| 1. |
- Arvizu, Miguel A, et al.
(författare)
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Electrochromism in sputter-deposited W-Ti oxide films : Durability enhancement due to Ti
- 2014
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 125, s. 184-189
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Tidskriftsartikel (refereegranskat)abstract
- Thin films of W-Ti oxide were prepared by reactive DC magnetron sputtering and were characterized by Rutherford bathcattering spectrometry, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The electrochromic properties were studied by cyclic voltammetry in an electrolyte of lithium perchlorate in propylene carbonate and by optical transmittance measurements. The addition of Ti significantly promoted the amorphous nature of the films and stabilized their electrochemical cycling performance and dynamic range for electrochromism. (C) 2014 Elsevier B.V. All rights reserved.
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| 2. |
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| 3. |
- Stefanov, Bozhidar, et al.
(författare)
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Fine control of the amount of preferential <001> orientation in DC magnetron sputtered nanocrystalline TiO2 film
- 2014
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Ingår i: INERA Workshop. - : Institute of Physics (IOP). ; , s. UNSP 012011-
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Konferensbidrag (refereegranskat)abstract
- Different crystal facets of anatase TiO2 are known to have different chemical reactivity; in particular the {001} facets which truncates the bi-tetrahedral anatase morphology are reported to be more reactive than the usually dominant {101} facets. Anatase TiO2 thin films were deposited by reactive DC magnetron sputtering in Ar/O2 atmosphere and were characterized using Rietveld refined grazing incidence X-ray diffraction, atomic force microscopy and UV/Vis spectroscopy. By varying the partial O2 pressure in the deposition chamber, the degree of orientation of the grains in the film could be systematically varied with preferred <001> orientation changing from random upto 39% as determined by March-Dollase method. The orientation of the films is shown to correlate with their reactivity, as measured by photo-degradation of methylene blue in water solutions. The results have implications for fabrication of purposefully chemically reactive thin TiO2 films prepared by sputtering methods.
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| 4. |
- Stefanov, Bozhidar, et al.
(författare)
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Gas-phase photocatalytic activity of sputter-deposited anatase TiO2 films : Effect of <001> preferential orientation, surface temperature and humidity
- 2016
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 335, s. 187-196
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Stefanov, Bozhidar I, et al.
(författare)
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Acetaldehyde adsorption and condensation on anatase TiO2 : Influence of acetaldehyde dimerization
- 2014
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Ingår i: Journal of Molecular Catalysis A. - : Elsevier. - 1381-1169 .- 1873-314X. ; 381, s. 77-88
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Tidskriftsartikel (refereegranskat)abstract
- Conversion of acetaldehyde to crotonaldehyde on anatase TiO2 films was studied by in situ Fourier transform infrared spectroscopy (FTIR) and by density functional theory (DFT) calculations. In situ FTIR showed that acetaldehyde adsorption is accompanied by the appearance of a hitherto non-assigned absorption band at 1643 cm−1, which is shown to be due to acetaldehyde dimers. The results were supported by DFT calculations. Vibrational frequencies calculated within a partially relaxed cluster model for molecular acetaldehyde and its dimer, and for the corresponding adsorbed species on the anatase (101) surface, were in good agreement with experimental results. A kinetic model was constructed based on the combined FTIR and DFT results, and was shown to explain the essential features of the acetaldehyde condensation reaction.
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| 6. |
- Stefanov, Bozhidar I., et al.
(författare)
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Demonstrating Online Monitoring of Air Pollutant Photodegradation in a 3D Printed Gas-Phase Photocatalysis Reactor
- 2015
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Ingår i: Journal of Chemical Education. - : American Chemical Society (ACS). - 0021-9584 .- 1938-1328. ; 92:4, s. 678-682
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Tidskriftsartikel (refereegranskat)abstract
- We present a demonstration of online monitoring of gas-phase photocatalytic reactions. A cotton cloth impregnated with commercial titanium dioxide nanoparticles is used as a photocatalytic filter to clean air contaminated with a model pollutant. A fan forces air through the filter while it is irradiated by UV diodes. The concentration of the air pollutant is measured online by an inexpensive, commercially available semiconductor air quality sensor. The structural parts of the reactor were 3D printed in polylactide bioplastic. We provide all schematics, 3D printed model parts, hardware, firmware, and computer code of the reactor and control units. The device can be used for interactive learning of both gas phase photocatalysis and gas sensing, as well as in student laboratory classes for measuring air pollutants and their photodegradation. The experimental setup can also form the basis for a project work for chemical engineering university students, and it can be employed as a building block for development of other gas phase chemical reaction demonstrations.
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| 7. |
- Stefanov, Bozhidar, 1988-
(författare)
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Photocatalytic TiO2 thin films for air cleaning : Effect of facet orientation, chemical functionalization, and reaction conditions
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 8. |
- Stefanov, Bozhidar, et al.
(författare)
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Quantitative relation between photocatalytic activity and degree of〈001〉orientation for anatase TiO2 thin films
- 2015
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry. - 2050-7488. ; 3:33, s. 17369-17375
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a quantitative relation between exposed crystal surfaces and photocatalytic activity of nanocrystalline anatase TiO2. Thin films with controlled amount of 〈001〉 preferential orientation were prepared by reactive DC magnetron sputtering in Ar/O2 atmosphere with the partial O2 pressure as control parameter. The samples were characterized with X-ray diffraction, transmission electron microscopy and atomic force microscopy, from which the degree of preferential 〈001〉 orientation and exposed facets were determined by an extension of the March–Dollase model. Photocatalytic degradation of methylene blue dye shows that the photocatalytic reaction rate increases approximately with the square of the fraction of 〈001〉 oriented surfaces, with about eight times higher rate on the {001} surfaces, than on {101}, thus quantifying the effect of crystal facet abundancy on the photocatalytic activity of anatase TiO2.
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| 9. |
- Topalian, Zareh, et al.
(författare)
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Adsorption and photo-oxidation of acetaldehyde on TiO2 and sulfate-modified TiO2 : Studies by in situ FTIR spectroscopy and micro-kinetic modeling
- 2013
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Ingår i: Journal of Catalysis. - : Elsevier. - 0021-9517 .- 1090-2694. ; 307, s. 265-274
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Tidskriftsartikel (refereegranskat)abstract
- Adsorption and photocatalytic oxidation of acetaldehyde have been investigated on TiO2 and sulfate-modified TiO2 films (denoted SO4TiO2). In situ Fourier transform infrared spectroscopy was used to study surface reactions as a function of time and number of experimental cycles. Spectral analysis and micro-kinetic modeling show that crotonaldehyde formation occurs spontaneously on TiO2 but is impeded on SO4TiO2, where instead acetaldehyde desorption is significant. Photo-oxidation yields significant amounts of formate on TiO2 and was identified as the rate-determining step and associated with site blocking. Significantly smaller amounts of formate were observed on SO4TiO2, which is due to the acidity of this surface resulting in weaker bonding of aldehyde and carboxylate intermediate species. Our results are of considerable interest for applications to photocatalytic air purification and to surfaces with controlled wettability.
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| 10. |
- Österlund, Lars, 1967-, et al.
(författare)
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Tuning the photocatalytic of TiO2 based films by orientated growth and surfacefunctinalization
- 2014
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Ingår i: 5th International Symposium on Transparent Conductive Materials. - : Mitos, Heraklion, Crete.
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Konferensbidrag (refereegranskat)abstract
- Anatase TiO2 is by far the most studied photocatalytic material.1 Normally anatase particlesare dominated by the low energy (101) surface planes with small fractions of (001) facets, whichgives the particles their characteristic truncated bi-pyramidal morphology. TiO2 thin films preparedby reactive DC magnetron sputtering in O2 atmosphere are initially amorphous, but after heattreatmentit has been reported that preferential alignment occurs for grains in the <004> direction,which is thought to expose reactive (001) planes at the film surface.2 Here, anatase TiO2 thin filmswere deposited by reactive DC magnetron sputtering in Ar/O2 atmosphere, and were characterizedusing Rietveld refined grazing-incidence XRD, AFM, spectrophotometry and surface profilometry.Upon heat-treatment for 1 h at 500ºC the as-deposited amorphous films crystallized in the anatasephase with grain size between 18 and 26 nm. By varying the partial O2 pressure in the depositionchamber, the degree of orientation of the grains in the film could be systematically varied withpreferred <001> orientation changing between 12 and 38%. Photocatalytic degradation ofmethylene blue dye was measured by in situ absorption spectroscopy in a home-built reactor.3 Aclear relation was found between the amount of preferred orientation and photocatalytic reactivityof the films. The rate increase is non-linear and follows approximately a square dependence on thedegree of <001> orientation.Moreover, we show that modifications of the acid–base properties of TiO2-basedphotocatalysts can be used to improve their sustained photocatalytic activity. Acid surfacefunctionalization was accomplished by a novel method, where SO2 gas molecules are adsorbed onsurface oxygen vacancy sites in a reactive atmosphere, and which subsequently are oxidized tosulphate groups.4 Here we have applied this method to DC magnetron sputtered films with variousdegree of <001> orientation, as well as on Doctor bladed TiO2 film from nanoparticle suspensions.In situ FTIR spectroscopy measurements of the photo-degradation of acetaldehyde shows thepresence of surface sulphate groups weakens bonding of intermediate aldehyde and carboxylatespecies to the surface, and lowers the formation rate of intermediate formate products. Repeatedadsorption/photo-oxidation cycles show that the acetaldehyde adsorption capacity is higher onsulphated TiO2 films, which can be explained by a reduced site-blocking effect of strongly bondedformate species which limit the complete oxidation on TiO2.The presented results point toward new strategies to purposefully improve the photocatalyticactivity of TiO2 films by industrially up-scalable technologies, and also show that appropriatemodification of the acid-base surface properties can help to avoid photocatalyst deactivation
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