Descriptor-assisted interpretation of VO2 polymorph formation in microwave-assisted hydrothermal synthesis

Monoclinic vanadium dioxide (VO 2 (M)) is a representative thermochromic material whose practical application requires precise control of its crystalline phase formation and crystallinity under hydrothermal conditions. However, the phase formation behavior of the VO 2 polymorphs remains difficult to interpret due to strong parameter coupling and phase coexistence. In this study, the crystal growth and phase formation behavior of VO 2 were systematically investigated using microwave-assisted hydrothermal synthesis and an L9 orthogonal experimental design. The effects of the oxalic acid ratio, precursor concentration, reaction temperature, and reaction time on phase formation were evaluated within a compact experimental space. Because phase coexistence complicates interpretation of mixed-phase X-ray diffraction (XRD) datasets, non-negative matrix factorization with clustering (NMFk) was employed to extract physically interpretable phase-related diffraction components. An NMF-derived descriptor associated with the VO 2 (M) phase was used as a semi-quantitative descriptor of the VO 2 (M)-related diffraction contribution. The descriptor showed a consistent ordering with the latent heat of the metal–insulator transition measured by differential scanning calorimetry. Within the investigated design space, statistical analysis identified precursor concentration as the factor producing the largest variation in the VO 2 (M)-related descriptor, followed by reaction temperature and time. Combined analysis of the phase-selection trends and precursor-concentration series revealed a concentration-dependent trade-off between VO 2 (M)-related structural development and phase selectivity, where dilution promotes crystallinity development of VO 2 (M) but also increases the contribution of competing VO 2 (A). The results further suggest that hydrothermal synthesis parameters influence not only phase selectivity but also the extent of polymorph evolution achieved during synthesis. These findings clarify how precursor concentration mediates the competing development of VO 2 (M) crystallinity and VO 2 (A) formation, thereby linking dilution-induced crystal growth with polymorph competition in hydrothermal VO 2 synthesis.