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Oum K.,University of Siegen | Lohse P.W.,University of Siegen | Lohse P.W.,Uppsala University | Flender O.,University of Siegen | And 6 more authors.
Physical Chemistry Chemical Physics | Year: 2012

The ultrafast photoinjection and subsequent relaxation steps of the indoline dye D149 were investigated in detail for a mesoporous electrodeposited ZnO thin film and compared with experiments on sintered TiO2 and ZrO2 thin films, all in contact with air, using pump-supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370-770 nm. D149 efficiently injects electrons into the ZnO surface with time constants from ≤70 fs (time-resolution-limited) up to 250 fs, without the presence of slower components. Subsequent spectral dynamics with a time constant of 20 ps and no accompanying change in the oscillator strength are assigned to a transient Stark shift of the electronic absorption spectrum of D149 molecules in the electronic ground state due to the local electric field exerted by the D149 •+ radical cations and conduction band electrons in ZnO. This interpretation is consistent with the shape of the relaxed PSCP spectrum at long times, which resembles the first derivative of the inverted steady-state absorption spectrum of D149. In addition, steady-state difference absorption spectra of D149•+ in solution from spectroelectrochemistry display a bleach band with distinctly different position, because no first-order Stark effect is present in that case. Interference features in the PSCP spectra probably arise from a change of the refractive index of ZnO caused by the injected electrons. The 20 ps component in the PSCP spectra is likely a manifestation of the transition from an initially formed bound D149 •+-electron complex to isolated D149•+ and mobile electrons in the ZnO conduction band (which changes the external electric field experienced by D149) and possibly also reorientational motion of D149 molecules in response to the electric field. We identify additional spectral dynamics on a similar timescale, arising from vibrational relaxation of D149•+ by interactions with ZnO. TiO2 exhibits similar dynamics to ZnO. In the case of ZrO2, electron injection accesses trap states, which exhibit a substantial probability for charge recombination. No Stark shift is observed in this case. In addition, the spectroelectrochemical experiments for D149•+ in dichloromethane and acetonitrile, which cover the spectral range up to 2000 nm, provide for the first time access to its complete D0 → D1 absorption band, with the peak located at 1250 and 1055 nm, respectively. Good agreement is obtained with results from DFT/TDDFT calculations of the D149 •+ spectrum employing the MPW1K functional. © 2012 the Owner Societies.

Ahmed A.Y.,Leibniz University of Hanover | Ahmed A.Y.,Sohag University | Kandiel T.A.,Leibniz University of Hanover | Kandiel T.A.,Sohag University | And 3 more authors.
Journal of Physical Chemistry Letters | Year: 2011

The photocatalytic activities of well-defined TiO2 single-crystal anatase (101) surfaces have been assessed by methanol oxidation and by terephthalic acid hydroxylation evincing the formation of OH • radicals and have been compared with that of rutile singlecrystal (001), (100), and (110) surfaces. The results showed that the anatase (101) surface exhibits a higher photocatalytic activity than all investigated rutile surfaces toward the oxidation of methanol and exhibits a comparable activity to that of the rutile (001) surface with respect of terephthalic acid hydroxylation. The rutile (001) surface shows a higher photocatalytic activity than both the rutile (110) and (100) surfaces for both photocatalytic test reactions. Because anatase (101) and rutile (110) are the thermodynamically most stable surfaces, anatase and rutile nanomaterials possess, thus, a large percentage of (101) and (110) surfaces, respectively. This offers a reasonable explanation why anatase nanoparticles usually exhibit higher photocatalytic activities than the respective rutile powders. (Chemical Equation Presented) © 2011 American Chemical Society.

Flender O.,University of Siegen | Lohse P.W.,University of Siegen | Du J.,Leibniz University of Hanover | Oekermann T.,Leibniz University of Hanover | And 4 more authors.
Zeitschrift fur Physikalische Chemie | Year: 2015

Ultrafast photoinduced electron injection and subsequent relaxation steps of the indoline dye D149 were investigated for mesoporous thin films using pump-supercontinuum probe (PSCP) transient absorption spectroscopy in the range 370-770 nm. Three types of mesoporous ZnO layers were compared: Electrodeposited ZnO thin films prepared by using the structure-directing agents (1) Eosin Y (EY) and (2) Coumarin 343 (C343), which are known to produce ZnO layers of different morphology; and (3) mesoporous ZnO thin films consisting of sintered nanoparticles with a diameter of 20 nm. As a reference system, we used a "non-injecting" mesoporous thin film with large band-gap, consisting of Al2O3 nanoparticles with a diameter of 50 nm. On all ZnO thin films, D149 efficiently injects electrons. We observed two components, a dominant one with τ ≤ 70 fs (time-resolution-limited) and a slower one with a time constant of 250-350 fs. Fast initial charge separation is also consistent with the immediate appearance of oscillatory structure due to a change in refractive index of ZnO upon electron injection. For all ZnO thin films, we observe a transient shift of the spectra with a time constant of ca. 20 ps which is assigned to a transient Stark effect (= electrochromism). The S0 → S1 absorption band of D149 is shifted due to the build-up of a local electric field between dye radical cations and ZnO conduction band electrons. On longer timescales, step-scan FTIR spectroscopy revealed a slightly faster cation-electron recombination on ZnO/C343 than on ZnO/EY thin films. On Al2O3, metastable close cation-electron pairs are formed initially which recombine afterwards, and no transient Stark effect is observed. Complementary PSCP experiments on D149 in the ionic liquid [C2mim]+ [N(CN)2]- show that D149 is ideally suited for solar cell applications because of its long S1 lifetime of 590 ps. In addition, biphasic solvation dynamics are observed in this IL with a subpicosecond component and a much slower component related to the viscosity of the IL which is well described by a stretched exponential. © 2015 Walter de Gruyter Berlin/Boston.

Graaf H.,TU Chemnitz | Graaf H.,University of Kassel | Luttich F.,TU Chemnitz | Dunkel C.,Leibniz University of Hanover | And 4 more authors.
Journal of Physical Chemistry C | Year: 2012

Codeposition of the dye coumarin 343 (C343) in its monomeric form strongly influences the crystallographic orientation and electronic properties of electrodeposited ZnO films. This is opposed to the codeposition of Eosin Y (EY), which forms aggregates in ZnO/dye films, leading to much less influence on crystallographic orientation and electronic properties. Highly porous ZnO films are formed upon dye extraction in both cases, which is due to the action of the dye aggregates as pore templates in the case of EY and due to a blocking effect on ZnO electrodeposition, as opposed to the catalytic effect of EY, in the case of C343. © 2011 American Chemical Society.

Sallard S.,Paul Scherrer Institute | Sallard S.,Justus Liebig University | Schroder M.,Paul Scherrer Institute | Boissiere C.,Collège de France | And 10 more authors.
Nanoscale | Year: 2013

In the present paper, we report the synthesis of bimodal mesoporous anatase TiO2 films by the EISA (Evaporation-Induced Self-Assembly) method using sol-gel chemistry combining two porogen agents, a low molecular weight ionic template and a neutral block copolymer. The surfactant template (C 16mimCl) generates non-oriented worm-like pores (8 to 10 nm) which connect the regularly packed ellipsoidal mesopores (15 to 20 nm diameter) formed by an amphiphilic block copolymer of the type poly(isobutylene)-b-poly(ethylene oxide) (PIB-PEO). The surfactant template can also significantly influence the size and packing of the ellipsoidal mesopores. The mesostructural organization and mesoporosity of the films are studied by Environmental Ellipsometry- Porosimetry (EEP), Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) and electron microscopy techniques. Electrochemical characterization is performed to study the permeability of the films to liquid solutions, using two types of probe moieties (K3FeIII(CN)6 and Ru(bpy) 32+) by the wall-jet technique. An optimum ratio of C 16mimCl/PIB-PEO provides anatase films with a continuous bimodal mesopore structure, possessing a permeability up to two times higher than that of the mesoporous films templated by PIB-PEO only (with partially isolated mesopores). When C16mimCl is used in large quantities, up to 20% weight vs. PIB-PEO, large overall porous volume and surface area are obtained, but the mesostructure is increasingly disrupted, leading to a severe loss of permeability of the bimodal films. A dye-sensitized solar cell set-up is used with anatase films as the photoelectrode. The photosensitizer loading and the total energy conversion efficiency of the solar cells using the mesoporous films templated by an optimal ratio of the two porogen agents C16mimCl and PIB-PEO can be substantially increased in comparison with the solar cells using mesoporous films templated by PIB-PEO only. © 2013 The Royal Society of Chemistry.

Dunkel C.,Leibniz University of Hanover | Dunkel C.,Carl von Ossietzky University | von Graberg T.,Wiley VCH Verlag GmbH and Co. KGaA | von Graberg T.,Justus Liebig University | And 5 more authors.
Materials | Year: 2014

Well-ordered 3D mesoporous indium tin oxide (ITO) films obtained by a templated sol-gel route are discussed as conductive porous current collectors. This paper explores the use of such films modified by electrochemical deposition of zinc oxide (ZnO) on the pore walls to improve the electron transport in dye-sensitized solar cells (DSSCs). Mesoporous ITO film were dip-coated with pore sizes of 20-25 nm and 40-45 nm employing novel poly(isobutylene)-b-poly(ethylene oxide) block copolymers as structure-directors. After electrochemical deposition of ZnO and sensitization with the indoline dye D149 the films were tested as photoanodes in DSSCs. Short ZnO deposition times led to strong back reaction of photogenerated electrons from non-covered ITO to the electrolyte. ITO films with larger pores enabled longer ZnO deposition times before pore blocking occurred, resulting in higher efficiencies, which could be further increased by using thicker ITO films consisting of five layers, but were still lower compared to nanoporous ZnO films electrodeposited on flat ITO. The major factors that currently limit the application are the still low thickness of the mesoporous ITO films, too small pore sizes and non-ideal geometries that do not allow obtaining full coverage of the ITO surface with ZnO before pore blocking occurs. © 2014 by the authors.

Selk Y.,Leibniz University of Hanover | Minnermann M.,Leibniz University of Hanover | Oekermann T.,Leibniz University of Hanover | Oekermann T.,Friemann and Wolf Batterietechnik GmbH | And 2 more authors.
Journal of Applied Electrochemistry | Year: 2011

Solid-state dye-sensitized solar cells based on highly porous ZnO films prepared by template-assisted electrodeposition as electron collector, the indoline dye D149 as sensitizer and CuSCN as hole collector have been prepared using three different methods, namely impregnation with saturated CuSCN solution, successive ionic layer adsorption and reaction (SILAR) and electrodeposition, for filling the pores in the ZnO with CuSCN. The highest pore filling and the highest conversion efficiency of 0.46% were achieved with the impregnation method, while SILAR led to a very low pore filling, causing very low photocurrents, and electrodeposition led to short-circuiting between the CuSCN and the conducting substrate of the ZnO sample despite the presence of a compact ZnO bottom layer between the porous ZnO layer and the conducting layer, causing very low open-circuit voltages. © 2011 Springer Science+Business Media B.V.

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