Seed-Mediated Growth of a High-Nuclearity Titanium-Oxide Cluster with Enhanced Photocatalytic Activities in CO₂/Epoxide Cycloaddition

A high-nuclearity titanium-oxide cluster (TOC), Ti₃₁O₅₂(OⁱPr)₄Bz₂₄ (denoted as Ti₃₁; Bz=PhCO₂), was synthesized via a seed-mediated synthesis method during the solvothermal reaction of the super-Keggin cluster Ti₁₇O₂₄(OⁱPr)₂₀ (denoted as Ti₁₇) and Ti(OⁱPr)₄. Single-crystal X-ray diffraction analyses indicate that Ti₁₇ acts as a template for the addition of two Ti₇O₆ rings to form Ti₃₁. The optical absorption edge of Ti₃₁ slightly shifts to 446 nm, relative to the 419 nm absorption edge of Ti₁₇. Ti₃₁ has much better visible-light response and charge-separation properties than Ti₁₇, as confirmed by electrochemical impedance, photoluminescence spectroscopy, and transient photocurrent response. Ti₃₁ also shows much better activity and stability than Ti₁₇ in the solar synthesis of cyclocarbonate through CO₂/epoxide cycloaddition. This study not only provides a high-nuclearity member of the TOC family, but also suggests a potential method to enhance the photocatalytic activity of the known TOCs for more efficient solar energy harvesting.