Synthesis, Characterization, Electro-catalytic And Photo-catalytic Properties Of NiCo₂O₄ Nanomaterials

In this dissertation, NiCo₂O₄ nano-material which has an innovative nanostructure, namely the core-ring structure, is studied. As a p-type semiconductor and as a mixed valence oxide that adopts a spinel structure, NiCo₂O₄ seems to be the most promising anode materials for water electrolysis. The solid-state redox couples Co3+/Co2+ and Ni3+/Ni2+ are present in the structure, providing a notable electrocatalytic activity. Many researchers have focused on the study of the electrocatalytic activity of NiCo₂O₄ materials; however, to our knowledge, in only a few of these studies have the morphologies and nanostructure of the nano- NiCo₂O₄ been investigated, let alone the relationship between the nanostructure and the electrocatalytic activity.Since the discovery of photocatalytic H2 formation with TiO2 several decades ago, extensive efforts have been carried out to develop new types of semiconductor photocatalysts capable of splitting water into H2 and O2. Up to now, however, most of the photocatalysts developed only respond to the UV light which occupies only merely about 4% of the whole solar energy. Only several water splitting photocatalysts were developed for H2 evolution from pure water under visible light irradiation, and the efficiency is less than 1%. Therefore, the development of visible light-responsive photocatalysts appears to be urgently necessary in order to utilize the solar energy effectively.The research includes the synthesis of core-ring structured nano- NiCo₂O₄, the XRD, TEM and HRTEM etc. characterizations, the investigation of the formation of core-ring structure, the catalytic properties of the electrode coated by nano- NiCo₂O₄ in water electrolysis, and the study of UV-visible light absorption and the band structure of the nano- NiCo₂O₄.It is revealed by the EDS analysis that the cation concentration of Co (Ni) in the core-ring structure gradually increase (decrease) from the ring to the core. A mechanism for the core-ring structure formation is proposed, which explains the separation of the ring from the core and the cation distributions in the structure.The core-ring structured nano- NiCo₂O₄ is active anode catalyst for water electrolysis. Compared with the electrodes of ordinary NiCo₂O₄and Co3O4, or other NiCo₂O₄electrodes prepared by alternate methods, the electrode coated by core-ring NiCo₂O₄ nanoplatelets exhibits the greatest electrocatalytic properties, with an over-potential of 0.315V at a current density of 100mAcm-2. The core–ring nanostructure providing an increased active surface area and providing a large number of active surface Co atoms, which play a significant role in oxgen evolution reaction (OER), are probably the main reasons for the excellent electrolytic activity.The core-ring structured nano- NiCo₂O₄ are characteristic of photocatalysts able to respond to UV light and visible light, with the bandgap energies of 3.63 eV and 2.06 eV, which suggest that it is a promising candidate for photocatalysts under visible light irradiation.