Nanofabrication and functional characterization of Co<inf>9-x</inf>Ni<inf>x</inf>S<inf>8</inf> nanoparticles for optoelectronic applications

Abstract

Different ratios of Co9-xNixS8 nanoparticles (2.6 ≤ x ≤ 3.4) were synthesized via the solvothermal technique. Spinel single phase of Co9S8 was detected for x = 2.6 and 3.4, while the impurity phase of cubic Co3O4 was also detected for x = 3.0 due to the segregation of some Co ions causing an obvious change in the interatomic distances of the tetrahedra (short/long) and octahedra of the solid solution Co9-xNixS8. XPS results indicated that Ni3+ cations were only existed in the structure of Co6·4Ni2·6S8, while Ni2+ and Ni3+ cations were located in the Ni-rich samples at x = 3.0 and 3.4. FTIR absorption bands of NiS bonds around 668 and 1036 cm−1 showed larger broadening with increasing Ni content. SEM images of Co6·4Ni2·6S8 revealed clusters of nanospheres that are coated with nanoplatelet carbon sheets. The bandgap energy of Co9-xNixS8 nanoparticles is gradually decreased with increasing Ni content. The sample with the lowest Ni content (x = 2.6) displayed the highest values of refractive index (n) and optical conductivity. The intensity of photoluminescence (PL) spectra for x = 3.0 was the highest emission due to the presence of secondary phase Co3O4 which increases the number of defect levels for trapping more excited electrons and preventing their rapid recombination with holes. The Ni-rich sample with very small refractive index and photoconduction could be a good candidate for antireflection coating materials.