Photo-induced electron transfer at sensitised polarisable liquid|liquid interfaces

This thesis explores the nature of photo-induced charge transfer processes at sensitised polarisable interfaces between two immiscible electrolyte solutions (ITIES), specifically aqueous|α,α,α - trifluorotoluene (TFT) interfaces, towards solar energy conversion applications. The thesis is split into two primary sections that are differentiated by the method used to sensitise the liquid|liquid interface. In the first section, a film of titanium dioxide nanoparticles (NPs) sensitised with zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrin (ZnTPPc/TiO2 NPs) was assembled at the interface in the presence of electron donor species (a series of ferrocene-derivatives) in the organic phase. The ZnTPPc/TiO2 NPs were characterised by techniques such as scanning electron microscopy, zeta ()-potential measurements and dynamic light scattering (DLS). Properties of each ferrocene derivative, such as their standard ion transfer and redox potentials, and diffusion coefficients were determined using electrochemical and spectroscopic methods. Through extensive photoelectrochemical experiments, it was demonstrated that the biphasic system can facilitate the proton-coupled electron transfer (PCET) mediated oxygen reduction reaction (ORR) upon illumination by light with the ferrocene-derivative as a sacrificial electron donor. The optimal experimental conditions to maximise the photocurrent magnitude were determined, in terms of the applied interfacial Galvani potential difference (∆o w𝜙), aqueous pH, interfacial porphyrin concentration, photon flux, and nature of the donor or acceptor species, respectively. Key is that the applied ∆o w𝜙 is more positive than the potential of zero charge (PZC) to populate the interface with aqueous protons and enhance the rate of the ORR step. In the second section, the liquid|liquid interface was functionalised with a photoactive free-base 5,10,15,20-tetrakis(4-carboxyphenyl)- porphyrin (H2TPPc) film. Through voltammetric analysis, it was shown that the interfacial film undergoes ion exchange and intercalation/deintercalation processes with the organic electrolyte cations. pH studies revealed the reversible suppression of the electrochemical response of the H2TPPc film, shifting it outside of the polarisable potential window in a reversible manner. Finally, it was shown that these films generate photocurrent responses under illumination by visible light, either with electron donor or acceptor species in the organic phase. Electrons flow from the organic to aqueous phase for the donor and vice versa for the acceptor species, respectively. Photoelectrochemical studies probed the optimal conditions to maximise the photocurrent magnitude for interfacial H2TPPc films, as per the ZnTPPc/TiO2 NP experiments.