Synthesis and characterization of externally doped sulfonated polyaniline/multi-walled carbon nanotube composites

Composites of carbon nanotubes with attached carboxylic groups (c-MWCNTs) and water-soluble externally doped sulfonated polyaniline (ED-SPANI) were prepared by solution mixing of c-MWCNT and ED-SPANI aqueous colloids. Fourier-transform infrared spectroscopy, Raman spectroscopy, ultraviolet–visible (UV–Vis) absorption spectroscopy, field-emission scanning and high-resolution transmission electron microscopy were used to characterize their structure and morphology. Raman and UV–Vis spectra revealed the presence of electrostatic interaction between the C–N⁺ species of the ED-SPANI and the COO⁻ species of the c-MWCNTs. The addition of c-MWCNT to ED-SPANI can improve its thermal stability. The conductivity of 3 wt.% ED-SPANI/c-MWCNT composites at room temperature is sixteen times higher than that of ED-SPANI. These results demonstrate that the addition of a small number of c-MWCNTs to an ED-SPANI matrix can form a conducting network in well dispersed composites, thus increasing their electrical conductivity.     

Impedance characteristics and electrical modelling of multi-walled carbon nanotube/silicone rubber composites

Multi-walled carbon nanotube (MWCNT)-filled silicone rubber (SR) composites were prepared by solvent evaporation method, with different MWCNT concentrations from 0.5 wt% to 6.5 wt%. Alternating current (AC) electrical properties of samples with interdigital electrodes were measured in the frequency range from 20 Hz to 1 MHz. Impedance spectroscopy analysis reveals a frequency-independent percolation transition between 2.0 wt% and 2.9 wt%. Samples above the percolation threshold exhibit more regular variations: the magnitude of impedance decreases gradually with frequency in the low-frequency range, and then decreases as a power law beyond a critical frequency, with the exponent in a limited range indicating the AC universality of disordered solids; the plots of real and imaginary parts of impedance fit semicircles well in the complex plane, implying semiconductive behaviours. Over the concentration range tested, a multi-stage circuit model consisting of resistor–capacitor (RC) networks is proposed to simulate the electrical responses of samples. The validity of the modelling approach is verified by comparing simulation results to experimental results, and is further supported by the analysis of the characteristic frequency. The use of equivalent circuits in modelling provides a further insight into the conducting network inside nanocomposites and more valuable guidance for the design of correlative devices.     

Influence of multi-walled carbon nanotubes on electrochemical performance of transparent graphene electrodes

In this work, carbon-carbon nanocomposites as transparent electrodes were prepared by a chemical reduction of graphite oxide (GO) and multi-walled carbon nanotubes (MWNTs). The electric, optical, and electrochemical properties of graphene-MWNT nanocomposites (G-MCs) were investigated as a function of the MWNT content. It was found that chemically bonded G-MCs were successfully formed with a reduction of the functional groups of the GO and acid-treated MWNTs, resulting in the conjugation of 1D MWNTs onto a 2D graphene surface. The electrical conductivity of the graphene was significantly enhanced by introducing the MWNTs. In addition, the G-MCs showed improved current density and high efficiency compared with graphene alone. This indicated that the improved electrochemical performance of the G-MCs can be attributed to the increase in the activity and electrical conductivity enhanced by π-π interaction between graphene and MWNTs.     

Poly(vinylidene fluoride)-assisted melt-blending of multi-walled carbon nanotube/poly(methyl methacrylate) composites

Multi-walled carbon nanotubes (MWNTs) were sonicated in the dimethylformamide solution of poly(vinylidene fluoride) (PVDF). The PVDF-covered MWNTs were then melt-blended with poly(methyl methacrylate) (PMMA). The dynamic mechanical behavior of various composites was studied. The presence of a small amount of PVDF leads to a significant improvement in the storage moduli of the MWNT/PMMA composites at low temperatures. The storage modulus of a PVDF/MWNT/PMMA composite containing 0.5 wt.% PVDF is almost twice as that of a MWNT/PMMA composite at 50°C. However, a further increase in the PVDF content leads to a reduction of the storage modulus. The beneficial effect of PVDF diminishes at higher temperatures.     

Effects of carboxyl radical on electrical resistance of multi-walled carbon nanotube filled silicone rubber composite under pressure

The piezoresistance of a multi-walled carbon nanotube filled silicone rubber composite under uniaxial pressure was studied. The experimental results show that the active carboxyl radical on multi-walled carbon nanotubes can effectively improve the homogeneous distribution and alignment of conductive paths in the composite. As a result, the composite presented positive piezoresistance with improved sensitivity and sensing linearity for pressure, both of which are key parameters for sensor applications.     

Surface coating of multi-walled carbon nanotube nanopaper on shape-memory polymer for multifunctionalization

We are presenting a method of synthesizing three-dimensional self-assembled multi-walled carbon nanotube (MWCNT) nanopaper on hydrophilic polycarbonate membrane. The process is based on the very well-defined dispersion of nanotube and controlled pressure vacuum deposition procedure. The morphology and structure of the nanopaper are characterized with scanning electronic microscopy (SEM) over a wide range of scale sizes. A continuous and compact network observed from the microscopic images indicates that the MWCNT nanopaper could have highly conductive property. As a consequence, the sensing properties of conductive MWCNT nanopaper are characterized by functions of temperature and water content. Meanwhile, in combination with shape-memory polymer (SMP), the conductive MWCNT nanopaper facilitates the actuation in SMP nanocomposite induced by electrically resistive heating. Furthermore, the actuating capability of SMP nanocomposite is utilized to drive up a 5-gram mass from 0 to 30 mm in height.     

Effect of photocatalytic activities of nano-sized copper molybdate (CuMoO4)-doped bismuth titanate (Bi2Ti4O11) (CMBT) alloy

Different compositions of [CuMoO₄]_x-doped Bi₂Ti₄O₁₁ nanophotocatalyst (x = 0.05, 0.1, 0.5) have been prepared by chemical precursor decomposition (CPD) method using triethanolamine (TEA) and HNO₃. Cu(II) is one of reactive species on the catalyst surface and Mo(VI) ion helps to generate charge compensation of lattice having poor catalytic properties. The photocatalytic properties based on the prepared samples for photodecolorization of thymol blue (TB) solutions are examined by Hg-lamp. The crystal structures of the prepared nano-powders are characterized by XRD, EDAX, UV-vis spectra, specific surface area (BET), and HRTEM analyses. The average particle size of copper molybdate-doped bismuth titanate ranges 32 ± 5 nm measured from TEM. Results show doping of copper molybdate of 5 mol% with bismuth titanate can significantly increase the photoactivity of bismuth titanate compared all the compositions studied except degussa P25 titania. The observed increased photocatalytic activity of copper molybdate-doped bismuth titanate ((CuMoO4)_x(Bi₂Ti₄O₁₁)_1−x; CM_xBT_1−x) is attributed to the strong absorption of OH groups at the surface of the catalyst.     

Novel low temperature synthesis of ZnO nanostructures and its efficient field emission property

We have developed a novel, simple and cost effective wet chemical synthetic route for the production of ZnO nanoneedles and nanoflowers at low temperature. The synthesis process does not require any surfactant, template or pre-seeding. The synthesized ZnO nanoneedles have very sharp tips with their lengths in the range 2-3 μm, while for the case of nanoflowers, the nanoneedles were bunched together. X-ray diffraction study and X-ray photoelectron spectroscopic studies confirmed the formation of pure ZnO phase. Studies on the electron field emission property of the grown nanostructures showed that they are very efficient field emitter. The turn-on fields and the threshold fields are 3.6 V/μm, 4.4 V/μm and 5.4 V/μm, 6.8 V/μm for the ZnO nanoneedles and ZnO nanoflowers, respectively. The enhanced field emission property was attributed to the presence of sharp tips of the nanostructures.     

Thermoelectric behavior of aerogels based on graphene and multi-walled carbon nanotube nanocomposites

In this work, we presented that the Seebeck coefficient and electrical conductivity can be increased simultaneously in aerogels based on graphene and multi-walled carbon nanotube (graphene-MWCNT) nanocomposites, and at the same time the thermal conductivity is depressed due to 3D porous skeleton structure. As a result, graphene-MWCNT aerogels possess ultra-low thermal conductivities (∼0.056 W m⁻¹ K⁻¹) and apparent density (∼24 kg m⁻³), thereafter the figure of merit (ZT) of ∼0.001 is achieved. Although the ZT value is too low for practical application as a thermoelectric (TE) material, the unique structure in this project provides a potential way to overcome the challenge in bulk semiconductors that increasing electrical conductivity generally leads to decreased Seebeck coefficient and enhanced thermal conductivity.     

Novel silicon carbide/polypyrrole composites; preparation and physicochemical properties

Novel silicon carbide/polypyrrole (SiC/PPy) conducting composites were prepared using silicon carbide as inorganic substrate. The surface modification of SiC was performed in aqueous solution by oxidative polymerization of pyrrole using ferric chloride as oxidant. Elemental analysis was used to determine the mass loading of polypyrrole in the SiC/PPy composites. Scanning electron microscopy showed the surface modification of SiC by PPy. PPy in composites was confirmed by the presence of PPy bands in the infrared spectra of SiC/PPy containing various amounts of conducting polymer. The conductivity of SiC/PPy composites depends on PPy content on the surface. The composite containing 35 wt.% PPy showed conductivity about 2 S cm⁻¹, which is in the same range as the conductivity of pure polypyrrole powder prepared under the same conditions using the same oxidant. PPy in the composites was clearly detected by X-ray photoelectron spectroscopy (XPS) measurements by its N1s and Cl2p peaks. High resolution scans of the C1s regions distinguished between silicon carbide and polypyrrole carbons. The fraction of polypyrrole at the composite surface was estimated from the silicon and nitrogen levels. The combination of XPS and conductivity measurements suggests that the surface of the SiC/PPy composites is polypyrrole-rich for a conducting polymer mass loading of at least 12.6 wt.%.     

Microporous carbon derived from polyaniline base as anode material for lithium ion secondary battery

Microporous carbon with large surface area was prepared from polyaniline base using K₂CO₃ as an activating agent. The physicochemical properties of the carbon were characterized by scanning electron microscope, X-ray diffraction, Brunauer-Emmett-Teller, elemental analyses and X-ray photoelectron spectroscopy measurement. The electrochemical properties of the microporous carbon as anode material in lithium ion secondary battery were evaluated. The first discharge capacity of the microporous carbon was 1108 mAh g⁻¹, whose first charge capacity was 624 mAh g⁻¹, with a coulombic efficiency of 56.3%. After 20 cycling tests, the microporous carbon retains a reversible capacity of 603 mAh g⁻¹ at a current density of 100 mA g⁻¹. These results clearly demonstrated the potential role of microporous carbon as anode for high capacity lithium ion secondary battery.     

Enhancing the concentration of TiO2 photocatalyst on the external surface of activated carbon by MOCVD

Activated carbon (AC) modified by HNO₃, H₂O₂ and N₂ + O₂ was selected as the support during metal organic chemical vapor deposition (MOCVD) production of TiO₂ photocatalysts in order to enhance the concentration of TiO₂ on the external surface of AC. The catalytic performance of the HNO₃ modified AC supported TiO₂ catalyst was significantly higher than that of the original AC supported TiO₂ catalyst due to the increased concentration of TiO₂ on the external surface of AC confirmed by XPS and EDAX.     

Multi-scale investigation of electronic transport and electromechanical behavior in carbon nanotube materials

Using home-built experimental setups, electrical properties and electromechanical characterization of two systems based on multiwalled carbon nanotubes (MWNTs) were investigated at room temperature. The first system is formed by carbon nanotubes (CNTs) either isolated or in small groups on a gold substrate, while the second one concerns a macroscale three-dimensional entanglement of CNTs in powder form. The local electrical resistance on systems of the first type was measured using an atomic force microscopy with a conductive tip and showed a narrow distribution of resistance values as well for isolated CNTs as for small groups of them. However, in this latter case the average resistance value has been found to be one order of magnitude higher than that of individual CNTs, which was attributed to the contact resistance between CNTs. This parameter was then studied from a statistical viewpoint through electromechanical tests performed at a macroscopic scale. They consisted in applying an external compression to CNTs powder samples and measuring the evolution of the electrical resistance across the pressed material. These tests demonstrated an outstanding decrease of the electrical resistance resulting from the increasing number of random connections between CNTs under compression, and the experimental curves were fitted with an analytical model. Furthermore, it was deduced from this model that the elementary contact resistance between CNTs decreases under compression. The stability of this electrical contact was verified over several durations and under different constant applied loads.     

Super-low turn-on and threshold electric fields of plasma-treated partly Fe-filled carbon nanotube films

Field emission properties of partly Fe-filled carbon nanotubes (FCNTs) treated with different plasmas have been studied. Super low turn-on (0.24 V/μm) and threshold (0.6 V/μm) electric fields are identified upon nitrogen plasma treatment for 60 min. The field-enhancement factor, as high as 4.8 × 10⁴, can be attributed to the plasma-induced structural defects and the unique characteristics of thin walled FCNTs with filled Fe nanowires. A model has been proposed to understand the effects of plasma on the CNTs.     

Dissolution and reprecipitation of barium at the particulate BaTiO3-aqueous solution interface

The electrokinetic behavior of BaTO₃ particles in an aqueous medium exhibited a hysteresis loop with titration direction. As BaTiO₃ suspension was titrated toward acidic pH, the amount of Ba²⁺ dissolution strongly increased, and in the subsequent titration toward alkaline pH, Ba²⁺ adsorbed and/or reprecipitated on the Ba-depleted BaTiO₃ surface. This dissolution-reprecipitation cycle results in the observed hysteresis behavior. An attempt was made to identify the responsible species of the adsorbed/reprecipitated compound by XPS in conjunction with electrokinetic titrations.     

The effect of strain on nonlinear temperature dependence of resistivity in SrMoO3 and SrMoO3−xNx films

Highly oriented SrMoO₃ thin films have been fabricated by pulsed laser deposition of SrMoO₄ in hydrogen. The films are found to grow along the (1 0 0) direction on LaAlO₃ (1 0 0) and SrTiO₃ (1 0 0) substrates. The method has been extended for the fabrication of oxynitride thin films, using ammonia as the reducing medium. The resistivity measurements show nonlinear temperature dependent (Tⁿ) behaviour in the temperature interval of 10-300 K. The conduction mechanism is largely affected by the strain due to the substrate lattice. A combination of T and T² dependence of resistivity on temperature is observed for films having lesser lattice mismatch with the substrate. The X-ray photoelectron spectroscopic studies confirm the formation of SrMoO₃ and SrMoO_3−xN_x films.     

Electromagnetic interference shielding behavior of poly(trimethylene terephthalate)/multi-walled carbon nanotube composites

Poly(trimethylene terephthalate) [PTT]/multiwalled carbon nanotube [MWCNT] composites having varying amounts of MWCNTs were fabricated with an aim to investigate the potential of such composites as an effective light weight electromagnetic interference (EMI) shielding material in the frequency range of 12.4-18 GHz (Ku-band). PTT/MWCNT composite with shielding effectiveness (SE) of 36-42 dB was obtained at 10% (w/w) MWCNT loading. Shielding mechanism was studied by resolving the total SE into absorption (SE_A) and reflection loss (SE_R). PTT/MWCNT composite showed absorption dominated shielding; thus it can be used as microwave, radar absorbing and stealth material. The effect of MWCNT loadings on electrical conductivity (σ) and dielectric properties of PTT and the correlation among conductivity, tan δ, absorption loss and reflection loss were also studied.     

Effect of growth temperature on the CVD grown Fe filled multi-walled carbon nanotubes using a modified photoresist

Fe filled carbon nanotubes were synthesized by atmospheric pressure chemical vapor deposition using a simple mixture of iron(III) acetylacetonate (Fe(acac)₃) with a conventional photoresist and the effect of growth temperature (550-950 °C) on Fe filled nanotubes has been studied. Scanning electron microscopy results show that, as the growth temperature increases from 550 to 950 °C, the average diameter of the nanotubes increases while their number density decreases. High resolution transmission electron microscopy along with energy dispersive X-ray investigation shows that the nanotubes have a multi-walled structure with partial Fe filling for all growth temperatures. The graphitic nature of the nanotubes was observed via X-ray diffraction pattern. Raman analysis demonstrates that the degree of graphitization of the carbon nanotubes depends upon the growth temperature.     

Far infrared reflectance of sintered nickel manganite samples for negative temperature coefficient thermistors

Single phase complex spinel (Mn, Ni, Co, Fe)₃O₄ samples were sintered at 1050, 1200 and 1300 °C for 30 min and at 1200 °C for 120 min. Morphological changes of the obtained samples with the sintering temperature and time were analyzed by X-ray diffraction and scanning electron microscope (SEM). Room temperature far infrared reflectivity spectra for all samples were measured in the frequency range between 50 and 1200 cm⁻¹. The obtained spectra for all samples showed the presence of the same oscillators, but their intensities increased with the sintering temperature and time in correlation with the increase in sample density and microstructure changes during sintering. The measured spectra were numerically analyzed using the Kramers-Krönig method and the four-parameter model of coupled oscillators. Optical modes were calculated for six observed ionic oscillators belonging to the spinel structure of (Mn, Ni, Co, Fe)₃O₄ of which four were strong and two were weak.     

Metallization of La1/3NbO3 by lithium incorporation

Lithium ion was successfully introduced into La_1/3NbO₃ with an A-site-deficient perovskite-type structure. The crystal structure and transport properties of La_1/3Li_xNbO₃ were investigated as a function of Li content (x = 0-0.59). The lattice parameters of La_1/3Li_xNbO₃ with an orthorhombic cell were enlarged with increasing Li content for x ≤ 0.3, and the structure was transformed to a pseudo-tetragonal cell for x = 0.44. The temperature dependence of electrical resistivity gradually changed from insulating to metallic with increasing x, and thermoelectric power measurement indicated that the carriers were electrons. In X-ray photoelectron spectra of the incorporated samples, Nb_3d⁴⁺ peaks appeared in addition to Nb_3d⁵⁺ peaks, which was consistent with the change of the transport properties. In spite of the success of metallization, no diamagnetic signal indicative of supercondcutivity was observed in La_1/3Li_0.59NbO₃ down to 1.8 K.