Tailoring the Charge Transfer-Driven Oxidation in van der Waals Ferroelectric NbOI2 Through Hetero-Interface Engineering

Abstract

2D transition metal halide oxides (TMHOs) have attracted much interest due to their intriguing ferroelectrics and excellent nonlinear optics, however, their susceptibility to oxidation makes their basic research and practical application a challenge. Therefore, it is crucial to understand the oxidation mechanism and explore effective strategies for protection. Here, taking van der Waals (vdWs) ferroelectric NbOI₂ as an example, oxidation mechanisms and tuning the oxidation behaviors of NbOI₂ and its heterostructures by a variety of in situ experiments and first-principles calculations are discovered. The ambient-pressure X-ray photoelectron spectra reveal a self-limiting oxidation in isolated NbOI₂, driven by spontaneously formed iodine vacancies that react with oxygen molecules due to their lower formation and adsorption energies. For the heterostructures with a lower Fermi level (such as WSe₂) than transition state V_I@NbOI₂ (NbOI₂ rich in I-vacancies), the charge transfer from NbOI₂ to WSe₂ drives the continuous and complete oxidation of NbOI₂ flakes. Moreover, the heterostructures with a higher Fermi level (such as graphene) than V_I@NbOI₂ can weaken the oxidation of NbOI₂. By linking the energy band structures to oxidation behavior, the work offers a new oxidation mechanism of 2D air-sensitive materials and a crucial strategy for improving their chemical stability.