“…The composition and functional groups of CDs were studied by XPS. Figure S1a shows the XPS test spectrum, which indicates that the main components of CDs are C, O, N, and S. The XPS spectrum of C 1s in high resolution displayed four distinct peaks at 284.48, 284.80, 285.94, and 287.94 eV (Figure b), which were assigned to C–S, C–C, C–N/C–O, and CO bonds, respectively. − Besides, the high-resolution XPS spectrum of O 1s revealed two characteristic peaks at 530.88 and 531.98 eV (Figure c), and those were attributed to CO and C–O bonds, respectively . The N 1s spectrum exhibited two characteristic peaks at 399.48 and 401.18 eV (Figure d), corresponding to the C–N and N–H bonds, respectively .…”