Dept. of Chemistry - Ph.D. / Sc.D.

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  • ItemOpen Access
    Acid-salt-surfactant lyotropic liquid crystalline mesophases: synthesis, characterization, and electrochemical properties of mesoporous M2P2O7 and M2-xM’xP2O7 (M and M’= MN(II), CO(II) and NI(II)) powders and films
    (Bilkent University, 2023-10) Ulu, Işıl
    The mesoporous metal pyrophosphates (M2P2O7) are considered to be important as energy storage materials. This thesis proposes that a surfactant-assisted approach for the synthesis of the mesoporous M2P2O7 would be a good solution since the high surface area is crucial for energy storage materials. A novel synthesis method for the synthesis of mesoporous metal pyrophosphates (Ni2P2O7, Co2P2O7, Mn2P2O7, and binary metal pyrophosphates) is investigated by using a modified MASA (Molten Salt Assisted Self-Assembly) method using related acid; phosphoric acid (PA, H3PO4) or pyrophosphoric acid (PPA, H4P2O7), salts ([Mn(H2O)4](NO3)2, [Co(H2O)6](NO3)2, [Ni(H2O)6](NO3)2) and surfactant (pluronic P123 (EO20PO70EO20, where EO is ethylene oxide and PO is propylene oxide)). Firstly, homogeneous solutions using a broad range of inorganic ingredients are prepared. These solutions are then coated (spin-coating, drop-cast coating, or dip-coating) over a substrate to form lyotropic liquid crystalline (LLC) mesophases that can be calcined at various temperatures to synthesize the mesoporous metal pyrophosphates. The mesophases are characterized using small-angle XRD, ATR-FTIR, POM, and gravimetric measurements during the water evaporation from the solution phases. Aging the mesophases at room temperature forms an ordered (display diffraction(s) at small angles) mesostructured semi-solid (exhibit some cracks under POM with particle-like morphology) M2HxP2O7(NO3)x∙nH2O materials as a result of a polymerization reaction (followed by ATR-FTIR) between transition metal species and PPA. This process initiates in the solution phase and continues within the mesophase by releasing water and nitrate species and becomes stable in 24 h under ambient conditions. The mesostructured semi-solid M2HxP2O7(NO3)x∙nH2O materials are calcined at 300 oC to produce mesoporous spherical M2P2O7 with surface areas of 60, 111, and 41 m2/g for Ni(II), Co(II), and Mn(II) pyrophosphates, respectively. These mesoporous M2P2O7 materials, calcined at 300 oC and higher temperatures, are further characterized using wide-angle XRD, ATR-FTIR, XPS, SEM-EDX, TEM, N2 adsorption-desorption, and electrochemical characterization techniques. Both Co2P2O7 and Mn2P2O7 are amorphous up to 600 oC, then crystallizing at around 600 oC to their alpha and beta phases, respectively. In contrast, the crystallization temperature of Ni2P2O7 is around 700 oC, and it has mainly alpha and minimal delta phases. Mesoporous NiCoP2O7 and MnCoP2O7 with surface areas of 68 and 70 m2/g, respectively, become crystalline at 600 oC to α-NiCoP2O7 and β-MnCoP2O7 phases, and they form solid-solutions when the mole ratio of the metal species is varied. The clear solutions are spin-coated onto an FTO surface and then calcined to produce FTO-coated electrodes; however, those electrodes are not stable during the electrochemical measurements. Therefore, the diluted solutions from the mother liquor are dip-coated over a pure graphite rod (GR) and subsequently calcined to fabricate electrodes of mesoporous metal pyrophosphates. The GR-electrodes, which remain stable during the measurements, are tested using cyclic voltammetry (CV) and galvanostatic charge-discharge measurements with a 3-electrode system in a 3M KOH electrolyte. It is important to note that the metal pyrophosphates transform to their corresponding hydroxides in an alkaline solution during the electrochemical measurements. As a result, the collected data from the electrochemical measurements originate from the M(OH)2 species rather than M2P2O7. The mesoporous spherical Ni2P2O7 material is converted into a very thin needle-like β-Ni(OH)2 (1.5 nm thick and 7 nm wide) in alkaline media, maintaining its spherical morphology. In contrast, the mesoporous spherical Co2P2O7 and Mn2P2O7 particles transform into much thicker plate-like β-Co(OH)2 and β-Mn(OH)2 particles. The transformation time differs depending on the type of metal; the Co2P2O7 and Mn2P2O7 materials transform rapidly (about 30 sec), whereas the complete transformation of Ni2P2O7 to its hydroxide takes around 1 hour. The transformation time determines the particle size and morphology, consequently influencing the capacitance values. The β-Ni(OH)2 exhibits a high charge capacity and specific capacitance (102 mA.s and 368 mF/cm2 at a current density of 1 mA/cm2). However, these values are nearly 10 times smaller in the β-Mn(OH)2 and β-Co(OH)2 electrodes. The addition of nickel ions to the cobalt system in the preparation of binary metal pyrophosphates enhances the capacity and specific capacitance values, with the sample having β-Ni0.67Co0.33(OH)2 composition displaying the highest capacity value in alkali media (170 mA.s at a current density of 1 mA/cm2). Nevertheless, other binary systems (Mn1-xCox(OH)2 and Ni1-xMnx(OH)2) display almost similar capacity behavior to pure cobalt and manganese systems.
  • ItemOpen Access
    Investigation of cyanide-based catalysts and hybrid assemblies for artificial photosynthesis
    (Bilkent University, 2022-09) Akbari, Sina Sadigh
    The storage and conversion of solar energy appear to be a highly promising solution to the global energy dilemma due to the sustainability and abundance of sunlight. Among various strategies, artificial photosynthesis, in which solar energy is directly converted to chemical bonds, has gained a great deal of attraction over the past decades. Such a design requires a catalyst coupled with a semiconductor and/or a photosensitizer as light-harvesting components with proper band levels for efficient charge separation. Therefore, the development of an earth-abundant, robust, and visible-light absorbing photocatalytic assembly has been one of the bottlenecks for the advancement of scalable cells. This work aims to overcome this critical challenge by developing new cyanide-based hybrid assemblies for light-driven water splitting and CO2 reduction. CoFe Prussian blue analogues (CoFe-PBAs) have recently emerged as active water oxidation catalysts with excellent long-term stabilities. In the first study, a ZnCr layered double hydroxide (ZnCr-LDH) with a two-dimensional (2D) morphology and a CoFe-PBA are combined to afford a precious metal-free photocatalytic assembly involving a visible light-absorbing semiconductor (SC) and a water oxidation catalyst (WOC). The SC−WOC hybrid materials exhibit a threefold enhancement in activity compared to bare ZnCr-LDH, which is maintained for 6 h under photocatalytic conditions. The band energy diagram was extracted from optical and electrochemical studies to clarify the origin of the improved photocatalytic performance. The assembly is observed to have an appropriate band energy alignment that facilitates charge transfer from the valence band of ZnCr-LDH to the HOMO level of CoFe-PBA for the water oxidation process. In a follow-up study, we move one step forward by coupling exfoliated Dion−Jacobson type niobate nanosheets as a 2D semiconductor with the CoFe-PBA to construct an SC−WOC hybrid structure, which produces a p−n junction. The assembly exhibits a promoted activity (89.5 μmol g−1 h−1) with a proper band energy alignment for the photocatalytic water oxidation process, and it is stable throughout a 12 h photocatalytic study. These studies mark a straightforward pathway to developing low-cost and precious metal-free assemblies for light-driven water oxidation. Metal dicyanamides are another well-known cyanide-based materials, which can be employed as a catalyst for hydrogen evolution reaction (HER) and CO2 reduction due to the partial electron delocalization and the proper coordination environment of metal ions. Herein, we promote a cobalt dicyanamide coordination polymer, Co-dca, for the first time, as a selective catalyst to reduce CO2 to CO in the presence of a ruthenium photosensitizer (Ru PS) under visible light irradiation. A series of photocatalytic experiments under various reaction conditions were performed to reveal the role of the PS, the scavenger, and the solvent in the selectivity and the activity of the photocatalytic process. We find that Co-dca exhibits an activity of 254 µmol h−1 g−1 and a CO selectivity as high as 93%. Furthermore, cobalt dicyanamide also displayed enhanced H2 evolution activity (25000 µmol h−1 g−1), which is maintained for at least 12 h in a mixed aqueous solution containing a Ru-based photosensitizer and a sacrificial electron donor. The effect of various reaction parameters on the photocatalytic activity of the system was investigated. Overall, this thesis presents various low-cost and stable SC-WOC hybrid assemblies based on CoFe-PBA for the light-driven water oxidation process. Moreover, we suggest the use of Co-dca, for the first time, as catalysts for HER and CO2 reduction reactions.
  • ItemOpen Access
    Exothermic catalytic decomposition of energetic ionic liquids on iridium based catalysts
    (Bilkent University, 2022-01) Kurt, Merve
    Hydrazine (N2H4) is the most commonly used propellant for in-orbit spacecraft propulsion. However, utilization of hydrazine in space missions has challenges associated with health, environment and safety risks. Energetic ionic liquids (EILs) such as ammonium dinitramide (ADN) present themselves as environmentally friendly alternative fuels to hydrazine. EILs can be decomposed efficiently and safely in the presence of a heterogenous catalyst. In this context, monometallic catalysts containing Ir and Al2O3 were synthesized using both wetness impregnation and incipient to wetness impregnation methods, and the structural properties of these catalysts were investigated. Furthermore, the effects of the Al2O3 support material on Ir dispersion and catalytic performance of anaerobic ADN decomposition were studied. In order to improve the Ir active site dispersion on the Al2O3 support material, promoters such as La and Ce were added to the catalyst systems and different pretreatment conditions were applied to the synthesized catalysts. Furthermore, LaMnO3 (perovskite) promoted alumina catalysts with Ir active sites were also studied. Catalysts with high performance, 5Ir/TH100 (5Ir/Al2O3), 5Ir/L3 (5Ir/La-Al2O3), and 5Ir/Sir10 (5Ir/Si-Al2O3) were investigated with in-situ X-ray Absorption Near Edge Spectroscopy (XANES), in-situ Extended X-ray Absorption Fine Structure (EXAFS), in-situ Fourier Transform Infrared Spectroscopy (in-situ FTIR), Temperature Programmed Desorption (TPD), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray (EDX), Pyridine adsorption via FTIR, CO Chemisorption, X-ray Photoelectron Spectroscopy (XPS) and X-ray Fluorescence (XRF) analysis techniques. Our findings revealed that 5Ir/TH100 and 5Ir/L3 catalysts favorably lowered the onset temperature of the ADN decomposition reaction, whereas 5Ir/Sir10 boosted the pressure generation during the reaction. The formation of mostly metallic Ir nanoparticles on 5Ir/TH100 and 5Ir/L3 enables the lowering of the activation energy of the reaction. On the other hand, enhancement in the pressure generation for 5Ir/Sir10 catalyst is associated with the generation of small oxidic Irn x+ clusters which are strongly interacting with the SiOx-AlOx surface domains of the support material. The fundamental structure-functionality relationships unraveled in the current work may allow design of novel catalytic systems for aerospace monopropellant propulsion systems with higher performance by simultaneous exploitation of Ir active sites with different electronic properties.
  • ItemOpen Access
    N-O activation on precious metal-free metal oxide based NOx removal systems
    (Bilkent University, 2022-01) Ercan, Kerem Emre
    Elevated operational costs of platinum group metal (PGM) based environmental catalytic systems shift the focus of catalysis research towards cost-effective materials. In search for PGM-free alternative catalytic materials for NOx removal, high catalytic performance and long catalyst lifetime emerge as two important technical challenges. Within the scope of this dissertation, novel B-site mixed perovskites LaCoxMn1-xO3 (x = 0.1-0.9) and Fe and/or Co based CeO2 catalysts were synthesized, investigated and optimized as high performance, PGM-free, and durable catalyst alternatives for NOx removal systems. The perovskite based catalytic architectures can be utilized as diesel oxidation catalyst (DOC) oxidizing NO/CO to NO2/CO2, which can be coupled with selective catalytic reduction (SCR) catalysts to reduce NOx species to N2. On the other hand, Fe/Co based CeO2 systems can be exploited as catalyst candidates in SCR of NOx. In both of these NOx aftertreatment systems, NO activation is required. A simple and reproducible synthetic protocol was utilized to obtain perovskite-based DOC catalysts whose comprehensive structural characterization was carried out via XRD, N2 adsorption-desorption isotherm, ICP-MS, TEM, H2-TPR, ex-situ and in-situ XANES, EXAFS, in-situ FTIR, XPS, and TPD techniques. The oxidative catalytic performance of the perovskites for CO and NO oxidation was determined in flow-mode catalytic activity tests. It was demonstrated that bulk-oxygen vacancies have a strong influence on the redox activity of the B-site mixed perovskites with the ABO3 structure (where A = La, B = Co, Mn) allowing them to efficiently switch between high and low oxidation states in a reversible fashion under relatively moderate redox conditions without requiring elevated temperatures for regeneration, unlike conventional LaMnO3 and LaCoO3-based simple perovskite systems. La1.01Co0.75Mn0.24O2.97 and La1.04Co0.65Mn0.31O2.97 were found to reveal the best NO and CO oxidation performances among the currently investigated perovskites (La1.01Co0.75Mn0.24O2.97, La1.04Co0.65Mn0.31O2.97, La0.97Co1.03O2.91, and La0.97Mn1.03O3.17), which were on par with a conventional precious-metal benchmark catalyst (i.e., 1 wt. % Pt/Al2O3). Influence of Fe and Co loading on monometallic (Fe or Co) or bimetallic (Fe- Co) catalysts with different CeO2 support materials were studied in SCR of NO to N2. The flow-mode NO reduction experiments point out that 4 wt. % Co/CeO2 is the best catalyst in the studied group of catalysts based on its high N2 selectivity at relatively low temperatures. Detailed structural characterization experiments conducted via XRD, N2 adsorption-desorption isotherm, ATR-FTIR, Raman, and in-situ FTIR techniques indicate correlations between catalyst structure and SCR functionality. Our experimental findings indicate that 4 wt. % Co/CeO2 has relatively higher catalytic performance under excess H2(g) concentrations. The NO activation performance of both La1.01Co0.75Mn0.24O2.97 and La1.04Co0.65Mn0.31O2.97 B-site mixed perovskites and 4 wt. % Co/CeO2 were tested under significantly harsh conditions indicating their strong potential to be used not only in mobile applications but also in stationary NOx removal systems.
  • ItemOpen Access
    Electrochemical impedance spectroscopy based characterization and modeling of electrochemical energy storage systems
    (Bilkent University, 2021-10) Zabara, Mohammed Ahmed Mohammed
    In this thesis, first we demonstrate the characterization of the electrochemical processes in primary Lithium batteries utilizing Electrochemical Impedance Spectroscopy (EIS). We develop Galvanostatic-EIS at discharge technique which provide linear and stable impedance data of primary Li batteries in wide frequency range. The obtained data is further investigated by variation of the electrolyte composition. The results reveal the electrochemical processes associated with impedance response at different frequency regions. The impedance response is then assigned to the corresponding anodic and cathodic charge transfer plus the interfacial processes. Further, we investigate the temperature dependence of the impedance of the batteries which reveals the activated processes and allow for the calculation of their activation energies. Along with the linear impedance response, we also investigate the non-linear response obtained from the primary Li batteries. We show that non-linearity can be used to detect the degree of the passivation of the Li anode. We also show the non-linear response at different States-of-Charge and with temperature change. Second, we utilize linear impedance data in modeling the voltage response of the primary Li batteries. We apply previously developed EIS based Zero-free-parameter modeling approach to predict the voltage response of the primary Li batteries for the desired applications. We improve the method to account for the voltage delay phenomena which is an outcome of using metallic Li in the anode. We further utilize the same modeling method to predict the voltage response of hybrid unmanaged secondary Li-ion batteries supercapacitor systems under real-life setarious. We develop the method to first predict the current distribution among the parallel connected hybrid systems utilizing differential evolution optimization algorithm. Then with the accurate impedance of each system the voltage response is predicted. We validate the modeled results with experimental measurements which shows high accuracy for various hybrid Li-ion supercapacitor systems. Finally, utilizing the modeled results we present design rules for hybridization based on the gains obtained in different parameters such as, peak current, power and total energy. Moreover, the improvements in size and cost of hybridization with different supercapacitor capacities are studied which also contribute in determining the best combination of the Li-ion battery supercapacitor hybrid system.
  • ItemOpen Access
    Investigation on lithium salt - surfactant lyotropic liquid crystalline mesophases: characterization, role of water, and electrochemical behaviors
    (Bilkent University, 2021-09) Topuzlu, Ezgi Yılmaz
    In this thesis, lyotropic liquid crystalline (LLC) mesophases of lithium salts (LiCl, LiBr, LiNO3, LiSCN, LiI, LiI/I2, and LiH2PO4) and 10-lauryl ether (C12H25(OCH2CH2)10OH, C12EO10) have been investigated and the LiI/I2 LLC mesophases was used as gel electrolyte in a dye sensitized solar cell (DSSC). The LLC mesophases of LiCl, LiBr, LiNO3, LiSCN, and LiH2PO4 salts were prepared in a broad range of salt concentrations and found that the mesophases of LiCl, LiBr, LiNO3, and LiSCN salts are stable between 2 and 10 salt/surfactant mole ratios. The LiI-C12EO10 samples undergo meso-crystallization over a 3 mole ratio and not investigated at high salt mole ratios. The LiH2PO4-C12EO10 LLC samples are semi stable and leach out salt crystals upon aging because they cannot hold sufficient water; the water holding capacity and stability of the LiH2PO4 mesophases can be improved by adding various amount of H3PO4 that prevents salt crystallization by increasing the water content of the mesophase. Therefore, this thesis was divided into three main chapters, the first chapter is on the role of water in the LLC mesophases of LiCl, LiBr, LiNO3, and LiSCN. The second chapter is on the LiH2PO4 LLC mesophases, and addition of H3PO4 to prevent crystal formation. And the last one is about the use of the LiI/I2 LLC mesophase as gel electrolyte in DSSCs. The mesophases were obtained by evaporating clear solutions of all ingredients. First, they were fully characterized by using Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR), Raman, and UV-Vis absorption spectroscopy, X-ray diffraction (XRD), AC conductivity, gravimetric measurements, and polarized optical microscope (POM) imaging to identify structural and electrical properties and water up takes of the mesophases. The water evaporation has been monitored by gravimetric, spectroscopic, and conductivity measurements. The water evaporation rate follows a 2/3 power of the evaporated water weight. After complete water evaporation, the LiCl and LiBr systems keep a similar amount of water in their mesophases (typically 3-8 water/Li+, depending on the humidity condition). However, the LiSCN and LiNO3 systems keep a significantly lower amount of water inside their mesophase (1.5-5 water/Li+ depending on the humidity condition). Water has a critical role in forming the lithium salt-surfactant mesophases and their structural properties. To investigate the role of water, humidity-dependent measurements have been carried by using gravimetry, spectroscopy, AC conductivity, and POM techniques. The gravimetric and spectroscopic data show that the H2O/Li+ mole ratio increases linearly by 0.08-0.09 per humidity. Increasing water amount in the mesophases improves their ionic conductivities with at least a factor of two when the humidity was changed from 20 % to 40 %. The water/Li+ mole ratios, water-ion, water-water, water-surfactant, and ion-surfactant interactions strongly depend on the counter anions of the lithium salts and closely follows the Hofmeister series of anions, such that the water molecules are strongly held in the LiCl mesophase but weakly held in the LiNO3 and LiSCN. Controlling the humidity over the lithium salt-surfactant mesophases is a promising way of tuning the properties of the LLC mesophases as needed. Furthermore, mixing two different lithium salts may also improve/adjust the water amount in the gel-phases. For instance, adding LiCl to LiNO3 mesophase increases the final water content in the mesophase and, therefore, the conductivity. Controlling the water amount in the mesophase can be beneficial for the use of the lithium salt-surfactant mesophases in various electrochemical applications. The LiH2PO4 mesophase has also been extensively investigated by changing the salt/surfactant mole ratio and found out that it forms at high humidity but is unstable at lower humidity. The LiH2PO4-mesophase undergoes a phase change by leaching out first surfactant and then the salt crystals. Increasing the salt amount in the mesophase can postpone the salt crystallization but it cannot stop completely. However, the LiH2PO4 mesophases can also be stabilized by adding H3PO4 that holds an extensive amount of water. The LiH2PO4-H3PO4 mesophase forms a buffer LLC mesophase that can be used in electrochemical devices as gel-electrolytes where constant pH is needed. The LiI-I2 redox couple was also prepared in LLC mesophase, characterized by the above approaches, and used in a DSSC as gel-electrolyte. To investigate the importance of the water content in the mesophase different humidity levels were tested for gelation. It was shown that 40 % humidity is the best since it increases the efficiency of DSSC from 3.67 % to 5.36 %. Further increase in the humidity level causes dye detachment and free iodine formation; therefore, the efficiency of DSSC start to decline. Altering how to introduce dye and electrolyte over the working electrode, such as introducing dye and electrolyte simultaneously and carrying gelation at 40 % humidity, results a further increase in the cell efficiency to 7.32 %, which is the highest efficiency achieved for an LLC gel electrolyte in DSSC.
  • ItemOpen Access
    Towards therapeutic automata and hypoxia activated singlet oxygen generators
    (Bilkent University, 2019-08) Ayan, Seylan
    Photodynamic therapy (PDT) is a treatment modality depends on the efficient generation of singlet oxygen (1O2) through excitation of a particular chromophore (sensitizer) followed by an energy transfer to the dissolved oxygen in tumor tissues. Cytotoxic singlet oxygen and other secondary products (reactive oxygen species, ROS) are responsible for the apoptotic and necrotic deaths of the tumor cells. We present a molecular 1:2 demultiplexer (DEMUX) which acts as a "terminator" automaton: once powered up by photoexcitation, the agent releases singlet oxygen to kill cancer cells. Once the cancer cells start apoptosis, the agent interacts with the exposed phosphatidylserines on the external leaflet, and autonomously switches to the signaling mode, turning on a bright emission signal, and turning off singlet oxygen generation. So, the output can switch between singlet oxygen and a confirmatory fluorescence emission for apoptosis, which are mutually exclusive in this design. The automaton that we present here, is based on logic gate considerations and a sound photophysical understanding of the system, and should be a very convincing case of molecular logic with a clear path of progress towards practicality. In another project, we are very much interested in transforming PDT into a more manageable and broadly applicable therapeutic protocol. Our approach to achieve that is to separate photosensitization event from the delivery of singlet oxygen, which is the primary cytotoxic agent of PDT. Thus, a storage compound (endoperoxide) for singlet oxygen has to be designed, which can react with molecular oxygen under typical photosensitization conditions, and then the metastable compound has to be transferred to the tumor site which would release its cargo in response to a chemical or enzymatic cue. This approach assumes that singlet oxygen produced stoichiometrically (as opposed to catalytically through photosensitization) by the chemical transformation of the carrier molecule, would be enough to trigger apoptotic response in cancer cells.
  • ItemOpen Access
    Synthesis and characterization of cucurbituril based photoactive multifunctional assemblies
    (Bilkent University, 2019-01) Koç, Ahmet
    Preparation of cucurbituril based functional materials and their use in various applications ranging from biomedicine to optoelectronics have been studied intensely over the last decade. Supramolecular assemblies, networks and nanostructures constructed through noncovalent interactions of cucurbiturils with -conjugated, photoactive compounds have also been investigated and potential applications in the areas of theranostics, imaging, sensing and catalysis have been shown. In these cucurbituril based architectures, however, cucurbituril is disabled to act as a molecular receptor since they do not involve the covalent conjugation of cucurbituril directly to chromophore. The main motivation of this study is to synthesize multifunctional assemblies and nanostructures in which cucurbituril is covalently attached to various conjugated compounds including porphyrin, conjugated oligomers and polymers. A new multifunctional porphyrin-cucurbituril conjugate based on a photoactive mannosylated porphyrin and monoporpargyloxycucurbit[7]uril was synthesized. Azido-functionalized tetraphenylporphyrin (TPP) was used as a building block. TPP was first mannosylated by copper-catalyzed azide-alkyne cycloaddition (CuAAC), then a monoporpargyloxycucurbit[7]uril was covalently attached to the mannosylated TPP with a second CuAAC reaction. Singlet oxygen generation efficiency of the supramolecular assembly was measured and found to be significantly higher than that of unfunctionalized TPP. ¹H NMR experiments were performed using a suitable guest, bisimidazolium, to prove the availability of CB7 in the assembly as a host. Bisimidazolium guest was observed to form inclusion complex with CB7, which is a promising result for the potential use of this supramolecular assembly as a drug carrier in conjunction with photodynamic therapy. Conjugated oligomers and polymers were synthesized from suitably- functionalized monomers via Pd-catalyzed cross-coupling reactions and their characterizations were performed. Their assemblies and nanostructures with covalently attached functionalized cucurbiturils were investigated. Redox sensitive crosslinked conjugated oligomer nanoparticles (CONs) were synthesized from a conjugated oligomer, OFVBt-N3 and a disulfide bond- containing crosslinker via ultrasound-assisted copper-free click reaction in THF. These spherical and approximately 50 nm-sized CONs preserved their stability and size («60 nm) after dispersing them in water. The behavior of the CONs in the presence of glutathione (GSH) was studied in aqueous medium. It was observed that the CONs are rapidly disrupted by GSH, which is an effective SS bond cleaving biomolecule that is overexpressed in cancer cells. These results imply that when nanoparticles are loaded with an anticancer drug, targeted delivery of the drug to cancer cells can be achieved by cooperative action of enhanced permeability and retention (EPR) effect and S-S bond cleavage by GSH.
  • ItemOpen Access
    X-Ray photoelectron spectroscopy for chemical and electrical characterization of devices extended to liquid/solid interfaces
    (Bilkent University, 2018-12) Göktürk, Pınar Aydoğan
    Understanding of electrical and electrochemical devices in operating conditions is vital for development of new technologies. Many important characteristics that determine the performance of such devices lie on their surfaces and interfaces which significantly deviate from the bulk properties. However, particularly for the liquid based devices, carrying out surface analysis is challenging and requires highly sophisticated instrumentation. In this PhD. thesis, we aim to unravel the potential development on liquids, dielectrics as well as the liquid/solid interfaces during AC and DC excitation in a chemically resolved fashion using the UHV compatible non-aqueous liquids in a basic electrowetting on dielectrics configuration within X-Ray Photoelectron Spectroscopy (XPS) chamber. Low molecular weight Polyethylene glycol (PEG) and a particular ionic liquid Diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI] are used to represent two extreme cases as being non-ionic and fully-ionic liquids. Application of external electrical bias to these devices either from the top or the bottom electrode during data acquisition enabled us to investigate the electrowetting phenomenon, in a chemically addressed fashion In the first part of the thesis, geometrical changes that the drop undergoes during electrowetting have been monitored both by steady state areal maps and by dynamic XPS point analysis where the potential was altered periodically. In the second part, we have focused only on the DC electrowetting of liquids. We probed the potential developments in the dielectric layer and on the liquid by monitoring the changes in the binding energy of the representative XPS peaks with respect to the applied potential. We showed that the conductivity of the liquid has no influence on the potential and the entire potential drop occurs at the liquid/dielectric interface. Dielectric breakdown and its effect on the potential developments were also investigated in this part. In the third part, we have tried to understand the frequency dependent potential developments of the ionic liquid and the polyethylene glycol based EWOD devices by AC electrowetting. Our time dependent XPS measurements under AC excitation with sweeping frequency have demonstrated that EWOD devices exhibit two different behaviors separated by a critical frequency, which is dependent on the AC resistance (impedance) or ionic content of the liquid and also the electrical characteristics of the dielectric layer. Below the critical frequency, XPS spectra are mainly affected by the capacitive component of the dielectric, hence the liquid completely screens the applied electrical field. However, for frequencies above the critical, the resistive component of the liquid dominates and the drop behaves like a resistive strip, resulting in the formation of equipotential surface contours which are shown experimentally for the first time in this study. In the last part of the thesis, an equivalent circuit model was developed to electrically describe the electrowetting behavior of PEG on dielectric and also to generate a solid-state mimicking device to produce the same XPS spectral observations.
  • ItemOpen Access
    Nature of oxygen species on Au(111) and Ag(111) model catalysts and their role in O-H, C-H, C-C, N-H bond activation
    (Bilkent University, 2017-10) Karatok, Mustafa
    Metal-catalyzed heterogeneous oxidation reactions have high importance for the large-scale production of the commodity chemicals vastly used in the chemical industry. Controlling the selectivity in such processes to increase the product yield and minimize the production of undesired byproducts requires a molecular level understanding of the bond activation mechanisms. Thus, understanding the nature of oxygen species in various bond cleavage processes is critical. In the current work, nature of oxygen species was studied on the planar Au(111) and Ag(111) single crystal model catalyst surfaces via x-ray photoelectron spectroscopy (XPS), temperature programmed desorption/ temperature programmed reaction spectroscopy (TPD/TPRS), low energy electron diffraction (LEED) and infrared reflection absorption spectroscopy (IRAS) techniques under ultra-high vacuum (UHV) conditions. Ozone (O3) was utilized as the oxygen delivery agent providing atomic oxygen to the reacting surface. Various oxygen species were determined on both Au(111) and Ag(111) model catalysts and their role in O-H, C-H, C-C and N-H bond activation was investigated by using probe molecules such as methanol, acetaldehyde and ammonia. Three different oxygen species such as atomic oxygen (Oa), subsurface oxygen (Osub) and surface oxide (Oox) were determined on Au(111) single crystal. Oxygen accumulation on Au(111) surface at 140 K for O<1.0 MLE of oxygen coverage resulted in the surface atomic oxygen (Oa) formation while 2D surface oxide (Oox) started to grow for O>1.0 MLE of oxygen coverage at the same temperature. It was also shown that oxygen atoms dissolved (Osub) into the bulk of the Au(111) single crystal when oxygen was accumulated at 473 K. Atomic oxygen species (Oa) on Au(111) was found to be very active for the cleavage of O-H and C-H bonds in methanol; C-C bond in acetaldehyde; N-H bond in ammonia molecules. Surface oxide (Oox) overlayer was also active for methanol oxidation, however it showed very high selectivity towards CO2. Dissolved oxygen atoms (Osub) revealed almost no activity in methanol oxidation reactions on Au(111). In a similar manner, three different oxygen species were determined on the Ag(111) surface such as surface atomic oxygen (Oa), surface oxide (Oox) and bulk-like oxide (Obulk) species. Disordered atomic oxygen (Oa) and surface oxide (Oox) overlayers prepared at 140 K on Ag(111) for O 0.2 MLE were found to be very active for O-H and CH bond cleavage producing formaldehyde as the dominant product. Increasing oxygen quantity for both oxygen species (0.7 MLE O 1.3 MLE) resulted mostly CO2 formation. Oa ( O < 1.10 MLE) was also found to be highly active in N-H bond cleavage for ammonia and selective to N2 as the dominant product. On the other hand, ordered p(5×1) and c(4×8) surface oxide (Oox) overlayers on Ag(111) prepared 473 K were found to be almost entirely inactive for N-H cleavage. Extreme oxygen exposures on Ag(111) ( O > 1.93 MLE ) at 140 K led to bulk-like silver oxide (Obulk) species with poor N2 selectivity in ammonia oxidation and increasing extent of formation of toxic pollutants such as NO and N2O.
  • ItemOpen Access
    Energy transfer, photosensitization and sensing with novel bodipy compounds and their supramolecular assemblies
    (Bilkent University, 2017-06) Yeşilgül, Nisa
    Fluorescent dyes have been used for decades in many applications due to their versatility, sensitivity and many other useful properties. Since their discovery in 1968, BODIPY dyes have come forward and have been used in many fields of research such as photodynamic therapy, anion/cation sensing, dye-sensitized solar cells. In this thesis, novel applications of fluorescent dyes, mainly based on BODIPY fluorophores are reported. In the first project, a photosensitizer derived from erythrosine attached to a luminol derivative is presented. The main purpose was to achieve photosensitization without requiring external excitation with light. In another project, we synthesized and characterized a series of heavy atom substituted BODIPY based photosensitizers. In a related study, the photophysical properties of a BODIPY based chemosensor substituted with benzo-21-crown-7 units were studied in the presence of various -diamino alkanes. Then, we designed a BODIPY based probe sensitive to bioreductive conditions known to be prevalent in hypoxic cancer cells. In the final chapter, we present a mechanically interlocked energy transfer cassette consisted of a distyryl-BODIPY acceptor and two donor units.
  • ItemOpen Access
    Investigation of electronic properties of ionic liquid electrochemical devices by X-ray photoelectron spectroscopy
    (Bilkent University, 2016-12) Camcı, Merve
    Attention towards electrochemical energy storage devices assembled with innovative solvent-free electrolytes ‘ionic liquids’ (ILs) has been progressively rising over the last two decades. In order to design a particular electrochemical device it becomes crucial to understand the structure of interfacial region and the electrical response of ILs. Accordingly, this thesis focuses on Xray Photoelectron Spectroscopic (XPS) investigations of electrochemical devices containing ILs, that is compatible with ultra high vacuum condition needed for XPS. Towards better understanding the fundamental aspects of certain electrochemical issues, electrochemical devices consisting of two metalelectrodes, which contains N,N – Diethyl -N- methyl -N- (2-methoxyethyl) ammonium bis (trifluoromethanesulfonyl) imide, (DEME-TFSI) IL-electrolyte between them, have been investigated by XPS under external electrical stimuli control, as a novel analytical tool for elucidating; (i) charging/ discharging phenomena, (ii) electrical double layer (EDL) formation and (iii) electrochemical reaction products. In the first part, a co-planar electrochemical device, with two gold electrodes on porous polyethylene membrane (PEM) plus DEME-TFSI impregnated between the electrodes, has been studied using external DC bias, for recording the position dependent electrical potential variations. In addition, AC bias is used to harvest temporal behavior. For the AC bias a square wave excitation is used, for which two frequencies are adopted corresponding to slow (10 mHz) and fast (1 kHz) time scales, for probing the response of the system at infinite- and zero-time onset, respectively. In all cases XP spectra have been recorded at different lateral positions. As a result of these DC and AC applications a new understanding has surfaced. Accordingly, although at the metal-electrolyte interface the EDL formation is limited to lateral dimensions at the nanometer scale, its visualization through the analysis of the XPS-probed voltage transients can be extended to very large distances from the interface, in the millimeters scale. These responses have also been modeled using a simple equivalent circuit with two oppositely polarized electrodes and an ionic conducting medium in between. In the second part, re-arrangement of the DEME-TFSI’s ionic constituents at the Au electrode/IL-electrolyte interface has been monitored by the dynamic-XPS approach under application of electrical pulses in the form of a slow (1 mHz) triangular wave with an amplitude of 5V, while recording the intensity fluctuations of the two N1s peaks corresponding to the anionic and the cationic fragments. In the last part, the externally bias XPS analysis has been used for insitu and in-vacuo monitoring of anodically triggered electrochemical preparation and characterization of Au NPs in both a co-planar and also in a wire-plane electrode electrochemical geometries. The small sized Au NPs’ formation within the DEME-TFSI medium has been confirmed by the characteristic peak around 470 nm in the Visible spectrum and with the spherical and well-dispersed (~4 nm) particles in TEM images.
  • ItemOpen Access
    Synthesis and characterisation of mesoporous transition metal ion modified silica-zirconia and silica-sulfated zirconia materials towards NOx catalysis
    (Bilkent University, 2006) Samarskaya, Olga
    The purpose of this work is to design and investigate mesostructured material as a potential support for the reaction of the methane with surface NOx species. Several objectives have been pursued in achievement of the goals. The first objective is to develop a facile procedure for the synthesis of mesoporous silica-zirconia mixed oxide supports that are modified with the sulphate (SO4 2-), cobalt (Co2+) and palladium (Pd2+) ions. The support with requisite catalytic properties was obtained through the adjustment of the synthetic steps and optimisation of the composition. The second objective is to explore the effect of cobalt and zirconia loading in the reaction of the NOx species with methane over the Co-, Pd-, and Co-Pd-silica-sulfated zirconia (Si-SZr). A one-pot synthesis procedure has been developed to prepare the mesoporous silica-zirconia (Si-Zr), Si-SZr supports and the supermicroporous Co(II) incorporated Si-SZr catalysts with a wide range of zirconia loadings. Introduction of the Co(II) active sites by various post-synthesis methods leads to the modification of the surface, whereas the direct (co-precipitation) techniques have provided the modification of both surface and bulk of the supports. The palladium ions were introduced by the conventional impregnation methods onto the calcined solid materials. The detailed analysis of the materials has revealed that the silica and zirconia are well mixed in the framework, whereas the cobalt and sulfate ions are uniformly dispersed on the internal surface of the silica-zirconia supports. The materials prepared in this thesis possess sufficient stability, requisite catalytic properties, as well as good Bronsted and Lewis acidity. However, the high cobalt loading renders the catalytic performance of the Pd-Si-SZr catalysts. Among the investigated catalysts, the interaction of the NOx species with the CH4 takes place at the lowest temperature over the Co-, Pd-, Co-Pd-supported zirconia rich (Zr/Si = 28) Si-SZr catalysts.
  • ItemOpen Access
    Investigation of NO2 and SO2 adsorption/desorption properties of advanced ternary and quaternary mixed oxides for DENOx catalysis
    (Bilkent University, 2015-11) Say, Zafer
    The main premise of the current study is the design, synthesis and functional characterization of novel catalytic materials with superior resistance against sulfur poisoning without compromising NOx storage capacity (NSC) in their NOx Storage Reduction (NSR) catalytic applications. BaO/TiO2-based materials are well known systems in deNOx catalysis, exhibiting promising performance towards sulfur poisoning. However, they suffer from limitations due to poor NSC and high affinity towards unwanted solid state interactionsbetweenTiO2 and BaO storage domains leading to the formation of BaTiOx.The main emphasis of the current work is the design of a novel catalytic system where ZrO2 and Al2O3 act as diffusion barriers between BaO and TiO2 domains while allowing good dispersion and preservation of the individual characteristicsof these active sites within a wide operational temperature window. Along these lines, binary and ternary mixed oxide materials, ZrO2/TiO2 (ZT) and Al2O3/ZrO2/TiO2 (AZT), and their Pt, BaO and/or K2O functionalized counterparts in the form of Pt/ZT, Pt/AZT, Pt/BaO/AZT, Pt/K2O/AZT and Pt/K2O-BaO/AZT with different mass loadings (i.e. 8 and 20 wt. % 20 BaO and 2.7, 5.4 and 10 wt. % K2O) were synthesized via sol-gel synthesis. Surface structure and catalytic properties of the synthesized materials were comprehensively investigated at the molecular level as a function of calcination temperature, catalyst composition, nature of the gas phase adsorbates (e.g. NO2, SO2, O2, H2, N2, N2O C5H5N etc.) interacting with the catalyst surface at various operational temperatures by means of XRD, Raman spectroscopy, BET analysis, in-situ FTIR and TPD. Current results indicate no evidence for the formation of undesired BaTiOx and/or KTiOx. NSC of fresh monolithic catalysts was also quantitatively measured under realistic operational conditions in a tubular flow reactor system. These flow reactor measurements indicated that Pt/8BaO/AZT and Pt/20BaO/AZT materials revealed promising NOx storage and sulfur regeneration performance at low (i.e. 473 K) and moderate (i.e. 573 K) temperatures in comparison to the conventional Pt/20Ba/Al2O3 benchmark catalyst. However, they were found to be surpassed by the conventional Pt/20BaO/Al2O3 benchmark catalyst at higher operational temperatures (i.e. 673 K). Therefore, activity loss at high temperatures was alleviated by incorporating a high-temperature storage functionality (i.e. K2O) to the catalyst structure. Upon this structural enhancement, Pt/5.4K2O/AZT catalyst was found to reveal much higher NSC at high temperatures (i.e. 673 K) as compared to BaO-based materials. An overall assessment of the results presented in the current study suggests that there exists a delicate trade-off between NOx Storage Capacity (NSC) and sulfur uptake/poisoning in NSR systems which is strongly governed by the BaO and K2O loading/dispersion as well as the surface structure of the support material.
  • ItemOpen Access
    Radiochemical and spectroscopic studies of cesium, barium, and cobalt sorption on some natural clays
    (Bilkent University, 2000-08) Shahwan, Talal
    The wide growth in the nuclear activities results in an increasing subsequent influx of radioactive wastes into the environment. This problem has manifested a great deal of interest aiming at finding out ways through which those wastes can be harmlessly isolated from the human environment. Geological disposal is considered as one of the most promising solutions that ensures a safe storage of radioactive wastes as long as their activities are above the accepted levels. Clay minerals are proposed as backfill buffering materials in the geological repositories that can delay the migration of the radionuclides through sorption and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. In this study radiochemical, spectroscopic (ToF-SIMS, XPS), and X-ray diffraction techniques were applied to examine different aspects of the sorption behavior of cesium, barium and cobalt on three natural clay minerals containing primarily kaolinite, illite-chlorite, and bentonite. The elements cesium (Z=55), barium (Z=56), and cobalt (Z=27) have the radioactive isotopes superscript 137 Cs (half-life=30.17 years), superscript 140 Ba (half-life=12.79 days), and superscript 60 Co (half-life=5.3 y) which are important in radioactive waste management. The first two radionuclides are produced in high yields in nuclear fission, whereas the third is an activation product. The natural clay samples that were used in this study originated from natural mineralogical beds at Sindırgı, Afyon, and Giresun regions in Turkey. The characterization of these clay samples showed that the primary clay minerals were kaolinite in Sındırgı clay, chlorite and illite in Afyon clay, and montmorillonite in Giresun clay. Each of these clays possesses different structural properties that result in different sorption capabilities. Radiochemical batch experiments were carried out to examine the effects of time, concentration, and temperature on the sorption of cesium, barium and cobalt on clays. Solutions of these cations spiked with several microliters of the radionuclides 137 CS (half-life=30.1 y), 133 Ba (half-life=10.7 y), and 60 Co (half-life=5.3 y) were monitored using gamma-ray spectroscopy prior to and after each sorption experiment. These results showed that equilibrium is achieved within two days in all cases. The sorption data was adequately described by Freundlich and Dubinin-Radushkevich isotherm models. Based on the parameters of those isotherm models, it was found that sorption was nonlinear, and that bentonite showed the highest sorption affinity and sorption capacity towards the sorbed ions. The thermodynamic parameters indicated that while sorption of cesium and barium on the three clays is exothermic that of cobalt is endothermic. The obtained values of Gibbs free energy change, Delta G degrees, were generally in the 8-16 (kJ/mol) energy range that corresponds to ion exchange type sorption mechanism. Since sorption is mainly a surface phenomenon, part of our sorption studies were carried out using the surface sensitive techniques; Time of Flight- Secondary Ion Mass Spectroscopy (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). In addition, depth profiling up to 70 angstroms was performed using ToF-SIMS to investigate cesium, barium and cobalt concentrations through the clay surface. ToF-SIMS and XPS studies were helpful in figuring out the surface composition of different clays prior to and after sorption. Quantification of the depletion of different alkali and alkaline-earth metals initially contained within the analyzed clay surface showed that ion exchange plays a primary role in the sorption process. In addition, X-Ray Diffraction (XRD) technique was applied to figure out the mineralogical composition of the clay minerals used and examine any structural change a accompanying the sorption process. XRD spectra of the clay samples after sorption showed that -apart from some intensity reductions in some clay features-, no primary changes were detected in the sorption cases of cesium and cobalt. In barium sorption, however, features belonging to barium carbonate were present in the spectra corresponding to sorption on chlorite-illite and bentonite.
  • ItemOpen Access
    New solvents for surfactant self-assembly : molten hydrated salts and concentrated aqueous electrolyte solutions
    (Bilkent University, 2013) Albayrak, Cemal
    Lyotropic liquid crystalline (LLC) mesophases are formed by at least two components: a surfactant and a solvent. Common solvents in the surfactant self-assembly include water, organic liquids, and ionic liquids. In this work, we show that molten hydrated salts of the type [M(H2O)m](X)n (where, M is a transiton metal cation and X is a suitable anion such as NO3 - , Cl- , and ClO4 - ), which have melting points close to room temperature (RT), can organize surfactant molecules into LLC mesophases. As an example, we have focused on the [Zn(H2O)6](NO3)2-C12EO10 system (where, C12EO10 is decaethylene monododecyl ether; H3C-(CH2)11-(OCH2CH2)10-OH). A binary phase diagram was constructed between -190oC and 110oC using differential scanning calorimetry (DSC), polarized optical microscopy (POM), X-ray diffractometry (XRD), fourier transform infrared spectroscopy (FT-IR), and raman spectroscopy. The phase diagram closely resembles the phase diagram of H2O-CmEOn systems, exhibiting typical phases such as spherical cubic, hexagonal, and bicontinuous cubic. It is also observed that the phase transitions are dictated by the critical packing parameter (CPP) as the solvent concentration is changed. The mesophases are unusually stable at low temperatures, where a LLC to mesostructured solid transformation has been observed with a glass transiton at - 52oC. The mesostructured solid phase is also stable at -190oC. The confinement of the salt species in the LLC domains prevents the crystallization of the salt at low temperatures. In the second part, from the analogy between [M(H2O)m](X)n type salts and concentrated electrolyte solutions of alkali metal salts, the mixtures of concentrated aqueous solutions of some Li+ salts (LiCl, LiBr, LiI, LiNO3 and LiClO4) with C12EO10 surfactant, were investigated. The mixtures exhibited LLC mesophases in a broad range of compositions. A ternary phase diagram was constructed for the LiNO3-H2O-C12EO10 system at room temperature using XRD and POM tecniques. In the LLC mesophases formed with the Li+ salts, the water remains as hydrated under ambient conditions and open atmosphere. In addition, the effect of anions on the phase behaviour follows a Hofmeister series except for the ClO4 - ion. Ionic conductivty of the LiX-H2O-C12EO10 (where X is Cland NO3 - ) mesophases has been determined in a broad range of the salt concentrations (5 to 7 salt/surfactant mole ratio) and temperature (-13 to 100oC). The LiCl-H2OC12EO10 LLC samples have also been used as a gel-electrolyte to run a polymer electrochromic device. The mesophase shows excellent performance in this device. The investigations were further extended to include some of the Ca2+ salts, namely CaCl2 and Ca(NO3)2. The concentrated aqueous solutions of both salts with C12EO10 and water exhibited LLC mesophases similar to the molten hydrated salts and concentrated solutions of Li+ salts. In the CaCl2.xH2O-C12EO10 system, an LLC to mesocrystalline phase transformation was observed, for the first time, where the salt, water and surfactant species freezes to a mesocrystalline phase at RT. Lastly, many other salt.xH2O-surfactant LLC mesophases were investigated using the following salts: NaCl, NaBr, NaI, CH3COONa, NaSCN, NaClO4, NaNO3, KNO3, KCl, KSCN, KI, MgCl2, Mg(NO3)2 and NaOH. In addition, the LLC mesophases of concentrated H3PO4 acid and C12EO10 were also investigated. Among these compounds, H3PO4 systems exhibited air stable LLC mesophases at RT and 25% relative humdity (RH). The MgCl2 system was found to exhibit air stable LLC mesophases for a couple of hours. The NaI, KSCN and NaClO4 systems were found to be stable at low salt concentrations with little or no mesostructured order. Other salt systems were unstable and leached out salt crystals rapidly. The NaOH system is unstable because of a reaction with CO2 in the air. In summary, we have found a correlation between the deliquescent relative humidity value of the salt and its LLC mesophase formation ability under ambient conditions.
  • ItemOpen Access
    Photo-dynamic XPS for investigating photoinduced voltage changes in semiconducting materials
    (Bilkent University, 2011) Sezen, Hikmet
    The main motivation of this Ph.D. study is investigation of the photoinduced voltage changes in semiconductive materials with X-ray Photoelectron Spectroscopy (XPS). For this purpose, we have developed a technique for recording the shifts in the positions of the XPS peaks in response to different waveforms of electrical and/or optical stimuli for tracing dynamics of the developed potentials originating from intrinsic or extrinsic factors of the semiconductive materials such as charging/discharging, photoconductivity, surface photovoltage, band-bending/flattening/inversion, etc. Within this purpose, the surface photovoltage behaviors of n- and p-type doped Si and GaN samples are examined with the photo-dynamic XPS, to follow the behavior of the bandbending under photoillumination in both static and dynamic fashions. The band inversion effects are clearly observed on the n- and p-Si samples in the presence of a dielectric silica overlayer and on the p-GaN sample due to variation of the illuminating laser energies Moreover, the extent of the dopant dependent XPS peak shifts of the n- and p-Si samples are assessed after correction of their surface photovoltage values. A laser patterned silicon wafer with a high-power near infrared fiber laser is also investigated. While the patterned silica domains have identical chemical composition with the non-patterned regions, an investigation with dynamic XPS clearly reveals distinct dielectric characteristics of the patterned domains. Electrical parameters of CdS thin film are extracted by dynamic XPS with and without photoillumination. The photo-dynamic XPS technique has also provided useful information by disentanglement of processes; charging/discharging, photoconductivity, and surface photovoltage. Furthermore, location (space) dependent resistance and chemical profile of a CdS based Light Dependent Resistor (LDR) is also probed during realistic operational conditions, by utilizing spatially resolved XPS analysis (in the area mapping mode). In addition, with the XPS mapping analysis defects and malfunctioning sites/domains have been located under various experimental and preparation conditions.
  • ItemOpen Access
    Synthesis and characterization of mesoporous metal sulfide and metal selenide thin films using liquid crystalline mesophases
    (Bilkent University, 2012) Türker, Yurdanur
    In this thesis, synthesis of the mesoporous CdS and CdSe by using of liquid crystalline templating (LCT) approach has been investigated. In the first part of the thesis, the thermal and structural behavior of the [Cd(H2O)4](NO3)2/surfactant (P85 = ((PEO)26(PPO)40(PEO)26)) binary lyotropic liquid crystalline (LLC) systems have been investigated towards synthesis of the mesoporous cadmium sulfide, CdS, or cadmium selenide (CdSe) directly from the mesostructured CdS (or CdSe) thin films. However, the mesostructured CdS/P85 films (at low salt concentrations), which were obtained by reacting [Cd(H2O)4](NO3)2/P85 LLC thin films under H2S atmosphere, are not stable to calcination process and always produced bulk CdO and CdS domains over the thin films. More metal ion containing [Cd(H2O)4](NO3)2-C12EO10-CTAB mesostructured films produced vast amount of HNO3 under the H2S atmosphere and caused decomposition of CdS back to their nitrates. To overcome above problems, a polymerizing agent, such as titania or silica precursors have been added to salt/surfactant LLC mesophase. Both titania and silica overcame the collapse of the mesophase by rigidifying the structure into mesostructured solid and also by providing stability for a thermal removal of nitrates from the medium. For this investigation, both [Cd(H2O)4](NO3)2 and [Zn(H2O)6](NO3)2 salts and P123 ((PEO)20(PPO)70(PEO)20) and C12EO10-CTAB couple have been used. Well-ordered mesostructured Cd(II) titania films have been obtained up to 15.0 Cd(II)/P123 mole ratio for a 60 mole ratio of Ti(IV)/P123 by spin or dip coating of a mixture of 1-butanol-[Cd(H2O)4](NO3)2-P123-HNO3-Ti(OC4H9)4. Exposing the mesostructured Cd(II)-TiO2 films to H2Se under a N2 atmosphere gave stable CdSe nanoparticles in the channels of the mesostructured rigid titania walls up to 25 mole % Cd(II)/Ti(IV). To further increase the metal ion (Cd(II) and Zn(II)) content in the structure, the C12EO10-CTAB-salt mesophase has been employed. The two surfactant-salt systems, in the presence of a titania precursor, produced sponge like mesoporous CdTiO3 and Zn2TiO4 films up to a mole percent of 57 and 86, respectively, upon calcination. Exposing the mesoporous CdTiO3 to H2S or H2Se atmosphere at RT produced homogeneously distributed CdS or CdSe nanocrystallites on the nanocrystalline TiO2 pore walls, respectively. The reaction of mesoporous Zn2TiO4 with H2Se produced stable ZnSe nanocrystallites on the nanocrystalline TiO2 pore walls. The conversion of titania from CdTiO3 to an anatase and brookite phase under H2S and H2Se atmosphere, respectively, and from Zn2TiO4 to a rutile phase under H2Se were observed for the first time. Adding a silica precursor to the two surfactants (C12EO10-CTAB)-salt mesophase produced mesostructured salted-silica, and its calcination produced sponge-like mesoporous silica-metal oxide (dumped meso-SiO2-CdO and mesoSiO2-ZnO) thin films. Up to ~100 % and ~50 % surface coverage could be achieved by CdO and ZnO as nano-islands over the SiO2 pore walls. Exposing the mesoporous SiO2-CdO and SiO2-ZnO thin film precursors to H2S and H2Se at RT enabled the synthesis of mesoporous SiO2-CdS, SiO2-CdSe, SiO2-ZnS, and SiO2- ZnSe thin films. The MS or MSe nanoflakes could homogenously cover the pore walls of mesoporous silica by retaining the pore morphology of the MO precursors. The H2S and H2Se reactions are slow and can be monitored using UV-Vis absorption spectroscopy and EDS to elucidate the reaction mechanism and kinetics. These data showed that the reaction starts from the top surface of the MO domains and proceeds until Si-O-M bond break. Finally, the SiO2 walls were removed from the meso-SiO2-CdS and meso-SiO2-CdSe films through etching in a dilute HF solution to produce mesoporous CdS (meso-CdS) and mesoporous CdSe (meso-CdSe). Surface of the meso-CdS has been modified using PEI (polyethyleneimine) and photoluminescent meso-CdS were obtained.
  • ItemOpen Access
    Chemistry and structure of sputter deposited boron-carbon-nitrogen thin films
    (Bilkent University, 2012) Genişel, Mustafa Fatih
    There is a growing interest in synthesizing new materials with unique mechanical properties like hardness or electrical and optical properties. For this purpose, Boron-Carbon-Nitrogen (BCN) ternary phase diagram promises new materials with potentially unique properties, such as variable band gap semiconductors or phases with extreme hardness. On the other hand, the physical or mechanical properties of these new BCN materials strongly depend on the chemical environment of the atoms and their atomic structure. In this thesis, atomic structure and chemical environment of the atoms in BCN thin films were investigated. BCN films were synthesized by Reactive Magnetron Sputtering (RMS) technique from a B4C target. Various process parameters of synthesis were changed during deposition, such as the substrate bias, substrateto-target distance and N2 flow. The effect of process parameters are investigated with respect to their fundamental effects on the growing BCN films. Several sets of experiments were planned and conducted in order to gain insight as per their effect on the final chemistry and atomic structure. The characterization of the chemical composition of the films was done using data from Infrared Spectroscopy, Raman Spectroscopy, X-ray Photoelectron Spectroscopy, X-Ray Diffraction, and Electron Energy Loss Spectroscopy. Also, electron transparent thin crosssections from the BCN films were prepared using focused-ion beam technique for conducting High Resolution Transmission Electron Microscopy analysis for the verification of atomic structure. In the first series, named B series, the energy is supplied to growing film by applying a radio frequency generated d.c. bias on the substrate. Magnitude of the applied bias was changed throughout the series. In the second and third series, namely P and D series, the effect of substrate-to-target distance was investigated. In these series, BCN and BN films were deposited on substrates that were located at different distances from the target surface. In sub-series, effect of, i) the magnitude of applied bias, ii) type of applied substrate bias on the chemistry of the BCN films were scrutinized. In addition, the effect of atomic composition on the bonding preferences was studied. For this purpose, a series of BCN films were r.f. sputter deposited from B4C target with different N2 flow rate at the process gas. After the careful analysis of the data from mainly the spectroscopic techniques, several important results were obtained. First, a prevailing bonding preference, i.e. phase segregation, was observed in the films deposited regardless of the process parameters used, such that a dominant presence of B-N and C-C or C-N bonding were observed in the films. Furthermore, increasing the substrate bias or decreasing the substrate-to-target distance resulted in the atomic ordering and layered (turbostratic) BCN films. Examination of the spectroscopic data in detail also indicated that the individual layers were made out of separate domains of h-BN like and graphitic like carbon regions, which supports the phase-segregation assertion. Two main regimes are identified for the growth of BCN films; thermodynamically or kinetically controlled regimes. BCN films synthesized with large substrate bias or close to target surface were overall more ordered as the adatoms arriving on the substrate surface had enough energy to diffuse and find energetically most favorable sites. Such a case could be termed as thermodynamically controlled regime. In the opposite case, where adatoms were in a diffusion-limited environment, the final chemistry and structure was dictated by the kinetics. However, the prevalence of B-N bonding in both cases, and failure to observe hybridized chemistry suggests that bonding energy consideration is the major deciding factor for the chemistry of BCN films. As a conclusion, the work presented herein suggests that phase segregation in BCN films reveal as an innate character, while hybridization is not observed in the process parameter space explored. The main reason for this is the relative energies of the B-N and C-C bonding.
  • ItemOpen Access
    Preparation and characterization of polymer composites containing gold nanoparticles
    (Bilkent University, 2011) Yılmaz, Eda
    In this study, light-assisted synthesis of gold nanoparticles in polymer films is demonstrated and characterization of gold nanoparticle-polymer composites using various techniques is shown. There are various methods introduced for the synthesis of gold nanoparticles in solution and their integration to the polymer films afterwards. However, synthesizing gold nanoparticles directly inside the polymer matrix is more advantageous for the production of polymer-nanoparticle composites. An advantage of synthesizing gold nanoparticles within polymer films is the opportunity of photo-patterning. Films having patterns made of regions with and without gold nanoparticles can be produced, using masks designed to cut off the radiation at desired places. Such patterned films were investigated with scanning electron microscope (SEM) and dark regions between irradiated regions and masked regions were observed. These dark regions are shown to be “ion depleted regions”, where gold ions diffuse through irradiated regions during the irradiation. These regions of about 10 m width, suggests a very large distance for gold ions to diffuse through a rigid matrix like Poly(methyl methacrylate)(PMMA), which is very interesting. Supporting evidence for the existence of these regions was obtained from fluorescence studies with Rhodamine 6G molecule and x-ray electron spectroscopy (XPS). The observations made through the formation of ion depleted regions can be used to estimate the diffusion constant of gold ions inside the PMMA matrix. Also the presence of ion depleted regions indicate the stability of photo-patterns created on the polymer film against smearing during light exposure after the production, by setting an upper limit to the critical feature size. During the characterization of gold nanoparticle-polymer composites, the electrical properties of PMMA with and without gold nanoparticles were investigated using charge resolved XPS, while applying external bias to the films with and without gold nanoparticles to probe the charging properties of the films. An enhancement of conductivity of PMMA films containing gold nanoparticles was observed using this technique. Additionally charge resolved XPS technique was also used to determine the charge storage characteristics of the polymer surfaces, which is important for the identification of charging mechanisms during contact and other electrification processes. It was shown that the PMMA surface is very susceptible to negative charging and even native negative charges on the PMMA surface can be observed prior to any treatment. Also when the surface is charged carbon and oxygen atoms of the carbonyl and methoxy groups of PMMA were observed to behave differently from the backbone of the polymer, which shows the chemical specificity of the charge accumulating spots on the surface.