| 1. |
- Langhammer, David Michael, 1991-
(author)
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Capturing Air Pollutants : Photochemical Adsorption and Degradation of SO2, NO2 and CO2 on Titanium Dioxide
- 2021
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Doctoral thesis (other academic/artistic)abstract
- Titanium dioxide (TiO2) is a material with many useful properties. It is used most widely as a pigment in white paint, although in technological research it is better known for its ability to catalyze chemical reactions during light absorption. This process is referred to as photocatalysis, where the energy of the light is used to power the chemical reactions. This has enabled several interesting applications of TiO2, where it can for instance be applied to windows or façade walls to make their surfaces self-cleaning. Another implementation that has received much attention lately is artificial photosynthesis, where the light energy is used to transform CO2 and H2O into synthetic fuels. This thesis work contributes to the development of both these applications, although the main ambition is to show how three of the most common ambient air pollutant molecules, SO2, NO2 and CO2, can be captured at the surface of TiO2 by means of photocatalysis. Specifically, infrared (IR) spectroscopy and density functional theory (DFT) has been used as complementary tools of analysis to study the photocatalytic reactions that enable transformation of SO2, NO2 and CO2 into strongly bound sulfates, nitrates and carbonates, respectively. This combined experimental and theoretical approach has enabled a detailed description of how these reactions proceed and has further shown how the fundamental reactivity of the TiO2 surface changes upon light absorption.The work presented herein contributes to an increased understanding of photocatalysis and shows, quite generally, how molecules can be effectively captured at the surface of metal oxides by forming surface-integrated ionic adsorbates.
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| 2. |
- Mattsson, Andreas, 1976-
(author)
-
Formic acid adsorption and photodecomposition on rutile TiO2 (110) : An in situ infrared reflection absorption spectroscopy study
- 2014
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Licentiate thesis (other academic/artistic)abstract
- TiO2 based photocatalysis is an emerging green nanotechnology that can be used forremoval of pollutants from water and air. It has had an increased research interest, bothfrom an application and fundamental point of view, during the last decades. Despite thismany elementary processes that occur on the photocatalyst surface are not fullyunderstood and are thus limiting our ability to purposefully manufacture more efficientphotocatalytic materials.In this licentiate thesis, the adsorption geometry and photodecomposition of formicacid on differently prepared rutile TiO2 (110) surfaces were investigated. The surface wasprepared by repeated cycles of Argon ion sputtering and annealing. By modifying thisprocedure either reduced, stoichiometric or oxylated surfaces have been obtained. Thesedifferent surfaces are all well-ordered as evident from the obtained low energy electrondiffraction pattern. In addition, a totally disordered surface was also prepared by Argonsputtering alone. Grazing incidence infrared reflection-absorption spectroscopy (IRRAS)employing polarized light with different azimuthal orientations of the TiO2 single crystalwas used to investigate the binding geometry of formic acid (HCOOH) on the surface.Upon adsorption of formic acid on the TiO2 surface, the molecule is deprotonatedresulting in a formate (HCOO-) and a hydrogen (H+) molecule on the surface. The formatemolecules are mainly bridge-bonded to the Ti5c surface atoms with their molecular axisalong the [001] direction. A minority of the formate species was found to adsorb throughhydroxylated oxygen vacancies (or protonated oxygen atoms) and therefore have differentorientations on the surface. For the disordered surface, it was found that the orientation ofthe formate adsorbates are more or less random since no changes in the IRRAS spectraare seen for the different directions of the single crystal. The adsorption geometry for thedisordered surface is also changed, as seen in the shift of the peak positions in the IRRASspectra. This changed adsorption geometry is attributed to exposures of Ti3+ atoms on thesurface, and is a result of the Ar ion sputtering.Irradiation of the HCOO/TiO2 systems by UV light (365 nm, 2 mW/cm2) showed onlya small change in formate coverage after 100 minutes of illumination. The decrease waslargest on the disordered surface and miniscule on the ordered surface. These results werecompared with those obtained on rutile nanoparticles at ambient conditions. Thecomparison shows that the adsorption geometry for the nanoparticles is similar to that ofthe ordered single crystal surfaces and that the photodecomposition rate is about a factorof 30 higher on the nanoparticles than on the disordered surface. This difference isexpected as the single crystal experiments were performed in vacuum, where the supplyof O2 electron acceptors and OH/H2O donors from the gas phase is limited.These results shows that the rutile TiO2 (110) surface is a good model system forfundamental studies of nanoparticle systems and that the presence of hydroxylated oxygenvacancies and protonated oxygen atoms are important for the reactivity of the TiO2surface and must be included in the description of surface reactions on rutile surfaces.
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| 3. |
- Ji, Yuxia, 1988-
(author)
-
Thermochromic VO2-based materials for smart windows : Progress towards applications in buildings
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- Vanadium dioxide is a well-known thermochromic material, whose optical properties can be varied reversibly in response to fluctuation of temperature. It is attractive in various fields due to its unique properties as well as its prospective applications. Especially, it is the most favorite candidate for smart window applications which can significantly lower energy consumption. Ideally, VO2-based thermochromic windows can regulate solar radiation gain dynamically according to the exterior environment conditions. However, commercial utilization of VO2 thermochromic windows is still on the way. There are still a few issues needed to be overcome, such as high phase transition temperature, the unfavorable yellow-brown color, low luminous transmittance and weak solar energy modulation ability.Vanadium oxides are known to have rich polymorphs and devices using thermochromic effect often require absence of impurities, so that stringent process control is crucial for practical manufacturing of VO2 materials. In this work, a novel route for fabrication of VO2 thin films was developed thermodynamically and verified experimentally. Another concern related to VO2-based materials is the relatively short durability of their desirable properties as VO2 is not the most thermodynamic stable species. Hence the lifetimes of the thermochromic VO2 films under various environmental conditions were evaluated. Furthermore, studies have been made to investigate the impacts of substrates on VO2 film growth.For window coating applications, the light scattering is of importance. Therefore, the light scattering effect for particulate VO2 film was studied. Additionally, the low luminous transmittance of VO2 film can be substantially increased by use of a top coating with suitable refractive index. In our study a TiO2 top layer was used, which leads to improved thermochromic behavior. Moreover, incorporation of VO2 plasmonic pigments into a matrix is a useful way to overcome the unsatisfied thermochromic performance of conventional VO2 films. A composite film of VO2-SiO2 was fabricated and its optical properties were studied. Besides, phase-pure VO2 nanospheres were synthesized via chemical route and their thermochromic properties were investigated.In general, these studies promote development and progress of VO2-based material further to be used in heat and light regulation applications.
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| 4. |
- Ahlinder, Linnea, 1987-
(author)
-
Raman Spectroscopy and Hyperspectral Analysis of Living Cells Exposed to Nanoparticles
- 2015
-
Doctoral thesis (other academic/artistic)abstract
- Nanoparticles, i.e. particles with at least one dimension smaller than 100 nm, are present in large quantities in ambient air and can also be found in an increasing amount of consumer products. It is known that many nanomaterials have physicochemical properties that differ from physicochemical properties of the same material in bulk size. It is therefore important to characterize nanoparticles and to evaluate their toxicity. To understand mechanisms behind nanotoxicity, it is important to study the uptake of nanoparticles, and how they are accumulated. For these purposes model studies of cellular uptake are useful. In this thesis metal oxide and carbon-based nanoparticles have been studied in living cells using Raman spectroscopy. Raman spectroscopy is a method that facilitates a non-destructive analysis without using any fluorescent labels, or any other specific sample preparation. It is possible to collect Raman images, i.e. images where each pixel corresponds to a Raman spectrum, and to use the spectral information to detect nanoparticles, and to identify organelles in cells. In this thesis the question whether or not nanoparticles can enter the cell nucleus of lung epithelial cells has been addressed using hyperspectral analysis. It is shown that titanium dioxide nanoparticles and iron oxide nanoparticles are taken up by cells, and also in the cell nucleus. In contrast, graphene oxide nanoparticles are mainly found attached on the outside of the cell membrane and very few nanoparticles are found in the cell, and none have been detected in the nucleus. It is concluded that graphene oxide nanoparticles are not cytotoxic. However, a comparison of Raman spectra of biomolecules in cells exposed to graphene oxide, unexposed cells and apoptotic cells, shows that the graphene oxide nanoparticles do affect lipid and protein structures. In this thesis, several multivariate data analysis methods have been used to analyze Raman spectra and Raman images. In addition, super-resolution algorithms, which originally have been developed to improve the resolution in photographic images, were optimized and applied to Raman images of cells exposed to submicron polystyrene particles in living cells.
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| 5. |
- Lebrun, Delphine, 1985-
(author)
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Fabrication of inverse opal oxide structures for efficient light harvesting
- 2014
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Licentiate thesis (other academic/artistic)abstract
- Artificial opals are self-assembled face centered cubic (fcc) structures of spherically shaped beads, which interesting applications as photonic band gap materials. Inverse opals are photonic crystals consisting of fcc paced voids of a low refractive index material imbedded in a high refractive index material. Such structures has been used to enhance the photocatalytic effect of different materials and motivates further studies to improve the deposition process of the opal templates and their inversion. We state the fabrication method to design and model metal oxide inverse opals. We have successfully created alumina and alumina-titania inverse opals. With the help of simulations, we engineered inverse opals with self-assembly and atomic layer deposition.
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| 6. |
- Mattsson, Andreas, 1976-
(author)
-
Infrared spectroscopy studies of adsorption and photochemistry on TiO2 surfaces : From single crystals to nanostructured materials
- 2018
-
Doctoral thesis (other academic/artistic)abstract
- TiO2 based photocatalysis is a green nanotechnology that can be used for removal of pollutants from water and air, as well as making synthetic fuels from water and carbon dioxide. Said photocatalysis has received major research interests during the last decades. Despite these efforts, many elementary processes that occur on the photocatalyst surface are not fully understood and, therefore, limit our ability to purposefully manufacture more efficient photocatalytic materials. The objective of this thesis is to provide new understanding at a molecular level of important adsorbate species on the TiO2 surfaces.Fundamental properties of adsorption and photochemistry of primarily formic acid on different TiO2 surfaces, ranging from single crystals to nanoparticles, have been studied using infrared spectroscopy. A method to simulate IR spectra have been developed and, combined with experimental data, has been proven to be a powerful tool to identify different adsorbate geometries on the surface. In the presence of oxygen, a thermally activated and irreversible reaction between formate and oxygen adatoms takes place on the single crystal rutile (110) surface to yield hydrogen bicarbonate surface complexes. For disordered single crystal surfaces, the adsorption geometry of formate changes due to exposure of Ti3+ atoms on the surface, and the adsorption spectra shows resemblances with that observed for formate adsorption on nanocrystalline surfaces.Illumination with UV light results in small changes of the formate coverage on the disordered single crystal and nanocrystalline rutile surfaces, whereas on the rutile (110) surface only miniscule changes in formate coverages are seen. This is due to the lack of oxygen electron acceptors and OH/H2O electron donors in the vacuum environment, which results in a much lower degradation rate compared to measurements made at ambient conditions. Furthermore, it is shown that the coordination of the formate molecule on various TiO2 surfaces has a profound effect on the photocatalytic degradation rate, with bidentate coordinated formate molecules being most resilient towards oxidation.The results presented here shows that additional insight in the processes on the TiO2 photocatalyst surface can be obtained by combining spectroscopic studies of single crystals and nanocrystalline films and that it is possible to unravel adsorption geometries on surfaces by combining experimental and simulated IR spectra.
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| 7. |
- Skoglund, Sara, 1987-
(author)
-
Surface reactivity of metal nanoparticles : - importance of surface active agents and biomolecules from a transformation, mobility and toxicity perspective
- 2017
-
Doctoral thesis (other academic/artistic)abstract
- Metallic nanoparticles possess unique properties due to their size and are widely used in e.g. consumer products. From this follows a need to identify and assess potential risks of human and environmental exposure. Their size facilitates uptake in organisms and disruption of various biological processes. Together with a high reactivity, mainly due to their large surface area in solution, they are both commonly used in different applications and of a potential safety concern. Risk assessment requires hence in-depth knowledge on the particle characteristics and their behavior in solution but also how these properties change with time and exposure conditions and whether these characteristics can be linked to toxicity following nanoparticle exposure. This thesis addresses these aspects with a main focus on metal nanoparticles and elaborates on the importance of characterization, how such measurements can be done, and on interactions with surfactants and biomolecules and toxic effects.Silver nanoparticles are, due to their antibacterial properties, often used in sportswear to prevent sweat odor. During laundry they may be dispersed and interact with surfactants of the washing powder, influencing their properties and stability in solution. These aspects are addressed in Papers I, III and V on silver nanoparticles of different size and surface coatings. The stability was shown to depend on the surface charge and the concentration of the surfactant. The stability and extent of silver release were reduced upon sequential exposure, indicating the importance of the particle history on their bioaccessibility, mobility and potential toxicity. A mechanism was proposed for how silver nanoparticles are stabilized in surfactant solutions.Toxic effects of silver nanoparticles of different size and coatings on cultivated lung cells, Paper II, and effects of copper-containing nanoparticles on different blood cells, Paper IV, were studied in vitro. The smallest particles were most cytotoxic and the “Trojan horse” mechanism played an important role, meaning that the nanoparticles facilitate cellular uptake followed by ion-release.Difficulties in the determination and interpretation of the zeta potential, related to the surface charge, of metal nanoparticles in complex solutions are elucidated in Paper VI. Guidelines are provided on how to accurately assess this property.
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| 8. |
- Österlund, Catharina, 1965-
(author)
-
Extra- and intrafusal muscle fibre type compositions of the human masseter at young age. : In perspective of growth and functional maturation of the jaw-face motor system.
- 2011
-
Doctoral thesis (other academic/artistic)abstract
- Muscles control body posture and movement by extrafusal and intrafusal (muscle spindle) fibres. The purpose of this thesis was to provide insight into the muscular basis for human jaw function at young age. Extrafusal and intrafusal fibres in the young masseter, and for comparison young biceps, were examined for composition of fibre types and myosin heavy chain (MyHC) isoforms by means of morphological, enzyme-histochemical, biochemical and immuno-histochemical techniques. For evaluation of plasticity during life span the data for young muscles were compared with previous reported data for adult and elderly muscles.The results showed significant differences in extrafusal fibre types and MyHC expression between young masseter and young biceps and between young masseter and masseter in adults and elderly. Compared with young biceps, young masseter was more intricate in composition of extrafusal MyHC expression. Muscle spindles were larger and more frequent in the masseter than in the biceps. Masseter and biceps muscle spindles showed fundamental similarities but also marked differences in MyHC expression.The results suggest that the young masseter is specialized in fibre types already at young age and shows a unique fibre type growth pattern. Whereas masseter extrafusal fibres display marked plasticity in fibre types and MyHC isoforms during life span muscle spindles/intrafusal fibres are morphologically mature already at young age and precede extrafusal fibres in growth and maturation. Results showed similarities in intrafusal MyHC expression between young masseter and biceps, but also differences implying muscle specific proprioceptive control. Differences in fibre types and MyHC expression between young masseter and young biceps extrafusal fibres are proposed to reflect diverse evolutionary and developmental origins and accord with the masseter and biceps being separate allotypes of muscle.
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| 9. |
- Cindemir, Umut
(author)
-
Thin films for indoor air monitoring : Measurements of Volatile Organic Compounds
- 2016
-
Doctoral thesis (other academic/artistic)abstract
- Volatile organic compounds (VOCs) in the indoor air have adverse effects on the dwellers residing in a building or a vehicle. One of these effects is called sick building syndrome (SBS). SBS refers to situations in which the users of a building develop acute health effects and discomfort depending on the time they spend inside some buildings without having any specific illness. Furthermore, monitoring volatile organic compounds could lead to early diagnosis of specific illnesses through breath analysis. Among those VOCs formaldehyde, acetaldehyde can be listed.In this thesis, VOC detecting thin film sensors have been investigated. Such sensors have been manufactured using semiconducting metal oxides, ligand activated gold nanoparticles and Graphene/TiO2 mixtures. Advanced gas deposition unit, have been used to produce NiO thin films and Au nanoparticles. DC magnetron sputtering has been used to produce InSnO and VO2 thin film sensors. Graphene/TiO2 sensors have been manufactured using doctor-blading.While presenting the results, first, material characterization details are presented for each sensor, then, gas sensing results are presented. Morphologies, crystalline structures and chemical properties have been analyzed using scanning electron microscopy, X-ray diffraction and X-ray photo electron spectroscopy. Furthermore, more detailed analyses have been performed on NiO samples using extended X-ray absorption fine structure method and N2 adsorption measurements. Gas sensing measurements were focused on monitoring formaldehyde and acetaldehyde. However, responses ethanol and methane were measured in some cases to monitor selectivity. Graphene/TiO2 samples were used to monitor NO2 and NH3. For NiO thin film sensors and Au nano particles, fluctuation enhanced gas sensing is also presented in addition to conductometric measurements.
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| 10. |
- Johansson, Wilhelm, et al.
(author)
-
Transparent TiO2 and ZnO Thin Films on Glass for UV Protection of PV Modules
- 2020
-
In: American Ceramic Society Bulletin. - : American Ceramic Society Inc.. - 0002-7812 .- 1945-2705. ; 99:4, s. 26-29
-
Journal article (other academic/artistic)abstract
- To stabilize the global temperature and mitigate climate change, the emission of anthropogenic greenhouse gases will have to be greatly reduced. To make it possible, the energy sector will have to transfer from fossil energy to environmentally friendly and carbon neutral sources.
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| 11. |
- Karlsson, Stefan, 1984-, et al.
(author)
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Effects of deposition temperature on the mechanical and structural properties of amorphous Al-Si-O thin films prepared by radio frequency magnetron sputtering
- 2023
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In: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 787
-
Journal article (peer-reviewed)abstract
- Aluminosilicate (Al–Si–O) thin films containing up to 31 at.% Al and 23 at.% Si were prepared by reactive RF magnetron co-sputtering. Mechanical and structural properties were measured by indentation and specular reflectance infrared spectroscopy at varying Si sputtering target power and substrate temperature in the range 100 to 500 °C. It was found that an increased substrate temperature and Al/Si ratio give denser structure and consequently higher hardness (7.4 to 9.5 GPa) and higher reduced elastic modulus (85 to 93 GPa) while at the same time lower crack resistance (2.6 to 0.9 N). The intensity of the infrared Si-O-Si/Al asymmetric stretching vibrations shows a linear dependence with respect to Al concentration. The Al–O–Al vibrational band (at 1050 cm−1) shifts towards higher wavenumbers with increasing Al concentration which indicates a decrease of the bond length, evidencing denser structure and higher residual stress, which is supported by the increased hardness. The same Al–O–Al vibrational band (at 1050 cm−1) shifts towards lower wavenumber with increasing substrate temperature indicating an increase in the average coordination number of Al.
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| 12. |
- Langhammer, David, 1991-, et al.
(author)
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Bonding of CO2 to TiO2 : Chemical Activation During Artificial Photosynthesis
-
Other publication (other academic/artistic)abstract
- There is a growing interest to develop materials for artificial photosynthesis of fuels by CO2 reduction. Titanium dioxide (TiO2) is frequently used as a model system to study photosynthetic heterogenous catalytic reactions due to its well-characterized properties and wide-spread use within the field of photocatalysis. It is ideal for use in industrial applications due to its large abundance and low cost, although the wide band gap of this material has limited its use in solar-driven technologies. Apart from being used as a pigment in white paint, it is primarily used in industry as a photocatalyst for the degradation of surface contaminants and air pollutants, both of which appear in low concentrations. Despite this, there is great hope that the properties of TiO2 will be developed to enable large scale deployment in technological applications. Artificial photosynthesis is considered by many to be an attractive application of large-scale photochemistry, and TiO2 has been studied extensively for this purpose. The most crucial step in the process of reducing CO2 is the activation of the stable CO2 molecule through chemical bonding. In this article, the interaction between CO2 and various surfaces of TiO2 is investigated to evaluate its catalytic properties. The usefulness of TiO2 for CO2 reduction is critically discussed based on its photocatalytic ability and on previously reported efficiencies during different types of photochemical reactions.
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| 13. |
- Montero Amenedo, José, 1983-, et al.
(author)
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Photobleaching of dyes by CuOx-based heterojunction bi-catalysts
- 2019
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Conference paper (other academic/artistic)abstract
- We present the synthesis and characterization of thin film heterojunction, or bi-catalyst, comprising of photocatalytic coatings based on a combination of two different materials exhibiting p-type and n-type conductivity, respectively. Here we show results for the combination of CuOx together with zinc oxide (ZnO) and tungsten oxide (WO3). The purpose of such compounds is to create an intrinsic electrical field at the np-junction that helps to separate electron-hole pairs formed upon interband photon absorption. At the same time desired photochemical properties implies that the constituent catalysts have appropriate bandgap and band edge position. For this purpose, CuOx/WO3 compound films have been prepared onto glass substrates by co-sputtering of tungsten and copper targets in an argon and oxygen atmosphere. CuOx/ZnO bi-catalyst have also been prepared on glass substrates by a two-step process consisting of deposition of CuOx by reactive magnetron sputtering, followed by the ap plication of ZnO particles by drop coating. The obtained bi-catalyst have been characterized by SEM, XRD, XPS, Raman and spectrophotometry. Finally, the photocatalytic activity of the different compound films is assessed by studying their photobleaching rate of methylene blue and orange II in water solution.
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| 14. |
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| 15. |
- Sorar, Idris, et al.
(author)
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Electrochromism in Ni Oxide Thin Films Made by Advanced Gas Deposition and Sputtering : A Comparative Study Demonstrating the Significance of Surface Effects
- 2020
-
In: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 167:11
-
Journal article (peer-reviewed)abstract
- Films of electrochromic Ni oxide, with thickness in the ∼100–1000-nm range, were prepared by reactive advanced gas deposition (AGD) and, for comparison, also by reactive DC magnetron sputtering (MS). Voltammetric cycling was performed in an electrolyte of lithium perchlorate in propylene carbonate in the voltage range 2.0–4.1 V vs Li/Li+, and concurrent optical transmittance modulation was studied at a wavelength of 530 nm. For the thickest films, the optical modulation range was as large as ∼54% and the maximum transmittance was ∼82% for deposition by AGD, whereas the corresponding numbers were ∼45% and ∼87% for MS. Rutherford backscattering spectrometry together with measurements of film thickness demonstrated that the porosity of 400–500-nm-thick films made by AGD and MS were ∼84% and ∼45%, respectively. The charge exchange per mass unit was as high as 48–67 C g–1 for samples made by AGD. The corresponding number was much lower for MS, namely 13–18 C g–1. These results lend convincing support to the view that the electrochromism of Ni-oxide-based films in Li-ion-conducting electrolytes is dominated by surface effects.
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| 16. |
- Thyr, Jakob, 1979-, et al.
(author)
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Energy Alignment of Quantum-Confined ZnO Particles with Copper Oxides for Heterojunctions with Improved Photocatalytic Performance
- 2022
-
In: ACS Nanoscience Au. - : American Chemical Society (ACS). - 2694-2496. ; 2:2, s. 128-139
-
Journal article (peer-reviewed)abstract
- The ability to control electronic states by utilizing quantum confinement of one of the material components in heterojunctions is a promising approach to perform energy-level matching. In this work, we report the possibility to achieve optimum energy alignment in heterojunctions made from size-controlled quantum dots (Q-dots) of ZnO in combination with three copper oxides: Cu2O, Cu4O3, and CuO. Quantum confinement effects on the ZnO nanoparticles in the diameter range 2.6–7.4 nm showed that the direct optical band gap decreased from 3.99 to 3.41 eV, with a dominating shift occurring in the conduction band (CB) edge, and thus the possibility to obtain close to 0.6 eV CB edge shift by controlling the size of ZnO. The effect was utilized to align the electronic bands in the ZnO Q-dot/copper oxide heterojunctions to allow for charge transfer between the materials and to test the ability to improve the photocatalytic performance for the system, evaluated by the transformation of a dye molecule in water. The catalyst materials were investigated by X-ray diffraction, scanning electron microscopy, ultraviolet–visible (UV–vis), photoluminescence, and Raman spectroscopy. The most promising material combination was found to be the Cu2O copper oxide in combination with an energy aligned ZnO Q-dot system with approximately 7 nm diameter, showing strong synergy effects in good agreement with the energy-level analysis, outperforming the added effect of its individual components, ZnO-Q-dots and Cu2O, by about 140%. The results show that utilization of a heterojunction with controllable energy alignment can provide a drastically improved photocatalytic performance. Apart from increased photocatalytic activity, specific surface states of ZnO are quenched when the heterojunction is created. It is anticipated that the same approach can be utilized in several material combinations with the added benefit of a system with controllable overpotential and thus added specificity for the targeted reduction reaction.
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| 17. |
- Thyr, Jakob, 1979-, et al.
(author)
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Photocatalytic properties and polarized Raman of different ZnO crystal planes
- 2019
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Conference paper (other academic/artistic)abstract
- Zinc oxide (ZnO) is a well-studied wide band gap semiconductor photocatalyst. The activities of ZnO nanomaterials with different ratios of exposed crystal planes are however less understood. In this work, three different ZnO single crystals exposing different crystal planes were studied: (0001), (1-100), and (11-20). The ZnO samples were characterized with polarized Raman spectroscopy and XRD, and their photocatalytic activities were quantified by means of methylene blue degradation using in situ spectrophotometry. The ZnO (1-100) surface showed three times higher photocatalytic activity than the other two surfaces. The results are discussed in terms of crystal facet dependent reactivity due to differences in surface structure and surface potential. Since it is possible to synthesize ZnO particles and structures with different ratios of exposed crystal planes, this finding may be of importance to guide synthesis of more efficient, tailor-made ZnO photocatalysts for water cleaning.
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| 18. |
- Thyr, Jakob, 1979-, et al.
(author)
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Polarized and non-polarized Raman spectroscopy of ZnO crystals : Method for determination of crystal growth and crystal plane orientation for nanomaterials
- 2021
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In: Journal of Raman Spectroscopy. - : John Wiley & Sons. - 0377-0486 .- 1097-4555. ; 52:8, s. 1395-1405
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Journal article (peer-reviewed)abstract
- Analysis and determination of crystal orientation and exposed surface facets remain a challenge in nanomaterial science. In this work we show that polarized and non-polarized Raman spectroscopy can be useful tools to determine crystal plane orientation and conveniently be applied to spatial dimensions limited only by the diffraction limit of the excitation laser. The methodology is exemplified for wurtzite structured ZnO. Three different crystal facets, (0001), (1-100), and (11-20) of ZnO are investigated with angle resolved polarized Raman spectroscopy. The polarization direction dependences of the main Raman peaks are characterized and related to the experimental vibrational modes in the crystal lattice and corroborated by density functional theory (DFT) calculations using two different hybrid functionals. By exploiting the symmetry of the modes and differences in Raman intensity of the optically activated phonons, a simple model is derived for determining the relation between the polar and non-polar crystal orientation. The results are generalized to allow peak intensity ratio analysis using Raman spectroscopy with a non-polarized light source, making it compatible with Raman mapping, as well as to include a critical discussion on the ability to determine the crystal plane orientation and exposed crystal facets using this model for nano dimensional ZnO and equivalent models for other nanomaterials. As the approach allows for use of non-polarized light sources, near-field excitations and local plasmons can in an extension be utilized for determination of crystal orientation and exposed planes in dimensions much smaller than the diffraction limit.
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| 19. |
- Topalian, Zareh, 1973-
(author)
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Nanostructured Transition Metal Oxides in Cleantech Application : Gas Sensors, Photocatalysis, Self-cleaning Surfaces Based on TiO2, WO3 and NiO
- 2011
-
Doctoral thesis (other academic/artistic)abstract
- This thesis focuses on the application of nanocrystalline transition metal oxide TiO2, WO3 and NiO thin films in new “green” building technologies. Specifically, their physicochemical properties in photocatalytic, self-cleaning and gas sensing applications are studied. There is an intimate connection between comfort issues, health, with connections to energy efficiency, leading to a need for intelligent building materials and green architecture. The importance of good indoor environment is augmented by the fact that modern man in developed countries spends some 90 % of his time inside buildings and vehicles. Poor air quality may lead to discomfort of the person inhabiting a building and in ultimately cause adverse health effects. Thin films of nanocrystalline TiO2 were prepared using reactive DC magnetron sputtering. Crystalline mesoporous films of WO3 and NiO were prepared using advanced gas deposition technique (AGD). The crystal structure, morphology, optical and chemical properties of the films were characterized by using grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), UV/Vis spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic properties and adsorption of both organic and inorganic molecules on pure and functionalized films were probed by in situ Fourier transform infrared spectroscopy (FTIR). The gas sensing properties of sensors based on TiO2, WO3 and NiO were investigated by conductivity measurements and noise spectroscopy. It was found for the first time that NiO based thin film sensors can be used to detect H2S and NO2 at low temperatures – down to room temperature. Hybrid WO3 sensors functionalized with multiwalled carbon nanotubes (MWCNTs) were used to detect NO2, CO and NH3 gases. These hybrid gas sensors show improved recovery properties compared to unmodified WO3 sensors. TiO2 based gas sensors were able to detect low concentrations of H2S by noise spectroscopy provided that the sensors were irradiated by UV light. Furthermore we show that sulphur is photo-fixated in crystalline TiO2 films upon simultaneous SO2 gas exposure and UV irradiation. Studies of the kinetics and identity of the photo-fixated sulphur complexes show that these are formed by photo-induced reactions between oxygen and SO2 at oxygen surface vacancy sites in TiO2. The sulphur modified TiO2 films show interesting self-cleaning properties compared to the pure films.
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| 20. |
- Österlund, Lars, 1967-, et al.
(author)
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Spectrally selective nanocoatings with synergistically enhanced photocatalytic and solar light modulation properties
- 2019
-
Conference paper (other academic/artistic)abstract
- Spectrally selective nanocoatings that exhibit synergistically enhanced solar light modulation, luminous transmittance and catalytic properties can be made by combining dielectric film stacks with complementary optical and structural properties. Here we show two case studies:1)TiO2/VO2 luminous transparent bilayers that exhibits enhanced near-infrared light absorption and heats the TiO2 film by up to ~ 30°C resulting in ~ 2-fold increase of the photocatalytic reaction rate. The TiO2/VO2 bilayer stack exhibits anti-reflective properties, and enhanced solar light modulation (∼ 9%) compared to VO2, and ∼ 20 times% increased solar absorptance compared to TiO2. In addition the TiO2 chemically protects the VO2 layer avoiding oxidation to vanadium pentoxide. 2)TiO2/TiAlN solar absorber bilayers that yield an almost ~ 10-fold enhancement of the quantum yield for acetaldehyde removal (on par with state-of-the-art, heterojunction photocatalysts), and an associated temperature rise ~120 °C.
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