Thermally Driven Downsizing of (100)-Rich α-MoO₃ Under Reduced Pressure

The thermal evolution of molybdenum oxides reflects competition among surface reconstruction, reduction, and sublimation. Here, we investigate the thermal downsizing behavior of microporous α-MoO₃, with predominantly exposed (100) facets under reduced pressure. We combine variable-temperature powder X-ray diffraction (PXRD) and temperature-programmed desorption mass spectrometry with in situ transmission electron microscopy at 10^-5 Pa to correlate phase and morphological changes during heating. At 400 °C, the parent microcrystalline particles underwent pronounced downsizing to form α-MoO₃ nanocrystals. Their mean size subsequently increased from 18.5 ± 8.2 nm at 400 °C to 46.1 ± 24.1 nm at 550 °C, followed by coalescence and further growth above 550 °C. These observations are consistent with surface reconstruction coupled to sublimation and redeposition, with the exposed (100) facets serving as likely initiation sites. Although PXRD detected a minor Mo₄O₁₁ phase, the MoO₂-forming pathway previously reported under electron irradiation was not observed under low-dose imaging at 80 keV, supporting a predominantly thermally driven mechanism. Our results demonstrate that reduced-pressure heating provides a kinetically controlled pathway for the formation of nanosized α-MoO₃ and highlight the importance of facet-dependent surface reconstruction in tailoring oxide nanostructures.