Functionalizing Nanostructured Fe₂O₃ Electrode And Studies Of Its Behaviors Toward Water Splitting

In response to the global energy crisis and environmental pollution, the research and development of renewable clean energy has fueled up in recent years. Hydrogen is one of the cleanest energies. One of the efficient and low cost hydrogen production technologiesis based on a semiconductor electrode water splitting process. Due to its unique size, a nanomaterial has many unique properties, and thus the photoelectrochemi0 cal water splitting is mainly based on nanostructured semiconductor photoelectrodes. Compared with catalysts made from regular materials, the catalysts in nanoscales have remarkable catalytic properties, such as high catalytic activity and selectivity. However, there are many factors that can affect the catalytic performance of the nanocatalysts such as the size and shape of the nanomaterials. Thus, the photolysis efficiency of the existing semiconductor electrodes is still generally lower than 5%, and it is urgent to explore the new materials or/and novel electrode structures to further improve the water splitting efficiency for practical applications. Modification of semiconductor electrodes could be an approach to improve the water splitting efficiency.This dissertation comprehensively reviews the development history of water splitting, introduces the basic principles of water splitting, analyzes the methods to improve the performance of water splitting and summarizes the research progress of water splitting. The thesis research mainly focuses on the controllable synthesis of semiconductor nanostructure electrode and further modification for improvement of the photoelectrochemical splitting performance. The enhancement mechanism is investigated for scientific insights. The details are summarized briefly as follows: 1 Preparation of rGQDs/Fe₂O₃ and its PEC water splitting behavior.The precursor of β-FeOOH nanothin film was prepared on FTO substrate by hydrothermal method using FeCl3.6H2 O as raw material, then, we can get α-Fe₂O₃ by high temperature annealing, then the Fe₂O₃ was immersed in GOQDs aqueous solution, and the rGQDs/Fe₂O₃ was prepared through annealing treatment in argon stream. The anode of rGQDs/Fe₂O₃ composite was synthesized by self-assembly method and functional modified rGQDs. The self-assembly method can effectively control the morphology of the nanostructures. The rGQDs/Fe₂O₃ nanosheet thin film composite structure was successfully prepared and further confirmed by the morphology and structure characterization.It is discovered that the photoelectric efficiency of the composite photoanode with a concentration of GOQDs of 0.05g/mL was the best. Results indicate that compared with the Fe₂O₃ electrode, the rGQDs/Fe₂O₃ anode can significantly improve the photoelectric current density by 8 times at1.23 V（vs.RHE）, mainly due to the effective passivation of rGQDs/ Fe₂O₃ surface to inhibit the interfacial charge recombination while demonstrating that the self-assembly method is an effective technique for surface passivation. 2 Fe₂O₃@ Ni（OH）₂ composite electrode for electrical water splitting.Ni（OH）₂ nanofilms are grown on the surface of Fe₂O₃ electrode by electrodepositing method. The Fe₂O₃@Ni（OH）₂ composite electrode was further prepared for an effective oxygen evolution electrode. The electrodepositing method is simple and easy to use in the three electrode system. Under the SEM, It is clear that a hematite surface is uniformly grown with nanothin Ni（OH）₂. The optimal deposition time is 60 s by optimizing the deposition time. The behavior of Fe₂O₃@Ni（OH）₂ composite electrode for electrocatalytic water splitting was tested. Results show that the catalytic performance of Fe₂O₃@Ni（OH）₂ electrode was significantly improved in comparison to the Fe₂O₃ electrode. When the current density is 10mAcm-2, the voltage of Fe₂O₃@Ni（OH）₂ is smaller than Fe₂O₃ electrode. In addition, Fe₂O₃@Ni（OH）₂ electrode has a good stability. Further investigation and analysis disclose that the modification of Ni（OH）₂ on Fe₂O₃ generates significantly synergistic catalytic effect toward water splitting, and the mechanism is proposed. In addition, the modification also increases the reaction surface area of Fe₂O₃. The Fe₂O₃@Ni（OH）₂ electrode has high stability.In summary, the works performed in the thesis reveal that modification, assembly and composition can be used to delicately tailor the chemical composition and physicalstructures for unique water splitting properties that can significantly improve the kinetic and thermodynamic limits for efficient water splitting.