Yazar "Akyildiz, Hasan" seçeneğine göre listele
Listeleniyor 1 - 9 / 9
Sayfa Başına Sonuç
Sıralama seçenekleri
Öğe Characteristics of Fe- and Mg-doped CuCrO2 nanocrystals prepared by hydrothermal synthesis(SPRINGER, 2016) Kaya, Ismail Cihan; Sevindik, Mehmet Alper; Akyildiz, HasanA series of Cu(Cr1-xFex)O-2 and Cu(Cr1-yMgy)O-2 (0.0 <= x,y <= 0.05) nanocrystalline samples with high surface area were prepared using hydrothermal synthesis. The effect of Fe3+ substitution for Cr3+ on the structural, morphological, optical and electrical characteristics of CuCrO2 nanoparticles was investigated in comparison with Mg-doped samples. X-ray diffraction study showed that the solubility limit was around 3 at% for both dopants and beyond this concentration the formation of spinel phases was observed. The incorporation of the tri- and divalent dopants induced a slight expansion in a- and c-parameters. Transmission electron microscopy examination indicated that the average crystallite size (12 nm for undoped) decreased with increasing doping amount of up to x = 0.03 (8.5 nm) and y = 0.05 (7 nm). Moreover, the introduction of Fe and Mg led to an increase in the size distribution of the crystallites. All samples exhibited transmittance above 80 % at 700 nm wavelength and transmittance was enhanced for all doping concentrations except for x = 0.05. A similar trend was also observed for the direct band gaps, where only 5 at% Fe doping induced a red-shift of Eg. The direct band gaps were estimated to be 3.09 eV for x = 0.03 and 3.07 eV for y = 0.03. At room temperature, the minimum achieved electrical resistivity was measured to be 6.4 and 0.068 k Omega cm for the samples with x, y = 0.03, respectively. These values are lower by a factor of similar to 2 and 166 than that of the undoped CuCrO2 sample (11.8 k Omega cm). All samples behaved like semiconductors, and the thermally activated energy for Cu(Cr0.97Mg0.03)O-2 and Cu(Cr0.97Fe0.03)O-2 pellets were found to be 45.40 and 92.5 meV, respectively.Öğe Highly efficient tandem photoelectrochemical solar cells using coumarin6 dye-sensitized CuCrO2 delafossite oxide as photocathode(PERGAMON-ELSEVIER SCIENCE LTD, 2018) Kaya, Ismail Cihan; Akin, Seckin; Akyildiz, Hasan; Sonmezoglu, SavasIn this study, we introduce a new concept for the highly efficient tandem p-n photoelectrochemical cell consisting of coumarin6 organic dye-anchored p-type CuCrO2 delafossite semiconductor as photocathodes coupling with traditional n-type TiO2 based photoanodes. Also, we have systematically studied the photovoltaic performance of tandem cells as a function of post-annealing of CuCrO2 films before sensitization. Hydrothermally synthesized CuCrO2 nanocrystals exhibited a high surface area with small crystallite size of 12 nm, phase-pure and well-crystalline after the optimized post-annealing conditions. The experimental results indicated that the optimal post-annealing temperature was 450 degrees C for 1 h due to the larger active surface area, lower R-th and higher J(sc) and V-oc, values. This tandem cell, fabricated by employing the CuCrO2 photocathodes, iodide-based redox mediator and a coumarin6 organic dye, afforded an impressive efficiency of 2.33% with V-oc of 813 mV, J(sc) of 4.83 mA cm(-2), and fill factor of 0.59. The obtained parameters are acceptably high in comparison to NiO photocathode-based tandem cells previously reported in literature under similar experimental conditions. Therefore, this work opened the way for developing highly-efficient tandem photoelectrochemical solar cells.Öğe Photocatalytic activity and dielectric properties of hydrothermally derived tetragonal BaTiO3 nanoparticles using TiO2 nanofibers(ELSEVIER SCIENCE SA, 2018) Kucuk, Oznur; Teber, Seda; Kaya, Ismail Cihan; Akyildiz, Hasan; Kalem, VolkanTetragonal BaTiO3 nanoparticles have been hydrothermally synthesized using the electrospun TiO2 nanofibers as the Ti-source. Microstructural and compositional analyses have been carried out using XRD, SEM, TEM, XPS, and Raman spectroscopy. Characterization studies confirmed that the BaTiO3 nanoparticles had a tetragonal structure with an average particle size of 100 nm. Photocatalytic activities of the nanoparticles were investigated via the decolorization of methylene blue dye aqueous solutions under visible light and UV-A irradiations. The degradation efficiency reached to 13 and 35% after exposing to light for 60 min with visible and UV-sources, respectively. Effect of sintering temperatures on the dielectric properties was also investigated. Among all the sintering temperatures employed, 1473 K was the optimum sintering temperature for these ceramics in terms of high relative density (95.3%), high dielectric constant (3162 at 1 MHz), and low loss tangent (15 x 10(-3) at 1 MHz). Experimental results showed that electrospun TiO2 fibers can be used as a precursor template to produce nano-scale BaTiO3 particles which are suitable for various applications such as photocatalysis and capacitors. (C) 2018 Elsevier B.V. All rights reserved.Öğe Production and characterization of CuCrO2 nanofibers(ELSEVIER SCIENCE SA, 2016) Cetin, Cagla; Akyildiz, HasanComposite nanofibers were prepared from homogeneously mixed copper chloride, chromium nitrate and polyvinylpyrrolidone solution by electrospinning and were subsequently heat treated under isochronal and isothermal annealing conditions. In the isochronal experiments, CuCrO2 formation began at 773 K. Annealing at 973 K yielded single-phase, highly porous CuCrO2 nanofibers accompanied by the formation of large quasi-hexagonal grains. In contrast, isothermal annealing at 973 K resulted in the formation of phase-pure CuCrO2 nanofibers with belt-like morphology and relatively higher density. A substantial decrease was observed in the size and number of plate-like grains in these nanofibers. When the fibers were annealed isothermally at 973 K for 1 h, the average width and specific surface area of the synthesized fibers were 150 nm and 7.85 m(2) g(-1), respectively, and the direct bandgap was 2.82 eV. The CuCrO2 behaved like a semiconductor, and the thermally activated energy was approximately 0.157 eV. A simple formation mechanism was proposed for the quasi-hexagonal grains. (C) 2015 Elsevier B.V. All rights reserved.Öğe Production and Characterization of Magnesium-Doped Copper Chromite Fibers(WILEY-V C H VERLAG GMBH, 2018) Kaya, Ismail C.; Akyildiz, HasanIn this study, Mg-doped CuCr1-xMgxO2 (x=0, 0.01, 0.03, 0.05) fibers are synthesized via electrospinning process followed by a two-step heat treatment in air and Ar atmospheres. The produced samples are all crystalline with a fiber diameter ranging from 120 to 140nm. The effect of Mg2+ substitution for Cr3+ on the structural, morphological, and optical properties of CuCrO2 fibers is investigated systematically. Structural characterizations indicate that the solubility limit of Mg2+ is around 3at.% and beyond this concentration the formation of MgO phase is observed. This also leads to the formation of Cu cations with ((+)2) oxidation state. In addition, the incorporation of Mg into the CuCrO2 lattice hinders the formation of perfect hexagonal-shaped isolated grains and contribute to the preservation of fiber morphology after the final sintering step. Further, the optical transmittance in the visible range decreases with increasing Mg-doping ratio. The corresponding optical band gaps are determined to be 3.53, 3.47, 3.30, and 3.22eV, respectively, for x=0, 0.01, 0.03, 0.05.Öğe Selective removal of cationic micro-pollutants using disulfide-linked network structures(ROYAL SOC CHEMISTRY, 2017) Atas, Mehmet Sahin; Dursun, Sami; Akyildiz, Hasan; Citir, Murat; Yavuz, Cafer T.; Yavuz, Mustafa SelmanMicropollutants are found in all water sources, even after thorough treatments that include membrane filtration. New ones emerge as complex molecules are continuously produced and discarded after used. Treatment methods and sorbent designs are mainly based on non-specific interactions and, therefore, have been elusive. Here, we developed swellable covalent organic polymers (COP) with great affinity towards micropollutants, such as textile industry dyes. Surprisingly, only cationic dyes in aqueous solution were selectively and completely removed. Studies of the COPs surfaces led to a gating capture, where negatively charged layer attracts cationic dyes and moves them inside the swollen gel through diffusive and hydrophobic interaction of the hydrocarbon fragments. Despite its larger molecular size, crystal violet has been taken the most, 13.4 mg g(-1), surpassing all competing sorbents. The maximum adsorption capacity increased from 12.4 to 14.6 mg and from 8.9 to 11.4 mg when the temperature of dye solution was increased from 20 to 70 degrees C. The results indicated that disulfide-linked COPs are attractive candidates for selectively eliminating cationic dyes from industrial wastewater due to exceptional swelling behaviour, low-cost and easy synthesis.Öğe Synthesis of CuAlO2 from chemically precipitated nano-sized precursors(ELSEVIER SCI LTD, 2015) Akyildiz, HasanA study was carried out on the synthesis of CuAlO2 at temperatures below 1000 degrees C, using a simple approach based on mechanical mixing of chemically precipitated CuO and Al(OH)(3) nanoparticles. Synthesis of precursor powders was achieved at low temperatures (<100 degrees C) via precipitation from their corresponding aqueous solutions. After drying at room temperature, precursors were mixed in equimolar ratios by a combination of ball milling and sonication. Green pellets were obtained by filtration casting of the mixed powder slurry followed by dry compaction. X-ray diffraction study showed that the single phase CuAlO2 samples with high crystallinity formed only after 5 h of firing at 900 degrees C under flowing nitrogen. Electron microscopy images revealed a loosely packed microstructure with high surface area consisting of nanosized grains after the heat treatment. The achieved relative density was 45% of the theoretical density. The crystallite size was estimated to be 39 nm and the specific surface area of the sintered sample was measured as 9.67 m(2)/g. The bandgap was calculated as 3.07 eV with a transmittance of 60% in the visible region. The measured electrical conductivity was 4 x 10(-3) mS/cm at room temperature and showed an increase with increasing measurement temperature, which indicates semiconducting behavior. The conductivity was shown to increase by an order of magnitude with a higher degree of densification by sintering at 1150 degrees C. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.Öğe Thickness effects in hydrogen sorption of Mg/Pd thin films(ELSEVIER SCIENCE SA, 2013) Gharavi, Ayshe G.; Akyildiz, Hasan; Ozturk, TayfurMg thin films with various thicknesses ranging from 50 to 1000 nm capped with Pd have been prepared by thermal evaporation and their hydrogen sorption properties were examined following the electrical resistance change. The study showed that the hydrogenation and dehydrogenation temperatures correlate with the film thickness, thinner films reacting with hydrogen at reduced temperatures. It is concluded that films thinner than 200 nm react fully with hydrogen, while a considerable portion of the thicker films remain unreacted. The significance of this is discussed with reference to the design of hydrogen storage systems based on thin films or nanoparticles. (C) 2013 Elsevier B.V. All rights reserved.Öğe Ti modified Pd-Ag membranes for hydrogen separation(PERGAMON-ELSEVIER SCIENCE LTD, 2015) Piskin, Fatih; Akyildiz, Hasan; Ozturk, TayfurAn investigation was carried out into Pd-Ag separation membranes to check whether a reduction in their Pd content was possible through the incorporation of a third element, namely Ti. For this purpose a combinatorial thin film deposition system was developed incorporating three sputter targets arranged in triangular form. The system had a substrate in the form of a magazine, 6 inch in diameter, accommodating 21 discs arranged again in triangular form aligned with the targets underneath. With this geometry, a library of thin film membranes was obtained in a single experiment covering a wide compositional field, Pd content up to of approx. 75%. The thin film library was then screened with respect their tendency to react with hydrogen. This was accomplished by the resistivity measurements carried out on the membranes, while they are subjected to heating-cooling cycle under hydrogen, the values being compared with identical cycle carried out under argon. Since permeability is a product of hydrogen solubility and diffusivity, membranes that react with hydrogen, i.e. either forming a solid solution or a hydride, delineate compositions which are candidates for separation membranes. In the present work, this procedure was applied to a portion of Ag-Ti Pd ternary system aiming for separation membranes with f.c.c. crystal structure. Mapping based on the resistivity measurement indicated, Ag35Ti22Pd43, Ag13Ti25Pd62 and their near compositions as possible candidates for separation membranes. Copyright (c) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.