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Phase control of iron oxides grown in nano-scale cauliflower structures: hematite, maghemite and magnetite
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Helmholtz Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Str. 15, Berlin 12489, Germany.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We demonstrate that iron oxide in the form of hematite, suitable as absorber in photoelectrochemical cells, can be produced by pulsed chemical vapour deposition. By choosing carbon monoxide or nitrogen as carrier gases in the process the phase and granularity of the grown material can be controlled. The choice of carrier gas a ect the decomposition rate of iron pentacarbonyl used as iron precursor. The iron oxide phase is also dependent on the chosen substrate, here fluorine doped tin oxide and crystalline silicon have been used. Regardless of the substrate nitrogen yields hematite, whereas carbon monoxide gives magnetite on Si and maghemite on fluorine doped tin oxide. A combination of Raman spectroscopy, X-ray di raction, and hard X-ray photoelectron spectroscopy were used for characterization of the crystalline phase and chemical composition in the films. Scanning electron microscopy were used to visualise the deposited films’ nano-structure reminiscent of a cauliflower.

Keyword [en]
Hard X-ray Photoelectron Spectroscopy, HAXPES, Hematite, Thin Films, Pulsed Chemical Vapour Deposition, PEC
National Category
Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:uu:diva-232943OAI: oai:DiVA.org:uu-232943DiVA: diva2:750274
Available from: 2014-09-28 Created: 2014-09-28 Last updated: 2015-01-23
In thesis
1. Synthesis and Characterisation of Ultra Thin Film Oxides for Energy Applications
Open this publication in new window or tab >>Synthesis and Characterisation of Ultra Thin Film Oxides for Energy Applications
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis describes studies of materials which can be exploited for hydrogen production from water and sunlight. The materials investigated are maghemite (γ-Fe2O3), magnetite (Fe3O4) and especially hematite (α-Fe2O3), which is an iron oxide with most promising properties in this field. Hematite has been deposited using Atomic Layer Deposition (ALD) - a thin-film technique facilitating layer-by-layer growth with excellent thickness control and step coverage. The iron oxides were deposited using bis-cyclopentadienyl iron (Fe(Cp)2) or iron pentacarbonyl (Fe(CO)5) in combination with an O2 precursor. Since it is crucial to have good control of the deposition process, the influence of substrate, process temperature, precursor and carrier gas have been investigated systematically. By careful control of these deposition parameters, three polymorphs of iron oxide could be deposited: hematite (α-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4).

The deposited materials were characterized using X-ray Diffraction, Raman and UV-VIS Spectroscopy, and Scanning Electron Microscopy. Hard X-ray Photoelectron Spectroscopy (HAXPES) was also used, since it is a non-destructive, chemically specific, surface sensitive technique – the surface sensitivity resulting from the short mean escape depth of the photoelectrons. The depth probed can be controlled by varying the excitation energy; higher photoelectron energies increasing the inelastic mean-free-path in the material.

HAXPES studies of atomic diffusion from F-doped SnO2 substrates showed increased doping levels of Sn, Si and F in the deposited films. Diffusion from the substrate was detected at annealing temperatures between 550 °C and 800 °C. Films annealed in air exhibited improved photocatalytic behavior; a photocurrent of 0.23 mA/cm2 was observed for those films, while the as-deposited hematite films showed no photo-activity whatsoever.

The optical properties of low-dimensional hematite were studied in a series of ultra-thin films (thicknesses in the 2-70 nm range). The absorption maxima were shifted to higher energies for films thinner than 20 nm, revealing a different electronic structure in thin films.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 113 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1186
Keyword
Atomic Layer Deposition, Iron oxides, Hematite, Solar Water Splitting, Hard X-Ray Photoelectron Spectroscopy
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-232948 (URN)978-91-554-9048-5 (ISBN)
Public defence
2014-11-21, Polhemsalen, 10134, Ångström, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
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Supervisors
Available from: 2014-10-30 Created: 2014-09-28 Last updated: 2015-01-23

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