Reversible Rapid Hydrogen Doping of WO₃ in Non-Acid Solution

Hydrogenation, an effective way to tune the properties of transition metal oxide (TMO) thin films, has been long awaited to be performed safely and without an external energy input. Recently, metal-acid-TMO has been reported to be an effective approach for hydrogenation, but the requirement of acid limits its application. In this work, the reversible and rapid hydrogen doping of WO₃ in NaOH(aq) | Al(s) | WO₃(s) is revealed by structural and electrical measurements. Accompanied by the structural phase transition identified by in situ X-ray diffraction, the electric resistance of the WO₃ film is found to be able to change by 5 orders of magnitude. A significant electrical response of touching, 8-fold in amplitude and 3 s in a cycle, can be achieved in the low-resistance state. These reactions are reversible at room temperature. This study unambiguously proves that the hydrogenation-driven dynamic phase transition of WO₃ in metal-solution-WO₃ systems could occur not only in acid solutions but also in some non-acid environments. Unlike the monotonic increase of resistance revealed during H_δWO₃ to WO₃ transition, an intriguing non-monotonic evolution was found for crystal lattice parameter c, indicating that the mechanism of WO₃ hydrogenation involves a series of metastable states, more comprehensive and reasonable. This work sheds light on the potential applications of metal-solution-TMO hydrogenation in touching sensors, circuits survey, and information storage.