Добірка наукової літератури з теми "Seed-mediated growth"

After a brief review of anisotropy on the nanoscale, experiments in which nanorod core–shell–shell particles are grown are presented. These “nanomatryoshkas” consist of a gold nanorod core, a silica shell, and a final gold shell. Calculation of the near-field properties of these structures using the discrete dipole approximation uncovers the change in location of local electric fields upon gold outer shell growth. Electrochemical experiments of the weak reducing agents used to grow the gold nanorod cores suggest a correlation between the strength of the reducing agent and its ability to promote longer nanorod growth. The final nanostructures do not exhibit a smooth outer shell, unlike their spherical counterparts.

In this study, silver nanorods and nanospheroids were prepared both in aqueous solution and on the surface of glass slides through seed-mediated growth approach at room temperature and used as a surface enhanced Raman scattering (SERS) substrate. The synthesis of metallic nanorods was started with the production of silver nanospheres as seed utilizing sodium borohydride and trisodium citrate as reducing and capping agents, respectively. These seeds were then added to a growth solution containing additional silver salt, ascorbic acid and cetyltrimethylammonium bromide (CTAB.) Nanorod preparation conditions were first optimized in solution phase. The plasmon absorption of the formed nanocrystals was monitored by UV-Visible spectrometry. The largest red shift in the longitudinal plasmon resonance absorption of silver nanostructures was tried to be achieved in order to realize the highest electromagnetic enhancement in Raman measurements. The images of the formed nanorods were recorded using field emission scanning electron microscopy (FE-SEM). The optimized colloidal growth conditions were adopted for the growth of nanorods on the surface of the glass substrate. Sol-gel coated glass slides were used in order to increase the porosity on the surface for an effective seeding process. We reported the development of a novel SERS substrate prepared by growing silver nanorods directly on the surface of glass surface without using any linker molecule. The SERS performances of the nanorod growth surfaces were evaluated with crystal violet (CV), brilliant cresyl blue (BCB) and benzoic acid (BA). Some modifications such as the increase in the AgNO3 concentration in the growth solution and the addition of hydrocarbons to the growth solution were investigated for the enhancement of the SERS signal. The intense spectra obtained for the model compounds demonstrated the efficiency of the prepared substrate for the SERS enhancement and its potential as a SERS detection probe for chemical and biological analysis.

Parmi les différents nanocatalyseurs, ceux constitués de nanoparticules de métaux nobles méritent une attention particulière en raison de leurs propriétés électroniques, chimiques et même optiques (dans le cas de transformations renforcées par les plasmons). Le platine ou le palladium sont bien connus pour leurs remarquables propriétés catalytiques, mais ils sont chers et leurs ressources sont limitées. En outre, les nanocatalyseurs monométallique ne peuvent conduire qu'à une gamme limitée de réactions chimiques. Ainsi, notre stratégie a été de développer des nanocatalyseurs bimétalliques composés de deux éléments métalliques qui peuvent présenter des effets synergiques entre leurs propriétés physicochimiques et une activité catalytique accrue. Nous avons ainsi conçu des nanocatalyseurs bimétalliques de type cœur-coquille composés d'un cœur en argent et d'une coquille en platine. L'intérêt est de combiner les activités catalytiques élevées et efficaces de la coquille de platine avec le cœur d'argent hautement énergétique, capable de renforcer les activités de la coquille grâce à ses propriétés plasmoniques. En outre, ces nanoparticules bimétalliques présentent souvent une activité catalytique supérieure en raison de la modification de la distance inter-atomique Pt-Pt (c'est-à-dire l'effet de contrainte). Dans ce travail de thèse, les nanoparticules Ag@Pt ont été synthétisées via un processus en deux étapes utilisant d'une part des nanoparticules d'Ag synthétisées chimiquement comme germes et d'autre part des complexes platine-oleylamine qui sont ensuite réduits à la surface des germes à une température contrôlée. Différentes tailles de germes d'Ag de 8 à 14 nm avec une très faible distribution de taille (<10%) ont été obtenues en ajustant le temps de réaction, la rampe de température, la concentration en précurseur d'Ag et la température finale pendant la synthèse. Différentes épaisseurs de coquille (de 1 à 6 couches atomiques) ont été obtenues en ajustant le rapport entre les concentrations de précurseur de platine et de germe d'argent. L'activité catalytique des nanoparticules Ag@Pt a été testée en considérant une réaction modèle de réduction du 4-nitrophénol en 4-aminophénol par NaBH4 en phase aqueuse. Nous avons observé que l'épaisseur de la coquille de Pt et la taille du noyau d'Ag influençaient les propriétés catalytiques et conduisaient à une activité catalytique accrue par rapport à l'argent ou au platine pur. Ceci a été attribué à des effets synergiques. De plus, nous avons observé une augmentation de l'activité catalytique des nanoparticules Ag et Ag@Pt sous irradiation lumineuse. Ce phénomène a été corrélé à la génération d'électrons chauds dans les noyaux d'Ag. Afin de développer une plateforme de nanocatalyse supportée, nous avons fabriqué des auto-assemblages 3D appelés aussi supercristaux composés de nanoparticules d'Ag@Pt obtenus spontanément après dépôt sur un substrat solide en raison de leur distribution de taille étroite et de leur forme homogène. L'activité catalytique de ces supercristaux pour la réaction d'évolution de l’hydrogène (HER) a été étudiée en suivant in situ par microscopie optique la production de nanobulles de gaz H2. Trois comportements distincts dans l'activité photo-catalytique (activité, activité intermittente et non-activité) ont été observés sur les supercristaux dans la même région d'intérêt. En outre, 50 % des assemblages ont été déterminés comme étant actifs pour l'HER qui a été démontrée comme étant accompagnée par une corrosion oxydative de l’argent
Among several nanocatalysts, those based on noble metal NPs deserve particular attention because of their electronic, chemical and even optical properties (in the case of plasmonic-enhanced transformations). Platinum or palladium are well known for their remarkable catalytic properties, but they are expensive and their resources are limited. In addition, single component nanocatalysts can only lead to a limited range of chemical reactions. Thus, our strategy was to develop bimetallic nanocatalysts composed of two metal elements that can exhibit synergistic effects between their physicochemical properties and enhanced catalytic activity. We have thus designed bimetallic nanocatalysts of the core-shell type composed of a silver core and a platinum shell. The interest is to combine the high and efficient catalytic activities of the platinum shell surface with the highly energetic silver core capable of enhancing the activities of the shell through its plasmonic properties. In addition, these bimetallic NPs often exhibit superior catalytic activity due to the modification of the Pt-Pt atomic bonding distance (i.e. the strain effect). In this thesis work, Ag@Pt NPs have been synthesized via a two-step process using chemically synthesized spherical Ag NPs as seeds on the one hand and platinum complexes with oleylamine on the other hand which are then reduced on the surface of the seeds at a controlled temperature. Different Ag seed sizes from 8 to 14 nm with a very low size distribution (<10%) have been obtained by adjusting the reaction time, temperature ramp, Ag precursor concentration and final temperature during the synthesis. The control of the shell thicknesses (from 1 to 6 atomic layers) has been possible by adjusting the ratio of platinum precursor to silver seed concentrations. The catalytic activity of the core-shell Ag@Pt NPs was tested by a model reaction of reduction of 4-nitrophenol to 4-aminophenol by NaBH4 in aqueous phase. We have observed that the thickness of the Pt shell and the size of the Ag core influence the catalytic properties and led increased catalytic activity compared to pure silver or platinum. This was attributed to synergistic effects. Furthermore, we have observed an enhancement of the catalytic activity of Ag and Ag@Pt NPs under light irradiation. This is correlated to the generation of hot electrons in the Ag core. Finally, in order to develop a supported nanocatalysis platform, 3D self-assemblies also called supercrystals composed of Ag@Pt nanoparticles have been spontaneously obtained after deposition on a solid substrate due to their narrow size distribution and homogeneous shape. The catalytic activity of these supercrystals for the hydrogen evolution reaction (HER) has been studied by following in situ by optical microscopy the production of H2 gas nanobubbles. Three distinct behaviors in photo-catalytic activity (activity, intermittent activity and non-activity) have been observed on the supercrystals in the same region of interest. In addition, 50% of the assemblies were determined to be active for HER which was shown to be accompanied by oxidative corrosion of silver

碩士
國立成功大學
材料科學及工程學系碩博士班
98
Because ITO possesses high optical transparency and good conductivity, it has been widely utilized in electrodes of optoelectronic device. However, the mineral resources of indium are limited, the price is high and it is toxic, many researchers seek alternative materials for ITO. This research would like to use silver nanowires to fabricate transparent conductive film in order to replace ITO material. The seed-mediated growth method was used to synthesize silver nanowires. The Pd reduced from PdCl2 could serve as source of seeds for growth of silver nanowires because of their close match in crystal structure and lattice constants. AgNO3 could be reduced with EG at the temperature of 160℃. Besides, in the presence of poly(vinyl pyrrolidone) (PVP), it would help the formation of silver nanowires. The experimental results show that the parameters of molecular weight of PVP, the molar ratio of PVP and AgNO3, the PdCl2 seed concentration, and the total amount of reactants would influence the formation of silver nanowire. UV-visible absorption spectra indicate that the product is the silver nanowires, and SEM and TEM were employed to characterize the structures of silver nanowires. From experimental results, we synthesize silver nanowire with a diameter of about 200 nm and a length of about 14.9 um. Finally, we fabricated the transparent conductive films consisting of random meshes of silver nanowires and used the PEDOT to stabilize the mesh structure. Random meshes of silver nanowires could reduce the sheet resistance greatly after 350℃, 40min heat treatment. In order to maintain good transparency and low sheet resistance, we fabricated the film with an the area density of silver nanowires of 2.62 mg/cm2 and obtained the Ag NW/PEDOT film of 75% optical transmittance and 38.7 Ω/□ sheet resistance. key word:silver nanowire

碩士
國立臺灣科技大學
化學工程系
101
Part I In this study, we synthesized Ag2S as crystalline seeds through thermal deposition, then further deposited different secondary metal or cation exchanging on the surfaces of Ag2S to obtain heterostructure nanocrystals with various types of compositions, shapes and properties, such as Ag2S-ZnS, Ag2S/CdS, Ag2S/CdS-ZnS and Ag/AgInS2. Unlike other metal seeds, Ag+ showed a fluid-like characteristic and exhibited high degree of mobility in the Ag2S, leading to more defects of cation. Even in large lattice mismatch at the interface, the growth of heterostuctures benefited from those defects. By controlling the reaction time and the concentration of secondary metal, we were able to synthesize spherical, triangular and stick nanocrystals. Finally, the heterostructure materials were identified by transmission electron microscopy, X-ray diffraction patterns, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, fluorescence spectroscopy, UV/vis absorption spectroscopy and cyclic voltammetry. Part II In this research, we investigated the Methotrexate (MTX) anticancer drugs conjugating with the AgInS2/ZnS quantum dots. The AgInS2/ZnS quantum dots cannot dissolve in aqueous phase. For biological application, PMAO were linked to the surfaces of AgInS2/ZnS quantum dots to make AgInS2/ZnS quantum dots dissolve in aqueous phase. Subsequently, the MTX anticancer drugs conjugated with the modified AgInS2/ZnS quantum dots by straightforward EDC/NHS catalyzed coupling reaction in order to develop multifunctional quantum dots including cell imaging, targeting and drug therapy.

碩士
國立清華大學
化學系
100
In this study, we have used a simple seed-mediated synthesis method for the preparation of penta-twinned gold nanorods with average lengths from 200 to 4.5 µm. In series I, the gold nanorods can be synthesized from 200 to 400 nm with ~20 nm diameter by tuning the pH of growth solution from 3.0 to 9.1. As the solution pH increases, the length of nanorods becomes shorter but with nanorod yield was much higher. The percentage of nanoplate byproducts decrease substantially as the solution pH increases by comparing the absorbance values of the absorption band of nanoplates to that of the transverse SPR band of nanorods. In Series II, we have systematically extended the lengths of gold nanorods and nanowires from 580 nm to 2850 nm by mainly decreasing the concentrations of tetrachloroauric acid used and raising the amount of nitric acid added to the last growth solution. However, when the pH of the growth solution is very low, the products usually accompanies with lots of nanoplates. In Series III, by lowering the transfer volume of the second growth solution to the third growth solution from 2 mL to 0.2 mL and progressively increasing the amount of nitric acid added only to the third growth solution, ultralong gold nanorods and nanowires with tunable average lengths varying from 700 nm to 4450 nm can be synthesized. The longest nanowires can reach lengths of more than 6 µm. The high uniformity and relatively short lengths of the nanorods synthesized is primarily due to the rapid particle growth rate in a basic solution condition, as evidenced by the fast solution color changes from pink to light purple and then dark purple in less than five minutes. In an acidic condition, the particle growth rate is much slower and the growth progress takes much longer time, allowing the nanorods to grow and extend their lengths. The slow growth rate has been confirmed by examining the intermediate nanorods formed during the particle growth process. HCl was found to be as effective as HNO3 in producing long gold nanorods with similar lengths, showing that the presence nitrate ions is not important to the growth of long gold nanords. These ultralong gold nanorods and nanowires are highly useful materials for a wide range of applications. Their tunable length means that selection of nanorods and nanowires most suitable for a particular situation is possible.

碩士
國立清華大學
化學系
103
CHAPTER 1 Seed-Mediated Growth of Silver Nanocubess in Aqueous Solution In this work, we have developed a seed-mediated approach to synthesize Ag nanocubes with average edge lengths from 23 nm to 60 nm. Ag seed solution is added a growth solution containing CF3COOAg, cetyltrimethylammonium chloride (CTAC), and ascorbic acid. Ag nanocubes are synthesized by using ascorbic acid as reducing agent to reduce AgCl in the presence of CTAC. We have also used another method to synthesize Ag nanocubes. This approach involves adding the seed solution to a growth solution containing AgNO3, cetyltrimethylammonium bromide (CTAB), CuSO4 and ascorbic acid. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and powder X-ray diffraction (PXRD) patterns have been employed to characterize the Ag nanocubes. UV‒vis absorption spectra of the particles have been obtained. CHAPTER 2 Formation of Au Nanocages from Ag Nanocubes and Their Photothermal Effect In this study, we have used Ag nanocubes as templates to synthesize Au nanocages by adding HAuCl4 at 30 ºC for 10 minutes. This involves galvanic replacement reaction. Ag nanocubes are oxidized and AuCl4‒ is reduced to Au and deposited on the surface of Ag nanocubes. When different amounts of HAuCl4 were added to a growth solution including Ag nanocubes and CTAC, the morphology and Au/Ag ratio can vary. We used the cages to investigate photothermal effect. Using different cages, the solution temperature can rise to 90, 95 and 100ºC under the illumination of an 808 nm laser with 1.47 W. Use of a 1064 nm laser led to a lower heating efficiency. These cages may find applications including heat generation for as an energy source.

碩士
國立清華大學
化學系
95
Synthesis of Pd Nanorods and Branched Particles by a Seeding Growth Approach In this thesis work, we observed the effects of oxidative etching on the Pd seeds. Two synthetic methods of Pd seed solution were demonstrated to block the oxidative influence: the CTAB-capped seeds were prepared under nitrogen and the TSC-capped seed solution was generated by substituting the capping agent to trisodium citrate (TSC). Both the approaches can successfully extend the lifetime of Pd seeds from initially fifteen minutes to several days. Via the following seeding growth in the presence of Cu(OAc)2, we obtained Pd nanorods by using the CTAB-capped seeds. From the TEM characterization, we conclude that the uniform Pd nanorods have an average length of 130 nm and exhibit a penta-twinned structure. The introducing of Cu2+ cations has also been found the key factor for increasing the nanorod yield and this could be referred to the underpotential deposition (UPD). In addition, when the TSC-capped seeds were applied to the seeding growth, a rarely seen Pd branched particles with an average size of 30 nm were obtained. The formation of these unusual Pd nanostructures may be attributed to the strong binding of TSC on the surface of the Pd seeds. Seed-Mediated Synthesis of Controllable Ultrahigh Aspect Ratio Gold Nanorods The three-step seed-mediated growth method has been widely used for growing gold nanorods with high aspect ratios, however few reports are available for tailoring the nanorod length to micrometer scale. In this study, gold nanorods can be extended to 1 μm in length by growing the seeds in a growth solution containing nitric acid. By increasing the concentrations of nitric acid, gold nanorods with different aspect ratios of 23, 35, 38, 49 can be generated. The addition of nitric acid could apparently retard the reaction, especially in concentrated ones, and possibly allows a better control in early stage of the growth. In the presence of the worm-like micelle of CTAB created by using nitrate anions as counter ions, the gold seeds grow into long nanorods. Another possible factor for the growth of high aspect ratio nanorods may be contributed by the chloride－enhanced oxidizing strength of nitric acid. Because the particles may serve as primary structure for oxidative etching due to the twinned defects, as a result, they may re-dissolve into the solution. That’s why we observed that main byproducts converted from initially spherical particles to gold nanoplates for the nanorods prepared in concentrated nitric acid.

碩士
國立清華大學
化學系
95
Thermal Aqueous Solution Approach for the Synthesis of Ultra-Small Gold Nanoplates We report a study on the thermal aqueous solution approach of ultrasmall triangular gold nanoplates with average widths of 40 ± 7 and 58 ± 10 nm. Because of their relatively uniform sizes, these tiny nanoplates can spontaneously self-assemble into some ordered 2-dimensional structures such as the hexagonally arranged pattern. UV-vis absorption spectroscopy showed that these nanoplates exhibit a strong absorption band at 590-602 nm and a weak and broadband centered at ~775-900 nm. Controlled Synthesis of Bipyramid-Shaped Gold Nanoparticles by Seed-Mediated Growth and Their Transformation into Star Fruit-Shaped Branched Nanocrystals We report a study on the synthesis of bipyramid-shaped gold nanoparticles by a seed-mediated approach and their transformation into star fruit-shaped branched nanocrystals by adding a certain amount of Ag+ ions to different solutions in the process. The colloidal of AgCl and AgBr nearby the side surfaces of gold seeds make the growth slowly, and gold atoms deposit at the twin boundaries and led to their unsymmetrical growth, all of which is the source of forming star fruit-shaped branched nanocrystals.

There are many mechanisms whereby animal activity can directly or indirectly influence the species diversity of plant communities. Most obviously, herbivory can influence the species composition directly through plant mortality or indirectly by changing the outcome of interspecific competition (Hulme 1996). Animals may also affect plant species composition by modifying the physical structure of the environment such that the flow of resources required for plant growth is altered. Such physical effects mediated by the physical structure of the environment have been termed ‘ecosystem engineering’ (defined in table 11.1; Jones et al. 1994, 1997, Lawton 1994, Lawton and Jones 1995). Animals may have other functions that influence the persistence of populations (e.g., pollination) or the colonization of new sites (seed dispersal). In this chapter we aim to provide an overview of how these diverse effects of animals influence plant species diversity, and to this end, we propose community assembly theory as a conceptual framework. Community assembly theory provides us with a schematic representation of the vital steps involved in the determination of species presence or absence at a particular site. By asking how might animal activity influence each of the steps of assembly, we ensure a comprehensive outlook on how animals affect plant species diversity. That said, we restrict ourselves in this chapter to ecological mechanisms and we do not consider evolutionary effects that are of undoubted importance at higher levels of spatial and temporal scales. Following our discussion of animal effects on community assembly, we highlight two case studies of herbivores arising from recent research in Israel. We use insights provided by these examples to suggest mechanisms that are likely to be of particular importance in arid ecosystems. For example, we suggest that since the flow of water, the primary limiting factor in these systems, is so easily influenced by structural changes in the environment, physical ecosystem engineering may be a relatively important, though previously neglected, interaction type in arid ecosystems. The processes governing which species occur in a particular plant community can be summarized very simply.

This one year BARD project was funded to develop methods to monitor promoter activity a gene expression patterns in citrus fruit. To fulfill this goal, we divided the research tasks between both labs so that the Israeli side evaluated the use of microprojectile bombardment ; a tool to evaluate transient gene expression in various citrus fruit tissues, and the US side optimized technical parameters required for Agrobacterium-mediated transformation of various citrus cultivars. Microprojectile bombardment appeared to be a very efficient method for transient gene expression analysis in citrus leaf tissues but was somewhat less applicable in fruit tissues. Nevertheless, we did succeeded to achieve significant levels of 35S-GUS gene expression in young green flavedo tissue. However, only single random spots of 35S-GUS gene expression were detected mature flavedo and in juice sacs and albedo tissue. Overall, we assume that following some more technical improvements particle bombardment could provide a useful technique to rapidly analyze promoter activity at least in the flavedo tissue. For Agrobacterium-mediated transformation, we found that shoot cultures of 'Washington' navel oranges,'Fairchild' mandarins,'Eureca' lemons,'Troyer' citrange and various grapefruits provided a more reliable and consistent source of tissue for transformation than germinated seedlings. Moreover, various growth media's (McCown, Quoirin & Lepoivre, DCR) further improved shoot and root growth relative to MS mineral media, which is commonly used. Also pure white light (using bulbs which do not emit UV or blue light) improved shoot growth in various citrus varieties, and paromomycin appeared to be a more efficient antibiotic for the selection of transgenic plants than Kanamycin. Overall, these optimizations improve transformation efficacy and shoot growth and rooting capacity. In addition to the development of transformation methods, both Israeli and US labs achieved progress in the identification of citrus fruit-specific promoters. In Israel, we isolated a 3.6 kb promoter fragment of the thiamine biosynthesis c-thi gene, which is highly expressed in fruit peel tissue, whereas in the US we isolated a 1.5 kb promoter fragment of the citrus seed-specific cDNA CssH. The identification of more fruit-specific cDNAs and their corresponding promoter regions is currently in progress.