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Synthesis and Characterisation of Ultra Thin Film Oxides for Energy Applications
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
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 [en]
Atomic Layer Deposition, Iron oxides, Hematite, Solar Water Splitting, Hard X-Ray Photoelectron Spectroscopy
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:uu:diva-232948ISBN: 978-91-554-9048-5OAI: oai:DiVA.org:uu-232948DiVA: diva2:750323
Public defence
2014-11-21, Polhemsalen, 10134, Ångström, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-10-30 Created: 2014-09-28 Last updated: 2015-01-23
List of papers
1. Optical quantum confinement in low dimensional hematite
Open this publication in new window or tab >>Optical quantum confinement in low dimensional hematite
2014 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 10, 3352-3363 p.Article in journal (Refereed) Published
Abstract [en]

Hematite is considered to be a promising material for various applications, including for example photoelectrochemical cells for solar hydrogen production. Due to limitations in the charge transport properties hematite needs to be in the form of low-dimensional particles or thin films in several of these applications. This may however affect the optical properties, introducing additional complications for efficient design of photo-active devices. In this paper the optical absorption is analyzed in detail as a function of film thickness for 35 thin films of hematite ranging between 2 and 70 nm. Hematite was deposited by atomic layer deposition on FTO-substrates using Fe(CO)(5) and O-2 as precursors. It was found that for film thicknesses below 20 nm the optical properties are severely affected as a consequence of quantum confinement. One of the more marked effects is a blue shift of up to 0.3 eV for thinner films of both the indirect and direct transitions, as well as a 0.2 eV shift of the absorption maximum. The data show a difference in quantum confinement for the indirect and the direct transitions, where the probability for the indirect transition decreases markedly and essentially disappears for the thinnest films. Raman measurements showed no peak shift or change in relative intensity for vibrations for the thinnest films indicating that the decrease in indirect transition probability could not be assigned to depression of any specific phonon but instead seems to be a consequence of isotropic phonon confinement. The onset of the indirect transition is found at 1.75 eV for the thickest films and shifted to 2.0 eV for the thinner films. Two direct transitions are found at 2.15 eV and 2.45 eV, which are blue shifted 0.3 and 0.45 eV respectively, when decreasing the film thickness from 20 to 4 nm. Low dimensional hematite, with dimensions small enough for efficient charge transport, thus has a substantially lower absorption in the visible region than expected from bulk values. This knowledge of the intrinsic optical behavior of low dimensional hematite will be of importance in the design of efficient photo-active devices.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-221059 (URN)10.1039/c3ta14846g (DOI)000331249900012 ()
Available from: 2014-03-26 Created: 2014-03-25 Last updated: 2015-01-23Bibliographically approved
2. Phase control of iron oxides grown in nano-scale cauliflower structures: hematite, maghemite and magnetite
Open this publication in new window or tab >>Phase control of iron oxides grown in nano-scale cauliflower structures: hematite, maghemite and magnetite
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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
Hard X-ray Photoelectron Spectroscopy, HAXPES, Hematite, Thin Films, Pulsed Chemical Vapour Deposition, PEC
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:uu:diva-232943 (URN)
Available from: 2014-09-28 Created: 2014-09-28 Last updated: 2015-01-23
3. Temperature Induced Diffusion of Sn and Si in Hematite and Implications for Photocatalytic Water Splitting Applications
Open this publication in new window or tab >>Temperature Induced Diffusion of Sn and Si in Hematite and Implications for Photocatalytic Water Splitting Applications
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

The performance of Hematite (α-Fe2O3) for the water oxidation step in solar hydrogen production is dependent upon annealing of the material. In this study, these effects are investigated in terms of temperature induced diffusion of Sn and Si from the substrate into thin films of hematite by using hard X-ray photoelectron spectroscopy (HAXPES). Here, HAXPES is used for the first time to characterize a buried interface between a conducting substrate and a nanostructured thin film overlayer by diffusion upon annealing. This process is prototypical for the large class of photoelectrochemical devices that uses indium and fluorine doped tin oxide as substrates where the device is subsequently annealed. Indeed, we observe that: diffusion of Sn and Si is significant, already at 550 °C; the photocatalytic efficiency of the hematite films increased, from low values to 0.23 mA/cm2; annealing in air preserves the hematite phase, while annealing in vacuum induces a phase transition into magnetite, which impairs the photocatalytic performance. The increase in efficiency is explained in terms of an improvement of the quantum efficiency of the oxygen evolution reaction accompanied by a slight improvement in charge carrier transport.

Keyword
HAXPES, diffusion, hematite, PEC, water splitting
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-232946 (URN)
Available from: 2014-09-28 Created: 2014-09-28 Last updated: 2015-02-13
4. On the Structural and Optical Properties of Ultrathin Iron Oxide
Open this publication in new window or tab >>On the Structural and Optical Properties of Ultrathin Iron Oxide
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Nanostructured iron oxides and especially hematite are interesting for a wide range of applications ranging from gas sensors to renewable solar hydrogen production. A promising method for deposition of low dimensional films is atomic layer deposition, ALD. Although a potent technique, ALD of ultrathin films is sensitive to the substrate and temperature conditions where initial formation of islands and crystallites influences the properties of the films. In this work the optical and structural properties of iron oxide films in the thickness range of 6 nm to 50 nm have been investigated. Below 10 nm nominal film thickness we find island formation and phase dependent particle crystallization that impose difficulties for ALD deposition of phase pure iron oxides on non-lattice matching substrates. For films between 10-20 nm, post-annealing steps were found to be needed to recrystallize iron polymorphs to hematite whereas for films thicker than 20 nm, phase pure hematite can be formed directly with ALD with very low influence of the substrate. Analysis of the indirect and direct band gaps of the thinnest films show a quantum confined blue shift of the absorption edge.

Keyword
A. Thin films, B. Vapour deposition C. Raman spectroscopy, D. Crystal structure
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-232945 (URN)
Available from: 2014-09-28 Created: 2014-09-28 Last updated: 2015-01-23
5. Self-Supported Three-Dimensional Nanoelectrodes for Microbattery Applications
Open this publication in new window or tab >>Self-Supported Three-Dimensional Nanoelectrodes for Microbattery Applications
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2009 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 9, no 9, 3230-3233 p.Article in journal (Refereed) Published
Abstract [en]

A nanostructured three-dimensional (3D) microbattery has been produced and cycled in a Li-ion battery. It consists of a current collector of aluminum nanorods, a uniform layer of 17 nm TiO2 covering the nanorods made using ALD, an electrolyte and metallic lithium counter electrode. The battery is electrochemically cycled more than 50 times. The increase in total capacity is 10 times when using a 3D architechture compared to a 2D system for the same footprint area.

Place, publisher, year, edition, pages
American Chemical Society, 2009
National Category
Chemical Sciences Engineering and Technology
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-119450 (URN)10.1021/nl9014843 (DOI)000269654900025 ()
Available from: 2010-02-25 Created: 2010-02-25 Last updated: 2016-04-14
6. An HAXPES study of Sn, SnS, SnO and SnO2
Open this publication in new window or tab >>An HAXPES study of Sn, SnS, SnO and SnO2
2014 (English)In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, Vol. 195, 195-199 p.Article in journal (Refereed) Published
Abstract [en]

Hard X-ray photoelectron spectra have been recorded for Sn, SnO2, SnO and SnS. The binding energies of the core levels of elemental Sn from 2s up to, and including, 4d have been determined with least squares fitting and calibrated against an Au 4f standard. For the oxides and the sulphide data on Sn 3p, 3d core levels’ binding energies and relative intensities are presented together with the binding energies of O 1s, S 1s and 2p. This study thus serves as a picture of tin's core level spectra compared to those of some of its oxides and a sulphide taken at photon energies beyond Al Kα and Mg Kα.

Place, publisher, year, edition, pages
Mattis Fondell, 2014
Keyword
Hard X-ray photoelectron spectroscopy; Sn; Metal; Oxides; Sulphide; HAXPES; Synchrotron radiation; SnO; SnO2; SnS
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-232941 (URN)10.1016/j.elspec.2014.07.012 (DOI)000342872800028 ()
Available from: 2014-09-28 Created: 2014-09-28 Last updated: 2015-01-23Bibliographically approved
7. HAXPES study of Sn core levels and their plasmon loss features
Open this publication in new window or tab >>HAXPES study of Sn core levels and their plasmon loss features
2014 (English)In: Results in Physics, ISSN 2211-3797, Vol. 4, 168-169 p.Article in journal (Refereed) Published
Abstract [en]

Hard X-ray Photoelectron spectra have been recorded for elemental Sn. Electron loss features, prominent in all core level spectra of the metal, are analyzed at several photo energies for the 3p core level. For higher photoelectron kinetic energies the intensity of the plasmonic features follows a simple exponential law. The data and models presented here will aid the modeling of spectra where tin is present and especially if its spectrum overlaps with those from other sources.

Keyword
Hard X-ray Photoelectron Spectroscopy; Plasmons; Sn; Metal; HAXPES; Synchrotron radiation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-232942 (URN)10.1016/j.rinp.2014.08.006 (DOI)
Available from: 2014-09-28 Created: 2014-09-28 Last updated: 2015-01-23Bibliographically approved
8. Surface modification of iron oxides by ion bombardment – comparing depth profiling by HAXPES and Ar ion sputtering
Open this publication in new window or tab >>Surface modification of iron oxides by ion bombardment – comparing depth profiling by HAXPES and Ar ion sputtering
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Iron oxide in the form of maghemite  γ-Fe2O3  and hematite α-Fe2O3 has been studied with x-ray photoelectron spectroscopy. It is found that even low energy sputtering induces a reduction of the surface layer into FeO. Satellites in the Fe 2p  core level spectra are used to determine the oxidation state of iron. Depth profiling with changing photon energy shows that the unsputtered films are homogeneous and that the information obtained from sputtering thus, in this instance, represents sputter damages to the sample.

Keyword
Hard X-ray Photoelectron Spectroscopy, Depth profiling, Iron oxide, HAXPES, synchrotron radiation
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-232947 (URN)
Available from: 2014-09-28 Created: 2014-09-28 Last updated: 2015-01-23

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