Browsing by Subject "Chemical structure"

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Open Access
3d cobalt dicyanamide derived 2d- ayered co(oh)2 based catalyst for light driven hydrogen evolution
(American Chemical Society, 2024-02-09) Akbari, Sina Sadigh; Karadaş, Ferdi
Derivation of 3D coordination polymers to produce active catalysts has been a feasible strategy to achieve a precise coordination sphere for the catalytic site. This study demonstrates the partial conversion of a 3D cobalt dicyanamide coordination polymer, Co-dca, to a 2D layered hydroxide-oxyhydroxide structure under photocatalytic conditions. The catalyst exhibits an activity as high as 28.3 mmol h(-1) g(-1) in the presence of a [Ru(bpy)(3)](2+)/triethylamine (TEA) couple to maintain it for at least 12 h. Photocatalytic and characterization studies reveal that the dicyanamide ligand within the coordination polymer is crucial for governing modification and achieving a superior H-2 evolution rate. Moreover, we observed the critical role of TEA as the hydrolyzing agent for the transformation process. This study displays that the metal dicyanamides can be utilized as templates for preparing active and robust catalysts.
Open Access
Antibacterial electrospun nanofibers from triclosan/cyclodextrin inclusion complexes
(Elsevier, 2014) Celebioglu A.; Umu, O. C. O.; Tekinay, T.; Uyar, Tamer
The electrospinning of nanofibers (NF) from cyclodextrin inclusion complexes (CD-IC) with an antibacterial agent (triclosan) was achieved without using any carrier polymeric matrix. Polymer-free triclosan/CD-IC NF were electrospun from highly concentrated (160% CD, w/w) aqueous triclosan/CD-IC suspension by using two types of chemically modified CD; hydroxypropyl-beta-cyclodextrin (HPβCD) and hydroxypropyl-gamma-cyclodextrin (HPγCD). The morphological characterization of the electrospun triclosan/CD-IC NF by SEM elucidated that the triclosan/HPβCD-IC NF and triclosan/HPγCD-IC NF were bead-free having average fiber diameter of 520±250nm and 1100±660nm, respectively. The presence of triclosan and the formation of triclosan/CD-IC within the fiber structure were confirmed by 1H-NMR, FTIR, XRD, DSC, and TGA studies. The initial 1:1molar ratio of the triclosan:CD was kept for triclosan/HPβCD-IC NF after the electrospinning and whereas 0.7:1molar ratio was observed for triclosan/HPγCD-IC NF and some uncomplexed triclosan was detected suggesting that the complexation efficiency of triclosan with HPγCD was lower than that of HPβCD. The antibacterial properties of triclosan/CD-IC NF were tested against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. It was observed that triclosan/HPβCD-IC NF and triclosan/HPγCD-IC NF showed better antibacterial activity against both bacteria compared to uncomplexed pure triclosan.
Open Access
Cyclodextrin-functionalized mesostructured silica nanoparticles for removal of polycyclic aromatic hydrocarbons
(Academic Press Inc., 2017) Topuz, F.; Uyar, T.
Polycyclic aromatic hydrocarbons (PAHs) are the byproducts of the incomplete combustion of carbon-based fuels, and have high affinity towards DNA strands, ultimately exerting their carcinogenic effects. They are ubiquitous environmental contaminants, and can accumulate on tissues due to their lipophilic nature. In this article, we describe a novel concept for PAH removal from aqueous solutions using cyclodextrin-functionalized mesostructured silica nanoparticles (CDMSNs) and pristine mesostructured silica nanoparticles (MSNs). The adsorption applications of MSNs are greatly restricted due to the absence of surface functional groups on such particles. In this regard, cyclodextrins can serve as ideal functional molecules with their toroidal, cone-type structure, capable of inclusion-complex formation with many hydrophobic molecules, including genotoxic PAHs. The CDMSNs were synthesized by the surfactant-templated, NaOH-catalyzed condensation reactions of tetraethyl orthosilicate (TEOS) in the presence of two different types of cyclodextrin (i.e. hydroxypropyl-β-cyclodextrin (HP-β-CD) and native β-cyclodextrin (β-CD)). The physical incorporation of CD moieties was supported by XPS, FT-IR, NMR, TGA and solid-state 13C NMR. The CDMSNs were treated with aqueous solutions of five different PAHs (e.g. pyrene, anthracene, phenanthrene, fluorene and fluoranthene). The functionalization of MSNs with cyclodextrin moieties significantly boosted the sorption capacity (q) of the MSNs up to ∼2-fold, and the q ranged between 0.3 and 1.65 mg per gram CDMSNs, of which the performance was comparable to that of the activated carbon.
Open Access
Design strategies for ratiometric chemosensors: modulation of excitation energy transfer at the energy donor site
(2009) Guliyev, R.; Coskun, A.; Akkaya, E. U.
Excitation energy transfer, when coupled to an ion-modulated ICT chromophore, creates novel opportunities in sensing. The direction of energy transfer and the point of ICT modulation can be varied as desired. In our previous work, we have shown that energy transfer efficiency between two energetically coupled fluorophores will be altered by the metal ion binding to the ICT chromophore carrying a ligand. There are two beneficial results: increased pseudo-Stokes shift and expanded dynamic range. Here, we explored the consequences of the modulation of energy transfer efficiency at the energy donor site, in a molecular design which has an ICT type metal ion-sensitive chromophore placed as the energy donor in the dyad. Clear advantages emerge compared to the acceptor site modulation: unaltered emission wavelength in the red end of the visible spectrum, while keeping a large Stokes shift and the ratiometric character. © 2009 American Chemical Society.
Open Access
Dispersion of multi-walled carbon nanotubes in an aqueous medium by water-dispersible conjugated polymer nanoparticles
(2010) Baykal, B.; Ibrahimova, V.; Er, G.; Bengü, E.; Tuncel, D.
Vertically aligned multi-walled carbon nanotubes (MWCNTs) synthesized by the alcohol catalytic CVD (ACCVD) technique are dispersed in water with the aid of water-dispersible conjugated polymer nanoparticles (CPNs). The interactions between CPNs and CNTs are studied with spectroscopy (UV-Vis, fluorescence and Raman) and electron microscopy techniques are used to confirm attachment of CPNs to the CNT sidewalls.
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
Dynamic correlation effects on the plasmon dispersion in a two-dimensional electron gas
(The American Physical Society, 2003) Yurtsever, A.; Moldoveanu, V.; Tanatar, Bilal
The charge-density oscillations (plasmons) of a low-density two-dimensional uniform electron gas are studied within the framework of finite temperature and frequency dependent (dynamic) version of Singwi, Tosi, Land, and Sjölander theory and compared with the recent experimental results. The use of the Hartree-Fock approximation for the static structure factor leads to a finite temperature dynamical counterpart of the static Hubbard approximation. We observe important differences between dynamic and static local-field factors as well as between the corresponding plasmon dispersion laws. Our calculated plasmon energies that include dynamic correlations are in very-good agreement with the recent experimental results.
Open Access
Electronic structure of Te-and As-covered Si(211)
(American Physical Society, 2003) Sen, P.; Batra, I. P.; Sivananthan, S.; Grein, C. H.; Dhar, N.; Çıracı, Salim
Electronic and atomic structures of the clean and As- and Te-covered Si(211) surface are studied using pseudopotential density-functional method. The clean surface is found to have (2 x 1) and rebonded (1 x 1) reconstructions as stable surface structures, but no π-bonded chain reconstruction. Binding energies of As and Te adatoms at a number of symmetry sites on the ideal and (2 x 1) reconstructed surfaces have been calculated because of their importance in the epitaxial growth of CdTe and other materials on the Si(211) surface. The special symmetry sites on these surfaces having the highest binding energies for isolated As and Te adatoms are identified. But more significantly, several sites are found to be nearly degenerate in binding-energy values. This has important consequences for epitaxial growth processes. Optimal structures calculated for 0.5 monolayer of As and Te coverage reveal that the As adatoms dimerize on the surface while the Te adatoms do not. However, both As- and Te-covered surfaces are found to be metallic in nature.
Unknown
Highly stable multicrown heterostructures of type-II nanoplatelets for ultralow threshold optical gain
(American Chemical Society, 2019) Dede, Didem; Taghipour, Nima; Quliyeva, Ulviyya; Sak, Mustafa; Kelestemur, Yusuf; Güngör, Kıvanç; Demir, Hilmi Volkan
Solution-processed type-II quantum wells exhibit outstanding optical properties, which make them promising candidates for light-generating applications including lasers and LEDs. However, they may suffer from poor colloidal stability under ambient conditions and show strong tendency to assemble into face-to-face stacks. In this work, to resolve the colloidal stability and uncontrolled stacking issues, we proposed and synthesized CdSe/CdSe1–xTex/CdS core/multicrown heteronanoplatelets (NPLs), controlling the amount of Te up to 50% in the crown without changing their thicknesses, which significantly increases their colloidal and photostability under ambient conditions and at the same time preserving their attractive optical properties. Confirming the final lateral growth of CdS sidewalls with X-ray photoelectron spectroscopy, energy-dispersive analysis, and photoelectron excitation spectroscopy, we found that the successful coating of this CdS crown around the periphery of conventional type-II NPLs prevents the unwanted formation of needle-like stacks, which results in reduction of the undesired scattering losses in thin-film samples of these NPLs. Owing to highly efficient exciton funneling from the outmost CdS crown accompanied by the reduced scattering and very low waveguide loss coefficient (∼18 cm–1), ultralow optical gain thresholds of multicrown type-II NPLs were achieved to be as low as 4.15 μJ/cm2 and 2.48 mJ/cm2 under one- and two-photon absorption pumping, respectively. These findings indicate that the strategy of using engineered advanced heterostructures of nanoplatelets provides solutions for improved colloidal stability and enables enhanced photonic performance.
Open Access
Hydrogenated carbon monolayer in biphenylene network offers a potential paradigm for nanoelectronic devices
(American Chemical Society, 2022-09-15) Demirci, S.; Gorkan, T.; Çallıoǧlu, Şafak; Özçelik, V. O.; Barth, J.; Aktürk, E.; Çıracı, Salim
A metallic carbon monolayer in the biphenylene network (specified as C ohs) becomes an insulator upon hydrogenation (specified as CH ohs). Patterned dehydrogenation of this CH ohs can offer a variety of intriguing functionalities. Composite structures constituted by alternating stripes of C and CH ohs with different repeat periodicity and chirality display topological properties and can form heterostructures with a tunable band-lineup or Schottky barrier height. Alternating arrangements of these stripes of finite size enable one to also construct double barrier resonant tunneling structures and 2D, lateral nanocapacitors with high gravimetric capacitance for an efficient energy storage device. By controlled removal of H atom from a specific site or dehydrogenation of an extended zone, one can achieve antidoping or construct 0D quantum structures like antidots, antirings/loops, and supercrystals, the energy level spacing of which can be controlled with their geometry and size for optoelectronic applications. Conversely, all these device functions can be acquired also by controlled hydrogenation of a bare C ohs monolayer. Since all these processes are applied to a monolayer, the commensurability of electronically different materials is assured. These features pertain not only to CH ohs but also to fully hydrogenated Si ohs.
Open Access
Hydrogenation of naphthalene and methylnaphthalene: modeling and spectroscopy
(Elsevier BV, 2002) Sayan, Ş.; Paul, J.
In situ infrared spectra of 1-methylnaphthalene (1-MeNapht)hydrogenation, over sulfided NiMo/Al2O3-TiO2 catalysts, were compared with theoretically derived properties of methylnaphthalene and its bicyclic products: MeDilin, MeTetralin, MeOctalin and MeDecalin, and with conversion data from literature. Comparisons were also made between the un-substituted and methyl-substituted two-rings, and between the 1- and 5-methyl isomers of 1,4-dihydronaphthalene (dilin) and 1,2,3,4-tetrahydronaphthalene (tetralin). IR spectra of MeNapht adsorption, on the sulfided catalyst, were matched with data for adsorption on the catalyst without sulfidation and the empty support. Surface bound MeNapht is observed below 250°C on all catalysts. MeNapht adsorption suppresses OH groups nondiscriminatory on the empty support and the metal loaded catalyst. We relate the results to previous data on the interaction between the supported metal sulfides and titanium modified aluminas. Calculated total energies, and experimentally derived heats of formation, pointed at decahydronaphthalene (decalin) as the dominant product of naphthalene hydrogenation, with tetralin as an abundant intermediate, and dilin and 1,2,3,4,5,6,7,8-octahydronaphthalene (octalin) as short lived transient stages. The spectroscopic modeling showed that the orbital fingerprints of the five bicyclic compounds were not distinctly different, nor more than marginally modified by methyl substitution or isomerization. The only significant difference came at the highest occupied orbital, where a high naphthalene density of states (DoS) overlapped with the valence bands of metal or metal sulfide catalysts. The vibrational bands for naphthalene, dilin, tetralin and octalin were well separated. Octalin and decalin, alone, have similar vibrational spectra. Upheaval of ring degeneracy for methyl-substituted two-ring structures broadened all infrared bands in a characteristic way.
Open Access
Josephson effect in superconductive SNS heterostructures with barriers
(The American Physical Society, 2003) Cakir, O.; Kulik, I. O.
The dc Josephson effect in a planar superconductor-normal-metal-superconductor (SNS) junction is studied in the existence of a δ barrier in the normal region. The Green function of the structure is obtained by solving the Gorkov equations for the structure and then the current is calculated from the Green functions. The effect of the strength and position of the barrier is investigated. The current shows a weak dependence on the position of the barrier and it is seen to be maximum when the barrier is at the middle of the normal region. Also it is found that the current shows a stronger dependence on the strength of the barrier at low temperatures. A comparative discussion of three possible types of Josephson junctions, the SIS, SCS, and SNS contacts, is presented.
Open Access
Manipulation of atoms across a surface at room temperature
(Nature Publishing Group, 2000) Fishlock, T. W.; Oral, A.; Egdell, R. G.; Pethica, J. B.
Since the realization that the tips of scanning probe microscopes can interact with atoms at surfaces, there has been much interest in the possibility of building or modifying nanostructures or molecules directly from single atoms. Individual large molecules can be positioned on surfaces, and atoms can be transferred controllably between the sample and probe tip. The most complex structures are produced at cryogenic temperatures by sliding atoms across a surface to chosen sites. But there are problems in manipulating atoms laterally at higher temperatures - atoms that are sufficiently well bound to a surface to be stable at higher temperatures require a stronger tip interaction to be moved. This situation differs significantly from the idealized weakly interacting tips of scanning tunnelling or atomic force microscopes. Here we demonstrate that precise positioning of atoms on a copper surface is possible at room temperature. The triggering mechanism for the atomic motion unexpectedly depends on the tunnelling current density, rather than the electric field or proximity of tip and surface.
Open Access
Multilayer mXene heterostructures and nanohybrids for multifunctional applications: a review
(American Chemical Society, 2022-05-17) Tasnim Mahmud, S.; Bain, S; Hasan, Md Mehdi; Rahman, S.T.; Rhaman, M.; Hossain, M.M.; Ordu, Mustafa
MXenes (transition metal carbides and nitrides) have experienced exponential growth over the last two decades, thanks to their excellent physical, chemical, and mechanical properties. Intriguing properties like high conductivity, wear, and corrosion resistance while maintaining flexibility are the strong motivation behind the exploration of MXenes. Moreover, the large surface area and unique layered structure enhance the functionality of multilayer-MXene heterostructures and hybrids. This paper reviews the synthesis chemistry, structure properties of multilayer MXenes, and their multifunctional applications. MXene synthesis under different conditions, their hybrids and composites, intercalation, and structural geometries are discussed. The electrical, mechanical, optical, and magnetic properties of MXenes are briefly presented. Recent progress and development in MXene-based heterostructures and nanohybrids for supercapacitors, batteries, environmental and water treatment, antibacterial and tissue engineering, and electromagnetic absorption and shielding are systematically discussed. Finally, research challenges and a perspective in this specified area are addressed for potential developments.
Open Access
Novel metallic clathrates of group-IV elements and their compounds in a dense hexagonal lattice
(American Chemical Society, 2019) Görkan, T.; Özdemir, İ.; Bakır, M. Y.; Ersan, F.; Gökoğlu, G.; Aktürk, E.; Çıracı, Salim
Further, to recently introduced metallic NaSi6 and Si6 clathrate structures, we show that not only Si but also other group-IV elements, such as C, Ge, and Sn, can form stable and metallic clathrate structures with open channels at the corners of hexagons. These elemental clathrates of Si, Ge, and Sn can be viewed as if they are a combination of 2D metallic planes and perpendicular 1D metallic chains, the interplay of which can give rise to interesting physical effects. When free-standing, these atomic planes transform to 2D semiconducting, single-layer structures. The clathrate structure of C, which consists of weakly interacting, vertical hexagonal tubes situated at the corners of a 2D hexagonal lattice, is insulating in the plane but 1D metallic perpendicular to the planes. We also show that stable compound clathrate structures can form by hosting different alkali, alkaline earth, and light transition metal atoms in the open channels of elemental clathrates. These new metallic allotropes of group-IV elements predicted by first-principles calculations based on the density functional theory exhibit features that can be critical fundamentally and technologically.
Open Access
Pd nanocube decoration onto flexible nanofibrous mats of core-shell polymer-ZnO nanofibers for visible light photocatalysis
(Royal Society of Chemistry, 2017) Arslan, O.; Topuz, F.; Eren, H.; Bıyıklı, Necmi; Uyar, Tamer
Plasmonic enhancement for electron-hole separation efficiency and visible light photocatalysis was achieved by Pd nanocube decoration on a ZnO nanolayer coated onto electrospun polymeric (polyacrylonitrile (PAN)) nanofibers. Since exciton formation and sustainable electron-hole separation have a vital importance for realizing better solar energy in photovoltaic and photocatalytic devices, we achieved visible light photocatalysis by Pd nanocube decoration onto well designed core-shell nanofibers of ZnO@PAN-NF. By controlling the cubic Pd nanoparticle size and the thickness of the crystalline ZnO nanolayer deposited onto electrospun PAN nanofibers via atomic layer deposition (ALD), defect mediated visible light photocatalysis efficiency can be increased. By utilizing nanofabrication techniques such as thermal decomposition, electrospinning and ALD, this fabricated template became an efficient, defect mediated, Pd nanocube plasmon enhanced photocatalytic system. Due to the enhanced contact features of the Pd nanocubes, an increase was observed for the visible light photocatalytic activity of the flexible and nanofibrous mat of Pd@ZnO@PAN-NF.
Open Access
Persistent currents in helical structures
(American Physical Society, 2004) Iskin, M.; Kulik, I. O.
The recent discovery of mesoscopic electronic structures, in particular the carbon nanotubes, made necessary an investigation of what effect a helical symmetry of the conductor (metal or semiconductor) may have on the persistent current oscillations, We investigate persistent currents in helical structures which are nondecaying in time, not requiring a voltage bias, dissipationless stationary flow of electrons in a normal-metallic or semiconducting cylinder or circular wire of mesoscopic dimension. In the presence of magnetic flux along the toroidal structure, helical symmetry couples circular and longitudinal currents to each other. Our calculations suggest that circular persistent currents in these structures have two components with periods Φ0 and Φ0/s (s is an integer specific to any geometry). However, resultant circular persistent current oscillations have Φ0 period.
Open Access
Phenylethynyl-BODIPY oligomers: bright dyes and fluorescent building blocks
(2009) Cakmak, Y.; Akkaya, E. U.
Boradiazaindacene dyes were converted into phenylethynyl-BODIPY oligomers via a cycle of reactions, notably including Sonogashira couplings. As expected, as the number, n, of repeating units increases, peak absorption and emission wavelengths are shifted to the red end of the visible spectrum, albeit with smaller increments as n increases. Decyl groups help to keep the solubility remarkably high, and in addition to being very bright red-emitting fluorophores, their rigid rod-like structures could allow their use as functional building blocks. © 2009 American Chemical Society.
Open Access
Quercetin/β-cyclodextrin inclusion complex embedded nanofibres: slow release and high solubility
(Elsevier, 2016-04) Aytac Z.; Kusku, S. I.; Durgun, Engin; Uyar, Tamer
Electrospinning of polyacrylic acid (PAA) nanofibres (NF) incorporating β-cyclodextrin inclusion complex (β-CD-IC) of quercetin (QU) was performed. Here, β-CD was used as not only the crosslinking agent for PAA nanofibres but also as a host molecule for inclusion of QU. The phase solubility test showed enhanced solubility of QU due to the inclusion complexation; in addition, the stoichiometry of QU/β-CD-IC was determined to be 1:1. Computational modelling studies confirmed that 1:1 and 1:2 complex formation are desirable; 1:1 complex formation was chosen to have higher weight loading of QU. SEM images showed that PAA/QU/β-CD-IC-NF were bead-free and uniform. XRD indicated that PAA/QU/β-CD-IC-NF were amorphous in nature without the crystalline peaks of QU. Comparative results revealed that the release profile of QU from PAA/QU/β-CD-IC-NF was much slower but greater in total than from PAA/QU/β-CD-IC-film. Moreover, high antioxidant activity and photostability of QU was achieved in PAA/QU/β-CD-IC-NF.
Open Access
Raman enhancement on a broadband meta-surface
(American Chemical Society, 2012-07-30) Ayas S.; Güner, H.; Türker, B.; Ekiz, O. O.; Dirisaglik, F.; Okyay, Ali Kemal; Dâna, A.
Plasmonic metamaterials allow confinement of light to deep subwavelength dimensions, while allowing for the tailoring of dispersion and electromagnetic mode density to enhance specific photonic properties. Optical resonances of plasmonic molecules have been extensively investigated; however, benefits of strong coupling of dimers have been overlooked. Here, we construct a plasmonic meta-surface through coupling of diatomic plasmonic molecules which contain a heavy and light meta-atom. Presence and coupling of two distinct types of localized modes in the plasmonic molecule allow formation and engineering of a rich band structure in a seemingly simple and common geometry, resulting in a broadband and quasi-omni-directional meta-surface. Surface-enhanced Raman scattering benefits from the simultaneous presence of plasmonic resonances at the excitation and scattering frequencies, and by proper design of the band structure to satisfy this condition, highly repeatable and spatially uniform Raman enhancement is demonstrated. On the basis of calculations of the field enhancement distribution within a unit cell, spatial uniformity of the enhancement at the nanoscale is discussed. Raman scattering constitutes an example of nonlinear optical processes, where the wavelength conversion during scattering may be viewed as a photonic transition between the bands of the meta-material.