Browsing by Subject "Oxygen"

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Open Access
Atomistic structure simulation of silicon nanocrystals driven with suboxide penalty energies
(American Scientific Publishers, 2008) Yılmaz, Dündar E.; Bulutay, Ceyhun; Çağın, T.
The structural control of silicon nanocrystals embedded in amorphous oxide is currently an important technological problem. In this work, an approach is presented to simulate the structural behavior of silicon nanocrystals embedded in amorphous oxide matrix based on simple valence force fields as described by Keating-type potentials. After generating an amorphous silicon-rich-oxide, its evolution towards an embedded nanocrystal is driven by the oxygen diffusion process implemented in the form of a Metropolis algorithm based on the suboxide penalty energies. However, it is observed that such an approach cannot satisfactorily reproduce the shape of annealed nanocrystals. As a remedy, the asphericity and surface-to-volume minimization constraints are imposed. With the aid of such a multilevel approach, realistic-sized silicon nanocrystals can be simulated. Prediction for the nanocrystal size at a chosen oxygen molar fraction matches reasonably well with the experimental data when the interface region is also accounted. The necessity for additional shape constraints suggests the use of more involved force fields including long-range forces as well as accommodating different chemical environments such as the double bonds.
Open Access
A bifunctional photosensitizer for enhanced fractional photodynamic therapy: singlet oxygen generation in the presence and absence of light
(Wiley-VCH Verlag, 2016) Turan, I. S.; Yildiz, D.; Turksoy, A.; Gunaydin, G.; Akkaya, E. U.
The photosensitized generation of singlet oxygen within tumor tissues during photodynamic therapy (PDT) is self-limiting, as the already low oxygen concentrations within tumors is further diminished during the process. In certain applications, to minimize photoinduced hypoxia the light is introduced intermittently (fractional PDT) to allow time for the replenishment of cellular oxygen. This condition extends the time required for effective therapy. Herein, we demonstrated that a photosensitizer with an additional 2-pyridone module for trapping singlet oxygen would be useful in fractional PDT. Thus, in the light cycle, the endoperoxide of 2-pyridone is generated along with singlet oxygen. In the dark cycle, the endoperoxide undergoes thermal cycloreversion to produce singlet oxygen, regenerating the 2-pyridone module. As a result, the photodynamic process can continue in the dark as well as in the light cycles. Cell-culture studies validated this working principle in vitro.
Open Access
Carbon supported nano-sized Pt-Pd and Pt-Co electrocatalysts for proton exchange membrane fuel cells
(2009) Kadirgan, F.; Kannan, A. M.; Atilan, T.; Beyhan, S.; Ozenler, S. S.; Süzer, Şefik; Yörür, A.
Nano-sized Pt-Pd/C and Pt-Co/C electrocatalysts have been synthesized and characterized by an alcohol-reduction process using ethylene glycol as the solvent and Vulcan XC-72R as the supporting material. While the Pt-Pd/C electrodes were compared with Pt/C (20 wt.% E-TEK) in terms of electrocatalytic activity towards oxidation of H2, CO and H2-CO mixtures, the Pt-Co/C electrodes were evaluated towards oxygen reduction reaction (ORR) and compared with Pt/C (20 wt.% E-TEK) and Pt-Co/C (20 wt.% E-TEK) and Pt/C (46 wt.% TKK) in a single cell. In addition, the Pt-Pd/C and Pt-Co/C electrocatalyst samples were characterized by XRD, XPS, TEM and electroanalytical methods. The TEM images of the carbon supported platinum alloy electrocatalysts show homogenous catalyst distribution with a particle size of about 3-4 nm. It was found that while the Pt-Pd/C electrocatalyst has superior CO tolerance compared to commercial catalyst, Pt-Co/C synthesized by polyol method has shown better activity and stability up to 60 °C compared to commercial catalysts. Single cell tests using the alloy catalysts coated on Nafion-212 membranes with H2 and O2 gases showed that the fuel cell performance in the activation and the ohmic regions are almost similar comparing conventional electrodes to Pt-Pd anode electrodes. However, conventional electrodes give a better performance in the ohmic region comparing to Pt-Co cathode. It is worth mentioning that these catalysts are less expensive compared to the commercial catalysts if only the platinum contents were considered.
Open Access
Characterization of niobium-zirconium mixed oxide as a novel catalyst for selective catalytic reduction of NO x
(2009) Cayirtepe, I.; Naydenov, A.; Ivanov, G.; Kantcheva, M.
The performance of mixed niobium-zirconium oxide in the SCR of NO x with propene in excess oxygen has been studied. The mixed oxide is prepared by impregnation of hydrated zirconia with acidic solution (pH 0.5) of peroxoniobium(V) complex, [Nb2(O2)3] 4+, ensuring ZrO2:Nb2O5 mole ratio of 6:1. The calcined sample (denoted as 25NbZ-P) has the structure of Zr 6Nb2O17. According to the catalytic test, the conversion of NO x over the 25NbZ-P catalyst passes through a maximum at 220 °C. Based on the in situ FT-IR results, a reaction mechanism is proposed with nitroacetone and NCO species as the key reaction intermediates. The results of the investigation show that the catalytic properties of the Zr6Nb2O17 solid solution could be of interest regarding the development of low-temperature catalyst for the SCR of NO x with hydrocarbons. © 2009 Springer Science+Business Media, LLC.
Open Access
Combined component swapping modularity for a VCT engine controller
(ASME, 2010) Çakmakcı, Melih; Ulsoy, A.G.
The use of bi-directional communication provides additional design freedom which can be used to maximize the swapping modularity of networked smart components. In this paper, application of a design method for combined swapping modularity of two or more system components is discussed. Development of measures for combined swapping modularity is important to be able to analyze more realistic engineering cases. The combined modularity problem is a more difficult problem compared to the individual component swapping modularity problem. First, two approaches (simultaneous and sequential) for combining component swapping modularity of two or more components are presented. Then these combined modularity approaches are used to design controllers which maximize the component-swapping modularity of the Variable Camshaft Timing (VCT) component (i.e. actuator and sensor) and the Exhaust Gas Oxygen (EGO) sensor for an internal combustion engine. Copyright © 2009 by ASME.
Open Access
A comparative study of O2 adsorbed carbon nanotubes
(2003) Dag, S.; Gülseren, O.; Çıracı, Salim
First-principles, density functional calculations show that O2 adsorbed single-wall carbon nanotubes (SWNT) show dramatic differences depending on the type of the tube. Upon O2 physisorption, the zig-zag SWNT remains semiconducting, while the metallicity of the armchair is lifted for the spin-down bands. The spin-up bands continue to cross at the Fermi level, and make the system metallic only for one type of spin. The singlet bound state of O2 occurs at the bridge site of the (6,6) SWNT at small distance from the surface of the tube. However, for the hollow site, the molecule dissociates when it comes close to the surface. © 2003 Elsevier B.V. All rights reserved.
Open Access
Double perovskite structure induced by Co addition to PbTiO3: Insights from DFT and experimental solid-state NMR spectroscopy
(American Chemical Society, 2019) Mete, E.; Odabaşı, S.; Mao, H.; Chung, T.; Ellialtıoğlu, Ş.; Reimer, J. A.; Gülseren, Oğuz; Üner, D.
The effects of Co addition on the chemical and electronic structure of PbTiO3 were explored both by theory and through experiment. Cobalt was incorporated into PbTiO3 during the sol–gel process with the X-ray diffraction (XRD) data of the resulting compounds confirming a perovskite structure for the pure samples. The XRD lines broadened and showed emerging cubic structure features as the Co incorporation increased. The changes in the XRD pattern were interpreted as double perovskite structure formation. 207Pb NMR measurements revealed a growing isotropic component in the presence of Co. Consistent with the experiments, density functional theory (DFT)-calculated chemical-shift values corroborate isotropic coordination of Pb, suggesting the formation of cubic Pb2CoTiO6 domains in the prepared samples. Hybrid functional first-principles calculations indicate formation of Pb2CoTiO6 with cubic structure and confirm that Co addition can decrease oxygen binding energy significantly. Experimental UV–vis spectroscopy results indicate that upon addition of Co, the band gap is shifted toward visible wavelengths as confirmed by energy band and absorption spectrum calculations. The oxygen binding energies were determined by temperature-programmed reduction (TPR) measurements. Upon addition of Co, TPR lines shifted to lower temperatures and new features appeared in the TPR patterns. This shift was interpreted as weakening of the oxygen–cobalt bond strength. The change in the electronic structure by the alterations of oxygen vacancy formation energy and bond lengths upon Co insertion is determined by DFT calculations.
Open Access
Effect of six weeks aerobic training upon blood trace metals levels
(2006) Savaş, S.; Şenel, Ö.; Çelikkan, H.; Uǧraş, A.; Aksu, M. L.
This study was carried out to investigate the effects of 6-week aerobic exercise program upon blood Zn and Cu levels. There were 12 male university students with an average age of 21.67+/-0.89 years and no regular training habits participated in the study. The participants were subjected three days a week 1 hour a day continuous running program on treadmill with an intensity of 60-70% for a period of six weeks. They were fed with zinc and copper free diet throughout the study and it was made sure that they were not using copper or zinc containing vitamin tablets. The difference between the pre and post study period were found to be statistically significant as regards to both resting and maximal loading conditions (p<0.01). The pre and post training maxVO2 values were also found to be positively correlated with the copper and zinc levels in blood. Both the copper and zinc blood levels were found decreased after the training period p<0.05.
Open Access
Full‐brain coverage and high‐resolution imaging capabilities of passband b‐SSFP fMRI at 3T
(Wiley‐Liss, Inc., 2008) Lee, J. H.; Dumoulin, S.; Sarıtaş, Emine Ülkü; Glover, G.; Wandell, B.; Nishimura, D.; Pauly, J.
Passband balanced-steady-state free precession (b-SSFP)fMRI is a recently developed method that utilizes the passband(ﬂat portion) of the b-SSFP off-resonance response to measureMR signal changes elicited by changes in tissue oxygenationfollowing increases in neuronal activity. Rapid refocusing andshort readout durations of b-SSFP, combined with the relativelylarge ﬂat portion of the b-SSFP off-resonance spectrum allowsdistortion-free full-brain coverage with only two acquisitions.This allows for high-resolution functional imaging, without thespatial distortion frequently encountered in conventional high-resolution functional images. Finally, the 3D imaging compati-bility of the b-SSFP acquisitions permits isotropic-voxel-sizehigh-resolution acquisitions. In this study we address some ofthe major technical issues involved in obtaining passband b-SSFP-based functional brain images with practical imaging pa-rameters and demonstrate the advantages through breath-holding and visual ﬁeld mapping experiments. Magn ResonMed 59:1099 –1110, 2008.
Open Access
Gold catalysts supported on ceria doped by rare earth metals for water gas shift reaction: influence of the preparation method
(2009) Andreeva, D.; Ivanov, I.; Ilieva, L.; Abrashev, M. V.; Zanella, R.; Sobczak, J. W.; Lisowski, W.; Kantcheva, M.; Avdeev, G.; Petrov, K.
Gold catalysts based on ceria, doped by various RE metals (La, Sm, Gd, Yb, Y) were studied. The influence of the preparation methods on structure, properties and catalytic activity in the WGS reaction was investigated. The catalysts' supports were prepared using two different methods: co-precipitation (CP) and mechanochemical activation (MA). The catalysts were tested in a wide temperature interval without and after reactivation. All samples were characterized using a combination of X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy (RS) and X-ray photoelectron spectroscopy (XPS) and TPR. It was found that the catalytic activity of MA catalysts is higher than CP ones. The gold catalysts based on ceria doped by Yb and Sm exhibited the highest activity. After reactivation in air the MA samples almost kept the WGS activity same, while the CP catalysts increased it. The catalysts of a single- and double-phase structure are formed as a result of CP and MA preparation, respectively. There are no big differences in the gold particles size (2-3 nm) depending on dopants and on the preparation techniques. The RS spectra analysis indicates that most probably the oxygen vacancies are adjacent to Me3+ dopant and the ceria structure seems to be better ordered than in the case of alumina as a dopant. There is no distinct correlation between reducibility and WGS activity. The XPS analysis disclose positively charged gold particles in addition to metallic gold within a surface region of fresh samples and only metallic gold on the samples after catalytic processing. There is no simple correlation between the concentration of Ce3+ in the samples and their WGS activity.
Open Access
In situ synthesis of biomolecule encapsulated gold-cross-linked poly(ethylene glycol) nanocomposite as biosensing platform: A model study
(Elsevier BV, 2010) Odaci, D.; Kahveci, M.U.; Sahkulubey, E.L.; Ozdemir, C.; Uyar, Tamer; Timur, S.; Yagci Y.
In situ synthesis of poly(ethylene glycol) (PEG) hydrogels containing gold nanoparticles(AuNPs) and glucose oxidase (GOx) enzyme by photo-induced electron transfer process was reported here and applied in electrochemical glucose biosensing as the model system. Newly designed bionanocomposite matrix by simple one-step fabrication offered a good contact between the active site of the enzyme and AuNPs inside the network that caused the promotion in the electron transfer properties that was evidenced by cyclic voltammetryas well as higher amperometric biosensing responses in comparing with response signals obtained from the matrix without AuNPs. As well as some parameters important in the optimization studies such as optimum pH, enzyme loading and AuNP amount, the analytical characteristics of the biosensor (AuNP/GOx) were examined by the monitoring of chronoamperometric response due to the oxygen consumption through the enzymatic reaction at − 0.7 V under optimized conditions at sodium acetate buffer (50 mM, pH 4.0) and the linear graph was obtained in the range of 0.1–1.0 mM glucose. The detection limit (LOD) of the biosensor was calculated as 0.06 mM by using the signal to noise ratio of 3. Moreover, the presence of AuNPs was visualized by TEM. Finally, the biosensor was applied for glucose analysis for some beverages and obtained data were compared with HPLC as the reference method to test the possible matrix effect due to the nature of the samples.
Open Access
Investigation of native oxide removing from HCPA ALD grown GaN thin films surface utilizing HF solutions
(IEEE, 2016) Deminskyi, Petro; Haider, Ali; Bıyıklı, Necmi; Ovsianitsky, A.; Tsymbalenko, A.; Kotov, D.; Matkivskyi, V.; Liakhova, N.; Osinsky, V.
The paper consider oxygen contamination of HCPA ALD grown GaN films under an air conditioning and during different time duration. High resolution XPS analysis of HCPA ALD grown GaN films after diluted 1:10 HF(41 %) : H2O and undiluted HF (41 %) influence on oxygen impurities was investigated. Lesser oxygen impurities have been observed. Better resistivity to oxygen atoms of GaN thin films after diluted HF solution treatment was achieved compared to undiluted HF treatment and without treatment.
Open Access
Mesoporous metallic rhodium nanoparticles
(Nature Publishing Group, 2017) Jiang B.; Li C.; Dag, Ö.; Abe, H.; Takei, T.; Imai, T.; Hossain, M. S. A.; Islam, M. T.; Wood, K.; Henzie, J.; Yamauchi, Y.
Mesoporous noble metals are an emerging class of cutting-edge nanostructured catalysts due to their abundant exposed active sites and highly accessible surfaces. Although various noble metal (e.g. Pt, Pd and Au) structures have been synthesized by hard- and soft-templating methods, mesoporous rhodium (Rh) nanoparticles have never been generated via chemical reduction, in part due to the relatively high surface energy of rhodium (Rh) metal. Here we describe a simple, scalable route to generate mesoporous Rh by chemical reduction on polymeric micelle templates [poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA)]. The mesoporous Rh nanoparticles exhibited a ∼1/42.6 times enhancement for the electrocatalytic oxidation of methanol compared to commercially available Rh catalyst. Surprisingly, the high surface area mesoporous structure of the Rh catalyst was thermally stable up to 400 °C. The combination of high surface area and thermal stability also enables superior catalytic activity for the remediation of nitric oxide (NO) in lean-burn exhaust containing high concentrations of O 2.
Open Access
Near-IR-triggered, remote-controlled release of metal ions: A novel strategy for caged ions
(Wiley-VCH Verlag, 2014) Atilgan, A.; Eçik, E. T.; Guliyev, R.; Uyar, T. B.; Erbas-Cakmak, S.; Akkaya, E. U.
A ligand incorporating a dithioethenyl moiety is cleaved into fragments which have a lower metal-ion affinity upon irradiation with low-energy red/near-IR light. The cleavage is a result of singlet oxygen generation which occurs on excitation of the photosensitizer modules. The method has many tunable factors that could make it a satisfactory caging strategy for metal ions.
Open Access
One-step codoping of reduced graphene oxide using boric and nitric acid mixture and its use in metal-free electrocatalyst
(Elsevier, 2015) Tien H.N.; Kocabas, C.; Hur, S.H.
In this study, the preparation of a highly efficient metal-free electrocatalyst, boron and nitrogen codoped reduced graphene oxide (BN-rGO), with an excellent durability is reported. The BN-rGO were prepared in one step using boric and nitric acid mixture, exhibiting highly improved oxygen reduction reaction (ORR) activity than those of the pristine GO and single doped rGOs. The electrocatalyst also showed the excellent long-term durability and CO tolerance than those of the commercial Pt/C catalysts. © 2014 Elsevier B.V.All rights reserved.
Open Access
Oxygen partial pressure dependence of magnetic, optical and magneto-optical properties of epitaxial cobalt-substituted SrTiO3 films
(OSA - The Optical Society, 2015) Onbaşli, M.C.; Goto, T.; Tang, A.; Pan, A.; Battal, E.; Okyay, Ali Kemal; Dionne G.F.; Ross, C.A.
Cobalt-substituted SrTiO3 films (SrTi0.70Co0.30O3-δ) were grown on SrTiO3 substrates using pulsed laser deposition under oxygen pressures ranging from 1 μTorr to 20 mTorr. The effect of oxygen pressure on structural, magnetic, optical, and magneto-optical properties of the films was investigated. The film grown at 3 μTorr has the highest Faraday rotation (FR) and magnetic saturation moment (Ms). Increasing oxygen pressure during growth reduced Ms, FR and optical absorption in the nearinfrared. This trend is attributed to decreasing Co2+ ion concentration and oxygen vacancy concentration with higher oxygen partial pressure during growth. © 2015 Optical Society of America.
Open Access
Oxygen plasma modification of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) film surfaces for tissue engineering purposes
(John Wiley & Sons, Inc., 2003) Hasirci, V.; Tezcaner, A.; Hasirci, N.; Süzer, Ş.
Plasma glow-discharge application is known as a technique to coat or modify the surfaces of various materials. In this study, the influence of oxygen rf-plasma treatment on surface and bulk properties of a biological polyester, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), were studied by determining water content and water contact angle, and by using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The plasma-treated films absorbed more water than the untreated film, and the absorbance increased with the total power applied. The water contact angles decreased and O/C atomic ratio increased on treatment, indicating that the material became more hydrophilic due to increases in the oxygen-containing functional groups on the surface of the polymer. A direct relation could be observed when the O/C ratio was plotted against the total power applied (treatment duration x treatment power). SEM revealed a visual record of surface modification, the extent of which increased with increased total power. It was thus possible to alter the surface chemistry and relevant properties of the polymer film using oxygen plasma as a tool.
Open Access
Plasma-enhanced atomic layer deposition of III-nitride thin films
(Electrochemical Society Inc., 2013) Ozgit-Akgun, Çağla; Dönmez İnci; Bıyıklı, Necmi
AlN and GaN thin films were deposited by plasma-enhanced atomic layer deposition using trimethylmetal precursors. The films were found to have high oxygen incorporation, which was attributed to oxygen contamination related to the plasma system. The choice of nitrogen containing plasma gas (N2, N2/H2 or NH3) determined the severity of oxygen incorporation into deposited films. Lowest oxygen concentrations were attained for AlN and GaN thin films using NH3 and N2 plasma, respectively. Initial experiments have shown that GaN thin films with low impurity concentrations can be deposited when plasma-related oxygen contamination is avoided by the use of an alternative plasma source. © The Electrochemical Society.
Open Access
Rational synthesis of Na and S co-catalyst TiO2-based nanofibers: presence of surface-layered TiS3 shell grains and sulfur-induced defects for efficient visible-light driven photocatalysis
(Royal Society of Chemistry, 2017) Ranjith, K. S.; Uyar, Tamer
Surface-modified TiO2 nanofibers (NFs) with tunable visible-light photoactive catalysts were synthesised through electrospinning, followed by a sulfidation process. The utilization of sodium-based sulfidation precursors effectively led to the diffusion and integration of sulfur impurities into TiO2, modifying its band function. The optical band function of the sulfur-modified TiO2 NFs can be easily manipulated from 3.17 eV to 2.28 eV through surface modification, due to the creation of oxygen vacancies through the sulfidation process. Sulfidating TiO2 NFs introduces Ti-S-based nanograins and oxygen vacancies on the surface that favor the TiO2-TiS3 core-shell interface. These defect states extend the photocatalytic activity of the TiO2 NFs under visible irradiation and improve effective carrier separation and the production of reactive oxygen species. The surface oxygen vacancies and the Ti-S-based surface nanograins serve as charge traps and act as adsorption sites, improving the carrier mobility and avoiding charge recombination. The diffused S-modified TiO2 NFs exhibit a degradation rate of 0.0365 cm-1 for RhB dye solution, which is 4.8 times higher than that of pristine TiO2 NFs under visible irradiation. By benefiting from the sulfur states and oxygen vacancies, with a narrowed band gap of 2.3 eV, these nanofibers serve as suitable localized states for effective carrier separation.
Open Access
Remote-controlled release of singlet oxygen by the plasmonic heating of endoperoxide-modified gold nanorods: towards a paradigm change in photodynamic therapy
(Wiley-VCH Verlag, 2016) Kolemen, S.; Ozdemir, T.; Lee, D.; Kim, G. M.; Karatas, T.; Yoon, J.; Akkaya, E. U.
The photodynamic therapy of cancer is contingent upon the sustained generation of singlet oxygen in the tumor region. However, tumors of the most metastatic cancer types develop a region of severe hypoxia, which puts them beyond the reach of most therapeutic protocols. More troublesome, photodynamic action generates acute hypoxia as the process itself diminishes cellular oxygen reserves, which makes it a self-limiting method. Herein, we describe a new concept that could eventually lead to a change in the 100 year old paradigm of photodynamic therapy and potentially offer solutions to some of the lingering problems. When gold nanorods with tethered endoperoxides are irradiated at 808 nm, the endoperoxides undergo thermal cycloreversion, resulting in the generation of singlet oxygen. We demonstrate that the amount of singlet oxygen produced in this way is sufficient for triggering apoptosis in cell cultures. EPT sees the light: When gold nanorods with tethered endoperoxides are irradiated with near-infrared light, the endoperoxides undergo thermal cycloreversion, resulting in the generation of singlet oxygen. The amount of singlet oxygen generated by these nanocomposites is sufficient for triggering apoptosis in cell cultures.