RESEARCH ARTICLE Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 11, 1–5, 2011 Comparison Studies between Hydrogenation and Oxidation of MWNTs Followed by Acid Treatment I. Pelech and U. Narkiewicz ∗ Institute of Chemical and Environmental Engineering, West Pomeranian University of Technology in Szczecin, Pulaskiego 10, 70-310 Szczecin, Poland MWNTs obtained using iron catalyst and ethylene as a carbon source were submitted to a purifi- cation procedure. To purify these materials from amorphous carbon two parallel methods—based on either hydrogen or air treatment—were applied. At the second purification stage iron particles were removed using 1 M or 5 M nitric or hydrochloric acids. The phase composition of the samples was determined using X-ray diffraction method. The sample morphology was characterized using a high resolution transmission electron microscopy. The relative fraction of impurities in the samples was estimated by Raman spectroscopy and the quantitative analysis of metal impurity content was validated by means of thermogravimetric analysis. Keywords: Carbon Nanotubes Purification, Catalyst Removal, Hydrogenation, Oxidation, Acid Treatment. 1. INTRODUCTION Carbon nanotubes can be obtained using various methods. One of them is chemical vapor deposition, consisting in hydrocarbon decomposition on catalyst particles. Together with CNTs considerable amounts of amorphous carbon are also created. To obtain pure CNTs, carbon impurities and metal particles should be removed after synthesis. To separate CNTs from this contamination chemical methods which include: gas phase oxidation, gas phase reduction and liquid phase oxidation are applied. For gas phase oxi- dation most often air or oxygen 1–3 are used. Combustion temperature of various carbon forms can be estimated on the basis of thermogravimetric analysis. 4 An alternative method to remove carbon impurities is the reduction of carbon under hydrogen. 5–7 However, there is little avail- able data concerning purification of CNTs under hydrogen in comparison with oxidation method. Liquid phase oxi- dation methods are applied to remove carbon impurities as well as metals particles from carbon materials. Most frequently nitric, sulfuric or hydrofluoric acid treatment is performed. 8–11 In such purification procedures hydrochlo- ric acid can be also applied, but its application must be preceded by a preliminary oxidation of samples. 12 In this paper a comparison between two purifica- tion methods of multi walled carbon nanotubes based on hydrogenation and oxidation is presented. Systematic ∗ Author to whom correspondence should be addressed. investigation of the influence of gas phase purification on the removal degree of metal particles is presented. Addi- tionally, the influence of nitric and hydrochloric acid con- centration and reaction time on the elimination of catalyst is shown. 2. EXPERIMENTAL DETAILS The catalyst used for multi walled carbon nanotubes synthesis was obtained by a fusion of magnetite with a small amount of promoter oxides. The role of these promoters—calcium and aluminum oxides was to stabi- lize the nanocrystalline iron structure at elevated tem- peratures. The obtained alloy was crushed and sieved in order to obtain a fraction of 1.2–1.5 mm and next was reduced under hydrogen. After the reduction nanocrys- talline iron was obtained and the promoters remained in the oxide state. The catalyst composition was deter- mined using the AES-ICP method (Yvon-Jobin). The sam- ples contained iron and 2.92 wt% Al 2 O 3 and 2.97 wt% CaO. The mean crystallite size determined using the X-ray diffraction method and calculated using Scherrer’s equa- tion amounted to ca. 17 nm. Multi walled carbon nanotubes were synthesized in a high temperature furnace using ethylene—argon mixture (1:1) at 700  C under atmospheric pressure. A prelimi- nary polythermal treatment of the catalyst under hydrogen atmosphere was performed at the temperature rising from 20 to 500  C and next isothermally at 500  C for 1 h, J. Nanosci. Nanotechnol. 2011, Vol. 11, No. xx 1533-4880/2011/11/001/005 doi:10.1166/jnn.2011.4756 1