## 3 Results and Discussion
### XPS Analysis
To further analyze the copper species associated with molybden, Figure 10 displays the Cu 2p3/2 XPS spectra of molybden surfaces under various treatment. No characteristic Cu 2p3/2 peaks were observed on untreated or STG-treated molybden surfaces, confirming the absence of detectable copper species. However, after Cu\({}^{2+}\) treatment, a distinct Cu 2p3/2 peak emerged, which was split into three components: a peak at 933.03 eV attributed to Cu(I), a peak at 934.33 eV corresponding to Cu(II), and a weak satellite structure in the 940-945 eV range, characteristic of Cu(II) shake-up satellites [40; 41; 42]. The simultaneous presence of Cu(I) and Cu(II) peaks indicated that most of the Cu\({}^{2+}\) in solution was reduced to Cu\({}^{+}\), while a small amount of Cu\({}^{2+}\) co-adsorbed onto the molybden surface. Following further treatment with STG after Cu\({}^{2+}\) treatment, the Cu 2p3/2 XPS spectrum exhibited a notable shift, with the Cu(I) peak moving from 933.03 eV to 932.81 eV. This shift in binding energy suggested that the -SH or -COO\({}^{-}\) group in STG molecules interacts with Cu(I) sites present on molybden, altering the electronic structure of the copper species at the mineral interface. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Samples**} & \multicolumn{5}{c}{**Elements (Atomic\%)**} \\ \cline{2-6}  & **Cu** & **Mo** & **S** & **C** & **O** \\ \hline Untreated molybden & 0.19 & 26.83 & 50.88 & 12.90 & 9.19 \\ Cu\({}^{2+}\) molybden & 0.91 & 27.11 & 45.19 & 18.48 & 8.31 \\ STG+ molybden & 0.22 & 23.80 & 47.63 & 17.21 & 11.14 \\ Cu\({}^{2+}\)+STG+ molybden & 3.65 & 18.78 & 44.85 & 18.50 & 14.23 \\ \hline \end{tabular}
\end{table} 
Table 1: Elemental analysis of the samples. Figure 10: Cu 2p XPS spectra of molybden: (**a**) untreated, (**b**) exposed to Cu\({}^{2+}\), (**c**) exposed to STG, and (**d**) exposed to both Cu\({}^{2+}\) and STG. Significant changes were noted in the C 1s XPS spectrum of the molybden surface (Figure 11). On untreated, Cu\({}^{2+}\)-treated, and STG-treated molybdenite surfaces, only the C-C and C-O-C peaks were detected at 284.80 eV and around 285.83 eV [43], respectively. In contrast, in the STG system with Cu\({}^{2+}\), subsequent STG treatment induced substantial alterations in the C 1s XPS spectrum, with the emergence of new peaks at 286.10 eV (C-S) and 288.50 eV (O-C=O) [10]. These new peaks suggested that the thiol or carboxyl groups of STG molecules interacted with the Cu(I) sites formed on the Cu\({}^{2+}\)-treated molybdenite surface. Figure 12 presents the S 2p XPS spectra of molybdenite surfaces under different treatment. For untreated, Cu\({}^{2+}\)-treated, and STG-treated molybdenite surfaces, only the intrinsic S\({}^{2-}\) characteristic peak of molybdenite was detected at 162.68 eV, 162.64 eV, and 162.80 eV, respectively, indicating that STG alone did not interact with the molybdenite surface [44; 45]. However, in the STG system with Cu\({}^{2+}\), a new peak at 163.52 eV appeared  
Figure 11: C 1s XPS spectra of molybdenite: (**a**) untreated, (**b**) exposed to Cu\({}^{2+}\), (**c**) exposed to STG, and (**d**) exposed to both Cu\({}^{2+}\) and STG. alongside the S\({}^{2-}\) peak at 162.53 eV [46]. The presence of the S-C peak indicated that STG molecules chemically bonded to Cu(I) sites on molybdenum after treatment. Figure 13 presents the Mo 3d XPS spectra of molybden surfaces under different treatment. Regardless of whether the molybden surface was untreated, treated with Cu\({}^{2+}\) alone, treated with STG alone, or subjected to Cu\({}^{2+}\) pre-treatment followed by STG treatment, no significant shifts in the Mo 3d peaks were observed, suggesting that none of these treatments induced detectable reactions with the Mo sites in molybden. In conclusion, neither Cu\({}^{2+}\) nor STG alone exerts a noticeable effect on molybden. Cu\({}^{2+}\) was predominantly adsorbed as Cu(I) species on the molybden surface, and subsequent exposure to STG resulted in the generation of Cu(I)-STG complexes.