Influence of coating techniques on the optical and structural properties of hematite thin films

Hematite (alpha-Fe2O3) thin films were deposited by dip, spin and combined coating techniques on fluorine-doped tin oxide (FTO)/glass substrates. Structural properties observed from X-ray Diffraction (XRD) and Raman spectra analysis suggest better crystallinity for films prepared by dip and combined coating techniques as compared to the ones prepared by spin coating. The Raman spectra of films prepared by spin coating and combined techniques showed red shifted and more broadened peaks relative to the ones prepared by dip coating. The red shifts were attributed to increased strain and defects in the films. Field emission scanning electron microscopy (FESEM) showed spherical nanoparticles with some agglomeration into small larvae-shape nanostructures for all samples. Cross-sectional images of the films revealed that samples prepared by dip and spin coating had the lowest and highest thicknesses of 452 +/- 28 and 622 +/- 30 nm respectively. Ultraviolet-visible (UV-Vis) spectroscopy studies revealed that the films absorb light in the visible region due to their bandgap of 1.98 +/- 0.03 eV. However, films prepared using combined coating technique showed better light absorption than the ones produced by spin coating at wavelengths below 485 nm despite having less film thickness. This study showed that coating techniques influences crystallization and optical absorption behaviour of hematite films which may have impact on their performance when applied towards photoelectrochemical (PEC) water splitting.