Excited-state dynamics of alizarin-sensitized TiO2 nanoparticles from resonance Raman spectroscopy

Abstract

Resonance Raman spectra of alizarin-sensitized TiO2 nanoparticles have been obtained at excitation wavelengths throughout the 488-nm charge transfer absorption band. The resonance Raman spectrum of the alizarin-sensitized TiO2 nanoparticle is significantly different from the spectrum of free alizarin, consistent with a chemisorption-type interaction. This interaction is probably chelation of surface titanium ions by the hydroxy groups of alizarin, supported by the observed enhancement of bridging C–O modes at 1326 cm−1. In contrast to resonance Raman intensity analysis of homogeneous electron transfer where vibrations of both the donor and acceptor are observed, self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using the time-dependent wave packet propagation formalism show mode-specific reorganization along alizarin vibrations exclusively; no resonance-enhanced vibrations attributable to the TiO2 moiety are observed. Therefore, the total resonance Raman-derived reorganization energy is only 0.04 eV, significantly smaller than the observed outer-sphere reorganization energy of 0.2 eV for this system and inner-sphere reorganization energies measured for other molecular systems. The discrepancy is ascribed to a significant environmental component to the outer-sphere reorganization energy arising from rapid dephasing of surface TiO2 units involved in adsorption by strongly coupled interior bath vibrations.

Adhesion of Mussel Foot Protein-3 to TiO2 Surfaces: the Effect of pH

“…6, 11 Similarly, a strong feature at 1331 cm −1 , attributed to a C-O stretching mode, indicates the vital role of the catecholate functionality in providing bridging adsorption. 18 …”

A surface science perspective on TiO2 photocatalysis

“…However, it is not altogether straightforward to determine if such complexes exist and if their excitations (as opposed to excitation of an inherent local transition in the dye) contribute to electron injection. Taking the alizarin-TiO 2 system as an example, both Ramakrishna et al [675], and Shoute and Loppnow [657] proposed that a charge transfer complex between alizarin and the TiO 2 surface formed on adsorption and excitation of this complex led to the observed electron injection. The latter authors estimated that the TiO 2 nanoparticle surface used in their studies experienced the majority of the observed reorganization energy (∼0.32 eV total) that resulted from this process based on a fluorescence Stokes shift seen after excitation.…”

Initial Excited-State Structural Dynamics of 9-Methyladenine from UV Resonance Raman Spectroscopy

“…46 The general implementation of these equations for absorption and resonance Raman spectroscopy has been described in detail previously. 47,48 We have used the strongly overdamped and high temperature (pΛ , kT) limits in our analysis. The Journal of Physical Chemistry B ARTICLE We used initial guesses for the displacements along each normal coordinate (Δ) that were based on the assumption that the average relative resonance Raman intensities are proportional to Δ 2 , with the intensity of the 1343 cm À1 mode set arbitrarily to 1.…”