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

Abstract

Dialkyldiselenophosphinatocobalt(II), [Co(Se₂PR₂)₂] (R = ⁱPr, Ph and ^tBu), complexes were used to grow cobalt selenide and cobalt phosphide nanoparticles by decomposition in trioctylphosphine oxide (TOPO) or hexadecylamine (HDA) at 300 °C. Orthorhombic CoSe₂ nanoparticles are readily formed in HDA or TOPO, whereas, in TOP/TOPO or TOP/HDA only orthorhombic CoP or Co₂P 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. Co₂P 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(S₂P^tBu₂)₂], was decomposed in TOP/TOPO, TOP/HDA, TOPO or HDA. The thermolysis reaction in TOPO or HDA gave cubic Co₉S₈ 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 Co₂P nanoparticles with a size of 5 nm diameter, and CoSe₂ nanoparticles are close to cubes of ca. 10 nm size. Co₉S₈ nanoparticles are aggregates composed of numerous small cuboid Co₉S₈ nanoparticles with average diameters of 10–20 nm. The optical spectra show that Co₂P and CoP nanoparticles have a band gaps (E_g) = 3.14 and 1.71 eV, respectively, whilst CoSe₂ nanoparticles a band gap of 1.45 eV.