Enhancing H2 evolution performance of an immobilised cobalt catalyst by rational ligand design

Synergy of {Co(H2O)6}2+ with a Polyoxometalate Leads to Aqueous Homogeneous Hydrogen Evolution: Experiments and Computations

In this work, we have described a polyoxometalate (POM)-based inexpensive and easily synthesizable compound [Co(H2O)6]2[{K(H2O)}2V10O28]·2H2O (1), which exhibits electrocatalytic hydrogen evolution in its aqueous solution without its decomposition (or electrodeposition), acting as a rare homogeneous electrocatalyst. Even though the compound [Co(H2O)6]2[{K(H2O)}2V10O28]·2H2O (1) (soluble in water) shows electrocatalytic hydrogen evolution reaction (HER) activity because of the Coulombic attraction, including H-bonding interactions, between the [Co(H2O)6]2+ cationic species and [{K(H2O)}2V10O28]4-anionic species, the individual homogeneous solutions of [V10O28]6- (source: Na6[V10O28]·18H2O) and [Co(H2O)6]2+ (source: CoCl2·6H2O) do not show any electrocatalytic HER activity. We have thus established that the synergy of [V10O28]6- with [Co(H2O)6]2+ in crystal matrix as well as in the aqueous solution of 1 makes the compound 1 a stable and highly active electrocatalyst for homogeneous HER in an aqueous solution. In order to corroborate these homogeneous HER studies, we performed density functional theory (DFT) calculations to show that decavanadate cluster anion [V10O28]6- interacts with hexa-aqua complex cation [Co(H2O)6]2+ via strong H-bonding interactions, leading to a synergy effect that enables the cobalt center of [Co(H2O)6]2+ to be an active site of HER in the present work.

In situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow-Core Photonic Crystal Fiber Microreactors

Hollow-core photonic crystal fibers (HC-PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette-based UV/Vis spectroscopy can be used to detect and identify the CoI and CoII states in hydrogen-evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT-simulated spectra. Our findings show that there are two types of signals observed for these molecular catalysts; a transient signal and a steady-state signal, with the former being assigned to the CoI state and the latter being assigned to the CoII state. These observations lend support to a unimolecular pathway, rather than a bimolecular pathway, for hydrogen evolution. This study highlights the utility of fiber-based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future.

Improved Photocatalytic H2 Evolution by Cobaloxime-Tethered Imidazole-Functionalized Periodic Mesoporous Organosilica

Molecular cobaloxime-based heterogeneous systems have attracted great interest during the last decades in light-driven hydrogen production. Here, we present a novel cobaloxime-tethered periodic mesoporous organosilica (PMO) hybrid (Im-EtPMO-Co) prepared through the immobilization of a molecular cobaloxime complex on the imidazole groups present in ethylene-bridged PMO. The successful assembly of a molecular cobaloxime catalyst via cobalt-imidazole axial ligation has been evidenced by several techniques, such as 13C NMR, Raman spectroscopy, ICP-MS, and XPS. The catalytic performance of Im-EtPMO-Co catalyst was essayed on the hydrogen evolution reaction (HER) under visible light in presence of a photosensitizer (Eosin Y) and an electron donor (TEOA). It showed an excellent hydrogen production of 95 mmol hydrogen at 2.5 h, which corresponded to a TON of 138. These results reflect an improved photocatalytic activity with respect to its homogenous counterpart [Co(dmgH)2(Im)Cl] as well as a previous cobaloxime-PMO system with pyridine axial ligation to the cobaloxime complex.

Photocatalytic CO2 -to-Syngas Evolution with Molecular Catalyst Metal-Organic Framework Nanozymes

Syngas, a mixture of CO and H₂ , is a high-priority intermediate for producing several commodity chemicals, e.g., ammonia, methanol, and synthetic hydrocarbon fuels. Accordingly, parallel sunlight-driven catalytic conversion of CO₂ and protons to syngas is a key step toward a sustainable energy cycle. State-of-the-art catalytic systems and materials often fall short as application-oriented concurrent CO and H₂ evolution requires challenging reaction conditions which can hamper stability, selectivity, and efficiency. Here a light-harvesting metal-organic framework hosting two molecular catalysts is engineered to yield colloidal, water-stable, versatile nanoreactors for photocatalytic syngas generation with highly controllable product ratios. In-depth fluorescence, X-ray, and microscopic studies paired with kinetic analysis show that the host delivers energy efficiently to active sites, conceptually yielding nanozymes. This unlocked sustained CO₂ reduction and H₂ evolution with benchmark turnover numbers and record incident photon conversions up to 36%, showcasing a highly active and durable all-in-one material toward application in solar energy-driven syngas generation.

Polyampholytic Graft Copolymers as Matrix for TiO2 /Eosin Y/[Mo3 S13 ]2- Hybrid Materials and Light-Driven Catalysis

An effective strategy to enhance the performance of inorganic semiconductors is moving towards organic-inorganic hybrid materials. Here, we report the design of core-shell hybrid materials based on a TiO2 core functionalized with a polyampholytic (poly(dehydroalanine)-graft-(n-propyl phosphonic acid acrylamide) shell (PDha-g-PAA@TiO2 ). The PDha-g-PAA shell facilitates the efficient immobilization of the photosensitizer Eosin Y (EY) and enables electronic interactions between EY and the TiO2 core. This resulted in high visible-light-driven H2 generation. The enhanced light-driven catalytic activity is attributed to the unique core-shell design with the graft copolymer acting as bridge and facilitating electron and proton transfer, thereby also preventing the degradation of EY. Further catalytic enhancement of PDha-g-PAA@TiO2 was possible by introducing [Mo3 S13 ]2- cluster anions as hydrogen-evolution cocatalyst. This novel design approach is an example for a multi-component system in which reactivity can in future be independently tuned by selection of the desired molecular or polymeric species.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Efficient homogeneous electrocatalytic hydrogen evolution using a Ni-containing polyoxometalate catalyst

NiCl₂·6H₂O ([Ni(H₂O)₆]₂Cl₂) per se does not show electrocatalytic hydrogen evolution reaction activity (HER) in an acidic aqueous medium as well as in neutral water. Interestingly, when [Ni(H₂O)₆]²⁺ is present in a polyoxovanadate matrix, for example, in the compound K₂[Ni(H₂O)₆]₂[V₁₀O₂₈]·4H₂O (1), it exhibits homogeneous electrocatalytic HER activity in an acidic aqueous solution with a turn over frequency of 2.1 s^-1 and an effective low overpotential of 127 mV at pH 2.3. Compound 1 is the first nickel-containing polyoxometalate catalyst for hydrogen production via homogeneous electrocatalytic proton reduction without its decomposition under electrochemical conditions of HER.

Polypyridyl Co complex-based water reduction catalysts: why replace a pyridine group with isoquinoline rather than quinoline?

The electronic effect of the substituent has been fully leveraged to improve the activity of molecular water reduction catalysts (WRCs). However, the steric effect of the substituents has received less attention. In this work, a steric hindrance effect was observed in a quinoline-involved polypyridyl Co complex-based water reduction catalyst (WRC), which impedes the formation of Co(iii)-H from Co(i), two pivotal intermediates for H₂ evolution, leading to significantly impaired electrocatalytic and photocatalytic activity with respect to its parent complex, [Co(TPA)Cl]Cl (TPA = tris(2-pyridinylmethyl)-amine). In sharp contrast, two isoquinoline-involved polypyridyl Co complexes exhibited significantly improved H₂ evolution efficiencies compared to [Co(TPA)Cl]Cl, benefitting mainly from the more basic and conjugated features of isoquinoline over pyridine. The dramatically different influences caused by the replacement of a pyridine group in the TPA ligand by quinoline and isoquinoline fully demonstrates the important roles of both the electronic and steric effects of a substituent. Our results may provide novel insights for designing more efficient WRCs.

Potent oxidation of DNA by Ru(ii) tri(polypyridyl) complexes under visible light irradiation via a singlet oxygen-mediated mechanism

The irradiation of Ru(ii) tri(polypridyl) complexes with visible light can induce potent oxidation of DNA mediated by ¹O₂via a type II photosensitization mechanism.

Flexible to rigid: IR spectroscopic investigation of a rhenium-tricarbonyl-complex at a buried interface

This work explores the solid-liquid interface of a rhenium-tricarbonyl complex embedded in a layer of zirconium oxide deposited by atomic layer deposition (ALD). Time-resolved and steady state infrared spectroscopy were applied to reveal the correlations between the thickness of the ALD layer and the spectroscopic response of the system. We observed a transition of the molecular environment from flexible to rigid, as well as limitations to ligand exchange and excited state quenching on the embedded complexes, when the ALD layer is roughly of the same height as the molecules.

Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO2

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.

Understanding the Performance of NiO Photocathodes with Alkyl-Derivatized Cobalt Catalysts and a Push-Pull Dye

Mesoporous NiO photocathodes containing the push-pull dye PB6 and alkyl-derivatized cobaloxime catalysts were prepared using surface amide couplings and analyzed for photocatalytic proton reduction catalysis. The length of the alkyl linker used to derivatize the cobalt catalysts was found to correlate to the photocurrent with the highest photocurrent observed using shorter alkyl linkers but the lowest one for samples without linker. The alkyl linkers were also helpful in slowing dye-NiO charge recombination. Photoelectrochemical measurements and femtosecond transient absorption spectroscopic measurements suggested electron transfer to the surface-immobilized catalysts occurred; however, H₂ evolution was not observed. Based on UV-vis, X-ray fluorescence spectroscopy (XRF), and X-ray photoelectron spectroscopy (XPS) measurements, the cobalt catalyst appeared to be limiting the photocathode performance mainly via cobalt demetallation from the oxime ligand. This study highlights the need for a deeper understanding of the effect of catalyst molecular design on photocathode performance.

Fighting Deactivation: Classical and Emerging Strategies for Efficient Stabilization of Molecular Electrocatalysts

Development of highly active molecular electrocatalysts for fuel-forming reactions has relied heavily on understanding mechanistic aspects of the electrochemical transformations. Careful fine-tuning of the ligand environment oriented mechanistic pathways towards higher activity and optimal product distribution for several catalysts. Unfortunately, many catalysts deactivate in bulk electrolysis conditions, diminishing the impact of the plethora of highly tuned molecular electrocatalytic systems. This Minireview covers classical and emerging methods developed to circumvent catalyst deactivation and degradation, with an emphasis on successes with molecular electrocatalysts.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Making Use of the δ Electrons in K4Mo2(SO4)4 for Visible-Light-Induced Photocatalytic Hydrogen Production

Quadruply bonded dimolybdenum complexes with a σ²π⁴δ² electronic configuration for the ground state have rich metal-centered photochemistry. An earlier study showed that stoichiometric or less amount of molecular hydrogen was produced upon irradiation by ultraviolet light (λ = 254 nm) of K₄Mo₂(SO₄)₄ in sulfuric acid solution, which was attributed to the reductive capability of the ππ* excited state. To make use of the δ electrons for visible-light-induced photocatalytic hydrogen evolution, a multicomponent heterogeneous photocatalytic system containing K₄Mo₂(SO₄)₄ photosensitizer, TiO₂ electron relay, and MoS₂ cocatalyst is designed and tested. With ascorbic acid added as a sacrificial reagent, irradiation by artificial sunlight (AM 1.5) on the reaction in 5 M H₂SO₄ has produced 13 400 μmol g^-1 of molecular hydrogen (based on the Mo₂ complex), which is 30 times higher than the hydrogen yield obtained from the reaction of bare K₄Mo₂(SO₄)₄ with H₂SO₄ under ultraviolet light irradiation. Further improvement of hydrogen evolution is achieved by addition of oxalic acid, along with an electron donor, which gives an additional 50% increase in H₂ yield. Spectroscopic analyses indicate that, in this case, a junction between the Mo₂ complex and TiO₂ is built by the oxalate bridging ligand, which facilitates charge injection and separation from the Mo₂ core. This Mo₂-TiO₂-MoS₂ system has achieved a high hydrogen evolution rate up to 4570 μmol g^-1 h^-1. The efficiency of K₄Mo₂(SO₄)₄ as a metal-centered photosensitizer is also proved by parallel experiments with a dye chromophore, fluorescein, which presents comparable H₂ yields and hydrogen evolution rates. Most importantly, in this study, detailed analyses illustrate that the photocatalytic cycle with hydrogen gas as an outcome of the reaction is established by involvement of the δδ* excited state generated by visible light irradiation. Therefore, this work shows the potential of quadruply bonded Mo₂ complexes as photosensitizers for photocatalytic hydrogen evolution.

Distinguishing deposition, corrosion, and stripping of transient heterogeneous materials during molecular electrocatalysis

Understanding the behavior of molecular hydrogen-evolving electrocatalysts is often challenging, because secondary heterogeneous materials can form during reductive electrode polarization and contribute to catalysis. Here, we use an electrochemical quartz crystal microbalance to interrogate secondary heterogeneous materials formed in situ during hydrogen generation with the proton- and difluoroboryl-bridged dimethylglyoxime cobalt catalysts in acetonitrile electrolyte. Detrimental protonation reactions of the molecular precatalysts lead to formation of the transient heterogeneous materials, and the structure of the molecular cobaloxime precursors affects the potential dependence and kinetics of deposition and loss of heterogeneous material from the electrode surface. The strength of the acid in the electrolyte (needed as the proton source for H2 generation) also contributes to the stability and corrosion rate of the heterogeneous materials, as revealed in multistep polarization studies. Overall, these results support involvement of an interplay of electrodeposition, proton-driven (chemical) corrosion, and anodic (electrochemical) stripping in influencing the catalytic activity and the behavior of transient heterogeneous materials that may be generated during catalysis.

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.

Host–guest chemistry between cyclodextrin and a hydrogen evolution catalyst cobaloxime

We report the host–guest chemistry between cyclodextrin and a bisdimethylglyoximato cobalt complex, cobaloxime.

Solar electricity and fuel production with perylene monoimide dye-sensitised TiO2 in water

Anchor-bearing perylene monoimide dyes were synthesised and studied back-to-back in both aqueous dye-sensitised solar cells and semiconductor photocatalysis.

Dye-sensitisation of TiO₂ and other metal oxides is an established strategy to couple solar light harvesting with efficient charge separation for the production of electricity in dye-sensitised solar cells (DSCs) or fuels in dye-sensitised semiconductor photocatalysis (DSP). Perylene monoimide (PMI) dyes have emerged as promising organic dyes, but they have not previously been used in a functional assembly with TiO₂ in aqueous solution. Here, five novel PMI dyes bearing carboxylic acid, phosphonic acid, acetylacetone, hydroxyquinoline or dipicolinic acid anchoring groups for attachment onto TiO₂ are reported. We identified functional DSC and DSP systems with PMI-sensitised TiO₂ in aqueous solution, which permitted a side-by-side comparison with respect to performance between the two systems. Structure–activity relationships allowed us to suggest anchor-condition-system associations to suit specific anchoring groups at various pH values, and with different electron mediators (redox couple or sacrificial electron donor) and catalysts in DSC and DSP schemes. A DSC sensitised with the hydroxyquinoline-modified PMI dye reached the highest short-circuit current density (J_SC ≈ 1.4 mA cm^–2) in aqueous electrolyte solution during irradiation with simulated solar light. This dye also achieved a turnover number (TON_PMI) of approximately 4900 for sacrificial proton reduction after 24 h irradiation in a DSP scheme with Pt as a H₂-evolving co-catalyst at pH 4.5. This performance was only surpassed by the carboxylic acid-bearing dye, which reached a new benchmark turnover number (TON_PMI ≈ 1.1 × 10⁴ after 72 h) for an organic dye in nanoparticulate DSP for solar fuel production. At higher pH (8.5), our results showed that the phosphonic acid group allows for higher performance due to a stronger anchoring ability. This study provides a platform for aqueous PMI dye-sensitised TiO₂ chemistry and gives valuable insights into the performance of different anchoring groups in DSC and DSP systems.

Catalysis by design: development of a bifunctional water splitting catalyst through an operando measurement directed optimization cycle

A critical challenge in energy research is the development of earth abundant and cost-effective materials that catalyze the electrochemical splitting of water into hydrogen and oxygen at high rates and low overpotentials. Key to addressing this issue lies not only in the synthesis of new materials, but also in the elucidation of their active sites, their structure under operating conditions and ultimately, extraction of the structure-function relationships used to spearhead the next generation of catalyst development. In this work, we present a complete cycle of synthesis, operando characterization, and redesign of an amorphous cobalt phosphide (CoP x ) bifunctional catalyst. The research was driven by integrated electrochemical analysis, Raman spectroscopy and gravimetric measurements utilizing a novel quartz crystal microbalance spectroelectrochemical cell to uncover the catalytically active species of amorphous CoP x and subsequently modify the material to enhance the activity of the elucidated catalytic phases. Illustrating the power of our approach, the second generation cobalt-iron phosphide (CoFePx) catalyst, developed through an iteration of the operando measurement directed optimization cycle, is superior in both hydrogen and oxygen evolution reactivity over the previous material and is capable of overall water electrolysis at a current density of 10 mA cm-2 with 1.5 V applied bias in 1 M KOH electrolyte solution.

Light-driven hydrogen evolution catalyzed by a cobaloxime catalyst incorporated in a MIL-101(Cr) metal-organic framework

A cobaloxime H₂ evolution catalyst with a hydroxo-functionalized pyridine ligand, Co(dmgH)₂(4-HEP)Cl [dmgH = dimethylglyoxime, 4-HEP = 4-(2-hydroxyethyl)pyridine] was immobilized on a chromium terephthalate metal-organic framework (MOF), MIL-101(Cr), to construct a MOF-catalyst hybrid which displays good photocatalytic H₂ evolution activity. The longevity of the cobaloxime catalyst is increased by MOF incorporation, but limited by the stability of the cobalt-pyridine bond under turnover conditions.

Solar H2 generation in water with a CuCrO2 photocathode modified with an organic dye and molecular Ni catalyst

Dye-sensitised photoelectrochemical (DSPEC) cells have emerged in recent years as a route to solar fuel production. However, fuel-forming photocathodes are presently limited by photo-corrodible narrow band gap semiconductors or the small range of available wide bandgap p-type semiconductors such as NiO that display low performance with dyes. Here, we introduce CuCrO2 as a suitable p-type semiconductor for visible light-driven H2 generation upon co-immobilisation of a phosphonated diketopyrrolopyrrole dye with a Ni-bis(diphosphine) catalyst. The hybrid CuCrO2 photocathode displays an early photocurrent onset potential of +0.75 V vs. RHE and delivers a photocurrent of 15 μA cm-2 at 0.0 V vs. RHE in pH 3 aqueous electrolyte solution under UV-filtered simulated solar irradiation. Controlled potential photoelectrolysis at 0.0 V vs. RHE shows good stability and yields a Ni catalyst-based turnover number of 126 ± 13 towards H2 after 2 h. This precious metal-free system outperforms an analogous NiO|dye/catalyst assembly and therefore highlights the benefits of using CuCrO2 as a novel material for DSPEC applications.

Supramolecular assembly of cobaloxime on nanoring-coated carbon nanotubes: addressing the stability of the pyridine-cobalt linkage under hydrogen evolution turnover conditions

A carbon nanotube-cobaloxime nanohybrid was prepared through supramolecular assembly of tailored polymerizable amphiphiles, leading to the coordination of cobalt on pyridine-coated nanotubes. This material was used as a catalyst for hydrogen evolution in fully aqueous media. This study provides a definitive asset regarding the stability of the pyridine-cobalt axial bond under H₂ evolution turnover conditions.

Achieving efficient photodynamic therapy under both normoxia and hypoxia using cyclometalated Ru(ii) photosensitizer through type I photochemical process

Photodynamic therapy (PDT) through the generation of singlet oxygen utilizing photosensitizers (PSs) is significantly limited under hypoxic conditions in solid tumors. So it is meaningful to develop effective PSs which can maintain excellent therapeutic effects under hypoxia. Here we reported a coumarin-modified cyclometalated Ru(ii) photosensitizer (Ru2), which exhibits lower oxidation potential and stronger absorption in the visible region than the coumarin-free counterpart. The evaluation of the PDT effect was performed under both normoxia and hypoxia. The results showed that Ru2 has a better therapeutic effect than the coumarin-free counterpart in in vitro experiments. Especially under hypoxia, Ru2 still retained an excellent PDT effect, which can be attributed to the direct charge transfer between the excited PS and an adjacent substrate through a type I photochemical process, forming highly-oxidative hydroxyl radicals to damage tumor cells. The anti-tumor activity of Ru2 was further proven to be effective in tumor-bearing mice, and tumor growth was inhibited remarkably under PDT treatment.

A Functional Hydrogenase Mimic Chemisorbed onto Fluorine-Doped Tin Oxide Electrodes: A Strategy towards Water Splitting Devices

A diiron benzenedithiolate hydrogen-evolving catalyst immobilized onto fluorine-doped tin oxide (FTO) electrodes is prepared, characterized, and studied in the context of the development of water splitting devices based on molecular components. FTO was chosen as the preferred electrode material owing to its conductive properties and electrochemical stability. An FTO nanocrystalline layer is also used to greatly improve the surface area of commercially available FTO while preserving the properties of the material. Electrodes bearing a covalently anchored diiron catalyst are shown to be competent for electrocatalytic hydrogen evolution from acidic aqueous media at relatively low overpotential (500 mV) with a faradaic efficiency close to unity. Compared with bulk solution catalysts, the catalyst immobilized onto the electrode surface operates at roughly 160 mV lower overpotentials, yet with similar rates.

Dye-sensitized photocatalyst for effective water splitting catalyst

Renewable hydrogen production is a sustainable method for the development of next-generation energy technologies. Utilising solar energy and photocatalysts to split water is an ideal method to produce hydrogen. In this review, the fundamental principles and recent progress of hydrogen production by artificial photosynthesis are reviewed, focusing on hydrogen production from photocatalytic water splitting using organic-inorganic composite-based photocatalysts.

Impact of Photosensitizing Multilayered Structure on Ruthenium(II)-Dye-Sensitized TiO2-Nanoparticle Photocatalysts

To improve the efficiency of photoinduced charge separation on the surface of dye-sensitized TiO₂ nanoparticles, we synthesized the Ru(II)-photosensitizer-immobilized, Pt-cocatalyst-loaded TiO₂ nanoparticles RuCP ² @Pt-TiO₂, RuCP ² -Zr-RuP ⁶ @Pt-TiO₂, and RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂ (RuCP ² = [Ru(bpy)₂(mpbpy)]^2-, RuP ⁴ = [Ru(bpy)(pbpy)₂]^6-, RuP ⁶ = [Ru(pbpy)₃]^10-, H₄mpbpy = 2,2'-bipyridine-4,4'-bis(methanephosphonic acid), and H₄pbpy = 2,2'-bipyridine-4,4'-bis(phosphonic acid)) using phosphonate linkers with bridging Zr⁴⁺ ions. X-ray fluorescence and ultraviolet-visible absorption spectra revealed that a layered molecular structure composed of Ru(II) photosensitizers and Zr⁴⁺ ions (i.e., RuCP ² -Zr-RuP ⁶ and RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ ) was successfully formed on the surface of Pt-TiO₂ nanoparticles, which increased the surface coverage from 0.113 nmol/cm² for singly layered RuCP ² @Pt-TiO₂ to 0.330 nmol/cm² for triply layered RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂. The photocatalytic H₂ evolution activity of the doubly layered RuCP ² -Zr-RuP ⁶ @Pt-TiO₂ was three times higher than that of the singly layered RuCP ² @Pt-TiO₂, whereas the activity of triply layered RuCP ² -Zr-RuP ⁴ -Zr-RuP ⁶ @Pt-TiO₂ was less than half of that for RuCP ² @Pt-TiO₂. The photosensitizing efficiencies of these Ru(II)-photosensitizer-immobilized nanoparticles for the O₂ evolution reaction catalyzed by the Co(II)-containing Prussian blue analogue [Co^II(H₂O)₂]_1.31[{Co^III(CN)₆}_0.63{Pt^II(CN)₄}_0.37] decreased as the number of Ru(II)-photosensitizing layers increased. Thus, crucial aspects of the energy- and electron-transfer mechanism for the photocatalytic H₂ and O₂ evolution reactions involve not only the Ru(II)-complex-TiO₂ interface but also the multilayered structure of the Ru(II)-photosensitizers on the Pt-TiO₂ surface.

Photoelectrocatalytic H2 evolution in water with molecular catalysts immobilised on p-Si via a stabilising mesoporous TiO2 interlayer

A versatile platform for the immobilisation of molecular catalysts on a readily-prepared Si photocathode with a mesoporous TiO₂ layer is reported.

The development of photoelectrodes capable of light-driven hydrogen evolution from water is an important approach for the storage of solar energy in the form of a chemical energy carrier. However, molecular catalyst-based photocathodes remain scarcely reported and typically suffer from low efficiencies and/or stabilities due to inadequate strategies for interfacing the molecular component with the light-harvesting material. In this study, we report the straightforward preparation of a p-silicon|mesoporous titania|molecular catalyst photocathode assembly that is active towards proton reduction in aqueous media with an onset potential of +0.4 V vs. RHE. The mesoporous TiO₂ scaffold acts as an electron shuttle between the silicon and the catalyst, while also stabilising the silicon from passivation and enabling a high loading of molecular catalysts (>30 nmol (geometrical cm)^–2). When a Ni bis(diphosphine)-based catalyst is anchored on the surface of the electrode, a high turnover number of ∼1 × 10³ was obtained from photoelectrolysis under UV-filtered simulated solar irradiation at 1 Sun after 24 h at pH 4.5. Notwithstanding its aptitude for molecular catalyst immobilisation, the p-Si|TiO₂ photoelectrode showed great versatility towards different catalysts and pH conditions, with photoelectrocatalytic H₂ generation also being achieved with platinum and a hydrogenase as catalyst, highlighting the flexible platform it represents for many potential reductive catalysis transformations.

Surface Immobilization of Molecular Electrocatalysts for Energy Conversion

Electrocatalysts are critically important for a secure energy future, as they facilitate the conversion between electrical and chemical energy. Molecular catalysts offer precise control of structure that enables understanding of structure-reactivity relationships, which can be difficult to achieve with heterogeneous catalysts. Molecular electrocatalysts can be immobilized on surfaces by covalent bonds or through non-covalent interactions. Advantages of surface immobilization include the need for less catalyst, avoidance of bimolecular decomposition pathways, and easier determination of catalyst lifetime. This Minireview highlights surface immobilization of molecular electrocatalysts for reduction of O₂ , oxidation of H₂ O, production of H₂ , and reduction of CO₂ .

A Nickel Dithiolate Water Reduction Catalyst Providing Ligand-Based Proton-Coupled Electron-Transfer Pathways

A nickel pyrazinedithiolate ([Ni(dcpdt)₂ ]^2- ; dcpdt=5,6-dicyanopyrazine-2,3-dithiolate), bearing a NiS₄ core similar to the active center of [NiFe] hydrogenase, is shown to serve as an efficient molecular catalyst for the hydrogen evolution reaction (HER). This catalyst shows effectively low overpotentials for HER (330-400 mV at pH 4-6). Moreover, the turnover number of catalysis reaches 20 000 over the 24 h electrolysis with a high Faradaic efficiency, 92-100 %. The electrochemical and DFT studies reveal that diprotonated one-electron-reduced species (i.e., [Ni^II (dcpdt)(dcpdtH₂ )]^- or [Ni^II (dcpdtH)₂ ]^- ) forms at pH<6.4 via ligand-based proton-coupled electron-transfer (PCET) pathways, leading to electrocatalytic HER without applying the highly negative potential required to generate low-valent nickel intermediates. This is the first example of catalysts exhibiting such behavior.