Photosensitive Electrified Water/Oil Interfaces Studied By Scanning Electrochemical Microscopy
Authors: Rastgar Shokoufeh , Wittstock Gunther 
Published: February 2020

The interface formed between two immiscible electrolytes solutions (ITIES) is a defect-free (down to the molecular level), self-healing, renewable and conductive substrate for the preferential and controlled formation of molecular assemblies and nano-objects.1 Similar to solid electrodes, modifier layer(s) can be formed and characterized by charge transfer processes across the interfaces. The potential drop across the interface can be controlled externally by using a potentiostat or chemically by dissolving a common salt in both phases. The polarization can be used to drive charged semiconductor nanoparticles to the interface. Polarization could also effectively drive the photo-induced charge transfer reaction by e- or h+ to control evolution of H2 with molecular photosensitizer2 and O2 semiconductor nanomaterials assembled at the ITIES.3 The jump in hydrophobicity at the soft interface supports the spatial separation of charge carriers on either side which partially restricts the recombination processes. Mechanistic studies of charge transfer in photoactivity of semiconductors at polarized ITIES are crucial for evaluation of such interfaces for fluidic systems with the ultimate goal of linking such interfaces to a z-scheme for water splitting.
We probe the photo-induced electron transfer reaction at polarized soft interface by integrating it into scanning electrochemical microscope (SECM) as a tool to analyse the kinetics of the interfacial reaction at the nanostructured BiVO4|butyronitrile interphase under chemically controllable Galvani potential difference tuned by variation of ClO4
- concentration ration between the two liquid phases.4 This reaction occurs between electron in conduction band of BiVO4 and [Co(bpy)3]3+ in the organic electrolyte. SECM feedback approach curves are recorded based on the regeneration of the oxidized form of initial [Co(bpy)3]2+ by using the microelectrode (ME) in organic phase as probe. The current is enhanced under illumination with visible light (Scheme A). Furthermore, using substrate generation-tip/collection mode of SECM, O2 is probed as a main product of the photogenerated hole transfer in the aqueous phase under variable interfacial polarization (Scheme B).
We also present a novel application of surface interrogation (SI) mode of SECM for in situ evaluate the surface of BiVO4 in photo-assisted water oxidation reaction (WOR). A micropipette (MP) filled with aqueous solution of dispersed nanostructured BiVO4, as a well-defined photosensitized substrate, is immersed into an immiscible organic solution containing perchlorate as common anion to generate the hydroxyl radicals (OH•) as intermediates of WOR under chemically controlled polarization. By the help of a ME as probe, surface concentrations of intermediates (Γ) and kinetics of their decay by bimolecular reactions (k) are analysed by recording the ME chronoamperograms transients enhanced by intermittent illumination of the substrate.
Figure 1: Schematic of the SECM invetigation of photocatalysts asembled at liquid-liquid interfaces. A) Extraction of electons by SECM mediator, B) Detection of oxygen evolution, c) Surface interrogation of adobred intermediates.

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Figure 1