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Found in: ključnih besedah
Keywords: TiO2, dye-sensitized, GW, first-principles
Published: 15.10.2018; Views: 2356; Downloads: 0
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Abstract: The objective of this dissertation was to examine how copper modification can improve the photocatalytic activity of TiO2-SiO2 and to explain the correlation between Cu concentration and chemical state of Cu cations in the TiO2-SiO2 matrix, as well as the photocatalytic activity under the UV/solar irradiation. The Cu-modified TiO2-SiO2 photocatalysts were prepared by a low temperature sol-gel method based on organic copper, silicon and titanium precursors with varied Cu concentrations (from 0.05 to 3 mol%). The sol-gels were dried at 150 °C to obtain the photocatalysts in the powder form. To test thermal stability, additional set of photocatalysts was obtained by calcinating dried samples in air at 500 °C for 1 h. The photocatalytic activity was determined by a fluorescence-based method of terephthalic acid decomposition. Up to three times increase in photocatalytic activity of air-dried samples is obtained when TiO2-SiO2 matrix is modified with Cu in a narrow concentration range from 0.05 to 0.1 mol%. At higher Cu loadings the photocatalytic activity of Cu-modified photocatalyst is smaller than in the unmodified reference TiO2-SiO2 photocatalyst. Calcined samples showed significantly reduced photocatalytic activity compared to air-dried samples. XRD analysis showed that all Cu-modified TiO2-SiO2 nanocomposites with different Cu concentrations have the same crystalline structure as unmodified TiO2-SiO2 nanocomposites (air-dried or calcined). The addition of Cu does not change the relative ratio between the anatase and brookite phase or unit cell parameters of the two TiO2 crystalline structures. TEM analysis showed that the addition of Cu does not change the morphology of TiO2-SiO2 catalyst dried at 150 °C. The Cu K-edge XANES and EXAFS analysis were used to determine valence state and local structure of Cu cations in Cu-modified TiO2-SiO2 photocatalyst. The results elucidate the mechanism responsible for the improved or hindered photocatalytic activity. In the air-dried samples with low Cu content, which exhibit largest activity, Cu-O-Ti connections are formed, suggesting that the activity enhancement is due to Cu(II) cations attachment on the surface of the photocatalytically active TiO2 nanoparticles, so Cu(II) cations may act as free electron traps, reducing the intensity of recombination between electrons and holes at the TiO2 photocatalyst’s surface. At higher Cu loadings no additional Cu-O-Ti connections are formed, instead only Cu-O-Cu connections are established, indicating the formation of amorphous or nanocrystalline Cu(II) oxide, which hinders the photocatalytic activity of TiO2. Calcination of Cu-modified TiO2-SiO2 photocatalysts at 500 °C induces significant structural changes: Cu-O-Ti connections are lost, Cu partially incorporates into the SiO2 matrix and amorphous copper oxides, which again reduce the photocatalytic activity of the material, are formed.
Found in: ključnih besedah
Keywords: titanium dioxide, Cu-modified TiO2-SiO2 photocatalyst, photocatalytic activity, Cu K-edge XANES, EXAFS.
Published: 17.12.2018; Views: 3640; Downloads: 137
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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: ključnih besedah
Keywords: Anatase-rutile Sn-modified TiO2 XAS analysis Photocatalytic activity
Published: 10.02.2020; Views: 2266; Downloads: 0
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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: ključnih besedah
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: 2953; Downloads: 140
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Abstract: In recent years a classification of materials based on their topological order gained popularity due to the discovery of materials with special topological character – topological insulators (TI). TI have different band structure than regular insulators or conductors. They are characterized by a band gap in the bulk of the material, but at the surface they possess conductive topological surface states (TSS) that cross the Fermi level. TSS are a consequence of the non-trivial bulk band structure and have properties that differ from ordinary surface states. They are robust toward contamination and deformation of the surface. Additionally, they are also spin polarized, which means that an electron spin is locked to a crystal momentum and, therefore, backscattering during transport is suppressed [1]. Due to their specific properties the TI could be used in fields of spintronics, quantum computing and catalysis [2]. The investigation of the interfaces between metals and the TI has not been given much attention even though its characterization is interesting from fundamental physics and applicative point of view. (In)stability of the contacts with metal electrodes, in a form of a chemical reaction or diffusion, has to be taken into account since it can affect the transport properties of the material or increase the contact resistance. Our research is dedicated to the study of the metal/TI interfaces, in particular to Bi2Se3 with deposited metals that are relevant for electrical contacts (Au, Ag, Pt, Cr, Ti). The thermal and chemical stability of the interfaces are of fundamental importance for understanding the contact behavior, therefore, we focused our work to the characterization of these properties. The metal/TI interfaces are investigated mainly with an electron microscopy (SEM, TEM, STM), EDX microanalysis and XRD. Our previous studies showed that the interface between Bi2Se3, and Ag deposited either chemically or from a vapor phase, results in the formation of new phases already at room temperature [3]. On the contrary, Au deposited on the Bi2Se3 surface shows very limited reactivity and is stable at RT, but diffusion and coalescence of the metal are observed starting from 100 °C [4]. In this contribution, we will present further characterization on the evolution of the Ag/Bi2Se3 and Au/Bi2Se3 interfaces, show preliminary results about recently investigated systems (Pt/Bi2Se3, Ti/Bi2Se3) and compare the thermal and chemical stability of the systems under investigation.
Found in: ključnih besedah
Keywords: thermal lens spectrometry, photothermal beam deflection spectroscopy, dye remediation, photothermal technique, photocatalytic degradation, reactive blue 19, TiO2 modification
Published: 20.08.2021; Views: 1353; Downloads: 0
Fulltext (100,42 KB)