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J. Phys. Chem. C 2008, 112, 74-79

A Novel Heterostructure of Co3O4/ZnO Nanowire Array Fabricated by Photochemical Coating Method Youngjo Tak and Kijung Yong* Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang UniVersity of Science and Technology (POSTECH), Pohang 790-784, Korea ReceiVed: September 16, 2007; In Final Form: October 15, 2007

We developed a facile solution-based synthetic route for the fabrication of two different types of novel Co3O4/ ZnO nanowire heterostructures: tip-coated array type and fully coated horizontal (or colloidal) type. Twostep solution-based methods were used for the fabrication of the Co3O4/ZnO nanowire heterostructures. First, ZnO nanowires were grown by ammonia solution hydrothermal method. Afterward, Co3O4 was coated on the ZnO nanowires using a photochemical reaction. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed for the observation of the heterostructure morphology. Also, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to confirm the crystallinity and composition of the heterostructures. We found that the morphology of the heterostructures strongly depends on the photochemical reaction parameters such as the concentration of the cobalt ion solution, UV irradiation time, and geometrical alignment of the ZnO nanowires. The possible mechanism for the heterostructure formation was discussed.

1. Introduction Recently, one-dimensional (1D) heterostructure nanomaterials have attracted much research interest because of their applicability as versatile building blocks in nanoelectronic and nanophotonic devices.1 Various 1D heterostructure nanomaterials have been reported, including epitaxial2 and nonepitaxial heterostructures.3 To fabricate the desired heteronanostructures, several growth techniques have been explored, such as chemical vapor deposition (CVD),4 sputtering,5 solution method,6 and so on. Among these methods, solution method has merits of simple and low-temperature processing. Especially, photochemical coating method, on the basis of the redox reactions of aqueous chemical species on photocatalytic solid surfaces, has characteristics of site-specific growth of nanomaterials. However, only a few studies have been reported for the photochemical 1D heteronanostructure fabrication and, also, most of them have focused on metal coating.7 Zinc oxide (ZnO), an n-type semiconductor with wide direct band gap (3.4 eV) and large exciton binding energy (60 meV), has transparent, piezoelectric, and photocatalytic properties.8 ZnO nanowire is one of the most intensively studied nanomaterials because of its unique material properties and welldeveloped synthesis methods. By employing these characteristics, various kinds of nanodevices based on ZnO nanowire have been reported, for example, light-emitting diode (LED),9 field effect transistor (FET),10 chemical sensor,11 and solar cell.12 On the other hand, cobalt oxide (Co3O4), an important magnetic p-type semiconductor, has many interesting applications such as gas sensor,13 heterogeneous catalyst,14 electrochromic device,15 and Li-ion battery electrode.16 Recently, several studies have been reported for the synthesis of 1D Co3O4 nanostructures, including nanowires and nanotubes, using chemical bath deposition (CBD),17 electrospinning,18 and tem* Author to whom correspondence should be addressed. Phone: 82-54279-2278. Fax: 82-54-279-8298. E-mail: [email protected].

plate method.19 Although each individual nanostructure of ZnO and Co3O4 has been studied extensively, few studies of the ZnO-Co3O4 heteronanostructures have been reported. The heteronanostructure of the two important oxides will expand the potential application area of the heterogeneous nanodevices. In this article, we report a novel photochemical coating method for the fabrication of heteronanostructures consisting of ZnO nanowires and Co3O4 coating layer. First, we prepared ZnO nanowire array on a silicon substrate using our previously reported ammonia solution method.20 Subsequently, UV light irradiation was performed to induce photochemical reactions of aqueous cobalt ions on the surfaces of the ZnO nanowires. The Co3O4/ZnO heteronanostructures were characterized, and the effects of experimental parameters, for example, concentration of cobalt solution, UV irradiation time, and geometric aligning of the ZnO nanowire sample, on the heteronanostructure were investigated. 2. Experimental Section ZnO nanowire arrays were grown on a silicon substrate using our previously reported method.20 A 5∼10 nm ZnO buffer film was coated on the silicon substrate by sputtering a ZnO target at room temperature and then was air-annealed at 800 °C for 1 h. After cooling to room temperature, the substrates were immersed in a 10 mM Zn(NO3)2‚6H2O (98%, Aldrich) aqueous solution where pH was adjusted to 11 by adding the ammonia solution (28 wt % in water, Aldrich), and the solution was heated at 95 °C for 6 h. After growth, the substrate was removed from the solution, was rinsed with the deionized water, and was dried by nitrogen blow. For the photochemical coating process, the ZnO nanowire array was immersed in 0.1∼100 mM Co(NO3)2‚ 6H2O (98%, Aldrich) aqueous solution and was irradiated with UV light (ca. 325 nm) by a UV lamp (UVITEC, LF-215LM). The temperature of the photochemical process was room temperature, and the irradiation time was varied from a few minutes to 24 h. Scanning electron microscopy (SEM) images

10.1021/jp077443a CCC: $40.75 © 2008 American Chemical Society Published on Web 12/11/2007

Novel Heterostructure of Co3O4/ZnO Nanowire Array

J. Phys. Chem. C, Vol. 112, No. 1, 2008 75

Figure 2. The morphology change of the Co3O4 coated ZnO nanowire arrays depending on the concentration of the cobalt ion solutions. (a, b) SEM images of the top and tilt view of a bare ZnO nanowire array before UV irradiation. (c, d), (e, f), (g, h), (i, j) SEM images of Co3O4 coated ZnO nanowire arrays after UV irradiation in 0.1, 1, 10, and 100 mM Co(NO3)2‚6H2O solution, respectively. For all samples, UV irradiation time was fixed as 24 h. We could observe that Co3O4 heads became larger and eventually coalesced to form a roof film as the concentration of cobalt ion increased. The lengths of the scale bars are 1 µm for all images.

Figure 1. Typical SEM image of the (a) Co3O4 tip-coated ZnO nanowire array on a silicon substrate (24 h UV irradiation in 2 mM cobalt ion solution). (b) Magnified image of a, where we can observe top, tilt, and cross section images simultaneously. (c-e) More magnified images of the top, tilt, and cross section region in b, respectively. The lengths of the scale bars are 10, 2, and 1 µm, for a, b, and c-e, respectively. XRD patterns of Co3O4 tip-coated ZnO nanowire arrays: (f) reference pattern of Co3O4 powder (PDF #74-1657); (g, h) XRD patterns obtained from the as-coated and the 800 °C air-annealed samples, respectively.

were taken using a Philips XL30S field emission scanning electron microscope. X-ray diffraction (XRD) patterns were obtained on a Rigaku DMAX-1400 at fixed detector angle as 2°. Transmission electron microscopy (TEM) analysis was performed on a JEOL JEM-2100F with energy-dispersive X-ray spectrometer (EDX). 3. Results and Discussion Scanning electron microscopy (SEM) was employed to investigate the morphological changes of the ZnO nanowire array after the photochemical coating process. Figure 1a shows a low-magnification SEM image of a Co3O4 coated (the identity of the coated material, i.e., Co3O4, will be discussed in the following XRD, TEM, and X-ray photoelectron spectroscopy (XPS) analyses) ZnO nanowire array on a silicon substrate. This top-view image was taken at an edge of the cut sample, which shows that the nanowire array is grown uniformly on a large area. While the sample was prepared for SEM analysis, some cracks were formed because of the damage during cutting of the substrate. Fortunately, we could obtain the image, which

shows top, tilt, and cross section view of the sample simultaneously, at this special region. Figure 1b is a magnified image of the region marked in Figure 1a. From the image, we could notice that the ZnO nanowire array was about 5 µm long and was vertically well-aligned. However, it was difficult to observe a detailed morphology of the Co3O4/ZnO heteronanostructures in this magnification. Figure 1c, d, and e shows more magnified images corresponding to the top, tilt, and cross section region in Figure 1b, respectively. These images clearly indicate the Co3O4/ZnO heterojunctions having Co3O4 “heads” on the tips of each ZnO nanowire. The shape was similar to the “matchstick”. To obtain this heterojunction nanostructure, we performed a photochemical coating experiment at the conditions of 2 mM cobalt solution under a 24 h UV irradiation. The crystal structure of the Co3O4/ZnO heteronanostructures was investigated using X-ray diffraction (XRD) analysis. Figure 1g and h shows XRD patterns obtained from the as-coated and the air-annealed Co3O4-ZnO heteronanostructure, respectively. The results showed two main ZnO peaks corresponding to wurtzite (002) and (103) phases, respectively, from the ZnO nanowire array. Besides the ZnO peaks, the as-coated sample in Figure 1g showed two more peaks at ∼31° and ∼37° which can be assigned to the cubic spinel Co3O4 (220) and (311) phases, respectively.21 The broadness and weakness of the Co3O4 peaks can be explained by the small grain sizes and poor crystallinity of the as-coated Co3O4 material. For the airannealed sample at 800 °C, the intensities of the main Co3O4 peaks, (220) and (311) phases, became stronger than those of the as-coated sample (Figure 1h). Also, other minor phases of Co3O4 were detected after annealing of the sample. We confirmed that the annealed Co3O4 coating layer had randomly oriented crystal directions, because the XRD pattern was identical with the reference powder XRD data (Figure 1f, PDF #74-1657). The morphology of this interesting Co3O4/ZnO heteronanostructure could be controlled by experimental parameters such as the concentration of the cobalt ion solution and the UV

76 J. Phys. Chem. C, Vol. 112, No. 1, 2008

Tak and Yong

Figure 3. TEM analysis results of the Co3O4 tip-coated ZnO nanowire sample (prepared in 1 mM cobalt ion solution after 24 h irradiation): (a) A low-magnification TEM image of a Co3O4 tip-coated ZnO nanowire; (b) a high-resolution TEM image of Co3O4 coated region. Inset shows a corresponding FFT image; (c-e) magnified images taken from marked regions of C-E, respectively, in a; (f, g) EDX spectra taken from the tip (c) and the middle (e) of the nanowire, respectively; (h) EDX elemental mapping of the nanowire tip; (i-k) the intensity of each element corresponding to O, Co, Zn, respectively.

irradiation time. We studied the concentration effects of the cobalt ion solution on the morphology of the Co3O4/ZnO heteronanostructure. The concentration of the cobalt ion solution was varied from 0.1 to 100 mM at a fixed UV irradiation time, that is, 24 h. Figure 2a and b shows top and tilt view of the bare ZnO nanowire array, respectively, before the UV irradiation. The ZnO nanowire array was deposited on the ZnO buffer layer coated Si substrate using a solution growth method. A more detailed procedure of the ZnO sample preparation is described in the Experimental Section. Figure 2c-j is the images of the Co3O4/ZnO heteronanostructures obtained from cobalt ion solutions of 0.1 (c and, d), 1 (e and f), 10 (g and h), and 100 mM (i and j). From the images, we could notice that the Co3O4 was mainly deposited on the tips of the ZnO nanowires to make “matchstick-shape” for all concentration cases, and moreover, the sizes of the Co3O4 “heads” became larger as the

concentration of the cobalt ion solution increased. The Co3O4 heads had grown and eventually coalesced to form a “roof” film on the top of the ZnO nanowire array at the 100 mM condition. Compared to these concentration effects, similar morphological changes were also observed with the increase of the UV irradiation time at a fixed concentration, that is, 100 mM (not shown here). To investigate the effects of the wavelength of incident light, we performed photochemical process under visible light. However, we could not obtain any deposition on the ZnO nanowire. Transmission electron microscopy (TEM) was used to study further details of the heteronanostructure (Figure 3). Figure 3a shows a low-magnification image of the Co3O4 coated ZnO nanowire, and Figure 3c-e is magnified images taken from the marked squares in Figure 3a, following the direction from the tip to the root of the nanowire. From the figures, it is seen that

Novel Heterostructure of Co3O4/ZnO Nanowire Array

Figure 4. XPS spectra of the Co lines obtained from the Co3O4 coated ZnO nanowire arrays: (a) as-coated and (b) 800 °C air-annealed sample.

thin coating layer is covering mainly the upper part of the nanowire up to a certain length (∼1 µm) from the tip, although the thickness is decreasing. We could notice that the coated surfaces (3c, d) were rougher than that of the bare ZnO nanowire region (3e). To investigate the microcrystalline structure of the heteronanostructure, high-resolution transmission electron microscopy (HRTEM) was employed. Figure 3b reveals a polycrystalline layer with a few nanometer grain sizes deposited on the single crystalline ZnO nanowire. The inset shows a corresponding fast Fourier transform (FFT) image displaying a superimposition of two patterns: a rectangular dot pattern from the single crystalline ZnO nanowire and a ring pattern from the polycrystalline Co3O4 coating layer. Figure 3f and 3g is energy-dispersive X-ray (EDX) spectra taken from the coated tip (Figure 3c) and uncoated ZnO nanowire (Figure 3e), respectively. In the spectra, Co element peaks were detected only at the coated tip, whereas Zn and O peaks were observed in both spectra. Cu signals originate from the TEM grid. This reveals again that cobalt coating mostly occurs on the top part of the vertically aligned ZnO nanowire array. As supplement information for the chemical analysis of the heteronanostructure, EDX element maps were collected by scanning the tip area (Figure 3h). Figure 3i, j, and k shows the image of each element corresponding to O, Co, and Zn, respectively. This elemental mapping confirms that the coating material is composed of Co and O element. XPS analysis was performed to investigate the chemical binding states of the heteronanostructure. The oxidation states of Co can be determined from Co 2p XPS spectra. Figure 4a and 4b shows the Co 2p spectra of the as-coated and the 800 °C air-annealed sample, respectively. As it is seen, the shape and peak positions of the two spectra are almost identical. These results indicate that the oxidation states of Co in the as-coated sample are the same as those in the air-annealed one. The Co 2p spectra both showed two main 2p3/2 and 2p1/2 spin-orbit lines at 780.7 and 796.7 eV, respectively, with two shakeup satellite peaks located at ∼6 eV above the main peaks. From the literature of the cobalt oxide films deposited by MOCVD at 500 °C,22 which had shown very similar XPS spectra to our results, we concluded that our photochemically coated cobalt oxide has a spinel Co3O4 phase. Also, this conclusion is consistent with the XRD data clearly. On the vertically aligned, high-density ZnO nanowire array, the photochemical deposition of Co3O4 mainly occurred on the tip part of the nanowires, which is referred to as “site-specific”

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Figure 5. (a) SEM image of the Co3O4 coated ZnO nanowire bundles on the edge of the sample, where ZnO nanowires were grown sparsely and laterally. Inset shows a magnified image of the boxed area. (b) A more magnified image of a. In this magnification, we could see that Co3O4 fully coated not only the tip but also the side walls of the ZnO nanowires. (c, d) ZnO nanowires deposited on a silicon substrate laterally and after Co3O4 coating by UV irradiation in cobalt ion solution, respectively. Insets show magnified images.

deposition in some literature. However, we found that the deposition strongly depends on the geometry of the nanowire array from the accidental observation in our experiments. At the edge of the silicon substrate, there exists a boundary region between ZnO seed-coated surface (where ZnO nanowire array was grown) and bare silicon surface (where no growth was found). At this region, we found that ZnO nanowire bundles were sparsely grown, were not fully covering the substrate, and were lying down on the substrate. Figure 5a shows a lowmagnification SEM image of the boundary region after photochemical deposition of the Co3O4 layer. We can see that the ZnO nanowires are uniformly coated with Co3O4 layer on not only the tips but also on side surfaces. A magnified SEM image of Figure 5b shows the uniform coating morphology more clearly. A uniform coating is important for future applications of heteronanostructure in nanodevices. To confirm the effects of the geometrical posture of the ZnO nanowires on the Co3O4 deposition, we prepared a laterally deposited ZnO nanowire sample on a silicon substrate using the spin-coating method. Figure 5c shows a top-view SEM image of the ZnO nanowires deposited laterally on the silicon substrate. The prepared sample was immersed in 10 mM of Co(NO3)2‚6H2O aqueous solution and was irradiated with UV light for 24 h. Figure 5d shows the morphological change of the sample after UV light irradiation. Compared to Figure 5c (asgrown ZnO nanowires), the surface and diameter of the ZnO nanowires became rough and thick, indicating a uniform Co3O4 deposition, which was very similar to the results of Figure 5b. We also performed another photochemical deposition experiment on the colloidal ZnO nanowires dispersed in a Co(NO3)2‚ 6H2O aqueous solution. A similar full coating of Co3O4 was also observed in this case (not shown here). In the report of Pacholski et al.,23 who studied the site-specific deposition of Ag nanosphere at one end of the ZnO nanorod, they suggested possible hypotheses for the site-specific deposition phenomena: priority of the particular crystallographic planes, for example, ZnO (0001), on the adsorption of Ag+ and the nucleation of Ag nanosphere and localization of the electrons at the ends of the ZnO nanorods. However, we could not find any conclusive proof that the hypotheses can be applied to our

78 J. Phys. Chem. C, Vol. 112, No. 1, 2008 SCHEME 1: (a-c) Schematic Illustrations of the Photochemical Reactions Leading to the Formation of the Co3O4 Coated ZnO Nanowire Heterostructures: UV Light Creates Electron-Hole Pairsa

Tak and Yong dispersed ZnO nanowires (Figure 6c). A possible mechanism to explain these effects of the geometrical configuration on the coating is the mass transport of reactants. When the ZnO nanowires are placed horizontally on the substrate or are dispersed in the solution, the nanowire surface is fully exposed to the reactants without any hindrance in mass transport. Thus, a uniform coating proceeds on the surface through the photochemical reactions between reactants. However, in the case of the high-density aligned ZnO nanowire array, the spacing between nanowires is very narrow of a few tenths of a nanometer. This little spacing hinders the mass transport of the reactants through it to the bottom of the array, and thus these effects cause Co3O4 coating mainly to proceed on the upper part of the nanowires. To verify this proposed mechanism, the investigation of the reactant concentration profiles through the nanowire length in array is required. 4. Conclusions

a The electrons reduce cobalt ion into cobalt metal, and the holes oxidize water to oxygen. Co3O4 can be formed through the oxidation of Co metal by the dissolved oxygen at room temperature. Co3O4 coating occurs mainly (b) on the tip or (c) on whole surfaces of the nanowires depending on the geometry of the sample.

results. From our study, we suggest that the deposition of Co3O4 mainly depends on the geometrical arrangement of the ZnO nanowires in a similar way of the sputtering deposition, which is controlled by the mass transport of reactants. In this study, another important point is that the Co3O4 coating occurred only on the surface of the ZnO nanowires and no deposition was observed on the bare silicon substrate surfaces. This indicates that the deposition process is not due to the simple precipitation of the Co3O4 nanocrystals nucleated from the bulk solution phase but to the chemical reaction occurring on the effective photocatalytic surface. Figure 6a shows a proposed mechanism for the photochemical reactions leading to the formation of the Co3O4 coated ZnO nanowire heterostructures. UV light, with energy larger than the band gap of ZnO (3.4 eV), generates electron-hole pairs in ZnO nanowires. The photogenerated electrons in the conduction band (C.B.) reduce cobalt ions (Co2+) to cobalt metal (Co). In this situation, cobalt ions act as an electron acceptor and make the photogenerated holes rich. This high concentration of the holes in the valence band (V.B.) enhances the reaction through which the holes oxidize water (H2O) into oxygen (O2).24 In the meantime, the cobalt metal, photochemically deposited on the surface of ZnO nanowires, can be easily changed to Co3O4 through the oxidation by the generated and dissolved oxygen in aqueous solution at room temperature.25 Depending on the geometry of the ZnO nanowires, Co3O4 coating occurs mainly on the tip of the aligned ZnO nanowire array (Figure 6b) or on the whole surfaces of the laterally deposited or colloidal

We developed a facile solution-based synthetic route for the fabrication of two kinds of Co3O4 coated ZnO nanowire heterostructures, tip-coated array type and fully coated horizontal (or colloidal) type using a photochemical reaction. These Co3O4/ ZnO heteronanostructures have promising applications in Liion batteries, chemical sensors, electrochromic devices, and catalysts. The photochemical coating method presented in this study has merits of simple, low-temperature, and selectivedeposition process, which only occurs on the surface of the photocatalytic ZnO. Also, we believe that our method can be applicable to other photocatalytic materials (TiO2, CdS, etc.) or to other shapes of nanomaterials (nanobelts, nanoparticles, etc.) to fabricate various Co3O4 coated heterostructured nanomaterials. Acknowledgment. This work was supported by grant no. R01-2006-000-10230-0 (2006) from the Korea Science and Engineering Foundation (KOSEF) and the Korean Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-005-J13101) and grant no. RTI04-01-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry, and Energy (MOCIE). References and Notes (1) (a) Law, M.; Goldberger, J.; Yang, P. Annu. ReV. Mater. Res. 2004, 34, 83. (b) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K. H.; Lieber, C. M. Science 2001, 294, 1313. (c) Qian, F.; Gradecak, S.; Li, Y.; Wen, C. Y.; Lieber, C. M. Nano Lett. 2005, 5, 2287. (2) (a) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M. Nature 2002, 420, 57. (b) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (c) Park, W. I.; Yoo, J.; Kim, D. W.; Yi, G. C.; Kim, M. J. Phys. Chem. B 2006, 110, 1516. (3) (a) Liu, J.; Li, X.; Dai, L. AdV. Mater. 2006, 18, 1740. (b) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793. (4) (a) Bae, S. Y.; Seo, H. W.; Choi, H. C.; Park, J.; Park, J. J. Phys. Chem. B 2004, 108, 12318. (b) Zhou, J.; Liu, J.; Wang, X.; Song, J.; Tummala, R.; Xu, N. S.; Wang, Z. L. Small 2007, 3, 622. (5) (a) Jung, S. W.; Park, W. I.; Yi, G. C.; Kim, M. AdV. Mater. 2003, 15, 1358. (b) Hsueh, T. J.; Hsu, C. L.; Chang, S. J.; Guo, P. W.; Hsieh, J. H.; Chen, I. C. Scr. Mater. 2007, 57, 53. (6) (a) Liao, H. C.; Kuo, P. C.; Lin, C. C.; Chen, S. Y. J. Vac. Sci. Technol., B 2006, 24, 2198. (b) Zhang, R.; Wang, X. Chem. Mater. 2007, 19, 976. (c) Gu, F.; Li, C.; Wang, S. Inorg. Chem. 2007, 46, 5343. (d) Li, X.; Liu, Y.; Fu, L.; Cao, L.; Wei, D.; Wang, Y. AdV. Funct. Mater. 2006, 16, 2431. (7) (a) Wu, J. J.; Tseng, C. H. Appl. Catal., B: EnViron. 2006, 66, 51. (b) Li, D.; McCann, J. T.; Gratt, M.; Xia, Y. Chem. Phys. Lett. 2004, 394, 387. (8) (a) Klingshirn, C. Chem. Phys. Lett. 2007, 8, 782. (b) Wang, Z. L. Appl. Phys. A 2007, 88, 7.

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