Photostable p-type dye-sensitized photoelectrochemical cells for water reduction

Noble Metal Plasmon-Molecular Catalyst Hybrids for Renewable Energy Relevant Small Molecule Activation

Significant endeavors have been dedicated to the advancement of materials for artificial photosynthesis, aimed at efficiently harvesting light and catalyzing reactions such as hydrogen production and CO2 conversion. The application of plasmonic nanomaterials emerges as a promising option for this purpose, owing to their excellent light absorption properties and ability to confine solar energy at the nanoscale. In this regard, coupling plasmonic particles with molecular catalysts offers a pathway to create high-performance hybrid catalysts. In this review, we discuss the plasmonic-molecular complex hybrid catalysts where the plasmonic nanoparticles serve as the light-harvesting unit and promote interfacial charge transfer in tandem with the molecular catalyst which drives chemical transformation. In the initial section, we provide a concise overview of plasmonic nanomaterials and their photophysical properties. We then explore recent breakthroughs, highlighting examples from literature reports involving plasmonic-molecular complex hybrids in various catalytic processes. The utilization of plasmonic materials in conjunction with molecular catalysts represents a relatively unexplored area with substantial potential yet to be realized. This review sets a strong basis and motivation to explore the plasmon-induced hot-electron mediated photelectrochemical small molecule activation reactions. Utilizing in situ spectroscopic investigations and ultrafast transient absorption spectroscopy, it presents a comprehensive template for scalable and sustainable antenna-reactor systems.

Promoting H2 generation of BiFeO3 photocathodes by a catalyst-sensitizer dyad linked with Sn(dipicolinate)2

An innovative method for the fabrication of a catalyst-sensitizer dyad-based photoelectrode was developed by using the coordinated interaction between the pyridine-2,6-dicarboxylic group and Sn4+. A dyad (C1 + PDI) was loaded on the mesoporous BiFeO3 (BFO) photocathode for light-driven H2 generation. The dyad could expand the light absorption range and promote the surface charge separation of BFO, resulting in an enhanced photocurrent.

Light-harvesting iridium (III) complex-sensitized NiO photocathode for photoelectrochemical bioanalysis

A novel iridium (III) complex bearing boron dipyrromethene (Bodipy) as the light-harvesting antenna has been synthesized and is firstly employed as photosensitizer to assemble a dye-sensitized NiO photocathode. The assembled photocathode exhibits significantly improved photoelectrochemical (PEC) performance. Integrating the prepared photocathode with hybridization chain reaction (HCR)--based signal amplification strategy, a cathodic PEC biosensor is proposed for the detection of microRNA-133a (miRNA-133a). In the presence of the target, HCR is triggered to form long duplex concatamers on the photocathode, which allows numerous manganese porphyrins (MnPP) to bind in the dsDNA groove. With the help of H2O2, MnPP with peroxidase-like activity catalyzes 4--chloro-1-naphthol (4-CN) to produce benzo--4--chlorohexadienone (4-CD) precipitate on the electrode, leading to a significant decrease of photocurrent signal. The decreased photocurrent correlates linearly with the target concentration from 0.1 fM to 1 nM with a detection limit of 66.2 aM (S/N = 3). The proposed PEC strategy exhibits delightful selectivity, reproducibility and stability.

A Multi-Pyridine-Anchored and -Linked Bilayer Photocathode for Water Reduction

The development of efficient photocathodes is of critical importance for the constructions of promising tandem photo-electrochemical cells. Most known dye-sensitized photocathodes are prepared with the conventional carboxylic or phosphonic acid anchors and require the presence of other terminal linking groups to connect catalysts; they suffer from high synthetic difficulty and low adsorption stability in aqueous media. Here, a compact bilayer photocathode has been prepared by using a pyrene-based photosensitizer with multiple terminal pyridine moieties as both the anchoring and linking groups to connect a Co hydrogen-evolution catalyst to the NiO substrate. The catalyst and dye molecule are assembled in a layer-by-layer manner on NiO through the metal-pyridine coordination. This photocathode exhibits good dye adsorption stability in aqueous media. A stable cathodic photocurrent of 70 μA cm-2 was achieved, with H2 being generated at the photocathode under the visible-light irradiation. The Faraday efficiency of H2 evolution was estimated to be 9.1 %. Transient absorption spectral studies suggest that the interfacial hole transfer occurs within a few picoseconds. The integration of the organic photosensitizer with pyridine anchoring and linking groups is expected to provide a simple method for the fabrication of stable and efficient photocathodes.

Optimized H2-evolving dye-sensitized LaFeO3 photocathodes prepared via the layer-by-layer assembly of dyes and catalysts

A molecular dye and a molecular catalyst were loaded onto the surface of a mesoporous LaFeO₃ (LFO) film via layer-by-layer assembly relying on the coordination of phosphates and Zr⁴⁺. After assembling six layers of the dye and four layers of the catalyst, the (NiP-4 + PQA-6)@LFO photocathode exhibited a significant photocurrent for light-driven H₂ generation.

Synthesis and Theoretical and Photophysical Study on a Series of Neutral Ruthenium(II) Complexes with Donor-Metal-Accepter Configuration

In order to construct a new type of ruthenium(II) terpyridine complexes with activated triplet metal-centered (³MC) states, as well as stabilized triplet metal-to-ligand charge transfer (³MLCT) states, conducive to fine emissive performances, Ru-1, Ru-2, Ru-3, and Ru-4 were synthesized. Compared with the [Ru(terpyridine)₂]²⁺ prototype (0.25 ns), this series of ruthenium(II) terpyridine complexes exhibit lengthened excited state lifetime (43.3 ns for Ru-1, 52.7 ns for Ru-2, 43.6 ns for Ru-3, and 53.4 ns for Ru-4). Interfragment charge transfer analysis illustrates the electron transfer direction of the four complexes, manifesting their intramolecular charge transfer characteristic. When excited, their lowest-lying triplet states are assigned as ³MLCT based on spin-density surface distribution. The singlet excited states and ³MLCT states were thoroughly studied by UV-visual absorption and nanosecond time-resolved transient absorption spectra, respectively. Photoluminescence spectra revealed their weak broadband near-infrared emission at room temperature and red phosphorescence at 77 K. The low molecular weight and the good thermal stability make Ru-1 and Ru-2 suitable for vaporization coating, while the fine solubility in common organic solvents makes Ru-3 and Ru-4 suitable for solution processing. Furthermore, the intrinsic electroneutrality and favorable energy levels endow them with new potential to be applied in the optoelectronic field.

Gold nanoparticle-based supramolecular approach for dye-sensitized H2-evolving photocathodes

Solar conversion of water into the storable energy carrier H₂ can be achieved through photoelectrochemical water splitting using light adsorbing anodes and cathodes bearing O₂ and H₂ evolving catalysts, respectively. Herein a novel photocathode nanohybrid system is reported. This photocathode consists of a dye-sensitized p-type nickel oxide (NiO) with a perylene-based chromophore (PCA) and a tetra-adamantane modified cobaloxime reduction catalyst (Co) that photo-reduces aqueous protons to H₂. An original supramolecular approach was employed, using β-cyclodextrin functionalized gold nanoparticles (β-CD-AuNPs) to link the alkane chain of the PCA dye to the adamantane moieties of the cobaloxime catalyst (Co). This new architecture was investigated by photoelectrochemical measurements and via femtosecond-transient absorption spectroscopy. The results show that irradiation of the complete NiO|PCA|β-CD-AuNPs|Co electrode leads to ultrafast hole injection into NiO (π = 3 ps) from the excited dye, followed by rapid reduction of the catalyst, and finally H₂ evolution.

Design of MOF-Derived NiO-Carbon Nanohybrids Photocathodes Sensitized with Quantum Dots for Solar Hydrogen Production

Nickel oxide (NiO) is a promising p-type material for a wide range of optoelectronic devices, as well as photocathode for photoelectrochemical (PEC) water splitting. However, traditional NiO photoelectrodes exhibit a wide bandgap (3.6 eV), intrinsic poor electrical conductivity, and low surface area, leading to low PEC systems performance. Herein, the authors explore a Ni-based metal-organic framework (MOF) template method to obtain hierarchical hollow spheres of carbon/NiO nanostructure by successive carbonization and oxidation treatments. After sensitization with core and core-shell quantum dots (QDs), the optimized NiO-photocathode exhibits a maximum current density of -93.6 µA cm^-2 at 0 V versus RHE (reversible hydrogen electrode) in neutral pH (6.8) and -285 µA cm^-2 at -0.4 V versus RHE. Compared to pure NiO and single-core CdSe QDs, a 2.2-fold increase in photocurrent can be obtained. The improvement in the performance of this hybrid is not only due to the high surface area for loading QDs and light scattering, but also to the presence of a highly conductive carbon matrix that promotes fast charge transfer. The proposed MOFs-based NiO/carbon photocathode sensitized with QDs can be an effective strategy to improve the efficiency of metal oxide-based PEC systems for hydrogen generation.

Photoelectrochemical Hydrogen Evolution Using Dye-Sensitised Nickel Oxide : Environmental effects and photocatalyst design considerations

Photoelectrocatalysis offers a way to generate hydrogen and oxygen from water under ambient light. Here, a series of hydrogen evolving photocatalysts based on a ruthenium(II) bipyridyl sensitiser covalently linked to platinum or palladium catalytic centres were adsorbed onto mesoporous nickel oxide and tested for hydrogen evolution in a photoelectrochemical half-cell. The electrolyte buffer was varied and certain catalysts performed better at pH 7 than pH 3 (for example, PC3 with photocurrent density = 8 μA cm‐2), which is encouraging for coupling with an oxygen evolving photoanode in tandem water splitting devices. The molecular catalysts were surprisingly robust when integrated into devices, but the overall performance appears to be limited by rapid recombination at the photocatalyst|NiO interface. Our findings provide further insight towards basic design principles for hydrogen evolving photoelectrochemical systems and guidelines for further development.

Hydrogen Production at a NiO Photocathode Based on a Ruthenium Dye-Cobalt Diimine Dioxime Catalyst Assembly: Insights from Advanced Spectroscopy and Post-operando Characterization

The production of hydrogen by efficient, low-cost, and integrated photoelectrochemical water splitting processes represents an important target for the ecological transition. This challenge can be addressed thanks to bioinspired chemistry and artificial photosynthesis approaches by designing dye-sensitized photocathodes for hydrogen production, incorporating bioinspired first-row transition metal-based catalysts. The present work describes the preparation and photoelectrochemical characterization of a NiO photocathode sensitized with a phosphonate-derivatized ruthenium tris-diimine photosensitizer covalently linked to a cobalt diimine dioxime hydrogen-evolving catalyst. Under simulated AM 1.5G irradiation, hydrogen is produced with photocurrent densities reaching 84 ± 7 μA·cm^-2, which is among the highest values reported so far for dye-sensitized photocathodes with surface-immobilized catalysts. Thanks to the unique combination of advanced spectroscopy and surface characterization techniques, the fast desorption of the dyad from the NiO electrode and the low yield of electron transfer to the catalyst, resulting in the Co demetallation from the diimine dioxime framework, were identified as the main barriers limiting the performances and the stability of the system. This work therefore paves the way for a more rational design of molecular photocathodes for solar fuel production and represents a further step toward the development of sustainable processes for the production of hydrogen from sunlight and water.

Localizing the initial excitation - A case study on NiO photocathodes using Ruthenium dipyridophenazine complexes as sensitizers

We report on the localization of the initially excited electronic state within the molecular framework of a series of [Ru(bpy)₂dppz]²⁺ derivatives (bpy:2,2'-bipyridine, dppz: dipyrido-phenazine) as sensitizers in NiO based photocathodes. The introduction of conjugated linkers with phenylene and triazole moieties in the bpy ligand sphere separates the NiO surface from the metal center and hence is considered to stabilize the charge separated state, which results from light-driven hole injection. However, introduction of the conjugated linkers also alters the localization of the excess electron density in the excited state within the ligand sphere and impacts the extent to which the charge-separated state is formed. The study emphasizes that tuning the ligand with the lowest-energy π* orbital distal or proximal to the NiO surface significantly affects the initial charge-separation and the solar cell performance. The stability of the charge-separated state correlates with the observed photocurrents in dye-sensitized solar cells. Furthermore, the study challenges the widely accepted concept that the introduction of extended anchoring groups, i.e. increasing Ru - NiO distance, stabilizes the charge-separated state and suppresses charge recombination at the metal-oxide molecule interface.

Sustainable hydrogen production from water using tandem dye-sensitized photoelectrochemical cells

If generated from water using renewable energy, hydrogen could serve as a carbon-zero, environmentally benign fuel to meet the needs of modern society. Photoelectrochemical cells integrate the absorption and conversion of solar energy and chemical catalysis for the generation of high value products. Tandem photoelectrochemical devices have demonstrated impressive solar-to-hydrogen conversion efficiencies but have not become economically relevant due to high production cost. Dye-sensitized solar cells, those based on a monolayer of molecular dye adsorbed to a high surface area, optically transparent semiconductor electrode, offer a possible route to realizing tandem photochemical systems for H₂ production by water photolysis with lower overall material and processing costs. This review addresses the design and materials important to the development of tandem dye-sensitized photoelectrochemical cells for solar H₂ production and highlights current published reports detailing systems capable of spontaneous H₂ formation from water using only dye-sensitized interfaces for light capture.

Cathodic photoelectrochemical aptasensor based on NiO/BiOI/Au NP composite sensitized with CdSe for determination of exosomes

A cathodic photoelectrochemical sensor has been developed for the determination of exosomes, based on a dual-signal reduction strategy. A heterostructure of NiO/BiOI/Au NP/CdSe was synthesized as a photoelectrochemical sensing interface, which is able to suppress the recombination of electron-hole pairs and produce a higher photocurrent. The obtained materials were characterized, and the mechanism for the generation of the cathodic photocurrent was proposed. CdSe QDs (quantum dots) modified with DNA₂ were assembled on the electrode through the hybridization with EpCAM aptamer on the surface of ITO/NiO/BiOI/Au NP. The introduction of CdSe QDs to the electrode increases the photocurrent.The recognition of exosomes with aptamer DNA led to the separation of CdSe QDs from the electrode, which in turn caused the decrease of photocurrents. Meanwhile, the big volume of exosomes hinders the electron transfer between the electrode and electrolyte. Due to the dual reduction effect, a sensitive PEC sensor was obtained with a detection limit of 1.2 × 10² particles/μL exosomes (λ_ex = 430 nm, bias voltage = - 0.1 V). The cathodic photoelectrochemical sensor showed good selectivity, performed well in a complex biological environment and could be used to distinguishbreast cancer patients from healthy individuals.

Supramolecular strategies in artificial photosynthesis

Artificial photosynthesis is a major scientific endeavor aimed at converting solar power into a chemical fuel as a viable approach to sustainable energy production and storage. Photosynthesis requires three fundamental actions performed in order; light harvesting, charge-separation and redox catalysis. These actions span different timescales and require the integration of functional architectures developed in different fields of study. The development of artificial photosynthetic devices is therefore inherently complex and requires an interdisciplinary approach. Supramolecular chemistry has evolved to a mature scientific field in which programmed molecular components form larger functional structures by self-assembly processes. Supramolecular chemistry could provide important tools in preparing, integrating and optimizing artificial photosynthetic devices as it allows precise control over molecular components within such a device. This is illustrated in this perspective by discussing state-of-the-art devices and the current limiting factors - such as recombination and low stability of reactive intermediates - and providing exemplary supramolecular approaches to alleviate some of those problems. Inspiring supramolecular solutions such as those discussed herein will incite expansion of the supramolecular toolbox, which eventually may be needed for the development of applied artificial photosynthesis.

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.

Tailored protective groups for surface immobilization of ruthenium dyes

McKenna reaction conditions are applied to the [Ru(4,4'-(CH₂PO₃Et₂)₂(bpy)](PF₆)₂ model chromophore in order to obtain [Ru(4,4'-(CH₂PO₃TMS₂)₂(bpy)](Br_2-x)(PF₆)_x (x = 0-2) (2) by replacing the alkyl moieties of the phosphonates with TMS groups (TMS = trimethylsilyl). The model complex is immobilized onto both NiO powder and NiO electrodes on FTO (fluorine doped tin oxide) using organic solvents. The stability of surface binding in aqueous media and the DSC performance of 2 are tested and compared to those of a conventional dye of structure [Ru(4,4'-(CH₂PO₃TBA₂)₂(bpy)](PF₆)₂ (1) (TBA = tetrabutyl ammonium). This is the first example of a ruthenium based chromophore with a phosphonic acid silyl-ester being directly immobilized onto a NiO surface. In addition, complex 2 exhibits superior stability towards desorption in aqueous media and at the same time showing improved DSC performance and stability in acetonitrile and a slightly higher dye loading on the electrode surface compared to 1.

Directly Photoexcited Oxides for Photoelectrochemical Water Splitting

Artificial photosynthesis promises to become a sustainable way to harvest solar energy and store it in chemical fuels by means of photoelectrochemical (PEC) cells. Although it is intriguing to shift the fossil-fuel-based economy to a renewable carbon-neutral one, which will alleviate environmental problems, there is still a long way to go before it rivals traditional energy sources. Existing solar water-splitting devices can be sorted into three categories: photovoltaic-powered electrolysis, PEC water splitting, and photocatalysis (PC). PEC and PC systems hold the potential to further reduce the cost of devices due to their simple structures in which photoabsorbers and catalysts are closely integrated. PC is expected to be the least expensive approach; however, additional costs and concerns are brought about by the subsequent explosive gas separation. At the heart of all devices, semiconductor photoabsorbers should be efficient, robust, and cheap to satisfy the strict requirements on the market. Therefore, this Review intends to give readers an overview on PEC water splitting, with an emphasis on oxide material-based devices, which hold the potential to support global-scale production in the future.

Inverse Opal CuCrO2 Photocathodes for H2 Production Using Organic Dyes and a Molecular Ni Catalyst

Dye-sensitized photoelectrochemical (DSPEC) cells are an emerging approach to producing solar fuels. The recent development of delafossite CuCrO2 as a p-type semiconductor has enabled H2 generation through the coassembly of catalyst and dye components. Here, we present a CuCrO2 electrode based on a high-surface-area inverse opal (IO) architecture with benchmark performance in DSPEC H2 generation. Coimmobilization of a phosphonated diketopyrrolopyrrole (DPP-P) or perylene monoimide (PMI-P) dye with a phosphonated molecular Ni catalyst (NiP) demonstrates the ability of IO-CuCrO2 to photogenerate H2. A positive photocurrent onset potential of approximately +0.8 V vs RHE was achieved with these photocathodes. The DPP-P-based photoelectrodes delivered photocurrents of -18 μA cm-2 and generated 160 ± 24 nmol of H2 cm-2, whereas the PMI-P-based photocathodes displayed higher photocurrents of -25 μA cm-2 and produced 215 ± 10 nmol of H2 cm-2 at 0.0 V vs RHE over the course of 2 h under visible light illumination (100 mW cm-2, AM 1.5G, λ > 420 nm, 25 °C). The high performance of the PMI-constructed system is attributed to the well-suited molecular structure and photophysical properties for p-type sensitization. These precious-metal-free photocathodes highlight the benefits of using bespoke IO-CuCrO2 electrodes as well as the important role of the molecular dye structure in DSPEC fuel synthesis.

Molecular Catalysts Immobilized on Semiconductor Photosensitizers for Proton Reduction toward Visible-Light-Driven Overall Water Splitting

Photocatalytic or photoelectrochemical hydrogen production by water splitting is one of the key reactions for the development of an energy supply that enables a clean energy system for a future sustainable society. Utilization of solar photon energy for the uphill water splitting reaction is a promising technology, and therefore many systems using semiconductor photocatalysts and semiconductor photoelectrodes for the reaction producing hydrogen and dioxygen in a 2:1 stoichiometric ratio have been reported. In these systems, molecular catalysts are also considered to be feasible; recently, systems based on molecular catalysts conjugated with semiconductor photosensitizers have been used for photoinduced hydrogen generation by proton reduction. Additionally, there are reports that the so-called Z-scheme (two-step photoexcitation) mechanism realizes the solar-driven uphill reaction by overall water splitting. Although the number of these reports is still small compared to those of all-inorganic systems, the advantages of molecular cocatalysts and its immobilization on a semiconductor are attractive. This Minireview provides a brief overview of approaches and recent research progress toward molecular catalysts immobilized on semiconductor photocatalysts and photoelectrodes for solar-driven hydrogen production with the stoichiometric uphill reaction of hydrogen and oxygen generation.

Artificial photosynthesis: opportunities and challenges of molecular catalysts

Molecular catalysis plays an essential role in both natural and artificial photosynthesis (AP). However, the field of molecular catalysis for AP has gradually declined in recent years because of doubt about the long-term stability of molecular-catalyst-based devices. This review summarizes the development history of molecular-catalyst-based AP, including the fundamentals of AP, molecular catalysts for water oxidation, proton reduction and CO2 reduction, and molecular-catalyst-based AP devices, and it provides an analysis of the advantages, challenges, and stability of molecular catalysts. With this review, we aim to highlight the following points: (i) an investigation on molecular catalysis is one of the most promising ways to obtain atom-efficient catalysts with outstanding intrinsic activities; (ii) effective heterogenization of molecular catalysts is currently the primary challenge for the application of molecular catalysis in AP devices; (iii) development of molecular catalysts is a promising way to solve the problems of catalysis involved in practical solar fuel production. In molecular-catalysis-based AP, much has been attained, but more challenges remain with regard to long-term stability and heterogenization techniques.

A donor-chromophore-catalyst assembly for solar CO2 reduction

We describe here the preparation and characterization of a photocathode assembly for CO₂ reduction to CO in 0.1 M LiClO₄ acetonitrile.

We describe here the preparation and characterization of a photocathode assembly for CO₂ reduction to CO in 0.1 M LiClO₄ acetonitrile. The assembly was formed on 1.0 μm thick mesoporous films of NiO using a layer-by-layer procedure based on Zr(iv)–phosphonate bridging units. The structure of the Zr(iv) bridged assembly, abbreviated as NiO|-DA-RuCP₂²⁺-Re(i), where DA is the dianiline-based electron donor (N,N,N′,N′-((CH₂)₃PO₃H₂)₄-4,4′-dianiline), RuCP²⁺ is the light absorber [Ru((4,4′-(PO₃H₂CH₂)₂-2,2′-bipyridine)(2,2′-bipyridine))₂]²⁺, and Re(i) is the CO₂ reduction catalyst, Re^I((4,4′-PO₃H₂CH₂)₂-2,2′-bipyridine)(CO)₃Cl. Visible light excitation of the assembly in CO₂ saturated solution resulted in CO₂ reduction to CO. A steady-state photocurrent density of 65 μA cm^–2 was achieved under one sun illumination and an IPCE value of 1.9% was obtained with 450 nm illumination. The importance of the DA aniline donor in the assembly as an initial site for reduction of the RuCP²⁺ excited state was demonstrated by an 8 times higher photocurrent generated with DA present in the surface film compared to a control without DA. Nanosecond transient absorption measurements showed that the expected reduced one-electron intermediate, RuCP⁺, was formed on a sub-nanosecond time scale with back electron transfer to the electrode on the microsecond timescale which competes with forward electron transfer to the Re(i) catalyst at t_1/2 = 2.6 μs (k_ET = 2.7 × 10⁵ s^–1).

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.

Electrochemical and Photoelectrochemical Properties of Nickel Oxide (NiO) With Nanostructured Morphology for Photoconversion Applications

The cost-effective production of chemicals in electrolytic cells and the conversion of the radiation energy into electrical energy in photoelectrochemical cells (PECs) require the use of electrodes with large surface area, which possess either electrocatalytic or photoelectrocatalytic properties. In this context nanostructured semiconductors are electrodic materials of great relevance because of the possibility of varying their photoelectrocatalytic properties in a controlled fashion via doping, dye-sensitization or modification of the conditions of deposition. Among semiconductors for electrolysers and PECs the class of the transition metal oxides (TMOs) with a particular focus on NiO interests for the chemical-physical inertness in ambient conditions and the intrinsic electroactivity in the solid state. The latter aspect implies the existence of capacitive properties in TMO and NiO electrodes which thus act as charge storage systems. After a comparative analysis of the (photo)electrochemical properties of nanostructured TMO electrodes in the configuration of thin film the use of NiO and analogs for the specific applications of water photoelectrolysis and, secondly, photoelectrochemical conversion of carbon dioxide will be discussed.

Photoelectrocatalytic H2 evolution from integrated photocatalysts adsorbed on NiO

A new approach to increasing the faradaic efficiency of dye-sensitised photocathodes for H2 evolution from water, using integrated photocatalysts, furnished with ester groups on the peripheral ligands, [Ru(decb)2(bpt)PdCl(H2O)](PF6)2 (1) and [Ru(decb)2(2,5-bpp)PtI(CH3CN)](PF6)2 (2), (decb = 4,4'-diethylcarboxy-2,2'-bipyridine, bpp = 2,2':5',2''-terpyridine, bpt = 3,5-bis(2-pyridyl)-1,2,4-triazole) is described. Overall, 1|NiO is superior to previously reported photocathodes, producing photocurrent densities of 30-35 μA cm-2 at an applied bias of -0.2 V vs. Ag/AgCl over 1 hour of continuous white light irradiation, resulting in the generation of 0.41 μmol h-1 cm-2 of H2 with faradaic efficiencies of up to 90%. Furthermore, surface analysis of the photocathodes before and after photoelectrocatalysis revealed that the ruthenium bipyridyl chromophore and Pd catalytic centre (1) were photochemically stable, highlighting the benefits of the approach towards robust, hybrid solar-to-fuel devices.

NiO nanowires as hole-transfer layer for drastic enhancement of CdSe-sensitized photocathodes

Grass-like NiO nanowires as a hole-transfer layer to improve light capture efficiency and charge transfer rate for a CdSe-sensitized photocathode.

Adsorción de un complejo de hierro sobre nanocristales de dióxido de titanio utilizando un residuo piridina

En este trabajo se presenta el comportamiento espectroscópico y electroquímico del complejo [Fe(pytpy2)](PF6)2 (donde pytpy es 4'-(piridin-4-il)-2,2':6', 2''-terpiridina) en solución fluida y anclado sobre una película delgada de nanocristales de dióxido de titanio. La constante de formación del aducto se estimó utilizando el modelo isotérmico de Langmuir encontrándose un valor de 1,03x105 M-1 para dicho complejo. El anclaje del complejo sobre la superficie del óxido metálico semiconductor se debe a la interacción del residuo de piridina con los sitios ácidos de Lewis presentes en la superficie del TiO2, observándose que las propiedades espectroscópicas y electroquímicas del complejo no se ven modificadas por la adsorción.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Insights into the mechanism and aging of a noble-metal free H2-evolving dye-sensitized photocathode

Co-grafting of a cobalt diimine–dioxime catalyst and push–pull organic dye on NiO yields a photocathode evolving hydrogen from aqueous solution under sunlight, with equivalent performances compared to a dyad-based architecture using similar components.

Dye-sensitized photo-electrochemical cells (DS-PECs) form an emerging technology for the large-scale storage of solar energy in the form of (solar) fuels because of the low cost and ease of processing of their constitutive photoelectrode materials. Preparing such molecular photocathodes requires a well-controlled co-immobilization of molecular dyes and catalysts onto transparent semiconducting materials. Here we used a series of surface analysis techniques to describe the molecular assembly of a push–pull organic dye and a cobalt diimine–dioxime catalyst co-grafted on a p-type NiO electrode substrate. (Photo)electrochemical measurements allowed characterization of electron transfer processes within such an assembly and to demonstrate for the first time that a Co^I species is formed as the entry into the light-driven H₂ evolution mechanism of a dye-sensitized photocathode. This co-grafted noble-metal free H₂-evolving photocathode architecture displays similar performances to its covalent dye–catalyst counterpart based on the same catalytic moiety. Post-operando time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of these photoelectrodes after extensive photoelectrochemical operation suggested decomposition pathways of the dye and triazole linkage used to graft the catalyst onto NiO, providing grounds for the design of optimized molecular DS-PEC components with increased robustness upon turnover.

Fundamental Factors Impacting the Stability of Phosphonate-Derivatized Ruthenium Polypyridyl Sensitizers Adsorbed on Metal Oxide Surfaces

A series of 18 ruthenium(II) polypyridyl complexes were synthesized and evaluated under electrochemically oxidative conditions, which generates the Ru(III) oxidation state and mimics the harsh conditions experienced during the kinetically limited regime that can occur in dye-sensitized solar cells (DSSCs) and dye-sensitized photo-electrosynthesis cells, to further develop fundamental insights into the factors governing molecular sensitizer surface stability in aqueous 0.1 M HClO₄. Both desorption and oxidatively induced ligand substitution were observed on planar fluorine-doped tin oxide (FTO) electrodes, with a dependence on the E_1/2 Ru(III/II) redox potential dictating the comparative ratios of the processes. Complexes such as RuP4OMe ( E_1/2 = 0.91 vs Ag/AgCl) displayed virtually only desorption, while complexes such as RuPbpz ( E_1/2 > 1.62 V vs Ag/AgCl) displayed only chemical decomposition. Comparing isomers of 4,4'- and 5,5'-disubstituted-2,2'-bipyridine ancillary ligands, a dramatic increase in the rate of desorption of the Ru(III) complexes was observed for the 5,5'-ligands. Nanoscopic indium-doped tin oxide thin films (nanoITO) were also sensitized and analyzed with cyclic voltammetry, UV-vis absorption spectroscopy, and X-ray photoelectron spectroscopy, allowing for further distinction of desorption versus ligand-substitution processes. Desorption loss to bulk solution associated with the planar surface of FTO is essentially non-existent on nanoITO, where both desorption and ligand substitution are shut down with RuP4OMe. These results revealed that minimizing time spent in the oxidized form, incorporating electron-donating groups, maximizing hydrophobicity, and minimizing molecular bulk near the adsorbed ligand are critical to optimizing the performance of ruthenium(II) polypyridyl complexes in dye-sensitized devices.

Production of solar chemicals: gaining selectivity with hybrid molecule/semiconductor assemblies

Research on the production of solar fuels and chemicals has rocketed over the past decade, with a wide variety of systems proposed to harvest solar energy and drive chemical reactions. In this Feature Article we have focused on hybrid molecule/semiconductor assemblies in both powder and supported materials, summarising recent systems and highlighting the enormous possibilities offered by such assemblies to carry out highly demanding chemical reactions with industrial impact. Of relevance is the higher selectivity obtained in visible light-driven organic transformations when using molecular catalysts compared to photocatalytic materials.

A protocol for quantifying hydrogen evolution by dye-sensitized molecular photocathodes and its implementation for evaluating a new covalent architecture based on an optimized dye-catalyst dyad

A protocol that combines gas chromatography and a high-sensitivity micro Clark-type electrode is described to quantify hydrogen production across gas and solution phases for systems operating at very low currents such as dye-sensitized H₂-evolving photocathodes. Data indicate that a significant fraction of H₂ remains in aqueous solution even after several hours of experiments. Using this protocol, re-evaluation of a dye-sensitized H₂-evolving photocathode based on a dye-catalyst dyad showed a reproducible 66% increase of the faradaic efficiency compared with previously reported headspace GC measurements [Kaeffer et al., J. Am. Chem. Soc., 2016, 138, 12308-12311]. This dyad was based on an organic push-pull dye where donor and acceptor are separated by one thiophene group. Insertion of a second thiophene group between the donor and acceptor led to a more efficient system with 30% improved faradaic efficiency for H₂ evolution.

Direct evidence of catalyst reduction on dye and catalyst co-sensitized NiO photocathodes by mid-infrared transient absorption spectroscopy

Co-sensitization of molecular dyes and catalysts on semiconductor surfaces is a promising strategy to build photoelectrodes for solar fuel production.

Co-sensitization of molecular dyes and catalysts on semiconductor surfaces is a promising strategy to build photoelectrodes for solar fuel production. In such a photoelectrode, understanding the charge transfer reactions between the molecular dye, catalyst and semiconductor material is key to guide further improvement of their photocatalytic performance. Herein, femtosecond mid-infrared transient absorption spectroscopy is used, for the first time, to probe charge transfer reactions leading to catalyst reduction on co-sensitized nickel oxide (NiO) photocathodes. The NiO films were co-sensitized with a molecular dye and a proton reducing catalyst from the family of [FeFe](bdt)(CO)₆ (bdt = benzene-1,2-dithiolate) complexes. Two dyes were used: an organic push–pull dye denoted E2 with a triarylamine–oligothiophene–dicyanovinyl structure and a coumarin 343 dye. Upon photo-excitation of the dye, a clear spectroscopic signature of the reduced catalyst is observed a few picoseconds after excitation in all co-sensitized NiO films. However, kinetic analysis of the transient absorption signals of the dye and reduced catalyst reveal important mechanistic differences in the first reduction of the catalyst depending on the co-sensitized molecular dye (E2 or C343). While catalyst reduction is preceded by hole injection in NiO in C343-sensitized NiO films, the singly reduced catalyst is formed by direct electron transfer from the excited dye E2* to the catalyst in E2-sensitized NiO films. This change in mechanism also impacts the lifetime of the reduced catalyst, which is only ca. 50 ps in E2-sensitized NiO films but is >5 ns in C343-sensitized NiO films. Finally, the implication of this mechanistic study for the development of better co-sensitized photocathodes is discussed.

Recent Advances in Sensitized Photocathodes: From Molecular Dyes to Semiconducting Quantum Dots

The increasing demand for sustainable and environmentally benign energy has stimulated intense research to establish highly efficient photo-electrochemical (PEC) cells for direct solar-to-fuel conversion via water splitting. Light absorption, as the initial step of the catalytic process, is regarded as the foundation of establishing highly efficient PEC systems. To make full use of visible light, sensitization on photoelectrodes using either molecular dyes or semiconducting quantum dots provides a promising method. In this field, however, there remain many fundamental issues to be solved, which need in-depth study. Here, fundamental knowledge of PEC systems is introduced to enable readers a better understanding of this field. Then, the development history and current state in both molecular dye- and quantum dot-sensitized photocathodes for PEC water splitting are discussed. A systematical comparison between the two systems has been made. Special emphasis is placed on the research of quantum dot-sensitized photocathodes, which have shown superiority in both efficiency and durability towards PEC water splitting at the present stage. Finally, the opportunities and challenges in the future for sensitized PEC water-splitting systems are proposed.

Hierarchical CuInS2-based heterostructure: Application for photocathodic bioanalysis of sarcosine

In this study, on the basis of hierarchical CuInS₂-based heterostructure, a novel cathodic photoelectrochemical (PEC) enzymatic bioanalysis of the sarcosine detection was reported. Specifically, heterostructured CuInS₂/NiO/ITO photocathode was prepared and sarcosine oxidases (SOx) were integrated for the construction of the enzymatic biosensor. In the bioanalysis, the O₂-dependent suppression of the cathodic photocurrent can be observed due to the competition between the as-fabricated O₂-sensitive photocathode and the SOx-catalytic event toward O₂ reduction. Based on the sarcosine-controlled O₂ concentration, a novel photocathodic enzymatic biosensor could be realized for the sensitive and specific sarcosine detection. This work manifested the great potential of CuInS₂-based heterostructure as a novel platform for future PEC bioanalytical development and also a PEC method for sarcosine detection, which could be easily extended to numerous other enzymatic systems and to our knowledge has not been reported. This work is expected to stimulate more interest in the design and implementation of numerous CuInS₂-based heterostructured photocathodic enzymatic sensing.

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.