Researcher:
Birer, Özgür

Profile Picture
ORCID

Job Title

Researcher

First Name

Özgür

Last Name

Birer

Name

Name Variants

Birer, Özgür

Email Address

Birth Date

Filters

2009 - 200932010 - 201933

Advanced Search

Filter by

Settings

Researcher Of Publications: search.filters.isAuthorOfPublication.9581e5ef-67d3-4143-9cca-a896dbad4045

Search Results

Now showing 1 - 10 of 36
  • Placeholder
    PublicationMetadata only
    Rattling in the cage: Ions as probes of sub-picosecond water network dynamics
    (American Chemical Society (ACS), 2009) Schmidt, Diedrich A.; Funkner, Stefan; Born, Benjamin P.; Gnanasekaran, Ramachandran; Schwaab, Gerhard W.; Leitner, David M.; Havenith, Martina; Department of Chemistry; Birer, Özgür; Faculty Member; Department of Chemistry; College of Sciences; N/A
    We present terahertz (THz) measurements of salt solutions that shed new light on the controversy over whether salts act as kosmotropes (structure makers) or chaotropes (structure breakers), which enhance or reduce the solvent order, respectively. We have carried out precise measurements of the concentrationdependent THz absorption coefficient of 15 solvated alkali halide salts around 85 cm-1 (2.5 THz). In addition, we recorded overview spectra between 30 and 300 cm-1 using a THz Fourier transform spectrometer for six alkali halides. For all solutions we found a linear increase of THz absorption compared to pure water (THz excess) with increasing solute concentration. These results suggest that the ions may be treated as simple defects in an H-bond network. They therefore cannot be characterized as either kosmotropes or chaotropes. Below 200 cm-1, the observed THz excess of all salts can be described by a linear superposition of the water absorption and an additional absorption that is attributed to a rattling motion of the ions within the water network. By providing a comprehensive set of data for different salt solutions, we find that the solutions can all be very well described by a model that includes damped harmonic oscillations of the anions and cations within the water network. We find this model predicts the main features of THz spectra for a variety of salt solutions. The assumption of the existence of these ion rattling motions on sub-picosecond time scales is supported by THz Fourier transform spectroscopy of six alkali halides. Above 200 cm-1 the excess is interpreted in terms of a change in the wing of the water network librational mode. Accompanying molecular dynamics simulations using the TIP3P water model support our conclusion and show that the fast sub-picosecond motions of the ions and their surroundings are almost decoupled. These findings provide a complete description of the solute-induced changes in the THz solvation dynamics for the investigated salts. Our results show that THz spectroscopy is a powerful experimental tool to establish a new view on the contributions of anions and cations to the structuring of water. © 2009 American Chemical Society.
  • Placeholder
    PublicationMetadata only
    Structural changes in a Schiff base molecular assembly initiated by scanning tunneling microscopy tip
    (Institute of Physics (IOP) Publishing, 2016) Tomak, A.; Bacaksiz, C.; Mendirek, G.; Sahin, H.; Hur, D.; Gorgun, K.; Senger, R. T.; Peeters, F. M.; Zareie, H. M.; N/A; Birer, Özgür; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    We report the controlled self-organization and switching of newly designed Schiff base (E)-4-((4-(phenylethynyl) benzylidene) amino) benzenethiol (EPBB) molecules on a Au (111) surface at room temperature. Scanning tunneling microscopy and spectroscopy (STM/STS) were used to image and analyze the conformational changes of the EPBB molecules. The conformational change of the molecules was induced by using the STM tip while increasing the tunneling current. The switching of a domain or island of molecules was shown to be induced by the STM tip during scanning. Unambiguous fingerprints of the switching mechanism were observed via STM/STS measurements. Surface-enhanced Raman scattering was employed, to control and identify quantitatively the switching mechanism of molecules in a monolayer. Density functional theory calculations were also performed in order to understand the microscopic details of the switching mechanism. These calculations revealed that the molecular switching behavior stemmed from the strong interaction of the EPBB molecules with the STM tip. Our approach to controlling intermolecular mechanics provides a path towards the bottom-up assembly of more sophisticated molecular machines.
  • Placeholder
    PublicationMetadata only
    TiO2–Al2O3 binary mixed oxide surfaces for photocatalytic NOx abatement
    (Elsevier Science Bv, 2014) Soylu, Asli Melike; Polat, Meryem; Erdogan, Deniz Altunoz; Say, Zafer; Ozensoy, Emrah; N/A; Yıldırım, Cansu; Birer, Özgür; Master Student; Researcher; Graduate School of Sciences and Engineering; N/A
    TiO2-Al2O3 binary oxide surfaces were utilized in order to develop an alternative photocatalytic NOx abatement approach, where TiO2 sites were used for ambient photocatalytic oxidation of NO with O-2 and alumina sites were exploited for NOx storage. Chemical, crystallographic and electronic structure of the TiO2-Al2O3 binary oxide surfaces were characterized (via BET surface area measurements, XRD, Raman spectroscopy and DR-UV-Vis Spectroscopy) as a function of the TiO2 loading in the mixture as well as the calcination temperature used in the synthesis protocol. 0.5 Ti/Al-900 photocatalyst showed remarkable photocatalytic NOx oxidation and storage performance, which was found to be much superior to that of a Degussa P25 industrial benchmark photocatalyst (i.e. 160% higher NOx storage and 55% lower NO2(g) release to the atmosphere). Our results indicate that the onset of the photocatalytic NOx abatement activity is concomitant to the switch between amorphous to a crystalline phase with an electronic band gap within 3.05-3.10 eV; where the most active photocatalyst revealed predominantly rutile phase together and anatase as the minority phase.
  • Placeholder
    PublicationMetadata only
    Using rattling ions to probe SUB-PS water network dynamics
    (IEEE, 2010) Schmidt, Diedrich A.; Funkner, Stefan; Born, Benjamin P.; Gnanasekaran, Ramachandran; Schwaab, Gerhard W.; Leitner, David M.; Havenith, Martina; N/A; Birer, Özgür; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    We present THz measurements on fifteen solvated alkali halide salts using narrow band THz absorption and broad band THz Fourier transform spectroscopies in order to shed new light on the controversy of salts as kosmotropes (structure breakers) or chaotropes (structure makers). © 2010 IEEE.
  • Placeholder
    PublicationMetadata only
    Sonochemical zinc oxide and layered hydroxy zinc acetate synthesis in fenton-like reactions
    (Elsevier, 2017) Department of Chemistry; N/A; Haciu, Durata; Birer, Özgür; Teaching Faculty; Researcher; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Sciences; N/A; N/A; N/A
    Zinc acetate solution is sonicated at high power in water and in ethanol in the absence and presence of various peroxides. In the absence of peroxides, the products are zinc oxide and layered hydroxy zinc acetate in water and in ethanol, respectively. Layered basic zinc acetate are prepared for the first time using sonochemical methods. The addition of peroxides alters the reaction mechanisms. In water, insoluble peroxides produce zinc oxides while the water soluble peroxide, i.e. hydrogen peroxide, completely destroyed the structure and casted a doubt on the accepted peroxide initiated mechanism of reactions. In ethanol, peroxide addition caused the reaction mechanism to change and some oxide formation is observed. The reaction mechanism is sensitive to water/ethanol amounts as well as the peroxide to zinc ion mole ratio. Thin zinc oxide wafers (ca. 30 nm) with band gaps of 3.24 eV were obtained. (C) 2016 Elsevier B.V. All rights reserved.
  • Placeholder
    PublicationMetadata only
    Ultraviolet upconversion spectra of sonochemically synthesized doped NaYF4 crystals
    (Elsevier, 2014) N/A; Department of Chemistry; Yıldırım, Cansu; Birer, Özgür; Master Student; Researcher; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; College of Sciences
    A facile sonochemical method was developed for the production of alpha- and beta-forms of Yb3+(20%)/Tm3+(2%) or Er3+(2%) doubly doped NaYF4 crystals. The reactions took place in aqueous solutions at low temperatures and employed oxide or salt precursors. Micrometer sized beta-NaYF4 and nanometer sized alpha-NaYF4 were obtained with upconversion property. The required activation energy for the solid-solid phase transition was overcome with the energy provided by cavitations. Upconversion emission properties of the Yb3+(20%), Er3+(2%) or Tm3+(2%) doubly doped, alpha- and beta-NaYF4 were investigated. The alpha-form with emission in the ultraviolet region had higher efficiency contradicting the literature. Several new peaks in the spectra, most likely due to crystal defects, were temporarily assigned.
  • Placeholder
    PublicationMetadata only
    Sonochemical shape control of copper hydroxysulfates
    (Elsevier, 2012) N/A; N/A; N/A; Kaş, Recep; Birer, Özgür; Master Student; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; N/A; N/A; N/A
    Shape control of inorganic nanoparticles generally requires the use of surfactants or ligands to passivate certain crystallographic planes. Additive free shape control methods utilize the differences in the growth rates of crystallographic planes. We combined this approach with the sonochemical method to synthesize copper hydroxysulfate (Brochantite) with morphologies ranging from flowers, to bricks, belts and needles. Sodium peroxydisulfate, which was used as the sulfate and hydroxide source, was decomposed thermally and/or sonically under various pH and temperature conditions. The relative release rates of the sulfate and hydroxide anions determined the final form of the crystals. This technique yielded products even at acidic pH, marking a distinction from the literature reactions, which start with stoichiometric amounts of sulfate and hydroxide anions and yield only a single crystal morphology.
  • Placeholder
    PublicationMetadata only
    Reactivity zones around an atmospheric pressure plasma jet
    (Elsevier Science Bv, 2015) N/A; Birer, Özgür; Resercher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    The reactivity zones around an atmospheric pressure plasma jet are revealed by XPS mapping of chemical moieties on a polyethylene surface treated with a 3-mm plasma jet. The area directly hit by the helium plasma jet initially oxidizes and later etches away as the plasma treatment continues. The oxidation initially starts at the center and expands outwards as a ring pattern with different spatial potency. At the end of 10 min plasma jet treatment, distinct ring patterns for -NO, -COO, -CO and -NO3 species can be detected with respectively increasing diameters. The plasma jet can cause chemical changes at locations several millimeters away from the center. The spatial distribution of oxidized species suggests presence of chemical reactivity zones. Introduction of nitrogen into the helium plasma jet, not only increases the type of nitrogen moieties, but enriches the reactivity zones by generating nitrogen molecular ions within the plasma jet. The complex competing reaction mechanisms among the radicals, ions, metastable atoms and UV photons lead to unusual etching patterns on the surfaces. (C) 2015 Elsevier B.V. All rights reserved.
  • Placeholder
    PublicationMetadata only
    E. coli-derived carbon with nitrogen and phosphorus dual functionalities for oxygen reduction reaction
    (2015) Wei, Li; Yu, Dingshan; Karahan, H. Enis; Goh, Kunli; Yuan, Yang; Jiang, Wenchao; Liang, Wang; Chen, Yuan; N/A; Birer, Özgür; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A
    Nitrogen and phosphorus dual doped carbon materials have shown competing catalytic activity as metalfree catalysts for oxygen reduction reaction (ORR). Prevailing methods to synthesize such carbon catalysts require high purity chemicals and gases, excessive amount of strong acids and bases, or costly organic precursors. Here, we show that Escherichia coli (E. coli), a common bacterium, can be used as a novel precursor to synthesize heterogeneous carbon materials with nitrogen and phosphorus dual functionalities by direct carbonization. The E. coli derived carbon has a large surface area up to 636 m2/g, and is comprised of up to 7.64 at% nitrogen and 8.53 at% phosphorus. The carbon material obtained at an optimum carbonization condition of 1000◦C for 2 h shows excellent catalytic performances, comparable to that of a 20 wt% Pt-C catalyst, with ∼50% higher limiting current density, for ORR in near four-electron transfer processes and excellent durability. The outstanding electrocatalytic performances are credited to the fine tuning of nitrogen species by carbonization conditions and the synergy between nitrogen and phosphorus dual functionalities. Our work demonstrates the great potential of using bacterial cells as a novel green precursor to synthesize heterogeneous carbon materials with nitrogen and phosphorus dual functionalities as electrocatalytic catalysts for ORR.
  • Placeholder
    PublicationMetadata only
    Dimer formation of perylene: an ultracold spectroscopic and computational study
    (Elsevier, 2015) N/A; Department of Chemistry; Birer, Özgür; Yurtsever, İsmail Ersin; Researcher; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; College of Science; N/A; 7129
    The electronic spectra of perylene inside helium nanodroplets recorded by the depletion method are presented. The results show two broad peaks in addition to sharp monomer vibronic transitions due to dimer formation. In order to understand the details of the spectra, first the dimer formation is studied by DFT and SCS-MP2 calculations and then the electronic spectra are calculated at the minima of the potential energy surface (PES). Theoretical calculations show that there are two low-lying energetically degenerate dimer structures; namely a parallel displaced one and a rotated stacked one. PES around these minima is very flat with a number of local minima at higher energies which at the experimental temperatures cannot be populated. Even though thermodynamically these two structures are equally populated, dynamical considerations point out that in helium droplet the parallel displaced geometry is encouraged by the natural alignment of the molecules due to the acquired angular momentum following the pick-up process. The calculated spectrum of the parallel displaced geometry predicts the positions of the dimer transitions within 30 nm of the experimental spectrum. Furthermore, the difference between the two dimer transitions is accurately predicted to be about 25 nm while the experimental difference was about 20 nm. Such a small difference could only be detected due to the ultracold conditions helium nanodroplets provided.