Photoelectrochemical Cells with a Pyridine-Anchored Bilayer Photoanode for Water Splitting

A dye-sensitized photoanode is prepared by coassembling a Ru complex photosensitizer and a Ru water oxidation catalyst (WOC) on a TiO2 substrate, in which the WOC molecules are immobilized in a layer-by-layer fashion through metal-pyridine coordination with the aid of a bifunctional anchoring and bridging molecule containing multiple pyridine groups. Under visible-light irradiation, an anodic photocurrent of around 200 μA/cm2 has been achieved with O2 and H2 being generated at the photoanode and Pt counter electrode, respectively. The pyridine anchoring strategy provides a simple method to prepare photoelectrodes for applications in photoelectrochemical cells.

Photoelectrochemical water oxidation improved by pyridine N-oxide as a mimic of tyrosine-Z in photosystem II

Artificial photosynthesis provides a way to store solar energy in chemical bonds with water oxidation as a major challenge for creating highly efficient and robust photoanodes that mimic photosystem II. We report here an easily available pyridine N-oxide (PNO) derivative as an efficient electron transfer relay between an organic light absorber and molecular water oxidation catalyst on a nanoparticle TiO2 photoanode. Spectroscopic and kinetic studies revealed that the PNO/PNO+˙ couple closely mimics the redox behavior of the tyrosine/tyrosyl radical pair in PSII in improving light-driven charge separation via multi-step electron transfer. The integrated photoanode exhibited a 1 sun current density of 3 mA cm-2 in the presence of Na2SO3 and a highly stable photocurrent density of >0.5 mA cm-2 at 0.4 V vs. NHE over a period of 1 h for water oxidation at pH 7. The performance shown here is superior to those of previously reported organic dye-based photoanodes in terms of photocurrent and stability.

Fluorinated aluminum(III) porphyrins: Synthesis, spectroscopy, electrochemistry and photochemistry

A series of fluorinated free-base porphyrins (H2TPPF[Formula: see text], [Formula: see text] = 0, 8, 12, 20, 24) and the corresponding aluminum(III) porphyrin (AlTPPF[Formula: see text]-Ph, [Formula: see text] = 0, 8, 12, 20, 24) derivatives have been synthesized and their spectroscopic, redox and optical properties were investigated. The absorption studies show that the spectral shapes of investigated porphyrins are sensitive to the degree of fluorination on the meso-phenyl units. Analogously, the fluorescence quantum yields and singlet-state lifetimes depend on the number of fluorine atoms, and decrease by increasing the number of fluorine atoms. The H2TPPF[Formula: see text] and AlTPPF[Formula: see text]-Ph ([Formula: see text] = 8, 12, 20, 24) derivatives exhibited lower fluorescence intensities compared to the H2TPP and AlTPP, respectively. However, the AlTPPF[Formula: see text]-Ph ([Formula: see text] = 0, 8, 12, 20, 24) derivatives yield relatively a strong fluorescence compared to the well-known ZnTPP. As predicted, the redox potentials are shifted to the more positive side by increasing the fluorine atoms. The Lewis acidity of AlTPPF[Formula: see text]-Ph was quantified by using the absorption and fluorescence titrations with the Lewis base [Formula: see text]-methylimidazole (Me-Im). The titration data suggests that the Lewis acidity of the Al center rises when increasing the number of fluorine atoms on the porphyrin. Together, the high fluorescence quantum yields, high-potentials, unique optical and redox properties suggest that the investigated porphyrins could be potential sensitizers to mimic various components of artificial photosynthetic systems for the production of solar fuels.

Electron-Withdrawing Boron Dipyrromethene Dyes As Visible Light Absorber/Sensitizers on Semiconductor Oxide Surfaces

The synthesis, characterization, and electrochemical and photophysical properties of the phosphonate-derivatized carbazole (CBZ) and boron dipyrromethene (BODIPY) chromophores in the dyes, BODIPY(CBZ)₂PO₃H₂ (8) and BODIPY(Tol)₂PO₃H₂ (7), are described. The oxide-bound dyes have been explored as light absorbers in dye-sensitized photoelectrosynthesis cell (DSPEC) applications. The BODIPY-CBZ phosphonate ester (6) features a broad, intense UV-visible absorption spectrum with absorptions at 297 and 650 nm that arise from mixed transitions at the CBZ and BODIPY units. Electrochemical measurements on BODIPY(CBZ)₂Br (4) in 0.1 M [nBu₄N][PF₆] in dichloromethane, vs normal hydrogen electrode (NHE), reveal reversible oxidations at 1.19 and 1.41 V and a reversible reduction at -0.59 V. On indium tin oxide (ITO) and TiO₂, a reversible one-electron oxidation appears for 7 at 0.86 and 0.90 V vs NHE in dichloromethane, respectively, which demonstrates the redox stability on metal oxide surfaces. The results of nanosecond transient absorption measurements on SnO₂/TiO₂ electrodes provide direct evidence for excited-state electron injection into the conduction band of TiO₂ following 590 nm excitation. A longer lifetime for 8⁺ compared to 7⁺ is consistent with extensive intramolecular charge separation between the CBZ and BODIPY units on the surface. Photoelectrochemical studies on 8 on a SnO₂/TiO₂ photoanode resulted in sustained photocurrents with current maxima of ∼200 μA/cm² with hydroquinone added as a reductant under 1 sun (AM1.5 100 mW·cm^-2) illumination at pH 4.5 in 0.1 M acetate buffer and 0.4 M LiClO₄. On mixed SnO₂/TiO₂ electrode surfaces, with the added catalyst [Ru(Mebimpy)((4,4'-(OH)₂PO-CH₂)₂bpy)(OH₂)]²⁺ and chromophores 7 and 8, addition of 0.1 M benzyl alcohol resulted in sustained photocurrents of 12 and 35 μA/cm², consistent with oxidation to benzaldehyde.

Fluorinated ZnII Porphyrins for Dye-Sensitized Aqueous Photoelectrosynthetic Cells

Three perfluorinated Zn^II porphyrins were evaluated as n-type sensitizers in photoelectrosynthetic cells for HBr and water splitting. All the dyes are featured by the presence of pentafluorophenyl electron-withdrawing groups to increase the ground-state oxidation potential and differ for the nature and position of the π-conjugate linker between the core and anchoring group tasked to bind the metal oxide, in order to assess the best way of coupling with the semiconductor. A phenyl-triazole moiety was used to link the carboxylic anchoring group onto the meso position, while an ethynyl-phenyl linker was chosen to bridge carboxylic and cyanoacrylic groups onto the β-pyrrolic position. A combination of electrochemical, computational, and spectroscopic investigations confirmed the strong electron-withdrawing effect of the perfluorinated porphyrin core, which assures all the investigated dyes of the high oxidation potential required to the coupling with water oxidation catalysts (WOC). Such an electron-poor core, however, affects the charge separation character of the dyes, as demonstrated by the spatial distribution of the excited states, leading to a nonquantitative charge injection, although tilting of the molecules on the semiconductor surface could bring the porphyrin ring closer to the semiconductor, offering additional charge-transfer pathways. Indeed, all the dyes demonstrated successful in the splitting of both aqueous HBr and water, with the best results found for the SnO₂/TiO₂ photoanode sensitized with the β-substituted porphyrin equipped with a cyanoacrylic terminal group, achieving 0.4 and 0.1 mA/cm² photoanodic currents in HBr and water under visible light, respectively. The faradaic yield for oxygen evolution in the presence of an Ir^IV catalyst was over 95%, and the photoanode operation was stable for more than 1000 s. Thus, the perfluorinated porphyrins with a cyanoacrylic anchoring group at the β-position should be considered for further development to improve the charge-transfer character.

Visible light-driven water oxidation with a subporphyrin sensitizer and a water oxidation catalyst

A new subporphyrin was synthesized for use as a molecular sensitizer in electrochemical and dye-sensitized photoelectrochemical water oxidation. A photoelectrochemical cell with a TiO₂ electrode modified with the sensitizer and a molecular water oxidation catalyst generated higher photocurrent than reference cells that have electrodes modified with either the photosensitizer or the catalyst under visible light (λ > 500 nm) illumination. Oxygen evolution was confirmed after photolysis by GC and GC-MS analyses using isotope-labeling experiments. The large molar extinction coefficients of the ring-contracted porphyrin in the visible region enabled kinetic analysis by time-resolved transient absorption spectroscopy, which also supported the photocatalytic activity.

Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition

Visible-light-driven water splitting was investigated in a dye sensitized photoelectrosynthesis cell (DSPEC) based on a photoanode with a phosphonic acid-derivatized donor-π-acceptor (D-π-A) organic chromophore, 1, and the water oxidation catalyst [Ru(bda)(4-O(CH₂)₃P(O₃H₂)₂-pyr)₂], 2, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate). The photoanode was prepared by using a layering strategy beginning with the organic dye anchored to an FTO|core/shell electrode, atomic layer deposition (ALD) of a thin layer (<1 nm) of TiO₂, and catalyst binding through phosphonate linkage to the TiO₂ layer. Device performance was evaluated by photocurrent measurements for core/shell photoanodes, with either SnO₂ or nanoITO core materials, in acetate-buffered, aqueous solutions at pH 4.6 or 5.7. The absolute magnitudes of photocurrent changes with the core material, TiO₂ spacer layer thickness, or pH, observed photocurrents were 2.5-fold higher in the presence of catalyst. The results of transient absorption measurements and DFT calculations show that electron injection by the photoexcited organic dye is ultrafast promoted by electronic interactions enabled by orientation of the dye's molecular orbitals on the electrode surface. Rapid injection is followed by recombination with the oxidized dye which is 95% complete by 1.5 ns. Although chromophore decomposition limits the efficiency of the DSPEC devices toward O₂ production, the flexibility of the strategy presented here offers a new approach to photoanode design.

Modulating Hole Transport in Multilayered Photocathodes with Derivatized p-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting

For solar water splitting, dye-sensitized NiO photocathodes have been a primary target. Despite marginal improvement in performance, limitations remain arising from the intrinsic disadvantages of NiO and insufficient catalysis. We report here a new approach to modifying NiO photocathodes with doped NiO bilayers and an additional layer of macro-mesoporous ITO. The trilayered electrode is functionalized with a surface-attached ruthenium polypyridyl dye and a covalently bridged nickel-based hydrogen evolution catalyst. The NiO film, containing a 2% K⁺-doped NiO inner layer and a 2% Cu²⁺-doped NiO outer layer, provides sufficient driving force for hole transport following hole injection by the molecular assembly. Upon light irradiation, the resulting photocathode generates hydrogen from water sustainably with enhanced photocurrents and a Faradaic efficiency of ∼90%. This approach highlights the value of modifying both the internal and surface structure of NiO and provides insights into a new generation of dye-sensitized photocathodes for solar-driven water splitting cells.

Layer-by-Layer Molecular Assemblies for Dye-Sensitized Photoelectrosynthesis Cells Prepared by Atomic Layer Deposition

In a dye sensitized photoelectrosynthesis cell (DSPEC), the relative orientation of the catalyst and chromophore plays an important role in determining the device efficiency. Here we introduce a new, robust atomic layer deposition (ALD) procedure for the preparation of molecular chromophore-catalyst assemblies on wide bandgap semiconductors. In this procedure, solution deposited, phosphonate derivatized metal complexes on metal oxide surfaces are treated with reactive metal reagents in the gas phase by ALD to form an outer metal ion bridging group, which can bind a second phosphonate containing species from solution to establish a R₁-PO₂-O-M-O-PO₂-R₂ type surface assembly. With the ALD procedure, assemblies bridged by Al(III), Sn(IV), Ti(IV), or Zr(IV) metal oxide units have been prepared. To evaluate the performance of this new type of surface assembly, intra-assembly electron transfer was investigated by transient absorption spectroscopy, and light-driven water splitting experiments under steady-state illumination were conducted. A SnO₂ bridged assembly on SnO₂/TiO₂ core/shell electrodes undergoes light-driven water oxidation with an incident photon to current efficiency (IPCE) of 17.1% at 440 nm. Light-driven water reduction with a ruthenium trisbipyridine chromophore and molecular Ni(II) catalyst on NiO films was also used to produce H₂. Compared to conventional solution-based procedures, the ALD approach offers significant advantages in scope and flexibility for the preparation of stable surface structures.

Fluoropolymer-Stabilized Chromophore-Catalyst Assemblies in Aqueous Buffer Solutions for Water-Oxidation Catalysis

Here, the application of the fluorinated polymer [Dupont AF, a copolymer of 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole and tetrafluoroethylene] is described in stabilizing phosphonate-derivatized molecular assemblies on oxide electrodes. In the procedure, the polymer was dip-coated onto the surfaces of oxide electrodes with pre-bound, phosphonate-derivatized chromophores and assemblies, including assemblies for water oxidation. The results of the experiments showed a high degree of stabilization by the added polymer and a demonstration of its use in stabilizing surface-bound assemblies for water-oxidation catalysis.

Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine-Sensitized NiO Photocathodes

Understanding the structural and electronic factors governing the efficiency of dye-sensitized NiO photocathodes is essential to optimize solar fuel production in photoelectrochemical cells (PECs). For these purpose, three different ruthenium dyes, bearing either two or four methylphosphonate anchoring groups and either a bipyridine or a dipyridophenazine ancillary ligand, were synthesized and grafted onto NiO films. These photoelectrodes were fully characterized by XPS, ToF-SIMS, UV-vis absorption, time-resolved emission and femtosecond transient absorption spectroscopies. Increasing the number of anchoring groups from two to four proved beneficial for the grafting efficiency. No significant modification of the electronic properties compared to the parent photosensitizer was observed, in accordance with the non-conjugated nature of the grafted linker. The photoelectrochemical activity of the dye-sensitized NiO electrodes was assessed in fully aqueous medium in the presence of an irreversible electron acceptor and photocurrents reaching 190 μA.cm^-2 were recorded. The transient absorption study revealed the presence of two charge recombination pathways for each of the sensitizers and evidenced a stabilized charge separated state in the dppz derivative, supporting its superior photoelectrochemical activity.

O–O bond formation in ruthenium-catalyzed water oxidation: single-site nucleophilic attack vs. O–O radical coupling

A review of water oxidation by ruthenium-based molecular catalysts, with emphasis on the mechanism of O–O bond formation.

Energy and Electron Transfer Cascade in Self-Assembled Bilayer Dye-Sensitized Solar Cells

Current high efficiency dye-sensitized solar cells (DSSCs) rely on the incorporation of multiple chromophores, via either codeposition or preformed assemblies, as a means of increasing broad band light absorption. These strategies have some inherent limitations including decreased total light absorption by each of the dyes, low surface loadings, and complex synthetic procedures. In this report, we introduce an alternative strategy, self-assembled bilayers, as a simple, stepwise method of incorporating two complementary chromophores into a DSSC. The bilayer devices exhibit a 10% increase in J_sc, V_oc, and η over the monolayer devices due to increased incident photon-to-electron conversion efficiency across the entire visible spectrum and slowed recombination losses at the interface. Directional energy and electron transfer toward the metal oxide surface are key steps in the bilayer photon-to-current generation process. These results are important as they open the door to a new architecture for harnessing broadband light in dye-sensitized devices.

Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells

The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO₂ by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO₂ reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO₂ reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

Growth and Post-Deposition Treatments of SrTiO3 Films for Dye-Sensitized Photoelectrosynthesis Cell Applications

Sensitized SrTiO3 films were evaluated as potential photoanodes for dye-sensitized photoelectrosynthesis cells (DSPECs). The SrTiO3 films were grown via pulsed laser deposition (PLD) on a transparent conducting oxide (fluorine-doped tin oxide, FTO) substrate, annealed, and then loaded with zinc(II) 5,10,15-tris(mesityl)-20-[(dihydroxyphosphoryl)phenyl] porphyrin (MPZnP). When paired with a platinum wire counter electrode and an Ag/AgCl reference electrode these sensitized films exhibited photocurrent densities on the order of 350 nA/cm(2) under 0 V applied bias conditions versus a normal hydrogen electrode (NHE) and 75 mW/cm(2) illumination at a wavelength of 445 nm. The conditions of the post-deposition annealing step-namely, a high-temperature reducing atmosphere-proved to be the most important growth parameters for increasing photocurrent in these electrodes.

Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications

A series of phosphonate-derivatized, high redox potential porphyrins with mesityl, pentafluorophenyl, and heptafluoropropyl meso-substituents were synthesized by acid-catalyzed condensation reactions. Ground and excited state redox potentials in the series were varied systematically with the electron-donating or electron-accepting nature of the meso-substitutents. The extent of excitation and injection by porphyrin singlet excited states surface-bound to SnO2/TiO2 core/shell metal oxide nanoparticle films varies with the excited state reduction potential, E°(')(P(+)/P*). With the mesityl-substituted porphyrin, high current density and sustained photocurrents are observed at pH 7 with the addition of the electron transfer donor hydroquinone.

Enhanced performance of dye-sensitized solar cells with Y-shaped organic dyes containing di-anchoring groups

Introducing two carboxylic acid anchors by the benzene bridge in the design of sensitizers can effectively improve the performance of DSSCs.

Visible light-driven water oxidation using a covalently-linked molecular catalyst-sensitizer dyad assembled on a TiO2 electrode

The combination of porphyrin as a sensitizer and a ruthenium complex as a water oxidation catalyst (WOC) is promising to exploit highly efficient molecular artificial photosynthetic systems. A covalently-linked ruthenium-based WOC-zinc porphyrin (ZnP) sensitizer dyad was assembled on a TiO₂ electrode for visible-light driven water oxidation. The water oxidation activity was found to be improved in comparison to the reference systems with the simple combination of the individual WOC and ZnP as well as with ZnP solely, demonstrating the advantage of the covalent linking approach over the non-covalent one. More importantly, via vectorial multi-step electron transfer triggered by visible light, the dye-sensitized photoelectrochemical cell (DSPEC) achieved a broader PEC response in the visible region than DSPECs with conventional ruthenium-based sensitizers. Initial incident photon-to-current efficiencies of 18% at 424 nm and 6.4% at 564 nm were attained under monochromatic illumination and an external bias of -0.2 V vs. NHE. Fast electron transfer from the WOC to the photogenerated radical cation of the sensitizer through the covalent linkage may suppress undesirable charge recombination, realizing the moderate performance of water oxidation. X-ray photoelectron spectroscopic analysis of the photoanodes before and after the DSPEC operation suggested that most of the ruthenium species exist at higher oxidation states, implying that the insufficient oxidation potential of the ZnP moiety for further oxidizing the intermediate ruthenium species at the photoanode is at least the bottleneck of the system.

Successive light-induced two electron transfers in a Ru-Fe supramolecular assembly: from Ru-Fe(ii)-OH2 to Ru-Fe(iv)-oxo

In the present work we describe the synthesis and study of a Ru^II-Fe^II chromophore-catalyst assembly designed to perform the light-induced activation of an iron bound water molecule and subsequent photo-driven oxidation of a substrate. Using a series of spectroscopic techniques, we demonstrate that excitation of the chromophore unit with 450 nm light, in the presence of a sacrificial electron acceptor, triggers a cascade of electron transfers leading to the formation of a high valent iron(iv)-oxo center from an iron(ii)-bound water molecule. The activity of this catalytic center is illustrated by the oxidation of triphenyl phosphine.

Towards multielectron photocatalysis: a porphyrin array for lateral hole transfer and capture on a metal oxide surface

“An artificial photosynthetic model system is reported consisting of a porphyrin monolayer on a SnO₂ surface that enables oxidizing equivalents to be efficiently delivered to a thermodynamic trap via lateral hole transfer.”