaker peak. The surface jamming is not found under the low driving potential of −2.5 V, but ZnO nanoparticles cannot fill interstitial spaces completely. Some voids are still observed, as shown in Fig. 4(d). However, increase of nanoparticle concentration will solve this problem. When the concentration of ZnO nanoparticles is increased from 2 × 10−5 g/ml to 2 × 10−4 g/ml, the interstitial spaces can be filled completely at the lower driving potential of −2.5 V, as shown in Fig. 5. This is because the increase of the nanoparticle concentration will result in the flux and the recorded current values elevating. On the other hand, larger interstitial spaces of colloidal crystal consisting of 300 nm colloidal particles can be filled with ZnO nanoaprticles by providing a larger potential (−15 V). Although the small driving potential can also drive the nanoparticles into larger interstitial space completely, the process time is longer. According to previous description for colloidal crystals using 180 nm colloidal particles, the larger driving potential will lead to the jam of template surface. However, the curve (e) in Fig. 3 for EPD using a larger potential (−15 V) for colloidal crystals using 300 nm colloidal particles is similar to the shape of curve (c) using 180 nm colloidal particles at −5 V. This is an optimum condition for filling the interstitial spaces of colloidal crystal template with diameter of 300 nm by EPD process, where complete filling can be obtained at larger driving potential. Therefore, the ZnO nanoparticles can fill colloidal crystal template with the diameter of 300 nm successfully by manipulating a larger driving potential in 5 min. The EPD is a simple, efficient, low cost and highly reproducible process for nano-channel-filling. The driving potential and nanoparticle concentration will influence the EPD current curves. Moreover, the optimum filling-behavior of nano-channel can be determined by the trend of current curve. During the short process time, the driving potential and nanoparticle concentration must be controlled appropriately in order to avoid the jam of template surface. 3.3. Fabrication of ZnO inverse opal After removing colloidal crystal template by calcination in air at 330 ◦ C for 8 h, ZnO inverse opal can be obtained. Fig. 6(a) shows an SEM image of the top surface of a 3D well-ordered ZnO replica formed by deposition of ZnO nanoparticles into a colloidal crystal template. It is similar to the original structure of colloidal crystal
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
3681
Fig. 7. Absorptive spectrum of ZnO inverse opal.
Fig. 6. SEM images showing ZnO macroporous structure fabricated via a template-mediated electrophoresis process at (a) low magnification and (b) high magnification.
that each spherical void is surrounded by six spherical cavities. Hence, it is apparent that the topmost plane of cells in the replica exhibits a close-packed pattern. According to the results of Fig. 6(b) in high magnification, each void formed after removing the colloidal particles has three dark spots corresponding to the contact points of the three particles in the layer below. Hence, ZnO inverse opal films are reproduced accurately through the deposition of ZnO into colloidal crystal template via a template-mediated electrophoresis process. Moreover, this nanostructure is also a well-ordered macroporous structure. The inverse opals with well-ordered arrangement of macropores possess the unique optical properties of photonic crystals. There is a characteristic absorptive peak at the wavelength of 560 nm in the spectrum, as shown in Fig. 7. This optical phenomenon can be estimated according to the modified Bragg’s law [25] shown in Eq. (1): = 1.633dnaverage
(1)
where and d are the absorptive wavelength and center-tocenter distance between two neighboring spheres, respectively. The naverage is the average refractive index of materials and can be expressed [26] in Eq. (2): n2average = VZnO n2ZnO + Vair n2air
(2)
where VZnO and Vair are the volume fractions of the ZnO nanoparticles and the air voids, respectively. Because the colloidal crystal templates appear as the FCC structure, their replicas (inverse opals) also possess the same structure. Hence, Vair is equal to 0.74 and
the VZnO is 0.26. Moreover, the refractive index of ZnO nanoparticles [27] and air are 2.2 and 1.0, respectively. Finally, the average refractive index of inverse opal (naverage ) can be calculated to be 1.41 according to Eq. (2). From the result of Fig. 4(b), the diameter of spherical hole (d) is equal to 240 nm. To combine naverage with d value, the theoretical value for the absorptive wavelength of inverse opal can be estimated to be 551.6 nm according to Eq. (1). This value (551.6 nm) is close to the experimental value for the absorptive wavelength of inverse opal (560 nm), so the optical property of inverse opal can be characterized according to the modified Bragg’s law. Due to the structural defects in the inverse opal, including domain boundary, dislocations and point defects, the d value from the modified Bragg’s law will slightly change, which then causes the discrepancy between theoretical and experimental value. Further, compared with the absorptive wavelength of 654 nm for colloidal crystal (artificial opal) consisting of 300 nm colloidal particles, the blue-shift property can be obtained after fabricating inverse opal structure. Although ZnO inverse opal can be fabricated through using other dry processes, including atomic layer deposition [28] and chemical vapor deposition [29], both processes need special and expensive apparatus. Recently, the electrochemical deposition method [30] for forming ZnO inverse opal is also utilized. However, this process is only appropriate for the specific oxide material. The fabrication of ZnO inverse opal through employing EPD is a new and efficient process. Moreover, the EPD method [25] is also a versatile process, i.e., the materials which can be prepared in nanoparticles are able to be employed for constructing the inverse opal by EPD, including ceramic, semiconductor, polymeric and metallic materials. 4. Conclusion A high-quality colloidal crystal template with FCC structure can be formed efficiently during a 24-h period by employing the capillary-enhanced evaporation method. Although electrophoresis is an efficient process for filling nano-channels of colloidal crystals, jamming of nano-channel is a main challenge. According to the variation of current (I–t) curve, a better condition of electrophoresis process can be found. Finally, ZnO inverse opal can be fabricated after removing template by calcination, with a characteristic absorptive wavelength of photonic crystals at 560 nm. Therefore, high-quality ZnO inverse opal can be fabricated by using a template-mediated electrophoresis process. Also it is expected that the EPD can find extensive applications for preparing photonic crystals of various oxides only if their nanoparticles are available.
3682
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
Acknowledgement The authors thank Professor W.T. Jiang of the Department of Earth Sciences, National Cheng Kung University, for his assistance with the TEM experiment. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
A.W. Castleman, K.H. Bowen, J. Phys. Chem. 100 (1996) 12911. A.P. Alivisators, Science 271 (1996) 933. M. Sundaram, S.A. Chalmers, P.F. Hopkins, A.C. Gossard, Science 254 (1991) 1326. S.I. Matsushita, T. Miwa, D.A. Tryk, A. Fujishima, Langmuir 14 (1998) 6441. J.H. Holtz, S.A. Asher, Nature 389 (1997) 829. T. Yamada, H.S. Zhou, H. Uchida, M. Tomita, Y. Ueno, I. Honma, K. Asai, T. Katsube, Micropor. Mesopor. Mater. 54 (2002) 269. S.A. Pruzinsky, P.V. Braun, Adv. Funct. Mater. 15 (2005 1995). J. Galloro, M. Ginzburg, H. Miguez, S.M. Yang, N. Coombs, A.S. Sefat, J.E. Greedan, I. Manners, G.A. Ozin, Adv. Funct. Mater. 12 (2002) 382. T. Sumida, Y. Wada, T. Kitamura, S. Yanagida, Chem. Lett. 30 (2001) 38. Y.C. Wang, I.C. Leu, M.H. Hon, J. Cryst. Growth 564 (2002) 237. X.D. Wang, E. Graugnard, J.S. King, Z.L. Wang, C.J. Summers, Nano Lett. 4 (2004) 2223. Y.A. Vlasov, X.Z. Bo, J.C. Sturm, D.J. Norris, Nature 414 (2001) 289. S. Ono, M. Saito, M. Ishiguro, H. Asoh, J. Electrochem. Soc. 151 (2004) B473.
[14] A.A. Zakhidov, R.H. Baughman, Z. Iqbal, C.X. Cui, T. Khayrullin, S.O. Dantas, I. Marti, V.G. Ralchenko, Science 282 (1998) 897. [15] Z.Z. Gu, A. Fujishima, O. Sato, Chem. Mater. 14 (2002) 760. [16] M.T. Wu, I.C. Leu, J.H. Yen, M.H. Hon, J. Phys. Chem. B 109 (2005) 9575. [17] P.H. Wang, C.Y. Pan, Colloid Polym. Sci 279 (2001) 98. [18] Y.W. Chung, I.C. Leu, J.H. Lee, M.H. Hon, Appl. Phys. A 79 (2004 2089). [19] D.W. Bahnemann, C. Kormann, M.R. Hoffmann, J. Phys. Chem. 91 (1987) 3789. [20] H. Miguez, F. Meseguer, C. Lopez, A. Mifsud, J.S. Moya, L. Vazquez, Langmuir 13 (1997) 6009. [21] Y.C. Wang, I.C. Leu, M.H. Hon, J. Am. Ceram. Soc. 87 (2004) 84. [22] Y.C. Wang, I.C. Leu, M.H. Hon, J. Appl. Phys. 95 (2004) 1444. [23] J.A. Switzer, Am. Ceram. Soc. Bull. 66 (1987) 1521. [24] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., Wiley, New York, 2001, p. 164. [25] Z.Z. Gu, S. Hayami, S. Kubo, Q.B. Meng, Y. Einaga, D.A. Tryk, A. Fujishima, O. Sato, J. Am. Chem. Soc. 123 (2001) 175. [26] M.G. Leonid, W. Jurgen, S. Joachim, P. Bernd-R, G. Eckhard, Langmuir 18 (2002) 3319. [27] C. Lopez, Adv. Mater. 15 (2003) 1679. [28] M. Scharrer, X. Wu, A. Yamilov, H. Cao, R.P.H. Chang, Appl. Phys. Lett. 86 (2005) 151113. [29] B.H. Juarez, P.D. Garcia, D. Golmayo, A. Blanco, C. Lopez, Adv. Mater. 17 (2005 2761). [30] H. Yan, Y. Yang, Z. Fu, B. Yang, L. Xia, S. Fu, F. Li, Electrochem. Commun. 7 (2005) 1117.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Discussion
Filling behavior of ZnO nanoparticles into opal template via electrophoretic deposition and the fabrication of inverse opal Yi-Wen Chung a , Ing-Chi Leu b,∗ , Jian-Hong Lee c , Min-Hsiung Hon c a
Flexible Electronics Equipment Dep., Laser Application Technology Center, South Industrial Technology Research Institute, No. 8. Gongyan Rd., Liujia Shiang, Tainan County, 734, Taiwan, ROC b Department of Material Science, National University of Tainan, 33, Sec. 2, Shu-Lin St., Tainan, 360, Taiwan, ROC c Department of Materials Science and Engineering, National Cheng Kung University, 1, Ta-Hsueh Rd., Tainan, 701, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 6 September 2008 Received in revised form 3 December 2008 Accepted 1 January 2009 Available online 15 January 2009 Keywords: Electrophoretic deposition Colloidal template-mediated process Inverse opal ZnO photonic crystal ZnO nanoparticles
a b s t r a c t Combining colloidal crystal template (artificial opal) and electrophoretic deposition (EPD) process, wellordered ZnO inverse opal can be formed by finding the optimum driving potential of EPD. Through providing the various driving potentials from −25 V, −10 V, −5 V to −2.5 V, the different mechanism of electrophoretically depositing ZnO nanoparticles into the colloidal crystal template was determined by the SEM observation of the filled templates. Because the nano-channels of colloidal crystal template are the network type, the results of surface jam, incomplete filling and perfect filling are found under specific applied voltages. The high-quality ZnO inverse opal can be only fabricated under the perfect nano-channel-filling condition. The filling behavior can be monitored dynamically by tracing the current transients, and the optimum conditions for filling the interstitial spaces of templates constructed from colloidal particles with 180 nm and 300 nm diameter can be obtained by applying a voltage of −5 V and −15 V, respectively. After the complete filling of ZnO nanoparticles into the colloidal crystal template consisting of 300 nm colloids, high-quality ZnO photonic crystal possessing an absorptive peak at the wavelength of 560 nm can be fabricated by removing the template. It is expected that the EPD can find extensive applications for preparing photonic crystals of various oxides only if their nanoparticles are available. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction The atomic clusters [1], nanoparticles [2] and nanostructures [3] of materials exhibit size-dependent properties that are profoundly different from their corresponding bulk material counterparts. Even, there has been extensive interest in the creation of nano-porous materials with three-dimensional periodicity due to potential applications, such as catalysts [4], chemical [5] and gas [6] sensors, photonic [7] and magnetic devices [8]. When ZnO macroprous structure was fabricated, it is useful for the applications in sensors and semiconducting electrodes in virtue of the large internal surface area and improved mass transportation originated from size-controlled macropores [9]. The template-mediated process is a conventional and important technique in forming nanostructure, including 1D nano-fiber [10] and 2D [11] macroporous monolayer or 3D well-ordered macroporous structures (the so-called inverse opal) [12]. The entire template-mediated process contains three steps. Firstly, appropri-
∗ Corresponding author. E-mail address: [email protected] (I.-C. Leu). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.01.013
ate templates must be fabricated in order to act as a host. Secondly, the material of interest can be filled into the interstitial space of template via induced filling of precursors into nano-channel. In general, nano-channel-filling technologies include chemical vapor deposition [13], dipping process [14], electrophoresis [10] and electrodeposition [15]. Finally, the replica can be obtained by removing template. Among these filling methods, electrophoresis is a powerful tool for fabrication of nanostructures. As compared to other methods, the advantages of electrophoresis deposition process (EPD) are simplicity, efficient, material general, low cost and highly reproducibility. Besides, it has also been widely applied for coatings and laminated materials in the micrometer size region. However, the main disadvantage is the possibility of jamming of the nanochannels leading to the poor filling ratio during the material filling process [16]. An illustration of electrophoresis process is shown in Fig. 1. In this technique, an electric field is used to perform the filling of voids of the colloidal crystal template with nanoparticles. The entire process includes two steps. First, nanoparticles in a suspension are forced to move toward working electrode having an opposite charge by applying an electric field to the suspension. Second, the particles are collected at the electrode and fill the interstitial spaces of template, until complete infiltration is achieved.
3678
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
Fig. 2. High-quality colloidal crystal with 300 nm colloidal diameter fabricated via a capillary-enhanced process.
Fig. 1. A schematic diagram of electrophoresis process.
In this manuscript we report the preparation of well-ordered ZnO macroporous structure via a template-mediated electrophoresis process. Under the different potentials of electrophoresis, different behaviors for filling interstitial spaces will be described. Because the colloidal crystal template appears as face-centered cubic (FCC) structure as prepared via the capillary-enhanced evaporation process, a ZnO replica constructed by removing the template is expected to have the well-ordered macroporous structure (inverse opal), and also can be used as photonic crystals. 2. Experimental section 2.1. Preparation of opal template Monodispersed Poly(stylene-co-methacrylic acid) microshperes were synthesized according to an emulsifier-free-emulsion copolymerization process [17], where styrene (St) and methacrylic acid served as reactants. The process, capillary-enhanced evaporation method, for preparing high-quality colloidal crystals (the so-called artificial opal) is characterized by the presence of multi-capillary forces [18]. First, the substrate was placed horizontally at the bottom of a container, which was filled with an aqueous dispersion of the P(St-co-MAA) microspheres obtained by ultrasonication. Then, the self-assembly of microspheres and the formation of colloidal crystals were conducted at 45◦ C while maintaining high humidity (90%). After 24 h, evaporation was terminated and the substrate was covered with colloidal crystal films. 2.2. Electrophoresis process of ZnO nanoparticles The fabrication of ZnO nanoparticle suspensions followed the method of Bahenmann et al. [19]. The ZnO colloidal suspensions were synthesized from zinc acetate (ACS reagent grade) in 2propanol (ACS reagent grade) with NaOH (ACS reagent grade) added to adjust the pH. Then, a transparent suspension of ZnO nanoparticles was obtained and the particle diameter was determined by transmission electron microscopy to be about 5 nm.
The schematic illustration of electrophoresis process and equipment is shown in Fig. 1. The conductive ITO-glass substrate with deposited colloidal crystal and a platinum plate were used as the working electrode and the counter electrode, respectively. The distance between the two electrodes was 2 cm. The electrophoresis was performed at different voltages. After electrophoresis, calcinations at 330 ◦ C for 8 h were required to both remove the P(St-co-MAA) microspheres and sinter the oxide nanoparticles. Finally, 3D macroporous structure of ZnO can be formed on the substrate. 3. Results and discussion 3.1. Fabrication of colloidal crystal template Colloidal crystal template (also named artificial opal) for synthesizing macroporous structure studied in this work was prepared using an efficient technology described previously [18], with an entire process time in only 24 h. In general, the gathering behavior of colloidal particles generally depends on an appropriate driving force. Subsequently, the self-assembly of colloidal particles can then form a crystal-like structure, named colloidal crystal. The capillary-enhanced method relies on multiple capillary driving forces, including primary and secondary capillary forces by controlling the ambient humidity, to organize colloids. Finally, high-quality colloidal crystal will be fabricated. SEM image of the (1 1 1) surface of the synthetic crystal reveals a close-packed arrangement, as shown in Fig. 2. Each colloidal particle is surrounded by other six colloids. Only few vacancies and dislocations are found in this domain. As shown in the inset figure, the square pattern in (1 0 0) facet can be observed clearly. In general, this geometric arrangement of colloidal particles indicates the stack of face-centered cubic structure [20]. 3.2. The filling characteristics of ZnO nanoparticles via electrophoresis process Preparation of nanoparticles with a small diameter is an important step for template-mediated electrophoresis process. This is because smaller nanoparticles will migrate easier in the interstitial space of colloidal crystal template. Zeta-potential measurements show that the surface charge of the ZnO nanoparticles is positive. Thus, a negative voltage is applied to the working electrode in order
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
3679
Fig. 3. The relationship between current and time for ZnO electrophoresis in the interstitial space of colloidal crystal template with different diameter and voltage. (a) 180 nm, −25 V, (b) 180 nm, −10 V, (c) 180 nm, −5 V, (d) 180 nm, −2.5 V and (e) 300 nm, −15 V.
to infiltrate the colloidal crystal template with nanoparticles, as shown in Fig. 1. The main disadvantage of EPD is the jamming of nano-channels near the surface. Hence, the effect of varied voltages on the possibility of jamming, including −25 V, −10 V, −5 V and −2.5 V, are studied for the template constructing from 180 nm colloidal particles. The recorded current transient curves for the first 60 s are shown in Fig. 3. The trend of curve (a) is different from the other curves. When a larger voltage (−25 V) is employed in the process of filling interstitial space, a large nanoparticle flux will be induced near the template surface. This will cause jamming of the mouth of the interstitial space near the template surface with ZnO nanoparticles. After jamming, ZnO nanoparticles agglomerate rapidly on the surface and form a continuous layer. This phenomenon will cause an increase of the electrophoretic current values quickly for the first 7 s of deposition. P(St-co-MAA) microsphere is a polar macromolecule and possess a negative zeta-potential in the suspension. It will adsorb the ZnO nanparticles with positive zeta-potential before the interstitial spaces of colloidal template are filled by EPD process. Even though the surface of colloidal crystal template is jammed by the ZnO nanoparticles, electrons can still be transported from ITO-glass substrate through the path constituted by ZnO coatings possessing a certain degree of electrical conductivity, and finally reach the surface. Hence, the surface jamming cannot screen the electric field. The ZnO nanoparticles continuously migrate toward the working electrode under the application of an electrical field. Then, the thickness of coating will increase gradually with electrophoresis time, which in turn results in an increase of electric resistance. Hence, the trend of current values appears to decrease accordingly for a long time. As shown in Fig. 4(a), there is a coating consisting of ZnO nanoparticles on the template surface for a deposition time of 1 min. In brief, the entire process exhibits the deposition characteristics similar to films deposited on planar surface by electrophoresis [21]. As observed in the inset figure, the surface of P(St-co-MAA) colloidal particles had adsorbed many ZnO nanoparticles, which is due to the electrostatics-induced adsorption effect between the positively charged ZnO and negatively charged PS microsphere. These ZnO nanoparticles adsorbed onto the PS particle surface also act as the medium for conducting electrons to working electrode. The recorded current curves (b–d), as compared with curve (a), are shown to exhibit a peak in the initial stage of filling
Fig. 4. ZnO nanoparticles filled into interstitial spaces of colloidal crystal template by electrophoresis with different voltages of (a) −25 V, (b) −10 V, (c) −5 V and (d) −2.5 V.
3680
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
interstitial spaces via an electrophoresis process. This is the phenomenon of nano-channel-filling resulting from the presence of time-dependent diffusion regime [22]. When a driving potential is applied at the working electrode, current value will first increase rapidly due to a large number of ZnO nanoparticles moving to working electrode. Subsequently, the decrease of current indicates the consumption of electroactive species (i.e., salt ions and complexes) [23] and ZnO nanoparticles near the electrode and the formation of a diffusion layer near the electrode [24]. Hence, the migration characteristics of ZnO nanoparticles in the interstitial spaces will influence the current value. The curve (b) of Fig. 3 performed at −10 V possesses a larger current value and a weaker peak. When the large nanoparticle flux is caused by applying a potential (−10 V), both the filling of interstitial spaces and jamming of template surface occur in 5 min of deposition. As shown in Fig. 4(b), the inset figure shows a thick coating covering the surface. Moreover, the boundary between filled and unfilled interstitial spaces can be observed clearly, as indicated by a white arrow in Fig. 4(b). The large current values in the initial stage result from the large driving potential (−10 V). If the effect of time-dependent diffusion regime [22] can be more pronounced, the recorded current curve (b) will possess a strong peak in the initial stage of EPD. However, the jamming of template surface will occur under this condition. The turning point which takes place at 13 s in curve (b) means the onset of the jamming of the template surface. The ZnO nanoparticles in the solution not only deposit into the interstitial spaces, but also accumulate onto the template surface. Thereafter, recorded current values caused by the effect of ZnO nanoparticle accumulation on template surface are larger, which is similar to the curve (a) in Fig. 3. Hence, this phenomenon exhibits a weaker peak in the initial stage of EPD. Finally, the interstitial spaces of colloidal crystal template are filled incompletely with ZnO nanoparticles, and the surface of colloidal crystal template is then covered completely. In order to infiltrate the nanoparticles into the interstitial spaces, optimization of parameter had been tried. The jamming of surface nano-channels can be avoided by decreasing voltages in the EPD system. Fig. 4(c) shows the SEM micrograph of colloidal crystal that is filled with ZnO nanoparticles. The electrophoresis is performed at constant voltage of −5 V and for a deposition time of 5 min. Firstly, ZnO nanoparticles in the interstitial spaces of colloidal crystal template will move towards and accumulate onto the substrate surface. Subsequently, ZnO nanoparticles in the solution will also be continuously attracted and move into the interstitial spaces. Finally, the interstitial space among the P(St-co-MAA) microspheres can be filled completely from bottom to top, as depicted in insets of Fig. 4(c). The jamming of template surface observed at high electrophoretic voltage had disappeared. The curve (c) in Fig. 3 possesses a sharp peak, which is because the effect of time-dependent diffusion is more significant for the present case without surface jamming. The current values are smaller due to the only source for charge transport from the ZnO nanoparticle infiltrating into the interstitial space. This result demonstrates that the ZnO nanoparticles can move under a voltage condition of −5 V, and fill the interstitial spaces of colloidal crystal template completely. The deposit thickness fabricated at −10 V in 5 min via an electrophoresis process is about 14 m, as shown in Fig. 4(b). This value is lower than that (20 m) fabricated at −5 V in 5 min. This result is not anticipated, because a larger driving potential is expected to obtain a thicker film. The main reason is the jamming of template surface. The surface jamming decreases the efficiency of infiltration into the interstitial spaces, so a thinner coating (14 m) can be obtained at −10 V in 5 min via an electrophoretic process. Hence, according to the result of the filling thickness of the interstitial spaces, it is also demonstrated that the jamming of template surface and the infiltration of interstitial space occur in the same time.
Fig. 5. ZnO nanoparticles filled into interstitial spaces of colloidal crystal template by electrophoresis with a voltage of −2.5 V in a suspension with high nanoparticle concentration.
If a potential of −2.5 V is applied, a smaller nanoparticle flux can be caused. Because of lower driving potential, the curve (d) of Fig. 3 possesses a smaller current value with weaker peak. The surface jamming is not found under the low driving potential of −2.5 V, but ZnO nanoparticles cannot fill interstitial spaces completely. Some voids are still observed, as shown in Fig. 4(d). However, increase of nanoparticle concentration will solve this problem. When the concentration of ZnO nanoparticles is increased from 2 × 10−5 g/ml to 2 × 10−4 g/ml, the interstitial spaces can be filled completely at the lower driving potential of −2.5 V, as shown in Fig. 5. This is because the increase of the nanoparticle concentration will result in the flux and the recorded current values elevating. On the other hand, larger interstitial spaces of colloidal crystal consisting of 300 nm colloidal particles can be filled with ZnO nanoaprticles by providing a larger potential (−15 V). Although the small driving potential can also drive the nanoparticles into larger interstitial space completely, the process time is longer. According to previous description for colloidal crystals using 180 nm colloidal particles, the larger driving potential will lead to the jam of template surface. However, the curve (e) in Fig. 3 for EPD using a larger potential (−15 V) for colloidal crystals using 300 nm colloidal particles is similar to the shape of curve (c) using 180 nm colloidal particles at −5 V. This is an optimum condition for filling the interstitial spaces of colloidal crystal template with diameter of 300 nm by EPD process, where complete filling can be obtained at larger driving potential. Therefore, the ZnO nanoparticles can fill colloidal crystal template with the diameter of 300 nm successfully by manipulating a larger driving potential in 5 min. The EPD is a simple, efficient, low cost and highly reproducible process for nano-channel-filling. The driving potential and nanoparticle concentration will influence the EPD current curves. Moreover, the optimum filling-behavior of nano-channel can be determined by the trend of current curve. During the short process time, the driving potential and nanoparticle concentration must be controlled appropriately in order to avoid the jam of template surface. 3.3. Fabrication of ZnO inverse opal After removing colloidal crystal template by calcination in air at 330 ◦ C for 8 h, ZnO inverse opal can be obtained. Fig. 6(a) shows an SEM image of the top surface of a 3D well-ordered ZnO replica formed by deposition of ZnO nanoparticles into a colloidal crystal template. It is similar to the original structure of colloidal crystal
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
3681
Fig. 7. Absorptive spectrum of ZnO inverse opal.
Fig. 6. SEM images showing ZnO macroporous structure fabricated via a template-mediated electrophoresis process at (a) low magnification and (b) high magnification.
that each spherical void is surrounded by six spherical cavities. Hence, it is apparent that the topmost plane of cells in the replica exhibits a close-packed pattern. According to the results of Fig. 6(b) in high magnification, each void formed after removing the colloidal particles has three dark spots corresponding to the contact points of the three particles in the layer below. Hence, ZnO inverse opal films are reproduced accurately through the deposition of ZnO into colloidal crystal template via a template-mediated electrophoresis process. Moreover, this nanostructure is also a well-ordered macroporous structure. The inverse opals with well-ordered arrangement of macropores possess the unique optical properties of photonic crystals. There is a characteristic absorptive peak at the wavelength of 560 nm in the spectrum, as shown in Fig. 7. This optical phenomenon can be estimated according to the modified Bragg’s law [25] shown in Eq. (1): = 1.633dnaverage
(1)
where and d are the absorptive wavelength and center-tocenter distance between two neighboring spheres, respectively. The naverage is the average refractive index of materials and can be expressed [26] in Eq. (2): n2average = VZnO n2ZnO + Vair n2air
(2)
where VZnO and Vair are the volume fractions of the ZnO nanoparticles and the air voids, respectively. Because the colloidal crystal templates appear as the FCC structure, their replicas (inverse opals) also possess the same structure. Hence, Vair is equal to 0.74 and
the VZnO is 0.26. Moreover, the refractive index of ZnO nanoparticles [27] and air are 2.2 and 1.0, respectively. Finally, the average refractive index of inverse opal (naverage ) can be calculated to be 1.41 according to Eq. (2). From the result of Fig. 4(b), the diameter of spherical hole (d) is equal to 240 nm. To combine naverage with d value, the theoretical value for the absorptive wavelength of inverse opal can be estimated to be 551.6 nm according to Eq. (1). This value (551.6 nm) is close to the experimental value for the absorptive wavelength of inverse opal (560 nm), so the optical property of inverse opal can be characterized according to the modified Bragg’s law. Due to the structural defects in the inverse opal, including domain boundary, dislocations and point defects, the d value from the modified Bragg’s law will slightly change, which then causes the discrepancy between theoretical and experimental value. Further, compared with the absorptive wavelength of 654 nm for colloidal crystal (artificial opal) consisting of 300 nm colloidal particles, the blue-shift property can be obtained after fabricating inverse opal structure. Although ZnO inverse opal can be fabricated through using other dry processes, including atomic layer deposition [28] and chemical vapor deposition [29], both processes need special and expensive apparatus. Recently, the electrochemical deposition method [30] for forming ZnO inverse opal is also utilized. However, this process is only appropriate for the specific oxide material. The fabrication of ZnO inverse opal through employing EPD is a new and efficient process. Moreover, the EPD method [25] is also a versatile process, i.e., the materials which can be prepared in nanoparticles are able to be employed for constructing the inverse opal by EPD, including ceramic, semiconductor, polymeric and metallic materials. 4. Conclusion A high-quality colloidal crystal template with FCC structure can be formed efficiently during a 24-h period by employing the capillary-enhanced evaporation method. Although electrophoresis is an efficient process for filling nano-channels of colloidal crystals, jamming of nano-channel is a main challenge. According to the variation of current (I–t) curve, a better condition of electrophoresis process can be found. Finally, ZnO inverse opal can be fabricated after removing template by calcination, with a characteristic absorptive wavelength of photonic crystals at 560 nm. Therefore, high-quality ZnO inverse opal can be fabricated by using a template-mediated electrophoresis process. Also it is expected that the EPD can find extensive applications for preparing photonic crystals of various oxides only if their nanoparticles are available.
3682
Y.-W. Chung et al. / Electrochimica Acta 54 (2009) 3677–3682
Acknowledgement The authors thank Professor W.T. Jiang of the Department of Earth Sciences, National Cheng Kung University, for his assistance with the TEM experiment. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
A.W. Castleman, K.H. Bowen, J. Phys. Chem. 100 (1996) 12911. A.P. Alivisators, Science 271 (1996) 933. M. Sundaram, S.A. Chalmers, P.F. Hopkins, A.C. Gossard, Science 254 (1991) 1326. S.I. Matsushita, T. Miwa, D.A. Tryk, A. Fujishima, Langmuir 14 (1998) 6441. J.H. Holtz, S.A. Asher, Nature 389 (1997) 829. T. Yamada, H.S. Zhou, H. Uchida, M. Tomita, Y. Ueno, I. Honma, K. Asai, T. Katsube, Micropor. Mesopor. Mater. 54 (2002) 269. S.A. Pruzinsky, P.V. Braun, Adv. Funct. Mater. 15 (2005 1995). J. Galloro, M. Ginzburg, H. Miguez, S.M. Yang, N. Coombs, A.S. Sefat, J.E. Greedan, I. Manners, G.A. Ozin, Adv. Funct. Mater. 12 (2002) 382. T. Sumida, Y. Wada, T. Kitamura, S. Yanagida, Chem. Lett. 30 (2001) 38. Y.C. Wang, I.C. Leu, M.H. Hon, J. Cryst. Growth 564 (2002) 237. X.D. Wang, E. Graugnard, J.S. King, Z.L. Wang, C.J. Summers, Nano Lett. 4 (2004) 2223. Y.A. Vlasov, X.Z. Bo, J.C. Sturm, D.J. Norris, Nature 414 (2001) 289. S. Ono, M. Saito, M. Ishiguro, H. Asoh, J. Electrochem. Soc. 151 (2004) B473.
[14] A.A. Zakhidov, R.H. Baughman, Z. Iqbal, C.X. Cui, T. Khayrullin, S.O. Dantas, I. Marti, V.G. Ralchenko, Science 282 (1998) 897. [15] Z.Z. Gu, A. Fujishima, O. Sato, Chem. Mater. 14 (2002) 760. [16] M.T. Wu, I.C. Leu, J.H. Yen, M.H. Hon, J. Phys. Chem. B 109 (2005) 9575. [17] P.H. Wang, C.Y. Pan, Colloid Polym. Sci 279 (2001) 98. [18] Y.W. Chung, I.C. Leu, J.H. Lee, M.H. Hon, Appl. Phys. A 79 (2004 2089). [19] D.W. Bahnemann, C. Kormann, M.R. Hoffmann, J. Phys. Chem. 91 (1987) 3789. [20] H. Miguez, F. Meseguer, C. Lopez, A. Mifsud, J.S. Moya, L. Vazquez, Langmuir 13 (1997) 6009. [21] Y.C. Wang, I.C. Leu, M.H. Hon, J. Am. Ceram. Soc. 87 (2004) 84. [22] Y.C. Wang, I.C. Leu, M.H. Hon, J. Appl. Phys. 95 (2004) 1444. [23] J.A. Switzer, Am. Ceram. Soc. Bull. 66 (1987) 1521. [24] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, 2nd ed., Wiley, New York, 2001, p. 164. [25] Z.Z. Gu, S. Hayami, S. Kubo, Q.B. Meng, Y. Einaga, D.A. Tryk, A. Fujishima, O. Sato, J. Am. Chem. Soc. 123 (2001) 175. [26] M.G. Leonid, W. Jurgen, S. Joachim, P. Bernd-R, G. Eckhard, Langmuir 18 (2002) 3319. [27] C. Lopez, Adv. Mater. 15 (2003) 1679. [28] M. Scharrer, X. Wu, A. Yamilov, H. Cao, R.P.H. Chang, Appl. Phys. Lett. 86 (2005) 151113. [29] B.H. Juarez, P.D. Garcia, D. Golmayo, A. Blanco, C. Lopez, Adv. Mater. 17 (2005 2761). [30] H. Yan, Y. Yang, Z. Fu, B. Yang, L. Xia, S. Fu, F. Li, Electrochem. Commun. 7 (2005) 1117.