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1.
XAS studies of Sn modified TiO2 coatings
Urška Lavrenčič Štangar, Iztok Arčon, Ksenija Maver, 2018, published scientific conference contribution abstract

Abstract: Coatings based on TiO2 are extensively investigated material for the preparation of surfaces which are referred as self-cleaning. Under solar illumination these coatings catalyze pollutants degradation and enhance their removal from the surface due to photoinduced superhydrophilicity [1]. There are two main drawbacks of using pure TiO2 (anatase) as a photocatalyst; i) its band gap lies in the UVA region so it can exploit only a part of the sunlight spectrum, and ii) it has a high degree of recombination between photo generated electrons and holes on the surface. There are various strategies to improve the photocatalytic efficiency of TiO2, one of them is the modification with transition metals. Based on our previous experiences with sol-gel synthesis of low-temperature TiO2 thin films [2], we prepared a series of Sn modified TiO2 photocatalysts. As a starting material only organic (Ti and Sn alkoxide) precursors were used. The loadings of Sn cations were varied in the range of 0.05 to 20 mol.%. The coatings, deposited on glass substrates by dip-coating technique, were dried at 150 oC. In addition, another set of photocatalyst coatings was prepared by further calcination at 500 °C in air. The comparison of photocatalytic activities of Sn modified TiO2 to unmodified TiO2 showed that Sn loadings in the range of 1-10 mol.% improved photocatalytic activity up to 8 times. At lower loadings of Sn, the photocatalytic activity was improved only by 30 %. After the coatings are calcined, their photocatalytic activity was significantly reduced. The objective of the research was to examine the mechanism responsible for photocatalytic properties of Sn modified TiO2 and to to clarify the role of Sn cations in the TiO2 photocatalytic process. For this purpose, Sn and Ti K-edge XANES and EXAFS analysis was used to precisely determine the local structure and the site of incorporation of Sn cations on titania nanoparticles in the coatings. We examined the hypothesis that the solid-solid interface was a crucial structural feature that facilitates charge separation and enhances photocatalytic efficiency of titania.
Found in: osebi
Keywords: Sn, EXAFS, TiO2 fotokatalizator
Published: 12.09.2018; Views: 1718; Downloads: 0
.pdf Fulltext (97,78 KB)

2.
Improved photocatalytic activity of anatase-rutile nanocomposites induced by low-temperature sol-gel Sn-modification of TiO2
Ksenija Maver, Iztok Arčon, Urška Lavrenčič Štangar, Mattia Fanetti, Saim Emin, Matjaž Valant, 2020, original scientific article

Abstract: The Sn-modified TiO2 photocatalysts are prepared by low-temperature sol-gel processing based on organic titanium and tin precursors with varied Sn concentrations (from 0.1–20 mol .%). The role of Sn dopant as the promotor of the formation of TiO2 rutile crystalline phase is explained and the optimal Sn concentration for preparation of efficient Sn-modified titania photocatalyst is determined. Up to 40 % increase in photocatalytic activity is achieved in Sn-modified TiO2 photocatalytic thin films dried at 150 °C with low Sn concentrations in the range from 0.1 to 1 mol .%. At low Sn concentrations optimal ratio between anatase and rutile (nano)crystals is obtained, which facilitates charge separation at the TiO2 photocatalyst’s surface. When the concentration of Sn increases above 5 mol.% or when the films are calcined at 500 °C, the relative amount of rutile phase with inferior photocatalytic activity, increases and the nanocrystals of titania grow, leading to fewer active sites per unit mass and the reduction of activity in comparison to unmodified TiO2.
Found in: osebi
Keywords: Anatase-rutile Sn-modified TiO2 XAS analysis Photocatalytic activity
Published: 10.02.2020; Views: 1310; Downloads: 0
.pdf Fulltext (537,61 KB)

3.
Sn-modified TiO[sub]2 thin film photocatalysts prepared by low-temperature sol-gel processing
Ksenija Maver, 2021, doctoral dissertation

Abstract: Due to many advantageous physiochemical properties, titanium dioxide (TiO2) is the most widely used photocatalyst in numerous applications, such as wastewater treatment and air purification, self-cleaning surfaces and energy conversion (H2 generation). However, one of its disadvantages is the high electron-hole recombination rate, and coupling with other semiconductors is one of the strategies to improve it. The objective of this dissertation was to investigate how the photocatalytic activity of pure TiO2 can be improved by tin modification and to explain the mechanism of increased or hindered photoactivity in correlation with the structural properties of the modified TiO2 photocatalysts. A new low-temperature sol-gel synthesis route was developed to prepare Sn- or SnO2-modified TiO2 photocatalysts. In both cases, organic tin and titanium precursors were used. Tin in the form of Sn cations was used to prepare Sn-modified TiO2. In this case, the precursors went through the sol-gel reaction together to form a Sn-TiO2 sol. In the case of SnO2 modification, the SnO2 sol was prepared separately and additionally mixed with the TiO2 sol to form a TiO2/SnO2 bicomponent semiconductor system. Different molar ratios of tin to titanium were prepared to investigate the correlation between the tin concentration and the photocatalytic properties of the photocatalysts in the form of thin films. The results were used to optimize the synthesis conditions to obtain an improved activity of the modified TiO2 photocatalysts under UV-irradiation. The photocatalytic activity of the thin films was determined by measuring the degradation rate of an azo dye. An increase of up to 40 % in the photocatalytic activity of the dried samples (at 150 °C) was achieved when the TiO2 was modified with the Sn or SnO2 in a concentration range of 0.1 to 1 mol.%. At higher Sn or SnO2 loadings and after calcination of the samples at 500 °C, the photocatalytic activity of the photocatalyst was reduced compared to the unmodified TiO2. Different characterization techniques (UV-Vis, XRD, nitrogen physisorption, TEM, SEM and XAS) were employed to clarify the mechanism responsible for the enhanced and hindered photocatalytic performance of the Sn- and SnO2-modified TiO2 photocatalysts. The results showed that a nanocrystalline structure is already achieved in the samples by the low-temperature film treatment (drying at 150 °C) and that the photocatalytic efficiency is mainly influenced by the crystalline phase composition: anatase/rutile in the case of Sn-modified and TiO2/SnO2 in the case of SnO2-modified TiO2. The crystal size and specific surface area differ insignificantly between the equally thermally treated samples and partly explain the differences in photoefficiency of the calcined samples compared to the dried samples. The structural study at the atomic level, using the Sn K-edge EXAFS, revealed that Sn cations act as nucleation sites for the anatase to rutile transformation in the Sn-modified TiO2 photocatalysts, while in the SnO2-modified TiO2 samples the nanocrystalline cassiterite SnO2 is bound to the TiO2 nanocrystallites via the Sn-O-Ti bond. In both cases, the advantage of coupling the two semiconductors was achieved by separating the charge carriers and thus prolonging their lifetime for accessibility to participate in the redox reactions. The maximum activity enhancement was achieved in the low concentration range (0.1–1 mol.%), which means that an optimal ratio and contact of the two phases are obtained for the given physical parameters, such as particle size, shape and specific surface area of the catalyst.
Found in: osebi
Keywords: Sn-modified TiO2, SnO2-modified TiO2, low-temperature sol-gel, thin films, photocatalytic activity, anatase/rutile system, Sn K-edge EXAFS, dissertations
Published: 09.06.2021; Views: 766; Downloads: 48
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4.
Improved photocatalytic activity of SnO[sub]2-TiO[sub]2 nanocomposite thin films prepared by low-temperature sol-gel method
Urška Lavrenčič Štangar, Giovanni Palmisano, Matjaž Valant, Mattia Fanetti, Samar Al Jitan, Iztok Arčon, Ksenija Maver, 2021, original scientific article

Abstract: The objective of this research was to investigate how the photocatalytic activity of pure TiO2 can be improved by SnO2 modification. Different molar ratios of tin to titanium were prepared. The correlation between tin concentration and structural properties was investigated to explain the mechanism of photocatalytic efficiency and to optimize the synthesis conditions to obtain enhanced activity of the SnO2-modified TiO2 photocatalysts under UV-irradiation. The SnO2-modified TiO2 photocatalysts were prepared by a low-temperature sol-gel method based on organic tin and titanium precursors. The precursors underwent sol-gel reactions separately to form SnO2-TiO2 sol. The sol-gels were deposited on a glass substrate by a dip-coating technique and dried at 150 ◦C to obtain the photocatalysts in the form of a thin film. To test the thermal stability of the material, an additional set of photocatalysts was prepared by calcining the dried samples in air at 500 ◦C. The photocatalytic activity of the samples was determined by measuring the degradation rate of an azo dye. An increase of up to 30% in the photocatalytic activity of the air-dried samples was obtained when the TiO2 was modified with the SnO2 in a concentration range of 0.1–1 mol.%. At higher SnO2 loadings, the photocatalytic activity of the photocatalyst was reduced compared to the unmodified TiO2. The calcined samples showed an overall reduced photocatalytic activity compared to the air-dried samples. Various characterization techniques (UV-Vis, XRD, N2-physisorption, TEM, EDX, SEM, XAS and photoelectrochemical characterization) were used to explain the mechanism for the enhanced and hindered photocatalytic performances of the SnO2-modified TiO2 photocatalysts. The results showed that the nanocrystalline cassiterite SnO2 is attached to the TiO2 nanocrystallites through the Sn-O-Ti bonds. In this way, the coupling of two semiconductors, SnO2 and TiO2, was demonstrated. Compared to single-phase photocatalysts, the coupling of semiconductors has a beneficial effect on the separation of charge carriers, which prolongs their lifetime for accessibility to participate in the redox reactions. The maximum increase in activity of the thin films was achieved in the low concentration range (0.1–1 mol.%), which means that an optimal ratio and contact of the two phases is achieved for the given physical parameters such as particle size, shape and specific surface area of the catalyst.
Found in: osebi
Keywords: photocatalytic activity, Sn-modified TiO2, low-temperature, thin films, XAS analysis
Published: 05.07.2021; Views: 369; Downloads: 0
.pdf Fulltext (5,18 MB)

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