Browsing by Author "Yilmaz, E."
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Item Open Access Au nanoparticles in PMMA matrix: In situ synthesis and the effect of Au nanoparticles on PMMA conductivity(Elsevier, 2010-09-01) Yilmaz, E.; Süzer, ŞefikThin PMMA films with and without gold nanoparticles were subjected to +/-10V d.c. and a.c. (square wave) excitations in various frequencies while recording their XPS spectra, and the resulting differences due to charging were examined. Both pure PMMA films and films containing gold nanoparticles showed charging shifts, but those of pure PMMA were more extensive than of PMMA containing gold nanoparticles, suggesting enhanced conductivity, induced by the incorporated gold nanoparticles. Non-charging behavior for these films was also observed with the increase of gold nanoparticle concentration. Gold nanoparticles were in situ synthesized and photo-patterned within the polymer films by UV irradiation.Item Open Access Catalytic properties of vanadium diselenide: a comprehensive study on ıts electrocatalytic performance in alkaline, neutral, and acidic media(American Chemical Society, 2017) Ghobadi, T. G. U.; Patil, B.; Karadas, F.; Okyay, Ali Kemal; Yilmaz, E.Here, we report the synthesis of vanadium diselenide (VSe2) three-dimensional nanoparticles (NPs) and two-dimensional (2D) nanosheets (NSs) utilizing nanosecond pulsed laser ablation technique followed by liquid-phase exfoliation. Furthermore, a systematic study has been conducted on the effect of NP and NS morphologies of VSe2 in their catalytic activities toward oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline, neutral, and acidic conditions. Research on VSe2 clearly demonstrates that these morphologies do not have a significant difference for ORR and OER; however, a drastic effect of morphology was observed for HER. The ORR activity of both NSs and NPs involves ∼2.85 numbers of electrons with the Tafel slope of 120 mV/dec in alkaline and neutral pH. In alkaline solution, NPs are proved to be an efficient catalyst for OER with an onset potential 1.5 V; however, for HER, NSs have a better onset potential of −0.25 V. Moreover, the obtained NPs have also better catalytic activity with a 400 mV anodic shift in the onset potential compared to NSs. These results provide a reference point for the future application of VSe2 in energy storage and conversion devices and mass production of other 2D materials.Item Open Access Constructing Turkey's "western" identity during the Cold War: discourses of the intellectuals of statecraft(Sage Publications Ltd., 2005) Yilmaz, E.; Bilgin, P.[No abstract available]Item Open Access Differentially private binary- and matrix-valued data query: an XOR mechanism(Association for Computing Machinery, 2021-01) Ji, T.; Li, P.; Yilmaz, E.; Ayday, Erman; Ye, Y. F.; Sun, J.Differential privacy has been widely adopted to release continuous- and scalar-valued information on a database without compromising the privacy of individual data records in it. The problem of querying binary- and matrix-valued information on a database in a differentially private manner has rarely been studied. However, binary- and matrix-valued data are ubiquitous in real-world applications, whose privacy concerns may arise under a variety of circumstances. In this paper, we devise an exclusive or (XOR) mechanism that perturbs binary- and matrix-valued query result by conducting an XOR operation on the query result with calibrated noises attributed to a matrix-valued Bernoulli distribution. We first rigorously analyze the privacy and utility guarantee of the proposed XOR mechanism. Then, to generate the parameters in the matrix-valued Bernoulli distribution, we develop a heuristic approach to minimize the expected square query error rate under ϵ-differential privacy constraint. Additionally, to address the intractability of calculating the probability density function (PDF) of this distribution and efficiently generate samples from it, we adapt an Exact Hamiltonian Monte Carlo based sampling scheme. Finally, we experimentally demonstrate the efficacy of the XOR mechanism by considering binary data classification and social network analysis, all in a differentially private manner. Experiment results show that the XOR mechanism notably outperforms other state-of-the-art differentially private methods in terms of utility (such as classification accuracy and F1 score), and even achieves comparable utility to the non-private mechanisms.Item Open Access Highly conducting lyotropic liquid crystalline mesophases of pluronics (P65, P85, P103, and P123) and hydrated lithium salts (LiCl and LiNO3)(American Chemical Society, 2014) Barım, G.; Albayrak, C.; Yilmaz, E.; Dag, Ö.Demand for ionically conducting materials, as membranes and electrodes, is one of the driving forces of current research in chemistry, physics, and engineering. The lithium ion is a key element of these materials, and its assembly into nanostructures and mesophases is important for the membrane and electrode technologies. In this investigation, we show that hydrated lithium salts (such as LiCl·xH2O and LiNO3· xH2O, x is as low as 1.5 and 3.0, respectively) and pluronics (triblock copolymer such as PX where X is 65, 85, 103, and 123) form lyotropic liquid crystalline mesophases (LLCM), denoted as LiY·xH2O-PX-n (Y is Cl− or NO3 −, and n is the salt/PX mole ratio). The structure of the mesophase is hexagonal over a broad salt concentration and transforms to a cubic mesophase and then to disordered gel phase with an increasing salt content of the mixtures. The mesophases are unstable at low salt contents and undergo a phase separation into pure pluronics and salt-rich LLCMs. The salt content of the ordered mesophase can be as high as 30 mole ratio for each pluronic, which is a record high for any known salted phases. The mesophases also display high ac ionic conductivities, reaching up to 21 mS/cm at room temperature (RT), and are sensitive to the water content. These mesophases can be useful as ion-conducting membranes and can be used as media for the synthesis of lithium-containing nanoporous materials.Item Open Access Impact of Li2O/metal mole ratio on lithium-ion battery anode performance(Electrochemical Science Group, 2018) Kunduraci, M.; Ghobadi, T. G. U.; Yilmaz, E.In this study the electrochemical impact of Li2O/metal mole ratio on the cycle life of lithium-ion battery anode materials is demonstrated. For this purpose, nanostructured layered LiNi1/3Mn1/3Co1/3O2 (LiNMC) and spinel LiMn1.5Ni0.5O4 (LiMNO) materials, traditionally known as cathode materials, are evaluated as anode materials and compared against their lithium-free versions NMC (Ni:Mn:Co=1:1:1) and MNO (Mn:Ni=3:1). The Li2O/metal ratio in fully lithiated states are 2.0 for lithium containing (LiNMC and LiMNO) and 1.3 for lithium-free (NMC and MNO) samples. Battery tests show that capacity fading of lithium containing samples is 3 to 4 times larger than lithium-free samples. The differences in the electronic conductivities and voltages profiles of lithium containing and lithium-free anode materials are suggested to be the origin of such electrochemical disparity.Item Open Access Photopatterning of PMMA films with gold nanoparticles: diffusion of AuCl4-ions(2010) Yilmaz, E.; Ertas, G.; Bengu, E.; Süzer, ŞefikPhotopatterning of poly(methyl methacrylate) (PMMA) films is performed by UV irradiation of the polymer films containing uniformly distributed AuCl 4 - ions. The process reduces the gold ions and leads to production of Au nanoparticles in the irradiated regions at room temperature (RT). Resulting films are investigated with scanning electron microscopy, which revealed, in addition to regions with gold nanoparticles, the presence of "ion-depleted regions". These regions are formed at RT and within the rigid polymer matrix by diffusion of gold ions toward the irradiated regions, ending up with no or very little gold moieties, which are important for prevention of delayed processes for postgeneration of unwanted features, if and when such materials are utilized for device production. Further investigations performed by fluorescence and Raman measurements and XPS mapping give additional evidence supporting the existence of such regions. Similar regions are also observed within the poly(vinyl alcohol) (PVAL) films. The ion-depleted regions are about 10 μm wide, which is a significant length for the metal ions to travel through a rigid matrix like PMMA (or PVAL) at room temperature and raises important questions as to the diffusion mechanism(s) of the metal ions and to the nature of the driving force(s).Item Open Access Photoresponse of PbS nanoparticles – quaterthiophene films prepared by gaseous deposition as probed by XPS(American Vacuum Society, 2012) Majeski, M. W.; Pleticha, F. D.; Bolotin, I. L.; Hanley, L.; Yilmaz, E.; Süzer, ŞefikSemiconducting lead sulfide (PbS) nanoparticles were cluster beam deposited into evaporated quaterthiophene (4T) organic films, which in some cases were additionally modified by simultaneous 50 eV acetylene ion bombardment. Surface chemistry of these nanocomposite films was first examined using standard x-ray photoelectron spectroscopy (XPS). XPS was also used to probe photoinduced shifts in peak binding energies upon illumination with a continuous wave green laser and the magnitudes of these peak shifts were interpreted as changes in relative photoconductivity. The four types of films examined all displayed photoconductivity: 4T only, 4T with acetylene ions, 4T with PbS nanoparticles, and 4T with both PbS nanoparticles and acetylene ions. Furthermore, the ion-modified films displayed higher photoconductivity, which was consistent with enhanced bonding within the 4T organic matrix and between 4T and PbS nanoparticles. PbS nanoparticles displayed higher photoconductivity than the 4T component, regardless of ion modification.Item Open Access Probing the charge build-up and dissipation on thin PMMA film surfaces at the molecular level by XPS(Wiley, 2012) Yilmaz, E.; Sezen, H.; Süzer, ŞefikWhat's the charge? X‐ray photoelectron spectroscopy was used to determine the charge state and dynamics of charge build‐up and decay on a thin poly(methyl methacrylate) film. The film is initially negatively charged to around −2 V and becomes progressively positively charged during the course of the XPS analysis.Item Open Access Visible light photocatalytic reduction of Cr(VI) by surface modified CNT/titanium dioxide composites nanofibers(Elsevier B.V., 2016) Mohamed, A.; Osman, T. A.; Toprak, M. S.; Muhammed, M.; Yilmaz, E.; Uheida, A.In this work we report a highly efficient photocatalytic reduction of Cr(VI) based on PAN-CNT/TiO2-NH2 composite nanofibers fabricated by using electrospinning technique followed by chemical crosslinking of surface modified TiO2 NPs functionalized with amino group. The structure and morphology of the fabricated composite nanofibers were characterized by FTIR, SEM, TEM, TGA, and XPS. The results indicate that the composite nanofibers possess excellent photoreduction performance for Cr(VI) under visible light (125 W) after 30 min, which is much faster than previous reports. The effects of various experimental parameters such as catalyst dose, irradiation time, initial concentration of Cr(VI), and pH on the photoreduction efficiency of Cr(VI) were investigated. The highest photoreduction efficiency of Cr (VI) was obtained at low acidity and low amount of TiO2/CNT photocatalyst. The kinetic experimental data was attained and fitted well with a pseudo-first-order model. The UV–vis spectrophotometer and XPS analyses proved that chromate Cr(VI) was reduced to Cr(III). In addition, it can be concluded that the addition of the phenol enhances the photocatalytic reduction of Cr(VI). Furthermore, the photoreduction mechanism has also been discussed. Finally, the fabricated composite nanofibers were found to be stable after at least five regeneration cycles. © 2016Item Open Access X-ray Raman spectroscopy of lithium-ion battery electrolyte solutions in a flow cell(Wiley-Blackwell, 2018) Ketenoglu, D.; Spiekermann, G.; Harder, M.; Oz, E.; Koz, C.; Yagci, M. C.; Yilmaz, E.; Yin, Z.; Sahle, C. J.; Detlefs, B.; Yavaş, H.The effects of varying LiPF6 salt concentration and the presence of lithium bis(oxalate)borate additive on the electronic structure of commonly used lithium-ion battery electrolyte solvents (ethylene carbonate-dimethyl carbonate and propylene carbonate) have been investigated. X-ray Raman scattering spectroscopy (a non-resonant inelastic X-ray scattering method) was utilized together with a closed-circle flow cell. Carbon and oxygen K-edges provide characteristic information on the electronic structure of the electrolyte solutions, which are sensitive to local chemistry. Higher Li+ ion concentration in the solvent manifests itself as a blue-shift of both the π∗ feature in the carbon edge and the carbonyl π∗ feature in the oxygen edge. While these oxygen K-edge results agree with previous soft X-ray absorption studies on LiBF4 salt concentration in propylene carbonate, carbon K-edge spectra reveal a shift in energy, which can be explained with differing ionic conductivities of the electrolyte solutions.Electronic structures of commonly used lithium-ion battery electrolyte solutions measured by non-resonant inelastic X-ray scattering method are presented.