The popularity of graphene owing to its unique properties has triggered huge interest in other two-dimensional (2D) nanomaterials. Among them, silicene shows considerable promise for electronic devices due to the expected compatibility with silicon electronics. However, the high-end potential application of silicene in electronic devices is limited owing to the lack of an energy band gap. Hence, the principal objective of this research is to tune the electronic and magnetic properties of silicene related nanomaterials through first-principles models. I first explored the impact of edge functionalization and doping on the stabilities, electronic, and magnetic properties of silicene nanoribbons (SiNRs) and revealed that the modified structures indicate remarkable spin gapless semiconductor and half-metal behaviors. In order to open and tune a band gap in silicene, SiNRs were perforated with periodic nanoholes. It was found that the band gap varies based on the nanoribbon’s width, nanohole’s repeat periodicity, and nanohole’s position due to the quantum confinement effect. To continue to take advantage of quantum confinement, I also studied the electronic and magnetic properties of hydrogenated silicene nanoflakes (SiNFs). It was discovered that half-hydrogenated SiNFs produce a large spin moment that is directly proportional to the square of the flake’s size. Next, I studied the adsorption behavior of various gas molecules on SiNRs. Based on my results, the SiNR could serve as a highly sensitive gas sensor for CO and NH3 detection and a disposable gas sensor for NO, NO2, and SO2. I also considered adsorption behavior of toxic gas molecules on boron carbide (BC3) and found that unlike graphene, BC3 has good sensitivity to the gas molecules due to the presence of active B atoms. My findings divulged the promising potential of BC3 as a highly sensitive molecular sensor for NO and NH3 detection and a catalyst for NO2 dissociation. Finally, I scrutinized the interactions of CO2 with lithium-functionalized germanene. It was discovered that although a single CO2 molecule was weakly physisorbed on pristine germanene, a significant improvement on its adsorption energy was found by utilizing Li-functionalized germanene as the adsorbent. My results suggest that Li-functionalized germanene shows promise for CO2 capture.

Os nanotubos de carbono (CNTs) possuem várias aplicações devido às suas propriedades com destaque à alta resistência química, baixa densidade e elevada condutividade elétrica. Dentro da Química Analítica, destacam-se as propriedades eletrocatalíticas destes nanomateriais para a modificação da superfície de eletrodos de trabalho, e assim atuarem como sensores eletroquímicos. Este trabalho investiga a utilização de três tipos de CNTs, um de paredes simples (SWCNT) e dois de paredes duplas (DWCNT), sendo um destes de menor comprimento (S-DWCNT), para a modificação do eletrodo de carbono vítreo (GCE). Inicialmente, a caracterização dos CNTs não funcionalizados e funcionalizados foi realizada com o intuito principal de analisar as possíveis mudanças obtidas pelo processo de funcionalização. As caracterizações foram feitas através de espectroscopia na região do infravermelho (IR) e espectroscopia Raman. As morfologias destes materiais foram avaliadas por microscopia eletrônica de varredura (SEM). A estabilidade térmica foi observada pela análise termogravimétrica. Posteriormente à etapa de caracterização dos CNTs, investigou-se o comportamento eletroquímico dos compostos fenólicos catecol (CT) e dopamina (DP) em eletrodos modificados com os três diferentes CNTs. O estudo do pH do eletrólito indicou melhores resultados em termos de corrente com a solução de HClO4. O estudo da velocidade de varredura de voltamogramas cíclicos indicou que os processos de oxidação eletroquímica são controlados ora por difusão das espécies ao eletrodo ora por processos adsortivos. A detecção amperométrica das espécies em sistema de análise por injeção em batelada (BIA) foi avaliada utilizando eletrodos modificados com os três CNTs. Obtiveram-se amplas faixas lineares de 1-1000 μmol L- 1 tanto para CT como para DP, em todos os casos. Para CT e DP, baixos desvios padrão foram obtidos para injeções consecutivas (n=15) de aproximadamente 1,0% para GCE, SWCNT, DWCNT e S-DWCNT. Frequências analíticas de 135 h-1 foram obtidas empregando-se os eletrodos modificados, enquanto que em GCE obteve-se 105 h-1. Baixos limites de detecção para CT (0,0093; 0,0136; 0,0715 μmol L-1) e DP (0,0145; 0,0493; 0,1743 μmol L-1) foram obtidos em SWCNT, DWCNT e S-DWCNT, respectivamente, enquanto que em GCE foram obtidos 0,867 e 1,034 para CT e DP, respectivamente. Todos resultados obtidos mostraram-se promissores, com faixas lineares amplas e DLs baixos, manifestando assim que as modificações em GCE com os CNTs foram eficientes e resultaram em sensores eletroquímicos mais sensíveis para os analitos investigados. / Carbon nanotubes (CNTs) have various applications due to their properties with emphasis to high chemical resistance, low density and high electrical conductivity. Within Analytical Chemistry, the electrocatalytic properties of these nanomaterials are highlighted towards the the surface modification of working electrodes, and thus act as electrochemical sensors. This work investigates the use of three types of CNTs, a single-walled (SWCNT) and two double walled carbon nanotubes (DWCNT), one of these of smaller length (S-DWCNT), for the modification of a glassy carbon electrode (GCE). Initially, the functionalized and non-functionalized CNTs were characterized aiming to analyze the possible changes obtained by functionalization process. The characterizations were made through infrared (IR) and Raman spectroscopies. The morphologies of these materials were evaluated by scanning electron microscopy (SEM). The thermal stability was observed by thermogravimetric analysis. After the step of characterizing the CNTs, it was investigated the electrochemical behavior of the phenolic compounds catechol (CT) and dopamine (DP) on electrodes modified with three different CNTs. The study of pH of the electrolyte showed best results in terms of current for HClO4 solution. Study of cyclic voltammetry scan rate indicated that the electrochemical oxidation processes are controlled either by diffusion of the species to the electrode or by adsorptive processes. The amperometric detection analysis of the species using a batch injection system (BIA) was carried out using three CNTs modified electrodes. It was obtained wide linear ranges of 1-1000 μmol L- 1 for both CT and DP in all cases. For CT and DP, low standard deviations were obtained for consecutive injections (n = 15) of approximately 1.0% on GCE, SWCNT, DWCNT and S-DWCNT. Analytical frequencies of 135 h-1 were obtained employing the modified electrodes, while in GCE was obtained 105 h-1. Low detection limits for CT (0.0093; 0.0136; 0.0715 μmol L-1) and DP (0.0145; 0.0493; 0.1743 μmol L-1) were obtained in SWCNT, DWCNT and S-DWCNT, respectively, while in GCE 0.867 and 1.034 μmol L-1 were obtained for CT and DP, respectively. All results proved to be promising, with wide linear ranges and low DLs, thus manifesting that the modifications of GCE with CNTs were efficient and resulted in more sensitive electrochemical sensors for the analytes investigated. / Dissertação (Mestrado)

Foram realizados testes de toxicidade ao longo de dois anos, com amostras de água e sedimento do rio Corumbataí, entre Novembro de 2004 e Setembro de 2006, com o objetivo de avaliar a qualidade da água e do sedimento. Os testes de toxicidade aguda realizados com os organismos Chironomus xanthus, Daphnia magna, Hydra attenuata, Lactuca sativa e Pseudokirchneriella subcapitata indicaram variados graus de toxicidade nos diferentes pontos de coleta do rio Corumbataí, com maior ocorrência de toxicidade nos pontos de coleta mais próximos à foz do rio, principalmente devido à afluência do Ribeirão Claro. Todas as amostras de sedimento se mostraram mais tóxicas do que as amostras de água dos mesmos locais, para todos os organismos utilizados, em todas as coletas realizadas. Para todos os organismos, foram realizados testes de sensibilidade, com substância de referência (NaCl), mensalmente, a fim de garantir a qualidade dos cultivos em laboratório e dos bioensaios realizados, com resultados regulares e satisfatórios. Os efeitos produzidos pelas amostras ambientais sobre as sementes de alface (Lactuca sativa) foram bastante variáveis e irregulares, não possibilitando os cálculos de CI50. Decidiu-se por bem suspender a realização deste teste após um ano. Dentre todos os endpoints avaliados, os dos testes de Hydra attenuata e Pseudokirchneriella subcapitata se mostraram mais sensíveis, e o de Daphnia magna o menos sensível às amostras testadas. A avaliação da toxicidade através de efeitos subletais em H. attenuata se mostrou um bom parâmetro de avaliação das condições ambientais do rio Corumbataí, sendo um dos endpoints mais sensíveis utilizados. Outros testes de toxicidade foram realizados em algumas das oito coletas realizadas neste estudo, em caráter complementar: teste de toxicidade crônica com D. magna, teste de toxicidade LuminoTox, que utiliza sistemas enzimáticos vegetais, e o teste de genotoxicidade com Allium cepa, com o objetivo de avaliar suas sensibilidades e contribuições ao estudo. Estes bioensaios adicionaram informações a respeito dos níveis de poluição e toxicidade do rio Corumbataí, contribuindo com os bioensaios principais. Informações como parâmetros físicos e químicos, quantificação de elementos químicos e herbicidas ofereceram subsídios para a interpretação dos resultados obtidos com os testes de toxicidade. São apresentados também resultados das atividades referentes ao estágio de doutorado realizado no exterior (doutorado-sanduíche), no período de Setembro a Dezembro de 2007, na agência de proteção ambiental Environment Canada, no qual foi aplicado um teste de toxicidade com linhagens microbianas (MARA assay) em amostras de água e sedimento, juntamente com nanomateriais / Toxicity tests were conducted for two years with water and sediment samples from Corumbataí River, between November 2004 and September 2006, in order to assess the water and sediment quality. Acute toxicity tests conducted with Chironomus xanthus, Daphnia magna, Hydra attenuata, Lactuca sativa and Pseudokirchneriella subcapitata showed varying degrees of toxicity along the river course, with more toxic results in the sampling sites near the river mouth, due mainly to the inflow of Ribeirão Claro. All sediment samples were more toxic than the water samples (from the corresponding sites), for all test organisms, and for all samplings. Sensitivity tests were performed monthly, with NaCl as the reference toxicant, in order to guarantee both the laboratory culture quality and the bioassays conducted with these organisms. Results with NaCl were regular and satisfactory. The effects of the samples on lettuce seeds (L. sativa) were irregular, thus not allowing the estimative of IC50. It was decided to suspend the performance of this bioassay after one year of testing. Among all studied tests and endpoints, the tests with H. attenuata and P. subcapitata had the most sensitive endpoints, and the acute toxicity test with D. magna had the less sensitive one. Toxicity assessment of Corumbataí River based on Hydra sub-lethality endpoint was one of the most sensitive among the tested ones. Other bioassays were conducted at least once as part of this study, as a complementary assessment: the 21-day chronic toxicity test with D. magna, the LuminoTox bioassay, and the genotoxicity test with Allium cepa, in order to assess their sensitivity and contribution to the present study. These bioassays added information about the contamination levels and toxicity of Corumbataí River, contributing with the main bioassays performed. Data such as physical and chemical parameters, metals and herbicides determinations had a contribution to bioassay results interpretation. Results from a research project developed during a four-month stay (from September 2007 to December 2007) at Environment Canada Protection Agency are further described, regarding the application of the MARA bioassay, (conducted with 11 microbial strains), testing water and sediment samples, along with nanomaterials

There are many challenges in materializing the applications utilizing inorganic nanoparticles. The primary drawback is the degradation of properties due to aggregation and sintering either due to elevated temperatures or prevailing chemical/electrochemical conditions. In this thesis, various wet chemical synthesis methods are developed to obtain metal nanostructures with enhanced thermal stability. The thesis is organized as below: Chapter 1 presents the problems and challenges in materializing the application of nanomaterials associated with the thermal stability of nanomaterials. A review of the existing techniques to improve the thermal stability and the scope of the thesis are presented. Chapter 2 gives a summary of the various materials synthesized, the method adopted for the synthesis and the characterization techniques used in the material characterization. Chapter 3 presents a general template-less strategy for the synthesis of nanoporous alloy aggregates by controlled aggregation of nanoparticles in the solution phase with excellent control over morphology and composition as illustrated using PdPt and PtRu systems as examples. The Pt-based nanoporous clusters exhibit excellent activity for methanol oxidation with good long term stability and CO tolerance. Chapter 4 presents a detailed study on the thermal stability of spherical mesoporous aggregates consisting of nanoparticles. The thermal stability study leads to a general conclusion that nanoporous structures transform to hollow structures on heating to elevated temperatures before undergoing complete densification. Chapter 5 presents a simple and facile method for the synthesis of single crystalline intermetallic PtBi hollow nanoparticles. A mechanism is proposed for the formation of intermetallic PtBi hollow structures. The intermetallic PtBi hollow structures synthesised show excellent electrocatalytic activity for formic acid oxidation reaction. Chapter 6 presents a robust strategy for obtaining a high dispersion of ultrafine Pt and PtRu nanoparticles on graphene. The method involves the nucleation of a metal precursor phase on graphite oxide surfaces and subsequent reduction with a strong reducing agent. The electrocatalytic activity of the composites is investigated for methanol oxidation reaction. Chapter 7 presents a microwave-assisted synthesis method for selective heterogeneous nucleation of metal nanoparticles on oxide supports leading to the synthesis of high activity catalysts. The catalytic activity of the hybrids synthesized by this method for investigated for H2 combustion. Chapter 8 presents thermal stability studies carried out on nanostructures by in-situ heating in transmission electron microscope. The microstructural changes during the sintering process are observed in real time and the observations lead to the understanding of the mechanism of particle growth and sintering. At the end, the results of the investigations were summarized with conclusions drawn.

No description available.

The properties of bulk 3D materials of metals or semiconductors are manifested with various length scales(e.g., Bohr excitonic radius, magnetic correlation length, mean free path etc.) and are important in controlling their properties. When the size of the material is smaller than these characteristics length scales, the confinement effects operate reflecting changes in their physical behavior. Materials with such confinement effects can be designated as low dimensional materials. There are exceedingly large numbers of low dimensional materials and the last half a century has probably seen the maximum evolution of such materials in terms of synthesis, characterization, understanding and modification of their properties and applications. The field of” nanoscience and nanotechnology”, have become a mature field within the last three decades where, for certain application, synthesis of materials of sizes in the nanometer range can be designed and controlled. Interface plays a very important role in controlling properties of heterogeneous material of every dimensionality. For example, the interface forms in 2D thin films or interface of heterogeneous nanoparticles(0D). In recent times, a large number of remarkable phenomena have triggered understanding and controlling properties arises due to nature of certain interface. In the field of nanoparticles, it is well known that the photoluminescence property depends very strongly on the nature of interface in heterostructured nanoparticles. In the recent time a large variety of heterostructured nanoparticles starting from core-shell to quantum dot-quantum well kind has been synthesized to increase the photoluminescence efficiency up to 80%. Along with improvement of certain properties due to heterostructure formation inside the nanoparticles, the techniques to understand the nature of those interfaces have improved side by side. It has been recently shown that variable energy X-ray Photoemission Spectroscopy (XPS) can be employed to understand the nature of interfaces (internal structure) of such heterostructure nanoparticles in great detail with high accuracy. While most of the previous studies of variable energy XPS, uses photonenergies sensitive to smaller sized particle, we have extended the idea of such nondestructive approach of understanding the nature of buried interfaces to bigger sized nanoparticles by using photon energy as high as 8000eV, easily available in various 3rd generation synchrotron centers. The nature of the interface also plays an important role in multilayer thin films. Major components of various electronic devices, like read head memory devices, field effect transistors etc., rely on interface properties of certain multilayer thin film materials. In recent time wide range of unusual phenomenon such as high mobility metallic behavior between two insulating oxide, superconductivity, interface ferroelectricity, unusual magnetism, multiferroicity etc. has been observed at oxide interface making it an interesting field of study. We have shown that variable energy photoemission spectroscopy with high photon energies, can be a useful tool to realize such interfaces and controlling the properties of multilayered devices, as well as to understand the origin of unusual phenomenon exists at several multilayer interfaces. Chapter1 provides a brief description of low dimensional materials, overall perspective of interesting properties in materials with reduced dimensionality. We have emphasized on the importance of determining the internal structure of buried interface of different dimensionalities. We have given a brief overview and importance of different interfaces that we have studied in the subsequent chapters dealing with specific interfaces. Chapter 2 describes experimental and theoretical methods used for the study of interface and self-assembly reported in this thesis. These methods are divided into two categories. The first section deals with different experimental techniques, like, UV-Visible absorption and photoluminescence spectroscopy, X-Photoelectron Spectroscopy(XPS), X-Ray diffraction, Transmission Electron Microscopy(TEM) etc. This section also includes brief overview on synchrotron radiation and methods used for detail analysis of interface structure using variable energy XPS. In the second part of this chapter, we have discussed theoretical methods used in the present study. \ In Chapter 3A we have combined low energy XPS, useful to extract information of the surface of the nanoparticles, with high energy XPS, important to extract bulk information and have characterized the internal structure of nanoparticle system of different heterogeneity. We have chosen two important heterostructure systems namely, inverted core-shell(CdScore-CdSeshell) type nanoparticles and homogeneous alloy(CdSeS)type nanoparticles. Such internal structure study revealed that the actual internal structure of certain nanomaterial can be widely different from the aim of the synthesis and knowledge of internal structure is a prerequisite in understanding their property. We were able to extend the idea of variable energy XPS to higher energy limit. Many speculations have been made about the probable role of interface in controlling properties, like blinking behavior of bigger sized core-shell nanoparticles, but no conclusive support has yet been given about the nature of such interface. After successfully extending the technique to determine the internal structure of heterostructured nanoparticles to very high photon energy region, we took the opportunity to determine the internal structure of nanoparticles of sizes as large as 12nm with high energy photoemission spectroscopy for the first time. In Chapter 3B we emphasize on the importance of interface structure in controlling the behavior of bigger sized nanoparticles systems, the unsettled issues regarding their internal structure, and described the usefulness of high energy XPS in elucidating the internal structure of such big particles with grate accuracy to solve such controversies. The existence of high density storage media relies on the existence of highly sensitive magnetic sensors with large magnetoresistance. Today almost all sensor technologies used in modern hard disk drives rely on tunnel magnetoresistance (TMR) CoFeB-MgO-CoFeB structures. Though device fabrication is refined to meet satisfactory quality assurance demands, fundamental understanding of the refinement in terms of its effect on the nature of the interfaces and the MgO tunnel barrier leading to improved TMR is still missing. Where, the annealing condition required to improve the TMR ratio is itself not confirmatory its effect on the interface structure is highly debatable. In particular, it has been anticipated that under the proposed exotic conditions highly mobile B will move into the MgO barrier and will form boron oxide. In Chapter 4 we are able to shed definite insights to heart of this problem. We have used high energy photoemission to investigate a series of TMR structures and able to provide a systematic understanding of the driving mechanisms of B diffusion in CoFeBTMR structures. We have solved the mix-up of annealing temperature required and have shown that boron diffusion is limited merely to a sub-nanometer thick layer at the interface and does not progress beyond this point under typical conditions required for device fabrication. We have given a brief overview on the evolution of magnetic storage device and have described various concepts relevant for the study of such systems. The interface between two nonmagnetic insulators LaAlO3 and SrTiO3 has shown a variety of interface phenomena in the recent times. In spite of a large number of high profile studies on the interface LaAlO3 and SrTiO3 there is still a raging debate on the nature, origin and the distribution of the two dimensional electron gas that is supposed to be responsible for its exotic physical properties, ranging from unusual transport properties to its diverse ground states, such as metallic, magnetic and superconducting ones, depending on the specific synthesis. The polar discontinuity present across the SrTiO3-LaAlO3 interface is expected to result in half an electron transfer from the top of the LaAlO 3 layer to each TiofSrTiO3 at the interface, but, the extent of localization that can make it behave like delocalized with very high mobility as well as localized with magnetic moments is not yet clear. In Chapter 5 we have given a description of this highly interesting system as well as presented the outcome of our depth resolved XPS investigation on several such samples synthesized under different oxygen pressure. We were able to describe successfully the distribution of charge carriers. While synthesizing and understanding properties of nanoparticles is one issue, using them for device fabrication is another. For example, to make a certain device often requires specific arrangements of nanoparticles in a suitable substrate. Self-assembly formation can be a potential tool in these regards. Just like atom or ions, both nano and colloidal particles also assemble by themselves in ordered or disordered structure under certain conditions, e.g., the drying of a drop of suspension containing the colloid particles over a TEM grid. This phenomenon is known as self-assembly. Though, the process of assembly formation can be a very easy and cost-effective technique to manipulate the properties in the nano region, than the existing ones like lithography but, the lack of systematic study and poor understanding of these phenomena at microscopic level has led to a situation that, there is no precise information available in literature to say about the nature of such assembly. In Chapter 6 we have described experiments that eliminate the dependence of the self-assembly process on many complicating factors like substrate-particle interaction, substrate-solvent interaction etc., making the process of ordering governed by minimum numbers of experimental parameter that can be easily controlled. Under simplified conditions, our experiments unveil an interesting competition between ordering and jamming in drying colloid systems similar to glass transition phenomenon Resulting in the typical phase behavior of the particles. We establish a re-entrant behavior in the order-disorder phase diagram as a function of particle density such that there is an optimal range of particle density to realize the long-range ordering. The results are explained with the help of simulations and phenomenological theory. In summary, we were able to extend the idea of variable energy XPS to higher energy limit advantageous for investigating internal structure of nonmaterial of various dimensionalities and sizes. We were able to comprehend nature of buried interface indicating properties of heterostructures quantum dots and thin films. Our study revealed that depth resolved XPS combined with accessibility of high and variable energies at synchrotron centers can be a very general and effective tool for understanding buried interface. Finally, we have given insight to the mechanism of spontaneous ordering of nanoparticles over a suitable substrate.

Ce travail de thèse propose d'étudier les effets de taille et de microstructure sur les propriétés mécaniques de films minces d'or nanostructurés déposés sur des substrats flexibles lors d'essais de traction bi-axiale. Les couches minces d'or sont déposées sur du polyimide par pulvérisation ionique, technique qui permet de contrôler la taille des grains selon la direction de croissance dans les films minces en contrôlant l'épaisseur de ces derniers. Nous avons ensuite réalisé des expériences de déformation in-situ sur ces couches minces grâce à la machine de traction bi-axiale installée sur la ligne de lumière DiffAbs du synchrotron SOLEIL, source de rayons X intense qui permet de mesurer par diffraction les déformations dans les films minces polycristallins. La première étape de ce travail a été d'effectuer des expériences de traction bi-axiale pour des chargements dits « pas à pas » en imposant différents ratios de force sur deux séries de couches minces d’or d'épaisseurs différentes afin d'étudier la limite d'élasticité en fonction du chemin de chargement choisi et de tracer une surface de charge pour les deux séries d'échantillons d'or étudiés. La deuxième étape de ce travail a consisté à valider un mode de chargement dit « continu » en comparant les propriétés mécaniques d'une même série d'échantillons d'or obtenus avec ces deux types de chargements : « pas à pas » et « continu ». Une fois validé, nous avons réalisé des expériences de traction bi-axiale sur différentes séries d'échantillons d'or possédant différentes tailles grains et architecture afin de mettre en évidence un effet de taille sur les propriétés mécaniques de films minces nanométriques. / The main purpose of this thesis is to study the size and microstructure effects on the mechanical response of gold nanostructured thin films deposited on flexible substrates during biaxial tensile tests. Gold thin films are deposited onto polyimide substrates by sequenced ion sputtering technique in order to control the grain size in the growth direction. We have carried out in situ deformation experiments using the biaxial tensile device installed on the diffractometer of the DiffAbs beamline at synchrotron SOLEIL (Saint-Aubin, France), an intense X-rays source which allows to determine applied strains in polycrystalline thin films thanks to x-ray diffraction measurements. In a first step, we performed tensile biaxial tests for different load ratio using “step by step” procedure on two series of gold thin films showing different thicknesses in order to study the mechanical response analyzing the yield surface that can be extracted with the biaxial device. In a second step, we validated a continuous loading procedure which allows gaining a factor of 10 in the time frame. Validation is made by comparing the mechanical properties of two series of gold thin films investigated using “step by step” loading and “continuous” loading. After validation of the continuous loading procedure, tensile biaxial tests have been performed on different series of gold thin films with different grain size and architecture in order to put in highlight a size effect on the mechanical behavior of nanolayered thin films.

Abstract The energy consumption around the globe is on the rise due to the exponential population growth and urbanization. There is a need for alternative and non-conventional energy sources, which are CO2-neutral, and a need to produce less or no environmental pollutants and to have high energy efficiency. One of the alternative approaches is hydrogen economy with the fuel cell (FC) technology which is forecasted to lead to a sustainable society. Hydrogen (H2) is recognized as a potential fuel and clean energy carrier being at the same time a carbon-free element. Moreover, H2 is utilized in many processes in chemical, food, metallurgical, and pharmaceutical industry and it is also a valuable chemical in many reactions (e.g. refineries). Non-renewable resources have been the major feedstock for H2 production for many years. At present, ~50% of H2 is produced via catalytic steam reforming of natural gas followed by various down-stream purification steps to produce ~99.99% H2, the process being highly energy intensive. Henceforth, bio-fuels like biomass derived alcohols (e.g. bio-ethanol and bio-glycerol), can be viable raw materials for the H2 production. In a membrane based reactor, the reaction and selective separation of H2 occur simultaneously in one unit, thus improving the overall reactor efficiency. The main motivation of this work is to produce H2 more efficiently and in an environmentally friendly way from bio-alcohols with a high H2 selectivity, purity and yield. In this thesis, the work was divided into two research areas, the first being the catalytic studies using metal decorated carbon nanotube (CNT) based catalysts in steam reforming of ethanol (SRE) at low temperatures (<450 °C). The second part was the study of steam reforming (SR) and the water-gas-shift (WGS) reactions in a membrane reactor (MR) using dense and composite Pd-based membranes to produce high purity H2. CNTs were found to be promising support materials for the low temperature reforming compared to conventional catalyst supports, e.g. Al2O3. The metal/metal oxide decorated CNTs presented active particles with narrow size distribution and small size (~2–5 nm). The ZnO promoted Ni/CNT based catalysts showed the highest H2 selectivity of ~76% with very low CO selectivity <1%. Ethanol was shown to be a more suitable and viable source for H2 than glycerol. The dense Pd-Ag membrane had higher selectivity but a lower permeating flux than the composite membrane. The MR performance is also dependent on the active catalyst materials and thus, both the catalyst and membrane play an important role. Overall, the membrane–assisted reformer outperforms the conventional reformer and it is a potential technology in pure H2 production. The high purity of H2 gas with a CO-free reformate for fuel cell applications can be gained using the MR system. / Tiivistelmä Maailman energiankulutus on kasvussa räjähdysmäisen väestönkasvun ja voimakkaan kaupungistumisen myötä. Tällä hetkellä energian tuottamisen aiheuttamat ympäristöongelmat ja taloudellinen epävarmuus ovat seikkoja, joiden ratkaisemiseksi tarvitaan vaihtoehtoisia ja ei-perinteisiä energialähteitä, joilla on korkea energiasisältö ja jotka tuottavat vähän hiilidioksidipäästöjä. Eräs vaihtoehtoisista lähestymistavoista on vetytalous yhdistettynä polttokennotekniikkaan, minkä on esitetty helpottavan siirtymistä kestävään yhteiskuntaan. Vety on puhdas ja hiilivapaa polttoaine ja energian kantaja. Lisäksi vetyä käytetään monissa prosesseissa kemian-, elintarvike-, metalli- ja lääketeollisuudessa ja se on arvokas kemikaali monissa prosesseissa (mm. öljynjalostamoissa). Uusiutumattomat luonnonvarat ovat olleet tähän saakka merkittävin vedyn tuotannon raaka-aine. Tällä hetkellä noin 50 % vedystä tuotetaan maakaasun katalyyttisellä höyryreformoinilla. Puhtaan (yli 99,99 %) vedyn tuotanto vaatii kuitenkin useita puhdistusvaiheita, jotka ovat erittäin energiaintensiivisiä. Integroimalla reaktio- ja puhdistusvaihe samaan yksikköön (membraanireaktori) saavutetaan huomattavia kustannussäästöjä. Biopolttoaineet, kuten biomassapohjaiset alkoholit (bioetanoli ja bioglyseroli), ovat vaihtoehtoisia lähtöaineita vedyn valmistuksessa. Tämän työn tavoitteena on tuottaa vetyä bioalkoholeista tehokkaasti (korkea selektiivisyys ja saanto) ja ympäristöystävällisesti. Tutkimus on jaettu kahteen osaan, joista ensimmäisessä tutkittiin etanolin katalyyttistä höyryreformointia matalissa lämpötiloissa (<450 °C) hyödyntämällä metallipinnoitettuja hiilinanoputkia. Työn toisessa osassa höyryreformointia ja vesikaasun siirtoreaktioa tutkittiin membraanireaktorissa käyttämällä vedyn tuotantoon tiheitä palladiumpohjaisia kalvoja sekä huokoisia palladiumkomposiittikalvoja. Hiilinanoputket (CNT) havaittiin lupaaviksi katalyyttien tukimateriaaleiksi verrattuna tavanomaisesti valmistettuihin tukiaineisiin, kuten Al2O3. CNT-tukiaineelle pinnoitetuilla aktiivisilla aineilla (metalli-/metallioksidit) todettiin olevan pieni partikkelikoko (~2–5 nm) ja kapea partikkelikokojakauma. Sinkkioksidin (ZnO) lisäyksellä Ni/CNT-katalyytteihin saavutettiin korkea vetyselektiivisyys (~76 %) ja erittäin alhainen hiilimoksidiselektiivisyys (<1 %). Etanolin todettiin olevan parempi vedyn raaka-aine kuin glyserolin. Tiheillä Pd-Ag-kalvoilla havaittiin olevan vedyn suhteen korkeampi selektiivisyys mutta matalampi vuo verrattuna palladiumkomposiittikalvoihin. Membraanireaktorin suorituskyky oli riippuvainen myös katalyytin aktiivisuudesta, joten sekä kalvolla että katalyyttimateriaalilla oli merkittävä rooli kyseisessä reaktorirakenteessa. Yhteenvetona voidaan todeta, että membraanierotukseen perustuva reformointiyksikkö on huomattavasti perinteistä reformeriyksikköä suorituskykyisempi mahdollistaen tehokkaan teknologian puhtaan vedyn tuottamiseksi. Membraanitekniikalla tuotettua puhdasta vetyä voidaan hyödyntää mm. polttokennojen polttoaineena.

This thesis reports the successful rational design of three highly active photocatalytic semiconductor nanocrystal (SNC) systems by exploiting morphology effects and the electronic properties of type II semiconductor heterojunctions. Novel architectures of colloidal SNCs are produced with the aim of suppressing exciton recombination and improving charge extraction for the successful initiation of desirable redox chemistry. Rod-shaped niobium pentoxide Nb₂O₅ nanocrystals (NCs) are shown to exhibit significantly enhanced activity (10-fold increase in rate constant) relative to spherical-shaped NCs of the same material. The increase is attributed to Nb5⁺ Lewis acid site rich (001) surfaces, present in higher proportions in the rod morphology, which bind organic substrates from solution resulting in direct interaction with photogenerated charges on the surface of the NC. Building on the insights into morphology-activity dependence, type II semiconductor heterojunctions are exploited for their ability to increase exciton lifetimes and spatially separate charges. Two novel II-VI heterostructured semiconductor nanocrystals (HSNCs) systems are investigated: a series of CdX/ZnO (X = S, Se, Te) HSNCs and ZnS/ZnO HSNCs capped with two different surface ligands. In the first case, substantial photocatalytic activity improvement is observed for HSNCs (relative to pure ZnO analogues) according to the following trend: CdTe/ZnO > CdS/ZnO > CdSe/ZnO. The observed trend is explained in terms of heterojunction structure and fundamental chalcogenide chemistry. In the second case, both ZnS/ZnO HSNCs exhibit activity enhancement over analogous pure ZnO, but the degree of enhancement is found to be a function of surface ligand chemistry. Photocatalytic activity testing of all the materials investigated in this work is performed via the photodecomposition of methylene blue dye in aerated aqueous conditions under UVA (350 nm) irradiation. The synthetic techniques employed for the synthesis of colloidal SNCs investigated in this thesis range from chemical precipitation and solvothermal techniques to several different organometallic approaches. A wide variety of analytical techniques are employed for the chemical, structural and optical characterisation of SNC photocatalysts including: XRD, XPS, TEM, UV-vis absorption, PL spectroscopy and FTIR. Atom Probe Tomography (APT) is employed for the first time in the structural characterisation of II-VI heterojunctions in colloidal HSNCs. Overall, this thesis provides a useful contribution to the growing body of knowledge pertaining to the enhancement of photocatalytic SNCs for useful applications including: solar energy conversion to chemical fuels, the photodecomposition of pollutants and light-driven synthetic chemistry.

Le remplissage des nanotubes de carbone (NTC) est considéré comme une approche relativement simple permettant de synthétiser des nanocristaux du fait de l'effet de confinement 1D imposé par la cavité centrale des NTC, qui peut être seulement de l'ordre du nanomètre ou moins, notamment dans le cas des NTC monoparoi et en particulier des NTC biparois (DWCNT) sur lesquels nous avons concentré nos efforts. De tels nanocristaux devraient avoir des propriétés physiques (électriques, magnétiques) différentes de celles de leurs équivalents à l'état massif du fait de la modification de la coordinence des atomes ou des ions les composant. Parmi les différentes méthodes existantes pour le remplissage des NTC, (in situ pendant la synthèse, a posteriori à partir de solutions), la méthode faisant intervenir des matériaux fondus est la plus populaire pour le remplissage par des matériaux inorganiques. Elle permet en effet d'atteindre des taux de remplissage raisonnablement élevés et demeure assez simple à mettre en œuvre. Cependant, elle fait preuve d'un certain nombre de limitations (techniques) qui posent problème dans le cas de matériaux à haut point de fusion (typiquement > 1000°C), dont la réactivité avec le carbone à haute température pourrait être gênante (carboréduction des oxydes par exemples), ou encore dont la faible mouillabilité vis-à-vis du carbone à l'état fondu est rédhibitoire (métaux par exemple). Il est possible de palier à cette difficulté en procédant par étapes successives et en remplissant d'abord les NTC avec un précurseur puis d'utiliser la cavité interne des NTC comme des nanoréacteurs afin de procéder dans un second temps à une transformation in situ. Dans ces travaux, nous avons étudié (1) le remplissage de DWCNT avec de l'iode ainsi qu'avec différents iodures métalliques en mettant en œuvre essentiellement la méthode des sels fondus. Nous avons étudié en détails l'influence des paramètres physico-chimiques du matériau de remplissage (réactivité chimique sous la forme en particulier du potentiel rédox du couple iodure métallique / métal, mais aussi viscosité, tension de surface, pression de vapeur saturante en milieu fondu) mais aussi du NTC (texture cylindrique ou "en arrêtes de poisson", diamètre, nombre de parois) sur le taux de remplissage. (2) Nous avons étudié en détail un certain nombre de structures inhabituelles de nanomatériaux confinés dans les DWCNT, en faisant appel à la modélisation structurale et à la simulation d'images de microscopie électronique sur la base de ces modèles pour guider notre analyse. (3) Nous avons enfin étudié différentes réactions in situ dans les DWCNT telles que la sulfuration, la réduction sous hydrogène ou encore la fluoration afin de synthétiser des nanocristaux originaux et de les caractériser en détails à l'aide d'outils tels que par exemple le MET Haute Résolution et la spectroscopie de perte d'énergie des électrons (EELS). / Filling carbon nanotubes (CNTs) has been considered as an easy approach to synthesize various nanocrystals since the inserted materials are forced to adopt a nearly one-dimensional morphology arising from their very high aspect ratio, especially in the case of single-walled CNTs (SWCNTs) or double-walled CNTs (DWCNTs). Nanocrystals/nanowires of transition metals, especially those with very narrow diameters, are predicted to exhibit peculiar magnetic property differing from the bulk metals. Filling CNTs provides a possible way for the synthesis of such metal nanocrystals/nanowires. There are several methods for filling CNTs including the in situ method, gas phase method, molten phase method, solution method, etc. Among them, molten phase has been very popular for filling various types of nanotubes due to the possibility to reach high filling rates, simplicity and versatility. However, for materials with high melting point such as metals, it is difficult to insert them into CNTs directly. To solve this problem, we also took advantage of the inner cavity of CNTs which not only templates the growth but also acts as a nanoreactor in order to perform chemical reactions. The insertion of materials with high melting point is typically achieved by first filling CNTs with a precursor, and then transforming the precursor into the desired 1D nanostructure by post-treatments. In this thesis, (i) filling DWCNTs with iodine and various halides via the molten phase method was performed and the influence of the relevant physical and chemical properties of the halides on the filling rate was investigated. The role of the redox potential as a main parameter driving the filling efficiency is pointed out, and explained; (ii) peculiar structures of the nanocrystals confined within DWCNTs were imaged by transmission electron microscopy (TEM) and corresponding modeling of the observed crystal nanostructures and related TEM images were proposed; (iii) different in situ transformations on the iodide-filled DWCNTs were attempted and the chemical composition of the encapsulated 1D nanocrystals before and after post-filling treatments was systematically identified by means of electron energy loss spectroscopy (EELS).