Comparison of electron injection dynamics from rhodamine B to In2O3, SnO2, and ZnO nanocrystalline thin films

Interfacial electron transfer (ET) dynamics from excited rhodamine B (RhB) to three semiconductor nanocrystalline thin films (In2O3, SnO2, and ZnO) was investigated to examine their dependence on semiconductors. The injected electrons in the semiconductors were directly measured by their transient absorption in the mid-IR region (similar to 5 mu m), and the evolution of the adsorbate in its ground, excited, and cation states was monitored by its transient absorption in the visible region. The formation of the IR absorption of injected electrons correlates well with the decay of the RhB excited state, allowing an unambiguous determination of electron injection rates from the adsorbate excited state to the semiconductor nanoparticles. The recombination processes were monitored by following the decay of injected electrons and RhB cations and the recovery of the RhB molecules in the ground state. The effects of dye aggregation and excited-state quenching on the injection dynamics were also examined and were shown to be negligible under low dye coverage and excitation power density. Injection times to In2O3 and SnO2 are similar (1.2 ps) and are similar to 6 times faster than to ZnO (7.0 ps). Possible reasons for the semiconductor dependent injection rates are discussed.