Polyoxometalate complexes of anatase-TiO2 cores in water

Ira Weinstock, Chemistry, Ben-Gurion University of the Negev, Beer Sheva, Israel
Manoj Raula, Chemistry, Ben-Gurion University of the Negev, Beer Sheva, Israel
Gal Gan OR, Chemistry, Ben-Gurion University of the Negev, Beer Sheva, Israel
Marina Saganovich, Chemistry, Ben-Gurion University of the Negev, Beer Sheva, Israel

A general feature of oxophilic Ti(IV) centers in molecular complexes is their tendency to form high-nuclearity oxide-bridged cores. Reported examples include entirely inorganic metal-oxide cluster-anion (polyoxometalate, or POM) complexes, such as [(α-1,2,3-P₂W₁₅Ti₃O₆₂)₄{μ₃-Ti(OH)₃}₄Cl]^45–, with a core of 16 Ti(IV) atoms, while up to 34 Ti atoms are found in molecular titanium-oxide clusters capped by alkoxide and other organic anions. Notably, such ligand-capped titanium-oxide clusters are fundamentally important molecular models for TiO₂ semiconductors. Building on our recent investigations of polyoxometalate ligand shells on silver and gold nanoparticles, we now report a conceptually new role for POM cluster-anions as covalently coordinated redox-active ligands in polyanionic “complexes” of TiO₂-semiconductor nanocrystals themselves. In these assemblies, numerous Ti(IV)-substituted POM capping ligands, “[α-PW₁₁O₃₉Ti]–O^–”, are covalently attached to ca. 6-nm anatase-TiO₂ cores, resulting in isolable, water-soluble nanostructures uniquely positioned between molecular macroanions and traditional—electrostatically stabilized—colloidal metal oxides. Using cryogenic sample preparation for transmission electron microscopy (cryo-TEM), in combination with the large electron densities of the capping-ligand W (Z =74) atoms, we provide—to our knowledge—the first direct images ever obtained of protecting ligands on colloidal metal-oxide nanocrystals. Moreover, the covalently attached POMs serve as tunable electron-acceptors, whose reduction potentials can be adjusted to span the flatband potentials of reduced TiO₂ itself, in principle fascilitating charge separation under photocatalytic conditions by efficiently “trapping” photoexcited electrons. Hence, just as traditional ligands control catalytically active metal centers in molecular complexes, the tunable redox chemistry of the covalently attached POM capping ligands could provide new options for rationally controlling metal-oxide-semiconductor mediated electron-transfer procecesses.