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De laatste decennia is er een toenemende belangstelling voor fotokatalyse. Eén van de voordelen is de mogelijkheid om zonlicht te gebruiken om bijvoorbeeld CO2 om te zetten in hernieuwbare chemische brandstoffen. TiO2 is een algemeen gebruikte fotokatalysator omdat het een grote band gap-energie, hoge stabiliteit en lage kosten heeft, maar de lichtabsorptie is beperkt tot het UV-gebied. Om de lichtabsorptie in het zichtbare lichtgebied te optimaliseren en de recombinatiesnelheid te verlagen, worden heterojunctie fotokatalysatoren gebruikt. De heterojunctievorming tussen de twee halfgeleiders in de fotokatalysator hangt af van de materiaaleigenschappen (bandgap, kristalstructuur, etc.) van de halfgeleiders. Het Type II- en Z-schema heterojuncties zijn de meest onderzochte typen heterojuncties, elk met zijn eigen voordelen en beperkingen. Het Z-schema vertoont een goed oxidatief en reducerend vermogen en ladingsscheiding, maar dit gaat ten koste van de helft van de ladingsdragers als gevolg van recombinatie aan het grensvlak tussen de twee halfgeleiders. In Type II vindt deze grensvlakrecombinatie niet plaats, waarbij de hoeveelheid beschikbare licht-gegenereerde ladingsdragers wordt gemaximaliseerd, maar ten koste van het oxidatieve en reductieve vermogen van respectievelijk de gaten en elektronen. In dit masterproefproject wordt de ladingsoverdracht in de heterojuncties met verschillende verhoudingen van CsPbBr3 nanokristallen en Bi2MoO6 nanosheets onderzocht. Eerst werd de synthesemethode bestudeerd om de vorming van de heterojunctie te verzekeren. De hot-injectie-methode is een gemakkelijke manier om perovskiet-nanokristallen met een kleine grootteverdeling op de Bi2MoO6-nanosheets te vormen. Om de gesynthetiseerde heterojunctie en de twee materialen in de heterojunctie te onderzoeken, werden de relevante materiaaleigenschappen geanalyseerd: (1) de bandgap van de halfgeleiders in de heterojuncties werd bestudeerd door Diffuse Reflectance Spectroscopie (DRS), (2) de kristalstructuur van de halfgeleiders geanalyseerd met röntgendiffractie (XRD), (3) de kristalgroottes gemeten met scanning elektronenmicroscopie (SEM) en berekend via de verandering in bandgap en de PL maxima en (4) de atomaire samenstelling van de heterojunctie werd bepaald door inductief gekoppelde plasma-optische emissiespectrometrie (ICP-OES). Om de licht-gegenereerde dynamiek van ladingsdragers in de composietmaterialen te analyseren, werden verschillende fotokatalytische tests met de gesynthetiseerde fotokatalysator uitgevoerd. De oxidatie van benzylalcohol tot benzaldehyde en de reductie van CO2 tot CO werden gebruikt voor de fotokatalyse met de heterojunctie. De redoxreacties zullen alleen plaatsvinden als de ladingsoverdracht de Z-schemaroute volgt, vanwege hun overeenkomstige redoxpotentialen. Er werd waargenomen dat de gecombineerde reactie, oxidatie van benzylalcohol en reductie van CO2, plaatsvond via de gewenste Z-schema-route, maar deze leidde tot een lage vorming van BAD en CO. Dit geeft aan dat de ladingsdragerroute veranderde van Z-schema naar Type II tijdens de fotokatalyse.
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The daily use of plastic causes environmental pollution, with microplastics posing a potential problem to the environment and human health. A reliable and efficient detection and characterisation technique can help in the analysis of the distribution of microplastics in the environment. This study aims to find a label-free method for detecting, identifying and characterising microplastics. Specifically, Raman microspectroscopy is used to identify and characterise microplastics by creating a Raman spectral database of several types of plastic and a correlation assessment technique. To create this database and correlation assessment technique, Raman spectra of macroscopic and microscopic sized plastics were obtained and analysed for the database to compare subsequent spectra to. Barcodes were created from the Raman spectra to easily compare different spectra. These barcodes represent the peak widths and peak positions of the spectrum in bars. Using these barcodes, the correlation between a sample spectrum and reference spectra is calculated to identify the sample. Weathering processes were simulated to test the use of the database for weathered microplastics. The results indicate a significantly higher correlation with the matching type of plastic than with other types of plastic. The identification technique succeeded in identifying plastics in a mixture of the different types of plastic used in this project. There were no notable differences in the Raman spectra of the artificially weathered microplastics and the samples could still be identified using the database. Either no weathering process occurred, it was not sufficiently impactful, or the weathering did not cause the expected chemical changes. In conclusion, Raman spectroscopy can be used to identify microplastics in a laboratory environment using the composed database with the help of the developed identification technique. More research is needed for the identification of weathered microplastics.
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The richness and complexity of physical and chemical processes in perovskites make correlative investigations necessary for a comprehensive understanding on perovskites and their optoelectronic devices. That includes surface, morphology, chemical composition, photoluminescence, electroluminescence, electrical responses and so on. A major drawback of most perovskite materials, however, lays in their instability under operating conditions, which is not a secret. Next to the great importance of surface engineering to in stabilizing perovskites, it is also beneficial in improving their physical properties.Besides the importance of surfaces and interfaces, crystal morphology (in particular at the nanoscale) and local stoichiometry also greatly influence the material properties. By doing the work on surface and interface engineering for perovskite optoelectronic nanomaterials, revenues in stability improvements and in property engineering can be envisaged.
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Ammonia is a key chemical in our society and it is produced at an enormous scale. Its primary use is in fertilizers, but ammonia also plays a vital role in the chemical industry as a precursor in other processes. Currently, ammonia is produced primarily using the Haber-Bosch process which uses fossil fuels to generate hydrogen causing it to contribute significantly (1.6%) to anthropogenic CO2 emissions. The Haber-Bosch process’ reaction conditions also limit the possibility of decentralized ammonia production, raising the price of nitrogen fertilizers in critical areas. Alternatives are thus needed to circumvent these shortcomings, one of which is the PECAN process. In this process nitrogen, from air, gets transformed into nitrate using a plasma reactor and this nitrate is subsequently electrochemically reduced to ammonia. For an energy-efficient operation of this last step, an effective electrocatalyst for the reduction of nitrate to ammonia has yet to be identified. This thesis set out to find such a catalyst and evaluate its applicability for this process. Previous literature pointed to copper as having an above-average performance for this reaction which is why first these were tested using bulk experiments in 1M HNO3. Although, it was found copper did not prove to be active or stable enough to be effective. Thus different metals were screened and it was found a copper-nickel alloy demonstrated to be more active. Alloying of copper with other metals was therefore deemed a promising strategy. Further optimization of the copper-nickel alloy was performed which indicated a 50% nickel content alloy was the most energy-efficient. A decrease in required energy of 0.145MJ/mol NH3 compared to pure copper was found. Although, no increased stability could be identified. Finally, copper-cobalt alloys were tested due to the high reported activity of cobalt. However, it was found cobalt was not stable in the reaction mixture and thus no increased performance could be identified.
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