Driven Water Oxidation Research Articles

High temperature activation of hematite nanorods for sunlight driven water oxidation reaction.

Here we show that chlorine species originating from commonly used iron precursors annihilate the hematite nanorod photocurrent by providing recombination pathways. Although hematite nanorod films could be obtained by thermal decomposition of the iron oxyhydroxide phase (β-FeOOH), indistinguishable photocurrent responses under dark and sunlight irradiation conditions were observed until the nanorods were annealed (activated) at 750 °C. The annealing led to the elimination of observable chlorine species and allowed photocurrent responses of 1.3 mA cm-2 at 1.23 V vs. RHE, which is comparable to the best results found in the literature, suggesting that residual chlorine species from the synthesis can act as electron traps and recombination sites for photogenerated holes.

Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine-Dicarboxylate Complexes.

In order to gain a deeper mechanistic understanding of water oxidation by [(bda)Ru(L)2] catalysts (bdaH2 = [2,2'-bipyridine]-6,6'-dicarboxylic acid; L = pyridine-type ligand), a series of modified catalysts with one and two trifluoromethyl groups in the 4 position of the bda2- ligand was synthesized and studied using stopped-flow kinetics. The additional -CF3 groups increased the oxidation potentials for the catalysts and enhanced the rate of electrocatalytic water oxidation at low pH. Stopped-flow measurements of cerium(IV)-driven water oxidation at pH 1 revealed two distinct kinetic regimes depending on catalyst concentration. At relatively high catalyst concentration (ca. ≥10-4 M), the rate-determining step (RDS) was a proton-coupled oxidation of the catalyst by cerium(IV) with direct kinetic isotope effects (KIE > 1). At low catalyst concentration (ca. ≤10-6 M), the RDS was a bimolecular step with kH/kD ≈ 0.8. The results support a catalytic mechanism involving coupling of two catalyst molecules. The rate constants for both RDSs were determined for all six catalysts studied. The presence of -CF3 groups had inverse effects on the two steps, with the oxidation step being fastest for the unsubstituted complexes and the bimolecular step being faster for the most electron-deficient complexes. Though the axial ligands studied here did not significantly affect the oxidation potentials of the catalysts, the nature of the ligand was found to be important not only in the bimolecular step but also in facilitating electron transfer from the metal center to the sacrificial oxidant.

Rutile TiO2 as an Anode Material for Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use a wide bandgap metal oxide semiconductor functionalized with a light-absorbing dye and water-oxidation catalyst to harvest light and drive water oxidation, respectively. We demonstrate here that the rutile polymorph of TiO2 (r-TiO2) is a promising anode material for WS-DSPECs. Recombination between the injected electron and oxidized sensitizer with r-TiO2 is an order of magnitude slower than with anatase TiO2 (a-TiO2), with injection yields approaching 100%. Studies with a reductive quencher demonstrate that r-TiO2 is significantly more efficient than a-TiO2, while exhibiting greater dye stability. Furthermore, comparison of direct band gap excitation photocurrent generation for bare and sensitized r-TiO2 suggests that the sensitizer functions as a light harvester and redox mediator.

Treated Nanolayered Mn Oxide by Oxidizable Compounds: A Strategy To Improve the Catalytic Activity toward Water Oxidation.

Herein, we investigate the effect of post-treatment of nanolayered manganese oxide by different inorganic and organic compounds. We use the fact that nanolayered manganese oxides are among the strongest naturally occurring oxidants, capable of oxidizing a wide range of organic molecules. Post-treatment of the synthetic Mn oxides with oxidizable compounds increases the cerium(IV)-driven water oxidation catalyzed by treated layered manganese oxides more than 25 times. On the basis of X-ray absorption investigations, we attribute this effect to the increased amount of manganese(III) ions. This finding can explain some puzzles in water oxidation by manganese oxides and may help to advance toward an efficient design strategy of water-oxidizing catalyst in artificial photosynthetic systems.

Ruthenium Complexes of Substituted Terpyridine and Pyridyl-quinoline Based Ligands with Ancillary Ligands: Synthesis, Characterization, Electrochemical Study and DFT Calculation

Herein we report three novel ruthenium(II) complexes of general formula [Ru(LL′L“)](PF6)n (where L= 4’-(9-Anthryl)-2,2’:6’,2”-terpyridine, L′=2-(2-pyridyl)-4-carboxyquinoline (pcqH) and L”=Cl, n=1 (for 1), L“=SCN, n=1 (for 2) and L”=H2O, n=2 (for 3)). All the complexes were characterized by routine spectroscopic techniques e. g., FTIR, UV-Vis, mass and NMR. A detailed pH based spectroscopic and electrochemical analysis for the complexes 1,2 and 3 were carried out to determine the pka of carboxyl and aqua moiety coordinated to ruthenium centre. The COOH group in complex 2 (pkCOOH=3.13) is slightly more acidic than in complex 3 (pkCOOH=3.98). Whereas pkH2O for complex 1 (12.33) and 3 (12.27) are comparable as expected. A Pourbaix diagram for the aqua complex has been reported. The lability of the Chloro moiety in aqueous medium has been corroborated with D2O exchange 1H-NMR study. Photovoltaic measurements for the Chloro and thiocyanato complex are also carried out. During the pH based spectrochemical and electrochemical study we find in-situ conversion of the aqua complex into hydroxo complex. A detailed TDDFT analysis of all the three synthesized complexes and in-situ generated hydroxo complex in the aqueous medium at higher pH has been performed to explain the various experimental findings. In the electronic spectrum all the molecules show a MLCT transition in the visible region (1: 529 nm, 2: 511 nm, 3: 488 nm). The remarkable blue shift on going from 1 to 2 to 3 is because of the decreasing stabilizing effect of the dπ (Ru) orbital (H2O > SCN > Cl). All the electronic transitions have been nicely correlated with the TDDFT analysis. Preliminary electrochemical experiment indicates that the aqua complex 3, may act as a promising catalyst for light driven water oxidation.

Heterogenized Iridium Water-Oxidation Catalyst from a Silatrane Precursor

A pentamethylcyclopentadienyl (Cp*) iridium water-oxidation precatalyst was modified to include a silatrane functional group for covalent attachment to metal oxide semiconductor surfaces. The heterogenized catalyst was found to perform electrochemically driven water oxidation at an overpotential of 462 mV with a turnover number of 304 and turnover frequency of 0.035 s–1 in a 0.1 M KNO3 electrolyte at pH 5.8. Computational modeling of the experimental IR spectra suggests that the catalyst retains its Cp* group during the first hour of catalysis and likely remains monomeric.

The PsbY protein of Arabidopsis Photosystem II is important for the redox control of cytochrome b559

Photosystem II is a protein complex embedded in the thylakoid membrane of photosynthetic organisms and performs the light driven water oxidation into electrons and molecular oxygen that initiate the photosynthetic process. This important complex is composed of more than two dozen of intrinsic and peripheral subunits, of those half are low molecular mass proteins. PsbY is one of those low molecular mass proteins; this 4.7–4.9kDa intrinsic protein seems not to bind any cofactors. Based on structural data from cyanobacterial and red algal Photosystem II PsbY is located closely or in direct contact with cytochrome b559. Cytb559 consists of two protein subunits (PsbE and PsbF) ligating a heme-group in-between them. While the exact function of this component in Photosystem II has not yet been clarified, a crucial role for assembly and photo-protection in prokaryotic complexes has been suggested. One unique feature of Cytb559 is its redox-heterogeneity, forming high, medium and low potential, however, neither origin nor mechanism are known. To reveal the function of PsbY within Photosystem II of Arabidopsis we have analysed PsbY knock-out plants and compared them to wild type and to complemented mutant lines. We show that in the absence of PsbY protein Cytb559 is only present in its oxidized, low potential form and plants depleted of PsbY were found to be more susceptible to photoinhibition.

Mesoporous Mn1.8Fe1.2O4 nanocubes as robust catalysts for water oxidation

We report mesoporous Mn1.8Fe1.2O4 nanocubes that are synthesized through simple self-assembly and calcination. By changing the calcination temperature, we investigated the correlation between the nanostructure and catalytic performance of mesoporous Mn1.8Fe1.2O4 catalysts. Mesoporous Mn1.8Fe1.2O4 exhibited high oxygen evolution activity in both the photochemical and cerium (IV)-driven water oxidation systems. The highest initial turnover frequency (TOF) of 4.6×10−4s−1 per metal atom is obtained at neutral pH in photocatalytic water oxidation reaction, compared with those nonporous MnFe2O4, Fe3O4, and Mn3O4. In a cerium (IV)-driven environment, a high TOF of 1.8×10−3s−1 per metal atom is achieved, which is at least two orders of magnitude more active than that of nonporous MnFe2O4. Multiple experimental results (e.g., XRD, FT-IR, SEM, TEM, and XPS) confirmed that mesoporous Mn1.8Fe1.2O4 materials are highly stable. Our results provide a facile method for synthesis of an inexpensive and highly active heterogeneous catalyst for the oxygen evolution reaction.

Reaction Progress Kinetic Analysis as a Tool To Reveal Ligand Effects in Ce(IV)-Driven IrCp*-Catalyzed Water Oxidation

A series of iridium-based complexes have been evaluated in Ce(IV)-driven water oxidation catalysis. Detailed kinetic data have been obtained from UV–vis stopped-flow experiments, and these data have been analyzed using reaction progress kinetic analysis. The graphical plots show that there are three clear phases in the reaction: catalyst activation, water oxidation catalysis, and cerium concentration controlled catalysis at the end of the reaction. The ligand attached to the IrCp* complex has a clear influence on both the activation as well as the catalysis. Some bidentate ligands result in relatively slow catalysis, and the first-order in iridium supports the presence of mononuclear active species; however, other bidentates form the more active dinuclear species. Monodentate ligands allow the formation of bis-μ-oxo bridged dimeric species, supported by kinetics displaying 1.6-order in [Ir], leading to high reaction rates.

Ruthenium based photosensitizer/catalyst supramolecular architectures in light driven water oxidation

Light driven water oxidation is a key step in artificial photosynthesis, aimed at splitting water into hydrogen and oxygen with sunlight. In such process, the interactions between a photosensitizer (PS) and a water oxidation catalyst (WOC) play a crucial role in the rates of photoinduced electron transfers, determining the overall quantum efficiency of the system. In this work, by means of Small Angle X-ray Scattering (SAXS) we investigate the nature of the aggregates between ruthenium polypyridine photosensitizers (Rubpy and Ru4dend) and a tetraruthenium polyoxometalate (Ru4POM) water oxidation catalyst. Aggregate scattering is confirmed by the strong intensity-increase in the low-q regime, whereas the power law-fit of this region show slopes between −3 and −4, suggesting globular and porous aggregates. Intermolecular PS/WOC distances lower than 3nm support the observed fast photoinduced electron transfers (<120ps), however the proximity of the two components in the hybrids is also responsible for fast charge recombination. Approaches for inhibiting such undesired process are discussed.

Highly efficient electrochemically driven water oxidation by graphene-supported mixed-valent Mn16-containing polyoxometalate

A highly efficient catalyst of graphene-supported mixed-valent Mn16-containing polyoxometalate is reported here by electrochemical strategy. The modified electrode with the catalyst exhibits an excellent electrocatalytic performance for water oxidation, which will contribute to the development of highly efficient catalysts for oxygen evolution.

A Molecular Tetrad That Generates a High-Energy Charge-Separated State by Mimicking the Photosynthetic Z-Scheme.

The oxygenic photosynthesis of green plants, green algae, and cyanobacteria is the major provider of energy-rich compounds in the biosphere. The so-called "Z-scheme" is at the heart of this "engine of life". Two photosystems (photosystem I and II) work in series to build up a higher redox ability than each photosystem alone can provide, which is necessary to drive water oxidation into oxygen and NADP(+) reduction into NADPH with visible light. Here we show a mimic of the Z-scheme with a molecular tetrad. The tetrad Bodipy-NDI-TAPD-Ru is composed of two different dyes-4,4-difluoro-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (Bodipy) and a Ru(II)(bipyridine)3 (Ru) derivative-which are connected to a naphthalene diimide (NDI) electron acceptor and tetraalkylphenyldiamine (TAPD) playing the role of electron donor. A strong laser pulse excitation of visible light where the two dye molecules (Ru and Bodipy) absorb with equal probability leads to the cooperative formation of a highly energetic charge-separated state composed of an oxidized Bodipy and a reduced Ru. The latter state cannot be reached by one single-photon absorption. The energy of the final charge-separated state (oxidized Bodipy/reduced Ru) in the tetrad lies higher than that in the reference dyads (Bodipy-NDI and TAPD-Ru), leading to the energy efficiency of the tetrad being 47% of the sum of the photon threshold energies. Its lifetime was increased by several orders of magnitude compared to that in the reference dyads Bodipy-NDI and TAPD-Ru, as it passes from about 3 ns in each dyad to 850 ns in the tetrad. The overall quantum yield formation of this extended charge-separated state is estimated to be 24%. Our proof-of-concept result demonstrates the capability to translate a crucial photosynthetic energy conversion principle into man-made molecular systems for solar fuel formation, to obtain products of higher energy content than those produced by a single photon absorption.

Visible light-driven water oxidation using a covalently-linked molecular catalyst-sensitizer dyad assembled on a TiO2 electrode.

The combination of porphyrin as a sensitizer and a ruthenium complex as a water oxidation catalyst (WOC) is promising to exploit highly efficient molecular artificial photosynthetic systems. A covalently-linked ruthenium-based WOC-zinc porphyrin (ZnP) sensitizer dyad was assembled on a TiO2 electrode for visible-light driven water oxidation. The water oxidation activity was found to be improved in comparison to the reference systems with the simple combination of the individual WOC and ZnP as well as with ZnP solely, demonstrating the advantage of the covalent linking approach over the non-covalent one. More importantly, via vectorial multi-step electron transfer triggered by visible light, the dye-sensitized photoelectrochemical cell (DSPEC) achieved a broader PEC response in the visible region than DSPECs with conventional ruthenium-based sensitizers. Initial incident photon-to-current efficiencies of 18% at 424 nm and 6.4% at 564 nm were attained under monochromatic illumination and an external bias of -0.2 V vs. NHE. Fast electron transfer from the WOC to the photogenerated radical cation of the sensitizer through the covalent linkage may suppress undesirable charge recombination, realizing the moderate performance of water oxidation. X-ray photoelectron spectroscopic analysis of the photoanodes before and after the DSPEC operation suggested that most of the ruthenium species exist at higher oxidation states, implying that the insufficient oxidation potential of the ZnP moiety for further oxidizing the intermediate ruthenium species at the photoanode is at least the bottleneck of the system.

Tantalum nitride films integrated with transparent conductive oxide substrates via atomic layer deposition for photoelectrochemical water splitting.

Tantalum nitride, Ta3N5, is one of the most promising materials for solar energy driven water oxidation. One significant challenge of this material is the high temperature and long duration of ammonolysis previously required to synthesize it, which has so far prevented the use of transparent conductive oxide (TCO) substrates to be used which would allow sub-bandgap light to be transmitted to a photocathode. Here, we overcome this challenge by utilizing atomic layer deposition (ALD) to directly deposit tantalum oxynitride thin films, which can be fully converted to Ta3N5via ammonolysis at 750 °C for 30 minutes. This synthesis employs far more moderate conditions than previous reports of efficient Ta3N5 photoanodes. Further, we report the first ALD of Ta-doped TiO2 which we show is a viable TCO material that is stable under the relatively mild ammonolysis conditions employed. As a result, we report the first example of a Ta3N5 electrode deposited on a TCO substrate, and the photoelectrochemical behavior. These results open the door to achieve efficient overall water splitting using a Ta3N5 photoanode.

Efficient Photochemical, Thermal, and Electrochemical Water Oxidation Catalyzed by a Porous Iron-Based Oxide Derived Metal–Organic Framework

Iron-based catalysts are of particular interest for water oxidation because of their high abundance, low toxicity, and rich redox properties. Herein, we report low cost porous iron-based oxides derived from calcining precursors of Prussian blue analogue (PBA) Mx[Fe(CN)6]y (M = Fe, Co, Ni). This synthesis approach involves a simple self-assembly technology and a low-temperature annealing procedure. These catalysts were investigated for photocatalytic, cerium(IV)-driven, and electrochemical water oxidation, and they exhibited superior activity. It is noteworthy that this photocatalytic water oxidation was conducted under neutral conditions that are similar to the natural photosystem II. The high initial turnover frequency (TOF) of ∼5.4 × 10–4 s–1 per transition metal atom at the first 60 s is obtained under neutral pH using porous CoxFe3–xO4 in photocatalytic water oxidation reaction, which is comparable with those published iron-based catalysts. Under cerium(IV)-driven water oxidation conditions, the TOF o...

Single Site Isomeric Ru WOCs with an Electron-Withdrawing Group: Synthesis, Electrochemical Characterization, and Reactivity.

The synthetic intermediate cis(out),cis-[Ru(Cl)2(HL)(DMSO)2], 1 (DMSO = dimethyl sulfoxide), and four new mononuclear ruthenium complexes with general formula out/in-[Ru(HL)(trpy)(X)](m+) (trpy = 4-tert-butylpyridine; X = Cl(-), m = 1, 2a(+) and 2b(+); X = H2O, m = 2, 3a(2+) and 3b(2+)) based on the ligand 1H-pyrazole-3-carboxylic acid, 5-(2-pyridinil)-, ethyl ester (HL), are synthesized and characterized by analytical, spectroscopic, and electrochemical methods. A linkage isomerism is observed for a DMSO moiety of 1, and relevant thermodynamics and kinetics values are obtained through electrochemical experiments and compared to literature. Different synthetic routes are developed to obtain isomeric 2a(+) and 2b(+), with different relative yields. Water oxidation activity of 3a(2+) and 3b(2+) is analyzed by means of electrochemical methods, through foot of the wave analysis, yielding kobs values of 1.00 and 2.23 s(-1), respectively. Chemically driven water oxidation activity is tested using [(NH4)2Ce(NO3)6] as sacrificial electron acceptor, and turnover number (TON) and turnover frequency (TOF) values of TON = 10.8 and TOFi = 58.2 × 10(-3) s(-1) for 3a(2+) and TON = 4.2 and TOFi = 15.4 × 10(-3) s(-1) for 3b(2+) are obtained.

Metal-Organic Frameworks as Promising Photosensitizers for Photoelectrochemical Water Splitting.

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.

Ultrahigh efficient water oxidation under visible light: Using Fe dopants to integrate nanostructure and cocatalyst in LaTiO2N system

Solar light driven water oxidation is the key raction for artificial photocatalysis. The light driven water oxidation on LaTiO2N nanostructures do not exhibit high efficiency. In this work, we design a new strategy that using the Fe dopants, which could create empty mid-gap states, to tune the energy band matching between the cocatalysts and LaTiO2N nanostructures. This strategy significantly increase the photo-generated carriers’ separation and it makes the CoOx/Fe doped LaTiO2N nanostructures show 55% quantum efficiency for O2 generation under visible light irradiation.

Iridium(III) Bis-Pyridine-2-Sulfonamide Complexes as Efficient and Durable Catalysts for Homogeneous Water Oxidation.

A family of tetradentate bis(pyridine-2-sulfonamide) (bpsa) compounds was synthesized as a ligand platform for designing resilient and electronically tunable catalysts capable of performing water oxidation catalysis and other processes in highly oxidizing environments. These wrap-around ligands were coordinated to Ir(III) octahedrally, forming an anionic complex with chloride ions bound to the two remaining coordination sites. NMR spectroscopy documented that the more rigid ligand frameworks-[Ir(bpsa-Cy)Cl2](-) and [Ir(bpsa-Ph)Cl2](-)-produced C1-symmetric complexes, while the complex with the more flexible ethylene linker in [Ir(bpsa-en)Cl2](-) displays C2 symmetry. Their electronic structure was explored with DFT calculations and cyclic voltammetry in nonaqueous environments, which unveiled highly reversible Ir(III)/Ir(IV) redox processes and more complex, irreversible reduction chemistry. Addition of water to the electrolyte revealed the ability of these complexes to catalyze the water oxidation reaction efficiently. Electrochemical quartz crystal microbalance studies confirmed that a molecular species is responsible for the observed electrocatalytic behavior and ruled out the formation of active IrOx. The electrochemical studies were complemented by work on chemically driven water oxidation, where the catalytic activity of the iridium complexes was studied upon exposure to ceric ammonium nitrate, a strong, one-electron oxidant. Variation of the catalyst concentrations helped to illuminate the kinetics of these water oxidation processes and highlighted the robustness of these systems. Stable performance for over 10 days with thousands of catalyst turnovers was observed with the C1-symmetric catalysts. Dynamic light scattering experiments ascertained that a molecular species is responsible for the catalytic activity and excluded the formation of IrOx particles.

P-Type Transparent Conducting Oxide/n-Type Semiconductor Heterojunctions for Efficient and Stable Solar Water Oxidation.

Achieving stable operation of photoanodes used as components of solar water splitting devices is critical to realizing the promise of this renewable energy technology. It is shown that p-type transparent conducting oxides (p-TCOs) can function both as a selective hole contact and corrosion protection layer for photoanodes used in light-driven water oxidation. Using NiCo2O4 as the p-TCO and n-type Si as a prototypical light absorber, a rectifying heterojunction capable of light driven water oxidation was created. By placing the charge separating junction in the Si using a np(+) structure and by incorporating a highly active heterogeneous Ni-Fe oxygen evolution catalyst, efficient light-driven water oxidation can be achieved. In this structure, oxygen evolution under AM1.5G illumination occurs at 0.95 V vs RHE, and the current density at the reversible potential for water oxidation (1.23 V vs RHE) is >25 mA cm(-2). Stable operation was confirmed by observing a constant current density over 72 h and by sensitive measurements of corrosion products in the electrolyte. In situ Raman spectroscopy was employed to investigate structural transformation of NiCo2O4 during electrochemical oxidation. The interface between the light absorber and p-TCO is crucial to produce selective hole conduction to the surface under illumination. For example, annealing to produce more crystalline NiCo2O4 produces only small changes in its hole conductivity, while a thicker SiOx layer is formed at the n-Si/p-NiCo2O4 interface, greatly reducing the PEC performance. The generality of the p-TCO protection approach is demonstrated by multihour, stable, water oxidation with n-InP/p-NiCo2O4 heterojunction photoanodes.