The Soft Chemistry of MoAlB

Author:

Alameda, Lucas Thomas

Graduate Program:

Chemistry

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

November 20, 2020

Committee Members:

Raymond Edward Schaak, Dissertation Advisor/Co-Advisor
Raymond Edward Schaak, Committee Chair/Co-Chair
Benjamin James Lear, Committee Member
Elizabeth A Elacqua, Committee Member
Joshua Alexander Robinson, Outside Member
Philip C Bevilacqua, Program Head/Chair

Keywords:

MoAlB
MBenes
Solid-State Synthesis
Soft Chemistry
Topochemical
Hydrogen Evolution Reaction
Mesoporosity
Metal Borides
Two-Dimensional Materials
Heterostructures

Abstract:

The synthesis of solid-state inorganic materials is limited by the diffusion requirements inherent to their bulk nature. Metastable materials are particularly challenging to synthesize, because the rate-limiting nature of solid-state diffusion requires the synthetic chemist to supply a large amount of thermal energy to drive diffusion and thoroughly mix the atoms. Without thorough mixing, there can be no reaction at all. However, thermal energy is indiscriminate. If abundant thermal energy is present, phases will nucleate with a propensity to favor the thermodynamically-stable phase. Thus, an interplay exists between thermal energy, diffusion, and nucleation that does not favor metastable phases. The diffusion limitation is the primary reason that the maturation of solid-state synthesis is occurring slowly relative to solution-phase processes. Instead, we are heavily reliant on empirical methods or straight serendipity. This was perhaps best expressed by Tom Mallouk in the early 1990’s when he said that many solid-state materials are synthesized “the old-fashioned way (by accident).” The year is now 2020 and this quote is still accurate. In this thesis, I tackle the issue of diffusion-limited solid-state synthesis. I developed two strategies that overcome diffusion limitations in metal boride systems to synthesize Mo2AlB2, a material with a metastable crystal structure, and mesoporous MoB, a material with a metastable morphology. The first synthetic strategy begins in Chapter 2, where NaOH is identified as a suitable etchant for partial removal of Al from MoAlB. In Chapter 3, I show that Al deintercalation from MoAlB induces strain in the structure. In Chapter 4, this strain is then relaxed with moderate annealing to yield metastable Mo2AlB2, a material targeted as a precursor to 2D MBenes. The second synthetic strategy, which is described in Chapter 5, uses the thermal decomposition of heterostructured, and chemically-dissimilar, nanosheets to engineer mesoporosity into MoB. Finally, the strategies are presented in context of the materials development cycle.