Ultrafast electron transfer dynamics from molecular adsorbates to semiconductor nanocrystalline thin films

Abstract

Interfacial electron transfer (ET) between semiconductor nanomaterials and molecular adsorbates is an important fundamental process that is relevant to applications of these materials. Using femtosecond midinfrared spectroscopy, we have simultaneously measured the dynamics of injected electrons and adsorbates by directly monitoring the mid-IR absorption of electrons in the semiconductor and the change in adsorbate vibrational spectrum, respectively. We report on a series of studies designed to understand how the interfacial ET dynamics depends on the properties of the adsorbates, semiconductors, and their interaction. In Ru(dcbpy)₂(SCN)₂ (dcbpy = 2,2'-bipyridine-4,4'-dicarboxylate) sensitized TiO₂ thin films, 400 nm excitation of the molecule promotes an electron to the metal-to-ligand charge transfer (MLCT) excited state, from which it is injected into TiO₂. The injection process was characterized by a fast component, with a time constant of < 100 fs, and a slower component that is sensitive to sample condition. Similar ultrafast electron injection times were measured in TiO₂ films sensitized by Ru(dcbpy)₂(X)₂ (X₂ = 2CN^- and dcbpy). Electron injection in these systems was found to compete with the vibrational energy relaxation process within the excited state of the molecules, leading to an injection yield that depends on the excited-state redox potential of the adsorbate. The injection rate from Ru(dcbpy)₂(SCN)₂ to different semiconductors was found to obey the trend TiO₂ > SnO₂ > ZnO, indicating a strong dependence on the nature of the semiconductor. To understand these observations, various factors, such as electronic coupling, density of states, and driving force, that control the interfacial ET rate were examined separately. The effect of electronic coupling on the ET rate was studied in TiO₂ sensitized by three adsorbates, Re(L_n)(CO)₃Cl [L_n is a modified dcbpy ligand with n (=0, 1, 3) CH₂ units between the bipyridine and carboxylate groups]. We found that the ET rate decreased with increasing number of CH₂ units (or decreasing electronic coupling strength). The effect of driving force was investigated in Ru(dcbpy)₂ X₂ (X₂ = 2SCN^-, 2CN^-, and dcbpy) sensitized SnO₂ thin films. In this case, we observed that the ET rate increased with the excited-state redox potential of the adsorbates, agreeing qualitatively with the theoretical prediction for a nonadiabatic interfacial ET process.