Morphology and surface structure of nanocarbon allotropes: A comparative study

Different carbon allotropes, including vulcan carbon, multiwall carbon nanotubes, graphene, and nanodiamonds, were processed by chemical purification and treated in a mixture of H₂SO₄–HNO₃. The materials were characterized by infrared and Raman spectroscopy as well as by scanning and transmission electron microscopy. Oxidative differences are indicated by Raman through the G band (～1570 cm^?1), D band (～1340 cm^?1), and G’ band (～2684 cm^?1). The crystal size (L_a) and purity, relative to the amorphous carbonaceous material, were studied as well, along with the morphological changes induced by the treatments.     

Preparation of graphene nanosheets/SnO2 composites by pre-reduction followed by in-situ reduction and their electrochemical performances

The Graphene nanosheets/SnO₂ composites were synthesized using stannous chloride to restore the semi-reduction graphene oxide (SRGO) under a simple hydrothermal reduction procedure. First graphene oxide was pre-reduced by glucose for a certain time to get SRGO, which keeps the good water-solubility of graphite oxide (GO) and has a good conductivity like graphene nanosheets. The higher electrostatic attraction between SRGO and Sn²⁺ makes SnO₂ nanoparticles tightly anchor on the graphene sheets in the hydrothermal reduction process. The formation mechanism of the composite was investigated by SEM, TEM, XRD, AFM and Raman. Moreover, the electrochemical behaviors of the Graphene nanosheets/SnO₂ nanocomposites were studied by cyclic voltammogram, electrical impedance spectroscopy (EIS) and chronopotentiometry. Results showed that the Graphene nanosheets/SnO₂ composites have excellent supercapacitor performances: the specific capacitance reached 368 F g⁻¹ at a current density of 5 mA cm⁻², and the energy density was much improved to 184 Wh kg⁻¹ with a power density of 16 kW kg⁻¹, and capacity retention was more than 95% after cycling 500 cycles with a constant current density of 50 mA cm⁻². The experimental results and the thorough analysis described in this work not only provide a potential electrode material for supercapacitors but also give us a new way to solve the reunification of the graphene sheets.     

Charge Transfer at Junctions of a Single Layer of Graphene and a Metallic Single Walled Carbon Nanotube

Junctions between a single walled carbon nanotube (SWNT) and a monolayer of graphene are fabricated and studied for the first time. A single layer graphene (SLG) sheet grown by chemical vapor deposition (CVD) is transferred onto a SiO₂/Si wafer with aligned CVD‐grown SWNTs. Raman spectroscopy is used to identify metallic‐SWNT/SLG junctions, and a method for spectroscopic deconvolution of the overlapping G peaks of the SWNT and the SLG is reported, making use of the polarization dependence of the SWNT. A comparison of the Raman peak positions and intensities of the individual SWNT and graphene to those of the SWNT‐graphene junction indicates an electron transfer of 1.12 × 10¹³ cm^?2 from the SWNT to the graphene. This direction of charge transfer is in agreement with the work functions of the SWNT and graphene. The compression of the SWNT by the graphene increases the broadening of the radial breathing mode (RBM) peak from 3.6 ± 0.3 to 4.6 ± 0.5 cm^?1 and of the G peak from 13 ± 1 to 18 ± 1 cm^?1, in reasonable agreement with molecular dynamics simulations. However, the RBM and G peak position shifts are primarily due to charge transfer with minimal contributions from strain. With this method, the ability to dope graphene with nanometer resolution is demonstrated.     

A Quinonoid‐Imine‐Enriched Nanostructured Polymer Mediator for Lithium–Sulfur Batteries

The reversible formation of chemical bonds has potential for tuning multi‐electron redox reactions in emerging energy‐storage applications, such as lithium?sulfur batteries. The dissolution of polysulfide intermediates, however, results in severe shuttle effect and sluggish electrochemical kinetics. In this study, quinonoid imine is proposed to anchor polysulfides and to facilitate the formation of Li₂S₂/Li₂S through the reversible chemical transition between protonated state (? NH⁺ ?) and deprotonated state (? N?). When serving as the sulfur host, the quinonoid imine‐doped graphene affords a very tiny shuttle current of 2.60 × 10^?4 mA cm^?2, a rapid redox reaction of polysulfide, and therefore improved sulfur utilization and enhanced rate performance. A high areal specific capacity of 3.72 mAh cm^?2 is achieved at 5.50 mA cm^?2 on the quinonoid imine‐doped graphene based electrode with a high sulfur loading of 3.3 mg cm^?2. This strategy sheds a new light on the organic redox mediators for reversible modulation of electrochemical reactions.     

Photoinduced changes in Raman spectra of YBa2Cu3O6.4 films

The evolution of Raman spectra with illumination has been studied in YBa₂Cu₃O_6.4 films at temperatures between 5–300 K. Low laser power has always been used to avoid local overheating, which was controlled by measuring the local temperature by the Stokes/anti-Stokes ratio. Three important photoinduced effects have been found: (i) the enhancement of the intensity of the observed phonon modes: (Cu(2) at 141 cm^?1, O(2)-O(3) at 338 cm^?1, and O(4) at 488 cm^?1), which may be related to the ordering of oxygen vacancies, (ii) the increase of the electronic scattering background for low Raman frequenciesω, which is in agreement with the enhancement of the static conductivityσ(ω→0) after illumination, and (iii) the suppression of the intensity of the two-magnon band, which may be caused by the increase of charge carriers due to photodoping.     

Growth of cobalt–nickel layered double hydroxide on nitrogen-doped graphene by simple co-precipitation method for supercapacitor electrodes

Nitrogen-doped graphene/Co–Ni layered double hydroxide (RGN/Co–Ni LDH) is synthesized by a facile co-precipitation method. Transmission electron microscopy images indicated that the formation of Co–Ni(OH)₂ nanoflakes with the good dispersion anchored on the surfaces of the nitrogen-doped graphene sheets. The nitrogen-doped graphene composites delivered the enhanced electrochemical performances compared to the pure Co–Ni LDH due to the improved electronic conductivity and its hierarchical layer structures. The high specific capacitance of 2092 F g^?1 at current density of 5 mA cm^?2 and the rate retention of 86.5% at current density of 5–50 mA cm^?2 are achieved by RGN/Co–Ni LDH, higher than that of pure Co–Ni LDH (1479 F g^?1 and 76.5%). Moreover, the two-electrode asymmetric supercapacitor, with the RGN/Co–Ni LDH composites as the positive electrode and active carbon as the negative electrode material, exhibits energy density of 49.4 Wh kg^?1 and power density of 101.97 W kg^?1 at the current density of 5 mA cm^?2, indicating the composite has better capacitive behavior.     

Effects of MgO on properties of Li2O–Al2O3–SiO2 glass–ceramics for LTCC applications

Li₂O–Al₂O₃–SiO₂ (LAS) glass–ceramics for low temperature co-fired ceramics (LTCC) application were prepared by melting method, and the effects of MgO on the sinterability, microstructure, dielectric property, thermal expansion coefficient (CTE) and mechanical character of this glass–ceramics have been studied. The X-ray diffraction images represent that the main phase is β-spodumene solid solutions. And some ZrO₂ and CaMgSi₂O₆ phases in LAS glass–ceramics are detected. The LAS glass–ceramics without additive (MgO) sintered at 800° had the dielectric properties: dielectric constant (ε_r) of 5.3, dielectric loss (tanδ) of 2.97 × 10^?3 at 1 MHz, CTE value of 1.06 × 10^?6 K^?1, bulk density of 2.17 g/cm³, and flexural strength of 73 MPa. 5.5 wt% MgO-added LAS glass–ceramic achieves densification at 800° exhibited excellent properties: low dielectric constant and loss (ε_r = 7.1, tanδ = 2.02 × 10^?3 at 1 MHz), low CTE (2.89 × 10^?6 K^?1), bulk density = 2.65 g/cm³ as well as high flexural strength (145 MPa). The results indicate that the addition of MgO is helpful to improve the dielectric and mechanical properties. The formation of CaMgSi₂O₆ crystal phase with higher CTE leads to the increase of CTE value of LAS glass–ceramics due to the increasing MgO content, and the increase of CTE is favourable for matching with silicon (3.1 × 10^?6 K^?1). The prepared LAS glass–ceramics have the potential for LTCC application.     

Ionic liquid-assisted hydrothermal synthesis of TiO2 nanoparticles and its application in photocatalysis

Anatase TiO₂ nanoparticles have been successfully synthesized at 130 °C for 2 days via ionic liquid-assisted hydrothermal method. The obtained products are characterized using various techniques. The X-ray diffraction data reveal that the nanoparticles are anatase TiO₂. FTIR spectrum shows that the presence of ionic liquid and indicates Ti–O–Ti peak at around 398 cm^?1, and the bands at 1500 and 1600 cm^?1 indicates C–H in-plane vibrations and stretching of imidazolium ring. Raman spectroscopy show bands at 142, 393, 513, and 636 cm^?1 reveal crystalline anatase phase. UV–Vis spectroscopy shows the λ _max at 355 nm corresponding to a band gap of 3.49 eV. TEM images reveal that the average diameters of anatase TiO₂ nanoparticles are in the range 50–100 nm. Anatase TiO₂ exhibited excellent photocatalysis for the degradation of organic dye.     

Synthesis of carbon nanostructures with unique morphologies via a reduction-catalysis reaction route

Large-scale carbon nanostructures with unique morphologies were successfully synthesized by a reduction-catalysis reaction route. The as-synthesized products, characterized by XRD, SEM and TEM, revealed that hollow carbon nanospheres with diameters in the range of 100–200 nm can be formed at 500 °C while the tetrapod-like carbon nanotubes with bamboo structure can be synthesized with the typical diameters of about 100 nm and length of over 1 μm. Two strong and wide Raman peaks at 1600 cm⁻¹ (G-band) and at 1347 cm⁻¹ (D-band) are observed at room temperature and their mechanism of formation is discussed. These unique carbon nanostructures offer potential applications, such as nanoscale transistors, amplifiers, switches and ballistic rectifiers and so on.     

Fabrication and characteristics of porous germanium films

Abstract

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm^?3) and a maximum of Hall mobility at ～225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Simultaneous functionalization and reduction of graphene oxide with diatom silica

The simultaneous coupling and reduction of graphene oxide (GO) with diatom silica (Amphora sp., Navicula ramossisira and Skeletonema sp.) were demonstrated in this work. Binding of GO with diatom silica via direct esterification reaction at 100 °C was observed as well as the reduction of GO. The Raman spectra of GO-diatom silica revealed the typical peaks for reduced graphene oxide at 1350 cm^?1 (D band) and 1585 cm^?1 (G band). Infrared spectroscopy also showed the presence of a unique peak at 1260–1300 cm^?1 indicative of Si–O–C=O bond formation. This confirms the successful functionalization of GO with silica. Scanning electron microscopy showed the presence of GO on the diatom. For the pennate diatoms, Amphora sp. and N. ramossisira, their pores were closed demonstrating that GO was able to cover the surface of the diatom via the Si–O–C bond formation. For the centric diatom, Skeletonema sp., GO was found to be on its rib cage-like body structure and on its centric top. Electrochemical measurements by cyclic voltammetry using a redox probe, K₃[Fe(CN)₆], showed that GO-Amphora and GO-Navicula had more surface negative charge compared with bare GO or bare diatom silica. Furthermore, they demonstrated similar surface charge characteristics as the chemically reduced GO (by hydrazine hydrate). This implies that the composite (reduced GO-diatom) can possibly replace chemically reduced GO (by exposure to hydrazine vapor) and it could probably function as an electrode in sensing cationic biomolecules.     

Temperature optimisation of CNT synthesis by spray pyrolysis of alpha-pinene as the carbon source

Multi-wall carbon nanotubes (MWCNTs) were prepared by spray-pyrolysis of a botanical hydrocarbon, alpha-pinene and ferrocene as the catalyst at 700–1000°C. The MWCNTs were analysed by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and X-ray diffraction. The microscopy studies show the formation of carbon nanotubes with diameters between 20 and 30 nm and length greater than 100 µm. Raman spectroscopy revealed that the alpha-pinene-grown carbon nanotubes were graphitised showing both the D and G bands at 1330 and 1590 cm^?1, respectively, and that the corresponding intensity band ratio (I _D/I _G) varied with respect to temperature formation.     

Topological Design of Ultrastrong and Highly Conductive Graphene Films

Nacre‐like graphene films are prepared by evaporation‐induced assembly of graphene oxide dispersions containing small amounts of cellulose nanocrystal (CNC), followed by chemical reduction with hydroiodic acid. CNC induces the formation of wrinkles on graphene sheets, greatly enhancing the mechanical properties of the resultant graphene films. The graphene films deliver an ultrahigh tensile strength of 765 ± 43 MPa (up to 800 MPa in some cases), a large failure strain of 6.22 ± 0.19%, and a superior toughness of 15.64 ± 2.20 MJ m^?3, as well as a high electrical conductivity of 1105 ± 17 S cm^?1. They have a great potential for applications in flexible electronics because of their combined excellent mechanical and electrical properties.     

Achieving High Capacitance of Paper‐Like Graphene Films by Adsorbing Molecules from Hydrolyzed Polyimide

To date, graphene‐based electric double layer supercapacitors have not shown the remarkable specific capacitance as theoretically predicted. An efficient strategy toward boosting the overall capacitance is to endow graphene with pseudocapacitance. Herein, molecules of hydrolyzed polyimide (HPI) are used to functionalize N‐doped graphene (NG) via π–π interaction and the resulting enhanced electrochemical energy storage is reported. These aromatic molecules in monolayer form on graphene contribute strong pseudocapacitance. Paper‐like NG films with different areal mass loadings ranging from 0.5 to 4.8 mg cm^?2 are prepared for supercapacitor electrodes. It is shown that the gravimetric capacitance can be increased by 50–60% after the surface functionalization by HPI molecules. A high specific capacitance of 553 F g^?1 at 5 mV s^?1 is achieved by the HPI‐NG film with a graphene mass loading of 0.5 mg cm^?2 in H₂SO₄ aqueous electrolyte. For the HPI‐NG film with highest mass loading, the gravimetric specific capacitance drops to 340 F g^?1 while the areal specific capacitance reaches a high value of 1.7 F cm^?2. HPI‐NG films are also tested in Li₂SO₄ aqueous electrolyte, over an extended voltage window of 1.6 V. High specific energy densities up to 40 Wh kg^?1 are achieved with the Li₂SO₄ electrolyte.     

Strong Charge Transfer at 2H–1T Phase Boundary of MoS2 for Superb High‐Performance Energy Storage

Transition metal dichalcogenides exhibit several different phases (e.g., semiconducting 2H, metallic 1T, 1T′) arising from the collective and sluggish atomic displacements rooted in the charge‐lattice interaction. The coexistence of multiphase in a single sheet enables ubiquitous heterophase and inhomogeneous charge distribution. Herein, by combining the first‐principles calculations and experimental investigations, a strong charge transfer ability at the heterophase boundary of molybdenum disulfide (MoS₂) assembled together with graphene is reported. By modulating the phase composition in MoS₂, the performance of the nanohybrid for energy storage can be modulated, whereby remarkable gravimetric and volumetric capacitances of 272 F g^?1 and 685 F cm^?3 are demonstrated. As a proof of concept for energy application, a flexible solid‐state asymmetric supercapacitor is constructed with the MoS₂‐graphene heterolayers, which shows superb energy and power densities (46.3 mWh cm^?3 and 3.013 W cm^?3, respectively). The present work demonstrates a new pathway for efficient charge flow and application in energy storage by engineering the phase boundary and interface in 2D materials of transition metal dichalcogenides.     

Structural, Spectroscopic Studies and Magnetic Properties of Doped Sillenites-Type Oxide Bi12[M]O20 M=Fe, Co

Layered structure of sillenites-type oxides Bi₁₂MO₂₀, the M position can be occupied by tetravalent or trivalent cations. This study focuses in Bi₁₂MO₂₀, M=Co, Fe, and Co/Fe, polycrystalline samples which are prepared by the solid-state reaction method. Infrared (IR) optical absorption, Raman scattering, and Foner magnetometer (BS2) techniques were used for systematic characterization of sillenite type oxide. IR and Raman scattering results showed the appearance of a band, at 850 cm^?1 and at 680 cm^?1, attributed to the (MO₄)^?3. Magnetic susceptibility measurements of all samples were done in a temperature range 2–300 K. The interaction of the M cations with each other through M–O–M linkages of approximately 180^° is expected to be dominant, and this would be paramagnetic in nature.     

Green synthesis of ZnO and Mg doped ZnO nanoparticles,and its optical properties

The Zinc oxide nanoparticles (ZnO NPs) and Magnesium doped ZnO nanoparticles (Mg doped ZnO NPs) are synthesized by Psidium guajava leaf extract. X-ray diffraction studies confirmed that, synthesized nanoparticles were retained the wurtzite hexagonal structure. In FESEM and HRTEM image analysis, ZnO and Mg doped ZnO NPs morphology were trigonal and spherical shape. Elemental compositions were identified by EDAX analysis. From FTIR result, the Zn–O stretching was observed at 453 and 448 cm^?1 for both ZnO samples. In Raman spectra, the high intensive E₂ ^high mode observed for 438 cm^?1 for ZnO NPs. But Mg doped ZnO NPs intensity of E₂ ^high mode decreased as compared to the pure ZnO NPs, it is due to the Mg²⁺ ion in to ZnO lattice site. The photoluminescence measurements revealed that the broad emission was composed of seven different bands due to zinc vacancies, oxygen vacancies and surface defects.     

Synthesis, characterization and electrochemical studies of LiNi0·8M0·2O2 cathode material for rechargeable lithium batteries

LiNiO₂ and substituted nickel oxides, LiNi_0·8M_0·2O₂ and LiCo_0·8M_0·2O₂ (M = Mg²⁺, Ca²⁺, Ba²⁺), have been synthesized using simple solid state technique and used as cathode active materials for lithium rechargeable cells. Physical properties of the synthesized products are discussed in the structural (XRD, TEM, SEM with EDAX) and spectroscopic (FTIR) measurements. XRD results show that the compounds are similar to LiNiO₂ in structure. TEM and SEM analyses were used to examine the particle size, nature and morphological aspects of the synthesized oxides. The composition of the materials was explored by EDAX analysis. Electrochemical studies were carried out in the range 3–4·5 V (vs Li metal) using 1 M LiBF₄ in ethylene carbonate/dimethyl carbonate as the electrolyte. The doping involving 20% Mg resulted in a discharge capacity of 185 mAhg⁻¹ at 0·1 mA/cm² and remained stable even after 25 cycles. Discharge capacity retention for Mg doped lithium nickelate at 25th cycle was noted to be nearly 7% higher than for the undoped material.     

Vertically Oriented Graphene Nanoribbon Fibers for High‐Volumetric Energy Density All‐Solid‐State Asymmetric Supercapacitors

Graphene fiber based micro‐supercapacitors (GF micro‐SCs) have attracted great attention for their potential applications in portable and wearable electronics. However, due to strong π–π stacking of nanosheets for graphene fibers, the limited ion accessible surface area and slow ion diffusion rate leads to low specific capacitance and poor rate performance. Here, the authors report a strategy for the synthesis of a vertically oriented graphene nanoribbon fiber with highly exposed surface area through confined‐hydrothermal treatment of interconnected graphene oxide nanoribbons and consequent laser irradiation process. As a result, the as‐obtained fiber shows high length specific capacitance of 3.2 mF cm^?1 and volumetric capacitance of 234.8 F cm^?3 at 2 mV s^?1, as well as excellent rate capability and outstanding cycling performance (96% capacitance retention after 10 000 cycles). Moreover, an all‐solid‐state asymmetric supercapacitor based on graphene nanoribbon fiber as negative electrode and MnO₂ coated graphene ribbon fiber as positive electrode, shows high volumetric capacitance and energy density of 12.8 F cm^?3 and 5.7 mWh cm^?3 (normalized to the device volume), respectively, much higher than those of previously reported GF micro‐SCs, as well as a long cycle life with 88% of capacitance retention after 10 000 cycles.     

Field emission from diamond and diamond-like carbon films

Field emission from diamond and diamond-like carbon thin films deposited on silicon substrates has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. The diamond-like carbon films were deposited using the radio frequency chemical vapor deposition method. Field emission studies were carried out using a sphere-to-plane electrode configuration. The results of field emission were analyzed using the Fowler-Nordheim model. It was found that the diamond nucleation density affected the field emission properties. The films were characterized using standard scanning electron microscopy, Raman spectroscopy, and electron spin resonance techniques. Raman spectra of both diamond and diamond-like films exhibit spectral features characteristic of these structures. Raman spectrum for diamond films exhibit a well-defined peak at 1333cm^?1. Asymmetric broad peak formed in diamond-like carbon films consists of D-band and G-band around 1550 cm^?1 showing the existence of both diamond (sp³ phase) and graphite (sp² phase) in diamond-like carbon films.