Liquid-phase exfoliation method to access cobalt oxide nanosheets in pH-neutral solutions

Two-dimensional materials have gained significant attention across the materials community due to their remarkable physical, chemical, electronic, and optical properties. With many two-dimensional materials discovered each year, investigations into their processing, scalability, and resulting properties are important to fully realize their potential in next-generation technologies. While numerous exfoliation techniques are available for a variety of two-dimensional materials, liquid-phase exfoliation techniques offer many advantages, particularly high throughput and scalability. Herein, we report a liquid-phase exfoliation method to access multilayer cobalt oxide nanosheets in pH-neutral aqueous solutions varying in size, concentration, and application. The size, thickness, and morphology of the multilayer nanosheets were confirmed using atomic force microscopy and transmission electron microscopy. Finally, selected cobalt oxide nanosheets underwent additional analysis of the crystallinity, structure, and cobalt valance.     

Correlation of exfoliation performance with interlayer cations of montmorillonite in the preparation of two-dimensional nanosheets

Montmorillonite (MMT) can be exfoliated into nanosheets used in a wide range of applications as two-dimensional (2D) material. The exfoliated MMT nanosheets with high diameter–thickness ratio appear to show superior properties in preparation of nanocomposites and other functional materials. In this work, the correlation of exfoliation performance with interlayer cations of MMT in the preparation of 2D nanosheets was investigated. The thickness and lateral diameter of MMT nanosheets were quantitatively measured by atomic force microscopy (AFM) technique. AFM results showed the exfoliated K-MMT had larger lateral diameter and thinner thickness than the exfoliated Ca-MMT under the same exfoliation conditions. Molecular simulation was applied to investigate the interlayer-binding energy (IBE), population analysis and density of states. K-MMT structures tend to have higher in-plane chemical bond intensity than Ca-MMT because of the impact of antibonding. The IBE of K-MMT was higher than that of Ca-MMT after the sorption of water molecules into the neighboring monolayers, indicating that thinner naonosheets tend to be easier obtained from Ca-MMT than K-MMT in liquid exfoliation. These findings might be helpful for the preparation and application of 2D MMTNS through controlling interlayer cation species and the exfoliation properties of MMT.     

An assessment of zinc oxide nanosheets as a selective adsorbent for cadmium

Zinc oxide nanosheet is assessed as a selective adsorbent for the detection and adsorption of cadmium using simple eco-friendly extraction method. Pure zinc oxide nanosheet powders were characterized using field emission scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The zinc oxide nanosheets were applied to different metal ions, including Cd(II), Cu(II), Hg(II), La(III), Mn(II), Pb(II), Pd(II), and Y(III). Zinc oxide nanosheets were found to be selective for cadmium among these metal ions when determined by inductively coupled plasma-optical emission spectrometry. Moreover, adsorption isotherm data provided that the adsorption process was mainly monolayer on zinc oxide nanosheets.     

Non-platinum oxygen reduction electrocatalysts based on pyrolyzed transition metal macrocycles

In this work pyrolyzed porphyrins were investigated for oxygen reduction electrocatalysis. Pyrolysis of non-supported cobalt and iron tetraphenylporphyrins in the temperature range of 500-800 °C generates high surface area catalysts with high degree of exposure of active sites to the reacting species. This is achieved through templating porphyrins on fumed amorphous silica that is removed after pyrolysis by etching with concentrated KOH.Detailed material characterization of the pyrolyzed materials is presented here. X-ray photoelectron spectroscopy (XPS) analysis of cobalt and iron porphyrins was used to elucidate the transformations of nitrogen, carbon, cobalt and iron species resulting from the heat treatment. Partial decomposition of the precursor material and formation of polymer-like network decorated by metal oxide particles are identified. Differences in the chemical composition of products of pyrolysis of FeTPP, CoTPP and Co/FeTPP are discussed. Transmission electron microscopy (TEM) imaging revealed the structure of the pyrolyzed porphyrins and was used to gain insight into the size of the metal crystals formed in the bulk. X-ray diffraction spectra (XRD) provided information about the type of crystals formed in the different formulations of the precursor porphyrins. Further, steady state polarization curves were obtained utilizing gas diffusion type electrodes in 0.5 M sulfuric acid and membrane electrode assembly (MEA) configurations under working PEM fuel cell conditions. This work revealed the necessity of the metal phases for the oxygen reduction process.     

Fabrication of a hybrid graphene/layered double hydroxide material

A hybrid graphene/Ni²⁺–Fe³⁺ layered double hydroxide material has been fabricated by the hydrothermal treatment of a mixed suspension of the exfoliated graphite oxide, Ni(NO₃)₂ 6H₂O, Fe(NO₃)₃·9H₂O, urea and trisodium citrate. The Ni²⁺–Fe³⁺ layered double hydroxide platelets are first homogeneously grown on the surface of GO nanosheets which are then reduced to graphene under a mild hydrothermal treatment. In the hybrid graphene/Ni²⁺–Fe³⁺ layered double hydroxide material, the restacking of graphene nanosheets is effectively prevented by the formation of Ni²⁺–Fe³⁺ layered double hydroxide platelets, and the graphene nanosheets exist in a complete exfoliation state.     

Synthesis and Characterization of Functionalized Multi-walled Carbon Nanotubes/Exfoliated Layered Double Hydroxide Nanosheets Hybrids via Electrostatic Force

In this paper, functionalized multi-walled carbon nanotubes (F-MWCNTs) were prepared through grafting melamine and 4, 4′-diphenylether dicarboxylic acid in proper sequence. The F-MWCNTs were combined with exfoliated Zn–Al layered double hydroxide (LDH) nanosheets via electrostatic force to form three-dimensional (3D) hybrids. The structural characteristics of the 3D hybrid composites were investigated by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis (TGA). It was found that F-MWCNTs were successfully combined with exfoliated LDH. TGA results demonstrated that the thermal stability of F-MWCNTs/exfoliated LDH nanosheets hybrids was decreased but the 3D hybrids produced a large amount of char.     

TEA协助层状LZH到ZnO纳米材料控制生长机制研究

三乙醇胺(TEA)协助下类水滑石氢氧化物碱式硝酸锌(LZH)结构到二维层状氧化锌纳米片(ZnO)转变及其性能研究相对较少。以Zn(NO₃)₂为锌源,借助于三乙醇胺/水体系,研究制备LZH和二维氧化锌纳米片工艺条件及影响合成层状LZH和氧化锌纳米材料的各种因素。利用X-射线粉末衍射(PXRD)、扫描电镜(SEM)、红外光谱(FT-IR)、荧光光谱(PL)等技术手段对其进行解析和表征,探讨层状碱式硝酸锌和氧化锌纳米材料之间的转变机制。结果表明:在研究范围内,温度和反应时间的改变不会影响产物的组成和结构,TEA用量直接影响到合成的目标产物。首先生成Zn(OH)₂,其次转化为Zn₅(OH)₈(NO₃)₂·2H₂O,过渡生成[Zn₅(OH)_10-x·2H₂O]^x+,最终自组装形成层状ZnO二维纳米材料,从而实现无需焙烧,通过控制TEA用...     

Tuning and thermal exfoliation graphene-like carbon nitride nanosheets for superior photocatalytic activity

In this work, ultrathin graphene-like carbon nitride nanosheets with rich nanoporous and excellent hydrophilic characteristics were synthesized by a simple and effective thermal exfoliation of bulk g-C₃N₄. In order to fully understand the effect of thermal exfoliation conditions on the texture, surface state, and photocatalytic activity of the resulting g-C₃N₄, a series of exfoliated g-C₃N₄ were prepared by adjusting the thermal exfoliation temperature and time. The detailed characterization and analysis distinctly suggested that increasing exfoliation temperature led to a large number of nitrogen vacancies and increased specific surface area, further prolonging exfoliation time, the thermal exfoliation degree was enhanced, more carbon vacancies and enlarged pore volume formed in the resulting products. Further, the exfoliation degree and photocatalytic ability of the resultant products were enhanced by increasing thermal exfoliation temperature and time. The optimized ultrathin graphene-like carbon nitride nanosheets exhibited a 89.6% degradation efficiency for Rh6G only in 10 min, which was much faster than other such nanosheets reported in previous literature.     

RESS制备石墨烯纳米片及防止其再团聚的新方法

本文以天然鳞片石墨为原料,利用超临界状态下二氧化碳的快速膨胀（RESS）来剥离石墨产生石墨烯纳米片。电子显微镜（SEM）表征证实RESS可有效地实现石墨的剥离,并产生了一些石墨烯纳米片层。同时,为了解决再团聚难题,提出利用碳纳米管在产生石墨烯纳米片间穿层的方法和利用小分子包覆法来防止其再团聚,实验证实都起到良好的效果。     

The effect of heat treatment on formation of graphene thin films from graphene oxide nanosheets

Graphene thin films with very low concentration of oxygen-containing functional groups were produced by reduction of graphene oxide nanosheets (prepared by using a chemical exfoliation) in a reducing environment and using two different heat treatment procedures (called one and two-step heat treatment procedures). The effects of heat treatment procedure and temperature on thickness variation of graphene platelets and also on reduction of the oxygen-containing functional groups of the graphene oxide nanosheets were studied by atomic force microscopy and X-ray photoelectron spectroscopy. While formation of the thin films composed of single-layer graphene nanosheets with minimum thickness of 0.37 nm and nearly without any functional group bonds was observed at the high temperature of 1000 °C in the one-step reducing procedure, similar high quality graphene thin films were obtained at the lower temperature of 500 °C in our two-step reducing temperature. The results also indicated possibility of efficient reduction of the graphene oxide thin films at even lower heat treatment temperatures (?500 °C).     

Utilization of multiple graphene layers in fuel cells. 1. An improved technique for the exfoliation of graphene-based nanosheets from graphite

An improved, safer and mild method was proposed for the exfoliation of graphene like sheets from graphite to be used in fuel cells. The major aim in the proposed method is to reduce the number of layers in the graphite material and to produce large quantities of graphene bundles to be used as catalyst support in polymer electrolyte membrane fuel cells. Graphite oxide was prepared using potassium dichromate/sulfuric acid as oxidant and acetic anhydride as intercalating agent. The oxidation process seemed to create expanded and leafy structures of graphite oxide layers. Heat treatment of samples led to the thermal decomposition of acetic anhydride into carbondioxide and water vapor which further swelled the layered graphitic structure. Sonication of graphite oxide samples created more separated structures. Morphology of the sonicated graphite oxide samples exhibited expanded the layer structures and formed some tulle-like translucent and crumpled graphite oxide sheets. The mild procedure applied was capable of reducing the average number of graphene sheets from 86 in the raw graphite to nine in graphene-based nanosheets. Raman spectroscopy analysis showed the significant reduction in size of the in-plane sp² domains of graphene nanosheets obtained after the reduction of graphite oxide.     

Ferromagnetism in exfoliated tungsten disulfide nanosheets

Two-dimensional-layered transition metal dichalcogenides nanosheets have attracted tremendous attention for their promising applications in spintronics because the atomic-thick nanosheets can not only enhance the intrinsic properties of their bulk counterparts, but also give birth to new promising properties. In this paper, ultrathin tungsten disulfide (WS₂) nanosheets were gotten by liquid exfoliation route from its bulk form using dimethylformamide (DMF). Compared to the antiferromagnetism bulk WS₂, ultrathin WS₂ nanosheets show intrinsic room-temperature ferromagnetism (FM) with the maximized saturation magnetization of 0.004 emu/g at 10 K, where the appearance of FM in the nanosheets is partly due to the presence of zigzag edges in the magnetic ground state at the grain boundaries.     

Synthesis and characterisation of hydrophilic and organophilic graphene nanosheets

Hydrophilic graphene nanosheets were rapidly synthesized by reacting graphene oxide nanosheets with poly(sodium 4-styrene sulfonate) and simultaneously reducing by hydrazine hydrate under hydrothermal conditions. Organophilic graphene nanosheets were prepared by reacting with octadecylamine and reduction by hydroquinone through a reflux process. Ultraviolet-visible spectroscopy and Fourier transform infrared spectroscopy measurements confirmed the attachment of organic molecules to the graphene nanosheets to achieve hydrophilic and organophilic affinity. X-ray diffraction, Raman spectroscopy, and transmission electron microscopy analysis indicated that the crystal structure of the graphene nanosheets was maintained intact after chemical functionalisation.     

Ammonia borane assisted solid exfoliation of graphite fluoride for facile preparation of fluorinated graphene nanosheets

Fluorinated graphene, which combines the unique properties of graphite fluoride and graphene, has attracted considerable attention in recent years. Here, we developed a facile, efficient, and scalable method for high-yield exfoliation of graphite fluoride into fluorinated graphene (fluorographene) nanosheets. The exfoliation approach consists of solid ball milling of graphite fluoride with ammonia borane and followed washing with ethanol to get rid of ammonia borane from the products. The majority of the as-synthesized fluorographene nanosheets consist of 1–6 atomic layers with grain sizes in the range of 0.3–1 μm. X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy demonstrated that fluorographene has the same structure as pristine graphite fluoride.     

Few-layer boron nitride nanosheets: Preparation,characterization and application in epoxy resin

Large-scale exfoliated boron nitride nanosheets were achieved via a liquid exfoliation sonication method by using sodium fluoride and ammonium hydroxide as Lewis base compounds. The crystal structure, surface functional groups, morphology and thickness of the as-prepared samples were characterized by X-ray diffraction, fourier transform infrared spectroscopy, transmission electron microscopy and atomic force microscopy, respectively. The as-prepared samples were introduced into epoxy resin to fabricate the polymer-based composites. Experimental results showed that the layer thickness of the as-prepared nanosheets was in the range of 1 to 3 layers. Moreover, it could improve the tensile properties of the matrix. When the loading of the as-prepared nanoparticles was 0.4 wt%, the tensile strength and elongation at break of the composites reached to their maximum values 65.6 MPa and 25.9%, which were increased by 118% and 192% more than that of pure resin. In addition, the as-prepared boron nitride samples could improve the thermostability and promote the curing of the matrix.     

Characterization of structure and electrochemical properties of lithium manganese oxides for lithium secondary batteries hydrothermally synthesized from δ-KxMnO2

Lithium manganese oxides have attracted much attention as cathode materials for lithium secondary batteries in view of their high capacity and low toxicity. In this study, layered manganese oxide (δ-K_xMnO₂) has been synthesized by thermal decomposition of KMnO₄, and four lithium manganese oxide phases have been synthesized for the first time by mild hydrothermal reactions of this material with different lithium compounds. The lithium manganese oxides were characterized by powder X-ray diffraction (XRD), inductively coupled plasma emission (ICPE) spectroscopy, and chemical redox titration. The four materials obtained are rock salt structure Li₂MnO₃, hollandite (BaMn₈O₁₆) structure α-MnO₂, spinel structure LiMn₂O₄, and birnessite structure Li_xMnO₂. Their electrochemical properties used as cathode material for secondary lithium batteries have been investigated. Of the four lithium manganese oxides, birnessite structure Li_xMnO₂ demonstrated the most stable cycling behavior with high Coulombic efficiency. Its reversible capacity reaches 155 mAh g⁻¹, indicating that it is a viable cathode material for lithium secondary batteries.     

体相掺镁球形钴酸锂前驱体的制备及表征

文章通过沉淀法合成了体相掺杂镁元素的球形钴酸锂正极材料前驱体。通过SEM形貌、密度测量、粒度表征等手段研究了共沉淀工艺对合成体相掺杂镁元素钴酸锂前驱体的影响,并通过EDS及ICP测试表征了镁元素掺杂的均匀性。研究结果表明:共结晶的反应时间对颗粒密度、颗粒形貌、粒度大小及分布有较大的影响,随着时间延长密度逐渐增加,颗粒形貌变好,粒度变大且趋于集中;共结晶体相掺杂镁的钴酸锂前驱体,镁元素能达到原子级混合均匀的要求;掺杂镁后,钴酸锂前驱体四氧化三钴物相未发生变化。掺镁钴酸锂前驱体在高压实密度钴酸锂得到了广阔的应用。     

Excellent electrical conductivity of the exfoliated and fluorinated hexagonal boron nitride nanosheets

The insulator characteristic of hexagonal boron nitride limits its applications in microelectronics. In this paper, the fluorinated hexagonal boron nitride nanosheets were prepared by doping fluorine into the boron nitride nanosheets exfoliated from the bulk boron nitride in isopropanol via a facile chemical solution method with fluoboric acid; interestingly, these boron nitride nanosheets demonstrate a typical semiconductor characteristic which were studied on a new scanning tunneling microscope-transmission electron microscope holder. Since this property changes from an insulator to a semiconductor of the boron nitride, these nanosheets will be able to extend their applications in designing and fabricating electronic nanodevices.     

Sol–gel preparation of cobalt manganese mixed oxides for their use as electrode materials in lithium cells

An ethanol dehydration procedure has been used to precipitate gel-like citrate precursors containing cobalt and manganese transition metal ions. Further annealing led to the Mn_xCo_3−xO₄ spinel oxide series (x: 1, 1.5, 2, 3). Annealing temperature and treatment time were also evaluated to optimize the performance of the oxides as active electrode materials in lithium cells. The manganese–cobalt mixed oxides obtained by this procedure were cubic or tetragonal phases depending on the cobalt content. SEM images showed spherical macroporous aggregates for MnCo₂O₄ and hollow spheres for manganese oxides. The galvanostatic cycling of lithium cells assembled with these materials demonstrated a simultaneous reduction of cobalt and manganese during the first discharge and separation of cobalt- and manganese-based products on further cycling. As compared with binary manganese oxides, a notorious electrochemical improvement was observed in the mixed oxides. This behavior is a consequence of the synergistic effect of both transition metal elements, associated with the in-situ formation of a nanocomposite electrode material when cobalt is introduced in the manganese oxide composition. Values higher than 400 mAh/g were sustained after 50 cycles for MnCo₂O₄.     

Ferromagnetism in freestanding MoS2 nanosheets

Freestanding MoS₂ nanosheets with different sizes were prepared through a simple exfoliated method by tuning the ultrasonic time in the organic solvent. Magnetic measurement results reveal the clear room-temperature ferromagnetism for all the MoS₂ nanosheets, in contrast to the pristine MoS₂ in its bulk form which shows diamagnetism only. Furthermore, results indicate that the saturation magnetizations of the nanosheets increase as the size decreases. Combining the X-ray photoelectron spectroscopy, transmission electron microscopy, and electron spin resonance results, it is suggested that the observed magnetization is related to the presence of edge spins on the edges of the nanosheets. These MoS₂ nanosheets may find applications in nanodevices and spintronics by controlling the edge structures.