Afterward, the as-prepared sample was rinsed with deionized water, dried in air, and annealed at 500 °C for 0.5 h. The obtained sample was denoted as WO 3 -001. Hexagonal phase WO 3 nanosheets without dominant (0 0 1) facets were prepared using the similar procedure, except that the hydrothermal solution was prepared by mixing 3.0 mL of H 2 WO 4 solution, 1.5 mL of HCl (5 M) and 10.5 mL of acetonitrile together, and the obtained sample was denoted as WO 3 .