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 Bi3O4BrxI1-x solid solution containing oxygen vacancies (OVs) was fabricated via solvothermal reaction by altering the Br/I molar ratio which Bi3O4Br0.5I0.5 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 Bi3O4BrxI1-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 Bi3O4Br0.5I0.5 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, Bi3O4BrxI1-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.