J. Phys. Chem. B 2005, 109, 14011-14016
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Formation of Uniform CuO Nanorods by Spontaneous Aggregation: Selective Synthesis of CuO, Cu2O, and Cu Nanoparticles by a Solid-Liquid Phase Arc Discharge Process Wei-Tang Yao,† Shu-Hong Yu,*,† Yong Zhou,‡ Jun Jiang,† Qing-Song Wu,† Lin Zhang,† and Jie Jiang† DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Structural Research Laboratory of the Chinese Academy of Sciences and Department of Materials Science and Engineering, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China, and Nanoarchitectonics Research Center, AdVanced Industrial Science and Technology, Tsukuba Center 5, 1-1-1 Higashi, Tsukuba, Japan ReceiVed: April 6, 2005; In Final Form: May 26, 2005
Uniform and monodisperse CuO nanorods have been synthesized by directional aggregation and crystallization of tiny CuO nanoparticles generated from a solid-liquid arc discharge process under ambient conditions in the absence of any surfactants. Uniform CuO nanorods with sharp ends are formed from tiny nanoparticles via a process that involves the rapid oxidation of Cu nanoclusters, the spontaneous aggregation of CuO nanoparticles, and the Ostawald ripening process. The spontaneous aggregation and oriented attachment of tiny CuO nanoparticles contributed obviously to the formation of these kinds of nanostructures. By choice of suitable reducing agent to prevent the oxidation of Cu nanoclusters, Cu and Cu2O nanoparticles can be selectively synthesized.
Introduction The oxides of transition metals such as iron, nickel, cobalt, zinc, and copper have many important applications. Among these metal oxides, cupric oxide (CuO) has been extensively studied due to its important properties and applications, such as high-critical-temperature superconductors,1 heterogeneous catalysts,2 complex magnetic phases,3 p-type semiconductors (Eg ) 1.2 eV),4 and lithium-copper oxide electrochemical cells.5 Due to the importance and potential applications of CuO, many methods have been used to fabricate CuO nanostructures such as the double-jet precipitation method,6 solid-state or wetchemical methods,7 sonochemical methods,8 templating methods,9 and thermal decomposition methods.10 CuO nanoparticles with different morphologies have been observed such as needlelike nanoparticles, nanowhiskers, nanorods/nanowires, nanoribbons, and nanotubes.6,7,11 Recently, CuO nanowires have been formed by direct oxidation of metallic copper substrates at high temperatures.12 In addition, CuO nanorods and nanoribbons can also be prepared via “soft chemistry” synthetic routes at elevated temperatures.13 Through the use of an arc discharge technique, a large number of materials such as carbon nanotubes,14 SiC whiskers,15 and β-Ga2O3 nanowires16 have been synthesized. Recently, a socalled solid-liquid phase arc discharge (SLPAD) method has emerged as a general route for the synthesis of nanoparticles such as nanocarbons,17 metal nanowires,18 and Mg(OH)2 nanorods.19 In this paper, we report a simple route for the selective synthesis of CuO, Cu2O, and Cu nanoparticles by the SLPAD * Author to whom correspondence should be addressed. Fax: + 86 551 3603040. E-mail:
[email protected]. † University of Science and Technology of China. ‡ Advanced Industrial Science and Technology.
Figure 1. XRD patterns of CuO nanoparticles obtained by the arc discharge method after aging for different periods without the protection of a reducing reagent: (a) 3 days, (b) 5 days, (c) 1 week, and (d) 2 weeks. (e) Formation of Cu2O nanoparticles and (f) Cu nanoparticles by the arc discharge method after aging for 2 weeks in the presence of the reducing agents ascorbic acid and hydrazine hydrate, respectively.
method. Uniform and size controllable CuO nanorods can be obtained via a spontaneous aggregation and crystallization processes in the absence of any surfactants or additives. It has been found that newly formed Cu nanoclusters generated by the SLPAD method rapidly oxidize into CuO nanoparticles, and then these CuO nanoparticles will spontaneously aggregate or self-organize and crystallize into uniform CuO nanorods via prolonging aging time under ambient conditions. Selective synthesis of CuO, Cu2O, and Cu phases can be easily achieved by controlling the reducing and oxidizing atmosphere of the initial solution.
10.1021/jp0517605 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/01/2005
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Figure 2. TEM images of the products obtained by the SLPAD method after aging for different times: (a) 30 min, (b) 1 day, (c) 3 days, (d) 5 days, (e) 1 week, and (f) 2 weeks. (g) Electron diffraction pattern taken after 2 weeks for CuO nanorods.
Experimental Section Apparatus and Preparation Methods. The apparatus of the solid-liquid phase arc discharge (SLPAD) method has been previously described.18a High-purity copper filaments (ca.1.5 mm in diameter) were used as two electrodes. One of the electrodes was dipped into the 0.1 mol/L NaNO3 solution. The end of another copper electrode was momentarily brought into contact with the surface of the NaNO3 solution while a certain voltage (ca. 150 V) was used between the two electrodes using an alternating current (AC) step-down circuit. It formed the instantaneous circulation between the two electrodes, and arc discharge sparks at the point end of the latter copper electrode. This resulted in the continuous dissolution of the copper electrode into the solution due to the great exothermic heat released during the arc discharge, and the newly formed Cu clusters will rapidly be oxidized and form a CuO colloidal solution. To synthesize Cu2O and Cu nanoparticles, 0.1 mol/L ascorbic acid and 1 mL of hydrazine hydrate (35 wt %) were added, respectively, into the initial solution. The apparatus was cooled with water. After the colloidal solution
was aged for several days, precipitates obviously appeared. The precipitate was washed with distilled water and absolute ethanol and dried in a vacuum at 60 °C for 2 h. Characterization. The X-ray diffraction (XRD) analysis was performed using a Rigaku (Japan) D/max-γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). The transmission electron microscopy (TEM) images were taken with a Hitachi model H-800 transmission electron microscope, using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) photographs and selected area electron diffraction (SAED) patterns were performed on a JEOL-2010 transmission electron microscope. The Raman spectra were recorded with 488-nm laser excitation with a micro-Raman system, which was modified by coupling an Olympus microscope to a Spex 1740 spectrometer with a CCD detector. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCALAB M KII X-ray photoelectron spectrometer using Mg KR X-rays as the excitation source. The UV-vis absorption spectrum was recorded with a Shimadzu UV-2501 spectrophotometer in the wavelength range of 200-800 nm at room temperature.
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Figure 3. (a-e) HRTEM images and electron diffraction patterns of CuO nanorods obtained after aging for 2 weeks. (a) Typical view of CuO nanorods. (b and d) HRTEM images of the two parts marked in part a showing the [1h11] and [111] growth directions of of the nanorod shown in part c, respectively. (c) Enlarged TEM image of the single CuO nanorod with the sharper end marked in the white block of part a, indicating that the nanorod has two growth directions, [1h11] and [111], respectively. (e) Electron diffraction pattern on a single CuO nanorod.
Figure 4. (a-d) HRTEM images of CuO nanoparticles with rough surfaces that are obtained after aging for 3 days, showing the aggregation of the nanoparticles and oriented growth stages. (a) CuO nanoparticles and nanorods. (b) Growth along the [1h11] direction. (c) Self-aggregation of CuO nanoparticles, indicating that the nanoparticles have two growth directions, which are along [1h11] and [111], respectively. (d) Growth along the [111] direction.
Results and Discussion Selective Synthesis of CuO Nanorods and Cu and Cu2O Nanoparticles. The continuous dissolution of the copper electrode into the solution was due to the great amount of exothermic heat released during the arc discharge process. The rapid oxidation of Cu nanoclusters will result in the formation of CuO clusters. Figure 1 shows the XRD patterns of the products obtained by aging the solution for different periods of time. Figure 1a shows that two broadening peaks (1h11) and (111) are observed for the product obtained after aging for 3 days. Prolonging the aging time will improve the crystallinity of the product. All XRD peaks can be indexed to the monoclinic CuO crystal structure (JCPDS Card No. 45-0937, a ) 4.69 Å, b ) 3.43 Å, c ) 5.13 Å, β ) 99.55°),20 the major peaks located at 2θ values of 20-70° clearly indicate that the CuO product is a pure phase, and the lattice constants were calculated to be a ) 4.68 Å, b ) 3.44 Å, and c ) 5.15 Å, respectively, comparable with those reported in the literature.12 The observation of the resulting solution reveals that no obviously visible precipitate appeared at the beginning of the aging process, suggesting that the tiny nanoparticles are formed as confirmed further by TEM results. TEM analysis on the precipitates obtained after aging for different periods provides more insight into the special details
of the growth process of CuO nanorods. The solution aged for 30 min contains many tiny amorphous particles with average sizes of 1-2 nm as shown in Figure 2a. If the aging time is prolonged from 3 days to 2 weeks, then the brown-black precipitate formed. Figures 2b-d display the typical morphologies of the particles obtained by aging the solution for 1, 3, and 5 days, respectively, which demonstrates that CuO nanorods with diameters of 1-3 nm and lengths up to 7-10 nm have formed. Many nanoparticles tend to aggregate and attach to these tiny nanorods (Figures 2c-d) for the samples grown for 3 and 5 days. More nanorods appeared (Figure 2c-d) compared with the very beginning stage (Figure 2a-b). When aging time is prolonged to 1 and 2 weeks, all tiny nanoparticles disappeared, and all of the particles are nanorods with sizes of 2-4 nm in diameter and 14-16 nm in length, as shown in Figures 2e-f. The electron diffraction pattern recorded on randomly sitting nanorods shows diffraction rings, indicating the nanorods are polycrystalline, which can be readily indexed as the CuO phase. An enlarged HRTEM observation of a sample obtained after aging for 2 weeks is shown in Figure 3. The nanorods with rough surface structures were observed and were polycrystalline (Figures 3a and 3e). Figure 3c shows a single CuO nanorod, in which a lattice spacing of about 2.52 Å of the left side of the nanorod was observed corresponding to the spacing for the [1h11]
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Figure 5. TEM images of the products obtained by the SLPAD method after aging for different periods of time: (a) 30 min and (b) 2 weeks, Cu nanoparticles prepared in the presence of hydrazine hydrate, and (c) 30 min and (d) 2 weeks, Cu2O nanoparticles prepared in the presence of ascorbic acid. The insets of parts b and d are the corresponding electron diffraction patterns of single-crystal Cu and Cu2O nanoparticles, respectively.
planes of monoclinic CuO (Figure 3b), while the lattice spacing of the right side is about 2.32 Å, corresponding to the spacing for the [111] planes (Figure 3d). It is noted that this highly anisotropic growth feature is very similar to that reported previously12 where the twin boundary of a nanowire grown by a high-temperature method was observed. However, the growth directions along [1h11] and [111] have also been observed in an intermediate coexisting growth stage of the CuO nanoparticles/ nanorods (Figure 4). Recently, the directional aggregation-based growth mechanism, which is called the oriented attachment mechanism,21 has been found to be an important and particular aggregation process happening in the solution system. The directional alignment of primary nanoparticles through the oriented attachment process due to the tendency for the minimization of their surface areas associated with the high-energy facets, possibly mediated by surface oxygen (or hydroxyl) molecules, which will occur at high-energy facets, results in the elimination of these surfaces of high surface energies.22,23 In the present system, crystallization and anisotropic growth of CuO nanorods from primary nano-
particles can happen under ambient conditions in the absence of any templates or organic additives. This spontaneous aggregation for the formation of CuO nanrods under ambient conditions is consistent with that found in the formation of ZnO nanorods by the evaporation and reflux of a solution containing 3-5 nm nanoparticles at elevated temperature.22a Self-assembly of nanoparticles capped by ligands was mainly driven by the interactions of the organic ligands rather than by the interaction of the particle cores.22a In addition, such spontaneous aggregation of small nanoparticles into much elongated nanorods/nanowires has been achieved at room temperature in the case of CdTe when the protective shell of organic stabilizer thioglycolic acid on the surface of the initial CdTe nanoparticles was partially removed.22b The whole formation process of CuO nanorods in the present case was suggested as the following. First, Cu nanoclusters are formed by the SLPAD method. Second, the nanoclusters are rapidly oxidized and change into primary CuO nanoparticles. Third, these tiny CuO nanoparticles tend to directionally selfaggregate or self-organize slowly to form nanorods through a
Formation of CuO Nanorods by SLPAD
J. Phys. Chem. B, Vol. 109, No. 29, 2005 14015 SCHEME 1: Illustration of the Selective Synthesis of CuO, Cu2O, and Cu Nanoparticles by the SLPAD Method
Figure 6. Raman spectrum of CuO nanorods.
possible oriented attachment process and start the anisotropic growth. Fourth, the uniform CuO nanorods are formed through the Ostawald ripening process. Our results demonstrated that self-assembly and crystallization of nanoparticles in a templatefree solution system could offer a new strategy to generate more complex ensembles of nanoparticles than the conventional crystal growth process. The formation of Cu nanoparticles in the very early stage was confirmed if a strong reducing agent such as hydrazine hydrate is present in an initial solution (Figure 1f). TEM images confirmed that the tiny nanoparticles with sizes of several nanometers are formed at very beginning (Figure 5a). When the aging time is prolonged, Cu nanoparticles (with sizes of 30-50 nm) will grow and crystallize as well, as shown in Figure 5b. If ascorbic acid was used as the reducing agent in the initial solution, then the product was found to be the Cu2O phase (Figure 1e). Again, the Cu2O nanoparticles with sizes of 4-10 nm are formed at the very beginning stage, and then they will grow into large particles with sizes of 300-500 nm and good crystallinity (Figures 5c-d). These results suggested that the Cu, Cu2O, and CuO phases can be selectively synthesized by controlling the oxidization and reduction atmosphere of the solution as summarized in Scheme 1. Raman Spectrum and XPS Analysis. The structural formation of CuO nanorods was studied by Raman spectroscopy (Figure 6). Three Raman-active modes (Ag + 2Bg) were observed.24 We can assign the peak at 282 cm-1 to the Ag mode and the peaks at 326 and 615 cm-1 to the Bg modes. In
comparison with the Raman vibrational spectrum of a CuO single crystal,21 the Raman peaks of the CuO nanorods are broadened and shift toward high wavenumbers due to the size effects of the CuO nanorods. The purity and the composition of the CuO nanorods were further investigated by X-ray photoelectron spectroscopy (Figure 7a). The peaks at about 934.5 and 954.5 eV are attributed to Cu 2p3/2 and 2p1/2 (Figure 7b), respectively, which are consistent with those observed in CuO.25,7c As shown in Figure 7c, the O1s core-level spectrum is broad, and three Gaussians (marked as I, II, and III) were resolved by using a curve-fitting procedure. Peak I, at the lower energy of 530.69 eV, is in agreement with that for O2- in CuO,25,7c while peaks II and III, at the high energies of 531.46 and 533.23 eV, respectively, are attributed to oxygen absorbed onto the surface of the CuO nanorods. Thus, the XPS results support the conclusion that the sample is composed only of CuO. The quantitative analysis of the signals based on the areas of the peaks gives the atomic ratio of Cu/O as 1:1.14. Conclusion In summary, CuO, Cu2O, and Cu nanoparticles can be selectively synthesized by the choice of a suitable reducing agent to control the oxidation of Cu nanoclusters, generated from a solid-liquid arc discharge process under ambient conditions in the absence of any surfactants. Uniform and monodisperse CuO nanorods with sharper ends can be prepared by spontaneous aggregation and crystallization of tiny CuO nanoparticles in a surfactant-free solution after aging under ambient conditions. The formation process of highly anisotropic CuO nanorods was studied, which involves the rapid oxidation of Cu nanoclusters into CuO primary nanoparticles and the self-aggregation of CuO nanoparticles, accompanied by the Ostawald ripening process. The results suggested that the combination of the SLPAD, which generates “clean” primary nanoparticles in a template-free solution system, with the ambient aging and
Figure 7. XPS spectra of CuO nanorods: (a) survey, (b) Cu2p, and (c) O1s.
14016 J. Phys. Chem. B, Vol. 109, No. 29, 2005 crystallization process can produce high-quality nanostructures in the absence of any surfactants/templates. In addition, CuO nanorods can be used as heterogeneous catalysts or as precursors for the preparation of various novel nanomaterials. This system can be used as a model system to study how nanostructures are formed through spontaneous aggregation and organization of primary building blocks. Acknowledgment. S.H.Y. thanks the Centurial Program of the Chinese Academy of Sciences, the National Science Foundation of China (Grant Nos. 20325104, 20321101, and 50372065), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, for funding support. We thank the referees for their suggestions during revision. References and Notes (1) (a) Bednorz, J. G.; Muller, K. A. Z. Phys. B: Condens. Matter 1986, 64, 189. (b) Wu, M. K.; Ashburn, J. R.; Torng, C. J.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang, Y. Q.; Chu, C. W. Phys. ReV. Lett. 1987, 58, 908-910. (2) (a) Reitz, J. B.; Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 11467-11478. (b) Ramirez-Ortiz, J.; Ogura, T.; Medina-Valtierra, J.; Acosta-Oritz, S. E.; Bosch, P.; de los Reyes, J. A.; Lara, V. H. Appl. Surf. Sci. 2001, 174, 177-184. (c) Wang, H.; Xu, J. Z.; Zhu, J. J.; Chen, H. Y. J. Cryst. Growth 2002, 244, 88-94. (3) (a) Forsyth, J. B.; Brown, P. J.; Wanklyn, B. M. J. Phys. C: Solid State Phys. 1988, 21, 2917-2929. (b) Yang, B. X.; Thurston, T. R.; Tranquada, J. M.; Shirane, G. Phys. ReV. B 1989, 39, 4343-4349. (c) Sukhorukov, Y. P.; Loshkareva, N. N.; Samokhvalov, A. A.; Naumov, S. V.; Moskvin, A. S.; Ovchinnikov, A. S. J. Magn. Magn. Mater. 1998, 183, 356-358. (4) (a) Rakhshani, A. E. Solid-State Electron. 1986, 29, 7-17. (b) Musa, A. O.; Akomolafe, T.; Carter, M. J. Sol. Energy Mater. Sol. Cells 1998, 51, 305-316. (5) (a) Podhajecky, P.; Zabransky, Z.; Novak, P.; Dobiasova, Z.; Cerny, R.; Valvoda, V. Electrochim. Acta 1990, 35, 245-249. (b) Lanza, F.; Feduzi, R.; Fuger, J. J. Mater. Res. 1990, 5, 1739-1744. (6) Lee, S. H.; Her, Y. S.; Matijevic´, E. J. Colloid Interface Sci. 1997, 186, 193-202. (7) (a) Wang, W.; Zhan, Y.; Wang G. Chem. Commun. 2001, 727728. (b) Wang, W.; Zhan, Y.; Wang, X.; Liu, Y.; Zheng, C.; Wang, G. Mater. Res. Bull. 2002, 37, 1093-1100. (c) Wang, W.; Liu, Z.; Liu, Y.; Xu, C.; Zheng, C.; Wang, G. Appl. Phys. A 2003, 76, 417-420. (8) Kumar, R. V.; Elgamiel, R.; Diamant, Y.; Gedanken, A.; Norwig, J. Langmuir 2001, 17, 1406-1410. (9) (a) Wang, S. H.; Huang, Q. J.; Wen, X. G.; Li, X. Y.; Yang, S. H. Phys. Chem. Chem. Phys. 2002, 4, 3425-3429. (b) Hsieh, C. T.; Chen, J. M.; Lin, H. H.; Shih, H. C. Appl. Phys. Lett. 2003, 82, 3316-3318. (10) (a) Hong, Z. S.; Cao, Y.; Deng, J. F. Mater. Lett. 2002, 52, 3438. (b) Xu, C. K.; Liu, Y. K.; Xu, G. D.; Wang, G. H. Mater. Res. Bull. 2002, 37, 2365-2372. (11) (a) Nabarro, F. R. N.; Jackson, P. J. Growth of Crystal Whiskers. In Growth and Perfection of Crystal Growth; Doremus, R. H., Roberts, B.
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