Highly Efficient Electrochemical Responses on Single Crystalline

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Highly efficient electrochemical responses on single crystalline ruthenium-vanadium mixed metal oxide nanowires Sung Hee Chun, Hyun-A Choi, Minkyung Kang, Moonjee Koh, Nam-Suk Lee, Lee Sang Cheol, Minyung Lee, Youngmi Lee, Chongmok Lee, and Myung Hwa Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/am4016445 • Publication Date (Web): 13 Aug 2013 Downloaded from http://pubs.acs.org on August 14, 2013

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ACS Applied Materials & Interfaces

Highly efficient electrochemical responses on single crystalline ruthenium-vanadium mixed metal oxide nanowires Sung Hee Chun†║, Hyun-A Choi†║, Minkyung Kang†, Moonjee Koh†, Nam-Suk Lee‡, Sang Cheol Lee§, Minyung Lee†, Youngmi Lee†, Chongmok Lee†*, and Myung Hwa Kim†* †

Department of Chemistry & Nano Science, Ewha Womans University Global Top5 Research Program, Seoul, 120-750, Korea ‡

National Center for Nanomaterials Technology (NCNT), Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Korea § Robotic Research Division, Daegu Gyeongbok Insititute of Science & Technology (DGIST), Dalseong, 711-873, Korea ║

Authors are equally contributed to this work.

*

To whom all correspondence should be addressed: [email protected],

[email protected] ABSTRACT The highly single crystalline ruthenium-vanadium mixed metal oxide (Ru1-xVxO2, 0 ≤ x ≤ 1) nanowires were prepared on a SiO2 substrate and a commercial Au microelectrode through a vapor phase transport process by simply adjusting the mixing ratios of RuO2 and VO2 precursors for the first time. Single crystalline Ru1-xVxO2 nanowires show the homogeneous solid solution characteristics as well as the distinct feature with the remarkably narrow dimensional distributions. The electrochemical observations of a Ru1-xVxO2 (x = 0.28 and 0.66) decorated-Au microelectrode, using the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), represent favorable charge-transfer kinetics of [Fe(CN)6]3–/4– and Ru(NH3)63+/2+ couples compared

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to a bare Au microelectrode. The catalytic activity of Ru1-xVxO2 for the oxygen reduction and H2O2 reduction at neutral pH increases as the fraction of vanadium increases within our experimental conditions, which might be useful in the area of biofuel cells and biosensors.

KEYWORDS: Ruthenium-vanadium mixed oxide, Nanowire, Electrocatalyst, H2O2 reduction, Oxygen reduction reaction


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INTRODUCTION Mixing two different metals with the high catalytic activity to form nanostructured metal oxide materials is one of intensive research directions for electrocatalytic applications due to the better performances in chemical, mechanical, and electrical, optical properties compared to those of the pure metal oxides.1-6 Especially the nanostructured mixed oxide systems, ruthenium dioxide (RuO2) or iridium dioxide (IrO2) with other metal oxides such as SnO2, TiO2, Ta2O5, VO2 etc. are one of most fascinating materials in the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER) owing to the extremely high performances as electrocatalysts7-12. In fact, major issue in developing the mixed metal oxide catalysts is to find out favorable alternatives with satisfactory activity and durability to reduce the high cost of novel metal-based electrocatalysts such as Pt, Ir, Ru, Pd, and Rh in real applications. However, very limited processes have been previously used to obtain the thin film or nanoparticle systems for enhancing the electrochemical activity as well as stability.13,14 Unfortunately, mixed metal oxides used often represent significant local inhomogeneity in the crystal structure which can be poorly defined crystal structures compared to those of single phases. Thus, it is quite challenge to obtain well defined single crystalline mixed metal oxides for a variety of applications so far.



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On the other hand, very recently, we reported a facile synthetic approach for growing highly single crystalline quasi 1-dimensional (1-D) IrxRu1-xO2 nanostructures via a simple vapor phase transport process as well as their characterizations by Raman scattering for the first time.15 In our previous work, since both metal oxides have the same tetragonal crystal structure and the similar ionic radius in the tetravalent ionic state, a binary mixture system of IrO2-RuO2 readily formed a continuous solid solution in the whole range of the composition.15 Interestingly, well defined 1-D nanostructures in the mixed forms between metal oxides are greatly interesting due to their unique advantages 16,17

compared to thin film or nanoparticle approaches. While a binary mixture system of

IrO2-RuO2 is remarkably attractive owing to the highest performance of the electrocatalytic activity in the field of electrochemical catalysts, it still suffers from very high cost to make the single crystalline IrxRu1-xO2 nanostructures. One of alternative strategies to overcome this issue is thus to incorporate other cheaper metals (Ta, Ti, Sn, V, Bi, etc..) to IrO2 or RuO2 metal oxide with retaining their fundamental electrochemical properties. Along this research direction, the rutile type RuO2-VO2 solid solutions have been also suggested as one of the promising candidates for electrochemical catalysts. For the binary system of Ru1-xVxO2, however, only a couple of traditional synthetic methods


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such as polymerizable-complex method and sol-gel method were reported until now for just electrochemical supercapacitors.18,19 Although Ru1-xVxO2 nanoparticles even showed excellent charge storage capacity18,19 comparable to well-known pure amorphous hydrated RuO2 nanoparticle systems, their fundamental properties in the single phase such as the crystal structure, composition, and electrochemical behaviors were not well described due to the lack of high quality of single crystalline Ru1-xVxO2 nature. Furthermore, no one reported yet for the synthesis of highly single crystalline Ru1-xVxO2 nanowires and the study of their fundamental electrochemical behaviors as a candidate of efficient electrocatalysts so far. In the work presented here, therefore, we have carried out the first attempts to introduce a simple process for growing single crystalline Ru1-xVxO2 nanowires on a SiO2 substrate as well as a commercial Au microwire under the broad range of Ru/V metal ratios. We have also studied their fundamental electrochemical responses including H2O2 reduction as well as oxygen reduction reaction (ORR).

EXPERIMENTAL Single crystalline Ru1-xVxO2 mixed oxide nanowires were directly prepared on a Si(001) wafer as well as a commercial Au microwire (diameter of 25µm) via a vapor



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phase transport process by using RuO2 and VO2 without any foreign catalyst at the atmospheric pressure 15,20 Since the vapor phase transport process was well described in detail with our recently published works15, 20-22, the differences are only given in here. In this study, particularly, Ru1-xVxO2 nanostructures were obtained using homogenous mixtures by adjusting of the relative ratios of RuO2 and VO2 (99.9%, Aldrich) powders. The growth was proceeded by using helium (99.999%, 400 ~ 450 sccm) and oxygen (99.9%, 1 ~ 5 sccm) as carrier gases for one hour. The location of the growth substrate was placed in ~14 cm from the downstream of the source region which is the temperature region of 1000oC under the growth condition. On the other hand, the temperature near the growth region was approximately measured by 870oC. Scanning electron microscopy (SEM) and micro Raman spectroscopy were used to characterize the nanostructures obtained from our growth processes. To explore the crystal structures, X-ray diffraction pattern was recorded from Rigaku diffractometer using Ni filtered Cu Kα radiation, λ = 0. 15418 nm at the room temperature. In addition, high resolution transmission electron microscopy (HRTEM, Cs-corrected STEM, JEOL JEM-2100F) at 200 kV was employed to view the images of the crystal structures for a single Ru1-xVxO2 mixed oxide nanowires. Topographical images were taken by an AFM (Park Systems XE-70) in an acoustic box at room temperature. The AFM tip used was


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one of the PointProbe series from Nanosensors, Inc. (910M-NCHR), having a resonance frequency of 330 kHz and a spring constant of 42 N/m. The radius of curvature at the tip apex was nominally less than 10 nm. The AFM images were obtained by the noncontact mode at 512×512 pixels at the 0.2 Hz scan rate. To investigate the electrochemical properties, the prepared Ru1-xVxO2 (x = 0.28 and 0.66) decorated-Au was glued to a W wire with silver conductive paste, which was immersed vertically in electrolyte solution up to 300 µm-deep as our previous works.2123

A CHI 920C scanning electrochemical microscopy (SECM, CH Instruments, Austin,

TX) was used for the electrochemical experiments where the electrode was positioned via the z-axis controller of the SECM. For the auxiliary and reference electrodes, a Pt coil and saturated calomel electrode (SCE) were used, respectively. The oxygen reduction was examined in an O2-saturated phosphate buffer solution (0.10 M PBS, pH 7.40) using linear sweep voltammetry (LSV) technique. The amperometric response of the Ru0.34V0.66O2 decorated-Au (or bare Au) against the concentration of H2O2 was monitored at Eapp = −0.20 V (vs. SCE), where the various concentration of H2O2 was achieved by standard H2O2 addition into PBS solution while stirring.21,22 Most of chemicals were purchased at Sigma-Aldrich (St. Louis, MO). All



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other reagents and solvents were analytical grade and used as received. Deionized water (resistivity ≥ 18 MΩ·cm) was used to prepare all aqueous solutions.

RESULTS AND DISCUSSION Figure 1 presents typical SEM images for Ru1-xVxO2 (x= 0.39) nanowires grown on a SiO2 substrate. Interestingly, it is immediately apparent that most of nanowires were grown in the plane of substrate, resembling with the previous pure VO2 nanowire growths25,26 on the SiO2 substrate. SEM images reveal that nanowires have the typical rectangular shape as well as sharp facets from the end of tip of nanowires (Figure 1b). Furthermore, it is clear that well defined 1-D nanostructure architectures can be directly grown on the surface of Au microelectrode for facile electrochemical applications as shown Figure S1.21-23 The growth of nanowires represents quite similar behavior, indicating almost in plane growth on a Au microelectrode. Particularly, it should be noticed that the dimensional distributions for as-grown nanowires on a SiO2 substrate are remarkably narrow as shown in Figure 2. The average lateral dimension and the length of nanowires are carefully estimated by high magnification SEM images, 182±18 nm and 3.2±0.08 µm, respectively. In addition, atomic force microscopy (AFM) measurements in Figure 2c clearly confirm that the height distribution of nanowires is


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extremely narrow with the average height of ~ 80 nm along the line scan profile. On the other hand, both the dimension and the dimensional distribution of Ru1-xVxO2 nanowires on a Au microwire are relatively much bigger as well as much broader than those of a SiO2 substrate probably due to the difference of the surface energy in the growth mode (Figure S1). Additionally, since the grown nanowires do not have any foreign catalytic particles at the end of tip from Figure 1b, the well-known vapor-liquid-solid (VLS) growth mechanism is not appropriate for explaining the growth behavior of Ru1-xVxO2 nanowires in our study.21 On the other hand, it is clearly seen that there are many droplet-like structures near nanowires from Figure S2. Also, the droplet density is dramatically depleted near nanowires, implying that the droplet-like structures are being consumed into the formation of nanowires in the period of the growth process.25 This kind of behavior was recently observed from the pure VO2 nanowire growth induced by super-cooled liquid nanodroplets, which was carefully probed by in situ GISAXS (grazing incident small angle X-ray scattering) measurements.25

Based on careful

inspections, although more detailed mechanistic study should be required further, it could be proposed that the plausible growth mechanism is quite similar with that of VO2 nanowires so that nanodroplets might be involved in the growth process of the



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formation of Ru1-xVxO2 nanowires. For a binary mixture system of RuO2-VO2, in fact, it was reported that a continuous solid solution can be readily obtained over the broad range of compositions between two metal oxides.16 It is likely due to the similar ionic sizes for Ru4+ (0. 0600 nm) and V4+ (0.0620 nm) ions which can be shared the same sub-lattice in a tetragonal phase.27 In our growth process, since the gas phase intermediates of RuO4 and V2O5 with much low melting points can be readily formed by the oxidation from the tiny amount of O2 gas flow at such a high temperature, the growth could be explained by the preferential nucleation for VOx and RuOx intermediates in the gas phase.20,25 Interestingly, the nanowire growth was highly dependent of the content of the oxygen gas at the growth temperature from Figure S3.15,22 Without oxygen gas flow, the nanowire growth was rarely obtained. As a consequence, morphologies of Ru1-xVxO2 in high oxygen flow (20 sccm) were close to microsized crystals with sharp crystal facets (Figure S3). In our work, thus, high aspect ratios of Ru1-xVxO2 nanowires were only obtained under the limited oxygen flow rate between 1 and 5 sccm. It would be thus expected that these volatile species could be the form of nanodroplets initially, which are strongly suppressed in the freezing point due to smaller sizes and strongly bounded nature with the substrate.25 In the next process, two precursors in nanodroplets could be completely


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dissolved each other to make a liquid solution. These mixed nanodroplets could be grown by Ostwald ripening and then super-cooled before starting crystallization on the substrate. From this process, a variety of compositions of Ru1-xVxO2 mixed metal oxide nanowires can be produced.25 According to rigorous EDX measurements in Table 1, the chemical compositions of present Ru1-xVxO2 nanowires were quantitatively determined. However, it should be noted that well-defined Ru1-xVxO2 mixed metal oxide nanowires with the higher vanadium content than x = 0.70 were rarely obtained in our study. X-ray diffraction patterns prove that present Ru1-xVxO2 nanowires possess the highly single crystalline nature for various compositions as shown in Figure S4. All X-ray diffraction patterns for Ru1-xVxO2 mixed metal oxide nanowires only exhibit two sharp major peaks, around 28o and 57o with a minor satellite peak at the position of near 40o, which could be attributed to (110), (220), and (200) indexed crystallographic planes, closely consistent with the rutile structure of both pure RuO2 and VO2 in the tetragonal phase from the standard data (JCPDS 43-1027 for RuO2 and 44-0253 for VO2).20,25 Only a few indexed peaks for Ru1-xVxO2 nanowires are mainly originated from the top crystal plane of nanowires with the rectangular cross-section owing to the preferential growth in the plane of a substrate. Furthermore, Figure 3a clearly represents the continuous variation of the peak position for a (110) indexed crystal plane with respect



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to the relative contents of two metals in the mixed oxide nanowires, indicating the homogeneous formation of the continuous solid solution from both pure RuO2 and VO2 crystal structures. We have quantitatively estimated the lattice parameters from our XRD data using the Bragg’s equation as a function of the V atom content. Although the lattice constants of a were easily calculated from XRD data, the lattice constant of c could not be extracted from our measured XRD data because of the absence of the c axis containing crystallographic plane due to highly oriented growth behavior of Ru1xVxO2

nanowires in the plane on a SiO2 substrate. To obtain the lattice constant of c,

thus, we have mechanically grinded the Ru1-xVxO2 nanowires on a SiO2 substrate in order to break them up from the substrate. As a consequence, XRD patterns after above process showed an additional indexed peak with relatively weak intensity corresponding to (011) or (101) as shown in Figure S5. Figure 3b thus indicates that the lattice parameters of Ru1-xVxO2 nanowires linearly vary with the vanadium content for a continuous substitutional solid solution, closely following the prediction of Vegard’s rule28 for ideal solid solution. Figure 4a and 4b represent typical HRTEM images of a single Ru59V41O2 nanowire along the zone axis of [1-10], indicating that a nanowire is the single crystalline nature. Also, there is no evidences for alternative structures such as (VO2)x/(RuO2)1 -x core/shell


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or layered structures. The distances of adjacent crystal planes are estimated by about 0.3124 nm and 0.2899 nm from the lattice resolved HRTEM image, corresponding to the (110) and (001) crystallographic interplanar distances of the tetragonal RuO2 and VO2 unit cells, respectively. Consequently, our result clearly indicates that the spacing of the (110) and (001) planes are slightly shifted along the incorporation of the relative amounts of two metal ions into the tetragonal lattice from two pure metal oxides, consistent with the result of X-ray diffraction patterns. In addition, the mixed oxide nanowire grows along the [001] direction from the Fast Fourier Transform (FFT) of the lattice resolved image in Figure 4a (inset). Furthermore, the spatial distributions of all elements on a single Ru1-xVxO2 mixed oxide nanowire shown in high angle annular dark field (HAADF) STEM image represent highly homogeneous distributions in the whole part of the nanowire in Figure 4b. This observation also supports the formation of a homogenous solid solution of RuO2-VO2 mixed metal oxide system. To investigate the electrochemical behavior of the prepared Ru1-xVxO2 raised on a single piece of Au microwire (Ru1-xVxO2-Au microelectrode), CV voltammograms of Ru1-xVxO2-Au microelectrodes were obtained immersing 300 µm-deep in an aqueous electrolyte solution (1.0 M KCl) containing cationic or anionic electroactive species (Ru(NH3)63+ or [Fe(CN)6]3-). The CV pattern of the Ru0.34V0.66O2-Au in 10 mM



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[Fe(CN)6]3- solution shows gradual change from a peak-shaped wave to a steady-state one as the potential scan rate (v) decreases due to the time-dependent growth pattern of the diffusion layer as discussed in our previous works21-23 (Figure 5a). In Figure 5b quasi steady-state current values at –0.1 V (with v of 10 mV s-1) shows linear relationship to the concentration of [Fe(CN)6]3- (1, 5 and 10 mM) using Ru0.34V0.66O2 – Au. The general electrochemical activity of the prepared Ru1-xVxO2-Au depending on the composition was compared with CV waves of bare microwire electrodes (Au or Pt, diameter = 25 µm) at v of 0.1 V s-1. The CV curves of Ru0.72V0.28O2–Au and Ru0.34V0.66O2–Au show decent electrochemical behaviors as that of bare Pt or Au electrode for both negatively and positively charged redox couples, i.e., [Fe(CN)6] 3-/4and Ru(NH3)63+/2+ (Figure 5c and d). It supports favorable electrode kinetics implying that Ru0.34V0.66O2, up to about 70% of metal exchange from RuO2, still shows good electrical conductivity as an electrode material. This metallic properties of Ru1-xVxO2 also support that VO2 has rutile-type structure along with electron microscopy data.18 To confirm the electron transfer kinetics of Ru(NH3)63+/2+ and [Fe(CN)6]3–/4– couple at the electrode surface, electrochemical impedance spectroscopy (EIS) experiments were also carried out under the same condition as the CV experiments. Nyquist plots of Ru1xVxO2-Au

(x = 0.28 and 0.66) for both in [Fe(CN)6]3- and Ru(NH3)63+ solution within


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the frequency range from 1 Hz to 10 kHz reveal straight lines, which implies Nernstian reversibility as those of bare Pt (Figure 5e and f). Meanwhile, bare Au showed a semicircle at higher frequencies, especially in [Fe(CN)6]3-solution, unlike Ru1-xVxO2-Au or bare probably due to relatively high charge-transfer resistance. The EIS result also supports the high electroactivity of Ru1-xVxO2-Au (x = 0.28 and 0.66) for the ferri/ferrocyanide redox couple. The electrocatalytic activity of Ru1-xVxO2-Au (x = 0.28 and 0.66) toward the oxygen reduction reaction (ORR) was investigated in an O2-saturated 0.10 M PBS (pH 7.40) using LSV technique. As shown in Figure 6a, Ru0.34V0.66O2 –Au revealed higher ORR activity than any other electrode except Pt, which could be applicable to biofuel cell cathode. Since Ru0.34V0.66O2–Au showed decent ORR activity, we further examined electrochemical sensing of H2O2 which is applicable to the broad fields of enzymebased biosensors. In the LSV measurement (Figure 6b), negligible H2O2 redox reaction was observed at bare Au and Ru0.72V0.28O2-Au but significant cathodic current of H2O2 reduction was observed at Ru0.34V0.66O2-Au, which was different from significant H2O2 oxidation at RuO2 nanorod.23 Figure 6c presents typical amperometric response curves (i-t curves) of Ru0.34V0.66O2-Au electrode to consecutive increments of H2O2 at –0.20 V in a deoxygenated 0.10 M PBS (pH 7.40). The amperometric response is fast enough (

Highly Efficient Electrochemical Responses on Single Crystalline

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