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Title:Surface chemistry to control bulk reaction dynamics of native point defects in rutile titanium dioxide
Author(s):Gilliard, Kandis Leslie
Director of Research:Seebauer, Edmund
Doctoral Committee Chair(s):Hirata, So
Doctoral Committee Member(s):Makri, Nancy; Jain, Prashant
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Point defects
Abstract:The properties of semiconductor materials depends on the type, concentration and spatial distribution of the point and extended defects it contains. For ceramic oxide semiconductor materials, the concentration and diffusion of cations and anions, in the form of interstitials and vacancies, plays a large role in the performance of these materials for gas sensing, photocatalysis, microelectronics and photovoltaic cells. The ability to control the properties of semiconductors through defect manipulation, or “defect engineering”, has been studied and applied extensively in non-ceramic semiconductors such as Silicon. However, the use of defect engineering techniques to control the properties of ceramic oxide semiconductors is still in its nascency. The dangling bonds on surfaces can offer efficient pathways for point defect injection and annihilation. A challenge to surface-based manipulation of defects in ceramic oxide semiconductors is elucidating the defect transport mechanism of the cation and anion self point defects at the surface and in the bulk. Developing suitable surface manipulation techniques to control cation and anion bulk defect dynamics will be crucial for tailoring the properties of ceramic oxide semiconductor materials. The present work uses isotopic diffusion experiments and microkinetic mathematical models to elucidate the (1) diffusion-reaction network of oxygen and titanium interstitials in rutile titanium dioxide and (2) determine the role of surfaces in changing the kinetics for the sequestration of oxygen and titanium interstitials at bulk extended defects. The diffusion-reaction networks of oxygen and titanium interstitials are mainly influenced by the activity of the surface (i.e density of active sites, surface configuration, foreign adsorbates) and Ostwald ripening kinetics of bulk extended defects. Oxygen and titanium interstitials in rutile titanium dioxide are primarily sequestered at bulk extended defects that are sometimes distributed in a spatially-dependent way. Pertinent kinetic quantities were determined from the isotopic diffusion experiments such as the estimated barrier for interstitial surface injection, EFlux. The barrier for oxygen and titanium interstitials surface injection is 0.76 ± 0.27 eV and 0.17 ± 0.10 eV, respectively. Microkinetic models were developed to understand the key elementary-step reactions for oxygen and titanium interstitials defect transport in rutile titanium dioxide. This work gives the most comprehensive quantitative and qualitative description of the self-point defect diffusion-reaction network of near-stoichiometric rutile that has yet been devised. Major findings from the model determined that gaseous Ti-flux proliferates the growth of incipient extended defects while sulfur-adsorbate retards the Ostwald ripening of bulk extended defects at low temperatures. The association of oxygen and titanium interstitials to bulk extended defects is determined by a diffusion-limited reaction while dissociation follows an Arrhenius-like behavior with a barrier of 3.5 eV and 3.7 eV, respectively. This work has reiterated the importance of clean surfaces for injecting oxygen interstitials and absorbing titanium interstitials, with the benefits of getting rid of oxygen vacancies and reducing the concentration of extended defects. Foreign adsorbates like sulfur seem to inhibit annihilation of titanium interstitials and inhibit oxygen interstitials injection. The presence of titanium gas flux aids oxygen interstitial injection but more than compensates by keeping the bulk concentration of extended defects high and creating new ones at the surface. A potential defect engineering strategy in the future will find other sets of conditions (e.g., temperature, pressure) that yield the favorable surface reconstruction that titanium gas flux seems to induce. The manufacturing of optoelectronics, sensors, and other devices require a sizable number of sequential steps, high temperature annealing for surface – based defect engineering may pose problems for integration into a process flow with tightly constrained thermal budgets. Discovering injection mechanisms that operate at or near room temperature are much preferred, as many of the defects themselves are mobile in the bulk under these conditions. For example, this work has determined that oxygen and titanium interstitial diffusion in TiO2 has an activation barrier of 0.65 eV and 0.5 eV, respectively. At room temperature, the corresponding diffusivities permit diffusion lengths in the range of 0.1-10 μm in 30 min, which is quite suitable for manufacturing. This work has investigated the feasibility of room-temperature defect engineering of oxygen bulk defects of ceramic oxide semiconductors using liquid interfacial chemistry. Preliminary results suggest that Oi or OHi may be injected in the bulk of rutile titanium dioxide. External stimuli such as UV illumination (to increase carrier concentration) and application of anodic potential (to promote surface oxidation reactions) may enhance the injection of mobile oxygen defects in the bulk of rutile titanium dioxide.
Issue Date:2017-06-13
Rights Information:Copyright 2017 Kandis Leslie Gilliard
Date Available in IDEALS:2017-09-29
Date Deposited:2017-08

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