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Structural and optical properties of visible active photocatalytic WO3 thin films prepared by reactive dc magnetron sputtering
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.ORCID iD: 0000-0002-8279-5163
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
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.

Place, publisher, year, edition, pages
2012. Vol. 27, no 24, 3130-3140 p.
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: urn:nbn:se:uu:diva-187494DOI: 10.1557/jmr.2012.384ISI: 000312394400012OAI: oai:DiVA.org:uu-187494DiVA: diva2:574883
Funder
Swedish Research Council, 2010-3514
Available from: 2012-12-06 Created: 2012-12-06 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Nanocrystalline Tungsten Trioxide Thin Films: Structural, Optical and Electronic Characterization
Open this publication in new window or tab >>Nanocrystalline Tungsten Trioxide Thin Films: Structural, Optical and Electronic Characterization
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
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:nbn:se:uu:diva-211855 (URN)978-91-554-8824-6 (ISBN)
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

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Johansson, Malin BNiklasson, Gunnar A.Österlund, Lars

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