One-step hydrothermal synthesis of rare earth/W-codoped VO₂ nanoparticles: Reduced phase transition temperature and improved thermochromic properties

As a reversible thermochromic material, vanadium dioxide (VO₂) is a promising candidate for smart window applications. The trade-off between the integrated visible transmission (T_lum) and the solar modulating ability (ΔT_sol), as well as the high phase transition temperature (τ_c∼68 °C) are regarded as the main obstacle for practical applications of pure VO₂ nanomaterials. The combination of both high τ_c reducing efficiency of W and improving T_lum/ΔT_sol properties of RE (rare earth: Eu, Tb), herein lies the purpose of RE/W-codoping to enhance the thermochromic performance. The RE/W-codoped VO₂ nanoparticles were synthesized under hydrothermal conditions, and exhibited grain size of less than 100 nm. The smart window which was fabricated by coating RE/W-codoped VO₂ nanoparticles onto glass, exhibits a thermochromic performance with a combination T_lum = 40%, ΔT_sol = 6.3%, τ_c = 40.8 °C or T_lum = 63%, ΔT_sol = 3.6%, τ_c = 31.9 °C, indicating the largely reduced absorption compared with the single W doping. Under the RE/W-codoping conditions, it was found that the ionic radius of the RE³⁺ cations controlled the crystallinity of the VO₂ particles and the electron/hole carrier counteraction as well as the competition between the strain and the hole carrier played a vital role in modulating the τ_c of the VO₂ products. The findings should be meaningful for investigating the codoping mechanisms for VO₂ nanomaterials.