Electrochemical studies of hematite-based thin films for photoelectrochemical water splitting

Abstract

In this dissertation, α-Fe2O3 thin film deposition techniques were first evaluated to understand their effects on the structural, optical and photoelectrochemical (PEC) properties of the films. α-Fe2O3 films were deposited by dip, spin and combined dip/spin coating techniques on fluorine-doped tin oxide (FTO) substrates at an annealing temperature of 500°C. Structural properties suggest better crystallinity for films prepared by dip and combined dip/spin coating techniques as compared to spin coated films. Field emission scanning electron microscopy showed spherical nanoparticles with some agglomeration into small larvae-shape nanostructures for all the films. All films absorb in the visible region due to their bandgap of 1.98 ± 0.03 eV. Maximum photocurrent densities of 34.6, 7.8, and 13.5 µA/cm2 were obtained at 1.23 V vs reversible hydrogen electrode (RHE) for dip, spin and combined dip/spin coated films with the thickness of 740-800 ± 30 nm respectively. Improved crystallization, low charge transfer resistance at the solid/electrolyte junction, high surface states capacitance, and a more negative flat band potential values obtained for dip coated films using electrochemical techniques, have been associated to their improved photocurrent response. Furthermore, the annealing approach for preparing multi-layered α-Fe2O3 films using the dip coating technique was modified to enhanced their PEC performance. The first three layers of the films were annealed at 500°C and the fourth layer at 500, 600, 700, 750 and 800°C respectively. Films annealed at 750°C recorded the best performance, producing 0.19 mA/cm2 photocurrent at 1.23 V vs RHE; 5.3 times more than what was recorded for films sintered at 500°C, and the onset potential yielded a cathodic shift of 300 mV. The enhanced performance was linked to improved crystallization and absorption coefficient, lowered flat band potential, increased charge carrier density, decreased charge transfer resistance at the solid/liquid interface and increased surface states capacitance for films annealed at 750°C. Also, nanostructured heterojunction of α-Fe2O3 and porous copper (II) oxide (CuO) composites represented as α-Fe2O3/CuO was prepared for the enhancement of PEC water splitting. Structural studies confirmed the high purity of α-Fe2O3/CuO heterostructures produced. Enhanced photocurrent density of 0.53 mA/cm2 at 1.0 V vs RHE was achieved for α-Fe2O3/CuO photoanodes, representing a 19-fold increase compared to the value recorded for α-Fe2O3. The formation of a heterojunction coupled with the porous surface morphology of α-Fe2O3/CuO facilitated charge separation of photogenerated electron-hole pairs, reduced the bandgap and increased the charge carrier density of the heterostructure, enhancing PEC water splitting.