Architecture of Metallic Nanoparticles by a Galvanic … of Metallic Nanoparticles by a Galvanic...
3
Architecture of Metallic Nanoparticles by a Galvanic Redox Reaction The quest for nanoscale architecture has demanded newer synthetic strategies for generating and manipulating nanoscopic metal crystals. This article reports the feasibility of a galvanic redox reaction as a novel synthesis strategy to architect metallic nanocrystals (NCs) with well-defined structures. Herein, we demonstrate galvanic redox reaction strategy to generate Fe 1-x PtRu x ternary metal NCs from FePt NCs and monometallic Pd NCs from Ag NCs. However, this methodology can easily be extended to generate other kinds of metallic NCs in solution. The unique application of X-ray absorption spectroscopy (XAS) in confirming the chemical transformation and getting structural insights during the architecture of metal NCs has also been demonstrated. Our method has opened a new route to easily and rapidly prepare a solid-solution type of metal NCs for potential catalytic applications. Materials Chemistry 39 Compared to conventional sequential reduction strategies, galvanic redox process has several advantages for the fabrication of core-shell type nanocrystals (NCs): (1) no additional reducing agent is needed and (2) spontaneous shell layer deposition occurs on top of the core particle surface. It suggests that the galvanic redox reaction of nanocrystals would be a quite useful method for the preparation of other new types of nanostructures. In case of Ag-Pd nanoparticles, the studies on Ag nanocrystals in the presence of Pd 2+ ions in AOT reverse micelles have shown that the replacement reaction resulted in Pd NCs with a significant structure transformation. The driving force of this specific reaction of the nanocrystals was attributed to the differences of the reduction potentials between Ag and Pd. The PtRu nanocrystals have been the subject of intense investi- gations because of their unique catalytic properties for fuel cell appli- cations. The change in atomic distribution of PtRu NCs has great influence on their electrocatalytic activity. It is of great importance to generate a new type of metal NCs with improved catalytic perfor- mance to replace expensive Pt and Ru metals and to increase their atomic distribution for the commercial purpose. In this report, we selected FePt nanocrystals (NCs) and the Ru 3+ ions as reactants to produce ternary metal NCs of Fe 1-x PtRu x using a galvanic redox reac- tion in a solution (Figure 1). The XAS results suggested that iron atoms of FePt lattices were oxidized to be Fe 2+ and Fe 3+ ions and were replaced by Ru atoms obtained from the reduction of Ru 3+ ions Beamlines 01C Extended X-ray Absorption Fine Structure SP12B2 Protein X-ray Crystallography 17C Extended X-ray Absorption Fine Structure Authors D. Y. Wang and C. C. C hen National Taiwan Normal University, Taipei, Taiwan B. J. Hw ang, C. H. C hen, L. S. S arma, J. M. C hen, S. C. S hih and G. R. W ang National Taiwan University of Science and Technology, Taipei, Taiwan B. J. Hw ang, J. F. Lee, M. T . Tang, H. S. S heu, and D. G. L iu National Synchrotron Radiation Research Center, Hsinchu, Taiwan
Transcript of Architecture of Metallic Nanoparticles by a Galvanic … of Metallic Nanoparticles by a Galvanic...
Architecture of Metallic Nanoparticles by a Galvanic Redox Reaction
The quest for nanoscale architecture has demanded newer synthetic strategies for generating and manipulatingnanoscopic metal crystals. This article reports the feasibility of a galvanic redox reaction as a novel synthesis strategyto architect metallic nanocrystals (NCs) with well-defined structures. Herein, we demonstrate galvanic redox reactionstrategy to generate Fe1-xPtRux ternary metal NCs from FePt NCs and monometallic Pd NCs from Ag NCs. However,this methodology can easily be extended to generate other kinds of metallic NCs in solution. The unique applicationof X-ray absorption spectroscopy (XAS) in confirming the chemical transformation and getting structural insightsduring the architecture of metal NCs has also been demonstrated. Our method has opened a new route to easily andrapidly prepare a solid-solution type of metal NCs for potential catalytic applications.
Materials Chemistry
39
Compared to conventional sequential reduction strategies,galvanic redox process has several advantages for the fabrication ofcore-shell type nanocrystals (NCs): (1) no additional reducing agent isneeded and (2) spontaneous shell layer deposition occurs on top ofthe core particle surface. It suggests that the galvanic redox reactionof nanocrystals would be a quite useful method for the preparationof other new types of nanostructures. In case of Ag-Pd nanoparticles,the studies on Ag nanocrystals in the presence of Pd2+ ions in AOTreverse micelles have shown that the replacement reaction resultedin Pd NCs with a significant structure transformation. The drivingforce of this specific reaction of the nanocrystals was attributed tothe differences of the reduction potentials between Ag and Pd.
The PtRu nanocrystals have been the subject of intense investi-gations because of their unique catalytic properties for fuel cell appli-cations. The change in atomic distribution of PtRu NCs has greatinfluence on their electrocatalytic activity. It is of great importance togenerate a new type of metal NCs with improved catalytic perfor-mance to replace expensive Pt and Ru metals and to increase theiratomic distribution for the commercial purpose. In this report, weselected FePt nanocrystals (NCs) and the Ru3+ ions as reactants toproduce ternary metal NCs of Fe1-xPtRux using a galvanic redox reac-tion in a solution (Figure 1). The XAS results suggested that ironatoms of FePt lattices were oxidized to be Fe2+ and Fe3+ ions andwere replaced by Ru atoms obtained from the reduction of Ru3+ ions
Beamlines
01C Extended X-ray Absorption Fine Structure
SP12B2 Protein X-ray Crystallography
17C Extended X-ray Absorption Fine Structure
Authors
D. Y. Wang and C. C. C henNational Taiwan Normal University, Taipei, Taiwan
B. J. Hw ang, C. H. C hen, L. S. S arma, J. M. C hen, S. C. S hih and G. R. W angNational Taiwan University of Science and Technology,Taipei, Taiwan
B. J. Hw ang, J. F. Lee, M. T . Tang, H. S. S heu, and D. G. L iuNational Synchrotron Radiation Research Center,Hsinchu, Taiwan
in solution to form Fe1-xPtRux lattices.
The starting materials, FePt NCs, were synthesized byfollowing the previous report with some modifications.High-resolution TEM images of resulting FePt NCs inFigure 2a have shown that the nanocrystals werecrystalline with clear lattice structures. The nanocrystalswere uniform and well-dispersed on the TEM grid with anaverage diameter of ~4 nm. The solution of FePt NCs wasthen allowed to react with the Ru3+ ions at 85˚C. Thetypical TEM image (Fig. 2b) of the resulting Fe1-xPtRux NCsrevealed that there was no significant change on theaveraged size and shape of Fe1-xPtRux NCs in comparisonto those of original FePt NCs (Fig. 2a).
The Fe/Ru ratio of Fe1-xPtRux NCs formed undertypical reaction time of 1 hour is calculated from XAS bymeasuring the edge jump is 0.27/0.73. The overall resultssuggested that metal ions, Ru3+, were indeed reduced tobe metal atoms of Fe1-xPtRux NCs and iron atoms of FePtNCs were oxidized and released as Fe2+ and Fe3+ in the
solution.
To further understand the redox reaction betweenthe FePt NCs and the Ru3+ ions, synchrotron XAS wasperformed on the samples of FePt and Fe1-xPtRux NCs. TheFe K-edge XANES spectra of FePt powder and referencecompounds (Fe foil, FeC2O4, and FePO4) are shown inFigure 3a. The edge energy position of the FePt is foundto be located between the Fe foil and FeC2O4 referencecompounds, indicating that the iron element in FePt NCsis a mixture of Fe0 and Fe2+. Figure 3b shows that thewhite line features of FePt and Fe1-xPtRux samples aresimilar to that of Pt foil, implying that the oxidation stateof Pt in the FePt and Fe1-xPtRux NCs retained to be Pt0. Thecomposition of iron in Fe1-xPtRux NCs is too low to get agood spectral resolution of EXAFS at Fe K-edge, indicatingthat a large number of the Fe atoms have been replacedby the Ru atoms. However, Ru elements can be observedat Ru K-edge for the Fe1-xPtRux sample.
The XANES feature of Ru K-edge for the Fe1-xPtRux
NCs is close to that of the Ru reference as shown in Figure3c, indicating that most of the Ru species have beenreduced to Ru0 state. It is suggested that the Fe atoms inthe bimetallic NCs can react with Ru3+ via the replace-ment reaction. The electrons required for the reduction ofRu3+ ions are contributed from the oxidation of Fe0 to Fe3+
or Fe2+ to Fe3+.
The structural parameters extracted from the EXAFSspectra for the FePt and Fe1-xPtRux NCs were listed inTable 1, in which the fitting was on the basis of the fccstructure and the FeO crystal structure, respectively. TheFe K-edge EXAFS of the FePt NCs (not shown here) showsFe-O peak between 1.3 and 2.0 Å. It is believed that theFeO species would be dispersed on the surface of theFePt NCs. The coordination number of NPt-Fe and NPt-Pt isfound to be 1.7 and 6.8 for the FePt NCs and 0.4 and 6.8for the Fe1-xPtRux NCs, respectively (Table 1). The decreasein NPt-Fe from 1.7 in the FePt sample to 0.4 in the Fe1-x
PtRux sample, implied that most of the iron atoms aredissolved after RuCl3 addition consistent with XANESobservations. The total coordination number of metallicFe atoms (5.6) is much smaller than that of metallic Ptatoms (8.5) in the case of FePt, indicating that the Ptatoms and Fe atoms are rich in the core region and inshell region, respectively, in the nanocrystals. For thestructural model of the FePt NCs, the overall resultssuggest that most of the Pt atoms are preferentiallylocated in the core region, the Fe atoms are preferentiallylocated in the shell region, and small amount of the FeO
Materials Chemistry
40
Fig. 1: Schematic illustration of chemical transformation fromFePt to Fe1-xPtRux NCs. (1) Ru3+ ions and FePt NCs. (2) galvanicredox reaction between Ru3+ ions and Fe species. (3) produc-tion of ternary Fe1-xPtRux NCs.
Fig. 2: High-resolution TEM images of (a) FePt and (b) Fe1-x
PtRux NCs.
species are dispersed on the FePt NC's surface. For a Pcore-Rshell nanocrystal model, if all the P atoms are occupied inthe core region, the total coordination number of P coreatoms should be 12 based on the fcc structure. However,the total coordination number of Pt atoms is 8.5 and 7.2for the FePt and Fe1-xPtRux NCs, respectively, indicatingthat some of surface sites are occupied by Pt atoms in theshell regions for both samples.
The fitting result shows that the coordinationnumbers of Cl, Fe, and Ru around Ru are found to be 2.7,1.2, and 3.3 respectively. The Cl coordination would beattributed to the residual RuCl3 after the galvanic redoxreaction. Both NPt-Ru and NRu-Pt cannot be extracted andonly shows the Pt-Fe and Ru-Fe bonds, indicating a three-stacking region in the Fe1-xPtRux NCs (Ru-Fe/Fe/Fe-Pt). Forthe structural model of Fe1-xPtRux NCs, these observationsindicate that most of the Pt atoms are preferentiallylocated in the core region and the Ru atoms are located inthe shell region with an intermediate iron-layer betweenthe Pt and Ru atoms. In conclusion, our results havedemonstrated a simple and rapid route for the synthesesof new catalysts based on metal alloy nanocrystals.
Experimental Stations
X-ray absorption spectroscopy stationX-ray powder diffraction stationProtein crystallography station
Publications
．�B.-J. Hwang, C.-H. C hen, L. S. S arma, J.-M. C hen, G.-R. Wang, M.-T. Tang, D.-G. Liu, and J.-F. Lee, J. Phys. Chem. B, 110, 6475 (2006).．�C.-H. Chen, L. S. S arma, G.-R. W ang, J.-M. C hen,
S.-C. Shih, M.-T. Tang, D.-G. Liu, J.-F. Lee, and B.-J. Hwang,J. Phys. Chem. B, 110, 10287 (2006).．�D.-Y. Wang, C.-H. C hen, H.-J. Y an, Y.-L. Lin, P.-Y. Huang,
B.-J. Hwang, C.-C. C hen, J. Am. Chem. Soc., 129, 1538(2007).．�L. S. S arma, C.-H. C hen, S.-C. Y en, G.-R. W ang,
S. M. S. K umar, K.-L. Y u, D.-G. Liu, H.-S. S heu, M.-T. Tang,J.-F. Lee, C. B ock, K. H. C hen, and B.-J. Hwang, Langmuir,DOI: 10.1021/la0637418 (2007).．�B.-J. Hwang, L. S. S arma, G.-R. W ang, C.-H. C hen,
D.-G. Liu, H.-S. S heu, and J.-F. Lee, Chemistry - AEuropean Journal (2007). (in press)
Contact E-mail
Materials Chemistry
41
Fig. 3: Normalized XANES spectra near the (a) Fe K-edge, (b) Pt LIII-edge, and (c) Ru K-edge.
Edge
FeK-edge
PtLIII-edge
PtLIII-edge
RuK-edge
Sample
FePt
Fe1-xPtRux
shell
Fe-OFe-PtFe-FePt-FePt-PtPt-FePt-PtRu-ClRu-FeRu-Ru
N
3.25.63.81.76.80.46.82.71.23.3
Rj(Å)
1.8992.6833.1902.6832.7362.7242.7362.3472.5732.717
∆E0(eV)
-18.02.7
13.73.94.13.93.8
17.312.2-13.6
σ�2(x10
-3)(Å
2)
5.07.90.07.26.00.06.20.08.00.0
Tab. 1: Structural parameters derived from the Fe K-edge, PtLIII-edge, and Ru K-edge XAS.
N: coordination number, Rj: bonding distance, σ�2: Debye-Waller factor, ∆E0: Inner potential shift.
