Effect of trap states on interfacial electron transfer between molecular absorbates and semiconductor nanoparticles

TiO2 and ZrO2 nanocrystals synthesized by the thermal decomposition of molecular precursors in hot surfactant solvents show much improved crystallinity compared with nanoparticles prepared via hydrolysis of metal alkoxides in water (Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613). In this work, we examine the effect of crystallinity and therefore the trap-state density on the dynamics of interfacial electron transfer between adsorbates and nanoparticles. Subpicosecond mid-IR and visible transient absorption spectroscopic techniques were used to study electron-injection and recombination dynamics of TiO2 and ZrO2 nanoparticles and nanocrystalline thin films sensitized with catechol and coumarin-343. Comparison of dynamics in materials with different crystallinity demonstrates that electron recombination dynamics are strongly dependent on the preparation method, and the effect on injection dynamics is significant when the injection energy level is near the conduction band edge.