Ultrathin MoS2 Nanosheets as Anode Materials for Sodium‐Ion Batteries with Superior Performance

Few‐layer MoS₂ nanosheets are successfully synthesized using a simple and scalable ultrasonic exfoliation technique. The thicknesses of the MoS₂ nanosheets ares about 10 nm as measured by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The unique nanosheet architecture renders the high‐rate transportation of sodium ions due to the short diffusion paths provided by ultrathin thickness and the large interlayer space within the MoS₂ crystal structure (d₍₀₀₂₎ = 6.38 Å). When applied as anode materials in sodium‐ion batteries, MoS₂ nanosheets exhibit a high, reversible sodium storage capacity and excellent cyclability. The MoS₂ nanosheets also demonstrate good electrochemical performance at high current densities.     

MoS2/Graphene Nanosheets from Commercial Bulky MoS2 and Graphite as Anode Materials for High Rate Sodium‐Ion Batteries

Tuning heterointerfaces between hybrid phases is a very promising strategy for designing advanced energy storage materials. Herein, a low‐cost, high‐yield, and scalable two‐step approach is reported to prepare a new type of hybrid material containing MoS₂/graphene nanosheets prepared from ball‐milling and exfoliation of commercial bulky MoS₂ and graphite. When tested as an anode material for a sodium‐ion battery, the as‐prepared MoS₂/graphene nanosheets exhibit remarkably high rate capability (284 mA h g^?1 at 20 A g^?1 (≈30C) and 201 mA h g^?1 at 50 A g^?1 (≈75C)) and excellent cycling stability (capacity retention of 95% after 250 cycles at 0.3 A g^?1). Detailed experimental measurements and density functional theory calculation reveal that the functional groups in 2D MoS₂/graphene heterostructures can be well tuned. The impressive rate capacity of the as‐prepared MoS₂/graphene hybrids should be attributed to the heterostructures with a low degree of defects and residual oxygen containing groups in graphene, which subsequently improve the electronic conductivity of graphene and decrease the Na⁺ diffusion barrier at the MoS₂/graphene interfaces in comparison with the acid treated one.     

Solvent Effect on the Photolysis of Riboflavin

The kinetics of photolysis of riboflavin (RF) in water (pH 7.0) and in organic solvents (acetonitrile, methanol, ethanol, 1-propanol, 1-butanol, ethyl acetate) has been studied using a multicomponent spectrometric method for the assay of RF and its major photoproducts, formylmethylflavin and lumichrome. The apparent first-order rate constants (k_obs) for the reaction range from 3.19 (ethyl acetate) to 4.61 × 10⁻³ min⁻¹ (water). The values of k_obs have been found to be a linear function of solvent dielectric constant implying the participation of a dipolar intermediate along the reaction pathway. The degradation of this intermediate is promoted by the polarity of the medium. This indicates a greater stabilization of the excited-triplet states of RF with an increase in solvent polarity to facilitate its reduction. The rate constants for the reaction show a linear relation with the solvent acceptor number indicating the degree of solute–solvent interaction in different solvents. It would depend on the electron-donating capacity of RF molecule in organic solvents. The values of k_obs are inversely proportional to the viscosity of the medium as a result of diffusion-controlled processes.KEY WORDS: dielectric constant, kinetics, photolysis, riboflavin, solvent effect, viscosity     

Exfoliation of Egyptian Blue and Han Blue,Two Alkali Earth Copper Silicate-based Pigments

In a visualized example of the ancient past connecting with modern times, we describe the preparation and exfoliation of CaCuSi₄O₁₀ and BaCuSi₄O₁₀, the colored components of the historic Egyptian blue and Han blue pigments. The bulk forms of these materials are synthesized by both melt flux and solid-state routes, which provide some control over the crystallite size of the product. The melt flux process is time intensive, but it produces relatively large crystals at lower reaction temperatures. In comparison, the solid-state method is quicker yet requires higher reaction temperatures and yields smaller crystallites. Upon stirring in hot water, CaCuSi₄O₁₀ spontaneously exfoliates into monolayer nanosheets, which are characterized by TEM and PXRD. BaCuSi₄O₁₀ on the other hand requires ultrasonication in organic solvents to achieve exfoliation. Near infrared imaging illustrates that both the bulk and nanosheet forms of CaCuSi₄O₁₀ and BaCuSi₄O₁₀ are strong near infrared emitters. Aqueous CaCuSi₄O₁₀ and BaCuSi₄O₁₀ nanosheet dispersions are useful because they provide a new way to handle, characterize, and process these materials in colloidal form.     

Scalable Water‐Based Production of Highly Conductive 2D Nanosheets with Ultrahigh Volumetric Capacitance and Rate Capability

Highly conductive and ultrathin 2D nanosheets are of importance for the development of portable electronics and electric vehicles. However, scalable production and rational design for highly electronic and ionic conductive 2D nanosheets still remain a challenge. Herein, an industrially adoptable fluid dynamic exfoliation process is reported to produce large quantities of ionic liquid (IL)‐functionalized metallic phase MoS₂ (m‐MoS₂) and defect‐free graphene (Gr) sheets. Hybrid 2D–2D layered films are also fabricated by incorporating Gr sheets into compact m‐MoS₂ films. The incorporated IL functionalities and Gr sheets prevent aggregation and restacking of the m‐MoS₂ sheets, thereby creating efficient and rapid ion and electron pathways in the hybrid films. The hybrid film with a high packing density of 2.02 g cm^?3 has an outstanding volumetric capacitance of 1430.5 F cm^?3 at 1 A g^?1 and an extremely high rate capability of 80% retention at 1000 A g^?1. The flexible supercapacitor assembled using a polymer‐gel electrolyte exhibits excellent resilience to harsh electrochemical and mechanical conditions while maintaining an impressive rate performance and long cycle life. Successful achievement of an ultrahigh volumetric energy density (1.14 W h cm^?3) using an organic electrolyte with a wide cell voltage of ≈3.5 V is demonstrated.     

Vertically Aligned MoS2 Nanosheets Patterned on Electrochemically Exfoliated Graphene for High‐Performance Lithium and Sodium Storage

Molybdenum disulfide (MoS₂) has been recognized as a promising anode material for high‐energy Li‐ion (LIBs) and Na‐ion batteries (SIBs) due to its apparently high capacity and intriguing 2D‐layered structure. The low conductivity, unsatisfied mechanical stability, and limited active material utilization are three key challenges associated with MoS₂ electrodes especially at high current rates and mass active material loading. Here, vertical MoS₂ nanosheets are controllably patterned onto electrochemically exfoliated graphene (EG). Within the achieved hierarchical architecture, the intimate contact between EG and MoS₂ nanosheets, interconnected network, and effective exposure of active materials by vertical channels simultaneously overcomes the above three problems, enabling high mechanical integrity and fast charge transport kinetics. Serving as anode material for LIBs, EG‐MoS₂ with 95 wt% MoS₂ content delivered an ultrahigh‐specific capacity of 1250 mA h g^?1 after 150 stable cycles at 1 A g^?1, which is among the highest values in all reported MoS₂ electrodes, and excellent rate performance (970 mA h g^?1 at 5 A g^?1). Moreover, impressive cycling stability (509 mA h g^?1 at 1 A g^?1 after 250 cycles) and rate capability (423 mA h g^?1 at 2 A g^?1) were also achieved for SIBs. The area capacities reached 1.27 and 0.49 mA h cm^?2 at ≈1 mA cm^?2 for LIBs and SIBs, respectively. This work may inspire the development of new 2D hierarchical structures for high efficiency energy storage and conversion.     

Effects of water miscible organic solvents on the activity and conformation of the Baeyer-Villiger monooxygenases from Thermobifida fusca and Acinetobacter calcoaceticus: a comparative study

A broader exploitation of enzymes in organic synthesis can be achieved by increasing their tolerance toward organic solvents. In this study, the stability and activity of Baeyer–Villiger monooxygenases from Thermobifida fusca (PAMO) and Acinetobacter sp. (CHMO) in the presence of water miscible organic solvents were compared. PAMO was more stable than CHMO. The concentration of solvent (v/v) at which it halved its activity (C₅₀) was 4‐ to 16‐fold higher than that observed for CHMO. For PAMO, the C₅₀ varied from 16% to 55% of solvent and followed the destabilizing order methanol < ethanol < 1,4‐dioxane < acetonitrile < trifluoroethanol. In the case of CHMO, the maximal C₅₀ was 7% with methanol and even lower with the other solvents. Therefore, methanol was the most tolerated solvent. In the case of PAMO, methanol induced a significant increase of enzyme activity (up to fivefold), which was optimal at 20% (v/v) solvent. Only minor spectral variations were observed with PAMO in 20% methanol, suggesting that the increase of activity observed in this condition is not due to marked conformational changes. Fluorescence and circular dichroism analyses showed that the lower stability of CHMO toward organic solvent correlates with a more pronounced destructive effect on its secondary and tertiary structure. A possible rationale for the higher stability of PAMO could be inferred from inspection of the PAMO and CHMO (two enzymes of similar size) structure, which revealed a higher (up to twofold) number of ionic bridges in PAMO with respect to CHMO. Biotechnol. Bioeng. 2011; 108:491–499. © 2010 Wiley Periodicals, Inc.     

Organic solvents for bioorganic synthesis

Summary The influence of solvents on enzymatic activity and stability was investigated. As a model reaction the -chymotrypsin-catalyzed esterification of N-acetyl-l-phenylalanine with ethanol was used. The enzyme was adsorbed on porous glass beads and used in various solvents. Small amounts of water were added to increase the enzymatic activity. These enzyme preparations obeyed. Michaelis-Menten kinetics. K _m,app decreased slightly with the log P value of the solvent while V _app increased markedly with the log P value. Log P values were also useful for generalizing the influence of solvents on enzyme stability. The enzyme preparations showed a markedly higher thermostability in dry solvents having log P values >0.7 than in less hydrophobic solvents.Also the operational stability was better in the more hydrophobic solvents. The amount of water added to the enzyme preparations greatly influenced the initial reaction rates. For some solvents optimal water contents were determined. The thermostability decreased with increasing water content.The observations are summarized in the conclusion that more hydrophobic solvents are preferable to less hydrophobic ones. The log P value gives a good guidance when selecting an organic solvent for enzymatic conversions.     

Development of MoS2–CNT Composite Thin Film from Layered MoS2 for Lithium Batteries

Layered MoS₂ prepared by liquid‐phase exfoliation has been blended with single‐walled carbon nanotubes (SWNTs) to form novel composite thin films for lithium battery applications. The films were formed by vacuum filtration of blended dispersions onto nitrocellulose membranes. The resulting composite films were transferred onto Cu foil electrodes via a facile filtration/wet transfer technique from nitrocellulose membranes. The morphology of the film was characterised by field emission scanning electron microscopy, which suggests that the MoS₂‐SWNT composite film shows good adherence to the Cu foil substrate. The MoS₂‐SWNT composite thin films show strong electrochemical performance at different charge‐discharge rates. The capacity of a MoS₂‐SWNT composite film with thickness of 1 μm is approximately 992 mAh g^?1 after 100 cycles. The morphology study showed that the MoS₂‐SWNT thin film retains structural integrity after 100 cycles, while the MoS₂ thin film without SWNTs displays significant cracking. In addition, the novel composite thin film preparation and transfer protocols developed in this study could be extended to the preparation of various layered‐material‐based composite films, with the potential for new device designs for energy applications.     

Ion Transport Nanotube Assembled with Vertically Aligned Metallic MoS2 for High Rate Lithium‐Ion Batteries

Metallic phase molybdenum disulfide (MoS₂) is well known for orders of magnitude higher conductivity than 2H semiconducting phase MoS₂. Herein, for the first time, the authors design and fabricate a novel porous nanotube assembled with vertically aligned metallic MoS₂ nanosheets by using the scalable solvothermal method. This metallic nanotube has the following advantages: (i) intrinsic high electrical conductivity that promotes the rate performance of battery and eliminates the using of conductive additive; (ii) hierarchical, hollow, porous, and aligned structure that assists the electrolyte transportation and diffusion; (iii) tubular structure that avoids restacking of 2D nanosheets, and therefore maintains the electrochemistry cycling stability; and (iv) a shortened ion diffusion path, that improves the rate performance. This 1D metallic MoS₂ nanotube is demonstrated to be a promising anode material for lithium‐ion batteries. The unique structure delivers an excellent reversible capacity of 1100 mA h g^?1 under a current density of 5 A g^?1 after 350 cycles, and an outstanding rate performance of 589 mA h g^?1 at a current density of 20 A g^?1. Furthermore, attributed to the material's metallic properties, the electrode comprising 100% pure material without any additive provides an ideal system for the fundamental electrochemical study of metallic MoS₂. This study first reveals the characteristic anodic peak at 1.5 V in cyclic voltammetry of metallic MoS_2. This research sheds light on the fabrication of metallic 1D, 2D, or even 3D structures with 2D nanosheets as building blocks for various applications.     

Urease-catalyzed urea synthesis.

The equilibrium of hydrolytic reactions can be shifted toward condensation by carrying out the reaction at low water concentration. The rate and yield of urease-catalyzed urea synthesis from (NH₄)₂CO₃ or NH₄HCO₃ has been examined as a function of water concentration (in mixtures with organic solvents), substrate and H⁺ concentration, and polarity of the nonaqueous component of the solvent. Similar effects of organic solvents are observed on the reaction rate in both directions; the results suggest that at least in some conditions the reaction proceeds through nonenzymically formed carbamate. The equilibrium concentration of urea, in 50% ($\text{v}\text{v}$) water, varies over 10-fold, depending on the nature of the nonaqueous component of the solvent; nonhydroxylic solvents such as acetone given the highest yield. Solubility measurements suggest that the interactions of the solvent mixtures with (NH₄)₂CO₃ (or carbamate), rather than urea, are responsible for the variations in urea yield. Activities of water and the ionic components of the equilibrium are strongly influenced by the nature of the nonaqueous component of the solvent, as well as its concentration.     

Structure and dynamics of Candida rugosa lipase: the role of organic solvent

The effect of organic solvent on the structure and dynamics of proteins was investigated by multiple molecular dynamics simulations (1 ns each) of Candida rugosa lipase in water and in carbon tetrachloride. The choice of solvent had only a minor structural effect. For both solvents the open and the closed conformation of the lipase were near to their experimental X-ray structures (C rms deviation 1–1.3 Å). However, the solvents had a highly specific effect on the flexibility of solvent-exposed side chains: polar side chains were more flexible in water, but less flexible in organic solvent. In contrast, hydrophobic residues were more flexible in organic solvent, but less flexible in water. As a major effect solvent changed the dynamics of the lid, a mobile element involved in activation of the lipase, which fluctuated as a rigid body about its average position. While in water the deviations were about 1.6 Å, organic solvent reduced flexibility to 0.9 Å. This increase rigidity was caused by two salt bridges (Lys85–Asp284, Lys75–Asp79) and a stable hydrogen bond (Lys75–Asn 292) in organic solvent. Thus, organic solvents stabilize the lid but render the side chains in the hydrophobic substrate-binding site more mobile. Figure Superimposition of open (black, PDB entry 1CRL) and closed (gray, PDB entry 1TRH) conformers of C. rugosa lipase. The mobile lid is indicatedThis revised version was published online in October 2004 with corrections to the Graphical Abstract.     

Thermal stability enhancement of Candida rugosa lipase using ionic liquids

The thermal stability of Candida rugosa (C. rugosa) lipase was investigated and compared in n-hexane, benzene, dibutyl-ether as well as [bmim]PF₆ and [omim]PF₆ ionic liquids and the effect of solvent polarity and water activity were evaluated. Deactivation of the enzyme followed a series-type kinetic model. First order deactivation rate constants and the ratios of specific activities were determined and the kinetics of deactivation were studied. Among the organic solvents, the best stability was observed in n-hexane with a half-life of 6.5?h at water activity of 0.51. In ionic liquids, however, even longer half lives were obtained, and the enzyme was stable in these solvents at 50°C. The highest half-life times were obtained in [bmim]PF₆ (12.3?h) and [omim]PF₆ (10.6?h). A direct correlation was found between solvent polarity and thermal stability since the higher the polarity of the solvent, the lower was the stability decrease at 50°C comparing to that at 30°C.     

Dual‐Functional N Dopants in Edges and Basal Plane of MoS2 Nanosheets Toward Efficient and Durable Hydrogen Evolution

Herein, the authors explicitly reveal the dual‐functions of N dopants in molybdenum disulfide (MoS₂) catalyst through a combined experimental and first‐principles approach. The authors achieve an economical, ecofriendly, and most efficient MoS₂‐based hydrogen evolution reaction (HER) catalyst of N‐doped MoS₂ nanosheets, exhibiting an onset overpotential of 35 mV, an overpotential of 121 mV at 100 mA cm^?2 and a Tafel slope of 41 mV dec^?1. The dual‐functions of N dopants are (1) activating the HER catalytic activity of MoS₂ S‐edge and (2) enhancing the conductivity of MoS₂ basal plane to promote rapid charge transfer. Comprehensive electrochemical measurements prove that both the amount of active HER sites and the conductivity of N‐doped MoS₂ increase as a result of doping N. Systematic first‐principles calculations identify the active HER sites in N‐doped MoS₂ edges and also illustrate the conducting charges spreading over N‐doped basal plane induced by strong Mo 3d –S 2p –N 2p hybridizations at Fermi level. The experimental and theoretical research on the efficient HER catalysis of N‐doped MoS₂ nanosheets possesses great potential for future sustainable hydrogen production via water electrolysis and will stimulate further development on nonmetal‐doped MoS₂ systems to bring about novel high‐performance HER catalysts.     

Bacteria tolerant to organic solvents

The toxic effects that organic solvents have on whole cells is an important drawback in the application of these solvents in environmental biotechnology and in the production of fine chemicals by whole-cell biotransformations. Hydrophobic organic solvents, such as toluene, are toxic for living organisms because they accumulate in and disrupt cell membranes. The toxicity of a compound correlates with the logarithm of its partition coefficient with octanol and water (log P _ow). Substances with a log P _ow value between 1 and 5 are, in general, toxic for whole cells. However, in recent years different bacterial strains have been isolated and characterized that can adapt to the presence of organic solvents. These strains grow in the presence of a second phase of solvents previously believed to be lethal. Different mechanisms contributing to the solvent tolerance of these strains have been found. Alterations in the composition of the cytoplasmic and outer membrane have been described. These adaptations suppress the effects of the solvents on the membrane stability or limit the rate of diffusion into the membrane. Furthermore, changes in the rate of the biosynthesis of the phospholipids were reported to accelerate repair processes. In addition to these adaptation mechanisms compensating the toxic effect of the organic solvents, mechanisms do exist that actively decrease the amount of the toxic solvent in the cells. An efflux system actively decreasing the amount of solvents in the cell has been described recently. We review here the current knowledge about exceptional strains that can grow in the presence of toxic solvents and the mechanisms responsible for their survival. Received: January 22, 1998 / Accepted: February 16, 1998     

Lipase in aqueous‐polar organic solvents: Activity,structure, and stability

Studying alterations in biophysical and biochemical behavior of enzymes in the presence of organic solvents and the underlying cause(s) has important implications in biotechnology. We investigated the effects of aqueous solutions of polar organic solvents on ester hydrolytic activity, structure and stability of a lipase. Relative activity of the lipase monotonically decreased with increasing concentration of acetone, acetonitrile, and DMF but increased at lower concentrations (upto ～20% v/v) of dimethylsulfoxide, isopropanol, and methanol. None of the organic solvents caused any appreciable structural change as evident from circular dichorism and NMR studies, thus do not support any significant role of enzyme denaturation in activity change. Change in 2D [15N, 1H]‐HSQC chemical shifts suggested that all the organic solvents preferentially localize to a hydrophobic patch in the active‐site vicinity and no chemical shift perturbation was observed for residues present in protein's core. This suggests that activity alteration might be directly linked to change in active site environment only. All organic solvents decreased the apparent binding of substrate to the enzyme (increased K_m); however significantly enhanced the k_cat. Melting temperature (T_m) of lipase, measured by circular dichroism and differential scanning calorimetry, altered in all solvents, albeit to a variable extent. Interestingly, although the effect of all organic solvents on various properties on lipase is qualitatively similar, our study suggest that magnitudes of effects do not appear to follow bulk solvent properties like polarity and the solvent effects are apparently dictated by specific and local interactions of solvent molecule(s) with the protein.     

New technique for kla measurement between gas and water in aerated solvent-in-water dispersions

Summary A new technique is presented to determine gas-to-water overall volumetric mass transfer coefficients (k _l a) in a stirred-tank reactor containing solvent-in-water dispersions. The compound to be transferred from the gas to the water was toluene; the water-immiscible organic solvent was FC40, a perfluorocarbon. The k _l a was determined in steady-state conditions in the absence of biological consumption. Toluene removal was achieved by passing a continuous flow of toluene-free water through the reactor. When solvent was present it was separated from the water at the reactor outlet by means of a small settler and recycled back to the vessel. The k _l a was found to increase with the FC40 volume fraction. An enhancement of 1.9 times on an aqueous-phase-volume basis was found at 15 % (v/v) FC40.     

Thermostability of α-chymotrypsin in water/organic solvent systems

Summary The influence of pH, temperature, substrate concentration and organic solvents (dimethylformamide, dimethylsulfoxide) on the -chymotrypsin stability in a water/organic solvent system was studied. The enzyme activity was measured as the dipeptide, AcPheLeuNH₂ synthesis and the ester substrate hydrolysis. Enzyme stability was enhanced by lower pH and temperature values and higher substrate concentrations. Dimethylsulfoxide allowed an higher enzyme stability than dimethylformamide. -Chymotrypsin displayed an higher stability in the water medium when it was compared to the organic system.     

Molybdenum Disulfide/Nitrogen‐Doped Reduced Graphene Oxide Nanocomposite with Enlarged Interlayer Spacing for Electrocatalytic Hydrogen Evolution

Facile design of low‐cost and highly active catalysts from earth‐abundant elements is favorable for the industrial application of water splitting. Here, a simple strategy to synthesize an ultrathin molybdenum disulfide/nitrogen‐doped reduced graphene oxide (MoS₂/N‐RGO‐180) nanocomposite with the enlarged interlayer spacing of 9.5 Å by a one‐step hydrothermal method is reported. The synergistic effects between the layered MoS₂ nanosheets and N‐doped RGO films contribute to the high activity for hydrogen evolution reaction (HER). MoS₂/N‐RGO‐180 exhibits the excellent catalytic activity with a low onset potential of ?5 mV versus reversible hydrogen elelctrode (RHE), a small Tafel slope of 41.3 mV dec^?1, a high exchange current density of 7.4 × 10^?4 A cm^?2, and good stability over 5 000 cycles under acidic conditions. The HER performance of MoS₂/N‐RGO‐180 nanocomposite is superior to the most reported MoS₂‐based catalysts, especially its onset potential and exchange current density. In this work, a novel and simple method to the preparation of low‐cost MoS₂‐based electrocatalysts with the extraordinary HER performance is presented.     

Systematic assessment of the stability of benzaldehyde lyase in aqueous–organic biphasic systems and its stabilization by modification with methoxy-poly(ethylene) glycol

Benzaldehyde lyase from Pseudomonas fluorescens Biovar I [BAL; E.C.4.1.2.38] catalyzes the stereoselective formation of C–C bonds coupling aldehydes to generate alpha-hydroxy ketones. A broad range of poorly water-soluble substrates are accepted in forward and reverse reactions. In this study, the stability of BAL in aqueous–organic biphasic systems as promising reaction media was systematically investigated using methyl-tert-butylether, 2-octanone, and toluene as the organic phase. Surprisingly, a strong individual molecular toxicity of these water-immiscible solvents was observed along with the interfacial toxicity exerted by the aqueous–organic interfaces. They could be considerably reduced by covalent attachment of methoxy-poly(ethylene) glycol (mPEG₇₅₀ and mPEG₂₀₀₀) to the enzyme surface increasing the half-life by a factor of up to 18. However, under reactive conditions solvent effects were strongly superimposed by an additional deactivating effect, possibly caused by the aldehyde substrate, and no differences between unmodified and modified BAL were detectable. For technical application of the enzyme in aqueous–organic biphasic media additional strategies for stabilization will therefore be desirable.