Mechanistic Studies on the Conversion of Dicobalt Octacarbonyl into Colloidal Cobalt Nanoparticles Anna Lagunas, ² Ciril Jimeno, ² Daniel Font, ² Lluı ´s Sola`, ² and Miquel A. Perica`s* ,²,‡ Institute of Chemical Research of Catalonia (ICIQ), AV. Paı ¨sos Catalans 16, 43007 Tarragona, Spain and Departament de Quı ´mica Orga` nica, UniVersitat de Barcelona, 08028 Barcelona, Spain ReceiVed NoVember 9, 2005. In Final Form: February 15, 2006 In situ ATR-FTIR monitoring has allowed the direct study of the effect of additives (trioctylphosphine oxide [TOPO] and oleic acid) on the kinetics and rate of the thermal decomposition of dicobalt octacarbonyl leading to the formation of colloidal cobalt nanoparticles (CoCNPs). The study has shown that additives usually considered as simple surfactants influence the rate and kinetics of the decomposition of dicobalt octacarbonyl. Several of the initial intermediates connecting Co 2 (CO) 8 with CoCNPs have been identified, and a tentative mechanism for the formation of the colloidal nanoparticles has been proposed. Introduction There is an increasing interest in the research on metal nanoparticles due to the multiple applications involving their physical 1 and chemical 2 properties. These properties, in turn, depend on the particles size and shape, their surface and the ligands coordinated onto it, and the way the nanoparticles are organized either in suspension or supported on polymeric or inorganic materials. 3 Therefore, a precise and accurate control in the synthesis of nanoparticles is needed to guarantee that the desired properties are achieved. In this context, we have recently initiated a project devoted to study the catalytic properties of colloidal cobalt nanoparticles (CoCNPs). 4 Among the methods available for their preparation, we selected the Puntes-Alivisatos procedure 5,6 which is based on the thermal decomposition of dicobalt octacarbonyl in boiling o-dichlorobenzene (179-180 °C) in the presence of a mixture of oleic acid and trioctylphosphine oxide (TOPO) as surfactants (eq 1). It is known that the generation of metal nanoparticles in solution requires the presence of stabilizers to avoid aggregation, specially in the case of magnetic ones. The nature of the surfactant or surfactants mixture and its ratio with the precursor allows us to control the size and shape of the growing particles. In the case of cobalt, it is believed that the additives stabilize the colloidal -Co phase formed as monodisperse spherical nanoparticles 5,7 Although the reproducibility of this procedure is well established, the role of the different additives participating in the process is still not completely understood. Thus, it is not clear if they act as simple surfactants by preventing further growth of the forming nanoparticles, and contributing to their stabilization, or if they really participate in the nanoparticle formation process from the early stages by, for example, coordinating to Co 2 (CO) 8 or to organometallic intermediates involved in the nanoparticle forma- tion. Since we were primarily interested in incorporating functional ligands onto the metallic surface of the particles, and we wanted to explore the possibility of performing this incorporation simultaneously with the generation of the nano- particles, we decided to investigate the role of these additives on the nanoparticle formation process. The information gathered from this study could help in the development of methods for the preparation of catalytic nanoparticles by coordination of functional ligands. Although monitorization of nanoparticle formation is usually carried out by taking samples at different times along the process leading to them and checking by (HR)TEM the presence of nanostructured materials in these samples, 5 we considered that a spectroscopic study of the solution where the nanoparticle formation takes place could provide rich and more detailed, yet complementary information. Given the nature of the chemical process under study (a decarbonylation), infrared spectroscopy was ideal for our purpose. We present herein an in situ and real time ATR-FTIR (attenuated total reflectance-fourier transform infrared spectros- copy) study of the thermal decarbonylation of Co 2 (CO) 8 in the presence of different additives to form colloidal nanoparticles, * To whom correspondence should be addressed. E-mail: mapericas@iciq.es. ² ICIQ. ‡ Universitat de Barcelona. (1) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Fendler, J. H. Chem. Mater. 1996, 8, 1616. (c) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. ReV. 2000, 29, 27. (d) Wang, Z. L. AdV. Mater. 1998, 10, 13. (e) El-Sayed, M. A. Acc. Chem. Res. 2001, 257. (2) (a) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (b) Lewis, L. N. Chem. ReV. 1993, 93, 2693. (c) Studer, M.; Blaser, H.-U.; Exner, C. AdV. Synth. Catal. 2003, 345, 45. (d) Schmidt, H. Appl. Organomet. Chem. 2001, 15, 331. (3) For example, see: (a) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (b) Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811. (c) Hyeon, T. Chem. Commun. 2003, 927. (4) For examples of catalytic applications of cobalt nanoparticles, see: (a) Son, S. U.; Lee, S. I.; Chung, Y. K.; Kim, S.-W.; Hyeon, T. Org. Lett. 2002, 4, 277. (b) Son, S. U.; Park, K. H.; Chung, Y. K. Org. Lett. 2002, 4, 3983. (c) Son, S. U.; Park, K. H.; Chung, Y. K. J. Am. Chem. Soc. 2002, 124, 6838. (d) Kim, S.-W.; Son, S. U.; Lee, S. S.; Hyeon, T.; Chung, Y. K. Chem. Commun. 2001, 2212. (5) (a) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (b) Puntes, V. F.; Zanchet, Z.; Erdonmez, C. K.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 12874. (6) Synthesis and magnetic properties of other cobalt nanoparticles: (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. (c) Sun, S.; Murray, C. B. J. Appl. Phys. 1999, 85, 4325. (d) Spasova, M.; Wiedwald, U.; Farle, M.; Radetic, T.; Dahmen, U.; Hilgendorff, M.; Giersig, M. J. Magnet. Magnet. Mater. 2004, 272-276, 1508. (e) Spasova, M.; Wiedwald, U.; Ramchal, R.; Farle, M..; Hilgendorff, M.; Giersig, M. J. Magnet. Magnet. Mater. 2002, 240, 40. (f) Giersig, M.; Hilgendorff, M. J. Phys. D: Appl. Phys. 1999, 32, L111. (7) Dinega, D. P.; Bawendi, M. G. Angew. Chem., Int. Ed. Engl. 1999, 38, 1788. Co 2 (CO) 8 98 o-Cl 2 C 6 H 4 , 180 °C TOPO, oleic acid Co (CNPs) (1) 3823 Langmuir 2006, 22, 3823-3829 10.1021/la053016h CCC: $33.50 © 2006 American Chemical Society Published on Web 03/18/2006