Single-layer Nanosheets Research Articles

Tailoring the hardness of aluminum surface reinforced with graphene and C3N nanosheets

In this study, the performance of C3N, a single-layer crystalline carbon nitride, under out-of-plane loading is investigated and compared with graphene. For this purpose, molecular dynamics simulations are performed to simulate the nanoindentation process on a single-layer C3N and graphene nanosheet. A significant increase in the penetrating force is observed by adding nanomaterials to the aluminum surface. Comparing the effect of graphene and C3N shows more considerable hysteresis of C3N coating. This is mainly due to the higher penetration force of the C3N coating on the retraction path. C3N nanosheet was found 2.72% harder than graphene in the indentation test. Voronoi analysis shows that when nanomaterials are coated on aluminum, the relationship of the Voronoi surfaces of the atoms at the surface with the aluminum atoms as the substrate is generally different from that of the substrate Voronoi surfaces. The frequency of the imposed stress wave of indenter onto the midline of the substrate when the substrate coated with C3N is higher than graphene. This can be seen as evidence for the distribution of Voronoi vectors and the larger C3N hysteresis loop.

PLLA/Graphene Nanocomposites Membranes with Improved Biocompatibility and Mechanical Properties

In this work, nanocomposite membranes based on graphene and polylactide were evaluated for mechanical properties and biocompatibility. Single-layer graphene (SLG), graphene nanosheets (GNS), and poly L-lactic acid (PLLA) were prepared through layer-by-layer deposition and homogeneous mixing. The results revealed that PLLA/SLG nanocomposites and PLLA/GNS nanocomposites could show enhanced mechanical properties and biocompatibility. The addition of a tiny amount of SLG significantly improved Young’s modulus and tensile strength of the PLLA matrix by 15.9% and 32.8% respectively, while the addition of the same mass ratio of GNS boosted the elongation at break of the PLLA matrix by 79.7%. These results were ascribed to the crystallinity and interfacial interaction differences resulting from graphene incorporation. Also, improved biocompatibility was observed with graphene incorporation. Such nanocomposites membranes showed a lot of potential as environment-friendly and biomedical materials.

Nematic suspension of a microporous layered silicate obtained by forceless spontaneous delamination via repulsive osmotic swelling for casting high-barrier all-inorganic films

Exploiting the full potential of layered materials for a broad range of applications requires delamination into functional nanosheets. Delamination via repulsive osmotic swelling is driven by thermodynamics and represents the most gentle route to obtain nematic liquid crystals consisting exclusively of single-layer nanosheets. This mechanism was, however, long limited to very few compounds, including 2:1-type clay minerals, layered titanates, or niobates. Despite the great potential of zeolites and their microporous layered counterparts, nanosheet production is challenging and troublesome, and published procedures implied the use of some shearing forces. Here, we present a scalable, eco-friendly, and utter delamination of the microporous layered silicate ilerite into single-layer nanosheets that extends repulsive delamination to the class of layered zeolites. As the sheet diameter is preserved, nematic suspensions with cofacial nanosheets of ≈9000 aspect ratio are obtained that can be cast into oriented films, e.g., for barrier applications.

Li+ additive accelerated structural transformation of MoS2 cathodes for performance-enhancing rechargeable Mg2+ batteries

Rechargeable magnesium ion batteries (MIBs) with intercalation materials as the cathode are found to offer better performance in Li+ based electrolytes. Herein, we compare the electrochemical performance of MIBs using all-phenyl complex (APC) electrolytes before and after adding Li+ additives with layered MoS2 cathodes. We coincidentally describe the lithiation mechanism that leads to a phase transition whereby Li+ additives accelerate a MoS2 structural transformation from 2H to 1T which accounts for the superior battery performance (150 mAh/g, around two times the capacity of that without Li+ additives). In this process, MoS2 is partly exfoliated into single-layer nanosheets during cycle as evidenced by operando electrochemical synchrotron X-ray diffraction, ex situ X-ray photoelectron spectroscopy and scanning electron microscopy analyses. Accordingly, electrochemical cells with pre-exfoliated MoS2 nanosheets (liquid phase exfoliation) require fewer activation cycles (50 cycles, a quart of the cycles that using pristine MoS2) in APC electrolyte and better rate performance. Our work thus provides new insight in designing high-performance rechargeable MIBs by optimizing the intercalation cathodes using Li+ additives.

"On/Off" Switchable Sequential Light-Harvesting Systems Based on Controllable Protein Nanosheets for Regulation of Photocatalysis.

A controllable protein nanostructures-based "On/Off" switchable artificial light-harvesting system (LHS) with sequential multistep energy transfer and photocatalysis was reported herein for mimicking the natural LHS in both structure and function. Single-layered protein nanosheets were first constructed via a reversible covalent self-assembly strategy using cricoid stable protein one (SP1) as building blocks to realize an ordered arrangement of pigments. Fluorescent chromophores like carbon dots (CDs) can be precisely distributed on the protein nanosheets superficially via electrostatic interactions and make the ratio between donors and acceptors adjustable. After being anchored with a photocatalysis center (eosin-5-isothiocyanate, EY), the constructed LHS could sequentially transfer energy between two kinds of chromophores (CD1 and CD2), and further transfer to EY center with a high efficiency of 84%. Interestingly, the Förster resonance energy transfer (FRET) process of our LHS could be reversibly "On/Off" switched by the redox regulated assembly and disassembly of SP1 building blocks. Moreover, the LHS has been further proved to promote the yield of a model cross-coupling hydrogen evolution reaction and regulate the process of the reaction with the FRET process "On/Off" state.

Assembly of two-dimensional nanofluidic channel with high proton conductivity using single-layer MnO2 nanosheets

Two-dimensional (2D) single-layer δ-MnO2 (SLMO) with cations intercalated in the interlayers demonstrates specific structural characteristics, possessing superiority in the fields such as energy storage, catalysis, and sensor. However, the synthesis technology of large-lateral-size SLMO nanosheets with high uniformity is rare, which hinders their correlated research. Herein, we report an intercalation-assisted exfoliation approach to produce large-sized SLMO nanosheets with high dispersity in aqueous solution. Few-layer K+-intercalated δ-MnO2 (KMnO) nanosheets were used as a precursor to ensure complete exfoliation. On account of high dispersion and ultrathin 2D morphology, SLMO nanosheets self-assembled into a flexible and free-standing film to construct ordered nanochannels. A high surface charge density of 1.71 mC m−2 and proton mobility of 2.59 × 10−3 cm2 V−1 s−1 were achieved in the free-standing SLMO film. With the extraordinary properties and easily scaled fabrication of the 2D SLMO film, this approach will pave the way for the study of confined ion transportation and enable the easy construction of nanofluidic devices.

Prediction of spin-dependent electronic structures of 3d transition metals doped Hittorf's violet phosphorene towards spintronics

Modulation of spin-dependent properties in semiconductor is of importance in exploring the potential application in spintronics. Here, we theoretically investigated the structural deformation, electronic characters, and magnetic properties of the single-layered Hittorf's violet phosphorene nanosheet with 3 d transition metal atoms (ranging from Sc to Ni) doping and under external electric field. The electronic band structures and magnetic properties was found to be obviously modulated by the dopants: the existence of a large spin splitting of the dopant 3 d states tailors the semiconducting phosphorene into simple magnetic semiconductor or bipolar magnetic semiconductor, except for the case of Sc. The application of an appropriate electric field to V-doped system could induce a transition from bipolar magnetic semiconductor to half metal with the carriers fully spin-polarized in alternative spin channel. Moreover, Hittorf's violet phosphorene-based field-effect transistor was proposed with the aim of realizing gate-voltage control on the carriers' spin orientation. Our findings pointed out the possibilities for realizing spin-dependent field-effect transistor in doped Hittorf's violet phosphorene nanosheet and opened a window for the burgeoning field of multifunctional 2D nanoelectronics and spintronics devices. • Ranging from Ti to Ni dopants can behave (bipolar) magnetic semiconductors. • Sc-doped Hittorfene is a nonmagnetic semiconductor. • The application of the electric field to V-doped Hittorfene can induce half metal transition. • The reversed carriers' spin orientation would be potential for spintronic devices.

Edge stimulated hydrogen evolution reaction on monodispersed MXene quantum dots

Nanostructuring of 2D MXene has promised as a non-noble electrocatalyst for hydrogen evolution reaction. Nevertheless, the insufficient density and activity of active sites on its basal plane are largely limiting the catalytic performance. Here, we show that by tailoring the 2D MXene nanosheets into their 0D quantum dots (QDs), active sites from edges are enriched for hydrogen evolution reaction (HER). We also demonstrate the generality of such an active trend by expanding the synthetic methodology to other MXene-based QDs, i.e., M−Ti3C2 QDs, M−V2C QDs, S−Nb2C QDs, M−Nb2C QDs, from their corresponded multi-layer (M) or single-layer (S) MXene nanosheets. Taking Ti-based MXene as a typical example, its monodispersed QDs exhibit 13.7 times higher activity than that of their pristine nanosheets, while remaining a good stability. We find that the metal atoms at the edge of QDs bind the intermediate H* species more moderately and therefore deliver a much lower energy barrier, especially for those terminated with hydroxyl groups, and rationalize this finding by theoretical calculations, demonstrating a more active role on the edge sites than those on the basal planes.

Interlayer Modification of a Layered Silicate RUB-18 with 4-Phosphonophenylsilane and Its Surface Acidic Functions.

The interlayer silylation of a layered silicate H-RUB-18 (Si4O7(OH)2) using a new aromatic silylating reagent containing a phosphonic acid group (4-phosphonophenylsilane: PPS) was demonstrated (H-PPS-RUB-18). The phosphonic acid groups were attached to the silicate layers through the reaction of H-RUB-18 with (4-diethoxyphosphorylphenyl)-triethoxysilane (p-PPS-E), and the ester moieties were subsequently hydrolyzed with hydrochloric acid. H-PPS-RUB-18 is a solid acid, as indicated by the intercalation of various alkylamines and the catalytic acetalization of ketones. A systematic increase in interlayer spacing leading to surface acidic properties was obtained through intercalation with a series of alkylamines. In addition, H-PPS-RUB-18 was exfoliated, resulting in single-layer nanosheets with ca. 2.0 nm thickness. The catalytic acetalization of ketones was related to the interlayer spacing of the modified RUB-18.

Nickel-Iron Layered Double Hydroxide Dispersions in Ethanol Stabilized by Acetate Anions.

This work reports a method to obtain stable dispersions of nickel-iron layered double hydroxide (NiFe-LDH) nanosheets in ethanol by exposing the as-synthetized bulk NiFe-LDH to a sodium acetate solution or by adding acetate and citrate anions inside the reaction mixture. In the case of citrate-containing NiFe-LDH, the formation of single-layer nanosheets is confirmed by X-ray diffraction and atomic force microscopy measurements. Lastly, the effect of acetate ions on the electrocatalytic activity of NiFe-LDH is discussed for the oxygen evolution reaction. Our results provide useful information to improve the existing LDH exfoliation routes based on the use of green solvent alternatives to the mostly used formamide.

Ultrafine Co-MoS2 monolayer catalyst derived from oil-soluble single-molecule polyoxometalates for slurry phase hydrocracking

Herein, a novel single-molecule confinement-sulfurization strategy was proposed to construct ultrafine Co-MoS2 monolayer catalysts by using oil-soluble single-molecule polyoxometalates (POMs), (DODA)6Co2Mo10, as precursors. Well-defined Co2Mo10-POM, acting as molecular pre-assembling platform, is introduced for the atomically precise construction of bimetallic sites. Highly efficient conversion of VR under severe condition is realized with (DODA)6Co2Mo10. The origin of prominent catalytic performance can be attributed to the in-situ formation of ultrafine single-layered Mo-based nanosheets with abundant exposed CoMoS active sites. By combining synchrotron radiation-based X-ray absorption spectroscopy and density functional theory calculations, we further discover that there is clearly intimate interaction between Mo and Co atoms and the dissociation of molecular hydrogen at CoMoS phases is energetically preferred, leading to remarkable hydrogenation and coke suppression activity. This work highlights an idea on the rational design and targeted synthesis of ultrafine bimetallic catalysts for slurry phase HCK with dramatically synergistic catalysis.

Preparation of double-layered nanosheets containing pH-responsive polymer networks in the interlayers and their conversion into single-layered nanosheets through the cleavage of cross-linking points.

Double-layered nanosheets containing pH-cleavable polymer networks between two niobate layers were prepared by copolymerization of N-isopropylacrylamide and an acid-degradable crosslinker via surface-initiated atom transfer radical polymerization on the surface of hydrated interlayers (interlayer I) of K4Nb6O17·3H2O and subsequent exfoliation by the introduction of tetra-n-butylammonium (TBA) ions into anhydrous interlayers (interlayer II). Moreover, the double-layered nanosheets were converted into single-layered nanosheets by the cleavage of cross-linking points in polymer networks by lowering pH. Fourier transform infrared spectroscopy (FTIR) and thermogravimetry (TG) results showed that polymer networks were present, and nanosheets with a thickness of 10.8 ± 1.6 nm were observed by using an atomic force microscope (AFM) after exfoliation using TBA ions. The thickness of the nanosheets was decreased to 6.1 ± 0.9 nm by lowering the pH, and proton nuclear magnetic resonance (1H NMR) and UV-vis spectroscopy showed that the degradation of the cross-linkers proceeded, suggesting that the cleavage of the cross-linking points led to the conversion of double-layered nanosheets into single-layered nanosheets.

Dispersing Single-Layered Ti3c2tx Nanosheets in Hierarchically-Porous Membrane for High-Efficiency Li+ Transporting and Polysulfide Anchoring in Li-S Batteries

Dispersing Single-Layered Ti3c2tx Nanosheets in Hierarchically-Porous Membrane for High-Efficiency Li+ Transporting and Polysulfide Anchoring in Li-S Batteries

Ballistic Impact Analysis of Graphene Reinforced Kevlar-29 and Kevlar-129 Laminated Composites

Aircraft goes through various high-speed hindrances like bird attacks, etc. Kevlar is increasingly used in the aviation industry to overcome such hindrances. Kevlar, an organic fibre from the family of aromatic polyamides consists of Kevlar 29, Kevlar 129, Kevlar 49 and Kevlar 149. This paper focuses on the use of Kevlar 29 and Kevlar 129 due to its low stiffness in comparison with Kevlar 49 and Kevlar 149. When pure Kevlar 29 and Kevlar 129 with 20 layers each are impacted with a 9g bullet at 900m/s velocity, the bullet perforates the laminate completely. Hence, the Kevlar layers are reinforced with graphene nanoplates. The bullet impact simulation of the ‘single-layer Kevlar 29 reinforced with single-layer graphene nanosheet’ (K29G), ‘single layer Kevlar 129 reinforced with single-layer graphene nanosheet’ (K129G), ‘double layer Kevlar 29 reinforced with double-layer graphene nanosheets’ (KK29GG) and ‘double layer Kevlar 129 reinforced with double-layer graphene nanosheets’ (KK129GG) is carried out using LS-DYNA. Maximum principal stress, effective stress, absorbed energy and displacement of the composite laminates (K29G, K129G, KK29GG and KK129GG) are used to compare the performance of these four laminates in the targeted applications. Due to its higher maximum principal stress, effective stress and lower displacement, the composite laminate K29G proves to be the apt laminate for its usage in aircrafts.

Effect of point defects and nanopores on the fracture behaviors in single-layer MoS2 nanosheets

Point defects and nanopores are inevitable and particularly noticeable in single-layer (SL) MoS2. Molecular dynamics (MD) simulations have been done to comprehensively study the influences of point defects and nanopores on tensile deformation behaviors of SLMoS2 nanosheets, and the dependences of fracture properties on defect type and concentration, pore size, temperature and strain rate are discussed. The formation energy of S vacancy (VS) is the lowest one, but that of VMoS6 is the highest one, corresponding to the highest and lowest fracture stress, respectively. The local stress concentration around point defects and nanopores might lead to the early bond breaking and subsequent nucleation of cracks and brittle fracture upon tensile loading. A modified Griffith criterion is proposed to describe the defect concentration and pore size dependent fracture stress and strain. These findings provide us an important guideline for the structural design of 2D materials in future applications.

Standalone 2-D Nanosheets and the Consequent Hydrogel and Coacervate Phases Formed by 2.5 nm Spherical U60 Molecular Clusters in Dilute Aqueous Solution.

Unexpected hydrogel and coacervate are observed for dilute (1 mM) uranyl peroxide molecular cluster (Li68K12(OH)20[UO2(O2)(OH)]60, U60) solution in the presence of di- or trivalent salts. We report the mechanism as the formation of anisotropic two-dimensional (2-D) single-layer nanosheets, driven by counterion-mediated attraction due to the size disparity between U60 and small counterions. With weak monovalent cations, the nanosheets are bendable, resulting in hollow, spherical blackberry-type supramolecular assemblies in a homogeneous solution. With extra strong divalent or trivalent cations, the tough, free-standing sheets lead to gelation at ∼1 mM U60. These stiff nanosheets are difficult to bend into spherical blackberry-type structures; instead, they stay in solution and form hydrogel based on their significant excluded volumes. At higher ionic strength, the large, thin filmlike nanosheet structures stack together more compactly and consequently lead to the transition from gel phase to a coacervate phase, another surprise since it was formed without the presence of bulky polycations.

Direct Z-scheme construction of g-C3N4 quantum dots / TiO2 nanoflakes for efficient photocatalysis

The rapid recombination rate of photogenerated carriers in TiO2 has been limiting the photocatalytic performance. Herein, TiO2 thin flakes modified by g-C3N4 quantum dots (g-C3N4 QDs) were fabricated successfully through a facile thermal treatment of restacked single-layer nanosheets of Ti1.73O41.07- in the presence of urea as a source of g-C3N4 QDs. Characterizations showed that g-C3N4 QDs with a size of ~10 nm were homogeneously deposited on the surface of TiO2 thin flakes. Quenching experiments of •OH radicals and the detection of radicals by EPR certified the direct Z-scheme heterojunctions between g-C3N4 QDs and TiO2 flakes. The TiO2 nanoflakes/g-C3N4 QDs hybrid exhibited excellent activity for the photocatalytic hydrogen evolution from a methanol solution and the degradation of RhB due to the enhanced charge separation efficiency and improved light absorption in the direct Z-scheme heterojunctions. This study demonstrates that the rational design of heterojunction is effective for attaining the superior photocatalytic performance.

Intercalation-induced partial exfoliation of NiFe LDHs with abundant active edge sites for highly enhanced oxygen evolution reaction

Edge sites and interlayer space of NiFe layered double hydroxides (LDHs) play an important role in water oxidation. However, the combined effect of interlayer expansion and partial exfoliation on the catalytic activity is yet to be investigated. Herein, scalable synthesis of partially exfoliated citrate-intercalated NiFe LDHs with tunable interlayer space have been achieved. The effect of citrate concentration on the phase, morphology, surface elemental composition, electronic states of surface metals, and electrochemical properties are comprehensively studied. The unique structure results in improved intrinsic catalytic activity and abundant active edge sites for oxygen evolution reaction. The optimal NiFe LDHs show an overpotential of 225 mV at 10 mA cm−2, which is much smaller than that (∼305 mV) of the single-layer NiFe LDH nanosheets reported in the literature. The high catalytic activity can be mainly attributed to the combined effect between the enlarged interlayer space and the partial exfoliation/nanosheet thickness. That is, the interlayer space is related to the reaction kinetics/mechanism, while the degree of exfoliation affects the magnitude of the current density at a certain potential.

Exfoliation and in situ functionalization of MoS2 nanosheets for MRI-guided combined low-temperature photothermal therapy and chemotherapy

Traditional photothermal therapy (PTT) usually requires hyperthermia (greater than50 °C) to mediate effective tumor ablation, which could inevitably induce heating damage to the adjacent normal tissues. Therefore, to achieve effective tumor killing at relatively low temperature is crucial to improve the application of PTT. MoS2, as an attractive photothermal agents, have attracted much attention in biomedical applications. But the separation of exfoliation and functionalization of MoS2 nanosheets (NSs) partly complicates the preparation process of MoS2 NSs-based theranostic nanomaterials. Herein, we formulated a facile one-pot approach for the exfoliation and in situ functionalization of single-layer MoS2 nanosheets by using bovine serum albumin template gadolinium oxide (BSA-Gd2O3) nanoparticles as the exfoliating agent and magnetic resonance imaging (MRI) T1 contrast agent. On this basis, hyaluronic acid (HA) was further conjugated to target cancer cells with overexpression of CD44, and then gambogic acid (GA), a natural heat shock protein 90 inhibitor and antitumor drug was loaded, which could reduce the thermo-resistance of tumor cells and achieve effective PTT at a mild temperature (43–45 °C). In this research, both in vitro and in vivo experiments have confirmed that the GA/MoS2/BSA-Gd2O3-HA nanocomposite possesses good biocompatibility and is promising for the application in MRI-guided combined low-temperature PTT and chemotherapy.

MXene@Fe3O4 microspheres/fibers composite microwave absorbing materials: Optimum composition and performance evaluation

In order to fully utilize the advantages that one-dimensional (1D) materials are conducive to form a conductive network and surface wrinkled microspheres can cause multiple reflection. In this paper, two MXene-based magnetic composites have been innovatively fabricated. 1D MXene@Fe3O4 magnetic nanofibers with a diameter of 25 nm are obtained by the self-assembly of single-layer Ti3C2Tx MXene nanosheets and coating Fe3O4 nanoparticles via thermal decomposition method. The micro-scale surface wrinkled porous MXene@Fe3O4 composite microspheres are prepared by simple ultrasonic atomization technology. A high efficiency microwave absorbing system is obtained by blending these two composites. Through the evaluation of microwave absorption performance, the optimal ratio of composite microspheres and nanofibers is conformed to be 7:3. The minimum reflection loss (RLmin) is −63.3 dB at a filler content of 40%. The corresponding matching thickness is 1.8 mm. The maximum effective absorption bandwidth (EABmax) is 5.2 GHz (12.8–18 GHz). Microwave absorbing mechanism study shows that the attenuation approaches include dielectric loss, magnetic loss, multiple reflection and conductive loss from the unique structure and impedance matching.