The spatial structures of iron oxide formed on an ordered Pt(111) electrode in pH 3 sulfate and chloride media

Platinum (Pt) electrodes modified with iron (Fe) have been used as electrocatalysts to split water, to reduce oxygen, and to oxidize methanol. The current study employed cyclic voltammetry and in situ scanning tunneling microscopy (STM) to probe the redox chemistry and spatial structure of an iron hydroxide thin film adsorbed on an ordered Pt(111) electrode in pH 3 sulfate solution (1 mM H₂SO₄ + 0.1 M K₂SO₄) containing 10 mM FeSO₄. The Pt(111) electrode was pretreated with the conventional annealing-and-quenching method, which resulted in a sub-monolayer native oxide. After this native oxide was stripped off, the adsorption of Fe²⁺ ions and the reduction of oxygen concurred to yield an ordered FeOH film on the Pt(111) electrode. In situ STM imaging revealed a unique spoke – wheel (SW) structure between 0.1 and -0.1 V (vs. Ag/AgCl). As opposed to the hexagonal FeO(111) bilayer structure observed on Pt(111) in the vacuum, a distorted hexagonal adlattice was seen with this SW pattern. Shifting the potential positively and negatively resulted in transformations of the SW structure into distorted moiré patterns. The same STM experiment performed with Fe₂(SO₄)₃ led to a disordered film, indicating that ferrous, not ferric, ions were orderly adsorbed on the Pt(111) electrode. The role of anion in the formation of the FeOH film on the Pt(111) electrode was also conducted in pH 3 chloride solution (1 mM HCl + 0.1 M KCl + FeCl₂).