  In the present work, multiwalled carbon nanotubes were decorated with rhodium
nanoparticles using a colloidal solution in the post-discharge of an RF
atmospheric plasma of argon (Ar) or argon/oxygen (Ar:O$_2$). The properties of
these hybrid materials towards the room temperature detection of NO$_2$,
C$_2$H$_4$, CO, C$_6$H$_6$ and moisture were investigated and discussed in view
of compositional and morphological studies. It was found that the presence of
oxygen in the plasma treatment is essential to significantly enhance the gas
response of Rh-decorated multiwalled carbon nanotubes and to avoid response
saturation even at low gas/vapor concentrations. These desirable effects are
attributed to the presence of oxygen during the CNT plasma treatment since
oxygenated vacancies act both as active adsorption sites for gases and as
anchoring sites for Rh nanoparticles (the presence of Rh nanoclusters is nearly
doubled in Ar-O$_2$ treated samples as compared to Ar treated samples). The
oxygen treatment also makes easier the charge transfer between Rh nanoparticles
and carbon nanotubes upon gas adsorption. The method for treating and
decorating multiwalled carbon nanotubes used here is simple, fast and scalable
for producing gas sensitive nanohybrid materials with uniform and
well-controlled properties.
