Strategies for Stabilization of Electrodeposited Metal Particles in Electropolymerized Films for H₂O Oxidation and H⁺ Reduction

Metal particles were electrodeposited on a variety of conducting substrates, and their electrocatalytic activity toward H₂O oxidation to O₂ and H⁺ reduction to H₂ was evaluated. Co, Ni, Cu, Pd, Ag, and Pt were all electrodeposited on fluorine-doped tin oxide (FTO) electrodes. Particularly active were Pd and Pt for H⁺ reduction and Co and Ag for H₂O oxidation. When cycled reductively in 0.1 M HClO₄, FTO electrodes derivatized with Pt and Pd reached current densities for hydrogen evolution of 18.3 and 13.2 mA/cm², respectively, at −0.6 V vs normal hydrogen electrode (NHE). FTO electrodes with electrodeposited Co or Ag were cycled oxidatively in H₂O buffered to pH 7 with phosphate buffer. Current densities of 10.5 and 8.70 mA/cm², respectively, were reached at +1.8 V vs NHE with H₂O oxidation onsets at +1.3 and +1.4 V, respectively. The impacts on catalytic stability and performance of electrodeposited metals in/on an electrically conductive polymer support were also investigated. Films of poly-[Fe­(vbpy)₃]­(PF₆)₂ (vbpy is 4-methyl-4′-vinyl-2,2′-bipyridine) were generated on FTO by reductive electropolymerization. Significant improvements to the long-term stability of electrodeposited Ag and Pt particles were observed in the poly-[Fe­(vbpy)₃]­(PF₆)₂ support. Films of poly-[M­(vbpy)₃]­(PF₆)₂ with M = Co­(II) or Cu­(II) were also prepared and evaluated as electrocatalysts for H₂O oxidation. Films containing Co­(II) reached current densities of 6.0 mA/cm² at +1.8 V vs NHE in H₂O

Ultrafast Recombination Dynamics in Dye-Sensitized SnO₂/TiO₂ Core/Shell Films

Interfacial dynamics are investigated in SnO₂/TiO₂ core/shell films derivatized with a Ru­(II)-polypyridyl chromophore ([Ru^II(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺, <b>RuP</b>) using transient absorption methods. Electron injection from the chromophore into the TiO₂ shell occurs within a few picoseconds after photoexcitation. Loss of the oxidized dye through recombination occurs across time scales spanning 10 orders of magnitude. The majority (60%) of charge recombination events occur shortly after injection (τ = 220 ps), while a small fraction (≤20%) of the oxidized chromophores persists for milliseconds. The lifetime of long-lived charge-separated states (CSS) depends exponentially on shell thickness, suggesting that the injected electrons reside in the SnO₂ core and must tunnel through the TiO₂ shell to recombine with oxidized dyes. While the core/shell architecture extends the lifetime in a small fraction of the CSS, making water oxidation possible, the subnanosecond recombination process has profound implications for the overall efficiencies of dye-sensitized photoelectrosynthesis cells (DSPECs)

Light-Driven Water Splitting with a Molecular Electroassembly-Based Core/Shell Photoanode

An electrochemical procedure for preparing chromophore-catalyst assemblies on oxide electrode surfaces by reductive vinyl coupling is described. On core/shell SnO₂/TiO₂ nanoparticle oxide films, excitation of the assembly with 1 sun (100 mW cm^–2) illumination in 0.1 M H₂PO₄^–/HPO₄^2– at pH 7 with an applied bias of 0.4 V versus SCE leads to water splitting in a DSPEC with a Pt cathode. Over a 5 min photolysis period, the core/shell photoanode produced O₂ with a faradaic efficiency of 22%. Instability of the surface bound chromophore in its oxidized state in the phosphate buffer leads to a gradual decrease in photocurrent and to the relatively modest faradaic efficiencies

Stabilization of Ruthenium(II) Polypyridyl Chromophores on Nanoparticle Metal-Oxide Electrodes in Water by Hydrophobic PMMA Overlayers

We describe a poly­(methyl methacrylate) (PMMA) dip-coating procedure, which results in surface stabilization of phosphonate and carboxylate derivatives of Ru­(II)-polypyridyl complexes surface-bound to mesoporous nanoparticle TiO₂ and nanoITO films in aqueous solutions. As shown by contact angle and transmission electron microscopy (TEM) measurements, PMMA oligomers conformally coat the metal-oxide nanoparticles changing the mesoporous films from hydrophilic to hydrophobic. The thickness of the PMMA overlayer on TiO₂–Ru­(II) can be controlled by changing the wt % of PMMA in the dipcoating solution. There are insignificant perturbations in electrochemical or spectral properties at thicknesses of up to 2.1 nm with the Ru­(III/II) couple remaining electrochemically reversible and <i>E</i>_1/2 values and current densities nearly unaffected. Surface binding by PMMA overlayers results in stable surface binding even at pH 12 with up to a ∼100-fold enhancement in photo­stability. As shown by transient absorption measurements, the MLCT excited state(s) of phosphonate derivatized [Ru­(bpy)₂((4,4′-(OH)₂PO)₂bpy)]²⁺ undergo efficient injection and back electron transfer with pH independent kinetics characteristic of the local pH in the initial loading solution

Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications

A series of phosphonate-derivatized, high redox potential porphyrins with mesityl, pentafluorophenyl, and heptafluoropropyl meso-substituents were synthesized by acid-catalyzed condensation reactions. Ground and excited state redox potentials in the series were varied systematically with the electron-donating or electron-accepting nature of the meso-substitutents. The extent of excitation and injection by porphyrin singlet excited states surface-bound to SnO₂/TiO₂ core/shell metal oxide nanoparticle films varies with the excited state reduction potential, <i>E</i>°^′(P⁺/P*). With the mesityl-substituted porphyrin, high current density and sustained photocurrents are observed at pH 7 with the addition of the electron transfer donor hydroquinone

Electrochemical Instability of Phosphonate-Derivatized, Ruthenium(III) Polypyridyl Complexes on Metal Oxide Surfaces

The oxidative stability of the molecular components of dye-sensitized photoelectrosynthesis cells for solar water splitting remains to be explored systematically. We report here the results of an electrochemical study on the oxidative stability of ruthenium­(II) polypyridyl complexes surface-bound to fluorine-doped tin oxide electrodes in acidic solutions and, to a lesser extent, as a function of pH and solvent with electrochemical monitoring. Desorption occurs for the Ru­(II) forms of the surface-bound complexes with oxidation to Ru­(III) enhancing both desorption and decomposition. Based on the results of long-term potential hold experiments with cyclic voltammetry monitoring, electrochemical oxidation to Ru­(III) results in slow decomposition of the complex by 2,2′-bipyridine ligand loss and aquation and/or anation. A similar pattern of ligand loss was also observed for a known chromophore–catalyst assembly for both electrochemical water oxidation and photoelectrochemical water splitting. Our results are significant in identifying the importance of enhancing chromophore stability, or at least transient stability, in oxidized forms in order to achieve stable performance in aqueous environments in photoelectrochemical devices

Water Oxidation by an Electropolymerized Catalyst on Derivatized Mesoporous Metal Oxide Electrodes

A general electropolymerization/electro-oligomerization strategy is described for preparing spatially controlled, multicomponent films and surface assemblies having both light harvesting chromophores and water oxidation catalysts on metal oxide electrodes for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The chromophore/catalyst ratio is controlled by the number of reductive electrochemical cycles. Catalytic rate constants for water oxidation by the polymer films are similar to those for the phosphonated molecular catalyst on metal oxide electrodes, indicating that the physical properties of the catalysts are not significantly altered in the polymer films. Controlled potential electrolysis shows sustained water oxidation over multiple hours with no decrease in the catalytic current

Ultrafast, Light-Induced Electron Transfer in a Perylene Diimide Chromophore-Donor Assembly on TiO₂

Surface-bound, perylenediimide (PDI)-based molecular assemblies have been synthesized on nanocrystalline TiO₂ by reaction of a dianhydride with a surface-bound aniline and succinimide bonding. In a second step, the Fe­(II) polypyridyl complex [Fe^II(tpy-PhNH₂)₂]²⁺ was added to the outside of the film, also by succinimide bonding. Ultrafast transient absorption measurements in 0.1 M HClO₄ reveal that electron injection into TiO₂ by ¹PDI* does not occur, but rather leads to the ultrafast formation of the redox-separated pair PDI^•+,PDI^•–, which decays with complex kinetics (τ₁ = 0.8 ps, τ₂ = 15 ps, and τ₃ = 1500 ps). With the added Fe­(II) polypyridyl complex, rapid (<25 ps) oxidation of Fe­(II) by the PDI^•+,PDI^•– redox pair occurs to give Fe­(III),PDI^•– persisting for >400 μs in the film environment

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

Water Oxidation by an Electropolymerized Catalyst on Derivatized Mesoporous Metal Oxide Electrodes

A general electropolymerization/electro-oligomerization strategy is described for preparing spatially controlled, multicomponent films and surface assemblies having both light harvesting chromophores and water oxidation catalysts on metal oxide electrodes for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The chromophore/catalyst ratio is controlled by the number of reductive electrochemical cycles. Catalytic rate constants for water oxidation by the polymer films are similar to those for the phosphonated molecular catalyst on metal oxide electrodes, indicating that the physical properties of the catalysts are not significantly altered in the polymer films. Controlled potential electrolysis shows sustained water oxidation over multiple hours with no decrease in the catalytic current