All chemicals used in the experimen

All chemicals used in the experiments were of analytical grade and used without furtherpurification. Hexagonal WO 3 nanosheets with and without dominant (0 0 1) facets were bothsynthesized on the fluorine-doped tin oxide (FTO) glass substrates through seed-mediatedhydrothermal methods.[27] A H 2 WO 4 solution for the seed layer and hydrothermal growthwas prepared by dissolving 1.25 g of H 2 WO 4 into 30 wt% H 2 O 2 and heated at 95 °C for 2 h.Before the hydrothermal growth, a seed layer was deposited onto the FTO substrate byspinning the above H 2 WO 4 solution with subsequent annealing treatment at 500 °C in air for2 h. In a typical synthesis of WO 3 nanosheets with dominant (0 0 1) facets, a seeded FTOsubstrate was put into a Teflon-lined stainless steel autoclave with vertically orientation.Then, a mixed solution containing 3.0 mL of H 2 WO 4 solution, 0.5 mL of HCl (6 M) and 12.5mL of acetonitrile was added into the reactor for the hydrothermal reaction at 180 °C for 6 h.Afterward, the as-prepared sample was rinsed with deionized water, dried in air, and annealedat 500 °C for 0.5 h. The obtained sample was denoted as WO 3 -001. Hexagonal phase WO 3nanosheets without dominant (0 0 1) facets were prepared using the similar procedure, exceptthat the hydrothermal solution was prepared by mixing 3.0 mL of H 2 WO 4 solution, 1.5 mL ofHCl (5 M) and 10.5 mL of acetonitrile together, and the obtained sample was denoted asWO 3