Thesis Defence: Towards fundamental understanding of interfaces in carbon nitride photocatalytic and (photo)electrochemical systems

Chang Liu, supervised by Dr. Robert Godin, will defend their thesis titled “Towards fundamental understanding of interfaces in carbon nitride photocatalytic and (photo)electrochemical systems” in partial fulfillment of the requirements for the degree of Master of Science in Chemistry.

An abstract for Chang Liu’s thesis is included below.

Defences are open to all members of the campus community as well as the general public. Registration is not required for in person defences.

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ABSTRACT

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 stands in the way of even 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.