Immobilization of molecular catalysts on electrode surfaces using host-guest interactions

Anchoring molecular catalysts on electrode surfaces combines the high selectivity and activity of molecular systems with the practicality of heterogeneous systems. Molecular catalysts, however, are far less stable than traditional heterogeneous electrocatalysts, and therefore a method to easily replace anchored molecular catalysts that have degraded could make such electrosynthetic systems more attractive. Here we applied a non-covalent 'click' chemistry approach to reversibly bind molecular electrocatalysts to electrode surfaces through host-guest complexation with surface-anchored cyclodextrins. The host-guest interaction is remarkably strong and enables the flow of electrons between the electrode and the guest catalyst. Electrosynthesis in both organic and aqueous media was demonstrated on metal oxide electrodes, with stability on the order of hours. The catalytic surfaces can be recycled by controlled release of the guest from the host cavities and the readsorption of fresh guest.

Carbazole-Cyanobenzene Dyes Electrografted to Carbon or Indium-Doped Tin Oxide Supports for Visible Light-Driven Photoanodes and Olefin Isomerizations

The organic carbazole-cyanobenzene push-pull dye 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene was derivatized and attached to carbon or indium-doped tin oxide (ITO) electrodes by simple diazonium electrografting. The surface-bound dye is active and stable for the visible light photosynthetic isomerization of a wide range of functionalized stilbene and cinnamic acid derivatives. Up to 87,000 net turnovers were obtained for the isomerization of trans-stilbene. The isomerizations can be carried out in air with a 33% reduction in the rate. The ITO photoelectrodes are also active and stable toward photo-oxidations under basic and acidic conditions.

Plasma-Initiated Graft Polymerization of Acrylic Acid onto Fluorine-Doped Tin Oxide as a Platform for Immobilization of Water-Oxidation Catalysts

The discovery of new and versatile strategies for the immobilization of molecular water-oxidation catalysts (WOCs) is crucial for developing clean energy conversion devices [e.g., (photo)electrocatalytic cells for water splitting]. The traditional approach for surface attachment to transparent conductive oxides [e.g., fluorine doped tin oxide (FTO)] is via synthetic modification of the ligand architecture to incorporate functional groups such as carboxylic acids (-COOH) or phosphonates (-PO₃H₂) prior to immobilization. However, challenges arising from desorption and the cumbersome derivatizations steps have limited the scope and applications of surface-bound WOCs. Herein, we report the successful immobilization of underivatized Ru(II)-based WOCs (Ru-Cat1 = [Ru(tpy) (bpy) (H₂O)]²⁺ (tpy = 2,2':6'2″-terpyridine and bpy = 2,2'-bipyridine) and Ru-Cat2 = [Ru(Mebimpy) (bpy) (H₂O)]²⁺ (Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) pyridine)) and the Ru(II) polypyridyl chromophore Ru-C3 = [Ru(bpy)₃]²⁺ onto a FTO plasma-grafted poly(acrylic acid) surface (PAA|FTO). Various characterization techniques such as attenuated total reflectance Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, and cyclic voltammetry measurements provide evidence for the plasma-induced grafted PAA|FTO film and immobilization. Surface stability and electrocatalytic properties of these new hybrid composite films upon cycling were investigated at different pH values. Immobilized Ru-Cat1 and Ru-Cat2 onto PAA|FTO displayed pH-dependent (Ru^III/Ru^II) couples and onset potentials indicative of PCET (proton-coupled electron transfer) reactions. Based on cyclic voltammetry results and spectroscopic monitoring, the immobilized WOCs Ru-Cat1 and Ru-Cat2 exhibited a higher surface stability in neutral aqueous solutions relative to Ru-C3 upon electrochemical oxidation. We attribute the surface PCET and stability to the presence of a water ligand in the coordination sphere of immobilized Ru-Cat1 and Ru-Cat2 which can H-bond with negatively charged carboxylate groups of the cross-linked PAA brushes. Our findings demonstrate that the plasma-grafted polymeric network onto FTO offers a versatile platform to directly anchor unmodified homogeneous WOCs or chromophores for potential applications in solar-to-fuel energy conversion.

Homogeneous Catalysts Based on First-Row Transition-Metals for Electrochemical Water Oxidation

Strategies that enable the renewable production of storable fuels (i. e. hydrogen or hydrocarbons) through electrocatalysis continue to generate interest in the scientific community. Of central importance to this pursuit is obtaining the requisite chemical (H+ ) and electronic (e- ) inputs for fuel-forming reduction reactions, which can be met sustainably by water oxidation catalysis. Further possibility exists to couple these redox transformations to renewable energy sources (i. e. solar), thus creating a carbon neutral solution for long-term energy storage. Nature uses a Mn-Ca cluster for water oxidation catalysis via multiple proton-coupled electron-transfers (PCETs) with a photogenerated bias to perform this process with TOF 100∼300 s-1 . Synthetic molecular catalysts that efficiently perform this conversion commonly utilize rare metals (e. g., Ru, Ir), whose low abundance are associated to higher costs and scalability limitations. Inspired by nature's use of 1st row transition metal (TM) complexes for water oxidation catalysts (WOCs), attempts to use these abundant metals have been intensively explored but met with limited success. The smaller atomic size of 1st row TM ions lowers its ability to accommodate the oxidative equivalents required in the 4e- /4H+ water oxidation catalysis process, unlike noble metal catalysts that perform single-site electrocatalysis at lower overpotentials (η). Overcoming the limitations of 1st row TMs requires developing molecular catalysts that exploit biomimetic phenomena - multiple-metal redox-cooperativity, PCET and second-sphere interactions - to lower the overpotential, preorganize substrates and maintain stability. Thus, the ultimate goal of developing efficient, robust and scalable WOCs remains a challenge. This Review provides a summary of previous research works highlighting 1st row TM-based homogeneous WOCs, catalytic mechanisms, followed by strategies for catalytic activity improvements, before closing with a future outlook for this field.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Modulating the water oxidation catalytic activity of iridium complexes by functionalizing the Cp*-ancillary ligand: hints on the nature of the active species

A comparative study on the behavior of a series of iridium dimeric WOCs with modified Cp* ligands reveals the key role played by the variable substituent.

Investigation of Immobilization Effects on Ni(P2N2)2 Electrocatalysts

A new synthetic route to complexes of the type Ni(P₂N₂)₂²⁺ with highly functionalized phosphine substituents and the investigation of immobilization effects on these catalysts is reported. Ni(P₂N₂)₂²⁺ complexes have been extensively studied as homogeneous and surface-attached molecular electrocatalysts for the hydrogen evolution reaction (HER). A synthesis based on postsynthetic modification of P^ArBr₂N^Ph₂ was developed and is described here. Phosphonate-modified ligands and their corresponding nickel complexes were isolated and characterized. Subsequent deprotection of the phosphonic ester derivatives provided the first Ni(P₂N₂)₂²⁺ catalyst that can be covalently attached via pendent phosphonate groups to an electrode without involvement of the important pendent amine groups. Mesoporous TiO₂ electrodes were surface modified by attachment of the new phosphonate functionalized Ni(P₂N₂)₂²⁺ complexes, and these provided electrocatalytic materials that proved to be competent and stable for sustained HER in aqueous solution at mild pH and low overpotential. We directly compared the new ligand to a previously reported complex that utilized the amine moiety for surface attachment. Using HER as the benchmark reaction, the P-attached catalyst showed a marginally (9-14%) higher turnover number than its N-attached counterpart.

Promotion of the water oxidation activity of iridium oxide by a nitrogen coordination strategy

The water oxidation reaction is the pivotal half-reaction for photo-/electro-catalytic water splitting. Fabrication of high-efficiency and robust water oxidation is essential to realize wide-scale artificial photosynthesis. Here, we report an efficient strategy to improve the water oxidation activity of iridium oxide by a nitrogen-coordination method. Due to the coordination effect, the iridium oxide can be well dispersed to generate ultra-small nanoparticles and the intrinsic activity can be improved for the water oxidation reaction. This study suggests that high-performance water oxidation catalysts can be constructed based on a nitrogen-coordination strategy.

Molecular and heterogenized dinuclear Ir-Cp* water oxidation catalysts bearing EDTA or EDTMP as bridging and anchoring ligands

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir)₂(μ-κ³-O,N,O-H₄-EDTMP)] (Ir-H₄-EDTMP, H₄-EDTMP^4- = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)₂(μ-κ³-O,N,O-EDTA)] (Ir-EDTA, EDTA^4- = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR spectroscopy, and in the solid state, by single crystal X-Ray diffraction. They were supported onto rutile TiO₂ nanocrystals obtaining Ir-H₄-EDTMP@TiO₂ and Ir-EDTA@TiO₂ hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO₄, providing almost quantitative yields, and TON values only limited by the amount of NaIO₄ used. As for the molecular catalysts, Ir-H₄-EDTMP (TOF up to 184 min^-1) exhibited much higher activity than Ir-EDTA (TOF up to 19 min^-1), likely owing to the higher propensity of the former to generate a coordination vacancy through the dissociation of a Ir-OP bond (2.123 Å, significantly longer than Ir-OC, 2.0913 Å), which is a necessary step to activate these saturated complexes. Ir-H₄-EDTMP@TiO₂ (up to 33 min^-1) and Ir-EDTA@TiO₂ (up to 41 min^-1) hybrid materials showed similar activity that was only marginally reduced in the second and third catalytic runs carried out after having separated the supernatant, which did not show any sign of activity, instead. The observed TOF values for hybrid materials are higher than those reported for analogous systems deriving from heterogenized mononuclear complexes. This suggests that supporting dinuclear molecular precursors could be a successful strategy to obtain efficient heterogenized water oxidation catalysts.

A diketopyrrolopyrrole dye-based dyad on a porous TiO2 photoanode for solar-driven water oxidation

Dye-sensitised photoanodes modified with a water oxidation catalyst allow for solar-driven O2 evolution in photoelectrochemical cells. However, organic chromophores are generally considered unsuitable to drive the thermodynamically demanding water oxidation reaction, mainly due to their lack of stability upon photoexcitation. Here, the synthesis of a dyad photocatalyst (DPP-Ru) consisting of a diketopyrrolopyrrole chromophore (DPPdye) and ruthenium-based water oxidation catalyst (RuWOC) is described. The DPP-Ru dyad features a cyanoacrylic acid anchoring group for immobilisation on metal oxides, strong absorption in the visible region of the electromagnetic spectrum, and photoinduced hole transfer from the dye to the catalyst unit. Immobilisation of the dyad on a mesoporous TiO2 scaffold was optimised, including the use of a TiCl4 pretreatment method as well as employing chenodeoxycholic acid as a co-adsorbent, and the assembled dyad-sensitised photoanode achieved O2 evolution using visible light (100 mW cm-2, AM 1.5G, λ > 420 nm). An initial photocurrent of 140 μA cm-2 was generated in aqueous electrolyte solution (pH 5.6) under an applied potential of +0.2 V vs. NHE. The production of O2 has been confirmed by controlled potential electrolysis with a faradaic efficiency of 44%. This study demonstrates that metal-free dyes are suitable light absorbers in dyadic systems for the assembly of water oxidising photoanodes.

Stabilization of a molecular water oxidation catalyst on a dye-sensitized photoanode by a pyridyl anchor

Understanding and controlling the properties of water-splitting assemblies in dye-sensitized photoelectrosynthesis cells is a key to the exploitation of their properties. We demonstrate here that, following surface loading of a [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) chromophore on nanoparticle electrodes, addition of the molecular catalysts, Ru(bda)(L)₂ (bda  =  2,2′-bipyridine-6,6′-dicarboxylate) with phosphonate or pyridyl sites for water oxidation, gives surfaces with a 5:1 chromophore to catalyst ratio. Addition of the surface-bound phosphonate derivatives with L = 4-pyridyl phosphonic acid or diethyl 3-(pyridin-4-yloxy)decyl-phosphonic acid, leads to well-defined surfaces but, following oxidation to Ru(III), they undergo facile, on-surface dimerization to give surface-bound, oxo-bridged dimers. The dimers have a diminished reactivity toward water oxidation compared to related monomers in solution. By contrast, immobilization of the Ru-bda catalyst on TiO₂ with the 4,4′-dipyridyl anchoring ligand can maintain the monomeric structure of catalyst and gives relatively stable photoanodes with photocurrents that reach to 1.7 mA cm⁻² with an optimized, applied bias photon-to-current efficiency of 1.5%.

Understanding the properties of water-splitting assemblies in dye-sensitized photoelectrochemical cells is a key challenge in artificial photosynthesis. Here, the authors report the importance of anchoring groups on a water oxidation catalyst in determining active species on metal oxide surfaces.

A Precious-Metal-Free Hybrid Electrolyzer for Alcohol Oxidation Coupled to CO2 -to-Syngas Conversion

Electrolyzers combining CO2 reduction (CO2 R) with organic substrate oxidation can produce fuel and chemical feedstocks with a relatively low energy requirement when compared to systems that source electrons from water oxidation. Here, we report an anodic hybrid assembly based on a (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) electrocatalyst modified with a silatrane-anchor (STEMPO), which is covalently immobilized on a mesoporous indium tin oxide (mesoITO) scaffold for efficient alcohol oxidation (AlcOx). This molecular anode was subsequently combined with a cathode consisting of a polymeric cobalt phthalocyanine on carbon nanotubes to construct a hybrid, precious-metal-free coupled AlcOx-CO2 R electrolyzer. After three-hour electrolysis, glycerol is selectively oxidized to glyceraldehyde with a turnover number (TON) of ≈1000 and Faradaic efficiency (FE) of 83 %. The cathode generated a stoichiometric amount of syngas with a CO:H2 ratio of 1.25±0.25 and an overall cobalt-based TON of 894 with a FE of 82 %. This prototype device inspires the design and implementation of nonconventional strategies for coupling CO2 R to less energy demanding, and value-added, oxidative chemistry.

Diazo coupling for surface attachment of small molecules to TiO2 nanoparticles

Robust surface attachment of molecular species to metal oxide semiconductors is desirable for many applications. Here, we report the interfacial diazo coupling of surface-bound amines with aromatics to bind them to the surface of TiO2 nanoparticles via a siloxane anchor and a diazo linker. The technique shows potential for the inexpensive, stable, modular and tunable attachment of molecules to metal oxide surfaces.

Solid-phase synthesis and photoactivity of Ru-polypyridyl visible light chromophores bonded through carbon to semiconductor surfaces

1,10-Phenanthroline (phen) was grafted to either indium tin oxide (ITO), fluorine-doped tin oxide (FTO), or titanium dioxide (TiO₂) semiconductors (SC's) by electrochemical reduction of 5-diazo-phen. The phen ligand is bonded to the semiconductor at C5, and it can be handled in air. The semiconductor-phen (SC-phen) complexes displace both CH₃CN ligands from either cis-[Ru(Mebipy)₂(CH₃CN)₂]²⁺ (Mebipy = 4,4'-methyl-2,2'-bipyridine), cis-[Ru(^tBubipy)₂(CH₃CN)₂]²⁺ (^tBubipy = 4,4'-tert-butyl-2,2'-bipyridine), or cis-[Ru(pheno)(bipy)(CH₃CN)₂]²⁺ (bipy = 2,2'-bipyridine; pheno = 1,10-phenanthroline-5,6-dione) dissolved in DCM/THF (4 h, 70 °C) to form the corresponding surface-bound SC-[(phen)Ru(bipyridyl)₂]²⁺ chromophores. The identities of the SC-[(phen)Ru(Mebipy)₂]²⁺, SC-[(phen)Ru(^tBubipy)₂]²⁺, and SC-[(phen)Ru(pheno)(bipy)]²⁺ (SC = ITO, FTO or TiO₂) chromophores were confirmed by X-ray photoelectron spectroscopy (XPS); inductively coupled plasma mass spectrometry (ICP-MS); UV-vis and reflectance infrared spectroscopies; and cyclic voltammetry (CV). The data were compared to analogous Ru-polypyridyl control compounds dissolved in solution. A facile ketone-amine condensation solid-phase synthesis reaction between SC-[(phen)Ru(pheno)(bipy)]²⁺ and [Ru(1,10-phenthroline-5,6-diamine)(bipy)₂]²⁺ in ethanol (80 °C, 1 h) formed the dinuclear, bound chromophore SC-[(phen)(bipy)Ru(tpphz)Ru(bipy)₂]⁴⁺ (tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine). Photoelectrochemical oxidation of hydroquinone and triethylamine under acidic, neutral, or basic conditions showed that the SC-chromophore photoanodes are active, and that TiO₂-[(phen)Ru(Mebipy)₂]²⁺ is the most active and stable under basic- and neutral conditions. The dinuclear chromophore SC-[(phen)(bipy)Ru(tpphz)Ru(bipy)₂]⁴⁺ was most active and stable under potentiostatic conditions in acid.

Molecular catalysis of CO2 reduction: recent advances and perspectives in electrochemical and light-driven processes with selected Fe, Ni and Co aza macrocyclic and polypyridine complexes

Earth-abundant Fe, Ni, and Co aza macrocyclic and polypyridine complexes have been thoroughly investigated for CO2 electrochemical and visible-light-driven reduction. Since the first reports in the 1970s, an enormous body of work has been accumulated regarding the two-electron two-proton reduction of the gas, along with mechanistic and spectroscopic efforts to rationalize the reactivity and establish guidelines for structure-reactivity relationships. The ability to fine tune the ligand structure and the almost unlimited possibilities of designing new complexes have led to highly selective and efficient catalysts. Recent efforts toward developing hybrid systems upon combining molecular catalysts with conductive or semi-conductive materials have converged to high catalytic performances in water solutions, to the inclusion of these catalysts into CO2 electrolyzers and photo-electrochemical devices, and to the discovery of catalytic pathways beyond two electrons. Combined with the continuous mechanistic efforts and new developments for in situ and in operando spectroscopic studies, molecular catalysis of CO2 reduction remains a highly creative approach.

Iridium-Doped Nanosized Zn-Al Layered Double Hydroxides as Efficient Water Oxidation Catalysts

Layered double hydroxides (LDHs) are an ideal platform to host catalytic metal centers for water oxidation (WO) owing to the high accessibility of water to the interlayer region, which makes all centers potentially reachable and activated. Herein, we report the syntheses of three iridium-doped zinc-aluminum LDHs (Ir-LDHs) nanomaterials (1-3, with about 80 nm of planar size and a thickness of 8 nm as derived by field emission scanning electron microscopy and powder X-ray diffraction studies, respectively), carried out in the confined aqueous environment of reverse micelles, through a very simple and versatile procedure. These materials exhibit excellent catalytic performances in WO driven by NaIO4 at neutral pH and 25 °C, with an iridium content as low as 0.5 mol % (∼0.8 wt %), leading to quantitative oxygen yields (based on utilized NaIO4, turnover number up to ∼10,000). Nanomaterials 1-3 display the highest ever reported turnover frequency values (up to 402 min-1) for any heterogeneous and heterogenized catalyst, comparable only to those of the most efficient molecular iridium catalysts, tested under similar reaction conditions. The boost in activity can be traced to the increased surface area and pore volume (>5 times and 1 order of magnitude, respectively, higher than those of micrometric materials of size 0.3-1 μm) estimated for the nanosized particles, which guarantee higher noble metal accessibility. X-ray absorption spectroscopy (XAS) studies suggest that 1-3 nanomaterials, as-prepared and after catalysis, contain a mixture of isolated, single octahedral Ir(III) sites, with no evidence of Ir-Ir scattering from second-nearest neighbors, excluding the presence of IrO2 nanoparticles. The combination of the results obtained from XAS, elemental analysis, and ionic chromatography strongly suggests that iridium is embedded in the brucite-like structure of LDHs, having four hydroxyls and two chlorides as first neighbors. These results demonstrate that nanometric LDHs can be successfully exploited to engineer efficient WOCs, minimizing the amount of iridium used, consistent with the principle of the noble-metal atom economy.

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.

Photocathode functionalized with a molecular cobalt catalyst for selective carbon dioxide reduction in water

Artificial photosynthesis is a vibrant field of research aiming at converting abundant, low energy molecules such as water, nitrogen or carbon dioxide into fuels or useful chemicals by means of solar energy input. Photo-electrochemical reduction of carbon dioxide is an appealing strategy, aiming at reducing the greenhouse gas into valuable products such as carbon monoxide at low or without bias voltage. Yet, in such configuration, there is no catalytic system able to produce carbon monoxide selectively in aqueous media with high activity, and using earth-abundant molecular catalyst. Upon associating a p-type Cu(In,Ga)Se2 semi-conductor with cobalt quaterpyridine complex, we herein report a photocathode complying with the aforementioned requirements. Pure carbon dioxide dissolved in aqueous solution (pH 6.8) is converted to carbon monoxide under visible light illumination with partial current density above 3 mA cm-2 and 97% selectivity, showing good stability over time.

Infrared driven hot electron generation and transfer from non-noble metal plasmonic nanocrystals

Non-noble metal plasmonic materials, e.g. doped semiconductor nanocrystals, compared to their noble metal counterparts, have shown unique advantages, including broadly tunable plasmon frequency (from visible to infrared) and rich surface chemistry. However, the fate and harvesting of hot electrons from these non-noble metal plasmons have been much less explored. Here we report plasmon driven hot electron generation and transfer from plasmonic metal oxide nanocrystals to surface adsorbed molecules by ultrafast transient absorption spectroscopy. We show unambiguously that under infrared light excitation, hot electron transfers in ultrafast timescale (<50 fs) with an efficiency of 1.4%. The excitation wavelength and fluence dependent study indicates that hot electron transfers right after Landau damping before electron thermalization. We revealed the efficiency-limiting factors and provided improvement strategies. This study paves the way for designing efficient infrared light absorption and photochemical conversion applications based on non-noble metal plasmonic materials.

A Comparative Investigation of the Role of the Anchoring Group on Perylene Monoimide Dyes in NiO-Based Dye-Sensitized Solar Cells

The anchoring group of a sensitizer may strongly affect the overall properties and stability of the resulting dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthetic solar cells (DSPECs). The properties of seven perylene monoimide (PMI) dyes have been comprehensively studied for their immobilization on nanocrystalline NiO film. The PMI dyes differ only by the nature of the anchoring group, which are: carboxylic acid (PMI-CO₂ H), phosphonic acid (PMI-PO₃ H₂ ), acetyl acetone (PMI-acac), pyridine (PMI-Py), aniline (PMI-NH₂ ), hydroxyquinoline (PMI-HQ), and dipicolinic acid (PMI-DPA). The dyes are investigated by cyclic voltammetry and spectroelectrochemistry and modeled by TD-DFT quantum chemical calculations. The mode of binding of these anchoring groups is investigated by infrared spectroscopy and the stability of the binding to NiO surface is studied by desorption experiments in acidic and basic media. The phosphonic acid group is found to offer the strongest binding to the NiO surface in terms of stability and dye loading. Finally, a photophysical study by ultrafast transient absorption spectroscopy shows that all dyes inject a hole in NiO with rate constants on a subpicosecond timescale and display similar charge recombination kinetics. The photovoltaic properties of the dyes show that PMI-HQ and PMI-acac give the highest photovoltaic performances, owing to a lower degree of aggregation on the surface.

Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications

A facile surface amide-coupling method was examined to attach dye and catalyst molecules to silatrane-decorated NiO electrodes. Using this method, electrodes with a push-pull dye were assembled and characterized by photoelectrochemistry and transient absorption spectroscopy. The dye-sensitized electrodes exhibited hole injection into NiO and good photoelectrochemical stability in water, highlighting the stability of the silatrane anchoring group and the amide linkage. The amide-coupling protocol was further applied to electrodes that contain a molecular proton reduction catalyst for use in photocathode architectures. Evidence for catalyst reduction was observed during photoelectrochemical measurements and via femtosecond-transient absorption spectroscopy demonstrating the possibility for application in photocathodes.

Photoanodes for water oxidation with visible light based on a pentacyclic quinoid organic dye enabling proton-coupled electron transfer

A novel pentacyclic quinoid photosensitizer with extended absorption in the visible region and enabling proton-coupled electron transfer is employed in photoelectrodes for water oxidation in combination with a ruthenium polyoxometalate catalyst.

Ir- and Ru-doped layered double hydroxides as affordable heterogeneous catalysts for electrochemical water oxidation

Doping low-cost LDHs with noble metal atoms represents a promising approach to develop effective heterogeneous Water Oxidation Catalysts.

Facile Orientational Control of M2L2P SURMOFs on ⟨100⟩ Silicon Substrates and Growth Mechanism Insights for Defective MOFs

Layer-by-layer growth of Cu₂(bdc)₂(dabco) surface-mounted metal-organic frameworks (SURMOFs) was investigated on silicon wafers treated with different surface anchoring molecules. Well-oriented growth along the [100] and [001] directions could be achieved with simple protocols: growth along the [100] direction was achieved by substrate pretreatment with 80 °C piranha, while growth along the [001] direction was enabled by only rinsing silicon with absolute ethanol. Growth along the [001] direction produced more homogeneous SURMOF films. Optimization to enhance [001]-preferred orientation growth revealed that small changes in the SURMOF growth sequence (the number of rinse steps and linker concentrations) have a noticeable impact on the final film quality and the number of misaligned crystals. This new straightforward protocol was used to successfully grow other layer pillar-type SURMOFs, including the growth of Cu₂(bdc)₂(bipy) with simultaneous suppression of framework interpenetration.

Post Synthetic Defect Engineering of UiO-66 Metal–Organic Framework with An Iridium(III)-HEDTA Complex and Application in Water Oxidation Catalysis

Clean production of renewable fuels is a great challenge of our scientific community. Iridium complexes have demonstrated a superior catalytic activity in the water oxidation (WO) reaction, which is a crucial step in water splitting process. Herein, we have used a defective zirconium metal–organic framework (MOF) with UiO-66 structure as support of a highly active Ir complex based on EDTA with the formula [Ir(HEDTA)Cl]Na. The defects are induced by the partial substitution of terephthalic acid with smaller formate groups. Anchoring of the complex occurs through a post-synthetic exchange of formate anions, coordinated at the zirconium clusters of the MOF, with the free carboxylate group of the [Ir(HEDTA)Cl]− complex. The modified material was tested as a heterogeneous catalyst for the WO reaction by using cerium ammonium nitrate (CAN) as the sacrificial agent. Although turnover frequency (TOF) and turnover number (TON) values are comparable to those of other iridium heterogenized catalysts, the MOF exhibits iridium leaching not limited at the first catalytic run, as usually observed, suggesting a lack of stability of the hybrid system under strong oxidative conditions.

Influence of Anchoring Groups on the Charge Transfer and Performance of p-Si/TiO2/Cobaloxime Hybrid Photocathodes for Photoelectrochemical H2 Production

Although hybrid photocathodes built by immobilizing molecular catalysts to the surface of semiconductors through chemical linkages have been reported in recent years, systematic and comparative studies remain scarce about the impact of various anchoring groups on the performance, stability, and charge-transfer kinetics of molecular catalyst-decorated hybrid photocathodes for photoelectrochemical (PEC) H₂ production. In this study, the molecular cobaloxime catalysts, CoPy-4-X (Py = pyridine, X = PO₃H₂, COOH, and CONH(OH)), bearing different anchoring groups were synthesized and covalently immobilized to the surface of the porous TiO₂ layer coated on a p-Si plate or a fluorine-doped tin oxide glass. The influence of the anchoring groups on the performance of p-Si/TiO₂/CoPy-4-X photocathodes was comparatively studied for PEC H₂ evolution. Among the tested hybrid photocathodes, the one with a hydroxamate as an anchoring group displayed higher activity and lower charge-transfer resistance than that observed for the electrode with a carboxylate or a phosphonate as the anchoring group. Notably, the catalytic current of p-Si/TiO₂/CoPy-4-CONH(OH) was attenuated only by 2.9% in the controlled potential photoelectrolysis tests in borate buffer solution at pH 9 at 0 V versus a reversible hydrogen electrode over 6 h. Moreover, the influence of anchoring groups on the interfacial electron transfer from the TiO₂ layer to the immobilized cobaloxime catalyst and electron-hole recombination was studied by transient absorption spectroscopy. These results revealed that the hydroxamate as an anchoring group is superior to the carboxylate and phosphonate groups for speeding up the interfacial electron transfer and firmly immobilizing the molecular catalysts to the metal oxide semiconductors to build efficient and stable hybrid photoelectrodes.

Use of Selective Redox Cross-Inhibitors for the Control of Organic Layer Formation Obtained via Diazonium Salt Reduction

The controlled electrochemical deposition of a series of four diazonium salts (4-bromobenzene, 4-iodobenzene, 4-methoxybenzene, and 4-diethylaminobenzene diazonium) on carbon surfaces has been achieved by exploiting the use of three redox mediators: 2,2-diphenyl-1-picrylhydrazyl, chloranil, and dichlone. The efficiency of the method rests on a fast redox cross-reaction in the diffusion layer between the diazonium compound and the reduced form of the selected inhibitor, characterized by an outer-sphere electron transfer. The effect of the inhibitor addition in the deposition solution was characterized using electrochemical techniques, X-ray photoelectron spectroscopy, and atomic force microscopy. Near-monolayers are obtained when the potential of the redox mediator is at least 100 mV lower than the reduction potential of the diazonium salt concerned. A judicious choice of the redox entity can allow, via a fine control of the experimental conditions, to modulate the thickness of organic layers by varying the grafting potential.

Self-Assembled Bilayers as an Anchoring Strategy: Catalysts, Chromophores, and Chromophore-Catalyst Assemblies

Anchoring strategies for immobilization of molecular catalysts, chromophores, and chromophore-catalyst assemblies on electrode surfaces play an important role in solar energy conversion devices such as dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. They are also important in interfacial studies with surface-bound molecules including electron-transfer dynamics and mechanistic studies related to small molecule activation catalysis. Significant progress has been made in this area, but many challenges remain in terms of stability, synthetic complexity, and versatility. We report here a new anchoring strategy based on self-assembled bilayers. This strategy takes advantage of noncovalent interactions between long alkyl chains chemically bound to a metal-oxide electrode surface and long alkyl chains on the molecule being anchored. The new methodology is applicable to the heterogenization of both catalysts and chromophores as well as to the in situ "synthesis" of chromophore-catalyst assemblies on the electrode surface.

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.

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.

Silatrane Anchors for Metal Oxide Surfaces: Optimization for Potential Photocatalytic and Electrocatalytic Applications

Silatrane surface anchors are protected siloxanes that are known to bond firmly (from pH 2-11) to metal oxide electrodes under heating. However, these conditions are not always compatible with the other functionality present. A silatrane-containing porphyrin molecule and a silatrane-containing ruthenium complex have now been designed, synthesized and optimized conditions have been identified for surface binding. Two mild, room-temperature surface binding methods were explored: binding with or without an acidic pretreatment; these methods were compared to the traditional, harsher binding conditions involving strong heating. We find that a preacidified electrode gave comparable surface loadings at room temperature compared to sensitization by using the previous strong heating method. This was also true on TiO₂, SnO₂, and nanoITO electrodes and thus may be generalizable. The new, milder binding methods also resulted in excellent aqueous and electrochemical stability from pH 2-11. Using a water-insoluble porphyrin with a silatrane anchor further increased the aqueous stability of the deposit, aided by the insolubility of the porphyrin. Finally, X-ray photoelectron spectroscopy (XPS) data confirmed for the first time that the triethanolamine released from the silatrane on deprotection/binding in turn binds to TiO₂, SnO₂, and nanoITO electrodes. This undesired triethanolamine deposit was easily removed from the surface by electrochemical voltage cycling or with an aqueous acidic wash for 1 h.

Nanocrystalline Sb-doped SnO2 films modified with cyclometalated ruthenium complexes for two-step electrochromism

Sb-Doped nanocrystalline SnO2 (SnO2:Sb) thin films functionalized with cyclometalated ruthenium complexes 1 or 2 on FTO conductive glasses have been prepared and characterized. These complexes contain a redox-active amine unit separated from the ruthenium ion by a phenyl or biphenyl linker, respectively, to modify the absorption wavelengths at different redox states. Near-infrared electrochromism of both films has been examined by oxidative spectroelectrochemical measurements and double-potential-step chronoamperometry. A contrast ratio (ΔT%) of 33% at 1070 nm and 63% at 696 nm has been achieved for the SnO2:Sb/1 film in two stepwise oxidation processes, respectively. The other film with complex 2 shows two-step electrochromism at 1310 and 806 nm with ΔT% of 36% and 76%, respectively. The response time of electrochromic switching is around a few seconds. Taking advantage of the good contrast ratio, the rapid response, and the long retention time of each oxidation state, these films have been successfully used to demonstrate surface-confined flip-flop memory functions with a high ON/OFF ratio.