Adsorption of Molecular Gases on Silver/Carbon Nanotube Composites at Low Temperatures and Low Pressures

We present an experimental study adsorption of molecular gases (N2, H2, O2, CH4, C2H4, and C2H6) on multiwalled carbon nanotubes (MWCNTs) and MWCNT doped with Ag at low temperatures (35？K) and pressures (10？6？Torr) using the temperature programmed desorption technique. Our results show that the desorption kinetics is of the first order; furthermore comparative measurements indicate that Ag/MWCNTs have an adsorption capacity higher than that of a pure sample suggesting that these composites are good candidates as gas cryosorbers for applications in cryopumps or sensor of latest generation. 1. Introduction In last decades, carbon nanotubes have been intensively studied because of their importance as building blocks in nanotechnology [1–3]. In particular, gas adsorption on carbon nanotubes and nanotubes bundles is an important issue for both fundamental research in nanotubes and their technical applications [4–6]. Considerable experimental and theoretical efforts have been dedicated to hydrogen storage in nanotube based materials [4–6], but the changes in electronic properties of carbon nanotubes upon exposure to gases as O2, NO2, NH3, or CH4 are also object of several studies for application to sensor. Recently we proposed to use multiwalled carbon nanotubes (MWCNTs) as hydrogen cryosorber in cryopumps and accelerators of latest generation [7]. We have demonstrated that MWCNT has, in conditions of very low temperature and pressure, an adsorption capacity about two orders of magnitude higher than that of activated carbon (charcoal) [7]. A means to achieve a high uptake of gas on a surface is to vary the porosity and the specific area of the adsorption surfaces, so that trapping and condensation of molecules inside the pores increase. Carbon nanotubes are good candidates for gas storage because of their high specific surface area and large volume of pores. Therefore it can be interesting to study the doping effects of nanoparticles on specific surface of carbon nanotubes. In fact, metallic nanoparticles possess a large surface area which makes them highly reactive in interactions with gas molecules. So, the insertion of metallic nanoparticles in nanotube networks can lead to the formation of composites with very great surface area. Recently we introduced a simple method to obtain composites based on carbon nanotubes doped with Ag nanoparticles [8]. We obtained that the Ag doping produces strong changes in electronic and optical properties of carbon nanotubes at very low nanoparticle concentration [8]. In this work we investigate the possibility to use these