Table 3. Proportion of Cu Species and Cu⁺/Cu²⁺ Atomic Ratios of the Samples Based on XPS Characterization.

	Cu 2p3/2 (%)
samples	Cu0	Cu+	Cu2+	Cu+/Cu2+
S0	0.8	6.2	93.0	0
S2	3.5	34.3	62.2	63.6
S4	2.9	25.1	72.0	47.0
S6	2.5	24.7	72.8	39.7
S8	3.9	23.1	73.0	35.0
S10	2.1	22.6	75.3	31.2