Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows.

Vanadium dioxide (VO2 ) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τc ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO2 a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long-standing issues that hindered its application in energy efficient windows: high τc , low luminous transmittance (Tlum ), and undesirable solar modulation ability (ΔTsol ). Many approaches, including nano-thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach-nano-thermochromism-which is to integrate VO2 nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high-quality VO2 nanoparticles, and has its own advantages of large-scale synthesis and precise phase control of VO2 . This Review focuses on hydrothermal synthesis, physical properties of VO2 polymorphs, and their transformation to thermochromic VO2 (M), and discusses the advantages, challenges, and prospects of VO2 (M) in energy-efficient smart windows application.