1,720,967 research outputs found
Nanoparticle synthesis and self-assembly: Tailoring supraparticles for catalysis
No full textHeterogenous catalysts are important for the production of everyday life products, ranging from plastics to pharmaceuticals and energy. Understanding how to improve these catalyst materials allows for the optimization of these current processes. Therefore, studying catalysts in detail and preparing them with high accuracy is essential. This PhD work focuses on the controlled design of catalytic (support) materials with nanometer-precision. To achieve this level of control, nanoparticles are synthesized and self-assembled into ordered structures. This self-assembly approach offers significant advantages over conventional methods, particularly in controlling the composition, size, location, and distribution of nanoparticles within the catalyst.
In this project, ordered silica and graphene mesoporous support materials were synthesized, and nickel/silica catalyst prepared. For the preparation of self-assembled mesoporous silica, monodisperse silica nanoparticles were prepared with sizes ranging from 5 to 100 nm. The self-assembly of these nanoparticles within drying emulsion droplets resulted in the formation of "supraparticles," which are spherical 3D assemblies of nanoparticles. These supraparticles feature an interconnected 3D mesoporous network with tunable pore size, symmetry, porosity, and surface chemistry. To demonstrate the applicability of mesoporous supraparticles in catalyst preparation, silica supraparticles were impregnated with nickel to create a nickel/silica catalyst for hydrogenation reactions. Additional synthesis control was achieved through colloidal synthesis of catalytically active nickel nanoparticles. For example, bimetallic nickel-palladium nanoparticles were studied using in situ electron microscopy, revealing that the atoms within these nanoparticles could shift between alloy and core-shell arrangements in response to reducing and oxidizing environments. Such rearrangement of atoms strongly effects the catalytic properties. Another material that was prepared is porous graphene, made using supraparticles of iron oxide nanoparticles as templates in the production ordered mesoporous graphene. The pore properties were controlled by the size and shape of the iron oxide nanoparticles used, and resulted in the production of new graphene materials.
The ordered catalyst (support) materials presented in this thesis are highly suitable for systematic catalyst optimization. The detailed synthesis and electron microscopy characterization moreover demonstrates that nanoparticle synthesis and self-assembly enable the precise tailoring of catalysts at the nanometer scale
Exploration of Gold Nanoplatelets: Synthesis, Structures, and Plasmonic Properties
No full textIn Chapter 1, a general introduction is given on the synthesis, morphologies, surface modifications, and optical properties of metal nanocrystals (NCs).
In Chapter 2, however, a single-step coating method is presented for synthesizing Au NT@hollow@mSiO2 yolk-shell nanoparticles (NPs) with tunable diameters, hollow space between core and shell, and opportunities for scale-up synthesis.
In Chapter 3, a study on the thermal stability of Au NTs with different surface coatings is presented. For the Au NTs without surface coatings, noticeable deformations occur at 200 ℃ for one-hour heating. The deformation took place via surface diffusion of Au atoms and started from corners and edges where the atoms` coordination numbers are lower than those at other positions, as these atoms have lower energy barriers for diffusion. Upon raising the heating temperature, Au NTs tended to become more sphere-like particles via surface diffusion of Au atoms to reduce the surface energy of the overall particle. Surprisingly, we found that Au NT@mSiO2 YS NPs with a yolk-shell morphology, where not all sides of the Au NP were covered by silica, were more stable than Au NT@mSiO2 core-shell (CS) NPs with a core-shell morphology where all surface Au was covered by the silica coating.
In Chapter 4, the mSiO2 coated Au NTs core-shell particles were reshaped by various means; by femtosecond (fs-) laser exposure, oven heating, and oxidative chemical etching. Moreover, the relationships between the LSPR bands and geometrical shape parameters were revealed which allowed to derive expressions for estimating the LSPR peak wavelengths of the Au NTs and Au NDs.
In Chapter 5, we studied the symmetric and asymmetric growth of Ag, Pd, and Pt onto Au NTs in aqueous solution using L-ascorbic acid (AA) and/or salicylic acid (SA) as reductants.
In Chapter 6, alloyed Au-M (Ag, Pd, and Pt) NPLs with well-defined triangular shapes were successfully obtained by heating the mSiO2 shell coated Au NT-M core-shell NPLs to a proper temperature. For Au NT-Pd and Au NT-Pt core-shell NPLs, the uneven or even spiky Pd and Pt shells tuned into smooth shells covering the Au NT cores when the particles were heated to 500 ℃, and, interestingly, phase segregation was found to occur in these two systems at 1100 ℃, leading to the formation of Au-Pt Janus NPLs, wherein the case of Pt for some particles even a sharp triangular shape was retained. The dynamic process, monitored by in-situ scanning transmission electron microscopy (STEM) imaging, showed that the overall structural evolution started with a core-shell morphology that changed into an alloyed configuration and subsequently changed to Janus-like NPLs. The solid solution alloying is driven by entropy changes that become more important at elevated temperature, while the subsequent phase separation at even higher temperature is a rather surprising result, since this is entropically unfavorable and must be driven by temperature-dependent changes to the cohesive, surface, and interface energies of the Au-M NPLs, where it should also be realized that all these changes take place within the confinement by the silica shell.
In Chapter 7 a diverse strategy combining the seed-mediated method, co-reduction, and Galvanic replacement to fabricate Au NT-Ag-Pt ternary nanoparticles with various well-designed structures. The structure of trimetallic NCs can be manipulated from yolk-shell to core-shell by varying the concentration of AgNO3 or Au NTs. Furthermore, by means of time-resolved UV-VIS spectra and STEM images, it was found that Au NT-Ag-Pt yolk-shell NCs were formed via a growth-Galvanic replacement synergistic mechanism
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
- …
