Atmosphere-Driven Phase Engineering in MoO x : From α-MoO3 Microbelts to MoO2 Revealed by Atomic-Scale Hyperfine Probes

This work reports a comprehensive investigation of the structural, morphological, and local-scale evolution of molybdenum oxides (MoOx) derived from a chelated precursor obtained via thermal decomposition of molybdenum(II) acetate. By systematically controlling annealing temperature and atmosphere, we demonstrate a clear pathway for phase and morphology engineering across the Mo–O system. Under oxidizing conditions, α-MoO3 microbelts are stabilized up to 800 °C and undergo a morphology-driven transformation into nanosheets without phase alteration. In contrast, vacuum annealing induces reduction processes, leading to the formation of β-MoO3 and its subsequent irreversible transformation into MoO2. The β-MoO3 → MoO2 transition is probed at the atomic scale using time-differential perturbed angular correlation (TDPAC) spectroscopy with the 111In(111Cd) probe nucleus over the temperature range 25–600 °C. The evolution of the hyperfine parameters reveals two distinct regimes associated with progressive local ordering and with the emergence of a likely associated with probable MoO2-related environment, characterized by a quadrupole frequency of ∼149 MHz. Correlation analysis and hierarchical clustering provide quantitative evidence of an irreversible transition above 600 °C. Structural characterization after TDPAC measurements as a function of temperature confirms the predominance of monoclinic MoO2 (∼90%) with residual β-MoO3, establishing a direct link between local hyperfine signatures and macroscopic phase composition. These results highlight the critical role of atmosphere in dictating phase transformation pathways and demonstrate that TDPAC is a powerful tool for resolving early stage local structural evolution. The findings provide a robust framework for the rational design of MoOx-based materials with tunable structural, electronic, and catalytic properties.