8 research outputs found
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 TiO<sub>2</sub> 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 TiO<sub>2</sub> 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>-polyÂ(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 TiO<sub>2</sub> deposited by atomic layer deposition (ALD) and
a surface-bound RuÂ(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 TiO<sub>2</sub>
To probe light-induced redox equivalent separation and accumulation, we prepared ruthenium polypyridyl molecular assembly [(dcb)<sub>2</sub>Ru(bpy-Mebim<sub>2</sub>py)Ru(bpy)(OH<sub>2</sub>)]<sup>4+</sup> (Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>II</sup>–OH<sub>2</sub>) with Ru<sub>a</sub> as light-harvesting chromophore and Ru<sub>b</sub> as water oxidation catalyst (dcb = 4,4′-dicarboxylic acid-2,2′-bipyridine; bpy-Mebim<sub>2</sub>py = 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 TiO<sub>2</sub> in nanoparticle films, it undergoes MLCT excitation, electron injection, and oxidation of the remote −Ru<sub>b</sub><sup>II</sup>–OH<sub>2</sub> site to give TiO<sub>2</sub>(e<sup>–</sup>)–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>III</sup>–OH<sub>2</sub><sup>3+</sup> as a redox-separated transient. The oxidized assembly, TiO<sub>2</sub>–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>III</sup>–OH<sub>2</sub><sup>3+</sup>, similarly undergoes excitation and electron injection to give TiO<sub>2</sub>(e<sup>–</sup>)–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>IV</sup>O<sup>2+</sup>, with Ru<sub>b</sub><sup>IV</sup>O<sup>2+</sup> a known water oxidation catalyst precursor. Injection efficiencies for both forms of the assembly are lower than those for [Ru(bpy)<sub>2</sub>(4,4′-(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>bpy)]<sup>2+</sup> bound to TiO<sub>2</sub> (TiO<sub>2</sub>–Ru<sup>2+</sup>), whereas the rates of back electron transfer, TiO<sub>2</sub>(e<sup>–</sup>) → Ru<sub>b</sub><sup>III</sup>–OH<sub>2</sub><sup>3+</sup> and TiO<sub>2</sub>(e<sup>–</sup>) → Ru<sub>b</sub><sup>IV</sup>O<sup>2+</sup>, are significantly decreased compared with TiO<sub>2</sub>(e<sup>–</sup>) → Ru<sup>3+</sup> 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-CH<sub>2</sub>NH<sub>2</sub>-4′-CH<sub>3</sub>-bpy)(bpy)<sub>2</sub>]<sup>2+</sup> (4-CH<sub>2</sub>NH<sub>2</sub>-4′-CH<sub>3</sub>-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/PC<sub>60</sub>BM/Al, exhibited enhanced photocurrent and power conversion efficiencies (∼5 fold) relative to devices with the configuration, ITO/PEDOT:PSS/PSRu/PC<sub>60</sub>BM/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 −CÂ(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>(OH) in the
metal complex [RuÂ(bpy)<sub>2</sub>(4,4′-(CÂ(OH)Â(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>bpy)]<sup>2+</sup> are converted into −COOH
and H<sub>3</sub>PO<sub>4</sub>. The reaction occurs by sensitized
formation of <sup>1</sup>O<sub>2</sub> by the lowest metal-to-ligand
charge transfer excited state(s) of [RuÂ(bpy)<sub>2</sub>(4,4′-(CÂ(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>(OH))<sub>2</sub>(bpy))]<sup>2+</sup>* followed by <sup>1</sup>O<sub>2</sub> oxidation of the bisphosphonate
substituent. A related reaction occurs for the bisphosphonate-based
drug, risedronic acid, in the presence of O<sub>2</sub>, light, and
a singlet oxygen sensitizer ([RuÂ(bpy)<sub>3</sub>]<sup>2+</sup> or
Rose Bengal)
A Sensitized Nb<sub>2</sub>O<sub>5</sub> Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell
Orthorhombic Nb<sub>2</sub>O<sub>5</sub> nanocrystalline
films
functionalized with [RuÂ(bpy)<sub>2</sub>(4,4′-(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>bpy)]<sup>2+</sup> were used as the photoanode
in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen
generation. A set of experiments to establish key propertiesî—¸conduction
band, trap state distribution, interfacial electron transfer dynamics,
and DSPEC efficiencyî—¸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
Nb<sub>2</sub>O<sub>5</sub> nanocrystalline film, the conduction band
potential is slightly positive (<0.1 eV), relative to that for
anatase TiO<sub>2</sub>. Anatase TiO<sub>2</sub> has a wide distribution
of trap states including deep trap and band-tail trap states. Orthorhombic
Nb<sub>2</sub>O<sub>5</sub> 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><sub>oc</sub>) for orthorhombic Nb<sub>2</sub>O<sub>5</sub>, compared to anatase TiO<sub>2</sub>. In an
operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA<sup>4–</sup>) added as a reductive scavenger, H<sub>2</sub> quantum
yield and photostability measurements show that Nb<sub>2</sub>O<sub>5</sub> is comparable, but not superior, to TiO<sub>2</sub>
Structure–Property Relationships in Phosphonate-Derivatized, Ru<sup>II</sup> 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 [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup> complexes
in aqueous solution and bound to ZrO<sub>2</sub> and TiO<sub>2</sub> 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 TiO<sub>2</sub> to the interface. With all results considered, the ruthenium
polypyridyl derivatives with one or two 4,4′-(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>bpy ligands provide the best balance of
electron injection efficiency and stability for application in solar
energy conversion devices