Binding energy (B.E) and assignments of GBP films^a.

Element	Thick GBP B.E	Chemical bonds, expected material	Thin GBP B.E	Chemical bonds, expected material	Reference
P 2p3/2	—	—	134.1 ± 0.1	Metal phosphate	36 and 37
S 2p3/2	168.7 ± 0.1	Metal sulphate, CaSO4	—	—	35 and 36
(Cl 2p3/2)	198.2 ± 0.0	Metal chloride, KCl	—	—	35
C 1s	285.0 ± 0.1	C–C	285.0 ± 0.1	C–C	35, 36 and 38
	286.5 ± 0.1	C–OH, C–O–C	286.6 ± 0.1	C–OH, C–O–C	36, 38 and 39
	288.3 ± 0.0	C O	288.2 ± 0.2	C O	36 and 38
	289.1 ± 0.1	O C–O	289.3 ± 0.4	O C–O	35, 36 and 38
Ca 2p3/2	347.6 ± 0.1	CaSO4	—	—	36
N 1s	400.1 ± 0.0	Organic matrix, C–NH2	400.4 ± 0.0	Organic matrix, C–NH2	35 and 36
O 1s	—	—	530.1 ± 0.1	Metal oxides, Fe2O3	35 and 36
	531.5 ± 0.1	Organic C–O	531.5 ± 0.1	Organic C–O	35
	532.9 ± 0.1	Organic C O	532.5 ± 0.0	Organic C O, O C–O	35 and 39
	—	—	533.4 ± 0.0	O C–O	39
	534.4 ± 0.2	Water, oxidised graphene	534.5 ± 0.0	Water, oxidised graphene	40–42
Mn 2p3/2	—	—	641.8 ± 0.2	Mn with O	36
Fe 2p3/2	710.2 ± 0.3	Iron oxides, Fe2+ and Fe3+	711.2 ± 0.1	Iron oxides, Fe3+	35, 36, 43 and 44
(Fe 2p1/2)	723.6 ± 0.2		724.8 ± 0.2

^aBracket means a reference value: measured only one time or not useful for the determination.