| 1. |
- Fondell, Mattis, 1984-
(författare)
-
Synthesis and Characterisation of Ultra Thin Film Oxides for Energy Applications
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes studies of materials which can be exploited for hydrogen production from water and sunlight. The materials investigated are maghemite (γ-Fe2O3), magnetite (Fe3O4) and especially hematite (α-Fe2O3), which is an iron oxide with most promising properties in this field. Hematite has been deposited using Atomic Layer Deposition (ALD) - a thin-film technique facilitating layer-by-layer growth with excellent thickness control and step coverage. The iron oxides were deposited using bis-cyclopentadienyl iron (Fe(Cp)2) or iron pentacarbonyl (Fe(CO)5) in combination with an O2 precursor. Since it is crucial to have good control of the deposition process, the influence of substrate, process temperature, precursor and carrier gas have been investigated systematically. By careful control of these deposition parameters, three polymorphs of iron oxide could be deposited: hematite (α-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4).The deposited materials were characterized using X-ray Diffraction, Raman and UV-VIS Spectroscopy, and Scanning Electron Microscopy. Hard X-ray Photoelectron Spectroscopy (HAXPES) was also used, since it is a non-destructive, chemically specific, surface sensitive technique – the surface sensitivity resulting from the short mean escape depth of the photoelectrons. The depth probed can be controlled by varying the excitation energy; higher photoelectron energies increasing the inelastic mean-free-path in the material.HAXPES studies of atomic diffusion from F-doped SnO2 substrates showed increased doping levels of Sn, Si and F in the deposited films. Diffusion from the substrate was detected at annealing temperatures between 550 °C and 800 °C. Films annealed in air exhibited improved photocatalytic behavior; a photocurrent of 0.23 mA/cm2 was observed for those films, while the as-deposited hematite films showed no photo-activity whatsoever.The optical properties of low-dimensional hematite were studied in a series of ultra-thin films (thicknesses in the 2-70 nm range). The absorption maxima were shifted to higher energies for films thinner than 20 nm, revealing a different electronic structure in thin films.
|
|
| 2. |
- Johansson, Fredrik
(författare)
-
Core-hole Clock Spectroscopy Using Hard X-rays : Exciting States in Condensed Matter
- 2020
-
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.
|
|
| 3. |
- Ahmed, Taha, 1984-
(författare)
-
Nanostructured ZnO and metal chalcogenide films for solar photocatalysis
- 2023
-
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.
|
|
| 4. |
- Svengren, Henrik, 1974-
(författare)
-
Water splitting by heterogeneous catalysis
- 2017
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A sustainable solution for meeting the energy demands at our planet is by utilizing wind-, solar-, wave-, thermal-, biomass- and hydroelectric power. These renewable and CO2 emission-free energy sources are highly variable in terms of spatial and temporal availability over the Earth, introducing the need for an appropriate method of storing and carrying energy. Hydrogen has gained significant attention as an energy storage- and carrier media because of the high energy density that is exploited within the ‘power-to-gas’ process chain. A robust way of producing sustainable hydrogen is via electrochemical water splitting.In this work the search for new heterogeneous catalyst materials with the aim of increasing energy efficiency in water splitting has involved methods of both electrochemical water splitting and chemical water oxidation. Some 21 compounds including metal- oxides, oxofluorides, oxochlorides, hydroxide and metals have been evaluated as catalysts. Two of these were synthesized directly onto conductive backbones by hydrothermal methods. Dedicated electrochemical cells were constructed for appropriate analysis of reactions, with one cell simulating an upscale unit accounting for realistic large scale applications; in this cell gaseous products are quantified by use of mass spectrometry. Parameters such as real time faradaic efficiency, production of H2 and O2 in relation to power input or overpotentials, Tafel slopes, exchange current density and electrochemical active surface area as well as turnover numbers and turnover frequencies have been evaluated.Solubility, possible side reactions, the role of the oxidation state of catalytically active elements and the nature of the outermost active surface layer of the catalyst are discussed. It was concluded that metal oxides are less efficient than metal based catalysts, both in terms of energy efficiency and in terms of electrode preparation methods intended for long time operation. The most efficient material was Ni-Fe hydroxide electrodeposited onto Ni metal foam as conductive backbone. Among the other catalysts, Co3Sb4O6F6 was of particular interest because the compound incorporate a metalloid (Sb) and redox inert F and yet show pronounced catalytic performance.In addition, performance of materials in water splitting catalysis has been discussed on the basis of results from electron microscopy, solubility experiments and X-ray diffraction data.
|
|
