Oxygen Vacancy Doped Hematite Nanostructures For Solar Water Splitting

According to theoretical prediction, the solar-to-hydrogen efficiency of hematitecan be16.8%and the water splitting photocurrent can be12.6mA cm-2. Comparedwith other semiconductor photocatalysts such as TiO2, ZnO and WO3, α-Fe2O3hasemerged as a good photocatalyst for efficient solar water splitting due to its favorableoptical band gap (2.1–2.2eV), extraordinary chemical stability in an oxidativeenvironment, abundant source and low cost. However, the practical performance ofα-Fe2O3for solar water splitting is far from the ideal conditions which was limited byseveral factors such as poor conductivity, short lifetime of the excited-state carrier(10-12s), poor oxygen evolution reaction (OER) kinetics, short hole diffusion length(2–4nm), and improper band position for unassisted water splitting. Enormous effortssuch as elemental doping, morphology control and surface treatment have been used toimprove the performance of hematite photoelectrode.In the second section of this thesis, by controlling the oxygen pressure in thesintering process, the contents of oxygen vacancies in α-Fe2O3can be effectivelycontrolled to achieve an optimized performance. By coupling Ti-doping and oxygenvacancies in α-Fe2O3nanostructures, an efficient photoelectrode for solar wateroxidation was prepared which showed a high photocurrent of2.25mA·cm-2at1.23Vvs. RHE and a remarkable maximum value of4.56mA·cm-2at1.6V vs. RHE at arelatively low activation temperature of550℃. In addition, the partial oxygen pressurerange suitable to produce oxygen vacancies in Ti-doped α-Fe2O3could be expanded toa wide region compared to that in undoped α-Fe2O3, which was critical to thephotoelectrode production in practical applications.Compared with other methods to prepare hematite nanostructures, hydrothermalprocess has more advantages such as its low requirements of instruments and source, low cost and strong operability in practical application. In the third section of thisthesis, we optimize the growth conditions of hydrothermal method to enhance the solarwater splitting activity of α-Fe2O3. By changing the amount of HCl in the precursorsolution, α-Fe2O3photoelectrode with nanorod structures was prepared with excellentperformance. Although the improvement of water splitting efficiency is not very strong,the further understanding of the basic conditions in hematite synthesis process is veryimportant for the fabrication of excellent and stable α-Fe2O3photocatalysts.In the fourth part of the thesis, Co-Pi was used to improve the performance ofTi-doped α-Fe2O3through electroplating process and synergistic doping method. Bythe way, FeNO3·9H2O was selected as Fe source in the synthesis process. Oxygenvacancies were also introduced by the annealing treatment in poor oxygen conditions.The samples showed excellent photocatalytic performance without any optimization.