Highly Single Crystalline IrxRu1–xO2 Mixed Metal Oxide Nanowires

Article pubs.acs.org/JPCC

Highly Single Crystalline IrxRu1−xO2 Mixed Metal Oxide Nanowires

Yumin Lee,†,∥ Hae-Young Shin,‡,∥ Sung Hee Chun,† Jaeyeon Lee,† Won Jeong Park,§ Jeong Min Baik,§ Seokhyun Yoon,*,‡ and Myung Hwa 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, 689-798, Korea S Supporting Information *

ABSTRACT: The nanostructured iridium−ruthenium mixed metal oxide (IrxRu1−xO2, 0 ≤ x ≤ 1) is the most promising candidate as a highly efficient electrode material for the electrolysis of water. We present a facile synthesis and structural characterization of highly single crystalline IrxRu1−xO2 mixed metal oxide nanowires for the first time via a simple vapor phase transport process by carefully controlling the relative ratios of IrO2 and RuO2 precursors. We also observed that an Eg mode of an IrxRu1−xO2 nanowire measured from Raman scattering is being linearly blue shifted and its line width is asymmetrically broadened with respect to the increase of the Ir contents, indicating that we could quantitatively determine stoichiometry information as well as crystal structure for mixed metal oxide nanowires.

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Here, for the first time, we successfully demonstrated that highly single crystalline IrxRu1−xO2 mixed metal oxide nanowires can be prepared via a simple physical vapor transport process by controlling the relative ratios of the two precursors, IrO2 and RuO2, respectively. In addition, we describe the influence of a distinct Eg phonon mode of Raman scattering on various compositions between Ir and Ru in a single IrxRu1−xO2 mixed metal oxide nanowire in order to elucidate the crystal structures and stoichiometry information for these novel mixed metal oxide nanowires.

INTRODUCTION Mixed metal oxide nanostructure materials are recently part of fascinating research directions toward future technological advances as building blocks in a variety of industrial electrochemical processes for alternative energy applications and as heterogeneous catalysts owing to favorable enhancements of their chemical, mechanical, electrical, and optical properties from those of single component metal oxides.1−6 Among the mixed metal oxides, nanostructured iridium− ruthenium mixed metal oxide (IrxRu1−xO2) is currently one of the extremely promising candidates as a highly efficient electrode material and electrocatalyst in electrochemical devices such as the oxygen evolution reaction (OER),7−9 hydrogen evolution reaction (HER)10 for the electrolysis of water, and the oxygen reduction reaction (ORR)11 for fuel cells due to its superior electrocatalytic activity, high electrical conductivity, chemical stability, and excellent diffusion barrier properties. In particular, IrxRu1−xO2 mixed metal oxide is very well-known for being the most resistant material to O2 evolution even in a strong acidic environment.7 However, only a few traditional methods such as the sol−gel process, thermal decomposition, and reactive sputtering have been previously employed to prepare IrxRu1−xO2 mixed metal oxide thin films or nanoparticles in order to enhance the electrochemical activity and stability in electrochemical devices.12,13 Although one-dimensional nanostructures of the mixed metal oxides are particularly appealing owing to their unique advantages14,15 over thin films or nanoparticles made by previous synthetic approaches, to the best of our knowledge, there have been no studies regarding the synthesis of welldefined quasi one-dimensional (1-D) IrxRu1−xO2 mixed metal oxide nanowires and their characterizations so far. © 2012 American Chemical Society

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EXPERIMENTAL SECTION We prepared highly single crystalline IrxRu1−xO2 mixed metal oxide nanowires directly on a Si(001) wafer by a vapor phase transport process starting from the mixtures of IrO2 and RuO2 powders without catalyst at 700 °C and under atmospheric pressure.16,17 Specifically, IrxRu1−xO2 nanowires (NW) were synthesized by the vapor phase transport process in a single zone horizontal quartz tube furnace, 2.5 cm in diameter and 60 cm long, under atmospheric pressure. First, various weight percents of fine meshed RuO2 and IrO2 (99.9%, Aldrich) powders were mixed and then sonicated for homogeneous mixing of the two precursors. Then 10 mg samples of various compositions of the mixed metal oxide precursors were taken and loaded at the center of a 6 cm long quartz boat without further purification. The substrate on a quartz boat was introduced into the furnace at a point ∼15 cm downstream of the mixture powder source. The quartz boat and mixed oxide Received: May 18, 2012 Revised: June 26, 2012 Published: July 12, 2012 16300

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charge were cleaned to get rid of impurities by first placing them at the center of the quartz tube furnace under He (99.999%) carrier gas flowing for ∼10 min at a gas flow rate of 300 sccm before heating. After that, the furnace temperature was rapidly increased to a temperature in the range of 1050 °C with flowing He (99.999%) carrier gas of 400 sccm at the rate of 100 °C/min. The nanowire growth proceeded with flowing high purity He (99.999%, 300 sccm) and O2 (99.9%, 10 sccm) for 1 h. The temperature of the region which obtained the nanowires on the substrate was measured as ∼700 °C. The furnace was finally allowed to cool to room temperature under the same flow rate of He. The product collected on the Si(001) substrate was collected and characterized by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The X-ray diffraction pattern was obtained using a Rigaku diffractometer with Ni filtered Cu Kα radiation, λ = 0.154 18 nm at 25 °C, and room temperature Raman scattering spectra were measured using a McPherson 207 spectrometer equipped with a nitrogen-cooled charge coupled device (CCD) array detector. The samples were excited with 0.6 mW of a 632.8 nm He−Ne laser, focused to ∼1 μm diameter spots using a microscope objective lens. IrxRu1−xO2 nanowires were also imaged 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 nanowirecovered substrate to a TEM grid, thereby transferring some of nanowires to the grid.

much smaller than that of the root of a nanowire (Figure S1a in the Supporting Information). As shown in Figure 1d, the nanowires also do not show the catalyst particles, suggesting that the growth mechanism may resemble sublimation followed by recrystallization, a process referred to as vapor−solid (VS) growth.18 For the binary mixture system of IrO2−RuO2, interestingly, it has been well-known that even a continuous solid solution over the whole compositional range between two metal oxides can be readily formed.13 It is most likely that Ru4+ and Ir4+ ions can share the same site on the cationic sublattice of a tetragonal phase in the crystal structure due to similar ionic radii, 0.0625 nm for Ir4+ and 0.0620 nm for Ru4+, respectively.8 Thus, the formation of IrxRu1−xO2 mixed metal oxide nanowires can be rationalized in terms of the preferential nucleation of gas phase IrOx and RuOx precursors with different ratios on a Si substrate. In fact, gas phase RuO4 and IrO3 species, which are much more volatile than RuO2 and IrO2, can be produced at high temperature. It would be thus expected that the two precursors are completely dissolved to make a solid solution before starting crystallization on the substrate. Through this process, we simply obtained a variety of IrxRu1−xO2 mixed metal oxide nanowires with different contents of Ir ions by just controlling relative weight ratios of the two precursors before the growth process. Energy dispersive spectroscopy (EDS) analysis for various compositions confirms that all the mixed metal oxide nanowires are composed of Ru, Ir, and O only. Furthermore, the line-scan profiles of each element show that all elements are uniformly distributed along the cross section of a nanowire (Figure S2 in the Supporting Information). According to rigorous EDS analysis, the chemical compositions of present IrxRu1−xO2 mixed metal oxide nanowires were quantitatively determined as shown in Table 1.

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RESULTS AND DISCUSSION Figure 1 presents typical SEM images for a high density of IrxRu1−xO2 (x = 0.69) nanowires grown from out of the plane

Table 1. Atomic Compositions of Various IrxRu1−xO2 (0 ≤ x ≤ 1) Mixed Metal Oxide Nanowires Estimated by SEM− EDS Measurementsa atomic % Ir 5.03 2.43 3.77 3.40 1.54 2.56 0.35

IrxRu1−xO2 Ru

0.44 1.09 2.96 3.26 2.87 8.07 2.38

xIr

Ir/Ru 11.46 2.24 1.28 1.13 0.56 0.32 0.14

1 0.92 0.69 0.56 0.53 0.36 0.24 0.12 0

(±0.01) (±0.04) (±0.03) (±0.05) (±0.03) (±0.01) (±0.03)

(1 − x)Ru 0 0.08 0.31 0.44 0.47 0.64 0.76 0.88 1

(±0.01) (±0.04) (±0.03) (±0.05) (±0.03) (±0.01) (±0.08)

a

Errors were carefully estimated by measuring 15 different nanowires for each composition.

Figure 1. SEM images of as-grown Ir0.69Ru0.31O2 mixed metal oxide nanowires on a Si(001) substrate by a vapor phase transport process.

To confirm the crystalline fine structure of IrxRu1−xO2 mixed metal oxide nanowires, HRTEM and EDS elemental mapping analysis were performed for a single IrxRu1−xO2 mixed oxide nanowire. As shown in Figure 2a, a typical HRTEM image of a single IrxRu1−xO2 (x = 0.69) nanowire along the [100] zone axis indicates that the nanowire clearly possesses a single crystalline nature with no sign of any defects, dislocations, or amorphous overlayer. In addition, there is no evidence for the existence of the (IrO2)x/(RuO2)1−x core/shell structures3 or alternative layer structures. The lattice resolved HRTEM image near the tip of a nanowire represents that the interplanar

with a length of more than 5 μm, lateral dimensions of 50 −200 nm, and random orientations on a Si substrate. The morphologies of most grown nanowires have a straight shape along the growth direction and a rectangular cross section with well-defined sharp facets at the ends of the tips of nanowires (Figure S1 in the Supporting Information). On the other hand, the lateral dimensions of pure IrO2 nanowires are not uniform along the growth direction, presenting a needlelike shape so that the lateral dimension of the end of a nanowire is generally 16301

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Figure 2. (a) Lattice resolved HRTEM image of a single Ir0.69Ru0.31O2 mixed metal oxide nanowire. Inset represents the fast Fourier transform (FFT) of the lattice resolved image. (b) Bright field (BF) STEM image and EDS elemental mapping analysis for Ru(L) and Ir(M) atoms of a single Ir0.69Ru0.31O2 mixed metal oxide nanowire.

spacing of adjacent planes is about 0.272 nm, close to that of between the (101) crystallographic interplanar distance of the tetragonal IrO2 unit cell. In general, it is very difficult to distinguish the interplanar spacing for (101) between pure IrO2 of 0.258 nm and pure RuO2 of 0.255 nm. On the other hand, our result for the mixed metal oxide nanowire represents that the spacing of the (101) planes is slightly increased compared with those of two pure metal oxides. It might be possibly attributed to inhomogenieties in the IrxRu1−xO2 solid solution. The nanowire grows along the [001] direction from the fast Fourier transform (FFT) of the lattice resolved image in Figure 2a (inset). Interestingly, the sharp facets with (111) and (111̅) planes seen at the tip of the nanowire imply that these lowindex facets arise from the low surface energy surfaces during the growth process. Furthermore, the spatial distribution of Ir, Ru, and O ions on a single IrxRu1−xO2 mixed oxide nanowire shown in a bright field (BF) STEM image is examined with EDS element mapping analysis. As illustrated in Figure 2b, all of the Ir, Ru, and O elements are homogeneously distributed along the entire nanowire, consistent with the results of the line-scan profiles in SEM−EDS measurements. This finding can be readily regarded as clear evidence of the formation of a homogeneous solid solution of a nanoscale IrO2−RuO2 mixed metal oxide system. X-ray diffraction patterns also support the highly single crystalline nature for the mixed metal oxide nanowires for various compositions of the IrO2−RuO2 system (Figure S3 in the Supporting Information). Both RuO2 and IrO2 as the precursors for growing single crystalline IrxRu1−xO2 mixed oxide nanowires are known to have the tetragonal rutile structure.19 While the Raman bands at 525, 645, and 714 cm−1 have previously been assigned to the first order Eg, A1g, and B2g phonon bands of pure rutile RuO2, the Raman bands measured at 561, 729, and 751 cm−1 are assigned to the first order Eg, B2g, and A1g phonon bands of the pure rutile IrO2 structure, respectively.19,20 Although A1g and B2g phonon bands for a pure IrO2 nanowire are not clearly resolved in our spectrum, measured band frequencies for pure RuO2 nanowire and IrO2 nanowire correspond closely to those observed with bulk single crystal as shown in Figure 3. On the other hand, it is readily observed that Raman spectra of a series of IrxRu1−xO2 mixed oxide nanowires as a function of the Ir contents (x) are markedly changed as shown in Figure 3. Particularly, it is immediately apparent that the Eg modes are well separated from the other two modes and their intensities are much higher. Furthermore, since two Eg modes for pure

Figure 3. Raman spectra of a single IrxRu1−xO2 (0 ≤ x ≤ 1) mixed metal oxide nanowire with a variety of compositions measured at the laser excitation wavelength of 632.8 nm.

nanowires have distinct band frequencies with a difference of about 31 cm−1, it thus allows us to thoroughly investigate the shift of the peak position and the change of the line width of the Eg mode upon varying the compositions of IrxRu1−xO2 mixed oxide nanowires. As shown in Figure 4, it is obvious that the peak maximum of the Eg mode from pure RuO2 nanowire linearly blue shifted with incorporating Ir contents. This result can thus allow the determination of accurate compositions in the IrO2−RuO2 mixed oxide systems by simply measuring the band frequency of the Eg mode. This resembles so-called one-mode behavior in mixed crystals of the form of AxB1−xC.21,22 It is also noted that no evidence of additional Raman features such as the local mode is seen, which again supports the idea that the mixture is homogeneous and maintains good crystalline quality. In addition, there is a notable broadening of the Eg phonon line width, which is maximized near x = 0.5. Similar broadening behavior of a phonon mode has also been observed in many other mixed crystalline systems.23−26 The line width broadening in our case is most likely associated with the substitutional disorder caused by adding Ir to RuO2 which results in the activation of phonons with nonzero wavevectors by lifting the k = 0 selection rule for Raman scattering in crystalline materials. It is well-known that the k = 0 selection rule for crystals reflects the smallness of a wavevector of (visible) laser light compared to that of an excitation in the material which in turn is based on the translational symmetry of a crystal. Translational invariance, thus the k = 0 selection rule, can be broken due to either disorder or defects.23−25 It can also be broken when the size of the sample becomes very small (usually ∼10 nm or less) so that the confinement effect has to be taken into account.26,27 Usually, confinement of phonon modes manifests itself as a frequency shift and asymmetric broadening.26−30 In our case, on the other hand, the dimension 16302

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ASSOCIATED CONTENT

S Supporting Information *

Additional SEM images of various compositions for IrxRu1−xO2 mixed metal oxide nanowires; SEM−EDS element mapping across a single nanowire and EDS spectrum for a single Ir0.69Ru0.31O2 mixed metal oxide nanowire; X-ray diffraction patterns for various compositions of IrxRu1−xO2 mixed metal oxide nanowires. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.H.K.); [email protected] (S.Y.). Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea Grants funded by the Korean Government (20100022028 and 2008-0062237) and by the Korean Government (MOEHRD) (2009-0073752). This work was also supported by MEST & DGIST (10-BD-0101, Convergence Technology with New Renewable Energy and Intelligent Robot). This work was supported by the converging research center program through the Ministry of Education, Science & Technology (2011K000589). This work was also supported by the Ewha Global Top 5 Grant 2011 of Ewha Womans University.

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Figure 4. (a) Maximum peak positions of Eg modes for various compositions of a single IrxRu1−xO2 (0 ≤ x ≤ 1) mixed metal oxide nanowire in Raman spectra at 632.8 nm. (b) Full width half-maximum (fwhm) of Eg modes for various compositions of a single IrxRu1−xO2 (0 ≤ x ≤ 1) mixed metal oxide nanowire in Raman spectra at 632.8 nm. Note that error bars were carefully estimated by measuring 12 different positions on a single nanowire.

of nanowires is approximately in the range from 50 to 200 nm and, furthermore, the frequency is measured to be almost the same as that of single crystalline bulk which implies the size effect is very small, if there is any effect.

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CONCLUSIONS

We have described the novel synthesis of highly crystalline IrxRu1−xO2 mixed metal oxide nanowires over the entire range of 0 ≤ x ≤ 1 via a vapor phase transport process preparing from IrO2 and RuO2 powder precursors. From the dependence of the phonon mode of the nanowire on the atomic composition, we also show that Raman scattering spectroscopy can provide a simple, prompt, and effective means to measure the stoichiometry and crystalline quality of mixed metal oxide nanowires. This work appears to be the first report of IrO2− RuO2 mixed metal oxide nanowires and can be then extended to other promising mixed metal oxide systems toward practical applications. 16303

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