L. Valentini

Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films

Carbon nanotubes (CNTs) deposited by plasma-enhanced chemical vapor deposition on Si3N4/Si substrates have been investigated as resistive gas sensors for NO2. Upon exposure to NO2, the electrical resistance of the CNTs was found to decrease. The maximum variation of resistance to NO2 was found at an operating temperature of around 165 °C. The sensor exhibited high sensitivity to NO2 gas at concentrations as low as 10 ppb, fast response time, and good selectivity. A thermal treatment method, based on repeated heating and cooling of the films, adjusted the resistance of the sensor film and optimized the sensor response to NO2.

High concentration few-layer graphene sheets obtained by liquid phase exfoliation of graphite in ionic liquid

In the present work, the use of a commercial ionic liquid as a convenient solvent medium for graphite exfoliation in mild and easy conditions without any chemical modification is presented. To confirm the presence of few layer graphene, its dispersion, which exhibits Tyndall effect, was characterized by Raman and UV spectroscopies, and atomic force and field emission electron microscopies. It is noteworthy that, by gravimetric analysis, a graphene concentration as high as 5.33 mg ml−1 was determined, which is the highest value reported so far in any solvent.

MORPHOLOGICAL CHARACTERIZATION OF SINGLE-WALLED CARBON NANOTUBES-PP COMPOSITES

Abstract Polypropylene matrix composites reinforced with single-walled nanotubes (PP-SWNTs) were produced using different nanotubes concentrations. The characterization of these new materials was performed by differential scanning calorimetry and Raman spectroscopy in order to obtain information on the matrix–nanotube interaction, on the crystallization kinetics of PP and, especially, on the macrostructure and organization of the nanotubes in the composite. On the one hand, the results confirm the expected nucleant effect of nanotubes on the crystallization of polypropylene but on the other, this effect is not linearly dependent on the SWNTs content, showing a saturation of the nucleant effect at low nanotube concentrations. Raman spectroscopy is successfully applied to demonstrate that in the composite films, the crystallization kinetics is strongly affected by the distance between the nanotube bundles as a consequence of a different intercalation of the polymer.

NO2 and CO gas adsorption on carbon nanotubes: Experiment and theory

In this work a combined experimental and theoretical study on carbon nanotube (CNT) based system for gas sensing applications is reported. Carbon nanotubes thin films have been deposited by plasma-enhanced chemical vapor deposition on Si3N4/Si substrates provided with Pt electrodes. Microstructural features as determined by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy highlight the growth of defective tubular carbon structures. The electrical resistivity of the CNT film shows a semiconductinglike temperature dependence and a p-type response with decreasing electrical resistance upon exposure to NO2 gas (100 ppb). No response has been found by exposing the film to CO gas in the temperature range between 25 and 250 °C. In order to obtain a theoretical validation of the experimental results, the equilibrium position, charge transfer, and density of states are calculated from first principles for the CNT+CO and CNT+NO2 systems. Our spin-unrestricted density functional...

NO2 gas sensitivity of carbon nanotubes obtained by plasma enhanced chemical vapor deposition

Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition on Si/Si3N4 substrates provided with platinum interdigital electrodes have been investigated as resistive gas sensors towards NO2 oxidizing gas.The electrical response has been measured exposing the films to sub-ppm NO2 concentrations (10‐100 ppb in dry air) at different operating temperatures ranging between 25 and 215 8C. The response to NO2 has been found to be at maximum at around 165 8C. Upon exposure to NO2 the electrical resistance of randomly oriented CNTs is found to decrease. Gas sensitivity, response time and reproducibility of the electrical response resulted to be dependant from the preparation conditions and film thickness. The prepared films show fast dynamic of the electrical response and high reproducibility of the electrical properties. The resistance decrease of the CNTs when exposed to NO2 gas and the sensor response to concentrations as low as 10 ppb NO2, suggest the possibility to utilize CNTs as new sensors for air-quality monitoring. # 2003 Elsevier Science B.V. All rights reserved.

Effects of single-walled carbon nanotube incorporation on the cure reaction of epoxy resin and its detection by Raman spectroscopy

The effects of the incorporation of single-walled carbon nanotubes (SWNTs) on the cure reaction of a diglycidyl ether of bisphenol A-based (DGEBA) epoxy resin is investigated by thermal analysis and Raman spectroscopy. The results of the investigation show that SWNTs act as a strong catalyst. A shift of the exothermic reaction peak to lower temperatures is in fact observed in the presence of SWNTs. Moreover, these effects are already noticeable at the lowest SWNT content investigated (5%) with slightly further effects at higher concentrations, suggesting a saturation of the catalysing action at the higher concentrations studied (10%). The thermal stability of cured DGEBA and DGEBA/SWNT composites was examined by thermogravimetry, showing a faster thermal degradation for DGEBA-SWNT composites. Raman spectroscopy was successfully applied to demonstrate that the changes observed in the cure reaction of the composites lead to a different residual strain on the SWNT bundles, following a different intercalation of the epoxy matrix.

Use of butylamine modified graphene sheets in polymer solar cells

We describe a facile method to use soluble chemically derived few-layer graphene sheets (GSs) as part of a transparent electrode for the preparation of polymer solar cells. Chemically functionalized GSs were obtained by first covalently attaching fluorine and then exposing the obtained fluorinated graphene sheets to an aliphatic amine at room temperature. Scanning electron microscopy, atomic force microscopy and UV-Vis analyses confirmed that transparent graphene sheets with an average thickness of 0.7–0.9 nm had been obtained. Their application in a polymer solar cell is demonstrated. Such functionalized graphene, which is cheap and easily prepared, is expected to be used as hole acceptor material in polymer photovoltaic applications.

Dielectric behavior of epoxy matrix/single-walled carbon nanotube composites

Abstract A study of the ac electrical transport properties of a diglycidyl ether of bisphenol A-based epoxy resin (DGEBA) polymerized with a diethylene triamine (DETA) and reinforced with single wall carbon nanotubes (SWNTs) is presented. The main objective is the investigation of the particular electrical behavior of the conductive filler in the composite and the development of new nanocomposite materials based on epoxy resins with controlled structural and electrical properties. The structural and electrical characterization of the SWNT–DGEBA/DETA hybrid system, performed by differential scanning calorimetry, Raman and ac impedance spectroscopy show interesting effects, including the particular interaction between the polymer and nanotubes, the tendency of the nanotube structure to increase the rate of reaction and substantial effects of the nanotube bundle conformation, dependent on matrix intercalation, on the dielectric behavior of the composite.

Ozone adsorption on carbon nanotubes: The role of Stone–Wales defects

First-principles calculations within the density functional theory have been performed in order to investigate ozone adsorption on carbon nanotubes. Particular emphasis is placed on the effects of Stone-Wales-like defects on the structural and electronic properties of (i) ideal tubes and (ii) tubes in the presence of ozone. Our results show that structural deformations induced on the pure carbon nanotubes by Stone-Wales defects are similar, as expected, to those induced on graphite; for the (10,0) tube, the semiconducting character is kept, though with a small reduction of the band gap. As for the ozone adsorption, the process on ideal nanotubes is most likely physisorption, though slightly stronger if compared to other previously studied molecules and consistent with the strong oxydizing nature of O(3). However, when ozone adsorbs on Stone-Wales defects, a strong chemisorption occurs, leading to relevant structural relaxations and to the formation of a CO covalent bond; this is consistent with experimental observations of CO functional groups, as well as of the liberation of CO gas phase and of the formation of C vacancies, thus explaining the consumption of the nanotube film upon ozone exposure.

Effects of oxygen annealing on gas sensing properties of carbon nanotube thin films

Carbon nanotubes (CNTs) thin films deposited by plasma enhanced chemical vapor deposition have been investigated as resistive gas sensors towards NO2 oxidizing gas. Effects of air oxidative treatment dramatically influence the nanotubes’ electrical resistance as determined by volt-amperometric measurements. In particular the electrical measurements show that electrical behavior of the CNT films can be converted from semiconducting to metallic through thermal treatments in oxygen. The electrical response was then measured exposing the films to sub-ppm NO2 concentrations (100 ppb in air) at 165 °C. Upon exposure to NO2, the electrical resistance of CNTs was found to decrease. The obtained results demonstrate that nanotubes could find use as a sensitive chemical gas sensor for (a) the fast response accompanied by a high sensitivity to sub-ppm NO2 exposure, and (b) the precise recover of the base resistance value in absence of NO2 at a fixed operating temperature, likewise indicating that intrinsic properties measured on as prepared nanotubes may be severely changed by extrinsic oxidative treatment effects.