Single-layer Nanosheets Research Articles

Synthesis of large monolayer titania nanosheets through flux method

ABSTRACT Herein, we present the structural and electrical properties of layered titanate, the precursor of an ideal two-dimensional (2D) nanosheet. A K0.8Ti1.73Li0.27O4 (KTLO) sample was synthesized by the flux method to maximize the area of the, relative to that under conventional methods, and to assess the oriented growth direction of the synthesis process. The structure of the protonated layered titanate H1.07Ti1.73O4 (HTO), obtained by replacing the intercalated K+ cations with H3O+, was analyzed to observe the effect of variables in the acid exchange process. Notably, the electrical characteristics of both layered titanates in the bulk state have not been previously reported. Herein, we measured the dielectric permittivity of bulk layered titanates without thermal treatment, which was observed to be lower than that of the nanosheet form. Additionally, these titanates were successfully exfoliated into large single-layer nanosheets. Overall, these results provide insights to clarify the properties of layered materials prior to nanosheet exfoliation.

Hydrolysis of Methoxylated Nickel Hydroxide Leading to Single-Layer Ni(OH)2 Nanosheets

Various methods for the preparation of inorganic nanosheets have been established and they have contributed to the substantial development of the research on diverse two-dimensional materials. Covalent surface modification of layered metal hydroxides with alkoxy groups is known to effectively weaken the interactions between layers, although the modified ligands are irreversibly immobilized. This study proposes the use of methanol as a removable surface modifier forming monodentate alkoxy bonds to prepare nickel hydroxide nanosheets through hydrolysis. Methoxylated layered nickel hydroxide, consisting of randomly stacked nano-sized nickel hydroxide sheets (10-20 nm in size) having Ni-OCH3 groups on its surface, was synthesized in a powder form through the precipitation reaction of a nickel salt in methanol at room temperature. After dispersing the aggregated methoxylated nickel hydroxide in water, single-layer nickel hydroxide nanosheets with a thickness of 1.2 nm and a lateral size of 460 nm at maximum, which is larger than the size of original methoxylated nickel hydroxide were found in the suspension. The time-course experiments during hydrolysis suggested that two-dimensional crystal growth of exfoliated nickel hydroxide sheets proceeded, resulting in the formation of the nanosheets. Moreover, single-layer and nano-sized cobalt hydroxide was prepared through a similar manner. This work demonstrates that two-dimensional alkoxides consisting of polymeric M-O-M bonds are useful precursors for the design of metal-hydroxide-based nanomaterials.

Cobalt(II)-dianthracene Frameworks: Assembly, Exfoliation and Properties.

Metal-organic frameworks containing responsive organic linkers are attractive for potential applications in sensors and molecular devices. Herein we report three cobalt(II) phosphonates incorporating responsive dianthracene linkers, namely, Co2 (amp2 H2 )2 (H2 O)4 ⋅ 6H2 O (MDAF-1), Co2 (amp2 )(H2 O)4 ⋅ 2H2 O (MDAF-2) and Co(amp2 H2 ) ⋅ 2H2 O ⋅ 0.5DMF (MDAF-3), where amp2 H4 is pre-photodimerized 9-anthrylmethylphosphonic acid. MDAF-1 shows a layer structure in which dinuclear Co2 (PO3 H)2 units are inter-connected by dianthracene ligands. In MDAF-2 and MDAF-3, inorganic chains of corner-sharing {CoO4 } (or {CoO6 }) and {PO3 C} are cross-linked by dianthracene ligands into 3D frameworks. All compounds underwent thermo-induced phase transitions, first the de-solvation and then the de-dimerization of dianthracene (as well as the release of the remaining solvent molecules for MDAF-2 and -3), associated with magnetic changes. MDAF-1 can be exfoliated into single-layer nanosheets in water which show light-triggered luminescent changes.

Simultaneous Recognition of Dopamine and Uric Acid in the Presence of Ascorbic Acid via an Intercalated MXene/PPy Nanocomposite.

Two-dimensional (2D) MXenes have shown a great potential for chemical sensing due to their electric properties. In this work, a Ti3C2Tx/polypyrrole (MXene/PPy) nanocomposite has been synthesized and immobilized into a glassy carbon electrode to enable the simultaneous recognition of dopamine (DA) and uric acid (UA) under the interference of ascorbic acid (AA). The multilayer Ti3C2Tx MXene was prepared via the aqueous acid etching method and delaminated to a single layer nanosheet, benefiting the in-situ growth of PPy nanowires. The controllable preparation strategy and the compounding of PPy material remain great challenges for further practical application. A facile chemical oxidation method was proposed to regulate magnitude and density during the forming process of PPy nanowire, which promotes the conductivity and the electrochemical active site of this as-prepared nanomaterial. The MXene/PPy nanocomposite-modified electrode exhibited the selective determination of DA and UA in the presence of a high concentration of AA, as well as a wide linear range (DA: 12.5–125 μM, UA: 50–500 μM) and a low detection limit (DA: 0.37 μM, UA: 0.15 μM). More importantly, the simultaneous sensing for the co-existence of DA and UA was successfully achieved via the as-prepared sensor.

Resolving Single-layer Nanosheets as Short-lived Intermediates in the Solution Synthesis of FeS

Short-lived reaction intermediates play a critical role in mediating material synthesis. Such short-lived species often elude characterization because of the mismatch between the time scale of measurements capable of describing them and their lifetimes. Thus, we have limited ability to probe, understand, and control the mechanism for material synthesis. Here we demonstrate a new approach to in situ X-ray pair distribution function (PDF) measurements of dynamic nanomaterials structure that yields an unprecedented combination of reaction time resolution and sensitivity. Reaction time is resolved spatially as a function of position along a flow path. By applying this approach to the well-studied aqueous reaction leading to FeS, mackinawite, we identify a novel metastable intermediate, FeSlayer, that forms in the first second of the reaction and which can be described as individual FeS nanosheets. Recognizing these nanosheets as synthons in the reaction opens up the possibility to deliberately redirect this assembly of the nanosheet toward different phases, including novel heterostructures.

Electronic Properties and Scanning Tunneling Microscopy Simulation of MoS2 Nanosheets by Using Density Functional Theory

The ab-initio Density Functional Theory (DFT) approach is used to study the electronic properties of bulk and layered MoS2 nanosheets. For the layered structures mono, bi, tri, tetra and penta layered structure is used. The direct to indirect transition of bandgap is observed as the number of layers is increasing. This transition of bandgap is attributed to the van der Waals interlayer interaction between two layers of MoS2 nanosheets. The indirect bandgap in the bulk MoS2 is found to be 0.94 eV, whereas for a single layered nanosheet is found to be direct bandgap with the value of 1.83 eV. To confirm the surface termination and understand the surface morphology of MoS2 the scanning tunneling microscopy (STM) simulation is performed in constant height mode. It is found that the detection of surface atoms via STM depends on the tip atom of the STM.
 Dhaka Univ. J. Sci. 69(1): 53-57, 2021 (January)

Single Atom Ruthenium-Doped CoP/CDs Nanosheets via Splicing of Carbon-Dots for Robust Hydrogen Production.

Ultrathin two-dimensional catalysts are attracting attention in the field of electrocatalytic hydrogen evolution. This work describe a composite material design in which CoP nanoparticles doped with Ru single-atom sites supported on carbon dots (CDs) single-layer nanosheets formed by splicing CDs (Ru1 CoP/CDs). Small CD fragments bore abundant functional groups, analogous to pieces of a jigsaw puzzle, and could provide a high density of binding sites to immobilize Ru1 CoP. The single-particle-thick nanosheets formed by splicing CDs acted as supports, which improved the conductivity of the electrocatalyst and the stability of the catalyst during operation. The Ru1 CoP/CDs formed from doping atomic Ru dispersed on CoP showed very high efficiency for the hydrogen evolution reaction (HER) over a wide pH range. The catalyst prepared under optimized conditions displayed outstanding stability and activity: the overpotential for the HER at a current density of 10 mA cm-2 was as low as 51 and 49 mV under alkaline and acidic conditions, respectively. Density functional theory calculations showed that the substituted Ru single atoms lowered the proton-coupled electron transfer energy barrier and promoted H-H bond formation, thereby enhancing catalytic performance for the HER. The findings open a new avenue for developing carbon-based hybridization materials with integrated electrocatalytic performance for water splitting.

Highly efficient and selectivity removal of heavy metal ions using single-layer NaxKyMnO2 nanosheet: A combination of experimental and theoretical study

Manganese oxides (MnO2) are widely applied in heavy metal ions removal due to their low-cost, environmental-friendly and biocompatibility. However, the adsorption capacity of MnO2 need to be further improved to satisfy the demand of practical application. Herein, a highly dispersed single layer NaxKyMnO2 nanosheet was synthesized by a facile wet-chemical method with sodium dodecyl sulfonate as surfactant. The high surface specific area, excellent dispersibility and abundant oxygen vacancies endowed NaxKyMnO2 nanosheets with potential in heavy metal ions adsorption. The adsorption experiments results showed that NaxKyMnO2 nanosheets possessed high efficiency and selectivity towards lead ion (Pb2+) with a high adsorption capacity of 2091.8 μmol g−1. The NaxKyMnO2 also showed an excellent reusability with the removal rate of 95.4% for Pb2+ even after five cycles. Moreover, both the theoretical calculation and experimental data illustrated that the single layer NaxKyMnO2 nanosheets possess high selectivity to Pb2+ adsorption.

Single-Layered MXene Nanosheets Doping TiO2 for Efficient and Stable Double Perovskite Solar Cells.

The inorganic lead-free Cs2AgBiBr6 double perovskite structure is the promising development direction in perovskite solar cells (PSCs) to solve the problem of the instability of the APbX3 structure and lead toxicity. However, the low short-circuit current and power conversion efficiency (PCE) caused by the low crystallization of Cs2AgBiBr6 greatly limit the optoelectronic application. Herein, we adopt a simple strategy to dope single-layered MXene nanosheets into titania (Ti3C2Tx@TiO2) as a multifunctional electron transport layer for stable and efficient Cs2AgBiBr6 double PSCs. The single-layered MXene nanosheets significantly improve the electrical conductivity and electron extraction rate of TiO2; meanwhile, the single-layered MXene nanosheets change the surface wettability of the electron transport layer and promote the crystallization of the Cs2AgBiBr6 double perovskite in solar cell devices. Therefore, the PCE went up by more than 40% to 2.81% compared to that of a TiO2 based device, and the hysteresis was greatly suppressed. Furthermore, the device based on Ti3C2Tx@TiO2 showed the long-term operating stability. After storing the device for 15 days under ambient air conditions, the PCE still remained a retention rate of 93% of the initial one. Our finding demonstrates the potential of Ti3C2Tx@TiO2 in electron transfer material of high-performance double PSCs.

Incorporation of open-pore MFI zeolite nanosheets in polydimethylsiloxane (PDMS) to isomer-selective mixed matrix membranes

A mixture of H2SO4 and H2O2 (known as piranha solution) was used to remove organic structure-directing agents (OSDA) from multilamellar MFI zeolite (ML-MFI). An aqueous solution of MFI zeolite nanosheets was obtained via exfoliation of the OSDA-free ML-MFI zeolite through sonication in water followed by purification through sedimentation. The exfoliated MFI zeolite nanosheets were characterized by various techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and porosimetry by cryogenic N2 adsorption-desorption, indicating that single-layer MFI nanosheets were obtained and the micropores were opened. MFI nanosheets/polydimethylsiloxane (PDMS) mixed matrix membranes (MMMs) were prepared by the solution casting method using the open-pore MFI zeolite nanosheets as the filler and PDMS as the continuous phase. The obtained MMMs exhibited greatly improved butane isomer permeabilities and ideal selectivities for n-butane over i-butane compared to the pure PDMS membrane. An ideal selectivity of 15.7 for n-butane/i-butane was achieved on the MMM at 1 wt% loading of MFI nanosheets, with a n-butane permeability up to 15615 Barrer.

Adsorption of Organic Molecules and Surfactants on Graphene: A Coarse-Grained Study.

The research studies on the adsorption of surfactants on graphene help us to know how to use surfactants to exfoliate graphene from graphite or functionalize the graphene surface. Among them, molecular dynamics (MD) simulation has been widely used to investigate the adsorption of organic molecules and surfactants on graphene. In particular, coarse-grained (CG) MD simulation greatly improves the computational efficiency by simplifying the complexity of the studied systems, allowing us to explore the structure and dynamics of complex systems on larger spatial scales and longer time scales. However, an accurate prediction of the adsorption of surfactants on graphene is required by optimizing the interaction between surfactants and graphene, which is often overlooked by some CG models. In this work, we found that an accurate prediction of the adsorption enthalpies of organic molecules on graphene can be achieved by optimizing the interactions between organic molecules and benzene. Meanwhile, we simulated the adsorption of a surfactant on single-layer and double-layer graphene nanosheets, respectively. Our results revealed that increasing the temperature would favor the interactions between hydrophilic groups of surfactants. In addition, we discovered that the surfactant prefers to be adsorbed on the inner surfaces of double-layer graphene compared with the outer surfaces, and this is owing to the dehydration in the middle of double-layer graphene, which is beneficial to the hydrophilic interactions between surfactant molecules inside the double-layer graphene.

Direct bottom-up synthesis of size-controlled monodispersed single-layer magnesium hydroxide nanosheets modified with tripodal ligands.

Conventional top-down methods for preparing inorganic nanosheets possess fundamental challenges of morphological control. Herein, the direct synthesis of organically modified single-layer magnesium hydroxide nanosheets with narrow size distribution was achieved by the in situ modification of magnesium hydroxide with a tripodal ligand, tris(hydroxymethyl)aminomethane.

Size-selective synthesis of platinum nanoparticles on transition-metal dichalcogenides for the hydrogen evolution reaction.

We report a micellar system to prepare Pt-TMDs composites with tunable Pt nanoparticles (NPs, 2-6 nm in size) on single-layer TMDs (MoS2, TiS2, TaS2) nanosheets. The Pt-MoS2 composites have shown excellent performance for the hydrogen evolution reaction (HER) with the Pt NPs exhibiting a volcano-type size effect toward HER activity due to the synergistic effects between the Pt NPs and MoS2.

Direct synthesis of 1T-phase MoS2 nanosheets with abundant sulfur-vacancies through (CH3)4N+ cation-intercalation for the hydrogen evolution reaction

An efficient route based on in situ TMA cation ((CH3)4N+) intercalation is reported for direct synthesis of single-layer 1T-MoS2 nanosheets with abundant sulfur vacancies for the hydrogen evolution reaction.

Preparation and application of layered double hydroxide nanosheets.

Layered double hydroxides (LDH) with unique structure and excellent properties have been widely studied in recent years. LDH have found widespread applications in catalysts, polymer/LDH nanocomposites, anion exchange materials, supercapacitors, and fire retardants. The exfoliated LDH ultrathin nanosheets with a thickness of a few atomic layers enable a series of new opportunities in both fundamental research and applications. In this review, we mainly summarize the LDH exfoliation methods developed in recent years, the recent developments for the direct synthesis of LDH single-layer nanosheets, and the applications of LDH nanosheets in catalyzing oxygen evolution reactions, crosslinkers, supercapacitors and delivery carriers.

Low humid transport of anions in layered double hydroxides membranes using polydopamine coating

Lithium carbonate and lithium hydroxide are two compounds used for producing battery cathodes, however, lithium hydroxide is favoured in producing battery compounds as it possesses very high electrochemical potential and low density. As a result, there is substantial demand for development of anion-selective membranes capable of selecting hydroxide over carbonate. Although single-layer nanosheets of Layered Double Hydroxides (LDHs) have shown anion conductivity, the reassembling of the building blocks in a membrane-like morphology will not result in a conductive membrane. Here we show that an appropriate size of LDH nanosheets with post-treatment with polydopamine (PDA) can form a highly conductive hydroxide-selective membrane. Our experimental results show that the in-plane ion conductance of the membrane for different ions is OH− > Br− > I− > NO3− > Cl− > CO32− with corresponding selectivity ratios of OH− to Br−, I−, NO3−, Cl−, CO32− found to be 2.30, 2.83, 2.99, 3.11 and 4.38 respectively, showing the membrane's excellent hydroxide selectivity. Molecular dynamics (MD) simulations revealed that the small nanochannels of LDH increase its diffusion barrier against ion permeation and cause the partial dehydration of ions. We showed that ions move inside the LDH nanochannels in a semi-dry transport manner so that they attract to the surface more than water molecules inside the nanochannels. Selectivity predictions from MD simulations were in excellent agreement with the experimental data, confirming our hypothesis about ion transport in LDH nanochannels. This work takes advantage of the semi-dry transport mechanism of anions in the narrow nanochannels of LDHs to produce a hydroxide selective membrane applicable in the production of LiOH for the new generation of Li-ion batteries.

Anticorrosive and UV-blocking waterborne polyurethane composite coating containing novel two-dimensional Ti3C2 MXene nanosheets

In this work, Ti3C2 MXene, a novel two-dimensional nanosheet, was introduced to waterborne polyurethane (WPU) coatings to prepare a composite coating. First, MAX phase materials were in situ etched by HF acid and further intercalated by water molecules to obtain exfoliated single-layer MXene nanosheet. And then, composite coatings were prepared via solution-blending low addition (0–0.4 wt%) of MXene, self-prepared waterborne polyacrylate emulsion (PAE) and isocyanate hardener, applying on Q235 mild steel. Results of AFM, XRD SEM and SEM–EDS confirm that single-layer MXene nanosheets with large lateral-to-thickness ratio are successfully prepared and achieved homogenous distribution within WPU matrix. With 0.4 wt% MXene incorporated, the WPU/Ti3C2 MXene composite coatings reach a lowest corrosion current of 2.143 × 10–6 A/cm2, a decrease of one order of magnitude compared with blank WPU (1.599 × 10–5 A/cm2) and own an excellent UV-blocking property (almost block the whole UV light).

Fe doped amorphous single layered vanadyl phosphate nanosheets as highly efficient electrocatalyst for water oxidation

The search for earth-abundant water oxidation electrocatalysts with low-cost and high-performance is essential to the energy conversion field. Well defined, rational designed two-dimensional materials have attracted enormous interest in light of much more specific surface areas and unique electronic properties. Herein, we report a facile two-phase solvothermal approach for the synthesis of Fe doped amorphous single-layered (~0.85 nm) vanadyl phosphate nanosheets (Fe-Am VOPO4). The obtained electrocatalyst exhibits excellent OER electrocatalytic performance, only require overpotential of 215 mV and 270 mV to reach current densities of 10 and 100 mA cm−2 in 1.0 M KOH electrolyte, and long-term electrochemical stability of 40 h. This work strikes out a path of synthesis of graphene-like materials with amorphous phase, and explores a new type of phosphate for efficient OER electrocatalysts.

Designing novel organic inhibitor loaded MgAl-LDHs nanocontainer for enhanced corrosion resistance

The combination of LDHs nanocontainer and organic inhibitor demonstrates a great prospect in the field of corrosion protection. Whereas the traditional ion exchange method has a low efficiency owing to the small space and the firm force between the LDHs laminates. A high-efficiency approach of intercalating organic inhibitor into the LDHs is delivered in this work. The MgAl-LDHs loaded with 5-aminoindazole (AIA) were synthesized by separating the layered structures of hydrotalcites into single-layer nanosheets and then restructuring the exfoliated nanosheets with organic inhibitor. The as-synthesized LDHs-AIA− displayed a controlled release behavior and excellent anti-corrosion performance on the copper in 3.5 wt% NaCl solution, which were characterized by UV–vis spectra and electrochemical tests. Furthermore, a series of structure and morphology tests involving SEM, TEM, XRD, FTIR, XPS and TGA were also used for further analysis the LDHs. And the analysis of corrosion products and the anti-corrosion mechanism were carried out by SEM, XRD tests. The theoretical calculations concerning the adsorption of corrosion inhibitors on the copper surface were also investigated. It can be concluded that the inhibitor-loaded LDHs have the ability to adjustably release the corrosion inhibitor in corrosive medium, thereafter the corrosion inhibitor can adsorb onto the active sites of copper surface to prevent corrosion.

Ferromagnetic metal conversion directly from two-dimensional nickel hydroxide

We have demonstrated a direct metallic conversion from nickel hydroxide nanosheets to nickel metal nanostructures by thermal annealing in vacuum. The metal transition of the single-layer nanosheets deposited on a Si substrate was revealed by x-ray absorption near edge structure (XANES) measurements. The XANES signal significantly changed at annealing temperatures above 250 °C. The metal transition temperature coincides with the reported temperatures at which layered nickel hydroxide nanosheets are converted to nickel oxide nanosheets by calcination in air. Auger measurements confirmed that a dissociation of oxygen from the hydroxide nanosheet induces the metallic conversion. The converted nickel metallic structures exhibit ferromagnetic behavior revealed by x-ray magnetic circular dichroism (XMCD) measurement. Atomic force microscopy measurements indicate that diffusions of nickel atoms on the substrates leads to a structural change from a 2D-like structure to a particle-like structure.