Effects of Annealing on Structural, Morphological, and Corrosion Properties of α-Fe2O3 Thin Films

Hematite (Fe2O3) thin films have been synthesized on indium tin oxide (ITO)-coated glass substrate using an electrodeposition technique and the changes in their structure due to subsequent annealing at different temperatures investigated. The structural, optical, and morphological properties of Fe2O3 thin films formed after annealing at temperatures of 500°C, 550°C, and 600°C after deposition were determined by x-ray diffraction (XRD) analysis, absorption spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). XRD analysis of the annealed electrodeposited samples revealed Fe2O3 thin films with rhombohedral crystal structure. The energy bandgap of the Fe2O3 films grown on ITO and annealed at different temperatures was calculated based on absorption measurements, yielding values between 2.0 eV and 2.2 eV. Raman peaks were observed between 210 cm−1 and 1305 cm−1 depending on the annealing temperature. SEM analysis clearly revealed the crystal structure belonging to Fe2O3 and an increase in the particle size as the annealing temperature was increased. The surface roughness values (in the range from 11 nm to 46 nm) obtained from AFM images of the samples annealed at different temperatures were compatible with the average roughness values (in the range from 14 nm to 51 nm). The results of these analyses confirm that the structure of the α-Fe2O3 thin films was improved by increasing the annealing temperature. Electrochemical impedance spectroscopy (EIS) and Tafel measurements enabled detailed examination of the corrosion behavior of the α-Fe2O3 films deposited on the ITO substrate. Nyquist and Bode analyses were applied to determine the structural changes to the Fe2O3 and its corrosion behavior. The Rs values calculated with the help of Nyquist plots increased from 65.10 Ω cm2 to 540.5 Ω cm2 with increase of the annealing temperature, while the Rct values decreased from 64.29 × 109 Ω cm2 to 1.142 Ω cm2 after annealing. The corrosion current and potential parameters were also calculated from Tafel measurements after deposition and annealing at different temperatures.