Scalable Synthesis of Graphene Based Heterostructures and their use in Energy Sensing, Conversion and Storage

Open Access
Author:
Bhimanapati, Ganesh Rahul
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 12, 2016
Committee Members:
  • Joshua A Robinson, Dissertation Advisor
  • Joshua A Robinson, Committee Chair
  • James Adair, Committee Member
  • Mauricio Terrones, Committee Member
  • Thomas E Mallouk, Outside Member
Keywords:
  • graphene
  • molybdenum disulfide
  • heterostructures
  • radiation
  • hydrogen evolution reactions
  • HERs
  • lithium ion batteries
  • LiBs
  • transition metal dichalcogenides
  • tungsten diselenide
Abstract:
2D materials are a unique class of materials system which has spread across the entire spectrum of materials including semi-metallic graphene to insulating boron nitride. Since graphene there has been many other 2D material systems (such as boron nitride (hBN), transition metal dichalcogenides (TMDs)) that provide a wider array of unique chemistries and properties to explore for applications specifically in optoelectronics, mechanical and energy applications. Specifically tailored heterostructures can be made which can retain the character of single-atom thick sheets while having an entirely different optical and mechanical properties compared to the parent materials. In the current work, heterostructures based on graphene, hBN and TMDs have been made, which were used to study the fundamental process-property relations and their use in energy conversion and storage have been studied. The first part of this dissertation focusses on scalable approach for liquid phase exfoliation of graphene oxide (GO) and hBN (Chapter 2). The current work successfully shows an exfoliation efficiency of ~25% monolayer material for hBN, which was not previously achieved. These exfoliated materials were further mixed in the liquid environment to form a new heterostructure BCON (Chapter 3). This newly formed heterostructure was studied in detail for its process-property relations. At pH 4-8, BCON was highly stable and can be dried to form paper or ribbon like material. New bonds were observed in BCON which could be linked to the GO linkage at the nitrogen sites of the hBN. This free standing BCON was tested under various radiation sources like x-rays, alpha, beta, gamma sources and heavy ion like Ar particles and was found that it is very robust to radiation (Chapter 5). By understanding the chemistry, stability and properties of these materials, this could lay a foundation in using these materials for integration in conductive and insulating ink development, polymer composite development to improve the thermal and mechanical properties. Another major focus of this dissertation work is combining TMDs and graphene for energy applications specifically hydrogen evolution reactions (HERs) and Lithium ion batteries (LiBs). TMD’s specifically MoS2 and WSe2 were grown on graphite paper using powder vaporization and metal organic chemical vapor deposition (MOCVD) (Chapter 4). Control over the architecture of the MoS2 and WSe2 was achieved by varying the precursor concentration and pressure, which was observed by using scanning electron microscopy. These samples were further characterized using cross-sectional transmission electron microscopy, x-ray photoelectron spectroscopy and raman microscopy confirming the high quality of the material that was grown. The MoS2/graphite flowers were tested for hydrogen evolution reactions and were found that they are highly active for catalysis and by modifying the surface using simple UV-Ozone treatments, this activity can be increased by 4x (reducing the Tafel slope from 185 to 54 mV/Dec). Similar performance was observed for WSe2/Graphite heterostructure where the tiny 100 nm vertical flakes on graphite paper showed one of the lowest reported Tafel slope of 64 mV/Dec (Chapter 6). MoS2/Graphite was further tested for lithium ion batteries and was found that it had a higher cyclic capacity of 750 mV/Dec. This enhanced stability and performance for energy applications was achieved because of the direct growth technique on graphite. Hence this technique could be used as a scalable alternative to make anodes for lithium ion batteries.