Publications by authors named "Dogan Ozkaya"

12 Publications

  • Page 1 of 1

Strain effects in core-shell PtCo nanoparticles: a comparison of experimental observations and computational modelling.

Phys Chem Chem Phys 2020 Nov 27;22(42):24784-24795. Epub 2020 Oct 27.

Department of Chemistry, University of Southampton, Southampton, UK.

Strain in Pt nanoalloys induced by the secondary metal has long been suggested as a major contributor to the modification of catalytic properties. Here, we investigate strain in PtCo nanoparticles using a combination of computational modelling and microscopy experiments. We have used a combination of molecular dynamics (MD) and large-scale density functional theory (DFT) for our models, alongside experimental work using annular dark field scanning transmission electron microscopy (ADF-STEM). We have performed extensive validation of the interatomic potential against DFT using a PtCo nanoparticle. Modelling gives access to 3 dimensional structures that can be compared to the 2D ADF-STEM images, which we use to build an understanding of nanoparticle structure and composition. Strain has been measured for PtCo and pure Pt nanoparticles, with MD annealed models compared to ADF-STEM images. Our analysis was performed on a layer by layer basis, where distinct trends between the Pt and PtCo alloy nanoparticles are observed. To our knowledge, we show for the first time a way in which detailed atomistic simulations can be used to augment and help interpret the results of ADF-STEM strain mapping experiments, which will enhance their use in characterisation towards the development of improved catalysts.
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http://dx.doi.org/10.1039/d0cp04318dDOI Listing
November 2020

Visualization and Chemical Characterization of the Cathode Electrolyte Interphase Using He-Ion Microscopy and Time-of-Flight Secondary Ion Mass Spectrometry.

ACS Appl Energy Mater 2020 Sep 25;3(9):8822-8832. Epub 2020 Aug 25.

Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, U.K.

Unstable cathode electrolyte interphase (CEI) formation increases degradation in high voltage Li-ion battery materials. Few techniques couple characterization of nano-scale CEI layers on the macroscale with chemical characterization, and thus, information on how the underlying microstructure affects CEI formation is lost. Here, the process of CEI formation in a high voltage cathode material, LiCoPO, has been investigated for the first time using helium ion microscopy (HIM) and time-of-flight (ToF) secondary ion mass spectrometry (SIMS). The combination of HIM and Ne-ion ToF-SIMS has been used to correlate the cycle-dependent morphology of the CEI layer on LiCoPO with a local cathode microstructure, including position, thickness, and chemistry. HIM imaging identified partial dissolution of the CEI layer on discharge resulting in in-homogenous CEI coverage on larger LiCoPO agglomerates. Ne-ion ToF-SIMS characterization identified oxyfluorophosphates from HF attack by the electrolyte and a Li-rich surface region. Variable thickness of the CEI layer coupled with inactive Li on the surface of LiCoPO electrodes contributes to severe degradation over the course of 10 cycles. The HIM-SIMS technique has potential to further investigate the effect of microstructures on CEI formation in cathode materials or solid electrolyte interphase formation in anodes, thus aiding future electrode development.
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http://dx.doi.org/10.1021/acsaem.0c01333DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7525808PMC
September 2020

Understanding the mechanochemical synthesis of the perovskite LaMnO and its catalytic behaviour.

Dalton Trans 2020 Jan 9;49(1):232-240. Epub 2019 Dec 9.

UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratories, Harwell Science & Innovation Campus, Didcot, Oxfordshire OX11 0FA, UK.

Mechanochemistry offers a solventless, 'waste free' route to preparing metal oxide catalysts, however, there is limited information on the chemical steps involved. In this work, the perovskite LaMnO has been successfully synthesized via mechanochemistry from metal oxide powders, LaO and MnO, at room temperature, using a planetary ball mill. Separate ex situ'time slices' were taken during the milling procedure to provide insights into the underlying chemistry. The crystalline material was assessed using XRD, which identified 100% perovskite phase after 3 h of milling. Conversely, characterization by X-ray absorption spectroscopy (XAS) at both the Mn K-edge and La L-edge provides a very different picture. The XAS data shows that there are significant structural alterations as early as 30 min of milling, with the La precursor dispersed over MnO. Increasing milling time then allows for mechanical activation of both precursors and the formation of powdered LaMnO, with no calcination step required. The XAS highlights that there is a significant amount of amorphous, oxygen deficient, content even when XRD has identified 100% perovskite phase. The samples were tested for the decomposition of the environmental pollutant NO; at a milling time of 3 h, the LaMnO catalyst displays a much early onset production of N compared to a traditional sol-gel synthesized LaMnO, resulting from increased oxygen deficiency at the surface, confirmed by XPS and STEM-EELS. This is an encouraging sign that mechanochemical routes can be harnessed to provide a sustainable route to preparing mixed metal oxide catalysts with enhanced catalytic performance.
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http://dx.doi.org/10.1039/c9dt03590gDOI Listing
January 2020

Determining EDS and EELS partial cross-sections from multiple calibration standards to accurately quantify bi-metallic nanoparticles using STEM.

Micron 2018 10 28;113:69-82. Epub 2018 Jun 28.

Department of Materials, University of Oxford, Parks Road, OX1 3PH, UK. Electronic address:

Spectroscopic signals such as EDS and EELS provide an effective way of characterising multi-element samples such as Pt-Co nanoparticles in STEM. The advantage of spectroscopy over imaging is the ability to decouple composition and mass-thickness effects for thin samples, into the number of various types of atoms in a sample. This is currently not possible for multi element samples using conventional ADF quantification techniques alone. With recent developments in microscope hardware and software, it is now possible to acquire the ADF, EDS and EELS signals simultaneously and at high speed. However, the methods of quantifying the signals emitted from the sample vary greatly. Most approaches use pure-element standards in the form of needles, nanoparticles and wedges to quantify the spectroscopic signal into either partial scattering cross-sections, zeta-factors or k-factors. But self-consistency between the different methods has not been verified and the units of the quantification are not standardised. We present a robust approach for measuring and combining ADF, EDS and EELS signals using needle and nanoparticle standards in units of the partial scattering cross-section. The partial scattering cross-section allows an easy interpretation of the signals emitted from the sample and enables accurate atom-counting of the sample.
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http://dx.doi.org/10.1016/j.micron.2018.06.015DOI Listing
October 2018

Ideal versus real: simulated annealing of experimentally derived and geometric platinum nanoparticles.

J Phys Condens Matter 2018 Apr 26;30(15):155301. Epub 2018 Feb 26.

Department of Chemistry, University of Southampton, Highfield, SO17 1BJ, United Kingdom.

Platinum nanoparticles find significant use as catalysts in industrial applications such as fuel cells. Research into their design has focussed heavily on nanoparticle size and shape as they greatly influence activity. Using high throughput, high precision electron microscopy, the structures of commercially available Pt catalysts have been determined, and we have used classical and quantum atomistic simulations to examine and compare them with geometric cuboctahedral and truncated octahedral structures. A simulated annealing procedure was used both to explore the potential energy surface at different temperatures, and also to assess the effect on catalytic activity that annealing would have on nanoparticles with different geometries and sizes. The differences in response to annealing between the real and geometric nanoparticles are discussed in terms of thermal stability, coordination number and the proportion of optimal binding sites on the surface of the nanoparticles. We find that annealing both experimental and geometric nanoparticles results in structures that appear similar in shape and predicted activity, using oxygen adsorption as a measure. Annealing is predicted to increase the catalytic activity in all cases except the truncated octahedra, where it has the opposite effect. As our simulations have been performed with a classical force field, we also assess its suitability to describe the potential energy of such nanoparticles by comparing with large scale density functional theory calculations.
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http://dx.doi.org/10.1088/1361-648X/aab251DOI Listing
April 2018

Predicting the Oxygen-Binding Properties of Platinum Nanoparticle Ensembles by Combining High-Precision Electron Microscopy and Density Functional Theory.

Nano Lett 2017 07 28;17(7):4003-4012. Epub 2017 Jun 28.

Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, United Kingdom.

Many studies of heterogeneous catalysis, both experimental and computational, make use of idealized structures such as extended surfaces or regular polyhedral nanoparticles. This simplification neglects the morphological diversity in real commercial oxygen reduction reaction (ORR) catalysts used in fuel-cell cathodes. Here we introduce an approach that combines 3D nanoparticle structures obtained from high-throughput high-precision electron microscopy with density functional theory. Discrepancies between experimental observations and cuboctahedral/truncated-octahedral particles are revealed and discussed using a range of widely used descriptors, such as electron-density, d-band centers, and generalized coordination numbers. We use this new approach to determine the optimum particle size for which both detrimental surface roughness and particle shape effects are minimized.
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http://dx.doi.org/10.1021/acs.nanolett.6b04799DOI Listing
July 2017

Quantitative Energy-Dispersive X-Ray Analysis of Catalyst Nanoparticles Using a Partial Cross Section Approach.

Microsc Microanal 2016 Feb 12;22(1):71-81. Epub 2016 Jan 12.

1Department of Materials,University of Oxford,Parks Road,Oxford OX1 3PH,UK.

The new generation of energy-dispersive X-ray (EDX) detectors with higher count rates than ever before, paves the way for a new approach to quantitative elemental analysis in the scanning transmission electron microscope. Here we demonstrate a method of calculating partial cross sections for use in quantifying EDX data, beneficial especially because of the simplicity of its implementation. Applying this approach to acid-leached PtCo catalyst nanoparticles leads to quantitative determination of the Pt surface enrichment.
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http://dx.doi.org/10.1017/S1431927615015494DOI Listing
February 2016

Tomographic heating holder for in situ TEM: study of Pt/C and PtPd/Al2O3 catalysts as a function of temperature.

Microsc Microanal 2014 Jun 18;20(3):982-90. Epub 2014 Mar 18.

4Center for Electron Nanoscopy,Technical University of Denmark,DK-2800 Kgs. Lyngby,Denmark.

A tomographic heating holder for transmission electron microscopy that can be used to study supported catalysts at temperatures of up to ~1,500°C is described. The specimen is placed in direct thermal contact with a tungsten filament that is oriented perpendicular to the axis of the holder without using a support film, allowing tomographic image acquisition at high specimen tilt angles with minimum optical shadowing. We use the holder to illustrate the evolution of the active phases of Pt nanoparticles on carbon black and PtPd nanoparticles on γ-alumina with temperature. Particle size distributions and changes in active surface area are quantified from tilt series of images acquired after subjecting the specimens to increasing temperatures. The porosity of the alumina support and the sintering mechanisms of the catalysts are shown to depend on distance from the heating filament.
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http://dx.doi.org/10.1017/S1431927614000373DOI Listing
June 2014

[email protected] bimetallic nanoparticle synthesis via anion coordination.

Nat Chem 2011 Jun 24;3(6):478-83. Epub 2011 Apr 24.

Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX13QR, UK.

[email protected] structured bimetallic nanoparticles are currently of immense interest due to their unique electronic, optical and catalytic properties. However, their synthesis is non-trivial. We report a new supramolecular route for the synthesis of [email protected] nanoparticles, based on an anion coordination protocol--the first to function by binding the shell metal to the surface of the pre-formed primary metal core before reduction. The resultant gold/palladium and platinum/palladium [email protected] nanoparticles have been characterized by aberration-corrected scanning transmission electron microscopy (as well as other techniques), giving striking atomic-resolution images of the [email protected] architecture, and the unique catalytic properties of the structured nanoparticles have been demonstrated in a remarkable improvement of the selective production of industrially valuable chloroaniline from chloronitrobenzene.
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http://dx.doi.org/10.1038/nchem.1030DOI Listing
June 2011

A simple algorithm for measuring particle size distributions on an uneven background from TEM images.

Ultramicroscopy 2011 Jan 26;111(2):101-6. Epub 2010 Oct 26.

Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark.

Nanoparticles have a wide range of applications in science and technology. Their sizes are often measured using transmission electron microscopy (TEM) or X-ray diffraction. Here, we describe a simple computer algorithm for measuring particle size distributions from TEM images in the presence of an uneven background. The approach is based on adaptive thresholding, making use of local threshold values that change with spatial coordinate. The algorithm allows particles to be detected and characterized with greater accuracy than using more conventional methods, in which a global threshold is used. Its application to images of heterogeneous catalysts is presented.
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http://dx.doi.org/10.1016/j.ultramic.2010.10.011DOI Listing
January 2011

Three-dimensional shapes and structures of lamellar-twinned fcc nanoparticles using ADF STEM.

J Electron Microsc (Tokyo) 2009 Jun 11;58(3):167-74. Epub 2009 Feb 11.

Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark.

Small particles with face-centred cubic structures can have non-single-crystallographic shapes. Here, an approach based on annular dark-field scanning transmission electron microscopy (STEM) is used to obtain information about the crystal sub-units that make up supported and unsupported twinned Pt, Pt alloy and Au nanoparticles. The three-dimensional shapes of two types of lamellar-twinned particles (LTPs) of Pt are obtained using high-angle annular dark-field STEM. Possible growth mechanisms of the LTPs and origins for the contrast features in the recorded images are discussed.
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http://dx.doi.org/10.1093/jmicro/dfp003DOI Listing
June 2009

Aberration-corrected imaging of active sites on industrial catalyst nanoparticles.

Angew Chem Int Ed Engl 2007 ;46(20):3683-5

Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK.

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http://dx.doi.org/10.1002/anie.200604811DOI Listing
August 2007
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