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Preparation and characterization of Pd2Sn nanoparticles
Materials Research Bulletin 42 (2007) 1969–1975 www.elsevier.com/locate/matresbu
Preparation and characterization of Pd2Sn nanoparticles Katharine Page a, Christina S. Schade a,b, Jinping Zhang a, Peter J. Chupas c, Karena W. Chapman c, Thomas Proffen d, Anthony K. Cheetham a, Ram Seshadri a,* a
Materials Department and Materials Research Laboratory, University of California, Santa Barbara, CA 93106, USA b Institut fu¨r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universita¨t Mainz, Duesbergweg 10-14, Mainz D55099, Germany c Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA d Los Alamos National Laboratory, Lujan Neutron Scattering Center LANSCE-12, MS H805, Los Alamos, NM 87545, USA Received 1 May 2007; accepted 3 May 2007 Available online 10 May 2007
Abstract We report a non-aqueous solution preparation of Pd2Sn nanoparticles with sizes near 20 nm. The intermetallic compound with the Co2Si structure has been characterized using transmission electron microscopy, Rietveld refinement of synchrotron X-ray and neutron powder diffraction, and real-space pair distribution function analysis of high-energy synchrotron X-ray scattering. We also present a description of the electronic structure of this covalent intermetallic using density functional calculations of the electronic structure. # 2007 Published by Elsevier Ltd. Keywords: A. Intermetallic compounds; C. Electron microscopy; C. Neutron scattering; C. X-ray diffraction
1. Introduction The preparation and characterization of nanostructured intermetallic materials has been receiving growing attention. The wide functionality of intermetallics combined with the materials characteristics of nanomaterials (new electronic, magnetic, and catalytic properties) promises an exciting future for this emerging class of materials. Here we report the preparation and characterization of Pd2Sn particles stabilized by polyvinylpyrrolidone. These are materials of interest for their catalytic properties. Palladium–tin catalysts in impregnated alumina supports have been studied for use in the selective hydrogenation of hexadienes [1]. Pd2Sn can be thought a chemical (but not structural) subunit of Heusler compounds with general composition RPd2Sn. For example, the coexistence of magnetism and superconductivity in the Heusler compounds ErPd2Sn is of great interest [2]. Bulk Pd2Sn [3] crystallizes in the structure of the mineral Paolovite Co2Si [4], which in turn is structurally similar to the cottunite structure adopted by many covalent and intermetallic A2B compounds. In the Paolovite structure, shown in Fig. 1, the A atom (Pd) has eight B (Sn) atom neighbors and five A atom neighbors, whereas the B atoms have ten A atom neighbors. Interestingly, this is a structure adopted by some alkaline earth halides under high pressure [5].
* Corresponding author. Tel.: +1 805 893 6129; fax: +1 805 893 8797. E-mail address: [email protected] (R. Seshadri). 0025-5408/$ – see front matter # 2007 Published by Elsevier Ltd. doi:10.1016/j.materresbull.2007.05.010
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Fig. 1. Crystal structure of Pd2Sn. The small black spheres are Pd, and the larger gray spheres are Sn.
In this contribution, we describe a non-aqueous solution preparation of small particles (sub 30 nm) of Pd2Sn starting from the metal halides and using an organic borohydride as the reducing agent. We present the structure as obtained from Rietveld refinement of synchrotron X-ray and neutron powder diffraction data and atomic pair distribution function (PDF) analysis of synchrotron X-ray diffraction data. We also describe the morphology of the particles obtained by high-resolution electron microscopy. This contribution adds to the recent, growing literature on the preparation of complex intermetallic phases through solution techniques [6]. 2. Experimental The compound was prepared at relatively high (for solution) temperatures, between 250 and 290 8C, in either refluxing benzyl ether or phenyl ether. We used the metal chlorides, PdCl2 and SnCl2, as starting materials, polyvinylpyrrolidone (PVP) as a stabilizer and tetrabutylammoniumborohydride, which is soluble in the ethers, as the reducing agent. All chemicals were purchased from Aldrich and used as received. All chemicals were opened and handled in a nitrogen-filled glove box. A typical preparation involved mixing a solution of PdCl2 (1 mmol), SnCl2 (2 mmol) and PVP (0.15 g) in 30 ml benzyl ether with a solution of (C4H9)4NBH4 (0.51 g) in 10 ml benzyl ether. After adding the reducing agent and stirring for about a minute the brownish suspension turned black. It was heated to reflux under flowing N2 and held there for about 1 h. After cooling down to room temperature the black solid was collected by centrifugation and washed two to three times with ethanol until a dry powder was obtained. X-ray diffraction (XRD) data were collected on powder samples using a Philips MRD diffractometer with Cu Ka; radiation ˚ ) and a graphite monochromator. Data were acquired over the angular range 20–908 2u, with a step size (l = 1.5418 A of 0.0178 (2u). XRD data were subject to Rietveld refinement using the XND Rietveld code [7]. Time-of-flight neutron diffraction data were collected at room temperature on NPDF [8] at the Lujan Neutron Scattering Center. Approximately 0.6 g were contained in a vanadium can for the experiments. The diffraction data were analyzed with the Rietveld method as embodied in the GSAS-EXPGUI [9] suite of programs. Synchrotron powder diffraction data was collected on 11-ID-B at the Advanced Photon Source of Argonne National Laboratory utilizing high-energy Xrays (90.6 kV) Data was collected on a 2D amorphous Si detector from General Electric Healthcare and processed using the program Fit-2D [10]. The pair distribution function: GðrÞ ¼ 4pr½rðrÞ r0 where r is the length of the vector in real space and r(r) is the radial density, was extracted as described previously [11] ˚ . Full structure profile refinements are carried out in the program with a maximum momentum transfer of Q = 28 A PDFFIT [12]. For HRTEM analysis, nanoparticle samples were dispersed in ethanol and loaded on a holey carbon grid by dropping two drops of the solution on to the grid. High-resolution images were obtained from a field emission transmission electron microscope (TEM) operated at 300 kV (FEI Tecnai F30UT, CS = 0.52 mm) in conventional high-resolution TEM and STEM modes. A high-angle annular dark-field detector was used for STEM imaging, and combined with an electron-dispersive X-ray detector (EDAX-R, 135 eV) for chemical analysis of the particles.
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Fig. 2. X-ray diffraction data (points) and Rietveld profile fits (solid lines) of the nanoparticles. Panel (a) displays a sample that contained fcc-Pd as a second phase. Panel (b) shows a sample prepared with 1:2 Pd:Sn starting ratios, and does not have any second phase. Vertical lines at the top of the figure display expected peak positions for the two phases.
3. Results and discussion Using the correct stoichiometry of Pd:Sn of 2:1 resulted in XRD patterns that showed evidence for fcc-Pd. Fig. 2(a) displays powder XRD data as points and a gray line as Rietveld fit to a mixture of two phases, Pd2Sn [13] as well as
Fig. 3. Rietveld refinement of synchrotron X-ray powder diffraction profiles showing data (points) the Rietveld fit (line) and the difference profile at the bottom. The wavelength was refined against lab diffraction data for a standard sample of ZnO. Vertical lines at the top of the figure display expected peak positions for Pd2Sn.
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˚ . The inset shows the experimental Fig. 4. Experimental G(r) (points) and fit (solid line) of the phase-pure particles to the Pd2Sn structure to 15 A data out to higher r, displaying the discrete size of the particles.
fcc-Pd metal. Quantitative phase analysis using Rietveld scale factors suggest that the two phases, Pd2Sn and Pd, occur in the XRD pattern in Fig. 2(a) in the mole fractions 58% and 42%, respectfully. We believe the problem of the two-phase product arises from the much more rapid rate of reduction of PdCl2 compared with SnCl2. Curiously, when we attempted to prepare the bulk Pd2Sn phase using standard arc-melting procedures starting with stoichiometric 2:1 mole ratios of elemental Pd and Sn, we similarly obtained a two-phase mixture of fcc-Pd and Pd2Sn. Starting with a 1:2 ratio of the chlorides for the solution synthesis resulted in phase-pure XRD patterns as seen in Fig. 2(b). Scherrer broadening analysis of the Pd2Sn 2 0 0 reflection from Fig. 2(b) suggests a correlation length near 20 nm. Synchrotron X-ray powder diffraction data (SXPD) was also subject to Rietveld refinement and this is displayed in Fig. 3. The small size of the particles and possible structure disorder, such as the presence of stacking variants, make it hard to model the data effectively. Displacement parameters could not be obtained reliably from these refinements, but this could in part be due to the manner in which the background is extracted in the process of integrating the 2D Debye–Scherrer rings. The time-of-flight neutron diffraction data were also similarly treated using the Rietveld method. Pd and Sn are neighboring elements in the periodic table and have similar
Table 1 Atom positions of orthorhombic Pnma Pd2Sn obtained from Rietveld refinement of synchrotron X-ray powder diffraction data (SXPD), time-of˚ flight neutron diffraction (N) data, and real-space refinement of powder X-ray (XPDF) data to 15 A Atom
x
y
z
˚ 2) B (A
Pd1
SXPD N PDF
0.053(2) 0.054(2) 0.05172(10)
1/4 1/4 1/4
0.218(1) 0.201(1) 0.22227(8)
0.66(9) 1.0
SXPD N PDF
0.177(2) 0.183(2) 0.18342(14)
1/4 1/4 1/4
0.5715(6) 0.566(1) 0.57806(6)
0.66(9) 1.0
SXPD N PDF
0.701(1) 0.703(2) 0.70670(10)
1/4 1/4 1/4
0.604(1) 0.614(1) 0.61737(5)
0.36(9) 0.67
Pd2
Sn
˚ ; b = 4.283(3) A ˚ ; c = 8.091(6) A ˚ . N: a = 5.647(1) A ˚ ; b = 4.294(1) A ˚ ; c = 8.103(2) A ˚. Cell parameters are SXPD: a = 5.635(4) A
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neutron scattering cross-sections [14] so neither the SXPD nor neutron could be employed for obtaining Pd:Sn ratios. The PDF method [15], in contrast to conventional diffraction techniques, utilizes both average (Bragg) diffraction and diffuse scattering, containing data pertaining to limited structural coherence. The method has traditionally been applied to crystals, glasses, and liquids, and has recently been implemented to quantitatively describe the structures of ˚ , in real a number of nanoparticle systems [16–18]. Fig. 4 displays the experimental nanoparticle PDF, G(r), fit to 15 A space. The refined Pd2Sn structure captures all the features of the experimental G(r), with the exception of a small ˚ , most likely corresponding to Pd–O bonding between the intermetallic particle and the PVP structural peak at 2.0 A stabilizing agent. The inset of Fig. 4 confirms the rapid disappearance of correlated structure associated with the small size of the Pd2Sn particles. Table 1 summarizes the structural results of the Rietveld refinement of synchrotron X-ray powder diffraction (SXPD) and time-of-flight neutron diffraction data (N), as well as pair distribution function analysis of synchrotron
Fig. 5. (Top) TEM image of Pd2Sn nanoparticles. The inset shows the bimodal size distribution obtained from the particles. (Bottom) Lattice image of Pd2Sn. The fringes in the large particle correspond to the 2 1 0 planes of Pd2Sn.
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X-ray scattering (XPDF). The X-ray and neutron Rietveld refinements are in excellent agreement. The ability of the PDF to more sensitively probe local structure is manifested in the greater precision with which the atomic positions are obtained in the PDFFIT refinement of G(r). Fig. 5 displays in the top panel, a transmission electron micrograph of the Pd2Sn particles without Pd impurities. We find a bimodal size distribution with many particles clustered around 5 nm in diameter, and a few larger particles near 20 nm. An analysis of particle size is displayed in the inset. The XRD patterns are clearly strongly weighted by the larger particles. It is possible that the dispersion of the sample on the TEM grid does not properly represent all sizes of the particles. In the bottom panel of Fig. 5a region of the particles is examined at higher resolution. The image shows the lattice fringes of Pd2Sn particles. The fringes running diagonally across the large particle in the image are separated ˚ spacings which correspond to 2 1 0 planes in the structure. Energy-dispersive X-ray analysis in the TEM by 2.35 A confirmed the presence of Pd and Sn in the particles. Given the quality of structural data and atom positions obtained from the Rietveld refinements, we considered it of interest to describe the electronic structure of this compound, and in particular, establish the nature of the bonding near the Fermi energy. Density functional calculations of the electronic structure were carried out using the linear muffin– tin orbital (LMTO) method within the atomic sphere approximation (ASA) as implemented in the Stuttgart TBLMTO-ASA program. [19] The calculations were performed on 388 irreducible k points in the primitive wedge of the Brillouin zone. Fig. 6(a) shows the projected densities of state of Pd (both Pd1 and Pd2) d densities of state, and Sn s and p densities of state. It is seen that the two Pd atoms in the formula unit are very similar in their chemical environment, and both have an almost filled d-shell with some states of rather small density at the Fermi energy. Given the small total densities of state at the Fermi energy, Pd2Sn is clearly not near any magnetic instability. Sn s and p states are centered near 9 eV with respect to EF and 5 eV, so there is little evidence for charge transfer from one moiety to another in this compound. The crystal orbital Hamiltonian population (COHP) allows densities of state to be recast in terms of the strengths of pairwise bonding and antibonding interactions, as a function of the energy, between the atoms in the cell, with negative COHPs signifying a bonding interaction and positive COHPs signifying an antibonding interaction [20]. In Fig. 6(b) Pd–Pd COHPs in Pd2Sn display d–d interactions typical of late transition metals, with the nearly filled d shells resulting in filled bonding states as well as filled antibonding states. Conduction arises (from the states crossing EF) due to antibonding Pd–Pd interactions. The Pd–Sn interaction is interesting, as the EF is precisely where the interaction switches from being bonding to being antibonding. This in a sense rationalizes the structure and composition of this compound, and also suggests why the stoichiometry is so stable.
Fig. 6. (a) Projected densities of Pd d states and Sn s and p states in Pd2Sn. (b) COHPs for Pd–Pd interactions and for Pd–Sn interactions. The origin on the energy axis is the Fermi energy.
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In conclusion, we have described a new route to the preparation of sub-20 nm particles of the intermetallic compound Pd2Sn, and characterized its crystal and electronic structure. We believe the route to hold promise for the preparation of ternary compounds such as the Heusler compounds XPd2Sn where X is a transition metal. Acknowledgements We gratefully acknowledge the National Science Foundation for support through a Career Award (NSF DMR0449354). KP has been supported by the NSF through IGERT and Graduate Student Fellowships. The work at UCSB made use of facilities of the Materials Research Laboratory, supported by the NSF (DMR05-20415). Work at Argonne National Labs and the Advanced Photon Source was supported by DOE Office of Basic Energy Sciences, under contract W-31-109-Eng.-38. This work has benefited from the use of NPDF at the Lujan Center at Los Alamos Neutron Science Center, funded by DOE Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE contract DE-AC52-06NA25396. The upgrade of NPDF has been funded by the NSF through grant DMR 00-76488. References [1] E.A. Sales, J. Jose, M.J. Mendes, F. Bozon-Verduraz, J. Catal. 195 (2000) 88. [2] R.N. Shelton, L.S. Hausermann-Berg, M.J. Johnson, P. Klavins, H.D. Yang, Phys. Rev. B 34 (1986) 199. [3] K. Schubert, H. Briemer, W. Burckhardt, E. Gu¨nzel, R. Haufler, H.L. Lukas, H. Vetter, J. Wegst, M. Wilkins, Naturwissenschaften 44 (1957) 229. [4] S. Geller, Acta Crystallogr. 8 (1955) 83. [5] J.M. Le´ger, J. Haines, A. Atouf, J. Appl. Crystallogr. 28 (1995) 416. [6] R.E. Schaak, A.K. Sra, B.M. Leonard, R.E. Cable, J.C. Bauer, Y.-F. Han, J. Means, W. Teizer, Y. Vasquez, E.S. Funck, J. Am. Chem. Soc. 127 (2005) 3506–3515; S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989–1992; E. Shevchenko, D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, J. Am. Chem. Soc. 124 (2002) 11480. [7] J.F. Be´rar, P. Garnier, NIST Spec. Publ. 846 (1992) 212. [8] Th. Proffen, T. Egami, S.J.L. Billinge, A.K. Cheetham, D. Louca, J.B. Parise, Appl. Phys. A 74 (2002) S163. [9] A.C. Larson, R.B. Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 1994, pp. 86–748. B.H. Toby, J. Appl. Crystallogr. 34 (2001) 210. [10] A.P. Hammersley, S.O. Svensson, M. Hanfland, A.N. Fitch, D. Hausermann, High Pressure Res. 14 (1996) 235. [11] P.J. Chupas, X. Qui, J.C. Hanson, P.L. Lee, C.P. Grey, S.J.L. Billinge, J. Appl. Cryst. 36 (2003) 1342. [12] Th. Proffen, S.J.L. Billinge, J. Appl. Cryst. 32 (1999) 572. [13] K. Schubert, H.L. Lukas, H.-G. Meissner, S. Bhan, Z. Metalkund. 50 (1959) 534. [14] V.F. Sears, Neutron News 3 (1992) 26. [15] T. Egami, S.J.L. Billinge, Underneath the Bragg Peaks: Structural Analysis of Complex Materials, Pergamon Press Elsevier, Oxford England, 2003. [16] K. Page, Th. Proffen, H. Terrones, M. Terrones, L. Lee, Y. Yang, S. Stemmer, R. Seshadri, A.K. Cheetham, Chem. Phys. Lett. 393 (2004) 385. [17] V. Petkov, M. Gateshki, J. Choi, E.G. Gillan, Y. Ren, J. Mater. Chem. 15 (2005) 4654. [18] R.B. Neder, V.I. Korsunskiy, J. Phys. Condens. Matter 17 (2005) S125. [19] O. Jepsen, O.K. Andersen, The STUTTGART TB-LMTO program, Max-Planck-Institut fu¨r Festko¨rperforschung, Stuttgart, Germany. [20] R. Dronskowski, P.E. Blo¨chl, J. Phys. Chem. 97 (1993) 8617.
Preparation and characterization of Pd2Sn nanoparticles Katharine Page a, Christina S. Schade a,b, Jinping Zhang a, Peter J. Chupas c, Karena W. Chapman c, Thomas Proffen d, Anthony K. Cheetham a, Ram Seshadri a,* a
Materials Department and Materials Research Laboratory, University of California, Santa Barbara, CA 93106, USA b Institut fu¨r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universita¨t Mainz, Duesbergweg 10-14, Mainz D55099, Germany c Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA d Los Alamos National Laboratory, Lujan Neutron Scattering Center LANSCE-12, MS H805, Los Alamos, NM 87545, USA Received 1 May 2007; accepted 3 May 2007 Available online 10 May 2007
Abstract We report a non-aqueous solution preparation of Pd2Sn nanoparticles with sizes near 20 nm. The intermetallic compound with the Co2Si structure has been characterized using transmission electron microscopy, Rietveld refinement of synchrotron X-ray and neutron powder diffraction, and real-space pair distribution function analysis of high-energy synchrotron X-ray scattering. We also present a description of the electronic structure of this covalent intermetallic using density functional calculations of the electronic structure. # 2007 Published by Elsevier Ltd. Keywords: A. Intermetallic compounds; C. Electron microscopy; C. Neutron scattering; C. X-ray diffraction
1. Introduction The preparation and characterization of nanostructured intermetallic materials has been receiving growing attention. The wide functionality of intermetallics combined with the materials characteristics of nanomaterials (new electronic, magnetic, and catalytic properties) promises an exciting future for this emerging class of materials. Here we report the preparation and characterization of Pd2Sn particles stabilized by polyvinylpyrrolidone. These are materials of interest for their catalytic properties. Palladium–tin catalysts in impregnated alumina supports have been studied for use in the selective hydrogenation of hexadienes [1]. Pd2Sn can be thought a chemical (but not structural) subunit of Heusler compounds with general composition RPd2Sn. For example, the coexistence of magnetism and superconductivity in the Heusler compounds ErPd2Sn is of great interest [2]. Bulk Pd2Sn [3] crystallizes in the structure of the mineral Paolovite Co2Si [4], which in turn is structurally similar to the cottunite structure adopted by many covalent and intermetallic A2B compounds. In the Paolovite structure, shown in Fig. 1, the A atom (Pd) has eight B (Sn) atom neighbors and five A atom neighbors, whereas the B atoms have ten A atom neighbors. Interestingly, this is a structure adopted by some alkaline earth halides under high pressure [5].
* Corresponding author. Tel.: +1 805 893 6129; fax: +1 805 893 8797. E-mail address: [email protected] (R. Seshadri). 0025-5408/$ – see front matter # 2007 Published by Elsevier Ltd. doi:10.1016/j.materresbull.2007.05.010
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Fig. 1. Crystal structure of Pd2Sn. The small black spheres are Pd, and the larger gray spheres are Sn.
In this contribution, we describe a non-aqueous solution preparation of small particles (sub 30 nm) of Pd2Sn starting from the metal halides and using an organic borohydride as the reducing agent. We present the structure as obtained from Rietveld refinement of synchrotron X-ray and neutron powder diffraction data and atomic pair distribution function (PDF) analysis of synchrotron X-ray diffraction data. We also describe the morphology of the particles obtained by high-resolution electron microscopy. This contribution adds to the recent, growing literature on the preparation of complex intermetallic phases through solution techniques [6]. 2. Experimental The compound was prepared at relatively high (for solution) temperatures, between 250 and 290 8C, in either refluxing benzyl ether or phenyl ether. We used the metal chlorides, PdCl2 and SnCl2, as starting materials, polyvinylpyrrolidone (PVP) as a stabilizer and tetrabutylammoniumborohydride, which is soluble in the ethers, as the reducing agent. All chemicals were purchased from Aldrich and used as received. All chemicals were opened and handled in a nitrogen-filled glove box. A typical preparation involved mixing a solution of PdCl2 (1 mmol), SnCl2 (2 mmol) and PVP (0.15 g) in 30 ml benzyl ether with a solution of (C4H9)4NBH4 (0.51 g) in 10 ml benzyl ether. After adding the reducing agent and stirring for about a minute the brownish suspension turned black. It was heated to reflux under flowing N2 and held there for about 1 h. After cooling down to room temperature the black solid was collected by centrifugation and washed two to three times with ethanol until a dry powder was obtained. X-ray diffraction (XRD) data were collected on powder samples using a Philips MRD diffractometer with Cu Ka; radiation ˚ ) and a graphite monochromator. Data were acquired over the angular range 20–908 2u, with a step size (l = 1.5418 A of 0.0178 (2u). XRD data were subject to Rietveld refinement using the XND Rietveld code [7]. Time-of-flight neutron diffraction data were collected at room temperature on NPDF [8] at the Lujan Neutron Scattering Center. Approximately 0.6 g were contained in a vanadium can for the experiments. The diffraction data were analyzed with the Rietveld method as embodied in the GSAS-EXPGUI [9] suite of programs. Synchrotron powder diffraction data was collected on 11-ID-B at the Advanced Photon Source of Argonne National Laboratory utilizing high-energy Xrays (90.6 kV) Data was collected on a 2D amorphous Si detector from General Electric Healthcare and processed using the program Fit-2D [10]. The pair distribution function: GðrÞ ¼ 4pr½rðrÞ r0 where r is the length of the vector in real space and r(r) is the radial density, was extracted as described previously [11] ˚ . Full structure profile refinements are carried out in the program with a maximum momentum transfer of Q = 28 A PDFFIT [12]. For HRTEM analysis, nanoparticle samples were dispersed in ethanol and loaded on a holey carbon grid by dropping two drops of the solution on to the grid. High-resolution images were obtained from a field emission transmission electron microscope (TEM) operated at 300 kV (FEI Tecnai F30UT, CS = 0.52 mm) in conventional high-resolution TEM and STEM modes. A high-angle annular dark-field detector was used for STEM imaging, and combined with an electron-dispersive X-ray detector (EDAX-R, 135 eV) for chemical analysis of the particles.
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Fig. 2. X-ray diffraction data (points) and Rietveld profile fits (solid lines) of the nanoparticles. Panel (a) displays a sample that contained fcc-Pd as a second phase. Panel (b) shows a sample prepared with 1:2 Pd:Sn starting ratios, and does not have any second phase. Vertical lines at the top of the figure display expected peak positions for the two phases.
3. Results and discussion Using the correct stoichiometry of Pd:Sn of 2:1 resulted in XRD patterns that showed evidence for fcc-Pd. Fig. 2(a) displays powder XRD data as points and a gray line as Rietveld fit to a mixture of two phases, Pd2Sn [13] as well as
Fig. 3. Rietveld refinement of synchrotron X-ray powder diffraction profiles showing data (points) the Rietveld fit (line) and the difference profile at the bottom. The wavelength was refined against lab diffraction data for a standard sample of ZnO. Vertical lines at the top of the figure display expected peak positions for Pd2Sn.
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˚ . The inset shows the experimental Fig. 4. Experimental G(r) (points) and fit (solid line) of the phase-pure particles to the Pd2Sn structure to 15 A data out to higher r, displaying the discrete size of the particles.
fcc-Pd metal. Quantitative phase analysis using Rietveld scale factors suggest that the two phases, Pd2Sn and Pd, occur in the XRD pattern in Fig. 2(a) in the mole fractions 58% and 42%, respectfully. We believe the problem of the two-phase product arises from the much more rapid rate of reduction of PdCl2 compared with SnCl2. Curiously, when we attempted to prepare the bulk Pd2Sn phase using standard arc-melting procedures starting with stoichiometric 2:1 mole ratios of elemental Pd and Sn, we similarly obtained a two-phase mixture of fcc-Pd and Pd2Sn. Starting with a 1:2 ratio of the chlorides for the solution synthesis resulted in phase-pure XRD patterns as seen in Fig. 2(b). Scherrer broadening analysis of the Pd2Sn 2 0 0 reflection from Fig. 2(b) suggests a correlation length near 20 nm. Synchrotron X-ray powder diffraction data (SXPD) was also subject to Rietveld refinement and this is displayed in Fig. 3. The small size of the particles and possible structure disorder, such as the presence of stacking variants, make it hard to model the data effectively. Displacement parameters could not be obtained reliably from these refinements, but this could in part be due to the manner in which the background is extracted in the process of integrating the 2D Debye–Scherrer rings. The time-of-flight neutron diffraction data were also similarly treated using the Rietveld method. Pd and Sn are neighboring elements in the periodic table and have similar
Table 1 Atom positions of orthorhombic Pnma Pd2Sn obtained from Rietveld refinement of synchrotron X-ray powder diffraction data (SXPD), time-of˚ flight neutron diffraction (N) data, and real-space refinement of powder X-ray (XPDF) data to 15 A Atom
x
y
z
˚ 2) B (A
Pd1
SXPD N PDF
0.053(2) 0.054(2) 0.05172(10)
1/4 1/4 1/4
0.218(1) 0.201(1) 0.22227(8)
0.66(9) 1.0
SXPD N PDF
0.177(2) 0.183(2) 0.18342(14)
1/4 1/4 1/4
0.5715(6) 0.566(1) 0.57806(6)
0.66(9) 1.0
SXPD N PDF
0.701(1) 0.703(2) 0.70670(10)
1/4 1/4 1/4
0.604(1) 0.614(1) 0.61737(5)
0.36(9) 0.67
Pd2
Sn
˚ ; b = 4.283(3) A ˚ ; c = 8.091(6) A ˚ . N: a = 5.647(1) A ˚ ; b = 4.294(1) A ˚ ; c = 8.103(2) A ˚. Cell parameters are SXPD: a = 5.635(4) A
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neutron scattering cross-sections [14] so neither the SXPD nor neutron could be employed for obtaining Pd:Sn ratios. The PDF method [15], in contrast to conventional diffraction techniques, utilizes both average (Bragg) diffraction and diffuse scattering, containing data pertaining to limited structural coherence. The method has traditionally been applied to crystals, glasses, and liquids, and has recently been implemented to quantitatively describe the structures of ˚ , in real a number of nanoparticle systems [16–18]. Fig. 4 displays the experimental nanoparticle PDF, G(r), fit to 15 A space. The refined Pd2Sn structure captures all the features of the experimental G(r), with the exception of a small ˚ , most likely corresponding to Pd–O bonding between the intermetallic particle and the PVP structural peak at 2.0 A stabilizing agent. The inset of Fig. 4 confirms the rapid disappearance of correlated structure associated with the small size of the Pd2Sn particles. Table 1 summarizes the structural results of the Rietveld refinement of synchrotron X-ray powder diffraction (SXPD) and time-of-flight neutron diffraction data (N), as well as pair distribution function analysis of synchrotron
Fig. 5. (Top) TEM image of Pd2Sn nanoparticles. The inset shows the bimodal size distribution obtained from the particles. (Bottom) Lattice image of Pd2Sn. The fringes in the large particle correspond to the 2 1 0 planes of Pd2Sn.
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X-ray scattering (XPDF). The X-ray and neutron Rietveld refinements are in excellent agreement. The ability of the PDF to more sensitively probe local structure is manifested in the greater precision with which the atomic positions are obtained in the PDFFIT refinement of G(r). Fig. 5 displays in the top panel, a transmission electron micrograph of the Pd2Sn particles without Pd impurities. We find a bimodal size distribution with many particles clustered around 5 nm in diameter, and a few larger particles near 20 nm. An analysis of particle size is displayed in the inset. The XRD patterns are clearly strongly weighted by the larger particles. It is possible that the dispersion of the sample on the TEM grid does not properly represent all sizes of the particles. In the bottom panel of Fig. 5a region of the particles is examined at higher resolution. The image shows the lattice fringes of Pd2Sn particles. The fringes running diagonally across the large particle in the image are separated ˚ spacings which correspond to 2 1 0 planes in the structure. Energy-dispersive X-ray analysis in the TEM by 2.35 A confirmed the presence of Pd and Sn in the particles. Given the quality of structural data and atom positions obtained from the Rietveld refinements, we considered it of interest to describe the electronic structure of this compound, and in particular, establish the nature of the bonding near the Fermi energy. Density functional calculations of the electronic structure were carried out using the linear muffin– tin orbital (LMTO) method within the atomic sphere approximation (ASA) as implemented in the Stuttgart TBLMTO-ASA program. [19] The calculations were performed on 388 irreducible k points in the primitive wedge of the Brillouin zone. Fig. 6(a) shows the projected densities of state of Pd (both Pd1 and Pd2) d densities of state, and Sn s and p densities of state. It is seen that the two Pd atoms in the formula unit are very similar in their chemical environment, and both have an almost filled d-shell with some states of rather small density at the Fermi energy. Given the small total densities of state at the Fermi energy, Pd2Sn is clearly not near any magnetic instability. Sn s and p states are centered near 9 eV with respect to EF and 5 eV, so there is little evidence for charge transfer from one moiety to another in this compound. The crystal orbital Hamiltonian population (COHP) allows densities of state to be recast in terms of the strengths of pairwise bonding and antibonding interactions, as a function of the energy, between the atoms in the cell, with negative COHPs signifying a bonding interaction and positive COHPs signifying an antibonding interaction [20]. In Fig. 6(b) Pd–Pd COHPs in Pd2Sn display d–d interactions typical of late transition metals, with the nearly filled d shells resulting in filled bonding states as well as filled antibonding states. Conduction arises (from the states crossing EF) due to antibonding Pd–Pd interactions. The Pd–Sn interaction is interesting, as the EF is precisely where the interaction switches from being bonding to being antibonding. This in a sense rationalizes the structure and composition of this compound, and also suggests why the stoichiometry is so stable.
Fig. 6. (a) Projected densities of Pd d states and Sn s and p states in Pd2Sn. (b) COHPs for Pd–Pd interactions and for Pd–Sn interactions. The origin on the energy axis is the Fermi energy.
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In conclusion, we have described a new route to the preparation of sub-20 nm particles of the intermetallic compound Pd2Sn, and characterized its crystal and electronic structure. We believe the route to hold promise for the preparation of ternary compounds such as the Heusler compounds XPd2Sn where X is a transition metal. Acknowledgements We gratefully acknowledge the National Science Foundation for support through a Career Award (NSF DMR0449354). KP has been supported by the NSF through IGERT and Graduate Student Fellowships. The work at UCSB made use of facilities of the Materials Research Laboratory, supported by the NSF (DMR05-20415). Work at Argonne National Labs and the Advanced Photon Source was supported by DOE Office of Basic Energy Sciences, under contract W-31-109-Eng.-38. This work has benefited from the use of NPDF at the Lujan Center at Los Alamos Neutron Science Center, funded by DOE Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE contract DE-AC52-06NA25396. The upgrade of NPDF has been funded by the NSF through grant DMR 00-76488. References [1] E.A. Sales, J. Jose, M.J. Mendes, F. Bozon-Verduraz, J. Catal. 195 (2000) 88. [2] R.N. Shelton, L.S. Hausermann-Berg, M.J. Johnson, P. Klavins, H.D. Yang, Phys. Rev. B 34 (1986) 199. [3] K. Schubert, H. Briemer, W. Burckhardt, E. Gu¨nzel, R. Haufler, H.L. Lukas, H. Vetter, J. Wegst, M. Wilkins, Naturwissenschaften 44 (1957) 229. [4] S. Geller, Acta Crystallogr. 8 (1955) 83. [5] J.M. Le´ger, J. Haines, A. Atouf, J. Appl. Crystallogr. 28 (1995) 416. [6] R.E. Schaak, A.K. Sra, B.M. Leonard, R.E. Cable, J.C. Bauer, Y.-F. Han, J. Means, W. Teizer, Y. Vasquez, E.S. Funck, J. Am. Chem. Soc. 127 (2005) 3506–3515; S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989–1992; E. Shevchenko, D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haase, H. Weller, J. Am. Chem. Soc. 124 (2002) 11480. [7] J.F. Be´rar, P. Garnier, NIST Spec. Publ. 846 (1992) 212. [8] Th. Proffen, T. Egami, S.J.L. Billinge, A.K. Cheetham, D. Louca, J.B. Parise, Appl. Phys. A 74 (2002) S163. [9] A.C. Larson, R.B. Von Dreele, General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 1994, pp. 86–748. B.H. Toby, J. Appl. Crystallogr. 34 (2001) 210. [10] A.P. Hammersley, S.O. Svensson, M. Hanfland, A.N. Fitch, D. Hausermann, High Pressure Res. 14 (1996) 235. [11] P.J. Chupas, X. Qui, J.C. Hanson, P.L. Lee, C.P. Grey, S.J.L. Billinge, J. Appl. Cryst. 36 (2003) 1342. [12] Th. Proffen, S.J.L. Billinge, J. Appl. Cryst. 32 (1999) 572. [13] K. Schubert, H.L. Lukas, H.-G. Meissner, S. Bhan, Z. Metalkund. 50 (1959) 534. [14] V.F. Sears, Neutron News 3 (1992) 26. [15] T. Egami, S.J.L. Billinge, Underneath the Bragg Peaks: Structural Analysis of Complex Materials, Pergamon Press Elsevier, Oxford England, 2003. [16] K. Page, Th. Proffen, H. Terrones, M. Terrones, L. Lee, Y. Yang, S. Stemmer, R. Seshadri, A.K. Cheetham, Chem. Phys. Lett. 393 (2004) 385. [17] V. Petkov, M. Gateshki, J. Choi, E.G. Gillan, Y. Ren, J. Mater. Chem. 15 (2005) 4654. [18] R.B. Neder, V.I. Korsunskiy, J. Phys. Condens. Matter 17 (2005) S125. [19] O. Jepsen, O.K. Andersen, The STUTTGART TB-LMTO program, Max-Planck-Institut fu¨r Festko¨rperforschung, Stuttgart, Germany. [20] R. Dronskowski, P.E. Blo¨chl, J. Phys. Chem. 97 (1993) 8617.