High crystallinity quasi-spherical single-phase vanadium dioxide nanoparticles for thermochromic applications

Vanadium dioxide (VO₂) has attracted significant attention as a promising material for energy-saving smart windows because of its reversible metal–insulator transition, which is accompanied by pronounced changes in optical properties, particularly in near-infrared (NIR) light transmittance. This thermochromic property of VO₂ is broadly influenced by various factors, such as the phase, morphology, and crystallinity of the film. Despite numerous attempts to synthesize high-quality VO₂ nanoparticles (NPs), there remains a dearth of literature reporting the successful formation of spherical VO₂ NPs (∼25 nm) that exhibit both high monoclinic P2₁/c (M1) phase purity and crystallinity. In this study, we present the synthesis of highly crystalline VO₂(M1) NPs with a size of ∼25 nm achieved through precise control of reaction temperature and pH using a hydrothermal method. We identified specific temperature–pH conditions under which the synthesis exclusively yielded single-phase VO₂(M1) NPs. Within these temperature–pH conditions, a higher temperature and lower pH resulted in a larger particle size, higher crystallinity, and improved thermochromic properties. The synthesis condition of 240 °C-pH 6.5 resulted in optimal thermochromic performance with a single-layered VO₂ film: achieving an integrated luminous transmittance (T_lum) of 53.8 % and a solar modulation ability (△T_sol) of 12.2 %. Furthermore, when forming a composite film using the optimized VO₂ NPs and a UV-curable adhesive, △T_sol was improved while a similar T_lum was maintained. We also demonstrated well-performing sandwich-type thermochromic windows (comprising glass/VO₂ composite/glass or plastic/VO₂ composite/plastic) for practical applications.