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Perovskites are an important group of ceramic materials with a structural formula ABO3 and wide array of potential applications in electronics, superconductors, catalysis, etc. CaTiO3, by which the whole group was named for, is particularly significant due to its use in catalysis, but its photocatalytic activity is limited by a large band gap value (~3.5 eV). A possible solution is the substitution of A and B cations with foreign cations which causes the alteration of properties, including photocatalytic efficiency. The aim of this work was the sol-gel synthesis of lithium and cerium codoped CaTiO3, characterization of the prepared gel and ceramics obtained by its thermal treatment. Samples of codoped perovskite, Ca1-xLixCexTiO3, where x = 0, 0.01, 0.02, 0.03 and 0.04, were prepared and characterized using powder X–ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), differential thermal and thermo-gravimetric analysis (DTA-TGA), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Photocatalytic activity was evaluated through the study of methylene blue photocatalytic degradation. XRD analysis showed that the prepared samples consisted of calcium nitrate and titanium chelate. In accordance with the established thermal evolution path, all samples were thermally treated at 500 °C for 2 hours. Beside perovskite, Ca2Ti2O6 appeared as a secondary phase in all thermally treated samples. SEM analysis of thermally treated samples showed the presence of agglomerates of irregular morphology and the decrease of primary particles size with the increase of dopants concentration. The sample with x=0.04 showed an increased photocatalytic activity.

The production of spatially ordered arrays of nanopores in alumina is attracting considerable attention due to their valuable filtering and sensing applications. Here we investigate the production conditions of thin films consisting of a continuous 3D network of nanopores in alumina, with the network nodes arranged in a body-centered tetragonal lattice, and the pore size about 1 nm. The emphasis is on controlling the nanopores length, radius, and geometry of their network by the preparation parameters. The material is produced from 3D network of 1D Ge nanowires in alumina by a dedicated annealing treatment that causes Ge to evaporate. We show that the main limiting factors for the production of the nanopore networks are the fraction of Ge nanowires before annealing combined with the unit cell parameters of their network. The 3D networks of nanopores form easily when the Ge atomic fraction in the film is less than 40 %. For larger Ge percentages, the alumina matrix collapses because the interconnection to the neighbouring unit cells in the vertical direction is lost. The refractive index of the materials can be tuned in a wide range by varying the porosity, i.e. the parameters of the Ge nanowire network.

Thin films comprising three dimensional germanium (Ge) quantum dot lattices formed by nitrogen (N) assisted magnetron sputtering deposition in alumina (Al2O3) matrix have been studied for light harvesting purposes. In order to expand the application of this material it is necessary to reduce germanium oxidation and to achieve stabilization of the germanium/alumina interface. Effects of tuning the N concentration, substrate temperature and Ge sputtering power during the films preparation are monitored. It is shown that the N presence not only reduces Ge oxidation during annealing but also affects the internal structure, size and arrangement of Ge quantum dots. Additionally, the deposition temperature and Ge sputtering power are used to tune the Ge quantum dot size, separation and the regularity of their positions. It is shown that the optical and electrical properties of the films, especially their photo-induced current, and quantum efficiency are strongly tunable by the deposition conditions. Moreover, a significant photo-response and effect of multiple exciton generation effect is observed. The materials presented could be used as a sensitive layer for photodetectors or photovoltaic light harvesting devices. The presented tools can be used for future fine tuning of the material to achieve the optimal quantum efficiency.

U radu je dan pregled pojmova i jednadžbi vezanih uz određivanje zabranjene zone metodom difuzne refleksijske spektroskopije i uporabu Taucova grafičkog prikaza. Kako bi se demonstrirao postupak i vrednovala sama metoda, primjenom metode određena je zabranjena zona komercijalnih uzoraka anatasa, rutila, cinkita i hematita. Na temelju eksperimentalno dobivenih vrijednosti širine zabranjene zone komentirana je točnost metode te osjetljivost pri razlikovanju poluvodiča s direktnim i indirektnim prijelazima. Pokazano je da Taucova metoda nije besprijekorna niti u pogledu točnosti niti razlikovanja indirektnih i direktnih elektronskih prijelaza u poluvodičkim materijalima, ali je vrlo praktičan način određivanja širine zabranjene zone poluvodiča budući da ne zahtijeva pretjerano skupu instrumentaciju a obrada eksperimentalnih podataka relativno je jednostavna. For the application of semiconductors, an important factor is the band gap, i.e., the minimum energy required for the transfer of electrons from the valence to the conduction band. One of the possible methods for band gap determination is diffuse reflectance spectroscopy and Tauc plot. In this paper, an overview of the terms and equations related to the said method is given, as well as its utilization in the determination of band gaps of commercial samples of various metal oxides. Thus, the procedure is demonstrated and evaluated through the determination of indirect and direct band gap values of anatase (TiO2), rutile (TiO2), zincite (ZnO), and hematite (Fe2O3). All samples were beforehand analysed and identified by X-ray powder diffraction on Shimadzu XRD 6000 diffractometer with CuKα radiation working in a step scan mode with steps of 0.02° and counting time of 0.6 s. It was determined that all samples are well crystallized with relatively large crystallite sizes. UV-Vis spectra of the samples, as well as barite, which was used as a reference, were obtained on the UV-Vis spectrometer with an integrating sphere in total reflectance mode. The UV-Vis DRS spectra were transformed to Kubelka-Munk function, after which Tauc plot was used for the determination of the indirect and direct band gap values of all samples. The obtained values for anatase were 3.20 eV for indirect transition and 3.41 eV for direct transition, and for rutile 3.00 eV for indirect transition and 3.11 eV for direct transition. The zincite sample showed an indirect band gap of 3.19 eV and direct band gap of 3.25 eV, while the obtained indirect band gap value for hematite was 1.96 eV and direct band gap value 2.15 eV. As may be seen, the method is not particularly useful when distinguishing direct from indirect semiconductors, since, for all samples, the curves in Tauc plot for both indirect and direct electron transitions possess a linear dependence region from which the band gap value is estimated. However, the obtained band gap values for all the studied semiconductors are in relatively good concordance with literature references. The method is perhaps most useful in monitoring the variation of band gap depending on the dopant content. Namely, the studied metal oxides are used in photocatalysis where the addition of dopants is expected to reduce the band gap to visible light area, and thus improve the photocatalytic activity of the semiconductor. It can be concluded that the Tauc method is not perfect in terms of accuracy and differentiation between indirect and direct electron transitions in semiconductors. Nevertheless, it is a very practical way of band gap assessment for semiconducting materials, because it requires no excessively expensive instrumentation, and the processing of experimental data is rather simple.