Deposition of an oxomanganese water oxidation catalyst on TiO₂ nanoparticles: computational modeling, assembly and characterization

Inexpensive water oxidation catalysts are needed to develop photocatalytic solar cells that mimic photosynthesis and produce fuel from sunlight and water. This paper reports the successful attachment of a dinuclear di-µ-oxo manganese water oxidation catalyst [H₂O(terpy)Mn^III(µ-O)₂ Mn^IV(terpy)H₂O](NO₃)₃ (1, terpy = 2,2′:6′2″-terpyridine) onto TiO₂ nanoparticles (NPs) via direct adsorption, or in situ synthesis. The resulting surface complexes are characterized by EPR and UV-visible spectroscopy, electrochemical measurements and computational modeling. We conclude that the mixed-valence (III,IV) state of 1 attaches to near-amorphous TiO₂ NPs by substituting one of its water ligands by the TiO₂ NP, as suggested by low-temperature (7 K) EPR data. In contrast, the analogous attachment onto well-crystallized TiO₂ NPs leads to dimerization of 1 forming Mn(IV) tetramers on the TiO₂ surface as suggested by EPR spectroscopy and electrochemical studies.