Дисертації з теми "Carbon encapsulated"

Nanotube-encapsulated ionic materials can be usefully modelled using ab initio methods to inform and validate experiment. This thesis models these materials using density functional theory, explores the specific challenges these materials pose, and presents results only visible once these challenges are addressed. Chemical potentials and ab initio calculations are used to predict tube radii at which filling occurs for a range of filling conditions, which informs our choice of tube. The practice of fitting both ionic core and encapsulating tube into the same unit cell causes artificial strain. A method of quantifying this mismatch is developed and the associated strain energy and distortions predicted. Current methods of approximating the nanotube treat it as a smooth cylinder. We find significant texture which can resist core movement, allow metastable twisted states, and be used to determine which supercells have physically realistic mismatch strain. Cores have previously been relaxed in vacuo with limited justification. We find that, in general, cores are not stable without constraints. We guide three bare silver iodide ansatze to metastable saddle-points by applying artificial symmetry constraints during in vacuo relaxation. An investigation into their unconstrained unstable phonon modes reveals them to be stabilised by an encapsulating tube. Two classes of stabilisation are identified, one due to radial encapsulation and a second due to mismatch-like effects. Charge of a few tenths of e per core ion is seen to transfer from core to tube, with magnitude depending on tube radius. In order to approximate the charge field by atomic charges, four population analysis schemes are investigated. The Bader and Density Derived Electrostatic and Chemical (DDEC) population analyses agree to physically sensible values while using different approaches. A machine-learned model is trained using local chemical-environment descriptors to transferrably predict atomic charges to within 0:004 e of DDEC values with an orders-of-magnitude speedup.

The report of the presence of LaC₂ and YC₂ encapsulated in carbon shell layers found in a carbonaceous deposit grown on a cathode surface of a carbon arc has attracted particularly great interest because encapsulating materials in the hollow core of carbon nanostructures (nanocapsules/nanotubes) had changed the physical properties of the encapsulated materials. Not only does the process offer an opportunity to investigate dimensionally confined systems, but also the encapsulated materials are likely to be immune to environmental effects or degradation, because of the protective carbon sheets around them. The formation of carbon encapsulated ferromagnetic nanoparticles and nanowires has been the goal of this research. Three novel techniques for the formation of carbon encapsulated ferromagnetic nanoparticles are introduced in this thesis: Carbon Arc in Localized Gas Pressure (CA), Water Arc Discharge (WA) and Pulsed Laser Deposition (PLD). In addition, a new technique of producing encapsulated ferromagnetic nanowires by nanotube-assisted oriented attachment is also introduced. The resultant materials have had their structural and magnetic properties assessed by High Resolution Transmission Electron Microscopy (HRTEM), X-Ray Diffraction (XRD) and Vibrating Sample Magnetometer (VSM) respectively. A carbon arc technique has been modified by creating a N₂ gas jet through the office of the anode, which resulted in a localized gas pressure in the arc region. The second technique successfully implemented has been that of the electric arc submerged in deionised water inside a 2.5 litre breaker. The third technique that has been explored is PLD. In our experiments, a KrF 248 nm Lambda Physique excimer laser was used to ablate two composite targets composed of Co + C (80 wt% Co) and Ni + C (80 wt% Ni) under the pressure of N₂ gas jet respectively. Pulsed laser respectively. Pulsed laser deposition coupled with the addition of N₂ gas jet has successfully been employed to grow carbon encapsulated Co and Ni nanoparticles with small means particle sizes of 12.4 and 15.5 nm respectively. Finally, a new technique in growing ferromagnetic nanowires encapsulated in carbon tubules through oriented attachment is presented. The process is based on the evaporation of fluorocarbon capsule (PCTFE in this case) over ferromagnetic particles dissolved in toluene onto lacey carbon TEM grids at 375°C for 72 hours.

Magnetic fluid hyperthermia is a potential therapy for achieving interstitial hyperthermia and is currently under clinical trials. This approach is based on the instillation of magnetic nanoparticles at the tumour site, which dissipate heat when exposed to an alternating magnetic field. This procedure leads to a local increase of temperature and induction of tumour death or regression. Nanoparticles of metallic iron are potential heating agents for this therapy, but rely on the presence of a protecting coat that avoids reactions with their environment. In this work, iron nanospheres and iron nanowires with a graphite coat are explored for this purpose. From these two nanostructures, the nanospheres are shown to have a greater potential in terms of heat dissipation. The graphite shell is further investigated as an interface for conjugation with other molecules of relevance such as drugs and fluorescent probes. The effect of acidic treatments on the magnetic and surface properties of the nanospheres is systematically studied and a suitable method to generate carboxylic functionalities on the nanoparticle surface alongside with a good preservation of the magnetic properties is developed. These carboxylic groups are shown to work as a bridge for conjugation with a model molecule, methylamine, as well as with a fluorescent dye, allowing the detection of the nanoparticles in cells by means of optical methods. The carboxylic functionalities are further explored for the conjugation with the anti-cancer drug cisplatin, where the amount of drug loaded per particle is found to be dependent on the density of free carboxylic groups. The release of the drug in physiological salt solutions is time and temperature dependent, making them particularly interesting for multi-modal anti-cancer therapies, where concomitant hyperthermia and chemotherapy could be achieved. Their potential for such therapies is shown in vitro by inducing hyperthermia in cell suspensions containing these nanoparticles. These results are finally translated to a three dimensional cell culture model where the in vitro growth of tumour spheroids is inhibited. The developed nanostructures have a great potential for therapeutic approaches based on the synergistic effects of hyperthermia and chemotherapy.

Carbon-encapsulated iron nanoparticles have being researched heavily, since they present advantageous properties over other protective coatings such as polymer or silica. The carbon coating protects the iron core from oxidation, chemical and thermal degradation and hence, magnetic cores present stable magnetic properties when nanoparticles are exhibited in air or other environments. Several studies about carbon-encapsulated magnetic nanoparticles were already reported. However, nanoparticles are obtained rather polydisperse and not very uniform in composition, making very difficult their use for several applications. The aim of this thesis is the production and characterisation of carbon-encapsulated iron nanoparticles showing very narrow size distributions with well-characterised magnetic properties for several applications, in particular, those related to the biomedical field (hyperthermia, drug delivery or as agents contrast in MRI). However, the systematic study of these applications was not the framework of this thesis. The content of this dissertation comprises the design of two arc-discharge plasma (ADP) reactors (a conventional and a modified one); the experimental study of the different reactor parameters involved; the morphological, structural and magnetic characterisation of the obtained nanoparticles; the comprehension of the mechanisms involve in the formation of this kind of nanoparticles in comparison with nanoparticles obtained by other methods; and finally, a first approach to the functionalisation of the nanoparticles for the biomedical applications. This thesis is structured in four parts: Backgrounds (Chapter 1), Nanotools (Chapter 2 and Chapter 3), Results (from Chapter 4 to Chapter 8) and finally, the Conclusions. Chapter 1 - Basis of carbon-encapsulated iron nanoparticles: A general introduction of the nanoparticle properties derived from their nanometric dimensions is presented in this chapter. The state of the art about the formation mechanisms and techniques used for the generation of carbon-encapsulated iron nanoparticles is described. Several applications of this kind of nanoparticles in fields such as biomedicine, electronics or food and environmental, are also presented. Chapter 2 - Characterisation methods: The most common characterisation techniques used to investigate the morphological, composition, structural and magnetic properties are described within this chapter. Details about the equipments and conditions used during this thesis for the characterisation of the nanoparticles are also reported. Chapter 3 - Experimental set-up: In this chapter, the description of two different arc-discharge reactors used during this thesis is presented. A first reactor (the conventional ADP) was developed by following similar experimental setups described in the literature. Second reactor (a modified ADP) was designed to overcome the disadvantages from the first reactor and to obtain high quality nanoparticles (narrower size distribution, uniform composition). Chapter 4 - Preliminary studies from conventional ADP reactor: This chapter presents a design of experiments (DOE) based on the Plackett-Burman design in order to evaluate the reactor parameters that influence the most the final characteristics of the nanoparticles. The study was performed using the conventional ADP reactor and the preliminary results were very useful for the development of next generation of experiments using the second reactor, the modified ADP. Chapter 5 - Generation of nanoparticles by a modified ADP reactor: Morphological and structural properties of the nanoparticles obtained by the modified ADP reactor are presented. The discussion of the effect of the most relevant parameters on the formation of the nanoparticles was reported. Iron core diameters with corresponding size distribution as well as the carbon shell formation obtained under different parameter conditions were investigated. Chapter 6 - Magnetic properties of the nanoparticles: A systematic study of the magnetic properties of the nanoparticles obtained in Chapter 5 is performed. Size-dependent variables such as magnetic moments, coercivity values, blocking temperature and anisotropy energies were presented. Magnetic properties were in agreement with the morphological characteristics of the nanoparticles. Chapter 7 – Thermally induced structural evolution of the nanoparticles: The comparison of annealed nanoparticles obtained by mADP and chemical vapour deposition (CVD) method is presented in this chapter. Differences on the morphological, structural and magnetic properties were studied. Structural evolution of nanoparticles during annealing under in-situ TEM observations was investigated. Chapter 8 - First approach to biomedical applications: As a first approach to biomedical applications, the stabilisation of the nanoparticles in aqueous solution by using polyvinyl-alcohol was investigated. Results of the internalisation of the nanoparticles into HeLa cells are presented.
Les nanopartícules magnètiques de ferro recobertes de carboni s’estan investigant en gran mesura, ja que presenten avantatjoses propietats sobre d’altres recobriments protectors del nucli magnètic com els polímers o la sílice. El recobriment de carboni protegeix el nucli de ferro de l’oxidació, la degració química i tèrmica, d’aquesta manera els nuclis presenten propietats magnètiques estables quan les nanopartícules s’exhibeixen en aire o en un altre medi. S’han realitzat diversos estudis sobre aquest tipus de nanopartícules, però aquest tipus de nanopartícules s’obtenen amb gran dispersió de grandàries i poca uniformitat en les seves característiques. És encara un repte en aquest camp la producció de nanopartícules de ferro recobertes de carbon amb propietats morfològiques i estructurals, així com l’estudi sistemàtic de les seves propietats magnètiques. Per aquest motiu, l’objectiu d’aquesta tesi es centra en la producció i caracterització de nanopartícules superparamagnètiques de ferro recobertes de carboni amb estreta distribució de mides i amb propietats magnètiques ben caracteritzades per diverses aplicacions, en particular, les relacionades amb el camp de la biomedicina. No obstant això, l’estudi sistemàtic d’aquestes aplicacions es troba fora del marc d’aquesta tesi. El contingut s’estructura en quatre parts: • La primera part d’introducció conté els aspectos bàsics sobre aquest tipus de nanopartícules, així com les propietats derivades de la seva mida nanomètrica, les tecnologies que s’utilitzen per generar aquest tipus de nanopartícules, una explicació sobre els possibles mecanismes responsables de la seva formació i les principals aplicacions d’aquestes nanopartícules. • La segona part descriu les tècniques utilitzades per la seva caracterització que engloben tècniques de microscopia, de difracció de raigs-X, d’espectroscòpia Raman, per la caracterització col•loidal de les nanopartícules fins la seva caracterització magnètica. També inclou la descripció detallada dels equips basats en la descàrrega d’arc utilitzats per la seva producció. El primer equip es va dissenyar seguint les característiques d’un reactor convencional (conventional ADP reactor). El segon equip basat en la mateixa tecnologia de descàrrega d’arc, però modificat (mADP reactor) i dissenyat especialment amb l’objectiu de millorar les característiques del producte final. • La tercera part exposa els resultats obtinguts durant aquesta tesi. L’estudi previ del reactor convencional basat en un disseny d’experiments de Plackett-Burman per avaluar l’efecte dels diferents paràmetres del reactor en la grandària dels nuclis de ferro. A partir d’aquest estudi, es va realitzar un estudi més específic en el nou reactor modificat on es van estudiar l’efecte del corrent d’arc utilizat, la velocitat del flux d’heli i el contigut de ferrocè com a matèria prima del ferro. Després es va realitzar l’estudi sistemàtic de les seves propietats magnètiques observant la dependència d’aquestes propietats amb la grandària dels nuclis de ferro. A continuació, es va presentar la comparació d’aquestes nanopartícules amb d’altres obtingudes mitjantçant el mètode de dipòsit químic en fase vapor (CVD). A partir d’aquesta comparació es va estudiar l’evolució estructural d’aquestes nanopartícules sotmetes a un tractament tèrmic en observació in-situ d’un microscopi de transmissió electrònica. Finalment, es va presentar un primer estudi de les propietats col•loidals en suspensió d’aquestes nanopartícules recobertes amb un polímer d’alcohol de polivinil (PVA). Es presenta un primer estudi de l’internalització d’aquestes nanopartícules en cèl•lules tumorals HeLa. • Per acabar es presenten les conclusions i l’apèndix que conté informació sobre les mostres produïdes i un llistat de publicacions, congressos, patents resultants d’aquest treball.

The crystalline iron-based nanowires encapsulated by multiwalled carbon nanotubes have been the subject of numerous studies owing to the range of potential applications. The presence of a-Fe (bcc)/y -Fe(fcc) junctions o ers the possibility of exploitation of the exchange bias effect, an interfacial magnetic phenomenon that plays a major role in magnetocaloric cooling, spintronic and high-density magnetic storage devices. This work is concerned with the synthesis and microstructural characterization of Fe-based and NiFe nanowires encapsulated by multiwall carbon nanotube radial structures. The known attributes of these structures are well matched to the magnetocaloric application. The primary aim of this work was to determine the unknown microstructural details of the encapsulated nanowire that are of relevance to the magnetocaloric application (junction types, location and orientation relative to the nanotube axis). The secondary aim was to explore the modi cation of the synthesis route to promote desirable attributes. This is the first report of a-Fe/y -Fe sequential junctions and a-Fe/Fe3C concentric junctions in encapsulated Fe-based nanowires. The presence of a-Fe/y -Fe junctions was inferred from the observation of a-Fe nanowires terminated by a ~100 nm length y-Fe crystallites of larger diameter. The a-Fe/Fe3C junctions exhibit the Bagaryatski orientation relationship: [110 ]bcck[100 ]orth. The degree of substrate roughness was found to be a means of tailoring details of the structure and composition of the encapsulated nanowires. NiFe encapsulated nanowires were found to contain crystallites of a-NiFe, y-NiFe and Ni3Fe and the sequential junctions -NiFe/Ni3Fe and a-NiFe/y-NiFe junctions.

碩士
國立清華大學
材料科學工程學系
93
Carbon nanotubes（CNTs）are graphene sheets rolled up into cylindrical structure and electron transport via tube surface lattice is essentially isotropic. This description is based on a 2D lattice model. In a 3D approach, electron wave function near to the Fermi level is extended by 1 Å along the 2Pz orbit and electron transport thus becomes sensitive to electron field. Reports indicate two possible paths of charge carrier in a small CNT, i.e. along tube axis and circumference. When electric field is applied in parallel with tube axis the electron flux along circumference is diverted into a helix current, similar to nano-coils. This spiral current becomes apparent when angular frequency is smaller than stark frequency in an AC field. To date, explore of magnetic field within a CNT remains as challenge and a straightforward verification is to encapsulate magnetic materials in CNTs so interior magnetic field is enhanced and becomes measurable. In this work, individual Fe-encapsulated CNTs bridging tungsten electrodes are verified and measured by AC impedance technique. Inductance at different frequency is detected and is on the order of mH.

碩士
國立交通大學
材料科學與工程系
90
To enlarge the application areas of the nano-structured materials, such as applications in magnetic recording media, the well-aligned carbon nano-structures encapsulating with magnetic catalyst particles were successfully synthesized on Si wafer by ECR-CVD method with CH4 and H2 as gas sources. The magnetic catalysts, including FePt, CoPt, Nd2Fe14B and Fe thin films, and FeNi thick film, were studied. The main process parameters include hydrogen content in the gas sources, hydrogen plasma catalyst pretreatment, substrate bias, deposition temperature and plasma flow guiding. The magnetic properties, morphologies, microstructures and bonding structures of the magnetic catalyst-assisted carbon nanostructures were characterized by VSM, MFM, AFM, SEM, TEM, HRTEM and Raman spectroscopy. The adhesion properties of nanostructures with the substrates were qualitatively compared by ultrasonic agitation in acetone bath. Regarding effects of catalyst materials, the results show that at the same deposition conditions, different catalysts can produce carbon nanotubes (CNTs) with different tube number density, tube length, carbon film formation, bonding between catalyst and CNTs, growth mechanism and type of CNTs. These differences in structures or properties may relate to the solubility difference of carbon in catalysts, etching rate difference between CNTs and carbon films by hydrogen plasma. In the present conditions, the maximum tube number density can go up to 134 Gtubes/inch2 for Fe-assisted CNTs. For Nd2Fe14B—assisted CNTs, the longest tube length can reach 2100 nm for 15 min deposition time, which is roughly corresponding to the highest growth rate. For certain applications, if the removal of catalysts from tips of CNTs is required, it can easily be achieved by selecting proper catalyst and combining with ultrasonic agitation in acetone bath. About effect of plasma flow guiding, the 90°-inclined CNTs was successfully modified to 45°-inclined CNTs by positioning a negatively-biased metal plate above the Si substrate surface to vary the plasma flow pattern. The results also show that the plasma flow guiding may be used to modify the seaweed-like nano-sheets from random orientations to parallel alignment. For effects of other process parameters, the results indicate that the hydrogen flow rate and substrate bias are essentially the factors governing the differential etching effect to different nanostructures, e.g. carbon film and CNTs. However, the etching effect is more directional for bias, and more isotropic for hydrogen plasma. A lower hydrogen flow rate favors formation of the seaweed-like carbon nanostructures, or CNTs surrounding with other carbon nanostructures. A lower negative bias voltage favors formation of the additional thicker carbon films. The results also show that the hydrogen plasma pretreatment of the catalyst-coated substrates is basically to attack the catalyst film to become the well-distributed nano-particles to act as catalysts of CNTs. Regard to the magnetic properties of the magnetic metal-encapsulated carbon nanostructures, the grain sizes of the magnetic particles (35 nm, or 10 ~ 100 nm in diameter) are greater than but close to the critical optimum size or single domain size, which favor a higher coercive force. A higher deposition temperature of CNTs results in a greater coercive force due to a smaller catalyst size, and the greatest coercive force can go up to 750 Oe for Fe-assisted CNTs at 715℃ deposition temperature, which is comparable with the reported values in the literature. The process also takes advantages of higher shape and induced anisotropy due to its higher aspect ratio and magnetic annealing effect. The coercive force difference between vertical and horizontal direction can reach 300 Oe in the present conditions. The results also demonstrate the potential applications in magnetic recording media that the isolated and well-distributed magnetic particles in the magnetic metal-encapsulated carbon nanostructures can be imaged by MFM micrographs.

碩士
國立中正大學
化學研究所
88
We described a simple method to prepare large-scale GaN nanowires from the reaction of gallium and ammonia using polycrystalline metal alloy compound as a catalyst. Scanning electron microscopy indicated that almost all the resulting materials exhibited wire-like structures with diameters in the range of 20 to 200 nm and lengths up to several micrometers. The bulk wire-like materials had the GaN wurtzite structure as characterized using X-ray diffraction. We also described a simple method to prepare large-scale GaN nanowires encapsulated in carbon nanotubes from the reaction of gallium ﹑ammonia and methane using polycrystalline metal alloy compound as a catalyst. High-resolution transmission electron microscopy have been employed to determine the structure and stoichiometry of the individual GaN nanowire encapsulated in carbon nanotube. The new synthetic technique, which makes possible simple and large-scale production of GaN nanowires and GaN nanowire encapsulated in carbon nanotube, opens up applications of the nanowires for high efficiency optoelectronic devices.

碩士
國立暨南國際大學
應用化學系
97
Abstract Carbon encapsulated magnetic nanoparticles have wide applications such as, serving as contrast reagents for magnetic resonance imaging (MRI), magnetic data storage medium, and magnetic hyperthermia. However, compared to the intense studies in the synthetic strategies for fabricating such core/shell materials, little effort has been devoted to the surface functionalziation strategies. In the report, we produced polyarginine-immobilized and titanium dioxide-immobilized carbon encapsulated magnetic iron nanoparticles (Fe@CNPs) as high affinity probes to extract and enrich phosphopeptides and phosphoprotein from a number of enzymatic digested products including α- and β-casein and milk sample. The poly(arcylic acid)-coated Fe@CNPs can be easily grafted with affinity ligands such as polyarginine (PA) or titanium dioxide via carbodiimide chemistry or sol-gel chemistry, respectively. For the capture of phosphopeptides, only a 10-s incubation time with affinity probes is sufficient to prove faithful information. The affinity probes with trapped phosphopeptides are then isolated by magnetic separation, cleaned with washing solvents to remove nonspecifically bound peptides. Finally, the trapped phosphopeptides were eluted with 2,5-dihydroxybenzoic acid (2,5-DHB) containing 5% phosphoric acid and deposited onto the MALDI target plate for analysis by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). After enrichment, the ion suppression caused by abundant nonphosphopeptides is eliminated, and the detection sensitivity of phosphopeptides by MALDI MS analysis is in the low femtomol range Both polyarginine and TiO2-coated Fe@CNPs exhibits good specificity and sensitivity toward phosphopeptides. These two types of affinity probes process complementary properties for phosphopeptide mapping by MALID MS and can be combined to provide rich information for phosphoproteomic analysis.

碩士
國立臺灣大學
地質科學研究所
105
Graphite encapsulated metal (GEM) nanoparticles are core-shell nanostructured materials with a diameter ranging from 5-100 nm. Due to the protection provided by the outer graphitic shells, GEM can survive in severely corrosive environments, under acid erosion and oxidation. The encapsulation efficiency increased substantially after replacing the diamond powder with liquid alcohol as the carbon source by using Tungsten arc-discharge. The encapsulation efficiency of GEM nanoparticles increased from lower than 20% to about 80%. In this study, five types of liquid compounds were used: ethanol, 2-propanol, n-propanol, cyclohexane and benzene; the encapsulation efficiencies were 54.7±7.0%, 69.5±4.4%, 61.8±4.2%, 76.5±7.3%, 74.7±5.9%, respectively. The results shown that the encapsulation efficiency will increase along with the carbon content, but when the carbon content is higher than cyclohexane, the encapsulation efficiency is maintained at the same level. The most possible reason is that the supply of carbon has been sufficient. From TEM images, Ni-GEM particles have a spherical surface which can be referred to as one of condition of hypothesis for particle size analysis. Two particle size analysis methods were used: one is using X-ray powder diffraction method and Scherrer equation to calculate the particle size; the other one is combining the results of thermogravimetric analysis and specific surface area analysis, then computing the particle size. Moreover, from X-ray powder diffraction analysis, it was determined that the oxide was NiO rather than Ni2O3. Comparing the relationship between dissociation energy and change of particle size, after obtaining the particle size of GEM which synthesized by different types of liquid carbon source. Finally, according to the theoretical calculation results of hydrogen content, hydrogen concentration after reaction with oxygen, and carbon content after reaction with oxygen, the factor that the residue hydrogen concentration at the coalescence region plays an important role in the change of particle size when using high-carbon-content carbon source.

碩士
國立交通大學
材料科學與工程系所
93
To develop the potential applications as magnetic media with nano-resolution, the well-aligned Fe-encapsulated carbon nanostructures were synthesized on Si wafer by ECR-CVD with H2 and CH4 as gas sources and Fe as the catalyst and under 875 Gauss magnetic field to maintain the ECR condition. The as-grown nanostructures were further post-annealed in vacuum (10-3 Torr) under a magnetic field of 875 Gauss at 640℃ for 4 hr. The main process parameters include the source gas ratio of H2/CH4 and the substrate bias. The morphologies, structures, bonding structures and magnetic properties of the nanostructures at each processing step were characterized by SEM, HRTEM, XRD, Raman spectroscopy, EDS, AFM, MFM, SQUID and field emission J-E measurements. From the experimental results, the following conclusions can be drawn. Regarding effects of the source gas ratio of H2/CH4, the results show that an increase in H2 flow rate at constant CH4 flow rate or a decrease in CH4 flow rate at constant H2 flow rate are more favor to produce CNTs with shorter length and smaller tube diameter. The effect of H-plasma is essentially to etch preferentially the amorphous carbon than graphite layers of CNTs. Therefore, at higher H-concentration, it implies a lower carbon concentration, lower growth rate and stronger etching effect; thus it results in formation of a shorter and smaller CNTs. In terms of memory density and rigidity of CNTs in perpendicular direction, the smaller and shorter CNTs or particle-like nanostructures are better to be applied in applications for magnetic storage media. Effect of the substrate negative bias is basically to accelerate the positive species, such as H-plasma, to bombard the substrate and also to align the carbon species in an ordered fashion vertically. Therefore, a higher negative bias (> -150 V) will result in a greater etching effect and forming a shorter and smaller well-aligned CNTs or particle-like nanostructures. On the contrary, under the conditions of no or smaller negative bias, the results indicate that carbon films or carbon nanostructures in non-uniform granularity may be formed. Under the present process conditions, the maximum tube number density of CNTs and the maximum number density of nano-particles can go up to 20.6 Gtubes/inch2 with 550 nm in length and 23.2 G/inch2 with 178 nm in height, respectively. Instead of CNTs, it is interesting to note that the particle-like Fe-encapsulated nanostructures could be melted together to become a big particle under focusing electron beam during TEM examination. This may be due to magnetic attraction among Fe catalysts at the tips and melting point decrease of nano-sized particles. Regard to effect of post magnetic annealing treatment at 640oC, the SEM examination shows that the CNTs become cleaner after treatment, which can be manifested from a decrease in Raman ID/IG ratio of CNTs. Furthermore, the XRD patterns indicate that the as-grown CNTs with diffraction signals of simple orthorhombic Fe3C, bcc-Fe and fcc-diamond will become CNTs without Fe signals after annealing treatment, so it gives rise to a decline in coercive force. The results also depict that the coercive force and the shift in hyseresis loop are a decline function of the measuring temperature and are linearly related to each other for both as-grown CNTs and CNTs after annealing treatment. In contrast, the magnetization intensity is also a decline function of the measuring temperature but is not linearly related to each other. The shift of hyseresis loop due to temperature is related to the exchange anisotropy. The average size of the encapsulated Fe catalysts in carbon nanostructures is 66 nm, which is much larger than the critical size (~ 14 nm) for the maximum coercive force of Fe particles. In other words, there are many spaces to improve the process to enhance the coercive force further by decreasing the sizes of CNTs or nano-particles. Although the post annealing treatment is not good for magnetic property improvement, however, it could benefit the field emission properties of CNTs with an increase in current density of three-order in magnitude. This may relate to a decrease in amorphous carbon after magnetic annealing in vacuum. From the AFM and MFM micrographs, it indicates that the Fe-encapsulated nanostructures can be imaged by MFM, indicating the higher possibility to be used for magnetic storage media applications with nano-resolution.

碩士
國立臺灣大學
地質科學研究所
94
Graphite Encapsulated Metal (GEM) nanoparticle is a new spherical composite material with a diameter ranging between 5 and 100 nm. It has a core/shell structure, where the core is metal and the shell is graphite. Having the special structure and nanosizes, GEM has become an interesting research subject for the academic community. For the last several years, we have been using a tungsten arc-discharge method to synthesize GEM, and have successfully increased the yield and the recovery ratio of the GEM. However, we still know very little about the changes of the raw materials inside the crucible during the runs. The lack of knowledge could be a barrier keeping us from both to improve the synthesis process and to develop a mathematic model for the system. To fill up the gap, this study specifically designed to investigate what has been happening to the raw materials in the graphite crucible. The work can be divided into two parts: First is to design and do a number of synthesis experiments that will produce suitable samples for the later analysis. Second is to analyze the metal blocks left in the crucible after the synthesis experiments. The metal blocks were first cut, polished and observed under the microscope; finally, the blocks were put into an acid-bath and fully dissolved. Only the insoluble graphite flakes stayed intact in the solution for the size analysis. The morphology on the upper surface of the metal block is closely related to the arc-discharge during the runs. The center smooth and shining area directly contacts with the arc plasma, while the surrounding rough and foggy area, on which the carbon and metal vapor condensed and deposited, stays outside of the arc. Chemical analysis shows both areas are covered with carbon (i.e., graphite.) In addition, many radial oriented small metal blobs can be found at the rough area. Many graphite flakes existed inside the metal blocks, and their distribution and shapes are related to the temperature gradient caused by the arc-discharge. The temperature at the center portion of the block is the highest, therefore only smaller pieces and less amount of graphite can be found. The temperature at the bottom and wall portions is relatively lower, where many larger graphite flakes can be found. The log-normal size distribution of the graphite flakes in the metal blocks indicates the graphite formed by a nucleation and growth process. Using diamond carbon source has greatly improved the synthesis efficiency. Evidence shows that small graphite flakes were formed and come off from the surface of the diamond powder. The existence of these small graphite flakes has significantly increased the contact area between graphite and metal, thus increased the amount of carbon dissolved into the metal. When evaporated from the melting metal pool, the carbon and metal vapor mixed more uniformly, and therefore synthesize more well-encapsulated GEM.

博士
國立交通大學
材料科學與工程系所
94
To examine effects of processing parameters, such as catalyst application methods, pretreatment atmospheres and nanostructure deposition methods, on the nanostructure formation, processes to synthesize metal-encapsulated carbon nanostructured materials by both ECR-CVD and MPCVD methods were designed, using CH4, C2H2, H2, N2, NH3 and CO2 as source gases or pretreatment atmospheres, and using Fe，Co，CoSix，Ni，Cu as catalysts. The catalysts were deposited on Si wafer by spin coating the catalyst precursor solutions and/or sputtering the metal targets. The pre-coated catalysts or their precursors were followed by H- or (H+N)-plasma pretreatment to obtain various catalyst nanoparticles distribution. The pretreated specimens were then deposited with various carbon nanostructures in ECR-CVD or MPCVD system. The nanostructures and their properties after each processing step were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, secondary ion mass spectroscopy (SIMS) and field emission I-V measurements. The following conclusions can be drawn from these studies. On studying growth mechanisms of various nanostructures, the results show the typical nanostructures by ECR-CVD with CH4 as source gas include the vertically aligned carbon nanotubes (VACNTs), bamboo-like CNTs (BLCNTs), rattan-like CNTs (RLCNTs) and seaweed-like nano-sheets (SLNSs). The essential condition to form VACNTs is enough higher substrate bias ( > -100 V). In contrast, a lower substrate bias (< 50 V) will give rise to SLNSs formation. However, the RLCNTs will appeal by prolonging the VACNTs deposition time over 10 min. It is noted that the presence of nitrogen and/or a lower deposition pressure, such as in ECR-CVD system, are the favor conditions forming BLCNTs. The replacement of hydrogen with nitrogen in the reaction chamber is essentially to increase the bombardment effect of plasma to prolong the catalyst-plasma surface from being poisoned by the carbon film. In case of plasma pretreatment process or in the initial stage of nanostructure formation, introduction of nitrogen is also basically to increase the bombardment effect to promote the agglomeration effect due to a higher temperature, which gives rise to bigger catalyst particle sizes and so bigger nanostructure diameters. The possible growth mechanisms to form these nanostructures may be able to be explained from the following points: (1) the catalysts with higher C solubility, such as transition metals or alloys, can promote tube-like nanostructure formation; (2) formation of the graphene layers of the nanostructures is mainly through carbon bulk diffusion route in the catalysts; (3) the sizes of the catalyst nanoparticles after initial nanostructure deposition stage basically determine the final diameters of the nanostructures; (4) the difference in carbon bulk diffusion rates around the center and the circumferential regions of the catalysts may determine the types of nanostructures; a progressive increase in rate difference can give rise a change in nanostructures from filament-like, bamboo-like to hollow-like. In other words, if the catalyst surface on the plasma side is partially poisoned by carbon films during deposition may be more favor to form hollow-like nanostructures; and (5) the growth orientation of the nanostructures is determined by the flow direction of carbon species near the catalyst surface. Regarding influence of catalyst application methods on the nanostructure growth, it is essentially depending on the differences in film thickness and, uniformity of the coated films, independent of application methods. However, the catalyst spin coating method has the advantages of large area, lower cost and mass production, but the drawbacks of poor uniformity, environmental pollution and difficultly to control the thickness of the film. To examine effect of catalyst materials, the Co and Ni catalyst-assisted nanostructures are typically VACNTs or RLCNTs by ECR-CVD. In contrast, the nanostructures are mainly carbon films or SLNSs for the Fe catalyst, and are SLNSs for Si substrate without catalyst or with Cu catalyst. It seems that the types of nanostructures are basically resulting from the competition between the carbon deposition and plasma etching rates. The deposition rate of the Fe-assisted nanostructures may be relatively faster than for Co and Ni catalysts due to its lower eutectic temperature. As to Cu catalyst, the solubility of carbon in Cu is very limited, which causes carbon from the plasma to deposit directly on the catalyst surface to form SLNSs. To study field emission properties for various catalyst-assisted nanostructures by ECR-CVD with CH4 as source gas, the results show that the field emission properties in terms of current density at 10 V/�慆 and the turn-on-voltage at 10 nA/cm2 are Co (> 32, 3.0), Ni (19.8, 1.1), Fe (7.1, 4.6), no catalyst (2.5, 4.6) (mA/cm2, V/�n�慆) for the Co- and Ni-assisted VACNTs or RLCNTs , and the Fe-assisted and no-catalyst- assisted SLNSs, respectively. The corresponding IG/ID values are 0.57, 0.55 , 0.59 and 0.45, respectively. It seems to indicate that IG/ID values are not the main factor to determine the field emission properties. Effects of geometrical features of various nanostructures on field emission properties are compared: the corresponding tube diameter, length and tube number density for the Co- and Ni-assisted VACNTs are (30~80 nm, 1.8~2.5�慆, 29~32 Gtubes/in2), (30~60 nm, 2.1~2.7�慆, 36~39 Gtubes/in2), respectively. It appeals that the field emission properties are favor for the nanostructures with higher aspect ratio and proper tube number density (also called the decreasing of screening effect). To examine effect of U-shaped covering plate to cover a part of the specimen on nanostructure formation, the results show that the plate did not change the type, but change the orientation of the nanostructures. The possible mechanism for this effect proposed in this work is explained from the flow pattern variation by a change in electric field around the covering plate, though some investigators explained by a guiding flow of covering plate.

碩士
國立臺灣大學
地質科學研究所
104
Graphite Encapsulated Metal (GEM) nanoparticles are spherical core-shell structured composite material with a diameter ranging from 5–100 nm. The core of GEM is metal, and its outer shell is composed of several layers of graphite/graphene which can preserve the inner core in a severe environment, such as from acid erosion and oxidation. It is well known that many different functional groups, including carboxyl and hydroxyl, can be easily attached to the surface of carbon materials. Recently, several studies have revealed that GEM has a great potential to become a novel material including, for example, in hydrogen storage and biomedical materials due to its unique properties. For instance, Wu et al.(2007) used polyethylene glycol and folic acid grafted on Fe-GEM for the heat treatment of cancer, and Chung et al. (2009) used Co-GEM as an electrochemical hydrogenation material. The modified tungsten arc-discharge method was developed by Teng et al. and Dravid et al. in 1995. This is the most practical method for producing a large quantity of GEM because it reduces the amount of carbon debris origin compared to the Krätschmer–Huffman method. However, the encapsulation efficiency of GEM remains low. Until 2012, with the help of the two-step mechanism model, we used n-propanol as the liquid carbon source to synthesize GEM, significantly increasing its encapsulation efficiency from 20–30 wt% to around 80 wt%, and presenting a preliminary method for controlling the particle size of GEM through different liquid carbon sources. However, we faced two difficult issues after switching the carbon source from solid to liquid. First, this method could disturb the arc discharge which causes the discontinuity of the experiment, leading to the lockout unsustainable injection. Second, the consumption rate of the tungsten rod rose from 1 mm/h to 420 mm/h, making it difficult to synthesize large quantities of well-encapsulated GEM. In addition, the detailed mechanism, after entering the liquid carbon source, still remains unclear. The purposes of this study are to realize the changes of arc in the cabin and to resolve problems after using the liquid carbon source. In order to solve the problems, this research has installed a liquid metering pump to regulate the amount and direction of each injection, so that the carbon source can be directed to mainly spray the synthetic region of GEM, which is called the coalescence region. This method avoids the resistance caused by dripping liquid along the tungsten rod, and successfully sustained the experiment. The TEM images show that the synthesized GEMs, using a liquid metering pump, retain a complete core-shell structure, and the utilization of carbon source calculated by TGA data shows significant improvement, from 20% to 64%. Furthermore, we listed the possible reasons causing the high consumption rate of tungsten rod, and verified them by theoretical calculation and manner of experiments, one by one. It can be speculated by OES data that the dominant gas in the center of the arc changed from helium to hydrogen. In the meantime, the arc temperature rose show by the color changing into blue and white, representing the higher arc temperature is the main reason causing the tungsten melting rate to increase 420-fold. After calculating the heat conduction, we confirmed that increasing the diameter of the tungsten rod can immediately solve this problem. Since it is feasible to control the injection rate through the use of a liquid metering pump, we tried to figure out the encapsulation efficiency of GEM over time when synthesizing GEM. Under the same experimental parameters and total liquid injection volume, we compared the results of two different injection rates, 10μL/min and 100μL/min, and found that using the former injection rate can result in 5-fold higher encapsulation efficiency. According to the two-step mechanism of GEM, we speculated that adding liquid carbon source during arc discharge would rapidly increase the carbon proportionate of the coalescence region; however, the carbon vapor will quickly leave the coalescence region via convection. Thus, for the same total liquid injection volume, taking a small amount and injecting it a few times is the best way to inject the liquid carbon source; it can significantly improve the encapsulation efficiency and the utilization rate of the carbon source. 　　Lastly, based on the experiment results, we proposed a model that can explain the transformation of the arc body from bell-shaped to columnar, after injecting the liquid carbon sources. Furthermore, our model raises the potential of employing GEM to fundamental science and applied material fields.

碩士
國立臺灣大學
地質科學研究所
107
Graphite encapsulated metal (GEM) nanoparticle is a core-shell structured nanocomposite material composed of a stable outer graphitic shell and inner nano-scaled metal core; its size is about 5 to 100 nm. Due to its core-shell structure, GEM can simultaneously exhibit the properties of both graphite and the metal core, such as the biocompatibility, ferromagnetism, and microwave adsorption ability. Therefore, GEM has huge potential for applications in various fields, such as a microwave absorbent in the electronics industry and military applications, groundwater tracer in geosciences, drug carrier in biomedical engineering, hydrogen storage in energy engineering, etc. However, GEM has so far been unable to serve in mass production due to various difficulties in the synthesis processes, especially for the Fe-GEM. Even though our research team has significantly increased the yield of GEM by using alcohols as carbon sources in 2012, the mechanism behind the processes (conventional droplet method) is still not fully understood. In addition, there are many disadvantages in using this method, such as the arc being prone to extinction when droplets fall into arc plasma, and it’s difficult to calculate how much carbon sources actually drop into the crucible. To further understand the mechanism of using liquid carbon sources, a new design (vapor method) for using vapor-form carbon sources has used the synthesis of GEM. In this study, the new design method succeeds in overcoming the problems caused by employing the conventional method, and enhancing the yield of various GEMs. Futhermore, some interesting results can be observed when using vapor-form carbon sources to synthesize various GEM in a modified tungsten arc-discharge system. For example, the encapsulation efficiency of Fe-GEM can be raised from less than 50% to nearly 90%, and the production rate of Ni-GEM is twice as high as that of droplet method. Additionally, by observing the products form of GEM, the graphite catalytic ability of ferromagnetic metal in the arc system is Ni > Co >> Fe when using the vapor method in an arc discharge system. The results also reveal the reason why Ni-GEM has an extremely high yield via the vapor method. This study further proposes a reaction hypothesis model called the "catalytic heating cycle". The catalytic heating cycle model can also provide the fundamental framework for a new design to greatly improve the production rate. Finally, by means of comparing the droplet method and the vapor method, the encapsulation efficiency, morphology of the graphite shell, and the remaining metal in the crucible, we can successfully establish a new working model. It shows that the mechanism of the droplet method is dominated by the "phase segregation", while the mechanism of vapor method is dominated by the “catalysis reaction”. The new working model can help to resolve the problems which were still not fully explained in the previous study, such as the high encapsulation efficiency by using gas carbon source (2002) and the high production rate by minor injection (2016).

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment for the degree of Master of Science in Chemistry. Johannesburg 2016.
In the current study, different platinum-hollow carbon sphere catalysts were synthesized for use as electrocatalysts in low temperature fuel cells such as proton exchange membrane fuel cells (PEMFCs). The support material was synthesized via a hard templating method using mesoporous silica (synthesized using a modified Stöber method) as a sacrificial template. In fuel cells, one aim is to ensure that as much platinum as possible is present on a given electrode while keeping the entirety of the catalytic layer as thin as possible (i.e. with the minimum amount of carbon). One approach to achieving this was to make the hollow carbon spheres as small as possible, starting of course with the templating material. It was found that tailoring the molar ratios between the two co-solvents (that is water and ethanol) during Stöber synthesis was the key to achieving particles as small as approximately 150 nm with a uniform shape, size, and significant yields of up to 5.00 g. Another focal point in terms of the template material was achieving a silica structure that consisted of a solid core, and a distinctly mesoporous shell. Two different surfactants were explored in order to fabricate these structures; namely octadecyltrimethoxysilane (C18TMS) and cetyltrimethylammonium bromide (CTAB). It was found that of the two, the C18TMS resulted in more distinctly formed mesoporous silica layers with higher measured specific surface areas. Because the type of support material greatly influences the catalytic behaviour of the loaded catalysts, two different carbonization techniques were explored; namely the bubbling method using toluene as a carbon source, and a nanocasting method where resorcinol formaldehyde (RF) was the carbon source. The toluene-synthesized hollow carbon spheres had advantages over their RF-synthesized counterparts in that they were more thermally stable and had a more graphitic crystalline carbon framework. The RF-synthesized carbon, however, possessed a pseudo-capacitance due to surface carbon-oxygen groups, as well as a higher specific surface area, which resulted in the RF-carbon cyclic voltammetry profile spanning a larger current range in microampere per square centimetre. The effect of the size of the support materials was also explored; comparing 350 nm and 150 nm hollow carbon spheres. Besides the type of carbon, the metal precursor used to synthesize the catalyst nanoparticles was also explored, with either platinum(II)chloride (PtCl2) or platinum(II)acetylacetonate [Pt(acac)2] being used as the platinum source. It is also known that achieving high metal yields using conventional methods is quite difficult, and so an easier, quicker and less wasteful method was also explored; comparing the traditional wet-impregnation (WI) method with a chemical vapour deposition (CVD) method. Ultimately, it was found that platinum loaded on top of small-sized toluene-synthesized hollow carbon spheres using the CVD method and Pt(acac)2 as the metal precursor was the better catalyst in terms of oxygen reduction (determined using linear sweep voltammetry measurements); outperforming even commercial Pt/C catalysts as a result of improved mass transfer afforded by the voided cores of the hollow carbon spheres. The ability of a catalyst to withstand the reaction conditions present in a PEM fuel cell (i.e. oxygen-rich environments) was also considered. The stability of the catalysts was tested using chronoamperometry measurements in an oxygen-saturated perchloric acid solution. It was evident that the platinum loaded on the inner shells of the hollow carbon spheres showed far superior stability to those loaded on the outside surface. This was attributed to the qualities bestowed by the carbon shell around the platinum nanoparticles, protecting said platinum against the consequences of support corrosion due to the oxygenated environment; consequences such as metal sintering and interaction with surrounding carbon supports for example. These encapsulated catalysts, however, displayed a decrease in electrocatalytic activity compared to the catalysts with top-loaded platinum. In conclusion, the study of different platinum-carbon catalysts studied in the current work revealed that (a) loading platinum on top of small sized toluene-synthesized hollow carbon spheres using a CVD method and Pt(acac)2 as a metal precursor resulted in a highly active oxygen reduction catalyst, while (b) loading platinum on the inside surface of the hollow carbon spheres under the dame conditions resulted in a more electrocatalytically stable catalyst.
LG2017

Magnetic nanomaterials have received significant attention because of their remarkable properties enabling applications in various fields such as catalysis, as contrast agents for magnetic resonance imaging, in sensing applications, as environmental catalysts and as adsorbents. Magnetic properties of materials depend on certain factors such as size, shape, crystallinity, composition, crystal structure, and synthetic methodology. Most of the pristine magnetic nanoparticles are highly pyrophoric in nature which poses difficulties in handling these materials. In addition, the ease of oxidation and potential toxicity of these materials preclude their practical applications. Further, these magnetic nanoparticles have strong magnetic interactions between them which leads to the aggregation of particles. These features affect the magnetic behaviour as well as the other characteristics of the material. In this context, fabricating magnetic nanomaterials with desired magnetic properties, chemical stability and surface chemistry is quite challenging. Similarly, plasmonic metal nanoparticles such as Ag and Cu also suffer from issues pertaining to oxidative instability upon air exposure under ambient conditions. Therefore, it is crucial to develop protection strategies to stabilize nanoparticles surface against oxidation. This thesis describes the synthesis of carbon encapsulated mono- and bi-metallic nanoparticles using solvated metal atom dispersion method in conjunction with digestive ripening approach followed by thermal annealing. The aim of the work is to understand the effect of size, shape, composition, and the nature of surface on the properties of these metallic nanomaterials. In this direction, nanosystems of Fe, Ag, Cu, Fe3C, and FeCo have been studied.

碩士
國立暨南國際大學
應用化學系
107
Polycyclic aromatic hydrocarbons ( PAHs ) are notorious environmental pollutants generated primarily during the incomplete combustion of organic materials. Due to the relatively low volatility of PAHs, they are adsorbed on suspended particulates ( e.g. PM2.5 ) and gradually aggregate in the air. Many PAHs have toxic, mutagenic and/or carcinogenic properties. Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry ( MALDI-TOF MS ) has the advantages of good sensitivity, high accuracy, wide detection quality, and easy analysis. It is used to quickly analyze molecular information contained in environmental or biological samples. However, organic compound-based substrates conventionally used are not suitable for analyzing low molecular weight small molecules such as polycyclic aromatic hydrocarbon molecules. In recent years, many research efforts have proposed the use of new carbon-based materials for small molecule detection such as carbon nanotubes, nanocarbon dots, colloidal graphite, graphene, etc., because these materials have good laser energy absorption characteristics. At the same time, there is almost no matrix background interference when detecting small molecular weight samples. In this work, we use a simple and rapid method by using magnetic carbon encapsulated iron nanoparticle ( Fe@C ) as both MALDI matrix and PAH adsorbents for Matrix-Assisted Laser Desorption/ Ionization Time of Flight Mass Spectrometry ( MALDI-TOF MS ) analysis of PAHs such as Benzo[a]pyrene ( BaP ), Benzo[g]perylene( BgP ), and Pyrene. The experimental results show that using Fe@C as MALDI matrix can effectively improve the intensity of PAHs and be successfully applied to detect polycyclic aromatic hydrocarbons on suspended particles. This method expands the application of Fe@C and provides a new matrix material selection for the detection of polycyclic aromatic hydrocarbons in air pollutants.