The effect of hydrophobicity of ruthenium complex dyes on photocatalytic water electrolysis

Bis(2,2′-bipyridine)bis(isothiocyanato)ruthenium complexes are organo-metallic dyes having broad absorption properties in the visible region that are widely used in dye-sensitized opto-electronic devices. Here, we report the photocurrent measurement and stability studies of three Ru-complexes, cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N3), its hydrophobic analogues cis-bis(isothiocyanato)(2,2’-bipyridyl-4,4’-dicarboxylato)(4,4’-di-nonyl-2’-bipyridyl)ruthenium(II) (Z-907) and hydrophilic analogues (di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II), (N719). Z907 shows higher stability and photoelectric conversion efficiency after 2 h analysis than its hydrophilic counter-molecules, N3 and N719. The Z907-TiO₂ system exhibited a 1.09 mA/cm² photocurrent density after 2 h of analysis, with 78% of its performance retained even after an extended 48 h of analysis. The activity of N719-TiO₂ and N3-TiO₂ sharply decreased after 2 h of analysis, and significant desorption of the dye present on titanium oxide was also observed. This study suggests that hydrophobicity at the electrode-dye interface is a major factor supporting the stability of the dye-sensitized photoanode.