Researcher:
Ünal, Uğur

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Uğur

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Ünal

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Now showing 1 - 10 of 59
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    Publication
    Electrochemical CO2 reduction to gaseous methane and carbon monoxide using plasma-synthesized copper nanowires
    (Springer, 2023) Butt, Faaz Ahmed; Alshahrani, Thamraa; Awan, Zahoor Ul Hussain; Christy, Maria; Khan, Firoz; Alanazi, Abdulaziz M.; Department of Chemistry; Department of Chemistry; Ünal, Uğur; College of Sciences
    There is a growing interest among environmental researchers to synthesize a sustainable catalyst for CO2 conversion process. Copper and modified copper offer a wide window for such catalysts. In this study, we report the use of plasma-treated copper/copper oxide nanowires for electrochemical CO2 reduction for the first time. Plasma-treated Cu nanowires (CuO-P NWs) were comparatively evaluated with bulk copper surface in CO2 saturated test solutions, namely, KHCO3, KCl, and NaCl, with 0.1 and 1 M concentrations. CuO-P NWs demonstrate 50% increase in selectivity towards CO2 reduced gaseous products (CO and CH4) at a higher applied potential of - 1.1 V vs reversible hydrogen electrode. This increased selectivity is associated with decreased binding energy of the intermediate species on plasma-treated nanowires compared to bulk surface. Both CO and CH4 were the main products detected in the gaseous state, and CO is suggested as the main intermediate species. The effects of different cations and anions and their concentrations in the solutions were also analyzed. It was observed that dilute solutions in 0.1 M are optimal for electrochemical CO2 reduction (ECO2R) and that hydration energy of cations plays a significant role in ECO2R selectivity, while hydrogen evolution reaction was the competing reaction.
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    Disulfonated polyarylene ether sulfone membrane for graphitic carbon nitride/zinc oxide based photo-supercapacitors
    (Pergamon-Elsevier Science Ltd, 2023) Altaf, Cigdem Tuc; Colak, Tuluhan Olcayto; Erdem, Emre; Misirlioglu, Feray Bakan; Condorelli, Guglielmo Guido; Sankir, Nurdan Demirci; Sankir, Mehmet; Department of Chemistry; Department of Chemistry; Ünal, Uğur; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences
    Photo-supercapacitors (PSCs) are environmentally friendly devices that directly convert and store solar energy into electricity and have a high potential to eliminate the need for grid electricity for a sustainable future. In this study, lithiated (Li+) biphenol-based disulfonated poly (arylene ether sulfone) random copolymer (BPS) mem-brane has been successfully integrated into graphitic carbon nitride/zinc oxide nanowire composite-based PSC to increase the efficiency and to offer simpler and more cost-effective designs. It has been observed that after UV illumination specific capacitance (C-p) and energy density (E-d) increased 2.8 and 2.7-fold, respectively, indicating that the PSC with BPS-Li(+)performs approximately 3 times better under illumination than dark conditions. Furthermore, at elevated temperatures and 100% relative humidity C-p and E-d of the PSC increased to 23.61 Fg(-1) and 47.22 Whkg(-1) at 85 degrees C, respectively. This enhancement can be linked to the temperature-boosted ionic conductivity of the membrane.
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    Bidisperse magnetorheological fluids with strong magnetorheological response, long-term stability and excellent in-use performance
    (IOP Publishing Ltd, 2024) Department of Chemistry;Department of Mechanical Engineering; Nejatpour, Mona; Saleh, Mostafa Khalil Abdou; Ulasyar, Abasin; Ünal, Uğur; Lazoğlu, İsmail; Acar, Havva Funda Yağcı; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Manufacturing and Automation Research Center (MARC); Graduate School of Sciences and Engineering; College of Sciences; College of Engineering
    There is a critical demand for magnetorheological fluids (MRFs) with high particle loading, long-term stability, and high magneto-viscous properties to be used in industrial MRF devices. Bidisperse MRFs composed of highly magnetizable micron-sized carbonyl iron particles and poly(acrylic acid) coated superparamagnetic iron oxide nanoparticles (SPIONs-PAA) that can chemically interact are proposed to achieve such MRFs, here. Coating bare, commercial CI with lauric acid (LA) enhanced its dispersibility in a hydrophobic carrier fluid, allowed high magnetic loading and significantly prevented the sedimentation of the particles when mixed with 9-12 wt% SPION. Different carrier fluids (mineral oil, paraffin, and hydraulic oil) were tested, and hydraulic oil was determined as the best for this particle combination. The most stable bidisperse MRF was achieved at 83%-84% magnetic content with 12 wt-%SPION-PAA, LA-coated-CI and 3 wt% polyvinyl alcohol. Such MRFs outperformed the commercial benchmark, 140-CG (R) from Lord Corp., in long-term stability (4 months) and stability under dynamic loading. Bidisperse MRFs were stable between 20 degrees C and 60 degrees C. Most importantly, the excellent performance of the bidisperse MRFs in dampers designed for washing machines suggests that these MRFs may provide comparable damping forces with much better stability, ensuring longer shelf-life and longer lifetime in use.
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    Active sites and their individual turnover frequencies for ethylene hydrogenation on reduced graphene aerogel
    (Amer Chemical Soc, 2024) Department of Chemistry;Department of Chemical and Biological Engineering; Yalçın, Kaan; Öztulum, Samira Fatma Kurtoğlu; Öztuna, Feriha Eylül Saraç; Kanat, Gizem Hasibe; Ünal, Uğur; Uzun, Alper; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; College of Engineering
    Graphene aerogel (GA) was reduced at various temperatures to prepare a series of reduced graphene aerogels (rGAs) with different surface characteristics. Detailed characterization demonstrated that an increase in the thermal reduction temperature leads to an increase in surface area accompanied by an increase in surface density of defect sites formed by the removal of the oxygen-containing functional groups. rGA samples were then tested for ethylene hydrogenation under identical conditions. A comparison of catalytic performances of each catalyst demonstrated that the rGA sample prepared by reduction in Ar at 900 degrees C (rGA-900) provides the highest performance compared with others prepared at lower temperatures. Next, we analyzed the per-gram activity of each catalyst as a sum of individual contributions from different defect sites quantified by Raman spectroscopy and CHNS-O analysis to determine the individual turnover frequencies (TOFs) of each active site. This analysis identified polyene-like structures and interstitial defects associated with amorphous sp(2) bonded carbon atoms as the dominant active sites responsible for hydrogenation. A comparison of their TOFs further indicated that the polyene-like structures provide approximately ten times higher TOF compared to those associated with the amorphous carbon defects. These results, identifying the dominant active centers and quantifying their corresponding TOFs, provide opportunities toward the rational design of GA-based carbocatalysts.
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    Machine learning – informed development of high entropy alloys with enhanced corrosion resistance
    (Elsevier Ltd, 2024) Yılmaz, R.; Maier, H.J.; Department of Mechanical Engineering;Department of Chemistry; Özdemir, Hüseyin Can; Nazarahari, Alireza; Yılmaz, Bengisu; Canadinç, Demircan; Kılıç, Elif Bedir; Ünal, Uğur; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Engineering; College of Sciences
    This study demonstrates the use of machine learning as a potential tool to efficiently develop new biomedical alloys with improved corrosion resistance by exploring the whole compositional space in the HfNbTaTiZr system. Owing to the small volume and inherited uncertainty of available corrosion data in the literature, k-fold cross-validation and bootstrapping were used to quantify the uncertainty of models and select a robust one. Potentiodynamic polarization experiments were performed on the predicted composition in simulated body fluid at 37 ± 1 °C for validation, demonstrating the new alloy's superior corrosion properties with a homogeneous microstructure as opposed to the dendritic structure. © 2023
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    PublicationOpen Access
    Optimization of laser-wavelength dependence for open-air atmospheric pressure pulsed laser deposition of AlCrFeMnTi high-entropy alloy for tailored surface properties
    (American Chemical Society, 2024) Department of Chemistry; Department of Chemistry; Mahdavi, Hossein; Alamdari, Armin Asghari; Kepçeoğlu, Abdullah; Yağcı, Mustafa Barış; Ünal, Uğur; Jahangiri, Hadi; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences
    High-entropy alloys (HEAs) have garnered significant attention in different fields due to their exceptional mechanical and physical properties, making them promising candidates for various applications. Several techniques, including physical vapor deposition and pulsed laser deposition (PLD), have been employed for the fabrication of HEA thin films. In this study, we explore a novel approach to synthesizing the lightweight HEA (LWHEA) AlCrFeMnTi using PLD in air at atmospheric pressure with a particular focus on the influence of the laser wavelength on the deposition process and the resulting alloy characteristics. This research investigates the impact of different laser wavelengths on the LWHEA's characterization and the optimization of laser wavelength dependence in air at atmospheric pressure PLD of LWHEA AlCrFeMnTi for tailored surface properties such as phase composition, microstructure, and corrosion resistance. Systematically varying the laser wavelength was attempted to optimize the deposition conditions. This was aimed at achieving enhanced properties and precise control over the alloy's composition. This work contributes to a deeper understanding of the open air PLD process for LWHEAs and sheds light on the role of the laser wavelength in tailoring their properties, which can have significant implications for the development of advanced materials for aerospace, automotive, and other high-performance applications. Ultimately, this research aims to provide valuable insights into the design and fabrication of LWHEAs with tailored properties through laser-based deposition techniques.
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    Co-sensitization of Copper Indium Gallium Disulfide and Indium Sulfide on Zinc Oxide nanostructures: effect of morphology in electrochemical carbon dioxide reduction
    (American Chemical Society, 2024) Altaf, Cigdem Tuc; Colak, Tuluhan Olcayto; Karagoz, Emine; Wang, Jiayi; Liu, Ya; Chen, Yubin; Liu, Maochang; Sankir, Nurdan Demirci; Sankir, Mehmet; Department of Chemistry; Department of Chemistry; Ünal, Uğur; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences
    Recent advances in nanoparticle materials can facilitate the electro-reduction of carbon dioxide (CO2) to form valuable products with high selectivity. Copper (Cu)-based electrodes are promising candidates to drive efficient and selective CO2 reduction. However, the application of Cu-based chalcopyrite semiconductors in the electrocatalytic reduction of CO2 is still limited. This study demonstrated that novel zinc oxide (ZnO)/copper indium gallium sulfide (CIGS)/indium sulfide (InS) heterojunction electrodes could be used in effective CO2 reduction for formic acid production. It has been determined that Faradaic efficiencies for formic acid production using ZnO nanowire (NW) and nanoflower (NF) structures vary due to structural and morphological differences. A ZnO NW/CIGS/InS heterojunction electrode resulted in the highest efficiency of 77.2% and 0.35 mA cm-2 of current density at a −0.24 V (vs. reversible hydrogen electrode) bias potential. Adding a ZTO intermediate layer by the spray pyrolysis method decreased the yield of formic acid and increased the yield of H2. Our work offers a new heterojunction electrode for efficient formic acid production via cost-effective and scalable CO2 reduction. © 2024 The Authors. Published by American Chemical Society.
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    Enhanced electrochemical performance and cyclic stability of Li-Ion batteries by employing nanostructured Bi2Te3 particles with amorphous ZrO2 nanocoating
    (American Chemical Society, 2024) Taghizadegan, Parham; Bolghanabadi, Nafiseh; Mohebi, Matin; Angizi, Shayan; Simchi, Abdolreza; Department of Chemistry; Department of Chemistry; Khodabakhsh, Mohammadreza; Ünal, Uğur; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences
    The interest in bismuth-based anodes for energy storage devices has recently been heightened due to their high specific capacity, low working potential, excellent electrical conductivity, and environmentally friendly nature. However, capacity fading during early battery cycling is associated with solid electrolyte interphase forming and volume changes that result in performance loss and cyclic instability. In this study, we propose a strategy to engineer the interface of nanostructured Bi2Te3 particles with amorphous zirconium oxide nanocoating to enhance the performance of lithium-ion batteries (LIBs). The active nanomaterial was synthesized by mechanical alloying followed by low-temperature calcination of zirconium(IV) oxynitrate at 280 C-degrees, which was predeposited on the particle surfaces via facile wet chemistry. X-ray photoelectron spectroscopy and transmission electron microscopy revealed that an amorphous ZrO2 shell with few nanometer thicknesses uniformly formed and covered the particle surfaces. It was also found that Te and Bi oxides were formed at the surface, which facilitated stable interfacial bonds with the oxide nanocoating. Electrochemical studies determined that the amorphous oxide ceramic slightly increased the electrical resistance but significantly reduced the Li+ diffusion coefficient (by an order of magnitude) and the formation of solid electrolyte phases. As a result, the discharge capacity of the nanostructured Bi2Te3 anode enhanced from 145.8 mAh g(-1) to 245.1 mAh g(-1) after interfacial engineering by 2-nm-thick amorphous ZrO2 nanocoating, while the stability was enhanced by three folds after 100 cycles at a current density of 0.5 C. The facile synthesis of amorphous nanocoating to engineer the interfacial of nanostructured anode materials paves the way to fabricate high-performance energy storage materials.
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    Exploring the role of Mo and Mn in Improving the OER and HER performance of CoCuFeNi-Based high-entropy alloys
    (Amer Chemical Soc, 2024) Igarashi, Keisuke; Matsumoto, Hiroaki; Department of Chemistry; Department of Chemistry; Alamdari, Armin Asghari; Jahangiri, Hadi; Yağcı, Mustafa Barış; Motallebzadeh, Amir; Ünal, Uğur; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of Sciences
    High-entropy alloys (HEAs) are a class of metallic materials composed of solid solutions of five or more elements in equi- or near-equiatomic proportions. The fascinating properties of HEAs have recently attracted considerable attention for water-splitting applications. Mechanical alloying (MA) is a method for preparing HEAs that results in crystalline, homogeneous materials at room temperature. In this work, several CoCuFeNi-based HEAs were prepared through MA and evaluated as electrocatalysts for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting in 1 M KOH. The results showed that CoCuFeNiMnMo1.5 with the highest amount of molybdenum exhibited the best OER performance (375 +/- 15 mV at the current density of 10 mA cm(-2)), and CoCuFeNiMnMo0.5 with the lowest amount of molybdenum exhibited the best HER activity with lower overpotentials (275 +/- 12 mV at the current density of 10 mA cm(-2)) and over 72 h of stability. The assembled CoCuFeNiMnMo1.5 (anode)parallel to CoCuFeNiMnMo0.5 (cathode) couple required 1.76 V to produce 10 mA cm(-2), and the Faradaic efficiency for generated H-2 was determined to be more than 80%.
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    Synthesis of upconverting nanosheets derived from Er-Yb and Tm-Yb Co-doped layered perovskites and their layer-by-layer assembled films
    (Elsevier, 2022) Gunay, Bensu; Suer, Ozge; Doger, Hilal; Arslan, Ozlem; Saglam, Ozge; Department of Chemistry; Department of Chemistry; Ünal, Uğur; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; 42079
    Here, we investigated the structure and upconversion (UC) properties of new-type of single oxide nanosheets, derived from the Er3+/Yb3+ and Tm3+/Yb3+ co-doped Ruddlesden-Popper type layered perovskites, and their layer-by-layer (LBL) self-assembled nanofilms. The single oxide nanosheets, obtained by exfoliation of the proton-exchanged K2La2Ti3O10, had the thickness in the range of 2-3 nm indicating good consistency with the theoretical thickness and lateral size from 500 nm up to 2 mu m. Er3+/Yb3+, Tm3+/Yb3+ and Tm3+/Er3+co-doped nanosheets were used as building blocks of the multilayer films deposited by layer-by-layer procedure. The LBL films composed of 2.5 % Er3+ + 5 % Yb3+, 2.5 % Tm3+ + 20 % Yb3+, 2.5 % Tm3+ + 20 % Er3+ after 60 sequences have shown a white emission confirmed by the CIE chromaticity diagram. The possible UC energy transfer of LBL films fabricated after 30 sequences using the nanosheets derived from the 2.5 % Er3+ + 5 % Yb3+ co-doped layered perovskites was also suggested. The number of photons participating in the UC process was confirmed as two-photon for both green and red UC emissions due to the F-4(9/2) -> I-4(15/2) and H-2(11/2), S-4(3/2) -> I-4(15/2) transitions, respectively.