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Structural and morphological evaluation of Ru–Pd bimetallic nanocrystals
Materials Chemistry and Physics xxx (2016) 1e6
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Structural and morphological evaluation of RuePd bimetallic nanocrystals Xianfeng Ma a, c, Rui Lin a, Robert Y. Ofoli a, Zhi Mei b, **, James E. Jackson c, * a
Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA Department of Chemistry, Wayne State University, Detroit, MI 48202, USA c Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
1. Polyol reduction method generates well-controlled RuePd alloy nanocrystals. Ru precursor types play a significant role in tuning particle morphology and structures. Pd to Ru precursor molar ratio controls final particle size and composition. RuePd bimetallic nanocrystals display alloyed structures over full composition space.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2015 Received in revised form 20 October 2015 Accepted 2 February 2016 Available online xxx
RuePd bimetallic nanocrystals are successfully synthesized via a facile polyol co-reduction method. The resulting nanocrystals show spheres, triangular nanoplates, decahedra, nanorods, and irregular shapes. A combination of PdII and RuIII precursors tends to yield RuePd bimetallic nanocrystals of higher shape monodispersity than those from PdII and RuII precursors. The mole ratio between Ru and Pd components in the precursor solution also plays a key role in determining the size/shape distribution of the nanocrystals, with higher Pd/Ru ratios generating products of more uniform size. Elemental analyses and electron microscopy studies suggest that the obtained nanocrystals have alloyed structures over the full composition space and that they form through either monomer addition or coalescence mechanisms. © 2016 Elsevier B.V. All rights reserved.
Keywords: Alloys Nanostructures Chemical synthesis Crystal growth Energy dispersive analysis of X-rays (EDS or EDAX) Electron microscopy (STEM, TEM and SEM)
1. Introduction Bimetallic nanostructures have attracted great interest for their electronic, optical, and chemical properties which may be distinct
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Mei), [email protected] (J.E. Jackson).
from those of either of their pure constituent metals [1e6]. Control over composition, morphology, and structure of bimetallic nanocrystals is desirable as these factors dictate their properties and function. Bimetallic nanocrystal structures may be generally categorized as hetero, core/shell, and alloyed [7,8], and have traditionally been prepared by gaseous or solid state methods [9,10], which require harsh or complicated synthesis conditions with poor control of final crystal composition and structure. However, several controllable
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Please cite this article in press as: X. Ma, et al., Structural and morphological evaluation of RuePd bimetallic nanocrystals, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.003
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X. Ma et al. / Materials Chemistry and Physics xxx (2016) 1e6
solution-based synthesis methods for bimetallic nanocrystals have emerged in the last decade; these include co-reduction, thermal decomposition, seeded-growth, and galvanic replacement reactions [7,11,12]. These simple liquid-phase methods offer flexibility in control of morphology, composition and structure of the products via easily adjusted reaction parameters. Ru and Pd nanocrystals have properties distinct from those of their bulk counterparts. These differences are manifest in applications such as photonics, sensing, imaging, drug delivery, and energy conversion [13e18]. For example, the characteristic optical and electrical properties of Ru and Pd nanocrystals have facilitated their application in oxygen and hydrogen sensing [14e16]. They are also widely utilized in many catalytic processes for energy storage and conversion, though Ru and Pd often exhibit distinct catalytic behavior [19e24]. It is thus of intrinsic interest to explore the structures and properties of Ru and Pd alloys based on the hypothesis that the alloy may possess desirable synergistic properties, leading to potentially novel applications. However, simple RuePd alloys are not easily made over a broad range of elemental ratios because Ru and Pd are thermodynamically immiscible as bulk materials [14,25]. Thus, traditional gaseous or solid state methods under thermodynamic control are not suitable. Herein we demonstrate that RuePd alloyed nanocrystals can be successfully synthesized via the simple liquid phase polyol co-reduction method. Extensive characterizations of the as-synthesized RuePd nanocrystals help elucidate their formation mechanisms, enabling structure-property correlations for their applications. 2. Experimental section 2.1. Materials Palladium acetylacetonate([Pd(acac)2], 99%), Ruthenium acetylacetonate(Ru(acac)3, 99%), and dichlorotricarbonylruthenium (II) dimer([RuCl2(CO)3]2, 98%) were purchased from Strem Chemicals (Newburyport, MA). Poly-N-vinyl-2-pyrrolidone (PVP, MW ¼ 55 K) and 1,4-butanediol were purchased from Sigma-Aldrich (St. Louis, MO). High purity argon (99.999%) was purchased from Airgas (Lansing, MI). All chemicals were used without further purification.
temperature to 65 C with magnetic stirring to completely dissolve the salt, followed by 10 min of evacuation and introduction of argon into the system. The solution was then heated from 65 C to 180 C at a rate of 10 C/min. To monitor the progress of each reaction, 0.2 ml of sample was retrieved periodically from the reaction mixture using a long needle syringe, mixed with acetone to form a suspension, and centrifuged to observe the color of the supernatant. The reaction was terminated when the supernatant became clear. When the reaction was complete, the product mixture was cooled to room temperature and acetone (seven times the volume of liquid product mixture) was poured into the mixture to induce a cloudy brown suspension. This suspension was centrifuged at 5000 RPM for 8 min and the black product was collected after discarding the colorless supernatant. The precipitated nanocrystals were washed once with acetone, and then re-dispersed in methanol prior to any analysis. The concentrations of precursors used in each synthesis are listed in Table 1. The procedure used to synthesize the Pd nanocrystals was similar to that used to synthesize the bimetallic nanocrystals, except that 0.2 mmol Pd(acac)2 was added to the reaction mixture (Table 1), and the reaction temperature was 110 C. 2.3. Characterization High-resolution transmission electron microscopy (HRTEM) was used for morphology and structure studies of bimetallic nanocrystals. Prior to TEM characterization, each sample of colloidal nanocrystals was diluted 40 fold with ethanol and deposited onto a 3 nm carbon-coated copper grid (Ted Pella, Inc. Redding, CA). The grid was first dried overnight in vacuum desiccators at room temperature, and then dried again at 80 C for about 12 h. TEM characterization was performed on a JEOL 2200FS electron microscope (Tokyo, Japan) equipped with an energy dispersive X-ray (EDS) spectrometer (Oxford Instrument, UK). All characterizations were done at an acceleration voltage of 200 kV. The line scan, point spectra, and elemental mapping of each sample were obtained with the scanning model (STEM). The bulk metal composition in each sample was determined by Inductively coupled plasma atomic emission spectroscopy (ICP-AES. Vista Pro, Varian, Cary, NC).
2.2. Synthesis of colloidal RuePd bimetallic nanocrystals 3. Results and discussion The RuePd bimetallic nanocrystals were synthesized by polyol co-reduction of Ru and Pd metal precursors, using PVP as the stabilizing agent. In a typical synthesis, a total amount of 0.2 mmol metal precursors consisting of Ru(acac)3 (or [RuCl2(CO)3]2) and Pd(acac)2 was transferred into a 50 mL round bottom Schlenk flask equipped with a reflux condenser and a Teflon-coated magnetic stirring bar, followed by addition of 0.222 g PVP. Ten mL of 1,4butanediol was added at room temperature to dissolve the precursors and stabilizer. The mixture was heated from room
3.1. Metal precursor composition effects on RuePd nanocrystals Using mixtures of metal precursors of different oxidation states is one way to tailor the morphology of the alloyed nanocrystals in the co-reduction method. We used both RuIII and RuII as precursors, keeping other reaction parameters constant, to assess their effects on the resulting RuePd nanocrystals’ structure and morphology. Fig. 1 shows typical TEM images of bimetallic RuePd, Pd and Ru
Table 1 Metal concentrations in precursor solution for synthesis of RuePd bimetallic nanocrystals. Sample IDa
Ru concentration in precursor solution (mM)
Pd concentration in precursor solution (mM)
Pd fraction in metal precursor (mol%Pd)
Pd00Ru(II/III) Pd20Ru(II/III) Pd40Ru(II/III) Pd50Ru(II/III) Pd60Ru(II/III) Pd80Ru(II/III) Pd100Ru(II/III)
20.0 16.0 12.0 10.0 8.0 4.0 0.0
0.0 4.0 8.0 10.0 12.0 16.0 20.0
0 20 40 50 60 80 100
Each sample was labeled as Pd#RuII or Pd#RuIII, where # represents the molar percentage of Pd in the precursor mixture. a The Roman numerals represent the type of Ru precursor used. RuIII represents the precursor Ru(acac)3; RuII represents the precursor [RuCl2(CO)3]2.
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Fig. 2. TEM images of Ru-Pd nanocrystals synthesized with RuII and PdII as metal precursors as well as the monometallic nanocrystals. (a) monometallic Ru nanocrystals; (bef) Ru-Pd nanocrystals synthesized with Pd mole fractions of 20%, 40%, 50%, 60%, and 80%, respectively.
Fig. 1. TEM images and bulk composition of Ru-Pd nanocrystals synthesized with a mixture of RuIII and PdII as metal precursors as well as the monometallic nanocrystals. (a) monometallic Ru nanocrystals; (bef) Ru-Pd nanocrystals synthesized with Pd mole fractions of 20%, 40%, 50%, 60%, and 80%, respectively; (g) monometallic Pd nanocrystals; (h) The plot of measured metal composition vs. theoretical metal composition. The composition was determined by ICP-AES.
nanocrystals synthesized from RuIII and PdII precursors. The monometallic Ru nanocrystals formed from the RuIII precursor displayed a spherical shape with a diameter of ~4.0 nm (Fig. 1a). The average particle diameter increased to ~8 nm with 20 mol% Pd in the metal precursor mixture, and a higher percentage of irregular shaped particles were seen (Fig. 1b). The percentage of irregular particles decreased as the Pd precursor was increased to 40 mol%, with a mixture of truncated polyhedra, including hexagonal, pentagonal, cubic and triangular shapes, (Fig. 1c), along with a
small amount of branched and multiply twinned structures. As the mole fraction of Pd precursor increased, the shape of the RuePd nanocrystals tended to become more uniform (Fig. 1def), with mostly monodisperse spherical nanocrystals produced at 80 mol% Pd (Fig. 1f). The monometallic Pd nanocrystals (Fig. 1g) were primarily spheres with an average diameter of ~10 nm together with a few triangular plates and decahedra. The average bulk composition(s) determined by ICP-AES in all obtained samples were in good agreement with the initial components’ proportions (Fig. 1h), suggesting similar reduction rates for RuIII and PdII ions during nanocrystal formation. In contrast to the above findings for RuIII, when the more-easilyreduced RuII was used as the Ru precursor, the resulting RuePd bimetallic nanocrystals showed drastic morphology variation (Fig. 2). Monometallic Ru nanocrystals made with RuII as precursor were spherical with an average particle diameter of 129 nm (Fig. 2a), one order of magnitude larger than those from RuIII and Pd nanocrystals. At 20 mol% Pd in the metal precursor mixture, a large number of rod-shaped nanocrystals were visible (Fig. 2b). Also seen were some small irregular nanocrystals, which may play a role as building blocks for further particle growth. The generation of such polydisperse nanocrystals from the RuII precursor was possibly due to the significant difference in redox potentials between the two
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Fig. 6. HRTEM images and corresponding FFT patterns of typical-I (a) and typical-II (b) Ru-Pd triangular nanoplates.
Fig. 3. Representative STEM images and EDS line scan spectra of Ru-Pd nanocrystals (scale bar 10 nm in each image). (a) Ru-Pd spherical polyhedron; (b) RuPd decahedron; (c) Ru-Pd triangular plate; (d) Ru-Pd nanorod. The consistent distribution of EDS signals for Ru (red) and Pd (cyan) in each sample suggests the formation of alloyed nanostructures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. HRTEM images and corresponding FFT patterns of typical-I (a) and typical-II (b) Ru-Pd nanorods.
metal precursors (0.455 V for RuII/Ru0 vs. 0.915 V for PdII/Pd0) [26,27]. At 40 mol% Pd in the metal precursors, the RuePd nanocrystals contained nanorod, nanoplate, star-shaped decahedra, icosahedra, and irregular dendrites (Fig. 2c); at 50 mol% Pd, fewer nanorods were observed (Fig. 2d); at 60 mol% Pd (Fig. 2e), particle uniformity further improved, while mostly monodisperse spherical nanocrystals of ~14 ± 2 nm diameter were formed at 80 mol% Pd (Fig. 2f). 3.2. The structure and compositions of RuePd nanocrystals with different morphologies Fig. 4. HRTEM images and corresponding FFT patterns of typical-I (a) and typical-II (b) Ru-Pd nanocrystals with near-spherical shape.
Fig. 5. HRTEM images and corresponding FFT patterns of typical-I (a) and typical-II (b) Ru-Pd nanocrystals in decahedral shape.
RuePd bimetallic nanocrystals with different shapes were studied by HRTEM and EDS to evaluate their structural character and element distribution. The EDS line scans and mapping were carried out on spherical polyhedron, star-shaped decahedron, and triangular-plate particles from samples synthesized using RuIII and PdII as metal precursors; here, the metal composition of the precursor mix was 50 mol% of PdII, 50 mol% RuIII (sample name Pd50RuIII) (Fig. 3aec). All Ru-Pd nanocrystals showed single Gaussian distributions of signals across the particle for both elements, demonstrating the random arrangement of atoms on the surface and in the bulk of the particle. Stray monometallic particles or regions were not observed in any of the RuPd particles of various shapes. This indicated that all particles were true bimetallic alloys. The EDS line scan and mapping profiles of a nanorod from sample Pd40RuII were also recorded (Fig. 3d), confirming that within the resolution of the method, Ru and Pd appeared to be distributed homogeneously in a single alloy phase. Fig. 4 shows the HRTEM images and corresponding fast Fourier transform (FFT) patterns of commonly-observed Ru-Pd particles of spherical polyhedron shape. Sometimes the icosahedral particles
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showed truncation at the vertices (Fig. 4a). The icosahedra had hexagonal projections, corresponding to orientations with either a 2-fold or 3-fold axis parallel to the electron beam [28]. Their FFT patterns (e.g. Fig. 4a, inset) showed a single roughly spherical crystal, with stacking faults inside its center. An icosahedron with 3-fold symmetry is displayed in Fig. 4b. Six twin planes could be observed. Each plane extended from the center of the particle to the edge along the twin boundary. The imperfect alignment suggested a poly-crystalline multi-twinned structure for this particle. However, well-defined fringes with lattice spacing of 0.22 nm could be observed in each domain of the particle. Fig. 5 shows representative HRTEM images and the corresponding FFT patterns of two typical star-shaped Ru-Pd decahedrons (approximate diameter 18 nm). In the FFT patterns (see inset, Fig. 5), the diffraction spots were interpreted as the superposition of five twin-related electron diffraction patterns, indicating that the decahedrons comprised five single crystal domains [29]. Visible also in the HRTEM images, the five domains shared twin-based adjoining planes extending from the edges to the center of the particles. These twin boundaries were coherent and free from stacking faults (SF), but some were more sharply defined than others, and some displayed complex contrast with evident structural distortions, especially at the central convergence region. They did not meet at one single point; rather, they converged in a small central area, suggesting a degree of defect of the five-fold symmetry. Also, between two adjacent twin planes (Fig. 5b), a 3-nm wide gap was seen mainly consisting of stacking faults presumably induced by lattice strain associated with the formation of the decahedron. Fig. 6a shows a typical HRTEM image of a Ru-Pd bimetallic nanocrystal of triangular-plate shape. The average side length of the triangular nanoplate was about 16 nm. The whole triangular plate (Fig. 6a) suggested that growth was complete. Smaller triangular plates, on the other hand, continued to grow from the two tips (Fig. 6b). From the FFT patterns (the insets in Fig. 6), the triangular plates were all single crystals. According to classical monomeraddition crystal growth theory, nanocrystals typically undergo continuous and near isotropic growth to form spherical shapes whose uniform diffraction confirms their identity as single crystals. Thus it appears reasonable to postulate that the triangular plates were similarly formed by atomic monomer addition [30]. The representative HRTEM images and the corresponding FFT patterns of nanorods, the primary products in the sample Pd20RuII, are shown in Fig. 7. The nanorod was about 2.7 nm wide and 15e25 nm in length. There were two structures of the nanorods resulting from different growth mechanisms. One was the extension along the specific facets of a small single-crystalline seed, indicated by the parallel fringes with a lattice spacing of 0.19 nm (Fig. 7a). The single crystal domain suggested that the reduced metal atoms nucleated and grew epitaxially from the end facets in a continuous manner, consistent with the classical atomic monomer addition pathway [30]. The other was a conjugated structure, composed of two or more single-crystalline domains, joined by coherent boundaries with lattice distortions between the short sections (Fig. 7b). Most of those single-crystalline domains were interconnected in a head-to-head manner to extend the nanorod, while a few of them were connected perpendicularly to the axis of the nanorod, forming branches. Such conjugated structures appeared to be results of multiple coalescence of smaller particles. 4. Conclusions In summary, we have demonstrated the ability to synthesize RuePd bimetallic nanocrystals by a facile polyol co-reduction of corresponding metal precursors. We found that the oxidation state
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of the Ru precursor has a strong influence on the final morphology and structure of the particles; RuII as precursor tends to yield nanocrystals with high shape polydispersity, including spheres, triangular nanoplates, decahedra, nanorods, and irregular shapes, as compared to the case with RuIII as precursor. The molar ratio between Ru and Pd metal precursors in the precursor solution also plays a key role in determining the size/shape distribution of the nanocrystals, with increases in the Pd/Ru ratio leading to more monodisperse products. Spectroscopic elemental composition studies indicated that Ru-Pd nanocrystals so obtained are true alloys as opposed to aggregates of different metal grains. Based on the combined HRTEM and FFT analyses on the structures of RuePd nanocrystals with different morphologies, including nanorod, decahedron, spherical polyhedron and triangular-plate, monomer addition and multiple coalescence of small particles are proposed to be the major growth mechanisms. Acknowledgments This work was supported by the 21st Century Jobs Fund of the Michigan Economic Development Corporation. References [1] Y. Tang, M. Ouyang, Tailoring properties and functionalities of metal nanoparticles through crystallinity engineering, Nat. Mater. 6 (2007) 754e759. [2] S. Alayoglu, A.U. Nilekar, M. Mavrikakis, B. Eichhorn, Ru-Pt core-shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen, Nat. Mater. 7 (2008) 333e338. [3] J. Bao, D.C. Bell, F. Capasso, J.B. Wagner, T. Mårtensson, J. Tr€ agårdh, L. Samuelson, Optical properties of rotationally twinned InP nanowire heterostructure, Nano. Lett. 8 (2008) 836e841. [4] A.M. Smith, A.M. Mohs, S. Nie, Tuning the optical and electronic properties of colloidal nanocrystals by lattice strain, Nat. Nano 4 (2009) 56e63. [5] S. Zhou, B. Varughese, B. Eichhorn, G. Jackson, K. McIlwrath, PteCu coreeshell and alloy nanoparticles for heterogeneous NOx reduction: anomalous stability and reactivity of a coreeshell nanostructure, Angew. Chem. Int. Ed. 44 (2005) 4539e4543. [6] H. Zhang, M. Jin, H. Liu, J. Wang, M.J. Kim, D. Yang, Z. Xie, J. Liu, Y. Xia, Facile synthesis of PdePt alloy nanocage and their enhanced performance for preferential oxidation of CO in excess hydrogen, ACS Nano 5 (2011) 8212e8222. [7] D. Wang, Y. Li, Bimetallic nanocrystals: liquid-phase synthesis and catalytic applications, Adv. Mater. 23 (2011) 1044e1060. [8] S. Alayoglu, B. Eichhorn, RhPt bimetallic catalysts: synthesis, characterization, and catalysis of coreshell, alloy, and monometallic nanoparticles, J. Am. Chem. Soc. 130 (2008) 17479e17486. [9] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1e184. [10] C. Binns, Nanocluster deposited on surfaces, Surf. Sci. Rep. 44 (2001) 1e49. [11] L. Guczi, G. Boskovic, E. Kiss, Bimetallic cobalt based catalysts, Catal. Rev. Sci. Eng. 52 (2010) 133e203. [12] C.J. Jia, F. Schuth, Colloidal metal nanoparticles as a component of designed catalyst, Phys. Chem. Chem. Phys. 13 (2011) 2457e2487. [13] M.B. Cortie, A.M. McDonagh, Synthesis and optical properties of hybrid and alloy plasmonic nanoparticles, Chem. Rev. 111 (2011) 3713e3735. [14] K. Kusada, H. Kobayashi, R. Ikeda, Y. Kubota, M. Takata, S. Toh, T. Yamamoto, S. Matsumura, N. Sumi, K. Sato, K. Nagaoka, H. Kitagawa, Solid solution alloy nanoparticles of immiscible Pd and Ru elements neighboring on Rh: changeover of the thermodynamic behavior for hydrogen storage and enhanced CO-oxidizing ability, J. Am. Chem. Soc. 136 (2014) 1864e1871. [15] D. Sil, J. Hines, U. Udeoyo, E. Borguet, Palladium nanoparticle-based surface acoustic wave hydrogen sensor, ACS Appl. Mater. Interfaces 7 (2015) 5709e5714. [16] M. Zolkapli, Z. Mahmud, S.H. Herman, W.F.H. Abdullah, U.M. Noorl, S. Saharudin, Fluorescence Characteristic of Ruthenium Nanoparticles as a Dissolved Oxygen Sensing Material in Gas and Aqueos Phase, Signal Processing & its Applications (CSPA), 2014, pp. 195e198. [17] M. Segev-Bar, H. Haick, Flexible sensors based on nanoparticles, ACS Nano 7 (2013) 8366e8378. [18] R.A. Ganeev, M. Suzuki, M. Baba, M. Ichihara, H. Kuroda, Low- and high-order nonlinear optical properties of Au, Pt, Pd, and Ru nanoparticles, J. Appl. Phys. 103 (2008), 063102e063102. [19] W.P. Deng, X.S. Tan, W.H. Fang, Q.H. Zhang, Y. Wang, Conversion of cellulose into sorbitol over carbon nanotube-supported ruthenium catalyst, Catal. Lett. 133 (2009) 167e174. [20] M. Lashdaf, A.O.I. Krause, M. Lindblad, M. Tiitta, T. Venalainen, Behavior of
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Structural and morphological evaluation of RuePd bimetallic nanocrystals Xianfeng Ma a, c, Rui Lin a, Robert Y. Ofoli a, Zhi Mei b, **, James E. Jackson c, * a
Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA Department of Chemistry, Wayne State University, Detroit, MI 48202, USA c Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
1. Polyol reduction method generates well-controlled RuePd alloy nanocrystals. Ru precursor types play a significant role in tuning particle morphology and structures. Pd to Ru precursor molar ratio controls final particle size and composition. RuePd bimetallic nanocrystals display alloyed structures over full composition space.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2015 Received in revised form 20 October 2015 Accepted 2 February 2016 Available online xxx
RuePd bimetallic nanocrystals are successfully synthesized via a facile polyol co-reduction method. The resulting nanocrystals show spheres, triangular nanoplates, decahedra, nanorods, and irregular shapes. A combination of PdII and RuIII precursors tends to yield RuePd bimetallic nanocrystals of higher shape monodispersity than those from PdII and RuII precursors. The mole ratio between Ru and Pd components in the precursor solution also plays a key role in determining the size/shape distribution of the nanocrystals, with higher Pd/Ru ratios generating products of more uniform size. Elemental analyses and electron microscopy studies suggest that the obtained nanocrystals have alloyed structures over the full composition space and that they form through either monomer addition or coalescence mechanisms. © 2016 Elsevier B.V. All rights reserved.
Keywords: Alloys Nanostructures Chemical synthesis Crystal growth Energy dispersive analysis of X-rays (EDS or EDAX) Electron microscopy (STEM, TEM and SEM)
1. Introduction Bimetallic nanostructures have attracted great interest for their electronic, optical, and chemical properties which may be distinct
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Mei), [email protected] (J.E. Jackson).
from those of either of their pure constituent metals [1e6]. Control over composition, morphology, and structure of bimetallic nanocrystals is desirable as these factors dictate their properties and function. Bimetallic nanocrystal structures may be generally categorized as hetero, core/shell, and alloyed [7,8], and have traditionally been prepared by gaseous or solid state methods [9,10], which require harsh or complicated synthesis conditions with poor control of final crystal composition and structure. However, several controllable
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solution-based synthesis methods for bimetallic nanocrystals have emerged in the last decade; these include co-reduction, thermal decomposition, seeded-growth, and galvanic replacement reactions [7,11,12]. These simple liquid-phase methods offer flexibility in control of morphology, composition and structure of the products via easily adjusted reaction parameters. Ru and Pd nanocrystals have properties distinct from those of their bulk counterparts. These differences are manifest in applications such as photonics, sensing, imaging, drug delivery, and energy conversion [13e18]. For example, the characteristic optical and electrical properties of Ru and Pd nanocrystals have facilitated their application in oxygen and hydrogen sensing [14e16]. They are also widely utilized in many catalytic processes for energy storage and conversion, though Ru and Pd often exhibit distinct catalytic behavior [19e24]. It is thus of intrinsic interest to explore the structures and properties of Ru and Pd alloys based on the hypothesis that the alloy may possess desirable synergistic properties, leading to potentially novel applications. However, simple RuePd alloys are not easily made over a broad range of elemental ratios because Ru and Pd are thermodynamically immiscible as bulk materials [14,25]. Thus, traditional gaseous or solid state methods under thermodynamic control are not suitable. Herein we demonstrate that RuePd alloyed nanocrystals can be successfully synthesized via the simple liquid phase polyol co-reduction method. Extensive characterizations of the as-synthesized RuePd nanocrystals help elucidate their formation mechanisms, enabling structure-property correlations for their applications. 2. Experimental section 2.1. Materials Palladium acetylacetonate([Pd(acac)2], 99%), Ruthenium acetylacetonate(Ru(acac)3, 99%), and dichlorotricarbonylruthenium (II) dimer([RuCl2(CO)3]2, 98%) were purchased from Strem Chemicals (Newburyport, MA). Poly-N-vinyl-2-pyrrolidone (PVP, MW ¼ 55 K) and 1,4-butanediol were purchased from Sigma-Aldrich (St. Louis, MO). High purity argon (99.999%) was purchased from Airgas (Lansing, MI). All chemicals were used without further purification.
temperature to 65 C with magnetic stirring to completely dissolve the salt, followed by 10 min of evacuation and introduction of argon into the system. The solution was then heated from 65 C to 180 C at a rate of 10 C/min. To monitor the progress of each reaction, 0.2 ml of sample was retrieved periodically from the reaction mixture using a long needle syringe, mixed with acetone to form a suspension, and centrifuged to observe the color of the supernatant. The reaction was terminated when the supernatant became clear. When the reaction was complete, the product mixture was cooled to room temperature and acetone (seven times the volume of liquid product mixture) was poured into the mixture to induce a cloudy brown suspension. This suspension was centrifuged at 5000 RPM for 8 min and the black product was collected after discarding the colorless supernatant. The precipitated nanocrystals were washed once with acetone, and then re-dispersed in methanol prior to any analysis. The concentrations of precursors used in each synthesis are listed in Table 1. The procedure used to synthesize the Pd nanocrystals was similar to that used to synthesize the bimetallic nanocrystals, except that 0.2 mmol Pd(acac)2 was added to the reaction mixture (Table 1), and the reaction temperature was 110 C. 2.3. Characterization High-resolution transmission electron microscopy (HRTEM) was used for morphology and structure studies of bimetallic nanocrystals. Prior to TEM characterization, each sample of colloidal nanocrystals was diluted 40 fold with ethanol and deposited onto a 3 nm carbon-coated copper grid (Ted Pella, Inc. Redding, CA). The grid was first dried overnight in vacuum desiccators at room temperature, and then dried again at 80 C for about 12 h. TEM characterization was performed on a JEOL 2200FS electron microscope (Tokyo, Japan) equipped with an energy dispersive X-ray (EDS) spectrometer (Oxford Instrument, UK). All characterizations were done at an acceleration voltage of 200 kV. The line scan, point spectra, and elemental mapping of each sample were obtained with the scanning model (STEM). The bulk metal composition in each sample was determined by Inductively coupled plasma atomic emission spectroscopy (ICP-AES. Vista Pro, Varian, Cary, NC).
2.2. Synthesis of colloidal RuePd bimetallic nanocrystals 3. Results and discussion The RuePd bimetallic nanocrystals were synthesized by polyol co-reduction of Ru and Pd metal precursors, using PVP as the stabilizing agent. In a typical synthesis, a total amount of 0.2 mmol metal precursors consisting of Ru(acac)3 (or [RuCl2(CO)3]2) and Pd(acac)2 was transferred into a 50 mL round bottom Schlenk flask equipped with a reflux condenser and a Teflon-coated magnetic stirring bar, followed by addition of 0.222 g PVP. Ten mL of 1,4butanediol was added at room temperature to dissolve the precursors and stabilizer. The mixture was heated from room
3.1. Metal precursor composition effects on RuePd nanocrystals Using mixtures of metal precursors of different oxidation states is one way to tailor the morphology of the alloyed nanocrystals in the co-reduction method. We used both RuIII and RuII as precursors, keeping other reaction parameters constant, to assess their effects on the resulting RuePd nanocrystals’ structure and morphology. Fig. 1 shows typical TEM images of bimetallic RuePd, Pd and Ru
Table 1 Metal concentrations in precursor solution for synthesis of RuePd bimetallic nanocrystals. Sample IDa
Ru concentration in precursor solution (mM)
Pd concentration in precursor solution (mM)
Pd fraction in metal precursor (mol%Pd)
Pd00Ru(II/III) Pd20Ru(II/III) Pd40Ru(II/III) Pd50Ru(II/III) Pd60Ru(II/III) Pd80Ru(II/III) Pd100Ru(II/III)
20.0 16.0 12.0 10.0 8.0 4.0 0.0
0.0 4.0 8.0 10.0 12.0 16.0 20.0
0 20 40 50 60 80 100
Each sample was labeled as Pd#RuII or Pd#RuIII, where # represents the molar percentage of Pd in the precursor mixture. a The Roman numerals represent the type of Ru precursor used. RuIII represents the precursor Ru(acac)3; RuII represents the precursor [RuCl2(CO)3]2.
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Fig. 2. TEM images of Ru-Pd nanocrystals synthesized with RuII and PdII as metal precursors as well as the monometallic nanocrystals. (a) monometallic Ru nanocrystals; (bef) Ru-Pd nanocrystals synthesized with Pd mole fractions of 20%, 40%, 50%, 60%, and 80%, respectively.
Fig. 1. TEM images and bulk composition of Ru-Pd nanocrystals synthesized with a mixture of RuIII and PdII as metal precursors as well as the monometallic nanocrystals. (a) monometallic Ru nanocrystals; (bef) Ru-Pd nanocrystals synthesized with Pd mole fractions of 20%, 40%, 50%, 60%, and 80%, respectively; (g) monometallic Pd nanocrystals; (h) The plot of measured metal composition vs. theoretical metal composition. The composition was determined by ICP-AES.
nanocrystals synthesized from RuIII and PdII precursors. The monometallic Ru nanocrystals formed from the RuIII precursor displayed a spherical shape with a diameter of ~4.0 nm (Fig. 1a). The average particle diameter increased to ~8 nm with 20 mol% Pd in the metal precursor mixture, and a higher percentage of irregular shaped particles were seen (Fig. 1b). The percentage of irregular particles decreased as the Pd precursor was increased to 40 mol%, with a mixture of truncated polyhedra, including hexagonal, pentagonal, cubic and triangular shapes, (Fig. 1c), along with a
small amount of branched and multiply twinned structures. As the mole fraction of Pd precursor increased, the shape of the RuePd nanocrystals tended to become more uniform (Fig. 1def), with mostly monodisperse spherical nanocrystals produced at 80 mol% Pd (Fig. 1f). The monometallic Pd nanocrystals (Fig. 1g) were primarily spheres with an average diameter of ~10 nm together with a few triangular plates and decahedra. The average bulk composition(s) determined by ICP-AES in all obtained samples were in good agreement with the initial components’ proportions (Fig. 1h), suggesting similar reduction rates for RuIII and PdII ions during nanocrystal formation. In contrast to the above findings for RuIII, when the more-easilyreduced RuII was used as the Ru precursor, the resulting RuePd bimetallic nanocrystals showed drastic morphology variation (Fig. 2). Monometallic Ru nanocrystals made with RuII as precursor were spherical with an average particle diameter of 129 nm (Fig. 2a), one order of magnitude larger than those from RuIII and Pd nanocrystals. At 20 mol% Pd in the metal precursor mixture, a large number of rod-shaped nanocrystals were visible (Fig. 2b). Also seen were some small irregular nanocrystals, which may play a role as building blocks for further particle growth. The generation of such polydisperse nanocrystals from the RuII precursor was possibly due to the significant difference in redox potentials between the two
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Fig. 6. HRTEM images and corresponding FFT patterns of typical-I (a) and typical-II (b) Ru-Pd triangular nanoplates.
Fig. 3. Representative STEM images and EDS line scan spectra of Ru-Pd nanocrystals (scale bar 10 nm in each image). (a) Ru-Pd spherical polyhedron; (b) RuPd decahedron; (c) Ru-Pd triangular plate; (d) Ru-Pd nanorod. The consistent distribution of EDS signals for Ru (red) and Pd (cyan) in each sample suggests the formation of alloyed nanostructures. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. HRTEM images and corresponding FFT patterns of typical-I (a) and typical-II (b) Ru-Pd nanorods.
metal precursors (0.455 V for RuII/Ru0 vs. 0.915 V for PdII/Pd0) [26,27]. At 40 mol% Pd in the metal precursors, the RuePd nanocrystals contained nanorod, nanoplate, star-shaped decahedra, icosahedra, and irregular dendrites (Fig. 2c); at 50 mol% Pd, fewer nanorods were observed (Fig. 2d); at 60 mol% Pd (Fig. 2e), particle uniformity further improved, while mostly monodisperse spherical nanocrystals of ~14 ± 2 nm diameter were formed at 80 mol% Pd (Fig. 2f). 3.2. The structure and compositions of RuePd nanocrystals with different morphologies Fig. 4. HRTEM images and corresponding FFT patterns of typical-I (a) and typical-II (b) Ru-Pd nanocrystals with near-spherical shape.
Fig. 5. HRTEM images and corresponding FFT patterns of typical-I (a) and typical-II (b) Ru-Pd nanocrystals in decahedral shape.
RuePd bimetallic nanocrystals with different shapes were studied by HRTEM and EDS to evaluate their structural character and element distribution. The EDS line scans and mapping were carried out on spherical polyhedron, star-shaped decahedron, and triangular-plate particles from samples synthesized using RuIII and PdII as metal precursors; here, the metal composition of the precursor mix was 50 mol% of PdII, 50 mol% RuIII (sample name Pd50RuIII) (Fig. 3aec). All Ru-Pd nanocrystals showed single Gaussian distributions of signals across the particle for both elements, demonstrating the random arrangement of atoms on the surface and in the bulk of the particle. Stray monometallic particles or regions were not observed in any of the RuPd particles of various shapes. This indicated that all particles were true bimetallic alloys. The EDS line scan and mapping profiles of a nanorod from sample Pd40RuII were also recorded (Fig. 3d), confirming that within the resolution of the method, Ru and Pd appeared to be distributed homogeneously in a single alloy phase. Fig. 4 shows the HRTEM images and corresponding fast Fourier transform (FFT) patterns of commonly-observed Ru-Pd particles of spherical polyhedron shape. Sometimes the icosahedral particles
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showed truncation at the vertices (Fig. 4a). The icosahedra had hexagonal projections, corresponding to orientations with either a 2-fold or 3-fold axis parallel to the electron beam [28]. Their FFT patterns (e.g. Fig. 4a, inset) showed a single roughly spherical crystal, with stacking faults inside its center. An icosahedron with 3-fold symmetry is displayed in Fig. 4b. Six twin planes could be observed. Each plane extended from the center of the particle to the edge along the twin boundary. The imperfect alignment suggested a poly-crystalline multi-twinned structure for this particle. However, well-defined fringes with lattice spacing of 0.22 nm could be observed in each domain of the particle. Fig. 5 shows representative HRTEM images and the corresponding FFT patterns of two typical star-shaped Ru-Pd decahedrons (approximate diameter 18 nm). In the FFT patterns (see inset, Fig. 5), the diffraction spots were interpreted as the superposition of five twin-related electron diffraction patterns, indicating that the decahedrons comprised five single crystal domains [29]. Visible also in the HRTEM images, the five domains shared twin-based adjoining planes extending from the edges to the center of the particles. These twin boundaries were coherent and free from stacking faults (SF), but some were more sharply defined than others, and some displayed complex contrast with evident structural distortions, especially at the central convergence region. They did not meet at one single point; rather, they converged in a small central area, suggesting a degree of defect of the five-fold symmetry. Also, between two adjacent twin planes (Fig. 5b), a 3-nm wide gap was seen mainly consisting of stacking faults presumably induced by lattice strain associated with the formation of the decahedron. Fig. 6a shows a typical HRTEM image of a Ru-Pd bimetallic nanocrystal of triangular-plate shape. The average side length of the triangular nanoplate was about 16 nm. The whole triangular plate (Fig. 6a) suggested that growth was complete. Smaller triangular plates, on the other hand, continued to grow from the two tips (Fig. 6b). From the FFT patterns (the insets in Fig. 6), the triangular plates were all single crystals. According to classical monomeraddition crystal growth theory, nanocrystals typically undergo continuous and near isotropic growth to form spherical shapes whose uniform diffraction confirms their identity as single crystals. Thus it appears reasonable to postulate that the triangular plates were similarly formed by atomic monomer addition [30]. The representative HRTEM images and the corresponding FFT patterns of nanorods, the primary products in the sample Pd20RuII, are shown in Fig. 7. The nanorod was about 2.7 nm wide and 15e25 nm in length. There were two structures of the nanorods resulting from different growth mechanisms. One was the extension along the specific facets of a small single-crystalline seed, indicated by the parallel fringes with a lattice spacing of 0.19 nm (Fig. 7a). The single crystal domain suggested that the reduced metal atoms nucleated and grew epitaxially from the end facets in a continuous manner, consistent with the classical atomic monomer addition pathway [30]. The other was a conjugated structure, composed of two or more single-crystalline domains, joined by coherent boundaries with lattice distortions between the short sections (Fig. 7b). Most of those single-crystalline domains were interconnected in a head-to-head manner to extend the nanorod, while a few of them were connected perpendicularly to the axis of the nanorod, forming branches. Such conjugated structures appeared to be results of multiple coalescence of smaller particles. 4. Conclusions In summary, we have demonstrated the ability to synthesize RuePd bimetallic nanocrystals by a facile polyol co-reduction of corresponding metal precursors. We found that the oxidation state
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of the Ru precursor has a strong influence on the final morphology and structure of the particles; RuII as precursor tends to yield nanocrystals with high shape polydispersity, including spheres, triangular nanoplates, decahedra, nanorods, and irregular shapes, as compared to the case with RuIII as precursor. The molar ratio between Ru and Pd metal precursors in the precursor solution also plays a key role in determining the size/shape distribution of the nanocrystals, with increases in the Pd/Ru ratio leading to more monodisperse products. Spectroscopic elemental composition studies indicated that Ru-Pd nanocrystals so obtained are true alloys as opposed to aggregates of different metal grains. Based on the combined HRTEM and FFT analyses on the structures of RuePd nanocrystals with different morphologies, including nanorod, decahedron, spherical polyhedron and triangular-plate, monomer addition and multiple coalescence of small particles are proposed to be the major growth mechanisms. Acknowledgments This work was supported by the 21st Century Jobs Fund of the Michigan Economic Development Corporation. References [1] Y. Tang, M. Ouyang, Tailoring properties and functionalities of metal nanoparticles through crystallinity engineering, Nat. Mater. 6 (2007) 754e759. [2] S. Alayoglu, A.U. Nilekar, M. Mavrikakis, B. Eichhorn, Ru-Pt core-shell nanoparticles for preferential oxidation of carbon monoxide in hydrogen, Nat. Mater. 7 (2008) 333e338. [3] J. Bao, D.C. Bell, F. Capasso, J.B. Wagner, T. Mårtensson, J. Tr€ agårdh, L. Samuelson, Optical properties of rotationally twinned InP nanowire heterostructure, Nano. Lett. 8 (2008) 836e841. [4] A.M. Smith, A.M. Mohs, S. Nie, Tuning the optical and electronic properties of colloidal nanocrystals by lattice strain, Nat. Nano 4 (2009) 56e63. [5] S. Zhou, B. Varughese, B. Eichhorn, G. Jackson, K. McIlwrath, PteCu coreeshell and alloy nanoparticles for heterogeneous NOx reduction: anomalous stability and reactivity of a coreeshell nanostructure, Angew. Chem. Int. Ed. 44 (2005) 4539e4543. [6] H. Zhang, M. Jin, H. Liu, J. Wang, M.J. Kim, D. Yang, Z. Xie, J. Liu, Y. Xia, Facile synthesis of PdePt alloy nanocage and their enhanced performance for preferential oxidation of CO in excess hydrogen, ACS Nano 5 (2011) 8212e8222. [7] D. Wang, Y. Li, Bimetallic nanocrystals: liquid-phase synthesis and catalytic applications, Adv. Mater. 23 (2011) 1044e1060. [8] S. Alayoglu, B. Eichhorn, RhPt bimetallic catalysts: synthesis, characterization, and catalysis of coreshell, alloy, and monometallic nanoparticles, J. Am. Chem. Soc. 130 (2008) 17479e17486. [9] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (2001) 1e184. [10] C. Binns, Nanocluster deposited on surfaces, Surf. Sci. Rep. 44 (2001) 1e49. [11] L. Guczi, G. Boskovic, E. Kiss, Bimetallic cobalt based catalysts, Catal. Rev. Sci. Eng. 52 (2010) 133e203. [12] C.J. Jia, F. Schuth, Colloidal metal nanoparticles as a component of designed catalyst, Phys. Chem. Chem. Phys. 13 (2011) 2457e2487. [13] M.B. Cortie, A.M. McDonagh, Synthesis and optical properties of hybrid and alloy plasmonic nanoparticles, Chem. Rev. 111 (2011) 3713e3735. [14] K. Kusada, H. Kobayashi, R. Ikeda, Y. Kubota, M. Takata, S. Toh, T. Yamamoto, S. Matsumura, N. Sumi, K. Sato, K. Nagaoka, H. Kitagawa, Solid solution alloy nanoparticles of immiscible Pd and Ru elements neighboring on Rh: changeover of the thermodynamic behavior for hydrogen storage and enhanced CO-oxidizing ability, J. Am. Chem. Soc. 136 (2014) 1864e1871. [15] D. Sil, J. Hines, U. Udeoyo, E. Borguet, Palladium nanoparticle-based surface acoustic wave hydrogen sensor, ACS Appl. Mater. Interfaces 7 (2015) 5709e5714. [16] M. Zolkapli, Z. Mahmud, S.H. Herman, W.F.H. Abdullah, U.M. Noorl, S. Saharudin, Fluorescence Characteristic of Ruthenium Nanoparticles as a Dissolved Oxygen Sensing Material in Gas and Aqueos Phase, Signal Processing & its Applications (CSPA), 2014, pp. 195e198. [17] M. Segev-Bar, H. Haick, Flexible sensors based on nanoparticles, ACS Nano 7 (2013) 8366e8378. [18] R.A. Ganeev, M. Suzuki, M. Baba, M. Ichihara, H. Kuroda, Low- and high-order nonlinear optical properties of Au, Pt, Pd, and Ru nanoparticles, J. Appl. Phys. 103 (2008), 063102e063102. [19] W.P. Deng, X.S. Tan, W.H. Fang, Q.H. Zhang, Y. Wang, Conversion of cellulose into sorbitol over carbon nanotube-supported ruthenium catalyst, Catal. Lett. 133 (2009) 167e174. [20] M. Lashdaf, A.O.I. Krause, M. Lindblad, M. Tiitta, T. Venalainen, Behavior of
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