2. Experimental Section
2.1 Synthesis of SCNTs
Pristine CNTs were prepared by a previously-reported chemical vapor deposition process,23,24 using a FeMo/Al2O3 catalyst. As a simplified synthesis process, SCNTs were prepared in a quartz fixed bed reactor using a SO2 and N2 mixture as S source. In a typical run, CNTs (5 g) were added into the vertical quartz reactor and heated to 900 ℃ in an Ar flow (0.4 L/min). Then, a gas mixture (0.1 L/min) containing 84 wt% N2 and 16 wt% SO2 was introduced and maintained for 30 min. After cooled to room temperature in Ar flow, the product was collected and labeled as SCNT-SO2. As a control sample, SCNTs were also prepared by a post-processing using MgSO4 as S source,22 labeled as SCNT-M.