NANO LETTERS
Facile Synthesis of Highly Faceted Multioctahedral Pt Nanocrystals through Controlled Overgrowth
2008 Vol. 8, No. 11 4043-4047
Byungkwon Lim,†,‡ Xianmao Lu,†,‡ Majiong Jiang,§ Pedro H. C. Camargo,‡ Eun Chul Cho,‡ Eric P. Lee,‡ and Younan Xia*,‡ Department of Biomedical Engineering and Department of Chemistry, Washington UniVersity, St. Louis, Missouri 63130 Received September 29, 2008; Revised Manuscript Received October 11, 2008
ABSTRACT Highly faceted Pt nanocrystals with a large number of interconnected arms in a quasi-octahedral shape were synthesized simply by reducing H2PtCl6 precursor with poly(vinyl pyrrolidone) in aqueous solutions containing a trace amount of FeCl3. The iron species (Fe3+ or Fe2+) play a key role in inducing the formation of the multioctahedral structure by decreasing the concentration of Pt atoms and keeping a low concentration for the Pt seeds during the reaction. This condition favors the overgrowth of Pt seeds along their corners and thus the formation of multiarmed nanocrystals. Electron microscopy studies revealed that the multioctahedral Pt nanocrystals exhibit a large number of edge, corner, and surface step atoms. The size of the multioctahedral Pt nanocrystals can be controlled by varying the concentration of FeCl3 added to the reaction and/or the reaction temperature. These multioctahedral Pt nanocrystals were tested as electrocatalysts for the oxygen reduction reaction in a proton exchange membrane fuel cell and exhibited improved specific activity and durability compared to commercial Pt/C catalyst.
Platinum is a key catalyst invaluable to many industrial processes; notable examples include CO/NOx oxidation in catalytic converters, hydrogen (or alcohol) oxidation and oxygen reduction reactions in fuel cells, nitric acid production, and petroleum cracking.1 All of these applications require the use of Pt in a finely divided state whose catalytic activity and selectivity can be tailored by controlling the shape or morphology. It has also been shown that the presence of a large number of edge and corner atoms holds the key to improving the catalytic performance of Pt nanoparticles.2 To date, a variety of chemical protocols have been developed for preparing Pt nanoparticles with controllable morphologies, which typically involved the reduction of a Pt4+ or Pt2+ precursor in the presence of a polymeric stabilizer or organic surfactant by using a reductant such as alcohol, sodium borohydride, or hydrogen gas. The products of most of these studies are, however, limited to polyhedrons (e.g., tetrahedrons, octahedrons, and cubes) that are enclosed by {111} or {100} facets and contain low percentages of edge and corner atoms.3 Sun and co-workers demonstrated the synthesis of tetrahexahedral (THH) Pt nanocrystals with high-index facets by applying a square-wave potential to polycrystalline Pt spheres deposited on a substrate of glassy carbon.2b These nanocrystals showed high catalytic activity * Corresponding author,
[email protected]. † These authors contributed equally to this work. ‡ Department of Biomedical Engineering. § Department of Chemistry. 10.1021/nl802959b CCC: $40.75 Published on Web 10/23/2008
2008 American Chemical Society
for the oxidation of ethanol and formic acid owing to the high density of stepped atoms on their surfaces. Recently, El-Sayed and co-workers used Pt tetrahedrons as seeds to prepare multiarmed or star-shaped Pt nanocrystals, which exhibited more edges and corners and thus higher activity for the reduction of ferricyanide by thiosulfate than the Pt tetrahedral seeds.2c Despite these nice demonstrations, it still remains a grand challenge to directly prepare highly branched Pt nanocrystals with a large number of edge/corner atoms on the surface via a simple chemical route. We have discovered that the addition of a trace amount of iron species (Fe3+ or Fe2+) to a polyol process could significantly alter the reduction kinetics involved in the formation of Pt nanocrystals.4 In this case, Fe3+ ions can oxidize both Pt atoms and nuclei back to Pt2+ species, significantly reducing the net rate at which the Pt atoms are generated. This approach provides a convenient means for manipulating the reduction kinetics of a polyol process and has enabled the synthesis of Pt nanocrystals with various morphologies including nanowires and branched multipods. Here we demonstrate that highly branched and faceted Pt nanocrystals consisting of a large number of interconnected arms in a quasi-octahedral shape can be synthesized simply by reducing a H2PtCl6 precursor with poly(vinyl pyrrolidone) (PVP) in an aqueous solution containing a trace amount of Fe3+ ions. Compared to the polyol synthesis, a water-based system can provide a more environmentally benign route to
Figure 1. Electron microscopy characterization of multioctahedral Pt nanocrystals. (A) TEM and (B, C) high-resolution TEM (HRTEM) images of Pt nanocrystals synthesized by heating 11 mL of an aqueous solution containing 7.4 mM H2PtCl6, 37 mM PVP (MW ≈ 55000, in terms of the repeating unit), and 36.4 µM FeCl3 at 100 °C for 24 h. In (C), the atomic steps on the Pt {111} surfaces are indicated by red arrows. (D) TEM and scanning electron microscopy (SEM) (inset) images of Pt nanocrystals prepared under the same conditions as in (A) except that the concentration of FeCl3 was increased to 91 µM.
the synthesis of metal nanocrystals, because it does not involve toxic organic solvents.5 In this system, PVP serves as a reducing agent thanks to its hydroxyl (-OH) end groups6 and the reduction kinetics can be manipulated by introducing Fe3+ ions at different concentrations and/or adjusting the reaction temperature. Using this simple approach, we have been able to produce the multioctahedral Pt nanocrystals with controllable sizes, in high yields, and without the need of organic solvents, preformed seeds, and substrates. Importantly, these nanocrystals display a large number of edge and corner atoms as well as surface atomic steps due to their unique structures and thus high specific activity and durability toward oxygen reduction reaction (ORR). The multioctahedral Pt nanocrystals were synthesized by heating 11 mL of an aqueous solution containing 7.4 mM H2PtCl6, 37 mM PVP (MW ≈ 55000, in terms of the repeating unit), and 36.4 µM FeCl3 at 100 °C for 24 h. After the solution was heated and stirred, no notable color change was observed until t ) 14 h. After 14 h, the color of the reaction solution rapidly changed from light yellow to dark brown within 30 min, indicating the formation of Pt nanocrystals. Figure 1A shows a typical transmission electron microscopy (TEM) image of the product that mainly contained Pt nanocrystals in a multiarmed morphology (>95%). An average particle size was measured to be 20 ( 3 nm, and the number of arms on each nanocrystal ranged from a few to over 10. The inset of Figure 1A shows the 4044
TEM image of a single Pt nanocrystal with six arms. It can be seen that the arms take a quasi-octahedral shape and are perpendicular to each other. Figure 1B shows a highresolution TEM (HRTEM) image of a single multioctahedral Pt nanocrystal recorded along the [011] zone axis. Both the HRTEM image and the corresponding Fourier transform (FT) pattern indicate that the multioctahedral Pt nanocrystal was a piece of single crystal. Most of the exposed facets are {111} although some of the corners are truncated by {100} facets (see also Figure 1C). The fringes with lattice spacings of 0.19 and 0.23 nm can be indexed as {200} and {111} of face-centered cubic (fcc) Pt, respectively. The singlecrystalline nature of the multioctahedral Pt nanocrystals suggests that these nanocrystals were formed through an overgrowth mechanism rather than the random aggregation of small nanocrystals. It is worth pointing out that the multioctahedral Pt nanocrystals exhibited rough surfaces with a large number of atomic steps as shown in Figure 1C. As we have observed in the polyol synthesis of Pt nanocrystals,4 the iron species could also significantly retard the reduction of H2PtCl6 in the current system. In order to further manipulate the reduction kinetics, we introduced a higher concentration of Fe3+ ions into the synthesis. When the reaction was conducted in the presence of 91 µM FeCl3 while maintaining the same concentrations for H2PtCl6 and PVP, the reaction was further slowed down so that it took more than 16 h for the solution to exhibit a visible color Nano Lett., Vol. 8, No. 11, 2008
Figure 2. (A, B) TEM images of Pt nanocrystals prepared under the same reaction conditions as in Figure 1, panels A and D, respectively, except that the reactions were conducted at 90 °C. (C) TEM and (D) HRTEM images of Pt nanocrystals prepared under the same condition as in Figure 1A except that the reaction was conducted in the absence of FeCl3. The resulting morphology was the octahedron. In (D), the fringes with a lattice spacing of 0.23 nm can be indexed as Pt {111}.
change from light yellow to dark brown. Interestingly, TEM and SEM images of the resulting product revealed that the sample contained multioctahedral Pt nanocrystals with an increased average size of 40 ( 4 nm (Figure 1D). It can be seen that the number of arms in the Pt nanocrystals increased drastically up to 30-40. We also conducted the reactions at a lower temperature (90 °C), with other reaction parameters being kept the same. In this case, no color change was observed for the solution until 20-22 h into the reaction, indicating that the nucleation and growth were greatly retarded. The resulting Pt nanocrystals retained the multioctahedral morphology but with a further increase in size (Figure 2A,B). These results indicate that the overgrowth of Pt nanocrystals is more favorable at a slower reduction rate. In this way, multioctahedral structures of Pt with large numbers of edge and corner sites can be formed without the need of exotic seeds. The evolution of multioctahedral morphology could be attributed to overgrowth of initially formed octahedral Pt seeds at their corner sites. The overgrowth of Pt nanocrystals has been observed at a low concentration of seeds and high concentration of Pt atoms.3c,4b In the present system, the presence of iron species was found to be critical to the formation of multioctahedral Pt nanocrystals. When the synthesis was performed in the absence of Fe3+ ions, the reduction was faster than that in the presence of Fe3+ ions, with a color of the solution changed about 12 h into the reaction, and only discrete octahedral Pt nanocrystals were obtained (Figure 2C,D). This observation implies that Fe3+ Nano Lett., Vol. 8, No. 11, 2008
ions could significantly slow down the reduction by oxidizing both Pt atoms and nuclei back to Pt2+ species. Once some Pt seeds had been formed in the solution, further reduction of the Pt2+ species could be accelerated via an autocatalytic process,7 and thereby a high concentration of Pt atoms could be generated and persist. Since the concentration of Pt seeds is kept low in the presence of iron species, these seeds could grow into multioctahedral nanocrystals via an overgrowth mechanism. The formation of multioctahedral nanocrystals with a larger size at a slower reduction rate could be attributed to the presence of an even lower concentration of Pt seeds, which could promote the overgrowth of each seed. The formation of atomic steps on the {111} surfaces of the multioctahedral Pt nanocrystals is very interesting. It might be caused by the localized oxidative etching of Pt {111} in the presence of Fe3+ ions, in which some of Pt atoms on the {111} facets could be oxidized back to Pt2+ species by Fe3+ ions and dissolved into the solution. In this case, the ordered surface structure of Pt{111} could be partially destroyed, resulting in the formation of atomic steps on the surfaces of the multioctahedral Pt nanocrystals. It should be pointed out that the discrete octahedral Pt nanocrystals synthesized in the absence of Fe3+ ions exhibited rather smooth surfaces, which supports our hypothesis that the formation of atomic steps on Pt{111} was due to the localized oxidative etching by Fe3+ ions. These results also demonstrate that atomic steps with high surface energy can be formed on Pt surfaces during a chemical synthesis. 4045
Figure 4. ORR polarization curves for 20 nm Pt multioctahedrons and Pt/C (E-TEK) catalyst recorded at room temperature with a sweeping rate of 10 mV/s in O2-saturated HClO4 solution (0.1 M). The inset compares the specific activity (left) and mass activity (right) for these two catalysts at 0.85 V. The kinetic currents were calculated using mass-transport correction from ORR polarization curves. The loading of 20 nm Pt multioctahedrons and Pt/C (ETEK) catalyst on the rotating disk electrode was the same, 15 µg (Pt)/cm2.
Figure 3. Cyclic voltammograms for (A) 20 nm Pt multioctahedrons and (B) Pt/C (E-TEK) catalyst before and after 4000 cycles of accelerated durability test. The durability tests were carried out in O2-saturated 0.1 M HClO4 solutions with the cyclic potential sweeping between 0.6 and 1.1 V at a rate of 50 mV/s.
The multioctahedral Pt nanocrystals were tested as electrocatalysts for ORR in a proton exchange membrane fuel cell.8 To evaluate the durability of the Pt multioctahedrons as cathode catalysts for ORR, we conducted accelerated durability tests by applying linear potential sweeps between 0.6 and 1.1 V at 50 mV/s in 0.1 M HClO4 solution bubbled with O2 at room temperature. After 4000 cycles, the cyclic voltammograms for 20 nm Pt multioctahedrons and commercial Pt/C catalyst (20 wt % 3.2 nm Pt nanoparticles on Vulcan XC-72 carbon support; E-TEK, Figure S1) showed a slight loss of 5.7% in electrochemical surface area (ECSA) for 20 nm Pt multioctahedrons (from 1.05 to 0.99 cm2, Figure 3A), but a significant loss of 34% for Pt/C catalyst (from 2.9 to 1.9 cm2, Figure 3B). The higher durability of Pt multioctahedrons can be attributed to the unique morphology, i.e., a highly branched structure consisting of interconnected octahedral arms. It is known that Pt-nanoparticle coarsening and corrosion of carbon support are the major contributions for ECSA loss of Pt/C catalyst.9 The HRTEM investigation of 20 nm Pt multioctahedrons after durability test revealed that they retained the branched structure without significant particle coarsening (Figure S2), which also enables their use as a catalyst without carbon support. Therefore, carbon corrosion could be avoided in multioctahedral Pt nanocatalysts. Figure 4 shows the polarization curves of 20 nm Pt multioctahedrons and Pt/C catalyst recorded at room temperature in O2-saturated 0.1 M HClO4 solution using a 4046
rotating disk electrode at a sweeping rate of 10 mV/s and a rotation speed of 1600 rpm. The half-wave potentials for the 20 nm Pt multioctahedrons and Pt/C catalyst were 0.843 and 0.840 V, respectively. The inset in Figure 4 shows that specific activities (left) and mass activities (right) at 0.85 V for these two catalysts. The specific activity of 20 nm Pt multioctahedrons was 2.7 times higher than that of Pt/C catalyst. It has been shown that for the three low indices, the activity of ORR increases in the order of (100) < (111) < (110) when a nonabsorbing electrolyte such as perchloric acid was used.10 In addition, it was also reported that stepped Pt surfaces exhibited higher ORR activity than all three lowindex planes in both sulfuric and perchloric acids.11 The higher specific activity of the 20 nm Pt multioctahedrons could be attributed to the preferential exposure of {111} facets rich of surface steps on their surfaces compared to Pt nanoparticles on Pt/C catalyst which usually take the shape of a truncated octahedron with mixed {111} and {100} facets. However, the mass activity of 20 nm Pt multioctahedrons was nearly the same as that of 3.2 nm Pt nanoparticles on Pt/C catalyst, presumably due to smaller ECSA per unit weight of Pt associated with their larger particle size. In summary, we have demonstrated the synthesis of highly faceted Pt nanocrystals with multioctahedral structures simply by reducing H2PtCl6 with PVP in aqueous solutions containing a trace amount of iron species. Both the presence of iron species and the reaction temperature were found to play a critical role in controlling the reduction kinetics and thus the final morphology of the Pt nanocrystals. These multioctahedral Pt nanocrystals consisted of interconnected arms with a quasi-octahedral shape and exhibited a large number of edge, corner, and surface stepped atoms in their surfaces. Due to the high ratio between exposed {111} and {100} facets and presence of surface steps on the Pt multioctahedrons, higher specific activity for ORR was observed than that for commercial Pt/C catalyst. Accelerated tests of Pt Nano Lett., Vol. 8, No. 11, 2008
multioctahedrons revealed their improved ORR durability, which was enabled by their unique structure and absence of carbon support. Acknowledgment. This work was supported in part by the ACS (PRF 44353-AC10) and the NSF (both DMR0451788 and DMR-0804088), as well as a seed grant from the I-CARES at Washington University. B.L. was also partially supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2006-352-D00067). Supporting Information Available: TEM and HRTEM images of the Pt/C (E-TEK) catalyst which contained 3.2 nm Pt nanoparticles on carbon support and HRTEM image of 20 nm Pt multioctahedrons after ORR durability test (4000 cycles). This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (a) Greeley, J.; Nørskov, J. K.; Mavrikakis, M. Annu. ReV. Phys. Chem. 2002, 53, 319. (b) Wang, X.; Waje, M.; Yan, Y. J. Electrochem. Soc. 2004, 151, A2183. (c) Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Angew. Chem., Int. Ed. 2005, 44, 2132. (d) Teng, X.; Liang, X.; Maksimuk, S.; Yang, H. Small 2006, 2, 249. (e) Formo, E.; Peng, Z.; Lee, E. P.; Lu, X.; Yang, H.; Xia, Y. J. Phys. Chem. C 2008, 112, 9970. (f) Chen, W.; Kim, J.; Sun, S.; Chen, S. J. Phys. Chem. C 2008, 112, 3891. (g) Ertl, G. Handbook of Heterogeneous Catalysis; Wiley-VCH: Weinheim, 2008. (2) (a) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (b) Tian, N.; Zhou, Z.-Y.; Sun, S.-G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (c) Mahmoud, M. A.; Tabor, C. E.; Ding, Y.; Wang, Z. L.; El-Sayed, M. A. J. Am. Chem. Soc. 2008, 130, 4590. (3) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (b) Lee, H.; Habas, S. E.; Kweskin, S.; Butcher, D.; Somorjai, G. A.; Yang, P. Angew. Chem., Int. Ed. 2006, 45, 7824. (c) Ren, J.; Tilley, R. D. J. Am. Chem. Soc. 2007, 129, 3287. (d) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974. (e) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 3588. (f) Bratlie, K. M.; Lee, H.; Komvopoulos, K.; Yang, P.; Somorjai, G. A. Nano Lett. 2007, 7, 3097. (4) (a) Chen, J.; Herricks, T.; Geissler, M.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 10854. (b) Chen, J.; Herricks, T.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2589. (c) Lee, E. P.; Peng, Z.; Cate, D. M.; Yang,
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H.; Campbell, C. T.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 10634. (5) Lim, B.; Xiong, Y.; Xia, Y. Angew. Chem., Int. Ed. 2007, 46, 9279. (6) Washio, I.; Xiong, Y.; Yin, Y.; Xia, Y. AdV. Mater. 2006, 18, 1745. (7) Petroski, J. M.; Wang, Z.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (8) Electrochemical measurements were performed at room temperature or 60 °C using a rotating disk electrode (Pine Research Instrumentation) connected to a PARSTAT 283 potentialstat (Princeton Applied Research). A leak-free AgCl/Ag/KCl (3 M) electrode (Warner Instrument) was used as the reference. All potentials were converted to reversible hydrogen electrode (RHE). The counter electrode was a platinum mesh (1 × 1 cm2) attached to a platinum wire. To prepare the working electrode, the Pt/C catalyst (20 wt % 3.2 nm Pt nanoparticles on Vulcan XC-72 carbon support; E-TEK) dispersed in water (1 mg/mL) was sonicated for 5 min. Fifteen microliters of the dispersion was then transferred to the glassy carbon rotating disk electrode (RDE, 0.196 cm2). The 20 nm Pt multioctahedral sample was diluted to 0.15 mg/mL (based on inductively coupled plasma mass spectrometry measurement) and 20 µL of the dispersion was transferred to the RDE. Therefore, the Pt loadings for both Pt/C catalyst and 20 nm Pt multioctahedrons were 3 µg. Upon drying under air for 2 h, the electrode was covered with 15 µL of Nafion dispersed in (0.05%). After evaporation of water, the electrode was put under vacuum for 30 min before measurement. The electrolyte was 0.1 M perchloric acid diluted from 70% (ACS Reagent grade, Baker) using Millipore ultrapure water. All the ORR measurements were done at a sweep rate of 10 mV/s from 0.05 to 1.1 V under flow of O2 (Research grade, Airgas), while for cyclic voltammetry the sweep rate was 50 mV/s under flow of N2 or Ar (ultrahigh purity, Airgas). Due to the existence of organic species (such as PVP) on the particle surfaces, the ORR polarization scans and cyclic voltammograms were recorded after applying a number of potential sweeps between-0.05 and 1.3 V. The accelerated durability tests were carried out in O2-saturated 0.1 M HClO4 solution with cyclic potential sweeps between 0.6 and 1.1 V at 50 mV/s. ECSAs were estimated from the charge collected in the hydrogen adsorption regions using 210 µC/cm2 for monolayer adsorption of hydrogen on a Pt surface. (9) (a) Chen, Z.; Waje, M.; Li, W.; Yan, Y. Angew. Chem., Int. Ed. 2007, 46, 4060. (b) Smith, M. C.; Gilbert, J. A.; Mawdsley, J. R.; Seifert, S.; Myers, D. J. J. Am. Chem. Soc. 2008, 130, 8112. (10) (a) Markovic, N. M.; Adzic, R. R.; Cahan, B. D.; Yeager, E. B. J. Electroanal. Chem. 1994, 377, 249. (b) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (11) (a) Kuzume, A.; Herrero, E.; Feliu, J. M. J. Electroanal. Chem. 2007, 599, 333. (b) Macia, M. D.; Campina, J. M.; Herrero, E.; Feliu, J. M. J. Electroanal. Chem. 2004, 564, 141.
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