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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Theoretical maximum thermoelectric performance of p-type Hf- and Zr-Doped NbFeSb Half-Heusler compounds(Wiley, 2024) Park, Hyunjin; Kim, Sang-il; Kim, Jeong-Yeon; Shin, Weon Ho; Kim, Hyun-Sik; Department of Chemistry; Aydemir, Umut; Department of Chemistry; Koç University Boron and Advanced Materials Application and Research Center (KUBAM) / Koç Üniversitesi Bor ve İleri Malzemeler Uygulama ve Araştırma Merkezi (KUBAM); College of SciencesHalf-Heusler compounds are promising materials for thermoelectric applications due to their high zT at elevated temperatures. However, their intrinsic high thermal conductivity limits their efficiency. Doping with Hf or Zr can improve the zT of these materials. Recently, a high zT of 1.5 at 1200 K achieved in p-type Nb1-xHfxFeSb has attracted much attention. While the effect of doping Hf in thermal conductivity is studied thoroughly, the effect of Hf doping on band parameters is not fully evaluated. This study investigates the effect of Hf and Zr doping on the electronic band parameters and thermoelectric properties of NbFeSb using the Single Parabolic Band model. The results show that Hf doping increases the weighted mobility of the samples, while Zr doping has no significant effect. Hf doping with x = 0.14 is predicted to improve the zT of NbFeSb by 35% at 300 K (0.19 -> 0.26). These results show the intricate effects of Hf and Zr doping on the electronic and thermal properties of NbFeSb.Publication Metadata only Anticorrosion efficiency of ultrasonically deposited silica coatings on titanium(Elsevier Science Bv, 2013) N/A; N/A; Department of Chemistry; N/A; Ertan, Fatoş Sibel; Kaş, Recep; Miko, Annamaria; Birer, Özgür; Master Student; Master Student; Teaching Faculty; Researcher; Department of Chemistry; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A; 163509; N/AWe utilized high intensity ultrasound to prepare coatings of silica and organically modified silica composed of multiple layers of densely packed nanoparticles. Ultrasound was used to collide nanoparticles onto an activated titanium surface with high speed. Large areas could be homogeneously coated by this method. These coatings were characterized by spectroscopy and microscopy methods and the anticorrosion efficiency in NaCl solution was evaluated by electrochemical measurements. The results indicated that the composite coatings provided good quality barrier layer on bare titanium and decreased the anodic corrosion rate. It was found that increase in the organic content of the coating shifted the passivation potential towards more positive direction. The comparison of the impedance results recorded at the corrosion potential pointed out that in each case a good quality barrier layer was formed on the titanium surface. The outstanding corrosion resistance of the composite coatings with only similar to 200 nm thickness shows that ultrasound assisted deposition can be a competitive method to obtain corrosion protective layers. (c) 2013 Elsevier B.V. All rights reserved.Publication Metadata only Enhancing biocompatibility of NiTi shape memory alloys by simple NH3 treatments(Elsevier, 2020) N/A; N/A; N/A; Department of Chemical and Biological Engineering; Department of Chemistry; Department of Mechanical Engineering; Öztulum, Samira Fatma Kurtoğlu; Yağcı, Mustafa Barış; Uzun, Alper; Ünal, Uğur; Canadinç, Demircan; PhD Student; Researcher; Faculty Member; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; Department of Chemistry; Department of Mechanical Engineering; 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; N/A; College of Engineering; College of Sciences; College of Engineering; 384798; N/A; 59917; 42079; 23433This paper presents the treatment of NiTi shape memory alloys (SMAs) in flowing ammonia at 700 degrees C as a simple and cost-effective nitriding process to provide a protective surface layer hindering Ni ion release in biological environments. Experimental results demonstrated that a smooth protective TiN layer on the NiTi SMAs along with TiOxNy and TiO2 formed on the surface upon treating the as-received NiTi SMA in ammonia at 700 degrees C. The protective TiN layer and the smooth surface hinder the amount of Ni ion release to artificial saliva (AS) after 28 days of immersion, while the dry air treatment at similar conditions results in a significantly rough surface, leading to about 20 times higher Ni ion release. Overall, the findings presented herein demonstrate that NH3 nitriding is an effective method to eliminate the Ni presence from the surface and to obtain a smooth final surface, which, in turn, restricts the Ni ion release from the NiTi SMA into AS. Consequently, nitriding the surface of NiTi under NH3 at 700 degrees C turned out as a promising method to lower Ni ion release and thereby contribute to the biocompatibility of NiTi SMAs, which, however; needs to be further validated through further experimentation.Publication Metadata only Residual stress gradients in electroplated nickel thin films(Elsevier Science Bv, 2015) N/A; N/A; Department of Chemistry; Department of Mechanical Engineering; Kılınç, Yasin; Ünal, Uğur; Alaca, Burhanettin Erdem; PhD Student; Faculty Member; Faculty Member; Department of Chemistry; Department of Mechanical Engineering; 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; N/A; 42079; 115108Residual stress gradients in electroplated nickel films of 1 mu m thickness are characterized for a wide range of current densities (1-20 mA/cm(2)) and electroplating temperatures (30-60 degrees C) in a nickel sulfamate bath. Although a variety of stress measurements is available, exploration of stress gradients remain unstudied at the scale of 1 mu m. Stress gradients - unlike uniform stresses - can cause significant bending even in monolayered released structures. Moreover, examples of misinterpretation of wafer curvature data as a measure of stress gradients exist in the literature. Based on these motivations, monolayered Ni microcantilevers are employed in this work as mechanical transducers for the characterization of stress gradients within the nickel film. Experiments are supported with finite element simulations. Residual stress gradient is found to vary in the range of about 130 to 70 MP/mu m with the sign change indicating a transition from downward to upward deflection of the microcantilever. Thus, a window of electroplating parameters is established yielding zero residual stress gradients, i.e. straight cantilevers, without the use of any additive agents.Publication Metadata only Development of color tunable aqueous cds-cysteine quantum dots with improved efficiency and investigation of cytotoxicity(Amer Scientific Publishers, 2010) N/A; N/A; Department of Chemistry; Department of Chemistry; Öztürk, Sinan S.; Selçukbiricik, Fatih; Acar, Havva Funda Yağcı; Master Student; N/A; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; N/A; N/A; 178902Cysteine capped aqueous CdS quantum dots with improved luminescence and excellent colloidal-luminescence stability were developed in a simple one pot aqueous method from safer precursors at low temperatures. Investigation of size and luminescence as a function of cysteine amount, pH and temperature revealed an optimum value for all these variables to maximize the quantum yield. Cysteine:Cd ratio of 2, reaction pH of 9.5 and synthesis at room temperature-30 degrees C emerged as the best conditions for the highest QY of 19%. Yet, QY can be improved up to 55% if QDs are cleaned from excess cysteine and ions and redispersed in pH 7 medium. Size of the QDs, therefore the color of luminescence, can be tuned by the reaction temperature in this simple process. Higher temperatures provide larger particles. Cell uptake and cell viability studies in a wide range of doses and different incubation times with MCF-7 and HeLa cell lines revealed cell dependent differences. MCF-7 cells uptake more ODs but are much more viable than HeLa cells. At low doses such as 0.025 mg QD/ml all cells are viable. At 24 h incubation times MCF-7 cells demonstrate viability above 75% up to 0.15 mg QD/ml. On the other hand HeLa cells loose viability with increasing dose.Publication Metadata only Single water microdroplets resting on a superhydrophobic surface: largely tunable optical microcavities(IEEE, 2007) N/A; Department of Physics; Department of Physics; N/A; Department of Chemistry; Kiraz, Alper; Kurt, Adnan; Dündar, Mehmet Ali; Demirel, Adem Levent; Faculty Member; Teaching Faculty; Master Student; Faculty Member; Department of Physics; Department of Chemistry; College of Sciences; College of Sciences; Graduate School of Sciences and Engineering; College of Sciences; 22542; 194455; N/A; 6568More than 9 nm tunability of the whispering gallery modes of water microdroplets resting on a superhydrophobic surface is demonstrated. Tunability was achieved by introducing evaporation or condensation in microdroplets in a mini humidity chamber.Publication Metadata only Enhanced hydrogen evolution via in situ generated 2D black phosphorous nanocomposites at the liquid/liquid interfaces(Elsevier, 2022) Aslan, Emre; Yanalak, Gizem; Patır, İmren Hatay; Department of Chemistry; Department of Chemistry; Eroğlu, Zafer; Metin, Önder; Researcher; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; College of Sciences; N/A; 46962The mimicry of bio-membrane with a liquid/liquid interface between two immiscible electrolyte solutions is intrinsically defect-free to study catalysis of energy conversion reactions i.e., CO2 reduction, oxygen reduction, and hydrogen evolution. Herein, we report the in-situ generation of electrodeposited black phosphorous (BP) based nanocomposites at the liquid/liquid interface for the first time and their catalysis in hydrogen evolution reaction (HER). The catalytic HER activities of these catalysts have been investigated electrochemically and also chemically by two-phase reactions. The BP/MoSx, BP/Cu, and BP/Pt nanocomposites were formed by reducing the catalyst precursors such as (NH4)(2)MoS4, (NH4)(2)PtCl4, and CuCl2 salts, respectively on the BP nanosheets by decamethylferrocene (DMFc) electron donor during the catalytic HER. The electrodeposited nanocomposites were collected from the interface and characterized by using advanced analytical techniques. Among them, the BP/MoSx nanocomposites showed the highest HER activity with a reaction rate constant of 0.202 min(-1) was about 230- and 7-times greater than the ones obtained by non-catalytic reaction and the free-MoSx catalyst. Moreover, the nucleation of the catalysts and the HER mechanisms were also explained in detail. The BP/MoSx also showed higher HER activity compared to that of carbon nanotubes CNTMoSx and reduced graphene oxide rGOMoS(x) nanocomposites.Publication Metadata only Ternary nanocomposites of mesoporous graphitic carbon nitride/black phosphorus/gold nanoparticles (mpg-CN/BP-Au) for photocatalytic hydrogen evolution and electrochemical sensing of paracetamol(Elsevier, 2021) Yanalak, Gizem; Doganay, Fatmanur; Aslan, Emre; Ozmen, Mustafa; Bas, Salih Zeki; Patir, Imren Hatay; Department of Chemistry; Department of Chemistry; Department of Chemistry; Eroğlu, Zafer; Küçükkeçeci, Hüseyin; Metin, Önder; Researcher; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; College of Sciences; N/A; N/A; 46962We report herein the fabrication of a novel ternary nanocomposite of mesoporous graphitic carbon nitride/black phosphorus-gold nanoparticles (mpg-CN/BP-Au) and its catalytic performance in the photocatalytic hydrogen evolution reaction (HER) and electrochemical detection of paracetamol. The photocatalytic hydrogen production rate of mpg-CN/BP-Au nanocomposite (1024 mu mol g-1 for 8 h) is compared with mpg-CN, mpg-CN/BP and mpgCN/Au in the presence of triethanolamine (TEOA) as a hole scavenger under the visible light. In addition to the photocatalytic HER application, as-prepared mpg-CN/BP-Au nanocomposite was deposited on modified glassy carbon electrode (mpg-CN/BP-Au/GCE) and for the first time tested for the detection of paracetamol (PA). Under the optimum conditions, linear range of paracetamol detection was found to be in the range of 0.3-120 mu M with a detection limit of 0.0425 mu M. mpg-CN/BP-Au/GCE provided higher electrocatalytic activity than pristine mpgCN and all other tested binary nanocomposites. The enhanced photo- and electrochemical activity of mpgCN/BP-Au/GCE are attributed to formation of heterojunction between BP and mpg-CN materials. Additionally, Au nanoparticles increase the rate of adsorption of mpg-CN/BP due to the excellent electrical properties and spillover effect. We believe that the presented design and catalysis of the ternary nanocomposite will pave a way in many photo- and electrochemical applications.