Fluidized-bed synthesis of sub-millimeter-long single walled carbon nanotube arrays

The rapid growth method for vertically aligned, single walled carbon nanotube (SWCNT) arrays on flat substrates was applied to a fluidized-bed, using ceramic beads as catalyst supports as a means to mass produce sub-millimeter-long SWCNT arrays. Fe/Al₂O_x catalysts were deposited on the surface of Al₂O₃ beads by sputtering and SWCNTs were grown on the beads by chemical vapor deposition (CVD) using C₂H₂ as a feedstock. Scanning electron microscopy and transmission electron microscopy showed that SWCNTs of 2–4 nm in diameter grew and formed vertically aligned arrays of 0.5 mm in height. Thermogravimetric analysis showed that the SWCNTs had a catalyst impurity level below 1 wt.%. Furthermore, they were synthesized at a carbon yield as high as 65 at.% with a gas residence time as short as <0.2 s. Our fluidized-bed CVD, which efficiently utilizes the three-dimensional space of the reactor volume while retaining the characteristics of SWCNTs on substrates, is a promising option for mass-production of high-purity, sub-millimeter-long SWCNT arrays.     

Effect of plasma modification of single wall carbon nanotubes on ethanol vapor sensing

The oxygen and fluorine modified single wall carbon nanotubes (SWCNTs) were manufactured by microwave plasma enhanced chemical vapor deposition (MPECVD), and were developed as novel gas sensor materials. The sensor characteristic has shown a p-type response with resistance enhancement upon exposure to 100 ppm ethanol at room temperature. Oxygen plasma modification can increase the sensor response from 1.13 to 1.74 on process duration of 30 s due to the apparent elimination of amorphous carbon, as demonstrated by FESEM and Raman results. However, oxygen plasma modification has no effective assistance in decreasing the response and recovery time. By applying fluorine plasma modification, the sensor response only increases from 1.13 to 1.51 on process duration of 60 s, but the response and recovery time can decrease apparently from 178 to 54 s and 364 to 97 s due to the existence of numerous fluorine-included functional groups, as demonstrated by XPS and AES results. Therefore, the plasma modified SWCNT can elevate the sensitivity and reactivity for room temperature ethanol sensing.     

Dispersant-assisted low frequency electrophoretically deposited TiO2 nanoparticles in non-aqueous suspensions for gas sensing applications

The effect of dispersant on deposition mechanism of TiO₂ nanoparticles at 1 Hz under non-uniform AC fields was investigated. It was found that by adding Dolapix to suspension, deposition pattern is drastically changed enabling particles to enter the gap leaving the electrodes intact. Using low frequency AC electrophoretic deposition technique in the presence of dispersant, we succeeded in fabricating gas sensor in less than 2 min. Gas sensing measurements were performed in the temperature range of 450–550 °C. The results explained that the sensor has good stability in time and repeatability performance toward high response. The maximum sensitivity of about 180 for the TiO₂ nanoparticles sensor is observed with 47 ppm NO₂ gas and the response and recovery times is about 60–150 s. The optimum temperature of the gas sensor was obtained in 450 °C where sensor showed a linear trend up to 50 ppm of NO₂ gas. This sensing behavior in un-doped TiO₂ as NO₂ sensor can be mainly ascribed to the porous structure of the sensing film and its good contacts to the substrate and electrode assembly.     

A micro sensor based on TiO2 nanorod arrays for the detection of oxygen at room temperature

A new micro gas sensor based on the TiO₂ nanorod arrays (NRAs) was developed and its response properties to oxygen (O₂) at room temperature were investigated. The micro sensor combined a pair of micro interdigitated electrodes realized by the MEMS process and sensing materials based on the TiO₂ NRAs. The TiO₂ NRAs were selectively grown on the patterned straps of Ti/Pd films through the acid vapor oxidation (AVO) process. Relationship between the morphology of the TiO₂ NRAs and reaction temperatures was analyzed with the scanning electronic microscopy (SEM) and X-ray diffraction (XRD). The results indicate that the diameters of the TiO₂ NRs enlarged as the reaction temperature increased from 140 °C to 180 °C. The TiO₂ NRAs sensors showed a good response to O₂ at room temperature (25 °C) due to the large specific surface areas of the TiO₂ NRs and the TiO₂ NR/NR junctions. The TiO₂ NRAs sensors prepared at 140 °C for 3 h exhibited better response properties to O₂ at room temperature with a fast response and recovery time. The research indicates that the TiO₂ NRAs prepared by the simple AVO process is a good choice for detecting O₂ at room temperature.     

Evidence for large hydrogen capacity in single-walled carbon nanotubes encapsulated by thin Pd layers onto a Pd substrate

We report a study of hydrogen storage and its mechanism in a novel material, representing single-walled carbon nanotubes (SWCNTs) encapsulated by thin Pd layers onto a Pd substrate. A synergetic effect resulting in combination of the Pd and the SWCNT properties with regard to hydrogen has been achieved. We showed that adding SWCNTs increases the H₂-capacity of the Pd–SWCNT composite under electrochemical loading only by up to 25% relative to the Pd metal alone. At the same time, with regard to the added SWCNTs, such synergetic approach (providing high H₂ pressure from highly H-loaded massive Pd substrate into a small fraction of deposited SWCNT) allowed us to achieve a net capacity of 8–12 wt.%. H₂, thus, bringing a unique chance to study hydrogen storage mechanism in highly H-loaded SWCNT. Using ESR technique it was established that the Pd–C_x π-complexes forming at the openings of SWCNTs could be considered as hydrogen adsorption sites, providing both high gravimetric capacity (H/C > 1) and low hydrogen binding energy in the Pd encapsulated SWCNT.     

Towards high purity graphene/single-walled carbon nanotube hybrids with improved electrochemical capacitive performance

CoMgAl layered double hydroxides were prepared as catalysts for the in situ synchronous growth of graphene and single-walled carbon nanotubes (SWCNTs) from methane by chemical vapor deposition. The as-calcined CoMgAl layered double oxide (LDO) flakes served as the template for the deposition of graphene, and Co nanoparticles (NPs) embedded on the LDOs catalyzed the growth of SWCNTs. After the removal of CoMgAl LDO flakes, graphene (G)/SWCNT/Co₃O₄ hybrids with SWCNTs directly grown on the surface of graphene and 27.3 wt.% Co₃O₄ NPs encapsulated in graphene layers were available. Further removal of the Co₃O₄ NPs by a CO₂-oxidation assistant purification method induced the formation of G/SWCNT hybrids with a high carbon purity of 98.4 wt.% and a high specific surface area of 807.0 m²/g. The G/SWCNT/Co₃O₄ hybrids exhibited good electrochemical performance for pseudo-capacitors due to their high Co₃O₄ concentration and the high electrical conductivity of SWCNTs and graphene. In another aspect, the G/SWCNT hybrids can be used as excellent electrode materials for double-layer capacitors. A high capacity of 98.5 F/g_electrode was obtained at a scan rate of 10 mV/s, 78.2% of which was retained even when the scan rate increased to 500 mV/s.     

Structure–property–processing relationships of single-wall carbon nanotube thin film piezoresistive sensors

In an investigation of structure–property–processing relationships for SWCNT thin film piezoresistive sensors, the gauge factor of the sensors for a small tensile deformation (less than 2% strain) was found to be close to unity and showed negligible dependence on the film thickness and SWCNT bundle length (L) and diameter (d). However, for a large tensile deformation (20–30% strain), the film thickness and the microstructure of SWCNTs had a compounding effect on the piezoresistive behavior. A gauge factor of ∼5 was obtained for the sensors fabricated with SWCNT bundles of short length and thin diameter (L = 549 nm and d = 3.7 nm) with thicker films. Furthermore, the gauge factor of the sensors was found inversely proportional to the excluded volume V_ex of SWCNT bundles (V_ex ∝ 1/L² d).     

Single-walled carbon nanotube/silicone rubber composites for compliant electrodes

Single-walled carbon nanotube (SWCNT)/silicone rubber composites that can be used in fabricating compliant electrodes are prepared by spraying a mixed solution of ionic-liquid-based SWCNT gel and silicone rubber onto an elastic substrate. Subsequently, the composites are exposed to nitric acid vapor. Scanning electron microscopy and atomic force microscopy images of the composites show that the SWCNTs are finely dispersed in the polymer matrix due to the addition of the ionic liquid. Doping of the SWCNTs by nitric acid can significantly lower the sheet resistance (R_s) of the composites; samples with 4 wt% of SWCNT content exhibit the lowest R_s value (50 Ω sq^?1). This sheet resistance corresponds to a conductivity value of 63 S cm^?1. In addition, the composites retain a high conductivity after several tensile strains are applied. Stretching the composite sample to 300% of the original length increased the R_s value to 320 Ω sq^?1 (19 S cm^?1). Even after 20th stretch/release/stretch cycle, the conductivity remains constant at a value of 18 S cm^?1. These results provide a scalable route for preparing highly stretchable and conductive SWCNT composites with relatively low SWCNT concentrations.     

Increasing the thermoelectric power generated by composite films using chemically functionalized single-walled carbon nanotubes

We prepared and characterized flexible thermoelectric (TE) materials based on thin films of single-walled carbon nanotube (SWCNT) composites with polyvinylalcohol. While pristine SWCNTs incorporated in a polymer matrix generated a p-type TE material, chemical functionalization of SWCNTs by using polyethyleneimine produced an n-type TE material. TE modules made of both p- and n-type composite were fabricated to demonstrate TE voltage and power generation. A single p–n junction made of two composite strips containing 20 wt.% of SWCNTs generated a high TE voltage of 92 μV per 1 K temperature gradient (ΔT). By combining five electrically connected p–n junctions an output voltage of 25 mV was obtained upon the applying ΔT = 50 K. Furthermore, this module generated a power of 4.5 nW when a load resistance matched the internal module resistance of 30 kΩ. These promising results show the potential of TE energy conversion provided by the SWCNT composite films connected in scalable modules for applications that require light weight and mechanical flexibility.     

Bias dependent NO2 sensitivity of SnO2 thin films at room temperature

Amorphous granular SnO₂ thin films were investigated from a standpoint of an NO₂ gas sensor working at room temperature. The films were deposited using pulsed laser deposition method with substrate at room temperature and ～90 nm thick SnO₂ films with amorphous structure were obtained as a result. SnO₂ films deposited on Pt electrode substrates formed a sensor structure that showed response I_air/I_gas to 4 ppm NO₂ up to ～8000. I–V characteristics of the sensor structure were described by the power law dependence, whereas the power indexes were different for measurements in pure air and in the presence of NO₂. As a result, the sensor response was highly dependent on bias voltage between the sensor electrodes. It was demonstrated that the nonlinear electrical characteristics and bias dependent gas sensitivity were the inherent properties of thin films and the contacts were ohmic.     

Development of a detection sensor for mixed trimethylamine and ammonia gas

This study investigated the gas sensor fabricated with TiO₂ added with metal oxide and catalyst to detect the main mixed gases, usually generated when the meat begins to decay, in order to measure the change in freshness according to protein denaturation. In particular, when fish begins to decompose, TMAO (trimethylamine oxide) is transformed into TMA (trimethylamine) by enzymes to regulate the salinity in fish, which is the major cause of fishy smell. The thick-film semi-conductor gas sensor for trimethylamine and ammonia mixed gas was fabricated with WO₃–TiO₂ prepared by sol–gel and precipitation methods. The nanosized TiO₂ was mixed with WO₃ and doped with transition metals (Pt, Ru, Pd and In). Particle sizes, phases and specific surface areas of sensor materials were investigated by SEM, XRD and BET analyses. The metal-TiO₂ thick films were prepared by screen-printing method onto Al₂O₃ substrates with platinum electrode. It was shown that the highest sensitivity and selectivity of the sensor for trimethylamine and ammonia mixed gas by doping with 1 wt.%In and 15 wt.% WO₃ was reached at the optimum operating temperature of 350 °C.     

The structure of 1D and 3D CuI nanocrystals grown within 1.5–2.5 nm single wall carbon nanotubes obtained by catalyzed chemical vapor deposition

Catalyzed chemical vapor deposition (CCVD) grown single wall carbon nanotubes (SWCNT) with diameter of D_m = 1.5–2.5 nm were used as templates to host one-dimensional nanocrystals of CuI. The CuI@SWCNT nanocomposite was obtained using capillary filling of preopened SWCNTs by CuI melt at 650 °C. Nanocomposite structural studies were performed on a FEI Titan 60–300 at 80 kV. According to the model and image simulation CuI crystallizes within 1.5–2.0 nm SWCNTs in the form of one-dimensional crystals with zinc blende or rock salt type unit cell connected by [0 0 1] edges and translated along 〈1 1 0〉. Copper cations occupy tetrahedral or octahedral sites in the lattice. In SWCNTs with D_m > 2.0 nm 3DCuI@SWCNTs were generated. The crystals of copper halides exhibit acceptor behavior as supported by Raman spectroscopy.     

Dielectric and Magnetic properties of (1 − x)BiFeO3–xBa0.8Sr0.2TiO3 ceramics

The polycrystalline samples of (1 ? x)BiFeO₃–xBa_0.8Sr_0.2TiO₃ (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4 and x = 1) were prepared by the conventional solid state reaction method. The effect of substitution in BiFeO₃ by Ba_0.8Sr_0.2TiO₃ on the structural, dielectric and magnetic properties was investigated. X-ray diffraction study showed that these compounds crystallized at room temperature in the rhombohedral distorted perovskite structure for x ≤ 0.3 and in cubic one for x = 0.4. As Ba_0.8Sr_0.2TiO₃ content increases, the dielectric permittivity increases. This work suggests also that the Ba_0.8Sr_0.2TiO₃ substitution can enhance the magnetic response at room temperature. A remanent magnetization M_r and a coercive magnetic field H_C of about 0.971 emu/g and 2.616 kOe, respectively were obtained in specimen with composition x = 0.1 at room temperature.     

Dielectric tunable properties of high dielectric breakdown Ba0.5Sr0.5TiO3–Zn2P2O7 composite ceramics

Dielectric properties of Ba_0.5Sr_0.5TiO₃–xZn₂P₂O₇ (x = 1, 3, 5, 10, 15 wt%) composite ceramics, which were prepared by solid-state reaction process, were intensively investigated. The results showed that the Curie temperature (T_c) of composites gradually shifted to lower temperature (?140 °C) with increasing the content of Zn₂P₂O₇, and the dielectric constant were tuned effectively from 2020 to 107, while maintaining a relatively high tunability. Zn₂P₂O₇ additions remarkably inhibited the grain growth of Ba_0.5Sr_0.5TiO₃ phases, and improved the breakdown strength of samples up to 385 kV/cm. The sample with x = 10 wt% exhibited good dielectric properties (?_r = 290, tg δ = 0.0006, T = 20.5%, BDS = 297 kV/cm). Meanwhile Zn₂P₂O₇ addition also made the T_c far away from the room temperature, which reduced the sensitivity of the dielectric constant to temperature change and simultaneously improved the stability of materials.     

X-ray diffraction,dielectric, pyroelectric,piezoelectric and Raman spectroscopy studies on (Ba0.95Ca0.05)0.8875Bi0.075TiO3 ceramic

The (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ ceramic composition was prepared using the conventional mixed-oxide technique. X-ray diffraction at room temperature and dielectric permittivity in the temperature range from 85 to 450 K and frequency range from 10² to 2 × 10⁵ Hz, respectively, were studied. The X-ray spectra were investigated by profile refinement technique with the use of specialized software at room temperature, the (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ composition crystallizes in quadratic perovskite structure. The dielectric measurements show classical ferroelectric behavior. The pyroelectric and piezoelectric results confirm the dielectric measurements. The pyroelectric coefficient is about 69.2 nC/cm² K at the transition temperature (T_C = 367 K). The piezoelectric constant is d₃₁ = 31.1 pC/N and the electromechanical coupling factor is k_P = 0.14679. Raman spectra of (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ ceramic were taken at various temperatures and measured over the wave number range from 50 to 1000 cm^?1. All the Raman bands were assigned as the transitional modes of Ba²⁺, Ca²⁺, Bi³⁺ and Ti⁴⁺ cations. The temperature evolution of Raman spectra across the transition shows an important evolution characterizing the disorder of the high temperature phase.     

Resist-assisted assembly of single-walled carbon nanotube devices with nanoscale precision

We report a novel resist-assisted dielectrophoresis method for single-walled carbon nanotube (SWCNT) assembly. It provides nanoscale control of the location, density, orientation and shape of individual SWCNTs. Sub-50 nm accuracy and a yield higher than 85% have been achieved. Using the method, we demonstrate suspended-body SWCNT field-effect transistors (FETs) with back-gate and sub-100 nm air-gap lateral-gate configurations. The suspended-body SWCNT FETs show excellent electrical characteristics with I_on/I_off ～ 10⁷, ultra-low off currents ～10^?14 A and small subthreshold swings. The technique contributes to the ultimate solution for bottom-up fabrication of a broad field of CNT-based devices, such as: complementary metal–oxide-semiconductor and nano-electrical–mechanical-system devices for sensing and radio-frequency applications. Moreover, the versatile method could be applied to the assembly of many other promising materials, such as: nanowires and graphene flakes.     

Polyimide/nano-TiO2 hybrid films having benzoxazole pendent groups: In situ sol–gel preparation and evaluation of properties

Polyimide/titania (PI/TiO₂) nanocomposite films have been successfully fabricated through the in situ formation of TiO₂ within a PI matrix via sol–gel method. Poly(amic acid) (PAA), which is the precursor of PI, was successfully synthesized by mixing pyromellitic dianhydride (PMDA), with equimolar amount of a diamine monomer having a pendent benzoxazole unit and two flexible ether linkages in N,N-dimethylformamide (DMF) solvent. Tetraethyl orthotitanate [Ti(OEt)₄] and acetylacetone were then added to the resulted PAA. After imidization at high temperature, PI/TiO₂ hybrid films were formed. The structure and morphology of the hybrid nanocomposites with different titania contents (0 wt%, 5 wt%, 10 wt%, and 15 wt%) were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and transmission electron microscopy. The results indicate that the TiO₂ nanoparticles were homogeneously dispersed in the hybrid films. The thermogravimetric analysis of nanocomposites confirms the improvement in the thermal stability with the increase in the percentage of titania nanoparticle. Transmission electron microscopy showed that the nanoparticles with an average diameter of 25–40 nm were dispersed in the polymer matrix.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Preparation of Pt-loaded TiO2 nanofibers by electrospinning and their application for WGS reactions

Preparation of Pt-loaded TiO₂ nanofibers and their catalytic performance for water gas shift (WGS) reactions have been explained in this work. The Pt-loaded TiO₂ nanofibers were obtained by electrospinning poly-ethylene oxide (PEO) aqueous solutions containing Ti(OH)_n slurry and Pt nanoparticles at room temperature, followed by calcination at 773 K for 4 h. The calcined nanofibers were rougher than the nanofibers of PEO/Ti(OH)_n/Pt due to the PEO degradation and oxidation of Ti(OH)_n to TiO₂. Diameters of the Pt-loaded TiO₂ nanofibers ranged between 200 and 900 nm. Catalytic activity of the Pt-loaded TiO₂ nanofibers for water gas shift (WGS) reactions was evaluated and it was observed that their activity was 5–7 times higher than that of a bulk catalyst. Such improvement is attributed to the larger surface area of the nanofiber catalyst compared to that of the bulk catalyst. To the best of our knowledge, this is the first demonstration of a synthesis of Pt-loaded TiO₂ nanofibers from a Ti(OH)_n nanoparticle slurry using electrospinning and its application to WGS reactions.     

Free-standing single-walled carbon nanotube/SnO2 anode paper for flexible lithium-ion batteries

Free-standing single-walled carbon nanotube/SnO₂ (SWCNT/SnO₂) anode paper was prepared by vacuum filtration of SWCNT/SnO₂ hybrid material which was synthesized by the polyol method. From field emission scanning electron microscopy and transmission electron microscopy, the CNTs form a three-dimensional nanoporous network, in which ultra-fine SnO₂ nanoparticles, which had crystallite sizes of less than 5 nm, were distributed, predominately as groups of nanoparticles on the surfaces of single walled CNT bundles. Electrochemical measurements demonstrated that the anode paper with 34 wt.% SnO₂ had excellent cyclic retention, with the high specific capacity of 454 mAh g^?1 beyond 100 cycles at a current density of 25 mA g^?1, much higher than that of the corresponding pristine CNT paper. The SWCNTs could act as a flexible mechanical support for strain release, offering an efficient electrically conducting channel, while the nanosized SnO₂ provides the high capacity. The SWCNT/SnO₂ flexible electrodes can be bent to extremely small radii of curvature and still function well, despite a marginal decrease in the conductivity of the cell. The electrochemical response is maintained in the initial and further cycling process. Such capabilities demonstrate that this model hold great promise for applications requiring flexible and bendable Li-ion batteries.