6 research outputs found
Organization of Alkane Amines on a Gold Surface: Structure, Surface Dipole, and Electron Transfer
Surface
molecular self-assembly is a fast advancing field with
broad applications in molecular electronics, sensing and advanced
materials. Although a large number of practical systems utilize alkanethiols,
there is increasing interest in alkylamine self-assembled monolayers
(SAMs). In this article, the molecular and electronic structure of
alkylamine SAMs on Au surfaces was studied. It was found that amine-terminated
alkanes self-assemble, forming a compact layer with the amine headgroup
interacting directly with the Au surface and the hydrocarbon backbone
tilted by around 30Ā° with respect to the surface normal. The
dense layers formed substantially decrease electron tunneling across
the metal/solution interface and form a dipole layer with positive
charges residing at the monolayer/vacuum interface
Molecular and Electronic Structure of Self-Assembled Monolayers Containing Ruthenium(II) Complexes on Gold Surfaces
RuĀ(II) bipyridyl complexes were covalently
bonded to self-assembled monolayers (SAM) on Au surfaces. Their molecular
and electronic structure was studied by means of polarization modulation
infrared reflection absorption spectroscopy (PM-IRRAS), photoelectron
spectroscopies, scanning tunneling microscopy (STM) and density functional
theory (DFT) calculations. We found that attaching the Ru complex
to the SAM does not cause great modifications to its molecular structure,
which retains the alkyl chain 30 deg tilted with respect to the surface
normal. Furthermore, the Ru center is located 20 Ć
away from
the metal surface, i.e., at a sufficient distance to prevent direct
electronic interaction with the substrate. Indeed the electronic structure
of the Ru complex is similar to that of the free molecule with a HOMO
molecular orbital mainly based on the Ru center located 2.1 eV below
the Fermi edge and the LUMO molecular orbital based on the bipyridine
groups located 1 eV above the Fermi level
Self-Assembled Monolayers of NH<sub>2</sub>āTerminated Thiolates: Order, p<i>K</i><sub>a</sub>, and Specific Adsorption
Self-assembled
monolayers (SAMs) of amino-terminated alkanethiols
on Au were characterized by a combination of electrochemical (LSV,
CV, and EIS) and spectroscopic (XPS and SER) techniques. Clear correlations
were obtained between the apparent surface p<i>K</i><sub>a</sub> values determined by impedimetric titrations and order parameters
such as the content of trans conformers in the SAMs. These results
contrast with previous studies that exhibit dispersions of up to 6
pH units in the reported p<i>K</i><sub>a</sub> values. In
addition, we determined that inorganic and organic phosphate species
bind specifically to these SAMs mediating adsorption and heterogeneous
electron transfer of positively charged macromolecules such as cytochrome <i>c</i>
Structure, Dynamics, and Phase Behavior of Water in TiO<sub>2</sub> Nanopores
Mesoporous titania is a highly studied material due to
its energy
and environment-related applications, which depend on its tailored
surface and electronic properties. Understanding the behavior of water
in titania pores is a central issue for practical purposes in photocatalysis,
solar cells, bone implants, or optical sensors. In particular, the
mechanisms of capillary condensation of water in titania mesopores
and the organization and mobility of water as a function of pore filling
fraction are not yet known. In this work, molecular dynamics simulations
of water confined in TiO<sub>2</sub>-rutile pores of diameters 1.3,
2.8, and 5.1 nm were carried out at various water contents. Water
density and diffusion coefficients were obtained as a function of
the distance from the surface. The proximity to the interface affects
density and diffusivity within a distance of around 10 Ć
from
the walls, beyond which all properties tend to converge. The densities
of the confined liquid in the 2.8 and the 5.1 nm pores decrease, respectively,
7% and 4% with respect to bulk water. This decrease causes the water
translational mobility in the center of the 2.8 nm pore to be appreciably
larger than in bulk. Capillary condensation takes place in equilibrium
for a filling of 71% in the 2.8 nm pore and in conditions of high
supersaturation in the 5.1 nm pore, at a filling of 65%. In the former
case, the surface density increases uniformly with filling until condensation,
whereas in the larger nanopore, a cluster of water molecules develops
on a localized spot on the surface for fillings just below the transition.
No phase transition is detected in the smaller pore. For all the systems
studied, the first monolayer of water is strongly immobilized on the
interface, thus reducing the accessible or effective diameter of the
pore by around 0.6 nm. As a consequence, the behavior of water in
these pores turns out to be comparable to its behavior in less hydrophilic
pores of smaller size
Electrochemical and Diffusional Investigation of Na<sub>2</sub>Fe<sup>II</sup>PO<sub>4</sub>F Fluorophosphate Sodium Insertion Material Obtained from Fe<sup>III</sup> Precursor
Sodium iron fluorophosphate
(Na<sub>2</sub>Fe<sup>II</sup>PO<sub>4</sub>F) was synthesized by
economic solvothermal combustion technique
using Fe<sup>III</sup> precursors, developing one-step carbon-coated
homogeneous product. Synchrotron diffraction and MoĢssbauer
spectroscopy revealed the formation of single-phase product assuming
an orthorhombic structure (s.g. <i>Pbcn</i>) with Fe<sup>II</sup> species. This Fe<sup>III</sup> precursor derived Na<sub>2</sub>Fe<sup>II</sup>PO<sub>4</sub>F exhibited reversible Na<sup>+</sup> (de)Āintercalation with discharge capacity of 100 mAh/g at
a rate of C/10 involving flat Fe<sup>III</sup>/Fe<sup>II</sup> redox
plateaus located at 2.92 and 3.05 V (vs Na/Na<sup>+</sup>). It delivered
good cycling stability and rate kinetics at room temperature. The
stability of Na<sub>2</sub>FePO<sub>4</sub>F cathode was further verified
by electrochemical impedance spectroscopy at different stages of galvanostatic
analysis. Bond valence site energy (BVSE) calculations revealed the
existence of 2-dimensional Na<sup>+</sup> percolation pathways in
the <i>aāc</i> plane with a moderate migration barrier
of 0.6 eV. Combustion synthesized Na<sub>2</sub>Fe<sup>II</sup>PO<sub>4</sub>F forms an economically viable sodium battery material. Although
the capacity of this cathode is relatively low, this study continues
systematic work, which attempts to broaden the scope of reversible
sodium insertion materials
Improving Energy Density and Structural Stability of Manganese Oxide Cathodes for Na-Ion Batteries by Structural Lithium Substitution
We
report excellent cycling performance for P2āNa<sub>0.6</sub>Li<sub>0.2</sub>Mn<sub>0.8</sub>O<sub>2</sub>, an auspicious cathode
material for sodium-ion batteries. This material, which contains mainly
Mn<sup>4+</sup>, exhibits a long voltage plateau on the first charge,
similar to that of high-capacity lithium and manganese-rich metal
oxides. Electrochemical measurements, X-ray diffraction, and elemental
analysis of the cycled electrodes suggest an activation process that
includes the extraction of lithium from the material. The āactivatedā
material delivers a stable, high specific capacity up to ā¼190
mAh/g after 100 cycles in the voltage window between 4.6ā2.0
V versus Na/Na<sup>+</sup>. DFT calculations locate the energy states
of oxygen atoms near the Fermi level, suggesting the possible contribution
of oxide ions to the redox process. The addition of Li to the lattice
improves structural stability compared to many previously reported
sodiated transition-metal oxide electrode materials, by inhibiting
the detrimental structural transformation ubiquitously observed with
sodium manganese oxides during cycling. This research demonstrates
the prospect of intercalation materials for Na-ion battery technology
that are active based on both cationic and anionic redox moieties