| Abstract |
Carbon-based nanostructured materials (CNMs) exhibit fundamental interest and are promising
candidates for numerous applications in hydrogen production, storage, and use in clean energy
applications. Extensive research on carbon nanotubes, fullerenes and graphene has dramatically
improved our knowledge about these materials. However, a wealth of other CNMs provide
opportunities for technological advances while also presenting scientific challenges. There is
strong evidence that complex nanostructures, nanoporous and disordered carbon phases, with
or without other chemical substitutions, adsorb hydrogen more efficiently. Difficulties in controlling
synthesis conditions, in characterization, and the complexity of these materials make
their detailed theoretical study imperative.
Among these CNMs, the mixed phase of amorphous carbon (a-C) and diamond nanocrystals
(n-D) has been less studied and characterized, with many of its properties remaining unexplored.
We perform atomistic simulations with empirical potentials in order to create several a-C–n-D
samples with different n-D sizes and a-C densities, and samples of ultra-nanocrystalline diamond
(UNCD) with various grain sizes. We analyze the structure, stability, and mechanical properties
of these nanocomposite materials and our results compare well with experiment and previous
simulations. Furthermore, we study their dynamical properties to find that some pronounced
features of their vibrational spectra may be observed in experiments. Finally, the effects of hydrogen
in the structural and mechanical properties of these materials are investigated.
We also investigate CNMs for the adsorption and desorption of hydrogen. Recent studies
have shown that transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se, Te) are
rising candidates in the replacement of Pt as catalysts in the water splitting process. We focus on
the hydrogen evolution reaction (HER) part of this process and on how Hydrogen (H) interacts
with the TMDs. Specifically, we perform Density Functional Theory (DFT) calculations for MoS2 as
free standing nanostructures or positioned on a graphene substrate. These MoS2 nanostructures
as well as MoS2/Graphene hybrid systems are investigated for their stability. Our calculations of
the adsorption of H on the MoS2/Graphene hybrid systems indicate that the effect of graphene
in the adsorption process of H on MoS2 nanostructures is quite significant.
Strain in the hybrid MoS2/Graphene systems, inherent due to lattice mismatch, plays an
important role in their properties. This leads into a theoretical investigation of the structural,
electronic and dielectric properties of single-layer TMDs under various types of strain. We find
that electronic band gaps decrease while dielectric constants increase for heavier chalcogens.
The direct gaps of equilibrium structures often become indirect under certain types of strain. The
effects of strain and of broken symmetry on the band structure are discussed. The DFT results
concerning the effect of strain in the dielectric properties are theoretically explained using only
structural parameters and equilibrium dielectric constants.
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