Flake Morphology Research Articles

Basal plane oxygen exchange of epitaxial MoS2 without edge oxidation

The intentional formation of defects in transition-metal dichalcogenides, such as MoS2, is an attractive way to modify the electronic and chemical properties of this class of 2D materials. However, the mechanisms and methods available for selective doping or modification of the basal plane must be improved. Here we investigate the process of O defect formation in epitaxial single-layer MoS2 on Au(1 1 1) using scanning tunneling microscopy (STM) and ambient pressure x-ray photoelectron spectroscopy (AP-XPS) during oxidation with O2 and H2O gas from low vacuum to the mbar range. Both oxidants result in exchange of S in the upper part of the basal plane with O, in line with air exposure experiments. Temperature-dependent measurements show that this is an activated process with an experimentally estimated reaction barrier of ~0.79 ± 0.20 eV. We surprisingly find that the morphology of the MoS2 flakes and their edges remain intact in O2, even for relatively high degrees of basal plane O exchange, in contrast to the oxidation behavior of exfoliated single-layer MoS2. From analysis of atom-resolved STM images of the MoS2 edges, we can attribute this unusual stability to a passivating effect of excess edge sulfur species adsorbed under the sulfiding conditions of the MoS2 synthesis in H2S gas. We thus demonstrate that control over pre-sulfidation of the edges, temperature and pressure during oxidation can be used in a fast process to form strongly O doped single-layer MoS2 with no degradation of the initial shape and edge structure of the epitaxial MoS2 sheet.

Atomic Insight into Thermolysis‐Driven Growth of 2D MoS2

AbstractUnderstanding and controlling the transformations of transition metal dichalcogenides (TMDs) from amorphous precursors into two‐dimensional (2D) materials is important for guiding synthesis, directing fabrication, and tailoring functional properties. Here, the combined effects of thermal energy and electron beam irradiation are explored on the structural evolution of 2D MoS2 flakes through the thermal decomposition of a (NH4)2MoS4 precursor inside an ultrahigh vacuum (10−9 Torr) scanning transmission electron microscope (STEM). The influence of reaction temperature, growth substrate, and the initial precursor morphology on the resulting 2D MoS2 flake morphology, edge structures, and point defects are explored. Although thermal decomposition occurs extremely fast at elevated temperatures and is difficult to capture using current STEM techniques, electron beam irradiation can induce local transformations at lower temperatures, enabling direct observation and interpretation of critical growth steps including oriented attachment and transition from single‐ to multilayer structures at atomic resolution. An increase in the number of layers of the MoS2 flakes from island growth is investigated using electron beam irradiation. These findings provide insight into the growth mechanisms and factors that control the synthesis of few‐layer MoS2 flakes through thermolysis and toward the prospect of atomically precise control and growth of 2D TMDs.

Thermal properties of multilayer graphene and hBN reinforced copper matrix composites

The unique properties of graphene make it a very attractive application, although there are still no commercial products in which graphene would play a key role. Good thermal conductivity is undoubtedly one of the attributes which can be easily used both in materials involving large monoatomic layers, that are very difficult to obtain, as well as multilayer graphene flakes, which have been commercially available on the market for several years. The article presents the results of tests on the characteristic thermal properties of composites with the addition of 2–15% of multilayer graphene (MLG) crystals. The motivation of the study was literature reports showing the possibility of increasing the thermal conductivity of composites with MLG participation in the copper matrix. Since the production of composites with increased properties is associated with obtaining a strong orientation of the flakes in the structure, composites with hBN flakes exhibiting significantly worse but also directional thermal properties were produced for comparison. The paper showed a strong influence of flake morphology on the possibility of creating a directional structure. The obtained Cu/MLG composites with the addition of only 2% MLG were characterized by an increase in the thermal conductivity coefficient of about 30% in relation to sinters without the participation of MLG.

Surface modification of Al6061-SiC surface composite through impregnation of graphene, graphite & carbon nanotubes via FSP: A tribological study

Fabricating high wear resistance Al matrix composite without sacrificing its ductility and thermal properties is a critical issue in the designing of Al-based composites. In present study, a detailed wear mechanism of Al-SiC surface composite modified by the impregnation of Graphene, Graphite & carbon nanotubes through friction stir processing (FSP) is documented. The results revealed that with SiC and graphene nanoplatelets (GNP) reinforcements the friction coefficient and specific wear rate decreased by ~34% and ~50% as compared to as-received Al6061 alloy respectively. On the other hand, combination of SiC and carbon nanotubes (CNT) reinforcements severely deteriorates the wear resistance of the composite. The layered structure, large specific surface area and the wrinkled morphology of the graphene flakes are the primary reasons for the increased wear resistance. The exfoliation of GNP to few layered graphene (FLG) under the effect of plastic flow stresses during FSP also contributes significantly to the improvement of surface properties. The abrasion is identified as the dominant wear mechanism in Al-SiC-Graphite & Al-SiC-GNP hybrid composite. Whereas, delamination due to adhesion with the counter surface is the leading mechanism in Al-SiC-CNT hybrid composite. Various characterization strategies such as microstructural characterization through SEM, interfacial study through TEM and phase analysis through XRD corroborate the results. Finally, GNP is identified as the best reinforcement among the carbon family for improvement in wear resistance of Al6061-SiC surface composite.

MOF-Derived FeS/C Nanosheets for High Performance Lithium Ion Batteries.

In recent years, transitional metal sulfides have received much attention as lithium ion batteries (LIBs) anode. In this paper, FeS/C nanosheets are prepared through Fe-based metal organic frameworks (Fe-MOFs) as a precursor. The electrochemical performance of FeS/C nanosheets has been obviously improved due to the synergistic effect of the oxygen doped carbon and the special flake morphology. When the test current density is 0.1 A/g, the initial discharge capacities of FeS/C nanosheets is up to 1702 mAh/g and can retain reversible capacities of about 830 mAh/g over 150 cycles with the voltage ranging from 0.01 V to 3 V. Moreover, these composite materials are proved to have a good rate performance and the capacities reach 460 mAh/g even at a higher current density of 5 A/g. This work suggests that FeS/C nanosheets are excellent anode materials for LIBs.

CuxCe1-xO2 nanoflakes with improved catalytic activity and thermal stability for diesel soot combustion

In the present study, nanoflakes (NF) of CeO2-NF and CuxCe1-xO2-NF (x = 0.1, 0.2, 0.3 and 0.4) were prepared with the polyvinylpyrrolidone(PVP)-assisted co-precipitation method. The prepared samples were characterized with FE-SEM, HR-TEM, XRD, N2 adsorption, O2-TPD, H2-TPR, Raman and XPS. It was found that doping of Cu in CeO2-NF significantly promoted the formation of oxygen species that replenished the vacancies. The CuxCe1-xO2-NF exhibited much better activity than CeO2-NF that has no Cu doping both in tight contact and NO-assistant loose contact modes. The highest activity was observed for the sample of Cu0.2Ce0.8O2-NF, which was most likely caused by its moderate doping that induced both a large amount of surface adsorbed oxygen species and a special flake morphology, thus providing effective contact area. Both the cyclic and accelerated aging tests demonstrated that Cu0.2Ce0.8O2-NF was able to keep a good thermal stability.

Delamination of 2D coordination polymers: The role of solvent and ultrasound

Two novel cadmium-based 2D coordination polymers have been synthesized and characterized. Experimental results evidence that the best delamination processes occurs when weak interactions dominate the cohesion between layers and solvent molecules are occluded within the crystalline network. In this case, the delamination of the crystals occurs spontaneously in water. On top of that, and thanks to the high stability of the resulting (flake) colloidal dispersions, we have completed a detailed study of the sonication assisted delamination impact by: I) comparison of two different sonication approaches (bath vs. tip sonication) and II) optimization of final flake morphology and yield by controlling solvent and sonication time. Our results definitely pave the way for the fabrication and implementation of 2D coordination polymers using ultrasound.

Confined growth of NiCo2S4 nanosheets on carbon flakes derived from eggplant with enhanced performance for asymmetric supercapacitors

Transition metal oxides and compounds have been extensively investigated as electrode materials for high-performance supercapacitors. However, the poor rate performance and cyclic stability of these materials have hindered their practical applications. Herein, carbon flakes with an ultrahigh surface area (3608.4 m2 g−1) prepared from eggplant were used as substrates to enhance the electrical conductivity and overall capacitive performance of NiCo2S4 nanosheets. The composite exhibited a high specific capacitance (1394.5 F g−1 at 1 A g−1) which was 50% higher than that of the bare NiCo2S4 (989.8 F g−1 at 1 A g−1) and four times that of the eggplant-derived carbon (327 F g−1 at 1 A g−1). It also exhibited excellent rate capability (80.2% retention from 1 to 20 A g−1) and cyclic stability (124% retention after 10,000 cycles). These results were achieved by optimizing the pyrolysis of eggplant and the hydrothermal process which led to the confined growth of NiCo2S4 nanosheets on the carbon flakes due to coordination of Ni2+ and Co3+ cations to the polar groups of the carbon. The flake morphology, oxygen and nitrogen-rich surface and the ultrahigh surface area of the eggplant-derived carbon were found to be crucial for the well separation of NiCo2S4 nanosheets and consequently the excellent overall performance. Furthermore, asymmetric supercapacitors were constructed using the composite as the cathode and eggplant-derived carbon as the anode, which delivered a maximum energy density of 46.5 W h kg−1 at a power density of 801 W kg−1 and retained an energy density of 26.2 W h kg−1 at a maximum power density of 16 kW kg−1.

Preparation and properties of thermally conductive PLA/PA 610 biomass composites

Thermally conductive biomass composites were prepared using poly(lactic acid) (PLA)/polyamide 610 (PA 610) blends as the matrix and a hexagonal boron nitride (BN) as the thermally conductive filler via a melt-blending process. The experimental results showed that BN was dispersed in the blends as the flake type morphology. Tg (glass transition temperature) and Tm (melting temperature) of both PLA and PA 610 remained unchanged, but the thermal degradation temperature increased up to 30°C with the addition of 50 phr BN in the blends. The tensile strength and tensile modulus increased with the incorporation of BN in the blends. The thermal conductivity of the blends could be improved with the addition of BN, reaching the highest level of 0.86 W m−1·K−1 for the 50 phr-filled composite. The thermal conductivities of the composites were successfully predicted via a semi-theoretical mathematical model proposed by Lewis and Nielsen.

Ternary doped polyaniline-metal nanocomposite as high performance supercapacitive material

Polyaniline composites incorporating transition metals are still an burgeoning interest as a supercapacitive material due to the formation of metal-PANI hetero junctions. Herein we demonstrate a facile chemical synthesis of ternary doped supercapacitive polyaniline using transition metal as a codopant with an inorganic acid dopant utilizing the oxidant ferric chloride, which itself acts as a dopant. More interestingly the rational combination of external binary dopants can successfully modify PANI morphology. The as synthesized transition metals incorporated PANI (0.7 M Cu2+ + 1 M HCl) with a flake like morphology outperform as an energy storage material due to its high surface to volume ratio. Thus the synergistic effect of transitions metals (Cu & Fe) with HCl contribute towards the enhanced electrochemical performance of ternary doped PANI. The frequency dependent dielectric properties, ac conductivity and electrochemical characterizations using cyclic voltammetry and Electrochemical Impedance Spectroscopy of ternary doped PANI has been systematically studied. The ternary doped polyaniline demonstrated a high specific capacitance of 880 F/g and good cyclic stability in 0.1 M H2SO4 electrolyte.

Inkjet-Printed Reduced Graphene Oxide (rGO) Films For Electrocatalytic Applications

Reduced graphene oxide (rGO) films for application as electrocatalyst have been deposited using an inkjet materials printer, with excellent control over the film morphology and properties. The dispersions were prepared by the ultrasound-assisted liquid exfoliation method using polyvinylpyrrolidone (PVP) as stabilizer. The environmentally friendly ink formulation exhibits a stability for at least 6 months. The inkjet-printed rGO/PVP composite films were thermally treated at different temperatures, and it was found that the fraction of remaining PVP considerably affects the final structural, textural, optical and electrocatalytic characteristics. After annealing at 400°C, the rGO film exhibits a morphology of interconnected flakes and a very good transparency with a transmittance >90% in the entire visible spectrum. The inkjet-printed rGO films have been applied as electrocatalytic coating on fluorine-doped tin oxide (FTO) and tested for activity for the reduction of Co(bpy)33+ in acetonitrile. Dye-sensitized solar cells (DSSCs) fabricated with an rGO film on FTO annealed at 400°C as counter-electrode achieved 87% of the efficiency with respect to DSSCs with a Pt-catalyzed counter electrode. Hence, the fabrication of active and inexpensive rGO films using a high-throughput technique such as inkjet printing is an interesting approach to accelerate the development of competitive electrocatalytic nanomaterials.

Synthesis, Characterization and studies on photocatalytic and dielectric properties of Copper –Poly (1- Naphthylamine) Nanocomposite

Copper incorporated Poly 1- Naphthylamine nano composites with high photocatalytic activity has been prepared in a simple one step route .The composite was prepared by the chemical oxidative polymerization of 1-Naphthylamine in nitric acid medium with Ammonium peroxo disulphate in presence of copper ion.The copper incorporation was confirmed by the X-ray diffraction studies and SEM images shows a flake like morphology compared to the morphology of pristine PNA. The prepared composites shows good catalytic activity toward the borohydride assisted reduction of Methylene blue under sun light irradiation .The dielectric permittivity and A.C. conductivity values shows improvement upon copper incorporation.

Ammonia sensing properties of two-dimensional tin disulphide/tin oxides (SnS2/SnO2-x) mixed phases

2D-SnS2 flakes were synthesized via a wet chemical route and deposited as a thin film onto the Pt-interdigitated contacts of a ceramic substrate with aim to fabricate a conductometric sensor. The 2D-SnS2 sensing film was then annealed “in situ” under controlled conditions at different temperatures up to 400 °C. The morphological, microstructural and electrical properties of the sensing film, before and after the thermal treatment, were characterized by “in situ” techniques. SEM analysis has shown that the 2D flake morphology was maintained after thermal treatments, whereas Raman and EDX analysis have highlighted the partial/total modification of the SnS2 phase towards the SnO2. As a consequence of the formation of 2D-SnS2/SnO2-x mixed phases, a dramatic change of the electrical properties of the sensing film has been also observed.Conductometric gas sensors based on hybrid SnS2-SnO2-x nanoflakes were then obtained by simple self-annealing treatments of 2D-SnS2 film directly on the sensor substrate. The fabricated sensors were tested toward ammonia (NH3) as target gas. Results evidence that increasing the annealing temperature of the 2D-SnS2 sensitive film, the electrical resistance of the sensing layer decreases while the response towards ammonia increases, showing the maximum at an annealing treatment of 250 °C. This behaviour was explained on the basis of the morphological, microstructural and electrical modifications observed, highlighting the role of 2D-SnS2/SnO2-x mixed phases in determining the sensing properties towards ammonia.

Investigation on tribological and corrosion characteristics of oxide-coated steel and mild steel fiber-based brake friction composites

PurposeThe purpose of this work is to investigate the effect of oxide-coated steel in comparison with mild steel fibers on the tribological and corrosion performances of friction composites.Design/methodology/approachIn this study, the friction composites were developed in the form of standard brake pads by using oxide-coated steel and compared with mild steel fibers-based one without varying the other ingredients. The brake pads were developed as per the industrial procedure. The physical, mechanical, thermal properties of the developed brake pads were analyzed as per the industrial standards. The tribological properties were analyzed using the Chase test. The worn surface analysis was done using scanning electron microscope. Corrosion behavior was also analyzed in both salt and normal water conditions.FindingsThe experimental results indicate that the oxide-coated steel-based friction composites brake pads possess good physical, chemical, thermal, corrosion resistance and mechanical properties with stable fade and recovery characteristics because of its oxide coating and flake morphology.Originality/valueThis paper explains the influence of oxide-coated steel in friction composites for enhancing the tribological performance and corrosion resistance by its oxide coating and flake morphology which could potentially replace mild steel fibers-based problems in friction composites.

Comparative study of effective photoabsorber CuO thin films prepared via different precursors using chemical spray pyrolysis for solar cell application

Copper oxide (CuO) thin film is a promising material used as a photo-absorber layer in solar cell application due to its narrow bandgap, low cost and high abundance. In this work, the CuO thin films have been prepared using various source materials such as Cu(NO3)2·3H2O, CuCl2·2H2O and Cu(CH3COO)2·H2O by chemical spray pyrolysis technique. The XRD result shows an increased crystallite size for Cu–N compared with Cu–Cl and Cu–A films. Good optical absorption in the visible region is observed for all CuO films and Cu–N film illustrates high absorption coefficient in the order of 5.7 × 105 cm−1. From SEM analysis, flake like morphology is observed for Cu–N and Cu–A films. The good electrical property i.e. high conductivity (0.0611 S cm−1) and carrier concentration (2.287 × 1017 cm−3) is observed for Cu–N films. All CuO films exhibit a single semicircle impedance nature and smaller diameter of semicircle corresponding to Cu–N film indicates low electrical resistivity present in the sample. From I–V measurement, it is observed that the high current for Cu–N (16.6 µA) than Cu–Cl (2.3 µA) and Cu–A (10.7 µA). All CuO films are photo-responsive under solar light exposure, which is due to high absorption coefficient that leads to higher photocurrent (20.5 µA) for Cu–N film. The best solar cell performance is obtained for Cu–N film which shows an efficiency of 0.31%. From the above results, we conclude that Cu–N is an optimum precursor for fabricating CuO based thin film solar cell.

Interference in edge-scattering from monocrystalline gold flakes [Invited

We observe strongly dissimilar scattering from two types of edges in hexagonal quasi-monocrystalline gold flakes with thicknesses around 1 micron. We identify as the origin the interference between a direct, quasi-specular scattering and an indirect scattering process involving an intermediate surface-plasmon state. The dissimilarity between the two types of edges is a direct consequence of the three-fold symmetry around the [111]-axis and the intrinsic chirality of a face-centered cubic lattice. We propose that this effect can be used to estimate flake thickness, crystal morphology, and surface contamination.

Hierarchical catalytic electrodes of cobalt-embedded carbon nanotube/carbon flakes arrays for flexible solid-state zinc-air batteries

Flexible air cathodes, with active and stable catalytic activities for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are key components and urgently required for realizing wearable metal-air batteries. Herein, a facile solution impregnation strategy to construct metal-organic frameworks on electrospun nanofibers following melamine-assisted carbonization synthetic approach is reported to prepare low-cost, scalable, and efficient bifunctional air electrodes. The flexible electrode comprises carbon nanofibers as the backbone that supports nitrogen-doped, cobalt embedded carbon nanotubes/porous carbon flakes arrays hierarchical structures (CoNCNTF/CNFs). The combined advantages of optimized chemical compositions and CNT/carbon flakes morphology endow CoNCNTF/CNFs with a promising reversible oxygen overpotential of 0.76 V. The flexible solid-state zinc-air battery using CoNCNTF/CNFs film as the additive-free air electrode exhibit favorable battery capacity and power density, durable stability and good bendability, surpassing that of the precious metal-based air electrodes.

The taxonomy of graphite nanoplatelets and the influence of nanocomposite processing

The reinforcement efficiency of graphene in a nanocomposite relies on the size, morphology, defects and agglomeration of flakes. However, the characterisation is usually undertaken only for the raw materials and any changes that take place during processing are not taken into consideration. In this work, epoxy nanocomposites reinforced by graphite nanoplatelet (GNP) were prepared and nano-scale X-ray computed tomography was used to visualize the geometry, morphology and defects of the flakes, as well as the three dimensional agglomerates that are normally difficult to characterise by other techniques. In combination with micromechanical analysis, the taxonomy of the nanoplatelets is shown to be of great importance in controlling the mechanical properties of nanocomposites, and this has been shown to explain the deviations of the predictions of micromechanical models from the measured values. Particularly, it is shown that taking single average values of flake size may not be appropriate and the entire distribution of flake size need to be taken into consideration. Furthermore, it is shown that the Young's modulus of a nanocomposite is controlled principally by a small number of large flakes and that volume average distributions of flake size are more appropriate to use rather than number average ones.

A Facile Synthesis Procedure for Sulfonated Aniline Oligomers with Distinct Microstructures.

Well-defined sulfonated aniline oligomer (SAO) microstructures with rod and flake morphologies were successfully synthesized using an aniline and oxidant with a molar ratio of 10:1 in ethanol and acidic conditions (pH 4.8). The synthesized oligomers showed excellent dispersibility and assembled as well-defined structures in contrast to the shapeless aggregated material produced in a water medium. The synergistic effects among the monomer concentration, oxidant concentration, pH, and reaction medium are shown to be controlling parameters to generate SAO microstructures with distinct morphologies, whether micro sheets or micro rods.

Facile synthesis of potassium intercalated p-terphenyl and signatures of a possible high Tc phase

A new facile synthesis methodology for making flakes of p-terphenyl through sublimation under inert atmosphere of argon is presented. Flake morphology of p-terphenyl provides a favourable environment for efficient intercalation of potassium. The ratio of potassium and p-terphenyl is adjusted so as to obtain the reported superconducting phase i.e. potassium doped p-terphenyl (K3C18H14). A magnetic transition is observed around 107 K under Zero Field Cooled (ZFC) and Field Cooled (FC) mode. But overall moment is positive, possibly due to paramagnetic phase dominating characteristics in the presence of very small superconducting volume fraction. The background subtracted M-H loop taken at 20 K shows typical characteristics of a superconductor.