Bismuth Oxyhalide Nanosheets with Adjustable Band Edge Potentials for Photocatalytic Ammonia Generation

Ammonia is a crucial compound and precursor in industry and agriculture. It can be easily liquified and transported. Thus, it could be an exciting way to store intermittent renewable energies. However, the current industrial process is energy demanding and polluting. Recently, photocatalytic nitrogen fixation for ammonia generation attracted much attention to be a promising and sustainable alternative to the traditional Haber-Bosch process. Although many proposed photocatalysts are shown to work for ammonia generation, there are still several challenges to achieving high production rates and energy conversion efficiency. Among the studied photocatalysts, bismuth iodide (BiOI) showed promising properties such as visible light absorption capability, adjustable band edge potentials, and abundant surface oxygen vacancies to activate nitrogen molecules. In the third chapter, we fabricated ultrathin BiOI nanosheets by a surfactant-assisted hydrothermal method. Unlike bulk BiOI, ultrathin nanosheets were active for water splitting and nitrogen reduction. It is shown that functional groups (polyvinyl pyrrolidone) on the surface of BiOI induced an electric dipole and upshifted the band edge potentials. Therefore, enhancing redox overpotential, reducing the particle size, and generating a significant amount of oxygen vacancies could enable the photocatalyst to efficiently reduce nitrogen to ammonia in pure water. Another challenge toward efficient ammonia generation is providing a sufficient number of electrons and protons from the water oxidation half-reaction. However, almost all previous studies focused solely on nitrogen reduction reaction, and its counterpart (OER) was neglected. In chapter 4, a new strategy was employed to eliminate the need for using organic sacrificial reagents for ammonia generation. We proposed using cobalt oxyhydroxide (CoOOH) as an OER co-catalyst for a well-known nitrogen reduction photocatalyst, bismuth oxychloride (BiOCl). A series of cobalt-doped BiOCl (Co-BiOCl) nano-platelets were synthesized through a combination of co-precipitation and hydrothermal synthesis with systematic variation in the percentage of added co-catalyst. The formation of CoOOH was verified via X-ray diffraction, X-ray photoelectron spectroscopy, electron microscopy, Raman, and infrared spectroscopy. A 4.6-fold improvement in the ammonia production rate was realized with 5% Co-BiOCl. The OER performed and correlated well with the ammonia generation. Consequently, improving water oxidation by loading OER co-catalyst points to a promising and applicable method for better N2 fixation photocatalyst design.