A FACILE SYNTHESIS OF TRANSITION METAL OXIDE AND CHALCOGENIDES NANOCRYSTALS FOR ELECTROCATALYTIC APPLICA TIONS

Cobalt-manganese oxide spinel nanocrystal are well known for its excellent performance as supercapacitor electrode and electrocatalyst. The high surface to volume ratio of spinel nanoparticles enables high charge storage while the interactions between mixed valence transition metal elements in the spinel structure contribute to its exceptional catalytic performance for oxygen reduction/evolution reactions. However, nanocrystals by conventional synthesis methods are either highly aggregated or capped with excessive insulating surfactant, resulting in inconsistent electrochemical performance and capacity lost in charge/discharge cycling. In this thesis, we present a general colloidal synthesis route for synthesizing single phase, monodisperse CoMn2O4 nanocrystal with low organics loading and be able to tune the ratio of Co from 0% to 56% while maintaining small, low dispersity, colloidally stable nanocrystal. We proposed that the formation for the core-shell structure in the nanocrystals originates from the faster conversion rates of Mn precursor to its monomer. Using NMR and FTIR spectroscopy to study surface chemistry, we present a systematic method to prevent aggregation of nanoparticles by re-passivate the surface with X-type ligand and remove excessive surface organics by using short chain alkylamine as solvent. The optimization leads to excellent catalytic performance in oxygen reduction reaction, which is only 20mV lower in half wave overpotential than commercial Pt/C. Our synthesis method is economical and highly scalable, providing insight for future research on creating monodisperse transition nanoparticles under reducing environment .The second part of the thesis, we present a cation exchange method for Cu2-xS nanocrystal in which the top and bottom layers of Cu2-xS are exchanged to ZnS with the assistance of tri-octylphosphine and the thickness of exchanged layer is expected be tuned from partially exchanged (sandwich like ZnS/Cu2-xS/ZnS heterostructure) to full exchange(ZnS) by limiting the reaction kinetics. We believe the kinetics is controlled by the intercalation of Zn2+ cations to copper sulfide. Constructing sandwich-like heterostructure allows us to systematically control the level of strain within the nanocrystal and induces band structure reconstruction of surface cations which will inform the binding energies of reaction intermediates in multi-steps catalytic reactions. We expect the heterostructure can be used as an excellent testbed for revealing the correlation between strain and electrocatalytic activities in reactions like CO or CO2 reduction for metal chalcogenide nanocrystals.

Name thesis:degree_name

M.S., Materials Science and Engineering

Level thesis:degree_level

Master of Science

Discipline thesis:degree_discipline

Materials Science and Engineering

Grantor

Cornell University

Year dc:date.issued

2023

Author dc:creator

• Jia, Yafu

Committee member dc:contributor.committeemember

• Suntivich, Jin

Language dc:language.iso

en

DOI

https://doi.org/10.7298/vdtg-6r33

Dc Identifier Other

ProQuest Submission ID: 11905
ProQuest Publication ID: 30575917

OAI identifier oai:identifier

oai:ecommons.cornell.edu:1813/114424