“…As shown in Figure 8 B, S 2p doublets appear at the binding energies of 161.76 eV (2p 3/2 ) and 162.93 eV (2p 1/2 ), where the dominant 2p 3/2 line agrees well with most of the sulfides, such as Cu x S (161.7 eV; Larson, 1974 ) and SnS x (161.7 eV; Cruz et al., 2003 ), indicating S 2– in the precursor. Besides, we do not observe any XPS peak at the higher binding energy from 172 to 167 eV coming from the oxidized sulfur (S 4+ or S 6+ ) ( Hernández-Rodríguez et al., 2015 , Meysing et al., 2015 , Thomas et al., 1997 ), suggesting that no (SO 3 ) 2– , (SO 4 ) 2– , or (S 2 O 3 ) 2– species is present in the precursor. The high-resolution C 1s spectrum in Figure 8 C shows three visible peaks at the binding energies of 284.63, 286.63, and 288.76 eV, which can be assigned to C–C, C–O, and O–C=O bonds from the tartrate and citrate species, respectively ( Cano et al., 2001 ).…”