| dc.description.abstract | Rapidly growing energy demand leads to research on the development of sustainable energy resources. Photocatalytic (PC) or photoelectrochemical (PEC) processes are considered a potential route for producing sustainable hydrogen directly from solar illumination. Many studies have been performed for photochemical water splitting utilising conventional semiconductors as photoelectrodes. However, the high recombination rate of photo-generated charges of conventional semiconductors hinders the development of PEC applications. In this context, ferroelectric materials having spontaneous polarisation response have attracted much attention. The remanent polarisation after poling can favour the separation of photo-generated charges. BiFeO₃ (also named as 'BFO‘ in this thesis) films, which possesses a bandgap around 2.2 – 2.7 eV along with significant polarisation response (Pr ranges from 60 – 150 µC/cm²) has been considered as a promising candidate as photoelectrode for water splitting. This thesis therefore focuses on the development of BiFeO₃-based ferroelectric films for PEC water splitting. AACVD was explored as a promising synthetic method for BiFeO₃ films. Simple and low-cost precursors were used to successfully achieve pure as-prepared BiFeO₃ by AACVD method on the FTO substrate for the first time. With post-annealing, it could fully transit into Bi₂Fe₄O₉. However, it was found difficult to reproduce the synthesis of phase-pure BiFeO₃ using AACVD. Since the desired BiFeO₃ films failed to be obtained via AACVD, a traditional chemical solution method was used for the further studies. Stoichiometric BiFeO₃ films was successfully achieved with 10% excess bismuth in the precursor solution to compensate for the loss of Bi during heat treatment. The as-prepared films by spin coating were further annealed in argon, air and oxygen conditions respectively to explore the effects of annealing atmospheres on the structure, optical properties, morphology, defects, electrical properties and PEC performance. Pure rhombohedral BiFeO₃ was confirmed without any presence of by-products or impurities. The enhanced photocathodic current in oxygen annealed BiFeO₃ films can be ascribed to the reduced oxygen vacancies, which emphasizes the importance of defect control to achieve good PEC performance for further exploration of BiFeO₃ films. Bismuth ions in the BiFeO₃ lattice is widely known as volatile, which is one of the defect origins, so that excess bismuth is usually used to compensate for the Bi loss during heat treatment. Bi₁₊ₓFeO₃ films were prepared using different amounts of excess bismuth added into the precursor. Bi-deficient, stoichiometric and Bi-rich BiFeO₃ films were studied. It was found that the PEC performance of BiFeO₃ films can be enhanced by a moderate amount of excess Bi, which was believed to be a consequence of several factors combined including structure, morphology, defects and electrical properties. It provides a comprehensive understanding of the effects of excess Bi on PEC performance of BiFeO₃. And it indicates that the bismuth amount should be carefully controlled, which has not been paid much attention in the literature. Since defect control is possibly an effective way to improve PEC performance of BiFeO₃ films, doping was used for further study this. Apart from the investigation of defects, the structure, optical, electrical properties can be affected by doping as well. BiFe₁−ₓCoₓO3 films were prepared and studied. The results confirm that defects level of oxygen vacancies and Fe²⁺ can be effectively modified by doping. However, the transition in structure and presence of impurity phases resulted in a reduced PEC performance. This research reveals that photocathodic performance of BiFeO₃-based films can be effectively improved by defect control. However, defect engineering also affects other factors such as structure, phase transition, morphology, optical properties and electrical properties. In order to achieve the best PEC performance, these factors should be considered and controlled carefully during the preparation process. In addition, it is confirmed that the ferroelectricity of BiFeO₃ can be utilized to improve the separation efficiency of photo-generated charges and hence enhance the PEC performance. In the case of BiFeO₃photocathodes, the poling with right direction which drives electrons to the electrode/electrolyte interface and holes to the substrate can result in a considerable enhancement in the photocurrent. It will benefit the future work on BiFeO₃ photoelectrodes. Although the strategies above are beneficial to the promotion of BiFeO₃ photocathodes, the maximum photocurrent achieved in this thesis under AM1.5G 1.0 sun radiation is around 52 µA/cm² at 0.4 V vs. RHE, which is far from the expected theoretical short-circuit photocurrent of 4 mA/cm2. The widely studied Cu-based photocathodes such as CuO and Cu₂O also exhibit the photocurrent at the magnitude of a few mA. Thus, there is great potential for the development of BiFeO₃ photocathodes and more efforts should be made in the future. | en_US |
