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Dai S, You Y, Zhang S, Cai W, Xu M, Xie L, Wu R, Graham GW, Pan X. In situ atomic-scale observation of oxygen-driven core-shell formation in Pt 3Co nanoparticles. Nat Commun 2017; 8:204. [PMID: 28785077 PMCID: PMC5547122 DOI: 10.1038/s41467-017-00161-y] [Citation(s) in RCA: 57] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2016] [Accepted: 06/06/2017] [Indexed: 11/09/2022] Open
Abstract
The catalytic performance of core-shell platinum alloy nanoparticles is typically superior to that of pure platinum nanoparticles for the oxygen reduction reaction in fuel cell cathodes. Thorough understanding of core-shell formation is critical for atomic-scale design and control of the platinum shell, which is known to be the structural feature responsible for the enhancement. Here we reveal details of a counter-intuitive core-shell formation process in platinum-cobalt nanoparticles at elevated temperature under oxygen at atmospheric pressure, by using advanced in situ electron microscopy. Initial segregation of a thin platinum, rather than cobalt oxide, surface layer occurs concurrently with ordering of the intermetallic core, followed by the layer-by-layer growth of a platinum shell via Ostwald ripening during the oxygen annealing treatment. Calculations based on density functional theory demonstrate that this process follows an energetically favourable path. These findings are expected to be useful for the future design of structured platinum alloy nanocatalysts. Core-shell platinum alloy nanoparticles are promising catalysts for oxygen reduction, however a deeper understanding of core-shell formation is still required. Here the authors report oxygen-driven formation of core-shell Pt3Co nanoparticles, seen at the atomic scale with in situ electron microscopy at ambient pressure.
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Affiliation(s)
- Sheng Dai
- Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, USA
| | - Yuan You
- Department of Physics and Astronomy, University of California Irvine, Irvine, CA, 92697, USA.,School of Physics and Electronics, Yancheng Teachers University, Yancheng, Jiangsu, 224002, China
| | - Shuyi Zhang
- Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, USA.,Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48109, USA
| | - Wei Cai
- Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48109, USA
| | - Mingjie Xu
- Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, USA.,Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48109, USA
| | - Lin Xie
- Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, USA.,National Laboratory of Solid State Microstructures and College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu, 210094, P.R. China
| | - Ruqian Wu
- Department of Physics and Astronomy, University of California Irvine, Irvine, CA, 92697, USA
| | - George W Graham
- Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, USA.,Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, 48109, USA
| | - Xiaoqing Pan
- Department of Chemical Engineering and Materials Science, University of California Irvine, Irvine, CA, 92697, USA. .,Department of Physics and Astronomy, University of California Irvine, Irvine, CA, 92697, USA.
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Ahmadi M, Cui C, Mistry H, Strasser P, Cuenya BR. Carbon Monoxide-Induced Stability and Atomic Segregation Phenomena in Shape-Selected Octahedral PtNi Nanoparticles. ACS NANO 2015; 9:10686-10694. [PMID: 26418831 DOI: 10.1021/acsnano.5b01807] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
The chemical and morphological stability of size- and shape-selected octahedral PtNi nanoparticles (NP) were investigated after different annealing treatments up to a maximum temperature of 700 °C in a vacuum and under 1 bar of CO. Atomic force microscopy was used to examine the mobility of the NPs and their stability against coarsening, and X-ray photoelectron spectroscopy to study the surface composition, chemical state of Pt and Ni in the NPs, and thermally and CO-induced atomic segregation trends. Exposing the samples to 1 bar of CO at room temperature before annealing in a vacuum was found to be effective at enhancing the stability of the NPs against coarsening. In contrast, significant coarsening was observed when the sample was annealed in 1 bar of CO, most likely as a result of Ni(CO)4 formation and their enhanced mobility on the support surface. Sample exposure to CO at room temperature prior to annealing led to the segregation of Pt to the NP surface. Nevertheless, oxidic PtOx and NiOx species still remained at the NP surface, and, irrespective of the initial sample pretreatment, Ni surface segregation was observed upon annealing in a vacuum at moderate temperature (T < 300 °C). Interestingly, a distinct atomic segregation trend was detected between 300 and 500 °C for the sample pre-exposed to CO; namely, Ni surface segregation was partially hindered. This might be attributed to the higher bonding energy of CO to Pt as compared to Ni. Annealing in the presence of 1 bar CO also resulted in the initial surface segregation of Ni (T < 400 °C) as long as PtOx and NiOx species were available on the surface as a result of the higher affinity of Ni for oxygen. Above 500 °C, and regardless of the sample pretreatment, the diffusion of Pt atoms to the NP surface and the formation of a Ni-Pt alloy are observed.
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Affiliation(s)
- Mahdi Ahmadi
- Department of Physics, University of Central Florida , Orlando, Florida 32816, United States
| | - Chunhua Cui
- Department of Chemistry, Chemical Engineering Division, Technical University Berlin , Berlin, Germany
| | - Hemma Mistry
- Department of Physics, University of Central Florida , Orlando, Florida 32816, United States
- Department of Physics, Ruhr-University Bochum , 44780 Bochum, 10623 Berlin, Germany
| | - Peter Strasser
- Department of Chemistry, Chemical Engineering Division, Technical University Berlin , Berlin, Germany
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Chung SY, Choi SY, Kim TH, Lee S. Surface-orientation-dependent distribution of subsurface cation-exchange defects in olivine-phosphate nanocrystals. ACS NANO 2015; 9:850-9. [PMID: 25565086 DOI: 10.1021/nn506495x] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
Atomic-scale exchange between two different cations of similar size in crystalline oxides is one of the major types of point defects when multiple cations in oxygen interstitials are arrayed in an ordered manner. Although a number of studies have been performed on a variety of Li-intercalation olivine phosphates to determine the distribution of exchange defects in bulk, understanding of the thermodynamic stability of the defects in subsurface regions and its dependency on the crystallographic orientation at the surface has remained elusive. Through a combination of small-angle neutron scattering, atomic-scale direct probing with scanning transmission electron microscopy, and theoretical ab initio calculations, we directly demonstrate that the antisite exchange defects are distributed in a highly anisotropic manner near the surfaces of LiFePO4 crystals. Moreover, a substantial amount of cation exchanges between Li and Fe sites is identified as an energetically favorable configuration in some surface regions, showing excellent agreement with the calculation results of negative defect formation energies. The findings in this study provide insight into developing better ways to avoid degradation of lithium mobility through the surface as well as scientifically notable features regarding the distribution of exchange defects in olivine phosphates.
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Affiliation(s)
- Sung-Yoon Chung
- Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST) , Daejeon 305-701, Korea
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Ahmadi M, Behafarid F, Cui C, Strasser P, Cuenya BR. Long-range segregation phenomena in shape-selected bimetallic nanoparticles: chemical state effects. ACS NANO 2013; 7:9195-9204. [PMID: 24015721 DOI: 10.1021/nn403793a] [Citation(s) in RCA: 73] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
A study of the morphological and chemical stability of shape-selected octahedral Pt0.5Ni0.5 nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG) is presented. Ex situ atomic force microscopy (AFM) and in situ X-ray photoelectron spectroscopy (XPS) measurements were used to monitor the mobility of Pt0.5Ni0.5 NPs and to study long-range atomic segregation and alloy formation phenomena under vacuum, H2, and O2 environments. The chemical state of the NPs was found to play a pivotal role in their surface composition after different thermal treatments. In particular, for these ex situ synthesized NPs, Ni segregation to the NP surface was observed in all environments as long as PtOx species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (<200-270 °C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion toward the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established.
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Affiliation(s)
- Mahdi Ahmadi
- Department of Physics, University of Central Florida , Orlando, Florida 32816, United States
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Schönecker S, Kwon SK, Johansson B, Vitos L. Surface parameters of ferritic iron-rich Fe-Cr alloy. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2013; 25:305002. [PMID: 23779265 DOI: 10.1088/0953-8984/25/30/305002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Using first-principles density functional theory in the implementation of the exact muffin-tin orbitals method and the coherent potential approximation, we studied the surface energy and the surface stress of the thermodynamically most stable surface facet (100) of the homogeneous disordered body-centred cubic iron-chromium system in the concentration interval up to 20 at.% Cr. For the low-index surface facets of Fe and Cr, the surface energy of Cr is slightly larger than that of Fe, while the surface stress of Cr is considerably smaller than that of Fe. We find that Cr addition to Fe generally increases the surface energy of the Fe-Cr alloy; however, an increase of the bulk amount of Cr also increases the surface stress. As a result of this unexpected trend, the (100) surface of Fe-Cr becomes more stable against reconstruction with increasing Cr concentration. We show that the observed trends are of magnetic origin. In addition to the homogeneous alloy case, we also investigated the impact of surface segregation on both surface parameters.
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Affiliation(s)
- S Schönecker
- Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm SE-10044, Sweden.
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Yuge K. Cluster expansion approach for transmutative lattice systems. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2010; 22:125402. [PMID: 21389487 DOI: 10.1088/0953-8984/22/12/125402] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
We propose a cluster expansion (CE) technique that can express any function of atomic arrangement on any given lattice with the same number of lattice points in a single formalism. In the proposed CE, two types of spin variable, σ and τ, on the base lattice and virtual lattice, respectively, are introduced. The former spin variable specifies the occupation of the constituent elements for each lattice point. The latter specifies the positions of each lattice point. Basis functions constructed from the two types of spin variable satisfy completeness and orthonormality for any atomic arrangement on given lattices. As examples, the proposed CE is applied to one- and three-dimensional lattices in a binary system, which clarifies the concept of base and virtual lattices, how the functions of atomic arrangements are expressed in terms of the two types of spin variable, and the efficiency and convergence of the proposed CE with a finite number of clusters and input structures.
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Affiliation(s)
- Koretaka Yuge
- Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan
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Studt F, Abild-Pedersen F, Bligaard T, Sørensen RZ, Christensen CH, Nørskov JK. Identification of non-precious metal alloy catalysts for selective hydrogenation of acetylene. Science 2008; 320:1320-2. [PMID: 18535238 DOI: 10.1126/science.1156660] [Citation(s) in RCA: 657] [Impact Index Per Article: 41.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
The removal of trace acetylene from ethylene is performed industrially by palladium hydrogenation catalysts (often modified with silver) that avoid the hydrogenation of ethylene to ethane. In an effort to identify catalysts based on less expensive and more available metals, density functional calculations were performed that identified relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces. This analysis not only verified the facility of known catalysts but identified nickel-zinc alloys as alternatives. Experimental studies demonstrated that these alloys dispersed on an oxide support were selective for acetylene hydrogenation at low pressures.
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Affiliation(s)
- Felix Studt
- Center for Atomic-scale Materials Design, Department of Physics, Building 311, Technical University of Denmark, DK-2800 Lyngby, Denmark
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