Certain soft interfaces formed between aqueous and organic electrolyte solutions of low miscibility (e.g., trifluorotoluene) are electrochemically active in the sense that it is possible to precisely control the Galvani potential difference between the two adjacent liquids (i.e., to “polarise” or electrify the interface), and thus drive charge transfer reactions. In comparison to solid electrode-electrolyte interfaces, where typically only electron transfer may be probed, charge transfer reactions at soft interfaces may include simple ion transfer, assisted ion transfer, heterogeneous electron transfer, or photoinduced heterogeneous electron transfer (PET), see Figure 1 [1].

PET represents a novel method of potentially harnessing solar energy at electrified soft interfaces. By exploiting (i) energy trapped from visible light by a photoactive interfacial film (e.g., a porphyrin dye or a dye-sensitized TiO₂ nanoparticle) and (ii) the electrochemical driving force provided by electrification of the soft interface, a solar fuel such as H₂ or H₂O₂ may be evolved by PET across the soft interface using a weak electron donor in the oil phase, e.g., ferrocene. Crucially, all photo-electrochemical techniques applied at traditional solid electrode-electrolyte interfaces (voltammetry, impedance, photo-current transients, etc.) are applicable at polarised soft interfaces [2-3].

The photochemical properties of TiO₂ nanoparticles have been extensively studied in homogeneous phases for a variety of processes including water splitting and the photocatalytic destruction of pollutants. Mesoporous films based on TiO₂ nanoparticles have also been studied as photoactive materials. In this regard, dye sensitization of wide bandgap semiconductors has been extensively studied in the field of photovoltaics. Despite the overwhelming literature on the photochemistry of TiO2 colloids, little is yet known about the properties of these particles floating as “photo-electrodes” at molecular water-oil interfaces, with only a small number of published articles on the topic [4-7].

In this presentation the photo-electrochemistry of TiO₂ nanoparticles floating at electrified water-oil interfaces under blue LED illumination will be detailed. The experimental techniques employed (cyclic voltammetry, photocurrent transient data) will be able to provide valuable fundamental information on the rates of charge transfer between redox species in solution and the semiconductors floating at the interface. The studies will explore new possibilities for contactless photo-electrochemistry of nanoparticles in the presence of a static electric field, with potential applications in solar energy conversion to solar fuels and storage (ECS) and photocatalytic pollutant degradation.

References
[1] Samec, Z. Pure Appl. Chem., 76, (2004), 2147.
[2] Fermin, D.J. et al., Phys. Chem. Chem. Phys., 1, (1999), 1461.
[3] Eugster, N. et al., J. Phys. Chem. B., 106, (2002), 3428.
[4] Plana et al., Phys. Chem. Chem. Phys., 18, (2016), 12428.
[5] Plana et al., J. Electroanal. Chem., 780, (2016), 373.
[6] Jensen et al., J. Phys. Chem. B, 106, (2002), 10908.
[7] Fermin et al., ChemPhysChem, 4,(2003), 85.

Figure 1: Summary of possible charge transfer reactions at polarised soft interfaces. A solar fuel such as H₂ or H₂O₂ may be generated at the soft interface via photo-induced electron transfer from a lipophilic electron donor in the oil phase.