Electrochemical measurements were carried out using an Autolab Potentiostat PGSTAT 302N from Metrohm (Utrecht, The Netherlands). A double junction Ag/AgCl (1 M LiCl, Metrohm) electrode and a platinum wire were used as the reference and the counter electrodes respectively. Graphite rods (spectroscopically pure graphite, 3 mm in diameter, SPI Supplies Inc.) insulated from side walls with epoxy resin were used as working electrode. The diode lasers of wavelength 659 nm, and 532 nm (Roithner Lasertechnik, Austria) were used to illuminate the working electrode with an intensity of 30 mW (pre-adjusted using a light power meter). The illuminated area was 0.07 cm 2 (a circle of 3 mm in diameter) and corresponded to the area of the working electrode.