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Solution-Chemical Synthesis of Cobalt and Iron:Zinc Oxide Nanocomposite Films
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The potentially most important challenges today are related to energy and the environment. New materials and methods are needed in order to, in a sustainable way, convert and store energy, reduce pollution, and clean the air and water from contaminations. In this, nanomaterials and nanocomposites play a key role, and hence knowledge about the relation between synthesis, structure, and properties of nanosystems is paramount.

This thesis demonstrates that solution-chemical synthesis, using amine-modified acetates and nitrates, can be used to prepare widely different nanostructured films. By adjusting the synthesis parameters, metals, oxides, and metal–oxide or oxide–oxide nanocomposites were prepared for two systems based on Co and Zn:Fe, respectively, and the films were characterised using diffraction, spectroscopy, and microscopy techniques, and SQUID magnetometry.

A variety of crystalline cobalt films—Co metal, CoO, Co3O4, and composites with different metal:oxide ratios—were synthesised. Heat-treatment parameters and control of the film thickness enabled tuning of the phase ratios. Random and layered Co–CoO composites were prepared by utilising different heating rates and gas flow rates together with a morphology effect associated with the furnace tube. The Co–CoO films exhibited exchange bias due to the ferromagnetic–antiferromagnetic interaction between the Co and CoO, whereas variations in e.g. coercivity and exchange bias field were attributed to differences in the structure and phase distribution.

Ordered structures of wurtzite ZnO surrounded by amorphous ZnxFeyO were prepared through controlled phase segregation during the heating, which after multiple coating and heating cycles yielded ZnO–ZnxFeyO superlattices. The amorphous ZnxFeyO was a prerequisite for superlattice formation, and it profoundly affected the ZnO phase, inhibiting grain growth and texture, already from 1% Fe. In addition, ZnO–ZnxFeyO exhibited a photocatalytic activity for the oxidation of water that was higher than results reported for pure ZnO, and comparable to recent results reported for graphene-modified ZnO.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. , 85 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1356
Keyword [en]
Fe:Zn oxide, Co-CoO, Solution-chemical synthesis, Heat treatment parameters, Nanocomposite, Film, Multilayer, Phase ratio, Phase distribution, Exchange bias, Magnetism, Photocatalysis
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-280619ISBN: 978-91-554-9520-6 (print)OAI: oai:DiVA.org:uu-280619DiVA: diva2:913246
Public defence
2016-06-09, Å 2005, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2016-05-17 Created: 2016-03-11 Last updated: 2016-06-01
List of papers
1. Synthesis and characterization of cobalt oxide and composite thin films
Open this publication in new window or tab >>Synthesis and characterization of cobalt oxide and composite thin films
2014 (English)In: Advances in Materials, ISSN 2327-2503, Vol. 3, no 5, 52-57 p.Article in journal (Refereed) Published
Abstract [en]

Cobalt oxide and composite thin films were synthesized by spin-coating technique, followed by heating to 500°C in oxidizing, inert, or reducing atmospheres. Methanolic solutions of triethanolamine complexes of cobalt acetates and nitrates were spin-coated at 1000, 2000, and 3000 rpm. The influence of heating parameters and film thickness on the phase content of the films were investigated, using grazing incidence X-ray diffraction, X-ray reflectivity, and scanning electron microscopy. By tuning the synthesis parameters, Co3O4, CoO and Co films were obtained, as well as CoO-Co and Co3O4-CoO composite films of varying phase ratios.

Place, publisher, year, edition, pages
Science Publishing Group, 2014
Keyword
Cobalt Oxides, CoO-Co Composites, Solution Synthesis, Thin Films, Tailor Composition
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-280377 (URN)10.11648/j.am.20140305.14 (DOI)
Available from: 2016-03-09 Created: 2016-03-09 Last updated: 2016-06-01
2. Morphology effects on exchange anisotropy in Co-CoO nanocomposite films
Open this publication in new window or tab >>Morphology effects on exchange anisotropy in Co-CoO nanocomposite films
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2015 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 576, 11-18 p.Article in journal (Refereed) Published
Abstract [en]

Co-CoO composite films were prepared by solution chemical technique using amine-modified nitrates and acetates in methanol. We study how particle size and porosity can be tuned through the synthesis parameters and how this influences the magnetic properties. Phase content and microstructure were characterised with grazing incidence X-ray diffraction and electron microscopy, and the magnetic properties were studied by magnetometry and magnetic force microscopy. Composite films were obtained by heating spin-coated films in Ar followed by oxidation in air at room temperature, and the porosity and particle size of the films were controlled by gas flow and heating rate. The synthesis yielded dense films with a random distribution of metal and oxide nanoparticles, and layered films with porosity and sintered primary particles. Exchange anisotropy, revealed as a shift towards negative fields of the magnetic hysteresis curve, was found in all films. The films with a random distribution of metal and oxide nanoparticles displayed a significantly larger coercivity and exchange anisotropy field compared to the films with a layered structure, whereas the layered films displayed a larger nominal saturation magnetisation. The magnitude of the coercivity decreased with increasing Co grain size, whereas increased porosity caused an increased tilt of the magnetic hysteresis curve. (C) 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

Keyword
Co-CoO composite, Thin film, Solution chemical synthesis, Morphology effect, Magnetism, Exchange anisotropy, Magnetic stray field
National Category
Physical Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-246807 (URN)10.1016/j.tsf.2014.11.064 (DOI)000349373300002 ()
Available from: 2015-03-16 Created: 2015-03-10 Last updated: 2017-12-04Bibliographically approved
3. ZnO-ZnxFeyO Superlattices - Influence of Metastable Phases on Structure and Properties in Fe:Zn:O
Open this publication in new window or tab >>ZnO-ZnxFeyO Superlattices - Influence of Metastable Phases on Structure and Properties in Fe:Zn:O
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(English)Manuscript (preprint) (Other academic)
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-280379 (URN)
Available from: 2016-03-11 Created: 2016-03-09 Last updated: 2016-06-01
4. Photoinduced reduction of surface states in Fe:ZnO
Open this publication in new window or tab >>Photoinduced reduction of surface states in Fe:ZnO
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2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 20, 204703Article in journal (Refereed) Published
Abstract [en]

We report on the electronic structure of nano-crystalline Fe:ZnO, which has recently been found to be an efficient photocatalyst. Using resonant photoemission spectroscopy, we determine the binding energy of Fe 3d states corresponding to different valencies and coordination of the Fe atoms. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the surface region of the nano-crystalline material due to the formation of oxygen vacancies. Electronic states corresponding to low-spin Fe2+ are observed and attributed to crystal field modification at the surface. These states are potentially important for the photocatalytic sensitivity to visible light due to their location deep in the ZnO bandgap. X-ray absorption and x-ray photoemission spectroscopy suggest that Fe is only homogeneously distributed for concentrations up to 3%. Increased concentrations does not result in a higher concentration of Fe ions in the surface region. This is limiting the photocatalytic functionality of ZnO, where the most efficient Fe doping concentration has been shown to be 1%-4%.

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-258046 (URN)10.1063/1.4921570 (DOI)000355919300030 ()26026457 (PubMedID)
Available from: 2015-07-13 Created: 2015-07-10 Last updated: 2017-12-04
5. Optical and Photocatalytic Properties of Zinc Oxide and Iron-Zinc Oxide Heterojunctions
Open this publication in new window or tab >>Optical and Photocatalytic Properties of Zinc Oxide and Iron-Zinc Oxide Heterojunctions
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(English)Manuscript (preprint) (Other academic)
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-280378 (URN)
Available from: 2016-03-11 Created: 2016-03-09 Last updated: 2016-06-01

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