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Nanostructured Transition Metal Oxides in Cleantech Application: Gas Sensors, Photocatalysis, Self-cleaning Surfaces Based on TiO2, WO3 and NiO
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 809
Keyword [en]
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
Identifiers
URN: urn:nbn:se:uu:diva-148872ISBN: 978-91-554-8026-4OAI: oai:DiVA.org:uu-148872DiVA: diva2:403225
Public defence
2011-03-25, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Note
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
List of papers
1. Resistance Noise in TiO2-Based Thin Film Gas Sensors under Ultraviolet Irradiation
Open this publication in new window or tab >>Resistance Noise in TiO2-Based Thin Film Gas Sensors under Ultraviolet Irradiation
2007 (English)Conference paper (Refereed)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-14568 (URN)
Conference
Journal of Physics: Conf. Series 76, 012056
Available from: 2008-01-30 Created: 2008-01-30 Last updated: 2016-04-11
2. Photo-Fixation of SO2 in Nanocrystalline TiO2 Films Prepared by Reactive DC Magnetron Sputtering
Open this publication in new window or tab >>Photo-Fixation of SO2 in Nanocrystalline TiO2 Films Prepared by Reactive DC Magnetron Sputtering
2009 (English)In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 518, no 4, 1341-1344 p.Article in journal (Refereed) Published
Abstract [en]

We report on photo-fixation of SO2 onto nanostructured TiO2 thin films prepared by reactive DC magnetron sputtering. The films were exposed to 50 PPM SO2 gas mixed in synthetic air and illuminated with UV light at 298 and 473 K. The evolution of the adsorbed SOx species was monitored by in situ Fourier transform infrared specular reflection spectroscopy. Significant photo-fixation occurred only in the presence of UV illumination. The SO2 uptake was dramatically enhanced at elevated temperatures and then produced strongly bonded surface-coordinated SO, complexes. The total SO, uptake is consistent with Langmuir adsorption kinetics. The sulfur doping at saturation was estimated from X-ray photoelectron spectroscopy to be similar to 2.2 at.% at 473 K These films were pale yellowish and had an optical absorption coefficient being similar to 3 times higher than in undoped film. The S-doped films exhibit interesting oleophobic properties, exemplified by the poor adherence of stearic acid. Our results suggest a new method for sulfur doping of TiO2 to achieve combined anti-grease and photocatalytic properties.

National Category
Engineering and Technology
Research subject
Solid State Physics
Identifiers
urn:nbn:se:uu:diva-122160 (URN)10.1016/j.tsf.2009.03.230 (DOI)000272733200072 ()
Available from: 2010-04-06 Created: 2010-04-06 Last updated: 2016-04-14Bibliographically approved
3. Gas Sensing Response of NiO Nanoparticle Films Made by Reactive Gas Deposition
Open this publication in new window or tab >>Gas Sensing Response of NiO Nanoparticle Films Made by Reactive Gas Deposition
Show others...
2009 (English)In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 138, no 1, 14-20 p.Article in journal (Refereed) Published
Abstract [en]

Nanocrystalline films comprised of NiO particles were prepared by advanced reactive gas evaporation and were investigated for gas sensing applications, specifically for detecting reducing (H2S) and oxidizing (NO2) species. The films were studied in as-deposited state and after annealing at temperatures up to 400,C, and the sensors were operated   from room temperature to 325 degrees C. The optimal operating temperature was in the 150-162 degrees C range for H2S and in the 100-125 degrees C range for NO2. We believe that this is the first report of high-sensitivity detection of H2S with NiO-based sensors, as well as the first proof that such sensors can work at room temperature.

Keyword
Nanoparticles, Nickel oxide, Gas sensing
National Category
Engineering and Technology
Research subject
Solid State Physics
Identifiers
urn:nbn:se:uu:diva-122141 (URN)10.1016/j.snb.2008.11.057 (DOI)000265656300004 ()
Available from: 2010-04-06 Created: 2010-04-06 Last updated: 2016-04-14Bibliographically approved
4. Highly Selective NO2 Gas Sensors made of MWCNTs and WO3 Hybrid Layers
Open this publication in new window or tab >>Highly Selective NO2 Gas Sensors made of MWCNTs and WO3 Hybrid Layers
Show others...
2007 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 154, no 5, J141-J149 p.Article in journal (Refereed) Published
Abstract [en]

Hybrid gas sensors were fabricated by means of multiwalled carbon nanotubes (MWCNTs) covered by W O3 deposited by an advanced reactive gas deposition method. In order to increase the dispersion of nanotubes and attach functional groups to their surface so as to enhance their compatibility with other compounds, the MWCNTs were functionalized in two different radio-frequency plasmas (oxygen or hydrogen) under different operating conditions. X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy were employed to analyze the composition and morphology of the hybrid films. Gas sensors based on such films were found to be very selective to N O2 when operated at room temperature. No cross-sensitivity was found to other hazardous gases such as N H3 or CO.

Keyword
carbon nanotubes, tungsten compounds, thin films, gas sensors, sputter deposition, X-ray photoelectron spectra, scanning electron microscopy, atomic force microscopy
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-14397 (URN)10.1149/1.2667855 (DOI)000245371700055 ()
Available from: 2008-01-30 Created: 2008-01-30 Last updated: 2016-04-11Bibliographically approved
5. Spectroscopic study of the photo-fixation of SO2 on anatase TiO2 thin films
Open this publication in new window or tab >>Spectroscopic study of the photo-fixation of SO2 on anatase TiO2 thin films
(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
TiO2, SO2, photo-reactions, FTIR, XPS
Identifiers
urn:nbn:se:uu:diva-148869 (URN)
Available from: 2011-03-11 Created: 2011-03-11 Last updated: 2011-12-07

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