Growth of Highly Single Crystalline IrO2 Nanowires ... - ACS Publications

Aug 9, 2012 - Sung Hee Chun , Hyun Yeong Kim , Hyesu Jang , Yejung Lee , Ara Jo , Nam-Suk Lee , Hak Ki Yu , Youngmi Lee , Myung Hwa Kim , Chongmok ...
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Growth of Highly Single Crystalline IrO2 Nanowires and Their Electrochemical Applications Yumin Lee,†,⊥ Minkyung Kang,†,⊥ Jun Ho Shim,‡ Nam-Suk Lee,§ Jeong Min Baik,∥ Youngmi Lee,† Chongmok Lee,*,† and Myung Hwa Kim*,† †

Department of Chemistry & Nano Science, Ewha Womans University, Seoul 120-750, Korea Department of Chemistry, Daegu University, Gyeongsan 712-714, Korea § National Center for Nanomaterials Technology (NCNT), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ∥ School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 698-805, Korea ‡

S Supporting Information *

ABSTRACT: We present the facile growth of highly single crystalline iridium dioxide (IrO2) nanowires on SiO2/Si and Au substrates via a simple vapor phase transport process under atmospheric pressure without any catalyst. Particularly, high-density needle-like IrO2 nanowires were readily obtained on a single Au microwire, suggesting that the melted surface layer of Au might effectively enhance the nucleation of gaseous IrO3 precursors at the growth temperature. In addition, all the electrochemical observations of the directly grown IrO2 nanowires on a single Au microwire support favorable electrontransfer kinetics of [Fe(CN6)]4−/3− as well as Ru(NH3)63+/2+ at the highly oriented crystalline IrO2 nanowire surface. Furthermore, stable pH response is shown, revealing potential for use as a miniaturized pH sensor, confirmed by the calibration curve exhibiting super-Nernstian behavior with a slope of 71.6 mV pH−1.

1. INTRODUCTION In current technological advances, quasi one-dimensional nanostructures are extremely versatile materials to be carefully considered as promising candidates in a variety of applications due to their unique nanoscale physicochemical properties. Particularly, one-dimensional (1-D) nanostructure metal oxides such as nanowires, nanorods, nanotubes, and nanobelts have been studied extensively as building blocks in optoelectronic devices for alternative renewable energy applications and as heterogeneous catalysts owing to the modification of their chemical, mechanical, electrical, and optical properties from those of the bulk.1−4 Very recently, 1-D nanostructure materials have been thus highlighted as the most promising architectures to enhance electrocatalytic activity due to their much higher surface area and amendable control of more complex hybrid nanostructures.5,6 Although a variety of methods for the synthesis of these materials have been developed so far such as vapor−liquid−solid (VLS)/vapor−solid (VS) growth methods, sol−gel techniques, and template-based approaches, it is still challenging to make some important metal oxide nanostructures with well-defined crystal structures with desirable density in real applications.7 Among the most synthetically challenging one-dimensional metal oxide nanostructures, iridium dioxide (IrO2) has a rutiletype crystal structure with space group P42/mnm and lattice © 2012 American Chemical Society

parameters a = b = 0.44983 nm, c = 0.31544 nm. Owing to the 5d band electrons of Ir, IrO2 is metallically conducting with a resistance of ∼50 μΩ·cm in bulk at room temperature.8−10 Although IrO2 is rare and expensive, it is of considerable interest as a promising candidate for electrodes in a variety of applications due to high catalytic activity, low resistivity, and superior chemical and thermal stability. Particularly, the chemical and electrical stability of IrO2 makes it possible to be used for excellent chemical sensing applications like pH sensors.11,12 It has also been known as one of the best electrocatalysts for highly effective light driven water oxidation.13,14 In addition, the most beneficial advantages for biological applications is that it is an exceptionally biocompatible metal oxide approved by the Food and Drug Administration (FDA) for stimulating and recording electrodes, with superior charge storing capacity as well as structural stability over commercial electrodes.15 On the other hand, it is worthwhile to note that onedimensional single crystalline IrO2 nanostructures are still a great challenge to prepare, indicating that there are only a few reports on their successful growth, more like nanorods. Most Received: July 11, 2012 Revised: August 8, 2012 Published: August 9, 2012 18550

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of the nanowires. The temperature of the region which obtained the nanowires on the substrate was measured to be 700 °C. The furnace was then allowed to cool to room temperature under flowing He. The Au microwires and the SiO 2 substrates were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The crystal structures of IrO2 nanowires on both a SiO2/Si substrate and a Au microwire were also imaged by high resolution transmission electron microscopy (HRTEM, Cscorrected STEM, JEOL JEM-2100F) at 200 kV. Samples for TEM imaging were prepared by touching the nanowire-covered SiO2/Si wafer and Au microwire to a TEM grid, thereby transferring some of the nanowires to the grid. Raman scattering measurements were directly carried out on both SiO2/Si substrate and Au microwire with IrO2 nanowires transferred to a Pyrex glass slide. Raman spectra were recorded in the backscattering configuration using a confocal microscope (Renishaw InVia System), using a 100× (0.9 NA) microscope objective, focusing a laser beam (∼1 μm) through both and collecting the backscattered light. Raman spectra were excited with a 632.8 nm He−Ne laser light. Low power was used to ensure that the nanowires did not decompose by localized laser heating. Optimal results were obtained with 50 mW laser power and 300 s integration times. The prepared IrO2 nanowire-Au or bare Au microwire was connected to a W rod using Ag conductive epoxy. The IrO2 nanowire-Au/W or Au/W was heated in an oven to cure the Ag epoxy. Cyclic voltammetry (CV) and potentiometry were performed using CHI 920C scanning electrochemical microscopy (SECM, CH Instruments, Austin, TX). A three-electrode system was employed for the electrochemical measurements, including a saturated calomel electrode (SCE) as the reference electrode and a Pt wire as the counter electrode. The Au microwires (on which with or without IrO2 nanowires) were immersed in electrolyte solution followed by regulation of the vertical position relative to the solution surface using a piezoelectric step motor of the SECM instrument until the electrode gently touched the solution surface. Assuming the contact position as distance zero, the microelectrode was immersed with the desired depths into the solution. For potentiometric experiments, the pH responses of Au microwires (on which with or without IrO2 nanowires) were obtained by titrating a universal buffer composed of 11.4 mM boric acid, 6.7 mM citric acid, and 10.0 mM NaH2PO4 with small aliquots of NaOH and HCl while monitoring the electrode potentials (vs Ag/AgCl reference electrode).12,23 The solutions were stirred magnetically, and the equilibrium potentials were recorded. K3[Fe(CN)6], Ru(NH3)6Cl3, H2O2, NADH, NaH2PO4·H2O, and Na2HPO4 were supplied by Sigma-Aldrich (St. Louis, MO). Au microwire (25 μm in diameter, 99.99%) and W rod (0.05 in. × 3 in.) were acquired from Good Fellow (Oakdale, PA) and FHC (Bowdoin, ME), respectively. Silver conductive epoxy was purchased from Alfa Aesar (Haverhill, MA). All other analytical grade reagents and solvents were used as received. All aqueous solutions were prepared with deionized H2O (resistivity ≥18 MΩ·cm).

studies of the growth of IrO2 one-dimensional nanostructures have thus employed the MOCVD (metal organic chemical vapor deposition) process16−18 using the iridium precursors under low vacuum pressure, which is very expensive and hard to synthesize. Furthermore, previous growths of IrO2 onedimensional nanostructures by the MOCVD process have been highly sensitive to the substrate. For example, Zhang et al.15 recently described that IrO2 nanowires were difficult to grow on bare Si, Pt, Au, and Ir noble metals compared to TiN (titanium nitride) and ITO (indium tin oxide) under similar growth conditions. This might be likely due to the nucleation barrier difference between substrates in its initial growth period. However, very recently, we introduced a successful fabrication method for a Pt microelectrode sensor densely decorated with IrO2 nanowires for direct sensing of H2O2 and NADH with high reproducibility as well as electrochemically enhanced catalytic activity for the first time.19 As a consequence, a direct vapor phase transport process at atmospheric pressure could be possible to conveniently and effectively grow IrO2 onedimensional nanostructures on various substrates for electrochemical applications. There have been no attempts reported so far in growing highly single crystalline IrO2 nanowires directly on the surface of Au, which is a much more favorable direction in electrochemical devices with regard to well-defined nanoscale architectures. Here we present a general synthetic strategy and characterizations of growing IrO2 nanowires on SiO2/Si and Au substrates via a simple vapor phase transport process under atmospheric pressure without any catalyst. Furthermore, we carefully investigate the fundamental electrochemical performances of IrO2 nanowires on a Au microwire and then the possibility for its use as an improved pH microsensor. The simple mode of operation is greatly promising for microelectronic applications including high performance amperometric sensors where the diffusion-controlled limiting current is directly proportional to the concentration of the electroactive analyte and miniaturized energy devices.20,21

2. EXPERIMENTAL SECTION The growth of IrO2 nanowires was directly carried out on a SiO2/Si wafer and a commercial Au microwire (diameter = 25 μm) by vapor transport without catalyst in a single zone quartz tube furnace, 2.5 cm in diameter and 60 cm long under atmospheric pressure. A 10 mg portion of fine meshed IrO2 (99.9%, Aldrich) powder was first loaded at the center of a 6 cm long quartz boat without further purification.22 Single crystalline Si(001), a 200-nm-silica-covered Si(001) wafer, and several Au microwires on SiO2/Si substrate were then introduced into a furnace at a point ∼15 cm downstream of the IrO2 powder source. In order to achieve the homogeneous growth of IrO2 nanowires on the surface of an entire Au microwire, the end part of the Au microwire was raised to the surface normal of the SiO2 substrate. The quartz boat and its IrO2 charge were cleansed of impurities by first placing it at the center of the quartz tube furnace under He (99.999%) carrier gas flowing for ∼10 min at a gas flow rate of 375 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 with 375 sccm at a rate of 100 °C min−1. The nanowire growth was proceeded as flowing high purity He (99.999%, 375 sccm) and O2 (99.9%, 10 sccm) for 1 h. The flow rates of O2 were carefully controlled from 0 to 50 sccm to explore the influence of oxygen contents on the growth

3. RESULTS AND DISCUSSION The growth of IrO2 nanowires was directly carried out on both SiO2/Si substrate and a Au microwire by vapor transport in the absence of a catalyst at 700 °C under atmospheric pressure. 18551

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IrO2 nanowires on a 25 μm Au microwire under growth conditions similar to those grown on a SiO2/Si substrate. The shape of the nanowires was similar to those grown on a SiO2/Si substrate, but the density of the IrO2 nanowires was much higher, with a uniform distribution on the entire Au microwire surface. As seen on the SiO2/Si substrate, the IrO2 nanowires were also more likely to be needle shaped, with the dimensions at the tip of the IrO2 nanowires much smaller than at the root. The high magnification SEM image (Figure 2) reveals that most IrO2 nanowires possess a rectangular cross section but some of them have a polyhedral shape. EDS analysis for asgrown nanowires on a single Au microwire confirmed that all the nanowires were composed of Ir and O only, as shown in Figure S1 (Supporting Information). It should be noted that the absence of the Au element on the dense nanowires from rigorous EDS analysis is again consistent with the absence of catalytic/liquid droplets at the ends of nanowire tips from SEM images which is responsible for the vapor−liquid−solid growth process with Au catalysts. It was also observed that the growth of nanowires was affected by controlling the content of the oxygen gas at the growth temperature, as shown in Figure S2 (Supporting Information). Without oxygen gas low, nanowire growth on both substrates was not observed. On the other hand, morphologies of the IrO2 under high oxygen flow (50 sccm) were rather close to those of microsized crystals with welldefined crystal facets and polyhedral shapes (Figure S2, Supporting Information). The growth of the IrO2 nanowires with high aspect ratios was only achieved within a limited range of the oxygen flow rate in the range from 10 to 15 sccm in our study. Thus, it is immediately apparent that the formation of gas phase IrOx precursors under oxidative conditions can play a critical role in the preferential nucleation of the IrO 2 nanostructures. Since IrO2 has a very low vapor pressure, it is well-known that it is hardly vaporized at even high temperature. In our growth mode with appropriate oxygen content, however, gaseous volatile IrO3 species can be readily formed by the oxidation of IrO2 at high temperature and then they can be transferred into the region of lower temperature on a substrate by carrier gas.24 Therefore, the IrO2 nanostructures could be rationalized in terms of the preferential nucleation of IrO2 species followed by the decomposition of IrO3 precursors at the region of lower temperature. XRD patterns of the IrO2 nanowires on a SiO2/Si substrate and a single Au microelectrode confirm the presence of highly crystalline IrO2 structures, exhibiting indexed peaks, (110), (101), (200), and (211), consistent with the rutile IrO2 structure in the tetragonal phase which is in agreement with the standard data (JCPDS 88-0288), as shown in Figure 3a and b.16 In addition to XRD data on the nanowires, evidence regarding the chemical composition of the nanowires was obtained from Raman microscopy in Figure 3c and d. While substrate features dominate the spectrum on SiO2/Si substrate, the Raman spectrum of a single Au microwire decorated with the IrO2 nanowires clearly exhibits three characteristic peaks measured at 557, ∼729, and 750 cm−1 which are attributed to the first order Eg, B2g, and A1g phonon bands of rutile IrO2 structure (Figure 3c and d), based on previous measurements.25 Our measured band frequencies correspond closely to those observed with bulk samples, implying high crystalline quality for IrO2 nanowires grown on a single Au microwire. The detailed crystal structure of the grown IrO2 nanowires was investigated by TEM images, as shown in Figure 4. It is

SEM images (Figure 1) clearly show a high density of IrO2 nanowires on the SiO2/Si substrate. High magnification SEM

Figure 1. As-grown IrO2 nanowires on a SiO2/Si substrate. (a−d) SEM images with different magnifications showing grown IrO2 nanowires on a SiO2/Si substrate.

images indicate IrO2 nanowires were well grown from out of the plane of the substrate with random orientations. The IrO2 nanowires have lateral dimensions ranging from 20 to 100 nm near the nanowire tip, with the length extending up to tens of micrometers. The shapes of nanowires were quite straight, but their lateral dimensions were not uniform along the growth direction, indicating that needle-like shape was predominant. Thus, the lateral dimension of the end of a nanowire is generally much smaller than that of the root of a nanowire (Figure 1c and d). According to extensive microscopic investigations, one interesting phenomenon is that multiple nanowires predominantly originated from the same nucleation sites, forming a lawn-like shape. In addition, the absence of catalytic/liquid droplets at the ends of nanowire tips strongly suggest that the growth mechanism may resemble sublimation followed by recrystallization, a process referred to as vapor− solid (VS) growth.4 Figure 2 also represents SEM images of the

Figure 2. As-grown IrO2 nanowires on a single Au microwire (diameter = 25 μm). (a−d) SEM images with different magnifications showing grown IrO2 nanowires on a 25 μm Au microwire. 18552

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Figure 3. X-ray diffraction patterns of IrO2 nanowires grown on SiO2/Si substrate (a) and a single Au microwire (b). Raman spectra taken from the excitation wavelength at 632.8 nm grown IrO2 nanowires on a SiO2/Si substrate (c) and a single Au microwire (d). Note that the peak at 33.2° represents the Si wafer in part a.

process. It is well-known that the growth rate is strongly dependent on the surface energies of crystal planes. The growth characteristics and crystal structures of the IrO2 nanowires grown on a SiO2/Si substrate are thus similar to those of previous results.16−18 On the other hand, we found that some of the IrO2 nanowires on Au microwire were also grown along the [110] crystallographic direction, which had not been reported yet, as shown in Figure S3 (Supporting Information). Although the growth mechanism was similar with the vapor− solid (VS) growth process due to the absence of the catalytic particle at the tips of nanowires as mentioned above, it is necessary to explore the IrO2 nanowires for deep understanding of the crystal growth. Figure 5 shows SEM images of the IrO2 nanostructures on a Au microwire taken as a function of time for evaluating the time dependence of the growth behavior. By keeping the constant growth temperature, the nucleation of the IrO2 structures on a Au microelectrode was just seen within 10 min. Figure 5b indicates that the IrO2 nanostructures were preferentially deposited on the surface of the Au microwire from the gaseous IrO3 species at the beginning of the growth process. At this moment, additionally, it would be expected that the surface layer of a Au microwire might be partly melted at such a high temperature and then it could enhance the nucleation of gaseous IrO3 precursors onto the surface of the Au microwire.26,27 When a Au microwire was placed with the same He/O2 flow rate but the absence of IrO2 powder in our growth process, in fact, the surface morphology of it was clearly changed like the recrystallization phenomenon after the melting and cooling cycle of it, as shown in Figure S4 (Supporting Information). After 20 min (Figure 5c), although the density was relatively low and the length was short, the IrO2 nanowires were already grown from out of the surface of the Au

Figure 4. Detailed crystal structures of as-grown IrO2 nanowires. The low and high magnification TEM images of a representative IrO2 nanowire and the fast Fourier transform (FFT) of the lattice-resolved image.

clearly seen that an IrO2 nanowire grown on SiO2/Si substrate is terminated by a prismatic form the rectangular cross sections and each well-defined facet (Figure 4). As shown in Figure 4b, a typical HRTEM image of a single IrO2 nanowire along the [010] zone axis indicates that a nanowire clearly possesses the single crystalline nature with no sign of any defects, dislocations, or amorphous overlayer. An HRTEM image (Figure 4b) of a tip of a IrO2 nanowire grown on SiO2/Si substrate represents that the lattice spacing of adjacent planes is about 0.2249 nm corresponding to that between the (200) planes of the tetragonal IrO2 phase. The nanowires grow along the [001] direction parallel to the {110} family planes, in exact agreement with the fast Fourier transform (FFT) of the latticeresolved image (Figure 4, inset). Interestingly, the sharp facets seen at the tip of the nanowire imply that these low-index facets arise from the low surface energy surfaces during the growth 18553

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Figure 5. SEM images of IrO2 nanowires grown on a single Au microwire taken from the different growth times: (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 45 min, (f) 60 min, (g) 75 min, and (h) 90 min.

microwire. After 30 min, the density of the IrO2 nanowires was greatly increased and the length was much longer, evidently covering the entire Au microwire region. It is also noteworthy that the morphology of the IrO2 nanowires was gradually changed into needle-like shape, since 20 min (Figures 5 and S5, Supporting Information). According to the results of the time dependence for the growth process, it is thus reasonably suggested that the surface melting of a Au microwire would allow for the rapid nucleation of IrO2 nanostructures, leading to and hence the nanowire growth proceeding by the recrystallization of continuous supplies of the gaseous IrO3 precursors. Furthermore, the formation of needle-like shape could be partly attributed to the distinct growth rate difference between the tip of the nanowire and root during the growth process. However, it should be pointed out that in situ experiments such as TEM or GISAXS (grazing incident small angle X-ray scattering) in real time would be greatly beneficial to fully understand this kind of growth mechanism.7 To evaluate the electrochemical behavior of the IrO2 nanowires grown on a single Au microwire (IrO2−Au microelectrode), CV curves of the IrO2−Au electrode were recorded with an immersion depth of 100 μm in an aqueous solution containing various concentrations of electroactive species (1, 5, and 10 mM [Fe(CN)6]3− or Ru(NH3)63+) and 1.0 M KCl, which were compared with CVs of a bare Au microwire (diameter = 25 μm) (Figure 6). With a potential scan rate (v) of 10 mV s−1, the shape of the CVs of the IrO2−Au microelectrode becomes a typical steady-state sigmoidal one with a hint of a peak-shaped one, indicating changes in the diffusion layer growth pattern; i.e., the so-called “quasi steady state”19,28 was obtained with slower v (Figure 6b and d). This observation indicates that the diffusion layer grows along with the smooth cylindrical geometric surface plane for longer experimental time, similarly to the one at a Au microwire substrate with the diameter extended from the original Au microwire by the length of IrO2 nanowires.19,28 The CV experiments for both a negatively charged [Fe(CN)6]3−/4− couple and a positively charged Ru(NH3)63+/2+ couple support favorable electrode kinetics with a linear proportionality to the concentration of electroactive species at the highly oriented crystalline IrO2 nanowire surface (Figure S6, Supporting Information). Note that the CV responses of [Fe(CN)6] 3−/4− on Au microwire were drawn out compared to those of the IrO2−Au microelectrode (Figure 6a and b) and the

Figure 6. CVs of 25 μm Au microwire (a) and IrO2−Au microelectrode (b) in aqueous solutions containing [Fe(CN)6]3− at various concentrations (1, 5, and 10 mM) and 1.0 M KCl with a scan rate of 10 mV s−1. CVs of 25 μm Au microwire (c) and IrO2−Au microelectrode (d) in aqueous solutions containing Ru(NH3)63+ at various concentrations (1, 5, and 10 mM) and 1.0 M KCl with a scan rate of 10 mV s−1. For all, the immersion depth was 100 μm.

reduction of [Fe(CN)6]3− on the Au microwire was suppressed in the absence of supporting electrolyte (Figure S7, Supporting Information).29 To estimate the real surface area (RSA) of the electrode with and without IrO2 decoration, CV experiments were performed in 1.0 M KCl between 0.7 and 0.1 V with various potential scan rates (v = 10−200 mV s−1). The difference of nonfaradaic capacitive currents (Δi) between forward and reverse scan were measured at 0.5 V and plotted against scan rate for the Au microwire (diameter = 25 μm) and IrO2−Au microelectrode (Figure S8, Supporting Information), where the apparent RSA increased by 60 times using the differences in slopes and double-layer capacitance (Cd) values. The potentiometric responses of the IrO2−Au microelectrode to pH were examined from pH 2 to 12 by adding aliquots of NaOH to a universal buffer solution, as illustrated in 18554

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Figure 7. A schematic diagram of the experiments for potentiometric pH sensing using single crystalline IrO2 1-D nanostructures on a single Au microwire (a) and comparison of the pH responses between a glass pH electrode and a Au microwire as well as an IrO2−Au microelectrode: dynamic potentiometric responses toward varying pH values (b) and calibration curves (c) corresponding to part b. The pH changes were realized by additions of NaOH to a universal buffer solution.

calibration curve in buffer solution exhibiting a super-Nernstian behavior with a slope of 71.6 mV/pH. This rational engineering of nanoscale architecture based on the direct formation of hierarchical 1-D nanostructures on an electrode can offer a useful platform for high performance electrochemical biosensors, enabling the efficient, sensitive detection of biologically important molecules. Attractive features are thus simplicity, low cost, and high throughput as well as high crystalline properties of 1-D nanostructures with unique multiscale architectures.

Figure 7a. Figure 7b and c shows the typical potentiometric response curves for IrO2−Au microelectrode and glass pH electrode. The IrO2−Au electrode showed a reliable potentiometric pH response including super-Nernstian behavior (slope = −71.6 mV pH−1, r = 0.9996) and a reasonable response time (t90% = ca. 5.3 ± 3.9 s), which showed the possibility of using the miniaturized sensor for local pH sensing. The superNernstian slopes have been reported for IrO2-based pH sensors and explained by the stoichiometric ratio of protons and electrons that balance the iridium oxyhydroxide compositions in the IrO2 nanowires.12,30 The reasonable response time seems to be ascribed to the facile equilibration due to the increase of the electrode surface area together with the nanoconfined structure of the IrO2−Au microelectrode.31

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ASSOCIATED CONTENT

S Supporting Information *

Additional SEM images of IrO2 nanowires; SEM−EDS for IrO2 nanowires; TEM images for a single IrO2 nanowire on a Au microwire; additional electrochemical measurement data. This material is available free of charge via the Internet at http:// pubs.acs.org.

4. CONCLUSION Here we report the direct growth of IrO2 nanowires on SiO2/Si substrate and a Au microelectrode in a single step by a simple vapor phase transport method. We demonstrated that extremely dense IrO2 nanowires on the surface of a single Au microwire were readily obtained on a single Au microwire, suggesting that the melted surface layer of Au might effectively enhance the nucleation of gaseous IrO3 precursors at the growth temperature. All the electrochemical observations of the directly grown IrO2 nanowires on a single Au microwire support favorable electron-transfer kinetics of [Fe(CN6)]4−/3− as well as Ru(NH3)63+/2+ at the highly oriented crystalline IrO2 nanowire surface. Furthermore, it shows a stable pH response as a potential miniaturized pH sensor, confirmed by the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.H.K.); [email protected] (C.L.). Author Contributions ⊥

These authors equally contributed to this work.

Notes

The authors declare no competing financial interest. 18555

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(29) Lee, C.; Anson, F. C. J. Electroanal. Chem. 1992, 323, 381−389. (30) Pásztor, K.; Sekiguchi, A.; Shino, N.; Kitamura, N.; Masuhara, H. Sens. Actuators, B 1993, 12, 225−230. (31) Han, J.-H.; Lee, E.; Park, S.; Chang, R.; Chung, T. D. J. Phys. Chem. C 2010, 114, 9546−9553.

ACKNOWLEDGMENTS This research was supported by Basic Science Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Nos. 2012-0001466, 2012-0005422, and 2010-0022028) and by MEST & DGIST (10-BD-0101, Convergence Technology with New Renewable Energy and Intelligent Robot).

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