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 (τ_S) and longer lifetime (τ_L). 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 Cd₃S₅ 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 CoO₅ Square-Based Pyramids with IS-Co³⁺ in Li-Overstoichiometric LiCoO₂

LiCoO₂, 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) Co³⁺ 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 Li₄Co₂O₅ 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 Co³⁺ 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 Co–Co 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 LiCoO₂, 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 Co³⁺ defects in Li-overstoichiometric LiCoO₂

Directly Coating a Multifunctional Interlayer on the Cathode via Electrospinning for Advanced Lithium–Sulfur Batteries

The lithium–sulfur 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^–1 and maintains the reversible capacity of 1030 mAh g^–1 with capacity fading of 0.05% per cycle at 200 mA g^–1 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 ∼1.44 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_2–<i>x</i>Se Nanorods

In this study, one-dimensional Cu_2–<i>x</i>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_2–<i>x</i>Se electrodes express outstanding electrochemical properties. The initial discharge capacity is 149.3 mA h g^–1 at a current density of 100 mA g^–1, and the discharge capacity can remain at 106.2 mA h g^–1 after 400 cycles. Even at a high current density of 2000 mA g^–1, the discharge capacity of the Cu_2–<i>x</i>Se electrode still remains at 62.8 mA h g^–1, showing excellent rate performance. Owing to the excellent electronic conductivity and one-dimensional structure of Cu_2–<i>x</i>Se, the Cu_2–<i>x</i>Se electrodes manifest fast Na⁺ ion diffusion rate. Moreover, detailed Na⁺ 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 ‘grafting to’ and ‘grafting from’ strategies in combination with reversible chain transfer and click chemistry. The use of the ‘grafting to’ 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 ‘grafting from’ 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 ‘grafting to’ and ‘grafting from’ 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 Al₂O₃ 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 ALD–VC 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^–1 after 200 cycles running at 200 mA g^–1, and the bare anode without VC showed a capacity of 400 mAh g^–1 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 Co³⁺ and Mn⁴⁺ in P2-Na_2/3Co_2/3Mn_1/3O₂ As Evidenced by X‑ray Absorption Spectroscopy during Electrochemical Sodium Intercalation

Sodium intercalation in P2-Na_2/3Co_2/3Mn_1/3O₂ (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, Co³⁺/Co²⁺ and Mn⁴⁺/Mn³⁺, are both involved in the reaction

Facile Synthesis of [101]-Oriented Rutile TiO₂ Nanorod Array on FTO Substrate with a Tunable Anatase–Rutile Heterojunction for Efficient Solar Water Splitting

Generating a sustainable energy source through photoelectrochemical (PEC) water splitting requires a suitable photocatalyst. A [101]-oriented rutile TiO₂ 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 NRs’ surface is confirmed by transmission electron microscopy and tip-enhanced Raman spectroscopy. Solar water-splitting performances of anatase–rutile heterojunction with low energy (101) and high energy (001) rutile facets are compared. The low energy (101) facet rutile–anatase heterojunction shows higher photoconversion efficiency of 1.39% at 0.49 V_RHE than the high energy (001) facet rutile–anatase heterojunction (0.37% at 0.73 V_RHE). The mechanism for enhanced photocatalytic activity of the low energy (101) facet rutile–anatase 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 1‑(Trimethylsilyl)imidazole as New Electrolyte Additive To Suppress LiPF₆ Decomposition

Trace water content in the electrolyte causes the degradation of LiPF₆, 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 N–Si bond, 1-(trimethylsilyl)­imidazole (1-TMSI) is proposed as a novel water scavenging electrolyte additive to suppress LiPF₆ 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 LiPF₆ 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 LiPF₆ 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 solid–electrolyte 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