Phase- and Size-Dependent Optical and Magnetic Properties of CoO Nanoparticles

Abstract

A simple pyrolysis method has been developed to synthesize microstructure-controlled CoO nanoparticles from cobalt acetylacetonate in oleylamine at or above 200 °C. XRD, SEM, and HRTEM analyses indicate that the cubic and hexagonal CoO nanoparticles with different morphologies, viz. spherical, quasi-cubic, and pyramidal, could be obtained via varying the precursor concentration, and the average size of hexagonal CoO nanoparticles increases with increasing reaction time or reaction temperature. XPS, TG-DTA, and FTIR analyses reveal that the as-synthesized nanoparticles are pure CoO with good thermal stability. Raman and UV–vis absorption spectra show that the optical properties of CoO nanoparticles are of obvious size effect, which revealed their characteristic feature. Whatever the crystal structure and particle shape are, the CoO nanoparticles with sizes of 33, 59, and 85 nm exhibit two band gaps, and the corresponding band gap differences are 1.84, 1.62, and 1.42 eV, respectively. The pure hexagonal CoO nanoparticles display complete room temperature paramagnetism, while the CoO nanoparticles that contain cubic phase show interesting magnetic behavior due to intrinsic antiferromagnetic structure and uncompensated surface spins, which were confirmed by VSM and ESR studies.

Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

“…The values for a and c are in line with previously reported lattice parameters of wurtzite‐CoO (JCPDS no. 80‐0075) . On the other hand, the sample obtained in a large batch shows a mixture of wurtzite‐CoO ( a = 3.2526(1) Å, c = 5.1976(1) Å; < D > W = 32.0(5) nm) and cubic zinc blende‐CoO structures (space group ; a = 4.5533(1) Å, < D > ZB = 13.0(5) nm – Figure b).…”

“…Additionally, it has been shown that surface effects and bulk defects (e.g., vacancies) may lead to uncompensated spins resulting in a ferromagnetic‐like response . Experimentally, opposing magnetic behaviors have been reported for wurtzite‐CoO, ranging from antiferromagnetic or ferromagnetic‐like to paramagnetic . This probably stems from the strong magnetic signal of the uncompensated spins which smears out any intrinsic magnetic properties of wurtzite‐CoO.…”

Magnetic and phonon transitions in B-site Co doped BiFeO 3 ceramics

“…Back-scattering Raman spectroscopy was used to study phonon anomalies and spin-phonon couplings. [48]. The powders were mixed in an agate mortar with alcohol as a medium for more than 24 h. The dried mixtures were calcined at 800°C (3 h) for all compounds and then sintered at 830°C (10 h) for BFO, and 850°C (3 h) for BFO5Co and BFO15Co.…”

Effect of cobalt alloying on the electrochemical performance of manganese oxide nanoparticles nucleated on multiwalled carbon nanotubes

“…Peaks located at 297.95, 353.35 and 640.31 cm −1 for x=0.2 are red shifted compared to those of the un-doped material, due to the alloying of CoO with the MnO host structure. The small peaks located at 172.04 and 461.42 cm −1 were assigned to CoO [44] in the x=0.2 sample. Highresolution TEM analysis (figures 1(c), (d); and S1, supporting information is available at stacks.iop.org/NANO/28/155403/ mmedia) confirms the nucleation and growth of MnO and Mn 0.8 Co 0.2 O nanoparticles onto the outer walls of the MWCNTs.…”