Electron Transfer Mediator Effects in the Oxidative Activation of a Ruthenium Dicarboxylate Water Oxidation Catalyst

The mechanism of electrocatalytic water oxidation by the water oxidation catalyst, ruthenium 2,2′-bipyridine-6,6′-dicarboxylate (bda) bis-isoquinoline (isoq), [Ru­(bda)­(isoq)₂], <b>1</b>, at metal oxide electrodes has been investigated. At indium-doped tin oxide (ITO), diminished catalytic currents and increased overpotentials are observed compared to glassy carbon (GC). At pH 7.2 in 0.5 M NaClO₄, catalytic activity is enhanced by the addition of [Ru­(bpy)₃]²⁺ (bpy = bipyridine) as a redox mediator. Enhanced catalytic rates are also observed at ITO electrodes derivatized with the surface-bound phosphonic acid derivative [Ru­(4,4′-(PO₃H₂)₂bpy)­(bpy)₂]²⁺, <b>RuP</b>²⁺. Controlled potential electrolysis with measurement of O₂ at ITO with and without surface-bound RuP²⁺ confirm that water oxidation catalysis occurs. Remarkable rate enhancements are observed with added acetate and phosphate, consistent with an important mechanistic role for atom-proton transfer (APT) in the rate-limiting step as described previously at GC electrodes

Photochemical Synthesis of a Water Oxidation Catalyst Based on Cobalt Nanostructures

New cobalt-based nanocomposites have been prepared by photoreduction of Co²⁺ salts to generate cobalt nanoparticles deposited on carbon-based materials such as nanocyrstalline diamond and carbon felt. Spontaneous air oxidation converts the metal to Co₂O₃ which has been tested as a water oxidation catalyst. This work demonstrates that the cobalt oxide nanostructures can be deposited on various carbon surfaces and can catalyze the four-electron oxidation of water to oxygen under anodic bias

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

Polymer Chromophore-Catalyst Assembly for Solar Fuel Generation

A polystyrene-based chromophore-catalyst assembly (poly-<b>2</b>) has been synthesized and assembled at a mesoporous metal oxide photoanode. The assembly contains water oxidation catalyst centers based on [Ru­(trpy) (phenq)]²⁺ (Ru-Cat) and [Ru­(bpy)₃]²⁺ derivatives (Ru-C) as chromophores (trpy= 2,2′;6,2″- terpyridine, phenq = 2-(quinol-8′-yl)-1,10-phenanthroline and bpy = 2,2′-bipyridine). The photophysical and electrochemical properties of the polychromophore-oxidation catalyst assembly were investigated in solution and at the surface of mesoporous metal oxide films. The layer-by-layer (LbL) method was utilized to construct multilayer films with cationic poly-<b>2</b> and anionic poly­(acrylic acid) (PAA) for light-driven photochemical oxidations. Photocurrent measurements of (PAA/poly-<b>2</b>)₁₀ LbL films on mesoporous TiO₂ demonstrate light-driven oxidation of phenol and benzyl alcohol in aqueous solution. Interestingly, illumination of (PAA/poly-<b>2</b>)₅ LbL films on a fluorine doped SnO₂/TiO₂ core/shell photoanode in aqueous solution gives rise to an initial photocurrent (∼18.5 μA·cm^–2) that is in part ascribed to light driven water oxidation

All-in-One Derivatized Tandem p⁺n‑Silicon–SnO₂/TiO₂ Water Splitting Photoelectrochemical Cell

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 μm thick SnO₂ films with an outer 4.3 nm shell of TiO₂ added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and p⁺n type silicon substrates. Modified electrodes were derivatized by addition of the water oxidation catalyst, [Ru­(bda)­(4-O­(CH₂)₃PO₃H₂)-pyr)₂], <b>1</b>, (pyr = pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate), and chromophore, [Ru­(4,4′-PO₃H₂-bpy) (bpy)₂]²⁺, <b>RuP</b>²⁺, (bpy = 2,2′-bipyridine), which form 2:1 <b>RuP</b>²⁺/<b>1</b> assemblies on the surface. At pH 5.7 in 0.1 M acetate buffer, these electrodes with a fluorine-doped tin oxide (FTO) back contact under ∼1 sun illumination (100 mW/cm²; white light source) perform efficient water oxidation with a photocurrent of 1.5 mA/cm² with an 88% Faradaic efficiency (FE) for O₂ production at an applied bias of 600 mV versus RHE (ACS Energy Lett., 2016, 1, 231−236). The SnO₂/TiO₂–chromophore–catalyst assembly was integrated with the Si electrodes by a thin layer of titanium followed by an amorphous TiO₂ (Ti/<i>a-</i>TiO₂) coating as an interconnect. In the integrated electrode, p⁺n-Si–Ti/<i>a</i>-TiO₂–SnO₂/TiO₂|-2<b>RuP</b>²⁺/<b>1</b>, the p⁺n-Si junction provided about 350 mV in added potential to the half cell. In photolysis experiments at pH 5.7 in 0.1 M acetate buffer, bias-free photocurrents approaching 100 μA/cm² were obtained for water splitting, 2H₂O → 2H₂ + O₂. The FE for water oxidation was 79% with a hydrogen efficiency of ∼100% at the Pt cathode

Efficient Light-Driven Oxidation of Alcohols Using an Organic Chromophore–Catalyst Assembly Anchored to TiO₂

The ligand 5-PO₃H₂-2,2′:5′,2″-terthiophene-5-trpy, <b>T3</b> (trpy = 2,2′:6′,2″-terpyridine), was prepared and studied in aqueous solutions along with its metal complex assembly [Ru­(<b>T3</b>)­(bpy)­(OH₂)]²⁺ (<b>T3</b>-Ru-OH₂, bpy = 2,2′-bipyridine). <b>T3</b> contains a phosphonic acid group for anchoring to a TiO₂ photoanode under aqueous conditions, a terthiophene fragment for light absorption and electron injection into TiO₂, and a terminal trpy ligand for the construction of assemblies comprising a molecular oxidation catalyst. At a TiO₂ photoanode, <b>T3</b> displays efficient injection at pH 4.35 as evidenced by the high photocurrents (∼350 uA/cm²) arising from hydroquinone oxidation. Addition of [Ru­(bpy)­(OTf)]­[OTf]₂ (bpy = 2,2′-bipyridine, OTf^– = triflate) to <b>T3</b> at the free trpy ligand forms the molecular assembly, <b>T3</b>-Ru-OH₂, with the oxidative catalyst fragment: [Ru­(trpy)­(bpy)­(OH₂)]²⁺. The new assembly, <b>T3</b>-Ru-OH₂, was used to perform efficient light-driven oxidation of phenol (230 μA/cm²) and benzyl alcohol (25 μA/cm²) in a dye-sensitized photoelectrosynthesis cell

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

Evaluation of Chromophore and Assembly Design in Light-Driven Water Splitting with a Molecular Water Oxidation Catalyst

The water oxidation catalyst [Ru­(bda)­(4-O­(CH₂)₃P­(O₃H₂)₂-pyr)₂], <b>1</b>, (pyr = pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate) was investigated as the catalyst part of a series of chromophore–​catalyst assemblies for light-driven water splitting in dye-sensitized photoelectrosynthesis cells (DSPECs). Both coloaded and layer-by-layer assemblies with phosphonate-Zr­(IV) bridging were investigated on SnO₂/TiO₂ core–shell electrodes. Device performance was evaluated by both photocurrent and direct O₂ measurements in collector–generator cells. The photoelectrodes displayed impressive photocurrents (0.72–1.5 mA/cm²) and Faradaic efficiencies for O₂ generation (71–97%) under 1 sun illumination in pH 5.7, 0.1 M acetate buffer solutions and provided important mechanistic insights into the microscopic details for DSPEC water splitting

Controlling Surface Defects and Photophysics in TiO₂ Nanoparticles

Titanium dioxide (TiO₂) is widely used for photocatalysis and solar cell applications, and the electronic structure of bulk TiO₂ is well understood. However, the surface structure of nanoparticulate TiO₂, which has a key role in properties such as solubility and catalytic activity, still remains controversial. Detailed understanding of surface defect structures may help explain reactivity and overall materials performance in a wide range of applications. In this work we address the solubility problem and surface defects control on TiO₂ nanoparticles. We report the synthesis and characterization of ∼4 nm TiO₂ anatase spherical nanoparticles that are soluble and stable in a wide range of organic solvents and water. By controlling the temperature during the synthesis, we are able to tailor the density of defect states on the surface of the TiO₂ nanoparticles without affecting parameters such as size, shape, core crystallinity, and solubility. The morphology of both kinds of nanoparticles was determined by TEM. EPR experiments were used to characterize the surface defects, and transient absorption measurements demonstrate the influence of the TiO₂ defect states on photoinduced electron transfer dynamics

Interfacial Deposition of Ru(II) Bipyridine-Dicarboxylate Complexes by Ligand Substitution for Applications in Water Oxidation Catalysis

Water oxidation is a critical step in artificial photosynthesis and provides the protons and electrons used in reduction reactions to make solar fuels. Significant advances have been made in the area of molecular water oxidation catalysts with a notable breakthrough in the development of Ru­(II) complexes that use a planar “bda” ligand (bda is 2,2′-bipyridine-6,6′-dicarboxylate). These Ru­(II)­(bda) complexes show lower overpotentials for driving water oxidation making them ideal for light-driven applications with a suitable chromophore. Nevertheless, synthesis of heterogeneous Ru­(II)­(bda) complexes remains challenging. We discuss here a new “bottom-up” synthetic method for immobilizing these catalysts at the surface of a photoanode for use in a dye-sensitized photoelectrosynthesis cell (DSPEC). The procedure provides a basis for rapidly screening the role of ligand variations at the catalyst in order to understand the impact on device performance. The best results of a water-oxidation DSPEC photoanode based on this procedure reached 1.4 mA/cm² at pH 7 in 0.1 M [PO₄H₂]^-/[PO₄H]^2-solution with minimal loss in catalytic behavior over 30 min, and produced an incident photon to current efficiency (IPCE) of 24.8% at 440 nm