Comparison of Electron-Transfer Dynamics from Coumarin 343 to TiO2, SnO2, and ZnO Nanocrystalline Thin Films: Role of Interface-Bound Charge-Separated Pairs

摘要

The role of the interface-bound charge-separated pair (IBCSP) in electron transfer at molecule/inorganic semiconductor interface remains poorly understood despite the importance of its counterpart in the solution-phase charge-separation process. To probe their role, we have compared the dynamics of electron transfer from C343 to TiO2, SnO2, and ZnO nanocrystalline thin films. The decay of the C343 excited state and the formation of oxidized C343 are measured by transient visible absorption to follow the rate of charge separation across the interface to form the IBCSPs. The dissociation of IBCSPs to form free electrons in semiconductor is probed by the free-carrier absorption in the mid-IR. For C343 on TiO2 and SnO2, the rate of the overall electron-transfer process is determined by the rate of the initial charge separation across the interface to form the IBCSP, which dissociates with negligible lifetime. The charge-separation rates are instrument-response-function limited (<150 fs) and ~1 ps for C343 on TiO2 and SnO2, respectively. The faster electron-transfer rate to TiO2 than to SnO2 is attributed to the higher density of conduction-band states in the former. For C343 on ZnO, the initial charge-separation rate across the interface was shown to be instrument-response limited (<150 fs), but the IBCSP dissociates in ~12 ps, which determines the overall electron transfer rate. We discuss possible reasons for the different lifetimes of IBCSP intermediates in interfacial electron transfer from C343 to TiO2, SnO2, and ZnO.