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Spectroscopic study of the photo-fixation of SO2 on anatase TiO2 thin films
Uppsala University, Disciplinary Domain of Science and Technology.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Photo-induced SO2 fixation on anatase TiO2 films was studied by in situ Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). The TiO2 films were prepared by reactive DC magnetron sputtering and were subsequently exposed to 50 ppm SO2 gas mixed in synthetic air and irradiated with UV light at substrate temperatures between 298 and 673 K. Simultaneous UV irradiation and SO2 exposure between 373 and 523 K resulted in significant sulfur deposits on crystalline TiO2 films as determined by XPS, whereas amorphous films contained negligible amounts of S. At substrate temperatures above 523 K, the S deposits readily desorbed from TiO2. The oxidation state of sulfur successively changed from S4+ for SO2 adsorbed on crystalline TiO2 films at room temperature without irradiation to S6+ for films exposed to SO2 at elevated temperatures with simultaneous irradiation. In situ FTIR was used to monitor the temporal evolution of the photo-induced surface reaction products formed on the TiO2 surfaces. It is shown that band gap excitation of TiO2 results in photo-induced oxidation of SO2 to form sulfide and sulfate ions, which at elevated temperatures become coordinated to the TiO2 lattice through interactions with O vacancies to form  and  species. These species makes the surface acidic, which is manifested in weak adherence of stearic acid. The films show good chemical stability as evidenced by sonication experiments. This suggests that photo-induced surface treatment in reactive sulfur gases may be interesting for fabrication of oleophobic and anti-greasing coatings.

Keyword [en]
TiO2, SO2, photo-reactions, FTIR, XPS
URN: urn:nbn:se:uu:diva-148869OAI: oai:DiVA.org:uu-148869DiVA: diva2:403195
Available from: 2011-03-11 Created: 2011-03-11 Last updated: 2011-12-07
In thesis
1. Nanostructured Transition Metal Oxides in Cleantech Application: Gas Sensors, Photocatalysis, Self-cleaning Surfaces Based on TiO2, WO3 and NiO
Open this publication in new window or tab >>Nanostructured Transition Metal Oxides in Cleantech Application: Gas Sensors, Photocatalysis, Self-cleaning Surfaces Based on TiO2, WO3 and NiO
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the application of nanocrystalline transition metal oxide TiO2, WO3 and NiO thin films in new “green” building technologies. Specifically, their physicochemical properties in photocatalytic, self-cleaning and gas sensing applications are studied. There is an intimate connection between comfort issues, health, with connections to energy efficiency, leading to a need for intelligent building materials and green architecture. The importance of good indoor environment is augmented by the fact that modern man in developed countries spends some 90 % of his time inside buildings and vehicles. Poor air quality may lead to discomfort of the person inhabiting a building and in ultimately cause adverse health effects.

Thin films of nanocrystalline TiO2 were prepared using reactive DC magnetron sputtering. Crystalline mesoporous films of WO3 and NiO were prepared using advanced gas deposition technique (AGD). The crystal structure, morphology, optical and chemical properties of the films were characterized by using grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), UV/Vis spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic properties and adsorption of both organic and inorganic molecules on pure and functionalized films were probed by in situ Fourier transform infrared spectroscopy (FTIR). The gas sensing properties of sensors based on TiO2, WO3 and NiO were investigated by conductivity measurements and noise spectroscopy.

It was found for the first time that NiO based thin film sensors can be used to detect H2S and NO2 at low temperatures – down to room temperature. Hybrid WO3 sensors functionalized with multiwalled carbon nanotubes (MWCNTs) were used to detect NO2, CO and NH3 gases. These hybrid gas sensors show improved recovery properties compared to unmodified WO3 sensors. TiO2 based gas sensors were able to detect low concentrations of H2S by noise spectroscopy provided that the sensors were irradiated by UV light. Furthermore we show that sulphur is photo-fixated in crystalline TiO2 films upon simultaneous SO2 gas exposure and UV irradiation. Studies of the kinetics and identity of the photo-fixated sulphur complexes show that these are formed by photo-induced reactions between oxygen and SO2 at oxygen surface vacancy sites in TiO2. The sulphur modified TiO2 films show interesting self-cleaning properties compared to the pure films.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 69 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 809
Gas sensing, Self-cleaning, photo-fixation, photocatalysis, TiO2, WO3, NiO
National Category
Manufacturing, Surface and Joining Technology Other Materials Engineering
Research subject
Engineering Science with specialization in Solid State Physics
urn:nbn:se:uu:diva-148872 (URN)978-91-554-8026-4 (ISBN)
Public defence
2011-03-25, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 739Available from: 2011-03-17 Created: 2011-03-11 Last updated: 2011-05-04

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