Defect complexes interplay and its influence on the hyperfine structure of hydrogenated TiO2

Determination of factors, that govern the kinetics of photocatalytic processes and the knowledge of their roles in transition metal oxides is a critical component to consider when devising efficient materials. Previous studies have shown that point defects, such as substitutional atoms, interstitials and vacancies account remarkably for the electronic structure and chemical properties, that influence the performance of these material in various applications. The expectation that a fundamental understanding of such defects will benefit to elucidate the influence that they have on the system’s functionality is the driving force for pursuing experimental and theoretical research on doped and reduced materials. This dissertation aims to ascertain how electronic structure of rutile and anatase TiO2, which is considered the model system in transition metal oxides, changes when the material contains even subtle amount of dopants (H, Fe and Cd) and point defects. Consequently, time-differential y-y perturbed angular correlation technique, along with Mössbauer spectroscopy and tracer diffusion studies have been applied and mostly complemented with theoretical studies and standard techniques. Furthermore, the author presents a newly developed emission Mössbauer set-up (eMIL), which has been developed and constructed during the current study. Obtained results demonstrate that dopant behaviour is not straight-forward. In monocrystalline rutile, Cd resides not only at the cationic site, but also at impurity-vacancy configurations near the probe. The probe in both environments seem to withstand high temperature annealing/measurements, although associated with alterations in fractions ratio. During emission Mössbauer studies, thin films of anatase show the temperature dependent behaviour throughout the whole temperature range. This results in two annealing stages caused by the vacancy movements and their interaction with Ti interstitials. Substitutional Fe3+, at Ti sites showing spin-lattice relaxation transforms upon hydrogenation, implying that hydrogen behaves as a shallow donor. eMS experiments performed in a temperature range of 300 - 700 K reveal that vacancies and their agglomerations may govern the hydrogen motion. Further perturbed angular correlation studies at the temperature when the hydrogen motion starts, show that depending on the hydrogenation degree the dopant (hydrogen) acts differently and could form a unique coupling with Cd. Hydrogenation for prolonged times demonstrates that doping/reduction is followed by the recovery processes. The current work clearly indicates that experimental techniques based on hyperfine interactions can unravel a wealth of information about the nature and behaviour of defects in transition metal oxides, that may be readily evaluated and complemented with detailed density functional theory calculations.Die Bestimmung von Faktoren, welche die Kinetik von photokatalytischen Prozessen beeinflussen und das Nachvollziehen von deren Funktion in Übergangsmetalloxiden, ist das Hauptthema beim Design von effizienten Materialien. Vorhergehende Studien haben gezeigt, dass Punktdefekte wie zum Beispiel Substitutionsatome, Zwischengitteratome und Leerstellen einen wesentlichen Beitrag zur Bandstruktur und zu den chemischen Eigenschaften eines Materials beitragen. Die Veränderung dieser Eigenschaften beeinflusst die Performance von Materialien in gewissen Anwendungsbereichen stark. Die Erwartung, dass ein fundamentales Verständnis von solchen Gitterdefekten positiv zur Aufklärung von deren Wirkung auf die Eigenschaften eines Materials beiträgt, ist die treibende Kraft hinter theoretischer und praktischer Forschung an dotierten und reduzierten Materialien. Das Ziel dieser Dissertation ist die Veränderung der elektronischen Struktur von Rutil und Anatas TiO2, welche die Modellsysteme von Übergangsmetalloxiden darstellen, unter der Beeinflussung von niedrigen Dotierungen mit Wasserstoff, Eisen und Cadmium sowie den Einfluss von Punktdefekten, zu untersuchen. Hierfür wurden Untersuchungen mir der Gestörte y-y Winkelkorrelation (englisch: time-differential perturbed angular correlation oder PAC) Methode, gemeinsam mit Mössbauer Spektroskopie und Tracer Diffusionsexperimenten durchgeführt, welche durch theoretische Studien und Untersuchungen mit Standardmethoden ergänzt wurden. Des Weiteren präsentiert der Autor einen neuen Versuchsaufbau zur Durchführung von Emissions-Mössbauer Messungen (eMIL), welches während dieser Arbeit entwickelt und konstruiert wurde. Die beobachteten Ergebnisse zeigen, dass das Dotandenverhalten nicht leicht voraussehbar ist und das Cd als Dotand in monokristallinem Rutil nicht nur am Kationenplatz sitzt, sondern dass Anteile davon auch andere Umgebungen, die von Leerstellenkonfigurationen beeinflusst werden, besetzen. Beide entdeckten Probenumgebungen scheinen hohen Temperaturen zu wiederstehen, allerdings sind Änderungen im Anteilverhalten sichtbar. Bei Emissions-Mössbauer Messungen zeigen dünne Schichten von Anastase ein temperaturabhängiges Verhalten im gesamten Messbereich. Dies hat zur Folge, dass zwei Anlass Stufen entstehen, welche durch Leerstellenbewegung und deren Interaktion mit Ti Zwischengitteratomen hervorgerufen werden. Die Oxidationsstufe Fe3+, welche eine Spin-Gitter-Relaxation zeigt, ändert sich durch Hydrierung. Dies impliziert, dass Wasserstoff ein sogenannter flacher Donator ist. Experimente in einem bestimmten Temperaturbereich zeigen, dass Leerstellen und Leerstellenansammlungen die Wasserstoffbewegung beeinflussen. Weitere PAC Experimente an den Temperaturwerten, an denen die Wasserstoffbewegung startet, zeigen eine Abhängigkeit des Verhaltens des Dotanden (Wasserstoff), zum Beispiel eine Kopplung mit Cd, von der Hydrierungsstärke. Längere Hydrierungsdauern zeigen, dass nach Dotierungs- bzw. Reduktionsprozessen Erholungseffekte folgen können. Die vorliegende Arbeit zeigt deutlich, dass die Methoden der Hyperfeinwechselwirkungen Informationen über die Natur und das Verhalten von Defekten in Übergangsmetalloxiden bereitstellen können. Diese Ergebnisse können einfach evaluiert und mit detaillierten Dichtefunktionaltheoriesimulationen verglichen werden

eMIL: advanced emission Mössbauer spectrometer for measurements in versatile conditions

The current work presents a contemporary design of an advanced emission Mössbauer Spectrometer: eMIL equipped with a parallel-plate avalanche detector, which has been devised and built for the Mössbauer collaboration at ISOLDE/CERN. The setup is based on emission geometry, combined with on-line/off-line isotope implantation and provides numerous advantages over conversion electron, common emission (where isotope is deposited chemically on a sample) or transmission Mössbauer spectroscopy. eMIL is designed to measure hyperfine interactions in solids under various exposures. The implemented design overcomes limitations and improves performance and handling. In the current revision, the chamber is supplied with an UV extension — allowing to perform studies of photo-catalytic materials under external light exposure. A specifically designed motorized lid-samples-holder is fully automatized, and makes it possible to study up to 4 samples loaded in a magazine within a temperature range from RT up to 1100 K and to perform angular dependent measurements in high vacuum. This work additionally briefly describes data acquisition with additional electronic blocks, vacuum and data-acquisition system construction

Investigation of the local environment of SnO2 in an applied magnetic field

This paper presents the results of time-differential perturbed gamma–gamma angular correlation measurements of SnO2 thin films carried out in an applied magnetic field. The measurements were performed upon the implantation of Fe at 80 keV and 111In (111Cd) at 160 keV. The samples were further characterized by energydispersive X-ray spectroscopy. The hyperfine parameters were studied at room temperature with and without an applied magnetic field. The results indicate the presence of two distinct local environments for the probe nuclei. Both occupy a paramagnetic state and correspond to a substitutional Sn site in the rutile phase of SnO2 with different numbers of electrons added to SnO2:Cd0. In addition, the crystal homogeneity of the site 1 increases upon applying the magnetic field

Confirming the unusual temperature dependence of the electric-field gradient in Zn

The electric-field gradient (EFG) at nuclei in solids is a sensitive probe of the charge distribution. Experimental data, which previously only existed in insulators, have been available for metals with the development of nuclear measuring techniques since about 1970. An early, systematic investigation of the temperature dependence of the EFG in metals, originally based on results for Cd, but then also extended to various other systems, has suggested a proportionality to T 3/2 . However, later measurements in the structurally and electronically similar material Zn, which demonstrated much more complex behavior, were largely ignored at the time. The present experimental effort has confirmed the reliability of this unexpected behavior, which was previously unexplained

A hyperfine look at titanium dioxide

Titanium dioxide is a commonly used material in a wide range of applications, due to its low price, and the increasing demand for it in the food- and pharmaceutical industries, and for low- and high-tech applications. Time-differential perturbed angular correlation (TDPAC) and Mössbauer spectroscopy measurements have a local character and can provide important and new information on the hyperfine interactions in titanium dioxide. With the application of characterization techniques and radioactive beams, these methods have become very powerful, especially for the determination of temperature dependence of hyperfine parameters, even at elevated temperatures. Such measurements lead to a better understanding of lattice defects and irregularities, including local environments with low fractions of particular defect configurations that affect electric quadrupole interactions. At ISOLDE-CERN, physicists benefit from the many beams available for the investigation of new doping configurations in titanium dioxide. We report the annealing study of titanium dioxide by means of the time differential perturbed γ-γ angular correlation of 111mCd/111Cd in order to study the possible effects of vacancies in hyperfine parameters. This paper also provides an overview of TDPAC measurements and gives future perspectives

Compositional dependence of epitaxial L10-Mnx Ga magnetic properties as probed by 57Mn/Fe and 119In/Sn emission Mössbauer spectroscopy

The magnetic properties of Mn x Ga alloys critically depend on composition x, and the atomic-scale origin of those dependences is still not fully disclosed. Molecular beam epitaxy has been used to produce a set of Mn x Ga samples (x = 0.7 ÷ 1.9) with strong perpendicular magnetic anisotropy, and controllable saturation magnetization and coercive field depending on x. By conducting 57Mn/Fe and 119In/Sn emission Mössbauer spectroscopy at ISOLDE/CERN, the Mn and Ga site-specific chemical, structural, and magnetic properties of Mn x Ga are investigated as a function of x, and correlated with the magnetic properties as measured by superconducting quantum interference device magnetometry. Hyperfine magnetic fields of Mn/Fe (either at Mn or Ga sites) are found to be greatly influenced by the local strain induced by the implantation. However, In/Sn probes show clear angular dependence, demonstrating a huge transferred dipolar hyperfine field to the Ga sites. A clear increase of the occupancy of Ga lattice sites by Mn for x > 1 is observed, and identified as the origin for the increased antiferromagnetic coupling between Mn and Mn at Ga sites that lowers the samples' magnetization. The results shed further light on the atomic-scale mechanisms driving the compositional dependence of magnetism in Mn x Ga

Anisotropy of the electric field gradient in two-dimensional alpha-MoO_(3) investigated by (57)^Mn((57)^Fe) emission mossbauer spectroscopy

Van der Waals alpha-MoO_(3) samples offer awide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mossbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing alpha-MoO_(3) samples. Although alpha-MoO3 is a twodimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5-5-mu m thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of (57)^Mn (t(1/ 2) = 1.5 min), which decays to the (57)^Fe, 14.4 keV Mossbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe^(2+) probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and weak out-of-plane van derWaals interactions in the 2D material. Moreover, a second broad component, D2, can be assigned to Fe^(3+) defects that are dynamically generated during the online measurements. The results are compared to ab initio simulations and are discussed in terms of the in-plane and out-of-plane interactions in the syste

Temperature Dependence of the Hyperfine Magnetic Field at Fe Sites in Ba-Doped BiFeO3 Thin Films Studied by Emission Mössbauer Spectroscopy

Emission 57Fe Mössbauer spectroscopy (eMS), following the implantation of radioactive 57Mn+ ions, has been used to study the temperature dependence of the hyperfine magnetic field at Fe sites in Ba-doped BiFeO3 (BFO) thin films. 57Mn β decays (t1/2 = 90 s) to the 14.4 keV Mössbauer state of 57Fe, thus allowing online eMS measurements at a selection of sample temperatures during Mn implantation. The eMS measurements were performed on two thin film BFO samples, 88 nm and 300 nm thick, and doped to 15% with Ba ions. The samples were prepared by pulsed laser deposition on SrTiO3 substrates. X-ray diffraction analyses of the samples showed that the films grew in a tetragonal distorted structure. The Mössbauer spectra of the two films, measured at absorber temperatures in the range 301 K–700 K, comprised a central pair of paramagnetic doublets and a magnetic sextet feature in the wings. The magnetic component was resolved into (i) a component attributed to hyperfine interactions at Fe3+ ions located in octahedral sites (Bhf); and (ii) to Fe3+ ions in implantation induced lattice defects, which were characterized by a distribution of the magnetic field BDistr. The hyperfine magnetic field at the Fe probes in the octahedral site has a room temperature value of Bhf = 44.5(9) T. At higher sample temperatures, the Bhf becomes much weaker, with the Fe3+ hyperfine magnetic contribution disappearing above 700 K. Simultaneous analysis of the Ba–BFO eMS spectra shows that the variation of the hyperfine field with temperature follows the Brillouin curve for S = 5/2.This work has received the financial support from the Federal Ministry of Education and Research (BMBF) through grants 05K16PGA, 05K22PGA, 05K16SI1, 05K19SI1 “eMIL” and “eMMA”; from the European Union’s Horizon 2020 Framework research and innovation program under grant agreement no. 654002 (ENSAR2) and 101057511 (EURO-LABS); from the Ministry of Economy and Competitiveness Consolider—Ingenio Project CSD2009 0013 “IMAGINE” Spain, and Banco Santander-UCM, project PR87/19-22613; from the Austrian Science Fund (FWF) through projects P26830 and P31423, from the Icelandic University Research Fund; from the National Research Foundation (South Africa); and from the Ministry of Economy and Competitiveness (MINECO/FEDER) for the Grant No. RTI2018-094683-B-C55 C55 and Basque Government Grant No. IT-1500-22

Anisotropy of the electric field gradient in two-dimensional α-MoO3 investigated by 57Mn(57Fe) emission Mössbauer spectroscopy

Van der Waals α-MoO3 samples offer a wide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mössbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing α-MoO3 samples. Although α-MoO3 is a two-dimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5-5-µm thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of 57Mn (t1/2 = 1.5 min), which decays to the 57Fe, 14.4 keV Mössbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe2+ probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and weak out-of-plane van der Waals interactions in the 2D material. Moreover, a second broad component, D2, can be assigned to Fe3+ defects that are dynamically generated during the online measurements. The results are compared to ab initio simulations and are discussed in terms of the in-plane and out-of-plane interactions in the system

Defect Complexes Interplay and its Influence on the Hyperfine Structure of Hydrogenated TiO2_2

Determination of factors, that govern the kinetics of photocatalytic processes and the knowledge of their roles in transition metal oxides is a critical component to consider when devising efficient materials. Previous studies have shown that point defects, such as substitutional atoms, interstitials and vacancies account remarkably for the electronic structure and chemical properties, that influence the performance of these material in various applications. The expectation that a fundamental understanding of such defects will benefit to elucidate the influence that they have on the system’s functionality is the driving force for pursuing experimental and theoretical research on doped and reduced materials. This dissertation aims to ascertain how electronic structure of rutile and anatase TiO$_2$, which is considered the model system in transition metal oxides, changes when the material contains even subtle amount of dopants (H, Fe and Cd) and point defects. Consequently, time-differential $\gamma-\gamma$ perturbed angular correlation technique, along with Mössbauer spectroscopy and tracer diffusion studies have been applied and mostly complemented with theoretical studies and standard techniques. Furthermore, the author presents a newly developed emission Mössbauer set-up (eMIL), which has been developed and constructed during the current study. Obtained results demonstrate that dopant behaviour is not straight-forward. In monocrystalline rutile, Cd resides not only at the cationic site, but also at impurity-vacancy configurations near the probe. The probe in both environments seem to withstand high temperature annealing/measurements, although associated with alterations in fractions ratio. During emission Mössbauer studies, thin films of anatase show the temperature dependent behaviour throughout the whole temperature range. This results in two annealing stages caused by the vacancy movements and their interaction with Ti interstitials. Substitutional Fe$^{3+}$, at Ti sites showing spin-lattice relaxation transforms upon hydrogenation, implying that hydrogen behaves as a shallow donor. eMS experiments performed in a temperature range of 300 - 700 K reveal that vacancies and their agglomerations may govern the hydrogen motion. Further perturbed angular correlation studies at the temperature when the hydrogen motion starts, show that depending on the hydrogenation degree the dopant (hydrogen) acts differently and could form a unique coupling with Cd. Hydrogenation for prolonged times demonstrates that doping/reduction is followed by the recovery processes. The current work clearly indicates that experimental techniques based on hyperfine interactions can unravel a wealth of information about the nature and behaviour of defects in transition metal oxides, that may be readily evaluated and complemented with detailed density functional theory calculations