Dual Microenvironment Modulation of Pd Nanoparticles in Covalent Organic Frameworks for Semihydrogenation of Alkynes.

The chemical microenvironment modulation of metal nanoparticles (NPs) holds promise for tackling the long-lasting challenge of the trade-off effect between activity and selectivity in catalysis. Herein, ultrafine PdCu2 NPs incorporated into covalent organic frameworks (COFs) with diverse groups on their pore walls have been fabricated for the semihydrogenation of alkynes. The Cu species, as the primary microenvironment of Pd active sites, greatly improves the selectivity. The functional groups as the secondary microenvironment around PdCu2 NPs effectively regulate the activity, in which PdCu2 NPs encapsulated in the COF bearing -CH3 groups exhibit the highest activity with >99 % conversion and 97 % selectivity. Both experimental and calculation results suggest that the functional group affects the electron-donating ability of the COFs, which successively impacts the charge transfer between COFs and Pd sites, giving rise to a modulated Pd electronic state and excellent catalytic performance.

Freeze-Tolerant Hydrogel Electrolyte with High Strength for Stable Operation of Flexible Zinc-Ion Hybrid Supercapacitors.

Constructing ionic conductive hydrogels with diversified properties is crucial for portable zinc-ion hybrid supercapacitors (ZHSCs). Herein, a freeze-tolerant hydrogel electrolyte (AF PVA-CMC/Zn(CF3 SO3 )2 ) is developed by forming a semi-interpenetrating anti-freezing polyvinyl alcohol-carboxymethyl cellulose (AF PVA-CMC) network filled with the ethylene glycol (EG)-containing Zn(CF3 SO3 )2 aqueous solution. The semi-interpenetrating AF PVA-CMC/Zn(CF3 SO3 )2 possesses enhanced mechanical properties, realizes the uniform zinc deposition, and impedes the dendrite growth. Notably, the interaction between PVA and EG suppresses the ice crystal formation and prevents freezing at -20 °C. Due to these advantages, the designed hydrogel owns high ionic conductivity of 1.73/0.75 S m-1 at 20/-20 °C with excellent tensile/compression strength at 20 °C. Impressively, the flexible AF quasi-solid-state ZHSC employing the hydrogel electrolyte achieves a superior energy density at 20/-20 °C (87.9/60.7 Wh kg-1 ). It maintains nearly 84.8% of the initial capacity after 10 000 cycles and a low self-discharge rate (1.77 mV h-1 ) at 20 °C, together with great tolerance to corrosion. Moreover, this device demonstrates a stable electrochemical performance at -20 °C under deformation. The obtained results provide valuable insights for constructing durable hydrogel electrolytes in cold environments.

Toward Flexible Zinc-Ion Hybrid Capacitors with Superhigh Energy Density and Ultralong Cycling Life: The Pivotal Role of ZnCl2 Salt-Based Electrolytes.

Zinc ion hybrid capacitors (ZIHCs) are promising energy storage devices for emerging flexible electronics, but they still suffer from trade-off in energy density and cycling life. Herein, we show that such a dilemma can be well-addressed by deploying ZnCl2 based electrolytes. Combining experimental studies and density functional theory (DFT) calculations, for the first time, we demonstrate an intriguing chloride ion (Cl- ) facilitated desolvation mechanism in hydrated [ZnCl]+ (H2 O)n-1 (with n=1-6) clusters. Based on this mechanism, a water-in-salt type hydrogel electrolyte filled with ZnCl2 was developed to concurrently improve the energy storage capacity of porous carbon materials and the reversibility of Zn metal electrode. The resulting ZIHCs deliver a battery-level energy density up to 217 Wh kg-1 at a power density of 450 W kg-1 , an unprecedented cycling life of 100 000 cycles, together with excellent low-temperature adaptability and mechanical flexibility.

One-Step Synthesis of a Nanosized Cubic Li2TiO3-Coated Br, C, and N Co-Doped Li4Ti5O12 Anode Material for Stable High-Rate Lithium-Ion Batteries.

Nanosized Li4Ti5O12 with both a Li2TiO3 coating and C-N-Br co-doping (CLLTO) was successfully synthesized via a facile reverse microemulsion method in one step using hexadecyl trimethyl ammonium bromide as a surface control agent and as a carbon, nitrogen, and bromine source. A uniform Li2TiO3 layer was formed on the surface and strongly adhered to the host material Li4Ti5O12 (LTO), which played an important role in improving the cyclic stability of CLLTO. The thin and stable Li2TiO3 layer has the same cubic structure as LTO, which provides many three-dimensional channels for ion transport. C, N, and Br co-doping in CLLTO promoted the transition of Ti4+ to Ti3+ in Li4Ti5O12, which could improve the capacity and facilitate the Li+ ion and electron transfer at the interface. The conductive behavior induced by co-doping was estimated by UV-vis diffuse reflectance spectra and further supported by theoretical calculations. The electrical conductivity of both p-type and n-type LTO can be well improved by co-doping C, N, and Br. This improvement may be due to the band gap reduction and the increased n-type electronic modification of the entire LTO. Owing to the synergistic effect of coating, co-doping, and nanosizing at one time, the CLLTO exhibits a high discharge capacity of 177.3-153.9 mA h g-1 at the working rate of 0.1C-20C, with a capacity retention of 86%. The stable cycling of CLLTO is also obtained after 500 cycles at 20C, with a capacity retention of 95.5% (approximately 8 times higher than that of pure LTO) and almost 100% Coulombic efficiency. With high capacity, excellent rate performance, and good cycling stability, CLLTO can be applied in high-power lithium-ion batteries.

Nanotoxicity of Boron Nitride Nanosheet to Bacterial Membranes.

Boron nitride (BN) nanosheet is a novel material with great potential in biomedical applications. A deep understanding of the basic interaction mechanisms between biosystems and foreign BN nanosheets can help to better clarify the potential risks of these nanomaterials and provide guidance on their safe design. In this paper, we show that BN nanosheets can cause degradation of bacterial cell membranes via experimental and simulation-based approaches. Our extensive molecular dynamics simulations results reveal that BN nanosheets cause toxicity to both bacterial outer and inner membranes in which hydrophobic effect plays an important role. The spontaneous lipid extraction by BN nanosheets is in agreement with the free-energy calculations. A liquid-to-gel phase transition is induced by the BN nanosheet in the outer model membrane of bacteria, indicating that the BN nanosheet may cause higher toxicity to the outer membrane than to the inner membrane. Our findings may offer new insights into the molecular basis of BN's cytotoxicity and antibacterial activity.

Lattice constant-dependent anchoring effect of MXenes for lithium-sulfur (Li-S) batteries: a DFT study.

The anchoring effect of the cathode plays a significant role in improving the performance of lithium-sulfur (Li-S) batteries. MXenes, a new class of two-dimensional materials, have been reported to be effective sulfur hosts for Li-S batteries. However, previous studies mainly focused on Ti-based MXenes, while other potential transition metal MXenes have not been systematically explored. In the present work, we thoroughly investigated the interactions between lithium polysulfides (LiPSs) and a Ti2CO2 substrate, as well as six other M3C2O2 (M = Cr, V, Ti, Nb, Hf and Zr) MXenes using density functional theory (DFT) calculations. It is found that all six M3C2O2 systems possess trapping ability towards soluble LiPSs, largely attributed to the strong Li-O interactions between the LiPSs and the surface of the M3C2O2. Among them, Cr3C2O2 exhibited the strongest anchoring effect with the largest Eb. More importantly, a monotonical relationship between the binding energies and the lattice constants of M3C2O2 was identified, which indicated that M3C2O2 MXenes with a smaller lattice constant tend to exhibit a stronger anchoring effect. Furthermore, all six M3C2O2 MXenes showed metallic properties during the whole process. Our results shed light on the future rational selection and design of MXenes acting as sulfur hosts in Li-S batteries and on the potential to improve host-guest interactions in other energy storage systems.

VC2 and V1/2Mn1/2C2 nanosheets with robust mechanical and thermal properties as promising materials for Li-ion batteries.

In this paper, vanadium carbides VC2 and bi-transition-metal carbides V1/2Mn1/2C2 are predicted to be stable metallic nanosheets showing promising mechanical properties and their Young's moduli are 70.8 N m-1 and 83.7 N m-1, respectively. Ab initio molecular dynamics results showed that both VC2 and V1/2Mn1/2C2 can tolerate temperatures up to 1000 K showing favorable thermal properties. The excellent Li-ion specific energy storage capacity and low diffusion barrier also make them promising candidates as anode materials. In addition, nonmagnetic-ferromagnetic (-ferrimagnetic) transitions in VC2 and V1/2Mn1/2C2 can be tuned easily by oxygen or fluorine passivation.

Modeling Interactions between Liposomes and Hydrophobic Nanosheets.

2D nanomaterials could cause structural disruption and cytotoxic effects to cells, which greatly challenges their promising biomedical applications including biosensing, bioimaging, and drug delivery. Here, the physical and mechanical interaction between lipid liposomes and hydrophobic nanosheets is studied utilizing coarse-grained (CG) molecular dynamics (MD) simulations. The simulations reveal a variety of characteristic interaction morphologies that depend on the size and the orientation of nanosheets. Dynamic and thermodynamic analyses on the morphologic evolution provide insights into molecular motions such as "nanosheet rotation," "lipid extraction," "lipid flip-flop," and "lipid spreading." Driven by these molecular motions, hydrophobic nanosheets cause morphologic changes of liposomes. The lipid bilayer structure can be corrugated, and the overall liposome sphere can be split or collapsed by large nanosheets. In addition, nanosheets embedded into lipid bilayers greatly weaken the fluidity of lipids, and this effect can be cumulatively enhanced as nanosheets continuously intrude. These results could facilitate molecular-level understanding on the cytotoxicity of nanomaterials, and help future nanotoxicology studies associating computational modeling with experiments.

Two-dimensional stable transition metal carbides (MnC and NbC) with prediction and novel functionalizations.

In this paper, manganese carbide (MnC) and niobium carbide (NbC) are predicted as stable monolayer metallic materials, whose Young's moduli are 50.06 N m-1 and 44.07 N m-1, respectively. The ab initio molecular dynamics (AIMD) results show that both MnC and NbC could hold their structure up to 1000 K, showing favorable thermal properties. These monolayers also show good properties for promising application in Li ion batteries because of their high specific capacities and low diffusion barriers. The MnC monolayer is ferromagnetic and the Curie temperature simulated by the Monte-Carlo method is about 205 K. The electronic band of MnC shows a metal to half-metal transition by passivation of Cl or Br atoms, and the functionalization methods also cause the metallic NbC monolayer to exhibit the quantum spin Hall effect (QSHE). These novel transition metal carbide monolayers hold great promise for 2D spintronic and electronic device applications.

Lipid extraction by boron nitride nanosheets from liquid-ordered and liquid-disordered nanodomains.

Atomically thin boron nitride nanosheets are important two-dimensional nanomaterials with great potential in biomedical applications. Understanding the basic interaction mechanisms between lipid domains and boron nitride nanosheets can help clarify the potential risks of these nanomaterials and thus provide guidance on the design of safe biomedical applications. Using molecular dynamics simulations, we demonstrate that the BNNS can disrupt the liquid disordered lipid bilayers much more easily compared to the liquid ordered phases. The potential of mean force profiles calculated from umbrella sampling further explain this adsorption preference. When the BNNS is placed at the boundary of the liquid ordered and liquid disordered nanodomains, disruption of the liquid ordered domains becomes much easier due to the presence of adjacent liquid disordered domains. Our findings provide new insights into the cytotoxicity of boron nitride nanosheets interacting with cellular membranes.

Charge driven lateral structural evolution of ions in electric double layer capacitors strongly correlates with differential capacitance.

Electric double layers (EDLs) play a decisive role in the energy storage of supercapacitors. Recently, voltage/charge driven ordering transitions of ions in the lateral direction of the EDL were found to dramatically affect the capacitance in experiments and molecular dynamics (MD) simulations. However, the correlation between the lateral structure of the ions and the capacitance was not well understood. In this work, all-atom MD simulations were applied to investigate the lateral ordering of the [BMIM][PF6] ionic liquid on both anode and cathode surfaces. The lateral ordering of ions was systematically characterized using the 2D structure factor, radial distribution function and coordination number. It was found that the disorder to order transition of PF6- ions on the anode occurs when the number of first nearest neighboring ions converges to six. What's more, the local maximum of the differential capacitance profile not only appears at the disorder-order transition point, but also occurs at the splitting point of the radial distribution function peak and where the number of second and third nearest neighboring ions become converged and stable. On the cathode side, a long range ordered phase of BMIM+ ions does not exist due to its multi-adsorption states on the electrode. To understand the origin of the correlation between the lateral structure and the differential capacitance, the correlation between the structures in the lateral and normal directions was investigated. Such a structural correlation is closely related to the three-dimensional characteristics of the EDL structure and the over-screening phenomenon.

The S-functionalized Ti3C2 Mxene as a high capacity electrode material for Na-ion batteries: a DFT study.

MXenes are attracting much attention as electrode materials due to their excellent energy storage properties and electrical conductivity, and the energy storage capacities were found to strongly depend on the surface terminal groups. Here S-functionalized Ti3C2 as a representative MXene material is designed. Our density functional theory (DFT) calculations are performed to investigate the geometric and electronic properties, dynamic stability, and Na storage capability of Ti3C2, Ti3C2O2 and Ti3C2S2 systems. The Ti3C2S2 monolayer is proved to show metallic behavior and has a stable structure, and meanwhile it also exhibits a low diffusion barrier and high storage capacity (up to Ti3C2S2Na4 stoichiometry) for Na ion batteries (NIBs). The superior properties such as good electrical conductivity, fast charge-discharge rates, low open circuit voltage (OCV), and high theoretical Na storage capacity, make the Ti3C2S2 monolayer a promising anode material for NIBs compared to the Ti3C2O2 monolayer. More importantly, similar to the Ti3C2S2 monolayer, other MXenes with a high charge density difference and suitable lattice constant can be formed, and thus the energy storage properties are worth further study. This finding will be useful to the design of anode materials for NIBs.

Tuning the indirect-direct band gap transition in the MoS2-xSex armchair nanotube by diameter modulation.

The application of the reported armchair transition-metal dichalcogenide (MoS2, MoTe2, MoSTe and WS2, etc.) nanotube is hindered for the optoelectronic devices due to the indirect band gap. By using first-principles calculations, the electronic structures of MoS2-xSex single-wall armchair nanotubes with respect to different diameters are investigated. The MoS2 armchair nanotube exhibits an indirect band gap as a function of nanotube diameters from 10 Å to 50 Å, whereas MoSSe and MoSe2 exhibit a surprising diameter-induced indirect-direct band gap crossover at the diameters of 25 Å and 33 Å, respectively. We also find that the optical properties of MoS2-xSex armchair nanotubes are anisotropic and strongly depend on the diameter.

Revealing the role of oxygen vacancies on the phase transition of VO2 film from the optical-constant measurements.

Vanadium dioxide (VO2) material shows a distinct metal-insulator transition (MIT) at the critical temperature of ∼340 K. Similar to other correlated oxides, the MIT properties of VO2 is always sensitive to those crystal defects such as oxygen vacancies. In this study, we investigated the oxygen vacancies related phase transition behavior of VO2 crystal film and systematically examined the effect of oxygen vacancies from the optical constant measurements. The results indicated that the oxygen vacancies changed not only the electron occupancy on V 3d-O 2p hybrid-orbitals, but also the electron-electron correlation energy and the related band gap, which modulated the MIT behavior and decreased the critical temperature resultantly. Our work not only provided a facile way to modulate the MIT behavior of VO2 crystal film, but also revealed the effects of the oxygen vacancies on the electronic inter-band transitions as well as the electronic correlations in driving this MIT process.

Theoretical prediction of MXene-like structured Ti3C4 as a high capacity electrode material for Na ion batteries.

MXenes are attracting much attention as electrode materials due to their excellent energy storage properties and good electrical conductivity. Here a carbonized derivative of Ti3C2 (one representative MXene material), a Ti3C4 monolayer, is designed. Density functional theory (DFT) calculations were performed to investigate the geometric and electronic properties, dynamic stability, and Li/Na storage capability of Ti3C4. The Ti3C4 monolayer is proved to be a structurally stable material showing the nature of the metal with C2 dimers rather than the individual C atom. Moreover, the Ti3C4 monolayer exhibits a low diffusion barrier and high storage capacity (up to Ti3C4Na4 stoichiometry) in Na ion batteries (NIBs) compared with Li ion batteries (LIBs). Its superior properties, such as good electronic conductivity, fast Na diffusion, low open circuit voltage (OCV), and high theoretical Na storage capacity, make the Ti3C4 monolayer a promising anode material for NIBs. More importantly, similar to MXene Ti3C2, new M3C4 monolayers with C2 dimers can be formed by replacing M with other transition metal elements, and the properties of these monolayers are worthy of further study.

Infrared Response and Optoelectronic Memory Device Fabrication Based on Epitaxial VO2 Film.

In this work, high-quality VO2 epitaxial films were prepared on high-conductivity n-GaN (0001) crystal substrates via an oxide molecular beam epitaxy method. By fabricating a two-terminal VO2/GaN film device, we observed that the infrared transmittance and resistance of VO2 films could be dynamically controlled by an external bias voltage. Based on the hysteretic switching effect of VO2 in infrared range, an optoelectronic memory device was achieved. This memory device was operated under the "electrical writing-optical reading" mode, which shows promising applications in VO2-based optoelectronic device in the future.

Co3 O4 Hexagonal Platelets with Controllable Facets Enabling Highly Efficient Visible-Light Photocatalytic Reduction of CO2.

A heterogeneous catalyst made of well-defined Co3 O4 hexagonal platelets with varied exposed facets is coupled with [Ru(bpy)3 ]Cl2 photosensitizers to effectively and efficiently reduce CO2 under visible-light irradiation. Systematic investigation based on both experiment and theory discloses that the exposed {112} facets are crucial for activating CO2 molecules, giving rise to significant enhancement of photocatalytic CO2 reduction efficiency.

Sub-20 nm-Fe3O4 square and circular nanoplates: synthesis and facet-dependent magnetic and electrochemical properties.

We present an effective method to synthesize 15 nm magnetite nanocrystals with the morphology of square and circular nanoplates, which expose (001) facet and (111) facet, respectively. The magnetic property and electrochemical behavior towards As(III) exhibit strong facet-dependent characteristics. Theoretical calculations confirm the facet-dependent characteristics and provide the corresponding explanations.

Origin of high photocatalytic properties in the mixed-phase TiO2: a first-principles theoretical study.

We present a step-by-step theoretical protocol based on the first-principles methods to reveal the insight into the origin of the high photocatalytic activity achieved by the mixed-phase TiO2, consisting of anatase and rutile. The interfacial geometries, density of states, charge densities, optical absorption spectrum, electrostatic potential, and band offsets have been calculated. The most stable mixed-phase structures have been identified, the interfacial tensile strain-dependent electronic structures have been observed, and the energy level diagram of band alignment has been given. We find that the geometrical reconstruction around the interfacial area has a negligible influence on the light absorption of the heterojunction and the interfacial sites seem not to dominantly contribute to the band-edge states. For the most stable heterojunction, the calculated valence-band maximum and conduction-band minimum of rutile, respectively, lie 0.52 and 0.22 eV above those of anatase, which agrees well with the experimental measurements and other theoretical predications. The good match of band energies to reaction requirements, large driving force for the charge immigration across the interface, and the difference of electrostatic potentials around the interface successfully explain the high photocatalytic activity achieved by the mixed-phase TiO2.

Band structure engineering of anatase TiO2 by metal-assisted P-O coupling.

In this work, we demonstrate that the metal-assisted P-O coupling is an effective approach to improve the photoelectrochemical properties of TiO2. The (Sc + P) and (In + P) codoping effects on electronic structures and photocatalytic activities of anatase TiO2 are examined by performing hybrid density functional theory calculations. It is found that the coupling of P dopant with the second-nearest neighboring O atom assisted by acceptor metals (Sc/In) leads to the fully occupied and delocalized intermediate bands within the band gap of anatase TiO2, which is driven by the P-O antibonding states (π*). This metal-assisted P-O coupling can prevent the recombination of photogenerated electron-hole pairs and effectively reduce the band gap of TiO2. Moreover, the band edge alignments in (Sc + P) and (In + P) codoped anatase TiO2 are desirable for water-splitting. The calculated optical absorption curves indicate that (Sc + P) and (In + P) codoping in anatase TiO2 can also effectively enhance the visible light absorption.