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1.
OXYGEN-EXCESS RELATED DEFECTS IN SiO2-BASED MATERIALS: COUPLING THEORY AND EXPERIMENTS
Blaž Winkler, doctoral dissertation

Abstract: This work is primarily focused on application of standard first-principle computational approaches to model oxygen excess related point defects in amorphous silica. Atomic models with their respective electronic and optical properties are explored together with some conversion mechanisms between defect models. The first chapter overviews extensive literature about the already known properties of oxygen related defects. Second chapter briefly introduces main methods that have been used in this research, in particular Density Functional Theory (DFT) as energy and force engine with short description of minimal energy path (MEP) algorithm used for modeling chemical/migration reactions, GW approximation for charged electronic excitations (band structure) and Bethe-Salpeter Equation (BSE) for neutral excitations (optical absorption and excitonic structure including electron hole interaction). The third chapter is devoted to the presentation of results. Thanks to the calculation of optical properties of peroxy bridge (POL), a correlation has been found between structural disorder, specifically dihedral angle dispersion, and low coupling with light, which has been identified as main reason why no clear absorption bands have been assigned to the POL. Structure and stability of some other defects, like interstitial ozone molecule (ozonyl) and dioxasilirane (silicon analogy of dioxirane), have been studied. These defects are usually not considered as most important species, however their calculated formation energies are lower compared to some known defects, which indicates they might be present in silica. From a detailed study on possible reaction mechanisms, it has been found that ozonyl might be one of the most important intermediate steps for oxygen exchange reactions. Results also show that dioxasilirane can be spontaneously created during the interaction of oxygen with lone pair defects. By exploring different reactions between oxygen and pre-existing oxygen deficiency centers (ODCs), calculations predict two kinds of passivation behaviors: single-barrier reversible mechanisms with the formation of dioxasilirane-like groups, for which the network keeps the memory of the precursory lone pair defects, and single or multiple-barrier mechanisms, for which the network loses its memory, either because of the high reverse barrier or because of a reconstruction. Final part of this research has been devoted to experimental characterization of the response and tolerance of optical fibers loaded with oxygen under irradiation. These include experiments on commercial fiber along with canonical samples (Optical fibers developed with the intention of studying correlations between different fabrication parameters, dopant/impurity concentration and doping concentrations). Studied fibers also include rare-earth doped fibers.
Keywords: Silica, DFT, GW-approximation, Bethe-Salpeter equation, NEB, defect, oxygen, oxygen excess centers, oxygen deficiency centers, optical absorption, optical fibers, radiation induced attenuation.
Published: 07.05.2019; Views: 129; Downloads: 9
.pdf Fulltext (13,18 MB)

2.
Modelling of Macrosegregation of a Low-Frequency Electromagnetic Direct Chill Casting by a Meshless Method
Vanja Hatić, 2019, doctoral dissertation

Abstract: The main aim of the dissertation is to develop a meshless model that describes the solidification and macrosegregation phenomena during the direct chill casting (DCC) of aluminium alloys under the influence of a low-frequency electromagnetic field. Macrosegregation is an undesired consequence of alloy solidification. It represents one of the major casting defects and substantially reduces the quality of the finished product. On the other hand, low-frequency electromagnetic casting (LFEC) is a process that promises to increase greatly the product quality, including the reduction of macrosegregation. The modelling of both processes is of tremendous importance to the metallurgical industry, due to the high costs of experiments during production. The volume-averaging formulation is used for the modelling of the solid-liquid interaction. The conservation equations for mass, energy, momentum, and species are used to model the solidification of aluminium-alloy billets in axysimmetry. The electromagnetic-induction equation is coupled with the melt flow. It is used to calculate the magnetic vector potential and the Lorentz force. The Lorentz force is time-averaged and included in the momentum-conservation equation, which intensifies the melt flow. The effect of Joule heating is neglected in the energy conservation due to its insignificant contribution. The semi-continuous casting process is modelled with the Eulerian approach. This implies that the global computational domain is fixed in space. The inflow of the liquid melt is assumed at the top boundary and the outflow of the solid metal is assumed at the bottom. It is assumed that the whole mushy area is a rigid porous media, which is modelled with the Darcy law. The Kozeny-Carman relation is used for the permeability definition. The incompressible mass conservation is ensured by the pressure correction, which is performed with the fractional step method. The conservation equations and the induction equation are posed in the cylindrical coordinate system. A linearised eutectic binary phase diagram is used to predict the solute redistribution in the solid and liquid phases. The micro model uses the lever rule to determine the temperature and the liquid fraction field from the transport equations. The partial differential equations are solved with the meshless-diffuse-approximate method (DAM). The DAM uses weighted least squares to determine a locally smooth approximation from a discrete set of data. The second-order polynomials are used as the trial functions, while the Gaussian function is used as the weight function. The method is localised by defining a smooth approximation for each computational node separately. This is performed by associating each node with a unique local neighbourhood, which is used for the minimisation. There are 14 nodes included in the local subdomains for the DCC and LFEC simulations. The stability of the advective term is achieved with a shift of the Gaussian weight in the upwind direction. This approach is called the adaptive upwind weight function and is used in the DAM for the first time. The Explicit-Euler scheme is used for temporal discretisation. The use of a meshless method and the automatic node-arrangement generation makes it possible to investigate the complicated flow structures, which are formed in geometrically complex inflow conditions in a straightforward way. A realistic inflow geometry and mould can therefore be included in the model. The number of computational nodes is increased in the mushy zone and decreased in the solid phase, due to the optimisation of the computational time and memory. The computational node arrangement is automatically adapted with time, as the position of the mushy zone is changed in shape and position.
Keywords: low-frequency electromagnetic casting, direct chill casting, macrosegregation, electromagnetic stirring, aluminium alloys, meshless methods, diffuse-approximate method, multiphysics model, solidification
Published: 25.04.2019; Views: 134; Downloads: 3
.pdf Fulltext (28,80 MB)

3.
MINERALOGY AND GEOMICROBIOLOGY IN ACTIVE VOLCANIC CAVE ENVIRONMENTS IN CENTRAL AMERICA
Andres Ulloa Carmiol, 2019, doctoral dissertation

Abstract: Recently, the study of mineralogical and geomicrobiological interactions in volcanic caves is gaining relevance, because there are many factors to consider them as Mars analogues for astrobiology and planetary sciences. In addition, sulfuric acid caves have also shown to be an important field of study for the understanding of chemolithoautotrophic metabolic pathways, especially in regard to the biogeochemical cycle of sulfur compounds. Hitherto, sulfur-rich volcanic caves found in Central America were almost unknown to the scientific community. This work presents the mineralogical and microbiological diversity and the potential geomicrobiological links found in active volcanic settings, such as Cueva los Minerales (CMI) and Cueva los Mucolitos (CMU) at Irazú volcano (Costa Rica), and Cueva Hoyo de Koppen (CHK) at El Hoyo volcano (Nicaragua). The volcanic caves at Irazú volcano (Costa Rica) are located in the northwest (NW) foothills of the main crater. These caves became accessible after the partial collapse of the NW sector of the Irazú volcano in 1994, offering the opportunity to investigate in situ active minerogenetic processes. Detailed mineralogical and geochemical analyses were performed to study the speleothems at CMI and CMU. Mineralogical analyses included X-ray diffraction (XRD) and Raman spectroscopy, while geochemical characterization was done using Energy Dispersive X-ray spectroscopy (EDX) coupled to Scanning Electron Microscopy (SEM). In noveladdition, measurements of cave environmental parameters, cave drip water, and a compilation of geochemical analyses of the Irazú volcanic lake (located ca. 150 m above the cave level) and fumarole analyses, were conducted between 1991 and 2014. Forty-eight different mineral phases were identified, mostly rare hydrated sulfates of the alunite, halotrichite, copiapite, kieserite and rozenite groups; thirteen of which are described here, as cave minerals, for the first time, i.e. aplowite, bieberite, boyleite, dietrichite, ferricopiapite, ferrinatrite, lausenite, lishizhenite, magnesiocopiapite, marinellite, pentahydrite, szomolnokite, and wupatkiite. The presence of other novel cave minerals, such as tolbachite, mercallite, rhomboclase, cyanochroite, and retgersite, is likely, but this was not possible to confirm by the various mineralogical techniques employed in this study. It was determined that uplifting of sulfurous gases, water percolation from the Irazú volcanic lake, and hydrothermal interactions with the volcanic host rock are responsible for such extreme mineralogical diversity. Moreover, acidic (pH < 2) viscous biofilms, known as snottites, were observed hanging from both, walls and ceiling, of the caves at Irazú volcano, in close relation with the presence of sulfate minerals. Knowledge about snottites in volcanic caves is scarce, being biofilms present in carbonated rocks the most studied, e.g. at Frasassi and Acquasanta caves (Italy), and Cueva de Villa Luz and Luna Azufre (Mexico). 16S rRNA techniques (with primers for target Bacteria and Archaea), together with bioinformatics analyses, were used to investigate the snottites from CMU and CMI. The results indicated that both prokaryotic groups in the snottites offer a vast metabolic potential to execute various reactions, including redox reactions. The phylogenetic findings revealed that approximately 65% of the identified taxa corresponded to species related to sulfur-oxidizing metabolic pathways (e.g., Leptospirillum, Mycobacterium, Acidithiobacillus and Acidiphilium), while just 0.04% corresponded to sulfur-reducing species (Desulfosporosinus). Since sulfur-oxidizing microorganisms appeared to be dominant in the acidic snottites, induced-gypsum precipitation by changes in micro environmental conditions, is strongly suggested as the path for biomineralization at the studied caves. 34S isotope signature of sulfur minerals presented negative values (from -16.2 to -3.4 ‰), comparable with data obtained...
Keywords: Volcanic caves, Costa Rica, Irazú, Cueva los Mucolitos, Cueva los Minerales, Nicaragua, Hoyo, Cueva Hoyo de Koppen, mineralogy, sulfur isotopes, hydrated sulfates, active volcano, microbiology, snottites, geomicrobiology, astrobiology, Mars analogue.
Published: 15.04.2019; Views: 128; Downloads: 4
.pdf Fulltext (16,65 MB)

4.
Mass composition of ultra-high energy cosmic rays at the Pierre Auger Observatory
Gašper Kukec Mezek, 2019, doctoral dissertation

Abstract: Cosmic rays with energies above 10^18 eV, usually referred to as ultra-high energy cosmic rays (UHECR), have been a mystery from the moment they have been discovered. Although we have now more information on their extragalactic origin, their direct sources still remain hidden due to deviations caused by galactic magnetic fields. Another mystery, apart from their production sites, is their nature. Their mass composition, still uncertain at these energies, would give us a better understanding on their production, acceleration, propagation and capacity to produce extensive air showers in the Earth's atmosphere. Mass composition studies of UHECR try to determine their nature from the difference in development of their extensive air showers. In this work, observational parameters from the hybrid detection system of the Pierre Auger Observatory are used in a multivariate analysis to obtain the mass composition of UHECR. The multivariate analysis (MVA) approach combines a number of mass composition sensitive variables and tries to improve the separation between different UHECR particle masses. Simulated distributions of different primary particles are fitted to measured observable distributions in order to determine individual elemental fractions of the composition. When including observables from the surface detector, we find a discrepancy in the estimated mass composition between a mixed simulation sample and the Pierre Auger data. Our analysis results from the Pierre Auger data are to a great degree independent on hadronic interaction models. Although they differ at higher primary masses, the different models are more consistent, when combining fractions of oxygen and iron. Compared to previously published results, the systematic uncertainty from hadronic interaction models is roughly four times smaller. Our analysis reports a predominantly heavy composition of UHECR, with more than a 50% fraction of oxygen and iron at low energies. The composition is then becoming heavier with increasing energy, with a fraction of oxygen and iron above 80% at the highest energies.
Keywords: astroparticle physics, ultra-high energy cosmic rays, extensive air showers, mass composition, Pierre Auger Observatory, machine learning, multivariate analysis
Published: 03.04.2019; Views: 170; Downloads: 13
.pdf Fulltext (17,53 MB)

5.
6.
Urban Conservation System in China and Its Improvement by Using Historic Urban Landscape Approach
Chen Shujie, 2018, doctoral dissertation

Abstract: In the last few decades, the Modern Conservation Movement has developed from European-limited practices into a global movement with universal common views and practical measures for managing heritage resources in different cultural contexts. As an innovative idea of this movement, the Historic Urban Landscape approach aims to protect and manage historic urban environments with respect to both the fundamental principles in the international doctrines and the local social/cultural/historical contexts. It recommends local authorities to use the HUL toolkit to identify, conserve and manage the overall landscape of their historic cities. In the case of China, who is an old civilization, a modern nation and a socialist country at the same time, the conservation practice needs to follow the basic and common conservation principles in the international doctrines, and meanwhile, it shall make its initiatives based on the actual social, cultural and political situations. The establishment of Historically and Culturally Famous City (HCF City in short) system is a positive attempt for such a purpose. The system manages various urban elements relating to the city’s historical and cultural features under a comprehensive notion of HCF City. However, the system is not prefect because it depends excessively on the top-down management of local governments, and also because it overlooks the spatial and spiritual relationships among the protected elements... The thesis provides a big picture of architectural and urban conservation practices in China. It introduces the forming process and the characteristics of historic urban fabric, as well as the history of urban conservation. Then, it takes a deep look at the existing HCF City system, including its basic ideas, structures and mechanism. It analyses the system’s initiatives and deficiencies. Finally, it provides feasible advices to improve the current system by using the HUL toolkit.
Keywords: Urban, History, Heritage, Conservation, Management International Principles, Modern Conservation Movement, Globalization, Localization Historic Urban Area, Historically and Culturally Famous Cities, Conservation System, Historic Urban Landscape Chinese Architecture, Chinese Urban Planning, Conservation History, Policy
Published: 11.02.2019; Views: 230; Downloads: 4
.pdf Fulltext (217,96 MB)

7.
REVITALISATION OF RUINS AND THE IMPACT ON CONSERVATION POLICIES IN SLOVENIA. CASE STUDY OF THE CARTHUSIAN MONASTERY AT ŽIČE, SLOVENIA
Mateja Golež, 2019, doctoral dissertation

Abstract: The doctoral dissertation in question presents in detail the issue of protecting architectural heritage as ruins on the example of the Žiče Charterhouse complex in Slovenia. The author based her work on the history of conservation, internationally applicable charters related to architectural heritage protection and on examples taken from international conservation practice. Although the Slovenian conservation profession, as an independent technical and scientific discipline, received its formal legal status through the emergence of independent Slovenia merely a couple of decades ago, it is possible to claim that Slovenia, with its first academically qualified conservation specialists Avguštin Stegenšek and France Stele, was in contact with active policy on heritage protection as early as before World War I, when the Slovenian territory was part of the Austro-Hungarian Empire, and also after the War, when the territory was annexed to the Kingdom of Yugoslavia. Despite long-standing efforts, Slovenians did not achieve the legal protection of architectural heritage until early 1980s, when an independent act on the protection of natural and cultural heritage was adopted in former Yugoslavia. Because of this, the tasks carried out by conservation specialists until that time primarily focused on documenting heritage and carrying out the most pressing maintenance works. Since all major international charters related to cultural heritage protection were signed when Slovenia was part of former Yugoslavia and after it became independent, it is not possible to claim that the profession did not follow the international legal order in this field. This is why it is sometimes difficult to understand the large gap between international technical criteria and the conservation decisions made by Slovenian conservation experts when performing intervention works on structures or sites of cultural value in Slovenia. To improve this condition, it is therefore vital that buildings be treated comprehensively prior to interventions, including in terms of carrying out natural science and technical research studies that provide an insight into the materials, structural frame and building physics of a building, as shown in the doctoral dissertation on the example of the ruin of the Church of St. John the Baptist at Žiče Charterhouse. Only by carefully analysing historic materials, it is possible to make the right decision on the use of adequate substitute materials for the needs of maintaining a ruin and, only on the basis of preliminary research made into structural frames, it will be possible to monitor the vitality of ruins in future, whereby using state-of-the-art research methods from conservation science. Since the Church of St. John the Baptist has lost its original intended use and also the possibility to get it back, the author of the dissertation proposes that the structure not be reconstructed, since this would imply a major deviation from original architecture, with a shortage of adequate documentation that would provide a basis for credible reconstruction. For this reason, the author of the doctoral dissertation defends the position that the Church be protected as a ruin. The operator of the monastery complex and the entire valley of St. John, where the remains of the lower and upper monastic houses of the Žiče Charterhouse are located, now faces quite a challenging task. It will have to prepare a management plan that will foresee expert bases for sustainable use of natural and cultural values of this site and a suitable development policy, whereby giving the key role to the local community.
Keywords: ruins, architectural history, legislation, international charters, revitalization, natural science research, Žiče monastery
Published: 22.01.2019; Views: 311; Downloads: 14
.pdf Fulltext (12,67 MB)

8.
HAS EVOLUTION BEEN INTERRUPTED IN THE AFRICAN OIKOS?
Tokunbo Laotan-Brown, 2019, doctoral dissertation

Abstract: In essence, a qualitative study revealing what the inhabitants reflect as a deep and personal relationship with and within the region, which contributes significantly to their sense of emotional security and self identity. This includes a strong conservation-orientated attitude with regard to the natural rural character of a clan, tribe or unbounded region using linguistic families, especially in the inner core. It acknowledges the journey though time of the dwellers, changes in requirements in a natural way. The principle behind this thesis is to identify the regional, cultural and environmental demands of where and how a fractal spatial function can be observed as well as how a community can be organised to maintain and preserve its culture in a natural way. Thus, responding to specific characteristics of the local environment and climatic conditions. This lifelong interaction between the cognitive and physical realms has existed overtime. During the evolution of values inhabitants adapted form and materials to the conditions of nature; working with natural forms and climatic cycles rather than considering forces as obstacles to overcome has hermeneutic and practical values; used by intentional makers. The cultural identity in the inhabitant made the home, and then the process of home-making ‘made’ the inhabitant; a reciprocal reward. This will include their connection to the culture, region and environment while proposing a self-organizational solution. The future of conserving African tangible and intangible values, need to take lessons from the past into the future through present resolutions while documenting what maybe an interrupted progression. To achieve the above, I modified an approach using cultural fractal organizational approach as a basis for defining; 1) The structure of research framework using computer simulations and thus establishing linkages between space, region and communal activities. 2) Using African linguistic families (Niger-Congo: Yoruba), as a distinct way of documenting within the context of co-existing social and discursive practices. 3) By generating a background in ethnographic and sensory archaeological data on Yoruba regional cities, focusing on the fractal structure of settlements in relation to the social activities that take place in and around them.
Keywords: Fractals, Self Similarity, Conservation, Self Organizational Approach, Cultural Heritage, Spatial Patterns, IFA, Oikos, Cultural Landscapes, Yoruba Cities, Circularity.
Published: 18.01.2019; Views: 245; Downloads: 5
.pdf Fulltext (24,92 MB)

9.
Groundwater distribution in the recharge area of Ljubljanica springs
Matej Blatnik, 2019, doctoral dissertation

Abstract: Porečje Ljubljanice je s svojo kraško hidrologijo že stoletja predmet proučevanja. V začetku 19. stoletja so bila dela v največji meri posvečena preprečevanju ali vsaj omilitvi poplav na Planinskem polju. Obsežne raziskave v 70. letih 20. stoletja so na območju porečja Ljubljanice razkrile okvirne poti toka podzemne vode. Bolj natančno proučevanje vodonosnika z ugotavljanjem dimenzij in razporeditve kanalov ter dinamike pretakanja pa do zdaj ni bilo izvedeno. Nove speleološke raziskave in razvoj samodejnih merilnikov ponujajo možnost postavitve kakovostne merilne mreže tudi na težje dostopnih mestih. Namen pričujočega dela je zato nadgraditi poznavanje dinamike podzemne vode na območju porečja Ljubljanice z uporabo naprednih tehnik merjenja in interpretacij. Območje proučevanja je omejeno na severni del porečja Ljubljanice, in sicer med Planinskim poljem na jugu in izviri Ljubljanice na severu. Na tem območju je bila vzpostavljena merilna mreža s samodejnimi meritvami višine, temperature in specifične električne prevodnosti vode. Meritve so potekale na štirih požiralnikih na Planinskem polju, na treh izvirih Ljubljanice in osmih jamah z dostopom do podzemne vode. Poleg tega so bile opravljene še številne ročne meritve, na Agenciji RS za okolje (ARSO) pa so bili pridobljeni hidrološki in meteorološki podatki iz uradnih postaj. Na novo pridobljeni podatki so bili obravnavani glede na dotedanje znanje o vodonosniku. Mnoge pretekle ugotovitve so bile potrjene, številne interpretacije pa so bile zgrajene na novo. Te so bile povečini navezane na obliko vodonosnika (razporeditve in dimenzije kanalov) in njihov vpliv na nihanje vodne gladine, kar je bilo v nadaljevanju preverjeno z izgradnjo poenostavljenih konceptualnih in numeričnih modelov. Slednji so bili zgrajeni v programu EPA SWMM (Storm Water Management Model). V 3,5 letnem obdobju meritev je bilo veliko dogodkov s povišanim vodostajem, med njimi prek 15 takih, ko je bilo Planinsko polje poplavljeno. Dogodki so se med seboj zelo razlikovali tako po intenzivnosti kot tudi po trajanju, kar je predstavljalo raznovrstno zbirko podatkov. Najdaljši poplavni dogodek je trajal okoli tri mesece, v tem času pa se je vodna gladina v proučevanem vodonosniku dvignila do 66 m. Raziskave so potrdile pretekle ugotovitve, da se tok vode v vodonosniku prepleta (cepi in ponovno združuje) in da voda prek severnih požiralnikov Planinskega polja povečini teče proti zahodnim izvirom Ljubljanice, voda z vzhodnih požiralnikov Planinskega polja pa povečini proti vzhodnim izvirom Ljubljanice. Ugotovitve so pokazale tudi to, da na vzhodne izvire Ljubljanice pomembno vpliva tudi neposreden tok s Cerkniškega polja, kar se odraža zlasti s temperaturnimi odstopanji. Analiza višin podzemne vode je omogočila nove interpretacije o obliki kraškega vodonosnika. Na skoraj vseh merilnih mestih se je pokazalo obdobno upočasnjeno dviganje in spuščanje vode, kar nakazuje na prisotnost prelivnih kanalov. Takšni kanali so bili v nekaterih jamah že predhodno poznani (npr. Šerkov rov v Gradišnici, Skalni rov v Logarčku, Levi rov v Lijaku v Najdeni jami), medtem ko so v nekaterih jamah le predvideni (Veliko brezno v Grudnovi dolini, Andrejevo brezno 1). Obsežna merilna mreža, večletno obdobje meritev, stalne in samodejne meritve, uporaba naprednih tehnik interpretacij in preverjanje z modeli so omogočili raznovrstne analize in številne ugotovitve. Postopek se je izkazal kot primeren in bi bil uporaben tudi v drugih podobnih vodonosnikih. Podobno kot pretekle raziskave pa tudi ta pušča številna odprta vprašanja in možnosti za nadaljnje raziskave. Smiselne bi bile nadaljnje in bolj podrobne raziskave vpliva geoloških struktur, raziskave freatičnih in globokih regionalnih tokov, proučevanje vodne bilance in kakovosti vode, in nenazadnje tudi v splošnem boljše razumevanje poplavljanja Planinskega polja.
Keywords: kraški vodonosnik, dinamika podzemne vode, poplavljanje, modeliranje, Planinsko polje, Ljubljanica
Published: 16.01.2019; Views: 281; Downloads: 27
.pdf Fulltext (21,14 MB)

10.
Correlations between photocatalytic activity and chemical structure of Cu-modified TiO2-SiO2 nanoparticle composites
T. Čižmar, doctoral dissertation

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.
Keywords: titanium dioxide, Cu-modified TiO2-SiO2 photocatalyst, photocatalytic activity, Cu K-edge XANES, EXAFS.
Published: 17.12.2018; Views: 294; Downloads: 22
.pdf Fulltext (3,05 MB)

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