Driving Force Dependent, Photoinduced Electron Transfer at Degenerately Doped, Optically Transparent Semiconductor Nanoparticle Interfaces

Photoinduced, interfacial electron injection and back electron transfer between surface-bound [Ru^II(bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺ and degenerately doped In₂O₃:Sn nanoparticles, present in mesoporous thin films (nanoITO), have been studied as a function of applied external bias. Due to the metallic behavior of the nanoITO films, application of an external bias was used to vary the Fermi level in the oxide and, with it, the driving force for electron transfer (Δ<i>G</i>^o′). By controlling the external bias, Δ<i>G</i>^o′ was varied from 0 to −1.8 eV for electron injection and from −0.3 to −1.3 eV for back electron transfer. Analysis of the back electron-transfer data, obtained from transient absorption measurements, using Marcus–Gerischer theory gave an experimental estimate of λ = 0.56 eV for the reorganization energy of the surface-bound Ru^III/II couple in acetonitrile with 0.1 M LiClO₄ electrolyte

Application of Degenerately Doped Metal Oxides in the Study of Photoinduced Interfacial Electron Transfer

Degenerately doped In₂O₃:Sn semiconductor nanoparticles (<i>nano</i>ITO) have been used to study the photoinduced interfacial electron-transfer reactivity of surface-bound [Ru^II(bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺ (RuP²⁺) molecules as a function of driving force over a range of 1.8 eV. The metallic properties of the ITO nanoparticles, present within an interconnected mesoporous film, allowed for the driving force to be tuned by controlling their Fermi level with an external bias while their optical transparency allowed for transient absorption spectroscopy to be used to monitor electron-transfer kinetics. Photoinduced electron transfer from excited-state -RuP^2+* molecules to <i>nano</i>ITO was found to be dependent on applied bias and competitive with nonradiative energy transfer to <i>nano</i>ITO. Back electron transfer from <i>nano</i>ITO to oxidized -RuP³⁺ was also dependent on the applied bias but without complication from inter- or intraparticle electron diffusion in the oxide nanoparticles. Analysis of the electron injection kinetics as a function of driving force using Marcus–Gerischer theory resulted in an experimental estimate of the reorganization energy for the excited-state -RuP^3+/2+* redox couple of λ* = 0.83 eV and an electronic coupling matrix element, arising from electronic wave function overlap between the donor orbital in the molecule and the acceptor orbital(s) in the <i>nano</i>ITO electrode, of <i>H</i>_ab = 20–45 cm^–1. Similar analysis of the back electron-transfer kinetics yielded λ = 0.56 eV for the ground-state -RuP^3+/2+ redox couple and <i>H</i>_ab = 2–4 cm^–1. The use of these wide band gap, degenerately doped materials provides a unique experimental approach for investigating single-site electron transfer at the surface of oxide nanoparticles

Interfacial Dynamics within an Organic Chromophore-Based Water Oxidation Molecular Assembly

Photoinduced electron injection, intra-assembly electron transfer, and back-electron transfer are investigated in a single-site molecular assembly formed by covalently linking a phosphonated terthiophene (T₃) chromophore to a Ru­(terpyridine)­(bipyridine)­(L)²⁺ (L = MeCN or H₂O) water oxidation catalyst adsorbed onto a mesoporous metal-oxide (MO<i>_x</i>) film. Density functional theory calculations of the T₃-trpy-Ru-L assembly indicate that the molecular components are strongly coupled with enhanced low-energy absorptions owing to the presence of an intraligand charge transfer (ILCT) transition between the T₃ and trpy moieties. Ultrafast spectroscopy of the MO<i>_x</i>//T₃-trpy-Ru-L assemblies reveals that excitation of the surface-bound T₃ chromophore results in ps–ns electron injection into the metal-oxide conduction band. Electron injection is followed by rapid (<35 ps) intra-assembly electron transfer from the Ru^II catalyst to regenerate the T₃ chromophore with subsequent back-electron transfer on the microsecond time scale

Photoinduced Interfacial Electron Transfer within a Mesoporous Transparent Conducting Oxide Film

Interfacial electron transfer to and from conductive Sn-doped In₂O₃ (ITO) nanoparticles (NPs) in mesoporous thin films has been investigated by transient absorption measurements using surface-bound [Ru^II(bpy)₂(dcb)]²⁺ (bpy is 2,2′-bipyridyl and dcb is 4,4′-(COOH)₂-2,2′-bipyridyl). Metal-to-ligand charge transfer excitation in 0.1 M LiClO₄ MeCN results in efficient electron injection into the ITO NPs on the picosecond time scale followed by back electron transfer on the nanosecond time scale. Rates of back electron transfer are dependent on thermal annealing conditions with the rate constant increasing from 1.8 × 10⁸ s^–1 for oxidizing annealing conditions to 8.0 × 10⁸ s^–1 for reducing conditions, presumably due to an enhanced electron concentration in the latter

Light-Driven Water Oxidation Using Polyelectrolyte Layer-by-Layer Chromophore–Catalyst Assemblies

Layer-by-Layer (LbL) polyelectrolyte self-assembly occurs by the alternate exposure of a substrate to solutions of oppositely charged polyelectrolytes or polyions. Here, we report the application of LbL to construct chromophore–catalyst assemblies consisting of a cationic polystyrene-based Ru polychromophore (PS-Ru) and a [Ru­(tpy)­(2-pyridyl-<i>N</i>-methyl­benzimidazole) (OH₂)]²⁺ water oxidation catalyst (RuC), codeposited with poly­(acrylic acid) (PAA) as an inert polyanion. These assemblies are deposited onto planar indium tin oxide (ITO, Sn:In₂O₃) substrates for electrochemical characterization and onto mesoporous substrates consisting of a SnO₂/TiO₂ core/shell structure atop fluorine doped tin oxide (FTO) for application to light-driven water oxidation in a dye-sensitized photoelectrosynthesis cell. Cyclic voltammetry and ultraviolet–visible absorption spectroscopy reveal that multilayer deposition progressively increases the film thickness on ITO glass substrates. Under an applied bias, photocurrent measurements of the (PAA/PS-Ru)₅/(PAA/RuC)₅ LbL films formed on FTO//SnO₂/TiO₂ mesoporous core–shell electrodes demonstrate a clear anodic photocurrent response. Prolonged photoelectrolysis experiments, with the use of a dual working electrode collector–generator cell, reveal production of O₂ from the illuminated photoanode with a Faradaic efficiency of 22%. This is the first report to demonstrate the use of polyelectrolyte LbL to construct chromophore–catalyst assemblies for water oxidation

Disentangling the Physical Processes Responsible for the Kinetic Complexity in Interfacial Electron Transfer of Excited Ru(II) Polypyridyl Dyes on TiO₂

Interfacial electron transfer at titanium dioxide (TiO₂) is investigated for a series of surface bound ruthenium-polypyridyl dyes whose metal-to-ligand charge-transfer state (MLCT) energetics are tuned through chemical modification. The 12 complexes are of the form Ru^II(bpy-A)­(L)₂²⁺, where bpy-A is a bipyridine ligand functionalized with phosphonate groups for surface attachment to TiO₂. Functionalization of ancillary bipyridine ligands (L) enables the potential of the excited state Ru^III/* couple, <i>E</i>^<i>+</i>/*, in 0.1 M perchloric acid (HClO₄(aq)) to be tuned from −0.69 to −1.03 V vs NHE. Each dye is excited by a 200 fs pulse of light in the visible region of the spectrum and probed with a time-delayed supercontiuum pulse (350–800 nm). Decay of the MLCT excited-state absorption at 376 nm is observed without loss of the ground-state bleach, which is a clear signature of electron injection and formation of the oxidized dye. The dye-dependent decays are biphasic with time constants in the 3–30 and 30–500 ps range. The slower injection rate constant for each dye is exponentially distributed relative to <i>E</i>^<i>+</i>/*. The correlation between the exponentially diminishing density of TiO₂ sub-band acceptor levels and injection rate is well described using Marcus–Gerischer theory, with the slower decay components being assigned to injection from the thermally equilibrated state and the faster components corresponding to injection from higher energy states within the ³MLCT manifold. These results and detailed analyses incorporating molecular photophysics and semiconductor density of states measurements indicate that the multiexponential behavior that is often observed in interfacial injection studies is not due to sample heterogeneity. Rather, this work shows that the kinetic heterogeneity results from competition between excited-state relaxation and injection as the photoexcited dye relaxes through the ³MLCT manifold to the thermally equilibrated state, underscoring the potential for a simple kinetic model to reproduce the complex kinetic behavior often observed at the interface of mesoporous metal oxide materials