The WO3 nanobricks were synthesized by a simple hydrothermal method using a modified version of a previously described procedure . In a typical procedure, 0.4 g of sodium tungsten (Na2WO4.2H2O, 95%, Alfa Aesar) and 0.2 g of oxalic acid (C2H2O4, 98%, Lachner), which acted as a stabilizer, were fully dissolved into 50 mL of deionized water with continuous stirring at room temperature. The growth solution was acidified to pH 2.5 by adding 2 mL of 3 M hydrochloric acid (HCl, 99%, mikroCHEM), resulting in the formation of a light-yellow precipitate after almost 10 min later. The mixture was then poured into a 100 mL Teflon-lined stainless-steel autoclave where a cleaned FTO substrate is obliquely positioned with its conductive side facing downward.