A Noble‐Metal‐Free System for Photocatalytic Hydrogen Production from Water

A series of heteroleptic copper(I) complexes with bidentate $\widehat{PP}$ and $\widehat{NN}$ chelate ligands was prepared and successfully applied as photosensitizers in the light‐driven production of hydrogen, by using [Fe₃(CO)₁₂] as a water‐reduction catalyst (WRC). These systems efficiently reduces protons from water/THF/triethylamine mixtures, in which the amine serves as a sacrificial electron donor (SR). Turnover numbers (for H) up to 1330 were obtained with these fully noble‐metal‐free systems. The new complexes were electrochemically and photophysically characterized. They exhibited a correlation between the lifetimes of the MLCT excited state and their efficiency as photosensitizers in proton‐reduction systems. Within these experiments, considerably long excited‐state lifetimes of up to 54 μs were observed. Quenching studies with the SR, in the presence and absence of the WRC, showed that intramolecular deactivation was more efficient in the former case, thus suggesting the predominance of an oxidative quenching pathway.     

[RhIII(dmbpy)2Cl2]+ as a Highly Efficient Catalyst for Visible‐Light‐Driven Hydrogen Production in Pure Water: Comparison with Other Rhodium Catalysts

We report a very efficient homogeneous system for the visible‐light‐driven hydrogen production in pure aqueous solution at room temperature. This comprises [Rh^III(dmbpy)₂Cl₂]Cl ( 1 ) as catalyst, [Ru(bpy)₃]Cl₂ ( PS1 ) as photosensitizer, and ascorbate as sacrificial electron donor. Comparative studies in aqueous solutions also performed with other known rhodium catalysts, or with an iridium photosensitizer, show that 1) the PS1 / 1 /ascorbate/ascorbic acid system is by far the most active rhodium‐based homogeneous photocatalytic system for hydrogen production in a purely aqueous medium when compared to the previously reported rhodium catalysts, Na₃[Rh^I(dpm)₃Cl] and [Rh^III(bpy)Cp*(H₂O)]SO₄ and 2) the system is less efficient when [Ir^III(ppy)₂(bpy)]Cl ( PS2 ) is used as photosensitizer. Because catalyst 1 is the most efficient rhodium‐based H₂‐evolving catalyst in water, the performance limits of this complex were further investigated by varying the PS1 / 1 ratio at pH 4.0. Under optimal conditions, the system gives up to 1010 turnovers versus the catalyst with an initial turnover frequency as high as 857 TON h^?1. Nanosecond transient absorption spectroscopy measurements show that the initial step of the photocatalytic H₂‐evolution mechanism is a reductive quenching of the PS1 excited state by ascorbate, leading to the reduced form of PS1 , which is then able to reduce [Rh^III(dmbpy)₂Cl₂]⁺ to [Rh^I(dmbpy)₂]⁺. This reduced species can react with protons to yield the hydride [Rh^III(H)(dmbpy)₂(H₂O)]²⁺, which is the key intermediate for the H₂ production.     

An Efficient RuII–RhIII–RuII Polypyridyl Photocatalyst for Visible‐Light‐Driven Hydrogen Production in Aqueous Solution

The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single‐component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen‐evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy‐conversion applications. Herein, we introduce a new ruthenium–rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H₂ production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components.     

A Crystalline Copper(II) Coordination Polymer for the Efficient Visible‐Light‐Driven Generation of Hydrogen

A crystalline coordination polymer (CP) photocatalyst (Cu‐RSH) which combines redox‐active copper centers with photoactive rhodamine‐derived ligands remains stable in acid and basic solutions from pH 2 to 14, and efficiently catalyzes dihydrogen evolution at a maximum rate of 7.88 mmol g^?1 h^?1 in the absence of a mediator and a co‐catalyst. Cyclic voltammetry, control experiments, and DFT calculations established that copper nodes with open coordination sites and favorable redox potentials, aided by spatially ordered stacking of rhodamine‐based linkers, account for the high catalytic performance of Cu‐RSH. Emission quenching, time‐resolved fluorescence decay, and transient photocurrent experiments disclosed the charge separation and transfer process in the catalytic system. The present study demonstrates the potential of crystalline copper CPs for the practical utilization of light.     

Visible‐Light‐Driven Hydrogen Production and Polymerization using Triarylboron‐Functionalized Iridium(III) Complexes

The development of novel iridium(III) complexes has continued as an important area of research owing to their highly tunable photophysical properties and versatile applications. In this report, three heteroleptic dimesitylboron‐containing iridium(III) complexes, [Ir(p‐B‐ppy)₂(N^N)]⁺ {p‐B‐ppy=2‐(4‐dimesitylborylphenyl)pyridine; N^N=dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz) ( 1 ), dipyrido[3,2‐d:2′,3′‐f]quinoxaline (dpq) ( 2 ), and 1,10‐phenanthroline (phen) ( 3 )}, were prepared and fully characterized electrochemically, photophysically, and computationally. Altering the conjugated length of the N^N ligands allowed us to tailor the photophysical properties of these complexes, especially their luminescence wavelength, which could be adjusted from λ=583 to 631 nm in CH₂Cl₂. All three complexes were evaluated as visible‐light‐absorbing sensitizers for the photogeneration of hydrogen from water and as photocatalysts for the photopolymerization of methyl methacrylate. The results showed that all of them were active in both photochemical reactions. High activity for the photosensitizer (over 1158 turnover numbers with 1 ) was observed, and the system generated hydrogen even after 20 h. Additionally, poly(methyl methacrylate) with a relatively narrow molecular‐weight distribution was obtained if an initiator (i.e., ethyl α‐bromophenylacetate) was used. The living character of the photoinduced polymerization was confirmed on the basis of successful chain‐extension experiments.     

Highly Dispersed and Small‐Sized Nickel(II) Hydroxide Co‐Catalyst Prepared by Photodeposition for Hydrogen Production

Photodeposition has been widely used as a mild and efficient synthetic method to deposit co‐catalysts. It is also worth studying how to synthesize non‐noble metal photocatalysts with uniform dispersion. Different synthetic conditions in photodeposition have a certain influence on particle size distribution and photocatalytic activity. Therefore, we designed experiments to prepare the inexpensive composite photocatalyst Ni(OH)₂/g‐C₃N₄ by photodeposition. The Ni(OH)₂ co‐catalysts disperse uniformly with particle sizes of about 10 nm. The photocatalytic hydrogen production rate of Ni(OH)₂/g‐C₃N₄ reached about 19 mmol g^?1 h^?1, with the Ni(OH)₂ deposition amount about 1.57 %. During 16 h stability testing, the rate of hydrogen production did not decrease significantly. The composite catalyst also revealed a good hydrogen production performance under sunlight. The Ni(OH)₂ co‐catalyst enhanced the separation ability of photogenerated carriers, which was proved by surface photovoltage and fluorescence analysis.     

Sunlight‐Driven Hydrogen Peroxide Production from Water and Molecular Oxygen by Metal‐Free Photocatalysts

Design of green, safe, and sustainable process for the synthesis of hydrogen peroxide (H₂O₂) is a very important subject. Early reported processes, however, require hydrogen (H₂) and palladium‐based catalysts. Herein we propose a photocatalytic process for H₂O₂ synthesis driven by metal‐free catalysts with earth‐abundant water and molecular oxygen (O₂) as resources under sunlight irradiation (λ>400 nm). We use graphitic carbon nitride (g‐C₃N₄) containing electron‐deficient aromatic diimide units as catalysts. Incorporating the diimide units positively shifts the valence‐band potential of the catalysts, while maintaining sufficient conduction‐band potential for O₂ reduction. Visible light irradiation of the catalysts in pure water with O₂ successfully produces H₂O₂ by oxidation of water by the photoformed valence‐band holes and selective two‐electron reduction of O₂ by the conduction band electrons.     

Hierarchical Integration of Photosensitizing Metal–Organic Frameworks and Nickel‐Containing Polyoxometalates for Efficient Visible‐Light‐Driven Hydrogen Evolution

Metal–organic frameworks (MOFs) provide a tunable platform for hierarchically integrating multiple components to effect synergistic functions that cannot be achieved in solution. Here we report the encapsulation of a Ni‐containing polyoxometalate (POM) [Ni₄(H₂O)₂(PW₉O₃₄)₂]^10? ( Ni₄P₂ ) into two highly stable and porous phosphorescent MOFs. The proximity of Ni₄P₂ to multiple photosensitizers in Ni₄P₂ @MOF allows for facile multi‐electron transfer to enable efficient visible‐light‐driven hydrogen evolution reaction (HER) with turnover numbers as high as 1476. Photophysical and electrochemical studies established the oxidative quenching of the excited photosensitizer by Ni₄P₂ as the initiating step of HER and explained the drastic catalytic activity difference of the two POM@MOFs. Our work shows that POM@MOF assemblies not only provide a tunable platform for designing highly effective photocatalytic HER catalysts but also facilitate detailed mechanistic understanding of HER processes.     

Design of BODIPY Dyes as Photosensitisers in Multicomponent Catalyst Systems for Light‐Driven Hydrogen Production

A study of visible‐light‐driven hydrogen production using a multicomponent system consisting of different boron dipyrromethene (BODIPY) dyes, triethylamine and [{Pd(PPh₃)Cl₂}₂] from THF/water mixtures is presented. A trio of meso‐mesityl BODIPY dyes display the best activities and long‐term stabilities of more than ten days with the 2,6‐diiodo derivative showing the best performance.     

Photocatalyzed Hydrogen Evolution from Water by a Composite Catalyst of NH2‐MIL‐125(Ti) and Surface Nickel(II) Species

A composite of the metal–organic framework (MOF) NH₂‐MIL‐125(Ti) and molecular and ionic nickel(II) species, catalyzed hydrogen evolution from water under UV light. In 95 v/v % aqueous conditions the composite produced hydrogen in quantities two orders of magnitude higher than that of the virgin framework and an order of magnitude greater than that of the molecular catalyst. In a 2 v/v % water and acetonitrile mixture, the composite demonstrated a TOF of 28 mol H₂ g(Ni)^?1 h^?1 and remained active for up to 50 h, sustaining catalysis for three times longer and yielding 20‐fold the amount of hydrogen. Appraisal of physical mixtures of the MOF and each of the nickel species under identical photocatalytic conditions suggest that similar surface localized light sensitization and proton reduction processes operate in the composite catalyst. Both nickel species contribute to catalytic conversion, although different activation behaviors are observed.     

A Covalent Organic Framework–Cadmium Sulfide Hybrid as a Prototype Photocatalyst for Visible‐Light‐Driven Hydrogen Production

CdS nanoparticles were deposited on a highly stable, two‐dimensional (2D) covalent organic framework (COF) matrix and the hybrid was tested for photocatalytic hydrogen production. The efficiency of CdS‐COF hybrid was investigated by varying the COF content. On the introduction of just 1 wt % of COF, a dramatic tenfold increase in the overall photocatalytic activity of the hybrid was observed. Among the various hybrids synthesized, that with 10 wt % COF, named CdS‐COF (90:10), was found to exhibit a steep H₂ production amounting to 3678 μmol h^?1 g^?1, which is significantly higher than that of bulk CdS particles (124 μmol h^?1 g^?1). The presence of a π‐conjugated backbone, high surface area, and occurrence of abundant 2D hetero‐interface highlight the usage of COF as an effective support for stabilizing the generated photoelectrons, thereby resulting in an efficient and high photocatalytic activity.     

An Efficient,Visible‐Light‐Driven,Hydrogen Evolution Catalyst NiS/ZnxCd1−xS Nanocrystal Derived from a Metal–Organic Framework

Photocatalytic water splitting for hydrogen production using sustainable sunlight is a promising alternative to industrial hydrogen production. However, the scarcity of highly active, recyclable, inexpensive photocatalysts impedes the development of photocatalytic hydrogen evolution reaction (HER) schemes. Herein, a metal–organic framework (MOF)‐template strategy was developed to prepare non‐noble metal co‐catalyst/solid solution heterojunction NiS/Zn_xCd_1?xS with superior photocatalytic HER activity. By adjusting the doping metal concentration in MOFs, the chemical compositions and band gaps of the heterojunctions can be fine‐tuned, and the light absorption capacity and photocatalytic activity were further optimized. NiS/Zn_0.5Cd_0.5S exhibits an optimal HER rate of 16.78 mmol g^?1 h^?1 and high stability and recyclability under visible‐light irradiation (λ>420 nm). Detailed characterizations and in‐depth DFT calculations reveal the relationship between the heterojunction and photocatalytic activity and confirm the importance of NiS in accelerating the water dissociation kinetics, which is a crucial factor for photocatalytic HER.     

Narrow‐Bandgap Chalcogenoviologens for Electrochromism and Visible‐Light‐Driven Hydrogen Evolution

A series of electron‐accepting chalcogen‐bridged viologens with narrow HOMO–LUMO bandgaps and low LUMO levels is reported. The optoelectronic properties of chalcogenoviologens can be readily tuned through heavy atom substitution (S, Se and Te). Herein, in situ electrochemical spectroscopy was performed on the proof‐of‐concept electrochromic devices (ECD). E‐BnV²⁺ (E=Se, Te; BnV²⁺=benzyl viologen) was used for the visible‐light‐driven hydrogen evolution due to the strong visible‐light absorption. Remarkably, E‐BnV²⁺ was not only used as a photosensitizer, but also as an electron mediator, providing a new strategy to explore photocatalysts. The higher apparent quantum yield of Se‐BnV²⁺ could be interpreted in terms of different energy levels, faster electron‐transfer rates and faster formation of radical species.     

A Titanium Metal–Organic Framework with Visible‐Light‐Responsive Photocatalytic Activity

The one‐step synthesis and characterization of a new and robust titanium‐based metal–organic framework, ACM‐1 , is reported. In this structure, which is based on infinite Ti?O chains and 4,4′,4′′,4′′′‐(pyrene‐1,3,6,8‐tetrayl) tetrabenzoic acid as a photosensitizer ligand, the combination of highly mobile photogenerated electrons and a strong hole localization at the organic linker results in large charge‐separation lifetimes. The suitable energies for band gap and conduction band minimum (CBM) offer great potential for a wide range of photocatalytic reactions, from hydrogen evolution to the selective oxidation of organic substrates.     

Electrochemical Properties of a Rhodium(III) Mono-Terpyridyl Complex and Use as a Catalyst for Light-Driven Hydrogen Evolution in Water

Molecular hydrogen (H₂) is considered one of the most promising fuels to decarbonize the industrial and transportation sectors, and its photocatalytic production from molecular catalysts is a research field that is still abounding. The search for new molecular catalysts for H₂ production with simple and easily synthesized ligands is still ongoing, and the terpyridine ligand with its particular electronic and coordination properties, is a good candidate to design new catalysts meeting these requirements. Herein, we have isolated the new mono-terpyridyl rhodium complex, [Rh^III(tpy)(CH₃CN)Cl₂](CF₃SO₃) (Rh-tpy), and shown that it can act as a catalyst for the light-induced proton reduction into H₂ in water in the presence of the [Ru(bpy)₃]Cl₂ (Ru) photosensitizer and ascorbate as sacrificial electron donor. Under photocatalytic conditions, in acetate buffer at pH 4.5 with 0.1 M of ascorbate and 530 μM of Ru, the Rh-tpy catalyst produces H₂ with turnover number versus catalyst (TON_Cat*) of 300 at a Rh concentration of 10 μM, and up to 1000 at a concentration of 1 μM. The photocatalytic performance of Ru/Rh-tpy/HA^–/H₂A has been also compared with that obtained with the bis-dimethyl-bipyridyl complex [Rh^III(dmbpy)₂Cl₂]⁺ (Rh2) as a catalyst in the same experimental conditions. The investigation of the electrochemical properties of Rh-tpy in DMF solvent reveals that the two-electrons reduced state of the complex, the square-planar [Rh^I(tpy)Cl] (Rh^I-tpy), is quantitatively electrogenerated by bulk electrolysis. This complex is stable for hours under an inert atmosphere owing to the π-acceptor property of the terpyridine ligand that stabilizes the low oxidation states of the rhodium, making this catalyst less prone to degrade during photocatalysis. The π-acceptor property of terpyridine also confers to the Rh-tpy catalyst a moderately negative reduction potential (Ep_c(Rh^III/Rh^I) = −0.83 V vs. SCE in DMF), making possible its reduction by the reduced state of Ru, [Ru^II(bpy)(bpy^•−)]⁺ (Ru⁻) (E_1/2(Ru^II/Ru⁻) = −1.50 V vs. SCE) generated by a reductive quenching of the Ru excited state (*Ru) by ascorbate during photocatalysis. A Stern–Volmer plot and transient absorption spectroscopy confirmed that the first step of the photocatalytic process is the reductive quenching of *Ru by ascorbate. The resulting reduced Ru species (Ru⁻) were then able to activate the Rh^III-tpy H₂-evolving catalyst by reduction generating Rh^I-tpy, which can react with a proton on a sub-nanosecond time scale to form a Rh^III(H)-tpy hydride, the key intermediate for H₂ evolution.     

Solar Hydrogen from an Aqueous,Noble‐Metal‐Free Hybrid System in a Continuous‐Flow Sampling Reaction System

We introduce the visible‐light photocatalytic H₂ evolution reaction as catalyzed by a cobaloxime/carbon nitride (C₃N₄) noble‐metal‐free hybrid photosystem by using a continuous‐flow sampling reaction system. The photocatalytic H₂ evolution rate is highly dependent on the structure of C₃N₄, in which porous C₃N₄ shows the best activity compared with bulk C₃N₄ (lamellar) and C₃N₄ nanosheets. When using porous C₃N₄, the system is neither affected by the solution pH, nor the C₃N₄ concentration, nor the structure of the cobaloxime complex.