Structure And Performance Manipulation On The Low-dimension Iron-oxide Functional Materials

Due to the excellent stability, earth-abundance, fascinating electronic and magnetic properties, transition metal oxides has played an important role in the research field and application of optoelectron and magnetism as well as the development of society and human civilization. Howerer their charge carrier transport and ferromagnetic interaction has been severely limited by the indirect interact mechanism of metal-xygen-metal between neighbored metal atoms, thereby how to manipulate electronic interaction of transition metal oxide is an important fundamental scientic topic. As a typical transition metal oxide, iron oxide possesses luxuriant properties of optoelectron, magnetism etc and corresponding study on modulation of structure and performance via dimensional control is helpful for cognition of the structure-function relationship and their broad application as photoelectronic and spintronic devices.Since the discovery of graphene, ultrathin transition metal oxide nanosheets have attracted much attention and their unique capabilities have been highly desired, but so far there isn’t any report about atomic iron oxide nanosheet yet. In this dissertation, half-unit-cell iron oxide nanosheets (-0.7nm) were firstly synthesized by CuO-template assisted growth method and the peculiar magnetic transformation from anti-ferromagnetism to ferromagnetism was observed, which could be well attributed to distorted atomic structure of nanosheets; A multi-layer electrode model was proposed and both double-layer a-Fe2O3/NixFe2-xO3and triple-layer Fe2TiO5-TiO2/Coox nanotube electrodes were synthesized, sucessfully conquering the charge transport issue and significantly increasing the utilization of visivble light; Besides, the band structure of ultrathin iron oxide nanosheets was changed by two-dimension (2D) confinement, realizing the automatic visible-light water splitting in～0.7nm γ-FeOOH nanosheets. The investigation of low-dimension iron-oxide materials was carried out in order to deeply cognize the intrinsic relationship between structure and performance via combining structure measures, such as transmission electron microscope (TEM), X-ray diffraction spectrum (XRD), X-ray absorption fine structure (XAFS) etc,with performance tests, for example photoelectron-catalyst, magnetism, and theory calculation. The main content of this dissertation is as following: 1. Magnetic study of half-unit-cell α-Fe2O3nanosheetsBy means of a CuO template-assisted oriented growth strategy, we have realized atomically thin a-Fe2O3nanosheets with half-unit-cell thickness for the first time. The half-unit-cell α-Fe2O3semiconductor nanosheet samples exhibit the intrinsic and robust erromagnetism of0.6μB saturation magnetic moment at100K and retained a ferromagnetic response at room temperature, comparable with that of traditional half metal magnets. Since the α-Fe2O3nanocrystal is antiferromagnetic, the observed large magnetic moment can be attributed to the antiferromagnetism to ferromagnetism transformation in2D α-Fe2O3nanosheets. Structural analyses revealed that the surface distortion of the2D nanosheets makes a significant proportion (ca.30%) of five-coordinated Fe (Fe5-co) ions in ideal nanosheets into Fe (Fe6-co) coordinated by six O atoms. The coexistence of Fe5-co and Fe6-co ions breaks the quantum degeneracy of Fe3d energy states in bulk α-Fe2O3and permits an effective electron exchange to activate the ferromagnetic exchange interaction in the α-Fe2O3nanosheets.2. Photoelectrochemical study of α-Fe2O3/NixFe2-xO3nanotubeWe have synthesized hematite nanotube arrays with a nickel oxide overlayer by anodic oxidation and subsequent processing. XAFS analysis and TEM results shown the nickel oxide overlayer is in the form of NixFe2-xO3with～lnm thickness. The incident photo to electron conversion efficiency (IPCEs) analysis shows that covering the thin NixFe2-xO3layer leads to greatly enhanced IPCEs in the400-550nm wavelength range with average efficiency of X due to the full utilization of visible light, resulting in a pronounced photocurrent density of3.3mA/cm2at0.45V vs Ag/AgCl, corresponding to the photoconversion efficiency of1.37%. Simultaneously, negative shift of100mV was observed for the corresponding onset potential. The enhancement of photocurrent is attributed to the extension structure of NixFe2-xO3layer as well as the nanotube structure. Electron injection of the thin NixFe2-xO3layer improves photocarriers migration ability in bulk, resulting in-70%raised arrival photocarriers at surface. Simultaneously, surface catalysis of this thin layer is useful for accelerating Oxygen evolution reaction (OER) kinetics at semiconductor liquid junction (SCLJ), leading to full utilization of photocarriers with low energy. Besides the triple-layer Fe2TiO5-TiO2/Coox nanotube electrodes were prepared and realized the absorption photo-to electron conversion efficiency (APCE) of40-50%in the range of400-600nm with the corresponding total energy conversion efficiency of2.7%. The impendence analysis confirmed the enhanced utilization of visible light was due to the anisotropic charge transportation and the elongated charge diffusion length.3. Photo-catalyst study of half-unit-cell a-Fe2O3nanosheetsWe have synthesized the γ-FeOOH ultrathin nanosheets with-0.7nm as a highly active photocatalyst to realize overall visible-light water splitting via CuO template-assisted oriented growth strategy combined with topology transform process. The half-unit-cell thickness of y-FeOOH ultrathin nanosheet ensures all of Fe atoms on surface which could directly allow the as-excited electron-hole pairs from Fe3d states to participate into the water redox reaction. Owing to2D confinement along [010] direction, the CBM of y-FeOOH nanosheets is0.8V above that of bulk material, straddling the redox potential of water. This superior feature leads to the achievement of2.4%and0.6%quantum efficiency at420nm and longest visible light wavelength of550nm, respectively, and an averaged quantum efficiency of～2%in the wavelength region of400-500nm for freestanding suspension. The high efficiency can be ascribed to a direct bandgap of2.4eV, exposed hydroxyl groups, as well as the high-rate charge separation. This novel photocatalyst holds the potential for practical applications owing to several advantages, including:(i) a direct bandgap of2.4eV suitable for the utilization of nearly half portion of the visible light;(ii) high activity for both photolysis water oxidation and reduction;(iii) the freestanding character for the unrestricted usage in a simple photocatalytic device;(iv) the earth-abundant of Fe oxides/hydroxide.