Effect of surface cleaning and functionalization of nanotubes on gas adsorption D. S. Rawat, 1 N. Taylor, 1 S. Talapatra, 2 S. K. Dhali, 3 P. M. Ajayan, 2 and A. D. Migone 1, * 1 Department of Physics, Southern Illinois University, Carbondale, Illinois 62901, USA 2 Rensselaer Nanotechnology Center, Rensselaer Polytechnic Institute, Troy, New York 12180, USA 3 Department of Electrical Engineering, Southern Illinois University, Carbondale, Illinois 62901, USA Received 18 April 2006; revised manuscript received 7 June 2006; published 12 September 2006 We present adsorption isotherm results for methane at 77 K on multiwalled carbon nanotubes exposed to oxygen plasma for varying periods of time. We found a sharpening of the adsorption steps in the high pressure region of isotherms measured on nanotubes exposed to oxygen plasma for 24 h. Transmission electron mi- croscopy characterization and Raman spectroscopic measurements performed on the nanotube samples provide evidence that the appearance of the step is a consequence of surface modification of the nanotubes resulting from the removal of amorphous carbon, generation of defects, and surface functionalization of the nanotubes due to prolonged plasma exposure. Our results suggest that application of this procedure could lead to tunable nanotube surfaces with controllable adsorption properties. DOI: 10.1103/PhysRevB.74.113403 PACS numbers: 61.46.Fg, 68.43.-h, 78.30-j The possibility of experimentally realizing matter in one dimensional 1D through the adsorption of gases on the sur- faces and inside carbon nanotubes has resulted in numerous theoretical and experimental investigations of these systems. 1–10 Interest in these systems also stems from their potential practical applications, since nanotubes present some characteristics that are favorable for gas storage how- ever, some of the initial exceedingly promising results in this area have failed to be confirmed 11,12 . Extensive theoretical and experimental studies have shown that adsorption on nanotubes is characterized by the presence of large specific surface areas and the availability of different groups of binding energy adsorption sites on the nanotubes. Some of the sorptive properties of the nanotubes can be altered significantly by subjecting them to treatments. Procedures that result in opening of the nanotubes thus mak- ing available the interior of the tubes for adsorption have been discussed extensively in the literature. 13–21 Purification of nanotubes with acids leads to the enhancement of the available specific surface area for adsorption. 22 Such sorptive capacity enhancement is one of the important factors in the development of this material as a suitable gas storage me- dium. We note, however, that merely achieving an enhanced specific surface area is not sufficient to turn nanotubes into a commercially usable gas storage medium. The key additional challenge is to tailor the nanotube surfaces so that the result- ing binding sites lead to enhanced gas storage at practical temperatures essentially, near room temperature, and, allow easy control of the kinetics involved in the adsorption/release of the adsorbate to and from the tubes. Therefore, a funda- mental understanding of the nanotube surfaces and the effect that their modification has on the adsorption characteristics of the material are essential to develop them into a useful gas storage medium. Some of the fundamental aspects of gas adsorption that were probed in the current study are: i How does oxygen plasma modify the nanotube surface? ii How are the multilayer adsorption characteristics of methane af- fected by the surface modification of nanotubes? iii How does the adsorbate-adsorbent interaction change with surface modification of the nanotubes? iv What are possible sur- face, temperature, and pressure conditions for enhanced multilayer adsorption? Here we demonstrate a method by which the surface of multiwalled nanotubes can be modified to alter their adsorp- tion characteristics. Surface modification of the nanotubes is achieved through controlled exposure to oxygen plasma. The effect of the surface modification is explored through the performance of methane adsorption isotherms at 77 K on the treated nanotubes. We found a sharpening of the adsorption features in the samples with plasma exposure time of 24 h. The features are not as well defined for untreated samples. Extensive Raman spectroscopy investigations confirm that these sharper adsorption features are due to surface modifi- cation of the nanotubes subjected to extensive plasma expo- sure. TEM analysis of exposed and unexposed samples also indicates that the surface of the MWNTs is modified as a result of increasing exposure to oxygen plasma. Arc-produced multiwalled nanotubes MWNT-A were purchased from Nanocraft, Inc. The sample consists of nano- tubes 20 to 30 %, nano-onionlike and nanopolygonal 50– 70 %, with graphite platelet accounting for the remainder. Except for oxygen plasma exposure for varying periods, the sample was not subjected to any post-production treatment. A Plasma Prep II setup purchased from SPI supplies di- vision of Structure Probe Inc. was used for our plasma treat- ments. The plasma process was accomplished through the use of a low pressure, rf-induced gaseous discharge. In this process, electrons are produced by ionization of a gas; the electrons gain energy in the electric field. Subsequent colli- sions between these energetic electrons and neutral gas mol- ecules result in energy transfer to the molecules producing chemically active atoms, free radicals, ions, and free elec- trons. An attractive feature of this process is that it occurs near ambient temperatures and does not require the use of toxic chemicals. The as-received nanotube samples were loaded in the reaction chamber and were evacuated to a mild vacuum 100 mTorr by a mechanical vacuum pump. Oxy- gen was drawn through the chamber over the specimen. Ra- dio frequency power is applied at 13.56 MHz. This process leads to oxidation of the samples. Our samples were exposed to the oxygen plasma for 3 h and 24 h. PHYSICAL REVIEW B 74, 113403 2006 1098-0121/2006/7411/1134034 ©2006 The American Physical Society 113403-1