Solution grown silicon and germanium nanostructures : characterization and application as lithium ion battery anode materials
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Solution-grown silicon and germanium nanowires were produced using various solvents and nanocrystalline seed materials. Silicon nanowires grown using monophenylsilane as the silicon source and gold catalyst seeds were made into a freestanding, lightweight, mechanically robust fabric and tested as a negative electrode material in lithium ion batteries. Annealing the fabric under reducing atmosphere converts the intrinsic poly(phenylsilane) shell into a highly conductive carbonaceous coating, improving Li storage behavior. Reduced graphite oxide (graphene) was studied as a freestanding support for gold-seeded germanium and silicon nanowires, the latter grown using trisilane. Graphene improves capacity retention for germanium nanowires but shows little improvement for silicon. Slurry-cast films of nanowires were also tested as negative electrodes in lithium ion batteries using a variety of electrolyte solvent / binder combinations. Gold is detrimental to performance of silicon nanowires grown using trisilane. Removing gold through a simple wet etching procedure dramatically improves capacity retention. Silicon nanowires were also synthesized using in-situ formed tin seeds. Tin-seeded nanowires are easier to produce and outperform gold-seeded wires in lithium ion batteries. Germanium nanowires perform exceptionally well under high current loads when cycled using electrolyte solutions that contain fluoroethylene carbonate and show promise for high- power applications. Controlled synthesis of solution-grown germanium nanorods is demonstrated using nanocrystalline bismuth seeds. The addition of poly(vinylpyrrolidinone) / hexadecene copolymer leads to branched nanorods. Absorbance spectra were calculated using the discrete dipole approximation to compare against spectra obtained experimentally. The absorbance spectra and electric field internal to the nanorods depend highly on nanorod orientation. The presence of bismuth or gold at the tip of the nanorods also significantly alters the spectra and electric fields. Ligand and surface chemistry of solution grown indium phosphide nanowires is also examined. Octylphosphonic acid and hexadecylamine are both essential for the growth of single crystalline indium phosphide nanowires. Potential solution synthesis routes to indium (III) oxide nanowires and indium phosphide nanowires with twinning superlattice structure are presented. Various phosphoric acid derivatives were tested in place of octylphosphonic acid and the efficacy of each is discussed.