Scalable flow hydrothermal synthesis of monoclinic vanadium dioxide nanoparticles without reducing agents for thermochromic film

• Continuous hydrothermal flow synthesis enables rapid formation of high-crystallinity VO 2 nanoparticles within seconds. • Reaction temperature and residence time play key roles in controlling particle size and crystallinity. • The thermochromic performance of composite films is governed by nanoparticle microstructure. • The developed flow-based approach offers a scalable route toward thermochromic smart window materials. Monoclinic vanadium dioxide (VO 2 (M)) is a promising thermochromic material for smart windows due to its reversible metal–insulator transition near 68 °C and strong near-infrared modulation. Conventional fabrication methods, such as batch hydrothermal synthesis, are limited by a high cost, long processing times, and reliance on hazardous reducing agents, thus hindering large-scale implementation. In this study, a reducing-agent-free continuous hydrothermal flow synthesis (CHFS) route was developed using a commercial T-junction reactor to rapidly and safely produce VO 2 (M) nanoparticles. The effects of the reaction temperature (340–400 °C) and residence time (0.5–10 s) on the particle formation and crystallinity were systematically investigated. VO 2 (M) nanoparticles with an average size of around 20 nm were obtained at 400 °C within 5 s, while an extended residence time promoted coarsening to 32 nm. Differential scanning calorimetry revealed phase transition enthalpies of 24–58 J g −1 , indicating a strong dependence on the reaction conditions. Composite films fabricated from optimized nanoparticles exhibited an excellent thermochromic performance, achieving a transmittance modulation (ΔT 1200 nm) of 47% with repeatable switching. These findings demonstrated a scalable, reducing-agent-free CHFS process for high-quality VO 2 (M) nanoparticles and their integration into thermochromic films, thus offering a practical route toward energy-efficient smart window applications.