Bismuth oxybromide-based photocatalysts for photocatalytic nitrogen fixation to ammonia

Ammonia (NH₃) is vital for producing chemicals and fertilisers. The Haber Bosch process, which is responsible for 2 % of world energy usage and global annual CO₂ emissions, is an industrial process to synthesize ammonia. Thus, the need for low-cost, environmentally benign methods of artificial ammonia generation under room conditions is critical. One of the promising alternatives is photocatalytic nitrogen (N₂) fixation. In this thesis, various nanomaterials were fabricated through a bismuth-rich, solid solution, bimetallic loading, and heterojunction for photocatalytic NH₃ synthesis under ambient conditions.

Firstly, a range of bismuth oxybromide (BiOBr) with different ratios of Bi/Br were fabricated via a facile method. The photoactivity results show that the higher the Bi/Br ratio, the higher the photocatalytic activity. Thorough characterisation results reveals that the enhanced photocatalytic activity of Bi₃O₄Br may be credited to the higher light absorption capacity, more negative conduction band, effective charge carrier separation and migration, and longer charge carrier lifespan. More importantly, the oxygen vacancy of Bi₃O₄Br can enhance the cleavage of N₂ molecules as the rate-determining step of N₂ fixation.

A novel bismuth-rich Bi₃O₄Br_xI_1-x solid solution containing oxygen vacancies (OVs) was fabricated via solvothermal reaction by altering the Br/I molar ratio which Bi₃O₄Br_0.5I_0.5 shows the highest photocatalytic NH₃ generation. This improved photocatalytic activity contributes to the more effective separation of photogenerated electron-hole pairs and faster charge transfer. By changing x from 1 to 0.25 in Bi₃O₄Br_xI_1-x samples, the bandgap value and morphology of samples would be altered, narrowing the bandgap, and shifting the nanosheet structure to nanorods one. The spectroelectrochemical measurements reveal that Bi₃O₄Br_0.5I_0.5 has the highest concentration of accessible states to the reactions and shifts the conduction band edge toward a lower potential.

To further extend the photocatalyst activity of Bi₃O₄Br, Co and Ni nanoparticles were co-deposited on the Bi₃O₄Br surface for photocatalytic nitrogen reduction by a one-step hydrothermal technique. The bimetallic Co-Ni/Bi₃O₄Br photocatalyst presents a higher photocatalytic activity in comparison with the monometallic Ni/Bi₃O₄Br and Co/Bi₃O₄Br. This finding could be ascribed to the beneficial synergistic effect between the two metals leading to remarkable photocatalytic properties. Photoelectrochemical analysis indicates that Co-Ni/Bi₃O₄Br possesses the highest charge separation and inhibited charge recombination, leading to a high N₂ photofixation performance.

At the last, LaFeO₃ and Bi₃O₄Br heterojunctions were synthesis for N₂ photofixation. The highest enhancement was observed for 10 wt% LaFeO₃/Bi₃O₄Br, whose activity is 2.3 times higher than that of Bi₃O₄Br. This improvement would be credited to the synergistic effect between Bi₃O₄Br and LaFeO₃ in the photocatalysis composite, which resulted in advanced visible light harvesting, successful charge separation and the longer lifetime of excited electrons based on comprehensive characterisation results. Reactive species trapping experiments were proposed the Z-scheme mechanism over LaFeO₃/Bi₃O₄Br.

In summary, this thesis explores the synthesis of various nanomaterials for photocatalytic NH₃ synthesis under ambient conditions. Bismuth-rich solid solution, bimetallic loading, and heterojunction were used to fabricate Bi₃O₄Br, Bi₃O₄Br_xI_1-x, Co-Ni/Bi₃O₄Br, and LaFeO₃/Bi₃O₄Br photocatalysts, which demonstrated enhanced photocatalytic activity due to effective charge carrier separation and migration, longer charge carrier lifespan, and advanced visible light harvesting.