Efficient solar-driven catalytic water oxidation is one of the main challenges in solar-to-fuel conversion. In this
proceeding, we investigate three approaches for constructing electron acceptor -sensitizer - catalysts systems for
photocatalytic water oxidation and our current understanding of the relevant fundamental processes involved. We
demonstrated that an all-inorganic molecular water oxidation catalyst (WOC), [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10-
(or Ru4POM), catalyzed a homogenous O2 evolution system with 27% quantum efficiency in homogeneous solution in
the presence of sensitizer (Ru(bpy)3) and sacrificial electron donor.1 This suggests the feasibility of a heterogeneous
photoelectrochemical system in which the photoanode integrates all three components: electron acceptor,
photosensitizer, and WOC. We prepare a photocatalytic electrode based on Ru4POM and a dye-sensitized nanoporous
TiO2 film for efficient light-harvesting and charge separation. Ultrafast spectroscopic studies of this triadic
nanocomposite indicate efficient charge separation from the excited sensitizer to TiO2 and efficient regeneration of the
ground state of the dye. The latter can be attributed to Ru4POM oxidation by the photogenerated dye cation and has a
yield of > 80% within 1 ns.
In addition to the size dependent optical and electronic properties of semiconductor quantum dots (QDs),
quantum confinement also affects the charge separation and recombination dynamics in QD - charge
acceptor complexes. It leads to enhanced amplitudes of electron and hole wave functions at the surface,
enabling ultrafast interfacial charge transfer, an important property for the application of QDs in
photovoltaic and photocatalytic devices. In this proceeding, we show that both charge separation and
recombination are ultrafast in strongly quantum confined PbS QDs adsorbed with electron acceptors. Using
CdSe/ZnS type I and CdTe/CdSe type II core/shell QDs as model systems, we show that the spatial
distributions of electron and wave functions can be optimized to simultaneously achieve ultrafast charge
separation and retard charge recombination.
The influence of surface hydration on the time-scales and mechanisms of interfacial electron transfer from rhodamine B
into SnO2 is investigated. We combine molecular dynamics simulations and quantum dynamics propagation of transient
electronic excitations to analyze the regulatory role of water molecules affecting the adsorbate-semiconductor
interactions and the underlying electronic couplings that determine the electron injection times. The reported results are
essential to advance our understanding of interfacial electron transfer dynamics in dye sensitized semiconductor surfaces
at the molecule level, including fundamental interactions that affect the efficiency of interfacial electronic processes in
dye-sensitized solar cells as well as in a wide range of other technological applications.
Single rhodamine B (RhB) molecule-Sb:SnO2(ATO) nanoparticle junctions were studied using two-photon excitation single molecule fluorescence spectroscopy. For each molecule-nanoparticle junction, fluorescence decay of RhB was found to be single exponential, indicating a static heterogeneous distribution of junction properties. The measured fluorescence lifetime is shorter than RhB on inert substrates but longer than that expected on ATO based on ensemble averaged electron transfer rates. Possible origins for the shorten single molecule lifetimes and distribution are discussed.
Electron transfer (ET) dynamics between molecular adsorbates and semiconductor nanoparticles has been a subject of intense recent interest because of relevance to many applications of nanomaterials, such as dye-sensitized solar cells, molecular electronics and sensors. However, it is still unclear how the charge transfer rate depends on the properties of molecules and semiconductors. In this paper we examine electron injection from Ru and Re polypyridyl
complexes to metal oxide (TiO2, SnO2 and ZnO) nanocrystalline thin films. Adsorbates with different energetics and electronic coupling are compared to identify molecular properties that influence ET dynamics. Different semiconductor nanomaterials are compared to understand the dependence on conduction band composition and energetics. ET dynamics were found to be biphasic consisting of ultrafast (<100fs) and slower components, with varying partitioning between them and rates of slow components. These kinetics can be well described by a two-state injection model, which includes injection from both unthermalized and thermalized excited states and competition between electron injection and intramolecular relaxation from the unthermalized state. The dependence of ET rates on various molecular and semiconductor properties is also discussed.
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