Lattice Strain Distributions in Individual Dealloyed Pt–Fe Catalyst

Letter pubs.acs.org/JPCL

Lattice Strain Distributions in Individual Dealloyed Pt−Fe Catalyst Nanoparticles Lin Gan, Rong Yu, Jun Luo, Zhiying Cheng, and Jing Zhu* Beijing National Center for Electron Microscopy, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Lattice strain is considered to play an important role in the oxygen reduction catalysis on Pt-based catalysts. However, so far, direct evidence of the lattice strain in the catalyst nanoparticles has not been achieved. By using aberration-corrected high-resolution transmission electron microscopy combined with image simulations, a unique core−shell structure, that is, a percolated lattice-contracted Pt−Fe alloy core and a Pt-rich surface with a gradient compressive strain, was directly demonstrated within individual dealloyed Pt−Fe nanoparticles and thus provides direct evidence for the strain effect on their enhanced oxygen reduction activity. SECTION: Surfaces, Interfaces, Catalysis

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The dealloyed Pt−Fe catalyst was synthesized by the impregnation method using a carbon-supported Pt catalyst (denoted as Pt/C) as the starting material followed by an acid leaching process, which is widely used in industry and recent studies on fuel cell catalysts.10,13,14 Figure 1a shows a TEM image of the dealloyed Pt−Fe/C catalyst, which exhibits an average particle size of 4.5 nm based on particle size distribution analysis (Figure S1, Supporting Information), larger than that of the Pt/C catalyst (2.8 nm). The X-ray diffraction pattern (XRD) (Figure 1b) shows that the dealloyed Pt−Fe catalyst exhibits a homogeneous single phase, and the average composition is estimated to be Pt85Fe15 based on Vegard’s law, which is consistent with energy dispersive X-ray analysis (Pt84±3Fe16±3). The electrochemical surface area of Pt in the catalysts could be estimated from the charge associated with hydrogen adsorption/desorption in cyclic voltammetry (see Figure 1c), which is 63 and 54 m2/gPt for the Pt/C catalyst and dealloyed Pt−Fe/C catalyst, respectively. Despite the lower Pt surface area, oxygen reduction activity tests (Figures 1d) show that the dealloyed Pt−Fe catalyst exhibits a mass activity of 0.159 A/mgPt at 0.9 V (versus the reversible hydrogen electrode, RHE), which is 1.8 times that of the Pt/C catalyst (0.089 A/mgPt). To study a lattice strain effect in the catalyst nanoparticles, we performed aberration-corrected HRTEM observations using the negative-Cs imaging technique,16 which was shown to give high contrast and low noise. Although there are few particles showing a larger particle size than 10 nm, we focused on the particles with the size of ∼4−5 nm that are dominant in the dealloyed Pt−Fe/C catalyst, and therefore, their fine structure is the most interesting. Figure 2a and b shows the typical

he slow kinetics of the cathode oxygen reduction reaction (ORR) is considered to be a major barrier for the development of polymer−electrolyte membrane fuel cells.1 Although Pt shows relatively high catalytic activity on the ORR compared to other pure metals, its application is greatly limited by the high price and limited storage on the earth. Recently, catalyst designs using Pt bimetallic or multimetallic structures, for example, Pt monolayer catalysts2−6 and dealloyed Pt-based catalysts,7−11 have shown great progress in enhancing the ORR activities and reducing the use of Pt. Regarding the origin of their enhanced activities, a Pt-rich surface with a moderate compression is proposed to be an important reason because it could lead to a lowered d-band center of Pt and reduced adsorption of oxygen-containing species.3,9,12 For instance, in the dealloyed catalysts, the Pt-rich surface was obtained by dissolution of the transition metals in Pt alloys (e.g., Pt−Cu, Pt−Co, Pt−Cu−Co, etc.) in acid solutions, which has been extensively studied by using high-angle annular dark field (HAADF) scanning transmission electron microscope (STEM) images with strong mass−thickness contrast.9,10,13,14 However, how the lattice strain distributes in the nanoparticles at the atomic scale is still unclear. To fully understand the strain effect in the catalysts, it is therefore necessary to perform a direct strain analysis in the catalyst nanoparticles. Due to the high resolution, high accuracy, and reduced contrast delocalization, Cs-corrected high-resolution transmission electron microscopy (HRTEM)15,16 has been shown to be an indispensable tool for atomic-scale studies of nanoparticles.17−21 For instance, surface steps on Pt nanoparticles19 and surface relaxing on Co3O4 nanoparticles21 have been quantitatively studied. In this work, we demonstrate that lattice strain distributions within individual nanoparticles could be also analyzed by using this technique, from which a latticecontracted Pt alloy core and a compressive Pt-rich surface were directly revealed in dealloyed Pt−Fe catalyst nanoparticles. © 2012 American Chemical Society

Received: February 14, 2012 Accepted: March 13, 2012 Published: March 13, 2012 934

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Figure 1. (a) TEM image of the dealloyed Pt−Fe/C catalyst. (b) XRD patterns of the dealloyed Pt−Fe/C catalyst and the Pt/C catalyst. (c) Cyclic voltammetry of the catalysts in N2-saturated 0.1 mol/L HClO4 at a scanning rate of 50 mV/s. (d) Voltammograms on a rotating disk electrode at 1600 rpm with a scanning rate of 20 mV/s in O2-saturated 0.1 mol/L HClO4; the inset shows a comparison of the mass activities at 0.9 V versus RHE.

dashed rectangle is shown in Figure 2f. The result shows that there is a random lattice contraction or expansion in the particle especially at the surface, which may be ascribed to a random displacement of atoms at the surface steps and edges.19 After all, no continuous lattice-contracted region at the particle core could be found. Therefore, the lattice-contracted region at the inner part of dealloyed Pt−Fe nanoparticles could be directly related to the formation of a Pt−Fe alloy core according to Vegard’s law, while the surface region could be regarded as a Pt shell. It is expected that the lattice-contracted Pt−Fe core could result in a gradient compressive strain over the Pt-rich shell. Density functional calculations show that a moderate compressive strain up to ∼2% at the Pt surface could significantly enhance the oxygen reduction catalysis.9 From the lattice strain mapping, the outmost Pt surface in Figure 2a and b generally shows a small compressive lattice strain relative to bulk Pt (up to 4 and 2%, respectively), though part of the surface at the edges and corners does not. However, it should be noted that the accuracy of the magnification of a TEM is normally in the rang of ±5%;22 thus, it is unwise to state an absolute value of such a small lattice strain at the outmost surface. In contrast, the relative value of the lattice strain in the same image is more meaningful, which clearly shows a gradient compressive strain from the core to the Pt shell. Image simulations were further carried out and compared with the experimental images to help understand the lattice strain within the dealloyed Pt−Fe nanoparticles. Three types of structural models of a cuboctahedral Pt87Fe13 nanoparticle were constructed, (1) a uniform Pt87Fe13 alloy nanoparticle with a lattice parameter of 3.876 Å (Figure 3a), (2) a Pt50Fe50 alloy core surrounded by a Pt shell without compressive strain

images of the dealloyed Pt−Fe nanoparticles, which show a single crystal structure and cuboctahedral shape with the surfaces bounded by {111} and {200} planes along the [110] zone axis. Due to the reduced contrast delocalization in aberration-corrected HRTEM, the surface atoms of the nanoparticles are clearly discernible. To quantitatively analyze the lattice strain in the nanoparticles, first, the positions of each atomic column were measured by fitting the image areas with an intensity maximum to two-dimensional Gaussian functions with an error of around 2 pm.21 To avoid interference by graphitic lattice fringes of the carbon support, only the atoms in the dashed rectangles were analyzed. Using the positions of the atomic columns, then, we could obtain the area of each triangle unit bounded by the nearest distance along [−112], [1−12], and [1−10] directions (denoted as Stri), as indicated in Figure 2a. The value of Stri is proportional to the square of the local lattice parameter (a). Therefore, by converting Stri to the lattice parameter a that was further converted to the contraction percentage related to an ideal Pt lattice parameter (i.e., (a − aPt)/aPt × 100%), we could obtain a two-dimensional mapping of the lattice strain relative to the bulk Pt lattice on the Ångstrom scale (for more details, see Figure S2 (Supporting Information)). Figure 2d and e shows the mapping result of the lattice strain relative to bulk Pt for the nanoparticle in Figure 2a and b, respectively. It can be seen that there is a percolated lattice-contracted region colored in blue at the core region of the particles. Moreover, the compressive lattice strain relative to Pt shows a stepped increase from the surface toward the inner part of the nanoparticles. For comparison, we also take the HRTEM image of the pure Pt nanoparticles, as shown in Figure 2c. Similarly, the map of the lattice contraction relative to that of an ideal Pt lattice in the 935

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Figure 2. Aberration-corrected HRTEM images of dealloyed Pt−Fe nanoparticles (a, b) and the pure Pt nanoparticle (c). The maps of the lattice contraction relative to the bulk Pt lattice (i.e., (a − aPt)/aPt × 100%) in the dashed rectangle of (a−c) are shown in (d−f), respectively.

shown in the experimental results. In contrast, when a gradient compressive strain was applied in the Pt shell in the third structural model, the obtained map (Figure 3i) shows a gradient decrease of lattice strain relative to that of bulk Pt from the core to the surface, which matches the best with the experimental results. As a result, it can be concluded that the dealloyed Pt−Fe nanoparticles consist of a Pt−Fe alloy core and a Pt shell with a compressive strain. Compared with the simulated result in Figure 3i, the experimental result (Figure 2d and e) still shows a little difference, that is, the lattice-contracted Pt−Fe region exhibits a percolated structure rather than a regularly shaped homogeneous core. Previously, Chen et al. proposed a percolated Ptpoor core and Pt-rich surface in acid-leached “Pt3Co”

(Figure 3b), which have lattice parameters of 3.743 and 3.923 Å, respectively, and (3) a Pt50Fe50 alloy core surrounded a Pt shell, in which a gradient compressive strain related to bulk Pt was applied, as an example, from 4% of the lattice contraction in the third subsurface layer to 2% in the outmost layer (Figure 3c). Figure 3d−f shows the simulated HRTEM images for the above structural models, and Figure 3g−i shows the corresponding maps of the lattice contraction percentage, respectively. It can be seen that, for the first type of model (i.e., a uniform alloy nanoparticle), the lattice contraction relative to Pt is generally quite uniform (Figure 3g). For the second type of model, an obvious lattice contraction occurs at the particle core; however, lattice expansion also occurs at the interface between the core and the shell (Figure 3h), which was never 936

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Figure 3. Structural model of cuboctahedral nanoparticles for (a) a uniform alloy nanoparticle, (b) a Pt50Fe50 alloy core surrounded by a Pt shell without compressive strain, and (c) a Pt50Fe50 alloy core surrounded by a Pt shell with a gradient compressive strain from 4 to 2% relative to bulk Pt. All of the nanoparticles contain 2120 Pt atoms and 321 Fe atoms in total and thus have a composition of Pt87Fe13. (d−f) Simulated HRTEM image of (a-c) using an overfocus of 8 nm and a sample tilt of 10 mard; (g−i) the map of the lattice contraction relative to bulk Pt (i.e., (a − aPt)/aPt × 100%) in (d−f), respectively.

method. In brief, 100 mg of commercial Pt catalyst supported on Vulcan XC-72 carbon black (denoted as Pt/C, 20 wt % of Pt, from Johnson Matthey Corp.) was impregnated with 200 μL of 0.5 mol/L−1 Fe(NO3)3 aqueous solution and stirred for 12 h. After that, the mixture was dried at 60 °C and then reduced in H2/Ar at 500 °C for 3 h. The obtained Pt−Fe alloy catalyst was then acid-leached in 0.5 mol/L−1 H2SO4 solution in air at 60 °C for 12 h. After that, the catalyst was filtered, washed by deionized water, and then dried completely. The product was denoted as dealloyed Pt−Fe/C catalyst. Aberration-Corrected HRTEM Analysis. HRTEM observations were performed on an FEI-Titan 80-300 microscope with a spherical aberration (Cs) corrector for the objective lens. The experiment was operated at a voltage of 300 KV and using the negative-Cs imaging technique (with Cs set at around −13 μm and the defocus at around +8 nm), which allows high contrast and low noise. Other residual aberrations are two-fold astigmatism A1 < 2 nm, three-fold astigmatism A2 < 20 nm, and comma B2 < 20 nm. Quantitative measurement of the positions of atom columns was performed by fitting the image areas with intensity maxima to two-dimensional Gaussian functions. Image simulations were carried out using the multislice method implemented in the commercial software package MacTempas X.

nanoparticles by using HAADF-STEM images with strong mass−thickness contrast.10,14 It is noted that the Pt-poor core may originate from Co atoms or Co vacancies formed by acid leaching, and it is difficult to distinguish between them with a Pt background in the HAADF images. In this study, we used aberration-corrected HRTEM images, where the demonstrated lattice-contracted region can be solely attributed to the Fe-rich region. Furthermore, we obtained a two-dimensional mapping of the lattice-contracted Pt−Fe region at the Ångstrom scale in the dealloyed Pt−Fe nanoparticles, which clearly shows how the percolated lattice-contracted Pt−Fe domain contributes a compressive strain over the Pt-rich surface. In summary, by using aberration-corrected TEM combined with image simulations, a percolated lattice-contracted Pt−Fe core and a Pt shell were directly demonstrated in individual dealloyed Pt−Fe nanoparticles with the size between ∼4 and 5 nm. The percolated lattice-contracted Pt−Fe core resulted in a gradient compressive strain in the Pt shell, which provides direct evidence for the strain effect in the enhanced ORR activity. The analysis technique by using aberration-corrected HRTEM shown here can be also applied to other studies of catalyst nanoparticles where the lattice-strain-controlled activity is important.

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EXPERIMENTAL METHODS Sample Preparation. First, carbon-supported Pt50Fe50 alloy catalyst was synthesized by the conventional impregnation 937

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(22) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science, 2nd ed.; Plenum Publishing Corporation: New York, 2009.

ASSOCIATED CONTENT

S Supporting Information *

Methods for XRD analysis and electrochemical measurement, particle size analysis of the dealloyed Pt−Fe/C catalyst and Pt/ C catalyst, and method for the aberration-corrected HRTEM image analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support from the National 973 Project of China and National Science Foundation of China (21003080). This work made use of the resources at Beijing National Center for Electron Microscopy.

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