Tunneling and Thermally Activated Electron Transfer in Dye-Sensitized SnO2|TiO2 Core|Shell Nanostructures

The mechanism for interfacial electron transfer (ET) from a metal oxide core|shell nanostructure to a [RuIII(2,2′-bipyridine)2(4,4′-(PO3H2)2-2,2′-bipyridine)]3+ sensitizer was probed through spectroscopic quantification of the ET kinetics over a 70 °C temperature range in an aqueous 0.1 M HClO4 solution. Mesoporous thin films of rutile SnO2 or insulating ZrO2 nanocrystals were coated through atomic layer deposition (ALD) with TiO2 shells of variable thickness to comprise the core|shell nanostructures. In agreement with previous research, Raman spectroscopy and transmission electron microscopy provided evidence that annealing the mesoporous SnO2|TiO2 materials at 450 °C resulted in rutile TiO2 shells. Materials heated to only 200 °C, termed “unannealed”, exhibited no evidence of crystallinity. Annealed and unannealed materials resulted in dissimilar interfacial ET kinetics. While temperature-independent kinetics indicated that ET occurred through tunneling in the thickest unannealed shells, materials with annealed shells underwent thermally activated ET. Further, thermally activated ET and tunneling competed in materials with unannealed shells less than 50 ALD-cycles thick. Arrhenius analysis of the thermally activated ET revealed large barriers, consistent with slow ET reactions for SnO2|TiO2 relative to SnO2 or TiO2 alone. Barriers were factors of 3–4 larger for unannealed SnO2|TiO2 materials than those observed upon annealing. Eyring analysis revealed that Gibbs free energies of activation were largely insensitive to heat treatment, ΔG‡ = 47 ± 3 kJ mol–1, and that ET was entropically favored for unannealed SnO2|TiO2 and entropically costly for annealed materials. A model is proposed wherein ET in annealed SnO2|TiO2 is rate-limited by electron transport in the shell, while ET in unannealed SnO2|TiO2 is rate-limited by electron escape from the core. The model is consistent with a comparative study of ZrO2|TiO2 materials for which insulating ZrO2 cores are energetically inaccessible to electrons. These mechanistic insights provide guidance on how to manipulate core|shell nanostructures for applications in solar water splitting

Evidence that ΔS‡ Controls Interfacial Electron Transfer Dynamics from Anatase TiO2 to Molecular Acceptors

Recombination of electrons injected into TiO2 with molecular acceptors present at the interface represents an important loss mechanism in dye-sensitized water oxidation and electrical power generation. Herein, the kinetics for this interfacial electron transfer reaction to oxidized triphenylamine (TPA) acceptors was quantified over a 70° temperature range for para-methyl-TPA (Me-TPA) dissolved in acetonitrile solution, 4-[N,N-di(p-tolyl)amino]benzylphosphonic acid (a-TPA) anchored to the TiO2, and a TPA covalently bound to a ruthenium sensitizer, [Ru(tpy-C6H4-PO3H2)(tpy-TPA)]2+ “RuTPA”, where tpy is 2,2′:6′,2′′-terpyridine. Activation energies extracted from an Arrhenius analysis were found to be 11 ± 1 kJ mol–1 for Me-TPA and 22 ± 1 kJ mol–1 for a-TPA, values that were insensitive to the identity of different sensitizers. Recombination to RuTPA+ proceeded with Ea = 27 ± 1 kJ mol–1 that decreased to 19 ± 1 kJ mol–1 when recombination occurred to an oxidized para-methoxy TPA (MeO-TPA) dissolved in CH3CN. Eyring analysis revealed a smaller entropy of activation |ΔS‡| when the a-TPA was anchored to the surface or covalently linked to the sensitizer, compared to that when Me-TPA was dissolved in CH3CN. In all cases, Eyring analysis provided large and negative ΔS‡ values that point toward unfavorable entropic factors as the key contributor to the barrier that underlies the slow recombination kinetics that are generally observed at dye-sensitized TiO2 interfaces

Evidence that Δ<i>S</i>^‡ Controls Interfacial Electron Transfer Dynamics from Anatase TiO₂ to Molecular Acceptors

Recombination of electrons injected into TiO₂ with molecular acceptors present at the interface represents an important loss mechanism in dye-sensitized water oxidation and electrical power generation. Herein, the kinetics for this interfacial electron transfer reaction to oxidized triphenylamine (TPA) acceptors was quantified over a 70° temperature range for <i>para</i>-methyl-TPA (<b>Me</b>-TPA) dissolved in acetonitrile solution, 4-[<i>N</i>,<i>N</i>-di­(<i>p</i>-tolyl)­amino]­benzylphosphonic acid (<b><i>a</i>-TPA</b>) anchored to the TiO₂, and a TPA covalently bound to a ruthenium sensitizer, [Ru­(tpy-C₆H₄-PO₃H₂)­(tpy-TPA)]²⁺ “<b>RuTPA</b>”, where tpy is 2,2′:6′,2′′-terpyridine. Activation energies extracted from an Arrhenius analysis were found to be 11 ± 1 kJ mol^–1 for <b>Me</b>-TPA and 22 ± 1 kJ mol^–1 for <b><i>a</i>-TPA</b>, values that were insensitive to the identity of different sensitizers. Recombination to <b>RuTPA</b>^<b>+</b> proceeded with <i>E</i>_a = 27 ± 1 kJ mol^–1 that decreased to 19 ± 1 kJ mol^–1 when recombination occurred to an oxidized <i>para</i>-methoxy TPA (<b>MeO</b>-TPA) dissolved in CH₃CN. Eyring analysis revealed a smaller entropy of activation |Δ<i>S</i>^‡| when the <b><i>a</i>-TPA</b> was anchored to the surface or covalently linked to the sensitizer, compared to that when <b>Me</b>-TPA was dissolved in CH₃CN. In all cases, Eyring analysis provided large and negative Δ<i>S</i>^‡ values that point toward unfavorable entropic factors as the key contributor to the barrier that underlies the slow recombination kinetics that are generally observed at dye-sensitized TiO₂ interfaces

Photophysical characterization of new osmium (II) photocatalysts for hydrohalic acid splitting

ABSTRACT Two osmium(II) photocatalysts bearing a dicationic 4,4′-bis-(trimethylaminomethyl)-2,2′-bipyridine (tmam) ligand and 2,2′-bipyridine {[Os(bpy)2(tmam)]4+} or 4,4′-(CF3)2-2,2′-bipyridine {[Os((CF3)2bpy)2(tmam)]4+} ancillary ligands were synthesized and characterized for application in HX splitting. Iodide titration studies in acetone solutions provided evidence for an in situ formed terionic complex with two iodide ions as evidenced by 1H NMR and UV-visible absorption spectroscopies, as well as by density functional theory calculations and natural bond order analysis. The photocatalyst [Os(bpy)2(tmam)]4+ was shown to be inefficient in iodide oxidation. In contrast, visible light excitation of [Os((CF3)2bpy)2(tmam)]4+ led to rapid iodide oxidation, kq = 2.8 × 1011 M−1 s−1. The data reveal that Os(II) photocatalysts can be fine-tuned for application in HX splitting

Interfacial Deposition of Ru(II) Bipyridine-Dicarboxylate Complexes by Ligand Substitution for Applications in Water Oxidation Catalysis

Water oxidation is a critical step in artificial photosynthesis and provides the protons and electrons used in reduction reactions to make solar fuels. Significant advances have been made in the area of molecular water oxidation catalysts with a notable breakthrough in the development of Ru(II) complexes that use a planar “bda” ligand (bda is 2,2′-bipyridine-6,6′-dicarboxylate). These Ru(II)(bda) complexes show lower overpotentials for driving water oxidation making them ideal for light-driven applications with a suitable chromophore. Nevertheless, synthesis of heterogeneous Ru(II)(bda) complexes remains challenging. We discuss here a new “bottom-up” synthetic method for immobilizing these catalysts at the surface of a photoanode for use in a dye-sensitized photoelectrosynthesis cell (DSPEC). The procedure provides a basis for rapidly screening the role of ligand variations at the catalyst in order to understand the impact on device performance. The best results of a water-oxidation DSPEC photoanode based on this procedure reached 1.4 mA/cm2 at pH 7 in 0.1 M [PO4H2]-/[PO4H]2-solution with minimal loss in catalytic behavior over 30 min, and produced an incident photon to current efficiency (IPCE) of 24.8% at 440 nm

Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light

Efficient excited-state electron transfer between an iron(III) photosensitizer and organic electron donors was realized with green light irradiation. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe(phtmeimb)2]+ (phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}, that exhibited long-lived and luminescent ligand-to-metal charge-transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with yields that exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru(bpy)3]2+. The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer, which is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectorial and may be a general advantage of Fe(III) photosensitizers. In-depth time-resolved spectroscopic methods, including transient absorption characterization from the ultraviolet to the infrared regions, provided a quantitative description of the catalytic mechanism with associated rate constants and yields

Molecular Photoelectrode for Water Oxidation Inspired by Photosystem II

In artificial photosynthesis, the sun drives water splitting into H2 and O2 or converts CO2 into a useful form of carbon. In most schemes, water oxidation is typically the limiting half-reaction. Here, we introduce a molecular approach to the design of a photoanode that incorporates an electron acceptor, a sensitizer, an electron donor, and a water oxidation catalyst in a single molecular assembly. The strategy mimics the key elements in Photosystem II by initiating light-driven water oxidation with integration of a light absorber, an electron acceptor, an electron donor, and a catalyst in a controlled molecular environment on the surface of a conducting oxide electrode. Visible excitation of the assembly results in the appearance of reductive equivalents at the electrode and oxidative equivalents at a catalyst that persist for seconds in aqueous solutions. Steady-state illumination of the assembly with 440 nm light with an applied bias results in photoelectrochemical water oxidation with a per-photon absorbed efficiency of 2.3%. The results are notable in demonstrating that light-driven water oxidation can be carried out at a conductive electrode in a structure with the functional elements of Photosystem II including charge separation and water oxidation

Mechanistic investigation of a visible light mediated dehalogenation/cyclisation reaction using iron(iii), iridium(iii) and ruthenium(ii) photosensitizers

The mechanism of a visible light-driven dehalogenation/cyclization reaction was investigated using ruthenium(II), iridium(III) and iron(III) photosensitizers by means of steady-state photoluminescence, time-resolved infrared spectroscopy, and nanosecond/femtosecond transient absorption spectroscopy. The nature of the photosensitizer was found to influence the product distribution such that the dehalogenated, non-cyclized products were only detected for the iron photosensitizer. Strikingly, with the iron photosensitizer, large catalytic yields required a low dielectric solvent such as dichloromethane, consistent with a previous publication. This low dielectric solvent allowed ultrafast charge-separation to outcompete geminate charge recombination and improved cage escape efficiency. Further, the identification of reaction mechanisms unique to the iron, ruthenium, and iridium photosensitizer represents progress towards the long-sought goal of utilizing earth-abundant, first-row transition metals for emerging energy and environmental applications

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 E1/2 Ru(III/II) redox potential dictating the comparative ratios of the processes. Complexes such as RuP4OMe (E1/2 = 0.91 vs Ag/AgCl) displayed virtually only desorption, while complexes such as RuPbpz (E1/2 > 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

Interfacial Deposition of Ru(II) Bipyridine-Dicarboxylate Complexes by Ligand Substitution for Applications in Water Oxidation Catalysis

Water oxidation is a critical step in artificial photosynthesis and provides the protons and electrons used in reduction reactions to make solar fuels. Significant advances have been made in the area of molecular water oxidation catalysts with a notable breakthrough in the development of Ru­(II) complexes that use a planar “bda” ligand (bda is 2,2′-bipyridine-6,6′-dicarboxylate). These Ru­(II)­(bda) complexes show lower overpotentials for driving water oxidation making them ideal for light-driven applications with a suitable chromophore. Nevertheless, synthesis of heterogeneous Ru­(II)­(bda) complexes remains challenging. We discuss here a new “bottom-up” synthetic method for immobilizing these catalysts at the surface of a photoanode for use in a dye-sensitized photoelectrosynthesis cell (DSPEC). The procedure provides a basis for rapidly screening the role of ligand variations at the catalyst in order to understand the impact on device performance. The best results of a water-oxidation DSPEC photoanode based on this procedure reached 1.4 mA/cm² at pH 7 in 0.1 M [PO₄H₂]^-/[PO₄H]^2-solution with minimal loss in catalytic behavior over 30 min, and produced an incident photon to current efficiency (IPCE) of 24.8% at 440 nm