Probing the Growth Habit of Highly Single Crystalline Twinned V

Article pubs.acs.org/JPCC

Probing the Growth Habit of Highly Single Crystalline Twinned V‑Shape RuO2 Nanowires by Polarized Raman Scattering Hae-Young Shin,† Jaeyeon Lee,‡ Yumin Lee,‡ Sewon Jeong,† Hayoung Jung,‡ Hak Ki Yu,§ Jeong Min Baik,⊥ Myung Hwa Kim,*,‡ and Seokhyun Yoon*,† †

Department of Physics, Ewha Womans University, Seoul, 120-750, Korea Department of Chemistry & Nano Science, Global Top 5 Research Program, Ewha Womans University, Seoul, 120-750, Korea § Department of Material Science & Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Korea ⊥ School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, Korea ‡

ABSTRACT: Polarized Raman scattering measurements have been carefully employed to explore the growth orientation and direction of a highly single crystalline twinned V-shaped ruthenium dioxide nanowire which were directly grown on a Si wafer with the thin SiO2 layer at 750 °C without any catalyst via a vapor phase transport process. Interestingly, the morphology of most of nanowires represents a well-defined twinned V-shape with a specific angle of approximately 52° between the two branches under a specific growth condition. We measured polarized Raman spectra of a single V-shaped nanowire and calculated the Raman intensity of RuO2 nanowire by considering a crystal coordinate. We observed that each branch of a V-shaped RuO2 nanowire grows into [001] direction, exactly consistent with the prediction of the growth direction by high resolution transmission electron microscopy (HRTEM).

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INTRODUCTION Micro Raman scattering spectroscopy has been used to study properties such as crystal structure, composition, growth direction, and so on, of nanoscaled materials, by measuring elementary excitations including optical phonons.1−9 The growth direction of nanosized materials is especially important because it directly influences electronic, magnetic, photonic, and/or mechanical characteristics. The growth direction strongly depends on the symmetry of the material, and hence deeply affects the elementary excitations. Using polarized Raman scattering spectroscopy, the symmetry of optical phonon modes associated with the crystalline structure can be studied. Moreover, the relative intensity of a phonon mode can be calculated by using Raman tensors and the factor group analysis (FGA). Recently, the growth direction of one-dimensional nanostructures has been carefully studied by polarized Raman scattering spectroscopy.1−4 It is seen that Raman spectroscopy can be performed relatively easily compared to, e.g., highresolution transmission electron microscopy (HRTEM) measurements and yet can provide similar information regarding the growth direction. Raman spectroscopy also has unique advantages such as being a nondestructive and a noncontact method. In 2006, Livneh et al. determined the structure of GaN nanowire and reported that Raman spectra are sensitive to the relative orientation of the k vector with respect to the crystallographic direction.1 In 2009, Munisso et al. studied the various crystallographic orientations of sapphire single crystal using Raman tensor analysis and polarized Raman spectroscopy.10 In 2010, Wu et al. also measured different © 2014 American Chemical Society

growth directions of GaP nanowires by analyses of phonon intensity variations.3 Ruthenium dioxide (RuO2) has recently drawn considerable attention due to its great potentials as highly active catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), and as optoelectronic devices due to high conductivity (∼2 × 104 S cm−1 in bulk), good chemical and thermal stability.11−17 In spite of its great potential toward future real applications, on the other hand, the growth of highly single crystalline RuO2 nanowires has been recently reported by thermal oxidation or chemical vapor deposition of appropriate Ru-based precursors and reactive sputtering using pure Ru metal targets.16,18,19 Very recently, we introduced a facile approach for synthesizing single crystalline RuO2 nanowires without catalyst, by atmospheric pressure chemical vapor deposition (APCVD), based on the thermal conversion from supercooled liquid nanodroplets.20,21 The attractive features of this method are simplicity, low cost, and high throughput as well as the high crystalline properties of the nanowires. The morphologies of the nanowires can be also controlled by varying only the He/O2 ratio and temperature.22,23 Interestingly, RuO2 nanowires can be also grown in the form of preferentially well-defined twinned V-shape nanostructures depending on different substrates or growth conditions such as the He/O2 ratio and temperature. In this study, we report the first demonstration of the determination of the growth habit of a single twinned V-shape RuO2 nanowire using Received: July 11, 2014 Revised: August 5, 2014 Published: August 13, 2014 20716

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polarized Raman scattering spectroscopy. Polarized Raman spectroscopy is much easier to perform and to analyze compared to a technique that provides similar information, e.g., HRTEM. Moreover, Raman scattering spectroscopy is a noncontact and nondestructive technique and there is no need of special preparation for a sample to be measured. Also, all measurements are made in an ambient condition. All of the above properties suggest that polarized Raman scattering spectroscopy is an effective research tool to study single nanowires.

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EXPERIMENTAL SECTION The twinned V-shape RuO2 nanowires were grown on a 200 nm silica-covered Si (001) wafer, by vapor transport at 750 °C and APCVD without catalyst.21 Fine meshed RuO2 (99.9% Aldrich) powder was loaded at the center of a quartz boat without further purification. Si substrate was introduced into the furnace at a point approximately ∼15 cm downstream of the RuO2 powder source, where the temperature was 750 °C. Gas consisting of He and O2 flowed for 2h 30 min at a gas flow rate of 500 and 15 sccm, respectively. The product that was collected on several Si substrates was characterized by scanning electron microscopy (SEM) in conjunction with energy dispersive X-ray spectroscopy and X-ray diffraction (XRD). The crystal structures of RuO2 nanowires were also imaged by HRTEM (FEI Titan TEM/STEM at 300 kV) at room temperature. Samples for TEM imaging were prepared by touching the nanowire-covered substrate to a TEM grid, thereby transferring some of nanowires to the grid. Room temperature polarized Raman scattering spectra of RuO2 nanowires were measured by using a McPherson 207 spectrometer equipped with a nitrogen-cooled charge-coupleddevice (CCD) array detector. The samples were excited with a 632.8 nm He−Ne laser and a 488 nm diode laser, focused to ∼1 μm diameter spot using a microscope objective (x100). The excitation power was 0.5 mW. The spectra were obtained with the incident and scattered light polarized in the following configurations in order to identify the symmetries of the phonon modes studied: (Ei,Es) = (x,x), A1g; (Ei,Es) = (x,y), B2g; (Ei,Es) = (x,z); (Ei,Es) = (y,z), Eg; where Ei and Es are the incident and scattered electric-field polarizations, respectively, x, y, and z are the [100], [010], and [001] crystal directions, respectively, and where A1g and B2g are the singly degenerate and Eg is the doubly degenerate irreducible representations of the RuO2 space group (D14 4h−P42/mnm). In our polarized Raman measurements, the angle between the polarization of the incident light and a nanowire was changed by rotating the incident polarization. When the polarization of the scattered light is parallel (perpendicular) to that of the incident light, we name that configuration parallel (perpendicular) polarized Raman measurement.

Figure 1. (a) FE-SEM image of twinned V-shape RuO2 nanowires. The angle between each branch is about 52°. (b) XRD pattern of as grown of twinned V-shape RuO2 nanowires.

closely consistent with those of the highly crystalline tetragonal rutile structure of the RuO2 crystal (JCPDS-40-1290).20 The most intense peak of XRD pattern clearly indicates that the crystallographic plane of (200) would be one of preferential crystal orientations of the twinned V-shape RuO2 nanowires. A Raman spectrum of a RuO2 nanowire with a 488 nm excitation is shown in Figure 2a. The symmetries of the Raman modes at 529, 647, and 717 cm−1 are readily assigned to be Eg, A1g, and B2g, respectively.7 These phonon modes are associated with the oxygen vibrations that are parallel to (Eg), or perpendicular to (A1g and B2g) the c-axis of the crystalline direction.7 The Raman tensors of each mode are as follows. ⎛ R Eg = ⎜⎜ ⎝d

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d⎞ ⎟, ⎟ ⎠

⎛ ⎞ d⎟ ⎜ ⎜ ⎟ ⎝ d ⎠

⎛a ⎞ R A1g = ⎜⎜ a ⎟⎟ ⎝ c⎠

RESULTS AND DISCUSSION Figure 1a illustrates the SEM images of the twinned V-shape RuO2 nanowires characterized by rectangular and polyhedral cross sections with the lateral dimension of several hundred nanometers and the length of up to 10 μm. Also, the angle between the two branches of the twinned V-shape RuO2 nanowires is ∼52° which is the same for all twinned V-shape nanowires. Figure 1b shows a XRD pattern of as grown nanowires indexed by crystallographic planes of (110), (101), and (200) with relatively weak intensities of (211) and (002),

⎛ b ⎞ ⎟ R B2g = ⎜⎜ b ⎟ ⎝ ⎠

(1)

Utilizing group theory, we can estimate the relative intensity of three Raman-active modes as follows:24 I ∝ |ei ·R ·es|2 20717

(2)

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̃ R jxyz = Φxyz ·R j·Φxyz

Here I is the Raman intensity, R is the Raman tensor, and ei and es are the unit polarization vectors of the incident and scattered light, respectively. To consider the discrepancy between the crystal coordinates and the laboratory (experimental) coordinates, we transformed the Raman tensors by using the Euler’s angles as seen in Figure 2.10 The transformed Raman tensor is given by

Φxyz

where j corresponds to each Raman-active mode and Φ is the Euler’s transformation matrix that is given as

⎛ cos ψ cos φ − cos θ sin φ sin ψ cos ψ sin φ + cos θ cos φ sin ψ sin θ sin ψ ⎞ ⎜ ⎟ = ⎜−sin ψ cos φ − cos θ sin φ cos ψ −sin ψ sin φ + cos θ cos φ cos ψ sin θ cos ψ ⎟ ⎜ ⎟ − sin θ cos φ sin θ sin φ cos θ ⎠ ⎝

The angles, θ, φ, and ψ represent the Euler’s angles (Figure 2b). Then the Raman intensity can be written as ̃ ·es|2 I R jxyz ∝ |ei ·Φxyz ·R j·Φxyz

(3)

(4)

are calculated results, respectively. The filled squares and the open circles represent the A and the B branch of a twinned V-shape nanowire in the optical image in Figure 3a inset. In

(5)

where we set ei [010], and es//, es⊥ [010], [100], respectively (the subscript // denotes the parallel configuration and ⊥ refers to the perpendicular one). Figure 2b describes the crystal coordinates transformed by the Euler’s matrix. The unprimed coordinates represent the original crystal coordinates (xyz) and the primed coordinates denote the final crystal coordinates (x′y′z′) that correspond to the experimental ones. The angle ψ is the polarization angle of the light. Initially, ψ is set to be zero, that is, the incident polarization direction coincides with the orientation of a single nanowire. The angle θg is the growth angle of a RuO2 nanowire. Parallel and perpendicular polarization configurations are shown in Figure 2c. For the convenience of calculation, let φ be 90°. This can be done without losing generality. In this case, the angle θ corresponds to the growth angle, θg. Using eqs 1, 4, and 5, we can write down the general Raman intensity equations of three Raman-active modes as a function of the growth angle θ (= −θg) as follows, I(Eg//) ∝

1 2 2 d [sin (2θ ) cos 4 ψ + sin 2 θ sin 2(2ψ )] 2

1 2⎡ 1 2 d ⎢ sin (2θ ) sin 2(2ψ ) 2 ⎣4 ⎤ + sin 2 θ(2 sin 2ψ − 1)2 ⎥ ⎦

I(Eg ⊥) ∝

I(A1g//) ∝ {a + (c − a)sin 2θ cos2ψ }2

⎧ (c − a)2 4 ⎫ sin θ sin 2(2ψ )⎬ I(A1g ⊥) ∝ ⎨ 4 ⎭ ⎩ I(B2g ) ∝ b2 cos2 θ sin 2(2ψ ) I(B2g ⊥) ∝ b2[cos θ(2 cos2 ψ − 1)]2

(6) Figure 2. (a) Raman spectrum of a twinned V-shape RuO2 nanowire excited by a 488 nm laser. The peaks at 529, 647, and 717 cm−1 are assigned to Eg, A1g, and B2g, respectively. (b) Crystal coordinates transformed by the Euler’s matrix. The black coordinates are the original crystal coordinates (xyz) and blue ones are the final crystal coordinates (x′y′z′) that correspond to the experimental ones. The angle ψ is the polarization angle of the light. (c) The polarization configuration at parallel (left) and perpendicular (right) polarized Raman measurements. ei and es are the unit polarization vectors of the incident and scattered light, respectively.

Figure 3a shows the Raman spectra of a twinned V-shape RuO2 nanowire in the parallel configuration. All of spectra in Figure 3 are measured with a He−Ne laser with wavelength of 632.8 nm. There are two Raman peaks, Eg (∼526 cm−1) and A1g (∼650 cm−1) seen in the spectra. The B2g mode is too weak to be observed. Figures 3b and 3c are the intensity variations of parallel and perpendicular configurations of Eg mode, respectively. Figure 3d is the intensity variation of A1g mode in the case of the parallel one. The symbols are experimental data and the lines 20718

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Figure 3a, the intensity of the Eg mode is much smaller than that of the A1g mode. However, for ψ = 150° in Figure 3a, the intensity of the Eg mode is similar to or larger than that of the A1g mode. Figure 4 shows the Raman images of the same

Figure 3. (a) Parallel polarized Raman spectra of a twinned V-shape RuO2 nanowire excited by a 632.8 nm laser. Inset: the optical image of a twinned V-shape RuO2 nanowire. (b and c) Intensity variations of parallel and perpendicular configurations of Eg mode, respectively. (d) Intensity variation of the A1g mode in the case of a parallel one. The symbols and lines represent the experimental and calculation results of Raman intensity, respectively. The filled and open symbols denote data taken at the different sides of a twinned V-shape nanowire. Figure 4. Raman images of the relative intensity ratio between Eg and A1g of a twinned V-shape RuO2 nanowires excited by 632.8 (a) and 488 nm (b). The intensity ratio between Eg and A1g modes is calculated by using Raman spectra in parallel configuration. (Inset: the optical image of a twinned V-shape RuO2 nanowire.)

Figure 3b, there are four nodes seen at polarization angles of 0°, 90°, 180°, and 270°, which are completely consistent with the calculation result of the growth angle of 90°, shown as a line.4 From these data, we can conclude that the growth angle of a twinned V-shape RuO2 nanowire is 90°, i.e., the [001] direction.4 Note that maximum intensities at the “antinodes” in the measured intensities of the Eg mode both in parallel and perpendicular configurations show variations, whereas maxima of the calculated intensities are all the same. This might be due to the possible position change of the focused light on the sample. As mentioned before, the polarization angle is changed by rotating a linear polarizer and a λ/2 plate, and slight change of the beam path which would be periodic can always happen during the process. Another reason for the intensity variations is the slight refraction of incident beam inside the sample.25 In any case, number and the location of the nodes are the same for the measured and the calculated intensities which inarguably indicate that the growth angle is 90°. Let us denote the angle ψ = 26° on the A branch as ψ′, which is bisecting the twinned V-shape nanowire on the laboratory coordinate system. This angle, ψ′ is identical to ψ = 154° on the B branch. In Figure 3a, the Raman spectra for ψ = 30° on the A branch and ψ = 150° on the B branch are the same as that of ψ′ on the A and the B branches, respectively. Since the A and the B branches have the same growth direction, we can consider ψ = 150° on the B branch as ψ = 150° on the A branch for convenience. As shown in Figure 3a, the relative intensities between the Eg and the A1g modes are different for the Raman spectra for ψ = 30° and 150° on the A branch. For ψ = 30° in

Figure 5. HRTEM images (a) and a diffraction pattern (b) of a single twinned V-shape RuO2 nanowire. The rotation angle between top (A) and bottom (B) branch is approximately 52°. Growth orientation of each branch is seen to be [002] and symmetric twins are divided by [301] direction.

nanowire as in Figure 3, which are excited with 632.8 and 488 nm. The color bar denotes the intensity ratio between the Eg and the A1g mode, I(Eg/A1g), in (ψ′ψ′) configuration. As clearly shown in Figure 4, the A and the B branches are divided from the bisecting plane of a twinned V-shape RuO2 nanowire. To confirm our Raman scattering results, we carefully measured HRTEM images in the vicinity of a twinned V-shape 20719

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(7) Korotcov, A. V.; Huang, Y.-S.; Tiong, K.-K.; Tsai, D. S. Raman scattering characterization of well-aligned RuO2 and IrO2 nanocrystals. J. Raman Spectrosc. 2007, 38, 737−749. (8) In, J.; Yoon, I.; Seo, K.; Park, J.; Choo, J.; Lee, Y.; Kim, B. Polymorph-tuned synthesis of α- and β-Bi2O3 nanowires and determination of their growth direction from polarized raman single nanowire microscopy. Chem.Eur. J. 2011, 17, 1304−1309. (9) Möller, M.; de Lima, M. M., Jr.; Cantarero, A.; Dacal, L. C. O.; Madureira, J. R.; Iikawa, F.; Chiaramonte, T.; Cotta, M. A. Polarized and resonant Raman spectroscopy on single InAs nanowires. Phys. Rev. B 2011, 84, 085318−1−1085318−8. (10) Munisso, M. C.; Zhu, W.; Pezzotti, G. Raman tensor analysis of sapphire single crystal and its application to define crystallographic orientation in polycrystalline alumina. Phys. Status Solidi B 2009, 246, 1893−1900. (11) Trasatti, S. Physical electrochemistry of ceramic oxides. Electrochim. Acta 1991, 36, 225−241. (12) Petrykin, V.; Macounova, K.; Shlyakhtin, O. A.; Krtil, P. Tailoring the selectivity for electrocatalytic oxygen evolution on ruthenium oxides by zinc substitution. Angew. Chem., Int. Ed. 2010, 49, 4813−4815. (13) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and activities of rutile IrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline solutions. J. Phys. Chem. Lett. 2012, 3, 399−404. (14) Over, H. Surface chemistry of ruthenium dioxide in heterogeneous catalysis and electrocatalysis: From fundamental to applied research. Chem. Rev. 2012, 112, 3356−3426. (15) Lin, Y.-T.; Chen, C.-Y.; Hsiung, C.-P.; Cheng, K.-W.; Gan, J.-Y. Growth of RuO2 nanorods in reactive sputtering. Appl. Phys. Lett. 2006, 89, 063123−1−063123−3. (16) Chueh, Y.-L.; Hsieh, C.-H.; Chang, M.-T.; Chou, L.-J.; Gan, J.Y.; Lao, C. S.; Song, J. H.; Wang, Z. L. RuO2 nanowires and RuO2/ TiO2 core-shell nanowires: from synthesis to mechanical, optical, electrical and photoconductive properties. Adv. Mater. 2007, 19, 143− 149. (17) Lian, H.-B.; Lee, K.-Y.; Chen, K.-Y.; Huang, Y.-S. Growth of needle-like RuO2 nanocrystals on carbon nanotubes and their field emission characteristics. Diamond Relat. Mater. 2009, 18, 541−543. (18) Hsieh, C. S.; Tsai, D. S.; Chen, R. S.; Huang, Y. S. Preparation of ruthenium dioxide nanorods and their field emission characteristics. Appl. Phys. Lett. 2004, 85, 3860. (19) Subhramannia, M.; Balan, B. K.; Sathe, B. R.; Mulla, I. S.; Pillai, V. K. Template-assisted synthesis of ruthenium oxide nanoneedles: Electrical and electrochemical properties. J. Phys. Chem. C 2007, 111, 16593−16600. (20) Lee, Y.; Ye, B. U.; Yu, H. K.; Lee, J. L.; Kim, M. H.; Baik, J. M. Facile synthesis of single crystalline metallic RuO2 nanowires and electromigration-induced transport properties. J. Phys. Chem. C 2011, 115, 4611−4615. (21) Kim, M. H.; Lee, B.; Lee, S.; Larson, C.; Baik, J. M.; Yavuz, C. T.; Seifert, S.; Vadja, S.; Winans, R. E.; Moskovits, M.; Stucky, G. D.; Wodtke, A. M. Growth of metal oxide nanowires from supercooled liquid nanodroplets. Nano Lett. 2009, 9, 4138−4146. (22) Kang, M.; Lee, Y.; Jung, H.; Shim, J. H.; Lee, N. S.; Baik, J. M.; Lee, S. C.; Lee, C.; Lee, Y.; Kim, M. H. Single carbon fiber decorated with RuO2 nanorods as a highly electrocatalytic sensing element. Anal. Chem. 2012, 84, 9485−9491. (23) Kim, S. J.; Jung, H.; Lee, C.; Kim, M. H.; Lee, Y. Biological application of RuO2 nanorods grown on a single carbon fiber for the real-time direct nitric oxide sensing. Sens. Actuators B: Chem. 2014, 191, 298−304. (24) Loudon, R. Theory of the resonance Raman effect in crystals. J. Phys. (Paris) 1965, 26, 677−683. (25) Livneh, T.; Lilach, Y.; Popov, I.; Kolmakov, A.; Moskovits, M. Polarized raman scattering from a single, segmented SnO2 Wire. J. Phys. Chem. C 2011, 115, 17270−17277.

junction regime of a single RuO2 nanowire. Figure 5 shows HRTEM images (a) of a twinned V-shape RuO2 nanowire whose two branches make an angle of approximately 52°. The preferential growth direction of each branch is readily identified by the [002] direction relative to a symmetric twin by the [301] direction from the diffraction pattern (b) of a twinned V-shape RuO2 nanowire. Since the theoretical angle between [002] and [301] crystallographic direction for the RuO2 tetragonal structure (a = b = 0.4499 nm, c = 0.3107 nm) is calculated by 51.5°; thus, the experimentally measured value from HRTEM is well matched with the calculated value.5,6

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CONCLUSIONS In conclusion, we accurately determined the growth direction of V-shaped RuO2 nanowires using polarized Raman spectroscopy. We calculated intensities of the Eg and the A1g mode by using the Euler’s matrix and Raman tensors. We also measured the polarized Raman spectra of a twinned V-shape RuO2 nanowire with two different polarization configurations. Compared to our calculation results, the experimental data suggest that the growth direction is 90 degree or the [001] direction. We independently confirmed the growth direction of a twinned V-shape nanowire by using HRTEM. We thus show that polarized Raman scattering spectroscopy can provide a relatively simple and prompt mean to determine the growth habit of well-defined one-dimensional nanostructures based on the anisotropic response of Raman scattering signals.

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AUTHOR INFORMATION

Corresponding Authors

*(S.Y.) E-mail: [email protected]. *(M.H.K.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean Government (MSIP) (2008-0062237 and 2013R1A1A2007951 for S.Y. and 2010-0022028 for M.H.K.).

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