A series of monomeric ruthenium polypyridyl complexes have been synthesized and characterized, and their performance as water oxidation catalysts has been evaluated. The diversity of ligand environments and how they influence rates and reaction thermodynamics create a platform for catalyst design with controllable reactivity based on ligand variations.
The performance of dye-sensitized solar and photoelectrochemical cells is strongly dependent on the light absorption and electron transfer events at the semiconductor−small molecule interface. These processes as well as photo/electrochemical stability are dictated not only by the properties of the chromophore and metal oxide but also by the structure of the dye molecule, the number of surface binding groups, and their mode of binding to the surface. In this article, we report the photophysical and electrochemical properties of a series of six phosphonate-derivatized [Ru(bpy) 3 ] 2+ complexes in aqueous solution and bound to ZrO 2 and TiO 2 surfaces. A decrease in injection yield and cross surface electron-transfer rate with increased number of diphosphonated ligands was observed. Additional phosphonate groups for surface binding did impart increased electrochemical and photostability. All complexes exhibit similar back-electron-transfer kinetics, suggesting an electron-transfer process rate-limited by electron transport through the interior of TiO 2 to the interface. With all results considered, the ruthenium polypyridyl derivatives with one or two 4,4′-(PO 3 H 2 ) 2 bpy ligands provide the best balance of electron injection efficiency and stability for application in solar energy conversion devices.
Artificial photosynthesis and the production of solar fuels could be a key element in a future renewable energy economy providing a solution to the energy storage problem in solar energy conversion. We describe a hybrid strategy for solar water splitting based on a dye sensitized photoelectrosynthesis cell. It uses a derivatized, core-shell nanostructured photoanode with the core a high surface area conductive metal oxide film--indium tin oxide or antimony tin oxide--coated with a thin outer shell of TiO 2 formed by atomic layer deposition. A "chromophore-catalyst assembly" 1, [(PO 3 H 2 ) 2 bpy) 2 Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH 2 )] 4+ , which combines both light absorber and water oxidation catalyst in a single molecule, was attached to the TiO 2 shell. Visible photolysis of the resulting core-shell assembly structure with a Pt cathode resulted in water splitting into hydrogen and oxygen with an absorbed photon conversion efficiency of 4.4% at peak photocurrent.P hotosynthesis uses the energy of the sun with water as the reducing agent to drive the reduction of carbon dioxide to carbohydrates with oxygen as a coproduct through a remarkably complex process. At photosystem II, a subsystem imbedded in the thylakoid membrane where O 2 is produced, light absorption, energy migration, electron transfer, proton transfer, and catalysis are all used in multiple stepwise chemical reactions which are carefully orchestrated at the molecular level (1, 2).Photosynthesis solves the problem of energy storage by biomass production but with low solar efficiencies, typically <1%. In artificial photosynthesis with solar fuels production, the goal is similar but the targets are either hydrogen production from water splitting, Eq. 1, or reduction of carbon dioxide to a carbon-based fuel, Eq. 2 (3, 4). Different strategies for solar fuels have evolved (5, 6). In one, direct bandgap excitation of semiconductors creates electron-hole pairs which are then used to drive separate halfreactions for water oxidation (2H 2 O → O 2 + 4H + + 4e − ) and water/proton reduction (2H + + 2e − → H 2 ) (7-9).Here, we report a hybrid strategy for solar water splitting, the dye sensitized photoelectrosynthesis cell (DSPEC). It combines the electron transport properties of semiconductor nanocrystalline thin films with molecular-level reactions (10). In this approach, a chromophore-catalyst molecular assembly acts as both light absorber and catalyst. It is bound to the surface of a "core-shell," nanostructured, transparent conducting oxide film. The core structure consists of a nanoparticle film of either tin-doped indium oxide (nanoITO), or antimony-doped tin oxide (nanoATO), deposited on a fluoride-doped tin oxide (FTO) glass substrate. The shell consists of a conformal TiO 2 nanolayer applied by atomic layer deposition (ALD). The resulting "photoanode," where water oxidation occurs, is connected to a Pt cathode for proton reduction to complete the water splitting cell. A diagram for the photoanode in the DSPEC device is shown in Fig. 1. It illustrates...
A hybrid strategy for solar water splitting is exploited here based on a dye-sensitized photoelectrosynthesis cell (DSPEC) with a mesoporous SnO 2 /TiO 2 core/shell nanostructured electrode derivatized with a surface-bound Ru(II) polypyridyl-based chromophore-catalyst assembly. The assembly, [(4, H 2 ) 2 bpy) 2 Ru(4-Mebpy-4'-bimpy)Ru (tpy) (OH 2 dye-sensitized photoelectrosynthesis cell | water oxidation | core/shell A lthough promising, significant challenges remain in the search for successful strategies for artificial photosynthesis by water splitting into oxygen and hydrogen or reduction of CO 2 to reduced forms of carbon (1-5). In a dye-sensitized photoelectrosynthesis cell (DSPEC), a wide band gap, nanoparticle oxide film, typically TiO 2 , is derivatized with a surface-bound molecular assembly or assemblies for light absorption and catalysis (6-8). In a DSPEC, visible light is absorbed by a chromophore, initiating a series of events that culminate in water splitting: injection, intraassembly electron transfer, catalyst activation, and electron transfer to a cathode or photocathode for H 2 production. Sun and coworkers have recently demonstrated visible-light-driven water splitting with a coloading approach combining Ru(II) polypyridyl-based light absorbers and catalysts on TiO 2 (9). The efficiency of DSPEC devices is dependent on interfacial dynamics and competing kinetic processes. A major limiting factor is the requirement for accumulating multiple oxidative equivalents at a catalyst site to meet the 4e − /4H + demands for oxidizing water to dioxygen (2H 2 O -4e − -4H + → O 2 ) in competition with back electron transfer of injected electrons to the oxidized assembly.One approach to achieving structural control of local electron transfer dynamics at the oxide interface in dye-sensitized devices is by use of nanostructured core/shell electrodes (10-12). In this approach, a mesoporous network of nanoparticles is uniformly coated with a thin oxide overlayer prepared by atomic layer deposition (ALD). We have used core/shell electrodes to demonstrate benzyl alcohol dehydrogenation (13). This approach has also been used to enhance the efficiency of dye-sensitized solar cells (14,15). Recently, we described the use of a core/shell consisting of an inner core of a nanoparticle transparent conducting oxide, tin-doped indium oxide (nanoITO), and a thin outer shell of TiO 2 for water splitting by visible light (16 Fig. 1A, provided the basis for a photoanode in a DSPEC application with a Pt cathode for H 2 generation with a small applied bias in an acetate buffer at pH 4.6.Application of the core/shell structure led to a greatly enhanced efficiency for water splitting compared with mesoscopic, nanoparticle TiO 2 but the per-photon absorbed efficiency of the resulting DSPEC was relatively low and problems arose from longterm instability due to loss of the assembly from the oxide surface in the acetate buffer at pH 4.6. The latter is problematic because the rate of water oxidation is enhanced by added buffer bases...
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].
Photoinduced formation, separation, and buildup of multiple redox equivalents are an integral part of cycles for producing solar fuels in dye-sensitized photoelectrosynthesis cells (DSPECs). Excitation wavelength-dependent electron injection, intra-assembly electron transfer, and pH-dependent back electron transfer on TiO(2) were investigated for the molecular assembly [((PO(3)H(2)-CH(2))-bpy)(2)Ru(a)(bpy-NH-CO-trpy)Ru(b)(bpy)(OH(2))](4+) ([TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+); ((PO(3)H(2)-CH(2))(2)-bpy = ([2,2'-bipyridine]-4,4'-diylbis(methylene))diphosphonic acid); bpy-ph-NH-CO-trpy = 4-([2,2':6',2″-terpyridin]-4'-yl)-N-((4'-methyl-[2,2'-bipyridin]-4-yl)methyl) benzamide); bpy = 2,2'-bipyridine). This assembly combines a light-harvesting chromophore and a water oxidation catalyst linked by a synthetically flexible saturated bridge designed to enable long-lived charge-separated states. Following excitation of the chromophore, rapid electron injection into TiO(2) and intra-assembly electron transfer occur on the subnanosecond time scale followed by microsecond-millisecond back electron transfer from the semiconductor to the oxidized catalyst, [TiO(2)(e(-))-Ru(a)(II)-Ru(b)(III)-OH(2)](4+)→[TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+).
Understanding why food chains are relatively short in length has been an area of research and debate for decades. We tested if progressive changes in the nutritional content of arthropods with trophic position limit the availability of a key nutrient, lipid, for carnivores. Arthropods at higher trophic levels had progressively less lipid and more protein in their bodies compared with arthropods at lower trophic levels. The nutrients present in arthropod bodies were directly related to the nutrients that predators extracted when feeding on those arthropods. As a consequence, nutrient assimilation shifted from lipid-biased to protein-biased as arthropods ascended trophic levels from herbivores to secondary carnivores. Successive changes in the nutritional consequences of predation may ultimately set an upper limit on the number of trophic levels in arthropod communities. Further work is needed to examine the influence of lipid and other nutrients on food web traits in a range of ecosystems.
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.
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