Growth of Magnetic Nanowires and Nanodumbbells in Liquid Polyol

Mar 15, 2007 - Mater. , 2007, 19 (8), pp 2084–2094 ... were successively observed when [NaOH] was increased in the range 0−0.2 M. For the .... The...
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Chem. Mater. 2007, 19, 2084-2094

Growth of Magnetic Nanowires and Nanodumbbells in Liquid Polyol D. Ung,† Y. Soumare,† N. Chakroune,† G. Viau,*,‡ M.-J. Vaulay,† V. Richard,§ and F. Fie´vet† ITODYS, UniVersite´ Paris 7-Denis Diderot, UMR CNRS 7086, case 7090, 2 place Jussieu, F-75251 Paris Cedex 05, France, Laboratoire de Physique et Chimie des Nano-Objets, INSA-Toulouse, 135 aVenue de Rangueil, 31077 Toulouse Cedex, France, and Laboratoire des Proprie´ te´ s Me´ caniques et Thermodynamiques des Mate´ riaux, CNRS UPR 9001, Institut Galile´ e, 99 aVenue J. B. Cle´ ment, 93430 Villetaneuse, France ReceiVed NoVember 17, 2006. ReVised Manuscript ReceiVed January 26, 2007

Cobalt and cobalt-nickel nanoparticles were synthesized by reducing mixtures of cobalt and nickel acetates in sodium hydroxide solution in 1,2-propanediol. The particle shape depends strongly on the sodium hydroxide concentration and the Co/Ni composition of the particles. For cobalt-rich content, agglomerated rods, nanowires with a mean diameter of about 8 nm, and platelets were successively observed when [NaOH] was increased in the range 0-0.2 M. For the Co50Ni50 composition and [NaOH] in the range 0.1-0.18 M, nanodumbbells are formed that consist of a central column richer in cobalt capped with two terminal platelets richer in nickel. The shape of the dumbbells strongly depends on the basicity; long dumbbells are obtained for the lowest NaOH concentration, and short dumbbells and diabolos for the highest. To understand the role of the sodium hydroxide concentration and the different reactivities of cobalt and nickel, we analyzed the equilibrium between the Co2+ and Ni2+ ions in solution and the intermediate unreduced solid phase. For the Co80Ni20 composition, we show that increasing the sodium hydroxide amount lowers the Co2+ and Ni2+ ions in solution through the precipitation of the intermediate solid phase, suggesting that the nanowires are obtained with a higher growth rate than the platelets. The analysis of the solid intermediate phases revealed a Co(II) alkoxide and a Ni(II) hydroxy-acetate showing strong differences in the chemistry of the these two ions in basic solutions of 1,2-propanediol. These differences can explain the two well-separated growth steps originating the Co50Ni50 nanodumbbells.

1. Introduction Magnetic nanoparticles present several applications in the field of permanent magnets and magnetic recording,1 ferrofluids,2 or magnetic hyperthermia,3 depending on their size, shape, and chemical composition. The major interest of ferromagnetic metals with respect to oxides is their higher saturation magnetization. Furthermore, elongated singledomain particles present high magnetic anisotropy. The combination of high anisotropy with high saturation magnetization makes this kind of particles very attractive as precursors for hard magnetic materials. The chemical synthesis of metal nanoparticles is interesting because of its simplicity and low cost, but anisotropic growth of metal particles by a liquid-phase process is not a straightforward route. The more developed way to obtain metallic wires is to fill the uniaxial pores of alumina or polycarbonate * Corresponding author. E-mail: [email protected]. † Universite ´ Paris 7-Denis Diderot. ‡ INSA-Toulouse. § Institut Galile ´ e.

(1) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (2) Odenbach, S. J. Phys.: Condens. Matter 2004, 16, R1135. (3) (a) Hergt, R.; Andra¨, W.; d’Ambly, C. G.; Hilger, I.; Kaiser, W. A.; Richter, U.; Schmidt, H.-G. IEEE Trans. Magn. 1998, 34, 3745. (b) Mornet, S.; Vasseur, S.; Grasset, F.; Duguet, E. J. Mater. Chem. 2004, 14, 2161.

membranes by electrodeposition.4 Iron and cobalt wires were prepared by this method, but despite a very good parallel orientation, the coercivity of these wires is generally much lower than the theoretical one expected for a magnetization curve by uniform rotation of a single-domain particle.4b Other solid templates have been studied, such as carbon nanotubes or mesoporous oxides.5 The other method is to use surfactants that induce an anisotropic growth. This method is particularly interesting because it affords a self-organization of the anisotropic particles in solution or as 3D supercrystals. Monodisperse nickel rods6 and cobalt wires7 were prepared by the organometallic way in the presence of mixtures of long chain acids and amines, and very high coercivity has been obtained with cobalt. The third way to obtain anisotropic growth is to induce different growth rates of the different crystallographic planes of the metal particles. Xia (4) (a) Fert, A.; Piraux, L. J. Magn. Magn. Mater. 1999, 200, 338.1999, 200. (b) Sellmyer, D. J.; Zheng M.; Skomski, R. J. Phys.: Condens. Matter 2001, 13, R433. (c) Zeng, H.; Skomski, R.; Menon, L.; Liu, Y.; Bandyopadhay, S.; Sellmyer, D. J. Phys. ReV. B 2002, 65 134, 426. (d) Wegrowe, J.-E.; Kelly, D.; Franck, A.; Gilbert, S. E.; Ansermet, J.-Ph. Phys. ReV. Lett. 1999, 82, 3681. (5) Zang, Z.; Dai, S.; Blom, D.; Shen, J. Chem. Mater. 2002, 14, 965. (6) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M.-J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. (7) (a) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M.-C.; Casanove, M.-J.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286. (b) Dumestre, F.; Chaudret, B.; Amiens, C.; Respaud, M.; Fejes, P.; Renaud, P.; Zurcher, P. Angew. Chem., Int. Ed. 2003, 42, 5213.

10.1021/cm0627387 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/15/2007

Magnetic Nanowires and Nanodumbbells in Liquid Polyol

et al. showed that the reduction of silver nitrate in polyvinylpyrrolidone (PVP) solution in ethyleneglycol affords the formation of cubes, spheres, or wires, depending on the PVP/ Ag molar ratio.8 The wires are the result of the anisotropic growth from decahedral seeds into pentagonal wires by a preferential addition of metal species at the twin faults of the seeds. This mechanism was also proposed for other metal wires with the fcc structure.9 Gold nanorods and nanowires were grown by reduction in CTAB aqueous solutions.10 An electric field-directed growth was proposed to explain the anisotropic growth. The precursors are AuCl2- ions bounded to cationic micelles. The mass transfer on a metal particle is favored in which the electrical double layer gradient is highest, namely at the highest curvature points of the seed surface and then at the tip of the rods.11 Several works have been performed that deal with the synthesis of cobalt and cobalt-nickel particles by reduction in liquid polyols. Monodisperse spherical particles were prepared with a mean size controlled in the 5-500 nm range by a heterogeneous nucleation with platinum.12 The role of the equilibrium between the metal cations in solution and an intermediate solid phase (eq 1) for the control of the growth step was pointed out.13 It was found that the precipitation of the unreduced solid phase limits the precursor concentration in solution and allows a good separation between the nucleation and growth steps. For this reason, all the syntheses of spherical particles by reduction in liquid polyol were realized with an excess of sodium hydroxide.12-14 nucleation

(MII)solid phase [\ ] MIIsolution 9 8 M0 (M ) Co, Ni) (1) growth OH

Several unreduced Co(II) and Ni(II) phases have been isolated by heating a solution of metal acetate in polyols and have been characterized. Poul et al. showed that cobalt and nickel layered hydroxy-salts (LHS) precipitate in liquid polyols when the hydrolysis ratio H ) [H2O]/[M] is high enough.15 The general formula of the CoII and NiII LHS hydroxyacetates is M(OH)1.5(CH3CO2)0.5‚xH2O, with the acetate ions and water occupying the interlayer spacing. The interlayer spacing is ca. 10 and 14 Å for the nickel and cobalt phase, respectively. This difference comes from a different structure of the layer: the Ni(II) hydroxyacetate presents a brucite like structure,16 whereas the Co(II) compound presents a hydrozincite like structure.15 For lower hydrolysis ratio and at higher temperatures, metal alkoxides can be obtained. The cobalt and nickel alkoxides of 1,2-ethanediol, (8) Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Chem.sEur. J. 2005, 11, 454. (9) (a) Lisieky, I.; Filamkembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M.-P. Phys. ReV. B 2000, 61, 4968. (b) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771. (10) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (11) Pe´rez-Juste, J.; Liz-Marza´n, L. M.; Carnie, S.; Chan, D. Y. C.; Mulvaney, P. AdV. Funct. Mater. 2004, 14, 571. (12) Toneguzzo, Ph.; Viau, G.; Acher, O.; Fie´vet-Vincent, F.; Fie´vet, F. AdV. Mater. 1998, 10, 1032. (13) Viau, G.; Fie´vet-Vincent F.; Fie´vet, F. Solid State Ionics 1996, 84, 259. (14) Luna, C.; Morales, M. P.; Serna, C. J.; Vasquez, M. Nanotechnology 2004, 15, S293. (15) Poul, L.; Jouini, N.; Fievet, F. Chem. Mater. 2000, 12, 3123. (16) Taibi, M.; Ammar, S.; Jouini, N.; Fie´vet, F.; Molinie´, P.; Drillon, M. J. Mater. Chem. 2002, 12, 3238.

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Co(C2H4O2)17 and Ni(C2H4O2),18 have been isolated and their structures described as a stacking of layers comprising CoO6 (NiO6) octahedra sharing an edge with the CH2-CH2 groups occupying the interlayer spacing. In this paper, we report the synthesis in sodium hydroxide solution of 1,2-propanediol of cobalt and cobalt-nickel particles with nonspherical shapes such as wires, dumbbells, or platelets. We show that the particle shape depends strongly on two parameters: the basicity of the medium and the CoNi composition. Structural and magnetic properties of some of these particles were reported in two previous studies.19,20 Here, we focus on the relation between the particle shape and the growth mechanism by analyzing the equilibrium involving the Co(II) and Ni(II) precursors in solution and the solid intermediate phase. We report the influence of basicity upon the concentration of cobalt and nickel ions in solution just before that the reduction occurs. We describe also the solid intermediate phases in the case of pure cobalt and pure nickel and for some Co/Ni mixtures. It must be stressed that there was no study available dealing with the characterization of unreduced solid phases of cobalt or nickel obtained in 1,2-propanediol. This detailed analysis allows us to describe the growth medium just before the reduction occurs and gives a better understanding of the cobalt and nickel chemistry in relation to the formation of the metal particles. 2. Experimental Section 2.1. Co-Ni Particle Synthesis. Mixtures of cobalt and nickel acetate tetrahydrates with the desired Co/Ni ratio were dissolved in 1,2-propanediol with suitable amounts of sodium hydroxide and ruthenium trichloride, RuCl3. The solution was heated at 170 °C with mechanical stirring for 15 min. After the mixture was cooled to room temperature, the black magnetic metal powder was recovered by centrifugation and washed in ethanol. The cobaltnickel concentration was 0.08 M in all experiments. The role of ruthenium is to seed the growth of cobalt and nickel. The molar ratio [Ru]/[Co+Ni] was fixed at 2.5% in all experiments described here. The NaOH concentration was varied from 0 to 0.25 M. Chemical analysis on metal powders showed a Co/Ni ratio equal to the initial one in the precursor mixture. Powder X-ray diffraction (XRD) patterns were recorded on a Philips PW1050/25 diffractometer (Fe KR1 and KR2 radiation). Transmission electron microscopy (TEM) observations were made on a Jeol CXII microscope operating at 100 kV and high-resolution transmission electron microscopy (HRTEM) images on a Jeol JEM 2010F UHR at 200 kV. Local energy-dispersive X-ray analysis (EDX) was performed using a PGT IMIX PC system with a 25 nm analysis spot. Electron energy-loss spectroscopy (EELS) analysis was performed using a Gatan Imaging Filter 2000. Magnetic measurements on compacted powders were performed with a Quantum Design MPMS-5S SQUID magnetometer. (17) Chakroune, N.; Viau, G.; Ammar, S.; Jouini, N.; Gredin, P.; Vaulay, M.-J.; Fie´vet, F. New J. Chem. 2005, 29, 355. (18) (a) Tekaia-Elhsissen, K.; Delahaye-Vidal, A.; Nowogrocki, G.; Figlarz, M. C. R. Acad. Sci. Paris 1989, 309, 349. (b) Tekaia-Elhsissen, K.; Delahaye-Vidal, A.; Nowogrocki, G.; Figlarz, M. C. R. Acad. Sci. Paris 1989, 309, 469. (19) Ung, D.; Viau, G.; Ricolleau, C.; Warmont, F.; Gredin, P.; Fie´vet, F. AdV. Mater. 2005, 17, 338. (20) Ung, D.; Viau, G.; Fie´vet-Vincent, F.; Herbst, F.; Richard, V.; Fie´vet, F. Prog. Solid State Chem. 2005, 33, 137.

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Figure 1. TEM images of Co-Ni particles prepared by reduction of a mixture of cobalt and nickel acetates in a 0.1 M NaOH solution in 1,2-propanediol: (a) Co wires, (b) Co80Ni20 wires, (c) Co50Ni50 wires, and (d) Ni platelets.

2.2. Growth Medium Analysis. To know more precisely the state of the system just before that the reduction occurs, we dissolved mixtures of cobalt and nickel acetate tetrahydrate with desired Co/Ni ratio in 1,2-propanediol with a suitable amount of sodium hydroxide. To avoid the nucleation of metal particles, we didn’t add any ruthenium chloride. The solution was heated at 170 °C and then readily cooled to room temperature. In these conditions, no reduction occurred. For sodium hydroxide concentrations higher than 0.05 M, a colored precipitate appeared. The precipitate was separated from the supernatant by centrifugation. Analysis of the Solid Intermediate Phase. Scanning electron microscope studies on the precipitate were performed on a Cambridge S-120 microscope. Thermal analysis (TGA-TDA) was performed on a Setaram 92-12 apparatus at 20-600 °C under an air flow. The transmission infrared spectra were recorded at 4004000 cm-1 using an Equinox 55 Bruker FT-IR spectrometer. The UV-visible-NIR spectroscopy was performed with a Varian Cary 5E spectrophotometer. Analysis of the Supernatant. The Co/Ni ratio in the supernatant was determined by EDX analysis on the solid that resulted from the complete precipitation of the metal cations by adding an excess of sodium carbonate to the solution. The supernatant was also analyzed by transmission UV-visible-NIR spectroscopy. The total concentration of the cations in solution ([Co2+] + [Ni2+]) was measured from the UV-visible spectroscopy absorbance at the wavelength λ ) 1170 nm using the Beer-Lambert relation. The respective cobalt and nickel concentrations, [Co2+] and [Ni2+], were inferred from the total concentration and the Co/Ni ratio.

3. Results and Discussion 3.1. Morphology and Structure of Co-Ni Particles. 3.1.1. Particle Shape. Influence of the Particle Composition. The cobalt-nickel particles prepared in 0.1 M sodium hydroxide solution in 1,2-propanediol present a shape that is very dependent on their chemical composition. Pure cobalt particles have a sea-urchin-like shape; they consist of an isotropic core from which several needles have grown (Figure 1a). When 20% nickel is added to the composition, the core is smaller and the needles are wires with a mean diameter of 8 nm and a length in the 100-500 nm range (Figure 1b). These wires always present a hexagonal head. For the Co50Ni50 composition, no more cores were observed, but isolated wires with a mean diameter of 8 nm and a mean length of 250 nm (Figure 1c). These wires present two ending platelets. More generally, for [NaOH] ) 0.1 M wires are observed for the composition range Co90Ni10-Co30Ni70; in this range, the wires grow from a metallic core for the cobalt-rich composition, whereas for nickel-rich composition, they are isolated. Pure nickel particles prepared in the same conditions are thin hexagonal platelets with a mean diameter of 250 nm and a mean thickness of 20 nm (Figure 1d). Influence of Basicity. For all compositions, the sodium hydroxide concentration governs the particle shape. This parameter has been studied precisely for the Co80Ni20 and Co50Ni50 particles. In both cases, agglomerated rods were observed when the sodium hydroxide concentration was

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Figure 2. TEM images of Co50Ni50particles prepared by reduction of a mixture of cobalt and nickel acetates in NaOH solution in 1,2-propanediol (the mean length, mean diameter of the wire, and mean diameter of the ending platelets describing the dimensions of the particles are noted as lm, dm, and Dm, respectively): (a) long dumbbells, [NaOH] ) 0.135 M, lm ) 150 nm, dm ) 5.7 nm, Dm ) 29 nm; (b) short dumbbells, [NaOH] ) 0.16 M, lm ) 54 nm, dm ) 13.5 nm, Dm ) 38 nm; (c) diabolos and platelets, [NaOH] ) 0.175 M, Dm ) 47 nm; (d) details of long and short dumbbells and a diabolo.

lower than 0.1 M and very fine isotropic nanoparticles when the sodium hydroxide concentration was higher than 0.25 M. Wires, dumbbells, and platelets were formed for the intermediate concentration range. Co80Ni20 wires were obtained for [NaOH] comprising between 0.08 and 0.15 M. In this narrow range, the diameter of the core, from which the wires grow, decreases when the sodium hydroxide concentration increases. Co50Ni50 particles with a hybrid shape were formed with [NaOH] in the range 0.1-0.18 M. These particles consist of a central column with two capping platelets (Figure 2). The length of the central column decreases steadily with [NaOH] while the diameter of the ending platelets increases, and long dumbbells (Figure 2a), short dumbbells (Figure 2b), diabolos, and platelets (Figure 2c) are successively observed when the basicity is increased within the narrow range 0.1-0.18 M. The influence of sodium hydroxide concentration on the particle shape is summarized for the Co80Ni20 and Co50Ni50 compositions in the Figure 3. More details about the average lengths of the Co80Ni20 and Co50Ni50 particles are given in refs 19 and 20, respectively. 3.1.2. Structure and Magnetic Properties. The pure cobalt particles prepared with [NaOH] ) 0.1 M crystallize with the hcp structure, the pure nickel platelets in the same conditions with the fcc one (Figure 4).

Figure 3. Different particle shapes of composition Co80Ni20 and Co50Ni50 obtained by reduction in sodium hydroxide solution in 1,2-propanediol with increasing concentration (not to scale).

The X-ray diffraction pattern of the Co80Ni20 nanowires shows mainly the hcp phase (Figure 4). The mean crystallite size for the [10.0] direction inferred from the broadening of the corresponding line is much lower than that for the [00.2] direction. The former, noted L10.0, is generally in the range 5-10 nm, whereas the latter, noted L00.2, comprises between

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Figure 4. XRD pattern of Co-Ni powders prepared by reduction of cobalt and nickel acetates in a 0.1 M NaOH solution in 1,2-propanediol: (a) Co urchin-like shape particles, (b) Co80Ni20 wires, (c) Co50Ni50 long dumbbells, and (d) Ni platelets.

15 and 25 nm. These values reflect that the growth of the wires takes place along the c-axis of the hcp structure. For the Co50Ni50 composition, a mixture of the hcp and fcc phase is observed, the proportion of which depends on the particle morphology. The hcp phase is predominant for the Co50Ni50 wires prepared with [NaOH] ) 0.1 M (Figure 4), whereas the hcp and fcc phases are found in similar amounts for the dumbbell-like particles. The proportion of

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the fcc phase increases when the volume fraction of the caps increases, consistent with the hcp phase being mainly located in the central column and the fcc one in the caps. These conclusions on the structure are consistent with the HRTEM observations on wires and Co50Ni50 dumbbells: (i) the wires crystallize with the hcp structure, and their long axis is parallel to the c-axis of this structure; (ii) the caps of the dumbbells crystallize with the fcc structure with a diffraction pattern indexed as the [011] zone axis.20 The X-ray diffraction pattern reflects also the growth of the hcp phase along the c-axis for the wires and the long dumbbells with L00.2 higher than L10.0. For the Co50Ni50 diabolos and platelets of Figure 2c, for which the central column is very short or has almost disappeared, a mixture of hcp and fcc phases in equal proportion is still found in their structure. In these samples, the mean size L10.0 is higher than L00.2, showing that the growth of the hcp phase occurs preferentially perpendicular to the c-axis.20 Previous EDX local analysis with a 25 nm diameter probe on the Co80Ni20 nanowires, prepared with [NaOH] ) 0.1 M, showed that the cobalt content is always higher in the wires with respect to the global composition and, on the contrary, a significant enrichment in nickel is measured in the hexagonal head.19 EELS analysis on Co50Ni50 dumbbells show that the central column is richer in cobalt and the two terminal platelets are richer in nickel (Figure 5). The cobalt and nickel maps in Co50Ni50 platelets obtained with [NaOH] ) 0.18 M show also a partial segregation of the two metals. Figure 6 presents a platelet viewed edgeways on the TEM

Figure 5. EELS analysis of a Co50Ni50 dumbbell-like particle: (a) bright-field TEM image, (b) mapping of Co, and (c) mapping of Ni.

Figure 6. EELS analysis of a Co50Ni50 edgewise platelet: (a) bright-field TEM image, (b) mapping of Co, and (c) mapping of Ni.

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Figure 7. Magnetization curves at T ) 300 K of cobalt-nickel wires prepared by reduction in a 0.1 M NaOH solution in 1,2-propanediol: (a) Co, (b) Co80Ni20, and (c) Co50Ni50. Remanence ratio, Mr/Ms, coercivity, Hc, and anisotropy field, Ha, are shown in the case of cobalt particles as a guideline for the text. Table 1. Magnetic Properties of Cobalt and Cobalt-Nickel Particles Prepared by Reduction in NaOH Solution in 1,2-Propanediola Co

Co80Ni20

NaOH (M)

0.1

0.1

0.1

Co50Ni50 0.16

0.175

Hc (Oe) Mr/Ms Ha (Oe) 2πMs (Oe)

2200 0.5 8000 8800

2000 0.5 7500 7650

1500 0.5 6000 5925

1400 0.47 4500 5925

325 0.14 2500 5925

a H , M /M , and H are experimental values inferred from the magnetizac r s a tion curves; 2πMs is calculated for pure bulk metal and alloys.

image that appears as a trilayer with an inner platelet richer in cobalt between two outer platelets richer in nickel. The cobalt-nickel particles are ferromagnetic at room temperature. The magnetization curves measured on powders reflect the particle anisotropy. A remanence to saturation ratio Mr/Ms ) 0.5 is found for the cobalt and cobalt-nickel nanowires prepared with [NaOH] ) 0.1 M (Figure 7). This value is characteristic of an assembly of randomly oriented single-domain uniaxial particles. The coercivity of the nanowires increases in the range 1500-2200 Oe when the cobalt content increases (Table 1). For the Co50Ni50 composition, the coercivity and the remanence of the diabolos and platelets prepared with higher NaOH concentration are much lower than for the wires and dumbbells (Table 1). These variations are in good agreement with magnetic properties governed by shape anisotropy. Indeed, for a magnetic wire, the shape anisotropy field is 2πMs, where Ms is the saturation magnetization. This field corresponds to the maximum of coercivity expected in the case of a uniform reversal of magnetization, when the applied field is parallel to the long axis of the wire and neglecting the magnetocrystalline anisotropy.4b The experimental values of coercivity are found to increase with the saturation magnetization of the cobaltnickel (Table 1), and the effective perpendicular anisotropy fields measured by extrapolating magnetization curves Ha (Figure 7) are found to be very close to the 2πMs value calculated for the corresponding bulk cobalt-nickel alloy (Table 1), as expected for a control by shape anisotropy. The Hc values are much lower than the Ha values because the wires are randomly oriented with respect to the applied field and probably because of structural and morphological defects. Nevertheless, it is interesting to note that despite

the absence of orientation, the Hc values are close to those observed on arrays of parallel cobalt nanowires prepared by electrodeposition.4 3.1.3. Nucleation and Growth Steps. The morphological and structural studies showed that the wires are the result of the anisotropic growth along the c-axis of particles crystallizing with the hcp structure. For this reason, wires are obtained when the cobalt content is high enough. With pure nickel particles that crystallize with the fcc structure, only platelets or spheres are formed. Nevertheless, the crystallization of the hcp phase is not a sufficient condition for the formation of wires; within the composition range for which the hcp phase is obtained, several kinds of shapes are observed. The growth of the hcp phase is strongly dependent on the sodium hydroxide concentration in the polyol. For example, for the Co80Ni20 composition the growth of the hcp phase can take place preferentially parallel to the basal plane, leading to platelets; parallel to the c-axis, leading to wires; or in a disordered manner, leading to agglomerated rods, depending on the sodium hydroxide concentration. The particle growth depends also on the cobalt-nickel composition. The pure cobalt particles with sea-urchin-like shape present the hcp structure but a core grows at first followed by the growth of needles. The core growth step is reduced when nickel is introduced in the particle composition and for a sufficient amount, only the wire growth step is observed. The differences between the shape of the pure cobalt and that of the cobalt-nickel particles can be interpreted by a different ratio, nucleation rate/growth rate. A low nucleation rate and a high growth rate can describe the formation of the pure cobalt particle shape. For the cobaltnickel particles, the diminution of the core size and the formation of wires can be explained by a higher nucleation rate and/or a slower growth rate with respect to pure cobalt. If we assume that the nucleation step consists of the formation of ruthenium ultrafine particles, for a given basicity, the nucleation rate should be constant. It is possible to conclude that the addition of nickel slows down the growth rate. The dumbbell-like particles strongly recall the shape of capped columnar snow crystals (also called tsuzumi snow crystals).21 These snow crystals are formed in two successive growth steps corresponding to different temperature and water supersaturation conditions. The formation of a hybrid shape such as dumbbells and diabolos can also be described by a growth with two well-separated steps: the first one involving mainly the cobalt gives the central rod or wire, and the second one involving mainly the nickel gives the terminal platelets. The overall aspect ratio of the Co50Ni50 particles depends mainly on the first step. With low NaOH concentration, the dumbbells are the result of the growth of the hcp phase parallel to the c-axis that gives the central column. The platelets formed with higher NaOH concentration, made of a trilayer, are also the result of two growth steps; however, in this case, the growth step of the hcp phase takes place perpendicular to the c-axis. In both cases, the nickel growth occurs after that of cobalt, showing a slower reduction rate. (21) Libbrecht, K.; Rasmussen P. The Snowflake: Winter’s Secret Beauty; Voyageur Press: Stillwater, MN, 2003; pp 78-80, also see http:// www.its.caltech.edu/∼atomic/snowcrystals/.

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Figure 8. (a) Cation concentration in basic solutions of 1,2-propanediol, at 170 °C just before the reduction, vs the sodium hydroxide concentration: (9) [Co2+] + [Ni2+]; (O) [Co2+]; (2) [Ni2+]. Inset: variation of the relative concentrations [M]/[M]0 with NaOH concentration (the initial concentrations were [Co2+]0 ) 0.064 M and [Ni2+]0 ) 0.016 M). (b) Relation between the total cation concentration at 170 °C just before the reduction occurs and the final Co80Ni20 particle shape.

3.2. Influence of Basicity on the Precursor Concentration in Solution. In this section, we describe the influence of [NaOH] on the concentration of the metal ions in solution just before that the reduction occurred. For that, solutions of cobalt and nickel acetates in basic solution in propanediol have been heated to 170 °C in the absence of ruthenium chloride, as has been described in the Experimental Section. The purpose is to show that the different regimes are related to different growth rates. The influence of [NaOH] on a solution containing a single cation is described first; we then study the case of Co/Ni mixtures. The UV-visible absorption spectrum of the cobalt acetate solution in 1,2-propanediol is characteristic of cobalt ions in the +II valence state, occupying a CoO6 octahedral site with a 4T1g(F) ground state. Two main absorption bands located at 1200 and 514 nm are attributed to the 4T1g(F) f 4 T2g and 4T1g(F) f 4T1g(P) transitions. An additional shoulder located near 610 nm is attributed to the 4T1g(F) f 4A2g transition.22 The UV-visible absorption spectrum of the nickel acetate solution in 1,2-propanediol is characteristic of nickel ions in the +II valence state, occupying a NiO6 octahedral site with a 3A2g ground state. The three main

absorption bands located at 1152, 730, and 396 nm are attributed to the 3A2g f 4T2g, 3A2g f 4T1g(F), and 3A2g f 4 T1g(P) transitions, respectively. When the cobalt or nickel acetate was heated to 170 °C in a sodium hydroxide solution in 1,2-propanediol, the energy of the transitions did not change in the UV-visible-NIR spectrum but an overall decreasing of the absorption was observed in the visible and NIR region. This decreasing is the result of the precipitation of a solid intermediate phase according to the equilibrium 1 (eq 1). The precipitation of the unreduced phase that lowers the Co2+ and Ni2+ concentration in solution is favored by increasing NaOH concentration. For a NaOH concentration higher than 0.15 and 0.175 M for Co2+ and Ni2+, respectively, the cation concentration in solution is almost zero at 170 °C. For solutions containing an 80/20 mixture of cobalt and nickel acetates, no precipitation occurred for a sodium hydroxide concentration below 0.05 M. Above this value, the metal ion concentration started to decrease (Figure 8a); (22) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, 1984.

Magnetic Nanowires and Nanodumbbells in Liquid Polyol

above [NaOH] ) 0.16 M. all the metal ions were precipitated. Figure 8 also shows that the cobalt ions precipitate at lower basicity than the nickel ions. The relative concentration [Co2+]/[Co2+]0 decreases more readily than the ratio[Ni2+]/ [Ni2+]0 when [NaOH] is increased. In agreement with this, the cobalt content in the unreduced solid phase is higher than 80% for low basicity and is again found equal to the nominal composition for NaOH concentration higher than 0.16 M. For the Co50Ni50 mixtures, the same kind of variation in the metal cation concentration with [NaOH] was observed. The sodium hydroxide concentration governs the particle shape. We described in the previous section that agglomerated rods, wires, and platelets are successively formed with increasing sodium hydroxide concentration (Figure 3). Figure 8b shows clearly, on the one hand, that the agglomerated rods are formed when the metal ion concentration is maximal and, on the other hand, that the platelets and the spheres are obtained when this concentration is almost zero. The wires appear in the intermediate range. As the reduction of the metal cations takes place in solution, increasing the basicity of the medium is equivalent to decreasing the growth rate. It must be concluded that the agglomerated rods are formed with the higher growth rates that lead to a noncontrolled shape. The shapes are much better controlled with a lower growth rate; the formation of wires requires an intermediate growth rate, whereas platelets are obtained with the lower one. The other point that is worth commenting on is the difference between the cobalt and nickel. The location of nickel in the head of the Co80Ni20 wires and in the caps of the Co50Ni50 dumbbells shows that the reduction of the nickel ions is slower than that of cobalt. This lower rate is observed despite the fact that the relative concentration of the nickel ions [Ni2+]/[Ni2+]0 is always higher than that of cobalt ions in solution. It must be concluded that the reduction of the nickel ions is intrinsically slower than the cobalt one in basic solution of 1,2-propanediol. 3.3. Characterization of the Solid Intermediate Phases. We present in this section the analysis of the solid intermediate phases involved in the equilibrium between nonreduced species before the reduction. The purpose is to find differences between the coordination chemistry of Co(II) and Ni(II) ions in basic solution of 1,2-propanediol. We present first the analysis of the solid phases isolated with pure cobalt and pure nickel and then the analysis of the solid phases isolated from mixtures of cobalt and nickel. 3.3.1. Pure Co(II) and Ni(II) Phases. A purple solid phase was recovered when cobalt acetate was heated in a sodium hydroxide solution with a concentration higher than 0.1 M. The formula weight calculated from the thermal analysis, 132 g mol-1, is very close to the formula weight, M(CoC3H6O2) ) 132.9 g mol-1. Thus, when cobalt(II) acetate is heated in a basic solution of 1,2-propanediol, the diol loses its two protons and the dianion complexes the metal center to give an alkoxide with the stoichiometry Co(II)(OCH2CH(-CH3)-O). The X-ray diffraction pattern of this compound shows a crystallized phase with a diffraction peak located at 9.1 Å (Figure 9a). Unfortunately, this diagram could not be indexed, but it is very different from the cobalt

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Figure 9. XRD pattern of the intermediate solid phases that precipitate at 170 °C in a 0.2 M NaOH solution in 1,2-propanediol: (a) CoII, (b) NiII.

alkoxides obtained with 1,2-ethanediol that have been previously characterized.17 The infrared spectrum of the cobalt alkoxide presents strong absorptions in the 2850-2950 cm-1 range corresponding to the stretching vibrations ν(C-H) and a strong bands at 1080 cm-1 attributed to the stretching vibration ν(C-O) (Figure 10). The comparison with the infrared spectrum of liquid 1,2-propanediol shows that the deformation bands δ(O-H) located in the 1300-1450 cm-1 range has disappeared in the IR spectrum of the cobalt alkoxide, in agreement with the loss of the polyol protons. Moreover, the ν(C-H) bands are split in the cobalt alkoxide, showing a chelating coordination to the cations.17 The broad and weak bands at 1595 and 1445 cm-1 are attributed to acetate ions as impurities. The UV-visible absorption spectrum of the cobalt alkoxide presents five main absorption bands located at 1800, 1160, 860, 580, and 485 nm (Figure 11) that are typical of d-d transitions of CoO5 chromophore with cobalt in the +II valence state. This kind of coordination is quite unusual for Co2+ ions in an oxygen environment but is identified unambiguously by the band at 860 nm.23 The green Ni(II) intermediate solid phase obtained by heating nickel acetate in 0.1 M NaOH solution in 1,2propanediol presents very different composition and structure. The X-ray diffraction pattern shows a poorly crystallized (23) (a) Lewis, J.; Nyholm, R. S.; Rodley, G. A. Nature 1965, 207, 73. (b) Ciampolini M.; Bertini, I. J. Chem. Soc., A 1968, 2241. (c) Baran, E. J.; Nord, A. G.; Diemann, E.; Ericsson, T. Acta Chem. Scand. 1999, 44, 513.

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Figure 10. Infrared absorption spectra of the intermediate solid phases that precipitate at 170 °C in a 0.2 M NaOH solution in 1,2-propanediol: (a) CoII, (b) NiII.

layered phase with a turbostratic disorder (Figure 9b). The main band indexed as the (001) line gives an interlayer spacing of 10.2 Å. This value is very close to the interlayer spacing of the Ni(II) LHS hydroxyacetate previously described.15 The UV-visible absorption spectrum of this phase presents three main absorption bands located at 1160, 675, and 390 nm (Figure 11b). These values are typical of d-d transitions of nickel in the +II valence state and occupying a NiO6 octahedron, in agreement with a brucitelike structure.15 Infrared absorption spectrum presents two intense bands at 1615 and 1420 cm-1 attributed to the asymmetric and symmetric stretching vibrations, νas(CdO) and νs(Cd O), of the acetate ions (Figure 10b). An intense band at 3400 cm-1 is also observed corresponding to the stretching vibration of OH groups linked by hydrogen bonds of hydroxide ions and/or water. The less intense bands at 1050 and 1134 cm-1 correspond to adsorbed molecules of propane diol. This infrared spectra is similar to those of the Ni(II) hydroxyacetate LHS salts.15 Thus, these analyses show that by heating acetate in a basic solution of 1,2-propanediol, the Ni(II) cations are linked to hydroxide and acetate ions to give an hydroxyacetate with a brucitelike structure; in contrast, the Co(II) cations are coordinated to chelating alkoxide anions (OCH2-CH(CH3)-O)2- to give a solid phase with 1/1 stoechiometry. The structure of this phase is not elucidated yet, but we know that Co(II) cations occupy CoO5 polyhedrons. The evidence of these solid phases shows that the coordination chemistry of the two metal ions in a basic solution of 1,2-propanediol is very different. It also certainly reflects that the environment of the Co(II) and N(II) ionic species in solution are different

Ung et al.

Figure 11. UV-visible-NIR absorption spectra of the intermediate solid phases that precipitate at 170 °C in a 0.2 M NaOH solution in 1,2propanediol: (a) CoII, (b) NiII.

and may explain the different reactivity of the two metal ions toward reduction. 3.3.2. Intermediate Solid Phases with Co/Ni Mixtures. The XRD pattern of the solid phase recovered by heating a 50/ 50 mixture of cobalt and nickel acetates in a 0.2 M sodium hydroxide solution in 1,2-propanediol presents a broad band at 10.2 Å and a narrow line at 9.1 Å corresponding to the distances observed in the nickel hydroxyacetate and the cobalt alkoxide, respectively. By centrifugation, a partial separation of two solid phases (a purple one and a greenish one) has been possible. The purple phase crystallized as micrometer-sized octahedron (Figure 12a). It is richer in cobalt; the EDX analysis showed a Co/Ni ratio of 90/10 instead of 50/50 in the initial mixture. Structural and spectroscopic analysis showed that this phase presents all the features of the cobalt alkoxide, in particular the intense line at 9.1 Å in the XRD pattern and the five absorption bands in the UV-visible-NIR absorption spectrum. The greenish phase consists of submicrometer-sized grains (Figure 12b) and is richer in nickel; the EDX analysis showed a Co/Ni ratio of 45/55. This phase presents the features of a hydroxyacetate LHS salt; the XRD pattern presents a very broad band at 10.2 Å, and the two intense bands attributed to the acetate ions are observed in the infrared absorption spectrum. The actual Co/Ni ratio in the two solid phases has been estimated by local EDX analysis. The error in the result is probably very important because of the difficulty in separating them. Nervetheless, the difference in the Co/Ni ratio

Magnetic Nanowires and Nanodumbbells in Liquid Polyol

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describe the dumbbell shape can be explained by the slower reduction rate of the Ni(II) ions linked to acetate and/or by the slower dissolution rate of the hydroxyacetate phase. 4. Summary and Conclusion

Figure 12. SEM images of the two intermediate solid phases that precipitated at 170 °C from a mixture of cobalt/nickel ) 50/50 in a 0.2 M NaOH solution in 1,2-propanediol: (a) purple phase, (b) greenish phase.

shows that the hydroxyacetate contains Co(II) species, whereas the Ni content in the alkoxide is very low. Solid solutions of cobalt-nickel hydroxyacetates were previously isolated.24 On the contrary, the absence of a Co-Ni solid solution for the alkoxyde phase is certainly due to the fact that Ni(II) ions do not bear the NiO5 coordination. This experiment on mixtures confirms the difference between the coordination chemistry of Ni(II) and Co(II) ions in a basic solution of 1,2-propanediol. The equilibrium between metal ions in solution and in the nonreduced solid phase that occurs before the reduction was described in the introduction by a single equilibrium (1). In the case of reduction of Co/Ni mixtures, this study shows that it must be replaced by two distinct equilibria to take into account that segregation occurs in the intermediate solid phases (CoII)alkoxide + nucleation

(CoII,NiII)hydroxyacetate [\ ] CoIIsolution + NiIIsolution 9 8 CoNi0 growth OH

v1,2-propanediol Co(CH3CO2)2 + Ni(CH3CO2)2

(2)

Despite the fact that the precursor is acetate for both metal ions, the binding between the acetate and the Ni(II) ions is stronger than that between acetate and Co(II) ions. Thus, the different reduction rates that have been put forward to (24) Taibi, M.; Ammar, S.; Jouini, N.; Fie´vet, F. J. Phys. Chem. Solids 2006, 67, 932.

The reduction of cobalt and nickel acetates in a basic solution of 1,2-propanediol leads to metal Co-Ni particles with a great variety of shape depending on the sodium hydroxide concentration and the Co-Ni composition. All these shapes result from a different nucleation/growth process. The particle analysis and the study of the growth medium allowed us to draw some conclusions about the mechanism of formation of these particles. 1. The measurement of the metal ion concentration, [Co(II)] and [Ni(II)], in solution just before that the reduction takes place showed that it is controlled by the sodium hydroxide concentration through the precipitation of an intermediate solid phase. As the reduction involves the cationic species, it suggests that the basicity controls the growth rate. 2. The second point is that within the bimetallic Co-Ni particles, an important part of nickel is located at the extremities of the particles, showing that the reduction rate of cobalt is higher than that of nickel. 3. The introduction of nickel in the particle composition also favors the formation of isolated nanoparticles (nanowires or nanodumbbells). Indeed, even if nanoneedles of pure cobalt were observed, they always grew from a core of diameter 100 nm. 4. Finally, strong differences between the Co(II) and Ni(II) chemistry in basic solution in 1,2-propanediol were evidenced: with pure metals, the intermediate solid phases are a Co(II) alkoxide and a Ni(II) hydroxy-acetate; in the case of the Co-Ni mixture, the precipitation of two solid phases was observed. These results on the solid phases suggest that the molecular complexes of Co(II) and Ni(II) in solution are distinct. Theses differences in coordination chemistry can explain the different behavior of the two metals toward reduction. Indeed, we expect different dissolution rates of the two solid phases and different reduction rates in solution for molecular species of a different nature. Thus, the various shapes presented at Figure 3 are explained by nucleation and growth rates controlled both by the cobalt/nickel composition and the sodium hydroxide concentration. The sea-urchin-like shaped particles of pure cobalt result in a low nucleation rate and a high growth rate. The growth is slowed down when 20% nickel is introduced in the composition. Wires result when the growth rate remains high for intermediate NaOH concentration and platelets result when the growth is slowed down even more by adding NaOH. When the nickel content is 50%, the hybrid shapes result in two well-separated growth steps: the faster involves mainly the cobalt and the slower mainly the nickel. The growth rate of the first step can be modulated by the NaOH concentration, giving long or short dumbbells, or even platelets. The growth rate of the hcp phase is also closely related to the crystallographic orientation. High rate favors the growth of the (10.0) planes of the hcp structure observed

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in the Co80Ni20 wires, whereas a lower rate favors the growth of the (00.2) dense planes observed in the platelets. Liquid polyols have been widely used as solvent and reducing agent for the synthesis of Co, Ni, and Co-Ni particles. Nevertheless, studies of the actual mechanism of formation of the metal particles are very scarce. We show clearly in this study that the particle shape is related to the equilibrium between cationic species and the intermediate solid phases. We also show that this solid phase is very

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different for cobalt and nickel, which explains the lower reduction rate of nickel in 1,2-propanediol. Acknowledgment. The authors acknowledge F. Herbst (ITODYS) and F. Warmont (LRS, Paris) for the electron microscopy. The authors acknowledge the French Ministry for Education and Research for financial support (ACI Nanoscience). CM0627387