Light-driven Hydrogen Generation Research Articles

Functionalized Vesicles with Co-Embedded CdSe Quantum Dots and [FeFe]-Hydrogenase Mimic for Light-Driven Hydrogen Production

A photocatalytic hybrid system for the production of hydrogen in purely aqueous solution was prepared by the co-embedding of hydrophobic oleic acid capped CdSe quantum dots (QD) for light harvesting and an amphiphilic [FeFe]-hydrogenase subunit mimic for proton reduction into phospholipid vesicle membranes. These bio-inspired functionalized vesicles concentrate the catalytic subunits in close proximity and show light-driven hydrogen generation in the presence of ascorbic acid resulting in 651 and 2.47 x 105 catalytic turnovers for the membrane-incorporated catalyst and the QDs, respectively.

In Situ Fluorine Doping of TiO2 Superstructures for Efficient Visible-Light Driven Hydrogen Generation.

With the aid of breakthroughs in nanoscience and nanotechnology, it is imperative to develop metal oxide semiconductors through visible light-driven hydrogen generation. In this study, TiOF2 was incorporated as an n-type F-dopant source to TiO2 mesocrystals (TMCs) with visible-light absorption during the topotactic transformation. The crystal growth, structural change, and dynamic morphological evolution, from the initial intermediate NH4 TiOF3 to HTiOF3, TiOF2, and F-doped TMCs, were verified through in situ temperature-dependent techniques to elucidate the doping mechanism from intermediate TiOF2. The visible-light efficiencies of photocatalytic hydrogen were dependent on the contents of the dopant as compared with the pure TMC and a controled reference. Using femtosecond time-resolved diffuse reflectance spectroscopy, the charge-transfer dynamics were monitored to confirm the improvement of charge separation after doping.

ZnS/CuS nanotubes for visible light-driven photocatalytic hydrogen generation

ZnS/CuS nanotubes exhibit visible-light photocatalytic activity for H2 evolution without cocatalysts, resulting from the heterojunctions between ZnS and CuS nanoparticles.

Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes

New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine dithiolate (PtN2S2) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H2 from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN2S2 CT chromophore. Each of the new dyads contains either a bipyridine (bpy) or phenanthroline (phen) diimine with an attached functional group that is used for binding directly to TiO2 nanoparticles, allowing rapid electron photoinjection into the semiconductor. The absorption spectra and cyclic voltammograms of the dyads show that the spectroscopic and electrochemical properties of the dyads are the sum of the individual chromophores (Bodipy and the PtN2S2 moieties), indicating little electronic coupling between them. Connection to TiO2 nanoparticles is carried out by sonication leading to in situ attachment to TiO2 without prior hydrolysis of the ester linking groups to acids. For H2 generation studies, the TiO2 particles are platinized (Pt-TiO2) so that the light absorber (the dyad), the electron conduit (TiO2), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H2O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO2 via a phosphonate or carboxylate attachment shows excellent light-driven H2 production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼ 40,000 turnover numbers of H2 over 12 d (with respect to dye).

Copper-based water reduction catalysts for efficient light-driven hydrogen generation

Although largely neglected, convenient and commercially available copper salts constitute appropriate precursors for efficient and stable water reduction catalysts (WRC). Various copper salts have been applied in the photocatalytic proton reduction together with [Ir(ppy)2(bpy)]PF6 as photosensitizer (PS) and triethylamine (TEA) as electron donor. Among them, copper(I) iodide showed the best productivity, resulting in a TONCu of 260 and a stability of 4 days. The addition of 2,2′-bipyridine (bpy) led to a significant improvement of the catalytic system. Application of 2equiv. of bpy and an increased amount of PS improved the stability as well as the productivity (TONCu 711). Remarkably, this catalytic system was reactivated seven times with only further addition of fresh PS and TEA. Consequently, this copper-based WRC was shown to be active for more than 48 days reaching a maximum TONCu of >3900, which is in the same order of magnitude as previously known noble metal WRCs. Aberration-corrected HAADF-STEM measurements revealed a part transformation of the molecularly-defined copper precursor into Cu containing nanoparticles of 5–10nm size. Additionally, by XAS measurements the conversion of the copper(I) salts into Cu(0) compounds under light irradiation was observed. With respect to low costs and high abundance of copper these WRCs are an appropriate alternative for noble metal based catalysts.

Investigation of the Ultrafast Dynamics Occurring during Unsensitized Photocatalytic H2 Evolution by an [FeFe]-Hydrogenase Subsite Analogue

Biomimetic compounds based upon the active subsite of the [FeFe]-hydrogenase enzyme system have been the focus of much attention as catalysts for hydrogen production: a clean energy vector. Until recently, use of hydrogenase subsite systems for light-driven hydrogen production has typically required the involvement of a photosensitizer, but the molecule [(μ-pdt)(μ-H)Fe2(CO)4(dppv)]+, (1; dppv = cis-1,2-C2H2(PPh2)2; pdt = 1,3-propanedithiolate) has been reported to catalyze the evolution of hydrogen gas under sensitizer-free conditions. Establishing the molecular mechanism that leads to photohydrogen production by 1 is thus an important step that may enable further development of this family of molecules as solar fuel platforms. Here, we report ultrafast UVpump–IRprobe spectroscopy of 1 at three different excitation wavelengths and in a range of solvents, including under the conditions required for H2 production. Combining spectroscopic measurements of the photochemistry and vibrational relaxation dynamics of 1 with ground-state density functional theory (DFT) calculations shows that, irrespective of experimental conditions, near-instantaneous carbonyl ligand loss is the main photochemical channel. No evidence for a long-lived excited electronic state was found. These results provide the first time-resolved data for the photochemistry of 1 and offer an alternative interpretation of the underlying mechanism of light-driven hydrogen generation.

Nickel Pyridinethiolate Complexes as Catalysts for the Light-Driven Production of Hydrogen from Aqueous Solutions in Noble-Metal-Free Systems

A series of mononuclear nickel(II) thiolate complexes (Et4N)Ni(X-pyS)3 (Et4N = tetraethylammonium; X = 5-H (1a), 5-Cl (1b), 5-CF3 (1c), 6-CH3 (1d); pyS = pyridine-2-thiolate), Ni(pySH)4(NO3)2 (2), (Et4N)Ni(4,6-Y2-pymS)3 (Y = H (3a), CH3 (3b); pymS = pyrimidine-2-thiolate), and Ni(4,4'-Z-2,2'-bpy)(pyS)2 (Z = H (4a), CH3 (4b), OCH3 (4c); bpy = bipyridine) have been synthesized in high yield and characterized. X-ray diffraction studies show that 2 is square planar, while the other complexes possess tris-chelated distorted-octahedral geometries. All of the complexes are active catalysts for both the photocatalytic and electrocatalytic production of hydrogen in 1/1 EtOH/H2O. When coupled with fluorescein (Fl) as the photosensitizer (PS) and triethylamine (TEA) as the sacrificial electron donor, these complexes exhibit activity for light-driven hydrogen generation that correlates with ligand electron donor ability. Complex 4c achieves over 7300 turnovers of H2 in 30 h, which is among the highest reported for a molecular noble metal-free system. The initial photochemical step is reductive quenching of Fl* by TEA because of the latter's greater concentration. When system concentrations are modified so that oxidative quenching of Fl* by catalyst becomes more dominant, system durability increases, with a system lifetime of over 60 h. System variations and cyclic voltammetry experiments are consistent with a CECE mechanism that is common to electrocatalytic and photocatalytic hydrogen production. This mechanism involves initial protonation of the catalyst followed by reduction and then additional protonation and reduction steps to give a key Ni-H(-)/N-H(+) intermediate that forms the H-H bond in the turnover-limiting step of the catalytic cycle. A key to the activity of these catalysts is the reversible dechelation and protonation of the pyridine N atoms, which enable an internal heterocoupling of a metal hydride and an N-bound proton to produce H2.

Graphene‐Based Materials for Hydrogen Generation from Light‐Driven Water Splitting

Hydrogen production from solar water splitting has been considered as an ultimate solution to the energy and environmental issues. Over the past few years, graphene has made great contribution to improving the light-driven hydrogen generation performance. This article provides a comprehensive overview of the recent research progress on graphene-based materials for hydrogen evolution from light-driven water splitting. It begins with a brief introduction of the current status and basic principles of hydrogen generation from solar water splitting, and tailoring properties of graphene for application in this area. Then, the roles of graphene in hydrogen generation reaction, including an electron acceptor and transporter, a cocatalyst, a photocatalyst, and a photosensitizer, are elaborated respectively. After that, the comparison between graphene and other carbon materials in solar water splitting is made. Last, this review is concluded with remarks on some challenges and perspectives in this emerging field.

High-Performance Bioassisted Nanophotocatalyst for Hydrogen Production

Nanophotocatalysis is one of the potentially efficient ways of capturing and storing solar energy. Biological energy systems that are intrinsically nanoscaled can be employed as building blocks for engineering nanobio-photocatalysts with tunable properties. Here, we report upon the application of light harvesting proton pump bacteriorhodopsin (bR) assembled on Pt/TiO2 nanocatalyst for visible light-driven hydrogen generation. The hybrid system produces 5275 μmole of H2 (μmole protein)(-1) h(-1) at pH 7 in the presence of methanol as a sacrificial electron donor under white light. Photoelectrochemical and transient absorption studies indicate efficient charge transfer between bR protein molecules and TiO2 nanoparticles.

Sacrificial Hydrogen Generation from Formaldehyde with Pt/TiO2 Photocatalyst in Solar Radiation

In this work, solar TiO2 photocatalysis was used for sacrificial hydrogen generation from formaldehyde. Platinum was loaded onto a TiO2 photocatalyst by a solar photodeposition method to suppress the electron/hole recombination process. The photocatalyst inside the reactor was irradiated from the top with a solar simulator. Photocatalytic hydrogen generation from formaldehyde was influenced by the solution pH, platinum loading (wt %) on TiO2, catalyst concentration, light intensity, and initial formaldehyde concentration. A Langmuir-type model fitted well with the experimental data, and the values of surface reaction rate constant, k, and the adsorption equilibrium constant, K, are 2.3598 × 10–6 mol min–1 and 17.73 M–1, respectively. Apparent quantum yield (QY) was higher for the UV light-driven hydrogen generation (10.91%) compared to the solar light-driven hydrogen generation (1.24%).

Structure and Electronic Configurations of the Intermediates of Water Oxidation in a Highly Active and Robust Molecular Ruthenium Catalyst

Photosynthesis is one of the most important chemical processes on our planet. Splitting water into O2 and H2 using sunlight is a clean and sustainable proposition for addressing energy and environmental problems encountered in our society. Mimicking this reaction in a manmade device will allow for sunlight-to-energy conversion with water providing electrons and protons for production of chemical fuels. About 30 years ago Meyer and coworkers reported the first ruthenium-based catalyst for water oxidation, known as the “blue dimer”. This catalyst may be considered as an artificial analog of the oxygen-evolving complex (OEC) in Photosystem II (PS II). Recently it has been demonstrated that single-site Ru and Ir catalysts are also active in water oxidation. These single-site catalysts are attractive model compounds for both experimental and theoretical studies of mechanism of water oxidation and show improved catalytic activity compared to “blue dimer”. A better understanding of this mechanism and identification of the rate-limiting steps could pave the way to light-driven generation of molecular hydrogen by water splitting.A mononuclear ruthenium complex [Ru(bda)(pic)2] (pic=4-picoline) was found to show high catalytic activity as well as high chemical stability. EPR, Raman and XAS characterization of the electronic structure and molecular geometry of Ru(III)-OH2,Ru(IV)=O and Ru(IV)-OO-Ru(IV) are reported in the single site water oxidizing complex. Formation of metal bound peroxides as the result of O-O coupling has been implicated in the mechanism of catalytic water oxidation by Photosystem II oxygen evolving complex (OEC) and in Ru-based catalysts. However, such intermediates were never isolated and their structural and electronic characterization has not been reported. We believe that the intermediates described here are direct products of the O-O bond formation step in the studied catalyst.

The influence of carbon content on the structure and properties of MoSxCyphotocatalysts for light-driven hydrogen generation

Carbon containing nano-sized molybdenum sulfide composites (MoS(x)C(y)) obtained by thermal decomposition reactions of (R(4)N)(2)MoS(4) (R = -H (C(0)), -CH(3) (C(1)), -C(3)H(7) (C(3)), and -C(6)H(13) (C(6))) show promising performance in visible-light driven photocatalytic hydrogen generation.

Caught in the act

Femtobiology freeze‐frames crucial split seconds of chemical reactions to investigate how enzymes function. The potential prize from this knowledge could be new avenues for drug development or ways to produce clean energy.

Photochemical hydrogen production with molecular devices comprising a zinc porphyrin and a cobaloxime catalyst

Two new noble-metal-free molecular devices, [{Co(dmgH)2Cl}{Zn(PyTPP)}] (1, dmgH = dimethyloxime, PyTPP = 5-(4-pyridyl)-10,15,20-triphenylporphyrin) and [{Co(dmgH)2Cl}{Zn(apPyTPP)}] (2, apPyTPP = 5-[4-(isonicotinamidyl)phenyl]-10,15,20-triphenylporphyrin), for light-driven hydrogen generation were prepared and spectroscopically characterized. The zinc porphyrin photosensitizer and the CoIII-based catalyst unit are linked by axial coordination of a pyridyl group in the periphery of zinc-porphyrin to the cobalt centre of catalyst with different lengths of bridges. The apparent fluorescence quenching and lifetime decays of 1 and 2 were observed in comparison with their reference chromophores, Zn(PyTPP) (3) and Zn(apPyTPP) (4), suggesting a possibility for an intramolecular electron transfer from the singlet excited state of zinc porphyrin unit to the cobalt centre in the molecular devices. Photochemical H2-evolving studies show that complexes 1 and 2 are efficient molecular photocatalysts for visible light-driven H2 generation from water with triethylamine as a sacrificial electron donor in THF/H2O, with turnover numbers up to 46 and 35 for 1 and 2, respectively. In contrast to these molecular devices, the multicomponent catalyst of zinc porphyrin and [Co(dmgH)2PyCl] did not show any fluorescence quenching and as a consequence, no H2 gas was detected by GC analysis in the presence of triethylamine with irradiation of visible light. The plausible mechanism for the photochemical H2 generation with these molecular devices is discussed.

Noncovalent Assembly of a Metalloporphyrin and an Iron Hydrogenase Active-Site Model: Photo-Induced Electron Transfer and Hydrogen Generation

A noncovalent assembly of a pyridyl-functionalized hydrogenase active-site model complex and zinc tetraphenylporphyrin has been obtained and characterized. Upon light irradiation, fluorescence quenching by electron transfer was observed from the singlet excited state of the porphyrin to the diiron center, and the mechanism was verified by fluorescence lifetime and transient absorption spectroscopic measurements. In contrast to molecular dyads linked by covalent bonds, the assembled system was designed to avoid charge recombination via complex dissociation after photo-induced electron transfer. Visible light-driven hydrogen generation was observed from this self-assembled system. The assembling strategy employed in this study has the potential to be used for any other hydrogenase models in the future.

Visible Light-Driven Electron Transfer and Hydrogen Generation Catalyzed by Bioinspired [2Fe2S] Complexes

Complexes [{(mu-SCH2)2NCH2C6H5}{Fe(CO)2L(1)}{Fe(CO)2L(2)}] (L(1) = CO, L(2) = P(Pyr) 3, 2; L(1) = L(2) = P(Pyr)3, 3) were prepared, which have the lowest reduction potentials for the mono- and double-CO-displaced diiron complexes reported so far. Hydrogen evolution, driven by visible light, was successfully observed for a three-component system, consisting of a ruthenium polypyridine complex, the biomimetic model complex 2 or 3, and ascorbic acid as both electron and proton donor in CH3CN/H2O. The electron transfer from photogenerated Ru(bpy)3(+) to 2 or 3 was detected by laser flash photolysis. Under optimal conditions, the total turnover number for hydrogen evolution was 4.3 based on 2 and 86 based on Ru(bpy)3(2+) in a three-hour photolysis.