Stacking Of Nanosheets Research Articles

Template synthesis of porous hierarchical Cu2ZnSnS4 nanostructures for photoelectrochemical water splitting

Environmentally friendly and low-cost Cu2ZnSnS4 (CZTS) is a promising light absorber for photoelectrochemical (PEC) hydrogen production from water splitting due to the earth-abundant elements, high absorption coefficient, and narrow bandgap. Herein, the hierarchical CZTS film with porous nanostructures was successfully synthesized by a template method. The hierarchical CZTS film was composed of flower-like particles, which were assembled with thin CZTS nanosheets. Macropores were generated owing to the aggregation of flower-like spheres, and mesopores were formed from the stacking of CZTS nanosheets. Compared to the dense CZTS film, the porous hierarchical CZTS film showed a much higher PEC property for water splitting. The improved performance could be attributed to three merits of the porous hierarchical morphology: enhanced light absorption, improved charge separation and transfer, and enlarged electrochemically active surface area. This study provides a useful idea to design efficient semiconductor photoelectrodes for water splitting with delicately controlled morphology.

Intercalation of Carbon Nanosheet into Layered TiO2 Grain for Highly Interfacial Lithium Storage.

Interfacial energy storage contributes a new mechanism to the emergence of energy storage devices with not only a high-energy density of batteries but also a high-power density of capacitors. In this study, success was achieved in preparing a highly ordered two-dimensional (2D) carbon/TiO2 (C/TiO2) nanosheet composite using commercially available organic molecules with multifunctional groups and taking advantage of the wedge effects, oxidative polymerization, and carbonization. An experiment was conducted to validate the excellent performance of this 2D composite with respect to interfacial energy storage. The coin cell with 2D C/TiO2 nanosheet composite demonstrates a specific capacity of as high as 510 mAh g-1 and a high specific energy of 390.9 Wh kg-1 at a specific power of 75.9 W kg-1 with a current density of 0.1 A g-1, and it also remains 39.0 Wh kg-1 at a specific power of 8.2 kW kg-1 with a high current density of 12.8 A g-1. The excellent electrochemical performance can be attributed to the superior artificial interface capacitive Li+ storage capability, which would bridge the energy and power density gap between batteries and capacitors. Meanwhile, there are two varieties of carbon derivatives, 2D carbon nanosheet stacks and exfoliated carbon nanosheets, which can be obtained by wet-chemical etching and mechanical peeling. The experimental route is simple from commercially available raw materials, and it could be scalable at a low cost and large scale, which makes it suitable for application in various fields such as energy storage, nanocatalysis, sensors, and so on.

Ti3C2Tx MXenes as thin broadband absorbers

Atomically multilayered two-dimensional transition-metal carbides have abundant interfaces, and are very promising as outstanding electromagnetic absorbing materials at thin thickness. Here, a Ti3C2Tx MXene was prepared by hydrofluoric acid etching method, and has typical multilayered morphology with stacks of nanosheets. The microwave dielectric behaviours of the Ti3C2Tx with efficient microwave absorption were investigated. The Ti3C2Tx presents good impedance matching, achieved with effective absorption bandwidth covering from 12.4 GHz to 17.1 GHz, with thickness of only 1.5 mm, which nearly covers the whole Ku band. The microwave absorption performance was adjusted, and the Ti3C2Tx has a minimum reflection loss of −34.4 dB at 12 GHz at only 1.7 mm. This study demonstrates the real potential of Ti3C2Tx MXene materials as electromagnetic wave thin broadband absorbers.

Colloidal Synthesis of NbS2 Nanosheets: From Large-Area Ultrathin Nanosheets to Hierarchical Structures.

Layered NbS2, a member of group-V transition metal dichalcogenides, was synthesized via a colloidal synthesis method and employed as a negative material for a supercapacitor. The morphologies of NbS2 can be tuned from ultrathin nanosheets to hierarchical structures through dynamics controls based on growth mechanisms. Electrochemical energy storage measurements present that the ultrathin NbS2 electrode exhibits the highest rate capability due to having the largest electrochemical surface area and its efficient ion diffusion. Meanwhile, the hierarchical NbS2 shows the highest specific capacitance at low current densities for small charge transfer resistance, displays 221.4 F g−1 at 1 A g−1 and 117.1 F g−1 at 10 A g−1, and cycling stability with 78.9% of the initial specific capacitance after 10,000 cycles. The aggregate or stacking of nanosheets can be suppressed effectively by constructing hierarchical structure NbS2 nanosheets.

A Hybrid Assembly of MXene with NH2−Si Nanoparticles Boosting Lithium Storage Performance

A facile hybrid assembly between Ti3 C2 Tx MXene nanosheets and (3-aminopropyl) triethoxylsilane-modified Si nanoparticles (NH2 -Si NPs) was developed to construct multilayer stacking of Ti3 C2 Tx nanosheets with NH2 -Si NPs assembling together (NH2 -Si/Ti3 C2 Tx ). NH2 -Si/Ti3 C2 Tx exhibits a significantly enhanced lithium storage performance compared to pristine Si, which is attributed to the robust crosslinking architecture and considerably improved electrical conductivity as well as shorter Li+ diffusion pathways. The optimized NH2 -Si/Ti3 C2 Tx anode with Ti3 C2 Tx : NH2 -Si mass ratio of 4 : 1 displays an enhanced capacity (864 mAh g-1 at 0.1 C) with robust capacity retention, which is significantly higher than those of NH2 -Si NPs and Ti3 C2 Tx anodes. Furthermore, this work demonstrates the important effect of the MXene-based electrode architecture on the electrochemical performance and can guide future work on designing high-performance Si/MXene hybrids for energy storage applications.

TiO2 Nanoparticles In Situ Formed on Ti3C2 Nanosheets by a One‐Step Ethanol‐Thermal Method for Enhanced Reversible Lithium‐Ion Storage

AbstractMXenes, a family of two‐dimensional (2D) carbides, nitrides and carbonitrides, have been considered as promising anode materials for lithium‐ion (Li+) batteries due to their good flexibility and conductivity. Herein, we report a one‐step ethanol‐thermal method to form titanium dioxide (TiO2) nanoparticles on the surface of Ti3C2 nanosheets with less oxidation and high 2D structural integrity, and meanwhile investigate the properties of reversible Li+ storage. It indicates that a reversible specific capacity of 257.3 mAh g−1 at the current density of 50 mA g−1 can be delivered. Moreover, the specific capacity of 141.4 mAh g−1 with the coulombic efficiency of ∼100 % can be obtained even after 250 cycles at the current density as high as 1 A g−1. The improved capacity can be attributed to the synergetic effect that prevents stacking of the Ti3C2 nanosheets by the surface‐anchored TiO2 nanoparticles and meanwhile avoids agglomerating of TiO2 nanoparticles due to the existence of Ti3C2 nanosheets.

A novel robust adsorbent for efficient oil/water separation: Magnetic carbon nanospheres/graphene composite aerogel

Recently, graphene aerogels (GAs) have attracted considerable research attention in oil/water separation owing to their remarkable properties. However, the serious stacking of graphene oxide nanosheets (GO) would lead to low adsorption capacity and poor recyclability. For the first time, with alkaline ammonium citrate as reducing agent and nitrogen source, the point-to-face contact between magnetic carbon nanospheres (MCNS) and graphene sheets was adopted to effectively inhibit the aggregation of graphene sheets. Nitrogen-doped magnetic carbon nanospheres/graphene composite aerogels (MCNS/NGA) were fabricated under weakly alkaline conditions by one-step hydrothermal in-situ electrostatic self-assembling strategy. The aerogels have low density, super-elasticity (up to 95 % compression), high specific surface area (787.92 m2 g−1) and good magnetic properties. Therefore, they exhibit adsorption capacity in the range of 187–537 g g−1 towards various organic solvents and oils, superior to most reported materials to date. In addition, thanks to their good mechanical properties, excellent thermal stability and flame retardancy, they can be regenerated by squeezing, distillation and combustion. More importantly, magnetic control technology can be adopted to realize oriented adsorption and facilitate recycling of organic solvents and oils in extreme environments.

Significantly Enhanced Thermoelectric Properties of Organic-Inorganic Hybrids with a Periodically Ordered Structure.

The deficient order in amorphous components severely affects the thermoelectric (TE) properties in polymers. Encouragingly, two-dimensional layered double hydroxides (LDHs) have been regarded as an efficient host material to tune the conformation of guest molecules and construct ordered hybrids. Herein, we report a facile construction of periodically ordered organic-inorganic TE hybrids by alternative stacking of inorganic LDH nanosheets and organic poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) molecules. The ordered structure of PEDOT:PSS-LDH gave rise to the extended molecular configuration of PEDOT:PSS, resulting in the improved carrier mobility in the hybrids. Moreover, the energy filtering was induced by such a periodically ordered structure, which blocked the low-energy carriers preferentially and improved the Seebeck coefficient in the hybrids. Therefore, the power factor of the PEDOT:PSS-LDH hybrid was 120-fold higher than that of pristine PEDOT:PSS. These results not only establish an effective method for the construction of periodically ordered TE materials but also address the significance of an ordered structure of molecules in TE materials.

Boron doping and structure control of carbon materials for supercapacitor application: the effect of freeze-drying and air-drying for porosity engineering

In the present work, we demonstrate a strategy of combining boron doping and porosity engineering for a highly modulated carbon component and pore structure, in which two contrasting drying methods (air-drying, freeze-drying) and their effects on supercapacitor performance are investigated in detail. Under freeze-drying and air-drying conditions, carbon nanoparticles and nanosheets are obtained, respectively. It is revealed that the carbon nanoparticles exhibit higher porosity (BET surface area of 1275 m2 g−1 and pore volume of 2.64 cm3 g−1) than the nanosheets (BET surface area of 1109 m2 g−1 and pore volume of 1.72 cm3 g−1); however, the boron content of the nanoparticles is lower (0.82 at.%) than that of the nanosheets (1.66 at.%). In the freeze-drying process, the direct sublimation of ice can prevent pore collapse, whereas stacking of nanosheets via Van der Waals interactions occurs during the air-drying process. With the air-drying method, the B-O-B structures produced by the evaporation process preferentially react with carbon, which promotes the production of more boron functional groups. As a result, the capacitive measurement indicates that the carbon nanoparticle electrode delivers larger capacitance of 129 F g−1 at 1 A g−1 and higher energy density of 41 Wh kg−1 in the two-electrode system, in contrast to those of the nanosheets (capacitance of 98 F g−1 and energy density 31 Wh kg−1) using EMIMBF4/AN as electrolyte. This kind of effect of freeze-drying and air-drying for porosity engineering is probably helpful for further supercapacitor applications.

Beneficial restacking of 2D nanomaterials for electrocatalysis: a case of MoS2 membranes.

Restacking of 2D nanomaterials is often deemed to be detrimental to their applications. In contrast to this common notion, here we demonstrate that tightly packed stacked MoS2 exhibits a higher electrocatalytic activity for hydrogen evolution than the more loosely stacked ones.

3D layered nano-flower MoSx anchored with CoP nanoparticles form double proton adsorption site for enhanced photocatalytic hydrogen evolution under visible light driven

Three-dimensional (3D) nanoflower can inhibit lamellar stacking to expose more edge active sites, showing larger surface areas and shorter electron transport paths. In this work, with the help of polyvinylpyrrolidone (PVP) morphology modifier, three-dimensional (3D) nanoflower MoSx was synthesized by hydrothermal method. The ZIF-9(Co)-derived CoP nanoparticles adhere to MoSx by means of a three-dimensional flower structure to form a MoSx/CoP photocatalyst. The 3D nano-flower of MoSx with layered structure can be observed in TEM images. This unique layered structure not only effectively inhibits the mutual stacking of MoSx nanosheets but also play a good role in dispersing CoP nanoparticles. In addition, the unsaturated Mo and S atoms exposed to the edge also promote the edge activity of sulfur, playing the role of active sites. Meanwhile, the CoP can also serve as active sites, which P as base sites can trap the positively charged protons. During the catalytic reaction, the Co atom and the P atom act as the double proton adsorption site in the CoP [P(δ−)-Co(δ+)], which could accelerate the water cracking. Besides, PL, EIS and Mott-Schottky show that MoSx and CoP play a synergistic role during the reaction, and the contact between them opens up a channel for accelerating the electron transfer to avoid electron aggregation. Thus, the MoSx/CoP is exhibited lower overpotential, larger apparent current density and higher conductivity. The MoSx/CoP-20 shows the highest photocatalytic activity of 8730 μmol g−1 h−1 under visible light irradiation. In this work, MoSx/CoP has excellent photocatalytic activity, which provides a reference for future energy innovation.

Unlocking the high redox activity of MoS2 on dual-doped graphene as a superior piezocatalyst

The need for highly efficient and environmentally friendly catalysts for pollutants removal sustainably has drawn extensive attention. In particular, piezocatalysis is promising for harnessing low-frequency vibration (e.g., wind) to enable high-performance degradation of organic pollutants. Herein, we synthesized a sandwich-like piezocatalyst by fabricating MoS2 nanosheets onto S, N-codoped graphene (MoS2@SNG) through a facile hydrothermal strategy. Results indicate that the MoS2@SNG exhibited ultrahigh piezocatalytic activities for both oxidation and reduction degradation of pollutants. Benefiting from the reduced stacking of MoS2 nanosheets and accelerated electron transfer of heterogeneous graphene, MoS2@SNG had a sharply enhanced electric field (i.e., 29.3 mV of MoS2@SNG versus 4.7 mV of MoS2). Moreover, the abundance of active sites of Mo4+ being exposed over MoS2@SNG remarkably promoted the redox reaction. This study provided evidence that layered piezoelectric materials represent an attractive strategy for environmental remediation.

Highly repeatable and sensitive three-dimensional γ-Fe2O3@reduced graphene oxide gas sensors by magnetic-field assisted assembly process

Reduced graphene oxide (RGO)/metal oxide hybrids are considered as an ideal candidate for high-performance gas sensors due to their large specific surface areas and heterostructures. However, they usually suffer from the poor and unrepeatable sensing performance due to the sheet stacking of RGO nanosheets and the uncontrollable drop-coating process. In this work, three-dimensional (3D) γ-Fe2O3@GO core-shell hybrids have been synthesized by an electrostatic self-assembly method. The sensing device based on uniform 3D γ-Fe2O3@RGO film can be fabricated using the magnetic-field assisted drop-coating and in-situ thermal reduction. Experimental results show that the as-fabricated 3D γ-Fe2O3@RGO sensor exhibits a good selectivity and high sensitivity of 3.43 toward 50 ppm NO2, which is about 2.5 times higher than that of the pure plane-stacked RGO sensor. The response value is still 1.23 when the NO2 concentration drops down to 100 ppb. The high performance can be attributed to the larger surface area, less agglomeration and the formed heterostructures which significantly promotes charge transfer. Importantly, the magnetic-field assisted drop-coating method results in a good stable and repeatable sensing performance due to the magnetic properties of γ-Fe2O3. The architecture strategy opens a new general route to large-scale practical fabrications of high-performance gas sensors based on RGO/magnetic oxide hybrids.

Oriented Graphenes from Plasma-Reformed Coconut Oil for Supercapacitor Electrodes.

The utilization of vertical graphene nanosheet (VGN) electrodes for energy storage in supercapacitors has long been desired yet remains challenging, mostly because of insufficient control of nanosheet stacking, density, surface functionality, and reactivity. Here, we report a single-step, scalable, and environment-friendly plasma-assisted process for the fabrication of densely packed yet accessible surfaces of forested VGNs (F-VGNs) using coconut oil as precursor. The morphology of F-VGNs could be controlled from a continuous thick structure to a hierarchical, cauliflower-like structure that was accessible by the electrolyte ions. The surface of individual F-VGNs was slightly oxygenated, while their interior remained oxygen-free. The fabricated thick (>10 μm) F-VGN electrodes presented specific capacitance up to 312 F/g at a voltage scan rate of 10 mV/s and 148 F/g at 500 mV/s with >99% retention after 1000 cycles. This versatile approach suggests realistic opportunities for further improvements, potentially leading to the integration of F-VGN electrodes in next-generation energy storage devices.

Effect of the carbon loading on the structural and photocatalytic properties of reduced graphene oxide-TiO2 nanocomposites prepared by hydrothermal synthesis

This work deals with the preparation of reduced graphene oxide (RGO)-TiO2 composites by a one-step hydrothermal treatment. The effect of the RGO loading on both the structural properties and photocatalytic behavior of RGO-TiO2 is deeply addressed herein. The hydrothermal treatment promoted the reduction of graphene oxide, crystallization of TiO2 into anatase, and anchoring of TiO2 nanoparticles on RGO sheets. It was observed that the prepared anatase particles showed sizes below 10nm, whereas the RGO sheets displayed thicknesses smaller than 1nm. The use of RGO at concentrations up to 15wt% greatly increased the specific surface area of RGO-TiO2. It was demonstrated that the combination of RGO and TiO2 gives rise to materials with improved photocatalytic properties and tailored structural properties. The composite with the highest photoactivity was the one containing an RGO loading of 1wt%; this composite displayed a photocatalytic rate constant about 9.5 times higher than that evaluated for pure TiO2. This behavior may be related to the stacking of RGO nanosheets when its concentration is above 1wt%. Moreover, the addition of RGO in excess may prevent the activation of the TiO2 surface by UV light and also decrease the lifetime of the photogenerated electron-hole pairs. Therefore, it appears that 1wt% is the optimal loading of RGO to obtain a close interfacial contact between RGO and TiO2, leading to both an effective activation of TiO2 by UV radiation and an enhanced charge transfer between RGO and TiO2.

High-Rate Structure-Gradient Ni-Rich Cathode Material for Lithium-Ion Batteries

To simultaneously achieve high compaction density and superior rate performance, a structure-gradient LiNi0.8Co0.1Mn0.1O2 cathode material composed by a compacted core and an active-plane-exposing shell was designed and synthesized via a secondary co-precipitation method successfully. The tight stacking of primary particles in the core part ensures high compaction density of the material, whereas the exposed active planes, resulting from the stacking of primary nanosheets along the [001] crystal axis predominantly, in the shell region afford enhanced Li+ transport. Thus, this structure-gradient Ni-rich cathode material shows a high compaction density with excellent electrochemical performances, especially the rate performance, exhibiting excellent rate capability (160 mA h g-1 at 10 C), which is 62% larger than that of the pristine material within 2.75-4.3 V (vs Li+/Li). Our work proposes a possible strategy for designing and synthesizing layered cathode materials with the required hierarchical structure to meet different application requirements.

Effect of Si/Al Ratio on the Catalytic Activity of Two‐Dimensional MFI Nanosheets in Aromatic Alkylation and Alcohol Etherification

AbstractZeolite catalysts with pronounced two‐dimensional (2D) morphologies, specifically 2D MFI nanosheets composed of multilamellar stacks of MFI nanosheets, are hypothesized to offer accessibility advantages in catalytic reactions involving large, bulky molecules. The Si/Al ratio of zeolite catalysts is an important parameter determining their performance. However, the effect of Si/Al ratio on catalytic reactivity is not well understood in 2D zeolite catalysts. To provide further insight into this issue, the catalytic performance of 2D MFI nanosheet materials with different Si/Al ratios is compared using the liquid phase Friedel‐Crafts alkylation of mesitylene with benzyl alcohol & the self‐etherification of benzyl alcohol that occurs in parallel, both of which are catalyzed by Brønsted acid sites. The turnover frequency (TOF) of the catalysts is found to decrease with decreasing Si/Al ratio, with the etherification reaction being the main contributor to this trend. When the same reaction is carried out in the presence of a bulky poison, 2,6 di‐tert‐butylpyridine (DTBP), to selectively deactivate the external acid sites, only the etherification reaction of benzyl alcohol takes place in the micropores of the 2D MFI catalyst and the effectiveness factor is found to decrease with decreasing Si/Al ratio. The single component adsorption isotherms of benzyl alcohol show that the uptake normalized per acid site decreases with decreasing Si/Al ratio. Thus, increasing the density of acid sites in the micropores by decreasing the Si/Al ratio makes it more difficult for the reactant molecules to access them, as demonstrated by the decrease in TOF and the effectiveness factor. While the micropore reactions (benzene alkylation, alcohol etherification) showed a clear trend with Si/Al ratio, the reaction catalyzed on the external surface (mesitylene alkylation) did not, showing distinct differences in reactivity.

Three-Dimensional Fe3O4@Reduced Graphene Oxide Heterojunctions for High-Performance Room-Temperature NO2 Sensors

Metal oxide/reduced graphene oxide (RGO) heterojunctions have been widely used to fabricate room-temperature gas sensors due to large specific surface areas of RGO nanosheets and enhanced carrier separation efficiency at the interface. However, the sheet stacking of RGO nanosheets limits the full utilization of metal oxide/RGO heterojunctions. Herein, we demonstrate a high-performance room-temperature NO2 gas sensor based on 3D Fe3O4@RGO p-n heterojunctions with core-shell structure, which were synthesized by self-assembly method and further reduction. The effect of different Fe3O4/RGO ratios and the relative humidity on the sensing performances have been investigated. The experimental results suggest that the 3D Fe3O4@RGO sensor exhibits a good selectivity and high sensitivity of 183.1% for 50 ppm NO2, which is about 8.17 times higher than that of the pure 2D RGO sensor. When exposed to 50 ppb of NO2, the response value still reaches to 17.8%. This enhanced sensing performance is mainly ascribed to the formed heterojunctions and the larger surface area of RGO nanosheets. This 2D to 3D heterostructures strategy provides a general route to fabricate ultrahigh-performance room-temperature RGO-based gas sensors.

Emerging switchable ultraviolet photoluminescence in dehydrated Zn/Al layered double hydroxide nanoplatelets

Layered double hydroxides show intriguing physical and chemical properties arising by their intrinsic self-assembled stacking of molecular-thick 2D nanosheets, enhanced active surface area, hosting of guest species by intercalation and anion exchanging capabilities. Here, we report on the unprecedented emerging intense ultraviolet photoluminescence in Zn/Al layered double hydroxide high-aspect-ratio nanoplatelets, which we discovered to be fully activated by drying under vacuum condition and thermal desorption as well. Photoluminescence and its quenching were reproducibly switched by a dehydration–hydration process. Photoluminescence properties were comprehensively evaluated, such as temperature dependence of photoluminescence features and lifetime measurements. The role of 2D morphology and arrangement of hydroxide layers was demonstrated by evaluating the photoluminescence before and after exfoliation of a bulk phase synthetized by a coprecipitation method.

Auto-programmed heteroarchitecturing: Self-assembling ordered mesoporous carbon between two-dimensional Ti3C2Tx MXene layers

Two-dimensional (2D) materials have attracted significant research interests for energy-storage applications owing to their unique structural and electronic properties. The modification of 2D materials with ordered mesoporous carbon (OMC) offers great opportunities for increasing the ion-accessible surface area and improving the ion diffusion. Herein, we report an unprecedented type of heterostructured MXene (Ti3C2Tx)/nitrogen-doped OMC (NOMC) hybrid, which is prepared via the self-assembly of melamine resol-P123 micelles with exfoliated Ti3C2Tx nanosheets. The NOMC exhibits well-ordered mesopores which are aligned with Ti3C2Tx nanosheet surface. This novel structure not only prevents the stacking of MXene nanosheets, but also provides a fast ion-diffusion path. When used as an electrode material for supercapacitors, the Ti3C2Tx-NOMC hybrid exhibits a high gravimetric capacitance of 329 F g−1 and a volumetric capacitance of 823 F cm−3. Furthermore, the Ti3C2Tx-NOMC composite also displays excellent rate capability and good cycling stability, thus highlighting the benefit of hybridizing MXene with ordered mesoporous carbon for for enhancing the energy storage performance.