Working electrodes were prepared on FTO glass (Solaronix, TCO10-10, 10 ohm/sq.) by drop casting. In detail, for pre-treatment, all BiOCl samples were sieved with an 80 µm sieve plate and the smaller particles were kept for further preparation of working electrodes. FTO glass substrates were initially subsequently ultra-sonicated in distilled water, ethanol and acetone for 30 minutes each, followed by UV-ozone cleaning treatment for 20 minutes. After that, 100 mg of the above samples were dispersed in a mixed solution of 5 w/w% Nafion TM solution (200 µL) and ethanol (2 mL). Then, the solution was ultra-sonicated for 30 minutes to obtain a uniform white slurry. 100 µL of slurry was carefully dropped and spread out on the FTO substrates (2x2 cm 2 ).