Charge Carrier Dynamics of Graphitic Carbon-based Photocatalysts for Solar Chemical Production

Abstract

The earth has reached an anthropogenic era, as greenhouse gas emissions from human activities have now increased global temperatures to a record level of 1.5 <SUP>◦</SUP>C above the preindustrial era. Therefore, it has become urgent to reduce global reliance on energy sources from fossil fuels. Solar energy which is based on harnessing a portion of the immense energy from our closest star, presents a promising carbon-neutral alternative. However, current solar technologies are limited by their dependence on daytime periods with expensive battery energy storage for nighttime. This Thesis presents a comparatively cheaper, more versatile and integrated energy storage solution in the form of solar chemical production using graphitic carbon-based photocatalysts (graphitic carbon nitride and graphenes).

Graphitic carbon-based photocatalysts are stable and cost-effective but limited by high charge carrier recombination rates. Thus, four synthetic strategies were used to decipher their charge carrier dynamics monitored by photoluminescence spectroscopy and transient absorption spectroscopy and subsequently mitigate any mechanistic bottlenecks affecting their solar-to-chemical conversion efficiencies. The photocatalytic activity of N-doped graphene was due to an unprecedented long-lived optical response from carrier trapping at nitrogen defects. A heterojunction was made with improved photocurrent due to charge transfer from carbon nitride to titanium dioxide, which competes with the carbon nitride radiative recombination pathway. Mesoporous graphitic carbon nitride was shown to have a long charge carrier separation lifetime due to an increased active sites. Oxamide-modified potassium poly(heptazine imide) had an increased amount of cyano defects which altered its charge carrier dynamics for enhanced hydrogen peroxide production. Overall, nitrogen defects in graphitic carbon nitrides showed the most potential to enhance their solar chemical production efficiencies. But further research is required to determine the best synthetic strategies to harness these nitrogen defects for the rational design of integrated devices for solar energy storage based on solar chemical production.