Urea assisted electrochemical synthesis of flower-like platinum arrays with high electrocatalytic activity

Electrochimica Acta 123 (2014) 227–232

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Urea assisted electrochemical synthesis of flower-like platinum arrays with high electrocatalytic activity Ming Zhang a,c , Jing-Jing Lv a , Fang-Fang Li c , Ning Bao b , Ai-Jun Wang a,∗ , Jiu-Ju Feng a,∗ , Dan-Ling Zhou a a

College of Chemistry and Life Science, College of Geography and Environmental Science, Zhejiang Normal University, Jinhua, 321004, China School of Public Health, Nantong University, Nantong 226019, China c School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, 453007, China b

a r t i c l e

i n f o

Article history: Received 10 August 2013 Received in revised form 18 December 2013 Accepted 11 January 2014 Available online 23 January 2014 Keywords: Electrodeposition Flower-like platinum arrays Urea Electrocatalytic activity

a b s t r a c t In this paper, well-defined flower-like Pt arrays were prepared on the glassy carbon electrode by onestep electrodeposition at–0.3 V for 600 s in 0.5 M H2 SO4 containing 5 mM H2 PtCl6 and 150 mM urea. This method is simple, facile, and controllable, without using any template, seed or surfactant. The experimental parameters were investigated and found urea acted as a growth directing agent. The as-prepared Pt nanocrystals were preferentially growing along the (111) directions, which were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy dispersive X-ray (EDX). Moreover, the flower-like Pt nanoarrays exhibited a large effective surface area (EASA) and enhanced performance toward the oxidation of ethylene glycol and methanol in acid media, compared with Pt nanoparticles and commercial Pt black catalysts. This strategy can be extended to prepare other noble metal nanostructures as good electrocatalysts in fuel cells. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, nano-sized noble metal materials have aroused tremendous attention because of their unique physical and chemical properties [1–4]. Particularly, platinum (Pt) nanostructures play an important role in the cutting-edge applications of advanced technology [5–8], owing to their shape- and size-dependent properties [1,9,10]. Thus, much work has been focused on manipulating their morphology and size by finely adjusting the experimental conditions [11–17]. Up to now, a number of Pt nanostructures were prepared [18], including multipods [5,11,13], wires [14,15,19], dendrites [1,12,20], cubes [10,11,21], rods [1,21], and flowers [22–25]. Thus-prepared Pt nanomaterials were demonstrated the improved catalytic activity for electrocatalytic oxidation of small organic molecules such as formic acid [1,26,27], methanol [20,28,29], ethanol [5,11,30], methanol [26], and glucose [12,31]. Several good examples have been reported. Hsieh et al. electrodeposited Pt nanoflowers on ITO substrates [5], which displayed the enhanced catalytic ability toward ethanol oxidation. Tiwari’s group synthesized Pt nanoflowers with three-dimensional (3D)

structures on silicon substrates by potentiostatic pulse plating [2], which showed the improved electrocatalytic activity toward methanol oxidation and the adsorbed CO-based species. Yin and coworkers prepared monomorphic single-crystalline Pt nanoflowers assisted by iodine ions [20]. Chen et al. constructed 3D Pt nanoflowers with high yield and good monodispersity supported on GO nanosheets [22]. Nevertheless, some of them involved complicated procedures and expensive raw material [1,23,25]. It is more desirable to develop a controllable, economical, and effective approach to synthesize Pt nanomaterials. In this paper, a simple and facile method was developed for onestep electrodeposition of well-defined flower-like Pt nanoarrays on a glassy carbon electrode (GCE) at room temperature, where urea was used as a growth directing agent for the first time. This method did not need any seed, template, or surfactant. The electrocatalytic performances of the as-prepared Pt nanoarrays were investigated, using the oxidation of ethylene glycol (EG) and methanol in acid media as model systems. 2. Experimental 2.1. Chemicals

∗ Corresponding authors. Tel.: +86 579 8228 2269, fax: +86 579 8228 2269. E-mail addresses: [email protected], [email protected] (A.-J. Wang), [email protected] (J.-J. Feng). 0013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2014.01.054

Chloroplatinic acid (H2 PtCl6 ·6H2 O), ethylene glycol (EG), H2 SO4 , urea, NaOH, and commercial Pt black catalyst were purchased from Shanghai Aladdin Chemical Reagent Company (Shanghai, China)

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and used without further purification. All the aqueous solutions were prepared with twice-distilled water.

3. Results and discussion 3.1. Characterization

2.2. Preparation of flower-like Pt nanoarrays Typically, Pt nannoflowers were prepared by a one-step electrodeposition on a GCE in the electrolyte containing 0.5 M H2 SO4 , 5 mM H2 PtCl6 and 150 mM urea at–0.3 V for 600 s, followed by thoroughly washing with water and dried at room temperature. For comparison, Pt nanoparticles were prepared without urea, but using the same electrical charges, while other conditions were kept constant. 2.3. Characterization X-ray diffraction (XRD) analysis was performed by a BrukerD8-AXS diffractometer equipped with a Cu Ka source. Scanning electron microscopy (SEM) images were taken using a JEOL-JSM6390LV field emission scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were carried out by a JEOL JEM-2100F system at 200 kV accelerating voltage. The TEM sample was prepared by depositing a drop of the dispersion of the diluted sample on a copper grid coated with carbon film and dried in air. Energy dispersive X-ray (EDX) spectroscopy was performed by X-ray energy dispersive spectrometer at 15 kV (JEOL, JSM-7500F). Electrochemical experiments were conducted on a CHI 660D electrochemical workstation (CH Instruments, Chenhua Co., Shanghai, China) by using a conventional three-electrode cell, which included a saturated calomel electrode (SCE) as reference electrode, a platinum wire as counter electrode, and the bare or Pt deposits modified GCE as working electrode. N2 was bubbled into all the electrolytes for 30 min before cyclic voltammetric experiments, if not stated otherwise.

Fig. 1 displays typical SEM and TEM images of the flower-like Pt arrays prepared under the standard conditions. Low-magnification SEM image (Fig. 1A) displays monodispersed Pt nanoflowers with well distribution and high density on the GCE surface. These Pt nanoflowers have the mean diameter of ca. 500 nm, which are assembled by many nanosheets with rough fringes and further reassembled to petals (Fig. 1B). The thickness of each nanosheet is ca. 50 nm. Similar observations are observed by the TEM image (Fig. 1C). Furthermore, HRTEM image (Fig. 1D) reveals the uniform interplanar spacing with a value of 0.23 nm, which is in agreement with the (111) lattice spacing of face-centered-cubic (fcc) Pt [10,20,32]. The corresponding SAED pattern reflects the polycrystallinity of the flower-like Pt nanoarrays (inset in Fig. 1C). Fig. 2 shows the XRD patterns of the typical flower-like Pt arrays, where the characteristic diffraction peaks at 39.8◦ , 46.2◦ , 67.5◦ , 81.2◦ , and 85.7◦ are well matched with the (111), (200), (220), (311), and (222) planes of Pt (JCPDS card No. 65-2868), respectively. Meanwhile, no peaks of impurities are detected, suggesting high purity of the electrodeposited Pt nanoflowers, as again demonstrated by the EDX spectrum (Fig. 2B). Moreover, the diffraction peak of the (111) planes is stronger and sharper, compared with those of the (200), (220), (311), and (222) planes, revealing the preferential growth along the (111) directions. In order to investigate the electrochemical behaviors of the flower-like Pt nanoarrays, the cyclic voltammograms (CVs) of the bare (Fig. 3, curve a) and flower-like Pt arrays (Fig. 3, curve b) modified electrodes are recorded in 0.5 M H2 SO4 at a scan rate of 50 mV s−1 . There are three distinguished H adsorption-desorption peaks at −0.30 V, −0.19 V, and −0.10 V for the flower-like Pt arrays, respectively. And the reduction peak of the flower-like Pt arrays is detected at 0.48 V. Further augmenting the potential, a wide plateau is observed, which is assigned to the oxidation of the Pt

Fig. 1. Typical SEM images of the flower-like Pt arrays with low (A) and high (B) magnification. TEM image of the Pt nanoflowers (C). HRTEM image (D) of the tips from the marked sheet. Inset shows the corresponding SAED pattern.

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Fig. 2. X-ray diffraction (XRD) patterns (A) and EDX spectrum (B) of the flower-like Pt arrays. The asterisk represents the XRD patterns of the GCE.

nanoflowers. Similar observations are found for the monodispersed Pt nanoparticles under the same conditions [1], whereas no characteristic peak is detected for the bare GCE (Fig. 3, curve a). 3.2. Effects of the experimental parameters To illustrate the growth mechanism of the flower-like Pt arrays, control experiments were firstly performed by adjusting the applied potential at room temperature (Fig. S1, Supporting Information). When the potential is 0 V, monodispersed bumpy quasi-spherical Pt nanostructures are formed with an average diameter of 1.4 ␮m (Fig. S1A). Decreasing the applied potential to −0.2 V yields monodispersed quasi-spherical Pt nanoflowers composed of many nanosheets as subunits, with a mean diameter of 450 nm and increased density (Fig. S1B), accompanied with the thickness decrease of the nanosheets, compared with those prepared under the standard conditions with the applied potential of −0.3 V (Fig. 1A). Surprisingly, using the potential of −0.4 V produces monodispersed bumpy quasi-spherical Pt nanostructures again with the smaller diameter of 350 nm (Fig. S1C). When the potential further drops to −0.5 V, large-scale nanorices are generated on the GCE surface (Fig. S1D), with smaller size (i.e. 100 nm in diameter and 200 nm in length). Control experiments were carried out by changing the deposition time (Fig. 4), while the other conditions are remained unchanged. The products are mainly composed of uniform Pt branched particles with an average diameter of 200 nm when the electrodeposition time is 60 s (Fig. 4A). Using the electrodeposition time of 300 s, a large number of immature Pt nanoflowers are

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3.3. Formation mechanism of the flower-like Pt arrays

a

Taken together, the formation of flower-like Pt arrays in urea solutions can be explained by initial nuclei and subsequent anisotropic growth as a two-staged growth model, with the assistance of urea. Moreover, the number of Pt nuclei formed on the electrode surface is closely associated with the initial H2 PtCl6 concentration based on Scharifker–Hills model at the initial stage [33]. In this study, a large amount of PtCl6 2− is quickly reduced to Pt atoms at the very early stage, which preferentially group together with the adjacent ones, leading to random formation of many tiny Pt nuclei with well distribution on the electrode surface, because forming a nucleus with many atoms is energetically and kinetically unfavorable [34]. Meanwhile, the newly generated Pt atoms are preferred to grow on the original Pt nuclei instead of heterogeneous substrates. The adjacent urea molecules are rapidly and selectively

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generated with the size of 300 nm, containing many nanosheets as building blocks, with the thickness of ca. 70 nm (Fig. 4B). Furthermore, longer electrodeposition time such as 900 s (Fig. 4C) and 1200 s (Fig. 4D) produces much more complicated flowers with larger size, accompanied with the thickness increase of the nanosheets in the flower-like structures. Evidently, the crystal morphologies grow more and more complex and plentiful by extending the electrodeposition time, suggesting that the as-prepared superstructures stay at the evolutional stage of growth. Interestingly, the amount of urea plays a key role in control synthesis of the Pt nanocrystals (Fig. S2, Supporting Information). As illustrated in Fig. S2A, the product contains numerous Pt particles with broad size distribution in the absence of urea. When the concentration of urea is decreased from 150 to 50 mM, the product is mainly composed of many irregular Pt urchin-like microstructures with many sharp pricks outward (Fig. S2B). On the other hand, increasing the urea concentrations to 250 mM, many bumpy-like Pt particles with broad size distribution are emerged (Fig. S2 C). Therefore, the urea concentration has a critical impact on the growth of the Pt deposits. Additionally, the control experiments about the supply of H2 PtCl6 as a precursor are found to be essential for preparation of Pt nanocrystals(Fig. S3, Supporting Information). Lower H2 PtCl6 concentrations such as 1 mM (Fig. S3A) and 3 mM (Fig. S3B) increase the density of the flower-like superstructures with smaller size and less nanosheets as subunits. However, the size of the Pt nanoflowers is enlarged as the concentrations of H2 PtCl6 increased to 10 mM (Fig. S3C) or 15 mM (Fig. S3D), and the number of the nanosheets as subunits is increased, accompanied with the increase of the pricks on the Pt flowers.

0.9

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Fig. 3. Cyclic voltammograms of the bare (curve a) and flower-like Pt arrays (curve b) modified electrodes in 0.5 M H2 SO4 at 50 mV s−1 .

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Fig. 4. SEM images of the Pt deposits obtained at −0.3 V with different electrodeposition time: 60 s (A), 300 s (B), 900 s (C), and 1200 s (D).

adsorbed on the specific Pt crystal planes via self-assembly. The adsorbed urea prevents the newly generated Pt atoms from further aggregation, resulted into the emergence of flower-like Pt nanostructures. Evidently, the nucleation and adsorption of urea are fast processes, and hence the morphology adjustment is occurred at the later anisotropic growth stage dominated by the slow kinetic model. The corresponding chronoamperograms provide strong evidence on this assumption, where the reduced currents exponentially decrease at the initial stage (Fig. 5) by using the applied potential in the negative potential window, followed by increasing the currents and then gradually reaching a plateau. The chronoamperogram recorded at −0.3 V reveals the characteristic electrodeposition behavior: initial instantaneous nucleation confirmed by rapidly decreased currents, and subsequent progressive growth supported by gradually increasing the current, verifying the formation of the Pt nanoflowers with high quality. This trend is reversed in the case of 0 V (Fig. 5), where the current

Instantaneous nucleation process

0V -0.2V -0.3V -0.4V -0.5V

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Current / mA

Progressive grow process

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is exponentially increase at the very early stage, and then slowly reaches a plateau. These observations provide strong evidence in the synthesis of the Pt deposits with decreased size by negatively shifting the potential from 0 to −0.5 V (Fig. S1). 3.4. Electrocatalytic performance The flower-like Pt arrays with “clean” surfaces could be directly used in catalysis, because urea as a growth directing agent is easily removed from the reaction media, while no any surfactant is involved in the synthetic process. Fig. 6 shows the CO stripping cyclic voltammograms (CVs) of the flower-like Pt arrays (A) and Pt nanoparticles (B) modified electrodes in 0.5 M H2 SO4 , respectively. The experiments are carried out by bubbling with N2 (Fig. 6, curve a) and high purity CO (Fig. 6, curve b) for 30 min. The stripping peak area on the flower-like Pt arrays is obviously larger than that on the Pt nanoparticles, demonstrating more efficient active sites present on the flower-like Pt arrays. The electrochemically active surface area (EASA) can be estimated based on the CO stripping measurements [35]. The EASA is 11.72 m2 g−1 for the flower-like Pt arrays, which is almost two-fold larger than the Pt nanoparticles with a value of 5.05 m2 g−1 , showing the enhanced catalytic activity in theory that is associated with larger EASA under the same conditions [35]. The CVs of the flower-like Pt nanoarrays (curve a), Pt nanoparticles (curve b), and commercial Pt black (curve c) catalysts modified electrodes were recorded in 0.5 M H2 SO4 containing 0.25 M EG (Fig. 7A) or 1.0 M methanol (Fig. 7B). In the presence of 0.25 M EG, two obvious oxidation peaks are emerged at 0.29 and 0.61 V during the forward sweep on the flower-like Pt arrays modified electrode, which are ascribed to the oxidation of EG to Pt-adsorbed carbonaceous intermediates (such as CO and CO2 ) [3,12], followed by the emergence of another oxidation peak centered at 0.32 V during the reverse scan, which is assigned to the secondary oxidation of the carbonaceous intermediates into CO2 . Similar observations are found on the Pt nanoparticles and commercial Pt black catalysts modified electrodes, except smaller oxidation peak

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Fig. 6. CO stripping voltammograms of the flower-like Pt arrays (A) and Pt nanoparticles (B) modified electrodes in 0.5 M H2 SO4 at 50 mV s−1 with N2 and with CO for 30 min. Curve a, the first cycle; curve b, the second cycle.

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Potential / V Fig. 7. Cyclic voltammograms of the flower-like Pt arrays (curve a), Pt nanoparticles (curve b), and commercial Pt black (curve c) catalysts modified electrodes in the presence of 0.25 M EG (A) and 1.0 M methanol (B) in 0.5 M H2 SO4 at 50 mV s−1 , respectively.

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Fig. 8. Chronoamperometric curves of the flower-like Pt arrays (curve a), Pt nanoparticles (curve b), and commercial Pt black (curve c) catalysts modified electrodes in the presence of 0.25 M EG at 0.60 V (A) and 1.0 M methanol at 0.65 V (B) in 0.5 M H2 SO4 .

current densities. It indicates that the flower-like Pt arrays have an improved catalytic activity under the same conditions. The ratio between the oxidation peak current during the forward scan (jf ) and the peak current of the reverse scan (jr ) is a crucial index denoting the CO poisoning tolerance of electrocatalysts [3,12,20]. The ratio of the jf /jr is estimated to be 2.04 for the flower-like Pt arrays, which is larger than that of the Pt nanoparticles (1.75) and commercial Pt black (1.30) catalysts, verifying better electrocatalytic performance of the flower-like Pt arrays, together with less accumulation of carbonaceous intermediates and higher tolerance toward CO poisoning. Similar trend is observed by using 1.0 M methanol (Fig. 7B) instead of 0.25 M EG, except the disappearance of first oxidation peak during the forward sweep. These results demonstrate the improved catalytic activity of the flower-like Pt arrays for EG and methanol oxidation in acid media, compared with the Pt nanoparticles and commercial Pt black catalysts under the same conditions. The electrocatalytic stability of the flower-like Pt arrays (curve a), Pt nanoparticles (curve b) and commercial Pt black (curve c) catalysts modified electrodes were investigated by chronoamperometry in 0.5 M H2 SO4 containing 0.25 M EG at 0.60 V (Fig. 8A) and 1.0 M methanol at 0.65 V (Fig. 8B), respectively. As shown in Fig. 8A, the decay of the current densities on the flower-like Pt nanoarrays (curve a) is much slower than the Pt nanoparticles (curve b) and commercial Pt black (curve c) catalysts. By extending the time to 5000 s, the current densities are 0.63 mA cm−2 for the flower-like Pt nanoarrays, 0.33 mA cm−2 for Pt nanoparticles, and 0. 06 mA cm−2 for commercial Pt black catalysts, respectively. The initial currents of Pt nanocrystals drop very fast during EG oxidation, possibly due to the formation of the intermediate species. Similar results are observed in the presence of 1.0 M methanol at 0.65 V (Fig. 8B), rather than EG. These results confirm that the flower-like Pt arrays have better tolerance and stability toward EG and methanol oxidation in acid media, compared with Pt nanoparticles and

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commercial Pt black catalysts, owing to special structures of the flower-like Pt arrays. 4. Conclusions A simple and controllable method was developed for electrodeposition of well-dispersed flower-like Pt arrays on the GCE, with the assistance of urea as a growth directing agent, while no seed, template, or surfactant was involved. The obtained flower-like Pt arrays possess a larger electroactive surface area and show the enhanced electrocatalytic activity for the oxidation of EG and methanol, compared with the Pt nanoparticles and the commercial Pt black catalysts. This simple strategy demonstrates a promising way to fabricate other noble metal electrocatalysts with good performance in fuel cells. Acknowledgments This work has been financially supported by the NSFC (Nos. 21175118, 21275130 and 21375066) and Zhejiang province university young academic leaders of academic climbing project (No. pd2013055). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2014.01.054. References [1] J.J. Feng, A.Q. Li, A.J. Wang, Z. Lei, J.R. Chen, Electrodeposition of monodispersed platinum nanoparticles on a glassy carbon electrode for sensing methanol, Microchimica Acta 173 (2011) 383. [2] J.N. Tiwari, F.M. Pan, K.L. Lin, Facile approach to the synthesis of 3D platinum nanoflowers and their electrochemical characteristics, New Journal of Chemistry 33 (2009) 1482. [3] L. Wang, S. Guo, J. Zhai, S. Dong, Facile synthesis of platinum nanoelectrocatalyst with urchinlike morphology, Journal of Physical Chemistry C 112 (2008) 13372. [4] J. Yin, J. Wang, M. Li, C. Jin, T. Zhang, Iodine ions mediated formation of monomorphic single-crystalline platinum nanoflowers, Chemistry of Materials 24 (2012) 2645. [5] T.-L. Hsieh, H.-W. Chen, C.-W. Kung, C.-C. Wang, R. Vittal, K.-C. Ho, A highly efficient dye-sensitized solar cell with a platinum nanoflowers counter electrode, Journal of Materials Chemistry 22 (2012) 5550. [6] Z.D. Pozun, S.E. Rodenbusch, E. Keller, K. Tran, W. Tang, K.J. Stevenson, G. Henkelman, A systematic investigation of p-nitrophenol reduction by bimetallic dendrimer encapsulated nanoparticles, Journal of Physical Chemistry C 117 (2013) 7598. [7] X. Chen, W. Zang, K. Vimalanathan, K.S. Iyer, C.L. Raston, A versatile approach for decorating 2D nanomaterials with Pd or Pt nanoparticles, Chemical Communications 49 (2013) 1160. [8] T. Zhu, E.J. Hensen, R.A. Van Santen, N. Tian, S.-G. Sun, P. Kaghazchi, T. Jacob, Roughening of Pt nanoparticles induced by surface-oxide formation, Physical Chemistry Chemical Physics 15 (2013) 2268. [9] S. Adora, Y. Soldo-Olivier, R. Faure, R. Durand, E. Dartyge, F. Baudelet, Electrochemical preparation of platinum nanocrystallites on activated carbon studied by X-ray absorption spectroscopy, The Journal of Physical Chemistry B 105 (2001) 10489. [10] L. Wang, M. Imura, Y. Yamauchi, Tailored design of architecturally controlled Pt nanoparticles with huge surface areas toward superior unsupported Pt electrocatalysts, ACS Applied Materials & Interfaces 4 (2012) 2865.

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