Luminescence decays and spectra of tris(bipyridine)ruthenium(2+) adsorbed on titania in vacuo and in the presence of water vapor

Excited-State Dynamics of [Ru(bpy)3 ]2+ Thin Films on Sensitized TiO2 and ZrO2

The excited state dynamics of Tris(2,2'-bipyridine)ruthenium(II) hexafluorophosphate, [Ru(bpy)3 (PF6 )2 ], was investigated on the surface of bare and sensitized TiO2 and ZrO2 films. The organic dyes LEG4 and MKA253 were selected as sensitizers. A Stern-Volmer plot of LEG4-sensitized TiO2 substrates with a spin-coated [Ru(bpy)3 (PF6 )2 ] layer on top shows considerable quenching of the emission of the latter. Interestingly, time-resolved emission spectroscopy reveals the presence of a fast-decay time component (25±5 ns), which is absent when the anatase TiO2 semiconductor is replaced by ZrO2 . It should be specified that the positive redox potential of the ruthenium complex prevents electron transfer from the [Ru(bpy)3 (PF6 )2 ] ground state into the oxidized sensitizer. Therefore, we speculate that the fast-decay time component observed stems from excited-state electron transfer from [Ru(bpy)3 (PF6 )2 ] to the oxidized sensitizer. Solid-state dye sensitized solar cells (ssDSSCs) employing MKA253 and LEG4 dyes, with [Ru(bpy)3 (PF6 )2 ] as a hole-transporting material (HTM), exhibit 1.2 % and 1.1 % power conversion efficiency, respectively. This result illustrates the possibility of the hypothesized excited-state electron transfer.

Photocatalysts for Solar-Induced Water Disinfection: New Developments and Opportunities

Recent years have seen a surge of interest in the application of solar energy for water disinfection by using nanostructured photocatalysts elaborately designed and fabricated. Photocatalysis has its unique advantage for utilizing sunlight to drive the disinfection process. The highly reactive oxygen species (ROS) serve as the main oxidants and are capable of inactivating microorganisms, including viruses, bacteria, spores and protozoa. This chapter presents an overview of current research activities that center on the preparation, characterization and application of highly efficient photocatalysts for water disinfection under both UV and visible light irradiation. It is organized into two major parts. One is the development of TiO2-based photocatalysts including surface noble metal modified, ion doped, dye-sensitized, and composite TiO2. The other part is the introduction of new types of photocatalysts and advanced technologies that have recently fascinated the scientific community. Particular attention is given to the pioneering fields such as graphene-based photocatalysts, plasmonic-metal nanostructures and naturally occurring photocatalysts. Finally, we conclude with a discussion of what major advancements are needed to move the field of photocatalytic water disinfection forward.

Toward Exceeding the Shockley−Queisser Limit: Photoinduced Interfacial Charge Transfer Processes that Store Energy in Excess of the Equilibrated Excited State

Nanocrystalline (anatase), mesoporous TiO2 thin films were functionalized with [Ru(bpy)2(deebq)](PF6)2, [Ru(bq)2(deeb)](PF6)2, [Ru(deebq)2(bpy)](PF6)2, [Ru(bpy)(deebq)(NCS)2], or [Os(bpy)2(deebq)](PF6)2, where bpy is 2,2'-bipyridine, bq is 2,2'-biquinoline, and deeb and deebq are 4,4'-diethylester derivatives. These compounds bind to the nanocrystalline TiO2 films in their carboxylate forms with limiting surface coverages of 8 (+/- 2) x 10(-8) mol/cm2. Electrochemical measurements show that the first reduction of these compounds (-0.70 V vs SCE) occurs prior to TiO2 reduction. Steady state illumination in the presence of the sacrificial electron donor triethylamine leads to the appearance of the reduced sensitizer. The thermally equilibrated metal-to-ligand charge-transfer excited state and the reduced form of these compounds do not inject electrons into TiO2. Nanosecond transient absorption measurements demonstrate the formation of an extremely long-lived charge separated state based on equal concentrations of the reduced and oxidized compounds. The results are consistent with a mechanism of ultrafast excited-state injection into TiO2 followed by interfacial electron transfer to a ground-state compound. The quantum yield for this process was found to increase with excitation energy, a behavior attributed to stronger overlap between the excited sensitizer and the semiconductor acceptor states. For example, the quantum yields for [Os(bpy)2(dcbq)]/TiO2 were phi(417 nm) = 0.18 +/- 0.02, phi(532.5 nm) = 0.08 +/- 0.02, and phi(683 nm) = 0.05 +/- 0.01. Electron transfer to yield ground-state products occurs by lateral intermolecular charge transfer. The driving force for charge recombination was in excess of that stored in the photoluminescent excited state. Chronoabsorption measurements indicate that ligand-based intermolecular electron transfer was an order of magnitude faster than metal-centered intermolecular hole transfer. Charge recombination was quantified with the Kohlrausch-Williams-Watts model.

Luminescent Properties of Tris(2,2′-bipyridine)ruthenium(II) in Sol-Gel-Processed Dip-Coated Thin Films

The luminescence decay and spectral behavior of ruthenium(II)-tris-1,2-bipyridine dichloride dissolved in different organically modified silicate gel matrixes were investigated. Dip-coated thin films were synthesized from tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS), ethyltriethoxysilane (ETEOS), and methyl- trimethoxysilane (MTMOS). A blue shift in the ruthenium complex emission spectrum with respect to the aqueous solution was observed for all the films on the sol to gel conversion. This spectral shift was slightly dependent on the precursor used to obtain the films and independent of the reaction pH to prepare the "sol". In the data treatment of the time-resolved luminescence measurements, it was assumed that the distribution of the luminophore in the films was nonhomogeneous. The analysis of the luminescence decay profiles was based on a multisite model. All decay curves are best described by a double-exponential model. The parameters of the decay components depended principally on the thermal treatment used in the processing of the films. The lifetimes decreased and the emission espectra showed a red shift with the increase in the drying temperature. A luminescence quenching of the ruthenium complex in the films by dissolved oxygen in aqueous solution was also observed. The quenching rate constant obtained from the preexponential amplitude-weighted mean lifetimes (tau(M)) was in the order of 10(9) M(-1) s(-1). When a phenolic derivative was used as quencher the process rate was greatly reduced compared to the quenching in water. It would seem that the metallic complex sequestered within the film is placed either into a higher microviscosity microenvironment or in a location which the phenolic quencher cannot access. In both cases, the quenching plot based on tau(o)(M)/tau(M) could be fitted satisfactorily by a sum of two terms of Stern-Volmer. This fact is indicative of the matrix microheterogeneity for the films and is fully consistent with the biexponential nature of the luminescence intensity decay profiles.