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Alkylamine Stabilized Ruthenium Nanocrystals: Faceting and Branching Miranda Vanden Brink,† Matthea A. Peck,† Karren L. More,‡ and James D. Hoefelmeyer*,† Chemistry Department, UniVersity of South Dakota, 414 East Clark Street, Vermillion, South Dakota 57069, and Microscopy Group, MST DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: February 21, 2008; ReVised Manuscript ReceiVed: June 5, 2008

Ruthenium nanocrystals were prepared upon decomposition of Ru3(CO)12 at high temperature in the presence of alkylamine stabilizers. The reaction produced a dark brown colloid that was processed with toluene/ethanol as solvent/nonsolvent pair. The materials were characterized using high resolution electron microscopy and powder X-ray diffraction. The Ru nanocrystals were single crystals with a hcp structure, and displayed regular facets. At lower temperatures nucleation was followed by attachment to yield mesoscale polycrystalline branched ruthenium structures. Introduction Design of functional materials, devices, or metamaterials that span multiple length scales can be achieved using top-down and/or bottom-up approaches. In the latter, individual nanoscale components comprise highly complex systems through hierarchical self-assembly.1–4 These components must be easily synthesized and have uniform size and shape to be useful within modular systems. For this reason, synthesis of monodisperse nanocrystals has been a topic of intensive research. The synthesis of high-quality nanocrystals, which exhibit outstanding uniformity has improved significantly.5,6 Welldefined samples allow careful study of chemical and physical effects that arise from quantum confinement or geometry. Of particular interest are nanocrystals in the mesoscale range that display significantly varied properties in their electronic structure7,8 and surface topography9–11 as a result of the particle size and shape. The ability to tune these properties has led to improvements for applications12–14 such as optoelectronics, medicine, and catalysis.15–19 There are numerous examples of the synthesis of metal nanocrystals.20,21 Nanocrystals of the noble metal elements can be obtained upon reduction or thermolysis of a suitable precursor in the presence of stabilizers. Despite these recent advances, relatively few reports exist for the preparation of monodisperse ruthenium nanocrystals. Some of the preparations of Ru colloids are highlighted here. Addition of NaBH4 to RuCl3 · 3H2O in water in the presence of stearylchloride, sodium dodecylsulfonate, or polyethylene glycol-nonylphenylether led to stable Ru colloids.22 Ru(COD)(COT) subjected to 3 bar H2 in THF in the presence of alkylamines led to Ru nanoparticles.23 Longer reaction time led to ‘vermicular’ particles. Reduction of ruthenium nitrosyl nitrate in ethylene glycol/PVP mixture led to Ru nanoparticles 1.2-7.3 nm in diameter, depending on reaction conditions.24 PVP-stabilized Ru nanoparticles were also prepared using microwave-polyol synthesis25 or reduction of RuCl3 · 3H2O in low-boiling point alcohols.26 Uniform Ru nanocrystals were prepared from RuCl3 and sodium acetate heated in low molecular weight diols.27,28 These particles disperse in water due to electrostatic stabilization from acetate. * To whom correspondence should [email protected]. † University of South Dakota. ‡ Oak Ridge National Laboratory.

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They exhibited a high degree of uniformity, and could be assembled to form colloidal crystal thin films. Nonhydrolytic solvothermal preparation of n-alkylamine stabilized ruthenium nanocrystals using Ru3(CO)12 as precursor is reported here. Conditions that lead to branched Ru nanoparticles are also presented. Preparations of branching noble metal particles are limited, with increasing attention to the control of structural features such as number of branches as well as branch length, width, and direction.29 Advances could lead to designed synthesis of well-defined dendritic nanostructures or porous nanostructures. An analysis of the shape of alkylamine stabilized Ru is presented based on powder X-ray diffraction and electron microscopy data. Experimental Sectioin Chemicals and Materials. Ru3(CO)12, diphenyl ether, tri(noctyl)amine, hexadecylamine, octadecylamine, and oleic acid were purchased from Aldrich and used without further purification. Ethanol and toluene were purchased from Fisher Scientific and used without further purification. N2 was supplied from boiloff from a liquid nitrogen storage tank, and used without purification. All reactions were regulated with a temperature controller (J-KEM Scientific, Inc., Gemini Model), thermocouple (inserted directly into the reaction mixture), and fiberglass heating mantle as power output. An Eppendorf 5804 centrifuge was used to process nanocrystals. X-ray Diffraction. Powder diffraction patterns were collected at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory on a Panalytical X’Pert-Pro from 5-90° 2θ. X-rays were produced from a Cu anode (Cu KR, λ ) 1.54 Å), with the generator set to 45kV, 40mA. Ru nanocrystals in toluene were deposited dropwise on a single crystal silicon wafer cut off-axis for low background. The wafer was set on a hotplate at 100 °C, and the solvent was allowed to evaporate before addition of successive drops. Electron Microscopy. Samples were diluted in toluene then added dropwise to thin film carbon on 200 mesh Cu grids (Electron Microsopy Sciences). Images were collected using a Hitachi HD-2000 FEG-STEM (Scanning Transmission Electron Microsopy) at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory. The electron beam was operated at 200kV. Images were collected in transmission, scanning, and Z-contrast modes. High-resolution TEM (Transmission Electron

10.1021/jp801546c CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

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Figure 1. STEM image and powder XRD data of Ru nanoparticles obtained by injection of Ru3(CO)12 in tri-(n-octyl)amine at 275 °C. Reference pattern from ICDD file 06-0663.

Figure 2. STEM image and powder XRD data of Ru nanocrystals obtained by heating Ru3(CO)12 in tri-(n-octyl)amine/HDA to 275 °C. Reference pattern from ICDD file 06-0663.

Figure 3. Z-contrast STEM image of octadecylamine stabilized Ru nanocrystals dropcast onto a carbon support.

Microscopy) images were obtained using a Hitachi HF-2000 operated at 200kV. Images were acquired with digital camera and CCD (Gatan, Inc.), and processed using DigitalMicrograph and DIFPACK software (Gatan, Inc.). Synthesis of 1.6 nm Ru. A 25 mL three-neck round-bottom flask fitted with a water-cooled reflux condenser and thermocouple was purged with N2. The flask was filled with 4 mL tri(n-octyl)amine, and the temperature was increased to 275 °C. Separately, 30 mg (0.047 mmol) Ru3(CO)12 was dissolved in 1 mL toluene. The solution was rapidly injected into the hot tri(noctyl)amine. After 5 min reaction time, the contents were quenched by removal of the heat source and addition of 4 mL toluene. To this mixture water/ethanol/isopropanol was added to induce flocculation of the colloid without phase separation

of the liquid. The mixture was centrifuged at 6500 rpm for 10 min. The supernatant was separated, and 2 mL toluene was added to the remaining dark brown precipitate, which dispersed readily. The toluene dispersion was centrifuged at 6500 rpm for 10 min to remove trace insolubles. The nanocrystals were subjected to two more cycles of precipitation and redispersion using ethanol/toluene. Synthesis of 4.7-5.8 nm Ru. A 25 mL three-neck roundbottom flask fitted with a water-cooled reflux condenser and thermocouple was purged with N2. The flask was filled with 4 mL tri(n-octyl)amine, 1 g hexadecylamine (or 1 g octadecylamine, or 1 g hexadecylamine and 1 mL oleic acid), and 30 mg (0.047 mmol) Ru3(CO)12. [The surfactants in the three variants did not lead to significantly different product morphology. FTIR data (KBr pellet) of Ru nanocrystals obtained using the combination of hexadecylamine and oleic acid indicate the presence of both stabilizers: w(br) 3439 cm-1 (N-H); s 2959 cm-1, s 2925 cm-1, m 2855 cm-1 (C-H); w(br) 1612 cm-1 (COO-)asym, m 1462 cm-1 (COO-)sym.] The temperature was increased from 25 to 275 °C at a rate of 30 °C/minute. The reaction was heated for 20 min total time (including temperature ramp), then was rapidly quenched by removal of the heat source and addition of 4 mL toluene. To this mixture, ethanol was added to precipitate the nanocrystals (minimum amount required to induce flocculation of the colloid). The mixture was centrifuged at 6500 rpm for 5 min. The supernatant was separated, and 2 mL toluene was added to the remaining precipitate, which dispersed readily. The toluene dispersion was centrifuged at 6500 rpm for 5 min to remove trace insolubles. The nanocrystals were subjected to two more cycles of precipitation and redispersion using ethanol/toluene.

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Figure 4. HRTEM images of octadecylamine stabilized Ru nanocrystals. Top left: HRTEM and FFT (inset) of Ru nanocrystal aligned [0001], lower left: HRTEM and FFT (inset) of Ru nanocrystal aligned [2-1-10].

Synthesis of Branching Ru. A 25 or 50 mL three-neck round-bottom flask fitted with a water-cooled reflux condenser and thermocouple was purged with N2. The flask was filled with 4 mL tri(n-octyl)amine, and heated to 175 °C. Separately, a vial was charged with Ru3(CO)12 (0.03 g, 0.047 mmol) and hexadecylamine (1.6 mL), and heated to produce a homogeneous solution. The Ru3(CO)12 solution was injected slowly, dropwise (1 drop/30 s) in the hot tri(n-octyl)amine. The contents were kept at 175 °C for a total of 60 min (including the time of addition), at which point 4 mL of toluene was added to quench the reaction. Ethanol was added to the dark brown mixture to induce precipitation of a dark brown solid. The contents were centrifuged at 6500 rpm for 5 min. The supernatant was discarded, and the dark brown precipitate was redispersed in toluene. The toluene dispersion was centrifuged (6500 rpm, 5 min); the trace solids were discarded. The dark brown supernatant was subject to two additional precipitation/redispersion cycles for purification. Results and Discussion In this study, triruthenium dodecacarbonyl, Ru3(CO)12 was used as precursor. High boiling point compounds, diphenyl ether or tri(n-octyl)amine, were chosen as solvent for nanocrystal growth. The solvents possess Lewis basic atoms that could

potentially coordinate metal atoms. In diphenyl ether, decomposition of Ru3(CO)12 occurs immediately upon rapid injection (as a toluene solution) at 260 °C. A black precipitate forms as product, and cannot be dispersed in any solvent. Rapid injection of Ru3(CO)12 in tri(n-octyl)amine at 275 °C also led to decomposition; however, the product initially appeared as a dark brown colloid that gradually transformed to insoluble black powder and nearly colorless supernatant. The reaction could be quenched with toluene within five minutes of injection of Ru3(CO)12 and a brown colloid could be recovered before agglomeration ensued. Polar solvent mixture of water/ethanol/ isopropanol was added until the mixture reached flocculation without phase separation of the liquid. Upon centrifugation (6500 rpm), a brown-black precipitate accumulated with nearly colorless supernatant. The supernatant was discarded, and the precipitate was redispersed in toluene. Electron microscopy revealed the sample was composed of particles with average diameter of 1.6 nm (st. dev. ) 0.4 nm) (Figure 1). Powder X-ray diffraction (XRD) data from the sample showed broad reflections with d-spacing values consistent with bulk ruthenium. The intensity of the (002) reflection versus other reflections appeared greater than the expected ratio for bulk ruthenium. Addition of surfactants to the reaction should lead to significantly different morphologies in the product due to the

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Figure 5. Electron microscopy of branched Ru nanostructures.

ability of the molecules to preferentially stabilize facets. Alkyl amines and alkyl carboxylates, being of particular interest as somewhat weakly binding surfactants that could potentially undergo surface ligand substitution reactions, were attractive choices. Rapid addition of solid Ru3(CO)12 to diphenyl ether and oleic acid at 260 °C resulted in immediate decomposition of the precursor and formation of insoluble black precipitate. Oleic acid was not an effective ligand for stabilization of Ru nanoparticles at the high temperature required for decomposition of Ru3(CO)12. Decomposition of Ru3(CO)12 in the presence of amines, however, led to significantly different outcome. Ruthenium nanocrystals were obtained upon heating a mixture of either: a) hexadecylamine, tri(n-octyl)amine, and Ru3(CO)12 or b) hexadecylamine, oleic acid, tri(n-octyl)amine, and Ru3(CO)12 resulting in Ru nanoparticles having a mean diameter of 4.7 nm ( 0.6 nm. The contents were heated from 25 to 275 °C at 30 °C/min. followed by constant temperature of 275 °C and quenching with toluene (total reaction time 20-25 min). This method yielded a brown colloid dispersed in the reaction mixture. A precipitate could be isolated upon addition of the minimum amount of ethanol to induce flocculation, followed by centrifugation. The precipitate redispersed in toluene, and pure nanocrystals were obtained after three cycles of precipitation and redispersion (see STEM image and XRD data, Figure 2). Powder XRD data showed broad reflections at d-spacing values consistent with bulk ruthenium. The higher order reflections were more pronounced than observed for the 1.6 nm Ru. The intensity ratio of the (002) reflection appeared higher than expected for bulk Ru. Octadecylamine could be used in place of hexadecylamine with similar results (mean diameter of 5.2 ( 0.7 nm). The syntheses are reproducible, yielding nanocrystals having mean diameters in the range 4.7-5.8 nm and standard deviations of 10-16%. In the most uniform samples (st. dev. e10%), Ru nanocrystals spontaneously arranged to form an ordered closepacked thin film on continuous carbon support films. The thin film arrays exhibited Moire´ patterns as localized second and third layer terraces formed across the sample (Figure 3). A Ru-octadecylamine sample was prepared for high-resolution transmission electron microscopy (HRTEM) characterization

(Figure 4). Nanocrystals were observed with the electron beam aligned parallel with Ru-[0001] (c-axis), [21j1j0], [011j1], or [12j13j] axes, and typically exhibited a 2D cross-section with a hexagonal perimeter. When imaged with the crystal aligned along the [0001] axis, the Ru nanocrystals typically terminated with [01-10], [10-10], [11j00], [01j10], [1j010], and [1j100] edges or facets. When imaged with the crystal aligned along the [21j1j0] axis, the nanocrystals terminated with [0001], [0001j], [011j1], [011j], [01j11j], and [01j11] edges or facets. When imaged with the crystal aligned along the [12j13j] axis, the nanocrystals terminated with [01j11], [011j1j], [11j01], [1j101j], [101j0], and [1j010] edges or facets (Figure S2). The shapes of the nanocrystals were truncated hexagonal prismatic, consisting of {001}, {101} ≡ {011} ≡ {011j}, and {100} ≡ {11j0} ≡ {010} surfaces (Figure S0), with some statistical variance in the length of edges, or ratio of faces. The a1, a2, and a3 axes ([21j1j0] and equivalent axes) radiate toward and through the axial corners of the polyhedron. This type of Ru faceting has been observed in Ru/MgO supported catalyst materials.30 The observed nanocrystal shape is slightly different than the Wulff construction of hcp-Ru.31 The (112j1) and (101j2) facets were not observed. It may be concluded that the alkylamine preferentially stabilizes {101} ≡ {011} ≡ {01-1} at the expense of (112j1) and (101j2) facets. Typical faceting of the Ru nanocrystals are shown in Figure S1. The {001} surface is flat with hexagonal arrangement of atoms. The {100} and {101} surfaces are grooved. The former resembles the {110} surface of an fcc lattice; the latter exhibits 3-fold and 4-fold hollow binding sites on the surface. The combination of XRD and electron microscopy data confirms the presence of crystalline ruthenium. The d-spacings measured using both methods matched values of hcp ruthenium. The intensity ratio of the (002) reflection, observed from XRD, in Ru nanocrystal samples indicates greater crystal coherence length in this direction, though the nanocrystals are not anisotropic. The apparent discrepancy can be explained by the fact that many of the nanocrystals have one or more twin planes. This is evident in the HRTEM data (see Figure S3) that show the presence of twin planes within individual nanocrystals. No twin planes were observed when nanocrystals were viewed through the c-axis. The twin plane is clearly visible when the nanocrystal is viewed through the [21j1j0] axis, and sometimes multiple parallel twin planes exist within a single nanocrystal.

12126 J. Phys. Chem. C, Vol. 112, No. 32, 2008 This defect is the result of a stacking fault in the c direction in which the ABABAB... sequence of close-packed layers within the hcp lattice is disrupted. The twin plane is parallel to the [21j1j0] axis and perpendicular to the c-axis [0001]. As a result, the crystal coherence is retained in the [0001] direction but disrupted in the others, and the (002) reflection intensity is enhanced in the XRD data. The alkylamine-stabilized Ru nanocrystals exhibit similar faceting as acetate-stabilized Ru reported by Viau and coworkers.27 Electrostatic stabilization prevents agglomeration in the acetate-stabilized Ru colloid, which disperses in water. Using phase-transfer techniques, the acetate-stabilized Ru colloid could be extracted into nonpolar solvent containing alkanethiol. Interestingly, HRTEM data of the thiol-substituted Ru revealed a highly disorganized atomic structure after the phase-transfer reaction. The alkanethiol appears to have induced a surface reconstruction that permeated throughout the entire nanocrystal. Alkylamine stabilized Ru, described herein, consist of highly ordered hcp Ru domains within a hydrophobic exterior. Alkylamine appears only to introduce occasional planar defects within Ru nanocrystals, whereas alkanethiol produces multiply twinned Ru nanocrystals upon ligand substitution with acetatestabilized Ru. The reaction temperature influences the morphology of the final product obtained upon quenching the reaction. Reactions kept at 275 °C, followed by toluene quench, yielded isotropic nanocrystals. Reactions kept at 175 °C, followed by toluene quench, led to branching Ru nanostructures (Figure 5). The particles each have a different number of irregular-shaped branches (crystals) with random directionality. The nanostructures are well separated and show no sign of agglomeration. In addition, the particles disperse readily in nonpolar solvents. The HRTEM data indicate that the branching structures are polycrystalline, which indicates an attachment mechanism and not anisotropic growth. The branched Ru nanostructures resemble Pt ‘flowers’ previously reported.29 Analysis of an individual Ru flower shows that the nanoparticle contains multiple nonparallel stacking faults that result in irregularly shaped nanoparticles built from small crystal domains. The Ru flowers do not resemble the beautiful, high-symmetry branching Pt structures reported by the Hong Yang group in which propagation of branches occurs in the direction of the twin plane.29 Conclusion In conclusion, alkylamine-stabilized Ru nanocrystals were reproducibly synthesized. The nanocrystals were uniform to within (15% of the mean diameter, exhibited a high degree of crystallinity, and easily separated using organic solvents. The faceting behavior of the nanocrystals was established from HRTEM images. Branching Ru-nanostructures were obtained at lower reaction temperatures, with irregular number and orientation of the branches. The branching Ru-nanostructures were polycrystalline, suggesting an attachment mechanism during growth. Future studies will focus on improved methods to control the morphology of the nanostructures, especially toward controlling the geometry of the branching structures. Compositionally more complex materials that contain Ru will also be the subject of future work. Acknowledgment. Research at the Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences and

Vanden Brink et al. SHaRE User Facility was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. The project was supported by funds from Research Corporation (Cottrell College Science Award CC6960), the National Science Foundation (EPS-0554609), the South Dakota Center for Research and Development of Lightactivated Materials, and start-up funding from The University of South Dakota. The authors express gratitude for the assistance of Michelle D. Pawel (ORNL). Supporting Information Available: Shape analysis of isotropic Ru nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature 2006, 439, 55–59. (2) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3248–3255. (3) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393–395. (4) van Blaaderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321– 324. (5) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545–610. (6) Jun, Y. W.; Lee, J. H.; Choi, J. S.; Cheon, J. J. Phys. Chem. B 2005, 109, 14795–14806. (7) Johnston, R. L. Phil. Trans. R. Soc. London A 1998, 356, 211– 230. (8) Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annu. ReV. Phys. Chem. 1990, 41, 477–496. (9) Van Hardeveld, R.; Hartog, F. Surf. Sci. 1969, 15, 189–230. (10) Marks, L. D. Surf. Sci. 1985, 150, 358–366. (11) Marks, L. D. Rep. Prog. Phys. 1994, 57, 603–649. (12) Ratner, M.; Ratner, D. Nanotechnology. A Gentle Introduction to the Next Big Idea: Prentice Hall, NJ, 2003. (13) Poole, C. P.; Jr. Owens, F. J. Introduction to Nanotechnology; John Wiley and Sons, Inc.: Hoboken, NJ 2003. (14) Kumar, C. Biofunctionalization of Nanomaterials; Wiley-VCH Verlag GmbH and Co.: Weinheim 2005. (15) Rioux, R. M.; Song, H.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 2192–2202. (16) Song, H.; Rioux, R. M.; Hoefelmeyer, J. D.; Komor, R.; Niesz, K.; Grass, M.; Yang, P.; Somorjai, G. A. J. Am. Chem. Soc. 2006, 128, 3027–3037. (17) Rioux, R. M.; Song, H.; Grass, M.; Habas, S.; Niesz, K.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. Top. Catal. 2006, 39, 167– 174. (18) Schlogl, R.; Hamid, S. B. A. Angew. Chem., Int. Ed. 2004, 43, 1628–1637. (19) El-Sayed, M. A. Acc. Chem. Res. 2004, 37, 326–333. (20) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630–4660. (21) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757– 3778. (22) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1986, 110, 82–87. (23) Pan, C.; Pelzer, K.; Philippot, K.; Chaudret, B.; Dassenoy, F.; Lecante, P.; Casanove, M. J. J. Am. Chem. Soc. 2001, 123, 7584–7593. (24) Chen, Y.; Yong, K.; Li, J. Mater. Lett. 2007, 62, 1018–1021. (25) Harpeness, R.; Peng, Z.; Liu, X.; Pol, V. G.; Koltypin, Y.; Gedanken, A. J. Colloid Interface Sci. 2005, 287, 678–684. (26) Zhang, Y.; Yu, J.; Niu, H.; Liu, H. J. Colloid Interface Sci. 2007, 313, 503–510. (27) Viau, G.; Brayner, R.; Poul, L.; Chakroune, N.; Lacaze, E.; FievetVincent, F.; Fievet, F. Chem. Mater. 2003, 15, 486–494. (28) Chakroune, N.; Viau, G.; Ammar, S.; Poul, L.; Veautier, D.; Chehimi, M. M.; Mangeney, C.; Villain, F.; Fievet, F. Langmuir 2005, 21, 6788–6796. (29) Maksimuk, S.; Teng, X.; Yang, H. J. Phys. Chem. C 2007, 111, 14312–14319. (30) Datye, A. K.; Logan, A. D.; Long, N. J. J. Catal. 1988, 109, 76–88. (31) Aβmann, J.; Crihan, D.; Knapp, M.; Lundgren, E.; Lo¨ffler, E.; Muhler, M.; Narkhede, V.; Over, H.; Schmid, M.; Seitsonen, A. P.; Varga, P. Angew. Chem., Int. Ed. 2005, 44, 917–920.

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