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- Ahmed, Taha, 1984-
(författare)
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Nanostructured ZnO and metal chalcogenide films for solar photocatalysis
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increasing demand for clean energy and safe water resources has driven the development of efficient and sustainable technologies. Among these technologies, photocatalysis using semiconducting materials has emerged as a promising solution for both solar hydrogen generation and water purification. Low-dimensional ZnO, including nanorods, nanoparticles, and quantum confined particles (so called quantum dots), has demonstrated excellent photocatalytic properties due to their large surface area, high electron mobility, and tunable band gap.The work in this thesis aims to investigate the potential of low-dimensional ZnO alone and in combination with CdS and Fe2O3 for solar hydrogen generation and photocatalytic water purification. The thesis includes a comprehensive analysis of the synthesis, characterization, and optimization of low-dimensional ZnO-based photocatalyst systems for solar hydrogen generation and photocatalytic water purification. Additionally, the thesis will evaluate the performance of the ZnO-based photocatalysts under different experimental conditions, either as photoelectrodes or as distributed particle systems for water purification. The work includes detailed size control of ZnO by itself in dimensions below 10 nm using a hydrothermal method, to provide an increased total surface area and introduce quantum confinement effects that increase the band gap to enable degradation of chemical bonds in a model pollutant in a distributed system for water purification. The work also includes a relatively detailed study of the phonon–phonon and electron–phonon coupling as a function of dimension from 10 nm to 150 nm for ZnO using non-resonant and resonant Raman spectroscopy. Ultimately, the thesis aims to provide insight into the potential of low-dimensional ZnO alone and in combination with other inorganic materials for solar hydrogen generation and photocatalytic water purification and pave the way for the development of efficient and sustainable technologies for clean energy and safe water resources.
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| 2. |
- Johansson, Fredrik
(författare)
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Core-hole Clock Spectroscopy Using Hard X-rays : Exciting States in Condensed Matter
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is about how electrons move from one place to another, that is charge transfer dynamics. Charge transfer dynamics is an important property governing chemical and physical changes that form the base for many applications such as electronics, optoelectronics and catalysis. The fundamental aspect is how charge transfer manifests in the constituent materials and their interfaces building up these devices. The basic method used is synchrotron radiation based electron spectroscopies.Using core-hole clock spectroscopy it is possible to study dynamic processes in the femtosecond and attosecond regimes - here we study the if the core-excited electron decays back into the core hole (local decays), or if the core excited electron have been tunneled away from the atomic site before the core-hole decays. Spectroscopically we can discern the two situations since one of the processes is photon energy dependent and one is not. Knowledge of the life-time of the core hole, and measuring the probability of the core-excited system decaying one way or the other makes it possible to calculate a charge transfer time. Using hard X-rays to create excited state with deep core-holes allow us to study high kinetic energy Auger electrons, also deep core-holes tend to be short lived, which gives access to short time-scales.Bulk crystals of 2D materials have been used as model systems here owing to their well-known properties. Using those it has been demonstrated that the regime of observable times using the mentioned method can be extended with an order of magnitude compared to previous studies. Our results present themselves on time-scales on par with the atomic unit of time. The highly selective nature of resonant X-ray excitations allows the anisotropic unoccupied electronic structure of bulk 2D crystals to be mapped out, here the example of SnS2 is presented. This shows that this is a direct probe of the unoccupied band structure.With core-hole clock spectroscopy the charge transfer time dependence on relative concentrations of blends between the low band-gap polymer PCPDTBT, with PCBM (functionalized fullerenes). This is a common prototypical system for organic photovoltaics. The charge transfer time decreases with increasing intermixing, up to a point where is starts getting slower, the same trend as the efficiency of solar cell devices made with the same mixing. The method employed here is chemically specific and probes the local surrounding energy landscape at the site of excitation – this is different from other techniques that utilize optical excitations which are non-local in character.The synthetization of bulk heterostructures and thin films, and the disentanglement of core-ionized states are also investigated using spectroscopic and scattering techniques.
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| 3. |
- Langhammer, David Michael, 1991-
(författare)
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Capturing Air Pollutants : Photochemical Adsorption and Degradation of SO2, NO2 and CO2 on Titanium Dioxide
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)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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| 4. |
- Thyr, Jakob, 1979-
(författare)
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Low dimensional Zinc- and Copper Oxides and their Electronic, Vibrational and Photocatalytic Properties
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Pollution of water resources is a growing problem in the world and this has drawn the attention to photocatalysis, which is an emerging technology for water purification. In this thesis, low dimensional zinc oxide and copper oxides, which are promising photocatalytic materials, have been studied. In the initial work, an approach for determining the crystal orientation in ZnO nanomaterials was developed based on polarized Raman spectroscopy. The approach was extended to non-polarized Raman spectroscopy for convenient crystal orientation determination. The results were corroborated by density functional theory (DFT) calculations providing a full vibrational mode analysis of ZnO, including higher-order Raman scattering. Photocatalyst materials based on both ZnO and copper oxides were synthesized, starting with visible light absorbing Cu2O prepared by low temperature thermal oxidation of flat and 3D structured Cu-foils. Defect induced Raman scattering revealed Raman activity in modes that are only IR active or optically silent in pristine Cu2O, with mode assignments supported by DFT calculations. Experiment with solar light illuminated Cu2O showed efficient degradation of organic water-soluble molecules and degradation rates could be further increased by 3D structuring into nanopillars. With the aim of creating a combined photocatalyst that use favourable properties from several materials, nanoparticles of ZnO were synthesized and deposited onto Cu2O, Cu4O3 and CuO. ZnO of sufficiently small size exhibit quantum confinement, which allowed for tuning of the electronic and optical properties of ZnO and this was utilized for energy level alignment in heterojunctions with copper oxides. The heterojunctions were shown to facilitate charge transfer which improved the photocatalytic properties of the dual catalysts compared to the single components. The quantum confinement effects in ZnO nanoparticles were further investigated by more detailed electrochemical measurements. The main finding was that quantum confinement results in a large decrease in the available electronic density of states which has clear implications on the capacitance and photon absorption in the material. Raman spectroscopy has been a central tool in all work, and the thesis ends with a study that goes through and explain spurious Raman signals. The contribution shows how to identify and avoid spectral artefacts and other light generating processes that compete with the Raman signal and guide the acquisition of good quality spectra.
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