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.