Probing the Excited State Dynamics of Aluminum Clusters as a Means of Identifying Metallicity

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Description
Pure metal clusters serve as model systems by providing an avenue for the study of fundamental phenomena, specifically the interaction between light and matter. Bulk metal materials are known to display defining characteristics, namely thermal conductivity, electrical conductivity, and luster,

Pure metal clusters serve as model systems by providing an avenue for the study of fundamental phenomena, specifically the interaction between light and matter. Bulk metal materials are known to display defining characteristics, namely thermal conductivity, electrical conductivity, and luster, which provide a quantifiable measure of their metallicity. These properties are all due to the electron delocalization throughout the metal. Nanoscale materials lack the ability to measure these properties, leading to the need for a manner of quantifying the metallic character at the nanoscale size regime.Excited state lifetimes vary for semiconducting and metallic systems, specifically metals relax to a ground state at a faster rate than semiconducting materials. Aluminum clusters have received decades of attention regarding their metallicity. Moreover, Al clusters have been debated to fit into the jellium model. The jellium model seeks to describe a cluster as a “superatom” where all electrons are delocalized around the positively charged metal center, like that of an atom. With three valence electrons, jellium shell closings can be met if the electrons involved in cluster bonding varies. This variance leads to a localization of electrons for instances in which all three electrons do not contribute to bonding. Localized electrons aren’t characteristic of the jellium model or metals more broadly. Tracking the excited state lifetimes of Al clusters produced through laser ablation seeks to uncover the onset of metallic character. Femtosecond pump-probe spectroscopy coupled with time-of-flight mass spectrometry has resolved the time dynamics for atomically precise Al clusters ranging in size from 1-43 atoms. At a size greater than 9 atoms, it’s identified that Al clusters show metallic character. This finding is supported by previous literature results and the fact that, above 9 atoms, Al cluster excited state lifetimes match that of the bulk scale Al excited state lifetime of ~300 fs.
Date Created
2024
Agent

The Morphology and Biochemical Structure of Spider Egg Cases: Potential Arachnid- based Silk Nanosealants in Medicine

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Description
The egg cases of spiders are commonly multilayered, complex structures that contain several silk fibers. This study uses optical and polarized microscopy, scanning electron microscopy, and infrared spectroscopy to compare the morphology and secondary protein structure of egg case silk

The egg cases of spiders are commonly multilayered, complex structures that contain several silk fibers. This study uses optical and polarized microscopy, scanning electron microscopy, and infrared spectroscopy to compare the morphology and secondary protein structure of egg case silk of two orb-web spider species (A. aurantia and A. trifascita), two cobweb species (L. hesperus and L. geometricus), and one nursery web species (D. okefinokensis). A common feature of all six spiders' egg cases was a more dense and rigid outer layer, which was typically comprised of both tubiliform and aciniform silk fibers, along with a less dense inner layer of pure tubiliform silk. Infrared spectroscopy revealed that tubiliform silk from all egg cases contain a significant proportion (30-50%) of beta-sheet nanocrystalline aligned regions that are embedded in an amorphous random coil matrix, which does not change appreciably with hydration. While the native as-spun aciniform silk fibers primarily incorporated into the outer shell layer of egg cases are observed to be dominated by alpha-helical and random coil secondary structures, where the alpha-helical component undergoes a partial hydration-induced conversion to beta-sheet. Akin to egg case silk’s biochemical structure, its potential uses encompass a wide variety of industries, especially medicine. Synthetic materials have served in roles where silk often caters best to with its high mechanical/chemical robustness and biocompatability while also ushering in novel treatment avenues. An arachnid-based film hybridized with a photothermal converter nanoparticle such as copper salt or silver nanoprisms, which serve to weld the suture to the dermal tissue, is a promising strategy in the goal of ever improving patient outcomes.These two studies in parallel, one of a fundamental focus and one of an applied outlook, seek to understand and exploit the properties of spider silk in order to advance our knowledge of this amazing material and harness its potential for a wide range of practical applications.
Date Created
2024
Agent

New Syntheses of Low-Dimensional Metal Oxide Hybrids through Destructive Exfoliation

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Description
Due to the potential synergistic properties from combining inorganic and organic moieties, inorganic/organic hybrids materials have recently attracted great attention. These hybrids are critical components in coating and nanocomposite additive technologies and have potential for future application in catalysis, energy

Due to the potential synergistic properties from combining inorganic and organic moieties, inorganic/organic hybrids materials have recently attracted great attention. These hybrids are critical components in coating and nanocomposite additive technologies and have potential for future application in catalysis, energy production or storage, environmental remediation, electronic, and sensing technologies. Most of these hybrids utilize low dimensional metal oxides as a key ingredient for the inorganic part. Generally, clay materials are used as inorganic components, however, the use of low dimensional transition metal oxides may provide additional properties not possible with clays. Despite their potential, few methods are known for the use of low dimensional transition metal oxides in the construction of inorganic/organic hybrid materials.Herein, new synthetic routes to produce hybrid materials from low dimensional early transition metal oxides are presented. Included in this thesis is a report on a destructive, chemical exfoliation method designed specifically to exploit the Brønsted acidity of hydrated early transition metal oxides. The method takes advantage of (1) the simple acid-base reaction principle applied to strong two-dimensional Brønsted solid acids and mildly basic, high-polarity organic solvents, (2) the electrostatic repulsion among exfoliated nanosheets, and (3) the high polarity of the organic solvent to stabilize the macroanionic metal oxide nanosheets in the solvent medium. This exfoliation route was applied to tungstite (WO3∙H2O) and vanadium phosphate hydrate (VOPO4∙H2O) to produce stable dispersions of metal oxide nanosheets. The nanosheets were then functionalized by adduct formation or silane surface modification. Both functionalization methods resulted in materials with unique properties, which demonstrates the versatility of the new exfoliation methods in preparing novel hybrid materials. Further extension of the method to aqueous systems allowed discovery of a new synthetic method for electrically-conducting polyaniline-polyoxometalate hybrid materials. Namely, destructive dissolution of MoO2(HPO4)(H2O) in water produces protons and Strandberg-type phosphomolybdate clusters, and in the presence of aniline and an oxidizing agent, the clusters self-assemble with protonated anilines and selectively form polyaniline-phosphomolybdate hybrids on various types of surfaces through in situ oxidative chemical polymerization. New conductive nanocomposite materials were produced by selectively coating the surface of silica nanoparticles.
Date Created
2022
Agent

Development of Wet Chemical Synthesis Strategies for the Class of MAX Phases

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Description
MAX phases are an intriguing class of materials with exotic combinations of properties, essentially turning them into metallic ceramics. Despite this unique feature, no commercialization has been accomplished yet. Looking at the state of the art within the MAX phase

MAX phases are an intriguing class of materials with exotic combinations of properties, essentially turning them into metallic ceramics. Despite this unique feature, no commercialization has been accomplished yet. Looking at the state of the art within the MAX phase community, almost all published studies can be summarized using the term “traditional high temperature synthesis”. Contrasting the scientific interest that has been on the rise especially since the discovery of MXenes, the synthetic spectrum has been largely the same as it has been over the past decades.Herein, the newly-emerging sol-gel chemistry is being explored as an alternative non-conventional synthetic approach. Building on the successful sol-gel synthesis of Cr2GaC, this study focuses around the expansion of sol-gel chemistry for MAX phases. Starting with a thorough mechanistic investigation into the reaction pathway of sol-gel synthesized Cr2GaC, the chemical understanding of this system is drastically deepened. It is shown how the preliminary nano-structured metal-oxide species develop into bulk oxides, before the amorphous and disordered graphite partakes in the reaction and reduces the metals into the MAX phase. Furthermore, the technique is extended to the two Ge- based MAX phases V2GeC and Cr2GeC, a critical step needed to prove the viability and applicability of the newly developed technique. Additionally, by introducing Mn into the Cr-Ga-C system, a Mn-doping was achieved, and for the first time for (Cr1–xMnx)2GaC, a unit cell increase could be recorded. Based on magnetometry measurements, the currently widely accepted assumption of statistically distributed Mn in the M-layer is challenged. The versatility of wet chemistry is explored using the model system Cr2GaC. Firstly, the MAX phase can be obtained in a microwire shape leveraging the branched biopolymer dextran, eliminating the need for any post-synthesis machining. Via halide intercalation, the electrical transport properties could be purposefully engineered. Secondly, leveraging the unique and linear biopolymer chitosan, Cr2GaC was obtained as thick films and dense microspheres, drastically opening potential areas of application for MAX phases. Lastly, hollow microspheres with diameters of tens of μm were synthesized via carboxymethylated dextran. This shape once more opens the door to very specific applications requiring sophisticated structures.
Date Created
2022
Agent

Synthesis of Metal Ternary and Quaternary Nitrides with Applications in Renewable Ammonia Production

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Description

Ammonia is one of the most important chemicals for modern civilization as well as a potentially invaluable intermediary component of a future sustainable H2 economy, yet its current production is decidedly unsustainable. Accordingly, researchers are attempting to devise new paradigms

Ammonia is one of the most important chemicals for modern civilization as well as a potentially invaluable intermediary component of a future sustainable H2 economy, yet its current production is decidedly unsustainable. Accordingly, researchers are attempting to devise new paradigms for ammonia production, one of which would involve the cyclical reaction of H2 with a nitride compound and the renitridation of that compound with N2 - a thermochemical loop that would allow for ammonia production with renewable inputs and at relatively low pressures. In this paper, researchers identified several ternary and quaternary metal nitrides with the potential to exhibit relatively favorable thermodynamics for both the reduction and nitridation steps of that reaction cycle. These compounds were synthesized via co-precipitation and Pechini synthesis and several were tested under gas flows of 75% H2/Ar at 100-700 C and 75% H2/N2 at 700 C to determine their behavior under these conditions. As suggested by the available literature, Co3Mo3N was found to be a far better candidate for thermochemical looping than Fe3Mo3N or Ni2Mo3N - with higher mass loss and mass regain. Interestingly, quaternary nitrides containing Fe and Co in addition to Mo also demonstrated remarkable reduction and nitridation capability under ambient pressures. Ultimately, this paper demonstrates the feasibility of synthesizing a variety of single phase ternary and quaternary nitrides and the potential that several of these nitrides hold for producing ammonia sustainably via cyclic thermochemistry.

Date Created
2022-05
Agent

Exploring Synthesis Strategies Towards Spherical Cr2GaC

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Description

MAX phases are layered hexagonal early transition metal carbides, sometimes nitrides, where M is an early transition metal, A is an A group element, most prominently groups 13 or 14, and X is either carbon or nitrogen.1 They are gaining

MAX phases are layered hexagonal early transition metal carbides, sometimes nitrides, where M is an early transition metal, A is an A group element, most prominently groups 13 or 14, and X is either carbon or nitrogen.1 They are gaining a lot of attention because of their unusual properties. Particularly, their hardness, chemical stability at room temperature, and high melting points. These properties provide a material that is viable for a wide range of demanding applications.2,3 MAX phases display a combination of both ceramic and metallic characteristics. Furthermore, they also serve as a precursor for two-dimensional MXenes.4,5<br/>Generally, bulk synthesis of MAX phases is done through traditional solid state synthesis techniques. For example, three solid state synthesis techniques include solid state method, hot pressing and arc melting and annealing. During solid state method, the powder precursors are preheated between 350 and 400°C, allowing for decomposition of starting reagents and removal of volatile products leaving only the oxides. At this point the germination phase has completed, and the crystal growth phase begins. Under the effect of a concentration gradient and very high temperatures, cations migrate, forming well-ordered layers. Slow cooling rates are used in order to ensure crystallinity of the product.6 The second method, hot pressing, involves the mixing of powder precursors thoroughly and then cold pressed into a green body – a ceramic body powder pre-sintering. They are then heated under vacuum and often high pressure in order to form the product. Two variants of hot-pressing exits: reactive hot pressing, where the pressure during the reaction will vary throughout the reaction, and isostatic hot pressing, where the pressure is held constant throughout the entire reaction.7 Another solid-state technique is arc melting and annealing. During arc melting, alternating current is applied to the electrode inside an inert reactor, which is arranged as to generate an arc discharge. The heat produced by arcing causes rapid melting of the samples.8 While these methods are most common, they are not always viable due to the specialized equipment required in order to achieve the high temperature and pressure conditions. Furthermore, these specific techniques don’t allow for high control over particle size and morphology. <br/>Because of this, alternative, non-conventional synthesis techniques have been developed involving more readily available tube furnaces and microwaves, which lack the extreme pressures instead opting for ambient conditions.9 Sol-gel techniques have been developed by the group of Christina Birkel, and have successfully produced MAX phases through calcination of homogeneous citric acid-based gel-precursors. Some advantages of using these gel-precursors include shorter diffusion paths, and faster mass transport, thus, resulting in lower reaction temperatures and shorter reaction times. Ultimately, this allows for control over particle morphology and size.10<br/>The focus of this work is to discover optimal synthesis conditions to create spherical Cr2GaC. Spherical MAX phases have been briefly explored in existing literature using polymer-based hollow microsphere templates.10 These polymer microspheres have been used to synthesize spherical metal oxides. This is achieved by heating the metal oxide precursors which adhere to the spheres, then by thermal treatment, the template is then removed.11 <br/>Two different microsphere templates will be explored to study the advantages and disadvantages of different size distributions and surface conditions of the spheres. Furthermore, reaction temperature, reaction time, citric acid equivalents, and gel to microsphere ratio will be altered to determine optimal synthesis parameters for depositing Cr2GaC onto spherical templates. Cr2GaC serves as a model compound because it has been successfully synthesized through sol-gel chemistry in the past.10 This phase will be prepared through non-conventional sol-gel chemistry, with various heating profiles, both furnace and microwave, and will be characterized through X-ray diffraction (XRD), and Rietveld refinement. Further, the morphology and atomic composition will be analyzed through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).

Date Created
2021-05
Agent

Microwave-assisted Sol-gel Synthesis of the MAX Phases Cr2GaC, Cr2GeC, and V2GeC

Description

MAX phases are ternary carbides or nitrides that possess unique material characteristics, often simplified as a mix of metallic and ceramic properties. Many aspects of MAX phases are still being researched, but they have exciting potential applications in high-temperature structural

MAX phases are ternary carbides or nitrides that possess unique material characteristics, often simplified as a mix of metallic and ceramic properties. Many aspects of MAX phases are still being researched, but they have exciting potential applications in high-temperature structural systems, the next generation of nuclear power plants, and concentrated solar power. This project aims to benefit further research into these applications by validating a rapid unconventional synthesis method: microwave-assisted sol-gel synthesis. Three MAX phases (Cr2GaC, Cr2GeC, and V2GeC) were successfully synthesized via this route, which should open the door for more rapid prototyping and ultimately more efficient research.

Date Created
2021-05
Agent