UBC Theses and Dissertations
Solar fuel generation by graphitic carbon nitride composites Lachance, Robert
Solar fuels are an attractive medium for long term storage of solar energy, but the low efficiency of photocatalysts in visible light prevents large scale adoption. Graphitic carbon nitride (g-CN) is a promising photocatalyst for solar fuel generation with a medium bandgap and demonstrated ability for photocatalyst engineering to improve charge recombination and charge mobility. In this work the improvement of g-CN material is investigated via phosphorous doping and post synthesis techniques to stabilize the g-CN surface chemistry and prepare the material for heterojunction integration. Further, the improved visible light absorption and activity of a self- assembled 2D-2D precious metal free g-CN type II heterojunction z-scheme is presented, targeting photocatalytic CO2 reduction and hydrogen evolution for solar fuel applications. The synergistic combination of highly reducing electrons in the conduction band of g-CN and very oxidizing holes in the valence band of BiVO4 enables tunable, small bandgap, visible light active materials to perform artificial photosynthesis. Strong interfacial ohmic contact induced by the electrostatic attraction of z-scheme constituent materials to promote charge separation is investigated with the addition of reduced graphene oxide (rGO) acting as an electron transfer medium. A realistic flat plate circulating batch reactor operating in the vapor phase, illuminated by a Xe arc solar simulator and coupled with online gas chromatography is employed for the evaluation of photocatalyst materials. This z-scheme employs facile synthesis techniques to achieve a visible light active, tunable band edge constituent material with similar reaction overpotentials to iii single, UV active materials such as TiO₂. Photoreduction of CO₂ improves over 100% and water splitting improves nearly 40% for the g-CN/BiVO₄ composite and is attributed to strong interfacial z-scheme charge transfer. Protonation and phosphorous doping are shown to influence the amount and oxidation state of the amine functional groups present on the g-CN surface, with protonation demonstrating nearly 4X increase in CO₂ reduction compared to washed g-CN and attributed to increased CO₂ adsorption. The work presented provides a pathway to electrostatic self-assembly of 2D-2D z-scheme heterojunctions for improved photocatalytic solar fuel generation.
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