Photocatalytic Hydrogen Production using Polymeric Carbon Nitride with a Hydrogenase and a Bioinspired Synthetic Ni Catalyst

Sol-Gel Processing of Water-Soluble Carbon Nitride Enables High-Performance Photoanodes*

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based sol-gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol-gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm-2 under 2 sun) down to <0 V vs. RHE. This enables even effective PEC operation under zero-bias conditions and represents the very first example of a 'soft matter'-based PEC system capable of bias-free photoreforming. The robust binder-free films derived from sol-gel processing of water-soluble PCN thus constitute a new paradigm for high-performance 'soft matter' photoelectrocatalytic systems and pave the way for further applications in which high-quality PCN films are required.

Electric-Field-Mediated Electron Tunneling of Supramolecular Naphthalimide Nanostructures for Biomimetic H2 Production

The design and synthesis of two semiconducting bis (4-ethynyl-bridging 1, 8-naphthalimide) bolaamphiphiles (BENI-COO^- and BENI-NH₃ ⁺ ) to fabricate supramolecular metal-insulator-semiconductor (MIS) nanostructures for biomimetic hydrogen evolution under visible light irradiation is presented. A H₂ evolution rate of ca. 3.12 mmol g^-1 ⋅h^-1 and an apparent quantum efficiency (AQE) of ca. 1.63 % at 400 nm were achieved over the BENI-COO^- -NH₃ ⁺ -Ni MIS photosystem prepared by electrostatic self-assembly of BENI-COO^- with the opposite-charged DuBois-Ni catalysts. The hot electrons of photoexcited BENI-COO^- nanofibers were tunneled to the molecular Ni collectors across a salt bridge and an alkyl region of 2.2-2.5 nm length at a rate of 6.10×10⁸  s^-1 , which is five times larger than the BENI-NH₃ ⁺ nanoribbons (1.17×10⁸  s^-1 ). The electric field benefited significantly the electron tunneling dynamics and compensated the charge-separated states insufficient in the BENI-COO^- nanofibers.

Biuret-A Crucial Reaction Intermediate for Understanding Urea Pyrolysis To Form Carbon Nitrides: Crystal-Structure Elucidation and In Situ Diffractometric, Vibrational and Thermal Characterisation

The crystal structure of biuret was elucidated by means of XRD analysis of single crystals grown through slow evaporation from a solution in ethanol. It crystallises in its own structure type in space group C2/c (a=15.4135(8) Å, b=6.6042(3) Å, c=9.3055(4) Å, Z=8). Biuret decomposition was studied in situ by means of temperature-programmed powder XRD and FTIR spectroscopy, to identify a co-crystalline biuret-cyanuric acid phase as a previously unrecognised reaction intermediate. Extensive thermogravimetric studies of varying crucible geometry, heating rate and initial sample mass reveal that the concentration of reactive gases at the interface to the condensed sample residues is a crucial parameter for the prevailing decomposition pathway. Taking these findings into consideration, a study on the optimisation of carbon nitride synthesis from urea on the gram scale, with standard solid-state laboratory techniques, is presented. Finally, a serendipitously encountered self-coating of the crucible inner walls by graphite during repeated synthetic cycles, which prove to be highly beneficial for the obtained yields, is reported.

Molecular Junctions on Polymeric Carbon Nitrides with Enhanced Photocatalytic Performance

Molecular catalysts (MC), namely homogeneous catalysts, have demonstrated great promise for efficient solar-to-chemical energy conversion in the hybrid system. However, the poor interfacial interaction between MC and photosensitizers (PS) impedes the efficient and fast interfacial electron transfer. To promote interfacial communication between PS and MC, a proof-of-concept method was developed for the combination of polymeric carbon nitride (PCN) PS with bipyridine cobalt [Co(bpy)₃ ²⁺ ] MC by covalent bonds, creating molecular junctions to promote interfacial electron transfer as confirmed by transient photoluminescence lifetime and electrochemical measurements. As a result, the binary photocatalyst [Co(bpy)₃ ²⁺ /BINA₂ -CN] showed extensively enhanced photocatalytic activity such as H₂ and CO₂ reduction in comparison with the physical mixture of Co(bpy)₃ ²⁺ and PCN. This observation highlights the importance of construction of surface molecular junctions between PS and MC to accelerate the interfacial charge-carrier mobility and, consequently, improve the photocatalytic activity.

Controlled Decoration of Divalent Nickel onto CdS/CdSe Core/Shell Quantum Dots to Boost Visible-Light-Induced Hydrogen Generation in Water

The search for a low-cost, noble-metal-free cocatalyst to replace expensive Pt for hydrogen (H₂ ) photogeneration in water has become a hot research topic, and among these, Ni-based cocatalysts are promising and highly desired. Developing new strategies and protocols to obtain Ni-based cocatalysts with high activity is therefore vitally important. Herein, we develop a new method to efficiently decorate divalent Ni onto pre-synthesized CdS/CdSe core/shell quantum dots (QDs). The concentration of Ni on the QDs can be easily tuned by varying the amount of the Ni precursor introduced during the synthesis. Further analyses reveal that Ni²⁺ can be strongly decorated onto QDs. Impressively, the Ni-decorated QDs displayed a significantly enhanced H₂ photogeneration performance as compared to the two components prepared separately. Through the optimization of the Ni concentration on the QDs, the turnover frequency (TOF) with respect to Ni and quantum yield ( Φ H 2 ) at 520 nm for H₂ evolution from water could reach 322 h^-1 and 12.3 %, respectively. A possible mechanism has also been proposed and discussed in detail.

Synthesis of Ergosterol Peroxide Conjugates as Mitochondria Targeting Probes for Enhanced Anticancer Activity

Inspired by the significant bioactivity of ergosterol peroxide, we designed and synthesized four fluorescent coumarin and ergosterol peroxide conjugates 8a–d through the combination of ergosterol peroxide with 7-N,N-diethylamino coumarins fluorophore. The cytotoxicity of synthesized conjugates against three human cancer cells (HepG2, SK-Hep1, and MCF-7) was evaluated. The results of fluorescent imaging showed that the synthesized conjugates 8a–d localized and enriched mainly in mitochondria, leading to significantly enhanced cytotoxicity over ergosterol peroxide. Furthermore, the results of biological functions of 8d showed that it could suppress cell colony formation, invasion, and migration; induce G2/M phase arrest of HepG2 cells, and increase the intracellular ROS level.

Solar Water Splitting with a Hydrogenase Integrated in Photoelectrochemical Tandem Cells

Hydrogenases (H₂ases) are benchmark electrocatalysts for H₂ production, both in biology and (photo)catalysis in vitro. We report the tailoring of a p‐type Si photocathode for optimal loading and wiring of H₂ase through the introduction of a hierarchical inverse opal (IO) TiO₂ interlayer. This proton‐reducing Si|IO‐TiO₂|H₂ase photocathode is capable of driving overall water splitting in combination with a photoanode. We demonstrate unassisted (bias‐free) water splitting by wiring Si|IO‐TiO₂|H₂ase to a modified BiVO₄ photoanode in a photoelectrochemical (PEC) cell during several hours of irradiation. Connecting the Si|IO‐TiO₂|H₂ase to a photosystem II (PSII) photoanode provides proof of concept for an engineered Z‐scheme that replaces the non‐complementary, natural light absorber photosystem I with a complementary abiotic silicon photocathode.

Photobiocatalysis: Activating Redox Enzymes by Direct or Indirect Transfer of Photoinduced Electrons

Biocatalytic transformation has received increasing attention in the green synthesis of chemicals because of the diversity of enzymes, their high catalytic activities and specificities, and mild reaction conditions. The idea of solar energy utilization in chemical synthesis through the combination of photocatalysis and biocatalysis provides an opportunity to make the "green" process greener. Oxidoreductases catalyze redox transformation of substrates by exchanging electrons at the enzyme's active site, often with the aid of electron mediator(s) as a counterpart. Recent progress indicates that photoinduced electron transfer using organic (or inorganic) photosensitizers can activate a wide spectrum of redox enzymes to catalyze fuel-forming reactions (e.g., H₂ evolution, CO₂ reduction) and synthetically useful reductions (e.g., asymmetric reduction, oxygenation, hydroxylation, epoxidation, Baeyer-Villiger oxidation). This Review provides an overview of recent advances in light-driven activation of redox enzymes through direct or indirect transfer of photoinduced electrons.

Photoelectrochemical H2 Evolution with a Hydrogenase Immobilized on a TiO2 -Protected Silicon Electrode

The combination of enzymes with semiconductors enables the photoelectrochemical characterization of electron‐transfer processes at highly active and well‐defined catalytic sites on a light‐harvesting electrode surface. Herein, we report the integration of a hydrogenase on a TiO₂‐coated p‐Si photocathode for the photo‐reduction of protons to H₂. The immobilized hydrogenase exhibits activity on Si attributable to a bifunctional TiO₂ layer, which protects the Si electrode from oxidation and acts as a biocompatible support layer for the productive adsorption of the enzyme. The p‐Si|TiO₂|hydrogenase photocathode displays visible‐light driven production of H₂ at an energy‐storing, positive electrochemical potential and an essentially quantitative faradaic efficiency. We have thus established a widely applicable platform to wire redox enzymes in an active configuration on a p‐type semiconductor photocathode through the engineering of the enzyme–materials interface.

Photoelectrochemical H2 Evolution with a Hydrogenase Immobilized on a TiO2-Protected Silicon Electrode

The combination of enzymes with semiconductors enables the photoelectrochemical characterization of electron‐transfer processes at highly active and well‐defined catalytic sites on a light‐harvesting electrode surface. Herein, we report the integration of a hydrogenase on a TiO₂‐coated p‐Si photocathode for the photo‐reduction of protons to H₂. The immobilized hydrogenase exhibits activity on Si attributable to a bifunctional TiO₂ layer, which protects the Si electrode from oxidation and acts as a biocompatible support layer for the productive adsorption of the enzyme. The p‐Si|TiO₂|hydrogenase photocathode displays visible‐light driven production of H₂ at an energy‐storing, positive electrochemical potential and an essentially quantitative faradaic efficiency. We have thus established a widely applicable platform to wire redox enzymes in an active configuration on a p‐type semiconductor photocathode through the engineering of the enzyme–materials interface.

Enhancing electrocatalytic hydrogen evolution by nickel salicylaldimine complexes with alkali metal cations in aqueous media

The pendant, chelating ethers in the second coordination sphere of nickel salicylaldimine complexes bind alkali metals to promote hydrogen evolution electrocatalysis.

Synthetic Active Site Model of the [NiFeSe] Hydrogenase

A dinuclear synthetic model of the [NiFeSe] hydrogenase active site and a structural, spectroscopic and electrochemical analysis of this complex is reported. [NiFe(‘S₂Se₂’)(CO)₃] (H₂‘S₂Se₂’=1,2-bis(2-thiabutyl-3,3-dimethyl-4-selenol)benzene) has been synthesized by reacting the nickel selenolate complex [Ni(‘S₂Se₂’)] with [Fe(CO)₃bda] (bda=benzylideneacetone). X-ray crystal structure analysis confirms that [NiFe(‘S₂Se₂’)(CO)₃] mimics the key structural features of the enzyme active site, including a doubly bridged heterobimetallic nickel and iron center with a selenolate terminally coordinated to the nickel center. Comparison of [NiFe(‘S₂Se₂’)(CO)₃] with the previously reported thiolate analogue [NiFe(‘S₄’)(CO)₃] (H₂‘S₄’=H₂xbsms=1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene) showed that the selenolate groups in [NiFe(‘S₂Se₂’)(CO)₃] give lower carbonyl stretching frequencies in the IR spectrum. Electrochemical studies of [NiFe(‘S₂Se₂’)(CO)₃] and [NiFe(‘S₄’)(CO)₃] demonstrated that both complexes do not operate as homogenous H₂ evolution catalysts, but are precursors to a solid deposit on an electrode surface for H₂ evolution catalysis in organic and aqueous solution.