Photochemical Hydrogen Generation Research Articles

Management of Triplet States in Modified Mononuclear Ruthenium(II) Complexes for Enhanced Photocatalysis.

Controlling the interplay between relaxation and charge/energy transfer processes in the excited states of photocatalysts is crucial for the performance of artificial photosynthesis. Metal-to-ligand charge-transfer triplet states (3MLCT*) of ruthenium(II) complexes are broadly implemented for photocatalysis, but an effective means of managing the triplets for enhanced photocatalysis has been lacking. Herein, We proposed a strategy to considerably prolong the triplet excited-state lifetime by decorating a ruthenium(II) phosphine complex (RuP-1) with pendent polyaromatic hydrocarbons (PAHs). Systematic studies demonstrate that in RuP-4 decorated with anthracene, sub-picosecond electron transfer from anthracene to 3MLCT* leads to a charge-separated state that can mediate the formation of the intra-ligand triplet state (3IL) of anthracene, resulting in an exceptionally long excited-state up to several milliseconds. This triplet management strategy enables impressive photocatalytic reduction of CO2 to CO with a turnover number (TON) of 404, an optimized quantum yield of 43% and 100% selectivity, which is the highest reported performance for mononuclear photocatalysts without additional photosensitizers. RuP-4 also catalyzes photochemical hydrogen generation under argon. This work opens up an avenue for regulating the excited-state charge/energy flow for the development of long-lived 3IL multi-functional mononuclear photocatalysts to boost artificial photosynthesis.

Management of Triplet States in Modified Mononuclear Ruthenium(II) Complexes for Enhanced Photocatalysis

Controlling the interplay between relaxation and charge/energy transfer processes in the excited states of photocatalysts is crucial for the performance of artificial photosynthesis. Metal‐to‐ligand charge‐transfer triplet states (3MLCT*) of ruthenium(II) complexes are broadly implemented for photocatalysis, but an effective means of managing the triplets for enhanced photocatalysis has been lacking. Herein, We proposed a strategy to considerably prolong the triplet excited‐state lifetime by decorating a ruthenium(II) phosphine complex (RuP‐1) with pendent polyaromatic hydrocarbons (PAHs). Systematic studies demonstrate that in RuP‐4 decorated with anthracene, sub‐picosecond electron transfer from anthracene to 3MLCT* leads to a charge‐separated state that can mediate the formation of the intra‐ligand triplet state (3IL) of anthracene, resulting in an exceptionally long excited‐state up to several milliseconds. This triplet management strategy enables impressive photocatalytic reduction of CO2 to CO with a turnover number (TON) of 404, an optimized quantum yield of 43% and 100% selectivity, which is the highest reported performance for mononuclear photocatalysts without additional photosensitizers. RuP‐4 also catalyzes photochemical hydrogen generation under argon. This work opens up an avenue for regulating the excited‐state charge/energy flow for the development of long‐lived 3IL multi‐functional mononuclear photocatalysts to boost artificial photosynthesis.

Photocatalytic Hydrogen Production by the Sensitization of Sn(IV)-Porphyrin Embedded in a Nafion Matrix Coated on TiO2.

Efficient utilization of visible light for photocatalytic hydrogen production is one of the most important issues to address. This report describes a facile approach to immobilize visible-light sensitizers on TiO2 surfaces. To effectively utilize the sensitization of Sn(IV) porphyrin species for photocatalytic hydrogen production, perfluorosulfonate polymer (Nafion) matrix coated-TiO2 was fabricated. Nafion coated-TiO2 readily adsorbed trans-diaqua[meso-tetrakis(4-pyridinium)porphyrinato]tin(IV) cation [(TPyHP)Sn(OH2)2]6+ via an ion-exchange process. The uptake of [(TPyHP)Sn(OH2)2]6+ in an aqueous solution completed within 30 min, as determined by UV-vis spectroscopy. The existence of Sn(IV) porphyrin species embedded in the Nafion matrix coated on TiO2 was confirmed by zeta potential measurements, UV-vis absorption spectroscopy, TEM combined with energy dispersive X-ray spectroscopy, and thermogravimetric analysis. Sn(IV)-porphyrin cationic species embedded in the Nafion matrix were successfully used as visible-light sensitizer for photochemical hydrogen generation. This photocatalytic system performed 45% better than the uncoated TiO2 system. In addition, the performance at pH 7 was superior to that at pH 3 or 9. This work revealed that Nafion matrix coated-TiO2 can efficiently produce hydrogen with a consistent performance by utilizing a freshly supplied cationic Sn(IV)-porphyrin sensitizer in a neutral solution.

A Novel Donor-π-Acceptor Type Sensitizer for Dye Sensitized Photochemical Hydrogen Generation

A novel triphenylamine (TPA) based donor-π-acceptor (D-π-A) dye is synthesized and its optical and electrochemical properties are examined by UV-Vis absorption spectroscopy and cyclic voltammetry methods, respectively. The synthesized D-π-A dye plays a role as a visible light sensitizer to wide bandgap TiO2 photocatalyst. Hydrogen evolution reaction (HER) are carried out by using D-π-A dye sensitized TiO2 (Dye/TiO2) under visible light irradiation in the aqueous triethanolamine (TEOA) medium. Photoelectrochemical properties of Dye/TiO2 are investigated by using linear sweep voltammetry (LSV) and chronoamperometry (CA) techniques in the aqueous Na2SO4/TEOA solution and its transient photoelectrochemical response is reached 90 μA cm-2. In addition, phtocatalytic hydrogen evolution rates are found out as 0.52 mmol g-1 h-1 and 1.95 mmol g-1 h-1 by using of Dye/TiO2 and Dye/TiO2/Pt, respectively, which are obtained by in situ photoreduction of H2PtCl6 on the Dye/TiO2 photocatalyst. The mechanism of photochemical HER is explained by electrochemical band levels of the D-π-A dye and TiO2 photocatalyst.

In-situ construction of graphene oxide in microsphere ZnS photocatalyst for high-performance photochemical hydrogen generation

Exploring high-efficiency photocatalysts for hydrogen generation from water splitting has recently spurred enormous scientific interest. Herein, the formation of Pt/GO-ZnS photocatalysts via a step-wise strategy and the mechanism of improved hydrogen production efficiency are systematically studied. Thus-prepared optimal Pt/GO-ZnS composite (with the ratio of Pt and GO are 1 and 2 wt%, respectively) exhibits excellent photocatalytic hydrogen production performance of 1082 μmol h−1 g−1 under simulated sunlight irradiation, which stays 2.8 and 2.6 times higher than that of ZnS/GO (2 wt%) and ZnS/Pt (1 wt%), respectively. Exhaustive experimental studies reveal that the outstanding activity of Pt/GO-ZnS is attributed to the efficient interfacial charge transfer and enhanced visible light harvesting caused by the synergetic effects of GO and Pt. In further contexts, the photocatalytic mechanism is explored: Photo-generated electrons in the conduction band of ZnS can be immediately trapped through GO and rapidly transferred onto Pt nanoparticles thanks to the tight interface. This study represents an effective and facile strategy to rationally design advanced photocatalytic system in solar energy conversion.

Photocatalytic Hydrogen Generation by Vesicle-Embedded [FeFe]Hydrogenase Mimics: A Mechanistic Study.

Artificial photosynthesis—the direct photochemical generation of hydrogen from water—is a promising but scientifically challenging future technology. Because nature employs membranes for photodriven reactions, the aim of this work is to elucidate the effect of membranes on artificial photocatalysis. To do so, a combination of electrochemistry, photocatalysis, and time‐resolved spectroscopy on vesicle‐embedded [FeFe]hydrogenase mimics, driven by a ruthenium tris‐2,2′‐bipyridine photosensitizer, is reported. The membrane effects encountered can be summarized as follows: the presence of vesicles steers the reactivity of the [FeFe]‐benzodithiolate catalyst towards disproportionation, instead of protonation, due to membrane characteristics, such as providing a constant local effective pH, and concentrating and organizing species inside the membrane. The maximum turnover number is limited by photodegradation of the resting state in the catalytic cycle. Understanding these fundamental productive and destructive pathways in complex photochemical systems allows progress towards the development of efficient artificial leaves.

Diiron Dithiolate Complex Induced Helical Structure of Histone and Application in Photochemical Hydrogen Generation.

Very-lysine-rich calf thymus histone proteins form disordered structure and hydrophobic interaction-driven aggregates in weakly acidic solution. We reported that the conjugation of diiron dithiolate complex to the lysine residues induced formation of helical conformation and condensed nanoassemblies with a high loading capacity up to 18.7 wt %. The incorporated diiron dithiolate complex showed photocatalytic activity for hydrogen evolution in aqueous solutions, with a turnover number (based on [FeFe] catalyst moiety) up to 359 that was more than 6 times that of the free catalyst. The increase of helical conformation in proteins was well correlated to the increasing enhancement of photocatalytic activity. We demonstrated that the [FeFe]-hydrogenase-mimic biohybrid system based on the photocatalyst-induced protein conformational conversion and reassembly is efficient for hydrogen generation regardless of the relatively large size.

Corrigendum: Photochemical and Photoelectrochemical Hydrogen Generation by Splitting Seawater.

The authors have noticed a mistake in the key for Figure 8 in the manuscript. Incorrect labels were shown for UHP water and seawater. A revised figure with the correct key is shown below. The authors apologize for the error. Time course of hydrogen evolution of (a) few-layer 2H-MoS2 and (b) 1T-MoS2 in seawater and UHP water in triethanolamine solution with Eosin Y (EY) dye as a sensitizer.

Corrigendum: Photochemical and Photoelectrochemical Hydrogen Generation by Splitting Seawater

Corrigendum: Photochemical and Photoelectrochemical Hydrogen Generation by Splitting Seawater

Disordered layers on WO3 nanoparticles enable photochemical generation of hydrogen from water

A simple treatment with Li-ethylenediamine alters the surface of WO3 nanoparticles with localized defects that form a thin disordered layer and modifies the electronic structure suitable for hydrogen generation.

Frontispiece: Photochemical and Photoelectrochemical Hydrogen Generation by Splitting Seawater

Frontispiece: Photochemical and Photoelectrochemical Hydrogen Generation by Splitting Seawater

Photochemical and Photoelectrochemical Hydrogen Generation by Splitting Seawater.

Producing hydrogen from water in an efficient manner could significantly reduce consumption of fossil fuels. In this regard the abundant presence of water in oceans offers an important alternative approach for water splitting using seawater. Direct use of seawater for the generation of hydrogen is a difficult and complex process due to the presence of various ions in seawater, which affect the activity of the catalysts and makes the selectivity towards efficient water splitting a challenging task. Herein various ways are reported to efficiently reduce seawater to hydrogen under visible light irradiation by various catalysts already reported by this group. A better performance than pure water was observed in some cases, and in a few cases the opposite was observed, implying that with a proper approach seawater can be efficiently reduced to generate hydrogen.

Borocarbonitrides as Metal‐Free Catalysts for the Hydrogen Evolution Reaction

Hydrogen generation by water splitting is clearly a predominant and essential strategy to tackle the problems related to renewable energy. In this context, the discovery of proper catalysts for electrochemical and photochemical water splitting assumes great importance. There is also a serious intent to eliminate platinum and other noble metal catalysts. To replace Pt by a non-metallic catalyst with desirable characteristics is a challenge. Borocarbonitrides, (Bx Cy Nz ) which constitutes a new class of 2D material, offer great promise as non-metallic catalysts because of the easy tunability of bandgap, surface area, and other electronic properties with variation in composition. Recently, Bx Cy Nz composites with excellent electrochemical and photochemical hydrogen generation activities have been found, especially noteworthy being the observation that Bx Cy Nz with a carbon-rich composition or its nanocomposites with MoS2 come close to Pt in electrocatalytic properties, showing equally good photochemical activity.

H-doped TiO2-x prepared with MgH2 for highly efficient solar-driven hydrogen production

Efficient utilization of visible light with high stability remains a critical challenge for solar-driven photochemical generation of hydrogen (H2) using particulate photocatalysts. Black TiO2 was introduced with remarkable enhancement of visible light absorption, but its efficiency in the visible light has not reached the desired level for real-world applications. Here we report a gold-colored H-doped TiO2-x (H:TiO2-x) nanoparticles prepared by controlled reduction via simultaneous presence of [Mg] and [H], which are obtained from the decomposition of MgH2. The H-doped TiO2-x exhibits a significant activity (16.1 mmolg−1h−1) and remarkable stability after Pt deposition for solar-driven H2 generation from methanol-water. The excellent photoactivity of H-doped TiO2 can be attributed to oxygen vacancies and H doping at the reduced TiO2-x surface generated by [Mg] and [H]. The H-doped TiO2 is also producing H2 from methanol-seawater with a rate of 6.1 mmolg−1h−1 under simulated sunlight.

Electrochemical and photochemical hydrogen generation by a water soluble cobalt(II) complex of 2,2-bipyridine

A molecular catalyst based on a cobalt(II) complex, [(bpy)2Co(SCN)(Cl)] 1, has been prepared by the reaction of 2,2-bipyridine (bpy), CoCl2·6H2O, and KSCN. The complex has been investigated for both electrocatalytic hydrogen generation and photocatalytic hydrogen generation from purely aqueous solution. Under photoirradiation with blue light (λ max = 469 nm), with [Ru(bpy)3]Cl2 as a coupled photosensitizer and ascorbic acid (H2A) as a sacrificial electron donor in pH 5.0 aqueous solution, complex 1 possesses good activity with an initial turnover rate of 2.7 × 103 mol of H2 per mole of catalyst for the first 3 h of photolysis. This activity can be sustained for at least 10 h with a turnover number of 1.2 × 104 mol of H2 per mole of catalyst. Additionally, complex 1 electrocatalyzes hydrogen generation from neutral water (pH 7.0) with a turnover frequency of 809 mol of hydrogen per mole of catalyst per hour at an overpotential of 837.6 mV.

Engineering Reaction Kinetics by Tailoring the Metal Tips of Metal-Semiconductor Nanodumbbells.

Semiconductor-metal hybrid nanostructures are one of the best model catalysts for understanding photocatalytic hydrogen generation. To investigate the optimal structure of metal cocatalysts, metal-CdSe-metal nanodumbbells were synthesized with three distinct sets of metal tips, Pt-CdSe-Pt, Au-CdSe-Au, and Au-CdSe-Pt. Photoelectrochemical responses and transient absorption spectra showed that the competition between the charge recombination at the metal-CdSe interface and the water reduction on the metal surface is a detrimental factor for the apparent hydrogen evolution rate. For instance, a large recombination rate (krec) at the Pt-CdSe interface limits the quantum yield of hydrogen generation despite a superior water reduction rate (kWR) on the Pt surface. To suppress the recombination process, Pt was selectively deposited onto the Au tips of Au-CdSe-Au nanodumbbells in which the krec was diminished at the Au-CdSe interface, and the large kWR was maintained on the Pt surface. As a result, the optimal structure of the Pt-coated Au-CdSe-Au nanodumbbells reached a quantum yield of 4.84%. These findings successfully demonstrate that the rational design of a metal cocatalyst and metal-semiconductor interface can additionally enhance the catalytic performance of the photochemical hydrogen generation reactions.

Elucidating the Key Role of pH on Light-Driven Hydrogen Evolution by a Molecular Cobalt Catalyst

Photochemical hydrogen generation from aqueous solutions can be accomplished with a combination of at least three molecular components: namely, a photosensitizer, a hydrogen-evolving catalyst, and an electron donor. A parameter that plays a key role in the light to hydrogen efficiency of such three-component systems is the solution pH. While this evidence has been usually observed in several works aiming at identifying catalysts and optimizing their performances, detailed studies capable of shining light on this issue have been extremely rare. Hence, the pH dependence of a reference three-component system based on Ru(bpy)32+ (where bpy = 2,2′-bipyridine) as the sensitizer, a cobaloxime HEC, and ascorbic acid as the sacrificial donor has been studied with care by merging photocatalytic hydrogen evolution kinetic data and detailed time-resolved spectroscopy results. The photocatalytic activity shows a bell-shaped profile as a function of pH which peaks at around pH 5. While at acidic pH (pH <5) the hydrogen...

Mixed-dimensional TiO2 nanoparticles with MoSe2 nanosheets for photochemical hydrogen generation

Novel TiO2/MoSe2 catalyst has been synthesized via a hydrothermal method. The morphologies, microstructures and optical properties were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, UV–Vis absorption spectroscopy, the result show that a large quantity of MoSe2 nanosheets (NSs) were obtained from MoSe2 bulks by lithium intercalatation way and TiO2 nanoparticles has been grown on the NSs successfully. According to the photoelectrochemical measurements, the TiO2/MoSe2 hybrid nanostructures show the photocurrent density of 15 μA/cm2, which is much higher than bare TiO2 of 8 μA/cm2 and bare MoSe2 of 10 μA/cm2. It is proposed that type-II heteroshtucture of TiO2/MoSe2 can efficiently promote photocatalytic activity.

Effectiveness of NiO in replacing Pt in the photochemical generation of hydrogen by (TiO2)1−(NiO) /Cd0.8Zn0.2S heterostructures

Hydrogen evolution activity of semiconductor heterostructures of the type (TiO2)1−x(NiO)x/Cd0.8Zn0.2S has been investigated in detail to examine the role of NiO, used in the place of Pt by making use of nanoparticles and nanotubes of TiO2. Combination of NiO and TiO2 markedly changes the photocatalytic efficiency of the heterostructures. While the H2 evolution activity increases smoothly with NiO mole farction up to x~0.50, it increases further reaching a maximum at x~0.83, yielding 1.6 mmol h−1 g−1 (AQY ~2.9%) with Na2S–Na2SO3 as sacrificial agents, under visible-light irradiation, and 6.84 mmol h−1 g−1 (AQY ~11.7%) and 13.5 mmol h−1 g−1 (AQY ~10%) with benzyl alcohol-acetic acid as sacrificial agents under visible and UV–visible light irradiation respectively. High hydrogen evolution activity to the tune of three times that yielded by equivalent Pt containg heterostructures has been attained, with a NiO mole fraction of 0.83 in the heterostructures.

Photochemical Hydrogen Generation Initiated by Oxidative Quenching of the Excited Ru(bpy)3 (2+) * by a Bio-Inspired [2Fe2S] Complex.

A diiron dithiolate complex 1 containing 1,8-naphthalic anhydride bridge was prepared, which possessed the lowest reduction potential for the synthetic diiron complexes modeled on the active site of [FeFe] hydrogenase reported so far. For the first time, oxidative quenching of the excited Ru(bpy)3 (2+) * through electron transfer to a bio-inspired [2Fe2S] complex was corroborated. Hydrogen evolution, driven by visible light, was successfully observed for a three-component system, consisting of Ru(bpy)3 (2+) , complex 1, and EDTA as electron donor in aqueous/organic media. These results provide a basis and also opportunity to develop a photo water splitting system employing Fe-based catalysts without sacrificial electron donors.