16 research outputs found
Surface Related Emission in CdS Quantum Dots. DFT Simulation Studies
In general, organic capping molecules are applied to passivate the surface of semiconductor nanomaterials to modulate the optical properties of these nanostructures. In this work, two alkylamines (<i>n</i>-butylamine (<i>n</i>-BA) and <i>n</i>-hexylamine (<i>n</i>-HA)) and oleic acid (OA) were used to modify the surface of moderately high luminescent CdS quantum dots (QDs). From the photoluminescence (PL) spectra and the quantum yield (QY) analyses, we observed that the PL QY of the CdS QDs decreased after introduction of the alkylamine and oleic acid molecules. The PL decay kinetics for these CdS-capping molecule systems were followed by time-resolved photoluminescence (TRPL), and the spectra were analyzed in terms of a biexponential model identifying two lifetime values, shorter lifetime (Ļ<sub>S</sub>) and longer lifetime (Ļ<sub>L</sub>). Compared to bare CdS QDs, for the CdS QDs surface modified by alkylamine or fatty acid, both the shorter and the longer excited state lifetimes were decreased; the fractional contribution by the longer-lifetime component became reduced and the shorter-lifetime component accounted for most of the total PL. Density function theory (DFT) simulation was employed using a Cd<sub>3</sub>S<sub>5</sub> cluster to model the adsorption of organics to calculate the binding energy and the charge on Cd and S of CdS. By comparing the elemental charges of the bare CdS with those of the CdS modified by the organic molecules, it is suggested that <i>n</i>-BA, <i>n</i>-HA, and OA could decrease the surface related radiative charge-recombination process and the PL-QY of the CdS QDs
DFT+U Calculations and XAS Study: Further Confirmation of the Presence of CoO<sub>5</sub> Square-Based Pyramids with IS-Co<sup>3+</sup> in Li-Overstoichiometric LiCoO<sub>2</sub>
LiCoO<sub>2</sub>, one of the major
positive electrode materials
for Li-ion batteries, can be synthesized with excess Li. Previous
experimental work suggested the existence of intermediate spin (IS)
Co<sup>3+</sup> ions in square-based pyramids to account for the defect
in this material. We present here a theoretical study based on density
functional theory (DFT) calculations together with an X-ray absorption
spectroscopy (XAS) experimental study. In the theoretical study, a
hypothetical Li<sub>4</sub>Co<sub>2</sub>O<sub>5</sub> material, where
all the Co ions are in pyramids, was initially considered as a model
material. Using DFT+U, the intermediate spin state of the Co<sup>3+</sup> ions is found stable for U values around 1.5 eV. The crystal and
electronic structures are studied in detail, showing that the defect
must actually be considered as a pair of such square-based pyramids,
and that CoāCo bonding can explain the position of Co in the
basal plane. Using a supercell corresponding to more diluted defects
(as in the actual material), the calculations show that the IS state
is also stabilized. In order to investigate experimentally the change
in the electronic structure in the Li-overstoichiometric LiCoO<sub>2</sub>, we used X-ray absorption near edge structure (XANES) spectroscopy
and propose an interpretation of the O Kedge spectra based on the
DFT+U calculations, that fully supports the presence of pairs of intermediate
spin state Co<sup>3+</sup> defects in Li-overstoichiometric LiCoO<sub>2</sub>
Directly Coating a Multifunctional Interlayer on the Cathode via Electrospinning for Advanced LithiumāSulfur Batteries
The lithiumāsulfur
battery is considered as a prospective candidate for a high-energy-storage
system because of its high theoretical specific capacity and energy.
However, the dissolution and shutter of polysulfides lead to low active
material utilization and fast capacity fading. Electrospinning technology
is employed to directly coat an interlayer composed of polyacrylonitrile
(PAN) and nitrogen-doped carbon black (NC) fibers on the cathode.
Benefiting from electrospinning technology, the PAN-NC fibers possess
good electrolyte infiltration for fast lithium-ion transport and great
flexibility for adhering on the cathode. The NC particles provide
good affinity for polysufides and great conductivity. Thus, the polysulfides
can be trapped on the cathode and reutilized well. As a result, the
PAN-NC-coated sulfur cathode (PAN-NC@cathode) exhibits the initial
discharge capacity of 1279 mAh g<sup>ā1</sup> and maintains
the reversible capacity of 1030 mAh g<sup>ā1</sup> with capacity
fading of 0.05% per cycle at 200 mA g<sup>ā1</sup> after 100
cycles. Adopting electrospinning to directly form fibers on the cathode
shows a promising application
In Situ Diffuse Reflectance Infrared Fourier-Transform Spectroscopy Investigation of Fluoroethylene Carbonate and Lithium Difluorophosphate Dual Additives in SEI Formation over Cu Anode
The synergetic effect of fluoroethylene carbonate (FEC)
and lithium
difluorophosphate (LiPO2F2) dual additives on
the cycling stability of lithium metal batteries has been previously
reported. This study applies in situ diffuse reflectance infrared
Fourier-transform spectroscopy (DRIFTS) to examine the impact of these
two additives on SEI species formation over Cu anode using a base
electrolyte of LiPF6 in ethylene carbonate (EC) and diethyl
carbonate (DEC). The results indicate that all electrolyte components
and additives can be electrochemically reduced over the Cu anode following
a potential sequence of LiPO2F2 > FEC >
EC >
DEC. The results illustrate that LiPF6 likely interacts
with the Cu anode upon contact, resulting in LixPFy, which can lead to a reduction
peak at ā¼1.44 V in CV. With the base electrolyte, reduced species
from LixPFy lead to the formation of alkyl phosphorus fluorides (RPF), which
can be suppressed by the presence of FEC and/or LiPO2F2. Similar to previous reports, FEC reduction in the 1st lithiation
cycle leads to the continuous formation of poly(FEC), while EC is
electrochemically reduced to (CH2OCO2Li)2 and Li2CO3 and DEC is reduced to CH3CH2OCO2Li and Li2CO3. With only the LiPO2F2 additive, the redox
of LiPO2F2 can be found in CV with LixPOy as the possible reduced
product. In addition, Li2CO3 formation from
EC and DEC reduction was relatively suppressed by the presence of
LiPO2F2. The simultaneous presence of the FEC
additive can suppress the redox of LiPO2F2 and
partly the decomposition of LiPF6 likely via the preferential
adsorption of FEC on Cu. Similar DRIFTS observations are found over
the Li anode. The electrolyte with dual additives demonstrates a possible
advantage from poly(FEC) and LixPOy species formation, suppressing the reduction
of LixPFy,
EC, and DEC though not completely
Investigation of the Na Storage Property of One-Dimensional Cu<sub>2ā<i>x</i></sub>Se Nanorods
In
this study, one-dimensional Cu<sub>2ā<i>x</i></sub>Se nanorods synthesized by a simple water evaporation-induced self-assembly
approach are served as the anode material for Na-ion batteries for
the first time. Cu<sub>2ā<i>x</i></sub>Se electrodes
express outstanding electrochemical properties. The initial discharge
capacity is 149.3 mA h g<sup>ā1</sup> at a current density
of 100 mA g<sup>ā1</sup>, and the discharge capacity can remain
at 106.2 mA h g<sup>ā1</sup> after 400 cycles. Even at a high
current density of 2000 mA g<sup>ā1</sup>, the discharge capacity
of the Cu<sub>2ā<i>x</i></sub>Se electrode still
remains at 62.8 mA h g<sup>ā1</sup>, showing excellent rate
performance. Owing to the excellent electronic conductivity and one-dimensional
structure of Cu<sub>2ā<i>x</i></sub>Se, the Cu<sub>2ā<i>x</i></sub>Se electrodes manifest fast Na<sup>+</sup> ion diffusion rate. Moreover, detailed Na<sup>+</sup> insertion/extraction
mechanism is further investigated by ex situ measurements and theoretical
calculations
Versatile Grafting Approaches to Functionalizing Individually Dispersed Graphene Nanosheets Using RAFT Polymerization and Click Chemistry
Developing powerful and reliable strategies to covalently
functionalize
graphene for efficient grafting and achieving precise interface control
remains challenging due to the strong interlayer cohesive energy and
the surface inertia of graphene. Here, we present versatile and efficient
grafting strategies to functionalize graphene nanosheets. An alkyne-bearing
graphene core was used to prepare polymer-functionalized graphene
using āgrafting toā and āgrafting fromā
strategies in combination with reversible chain transfer and click
chemistry. The use of the āgrafting toā approach allows
full control over limited length grafted polymer chains, while permitting
a high grafting density to a single graphene face, resulting in good
solubility and processability. The āgrafting fromā approach
offers complementary advantages, such as the grafting of high molecular
weight polymer chains and a better coverage ratio on the graphene
surface; however, the extra steps introduced, the presence of initiating
groups, and difficulty in controlling the grafted polymer lead to
decreased processability. Various types of polymer chains have been
successful covalently tethered to graphene nanosheets using these
two approaches, producing various molecular brushes with multifunctional
arms resulting in water-soluble, oil-soluble, acidic, basic, polar,
apolar, and variously functionalized polymers. This work describes
versatile methodologies, using the āgrafting toā and
āgrafting fromā approaches, for the preparation of individually
dispersed graphene nanosheets having the desirable properties described
Stabilizing Nanosized Si Anodes with the Synergetic Usage of Atomic Layer Deposition and Electrolyte Additives for Li-Ion Batteries
A substantial increase in charging
capacity over long cycle periods
was made possible by the formation of a flexible weblike network via
the combination of Al<sub>2</sub>O<sub>3</sub> atomic layer deposition
(ALD) and the electrolyte additive vinylene carbonate (VC). Transmission
electron microscopy shows that a weblike network forms after cycling
when ALD and VC were used in combination that dramatically increases
the cycle stability for the Si composite anode. The ALDāVC
combination also showed reduced reactions with the lithium salt, forming
a more stable solid electrolyte interface (SEI) absent of fluorinated
silicon species, as evidenced by X-ray photoelectron spectroscopy. Although the bare Si composite
anode showed only an improvement from a 56% to a 45% loss after 50
cycles, when VC was introduced, the ALD-coated Si anode showed an
improvement from a 73% to a 11% capacity loss. Furthermore, the anode
with the ALD coating and VC had a capacity of 630 mAh g<sup>ā1</sup> after 200 cycles running at 200 mA g<sup>ā1</sup>, and the
bare anode without VC showed a capacity of 400 mAh g<sup>ā1</sup> after only 50 cycles. This approach can be extended to other Si
systems, and the formation of this SEI is dependent on the thickness
of the ALD that affects both capacity and stability
Simultaneous Reduction of Co<sup>3+</sup> and Mn<sup>4+</sup> in P2-Na<sub>2/3</sub>Co<sub>2/3</sub>Mn<sub>1/3</sub>O<sub>2</sub> As Evidenced by Xāray Absorption Spectroscopy during Electrochemical Sodium Intercalation
Sodium intercalation in P2-Na<sub>2/3</sub>Co<sub>2/3</sub>Mn<sub>1/3</sub>O<sub>2</sub> (obtained
by a coprecipitation method) was
investigated by ex situ and in situ X-ray absorption spectroscopy.
The electronic transitions at the O K-edge and the charge compensation
mechanism, during the sodium intercalation process, were elucidated
by combining Density Function Theory (DFT) calculations and X-ray
absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge
moves to higher energy while the integrated intensity dramatically
decreases, indicating that the population of holes in O 2p states
is reduced with increasing numbers of sodium ions. From the K-edge
and L-edge observations, the oxidation states of pristine Co and Mn
were determined to be +III and +IV, respectively. The absorption energy
shifts to lower positions during the discharging process for both
the Co and the Mn edges, suggesting that the redox pairs, that is,
Co<sup>3+</sup>/Co<sup>2+</sup> and Mn<sup>4+</sup>/Mn<sup>3+</sup>, are both involved in the reaction
Facile Synthesis of [101]-Oriented Rutile TiO<sub>2</sub> Nanorod Array on FTO Substrate with a Tunable AnataseāRutile Heterojunction for Efficient Solar Water Splitting
Generating
a sustainable energy source through photoelectrochemical
(PEC) water splitting requires a suitable photocatalyst. A [101]-oriented
rutile TiO<sub>2</sub> nanorod (NR) array in heterojunction with anatase
on a fluorine-doped tin oxide (FTO) substrate is successfully prepared
using a facile single-step hydrothermal process. The presence of anatase
phase over the predominant rutile NRsā surface is confirmed
by transmission electron microscopy and tip-enhanced Raman spectroscopy.
Solar water-splitting performances of anataseārutile heterojunction
with low energy (101) and high energy (001) rutile facets are compared.
The low energy (101) facet rutileāanatase heterojunction shows
higher photoconversion efficiency of 1.39% at 0.49 V<sub>RHE</sub> than the high energy (001) facet rutileāanatase heterojunction
(0.37% at 0.73 V<sub>RHE</sub>). The mechanism for enhanced photocatalytic
activity of the low energy (101) facet rutileāanatase heterojunction
has been proposed. The role of NaCl in tuning the anatase portion,
morphology, and PEC water-splitting performance has also been studied
Improved Interfacial Properties of MCMB Electrode by 1ā(Trimethylsilyl)imidazole as New Electrolyte Additive To Suppress LiPF<sub>6</sub> Decomposition
Trace water content
in the electrolyte causes the degradation of LiPF<sub>6</sub>, and
the decomposed products further react with water to produce HF, which
alters the surface of anode and cathode. As a result, the reaction
of HF and the deposition of decomposed products on electrode surface
cause significant capacity fading of cells. Avoiding these phenomena
is crucial for lithium ion batteries. Considering the Lewis-base feature
of the NāSi bond, 1-(trimethylsilyl)Āimidazole (1-TMSI)
is proposed as a novel water scavenging electrolyte additive to suppress
LiPF<sub>6</sub> decomposition. The scavenging ability of 1-TMSI and
beneficiary interfacial chemistry between the MCMB electrode and electrolyte
are studied through a combination of experiments and density functional
theory (DFT) calculations. NMR analysis indicated that LiPF<sub>6</sub> decomposition by water was effectively suppressed in the presence
of 0.2 vol % 1-TMSI. XPS surface analysis of MCMB electrode showed
that the presence of 1-TMSI reduced deposition of ionic insulating
products caused by LiPF<sub>6</sub> decomposition. The results showed
that the cells with 1-TMSI additive have better performance than the
cell without 1-TMSI by facilitating the formation of solidāelectrolyte
interphase (SEI) layer with better ionic conductivity. It is hoped
that the work can contribute to the understanding of SEI and the development
of electrolyte additives for prolonged cycle life with improved performance