In this work, we propose the synthesis and investigate the band edge properties of WO3 thin films having an optimum amount of oxygen vacancies for efficient PEC applications. A facile hydrothermal route is utilized to fabricate WO3 thin films, and a low, moderate, and high number of oxygen vacancies are introduced via annealing in O-2, air, and H-2, respectively. The optimum amount of oxygen vacancies in WO3 were created via the reconstructive transformation and phase transition during air annealing. Compared with the O-2 and H-2 annealed samples, the air annealed WO3 exhibited a significantly higher photocurrent (3.33 mA cm(-2) at 2 V vs. RHE). The systematic increase in oxygen vacancies in WO3 causes a gradual decrease in the band gap and increase in the carrier density with a significant upward shift in the Fermi level and conduction and valence band edge positions. Importantly, the air annealed WO3 showed valence band expansion along with a narrowing of the band gap, increase in carrier density that giving rise to an incident photon to current efficiency of 2-3% in the far visible and IR region (600-1100 nm). Based on these data, possible channels involved in the solar light harvesting process are proposed. (C) 2018 Elsevier Ltd. All rights reserved.