Matching Items (4)

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Nanoporous conducting materials

Description

Nanoporous electrically conducting materials can be prepared with high specific pore volumes and surface areas which make them well-suited for a wide variety of technologies including separation, catalysis and owing

Nanoporous electrically conducting materials can be prepared with high specific pore volumes and surface areas which make them well-suited for a wide variety of technologies including separation, catalysis and owing to their conductivity, energy related applications like solar cells, batteries and capacitors. General synthetic methods for nanoporous conducting materials that exhibit fine property control as well as facility and efficiency in their implementation continue to be highly sought after. Here, general methods for the synthesis of nanoporous conducting materials and their characterization are presented. Antimony-doped tin oxide (ATO), a transparent conducting oxide (TCO), and nanoporous conducting carbon can be prepared through the step-wise synthesis of interpenetrating inorganic/organic networks using well-established sol-gel methodology. The one-pot method produces an inorganic gel first that encompasses a solution of organic precursors. The surface of the inorganic gel subsequently catalyzes the formation of an organic gel network that interpenetrates throughout the inorganic gel network. These mutually supporting gel networks strengthen one another and allow for the use of evaporative drying methods and heat treatments that would usually destroy the porosity of an unsupported gel network. The composite gel is then selectively treated to either remove the organic network to provide a porous inorganic network, as is the case for antimony-doped tin oxide, or the inorganic network can be removed to generate a porous carbon material. The method exhibits flexibility in that the pore structure of the final porous material can be modified through the variation of the synthetic conditions. Additionally, porous carbons of hierarchical pore size distributions can be prepared by using wet alumina gel as a template dispersion medium and as a template itself. Alumina gels exhibit thixotropy, which is the ability of a solid to be sheared to a liquid state and upon removal of the shear force, return to a solid gel state. Alumina gels were prepared and blended with monomer solutions and sacrificial template particles to produce wet gel composites. These composites could then be treated to remove the alumina and template particles to generate hierarchically porous carbon.

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Date Created
  • 2012

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Nanoporous transparent conducting oxides and new solid acid catalysts

Description

New sol-gel routes were developed to fabricate transparent conducting oxide coatings for energy applications. Sol-gel synthesis was chosen because the metal oxide products have high surface area and porosity.

New sol-gel routes were developed to fabricate transparent conducting oxide coatings for energy applications. Sol-gel synthesis was chosen because the metal oxide products have high surface area and porosity. Titanium sol-gel chemistry was the main focus of the studies, and the synthesis of macroporous antimony-doped tin oxide was also explored. The surface chemistry and band characteristics of anatase TiO2 show promise for solar energy purposes as photoelectrodes in DSSCs and as photocatalysts to degrade organic dyes and to split water. Modifying the band structure by increasing the conduction band edge energy is specifically of interest for reducing protons in water. To this end, a new sol-gel method was developed for incorporating Zr-dopant in nanoporous anatase TiO2. The products follow Vegard’s law up to 20 atom%, exhibiting surface area of 79 m2/g and pore volume of 0.20 cm3/g with average pore diameter of 10.3 nm; the conduction band edge energy increased by 0.22 eV and the band gap increased by 0.1 eV.

In pursuit of a greener sol-gel route for TiO2 materials, a solution of TiOSO4 in water was explored. Success in obtaining a gel came by utilizing hydrogen peroxide as a ligand that suppressed precipitation reactions. Through modifying this sol-gel chemistry to obtain a solid acid, the new material hydrogen titanium phosphate sulfate, H1-xTi2(PO4)3-x(SO4)x, (0 < x < 0.5) was synthesized and characterized for the first time. From the reported synthetic route, this compound took the form of macroscopic agglomerates of nanoporous aggregates of nanoparticles around 20 nm and the product calcined at 600 °C exhibited surface area of 78 m2/g, pore volume of 0.22 cm3/g and an average pore width of 11 nm. This solid acid exhibits complete selectivity for the non-oxidative dehydrogenation of methanol to formaldehyde and hydrogen gas, with >50% conversion at 300 °C.

Finally, hierarchically meso-macroporous antimony doped tin oxide was synthesized with regular macropore size around 210 nm, determined by statistical dye trajectory tracking, and also with larger pores up to micrometers in size. The structure consisted of nanoparticles around 4 nm in size, with textural mesopores around 20 nm in diameter.

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Date Created
  • 2016

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Exploring Synthesis Strategies Towards Spherical Cr2GaC

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,

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).

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Created

Date Created
  • 2021-05

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New acid medium sol-gel synthesis of metal phosphates

Description

New sol-gel routes based on peroxo complexes of early transition metals in a highly acidic medium were developed, to prepare metal oxide phosphates that feature structural protons. A sol-gel synthetic

New sol-gel routes based on peroxo complexes of early transition metals in a highly acidic medium were developed, to prepare metal oxide phosphates that feature structural protons. A sol-gel synthetic route was chosen because it allows atomic level mixing of precursors and lower heating temperatures, which are preferable in exploring metastable phases. Titanium and molybdenum sol-gel chemistries were the focus of the initial studies and the synthesis of Ti1-xMoxP2O7 (x = 0 – 0.5) and Mo1-yTiyP2O8-y (y = 0 – 0.4) type metal oxide phosphates were explored. For the synthesis of the metal oxide phosphates, hydrogen peroxide was employed to prepare the respective precursor solutions. The peroxide ligand suppressed the immediate precipitation of metal cations in aqueous medium, by coordinating to Ti4+ and Mo6+ ions, and produced a soft wet-gel following polycondensation. Phosphoric acid was used to acidify the reaction medium and to provide protons and phosphate ions as structural components. From this synthetic route, a series of Ti1-xMoxP2O7 (x = 0 – 0.5) and Mo1-yTiyP2O8-y (y = 0 – 0.4) crystalline compounds, with various degrees of purity, were synthesized. For x = 0 and y = 0, the crystalline compounds TiP2O7 and MoP2O8 were produced, respectively, after calcining at 600 °C.

In pursuit of new metastable molybdenum oxide phosphate compounds, peroxo-molybdenum precursor mixtures with different molar ratios were treated gently by low-temperature heating. After controlled drying in a lab oven, MoO2(H2O)(HPO4) crystals were obtained as a highly crystalline pure product instead of a gel. The dissolution of MoO2(H2O)(HPO4) in water and precipitation with a CsCl solution produced a new crystalline compound with a cubic unit cell (a = 11.8(2) Å). Further studies will lead to crystal structure determination and elucidation of the aqueous chemistry of MoO2(H2O)(HPO4).

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Created

Date Created
  • 2018