Using Molecular Design to Control the Performance of Hydrogen-Producing Polymer-Brush-Modified Photocathodes

Attachment of difluoroborylcobaloxime catalysts to a polymer-brush-modified GaP semiconductor allows improved hydrogen production levels and photoelectrochemical performance under aqueous acidic conditions (pH = 4.5) as compared to the performance of electrodes without catalyst treatment. The catalytic assembly used in this work incorporates a boron difluoride (BF2) capping group on the glyoximate ligand of the catalyst, a synthetic modification previously used to enhance the stability of nonsurface-attached complexes toward acid hydrolysis and to shift the cobalt reduction potentials of the complex to less negative, and thus technologically more relevant, values. The pH-dependent photoresponses of the cobaloxime- and difluoroborylcobaloxime- modified semiconductors are shown to be consistent with those from analogous studies using non-surface-attached cobaloxime catalysts as well as catalysts supported on conductive electrodes. Thus, this work illustrates the potential to control and optimize the properties of visible-light-absorbing semiconductors using polymeric overcoating techniques coupled with the principles of synthetic molecular design.

Synthesis, electrochemical characterization, and photophysical studies of structurally tuned aryl-substituted terpyridyl ruthenium(II) complexes

Synthesis, electrochemical potentials, static emission, and temperature-dependent excited-state lifetimes of several 4'-aryl-substituted terpyridyl complexes of ruthenium(II) are reported. Synthetic tuning is explored within three conceptual series of complexes. The first series explores the impact of introducing a strong σ-donating 4,4',4″-tri-tert-butyl-2,2':6',2″-terpyridine (tbtpy) opposite to an arylated terpyridine ligand 4'-(4-methylphenyl)-2,2':6',2″-terpyridine (ttpy). It is found that (3)MLCT (triplet metal-to-ligand charge-transfer state) stabilization concomitant with (3)MC (triplet metal-centered state) destabilization in the heteroleptic parent complex [Ru(ttpy)(tbtpy)](2+) leads to an extended excited-state lifetime relative to the structurally related bis-homoleptic species [Ru(ttpy)2](2+). The second series explores the impact of introducing a carboxylic acid or a methyl ester moiety at the para-position of the arylterpyridyl ligand (R1 = R2 = H) within heteroleptic complexes as a platform for future semiconductor attachment studies. This substitution leads to further lifetime enhancements, understood as arising from (3)MLCT stabilization. Such complexes are referred to as [Ru(1)(tbtpy)](2+) (for the acid at R3) and [Ru(1')(tbtpy)](2+) (for the ester at R3). In the final series, methyl substituents are sequentially added at the R1 and R2 positions for both the acid ([Ru(2)(tbtpy)](2+) and [Ru(3)(tbtpy)](2+)) and ester ([Ru(2')(tbtpy)](2+) and [Ru(3')(tbtpy)](2+)) analogues to eventually explore dynamical electron transfer coupling at dye/semiconductor interfaces. In these complexes, sequential addition of steric bulk decreases excited state lifetimes. This can be understood to arise primarily from the increase of the (3)MLCT level, as excited-state electron delocalization is limited by inter-ring twisting in the lower-energy arylated ligand. The introduction of a dimethylated sterically encumbered ligand lead to a notable 14-fold increase in knr from [Ru(1')(tbtpy)](2+) to [Ru(3')(tbtpy)](2+) (or [Ru(1)(tbtpy)](2+) to [Ru(3)(tbtpy)](2+)).

Homogeneous electrocatalytic water oxidation at neutral pH by a robust macrocyclic nickel(II) complex

The development of an earth-abundant, first-row water oxidation catalyst that operates at neutral pH and low overpotential remains a fundamental chemical challenge. Herein, we report the first nickel-based robust homogeneous water oxidation catalyst, which can electrocatalyze water oxidation at neutral pH and low overpotential in phosphate buffer. The results of DFT calculations verify that the O-O bond formation in catalytic water oxidation prefers a HO-OH coupling mechanism from a cis-isomer of the catalyst.

Spectroscopic analysis of catalytic water oxidation by [Ru(II)(bpy)(tpy)H2O]2+ suggests that Ru(V)═O is not a rate-limiting intermediate

Modern chemistry's grand challenge is to significantly improve catalysts for water splitting. Further progress requires detailed spectroscopic and computational characterization of catalytic mechanisms. We analyzed one of the most studied homogeneous single-site Ru catalysts, [Ru(II)(bpy)(tpy)H2O](2+) (where bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine). Our results reveal that the [Ru(V)(bpy)(tpy)═O](3+) intermediate, reportedly detected in catalytic mixtures as a rate-limiting intermediate in water activation, is not present as such. Using a combination of electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy, we demonstrate that 95% of the Ru complex in the catalytic steady state is of the form [Ru(IV)(bpy)(tpy)═O](2+). [Ru(V)(bpy)(tpy)═O](3+) was not observed, and according to density functional theory (DFT) analysis, it might be thermodynamically inaccessible at our experimental conditions. A reaction product with unique EPR spectrum was detected in reaction mixtures at about 5% and assigned to Ru(III)-peroxo species with (-OOH or -OO- ligands). We also analyzed the [Ru(II)(bpy)(tpy)Cl](+) catalyst precursor and confirmed that this molecule is not a catalyst and its oxidation past Ru(III) state is impeded by a lack of proton-coupled electron transfer. Ru-Cl exchange with water is required to form active catalysts with the Ru-H2O fragment. [Ru(II)(bpy)(tpy)H2O](2+) is the simplest representative of a larger class of water oxidation catalysts with neutral, nitrogen containing heterocycles. We expect this class of catalysts to work mechanistically in a similar fashion via [Ru(IV)(bpy)(tpy)═O](2+) intermediate unless more electronegative (oxygen containing) ligands are introduced in the Ru coordination sphere, allowing the formation of more oxidized Ru(V) intermediate.

Effects of electron trapping and protonation on the efficiency of water-splitting dye-sensitized solar cells

Water-splitting dye-sensitized photoelectrochemical (WS-DSPECs) cells employ molecular sensitizers to absorb light and transport holes across the TiO2 surface to colloidal or molecular water oxidation catalysts. As hole diffusion occurs along the surface, electrons are transported through the mesoporous TiO2 film. In this paper we report the effects of electron trapping and protonation in the TiO2 film on the dynamics of electron and hole transport in WS-DSPECs. When the sensitizer bis(2,2'-bipyridine)(4,4'-diphosphonato-2,2'-bipyridine)ruthenium(II) is adsorbed from aqueous acid instead of from ethanol, there is more rapid hole transfer between photo-oxidized sensitizer molecules that are adsorbed from strong acid. However, the photocurrent and open-circuit photovoltage are dramatically lower with sensitizers adsorbed from acid because intercalated protons charge-compensate electron traps in the TiO2 film. Kinetic modeling of the photocurrent shows that electron trapping is responsible for the rapid electrode polarization that is observed in all WS-DSPECs. Electrochemical impedance spectroscopy suggests that proton intercalation also plays an important role in the slow degradation of WS-DSPECs, which generate protons at the anode as water is oxidized to oxygen.

Spatial location engineering of oxygen vacancies for optimized photocatalytic H2 evolution activity

Enhanced H2 evolution efficiency is achieved via manipulating the spatial location of oxygen vacancies in niobates. The ultrathin K4 Nb6O17 nanosheets which are rich in surface oxygen vacancies show enhanced optical absorption and band gap narrowing. Meanwhile, the fast charge separation effectively reduces the probability of hole-electron recombination, enabling 20 times hydrogen evolution rate compared with the defect-free bulk counterpart.

The use of a vanadium species as a catalyst in photoinduced water oxidation

The first water oxidation catalyst containing only vanadium atoms as metal centers is reported. The compound is the mixed-valence [(V(IV)5V(V)1)O7(OCH3)12](-) species, 1. Photoinduced water oxidation catalyzed by 1, in the presence of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) and Na2S2O8, in acetonitrile/aqueous phosphate buffer takes place with a quantum yield of 0.20. A hole scavenging reaction between the photochemically generated Ru(bpy)3(3+) and 1 occurs with a bimolecular rate constant of 2.5 × 10(8) M(-1) s(-1). The time-resolved formation of the oxidized molecular catalyst 1(+) in bimolecular reactions is also evidenced for the first time by transient absorption spectroscopy. This result opens the way to the use of less expensive vanadium clusters as water oxidation catalysts in artificial photosynthesis schemes.

Controlling ground and excited state properties through ligand changes in ruthenium polypyridyl complexes

The capture and storage of solar energy requires chromophores that absorb light throughout the solar spectrum. We report here the synthesis, characterization, electrochemical, and photophysical properties of a series of Ru(II) polypyridyl complexes of the type [Ru(bpy)2(N-N)](2+) (bpy = 2,2'-bipyridine; N-N is a bidentate polypyridyl ligand). In this series, the nature of the N-N ligand was altered, either through increased conjugation or incorporation of noncoordinating heteroatoms, as a way to use ligand electronic properties to tune redox potentials, absorption spectra, emission spectra, and excited state energies and lifetimes. Electrochemical measurements show that lowering the π* orbitals on the N-N ligand results in more positive Ru(3+/2+) redox potentials and more positive first ligand-based reduction potentials. The metal-to-ligand charge transfer absorptions of all of the new complexes are mostly red-shifted compared to Ru(bpy)3(2+) (λmax = 449 nm) with the lowest energy MLCT absorption appearing at λmax = 564 nm. Emission energies decrease from λmax = 650 nm to 885 nm across the series. One-mode Franck-Condon analysis of room-temperature emission spectra are used to calculate key excited state properties, including excited state redox potentials. The impacts of ligand changes on visible light absorption, excited state reduction potentials, and Ru(3+/2+) potentials are assessed in the context of preparing low energy light absorbers for application in dye-sensitized photoelectrosynthesis cells.

Ultrafast Carrier Dynamics in Nanostructures for Solar Fuels

Sunlight can be used to drive chemical reactions to produce fuels that store energy in chemical bonds. These fuels, such as hydrogen from splitting water, have much larger energy density than do electrical storage devices. The efficient conversion of clean, sustainable solar energy using photoelectrochemical and photocatalytic systems requires precise control over the thermodynamics, kinetics, and structural aspects of materials and molecules. Generation, thermalization, trapping, interfacial transfer, and recombination of photoexcited charge carriers often occur on femtosecond to picosecond timescales. These short timescales limit the transport of photoexcited carriers to nanometer-scale distances, but nanostructures with high surface-to-volume ratios can enable both significant light absorption and high quantum efficiency. This review highlights the importance of understanding ultrafast carrier dynamics for the generation of solar fuels, including case studies on colloidal nanostructures, nanostructured photoelectrodes, and photoelectrodes sensitized with molecular chromophores and catalysts.

Green-engineered all-substrate mesoporous TiO(2) photoanodes with superior light-harvesting structure and performance

Electrophoretic deposition (EPD) is employed successfully in a suspension of multicomponent TiO2 nanoparticulates of different sizes and morphologies to engineer a very robust bifunctional electrode structure for dye-sensitized solar cell (DSSC) applications that shows excellent light-harvesting and photoelectrochemical performance. Aqueous-synthesized anatase nanocrystallites and sub-micrometer-sized "sea urchin"-like rutile aggregates are formulated in a stable isopropanol suspension without resorting to binders or charging agents. Interestingly, extremely robust films are obtained because of the high surface reactivity, electrophoretic mobility, and unique morphology of the rutile aggregates. DSSCs built with the newly configured bifunctional electrode yielded a record efficiency (8.59 %) for EPD-fabricated devices without resorting to mechanical compression. Such green-engineered mesoporous electrode structures can be built on both metallic and plastic substrates and can find applications in various energy and environmental fields.

Energetics and efficiency analysis of a cobaloxime-modified semiconductor under simulated air mass 1.5 illumination

An energetics and efficiency analysis of a gallium phosphide semiconductor functionalized with molecular hydrogen production catalysts yields insights into the design of improved photocathodes.

Protein film photoelectrochemistry of the water oxidation enzyme photosystem II

This review describes key functions of the water oxidation enzyme photosystem II, protein film photoelectrochemistry of photosystem II and bio-inspired photoelectrochemical water oxidation systems.

Mechanistic analysis of water oxidation catalyzed by mononuclear copper in aqueous bicarbonate solutions

We characterize a mechanism for a monomeric copper catalyst reported to oxidize water in bicarbonate solution when subject to sufficiently high external potentials at near neutral pH values.

Substitution of a hydroxamic acid anchor into the MK-2 dye for enhanced photovoltaic performance and water stability in a DSSC

Substitution of a hydroxamic acid anchoring group into organic dyes such as MK-2 results in significantly improved water stability of DSSC devices.

A functionalized, ethynyl-decorated, tetracobalt(iii) cubane molecular catalyst for photoinduced water oxidation

A new tetracobalt(iii)-oxo cubane 1 was prepared. The ethynyl groups do not affect the photocatalytic properties of 1, which, in contrast, appear to be improved.

Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation

Enhancing the surface binding stability of chromophores, catalysts, and chromophore-catalyst assemblies attached to metal oxide surfaces is an important element in furthering the development of dye sensitized solar cells, photoelectrosynthesis cells, and interfacial molecular catalysis. Phosphonate-derivatized catalysts and molecular assemblies provide a basis for sustained water oxidation on these surfaces in acidic solution but are unstable toward hydrolysis and loss from surfaces as the pH is increased. Here, we report enhanced surface binding stability of a phosphonate-derivatized water oxidation catalyst over a wide pH range (1-12) by atomic layer deposition of an overlayer of TiO2. Increased stability of surface binding, and the reactivity of the bound catalyst, provides a hybrid approach to heterogeneous catalysis combining the advantages of systematic modifications possible by chemical synthesis with heterogeneous reactivity. For the surface-stabilized catalyst, greatly enhanced rates of water oxidation are observed upon addition of buffer bases -H2PO(-)(4)/HPO(2-)(4), B(OH)3/B(OH)2 O-, HPO(2-)4 /PO(3-)(4) - and with a pathway identified in which O-atom transfer to OH(-) occurs with a rate constant increase of 10(6) compared to water oxidation in acid.

Stabilizing small molecules on metal oxide surfaces using atomic layer deposition

Device lifetimes and commercial viability of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells (DSPECs) are dependent on the stability of the surface bound molecular chromophores and catalysts. Maintaining the integrity of the solution-metal oxide interface is especially challenging in DSPECs for water oxidation where it is necessary to perform high numbers of turnovers, under irradiation in an aqueous environment. In this study, we describe the atomic layer deposition (ALD) of TiO2 on nanocrystalline TiO2 prefunctionalized with the dye molecule [Ru(bpy)2(4,4'-(PO3H2)bpy)](2+) (RuP) as a strategy to stabilize surface bound molecules. The resulting films are over an order of magnitude more photostable than untreated films and the desorption rate constant exponentially decreases with increased thickness of ALD TiO2 overlayers. However, the injection yield for TiO2-RuP with ALD TiO2 also decreases with increasing overlayer thickness. The combination of decreased injection yield and 95% quenched emission suggests that the ALD TiO2 overlayer acts as a competitive electron acceptor from RuP*, effectively nonproductively quenching the excited state. The ALD TiO2 also increases back electron transfer rates, relative to the untreated film, but is independent of overlayer thickness. The results for TiO2-RuP with an ALD TiO2 overlayer are compared with similar films having ALD Al2O3 overlayers.

Photogeneration of hydrogen from water using CdSe nanocrystals demonstrating the importance of surface exchange

Unique tripodal S-donor capping agents with an attached carboxylate are found to bind tightly to the surface of CdSe nanocrystals (NCs), making the latter water soluble. Unlike that in similarly solubilized CdSe NCs with one-sulfur or two-sulfur capping agents, dissociation from the NC surface is greatly reduced. The impact of this behavior is seen in the photochemical generation of H2 in which the CdSe NCs function as the light absorber with metal complexes in aqueous solution as the H2-forming catalyst and ascorbic acid as the electron donor source. This precious-metal-free system for H2 generation from water using [Co(bdt)2](-) (bdt, benzene-1,2-dithiolate) as the catalyst exhibits excellent activity with a quantum yield for H2 formation of 24% at 520 nm light and durability with >300,000 turnovers relative to catalyst in 60 h.

Photofunctional construct that interfaces molecular cobalt-based catalysts for H2 production to a visible-light-absorbing semiconductor

Molecular cobalt-containing hydrogen production catalysts are grafted to a visible-light-absorbing semiconductor. The attachment procedure exploits the UV-induced immobilization chemistry of vinylpyridine to p-type (100) gallium phosphide (GaP). Single step surface-initiated photopolymerization yields a covalently attached polymer with pendent pyridyl groups that provide attachment points for assembling cobaloxime catalysts. Successful attachment is characterized by grazing angle attenuated total reflection Fourier transform infrared spectroscopy (GATR-FTIR), which shows distinct vibrational modes associated with the catalyst, as well as X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) that confirm the presence of intact Co(III) complex on the surface. The Co-functionalized photocathode shows significantly enhanced photoelectrochemical (PEC) performance in aqueous conditions at neutral pH, compared to results obtained on GaP without attached cobalt complex. PEC measurements, at 100 mW cm(-2) illumination, yield a 2.4 mA cm(-2) current density at a 310 mV underpotential.