A three electrode set-up was adopted for the ex-situ experiments with a gold disk electrode (∅=2 mm), a Pt mesh counter electrode and Ag|AgCl (3M KCl) as the reference electrode. Prior to electrode fabrication, the Au disk working electrode was polished with two grades of Al2O3 powders (0.3 µm and 0.1 µm) on wet polishing cloths, followed by sonication in water for 5 mins then in EtOH for another 5 mins. The working electrode was further activated by running cyclic voltammetry at 0.1 V s -1 in 0.5 M H2SO4 for 15 cycles between 0 and 1.6 V vs Ag|AgCl (3 M KCl).