pubs.acs.org/NanoLett
Size-Dependent Enhancement of Electrocatalytic Performance in Relatively Defect-Free, Processed Ultrathin Platinum Nanowires Christopher Koenigsmann,† Wei-ping Zhou,‡ Radoslav R. Adzic,‡ Eli Sutter,§ and Stanislaus S. Wong*,†,| †
Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, Chemistry Department, Brookhaven National Laboratory, Building 555, Upton, New York 11973, § Center for Functional Nanomaterials, Brookhaven National Laboratory, Building 735, Upton, New York 11973, and | Condensed Matter Physics and Materials Sciences Department, Brookhaven National Laboratory, Building 480, Upton, New York 11973 ‡
ABSTRACT We report on the synthesis, characterization, and electrocatalytic performance of ultrathin Pt nanowires with a diameter of less than 2 nm. An acid-wash protocol was employed in order to yield highly exfoliated, crystalline nanowires with a diameter of 1.3 ( 0.4 nm. The electrocatalytic activity of these nanowires toward the oxygen reduction reaction was studied in relation to the activity of both supported and unsupported Pt nanoparticles as well as with previously synthesized Pt nanotubes. Our ultrathin, acidtreated, unsupported nanowires displayed an electrochemical surface area activity of 1.45 mA/cm2, which was nearly 4 times greater than that of analogous, unsupported platinum nanotubes and 7 times greater than that of commercial supported platinum nanoparticles. KEYWORDS Platinum, nanowires, electrocatalysis, oxygen reduction reaction
A
loadings7 of 0.4-0.8 mg/cm2, rendering fuel cells impractical for widespread commercialization. By contrast, single-crystalline, inherently anisotropic 1-D structures possess (a) high aspect ratios, (b) fewer lattice boundaries, (c) long segments of smooth crystal planes, and (d) a low number of surface defect sites, all of which are desirable attributes for fuel cell catalysts.9-11 Moreover, the 1-D geometry allows for the preferential exposure of lowenergy crystal facets that are highly active toward the ORR.9 This host of factors has been shown to contribute to delaying surface oxidation toward higher potentials, thereby enhancing ORR kinetics.9,12,13 In addition, 1-D structures are widely known to maintain improved electron transport characteristics due to the path directing effects of the structural anisotropy.13,14 Not surprisingly, 1-D morphologies have previously been shown to lead to intrinsically improved performance as fuel cell electrocatalysts.14-17 Specifically, 1-D platinum nanostructures possess proportionally fewer defect sites and have higher numbers of surface atoms with the potential for higher degrees of coordination.18 In particular, one study found that small diameter Pt nanowires grown directly on an amorphous carbon support showed an area specific activity for the ORR of 275 µA/cm2, more than three times that of a commercial nanoparticle cathode.15 Similar improvements were observed with platinum nanotubes prepared by Yan and co-workers that displayed a 4-fold enhancement in area-specific activity when compared with
growing demand for efficient, low-cost renewable energy has sparked great scientific interest in the development of materials for use as electrocatalysts in the oxygen reduction reaction (ORR).1 State-of-the-art ORR electrocatalysts primarily consist of platinum nanoparticles supported on mesoporous carbon supports.2 Nevertheless, there are several inherent problems that are associated with zero-dimensional (0-D) nanoparticle morphologies. In general, nanoparticle morphologies maintain a proportionally larger number of lattice boundaries and defect sites on their surfaces as compared with associated one-dimensional (1-D)-type analogues.3 It should be noted that defect sites are less catalytically active than smooth crystal planes because of local differences in coordination geometry and surface energy, which can change the interface between exposed Pt atoms and the oxygen adsorbate, for instance.1 As a result, these nanoparticulate electrocatalysts possess slow kinetics due to an observed overpotential4,5 and catalytic inhibition due to adsorption of surface OH groups6 at potentials below 1 V. Moreover, 0-D electrocatalysts lack the durability for long-term applications in fuel cells due to irreversible oxidation of surface atoms in defect sites.7,8 These factors contribute to high precious metal
* To whom correspondence should be addressed,
[email protected] or
[email protected]. Received for review: 02/28/2010 Published on Web: 07/07/2010 © 2010 American Chemical Society
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FIGURE 1. A representative experimental X-ray powder diffraction of as-prepared nanowires with the corresponding JCPDS standard shown immediately below.
carbon-supported platinum nanoparticles.14 Moreover, our group has also demonstrated that unsupported, polycrystalline Pt nanotubes synthesized by a facile template directed method can achieve more than double the area-specific ORR activity as compared with commercial carbon-supported platinum nanoparticles.19 Thus, emerging literature on the use of Pt 1-D morphologies as ORR electrocatalysts has shown that structural anisotropy can give rise to higher intrinsic catalytic activity. Nevertheless, despite these tangible improvements in the activity of the catalysts, a continuing challenge has been to develop even more highly active catalysts while simultaneously minimizing the precious metal loading. In this paper, therefore, we report on the synthesis, characterization, and ORR activity of ultrathin (1-2 nm) platinum nanowires synthesized by a modification of the method previously reported by Miyake and co-workers.20 The use of ultrathin nanowires maximizes the surface area-to-volume ratio and therefore decreases the amount of catalytically inactive support material present within the interior of the wire.10 More importantly, previous analysis of small diameter platinum nanowires by DFT has predicted that the nanowire undergoes a surface contraction when the diameter is decreased below a critical value of approximately 2 nm.21-23 Surface contractions of platinum have been shown both experimentally24,25 and theoretically5,26 to improve the surface ORR activity because it is thought that such contractions weaken the binding of oxygen and increase the kinetics of O-H bond formation. Thus, we have set out to investigate the size dependence of an ultrathin nanowire with a diameter of less than 2 nm with respect to its intrinsic electrocatalytic activity, since, to the best of our knowledge, there have not been any previous reports exploring such a structure-property correlation. Our experimental and characterization protocols are described in the Supporting Information. In our studies, the crystallinity and purity of as-prepared ultrathin platinum nanowires were studied by means of X-ray powder diffraction. Crystallographic analysis (Figure 1) of the solid revealed that all of the peaks could be readily indexed to the (111), (200), (220), and (311) reflections of the face-centered cubic platinum (Fm3m, JCPDS #04-0802). © 2010 American Chemical Society
FIGURE 2. Low-resolution TEM images of acid-washed platinum nanoparticles (A) as well as prepared ultrathin nanowires both before (B) and after (C) washing with acid. A high-resolution TEM image (D) of a single acid-washed platinum nanowire is shown as an inset within an image containing a representative collection of these nanowires. Associated selected area electron diffraction (SAED) (E) and energy-dispersive X-ray spectroscopy (EDAX) (F) patterns of these 1-D nanostructures.
No detectable impurity peaks were observed in the X-ray diffraction pattern. The morphology of the nanowires and nanoparticles was characterized by transmission electron microscopy (TEM). Overview TEM images of the acid-treated nanoparticles (Figure 2A) prepared by the reduction of the precursor without the presence of toluene in the solvent system revealed nanoparticles with an average diameter of 2.8 ( 0.7 nm. The addition of toluene to the reaction resulted in a majority of nanowires (85-90%) (Figure 2B) forming a netlike structure with some nanoparticles distributed throughout the net. The average diameter of the wires was determined from the TEM images to be 1.8 ( 0.3 nm with an average length of 100 ( 25 nm. As-prepared nanoparticles possessed an average diameter of 2.3 ( 0.6 nm. Upon washing the as-prepared wires with acid, most of the nanoparticles were removed, and the wire diameters perceptibly decreased to 1.3 ( 0.4 nm (Figure 2C) with an associated drop in the level of aggregation. The TEM image in Figure 2C shows the presence of well-defined individual nanowires. The nanowires are polycrystalline, i.e., consist of multiple crystalline segments with an average length of 6 ( 2 nm that extends along the axis of the nanowire (high-resolution TEM (HRTEM) image in Figure 2D). Indeed, the majority of the 2807
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single crystalline segments making up the nanowires have lattice spacings of 0.23 and 0.19 nm, consistent with the (111) and (200) lattice planes, respectively. The selected area electron diffraction (SAED) pattern shown in Figure 2E highlights not only continuous rings that can be indexed to platinum’s (111), (220), (311), and (331) planes, respectively, but also discrete diffraction spots indicating that the high degree of substructure observed is representative of the whole ensemble of nanowires that contributed to the diffraction pattern. The composition of individual nanowires investigated by energy dispersive X-ray spectroscopy (Figure 2F) performed in scanning TEM mode with an electron beam size of 0.2 nm suggests only the presence of Pt, as expected, with Cu peaks emanating from the TEM grid. The novel use of an acid wash was critical in achieving suitably exfoliated nanowires maintaining ultrathin diameters. Acid washes are widely utilized in nanomaterial purification, especially with respect to the purification of platinum thin films after plasma etching.27 Figure S1 (Supporting Information) shows representative scanning electron microscopy (SEM) images of nanowires washed with water and with 6 M HCl. When washed in water (Figure S1A, Supporting Information), the nanowires are highly aggregated into a monolithic netlike structure and are encased in an amorphous residue. By comparison, acid-washed nanowires (Figure S1B, Supporting Information) displayed a much lower degree of aggregation and in fact, individual nanowires could be resolved. We attribute the improvement in dispersibility and nanowire exposure to the solubilization of residual amorphous platinum and platinum salts such as PtClx and PtOx.28 Moreover, the size of acid-washed nanowires was observed to decrease with increasing hydrochloric acid concentration. We attribute this change in wire diameter to a symmetrical etching of the exposed surfaces, which has been previously observed with anisotropic silver nanocrystals in acidic media.29 UV-visible spectroscopy (Figure S2, Supporting Information) was performed on various wash solutions used to purify the nanowires so as to explore the effect of the acid on the nanoscale platinum. Acid wash solutions, after precipitation of the nanowires, displayed a peak at 362 nm, consistent with the presence of dissolved H2PtCl6 under acidic conditions.30 Interestingly, the spectrum is bereft of absorption peaks from H2PtCl6, when the acid wash is substituted with a water wash. Because no H2PtCl6 is observed in the ethanol washes, we can attribute the presence of H2PtCl6 in the acid wash to the resolubilization of platinum and of amorphous platinum residues by HCl. The concentration of H2PtCl6 was determined from Beer’s law to be 0.52 mg/mL, indicating that approximately 2.5 mg of platinum residue has been redissolved in the acid wash. The cathodic ORR kinetics of the as-synthesized nanowire samples were probed by rotating disk cyclic voltammetry. Initially, stationary electrode cyclic voltammograms (CVs) were obtained of rotating disk electrodes (RDEs) loaded with © 2010 American Chemical Society
FIGURE 3. Cyclic voltammograms obtained for as-synthesized nanowires before and after acid washing as well as for unsupported platinum nanoparticles loaded onto a glassy carbon electrode in a 0.1 M HClO4 solution at 50 mV/s.
the platinum nanowires in deoxygenated 0.1 M HClO4 solution in order to retrieve the electrochemically addressable surface areas (Figure 3). For comparison, the sample of acid-washed platinum nanoparticles prepared by modification of the nanowire synthesis is shown. Initially, the active surface areas of the both electrodes were determined by the integrated hydrogen adsorption charge measured.31 The peaks associated with H adsorption (Hads) in the CVs in Figure 3 indicate the presence of the 110 and 100 oriented steps for the as-prepared nanowires. For the acid-treated nanoparticles, the number of low-coordinated sites is significantly lower as compared with that found with asprepared nanowires, as evidenced by the absence of distinct peaks in the 0-0.2 V region.32 It is also apparent that a considerable amount of low-coordinated atoms has been removed from the nanowire surface upon treating it with acid. Thus, it appears that the acid wash contributed to the removal of atoms occupying low coordination sites, in addition to getting rid of amorphous deposits. This assertion is consistent with previous reports that have shown that high-energy metallic surfaces and defect sites are preferentially etched by acidic solutions.29 Thus, we can attribute the lack of extraneous, undesirable low-coordination sites to the etching effect of the acid wash, leaving behind primarily desirable smooth, crystalline, and defect-free surfaces. Moreover, the shape of our acid-treated ultrathin nanowire CV curve is close to that of a characteristic bulk Pt(111) surface, which has a high intrinsic activity toward the ORR. Previous analysis of ORR kinetics at bulk single crystals of platinum has revealed that the most active facet is, in fact, the (111) facet.1 The ORR activity for the nanowire samples was measured electrochemically by obtaining cyclic voltammograms in an oxygen-saturated 0.1 M HClO4 solution (Figure 4). The nanowires show an ORR onset between 0.7 and 0.8 V, as expected, with nanostructured platinum electrocatalysts. The polarization curves also show that there is a slight shift in the acid-treated nanowire’s curve toward higher potentials 2808
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strictly derived from the ultrathin, one-dimensional motif of the nanowire morphology. This incredible improvement in activity arising from having only a smaller diameter wire coupled with an acid wash protocol raises very interesting issues. Specifically, what is the intrinsic electrocatalytic difference between 200 nm platinum nanotubes and ultrathin sub-2-nm platinum nanowires? We attribute the enhancement in activity of our ultrathin nanowires to a contraction of the surface of the ultrathin nanowires that promotes enhanced ORR activity by enhancing O-H bonding formation kinetics. Previous theoretical21-23 and experimental33 studies have demonstrated that small-diameter noble metal nanowires (Au, Pt, and Ir) undergo a surface contraction, as their diameter is decreased below a critical value of approximately 2 nm. Contraction of the platinum surface has been shown both experimentally24,25 and theoretically26 to improve the surface ORR activity because it is thought that such contractions weaken the binding of oxygen and increase the kinetics of O-H bond formation. Hence, we believe that the surface reconstruction of the small diameter wires contributes to the observed enhancement of their intrinsic activity toward ORR. Furthermore, we also attribute the improvement to the novel use of an acid wash protocol that was shown by cyclic voltammetry to remove surface atoms occupying low coordination sites and expose crystalline planes. The residual smooth lattice planes characterized by a low percentage of surface atoms occupying low coordination (defect) sites are much more active toward the ORR reaction.9 Smooth, defect-free, and crystalline surfaces display an ORR onset at higher potentials, which increases the underlying kinetics of the ORR reaction.34,35 Finally, the shape of the acidwashed, ultrathin nanowire CV is close to what is typically characteristic of a bulk Pt(111) surface. This observation corroborates the above discussion of the inherent activity of this surface, given the high intrinsic activity of (111)oriented surfaces. Furthermore, the increased electrocatalytic activity of the unsupported acid-treated nanoparticles with respect to that of the commercial supported nanoparticles provides for strong evidence that the acid treatment improves the electrocatalytic activity by removing undesirable defect sites. Thus, we propose that the effects of the acid wash are 3-fold: (i) it exfoliates the nanowires by resolubilizing platinum deposits, thereby exposing the active crystalline facets, (ii) it perceptibly decreases the nanowire diameters, and (iii) it removes undesirable defect sites from the surfaces of the platinum nanowires. To further explore the kinetics of ORR at the ultrathin Pt nanowire electrode, we constructed a Koutecky-Levich (KL) plot at various potentials (Figure 5B) and determined the experimental value of the B factor to be 0.096 mA/s1/2. A value of 0.092 mA/s1/2 for an ideal 4e- process was calculated using known values and constants from the literature (cf. Supporting Information). The close agreement between
FIGURE 4. The polarization curve (A) for the acid-washed ultrathin platinum nanowires as compared with commercial 3.3 nm platinum nanoparticles 46.4 wt % on a Vulcan carbon support, both on a glassy carbon RDE. Curves (anodic sweep direction) were obtained with a rotation rate of 1600 rpm in a 0.1 M HClO4 solution at 20 °C. (B) The electrochemical surface area activity (ECSA) at 0.9 V for acidtreated ultrathin nanowires (red) as compared with commercial supported nanoparticles (green), acid-treated platinum nanoparticles (orange), previously synthesized19 platinum nanotubes (black), and as-prepared ultrathin nanowires without acid treatment (blue), respectively.
when compared with commercial nanoparticles, a trend which suggests a lower ORR overpotential since the thermodynamic potential of ORR is 1.23 V. More importantly, the measured kinetic currents (Figure 4 inset) were normalized to the real surface areas so as to probe the intrinsic activity of each nanowire sample (cf. Supporting Information). On the basis of this protocol, the acid-washed ultrathin nanowires displayed an outstanding specific activity (JK) for an unsupported nanostructured material of 1.45 mA/cm2. In fact, our unsupported acid-washed nanowires showed an almost 4-fold higher intrinsic activity as compared with previously synthesized19 platinum nanotubes (0.38 mA/cm2) and a 7-fold increase in intrinsic activity as compared with commercial carbon-supported platinum nanoparticles (0.21 mA/cm2). In addition, we also observed that the unsupported acid-treated Pt nanoparticles displayed an enhanced activity as compared with their commercial carbon-supported nanoparticle analogues. However, the comparatively low activity displayed by the acid-treated Pt nanoparticles as compared with their 1-D counterparts is strong evidence that the superior performance observed in the nanowire system is © 2010 American Chemical Society
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In conclusion, we have synthesized crystalline platinum nanowires with an ultrathin diameter of 1.3 ( 0.4 nm. As-prepared nanowires were treated with an acid wash protocol in order to suitably exfoliate the nanowires and redissolve amorphous platinum deposits so as to expose the active surface areas of the wires themselves. Moreover, cyclic voltammetry revealed that the acidwashed nanowires displayed fewer undesirable surface defects, which was also attributed to the acid wash itself. The electrocatalytic surface area (ESCA) activity of the acid-treated nanowires was explored with respect to that of platinum nanoparticles and platinum nanotubes in order to explore the intrinsic relationship between structure and electrocatalytic activity. Our unsupported, ultrathin, acid-treated nanowires displayed an outstandingly high ESCA of 1.45 mA/cm2, which was a significantly larger value than that reported for both previously synthesized platinum nanotubes and platinum nanoparticles. Acknowledgment. We acknowledge the U.S. Department of Energy (DE-AC02-98CH10886) for facility and personnel (including PI) support and for all electrochemical experiments performed. Moreover, research carried out (in whole or in part, such as the transmission electron microscopy studies)attheCenterforFunctionalNanomaterials,Brookhaven National Laboratory, is also supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. S.S.W. also thanks the Alfred P. Sloan Foundation for experimental supplies necessary for the synthesis reactions. Supporting Information Available. Details of experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org.
FIGURE 5. Polarization curves (anodic sweep direction) of the platinum nanowire electrode (A) at various rates of rotation. Koutecky-Levich plots (B) at different potentials obtained from the data in Figure 5A. Tafel plots (inset) of commercial supported Pt nanoparticles and acid-treated unsupported ultrathin platinum nanowires, respectively, are also shown.
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these values suggests that the reaction mechanism of the nanowire system most closely follows the ideal 4e- process expected for oxygen reduction. Furthermore, the data at each potential closely fit the K-L linear relationship and a consistent slope (i.e., consistent B factor) is maintained over all of the potentials. Taken together, these observations suggest first-order kinetics36 with respect to molecular oxygen, which is highly desirable. In addition, the diffusioncurrent-corrected Tafel plots of acid-treated Pt nanowires and commercial Pt nanoparticles are also shown (inset to Figure 5), wherein the kinetic currents normalized to specific surface areas were obtained from the K-L plots.37,38 Indeed, the Tafel plots clearly reveal that the acid-treated nanowires maintain significantly higher kinetic currents over the entire range of operating potentials. Overall, these observations provide additional evidence that the ultrathin acid-treated nanowires display greatly enhanced ORR kinetics as compared with commercial Pt nanoparticles. © 2010 American Chemical Society
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