Wang, Yiou; (2019) The investigation into highly efficient heptazine-based polymeric photocatalysts for visible light-driven solar fuel synthesis. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Artificial photosynthesis has been regarded as a promising method to generate fuels in a much greener way by utilising inexhaustible solar energy via water splitting and CO2 conversion. Polymeric semiconductors have been recently identified as promising photocatalysts due to their comparatively low cost and ease modification of the electronic structure. However, the majority only respond to a limited wavelength region (<460 nm) and still suffer from fast charge recombination. Herein, a novel synthetic pathway has been developed to control the O and N linker/terminal species in polymeric photocatalysts, which highly influences the bandgap, band positions and charge separation. As such, the synthesised oxygen-doped C3N4 polymers can be excited from UV via visible to even near-IR (800 nm) wavelengths, resulting in one order of magnitude higher H2 evolution rate than the widely-reported polymeric g-C3N4 (λ > 420 nm), leading to a 10.3% apparent quantum yield (λ = 420 nm). Both theoretical calculations and spectroscopies have attributed such superior performance to enhanced charge separation and narrow bandgap. Such new polymer was then coupled with an inorganic photocatalyst to construct a Z-scheme system, which successfully splits water into both H2 and O2 in a stoichiometry ratio. Further, an efficient strategy was demonstrated to stepwise tailor the bandgap of polymeric photocatalysts from 2.7 to 1.9 eV by carefully manipulating the O/N linker/terminal atoms in the heptazine chains. These polymers work stably and efficiently for both H2 and O2 evolution (420 nm < λ < 710 nm), exhibiting nearly 20 times higher activity compared to g-C3N4 with high AQYs under visible light irradiation. Experimental and theoretical results have attributed the narrowed band gap and enhanced charge separation to the oxygen incorporation into the linker/terminal position. Based on this success, a more challenging multi-electron photochemical process of visible light-driven CO2 reduction in water was investigated using junctions consisting of the novel polymers and two kinds of carbon quantum dots (CQD) cocatalysts. The novel CQD was synthesised via a microwave-assisted method while the other CQD fabricated via sonication of glucose was reported as reduction cocatalysts (redCQD). In CO2 reduction reactions, the novel CQD/polymer junctions selectively produce methanol and O2 while the redCQD/polymer junction generates CO only. Ultrafast spectroscopies revealed that novel CQD works as a hole acceptor in the junctions, different from the redCQD as an electron acceptor. Electrons reach the surface of polymers to reduce CO2 to produce methanol while holes accumulate on CQD to oxidise water. Microwave-assisted CQD shows more favourable water adsorption instead of methanol adsorption compared with polymers, thus facilitating methanol production instead of CO. Therefore, the function of CQD is a key reason for such high selectivity.

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