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

초록

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 (similar to 25 nm) that exhibit both high monoclinic P2(1)/c (M1) phase purity and crystallinity. In this study, we present the synthesis of highly crystalline VO2(M1) NPs with a size of similar to 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 degrees 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 (triangle T-sol) of 12.2 %. Furthermore, when forming a composite film using the optimized VO2 NPs and a UV-curable adhesive, triangle 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.

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