Electron Paramagnetic Resonance Studies on Native and Photo-Induced Species in Carbon Nitride

The research described in this thesis concerns the investigation of the photophysical properties of carbon nitride. This material is a semiconductor studied as a photocatalyst, which has already proved active in several types of chemical conversions. However, its efficiency as a photocatalyst is limited by two main causes: i) rapid charge-carrier recombination and ii) the strong dependence of its photochemical properties on its morphology. A detailed knowledge of the processes underlying light harvesting and conversion into charge carriers and how these depend on the morphology of the material is crucial to improve its efficiency and subject to much debate in the scientific literature. Several studies suggested that in carbon nitride there exist a correlation between the catalytic efficiency and the number of native and photo-induced paramagnetic species. In this thesis, Electron Paramagnetic Resonance (EPR) spectroscopy is applied to track different stages that follow light excitation in carbon nitride. In particular, the metastable photo-excited triplet excitons are characterised by means of time-resolved EPR, while the structural and photo-responsive properties of localised charge-carriers are obtained through hyperfine and dipolar spectroscopies. The results obtained through EPR spectroscopy are complemented by optical spectroscopy data to derive comprehensive energy level diagrams that enrich the current description of the electronic structure of carbon nitride and help explain the characteristic behaviour observed in working systems. This thesis extends to investigate the role played by carbon nitride as a support for single-atom catalysts, in particular for nickel ions. EPR spectroscopy and hyperfine methodologies are employed to characterise the chemical reducibility and the local coordination environment of isolated Ni(I) ions, which have been implicated as active species in catalysis. Overall, the experimental work reported in this thesis illustrates how a careful combination of EPR methods gives i) the opportunity to cover the temporal range from nano- to millisecond, the time scale compatible with chemical reactivity and; ii) exquisite details both on the local structure (sub-nanometre range) and the extended (nanometre-range) distribution of paramagnetic species even in a complex and polycrystalline semiconductor.