Depth-reduction Of Hematite Photoanodes For Solar Water Oxidation

Hematite is a promising material for solar water splitting due to its low cost,abudance, high stability in water solution, and favorable band gap. However, its practical performance as a photoanode has been significantly limited by various factors such as poor conductivity, short lifetime of the excited-state carrier, poor oxygen evolution reaction(OER) kinetics, short hole diffusion length(2-4 nm), and improper band position for unassisted water splitting. Because of the improper band position of hematite, a bias about 0.4 V vs. RHE has to be applied for water oxidation. Moreover,the poor OER kinetics, the accumulation of photo-excited holes or many other reasons may lead to additional bias for water oxidation, which will significantly decrease the efficiency. A low onset potential for the water oxidation process is thus important for the performance of hematite. Various methods were used in this thesis to improve the performance of hematite for solar water splitting.We have modified the hydrothermal method to prepare repeatable hematite samples with good efficiency. By reducing the concentration of the two reactants and removing the HCl, the prepared hematite can be well repeated by the same method,which will be important for the further treatment to achieve high efficiency.Recently by introducing oxygen vacancies into hematite the performance could be significantly improved with increased conductivity. Here we extend our study of producing oxygen vacancies in hematite by the pyrolysis of NH3BH3(AB) solution on hematite in a crucible. The photocurrent of AB-treated hematite also shows obvious improvement. More interestingly, we find that the crucible as a cover may significantly affect the onset potential, which shows an obvious cathodic shift of the onset potential(up to 70 mV) compared to that without the cover. The seperated two processes ofincreasing the photocurrent and lowering the onset potential allow us to further understand the factors affecting the onset potential shift. Various concentrations of AB and annealing temperatures have been applied to study the cathodic shift and the results suggest that it can be attributed to a depth-reduction of hematite in the near surface region by increased pressure. The present work suggests that the treating depth of hematite is very important for the onset potential, which should be optimized to the near surface region.We also explored the Ni-doping in hematite. Experiments showed by both sputtering or surface deposition of Ni in hematite, the modified hematite exhibited decreased efficiency. The reason will be studied in our future work.