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Gas-phase photocatalytic activity of sputter-deposited anatase TiO2 films: Effect of <001> preferential orientation, surface temperature and humidity
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.
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.
2016 (English)In: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694, Vol. 335, 187-196 p.Article in journal (Refereed) Published
Abstract [en]

We present a systematic in situ study of the environmental reaction conditions on the photocatalytic activity of sputter deposited anatase TiO2 films with controlled amounts of preferential <001> orientation. In particular, the effects of relative humidity (RH) and substrate temperature ( ) are investigated. It is found that {001} facets, which are present in higher abundance on highly oriented samples, exhibit an order of magnitude higher reactivity for gas-phase photocatalytic oxidation of the indoor air pollutant acetaldehyde (CH3CHO) than {101} facets do, and a functional dependence of the reaction rate on facet orientation is determined. It is proposed that water adsorbed on the film contributes with two counteracting effects on the photocatalytic activity: (i) It provides hole acceptors to complete the photo-induced redox cycle and subsequent OH– radical formation for pollutant degradation, and (ii) it creates a diffusion barrier between the catalyst interface and pollutant molecules adsorbed in the water layer. As a consequence, increasing  at high RH has the beneficial effect of removing excess water and reducing the diffusion barrier, thereby improving the photocatalytic activity. A comparison is also made with a commercial anatase TiO2 film, with less developed surface crystallinity and random facet distribution, where the improvement is even more pronounced. Films with a higher degree of orientation exhibit much more stable performance over a range of operating conditions, which suggests that it is possible to tune the effects of water and exposed facet orientation to achieve optimum activity and making TiO2 films amenable to a larger (RH, ) parameter space for practical applications.

Place, publisher, year, edition, pages
2016. Vol. 335, 187-196 p.
National Category
Inorganic Chemistry Condensed Matter Physics Chemical Engineering Nano Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
URN: urn:nbn:se:uu:diva-265055DOI: 10.1016/j.jcat.2015.12.002ISI: 000371098200018OAI: oai:DiVA.org:uu-265055DiVA: diva2:862332
Projects
GRINDOOR
Funder
EU, FP7, Seventh Framework Programme, FP7/2007-2013
Available from: 2015-10-21 Created: 2015-10-21 Last updated: 2017-12-01Bibliographically approved
In thesis
1. Photocatalytic TiO2 thin films for air cleaning: Effect of facet orientation, chemical functionalization, and reaction conditions
Open this publication in new window or tab >>Photocatalytic TiO2 thin films for air cleaning: Effect of facet orientation, chemical functionalization, and reaction conditions
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Poor indoor air quality is a source of adverse health effects. TiO2 coatings deposited on well-illuminated surfaces, such as window panes, can be used to fully mineralize indoor air pollutants by photocatalysis. In such applications it is important to ensure stable photocatalytic activity for a wide range of operating conditions, such as relative humidity and temperature, and to avoid deactivation of the catalyst.

In this thesis photocatalytic removal of the indoor-pollutant acetaldehyde (CH3CHO) on nanostructured TiO2 films is investigated, and in particular it is proposed how such films can be modified and operated for maximum performance. Catalyst deactivation can be reduced by purposefully changing the surface acidity of TiO2 by covalently attaching SO4 to the surface. Moreover, the overall photocatalytic activity on anatase TiO2 films can be improved by increasing the fraction of exposed reactive {001} surfaces, which otherwise are dominated by {101} surfaces.

In the first part of the thesis mode-resolved in-situ FTIR is used to elucidate the reaction kinetics of CH3CHO adsorption and photo-oxidation on the TiO2 and SO4 – modified TiO2 surfaces. Surface concentrations of main products and corresponding reaction rates were determined. Formate is the major reaction product, whose further oxidation limits the complete oxidation to gaseous species, and is responsible for photocatalyst deactivation by site inhibition. The oxidation reaction is characterized by two reaction pathways, which are associated with two types of surface reaction sites. On the sulfate modified TiO2 catalyst fewer intermediates are accumulated, and this catalyst resists deactivation much better than pure TiO2. A hitherto unknown intermediate – surface-bound acetaldehyde dimer with an adsorption band at 1643 cm−1 was discovered, using interplay between FTIR spectroscopy and DFT calculations.

The second part of the thesis treats the effect of increasing the relative abundance of exposed {001} facets on the photocatalytic activity of anatase TiO2 films prepared by DC magnetron sputtering. A positive effect was observed both for liquid-phase photo-oxidation of methylene blue, and for gas-phase photocatalytic removal of CH3CHO. In both cases it was found that the exposed {001} surfaces were an order of magnitude more reactive, compared to the {101} ones. Furthermore, it was found that the reactive films were more resilient towards deactivation, and exhibited almost unchanged activity under varying reaction conditions. Finally, a synergetic effect of SO4 – modification and high fraction of exposed {001} surfaces was found, yielding photocatalysts with sustained high activity.

The results presented here for facet controlled and chemically modified TiO2 films are of interest for applications in the built environment for indoor air purification and as self-cleaning surfaces.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. 148 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1307
Keyword
titanium dioxide, photocatalysis, thin films, surface functionallization, acetaldehyde, indoor air cleaning, sputter deposition, crystallographic modifications, preferential orientation, self-cleaning surfaces
National Category
Chemical Engineering Materials Engineering Nano Technology Theoretical Chemistry Inorganic Chemistry
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-265056 (URN)978-91-554-9387-5 (ISBN)
Public defence
2015-12-11, Polhemssalen, Lägerhyddsv. 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Projects
GRINDOOR
Funder
EU, European Research Council, ERC Grant Agreement No. 267234 (“GRINDOOR”)
Available from: 2015-11-19 Created: 2015-10-21 Last updated: 2016-01-13

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The full text will be freely available from 2018-01-25 08:00
Available from 2018-01-25 08:00

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Stefanov, BozhidarNiklasson, Gunnar AGranqvist, Claes-GöranÖsterlund, Lars

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