Erosion of Electrochemically Generated Metastable Metal Surfaces (MMS) on Gold and Platinum Electrodes
Authors: Karthik Esakki , Phani Kanala Lakshminarashimha 
Published: February 2020

The metal surfaces with protruding high-energy metal atoms are referred to as Metastable Metal surfaces (MMS) [1]. The coverages and responses of MMS active site atoms, are usually quite low (possibly less than 1% of the surface metal atoms are involved), but these levels may be accentuated by disordering or energizing the surface, e.g., by electrochemical, abrasive or thermal pretreatment. The remaining smoothened surface is called Equilibrated Metal surface (EMS). Due to their high surface energy, MMS on noble metals like Au, Pt exhibit enhanced electrocatalytic activity than the EMS [1]. In electrocatalysis, the reactions like oxygen reduction (ORR), oxidation of methanol (MOR) and ethanol (EOR) are accelerated by the MMS surface. Hence, one needs to create MMS in order to attain high catalytic activity. The MMS state can be created electrochemically, viz., by (a) anodization and followed by chemical/electrochemical reduction; and (b) repetitive potential cycling. The latter is usually carried out in the range of high cathodic (upto HER) to high anodic (prior to OER) potentials for Au, i.e., 0.65 V to 1.7V vs. NHE and for Pt, 0V to 1.4V vs. NHE during which, the metal surface undergoes oxidation followed by reduction, allowing nucleation on certain sites to form MMS-type nano-sites [1].

A clear distinction between MMS and EMS states was demonstrated recently by Scholz et al. [2] in the case of Au in acid. Selective dissolution of small asperities (evidently the main MMS states) was observed in the presence of hydroxy radicals generated by Fenton’s reagent. Smooth EMS regions of the electrode surface remained undissolved with a passivating α–oxide film. The loss of active zones resulted in a decrease in the electrode surface area, surface smoothening and severe loss of electrocatalytic activity for reactions: oxygen reduction, hydrogen evolution and hydroquinone oxidation [2]. Clearly, the removal of the MMS states is responsible for the loss of catalytic activity of gold in acid. Similar effects were recently reported by Jo et al [3] when Au surface is sonicated at 125W power in water.

While it is known that water on sonication produces hydroxyl radicals [4], other reasons for these effects may be mechanical erosion/localized melting.

In the present work, we show that Au and Pt surfaces electrochemically activated in acid solutions, when treated with (boiling) water, are deactivated to the reactions mentioned earlier. The surfaces produced during electrochemical activation are of MMS-type. These are also characterized by their typical CV responses showing pre-monolayer oxidation. As gold particles are soft and fragile, they can readily be removed by soft treatments like a forceful jet (approx. 4 bar pressure) of cold water. Whereas, Pt particles detach from the surface only when boiling water is used. Gold is more fragile in nature compared to platinum and hence needs only simple water treatment enough to remove the MMS sites, but Pt needs some thermal energy considering its melting temperature. These particles are analyzed by TEM and selected area electron diffraction; the former showing a wide particle size-distribution and the latter confirming identity of particles to be Au and Pt. The removal of MMS surface is confirmed by the voltammetric features that are similar to those shown by Scholz et al and topographical AFM analysis. In the latter, the RMS roughness (Rq) factor decreases drastically when Au (Pt) surfaces are treated with (boiling) water and the values are: (a) 4.87 and 3.19 nm for the mechanically polished Au, Pt surfaces; (b) and the values increase to 15.8 & 8.12 upon electrochemical activation, (c) falling to 4.21 and 3.17 after treating with (boiling) water, respectively.

Surface smoothening caused by (hot)-water-jet treatments in the case of Au and Pt respectively is monitored by oxygen adsorption-desorption charge on Au and hydrogen-UPD on Pt. Moreover, decrease in the catalytic activity on MMS-free Au and Pt surfaces is reflected in the kinetics of oxygen reduction and Prussian blue film formation [as in ref.5]. We confirm instability of MMS sites as they are easily knocked out by simple mechanical erosion caused by a jet of (boiling) water.

Figure (1):
(Panel A) Cyclic voltammetric responses of Au in 0.5M H2SO4 at v=50 mV/s; (a) mechanically-polished surface (b) electrochemically activated surface (c) after boiling water treatment of (b); (Panel B) Platinum Surface under same conditions; (C) and (D) TEM images of Au and Pt extracts collected into boiling water after electrochemical activation.

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