Switchable Wettability in SnO2 Nanowires and SnO2@SnO2

Sep 30, 2011 - The switchable surface wettability of SnO2 nanostructures may develop the significant potential for industrial coating and self-cleanin...
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Switchable Wettability in SnO2 Nanowires and SnO2@SnO2 Heterostructures Jun Pan,†,‡ Xuefeng Song,† Jun Zhang,‡ Hao Shen,*,† and Qihua Xiong*,‡,§ †

Institute of Inorganic Chemistry, University of Cologne, D-50939 Cologne, Germany Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore § Division of Microelectronics, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 ‡

ABSTRACT: We present here the synthesis of SnO2 nanowires SnO2@SnO2 and SnO2@SnO2@SiOx heterostructures via the chemical vapor deposition (CVD) method. Surface wettability could be modulated from superhydrophilic to superhydrophobic by changing both chemical composition and the geometrical architecture of nanoheterostructures. The random SnO2 nanowires with a contact angle of 3° presented a superhydrophilic property. The contact angle of SnO2@SnO2 heterostructures synthesized by a two-step CVD process increased to 133°. The corresponding contact angle of SnO2@SnO2@SiOx heterostructures was 155.8°. Switchable surface wettability of the SnO2@SnO2@SiOx heterostructure was observed by alternation of UV irradiation, dark storage, and O2 annealing, indicating that geometric microstructure was the major determinant in the switchable wettability from superhydrophilic to superhydrophobic. The switchable surface wettability of SnO2 nanostructures may develop the significant potential for industrial coating and self-cleaning system.

’ INTRODUCTION Wettability is an important characteristic of a solid surface. It affects the degree of interaction on a surface and concerns the surrounding species including water vapor and other chemicals. It is also related to surface energy of a solid surface which is the basic driving force for many surface reactions and interactions. Hence, the knowledge regarding wettability of a surface and perhaps more significantly, the ability to control it, are important for various surface related technologies.1,2 Wettability is commonly quantified by measuring the water contact angle (CA). A surface with a water CA larger than 150° is defined as a superhydrophobic surface. Superhydrophobic surfaces are especially beneficial for applications in nanotechnology, anticorrosion, resisting water coalescence, self-cleaning systems, microelectromechanical systems (MEMS), and solid lubrication systems that require a hydrophobic surface to decrease adhesion and friction.3,4 In addition, the superhydrophobic surface offers much promise for the formation of high-performance nanostructured surfaces with multifunctionality that can be used in optical, photoelectric, microelectronic, and biomedical applications.5,6 On the other hand, extreme wetting surfaces, or superhydrophilic surfaces with a water CA close to 0°, may be beneficial for applications in catalysts and surface reactant systems.7,8 This study focuses on the synthesis of one-dimensional nanostructures based superhydrophilic and superhydrophobic surfaces and switchable surface wettability. r 2011 American Chemical Society

The study of controlling surface wettability has attracted considerable attention in the past. In controlling the wettability of a surface, the chemical composition and the geometrical structure on superhydrophobicity have been elucidated by Wenzel and Cassie models. Surface roughness increases the surface area of the solid, which geometrically enhances its hydrophobicity, when the smooth surface exhibits a CA larger than 90°. If the flat surface has a CA smaller than 90°, the rough one becomes more hydrophilic (Wenzel model).9 On the other hand, air becomes trapped below the water drop, which also leads to a superhydrophobic behavior, because the drop sits on air partially (Cassie model).10 However both the models have put emphasis on the geometrical structure of solid surfaces as an important factor in determining the wettability. Tin oxide (SnO2) with rutile structure is an important wide band gap (3.6 eV at 300 K) semiconductor, which is well-known for its excellent transparency and conductivity, and has been widely used in many fields such as optoelectronics,11 gas sensors,12 energy storage,13 and energy conversion.14 In the past several years, nanostructured SnO2 has attracted much attention due to its unique properties,15 these studies are mainly focused on its gas sensitivity16 and electricity,17 and few reports have concerned its surface wettability.18 Especially, many works on the Received: August 2, 2011 Revised: September 26, 2011 Published: September 30, 2011 22225

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The Journal of Physical Chemistry C wettability switching of superhydrophobicity and superhydrophilicity are limited on the nanorod films.19,20 However, to our knowledge, little is known on the wettability of SnO2 nanowire based branched structure surface.21 More importantly, the switching from superhydrophilic to superhydrophobic surface based on SnO2 nanowire by chemical vapor deposition (CVD) methods has not been reported; it is completely different with the widely used chemical modification to obtain the superhydrophobic surface.22 24 This study may develop the potential application in industrial coating and self-cleaning system. In this study, we reported an effective method to convert the SnO2 nanowire based surface from superhydrophilic to superhydrophobic by a three-step CVD growth and attempted to present a clear portrait of the key factor in determining the surface wettability. Moreover, we demonstrated the variation of the superhydrophobic surface by employing UV irradiation, dark storage and O2 annealing.

’ EXPERIMENTAL SECTION Synthesis and Characterizations of Nanostructures. Tin oxide nanostructures were grown by the decomposition of Sn(OtBu)4 at 650 750 °C, in a cold-walled quartz CVD reactor.25 A high-frequency field was used to inductively heat the substrates (Au-coated Al2O3) by placing them on a graphite holder for 30 90 min, with the precursor reservoir maintained at 25 °C to ensure a significant feedstock in the gas phase. Ultrasonic cleaning of the substrates was performed before Au deposition (Cressington Sputter Coater Type 108 Auto). Sn(OtBu)4 was introduced into the reactor through a glass flange by applying a dynamic vacuum (10 6 Torr). Growth of heterostructures SnO2@VOx and SnO2@SnO2 was obtained by decomposing VO(OiPr)3 and Sn(OtBu)4 at 600 650 and 650 750 °C, respectively, on pregrown SnO2 nanowires acting as semiconductor substrates for a second-step growth via CVD. The SiOx film was deposited on heterostructures by decomposing hexamethyldisiloxane (HMDSO) via plasma-enhanced CVD. The annealing treatment was performed under O2 atmosphere at 50 °C. Room-temperature powder X-ray diffraction (XRD) was performed with a STOE-STADI MP diffractometer using a Cu Kα radiation. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) analysis were performed with an EDS-coupled scanning electron microscope Nvision 40 (Zeiss). For transmission electron microscopy (TEM) analyses, nanostructures were mechanically transferred on carbon-coated copper grids for TEM measurements. In these studies, a Philips 200 FEG (200 kV) transmission electron microscope was used. Raman scattering spectroscopy was conducted at room temperature using a Micro-Raman spectrometer (Horiba-JY T64000) in a backscattering configuration. A solid state laser (λ = 532 nm) was used to excite the sample. The laser spot at sample surface was around 1 μm with a power 6 mW. X-ray photoelectron spectroscopic (XPS) measurements were performed on a PerkinElmer Model 5600 spectrometer. Aluminum Kα radiation (photon energy, 1486.6 eV) was used, and photoelectrons were collected at a takeoff angle of 45° with respect to the film surface normal. Argon ion sputtering was carried out in the same ultrahigh-vacuum chamber. CA Measurement. The CAs were measured on a DSA 100 Kr€uss instrument at 25 °C using the sessile drop fitting method for the static CAs and deionized water (5 μL) as test solution. CAs were determined using the tangent method. For each drop

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on a substrate, approximately 10 images were recorded and averaged to obtain a mean CA. At least five spots per substrate were averaged. The CAs were recorded after stabilization and were checked on several surfaces. Dynamic advancing and receding angles were recorded as water was added to and withdrawn from the drop through a flat-tipped probe.

’ RESULTS AND DISCUSSION CVD represented a cost-effective method to deposit functional layers and modification of microscale and/or nanoscale structures. This technique was used in our case to prepare SnO2 nanowires and SnO2@SnO2 heterostructures. Figure 1 showed typical top-view SEM and TEM images of SnO2 NWs and SnO2@SnO2 heterostructures, indicating that both samples consist of 1D structures. It was clearly seen that the brush-type SnO2@SnO2 heterostructures were fabricated on the SnO2 NWs with a diameter of ∼60 nm and length of ∼20 μm. The branched structures were about 3 μm in length and 20 nm in diameter. HR-TEM was used to investigate the structure of the junction of the SnO2@SnO2 heterostructure. Figure 2 revealed that a clear lattice fringes from “branch” and “backbone” of SnO2@SnO2 heterostructure were 0.26 and 0.34 nm, which was in good agreement with the lattice spacing of (101) and (110) planes of the rutile SnO2 lattice, respectively. This result can be confirmed by the fact that the resulted angle is ∼68°, in agreement with the angle between [110] and [101] crystalline orientations.17,26 The crystalline structures of SnO2 nanowires and SnO2@ SnO2 heterostructures were identified by XRD analysis, as shown in Figure 3a. Compared with the XRD data, all peaks can be indexed as the rutile crystalline structure with the lattice constants of a = 4.738 Å and c = 3.187 Å according to PDF No. 411445. The peaks intensity of SnO2@SnO2 heterostructures was stronger than that of SnO2 nanowires due to the better crystallization, and the diffraction peaks are broadened due to the size effects of branched structures (overlapping of peaks contributed from branched and framework structures). No peaks from other phases can be detected by XRD, which revealed that the obtained product contains of pure SnO2 rutile structure. Figure 3b showed the room-temperature Raman scattering spectra of SnO2 nanowires and SnO2@SnO2 heterostructures. Three Raman scattering peaks of SnO2 nanowires at 474, 633, and 775 cm 1 were identified, corresponding to the Eg, A1g, and B2g vibrational modes, respectively. Thus, these peaks further confirmed the rutile phase of SnO2 nanowires. The Raman peak was at 694 cm 1, which was assigned to the IR-active LO A2u mode of SnO2. In the Raman spectrum of SnO2@SnO2 heterostructures, broad peaks at the same position with SnO2 nanowires were observed. The relaxation of the k = 0 selection rule is progressive when the rate of disorder increases or the size decreases, and infrared (IR) modes can become weakly active when the structural changes induced by disorder and size effects take place.27 Here, the branched SnO2 nanowire was small enough (∼20 nm) and had the different crystalline orientation (101) with as-synthesized SnO2 nanowires (110), which resulted in the disorder of crystalline orientation. So, it seemed reasonable that broad peaks in Raman spectrum of SnO2@SnO2 heterostructures were induced by the disorder and size effects of the branched thin SnO2 nanowires. It is well-known that the unusual wetting characteristics of superhydrophobic surfaces are governed by both their chemical composition and geometric microstructure, especially, the geometrical 22226

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Figure 1. SEM and TEM images of (a, b) SnO2 NWs and (c, d) SnO2@SnO2 heterostructures. The inset c showed the individual SnO2@SnO2 heterostructure.

Figure 2. TEM image of (a) junction structure of the SnO2@SnO2 heterostructures, and HR-TEM images of indicated area in (b) branch and (c) backbone from SnO2@SnO2 heterostructure.

microstructure is an important factor in determining the wettability. To study the influence of nanoarchitecture on the wettability, the wettability of SnO2 nanostructures was investigated by measuring the water CA. Further criteria for the measure of hydrophobicity is given by CA hysteresis (Δθ), which is given by the difference between the advancing CA (θa) and the receding CA (θr), involves the adding or withdrawing the water from the drop and accurately measuring the CA. Parts a and b of Figure 4 showed the images of a water droplet on the surfaces of SnO2 nanowires and SnO2@SnO2 heterostructures respectively.

Figure 3. (a) XRD patterns and (b) Raman spectra of SnO2 NWs and SnO2@SnO2 heterostructures.

It is surprising that the CA on SnO2 nanowires was only 3.6 ( 1.2°. It can be considered the relatively superhydrophilic surface, which is 22227

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Figure 4. Optical images of water droplet on the surfaces of (a) SnO2 nanowires and (b) SnO2@SnO2 heterostructures. (c) SEM image of SnO2@VOx heterostructures. (d) The shape of water droplet on the corresponding surface.

Figure 5. SEM images (a), TEM images (b) and EDS spectra (c) of SiOx film coated SnO2@SnO2 heterostructures and (d) the corresponding shape of water droplet.

corresponding to the Wenzel model.9 In this case the distance between the nanowires was such that the radius of curvature of the liquid surface on the SnO2 wires was too large to support the liquid surface. This resulted in a total wetting of the surface. The entry of water into the crevices between the nanowires increased the effective contact area of the liquid with the nanowire surface and led to a moderately small CA. Hence, the CA was much smaller than the corresponding value on SnO2 films (∼20°).28 However, the CA on SnO2@SnO2 heterostructures increased drastically to 133.2 ( 1.2°, and the CA hysteresis was about 20°, corresponding to hydrophobicity. Heterostructures, which contain enough room to hold air in the troughs between separately heterostructures on which a water droplet sits, can efficiently increase the proportion of air/water interface and ensure that the surface contact area available to water was very low, while the branched structures prevented penetration of water into the grooves. Meanwhile, In comparison to smooth SnO2 nanowires, the backbone of SnO2@SnO2 heterostructure has been changed from smooth to rough surface (as shown in TEM images), suggesting more surface defects are formed; it also will lead to the hydrophobic surface. Namely, the water did not fill the grooves on the heterostructure surface and therefore forms nearly spherical droplets, leading to hydrophobic surface. According to the Cassie model,10 the CA for a composite surface is influenced greatly by the surface fraction of solid (f1) vs air pockets (f2): co sθr = f1 cos θ f2 (where θr and θ are the CAs on the SnO2@SnO2 heterostructures and on a smooth SnO2 surface, respectively, and f1 + f2 = 1). The alteration to the small surface fraction (f1) of SnO2 nanowires due to the adjustment of the surface energy by chemical modification leads to the increase of a large surface fraction (f2) of air pockets and this explains the gradual increase of CA. In this Cassie state, capillary effects make it unfavorable for droplets to penetrate into the structures.29,30 The SnO2@VOx heterostructure also got the similar result, the CA value was 121.1 ( 1.2°, and the CA hysteresis was about 25° (parts c and d of Figure 4). All these results agreed well with the theory. It is generally accepted that the unusual wetting characteristics of superhydrophobic surfaces were governed by both their chemical composition and geometric microstructure based on Cassie and

Baxter’s equation.10 Actually, the above results confirmed that the wettability of SnO2-based surface can be controlled from superhydrophilic to hydrophobic through the increasing geometric microstructure. This indicated that different chemical modifications may provide a general route to synthesize the SnO2 based heterostructure surface with superhydrophobic properties by avoiding the wetting of the surface. Hence, the SiOx film with hydrophobic property was used to modify the SnO2 based heterostructure surface.31,32 Figure 5 showed the SEM and TEM images of SiOx film coated SnO2@SnO2 heterostructures. The SnO2@SnO2 heterostructure was covered with a uniform SiOx shell with a thickness of ∼5 nm. The boundary between the core and the shell in the trunk was clearly displayed. The composition of this structure was verified using EDS (Figure 5c). The EDS pattern (bottom) recorded at the bottom indicated that the bottom of this structure was composed of Cu, O, Sn, and Si. The EDS spectrum (up) recorded along the surface indicated that the surface of this structure was only composed of Cu, Si, and O; the peak of Sn was not determined. The peak of copper was derived from Cu TEM grids. These results definitely showed that the SnO2@SnO2 heterostructure was coated with a SiOx film. The shape of water droplet on this surface was shown in Figure 4d (CA = 155.8 ( 1.1°), and the CA hysteresis was approximately closed to 1°, indicating a superhydrophobic surface. Hence, the switching from superhydrophilic to superhydrophobic surface based on SnO2 nanowire by CVD growth was achieved. We now refer to the schematic presentation in Figure 6 to understand our results. In the case of SnO2 nanowires, the liquid completely penetrated into the vacancies among individual nanowires surface without any air pockets being trapped. As a result, the water droplet was pushed to spread over the surface rapidly, and the surface showed superhydrophilic properties. Then this surface was modified to SnO2@SnO2 heterostructure surface, geometric microstructure may decrease the concrete contact between the water droplet and the solid surface and the triple phase contact line (water/solid/air), and prevent penetration of water into the grooves, leading to hydrophobic surface. For the SnO2@SnO2@SiOx heterostructure surface, synthesized by different chemical modifications, the large fraction of air trapped 22228

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Figure 6. Schematic presentation of the switching behaviors of SnO2 based surface.

Figure 7. Time dependence of water CAs of SnO2@SnO2 and SnO2@SnO2@SiOx heterostructure surfaces (a) under UV irradiation, (b) in dark storage, and (c) with O2 annealing. (d) XPS spectra of SnO2@SnO2 and SnO2@SnO2@SiOx heterostructure and changes of XPS spectra of SnO2@SnO2@SiOx heterostructure under UV irradiation and O2 annealing in the O 1s peak region.

within the geometric microstructure should greatly increase the air/water interface, which coated film with hydrophobic property to avoid the wetting of this surface, leading to superhydrophobicity. To study the switchable surface wettability and present a clear portrait of the key factor in determining the surface wettability, UV irritation, dark storage, and O2 annealing were explored. For the UV irritation-influenced wettability variation of the SnO2@SnO2@SiOx heterostructure, which had a stable superhydrophobicity, we measured the CAs as a function of UV irradiation time in ambient conditions (Figure 7a). It revealed that the water CA was reduced abruptly at the beginning with the increase of UV irradiation time, indicating the high-speed yielding of electron hole pairs on the initial surface. Interestingly, UV illumination time was over 10 min, and even longer than 30 min, the variation of CA was slightly, and kept closely at 133°. The water CA of SnO2@SnO2 heterostructure surface exhibited no obvious change around 133°. As is known, UV irradiation will generate electron hole pairs in the SiOx surface, and some of the holes react with lattice oxygen to form surface oxygen vacancies (defect sites).33 Water molecules may easily coordinate into the oxygen vacancy sites, leading to the increase of the water adsorption. However, the surface becomes energetically unstable after the adsorption.

Meanwhile, the oxygen adsorption is thermodynamically favored, and it is more strongly bonded on the defect sites than the hydroxyl group. This process is similar with the case of the TiO2 film.34 Oxygen atoms can replace the hydroxyl groups adsorbed on the defective sites gradually when the UV-irradiated films are placed in the dark. Subsequently, the surface evolves back to its original state (before UV irradiation), and the wettability is reconverted to superhydrophobic. As reported, after storing the sample in dark and annealing at O2 atmosphere for a certain time, defective sites will gradually disappear due to the atmospheric oxygen effect. Subsequently, the surface will recover to the original superhydrophobicity.35 Hence, we try to use this method to reconvert the surface wettability to superhydrophobicity. Figure 7b shows the typical reconversion processes of UV-irradiated SnO2@SnO2@SiOx heterostructure surface with dark storage time. During the first three storage days, water CA increased remarkably, because of the high-speed bonding of oxygen atoms to the surface defect sites at the beginning. In the next several days, the water CA increased slowly, because of the gradual decrease of the defect sites. Further increase in CA was not observed after the water CA reached the values close to those at their original states (about 22229

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The Journal of Physical Chemistry C 155°). On the other hand, there was no change for the case of SnO2@SnO2 heterostructure surface. Figure 7c showed the reconversion processes of UV-irradiated SnO2@SnO2@SiOx heterostructure surface with O2 annealing time. The water CA increased sharply in the beginning and fully recovered to the original states (about 155°) in less than 6 h. This result implied that the oxygen adsorption was thermodynamically favored, and it was more strongly bonded on the defect sites than the hydroxyl group. The hydroxyl groups adsorbed on the defective sites can be replaced gradually by oxygen atoms when the hydrophilic samples were annealed in oxygen atmosphere at low temperature. Subsequently, the surface evolved back to its original state (before UV irradiation), and the wettability was reconverted to superhydrophobicity. The result for the case of SnO2@SnO2 heterostructure surface was the same with in dark storage. In comparison, this method in the reconversion rate of SnO2@SnO2@SiOx heterostructure surface showed significant improvement than storing the sample in dark. XPS analysis was explored to characterize the surface defect status in order to study the CA variation. Figure 7d showed the XPS spectra of SnO2@SnO2 and SnO2@SnO2@SiOx heterostructures under UV irradiation and O2 annealing in the O 1s peak region, and the peak position was calibrated using the C 1s peak of 284.7 eV. It is clear that coated SiOx film leads to the peak position shifting to the lower binding energy side for 0.3 eV due to the dissociatively adsorbed water.36 The O 1s peak of the SnO2@SnO2@SiOx heterostructure further shifted to the lower binding energy side for 0.2 eV after UV irradiation due to the surface oxygen vacancies and recovered to the state before UV irradiation due to the oxygen adsorption. It should be noted that there were not any changes of the CA values of SnO2@SnO2 heterostructures surface under UV irradiation, in dark storage or O2 annealing. Meanwhile, the CA value of SnO2@SnO2@SiOx heterostructure was changed from superhydrophobic to hydrophobic under UV irradiation, and then the surface wettability was switched back to superhydrophobic under dark storage and O2 annealing. It revealed that the geometric microstructure, the factor influenced the superhydrophobic surface, was the major determinant in the switchable wettability from superhydrophilic to superhydrophobic. The different chemical compositions were the minor factor in this process.

’ CONCLUSION In summary, we have demonstrated the control of the wettability switching of SnO2 nanowire based surface modified by CVD. It was found that the SnO2 nanostructured surface greatly enhanced the wetting effects in comparison with the chemical composition and geometric microstructure, and the superhydrophobic surface was achieved. Also, switchable surface wettability of SnO2@SnO2@ SiOx heterostructure was observed by alternation of UV irradiation, dark storage, and O2 annealing, indicating that geometric microstructure was the major determinant in the switchable wettability from superhydrophilic to superhydrophobic. These studies provided valuable information for the design of a patterned superhydrophobic surface through a simple method, which has potential applications in industrial coating and self-cleaning system. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (H.S.); [email protected] (Q.X.).

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’ ACKNOWLEDGMENT H. Shen acknowledges the support of Federal Ministry of Education and Research in the frame of the priority program “BMBF-NanoFutur” (FKZ 03  5512) operating at Institute of Inorganic Chemistry, University of Cologne, Cologne, Germany. Q. Xiong acknowledges strong support from Singapore National Research Foundation through a NRF fellowship grant (Grant No. NRF-RF2009-06), start-up grant support (Grant No. M58113004), and New Iniative Fund (Grant No. M58110100) from Nanyang Technological University. ’ REFERENCES (1) Gau, H.; Herminghaus, S.; Lenz, P.; Lipowsky, R. Science 1999, 283, 46–49. (2) Majumder, M.; Chopra, N.; Andrews, R.; Hinds, B. J. Nature 2005, 438, 44–46. (3) Schmidt, D. L.; Coburn, C. E.; Benjamin, M. D. Nature 1994, 368, 39–41. (4) Guo, Z.; Zhou, F.; Hao, J.; Liu, W. J. Am. Chem. Soc. 2005, 127, 15670–15671. (5) Xu, Y.; Wu, D.; Sun, Y. H.; Huang, Z. X.; Jiang, X. D.; Wei, X. F.; Wei, Z. H.; Dong, B. Z.; Wu, Z. H. Appl. Opt. 2005, 44, 527–533. (6) Li, M.; Zhai, J.; Liu, H.; Song, Y. L.; Jiang, L.; Zhu, D. B. J. Phys. Chem. B 2003, 107, 9954–9957. (7) Verplanck, N.; Galopin, E.; Camart, J. C.; Thomy, V. Nano Lett. 2007, 7, 813–817. (8) Hayes, R. A.; Feenstra, B. J. Nature 2003, 425, 383–385. (9) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (10) Cassie, A. B. D.; Baxter, S. Trans. Faraday. Soc. 1944, 40, 546– 551. (11) Tatsuyama, C.; Ichimura, S. Jpn. J. Appl. Phys. 1976, 15, 843– 847. (12) Harrison, P.; Willett, M. Nature 1988, 332, 337–339. (13) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395–1397. (14) Gr€atzel, M. Nature 2001, 414, 338–344. (15) Pan, Z.; Dai, Z.; Wang, Z. Science 2001, 291, 1947–1949. (16) Pan, J.; Ganesan, R.; Shen, H.; Mathur, S. J. Phys. Chem. C 2010, 114, 8245–8250. (17) Pan, J.; Shen, H.; Werner, U.; Prades, J. D.; HernandezRamirez, F.; Soldera, F.; M€ucklich, F.; Mathur, S. J. Phys. Chem. C 2011, 115, 15191–15197. (18) Chen, A.; Peng, X.; Koczkur, K.; Miller, B. Chem. Commun. 2004, 17, 1964–1965. (19) Zhu, W. Q.; Feng, X. J.; Feng, L.; Jiang, L. Chem. Commun. 2006, 2753–2755. (20) Kwak, G.; Seol, M.; Tak, Y.; Yong, K. J. Phys. Chem. C 2009, 113, 12085–12089. (21) Wu, X.; Sui, J. H.; Cai, W.; Qu, F. Y. Mater. Chem. Phys. 2008, 112, 325–328. (22) Kuan, W. F.; Chen, L. J. Nanotechnology 2009, 20, 035605. (23) Cortese, B.; D’Amone, S.; Manca, M.; Viola, I.; Cingolani, R.; Gigli, G. Langmuir 2008, 24, 2712–2718. (24) Verplanck, N.; Coffinier, Yannick.; Thomy, Vincent.; Boukherroub, R. Nanoscale Res. Lett. 2007, 2, 577–596. (25) Pan, J.; Xiao, L. S.; Shen, H.; Mathur, S. Ceram. Eng. Sci. Proc. 2010, 30, 9–15. (26) Beltran, A.; Andres, J.; Longo, E.; Leite, E. R. Appl. Phys. Lett. 2003, 83, 635–637. (27) Abello, L.; Bochu, B.; Gaskov, A.; Koudryavtseva, S.; Lucazeau, G.; Roumyantseva, M. J. Solid State Chem. 1998, 135, 78–85. (28) Miyauchi, M.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Chem. Mater. 2002, 14, 2812–2816. (29) Tuteja, A.; Choi, W.; McKinley, G. H.; Cohen, R. E.; Rubner, M. F. MRS Bull. 2008, 33, 752–758. 22230

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