<p>Controlling the
surface chemistry of 2D materials is critical for the development of next
generation applications including nanoelectronics and organic photovoltaics
(OPVs). Further, next generation nanoelectronics devices require very specific
2D patterns of conductors and insulators with prescribed connectivity and
repeating patterns less than 10 nm. However, both top-down and bottom-up
approaches currently used lack the ability to pattern materials with sub 10-nm
precision over large scales. Nevertheless, a class of monolayer chemistry
offers a way to solve this problem through controlled long-range ordering with
superior sub-10 nm patterning resolution. Graphene is most often functionalized
noncovalently, which preserves most of its intrinsic properties (<i>i.e.,</i> electronic conductivity) and
allows spatial modulation of the surface. Phospholipids such as
1,2-bis(10,12-tricsadiynoyl)-<i>sn</i>-glycero-3-phosphoethanolamine
(diyne PE) form lying down lamellar phases on graphene where both the
hydrophilic head and hydrophobic tail are exposed to the interface and resemble
a repeating cross section of the cell membrane. Phospholipid is made up of a complex
headgroup structure and strong headgroup dipole which allows for a diverse
range of chemistry and docking of objects to occur at the nonpolar membrane,
these principals are equally as important at the nonpolar interface of 2D
materials. A key component in the development of nanoelectronics is the
integration of inorganic nanocrystals such as nanowires into materials at the
wafer scale. Nanocrystals can be integrated into materials through templated
growth on to surface of interest as well as through assembly processes (i.e.
interfacial adsorption). </p>
<p>In this work, I
have demonstrated that gold nanowires (AuNWs) can be templated on striped
phospholipid monolayers, which have an orientable headgroup dipoles that can
order and straighten flexible 2-nm diameter AuNWs with wire lengths of ~1 µm. While AuNWs in
solution experience bundling effects due to depletion attraction interactions,
wires adsorb to the surface in a well separated fashion with wire-wire
distances (e.g. 14 or 21 nm) matching multiples of the PE template pitch. This
suggests repulsive interactions between wires upon interaction with dipole
arrays on the surface. Although the reaction and templating of AuNWs is
completed in a nonpolar environment (cyclohexane), the ordering of wires varies
based on the hydration of the PE template in the presence of excess oleylamine,
which forms hemicylindrical micelles around the hydrated headgroups protecting
the polar environment. Results suggest that PE template experience
membrane-mimetic dipole orientation behaviors, which in turn influences the
orientation and ordering of objects in a nonpolar environment.</p>
<p>Another
promising material for bottom-up device applications is MoS<sub>2 </sub>substrates
due to their useful electronic properties. However, being able to control the
surface chemistry of different materials, like MoS<sub>2</sub>, is relatively
understudied, resulting in very limited examples of MoS<sub>2 </sub>substrates
used in bottom-up approaches for nanoelectronics devices. Diyne PE templates adsorb
on to MoS<sub>2 </sub>in an edge-on conformation in which the alkyl tails
stack on top of each other increasing the overall stability of the monolayer. A
decrease in lateral spacing results in high local concentrations of orientable
headgroups dipoles along with stacked tails which could affect the interactions
and adsorption of inorganic materials (i.e. AuNW) at the interface. </p>
<p>Here, I show
that both diyne PE/HOPG and diyne PE/MoS<sub>2</sub> substrates can template
AuNW of various lengths with long range ordering over areas up to 100 µm<sup>2</sup>. Wires on
both substrates experience repulsive interactions upon contact with the
headgroup dipole arrays resulting in wire-wire distances greater than the
template pitch (7 nm). As the wire length is shortened the measured distance
between wires become smaller eventually resulting in tight packed ribbon
phases. Wires within these ribbon phases have wire-wire distances equal to the
template. Ribbon phases occur on diyne
PE/HOPG substrates when the wire length is ~50 nm, whereas wire below ~600 nm
produce ribbon phases on diyne PE/MoS<sub>2 </sub>substrates. </p>
<p>Another
important aspect to future scientific development is the catalysis of organic
reactions, specifically oxygenation of organic sulfides. Sulfide oxygenation is
important for applications such as medicinal chemistry, petroleum
desulfurization, and nerve agent detoxification. Both reaction rates and the
use of inexpensive oxidants and catalysts are important for practical
applications. Hydrogen peroxide and <i>tert</i>-butyl
hydroperoxide are ideal oxidants due to being cost efficient and
environmentally friendly. Hydrogen peroxide can be activated through transition
metal base homogeneous catalysts. Some of the most common catalysts are homo-
and hetero-polyoxometalates (POMs) due their chemical robustness. Heptamolybdate
[Mo<sub>7</sub>O<sub>24</sub>]<sup>6-</sup><sub> </sub>is a member of the
isopolymolybdate family and its ammonium salt is commercially available and low
in cost.<sup>22</sup> Heteropolyoxometalates have
been widely studied as a catalyst for oxygenation reactions whereas heptamolybdate
has been rarely studied in oxygenation reactions. </p>
<p> Here
I report sulfide oxygenation activity of both heptamolybdate and its peroxo
derivate [Mo<sub>7</sub>O<sub>22</sub>(O<sub>2</sub>)<sub>2</sub>]<sup>6-</sup>.
Sulfide oxygenation of methyl phenyl sulfide (MPS) by H<sub>2</sub>O<sub>2 </sub>to
sulfoxide and sulfone occurs rapidly with 100 % utility of H<sub>2</sub>O<sub>2</sub>
in the presence of [Mo<sub>7</sub>O<sub>22</sub>(O<sub>2</sub>)<sub>2</sub>]<sup>6-</sup>,
suggesting the peroxo adduct is an efficient catalyst. However, heptamolybdate
is a faster catalyst compared to [Mo<sub>7</sub>O<sub>22</sub>(O<sub>2</sub>)<sub>2</sub>]<sup>6-</sup>
for MPS oxygenation and all other sulfides tested under identical conditions.
Pseudo-first order <i>k</i><sub>cat</sub>
constants from initial rate kinetics show that [Mo<sub>7</sub>O<sub>24</sub>]<sup>6-</sup><sub>
</sub>catalyzes sulfide oxygenation faster. The significant difference in the <i>k</i><sub>cat</sub> suggests differences in
the active catalytic species, which was characterized by both UV-Vis and
electrospray ionization mass spectrometry. ESI-MS suggest that the active
intermediate of [Mo<sub>7</sub>O<sub>24</sub>]<sup>6-</sup><sub> </sub>under
catalytic reaction conditions for sulfide oxygenation by H<sub>2</sub>O<sub>2</sub>
is [Mo<sub>2</sub>O<sub>11</sub>]<sup>2-</sup>. These results show that
heptamolybdate is a highly efficient catalyst for H<sub>2</sub>O<sub>2 </sub>oxygenation
of organic sulfides.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7982375 |
| Date | 12 October 2021 |
| Creators | Ashlin G Porter (6580085) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY-NC-SA 4.0 |
| Relation | https://figshare.com/articles/thesis/Controlled_Interfacial_Adsorption_of_AuNW_Along_1-Nm_Wide_Dipole_Arrays_on_Layered_Materials_and_The_Catalysis_of_Sulfide_Oxygenation/7982375 |
Page generated in 0.003 seconds