Tunable Defect Chemistry in Nonstoichiometric WO_x (0 < x < 3) Enables Diverse Catalytic Reactions

Transition metal oxides (TMOs) offer rich catalytic functionality, yet controlling nonstoichiometry across broad composition ranges remains a significant challenge due to their intrinsic structural instability. Here, we show that a potassium-assisted anodization approach, combined with controlled annealing, enables the deliberate synthesis of wide-range nonstoichiometric tungsten oxides (WRNS-WO_x, 0 < x < 3) exhibiting versatile catalytic properties. This approach stabilizes both oxygen-deficient WO_x (x < 2) and highly covalent WO_x (x > 2), as verified by atomic-scale structural and electronic characterization. Systematic tuning of the oxidation state leads to predictable changes in the crystal structure, evolving from a distorted octahedral WO₆ coordination toward bcc-like metallic features with progressive oxygen deficiency. This evolution concurrently reshapes the electronic band structure by modulating W-O hybridization and vacancy-derived states. The oxidation-state-dependent structural characteristics ultimately determine catalytic behavior, where strong W-O covalency in WO_x (x > 2) promotes photoelectrochemical oxygen evolution, while Bro̷nsted-acidic vacancy sites in WO_x (x < 2) favor selective electrochemical oxygen reduction reactions. This work establishes WRNS-WO_x as a systematic platform for elucidating how the nonstoichiometry of TMO catalysts influences their electronic structure and active site. Furthermore, it offers a promising strategy for endowing transition metal oxides with programmable catalytic selectivity for both oxidative and reductive reactions.