A Seed-Mediated Growth Method for Vertical Array of Single

Nov 10, 2007 - A Seed-Mediated Growth Method for Vertical Array of. Single-Crystalline CuO Nanowires on Surfaces. Akrajas Ali Umar* and Munetaka Oyama...
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CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2404–2409

Articles A Seed-Mediated Growth Method for Vertical Array of Single-Crystalline CuO Nanowires on Surfaces Akrajas Ali Umar* and Munetaka Oyama DiVision of Research InitiatiVes, International InnoVation Center, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8520, Japan ReceiVed April 6, 2006; ReVised Manuscript ReceiVed August 20, 2007

ABSTRACT: The preparation and characterization of a large-scale vertical array of single-crystalline CuO nanowires on different material surfaces is demonstrated for the first time. The procedure simply involved a room-temperatures liquid–solid growth process of attached CuO nanoseeds on ITO surface in the mixed aqueous solution of Cu(CH3COO)2 and NH3. By controlling the concentration of the chemical precursors in the growth solution, we can fabricate variable-shape 1D structures of CuO, such as nanowires and nanobelts. The FESEM image analysis indicated that these nanowires feature uniform size with tiny structures that have diameters and lengths in the range of 10 and 100 nm, respectively, and tend to form a bundlelike structure at the top end of the wires. The high-resolution transmission electron microscopy (HRTEM) study on individual CuO NWs reveals that the NWs are single-crystalline with growth orientation is [110]. The XRD analysis of the samples revealed that the nanostructures were a high-crystalline CuO without the presence of other phases of the Cu complexes. A possible mechanism that accounts for the growth of these nanostructures using the present techniques was described. 1. Introduction Copper oxide compounds are among well-known technologically important materials that exhibit potential for use in diverse application areas including in solar energy materials, electronics, sensors, magnetic storage media, optical, batteries, and catalysis.1–7 CuO (the common phase of copper oxide), in particular, is a p-type semiconducting material (energy gap of ∼1.4 eV) that exhibits photoconductive,2 field emission,3 and photovoltaic4 properties. These materials also demonstrate excellent characteristics as negative electrode in the lithium ions batteries5 and have been well-known for their outstanding catalytic behavior in the conversion of hydrocarbons into water and carbon dioxide.6 CuO is also renowned for its central function in cupricbased high Tc superconductors,1 in which its Cu valence electrons and their spin fluctuation play the key function in determining the superconductivity. Additionally, this compound is well-known for its excellent performance as a sensing material for hazardous gas detection.7 Because the properties of materials at nanoscale regime are strongly influenced by their shape and dimensional constraint,8 it is expected that the synthesis of the CuO compound into nanostructured materials, particularly those with one-dimensional (1D) structure such as nanowires, (NWs), nanotubes, nanobelts, etc., could enhance its intrinsic characteristics for use in existing applications. * Corresponding author. E-mail: [email protected].

Recently, several techniques for the synthesis of 1D structures of CuO have been made available. Among them, the solutionphase colloidal-based approach9 is the most widely used technique for this purpose. In a typical synthetic process, the CuO NWs were produced from a two-step synthesis process that was initiated with the synthesis of colloidal Cu(OH)2 NWs and followed by a phase transformation process of these copper complexes to obtain monoclinic CuO NW products, using either a thermal dehydration (typically at 100–150 °C) 9a,c,9d or a strong reductant,9b such as sodium borohydrate, treatment of Cu(OH)2 NWs. The CuO NW products produced from this approach were characterized by a highly crystalline structure, with growth mainly along the κ direction. A hydrothermal approach for the synthesis of quasi-1D CuO nanostructures was also reported by Yang et al.10 By following a hydrothermal reaction of CuSO4 and ethylene glycol in alkaline conditions at 200 °C, branched CuO nanorods with sizes up to several hundreds of nanometers can be normally produced. The nanorods were characterized with a single-crystalline structure. Despite these techniques providing an interesting synthetic procedure for producing highquality CuO NWs, another challenge, namely, finding the procedure to attach these nanostructures onto a certain solid support, particularly in a vertical array, should be resolved first in order to make this kind of nanostructure viable in the existing applications, which appear to require a lot of effort with limited success.

10.1021/cg0602008 CCC: $37.00  2007 American Chemical Society Published on Web 11/10/2007

Growth Method for Vertical Array of CuO Nanowires

Xia et al. presented a fascinating strategy for the realization of a vertical array of CuO NWs that grow on Cu substrate, which involved heating the Cu substrates at moderately high temperature (typically ∼400 °C) in an oxygen atmosphere via a vaporsolid process.11a The NWs are mainly characterized by a bicrystalline structure with a twin-plane oriented parallel to the longitudinal axis of the NW that grow along the κ direction. Yang et al. presented another potential method for growing the CuO NWs vertically on Cu substrates that was achieved by using a liquid–solid phase technique.12 In their two-step synthesis approach thatwas initiated by treating a clean Cu substrate with diluted NH3 solution at room temperature to produce Cu(OH)2 NWs templates followed by a thermal dehydration process of the as-prepared Cu(OH)2 NWs, a high-density vertical array of CuO NWs can be normally produced. Because these techniques could embed the formation of CuO NWs, particularly in vertical orientation, on certain solid substrate, the possibilities of employing this kinds of nanostructures in currently existing applications is widely open. However, despite both approaches providing an elegant strategy for producing a highly crystalline vertical array CuO NWs, the new characteristics that are probably acquired from these nanostructures might be strongly influenced by the bulk properties of Cu or CuO substrate, because they can be grown only on their mother Cu substrate. Hence, their functionalities in applications could be limited and restrained. However, in order to take vast benefit from the superior characteristics of these nanostructures and extend their application ranges, any efforts toward enabling the growth of CuO NWs on various kinds of solid surfaces should be demonstrated. Our group concentrates on the growth of metal nanostructures on solid surfaces such as glass, ITO, glassy carbon, etc., utilizing a seed-mediated growth method.13,14 By applying a simple seeding process of metal nanoseeds on a solid surface and following with a growth process in the growth solution, variable shapes, such as spherical, nanowire, and nanorod, of metal nanostructures, i.e. gold13 and silver,14 can normally be fabricated directly on the surface. Recently, nanowires, nanorods, and nanoparticles of several metaloxides, such as ZnO15 and Fe2O3,16 have also been prepared directly on the substrate by adopting the seed-mediated growth method. In this paper, by adopting this technique, we demonstrate for the first time the growth of a large-scale vertical array of CuO NWs on different materials surfaces, such as ITO, glassy carbon, etc. The fieldemission scanning electron microscopy (FESEM) analysis of the samples confirmed the formation of large-scale vertical array of small (diameter ca. ∼10 nm and length >100 nm) CuO NWs and nanobelts on the surface. The high-resolution transmission electron microscopy analysis revealed that the CuO nanowires are single crystalline in nature with a growth orientation along the [110] direction. The thin film X-ray diffraction results of these nanostructures further verified that these kinds of nanostructures were CuO NWs without the presence of any other phases of the Cu compounds. The CuO NWs grown on such surfaces should find use as an important component in photoconducting and photovoltaics applications. 2. Experimental Section The growth of CuO NWs on an ITO surface was carried out using a CuO nanoseed-mediated growth method. In a typical process, CuO nanoseed particles were first grown on the ITO surface via an alcohothermal method.17 A clean ITO sample prepared from a consecutive ultrasonication in acetone, ethanol, and pure water was wetted with a 0.01 M ethanoloic solution of copper acetate hydrate (Cu(CH3COO)2 · xH2O, Aldrich) for ∼20 s, washed with pure water,

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Figure 1. (A) FESEM images of CuO nanoseeds particles prepared using an alcohothermal method for 2 h at 250 °C. (B, C) FESEM images of CuO NWs grown at different growth periods in the mixed aqueous solution of 10 mM Cu(CH3COO)2 and 12 mM NH3 for (B) 2 and (C) 15 h at room temperature (25 °C). (D) High-magnification image of (C). Scale bars are 100 nm. and dried with a flow of nitrogen gas. This step was repeated at least three times to obtain high-density growth of CuO nanoparticles. After that, the sample was transferred into a regular laboratory oven for an annealing process at 250 °C in air for 2 h. The samples were then cooled to room temperature before being used further. A light bluish color formed on the surface after the annealing process was finished, indicating the formation of CuO nanoparticles. The samples were then removed and washed with a copious amount of pure water to remove any loosely bound particles on the surface. After that, the samples were subjected to the next drying process in air at 100 °C for 15 min. The CuO nanoparticles can also be grown on other solid support, such as glass, glassy carbon, etc., by following this process. The ITO surface in this work was chosen simply for convenience for further characterization in FESEM experiment and not for a special reason. To grow the CuO NWs from the attached CuO nanoseeds, the CuO seed-treated ITO samples were immersed in a vertical position into a 25 mL glass vial that contains 10 mL of a 0.01 M aqueous solution of Cu(CH3COO)2. After that, 60 µL of a 2 M ethanoloic solution of NH3 (WAKO Chemical, Japan) was added dropwise into the reaction using a micropipette while being vigorously stirred. The interval between the additions of NH3 drops was typically 1 min. The solution was then mildly stirred during the growth process, typically for 15 h at room temperature (∼20 °C). After the growth process was finished, the samples were removed and vigorously washed several times using a pure water to remove any precipitate on the surface. The samples were then put into oven for a brief drying process, typically 15 min, at 50 °C. The SEM images were obtained using field emission scanning electron microscope (FESEM) JEOL JSF 7400F, Japan, operated at an acceleration voltage of 5 kV. The high-resolution transmission electron microscope (HRTEM) images were taken using JEOL JEM2100F, Japan, operated at an acceleration voltage of 200 kV. The XRD analysis was performed using a RINT 2500 X-ray diffraction instrument with CuKR irradiation operated at 50 kV and 300 mA. The measurement was carried out at room temperature with a scan rate as low as 2°/min. The optical absorption of the CuO NWs on the ITO surface was characterized using Ocean Optics S2000, a UV–vis fiber optical spectrophotometer.

3. Results and Discussion Figure 1 shows typical FESEM images of CuO nanostructures grown on the ITO surface prepared at three different growth

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periods in a growth solution of a mixed aqueous solution of 0.01 M Cu(CH3COO)2 and 0.012 M NH3, namely, growth for 0, 2, and 15 h. Figure 1A shows a typical FESEM image of CuO nanoseeds particles that grow on an ITO surface prepared using an alcohothermal process showing high-density growth of CuO nanoparticles with a uniform particles size (ca. j 10 nm). The CuO nanoseeds were also found to regularly distribute throughout the surface, texturing all levels of the ITO nanocrystals surface. These nanoparticles grew into large-scale vertically oriented 1D nanostructures just after completing a short growth period in the growth solution, typically 2 h (Figure 1B). In this stage, small NWs with length on the order of 30-50 nm were obtained. Our observation on the high-magnification FESEM image of the NWs grown at the present growth stage revealed that the NWs are remarkably thin, with thicknesses in the range of 4-6 nm. From the image, it was also found that no CuO nanoseeds were observed on the surface, inferring effective growth of CuO nanoseeds into NW structures. In addition to such interesting NW structural growth prepared using the present method, the CuO NWs were found to exhibit the tendency to form bundlelike structures at the top-end of the NWs. These kinds of structures are likely typical for the CuO NW growth that prepared from a solution-phase approach as a result of coalescing among the NWs growth, which is also demonstrated in the currently existing literature.12 The NWs were found to grow and expand further into relatively longer and thicker structures when the growth time elongated. A typical FESEM image of the CuO NW growth prepared from a relatively longer growth period, i.e., 15 h, is shown in Figure 1C. On the basis of the high-magnification image (Figure 1D) of the samples prepared at this growth period, it can be worth noting that the NWs feature uniform diameter and length on the order of ∼10 and 100 nm, respectively. By making a comparison between the structural growth of CuO NWs prepared for short and relatively long growth periods, it can be understood that the growth process of the CuO NWs may be terminated in a relatively short period because of the presence of insignificant change in the nanocrystal size even though the growth time was extended to 15 h. This case could be understood as the effect of fast growth process of the CuO NWs, as presented in the short growth period (Figure 1B), that rapidly consumed all the available CuO NW precursor in the reaction so that the growth process immediately terminated. It was also found that the CuO nanoseed particles are particularly important for the growth process of the CuO NWs on the surface. Our investigation on the growth of the CuO nanostuctures in the absence of CuO nanoseed particles during the growth process revealed that no CuO nanowires or nanobelts as well as others geometries could be grown on the surface. Thus, it can be worth noting that the 1D structures of CuO were, indeed, formed from the attached CuO nanoseeds. On the basis of the experimental results, it can be concluded that the present approach may provide an effective strategy for the fabrication of vertical array of CuO NWs on different material surfaces via 1D structural growth of CuO nanoseed particles in a kind of liquid–solid growth process. The structure and crystallinity of the CuO NWs were further characterized using a TEM analysis. The TEM experiment was performed by scratching the CuO NWs that grew on the ITO surface or other substrate surface using a clean razor blade and then dispersing them in the ethanol solution. These nanostructures were then redeposited onto a carbon-coated Cu grid for a high-resolution TEM experiment. Figure 2 shows typical TEM images of the CuO nanostructures grown for 15 h in the growth solution taken at a different magnification. Figure 2A shows a

Umar and Oyama

low-magnification TEM image of CuO NWs, indicating aggregation of the CuO NWs on the TEM grid. As has been observed in the FESEM image as shown in Figure 1, the CuO NWs originally form a cluster with two or more nanowires at the top end of the NWs structures. However, when this structure dissolved into the solution and cast onto the surface, they might aggregate each other, forming a relatively larger threedimensional aggregates structure. Figure 2B shows a highresolution TEM image of the CuO NWs showing several individual CuO NWs. The NWs were found to be relatively shorter than they should be. It may be the cause of the scratching process of the NWs for TEM sample preparation. It can be clearly seen that all of the NWs comprised well-ordered lattice fringe pattern, inferring that these nanostructures were single crystalline in nature. A high-resolution TEM image on single NWs, Figure 2C, indeed confirms that these nanostructures are single crystalline. The interplanar spacing of these lattice fringe patterns was calculated to be 0.29 nm. This value corresponded well with the spacing calculated for {110} crystallographic planes for monoclinic CuO (cell constants a ) 0.469 nm, b ) 0.343 nm, c ) 0.513 nm, and β ) 99.55°; JCPDS 45-0937). Therefore, the long axis or the growth direction for the CuO NWs could be thought as [110]. We further characterized the structure and the phase of the CuO nanostructures using XRD experiment on the as prepared sample. Figure 3 shows the typical thin film X-ray diffraction result of the NWs. One strong peak at 38.5° and six small, broad peaks at 49, 52, 58, 62, 68, and 72° can be recognized from the spectrum as belonging to the CuO nanocrystals. The presence of few as well as small and broad Bragg peaks of CuO nanostructures in the current measurement could be related to the presence of small crystallite size on the surface because of the relatively thin structures of the NWs. All these Bragg peaks j j can be indexed to {111}, {202}, {020}, {202}, {113}, {113}, and {311} of monoclinic CuO crystal. It also can be found that the number of the diffraction peaks appearing in the XRD spectrum for CuO NWs prepared for the longer growth period (15 h) (curve c) is higher than that for CuO NWs prepared at a relatively shorter growth period (curve b) as well as CuO nanoseed particles (curve a), indicating the increase in nanocrystalites size. These results further confirmed that the NWs indeed grew from the attached nanoseed particles. From the XRD result, it was also found that the intensity ratio between the {111} plane and the other peaks is high, suggesting that the nanostructures were characterized by dominant {111} planes. This result is in good agreement with that obtained in the TEM characterization on the NWs, which found that the NWs were mainly bounded by (111) planes, which might be dominantly exposed toward the X-ray beam because of their imperfect vertical orientation on the ITO surface. Other peaks (indicated with asterisk) were also found in the spectrum. They could be easily assigned as the ITO crystal background substrate diffraction peaks. Furthermore, interestingly, no peaks related to the presence of another phase of the copper oxides as well as the other copper complexes appeared in the spectrum, suggesting that the nanostructures prepared using the present technique have phase purity of CuO nanocrystals. Despite the samples being subjected to a brief drying process at 50 °C for 15 min, the presence of pure CuO phase as recognized from the XRD result can not be attributed as a result of possible phase transformation of other Cu phase, such as Cu(OH)2, during the drying process; this is because the phase transformation requires a much higher temperature, typically 150 °C.9d

Growth Method for Vertical Array of CuO Nanowires

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Figure 2. TEM images of CuO NWs prepared from the liquid–solid process of attached CuO nanoseeds in the mixed aqueous solution of 10 mM Cu(CH3COO)2 and 12 mM NH3 for 15 h. (A) Low-magnification image showing aggregate structure of NWs. (B, C) HRTEM images showing individual NWs with a clear lattice fringe pattern. The NWs are single crystalline in nature. Scale bars: (A) 50, (B) 5, and (C) 2 nm.

Figure 3. XRD spectra of CuO nanostructures grown on ITO substrate. (a) CuO nanoseed particles and after being grown for (b) 2 and (c) 15 h in a mixed aqueous solution of 0.01 M Cu(CH3COO)2 and 0.012 M NH3.

The relative concentration ratio between Cu(CH3COO)2 and NH3 plays the key function in determining the growth morphology of the nanostructures. In a typical synthesis process, the growth solution contains 10 mM Cu(CH3COO)2 and 12 mM NH3, which correspond with the Cu(CH3COO)2 to NH3 (Cu: NH3 ratio of 0.83 (see Figure 4A)). When this Cu:NH3 ratio was reduced to 0.36, the CuO nanobelts were normally formed (Figure 4B). The nanobelts were characterized by a thin structure (j5 nm in width) and relatively longer (typically ∼200 nm) compared to the nanowire structures (Figure 4A). However, relatively smaller nanobelt structures (length of ∼100 nm) were

produced when the Cu:NH3 ratio was further lowered to 0.18 (Figure 4C). This could be due to the effect of the relatively higher concentration of NH3 present in the solution, which might hinder the nucleation of CuO precursors.12 We also investigated the growth of CuO nanostructures when the Cu:NH3 ratio was increased to 1.63. Typical results for the CuO nanostructures prepared in this condition are shown in Figure 4D. It was found that the main products were nanowires. However, the structure of the nanowires was relatively smaller, with a typical length of j50 nm, compared to that prepared from the growth solution with a concentration ratio of 0.83 (Figure 4A). This could be caused by either the retardation of nucleation process on the CuO nanoseeds particles or low rate formation of CuO precursor in solution as a result of steric effect in solution due to the presence of a large amount of Cu(CH3COO)2. In the present work, we also investigated the effect of growth solution concentration on the morphology of the nanostructures while preserving the Cu:NH3 ratio at 0.83. When the concentration of the growth solution was reduced to 0.6 mM (with respect to the Cu(CH3COO)2 concentration), no nanowires or nanobelts were formed. However, irregularly shaped nanostructures were produced instead (Figure 4E). In the present work, our study on the effect of using a high concentration of growth solution on the structural growth of CuO nanostructure was limited and avoided because the use of high content of NH3 in solution may cause a rapid formation of Cu(OH)2 in solution. Hence, the attached CuO nanoseed particles could not grow properly. On the basis of the experimental results, it can be concluded that the ratio between the Cu(CH3COO)2 and NH3 concentrations

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Figure 5. UV–vis optical absorption spectra for (a) CuO nanoseed particles and after being grown for (b) 2 and (c) 15 h in a mixed aqueous solution of 10 mM Cu(CH3COO)2 and 12 mM NH3.

Figure 4. FESEM image of CuO NWs grown on ITO surface prepared from a mixed aqueous solution of: (A) 10 mM Cu(CH3COO)2 and 12 mM NH3 (Cu:NH3 ratio is 0.83); (B) 4 mM Cu(CH3COO)2 and 12 mM NH3 (Cu:NH3 ratio is 0.36); (C) 2 mM Cu(CH3COO)2 and 12 mM NH3 (Cu:NH3 ratio is 0.18); (D) 18 mM Cu(CH3COO)2 and 12 mM NH3 (Cu:NH3 ratio is 1.63); (E) 0.6 mM Cu(CH3COO)2 and 0.7 mM NH3 (Cu-NH3 ratio is 0.83). Scale bars are 100 nm.

is important for the formation of nanowire and nanobelt morphology, for which the optimum value lay in the range of 0.5-0.8. Also, the concentration of the NH3 in the solution lay in a critical small window region, typically near ∼10 mM, for the promotion of CuO nanowire or nanobelt structures. Otherwise, irregularly shaped structures were formed instead. When the NH3 was added into the aqueous solution of Cu(CH3COO)2, the square-planar complexes of Cu[(NH3)4]2+ were immediately formed. In aqueous media, these complexes were rapidly transformed into the Cu[(OH)4]2- because of the nature of weak complexes of Cu[(NH3)4]2+. The color of the solution may then turn into a light hazy blue color, inferring the formation of Cu(OH)2 nanocrystals. The formation of Cu(OH)2 nanocrystals in solution can be simply understood as a result of fast transformation of Cu[(OH)4]2- into coordination self-assembly of the chain structure. A blue precipitate of Cu(OH)2 nanostructures was then obtained when a high concentration of NH3 was added into the reaction. However, in the present work, we decelerated the formation of chains in solution by the addition of as small an amount as possible of NH3 at one time so as to preserved the formation of Cu[(OH)4]2- complexes in solution. When a substrate that contains CuO nanoseed particles on its surface is

present in the reaction, the Cu[(OH)4]2- complexes coordinated and nucleated selectively on the CuO seed surface via a process called olation.9a At the same time, the dehydration process that produces square-planar complexes of CuO4 was believed to occur in order to facilitate the formation of O-Cu-O bridges with the CuO nuclei.9d In contrast to the growth process in the solution, in this process, the Cu(OH)2 complexes as the result of Cu[(OH)4]2- transformation were thought to be unformed or rapidly transformed into the CuO compound during the reaction. Therefore, the phase purity of CuO nanostructures can be achieved. The formation of nanowire or nanobelt structures of CuO can be related to the effect of NH3 on the structural growth of CuO during the growth process. When the concentration of NH3 is relatively high in the solution, they may easily complex with the CuO crystal on the surface of, short-axis planes {001} and grown-up CuO nanowires and further decelerate the NW growth in the [001] direction, supporting the formation of nanoribon structures. However, the growth rate of CuO nanostructures to the other direction, namely, the long axis of the nanowires, becomes faster because of the presence of a relatively high concentration of Cu[(NH3)4]2+, which might transport to the growing crystallographic planes, i.e., {110}, and then supply the precursors for the formation of Cu[(OH)4]2complexes at this planes. Therefore, the nanobelts are relatively longer compared to their nanowire counterparts. Vice versa, when the NH3 concentration is relatively low, the symmetrical growth toward the short axis of the CuO nanostructures can be achieved. Hence, the nanowires are produced. The UV–vis optical absorption spectra of the CuO nanostructures grown at three different growth periods on the ITO surface are shown in Figure 5. Curve a shows the optical absorption of the grown-up CuO nanoseed particles. A single absorption band centered at ∼393 nm was observed in this spectrum. The optical absorption spectrum of the CuO nanostructures was relatively unchanged when the CuO nanoseed particles were subjected to a short growth period (2 h) (curve b). However, the relative optical absorbance of this nanostructure, in particular at the absorption band, increased inferring the evolution of the NWs. A red shift and a higher increase in the optical absorption band of the CuO nanostructures were observed when the growth period was elongated to 15 h (curve c), further confirming the growth of the CuO NWs on the surface. Using this absorption spectrum, we can calculate the direct optical band gap energy of the CuO nanostructure by simply plotting the (RhV)2 versus hV, obtained from the relation of RhV ) A(hV - Eg)1/2, where R and A are the absorption coefficient and a constant, respectively. From this analysis, the direct optical band gap energy was calculated to be ∼1.9 eV (curve c), which is in good agreement with the results reported elsewhere.18 Because of their relatively low optical energy gap, the CuO

Growth Method for Vertical Array of CuO Nanowires

NW-attached ITO surface may find use in optoelectronic and photovoltaic applications.

Crystal Growth & Design, Vol. 7, No. 12, 2007 2409

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4. Conclusions A simple approach for growing large-scale, uniform vertically oriented CuO nanowires and nanobelts from the CuO nanoseed particles on different material surfaces has been demonstrated. The FESEM analysis on the as-prepared samples shows that the NWs exhibit a thin structure with uniform diameter and length (typically ∼10 and 100 nm, respectively). The HRTEM analysis on individual CuO NWs reveals that the nanowires are single crystalline in nature and grow mainly toward the [110] direction. The XRD analysis of the NWs revealed that the NWs feature a high crystallinity without the presence of other phases of the Cu complexes. The concentration of the NH3 in the growth solution was found to play the key function in determining the shape of the nanostructures, which are nanobelt and nanowire structures for relatively high and low concentrations, respectively. Because of its simplicity in realizing vertically oriented CuO nanowires on different material surfaces, this approach can be further used to explore the fascinating properties of the CuO nanostructures to be used in optoelectronic, superconductivity, catalyst, and sensor applications. Although the growth of CuO nanowires can be achieved straightforwardly using the present approach, the growth orientation of the nanostructures was found to be uncontrollable. A study on the aspect that influences the structural growth of CuO nanostructures on the surface is in progress. Acknowledgment. This work was supported by Kyoto Nanotechnology Cluster Project, a Grant for Regional Science and Technology Promotion from the Ministry of Education, Culture, Sports, Science and Technology, Japan. A.A.U. thanks the JSPS (Japan Society for the Promotion of Science) for the fellowship. We thank Mr. Gang Chang for assisting us with an XRD characterization.

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