| Abstract
| - The dependence of the interfacial electron transfer in alizarin-sensitized TiO2 nanoparticles on the sample pHhas been examined via transient absorbance spectroscopy in the visible spectral region (443−763 nm).Excitation of the alizarin/TiO2 system with visible pump pulses (λexc = 500 nm) leads to a very fast electroninjection (τinj< 100 fs) over a wide pH range. Back electron transfer shows complicated multiphasic kineticsand strongly depends on the acidity of the solution. The strong dependence of back-electron-transfer dynamicson the ambient pH value is explained by a Nernstian-type change in the semiconductor band energy. Indeed,a variation of pH values over 7 units leads to a ∼0.42 eV change of the conduction band edge position (i.e.,the nominal free energy of the electron in the electrode). Assuming a pH-independent redox potential of thedye, this change was sufficient to push the system to a condition where direct photoinitiated electron injectionto intraband gap surface states could be investigated. The existence of an electron-transfer pathway via surfacetrap states is supported by the similarity of the observed back-electron-transfer kinetics of alizarin/TiO2 at pH9 and alizarin/ZrO2 reported in earlier work (J. Phys. Chem. B2000, 104, 8995), where the conduction bandedge is approximately 1 eV above the excited state of the dye. The influence of surface trap states on interfacialelectron transfer has been studied, and a detailed analysis of their population, depopulation, and relaxationkinetics is performed. Therefore, alizarin adsorbed on the surface of TiO2 nanoparticles is an ideally suitedsystem, where pH-dependent investigations allow a detailed study of the electron dynamics in trap states ofTiO2 nanoparticles.
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