Synthesis Of α-Fe₂O₃ Films With Surface Modification And Their Photoelectrochemical Properties

Hematite(ɑ-Fe2O3) is a promising material for photoelectrochemical(PEC) water splitting due to its favorable optical band gap, extraordinary chemical stability in oxidative environment, abundance, and low cost. However, the efficiency of PEC water splitting of hematite is still low which limited by several key factors such as relatively poor absorptivity, and a short hole diffusion length(2-4 nm).Enormous efforts were focused on the modification of electronic structure to improve its PEC activity.In this thesis, we employed the surface modification and treatment of hematite nanostructures which have been synthesized by two different approaches to enhance its phtotoelectrochemical activity.Hematite nanostructures with the surface RGO modification.Hematite nanostructures have been synthesized from hydrothermal process in an aqueous solution, and the surface is modified by RGO. First of al, Hematite was synthesized by reacting Fe(acac)3 and NH3 ·H2O,The film thickness can be controlled by the number of deposition cycles.The results show that: The current density of hematite can be effected by thickness.The thicknessof best current density reach 178 nm when the number of deposition cycles is 7cycles.The photocurrent can be improved by coating a layer of reducing graphene oxide(RGO) through spin coating base on the best deposition cycles. The photocurrent density of ɑ-Fe2O3/RGO nanostructures reach 0.666 m A cm-2 at 1.5 V vs RHE. To identify the Material performance of ɑ-Fe2O3/RGO, scanning electron microscope(SEM), X-ray photoelectron spectroscopy(XPS), Electrochemical Impedance Spectroscopy(EIS), monochromatic incident photon-to-electron conversion efficiency(IPCE)analyses were carried out. The enhanced performance is attributed to the promoting effects of RGO which acted as hole collector and transporter in the electrodes. This research demonstrates that graphene can be used as an alternative co-catalyst to enhance photoelectrochemical water splitting activity of ultrathin hematite film.Hematite nanostructures with the surface phosphate modification(PM-Fe2O3) have been synthesized with hydrothermal method in an aqueous solution of Fe Cl3 and phosphate. For the sake of contrastive analysis, Hematite nanostructures without the surface phosphate modification and hematite nanostructures with the surface phosphate modification were prepared by means of soaking, then the three films were characterized by J-V, The photocurrent density of hematite nanostructures with the surface phosphate modification reach 0.958 m A cm-2 at 1.5 V vs RHE.When PM-Fe2O3 was Characterized by SEM we found that the morphology of hematite can be controlled by a small amount of phosphate from nanorods became nanoblocks structure.To identify the Material performance of hematite nanostructures with the surface phosphate modification, Transmission electron microscope(TEM), X-ray photoelectron spectroscopy(XPS), Electrochemical Impedance Spectroscopy(EIS) analyses were carried out. The surface negative charge could build an electrostatic field that facilitates the separation of photoexcited electron–hole pairs by extraction of photoexcited holes to the surface when the hematite photoanode was modified with phosphate ions on its surface. The surface charge were described via zeta potential analyzer. The results show that the surface phosphate group concentration of PS-Fe2O3 are lower than PM-Fe2O3. These phosphate groups mainly presented on the surface due to much OH groups on the surface of intermediate Fe OOH. Whileas PM Fe2O3 films were prepared. a small amount of phosphate not only control the morphology of hematite enlarge contact area with FTO substrate, and is beneficial to facilitating charge transfer and minimizing bulk charge recombination. It is important to minimizing Photogenerated holes and photo-generated electrons recombination, the facial modification at the ɑ-Fe2O3 for a better photoelectrochemical performance.