Electric Fields Detected on Dye-Sensitized TiO₂ Interfaces: Influence of Electrolyte Composition and Ruthenium Polypyridyl Anchoring Group Type

Abstract

Electric fields at the dye-sensitized interface of anatase TiO₂ nanocrystallites interconnected in a mesoporous thin film are reported using carboxylic acid-derivatized and phosphonic acid-derivatized ruthenium polypyridyl complexes. Systematic investigations with [Ru­(dtb)₂(dpb)]­(PF₆)₂, where dtb is 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine and dpb is 4,4′-bis-(PO₃H₂)-2,2′-bipyridine, were carried out in conjunction with its carboxylic acid structural analogue. Electric fields attributed to cation adsorption were measured from a bathochromic (red) shift of the sensitizer’s UV–visible absorption spectra upon replacement of neat acetonitrile solution with metal cation perchlorate acetonitrile electrolyte. Electric fields attributed to TiO₂ electrons were measured from the hypsochromic (blue) shift of the absorption spectra upon electrochemical reduction of the sensitized TiO₂ thin films. Electric fields, induced by either cation adsorption or electrochemically populated electrons, increase in magnitude following the same general cation-dependent trend (Na⁺ < Li⁺ < Ca²⁺ ≤ Mg²⁺ < Al³⁺), regardless of the sensitizer’s anchoring group type. For the first time, surface electric fields in the presence of trivalent cations (i.e., Al³⁺) were measured using [Ru­(dtb)₂(dpb)]­(PF₆)₂. The magnitude of electric fields detected by the carboxylic acid sensitizer was 3 times greater than that detected by the phosphonic acid structural analogue under the same experimental conditions. The influence of protons and water in the acetonitrile electrolyte was also quantified. The added water was found to decrease the electric field, whereas protons had a very similar influence as did metal cations