Site-Selective Excitation of Defects Promotes Ultrafast Hot-Electron Transfer at the Semiconductor Interface

Defect engineering is an effective strategy to manipulate light absorption and charge trapping in photocatalytic materials and improve their solar energy conversion efficiency. However, little is known about the mechanism of photoinduced charge transfer from these defects to surface-adsorbed species, a key step linking light absorption and surface chemical reactions. Thus far, hot-charge transfer from semiconductor photocatalysts to adsorbed molecules has not yet been directly detected. Combining time-resolved photoelectron spectroscopy and first-principles calculations, we demonstrate the ultrafast hot-electron transfer (∼15 fs) from rutile TiO 2 to acetone through the site-selective excitation (d-d transition) of Ti 3+ defects where acetone is adsorbed. The high-lying 3d excited states of the Ti 3+ ions (2.5–2.8 eV above the Fermi level) and their hybridization with adsorbate orbitals provide suitable interfacial level alignment and strong electronic coupling, thus promoting hot-electron transfer. Such a defect-mediated process may be a general phenomenon in adsorbate/semiconductor systems for light harvesting.