SYNTHESIS OF MULTI-WALLED CARBON NANOTUBES BY FLOATING CATALYST CHEMICAL VAPOR DEPOSITION AND THEIR UTILITY FOR HYDROGEN SENSING AT ROOM TEMPERATURE

D. ZILLI; P. BONELLI; A. L. CUKIERMAN

Carbon Nanotubes: Synthesis, Properties and Applications

Nova Science Publishers Inc.

Año: 2012; p. 297 - 332

Self-aligned muti-walled carbon nanotubes (CNTs) assemblies are synthesized by floating catalyst chemical vapor deposition. Iron phthalocyanine, as a precursor, different reaction temperatures (880-1050 °C), total gas flow rates (10-100 cm3/min), and H2 molar fractions (0.25-0.75) are used. Morphological characteristics of the CNTs are assessed by scanning and transmission electronic microscopy. Different multivariate analysis techniques are applied to relate the operating variables investigated and the morphological characteristics, the latter including inner and outer diameters of the individual CNTs building the assemblies, length of the CNTs, aspect ratio, and volume of the individual CNTs. It is found that the temperature is the operating variable which exerts the highest influence on the morphological characteristics, with values increasing with temperature raise. Lower effects are inferred for the gas flow rate and H2 composition, both in the same direction but in opposition to that determined for the reaction temperature. Besides, for certain sets of operating conditions, the synthesized CNTs assemblies evidence large scale occurrence of naturally grown Y-junctions-like CNTs, that are relevant to potential applications in nano-electronics. The orientation of the concavity of inner structures of the nanotubes indicates that the Y-junctions are formed by the merging of metal catalyst nanoparticles during a tip-growth scenario of the tubes. Formation and growth of these nanostructures are explained by an own previously reported mechanism. On the other hand, the CNTs synthesized under pre-established operating conditions, without further treatment and subjected to a three-stage purification protocol are successfully processed in the form of thin films (buckypapers). Their morphology and electrical behavior are characterized. Purification consists of gas-phase oxidation of the metallic nanoparticles under strictly controlled conditions, subsequent removal of metal oxide nanoparticles by acid treatment, and final dispersion in ethanol by ultrasonication. The buckypapers show bidimensional structures constituted by randomly oriented CNTs, room temperature resistivity of around 10-3 W·m, and semiconductor-type behaviour. Finally, nano-composite films composed of the pristine and purified CNTs and Pd nanoparticles are obtained by a simple process involving the reduction of palladium in solution and the subsequent filtering of the aqueous suspension of the decorated nanotubes. The nano-composite films show lower values of room temperature resistivity than the buckypapers, pointing to an increase in the density of electrical contact points in the structure of the films. The effect is more pronounced for the films based on the oxidized CNTs, showing the lowest resistivity value (3.2 x 10-4 Ù m). The viability of the nano-composite films as room temperature H2 sensors is evaluated from the change in their electrical resistance in H2 atmosphere. They all exhibit a reversible response towards H2 at low concentration, with repeatable characteristics. However, the sensing capacity of the Pd-decorated oxidized CNTs films is twice larger than that for the films based on the pristine or completely purified CNTs, likely because the surface functionalities generated during oxidation could act as anchor seeds for Pd nanoparticles deposition. The sensing behavior of the former is further examined for H2 average concentrations in the range 70-2100 ppm. The sensor magnitude of response at room temperature reaches a saturation-like state with a maximum associated value of 2.15% for an average H2 concentration above 350 ppm. The maximum value is 0.6% for an average H2 concentration of 70 ppm. Accordingly, the films based on the Pd-decorated oxidized CNTs could be used as nano-sensors of H2 at room temperature