J. Phys. Chem. C 2007, 111, 18475-18478
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ARTICLES Synthesis and Physical Properties of Co3O4 Nanowires Zhao Dong,† Yunyi Fu,‡ Qin Han,† Yingying Xu,† and Han Zhang*,† Materials Physics Laboratory, State Key Laboratory for Mesoscope Physics, Department of Physics, and Department of Microelectronics, Peking UniVersity, Beijing 100871, China ReceiVed: July 10, 2007; In Final Form: August 24, 2007
By carefully controlling the reacting time, temperature, and humidity, we have prepared Co3O4 nanowires by heating a pure cobalt foil in atmosphere. Scanning electronic microscopy demonstrates that the nanowires have a diameter ranging from 20 to 100 nm and their typical lengths are in the range of 10-20 µm. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectrum, and transmission electronic microscopy analyses demonstrate that the nanowires are Co3O4. The optical property of the nanowires is determined by photoluminescence spectrum. The magnetic behavior of it is investigated by a magnetic property measurement system. The nanowires exhibit some novel optical and magnetic properties, which are different from its bulk material.
Introduction Co3O4 is widely used as catalysts,1 gas sensors,2 and electronic devices.3 Because of potential applications of one dimensioned (1-D) nanostructured materials,4 some methods for preparing 1-D Co3O4 nanowires or nanotubes have been explored. Li et al.5 prepared polycrystal Co3O4 nanotubes via chemical thermal decomposition of Co(NO3)2‚6H2O within alumina templates. Li et al.6 fabricated Co3O4 nanowires by immersing the selected substrate in a reaction solution consisting of Co(NO3)2 and concentrated aqueous ammonia, and then postannealing the substrate after growth. Nam et al.7 obtained Co3O4 polycrystal nanowires using viruses as template. The above methods are a little complicated, and some of them may cause pollution. In this Article, we report a relatively simple method, by directly oxygenating pure cobalt foils, to prepare Co3O4 nanowires. Yu et al.8 have used a method similar to ours to prepare Co3O4 nanostucture, but they obtained only nanowalls, not wires. We improved the method and successfully obtained Co3O4 nanowires. On the other hand, some interesting magnetic and optical properties have been found in the nanosystems of Co3O4, including thin film, nanoparticles, and nanotubes.9-12 The magnetic properties and optical properties of the nanowires prepared by us have been investigated in this work, and some interesting phenomena have been observed. Experimental Section The substrates used in the experiment are cobalt foils (99.8 wt %) with thickness of about 2 mm and areas of 1 × 1.2 cm2. The Co3O4 nanowires were synthesized by the following process: first, each side of the substrate was burnished and carefully washed with alcohol in an ultrasonic cleaner. Next, * To whom correspondence should be addressed. Telephone: 86-1062754233. Fax: 86-10-62751615. E-mail:
[email protected]. † Department of Physics. ‡ Department of Microelectronics.
Figure 1. SEM micrographs of the sample. (a) An image of the surface covered with nanowalls. (b,c) Images of the edge covered with nanowires. (d) The typical TEM image of part of a single nanowire.
the substrate was placed into a quartz tube in a tube furnace. The reacting temperature and time were controlled. It was heated in air at 480-520 °C for about 10-12 h. The heating rate was about 200 °C/h. The water vapor produced from 1 g water per minute was flowing in the quartz tube from a humidifier to keep the reacting atmosphere wet. By adjusting the humidifier, the reacting humidity can be controlled. After reacting, the sample was cooled within the furnace at the cooling rate of 200 °C/h. As-synthesized samples were characterized and analyzed by X-ray diffraction (XRD) (Philips’ X’Pert MRD with Cu KR radiation), X-ray photoelectron spectroscopy (XPS) (Kratos Analytical Axis Ultra with Mono Al KR radiation), field emission scanning electron microscopy (SEM) (FEI Strata
10.1021/jp075365l CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2007
18476 J. Phys. Chem. C, Vol. 111, No. 50, 2007
Dong et al.
Figure 2. EDX pattern of the nanowire mentioned in Figure 1d.
DB235 FIB), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX) spectrometry (Tecnai F30 TEM). The Raman spectrum (Renishaw-inVia) of the nanowires was collected at room temperature with the excitation of the 514.5 nm wavelength of an Ar+. The photoluminescence (PL) spectrum (Jobin-Yvon-Spex) was also measured at room temperature using a Xe arc lamp as the excitation source. The excitation wavelength for the PL measurement was 325 nm. The magnetic measurement of the nanowires was conducted using a quantum design magnetic property measurement system (MPMS-XL7). Because the dimension of one single substrate is 1.0 × 1.2 cm2, wires of only one substrate were not enough to get strong signals for magnetic measurements. So, the nanowires were carefully peeled off several substrates by nonmagnetic adhesive tape and then put together for measurement. We may convincingly assert on the reproducibility of our results on wires clinched from different substrates and have, in fact, achieved mutually comparable signals from different groups of the nanowires prepared in this way.
Figure 3. XRD pattern of the cobalt foil heated at 520 °C for 12 h.
Figure 4. X-ray photoelectron spectra of the nanowires.
Results and Discussion After reaction, the substrates turned black, and no obvious change in their shapes and sizes is observed. Nanowires and nanowalls are found on the as-synthesized samples. Figure 1 shows the SEM images of the cobalt foil heated at 520 °C for 12 h. It is observed that many nanowires grow on the edge of the substrate and the nanowalls grow on the surface of the substrate. Figure 1a reveals a surface covered with the nanowalls, which is similar to the result in ref 8. Figure 1b and c shows the nanowires growing on the edge. The nanowires’ lengths are about 10-20 µm, with diameters of about 20-100 nm. From Figure 1b and c, it is also seen that most of the nanowires are very straight and perpendicular to the surface of the substrate. Figure 1d is a bright-field TEM micrograph, showing part of a nanowire with the diameter of about 30 nm. An EDX spectrum of the nanowire is shown in Figure 2. The peaks of Co and O can be clearly seen. The peaks of Cu and C come from the grid covered by carbon film. No peaks of other elements are found. It can be identified that the nanowire is composed of cobalt and oxygen. It is difficult to determine the Co/O ratio of the nanowire, because as a lighter element, the intensity of the EDX detection for oxygen is uncertain. The XRD pattern of the sample is shown in Figure 3. In the pattern, two phases of cobalt oxide, CoO (JCPDS No. 75-0418) and Co3O4 (JCPDS No. 80-1536), are found, CoO in just a tiny amount. To further identify the composition of the nanowires, Raman spectrum (see Figure 5) has been measured at room temperature. There are four peaks at 483, 522, 621, and 693 cm-1 in the pattern, just like the Raman spectra of the nanowalls shown in ref 8. They correspond respectively to one Eg, two F2g, and A1g models of Co3O4.8,13 Only peaks of Co3O4 and no peaks of CoO14 are found in the pattern. From the results of
Figure 5. Raman spectrum of the nanowires.
the XRD and Raman spectrum, it is reasonable to say that the nanowires are Co3O4. To further confirm the conclusion, XPS of the nanowires has been measured. Figure 4 shows the spectrum. The two forms of cobalt oxide, CoO and Co3O4, can be identified by different intensities of the shakeup satellites between the main peaks Co 2p3/2 and Co 2p1/2.15 A low satellite peak is at a binding energy (BE) of 789.6 eV, about 9.7 eV higher than 779.9 eV, where the peak of Co 2p3/2 appears. It is a typical satellite peak of Co3O4.15,16 The typical satellite peak of CoO, at about 2.1 eV, higher than the position of the main peak of Co 2p3/2,17 is not found in the spectrum. From the results of XPS, it also demonstrates that the nanowires are Co3O4. How do we explain the existence of CoO in the XRD pattern but not in the Raman spectrum and XPS? Comparing these results, we suggest that CoO found in the XRD pattern comes from a precursor for the growth of the Co3O4 nanowires. It is found that before growth of the nanostructure, a film composed of CoO is formed (see ref 8), and then the nanostructure grows out from this film. Because of the relatively deeper penetrating depth of X-ray diffraction, CoO is detected in the XRD pattern.
Synthesis and Physical Properties of Co3O4 Nanowires
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Figure 8. PL spectrum of the Co3O4 nanowires. Figure 6. HRTEM images and the corresponding SAED patterns (the inset) of the nanowire.
Figure 7. A sketch of atoms on edge. Black dots represent the atoms.
High-resolution transmission electron microscopy (HRTEM) images and the selected area electron diffraction (SAED) patterns of the nanowires are measured. Figure 6 shows a HRTEM image and its corresponding SAED pattern of a single nanowire. The SAED pattern (the inset of Figure 6) confirms that the Co3O4 nanowire is single crystalline. The HRTEM image shows that the lattice fringes are about 2.90 nm, which agrees well with the separations between the (220) planes. The HRTEM images and SAED patterns show that most of the nanowires are single crystals and the rest are twinned crystals. The HRTEM images also confirm that the nanowires grow along the [110] direction. Combining the above discussion about CoO as a precursor, it is reasonable to propose the mechanism roughly as follows: When the temperature goes up, the surface of cobalt foil is oxidized to form a CoO film first. Next, the film is further oxidized to form nanowalls on the surface and nanowires on the edge. Details about the mechanism of the nanowalls’ growth can be seen in ref 8. The mechanism of the nanowires’ growth should be different, as the atoms on the edge are different from those on the surface. Distances between atoms on the edge are always longer (see Figure 7). So their chemical activity would be higher than that of the ones on the surface. CoO here can be further oxidized to Co3O4 more easily. It would be the main reason for the nanowires’ growth on the edge. Experiment without the humidifier has also been done. Without water vapor, only the nanowalls can be found on the samples. This means that wet atmosphere plays a key role in the nanowires’ growth. However, the detailed mechanism about the nanowires’ growth is unclear and is still being studied.
Figure 9. (a) Temperature dependence of ZFC and FC magnetization for the nanowires under an applied field of 100 Oe. (b) Magnetic hysteresis curves measured at 30 and 280 K. (c) The partly enlarged hysteresis curves.
Some physical properties of the nanowires have also been preliminarily investigated. PL spectroscopy of the nanowires has been measured at room temperature. Figure 8 shows the result. The wide prominent peak may represent the band gap emission. From the pattern, the nanowires’ band gap is about
18478 J. Phys. Chem. C, Vol. 111, No. 50, 2007 2.07 eV. It is consistent with the band gap of the Co3O4 thin films (about 2.06-2.2 eV)11,12 and obviously higher than the bulks’ (about 1.6 eV).18 One reason for the blue-shift of the energy band gap may be the quantum size effect. Co3O4 is a p-type semiconductor.5 As the size of a semiconductor is sufficiently small, the electron and hole interactions with the interface become strong. That may cause carrier confinement. According to ref 19, the energy gap will become larger than the bulks’. In the nanoscale, the energy gap partly depends on the size and shape of the crystalline.19 There is a distribution of the diameters of the Co3O4 nanowires. So their energy gaps show a distribution, which makes the peak wider. Magnetic properties of Co3O4 nanowires have been investigated too. The temperature dependence of magnetization is shown in Figure 9a. The curves are acquired between 10 and 350 K using the zero-field-cooled (ZFC) and field-cooled (FC) procedures under a magnetic field of 100 Oe. The bulk Co3O4 is antiferromagnetic with the Neel temperature of about 40 K.20 However, the FC magnetization of the nanowires increases as the temperature decreases below about 40 K. A similar phenomenon is also found in the Co3O4 nanowires and nanotubes synthesized by templets.21,22 The weak ferromagnetism may be attributed to the surface spins, which is frozen in the external field. The ZFC and FC magnetizations increase with increasing temperature above 40 K. They also show the antiferromagnetic property. Unlike the nanostructures mentioned in refs 21 and 22, the FC and ZFC curves do not merge together until 350 K. We measured the sample several times, and the results are the same. The novel behaviors of the curves are being studied. To determine the magnetic properties more clearly, the hysteresis loops of the nanowires are measured at 30 and 280 K, respectively. The results are shown in Figure 9b and c. Both curves exhibit a little ferromagnetic property, as a little hysteresis and low remanent magnetization can be seen. The coercive force and remanent magnetic moment decrease with the temperature going up. The hysteresis is also found in other nanoscale systems of Co3O4, including thin films and nanoparticles.9,10 The hysteresis may be also caused by the surface spins. As temperature increases, more surface spins are free to the thermal fluctuations,21 so the sample exhibits weaker ferromagnetic properties at higher temperature. Because the nanowires can be peeled off the substrate by an adhesive tape, they are not very firmly fixed to the substrate. They can be used as some kinds of devices without fixing on the substrate, such as probes for the scanning probe microscope.
Dong et al. However, if they need to be solidly fixed on the substrate for application, further research should be carried out. Conclusion A new method for preparing Co3O4 nanowires, by heating cobalt foils in air, is reported. Most of the nanowires are very straight and grow along [110] direction. The PL spectrum of the nanowires is measured. It shows that the band gap is higher than that of the bulk. The magnetic properties of the nanowires are preliminarily investigated, and some novel properties are observed. Acknowledgment. The project was supported by the National Natural Science Foundation of China under grant no. 10434010. References and Notes (1) Liotta, L. F.; Carlo, G. D.; Pantaleo, G.; Deganello, G. Catal. Commun. 2005, 6, 329. (2) Nam, H.-J.; Sasaki, T.; Koshizaki, N. J. Phys. Chem. B 2006, 110, 23081. (3) Fu, L.; Liu, Z.; Liu, Y.; Han, B.; Hu, P.; Cao, L.; Zhu, D. AdV. Mater. 2005, 17, 217. (4) Im, Y.; Lee, C.; Vasquez, R. P.; Bangar, M. A.; Myung, N. V.; Menke, E. J.; Penner, R. M.; Yun, M. Small 2006, 2, 356. (5) Li, W.-Y.; Xu, L.-N.; Chen, J. AdV. Funct. Mater. 2005, 15, 851. (6) Li, Y.; Tan, B.; Wu, Y. J. Am. Chem. Soc. 2006, 128, 14258. (7) Nam, K. T.; Kim, D.-W.; Yoo, P. J.; Chiang, C.-Y.; Meethong, N.; Hammond, P. T.; Chiang, Y.-M.; Belcher, A. M. Science 2006, 312, 885. (8) Yu, T.; Zhu, Y.; Xu, X.; Shen, Z.; Chen, P.; Lim, C.-T.; Thong, J. T.-L.; Sow, C.-H. AdV. Mater. 2005, 17, 1595. (9) Apa´tiga, L. M.; Castan˜o, V. M. Thin Solid Films 2006, 496, 576. (10) Makhlouf, S. A. J. Magn. Magn. Mater. 2002, 246, 184. (11) Pejova, B.; Isahi, A.; Najdoski, M.; Grozdanov, I. Mater. Res. Bull. 2001, 36, 161. (12) Yamamoto, H.; Tanaka, S.; Natio, T.; Hirao, K. Appl. Phys. Lett. 2002, 81, 999. (13) Hadjiev, V. G.; Iliev, M. N.; Vergilov, I. V. J. Phys. C: Solid State Phys. 1988, 21, L199. (14) Chou, H.-h.; Fan, H. Y. Phys. ReV. B 1976, 13, 3924. (15) Comini, E.; Cusma`, A.; Kaciulis, S.; Kandasamy, S.; Padeletti, G.; Pandolfi, L.; Sberveglieri, G.; Trinchi, A.; Wlodarski, W. Surf. Interface Anal. 2006, 38, 736. (16) Wang, Y.; Fu, Z.-W.; Qin, Q.-Z. Thin Solid Films 2003, 441, 19. (17) Mclntyre, N. S.; Cook, M. G. Anal. Chem. 1975, 47, 2208. (18) van Elp, J.; Wieland, J. L.; Eskes, H.; Kuiper, P.; Sawatzky, G. A.; de Groot, F. M. F.; Turner, T. S. Phys. ReV. B 1991, 44, 6090. (19) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (20) Roth, W. L. J. Phys. Chem. Solids 1964, 25, 1. (21) Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Schu¨th, F. Nano Lett. 2006, 6, 2977. (22) Wang, R. M.; Liu, C. M.; Zhang, H. Z.; Chen, C. P.; Guo, L.; Xu, H. B.; Yang, S. H. Appl. Phys. Lett. 2004, 85, 2080.
