Synthesis of Carbon nanotubes by CVD: nanotubes grown on W-doped Fe nanoparticles

M. ESCOBAR; S. MORENO; S. GOYANES; M. C. MARCHI; G. H. RUBIOLO; R. J. CANDAL

San carlos de Bariloche, Argentina

Congreso; 13th international conference on solid films and surfaces; 2006

It is known that CNTs can be grown on few transition metals nanoparticles (Fe, Co, Ni) or with iron doped with transition metals. The dopant has different effects on the morphological and structural characteristics of the nanotubes. In this work we describe the effect of tungsten on the morphology on CNT grown on W-doped iron nanoparticles, included into a SiO2 matrix. Iron catalyst nanoparticles containing 0, 2, 5 or 10 % mol/mol tungsten, were prepared by sol-gel. The sol precursor was prepared by mixing an ethanol solution of iron nitrate with the adequate amount of tungsten (VI) ethoxide, tetra ethyl orto-silicate and water. The hydrolysis was catalyzed with hydrofluoric acid. The sol was dried at 80 ºC and firing at 450 º during 10 hr. The fired xerogel was reduced under a 10% H2/N2 atmosphere at 600 ºC. The CNTs were grown under the following general conditions: 2.5% C2H2/N2 with a flow rate of 110 cm3/min, 600 °C synthesis temperature, 180 torr over pressure. The products were characterized by HR-TEM, SEM, IR, UV-Vis TGA-DTA and DRX.                 CNT grown on pure iron nanoparticles were the multiple wall type (MWCNT). They displayed 11-19 nm external diameter, with an inner channel of 4-6 nm wide. The walls were 5-4 nm thick, well crystallized, and almost free of amorphous carbon. The CNT were several micrometers long. The presence of 2% mol/mol W(VI) is enough  to produce important changes in the yielding and morphology of the CNT. The yield was reduced more than ten times The nanotubes display walls with similar characteristics that those grown on pure iron nanoparticles, but the length of the nanotubes was considerably lower: no more than several hundred nanometres.