Chemistry and catalysis advances in organometallic chemistry and catalysis
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31.5 COBALT 31.5.1 Cobalt Nanoparticles and Nanorods Cobalt nanocrystals are also good candidates for many applications aiming at exploiting their magnetic properties [54]. This is due to their high magnetic moment and to their effective anisotropy which is much higher than that of iron. Furthermore, the possibility of cobalt in adopting different crystal phases facilitates a modulation of its magnetic properties through shape control.
430 ORGANOMETALLIC NANOPARTICLES 1.3nm,
5.4nm,
σ = 0.5 nm 6.2nm,
σ = 0.6 nm 7.7nm,
σ = 0.4 nm 9.4nm,
σ = 0.5 nm 13.3nm,
σ = 1.0 nm 20.8nm,
σ = 2.7 nm 0 1 1.1 1.2
1.3 1.4
1.5 100 nm
100 nm 200 nm
1 μm (a) (b) (c)
Figure 31.9 TEM images of Fe NPs. (a) Size evolution as a function of the acid concentration between 0 and 1.5 equiv. (b) 5.5 nm nanospheres and (c) 13.3 nm nanocubes obtained with 1 and 1.4 equiv of acid, respectively. Nanocubes self-organized into micrometric wires [67]. (a) (c)
(d) (e)
(b) 1 μm 1 μm ? 50 nm 50 nm
Anisotropic Nucleation Growth:coalescence Growth:ripening Isotropic G r o w t h Figure 31.10 TEM images of (a) organic super structure before decomposition and (b) self-assemblies obtained after 48 h; they are filled with (c) nanocubes while (d) nanospheres are found outside of these micrometric organizations. (e) Schematic view of the environment-dependent growth mechanism proposed. The “organometallic” approach has greatly contributed to the enrichment of the arsenal of Co nanocrystals available by chemical routes, affording highly crystalline, oxide-free nanocrystals of various sizes and shapes. The first precursor used by our group was the Co I organometallic complex [Co( η 3 -C 8 H 13 )( η 4 -C 8 H 12 )]. This compound in THF and in the presence of PVP affords, upon reaction with H 2 , NPs of very small size (1.6–2 nm) even at temperatures as low as 0 ◦ C [55]. An important characteristic of these objects was the enhanced magnetic moment per cobalt atom ( μCo = 1.94 ± 0.04 μB) COBALT 431 100 nm
50 nm 10 nm
(a) (b)
(c) (d)
Figure 31.11 (a) SEM image of nanostars. (Inset), TEM image of a single star. (b) TEM image of porous nanocubes, obtained with dodecylamine/lauric acid surfactants. (c), (d) 3D reconstruction of the nanocubes obtained from electron tomography study, the defects embedded within the objects are represented in blue, and the outer surface of the cubes are in light violet. (See insert for color representation
(b)
(a) (c)
H 1 H 2 [010]
[100] [001]
10 nm 15 15 20 25 30 35 40 45 20 25 30 35 40 45 20 30 40 z (nm)
x (nm) y (nm) 0 0.1 0.2 0.3
0.4 0.5 m 2 0.6
0.7 0.8
0.9 1
(a) TEM image of a 30 nm nanocube. (b) Mapping of the induction field obtained from electronic holography. (c) 3D visualization of the magnetic moment simulated within the nanocube. (See insert for color representation of the figure.) compared to the bulk value. This finding is attributed the high surface to volume ratio of the NPs. This enhancement, observed for the first time in NPs prepared by chemical methods, was found to cancel when CO was coordinated to the surface of the objects. A more detailed study of the magnetic properties of these particles and the effect of the high surface to volume ratio has shown that the influence of the surface atoms on the anisotropy is increased by decreasing the size and also that the magnetization enhancement increases with the applied magnetic field [56].
432 ORGANOMETALLIC NANOPARTICLES (a) (b)
(c) 5 nm
(10-10) Figure 31.13 (a,b) TEM images of self-organized nanorods (scale bar: 10 nm). (c) HRM image and electron diffraction pattern of some aligned rods. The use of the binary ligand system long-chain amine/long-chain acid with the same compound has permitted the synthesis of anisotropic Co nano-objects by the reduction of the above mentioned precursor in anisole at 150 ◦ C [12e, 57]. The role of dihydrogen was examined and found to be crucial for the formation of nanorods, since in its absence the formation of anisotropic nanorods is inhibited. The resulting objects varied from spherical NPs to long nanowires. The concentration in carboxylic acid had a strong influence on the shape of the final objects. For low acid concentrations (0.3 equiv), the resulting objects were spherical NPs, and for high concentrations long nanowires. Moreover, the length of the alkyl chain of the amine was shown to be critical for the dimensions of the objects obtained. Nano-objects with an aspect ratio between 1.7 and 22 could be formed by simply varying the nature of the amine. The nanorods obtained were found to be single crystalline with hcp structure and nonoxidized as revealed by high resolution electron microscopy (HREM) and WAXS studies. The impact of the shape anisotropy on the magnetic properties was evidenced by SQUID measurement (superconducting quantum interference device) of the hysteresis loops. The nanorods exhibited high coercive field (0.89 T) at 2 K. At the same time, the magnetic moment per atom was found to be identical to that of bulk cobalt (1.71 μB). These nanorods self-assemble in solution, forming superlattices in which the rods are arranged with their long axes next to each other [58]. The mechanism invoked for their formation is an oriented attachment of the initially spherical NPs in a template formed by the organic ligands. TEM micrographs of these superlattices and an HREM images are shown in Fig. 31.13. Cobalt nanodisks have also been prepared by the same method and the same precursor in the presence of rhodamine (RhB) and HDA. The bulkiness of RhB, which hinders its adsorption on the Co surface in the early growth stages as well as the specific adsorption of HDA at the {0001} facets, is responsible for this peculiar growth mode. An out-of-plane magnetization was observed. These nano-objects self-assemble into 1D chains with high magnetic anisotropy [59]. The Co
II coordination compound [Co {N(SiMe 3
2 } 2 ] is a precursor employed in more recent works. Being thermally more stable than [Co( η 3
8 H 13 )( η 4 -C 8 H 12 )], it has a longer “shelf-life”. Nevertheless, it is also readily reduced under H 2 , giving
rise to a great variety of cobalt nanocrystals. Similar reaction conditions give rise to qualitatively similar nanocrystals either when using [Co( η 3
8 H 13 )( η 4 -C 8 H 12 )] or [Co {N(SiMe 3
2 } 2 ]. Indeed, when an HDA/lauric acid (LA) mixture is used with [Co
{N(SiMe 3 ) 2 } 2 ], organized superlattices of several layers of arrays of vertically aligned nanorods are obtained in solution. The 3D arrays, the SEM (scanning electron microscopy) images of which are shown in Fig. 31.14, are larger than the ones obtained by [Co( η 3 -C 8 H 13 )( η 4 -C 8 H 12 )]. The nanorods are assembled in several layers of arrays in which the nanorods are arranged along their long axes [60]. Upon alignment under an external magnetic field in an organic matrix and after purification, these nanorods present increased coercive field values at room temperature (0.6 T), as well as a remnant magnetization value of about 0.90 Ms [61]. These characteristics make them good candidates for applications as permanent magnets or components for high-density magnetic recording devices.
An emerging direction in nanomaterials research concerns the synthesis of multifunctional nanocrystals [62]. “Seeded growth” [63] of a second material on already preformed nanocrystals has given access to nano-objects associating discrete, nonconcentric, and chemically different domains. Anisotropic nanocrystals due to their distinct reactivity along different facets offer the possibility to position the second domain on a specific facet [64]. Using as seeds cobalt nanorods we have
CONCLUSION 433 CAN
SEI 10.0kV
X100.000 100nm
WD 6.0mm WD 6.0mm
SEI 10.0kV
X150,000 100nm
WD 8.2mm Figure 31.14 SEM images of superlattices of Co nanorods prepared by [Co {N(SiMe 3
2 } 2 ], in a mixture of HDA/LA. (b)
(a) Figure 31.15 (a) Au-tipped Co nanorods and (b) Au-decorated Co nanorods (scale bars 50 nm). performed a secondary growth of gold toward Co–Au hybrid nano-objects. Control of the surface chemistry of the nanorods permits the control of the gold nucleation in a topologically selective way and growth of either gold-tipped or decorated nanorods, as shown in Fig. 31.15. The NPs on the lateral sides grow epitaxially, whereas the tips are strained because of lattice mismatch between the two metals across the (111) and (0002) planes of Au and Co, respectively [65]. Alternatively, we have used CdSe nanorods on which we have grown Co by reducing [Co( η 3
8 H 13 )( η 4 -C 8 H 12 )] or
[Co {N(SiMe
3 ) 2 } 2 ] under conditions milder than usual in order to avoid any homogeneous nucleation of Co not associated to CdSe [66]. Cobalt tips initially growing as spheres finally adopt an anisotropic growth mode as shown in Fig. 31.16. Interestingly four growth modes have been identified by HREM, characterized by different angles between the long axes of CdSe and Co. These angles are dictated by the need to minimize of the lattice mismatch between the two materials along the various crystallographic directions. In heterostructured objects, the magnetic properties of Co are unaffected by the presence of CdSe, whereas the luminescence properties of CdSe are affected compared to pure CdSe but not completely quenched.
To conclude, the techniques and concepts of organometallic chemistry can be fruitfully adopted to the synthesis of NPs of various metals and of various size and shapes. Bimetallic nano-objects can also be easily produced, whether alloys, core–shell systems or heterostructures. Many different stabilizers can be used, whose choice is guided by the similarity 434 ORGANOMETALLIC NANOPARTICLES (a) (b)
Figure 31.16 CdSe nanorods with (a) spherical Co tips and (b) Co nanorods at the tips (scale bar 100 nm). between the behavior of ligands on the molecular complexes and on NPs. Thus, hydrides and CO coordinate on the surface of these NPs, as well as olefins and methyl groups. The hydrides are always fluxional as determined by static D NMR whereas CO appears fluxional only when the surface of the particles is free. Amines are also fluxional but do not stabilize the particles unless they are added in excess. However, what is a drawback in molecular chemistry can be used here as an advantage since amines allow the growth of NPs and may lead to shape-controlled nano-objects. Phosphines and especially diphosphines are, as in molecular chemistry, the ligands of choice since their steric and electronic properties can be modulated. N-heterocyclic carbene ligands are definitely extremely well adapted to ruthenium and, if we want to look at complex ligands for, for example, asymmetric catalysis, we will have to consider them carefully. Ligands are also involved in the growth of the NPs, and the choice of acid/amine mixtures allows the dissociation of a nucleation reservoir form a growth reservoir and, therefore, with the help of a dihydrogen atmosphere, the growth of large monodisperse nano-objects, iron nanocubes, and cobalt nanorods, the monodispersity of which results from thermodynamic control [67]. The need in surface chemistry will be the development of complex systems accommodating a combination of ligands some of which are specific for NPs to achieve selective catalytic transformations as well as cascade reactions. Similarly, complex nano-objects combining different functionalities (magnetic, optic, catalytic, electric) will be required in fields as different as energy conversion or nanomedicine. In all cases, the precise control of the nature and positioning of the different species and ligands will be required, which means that all these different aspects are new challenges for organometallic chemists who are likely to become more and more involved in the field. ACKNOWLEDGMENTS The authors wish to acknowledge the financial support from CNRS, from the European Community’s Seventh Framework Programme (FP7/2007– 2013) under grant agreement no. 226716 and the ANR project Batmag.
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