Comparison of the photoelectrochemical oxidation of methanol on rutile TiO 2 (001) and (100) single crystal faces studied by intensity modulated photocurrent spectroscopy

The photooxidation of methanol as a model substance for pollutants on rutile TiO 2 (001) and (100) surfaces was investigated using intensity modulated photocurrent spectroscopy (IMPS). The results are analyzed in view of the influence of the surface structure, the methanol concentration and the electrode potential on the rate constants of charge transfer and recombination. The obtained results have been explained with a model combining the theory of IMPS for a bulk semiconductor surface and the nature of the surface-bound intermediates (alternatively mobile or immobile OH radicals). The results indicate that water photooxidation proceeds via mobile OH radicals on both surfaces, while methanol addition gives rise to the involvement of immobile OH radicals on the (100) surface. Detailed analysis in view of the surface structures suggests that the latter observation is due to efficient electron transfer from bridging OH radicals on the (100) surface to methanol, while coupling of two of these radicals occurs in the absence of methanol, making them appear as mobile OH radicals. In the case of the (001) surface, the coupling reaction dominates even in the presence of methanol due to the smaller distance between the bridging OH radicals, leading to more efficient water oxidation, but less efficient methanol photooxidation on this surface. © the Owner Societies 2012

Limits of ZnO electrodeposition in mesoporous tin doped indium oxide films in view of application in dye-sensitized solar cells

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.DFG/OE 420/5-1DFG/SM 199/6-1DFG/WA 1116/1

Ultrafast dynamics of the indoline dye D149 on electrodeposited ZnO and sintered ZrO2 and TiO2 thin films

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

Ultrafast photoinduced relaxation dynamics of the indoline dye D149 in organic solvents

The relaxation dynamics of the indoline dye D149, a well-known sensitizer for photoelectrochemical solar cells, have been extensively characterized in various organic solvents by combining results from ultrafast pump–supercontinuum probe (PSCP) spectroscopy, transient UV-pump VIS-probe spectroscopy, time-correlated single-photon counting (TCSPC) measurements as well as steady-state absorption and fluorescence. In the steady-state spectra, the position of the absorption maximum shows only a weak solvent dependence, whereas the fluorescence Stokes shift Δ[small nu, Greek, tilde]F correlates with solvent polarity. Photoexcitation at around 480 nm provides access to the S1 state of D149 which exhibits solvation dynamics on characteristic timescales, as monitored by a red-shift of the stimulated emission and spectral development of the excited-state absorption in the transient PSCP spectra. In all cases, the spectral dynamics can be modeled by a global kinetic analysis using a time-dependent S1spectrum. The lifetime τ1 of the S1 state roughly correlates with polarity [acetonitrile (280 ps) < acetone (540 ps) < THF (720 ps) < chloroform (800 ps)], yet in alcohols it is much shorter [methanol (99 ps) < ethanol (178 ps) < acetonitrile (280 ps)], suggesting an appreciable influence of hydrogen bonding on the dynamics. A minor component with a characteristic time constant in the range 19–30 ps, readily observed in the PSCP spectra of D149 in acetonitrile and THF, is likely due to removal of vibrational excess energy from the S1 state by collisions with solvent molecules. Additional weak fluorescence in the range 390–500 nm is observed upon excitation in the S0 → S2 band, which contains short-lived S2 → S0 emission of D149. Transient absorption signals after excitation at 377.5 nm yield an additional time constant in the subpicosecond range, representing the lifetime of the S2 state. S2 excitation also produces photoproducts

Bimodal mesoporous titanium dioxide anatase films templated by a block polymer and an ionic liquid: influence of the porosity on the permeability

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(16)mimCl) 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)(3)(2+)) by the wall-jet technique. An optimum ratio of C(16)mimCl/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 C(16)mimCl 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 C(16)mimCl and PIB-PEO can be substantially increased in comparison with the solar cells using mesoporous films templated by PIB-PEO only.DFG/SM 199/6-1DFG/OE 420/5-1BMBF/SOHyb/03X3525

Scale-Up of the Electrodeposition of ZnO/Eosin Y Hybrid Thin Films for the Fabrication of Flexible Dye-Sensitized Solar Cell Modules

The low-temperature fabrication of flexible ZnO photo-anodes for dye-sensitized solar cells (DSSCs) by templated electrochemical deposition of films was performed in an enlarged and technical simplified deposition setup to demonstrate the feasibility of the scale-up of the deposition process. After extraction of eosin Y (EY) from the initially deposited ZnO/EY hybrid films, mesoporous ZnO films with an area of about 40 cm2 were reproducibly obtained on fluorine doped tin oxide (FTO)-glass as well as flexible indium tin oxide (ITO)–polyethylenterephthalate (PET) substrates. With a film thickness of up to 9 µm and a high specific surface area of up to about 77 m2·cm−3 the ZnO films on the flexible substrates show suitable properties for DSSCs. Operative flexible DSSC modules proved the suitability of the ZnO films for use as DSSC photo-anodes. Under a low light intensity of about 0.007 sun these modules achieved decent performance parameters with conversion efficiencies of up to 2.58%. With rising light intensity the performance parameters deteriorated, leading to conversion efficiencies below 1% at light intensities above 0.5 sun. The poor performance of the modules under high light intensities can be attributed to their high series resistances