Communication pubs.acs.org/crystal
Graphene as a Highly Efficient Template for Growing OneDimensional RuO2 Nanostructures Ji-eun Park,†,# Yumin Lee,†,# Jaeyeon Lee,† Hye Soo Jang,† Hae-Young Shin,†,‡ Seokhyun Yoon,†,‡ Jeong Min Baik,§ Myung Hwa Kim,*,† and Sung-Jin Kim*,† †
Department of Chemistry & Nano Science and ‡Department of Physics, Ewha Womans University, Seoul, 120-750, Korea School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, Korea
§
ABSTRACT: We report a new strategy in the use of graphene layers as a template for remarkably enhancing the growth of single crystalline one-dimensional (1-D) nanostructures and for selectively controlling the growth area. Single crystalline RuO2 nanowires of high density were almost exclusively obtained over a large area of graphene layers that had been transferred onto a SiO2/Si(100) substrate within 20 min. This rational engineering of graphene layers is a promising technique for producing very dense 1-D nanomaterials over large areas for various applications.
G
raphene, with a structure composed of planes of honeycomb carbon lattice, is the most fascinating research subjects in the current science community due to its superior physicochemical properties arising from its unique two-dimensional (2-D) structure used for various potential applications.1−3 Many synthetic methods have been investigated for developing large-scale growth of high-quality graphene films and their chemical modification for practical applications such as transparent conductive electrodes, field effect transistors, sensors, catalysts, and clean energy devices. Fortunately, the cost-effectiveness and large-scale preparation of high quality graphene materials were successfully realized by chemical vapor deposition (CVD) growth on metal foils such as Ni or Cu, with even the potential for fine-tuning the number of graphene layers.4−6 With these improved synthetic approaches, graphene-based hybrid nanostructures with other inorganic nanomaterials are currently a promising research direction for accelerating the fabrication of new functional materials for a variety of applications. The growth of one-dimensional (1-D) semiconductor or conductive metal oxide nanowires directly on graphene layers is particularly interesting due to their additional benefits for the development of high performance optoelectronic devices. Kim et al.7,8 recently reported that ZnO nanoneedles were grown vertically on the graphene layer using catalyst-free, metal−organic vapor-phase epitaxy and were then used to fabricate transferable gallium nitrite thin film based light-emitting diodes as an intermediate layer. They attributed the formation of aligned ZnO nanoneedles to the enhanced nucleation on the graphene step edges and kinks.8 In addition, Hong and Fukui demonstrated the controlled van der Waals epitaxy method for high-hybrid and uniform InAs nanowire arrays on graphene layers.9 © 2012 American Chemical Society
Nanostructured ruthenium dioxide (RuO2) is a promising candidate as an electrode material in electrochemical devices, field-emission displays, and even biochemical sensors because of its high electrical conductivity, chemical stability, and excellent diffusion barrier properties.10,11 Additionally, it has a great potential as a chemical catalyst due to its excellent chemical properties such as HCl oxidation12 and CO oxidation reaction.13 Despite its extremely versatile properties, however, the growth of highly single crystalline RuO2 nanowires remains a challenge for the fabrication of 1-D nanostructures. Despite our very recent introduction of a simple route14 to grow highquality RuO2 nanowires via CVD under atmospheric pressure, it generally requires several hours to obtain nanowires with favorable dimensions. Furthermore, it is quite challenging to obtain nanowires of uniform density with a relatively large surface area. On the other hand, we here present that 2-D graphene layers can be effective template layers to greatly enhance the growth of 1-D nanostructures rather than being used to form hybrid structures with 1-D nanostructures. Accordingly, the effective and selective growth of high-quality single crystalline RuO2 nanowires can be achieved on graphene layers, which are grown by CVD and then transferred into a substrate, via a simple CVD process in much shorter growth time than previously reported results. A few layers of graphene were first synthesized by CVD on 35 μm copper foil prior to the nanowire growth.5 In the first step, graphene was directly grown on the copper foil by thermal CVD using CH4 gas at 1000 °C under atmospheric pressure for 15 min.7 The Cu foil wrapping around the inside of a quartz Received: April 15, 2012 Revised: June 19, 2012 Published: June 21, 2012 3829
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Figure 1. (a) A Raman spectrum of the graphene layer grown by CVD and an optical image of the taken area. (b) The line profile across the boundary between the graphene layer and the SiO2 wafer by atomic force microscopy (AFM) measurement.
by high resolution transmission electron microscopy (HRTEM; FEI Titan TEM/STEM at 300 kV) at room temperature. Samples for TEM imaging were prepared by simply touching the nanowire-covered substrate to a TEM grid, thereby transferring some of the nanowires to the grid. In Figure 1, both the Raman spectrum and atomic force microscopy (AFM) scanning data revealed that the graphene layer grown by CVD consisted of only a single layer, or a few layers, of graphene. Figure 2 shows SEM images of the asgrown RuO2 nanowires on a SiO2/Si(100) substrate. A distinct boundary is immediately apparent between the region of the densely grown nanowires and that of very rarely scattered nanowires, indicating that the RuO2 nanowires were exclusively formed on the originally prepared graphene layers. Generally, the nanowires on the graphene layers were grown from out of plane with random orientations, as shown in Figure 2c,d. The RuO2 nanowires were straight with uniform dimensions along the growth direction and did not show any catalytic particles at the end of tip. A high density of RuO2 nanowires was only obtained on the graphene layers with a diameter varying between 30 and 200 nm and a length of more than 10 μm, as shown in Figure 2c,d. In addition, the Raman spectrum of the as-grown nanowires in Figure 2e exhibits three wellcharacterized sharp peaks at 528, 647, and 714 cm−1, which were attributed to a tetragonal RuO2 structure with Eg, A1g, and B2g modes identified from the group theory prediction,15 respectively. The XRD pattern of the as-grown RuO 2 nanowires, shown in Figure 2f, revealed their high crystallinity with a tetragonal rutile structure that exactly matched the reference data.15 Figure 3a displays a TEM image focused on a single RuO2 nanowire. Figure 3c is a high resolution TEM image taken from the center of a RuO2 nanowire. The separation of two adjacent
tube was located at the center of a quartz tube and then heated to 1000 °C under Ar/H2 atmosphere. After reaching the growth temperature, flowing reaction gas mixtures (CH4/H2/ Ar = 5:10:500 sccm) were passed over the quartz tube for 15 min, which was subsequently rapidly cooled to room temperature at the rate of ∼10 °C s−1 with a flow of Ar gas. The graphene film grown on the copper foil was then isolated from the Cu foil using the etching process with an aqueous solution containing a high concentration of Fe(NO3)3. RuO2 nanowires were directly grown on the graphene layer which was simply transferred on a SiO2/Si(100) wafer, by vapor transport under atmospheric pressure, without any catalyst. Ten milligrams of fine meshed RuO2 (99.9%, Aldrich) powder was first loaded at the center of a 6 cm long quartz boat without further purification. The graphene layer on a Si wafer on a quartz boat was introduced into the furnace at a point approximately ∼15 cm downstream of the RuO2 powder source. The furnace temperature was then increased to 1000 °C at the rate of 100 °C/min with flowing He (99.999%, 300 sccm) carrier gas. The nanowires were grown under flowing He (99.999%, 300 sccm) and O2 (99.9%, 20 sccm) for 20 min. The temperature of the region in which the nanowires were grown on the substrate was measured as 650 °C. The furnace was finally allowed to cool down under only He flow. The as-grown products on the SiO2/Si substrate were characterized by scanning electron microscopy (SEM) and micro Raman spectroscopy. X-ray diffraction (XRD) patterns were also obtained using a Rigaku diffractometer with Ni filtered Cu Kα radiation, λ = 0.15418 nm at 25 °C. The Raman spectra were recorded using a confocal microscope (Renishaw InVia System) with a 100× (0.9 NA) microscope objective. A 632.8 nm He−Ne laser light was used as the excitation source with a laser power of 0.57 mW. RuO2 nanowires were imaged 3830
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Figure 2. (a−d) SEM images of RuO2 nanowires grown on the graphene layers transferred onto a SiO2/Si(100) substrate. (e) Raman spectrum of the as-grown nanowires on the graphene layers transferred onto a SiO2/Si(100) substrate. (f) An XRD pattern of the grown RuO2 nanowires revealing their high single crystallinity. All the diffraction peaks except the diffraction from Si wafer are indexed with the tetragonal rutile structure with a P42/mnm space group and unit cell of a = 4.4994 c = 3.1071 Å. The peak at 33.2° represents the Si wafer.
and 2-D bands, respectively. As shown in Figure 4, the G and 2D bands had almost completely disappeared after the growth process. Meanwhile, the relative intensity of the disorderinduced D band was greatly increased, indicating that the graphene layers underwent major degradation during the growth process. Although a detailed understanding of the surface structures is beyond the present description of our experimental results, it is most likely that the oxygen molecules selectively attack the carbon atoms at the edges, as well as at the defects of the graphene surface and then chemically react with the graphene structure at high temperature.9,17 The RuO2 nanowire growth can be thus rationalized in terms of the generation of many defect sites under highly oxidative condition on the graphene layer. The gas phase RuO3 or RuO4 species, which is much more volatile compared with RuO2, are rapidly deposited on many defect sites so that the nucleation of RuO2 nanostructures can be effectively enhanced in our growth process. Interestingly, the tips of the nanowires were clearly characterized by well-defined facets, as shown in Figure 3a,b, implying the presence of low energy surfaces in the growth process. These results may support a simple growth mechanism of RuO2 nanowires that resembles sublimation followed by recrystallization, a process referred to as vapor− solid (VS) growth.18 However, the mechanistic behaviors must be explored in great detail in order to fully demonstrate the preferential and unidirectional crystal growth of RuO 2 nanostructures.
planes is about 0.314 nm, corresponding to that of between the (001) planes of tetragonal RuO2 structure. The nanowires grew along the [001] direction parallel to the {110} family of planes, which was in excellent agreement with the fast Fourier transform (FFT) of the lattice-resolved image shown in Figure 3d. As recently reported7,8 for ZnO nanostructures, the preferential growth of nanowires on a few graphene layers may have resulted from the enhanced nucleation of the gas phase precursor at many graphene step edges, which in turn may have originated from the process of transferring the graphene layers onto a SiO2/Si substrate or the growth process. On the other hand, our results revealed a high density of nanowires along the boundary between the graphene layer and the pure SiO2/Si substrate, as well as on the entire graphene surface, as shown in Figure 2b. Compared to the previous 1-D ZnO nanostructures grown7,8,16 on a few layers of graphene, however, a distinct difference was the inevitability of the oxidation of the graphene layer under high growth temperature and oxygen environment for our growth condition. Hence, to explore the effect on the surface structure of the graphene layer during the growth process, the Raman spectra of the graphene layer were recorded before and after the growth processes at the same growth temperature and oxygen flow rate as shown in Figure 4. The result measured after the growth process represents a distinct behavior for relative Raman peaks at around 1360, 1590, and 2700 cm−1, corresponding to the D, G, 3831
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Figure 3. (a−d) TEM images focused on a single RuO2 nanowire with low and high magnification.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (M.H.K.),
[email protected] (S.-J.K.). Author Contributions #
These authors equally contributed to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by MEST & DGIST (10-BD-0101, Convergence Technology with New Renewable Energy and Intelligent Robot) and by National Research Foundation of Korea Grant funded by the Korean Government (20100022028). This work was also supported by the Defense Acquisition Program Administration & Agency for Defense Development and by the National Research Foundation of Korea (NRF) grant funded by the Korea government(MEST) (20110014219 & 2008-0062237). This work was also supported by the Ewha Global Top 5 Grant 2011 of Ewha Womans University.
Figure 4. Raman spectra of graphene layers before and after nanowire growth at the same temperature and the same carrier gas flow rate.
In conclusion, we have presented a new strategy using a few layers of graphene as a template layer to effectively enhance the nanowire growth and selectively control the growth area. A high density of single crystalline RuO2 nanowires was exclusively and very quickly grown on the graphene layers that had been transferred onto a SiO2/Si substrate with a lateral dimension of 30−200 nm and a length of more than 10 μm. We tentatively attributed this enhanced nanowire growth to the generation of many defect sites on the graphene layer. Indeed, the preferential nucleation process of the precursor gas in a growth process can occur rapidly from these highly activated defect sites. This rational engineering of graphene layers has a promising potential for the production of very dense 1-D nanomaterials over large areas for various applications.
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REFERENCES
(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (2) Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Nature 2005, 438, 201. (3) Miller, J. R.; Outlaw, R. A.; Holloway, B. C. Science 2010, 329, 1637. (4) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.; Kim, P.; Choi, J.; Hong, B. H. Nature 2009, 457, 706. 3832
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(5) Li, X. S.; Cai, W. W.; An, J.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312. (6) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozylimaz, B.; Ahn, J.H.; Hong, B. H.; Iijima, S. Nature Nanotechnol. 2010, 5, 574. (7) Chung, K; Lee, C.; Yi, G. Science 2010, 330, 655. (8) Kim, Y.; Lee, J.; Yi, G. Appl. Phys. Lett. 2009, 95, 213101. (9) Hong, Y. J.; Fukui, T. ACS Nano 2011, 5, 7576. (10) Chueh, Y.-L.; Hsieh, C.-H.; Chang, M.-T.; Chou, L.-J.; Gan, J.Y.; Lao, C. S.; Song, J. H.; Wang, Z. L. Adv. Mater. 2007, 19, 143. (11) Hsieh, C. S.; Tsai, D. S.; Chen, R. S.; Huang, Y. S. Appl. Phys. Lett. 2004, 85, 3860. (12) Crihan, D.; Knapp, M.; Zweidingey, S.; Lundgren, E.; Weststrate, C. J.; Andersen, J. N.; Seitsonen, A. P.; Over, H. Angew. Chem., Int. Ed. 2008, 47, 2131. (13) Over, H.; Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Lundgren, E.; Schmic, M.; Varga, P.; Morgante, A.; Ertl, G. Science 2000, 287, 1474. (14) Lee, Y.; Ye, B. U.; Yu, H. K.; Lee, J. L.; Kim, M. H.; Baik, J. M. J. Phys. Chem. C 2011, 115, 4611. (15) Korotcov, A. V.; Huang, Y. S.; Tiong, K. K.; Tsai, D. S. J. Raman Spectrosc. 2007, 38, 737. (16) Choi, W. M.; Shin, K. S.; Lee, H. S.; Choi, D.; Kim, K.; Kim, H. J.; Yoon, S. M.; Choi, J. Y.; Kim, S. W. Nano Res. 2011, 4, 440. (17) Malard, L. M.; Moreira, R. L.; Elias, D. C.; Plentz, F.; Alves, E. S.; Pimenta, M. A. J. Phys.: Condens. Matter 2010, 22, 334202. (18) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947.
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