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  • Küçük Resim Yok
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    Characteristics of Fe- and Mg-doped CuCrO2 nanocrystals prepared by hydrothermal synthesis
    (SPRINGER, 2016) Kaya, İsmail Cihan; Sevindik, Mehmet Alper; Akyıldız, Hasan
    A 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.
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    Highly efficient tandem photoelectrochemical solar cells using coumarin6 dye-sensitized CuCrO2 delafossite oxide as photocathode
    (PERGAMON-ELSEVIER SCIENCE LTD, 2018) Kaya, İsmail Cihan; Akın, Seçkin; Akyıldız, Hasan; Sönmezoğlu, Savaş
    In 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.
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    Mg-aşılanmış CuCrO2 parçacıklarının hidrotermal yöntemle üretimi ve karakterizasyonu
    (Selçuk Üniversitesi Fen Bilimleri Enstitüsü, 2016-01-08) Kaya, İsmail Cihan; Akyıldız, Hasan
    Bu çalışmada, optoelektronik, foto-katalitik ve gaz-algılama uygulamalarına yönelik olarak p-tipi elektrik iletkenliği gösteren ve delafosit yapıya sahip, CuCr1-xMgxO2 (x=0, 0.01, 0.03, 0.05) kompozisyonlarında nano-kristallerin hidrotermal yöntemle üretimi hedeflenmiştir. Mg-aşılamanın yapısal, morfolojik, optik ve elektriksel özelliklere etkisi belirlenmiştir. X-ışınları kırınımı incelemeleri, hidrotermal sentez ile CuCrO2'nin saf halde üretilebildiğini göstermektedir. Deneysel veriler optimum Mg-aşılama miktarının atomca % 3 olduğunu ortaya koymuştur. Bu miktar üzerindeki aşılama, MgCr2O4 spinel fazının oluşmasına neden olmaktadır. Saf numunelerin ortalama kristalit boyutu 12 nm olarak bulunmuştur. Artan Mg-aşılama miktarı ile boyut 7 nm'ye kadar düşmektedir. Ayrıca, aşılama ile kristalit boyut dağılımında da bir artış meydana gelmektedir. UV-Vis spektrofotometre ölçümleri sonucu saf numunenin optik geçirgenlik değeri 700 nm dalga boyunda % 83 olarak bulunmuştur. Bu değer atomca % 1, 3 ve 5 Mg-aşılanmış numuneler için sırası ile % 85, 87 ve 90'a artmaktadır. Benzer şekilde, direk bant aralık değerleri saf numune için 3.00 eV, aşılanmış numuneler için sırası ile 3.01, 3.06 ve 3.07 eV olarak hesaplanmıştır. Elektriksel ölçümler, kompozisyondan bağımsız olarak tüm numunelerin yarı iletken davranış sergilediğini ortaya koymuştur. Çalışma kapsamında üretilen tüm kompozisyonlar için en düşük oda sıcaklığı elektriksel özdirenç değeri % 3 Mg-aşılanmış numunede elde edilmiş ve 68 Ω.cm olarak kaydedilmiştir. Bu değer saf numune için oda sıcaklığında ölçülen özdirenç değerinden (11,8 kΩ.cm) ~170 kat daha düşüktür.
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    Photocatalytic activity and dielectric properties of hydrothermally derived tetragonal BaTiO3 nanoparticles using TiO2 nanofibers
    (ELSEVIER SCIENCE SA, 2018) Küçük, Öznur; Teber, Seda; Kaya, İsmail Cihan; Akyıldız, Hasan; Kalem, Volkan
    Tetragonal 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.