Vertically Aligned Double-Walled Platinum Nanotubes Decorated with

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Vertically Aligned Double-Walled Platinum Nanotubes Decorated with Inner Fibrils for Their Enhanced Electrocatalytic Properties Sang Min Kim, Lichun Liu, Dae Jin Kim, and Sungho Park J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07620 • Publication Date (Web): 15 Sep 2015 Downloaded from http://pubs.acs.org on September 17, 2015

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The Journal of Physical Chemistry

Vertically Aligned Double-Walled Platinum Nanotubes Decorated with Inner Fibrils for Their Enhanced Electrocatalytic Properties Sang Min Kim,† Lichun Liu, §,* Dae Jin Kim,‡ and Sungho Park †,‡,*

†Department of Energy Science and ‡Department of Chemistry, BK21 School of Chemical Materials Science, Sungkyunkwan University, Suwon, 440-746, South Korea § College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China

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ABSTRACT

Vertically aligned double-walled platinum nanotubes (Pt DNTs) with inner fibrils have been synthesized by means of homogenous coating of Pt onto a Pd thin-wall nanotube array template using electrochemical deposition, followed by chemical etching of the template. The resulting Pt DNTs with inner fibrils have advantageous morphological characteristics including high crystallinity of the coated Pt layers, high porosity of the inner nanotubes, the non-carbon supported substrate, and a three-dimensional orientation whereby the catalysts are directly and vertically connected with a conducting electrode. In addition, tube length can be controlled by tuning the total amount of charge injected during the electrochemical deposition. Electrochemical measurements have demonstrated that the resulting Pt DNTs have considerably greater electrochemically active surface area (ECSA) and higher electrocatalytic activity toward methanol oxidation and CO oxidation behavior compared to Pt single nanotube and Pt nanorod structures as well as other reported Pt-based catalysts under similar testing conditions. These results suggest that Pt DNTs are excellent nanocatalytic systems for low-temperature catalytic reactions and surface chemical processes.

KEYWORDS Electrodeposition, AAO, Platinum double-walled nanotubes, DMFC, Electrocatalysis


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Introduction Direct alcohol fuel cells (DAFCs), which mainly utilize methanol and ethanol, operate at relatively low operating temperatures and have several advantages compared to hydrogen fuel cells, including easy handling, storage, and transportation, as well as low cost and high electrical power density.1-3 There has been considerable research and engineering development of DAFCs, and Pt-based nanocatalysts have attracted great attention. Commercial Pt-based nanocatalysts, however, are strongly dependent on the shape and size,4 exposed surface facet,5-7 and surrounding environment of the nanocrystal (e.g., amorphous carbon or graphene support).8 To increase the exposed surface area of nanocatalysts, Pt-based plate,9-10 dendrite,11-13 wire,14 and tube15-16 nanostructures have been investigated because of their high active surface areas compared to spherical nanostructures. In particular, tubular nanostructures with nanometer-scale wall thickness provide high exposed surface area as extended plate structures. Although the commonly accepted model for nanotubular structures does not have enough pores to allow penetration of the electrolyte, most synthesized nanotubular structures possess active surface areas that are similar to or greater than those of commercial nanoparticles.17 In our previous work,18 a new electrocatalyst based on Pt nanotubes (Pt NTs) was prepared by means of a galvanic replacement reaction. By replacing vertically aligned Ni nanorods prepared using an AAO template, Pt NTs were formed that included single-crystalline platinum flakes on the tops of the nanotubes. These results suggest that the nanoflakes on the Pt NTs may contribute to the effective surface area, thereby potentially enhancing the catalysis of electrooxidation reactions. To date, three-dimensional noble metal nanostructures have attracted a great deal of interest due to their various energy applications such as in supercapacitors,19 Li-ion batteries,20-21 and fuel cells,22-23 as a result of their periodic structure, which provides short electron pathways24-25 and


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effective mass transport,26-28 as well as their relatively high surface area, as mentioned above. For catalytic applications, three-dimensionally high-ordered nanostructures that are directly connected to an electrode layer not only protect against carbon corrosion, but also distinctly improve catalytic activity. A carbon support as an electron transport material is necessary for nanoparticle catalysts to fully utilize the exposed surface of nanoparticles. However, the carbon support has serious drawbacks such as regrowth and degradation of the catalysts, giving rise to the significantly reduced electrocatalytic surface area and performance.29-30 Y. S. Yan and coworkers31 demonstrated that Pt nanotubes without carbon supports eliminate carbon corrosion problems and significantly reduce platinum dissolution and redeposition during fuel cell operation due to their micrometer-scale lengths. More recently, Y. E. Sung and coworkers32 reported an inverse opal structured Pt electrode. This modified Pt electrode has a robust and integrated configuration of catalyst layers, which minimizes catalyst loss and improves mass transfer. Herein we report the formation of highly ordered three-dimensional Pt nanostructures that include double walls and inner fibrils. Hierarchical nanostructures with double slender walls were readily fabricated by means of electrochemical deposition and chemical etching. Because of their hollow tubular nanostructure with inner and outer shells, the vertically aligned double nanotube structures enhanced the active surface area, mass transfer, and electrocatalytic activity compared to single-walled Pt nanotube (Pt SNT) and Pt nanorod (Pt NR) structures as anode materials for direct methanol fuel cells (DMFCs). Our approach for the synthesis of perpendicular nanocatalysts is expected to generate highly ordered tubular nanostructures produced by AAO templates, which have the following advantages as a functional catalyst: 1) a greater interfacial area of Pt, owing to the rough core metal template decorated with inner fibrils,


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2) shorter electron pathways more directly connected to the conductive bottom layer than in commercial Pt/C, 3) reduced vulnerability to dissolution and aggregation of Pt nanoparticles due to the micrometer-length structure,31 4) elimination of carbon corrosion problems due to the omission of the carbon support layer, and 5) effective mass transport influenced by convective flow.32 The highly ordered catalyst layer had higher mass transport efficiency, higher effective diffusivity, higher flux of gas molecules, and lower Pt loading than a disordered catalyst layer.33 Experimental Section Materials Anodic aluminum oxide (AAO) templates (Whatman International; diameter ~14 mm, pore size ~250 nm, thickness ~60 µm) were used as hard templates for nanostructure synthesis. An Au conducting layer was electrodeposited using Au plating solution (Technic Inc.). Each precursor reagent including PdCl2 (Sigma-Aldrich) and H2PtCl6·6H2O (Kojima Chemicals) was used without purification. Hydrochloric acid (60%), nitric acid (60%), and sulfuric acid (99.999%) were purchased from Sigma-Aldrich. We used deionized water (Millipore Milli-Q; resistivity > 18.2 MΩ cm) as the solvent for all aqueous solutions.

Preparation of Pt DNTs In a typical synthesis, highly ordered Pd nanotubes (Pd NTs) were prepared by means of an electrodeposition method that did not require any additional seeds, surfactants, or reducing agents. All nanostructures were synthesized at room temperature (24 °C). The diameter of the circular electrode substrate was ca. 1 cm. A predeposited Au film on the AAO for depositing the


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Pd and Pt bimetallic structure played a vital role as a conducting layer, which is helpful for developing supported catalyst systems. This facile and interesting synthetic strategy for Pd NTs has been described previously.34 The vertically aligned Pd NTs on the Au film were prepared within a commercial AAO membrane template. A precursor solution consisting of 50 mM PdCl2 and 100 mM HCl was introduced into the electrochemical cell using an AAO template to yield an array of thin-walled Pd nanotubes.35 The prepared Pd nanotubes were washed five times with deionized water, and then the AAO template was completely removed by treatment with a 3 M NaOH solution for 20 min. The Pt (20 mM H2PtCl6) was electrochemically coated on the asprepared Pd nanotubes array by passing 0.5 C of charge at −0.1 V (vs. Ag/AgCl). Then, Pt DNTs were obtained after dissolving the Pd nanotube core using 30% HNO3 solution for 10 min. The total lengths of the Pt DNTs were determined by the lengths of the Pd NTs, which could be controlled by controlling the charge passed through the working electrode. Pt SNTs were synthesized using the same strategy used to synthesize Pt DNTs, with the same amount of charge (0.5 C), but Pd nanorods (Pd NRs) were used as the templates instead of Pd NTs. Pure Pt NRs were directly grown in AAO nanochannels by an electrodeposition method using 20 mM H2PtCl6 electrolyte and 1 C of charge at −0.1 V. To prepare a standard commercial electrocatalyst in fuel cells, the total mass of Vulcan XC-72 carbon-supported Pt (10 wt%, 625 µg) catalyst was applied (Item NO: P10A100). To minimize error in measuring the weight of nanotubes, 5 sets of samples were prepared and were weighed together.

Instruments for physical characterization


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Scanning electron microscopy (SEM) images were obtained using a JEOL (model JSM-7100) field-emission scanning electron microscopy. Transmission electron microscopy (TEM, model JEM2100F) was conducted to examine the catalyst size and shape, and was used to obtain electron diffraction (ED) patterns and high-resolution images of the catalysts. Cross-sectional specimens were prepared for TEM observation by slicing Pt nanotubes embedded in epoxy (EpoFix Resin, Struers). Focused ion beam (FIB, model JIB-4601F) irradiation was conducted to figure out the completed Pt DNT structures. The FIB instrument was operated at an incident Ga ion energy of 30 keV and a current of 1 pA. An analysis area of 1×1 µm2 was randomly selected on the Pt DNT array. Electrochemical measurements and deposition were performed on an Autolab instrument (model PGSTAT12) using a three-compartment electrochemical system including a Pt mesh counter electrode and an Ag/AgCl (3 M KCl) reference electrode. The metal loadings of the Pt-based samples were determined by means of inductively coupled plasma optical emission spectroscopy (ICP-OES model 720-ES, VARIAN).

Electrochemical measurements Electrochemical deposition and measurements of the Pt DNTs, Pt SNTs, and Pt NRs were carried out at room temperature. The lengths (~2.4 µm) of vertically aligned Pt nanostructures were fixed by controlling the electric charges: 0.9 C for Pd NRs and 0.5 C for Pd NTs.15 Cyclic voltammetry (CV) was performed at a scan rate of 50 mV/s in a nitrogen-saturated 0.5 M H2SO4 solution. Methanol oxidation measurements were conducted at a scan rate of 20 mV/s in a nitrogen-saturated 0.5 M H2SO4 solution containing 0.1 M methanol. For CO stripping measurements, CO (>99.9% purity) was bubbled in the electrolyte for 20 min. Then, N2 gas was bubbled for 10 min at the same potential (0.0 V vs. Ag/AgCl) to remove the dissolved CO in


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solution. The electrochemical potential during anodic stripping was recorded over the scan range of −0.2 to +1.0 V for CO oxidation. Chronoamperometry measurements were performed by monitoring the current over a period of 3000 s at a constant potential of 0.4 V in a nitrogensaturated aqueous solution 0.1 M in methanol and 0.5 M in H2SO4. Results and Discussion Synthesis of vertically aligned Pt DNTs Figure 1 illustrates the fabrication process of Pt DNT arrays; our previously reported Pd nanotube35 was used as a template in this process. In this work, all nanostructures were synthesized by electrodeposition followed by chemical etching. The thin-walled Pd NTs were directly grown into the AAO nanochannels on the conducting substrate. To achieve both inner and outer Pt walls, the AAO template was gently removed by a 3 M NaOH solution after Pd nanotube deposition. Then, a thin homogeneous shell of Pt was coated on the perpendicularly aligned Pd nanotube structure by potentiostatic electrochemical deposition. Typically, the metallic conductive properties of the Pd nanostructure contributed to the homogeneity of the Pt shell. The electrons that were received through the working electrode reduced Pt ions from the bottom to the top of the surrounding Pd nanotube template. Finally, the Pd NT cores were dissolved by a nitric acid solution, leaving a vertical Pt nanostructure array with double slender walls on the substrate. The morphology of the Pt DNTs was investigated by SEM (Figure 2). The first step, the formation of Pd NTs (Figure 2A), was adapted from a method we previously reported.35 Briefly, Pd NTs were fabricated by means of an AAO template–assisted electrochemical deposition strategy. The hydroxyl groups (–OH) on the surface of the nanochannels interacted with H+ ions


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in the plating solution, causing the positive charge to transfer to the –OH groups, forming –OH2+ groups. The positive charge of the alumina surface led to the formation of a thin interfacial double layer that attracted negatively charged Pd metal ions [PdCln(n−2)− (2 ≤ n ≤ 4)]. After removal of the AAO membrane, the densely ordered Pd NTs played an important role as a metal template for the formation of Pt DNTs. Among the various available candidates as templates for the Pt architecture, Pd was selected as the core metal because of its good electrical conductivity, high surface area due to the thin-walled tubular structure in the AAO nanopores, low crystal lattice mismatch with Pt,10-11 and strong connection with the conducting substrate. In particular, the pre-deposited Pd NTs had numerous nanofibers within each Pd pore that acted as nanofiber templates for Pt DNTs and provided not only high surface area, but also sufficient porosity to allow permeation of the electrolyte through the Pt inner wall. After deposition of Pt on the asprepared Pd NTs, the resulting Pd–Pt core–shell nanotube structures (Pd@Pt NTs) maintained their tubular structure and were thicker than the Pd NTs (Figure 2B). Then, after removing the Pd core tubes, the Pt shells kept their vertically ordered structure due to the uniform coating of the Pt shells on the Pd NTs (Figure 2C). A cross-sectional SEM image (Figure 2D) prepared by FIB irradiation of the sample shown in Figure 2C demonstrated that the products possessed a unique double-tubular nanostructure; this image revealed that all inner Pt shells were vertically aligned with their outer shells, with nanometer-scale gaps between the shells. In addition to coating of nanometer-scale Pt shells in the inner cores of the metal nanotubes, another advantage of the present method was control over length. Because the metal nanotube templates can be synthesized by means of electrochemical deposition, the lengths of the Pt DNTs were dependent on the lengths of the fabricated Pd metal nanotemplates, which were easily controlled by the quantity of electric charge passed through the AAO membrane. For deposition


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charges passed through the Pd NT templates of 0.3, 1, and 2 C, the resulting Pt lengths were 1.9 ± 0.3, 2.4 ± 0.4, and 6.7 ± 1.9 µm, respectively (Figure S1, Supporting Information). The resulting perpendicular Pt DNTs showed homogeneous diameters over large areas of the samples. To further investigate the morphological characteristics of the tubular structure at each step, TEM images were obtained after each step (Figure 3A–C). These images showed the tubular geometrical shape of the synthesized nanostructure. Although the wall thickness of the Pd@Pt NTs (~40 nm) was greater than that of the Pd NTs (~26 nm), the lengths were very similar. No nanostructure was observed at the end of the nanotubes, indicating that the Pt atoms fully coated the inner and outer shells and also the tops of the Pd NTs. Interestingly, the wall thickness of the Pt DNTs (~21 nm) was less than that of the Pd NT template, leading to a mass effective surface area (Figure 3D). The typical length and thickness of the nanotube walls were determined from measurement of tens of nanotubes. The crystallinity of each nanotube structure was evaluated based on its electron diffraction pattern. Inset images in Figure 3 show representative selected area electron diffraction (SAED) patterns for each step: Pd NTs, Pd@Pt NTs, and Pt DNTs. The set of concentric rings evidences that the polycrystalline materials clearly remained after chemical etching of the Pd core using HNO3 solution. This demonstrates that the processes of coating with Pt and dissolving the as-prepared Pd did not damage the crystallinity of the resulting nanotube structures. The polycrystalline nature and overall composition of the bulk materials were also confirmed by X-ray diffraction (XRD) patterns. The results reflected the (111), (200), (220), and (311) facets of the series of Pd NTs, Pd@Pt NTs, and Pt DNTs (Figure S2A, Supporting Information), featuring the face-centered cubic (fcc) crystalline structure consistent with the SAED pattern in the Figure 3 inset. Because the lattice constants of Pt and Pd are very close, it was not possible to readily identify the patterns of the resulting nanotube


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structures. Nevertheless, it was clearly seen that the (111) peak of Pt DNTs (39.9°) shifted to that of pure Pt NRs when compared to that of Pd@Pt NTs (40.0°) in an expanded view of the (111) peaks (Figure S2A, Supporting Information), indicating that the as-prepared Pd NTs were completely eliminated during the chemical etching, without impairing the crystallinity of the Pt DNTs (Figure S2B). We performed TEM analysis to reveal the morphological properties of a Pt DNT product consisting of an inner tube and an outer tube. A TEM image of the product showed that both the inner and outer Pt layers were uniformly deposited from bottom to top over the as-prepared Pd template, indicating a totally tubular structure (Figure 4A). The straight Pt nanotube arrays can allow the electrons generated by electrooxidation reactions on both the inner and outer tubes to directly reach the bottom conducting electrode. The extended and micrometer-scale Pt layer connecting the inner and outer tubes at the top protects the aggregation, dissolution, and Ostwald ripening of Pt during fuel cell operation.31 We also investigated the two compositional line profiles along the side faces of the Pt DNTs by conducting energy-dispersive X-ray spectroscopy (EDS) line scanning analysis (Figure 4B). The results clearly demonstrated that the synthesized products were composed of Pt atoms only (cyan line; the atomic value of Pd, red line, is 0%). The positions of the four peaks and three valleys of the Pt trace obviously indicated that the products were made up of two nanotube structures, outer (ⓐ) and inner (ⓑ) layers. The response of Pt (which is represented with a cyan color) was maximized when the density of Pt was highest. The inner two peaks represented the sum of inner and outer Pt. Because the tube has an inner space where Pt is absent (space between “ⓐ” and “ⓑ” in the figure), the response came from the outer Pt shell only. Figure 4C shows a TEM image of a FIB cross section taken perpendicular to the middle faces of a Pt DNT. The synthesized morphology of the inner Pt tubes


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was rough and porous compared to the outer Pt tubes because the as-prepared Pd NT shape had a rough inner surface consisting of nanofibers. This also demonstrates that the Pt layer was uniformly applied and maintained after the chemical etching procedure because Pd is an ideal platform for homogenous coating of Pt shells in a mild reducing environment (with a small lattice mismatch of 0.77% between Pd and Pt).10, 36 Most of the exposed surfaces were (111) facets (Figure 4A and 4C insets).

Electrochemical performance of Pt DNT arrays We evaluated the electrocatalytic properties of the Pt DNTs toward methanol oxidation and compared the results to those for Pt NRs and Pt SNTs with same lengths (ca. 2.4 µm), grown by an electrodeposition method (Figure S3, Supporting Information). CVs acquired in 0.5 M H2SO4 electrolyte at room temperature showed distinct electrochemical behaviors in the hydrogen (Hupd) adsorption/desorption region (from −0.2 to +0.1 V vs. Ag/AgCl) and the surface oxide (OHads) formation/stripping region (Figure 5A).11 The electrochemically active surface area (ECSA per unit weight of metal, Supporting Information 1.1) was determined by mathematical integration of the hydrogen desorption region from the CV data shown in Figure 5A for the Pt NRs (250 µg), Pt SNTs (125 µg), Pt DNTs (125 µg), and Pt/C (625 µg, Vulcan XC-72). The ECSA of the newly developed Pt DNT catalysts (67.0 m2/g) was 4.7 and 1.6 times than that of the Pt NRs (14.3 m2/g) and Pt SNTs (39.6 m2/g), respectively (Figure 5B). This value was also about three times higher than the value of 24.1 m2/g recently reported for highly ordered structures without a carbon support,31 and considerably higher than the value of 46.9 m2/gPt obtained for conventional Pt/C. The CO oxidation properties (Figure 5C) also included a reliable charge integration ratio; the


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integration area of the Pt DNTs was 4.3 and 1.6 times that of Pt NRs and Pt SNTs of the same mass, respectively. The greater observed CO stripping peak area for the Pt DNTs demonstrated that the carbon monoxides dissolved in the electrolyte were able to penetrate into the inner Pt nanotube surface. Interestingly, the onset potential of the Pt DNTs appeared at 0.48 V, which is a lower potential compared to that of the Pt NRs (0.63 V) and Pt SNTs (0.55 V). It is well known that this lower potential is related to the surface roughness and irregularities of the Pt catalysts, which play important roles in facilitating OH adsorption on defect sites of the Pt surface.37 In line with previous findings, the SEM images shown in Figures 2 and S3 unambiguously illustrate that the Pt nanotube structures had rougher surfaces than the Pt NRs due to their different templates; whereas Pt NRs were deposited in an AAO nanochannel with smooth walls, the Pt single/double nanotubes were deposited on a metal template with a rough surface. The Pt DNTs have the most irregular surfaces owing to their inner tubes, resulting in strong oxide (Pt–OH and Pt–O) formation.38 Therefore, considering the anodic oxidation of CO to CO2 by Pt nanotubes, particularly Pt DNTs consisting of rough and irregular walls, the reactivity for CO oxidation is strongly promoted, induced by the lower onset potential of the Pt nanotubes as well as the CO oxidation peak position shift to 0.65 V, compared to the peak position of 0.75 V for the Pt nanorods.39-40 To demonstrate the enhanced electrocatalytic activity of the highly ordered Pt nanostructure arrays, they were used to catalyze the representative electrooxidation of methanol at 20 mV/s in an aqueous solution 0.1 M MeOH and 0.5 M H2SO4 (Figure 5D). Well-defined anodic peaks for the methanol oxidation reaction (MOR) in the presence of prepared Pt NRs, Pt SNTs, and Pt DNTs of the same lengths were observed at 0.70, 0.67, and 0.67 V in the forward sweep, respectively. Onset potential is another parameter for the MOR; it indicates the activation energy


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required to initiate catalysis. The MOR onset potential of the ordered Pt DNTs was obviously negatively shifted compared to those of the Pt SNTs and Pt NRs. Figure 5E shows the mass activity and specific activity obtained from Figure 5D; these are respectively the mass current density per unit mass of platinum and the specific current density per geometric surface area of the platinum film. The Pt DNTs had a high mass activity of 85.6 mA/mg, 1.5, 4.4, and 1.7 times that of Pt SNTs (58.4 mA/mg), Pt NRs (19.6 mA/mg), and Pt/C (51 mA/mg), respectively, and the specific peak current densities of the Pt DNTs, Pt SNTs, Pt NRs, and Pt/C were 10.7, 7.3, 4.9, and 3.2 mA/cm2, respectively. That is to say, both the specific and mass activities of the Pt DNTs were greater than those of the Pt NR, Pt SNT, or commercial Pt/C catalysts. These values were also higher than those obtained for previously reported nanostructures as well as Pt nanoparticle catalysts (4.2 mA/cm2),41 Pt-based nanowires (1.25 mA/cm2),42 Pt-based nanocubes (4.7 mA/cm2),43 graphene-supported Pt catalysts (

Vertically Aligned Double-Walled Platinum Nanotubes Decorated with

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