Dissertationen zum Thema „Films nanoporeux“

Dans le contexte de la mise au point de dispositifs photovoltaïques efficaces, bon marché et respectueux de l’environnement, la synthèse d’oxydes métalliques semi-conducteurs tels que SnO2, Zn2SnO4 et WO3 de morphologies et textures diverses a été développée afin d’élaborer des photoanodes poreuses pour cellules solaires à colorant. D’après les études réalisées par différentes méthodes (MEB, MET, DRX et BET), les matériaux obtenus présentent des caractéristiques texturales, morphologiques et structurales appropriées pour l’application visée. Des cellules solaires à colorant ont donc été réalisées à partir de ces oxydes, puis différents paramètres influençant leurs performances ont été optimisés afin d’améliorer l’efficacité de la conversion photovoltaïque. Notamment l’influence positive de différents traitements des photoanodes (i.e. solution aqueuse de TiCl4 ou traitement à l’eau) sur les rendements de conversion énergétique et la stabilité des dispositifs a été démontrée. Ainsi, des performances comparables ou supérieures à l’état de l’art ont été atteintes pour les systèmes à base de SnO2. Ces performances ont ensuite été interprétées en déterminant les processus électroniques et ioniques ayant lieu dans ces cellules par différentes méthodes physiques (mesures de tension de seuil et de décroissance de circuit-ouvert, spectroscopie d’impédance). Enfin, des électrodes réalisées à partir de WO3 déposé sur substrats flexibles ont démontré des propriétés électrochromes très prometteuses ce qui ouvre de nouvelles perspectives dans le domaine de l’affichage
In the context of the development of efficient, low-cost and environmentally friendly photovoltaic devices, the synthesis of metal-oxide semiconductors such as SnO2, Zn2SnO4 and WO3 with various textures and morphologies have been developed in order to achieve nanoporous photoanodes for dye-sensitized solar cells. According to studies carried out by different characterization methods (SEM, TEM, XRD and BET), the resulting materials show interesting features for the expected application. Dye solar cells were then fabricated from photoanodes processed with these oxides and several parameters influencing their performance were optimized to improve the overall conversion efficiency. In particular, the beneficial effect of different treatments of the photoanodes (ie aqueous TiCl4 or water treatment) on the power conversion efficiency and the stability of the devices has been evidenced. Thus, state-of-the art or, even, record efficiencies were reached in the case of SnO2-based systems. These performances were then rationalized by determining the electronic and ionic processes occurring in these devices by various physical methods (threshold voltage and open-circuit photovoltage decay measurements, electrochemical impedance spectroscopy). Finally, electrodes based on WO3 and deposited on flexible substrates have shown very promising electrochromic properties, which opens up new prospects in the field of smart displays

Lors des dix dernières années, les films multicouches ont suscité l’intérêt de la communauté scientifique pour leurs propriétés innovantes. Principalement issus de l’association de polyélectrolytes et/ou de nanoparticules de différentes morphologies, ils ont ouvert la voie à la fabrication d’une nouvelle catégorie de matériaux nanoporeux, possédant des propriétés optiques attractives telles que la coloration structurale et l’antireflet. Les films multicouches à base de deux nanoparticules de charges opposées sont plus rares et permettent de jumeler les propriétés des deux nanoparticules utilisées et d’en faire émerger de nouvelles. Dans cette étude, nous nous sommes intéressés à deux nanoparticules anisotropes, de facteurs d’aspects contrôlés et respectivement bio/geosourcées : les nanocristaux de cellulose (NCC) et des nanotubes d’imogolite. Le but de cette étude est d’étudier la possibilité de créer un film multicouche bio-géo inspiré à base de ces deux nanoparticules par immersion et d’en étudier les propriétés optiques. Dans un premier temps, nous avons comparé les films multicouches NCC/Ge-imogolites à ceux plus communément décrits dans la littérature, à savoir, des films à base de NCC ou d’imogolite associés à un polyélectrolyte de charge opposée. Les différents paramètres de trempage comme le temps d’immersion et la force ionique de la suspension ont été variés afin d’obtenir une densité de film optimale. Pour finir la porosité des films et leur comportement dans l’eau ont été étudiés par QCM-D, ainsi que leurs propriétés optiques par mesure de transmittance
In the past decade, multilayer thin films drew the scientific community attention for their unique properties. Indeed, principally made of an association of polyelectrolytes and/or nanoparticles, of various morphologies and chemistries, they allow the design of a range of porous nanomaterials with unique optical properties, such as structural colors or anti-reflectivity. Less commonly described, thin films made of two nanoparticles of opposite charges are gaining interest since they combine the properties of the two nanoparticles used, and generate new ones through their association. In this study, multilayer coatings were formed through the association of two anisotropic oppositely charged nanorods of well-controlled aspect ratio, i.e. bio-based anionic cellulose nanocrystals (CNC) and geo-based cationic Imogolites. This study deals with the feasibility to create a bio-geo-inspired multilayer thin film based on these two nanoparticles by dipping and characterize their optical properties. Firstly, elaboration of multilayered thin films from CNC and Ge-Imogolites nanorods, were studied in comparison with reference films incorporating CNC or Imogolites with polyelectrolytes bearing opposite charges of the nanorods. Multilayered thin films were assembled by the dipping procedure and various parameters (adsorption time, ionic strength, etc.) were varied to investigate the optimal density for the film. To finish, film porosities were investigated using QCM-D, and optical properties were investigated by transmittance measurements

Le travail de thèse a pour objectif la conception de micro-supercondensateurs tout-solide à base d’or nanoporeux, intégrés sur substrat de silicium. Dans un premier temps nous avons développé un procédé de formation de films d’or par réduction chimique auto-catalytique. Afin d’augmenter l’adhérence du film d’or sur le substrat de silicium, une couche d’accroche originale a été élaborée par procédé sol-gel. Il s’agit d’un film mince d’oxyde de zirconium (ZrO2) dopé par des nanoparticules d’or. La porosité de ces films d’or a été contrôlée par une méthode de templating à partir de microsphères de polystyrène (Ø ≈ 20 nm). Les films d’or nanoporeux peuvent atteindre 1,2 µm d’épaisseur en l’absence de délamination. La porosité est totalement interconnectée et la taille des pores (20 nm) a été choisie afin d’être compatible avec l’électrolyte utilisé. Le procédé fait uniquement intervenir des méthodes chimiques en solution et est totalement compatible avec les procédés classiques de micro-fabrication. Les films d’or nanoporeux constituant le matériau d’électrodes du micro-supercondensateur, ont été structurés par photolithographie sous la forme de peignes interdigités. L’imprégnation d’un électrolyte polymère gélifié (PVA / KOH) a permis de finaliser la fabrication du micro-supercondensateur tout-solide. Les caractérisations électrochimiques montrent que le micro-dispositif atteint une capacité surfacique de 240 µF/cm² à 20 mV/s, et peut endurer plus de 8000 cycles en ne perdant que 5% de sa capacité initiale. Ces performances sont comparables à celles des micro-supercondensateurs intégrés tout-solide reportées dans la littérature
The thesis work aims at the design of nanoporous gold-based all-solid state micro-supercapacitors, integrated on a silicon substrate. In a first step, we have developed a process for the formation of gold films by auto-catalytic chemical reduction. In order to enhance the adhesion of the gold film to the silicon substrate, an original seed layer was produced by a sol-gel process. It consists in a thin film of zirconium oxide (ZrO2) doped with gold nanoparticles. The porosity of these gold films was controlled by a templating method using polystyrene microspheres (Ø ≈ 20 nm). Nanoporous gold films can reach a 1.2 μm thickness in the absence of delamination. The porosity is completely interconnected and the pore size (20 nm) was chosen in order to be compatible with the used electrolyte. The method only involves wet chemistry processes and is fully compatible with conventional micro-manufacturing processes. The nanoporous gold films constituting the electrode material of the micro-supercapacitor have been structured by photolithography in the form of interdigitated combs. The impregnation of a gelled polymer electrolyte (PVA / KOH) made it possible to finalize the manufacture of the all-solid state micro-supercapacitor. Electrochemical characterizations show that the micro-device reaches a surface capacitance of 240 μF/cm² at 20 mV/s, and can endure more than 8000 cycles, while losing only 5% of its initial capacitance. These performances are comparable to those of the all-solid state integrated micro-supercapacitors reported in the literature

Les semi-conducteurs nanométriques (NSCs) sont des matériaux à haut potentiel pour diverses applications tels que l’optoélectronique, le photovoltaïque, et les capteurs chimiques. Dans cette étude, deux types de synthèses ont été développées pour la fabrication des NSCs dans des matériaux zéolitiques: une synthèse en une étape (OSS), et une synthèse par échange d’ion et irradiation (IEI). La méthode OSS permet de synthétiser directement des NSCs dans une zéolithe hydrophobe de silice pure de type MFI par co-condensation de précurseur de silice, de 3-mercaptopropyl-trimethoxysilane (MPTS), de précurseur métallique (Cd2+, Zn2+, Pb2+, Mo5+, Co2+) et de tetrapropylammonium hydroxyde (TPAOH). Pour la méthode IEI, le précurseur métallique est introduit dans les zéolithes des aluminosilicates (zéolithe du type LTL, FAU et MFI) par échange d’ion. La suspension de zéolithe, en présence de 2-mercaptoethanol, est irradiée avec différentes doses de rayon gamma pour favoriser la formation de métaux soufrés nanométriques. Les particules CdS, emprisonnées dans les canaux de la LTL, maintiennent leur taille dans la gamme sous-nanométrique. Tandis que dans les zéolithes du type FAU et MFI, 2 types de CdS sont formés. Les échantillons préparés par les approches OSS et IEI sont stables dans l’air et assemblés en films minces. Ainsi, une nouvelle voie de préparation de NSCs appliquée à la fabrication de capteur chimique, d’optoélectronique et de photovoltaïque est possible
Nanosized semiconductors (NSCs) are potential materials for a variety of optoelectronic, solar photovoltaic and sensor devices. In this study, two main approaches for preparation of stable NSCs in zeolitic materials, namely the one-step synthesis (OSS) and the ion-exchange-irradiation (IEI) have been developed. The OSS approach involves direct synthesis of NSCs in hydrophobic pure silica MFI-type zeolite via co-condensation of silica source, 3-mercaptopropyl-trimethoxysilane (MPTS), metal precursor (Cd2+, Zn2+, Pb2+, Mo5+, Co2+) and tetrapropylammonium hydroxide (TPAOH). For IEI approach, metal precursors were first introduced into the as-prepared aluminosilicate zeolites (LTL, FAU and MFI- type zeolites) by ion-exchange process. The ion-exchanged zeolite suspensions in the presence of 2-mercaptoethanol are irradiated with different doses of gamma ray to facilitate the formation of nanosized metal sulfides. The NSCs trapped in the LTL channels maintain their maximum particle size in the sub-nm range, while two different populations of clusters in FAU and MFI zeolites are formed. The NSCs samples prepared by both OSS and IEI approaches are stable in air and assembled into thin films. Thus, alternative routes for preparation of stable NSCs for fabrication of new sensing, optoelectronic and photovoltaic devices are disclosed

Les films nanoporeux présentent un grand intérêt en raison de leurs propriétés uniques et de leurs applications variées dans différents domaines. La caractérisation de ces films est cruciale pour comprendre et contrôler leurs propriétés (telles que la taille et la distribution des pores, l'aire de surface, la porosité et la résistance mécanique) et leur évolution dans des conditions de fonctionnement. A cet égard, parmi les méthodes spectroscopiques optiques, l'ellipsométrie, en particulier l'ellipsométrie in situ, est une technique puissante et non-destructive pour caractériser les films nanoporeux. Cette technique est bien adaptée pour contrôler l'épaisseur et les propriétés optiques du film dans différents environnements. Il s'agit d'un outil important pour les applications dans lesquelles des molécules invitées sont absorbées dans la porosité (détection, capture de gaz, récolte d'eau, refroidisseurs). Malgré ses nombreux avantages, l'ellipsométrie spectroscopique in situ ne parvient toujours pas à fournir certaines informations clés qui restent insaisissables ou n'ont pas été étudiées, telles que (i) l'évolution chimique dans les films, (ii) la réponse cinétique des films dans des conditions hors équilibre et (iii) la cartographie locale de processus d'adsorption. Dans cette thèse, nous avons abordé certaines des questions mentionnées ci-dessus en développant trois méthodologies optiques pour caractériser plus en profondeur les films nanoporeux. Nous avons d'abord introduit l'ellipsométrie IR in situ avec une chambre environnementale/thermique pour suivre l'évolution des propriétés structurelles, optiques et chimiques pendant la formation de films mésoporeux dérivés de sol-gel. A titre d'exemple, nous avons étudié la formation de TiO2 mésoporeux pendant les processus de calcination et de photo-recuit. De plus, nous introduisons une nouvelle approche appelée "ellipsométrie cyclique" pour étudier le comportement cinétique d'adsorption de différents films nanoporeux connus, y compris les films de TiO2, SiO2 et MOFs. Inspirée de la voltampérométrie cyclique (en électrochimie), cette méthode nous permet d'étudier la réponse d'adsorption dans les pores des films dans des conditions hors équilibre. Enfin, nous étudions la possibilité de visualiser les processus d'adsorption locaux en combinant la structure poreuse avec des nanoantennes plasmoniques. Nous avons d'abord développé une boîte à outils de simulation pour décrire les propriétés optiques des films plasmoniques composites. Ensuite, nous introduisons la microscopie hyperspectrale environnementale pour suivre les mécanismes d'adsorption au niveau d'une seule nanoantenne. Enfin, nous avons fabriqué un revêtement poreux plasmonique composite dans lequel un chauffage local est induit par irradiation lumineuse, une première étape vers de futurs dispositifs programmables. Plus largement, nos études soulignent le potentiel des spectroscopies optiques avec des chambres environnementales pour sonder la réponse et l'évolution des films nanoporeux dans diverses conditions (équilibre ou hors équilibre) et à différentes échelles (locale ou d'ensemble) avec des implications importantes au niveau fondamental et appliqué
Nanoporous films are of great interest due to their unique properties and various applications in different fields. Characterization of these films is crucial to understanding and controlling their properties (such as pore size and distribution, surface area, porosity, and mechanical strength) and their evolution in operating conditions. In this regard, among the optical spectroscopic methods, ellipsometry, especially in situ ellipsometry is a powerful and non-destructive technique for characterizing nanoporous films. This technique is well suited to monitor the film's thickness and optical properties in different environments. This is an important tool for applications in which guest molecules are uptaken into the porosity (sensing, gas capture, water harvesting, chillers). Despite the many advantages, the in situ spectroscopic ellipsometry still fails in providing some key insights that remain elusive or have not been investigated, such as (i) the chemical evolution in the films, (i) the kinetics response of the films under out-of-equilibrium conditions and (iii) the local mapping of sorption events. In this thesis, we addressed some of the abovementioned questions by developing three optical methodologies to characterized nanoporous films more in depth. We first introduce in situ IR ellipsometry with an environmental/thermal chamber to monitor the evolution of structural, optical, and chemical properties during the formation of sol-gel derived mesoporous films. As a case of study, we investigated mesoporous TiO2 formation during the calcination and photo-annealing processe. Furthermore, we introduce a new approach called "cyclic ellipsometry" to study the sorption kinetics behavior of different known nanoporous films, including TiO2, SiO2, and MOFs films. Inspired by cyclic voltammetry (in electrochemistry), this method allows us to investigate sorption response into the pores of the films in out-of-equilibrium conditions. At last, we investigate the possibility of visualizing local sorption events by the combination of porous structure with plasmonic nanoantennas. First, we developed a simulation toolbox to describe the optical properties of the composite plasmonic films. Then, we introduce environmental hyperspectral microscopy to follow sorption behaviors at the single antenna level. At last, a composite plasmonic porous coating was fabricated in which local heating is induced by light irradiation, a first step toward future programmable devices. More broadly, our studies highlight the potential of optical spectroscopies with environmental chambers to probe the response and the evolution of nanoporous films in various conditions (equilibrium vs out-of-equilibrium) and at different scales (local vs ensemble) with important implications at the fundamental and applied levels

Ces travaux de thèse ont eu deux objectifs: i) le développent de systèmes nanofluidique en utilisant une méthode non-lithographique, peu chère et facilement transposable à l'échelle industrielle ii) la compréhension des phénomènes nanofluidiques au travers des études expérimentales et de modélisation. Des couches minces mesoporeuses, en particulier des structures planaires avec des nanopiliers, ont été utilisé pour des études sur l'infiltration capillaire des liquides dans espaces confiné au niveau nanométrique. En plus des premiers tests pour des applications plus complexes comme des séparations et réactions nanoconfiné. Des structures mesoporeuses non-organisés ont aussi été étudiées pour déterminer la relation entre la nanostructure et la vitesse de remplissage capillaire. A été aussi démontré que pour des porosités avec des forts rétrécissements le remplissage capillaire se produit par l'intermédiaire d'une phase vapeur. Les échantillons ont été préparés par dip-coating. Une méthode de préparation basé sur une substitution de la plus grande parte de la solution à déposer par un fluide inerte a été développé. La méthode permet de réduire fortement le cout de procédé et, par conséquence, de faire des dépôts sur plus grande surface. Un effort dans la modélisation des phénomènes nanofluidiques a aussi été fait pendant cette thèse. Une méthode de simulation qui permet de décrire adéquatement les interactions hydrodynamiques dans un système nano a été utilisée pour simuler un flux électro-osmotique. La méthode, Stochastic Rotational Dynamics, a été valide par confrontation avec des résultats connus et l'influence des certains paramètres de simulation évaluée dans le détail
This thesis had a dual purpose: i) the development of nanofluidic devices through not lithographic, cheap and scalable bottom-up approach ii) the understanding of nanofluidic phenomena both through experiments and simulations. Mesoporous thin films, in particular Pillared Planar Nanochannels (PPNs), were prepared and utilized to study the capillary infiltration of liquids in nanostructures and have been tested for future nanofluidic applications like separations and nanoconfined reactions. Non organized mesoporous films have also been studied to determine the relationship between nanostructure characteristics and infiltration speed. It has been also demonstrated that in the case of porosities with reduced bottle-necks capillary penetration is performed through a vapor mediated mechanism The samples were prepared by dip-coating. A novel method of preparation based on the substitution of a large part of the deposing solution in dip-coating with an inert fluid has been developed in order to strongly reduce the fabrication costs and allow the preparation of larger samples. Moreover advancement in control of the dip-coating technique in “acceleration-mode” to produce thickness gradients has been developed and some potential application linked to fluidics shown. Finally a part of the effort of this thesis has been placed in the modeling of the electro-osmotic phenomenon in nanostructures through a rather novel simulation method, Stochastic Rotational Dynamics, which takes into account the hydrodynamics and the other interactions inside a nanofluidic system. Validations of the method and further investigations in particular nanofluidic conditions have been performed

Ces travaux de thèse ont eu deux objectifs: i) le développent de systèmes nanofluidique en utilisant une méthode non-lithographique, peu chère et facilement transposable à l'échelle industrielle ii) la compréhension des phénomènes nanofluidiques au travers des études expérimentales et de modélisation. Des couches minces mesoporeuses, en particulier des structures planaires avec des nanopiliers, ont été utilisé pour des études sur l'infiltration capillaire des liquides dans espaces confiné au niveau nanométrique. En plus des premiers tests pour des applications plus complexes comme des séparations et réactions nanoconfiné. Des structures mesoporeuses non-organisés ont aussi été étudiées pour déterminer la relation entre la nanostructure et la vitesse de remplissage capillaire. A été aussi démontré que pour des porosités avec des forts rétrécissements le remplissage capillaire se produit par l'intermédiaire d'une phase vapeur. Les échantillons ont été préparés par dip-coating. Une méthode de préparation basé sur une substitution de la plus grande parte de la solution à déposer par un fluide inerte a été développé. La méthode permet de réduire fortement le cout de procédé et, par conséquence, de faire des dépôts sur plus grande surface. Un effort dans la modélisation des phénomènes nanofluidiques a aussi été fait pendant cette thèse. Une méthode de simulation qui permet de décrire adéquatement les interactions hydrodynamiques dans un système nano a été utilisée pour simuler un flux électro-osmotique. La méthode, Stochastic Rotational Dynamics, a été valide par confrontation avec des résultats connus et l'influence des certains paramètres de simulation évaluée dans le détail
This thesis had a dual purpose: i) the development of nanofluidic devices through not lithographic, cheap and scalable bottom-up approach ii) the understanding of nanofluidic phenomena both through experiments and simulations. Mesoporous thin films, in particular Pillared Planar Nanochannels (PPNs), were prepared and utilized to study the capillary infiltration of liquids in nanostructures and have been tested for future nanofluidic applications like separations and nanoconfined reactions. Non organized mesoporous films have also been studied to determine the relationship between nanostructure characteristics and infiltration speed. It has been also demonstrated that in the case of porosities with reduced bottle-necks capillary penetration is performed through a vapor mediated mechanism The samples were prepared by dip-coating. A novel method of preparation based on the substitution of a large part of the deposing solution in dip-coating with an inert fluid has been developed in order to strongly reduce the fabrication costs and allow the preparation of larger samples. Moreover advancement in control of the dip-coating technique in “acceleration-mode” to produce thickness gradients has been developed and some potential application linked to fluidics shown. Finally a part of the effort of this thesis has been placed in the modeling of the electro-osmotic phenomenon in nanostructures through a rather novel simulation method, Stochastic Rotational Dynamics, which takes into account the hydrodynamics and the other interactions inside a nanofluidic system. Validations of the method and further investigations in particular nanofluidic conditions have been performed

Structure and dynamics of nanoconfined glass-forming oligomers and diblock coplymers (BPCs) are investigated by a combination of infrared transition moment orientational analysis (IR-TMOA), positron annihilation lifetime spectroscopy (PALS), grazing incidence small angle X-ray scattering (GISAXS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and broadband dielectric spectroscopy (BDS). The oligomers probed are the van der Waals type, tris(2-ethyhexyl)phosphate (TEHP) and the self-associating molecules of 2-ethyl-1-hexanol (2E1H). Symmetric and asymmetric poly(styrene-b-1,4-isoprene) P(S-b-I) are studied for the case of BCPs. The samples are confined either in one-dimensional (1D) in form of thin films or in 2D (nanopores) geometrical constraints. The molecular order of TEHP in nanopores as studied by IR-TMOA shows that about 7% of the molecules are preferentially oriented perpendicular to the long axis of the pores due to their interaction with the pore walls. PALS results reveal that 2E1H confined in nanopores exhibit larger free volume with respect to the bulk. In thin films (1D), P(S-b-I) having volume fraction of isoprene blocks f(PI)= 0.55 exhibits randomly oriented lamellae and their thicknesses are directly proportional to the film thickness d(film). For f(PI) = 0.73, perpendicular cylinders with respect to the substrate are observed for d(film)>50 nm but they lie along the substrate plane when d(film) < 50 nm. In AAO pores (2D) with average pore diameter d(pore) of 150 nm, straight nanorods are formed which change to helical structures in 18 nm pores. Molecular dynamics of 2E1H and TEHP constrained in nanopores (2D), is influenced by the interplay between confinement and surface effects. Confinement effects show up as an increase in the structural relaxation rate with decreasing pore sizes at the vicinity of the glass transition temperature. This is attributed to the reduced packing density of the molecules in pores as quantified by PALS results for 2E1H. Whereas the orientation and morphologies of the domains in P(S-b-I) and the chain dynamics of isoprene chains are influenced by the finite--size and dimensionality of confinement, the segmental motion, related to the dynamic glass transition (DGT) of both styrene and isoprene blocks remains unaffected-in its relaxation time-within experimental accuracy. Effects of nanoscale confinement on the molecular dynamics therefore depend on a number of factors: the type of molecules (polymers, low molecular liquids), interfacial interactions and the dimensionality of the constraining geometries.

Thesis (Ph.D.)--Chemical Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Prof. Sankar Nair; Committee Co-Chair: Prof. Samuel Graham; Committee Member: Prof. Amyn S. Teja; Committee Member: Prof. Mo Li; Committee Member: Prof. Peter Ludovice.

Nanoporous metal films have many applications in a great variety of scientific fields, and especially nanoporous gold films have many applica- tions in green nanotechnology. Hence, structural and optical properties of such materials were investigated. Local density functions and local per- colation functions were calculated by using scaning electron micrographs and the optical properties of the films were calculated using the Hilfer equation. The results are presented in the report as graphs and show how the materials optical properties depend on the structure of the gold films.

During energy generation, transportation and usage, large amounts of energy are lost as waste heat. With increasing energy consumption and environmental issues, exploiting this waste heat has drawn extensive attention. Thermoelectric energy conversion is an approach to take advantage of the ability of thermoelectric materials to convert waste heat into electricity. The thermoelectric effect was initially studied in the early 19th century with the discovery of the Seebeck effect. Thermoelectric materials and devices can directly convert thermal energy (temperature gradients) into electric energy (voltage) and vice versa. Thermoelectric devices have been used in space as energy generators and as coolers in small-scale instruments and devices. However, thermoelectrics remain limited in terms of applications. The traditional state-of-the-art thermoelectric materials, such as Bi2Te3, PbTe, and SnTe, exhibit high thermoelectric properties, but their disadvantages of toxicity, extreme rarity of tellurium, and oxidation when exposed to high temperature air restrict them from widespread use in applications. Compared to traditional thermoelectric materials, misfit-layered Ca3Co4O9 not only has the typical advantages of oxides including low cost, being environmentally friendly, and good chemical stability at high temperatures, but also has relatively high thermoelectric properties due to the complex structure which composed of CoO2 conductive layers and rock-salt type Ca2CoO3 insulating layers. Many strategies have been used to enhance the thermoelectric performance of Ca3Co4O9. Compared with bulk materials, thermoelectric thin films can exhibit improved thermoelectric properties and new application in flexible devices and miniaturization. Flexibility can be induced in Ca3Co4O9 by nanostructural tailoring to act as fully inorganic flexible thermoelectrics. In order to explore how to produce Ca3Co4O9 nanoporous thin film and control the porosity in the films, I have investigated the nanoporous Ca3Co4O9 system. Nanoporous Ca3Co4O9 thin films were synthesized using sequential reactive magnetron sputtering and post annealing to determine the key factors of nanoporous Ca3Co4O9 formation and tailoring of the porosity.
Under produktion, transport och användning av energi förloras stora mängder som spillvärme. Med ökande energiförbrukning och miljöfrågor har utnyttjande av spillvärme fått mer uppmärksamhet de senaste åren. Termoelektrisk omvandling av energi är ett tillvägagångssätt som utnyttjar förmågan hos termoelektriska material att omvandla spillvärme till el. Den termoelektriska effekten studerades ursprungligen i början av 1800-talet med upptäckten av Seebeck-effekten. Termoelektriska material och enheter kan direkt omvandla termisk energi (temperaturgradienter) till elektrisk energi (spänning) och vice versa. Termoelektriska komponenter har använts i rymden som energikällor och för kylning i småskaliga instrument och anordningar. Emellertid förblir termoelektriska komponenter begränsade när det gäller breda tillämpningar. Traditionella termoelektriska material som Bi2Te3, PbTe och SnTe, har bra termoelektriska egenskaper, men deras nackdelar med toxicitet och oxidation när de utsätts för luft vid hög temperatur begränsar dem från utbredd användning, liksom det faktum att råmaterialet tellur är mycket sällsynt. Jämfört med traditionella termoelektriska material har Ca3Co4O9 inte bara de typiska fördelarna med oxider som låg kostnad och kemisk stabilitet vid höga temperaturer utan har också relativt goda termoelektriska egenskaper på grund av den komplexa strukturen som består av ledande CoO2-skikt och isolerande Ca2CoO3-skikt. Många strategier har använts för att förbättra dess termoelektriska prestanda. Termoelektriska tunna filmer kan uppvisa förbättrade termoelektriska egenskaper och leda till nya tillämpningar i flexibla enheter och miniatyrisering. Mekanisk flexibilitet kan induceras i Ca3Co4O9 genom att styra nanostrukturen. För att utforska hur man producerar Ca3Co4O9 tunna filmer och kontrollerar porositeten i filmerna har jag undersökt det nanoporösa Ca3Co4O9-systemet. Nanoporösa tunna Ca3Co4O9 filmer syntetiserades med sputtring för att bestämma de viktiga faktorerna som påverkar bildning och porositet i Ca3Co4O9-filmer.

Funding agencies: Chinese Scholarship Council, The Knut and Alice Wallenberg Foundation, The Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971), The Swedish Energy Agency (Project 46519-1)

2010 - 2011
When the applied magnetic field is higher than the lower critical field but below the upper critical field, a type-II superconductor allows magnetic flux to penetrate it in the form of vortices, a tiny normal area surrounded by supercurrents. Driven by the Lorentz force of a passing external current or by thermal activation, vortices can move. Their motion induces energy dissipation and eventually can destroy the super- conductivity. Recent advances in nanofabrication have led to tremendous possibilities for implementing superconducting pinning structures and controlling the motion of vortices. The dynamics of vortices in confined superconducting geometries has gener- ated much interest, including studies of fundamental properties about vortex matter and devices based on the motion of the vortices. During the past decades, a lot of efforts have been devoted to introducing artificial pinning centers into superconductors to stabilize and pin the vortex lattice against the external driving force, thus giving rise to higher critical currents. This is of practical importance since super- conductors are required to maintain high critical currents for potential technological applications. Generally there are two different kinds of artificial pinning centers. [edited by the author]
X n.s.

In this work, a series of simple, rapid and effective photoelectrochemical methodologies have been developed and successfully applied to the study of kinetic and thermodynamic characteristics of photocatalytic oxidation processes at TiO2 nanoparticulate films. As an application of the systematic studies of photocatalytic processes by photoelectrochemical techniques, a rapid, direct, absolute, environmental-friendly and accurate COD analysis method was successfully developed. In this work, the TiO2 nanoparticles colloid was prepared by the sol-gel method. The TiO2 nanoparticles were immobilized onto ITO conducting glass slides by dip-coating method. Thermal treatment was carried out to obtain nanoporous TiO2 films of different structures. At low calcination temperature (below 600°C), nanoporous TiO2 films of pure anatase phase were prepared. At high calcination temperature (above 600°C), nanoporous TiO2 films of mixed anatase and rutile phases were obtained. At these film electrodes, the work was carried out. By employing steady state photocurrent method and choosing phthalic acid as the model compound, the photocatalytic activity of the TiO2 nanoporous films calcined at various temperatures and for different lengths of time was evaluated. It was found that the films with mixed anatase and rutile phases calcined at high temperature exhibited high photocatalytic activity. Based on semiconductor band theory, a model was proposed, which explained well this finding. By employing linear sweep voltammetry (under illumination) and choosing glucose (an effective photohole scavenger) as a model compound, the characteristics of the photocatalytic processes at nanoparticulate semiconductor electrodes were investigated. Characteristics of the nanoporous semiconductor electrodes markedly different from bulk semiconductor electrodes were observed. That is, within a large range of electrode potentials above the flat band potential the electrodes behaved as a pure resistance instead of exhibiting variable resistance expected for bulk semiconductor electrodes. The magnitude of the resistance was dependent on the properties of the electrodes and the maximum photocatalytic oxidation rate at TiO2 surface determined by the light intensity and substrate concentration. A model was proposed, which explained well the special characteristics of particulate semiconductor electrodes (nanoporous semiconductor electrodes). This is the first clear description of the overall photocatalytic process at nanoparticulate semiconductor electrodes. The investigation set a theoretical foundation for employing photoelectrochemical techniques to study photocatalytic processes. By using the transient technique (illumination step method analogous to potential step method in conventional electrochemistry), the adsorption of a number of strong adsorbates on both low temperature and high temperature calcined TiO2 nanoporous films was investigated. Similar adsorption characteristics for different adsorbates on different films were observed. In all the cases, three different surface bound complexes were identified, which was attributed to the heterogeneity of TiO2 surface. The photocatalytic degradation kinetics of the pre-adsorbed organic compounds of different chemical nature was also studied by processing the photocurrent-time profiles. Two different photocatalytic processes, exhibiting different rate characteristics, were observed. This was, again, attributed to the heterogeneity of the TiO2 surface corresponding to heterogeneous adsorption characteristics. The catalytic first order rate constants of both fast and slow processes were obtained for different organic compounds. It was found that for different adsorbates of different chemical nature the magnitudes of rate constant for the slow kinetic process were very similar, while the magnitudes of rate constant for the fast process were significantly affected by the photohole demand characteristics of different adsorbates. Photohole demand distribution that depends on the size and structure of the adsorbed molecules was believed to be responsible for the difference. By employing steady state photocurrent method, the photocatalytic degradation kinetic characteristics of both strong adsorbates and weak adsorbates of different chemical structures were compared at pure anatase TiO2 nanoporous TiO2 films as well as at anatase/rutile mixed phase TiO2 nanoporous film electrodes. At the former electrodes for all the different organic compounds studied, the photocatalytic reaction rate increased linearly with concentration at low concentrations. Under such conditions, it was demonstrated that the overall photocatalytic process was controlled by diffusion and was independent of the chemical nature of organic compounds. However, the linear concentration range and the maximum photocatalytic reaction rate at high concentrations were significantly dependent on the chemical nature of the substrates. This was explained by the difference in the interaction of different organic compounds with TiO2 surface, the difference in their photohole demand distributions at the TiO2 surface and the difference in their nature of intermediates formed during their photocatalytic mineralization. In contrast, at the latter electrodes for the photocatalytic oxidation of different organic compounds the linear ranges (diffusion control concentration range) and the maximum reaction rates at high concentration were much larger than at the former electrodes and much less dependent on the chemical nature of the organic compounds. The spatial separation of photoelectrons and photoholes (due to the coexistence of rutile phase and anatase phase) and the increase in the lifetime of photoelectrons and photoholes are responsible for the excellent photocatalytic activity of the electrodes. By employing the thin-layer photoelectrochemical technique (analogous to the thin-layer exhaustive electrolytic technique), the photocatalytic oxidation of different organic compounds at the mixed phase TiO2 nanoporous electrodes were investigated in a thin layer photoelectrochemical cell. It was found that the charge derived from exhaustive oxidation agreed well with theoretical charge expected for the mineralisation of a specific organic compound. This finding was true for all the compounds investigated and was also true for mixtures of different organic compounds. The photocatalytic degradation kinetics of different organic compounds of different chemical identities in the thin layer cell was also investigated by the photoelectrochemical method. Two kinetic processes of different decay time constants were identified, which were attributed to the degradation of preadsorbed compounds and the degradation of compounds in solution. For the degradation of compounds in solution, a change in the overall control step from substrate diffusion to heterogeneous surface reaction was observed. For different organic compounds, the variation of the rate constant was determined by the photohole demand rather than by the chemical identities of substrates. The kinetics of the fast kinetic process, on the other hand, was greatly affected by the adsorption properties of the substrates. For the strong adsorbates, the rate was much larger than for weak adsorbates. However, the rate constant of the process was independent of the chemical identities of the substrates and the variation of the constant was also determined by the photohole demand. Based on the principles of exhaustive photoelectrocatalytic degradation of organic matter in a thin layer cell, a novel, rapid, direct, environmental-friendly and absolute COD analysis method was developed. The method was tested on synthetic samples as well as real wastewater samples from a variety of industries. For synthetic samples with given compositions the COD values measured by my method agree very well with theoretical COD value. For real samples and synthetic samples the COD values measured by my method correlated very well with those measured by standard dichromate COD analysis method.

In this work, a series of simple, rapid and effective photoelectrochemical methodologies have been developed and successfully applied to the study of kinetic and thermodynamic characteristics of photocatalytic oxidation processes at TiO2 nanoparticulate films. As an application of the systematic studies of photocatalytic processes by photoelectrochemical techniques, a rapid, direct, absolute, environmental-friendly and accurate COD analysis method was successfully developed. In this work, the TiO2 nanoparticles colloid was prepared by the sol-gel method. The TiO2 nanoparticles were immobilized onto ITO conducting glass slides by dip-coating method. Thermal treatment was carried out to obtain nanoporous TiO2 films of different structures. At low calcination temperature (below 600°C), nanoporous TiO2 films of pure anatase phase were prepared. At high calcination temperature (above 600°C), nanoporous TiO2 films of mixed anatase and rutile phases were obtained. At these film electrodes, the work was carried out. By employing steady state photocurrent method and choosing phthalic acid as the model compound, the photocatalytic activity of the TiO2 nanoporous films calcined at various temperatures and for different lengths of time was evaluated. It was found that the films with mixed anatase and rutile phases calcined at high temperature exhibited high photocatalytic activity. Based on semiconductor band theory, a model was proposed, which explained well this finding. By employing linear sweep voltammetry (under illumination) and choosing glucose (an effective photohole scavenger) as a model compound, the characteristics of the photocatalytic processes at nanoparticulate semiconductor electrodes were investigated. Characteristics of the nanoporous semiconductor electrodes markedly different from bulk semiconductor electrodes were observed. That is, within a large range of electrode potentials above the flat band potential the electrodes behaved as a pure resistance instead of exhibiting variable resistance expected for bulk semiconductor electrodes. The magnitude of the resistance was dependent on the properties of the electrodes and the maximum photocatalytic oxidation rate at TiO2 surface determined by the light intensity and substrate concentration. A model was proposed, which explained well the special characteristics of particulate semiconductor electrodes (nanoporous semiconductor electrodes). This is the first clear description of the overall photocatalytic process at nanoparticulate semiconductor electrodes. The investigation set a theoretical foundation for employing photoelectrochemical techniques to study photocatalytic processes. By using the transient technique (illumination step method analogous to potential step method in conventional electrochemistry), the adsorption of a number of strong adsorbates on both low temperature and high temperature calcined TiO2 nanoporous films was investigated. Similar adsorption characteristics for different adsorbates on different films were observed. In all the cases, three different surface bound complexes were identified, which was attributed to the heterogeneity of TiO2 surface. The photocatalytic degradation kinetics of the pre-adsorbed organic compounds of different chemical nature was also studied by processing the photocurrent-time profiles. Two different photocatalytic processes, exhibiting different rate characteristics, were observed. This was, again, attributed to the heterogeneity of the TiO2 surface corresponding to heterogeneous adsorption characteristics. The catalytic first order rate constants of both fast and slow processes were obtained for different organic compounds. It was found that for different adsorbates of different chemical nature the magnitudes of rate constant for the slow kinetic process were very similar, while the magnitudes of rate constant for the fast process were significantly affected by the photohole demand characteristics of different adsorbates. Photohole demand distribution that depends on the size and structure of the adsorbed molecules was believed to be responsible for the difference. By employing steady state photocurrent method, the photocatalytic degradation kinetic characteristics of both strong adsorbates and weak adsorbates of different chemical structures were compared at pure anatase TiO2 nanoporous TiO2 films as well as at anatase/rutile mixed phase TiO2 nanoporous film electrodes. At the former electrodes for all the different organic compounds studied, the photocatalytic reaction rate increased linearly with concentration at low concentrations. Under such conditions, it was demonstrated that the overall photocatalytic process was controlled by diffusion and was independent of the chemical nature of organic compounds. However, the linear concentration range and the maximum photocatalytic reaction rate at high concentrations were significantly dependent on the chemical nature of the substrates. This was explained by the difference in the interaction of different organic compounds with TiO2 surface, the difference in their photohole demand distributions at the TiO2 surface and the difference in their nature of intermediates formed during their photocatalytic mineralization. In contrast, at the latter electrodes for the photocatalytic oxidation of different organic compounds the linear ranges (diffusion control concentration range) and the maximum reaction rates at high concentration were much larger than at the former electrodes and much less dependent on the chemical nature of the organic compounds. The spatial separation of photoelectrons and photoholes (due to the coexistence of rutile phase and anatase phase) and the increase in the lifetime of photoelectrons and photoholes are responsible for the excellent photocatalytic activity of the electrodes. By employing the thin-layer photoelectrochemical technique (analogous to the thin-layer exhaustive electrolytic technique), the photocatalytic oxidation of different organic compounds at the mixed phase TiO2 nanoporous electrodes were investigated in a thin layer photoelectrochemical cell. It was found that the charge derived from exhaustive oxidation agreed well with theoretical charge expected for the mineralisation of a specific organic compound. This finding was true for all the compounds investigated and was also true for mixtures of different organic compounds. The photocatalytic degradation kinetics of different organic compounds of different chemical identities in the thin layer cell was also investigated by the photoelectrochemical method. Two kinetic processes of different decay time constants were identified, which were attributed to the degradation of preadsorbed compounds and the degradation of compounds in solution. For the degradation of compounds in solution, a change in the overall control step from substrate diffusion to heterogeneous surface reaction was observed. For different organic compounds, the variation of the rate constant was determined by the photohole demand rather than by the chemical identities of substrates. The kinetics of the fast kinetic process, on the other hand, was greatly affected by the adsorption properties of the substrates. For the strong adsorbates, the rate was much larger than for weak adsorbates. However, the rate constant of the process was independent of the chemical identities of the substrates and the variation of the constant was also determined by the photohole demand. Based on the principles of exhaustive photoelectrocatalytic degradation of organic matter in a thin layer cell, a novel, rapid, direct, environmental-friendly and absolute COD analysis method was developed. The method was tested on synthetic samples as well as real wastewater samples from a variety of industries. For synthetic samples with given compositions the COD values measured by my method agree very well with theoretical COD value. For real samples and synthetic samples the COD values measured by my method correlated very well with those measured by standard dichromate COD analysis method.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environmental and Applied Science
Full Text

This dissertation mainly addresses the synthesis of well-ordered mesoporous titania thin films by dip coating with PEO-PPO-PEO triblock copolymer surfactant template P123. Because P123 is composed of poly(ethylene oxide) [PEO] and poly(propylene oxide) [PPO] blocks, concentrations of ingredients are adjusted to tune the films’ wall thickness, pore size and mesophase. Structural changes are consistent with partitioning of species among PEO blocks, PPO blocks, and the PEO/PPO interface. Titanates localize near PEO and increase wall thickness (by 5 nm to 7 nm). Depending on aging temperature, PPG either swells the PPO cores (when it is hydrophobic) or introduces large (>200 nm) voids (when it is hydrophilic but phase separates during heating). 1-butanol localizes at the PEO/PPO interface to favor a 3D hexagonal mesostructure. In another approach, anodizing Ti foils yields vertically aligned titania nanotubes arrays with exceptional stabilities as anodes in lithium ion batteries; they maintain capacities of 130-230 mAhg-1 over 200 cycles. No microstructural changes are induced by battery cycling and good electrical contact is maintained. A diffusion induced stress model suggests that thin-walled nanotubes arrays should be stable under testing conditions, and that ordered hexagonal columnar pore arrays should have both high charge/discharge rates and low stress development.

This dissertation addresses the synthesis of oriented mesoporous ceramic films by evaporation induced self-assembly of surfactants and ceramic precursors in films dip coated from ethanol-rich sols. First, the kinetics of silica polycondensation in surfactant templated sol-gel films is studied both before and after deposition using infrared spectroscopy. These observations reveal an induction time (with minimal condensation rate) before curing begins in certain surfactant-templated silica films, which can be utilized to perform post-synthesis modification. This induction time is maximized at high humidity, and by long nonionic surfactant headgroups (rather than, for instance, a trimethylammonium headgroup). The second part of the dissertation addresses lattice Monte Carlo (MC) simulation of the effects of confinement on the 2D hexagonally close packed (HCP) phase formed by 60 vol% surfactant in a polar solvent. The effects of size and type of confining geometry (slit, cylindrical and spherical cavities) and of surface chemistry are simulated. The HCP mesophase orients orthogonal to chemically neutral surfaces which attract both head and tail of the surfactant equally. Novel mesophase geometries are simulated including radially oriented micelles, concentric helices, and concentric porous shells. Utilizing fundamental insights from the kinetics and MC studies, the third part of the dissertation describes the synthesis of silica films with orthogonally tilted HCP mesophase on chemically neutral surfaces. Crosslinking a random copolymer of polyethylene oxide (PEO)-polyproplyene oxide (PPO) on glass slides results in chemically neutral surfaces for the PEO-PPO-PEO triblock copolymer template (P123) used here. The orthogonal orientation of the HCP channels is confirmed using advanced x-ray scattering techniques and electron microscopy. The final part of the dissertation discusses applications of ceramic films with orthogonally tilted (ortho-) HCP mesophase. Silica membranes with ortho-HCP pores are prepared on porous alumina supports, and show permeability of ethanol orders of magnitude greater than films with parallel-oriented HCP channels. Size-selective filtration of gold nanoparticles confirms the absence of any nanoscale cracks in the membranes. For a second application, we prepare titania films with ortho-HCP mesopores. Careful crystallization of the films followed by spinning on an organic hole conducting polymer (P3HT) leads to active bulk heterojunction solar cells.

This work investigates the fabrication of nanoporous templates as a part of the supercritical fluid electrodeposition (SCFED) project. The goals set for this investigation was to produce films with pore channels (diameter < 5 nm) orientated perpendicularly on top of an electrode. During the course of this work, two techniques were investigated, Stöber-derived method and electrochemical-assisted self-assembly (EASA). The Stöber films produced perpendicularly orientated pore structure through a hydrothermal process. EASA film were generated through electrodeposition, resulting in highly ordered vertically aligned pore channels. Both these techniques were transferred from indium-tin oxide (ITO) onto titanium nitride (TiN), which increased the potential window of the substrate. The pore diameters of the Stöber and EASA films were determined as 2.6 and 1.6 nm respectively. This could be increased with the addition of a swelling agent or decreased by using a surfactant with a shorter tail length. Helium-ion microscopy was shown to provide high-resolution images of silica films. It provided detailed images of the topography and pores structure at the surface of the films. Tin was deposited into the pores of an EASA film using SCFED method. The EASA films were also subjected to post-synthesis chemical modification based on grafting functionalised silane molecules. As a result, the pore size and chemical properties were altered using a range of functionalised silane based grafting agents. Trimethylchlorosilane was found to be most successful at coating the pore walls. Other larger grafting agents were shown only to partially coat the surface of the films. This was only possible due to the films being placed on the reflective surface of the TiN substrates, which allowed for changes in porosity to be analysed using ellipsometric porosimetry.

För att undersöka skillnad i prestationsförmåga mellan celler sensiterade med växtextraktsbaserad färg, och celler sensiterademed ruteniumkomplex-baserad färg, samt huruvida presskraften påverkar en cells prestationsförmåga, tillverkades icke-slutna fotoelektrokemiska färg-sensiterade solceller med tunnfilmsfotoelektroder av pressad, nanoporös titandioxid.

Cellerna pressades med tre olika presskrafter och sensiterades med växtextraktsfärg från rödkål, rödbeta, viol och henna, samt en ruteniumkomplex-baserad färg som fick utgöra kontrollbetingelse. För varje cell uppmättes IPCE- och iV-värde och motsvarande fyllnadsgrad (fill factor) och dessa jämfördes.

Ingen signifikant skillnad kunde fastställas mellan celler pressade med olika presstryck. Bland cellerna sensiterade med växtextraktbaserad färg presterade rödbeta bäst. Cellen med högst effektivitet hade fyllnadsgraden 70%. Emellertid uppvisade de växtfärgade cellerna genomgående sämre effektivitet än de rutenium-sensiterade och fotoströmmarna var mycket låga. IPCE-värdena var allmännt låga: den bäst presterande cellen hade ett IPCE-värde på något över 0,06 i våglängdsintervallet 440-470 nm. En förklaring till detta är de övriga ämnen som förutom pigment återfinns i de växtbaserade färgerna. Dessa hindrar pigmentmättnad och förhindrar att växtfärgen når ruteniumfärgens intensitet. En annan anledning består i svårigheten att passa ihop energinivåerna i cellens elektrolyt-halvledarsystem med energinivåerna hos pigmentet i växtfärgen.

Non-sealed photoelectrochemical dye sensitised solar cells (DSSC) with pressed nanoporous TiO2 thin film photoelectrodes were manufactured for the purposes of finding out whether plant extractbased dye sensitised cells can perform as well as ruthenium complex-based dye sensitised cells and whether the pressing force affects the cell performance.

The cells were pressed with three different pressing forces and sensitised with plant extracts from red cabbage, beetroot, violet and henna, as well as with a ruthenium complex-based dye for comparison. The IPCE and iV values and the corresponding fill factors of the cells were evaluated and compared.

No significant difference between the cells pressed with different pressing forces could be established. Among the plant extract-based dye sensitised cells the ones sensitised with beetroot extract performed best. The cell that achieved the highest efficiency had a fill factor of 70%. Compared to the ruthenium-sensitised cells the overall performance of the plant dye sensitised cells were very poor and the produced photocurrents very low. The IPCE values were generally low: one of the best-performing cells had an IPCE value of slightly over 0.06 in the 440-470 nm wavelength ranges. One reason for this is that it is difficult to obtain a plant extract dye as intense and deep in colour as ruthenium complex-based dyes, since pigment saturation is obstructed by the presence of other chemical compounds in the plant extracts. Another is that it is a delicate and difficult matter to match the energy levels in the electrolyte-semiconductor system with the energy levels of the pigments in the plant extract dye.

Esta tesis abarca el estudio de propiedades magnetoeléctricas en aleaciones magnéticas y metálicas nanoporosas, y en capas densas de óxidos de metales de transición. La naturaleza interfacial de los procesos magnetoeléctricos ha hecho que históricamente el estudio de estos fenómenos se abordara en sistemas de elevada relación superficie/volumen, limitándose muchas veces a capas ultradelgadas (1-2 nm). En esta tesis, se postula una nueva forma de afrontar el estudio de estos procesos, basada en el uso de materiales nanoporosos los cuales se caracterizan por tener una relación superficie/volumen muy elevada y una pared de poro o ligamento de pocos nanómetros. De esta forma, se han obtenido efectos magnetoeléctricos en materiales cuyo grosor global supera el centenar de nanometros. La síntesis de los materiales de interés se ha llevado a cabo por métodos de deposición electroquímica. Específicamente, se ha sacado partido de la formación de micelas al disolver copolímeros bloque en soluciones acuosas por encima de la concentración micelar crítica. Estas micelas quedan atrapadas durante el proceso de electrodeposición, actuando como agente moldeador. Usando este enfoque, se han podido sintetizar diferentes muestras de distintas morfologías y composiciones de la aleación cobre-níquel. La aplicación de voltaje se ha realizado haciendo uso de electrolitos, aprovechando la formación de una doble capa eléctrica. Con la intención de obtener efectos magnetoeléctricos puros (acumulación de carga) y evitar procesos oxidativos simultáneos, se ha utilizado un electrolito orgánico aprótico. Con este método, se pueden obtener campos eléctricos del orden de centenares de MV/cm. Gracias a este elevado campo eléctrico, junto con la enrome relación superficie/volumen de los materiales nanoporosos, se ha obtenido una disminución de la coercitividad de una muestra nanoporosa de Cu25Ni75 en un 32 %. Simulaciones ab-initio atribuyen estos cambios a modificaciones en la energía de anisotropía magnética adscritos a la acumulación de cargas electrostáticas en la aleación. En una segunda aproximación, se han realizado estudios de procesos de oxidación-reducción en medios acuosos (1M NaOH) controlados por voltaje, en este tipo de aleaciones. Después de aplicar potenciales positivos, se ha visto una modificación de un 33 % en la magnetización, debido a la oxidación selectiva del cobre en una muestra nanoporosa de Cu20Ni80. La oxidación resulta en una aleación enriquecida en níquel y, por ende, en una aleación con mayor momento magnético. En esta tesis, también se ha demostrado la idoneidad de la técnica de deposición por capas atómicas para producir recubrimientos conformales en materiales nanoporosos. Se ha visto que esta técnica permite preservar la integridad morfológica y estructural de la capa activa, asentando así las bases para aplicaciones en estado sólido. En la última parte de esta tesis, se ha demostrado la posibilidad de inducir ferromagnetismo mediante la aplicación de voltaje eléctrico en capas densas de Co3O4. El campo eléctrico aplicado da lugar a una migración iónica controlada, resultando en regiones ricas en oxígeno y otras en cobalto, estas últimas originando el ferromagnetismo. Este experimento es una de las primeras evidencias de movimiento iónico inducido por voltaje a temperatura ambiente y sin la necesidad de utilizar capas donadoras/aceptores de oxígeno (en otras palabras, sin fuentes o sumideros de oxígeno).
This Thesis covers the study of the magnetoelectric response in nanoporous metallic alloy and transition metal oxide dense films. The interfacial nature of magnetoelectric processes, independently of its origin, has limited its study to ultrathin film configurations (usually 1-2 nm). Here we propose a novel approach to overcome this thickness limitation, thus achieving magnetoelectric response in materials whose overall thickness is larger than 100 nm. To accomplish this, we have employed nanoporous materials, with pore walls and ligands of very few nanometers, which are characterized by a large surface to volume ratio. These materials have been synthesized by micelle assisted electrodeposition. Micelles get trapped during the electrodeposition process thus acting as a soft templating agent, allowing us to synthesize nanoporous copper-nickel alloy films with tunable composition and morphology. Voltage application has been performed through electrolyte-gating, taking advantage of the generation of an electrical double layer in aprotic organic electrolytes which helps to avoid spurious oxidation processes. This method allows for the application of electric fields as high as hundreds of MV/cm. Thanks to the high electric field achieved, together with the ultrahigh surface area of nanoporous materials, a 32 % reduction in the coercivity of a Cu25Ni75 nanoporous film has been achieved. Ab-initio simulations attribute this large effect to changes in the magnetic anisotropy energy due to charge accumulation in the sample|electrolyte interface. In a second approach, the voltage control of redox processes has been studied in aqueous electrolytes (1M NaOH). After positive bias application up to a 33 % reduction in the magnetization has been achieved in a Cu20Ni80 nanoporous sample thanks to the selective Cu oxidation. The controlled oxidation process resulted in an enriched Ni alloy which possesses a larger magnetic moment. Moreover, we have demonstrated the suitability of atomic layer deposition to conformally coat the nanoporous alloys, preserving the morphology and structure, thus setting the basis for future solid state applications. In the last part of this Thesis, it has been demonstrated that, upon electric field application, a ferromagnetic response arises in a paramagnetic single Co3O4 layer, at room temperature. The applied voltage promotes the ionic diffusion, resulting in oxygen rich and cobalt rich regions, being the latter the responsible of the induced magnetic signal. This experiment is one of the first evidences of ionic motion at room temperature without the assistance of oxygen buffer layers such as Gd2O3 or HfO2.

In the past two decades, phonon transport within nanoporous thin films has attracted enormous attention for their potential applications in thermoelectrics and thermal insulation. Various computational studies have been carried out to explain the thermal conductivity reduction within these thin films. Considering classical phonon size effects, the lattice thermal conductivity can be predicted assuming diffusive pore-edge scattering of phonons and bulk phonon mean free paths. Following this, detailed phonon transport can be simulated for a given porous structure to find the lattice thermal conductivity [Hao et al., J. Appl. Phys. 106, 114321 (2009)]. However, such simulations are intrinsically complicated and cannot be used for the data analysis of general samples. In this work, the characteristic length K-Pore of periodic nanoporous thin films is extracted by comparing the predictions of phonon Monte Carlo simulations and the kinetic relationship using bulk phonon mean free paths modified by K-Pore. Under strong ballistic phonon transport, K-Pore is also extracted by the Monte Carlo ray-tracing method for graphene with periodic nanopores. The presented model can be widely used to analyze the measured thermal conductivities of such nanoporous structures. Published by AIP Publishing.

Avec le développement des nanotechnologies, la nécessité d’élaborer des matériaux à très petite échelle s’est accrue. En particulier, les couches minces (du nanomètre au micromètre) nanoporeuses sont utilisées dans de nombreux domaines tels que l’optique, l’électronique, ou la détection. Leur utilisation principale en tant que revêtement anti-reflets est cruciale pour les panneaux solaires en permettant d’augmenter leur rendement global. Pour ce type d’application, les couches minces doivent être en mesure de supporter des phénomènes d’abrasion (sable, nettoyage) ou des attaques chimiques (pluie, pollution). La caractérisation de la porosité des couches minces est un prérequis pour ajuster leurs propriétés mais également suivre l’évolution de la couche lors de son vieillissement. L’ellipsométrie porosimétrie, technique basée sur la sorption de gaz dans la porosité, est un des outils les plus adaptées pour cette tâche. Dans cette thèse, de nouveaux revêtements anti-reflets pour panneaux solaires ont été élaborés et leur résistance à leur environnement éprouvée. La caractérisation de leur porosité a été améliorée grâce au développement de l’ellipsométrie porosimétrie au-delà de l’état de l’art actuel. La précision de celle-ci a été évaluée par comparaison avec une autre technique. De plus, une meilleure caractérisation des interconnexions entre les pores a pu être atteinte en ajoutant une nouvelle méthode d’analyse. Cela a également permis d’établir une meilleure compréhension fondamentale des phénomènes d’adsorption dans les couches minces nanoporeuses
With the development of nanotechnologies, the elaboration of materials at a very small scale has grown as an increasing necessity. In particular, nanoporous thin films (from nanometer to micrometer) are found in many different domains such as optics, electronics, protection or sensing. Their main use as antireflective coatings is of particular interest for photovoltaics, increasing their global yield. In such applications, thin films must be designed to survive various conditions, such as abrasion (sand blasting, cleaning) or chemical attacks (rain and pollution). The characterization of the nanoporosity of thin films is a prerequisite not only to adjust the properties, but also to follow the evolution of the porous structure upon aging in operating conditions. Ellipsometry porosimetry, a technique relying on the sorption of a gas inside the porosity, is one of the best candidates for this purpose. In this thesis, new antireflective coatings for photovoltaic top glass covers were elaborated and their resistance to their environment was tested. Their characterization, which can be extended to any nanoporous thin film, was improved by developing ellipsometry porosimetry above the current state of the art. The precision of the technique was assessed by comparing it with an independent method, and a better characterization of the pore interconnections was achieved by implementing an additional mode of analysis. By doing so, a better fundamental understanding of the sorption mechanisms in nanoporous thin films was established

Nanostructured noble metals exhibit novel physical, mechanical and chemical behavior, and hold promise for applications such as gas sensing and electron emission. A strong emphasis was placed on the processing and characterization of these materials, in the form of nanoporous or nanocrystalline thin films. Palladium-based and osmium-ruthenium alloys were investigated in this dissertation research and will be presented as follows: (1) Preparation and Characterization of Nanoporous Metal Thin Films (2) Characterization of Osmium-Ruthenium Coatings Nanoporous palladium (np-Pd) thin films were prepared by dealloying co-sputtered palladium-nickel precursor alloys. Nanoporous structures were created with 3-D interconnected ligaments and open pores. Size of ligaments and pores was ~5 nm, achieved with a novel processing method developed in this study. Hydrogen cycling tests performed with np-Pd films demonstrated a significant improvement in sensitivity to hydrogen and response time for sensing. Effects of alloying element (Ni), film thickness, local stress and pore/ligament size on hydrogen cycling behavior were investigated in detail. Additionally, nanoporous gold and gold-palladium thin films were studied to clarify the evolution of microstructure during dealloying, including the formation of nanoporous structure and effects of substrate curvature on dealloying behavior. The results from this project have yielded a new understanding of dealloying as well as an ideal coating material for hydrogen sensing. Nanocrystalline osmium-ruthenium (Os-Ru) thin films were deposited on porous tungsten substrates with varied sputtering parameters. These parameters were mapped to microstructure, film texture and film composition in samples that were comparable to commercial devices. Using this map, Os-Ru films can be produced with higher stability during annealing and/or high-temperature operation. These results should lead to Os-Ru top coatings that increase the lifetime and emission performance of dispenser cathodes.

2013 - 2014
Polymers can crystallize in different crystalline forms; polymorphism is the term to indicate this ability. It is known that processing and physical properties of polymer-based materials are strongly affected by the occurrence of ‘‘polymorphism’’ and ‘‘metamorphism’’ (i.e., the occurrence of ‘‘disordered’’ crystalline phases, characterized by a degree of structural organization that is intermediate between those identifying crystalline and amorphous phases). My PhD thesis is focused on the study and on the characterization of polymer films with co-crystalline and nanoporous crystalline phases. Many polymers are able to form co-crystals i.e. molecules of low molecular weight (guest) trapped in the crystalline polymer lattice (host). Over the past two decades it has been observed that some polymers, with co-crystalline phases, such as syndiotactic polystyrene (sPS) and poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) after guest removal can form nanoporous crystalline phases, able to absorb suitable guest molecules also at low activity. During this work, I have studied the possible molecular orientations that may be induced by solvents during cocrystallization process in polymeric films, (chapter 2); the development of chiro optical response, after co-crystallization with temporary chiral guest (chapter 3) and the possibility to realize photonic crystals by using polymers able to form nanoporous crystalline forms (chapter 4). In detail, in chapter 1 the procedure to obtain disordered nanoporous crystalline phases in sPS films and their possible application is reported. This disordered nanoporous crystalline phase rapidly absorb low molecular mass molecules, also from very dilute aqueous solutions. It is known in literature that nanoporous δ form of sPS is also able to absorb ethylene2b and carbon dioxide 2c-d, that have negatively effects for vegetable. Active packaging by nanoporous-crystalline films, based on the removal of molecules generated by the vegetables being detrimental for their preservation 2e, could be complemented by the slow release of antimicrobial molecules, which could be included as guest of the film crystalline cavities. Therefore the preparation of s-PS co-crystalline films that include guests with antimicrobial activity, in particular the carvacrol guest has been studied and reported in chapter 1. The kinetics of release, in variable concentrations of carvacrol in films with different thickness, has been analyzed. It was observed that the location of antimicrobial molecules mainly in the crystalline phase assure a decrease of desorption diffusivity and hence a long-term antimicrobial release. In chapter 2, the study of the possible molecular orientations that can be developed in polymer films able to form cocrystalline phases, are reported. This phenomenon has been observed only for sPS films until now. In particular, in my thesis has been shown that also other polymers, such as poly (2, 6-dimethyl-1, 4-phenylene oxide) (PPO) and poly (L-lactide) (PLLA), able to form co-crystalline phases, can develop orientations during the co-crystallization process with solvents. These orientations can be useful to the structural studies on PPO and PLLA co-crystalline forms. We have also investigated on the shrinkage behaviour developed in syndiotactic polystyrene (sPS) films after cocrystallization procedures leading to co-crystalline phases. High shrinkage values have been measured on sPS d cocrystalline phase showing a crystalline phase orientation. In order to minimize this effect, novel procedures have been developed. Another aspect of my work is focused on the study of chiro optical response of a racemic polymer crystallized with a temporary chiral guest, as reported in chapter 3. In particular, I evaluated the degree of circular polarization of different thickness sPS films, and of the achiral guests, such as azulene and 4-nitroaniline, included in the polymer crystalline phase after guest exchange procedure. These studies have been useful to investigate on the nature of this phenomenon. Finally, in chapter 4, a method to realize a photonic crystal (PhC) with polymeric materials is reported. A PhC is an object composed by two or more materials with different refractive index and an alternated periodicity. The main advantage to use polymers rather than inorganic materials is the ease and the speed to obtain thin films by spin coating and the low cost of materials. In order to realize a photonic crystal, by using thin layers of PPO presenting nanoporous crystalline phase, it has been necessary to characterize amorphous as well as crystalline phases for this purpose. Techniques such as IRRAS and ellipsometry have been used (as reported in section 4.3 of chapter 4). [edited by Author]
XIII n.s.

Nanoporous (np) materials with pore size below 100 nano-meters exist naturally in biological and mineral structures, and synthetic np materials have been used industrially for centuries. Np materials have attracted significant research interest in recent decades, as the development of new characterization techniques and nanotechnology allow the observation and design of np materials at a new level. This study focuses on two np materials: nanoporous silicon (np-Si) and nanoporous palladium (np-Pd). Silicon (Si), because of its high capacity to store lithium (Li), is increasingly becoming an attractive candidate as anode material for Li ion batteries (LIB). One significant problem with using Si as an anode is the large strain that accompanies charge-discharge cycling, due to swelling of the Si during Li insertion and deinsertion. Np-Si offers a large amount of free volume for Li absorption, which could allow the anode material to swell without cracking. A new method to fabricate thin films of high-purity (100% Si content) np-Si, which is promising as an anode material for LIB, is demonstrated and discussed in this study. Microstructural characterization, chemical analysis, battery performance testing and mechanical behavior of thin film np-Si are discussed here. Palladium (Pd) is considered an ideal and reliable hydrogen sensor and storage material, due to its fast response and selectivity for hydrogen gas. This research not only demonstrates a method to fabricate np-Pd thin films, but also proposes a method to fabricate bulk np-Pd. The uniformly crack-free and sponge-like np-Pd thin film provides high sensitivity to low concentrations of H2, showing promise as a hydrogen sensor material. Stress changes during hydrogenation/dehydrogenation were measured using wafer curvature. For bulk np-Pd, ultra-fine pore sizes were achieved by electrochemically dealloying bulk PdNi alloy. Mechanical behavior of bulk np-Pd was studied using in-situ transmission electron microscopy (TEM). Scanning electron microscopy (SEM) and x-ray diffraction were also used to characterize the structure and morphology of np-Pd. This doctoral research has involved the optimization of fabrication conditions and investigations of microstructural evolution during processing, yielding an improved understanding of the properties, mechanical behavior and potential applications of np-Si and np-Pd.

La structuration multi-échelle (à l’échelle micro- et nanométrique) de matériaux fonctionnels est un thème de recherche particulièrement actif en raison du grand potentiel des dispositifs miniatures utilisables en microélectronique, en optique (contrôle solaire, photonique) en microfluidique (lab-on-a-chip) ou pour de la détection de molécules cibles. Plusieurs techniques de micronanofabrication sont utilisées pour réaliser ces dispositifs miniaturisés. D’une part, des techniques dites « Top-Down » qui sont utilisées pour créer des formes complexes à l’échelle micro- et nanométriques à partir de matériaux massifs. Cette approche repose sur des techniques de lithographie qui offrent une grande liberté sur l’architecture finale du dispositif. Cependant, ce large choix de formes et de structures se fait au prix d’un faible rendement de fabrication qui diminue considérablement le transfert vers la production à grande échelle. A l’inverse, la fabrication de matériaux multi-échelle peut être faite par l’assemblage d’éléments moléculaires (« building blocks »). Cette seconde approche, dite « Bottom-Up » a le mérite de pouvoir être employée sur de grande surfaces mais permet seulement d’obtenir des architectures simples. La combinaison de ces deux approches ouvre un large champ d’exploration et permettrait d’obtenir des structures complexes inédites inaccessibles autrement. Le but de cette thèse est de combiner les approches « Top-Down » et « Bottom-Up » pour obtenir des matériaux à architecture hiérarchique et qui possèdent à la fois des propriétés chimiques et optiques originales. Concrètement, le cœur de ce travail de doctorat a été le dépôt par voie liquide (« Chemical Liquid Deposition ») de couches minces nanoporeuses grâce à une technique de trempage-retrait (« Dip-Coating »). Ce dépôt est suivi d’une structuration induite par évaporation ou bien par lithographie. La nanoporosité de ces couches minces, qui peuvent êtres inorganique ou organique-inorganique (hybride), est induite par un phénomène d’auto-assemblage se déroulant pendant la phase de séchage du film. La structure multi-échelle résultante possède une organisation périodique à l’échelle micro- voire submicrométrique (structure photonique) mais également une nanoporosité (<100 nm) structurée; le dispositif photonique est utilisé pour détecter des vapeurs de Composés Organiques Volatiles (COV). Un accent sera mis sur la compatibilité de ces capteurs optiques avec les caméras de smartphone. Le but final est en effet de fabriquer des capteurs à bas coût, dont la réponse optique est directement lisible par un smartphone
Multi-scale structuration of functional materials at nano- and micro- levels is an active scientific field driven by the tremendous potential of miniaturized devices in microelectronics, optics (light harvesting, photonics), sensing (selective sensors) or microfluidics (lab-on-a-chip). Diverse micro-nanofabrication techniques are exploited for device fabrication. On one hand, Top-Down techniques are developed to fabricate complex micro- and nano- structures from bulk materials; this approach relies on lithography which offers a wide flexibility on the final object architecture but suffers from low-throughput that hinders its use for large-scale production. On the other hand, Bottom-Up techniques based on the assembly of molecular building blocks are suited for the large-scale fabrication of nanostructured materials but are limited to simple architectures. The fruitful combination of both approaches is thus a vast field of investigation with promising technological outcomes.The scope of this thesis is to combine Bottom-Up and Top-Down approaches to obtain hierarchical architectures with original chemical characteristics and optical properties. In practical terms, the deposition by Chemical Liquid Deposition (dip-coating) of nanoporous inorganic or organic-inorganic (hybrid) films structured by self-assembly and the subsequent patterning by either lithographic or evaporation-driven patterning will be presented. The resulting multi-scale structures possess periodic micro- or submicro- organization and engineered nanopores (<100 nm) and are used as optical sensing devices for the detection of Volatile Organic Compounds (VOC). In the pursuit of simplicity, the compatibility of these sensors with Smartphone technology is emphasized; the final goal is to fabricate low-cost sensors with pronounced chemical selectivity that produce an optical signal directly readable by Smartphone cameras

With the growing demand of the photovoltaic market, high cost of the traditional silicon-based solar cells has been the biggest barrier for its widespread use of solar energy conversion. New generation solar cells, aiming at the low-cost and high- efficiency photovoltaic devices, have been researched for a couple of decades. Among the new generation solar technologies, dye-sensitized solar cell (DSSC) normally employing a nanoporous TiO2 photoanode draw a quite attention to meet future solar energy market demanding because its realizable low-cost property and high-efficiency achievement. Continuing efforts have been devoted to improve the performance of DSSCs. However, there are still several drawbacks need to be overcome. The major drawbacks mainly focus on the electron recombination and low-efficiency electron transport in the nanoporous TiO2 photoanodes, which significantly limit the practical application of DSSCs. Therefore, the main task of this thesis is to retard the electron recombination and to improve the electron transport efficiency in the nanoporous TiO2 photoanodes for DSSCs through interfacial and structural modification strategies.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
Science, Environment, Engineering and Technology
Full Text

Nanoporous copper (NPC) thin films with bimodal channel size distributions can be benignly fabricated by one-pot chemical dealloying of dual-phase Al 27 at  Cu alloy with hypereutectic structure in the NaOH solution. The microstructure of these NPC thin films was characterized using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis. The results show that these NPC thin films are composed of interconnected large-sized channels (100s of nm) with highly porous channel walls (10s of nm), in which large-sized channels resulting from entire dissolution of solid solution while small-sized those de-riving from part corrosion of intermetallics. Both large- and small-sized channels are 3D, open, and bicon-tinuous. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/35204

Thesis (Ph.D.)--University of Missouri-Columbia, 2007.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on March 24, 2009) Vita. Includes bibliographical references.

Le diamant est un matériau fascinant avec d'exceptionnelles propriétés physiques. Son application à divers domaines reste limitée parce que sa fabrication est difficile et nécessite des substrats et conditions spécifiques. En outre, les dispositifs de diamant tels que les capteurs nécessitent généralement la structuration et l'échelle micro ou nanométrique, et l'inertie chimique du diamant rend ce processus technologique plus difficile que celui des semiconducteurs réguliers. Il s'agit d'un besoin évident de la recherche fondamentale d’explorer de nouvelles façons de fabriquer des nanostructures de diamant, ce qui permet de nouvelles formes de capteurs et dispositifs. Dans ce contexte, le travail présenté est d'une grande importance pour la communauté de diamant et pour le développement futur de la technologie du diamant.Le manuscrit est divisé en huit parties: une introduction; 6 chapitres, une conclusion générale. Dans l'introduction le contexte de l'étude est brièvement présenté avec les deux objectifs. Le premier consistait à étudier la croissance des nanofils de diamant et à trouver des conditions appropriées pour obtenir des nanofils de façon reproductible. Le deuxième objectif était la mise au point du procédé de gravure du diamant avec des particules de catalyseur et de l'optimisation des paramètres du procédé.Le premier chapitre de ce manuscrit présente tout d'abord l'état de l’art en mat ière de propriétés et des technologies de croissance du diamant. Puis, dans le deuxième chapitre, en vue de la croissance des nanofils et des études de gravure de nanostructures utilisées catalyseurs métalliques, la base de l'interaction métal-carbone est présenté.Le chapitre trois contient l'instrumentation et principe de fonctionnement des techniques expérimentales et analytiques utilisées dans cette étude. Le chapitre suivant se concentre sur la recherche de conditions favorables à la croissance des nanofils de diamant, d'abord en étudiant en détail un processus signalé en 2005 qui a conduit à la nucléation des nanocristaux sur des nanotubes de carbone, puis la croissance de nanofils.Les conditions de croissance ont été soigneusement reproduites, sans succès reproductible. Il en est déduit déduit que d'un élément non a contribué à la croissance, comme une contamination du catalyseur. La combinaison avec le fait que le processus publiée en 2005 n'a jamais été reproduite, en dépit de son importance technologique élevé, ce qui suggère que la contamination s'est produite également dans cette oeuvre originale.Puis, à partir de cette première observation, l'effet d'un catalyseur a été étudié, et des résultats intéressants ont été obtenus. Les nanofils ont été obtenus de façon reproductible, mais le point important est que les nanofils à base de silicium sont très faciles à cultiver, et qu'un environnement deCarbone pur était nécessaire d'étudier la croissance de nanofils de carbone. Dans ces conditions, un continuum allant de diamant de gravure pour la croissance du diamant a été obtenue en fonction de l'apport de carbone, très intéressant pour la technologie du diamant. Dans le cinquième chapitre du mécanisme de gravure de diamant par des particules de catalyseur est explorée. La gravure à motifs a été proposée pour la fabrication de nano-ou micro-structures dans le diamant, et il est présenté dans la dernière partie de ce chapitre. Le chapitre 6 présente deux applications intéressantes du processus dedéveloppement. Les premières membranes poreuses préoccupations utilisés comme bio-capteurs, et les nanotubes de carbone second concerne la base neuro-capteurs.Malgré l'étude infructueuse de la croissance du diamant nanofil, le travail fait des progrès significatifs à la science de la croissance matérielle nanocarbone. Et elle a conduit à l'étude approfondie de gravure diamant, qui est également très important pour la technologie
One-dimensional structures with nanometre diameters, such as nanotubes and nanowires, have attracted extensive interest in recent years and form new family of materials that have characteristic of low weight with sometimes exceptional mechanical, electrical and thermal properties. Without any change in chemical composition, fundamental properties of bulk materials can be enhanced at the nanometre scale leading to extraordinary nanodevices.Since a few years, nanowires of different semiconducting materials have been grown. To mention few of these, Si, GaN, SnO, SiC and ZnO nanowires were all successfully demonstrated. However, the growth of diamond nanowires has not yet been demonstrated, despite the strong interest for this material. Bulk diamond combines various exceptional properties for a wide range of applications: Chemical inertness, radiation hardness, biocompatibility, high hole/electron mobility (2000/1000 cm2/V/s), high thermal conductivity (22 W/cm/K), wide bandgap (5.5 eV), and wide electric potential window (3.25 eV H-O evolutions).Since about 30 years, the growth of diamond thin film is well controlled either as insulator or as semiconductor with p- and n- type dopants. Fabrication of 25x25 mm2 monocrystalline diamond wafer has already been reported, and two inches wafers are expected in a couple of years demonstrating the growing interest for this material. Among present or short-term applications one can mention alpha-particle detectors, solar-blind UV sensors, high voltage electronic devices, bio-sensors and single photon source. The realization of nanowires should improve the performance of some of these devices and also open a range of new high performance applications.The stability of 0D (nanocrystals) and 1D (nanowires) diamond nanostructures has been extensively studied using ab initio modelling and indicates that for specific crystallographic orientations clusters of nanometric size are thermodynamically stable. One experimental indication for diamond nanowire growth has been published by Sun et al. in 2005, based on nanocrystal nucleation and growth on carbon nanotubes followed by 1D growth. This particular nucleation process on carbon nanotube has furthermore been explained theoretically in 2009.Based on these experimental and theoretical results, the first objective of this thesis was to explore the growth of diamond nanowire and find suitable conditions to obtain nanowires in a reproducible way. A wide range of process conditions were explored, first without any catalyst, then with metallic catalyst in order to promote Vapour-Liquid-Solid (VLS) growth. Although a comprehensive knowledge regarding carbon nanotube stability in hydrogen atmosphere and diamond-catalyst interaction has been obtained and some carbon nanostuctures were grown, no diamond nanowires were obtained in a reproducible way.However, the careful study of the diamond-catalyst interaction revealed a very interesting etching process that could be very useful for the fabrication of diamond nanostructures. A second objective was then defined: development of the etching process for diamond using transition metal as catalyst and optimization of the process parameters for specific applications such as the fabrication of porous diamond membranes for bio-sensors

Ces travaux de thèse s'inscrivent dans le cadre d'un vaste projet qui vise à construire des membranes hybrides constituée d'un support solide nanoporeux et de protéines canal-ionique biologiques. Nous nous intéressons ici à un film polymère nanoporeux en polycarbonate et à la Gramicidine-A. La membrane ainsi réalisée est étudiée par des mesures expérimentales. Ce travail peut être divisé en deux parties. Dans la première, nous rapportons l'étude du confinement de la protéine canal ionique, au sein des nanopores du film « track-etched » en polycarbonate. Après imprégnation de gA, la membrane est étudiée par Spectroscopie de Fluorescence Confocale. Les premiers résultats expérimentaux particulièrement encourageants montrent que la gA est localisée dans les nanopores et non pas à la surface de la membrane. Dans la deuxième, les propriétés de transport ionique de la membrane hybride sont caractérisées par le biais de deux grandeurs : d'une part les coefficients de diffusion mesurés à partir d'une cellule et d'autre part les conductivités via la Spectroscopie d'Impédance Complexe (S.I.C). Les électrolytes aqueux étudiées sont : XCl(2) où X=Na, K, Mg et Ca à des concentrations comprises entre 5.10-3 à 1M. Une étude statistique approfondie des données obtenues par la méthode de la variance permet de déterminer les effets relatifs des différentes variables : nature et concentration du sel, présence de la Gramicidine A et traitement à l'éthanol de la membrane. Cette analyse révèle clairement que la présence de Gramicidine A au sein des nanopores de 15nm modifie de façon positive le transport ionique. Il est, par contre, difficile de conclure sur la nature sélective du transport ionique en présence de cette protéine. Ce travail de thèse ouvre un champ de recherche très prometteur dans le domaine de la nanofiltration
This project of thesis is to build of a bio-inspired hybrid membrane made of a thin nanoporous polymer film in which a biological ionic channel is confined. Thus, this work may be divided in two parts. First, we report the confinement of the biological ionic channel, i.e. Gramicidin A, inside the nanopore of nanoporous thin film, i.e. a track etched polycarbonate film (Whatman NucleoporeTM). After impregnation with Gramicidine-A, the membrane is studied by means of confocal fluorescence spectroscopy. The results show the ionic channel is well located into the nanopores and not at the surface of the membrane. Secondly, ionic transport properties are measured by means of two experiments: on the one hand, ionic diffusion coefficients are measured using a cell and on the other hand, ionic dc conductivity is measured via Complex Impedance Spectroscopy (SIC). Various aqueous electrolytes (XCl(2) where X=Na,K, Mg et Ca) at different concentrations ranging from 5.10-3 à 1M are carried out. A statistical analysis of the data so-obtained allows to determine the relative effects of the different parameters: the nature and concentration of the electrolytes, the presence of Gramicidine A and the membrane pre-treatment with ethanol treatment. It is thus clearly pointed out that the presence of Gramicidine A inside the 15nm nanopores improves ion permeability. However, it is difficult to conclude about ionic selectivity of the hybrid membrane. Nevertheless, this work which is the first attempt ever to build such a bio-inspired system opens an extremely promising field of research in the domain of nanofiltration

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 44-46).
Heat dissipation is a limiting factor in the performance of integrated circuits, power electronics and laser diodes. State-of-the-art solutions typically use air-cooled heat sinks, which have limited performance owing to the use of air. One of the promising approaches to address these thermal management needs is liquid vapor phase-change. In this thesis, we present a study into the design and modeling of a cooling device based on thin film evaporation from a nanoporous membrane supported on microchannels. The concept utilizes the capillary pressure generated by the small pores to drive the liquid flow and largely reduces the viscous loss due to the thinness of the membrane. The interfacial transport has been re-investigated where we use the moment method to solve the Boltzmann Transport Equation. The pore-level transport has been modeled coupling liquid transport, vapor transport and the interfacial balance. The interfacial transport inside the pore also serves as a boundary condition for the device-level model. The heat transfer and pressure drop performance have been modeled and design guidelines are provided for the membrane-based cooling system. The optimized cooling device is able to dissipate 1 kW/cm² heat flux with a temperature rise less than 30 K from the vapor side. Future work will focus on more fundamental understanding of the mass and energy accommodation at the liquid vapor interface.
by Zhengmao Lu.
S.M.

Tethered polymer layers at solid-fluid interfaces are used widely in a variety of surface science applications. Although many of these applications require exposure to dynamic flow conditions, flow field penetration into densely grafted polymer brushes, is still a question open to debate despite the fact that it is a fundamental process crucial to mass transport through these polymer films. Although most theoretical work has indicated flow field penetration into polymer films, with varying predicted penetration depths predicted, the limited experimental attempts to investigate this phenomenon have resulted in inconsistent conclusions due to lack of a proper analytical method. To help resolve this controversy, in this Dissertation, a new spectroscopic method, FRET-TIRFM, based on a combination of Förster resonance energy transfer (FRET) and total internal reflectance fluorescence microscopy (TIRFM), is developed to provide the first direct, quantitative measurements on flow field penetration by measuring linear diffusion coefficients of small molecules through densely grafted, thin poly(N-isopropylacryl-amide) (pNIPAM) films. Decay curves from FRET of the acceptor with a donor covalently attached at the substrate surface were fit to a combined Taylor-Aris-Fickian diffusion model to obtain apparent linear diffusion coefficients of the acceptor molecules for different flow rates. These values can then be used to obtain quantitative estimates of flow field penetration depths. For a pNIPAM surface of 110 nm dry thickness, with a 0.6 chain/nm² grafting density, apparent diffusion coefficients ranging from 1.9-9.1 × 10-12 cm²/s were observed for flow rates ranging from 100 to 3000 μL/min. This increase in apparent diffusion coefficient with applied fluid flow rate is indicative of flow field penetration of the polymer film. The depth of penetration of the flow field is estimated to range from ~6% of the polymer film thickness to ~57% of the film thickness in going from 100 to 3000 μL/min flow rate of a good solvent. Factors other than flow rate that may impact flow field penetration were also investigated using this new FRET-TIRFM method. Solvent quality and polymer brush grafting density are the two most important parameters due to the fact that they influence changes in tethered polymer chain conformation. This work demonstrates that polymer films are most penetrable in a good solvent and least penetrable in a poor solvent under identical flow conditions. These findings are consistent with polymer chain conformational changes going from extended brushes to compact globules. For flow rates ranging from 100 to 3000 μL/min, flow field penetration depth ranges from ~6% of the film thickness to ~57% of film thickness for a good solvent compared to ~4% to ~19% for a poor solvent. Thus, by simply changing solvent quality from good to poor, flow field penetration decreases by about 38%. Grafting density has a less pronounced effect than solvent quality on penetration depth, probably due to the small range of grafting densities chosen for study. However, a roughly 10-20% difference in penetration depth was observed between high density (0.60 chain/nm²) and low density (0.27 chain/nm²) pNIPAM surfaces in the same solvent. Changes in grafting density have a less significant impact in a good solvent compared to a poor solvent. This is most likely caused by the fact that grafting density impacts polymer chain conformation mainly through polymer-polymer repulsion, which becomes less significant in a solvent that better solvates the polymer. For the two extreme cases studied here at flow rates ranging from 100 to 3000 μL/min, the penetration depth is estimated to range from ~4-19% of the original solvent-swollen film thickness for high density pNIPAM films in a poor solvent and from ~7-67% for low density films in a good solvent. One important assumption that underlies all of this work is that the dominant mass transport mechanism for small molecules in dense polymer brushes is diffusion. This assumption was further validated through the use of two different small molecule quenchers, RhB and 2-nitrobenzylalcohol. These molecules are significantly different in size, charge, and structure, and operate by different quenching mechanisms. Despite these differences, the results for flow field penetration are statistically the same for both. These observations validate the assumption of diffusive mass transport in these films.

Nanostructured porous materials generally, and nanoporous noble metals specifically, have received considerable attention due to their superior chemical and physical properties over nanoparticles and bulk counterparts. This dissertation work aims to develop well-established strategies for the preparation of multifunctional nanostructured porous materials based on the combination of inorganic-chemistry, organic-chemistry and electrochemistry. The preparation strategies involved one or more of the following processes: sol-gel synthesis, co-electrodeposition, metal ions reduction, electropolymerization and dealloying or chemical etching. The study did not stop at the preparation limits but extended to investigate the reaction mechanism behind the formation of these multifunctional nanoporous structures in order to determine the different factors controlling the nanoporous structures formation. First, gold-silica nanocomposites were prepared and used as a building blocks for the fabrication of high surface area gold coral electrodes. Well-controlled surface area enhancement, film thickness and morphology were achieved. An enhancement in the electrode’s surface area up to 57 times relative to the geometric area was achieved. A critical sol-gel monomer concentration was also noted at which the deposited silica around the gold coral was able to stabilize the gold corals and below which the deposited coral structures are not stable. Second, free-standing and transferable strata-like 3D porous polypyrrole nanostructures were obtained from chemical etching of the electrodeposited polypyrrole-silica nanocomposite films. A new reaction mechanism was developed and a new structural directing factor has been discovered for the first time. Finally, silver-rich platinum alloys were prepared and dealloyed in acidic medium to produce 3D bicontinuous nanoporous platinum nanorods and films with a nanoporous gold-like structure. The 3D-BC-NP-Pt displayed high surface area, typical electrochemical sensing properties in an aqueous medium, and exceptional electrochemical sensing capability in a complex biofouling environment containing fibrinogen. The 3D-BC-NP-Pt displayed high catalytic activity toward the methanol electro-oxidation that is 30 times higher that of planar platinum and high volumetric capacitance of 400 F/cm3. These findings will pave the way toward the development of high performance and reliable electrodes for catalysis, sensing, high power outputs fuel cells, battery-like supercapacitors and miniaturized device applications.

Doutoramento em Ciência e Engenharia de Materiais
Os materiais multiferróicos possuem simultaneamente pelo menos duas das três propriedades ferróicas: i) ferroelectricidade; ii) ferromagnetismo; e / ou iii) ferroelasticidade. Estes materiais têm despertado considerável interesse na indústria microeletrónica devido ao seu potencial para serem usados em dispositivos de armazenamento de informação com elevada capacidade e eficiência energética. A constante procura pela redução do tamanho e aumento da funcionalidade dos dispositivos, imposta pela Lei de Moore, exige materiais ferróicos, na forma de filmes finos e multifuncionalidade. Contudo, à medida que a espessura dos filmes diminui, as propriedades ferróicas ficam comprometidas em virtude de constrangimentos provocados pelo substrato ou outros efeitos. Neste contexto, esta tese estuda a possibilidade de utilizar a porosidade em filmes funcionais para criar sistemas compósitos multifuncionais. Assim, desenvolveram-se estratégias para a preparação de filmes de ferroeléctricos, ferromagnéticos e multiferroícos com porosidade uniforme e ordenada. O efeito dessa porosidade foi avaliado nas propriedades físicas locais e macroscópicas. Foram estudados óxidos multimetálicos com estrutura de perovisquite ou de espinela por serem promissores para aplicação em sensores; atuadores; condensadores; memórias; etc. Escolheu-se uma metodologia química em que os filmes são depositados por técnica de mergulho em soluções sol-gel contendo um copolímero em bloco que se organiza espontaneamente conjuntamente com os precursores durante o processo de evaporação. PbTiO3 foi a composição inicialmente escolhida para entender o efeito da nanoporosidade nas propriedades eléctricas locais por ser o material piezoeléctrico protótipo que possui o mais alto coeficiente piezoeléctrico conhecido. Assim, foram preparados filmes nanoporosos e densos de PbTiO3 com espessura de cerca de 100 nm e diâmetro de poro na ordem dos 50 nm. A presença da nanoporosidade contribui para a cristalização precoce da fase cristalina por aumento local da temperatura durante a decomposição do copolímero e / ou por funcionarem como núcleos de cristalização. Consequentemente, os fimes porosos exibem melhores coeficientes piezoeléctricos e baixo campo coercivo, sendo mais fácil inverter a direção da polarização por efeito do campo elétrico. Sendo a porosidade um meio para atingir propriedades melhoradas, esta pode funcionar como uma ferramenta para ajustar as propriedades ferroeléctricas à aplicação desejada. Todos os resultados de PFM foram previstos através de modelação teórica usando o modelo de elementos finitos. Foi também investigada a preparação de filmes porosos de titanado de bário enquanto protótipo de um ferroeléctrico sem chumbo. Neste contexto, foi avaliado o efeito de vários parâmetros, tais como: i) o aquecimento da solução de precursores; ii) adição de precursores inorgânicos / solventes orgânicos; e iii) envelhecimento da solução inicial, na estrutura final dos filmes.Verificou-se que o uso de uma solução fresca de precursores sem qualquer ciclo de aquecimento contribuía para uma melhor organização dos filmes porosos de BaTiO3. Verificou-se também que o tamanho dos blocos num copolímero à base de poliestireno e poli(óxido de etileno) era preponderante para a ordem e microestrutura cristalina dos filmes finais. Copolímeros em bloco com cadeias de bloco mais longas são preferíveis para obter uma estrutura ordenada e aparentemente desempenham um papel na cristalização precoce da fase ferroeléctrica tetragonal, contribuindo para uma melhoria da resposta piezoeléctrica. Em analogia com o PbTiO3, os resultados indicam que nos filmes nanoporosos de BaTiO3, a cristalização ocorre a temperaturas mais baixas do que nos filmes densos. Utilizou-se a deposição electroquímica para inserir nanopartículas metálicas de cobalto dentro dos poros dos filmes de BaTiO3. O carácter multiferróico dos filmes foi constatado através da avaliação nanoscópica das propriedades elétricas e pela medida das propriedades magnéticas macroscópicas à temperatura ambiente. Verificaram-se as dificuldades de conseguir um preenchimento uniforme dos poros e de otimizar a interface entre as duas fases ferróicas. Assim com vista a tentar ultrapassar estas dificuldades, prepararam-se filmes mais finos e em que a porosidade estivesse devidamente organizada, com poros perpendiculares à superfície. Conceberam-se filmes nanotexturados ordenados de óxidos multimetálicos com propriedades ferroelétricas, ferromagnéticas e multiferróicas com espessuras e texturas de dimensão inferior a 100 nm. As composições escolhidas foram PbTiO3, CoFe2O4 e BiFeO3. Os filmes finos porosos nanotexturados PbTiO3 apresentaram a fase cristalográfica tetragonal mesmo em espessuras de filme de 22 nm. Os filmes finos de CoFe2O4 apresentaram uma orientação preferencial no plano e elevadas magnetizações de saturação. Deduziu-se que os filmes teriam uma impureza ferromagnética compatível com uma liga metálica rica em platina. A presença desta impureza não só melhora o desempenho magnético dos filmes mas também fornece uma forte evidência para a potencial aplicabilidade dos filmes de CoFe2O4 como catalisadores para a oxidação de hidrocarbonetos através do mecanismo de Mars-Van-Krevelen. Foram também preparados filmes finos porosos nanotexturados de BiFeO3, com 66 nm de espessura e tamanho médio de diâmetro de 100 nm. Verificou-se o caráter multiferróico destes filmes e mais uma vez a melhoria clara das propriedades eléctricas locais induzida pela porosidade. A estrutura porosa também tem um efeito positivo nas propriedades magnéticas no plano, mostrando uma componente ferromagnética 50% maior que a medida em filmes densos. Verificou-se também que porosidade dos filmes de BiFeO3 pode ter interesse para aplicações fotocatalíticas, conjugando reduzido valor do hiato óptico direto (2.58 eV) com relativamente elevada área porosa (ca. 57 %). Para testar a aplicabilidade dos filmes nanotexturados na construção de um filme multiferróico compósito, uma matriz porosas ferroelétricos (BaTiO3) foi funcionalizada por preenchimento dos poros com nanopartículas ferromagnéticas de níquel. A estratégia de funcionalização dos poros foi a deposição por arrastamento com CO2 supercrítico, seguida de redução da espécie metálica a 250 ºC ativada por etanol. Pequenas nanopartículas de níquel com cerca de 21 nm foram depositadas dentro dos poros da matriz porosa, tendo-se verificado as propriedades estruturais e magnéticas do compósito. Esta tese, provou a adequação desta metodologia química de baixo custo na concepção de materiais multifuncionais, criando novas perspectivas para a indústria da microeletrónica na sua abordagem contínua de redução de tamanho e custo, enquanto aumenta a complexidade de funcionamento.
Multiferroic materials, exhibiting simultaneously at least two of the three ferroic properties: i) ferroelectricity; ii) ferromagnetism; and iii) ferroelasticity, have attracted considerable interest from the microelectronics industry. Due to their potential, these materials can be used in information storage applications with significantly high energetic efficiencies and elevated capacities. During the last decades and owing the increasing need for miniaturization of electronic devices, the ferroic materials, mainly in the format of thin films, have been extensively studied both theoretically and experimentally. However, as the film thickness decreases the ferroics properties progressively decreases due to the in-plane strain relaxation constrained by the substrate or others intrinsic and extrinsic effects. Within this context, here we exploit the role of nanoporosity on local and macroscopic properties of ferroelectrics, ferromagnetic and multiferroics thin films. Although, porosity is normally considered as a defect (or secondary phase) having usually a detrimental effect on the electrical macroscopic response; it can also be regarded as an asset, in terms of: i) density (light weight) and ii) capacity to host other functionality/ies. Oxides with perovskite and spinel structures are promising materials because they possess extraordinarily useful properties namely to be used as piezoelectrics sensors, as ferroelectric actuators, capacitors and memories, in high-strength dielectrics, for ferromagnetics or even multiferroics. Among the bottom-up approaches, the sol-gel method and evaporation-induced self-assembly methodology are the most suitable, low-cost and easy preparation method to prepare nanoporous and nanopatterned thin films of different compositions. PbTiO3 is the chosen composition to understand the role of the nanoporosity on the local electric properties. Thus, nanoporous and dense ferroelectric PbTiO3 thin films with 100 nm and ~ 50 nm pore size formed using a block polymer as a structure-directing agent are prepared. The presence of nanoporosity markedly affects the microstructure, crystallization and ferroelectric film properties. The crystallization of tetragonal phase is enhanced in nanoporous films. It seems that the decomposition of the block-copolymer in porous films triggers the crystallization of the perovskite phase at low temperatures via the local increase of temperature. Moreover, pores may work as initiators of the crystallization. Consequently, nanoporous films with improved tetragonality exhibit enhanced piezoelectric coefficients, switchable polarization and low local coercivity. In fact, the porosity induces instability in the dipole-dipole interactions and consequently the reverse polarization can be favoured for low bias values. By providing a means of achieving enhanced properties, nanoporosity may work as a tool to tune electric properties to the desired ferroelectric application. All the PFM results were supported by theoretical modelling using Finite Element Model. To have a more complete picture of the role of the nanoporosity on the crystallization and electric properties, the procedure is applied to prepare a nanoporous lead-free material, BaTiO3. However, this expantion was not trivial whereas the crystallization temperature of the tetragonal phase necessary for the ferroelectric properties is much higher than the decomposition temperature of the block-copolymer used as template. From this, several parameters such as: heating the solution, addition of inorganic precursors / organic solvent and aging time of solution are studied in order to understand the effect of these on the micellization process and consequently in the final porous BaTiO3 films. Based on the results of this study, for this specific multimetallic oxide system it is preferable to use a very fresh solution, without any heating cycles. In addition, block-copolymers based on polystyrene and poly(ethylene oxide) with different block sizes are used to investigate their influence on the order and crystalline microstructure of the final films. Blocks-copolymers with longer block chains are preferable to get an ordered structure and apparently play a role on the earliest crystallization of the tetragonal ferroelectric perovskite phase, contributing to an enhancement of the piezoelectric response. Similarly to PbTiO3, our results indicate that in nanoporous BaTiO3 films the crystallization occurs as well before in dense films. Moreover, besides providing a means of achieving enhanced properties, nanoporosity may work as a tool to tune electric properties to the desired ferroelectric application. BaTiO3 nanoporous films are tested as a kind of “golf course” full of holes to accommodate ferromagnetic particles. In this way, electrochemical deposition is used to insert the cobalt metal nanoparticles into the pores of BaTiO3 films. Films containing cobalt particles within the pores are obtained and piezoelectric and ferromagnetic properties are evaluated. For many applications would be a challenge to prepare ferroelectric thin films with lateral sizes well below 100 nm. Furthermore, the design of nanofeatures, uniformed in size and shape at a reasonable large-range order, i.e. “nanopatterning”, would extend their utility for electronic devices and integrated circuits, which require that each pixel feature can be individually addressable. Additionally, nanopatterned porous ferroelectric thin films may be interesting to develop vertical composite structures with perfect strain coupling at the interface. Thus, and using the chemical self-assembly method, different functional nanopatterned porous thin films: PbTiO3, BiFeO3 and CoFe2O4 are designed. Nanopatterned PbTiO3 thin films display the tetragonal ferroelectric crystallographic phase even when the films are as thin as 22 nm. CoFe2O4 thin films present a preferential in-plane orientation. High saturation magnetizations (close or even higher than in bulk CoFe2O4) are determined in all films, pointing to the presence of a ferromagnetic impurity compatible with a platinum-rich metal alloy. The presence of this impurity not only enhances the magnetic performance but also provides evidence for the catalytic activity of these CoFe2O4 films for hydrocarbon oxidation through a Mars-Van-Krevelen mechanism. For the BiFeO3 composition, crystalline nanopatterned BiFeO3 layers with 66 nm of thickness and average pore diameter of 100 nm at 600 ºC are obtained. The large vertical porosity markedly enhances the local electric and macroscopic magnetic properties when compared with the dense counterparts. The porous structure also has a positive effect on the parallel magnetic characteristics of the system, displaying a 50% larger ferromagnetic component and enhanced remanent magnetization when compared to the dense thin films counterpart. The porosity is also important for the photocatalytic applications conjugating the smallest direct band gap (2.58 eV) and extended porous area (ca. 57 %). The nanopatterned thin films allow the exploitation of a new concept to prepare multiferroic nanocomposite thin films. The multiferroic films based on in two chemical-based bottom-up steps, including: i) the formation of a porous ferroic matrix and ii) the accomodation of nanoparticles from another ferroic phase within the pores. Hexagonal-arranged pores with diameter of ca. 95 nm, running perpendicularly to the substrate are filled with nickel nanoparticles using the supercritical fluid deposition technique from reduction of hydrated nickel nitrate in a supercritical CO2-ethanol mixture at 250 ºC. Small nickel nanoparticles with ca. 20 nm are deposited inside the pores of the porous matrix. Structural and magnetic properties proved the coexistence of both phases. The chemical based methodology offers thus an excellent control of the physical and chemical properties of nanostructured materials such as: stoichiometry, thickness, size, array and porous distribution. Moreover the self-assembly of block-copolymers provides a versatile platform to prepare functional nanostructured materials, namely mesostructured oxide thin films, due to their capability to form large pores and thick walls, apart from being industrially available and hazard-free. Additionally, the chemical-assembly method can allow the direct nanopatterning of large substrate areas with a functional oxide at a cost-effective price, in the absence of expensive equipment or etching processes (which typically affect negatively the ferroic properties).Besides, the functional properties of the porous films by themselves, the porous films are extremely promising to achieve multiferroic composites.

A thorough study on the distribution of defect-related active energy levels has been performed on nanocrystalline TiO2. Films have been deposited on thick-alumina printed circuit boards equipped with electrical contacts, heater and temperature sensors, to carry out a detailed thermally stimulated currents analysis on a wide temperature range (5-630 K), in view to evidence contributions from shallow to deep energy levels within the gap. Data have been processed by numerically modelling electrical transport. The model considers both free and hopping contribution to conduction, a density of states characterized by an exponential tail of localized states below the conduction band and the convolution of standard Thermally Stimulated Currents (TSC) emissions with gaussian distributions to take into account the variability in energy due to local perturbations in the highly disordered network. Results show that in the low temperature range, up to 200 K, hopping within the exponential band tail represents the main contribution to electrical conduction. Above room temperature, electrical conduction is dominated by free carriers contribution and by emissions from deep energy levels, with a defect density ranging within 10(14)-10(18) cm(-3), associated with physio- and chemi-sorbed water vapour, OH groups and to oxygen vacancies.

Broadband Dielectric Spectroscopy (BDS) in combination with a nanostructured electrode arrangement – which circumvents the conventional need to evaporate metal electrodes onto soft matter – is used to study the molecular dynamics of several glass forming materials confined in nanometric (> 5 nm) layers. Other complementary experimental tools employed in this work include spectroscopic vis-Ellipsometry (SE), AC-chip calorimetry (ACC), X-ray reflectrometry (XRR), Differential Scanning Calorimetry (DSC) and Atomic Force Microscopy (AFM). The latter is used to characterize the topography of the samples and to determine their thicknesses. Under the conditions of annealing samples (Tg + 50K) in high oil-free vacuum (10E-6 mbars) for at least 12 h and carrying out measurements in inert (dry nitrogen or argon) atmosphere, it is found for all studied thin layers that the structural relaxation, and hence the dynamic glass transition – in its mean relaxation times – remains within a margin ±3 K from the respective bulk behaviour. It is revealed, inter alia, that the one-dimensional confinement of thin films introduces restrictions on other (slower) molecular relaxation processes which manifest, depending on the specific system under investigation, as (i) an interruption of the end-to-end (normal mode) fluctuation of the chains, or (ii) a slowing down of the delta-relaxation when the system is cooled towards glass-formation. Furthermore, (iii) evidence is provided to show that the dimensionality of confinement plays a significant role in determining the resulting dynamics. A molecular understanding of these findings is given, and the discussion presented with respect to the on-going international debate about dynamics in confinement.

Le but de ce travail a été la réalisation de cellules photoélectrochimiques de type "Grätzel" à base de dioxyde d'étain photosensibilisé par des dérivés organostanniques du pérylène-3,4-decarboximide, réalisant la conversion photovoltaïque. Ces chromophores ont été obtenus par une synthèse convergente en plusieurs étapes, l'étape clef consistant en un couplage de type Stille entre un dérivé du pérylène et un oméga-iodoalkyl- ou 4-bromophenyl-tricyclohexylétain. Les études physico-chimiques réalisées en solution sur ces nouveaux colorants ont montré qu'ils possédaint les propriétés photochimiques et électrochimiques requises pour photosensibiliser des oxdes semi-conducteurs. Par ailleurs, des films nanoporeux et nanocristallins de dioxyde d'étain d'épaisseur contrôlée ont été élaborés sur substrats conducteurs par différentes techniques à partir de suspension colloïdales de nanaoparticules de SnO2, une étude approfondie de la texture, de la structure et de la morphologie de ces couches ayant permis de sélectionner celles les plus appropriées pour la conversion photovoltaïque. Ces dernières ont ensuite été modifiées chimiquement en surface par les chromophores synthétisés puis l'aptitude des cellules ainsi réalisées à convertir la lumière en électricité a été évaluée. Le meilleur système a permis d'atteindre des rendements de conversion énergétique de l'ordre de 0,3 %.

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Nanoporous anodic alumina films (NAA) may present different behavior to reflectance and photoluminescence techniques, with Fabry-Pérot interferences and waveguide properties. This phenomenon makes possible the use of NAA as transducer signal in optical sensors. In this work, we investigated how the pre-treatment, the number of steps of anodizing, the anodizing electrolyte mode and temperature affect electrochemical characteristics, morphological and optical mainly NAA. As a result, it was noticeable that the realization of electropolishing is necessary for both the NAA with good regularity as to make it possible to obtain a spectrum with the Fabry-Pérot interference. If the fabrication of NAA is done in two steps, it is possible to obtain reflectance spectra and luminescence fringed with better amplitudes, areas and heights. Regarding the anodizing mode, both the NAA anodized in galvanostatic how potentiostatic showed similar morphologies and spectra with fringes, but the interferences were better defined when the galvanostatic mode was performed. Regarding the temperature, it was noticeable that the change of this parameter leads influences the porous oxide thickness. The spectrum of the luminescence and reflectance increasing the electrolyte temperature caused an increase in interference. However, the range and resolution of interference decreased with increasing temperature. The oxide thicknesses were estimated by energy variation (?E), graph slope between order and 1/? and fast Fourier transform (FFT) techniques. The ratio of film thickness and pore diameter (L/dp) was performed to validate the NAA films with better waveguides property. Moreover, the surface composition analysis of NAA films anodized in phosphoric acid, oxalic acid and mixtures thereof by backscattering spectroscopy Rutherford (RBS) was performed. From simulations it was possible to note that the amount of carbon in the porous oxide structure is practically zero, which may indicate that the origin of the luminescence is related to the presence of more centers F.
Filmes de alumina anódica porosa (AAP) podem apresentar, espectros de reflectância e luminescência com interferências de Fabry-Pérot e propriedades de guias de onda. Esse fenômeno possibilita que a AAP possa ser utilizada como plataforma em sensores ópticos. Neste trabalho, foi investigado como o pré-tratamento, o número de etapas de anodização, o modo de anodização e a temperatura do eletrólito afetam características eletroquímicas, morfológicas e principalmente ópticas da AAP. Como resultado, foi possível notar que a realização do eletropolimento é necessário para obter tanto a AAP com boa regularidade como para que seja possível obter um espectro com as interferências Fabry-Pérot. Se a fabricação da AAP for feita em duas etapas, é possível obter espectros de reflectância e luminescência com franjas com melhores amplitudes, áreas e alturas. Com relação ao modo de anodização, tanto as AAPs anodizadas em modo galvanostático como potenciostático apresentaram morfologias semelhantes e espectros com franjas, mas as interferências foram melhor definidas quando o modo galvanostático foi realizado. Com relação à temperatura, foi possível notar que a mudança desse parâmetro ocasiona influencia na espessura do óxido poroso. Quanto aos espectros de luminescência e reflectância, o aumento da temperatura do eletrólito ocasionou um aumento no número de interferências. Entretanto, a amplitude e a resolução das interferências diminuíram com o aumento da temperatura. A espessura do filme poroso foi estimada pelas técnicas de variação de energia (?E), coeficiente angular do gráfico entre ordem da interferência e 1/? e através da transformada rápida de Fourier (FFT). E foi utilizada a razão entre a espessura do filme e diâmetro do poro (Esp/Dp) para averiguar os filmes de AAP com guias de onda que pudessem ser utilizadas como substratos para sensores ópticos. Além disso, foi realizada a análise de composição superficial dos filmes de AAP anodizados em ácido fosfórico, oxálico e mistura destes ácidos pela técnica de espectroscopia de retroespalhamento de Rutherford (RBS). A partir das simulações realizadas foi possível notar que a quantidade que carbono na estrutura do oxido poroso é praticamente nula, o que pode indicar que a origem da luminescência está relacionada à presença dos centros F.

The work presented in this thesis investigates the mathematical modelling of charge transport in electrolyte solutions, within the nanoporous structures of electrochemical devices. We compare two approaches found in the literature, by developing onedimensional transport models based on the Nernst-Planck and Maxwell-Stefan equations. The development of the Nernst-Planck equations relies on the assumption that the solution is infinitely dilute. However, this is typically not the case for the electrolyte solutions found within electrochemical devices. Furthermore, ionic concentrations much higher than those of the bulk concentrations can be obtained near the electrode/electrolyte interfaces due to the development of an electric double layer. Hence, multicomponent interactions which are neglected by the Nernst-Planck equations may become important. The Maxwell-Stefan equations account for these multicomponent interactions, and thus they should provide a more accurate representation of transport in electrolyte solutions. To allow for the effects of the electric double layer in both the Nernst-Planck and Maxwell-Stefan equations, we do not assume local electroneutrality in the solution. Instead, we model the electrostatic potential as a continuously varying function, by way of Poisson’s equation. Importantly, we show that for a ternary electrolyte solution at high interfacial concentrations, the Maxwell-Stefan equations predict behaviour that is not recovered from the Nernst-Planck equations. The main difficulty in the application of the Maxwell-Stefan equations to charge transport in electrolyte solutions is knowledge of the transport parameters. In this work, we apply molecular dynamics simulations to obtain the required diffusivities, and thus we are able to incorporate microscopic behaviour into a continuum scale model. This is important due to the small size scales we are concerned with, as we are still able to retain the computational efficiency of continuum modelling. This approach provides an avenue by which the microscopic behaviour may ultimately be incorporated into a full device-scale model. The one-dimensional Maxwell-Stefan model is extended to two dimensions, representing an important first step for developing a fully-coupled interfacial charge transport model for electrochemical devices. It allows us to begin investigation into ambipolar diffusion effects, where the motion of the ions in the electrolyte is affected by the transport of electrons in the electrode. As we do not consider modelling in the solid phase in this work, this is simulated by applying a time-varying potential to one interface of our two-dimensional computational domain, thus allowing a flow field to develop in the electrolyte. Our model facilitates the observation of the transport of ions near the electrode/electrolyte interface. For the simulations considered in this work, we show that while there is some motion in the direction parallel to the interface, the interfacial coupling is not sufficient for the ions in solution to be "dragged" along the interface for long distances.

This thesis presents a mathematical model of the nanoporous anode within a dyesensitised solar cell (DSC). The main purpose of this work is to investigate interfacial charge transfer and charge transport within the porous anode of the DSC under both illuminated and non-illuminated conditions. Within the porous anode we consider many of the charge transfer reactions associated with the electrolyte species, adsorbed dye molecules and semiconductor electrons at the semiconductor-dye- electrolyte interface. Each reaction at this interface is modelled explicitly via an electrochemical equation, resulting in an interfacial model that consists of a coupled system of non-linear algebraic equations. We develop a general model framework for charge transfer at the semiconductor-dye-electrolyte interface and simplify this framework to produce a model based on the available interfacial kinetic data. We account for the charge transport mechanisms within the porous semiconductor and the electrolyte filled pores that constitute the anode of the DSC, through a one- dimensional model developed under steady-state conditions. The governing transport equations account for the diffusion and migration of charge species within the porous anode. The transport model consists of a coupled system of non-linear differential equations, and is coupled to the interfacial model via reaction terms within the mass-flux balance equations. An equivalent circuit model is developed to account for those components of the DSC not explicitly included in the mathematical model of the anode. To obtain solutions for our DSC mathematical model we develop code in FORTRAN for the numerical simulation of the governing equations. We additionally employ regular perturbation analysis to obtain analytic approximations to the solutions of the interfacial charge transfer model. These approximations facilitate a reduction in computation time for the coupled mathematical model with no significant loss of accuracy. To obtain predictions of the current generated by the cell we source kinetic and transport parameter values from the literature and from experimental measurements associated with the DSC commissioned for this study. The model solutions we obtain with these values correspond very favourably with experimental data measured from standard DSC configurations consisting of titanium dioxide porous films with iodide/triiodide redox couples within the electrolyte. The mathematical model within this thesis enables thorough investigation of the interfacial reactions and charge transport within the DSC.We investigate the effects of modified cell configurations on the efficiency of the cell by varying associated parameter values in our model. We find, given our model and the DSC configuration investigated, that the efficiency of the DSC is improved with increasing electron diffusion, decreasing internal resistances and with decreasing dark current. We conclude that transport within the electrolyte, as described by the model, appears to have no limiting effect on the current predicted by the model until large positive voltages. Additionally, we observe that the ultrafast injection from the excited dye molecules limits the interfacial reactions that affect the DSC current.

This thesis presents a mathematical model of the nanoporous anode within a dyesensitised solar cell (DSC). The main purpose of this work is to investigate interfacial charge transfer and charge transport within the porous anode of the DSC under both illuminated and non-illuminated conditions. Within the porous anode we consider many of the charge transfer reactions associated with the electrolyte species, adsorbed dye molecules and semiconductor electrons at the semiconductor-dye- electrolyte interface. Each reaction at this interface is modelled explicitly via an electrochemical equation, resulting in an interfacial model that consists of a coupled system of non-linear algebraic equations. We develop a general model framework for charge transfer at the semiconductor-dye-electrolyte interface and simplify this framework to produce a model based on the available interfacial kinetic data. We account for the charge transport mechanisms within the porous semiconductor and the electrolyte filled pores that constitute the anode of the DSC, through a one- dimensional model developed under steady-state conditions. The governing transport equations account for the diffusion and migration of charge species within the porous anode. The transport model consists of a coupled system of non-linear differential equations, and is coupled to the interfacial model via reaction terms within the mass-flux balance equations. An equivalent circuit model is developed to account for those components of the DSC not explicitly included in the mathematical model of the anode. To obtain solutions for our DSC mathematical model we develop code in FORTRAN for the numerical simulation of the governing equations. We additionally employ regular perturbation analysis to obtain analytic approximations to the solutions of the interfacial charge transfer model. These approximations facilitate a reduction in computation time for the coupled mathematical model with no significant loss of accuracy. To obtain predictions of the current generated by the cell we source kinetic and transport parameter values from the literature and from experimental measurements associated with the DSC commissioned for this study. The model solutions we obtain with these values correspond very favourably with experimental data measured from standard DSC configurations consisting of titanium dioxide porous films with iodide/triiodide redox couples within the electrolyte. The mathematical model within this thesis enables thorough investigation of the interfacial reactions and charge transport within the DSC.We investigate the effects of modified cell configurations on the efficiency of the cell by varying associated parameter values in our model. We find, given our model and the DSC configuration investigated, that the efficiency of the DSC is improved with increasing electron diffusion, decreasing internal resistances and with decreasing dark current. We conclude that transport within the electrolyte, as described by the model, appears to have no limiting effect on the current predicted by the model until large positive voltages. Additionally, we observe that the ultrafast injection from the excited dye molecules limits the interfacial reactions that affect the DSC current.