Power-Law Kinetics in the Photoluminescence of Dye-Sensitized Nanoparticle Films: Implications for Electron Injection and Charge Transport

Dye-sensitized solar cells have provided a model to inexpensively harness solar energy, but the underlying physics that limit their efficiency are still not well understood. We probe electron injection in sensitized nanocrystalline TiO₂ films using time-correlated single photon counting (TCSPC) to measure time-dependent chromophore photoluminescence quenching. The time-dependent emission exhibits kinetics that become faster and more dispersive with increasing ionic concentrations in both water and acetonitrile; we quantify these trends by fitting the data using several kinetic models. Even more notably, we show that the residual emission under conditions that favor efficient electron injection exhibits a power-law decay in time. We attribute this highly dispersive kinetic behavior to electron injection from the dye into localized acceptor states of the TiO₂ nanoparticle film, which exhibits a distribution of injection rate constants that depend on the energetic distribution of sub-band-gap trap states

High Surface Area Antimony-Doped Tin Oxide Electrodes Templated by Graft Copolymerization. Applications in Electrochemical and Photoelectrochemical Catalysis

Mesoporous ATO nanocrystalline electrodes of micrometer thicknesses have been prepared from ATO nanocrystals and the grafted copolymer templating agents poly vinyl chloride-<i>g</i>-poly­(oxyethylene methacrylate). As-obtained electrodes have high interfacial surface areas, large pore volumes, and rapid intraoxide electron transfer. The resulting high surface area materials are useful substrates for electrochemically catalyzed water oxidation. With thin added shells of TiO₂ deposited by atomic layer deposition (ALD) and a surface-bound Ru­(II) polypyridyl chromophore, they become photoanodes for hydrogen generation in the presence of a reductive scavenger

Photoinduced Stepwise Oxidative Activation of a Chromophore–Catalyst Assembly on TiO₂

To probe light-induced redox equivalent separation and accumulation, we prepared ruthenium polypyridyl molecular assembly [(dcb)₂Ru(bpy-Mebim₂py)Ru(bpy)(OH₂)]⁴⁺ (Ru_a^II–Ru_b^II–OH₂) with Ru_a as light-harvesting chromophore and Ru_b as water oxidation catalyst (dcb = 4,4′-dicarboxylic acid-2,2′-bipyridine; bpy-Mebim₂py = 2,2′-(4-methyl-[2,2′:4′,4″-terpyridine]-2″,6″-diyl)bis(1-methyl-1H-benzo[<i>d</i>]imidazole); bpy = 2,2′-bipyridine). When bound to TiO₂ in nanoparticle films, it undergoes MLCT excitation, electron injection, and oxidation of the remote −Ru_b^II–OH₂ site to give TiO₂(e^–)–Ru_a^II–Ru_b^III–OH₂³⁺ as a redox-separated transient. The oxidized assembly, TiO₂–Ru_a^II–Ru_b^III–OH₂³⁺, similarly undergoes excitation and electron injection to give TiO₂(e^–)–Ru_a^II–Ru_b^IVO²⁺, with Ru_b^IVO²⁺ a known water oxidation catalyst precursor. Injection efficiencies for both forms of the assembly are lower than those for [Ru(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺ bound to TiO₂ (TiO₂–Ru²⁺), whereas the rates of back electron transfer, TiO₂(e^–) → Ru_b^III–OH₂³⁺ and TiO₂(e^–) → Ru_b^IVO²⁺, are significantly decreased compared with TiO₂(e^–) → Ru³⁺ back electron transfer

Soluble Reduced Graphene Oxide Sheets Grafted with Polypyridylruthenium-Derivatized Polystyrene Brushes as Light Harvesting Antenna for Photovoltaic Applications

Soluble graphene nanosheets, prepared by grafting polystyrene-based polymer chains from the surface of reduced graphene oxide (RGO), have been functionalized with pendant Ru(II) polypyridine chromophores. <i>N</i>-Hydroxysuccinimide (NHS) derivatized <i>p</i>-vinylbenzoic acid polymer chains were grown from methyl bromoisobutyrate initiation sites on the surface of RGO by atom transfer radical polymerization (ATRP). Deprotection of the resulting NHS polystyrene chains followed by amide coupling with the amine-derivatized Ru(II) polypyridyl complex [Ru(4-CH₂NH₂-4′-CH₃-bpy)(bpy)₂]²⁺ (4-CH₂NH₂-4′-CH₃-bpy = 4-aminomethyl-4′-methyl 2,2′-bipyridine and bpy = 2,2′-bipyridine) afforded the covalently linked RGO-metallopolymer. The hybrid graphene-polymer assembly has been fully characterized with clear evidence for covalent attachment of the metallopolymer brushes to the graphene substrate. On the basis of thermal gravimetric analysis, one polymer strand is grafted to the surface of RGO for every hundred graphene carbons. The covalently linked polymer brushes feature controlled chain lengths of ∼30 repeat units with a small polydispersity index (PDI, ∼ 1.2). Photovoltaic cells based on the derivatized polymers and graphene-polymer assemblies were evaluated. The graphene-polymer assembly in the configuration, ITO/PEDOT:PSS/RGO-PSRu/PC₆₀BM/Al, exhibited enhanced photocurrent and power conversion efficiencies (∼5 fold) relative to devices with the configuration, ITO/PEDOT:PSS/PSRu/PC₆₀BM/Al

Sensitized Photodecomposition of Organic Bisphosphonates By Singlet Oxygen

During efforts to stabilize metal oxide bound chromophores for photoelectrochemical applications, a novel photochemical reaction has been discovered. In the reaction, the bisphosphonate functional groups −C­(PO₃H₂)₂(OH) in the metal complex [Ru­(bpy)₂(4,4′-(C­(OH)­(PO₃H₂)₂bpy)]²⁺ are converted into −COOH and H₃PO₄. The reaction occurs by sensitized formation of ¹O₂ by the lowest metal-to-ligand charge transfer excited state(s) of [Ru­(bpy)₂(4,4′-(C­(PO₃H₂)₂(OH))₂(bpy))]²⁺* followed by ¹O₂ oxidation of the bisphosphonate substituent. A related reaction occurs for the bisphosphonate-based drug, risedronic acid, in the presence of O₂, light, and a singlet oxygen sensitizer ([Ru­(bpy)₃]²⁺ or Rose Bengal)

A Sensitized Nb₂O₅ Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell

Orthorhombic Nb₂O₅ nanocrystalline films functionalized with [Ru­(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺ were used as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. A set of experiments to establish key propertiesconduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiencywere undertaken to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors. We have found that, for a T-phase orthorhombic Nb₂O₅ nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO₂. Anatase TiO₂ has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb₂O₅ is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage (<i>V</i>_oc) for orthorhombic Nb₂O₅, compared to anatase TiO₂. In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA^4–) added as a reductive scavenger, H₂ quantum yield and photostability measurements show that Nb₂O₅ is comparable, but not superior, to TiO₂

Structure–Property Relationships in Phosphonate-Derivatized, Ru^II Polypyridyl Dyes on Metal Oxide Surfaces in an Aqueous Environment

The performance of dye-sensitized solar and photoelectrochemical cells is strongly dependent on the light absorption and electron transfer events at the semiconductor–small molecule interface. These processes as well as photo/electrochemical stability are dictated not only by the properties of the chromophore and metal oxide but also by the structure of the dye molecule, the number of surface binding groups, and their mode of binding to the surface. In this article, we report the photophysical and electrochemical properties of a series of six phosphonate-derivatized [Ru­(bpy)₃]²⁺ complexes in aqueous solution and bound to ZrO₂ and TiO₂ surfaces. A decrease in injection yield and cross surface electron-transfer rate with increased number of diphosphonated ligands was observed. Additional phosphonate groups for surface binding did impart increased electrochemical and photostability. All complexes exhibit similar back-electron-transfer kinetics, suggesting an electron-transfer process rate-limited by electron transport through the interior of TiO₂ to the interface. With all results considered, the ruthenium polypyridyl derivatives with one or two 4,4′-(PO₃H₂)₂bpy ligands provide the best balance of electron injection efficiency and stability for application in solar energy conversion devices

Additional file 1 of Model containing sarcopenia and visceral adiposity can better predict the prognosis of hepatocellular carcinoma: a multicenter study

Additional file 1