Optimization of processing parameters in the preparation of hematite thin films for photoelectrochemical water splitting

Abstract

In this study, the influence of coating techniques and layering of hematite (α-Fe2O3) thin films were studied. Two colloidal-based coating techniques namely dip coating and thermal spray pyrolysis were used to synthesize nanostructured hematite thin films. All films were annealed at 500 °C for I hr to perform the calcination from ɣ- Fe2O3 (magnetite) to α-Fe2O3 (hematite). X-ray diffraction confirmed the calcination to hematite, it furthermore confirmed the hexagonal corundum structure of hematite. Raman spectroscopy confirmed the polycrystallinity of hematite with seven optical vibrational modes (two Eg and five A1g) observed in the first Brillouin zone. Ultraviolet-visible spectroscopy showed good absorbance in the visible region with absorbance onset ranging from 596.75 – 608.75 nm. Scanning electron microscopy depicted various nanoparticles ranging from agglomerated nanodiscs and nanorods to nanospheres. This study showed that coating techniques could attribute to the optical and structural properties of hematite thin films for photoelectrochemical water splitting. Furthermore, a modified annealing approach was used to determine the effects of annealing time and multiple layer formation on the structural, optical and electronic properties of hematite thin films. Scanning electron microscopy revealed an increase in film thickness with increased annealing time. Furthermore, it depicted the formation of four single layers when annealing between each coating. UV-Vis indicated a decrease in bandgap with prolonged annealing. Additionally, UV-Vis revealed better absorption for coherent layers when compared to the single layers annealed four times. The photocurrent increased two-fold from 1.65 x 10-4 Acm-2 to 4.77 x 10-4 Acm-2 when annealing time was increased from 30 mins to 1 hr. Similarly, a significant improvement from 2.26 x 10-4 Acm-2 to 4.35 x 10-4 Acm-2 was found with a decrease of multiple layer formation and annealing frequency. From this work it was determined that annealing time and multiple layer formation by increased annealing frequency influences the optical, structural and electrical properties of nanostructured hematite thin films.