In the first step, 500mg raw MWNTs were added in a mixture of 90ml H2SO4 and 30ml HNO3 [30,31] and the suspension was refluxed in a round-bottom flask at 120 1C for 90min. After the reaction, the suspension was cooled to room temperature. The residual acids were removed and the products were washed with reverse osmosis (RO) water in a vacuum filtration using the microporous PTFE membrane until the filtrate pH reached a constant value. The resulting acid-treated MWNTs (AT-MWNTs) were dispersed in RO water using a ultrasonication bath (FS-30, 130W, Fisher Scientific) until an aggregation-free dispersion (1wt% AT-MWNTs) was achieved. The AT-MWNTs concentration was determined by the drying method, i.e., weighing a certain amount of AT-MWNTs solution (about 1g), removing all the water at 105 1C for 3h, and weighing the dry mass.