LEE- Nano Bilim ve Nano Mühendislik-Doktora
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ÖgeMixed-anion photochromic yttrium oxyhydride and gadolinium oxyhydride: Research and applications(Graduate School, 2021-02-04) Baba, Elbruz Murat ; Zayim, Esra ; Karazhanov, Smagul ; 513152002 ; Nano Science and Nano Engineering ; Nano Bilim ve Nano Mühendislikare-earth metal oxides and hydrides are the focal point in literature, promising various high demand solutions in industry and everyday life. Recently, a multi-functional, multi-anion material class called "rare-earth metal oxyhydrides" were shown to be synthesized as a thin film under ambient temperatures with photochromic properties. Oxyhydrides present high level of flexibility for material development through various combination possibilities having multi-anions. The newly realized possibility of ambient temperature thin film synthesis possibility of rare-earth metal oxyhydrides attracted high attention from literature but, there is little to no data is available in conventional databases for rare-earth oxyhydrides. Combination of the oxyhydrides being an under-developed class of materials and the promise of presenting important solutions for a high demanding era we live in, oxyhydrides is an excellent topic to research. In 2011, the synthesis of yttrium hydride thin film in room temperature with wide-spectrum transparency using only one step deposition process was shown. Originally, this research started to search for an alternative material for solar cells but, evolved into another dimension when the material was realized to be a highly responsive photochromic material that can modulate in wide spectrum. The material attracted high attention from literature and initially named as "oxygen containing yttrium hydride". However, it was later found through synchrotron measurements that this material is belong to the emerging class of materials; rare-earth metal oxyhydrides. When exposed to air, yttrium dihydride (YH2) and gadolinium dihydride (GdH2) films turn into insulating and transparent yttrium oxyhydride (YHO) and gadolinium oxyhydride (GdHO), respectively. Oxidation in air, hence bandgap, can be controlled by deposition parameters. MHO (M; rare-earth metal) photodarkens when illuminated with light of adequate energy and intensity, recovers (bleaching) when stored in dark. Photochromic rare-earth metal oxyhydride knowledge and know-how was established around synthesis method, band-gap engineering, optical properties, anion sites in the lattice etc. but the photochromic mechanism and environmental effects either yet to be understood or never even investigated before the present thesis work has been started. Also, yttrium was the only rare-earth element that was shown to have photochromic properties and investigated. Especially the knowledge gap over the interaction of photochromic oxyhydrides with the environment, prevented the realization into a product that sought heavily in industry. Therefore, in the present PhD thesis the interactions of the photochromic rare-earth metal oxyhydride thin films with the environment were first and foremost investigated. This endeavor resulted with solutions that enabled product development. Furthermore, contribution to the knowledge of photochromic rare-earth metal oxyhydrides by developing and studying at least one more rare-earth element next to the yttrium was also targeted. The studies showed that photochromic rare-earth metal oxyhydride thin films interacts with environment heavily. Additionally, we have published one article in Physical Review Materials that tries to shine a light to the understanding of the rare-earth metal oxyhydride photochromic mechanism, related to the environmental interaction: "Light-induced breathing in photochromic yttrium oxyhydrides". The studies showed that during the photodarkening/bleaching cycle of yttrium oxyhydride, material releases/intakes oxygen following lattice contraction/expansion, respectively. We coined the term, breathing, after the accordion-like structural process of yttrium oxyhydride based on oxidation. The article was selected as an editor selection and featured in Physics magazine. Based on these studies, stable IGUs which has been long sought since 2016 was able to be manufactured. Another contribution for the explanation of the photochromic mechanism of yttrium oxyhydride thin films published in Physica Status Solidi Rapid Research Letters with the title of "Temperature-dependent photochromic performance of yttrium oxyhydride thin films". In this article, we have presented the photochromic kinetics of yttrium oxyhydride thin films studied between 5-250K and presented a new approach which would enable new questions. The second part of the PhD thesis plan was contributing to the knowledge of photochromic rare-earth metal oxyhydrides by at least one another rare-earth element. Gadolinium was selected as a member of the rare-earth elements for study for having similar chemical properties and widely accepted by the nuclear industry for large neutron capture diameter. In the span of a year, the production know-how and knowledge related to gadolinium oxyhydride thin films were elevated also. One article was published in the journal Molecules that shows the post-deposition oxidation is related with the preferential lattice orientation which controlled by the deposition parameters: "Preferential Orientation of Photochromic Gadolinium Oxyhydride Films". Additionally, another article based on the environmental interaction of gadolinium oxyhydride thin films will be submitted in an international journal in 2020. Environmental effect on rare-earth metal oxyhydrides was investigated further by systematic study of yttrium oxyhydride thin films under atmospheres with varying relative humidity levels. Correlation between the relative humidity levels and photochromic kinetics was observed and microstructure formation that causes the delamination was shown. One article based on the results is under progress and planned to be submitted in 2021. In the last part, applications developed during this thesis based on yttrium and gadolinium oxyhydride thin films were presented. First, photochromic kinetics of stable IGUs based on yttrium oxyhydride were presented. However, the properties photochromic rare-earth metal oxyhydrides present is much wider than only window applications as a result of their multi-anion nature. Lastly, photocatalytic properties of photochromic gadolinium oxyhydrides were also shown.