Adsorption Behavior of I3- and I- Ions at a Nanoporous NiO/Acetonitrile Interface Studied by X-ray Photoelectron Spectroscopy

The adsorption of I- and I-3(-) anions, i.e., the two species constituting the most common redox couple of dye-sensitized solar cells (DSCs), onto the surface of screen printed nanoporous NiO was studied by means of X-ray photoelectron spectroscopy (XPS). Nanoporous NiO films were deposited on transparent metallic fluorine-doped tin oxide (FTO) and polarized as working electrodes in a three electrode cell with differently concentrated I-/I-3(-) electrolytes to simulate the different conditions experienced by the NiO cathodes during the lifecycle of a p-type DSC (p-DSC) at those atomic sites not passivated by the dye. Bare NiO films were tested also as photocathodes of nonsensitized p-DSCs. The ex situ XPS analysis of I 4d ionization region of both reference and electrochemically treated NiO films showed that the presence of native and electrochemically generated Ni3+ and Ni4+ centers induces fast adsorption/desorption of r ions and catalyzes their oxidation to ions. The adsorption phenomena generated by I- and I-3(-) species on nanoporous NiO electrodes can also induce an effect of electrochemical passivation toward a fraction of charged Ni sites. Such an effect would render these sites inactive for the further realization of those photoelectrochemical processes at the basis of the operation of a p-DSC.