Photorefractivity of Organic-Inorganic Hybrid Composites Photosensitized with Nickel-Sulfide Nanocrystals

The photo refractivity of organic-inorganic hybrid composites doped with nickel sulfide (NiS) nanocrystals is described. The nanocrystals were characterized using visible-absorption spectroscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The nanocrystals were further modified by ligand exchange, lowering the over-modulation voltage without sacrificing diffraction efficiency as well as increasing the two-beam coupling gain coefficient. Photoconductivity measurements were used to determine the overall quantum efficiency of the photorefractive samples. The measurements represent a significant increase in photo refractivity when compared to other organic-inorganic hybrid photorefractive composites

Synthesis and characterization of vanadium oxide nanomaterials

This dissertation focuses on the synthesis and characterization of vanadium oxide nanomaterials and neutron-transparent Li-ion battery electrolytes. Vanadium oxide gels were synthesized via the epoxide-assisted gelation of VOCl3 using epichlorohydrin in ethanol. This represents the first epoxide-assisted gelation developed for vanadium oxide gels and serves as a potential economical replacement for the fabrication of vanadium oxide gels via the hydrolysis of vanadium alkoxides. Though epoxide-assisted gelation has been demonstrated for a number of transition metal oxides, the gelation mechanism observed here for VOCl3 appears to be unique. While investigating the electrochemical and structural properties of lithium vanadates, it became desirable to perform in situ neutron diffraction with electrochemical cycling. The inability to perform these experiments prompted the development of a series of perfluorocarboxylate esters which could potentially serve as neutron-transparent electrolyte solvents and help enable in situ neutron scattering characterizations. Electrochemical studies were carried out on Si anodes, and the solid-electrolyte-interphase was studied both ex situ and in situ. Results indicate that these electrolyte solvents improve neutron scattering data when used in place of standard deuterated carbonates and provide insight into fluorinated components proposed for next-generation electrolytes --Abstract, page iv

Photosensitization of Photoconductive and Photorefractive Composites through the Inclusion of Semiconductor Nanocrystals

A novel synthesis of NiS nanocrystals included in photorefractive polymeric composites is described. The nanocrystals were characterized using visible-absorption spectroscopy, energy dispersive X-ray spectroscopy, and transmission electron microscopy. We further demonstrate the ability to enhance aspects of the composites\u27 photorefractive performance by performing ligand exchange on the nanocrystals prior to their incorporation into the polymer composite. It was originally believed that this modification of the nanocrystals\u27 surface characteristics would result in an enhanced charge-transfer process between the nanocrystal and the charge transporting matrix. However, when the photo conductivities were determined and used in calculating the quantum efficiencies associated with the photorefractive devices, it was determined that this enhancement did not occur

Photorefractivity in a Polymeric Composite Photosensitized with NiS Nanocrystals

The photorefractive performance of a polymeric composite photosensitized through the inclusion of NiS nanocrystals is described. The nanocrystals were characterized using visible-absorption spectroscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. We further demonstrate the ability to enhance various aspects of the composite\u27s photorefractive performance by performing ligand exchange on the nanocrystals prior to their incorporation into the polymer composite. This procedure resulted in a lowering of the overmodulation voltage from ~70 to ~50 V/µm without affecting the maximum diffraction efficiency of ~40%. An increase in the two-beam-coupling gain coefficient was similarly observed, increasing from 38 to 79 cm−1. The photoconductivities were used in determining the overall quantum efficiencies associated with the photorefractive devices. All experiments were conducted at 633 nm and the data represent a significant improvement in the photorefractive performance of inorganic-organic hybrid photorefractive materials

Economical Synthesis of Vanadia Aerogels via Epoxide-Assisted Gelation of VOCl₃

Herein is reported the first synthesis of vanadium oxide (vanadia) aerogels via epoxide-assisted gelation. Vanadia aerogels fabricated via epoxide-assisted gelation using VOCl3 and epichlorohydrin mimic those fabricated via the hydrolysis of vanadium oxytripropoxide while reducing cost per mole by a factor of 10, on par with silica gels synthesized from alkyl orthosilicates. Typical vanadia aerogels possess a macroporous nanoworm morphology, a density of 0.103 g/cm3, 96.9 % porosity, a specific surface area of 102 m2/g, and ~18 % reversible hydration capacity by mass. 1H and 13C NMR reveal that epoxide ring-opening does not proceed through epoxide protonation, as is the typical case with other transition metal oxide gels, but rather through Lewis-acid-catalyzed ring-opening. a multi-step gelation mechanism is proposed involving a fast initial VV oxide gelation, driven by H+ and Cl- consumption during epoxide ring-opening, followed by partial vanadia dissolution, VV → VIV reduction, and secondary gelation of low-valent VIV/V oxide

A Study of Perfluorocarboxylate Ester Solvents for Lithium Ion Battery Electrolytes

Several high-purity methyl perfluorocarboxylates were prepared (\u3e99.5% purity by mole) and investigated as potential fluorine-rich electrolyte solvents in Li-ion batteries. the most conductive electrolyte, 0.1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethyl perfluoroglutarate (PF5M2) (ionic conductivity = 1.87 × 10-2 mS cm-1), is investigated in Si thin-film half-cells. the solid-electrolyte-interphase (SEI) formed by the PF5M2 electrolyte is composed of similar organic and inorganic moieties and at comparable concentrations as those formed by ethylene carbonate/dimethyl carbonate electrolytes containing LiPF6 and LiTFSI salts. However, the SEI formed by the PF5M2 electrolyte undergoes reversible electrochemical defluorination, contributing to the reversible capacity of the cell and compensating in part for capacity fade in the Si electrode. While far from ideal these electrolytes provide an opportunity to further develop predictions of suitable fluorinated molecules for use in battery solvents

Ultra-low-density silver aerogels via freeze-substitution

Herein is reported a method for fabricating <10 mg/cm3 silver aerogels via the freeze casting of aqueous nanowire suspensions followed by freeze substitution and supercritical drying. This method overcomes the limitations of traditional freeze drying and yields highly uniform, crack-free monoliths that undergo no measurable shrinkage with excellent mold reproduction. Significantly, freeze substitution enables the use of high concentrations of cryoprotectants to control the freezing process and, hence, the pore architecture of the resulting aerogels. Due to its physical nature, this method is applicable to the fabrication of materials with a broad range of compositions

Direct Determination of Solid-Electrolyte Interphase Thickness and Composition as a Function of State of Charge on a Silicon Anode

With the use of neutron reflectometry, we have determined the thickness and chemistry of the solid-electrolyte interphase (SEI) layer grown on a silicon anode as a function of state of charge and during cycling. We show the chemistry of this SEI layer becomes more LiF like with increasing lithiation and more Li–C–O–F like with delithiation. More importantly, the SEI layer thickness appears to increase (about 250 Å) as the electrode becomes less lithiated and thins to 180 Å with increasing Li content (Li_3.7Si). We attribute this “breathing” to the continual consumption of electrolyte with cycling