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)are-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.
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ÖgeDevelopment of antibacterial coatings on titanium based biomaterials(Lisansüstü Eğitim Enstitüsü, 2022)Since the average human life is getting longer, research on long-lasting and biocompatible biomaterials has become more common. A biomaterial is a material that can replace any damaged tissue or organ and therefore, it is in continuous interaction with body fluids. There are various biomaterials for different application areas. The main issues to be considered are the mechanical properties, design, and biocompatibility of the developed biomaterial. Metal and metal alloys are the most frequently preferred biomaterials and they have been used as hip, knee, and dental implants for many years. These biomaterials are also expected to show superiour biocompatibility, corrosion and wear resistance, along with non-toxicity. Especially the long-term stability of orthopedic and dental implants depends on the bonding properties at the implant-bone interface and being free of any post-operative infections due to the implant features. Titanium and titanium alloys stand out among other metallic implant materials (such as stainless steel and cobalt-chromium alloys) with their excellent mechanical properties, biocompatibility, low densitiy, high corrosion and wear resistance. The most important feature that separates titanium from the other metals is the natural oxide film layer on its surface. Even though, this stable, dense, and continuous layer provides corrosion resistance and biocompatibility to the material, but, its ability to bond to bone is quite weak. In addition, due to the toxic effect caused by alloying elements that can be released from some titanium alloys (such as Ti6Al4V), titanium alloys may fail in long-term implant applications. For this reason, numerous surface treatment methods are used to enhanced the surface features of titanium and its alloys. Micro-arc oxidation (MAO), also called plasma electrolytic oxidation, is a convenient technic that is used to produce ceramic coatings on titanium, aluminum, magnesium, and their alloys. With this method, it is possible to get thick, porous, firmly attached ceramic coatings on the surface of titanium and its alloys. In addition to these, antibacterial oxide coatings can also be obtained by adding appropriate antibacterial agents into the electrolyte used during the process. The ability to coat materials with complex shapes, using environmentally friendly chemicals, and being a cost-effective process are the prominent advantages of this method. In the scope of this study, the formation of bioactive and antibacterial oxide coatings on the surface of titanium and its alloys was carried out via MAO process. In the first chapter of the thesis, it was aimed to produce multi-layer bioactive and antibacterial coatings on the commercially pure titanium (grade 4 quality, Cp-Ti) surface, which is frequently preferred in biomedical applications, by applying the MAO process. For this purpose, samples were subjected to a base electrolyte (which is containing calcium acetate hydrate (Ca(CH3COO)2.H2O) and disodium hydrogen phosphate (Na2HPO4)) during the MAO process. To obtain antibacterial properties on the coating surface silver acetate (AgC2H3O2) was added into the base electrolyte (the amount of silver (Ag) on the coating was measured as 4.6 wt.%). After the MAO process, a multi-layered oxide coating consisting of TiO2 (dense rutile-anatase phases) on the inner layer, and biocompatible compounds such as hydroxyapatite (HA) and calcium titanate (CaTiO3) just above oxide layer was obtained. It was observed that MAO treated samples in the base electrolyte formed biomimetic apatite structure faster in the simulated body fluid (SBF), as well as showed higher antibacterial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacteria. It has been determined that the addition of AgC2H3O2 to the base electrolyte increased the antibacterial activity of the samples without sacrificing bioactivity. In the second chapter of the thesis, optimization of AgC2H3O2 amount which had been added into the base electrolyte was studied in order to avoid toxic effect of Ag without sacrificing the antibacterial effect. The MAO process has been conducted in the base electrolyte containing Na2HP04 and Ca(CH3COO)2.H2O and AgC2H3O2 was added at different concentrations. Samples treated with an electrolyte containing 0.005 mol/L AgC2H3O2 concentration showed poor antibacterial activity against S. aureus bacteria, while samples treated with the electrolyte containing 0.001 and 0.002 mol/L Ag concentrations was determined strong antibacterial activity. Based on these results, considering the possible toxic effect of Ag, 0.001 mol/L AgC2H3O2 concentration into the base electrolyte is sufficient for MAO process. It was determined that used optimum AgC2H3O2 concentration into the base electrolyte caused 1.14% Ag on the oxide coating surface after MAO process. Cell culture experiments were performed using SAOS-2 (a human primary osteogenic sarcoma cell line) to understand the effect of the amount of silver measured from the coating surface on the cell. As a result, it was observed that the amount of Ag determined in the oxide coating did not prevent cell growth however retarted it and also showed high antibacterial efficiency against S. aureus bacteria. In the third chapter of the thesis, after determining the optimum amount of AgC2H3O2 concentration in the base electrolyte containing Ca(CH3COO)2.H2O and Na2HPO4 in the previous section, the biological properties of the oxide coating formed in the MAO process using Ti6Al4V alloy were investigated by biofilm formation and cell culture experiments. After adding 0.001 mol/L AgC2H3O2 to the base electrolyte during the MAO process, it was determined that there was 0.76 wt.% Ag on the oxide coating surface. It was observed that the alloying elements in the Ti6Al4V alloy effect the structure of the oxide coating formed by the MAO process conditions. The low amount of Ag measured from the coating surface was explained by the precipitation of Ag particles mostly around the pores of the thick TiO2 layer, and the formation of a thick HA layer on it. The presence of Ag particles between TiO2 and HA layer effected Ag release behavior in simulated body fluid (SBF). Compared with Ag-free coatings, the presence of 0.76 wt. % Ag in oxide coatings exhibited antibacterial activity to some extent against Streptococcus mutans (S. mutans) bacteria and did not adversely effect the proliferation of SAOS-2 cells. However, in order to obtain enhanced antibacterial efficiency, higher amount of silver must be incorporated into the MAO coating. In the fourth chapter of the thesis, the structural features, Ag release behavior and bioactivity of HT treated oxide layer with different amounts of Ag nanoparticles formed via MAO process on Ti6Al7Nb alloy have been investigated. While MAO process was applied in the base electrolyte (containing (Ca(CH3COO)2.H2O) and (Na2HPO4)) with and without the addition of AgC2H3O2 to obtain oxide layer, HT treatment was performed in an alkaline solution (pH = 11) at 230 ºC to improve bioactivity. After the MAO process, HA structure with a low degree of crystallinity and TiO2 layer containing rutile and anatase structures was formed on the surface. Nano-sized Ag particles were detected on the coatings formed over Ag incorporated oxide coatings. Moreover, higher AgC2H3O2 concentration in the base electrolyte caused a higher number of Ag nano-particles in the MAO coating. Afterwards, application of the HT treatment fabricated an 1-2 m thick exterior surface layer that is composed of nano-rod TiO2 and hexagonal HA crystal morphologies on oxide surface and increased degree of HA crystallinity. When samples treated with MAO and MAO+HT are compared, it was observed that HT treatment not only accelerated biomimetic apatite accumulation on Ti6Al7Nb alloy but it also eliminated the negative effect of Ag, which delayed the apatite formation on the MAO coatings. In addition, unlike the oxide coatings formed with MAO, HT treatment considerably reduced the amount of Ag released from the oxide coating into the SBF solution. As a result, thick, microporous and multi-layered oxide coatings containing bioactive components have been successfully produced on the surface of titanium and its alloys, which are frequently prefered in implant applications. Generally, various additives such as ions or particles can be introduced into the electrolyte to fabricate antibacterial oxide coatings with biocampatible properties via MAO process. Within the scope of the thesis, studies have shown that oxide coatings which were fabricated using different amount of Ag agent in the base electrolyte exhibited antibacterial efficiency on bacterial cultures and bioactive components support the bioactivity. Especially in obtaining antibacterial coatings, the importance of Ag agent amount in the base electrolyte has been demonstrated by antibacterial tests. In addition to the MAO process, when MAO process is combined with HT treatment, it is possible to fabricate highly bioactive surfaces without obtaining multi-layered coatings on the substrates. Moreover, Ag agent has been introduced into the oxide coating to give antibacterial properties to the surface. The Ag agents and their amounts is still one of the biggest concerns for health. In the future studies, the living body applications (in vivo) will guide the evaluation and development of short and long term effects of TiO2 based bioactive and antibacterial coatings fabricated with MAO process and HT treatment.
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ÖgeDevelopment of microfluidic based single cell capturing systems for early detection of diseases(Fen Bilimleri Enstitüsü, 2020)It is known that cancer cells in the bloodstream are quite low compared to other cells in the blood. Microfluidic based systems have been studied for diagnosis, follow-up of the disease and new drug tests to be performed on this disease. A microfluidic based system with two successive regions for separation and analysis has been developed. In the first region, the target cell type is differentiated from a complex mixture containing multiple cells by dielectrophoresis, which allows an insulating particle to be polarized under an electric field. Since different cell types can be polarized at different rates under the same electric field, this method allows the separation of the cells from each other under suitable conditions. In this study, a microfluidic system consists of two consecutive regions, namely the separation and analysis regions are demonstrated. In the first region, the target cell type is separated by dielectrophoresis from a complex cell mixture. The target cells collected in the first region are continuously transferred to the second region and are captured in a single cell array formation at the hydrodynamic capture stations placed on the measuring electrodes. Impedance analysis was performed to establish a platform for detection and drug screening. While both regions were integrated on a single chip in the final device, each region were examined separately during our study. The results of impedance analysis obtained from different cells based on different medium conductivities with a frequency range of 0.1kHz – 500kHz are presented here. We recorded impedance measurements at stations where cells were individually captured before and after cell entrapment. Experimental results are divided into cases where the conductivity of the medium is higher and lower than the cell conductance. Overall magnitude of impedance shift is significantly higher when the medium conductivity is lower than the cell conductance. When all results are evaluated, it can be seen that depending on the target cell type, an optimum medium conductivity and frequency range can be selected so as to obtain the measurement result with the highest sensitivity. A microfluidic cell culture platform, named as organ-on-a-chip in the literature, has been increasingly studied over the last few years to mimic tissue and organ-level physiology, containing a membrane with a continuous and porous structure inhabited by living cells, and with microfluidic channels to mimic the mechanical effects and to supply the necessary nutrients. These platforms create tissue and organ environments that are not possible with traditional 2D or 3D culture systems, and enable real time imaging and analysis of biochemical, genetic and metabolic activities of living cells. In this project, present fabrication techniques of microfluidic devices are used for the fabrication of organ-on-a-chip platforms. The tissue structure was imitated by coating a single-layer cell on the upper and lower sides of the membrane in the structures of the renal chip tubules and lung alveoli on organ-on-a-chip platforms. The cell viability was characterized by MTT test and the cell viability was maintained by providing oxygen, carbon dioxide and nutrient exchange under incubation conditions by means of nutrient medium flow provided into the upper and lower channels, and the barrier property of the cell tissue was measured by electrical resistance (TEER) measurements. The viability of the renal tubules cultured in the microfluidic system between 0-48 hours was recorded by MTT assay. TEER results showed that the tight-junctions of cell tissue were different under static and dynamic conditions in the kidney-on-chip systems. The results obtained by MTT test to measure cell viability were in agreement with TEER and the viability of kidney cells was higher in 48 hours under dynamic conditions compared to static conditions. With the successful culturing of two different cell types under static conditions in lung-on-chip systems, their viability and cell barrier resistance values were recorded by TEER measurement for 0-48 hours. The results obtained by MTT test and TEER measurements showed that lung cells under shear stress and mechanical stress had higher viability than cells under static conditions.
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ÖgeFabrication of nanostructured metal oxide materials and their use in energy and environmental applications( 2020)Metal oxides are considered to be the most vital material class and they show unique chemical, physical and electronic properties when produced on the nanometer scale. In this context, metal oxide nanomaterials are of increasing importance in many industries and are used in applications such as sensors, medical technologies, energy, water treatment, and personal care products. In this thesis, the fabrication of nanostructured metal oxide materials and their use in energy and environmental applications which have strategic and vital importance are focused. The optimization of process parameters for the production of metal oxide nanostructures via industrial-scale production methods and application-oriented modifications of the properties of metal oxides via controlling their size and composition have been realized. Solar energy is an environmentally friendly technology that allows direct energy production from the sun. Perovskite solar cells (PSCs) have been studied intensively in the last decade and they constitute the energy leg of this thesis. In this context, the effects of metal oxide nanomaterials on perovskite cells performance were investigated. The performance of planar and mesostructured PSCs was compared, while all the experimental studies for the production of highly-efficient PSCs were given in detail. The mesoporous architecture allows the deposition of denser perovskite films than planar architecture due to the high porosity. Though higher efficiency was expected due to effective absorption of incoming light, XRD results showed that PbI2 - perovskite conversion in the mesoporous structure was more difficult. The average efficiency of the cells produced with mesoporous architecture was 15.07 %, which was just 0.9 % higher than the planar one. As can be deduced from the absorbance curves and IPCE analysis, this is because of the mesoporous structure showing more absorbance in the 400-600 nm wavelength range resulting in more photocurrent. However, due to the small difference in efficiency and fewer steps in the planar architecture, it was found more viable for industrial scale-up. Air pollution is one of the most critical environmental problems today, and filtration is one of the practical solutions to remove the pollutants, especially particulate matter within the air. However, exhaust gases might be at high temperatures and require high-temperature resistant filter materials. The use of ceramic-based, fibrous filter elements in filtration applications will enable the production of highly efficient filters with high-temperature stability. In this context, SiO2 nanofibrous mats were produced from sol-gel based solutions via centrifugal spinning (CS) and solution blowing (SB) methods. According to results, centrifugally spun SiO2 fibers were found more flexible where fibers have diameters between 1 and 1.5 microns. Solution blown silica fibrous mats consisted of thinner fibers but have denser bead and droplet defects. Besides, due to the fibrous mats obtained by SB had a dense-packed structure it showed more shrinkage during heat treatment. XRD results show that all fibers have an amorphous SiO2 structure after heat treatment at 600°C. According to the porosity analysis, the solution blown and centrifugally spun SiO2 samples had the lowest pore diameters of 5.2 and 10.5 microns, respectively. Moreover, the effects of SiO2 precursor solution concentration on spinnability in the CS method, the diameter of SiO2 fibers, and filtration efficiency were investigated. Contrary to expectations, the average diameter of the fibers has been found to decrease with increasing precursor concentration a result of reduced viscosity of the spinning solution. While all the produced fibers are incredibly flexible, the highest filtration efficiency (43.35 Pa pressure drop and 75% particle capture efficiency) was obtained from the sample that produced from 15 wt.% TEOS added solution. Due to excellent thermal stability and high mechanical performance of centrifugally spun SiO2 fibrous mats they have the potential as filter materials for hot air filtration applications. Photocatalyst-based purification techniques emerge as a solution for recovery of used water. While the studies focused on the development of photocatalyst material with visible light activity, there is also a need for the development of photocatalyst geometries that can be easily separated from treated water. Although nanoparticulate morphology offers high surface area, it is difficult to remove them from treated water. On the other hand, TiO2 is one of the most studied materials among the photocatalysts due to its high photocatalytic activity, photostability, chemical inertness, and low-cost. TiO2 fibers were fabricated via CS and subsequent calcination methods. The effects of precursor concentrations on fiber diameter, surface area, and photocatalytic activity were investigated. Results showed that the fiber diameter was increased from 0.65 to 1.2 µm with increasing precursor content. The calcined fibers consisted mainly of anatase and also a minor amount of rutile phases. PVP used as the carrier polymer for precursor solution also behaved as a nitrogen source for TiO2 fibers during calcination. The slight shift of peaks in XRD, the presence of nitrogen in XPS spectrum and EDX mapping, and the enhanced visible-light photocatalytic response were pieces of evidence for in-situ N-doped TiO2 NFs. Besides, nanoparticles (P25 NPs) were added into the spinning solution to increase the surface area by producing nanoparticle in nanofiber structure, and it was also used as a reference sample. According to the results of photocatalysis tests, the surface area is the dominant factor for photocatalysis under UV illumination and the optical bandgap is the critical factor for the tests performed under visible light illumination. Moreover, recycle analysis showed that fibrous photocatalysts were easily separated from the treated water. In this regard, the fibrous TiO2 was emphasized as the best visible-light photocatalyst, losing only 14% of its degradation performance after the 3rd use. The effect of Al and Li doping on the crystallinity, fiber diameter, optical bandgap, and photocatalytic activity of TiO2 fibers was investigated in the last part of this thesis. Al and Li doped N-TiO2 fibers were successfully produced via CS method and followed calcination. N- TiO2 showed a fiber diameter of 0.54 µm while Al- and Li-doped N- TiO2 had a diameter of 0.94 and 1.15 nm, respectively. While the crystal structure of N-TiO2 transformed from major anatase and minor rutile phases to the only anatase in the case of Al- and major rutile and minor anatase phases in the case of Li-doped N-TiO2. Additionally, band gap values were calculated as 3.00, 2.94, and 3.14 eV for N- TiO2, Li- and Al-doped N- TiO2, respectively. For the photocatalysis tests conducted under UV-light, the most efficient sample was the nanoparticulate TiO2 due to its high surface areas, while all-fibrous structures showed similar activities, which were nearly two times higher than the activity of nanoparticulate TiO2 under visible-light.