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

Vanadium dioxide (VO 2 ) 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 τ c ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO 2 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 τ c , low luminous transmittance ( T lum ), and undesirable solar modulation ability (Δ T sol ). 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 VO 2 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 VO 2 nanoparticles, and has its own advantages of large‐scale synthesis and precise phase control of VO 2 . This Review focuses on hydrothermal synthesis, physical properties of VO 2 polymorphs, and their transformation to thermochromic VO 2 (M), and discusses the advantages, challenges, and prospects of VO 2 (M) in energy‐efficient smart windows application.