Effect of annealing temperature on the thermal transformation to cobalt oxide of thin films obtained via chemical solution deposition

An ammonia-free chemical solution deposition formulation is employed to synthesize cobalt hydroxide thin films. The films are subsequently annealed at different temperatures to obtain cobalt oxide (Co₃O₄). The reaction solution is composed solely of cobalt sulfate and triethanolamine. The annealing temperature has a strong effect on the properties of thin films. X-ray diffraction shows that the films are amorphous despite the thermal treatment; the morphology for both as-deposited and annealed films are found to be homogeneous, compact, with no evident changes. X-ray photoelectron spectroscopy reveals that the as-deposited thin films are mainly composed of cobalt hydroxide. After annealing, the films transform into cobalt oxide (Co₃O₄) thin films. The optical spectra confirm the X-ray photoelectron spectra, due to the appearance of characteristic features of Co₃O₄ on both characterization methods. The energy band gap is estimated to lie between 2.1 and 3.0 eV. The resistivity of the annealed films ranges from 4.29 × 10³ to 1.46 × 10⁴ Ω⋅ cm. Finally, with the work function and ionization energy, we propose an experimental band diagram as a function of annealing temperature, showing a p-type nature for the cobalt oxide films.