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One pot synthesis of chain-like palladium nanocubes and their enhanced electrocatalytic activity for fuel-cell applications

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One pot synthesis of chain-like palladium nanocubes and their enhanced electrocatalytic activity for fuel-cell applications

  1. 1. Nano Energy (2013) 2, 677–687 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/nanoenergy RAPID COMMUNICATION One-pot synthesis of chain-like palladium nanocubes and their enhanced electrocatalytic activity for fuel-cell applications Palanisamy Kannana,n, Thandavarayan Maiyalaganb,nn, Marcin Opalloa a Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 ul. Kasprzaka, 01-224 Warszawa, Poland Materials Science and Engineering Program, The University of Texas at Austin, Austin, TX, USA b Received 7 May 2013; received in revised form 10 August 2013; accepted 11 August 2013 Available online 23 August 2013 KEYWORDS Abstract Chain-like nanoparticles; Cubic shape; Formic acid; Methanol and ethanol A simple approach has been developed for the synthesis of anisotropic cubic chain-like Pd nanostructures in an aqueous medium. The cubic chain-like Pd nanostructures show better performance toward oxidation of formic acid and methanol. The cubic chain-like Pd nanostructures show $ 11.5 times more activity on the basis of an equivalent noble metal mass for the formic acid and methanol than the spherical shaped Pd nanoparticles and commercial Pd/C catalysts. Further, the superior electrocatalytic performance of the present anisotropic cubic chain-like Pd nanostructures toward the oxidation of alcohols makes them excellent candidates as high performance multipurpose catalysts for direct formic acid fuel cells (DFAFC), direct methanol fuel cells (DMFC), direct ethanol fuel cells (DEFC), respectively. & 2013 Elsevier Ltd. All rights reserved. Introduction Controlling the morphology of noble metal nanocrystals has been one of the most recent and attractive research topics n Corresponding author. Tel.: +48 223 433 375; fax: +48 223 433 333. nn Corresponding author. Tel.: +1 512 772 3084 E-mail addresses: ktpkannan@gmail.com (P. Kannan), maiyalagan@gmail.com (T. Maiyalagan). 2211-2855/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2013.08.001 for the past few decades, especially in the field of catalysis, because the catalytic activity and stability of catalysts are strongly correlated with the size and shape of the nanocrystals in a variety of chemical reactions [1–3]. It has been demonstrated that the intrinsic properties of nanostructured material can be dramatically enhanced by shape and structural variation [4–6]. The one dimensional metal nanostructures like nanowires, nanobelts, and nanotubes have attracted significant interest due to their exotic technological applications [7–10]. The surface roughness and inherent high
  2. 2. 678 index facets of anisotropic metal nanoparticles stimulate impressive applications in electro-catalysis, sensors, SERS and many more [11–15]. At present, Pt is still the most commonly used electrocatalytic material for fuel cells in terms of both activity and stability. However, the high cost of Pt is one of the most important barriers that limits the large-scale commercialization of fuel cells [16]. Therefore, economical and effective alternative catalysts are required. Among the metal based catalysts studied to date, Palladium (Pd) is considerably less expensive than Pt, and Pd has been focusing on recent research as an attractive and alternative catalysts nanomaterial for a wide range of applications in catalysis, hydrogen storage and biosensing, reduction of automobile pollutants and so forth [17–20]. It has been found that upon appropriate modification of their surface atomic structure, Pd-based nanomaterials can become promising electrocatalysts by simultaneously decreasing the material cost and enhancing the performance [20,21]. Recently, Pd nanoparticles have been accepted as a better alternative material for polymer electrolyte membrane fuel cells towards catalytic oxidation of formic acid as well as oxygen reduction in a proton-exchange membrane (PEM) fuel cell [22,23]. Furthermore, Pd nanoparticles have been chosen as the best alternative material for solving many industrial process problems and demand the exploration of a new synthetic approach for production of well-defined shapes and structures. Attempts have been made to develop anisotropic Pd nanostructures mainly using surfactants and polymers or high temperature reactions in organic medium [24–31]. Although, these methods were successful in producing well-defined Pd nanostructures, the strong adsorption of these protecting agents resulted in a decrease of catalytic activity [22]. Nevertheless the aqueous medium based synthesis of metal nanoparticles is more promising from an environmental standpoint, adding an advantage over the use of toxic organic solvents [32–34]. Moreover these protocols are limited to producing nanoparticles with a smooth surface morphology. However, it has been observed that the interesting nanoarchitectures with high surface roughness and surface steps can contribute to the increased accessibility of reactant species and are more attractive for enhancing catalytic application [35]. Therefore, it is highly desirable to explore a facile approach to produce nanocatalysts with high surface areas and high degree of structural anisotropy for improved performance in technological applications. Herein, we explore a new approach for the synthesis of chain-like Pd (branches in a tree) nanostructures (PdCNs) with a cubic morphology in an aqueous medium. Here we use the neurotransmitter 5-hydroxytryptamine (HT) to address the environment-friendly and green perspectives. A notorious problem in the shape-controlled synthesis of metal nanoparticles is the shape heterogeneity of the synthesized product. This report clearly demonstrates the formation of uniform PdCNs with a unique morphology. To the best of our knowledge, this is the first report that describes rapid eco-friendly synthesis of cubic chain-like Pd nanostructures without any template, polymer, surfactant or without any heat treatment procedures. The cyclic voltammetry and chronoamperometry are employed to investigate the morpho-dependent electrocatalytic activity of these chain-like cubic Pd nanostructures toward the oxidations of formic acid, methanol and ethanol. P. Kannan et al. Experimental section Materials PdCl2 and 5-hydroxytryptamine were obtained from SigmaAldrich Chemical Company. The phosphate buffer solution (PBS; pH= 7.2) was prepared using Na2HPO4 and NaH2PO4. Double distilled water was used to prepare the solutions in this investigation. All other chemicals used in this investigation were of analytical grade. All the solutions were prepared with deionised water (18 mΩ) obtained from Millipore system. Synthesis of cubic chain-like Pd nanostructures Glasswares used for synthesis were well cleaned with freshly prepared aqua regia (3:1 HCl and HNO3), then rinsed thoroughly with water and dried prior to use (Caution! aqua regia is a powerful oxidizing agent and it should be handled with extreme care). In a typical synthesis, 10 ml aqueous solution of PdCl2 (0.1 mM) was taken in a beaker and then 0.1 ml of 5-hydroxytryptamine (5-HT) (10 mM) was rapidly injected into the solution, allowed for 30 min in static state. The resulting nanocolloid was stored at 4 1C for further use. Characterization The morphologies of the cubic chain-like, dendrites and fractal Pd nanostructures were characterized by a field emission-scanning electron microscopy (FE-SEM JEOL JSM6301F). The specimens were prepared by dropping 3 μl of colloidal solution onto silicon wafer substrate. The crystallographic information of the prepared Pd chain-like nanostructures were studied by the powder X-ray diffraction technique (XRD, Shimadzu XRD-6000, Ni filtered CuKα (λ = 1.54 Å) radiation operating at 30 kV/40 mA). Electrochemical measurements Electrochemical measurements were performed using two compartment three-electrode cell with a glassy carbon working electrode (geometric area = 0.07 cm2), a Pt wire auxiliary electrode and Ag/AgCl (3 M KCl) as reference electrode. Cyclic voltammograms were recorded using a computer controlled CHI643B electrochemical analyzer. All the electrochemical experiments are carried out in an argon atmosphere. The 3 μl of as-synthesized chain-like nanostructures are dispersed over glassy carbon electrode with nafion and dried prior to electrochemical experiments. Results and discussion The cubic chain-like Pd nanostructures have been characterized to establish their nanostructure and mechanism of shape evolution. Figure 1 shows the FE-SEM images obtained for the nanoparticle synthesized using 5-HT. The FE-SEM image shows that Pd nanoparticles were cubic morphology with chain like-structures (branches in a tree; Figure 1a–d). The cubic chain-like Pd nanocubes have an average size between 140 Â 160 nm and 140 Â 210 nm. Furthermore, we examine the stability of cubic chain like-structures PdCNs
  3. 3. One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes 679 Figure 1 FE-SEM images of (a) simple branched chain-like cubic Pd nanostructures using 5-HT (0.1 mM), (b) a portion of image “a”, (c) a portion of image “b” and (d) a portion of image “c”. prepared at a conc. of 0.1 mM 5-HT, the FE-SEM images were taken after 1 and 5 h time interval and presented in Figure 2a–c. From the FE-SEM measurements the density, length and number of cubic branches per nanochains were almost same over the growth period up to 5 h (Figure 2a and b). These time-dependent features can be ascribed either to the aggregation of nanoparticles or the growth of anisotropic nanostructures. The morphology of cubic chain-like Pd nanostructures remained the same even after 5 h, implying that the observed feature is not due to the aggregation of nanoparticles. The chain-like Pd nanocubes grow up to the size of $80 mm with more than 25 branches in 5 h (Figure 2d). The observed results suggest that the cubic chain-like morphology is highly stable by varying the time intervals (both 30 min, 1 and 5 h). To the best of our knowledge, this is the first report that describes rapid eco-friendly synthesis of cubic chain-like Pd nanostructures (branches in a tree) at room temperature without any heat treatment, template, polymer, or surfactant. The overview of the mechanism for formation of such a unique shape can be outlined in Scheme 1 as: (i) in the initial stage of reaction, Pd2 + ions are reduced by 5-HT to form nano-sized small Pd cubic shaped nanoparticles (around 5–10 min), (ii) a homogeneous nucleation of these cubic nanoparticles forms small, primary Pd cubic nano-chains by gradual assembly of these particles that grows into a one-dimensional interconnected nanostructure and a complete growth of cubic chainlike nanostructures was observed after 30 min of reaction (iii) these primary particles undergo nucleation/assemble due course over time into a 1-D nanostructure and reconstruction of these small nanocubes into cubic dendritic nanowires [36,37]. We propose that, 5-HT molecule (stabilising/reducing agent) plays a vital role on selective binding on the surface of 1-D nanostructure and reconstruct the tiny nanocubes into dendritic-like nanochains. The growth of nanostructured particles is highly perceptive to the concentration of 5-HT and precursor, Pd2 + (Figure 3). Here we demonstrate that cubic chain-like Pd nanostructures, cubic small dendrites of Pd nanostructures, cubic big-dendrites of Pd nanostructures and fractal Pd nanostructures could be selectively prepared by manipulating and controlling the concentration of 5-HT and Pd2 + (Figures 4, 5 and Figure S1). It should be noted here that this procedure establishes a universal protocol for synthesis of monodisperse Pd nanostructures with a variety of shapes without adopting different strategies, reaction conditions, or injection of foreign reagents. Minor change in the concentrations of stabilizer and a fixed concentration of precursor or vice versa results in the formation of different morphologies of the nanoparticles assembly (Figure 3). Recently, Willner and co-workers have reported the cataecholamine neurotransmitters mediated growth of Au nanoparticles and their quantification by optical method [38]. The catacholamine neurotransmitters and their metabolites induce the growth of spherical shape nanoparticles, and they do not induce the formation of any anisotropic nanoparticles [38]. In the present investigation, we observed cubic Pd nanochains, cubic nanostructures of big-dendrites, and fractal nanostructures by the indoleamine neurotransmitter. The formation of chain-like nanostructured particles is attributed to the oxidation of 5-HT by PdCl2. As shown in Scheme 2, reduction of PdCl2 occurs due to the transfer of electrons from HT to the metal ion, resulting in the formation of Pd0. The metallic Pd then undergoes nucleation and growth with time to form cubic chain-like Pd nanostructures, cubic nanostructures of dendrites and fractal nanostructures. It is well documented in the
  4. 4. 680 P. Kannan et al. Figure 2 FE-SEM images of simple branched chain-like cubic Pd nanostructures using 5-HT (0.1 mM) after 1 h (a), 5 h (b) and close view of image “b”. The image “c” represents the length of the branches in a sample. The image “d” represents maximum length of chain-like nanocubic particles. Nucleation 5-HT Pd cubes Self-assembly Primary cubic Pd chain-like nanostructure Reconstruction Scheme 1 Schematic presentation showing the formation/ growth process of cubic chain-like Pd nanostructures and dendritic nanochains. literature that the oxidation of 5-hydroxyindole generates a free radical [39–43]. The radical generated by the oxidation of HT can undergo a chemical reaction in aqueous solution to yield different products. The reaction of radical strongly depends on the experimental condition. The hydroxylated dimers (5,5'-Dihydroxy-4,4'-bitryptamine (DHB)) and tryptamine-4,5-dione (TAD) (Scheme 2) are the major products in neutral and acidic pH. These oxidation products are expected to adsorb on the surface of the nanoparticles (vide infra) by different orientations, resulted that different cubic chain-like Pd nanostructures. In Scheme 2, “a” and “b” representing the reversible redox reaction, these oxidized products were expected to adsorb on the surface of the nanoparticles through amino groups (–NH2), resulted the Pd nanoparticles. It is expected that the hydrogen bonding between above two compounds with PdNPs (cellulose type hydrogen bonding), resulting that chain-like nanostructures. We suggest that the balance act between the concentration of precursor and stabilizer determine their shape evolution process. The effect of Pd2 + concentration can be speculated as at high concentration, Pd2 + ions are floating around to seed the growth of the Pd nanocubes and speed up the nucleation to form compact large nanocubes. On the other hand at lower concentration the Pd2 + ions are diluted enough to decrease the seeding and small cubic chain-like nanostructures were formed. Those small nanocubes undergo controlled nucleation to form the dendritic morphology. When the concentration of 5-HT is increased, Pd nanostructures with a big dendritic and fractal shaped morphology were obtained. This may be due to the high concentration of 5-HT (reducing/stabilising agents) which not only expedites the reduction of Pd2 + to
  5. 5. One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes 681 HT (0.1 mM) Pd (0.1 mM) in 10 mL HT (0.3 mM) + 5-Hydroxytryptamine (HT) HT (0.5 mM) HT (0.75 mM) Figure 3 FE-SEM measurements of different morphologies of Pd nanoparticles induced at different concentrations of 5-HT and Pd(II). Figure 4 FE-SEM images of 5-HT-induced (0.3 mM) formation of cubic chain-like small dentries of Pd nanostructures.
  6. 6. 682 P. Kannan et al. Figure 5 FE-SEM images of 5-HT-induced (0.5 mM) formation of cubic chain-like big-dendrites of Pd nanostructures. Pd0, but also controls the nucleation process. Our examination concludes that the optimal metal precursor concentration for the preparation of Pd nanoparticles with uniform distribution of nanocubes was of 0.5 mM. When the concentration goes up to 0.75 mM, the chain-like cubic nanoparticles were aggregated. The sufficient numbers of 5-HT molecules present around the reaction direct the nucleation/assembly of the reduced primary Pd cubic nanoparticles in different arrays or unique direction to produce cubic chain-like Pd dendritic nanostructures, nanostructures of small, big-dendrites, and multiple chain-like fractal shapes. So it can be speculated that the concentration of 5-HT not only stabilizes the nano-sized particles but its structure plays a vital role in controlling their nucleation or growth process in a universal pattern to produce cubic chain-like Pd nanostructures, cubic chain-like dendritic or fractal shapes. It is well established that using surfactants or polymers with different functional groups having different binding strategies such as acids and amines regulate the morphology of nanopararticles [44]. The formation of different shapes and morphology is controlled by the faceting tendency of the stabilizer and the growth kinetics to crystallographic planes [6,29,30]. We anticipate here that the structure and functional groups of reducing/stabilizing agents play a vital role in shape and morphology evolution of nanoparticles. Our efforts are yet underway to establish the structure– function relationship of the stabilizer for shape evolution of Pd nanoparticles using the bi-products of 5-HT (DHB and TAD). We propose a reasonable explanation towards the balancing act between the concentration of precursor and stabilizer for shape evolution of Pd nanostructures in Schemes 1 and 2. The X-ray diffraction (XRD) patterns were recorded to confirm the lattice facets of cubic chain-like Pd nanostructures as shown in Figure 6. Peaks corresponding to Pd (111), (200), (220) and (311) were obtained. The relative peak intensities were compared using the peak area of (111) as a reference (JCPDS card number: 41-1021). The ratio of the relative peak intensity of (200) with respect to (111) is found to be 0.52 versus 0.6 of the standard value. However, the ratio of the relative peak intensities of high index planes (220) and (311) are higher than the standard values previously reported (0.73 versus 0.42) and (0.71 versus 0.55), respectively [17,45,46]. This observation reveals that the cubic chain-like Pd nanostructures were abundant in high index facets. Further, XRD patterns for cubic small dendrites Pd nanostructures, cubic chain-like big-dendrites Pd nanostructures and fractal nanostructures of Pd nanostructures were recorded to confirm their lattice facets (Figure 6 and Figure S2). For instance, the ratio of the relative peak intensities of high index planes (220) and (311) is 0.61 versus 0.42 and 0.65 versus 0.55 respectively for cubic small dendrites Pd nanostructures, which is lower than the values obtained for cubic chain-like Pd nanostructures (0.73 versus 0.42) and (0.71 versus 0.55). Moreover, the formations of Pd nanoparticles were also investigated in the presence of 5-hydroxyindoleacetic acid and N-aceltyserotonin, that are functionally correlated with respect to 5-HT. In similar conditions, both these structurally different molecules induce the formation of Pd nanostructures but the morphology differs (Figure S3). We anticipate here that the structure and functional groups of reducing/stabilizing agents affect the shape and morphological evolution of nanoparticles.
  7. 7. One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes 683 OH Pd2+ HN 5-hydroxytryptamine NH2 -H + O* Pd0 HN O H N 2 H HN N OH OH OH 2e2H+ b NH N O H N HO HN 2e2H+ PdNPs O N H OH N H N a HN N NH2 OH NH2 NH N PdNPs O HO NH NH NH2 HO OH N O NH2 O HO N H NH2 NH2 tryptamine-4,5-dione (TAD) N 5,5'-Dihydroxy-4,4'-bitryptamine (DHB) ‘a’ and ‘b’ represent the reversible redox reaction. Scheme 2 Mechanism for 5-HT induced formation of Pd nanostructures with different branches with cubic and fractal shapes. (111) (200) 20 40 (220) 60 (311) 80 2 Theta (Degree) Intensity (a.u.) Intensity (a.u.) (111) (200) (220) (311) 20 40 60 80 2 Theta (Degree) Figure 6 XRD pattern of as synthesized cubic chain-like Pd nanostructures (a) and cubic chain-like small dendrite of Pd nanostructures (b). Pd is well-known to absorb massive quantities of hydrogen in bulk to form hydrides and therefore has a great technological potential as a hydrogen storage material [47]. We have carried out cyclic voltammetric studies in acidic medium to examine the affinity of hydrogen toward all cubic chain-like Pd nanostructures. Figure 7 shows the voltammetric behavior of the cubic chain-like Pd nanostructures modified electrode in 0.5 M HClO4 at a scan rate of 50 mVsÀ1. The cyclic voltammogram (CVs) obtained for cubic chain-like Pd nanostructures in 0.5 M HClO4 shows sharp distinguished anodic peaks in between À0.25 and –0.15 V. This broad peak is due to the oxidation of both the absorbed and adsorbed hydrogen on the cubic chain-like Pd nanostructures and has been earlier reported for bulk Pd electrodes [48]. However, the CV for the cubic chain-like Pd nanostructures coated disk electrode shows two distinct
  8. 8. 684 P. Kannan et al. d Current Density (mA/cm2) Current Density (mA/cm2) 0.8 0.4 a 0 0.4 0.8 –0.5 0 0.5 Potential (V) vs. Ag/AgCl 1 0 a 0.05 d 0.5 Potential (V) vs. Ag/AgCl Figure 7 (A) CVs of fractal Pd nanostructures (a), cubic Pd big-dendrites (b), cubic chain-like small dendrites of Pd nanostructures (c) and cubic chain-like Pd branched nanostructures in N2-saturated 0.5 M HClO4 solution at a scan rate of 50 mV sÀ1. voltammetric peaks, one of which is a small shoulder peak in the negative potential region during the anodic sweep [48–51]. The first peak at À0.23 V corresponds to the oxidation of the adsorbed hydrogen (Had), while the subsequent peak at À0.18 V is due to the oxidation of the absorbed hydrogen (Hab) on Pd surface [49]. The cathodic peak during the reverse scan is due to both absorption and adsorption of hydrogen. It is known from the literature that the two wellresolved peaks for the oxidation of adsorbed and absorbed hydrogen in Pd are manifested exclusively in the case of Pd nanoparticles. This behavior is attributed to the larger number of surface sites available for adsorption on the nanoparticle's surface [48–51]. The large anodic peak corresponding to the desorption of hydrogen in the case of cubic chain-like Pd nanostructures modified electrode shows its potential as an excellent hydrogen storage material. The electroactive surface area of Pd has been measured from the palladium oxide stripping analysis [48]. The charge consumed during the reduction of Pd oxides has been estimated by integrating the area under the reduction wave. The electric charge of the hydrogen desorption of cubic chain-like Pd nanochains, small cubic nanostructures of dendrites, big-dendrites and fractal nanostructures were measured to be 132.4, 366.2, 509.4, and 784.2 μC (a–d) respectively. The electrochemically accessible area of cubic chain-like Pd nanostructures was calculated to be 1.476 cm2 using the reported value of 424 μC cmÀ2. This is a very large real surface area for the other chain-like Pd nanostructures such as cubic nanostructures of small dendrites, big-dendrites, and fractal nanostructures are 0.875 0.554, and 0.253 cmÀ2, respectively. This large surface area arises due to cubic chain-like Pd nanostructures in which the Pd nanocubes were highly dispersed making a branches-on a tree type of nanostructure. Since the Pd nanoparticles were directly modified on the electrode surface, the available effective active centers were also higher, which makes them an ideal electrocatalytic nanomaterial. Inspired by the attractive cubic chain-like structures and unique morphology of Pd nanostructures, we further proved these as nanoelectrocatalyst. Very recently Mohanty et al. observed a dramatic catalytic performance of dendritic noble metal nanoparticles towards the Suzuki–Miyaura and Heck coupling reaction [17]. Here we use methanol, ethanol and formic acid as model molecules for studying the electrocatalytic performance of cubic chain-like Pd nanostructures. The electrochemical oxidation of methanol and formic acid has attracted much attention due to their potential energy related applications for direct methanol fuel cells (DMFC) and direct formic acid fuel cells (DFAFC), respectively. It is generally accepted that noble Pt metal is the best catalyst for the formic acid oxidation but it suffers poisoning due to a strong adsorbed CO species generated as reaction intermediates [52]. A noble Pd-based catalyst was explored to be efficient and possesses superior performances for fuel cell applications because it is highly resistant to CO poisoning [22]. We have examined the electrocatalytic activity of as-synthesized chain-like cubic Pd nanostructures towards formic acid and methanol oxidation (Figure 8). Well-defined voltammograms with prominent peaks in the forward and reverse scans were obtained, respectively. The oxidation current has been normalized to the electroactive surface area of the cubic chain-like Pd nanostructures. This surface area was determined from the coulombic charge reduction of Pd oxide (vide supra), according to the reported value of 424 mC cmÀ2. As shown, the as-synthesized cubic chain-like Pd nanostructures of different branched morphologies exhibit different electrocatalytic activities. The oxidation of formic acid on Pt involves a dual path mechanism, a dehydrogenation path to the direct formation of CO2 and a dissociative adsorption to form poisoning CO species (a dehydration path) [53–55]. It can be seen that the formic acid electro-oxidation on these present cubic chainlike Pd nanostructures shows two anodic peaks in the forward scan. The onset potential and peak potential on cubic chain-like Pd nanostructures are À0.15 and 0.13 V, respectively (Figure 8A, curve b). The first small anodic peak is attributed to the direct oxidation of formic acid to CO2 on the remaining sites unblocked by intermediate species. At higher potentials, the current further increases and reaches another peak. The second peak is related to the oxidation of adsorbed intermediate species (CO), which releases the free surface sites for the subsequent direct oxidation of formic acid. In the reverse scan, the current reaches a high peak located at around 0.02 V, which is much higher than those in the positive scan, due to the bulk HCOOH oxidation and the absence of CO poison. However, with the potential scanning to more negative values, the
  9. 9. a b c d e f 10.0 8.0 6.0 Current Density (mA/cm2) Current Density (mA/cm2) One-pot synthesis and enhanced electrocatalytic activity of chain-like palladium nanocubes 4.0 2.0 0.0 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 685 a b c d e f 6.0 5.0 4.0 3.0 2.0 1.0 0.0 -0.1 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 Potential (V) vs. Ag/AgCl Potential (V) vs. Ag/AgCl surface was again poisoned by CO, resulting in the decrease of the oxidation current [56–59]. The oxidation of formic acid at cubic chain-like Pd nanostructures is higher than those on cubic chain-like small dendrites of Pd nanostructures (À0.10 and 0.16 V), cubic big-dendrites Pd nanostructures (À0.10 and 0.16 V) and fractal Pd nanostructures (À0.07 and 0.18 V), indicating that the formic acid is easier to oxidize on cubic chain-like Pd nanostructures. The peak current density for cubic chain-like Pd nanostructures is $ 760 mA/cm2, which is much higher than that for cubic small dendrites of Pd nanostructures ($ 480 mA), cubic big-dendrites Pd nanostructures ($460 mA) and fractal Pd nanostructures ($250 mA). This should be resulted from its highest ECSA. The specific surface activities of these catalysts could be obtained from the normalized peak current density with ECSA to further investigate the possibility of the enhancement effect of cubic branched nanostructures on the intrinsic electrocatalytic activity, which is 10.8, 6.8, 6.5 and 3.5 AmÀ2 for cubic chain-like Pd nanostructures, cubic small dendrites of Pd nanostructures, cubic big-dendrites Pd nanostructures and fractal Pd nanostructures, respectively. The low onset and oxidative peak potential value (less positive potential), much higher specific surface activities of the cubic chain-like Pd nanostructures catalysts towards formic acid oxidation may be attributed to the presence of branched (branches in a tree) morphology. On the other hand, the present Pd nanostructures also efficiently catalyze the oxidations of methanol (Figure 8B) and ethanol (Figure S4). We summarized the activity of Pd nanostructures in terms of oxidation potential and generated catalytic current-density and these data are provided in (Table S1). Further, we compared the electrocatalytic performance of the present branched cubic chain-like nanoparticles to commercial Pd/C and spherical PdNPs toward the oxidation of formic acid and methanol (Figures S5 and S6). The current densities were also normalized to the electroactive surface area, which were measured from the electric charge of hydrogen adsorption/desorption on Pd surfaces. Such normalization allowed the current density to apply directly for comparison of the catalytic activities of the different catalysts. The cubic chain-like Pd nanostructures show Z11.5 (comparison for both cases) times more activity on the basis of an equivalent noble metal mass for the formic acid and methanol than the spherical shaped PdNPs and Current Density (mA/cm2) Figure 8 Cyclic voltammograms show the oxidation of (A) formic acid (0.25 M) in 0.5 M HClO4 and (B) methanol (0.25 M) in 0.1 M KOH at (a) bare GC, (b) cubic chain-like Pd nanochains, (c) cubic chain-like Pd nanochains in the absence of both formic acid and methanol, (d) cubic small dendrites of Pd nanostructures, (e) cubic big-dendrites Pd nanostructures and (f) fractal Pd nanostructures modified electrodes. Scan rate: 50 mV sÀ1. 8.0 7.0 6.0 a 5.0 4.0 3.0 b c 2.0 1.0 0 d 0 2000 4000 6000 Time (Sec) Figure 9 Polarization current vs. time plots of branched cubic chain-like Pd nanostructures (a), cubic small dendrites of Pd nanostructures (b), cubic big-dendrites of Pd nanostructures (c) and fractal Pd nanostructures (d) modified electrodes measured in formic acid (0.25 M) in 0.5 M HClO4 solution. commercial Pd/C catalysts. This may be attributed to the high degree of structural anisotropy of cubic chain-like Pd nanostructures and branched tree-like morphology contributing to their superior electrocatalytic activities. The rich branched sharp edges and corner atoms of tree-like structures were highly preferred for improving the catalytic activity [60]. The long-term stability of these catalysts was evaluated by the chronoamperometry method and also the current decay, which is associated with the poisoning of intermediate species on the nanoparticles determining the fate of its practical application. The chronoamperometry profile is sufficiently stable for cubic chain-like Pd nanostructures and cubic nanostructures of dendrites morphologies. The chronoamperometric stabilities of cubic chain-like Pd nanostructures, cubic nanostructures of small dendrites, cubic big-dendrites and fractal nanostructures were investigated using towards formic acid oxidation and as shown in Figure 9. The polarization current for the formic acid oxidation reaction on these three supported Pd catalysts shows a significant decay initially and reaches a stable value after polarization at 0.20 V for $ 400–600 s. The current decay for the formic acid oxidation reaction indicates the slow deactivation of the nanostructued Pd-based electrocatalysts
  10. 10. 686 by the slow adsorption of CO or CO-like intermediates [61–63]. This is supported by the detection of the adsorption of CO species on Pd catalysts during formic acid oxidation by surface-enhanced infrared absorption spectroscopy [64]. However, the stable mass specific current for formic acid oxidation reaction on cubic chain-like Pd nanostructures is $540 mA, significantly higher than cubic nanostructures of small dendrites ($170 mA) and cubic big-dendrites ($90 mA). This indicates that cubic chain-like Pd nanostructures catalysts possess much better stability against the poisoning by adsorbed CO or CO-like intermediate species. The stability of these catalysts was also studied by the cyclic voltammetric method. The first peak currents at forward scan are recorded as a function of the cycle numbers, as shown in Figure S7. Using the peak current of the 10th cycle as the baseline, the peak current after 100 cycles on cubic chain-like Pd nanostructures is reduced by 29.6%, which is significantly lower than the reduction in activity on cubic nanostructures of small dendrites (38.5%) and cubic big-dendrites (57.2%) nanocatalysts, further indicating that cubic chain-like Pd nanostructures exhibit the highest catalytic stability and is in good agreement with the results of chronoamperometry curves. Therefore, the as-synthesized cubic chain-like Pd nanostructures were highly resistant to poisoning due to intermediates and were favorable for practical uses. We could also evidently argue that the presence of cubic chain-like Pd nanostructures significantly enhances the electrocalytic activity and diminishes the poisoning of the Pd catalysts for the highest stability. We anticipate that the cubic chain-like Pd nanostructures (Figure 1) expose a larger number of active sites for the adsorption of active oxygen atoms, which readily oxidizes the reaction intermediate species, protecting it from surface poisoning [65,66]. No change in the morphology of branched cubic chain-like Pd nanostructures was observed after several voltammetric cycles (Figure S8) and hence acclaims its robust nature. Our efforts are underway to modify these cubic chainlike Pd nanostructures on the graphene, carbon support and compare their enhanced electrocatalytic activity with the commercially available Pd/C. Conclusions In summary, we have explored a facile approach to tailor the shape and tree-like branched morphology of Pd nanostructures in aqueous medium. We propose that, 5-HT molecule (stabilising/reducing agent) plays a vital role on selective binding on the surface of 1-D nanostructure and reconstruct the tiny nanocubes into dendritic-like cubic nanochains. 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He received his Ph.D. in 2010, Gandhigram Rural University and completed the Post-Doctoral Program in School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore. His research interests are mainly on the development of novel nanomaterials for electrochemical biosensor and fuel cell applications. Thandavarayan Maiyalagan is currently working as Research Fellow in Texas Materials Institute, The University of Texas at Austin, United States. He received his Ph.D. in 2007, IIT Madras and completed the PostDoctoral Programs at New Castle University, United Kingdom and Nanyang Technological University, Singapore. His current research interest includes development of nanomaterials for energy conversion and storage devices, electro-catalysis, fuel cells and biosensors. Marcin Opallo is a Professor of Department of Electrode Process, Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland. He received his Ph.D. in 1987 completed the Post-Doctoral Programs at Tohoku University and University of California, Davis, United States. He has published more than 100 papers. Currently, his research focuses on developing new materials for energy conversion and sensing.

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