Abstract: Even at low temperatures, metal nanoparticles (NPs) possess atomic dynamics that are key for their properties but challenging to elucidate. Recent experimental advances allow obtaining atomic‐resolution snapshots of the NPs in realistic regimes, but data acquisition limitations hinder the experimental reconstruction of the atomic dynamics present within them. Molecular simulations have the advantage that these allow directly tracking the motion of atoms over time. However, these typically start from ideal/perfect NP structures and, suffering from sampling limits, provide results that are often dependent on the initial/putative structure and remain purely indicative. Here, by combining state‐of‐the‐art experimental and computational approaches, how it is possible to tackle the limitations of both approaches and resolve the atomistic dynamics present in metal NPs in realistic conditions is demonstrated. Annular dark‐field scanning transmission electron microscopy enables the acquisition of ten high‐resolution images of an Au NP at intervals of 0.6 s. These are used to reconstruct atomistic 3D models of the real NP used to run ten independent molecular dynamics simulations. Machine learning analyses of the simulation trajectories allows resolving the real‐time atomic dynamics present within the NP. This provides a robust combined experimental/computational approach to characterize the structural dynamics of metal NPs in realistic conditions.

Abstract: In the context of global challenges such as climate change and environmental pollution, photocatalysis evolved as one of the promising strategies for sustainable energy conversion and pollutant degradation. In this thesis, photocatalysis using gas phase deposited bimetallic nanoclusters (BNCs) on TiO2 supports is studied in the context of self-cleaning surfaces and photoelectrochemical (PEC) water splitting applications. Thanks to their plasmonic properties, BNCs made of coinage metals can serve as efficient cocatalysts for the degradation of organic pollutants and surface contaminants under light irradiation. They also hold great promise for PEC water splitting, a promising pathway for renewable hydrogen production, which can be used in hydrogen fuel cells or for the environmentally friendly production of fuels in, for example, CO2 hydrogenation processes. The small size and high surface-to-volume ratio of plasmonic BNCs play pivotal roles in influencing the efficiency and selectivity of photocatalytic processes. BNCs have unique optical, physical, chemical, and structural properties distinctly different from their bulk and monometallic counterparts. These properties can be fine-tuned at the single particle level by their size, composition, and atomic arrangement, but also by interaction with other particles through the coverage and through interaction with the support. To design better photocatalysts it is crucial to carefully understand the BNCs’ characteristic properties, especially at the atomic level where synergies between different elements are sought. To achieve this objective, BNCs with well-defined sizes and compositions are deposited on TiO2 supports and we studied their structural properties and their influence on the photocatalytic activity. The general procedure followed in this thesis is the production and deposition of BNCs on TiO2 by the cluster beam deposition (CBD) technique, followed by structural and optical characterization to understand their tailored properties, and photocatalytic testing either for photodecomposition of organic molecules or PEC water splitting. In a first study, AuxAg1-x (x = 1, 0.9, 0.7, 0.5, 0.3, and 0) alloy BNCs with different compositions are synthesized in the gas phase and deposited from a molecular beam on TiO2 P25 supports. The photocatalytic self-cleaning activity of as-prepared samples is tested under UV and visible light towards stearic acid (SA) degradation. SA is a widely accepted model contaminant, which represents the group of organic fouling compounds that typically contaminates glass surfaces. A composition-dependent activity is observed with the Au0.3Ag0.7 nanocluster modified TiO2 exhibiting the highest photoactivity. Scanning transmission electron microscopy (STEM) measurements reveal that, for a mass loading corresponding to an equivalent of 4 atomic monolayers (MLs), the BNCs are uniformly distributed over the surface. The clusters have an average size of 3.5 ± 0.5 nm and are crystalline in nature. The atomic structure is characterized by X-ray absorption fine structure (XAFS) spectroscopy and their electronic structure by X-ray photoelectron spectroscopy (XPS). These measurements demonstrate a charge redistribution between the Ag and Au atoms when alloyed at the nanoscale. The effect of this charge redistribution is likely the stabilization of Ag against oxidation and directly affects the catalytic properties of the clusters. It is suggested that the highest photoactivity of 4 ML loaded Au0.3Ag0.7 under solar light results from a combination of four main possible contributing factors: (i) injection in TiO2 of excited carriers that are generated by the localized surface plasmon resonance (LSPR) effect of the BNCs in the visible light wavelength range which overlaps with the sun’s irradiance spectrum. (ii) a strong near-field enhancement that increases the photoabsorption by the TiO2 for photons that have enough energy to overcome the high bandgap, (iii) the optimized total metal loading of 4 ML leaves enough of the TiO2 surface accessible for light absorption, and finally (iv) an effective charge distribution between Au and Ag. This study demonstrates that CBD is an efficient approach for fabricating well-defined, tunable AuAg plasmon-based photocatalysts for self-cleaning applications, outperforming their monometallic counterparts as well as bimetallic alternatives obtained through colloidal methods. In a second study, titania nanotubes (TNTs) are modified with a series of AuxCu1-x (x = 1, 0.75, 0.5, 0.25, and 0) BNCs using the CBD technique. Based on the results of the first study, we opted again for a loading of 4 ML. TNTs are known for their high surface area, fast charge transfer, and corrosion resistance, while keeping the inherent strengths of traditional TiO2 materials. They prove to be promising photoanodes, enhancing photocurrent in PEC applications for water oxidation. In this work the TNTs are grown via anodic oxidation of a titanium metal foil. The crystalline anatase phase of the grown TNTs is confirmed by the X-ray diffraction technique (XRD), while transmission electron microscopy (TEM) provides information about the size and composition of the deposited BNCs. XAFS provides further structural information, while XPS measurements reveal charge redistribution between Au and Cu, which can aid in the enhancement of the PEC activity. Oxidation of as-prepared electrodes over the time results in structural changes with CuxO at the outer shell functioning as a protective layer, while the majority of the core is an alloy. The optical properties, studied through UV-Vis spectroscopy confirm the extended absorption range of the cluster-modified TNTs towards the visible region. The charge carrier recombination rate is derived from photoluminescence (PL) measurements. The as-prepared electrodes are tested photoelectrochemically for the generation of an anodic photocurrent using simulated sunlight. It is found that the AuxCu1-x (x = 1, 0.75, 0.5, 0.25 and 0) BNC modified TNTs show a remarkable enhancement in the anodic photocurrent relative to pristine TNTs, with Au0.25Cu0.75 exhibiting the highest photocurrent. This is due to the combination of many possible factors. Firstly, the charge redistribution between Au and Cu and increase stability of the Au0.25Cu0.75 electrode as observed in XAFS, indicates that the electronic effect in the cluster is also one of the governing factors for PEC activity. Secondly, formation of a surface CuOx layer, protects against further corrosion of the metallic AuCu BNCs cores. Third, reduced recombination of charge carriers is indicated by lower photoluminescent (PL) intensity compared to pristine TNTs and all other electrodes except pure gold, as observed in PL spectra. This implies that the generated charge carriers are efficiently separated by Au0.25Cu0.75 NCs acting as electron sinks and easily available for redox reactions. Fourth, the highest interfacial charge transfer efficiency is evidenced by the electrochemical impedance spectroscopy (EIS), leading to more efficacious charge migration and separation, facilitating the water oxidation surface reaction. A final beneficial factor is the uniform deposition of well-defined, size- and composition-controlled, ligand-free BNCs. Such BNCs provide more effective surface sites to the reaction medium, in contrast to electrodes synthesized by e.g. sol-gel methods, where (in)organic residues on metal surfaces may decrease the efficiency.

Abstract: In recent decades, as crop production has increased in many areas where irrigation projects have been implemented, the global agricultural development community has promoted irrigation investments. However, due to the disappointing performance of irrigation farming in developing countries, irrigation intervention in Africa South of the Sahara including Ethiopia is an issue of debate. Moreover, several gaps exist in the Ethiopian irrigation farming literature. For instance, evidence about the direct and indirect effects of irrigation water on agriculture is not well documented. The irrigation farming literature has not disentangled the indirect effects of having access to irrigation water from the direct effect and the indirect effects have been underrepresented. Furthermore, most previous studies have applied either a quantitative or qualitative approach and have relied only on revealed data as main type of methodology, making studies that combine qualitative and quantitative research and that use both stated and revealed data underrepresented. In this study, different approaches have been applied to investigate how irrigation water impacts Ethiopia agriculture with special attention being given to disentangling the direct and indirect effects of irrigation water on Ethiopian agriculture. Using a structural equation model, a stochastic production frontier approach, and a discrete choice experiment, I drew evidence regarding the direct and indirect effects of irrigation water on crop revenue of smallholder farmers, the technical efficiency of irrigation user farmers, and the farmers’ willingness to pay to improve poor irrigation schemes from field observations, semi-structured interviews and focus group discussions with farmers, and key informant interviews with the local agricultural agents from the Koga and Fogera Districts of Amhara Region Ethiopia. The results indicate that irrigation water in general has both direct and indirect positive effects on agriculture, and the indirect effect is mediated by both improved farm inputs and the type of crops produced. The results also show that – due to poor extension services and backward agronomic practices, the mean technical efficiency of farmers in Ethiopia is very low, and that large-scale irrigation users are less technically efficient than small-scale irrigation users. Moreover, the results show that improving irrigation schemes shifts the frontier up, and smallholder farmers are strongly willing to contribute financially to the maintenance costs of irrigation schemes. The results offer relevant lessons for policymakers that providing irrigation water supply must be embedded in a comprehensive support package including access to extension services, improved input supply, and access to stable markets.

Abstract: Not much about the effect of thiocyanate addition on the sulphur ripening is known, although it is used for many applications in photographic practice. Via a combination of tracer analysis and diffuse reflectance spectroscopy the effect of thiocyanate addition on the sulphur and sulphur-plus-gold ripening could be unveiled. When thiocyanate is added prior to the sulphur addition, it appears to rearrange the silver halide surface in such way that the sulphur deposition rate is enhanced, but the supply of interstitials is limited. Addition of thiocyanate after the sulphur reaction results in the formation of thiocyanate complexes with silver, from which a silver ion is more easily deposited in a surface cell of the silver sulphide clusters thus enhancing the sensitization rate. For sulphur-plus-gold sensitized emulsions it was observed that part of the gold ions could be removed out of the Ag2-xAuxS clusters by addition of thiocyanate ions and subsequent washing. Hence, it was concluded that two different types of gold ions are present in the silver sulphide clusters; 1. gold ions which are substitutional for silver (bound between sulphur and bromide ions) 2. gold ions which bridge two or three sulphur atoms. Incorporation of gold ions into silver sulphide clusters suppresses their optical absorption in diffuse reflectance spectroscopy. Since the optical absorption at 505 nm can completely be restored by addition of thiocyanate, it is assumed that the entity absorbing at this wavelength is a monomer of silver sulphide.

Abstract: This dataset comprises a large collection of atom probe tomography datasets collected from DIN 1.4970 alloy that was irradiated with Fe ions at different conditions. The DIN 1.4970 alloy is an austenitic stainless steel with 15 wt% Cr, 15 wt% Ni, a small addition of Ti. The full composition and characterization of our material can be found published elsewhere [1,2]. Some of our material was subjected to ageing heat treatments at different temperatures for different times. Small samples of our original material and aged material was irradiated at the Michigan Ion Beam Laboratory in 2017 with 4.5 MeV Fe ions up to 40 dpa at an average dose rate of 2×10−4 dpa/s. This was done at three different temperatures: 300, 450, and 600 ºC. Atom probe samples were made of the irradiated layers (approximately 1.5 micron deep) with focused ion beam and mounted on Microtip coupons. APT measurements took place on three CAMECA LEAP-HR systems located at CAES in Idaho Falls, USA (files beginning with R33), at Montanuniversität Leoben in Leoben, Austria (R21) and at Friedrich–Alexander University in Erlangen, Germany (R56).

Abstract: The gravitational energy associated with variations in the radius of the Sun is huge: with a relative change of 1/2000 (the estimated difference between maximum and minimum solar activity) and taking only the convection zone to expand, corresponds to about 2 10(34)J, which is 1/10 of the total rotational energy of the Sun. Non-radial oscillations with a tiny nonlinearity can still yield energies comparable to or larger than the total magnetic energy of a full sunspot cycle or three or four orders more than the luminosity of the Sun, but that is not sufficient to account for the changes of the temperature on Earth. The expansion of the upper layers of the convection zone, however, may directly or indirectly affect the climate and the biological aspects on the Earth. Moreover this suits a qualitative explanation why the Sun expands during a minimum of the magnetic activity.