Controllable Strongly Electron-Correlated Properties of NdNiO₃ Induced by Large-Area Protonation with Metal–Acid Treatment

Unlike normal hydrogen doping in a H2 gas atmosphere assisted by precious metal catalysts at high temperatures, the cost-effective hydrogenation of rare-earth nickelate (RNiO3) thin films in acid solution with abundant protons supplies a tunable protonation-induced phase transition via the internal mechanism of electron–proton synergistic co-doping under ambient conditions. Here, we report the first ever large-area hydrogenation of a millimeter-scale NdNiO3 film by metal–acid treatment, which leads to marked changes in optical and electrical transport properties corresponding to the hydrogen-induced electronic phase transition. We found that reflectance and conductance modulations are governed by the concentration of protons in NNO, which is controlled by the crystal facet orientation, which in turn affects the proton doping speed. The effect of the crystal orientation on the speed of protonation may be due to anisotropic hydrogen diffusion owing to the existence of an energetically favored channel along the [001] direction in the NdNiO3 film. Our work provides a green route for the development of electronic devices based on nickelates with hydrogenation.