In situ STM observation of morphological changes of the Pt(111) electrode surface during potential cycling in 10 mM HF solution

In the present study, we have performed in situ STM observation of the surface oxidation-reduction process at Pt(111) surface in 10 mM HF solution under N-2 atmosphere in a stainless-steel chamber. We have for the first time demonstrated the dynamic process of the roughening of the Pt(111) surface during the potential cycles. At E < 0.8 V vs. RHE, no distinct change was observed in the positive-going potential sweep, even though surface oxidation is expected to commence in the so-called butterfly peak region at 0.6 V. At E >= 0.9 V, tiny spots with a height of 0.08 nm appeared on the terraces, which can be attributed to adsorbed oxygen species rather than Pt ad-atoms. When the electrode potential reached 1.3 V, the electrode surface became bumpy with small corrugations (< 0.1 nm) due to oxygen atoms becoming incorporated into the subsurface, without any additional layer formation. In the negative-going potential sweep, the electrode surface was suddenly covered with monoatomic islands, as well as pits, while the tiny spots disappeared at the moment that the electrode potential reached 0.5 V, at which the surface reduction was completed. During the repetitive potential cycles, the formation and growth of the Pt islands were found to occur only around 0.5 V in each negative-going potential sweep back from 1.3 V.