The growing interest of renewable fuel and energy sources has steadily increased over time due to climate changes. Research is being made around the world to find solutions for the different problems; one possible solution is to produce hydrogen gas to help phase out the usage of fossil fuels. So far, the technology for the hydrogen gas production is expensive for various reasons, one of the challenges is to minimize the energy usage for the production. Hydrogen could be used in fuel cells which can be used to fuel an electric car. In a fuel cell, hydrogen and oxygen gas are mixed to produce electrical energy as the main product, but it also forms thermal energy and water. Hydrogen gas can be produced from the reversed reaction; by electrolysis of water. This reaction requires energy and one way to minimize the energy usage for this is by using acatalyst. The goal with this master thesis was to see how the reaction rate of the oxygen evolution reaction can be affected by different single atom catalyst systems. The main structure for this catalyst in this thesis is aporphyrin molecule where different transition metals were tried as the active site. Different modifications on the structure were also made by exchanging some of the structures atoms and by adding different ligands.The purpose of this is to see how these modifications change the activity of the catalyst. The catalysts were optimized and calculated in a computational chemistry program called Gaussian 16. The calculations was made by using the DFT functional PBE0 and the basis sets Def2svp and Def2tzvpp. The results show that different modifications do affect the activity of the catalyst. The biggest variations in activity are from placing ligands under the active site while exchanging hydrogens to other substituents on the outer radial position can fine tune the results. The best active sites for this system came by using iridium, rhodium and cobalt which are all elements in group 9 of the periodic table. The lowest overpotential of 0.513 V was given by an iridium based system with four hydrogens exchanged by fluorides. / Runt om i världen finns ett ökat intresse för förnyelsebara energi och bränslekällor för att tackla klimat förändringarna. Stor del av forskningen som görs idag har i syfte att hitta nya lösningar för att minska klimatpåverkan i olika områden. Ett av forskningsområderna är hitta vägar till en miljövänligare vätgasproduktion där vätgasen skulle kunna användas i bränsleceller. Dessa celler kan sättas i elbilar och på så sätt fasa ut användingen av fossila bränslen. En av utmaningarna för vätgasproduktionen är att den idag är kostsam och kräver mycket energi. Forskare försöker hitta olika katalysatorer som kan minska energiåtgången som krävs vid elektrolys av vatten där syrgas och vätgas produceras. Målet med det här examensarbetet är att se hur en single atom catalyst kan påverka reaktionskinitiken för den syrgasbildande reaktionen vid elektrolys av vatten. Huvudstrukturen för katalysatorn som beräkningarna är gjorda på är en porphyrinmolekyl där olika övergångsmetaller kommer testas som det aktiva sätet i katalysatorn. Olika ligander kommer även tillsättas systemet samt utbyte av några väteatomer till olika substituenter i porfyrinstrukturen. Katalysatorn optimerades i det kvantkemiska beräkningsprogrammet Gaussian 16 med funktionalen PBE0 med basset Def2svp och Def2tzvpp. Resultaten visade att olika modifikationer på systemet hade en påverkan på katalysatorns aktivitet. Den största påverkan hade de olika liganderna som placerades under det aktiva sätet jämfört med de olika substituenterna. De bästa metallerna för katalysatorn var iridium, rhodium och kobolt vilket alla ligger i grupp nio i det periodiska systemet. Den lägsta överpotentialen på 0.513 V gavs av iridium systemet med fyra utbyta väten till fluor.

<p>Ytterbium-doped optical fibers have extensive applications in high-power fiber lasers, optical amplifiers, and amplified spontaneous emission light sources. In this thesis two sub-projects associated with ytterbium doped fibers are discussed.</p> <p>Numerical simulations have been used to model high-repetition rate ultrafast ytterbium-doped fiber amplifier systems assuming continuous-wave input signals under variable situations, such as one-sided and two-sided pumping. Different system configurations are also developed, such as a single-stage amplification system, a two-stage amplification system and a separated amplification system, providing alternative choices for experiments and applications. The simulation results are compared with experimental data and the simulation results from some other software. The influence of nonlinear effects in the fiber is also very briefly discussed in this thesis.</p> <p>In a second research activity, the distribution of ytterbium atoms is being investigated in a range of double-clad ytterbium-doped fibers. Using aberration-corrected electron microscopy, ytterbium atoms are directly observed from the wedge-shaped specimen, which was prepared from ytterbium-doped optical fibers by tripod polishing combined with ion milling. Challenges related to sample preparation and the interpretations of images are discussed, but the approach shows great potential to investigate the doping behaviors down to atomic scale in the fibers. The work is expected to help reveal mechanisms affecting the performance for the doped fibers, such as photodarkening which is potentially associated with clustering effects.</p> / Master of Applied Science (MASc)

Graphene and its functionalized derivatives, such as nitrogen-doped graphene, have recently become a popular substrate material for the proton exchange membrane fuel cell (PEMFC) due to its enhanced electrical conductivity, electrochemical stability, and increased surface area when compared to the conventional, carbon black. In order to further develop the alternative fuel industry, the Pt catalyst within the PEMFC must also be considered. Single Pt atoms have a higher surface area to volume ratio when compared to nanoparticles, thus offering the potential to create a more affordable and efficient PEMFC. In this thesis, electrode materials comprising single Pt atoms and clusters, produced using atomic layer deposition (ALD) on various C derivatives, including graphene, N-doped graphene, carbon nanotubes (CNTs), and N-doped CNTs (NCNTs) are investigated through the utilization of aberration corrected transmission electron microscopy. Structural and chemical analysis was performed on thermally exfoliated N-doped graphene and CVD-produced graphene that was exposed to N+ ion sources. It was determined that the thermally exfoliated N-doped graphene maintained the short-range order of the graphene lattice; however, local inhomogeneities existed for the total N concentration, and the specific N-dopants within and between graphene sheets. More importantly, Pt atoms and clusters were observed and determined to be primarily stabilized at the edge of the N-doped graphene sheets. The stabilization of the Pt atoms and clusters resulted in a significantly higher mass and specific activity for the hydrogen evolution reaction, when compared to the use of a graphene substrate and Pt nanoparticles on C black. The N+ ion implantation in the CVD graphene showed the incorporation of N-dopants; however, electron energy loss spectroscopy revealed structural damage to thin sheets. NCNTs were also characterized in this thesis as possible gas containers, and as a substrate material to examine the effects of varying ALD conditions. It was determined that the NCNTs were an effective N2 gas conduit, wherein a decreasing pressure was observed with an increase to the inner diameter of the nanotubes. Using similar NCNTs, the effect of dosing time, temperature, and substrate on the Pt size were analyzed using ALD. While no singular condition resulted in the sole production of single Pt atoms, modifying both the substrate and dosing time were shown to provide the greatest potential for producing individual Pt atom catalysts. / Thesis / Doctor of Philosophy (PhD)

Ce travail de thèse a consisté en l’étude de catalyseurs ultradispersés, composés de clusters nanométriques ou d’atomes isolés (single-atom catalysts, SACs) de métal, une nouvelle classe de catalyseurs faisant actuellement l’objet d’un engouement mondial. Les systèmes Pt/?-Al2O3 et Pt/TiO2 ont été préparés, caractérisés et testés en oxydation de CO et photogénération d’hydrogène, respectivement. Plusieurs méthodes d’imprégnation et de traitement thermique ont été comparées, notamment grâce à l’analyse de la dispersion du platine par microscopie électronique en transmission à balayage (STEM). Pour la préparation de SACs, notre choix s’est finalement porté sur l’imprégnation à humidité naissante d’une faible charge de précurseur Pt(NH3)4(NO3)2, suivie d’une calcination à l’air. L’étude des performances catalytiques et de l’évolution structurale des catalyseurs au cours des réactions a permis de montrer que les atomes isolés (cations) de platine étaient moins actifs que les clusters (réduits) pour les deux systèmes catalytiques étudiés. Dans le cas de Pt/?-Al2O3, des suivis par spectroscopie d’absorption X (XAS) operando en rayonnement synchrotron, spectroscopie infrarouge par réflexion diffuse (DRIFTS) operando et microscopie environnementale (E-STEM) ont montré la formation et la déstabilisation des SACs, cette dernière étant toutefois moindre en conditions oxydantes. En effet, l’oxygène stabilise le platine isolé via la formation de plusieurs liaisons Pt-O-Al comme montré par modélisation DFT, alors que la présence d’un composé réducteur (CO, H2) conduit à la formation de clusters, mobiles sur leur support. Ce travail met en évidence les limites possibles concernant la stabilisation et la mise en œuvre des SACs dans des réactions catalytiques impliquant des conditions réductrices / This PhD work is focused on the study of ultradispersed catalysts, composed of nanometer-sized clusters or isolated atoms (single-atom catalysts, SACs) of metal, a new class of catalysts which are currently the object of worldwide interest. The Pt/?-Al2O3 and Pt/TiO2 systems were prepared, characterized and evaluated for CO oxidation and hydrogen photogeneration, respectively.Several methods of impregnation and thermal treatment were compared, in particular through platinum dispersion analysis using scanning electron microscopy (STEM). For the preparation of SACs, our choice finally turned to the incipient wetness impregnation of Pt(NH3)4(NO3)2 precursor at low loading, followed by calcination in air.For the two catalytic systems of interest, the study of the performances and the structural evolution of the catalysts during the reactions shows that isolated Pt atoms (cations) are less active than their (reduced) cluster counterparts.In the case of Pt/?-Al2O3, operando X-ray absorption spectroscopy (XAS) using synchrotron radiation, operando diffuse reflectance infrared spectroscopy (DRIFTS), and environmental microscopy (E-STEM) allowed us to monitor the SAC formation and destabilization, the latter being however limited in oxidizing conditions. Indeed, the presence of oxygen stabilizes single Pt atoms via the formation of several Pt-O-Al bonds as shown by DFT modeling, whereas the presence of a reducing compound (CO, H2) leads to the formation of Pt clusters, mobile on their support.This work highlights the possible limitations in the stabilization and implementation of SACs for catalytic reactions involving reducing conditions

No description available.

Single-atom catalysts (SACs), as promising alternatives to Pt-based catalysts, suffer from the limited choice of center metals and low single-atom loading. Here, we report a pentacoordinated Zr-based SAC with nontrivial axial O ligands (denoted O−Zr−N−C) for oxygen reduction reaction (ORR). The O ligand downshifts the d-band center of Zr and confers Zr sites with stable local structure and proper adsorption capability for intermediates. Consequently, the ORR performance of O−Zr−N−C prominently surpasses that of commercial Pt/C, achieving a half-wave potential of 0.91 V vs. reversible hydrogen electrode and outstanding durability (92 % current retention after 130-hour operation). Moreover, the Zr site shows good resistance towards aggregation, enabling the synthesis of Zr-based SAC with high loading (9.1 wt%). With the high-loading catalyst, the zinc-air battery (ZAB) delivers a record-high power density of 324 mW cm−2 among those of SAC-based ZABs.

Excessive CO2 emissions can negatively impact society and our planet. Reduction of CO2 is one potential avenue for its abatement. One of the most significant challenges to reducing CO2 is its extremely stable linear form. Experimentally, Cu/TiO2 has shown promise for CO2 photocatalytic reduction. Dispersed atomic catalysts can achieve high catalytic efficiency on a per atom basis. Active sites also typically having lower coordination number, and therefore may be more reactive. Using density functional theory and experimental techniques, we have investigated the role of surface oxygen vacancies (Ov) and photoexcited electrons on supported single atom catalysts and CO2 reduction. Cu atoms with Ov have shown to aid in the process of bent, anionic CO2 formation. In the first step involving CO2 dissociation (CO2* --> CO* + O*), a single Cu atom in Ov lowered the activation barrier to 0.10 - 0.19 eV, which could enable fast reduction of CO2 even at room temperature, in agreement with experimental findings. A photoexcited electron model was shown to readily promote Cu binding to the surface vacancy, and CO2 adsorption and direct dissociation. Finally, we briefly compare our results to calculations of supported single Pt atoms to determine how metals besides Cu may behave as photocatalysts for CO2 reduction, and we found a single Pt with Ov can promote CO2 dissociation. Our results show that tailoring TiO2 surfaces with defects in conjunction with atomic catalysts may lead to useful catalysts in the photoreduction of CO2.

Propylene is the second most important starting chemical in the petrochemical industry after ethylene. Unlike ethylene, propylene readily undergoes substitution reactions including polymerisation, oxidation, halogenation, hydrohalogenation, alkylation, hydration, oligomerization and hydroformylation, which lead to a wide variety of important downstream products. One of the principal uses of propylene is to produce key chemicals from selective oxidation. In 2016, the world annual production of propylene is about 94 million tonnes, and the global proportion used to produce selective oxidation product is over 18%. They constitute a key part of the chemical industry and contribute towards substantial economic benefits. The application of Ag based heterogeneous catalysts to selective propylene oxidation is a key factor in the synthesis of nearly all downstream chemicals, however billions of pounds are lost every year due to unplanned reactor shutdown, safety control and environment unfriendly emission control as a results of inefficiency catalytic selectivity and activity. Despite, both theoretical and experimental research works have been intensively involved, the fundamental reason leading to these effects are not yet well understood. The work presented in this thesis explores a range of novel modification techniques that alter the activity of Ag nanocatalysts for selective propylene oxidation, especially in propylene epoxidation. Particular focus is placed on developing surface modified Ag catalysts through morphology control, surface architecture engineering with another sublayer metal. Using a combination of modelling, novel and traditional materials characterisation methods, it is found that these modification result in some significant electronic and/or geometric alterations to the Ag nanoparticles surface. The Ag-Ag bond distance can be dramatically enlarged by exposing a high-index Ag surface or a core-shell structure with monolayer Ag shell. When interacting with molecular oxygen, the molecular oxygen adsorption and dissociation behaviour is sensitive to the geometric changes in Ag surface. This leads to an enhanced selectivity toward propylene epoxidation than combustion resulting from preventing a C-H bond cleavage. Finally, be creating atomically dispersed Ag on zeolite, a completely different interaction between molecular oxygen and single atom Ag were discovered comparing to on a extensive silver surface. This leads to the observation of an excitingly new propylene oxidation reaction producing ethanol and CO₂ resulting from C=C bond cleavage. Overall, the research presented within this thesis demonstrated a number of methods for the intelligent design of novel heterogeneous Ag catalysts with remarkable activity and selectivity toward specific selective propylene oxidation. These modification methods are believed to be potentially applicable to a wide range of other catalytic reactions.

La réduction (« scaling ») continue des dimensions des transistors MOSFET nous a conduits à l'ère de la nanoélectronique. Le transistor à effet de champ multi-grilles (MultiGate FET, MuGFET) avec l'architecture «nanofil canal» est considéré comme un candidat possible pour le scaling des MOSFET jusqu'à la fin de la roadmap. Parallèlement au scaling des CMOS classiques ou scaling suivant la loi de Moore, de nombreuses propositions de nouveaux dispositifs, exploitant des phénomènes nanométriques, ont été faites. Ainsi, le transistor monoélectronique (SET), utilisant le phénomène de «blocage de Coulomb», et le transistor à atome unique (SAT), en tant que transistors de dimensions ultimes, sont les premiers dispositifs nanoélectroniques visant de nouvelles applications comme la logique à valeurs multiples ou l'informatique quantique. Bien que le SET a été initialement proposé comme un substitut au CMOS («Au-delà du dispositif CMOS»), il est maintenant largement considéré comme un complément à la technologie CMOS permettant de nouveaux circuits fonctionnels. Toutefois, la faible température de fonctionnement et la fabrication incompatible avec le procédé CMOS ont été des contraintes majeures pour l'intégration SET avec la technologie FET industrielle. Cette thèse répond à ce problème en combinant les technologies CMOS de dimensions réduites, SET et SAT par le biais d'un schéma d'intégration unique afin de fabriquer des transistors « Trigate » nanofil. Dans ce travail, pour la première fois, un SET fonctionnant à température ambiante et fabriqués à partir de technologies CMOS SOI à l'état de l'art (incluant high-k/grille métallique) est démontré. Le fonctionnement à température ambiante du SET nécessite une île (ou canal) de dimensions inférieures à 5 nm. Ce résultat est obtenu grâce à la réduction du canal nanofil ‘‘trigate'' à environ 5 nm de largeur. Une étude plus approfondie des mécanismes de transport mis en jeu dans le dispositif est réalisée au moyen de mesures cryogéniques de conductance. Des simulations NEGF tridimensionnelles sont également utilisées pour optimiser la conception du SET. De plus, la cointégration sur la même puce de MOSFET FDSOI et SET est réalisée. Des circuits hybrides SET-FET fonctionnant à température ambiante et permettant l'amplification du courant SET jusque dans la gamme des milliampères (appelé «dispositif SETMOS» dans la littérature) sont démontrés de même que de la résistance différentielle négative (NDR) et de la logique à valeurs multiples. Parallèlement, sur la même technologie, un transistor à atome unique fonctionnant à température cryogénique est également démontré. Ceci est obtenu par la réduction de la longueur de canal MOSFET à environ 10 nm, si bien qu'il ne comporte plus qu'un seul atome de dopant dans le canal (diffusée à partir de la source ou de drain). A basse température, le transport d'électrons à travers l'état d'énergie de ce dopant unique est étudié. Ces dispositifs fonctionnent également comme MOSFET à température ambiante. Par conséquent, une nouvelle méthode d'analyse est développée en corrélation avec des caractéristiques à 300K et des mesures cryogéniques pour comprendre l'impact du dopant unique sur l'échelle MOSFET à température ambiante. / Continuous scaling of MOSFET dimensions has led us to the era of nanoelectronics. Multigate FET (MuGFET) architecture with ‘nanowire channel' is being considered as one feasible enabler of MOSFET scaling to end-of-roadmap. Alongside classical CMOS or Moore's law scaling, many novel device proposals exploiting nanoscale phenomena have been made either. Single Electron Transistor (SET), with its unique ‘Coulomb Blockade' phenomena, and Single Atom Transistor (SAT), as an ultimately scaled transistor, are prime nanoelectronic devices for novel applications like multivalued logic, quantum computing etc. Though SET was initially proposed as a substitute for CMOS (‘Beyond CMOS device'), it is now widely considered as a compliment to CMOS technology to enable novel functional circuits. However, the low operation temperature and non-CMOS fabrication process have been major limitations for SET integration with FET. This thesis makes an effort at combining scaled CMOS, SET and SAT through a single integration scheme enabling trigate nanowire-FET, SET or SAT. In this work, for the first time, fabrication of room temperature operating SET on state-of-the-art SOI CMOS technology (featuring high-k/metal gate) is demonstrated. Room temperature operation of SET requires an island (or channel) with dimensions of 5 nm or less. This is achieved through reduction of trigated nanowire channel to around 5 nm in width. Further study of carrier transport mechanisms in the device is carried out through cryogenic conductance measurements. Three dimensional NEGF simulations are also employed to optimize SET design. As a step further, cointegration of FDSOI MOSFET and SET on the same die is carried out. Room temperature hybrid SET-FET circuits enabling amplification of SET current to micro-ampere range (proposed as ‘SETMOS device' in literature), negative differential resistance (NDR) and multivalued logic are shown. Alongside this, on the same technology, a Single Atom Transistor working at cryogenic temperature is also demonstrated. This is achieved through scaling of MOSFET channel length to around 10 nm that enables having a single dopant atom in channel (diffused from source or drain). At low temperature, electron transport through the energy state of this single dopant is studied. These devices also work as scaled MOSFETs at room temperature. Therefore, a novel analysis method is developed correlating 300 K characteristics with cryogenic measurements to understand the impact of single dopant on scaled MOSFET at room temperature.

Ruthenium (Ru) has been theoretically considered a viable alkaline hydrogen evolution reaction electrocatalyst due to its fast water dissociation kinetics. However, its strong affinity to the adsorbed hydroxyl (OHad) blocks the active sites, resulting in unsatisfactory performance during the practical HER process. Here, we first reported a competitive adsorption strategy for the construction of SnO2 nanoparticles doped with Ru single-atoms supported on carbon (Ru SAs-SnO2/C) via atomic galvanic replacement. SnO2 was introduced to regulate the strong interaction between Ru and OHad by the competitive adsorption of OHad between Ru and SnO2, which alleviated the poisoning of Ru sites. As a consequence, the Ru SAs-SnO2/C exhibited a low overpotential at 10 mAcm􀀀2 (10 mV) and a low Tafel slope of 25 mVdec􀀀1. This approach provides a new avenue to modulate the adsorption strength of active sites and intermediates, which paves the way for the development of highly active electrocatalysts.