Date of Degree

9-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Chemistry

Advisor

Stephen O'Brien

Committee Members

Glen Kowach

Alan Lyons

Ioannis Kymissis

Subject Categories

Inorganic Chemistry | Materials Chemistry | Physical Chemistry | Polymer Chemistry

Keywords

Sol-Gel, Dielectric Constant, BaTiO3, Perovskite Crystals, Nanoparticles

Abstract

The Synthesis of transition metal oxide nanoparticles has been studied in great detail over the many years. The most studied transition metal oxide nanoparticles are perovskites of the ABO3 stoichiometry (A and B = transition metal) and more recently double perovskite crystal structures of the AA’BO6 or A2BB’O6 stoichiometry due to the many different properties arising from the many different combinations of elements possible. These materials have proven potentially useful in many fields, but due to properties such as ferroelectricity and ferromagnetism, the desire to integrate these materials into electronics is ever growing. Many synthesis techniques require a high temperature (>800°C) calcination step that can prove troublesome for device integration. Here, a modified sol-gel synthesis was used in order to lower the calcination temperature to synthesized uniform nanocrystalline multifunctional transition metal oxides KNbO3, [KNbO3]1-x[BaNi0.5Nb0.5O3-d]x (where x = 0.1-0.3), BaTiO3, Ba(Ti1-xFex)O3 (where x = 0.1-0.75, and 1.0), Ba2NbTiO6, and Ba2TaTiO6. A chemical solution processing method based on sol-gel chemistry was used to obtain a set of perovskite compounds of the formula KNbO3, and [KNbO3]1-x[BaNi0.5Nb0.5O3-d]x, called KBNNO, a class of visible light absorbing ferroelectric photovoltaic materials, with a tunable bandgap as a function of x. The materials produced were fully crystallized with average v nanoparticle sizes of 15-20 nm (KNO) and 20 nm (KBNNO). Control over the composition of KBNNO was based on the synthesis of nanocrystalline potassium niobate KNbO3 (KNO) via potassium and niobium ethoxides, with subsequent chemical reaction of complimentary barium and nickel alkoxides and methoxyethoxides. Characterization by Raman, TEM, SEM, XRD, and EDS confirms structure and composition. Following the introduction of Ba and Ni, a transition from the original orthorhombic Amm2 unit cell (x = 0) to a more complex atomic arrangement in cubic Pm3m (x > 0.1) is observed. This synthetic route to KBNNO, previously only synthesized by solid state processing at 1050-1200 °C, provides a lower temperature (< 525 °C) approach to doping ferroelectric KNbO3 with Ba and Ni, which inserts Ba2+ onto the A-site, and Ni2+ onto the B-site with the addition of oxygen vacancies for charge compensation. Frequency dependent dielectric measurements, performed on KNO-PFA (poly furfuryl alcohol) and KBNNO-PFA nanocomposites, show stable effective dielectric constants of 41.2, 70.8, 94.0, and 108.3 for KNO, KBNNO x = 0.1, 0.2, and 0.3 respectively at 1 MHz. Using full error analysis and the modified interphase model, a Maxwell-Garnett based micromechanics approach, the dielectric constant of the individual nanoparticles of KNO, KBNNO x = 0.1, x = 0.2, and x = 0.3 were calculated to be 154, 180, 225, and 255 respectively. The decrease in observed values relative to bulk films is attributed to a potential particle size suppression of the ferroelectric behavior. A series of iron-substituted barium titanate nanocrystals (BaTi1−xFexO3) were synthesized at 60°C using a hybrid sol−gel chemical solution processing method. No further crystallization/calcination steps were required. The as-prepared nanocrystals were fully crystalline, uniform in size (∼8 nm by TEM), and dispersible in polar organic solvents, yielding nanocrystal/alcohol formulations. Complete consumption of the reactant precursors ensures adequate control over stoichiometry of the final product, over a full range of x (0, 0.1 to 0.75, 1.0). Pair distribution function (PDF) analysis enabled in-depth structural characterization (phase, space group, unit cell parameters, etc.) and shows that, in the case of x = 0, 0.1, 0.2, 0.3, BaTiO3, and BaTi1−xFexO3 nanocrystals, it is concluded that they are tetragonal noncentrosymmetric P4mm with lattice parameters increasing from, e.g., c = 4.01 to 4.08. XPS analysis confirms the presence of both Fe3+(d5) and Fe4+(d4), both candidates for multiferroicity in this system, given certain spin configurations in octahedral field splitting. The PDF cacluated lattice expansion is attributed to Fe3+(d5, HS) incorporation. The evidence of noncentrosymmetry, lattice expansion, and XPS conformation of Fe3+ provides support for the existence of multiferroicity in these sub 10 nm vi uniform dispersed nanocrystals. For x > 0.5, Fe impacts the structure but still produces dispersible, relatively monodisperse nanocrystals. XPS also shows an increasing amount of Fe4+ with increasing Fe, suggesting that Fe(IV) is evolving as charge compensation with decreasing Ti4+, while attempting to preserve the perovskite structure. A mixture of Fe3+/Fe4+ is thought to reside at the B site: Fe4+ helps stabilize the structure through charge balancing, while Fe3+ may be complimented with oxygen vacancies to some extent, especially at the surface. The structure may therefore be of the form BaTi1−xFexO3‑δ for increasing x. At higher concentrations (Fe > 0.5) the emergence of BaFeO3 and/or BaFe2O4 is offered as an explanation for competing phases, with BaFeO3 as the likeliest competing phase for x = 1.0. Because of the good dispersibility of the nanocrystals in solvents, spin coating of uniform 0−3 nanocomposite BaTi1-xFexO3/polyvinylpyrrolidone thin film capacitors (1−xFexO3 samples for x = 0−0.75, respectively. Loss tangent values at 1 MHz were ∼0.04, demonstrating the ability to prepare capacitors of magnetic Ba(Ti1−xFex)O3 with relatively high permittivity. Magnetic characterization by MPMS (both magnetic hysteresis loops and zero field and field cooling measurements) showed increased magnetization with increasing Fe ion concentration. Weak magnetic coercivity and a small remanence magnetization is observed (<5 >K), implying a weak ferromagnetic state at low temperatures (<5 >K). The synthesis of the nanocrystalline double perovskites Ba2NbTiO6 and Ba2TaTiO6 through a hybrid sol-gel synthesis followed by a solvothermal step at 200°C was accomplished. The powder XRD measurements show that a single phase nanocrystalline material was produced. Through TEM images it was confirmed that the material has a size range of 10-30 nm single crystalline nanoparticles. XPS measurements confirm the presence of Nb5+ and Ta5+ oxidations states throughout the material. Raman spectroscopy confirms that the material is polarizable and has a phase transition to a cubic crystalline structure between 270 and 300°C for both materials. 0-3 nanocomposites were made using polyfurfuryl alcohol (PFA) and the nanocrystalline powders. The effective dielectric measurements show high permittivity (~50) over a wide range of frequencies (200 Hz-2 MHz) and low dielectric loss tangent (

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