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Nanocrystalline Tungsten Trioxide Thin Films: Structural, Optical and Electronic Characterization
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis concerns experimental studies of nanocrystalline tungsten trioxide thin films. Functional properties of WO3 have interesting applications in research areas connected to energy efficiency and green nanotechnology. The studies in this thesis are focused on characterization of fundamental electronic and optical properties in the semiconducting transition metal oxide WO3. The thesis includes also applied studies of photocatalytic and photoelectrochemical properties of the material.

    All nanocrystalline WO3 thin films were prepared using DC magnetron sputtering. It was found that structures like hexagonal and triclinic phase with different properties can be produced with sputtering technique. Thin film deposition has been performed using different process parameters with emphasis on sputter pressure and films that mainly consist of monoclinic γ-phase, with small contributions of ε-phase. Changes in the pressure are shown to affect the number of oxygen vacancies in the WO3 thin film, with close to stoichiometric WO3 formed at high pressures (30 mTorr), and slightly sub-stochiometric WO3-x, x = 0.005 at lower pressures (10 mTorr). Both stoichiometric and sub-stoichiometric thin films have been characterized by several structural, optical and electronic techniques.

   The electronic structure and especially band gap states have been explored and optical properties of WO3 and WO3-x have been studied in detail. The band gap has been determined to be in the range 2.7-2.9 eV. Absorption due to polaron absorption (W5+  -W6+), oxygen vacancy sites (Vo -W6+), and due to differently charged oxygen vacancy states in the band gap have been determined by spectrophotometry and photoluminescence spectroscopy, in good agreement with resonant inelastic x-ray spectroscopy and theoretical calculations. The density of electronic states in the band gap was determined from cyclic voltammetry measurements, which correlate with O vacancy concentration as compared with near infrared absorption.  

   By combining different experimental methods a thorough characterization of the band gap states have been possible and this opens up the opportunity to tailor the WO3 functionalities. WO3 has been shown to be visible active photocatalyst, and a promising electrode material as inferred from photo-oxidation and water splitting measurements, respectively. Links between device performance in photoelectrochemical experiments, charge transport and the electronic structure have been elucidated.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , xii+129 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1103
Keyword [en]
tungsten trioxide, nanocrystallin structure, thin films, optical properties, electronic properties
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
URN: urn:nbn:se:uu:diva-211855ISBN: 978-91-554-8824-6 (print)OAI: oai:DiVA.org:uu-211855DiVA: diva2:668997
Public defence
2014-01-30, Polhemsalen, Ångdtrömlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2014-01-08 Created: 2013-12-02 Last updated: 2014-11-17
List of papers
1. Structural and optical properties of visible active photocatalytic WO3 thin films prepared by reactive dc magnetron sputtering
Open this publication in new window or tab >>Structural and optical properties of visible active photocatalytic WO3 thin films prepared by reactive dc magnetron sputtering
2012 (English)In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 27, no 24, 3130-3140 p.Article in journal (Refereed) Published
Abstract [en]

Nanostructured tungsten trioxide films were prepared by reactive dc magnetron sputteringat different working pressures P-tot = 1-4 Pa. The films were characterized by scanning electron microscopy, x-ray diffraction, Rutherford backscattering spectroscopy, Raman spectroscopy, and ultraviolet-visible spectrophotometry. The films were found to exhibit predominantly monoclinic structures and have similar band gap, E-g approximate to 2.8 eV, with a pronounced Urbach tail extending down to 2.5 eV. At low P-tot, strained film structures formed, which were slightly reduced and showed polaron absorption in the near-infrared region. The photodegradation rate of stearic acid was found to correlate with the stoichiometry and polaron absorption. This is explained by a recombination mechanism, whereby photoexcited electron-hole pairs recombine with polaron states in the band gap. The quantum yield decreased by 50% for photon energies close to E-g due to photoexcitations to band gap states lying below the O-2 affinity level.

National Category
Nano Technology Condensed Matter Physics Materials Chemistry
Research subject
Engineering Science with specialization in Nanotechnology and Functional Materials; Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-187494 (URN)10.1557/jmr.2012.384 (DOI)000312394400012 ()
Funder
Swedish Research Council, 2010-3514
Available from: 2012-12-06 Created: 2012-12-06 Last updated: 2017-12-07Bibliographically approved
2. Electronic and optical properties of nanocrystalline WO3 thin films studied by optical spectroscopy and density functional calculations
Open this publication in new window or tab >>Electronic and optical properties of nanocrystalline WO3 thin films studied by optical spectroscopy and density functional calculations
Show others...
2013 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 20, 205502- p.Article in journal (Refereed) Published
Abstract [en]

The optical and electronic properties of nanocrystalline WO3 thin films prepared by reactive dc magnetron sputtering at different total pressures (Ptot) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low Ptot show absorption in the near infrared due to polarons, which is attributed to a strained film structure. Analysis of the optical data yields band-gap energies Eg ≈ 3.1 eV, which increase with increasing Ptot by 0.1 eV, and correlate with the structural modifications of the films. The electronic structures of triclinic δ-WO3, and monoclinic γ- and ε-WO3 were calculated using the Green function with screened Coulomb interaction (GW approach), and the local density approximation. The δ-WO3 and γ-WO3 phases are found to have very similar electronic properties, with weak dispersion of the valence and conduction bands, consistent with a direct band-gap. Analysis of the joint density of states shows that the optical absorption around the band edge is composed of contributions from forbidden transitions (>3 eV) and allowed transitions (>3.8 eV). The calculations show that Eg in ε-WO3 is higher than in the δ-WO3 and γ-WO3 phases, which provides an explanation for the Ptot dependence of the optical data.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-201786 (URN)10.1088/0953-8984/25/20/205502 (DOI)000318556100013 ()
Available from: 2013-06-17 Created: 2013-06-17 Last updated: 2017-12-06Bibliographically approved
3. Band gap states in nanocrystalline WO3 thin films studied by soft x-ray spectroscopy, optical absorption spectroscopy and density functional calculations
Open this publication in new window or tab >>Band gap states in nanocrystalline WO3 thin films studied by soft x-ray spectroscopy, optical absorption spectroscopy and density functional calculations
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-211849 (URN)
Available from: 2013-12-02 Created: 2013-12-02 Last updated: 2015-06-24
4. Photoelectrochemical properties of nanocrystalline WO3 thin films prepared by DC magnetron sputtering
Open this publication in new window or tab >>Photoelectrochemical properties of nanocrystalline WO3 thin films prepared by DC magnetron sputtering
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-211852 (URN)
Available from: 2013-12-02 Created: 2013-12-02 Last updated: 2014-01-24
5. Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering
Open this publication in new window or tab >>Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering
2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, no 21, 213510- p.Article in journal (Refereed) Published
Abstract [en]

The optical properties of tungsten trioxide thin films prepared by DC magnetron sputtering, withdifferent oxygen vacancy (Vo) concentration, have been studied by spectrophotometry andphotoluminescence (PL) emission spectroscopy. Absorption and PL spectra show that the filmsexhibit similar band gap energies, Eg 2.9 eV. The absorption spectra of the films show twopronounced absorption bands in the near-infrared region. One peak (P1) is located atapproximately 0.7 eV, independent of Vo concentration. A second peak (P2) shifts from 0.96 eV to1.16 eV with decreasing Vo concentration. Peak P1 is assigned to polaron absorption due totransitions between tungsten sites (W5þ!W6þ), or an optical transition from a neutral vacancystate to the conduction band, Vo0!W6þ. The origin of peak P2 is more uncertain but may involveþ1 and þ2 charged vacancy sites. The PL spectra show several emission bands in the range 2.07 to3.10 eV in the more sub-stoichiometric and 2.40 to 3.02 eV in the less sub-stoichiometric films.The low energy emission bands agree well with calculated optical transition energies of oxygenvacancy sites, with dominant contribution from neutral and singly charged vacancies in the lesssub-stoichiometric films, and additional contributions from doubly charged vacancy sites in themore sub-stoichiometric films.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014
National Category
Physical Sciences Engineering and Technology Condensed Matter Physics
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-228539 (URN)10.1063/1.4880162 (DOI)000337161600016 ()
Available from: 2014-07-17 Created: 2014-07-16 Last updated: 2017-12-05Bibliographically approved

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