Effect of calcination on band gaps for electrospun titania nanofibers heated in air-argon mixtures

The relationship between the band gap in electrospun titania nanofibers at ambient temperature and the nature of the air-argon atmosphere in which the material has been heated non-isothermally to 900. °C was investigated by ultraviolet-visible absorption spectrometry at room temperature. The results for heating in different selected air-argon mixtures show that the UV-region band gap found in unheated as-spun amorphous nanofibers, 3.33. eV, may be shifted well into the visible region by calcining in the different air-argon mixtures. The band gap value found for heating in air, 3.09. eV, reduces systematically when the material is heated in an air-argon mixture, with the gap in pure argon being 2.18. eV. The progressive lowering of the band gap is attributed to the development of crystallinity in the fibers as the material is calcined and the associated development of oxygen vacancies when heated in argon, and therefore to the formation of defect states below the conduction band

Stability and Electronic Properties of TiO2 Nanostructures With and Without B and N Doping

We address one of the main challenges to TiO2-photocatalysis, namely band gap narrowing, by combining nanostructural changes with doping. With this aim we compare TiO2's electronic properties for small 0D clusters, 1D nanorods and nanotubes, 2D layers, and 3D surface and bulk phases using different approximations within density functional theory and GW calculations. In particular, we propose very small (R < 0.5 nm) but surprisingly stable nanotubes with promising properties. The nanotubes are initially formed from TiO2 layers with the PtO2 structure, with the smallest (2,2) nanotube relaxing to a rutile nanorod structure. We find that quantum confinement effects - as expected - generally lead to a widening of the energy gap. However, substitutional doping with boron or nitrogen is found to give rise to (meta-)stable structures and the introduction of dopant and mid-gap states which effectively reduce the band gap. Boron is seen to always give rise to n-type doping while depending on the local bonding geometry, nitrogen may give rise to n-type or p-type doping. For under coordinated TiO2 surface structures found in clusters, nanorods, nanotubes, layers and surfaces nitrogen gives rise to acceptor states while for larger clusters and bulk structures donor states are introduced

Photoelectrochemical and photocatalytic properties of N + S co-doped TiO2 nanotube array films under visible light irradiation

In this paper, we report on the co-doping nitrogen and sulfur has been achieved in the TiO2 nanotube array films by treatment with thiourea and calcination under vacuum at 500 {\deg}C for 3 h. The samples were characterized by scanning electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible diffuse reflectance spectroscopy. XPS spectra revealed that N might coexist in the forms of NTiO and NOTi, S was incorporated into the lattice of TiO2 through substituting oxygen atoms in the N + S co-doped TiO2 nanotube array films. XRD patterns indicated that improved crystallinity was obtained for N + S co-doped TiO2 nanotube arrays as compared to that of undoped TiO2 nanotube arrays. In photoelectrochemical measurements, the photocurrent of N + S co-doped TiO2 nanotube array films was greatly enhanced compared to that of undoped samples under visible light irradiation. And the photocatalytic activities of the samples were evaluated on the removal of methylene blue under visible light irradiation. The N + S co-doped TiO2 nanotube array films showed a better photocatalytic activity than the undoped sample due to the N, S doping. Keywords: Nanostructures; Oxide; Thin films; Electrochemical propertiesComment: 5 pages, 6 figure

Passivation and dissolution mechanisms in ordered anodic tantalum oxide nanostructures

Tantalum oxide (Ta2O5) nanostructures exhibit outstanding electrical and optical properties, as well as, high chemical resistance and stability. These materials have great potential for biomedical, catalysis, semiconductors and energy applications due to their large surface area and high specific charge, when arranged in nanoporous or nanotubular morphologies. In order to obtain these structures, an anodization process, which is inexpensive, reproducible and easy to scale up, is used. Yet, depending on the anodization conditions, the formation of a nanoporous or nanotubular layer is difficult to stabilize during the anodization process. In this regard, anodized tantalum oxide nanostructures were produced to understand the effect of the anodization conditions, including electrolyte concentration, potential and time. The nanopores or nanotubes morphologies, their chemical composition and structure were investigated by FIB-SEM, double-corrected TEM-STEM and EDS. We found that it is necessary to have high acid concentrations (mixture of H2SO4 with HF) to be able to form nanoporous or nanotubular structures. Despite the capacity of HF to dissolve and create anodic oxide nanostructures, the amount of H2SO4 concentration in the mixture is very important, leading to a dimple morphology. Furthermore, the increase of the anodization potential/electrical field clearly leads to an increase in the dimples diameter.This research is sponsored by FEDER funds through the program COMPETE -Programa Operacional Factores de Competitividade and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020, and UID/EMS/00285/2020 and with a PhD fellowship SFRH/BD/98199/2013.The authors thank the financial support in the framework of HEALTHYDENT -POCI-01-0145-FEDER-030708 and PTDC/CTM-NAN/4242/2014 projects.This work was supported by FCT, through IDMEC, under LAETA, project UIDB/50022/2020.The authors would like to acknowledge that this project has received funding from the EU Framework Programme for Research and Innovation H2020, scheme COFUND -Co-funding of Regional, National and International Programmes, under Grant Agreement 713640

Effect of Reynolds number and lithium cation insertion on titanium anodization

This work studies the influence of using hydrodynamic conditions (Reynolds number, Re = 0 to Re = 600) during Ti anodization and Li+ intercalation on anatase TiO2 nanotubes. The synthesized photocatalysts were characterized by using Field Emission Scanning Electron Microscope (FE-SEM), Raman Confocal Laser Microscopy, Electrochemical Impedance Spectroscopy (EIS), Mott-Schottky analysis (M-S), photoelectrochemical hydrogen production and resistance to photocorrosion tests. The obtained results showed that the conductivity of the NTs increases with Li+ intercalation and Re. The latter is due to the fact that the hydrodynamic conditions eliminate part of the initiation layer formed over the tube-tops, which is related to an increase of the photocurrent in the photoelectrochemical water splitting. Besides, the photogenerated electron-hole pairs are facilitated by Li+ intercalation. Finally, this work confirms that there is a synergistic effect between Re and Li+ intercalation

Elektronische und ionische Eigenschaften von TiO2 Nanoröhren

More and more, the well studied and understood properties of a bulk material are combined with the new properties obtained from its nanostructured counterpart, which in many cases makes possible improvement in applications and devices performance. A very successful example is titanium dioxide – where, by forming nanostructured films with a relative low costs and a high chemical stability, it was possible to apply it in solar energy conversion, Li-ion storage, liquid/gaseous photo-catalysis and biomedical devices. Many scientists worldwide have focused their attention on new approaches to produce nano-TiO2. Besides the classical approaches, such as hydrothermal synthesis, another solution for forming nano-architectures would be through a process of self-organization. Obtaining self-organized morphologies via simple methods would make the production costs decrease and allow new generation nano-devices to appear. In this work we employ the use of a single step electrochemical process to form highly self-ordered vertically-oriented metal oxide nanotubes on Ti and a Ti-based alloy (Ti45Nb). Their geometry (nanotube length, diameter and wall thickness), chemistry (modification with different species onto and into the nanotubes) and crystallography (amorphous, anatase, rutile or a mixture of them) can be tuned and adjusted to fit certain applications. The first part of the thesis will describe the use of TiO2 nanotubes in photo-electrochemical cells under UV light and discuss the differences and analogies to the classical nanoparticle system. Besides the influence of chemical and crystallographic structure, the nanotube morphology, such as tube diameter, length and wall thickness and organization order of the nanotubes, electronic properties will be investigated when used in photo-electrochemical applications. Techniques such as X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), photocurrent spectroscopy (PS), phototransient measurements and intensity modulated photocurrent spectroscopy (IMPS) were used in this study for a deeper understanding of the nanotube electronic and ionic structure. Titanium dioxide is a semiconductor-like material with a wide bandgap of 3.2 eV (in the anatase form) that limits its activity to the UV light range. In the second part of this work we show how to make TiO2 nanotubes active under visible light illumination by: i) modifying their electronic properties (by doping with N or Cr) or ii) sensitization with visible light absorbers such as CdS/CdSe quantum dots or Ru-complex dyes. Here, great attention is given to the use of a Ru-complex dye for nanotube sensitization with a successive dye-sensitized solar cell (DSSC) construction. Photovoltage decay, charge extraction, impedance spectroscopy, as well as intensity modulated photocurrent spectroscopy and intensity modulated photovoltage spectroscopy (IMVS) were used to investigate the specific electronic properties – particularly the electron diffusion constant, life time and diffusion length – in the 1D TiO2 nanotubes. Further on, in the third part of the work, the so called electrochromic effect that is exploited by electrochromic display devices (ECDs) when using TiO2 nanotubes as transparent electrodes is discussed. The transparent appearance of nanotubular electrodes can be changed to a dark-blue color by applying a cathodic bias in certain electrolytes; this is accompanied by the insertion of ions (from electrolyte) into the TiO2 nanotube layer. The charging capacity, kinetics, stability and color contrast under conditions of switching, from the transparent to the colored state, were investigated with respect to nanotube length and crystallographic structure by cyclic voltammograms (CVs), chronoamperometry, chronopotentiometry and reflectance/transmission measurements. Moreover, a new system is proposed with an improved switchability and higher charge capacity performance – Nb-doped TiO2 nanotubes. In the end, a small example how a dye-sensitized solar cell was combined with an electrochromic display device, both based on TiO2 nanotubes is shown. The working principle is as follows: by increasing light intensity on the dye-sensitized solar cell the output potential will overcome the onset potential of the electrochromic device and will change its color from transparent to dark. A decrease in the light intensity will decrease the output potential from the dye-sensitized solar cell below the onset potential of the electrochromic display device, which allows the nanotube layer in the electrochromic device to return to its initial transparent appearance.Ein wichtiges Gebiet der Nanotechnologie ist es bekannte mit neuen Materialeigenschaften durch Nanostrukturierung zu kombinieren. Ziel hierbei ist es bestehende Anwendungen und Prozesse zu verbessern bzw. völlig neue elektronische Bauelemente mit überlegenen Leistungsmerkmalen zu entwickeln. Eines der erfolgreichsten Beispiele hierfür ist TiO2, das durch seine geringen Herstellungskosten und seine ausgezeichnete Stabilität als nanostrukturierte Schicht breite Anwendung in der Solarenergieumwandlung, in Li-Ionen Speichern, als Photokatalysator für die Reinigung von Flüssigkeiten und Gasen und in der Biomedizintechnik findet. Weltweit arbeiten Forscher an neuen Methoden um nanostrukturiertes TiO2 in Form von Partikeln, Röhren, oder Fäden herzustellen. Neben den konventionellen Ansätzen wie z.B. der hydrothermalen Synthese, ist die Nanostrukturierung durch selbstorganisierte Prozesse eine weitere Möglichkeit. Solch eine homogenen und kostengünstige Oberflächenstrukturierung könnte der Durchbruch für neuartige Nano Bauelemente und deren Anwendungen sein. In dieser Arbeit benutzen wir einen elektrochemischen Prozess, die Anodisierung von Titan und Titan-Legierungen, um in einem Schritt selbstorganisierte Nanoröhrenschichten aus Titandioxid bzw. Titanmischoxiden zu erhalten. Dabei kann die Nanoröhrengeometrie (Länge, Durchmesser und Wandstärke), die Chemie (durch Einlagern von Ionen bzw durch das Ankoppeln von Molekülen) und die Kristallstruktur verändert und gezielt an die gewünschten Anforderungen angepasst werden. Der erste Teil der Arbeit beschreibt das Verhalten von TiO2 Nanoröhrenelektroden unter UV-Licht in der Photoelektrochemie, sowie die Gemeinsamkeiten und Unterschiede zu den klassischen gesinterten TiO2 Nanopartikel Systemen. Dabei wird neben dem Einfluss der chemischen und kristallinen Struktur auch der Einfluss der Nanoröhrengeometrie (Durchmesser, Länge, Wandstärke und Ordnungsgrad) auf die elektronischen Eigenschaften in photoelektrochemischen Anwendungen aufgezeigt. Hierzu werden verschiedenste Charakterisierungstechniken wie Photoelektronenspektroskopie (XPS), Röntgenbeugung (XRD), Rasterelektronenmikroskopie (SEM), Photostromspektroskopie (PS), Phototransientenmessungen und Intensitätsmodulierte Photostromspektroskopie (IMPS) herangezogen. Da Titandioxid ein n-Typ Halbleiter mit einer Bandlücke von 3.2 eV ist, ist die direkte Anwendung zur Energiegewinnung aus Sonnenlicht auf den UV-Anteil beschränkt. Der zweite Teil der Arbeit beschäftigt sich damit die Aktivität der TiO2 Nanoröhren auf den sichtbaren Bereich, durch i) Änderung der elektronischen Struktur (Dotierung mit Stickstoff oder Chrom) oder ii) Sensibilisierung mit geeigneten Lichtabsorbern (CdS/CdSe Quantenpunkten oder Ru-basierten Farbstoffen), auszudehnen. Hierbei wird der Schwerpunkt auf die Sensibilisierung mit Ru-Farbstoffen und das Konstruieren einer auf TiO2 Nanoröhren basierenden Farbstoff Solarzelle (DSSC) gelegt. Messungen zum Abklingverhalten der Photospannung und zur Ladungsträgerextraktion, sowie Impedanzspektroskopie, IMPS und Intensitätsmodulierte Photospannungspektroskopie werden verwendet um die wichtigsten elektronischen Eigenschaften der Nanoröhren DSSC, wie Diffusionskonstanten, Ladungsträgerlebenszeiten und Diffusionslängen zu bestimmen. Im dritten Teil der Arbeit wird die Verwendung von transparenten TiO2-Nanoröhrenschichten in elektrochromen Anwendungen (ECD) wie z.B. selbsttönenden Fensterscheiben gezeigt. Der sogenannte „elektrochrome Effekt“ beruht auf das Anlegen einer kathodischen Spannung an die TiO2-Elektrode wodurch Ionen (z.B. H+,Li+,...) von einem geeigneten Elektrolyten in das TiO2 Kristallgitter eingelagert werden. Dieser Vorgang wird begleitet von einer Farbänderung der Schicht von transparent nach tiefblau. Zu untersuchende Parameter sind Ladungskapazitäten, kinetische Aspekte, Langzeitstabilität bei Wechselbelastungen (Be- und Entladen) und der Farbkontrast in Abhängigkeit der Nanoröhrenlänge und Kristallstruktur durch zyklische Voltammetrie (CV), amperometrische und potentiostatische Messungen sowie Reflexions bzw. Transmissionsmessungen. Darüberhinaus wird ein neues System mit verbesserter Langzeitstabilität und höheren Ladungskapazitäten vorgestellt – Niob dotierte TiO2 Nanoröhren. Abschließend wird eine auf TiO2 Nanoröhren basierende Tandem Farbstoff Solarzelle elektrochromen Zelle vorgestellt. Durch eine höhere Lichtintensität auf der Farbstoff Solarze wird mehr Leistung generiert. Diese Leistung wird direkt auf eine elektrochrome Zelle übertragen, die dadurch nach Erreichen eines kritischen Spannungswertes abdunkelt. Eine Verminderung der Lichtintensität wiederum senkt die Leistungsabgabe der Farbstoff Solarze und die elektrochrome Zelle wird wieder transparent

ASPECTE DE RETELLIZARE EDUCAŢIONALĂ A TEXTULUI

În articol este abordată problematica retellizării ca fenomen general, dar şi ca unul specific în valorificarea educaţio­nală a textului, pornind de la diverse constatări din ştiinţele informaticii, matematicii, logicii etc.Formulând ideea că textul presupune o anumită ordine a părţilor, este analizat conceptul de formă, cel de sistem şi de gândire în sistem, de limbaj, interpretare etc., corelate semnificativ cu cel de educaţie textuală. Este desemnat analitic principiul retellizării şi rolul lui în vectorizarea calităţii rezultatelor educaţionale, fiind demonstrată importanţa retellizării în cunoaştere şi învăţare, în perceperea textului şi a lumii.ASPECTS OF EDUCATIONAL TEXT RETELLIZATIONThe article addresses the issue retellization as a general phenomenon and as one specific educational capitalization of the text from various findings, from computer science, mathematics, logic etc.By formulating the idea that the text requires a certain order to the parties, considering the concept of form, the system and the system of thought, language, interpretation, etc., correlated significantly with the summary education. Retellization analytical principle is designated and its role in the quality educational outcomes of the vectorization (vector potential). It illustrates the importance of knowledge and learning retellization in the perception of the text and the world.</p