Integration of organometallic complexes with semiconductors and other nanomaterials for photocatalytic H2 production

Advanced photocatalytic materials based degradation of micropollutants and their use in hydrogen production - a review

The use of pharmaceuticals, dyes, and pesticides in modern healthcare and agriculture, along with expanding industrialization, heavily contaminates aquatic environments. This leads to severe carcinogenic implications and critical health issues in living organisms. The photocatalytic methods provide an eco-friendly solution to mitigate the energy crisis and environmental pollution. Sunlight-driven photocatalytic wastewater treatment contributes to hydrogen production and valuable product generation. The removal of contaminants from wastewater through photocatalysis is a highly efficient method for enhancing the ecosystem and plays a crucial role in the dual-functional photocatalysis process. In this review, a wide range of catalysts are discussed, including heterojunction photocatalysts and various hybrid semiconductor photocatalysts like metal oxides, semiconductor adsorbents, and dual semiconductor photocatalysts, which are crucial in this dual function of degradation and green fuel production. The effects of micropollutants in the ecosystem, degradation efficacy of multi-component photocatalysts such as single-component, two-component, three-component, and four-component photocatalysts were discussed. Dual-functional photocatalysis stands out as an energy-efficient and cost-effective method. We have explored the challenges and difficulties associated with dual-functional photocatalysts. Multicomponent photocatalysts demonstrate superior efficiency in degrading pollutants and producing hydrogen compared to their single-component counterparts. Dual-functional photocatalysts, incorporating TiO2, g-C3N4, CeO2, metal organic frameworks (MOFs), layered double hydroxides (LDHs), and carbon quantum dots (CQDs)-based composites, exhibit remarkable performance. The future of synergistic photocatalysis envisions large-scale production facilitate integrating advanced 2D and 3D semiconductor photocatalysts, presenting a promising avenue for sustainable and efficient pollutant degradation and hydrogen production from environmental remediation technologies.

A Bifunctional Three-Dimensional Zn(II) Metal-Organic Framework with Strong Luminescence and Adsorption Cr(VI) Properties

The mixed ligand 3-amino-1,2,4-triazole (Hatz) and terephthalic acid (H2pta) reacted with Zn(NO3)2·6H2O to synthesize a three-dimensional binuclear Zn(II) metal-organic framework: {[Zn2·(atz)2·(pta)]·3H2O}n (3D-Zn-MOF). This 3D-Zn-MOF has two different types of pores (4.5 × 4.5 Å2, 5.7 × 5.7 Å2). The crystalline 3D-Zn-MOF could be prepared into nanomaterials (3D-N-Zn-MOF) with particles of approximately 100 nm by a cell fragmentation apparatus. Compared with the solid-state luminescence of Hatz and H2pta, it was found that 3D-N-Zn-MOF exhibited strong luminescence performance and significant red-shift phenomenon. Due to the decrease in electronegativity and rigidity of ligands, as well as the effect of ligand metal charge transfer (LMCT), the fluorescence lifetime and quantum yield of 3D-ZN-N-MOF were 2.7241 ns and 3.02%, respectively. The maximum experimental adsorption capacity of 3D-N-Zn-MOF could reach 125.52 mg/g, which was superior to the majority of MOF adsorbents under the optimal adsorption conditions (25 °C, pH = 7, and the adsorbent concentration is 0.2000 g/L). The thermodynamic analysis of adsorption showed that the adsorption of Cr(VI) by 3D-N-Zn-MOF was a spontaneous (△G < 0) and exothermic (△H < 0) process. It could be found that 3D-N-Zn-MOF was a bifunctional material with potential applications by comprehensive analysis of the fluorescence and adsorption Cr(VI) performance.

Fabrication of Noble-Metal-Free Mo2C/CdIn2S4 Heterojunction Composites with Elevated Carrier Separation for Photocatalytic Hydrogen Production

Molybdenum-based cocatalyst being used to construct heterojunctions for efficient photocatalytic H2 production is a promising research hotspot. In this work, CdIn2S4 was successfully closely supported on bulk Mo2C via the hydrothermal method. Based on their matching band structures, they formed a Type Ⅰ heterojunction after the combination of Mo2C (1.1 eV, −0.27 V, 0.83 V) and CdIn2S4 (2.3 eV, −0.74 V, 1.56 V). A series of characterizations proved that the heterojunction composite had higher charge separation efficiency compared to a single compound. Meanwhile, Mo2C in heterojunction could act as an active site for hydrogen production. The photocatalytic H2 production activity of the heterojunction composites was significantly improved, and the maximum activity was up to 1178.32 μmmol h−1 g−1 for 5Mo2C/CdIn2S4 composites. 5Mo2C/CdIn2S4 heterojunction composites possess excellent durability in three cycles (loss of 6%). Additionally, the mechanism of increased activity for composites was also investigated. This study provides a guide to designing noble-metal-free photocatalyst for highly efficient photocatalytic H2 evolution.

Post-cyclization of a bisimine-linked covalent organic framework to enhance the performance of visible-light photocatalytic hydrogen evolution

The photocatalytic hydrogen evolution rate of a bisimine-linked COF was enhanced 3 times when partially converted to an imidazolium-linked COF.

Rethinking Electronic Effects in Photochemical Hydrogen Evolution Using CuInS2@ZnS Quantum Dots Sensitizers

Molecular catalysts based on coordination complexes for the generation of hydrogen via photochemical water splitting exhibit a large versatility and tunability of the catalytic properties through chemical functionalization. In the present work, we report on light-driven hydrogen production in an aqueous solution using a series of cobalt polypyridine complexes as hydrogen evolving catalysts (HECs) in combination with CuInS₂@ZnS quantum dots (QDs) as sensitizers, and ascorbate as the electron donor. A peculiar trend in activity has been observed depending on the substituents present on the polypyridine ligand. This trend markedly differs from that previously recorded using [Ru(bpy)₃]²⁺ (where bpy = 2,2'-bipyridine) as the sensitizer and can be ascribed to different kinetically limiting pathways in the photochemical reaction (viz. protonation kinetics with the ruthenium chromophore, catalyst activation via electron transfer from the QDs in the present system). Hence, this work shows how the electronic effects on light-triggered molecular catalysis are not exclusive features of the catalyst unit but depend on the whole photochemical system.

Switching Excited State Distribution of Metal-Organic Framework for Dramatically Boosting Photocatalysis

Photosensitization associated with electron/energy transfer represents the central science of natural photosynthesis. Herein, we proposed a protocol to dramatically improve the sensitizing ability of metal-organic frameworks (MOFs) by switching their excited state distribution from ³ MLCT (metal-to-ligand charge transfer) to ³ IL (intraligand). The hierarchical organization of ³ IL MOFs and Co/Cu catalysts facilitates electron transfer for efficient photocatalytic H₂ evolution with a yield of 26 844.6 μmol g^-1 and CO₂ photoreduction with a record HCOOH yield of 4807.6 μmol g^-1 among all the MOF photocatalysts. Systematic investigations demonstrate that strong visible-light-absorption, long-lived excited state and ingenious multi-component synergy in the ³ IL MOFs can facilitate both interface and intra-framework electron transfer to boost photocatalysis. This work opens up an avenue to boost solar-energy conversion by engineering sensitizing centers at a molecular level.

A Semiconductor-Mediator-Catalyst Artificial Photosynthetic System for Photoelectrochemical Water Oxidation

In fabricating an artificial photosynthesis (AP) electrode for water oxidation, we have devised a semiconductor-mediator-catalyst structure that mimics photosystem II (PSII). It is based on a surface layer of vertically grown nanorods of Fe₂ O₃ on fluorine doped tin oxide (FTO) electrodes with a carbazole mediator base and a Ru(II) carbene complex on a nanolayer of TiO₂ as a water oxidation co-catalyst. The resulting hybrid assembly, FTO|Fe₂ O₃ |-carbazole|TiO₂ |-Ru(carbene), demonstrates an enhanced photoelectrochemical (PEC) water oxidation performance compared to an electrode without the added carbaozle base with an increase in photocurrent density of 2.2-fold at 0.95 V vs. NHE and a negatively shifted onset potential of 500 mV. The enhanced PEC performance is attributable to carbazole mediator accelerated interfacial hole transfer from Fe₂ O₃ to the Ru(II) carbene co-catalyst, with an improved effective surface area for the water oxidation reaction and reduced charge transfer resistance.

Enhancing Photocatalytic Hydrogen Production via the Construction of Robust Multivariate Ti-MOF/COF Composite

Titanium metal-organic frameworks (Ti-MOFs), as an appealing type of artificial photocatalysts, have shown great potentials in the field of solar energy conversion due to their well-studied photo-redox activity similar to TiO 2 and good optical responsiveness of linkers serving as the antenna to absorb visible-light. Although enormous efforts have been dedicated to developing Ti-MOFs with high photocatalytic activity, their solar energy conversion performances are still poor. Herein, a covalent-integrated strategy has been implemented to construct a series of multivariate Ti-MOF/COF hybrid materials, PdTCPP⸦PCN-415(NH 2 )/TpPa (composites 1, 2, and 3), featuring excellent visible-light utilization, suitable band gap, and high surface area for photocatalytic H 2 production. Notably, the resulting composites demonstrated remarkably enhanced visible-light-driven photocatalytic H 2 evolution performance, especially for the composite 2 with the maximum H 2 evolution rate of 13.98 mmol g -1 h -1 (turn-over frequency (TOF) = 227 h -1 ), which is much higher than the prototypical counterparts, PdTCPP⸦PCN-415(NH 2 ) (0.21 mmol g -1 h -1 ) and TpPa (6.51 mmol g -1 h -1 ). Our work thereby suggests a new approach to develop highly efficient photocatalysts for photocatalytic H 2 evolution reaction and beyond.

A semiconductor/molecular catalyst hybrid photoanode with FeOOH as an electron transfer relay

A Fe₂ O₃ /FeOOH/poly-Ru(bda)(vpy) (bda = 2,2'-bipyridine-6,6'-dicarboxylate,vpy = 4-vinylpyridine) photoanode has been fabricated by electropolymerization of molecular Ru(bda)(vpy) catalyst on FeOOH modified Fe₂ O₃ , in which a thin layer of FeOOH replicates the role of tyrosine residue in PSII as an efficient electron transfer mediator. The ternary hybrid photoanode produced a 2.4 times higher photocurrent density than that of previously reported Fe₂ O₃ /poly-Ru(bda)(vpy) under AM 1.5 G illumination and displayed a negative shift on the onset potential by 100 mV. In addition, the Fe₂ O₃ /FeOOH/poly-Ru(bda)(vpy) exhibited long-term stability for at least 10 h with a Faraday efficiency of ∼96%. The high performance shown here was attributed to the improved charge separation between excited semiconductor and the catalyst caused by FeOOH mediated electron transfer on the electrode surface.

Ultrafast Electron Transfer from CdSe Quantum Dots to an [FeFe]-Hydrogenase Mimic

The combination of CdSe nanoparticles as photosensitizers with [FeFe]-hydrogenase mimics is known to result in efficient systems for light-driven hydrogen generation with reported turnover numbers in the order of 10⁴-10⁶. Nevertheless, little is known about the details of the light-induced charge-transfer processes. Here, we investigate the time scale of light-induced electron transfer kinetics for a simple model system consisting of CdSe quantum dots (QDs) of 2.0 nm diameter and a simple [FeFe]-hydrogenase mimic adsorbed to the QD surface under noncatalytic conditions. Our (time-resolved) spectroscopic investigation shows that both hot electron transfer on a sub-ps time scale and band-edge electron transfer on a sub-10 ps time scale from photoexcited QDs to adsorbed [FeFe]-hydrogenase mimics occur. Fast recombination via back electron transfer is observed in the absence of a sacrificial agent or protons which, under real catalytic conditions, would quench remaining holes or could stabilize the charge separation, respectively.

Pentadecanuclear Fe-Containing Polyoxometalate Catalyst for Visible-Light-Driven Generation of Hydrogen

The structurally new, carbon-free pentadecanuclear Fe-containing polyoxometalate, Na₂₁[NaFe₁₅(OH)₁₂(PO₄)₄(A-α-SiW₉O₃₄)₄]·85H₂O (Na₂₁-Fe₁₅P₄(SiW₉)₄), was synthesized using a facile one-pot, solution-based synthetic approach and systematically characterized by various spectroscopic techniques. Single-crystal X-ray diffraction reveals that the title complex is composed of two [Fe₄(A-α-SiW₉O₃₄)] fragments and two [Fe_3.5(A-α-SiW₉O₃₄)] fragments stabilized by four PO₄ linkers in a tetrameric style with idealized T_d point group symmetry. When coupling with (4,4'-ditert-butyl-2,2'-dipyridyl)-bis(coumarin)-iridium(III) hexafluorophosphate ([Ir(coumarin)₂(dtbbpy)][PF₆]) photosensitizer and triethanolamine (TEOA) sacrificial electron donor, polyoxoanion Fe₁₅P₄(SiW₉)₄ effectively catalyzed hydrogen production with a minimally optimized TON of 986, which represents, to our knowledge, one of the highest values among known Fe-substituted POM-catalyzed hydrogen production systems. Both a mercury-poisoning test and FT-IR characterizations proved the structural stability of Fe₁₅P₄(SiW₉)₄ catalyst under photocatalytic conditions. The photocatalytic mechanism of the present hydrogen-evolving system was investigated by time-solved luminescence and static emission quenching measurements.

Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes

This review provides a comprehensive overview on characterisation techniques for light-driven redox-catalysts highlighting spectroscopic, microscopic, electrochemical and spectroelectrochemical approaches.

Bioinspired metal complexes for energy-related photocatalytic small molecule transformation

Bioinspired transformation of small-molecules to energy-related feedstocks is an attractive research area to overcome both the environmental issues and the depletion of fossil fuels. The highly effective metalloenzymes in nature provide blueprints for the utilization of bioinspired metal complexes for artificial photosynthesis. Through simpler structural and functional mimics, the representative herein is the pivotal development of several critical small molecule conversions catalyzed by metal complexes, e.g., water oxidation, proton and CO₂ reduction and organic chemical transformation of small molecules. Of great achievement is the establishment of bioinspired metal complexes as catalysts with high stability, specific selectivity and satisfactory efficiency to drive the multiple-electron and multiple-proton processes related to small molecule transformation. Also, potential opportunities and challenges for future development in these appealing areas are highlighted.

Controlling Pt co-catalyst loading in a WO3 quantum dot and MoS2 nanosheet composite Z-scheme system for enhanced photocatalytic H2 evolution

A solid-state Z-scheme system, with the synergistic integration of the advantages of various narrow-band semiconductors, is considered to be a potential strategy to develop efficient photocatalysts for operation under visible light illumination. However, the charge separation efficiency of these systems has always been reduced by disordered electron transfer between coupling semiconductors. In this work, we constructed a direct Z-scheme system WO₃-MoS₂-Pt through the loading of WO₃ quantum dots onto MoS₂ nanosheets and the selective depositing of a Pt co-catalyst onto MoS₂. X-ray diffraction, transmission electron microscopy, atomic electron microscopy and x-ray photoelectron spectroscopy, etc were used to confirm the successful preparation of the targeted photocatalyst. This photocatalytic system showed high visible-light-driven H₂ evolution activity (802.2 μmol · h^-1 · g^-1) and good photostability. Control experiment and mechanism analysis suggested that the remarkable performance can be attributed to the heterojunction formed WO₃ and MoS₂ and the vectorial electron transfer (WO₃ → MoS₂ → Pt) achieved by selectively loading the Pt co-catalyst.

Unveiling Catalytic Sites in a Typical Hydrogen Photogeneration System Consisting of Semiconductor Quantum Dots and 3d-Metal Ions

Semiconductor quantum dots (QDs) in conjunction with non-noble 3d-metal ions (e.g., Fe³⁺, Co²⁺, and Ni²⁺) have emerged as an extremely efficient, facile, and cost-effective means of solar-driven hydrogen (H₂) evolution. However, the exact structural change of the active sites under realistic conditions remains elusive, and the mechanism of H₂ evolution behind the remarkable activity is poorly understood. Here, we successfully track the structural variation of the catalytic sites in the typical H₂ photogeneration system consisting of CdSe/CdS QDs and 3d-metal ions (i.e., Ni²⁺ used here). That is, the nickel precursor of Ni(OAc)₂ changes to Ni(H₂O)₆²⁺ in neutral H₂O and eventually transforms to Ni(OH)₂ nanosheets in alkaline media. Furthermore, the in operando spectroscopic techniques of electron paramagnetic resonance and X-ray absorption spectroscopy reveal the photoinduced transformation of Ni(OH)₂ to a defective structure [Ni_x⁰/Ni_1-x(OH)₂], which acts as the real catalytic species of H₂ photogeneration. Density functional theory (DFT) calculations further indicate that the surface Ni-vacancies (V_Ni) on the Ni(OH)₂ nanosheets enhance the adsorption and dissociation of H₂O molecules to enhance the local proton concentration, while the Ni⁰ clusters behave as H₂-evolution sites, thereby synergistically promoting the activity of H₂ photogeneration in alkaline media.

Synthesis of water-soluble Ni(II) complexes and their role in photo-induced electron transfer with MPA-CdTe quantum dots

Photocatalytic water splitting using solar energy for hydrogen production offers a promising alternative form of storable and clean energy for the future. To design an artificial photosynthesis system that is cost-effective and scalable, earth abundant elements must be used to develop each of the components of the assembly. To develop artificial photosynthetic systems, we need to couple a catalyst for proton reduction to a photosensitizer and understand the mechanism of photo-induced electron transfer from the photosensitizer to the catalyst that serves as the fundamental step for photocatalysis. Therefore, our work is focused on the study of light driven electron transfer kinetics from the quantum dot systems made with inorganic chalcogenides in the presence of Ni-based reduction catalysts. Herein, we report the synthesis and characterization of four Ni(II) complexes of tetradentate ligands with amine and pyridine functionalities (N2/Py2) and their interactions with CdTe quantum dots stabilized by 3-mercaptopropionic acid. The lifetime of the quantum dots was investigated in the presence of the Ni complexes and absorbance, emission and electrochemical measurements were performed to gain a deeper understanding of the photo-induced electron transfer process.

Conjugated nanoporous polycarbazole bearing a cobalt complex for efficient visible-light driven hydrogen evolution

A conjugated nanoporous polycarbazole (CNP) cross-linked by pyridine and coordinated to Co(iii) displays high catalytic performance for visible light-driven H₂ generation.

A broadband and strong visible-light-absorbing photosensitizer boosts hydrogen evolution

Developing broadband and strong visible-light-absorbing photosensitizer is highly desired for dramatically improving the utilization of solar energy and boosting artificial photosynthesis. Herein, we develop a facile strategy to co-sensitize Ir-complex with Coumarins and boron dipyrromethene to explore photosensitizer with a broadband covering ca. 50% visible light region (Ir-4). This type of photosensitizer is firstly introduced into water splitting system, exhibiting significantly enhanced performance with over 21 times higher than that of typical Ir(ppy)2(bpy)+, and the turnover number towards Ir-4 reaches to 115840, representing the most active sensitizer among reported molecular photocatalytic systems. Experimental and theoretical investigations reveal that the Ir-mediation not only achieves a long-lived boron dipyrromethene-localized triplet state, but also makes an efficient excitation energy transfer from Coumarin to boron dipyrromethene to trigger the electron transfer. These findings provide an insight for developing broadband and strong visible-light-absorbing multicomponent arrays on molecular level for efficient artificial photosynthesis.

A bio-inspired strategy for enhanced hydrogen evolution: carbonate ions as hole vehicles to promote carrier separation

Natural photosynthesis involves a subtle electron transfer mechanism in which freely-moving electron transfer intermediates (plastoquinone and plastocyanin) are capable of effectively separating the photo-generated carriers, and therefore, it has high quantum efficiency. Inspired by this mechanism, in this study, carbonate (CO32-) ions were employed as hole vehicles to promote photo-generated carrier separation, and greatly improved the photocatalytic hydrogen evolution activity of K4Nb6O17 nanosheets. The hydrogen evolution rate at the optimal concentration of CO32- ions reached 2018 μmol h-1 g-1, which was 16.3 times that of the blank sample (124 μmol h-1 g-1). This marked enhancement was based on the transfer of holes from the photocatalyst to the sacrificial reagent (methanol) via CO32- ions; this process is faster than direct hole transfer between the photocatalyst and sacrificial reagent. This bio-inspired strategy provides a facile and cost-effective approach to improve the solar-to-fuel conversion efficiency of photocatalysts.

Control of the overpotential of a [FeFe] hydrogenase mimic by a synthetic second coordination sphere

Hydrogen as a renewable fuel is viable when produced sustainably via proton reduction catalysis (PRC). Many homogeneous electrocatalysts perform PRC with high rates, but they all require a large overpotential to drive the reaction. Natural hydrogenase enzymes achieve reversible PRC with potentials close to the thermodynamic equilibrium through confinement of the active site in a well-defined protein pocket. Inspired by nature, we report a strategy that relies on the selective encapsulation of a synthetic hydrogenase mimic in a novel supramolecular cage. Catalyst confinement decreases the PRC overpotential by 150 mV, and is proposed to originate from the cationic cage stabilizing anionic reaction intermediates within the catalytic cycle.

Photocatalytic Hydrogen Production: Role of Sacrificial Reagents on the Activity of Oxide, Carbon, and Sulfide Catalysts

Photocatalytic water splitting is a sustainable technology for the production of clean fuel in terms of hydrogen (H2). In the present study, hydrogen (H2) production efficiency of three promising photocatalysts (titania (TiO2-P25), graphitic carbon nitride (g-C3N4), and cadmium sulfide (CdS)) was evaluated in detail using various sacrificial agents. The effect of most commonly used sacrificial agents in the recent years, such as methanol, ethanol, isopropanol, ethylene glycol, glycerol, lactic acid, glucose, sodium sulfide, sodium sulfite, sodium sulfide/sodium sulfite mixture, and triethanolamine, were evaluated on TiO2-P25, g-C3N4, and CdS. H2 production experiments were carried out under simulated solar light irradiation in an immersion type photo-reactor. All the experiments were performed without any noble metal co-catalyst. Moreover, photolysis experiments were executed to study the H2 generation in the absence of a catalyst. The results were discussed specifically in terms of chemical reactions, pH of the reaction medium, hydroxyl groups, alpha hydrogen, and carbon chain length of sacrificial agents. The results revealed that glucose and glycerol are the most suitable sacrificial agents for an oxide photocatalyst. Triethanolamine is the ideal sacrificial agent for carbon and sulfide photocatalyst. A remarkable amount of H2 was produced from the photolysis of sodium sulfide and sodium sulfide/sodium sulfite mixture without any photocatalyst. The findings of this study would be highly beneficial for the selection of sacrificial agents for a particular photocatalyst.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Enantioselective synthesis of chiral heterocyclic biaryls via asymmetric Suzuki-Miyaura cross-coupling of 3-bromopyridine derivatives

A series of chiral heterocyclic biaryls with a pyridyl moiety were prepared in moderate to good yields with up to 92% ee via asymmetric Suzuki-Miyaura coupling. The chiral-bridged biphenyl monophosphine ligand L1 was found to be much more effective in the reaction enantioselection than its counterpart binaphthyl monophosphine ligands.

Metal–organic framework assisted and in situ synthesis of hollow CdS nanostructures with highly efficient photocatalytic hydrogen evolution

Hollow CdS nanoboxes with a specific surface area of 153 m² g⁻¹ are synthesized through in situ sulfurizing Cd-MOF-47 with thiourea, which exhibit a greatly improved photocatalytic activity in water splitting to hydrogen (21 654 μmol g⁻¹ h⁻¹).

Design of Carbon Dots for Metal-free Photoredox Catalysis

The photoreduction potential of a set of four different carbon dots (CDs) was investigated. The CDs were synthesized by using two different preparation methods-hydrothermal and pyrolytic-and two sets of reagents-neat citric acid and citric acid doped with diethylenetriamine. The hydrothermal syntheses yielded amorphous CDs, which were either nondoped (a-CDs) or nitrogen-doped (a-N-CDs), whereas the pyrolytic treatment afforded graphitic CDs, either non-doped (g-CDs) or nitrogen-doped (g-N-CDs). The morphology, structure, and optical properties of four different types of CDs revealed significant differences depending on the synthetic pathway. The photocatalytic activities of the CDs were investigated as such, that is, in the absence of any other redox mediators, on the model photoreduction reaction of methyl viologen. The observed photocatalytic reaction rates: a-N-CDs ≥ g-CDs > a-CDs ≥ g-N-CDs were correlated with the presence/absence of fluorophores, to the graphitic core, and to quenching interactions between the two. The results indicate that nitrogen doping reverses the photoredox reactivity between amorphous and graphitic CDs and that amorphous N-doped CDs are the most photoredox active, a yet unknown fact that demonstrates the tunable potential of CDs for ad hoc applications.

Photocatalytic H2 production using a hybrid assembly of an [FeFe]-hydrogenase model and CdSe quantum dot linked through a thiolato-functionalized cyclodextrin

It is a great challenge to develop iron-based highly-efficient and durable catalytic systems for the hydrogen evolution reaction (HER) by understanding and learning from [FeFe]-hydrogenases. Here we report photocatalytic H₂ production by a hybrid assembly of a sulfonate-functionalized [FeFe]-hydrogenase mimic (1) and CdSe quantum dot (QD), which is denoted as 1/β-CD-6-S-CdSe (β-CD-6-SH = 6-mercapto-β-cyclodextrin). In this assembly, thiolato-functionalized β-CD acts not only as a stabilizing reagent of CdSe QDs but also as a host compound for the diiron catalyst, so as to confine CdSe QDs to the space near the site of diiron catalyst. In addition, another two reference systems comprising MAA-CdSe QDs (HMAA = mercaptoacetic acid) and 1 in the presence and absence of β-CD, denoted as 1/β-CD/MAA-CdSe and 1/MAA-CdSe, were studied for photocatalytic H₂ evolution. The influences of β-CD and the stabilizing reagent β-CD-6-S^- on the stability of diiron catalyst, the fluorescence lifetime of CdSe QDs, the apparent electron transfer rate, and the photocatalytic H₂-evolving efficiency were explored by comparative studies of the three hybrid systems. The 1/β-CD-6-S-CdSe system displayed a faster apparent rate for electron transfer from CdSe QDs to the diiron catalyst compared to that observed for MAA-CdSe-based systems. The total TON for visible-light driven H₂ evolution by the 1/β-CD-6-S-CdSe QDs in water at pH 4.5 is about 2370, corresponding to a TOF of 150 h^-1 in the initial 10 h of illumination, which is 2.7- and 6.6-fold more than the amount of H₂ produced from the reference systems 1/β-CD/MAA-CdSe and 1/MAA-CdSe. Additionally, 1/β-CD-6-S-CdSe gave 2.4-5.1 fold enhancement in the apparent quantum yield and significantly improved the stability of the system for photocatalytic H₂ evolution.

Photocatalytic Hydrogen Evolution by a Synthetic [FeFe] Hydrogenase Mimic Encapsulated in a Porphyrin Cage

The design of a biomimetic and fully base metal photocatalytic system for photocatalytic proton reduction in a homogeneous medium is described. A synthetic pyridylphosphole-appended [FeFe] hydrogenase mimic was encapsulated inside a supramolecular zinc porphyrin-based metal-organic cage structure Fe4 (Zn-L)6 . The binding is driven by the selective pyridine-zinc porphyrin interaction and results in the catalyst being bound strongly inside the hydrophobic cavity of the cage. Excitation of the capsule-forming porphyrin ligands with visible light while probing the IR spectrum confirmed that electron transfer takes place from the excited porphyrin cage to the catalyst residing inside the capsule. Light-driven proton reduction was achieved by irradiation of an acidic solution of the caged catalyst with visible light.

Carbon dots as photosensitisers for solar-driven catalysis

Artificial photosynthesis is the mimicry of the natural process of solar energy conversion into chemical energy carriers. Photocatalytic systems that combine light-harvesting materials and catalysts in solution or suspension provide a promising route towards this goal. A key requirement for a sustainable solar fuel production system is a low-cost, stable and non-toxic light harvester. Photoluminescent carbon nanoparticles, carbon dots (CDs), are promising emerging light-harvesters for photocatalytic fuel production systems. CDs possess many desirable properties for this purpose, such as inexpensive, scalable synthetic routes, low-toxicity and tuneable surface chemistry. In this tutorial review, the integration of CDs in photocatalytic fuel generation systems with metallic, molecular and enzymatic catalysts is discussed. An overview of CD types, synthesis and properties is given along with a discussion of tuneable CD properties that can be optimised for applications in photocatalysis. Current understanding of the photophysical electron transfer processes present in CD photocatalytic systems is outlined and various avenues for their further development are highlighted.

Electron donor-free photoredox catalysis via an electron transfer cascade by cooperative organic photocatalysts

Electron transfer cascade in cooperative organic photocatalysts can prevent the use of sacrificial reagent for photoredox catalysis.