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Bazı yeni amfifilik tarak tipi graft kopolimerlerin sentezi, karakterizasyonu ve mikro/nanokürelerinin eldesi
Bu tez kapsamında, kitosan-MBSA kompleksinin metil metakrilat (MMA) ve stiren vinil monomerleri ile radikalik redoks polimerizasyonu yöntemi ile amfifilik graft kopolimer sentezi ve karakterizasyonu yapıldı. Sentezlenen kopolimerlerin boş ve deksametazon gibi kanser ilacı yüklü mikro/nanoküre eldesinde kullanılabilirliği araştırıldı. Bunun için kitosanın, 4-metilbenzensülfonik asit (tosilik asit) (MBSA) ile reaksiyonu sonucu reaktif olan amin gruplarından bağlanarak kitosan-MBSA kompleksi elde edildi. Farklı miktarlarda kitosanın-MBSA kompleksi ile MMA ve stiren ile hidroksimetil grubu üzerinden amonyum seryum nitrat (CAN) başlatıcısı kullanılarak redoks polimerizasyonu sonucunda kitosan-g-PMMA ve kitosan-g-PS graft kopolimerleri sentezlendi. Elde edilen kopolimerin yapısal analizi FT-IR, 1H-NMR, termal analizi DSC ve TGA teknikleri kullanılarak gerçekleştirildi. Bu graft kopolimerden model ilaç olarak deksametazon kullanımı ile ilaç yüklü/yüksüz mikro/nanoküreleri solvent buharlaşma tekniği kullanılarak hazırlandı. Kitosan-g-PMMA mikrokürelerine % 79.8 oranında ilaç yüklendi. İlaç yüksüz Kitosan-g-PMMA mikrokürelerinin boyutları 2.8 µm ile 11. µm arasında, ilaç yüklü Kitosan-g-PMMA mikrokürelerinin boyutlarının ise 5.6 µm ile 14.5 µm arasında değişmekte olduğu gözlemlendi
Kemikteki tutunma gücünü artırmak için geliştirilen vidaların ve ortamların biyomekanik karakterizasyonu
Plaka ? vida sistemleri; travma, skolyoz, spinal stenoz, tümör, enfeksiyon vb. sebeplerle ortaya çıkan spinal instabilitenin tedavisinde kullanılan en popüler araçlardır. Stabilize amacıyla takılan vidanın sıyrılma dayanımı birçok üretici ve cerrah tarafından üzerinde durulan en önemli noktalardan biridir.Bu çalışmada farklı tiplerde iki vida ve bunların uygulandığı iki farklı ortamın katı modeli oluşturulmuş ve biyomekanik analizleri sonlu elmanlar yöntemi kullanılarak çalışılmıştır.Elde edilen analiz sonuçları, biyomekanik deney sonuçları ile karşılaştırılmış ve uygunluğu tartışılmıştır.
Shape memory alloy design by machine learning for biomedical and high-temperature applications
Shape memory alloys (SMAs) are of great importance due to their extensive usage in biomedical applications, aerospace engineering, or robotics. In recent years, although there has been a considerable amount of research to achieve optimum compositions of SMAs for these applications, due to the high experimental costs, the demand for alloys with optimum properties has not been met for many applications yet. In this research, using the predictive power of artificial intelligence and machine learning, a systematic approach to predict the optimum composition of the SMAs was proposed to address two problems related to the binary NiTi and NiTi-based SMAs. In particular, in chapter two, the optimum chemical composition was proposed to minimize the Ni ion release in the binary NiTi SMA. The method to do so was to gather a database from the existing literature and using it to train a special algorithm that provides the information for predicting the desired compositions. In chapter three, using the same approach, two models were developed to predict the phase transformation temperatures and thermal hysteresis of multi-component NiTi-based SMAs. These models were used to predict the optimum alloy with the highest Phase transformation temperatures with the least possible thermal hysteresis.
Magnetic anisotropy control in rare earth iron garnet thin films for spintronic devices and all-optical ultrafast manipulation of magnetization
Spin-based memory and logic devices might provide a promising route for fast, nonvolatile and power-efficient operation using magnetic insulators with minimal Joule heating and ultrafast spin dynamics, which can be controlled all optically. There are very few magnetic insulator iron garnets, whose magnetic properties cannot be tuned easily and limit spintronic device applications. Thus, new iron garnets with perpendicular magnetic anisotropy and low saturation fields are needed for ultra-low power spintronics. In this thesis, we present theory, modelling and experimental studies on controlling magnetic anisotropy in epitaxial iron garnet thin films and all-optical control of magnetization dynamics in quantum-confined metallic nanolayers. First, we developed an anisotropy model for describing the magnetic anisotropy characteristics of insulating rare earth iron garnet (REIG) films (Re3Fe5O12, Re; rare-earth ions Y, Tm, Dy, Ho, Er, Yb, Tb, Gd, Sm, Eu). We construct an effective anisotropy energy using shape, magnetocrystalline, and magnetoelastic terms for ten different REIG films grown epitaxially and lattice-matched on five commercially-available garnet substrates. We calculated their magnetic easy axes and predict that 20 different pairs out of 50 to possess out-of-plane magnetic easy axis (PMA). Only 7 of them were experimentally tested and confirmed. We predict that the magnetic saturation fields of PMA garnets could span two orders of magnitude (300 Oe to 12.6 T), significantly expanding the available PMA garnet class. To test our predictions, we grew 420 and 67 nm-thick Holmium iron garnet films on Gd3Ga5O12 and Tb3Ga5O12 substrates using pulsed laser deposition at 800°C and 650°C. X-ray diffraction and magnetic hysteresis loop measurements indicate phase purity and PMA, respectively, for 67 nm HoIG, confirming our prediction. Using a modified microscopic three temperature model, we investigate the effect of laser pulse parameters and magnetic elemental metal thin film properties on the femto- and picosecond magnetization dynamics. We model the coupled energy transfer between electrons, phonons and spin baths. A magnetization quenching (in sub-200 fs) and recovery (a few ps) was found for metals with high Curie temperature (Fe, Co, and Ni). In the quantum-confined thickness regime (t < 50 Å), spin-phonon scattering in magnetic metals are significantly reduced due to the reduced density of states. Thus, THz spin wave emission might become feasible with three orders of magnitude lower laser fluence compared with the previously reported experimental values. Our models and experiments could expand the available PMA iron garnets with minimal Joule heating for spintronics and help control ultrafast spin dynamics much more efficiently.
Development of highly efficient platinum nanocatalysts for the dehydrogenation of ammonia borane via rational design of graphitic carbon nitride-based heterojunction photocatalysts
In the last decade, the use of hydrogen as an efficient energy carrier has already been started in fuel cells, portable devices, public transportations, and grid systems thanks to the advancements in the related technologic/scientific area. However, the safe storage and delivery of hydrogen are still on-going challenges in front of the hydrogen energy-based society. The implementation of solid chemical hydrogen storage compounds as the hydrogen energy carrier systems is one of the most promising solutions. Among the solid chemical hydrogen storage materials, ammonia borane (NH3BH3, AB) arises as one of the most convenient material for the chemical hydrogen storage owing to its advantageous properties. Among the several methods applied for H2 generation from AB, the hydrolysis route which results in the release of 3 equivalents of H2 gas in the presence of a suitable metal catalyst has been mostly preferred one for the mobile applications. Up to date, numerous catalysts have been tested in the HAB, among which platinum nanoparticles (Pt NPs) supported on high-surface-area and porous materials have been reported with one of the highest catalytic activities for the hydrolysis of AB (HAB). However, those Pt NPs were generally synthesized by following the methods comprising high-temperature surfactant-assisted decomposition and reduction of Pt precursors, and then the obtained highly monodisperse Pt NPs were deposited on a suitable support material to prevent their agglomeration. Supported Pt NPs can also be synthesized by the in-situ synthesis method involving the impregnation of Pt precursor into the support material followed by their reduction in catalytic reaction medium, which has distinct advantages over the initial one such as being sustainable, practical, green, atom-economical, cost-effective, and timesaving. Here, the selection of support material is so crucial to obtain stable and reusable nanocatalysts. Graphitic carbon nitride (g-CN) has been suggested as an appropriate substrate to stabilize Pt NPs due to its nitrogen-rich "six-fold interstices" between tri-s-triazine units, having capability of strong interaction with incorporated metal, and its adjustable surface area and morphology obtained by following different synthesis procedures. Besides, g-CN is a visible light active semiconductor material with suitable band positions for the construction of heterojunctions with Pt NPs and other suitable semiconductor materials. In this thesis, it was aimed to enhancement of the catalytic activity of Pt nanocatalysts in the HAB via the rational design of g-CN based heterojunction photocatalysts. To fulfill this aim, firstly, a novel method for the in-situ generation of Pt NPs supported on mesoporous g-CN (m-g-CN/Pt) during the catalytic HAB was realized under the white-light irradiation. Secondly, U-g-CN, which was synthesized by using urea as an amine precursor via an easy and green synthesis approach compared to m-g-CN, was used as a support material for the in-situ generation of Pt NPs. U-g-CN/Pt showed better catalytic performance than m-g-CN/Pt nanocatalysts under the same conditions. Next, to further enhancing the catalytic activity of the Pt-based nanocatalysts in the HAB, we turned our attention to the in-situ synthesis of the bimetallic MPt NPs supported on U-g-CN (U-g-CN/MPt). Indeed, U-g-CN/Pt92Au8 and U-g-CN/Pt85Au15 provided a higher catalytic activity (hydrogen production rate of 68.9 and 67.4 L H2 gPt-1 min-1, respectively) than the monometallic counterpart (60.4 L H2 gPt-1 min-1), which was attributed to the alloy effect, surface plasmon resonance (SPR) contribution, and the heterojunction formation. Thirdly, on the way to thesis's main goal, we tried to further enhance the activity of Pt nanocatalysts in the HAB by supporting them on U-g-CN/WOx binary nanocomposites by considering the optical properties and charge kinetics of U-g-CN can be improved via the construction of heterojunction with other semiconductors having proper band gap and band potentials. The results revealed that U-g-CN/a-WOx/Pt nanocatalysts showed a higher catalytic activity than U-g-CN/Pt by providing a maximum hydrogen production rate of 48.1 L H2 gPt-1 min-1. All the yielded Pt nanocatalysts (m-g-CN/Pt, U-g-CN/Pt, U-g-CN/MPt, and U-g-CN/a-WOx/Pt) were characterized via many advanced analytical techniques including TEM, HR-TEM, XPS, XRD, ICP-MS, FTIR, PL, and TRES techniques to clarify the reasons behind the enhanced photocatalytic activities in each step. Moreover, the rate law and the activation parameters were also derived by using the data of kinetic studies. Additionally, a reusability test for each developed Pt based nanocatalyst in HAB was also reported. In summary, this thesis demonstrates that g-CN is a appropriate substrate as support and stabilizer for the in-situ synthesis of catalytically active Pt or MPt NPs in hydrogen production from the HAB due to its availability for rational design of heterostructures.
Acid catalysed synthesis of mesoporous silica and its applications as adsorbents, catalyst support and micromotors
Micron-sized mesoporous silica particles have been used in various applications such as adsorption, separation, catalysis, and drug delivery due to their high specific surface area, pore size, high thermal/chemical stability. However, controlling the properties of mesoporous silica particles is difficult because of the need for different chemicals and additional steps required in the synthesis method. In this thesis , the properties of mesoporous silica particles have been investigated and tuned by controlling the process parameters of sol-gel synthesis method carried out by mixing Pluronic F127 surfactant and tetra ethyl orthosilicate (TEOS) in highly acidic solution at room temperature. Ternary phase diagram of mesoporous silica morphologies was determined by adjusting the compositions of three reactants in the synthesis solutions. The ternary phase diagram was an important development for the investigation of all possible morphologies that can be obtained with a simple and low-cost synthesis method. After analysis of over 150 silica samples, regions with different morphologies (sphere, monolith and polyhedron) and variable surface areas were determined. Mesoporous spherical silica particles (MSPs) with average particle sizes ranging from approximately 1.00 µm to 5.00 µm and pore sizes from 3 nm up to 15 nm were successfully synthesized. Furthermore, hydrothermal treatment process was used to improve the specific surface area (up to 1000 m2/g) and pore size (up to 15 nm) of MSPs. With hydrothermal treatment, the surface area of MSPs was increased by up to a factor of 1.76. The synthesized mesoporous silica structures were used in the purification of wastewater from textile dyes, photocatalysis and self-propelled micromotor applications and their performances were studied. The changes in the morphology and pore structures of the particles were successfully identified by using different acids (nitric acid (HNO3) and sulfuric acid (H2SO4)) compared to the hydrochloric acid (HCl) in the investigation of ternary phase diagram. Pore size of the particles was increased from 3 nm to 14 nm by using sulfuric acid without changing the morphology of MSPs. Wastewater treatment performance of mesoporous silica particles synthesized with different acids was tested with Rhodamine 6G as a model molecule and the performances were compared. The maximum absorption capacity of about 20 mg/g was reached by silica particles synthesized with sulfuric acid. The capacity of the particles for removal of dye molecules from water was investigated by changing the process parameters (pH, temperature, time and initial concentrations). Adsorption processes were analyzed with linearized Langmuir and Freundlich isotherms, to estimate the mechanism of adhesion of dye molecules to particles. In addition, the suitability of the adsorption processes to the pseudo-first-order and pseudo-second-order kinetic models was also investigated by varying the contact time. In addition, the changes in thermodynamic parameters (ΔG°, ΔH°, and ΔS°) of the adsorption processes were calculated with the experimental results obtained as a function of temperature at which adsorption took place. The results showed that mesoporous silica particles synthesized with a simple and cost-effective process can satisfy the requirements for adsorbents in wastewater treatment applications. The synthesis of metal (iron and copper) loaded porous silica particles as catalysis material has been successfully completed. The catalytic activity of the metal loaded particles was investigated by degradation of Methylene Blue (MB) textile dye selected as the model pollutant. The degradation efficiency observed by Fenton and photo-Fenton reactions was compared depending on the type of metals in the particles and the amount of loading. It was also observed that the iron-loaded mesoporous silica particles cleaned the MB solution at 100 mg/L concentration by 99% in 20 minutes under UV light. The experimental results showed that the use of catalyst particles produced by a simple synthesis method in advanced oxidation processes will provide high efficiency. Self-propelled mesoporous silica particles have been successfully synthesized by CaCO3 loading into the structure. The loading process was carried out by the formation reaction of the CaCO3 structure by the reaction of CaCl2 and NaCO3 and proved by EDX and SEM analyses. Optimization of the CaCO3 loading process has been completed and the synthesis parameters required for the highest loading yield have been determined. The ability of the loaded mesoporous silica particles to move in acidic aqueous solution (acetic acid) by bubble propulsion was observed under optical microscope. In addition, the velocities of silica particles with different particle size, surface area and pore size were compared. It has been observed that similar size particles having smaller pore diameters move faster. Moreover, it has been understood that pore size is as effective as particle size and surface area. The highest velocity was observed as 1.6 μm/s in particles with a diameter of about 1.5 μm and a surface area of 789 m2/g. Self-propelled mesoporous silica particles have the potential to be converted to Janus particles and used in biological applications as micromotors.
Metal-substituted ZrB2 and HfB2 as low-cost and high-performance bifunctional electrocatalysts for water splitting
Growing global energy demand, descending amount of fossil fuels, and severe climate changes have encouraged researchers to develop clean, sustainable, and efficient energy sources. Currently, hydrogen energy is one of the most promising solutions as the ideal energy carrier by virtue of the highest gravimetric energy density of any known fuel, zero carbon footprint, and earth abundance. In high purity hydrogen gas production, electrocatalytic water splitting has come to the fore; in this regard, earth-abundant, cost-efficient, corrosion-resistant, and bifunctional materials with remarkable electrocatalytic performances are needed. In this thesis, transition metal substituted ZrB2 and HfB2 families were investigated as promising electrocatalysts. They were synthesized by the carbothermal reduction method and employed as bifunctional catalyst in 1 M KOH alkaline medium for both hydrogen and oxygen evolution reactions (HER and OER, respectively). The family of ZrB2-based catalysts was studied with a general formula of Zr(1-x)TMxB2 (x = 0.05, 0.1, 0.2 and 0.3) (TM = Fe, Co, and Ni). In the case of OER, Zr0.8Ni0.2B2 sample led to an onset potential of 1.58 V at 10 mA cm–2 current density, indicating a remarkable performance with a very low overpotential of 350 mV. Besides, Zr0.8Ni0.2B2 displayed a value of 56.6 mV dec–1 regarding the Tafel slope, which was smaller than the commercial RuO2 (66.2 mV dec–1). In the case of HER, Zr0.8Co0.2B2 with 420 mV overpotential and 101.6mV dec–1 showed the best performance. The family of HfB2-based catalysts with a general formula of Hf(1-x)TMxB2 (x = 0.1, 0.2 and 0.3) (TM = Co, and Ni) was also investigated. In parallel with the ZrB2 case, Ni substitution enhanced the OER performance and Hf0.8Ni0.2B2 sample leading to an onset potential of 1.55 V at 10 mA cm–2 current density with a very low overpotential of 320 mV and infinitesimal value of 39.5 mV dec–1 as Tafel slope. For HER, Hf0.8Co0.2B2 with 430 mV overpotential and 105.4mV dec–1 showed the best performance. Both catalysts were examined for their long-term stability, resulting in excellent durability after 12 -20 h. As-prepared transition metal substituted diborides have great potential to be implemented in green energy applications.
Metalens integrated photonics
This thesis investigates the optical metasurfaces utilized for lensing (metalens) and their potential to integrate with photonic circuits. In recent years, a plethora of studies have been done on metalenses, especially in the visible region. However, there are still rooms to discover the metalenses working in the mid-IR and ultraviolet (UV) spectra. Due to the applications in these regions, from biosensing in mid-IR to lithography in UV, the development of metalenses operating in these ranges is essential. One aspect of metalenses that requires further investigation is to analyze the deviation of the measured focal spot from the simulated value, which can affect the focusing quality. As this deviation becomes smaller, a more homogeneous airy-disk pattern can be achieved. Unlike the previous studies that have suggested the Fresnel number is the only relevant factor for the possible focal shift, we numerically show that for mid-IR metalenses, the numerical aperture (NA) of metalenses is an essential parameter to predict the focal spot deviation from the simulated value. We also introduce a critical NA (between 0.55-0.65) in which the deviation of the simulated and measured focal spot can reach zero value. The other aspect of metalenses addressed in this thesis is to find a fine-tuning method to multiply the number of focal sites, which is a practical concept for polarization filtering applications. The single-material-base metalenses are numerically investigated in the UV regime. Moreover, a design method to have full spatial control on phase, polarization, and focal spots is carefully implemented to attain multifocal metalenses. These single-material metalenses are composed of Al$_2$O$_3$ nanoblocks on the Al$_2$O$_3$ substrates, which can facilitate the fabrication process. Finally, by taking advantage of focal shift prediction and multifocal design principle, we propose a metalens-integrated photonic device. On one side of such device, a metalens structure is incorporated, and a plasmonic antenna array is patterned on its other side. We numerically study this device and show that this new compact design could offer an excellent lens-free illumination mechanism for plasmonic structures. Also, as it is possible to focus the incident light on the plasmonic structure, the near field intensity is significantly increased in this device. In addition, utilizing multifocal metalenses on the backside also allows us to illuminate multiple chips on the surface with different polarization modes. Considering the plasmonic structure applications for biosensing purposes, we can increase the sensitivity and selectivity of biodetection in these integrated devices while avoiding complicated measurement techniques.
Mesoporous materials for electrochemical energy conversion
Cheap, abundant, and easily manufactured electrocatalysts with high efficiency and stability are required for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to make renewable hydrogen production widespread. Transition metal chalcogenides and oxides have shown to be promising and versatile materials for catalyzing HER and OER in recent decades. Specifically, transition metal sulfides such as NiS2 and MoS2 offer great potential, their defective surface and edge site compositions, conductivities, and surface areas impose a significant effect on their electrochemical activities towards HER. On the other hand, mixed iron-nickel oxide based electrocatalysts have been intensively studied for OER. Engineering of abundant metal oxide/sulfide nanoparticles with a porous network by tuning the composition, structure, and morphology is highly promising to explore the most active and stable electrocatalyst. For this purpose, in this study, we developed a soft templating method to produce mesoporous nickel sulfides, mixed iron-nickel oxides, and molybdenum sulfide/oxide thin films. All synthesized materials were investigated by advanced characterization tools to identify their compositions, morphologies, and structures. Our soft templating method enables the production of mesoporous NiS2, Ni3S2, NiS, Ni7S6, and mixed compositions by altering the starting precursor compositions. The performances of nickel sulfides were tested towards HER in both acidic and alkaline electrolytes. Electrochemical tests with tracking the samples by ex-situ advanced material characterization tools reveal that mesoporous nickel sulfides act as pre-catalyst that transforms into superior active sulfur deficient nickel sulfide with the collapse of the mesoporous structure under HER in an alkaline electrolyte. A similar observation was observed in electrochemical tests of mesoporous NiS2 for OER. NiS2 was converted to Ni(OH)2 under a positive bias during OER. We synthesized the mesoporous NiFeOx, NiFe2O4, and Fe2O3 thin films with tunable compositions by changing the Ni/Fe precursors for OER in alkaline electrolytes. In the analysis of intrinsic activities, a peak in oxygen evolution activity was observed below %5 Fe content, leading to the formation of NiFeOx, and it was noticed that futher Fe content decreases OER activity. The formation of NiFe2O4 slightly decreased OER activity, but mixed metal oxide was substantially more active than the pure NiO or Fe2O3. Fe impurities in KOH electrolyte electrochemical deposit on the electrocatalyst surface and enhance the OER activity of Ni-based electrocatalyst. With a cautious comparison of electrochemical tests results, Fe atoms in NiO lattice were found to boost OER activity substantially more compared to Fe deposition Ni-based metal oxide electrocatalyst. We also applied the soft templating strategy with minor modifications to produce mesoporous MoS3-MoS2-MoO3 thin films for HER in acidic media. With ex-situ analysis of structure and activity, MoS3 in the mesoporous framework was electrochemically reduced to more active MoS2. Overall, the soft templating strategy is a facile and scalable fabrication method to develop a cheap and efficient electrocatalyst with an integrated film structure. In addition, the mesoporous structure of electrocatalysts provides insights into understanding structure-activity relation, changes in the surface composition or morphology, and durability of electrocatalysts owing to the high surface area with ultra-small crystal sizes.
High-throughput computational screening of MOFs for carbon dioxide capture and hydrogen purification
Metal organic frameworks (MOFs) constitute a novel class of porous materials that are formed by the combination of inorganic nodes and organic linkers. The number of newly synthesized MOFs increases exponentially each year, and it is not feasible to experimentally test the gas separation performances of each MOF adsorbent and membrane. Therefore, a computational approach that can identify the promising MOFs out of thousands of materials is essential. In this thesis, by performing high-throughput computational screening, CO2/H2 separation performances of MOF adsorbents and membranes were investigated in detail, and the top promising MOFs were identified. In the first part of this thesis, molecular simulations were performed to identify adsorption- and membrane-based CO2/H2 separation performances of 3857 unique MOFs. Results showed that all 3857 MOFs were CO2 selective when considered as adsorbents, whereas 899 MOFs overcame the performances of polymeric membranes as H2 selective membranes. H2/CO2 selectivities and H2 permeabilities of MOF membranes varied between 2.1×10−5-6.3 and 2.30-1.7×106 Barrer, respectively. Structure-performance relationships revealed that MOFs with pore size <7.5 Å performed well as CO2 selective adsorbents whereas MOFs with pore size >15 Å were more suitable to be used as H2 selective membranes. In the second part of this thesis, Grand Canonical Monte Carlo (GCMC) and Equilibrium Molecular Dynamics (EMD) simulations were performed on the updated MOF database to identify adsorption and membrane-based CO2/H2 separation performances of 10221 unique MOFs. The applicability of Ideal Adsorbed Solution Theory to MOFs for CO2/H2 separation at temperature and pressure swing adsorption conditions, and the effects of inaccessible local pores, catenation in the frameworks, and presence of impurities (CO, CH4 and H2O) in gas mixture on the selectivity, working capacity, and regenerability of MOFs were examined. MOFs that are recently synthesized and added to the updated MOF database were shown to have higher CO2/H2 selectivities and working capacities than the previously reported MOFs. In the third part of this thesis, new MOFs were designed by using in-silico metal exchange techniques and CO2/H2 and CO2/CH4 separation performances of these MOFs were studied. Results showed that the type of the metal site affects the CO2/H2 selectivities of MOFs. As a result, CO2/H2 selectivity of a commonly studied MOF in the literature, HKUST-1, was significantly enhanced by 11% and 38% when Cu metal was exchanged with Cr and Cd metals, respectively. The exchange of Zn with V increased the selectivity of HIFTOG02 from 119 to 355. Overall, results showed that high-throughput computational screening techniques introduced in this thesis can be used to (i) shortlist potentially promising MOFs among thousands of MOFs for pre-combustion CO2 capture, (ii) identify the structural properties of the promising MOFs with high CO2/H2 separation performances, (iii) design new MOFs with exceptional CO2 capture and H2 purification properties. The results presented in this thesis will serve as a catalyst for future computational and experimental studies on MOFs which will efficiently capture and sequester CO2.
Wavelength dependent photothermal conversion efficiency of photosensitizers for photothermal therapy
Today, cancer treatment is one of the most important research fields since it is the second most lethal disease worldwide. Among the current treatment methods that have strong potential for future cancer therapies is photothermal therapy (PTT). Nanoparticles have become popular photosensitizers for photothermal therapy (PTT), as they can be targeted to specific cancer tissues and deliver a chemotherapeutic drug, providing a multimodal therapeutic approach. Photothermal conversion efficiency of nanoparticles is critical in the assessment of their therapeutic use in PTT. In this thesis, we describe an accurate calorimetric method for the determination of the photothermal conversion efficiency of nanoparticles in solution. A tightly focused continuous wave laser beam was used to irradiate a cuvette containing a solution of silver sulfide-glutathione quantum dots (Ag2S-GSH QDs), and the maximum steady-state temperature rise was measured with an infrared camera. The data were analyzed using two different photothermal conversion efficiencies, the intrinsic and external conversion efficiencies, to relate the induced heating power of the nanoparticles to the absorbed and incident optical powers, respectively. Measurements with a tunable Ti3+:sapphire laser showed that the intrinsic photothermal conversion efficiency of Ag2SGSH QDs exceeded 91% over the 720−810 nm wavelength range. The method was also used to analyze poly (acrylic acid)-coated superparamagnetic iron oxide nanoparticles (PAA/SPIONs), and the intrinsic photothermal conversion efficiency was determined to be 83.4% at 810 nm. At 640nm, PAA/SPIONs with 600 µg/ml of iron had intrinsic and extrinsic photothermal conversion efficiency of 76% and 63%, respectively. This approach is useful for the evaluation of various potential nanoparticles for photothermal therapy applications. In additional in vivo experiments, Ag2S-GSH Herceptin QDs were injected into mice grown with tumor SKBR3. Cisplatin was used as chemotherapeutic drug with the QDs. Both Cisplatin and Ag2S-GSH Herceptin QDs were injected subcutaneously into the mouse and investigated for different drug dozes of 10 mg/kg and 20 mg/kg. The mice were then irradiated with a fiber-coupled diode laser at 793 nm. 10 mg/kg of QDs gave 6.2℃ temperature rise whilst 20 mg/kg of QDs raised the surface temperature by almost 9 ℃ at 1.49 W/cm2 of laser intensity. When the laser intensity was increased to 1.59 W/cm2 and applied for 10 minutes, the temperature rise was recorded as 15℃. The experimental methods described in this thesis work should be useful in the characterization of new potentially important nanoparticles for photothermal therapy applications.
A numerical and experimental design of high entropy alloys for biomedical applications
High entropy alloys are a relatively new type of multi-component alloys that attracted researcher's attention because of their superior properties, which mainly come from their high mixing entropy. Some of their main features are excellent mechanical and corrosion properties that lead them to be a potential candidate for biomedical applications. In this study, both numerical and experimental methods were used to enhance the biocompatibility of high entropy alloys. A machine learning approach was implemented to design a high entropy alloy with desired properties. Two unsupervised machine learning clustering techniques were performed to design a high entropy alloy with excellent biocompatibility properties. The characteristics required for these alloys to be used in biomedical applications were used as the input dataset of the models. These features were mainly pure metal's physical, chemical, mechanical, and biocompatibility properties, such as elastic modulus or cell viability measurements. Furthermore, a magnetron sputtering method was utilized to coat the high entropy alloys with silver for introducing antibacterial properties. The parameters of experiments were adjusted to achieve a coating film with desirable characteristics. It was shown that by increasing the deposition duration, the coating thickness and grain size increased, which can enhance the antibacterial characteristics. The ion release of coated samples was then measured after immersing in simulated body fluid for fourteen days. The measurements demonstrated that the Ag ion release increased by increasing the coating thickness, but the ion release of substrate elements stayed almost the same. Consequently, the higher release of Ag ions may lead to a better antibacterial effect since it is the main factor controlling antibacterial characteristics.
Iron, Cobalt and Nickel-Based Metal Borides as Low-Cost Nanocatalysts for Highly Efficient Hydrolysis of Sodium Borohydride
In today's world, fossil fuels, commonly known as non-renewable energy sources, provide 80 percent of the energy required. Toxic gases such as CO, SO2, and NOx are produced as a result of the combustion of these fuels. These gases have a wide range of harmful consequences for the environment and human health, particularly in terms of climate change. Environmentally friendly solutions are being researched to meet the energy needs. Hydrogen energy, in particular, has received a lot of interest in recent years. Hydrogen could be extracted from a variety of substances. In terms of high gravimetric hydrogen density, controllable reaction kinetics, non-toxicity, non-combustibility, and ease of storage in the open air, sodium borohydride (NaBH4) is a good choice. However, the activation energy of the NaBH4 hydrolysis reaction is 217 kJ/mol, which is a relatively high value. In order to reduce this activation energy value and to obtain hydrogen more effectively, the reaction should be carried out in the presence of a suitable catalyst. Although traditionally known noble metal catalysts such as platinum, iridium, rhodium, and ruthenium show excellent performance, alternatives should be developed due to their limited reserves and very high costs. Due to their high strength, high hardness, high chemical stability, magnetic qualities, and superior wear/corrosion resistance, transition metal borides can be used in a variety of applications. Transition metal borides, which are less expensive and durable, are emerging as a viable alternative catalyst. In this thesis, among transition metal borides, iron, cobalt and nickel-based metal borides (Fe–Ni–B and Co–Ni–B systems) were chosen to investigate as promising catalysts for the hydrolysis reaction of NaBH4. Catalyst powders with varied mole ratios were synthesized using a mechanochemical method (followed by a wet milling step) in the Fe–Ni–B system, while inorganic molten salt technique was used in the Co–Ni–B system. Utilized methods enabled to prepare the powders with nanoscale size and a uniform particle distribution, and pure composition. In the Fe–Ni–B system, the powder having Ni3B and FeB semi-crystalline phases, homogenous shape, and 70 nm particle size displayed a remarkable catalytic performance in this direction. The availability of active iron, nickel and boron species on the surface was contributed to the enhancement of catalytic activity. It was able to produce 758 ml H2 min-1 gcat -1 of hydrogen at room temperature and reduce the activation energy of the reaction to 40.8 kJ/mol. In the Co–Ni–B system, the powder with CoB–Ni4B3 crystalline phases, which had a homogeneous morphology, approximately 60 nm particle size and pure content, exhibited an enhanced catalytic performance with a very low activation energy of the reaction of 32.7 kJ/mol. According to the recyclability tests, nanocatalyst powders in both systems exhibited catalytic activity even when used for 5 consecutive cycles. As-prepared catalysts that can compete with noble metals can be considered as recyclable, stable and low-cost materials for highly efficient hydrolysis of sodium borohydride.
Developing transparent polycrystalline zinc selenide (ZnSe) pellets for laser applications
Transparency in particular regions of the electromagnetic spectrum is well recognized and is utilized in many applications, including solid-state lasers, optical windows in optical spectroscopy instruments, and transparent armors. Improving the quality of transparency, mechanical endurance, thermal stability, and corrosion resistance in this material class has attracted academic and industrial interest. Nowadays, numerous transparent ceramics are available in various compositions employed in different applications due to their physical, chemical, and optical characteristics. Among many functional transparent ceramics, ZnSe, in particular, exhibits a number of unique properties: (i) it does not oxidize at temperatures up to 300 °C in the ambient atmosphere, (ii) its refractive index doesn't vary to a large extent with temperature, (iii) it exhibits homogeneity in refractive index at different wavelengths, (iv) it is non-toxic, (v) it has low phonon energy, (vi) it exhibits isotropic properties because of its cubic crystal structure, and (vii) it exhibits transparency in a broad range of the electromagnetic spectrum (0.6–21μm). ZnSe is widely chosen as a host material for lasers that operate in a broad spectrum, particularly in the mid-infrared range. Until now, the polycrystalline ZnSe materials having the highest light transmittance have been synthesized using chemical vapor deposition. However, this approach needs the use of very hazardous gases such as H2Se and has a relatively slow crystal growth rate and is an expensive process. Therefore, new synthesis methods should be developed to produce transparent ZnSe materials. In this thesis, polycrystalline ZnSe materials have been prepared via solid-state synthesis, co-precipitation, and mechanical alloying (high-energy ball milling) methods. Furthermore, single-crystalline ZnSe was obtained via the chemical vapor transport method. The polycrystalline ZnSe materials synthesized by one of the aforementioned methods have been consolidated using the spark plasma sintering (SPS) process with a variety of sintering parameters. Chemical, structural, and optical analyses have been performed on all as-obtained ZnSe pellets. According to the characterization results, the formation of a trace amount of impurity in the powder material to be sintered, a broad particle size distribution, and small crystallite sizes considerably lower the optical transmittance of the ZnSe pellets obtained by SPS. Additionally, carbon diffusion to the sintered material has been detected from the graphite die after SPS process, evidenced by the material's black/gray color. By optimizing the SPS parameters, it has been possible to hinder carbon contamination of the material to be sintered. Besides, by applying a purification step (heat treatment either under Argon or vacuum atmosphere) for the powder to be sintered and by tuning the SPS parameters, pellets with large grain size and less porous structure have been obtained. In this way, the transparency of the pellets has been largely improved. The best sintered ZnSe sample has been obtained by using the ZnSe powder prepared by the solid-state method for 150 minutes at 1200 °C. This sample provided a light transmittance of around 50% in the far-infrared range. To the best of our knowledge, to synthesize single-crystalline ZnSe materials, ZnCl2 was used as a transport agent for the first time in this thesis. Single-crystalline ZnSe synthesis up to 2 mm in length has been accomplished using this method in trials conducted under various heat gradients and with varying concentrations of transport agents.
Development of carbon dots for security inks
Carbon Dots (CDs) emerged as metal-free promising nanoparticles creating new opportunities in optoelectronics, bio-applications, catalysis, sensing, security applications, etc. However, a profound understanding of the physical and chemical properties still pose a significant challenge. The origin of optical properties and emission tunning is not straightforward. The fundamental problem of this thesis work is towards elucidating the relationship between the structural/chemical features and the optical properties of CDs. This thesis also addresses a daily problem: Authentication of valuable papers such as diplomas, banknotes, passports, etc. to prevent counterfeiting. Therefore, CD`s developed in this thesis was evaluated in inks and binders towards developing photostable optical tags. The documents were authenticated by measuring or reading these tags with special optical sensors or lamps.
Development of luminescent carbon nanoparticles for authentication of petroleum products and investigation of phototherapy potential of nanoparticles
Nanomaterials were exploited in many engineered materials since their properties can be controlled and modified by tunning their size. The relatively new member of the nanomaterials portfolio is the luminescent carbon dots (CD) which are more or less considered biocompatible, non-toxic, and environmentally friendly. However, the development of CDs with different luminescence properties can be challenging. In this thesis, the effects of small organic precursors, N-, O-, P- doping, and solvent (polarity and reducing power) on the emission wavelength of CDs were investigated. In literature, most CDs have blue emission. Examples to CDs with different emission colors exist, yet most of them have major drawbacks such as non-reproducibility, low colloidal stability. A portfolio of colloidally stable, reproducible, easily manufactured CDs to address these disadvantages was developed in this thesis. The second part of the thesis focuses on using CDs in the authentication of petroleum products as photostable optical tags. In the third part of the thesis, one of the most popular photosensitizers, BODIPY, was loaded to highly biocompatible superparamagnetic iron oxide nanoparticles for enhanced delivery of BODIPY to cancer cells and achieve highly selective and enhanced photodynamic therapy. Lastly, the phototherapy potential of aqueous red-emitting CD produced in this thesis was investigated.
Development of high purity lithium Bis(oxalate)borate, LiB(C2O4)2 (LiBOB)and its effect on the stability of standard and new generation electrode materials
Among other battery technologies, lithium-ion batteries have been extensively used in portable electronics, hybrid/electric vehicles (HEVs/EVs) and energy storage devices because of their high energy density, high capacity, long lifetime, low self-discharge rate, and design flexibility. Over the past decades, many attempts have been made to develop new battery materials towards higher energy density, longer life, safety, environmental friendliness, and sustainability. Currently, most commercial lithium-ion batteries have been used graphite as an anode, LiCoO2 as a cathode and LiPF6 in ethyl carbonate (EC) – dimethyl carbonate (DMC) as an electrolyte solution. However, graphite anode may suffer from its limited capacity and the dendrite formation, whereas LiPF6 salt is susceptible to decompose into dangerous byproducts resulting in safety issues and poor electrochemical performances at high operating voltages. Therefore, various electrolyte additives have been introduced to prevent electrolyte degradation, structural changes, HF attack and impedance rise at the electrode surface by developing a solid electrolyte interface (SEI). Among alternative electrolyte additives, lithium bis(oxalate)borate, LiB(C2O4)2 (LiBOB) has received significant interest owing to its high thermal stability, good solubility, high conductivity, low cost, and safety in a wide potential window. In this thesis, high purity LiBOB was developed as an electrolyte additive to enhance the electrochemical performances of standard and new generation electrode materials. First, LiBOB was synthesized using low-cost starting chemicals and fewer processing steps under a protective atmosphere. The detrimental effects of humidity and remaining impurities of LiBOB were evaluated through physical and chemical characterizations. Secondly, a practical recrystallization process was carried out to eliminate lithium oxalate (Li2C2O4) impurity, which is susceptible to precipitate in the electrolyte solution resulting in high cell impedance and poor cycling stability. After obtaining a high purity product, the desired amount of LiBOB was dissolved in the base electrolyte to increase the stability of LiCoO2 cathode at high potential. Thus, LiCoO2 showed superior cycling stability, rate capability, and high energy density related to the enhanced interfacial stability by LiBOB originated SEI layer. As a new generation anode material, surface modified TiO2 / reduced graphite oxide (RGO) nanocomposite was fabricated due to its promising features like structural stability, cycling stability, environmental friendliness, safety, and low cost. First, anatase TiO2 nanoparticles with an average crystallite size below 20 nm was synthesized by a sol-gel method. Coating with RGO as a conductive network and a surface modification process were applied to enhance overall electrochemical performance related to the better dispersion of nanoparticles and mechanical stability. Consequently, the surface modified TiO2/RGO anode showed high capacity, superior cycling stability, high coulombic efficiency (> 99 %) and outstanding rate capability compared to the pristine composite and TiO2 anodes. Finally, the effects of LiBOB on the stability of surface modified TiO2/RGO anode was studied via galvanostatic cycling tests and various post-mortem analyses. It was found that the applied current rate has a large impact on the correct formation of a SEI layer to ensure cycling stability. With the addition of LiBOB, surface modified TiO2/RGO anode showed improved capacity retention and high coulombic efficiency at lower current density. Ex-situ scanning electron microscopy (SEM) images and X-ray photoelectron spectroscopy (XPS) evaluated the development of a chemically and mechanically stable SEI on the anode surface. However, after first discharging, the huge irreversible capacity loss was occurred due to the preferential oxidation of LiBOB that could not be avoided even at low current densities. The promising results given in this study for the high voltage LiCoO2 cathodes and TiO2/RGO anodes with high purity LiBOB could open new pathways for other high voltage cathodes and new generation anodes resulting in higher energy densities, cycling stability and safety for state-of-the-art lithium-ion batteries.
Electrochemical investigations on functionalized 2D graphene, molybdenum disulfide, and nickel sulfide
Widespread implementation of electrochemical-based energy generation and storage techniques is the indispensable strategy for addressing the issues resulting from the constant use of fossil fuels. After decades of searching for the best candidates to be used as electrodes in the electrochemical-based devices, electrochemists have started to investigate the details of electrochemical reactions, such as mechanisms, reaction site, and electrode alterations under operando conditions. Despite invaluable findings about these issues, there are still unanswered or unclearly answered significant questions. Two dimensional (2D) materials provide unique features to design highly active electrodes and have comprehensive investigations about the details of electrochemical reactions. The distinctive structure of 2D materials delivers a high number of available sites for functionalization. This paves the way to alter the electrochemically essential properties such as surface morphologies, electronic structure, carrier concentration, charge transfer efficiency, etc. As a result, it is possible to design new electrodes with enhanced electrochemical activities. In addition, the combination of the availability of a high number of active sites and accessibility of these sites with the surface-sensitive characterization methods provide exceptional conditions to study the electrochemical reactions in detail. This issue has gained prime importance with recent findings about the structural and compositional alterations of electrodes during electrochemical measurements. In this thesis, the unique features of 2D graphene, MoS2, and NiSx derived from Ni(OH)2 combined with various physical and electrochemical characterization techniques have been used to study the alterations in electrode materials during electrochemical reactions. The initial part of the study has been focused on graphene, which is the simplest and the best-known 2D material. It has been shown that N-doping significantly enhances the electrochemical activity of graphene toward oxygen reduction reaction (ORR). After studying the effect of different N configurations on the activity of N-doped graphene, it has been shown that pyridinic N improves the activity of bilayer N-graphene by modulating the interaction between the layers. The increased π- π interaction in pyridinic N-doped graphene, promotes the electron transfer kinetics between layers and results in an improved activity for the N-doped bilayer graphene. In order to have a better insight into the role of different N configurations, active sites, and ORR mechanisms, bilayer pristine and N-doped graphene were investigated by the spectroelectrochemical Raman technique. It has been shown that based on the configuration of the N dopants, it is possible to have a competition between adsorption of H/OH and O2 groups. After showing Raman fingerprints for the possible reactions between O2 (or O2-) with H/OH, it has been proposed that the ORR could have multiple mechanisms with the same transferred electron numbers. Despite enormous efforts in advancing the hydrogen evolution reaction (HER) activity of MoS2, the presence of different phases and relatively complex structure of MoS2 has resulted in a significant number of controversial results in the published reports so far. Unfortunately, most of the operando techniques which have been used to answer some fundamental questions, such as the active site, the mechanism on different MoS2 structures, and the structural changes of MoS2 during HER, have been focused on amorphous MoS2. Here a systematic investigation has been performed to study the effect of modifying a synthesis parameter on the phase, composition, flake size, and defect density of MoS2. It has been shown that N doping improves the HER activity of MoS2. However, this improvement is not stable, and the strain and phase conversion induced by N dopants fade away when they abandon the MoS2 structure. Finally, the HER activity of different nickel sulfides phases in the alkaline electrolyte was investigated. Despite the 3D structure for NiSx, it has been shown that it is possible to synthesize pure 2D NiS2 using a Ni precursor, which has a layer-by-layer structure (Ni(OH)2). It has been shown that the electrochemical measurements performed on these samples have better results compared to the samples with 3D structures. The non-similar dynamic behavior in linear sweep voltammetry results of the samples was investigated by performing Raman spectroscopy and XPS investigations during the course of structural transition. It has been shown that the co-presence of hydroxylated Ni and NiSx promotes the HER activity of the electrodes.
Designing metal-substituted WB2 and MoB2 asefficient bifunctional electrocatalysts for hydrogenand oxygen evolution reactions
Over the past few years, the ever-increasing global energy demand, environmental problems, and climate change have fostered researchers to focus on designing economic and competent catalysts to produce pure energy originating from renewable energy sources. Transition metal diborides (TMDbs) have recently attracted the scientific community's attention because of their remarkable attributes as bifunctional catalysts for both oxygen and hydrogen evolution reactions (OER/HER) with excellent durability in alkaline media. The prominent features, such as superior electronic conductivity and a high density of active sites, are dominated in the 3D framework of MDbs due to the presence of due to the presence of borophene subunits as. In this thesis, metal substituted WB2 and MoB2 were investigated under the alkaline medium as promising electrocatalysts for OER/HER by comparing different substitution transition metals (Ni and Co), variety for concentration in the structure (x = 0, 0.1, 0.2, and 0.3 even higher for MoB2), and two alternative synthesis techniques (molten salt-assisted (ms) and carbothermal reduction (ct)). The aim was to control the crystal structure and morphology, and tailor the electronic structure to optimize adsorption-desorption features on the surface. The results unraveled a general fact that materials synthesized by the molten-salt technique turned to be relatively better catalysts toward OER/HER. In particular, W0.8Co0.2B2/ms demanded the lowest respective overpotentials of 340 and 363 mV to generate a current density of 10 mA cm-2 for OER/HER. While Mo0.8Co0.2B2/ms provided 340 mV to reach 10 mA cm-2 for OER, Mo0.9Ni0.1B2/ms ensured 220 mV overpotential for HER as a further improvement. Besides enhancement in overpotentials, W0.8Co0.2B2/ms displayed durability of 12 h against OER/HER. For MoB2 electrocatalysts, Mo0.9Ni0.1B2/ms showed excellent HER stability for 12 h. Thanks to controlling the nano-size of the particles, their homogeneous distribution throughout the samples, and keeping the layered structure for both metal diborides, the molten saltassisted technique led to electrocatalysts with better electrocatalytic activity compared to the carbothermal reduction. These results demonstrate that layered metal diborides possess great potential as electrocatalyst toward hydrogen and oxygen evolution reactions.
Photocatalytic hydrogen production using layered perovskite oxides
Fossil fuels have a known impact on environmental pollution, and there is a possible scarcity of these fuels in the future. Because of this reason, cleaner and more abundant fuels may be preferred for substitution. Hydrogen, an eco-friendly and promising fuel obtained from electrolysis, is a strong candidate for this purpose. Since solar energy is renewable, electrolyzing water using semiconductors is a sustainable approach. The photocatalytic water splitting reaction can be done by using perovskite oxides, and there are many examples of perovskite oxide materials that can be found in the literature for this purpose. Among these perovskite oxides, layered perovskite oxides stand out because of their chemical stability, tunable morphology, and low-cost precursor chemicals. Sr2TiO4 and KCa2NaNb4O13 are layered perovskite oxides, which have a limited number of studies in the literature, were inspected for their hydrogen evolution capability in this dissertation. The stated molecular structures were studied both with and without dopants to determine their hydrogen evolution rates. Adding additives to a photocatalyst is a widely used method for bandgap engineering. In the study, the effect of copper and nitrogen addition was tested for Sr2TiO4 without the addition of a cocatalyst. Noble metals (Pt, Pd, Ru, Rh) were added to KCa2NaNb4O13 during the synthesis instead of being added later, and hydrogen production rates were evaluated. In addition, proton exchange method was used to increase the surface area of KCa2NaNb4O13 and change its electronic structure. The structural analysis methods, optical measurements, and hydrogen evolution tests were performed on these materials to evaluate their properties. According to the results, both materials were successfully synthesized, their bandgap changes were determined, and their hydrogen production rates increased after the doping and proton exchange processes.
Investigations on the roles of surface states on BiVO4 photoanodes and CuBi2O4 photocathodes for photoelectrochemical water splitting
Climate change is an alarming issue, which has a devastating impact on our planet. Especially CO2 gas emissions due to the wide-spread utilization of fossil fuels play a major role in global warming. To remove fossil fuels from our lives, alternative energy sources are needed to satisfy the ever-increasing energy demand. Hydrogen is a one of the most promising green energy carriers due to its zero-carbon emission upon utilization, and its extremely high gravimetric energy density. Photoelectrochemical water splitting is a promising route to obtain green hydrogen directly from water and sunlight. Oxide semiconductors utilized as photoelectrodes offer significant potential in terms of light absorption and stability in aqueous electrolytes however their poor surface activities and ineffective charge carrier utilization properties hamper their widespread use. Understanding the origin of these poor activities is crucial to de-bottleneck the performance of photoelectrodes. BiVO4 photoanodes, one of the most promising materials for water oxidation reaction, suffer from overwhelming surface recombination of charge carriers which limit their activity. However, the recombination dynamics are complex and the detailed mechanisms of the recombination taking place in the bulk and the surface regions are typically not addressed. In the first part of this study, BiVO4 photoanodes with high surface area were synthesized using electrodeposition method. We show that the water oxidation activity of BiVO4 photoanode is significantly boosted by the TiO2 overlayer prepared by atomic layer deposition (ALD). With a TiO2 overlayer of an optimized thickness, the photocurrent at 1.23 VRHE increased from 0.64 to 1.1 mA.cm-2 under front illumination, corresponding to 72% enhancement. We attribute this substantial improvement to enhanced charge separation and suppression of surface recombination due to surface state passivation. We provide direct evidence via transient photocurrent (TPC) measurements that TiO2 overlayer significantly decreases the photogenerated electron-trapping process at the BiVO4 surface. Electron-trapping passivation leads to enhanced electron photoconductivity, which results in higher photocurrent enhancement under front illumination, rather than back illumination. Even though the electron trapping process is eliminated completely at higher TiO2 overlayer thicknesses, the charge transfer resistance at the surface also increases significantly, resulting in a diminished photocurrent. We demonstrate that ultrathin TiO2 overlayer can be used to fine-tune the surface properties of BiVO4. CuBi2O4 is one of the most promising photocathodes for HER due to its low band gap and high flat-band potential. However, compared to BiVO4, it is a lot less studied, so the roles of the charge carrier dynamics are not yet fully understood. In the second part of this study, we synthesized CuBi2O4 photocathodes via electrodeposition method. The synthesized photocathode films were characterized by XPS, XRD, FESEM, UV-vis-NIR absorption, and Raman spectroscopy. Using electrochemical impedance spectroscopy (EIS) and TPC measurements, we have identified the presence of surface-states, and photogenerated electron trapping process at these states. The results indicate that surface-states near the flat-band potential act as photogenerated electron traps, which results in a delay in the photocurrent onset to 0.9 VRHE -1.0 VRHE even though photocurrent is observed as early as 1.4 VRHE. The TiO2 overlayer was grown on CuBi2O4 surface via ALD with different thicknesses to tune the charge carrier dynamics at the surface and inhibit the electron trapping process. However, even with low coverage of TiO2 overlayer, CuBi2O4 activity significantly decreased. Furthermore, we have identified significant photocorrosion, which is detrimental for the stability of the photocathode. These results suggest that interfacial dynamics are important for the CuBi2O4 photocathode activity and stability. Thus, the surface charge carrier dynamics need to be tuned by other materials than TiO2 to passivate the electron trapping process to improve the performance of CuBi2O4.
Developing various design strategies for poly (ethyleneoxide) (PEO) based dry polymer electrolytes (SPEs)
Poly (ethylene oxide) (PEO) based electrolytes promise high ionic conductivity because of high solubility of various lithium salts and fast ion mobility coupled with segmental dynamics of the polymer. Due to its simple monomer structure with no bulky side groups, PEO crystallizes at room temperature, therefore impedes ion transportation. Furthermore, its low glass transition temperature (-55oC) results in poor mechanical performance. Obtaining highly conductive solvent-free polymer electrolytes without much sacrificing mechanical stability near room temperature remains as a major challenge. In this work, we systematically investigate the role of polymer architecture on crystallization, segmental dynamics, and ionic conductivity in various PEO-based solid polymer electrolyte types, including homopolymers, polymer blends, and nanocomposites. While the homopolymer electrolytes allowed to understand the effect of branching and nanoscale dynamics on Li-ion transport at the fundamental level, the blends of PEO of non-linear architectures of PEO,such as stars, hyperbranched, and bottlebrush, with linear PMMA and their nanocomposites with silica (SiO2) nanoparticles enabled us to decouple segmental dynamics from ionic conductivity. Our results showed a remarkable suppression of crystallization of PEO in the nonlinear blends. Furthermore, the ionic conductivity of the electrolytes which depends on the salt concentration and polymer architecture could be improved with increased mechanical performance by simply modifying the PEO architecture from linear to branched structures. Overall, our study provides a promising and compelling experimental evidence that macromolecular architecture could be a powerful new tool for tuning the ionic conductivity PEO-based electrolytes, opening a new research avenue in the field of polymer electrolytes for lithium-ion batteries.
Nanoparticle mediated diagnosis and combination therapy of bacterial infections
Treatment of bacterial infections has become one of the challenges over the decades. Adhesion on different surfaces of bacteria and biofilm formation have led to the bacteria species to obtain increased survival rates with high morbidity especially in hospital-related infections. Insufficient treatments result in recurrence of the infection or even death of the patient. Current antibiotic drugs are not capable of eradicating the complex biofilm structure due to the acquired resistance mechanisms of bacteria. Therefore studies have been conducted for alternative therapies against multi-drug resistance bacteria and their biofilm-related infections. Photodynamic therapy (PDT) is noninvasive, localized, and promising alternative method. There have been extensive studies of PDT for the treatment of cancer and bacterial infections. PDT is composed of a photosensitizer (PS), molecular oxygen, and the appropriate light source. PS is excited from ground state to the triplet state by the light source to produce reactive oxygen species (ROS) with the interaction of molecular oxygen in the environment. Following ROS production, cellular components are exposed to produced ROS and consequently cell death. PDT can be utilized alone or in combination with chemotherapy or photothermal therapy (PTT) for the treatment of bacterial infections. Photothermal therapy is another alternative, wherein the PS converts light-to-heat and induce local temperature increase, resulting in cell death. It is again controlled, highly-localized, and noninvasive method which has been combined with different therapeutics such as PDT, chemotherapy, immunotherapy or radiotherapy for infectious diseases. Some nanoparticles act as photothermal agents, namely as photothermal nanotherapeutics (PTN). Superparamagnetic iron oxide nanoparticles (SPIONs), mostly known as Magnetic Resonance Imaging (MRI) agents, have been also recognized as PTT agents recently. They are the most studied nanoparticles in nanotherapeutics because of the FDA-approval and high biocompability. SPIONs may also be used as nano-carriers by tailoring surface coating with targeting agents or ligands for the targeted delivery of drugs, genes, and DNA/RNA. Silver sulfide quantum dots (Ag2S-QDs) are semiconductor nanocrystals with broad absorption in UV-visible region and NIR-emission. Their luminescent properties are superior to commercial fluorescent dyes in particular with properties such as resistance to the photobleaching, higher quantum yield, and stability. They have also been recognized as PTT agents and extensively studied for the treatment of cancer. QDs are another class of nano-carriers which can be used for the targeted delivery of therapeutics with bioimaging properties. 5-Aminolevulinic acid (ALA) is an FDA-approved PS with strong fluorescence in visible region for diagnosis and fast accumulation in the tumor and infectious tissue. ALA, itself is not an active PS molecule but it is internalized and converted into active protoporphyrin IX (PpIX) molecule in cells. ALA is also an endogeneous molecule in heme biosynthetic pathway of cells. However, high solubility and rapid clearance from tissues reduces effect of PDT. Additionally, higher amounts of PpIX accumulation is needed for successful PDT applications. In this thesis, antibiotic free treatment of bacterial infections via combination of PTT and PDT is proposed. The combination is expected to bring about a synergestic effect at lower doses of PS and mild hyperthermia. In case of the biofilms, permeabilization of the biofilm via mild PTT is expected to take place and enhance the treatment efficiency. For this purpose, ALA loaded SPIONs were produced. Planktonic cells and biofilms of gram-positive (Staphylococcus epidermidis) and gram-negative (Pseudomonas aeruginosa) bacteria were treated with ALA loaded SPIONs and subjected to dual laser irradiation (640 + 808 nm). ALA-PDT takes place at 640 nm irradiation. SPION-PTT is usually achieved at 808 nm, but here it was determined that light-to-heat conversion efficiency of SPION at 640 nm is also 89.73\%. Combination therapy resulted in a higher growth inhibition rate with complete eradication of planktonic cells of P.aeruginosa. Significant growth inhibition rates were achieved for biofilms of S.epidermidis and P.aeruginosa (10-log and 13-log, respectively). Fluorescence images supported the enhanced delivery of ALA into P.aeruginosa biofilms. It is important to emphasize that SPIONs are the only FDA-approved nanoparticles, and they show low cytotoxicities. Therefore, they can be good candidates for the treatment of multi-drug resistant bacteria infections in vivo and in clinical studies by utilizing combination therapy of aPDT and aPTT. An alternative composition for combination therapy wherein a single wavelength irradiation would be sufficient is also proposed in this thesis: ALA loaded Ag2S-QDs coupled with 640 nm irradiation. PTT potential of QDs at 640 nm laser irradiation was investigated and reported as 65.7%. These QDs were mostly effective against gram-positive bacteria. Significant growth inhibition rates were observed as 8-and 6-log for P.aeruginosa planktonic cells and biofilms, respectively. Confocal microscopy images showed strong NIR emission highlighting the imaging potential of the QDs. It is also shown that these QDs do not have any cytotoxicity against healthy cell line, Vero. Thus, ALA loaded NAC coated Ag2S-QDs are promising candidates for in vivo and clinical phototherapies against gram-positive bacteria related infections with the advantage of imaging in the NIR region.
Transport properties of thermoelectric materials displaying structural phase transition
Developing renewable, sustainable, and environmentally friendly energy conversion technologies is crucial for addressing the global energy crisis. Waste heat recovery using thermoelectric materials has emerged as a promising solution to support energy sustainability. The efficiency of thermoelectric materials depends on their figure of merit, zT, which represents their ability to convert heat into electricity. However, achieving optimal zT values in thermoelectric materials is a complex task due to the interdependence of transport properties. This thesis study focused on phase-changing binary copper chalcogenides (Cu3-xTe2 and Cu2Se) and Li3Sb, as well as a p-type half-Heusler compound, NbCoSn, to investigate their synthesis, characterization, and thermoelectric properties. Synthetic rickardite mineral, Cu3-xTe2 samples exhibited phase transitions, with the second transition leading to improved thermoelectric performance. Zn- and Ag-substitution on the samples affected the phase transition temperature and transport properties. Cu2Se, another thermoelectric material, was studied for its mid-temperature thermoelectric performance. Two synthesis methods were employed, and the incorporation of nano-B or C-coated nano-B was investigated. The study on c-Li3Sb focused on its high-energy ball milling synthesis method and observed stress/pressure-induced phase transitions. The thermoelectric properties of c-Li3Sb showed degenerate semiconducting behavior, but its air sensitivity posed challenges for implementation in energy harvesting applications. Addressing the poor thermoelectric performance of p-type NbCoSn was investigated through Co-deficiency and Zr-substitution. Zr-substitution was found to be more effective in improving the thermoelectric properties of the material.
Femtosecond laser fabrication and optical characterization of low-loss diamond waveguides
In addition to its excellent mechanical and thermal properties, diamond also possesses favorable linear and nonlinear optical properties, including a wide spectral transparency window from the ultraviolet to infrared, a high refractive index, and a reasonably high nonlinear refractive index, among others. In recent years, diamond has also been demonstrated as a promising platform for quantum information and sensing applications due to the presence of spin-active nitrogen vacancy centers (NV) with long coherence times at room temperature. These NV centers generate spin-dependent fluorescence when excited at 532 nm and enable the measurement of numerous physical quantities such as strain, magnetic field, and temperature with high sensitivity. In such quantum sensing applications, optical waveguides play a crucial role, since they can be used for addressing and spatially linking NV centers or for guiding fluorescence between different locations inside the diamond crystal. In this thesis, fabrication and optical characterization of femtosecond (fs) laser written waveguides with varying geometries and parameters in a single crystal CVD grown diamond has been demonstrated. Depressed circular cladding, half-ring, and double-line waveguides were fabricated. Design parameters such as core size and number of written tracks were varied to minimize propagation losses. Characterization of the waveguides was performed at 633 nm, which is close to the peak fluorescence wavelength of the nitrogen NV centers in diamond. Important experimental results revealing the dependence of the propagation loss and refractive index contrast on the design parameters were obtained. The maximum refractive index contrast was estimated as 22.7x10-5 for the fabricated waveguides. The measured propagation loss values of 2.05 dB/cm and 1.20 dB/cm, obtained with circular depressed cladding and half-ring waveguides, respectively, are, to the best of our knowledge, the lowest propagation loss values reported so far among fs laser written diamond waveguides. It is forseen that the photonic devices based on the diamond waveguides examined in this thesis can find applications in quantum communication.
Modified layered titanates for enhanced photochemical and photoelectrochemical water splitting
The search for sustainable green energy sources is becoming essential due to the rising global energy demands while making use of sunlight and water as benign resources show the most potential. A notable method is the use of the energy of the sun to split water into hydrogen and oxygen. Highlighting its critical role in meeting the world's energy demands as a promising energy carrier, hydrogen, with its zero carbon emissions. Solar-driven photocatalytic and photoelectrochemical water splitting offer a compelling method to produce hydrogen and oxygen. Exfoliation and ion exchange methods were utilized to expose active sites in layered titanates, along with modifications like loading single atoms with photoactive species. The first part of this study involved exfoliating potassium lithium layered titanates (KTLO) and photo-deposition of single atom Sn onto the layer surfaces, as Sn atoms act as co-catalysts. The exfoliation is confirmed by using X-ray diffraction (XRD), showing the expansion of interlayer spaces. Exfoliated titanate showed a distinguished photocatalytic hydrogen evolution reaction (HER) from water and ammonia borane. Adding Sn single atoms to its surface significantly increases the photocatalytic activity, causing roughly a threefold increase in exfoliated titanate. It is also demonstrated that the optimal Sn loading is required for the best HER activity. A thorough investigation demonstrated by photoluminescence spectroscopy (PL) showed that exfoliated titanate showed an improved lifetime for charge carriers. The second part of this study focused on the photocatalytic and photoelectrochemical activities of cetyltrimethylammonium bromide (CTAB) intercalated layered cesium titanate (CsTO). Interlayer spacing served as proof of successful intercalation confirmed by the X-ray diffraction. Interlayer space expansion of CsTO results in morphological changes and a larger surface area, characterized by Scanning Electron Microscopy (SEM). Likely, the treatment with CTAB induced an ion exchange mechanism during the intercalation process, resulting in the removal of Cs+ ions, and this phenomenon was v confirmed through X-ray Photoelectron Spectroscopy (XPS) analysis. As an n-type material, CTAB intercalated CsTO samples are promising candidates to utilize surface electrons for photocatalytic HER, and photogenerated holes are utilized for photoelectrochemical oxygen evolution reaction (OER). Carbon content between interlayers had an impact on charge carrier utilization and served as a conductive layer for photoelectrochemical OER. Consistent with the photocatalytic HER findings, the linear sweep voltammetry (LSV) experiments conducted for the OER revealed a similar trend, indicating enhanced activity with increased CTAB concentrations. Overall, this study highlights the potential of modified layered titanate photocatalysts for effective water splitting driven by solar energy, with implications for HER and OER in the search for renewable energy sources.
Modified layered titanates for enhanced photochemical and photoelectrochemical water splitting
The search for sustainable green energy sources is becoming essential due to the rising global energy demands while making use of sunlight and water as benign resources show the most potential. A notable method is the use of the energy of the sun to split water into hydrogen and oxygen. Highlighting its critical role in meeting the world's energy demands as a promising energy carrier, hydrogen, with its zero carbon emissions. Solar-driven photocatalytic and photoelectrochemical water splitting offer a compelling method to produce hydrogen and oxygen. Exfoliation and ion exchange methods were utilized to expose active sites in layered titanates, along with modifications like loading single atoms with photoactive species. The first part of this study involved exfoliating potassium lithium layered titanates (KTLO) and photo-deposition of single atom Sn onto the layer surfaces, as Sn atoms act as co-catalysts. The exfoliation is confirmed by using X-ray diffraction (XRD), showing the expansion of interlayer spaces. Exfoliated titanate showed a distinguished photocatalytic hydrogen evolution reaction (HER) from water and ammonia borane. Adding Sn single atoms to its surface significantly increases the photocatalytic activity, causing roughly a threefold increase in exfoliated titanate. It is also demonstrated that the optimal Sn loading is required for the best HER activity. A thorough investigation demonstrated by photoluminescence spectroscopy (PL) showed that exfoliated titanate showed an improved lifetime for charge carriers. The second part of this study focused on the photocatalytic and photoelectrochemical activities of cetyltrimethylammonium bromide (CTAB) intercalated layered cesium titanate (CsTO). Interlayer spacing served as proof of successful intercalation confirmed by the X-ray diffraction. Interlayer space expansion of CsTO results in morphological changes and a larger surface area, characterized by Scanning Electron Microscopy (SEM). Likely, the treatment with CTAB induced an ion exchange mechanism during the intercalation process, resulting in the removal of Cs+ ions, and this phenomenon was confirmed through X-ray Photoelectron Spectroscopy (XPS) analysis. As an n-type material, CTAB intercalated CsTO samples are promising candidates to utilize surface electrons for photocatalytic HER, and photogenerated holes are utilized for photoelectrochemical oxygen evolution reaction (OER). Carbon content between interlayers had an impact on charge carrier utilization and served as a conductive layer for photoelectrochemical OER. Consistent with the photocatalytic HER findings, the linear sweep voltammetry (LSV) experiments conducted for the OER revealed a similar trend, indicating enhanced activity with increased CTAB concentrations. Overall, this study highlights the potential of modified layered titanate photocatalysts for effective water splitting driven by solar energy, with implications for HER and OER in the search for renewable energy sources.
Toward efficient electrochemical reduction of CO2 to CO: Decorating ZnO nanorods with CuxO
This thesis investigates CuxO-ZnO based electrocatalyst as a potential cost-effective electrocatalyst for electrochemical reduction of CO2 to CO (CO2RR). CO2RR is a viable solution that can offer a potential source of energy while also reducing the amount of CO2 in the atmosphere. CO2RR to CO presents a promising route due to having the highest economical visibility among other targeted products. In response, we have synthesized and examined two sets of CuxO-ZnO electrocatalysts derived from earth-abundant elements that have high Faradic efficiency (FE) for CO, exceeding 70%. We initially synthesized CuxO/ZnO electrocatalysts with different amounts of CuxO, using the electrodeposition technique. The optimized CuxO/ZnO electrode exhibited elevated selectivity towards CO, achieving a Faradaic efficiency (FE) of 75% at a low overpotential of -0.8 V vs. RHE. This, in itself, marked a clear improvement from the performance of bare ZnO, which only achieved a CO selectivity of 48% under identical conditions. However, an even more significant enhancement in CO selectivity was observed when CuxO was introduced using atomic layer deposition (ALD). Specifically, the CuxO-250/ZnO electrode, prepared with 250 ALD cycles, exhibited a remarkable CO selectivity of 88% at the same potential, demonstrating the superior performance of ALD CuxO/ZnO over both bare ZnO and the electrodeposited CuxO/ZnO electrodes. The addition of CuxO was found to enhance CO selectivity through the introduction of the new active phase ε-CuZn4 and a reduction in charge transfer resistance (Rct). Furthermore, the incorporation of CuxO significantly influenced the surface reconstruction process, resulting in a distinct surface morphology for the CuxO/ZnO electrodes after CO2RR. This led to an increased surface area compared to bare ZnO, suggesting a possible contribution to enhanced CO2RR performance. We also demonstrated that the applied potential substantially impacts the reconstruction process, affecting the Cu/Zn ratio at the surface, which in turn influences CO selectivity. We proved that after CO2RR at -0.8 V, the Cu/Zn atomic ratio was higher than that after -1.2 V of the same electrode, aligning with our findings on CO selectivity and highlighting the crucial role of surface Cu in CO production. We further noted that after CO2RR, the surface composition underwent significant changes, and the highest Cu/Zn ratio was not necessarily associated with the greatest initial amount of Cu. Rather, the maximum Cu/Zn ratio was observed in the CuxO- 250/ZnO electrode, which also exhibited the highest FE of CO. We attribute this variation to the surface reconstruction process and the galvanic effect between Cu and Zn atoms, both of which influence the surface composition and, by extension, electrode selectivity. In summary, this thesis presents a new earth-abundant electrocatalyst that is highly selective toward CO and provides new insights into the dynamic behavior and surface composition of Cu-Zn electrocatalysts during CO2RR. These insights will significantly contribute to future studies aiming to design more effective and efficient Cu-Zn based electrocatalysts.
Synthesis and characterization of electrocatalysts based on boron and earth-abundant transition metals for electrochemical water splitting
As the world grapples with growing environmental challenges and an ever-expanding population, the need for a sustainable alternative to fossil fuels has become an unprecedented challenge. The quest for sustainable energy production strategies has converged on the electrocatalytic water splitting process integrated with renewable energy resources. The successful realization of this process hinges on the development of competent, cost-effective, and earth-abundant electrocatalysts capable of driving both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) efficiently within the same electrolyte. Over the course of four years of research, this PhD thesis has dedicated extensive efforts to address this challenge by investigating novel electrocatalysts. The following key contributions have been made. 1) Novel Electrocatalysts: Synthesis, characterization, and electrochemical evaluation of newfangled electrocatalysts with the formal composition of Mg1−xTMxB2 (x = 0.025, 0.05, and 0.1; TM = Fe and Co). The study primarily focuses on their performance in both HER and OER under 1.0 M KOH medium. Among these catalysts, Mg0.95Co0.05B2 has demonstrated exceptional HER performance, boasting an overpotential of 470 mV at 10 mA cm−2. The findings suggest a promising direction in the development of highly competent and low-cost electrocatalysts. 2) Transformative Materials: Development of electrocatalysts with a formal composition of V1–xCoxB2 (x = 0, 0.05, 0.1, and 0.2) for use in the oxygen-evolving reaction. The incorporation of Co into the VB2 structure has led to a dramatic transformation in morphology, reducing the overpotential to 200 mV at 10 mA cm−2. This performance rivals that of the noble-metal catalyst RuO2 (290 mV) and demonstrates notable durability under 1.0 M KOH conditions. 3) Metal Organic Frameworks (MOFs): Exploration of nanocomposites based on NCM [N = Ni, C = Co, M = Fe, Cu, Zn]/Ni-BDC@NF, directly developed on nickel foam (NF), revealing unique electronic structure modulation. Notably, the NCF/Ni-BDC catalyst demonstrates outstanding OER performance, requiring only 1.35 V versus a reversible hydrogen electrode (RHE) to achieve 10 mA cm–2 current density. This research showcases the potential of 2D MOFs as versatile materials for efficient water splitting. 4) Heterostructures: Presentation of a high-performance and durable heterostructure of NiMo/CoMoO4 for the alkaline HER, constructed via a two-pot in situ growth strategy on a NF. The heterostructure exhibits low overpotential (102 mV at 10 mA cm–2) and high Faradaic efficiency. It highlights the synergy of metallic and oxide components, demonstrating superior performance in the HER, a crucial element of water electrolysis for carbon-neutral hydrogen production. 5) MOF-Derived Catalysts: Introduction of a nanostructured interface between NiMo/CuO@C derived from Cu-MOF, designed and developed on NF, as a competent HER electrocatalyst in alkaline media. This catalyst shows a low overpotential of 85 mV at 10 mA cm−2, rivalling Pt/C, and exhibits remarkable durability over an extended period of 50 h. This four-year research endeavor culminates in a comprehensive understanding of advanced electrocatalysts for sustainable water splitting. The thesis underscores the significance of these materials in addressing the global need for clean and efficient hydrogen production, offering a pathway towards a more sustainable and eco-friendly energy future.
Tailored doping strategies for photocatalytic CO2 reduction and water splitting on KCa2Nb3O10 layered perovskite
Global warming is defined as the long-term heating of the Earth's surface that has been observed since the pre-industrial period. This phenomenon is primarily driven by human activities, especially the use of fossil fuels, and is accompanied by various consequences such as the increasing global population and industrialization, leading to elevated greenhouse gas levels in the atmosphere and a reduction in green areas. The negative effects of global warming and related climate change, the world's most critical problem, are becoming more serious day by the day. Researchers and industries are pursuing two main approaches to address the challenge of reducing CO2 emissions. The primary approach is to tackle CO2 emissions at the production level by reducing reliance on fossil fuels and exploring alternative energy sources such as hydrogen energy. The second way is clearly to convert CO2 into useful products. Recently, photocatalytic reactions have gained popularity in converting CO2 to useful chemicals and producing H2 from water due to their features such as being environmentally friendly, simpler to set-up, more cost-effective, and more scalable. Yet, there is more to investigate about photocatalyst systems. The use of hole-scavengers to improve photocatalytic activity has been widely used, but the behavior of hole-scavengers on 2D nanosheets of perovskite photocatalysts has yet to be discovered. Another way to improve the photocatalytic activity is the utilization of co-catalyst particles on the surface of the photocatalyst. Given that only the surface atoms of particles actively participate in photocatalytic reactions while the inner atoms remain inactive, the sustainability of using co-catalyst particles, especially those incorporating noble metals, becomes a concern. Thus, it is crucial to disperse single-atom co-catalysts in a photocatalytic system. Hole scavenger study as a first chapter, aims to mechanistically investigate the photocatalytic performance of hole scavengers on nanosheets of 2D layered KCa2Nb3O10 perovskite oxide. A range of hole scavengers were added to the photocatalytic system to observe how they influence the charge carrier dynamics and overall photocatalytic efficiency. To analyze the behavior of hole scavengers on a [Ca2Nb3O10]- perovskite nanosheet, photoelectrochemical and photocatalytic experiments were utilized. Investigation of single site noble atom doped 2D layered perovskite for photocatalytic hydrogen evolution reactions is a second chapter. Pd noble metal was chosen for single-site atom doping. The incorporation of Pd as a single-site atom dopant altered the electrical band structure of the photocatalyst. Depending on the doping concentration, this caused the narrowing or shifting of the bandgap to visible spectrum. Also, it can serve as an active site for photocatalytic reactions, facilitating the transfer of charge carriers. Pd doping reduced the recombination rate of electron-hole pairs by providing additional reaction pathways, leading to more efficient charge utilization. In the last chapter, the photocatalytic CO2 reduction performances and photocatalytic activities were investigated by using ultrathin 2D layered Dion-Jacobson type perovskite oxide KCa2Nb3O10 with single-site ruthenium doping. To investigate the impact of ruthenium doping, a study on CO2 reduction was carried out using photoelectrochemical and photocatalytic methods. The nanosheets exhibited higher photocatalytic CO2 reduction activity. The main products obtained were methanol and ethanol. The findings presented in this study will shed light on the tremendous potential of ultrathin 2D layered Dion-Jacobson type perovskite oxide KCa2Nb3O10 with single-site ruthenium doping as a promising photocatalyst for CO2 reduction. This thesis holds significant implications for developing sustainable strategies to combat CO2 emissions and foster a greener future.
Graphitic carbon nitride/oxygen deficient tungsten oxide s-scheme heterojunctions for photocatalytic dye degradation and hydrogen peroxide generation under visible light irradiation
Synthetic dyes used in various sectors, especially textile and paper, are one of the most important causes of water pollution. These synthetic dyes, which are released into the water stream without being reduced to less toxic byproducts, seriously affect the aquatic ecosystem. For example, synthetic dye effluents in a lake absorb most of the sunlight, making it difficult for algae to photosynthesize. In addition, since dyes mix with groundwater, freshwater reserves are decreasing day by day. In this context, the release of synthetic dye effluents into water streams without being converted into harmless or less harmful by-products such as CO2 and H2O threatens the aquatic ecosystem in the short term and all living life in the long term. In parallel to that, another problem facing humanity is that fossil fuels are about to run out. As of the 21st century, alternative energy sources to fossil fuels are being considered in order to ensure the continuity of civilization. However, the alternative resources that are focused on should not strengthen the threat of global warming, which is accelerating today due to the uncontrolled use of fossil fuels and makes itself felt more like a climate crisis. Among the alternatives, hydrogen peroxide (H2O2), which possesses high energy density, as an energy carrier has attracted great attention from the scientific community since no harmful byproduct or emission is produced when H2O2 is used. Photocatalysis, which is based on the excitation of semiconducting material called photocatalyst through sunlight and its production of electrons and holes to be used in reduction/oxidation reactions, is one of the most sustainable and promising methods for degradation of dyes to harmless compounds and the generation of H2O2 from water. The most critical point in photocatalytic applications is an effective photocatalyst design that can meet the demands and provide high efficiency. Graphitic carbon nitride (g-CN) is a polymeric semiconductor with a generally two-dimensional and layered structure. Furthermore, it can be easily synthesized using nitrogen-rich materials such as melamine and urea. It can be separated into thinner layers by exfoliation. The fact that g-CN can be activated by visible light due to its approximately 2.7 eV band gap and suitable band positions makes it a frequently preferred material in photocatalytic applications. However, the fast and high rate of recombination of photo-generated electrons and holes along with extremely limited light absorption capacity in the visible region pose a major obstacle for the wide-scale use of g-CN. To overcome these disadvantages of g-CN, constructing heterojunctions with other semiconductor materials is a frequently used and successful strategy. In this regard, oxygen-deficient tungsten oxide (WO3-x) is one of the most attractive semiconductors to form a heterojunction with g-CN. First of all, the high valence band potential enables many photocatalytic reactions to occur. However, the most interesting point about this material is the optical properties caused by deficiency of oxygen in the structure. In this way, WO3-x gains a localized surface plasmon resonance property, which makes it possible to produce electrons and holes in the visible and infrared regions despite its wider band gap close to 3 eV. Consequently, the light absorption ability of the material increases, and additional electrons and holes are provided to the system to use in reduction and oxidation reactions. In the light of the information presented, in this thesis, graphitic carbon nitride/oxygen-deficient tungsten oxide (g-CN/WO3-x) heterojunction photocatalyst was synthesized for use in photocatalytic dye degradation and H2O2 production without any metal addition. In this thesis, g-CN/WO3-x heterojunction was synthesized via the solvothermal method, but the synthesis parameters including temperature, time, solvent and concentration (mg/mL), which affect the localized surface plasmon resonance intensity of WO3-x, were also evaluated to synthesize optimum g-CN/WO3-x photocatalyst. Then, many advanced characterization methods (Raman, XRD, SEM, TEM, XPS, ssNMR, UV-Vis DRS, PL, TRPL, TPC and EIS) were used to reveal the structural, chemical, photophysical and electrochemical properties of the photocatalyst. Characterization data revealed that heterojunction synthesis occurred successfully, and electron-hole recombination was greatly suppressed while the optical properties of the heterojunction improved. In the photocatalytic application, under white light (λ> 400 nm), the photocatalyst achieved 98% degradation of 5 ppm methyl orange in 25 minutes (k = 0.1787 min-1) and generated 45.9 mg/L H2O2 from 10% (v/v) methanol solution in 90 minutes. Additionally, in H2O2 production, the effect of adding various alcohols as hole scavengers to the reaction medium on photocatalytic activity was investigated and it was observed that adding 10% (v/v) isopropanol to the reaction environment resulted in the production of 87.44 mg/L H2O2. By using various characterization methods, band structures of semiconductors were elucidated and a plausible electron flow routes were proposed. Then, it was revealed that the formed g-CN/WO3-x heterostructure demonstrated S-scheme heterojunction properties. In the following step, H2O2 experiments were carried out at green light, and it was proved that hot electrons could be produced thanks to localized surface plasmon resonance of g-CN/WO3-x photocatalyst. Also, pathway followed by hot electrons was determined through the same experiment. At the next step, scavenger experiments were carried out to determine the reactive oxygen species present in the reaction environment under white light illumination, and it was observed that the photocatalyst had sufficient potential to produce superoxide and hydroxyl radicals as well as electrons and holes. Following all these results, a possible reaction mechanism was suggested for photocatalytic methyl orange degradation and hydrogen peroxide generation. High activity of the photocatalyst was associated with reasons as follows i) decreased electron-hole recombination due to the formation of S-scheme heterojunction ii) enhanced light absorption ability of photocatalyst towards visible and near-infrared region thanks to the localized surface plasmon resonance, iii) generated hot electrons to participate in redox reactions. Finally, recyclability measurements were performed, and it was observed that the photocatalyst maintained its high activity during the first 4 cycles, but there was a sharp decrease in activity at 5th cycle. Based on the XRD and TEM analysis applied after the 5th cycle, the loss of activity was explained by the conversion of WO3-x in the photocatalyst to stoichiometric WO3.
Development of theranostic hybrid nanoparticles for phototherapies
Treatment of bacterial infections, just like cancer is complicated. The effectiveness of traditional treatment methods is often poor. As an alternative, local phototherapies show great promise as a single modality or in combination with traditional therapies. This thesis describes a portfolio of superparamagnetic iron oxide (SPION), quantum dots (QD) and their combination, namely hybrid nanoparticles, as a new theranostic nanoparticle platform to deliver a combination of tracking/imaging and treatment, specifically, photodynamic therapy (PDT) and photothermal therapy (PTT) with enhanced efficiency. Both PTT and PDT and the treatment of cancer and bacterial infections are the targeted applications for such nanoparticles. A multitude of techniques is employed in the development of such theranostic hybrid nanoparticles considering translation to the clinic. Nanoparticles that can effectively convert light energy into heat are effective in PTT; but for PDT usually photosensitizers, PS, which produces toxic reactive oxygen species (ROS) upon excitation at a particular wavelength, are used. The combination of a PS with a nanoparticle capable of inducing PTT may provide improved therapeutic efficiency. 5-Aminolevulinic acid (ALA), a well-known and FDA-approved prodrug, is used as the, PS in this thesis. This thesis first describes the synthesis of high-quality SPION and Ag2S quantum dots, the formation of hybrid structures and the evaluation of their use as theranostics for imaging and enhanced PTT, thereafter. Yet, NIR-emitting and N-acetyl cystein-coated Ag2S quantum dots emerged as the most promising nanoparticle for imaging, PTT and were applied for the treatment of wound-related biofilm eradication in vivo, for the first time in the literature.
Synthesis and characterization of novel sustainable dual crosslinked sodium carboxymethyl cellulose aerogels
Aerogels are remarkable nanoporous materials with unique properties such as low density, high porosity, high specific surface area, and interconnected pore networks. In addition, their ability to be synthesized from various precursors such as inorganics, organics, or hybrid, and the tunability of their properties make them very attractive for many applications such as adsorption, thermal insulation, catalysts, tissue engineering, and drug delivery. The physical and chemical properties and pore structure of aerogels are crucial in determining their application areas. Moreover, it is possible to tailor the aerogel properties to meet the specific requirements of each application. Synthesis of novel carboxymethyl cellulose (CMC) aerogels for thermal insulation applications is an essential area of research in finding alternative sustainable insulation materials instead of petroleum-based ones. Previous studies have been mainly focused on polysaccharide aerogels, especially cellulose aerogels. Although extensive research has been carried out on cellulose ether hydrogels or films, cellulose ethers and the effects of their functional groups on gelation mechanism, aerogels' morphology, and thermal properties have not been widely investigated. Moreover, when these cellulose ethers are used to synthesize aerogels, their functional groups can play a crucial role in controlling the morphology of the resulting aerogel. Modifying the synthesis parameters such as solvent choice, concentration, pH, and temperature makes it possible to tune the interactions between cellulose ethers and the solvent, leading to the formation of aerogels with tailored morphologies. Furthermore, adding crosslinking agents or other additives can also influence cellulose ether-based aerogels' gelation kinetics and final morphology. This tunability makes cellulose ether-based aerogels promising materials for various applications, including insulation, drug delivery, and tissue engineering. The objective of this study is to synthesize CMC aerogels and also, at the same time, preserve their 3D porous structure and shape. To obtain CMC hydrogels with a 3D porous structure, the freeze-thaw induced gelation method was used and effecting factors were investigated. Since CMC aerogels obtained via supercritical drying have not been studied extensively in the literature, solvent exchange conditions for CMC hydrogels were determined. We aimed to elucidate the pore size distribution, specific surface area, and porosity of these novel materials, thereby paving the way for future exploration of their potential applications in diverse fields. Our work expands the available synthesis methods for CMC aerogels and provides crucial insights into their fundamental pore characteristics, contributing to the ongoing development of sustainable and environmentally friendly aerogels. In this study, sustainable maleic anhydride crosslinked CMC aerogels were synthesized by freeze-thaw induced gelation method. The effect of precursor concentration, crosslinker concentration, and gelation parameters on the aerogel properties, such as density, volumetric shrinkage, surface area, pore diameter, and thermal properties, were investigated. The prepared aerogels exhibited low density between 0.051 g/cm3 and 0.204 g/cm3, high porosity (> 93%), and high specific surface areas up to 214 m2/g. The crosslinking mechanisms in CMC aerogels were examined by Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) analysis. The lowest thermal conductivity was measured as 0.038 W/mK. A theoretical equation was also developed to calculate the thermal conductivity of aerogels. The results indicated that it is possible to tune the aerogel properties by adjusting biopolymer or crosslinker concentration and changing the freeze-thaw parameters. PVA was incorporated into CMC aerogels to promote intermolecular crosslinking and, therefore, reduce the pore diameter. CMC/PVA hybrid aerogels from 4 and 6 wt% aqueous solutions were prepared, and the porous structures of hybrid aerogels were investigated. CMC/PVA hybrid aerogels had low bulk density and high porosity (> 95%). FTIR and NMR analyses were conducted to confirm the esterification reaction. The thermal conductivity of CMC/PVA hybrid aerogel was measured as 0.055 W/mK. Finally, sulfuric acid catalyzed aerogels were synthesized, and it was seen that it is possible to reduce the gelation time with an acid catalyst. The effect of acid concentration on the aerogel properties, such as density, volumetric shrinkage, surface area, pore diameter, and thermal properties, was investigated. The crosslinking reaction was confirmed by FTIR analysis. Contrary to expectations, sulfuric acid catalyzed aerogels had low porosity, low surface area, and high pore size. However, these results shed light on the potential to tailor aerogel properties using acid catalysts precisely. These findings provide valuable insights for optimizing aerogel synthesis tailored to specific applications.
Development and characterisation of high-entropy alloys (HEAs)
This doctoral thesis presents a comprehensive investigation into the potential applications of High Entropy Alloys (HEAs) as protective coatings for biomedical implants and as electrocatalysts for water-splitting applications. The research spans five main chapters, each exploring different facets of HEAs, including their microstructure, mechanical properties, electrochemical behavior, biocompatibility, and catalytic performance. Chapter three highlights the deposition of Ti1.5ZrTa0.5Nb0.5Hf0.5 RHEA films on 316L, CoCrMo, and Ti6Al4V substrates, revealing amorphous, compact structures with superior mechanical properties and adhesion, particularly on the Ti6Al4V substrate, and demonstrating enhanced corrosion resistance in PBS solution. Chapter four focuses on different RHEA films on 316L substrates, identifying potential as protective coatings due to improved hardness, tribological performance, and biocorrosion resistance, with biocompatibility confirmed through in vitro tests. Chapter five discusses both undoped and Ag-doped Ti1.5ZrTa0.5Nb0.5W0.5 RHEA films, noting the mechanical and corrosion-resistant benefits of Ag nanoparticle inclusion. Chapter six details the development of antibacterial RHEA films doped with Ag nanoparticles, showing significant antibacterial efficacy against P. Aeruginosa and S. Aureus, and promising biocompatibility with C2C12 myoblast cells. Collectively, these studies underscore the potential of RHEA films as functional coatings for biomedical applications. The seventh chapter explores the preparation of CoCuFeNi-based HEAs through mechanical alloying (MA) and their evaluation as electrocatalysts for water splitting. The results show that CoCuFeNiMnMo1.5 exhibits the best OER performance, while CoCuFeNiMnMo0.5 demonstrates the best HER activity with lower overpotentials and excellent stability. The assembled CoCuFeNiMnMo1.5 (anode)∥CoCuFeNiMnMo0.5 (cathode) couple achieves a current density of 10 mA cm–2 at 1.76 V, with a Faradaic efficiency for generated H2 of more than 80%. In conclusion, this thesis provides valuable insights into the potential of HEAs as protective coatings for metallic biomaterials and as efficient electrocatalysts for water splitting, contributing to the advancement of HEA research and the development of novel materials with enhanced properties for biomedical and energy-related applications.
Doping strategy for enhanced photocatalytic hydrogen production on tantalum layered perovskite nanosheets
Rise in greenhouse gas levels due to human activity, such as burning fossil fuels, causes global warming and environmental problems. Thus, industries and researchers are investigating alternative energy sources to find solutions to environmental problems and slow down global warming. Hydrogen is one of the solutions to these problems. Semiconductor photocatalysis has been researched for hydrogen production as a sustainable energy source since the 18th century. Many photocatalysis materials, such as metal oxide, metal sulfide, and metal nitride, have been studied and used as photocatalysts. In addition to these materials, layered perovskite oxides have gained significant interest in the photocatalytic field over the last 20 years—the heightened interest results from its tunable band structure, flexible interlayer structure, and tunable morphology. Furthermore, exfoliating layered materials gives 2D building blocks of bulk materials. Resulting nanosheets with few atomic layers offer a high surface area, low migration distance, and high charge separation to improve photocatalytic activity. Although there is a vast interest for efficient H2 production with photocatalytic water splitting, research on H2 production activities needs a long way to go to achieve solar hydrogen efficiencies above 10%, raising questions about strategies to achieve effective photocatalytic activity. The common approach to achieve a high productivity from a photocatalyst is to use co-catalysts, which improves charge separation in photocatalysts. In general, noble metals like Pt are used as cocatalysts in nanoparticle form on the surface of photocatalyst. In this case, the inner atoms of the catalysts are inactive since only surface atoms play a role in catalytic activity. In this study, the effects of introducing Pd, Sn, and N into the structure were conducted for photocatalytic hydrogen production in an aqueous 10 vol.% methanol solution as a sacrificial agent without cocatalyst. After exfoliation of tantalum based perovskite oxide, PdO6 and SnO6 octahedra were formed when Ta was substituted iv with Pd or Sn, which acted as a single-atom catalyst site (SACs). These obtained octahedra structures reduce the original phase's electrical structure and serve as sites for trapping electrons, thereby reducing the recombination rate of photo-induced carriers. Furthermore, exfoliated materials have high surface areas, making them ideal platforms to disperse SACs uniformly. This allows for the use of all metal atoms added to the structure with nearly 100% photocatalytic activity. On the other hand, nitrogen, which is partly substituted for oxygen, has a great potential to minimize electronic band energies due to the formation of isolated electronic states positioned above the top of the oxygen (O) 2p valence band. Various structural, spectroscopic, and electrochemical characterization techniques were used to analyze materials' properties in detail. According to the results, all target materials were successfully synthesized, their bandgap changes were determined, and their hydrogen production rates increased after the doping and proton exchange processes
Designing n-type zintl phases for thermoelectric power generation applications
The advancement of technology and a growing global population have increased energy demand, with over 60% of generated energy lost as waste heat, highlighting inefficiencies. Recovering waste heat is crucial for sustainable energy solutions. Thermoelectric materials, which convert heat into electricity, are of significant interest due to their silent operation, lack of moving parts, and scalable solid-state design. Thermoelectric generators use temperature gradients for power generation, and their efficiency is measured by the dimensionless figure of merit, zT (zT = α²σT/( κlat + κe)), where α is the Seebeck coefficient, σ is electrical conductivity, κlat is lattice thermal conductivity, κe is electronic thermal conductivity, and T is temperature. Achieving optimal zT is challenging due to the interdependence of these properties. Certain ternary and binary metal phosphides, like GaP, NaSnP, and SrLiP, show promising potential as thermoelectric materials due to their superior electronic properties and low thermal conductivities, outperforming PbTe and Bi2Te3. Zintl phase thermoelectric materials, known for their tunable electronic transport properties, complex crystal structures, and low thermal conductivities, also exhibit high thermoelectric efficiencies. Notably, Mg3Sb2-based Zintl phases are particularly interesting for their excellent zT values as n-type materials, highlighting their potential in advancing thermoelectric technology. In this thesis, the synthesis and characterization of binary and ternary phosphides have been explored. These phosphides were synthesized through solid-state synthesis or high-energy ball milling (also known as mechanochemical synthesis), followed by spark plasma sintering to achieve densification for measurement of physical properties. Efforts were made to identify the most efficient synthesis routes for ternary phosphides. Despite rigorous efforts, our exploration encountered significant obstacles owing to the high air sensitivity of the ternary phases and substantial resistivity characteristic of the binary phases. These properties rendered the measurement of both electrical and thermal conductivities unfeasible. Subsequent investigations focused on the design and determination of electrical and thermal transport properties of innovative and efficient n-type Mg3Sb2-based Zintl phases, such as CaMg2Sb2 and CaMg2Bi2. This was achieved through band structure engineering, employing doping strategies with tellurium and lanthanum (Ca0.8La0.2Mg2.1Sb2, Ca0.9La0.1Mg2.1Sb1.95Te0.05, Ca0.8La0.2Mg2.1Bi2 and Ca0.9La0.1Mg2.1Bi1.95Te0.05). These samples were successfully synthesized using high-energy ball milling and sintered through the spark plasma sintering technique. With the help of microstructural characterization and WDX analysis determination of effective doping has been made. Transport measurements were conducted to determine the thermoelectric performance of doped and undoped samples between 300-700 K. After introducing La and Te doping, resistivity and thermal conductivity values had a decrease, Seebeck coefficient values shifted from positive to negative across the entire temperature range studied. Additionally, the observation of negative Hall mobility values indicated that the electrical conductivity in these materials is predominantly due to the movement of free electrons, suggesting that effective doping strategies have indeed increased the carrier concentrations. Also, the calculations made to determine the main contribution of thermal conductivity values revealed that the main contribution is coming from the lattice thermal conductivity part. In conclusion, the La and Te doped Mg3Sb2-based samples, specifically CaMg2Sb2 and CaMg2Bi2, demonstrate potential as efficient n-type thermoelectric materials. By increasing the electrical conductivity values and achieve a promising zT value these findings suggest their viability for incorporation into complete Zintl phase thermoelectric modules, offering promising pathways for enhancing thermoelectric technology's effectiveness and sustainability.
Investigation of Co decorated N, S doped reduced graphene oxide (Co/NSrGO) catalysts for oxygen reduction reaction (ORR) and nh3 electrosynthesis through nitrate reduction reaction (NO3RR)
One of humanity's most challenging problems in our century is enabling sustainable and carbon-zero energy, chemical, and fuel production and minimizing the use of fossil fuel sources to reduce CO2 emissions and decelerate climate change. Two crucial routes to decarbonize energy consumption are the widespread commercialization of proton exchange membrane fuel cells (PEMFC) for transportation and the environment- friendly NH3 electrosynthesis. It is essential to understand reaction kinetics, adsorption tendencies, and current performance of feasible catalysts to actualize the use of affordable and high-performance catalysts for oxygen reduction reaction (ORR) at PEMFCs' limiting cathode compartment and to provide a pathway for zero-emission NH3 electrosynthesis that compensate for NH3 production of carbon-intensive Haber- Bosch process. Comparably cheap and abundant Co and high-surface-area graphene- based Co decorated N, S Doped Reduced Graphene Oxide (Co/NS-rGO) catalysts with different metal compositions and heat treatments are investigated for NH3 electrosynthesis through nitrate reduction reaction (NO3RR) and ORR for their activity and selectivity. In the first part of the thesis, it is unraveled that the optimum composition of Co enables the formation of ORR-active CoS species and homogenous distribution of Co Single Atoms (SAs) steer the selectivity towards 4epathway that is desired for PEMFC and provide the lowest overpotential with the highest current output. With the help of rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques that eliminate the effect of mass-transfer limitations the number of electrons transferred (ne -) is measured for Co/NS-rGO as 4ethroughout the studied potential range as of industrial PEMFC catalyst of Pt/C while the 2Co/NS-rGO catalyst that has higher Co composition with CoO species, favored more HO2 - production. Furthermore, the origin of the catalysts' kinetic activity and the materials' electronic properties are investigated using electrochemical impedance (EIS) techniques. Co/NS-rGO catalyst that has the highest performance obtained a higher density of states (D(EF)) around the Fermi level for the electroreduction, higher charge carrier density concentration (ND) and the flatband potential. Also, electrochemical surface area (ECSA) normalized RDE and RRDE figures indicate the maximum utilization of electrochemically active sites for the Co/NS-rGO catalyst, which has the highest charge storage properties detected by EIS capacitive measurement. In the second part of the thesis, the electrochemical NO3RR-to-NH3 activity of NS-rGO and its Co incorporated and/or 900 °C pyrolyzed counterparts are investigated. It is observed that NS-rGO is inclined to generate high amounts of NO2 while unable to convert NO3 to NH3 with high efficiency. On the other hand, pyrolysis facilitates hydrogen evolution reaction (HER) for both metal and non-metal catalysts. Co incorporation is claimed to promote NH3 yield rate and Faradaic Efficiency (F.E.) significantly at lower overpotentials with higher current outputs. A mechanistic approach using X-ray Absorption Spectroscopy (XAS), EIS, EIS-driven Distribution of Relaxed Species (DRT), and Tafel analysis enabled us to find that the Co/NS-rGO follows a mass transfer limited mechanism. At the same time, Co/NS-rGO 900 is driven by charge transfer limitations and protonation properties. Furthermore, mechanistic studies show that NO2 desorption plays a significant role in NO3RR. To modulate and increase NH3 yield and F.E., a pulsed electrolysis strategy is inherited at both mass- transfer and charge-transfer limited potential regimes. Pulsed electrolysis results show that Co/NS-rGO catalyst with a very high NH3 yield rate and F.E. for NO2RR can increase its NH3 F.E. by two-fold and yield rate by 3-fold through pulsed electrolysis. In contrast, the pulsed strategy facilitates HER and suppresses NH3 production efficiency for the Co/NS-rGO 900 catalyst.
CO₂ elektrokimyasal indirgeme sürecinde elektrokatalizör üzerindeki arayüzey dinamiklerine yönelik içgörüler
The electrochemical reduction of carbon dioxide (CO2RR) into valuable hydrocarbons is a promising approach to address the environmental challenges posed by high atmospheric CO2 levels. CO2RR, as an inevitable pathway to achieve carbon neutrality, offers the conversion of CO2 into high-demand chemical feedstocks, such as carbon monoxide (CO), methane (CH4), and ethylene (C2H4). Despite numerous experimental and computational studies on this topic aimed at increasing the efficiency and selectivity of desired CO2RR products, a comprehensive understanding of the mechanisms and the role of multiple influencing parameters remains unclear. The product selectivity of CO2RR is highly dependent on the properties of the electrocatalysts, electrolyte conditions, and the electrocatalyst/electrolyte interfacial dynamics. Particularly on copper-based electrocatalysts, which are capable of forming multi-carbon products through multistep pathways, understanding the role of each factor controlling selectivity is essential to optimize performance and make CO2RR commercially viable. In this thesis, we seek to gain deeper insights into the mechanisms that favor the formation of specific products, such as hydrocarbons that contain two or more carbon atoms in their molecular structure (C2+) by investigating the structural and chemical changes that occur on the surface of the electrocatalysts during the reaction. The major part of this study focuses on the role of morphology and chemical state of copper-based electrocatalysts in promoting certain reaction pathways over others, such as hydrogen evolution reaction (HER). This has been attainable using the in-situ spectroscopy technique. We investigated CO2RR on two Cu oxide electrodes with distinct structures, finding that the compact structure achieved double faradaic efficiency for C2+ products (40%). Operando Raman spectroscopy revealed that the formation of a metastable malachite phase shifted local pH, potentially by consuming bicarbonate species, and hindered further reduction by preventing CO dimerization, thus limiting C2+ product formation. Additionally, we explore the impact of the reaction environment, particularly the influence of electrolyte composition. This study explores the impact of Cs+, K+, and Li+ cations on CO₂ reduction to CO over a ZnO nanorod electrode using the fast and facile technique of rotating ring disk electrode (RRDE). Our results show that Cs+ boost CO₂RR activity by regulating OH⁻ concentration and maintaining local pH, highlighting the role of cations in influencing CO2RR dynamics on oxide-derived Zn electrodes. Moreover, we investigate the structural changes and chemical states of synthesized Cu oxide nanocubes as the electrocatalyst, affected by the presence of Cs+, K+, and Li+ cations during CO2RR. Operando Raman spectroscopy and ex-situ XAS and XPS demonstrate that these cations induce structural rearrangements and alter the surface chemistry of the electrocatalysts, which eventually influence the product selectivity. Li+ promotes Cu dissolution, led to significant restructuring of the surface and a mainly metallic Cu phase, favoring CH4 production, while K+ and Cs+ stabilize oxide/hydroxide species on the surface, enhancing C2H4 formation. This research also addresses the stability of copper-based electrocatalysts, a critical factor for applying pulsed CO2RR technologies. We explore the mechanism of Cu dissolution during pulsed CO2RR using a rotating ring-disk electrode (RRDE) and an electrochemical scanning tunneling microscopy (STM) setups. Key factors such as anodic potential, surface roughness, CO concentration, electrolyte composition, and pH were analyzed. Our findings highlight the strong and direct role of higher anodic potential in further Cu ion oxidation and its dissolution, as well as the impact of CO2RR intermediates (CO and OH⁻) in forming a new surface structure that prevents further Cu species dissolution into the electrolyte. Insights from this research will help optimize pulsed CO2RR techniques for better Cu catalyst preservation. Throughout this thesis, we combine characterization techniques and electrochemical analysis to provide a better understanding of the main factors influencing CO2RR outcomes. The insights gained contribute to developing more efficient and durable electrocatalysts, paving the way for the practical performance of CO2RR technologies in sustainable chemical manufacturing.
Designing Mg3(Sb,Bi)2 and MgAgSb for low- and mid-temperature thermoelectric applications
In response to growing concerns over environmental pollution, global warming, and greenhouse gas emissions, many countries are striving to achieve net-zero emissions by 2050. Given that approximately 60% of the energy produced is lost as waste heat, thermoelectric (TE) materials present a promising solution for converting this waste heat into electricity. The conversion efficiency of thermoelectric materials is quantified by the dimensionless thermoelectric figure of merit, zT = S2σT/(κl+κe), where S, σ, κl, κe and T are the Seebeck coefficient, electrical conductivity, lattice thermal conductivity, electronic thermal conductivity and temperature, respectively. Achieving high zT values is challenging due to the interdependent nature of these transport properties. The first part of this thesis focuses on the synthesis, characterization, and transport property evaluation of Nb incorporated Mg3Sb2-Mg3Bi2 solid solutions for low temperatures. The grain boundary complexion phenomena and its effects on transport properties were investigated. As a result of a space charge region formed in grain boundaries, the conduction barrier in grain boundary region was lowered and scattering of charge carriers was inhibited. The achievement of low thermal conductivity coupled with low resistivity values and elevated carrier mobility, contributed to the attainment of high zT values, particularly in low-temperature regimes. For mid-temperature applications, the effectiveness of MgB2 incorporation on the thermoelectric properties of Mg3(Sb,Bi)2 system was investigated, inspired by the energy filtering effect. Despite negligible changes in carrier concentration, a significant increase in the Seebeck coefficient of composite materials is observed. This phenomenon is attributed to energy filtering, where carriers are selectively scattered. The addition of MgB2 induces a reduction in the electronic component of thermal conductivity, resulting in a low κ value at 673 K. The achievement of a high Seebeck coefficient, coupled with low thermal conductivity, contributes to high zT values. The second part of this thesis focuses on the synthesis, characterization, and thermoelectric performance of polycrystalline MgAg0.97Sbx (x = 1, 0.995, 0.975) materials. DSC analysis revealed phase transitions at approximately 588 K (α- to β-MgAgSb) and 655 K (β- to γ-MgAgSb). An additional annealing step was incorporated to eliminate the β-MgAgSb phase, improving the crystal structure stability and transport properties. The impact of Sb content on electronic transport characteristics was investigated. Further studies on sintering and annealing parameters revealed that Mg-Ag anti-site defects and strain-induced defects influence thermoelectric behavior, with high Seebeck coefficients, high Hall and weighted mobilities, and low lattice thermal conductivity, resulting in a zT of 0.82 at 330 K. Additionally, the effect of various dopants (Y and Nb on the Mg site, Zn and Co on the Ag site, and Ge and Te on the Sb site) was investigated.
Photovoltaic neural interfaces based on nanowires and quantum dots to restore vision
Optoelectronic biointerfaces have emerged as a promising platform for controlling the nervous system at the cellular, tissue, and organ levels with potential clinical applications via transduction of light energy to ionic currents. In this thesis, we presented a solution-processed photovoltaic nanoassembly comprising a ZnO nanowire (NW) array sensitized with AgBiS2 nanocrystals that enables efficient near-infrared (NIR) neural stimulation through capacitive photocurrents. Nanowires have served as a transformative platform for advanced neural and tissue interfaces. While their photovoltaic properties hold exceptional promise for neural modulation, existing photostimulation approaches predominantly rely on visible-light-activated photoelectrochemical mechanisms. By optimizing nanowire morphology and nanocrystal interdigitation, the platform achieved high charge injection densities (tens of μC cm-2) at low NIR intensities (<1 mW mm-2). The nanoassembly was subretinally placed in an ex-vivo blind rat retina, where it elicited repeatable and robust responses in retinal ganglion cells (RGCs) under NIR pulses. Notably, these responses were achieved at light intensities significantly below established ocular safety limits. We further demonstrated a bioelectronic design where AgBiS2 quantum dots (QDs) served as the photoabsorption material, hole transport medium, and pseudocapacitive electrode–electrolyte interface. The power-law behavior of the anodic and cathodic peaks suggested that diffusion-controlled and capacitive processes contributed to the charge storage mechanism. Furthermore, 3D Bode capacitance maps and phase angle responses indicated a high capacitance of 3.3 mF cm-2 at the half-wave potential (0.044 V vs Ag/AgCl) in artificial cerebrospinal fluid (aCSF). For efficient transduction of light to electrical stimulation, AgBiS2 QDs are embedded onto ZnO NWs in a photovoltaic device architecture, which produced twice the photocurrent (1.9 ± 0.3 mA cm-2) and nearly three times the charge injection (29 ± 2.3 μC cm-2) compared to the planar devices without NWs. Moreover, photostimulation of hippocampal neurons was demonstrated on the device without inducing significant oxidative stress. Collectively, these findings demonstrated an unconventional and efficient bioelectronic device via pseudocapacitive optoelectronic nanocrystals, as well as the nexus of neuronal systems and nanoassemblies, which offers significant potential for enabling unconventional visual prosthetics and advanced neuromodulation therapies. Notably, proven efficacy in eliciting retinal responses within ex vivo models of retinal degeneration underscores its potential for next-generation visual prosthetics and broader neuromodulation applications.
From phase-engineered MoAlB to MOB Mbene-MOF nanocomposites: a viable route to high-performance electrocatalysts for hydrogen evolution
The growing global energy crisis and the environmental consequences of fossil fuel consumption have accelerated the search for clean and sustainable energy sources. Among these, hydrogen has emerged as a promising energy carrier due to its high gravimetric energy density and zero-carbon emissions upon use. Electrocatalytic water splitting offers a green and efficient route to hydrogen generation, but its widespread application is hindered by the need for high-performance, earth-abundant electrocatalysts capable of facilitating both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Two-dimensional (2D) transition metal borides, known as MBenes, have recently gained attention as next-generation electrocatalysts owing to their high electrical conductivity, chemical stability, and large surface area. In this thesis, a two-step strategy was developed to design MBene-based nanostructures for efficient HER catalysis. In the first part of this thesis, the synthesis and chemical characterization of Mo2AlB2 were investigated using MoAlB, a member of the ternary transition metal boride (MAB phase) family. The primary objective of this study was to develop a novel approach for obtaining Mo2AlB2, which is considered the most suitable precursor phase for MoB MBene due to its favorable bonding characteristics. However, Mo2AlB2 is metastable and cannot be synthesized directly through conventional solid-state methods. Existing techniques reported in the literature typically require high temperatures, prolonged reaction times, and multiple processing steps. To address these limitations, a more efficient, low-temperature synthesis method was developed. The reaction was carried out at 450 °C for 2 hours using HCl gas, which was generated in situ through the thermal decomposition of NH4Cl. This process enabled the production of high-purity Mo2AlB2 in a single step. Structural and compositional analyses using XRD, SEM, TEM, EDX, ICP-MS and XPS confirmed the successful synthesis of Mo2AlB2 as a viable precursor for MoB MBene. In the second part of the thesis, MoB MBene was synthesized from the MoAlB phase using a Lewis acid molten salt etching method. The as-obtained MBene was then combined with Ni-BDC, a nickel-based metal-organic framework (MOF), and deposited directly onto nickel foam via a solvothermal process at varying loading ratios. The aim was to fabricate a nanocomposite with a well-integrated interface that could lower the overpotential for HER by harnessing the high surface area of Ni-BDC and the excellent electrical conductivity of MoB MBene. At a current density of 10 mA cm-2, the nanocomposite containing 7.5 wt.% MoB MBene exhibited an overpotential of 214 mV. Upon annealing at 400 °C, the formation of nickel nanoparticles further increased the surface area, resulting in a significantly reduced overpotential of 120 mV. Moreover, chronopotentiometric analysis demonstrated remarkable long-term stability, with the optimized catalyst maintaining consistent performance over 50 hours at 50 mA cm-2. This work introduces an efficient synthesis route for MBene-type materials and demonstrates their successful integration into hierarchical nanocomposites for electrocatalytic hydrogen production. The findings not only expand the synthetic toolbox for 2D boride materials but also underscore the promise of MBene-MOF architectures in green energy applications.
Fonksiyonel aşamalı SiC-TiB2-Al kompozitlerin spark plazma sinterleme yöntemi ile üretimi ve karakterizasyonu
Bu tez çalışmasında, fonksiyonel aşamalı SiC-TiB2-Al kompozit malzemelerin üretimi ve karakterizasyonu üzerine çalışılmıştır. Çalışmanın başlangıç aşamasında, fonksiyonel aşamalı malzeme yaklaşımı ile iki farklı tasarım yapılmıştır. Tasarlanan FAM lara ait tabaka bileşimleri SPS (spark plazma sinterlemesi) yöntemi kullanılarak 1800˚C' de 5 dakika süreyle sinterlenmiştir. Elde edilen fonksiyonel aşamalı kompozit malzemelerin SEM ve/veya optik mikroskop, XRD yöntemiyle içyapı incelemeleri ve faz analizleri yapılmış, aynı zamanda sertlikleri ve yoğunlukları ölçülmüştür. Yapılan çalışmalar sonucunda; TiB2 katkısının SiC'nin sinterlenmesine ve SiC-TiB2-Al kompozit malzemesinin sertliğine olumlu yönde etki yaptığı gözlemlenmiştir. Üretilen kompozit malzemelerin gözenekliliğin %1 in altında olduğu belirlenmiştir. Yapılan FAM tasarımlarında hedeflendiği gibi ön ve arka yüzey arasında aşamalı olarak sertlik ve kırılma tokluğu geçişleri sağlanmıştır.
İki boyutlu mxene kristallerinin mekanik ve dinamik özelliklerinin temel prensiplere dayalı yöntemler ile incelenmesi ve araştırılması
Mn+1Xn formülü ile ifade edilen MXene'ler, iki boyutlu malzeme ailesinin en yeni üyelerinden biridir. Kimyasal aşındırma işleminin doğası gereği, sentezlenmiş MXene bileşikleri, Mn+1Xn(Tm) formülü ile ifade edilen yeni ve gelişmiş işlevsellikler sağlayan fonksiyonel F ve O gruplarına sahiptir. Bu tez çalışmasında, MXene'lerin deneysel çalışmalarındaki son gelişmeler göz önünde bulundurularak, ilk prensipler hesaplamalarına dayanan dinamik ve mekanik kararlılıkları incelenerek kararlı MXene fazlarının belirlenmesini amaçlanmıştır. Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta ve W elementlerini içeren farklı 20 adet saf MXene yapıları (n = 1) ile F ve O (m =2) ile fonksiyonalize olmuş 160 farklı MXene yapıları incelenerek deneysel çalışmalar için en iyi adaylar belirlenmeye çalışılmıştır. Bunun yanında, bu 180 yapının elektronik özellikleri de incelenmiştir. Sonuç olarak, tüm saf yapıların dinamik olarak kararlı olduğu ve deneysel çalışmalar için iyi adaylar olduğu belirlenmiştir. Diğer taraftan fonksiyonelleştirme, MXene'lerin dinamik ve mekanik kararlılığı konusunda çok yönlülük sağlamaktadır. Bu çalışmadan elde edilen sonuçlara göre hem metalik hem de yarı iletken kararlı MXene yapılarının deneysel olarak üretilebileceğinin olası olduğunu göstermektedir.
Synthesis of designed Al- and Ga- doped zinc oxide (ZNO) particles and their application for antistatic coatings
Conductive polymer composites (CPCs) are generally used in products including floor coatings, antistatic-paints, ESD protecting packages, shoes and clothes where dissipation of electrostatic charges are desired. There are several polymeric materials and conductive fillers used in CPCs. In the literature, carbon-based materials or metal nano particles are added to these systems for obtaining desired conductivity. However, these filler materials exhibit some disadvantages such as uncontrolled agglomeration, aesthetically dark color and toxicity. The solution to these problems, developed by our research group, is utilization of Al and Ga doped ZnO particles with designed hexagonal platelet morphology synthesized by a modified solvothermal method. Such designed particles, denoted as Al and Ga doped MicNo (Micron + naNo) throughout the thesis, are composed of primary spherical nanoparticles agglomerated in a controlled manner to form micron sized hexagonal platelets. Accordingly, the research objectives of this study were to determine the role of Al and/or Ga doping on the structural and physicochemical properties of Al- and Ga-doped MicNo-ZnO particles and then to evaluate how the resulting properties affect the performance of antistatic resin in which such particles are used as fillers. The results showed that Al and Ga doped MicNo exhibit highly transparent characteristics in the visible range with low bulk resistivity values and they provide antistatic properties to polymer composites.
Çinko stanat (Zn2SnO4) tozlarının kimyasal kararlılığının belirlenmesi
Bu çalışmada, malzeme özelliklerinin (partikül yüzey alanı, partikül boyutu ve morfolojisi) ve ortam pH'ının, çinko stanatın (Zn2SnO4) sulu ortamdaki kimyasal kararlılığı üzerindeki etkileri araştırılmıştır. Farklı yüzey alanı ve partikül boyutuna sahip Zn2SnO4 tozları yüksek sıcaklık katı hal reaksiyonu ve hidrotermal yöntem ile sentezlenmiştir. Daha sonra Zn2SnO4 süspansiyonları sırasıyla sulu ortamda pH 3, 7 ve 9'da deiyonize suya HCl asidi, NH4OH bazı ve %10 NaCl çözeltileri eklenerek hazırlanmıştır. Süspansiyonlardan alınan süpernatant sıvıları 24 saat aralıklarla 30 gün boyunca toplanarak Zn+2 ve Sn+4 iyonu konsantrasyonları ICP-OES analizi ile belirlenmiştir. ICP-OES sonuçlarına göre, Zn2SnO4'ün katyonları nötr pH ve bazik ortamlarda çok düşük çözünürlük göstermesine rağmen, asidik ortamdan daha fazla etkilenmiştir. Ayrıca, hidrotermal yöntem ile sentezlenen Zn2SnO4 tozunun yüzey alanı (18,053 m2/g) katı hal ile sentezlenen Zn2SnO4 tozuna (1,99 m2/g) göre daha yüksek olduğu için pH 3'teki çözünme hızının daha fazla olduğu ortaya çıkmıştır. Sulu ortamda bekletilen Zn2SnO4 partiküllerinin su ile etkileşime girip girmediği TEM analizleriyle incelenmiştir ve suda bekletilen partiküllerin etrafında etkileşim tabakası oluşmadığı belirlenmiştir. Bu sonuçlar asidik ortamda 30 gün süre boyunca mevcut test koşullarında Zn2SnO4 partiküllerinin yapısının değişmediğini açıkça göstermiştir.
Şekil ve boyut kontrollü Fe ve Co katkılı çinko oksit tozların fotokatalitik özelliklerinin incelenmesi
Çinko oksit (ZnO) güneş kremi içeriklerinde kullanılan mineral esaslı inorganik 2 adaydan biridir. Bununla birlikte ZnO toksik değildir ve UV absorblama potansiyeli yüksektir. Fakat fotokatalitik karakteristik sergiler ve ZnO boyutu nano boyuta doğru gidildikçe fotokatalitik aktivite artar. Güneş kremi kompozisyonlarında yer alan fotokatalitik malzemelerin UV ışınları ile arasındaki etkileşimden dolayı cilt üzerinde oluşan serbest radikalleri azaltmak önemlidir. Bu tez çalışmasında, grubumuz tarafından geliştirilen solvotermal yöntem ile sentezlenen şekil ve boyut kontrollü hekzagonal şekile sahip nano birincil tanelerden oluşan MicNo® ZnO plakaların Fe ve Co katkısı ile yapısal ve performans özelliklerindeki değişikliklerin incelenmesi amaçlanmıştır. Çalışma bilimsel amaç bakımından i) Zn1-xMxO stokiometrisine sahip ağırlıkça %1, 2, 3 ve 4 Co katkılı ve ağırlıkça %0.5, 1, 1.5 ve 2 Fe katkılı MicNo® ZnO nanopartiküllerin sentezlenmesi sonucunda MicNo® ZnO yapısına etkilerinin araştırılması ve bu etkilerden özellikle optik özelliklerinden fotokatalitik aktivite davranışlarına odaklanılması, teknolojik amaç bakımından da ii) sentezlenen nanopartiküllerin optik uygulamalardan güneş kremleri içeriğinde belirli bir oranda yer alan fiziksel UV filtre olarak nano ve mikron boyutun avantajlarını birarada bulunduran MicNo® ZnO teknolojisinin nano birincil tanelerden oluşmasından kaynaklanabilecek olumsuz fotokatalitik aktivite etkileri azaltılarak farklı optik özelliklerde MicNo®ZnO ürünlerin geliştirilmesi olmak üzere 2 amaç üzerine yoğunlaşılmıştır. XRD analiz sonuçlarından %2'ye kadar Fe ve %4'e kadar Co katkılı ZnO tozların herhangi ikincil faz içermediği bulunmuştur. Rietveld analizi ve WD-XRF analizinden katkı elementlerinin ZnO kristal yapısına girdiği desteklenmiştir. SEM mikroyapılarından ve tane boyut analizinden ise Co ve Fe katkılamanın MicNo®ZnO şekil ve boyutunda herhangi bir değişikliğe neden olmadığı görülmüştür. Teknolojik amaç olarak belirlenen fotokatalitik aktivite UV-görünür spektrofotometre kullanılarak model kirletici olarak seçilen metilen mavisinin bozunumu ile belirlenmiştir. Referans MicNo®ZnO toz numunenin fotokatalitik aktivitesi %3 Co katkısı ile %85 oranında azaltılmıştır. %3 Co katkılı ZnO numunesinin tüm numunelerde en düşük değere sahip OH ve O2 serbest radikal üretme yeteneğine sahip olması ve böylelikle bu sonuçlar e- h+ rekombinasyonunda bir artışa neden olduğu XPS analizi ile açıklanmaya çalışılmıştır.
Kimyasal temperlenmiş soda-kireç camının balistik özelliklerinin incelenmesi
Bu çalışmada geleneksel transparan zırh sistemlerine alternatif olarak kullanılabilecek, daha hafif, ince ve aynı zamanda ticari olarak üretilmesine olanak sağlanabilecek kapasitede transparan zırh malzemesi geliştirilmesi hedeflenmiştir. Çalışmanın başlangıç aşamasında piyasadan kolayca elde edilebilecek soda-kireç camı 450 ºC sıcaklığındaki KNO3 tuzunda 8 saat, 16 saat, 1, 3, 5, ve 7 gün boyunca iyon değişimi için bekletilmiştir. Daha sonra her bir numune grubu için on adet sertlik ve beş adet nokta eğme testi uygulanmıştır. Ayrıca taramalı elektron mikroskobunda EDX yöntemiyle KNO3 tuzunun nüfuz etme derinliği ve konsantrasyonu gösterilmiştir. Elde edilen bilgiler doğrultusunda 6 âdeti kimyasal iyon değişim yöntemi ile mukavemet kazandırılmış olmak üzere toplam 16 adet balistik test numunesi üretilmiştir. MIL-STD-662F test standardına göre gerçekleştirilen V50 balistik testi sonucuna göre mukavemet kazandırılmış numunelerin %17 daha iyi performans gösterdiği tespit edilmiştir
Yeni nesil inorganik nanoparçacıkların sentezi ve boşluk taşıyıcı tabakası olarak perovskit güneş hücreleri uygulamaları
Son yıllarda fotovoltaik teknolojisinde önemli bir yer edinen perovskit güneş hücreleri (PGH), %23'ün üzerindeki verim değerleri ile ticarileşme yolundaki en önemli engellerden birini aşmış durumdadır. Ancak, yüksek verimli hücrelerde boşluk transfer tabakası (HTL) olarak kullanılan organik tabanlı spiro-OMeTAD'ın yüksek maliyeti ve içerdiği higroskopik katkılar sebebiyle uzun-dönem hücre kararlılığına olumsuz etkileri bu hücre türünün ticarileşme potansiyelini sınırlayan temel etkenlerdir. Bu çalışmada, spiro-OMeTAD ile kıyaslandığında çok daha ucuza elde edilebilme imkanı sunan ve katkı içermemesi sebebiyle aygıt kararlılığı üzerine olumsuz etkileri bulunmayan inorganik CuCrO2 (CCO) ve CuFeO2 (CFO) malzemelerinin HTL olarak n-i-p konfigürasyonlu PGH'lerine entegrasyonu başarılı bir şekilde gerçekleştirilmiştir. Hidrotermal yöntem ile üretilen HTL malzemelerinin spiro-OMeTAD ile kıyaslandığında; iyi boşluk mobilitesi sunduğu ve hem perovskit hem de metal üst elektrot ile uyumlu enerji seviyelerine sahip olduğu belirlenmiştir. Gerçekleştirilen optimizasyon çalışmaları sonucunda hücre verimleri ihmal edilebilir histeresis ile CCO için %16.7 (16.04 ± 0.40) ve CFO için %15.5 (15.12 ± 0.28) olarak elde edilmiştir. %19.4 (18.87 ± 0.34) verim sunan spiro-OMeTAD tabanlı hücre ile kıyaslandığında kısmen düşük olan bu değerler, arayüzeylerde meydana gelen olası rekombinasyon olaylarına ve akım kaçaklarına atfedilmiştir. Ancak, hem nem miktarına bağlı raf-ömrü (60 gün) hem de operasyonel kararlılık (500 saat) ölçümlerinden inorganik HTL tabanlı hücrelerin başarılı bir duruş sergilediği tespit edilmiştir. Tahmini maliyet analizlerinin de rapor edildiği çalışmada, yüksek verim/maliyet oranı ve uzun dönem kararlılıkları ile inorganik HTL malzemelerinin PGH'lerinin ticarileşme potansiyelinin pratiğe dönüşmesi açısından iyi bir alternatif olduğu söylenebilir.
Piezoelektrik seramik eyleyiciler kullanarak aktif akustik sönümleme
Titreşimler sistemlerin hem yapısal bütünlüğüne olumsuz yönde etki ederken hem de sistemlerin gürültü seviyelerini arttırmaktadır. Sistemlerde meydana gelen titreşimi kontrol edebilmek ve sönümlemek amacıyla pasif kontrol yöntemlerinin dışında yarı-aktif ve aktif titreşim kontrol sistemleri kullanılmaktadır. Bu tez çalışmasının amacı, farklı geometrilere sahip paslanmaz çelik plakalarda oluşturulan titreşimi, farklı sayıdaki pizeoelektrik malzeme kullanarak oluşturulan aktif titreşim sönümleme yöntemini kullanarak sönümlemektir. Aktif titreşim kontrolü için kullanılan malzemeler piezoelektrik seramik plakalardır. Bu çalışmada deneysel ve analitik metotlara ek olarak güncel sayısal yöntemlerden de yararlanılmıştır. Farklı geometrilerdeki plakalar üzerinde yapılan aktif titreşim sönümleme çalışmaları ile bu yöntemin hem iki boyutlu sistemlerdeki uygulanabilirliği ortaya çıkarılmış olacak hem de piezo sistem tasarımının şekillenmesinde iki boyutlu sistemin geometrisine uygunluğuna yönelik araştırmalara katkı sağlanacaktır.
Seramik sağlık gereçlerinde sır yüzeyinin iyileştirilmesi
Bu çalışmada, standart bir seramik sağlık gereçleri (SSG) sır reçetesi esas alınarak daha pürüzsüz, ışığı daha iyi yansıtan opak bir yüzey geliştirilmesi amaçlanmıştır. Sır yüzeyinin geliştirilmesinde etkin parametreler, sır süspansiyonun tane boyut dağılımı, sır yapısında opaklaştırıcı olarak kullanılan zirkonun ağırlıkça yüzdesel değişimi, kuvars hammaddesinin tane boyut dağılımı olarak belirlenmiştir. Bu kapsamda, pişmemiş numunelerin tane boyut dağılım analizi, yoğunluk, viskozite, ısı mikroskobu ölçümleri yapılmış, pişmiş numunelerin karakterizasyonu için XRD, XRF, SEM, spektrofotometre, yüzey pürüzlülük ölçümleri ve ısıl analizleri yapılmıştır. Tane boyut dağımı ölçümleri lazer kırınım tekniği ile ölçüm alan tane boyut cihazıyla (Mastersizer 2000, Malvern Instruments) ile gerçekleştirilmiştir Kaba tane boyutunda parlaklığın azaldığı, ince tane boyutunda daha parlak değerler elde edilmiştir. Kaba tane boyutunda kuvarsın mineralojik analizde tespit edilmesiyle yapıda erimeden kaldığı ve pürüzlülüğü arttırdığı gözlemlenmiştir. Dolayısıyla, optimum tane boyut dağılımı aralığı 7.5 – 8.5 µm bulunmuştur. İkinci olarak, Zirkon miktarının sır yüzeyine etkisini görmek amacıyla yapılan denemelerde standart bir reçete esas olarak alınmıştır. Bu reçeteye zirkon %3 ile %12 arasında ilave edilmiştir. Artan oranda zirkonun ilave edilmesiyle elde edilen beyazlık, pürüzlülük ve parlaklık ölçümlerine bakıldığında beyazlık ve parlaklık değerinin arttığı görülmektedir. Çalışmada optimum zirkon yüzdesinin %10-12 olduğu düşünülmektedir. Son olarak sırda cam yapıcı olarak kullanılan ve parlaklığa katkı sağlayan kuvarsın tane boyutu üzerinde çalışılmıştır. Sonuçlara bakıldığında ince taneli kuvarsın camsı yapıya katkısı daha fazla olduğu için parlaklık ve pürüzlülük değerlerini olumlu yönde etkilemiştir. Anahtar Kelimeler: Seramik, Opak Sır, Zirkon, Tane Boyut Dağılımı, Reoloji