Tuning the Surface Stability and Li/Na Storage of MXenes by Controlling the Surface Termination Coverage.

2D transition metal carbides and/or nitrides, MXenes, are a class of widely studied materials with great potential for energy storage applications. The control of surface chemistry is an effective approach for preparing novel MXenes and modifying their electrochemical properties. However, an in-depth and systematic atomic-scale study of the effect of surface termination on MXene stability and electrochemical performance is scarce and thus is highly desired. Here, through high-throughput first-principles calculations, 28 stable chalcogen-functionalized M2 CTz (M = V, Nb, and Ta, T = S, Se, and Te) under different chemical environments are identified. The reduction of termination coverage improves electrical conductivity but weakens in-plane stiffness. Intriguingly, based on charge transfer mechanism, the diffusion barrier of lithium/sodium atoms on the M2 CTz exhibits a volcano-like relationship with termination coverage, and the ion diffusion channel formed in half termination coverage greatly accelerates lithium ion diffusion and returns to or exceeds sodium ion diffusion rate at full termination coverage. V2 CSe2 /Nb2 CSz not only displays the large lithium/sodium capacity (592/409-466 mAhg-1 ) but also exhibits low barrier energy and open-circuit voltage, suggesting a promising candidate anode material for lithium/sodium-ion batteries. These findings provide insights into the design and fabrication of MXenes and tuning the electrochemical performance of MXenes by controlling termination coverage.

Structural, Electronic, and Mechanical Properties of Zr2SeB and Zr2SeN from First-Principle Investigations.

MAX phases have exhibited diverse physical properties, inspiring their promising applications in several important research fields. The introduction of a chalcogen atom into a phase of MAX has further facilitated the modulation of their physical properties and the extension of MAX family diversity. The physical characteristics of the novel chalcogen-containing MAX 211 phase Zr2SeB and Zr2SeN have been systematically investigated. The present investigation is conducted from a multi-faceted perspective that encompasses the stability, electronic structure, and mechanical properties of the system, via the employment of the first-principles density functional theory methodology. By replacing C with B/N in the chalcogen-containing MAX phase, it has been shown that their corresponding mechanical properties are appropriately tuned, which may offer a way to design novel MAX phase materials with enriched properties. In order to assess the dynamical and mechanical stability of the systems under investigation, a thorough evaluation has been carried out based on the analysis of phonon dispersions and elastic constants conditions. The predicted results reveal a strong interaction between zirconium and boron or nitrogen within the structures of Zr2SeB and Zr2SeN. The calculated band structures and electronic density of states for Zr2SeB and Zr2SeN demonstrate their metallic nature and anisotropic conductivity. The theoretically estimated Pugh and Poisson ratios imply that these phases are characterized by brittleness.

First-principles study on the structure and electronic properties of M2CSx (M = Sc, Ti, Y, Zr and Hf, x = 1, 2).

Two-dimensional (2D) transition metal carbides/nitrides, known as MXenes, have attracted extensive attention due to their rich elemental composition and diverse surface chemistry. In this study, the crystal structure, electronic, mechanical, and electronic transport properties of M2CSx (M = Sc, Ti, Y, Zr, and Hf, x = 1, 2) were investigated by density functional theory (DFT). Our results showed that the studied M2CSx except Y2CS2 are thermodynamically, dynamically, thermally, and mechanically stable. The p-d hybridization between the M-d state and the C/S-p state of M2CS is stronger than that of the corresponding M2CS2. However, the antibonding state would appear near the Fermi level and thus reduce the thermal stability of the material due to the introduction of sulfur vacancies in the Y-free MXenes studied. In contrast, sulfur vacancies would significantly enhance the bonding states of Y-C and Y-S bonds and improve the stability of Y2CSx. This provides an explanation for the experimentally observed formation of non-stoichiometric Ti2CS1.2. The room-temperature electron mobilities of semiconductor Sc2CS (Y2CS) along the x and y directions were determined to be 232.59 (818.51) and 628.22 (552.55) cm2 V-1 s-1, and the room-temperature hole mobilities are only 88.32 (1.64) and 61.75 (17.80) cm2 V-1 s-1. This work is expected to provide theoretical insights for the preparation and application of S-terminated MXenes.

Ab Initio Studies of Structural and Mechanical Properties of NH3, NO, and N2O Hydrates.

The investigation on the mechanical properties of clathrate hydrate is closely related to the exploitation of hydrates and gas transportation. In this article, the structural and mechanical properties of some nitride gas hydrates were studied using DFT calculations. First, the equilibrium lattice structure is obtained by geometric structure optimization; then, the complete second-order elastic constant is determined by energy-strain analysis, and the polycrystalline elasticity is predicted. It is found that the NH3, N2O, and NO hydrates all have high elastic isotropy but are different in shear characteristics. This work may lay a theoretical foundation for studying the structural evolution of clathrate hydrates under the mechanical field.

Engineering Biomimetic Nanostructured "Melanosome" Textiles for Advanced Solar-to-Thermal Devices.

In nature, deep-sea fish featured with close-packed melanosomes can remarkably lower light reflection, which have inspired us to design ultrablack coatings for enhanced solar-to-thermal conversion. Herein, a biomimetic ultrablack textile is developed enabled by the formation of hierarchical polypyrrole (Ppy) nanospheres. The fabricated textile exhibits prominently suppressed reflectance of lower than 4% and highly enhanced absorption of up to 96%. Further experimental results and molecular dynamics (MD) simulation evidence the formation process of hierarchical nanospheres. Based on high-efficient solar-to-thermal conversion, the biomimetic textile with desirable conductivity allows the development of a salt-free solar evaporator, enabling a sustainable seawater evaporation rate of up to 1.54 kg m-2 h-1 under 1 sun. Furthermore, the biomimetic hierarchical textile exhibits good superhydrophobicity, enhanced photothermal property, and high electrothermal conversion, demonstrating significant potential in wearable thermal management (rescue vests) in water conditions.

Boosting Oxygen Reduction for High-Efficiency H2 O2 Electrosynthesis on Oxygen-Coordinated CoNC Catalysts.

Atomically dispersed CoNC is a promising material for H2 O2 selective electrosynthesis via a two-electron oxygen reduction reaction. However, the performance of typical CoNC materials with routine CoN4 active center is insufficient and needs to be improved further. This can be done by fine-tuning its atomic coordination configuration. Here, a single-atom electrocatalyst (Co/NC) is reported that comprises a specifically penta-coordinated CoNC configuration (OCoN2 C2 ) with Co center coordinated by two nitrogen atoms, two carbon atoms, and one oxygen atom. Using a combination of theoretical predictions and experiments, it is confirmed that the unique atomic structure slightly increases the charge state of the cobalt center. This optimizes the adsorption energy towards *OOH intermediate, and therefore favors the two-electron ORR relevant for H2 O2 electrosynthesis. In neutral solution, the as-synthesized Co/NC exhibits a selectivity of over 90% over a potential ranging from 0.36 to 0.8 V, with a turnover frequency value of 11.48 s-1 ; thus outperforming the state-of-the-art carbon-based catalysts.

Resveratrol analog, triacetylresveratrol, a potential immunomodulator of lung adenocarcinoma immunotherapy combination therapies.

Introduction: Resveratrol, an activator for longevity regulatory genes-sirtuin family (SIRTs) and Sirtuin 2 (SIRT2) is an important factor of SIRTs which demonstrated biological function in cancers, but the underlying mechanism is unrevealed. Methods: We investigated the mRNA and protein levels of SIRT2 in a variety of cancers and the potential role for clinical prognosis, as well as analysed the association between the gene and immune infiltration in various cancers. And an analysis of two types of lung cancer was conducted to construct a systematic prognostic landscape. Finally, putative binding site of the triacetylresveratrol bound to SIRT2 was built from homology modeling. Results and discussion: We concluded that higher mRNA and protein levels of SIRT2 affected prognosis in various types of cancers, especially in LUAD cohorts. In addition, SIRT2 is linked with a better overall survival (OS) in LUAD patients. Further research suggested a possible explanation for this phenotype might be that SIRT2 mRNA levels are positively correlated with infiltrating status of multiple immunocytes in LU-AD but not LUSC, i.e. SIRT2 expression may contribute to the recruitment of CD8+T cell, CD4+ T cell, T cell CD4+ memory resting, Tregs, T cell NK and positively correlated to the expression of PD-1, also excluding neutrophil, T cell CD8+ naïve and B cell plasma cells in LUAD. We found that triacetyl-resveratrol demonstrated the most potent agonist efficiency to SIRT2 and the EC 50 as low as 142.79 nM. As a result, SIRT2 appears to be a promising novel biomarker for prognosis prediction in patients with LUAD and triacetylresveratrol might be a potential immunomodulator of LUAD to anti-PD-1 based immunotherapy combination therapies.

Helium-induced damage in U3Si5 by first-principles studies.

Uranium silicide U3Si5 has been explored as an advanced nuclear fuel component for light water reactor to enhance the accident tolerance. In this paper, in order to understand the fuel performance of U3Si5, the primary point defects, secondary point defects, and the dissolution of He gas were studied by first-principles methods. Compared with U atoms and another type of Si2 atoms, Si1 atoms far from intrinsic Si vacancies are more likely to form point defects, implying that Si vacancies are prone to form separate single vacancies rather than vacancy clusters in the initial stage. From the calculated anti-site defect energies, it can be predicted that non-stoichiometric U-rich phase of U3Si5 are more likely to be formed than Si-rich phase, which are consistent with the chemical analysis of experimentally sintered Si-lean U3Si5 sample. It can be found that a single He atom favors residence in the interstitial site in the U layer directly above/below the intrinsic vacancy. It can also be seen that Vac-U, Vac-Si1, and Vac-Si2 vacancies can energetically accommodate up to 4, 0, and 3 He atoms, respectively. The formation of secondary vacancy defects is strongly dependent on the helium concentration. The current results show that the He-filled vacancy can promote the formation of adjacent secondary vacancy, leading to the formation of gas bubbles. This work may provide theoretical insights into the He irradiation-induced damage in U3Si5 as well as provide valuable clues for improving the design of the UN-U3Si5 composite fuel.

Correction: Helium-induced damage in U3Si5 by first-principles studies.

[This corrects the article DOI: 10.1039/D1RA04031F.].

Two-dimensional semiconducting Lu2CT2 (T = F, OH) MXene with low work function and high carrier mobility.

As a new family of two-dimensional materials, MXenes have attracted increasing attention in recent years due to their widespread potential applications. In contrast to early transition metals in convention, here we expand the M element of MXene to the rare earth element lutetium. Based on the first-principles density functional calculations, the bare lutetium-based carbide MXene Lu2C is determined to be stabilized in the T-type configuration. Furthermore, both fluorine and hydroxyl terminated configurations are found to be semiconductors, and their band gaps are suitable for use in semiconductors and visible and near-infrared optical devices. The Lu2C(OH)2 configuration shows a direct band gap and possesses an ultralow work function of 1.4 eV. Both Lu2CT2 (T = F, OH) MXenes exhibit high carrier mobilities. Particularly, the electron mobility of the Lu2C(OH)2 MXene is found to be anisotropic at room temperature, with values as high as 95.19 × 103 and 217.1 × 103 cm2 V-1·s-1 in the zigzag and armchair directions, respectively, which makes Lu2C(OH)2 a promising material for nanodevices. Based on these predicted properties, our work widens the range of MXene materials and their applications in semiconducting devices.

Tillandsia-Inspired Hygroscopic Photothermal Organogels for Efficient Atmospheric Water Harvesting.

Tillandsia species with degenerated roots have evolved into hygroscopic leaves that absorb moisture from air. This interesting biological adaptability has inspired us to develop an integrated hygroscopic photothermal organogel (POG) to achieve a solar-powered atmospheric water harvesting (AWH). The well-designed hydrophilic co-polymeric skeleton is fabricated to accommodate hygroscopic glycerin medium, which enables the POG self-contained property, mechanically flexibility and synergistic enhancement of moisture sorption. The integration of interpenetrated photothermal component of poly-pyrrole-dopamine (P-Py-DA) can endow the POG an efficient solar-to-thermal property for controllable solar-driven interfacial water releasing. The integrated POG has an equilibrium moisture sorption of 16.01 kg m-2 at the RH of 90 %, and daily water production as high as 2.43 kg m-2 day-1 is achieved in actual outdoor experiments.

Predictions of the structures and properties of the substituted layered ternary compound series (Zr1-x T x )3Al3C5 (T = Hf, Nb, and V) through first-principles studies.

The (Zr1-x T x )3Al3C5 (T = Hf, Nb, and V) series can be potential candidates to extend the domain of MAX phases. In this work, the structures and properties of (Zr1-x T x )3Al3C5 (T = Hf, Nb, and V) are studied using first-principles calculations. The obtained structural parameters are in good accordance with previously reported data in the literature. Based on the phase stability investigations, the potentially stable layered (Zr1-x T x )3Al3C5 (T = Hf, Nb, and V) solid solutions are proposed, which confirms that Zr3Al3C5 and Hf3Al3C5 can be synthesized. The substitution, especially with group VB transitional metals, endows layered (Zr1-x T x )3Al3C5 series with greatly enhanced mechanical and thermal properties. It is indicated that the bulk modulus B of all the three systems increase with increasing substitution concentration x. A significant increase in the ratio of bulk modulus to shear modulus, B/G, is found with increasing V concentration, which corresponds to the improved ductility. Moreover, solid solutions with V substitution, (Zr1-x V x )3Al3C5, yield a larger effect on thermal conductivity with respect to x, indicating flexible modulation of the thermal conductivity of (Zr1-x V x )3Al3C5 by V substitution can be achieved, which may promote them as promising coating materials. The intrinsic mechanism involved in the enhancements of the mechanical and thermal properties were elucidated through thorough electronic band structure analysis. The difference in the electronic structure and bonding between Zr and T in (Zr1-x T x )3Al3C5 compounds are shown to account for the abnormal variations in their properties.

Manganese-Zeolitic Imidazolate Frameworks-90 with High Blood Circulation Stability for MRI-Guided Tumor Therapy.

Zeolitic imidazolate frameworks (ZIFs) as smart drug delivery systems with microenvironment-triggered release have attracted much attention for tumor therapy. However, the exploration of ZIFs in biomedicine still encounters many issues, such as inconvenient surface modification, fast drug release during blood circulation, undesired damage to major organs, and severe in vivo toxicity. To address the above issues, we developed an Mn-ZIF-90 nanosystem functionalized with an originally designed active-targeting and pH-responsive magnetic resonance imaging (MRI) Y1 receptor ligand [Asn28, Pro30, Trp32]-NPY (25-36) for imaging-guided tumor therapy. After Y1 receptor ligand modification, the Mn-ZIF-90 nanosystem exhibited high drug loading, better blood circulation stability, and dual breast cancer cell membrane and mitochondria targetability, further favoring specific microenvironment-triggered tumor therapy. Meanwhile, this nanosystem showed promising T1-weighted magnetic resonance imaging contrast in vivo in the tumor sites. Especially, this nanosystem with fast clean-up had almost no obvious toxicity and no damage occurred to the major organs in mice. Therefore, this nanosystem shows potential for use in imaging-guided tumor therapy.

Two-Dimensional Lamellar Mo2C for Electrochemical Hydrogen Production: Insights into the Origin of Hydrogen Evolution Reaction Activity in Acidic and Alkaline Electrolytes.

Developing high surface area Mo2C with certain crystal plane exposed is an efficient strategy but is an urgent challenge to optimize the hydrogen evolution reaction (HER) catalytic performances. In addition, the effects of certain crystal faces on catalytic performance have been limitedly understood. Toward this end, the (1 0 0) plane oriented two-dimensional lamellar Mo2C transformed from carbon fibers is synthesized successfully in a molten salt system. Subsequently, the electrocatalytic properties toward HER show that (1 0 0) plane oriented Mo2C functions well in both acidic and basic media. The density functional theory calculations show that the most stable Mo/C termination of the (1 0 0) plane contains multiple catalytically active centers. These close-to-zero Δ GH* values verify its better HER performance. Besides, the correlation between hydrogen adsorption behavior and the water dissociation process as well as their corresponding roles in the overall acid and alkaline HER rates have been discussed in depth. A simple mechanistic analysis is put forward to explain the favorable HER performance of the lamellar structure ß-Mo2C in alkaline other than acid electrolytes. The molten salt method may provide a new way for developing electrocatalysts with oriented crystal faces.

Grand Canonical Monte Carlo Simulations on Phase Equilibria of Methane, Carbon Dioxide, and Their Mixture Hydrates.

The cage occupancy plays a crucial role in the thermodynamic stability of clathrate hydrates and is an important quantity for understanding the CO2-CH4 replacement phenomenon. In this work, the occupancy isotherms of pure CH4, pure CO2, and their mixture in sI and sII hydrates are studied by GCMC + MD simulations. The adsorption of CH4 and CO2 + CH4 in the sI and sII hydrates can be categorized as the one-site Langmuir type. The calculated occupancy ratio θL/θS and the abundance ratio of CO2 to CH4 vary with the temperature and pressure, which provide the prerequisite information for the prediction of CH4 recovery yield at different conditions in the CO2-CH4 gas exchange process. The phase equilibria of clathrate hydrates of pure gases and mixtures are explored and the corresponding heat of dissociation and hydration numbers are determined. The current investigation provides new perspectives to understand the mechanism behind the gas adsorption behavior of clathrate hydrates.

Y1 receptor ligand-based nanomicelle as a novel nanoprobe for glioma-targeted imaging and therapy.

Due to the molecular and cellular heterogeneity of glioma, discovery of novel targeted sites and ligands for glioma imaging and therapy remains challenging. Neuropeptide Y (NPY) Y1 receptors (Y1Rs) are highly over expressed in various brain tumors including glioma, and can serve as potential targeting sites for glioma imaging and therapy. Here, we show by in vivo fluorescent imaging that a highly selective Y1R ligand, [Asn6, Pro34] NPY (AP-NPY), facilitated circumvention of the blood brain barrier (BBB) by nanomicelles specifically targeting glioma. Modification with AP-NPY stabilized doxorubicin-loaded nanomicelles in the normal physiological state and promoted drug release in the acidic tumor microenvironment. Furthermore, targeted delivery of AP-NPY nanomicelles improved the therapeutic efficacy of doxorubicin for glioma, producing a prolonged survival rate. These results suggest that Y1R is a novel targeted receptor, and its selective ligand AP-NPY improves BBB permeability and glioma targeting. Our study paves the way for developing a novel delivery system for diagnosis and treatment of glioma in which Y1Rs are over expressed.

Ab Initio Studies on the Clathrate Hydrates of Some Nitrogen- and Sulfur-Containing Gases.

Ab initio calculations are performed to investigate the host-guest interactions and multiple occupancies of some sulfur- (H2S, CS2) and nitrogen-containing (N2, NO, and NH3) molecules in dodecahedral, tetrakaidecahedral, and hexakaidecahedral water cages in this work. Five functionals in the framework of density functional theory are compared, and the M06-2X method appears to be the best to predict the binding energies as well as the geometries. Results show that N2 and NO molecules are more stable in the 51264 cage, while NH3 and H2S prefer to stabilize in the 51262 cage. This suggests that the sI hydrates of NH3 and H2S exhibit higher stability than the sII structures and that sII NO hydrate is more stable than sI NO hydrate. N2 is found to be more stable in type II structure with single occupancy and to form type I hydrate with multiple occupancy, which is consistent with the experimental observations. As to the guest molecule CS2, it may undergo severe structural deformation in the 512 and 51262 cage. For multiple occupancies, the 512, 51262, and 51264 water cages can trap up to two N2 molecules, and the 51264 water cage can accommodate two H2S molecules. This work is expected to provide new insight into the formation mechanism of clathrate hydrates for atmospherically important molecules.

Direct supramolecular interacted graphene oxide assembly on graphene as an active and defect-free functional platform.

Graphene oxide (GO), which is structurally analogous to graphene, was used as the ink to precisely tune the surface activity of graphene, without diminishing the outstanding properties of the 2D sp2 network, via microcontact printing-induced multiple π-π stacking interactions. The GO/graphene hybrids, thus, provide a robust platform for chemical/photo-active GO for further chemical modification with multifunctionality.

Cytocompatibility of Ti3AlC2, Ti3SiC2, and Ti2AlN: In Vitro Tests and First-Principles Calculations.

Herein, the cytocompatibility of selected MAX phases, Ti3AlC2, Ti3SiC2, and Ti2AlN, were systematically evaluated using in vitro tests for the first time. These phases were anoxic to preosteoblasts and fibroblasts. Compared with the strong viable fibroblasts, the different cellular responses of these materials were clearly distinguishable for the preosteoblasts. Under an osteoblastic environment, Ti2AlN exhibited better cell proliferation and differentiation performance than Ti3AlC2 and Ti3SiC2. Moreover, the performance was superior to that of a commercial Ti-6Al-4V alloy and comparable to that of pure Ti. A possible mechanism was suggested based on the different surface oxidation products, which were determined from the binding energy of adsorbed Ca2+ ions using first-principles calculations. Compared with the partially oxidized TiCxOy layer on Ti3AlC2 and Ti3SiC2, the partially oxidized TiNxOy layer on the Ti2AlN had a stronger affinity to the Ca2+ ions, which indicated the good cytocompatibility of Ti2AlN.

Tuning hydrogen storage in lithium-functionalized BC2N sheets by doping with boron and carbon.

First-principles calculations are used to explore the strong binding of lithium to boron- and carbon-doped BC2N monolayers (BC2NBC and BC2NCN, respectively) without the formation of lithium clusters. In comparison to BC2N and BC2NCB, lithium-decorated BC2NBC and BC2NCN systems possess stronger s-p and p-p hybridization and, hence, the binding energy is higher. Lithium becomes partially positively charged by donating electron density to the more electronegative atoms of the sheet. Attractive van der Waals interactions are responsible for binding hydrogen molecules around the lithium atoms. Each lithium atom can adsorb three hydrogen molecules on both sides of the sheet, with an average hydrogen binding energy of approximately 0.2 eV, which is in the range required for practical applications. The BC2NBC-Li and BC2NCN-Li complexes can serve as high-capacity hydrogen-storage media with gravimetric hydrogen capacities of 9.88 and 9.94 wt %, respectively.