Figure 3. Chemical states and filling ratios of 1D sulphur chains.

XPS spectra in the (a) S2s and (b) S2p core-level regions of empty SWCNTs, S@SWCNTs, empty DWCNTs and S@DWCNTs, in comparison with bulk sulphur (α-S₈). Filling ratios of the 1D sulphur chains, evaluated by XPS analysis, are 6.7 atom% (16 wt%) for S@SWCNTs and 3.5 atom% (8.8 wt%) for S@DWCNTs. TG curves of (c) empty SWCNTs and S@SWCNTs and (d) empty DWCNTs and S@DWCNTs, measured under He/O₂ gas flow (He:O₂=8:2). Insets show the differential TG curves. TG analysis reveals that the filling ratio is 12 wt% for S@SWCNTs, in good agreement with values obtained by XPS. As the oxidation reactions start at nearly the same temperature for DWCNTs and 1D chains of sulphur encapsulated in DWCNTs (~800 K), the filling ratio of S@DWCNTs could not be evaluated by TG analysis.