Date on Master's Thesis/Doctoral Dissertation

12-2015

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Sunkara, Mahendra

Committee Member

Starr, Thomas

Committee Member

Alphenaar, Bruce

Committee Member

Sumanasekera, Gamini

Committee Member

Fu, Xiao-An

Author's Keywords

Plasma catalysis, low melting point metals, gallium

Abstract

Plasma catalysis is emerging as one of the most promising alternatives to carry out several reactions of great environmental importance, from the synthesis of nanomaterials to chemicals of great interest. However, the combined effect of a catalyst and plasma is not clear. For the particular case of 1-D nanomaterials growth, the low temperatures synthesis is still a challenge to overcome for its scalable manufacturing on flexible substrates and thin metal foils. Herein, the use of low-melting-point metal clusters under plasma excitation was investigated to determine the effectiveness in their ability to catalyze the growth of 1-D nanomaterials. Specifically, plasma catalysis using Gallium (Ga) was studied for the growth of silicon nanowires. The synthesis experiments using silane in hydrogen flow over Ga droplets in the presence of plasma excitation yielded tip-led growth of silicon nanowires. In the absence of plasma, Ga droplets did not lead to silicon nanowire growth, indicating the plasma-catalyst synergistic effect when using Ga as catalyst. The resulting nanowires had a 1:1 droplet diameter to nanowire diameter relationship when the droplet diameters were less than 100 nm. From 100 nm to a micron, the ratio increased from 1:1 to 2:1 due to differences with wetting behavior as a function of droplet size. The growth experiments using Ga droplets derived from the reduction of Gallium oxide nanoparticles resulted in silicon nanowires with size distribution similar to that of Gallium oxide nanoparticles. Systematic experiments over 100 ºC – 500 ºC range suggest that the lowest temperature for the synthesis of silicon nanowires using the plasma-gallium system is 200 ºC. A set of experiments using Ga alloys with aluminum and gold was also conducted. The results show that both Ga rich alloys (Ga-Al and Ga-Au) allowed the growth of silicon nanowires at a temperature as low as 200 ºC. This temperature is the lowest reported when using either pure Al or Au. The estimated activation energy barrier for silicon nanowire growth kinetics using Al-Ga alloy (~48.6 kJ/mol) was higher compared to that using either pure Ga or Ga-Au alloy (~34 kJ/mol). The interaction between Ga and hydrogen was measured experimentally by monitoring pressure changes in a Ga packed batch reactor at constant temperature. The decrease of the pressure inside the reactor when the Ga was exposed to plasma indicated the absorption of hydrogen in Ga. The opposite effect is observed when the plasma is turned off suggesting that hydrogen desorbed from Ga. This experimental observation suggests that Ga acts as hydrogen sink in the presence of plasma. The formation of Ga-H species in the Ga surface and in the bulk as intermediate is suggested to be responsible for the dehydrogenation of silyl radicals from the gas phase and subsequently for selective dissolution of silicon into molten Ga. The proposed reaction mechanism is also consistent with the experimentally determined activation barrier for growth kinetics (~34 kJ/mol). In addition, theoretical simulations using VASP (Vienna Ab-initio Simulation Package) were used to study atomic hydrogen – molten Ga interactions. The simulation results suggest significant interaction of atomic hydrogen with molten Ga through formation of Ga-H species on the surface and fast diffusion through bulk Ga while supporting the proposed model to explain the Plasma-Ga synergistic effect. Finally, plasma synthesis of silicon nanotubes using sacrificial zinc oxide nanowire thin film as a template was investigated for lithium ion battery anode applications. The silicon nanotube anode showed high initial discharge capacity during the first cycle of 4600 mAh g−1 and good capacity retention (3600 mAh g−1 after 20 cyles). The silicon nanotubes preserved their morphology after cycling and the observed performance was attributed to the change in phase from nanocrystalline silicon hydrogenated (nc-Si:H) to amorphous silicon hydrogenated (a-Si:H) during lithiation. This dissertation demonstrated the plasma synergism with molten metals during vapor-liquid-solid growth of silicon nanowires. A model based on atomic hydrogen interactions with molten metals under plasma excitation has been proposed and validated through systematic experimental studies involving Ga and its alloys with gold and aluminum and theoretical studies involving first principles computations. Finally, the plasma-Ga system has been used to grow successfully silicon nanowires on various technologically useful substrates at temperatures as low as 200 ºC.

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