THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Friday, May 11, 2018

11:30 AM
in MRDC 3515

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Matthew N. Drexler

"Deconvolution of the effects of strain and ligands on catalytic performance in transition metals"

Committee Members:

Dr. Faisal Alamgir, Advisor, MSE

Dr. Seung Soon Jang, MSE

Dr. Matthew McDowell, MSE

Dr. Wenshan Cai, MSE/ECE

Dr. Phillip First, PHYS

Dr. Raymond Unocic, ORNL

Abstract:

Fuel cells offer a promising alternative to traditional power sources due to their environmentally benign emissions. However, fuel cells have yet to replace established technology due to their poor performance and durability. This lack of performance is mostly driven by the catalyst, which are made of rare and expensive transition metals such as Pt. The performance of a catalyst is highly governed by its electronic structure. This is exemplified by d-band center theory that states that catalytic activity in transition metals is directly governed by the center of the density of states in their d-band. The electronic structure of a transition metal can be modified through two different avenues termed the strain effect and the ligand effect. The strain effect is the change in charge density due to changes in interatomic spacing altering the degree of orbital overlap within a material. The ligand effect is the change in charge density due to the presence of other elements. Further improvements in catalyst performance will come from being able to control these two effects, but in practical implementation, these two effects are inseparable. Alloying two metals together changes their lattice constant, providing a simultaneous ligand and strain effect. As such, it is difficult to separate the two effects and predict how different catalyst geometries will perform.

Atomically thin nanofilms (ATNF), thin films only a few atoms thick, are a type of catalyst geometry that can help separate these effects. Previous work by our group has shown that transition metal ATNFs can be templated on graphene, assuming a structure based on the graphene, and that the graphene allows for electronic interaction across it. Since graphene can be transferred to multiple different substrates, it can be used to apply different ligand effects to and ATNF without changing its strain. It should also be possible to template ATNFs on curved graphitic materials, such as carbon nanotubes and fullerene, allowing for the variation of strain without changing the ligand. Two sets of experiments will be used to deconvolute the strain and ligand effects. The first will use ATNF-graphene-substrate sandwich composites to observe the ligand effect in isolation. The second set will use carbon nanotubes of varying thicknesses as an ATNF substrate to observe the strain effect in isolation. Six different substrates will be tested in the graphene studies and will be chosen based on the ATNF material to establish trends across its group and period in the periodic table. Pt, Pd, and Rh will be used as ATNF materials as they have enough neighbors to their left and right across the periodic table to establish trends. Electrochemical performance will be characterized through hydrogen and oxygen reduction reactions. ATNF thickness will be characterized directly by scanning transmission electron microscopy and indirectly through x-ray photoelectron spectroscopy and Raman spectroscopy. Lattice strain will be characterized by scanning transmission electron microscopy. Electronic structure will be characterized through ultraviolet photoelectron spectroscopy.