Surface Grafting of Ru(II) Diazonium-Based Sensitizers on Metal Oxides Enhances Alkaline Stability for Solar Energy Conversion

The electrografting of [Ru(ttt)(tpy-C6H4-N2+)]3+, where "ttt" is 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine, was investigated on several wide band gap metal oxide surfaces (TiO2, SnO2, ZrO2, ZnO, In2O3:Sn) and compared to structurally analogous sensitizers that differed only by the anchoring group, i.e., -PO3H2 and -COOH. An optimized procedure for diazonium electrografting to semiconductor metal oxides is presented that allowed surface coverages that ranged between 4.7 × 10-8 and 10.6 × 10-8 mol cm-2 depending on the nature of the metal oxide. FTIR analysis showed the disappearance of the diazonium stretch at 2266 cm-1 after electrografting. XPS analysis revealed a characteristic peak of Ru 3d at 285 eV as well as a peak at 531.6 eV that was attributed to O 1s in Ti-O-C bonds. Photocurrents were measured to assess electron injection efficiency of these modified surfaces. The electrografted sensitizers exhibited excellent stability across a range of pHs spanning from 1 to 14, where classical binding groups such as carboxylic and phosphonic derivatives were hydrolyzed.