Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films

Abstract

Recent advances in our mechanistic understanding of dye-sensitized electron transfer reactions occurring at metal oxide interfaces are described. These advances were enabled by the advent of mesoporous thin films, comprised of anatase TiO 2 nanocrystallites, that are amenable to spectroscopic and electrochemical characterization in unprecedented molecular-level detail. The metal-to-ligand charge transfer (MLCT) excited states of Ru polypyridyl compounds serve as the dye sensitizers. Excited-state injection often occurs on ultrafast time scales with yields that can be tuned from unity to near zero through modification of the sensitizer or the electrolyte composition. Transport of the injected electron and the oxidized sensitizer (hole hopping) are both operative in the composite mechanism for charge recombination between the injected electron and the oxidized sensitizer. Sensitizers that contain a pendant electron donor, as well as core/shell SnO 2 /TiO 2 nanostructures, often prolong the lifetime of the injected electron and provide fundamental insights into adiabatic and nonadiabatic electron transfer mechanisms. Regeneration of the oxidized sensitizer by iodide is enhanced through halogen bonding, orbital pathways, and ion pairing. A substantial ∼10 MV cm −1 electric field is created by electron injection into TiO 2 nanocrystallites that induces ion migration, reports on the sensitizer dipole orientation, and (in some cases) reorients or flips the sensitizer. Dye-sensitized conductive oxides also promote long-lived charge separation with bias dependent kinetics that provide insights into the reorganization energies associated with electron and proton-coupled electron transfer in the electric double layer.

Enhancement of Photocatalytic Activity for Hydrogen Production by Surface Modification of Pt‐TiO2 Nanoparticles with a Double Layer of Photosensitizers

“…Meyer et al suggested that, in addition to the iodide that injectsa ne lectron to Ru PS, another iodide anion should be close to stabilizet he iodine radical generated in the electron injection; they also demonstrated that hydrogen-bonding interactions with iodide by amide and amino functional groups is effective in improving electron injection. [44][45][46] In our Zr-RuCP 6 -Zr-RuP 6 @1 wt %Pt-TiO 2 nanoparticles, the surfaceZ r 4 + -phosphate groups could attract iodide anionsnear the surface, probablyb yelectrostatic interactions, leading to enhanced H 2 evolutionp hotocatalytic activity.…”

Tuning the Conduction Band for Interfacial Electron Transfer: Dye-Sensitized SnxTi1–xO2 Photoanodes for Water Splitting

“…In contrast, Sn25 and Sn50 exhibited the fastest 3 MLCT injection dynamics, owing to the greater driving force for injection and higher Ti DOS available as an acceptor. This is reasonable because a larger density of acceptor states is expected to increase the rate of electron transfer in the Marcus-Gerischer picture . While Sn75 also has a higher Ti DOS according to periodic DFT calculations, the slower relative 3 MLCT injection rates were more comparable to SnO 2 , which is in contrast to the high 1 MLCT injection amplitude observed in Sn75.…”

“…This is reasonable because a larger density of acceptor states is expected to increase the rate of electron transfer in the Marcus-Gerischer picture. 36 While Sn75 also has a higher Ti DOS according to periodic DFT calculations, the slower relative 3 MLCT injection rates were more comparable to SnO 2 , which is in contrast to the high 1 MLCT injection amplitude observed in Sn75. Although the reason for this dichotomy is not entirely clear, it is plausible that injection from the 1 MLCT state is largely mediated by discrete surface states, while injection from the 3 MLCT is mediated by the band structure of the metal oxide.…”

“…35 In metal oxides sensitized with a ruthenium polypyridyl complex such as RuP, IET is typically modeled as a two-state process where ultrafast injection occurs from the singlet metal-toligand charge transfer ( 1 MLCT) excited state, followed by a slower injection process from the triplet metal-to-ligand charge transfer ( 3 MLCT) excited state. 18,36 Injection from the 1 MLCT state is competitive with intersystem crossing (τ ISC < 30 fs), 37−39 and its detection is limited by the instrument response function (IRF) in the fs-TAS experiments discussed herein. Figure 1 shows a schematic representation of IET and photophysical processes in RuP depicting IET from both the 1 MLCT and 3 MLCT states, intersystem crossing (ISC), and vibrational relaxation (VR) in the 3 MLCT manifold.…”

“…This work investigates how IET dynamics can be modulated using Sn x Ti 1– x O 2 sensitized with a prototypical dye-sensitizer, ruthenium­(II) bis­(2,2′-bipyridine)­(4,4′-diphosphonato-2,2′-bipyridine) bromide or RuP , in aqueous electrolyte (0.1 M HClO 4 , pH 1) using a combination of femtosecond transient absorption spectroscopy (fs-TAS), linear absorption spectroscopy, and density functional theory (DFT) calculations . In metal oxides sensitized with a ruthenium polypyridyl complex such as RuP , IET is typically modeled as a two-state process where ultrafast injection occurs from the singlet metal-to-ligand charge transfer ( 1 MLCT) excited state, followed by a slower injection process from the triplet metal-to-ligand charge transfer ( 3 MLCT) excited state. , Injection from the 1 MLCT state is competitive with intersystem crossing ( τ ISC < 30 fs), − and its detection is limited by the instrument response function (IRF) in the fs-TAS experiments discussed herein. Figure shows a schematic representation of IET and photophysical processes in RuP depicting IET from both the 1 MLCT and 3 MLCT states, intersystem crossing (ISC), and vibrational relaxation (VR) in the 3 MLCT manifold.…”