No abstract

Carbon-coated hematite nanostructures for solar water splitting were prepared by a simple pyrolysis of ferrocene which showed a remarkable photocurrent of 2.1 mA cm À2 at 1.23 V vs. RHE, compared to a value of 0.5 mA cm À2 for hematite without the carbon layer. The carbon layer is a few nm thick covering the surface of hematite nanostructures. X-Ray photoelectron spectroscopy and X-ray absorption spectroscopy revealed that the electronic structure of hematite was significantly modified with the existence of oxygen vacancy, which was responsible for the remarkable photocurrent. The carbon layer plays an important role for the appearance of oxygen vacancy. The simple and cheap method could be scaled up easily which may pave the way for the practical application for efficient solar water splitting. Broader contextHematite has emerged as a good photocatalyst for efficient solar water splitting. Various methods have been used to prepare high efficiency hematite nanostructures. However, a facile and cheap synthesis method of hematite nanostructures with high efficiency is critical for future practical application. Here we report a facile synthesis of hematite nanostructures for solar water splitting via a simple pyrolysis of ferrocene under ambient pressure. The photocurrent of the carbon-coated hematite nanostructures can be 2.1 mA cm À2 at 1.23 V vs. RHE, compared to a value of 0.5 mA cm À2 for hematite without the carbon layer. The carbon layer is a few nm thick covering the surface of hematite nanostructures. X-Ray photoelectron spectroscopy and X-ray absorption spectroscopy revealed that the electronic structure of hematite was signicantly modied with the existence of oxygen vacancy, which was responsible for the remarkable photocurrent. The carbon layer plays an important role for the appearance of oxygen vacancy. The simple and cheap method could be scaled up easily which may pave the way for the practical application for efficient solar water splitting.

Hydrogen-treated hematite nanostructures were prepared by a simple pyrolysis of NaBH 4 in a crucible. The H 2 -treated hematite photoelectrode showed high efficiency for solar water oxidation with a photocurrent of 2.28 mA cm À2 at 1.23 V vs. RHE, which was over 2.5 times higher than that for pristine hematite (0.88 mA cm À2 ). The significant improvement of the photocurrent can be attributed to increased oxygen vacancies after the H 2 treatment. Moreover, the onset potential for H 2 -treated hematite was low and when compared to the hematite photoelectrode treated in an oxygen-deficient atmosphere to produce oxygen vacancies, a cathodic shift of the onset potential was observed by about 120 mV (from 0.99 to 0.87 V vs. RHE). The cathodic shift of the onset potential was attributed to the surface effect of H 2 treatment while the oxygen-deficiency treatment mainly affected the bulk, which was confirmed by X-ray absorption spectroscopy. The results also suggest that the presence of surface defect states of Fe 2+ in hematite is not the reason for high onset potential described in the literature. The H 2 -treated hematite with high efficiency could be used as a good starting material to achieve better performance for practical applications with further modifications such as surface catalysts or elemental doping.

Ti-doped hematite nanostructures have been synthesized for efficient solar water splitting by adding TiCN as the Ti precursor in a hydrothermal method. Ti-doped hematite nanostructures show an urchin-like morphology with nano feature size, which increases the effective surface area compared to undoped nanostructures. A remarkable plateau photocurrent density value of 3.76 mA/cm 2 has been observed for Ti-doped nanostructures under standard illumination conditions in 1 M NaOH electrolyte, which is 2.5 times higher than that for undoped nanostructures (1.48 mA/cm 2). The photocurrent at 1.23 V vs. RHE (1.91 mA/cm 2) is also enhanced to be over 2 times higher than that for undoped nanostructures (0.87 mA/cm 2). X-ray photoelectron spectroscopy and x-ray absorption spectroscopy have been used to investigate the electronic structure of Ti-doped hematite, which suggest the increased donor density of hematite by Ti doping. The remarkable plateau current density in Ti-doped hematite nanostructures can be attributed to both the favorable urchin-like morphology and the Ti doping. V

Permission has been granted to the Libnry of The University of Manitoba to lend or sen copies of this thesislpracticum, to the National Library of Canada to microfilm this thesis/practicum and to lend or seil copies of the film, and to Dissertations Abstracts international to pubiisb an abstract of this thesis/prrcticum.The author reserves other publication rights, and neither this thesis/prrcticum nor extensive extracts from it m a y be printed or otherwisc reproduccd without the author's written permission. Abstract ABSTRACTVacancy-type defects in the four main types of diamond (Ia, Ib, IIa and IIb) were investigated using positron lifetime, Doppler broadening and optical absorption spectroscopies. In unirradiated sarnples vacancy clusters were found in al1 types, synthetic as well as natural. These clusters are situated in highly defected regions, rather than homogeneously distributed, and their concentration varies significantly from sample to sample. For synthetic Tt, diamonds vacancy clusters were investigated as a function of nitrogen content. The bulk positron lifetime for diamond is calculated to be 98 + 2 ps and the bulk Doppler S parameter is estimated to be 25% lower than that for silicon. Electron irradiation (2.3 MeV) produced neutral monovacancies in IIa diamond and the positron data correlated well, as a function of dose, with the GR1 optical zero-phonon line (the optical absorption at 740nm arise fiom neutral monovacancies in diarnond); the introduction rate was estimated to be 0.5 k 0.2 cm-'. In Ib diamond monovacancies were found to be negatively charged whereas they were neutral in IIa diamonds. The positron lifetime for monovacancies was (40 t 6)% larger than the bulk lifetime and the Doppler S parameter increased by (8 + 1)%. At-temperature Doppler measurements between 30 and 770 K indicated that irradiation-produced neutral monovacancies can convert to the negatively charged state above 400 K but this was dependent on diamond type.Isochronal annealing of irradiated Ib diarnonds showed that the complex of a substitutional nitrogen and a vacancy, formed upon annealing close to 6 0 0°c , undergoes two detectable modifications between 600 and 870 OC reaching a configuration stable to 1 170 OC . Key conclusions based on positron and optical data are in mutual accord.