Synthesis, Characterization, and Magnetic Properties of Cobalt Nanoparticles from an Organometallic Precursor

THF solutions of the new organometallic precursor Co(η 3 -C 8 H 13 )(η 4 -C 8 H 12 ) react with 3 bar dihydrogen in the presence of polyvinylpyrrolidone (PVP) at three different temperatures, namely, 0, 20, and 60 °C to give colloidal solutions containing fcc cobalt particles of respectively ≤1 nm ( Coll. 1 ) and ca. 1.5 nm ( Coll. 2 and Coll. 3 ) mean sizes, as evidenced by HREM analysis. All colloids react with carbon monoxide to give new colloids ( Coll. 1 ‘ − 3 ‘ ) displaying broad CO stretches near 2000 and 1890 cm -1 on their infrared spectra recorded in KBr disks. The magnetic behavior of the colloids was studied by SQUID techniques. Coll. 1 − 3 display a typical superparamagnetic behavior with blocking temperatures in the range 9−10 K. From magnetization studies above and below the blocking temperature, it was possible to determine that Coll. 3 shows a mean size of 1.6 nm and a very narrow size distribution. Because of a magnetic surface effect, the magnetic moment per atom (μ Co = 1.94 ± 0.04 μ B ) appears significantly larger than the value admitted for bulk cobalt. In contrast, Coll. 2 ‘ shows a weak magnetization and disappearance of the blocking temperature as a result of CO coordination.