Activation of Inorganic Ligands by Interfacial Charge Transfer from a Complexation-Generated Valence Band at the Surface of a Metal Oxide Nanocrystal

Incorporation of colloidal metal oxides into multicomponent solid-state materials gives rise to heterojunctions that, in photocatalysis, can obviate the need for sacrificial reagents. In principle, however, metal-oxide nanocrystals (NCs) could be functionalized by ligands that, while stabilizing the particles in solution, interact with the metal-oxide surface to provide self-contained analogs of multicomponent solid-state photocatalysts. We now report that simulated solar-light irradiation of 3.2 nm rutile-SnO₂ NCs complexed by monolayers of approximately 50 hexaniobate anions, [Nb₆O₁₉]^8– (1) leads to the selective reduction of CO₂ to CO by water alone. Mott–Schottky analysis of 1 gives band energies typical of rutile SnO_2, which, in relation to the HOMO and LUMO energies of hexaniobate, rule out Z-scheme or type-II heterojunction mechanisms. Ultraviolet photoelectron spectroscopy (UPS), however, revealed a new valence band (VB) nearly 2 eV higher in energy than the canonical VB energy of rutile SnO₂. Topotactic mapping of Nb–O→Sn linkages from hexaniobate ligands to nearly all surface-accessible Sn atoms supports the generation of a network of electron-rich Sn-linked O atoms. Excitation of this surface-localized VB results in charge transfer (CT) conversion of the inorganic ligands into catalytic active sites. More generally, the findings introduce a new approach to interfacial engineering of metal-oxide NCs.