To obtain a relatively stable substrate during the measurements, all iron substrates were pre-conditioned in a 10 wt.% NaOH solution overnight to form a thin oxidized surface layer, followed by rinsing with 99.5% pure ethanol and drying with a gentle stream of nitrogen gas prior to use. The alkaline treatment of the iron surface results in formation of hematite (α-Fe2O3), which will be shown and discussed in relation to Fig. 9. The hematite surface contains different types of surface hydroxyl groups that differ by their coordination to the substrate [26]. The hematite surface is amphoteric due to the possibility of protonization and deprotonization of the surface hydroxyl groups. In neutral solution, the ξ-potential has been found to be slightly positive [27,28]. The water contact angle on our hematite surface is 69 ± 3°, and the open circuit potential relative to Ag/AgCl is −0.72 V, which is similar to that of iron due to the small thickness of the oxide layer.
solution
NaOH
10 wt.%
wt.%
pure ethanol
99.5%
%
water
contact angle
69 ± 3°
°
hematite surface
open circuit potential
−0.72 V
V