Spektroskopske raziskave kisikovih vrzeli v CeO[spodaj]2

A unique material, ceria (CeO2), which is widely applied in automobile exhaust catalysts, is functional due to presence of defects in its crystal structure. Furthermore, the structural defects dictate electrical and chemical properties of ceria. The creation of intrinsic oxygen vacancies in ceria is responsible for oxygen-ion conductivity in solid oxide fuel cells. This unfolds the keen interest in ceria defects. Using the analytical technique cathodoluminescence spectroscopy (CLS) we can characterize ceria for its band gap and the defect states within the band gap. Since CLS has a high spatial resolution, high sensitivity to low concentration of defects and ability to obtain depth resolved information it is an obvious technique of choice. The first part of the thesis is an introduction to the topic and description of the experimental techniques. Importance of ceria as a multifaceted material finding applications in areas spanning from energy production and conversion to biomedical applications is detailed. CLS as a tool to understand defect-related optical properties and advancement in the CL detection systems are discussed. To study the relationship between local structure and its impact on CL emission spectra, an X-ray absorption spectroscopy techniques were used. The X-ray absorption near edge structure (XANES) and the Extended x-ray absorption fine structure (EXAFS) techniques are summarized. The second part discusses CL emission from ceria. Initially, CL emission from reduced ceria and its dependence on oxygen vacancy concentration are presented. The origin of emission was attributed to different configurations of the oxygen vacancies and polarons. The recent F center description in ceria was adopted here. The intriguing observation of CL emission quenching as a function of oxygen vacancy concentration was explained on the basis of a relative change in population of F centers in ceria. This demonstrated the relevance of local structure for the CL emission in ceria. In order to have a better understanding of the system, La-doped ceria was proposed as a model system. A precise control over the stoichiometry helped to achieve a desired oxygen vacancy concentration. The CL emission behavior, as observed in reduced ceria, was replicated in the case of La-doped ceria and the analysis revealed that F+ centers favor CL emission whereas F0 centers are disadvantageous. The local structure investigation using EXAFS analysis of both cations Ce and La (K-Edge) showed distortion from the fluorite symmetry and corroborated the F center description of oxygen vacancies in ceria. Our results provide an experimental evidence for F center description involving oxygen vacancies and polarons.Uporaba cerijevega dioksid (CeO2) kot katalizatora v avtomobilskih izpušnih sistemih je pogojena s prisotnostjo napak v njegovi kristalni strukturi. Te napake določajo električne in kemične lastnosti tega materiala. Prisotnost lastnih kisikovih vrzeli v materialu je odgovorna za prevodnost kisikovih ionov v trdnih oksidnih gorivnih celicah. Z uporabo katodoluminescenčne spektroskopije (CLS) lahko karakteriziramo energijsko vrzel cerijevega dioksida in dodatne pasove znotraj energijske vrzeli, ki nastanejo zaradi strukturnih napak. Metoda CLS je očitna izbira, predvsem zaradi visoke prostorske ločljivosti, visoke občutljivosti na nizko koncentracijo napak in sposobnosti pridobivanja informacij o globinski strukturi materiala. Prvi del disertacije je uvod v temo in opis uporabljenih eksperimentalnih metod. Prav tako podamo podroben opis in pomembnost cerijevega dioksida kot vsestransko uporabnega materiala na področjih proizvodnje in pretvorbe energije ter za biomedicinsko uporabo. Opisali smo CLS kot orodje za razumevanje optičnih lastnosti, ki nastanejo zaradi strukturnih napak, ter napredek pri razvoju detekcijskih sistemov CL. Pri preučevanju razmerja med lokalno strukturo in njenim vplivom na emisijske spektre CL smo uporabili rentgensko absorpcijsko spektroskopijo. Od tega smo povzeli metodi analize obrobnih struktur rentgenske absorpcije (amg. X-ray absorption near edge structure - XANES) in analize finig struktur podaljšane rentgentske absorcije 8ang. Extended X-ray absorption fine structure - EXAFS). V drugem delu obravnavamo CL emisijo cerijevega dioksida. Predstavimo CL emisijo reduciranega cerijevega dioksida in njegovo odvisnost od koncentracije kisikovih vrzeli. Izvor emisije smo pripisali različnim konfiguracijam kisikovih vrzeli in polaronov. Pri tem smo uporabili nedavno sprejet opis F centra v cerijevem dioksidu. Na podlagi relativne spremembe populacije njegovih F centrov razložimo zanimivo dušenje CL emisije kot funkcijo koncentracije kisikovih vrzeli. To je pokazalo na pomemben vpliv lokalne strukture na CL emisijo cerijevega dioksida. V disertaciji smo uporabili La-dopiran cerijeve dioksida kot model, na osnovi katerega lahko bolje razumemo sistem. Z natančnim nadzorom nad stehiometrijo smo dosegli želeno koncentracijo kisikovih vrzeli. Preko CL emisije smo opazili, da se z La dopiran cerijev dioksid obnaša enako kot reduciran cerijev dioksid, analiza pa je pokazala, da F+ centri inducirajo CL emisijo, medtem ko jo F0 centri dušijo. Preiskava lokalne strukture obeh kationov, K-robu Ce in La, z uporabo EXAFS analize je pokazala popačenje strukture fluorita in potrjuje opis F centrov kisikovih vrzeli v cerijevem dioksidu. Naši rezultati zagotavljajo eksperimentalne dokaze za opis F centrov, ki vključujejo kisikova vrzeli in polarone