The role of water in reducing WO 3 film by hydrogen: Controlling the concentration of oxygen vacancies and improving the photoelectrochemical performance.

H2 O, as a product of reduced reaction by H2 , may affect the chemical equilibrium according to the changing of the pressure ratio of H2 O/H2 in the system. Meanwhile, the performance may also be influenced by adsorption of H2 O on the surface of the material. In this work, the effect of H2 O is studied by reducing plate-like array WO3 films under different pressure ratio of H2 O/H2 . It is controlled by changing the water temperature in the washing bottle through which the Ar/H2 (80:20) gas flows. The higher water vapor pressure not only decreases the content of W5+ but also increases the content of surface hydroxyl groups in the WO3 films. Moreover, the excess water vapor improves the crystallinity. The WO3 film shows hydrophobicity with adhesive property and high contact angle hysteresis after reduction, and the wettability increases with the increase of the pressure ratio of H2 O/H2 . Additionally, a built-in electric field may form by dissociation of the surface hydroxyl group and absorption of O- species, which promotes the charge separation, showing better photoelectrochemical (PEC) performance. Thus, water influences the coverage of chemical species on the surface of hydrogen reduced WO3 film, which affects the wettability and PEC performance.