Marketa Zukalova

Raman spectra of titanium dioxide (anatase, rutile) with identified oxygen isotopes (16, 17, 18)

Six representative isotope-labeled samples of titanium dioxide were synthesized: Ti(16)O(2), Ti(17)O(2) and Ti(18)O(2), each in anatase and rutile forms. Their Raman scattering was analyzed at temperatures down to 5 K. Spectral assignment was supported by numerical simulation using DFT calculations. The combination of experimental and theoretical Raman frequencies with the corresponding isotopic shifts allowed us to address various still-open questions about the second-order Raman scattering in rutile, and the analysis of overlapping features in the anatase spectrum.

Voltage enhancement in dye-sensitized solar cell using (001)-oriented anatase TiO2 nanosheets

A nanocrystalline TiO2 (anatase) nanosheet exposing mainly the (001) crystal faces was tested as photoanode material in dye-sensitized solar cells. The nanosheets were prepared by hydrothermal growth in HF medium. Good-quality thin films were deposited on F-doped SnO2 support from the TiO2 suspension in ethanolic or aqueous media. The anatase (001) face adsorbs a smaller amount of the used dye sensitizer (C101) per unit area than the (101) face which was tested as a reference. The corresponding solar cell with sensitized (001)-nanosheet photoanode exhibits a larger open-circuit voltage than the reference cell with (101)-terminated anatase nanocrystals. The voltage enhancement is attributed to the negative shift of flatband potential for the (001) face. This conclusion rationalizes earlier works on similar systems, and it indicates that careful control of experimental conditions is needed to extract the effect of band energetic on the current/voltage characteristics of dye-sensitized solar cell.

Lithium Insertion into Anatase Inverse Opal

Reference LPI-ARTICLE-2004-018doi:10.1149/1.1769273View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Polycrystalline TiO2 Anatase with a Large Proportion of Crystal Facets (001): Lithium Insertion Electrochemistry

The electrochemical behavior of TiO anatase with a predominant (001) face (ANA001) was studied by cyclic voltammetry of Li insertion and chronoamperometry. Both voltammetric and chronoamperometric diffusion coefficients and rate constants proved the higher activity of ANA001 toward Li insertion compared to that of a reference anatase material (C240) with dominating (101) facets. The enhanced activity of the anatase (001) face for Li insertion stems from synergic contributions of a faster interfacial charge transfer at this surface and a facile Li transport within a more open structure of the anatase lattice in the direction parallel to the c-axis. Despite the larger particle size of ANA001, the values of integral charge capacity and Li-insertion coefficient further confirmed its improved Li-insertion properties. The results of this study further complete the analogous data published on single-crystal anatase electrodes and evidence their validity for nanocrystalline materials too.

Two Positions of Potassium in Chemically Doped C(60) Peapods: An in situ Spectroelectrochemical Study.

The state of doping of fullerene peapods C60@SWCNT treated with K vapor was studied by in situ Raman spectroelectrochemistry. For all samples under study, a heavy chemical n doping was proved by the vanishing of the radial breathing mode and the downshift of tangential displacement mode. The K-treated peapods remain partly doped even if they are exposed to humid air. The Ag(2) mode of intratubular fullerene in K-doped peapods in contact with air was still redshifted as referred to its position in pristine peapods. Potassium inserted into the peapods is the reason for the air-insensitive residual doping, which can be removed only by electrochemical oxidation. This indicates the presence of two positions of potassium in doped sample.

Li Insertion into Li[sub 4]Ti[sub 5]O[sub 12] (Spinel)

Li 4 Ti 5 O 1 2 (spinel) materials were prepared with Brunauer-Emmett-Teller surface areas ranging from 1.3 to 196 m 2 /g. The corresponding average particle sizes varied from ca. 1 μm to ca. 9 nm. Twenty-five different materials were tested as Li insertion hosts in thin-film electrodes (2-4 μm) made from a pure spinel. Trace amounts of anatase in Li 4 Ti 5 O 1 2 were conveniently determined by cyclic voltammetry of Li insertion. Electrodes from nanocrystalline Li 4 Ti 5 O 1 2 exhibited excellent activity towards Li insertions even at charging rates as high as 250C. The charge capability at 50-250C was proportional to the logarithm of surface area for coarse particles (surface areas smaller than ca. 20 m 2 /g). With increasing charge/discharge rates, a narrowing plateau in performance was observed for materials with surface areas between ca. 20 to 100 m 2 /g. These materials can be charged/discharged nearly to the nominal capacity of L1 4 Ti 5 O 1 2 (175 mAh/g) within a wide range of the rates. Very small particles (surface areas > 100 m 2 /g) exhibit a growing decrease of charge capability at 50-250C. The Li-diffusion coefficients, calculated from chronoamperometry, decrease by orders of magnitude if the average particle size drops from ca. I μm to ca. 9 nm. However, the sluggish Li + transport in small particles is compensated by the increase in active electrode area. Materials having surface areas larger than ca. 100 m 2 /g also tend to show increased charge irreversibility. This could be caused by parasitic cathodic reactions, due to enhanced adsorption of reducible impurities (humidity) or the quality of the spinel crystalline lattice itself. The optimum performance of thin-film Li 4 Ti 5 O 1 2 electrodes is achieved, if the parent materials have surface areas between ca. 20 to 110 m 2 /g, with the maximum peak at 100 m 2 /g.Reference LPI-ARTICLE-2003-015doi:10.1149/1.1581262View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Li Insertion into Li4Ti5 O 12 (Spinel) Charge Capability vs. Particle Size in Thin-Film Electrodes

Li 4 Ti 5 O 1 2 (spinel) materials were prepared with Brunauer-Emmett-Teller surface areas ranging from 1.3 to 196 m 2 /g. The corresponding average particle sizes varied from ca. 1 μm to ca. 9 nm. Twenty-five different materials were tested as Li insertion hosts in thin-film electrodes (2-4 μm) made from a pure spinel. Trace amounts of anatase in Li 4 Ti 5 O 1 2 were conveniently determined by cyclic voltammetry of Li insertion. Electrodes from nanocrystalline Li 4 Ti 5 O 1 2 exhibited excellent activity towards Li insertions even at charging rates as high as 250C. The charge capability at 50-250C was proportional to the logarithm of surface area for coarse particles (surface areas smaller than ca. 20 m 2 /g). With increasing charge/discharge rates, a narrowing plateau in performance was observed for materials with surface areas between ca. 20 to 100 m 2 /g. These materials can be charged/discharged nearly to the nominal capacity of L1 4 Ti 5 O 1 2 (175 mAh/g) within a wide range of the rates. Very small particles (surface areas > 100 m 2 /g) exhibit a growing decrease of charge capability at 50-250C. The Li-diffusion coefficients, calculated from chronoamperometry, decrease by orders of magnitude if the average particle size drops from ca. I μm to ca. 9 nm. However, the sluggish Li + transport in small particles is compensated by the increase in active electrode area. Materials having surface areas larger than ca. 100 m 2 /g also tend to show increased charge irreversibility. This could be caused by parasitic cathodic reactions, due to enhanced adsorption of reducible impurities (humidity) or the quality of the spinel crystalline lattice itself. The optimum performance of thin-film Li 4 Ti 5 O 1 2 electrodes is achieved, if the parent materials have surface areas between ca. 20 to 110 m 2 /g, with the maximum peak at 100 m 2 /g.Reference LPI-ARTICLE-2003-015doi:10.1149/1.1581262View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Oxygen-isotope labeled titania: Ti18O2

(18)O-isotope labelled titania (anatase, rutile) was synthesized. The products were characterized by Raman spectra together with their quantum chemical modelling. The interaction with carbon dioxide was investigated using high-resolution FTIR spectroscopy, and the oxygen isotope exchange at the Ti(18)O(2)/C(16)O(2) interface was elucidated.

Organized Mesoporous TiO2 Films Stabilized by Phosphorus: Application for Dye-Sensitized Solar Cells

A synthetic protocol was developed for the preparation of thin mesoporous TiO2 films with enhanced thermal stability. This objective was achieved by a modification of the procedure of supramolecular templating by a Pluronic P-123 copolymer by an addition of a small amount of phosphoric acid to the precursor solution. Thin mesoporous TiO2 films up to 2.3 mu m thick were prepared via layer-by-layer deposition. The roughness factor exceeding 1300 was achieved on these films. After annealing at 540 degrees C, well-developed 5-6 nm anatase nanocrystals were present in the pore walls of a still perfect mesoporous structure. The pore coalescence and structure collapse during heat-treatment were effectively hindered by the presence of phosphorus. Our P-modified mesoporous TiO2 films were sensitized with the N-945 dye and used as photoanodes of the dye-sensitized solar cells. They reached the conversion efficiency of 5.03 or 5.05% for the film thickness of 1.8 or 2.3 mu m, respectively. Whereas the roughness factor scaled linearly with the number of layers, the solar conversion efficiency reached a constant value for films consisting of eight or more layers

Sol-gel titanium dioxide blocking layers for dye-sensitized solar cells: electrochemical characterization.

Compact, thin TiO2 films are grown on F-doped SnO2 (FTO) by dip-coating from precursor solutions containing poly(hexafluorobutyl methacrylate) or hexafluorobutyl methacrylate as the structure-directing agents. The films are quasi-amorphous, but crystallize to TiO2 (anatase) upon heat treatment at 500 °C in air. Cyclic voltammetry experiments performed using Fe(CN)6(3-/4-) or spiro-OMeTAD as model redox probes selectively indicate the pinholes, if any, in the layer. The pinhole-free films on FTO represent an excellent rectifying interface at which no anodic faradaic reactions occur in the depletion state. The flat-band potentials of the as-grown films are upshifted by 0.2-0.4 V against the values predicted for a perfect anatase single-crystal surface, but they still follow the Nernstian pH dependence. The optimized buffer layer is characterized by a combination of quasi-amorphous morphology (which is responsible for the blocking function) and calcination-induced crystallinity (which leads to fast electron injection and electron transport in the conduction band). The latter manifests itself by a reversible charging of the chemical capacitance of TiO2 in its accumulation state. The capacitive-charging capability and pinhole formation significantly depend on the post-deposition heat treatment.