The preparation of cobalt phosphide and cobalt chalcogenide (CoX, X = S, Se) nanoparticles from single source precursors

Dialkyldiselenophosphinatocobalt(II), [Co(Se(2)PR(2))2] (R = 'Pr, Ph and 'Bu), complexes were used to grow cobalt selenide and cobalt phosphide nanoparticles by decomposition in trioctylphosphine oxide (TOPO) or hexadecylamine (HDA) at 300 'C. Orthorhombic CoSe(2) nanoparticles are readily formed in HDA or TOPO, whereas, in TOP/TOPO or TOP/HDA only orthorhombic CoP or Co(2)P nanoparticles were formed. This observation suggests that TOP provides the phosphorus for nanoparticles. The composition of cobalt phosphide can be controlled by the reaction time. Co(2)P was obtained after 60 min of thermolysis at 300 'C whereas CoP was grown over 150 min. In order to confirm the effect of TOP on the thermolysis of the precursor, the related sulfur containing precursor, [Co(S(2)P'Bu(2))(2)], was decomposed in TOP/TOPO, TOP/HDA, TOPO or HDA. The thermolysis reaction in TOPO or HDA gave cubic Co(9)S(8) nanoparticles whereas in TOP/TOPO or TOP/HDA only CoP nanoparticles were formed as expected due to the presence of TOP. Characterization of these nanoparticles by transmission electron microscopy, X-ray diffraction, and electron diffraction shows highly crystalline spherical CoP and Co(2)P nanoparticles with a size of 5 nm diameter, and CoSe(2) nanoparticles are close to cubes of ca. 10 nm size. Co(9)S(8) nanoparticles are aggregates composed of numerous small cuboid Co(9)S(8) nanoparticles with average diameters of 10-20 nm. The optical spectra show that Co(2)P and CoP nanoparticles have a band gaps (E(g) = 3.14 and 1.71 eV, respectively, whilst CoSe(2)nanoparticles a band gap of 1.45 eV.