Modification Of Hematite Photoanodes For Highly Efficient Photoelectrochemical Water Splitting

Hematite（α-Fe₂O₃） has been considered to be a promising material for photoelectrochemical（PEC） water splitting due to its favorable optical band gap, good chemical stability in oxidative environment, abundance, and low cost. However, the performance of PEC water splitting for hematite photoanodes is limited by several key factors such as relatively poor conductivity, a short hole diffusion length（2-4 nm） and slow oxygen evolution reaction（OER） kinetics. Hence, recent studies have focused on improving their PEC activity via various modifications.In this thesis, different modification approaches to enhance the performance of hematite for PEC water splitting are discussed, including the formation of heterojunction, loading of oxygen evolution catalysts（OECs） and synthesis of TiO₂ underlayer. The contents are listed as below:（1） Fe₂TiO₅ coated hematite photoanodes were produced either by evaporating a TiCl₄ solution on FeOOH or by a simple HF-assisted Ti treatment of FeOOH, both followed by annealing. The prepared Fe₂TiO₅-hematite heterostructure showed a significant enhancement in photocurrent density compared to that of the pristine hematite. For example, the sample prepared via HF-assisted Ti treatment exhibited a significantly enhanced photocurrent of 2.0 mA/cm2 at 1.23 V vs. RHE.Synchrotron-based soft X-ray absorption spectroscopy analyses clearly revealed the existence of an Fe₂TiO₅ structure on hematite forming a heterojunction, which reduced the photogenerated hole accumulation and then improved the PEC performance.（2） FeNiOOH-decorated hematite photoanode has been prepared by a facile electrodeposition method, with a significant cathodic shift of the onset potential（up to 190 mV） compared to the pristine sample. The enhanced performance can be attributed to the better oxidation evolution reaction kinetics with the FeNiOOH cocatalyst. The FeNiOOH-decorated hematite is very stable in a long time.Moreover, the cocatalyst can be well coupled to the Pt-modified hematite photoanode achieving a high photocurrent of 2.21 mA/cm2 at 1.23 V vs. RHE. The good catalytic property and the facile preparation method suggest that the decoration of FeNiOOH is a favorable strategy to improve the performance of hematite.（3） TiO₂ underlayer thin films for hematite-based photoelectrochemical cells were prepared on FTO substrate by the hydrolysis of TiCl₄ solution with different concentrations and showed an enhanced photocurrent density.Upon high temperature annealing, Ti⁴⁺ can diffuse into hematite film from the TiO₂ underlayer and form the Fe₂O₃/Fe₂TiO₅ composited photoanodes, which has been confirmed by XPS, XAS, element mapping and EDS measurements. Therefore, the enhanced PEC activity can be attributed to the formation of Fe₂TiO₅ which facilitate the surface trapping of photogenerated holes and decreased the recombination rate of photogenerated carriers.