Investigation of the Influence of Organometallic Precursors on the Formation of Cobalt Nanoparticles Rohini M. de Silva, ² Vadim Palshin, ² Frank R. Fronczek, ‡ Josef Hormes, ² and Challa S. S. R. Kumar* ,² Centre for AdVanced Microstructures and DeVices, Louisiana State UniVersity, 6980 Jefferson Highway, Baton Rouge, Louisiana 70806, and Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803 ReceiVed: January 20, 2007; In Final Form: April 26, 2007 An investigation of the influence of organometallic precursors on the formation of cobalt nanoparticles is presented. The cobalt nanoparticles were obtained by decomposing two different organometallic cobalt complexes, [(Co 2 (µ-HCtCH)(CO) 6 ] and [Co 2 (CO) 8 ], under identical experimental conditions. Time-dependent FT-IR analysis revealed different decomposition routes, rates, and reaction intermediates leading to the formation of cobalt nanoparticles. Spectroscopic data in addition to X-ray crystallography confirmed the structures of reaction intermediates from [(Co 2 (µ-HCtCH)(CO) 6 ]. The crystal structure, particle size, size distribution, and magnetic properties of the cobalt nanoparticles from the two precursors were analyzed using synchrotron radiation based X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry, respectively, and showed significant differences. 1. Introduction A great deal of research continues to be focused on nanom- eter-sized materials in general and cobalt-based magnetic materials in particular due to their potential applications in the field of electronics, 1 high-density data storage media, 2 catalysis, 3 and biomedical sciences. 4 Numerous physical and chemical methods such as sputtering, 5 chemical vapor deposition, 6 reverse micelle synthesis, 7 mechanical milling, 8 solution-phase metal salt reduction, 9-13 and decomposition of neutral organometallic precursors 14 have already been well-established to produce cobalt nanoparticles (NPs). Of these, high-temperature wet- chemical reactions, as compared to other techniques, are particularly attractive as they are known to offer better control over size, size distribution, shape, and crystal structure of cobalt nanoparticles. For example, high-temperature reduction of salts such as CoCl 2 , 10 CoI 2 , 11 Co(CH 3 COO) 2 , 9,12 and Co(acac) 3 13 using lithium and sodium compounds in the presence of stabilizing agents is known to provide controlled particle sizes and avoid agglomeration of cobalt nanoparticles. The thermal decomposition of dicobalt octacarbonyl (DCO) under inert atmospheric conditions in the presence of surfactants is known to produce cobalt NPs of controlled size, shape, and crystal structure. 9-10,14-17 The effect of reaction conditions such as temperature, 18 time of addition of reagents, 16 solvents, 19 utiliza- tion of surfactants, 9-10,20 polymeric stabilizers, 21 and type of reaction vessels (for example, utilization of microreactors) 22 for control of cobalt nanoparticle formation is well-documented. It is also reported that the particle size and size distribution are affected by the actual position of injection of the precursor into the reaction vessel. 16 Surfactants, in particular, have the ability to control not only the particle size but also the shape and crystal structure. 14b,17,23 The decomposition of DCO in the presence of trioctylphosphine oxide (TOPO) resulted in obtaining ǫ-cobalt nanoparticles. 24 However, in the absence of TOPO, fcc cobalt nanoparticles were obtained. 24 The synthesis of ǫ-cobalt nano- particles by the thermal decomposition of DCO has been achieved using oleic acid and triphenyl phosphine 25 or a mixture of surfactants composed of oleic acid (OA), lauric acid, and trioctyl phosphine (TOP). 20 The synthesis of multiply twinned fcc cobalt nanoparticles was accomplished by thermal decom- position of DCO in the presence of OA and tributyl phosphine. 26 The ǫ-cobalt and fcc-cobalt phases require annealing at 300- 500 °C to convert into the hcp phase. 5b,10,27 Alivisatos et al. 16a have reported direct synthesis of hcp Co nanoparticles eliminat- ing the need for annealing at high temperatures. Chaudret’s group synthesized hcp Co nanoparticles by thermolysis of [Co- (η 3 -C 8 H 13 )(η 4 -C 8 H 12 )]. 28 In order to obtain high quality single- crystal cobalt NPs, careful selection of surfactants is essential, 14b in addition to manipulation of particle growth kinetics ac- complished by adjusting the capping agents and their ratios. 15a,16 It is believed that the capping molecules bind to crystal faces in a selective manner slowing down the growth of the capping- agent-bound crystal face relative to the other faces resulting in obtaining highly anisotropic crystal shapes such as discs, 16 rods, 23,28 and wires 29 of cobalt NPs. While the effect of surfactants, reaction conditions, and different types of reactors on the formation of Co nanoparticles has been investigated, the influence of different precursors has not been examined systematically. Such an investigation as- sumes more importance as it is well-established that properties of cobalt nanoparticles are extremely sensitive to the presence of even minute surface impurities. 7b,30,31 Recently, Gugliotti et al. 32 revealed the importance of the ligands bound to the metal center of organometallic precursors in controlling the particle shape and size of Pd and Pt nanoparticles. They observed that changing the ligand dibenzylideneacetone (DBA) bound to Pd and Pt metals with triphenyphosphine (PPh 3 ) resulted in particles ² Centre for Advanced Microstructures and Devices. ‡ Department of Chemistry. 10320 J. Phys. Chem. C 2007, 111, 10320-10328 10.1021/jp070499k CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007