53 research outputs found
Contribution of Dielectric Screening to the Total Capacitance of Few-Layer Graphene Electrodes
We apply joint density functional
theory (JDFT), which treats the
electrode/electrolyte interface self-consistently, to an electric
double-layer capacitor (EDLC) based on few-layer graphene electrodes.
The JDFT approach allows us to quantify a third contribution to the
total capacitance beyond quantum capacitance (<i>C</i><sub>Q</sub>) and EDL capacitance (<i>C</i><sub>EDL</sub>).
This contribution arises from the dielectric screening of the electric
field by the surface of the few-layer graphene electrode, and we therefore
term it the dielectric capacitance (<i>C</i><sub>Dielec</sub>). We find that <i>C</i><sub>Dielec</sub> becomes significant
in affecting the total capacitance when the number of graphene layers
in the electrode is more than three. Our investigation sheds new light
on the significance of the electrode dielectric screening on the capacitance
of few-layer graphene electrodes
Ligand-Conformation Energy Landscape of Thiolate-Protected Gold Nanoclusters
Although
several thiolate-protected Au nanoclusters have yielded
to total-structure determination, the ligand-conformation energy landscapes
and how they affect the relative stability of the whole clusters are
not well understood. In this work, we employ a force-field-based approach
to perform the ligand-conformation search for isolated thiolate-protected
Au nanoclusters using Au<sub>25</sub>(SR)<sub>18</sub> (R = C<sub>2</sub>H<sub>4</sub>Ph) as an example. We find that the ligand-conformation
energy landscape of Au<sub>25</sub>(SC<sub>2</sub>H<sub>4</sub>Ph)<sub>18</sub> comprises multiple low-energy funnels of similar stability
instead of a single global minimum. In fact, we find slightly more
stable conformations of isolated Au<sub>25</sub>(SC<sub>2</sub>H<sub>4</sub>Ph)<sub>18</sub> than those observed in the experiment from
a crystalline state, indicating that specific environments such as
crystal packing and solvents may all affect the ligand conformation.
This work reveals the role of ligand conformation in the cluster energy
landscape
Revisiting Structural Models for Au<sub>18</sub>(SR)<sub>14</sub>
Au<sub>18</sub>(SR)<sub>14</sub> (âSR being a thiolate group)
is a small, stable thiolated gold nanocluster experimentally identified
in 2005, but its structure remains elusive. A previously proposed
model for Au<sub>18</sub>(SR)<sub>14</sub> based on density functional
theory (DFT) structural optimization consists of a Au<sub>8</sub> core
protected by two âRSâAuâSRâAuâSRâ
(dimer) and two âRSâAuâSRâAuâSRâAuâSRâ
(trimer) motifs. Here we revisit structure prediction for Au<sub>18</sub>(SR)<sub>14</sub> from extensive exploration of the possible isomers
for Au<sub>18</sub>(SCH<sub>3</sub>)<sub>14</sub> by applying structural
hypotheses based on both âstaple motifsâ and âring
and staple motifsâ. Three isomers based on the staple motifs
are found to be more stable than the best previous model. The two
lowest-energy Au<sub>18</sub>(SCH<sub>3</sub>)<sub>14</sub> isomers
(I and II) also have a Au<sub>8</sub> core protected by two dimer
and two trimer motifs, but the core geometry and electronic properties
are different. The third lowest-energy isomer (III) consists of a
Au<sub>8</sub> core protected by two âRSâAuâSRâ
(monomer) and two âRSâAuâSRâAuâSRâAuâSRâAuâSRâ
(tetramer) motifs. By changing R from CH<sub>3</sub> to the experimentally
used cyclohexanyl group (C<sub>6</sub>H<sub>11</sub>), we found that
isomer III is the most stable for Au<sub>18</sub>(SC<sub>6</sub>H<sub>11</sub>)<sub>14</sub>. The computed X-ray diffraction (XRD) pattern
and optical spectrum of isomer III are in good agreement with the
experimental data. This work suggests that Au<sub>18</sub>(SR)<sub>14</sub> may have monomer and tetramer motifs in the protective layer
Mechanism of Hydrogen Evolution Reaction on 1T-MoS<sub>2</sub> from First Principles
The 1T phase of transition-metal
dichalcogenides (TMDs) has been
demonstrated in recent experiments to display excellent catalytic
activity for hydrogen evolution reaction (HER), but the catalytic
mechanism has not been elucidated so far. Herein, using 1T MoS<sub>2</sub> as the prototypical TMD material, we studied the HER activity
on its basal plane from periodic density functional theory (DFT) calculations.
Compared to the nonreactive basal plane of 2H phase MoS<sub>2</sub>, the catalytic activity of the basal plane of 1T phase MoS<sub>2</sub> mainly arises from its affinity for binding H at the surface S sites.
Using the binding free energy (Î<i>G</i><sub>H</sub>) of H as the descriptor, we found that the optimum evolution of
H<sub>2</sub> will proceed at surface H coverage of 12.5% âŒ
25%. Within this coverage, we examined the reaction energy and barrier
for the three elementary steps of the HER process. The Volmer step
was found to be facile, whereas the subsequent Heyrovsky reaction
is kinetically more favorable than the Tafel reaction. Our results
suggest that at low overpotential, HER can take place readily on the
basal plane of 1T MoS<sub>2</sub> via the VolmerâHeyrovsky
mechanism. We further screened the dopants for the HER activity and
found that substitutional doping of the Mo atom by metals such as
Mn, Cr, Cu, Ni, and Fe can make 1T MoS<sub>2</sub> a better HER catalyst
Comprehensive View of the LigandâGold Interface from First Principles
A large
number of ligands have been used to stabilize and functionalize
the gold surfaces and nanoclusters, but there has been no systematic
comparison about their binding strengths with gold. In this work,
we studied the interaction between 27 ligands of six different types
(thiolates, phosphines, amines, aryl radicals, alkynyls, and N-heterocyclic
carbenes) with the model Au surfaces by first-principles density functional
theory (DFT). On the perfect Au(111), we found the order of binding
strengths to be bulky N-heterocyclic carbenes (NHCs) â alkynyls
> thiolates â phosphines > aryls â less sterically
bulky
NHCs > alkylamines. The much stronger interaction of bulky carbenes
to Au than the less sterically bulky NHCs arises from the van der
Waals (vdW) attraction of bulky side groups with gold surface via
the short Au···HCH<sub>2</sub>R contact. Further, we
showed that the presence of a gold adatom on Au(111) leads to enhanced
binding and a similar order for most of the ligands examined. Overall,
bulky NHCs and alkynyl groups form the strongest interaction to both
Au(111) and Au<sub>ad</sub>âAuÂ(111). This suggests that NHCs
can be employed as alternatives to the currently widely used thiolates
and the emerging alkynyl ligands for the preparation of more stable
self-assembled monolayer structures on metal surfaces. Further, this
insight allowed us to design viable magic-number gold clusters with
NHCs as the protecting ligands
UranylâGlutardiamidoxime Binding from First-Principles Molecular Dynamics, Classical Molecular Dynamics, and Free-Energy Simulations
Exploring the structural
interplay of ligands with uranyl can provide
important knowledge for technology advances in uranium extraction
from seawater. However, obtaining such chemical information is not
an easy endeavor experimentally. From a plethora of computational
methods, this work provides both microscopic insights and free-energy
profiles of the binding between uranyl and deprotonated glutardiamidoxime
(H<sub>2</sub>B) for which experimental structural information is
not available, despite H<sub>2</sub>B being an important model ligand
with an open-chain conformation for understanding aqueous uranium
extraction chemistry. In our molecular dynamics (MD) simulations,
we explicitly accounted for the water solvent as well as the Na<sup>+</sup> and Cl<sup>â</sup> ions. We found that hydrogen bonding
plays a critical role in dictating the binding configurations of B<sup>2â</sup> and HB<sup>â</sup> with uranyl. Simulated
free energies of sequential ligand binding to form UO<sub>2</sub>B,
[UO<sub>2</sub>B<sub>2</sub>]<sup>2â</sup>, and [UO<sub>2</sub>(HB)ÂB]<sup>â</sup> show very good agreement with the experimental
values, lending support to our structural insights. The potential
of mean force simulations showed the common feature of an important
intermediate state where one end of the ligand binds to uranyl while
the other end is solvated in water. Bringing the loose end of the
ligand to bind with uranyl has a free-energy barrier of 15â25
kJ/mol. The present work shows that the combined simulation approach
can reveal key structural and thermodynamic insights toward a better
understanding of aqueous complexation chemistry for uranium extraction
from the sea
Site Partition: Turning One Site into Two for Adsorbing CO<sub>2</sub>
We
propose the concept of site partition to explain the role of
guest molecules in increasing CO<sub>2</sub> uptake in metalâorganic
frameworks and to design new covalent porous materials for CO<sub>2</sub> capture. From grand canonical Monte Carlo simulations of
CO<sub>2</sub> sorption in the recently synthesized CPM-33 MOFs, we
show that guest insertion divides one open metal site into two relatively
strong binding sites, hence dramatically increasing CO<sub>2</sub> uptake. Further, we extend the site partition concept to covalent
organic frameworks with large free volume. After insertion of a designed
geometry-matching guest, we show that the volumetric uptake of CO<sub>2</sub> doubles. Therefore, the concept of site partition can be
used to engineer the pore space of nanoporous materials for higher
gas uptake
UranylâGlutardiamidoxime Binding from First-Principles Molecular Dynamics, Classical Molecular Dynamics, and Free-Energy Simulations
Exploring the structural
interplay of ligands with uranyl can provide
important knowledge for technology advances in uranium extraction
from seawater. However, obtaining such chemical information is not
an easy endeavor experimentally. From a plethora of computational
methods, this work provides both microscopic insights and free-energy
profiles of the binding between uranyl and deprotonated glutardiamidoxime
(H<sub>2</sub>B) for which experimental structural information is
not available, despite H<sub>2</sub>B being an important model ligand
with an open-chain conformation for understanding aqueous uranium
extraction chemistry. In our molecular dynamics (MD) simulations,
we explicitly accounted for the water solvent as well as the Na<sup>+</sup> and Cl<sup>â</sup> ions. We found that hydrogen bonding
plays a critical role in dictating the binding configurations of B<sup>2â</sup> and HB<sup>â</sup> with uranyl. Simulated
free energies of sequential ligand binding to form UO<sub>2</sub>B,
[UO<sub>2</sub>B<sub>2</sub>]<sup>2â</sup>, and [UO<sub>2</sub>(HB)ÂB]<sup>â</sup> show very good agreement with the experimental
values, lending support to our structural insights. The potential
of mean force simulations showed the common feature of an important
intermediate state where one end of the ligand binds to uranyl while
the other end is solvated in water. Bringing the loose end of the
ligand to bind with uranyl has a free-energy barrier of 15â25
kJ/mol. The present work shows that the combined simulation approach
can reveal key structural and thermodynamic insights toward a better
understanding of aqueous complexation chemistry for uranium extraction
from the sea
Nitrogen-Doped Mesoporous Carbon for Carbon Capture â A Molecular Simulation Study
Using molecular simulation, we investigate the effect of nitrogen
doping on adsorption capacity and selectivity of CO<sub>2</sub> versus
N<sub>2</sub> in model mesoporous carbon. We show that nitrogen doping
greatly enhances CO<sub>2</sub> adsorption capacity; with a 7 wt %
dopant concentration, the adsorption capacity at 1 bar and 298 K increases
from 3 to 12 mmol/g (or 48% uptake by weight). This great enhancement
is due to the preferred interaction between CO<sub>2</sub> and the
electronegative nitrogen. The nitrogen doping coupled with the mesoporosity
also leads to a much higher working capacity for adsorption of the
CO<sub>2</sub>/N<sub>2</sub> mixture in nitrogen-doped mesoporous
carbon. In addition, the CO<sub>2</sub>/N<sub>2</sub> selectivity
is almost 5 times greater than in nondoped carbon at ambient conditions.
This work indicates that nitrogen doping is a promising strategy to
create mesoporous carbons for high-capacity, selective carbon capture
Solubility of Gases in a Common Ionic Liquid from Molecular Dynamics Based Free Energy Calculations
Solubility of eight common gases
in the 1-ethyl-3-methylimidazolium
bisÂ(trifluoromethylsulfonyl)Âimide, [emim]Â[Tf<sub>2</sub>N], ionic
liquid was systematically investigated based on alchemical free energy
calculations from molecular dynamics simulations. The simulated solubilities
and trend in terms of Henryâs law constants agree qualitatively
with the experiment. Polar gases such as H<sub>2</sub>S and nonpolar
gases with a large quadrupole moment such as CO<sub>2</sub> show the
highest solubility, while nonpolar gases of small quadrupole moments
(such as N<sub>2</sub> and H<sub>2</sub>) are least soluble. The soluteâionic
liquid interaction correlates with the observed solubility order.
We also examined the temperature dependence of solubility for CO<sub>2</sub> and N<sub>2</sub> and found that the CO<sub>2</sub> solvation
in IL is exothermic with a negative solvation enthalpy, while the
N<sub>2</sub> solvation is endothermic, in agreement with the experiment
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