Preparation of CoPOM-BiVO4/CoPOM (labelled Composite 2). The Composite 1 electrode was immersed in a 50 mL 5 mM aqueous CoPOM solution (1.35 g, 2.5×10 -4 mol) for 30 min, rinsed with distilled water and dried in air flow. Different immersing time thin films were synthesized via the same procedure, except immersing time (10 min, 60 min) were used instead of 30 min.