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Liu, Chang

University of British Columbia

Photo(electro)catalysis is one of the promising approaches for solar-to-chemical conversion, which includes the splitting of water to produce hydrogen as an alternative solar fuel to traditional fossil fuels. Carbon nitride (CNₓ) has been recognized as the privileged particulate photocatalyst and emerging photoelectrode material for its hydrogen evolution ability, but the low charge mobility, fast charge recombination, and charge trapping of the pristine CNₓ stand in the way of better performance. Overcoming these challenges motivates the diverse modifications made to CNₓ. Post-synthetic modifications (i.e., the loading of cocatalysts) create new interfaces, for example, CNₓ|cocatalyst compared to CNₓ|electrolyte, which impacts the photo(electro)catalytic performance by creating new interfacial charge transfer pathways. Pre-synthetic modifications can also modify the interface (i.e., substrate) where CNₓ is grown to tune the CNₓ structure and diversify its properties. The lack of fundamental understanding or at least systematic investigation of these interfaces motivates my thesis studies. In the particulate system, I confirmed that cocatalysts accelerate the interfacial charge transfer at the CNₓ|cocatalyst interface, and surprisingly found that the transition metal Ni exhibited comparable hydrogen production ability to Pt. Meanwhile, a Ni in situ photoreduction pathway, different from that observed with Pt, was revealed during the hydrogen evolution reaction mechanism. In the (photo)electrochemical system, I modified the substrate surface via aminosilanization. This interface modification produced a thinner and compact CNₓ layer, and different behavior in charge carrier dynamics (i.e., fewer deep traps) monitored by transient absorption spectroscopy, which is not seen when post-modifying the CNₓ with cocatalysts due to the high population of the traps in the bulk CNₓ. The faster charge transport with less charge trapping through the CNₓ layer as well as more efficient charge transfer at the CNₓ|substrate interface allow more charges to pass through the electrodes. The interface studies in two CNₓ systems concluded that the charge transfer at interfaces and charge transport in the bulk CNₓ are still challenging to address. Developing strategies to decrease or passivate the trap states in CNₓ or promote the rate of charge transfer across the interfaces remain promising directions to achieve optimized CNₓ-based photocatalytic and (photo)electrochemical systems.

Text

2023-08-10

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/85551

Chemistry

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/