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ZnO/CuO Hetero-Hierarchical Nanotrees Array: Hydrothermal Preparation and Self-Cleaning Properties Zheng Guo, Xing Chen, Jie Li, Jin-Huai Liu,* and Xing-Jiu Huang* Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, PR China
bS Supporting Information ABSTRACT: ZnO/CuO heterohierarchical nanotrees array has been prepared via a simple hydrothermal approach combined with thermal oxidation method on Cu substrates. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometer(XRD) are employed to characterize and analyze the as-synthesized samples. The results demonstrate that the secondary growth of ZnO nanorods enclose with CuO nanowires, leading to the formation of ZnO/CuO heterohierarchical nanotrees array. The hierarchical nanostructures have isotropic crystal symmetry and they have no 6-fold (or 4-fold or 2-fold) symmetry as general epitaxial growth. Enlightened by the similarity with microstructure of lotus, the wettability of ZnO/CuO heterohierarchical nanotrees array has been investigated. It is revealed that as-prepared ZnO/CuO nanotrees array after silanization present remarkable superhydrophobic performance, which is attributed to the trapped air and hierarchical roughness. Furthermore, their wettability could be manipulated by the morphologies of hierarchical ZnO nanorods. At the optimal condition, the greatest static angle of water droplet on the obtained heterohierarchical nanotrees array could reach almost 170°, and this substrate could be used as self-cleaning surface.
’ INTRODUCTION Due to its wide bandgap (∼3.37 eV), large exciton binding energy (∼60 meV), and a noncentral symmetric wurtzite crystal structure,1 zinc oxide (ZnO) has been explosively investigated for applications including light-emitting devices, UV photoluminescence (PL), piezoelectricity, and vibrational energy harvesters.25 In recent years, ZnO branched nanostructures, particularly hierarchical and/or hyper-branched structures have received much attention due to their rich architectures and promising applications in the field of optoelectronics. This type of hierarchical structures exhibits large surface areas that enhance their functionality. The ZnO heteronanostructures have been obtained by the secondary growth of well aligned arrays of ZnO nanorods on primary carbon nanotubes,6,7 ZnO,811 SnO2,12,13 TiO2,14 Ga2O3,15 In2O3,16 Si nanowires,17 GaN, GaP, SiC nanowires, and SiCC coaxial nanocables,7 and polymer micropillar array,18 whose syntheses were reported previously. Among these works, Fan et al.17 prepared a highly ordered treelike Si/ZnO hierarchical nanostructures array. Ali Javey et al.18 reported hierarchical polycarbonate micropillar array decorated with ZnO nanowires. However, photolithography having a complicated process19 needs to be used to fabricate Si nanopillars. And the polymer micropillar array was first fabricated by replica molding on microfabricated silicon templates containing hexagonal micropore arrays. Although much effort has been devoted to the synthesis of hierarchical nanostructures, it remains a big challenge to create regular ZnO heterohierarchical nanostructure array only using chemical approaches and develop the new hierarchical structures. r 2011 American Chemical Society
Although it is not a new topic, the wettability of solid surfaces is still raising great interest, not only because it can be used for potential applications, e.g., in the development of self-cleaning surfaces or in wettability driven microfluidics, but also because it is related to the biocompatibility of solid surfaces.2023 Two routes can be always considered for self-cleaning: superhydrophobic or superhydrophilic surfaces.24 The self-cleaning mechanism of superhydrophobic surfaces relies on the minuscule contact area of the drops with these surfaces. For the superhydrophilic route to self-cleaning, the flow of the liquid film is essential. Which case is used from a practical perspective, the first question to be addressed currently is how to fabricate surfaces with such controlled wettability. Inspired by the Wenzel25 (that is, the liquid is assumed to fill up the rough surface) and CassieBaxter26 (that is, hydrophobicity is enhanced by trapped air inside the topography) model and so-called “Lotus effect”, there is an enormous literature that describes research on studying of rough surface design and its wettability. Making arrayed-surfaces, together with the required surface chemistry, is an effective way to realize self-cleaning. In this work, we report the synthesis of ZnO/CuO heterohierarchical nanotrees array based on CuO nanowires array via a simple hydrothermal approach combined with thermal oxidation method. The secondary ZnO nanorods grow perpendicular to all Received: December 16, 2010 Revised: April 5, 2011 Published: April 14, 2011 6193
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Langmuir the facets of the backbone CuO nanowire. They have isotropic crystal symmetries, and the growth is not epitaxial with the primary CuO nanowire. The diameter, length, and density of the secondary ZnO nanorod branches can be tailored by changing the precursor concentration and reaction time. In order to take advantage of such exciting hierarchical heterostructures for a broad range of applications, such kind of substrate with nanotrees array was used to construct a superhydrophobic surface. The static and dynamic behavior, such as static contact angle, impact, and contact time, were carefully investigated. This hierarchical array, together with their surface silanization reaction, exhibited superhydrophobicity with a high water contact angle of nearly 170° and a very low contact angle hysteresis (sliding angle) of
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almost 0°. Of particular interest in this direction is that water drips off these surfaces, taking the dust particle along, like a lotus leaf does.
’ EXPERIMENTAL SECTION All the reagents and solvents, which were purchased from Shanghai Chemical Reagent Ltd. Co., were analytical grade and used as received without further purification. Synthesis of CuO Nanowires Array. CuO nanowires array was prepared according to the procedure previously reported.27 Briefly, the Cu foils were ultrasonicated in acetone and distilled water for about 10 min to refresh the surface, respectively. Then it was put into an oven and kept at 500 °C for 4 h in the air atmosphere. Finally, black CuO nanowire arrays on Cu foil were formed when the oven was cooled down to room temperature.
Synthesis of ZnO/CuO Heterohierarchical Nanotrees Array. A simple hydrothermal method was used in this experiment.
Figure 1. (a) SEM images corresponding to morphologies of CuO nanowires array at a tilt of 30°. (b) Side cross-sectional view of the product.
First, as-prepared CuO nanowires array with Cu foil were immersed into the 0.005 M ethanol solution of Zn(CH3COO)2 for about 15 s and then dried in air. After repeated for several times, it was annealed at 350 °C for 30 min in the air atmosphere, which leaded to the formation of ZnO seeds on the CuO nanowires.28 Subsequently, CuO nanowire arrays coated with ZnO seeds were put into Teflon-sealed autoclave containing 40 mL of an aqueous solution of equimolar Zn(NO3)2 3 6H2O and C6H12N4 and maintained at 95 °C for approximately 10 h. Afterward, the autoclave was cooled down naturally to room temperature, and the ZnO/CuO heterohierarchical nanostructure arrays on CuO foil were
Figure 2. Component analysis of ZnO/CuO heterohierarchical nanostructure. (a) SEM image. (b) TEM image (Ni grid used here). (ce) O, Cu, and Zn elemental mapping recorded from an individual heterohierarchical nanostructure, respectively. (f) EDX result. 6194
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Figure 3. Growth manipulation of the ZnO/CuO heterohierarchical nanostructures using different concentrations of Zn(NO3)2 salt for 10 h. (a) 0.01 M (sample a), (b) 0.025 M (sample b), (c) 0.05 M (sample c), (d) 0.1 M (sample d). The inset in panel d is a side cross-sectional view of an individual ZnO/CuO nanotree. obtained. Followed by washing with distilled water and ethanol for several times and finally dried in a vacuum oven, the as-prepared samples were used for characterization and further experiments concerning their wettabilities. Characterization. The morphologies and microstructures of assynthesized nanomaterials were investigated with a Quanta 200 FEG Environmental scanning electronic microscopy (ESEM) and JEOL 2010 transmission electron microscopy (TEM) equipped with an attached EDX system. XRD patterns of the samples were recorded on a Philips X’pert diffractometer (XRD, X’Pert Pro MPD) with Cu KR radiation (1.5418 Å). Photoluminescence (PL) spectra of the samples were carried out on a FluoRoLOG-3-TAU (Jobin Yvon, France) fluorescence spectrometer using Xenon lamp as the excitation light source. The excitation wavelength is 325 nm. The measurement of the contact angles of sessile drops were performed on an OCA20 system from Dataphysics GmbH, Germany, which was equipped with an high light transmitting capacity CCD camera and SCA 20 software. Furthermore, droplets of DI water in a volume of 4 μL were used to measure the static contact angle. Each contact angle was measured repeatedly ten times at the different places on the sample and statistics value was recorded. Prior to the measurements, the samples are chemically modified with fluoroalkysilane to decrease its surface free energy, which is preformed with 2 μL of ethonol solution of 20 mM 1H,1H,2H,2H-perfluorodecyltrichlorosilane (Alfa Aesar, USA) dropping on the substrates of the samples for 30 min, followed by washing the samples in ethanol. Finally, the samples were dried at room temperature for 30 min, inducing a layer of perfluorosilane on the surface of the samples.
’ RESULTS AND DISCUSSION SEM analysis of morphologies of CuO nanowires array is shown in Figure 1. A 30° side view of the arrays (Figure 1a) shows clearly the well-aligned growth of the CuO nanowires. A few bending nanowires are due to the long length. The side crosssectional view shown in Figure 1b obviously suggests that the CuO nanowires are approximately perpendicular to the substrate (that is, vertically) forming nanowires array, similar to that
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Figure 4. Growth manipulation of the ZnO/CuO heterohierarchical nanostructures using different reaction times for 0.025 M of Zn(NO3)2 salt. Key: (a) 2 h, (b) 4 h, (c) 6 h, and (d) 10 h.
reported previously.27 The diameter and length of the CuO nanowires are approximately 100200 nm and 10 μm, respectively. XRD result of CuO nanowire can be found in the following section. By employing solution of Zn(NO3)2 to the growth of ZnO, many braches were found to be appeared from the surface of CuO nanowires, forming a tree-like structure. A representative SEM image of ZnO/CuO heterohierarchical nanostructure is shown in Figure 2a. It is clear that ZnO branches grow virtually perpendicular to the surface of the CuO nanowire cores. The diameter of ZnO branches was observed to be approximately 100 nm, and their length ranges from 200 to 500 nm. This observation is strongly evidenced by the corresponding TEM image (Figure 2b). The broken parts of ZnO nanorodes in the picture (i.e., branches) were caused by the TEM sample preparation process. TEM phase mappings clearly reveal the heterofeature of ZnO/CuO nanostructure (Figure 2ce). To avoid the interference, Ni TEM grid is employed to support the sample. As seen, the shape of O element mapping is consistent with the TEM image of individual heterohierarchical nanostructure, clearly showing a uniform element distribution in the structure (Figure 2c). In the elemental mapping of Cu (Figure 2d), only the backbone corresponding to the main stem of the nanostructure is displayed. The distribution of Zn element (Figure 2e) fits well with the branched morphologies of individual heterohierarchical nanostructure, implying that the component of hierarchical nanorods is ZnO. This also gives a consistent picture of ZnO branches attached to the CuO surface. The elemental composition was further analyzed by EDX ananlysis, as shown in Figure 2f. In the EDX spectrum, the strong peaks of Cu and Zn come from synthesized ZnO/CuO heterohierarchical nanostructure; the Ni peak originates most likely from the Ni grid. To better understand the secondary growth of ZnO nanorods alone with CuO nanowires, the effect of the concentration of reactants was investigated. SEM images of as-synthesized samples are described in Figure 3. It is seen that, as the salt concentration is increased, the ZnO/CuO nanostructure becomes “stout”. The ZnO nanorods are thin and sparse at lower 6195
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Figure 5. Schematic growth process of the ZnO/CuO heterohierarchical nanotree. (a) Before the growth. The CuO nanowire surfaces are coated with ZnO seed nanoparticles. Subsequent growth of ZnO nanorods are seen on the side faces of CuO nanowires. (b) With a lower Zn(NO3)2 salt concentration or shorter reaction time, the branches are individual ZnO nanorod arrays, whereas (c) with a high salt concentration or longer reaction time, the nanorods tend to a fully developed complex heterohierarchical architecture.
Figure 6. (a) XRD patterns and (b) PL spectra of CuO nanowires and ZnO/CuO heterohierarchical nanostructure. Image of samples were shown in Figure 3. The inset in panel b is the enlarged spectrum of the band edge emission.
Figure 7. Sequential photographs in the droplet to be downed were recorded before and after the water droplet made contact with the silanemodified ZnO/CuO hierarchical nanotrees array substrate (sample c in Figure 3).
salt concentration (Figure 3a); their length is approximately several hundred nanometers. Each nanotree is slim in the array. Whereas at higher salt concentration (Figure 3d), the ZnO nanorods become longer and longer, the length reaches several micrometers; the diameter also increases. In contrast to Figure 3a, the ZnO nanorods turn to be more compact, as clearly shown in the inset of Figure 3d. It could also be noticed that the hierarchical nanostructures have isotropic crystal symmetry and they have no 6-fold (or 4-fold or 2-fold) symmetry as general epitaxial growth. A series of time-dependent experiments were conducted for a clear illustration of the growth process (Figure 4). SEM images of
Figure 8. Statistical distribution of static contact angle measured in 10 different locations for each sample (CuO nanowires array and ZnO/ CuO heterohierarchical nanotrees array).
the ZnO/CuO heterostructures were recorded at reaction times of 2, 4, 6, 10 h, respectively. From these images, it can be seen that the growth of ZnO nanorod begins with the enlargement of the ZnO nanoseeds (Figure 4a). A further increase in the reaction time leads to the gradual formation of ZnO nanosprouts (Figure 4b); followed by the growth of highly dense vertically oriented ZnO nanorods at longer reaction time (Figure 4c,d). Similar to the results observed at different salt concentrations, increasing the reaction time from 4 to 10 h results in an increase 6196
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Figure 9. Model of static contract angles evolving with the morphologies of heterohierarchical ZnO/CuO nanotrees array. (a) Schematics. Hierarchical rough surfaces are an intermediate case between regular and random rough surface. As for “Cassie-Baxter” model, such as CuO nanowires array, air is trapped inside the topography. (b) Side cross-sectional views of the product (sample d in Figure 3). The spacing between CuO nanowires were filled by fully developed ZnO nanorods. Hierarchical rough surface is responsible for the wetting behavior in this situation.
Figure 10. Sequence of droplet (R = 0.985 mm) impacting a superhydrophobic surface of ZnO/CuO heterohierarchical nanotrees array (Sample c in Figure 3). The snapshots are obtained using a high-speed camera (400 frames per second). The liquid is dropped from H0 = 2.81 mm, closes to the surface (a; t = 0 s), hits the surface (b; t = 2.5 ms), deforms (c, which shows the maximum deformation; t = 5.0 ms), lifts off (d; t = 7.5 ms) and rises up (e; t = 10.0 ms). The impact velocity is 0.235 m/s, and the Weber number is 0.75.
Figure 11. Contact time of a bouncing drop as a function of impact velocity (top) and drop radius (bottom). In the explored interval, Weber number, W, is from 0.08 to 5.4.
in the diameter, length and density of the ZnO nanorod branches. On the basis of the above observations, a schematic of the growth process is summarized in Figure 5. First, the CuO nanowire surfaces are coated with a layer of ZnO seed nanoparticles as specific nucleate sites. Subsequent growth of ZnO nanorods are seen on the surfaces of CuO nanowires. In the early stage, ZnO nanorods nucleate and crystallized along the ZnO seed sites via a hydrothermal method. And with the increasing of time or reactants concentration, the initial nanorods continuously crystallize, which leads to the increasing of length and diameter of ZnO nanorods. Finally, it tends to form a densely complex heterohierarchical architecture.
The XRD pattern (Figure 6a) reveals the crystal structure and phase purity of the ZnO/CuO heterohierarchical nanostructures. It could be found that the ZnO diffraction peaks are very weak for sample a, b and c, which is likely due to the small amount of ZnO nanorods grown onto the CuO nanowires surface. All ZnO diffraction peaks are clearly seen in sample d and match very well with those of wurtzite ZnO (a = 3.253 Å, c = 5.209 Å, JCPDS file No.800075), as previously seen by Yan et al.,8 who synthesized ZnO nanorods on primary ZnO backbone nanowires. Furthermore, it should be noted that for all as-prepared samples the diffraction peaks of CuO and Cu2O are ascribed from the nanowires and a dense film covered on Cu foil, respectively.27 Figure 6b is the PL spectra of ZnO/CuO products. In contrast to CuO nanowires array, strong emission at approximately 559.0 nm (i.e., defect emission) was observed for all products at different stages. The intensity increases as the amount (or density) of ZnO is increased (from samples a to d). The UV emission with a peak at around 380 nm corresponds to the band edge peak. Interestingly, the peak shifts from 377.7 to 384.0 nm when the diameter and length is increased (Inset in Figure 6b). This size-dependent PL results further demonstrate the control growth of ZnO/CuO heterohierarchical nanostructure.29 Enlightened by the novel morphology of ZnO/CuO heterohierarchical nanostructure arrays similar with the lotus microstructures, their wettability has been investigated. In Figure 7, a sequence of photographs of a water droplet (4 μL) approached onto the surface of ZnO/CuO hierarchical nanotrees array is presented, which indicates that the surface displayed remarkable superhydrophobic performance without vertical adhesive force between the droplet and surface. In the initial state, the droplet shows a normal shape owing to the self-gravity. When the droplet just touched the surface, that is the exact contacting state, the morphology of the droplet is not deformation compared with the initial state. Meanwhile, with substrate continuously heightened, 6197
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Figure 12. Schematic program of self-cleaning behavior for ZnO/CuO heterohierarchical nanotrees array. To guarantee the controllability of experiment, a water (not viscous) droplet slides (not rolls off) the surfaces.
the droplet would tightly and severely contact with the surface. The morphology of the droplet greatly would be deformed. Even more, the center of the droplet departed from the needle of the string. Similar phenomena are observed when the substrate is gradually removed from the droplet. As the substrate is completely removed, we could clearly observe that there is no water residue on the surface. Accordingly, the above results adequately prove the existence of a superhydrophobic surface and the adhesive force between the sample and the water droplet was extremely feeble and could be ignored. On such solids, their static contact angles have been measured and displayed in Figure 8. The contact angle on a surface of CuO nanowires array was found to be about 161.3 ( 0.9°. Compared with this result, the samples with ZnO/CuO hierarchical nanostructure present higher static contact angles. As expected, the static contact angle would increase with the increasing of density of hierarchical nanorods and reach a maximum value about 168.4 ( 0.5° for the sample c. However, continuously increasing the concentration of reactants to 0.1 M, the density, diameter, and length of ZnO nanorods also continuously increase, which would filled up the spacing between CuO nanowires (no air is trapped in this stage). Then it leads to the decrease of the contact angles. The proposed schematic diagram together with SEM images (Figure 9) would explain the change of static contact angle observed in Figure 8, which is believed to be due to the higher density of ZnO nanorods that is unfavorable to superhydrophobic behavior because air could not be trapped in this hierarchical roughness. Figure 9 shows schematically the morphologies of CuO nanowires array and heterohierarchical ZnO/CuO nanotrees array employed in wettability measurements together with a representative side cross-sectional view of ZnO/CuO nanotrees array (sample d). As seen from the schematic diagram of the samples (Figure 9a), CuO nanowires arrays shows a regular
rough surface (SEM image is shown in Figure 1), whereas the topography of ZnO/CuO nanotrees array switches from a regular hierarchical rough surface to completely hierarchical rough surface, as is evident in the side cross-sectional view (Figure 9b). The number of ZnO nanorods becomes greater, and the density, diameter and length of ZnO nanorods branch increase. This increase will eventually reach a limit as the spacing between the CuO nanowires is fully filled by ZnO nanorods. In the cross-section of CuO nanowires arrays, the forest-like feature is clearly visible. The “substrate” is mainly composed of air, which eventually leads to a strong reduction or elimination of the contact-angle hysteresis (CassieBaxter state26). Figure 9b shows the cross-section of ZnO/CuO nanotrees array (sample d). In this situation, hierarchical rough surface enters the Wenzel regime25 and is responsible for the wetting behavior. If the ZnO nanorods are not too denser, e.g., in sample c, air can be trapped in this structure and hierarchical roughness also makes great contributions to the superhydrophobicity. Therefore, contact angle of about 168.4 ( 0.5° was achieved. The second dynamic feature of drops on superhydrophobic surfaces concerns impact. When a drop is thrown at such a surface, it rebounds elastically with a velocity almost equal to impact velocity (Figure 10). As the drop (4.0 μL) hits (Figure 10b) the solid and it is deformed at most (Figure 10c), its kinetic energy of the impinging can be transferred to surface energy. And the drop can fully bounce (Figure 10d,e). More details can be seen in Movie S1 in the Supporting Information. It is observed that the droplet bounces numerous times before coming to rest. During the whole shock, there is no remaining water observed on the substrate. It indicates that the ZnO/CuO hierarchical surface has no apparent water adhesion and drag resistance as a droplet slide. Thus, these ZnO/CuO surfaces with hierarchical structure have almost no contact angle hysteresis, showing remarkable isotropic superhydrophobicity. 6198
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In an effort to gain further insight into the surface wettability, the contact time of water droplet was studied at a surface of sample c. Figure11 shows contact time of a bouncing drop as a function of impact velocity and drop radius. It is observed that the contact time decreases with V at small velocity, smaller than 0.2 m/s, which can be related to the vicinity of a threshold in velocity for bouncing. Continuously, the contact time does not depend on the impact velocity over a wide range of velocities (larger than 0.2 m/s), although both the deformation amplitude and the details of the intermediate stages largely depend on it. The Weber number is defined as the ratio of the drops kinetic energy to surface energy: We ¼
FV R γ 2
where F is the density of water (1000 kg/m3), V is the velocity of the droplet before impact, R is the radius of the water droplet, and γ is the surface tension of water with respect to air (0.07275 kg/s2). As seen in Figure11 (bottom), upon taking a power regression to the data, it was determined that the least-squares fit had an exponent of 1.89. This value is slightly different from Richard et al.,30 who additionally derived a power law relating the radius of the drop to its contact time, given by the following ratio: contact time radius (R) 3/2. We think that the error comes from our instrument (400 frames per second) and is tolerable. When the diameter is smaller than 1.261 mm in our experiments, the droplet bounces fast; it is difficult for the instrument to get a precise value due to the shorter contact time. However, we found that the contact time is well fitted by R1.55 over a range of radii (>1.261 mm), which is very close to the value of 1.5.30 Finally, extensions to higher Weber numbers for which break-up of the liquid drop is observed would also be very useful, particularly from the viewpoint of practical applications (achievement of socalled water-repellent surfaces). Based on its good superhydrophobic performance, the selfcleaning behavior of the ZnO/CuO heterohierarchical nanotrees array surface has also been investigated, as shown in Figure 12. Denoted by black real dots in the Figure 12a, dust particles were put on the ZnO/CuO heterohierarchical nanotrees array surface alone the red dash line, which are correlated with the protrusions above the baseline of substrate. When the surface of the sample was lifted to tightly contact a droplet (4.0 μL) suspended on a microsyringe and then moved toward the direction noted by the green arrow (shown in Figure 12a). From one end (Figure 12a) to the other one (Figure 12d) along the perpendicular direction, the water droplet remains intact without any apparent deformation, preserving a solid ball sliding on a smooth surface. Excitedly, the dust particles entered into the droplet and accompanied with the sliding of droplet without any dust particles remained. Obviously, these results demonstrate that novel ZnO/CuO heterohierarchical nanotrees array surface is of great selfcleaning performance. Concerning about the details of the selfcleaning process, it could be seen in the Movie S2 in the Supporting Information.
’ CONCLUSIONS The results of this study show that ZnO/CuO heterohierarchical nanotrees array is fabricated through a simple hydrothermal approach combined with thermal oxidation method. The density, diameter and length of hierarchical ZnO nanorods branch could be tuned by adjusting the reaction conditions, such
as Zn(NO3)2 concentration and reaction time. After silanization, the surface containing ZnO/CuO heterohierarchical nanotrees array exhibit impressive superhydrophobic behavior. Changing the structure from CuO nanowires array to ZnO/CuO nanotrees array at the surface causes an obviously increase in static contact angle and decrease then. These results demonstrate the switch between Wenzel and CassieBaxter model. When impinging such a solid surface, a water drop can fully bounce as a balloon. As a potential application study in lab, the nanotrees array surface is found to be self-cleanable.
’ ASSOCIATED CONTENT
bS
Supporting Information. Movies showing droplet bounce and self-cleaning process of the substrate. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (X.J.H);
[email protected] (J.H.L). Telephone: þ86-551-5591142. Fax: þ86-551-5592420.
’ ACKNOWLEDGMENT This work was supported by the One Hundred Person Project of the Chinese Academy of Sciences (CAS), China, and the National Natural Science Foundation of China (Grant No. 90923033) to X.-J. Huang. ’ REFERENCES (1) Schmidt-Mende, L.; MacManus-Driscoll, J. L. Mater. Today 2007, 10, 40. (2) Pauzauskie, P. J.; Yang, P. Mater. Today 2006, 9, 36. (3) Comini, E.; Sberveglieri, G. Mater. Today 2010, 13, 28. (4) Wang, Z. L. Mater. Today 2007, 10, 20. (5) Loos, J. Mater. Today 2010, 13, 14. (6) Ok, J. G.; Tawfick, S. H.; Juggernauth, K. A.; Sun, K.; Zhang, Y. Y.; Hart, A. J. Adv. Funct. Mater. 2010, 20, 2470. (7) Bae, S. Y.; Seo, H. W.; Choi, H. C.; Park, J.; Park, J. J. Phys. Chem. B 2004, 108, 12318. (8) Zhuo, R. F.; Feng, H. T.; Chen, J. T.; Yan, D.; Feng, J. J.; Li, H. J.; Geng, B. S.; Cheng, S.; Xu, X. Y.; Yan, P. X. J. Phys. Chem. C 2008, 112, 11767. (9) Wang, Z. Q.; Gong, J. F.; Su, Y.; Jiang, Y. W.; Yang, S. G. Crys. Growth Des. 2010, 10, 2455. (10) Kim, K. S.; Jeong, H.; Jeong, M. S.; Jung, G. Y. Adv. Funct. Mater. 2010, 20, 3055. (11) Zhao, F. H.; Zheng, J. G.; Yang, X. F.; Li, X. Y.; Wang, J.; Zhao, F. L.; Wong, K. S.; Liang, C. L.; Wu, M. M. Nanoscale 2010, 2, 1674. (12) Sun, S. H.; Meng, G. W.; Zhang, G. X.; Zhang, L. D. Crys. Growth Des. 2007, 7, 1988. (13) Cheng, C. W.; Liu, B.; Yang, H. Y.; Zhou, W. W.; Sun, L.; Chen, R.; Yu, S. F.; Zhang, J. X.; Gong, H.; Sun, H. D.; Fan, H. J. ACS Nano 2009, 3, 3069. (14) Wang, N. X.; Sun, C. H.; Zhao, Y.; Zhou, S. Y.; Chen, P.; Jiang, L. J. Mater. Chem. 2008, 18, 3909. (15) Mazeina, L.; Picard, Y. N.; Prokes, S. M. Crys. Growth Des. 2009, 9, 1164. (16) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (17) Cheng, C. W.; Yan, B.; Wong, S. M.; Li, X. L.; Zhou, W. W.; Yu, T.; Shen, Z. X.; Yu, H. Y.; Fan, H. J. ACS Appl. Mater. Interf. 2010, 2, 1824. (18) Ko, H.; Zhang, Z. X.; Takei, K.; Javey, A. Nanotechnology 2010, 21, 295305. 6199
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