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Publication Metadata only A post-HF study on the halogen bonding interaction of pyrene with diatomic halogen molecules(Wiley, 2016) Sütay, Berkay; Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129We present a detailed SCS-MP2 study on the potential energy curves (PEC) for interactions between diatomic halogen molecules and pyrene. BSSE corrected CCSD[T] energies at equilibrium distances are computed and compared to CCSD(T) energies. The most stable conformation of these weakly bound van der Waals complexes is almost linear in the perpendicular direction to the pyrene plane. The complexes of highly polarizable bromine and iodine molecules with pyrene are very stable and they carry rather large number of vibrational states. Despite its small size, F-2 also forms strong halogen bonding similar to Br-2 and I-2. The interaction between Cl-2 and pyrene is the weakest and it is attributed to the highest polarizability/molar mass ratio of chlorine among the others. I-2-pyrene is found to be the most stable complex due to the strongest mutual polarization effects and is carrying more than 60 vibrational states. Due to the rather large number of electrons in some complexes, the relativistic corrections are also considered.Publication Metadata only Alpha- and beta-na-2[BH4][NH2]: two modifications of a complex hydride in the system NaNH2-NaBH4; syntheses, crystal structures, thermal analyses, mass and vibrational spectra(Elsevier Science Sa, 2010) Höhn, Peter; Cardoso-Gil, Raul; Aydemir, Umut; Akselrud, Lev; Department of Chemistry; N/A; N/A; N/A; Somer, Mehmet Suat; Acar, Selçuk; Koz, Cevriye; Kokal, İlkin; Faculty Member; PhD Student; Master Student; Master Student; Department of Chemistry; College of Sciences; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 178882; N/A; N/A; N/AThe title compound Na-2[BH4][NH2] Na2BNH6 exists in two different modifications: the low-temperature (LT) beta- and the high-temperature (HT) alpha-phase. The HT alpha-Na2BNH6 is accessible by single heating of the binaries NaNH2 and NaBH4 (molar ratio = 1:1) to the melting point of 492 K for I h in evacuated Pyrex ampoules. on longer annealing (T = 342 K, 10 days), the HT alpha-phase converts completely to the LT beta-phase. The beta -> alpha transition at 371 K is very fast, but the conversion alpha -> beta is quite slow. No further phase formation was detected in the system up to a molar ratio of 5:1 (NaNH2:NaBH4). The cubic alpha-Na2BNH6 crystallizes in the K3SO4F-type of structure (space group: Pm (3) over barm, Z = 1) with a = 4.6950(1) angstrom. beta-Na2BNH6 crystallizes in orthorhombic symmetry (space group: Pbcm, Z = 4) with a = 6.5384(2) angstrom. b = 6.4960(1) angstrom, c=9.8512(2)angstrom. Both structures can directly be derived from the perovskite type structure ABO(3), with the [BH4](-) and [NH2](-) moieties in the A and B sites, respectively. The charge balancing Na ions are occupying 2/3 of the O positions. In both cases, the presence of the two different anions in the solids was also confirmed by vibrational spectra. According to DTA and high temperature X-ray diffraction (HT-XRD) measurements, the educts NaNH2 and NaBH4 react at 430 K to form alpha-Na2BNH6 which remains stable until the melting point of 492 K. Between 492 and 573 K, no significant effects were detected in DTA and TG. Above 573 K, however, a sudden decomposition takes place in two steps (659 and 689 K) causing a total mass loss of 8%. The gaseous species formed during the decomposition process were followed by mass spectra (MS). The mass spectra obtained from 1:1 and 2:1 mixtures of NaNH2 and NaBH4 in the range 492 K < T < 773 K prove that the main gaseous product evolved during the thermal decomposition is H-2. NH3 and H2O were also detected, but their concentrations are quite low. These findings confirm that the mixture of the complex hydrides NaNH2-NaBH4 (molar ratio >= 1:1) - comparable to the LiNH2-LiBH4 system - are potential candidates for solid hydrogen storage materials. The composition of the solid residues after the thermal treatment depends on the molar ratio of the starting binaries NaNH2 and NaBH4. While Na2BNH6 (1:1) decomposes to a mixture of NaH, Na and an unidentified amorphous solid, mixtures >= 2:1 yielded exclusively Na3BN2. The vibrational spectra of the title compounds Na2BNH6 have been measured and interpreted based on the T-d and C-2v symmetry of the relevant [BH4](-) and [NH2](-) groups. Both the (N-H) and (B-H) frequencies exhibit small but significant shifts with respect to the pure binaries.Publication Metadata only Bubble point pressures and densities of hexamethyldisiloxane-carbon dioxide binary mixture using a constant volume view cell(Elsevier Science Bv, 2013) N/A; Department of Chemical and Biological Engineering; Şanlı, Deniz; Erkey, Can; Researcher; Faculty Member; Department of Chemical and Biological Engineering; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of Engineering; N/A; 29633The phase behavior of hexamethyldisiloxane (HMDS)-carbon dioxide (CO2) binary mixture was investigated using a constant volume view cell. The accuracy of the measurement technique was inspected against the bubble point pressure data in the literature for ethanol (C2H5OH)-carbon dioxide (CO2) binary mixture. The bubble point pressures for C2H5OH-CO2 agreed well with the literature values. The bubble point pressures of HMDS-CO2 binary mixture were determined at five different temperatures (T=298.2 K, 308.2 K, 3132 K, 323.2 K, 333.2 K) and at various compositions. The bubble point pressures increased with increasing temperature and CO2 mole fraction in the binary mixture. The phase behavior of the binary mixture was modeled using the Peng-Robinson Stryjek-Vera equation of state (PRSVEoS). The binary interaction parameters were regressed from experimental bubble point pressures at each temperature and were found to exhibit a linear dependency on temperature. The HMDS-CO2 binary mixture was also found to exhibit Type II phase behavior. Additionally, P-T-rho measurements for the same binary system were conducted and excess molar volumes were calculated.Publication Metadata only Characterization of self-assembled films of NiGa layered double hydroxide nanosheets and their electrochemical properties(Academic Press Inc Elsevier Science, 2008) Ida, Shintaro; Goto, Motonobu; Matsumoto, Yasumichi; Altuntasoglu, Ozge; Department of Chemistry; Ünal, Uğur; Faculty Member; Department of Chemistry; College of Sciences; 42079In this study, we have demonstrated the synthesis and delamination of a rarely studied NiGa layered double hydroxide (LDH) system. Hydrothermal treatment under agitation conditions at 200 degrees C for 4 h resulted in the formation of highly crystalline NiGa LDHs in a shorter time than those synthesized without agitation. The LDH was delaminated into the individual nanosheets in formamide. The most significant finding in this study is the electrochemical behavior of interlayer ferricyanide anions intercalated with the layer-by-layer (LBL) assembly method. The morphology of LBL film with one layer is also monitored with atomic force microscopy. The cyclic voltammogram is similar to potassium metal hexacyanoferrate systems with its unique two-peak wave. Raman spectrum of the film revealed that the metal center of the interlayer cyano complex is in interaction with the Ni2+ of the host layer. It was concluded that the two-peak cyclic voltammogram of the film is a result of two different forms of the hexacyanoferrate in the interlayer.Publication Metadata only Computational studies of cyclobutadiene and benzocyclobutene fused to p- and o-quinone(American Chemical Society (ACS), 1998) McKee, Michael L.; Balcı, Metin; Kılıç, Hamdullah; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Cyclobutadiene and benzocyclobutenes fused to o- and p-quinone have been studied by computational methods. Geometries were optimized at the B3LYP/6-31G* level, and absolute NMR shielding values were calculated using the GIAO method with the HF/6-31G* basis set. NICS values of the compounds 8b,c and 9b,c indicate strong antiaromatic character for cyclobutadiene units. However, 8a and 9a show negative NICS values where the quinodal system reduces the antiaromaticity significantly by forcing these systems to possess a dimethylene-like structure. The calculated C-13 NMR chemical shifts of 6-9 and parent systems are in very good agreement with literature values.Publication Metadata only Demixing pressures of hydroxy-terminated poly(dimethylsiloxane)-carbon dioxide binary mixtures at 313.2 K, 323.2 K and 333.2 K(Elsevier Science Bv, 2014) N/A; Department of Chemical and Biological Engineering; Şanlı, Deniz; Erkey, Can; Researcher; Faculty Member; Department of Chemical and Biological Engineering; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of Engineering; N/A; 29633The phase behavior of PDMS(OH)-CO2 binary mixtures was investigated. Two different molecular weight PDMS(OH) were utilized and the demixing pressures were determined at three temperatures for a wide composition range. Both of these polymers were found to form miscible mixtures with CO2 at all compositions at pressures lower than 31 MPa in the temperature range 313.2-333.2 K. Depending on the composition of the binary mixtures, two types of phase separation was observed during depressurization; the bubble point and the cloud point. In addition, at specific weight fractions a color change was also observed which was attributed to the mixture critical point. The demixing pressures were observed to increase with temperature and decrease with increasing polymer weight fraction. In addition, higher demixing pressures were obtained for the higher molecular weight polymer mixtures. The bubble point data were modeled by using Sanchez-Lacombe equation of state (SLEoS) and the binary interaction parameters were regressed at the studied temperatures. It was observed that the binary interaction parameters decreased with increasing temperature.Publication Metadata only Efficient synthesis of perovskite-type oxide photocathode by nonhydrolytic sol-gel method with an enhanced photoelectrochemical activity(Elsevier, 2018) Tabari, Taymaz; Ebadi, Mehdi; Singh, Dheerendra; Caglar, Basar; N/A; Yağcı, Mustafa Barış; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/AThe photoelectrochemical activity of PbTiO3 (PTO) for water splitting was studied by linear sweeping voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) techniques. The nanohydrolytic sol-gel method was used to synthesise a crystalline PbTiO3 perovskite nanoparticles. The physical and chemical properties of nanoparticles such as crystal structure, surface area, reducibility, band gap energy, particle morphology and size, surface composition and valence states were investigated by X-Ray diffraction (XRD), BET, temperature-programmed reduction (TPR), UV diffuse reflectance spectroscopy (UV-DRS), high resolution scanning and transmission electron microscopy (HR-SEM and HR-TEM) along with X-Ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). PTO nanoparticles showed pure crystallinity, high surface area (14 m(2)/g) and high oxygen mobility. PTO has band gap energy of 2.66 eV, which makes it active under visible light irradiation. Moreover, nanoparticles vary in size and create a core-shell structure in a way that small particles surround large particles. The core-shell structure along with a free defected sites on the surface results in high photoelectrochemical activity for water splitting reaction. The I-V curve revealed that the PTO nanoparticles are a p-type electrode with the photocurrent efficiency of approximate to 19%. This suggests that the photoelectrode does not require external bias to initiate the water splitting and the reaction can be initiated simply by making a connection between the anode and the cathode. In addition, a great stability is observed for PTO electrodes during the reaction, as evidenced by no leaching to the reaction medium.Publication Metadata only Electroadhesion with application to touchscreens(Royal Soc Chemistry, 2019) Ayyıldız, Mehmet; Persson, Bo N. J.; N/A; Department of Mechanical Engineering; Şirin, Ömer; Başdoğan, Çağatay; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 125489There is growing interest in touchscreens displaying tactile feedback due to their tremendous potential in consumer electronics. In these systems, the friction between the user's fingerpad and the surface of the touchscreen is modulated to display tactile effects. One of the promising techniques used in this regard is electrostatic actuation. If, for example, an alternating voltage is applied to the conductive layer of a surface capacitive touchscreen, an attractive electrostatic force is generated between the finger and the surface, which results in an increase in frictional forces acting on the finger moving on the surface. By altering the amplitude, frequency, and waveform of this signal, a rich set of tactile effects can be generated on the touchscreen. Despite the ease of implementation and its powerful effect on our tactile sensation, the contact mechanics leading to an increase in friction due to electroadhesion has not been fully understood yet. In this paper, we present experimental results for how the friction between a finger and a touchscreen depends on the electrostatic attraction and the applied normal pressure. The dependency of the finger-touchscreen interaction on the applied voltage and on several other parameters is also investigated using a mean field theory based on multiscale contact mechanics. We present detailed theoretical analysis of how the area of real contact and the friction force depend on contact parameters, and show that it is possible to further augment the friction force, and hence the tactile feedback displayed to the user by carefully choosing those parameters.Publication Metadata only Evidence for reverse core-shell phase separation in mixed parahydrogen/orthodeuterium clusters with an ionic impurity(Elsevier, 2018) Calvo, F.; Batra, N.; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Using a flexible polarizable potential, the stable structures of H (H-2)(n)(D-2)(p) clusters with fixed total size n thorn p = 32 have been investigated by means of path- integral molecular dynamics simulations as a function of composition. The most stable phase is found to be reverse core-shell, in which the heavier deuterium molecules reside outside. This result contrasts with neutral clusters, in which deuterium preferentially occupies internal sites. This purely quantum effect is interpreted to originate from the significant weakening of hydrogen covalent bonds in the vicinity of the ionic impurity and its lowering of zero- point energy that affects H-2 more than D-2.Publication Metadata only High voltage LiCoO2 cathodes with high purity lithium Bis(oxalate) Borate (LiBOB) for lithium-ion batteries(American Chemical Society (ACS), 2022) Afyon, Semih; Department of Chemistry; Subaşı, Yaprak; Researcher; 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 Sciences; N/ALithium bis(oxalate) borate, LiB(C2O4)(2) (LiBOB) can be used as an electrolyte additive for lithium-ion batteries (LIBs) to prevent structural change and electrolyte decomposition by developing a protective solid electrolyte interphase (SEI) on the cathode surface. However, impurities present in LiBOB result in significant electrochemical performance decays related to higher full cell impedance. Here, a practical purification technique is performed to remove these impurities within the as-synthesized anhydrous LiBOB in which we further add 1 wt % in 1 M LiPF6 in EC:DMC (1:1) electrolytes to achieve a more stable cycling performance for high voltage applications of LiCoO2 (LCO) cathodes. The phase and purity of as-synthesized LiBOB and recrystallized LiBOB is determined by a combination of X-ray powder diffraction (XRPD), Fourier-transform infrared (FTIR) spectra, and scanning electron microscopy (SEM) measurements. The LIB performance with the addition of high purity LiBOB as an electrolyte additive is investigated via galvanostatic charge-discharge cycling, rate capability, and cyclic voltammetry (CV) measurements within a voltage range of 3.0-4.4 V. The cell containing 1 wt % recrystallized LiBOB shows superior cycling performance, rate capability with higher energy density, and Coulombic efficiency in comparison with the reference cell through the formation of a passivation layer on the LCO surface. Thus, for the LiBOB added cell, the crystal structure of LiCoO2 is well-maintained even at higher potentials after 100 cycles according to the ex situ XRPD and SEM analyses. Therefore, high-purity LiBOB improves the interfacial stability of the LCO cathode by inhibiting oxidative decomposition of electrolytes, undesirable structural changes, and cobalt dissolution bringing about safer cycling even at high operation voltages.
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