Spectroscopic Studies of Surface Chemistry on Cerium Oxide

Fundamental studies of three C1 molecules (CO, CO2, and CH3OH/CD3OD) adsorption on surfaces of ceria single crystals and/or nanocrystals have been carried out by using ultra-high vacuum Fourier transform infrared spectroscopy (UHV-FTIRS) or near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The oxidation states of cerium cations were determined by the core-level and valence band X-ray photoelectron spectroscopy (XPS)

Compositional Control and Catalytic Activity of Sodium Bismuth Titanate Materials

Piezoelectric materials, such as BaTiO3 (BT), are of considerable research interest owing to their capacity to couple electrical input/output with mechanical displacement. Materials based on Na0.5Bi0.5TiO3 (NBT) are environmentally friendly lead-free piezoelectric materials that exhibit electrical and piezoelectric properties that are sensitive to nonstoichiometry. Compositional regulation of the stoichiometric ratios is used widely to control the phase assemblage, defect equilibria, and resultant applications. NBT-based materials also have been reported to offer dual photocatalytic and piezocatalytic functionality for degradation of organic pollutants, catalytic water splitting, CO oxidation, and CO2 reduction. The present work is comprised of four discrete sections that report investigations of the effects of compositional regulation on NBT and the effects of defects on BaTiO3. The NBT was synthesized by solid-state reaction at 1000°C for 24 h (phase equilibria) and hydrothermal synthesis at 180°C for 24 h (Bi/Ti and Na/Ti compositional control) while defects in BT were induced in commercial material by the application of different atmospheres at 800°C for 12 h (comparable study). Characterisation included XRD, Raman, SEM, TEM, AFM, PFM/KPFM, XPS/ARXPS, ICP-MS, DSC/TGA, and UV/Vis spectrophotometry. Testing included various approaches to photocatalysis, piezocatalysis, and photoelectrochemistry. In chapter 4 Two revised binary phase diagrams and a complete isothermal section of the ternary system Na2O-Bi2O3-TiO2 at 1000°C were constructed on the basis of (1) comprehensive literature survey of published phases diagrams, (2) experimental XRD and SEM data, and (3) thermodynamic calculations. The latter were enabled by the generation of new thermodynamic data for the twelve phases for which these data were unavailable. In chapte 5 The study on the effects of variable Bi/Ti ratio during hydrothermal synthesis were conducted. As the Bi/Ti ratio decreased, there was a noticeable shift in the phase composition from principally BO (Bi2O3) to BTO (Bi4Ti3O12) to NBT. Additionally, the principal grain shapes altered from equiaxed to platy to spherical. Samples with Bi/Ti ratio of 12/12 exhibited the highest methylene blue (MB) photodegradation of 92% under UV irradiation for 2 h. This excellent photocatalytic performance was attributed to the formation of a heterojunction of BTO on NBT with advantageous band alignment. In chapter 6 Alteration of the Na/Ti ratio through control of the NaOH precursor concentration (2.5 M, 7.5 M, 12.5 M) revealed the formation of a spherical secondary phase Na0.5Bi4.5Ti4O15 (NBT4) dispersed in a continuous matrix of NBT, resulting in the formation of a type-Ⅱ heterojunction (confirmed by DFT modelling). Increasing NaOH concentration facilitated substitution of Bi3+ by Na+ in the NBT solid solution, resulting in the formation of charge-compensating oxygen vacancies (["V" _"O" ^"••" ]). The resultant energy band structure was favorable for the hydrogen evolution reaction (HER). Piezo/photocatalytic hydrogen production showed very efficient HER rates up to 140 μmol/g/h for DI water, 68 μmol/g/h for simulated seawater, and 58 μmol/g/h for natural seawater. In chapter 7 BT was used in a comparable study, albeit for a commercial product whose defect states were enhanced through exposure at O2, N2, Ar, or H2. Optimal results for maximal reduction by H2 were obtained. ARXPS revealed the formation of graded surface-to-bulk reduction, which exposed for the first time a discrete interface between the surface and subsurface. The associated "V" _"O" ^"••" formation resulted in the establishment of an energy band structure favorable to the HER by piezo/photocatalysis, yielding rates up to 132.4 μmol/g/h for DI water, 63.4 μmol/g/h for simulated seawater, and 48.7 μmol/g/h for natural seawater

Atomic Level Computational Studies of Ionic Defects and Transport Properties of Solid State Ionic Conductors

Solid state ionic conductors (or electrolytes) are a vital component for electrochemical devices or systems for chemical and energy transformation. The chemical composition, crystal structure, defects, morphology, and electronic structure of these materials greatly affect their electrochemical properties such as ionic and electronic conductivity. Similar to barium zirconate (BaZrO3), barium hafnate (BaHfO3) is one of the most promising proton-conducting electrolytes for solid oxide fuel cells (SOFCs) because of their high proton conductivity at 400~700 °C. In this study, I have investigated dopant solubility, proton concentration, mobility, and chemical stability of A/B-site co-doped BaHfO3 using density functional theory calculations coupled with statistical thermodynamics. Specifically, I have calculated defect formation energy in charged supercells, finite temperature vibrational energy via phonon calculations in the harmonic approximation, proton migration energy via transition state theory, and defect-defect interactions via cluster-expansion method. A wide range of relevant properties are predicted, including the degree of hydration governed by hydration Gibbs free energy, proton diffusion coefficient derived from proton migration barrier search, and defect-defect interactions using cluster expansion method. These properties are sensitive to the type and amount of chemical dopants in the lattice, including Li, Na, K, Rb, and Cs on A-site and Sc, Y, La, Gd, Lu, Al, Ga, and In on B-site. The mismatch in the size of the dopant and the host ion induces local strain or elastic interactions. However, the electrostatic interactions between them are much less dependent on the ionic radius of dopant ions. Accordingly, the dependence of the dopant-proton binding energy on ionic radius of dopant has a “volcano” shape. In addition, the electronegativity of dopant ions also affect the affinity of acceptor-type dopants with donor-type protons. Hydration is promoted by both the A-site and the B-site dopants, although the effect of the latter is less pronounced. In general, a “trade-off” relation between proton concentration and mobility is observed in all cases, regardless of the ionic radius or the lattice site (A- or B-site) of the dopants. Defects play an important role in ionic transport and in enhancing catalytic activities for chemical and energy transformation processes. Thus, it is crucial to understand how to effectively enhance ionic transport by rationally design preferred defect structures, including 0D (point defects such as vacancies), 1D (dislocation), and 2D (grain boundary) defects. For example, local ion segregation may result in a space charge region, leading to accumulation of mobile charge carriers or improved mobility near those 1D/2D defects. The effect of the space charge layer, strain near 1D/2D defects, as well as collective defect-defect interactions pose an extreme challenge for both experiments and computations. In this study, the effect of an edge dislocation in Y:BaZrO3 on oxygen ion transport is evaluated. To probe the ion mobility, a reactive molecular dynamics simulation based on ReaxFF is utilized to simulate the super-large Y:BaZrO3 supercell with two edge dislocations. Radial distribution functions and thermal/chemical expansion coefficients are used to benchmark the local and global structure properties, and mean-square displacements are used to calculate diffusivity and conductivity. Dislocation is found to lower the activation energy of ionic transport, possibly because of distinct oxygen cage structures locally at the dislocation core. However, optimal Y% for oxygen ion conductivity is shifted to higher levels with increasing temperature. This could be due to the weakening of Y’s electrostatic “trapping effect”. Besides materials chemistry and microstructural features, the mechanical strain is another factor affecting ionic properties. Ceria (or CeO2) is a prototypical ionic material for catalyst and electrolyte applications. Chemo-mechanical coupling in ceria significantly affect the bulk defect properties of ceria. In this study, the effect of chemo-mechanical coupling is extended from the bulk to the (111) surface of ceria. There have been extensive theoretical and experimental research on the configurations of vacancies and polarons on the (111) surface, the dominantly exposed surface, which is crucial to surface catalytic activity. It was reported that surface oxygen vacancy on ceria’s (111) surface is not necessarily the most stable vacancy; however, the sub-surface vacancy could be. Similarly, polarons are not necessarily at the 1st-nearest-neighbor (1NN) of the corresponding vacancy either; they could be at the 2nd-nearest-neighbor (2NN). All those counter-intuitive phenomena were unveiled and validated both theoretically and experimentally. Inspired by previous research, I have identified a unique way of tuning defect configurations by applying tensile and compressive epitaxial strain on (111) slab. Across the magnitude of the applied strain from -5% compression to +5% tension, stability relationships of the surface vs. the sub-surface vacancy, the 1NN vs. the 2NN polaron, and the vacancy monomer vs. the dimer are surprisingly reversed. Elastic, electrostatic and electronic excitation energies are found to be dependent on defect-configuration. This gives us a new perspective to interpret the various vacancy patterns observed on (111) surface of the prepared ceria samples.Ph.D

Annual Report 2021 - Institute of Ion Beam Physics and Materials Research

The year 2021 was still overshadowed by waves of the COVID-19 pandemic, although the arrival of efficient vaccinations together with the experience of the preceding year gave us a certain routine in handling the situation. By now the execution of meetings in an online mode using zoom and similar video conference systems has been recognized as actually being useful in certain situations, e.g. instead of flying across Europe to attend a three-hours meeting, but also to be able to attend seminars of distinguished scientists which otherwise would not be easily accessible. The scientific productivity of the institute has remained on a very high level, counting 190 publications with an unprecedented average impact factor of 8.0. Six outstanding and representative publications are reprinted in this Annual Report. 16 new third-party projects were granted, among them 7 DFG projects, but very remarkably also an EU funded project on nonlinear magnons for reservoir computing with industrial participation of Infineon Technologies Dresden and GlobalFoundries Dresden coordinated by Kathrin Schultheiß of our Institute. The scientific success was also reflected in two HZDR prizes awarded to the members of the Institute: Dr. Katrin Schultheiß received the HZDR Forschungspreis for her work on “Nonlinear magnonics as basis for a spin based neuromorphic computing architecture”, and Dr. Toni Hache was awarded the Doktorandenpreis for his thesis entitled “Frequency control of auto-oscillations of the magnetization in spin Hall nano-oscillators”. Our highly successful theoretician Dr. Arkady Krasheninnikov was quoted as Highly Cited Researcher 2021 by Clarivate. The new 1-MV facility for accelerator mass spectrometry (AMS) has been ordered from NEC (National Electrostatics Corporation). Design of a dedicated building to house the accelerator, the SIMS and including additional chemistry laboratories for enhanced sample preparation capabilities has started and construction is planned to be finished by mid 2023, when the majority of the AMS components are scheduled for delivery. In the course of developing a strategy for the HZDR - HZDR 2030+ Moving Research to the NEXT Level for the NEXT Gens - six research focus areas for our institute were identified. Concerning personalia, it should be mentioned that the long-time head of the spectroscopy department PD Dr. Harald Schneider went into retirement. His successor is Dr. Stephan Winnerl, who has been a key scientist in this department already for two decades. In addition, PD Dr. Sebastian Fähler was hired in the magnetism department who transferred several third-party projects with the associated PhD students to the Institute and strengthens our ties to the High Magnetic Field Laboratory, but also to the Institute of Fluid Dynamics. Finally, we would like to cordially thank all partners, friends, and organizations who supported our progress in 2021. First and foremost we thank the Executive Board of the Helmholtz-Zentrum Dresden-Rossendorf, the Minister of Science and Arts of the Free State of Saxony, and the Ministers of Education and Research, and of Economic Affairs and Climate Action of the Federal Government of Germany. Many partners from universities, industry and research institutes all around the world contributed essentially, and play a crucial role for the further development of the institute. Last but not least, the directors would like to thank all members of our institute for their efforts in these very special times and excellent contributions in 2021

Engineering Rare-Earth Based Color Centers in Wide Bandgap Semiconductors for Quantum and Nanoscale Applications

For many years, atomic point-defects have been readily used to tune the bulk properties of solid-state crystalline materials, for instance, through the inclusion of elemental impurities (doping) during growth, or post-processing treatments such as ion bombardment or high-energy irradiation. Such atomic point-defects introduce local ‘incompatible’ chemical interactions with the periodic atomic arrangement that makes up the crystal, resulting for example in localized electronic states due to dangling bonds or excess of electrons. When present in sufficient concentrations, the defects interact collectively to alter the overall bulk properties of the host material. In the low concentration limit, however, point-defects can serve as interesting nanoscale or quantum objects, giving rise to localized magnetic moments (unpaired electrons) or addressable optical and electronic transitions otherwise absent in the bulk material. These effects have been particularly promising in semiconductors and insulators, where isolated point-defects can behave as optically addressable ‘artificial atoms’ with narrow-band and tunable single-photon emission, and remarkable sensitivities to local fluctuations in temperature, magnetic or electric fields, strain, and/or surface effects, even at room temperature. Such atomic point-defects are naturally present in any semiconductor, but not all of these are optically active within the electromagnetic regions of interest (known as defect-related color centers), while typically only a fraction of them feature addressable unpaired electrons (an important element for nanoscale sensing and/or quantum applications). Further, identifying their underlying atomic structures is challenging and required to generate them on demand. The work presented in this thesis aims at contributing to the collective efforts of studying and identifying native defect-related color centers in relevant wide bandgap semiconductors, while providing an alternative approach towards engineering defect-related color centers with interesting optoelectronic properties for quantum and nanoscale applications. The approach towards defect engineering presented in this work builds on the well-known, robust, and high-quality atomic-like properties of the rare-earth ions, such as cerium and erbium, and the material advantages that 2-dimensional wide bandgap semiconductors, such as hexagonal boron nitride and tungsten disulfide, offer for device integration. First, confocal Raman and fluorescence spectroscopy techniques are combined to isolate single native color centers in cubic boron nitride nanocrystals that feature room-temperature narrow-band single-photon emission within the visible. Secondly, fluorescence and X-ray photoelectron spectroscopy are combined with first-principles calculations as an alternative approach towards the identification of point-defects and impurities present in highly fluorescent hexagonal boron nitride thin flakes. Lastly, cerium-doped hexagonal boron nitride and erbium-doped tungsten disulfide are studied via spectroscopic and first-principles techniques and proposed as alternative material platforms for nanoscale and quantum applications based on engineered rare-earth related color centers. Further considerations towards material integration, photonic interactions with small rare-earth based color center ensembles, and improved techniques for quantitative computational descriptions of rare-earth based color centers are also discussed

The Role of Graphene in Improving Photocatalytic Properties of Immobilized AgCl Photocatalysts and Biomimetic High Valent Iron-oxo Catalysts: Preparation, Characterisation and Evaluation towards Contaminants Degradation

Advanced oxidation processes have been employed to address environmental contamination issues related to the presence of synthetic industrial organic chemicals, pesticides, pharmaceutical and personal care products (PPCPs). Both photocatalysis and biomimetic catalysis have attracted great interest in this area due to their green catalyst credentials of sustainability, reusability and recyclability. Plasmonic photocatalysts consisting of silver nanoparticles anchored to silver halides (Ag@'AgX) have been increasingly investigated as potentially highly efficient visible-light photocatalysts. Enhanced performance of Ag@AgX has been observed when supported on graphene-based materials as graphene has zero banggap, is oxidatively robust, has high electron conductivity and high specific surface area. However, the relationship between catalytic performance and structure change during use for the three components AgNPs, AgCI and partially reduced graphene oxide (rGO) in the assemblages has not been previously elucidated. Strategies to control the identified problem of photo-corrosion have been explored, including wavelength adjustment, graphene oxide (GO) loading, GO reduction methods, photo-reduction period, ferric iron doping and cocatalyst modification using iron oxide (FeOx). The influence of various reaction conditions including oxygen concentration, pH, ionic strength and substrate concentration were also examined in order to understand their role in the process. The integration of photoactive and/or redox active building blocks onto carbon materials to yield multifunctional electron donor/acceptor conjugates has also recently attracted extensive interest. A fundamental understanding of how factors such as coupling with or without a linker and how the incorporation of various transition metal complexes affect the electron transfer in these systems is essential to the optimal design of "smart" molecular interfaces on the "green" catalyst. This work focused on the preparation, characterisation and evaluation of both covalently and non-covalently anchored redox active non-heme iron complexes on graphene-based nanomaterials, with the phenolate-hinged ligand 2,6- bis{[bis(2-pyridylmethyl)amino]methyl}-phenolato(1-) (denoted as bpbp in this work) and the tetraamido macrocyclic ligand (TAML) considered using various immobilization strategies (such as a microwave­assisted in situ diazonium grafting method and a normal heating-assisted diazonium grafting method). This work has progressed the development of novel sustainable catalysts for the remediation, transformation and mineralization of contaminants in the environment

Origins of limited electrical performance of polycrystalline Cu2O thin-film transistors

In this thesis, cuprous oxide Cu2O was investigated concerning its ability to function as ptype channel in thin-film transistors. The material was chosen for its promising electronic characteristics as bulk single crystal. In order to be competitive with other technological solutions for flexible thin-film electronics, the temperature during fabrication has to remain below 200 C. Following this approach, a tremendous gap between potential and actual electrical performance of Cu2O thin-film transistors is encountered. The aim of this thesis is to show the reasons for this discrepancy. Relevant stages during the fabrication process of a thin-film transistor were analyzed with respect to their impact on the cation oxidation state. These stages included thin film deposition, the study of interface formation to the dielectric layers as well as postdeposition annealing. Semiconducting and dielectric layers were deposited by reactive magnetron sputtering (Cu2O, Cu4O3, CuO, Bi2O3, Al2O3) and atomic layer deposition (Al2O3). An innovative approach for a thickness-dependent characterization of thin films was conducted by a combination of in situ X-ray photoelectron spectroscopy with in situ conductance measurement. Electrical properties of Cu2O films and thin-film transistors were analyzed in dependence of film thickness, temperature, oxygen partial pressure and time. It is shown, that the primary cause for the limited electrical performance is the polycrystalline morphology in conjunction with the material-inherent tendency to oxidation and reduction of the metal cation. On the one hand, metallic Cu(0) depletes the material from hole carriers and causes Fermi level pinning. On the other hand, a high conductivity in the grain boundary is caused by the presence of Cu(II). A model is presented to describe the conductivity at different film thicknesses as a function of grain size

Commemorative Issue in Honor of Professor Karlheinz Schwarz on the Occasion of His 80th Birthday

A collection of 18 scientific papers written in honor of Professor Karlheinz Schwarz's 80th birthday. The main topics include spectroscopy, excited states, DFT developments, results analysis, solid states, and surfaces