Rozprawy doktorskie na temat „Liquid Metal Films”

Perowskite und Perowskit-artige Materialien sind von großem Interesse, da sie eine Vielzahl von strukturellen und physikalischen Eigenschaften haben, welche die Möglichkeit bieten, sie für unterschiedliche Anwendungen einzusetzen. Die Methode der Liquid-Delivery Metal Organic Chemical Vapour Deposition (LD-MOCVD) wurde gewählt, da sie eine gute Kontrolle über die Zusammensetzung ternärer Oxide und eine hohe Homogenität der Filme ermöglicht. Darüber hinaus können mit dieser Methode Filme hergestellt werden, die aus Elementen bestehen, für welche nur feste Precursor oder welche mit niedrigem Dampfdruck zur Verfügung stehen. Ziel dieser Arbeit war es, mit Hilfe der LD-MOCVD Filme aus SrRuO3, Bi4Ti3O12 und (Na,Bi)4Ti3O12 abzuscheiden und den Einfluss der Wachstumsbedingungen auf die Eigenschaften der Filme zu untersuchen. Zusätzlich wurde die Wirkung der Verspannung, die durch die Gitterfehlanpassung zwischen Substrat und Film entsteht, auf die physikalischen Eigenschaften der Schichten untersucht. SrRuO3 Filme wurden auf gestuften SrTiO3(001), NdGaO3(110) und DyScO3(110) Substraten gewachsen, deren Oberflächenterminierung durch oberflächensensitive Proton-induzierte Auger-Elektronen-Spektroskopie (AES) bestimmt wurde. Die Substrate wurden unter verschiedenen Bedingungen durch Änderung der Temperdauer und -atmosphäre präpariert. Die systematische Untersuchung der Beziehung zwischen Verspannung und Curie-Temperatur von dünnen SrRuO3(100) Filmen erfolgte unter Verwendung von Substraten mit unterschiedlichen Gitterkonstanten. Die beobachtete Curie-Temperatur sank mit erhöhter kompressiver Verspannung und nahm mit erhöhter tensiler Verspannung zu. Um stöchiometrische und epitaktische Bi4Ti3O12(001) Filme zu wachsen, war aufgrund der Flüchtigkeit des Bismuts ein Bi Überschuss in der Precursor-Lösung notwendig. Die Substitution von Bi durch Na in Bi4Ti3O12 wurde zum ersten Mal in LD-MOCVD-Filmen erreicht.
Perovskites and perovskite-like materials are actually of great interest since they offer a wide range of structural and physical properties giving the opportunity to employ these materials for different applications. Liquid-Delivery Metal Organic Chemical Vapour deposition (LD-MOCVD) was chosen due to the easy composition control for ternary oxides, high uniformity and good conformal step coverage. Additionally, it allows growing the films, containing elements, for which only solid or low vapour pressure precursors, having mainly thermal stability problems over long heating periods, are available. The purpose of this work was to grow SrRuO3, Bi4Ti3O12 and (Na, Bi)4Ti3O12 films by LD-MOCVD and to investigate the influence of the deposition conditions on the properties of the films. Additionally, the effect of the strain due to the lattice mismatch between substrates and films on the physical properties of the films was also investigated. SrRuO3 films were grown on stepped SrTiO3(001), NdGaO3(110) and DyScO3(110) substrates, which were prepared under different conditions by changing the annealing time and atmosphere. The termination of the substrates was measured by surface sensitive proton-induced Auger Electron Spectroscopy (p-AES) technique. Another systematic study of the relation between epitaxial strain and Curie temperature of thin SrRuO3(100) films was performed by using substrates with different lattice constants. The observed Curie temperature decreased with compressive and increased with tensile strain. Thin films of Bi4Ti3O12 as well as (Na, Bi)4Ti3O12 were successfully deposited. In order to grow stoichiometric and epitaxial Bi4Ti3O12(001) films, Bi excess in the precursor solution was necessary, due to the volatility of Bi. Substitution of Bi with Na in Bi4Ti3O12 was achieved for the first time for the films deposited by LD-MOCVD.

Ce travail constitue une caractérisation des films de Langmuir-Blodgett par résonance paramagnétique électrique et par diffusion Raman résonante en lumière polarisée. Les films sont constitués de multicouches, mixtes ou alternées, de porphyrines amphiphiles et d'acide docosanoïque. Le sujet porte sur l'orientation des macrocycles porphyriniques par rapport au substrat supportant les couches

Deposition of noble metal and semiconductor nanocrystalline thin films has received much attention. CdS and CdSe are important semiconductors used in optical devices. A wet chemical route which uses the interface of two immiscible liquids to control the growth and deposition of nanocrystalline thin films forms the basis of the current study. In this method, a metal precursor dissolved in toluene or decane is held in contact with a water layer containing a reducing or sulphiding agent. The reaction proceeds at the interface of the liquids and results in deposits adhering to the interfacial region. The products of such reactions typically consist of nanocrystals forming a thin film. Stable sols of Au, Ag were found to metathesize on contact with alkylamine in oil to form monolayer films that spread across large areas at the water/oil interface. The nature and properties of interfacial thin films depend on the alkylamine. Nanocrystalline thin films consisting of CdS adhering to the interface starting with a polydispersed aqueous sol of crystallites and alkylamine were obtained. The optical band gaps of the films formed are dependent on the alkylamine chain length, with the shortest chain yielding the largest gap. A systematic increase in particle diameters following adsorption is responsible for changes in the electronic structure of films. The formation of nanocrystalline films of CdS adhering at the interface using a toluene solution of cadmium diethyldithiocarbamate and aqueous Na2S solution, in the presence of tetraoctylammonium bromide (TOAB) in the aqueous phase, was investigated under various reaction parameters, while CdSe was obtained using Na2SeSO3 solution and the influences of deposition temperature and solution concentration were studied. A ternary water/decane/2-butoxyethanol /salt system was used to grow deposits of CdSe and CdS. Nanostructured thin films were obtained at the upper interface of the ternary system, between the emulsive middle layer and oil rich top phase. The influence of deposition conditions such as precursor concentrations and temperature, as well as the nature of the medium on the properties of the deposits was studied. Deposits grown using the ternary system were compared with those obtained using water/decane and water/toluene systems. Reaction parameters such as temperature, solution concentration and the size of CdS and CdSe were controlled. A thin film of CdS and CdSe nanocrystals was formed at the interface. The grain size was found to be dependent on reaction temperature and solution concentration, with higher temperatures and solution concentration resulting in larger grains. The nature of thin films obtained at the interface of two immiscible liquids and of a water/decane/2-butoxyethanol/salt ternary system were studied using Scanning and Transmission electron microscopy, X-ray diffraction and UV-visible spectroscopy.

Le contact entre le jet d'eau et l'acier inoxydable fondu observe dans l'installation experimentale seizies provoque une pressurisation et une liberation de l'energie mecanique. On propose dans ce memoire une analyse et une interpretation de cette interaction a l'aide d'un modele thermodynamique et d'un modele parametrique. Les objectifs de ce travail sont: l'evaluation du terme source de l'interaction, a savoir l'energie reellement transmise dans l'interaction et celle transformee en travail mecanique et l'amelioration des connaissances sur l'interaction thermique metal fondu et notamment dans le cas d'une injection de l'eau sur le metal fondu. Les resultats importants sont: le travail mesure experimentalement est representatif du travail maximum liberable dans seizies, la masse d'acier participant a ete determine et certains mecanismes physiques ont ete valides

Dans le but d'etudier un materiau utilisable pour la fabrication de detecteurs pour les telecommunications par fibre optique a 1,3 et 1,55mu m, des echantillons de cd::(x) hg::(1-x) te(0,5 x 1) ont ete prepares par epitaxie en phase liquide en tube ouvert. Diverses methodes de caracterisation (microsonde electronique, topographie rx, effet hall, sonde ionique. . . ) ont montre la bonne qualite des couches. Le dopage effectue au cours de la croissance par in et cu a ete aborde. Etude des effets d'alliage dans cdhgte par photoluminescence

碩士
中原大學
奈米科技碩士學位學程
103
Abstract This thesis focuses on the Metal–Organic–Frameworks (MOFs) doped polymer films which improve the electrical properties of degraded liquid crystals (LCs) by taking the advantages of the absorption characteristics of MOFs. The research result shows that the pure polymer films are not good absorbers. However, doping MOF material in the polymer films can improve the absorbing ability of the films. And when immerse the doped films in the degraded liquid crystals (LCs), the electrical properties of the LCs can be recovered through the measurements of the dielectric spectroscopy and voltage holding ratio. By adding the electronegative material AMPS (2-acrylamido-2-methylpropane sulfonic acid) to the MOFs doped films, the purification is further improved. The doped polymer films can be separated easily from the purified LCs and reusable, that is, achieves the eco purposes.

Transfer of mass energy is at the heart of many physical processes that form the backbone of modern science and technology. Transport studies help in understanding these processes and reveal the underlying physics that govern the process dynamics. Electromigration, which at the heart of the thesis, is basically a material transport phenomenon driven by electric field

In recent times, researchers have found potential in using liquid metals in broad-spectrum applications such as processing ceramics, stretchable electronics, 3D printing, coolant technology, energy storage, and many others. Gallium and its alloys are preferred over conventional mercury in all these applications because of its low melting point, high boiling point, and non-toxicity. Given these exciting applications, there is a need to understand the flow behavior of low melting metals in the liquid state, especially at small length scales. The wetting of gallium on metal thin solid films is not well understood. Wetting of liquid metals on thin metal films is complex due to the nature of the interaction between the liquid and the substrate. This interaction involves solidification and chemical reactions, including oxidation of the liquid metal itself. The central theme of this thesis is to understand the liquid metal flow on thin solid films due to various driving forces, such as wetting, chemical reaction, and electromigration. This quest was pursued because of the versatile nature of the phenomenon, which produces patterns ranging from distinct islands to periodic patterns at microscale and nanoscale .Given its interesting and technologically important features, we pursued our research to understand the fundamentals of this phenomenon. In the first part of the work, we investigate the formation of patterns of liquid metals with the rippled surfaces on the metal thin solid films. Flow is observed in situ using a scanning electron microscope (SEM) at high magnifications to gain fundamental insights into the phenomenon of the spontaneous flow of the liquid metal and the formation of the unique surface features . SEM characterization indicates that the liquid-solid reaction and wetting front drives the entire flow and thereby pattern formation. To further understand the type of materials interaction during the liquid metal flow, characterization of the features formed in multiple systems, such as Ga-Pt and Sn-Pt, is performed using the energy dispersive spectroscopy (EDS) inside a scanning transmission electron microscope (STEM). All material systems demonstrated similar features of chemical reaction taking place between the liquid metal and solid thin film similar observations and enrichment of the top layer of the solidified flow pattern by the substrate metal film. Based on the observations, a finite element (FE) model is developed, whose predictions are compared with the experimental observations . The predictions match qualitatively well with the experimental findings, thereby confirming the important role of wetting, chemical reaction, and formation of semi-solidus membrane on top of the liquid film on the flow as well as ripple-pattern formation. Further, we performed an optimization study for controlling the features of surface ripples and implemented the inferred techniques for creating Ga ripple patterns with gaps as small as 100 nm. In the second part of the work, the electric current-induced liquid metals flow or liquid electromigration is studied. Liquid electromigration is a special case of liquid metal flow on thin solid films, wherein the applied electric field provides an additional force for flow, along with the liquid-solid wetting and chemical reaction at the flow front. Liquid electromigration is considered to be a diffusion-controlled phenomenon, and this work is conducted with an objective to improve the understanding of the phenomena by examining it from a continuum-based perspective. Herein, experiments are designed to observe the behaviors similar to pressure-driven liquid flow in the open channel, with the pressure replaced by the electric current (or electromigration “force”). The observations, with a custom-made experimental setup, are carried out in situ inside an SEM as well as an optical microscope. In situ studies indicate the formation of an immovable layer on the top of the flow. Flow splitting and selective wetting of the thin film are demonstrated via modulation of the current density. Liquid metal flow front velocity is characterized for different geometries of the metal track, and the obtained profile is correlated with the current density value. Although liquid electromigration has some remarkable similarities with the pressure-driven open-channel flow and intuition about electric-field induced liquid metal flow can be built using the continuum ideas, the order of magnitude calculation suggests that the driving force for liquid electromigration cannot be converted into an equivalent pressure or body force term that can be used in the continuum fluid mechanics equations (e.g., Navier-Stokes equation) to quantify liquid electromigration. Therefore, the diffusion treatment remains the most accurate way for quantifying the liquid electromigration. In the last segment of the work, we develop a new technique based on the principle of liquid electromigration and liquid metal wetting of thin solid films to transport liquid metal from a liquid metal pool over a metal-coated needle and then “back” to a location of choice by application of an electric current. The method is helpful in conformally coating the metal films, establishing electrical connections, and bridging a gap at the microscale and, potentially, at the nanoscale. The mechanism is analogous to a “liquid” dropper which uses suction to collect liquid and “release” to dispense a liquid, with, herein, the suction replaced by the liquid electromigration and wetting. The “sucked” liquid metal can be transported for the bridging process to form different electrical connections as well as standalone metallic structures. An experimental setup is developed, and the proof of concept for a method of bridging gaps at two different length scales is given. In summary, the work contributes to both the fundamental understanding of liquid metal flow on thin solid films and taking advantage of the flow physics for technology development at micro-and nano-scale.

碩士
高雄醫學大學
醫藥暨應用化學研究所
96
This thesis reports the electrochemical behavior of metals (such as manganese and silver) and conducting polymers (such as polycarbazole and polypyrrole) in room-temperature ionic liquids at tungsten, platinum and glassy carbon electrodes, respectively. Ionic liquids used in this thesis include BMI-PF6 (1-butyl-3-methylimidazolium hexafluorophosphate), BMP-Tf2N(N-butyl-N-methylpyrrolidinium bis((trifluoromethyl)sulfon- yl)imide), and BMP-DCA (N-butyl-N-methylpyrrolidinium dicyanamide). All the electrodeposition samples were analyzed with scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS). The electrochemistry of manganese was studied in BMP-DCA. It was found that dicyanamide anions exhibit good complexing ability that can assist MnCl2 to dissolve. The manganese coatings were prepared by controlled- potential electrodeposition at copper foil or wire. Highly pure and amorphous Mn coatings can be obtained in BMP-DCA. The high-quality electrochromic polycarbazole (PCz) film that is stable, strongly adhesive, and show different color depth between oxidative state (deep green) and reductive state (light green) was obtained in BMP-Tf2N and BMI-PF6, respectively. The electrochemical behavior of Cz monomer and the PCz films and the electrochromic behavior of the PCz films in both ionic liquids were also investigated. The PCz films were electrodeposited at indium tin oxide (ITO) electrode by cyclic voltammetry, controlled-potential and potential pulse techniques. It is expected that the stable, conductive, and strongly adhesive PCz films are appropriate materials to eucapsulate catalysts upon electrode surface. The electrochemistry of pyrrole (Py) and silver (Ag) was studied in BMP-Tf2N and BMI-PF6, respectively. The PPy/Ag composite films were electrodeposited at tungsten wire by controlled-potential and potential pulse techniques. A porous, rigid structure of PPy/Ag composite films can be prepared by potential pulse electrodeposition and this material is possible to be used for electrocatalysis.

碩士
明道大學
材料科學與工程學系碩士班
100
The goal of this study is to develop high quality and low cost metal oxide semiconductor fabrication technology. Up to now, there is less study in metal oxide semiconductor TFTs by liquid phase deposition (LPD) process. It has many advantages like low cost, good uniformity and adhesion, easy stoichiometry control, and a potential of large scale batch-type mass production. In preparation of indium titanium oxide thin film transistors, ammonium hexafluoro-titanate and boric acid were used as materials to produce titanium oxide semiconductor films with the liquid phase deposition method. In this process, indium nitrate was doped to improve the mobility. Then, aluminum electrodes were further plated on these films via a shadow mask with the thermal evaporation. The transfer characteristics of TFT device were measured by an HP-4145B semiconductor parameter analyzer. Finally, we proved that, in this preparation of TiO2 by LPD, indium nitrate is an effective doping to the TiO2. This doping made the optical energy gap down to 2.95 eV, and the average transmittance is 74.08 %, the Hall mobility as high as 7.45 cm2/Vs. The final product is a type of inverse staggered, the μsat is 0.799 cm2/Vs, threshold voltage is -5.27 V, subthreshold swing is 0.13 V/dec, and on/off current ratio is 1.39 x 106.

Numerous electric field induced phenomena have been studied, for long, at various length scales. In particular, a concentrated electric field applied across a conductor, or equivalently an electric current of very high density passing through a conductor, can manifest in form of both destructive and constructive processes, depending on the requirements of an application. For example, electromigration, which is a diffusion-controlled electric field directed mass transport phenomenon, often leads to the formation of voids and hillocks near the cathode and the anode, respectively, metal interconnects in microelectronic devices. This results in failure of the device and hence this “destructive” manifestation of the electric field is considered as a “villain” in microelectronic interconnects. On the other hand, recently discovered electromigration in liquid metals may pave the path for various useful applications, such as maskless conformal coating, pattern formation, surface modification, etc. Besides the exploitation of the capability of the electric field for transporting matter (e.g., in liquid metals) in controlled and directed fashion in various applications, harnessing the unique potential of the electric field in inducing a chemical reaction in a controlled fashion in a confined region also provides new avenues for constructive usage. In particular, the electric field induced chemical reaction has been exploited for patterning at extremely small length scale, using scanning probe microscopes, such as atomic force microscope (AFM) and scanning tunneling microscope (STM). It is imperative to unambiguously understand the fundamentals of the concerned phenomenon before the aforementioned electric current induced phenomenon can be exploited to bear numerous technologies and applications. Here, we have studied two different electric field induced phenomena, namely electromigration in liquid metals and electric field-induced chemical reaction in solid thin metals1. The presentation of the study in form of this thesis is divided into three main parts, (i) Theoretical modelling of electromigration in liquid metals (or liquid electromigration), (ii) Study of the electric field induced chemical reaction in Cr film, including a detailed investigation of effects of ambient conditions on reaction kinetics, and (iii) Development of a tool for pattern drawing by the means of electric field-induced chemical reaction. As mentioned earlier, electromigration, irrespective of whether it is in solids or liquids, is a diffusion-controlled directional mass transport phenomenon that is driven by the applied electric field. The direction of the mass transport, in general, depends on the net force experienced by the positively charged ions due to the applied electric field and the momentum transferred from the colliding free-electrons. Hence, it is critical to understand the direction of this effective force in metal. Often it is from the cathode to the anode in a solid metal; however, it is not that straightforward in liquid metals. For example, the direction of the net mass transport in most of the liquid metals, e.g., Ga, In, Ga, etc., is in the direction of the electric field (i.e., from the anode to the cathode, which is contrary to the solid metals), whereas the direction of the flow is reversed in the liquid Pb. The reason for the dichotomy of the directionality in the liquid metal flow was not completely understood, and hence we performed detailed analytical and experimental work to resolve it. Here, we developed a theoretical model based on the cell model of liquids and incorporating Lennard-Jones interaction potential. The model considers the short-range order in liquid metals and calculates the force on the ions due to the momentum transferred by electrons during electron-ion collisions. The model not only successfully predicts the flow direction of numerous liquid metals, including liquid Pb, Ga, etc., but it also gives the value of effective charge number of liquid metals as the function of the temperature. Experiments were performed on selected liquid metals in order to validate the model. As mentioned earlier, electrical interaction between the tip and the metal (e.g., Cr) film may induce a chemical reaction in the localized region around the probe tip on the film. If the probe is translated over the metal film along a predefined path, then the chemical reaction induced controlled patterning of the film can be achieved. In the second segment of our work, the focus has been to understand the mechanism of the phenomenon of electric field-induced chemical reaction in Cr film by performing a series of experiments using a custom-built experimental setup, so that we can later exploit the gained understanding for lithography. The phenomenon was studied using a W-tip with a diameter of 20 μm under stationary tip condition. Although this length scale is relatively larger than that of using AFM or STM tip, it provides a significant amount of reaction product and allows easy maneuverability as well as better control of the experiments: Both of these are critical for an unambiguous understanding of the nature and kinetics of the chemical reaction. The study includes confirmation of observation of a chemical reaction induced process in presence of electric field (as per Faraday’s law) and the identification of the reactants (as Cr and H2O – in the form of both vapor and liquid) and the product (as CrO3) at the cathode. The ambient conditions affect the reaction kinetics at the cathode probe tip and hence the dimensions (as well as quality) of the patterns. Therefore, the phenomenon was studied under different ambient conditions, such as vacuum, gaseous (e.g., N2, O2 and air) environments, variable humidity, high and low temperature, etc. It was observed that the reaction did not occur in an environment unless water vapor (or water) was present. Furthermore, the reaction occurred without the generation of a significant amount of heat (and hence the negligible rise in the temperature was associated with this process). Finally, the reaction was favored at the locations of high current densities at or near the cathode. A study on the understanding of the nature of the reaction product revealed that CrO3 is highly hygroscopic and it quickly absorbs water from the air to become liquid. As the reaction product is soluble in water, the region where reaction had taken place could be easily removed by dipping the sample into water. The use of water in the reaction was further exploited to develop a new SPM based lithography process that can preclude the need to keep the sample and the tip into contact and proffers spontaneous removal of the reaction product. Overall, the results obtained in this segment of the work paves the path for developing a new tip-based lithography technique that is better suited to meet the challenges of tip wear, debris collection, low throughput, etc., which are often associated with other SPM based lithography techniques. In the last segment of the work, the understanding of the electric current induced chemical reaction in Cr film was applied to develop a tool for drawing patterns at the micrometer length scale. A considerable amount of effort was made to assemble a standalone lithography unit that can work in ambient as well as submerged in water conditions. Here, a micro-positioner was used to place the sample at the desired location relative to the tip, and a W-tip was traversed over the sample. The tip was brought into contact with (and detached from) sample using an “electromagnet-based lever-type drive.” A software-hardware interface was developed using LabView software, which was also capable of importing drawings made in third-party software, such as CleWin. Tool parameters, such as tip velocity, tip force, etc., were observed to have a significant impact on the pattern dimensions. Finally, several patterns, including closely spaced parallel lines, were generated using the developed tool in Cr films of different thicknesses and statistical information was obtained. In summary, this work, which includes both explorations of fundamentals and application of the learned fundamentals to develop new technology for lithography, confirms the constructive potential of the electric current and invites researchers to explore this area further.

碩士
國立成功大學
環境醫學研究所
93
Shift workers have more mental health problems and sleep disturbances. Thin film transistor liquid crystal display (TFT-LCD) industry has become the leading industry in Taiwan. We conducted two cross-sectional studies on mental health and sleep disturbance in the TFT-LCD industry in Taiwan. The first study was designed to sketch the mental health condition of the employees of the TFT-LCD industry, and the second was on sleep quality of the workers. Data were collected by self-administered questionnaires. In the first study, after 9458 questionnaires were delivered, 5520 (58.2%) were completed and analyzed. Shift workers were mostly female, single, younger, and with lower educational level and shorter duration of employment. After adjusting for other factors, female gender (odds ratio [OR] 2.00, 95% confidence interval [CI] 1.67-2.39), married (OR 1.28, 95%CI 1.09-1.50), ever married but single now (OR 1.93, 95%CI 1.03-3.61), and the duration of current employment more than 1 year (OR 1.51, 95%CI 1.33-1.71) had significant associations with poor mental health condition. The risk of mental problems decreased with age (OR 0.98, 95%CI 0.96-1.00), and the employees of the entry level had less risk of mental problems (odds ratio 0.59, 95%CI 0.43-0.82). 　　In the second study, the 3640 (85.6%) of 4250 delivered questionnaires were finished and analyzed. After adjusting for other factors, the clean room workers (OR 1.60, 95%CI 1.09-2.33) and female gender (OR 2.37，95%CI 1.75-3.20) had significant associations with poor sleep quality. In stratified analyses, female clean room workers had the highest proportion of poor sleepers while male non-clean room workers had lowest proportion. Shift workers had different directions of association with individual items of sleep quality complaints.

Alla luce del recente allarmante tasso di esaurimento delle riserve di combustibili fossili e al contemporaneo drastico aumento dei livelli di CO2 nell'atmosfera, principale gas serra responsabile del riscaldamento globale e di cambiamenti climatici molto gravi, una delle priorità assolute nella ricerca a livello mondiale è quella di sfruttare il più possibile le fonti di energia rinnovabile. Una possibilità molto interessante è quella di realizzare un processo di riduzione della CO2 a combustibili liquidi che sfrutti energie rinnovabili, quale quella solare, mediante dispositivi più comunemente noti come celle fotosintetiche artificiali o foglie artificiali o celle foto-elettro-catalitiche (PEC). L'obiettivo principale di questo lavoro, è stato pertanto quello di condurre uno studio approfondito su due diversi sistemi elettrocatalitici di riduzione della CO2 a prodotti liquidi con un più alto valore aggiunto, uno operante in fase gassosa (cioè in assenza di elettrolita al catodo) e uno operante in fase liquida. In particolare, è stata progettata e utilizzata nel processo di conversione della CO2, un’innovativa cella in fase liquida operante su scala di laboratorio, sulla falsariga della cella in fase gas precedentemente sviluppata all’Università di Messina. Il lavoro è stato svolto principalmente presso il laboratorio CASPE/INSTM dell’Università degli Studi di Messina (Dipartimento di Ingegneria Elettronica, Chimica e Ingegneria Industriale). Un periodo di sei mesi è stato svolto invece, nel corso del secondo anno di dottorato, presso l’École supérieure de chimie, physique, électronique de Lyon (CPE Lyon). In tale periodo sono stati sintetizzati, mediante innovative tecniche di sintesi organometallica, materiali compositi da utilizzare come elettrocatalizzatori nel processo di riduzione della CO2. Sono state effettuate molteplici prove sperimentali utilizzando svariate tipologie di catalizzatori, sia in fase gas che in fase liquida, al fine di indagare la differente selettività, produttività e varietà di prodotti ottenuti. Il processo in fase liquida è infatti quello maggiormente studiato in letteratura, ma esistono alcune problematiche che devono essere superate per consentire un successivo semplice scale up. quali ad esempio, la scarsa solubilità della CO2 e la tipologia di prodotti ottenuti (principalmente acido formico). Lo scopo principale di questo lavoro è stato quello di preparare nuovi materiali a base di carboni dopati con metalli, catalizzatori questi molto diversi da quelli comunemente utilizzati nel processo di riduzione della CO2 (generalmente metalli in bulk), e di testarli sia in fase gas (per sfruttare i vantaggi di questa condizione, quali ad esempio facile recupero dei prodotti e alta qualità dei prodotti stessi) sia in fase liquida (per avere un miglior confronto con i dati ampiamente presenti in letteratura). Per gli studi sulla riduzione elettrocatalitica della CO2 nella cella operante in fase gassosa, sono stati preparati una serie di elettrodi (basati su nano particelle –NP- di Cu, Fe, Pt e Cu/Fe depositate su nanotubi di carbonio o carbon black e successivamente poste all'interfaccia tra una membrana di Nafion e uno strato a diffusione di gas –GDL-). I risultati ottenuti sono stati molto promettenti, sia in termini di tipologia di prodotti formati che di produttività. In fase gas (senza elettrolita) è stata osservata la formazione di prodotti ≥C1 quali etanolo, acetone e isopropanolo, in particolare utilizzando il Fe (seguito dal Pt), evidenziando che anche metalli non nobili possono essere usati in maniera efficiente in questo processo. Per migliorare la produttività nella reazione di riduzione della CO2, sono stati preparati elettrodi differenti, basati su coating con sostituti zeolitici imidazolici (SIM-1) tipo MOF. In particolare, i catalizzatori testati sono stati MOF modificati con Fe-CNT, Pt-CNT, e CuFe-CNT. E’ stato osservato un cambiamento significativo in termini di produttività e anche di selettività verso i prodotti finali. Nel dettaglio, in particolare per il catalizzatore a base di MOF modificato con Pt, è stato osservato un aumento nei prodotti carboniosi e anche una selettività più alta verso prodotti con un più elevato numero di atomi di C. Per quanto riguarda lo studio del processo di riduzione elettrocatalitica della CO2 utilizzando la cella operante in fase liquida, sono state preparate tipologie di elettrodi simili ai precedenti. Inizialmente infatti, sono stati studiati elettrodi a base di nanoparticelle metalliche (Cu, Fe, Pt, Ru, Co) depositate su nanotubi di carbonio o carbon black. L'ordine relativo della produttività nella riduzione elettrocatalitica della CO2 in questa serie di elettrodi, è però risultato essere diverso rispetto alla fase gassosa, indicando quindi un differente percorso di reazione. In termini di produttività totale, gli elettrodi a base di Pt hanno consentito di ottenere le migliori performance, seguiti da Ru e Cu, mentre il Fe ha dato risultati peggiori. Sulla base dei risultati sperimentali ottenuti, è stato inoltre ipotizzato un possibile meccanismo di reazione. Successivamente, per cercare di migliorare ulteriormente le prestazioni nel processo di riduzione della CO2 in fase liquida, è stato effettuato uno studio approfondito, volto ad indagare la dipendenza di tale processo dalle dimensioni delle nanoparticelle metalliche. A tale scopo sono stati utilizzati elettrodi a base di nanoparticelle metalliche (Ru, Fe, Pt e Cu) su nanotubi di carbonio (CNT) depositati su GDL. Sono state sintetizzate nanoparticelle metalliche di diverse dimensioni utilizzando molteplici tecniche di sintesi: (i) impregnazione che ha consentito di ottenere NP di dimensioni comprese tra 10-50 nm; (ii) sintesi organometallica che ha consentito di ottenere NP uniformi e ultrafine con dimensioni comprese tra 1-5 nm. (ad esempio sono state sintetizzate NP di Fe di 1-3 P nm) (iii) sintesi mediante nanowires che ha consentito di ottenere NP di rame ultrafine con dimensioni comprese tra 2-3,8 nm. In particolare, la novità dell’approccio mediante nanowires sta nella possibilità di ottenere particelle di dimensioni molto piccole sintetizzando inizialmente i Cu NWs, mettendoli poi a contatto con il supporto carbonioso e facilitandone il suo trasferimento, ciò grazie alle forze intermolecolari di attrazione dei gruppi funzionali presenti sui CNT parzialmente ossidati. Inoltre, a differenza della sintesi organometallica, tale approccio permette di condurre le reazioni in aria e non in atmosfera inerte. I valori di produttività ottenuti sono stati 5-30 volte più alti utilizzando nanoparticelle metalliche più piccole (ottenute via nanowires o mediante sintesi organometallica) rispetto alle nanoparticelle metalliche più grandi (ottenute per impregnazione). I risultati sperimentali indicano pertanto che le NP di dimensioni più piccole hanno un ruolo fondamentale nelle performance catalitiche. Inoltre, il carico di NP metalliche è stato significativamente ridotto dal 10% al 1-2% in peso consentendo di ottenere, per le NP più piccole, una produttività equivalente o addirittura superiore rispetto alle nanoparticelle più grandi. In seguito, è stato effettuato anche uno studio sul possibile riutilizzo degli elettrodi di lavoro e sulla disattivazione per tempi di reazione più lunghi. E’ stata infine preparata una diversa tipologia di elettrodi a base di nano-foams su lastrine metalliche, al fine di ottenere un ulteriore miglioramento nel processo di riduzione elettrocatalitica della CO2. Le nano-foams o dendriti, sono state preparate mediante la tecnica di deposizione elettrochimica ed è stato effettuato uno studio preliminare di ottimizzazione, al fine di determinare le condizioni di sintesi più adatte. In aggiunta, è stato eseguito uno studio specifico per ottimizzare il valore di Voltaggio da utilizzare nelle prove catalitiche, mediante sia test di voltammetria ciclica che test completi di riduzione della CO2. Sono stati testati nano-foams a base di Cu e Fe depositati su fogli di Cu Fe, Al, di Inconel e su una griglia di Al. L’aumento nella produttività usando queste tipologie di elettrodi, è stata nell’ordine di 2-10 volte rispetto alla massima produttività ottenuta utilizzando NP metalliche su materiali carboniosi. Svariate tecniche analitiche sono state poi utilizzate per caratterizzare in modo approfondito i materiali preparati tra cui, microscopia elettronica a trasmissione (TEM), microscopia elettronica a scansione (SEM), spettroscopia ad assorbimento atomico (AAS), diffrazione a raggi X (XRD), spettroscopia fotoelettronica a raggi X (XPS), determinazione dell’area superficiale mediante metodo Brunauer-Emmett-Teller (BET). La determinazione dei prodotti di reazione è stata effettuata invece mediante cromatografia ionica (IC), gas cromatografia con rivelatore a spettrometria di massa (GC-MS), gas cromatografia (GC) con rivelatore a termo conducibilità (TCD).
In view of the recent alarming rate of depletion of fossil fuel reserves and the drastic rise in the CO2 levels in the atmosphere leading to global warming and severe climate changes, tapping into all kinds of renewable energy sources has been among the top priorities in the research fields across the globe. One of the many such pathways is CO2 reduction to fuels using renewable energies, more commonly referred as artificial photosynthetic cells or artificial leaves or photo-electro-catalytic (PEC) cells. The key objective of the present PhD work was to conduct in-depth studies on two different electro-catalytic CO2 reduction systems: electrolyte-less cell (gas phase) and electrolytic cell (liquid phase). In particular, a novel lab scale liquid phase cell, on the similar lines of the previously realized gas phase cell at the University of Messina, was developed and used to convert electro-catalytically CO2 to more value-added products. The work was carried out at the Laboratory CASPE/INSTM of the University of Messina (Department of Electronic Engineering, Industrial Chemistry and Engineering). During the second year, a six-month period was spent at the École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), where organometallic routes were explored for the synthesis of novel composite materials to be used as electrocatalysts in the CO2 reduction process. Experimental tests were carried out on various types of catalysts in both the gas and liquid phase cells to understand the different selectivity, productivity and the reaction products obtained. Liquid phase, in fact, has been the most studied process in literature, but some issues mainly related to CO2 solubility and types of products formed (i.e. mainly formic acid), have never be allowed to pass the lab scale stage. The general aim of this PhD was to prepare novel metal doped nanocarbon substrates, which are very different with respect to the conventional metal bulk layers used as electrocatalysts in CO2 reduction, and test them both in gas phase (to take advantage of these conditions, i.e easy recovery and improved quality of the products) and in liquid phase (to have a better comparison with conditions typically adopted in literature). For the studies on the electro-catalytic reduction of CO2 in gas phase cell, a series of electrodes (based on Cu, Fe, Pt and Cu/Fe metal nanoparticles – NPs - deposited on carbon nanotubes – CNTs - or carbon black and then placed at the interface between a Nafion membrane and a gas diffusion-layer) were prepared. The results, evidencing the various types of products formed and their different productivities, are very promising. Under electrolyte-less conditions, the formation of ≥C1 products (such as ethanol, acetone and isopropanol) were observed, the highest being for Fe and closely followed by Pt, evidencing that also non-noble metals can be used as efficient catalysts under these conditions. To enhance the productivities of the CO2 reduction, a different set of electrodes were also prepared based on substituted Zeolitic Imidazolate (SIM-1) type MOF coatings during a stay at CPE Lyon and Institut de recherches sur la catalyse et l'environnement de Lyon (IRCELYON). Particularly, the catalysts tested were MOF-based Fe-CNTs, Pt-CNTs and Cu/Fe-CNTs. There was a significant change in the reaction products and in the selectivity towards the end-products. Particularly, especially for the MOF modified Pt based catalyst, there was an increase in the C-products and also a better selectivity towards higher C-products. Moving to the studies on the electro-catalytic reduction of CO2 in liquid phase cell, a similar set of electrodes were prepared. Initially, electrodes based on metal NPs of Cu, Fe, Pt, Ru and Co deposited on CNTs or carbon black were studied for their CO2 reduction capability. The relative order of productivity in CO2 electro-catalytic reduction in these series of electrodes was found to be different between the gas and liquid phase cells indicating the different reaction pathways. For liquid phase conditions, in terms of net C-products, catalytic electrodes based on Pt topped the class, closely followed by Ru and Cu, while Fe got the lowest position. The probable underlying reaction mechanism was also provided. In order to improve further the performances of the CO2 reduction in liquid phase conditions, a metal NPs size dependant study on the electro-catalytic reduction of CO2 to fuels was carried out. This study was performed using electrodes based on metal NPs of Ru, Fe, Pt and Cu loaded on CNTs and then transferred on a gas diffusion layers (GDL). Varied sized metal NPs have been synthesized using different techniques: (i) impregnation route to achieve NPs in the size range of 10-50 nm; (ii) organometallic approach to synthesize uniform and ultrafine NPs in the size range of 1-5 nm (i.e., Fe NPs were synthesized through a novel synthesis route to attain 13 nm NPs);(iii) Nanowire (NW) top-down approach to obtain ultrafine copper metal NPs in the size range of 2-3.8 nm. Particularly, the novelty of nanowire approach is the ability to obtain very small metal NPs starting from the synthesis of Cu NWs and then transferring the Cu onto the carbon surface, taking advantage of the different inter-forces of between Cu NWs and the functional groups present on the partially oxidized CNT surface. Furthermore, unlike the case of organo-metallic approach, this approach allows a preparation under air avoiding the use of potentially demanding inert atmospheric conditions. The enhancements in the fuel productivity were found to be 5-30 times higher for the smaller metal NPs obtained via organo-metallic route or nanowire route as compared to the larger metal NPs obtained via impregnation route. The results signify that the smaller sized metal NPs loading on the CNTs have a prevailing role in the catalytic performance and the selectivity towards different products. Moreover, the percentage of metal NPs loading was significantly reduced from 10 to 1-2 wt. % producing higher or equivalent fuels for small NPs as compared to the larger NPs. The reusability of the working electrodes and long reaction times (until 24 hours) were also probed. A different set of electrodes based on nano-foams on metal foils, were also investigated to achieve further improvements in the electro-reduction of CO2 to fuels. These nano-foams or dendrites were prepared by electrochemical deposition technique. Optimization studies on the deposition of these foams were performed initially to fix the set of preparation conditions. Moreover, voltage optimization study was performed using cyclic voltammetry and full CO2 reduction tests to find the optimum voltage for the process. The nano-foam electrodes tested include Cu and Fe foams on Cu foil, Fe foil, Al foil, Inconel foil and Al grid/mesh. The enhancements in the fuel productivity for various foams were in the range of 2-10 times greater as compared to the highest net fuel productivity achieved using metal NPs doped carbon catalytic electrodes, from all the previous studies. Various characterizations and analysis tools were used to analyse the catalysts qualitatively and quantitatively, which include Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Atomic Absorption Spectroscopy (AAS), X-ray diffraction (XRD), X-ray Photo-electron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET). To determine the fuel productivities, Ion Chromatography (IC), Gas Chromatography-Mass Spectrometer (GC-MS), Gas Chromatography (GC) were used.
Compte tenu du récent taux alarmant d'épuisement des réserves de combustibles fossiles et de l'augmentation drastique des niveaux de CO2 dans l'atmosphère qui a conduit au réchauffement de la planète et à des changements climatiques sévères, l'exploitation de toutes sortes d'énergies renouvelables a été la Parmi les principales priorités de la recherche Champs à travers le monde. L'une des nombreuses voies de ce genre est la réduction du CO2 aux combustibles utilisant des énergies renouvelables, plus communément appelées cellules photosynthétiques artificielles ou feuilles artificielles ou cellules photoélectro-catalytiques (PEC). L'objectif principal de ce travail était de réaliser des études approfondies sur les différents systèmes de réduction électro-catalytique du CO2, à savoir les cellules sans électrolyte (phase gazeuse) et les cellules électrolytiques (phase liquide). Dans ce processus, nous avons conçu une nouvelle cellule en phase liquide à échelle de laboratoire sur les lignes similaires de la cellule de phase gazeuse de modèle précédemment modélisée. Des essais expérimentaux sur la réduction du CO2 ont été réalisés sur différents types de catalyseurs dans les deux cellules afin de comprendre la sélectivité, la productivité et les produits de réaction obtenus. L'obtention de résultats de test dans les deux cellules nous a permis d'effectuer une comparaison décente avec les résultats de réduction électro-catalytique de CO2 existants dans la littérature. Des essais expérimentaux ont été réalisés sur différents types de catalyseurs à la fois dans les cellules en phase gazeuse et en phase liquide pour comprendre la sélectivité, la productivité et les produits de réaction obtenus. La phase liquide, en fait, a été le processus le plus étudié dans la littérature, mais certaines questions liées principalement à la solubilité du CO2 et aux types de produits formés (c'est-à-dire principalement l'acide formique) n'ont jamais été autorisées à franchir le stade de l'échelle du laboratoire. L'objectif général de ce doctorat était de préparer de nouveaux substrats de nanocarbone dopés par des métaux, qui sont très différents par rapport aux couches en vrac métalliques conventionnelles utilisées comme électrocatalyseurs dans la réduction de CO2, et de les tester en phase gazeuse (pour profiter de ces conditions, Une récupération facile et une qualité améliorée des produits) et en phase liquide (pour une meilleure comparaison avec les conditions typiquement adoptées dans la littérature). Pour les études sur la réduction électro-catalytique du CO2 en phase gazeuse, une série d'électrodes (à base de nanoparticules de Cu, Fe, Pt et CuFe déposées sur des nanotubes de carbone ou de noir de carbone puis placées à l'interface entre une membrane Nafion et Une électrode à couche de diffusion de gaz). Les résultats démontrent le type divers de produits formés et leurs productivités. Dans des conditions sans électrolyte, la formation de produits ≥C1 tels que l'éthanol, l'acétone et l'isopropanol a été observée la plus élevée étant pour Fe et suivie de près par Pt. Pour améliorer les productivités de la réduction du CO2, un ensemble différent d'électrodes a été préparé sur la base de revêtements MOF de type imidazolate de type zéolitique substitué (SIM-1) lors d'un séjour au CPE Lyon et à l'Institut de recherches sur la catalyse et l'environnement de Lyon (IRCELYON). Les catalyseurs testés étaient Fe-CNT, Pt-CNT et CuFe-CNT basés sur MOF. Il y a eu un changement significatif dans les produits de réaction et aussi, la sélectivité vis-à-vis des produits finaux. Pour le catalyseur à base de Pt modifié, MOF, il y avait une augmentation des produits C et également une sélectivité différente tandis que pour le catalyseur à base de Fe, il y avait une légère diminution des produits C. En se reportant aux études sur la réduction électro-catalytique du CO2 dans une cellule en phase liquide, un ensemble similaire d'électrodes a été préparé afin d'obtenir une bonne comparaison des résultats dans les expériences en phase gazeuse. Initialement, des électrodes à base de nanoparticules métalliques (Cu, Fe, Pt, Ru, Co) déposées sur des nanotubes de carbone ou du noir de carbone ont été étudiées pour leur capacité de réduction du CO2. L'ordre relatif de productivité dans la réduction électrocatalytique de CO2 dans ces séries d'électrodes a été trouvé différent entre les cellules en phase gazeuse et en phase liquide indiquant les différentes voies de réaction. Pour les conditions de phase liquide, en termes de produits C nets, les électrodes catalytiques à base de Pt sont en tête de la catégorie, suivies de près par Ru et Cu, tandis que Fe a obtenu la position la plus basse. Le mécanisme réactionnel sous-jacent probable a également été fourni. Afin d'améliorer encore les performances de la réduction du CO2 dans les conditions de phase liquide, une étude de la nanoparticules métalliques (NPs) dépendant de la taille de la réduction électro-catalytique du CO2 aux combustibles a été réalisée. Ceci a été réalisé à l'aide d'électrodes à base de nanoparticules métalliques (Ru, Fe, Pt et Cu) chargées sur les nanotubes de carbone (CNT) transférés sur les couches de diffusion gazeuse (GDL). On a synthétisé des nanoparticules de métal de différentes tailles en utilisant différentes techniques de synthèse: (i) l'itinéraire d'imprégnation pour obtenir des NP dans la plage de tailles de 10 à 50 nm; (Ii) Approche organométallique pour synthétiser des NPs uniformes et ultrafines dans la plage de tailles de 1-5 nm. Fe ont été synthétisés par une nouvelle voie de synthèse et des conditions pour atteindre des NP de 1 à 3 nm. (Iii) Approche de haut en bas de Nanowire pour obtenir des NP métalliques de cuivre ultrafin dans la plage de taille de 2-3,8 nm. En particulier, la nouveauté de l'aide de nanofils est la capacité à obtenir des particules de très petite taille d'abord la synthèse du Cu NFs, puis de les mettre en contact avec le support carboné et de faciliter son transfert, cela grâce à des forces d'attraction intermoléculaires des groupes fonctionnels présent sur le CNT partiellement oxydée. En outre, contrairement à la synthèse organométallique, cette approche permet d'effectuer les réactions dans l'air et non pas dans une atmosphère inerte. Les améliorations de la productivité du combustible ont été trouvées être au moins 5 à 30 fois plus élevées pour les NP métalliques de plus petite taille obtenus par voie organo-métallique ou par nanofil, par rapport aux NP métalliques plus grands obtenus par voie d'imprégnation. Les résultats indiquent que les NP métalliques de plus petite taille chargés sur les CNT jouent un rôle prédominant dans la performance catalytique et la sélectivité vis-à-vis de différents produits. En outre, le pourcentage de charge de NP métalliques a été réduit de façon significative de 10% à 1-2% en poids, produisant des carburants plus élevés ou équivalents pour de petites NP en comparaison avec les NP plus grandes. De plus, comme on a observé clairement la productivité en H2 qui a augmenté de nombreux facteurs pour les NP plus petits sur les plus grandes NP. La réutilisabilité des électrodes de travail et les longs temps de réaction ont également été sondés. Un ensemble différent d'électrodes à base de nano-mousses sur des feuilles métalliques a également été étudié afin d'obtenir des améliorations beaucoup plus importantes de l'électro-réduction de CO2 aux carburants. Ces nano-mousses ou dendrites ont été préparées par une technique de dépôt électrochimique. Des études d'optimisation sur le dépôt de ces mousses ont été effectuées initialement pour fixer l'ensemble des conditions de préparation. De plus, une étude d'optimisation de la tension a été réalisée en utilisant la voltamétrie cyclique et des tests de réduction de CO2 complets pour fixer une tension optimale pour les réactions. Les électrodes nano-mousses testées incluent (mousses Cu, Fe sur feuille Cu, feuille Fe, feuille Al, feuille Inconel et grille Al). Les améliorations de la productivité du combustible pour diverses mousses se situaient dans la plage de 2 à 10 fois par rapport à la productivité nette de combustible la plus élevée obtenue en utilisant des électrodes catalytiques en carbone dopé par des NP métalliques. Différentes caractérisations et outils d'analyse ont été utilisés pour analyser les catalyseurs qualitativement et quantitativement qui incluent la microscopie électronique à transmission (TEM), la microscopie électronique à balayage (SEM), la spectroscopie d'absorption atomique (AAS), la diffraction des rayons X (XRD) La spectroscopie électronique (XPS) et Brunauer-Emmett-Teller (BET) et pour déterminer les productivités des combustibles, chromatographie ionique (IC), chromatographie gazeuse-spectromètre de masse (GC-MS), chromatographie gazeuse.