Novel Hematite Nanostructure For Solar Water Oxidation

Hematite ??-Fe₂O₃? has been widely studied as a promising candidate for solar water splitting. Compared with other semiconductors, ?-Fe₂O₃ has some obvious advantages, such as low cost, chemical stability in solution and suitable band gap. However, by now the performance of hematite photoanodes is still not high enough ?the photocurrent density lower than 5 mA/cm2?, far away from the theoretical value ?12.6 mA/cm2 at 1.23 V vs. RHE?. The performance of hematite photoanodes has been limited by several factors such as low conductivity, short lifetime of the excited-state carrier ?10-12 s?, electron-hole recombination, poor oxygen evolution reaction ?OER? kinetics. Various modifications were thus applied to improve its photoelectrochemical ?PEC? performance. Typically, there are two most significant issues to evaluate the performance of hematite:the plateau photocurrent and the onset potential. In this paper, we have effectively improved the PEC performance of hematite nanostructures by coupling carbon material and using hydrogen treatment.In the second section of this thesis, the surface of FeOOH or Fe₂O₃ was modified by carbon materials ?CNTs or GO?. Firstly, we used a facile hydrothermal method to synthesize FeOOH. By spin coating CNTs or GO were loaded on the surface of FeOOH or Fe₂O₃. Then by using a simple annealing method the carbon materials modified hematite nanostructures have been prepared. Hematite was etched by HF to help the coupling. The final performance has been improved but it is not very good. Anyway, it could be a significant example for future work to couple carbon materials on hematite nanostructures for solar water oxidation.In the next section, hydrogenated hematite was prepared by a facilely method. We show that hydrogenated hematite nanostructures, because of its high photocurrent ability, low onset potential and good treated depth in the near-surface region, can serve as an excellent precursor to design high-performance catalysts for water splitting. Particularly, hydrogenated hematite can be facilely prepared at low cost by adding NH3BH3 ?AB? in the sintering process of a typical hydrothermal method. The one step annealing at a high temperature can reduce the surface defect sites and lower the onset potential. The final product exhibits a high photocurrent of 2.12 mA/cm2 at 1.23 V vs. RHE and shows a cathodic shift of the onset potential ?about 40 mV? compared to the sample prepared by the post treatment method. Moreover, it can be modified by Co-Pi catalyst to achieve a high photocurrent of 2.6 mA/cm2 at 1.23 V vs. RHE and a low onset potential of 0.62 V vs. RHE. The benefits of facile, low-cost preparation, and high performance promise the hydrogenated hematite to be an excellent precursor material for further modifications for solar water splitting applications.Finally, Nitrohydrochloric acid ?HCl:HNO₃= 3:1? and zinc acetate, magnesium nitrate, calcium nitrate were employed for the surface treatment of hematite to achieve high performance. The results show that the various treatments can significantly improve the photocurrent and lower the onset potential of hematite.