Ultrafast electron transfer dynamics in sensitised TiO₂ nanoparticles

We have studied electron transfer dynamics between TiO2 nanoparticles and molecular adsorbates using femtosecond mid-infrared spectroscopy. We have demonstrated that dynamics of the injected electrons in TiO2 could be directly monitored through their mid-IR absorption and those of the adsorbates could be measured by their vibrational spectral change. Ru(dcbpy)2(NCS)2 (dcbpy=2,2'-bypyridine-4,4'-dicarboxylate) sensitized TiO2 nanocrystalline films were studied as a model system for ultrafast electron injection from the excited state of the sensitizer to nanoparticles. Optical excitation of the MLCT band at 400 nm promotes an electron from a filled Ru d orbital to the π* orbital of the dcbpy ligand. The subsequent electron injection to TiO2 was found to occur with a time constant of ca 50 fs by directly measuring the transient IR absorption signal of the injected electrons in TiO2. These injection dynamics are as fast as, if not fast than, the electronic or vibrational relaxation within the excited states. Back electron transfer from nanoparticles to the adsorbates was studied in interfacial charge transfer complexes formed by Fe(II)(CN)64- and TiO2 colloidal nanoparticles. Optical excitation at 400 nm directly promotes an electron from Fe(Il)(CN)64- to TiO2 as indicated by the measured instrument-response—function limited appearance time of transient IR signal. The back electron transfer time from TiO2 to Fe(III)(CN)63- was measured by the bleach recovery of CN stretching mode. A highly non-single-exponential recombination process was observed and was tentatively attributed to different recombination rates for injected electrons trapped at different sites in TiO2. The measured decay of the IR absorption of electrons can be attributed to back electron transfer and electron trapping. Since the back electron transfer kinetics can be measured independently, the trapping dynamics can be determined. Electron trapping dynamics in a bulk crystal and nanocrystalline thin films were found to be similar in the first nanosecond, showing a »1 ns decay time. Trapping dynamics are much faster in the colloidal nanoparticles, indicating a much higher trap state density

Dielectric properties of ferroelectric thin films with surface transition layers

Abstract By taking into account surface transition layers (STL), the dielectric properties of ferroelectric thin films described by the transverse Ising model are discussed in the framework of the mean-field approximation. Functions of the intra-layer and interlayer couplings are introduced to characterize STL, which makes the model more realistic compared to the previous treatment of surface layers using uniform surface exchange interactions and a transverse field. The effects of physical parameters on the dielectric properties are quantified. The results obtained indicate that STL has a very strong influence on the dielectric properties of ferroelectric thin films. Some of our theoretical results are in accordance with the available experimental data

Sub-picosecond injection of electrons from excited [Ru(2,2′-bipy-4,4′-dicarboxy)₂(SCN)₂] into TiO₂ using transient mid-infrared spectroscopy

We have used femtosecond pump-probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4’-dicarboxy-2,2’-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge transfer adsorption peaks and probing the absorption of electrons injected into the TiO2 conduction band at 1.52 µm and in the range of 4.1 to 7.0 µm, we have directly observed the arrival of the injected electrons. Our measurements indicate an instrument-limited ~50-fs upper limit on the electron injection time under ambient conditions in air. We have compared the infrared transient absorption for noninjecting (blank) systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2, and found no indication of electron injection at probe wavelengths in the mid-IR (4.1 to 7.0 µm). At 1.52 µm interferences exist in the observed transient adsorption signal for the blanks

Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer

A series of heterobimetallic transition metal substituted polyoxometalates (TMSPs) have been synthesized based on the CoII-centered ligand [CoIIW11O39]10-. The eight complex series, [CoII(MxOHy)W11O39](12-x-y)- (MxOHy = VIVO, CrIII(OH2), MnII(OH2), FeIII(OH2), CoII(OH2), NiII(OH2), CuII(OH2), ZnII(OH2)), of which six are reported for the first time, was synthesized starting from [CoIIIW11O39]9- and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [CoIIW12O40]6- and demonstrate the same MPCT transition as the plenary complex, but have shorter excited state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited state lifetimes at least 50% of that of [CoIIW12O40]6- while more than tripling the absorbance at 400 nm

Robust Binding of Disulfide-Substituted Rhenium Bipyridyl Complexes for CO2 Reduction on Gold Electrodes

Heterogenization of homogenous catalysts on electrode surfaces provides a valuable approach for characterization of catalytic processes in operando conditions using surface selective spectroelectrochemistry methods. Ligand design plays a central role in the attachment mode and the resulting functionality of the heterogenized catalyst as determined by the orientation of the catalyst relative to the surface and the nature of specific interactions that modulate the redox properties under the heterogeneous electrode conditions. Here, we introduce new [Re(L)(CO)3Cl] catalysts for CO2 reduction with sulfur-based anchoring groups on a bipyridyl ligand, where L = 3,3′-disulfide-2,2′-bipyridine (SSbpy) and 3,3′-thio-2,2′-bipyridine (Sbpy). Spectroscopic and electrochemical analysis complemented by computational modeling at the density functional theory level identify the complex [Re(SSbpy)(CO)3Cl] as a multi-electron acceptor that combines the redox properties of both the rhenium tricarbonyl core and the disulfide functional group on the bipyridyl ligand. The first reduction at −0.85 V (vs. SCE) involves a two-electron process that breaks the disulfide bond, activating it for surface attachment. The heterogenized complex exhibits robust anchoring on gold surfaces, as probed by vibrational sum-frequency generation (SFG) spectroscopy. The binding configuration is normal to the surface, exposing the active site to the CO2 substrate in solution. The attachment mode is thus particularly suitable for electrocatalytic CO2 reduction.Fil: Cattaneo, Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Química del Noroeste. Universidad Nacional de Tucumán. Facultad de Bioquímica, Química y Farmacia. Instituto de Química del Noroeste; ArgentinaFil: Guo, Facheng. University of Yale; Estados UnidosFil: Kelly, H. Ray. University of Yale; Estados UnidosFil: Videla, Pablo E.. University of Yale; Estados UnidosFil: Kiefer, Laura. Emory University; Estados UnidosFil: Gebre, Sara. Emory University; Estados UnidosFil: Ge, Aimin. Emory University; Estados UnidosFil: Liu, Qiliang. Emory University; Estados UnidosFil: Wu, Shaoxiong. Emory University; Estados UnidosFil: Lian, Tianquan. Emory University; Estados UnidosFil: Batista, Víctor S.. University of Yale; Estados Unido

Anthracene Diphosphate Ligands for CdSe Quantum Dots; Molecular Design for Efficient Upconversion

Quantum dot (QD)-sensitized photon upconversion follows a multi-step energy transfer process from the QD to transmitter ligand to a soluble annihilator. Using a novel 10-R-anthracene-1,8-diphosphoric acid (R = octyl, 2-hexyldecyl, phenyl) ligand with high binding affinity for CdSe QD surfaces, we demonstrate a photon upconversion process that is limited by the transmitter to annihilator transfer efficiency. Using 1H NMR spectroscopy, we demonstrate that these bidentate diphosphate ligands rapidly and irreversibly displace two carboxylate ligands. These ligands mediate energy transfer from the photoexcited QDs to a triplet annihilator (1,10-diphenylanthracene), producing overall photon upconversion quantum efficiencies as high as 17%, the highest for QDs with no shells. Transient absorption spectroscopy shows that the anthracene dihydrogen phosphate (ADP) ligand supports a 3.4 fold longer triplet state lifetime compared to 9-anthracene carboxylic acid (299.9 ± 9.5 vs 88.2 ± 2.1 μs), increasing the probability of energy transfer

In situ probe of photocarrier dynamics in water-splitting hematite (α-Fe_(2)O_3) electrodes

The spectra and dynamics of photogenerated electrons and holes in excited hematite (α-Fe_(2)O_3) electrodes are investigated by transient absorption (from visible to infrared and from femto- to micro-seconds), bias-dependent differential absorption and Stark spectroscopy. Comparison of results from these techniques enables the assignment of the spectral signatures of photogenerated electrons and holes. Under the pulse illumination conditions of transient absorption (TA) measurement, the absorbed photon to electron conversion efficiency (APCE) of the films at 1.43 V (vs. reversible hydrogen electrode, RHE) is 0.69%, significantly lower than that at AM 1.5. TA kinetics shows that under these conditions, >98% of the photogenerated electrons and holes have recombined by 6 μs. Although APCE increases with more positive bias (from 0.90 to 1.43 V vs. RHE), the kinetics of holes up to 6 μs show negligible change, suggesting that the catalytic activity of the films is determined by holes with longer lifetimes