Point defects and their effect on dielectric permittivity in strontium titanate ceramics

The origin of dielectric properties of strontium titanate ceramics is investigated using DFT calculations in periodic system. It was determined that the main factors contributing to the increase in dielectric permittivity are: tetragonal distortion of the normally cubic lattice, and charge imbalance induced displacement of titanium center from its central position. Oxygen vacancies were determined to create significantly larger effects than other types of vacancies, like Ti and SrO. The extent of tetragonal distortion was found to be determined by oxygen vacancy distribution, rather than total concentration: relatively symmetrical distribution of oxygen vacancies resulted in smaller tetragonal distortion of the lattice, and, consequently, smaller increase in dielectric permittivity. Charge imbalance naturally destabilizes the cubic lattice, forcing the Ti-atom out of its central position, resulting in tetragonal lattice with increased dielectric permittivity. The process stabilizes the strontium titanate lattice, while increasing the c/a ratio. Therefore, the dielectric permittivity of strontium titanate can be increased by changes to the system that increase tetragonal distortion of the lattice and/or introduce additional negative charge

Modification of structure and photoactivity of titanium(IV) oxide nanotubes by doping and applying of photosensitive components

Ekološki izvori energije i ekonomične metode prečišćavanja vode i vazduha velika su potreba današnjeg tehnološki razvijenog sveta. Fotoaktivni materijali mogu da obezbede ekološku alternativu fosilnim gorivima kada je u pitanju proizvodnja energije i omoguće saniranje nastalih promena u životnoj sredini. Naime, fotoaktivni materijali apsorbuju svetlost i tada dolazi do pobuđivanja elektrona, koji su odgovorni za prenos električne struje ili učestvuju u procesu razgradnje zagađujućih materija. Postoji veliki broj fotoaktivnih materijala, među kojima se izdvaja TiO2, kojeg odlikuje hemijska inertnost, otpornost na fotoindukovanu i hemijsku koroziju i netoksičnost. Na fotoaktivnost TiO2 utiču mnogi faktori, kao što su morfologija, kristalna struktura i stepen rekombinacije nosilaca naelektrisanja. Cilj ove doktorske disertacije je poboljšanje fotokatalitičke i fotoelektrične aktivnosti TiO2 nanocevi. U tu svrhu, TiO2 nanocevi su sintetisane metodom elektrohemijske anodizacije pločice titana. Ispitivan je uticaj procesnih parametara anodizacije na morfologiju i fotokatalitičku aktivnost nanocevi TiO2 u procesu razgradnje boje. Utvrđeno je da sa porastom napona anodizacije rastu visina i prečnik nanocevi, što utiče na pomeraj apsorpcione granice ka vidljivom delu spektra i na poboljšanje fotokatalitičke aktivnosti. Određen je uticaj temperature žarenja na kristalnu strukturu, morfologiju i fotokatalitičku aktivnost TiO2 nanocevi. Žarenjem nanocevi na 450 °C dobijena je kristalna modifikacija anatasa sa veoma malim sadržajem faze rutila. Sa daljim porastom temperature žarenja (600 i 650 °C) uočeno je prisustvo obe faze, anatasa i rutila, u različitim odnosima, a do kompletne transformacije anatasa u rutil došlo je na 700 °C. Pored promene kristalne strukture, uočen je uticaj temperature žarenja na morfologiju nanocevi. Sa povišenjem temperature žarenja, visina nanocevi se smanjivala, kao i poroznost nanocevi, da bi na 700 °C došlo do kolapsa nanocevne morfologije. Sagledan je uticaj kristalne strukture i morfologije TiO2 nanocevi na fotokatalitičku razgradnju boje metil-oranž. Zaključeno je da istovremeno prisustvo anatasa i rutila obezbeđuje bolju fotokatalitičku aktivnost u odnosu na čist anatas i čist rutil. Kako bi se ispitao uticaj dopiranja TiO2 azotom na fotokatalitičku aktivnost, nanocevi TiO2 su žarene u atmosferi amonijaka, na temperaturi od 450 °C, pri različitim dužinama žarenja. Ispitivan je uticaj vremena žarenja na količinu i vrstu dopiranja. Uočeno je da žarenje u atmosferi amonijaka, pri različitim vremenima, nije uticalo na morfologiju nanocevi. Hemijska analiza površine TiO2 nanocevi rendgenskom fotoelektronskom spektroskopijom pokazala je da sa dužim vremenom žarenja opada ukupan sadržaj azota u nanocevima. Osim toga, dužina žarenja u amonijaku je uticala i na prirodu inkorporiranja azota u TiO2. Pri kraćem vremenu žarenja zapažen je veći sadržaj intersticijskog azota, a manji sadržaj supstitucijskog azota. S dužim vremenom žarenja u amonijaku, sadržaj intersticijskog azota opada, dok sadržaj supstitucijskog raste. Apsorpcioni spektri su pokazali da uzorak sa najvećom količinom azota i najvećim sadržajem intersticijskog azota pokazuje najveći pomeraj granice apsorpcije ka većim talasnim dužinama. Isto je uočeno i za fotokatalitičku efikasnost. Najveću fotokatalitičku razgradnju boje postigao je uzorak žaren u amonijaku pri najkraćem vremenu. Dalje istraživanje je išlo u smeru proširenja saznanja o pojedinačnim i kombinovanim uticajima dopiranja azotom i deponovanja kvatnih tačaka neorganskog senzivatora na fotokatalitičku aktivnost TiO2 nanocevi. Zapaženo je da deponovanje kvantnih tačaka CdS nije uticalo na morfologiju nanocevi, ali je na pojedinim mestima došlo do aglomeracije CdS nanočestica, što je dovelo do zatvaranja otvora nanocevi. Utvrđeno je da je fotokatalitička efikasnost azotom dopiranih nanocevi poboljšana nakon deponovanja CdS nanočestica. Najveću fotokatlitiču efikasnost pokazao je uzorak sa najvećim sadržajem ukupnog azota i intersticijskog azota u kombinaciji sa deponovanim CdS nanočesticama...Environmental friendly power sources and cost-effective methods for water and air purification, are a necessity in modern technologically developed world. Photoactive materials can provide an environmentally friendly alternative to fossil fuels in power production and enable the remediation of changes in the environment. Namely, photoactive materials absorb light, which excites electrons that are responsible for generating electricity or participate in the process of decomposition of pollutants. There are a large number of photoactive materials, among which TiO2 has attracted particular attention, due to its chemical inertness, resistance to photo and chemical corrosion and non-toxicity. The photoactivity of TiO2 depends on many factors, such as morphology, crystal structure and the degree of recombination of charge carriers. The aim of this doctoral dissertation is to improve the photocatalytic and photoelectric activity of TiO2 nanotubes. For this purpose, TiO2 nanotubes were synthesized by the electrochemical anodization of a titanium plate. The influence of the anodization process parameters on the morphology and photocatalytic activity of TiO2 nanotubes for color degradation was investigated. It was found that an increased anodization voltage leads to increased heights and diameters of nanotubes, causing the shift of the absorption limit towards the visible part of the spectrum and the improvement of the photocatalytic activity. The influence of the annealing temperature on the crystal structure, morphology and the photocatalytic activity of TiO2 nanotubes was determined. The anatase crystalline modification with a very low rutile phase content was obtained by annealing nanotubes at 450 °C. When the annealing temperature was further increased (600 and 650 °C), the presence of both phases, anatase and rutile, was observed in different ratios, and the complete transformation of anatase to rutile occurred at 700 °C. Besides the change in the crystal structure, the influence of the annealing temperature on the morphology of the nanotubes was observed as well. As the annealing temperature increased, the height and porosity of nanotubes decreased, so that the nanotube morphology collapsed at 700 °C. The influence of the crystal structure and morphology of TiO2 nanotubes on the photocatalytic degradation of methyl-orange was observed. It was concluded that the simultaneous presence of anatase and rutile provides better photocatalytic activity than pure anatase and pure rutile. In order to investigate the effect of nitrogen doping of TiO2 on photocatalytic activity, TiO2 nanotubes were annealed in an ammonia atmosphere, at a temperature of 450 °C, for different annealing times. The influence of the annealing time on the amount and type of doping was investigated. It was observed that annealing in an ammonia atmosphere, for different periods, did not affect the morphology of nanotubes. A chemical analysis of the surface of TiO2 nanotubes by Xray photoelectron spectroscopy showed that the total nitrogen content in nanotubes decreased with longer annealing times. In addition, the annealing duration in ammonia also influenced the nature of nitrogen incorporation into TiO2. With a shorter annealing time, a higher content of interstitial nitrogen and a lower amount of substitution nitrogen were observed. With a longer annealing time in ammonia, the amount of interstitial nitrogen decreased, while the substitution nitrogen increased. The absorption spectra showed that the sample with the highest amount of nitrogen and the highest content of interstitial nitrogen showed the largest shift towards higher wavelengths. The same applies to the photocatalytic efficiency. The greatest photocatalytic degradation of the dye was achieved in the sample annealed in ammonia for the shortest time. Further research was directed towards expanding the knowledge of individual and combined effects of nitrogen doping and deposition of the quantum dots of inorganic sensors on the photocatalytic activity of TiO2 nanotubes. It was observed that the deposition of CdS quantum dots did not affect the morphology of nanotubes, but in some places agglomeration of CdS nanoparticles occurred, due to which nanotube openings closed. The photocatalytic efficiency of nitrogen-doped nanotubes was improved after CdS nanoparticle deposition. The sample with the highest amount oftotal nitrogen and interstitial nitrogen in combination with deposited CdS nanoparticles exhibited the highest photocatalytic efficiency..

Microstructure changes caused by thermal etching of sintered ZnTiO3

Polishing and thermal etching was performed on ZnTiO3sintered ceramic. Two set of specimens were recorded, on higher 1200oC/1100oC sintered/etched, and lower 900oC/800oC sintered/etched. Phase composition, crystallite sizes and grain size distributions before and after etching were compared. Grain size enlargement was noted.\ud \ud Poster presented at the 12th International Conference on Fundamental and Applied Aspects of Physical Chemistry - Physical Chemistry 2014, Belgrade, September 22-26, 201

TiO2 based nanomaterials and nanostructures for green convergent technologies and environmental protection

Imajući u vidu značaj razvoja naprednih nanomaterijala za zaštitu prirodne okoline, u ovome radu su prezentovane primene nanomaterijala i nanostruktura na bazi TiO2 u oblastima obnovljivih izvora energije, hemijske i bio dekontaminacije. Objašnjena je uloga TiO2 u formiranju kompozitnih hibridnih struktura za prikupljanje solarne energije, kao i primena različitih antimikrobnih materijala na bazi TiO2 koji se koriste za fotokatalitičko uklanjanje polutanata. Naročita pažnja je posvećena strukturi i svojstvima titan dioksidnih nanotuba dobijenih primenom metode elektrohemijske anodizacije i njihova primena u bio dekontaminaciji.Taking into account the importance of development of the advanced nanomaterials for the environmental protection, in this article the application of TiO2 based nanomaterials and nanostructures in the fields of renewable energy, chemical and bio decontamination has been presented. The role of TiO2 in formation of composite hybrid structures for solar energy harvesting has been explained, as well as the use photocatalytic degradation of pollutants. properties of TiO2 nanotubes obtained by decontamination

TiO2 based nanomaterials and nanostructures for green convergent technologies and environmental protection

Imajući u vidu značaj razvoja naprednih nanomaterijala za zaštitu prirodne okoline, u ovome radu su prezentovane primene nanomaterijala i nanostruktura na bazi TiO2 u oblastima obnovljivih izvora energije, hemijske i bio dekontaminacije. Objašnjena je uloga TiO2 u formiranju kompozitnih hibridnih struktura za prikupljanje solarne energije, kao i primena različitih antimikrobnih materijala na bazi TiO2 koji se koriste za fotokatalitičko uklanjanje polutanata. Naročita pažnja je posvećena strukturi i svojstvima titan dioksidnih nanotuba dobijenih primenom metode elektrohemijske anodizacije i njihova primena u bio dekontaminaciji.Taking into account the importance of development of the advanced nanomaterials for the environmental protection, in this article the application of TiO2 based nanomaterials and nanostructures in the fields of renewable energy, chemical and bio decontamination has been presented. The role of TiO2 in formation of composite hybrid structures for solar energy harvesting has been explained, as well as the use photocatalytic degradation of pollutants. properties of TiO2 nanotubes obtained by decontamination

Application of Nanocrystalline Pseudobrookite (Fe2TiO5) Thick Films for Humidity Sensing

Pseudobrookite nanocrystalline thick films were screen printed on alumina substrate with small interdigitated PdAg electrodes (width 6 mm, length 12 mm, electrode spacing 0.2 mm) and fired at 600°C for 30 minutes. Scanning electron microscopy (SEM) of the thick film surface confirmed the formation of a porous structure consisting of agglomerated nanocrystalline grains of pseudobrookite. Impedance response of pseudobrookite thick film samples was measured in a humidity chamber at operating temperatures of 25 and 50°C in the relative humidity (RH) range 40-90% and frequency range 42 Hz-1 MHz. At the lowest frequency of 42 Hz at 25°C the impedance reduced 7 times (from 35.74 MΩ for RH 40% to 4.91 MΩ for RH 90%) and at 50°C 33 times (from 30.98 MΩ for RH 40% to 0.944MΩ for RH 90%). Low hysteresis (1.82 and 3.65%) was obtained at 25 and 50°C, respectively. Complex impedance was analyzed using an equivalent circuit consisting of parallel impedance and constant phase (CPE) element showing the dominant influence of grain boundaries. © 2018 IEEE

Influence of nitrogen and air atmosphere during thermal treatment on micro and nano sized powders and sintered TiO2 specimens

The influence of air and nitrogen atmosphere during heating on TiO2 nano and micro sized powders as well as sintered polycrystalline specimens was analyzed. Sintering of TiO2 nano and micro powders in air atmosphere was monitored in a dilatometer. Non compacted nano and micro powders were analyzed separately in air and nitrogen atmospheres during heating using thermo gravimetric (TG) and differential thermal analysis (DTA). The anatase to rutile phase transition temperature interval is influenced by the powder particle size and atmosphere change. At lower temperatures for nano TiO2 powder a second order phase transition was detected by both thermal techniques. Polycrystalline specimens obtained by sintering from nano powders were reheated in the dilatometer in nitrogen and air atmosphere, and their shrinkage is found to be different. Powder particle size influence, as well as the air and nitrogen atmosphere influence was discussed. [Projekat Ministarstva nauke Republike Srbije, br. OI172057 i br. III45014

Synthesis and deposition of MAPbBr3 perovskite on titania nanotube arrays

The organo-inorganic perovskites are materials that have recently revolutionized the field of photovoltaics due to their low-cost fabrication and high optical absorption. The hybrid organoinorganic perovskite absorbs the visible part of the spectrum resulting in the creation of electron-hole pair. To decrease the recombination of charge carriers, the construction of solar cells requires the existence of separate layers for holes and for electrons. TiO2 is usually used as an electron transport layer because its conduction band (CB) lies under the CB of perovskite. In that way, electrons diffuse from CB of perovskite to CB of TiO2. For these experiments, TiO2 nanotubular structure was used as an electron transport layer due to its advantages compared to nanoparticular TiO2. TiO2 nanotubes can provide a one-dimensional transmission channel for the charge carriers, so it will reduce the recombination rate of the carriers and provide a channel for fast carrier transport. However, there is a problem with the contact surface between perovskite and TiO2 nanotubes. The aim of this study is to increase the contact surface of perovskite and TiO2 nanotubes by filling the nanotubes with perovskite material in order to improve electron transport. Methylammonium lead bromide perovskite (MAPbBr3) was deposited on anodically synthesized TiO2 nanotubes which were annealed at 450 °C for 1 h. After degassation of the sample under high vacuum for 3 h at 200 °C, the cooled sample was put in a solution of MAPbBr3 in dimethylformamide (DMF) and it was treated with inert gas (N2), which enabled the filling of the nanotubes with perovskite material to some extent. FESEM and XRD analyses were used for morphological and chemical characterization of the sample. The diffuse reflectance spectroscopy measurement of the sample proved that deposition of MAPbBr3 improves the absorption properties of TiO2 nanotubes. By measuring the I-V characteristics of the sample in the dark and under visible light, a hysteresis curve was obtained

The effect of pH on visible-light photocatalytic properties of pseudobrookite nanoparticles

In this study, pseudobrookite (Fe2TiO5) nanoparticles were fabricated by a modified sol-gel method using Fe(NO3)39H2O and Ti(OC3H7)4 as starting reagents and ethanol as solvent. Oxalic acid was used as a chelating agent while cetyltrimethyammonium bromide (CTAB) and citric monohydrate were used as surfactants. Structral and morphological characterization using X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) analysis confirmed the formation of pseudobrookite nanoparticles. As synthetized Fe2TiO5 nanoparticles were utilized as photocatalysts for decolorisation of Methylene blue (MB) under visible light irradiation. It was observed that the adsorption of MB onto Fe2TiO5 nanoparticles is strongly dependent on the solution pH. Maximum decolorozation was observed for Fe2TiO5 nanoparticles prepared with CTAB under alcaline conditions (pH=10.5)

Nanocrystalline iron-manganite (FeMnO3) applied for humidity sensing

Nanocrystalline iron manganite was synthesized using a sol-gel self-combustion method with glycine as fuel, followed by calcination at 900 °C for 8 hours. Structural characterization was performed using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). It confirmed the formation of nanocrystalline iron-manganite with a perovskite structure. Humidity sensing properties of bulk and thick film samples of the obtained nanocrystalline iron manganite powder were analyzed. Organic vehicles were added to the powder to form a thick film paste that was screen printed on alumina substrate with test PdAg interdigitated electrodes. Impedance response of bulk and thick film samples was analyzed in a humidity chamber in the relative humidity range 30-90% in the frequency range 42 Hz to 1 MHz in view of applying iron-manganite for humidity sensing applications