Date on Master's Thesis/Doctoral Dissertation

12-2016

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Sunkara, Mahendra

Committee Co-Chair (if applicable)

Starr, Thomas

Committee Member

Starr, Thomas

Committee Member

Berson, R. Eric

Committee Member

Sumanasekera, Gamini

Committee Member

Druffel, Theodore

Author's Keywords

metal oxide; nanowires; scalable production; atmospheric plasma; solvo-plasma; batteries; catalyst

Abstract

Metal oxide nanowires are materials of interest in number of applications such as lithium ion batteries, solar cells, catalyst support, and gas sensing due to their unique charge transport properties and short diffusion length scales. To incorporate nanowires for any applications, one would need hundreds of grams to kilograms of these nanowires. However, state-of-the-art methods for producing metal oxide nanowires are limited to producing only milligrams to a gram in a batch. Hence, there is a need to develop scalable and cost effective processes and reactors to address this challenge. Direct gas phase oxidation of zinc metal powders using a downward atmospheric microwave plasma allowed producing 50-100 grams of zinc oxide nanowires per day. The downstream plasma reactor has certain limitations in terms of scalability: short residence time and plasma instability. Thus, a more improved reactor design is needed for continuous production of nanowire materials at commercially viable production rates. In this dissertation, a fluidized bed reactor is designed and studied for scalable production. The key feature of the reactor involves feed particles being fluidized in the flame to increase residence time. The reactor is equipped with cyclone and bag house filter housing enabling efficient powder collection and allows continuous production. Experiments using fluidized bed reactor produced single crystalline nanowires of about 30-200 nm in diameter and 0.5 – 2 mm in length. The nanowire morphology could be controlled by gas flow rate, powder feeding rate and flame type. A production rate of 1.2 kg of nanowires per hour and yield of about 90% has been demonstrated. The zinc oxide nanowires prepared in this work have been tested as catalyst support for hydro-desulfurization of diesel. The sulfur content was reduced from 200 ppm to less than 1 ppm and the catalyst was active for over 100 hours. Another concept termed as solvo-plasma oxidation has been demonstrated earlier with producing nanowires of titania and related transition metals. The technique involves the synthesis of nanowires using oxidation of metal containing precursors in the presence of alkali salts. The reaction time scales were on the order of few seconds to a minute which is about 3 to 4 orders of magnitude faster than that using a hydrothermal method. Growth rates higher than 1 mm/min were obtained. Here, the concept is studied with tin oxide first to see the ability to produce nanowires and then to understand the mechanism responsible for one-dimensional growth. Experiments reveal that the intermediate phase of potassium stannate could be held responsible for the 1D growth. In addition, experiments also confirm that the solvo-plasma technique is generic for synthesizing most of metal oxides nanowires including titania, cobalt oxide, manganese oxide, tungsten oxide, zinc oxide, and tin oxide nanowires. A simple lab-scale roll-to-roll setup can produce up to 300 grams per hour. Tin oxide nanowires find applications in lithium ion batteries. However, as-produced tin oxide nanowire powders are not chemically stable with respect to cycling. Previously, the nanowires were shown to be stable when they were reduced to produce decoration of tin clusters on their surfaces. Here, the tin oxide nanowires were treated with ultra-thin layers of titania or alumina coating as thin as 1 nm to understand the stability with respect to lithium ion cycling. No initial capacity loss due to SEI formation was found which increased the reversible capacity retention. Both titania- and alumina-coated tin oxide nanowires exhibited tin migration through the coatings to form tin nanoclusters. The compressive stress build-up during lithium intercalation and the enhanced diffusion of tin during lithium de-intercalation allowed for migration of tin to outside of coatings. The results obtained with tin should be applicable to other high capacity materials such as silicon. In summary, two types of scalable production for metal oxide nanowires were studied: A fluidized bed involving plasma and other types of flames for implementing direct oxidation of low-melting metals is studied with great success. In the second concept, the plasma oxidation of tin oxide materials dissolved in alkali salts is studied to understand the intermediate steps responsible for one-dimensional growth. Studies further showed that the second concept could also be implemented using fluidized bed reactor for scalable production. Finally, the bulk produced zinc oxide nanowires and tin oxide nanowires have been tested in hydro-desulfurization and lithium ion battery applications, respectively. In the case of lithium ion battery application, the bulk produced nanowires exhibited stability with cycling when coated with ultra-thin layers of tinania and alumina.

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