Investigation of Factors That Affect Excited-State Lifetime Distribution of Dye-Sensitized Nanoparticle Films

We investigate the influence of potential determining ions and applied electric potentials on the excited-state lifetime distribution of sensitized TiO₂ nanoparticle films by using time-correlated single photon counting to measure the time-dependent photoluminescence decay. The data are consistent with quenching by excited-state electron injection into localized semiconductor acceptor states that are distributed in energy. We show that the characteristic lifetime and the amount of dispersion in the lifetime distribution exhibit a strong correlation that is the same for all of the chemical additives and for the applied bias conditions. The universal nature of this correlation under conditions that affect the distribution of available acceptor states differently may be due to the exponential form of the TiO₂ sub-bandgap density of states

Power-Law Kinetics in the Photoluminescence of Dye-Sensitized Nanoparticle Films: Implications for Electron Injection and Charge Transport

Dye-sensitized solar cells have provided a model to inexpensively harness solar energy, but the underlying physics that limit their efficiency are still not well understood. We probe electron injection in sensitized nanocrystalline TiO₂ films using time-correlated single photon counting (TCSPC) to measure time-dependent chromophore photoluminescence quenching. The time-dependent emission exhibits kinetics that become faster and more dispersive with increasing ionic concentrations in both water and acetonitrile; we quantify these trends by fitting the data using several kinetic models. Even more notably, we show that the residual emission under conditions that favor efficient electron injection exhibits a power-law decay in time. We attribute this highly dispersive kinetic behavior to electron injection from the dye into localized acceptor states of the TiO₂ nanoparticle film, which exhibits a distribution of injection rate constants that depend on the energetic distribution of sub-band-gap trap states

Stabilization of a Ruthenium(II) Polypyridyl Dye on Nanocrystalline TiO₂ by an Electropolymerized Overlayer

The long-term performance of dye-sensitized solar and photoelectrochemical cells is strongly dependent on the stability of surface-bound chromophores and chromophore–catalyst assemblies at metal oxide interfaces. We report here electropolymerization as a strategy for increasing interfacial stability and as a simple synthetic route for preparing spatially controlled, multicomponent films at an interface. We demonstrate that [Fe­(v-tpy)₂]²⁺ (v-tpy = 4′-vinyl-2,2′:6′,2″-terpyridine) can be reductively electropolymerized on nanocrystalline TiO₂ functionalized with a phosphonate-derivatized Ru­(II) polypyridyl chromophore. The outer:inner Fe:Ru ratio can be controlled by the number of reductive electrochemical scan cycles as shown by UV–visible absorption and energy dispersive X-ray spectroscopy measurements. Overlayer electropolymerization results in up to 30-fold enhancements in photostability compared to the surface-bound dye alone. Transient absorbance measurements have been used to demonstrate that photoexcitation and electron injection by the MLCT excited state(s) of the surface-bound Ru^II complex is followed by directional, outside-to-inside, Fe^II → Ru^III electron transfer. This strategy is appealing in opening a new approach for synthesizing surface-stabilized chromophore–catalyst assemblies on nanocrystalline metal oxide films

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

Synthesis, Electrochemistry, and Excited-State Properties of Three Ru(II) Quaterpyridine Complexes

The complexes [Ru­(qpy)­LL′]²⁺ (qpy = 2,2′:6′,2″:6″,2‴-quaterpyridine), with <b>1</b>: L = acetonitrile, L′= chloride; <b>2</b>: L = L′= acetonitrile; and <b>3</b>: L = L′= vinylpyridine, have been prepared from [Ru­(qpy) (Cl)₂]. Their absorption spectra in CH₃CN exhibit broad metal-to-ligand charge transfer (MLCT) absorptions arising from overlapping ¹A₁ → ¹MLCT transitions. Photoluminescence is not observed at room temperature, but all three are weakly emissive in 4:1 ethanol/methanol glasses at 77 K with broad, featureless emissions observed between 600 and 1000 nm consistent with MLCT phosphorescence. Cyclic voltammograms in CH₃CN reveal the expected Ru^III/II redox couples. In 0.1 M trifluoroacetic acid (TFA), <b>1</b> and <b>2</b> undergo aquation to give [Ru^II(qpy)­(OH₂)₂]²⁺, as evidenced by the appearance of waves for the couples [Ru^III(qpy)­(OH₂)₂]³⁺/[Ru^II(qpy)­(OH₂)₂]²⁺, [Ru^IV(qpy)­(O)­(OH₂)]²⁺/[Ru^III(qpy)­(OH₂)₂]³⁺, and [Ru^VI(qpy)­(O)₂]²⁺/[Ru^IV(qpy)­(O)­(OH₂)]²⁺ in cyclic voltammograms

A Dye-Sensitized Photoelectrochemical Tandem Cell for Light Driven Hydrogen Production from Water

Tandem junction photoelectrochemical water-splitting devices, whereby two light absorbing electrodes targeting separate portions of the solar spectrum generate the voltage required to convert water to oxygen and hydrogen, enable much higher possible efficiencies than single absorber systems. We report here on the development of a tandem system consisting of a dye-sensitized photoelectrochemical cell (DSPEC) wired in series with a dye-sensitized solar cell (DSC). The DSPEC photoanode incorporates a tris­(bipyridine)­ruthenium­(II)-type chromophore and molecular ruthenium based water oxidation catalyst. The DSPEC was tested with two more-red absorbing DSC variations, one utilizing N719 dye with an I₃^–/I^– redox mediator solution and the other D35 dye with a tris­(bipyridine)­cobalt ([Co­(bpy)₃]^3+/2+) based mediator. The tandem configuration consisting of the DSPEC and D35/[Co­(bpy)₃]^3+/2+ based DSC gave the best overall performance and demonstrated the production of H₂ from H₂O with the only energy input from simulated solar illumination

Synthesis of Phosphonic Acid Derivatized Bipyridine Ligands and Their Ruthenium Complexes

Water-stable, surface-bound chromophores, catalysts, and assemblies are an essential element in dye-sensitized photoelectrosynthesis cells for the generation of solar fuels by water splitting and CO₂ reduction to CO, other oxygenates, or hydrocarbons. Phosphonic acid derivatives provide a basis for stable chemical binding on metal oxide surfaces. We report here the efficient synthesis of 4,4′-bis­(diethylphosphonomethyl)-2,2′-bipyridine and 4,4′-bis­(diethylphosphonate)-2,2′-bipyridine, as well as the mono-, bis-, and tris-substituted ruthenium complexes, [Ru­(bpy)₂(Pbpy)]²⁺, [Ru­(bpy)­(Pbpy)₂]²⁺, [Ru­(Pbpy)₃]²⁺, [Ru­(bpy)₂(CPbpy)]²⁺, [Ru­(bpy)­(CPbpy)₂]²⁺, and [Ru­(CPbpy)₃]²⁺ [bpy = 2,2′-bipyridine; Pbpy = 4,4′-bis­(phosphonic acid)-2,2′-bipyridine; CPbpy = 4,4′-bis­(methylphosphonic acid)-2,2′-bipyridine]

Redox Mediator Effect on Water Oxidation in a Ruthenium-Based Chromophore–Catalyst Assembly

The synthesis, characterization, and redox properties are described for a new ruthenium-based chromophore–catalyst assembly, [(bpy)₂Ru­(4-Mebpy-4′-bimpy)­Ru­(tpy)­(OH₂)]⁴⁺ (<b>1</b>, [Ru_a^II-Ru_b^II-OH₂]⁴⁺; bpy = 2,2′-bipyridine; 4-Mebpy-4′-bimpy = 4-(methylbipyridin-4′-yl)-<i>N</i>-benzimid-<i>N</i>′-pyridine; tpy = 2,2′:6′,2″-terpyridine), as its chloride salt. The assembly incorporates both a visible light absorber and a catalyst for water oxidation. With added ceric ammonium nitrate (Ce^IV, or CAN), both <b>1</b> and <b>2</b>, [Ru­(tpy)­(Mebim-py)­(OH₂)]²⁺ (Mebim-py = 2-pyridyl-<i>N</i>-methylbenzimidazole), catalyze water oxidation. Time-dependent UV/vis spectral monitoring following addition of 30 equiv of Ce^IV reveals that the rate of Ce^IV consumption is first order both in Ce^IV and in an oxidized form of the assembly. The rate-limiting step appears to arise from slow oxidation of this intermediate followed by rapid release of O₂. This is similar to isolated catalyst <b>2</b>, with redox potentials comparable to the [-Ru_b-OH₂]²⁺ site in <b>1</b>, but <b>1</b> is more reactive than <b>2</b> by a factor of 8 due to a redox mediator effect

Synthesis and Electrocatalytic Water Oxidation by Electrode-Bound Helical Peptide Chromophore–Catalyst Assemblies

Artificial photosynthesis based on dye-sensitized photoelectrosynthesis cells requires the assembly of a chromophore and catalyst in close proximity on the surface of a transparent, high band gap oxide semiconductor for integrated light absorption and catalysis. While there are a number of approaches to assemble mixtures of chromophores and catalysts on a surface for use in artificial photosynthesis based on dye-sensitized photoelectrosynthesis cells, the synthesis of discrete surface-bound chromophore–catalyst conjugates is a challenging task with few examples to date. Herein, a versatile synthetic approach and electrochemical characterization of a series of oligoproline-based light-harvesting chromophore–water-oxidation catalyst assemblies is described. This approach combines solid-phase peptide synthesis for systematic variation of the backbone, copper­(I)-catalyzed azide–alkyne cycloaddition (CuAAC) as an orthogonal approach to install the chromophore, and assembly of the water-oxidation catalyst in the final step. Importantly, the catalyst was found to be incompatible with the conditions both for amide bond formation and for the CuAAC reaction. The modular nature of the synthesis with late-stage assembly of the catalyst allows for systematic variation in the spatial arrangement of light-harvesting chromophore and water-oxidation catalyst and the role of intrastrand distance on chromophore–catalyst assembly properties. Controlled potential electrolysis experiments verified that the surface-bound assemblies function as water-oxidation electrocatalysts, and electrochemical kinetics data demonstrate that the assemblies exhibit greater than 10-fold rate enhancements compared to the homogeneous catalyst alone

Visible Photoelectrochemical Water Splitting Based on a Ru(II) Polypyridyl Chromophore and Iridium Oxide Nanoparticle Catalyst

Preparation of Ru­(II) polypyridyl–iridium oxide nanoparticle (IrO_X NP) chromophore–catalyst assemblies on an FTO|<i>nano</i>ITO|TiO₂ core/shell by a layer-by-layer procedure is described for application in dye-sensitized photoelectrosynthesis cells (DSPEC). Significantly enhanced, bias-dependent photocurrents with Lumencor 455 nm 14.5 mW/cm² irradiation are observed for core/shell structures compared to TiO₂ after derivatization with [Ru­(4,4′-PO₃H₂bpy)₂(bpy)]²⁺ (RuP₂) and uncapped IrO_X NPs at pH 5.8 in NaSiF₆ buffer with a Pt cathode. Photocurrents arising from photolysis of the resulting photoanodes, FTO|<i>nano</i>ITO|TiO₂|−RuP₂,IrO₂, are dependent on TiO₂ shell thickness and applied bias, reaching 0.2 mA/cm² at 0.5 V vs AgCl/Ag with a shell thickness of 6.6 nm. Long-term photolysis in the NaSiF₆ buffer results in a marked decrease in photocurrent over time due to surface hydrolysis and loss of the chromophore from the surface. Long-term stability, with sustained photocurrents, has been obtained by atomic layer deposition (ALD) of overlayers of TiO₂ to stabilize surface binding of −RuP₂ prior to the addition of the IrO_X NPs