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A new approach is described here for the preparation of water-oxidation photoanodes in a dye-sensitized photoelectrosynthesis cell for water splitting. It is based on a chromophore (C)–catalyst (Ca...

We describe here the preparation and characterization of a photocathode assembly for CO2 reduction to CO in 0.1 M LiClO4 acetonitrile. The assembly was formed on 1.0 μm thick mesoporous films of NiO using a layer-by-layer procedure based on Zr(IV)–phosphonate bridging units. The structure of the Zr(IV) bridged assembly, abbreviated as NiO|-DA-RuCP22+-Re(I), where DA is the dianiline-based electron donor (N,N,N′,N′-((CH2)3PO3H2)4-4,4′-dianiline), RuCP2+ is the light absorber [Ru((4,4′-(PO3H2CH2)2-2,2′-bipyridine)(2,2′-bipyridine))2]2+, and Re(I) is the CO2 reduction catalyst, ReI((4,4′-PO3H2CH2)2-2,2′-bipyridine)(CO)3Cl. Visible light excitation of the assembly in CO2 saturated solution resulted in CO2 reduction to CO. A steady-state photocurrent density of 65 μA cm−2 was achieved under one sun illumination and an IPCE value of 1.9% was obtained with 450 nm illumination. The importance of the DA aniline donor in the assembly as an initial site for reduction of the RuCP2+ excited state was demonstrated by an 8 times higher photocurrent generated with DA present in the surface film compared to a control without DA. Nanosecond transient absorption measurements showed that the expected reduced one-electron intermediate, RuCP+, was formed on a sub-nanosecond time scale with back electron transfer to the electrode on the microsecond timescale which competes with forward electron transfer to the Re(I) catalyst at t1/2 = 2.6 μs (kET = 2.7 × 105 s−1).

Significance Dye-sensitized photoelectrosynthesis cells (DSPECs) are a very promising approach to convert solar energy into chemical fuels as H 2 or reduced CO 2 species from water or CO 2 . Water oxidation occurring in the photoanode, involving 4e − process, is a critical reaction for the artificial photosynthesis. Here, we inserted an electron mediator between light harvester and water oxidation catalyst. In this role it acts as a mimic for the tyrosine (Yz) between the chromophore and catalyst in PSII. The resulting assembly structures were stable for extended periods (3 h) toward water oxidation to O 2 , and they also extended the chromophores used in the DSPECs for water oxidation to a phosphonated porphyrin dye.

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/cm

Significance Efficient reduction of carbon dioxide to useful fuels and chemicals is an important research goal in artificial photosynthesis. Significant progress has been made for the C1 products, CO and HCOO − . We report here a procedure based on the use of ultrasmall, monodispersed Cu and Ag bimetallic nanoparticles on thin, electrochemically polymerized poly-Fe(vbpy) 3 (PF 6 ) 2 films. They reduce CO 2 to acetate at pH 7 in aqueous HCO 3 − solutions at relatively high efficiencies with significant rate enhancements with added benzotriazole. In the sequence of clusters, the most efficient results for acetate production were obtained in films of (Cu) 2 ,(Ag) 3 with a faradaic efficiency of 21.2% for acetate from CO 2 at −1.33 V vs. reversible hydrogen electrode in 0.5 M KHCO 3 with 8 ppm of added benzotriazole at 0 °C.

Organic chromophores can be synthesized by established methods and offer an opportunity to expand overall solar spectrum utilization for dye-sensitized photoelectrosynthesis cells. However, there are complications in the use of organic chromophores arising from the instability of their oxidized forms, the inability of their oxidized forms to activate a water oxidation catalyst, or the absence of a sufficiently reducing excited state for electron injection into appropriate semiconductors. Three new triarylamine donor–acceptor organic dyes have been investigated here for visible-light-driven water oxidation. They offer highly oxidizing potentials (>1 V vs NHE in aqueous solution) that are sufficient to drive a water oxidation catalyst and excited-state potentials (∼−1.2 V vs NHE) sufficient to inject into TiO2. The oxidized form of one of the chromophores is sufficiently stable to exhibit reversible electrochemistry in aqueous solution. The chromophores also have favorable photophysics. Visible-light-driven...

The rate of reduction of hexavalent 243Am due to self-radiolysis was measured across a range of total americium and nitric acid concentrations. These so-called autoreduction rates exhibited zero-order kinetics with respect to the concentration of hexavalent americium, and pseudo-first-order kinetics with respect to the total concentration of americium. However, the rate constants did vary with nitric acid concentration, resulting in values of 0.0048 ± 0.0003, 0.0075 ± 0.0005, and 0.0054 ± 0.0003 h-1 for 1.0, 3.0, and 6.5 M HNO3, respectively. This indicates that reduction is due to reaction of hexavalent americium with the radiolysis products of total americium decay. The concentration changes of Am(III), Am(V), and Am(VI) were determined by UV-vis spectroscopy. The Am(III) molar extinction coefficients are known; however, the unknown values for the Am(V) and Am(VI) absorbances across the studied range of nitric acid concentrations were determined by sensitivity analysis in which a mass balance with the known total americium concentration was obtained. The new extinction coefficients and reduction rate constants have been tabulated here. Multiscale radiation chemical modeling using a reaction set with both known and optimized rate coefficients was employed to achieve excellent agreement with the experimental results, and indicates that radiolytically produced nitrous acid from nitric acid radiolysis and hydrogen peroxide from water radiolysis are the important reducing agents. Since these species also react with each other, modeling indicated that the highest concentrations of these species available for Am(VI) reduction occurred at 3.0 M HNO3. This is in agreement with the empirical finding that the highest rate constant for autoreduction occurred at the intermediate acid concentration.

Creating stable surface-bound molecular catalysts for CO2 reduction in aqueous solutions for either electrochemical or photoelectrochemical reduction is a continuing challenge. We report here the preparation and characterization of thin oxide/carbon nanotube electrodes on fluorine-doped tin oxide (FTO) electrodes. The electrodes were prepared by atomic layer deposition (ALD) of ∼15 nm of TiO2 followed by a layer of carbon nanotubes and a thin (2 nm) overlayer of TiO2 for surface stabilization and binding. The phosphonate-derivatized version of the known aqueous solution catalyst for H2/CO reduction, [RuII(tpy-Ph–CH2–PO3H2)(Mebim-py)(H2O)](PF6)2 (tpy-Ph–CH2–PO3H2 = (4-([2, 2′:6′,2″-terpyridin]-4′-yl)benzyl)phosphonic acid; Mebim-py = 3-methyl-1-pyridyl-benzimidazol-2-ylidene), was added to the surface and stabilized by addition of a thin overlayer of TiO2 by ALD. In the derivatized electrodes, the catalyst maintains its reactivity toward CO2 reduction in the short-term, giving mixtures of H2/CO that vary f...

The recent development of facile methods to oxidize trivalent americium to its higher valence states holds promise for the discovery of new chemistries and critical insight into the behavior of the 5f electrons. However, progress in understanding high-valent americium chemistry has been hampered by americium's inherent ionizing radiation field and its concomitant effects on americium redox chemistry. Any attempt to understand high-valent americium reduction and/or disproportionation must account for the effects of these radiolytic processes. Therefore, we present a complete, quantitative, mechanistic description of the radiation-induced redox chemistry of the americyl oxidation states in aerated, aqueous nitric acid, as a function of radiation quality (type and energy) and solution composition using multiscale modeling calculations supported by experiment. The reduction of Am(VI) to Am(V) was found to be most sensitive to the effects of ionizing radiation, undergoing rapid reductions with the steady-state products of aqueous HNO3 radiolysis, i.e., HNO2, H2O2, and HO2•, which dictated its practical lifetime under acidic conditions. In contrast, Am(V) is only susceptible to radiolytic oxidation, mainly through its reactions with NO3•, and is notably radiation-resistant with respect to direct one-electron reduction to produce Am(IV). Our multiscale modeling calculations predict that the lifetime of Am(V) is dictated by its rate of disproportionation, 2AmO2+ + 4Haq+ → AmO22+ + Am4+ + 2H2O, with a fourth-order dependence on [Haq+] in agreement with previous experimental findings, giving an optimized rate coefficient of k = 2.27 × 10-6 M-5 s-1. This disproportionation initially produces Am(IV) and Am(VI) species, but the lack of any spectroscopic evidence in our study for Am(IV) suggests that solvent reduction of this cation occurs rapidly. The ultimate product of all the Am(VI)/Am(V) irradiations is Am(III), which shows great stability in an irradiation field.

In artificial photosynthesis, the sun drives water splitting into H2 and O2 or converts CO2 into a useful form of carbon. In most schemes, water oxidation is typically the limiting half-reaction. H...

We present here the electrochemical oxidation of Am(iii) to AmVO2+ and AmVIO22+ in pH 1 nitric acid using a mesoporous tin-doped indium oxide electrode modified with a covalently attached dipyrazinylpyridine ligand. The applied potential affects the distribution of Am oxidation products. At potential 1.8 V, only Am(v) is observed, while increasing the potential to as much as 2.0 V, results in oxidation of Am(iii) to Am(v) and subsequent oxidation of Am(v) to Am(vi). At applied potentials >2.0 V, Am(iii) is oxidized to Am(v), while Am(vi) is reduced to Am(v). The latter reduction reaction is likely due to the increased rate of hydrogen peroxide formation from the 2-electron oxidation of water at the electrode at these high potentials. The development of future ligand modified electrodes for actinide oxidations must consider how they facilitate Am oxidations while disfavoring unwanted or competing reactions.

Four new structurally similar dinuclear oxorhenium(V) complexes, [{Re(O)X(PPh3)}2(μ-O)(μ-4-X′-pz)2], where pz = pyrazolate anion, X = X′ = Cl (1) and Br (4), X = Cl, X′ = Br (2), and X = Br, X′ = Cl (3), have been synthesized and characterized. Little variation in spectroscopic features – 1H NMR, IR, UV–Vis – exists among the four complexes. All complexes possess a bent Re-O-Re core as well as distorted octahedral coordination geometry around the rhenium centers. A reversible one-electron electrochemical process is observed at approximately 0.84 V vs. Fc+/Fc in all four complexes; however, changing the terminal halide from chloride to bromide slightly destabilizes the oxidized Re(VI) center.

The redox properties of a surface-bound phosphate flavin derivative (flavin mononucleotide, FMN) have been investigated on planar-FTO and nanoITO electrodes under acidic conditions in 1:1 CH3CN/H2O (V:V). On FTO, reversible 2e-/2H+ reduction of FTO|-FMN to FTO|-FMNH2 occurs with the pH and scan rate dependence expected for a 2e-/2H+ surface-bound couple. Addition of tetramethylbenzoquinone (Me4Q) results in rapid electrocatalyzed reduction to the hydroquinone by a pathway first order in quinone and first order in acid with kH = (2.6 ± 0.2) × 106 M–1 s–1. Electrocatalytic reduction of the quinone also occurs on derivatized, high surface area nanoITO electrodes with evidence for competitive rate-limiting diffusion of the quinone into the mesoporous nanostructure.

A dual working electrode technique for the in situ production and quantification of electrochemically or photoelectrochemically produced O2 is described. This technique, termed a collector-generator cell, utilizes a transparent fluorine doped tin oxide electrode to sense O2. This setup is specifically designed for detecting O2 in dye sensitized photoelectrosynthesis cells.

We describe here the preparation of a family of photoanodes for water oxidation that incorporate an electron acceptor–chromophore–catalyst in single molecular assemblies on nano-indium tin oxide (nanoITO) electrodes on fluorine-doped tin oxide (FTO). The assemblies were prepared by using a layer-by-layer, Atomic Layer Deposition (ALD), self-assembly approach. In the procedure, addition of an electron acceptor viologen derivative followed by a RuII(bpy) chromophore and a pyridyl derivative of the water oxidation catalyst [Ru(bda) (L)2] (bda = 2,2′-bipyridine-6,6′-dicarboxylate)2, were linked by ALD by addition of the bridge precursors TiO2, ZrO2, and Al2O3 as the bridging groups giving the assemblies, FTO|nanoITO|–MV2+–ALD MO2–RuP22+–ALD M′O2–WOC. In a series of devices, the most efficient gave water oxidation with an incident photon to current efficiency of 2.2% at 440 nm. Transient nanosecond absorption measurements on the assemblies demonstrated that the slow step in the intra-assembly electron transfer is the electron transfer from the chromophore through the viologen bridge to the nanoITO electrode.

Light-driven water splitting was achieved using a dye-sensitized mesoporous oxide film and the oxidation of bromide (Br- ) to bromine (Br2 ) or tribromide (Br3- ). The chemical oxidant (Br2 or Br3- ) is formed during illumination at the photoanode and used as a sacrificial oxidant to drive a water oxidation catalyst (WOC), here demonstrated using [Ru(bda)(pic)2 ], (1; pic=picoline, bda=2,2'-bipyridine-6,6'-dicarboxylate). The photochemical oxidation of bromide produces a chemical oxidant with a potential of 1.09 V vs. NHE for the Br2 /Br- couple or 1.05 V vs. NHE for the Br3- /Br- couple, which is sufficient to drive water oxidation at 1 (RuV/IV ≈1.0 V vs. NHE at pH 5.6). At pH 5.6, using a 0.2 m acetate buffer containing 40 mm LiBr and the [Ru(4,4'-PO3 H2 -bpy)(bpy)2 ]2+ (RuP2+ , bpy=2,2'-bipyridine) chromophore dye on a SnO2 /TiO2 core-shell electrode resulted in a photocurrent density of around 1.2 mA cm-2 under approximately 1 Sun illumination and a Faradaic efficiency upon addition of 1 of 77 % for oxygen evolution.

Significance Dye-sensitized photoelectrosynthesis cells (DSPECs) provide a basis for artificial photosynthesis and solar fuels production. By combining molecular chromophores and catalysts with high surface area, transparent semiconductor electrodes, a DSPEC provides the basis for light-driven conversion of water to O 2 and H 2 or for reduction of CO 2 to carbon-based fuels. The incorporation of plasmonic cubic silver nanoparticles, with a strongly localized surface plasmon absorbance near 450 nm, to a DSPEC photoanode induces a great increase in the efficiency of water oxidation to O 2 at a DSPEC photoanode. The improvement in performance by the molecular components in the photoanode highlights a remarkable advantage for the plasmonic effect in driving the 4e - /4H + oxidation of water to O 2 in the photoanode.

The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore–catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO2 by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO2 reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO2 reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

A water-soluble, hydroxyphenyl-substituted dipyrazinylpyridine (HOL) and its liposoluble, n-octylated derivative (C8OL) were prepared in short sequences and good yields. The monocarbonyldichlororuthenium(II) complex (C8OL)Ru(CO)Cl2 were obtained in cis and trans isomeric forms, which could be distinguished by their absorption, 13C NMR and 15N NMR spectra. The octahedral homoleptic complexes [(C8OL)2M]2+ (M = RuII, FeII) and five-coordinate (HOL)NiCl2 are also described. Density functional theory is used to derive optical geometries, optical spectra, a molecular orbital description and the electronic structure. The time-dependent calculations reveal that the strong visible-region absorption responsible for the color of the homoleptic FeII and RuII species arises mainly from an internal ligand charge-transfer process and only to a small degree from the expected metal-to-ligand CT process. The crystal structures of free C8OL, trans-(C8OL)Ru(CO)Cl2, [(C8OL)2Fe](FeCl4)2 and (HOL)NiCl2 were also obtained. Electrochemical data are also presented and discussed.

A collector-generator (C-G) technique has been applied to determine the Faradaic efficiencies for electrocatalytic O2 production by the homogeneous water oxidation catalysts Ru(bda)(isoq)2 (1; bda = 2,2'-bipyridine and isoq = isoquinoline) and [Ru(tpy)(bpz)(OH2)](2+) (2; tpy = 2,2':6',2″-terpyridine and bpz = 2,2'-bipyrazine). This technique uses a custom-fabricated cell consisting of two fluorine-doped tin oxide (FTO) working electrodes separated by 1 mm with the conductive sides facing each other. With a catalyst in solution, water oxidation occurs at one FTO electrode under a sufficient bias to drive O2 formation by the catalyst; the O2 formed then diffuses to the second FTO electrode poised at a potential sufficiently negative to drive O2 reduction. A comparison of the current versus time response at each electrode enables determination of the Faradaic efficiency for O2 production with high concentrations of supporting electrolyte important for avoiding capacitance effects between the electrodes. The C-G technique was applied to electrocatalytic water oxidation by 1 in the presence of the electron-transfer mediator Ru(bpy)3(2+) in both unbuffered aqueous solutions and with the added buffer bases HCO3(-), HPO4(2-), imidazole, 1-methylimidazole, and 4-methoxypyridine. HCO3(-) and HPO4(2-) facilitate water oxidation by atom-proton transfer (APT), which gave Faradaic yields of 100%. With imidazole as the buffer base, coordination to the catalyst inhibited water oxidation. 1-Methylimidazole and 4-methoxypyridine gave O2 yields of 55% and 76%, respectively, with the lower Faradaic efficiencies possibly due to competitive C-H oxidation of the bases. O2 evolution by catalyst 2 was evaluated at pH 12 with 0.1 M PO4(3-) and at pH 7 in a 0.1 M H2PO4(-)/HPO4(2-) buffer. At pH 12, at an applied potential of 0.8 V vs SCE, water oxidation by the Ru(IV)(O)(2+) form of the catalyst gave O2 in 73% yield. In a pH 7 solution, water oxidation at 1.4 V vs SCE, which is dominated by Ru(V)(O)(3+), gave O2 with an efficiency of 100%. The lower efficiency for Ru(IV)(O)(2+) at pH 12 may be due to competitive oxidation of a polypyridyl ligand.