I. Gregora

Notes on the photo-induced characteristics of transition metal-doped and undoped titanium dioxide thin films.

This study reports the preparation of thin nanoparticulate films of titanium dioxide and its modified version doped with a transition metal. The behavior of prepared films was described by means of their photocatalytic and photo-induced electrochemical properties. The TiO(2) and M/TiO(2) (M=Ag, Zr, Fe) thin films were produced via a standard sol-gel method using titanium n-butoxide, acetylacetone, and transition metal acetylacetonates as precursors. Prepared films were analyzed by a series of techniques involving XRD, Raman spectroscopy, SEM, AFM, and XPS. Their photocatalytic activity was monitored with the aid of decomposition of the model compound Rhodamine B in water. All films were then tested for their photo-induced electrochemical properties based on evaluation of polarization curves (photocurrents). The highest reaction rate constant (0.0101min(-1)), which was even higher than that for pure TiO(2), was obtained for the Ag/TiO(2) sample. The highest quantum yield of the charge collection was determined for the undoped TiO(2) film.

Amorphous nanostructuring in potassium niobium silicate glasses by SANS and SHG: a new mechanism for second-order optical non-linearity of glasses

Abstract Potassium niobium silicate (KNS) glasses xK2OxNb2O5(1−2x)SiO2 with x=0.167; 0.182; 0.200; 0.220 and 0.250 have been subjected to prolonged heat treatments in a wide temperature range above Tg. As a result, glasses exhibiting liquid-type phase separation phenomena have been isolated. Moreover for each glass composition, the temperature zones have been determined to produce transparent, opalescent or opaque materials which have been studied by X-ray diffraction (XRD), small-angle neutron scattering (SANS) and second harmonic generation (SHG) techniques. SANS data unambiguously point at nanostructuring of KNS glasses in the scale of 5–20 nm under appropriate heat treatments near Tg. In contrast to initial KNS glasses, nanostructured glasses exhibit SHG activity. For earliest stages of phase separation SHG-active glasses are characterized by fully amorphous XRD patterns. Further development of phase separation in glasses with increasing of their opalescence leads to diminishing SHG, and subsequently partial crystallization takes place giving opaque materials. Since relative maximum of SHG efficiency corresponds to non-crystalline nanostructured glasses, such new transparent second-order non-linear media may be of both scientific and practical interest. With regard to non-crystalline structure of nano-inhomogeneities, SHG mechanism in the glasses is supposed to be due to a combination of third-order non-linearity with a spatial modulation of linear polarizability.

Photoanodes with Fully Controllable Texture: The Enhanced Water Splitting Efficiency of Thin Hematite Films Exhibiting Solely (110) Crystal Orientation.

Hematite, α-Fe2O3, is considered as one of the most promising materials for sustainable hydrogen production via photoelectrochemical water splitting with a theoretical solar-to-hydrogen efficiency of 17%. However, the poor electrical conductivity of hematite is a substantial limitation reducing its efficiency in real experimental conditions. Despite of computing models suggesting that the electrical conductivity is extremely anisotropic, revealing up to 4 orders of magnitude higher electron transport with conduction along the (110) hematite crystal plane, synthetic approaches allowing the sole growth in that direction have not been reported yet. Here, we present a strategy for controlling the crystal orientation of very thin hematite films by adjusting energy of ion flux during advanced pulsed reactive magnetron sputtering technique. The texture and effect of the deposition mode on the film properties were monitored by XRD, conversion electron Mössbauer spectroscopy, XPS, SEM, AFM, PEC water splitting, IPCE, transient photocurrent measurements, and Mott-Schottky analysis. The precise control of the synthetic conditions allowed to fabricate hematite photoanodes exhibiting fully textured structures along (110) and (104) crystal planes with huge differences in photocurrents of 0.65 and 0.02 mA cm(-2) (both at 1.55 V versus RHE), respectively. The photocurrent registered for fully textured (110) film is among record values reported for thin planar films. Moreover, the developed fine-tuning of crystal orientation having a huge impact on the photoefficiency would induce further improvement of thin hematite films mainly if cation doping will be combined with the controllable texture.

Multiple Soft-Mode Vibrations of Lead Zirconate

Polarized Raman, IR, and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals have yielded a comprehensive picture of temperature-dependent quasiharmonic frequencies of its low-frequency phonon modes. It is argued that these modes primarily involve vibrations of Pb ions and librations of oxygen octahedra. Their relation to phonon modes of the parent cubic phase is proposed. Counts of the observed IR and Raman active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Analysis of the results yields insight into the phase transition mechanism, involving a soft ferroelectric branch coupled by a trilinear term to another two oxygen octahedra tilt modes.

Laser photolysis of liquid benzene and hexafluorobenzene: Graphitic and polymeric carbon formation at ambient temperature

Abstract ArF laser-induced liquid-phase photolysis of benzene and hexafluorobenzene afford similar products which are polyaromatic hydrocarbons or fluorocarbons with the preponderance of biphenyl or decafluorobiphenyl, graphite, and polymeric H-containing or F-containing carbon. The remarkable feature is that the graphite formation occurs at ambient temperature of the irradiated liquids; photolytic graphitization modes must therefore be different from those occurring in thermally-induced graphitization at temperatures above 2500 °C.

Raman spectroscopy and effective dielectric function in PLZT x/40/60

The Raman spectra of the ceramics PLZT x/40/60 have been measured in the temperature range 20‐800 K. An assignment of the Raman modes is presented using the results of the effective medium approximation for the IR data. Factor-group analysis is presented for the case of off-site position of cations or unit cell doubling, which allows a higher number of active phonons in the IR and Raman spectra. The presence of Raman-active modes in the cubic phase is associated with local unit cell doubling and symmetry breakdown due to the off-site cations. (Some figures in this article are in colour only in the electronic version)

Lattice dynamics and dielectric response of undoped, soft and hard PbZr0.42Ti0.58O3

In this study, ceramic samples of lead zirconate titanate Pb(Zr0.42Ti0.58)O3 (PZT 42/58) doped with Fe (hard) and Nb (soft) were studied by Raman, infrared (IR) and THz spectroscopy in the temperature range from 900 to 20 K. From the evaluation of the IR and Raman phonons in undoped ceramics, we conclude that tetragonal PZT ceramics undergo a low-temperature phase transition to a tilted phase, revealed by anomalies in phonon parameters and the appearance of new modes. Doped samples also presented similar behaviour. The main differences between undoped and doped ceramics lie in the dielectric behaviour below phonon frequency range, where several mechanisms appear: a soft central mode (CM) located near 1 THz and two relaxations in the GHz range. Both types of doping raise the permittivity values below the phonon frequencies, not only increasing the dielectric contribution of the CM, but also modifying the dielectric response near the GHz range. Soft ceramics show higher permittivity with logarithmic increase at low frequencies, corresponding to an almost frequency-independent value of the dielectric losses below 0.1 GHz.

Resolved E-symmetry zone-centre phonons in LiTaO3 and LiNbO3

E-symmetry optical phonons at the Γ point of LiNbO3 and LiTaO3 were experimentally resolved in spectra measured by infrared and Raman spectroscopy. For this purpose, congruent and nearly stoichiometric crystals of LiNbO3 and LiTaO3, and mixed LiNb1−xTaxO3, crystals were studied. The results show that some of the E modes have weak intensities in Raman or infrared spectra. Thus the complete assignment of E-symmetry modes has been achieved by comparing Raman and infrared data. In addition, this assignment has been confirmed by Raman measurements at low temperatures.

Optical Phonons and Ferroelectric Phase Transition in the LaBGeO5 Crystal

Infrared (IR) reflectivity and submillimeter transmission measurements of ferroelectric and paraelectric LaBGeO 5 single crystals were carried out in the temperature range of 300 to 870 K. From the classical oscillator fit to the spectra, the complex dielectric function in the range of 4 to 3000 cm -1 was calculated. In addition, room temperature polarized Raman spectra were recorded and evaluated which, together with the known high temperature Raman data, enabled us to identify and classify all of the 46 transverse optical modes predicted by the factor-group analysis and their longitudinal optical analogues. The order-disorder type of the ferroelectric transition at T c 800 K was confirmed by revealing a microwave critical relaxation near and above T c . However, this relaxation vanished below 700 K and an anomalous IR phonon mode was detected which softens partially from 87 cm -1 at 300 K down to 60 cm -1 near T c , where it becomes nearly overdamped and coupled to the relaxation. This represents a novel partial displacive feature which is revealed when the structure below the order-disorder transition becomes ordered.

Enhancement of tetragonality and role of strontium vacancies in heterovalent doped SrTiO3

The effect of Sr vacancies on the behavior of strontium titanate with trivalent dopants (La3+, Gd3+, and Y3+) substituting Sr2+ ions is reported. A remarkable shift of the antiferrodistortive transition temperature Ta is revealed by Raman spectroscopy for just a small content of dopant. It is shown that a unique linear dependence of Ta versus tolerance factor is obtained when Sr-vacancies are taken into account. A vacancy size value of ∼1.54 A is estimated, which is ∼7% larger than Sr2+ radius. This size difference enables explaining the unexpected increase of lattice parameter with increasing Bi3+ content in Sr1−1.5xBixTiO3.