Photoelectrochemical Behavior of a Molecular Ru-Based Water-Oxidation Catalyst Bound to TiO₂-Protected Si Photoanodes

A hybrid photoanode based on a molecular water oxidation precatalyst was prepared from TiO₂-protected n- or p⁺-Si coated with multiwalled carbon nanotubes (CNT) and the ruthenium-based water oxidation precatalyst [Ru^IV(tda)(py-pyr)₂(O)], 1(O) (tda^2- is [2,2′:6′,2″-terpyridine]-6,6″-dicarboxylato and py-pir is 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide). The Ru complex was immobilized by π-π stacking onto CNTs that had been deposited by drop casting onto Si electrodes coated with 60 nm of amorphous TiO₂ and 20 nm of a layer of sputtered C. At pH = 7 with 3 Sun illumination, the n-Si/TiO₂/C/CNT/[1+1(O)] electrodes exhibited current densities of 1 mA cm^-2 at 1.07 V vs NHE. The current density was maintained for >200 min at a constant potential while intermittently collecting voltammograms that indicated that over half of the Ru was still in molecular form after O₂ evolution.