An intermediate phase (NH4)2V4O9 and its effects on the hydrothermal synthesis of VO2 (M) nanoparticles

Abstract

The hydrothermal synthesis of VO₂ (M) nanoparticles is commonly considered as a result of the transformation of intermediate phase VO₂ (A) or VO₂ (B). Here, we found a new intermediate phase (NH₄)₂V₄O₉ that appeared in the hydrothermal synthesis and played a crucial role in the formation of VO₂ (M) nanoparticles. Then, the mechanism for the transformation of intermediate (NH₄)₂V₄O₉ to VO₂ (M) was described as a self-assembly–decomposition–nucleation–growth process. Pure crystalline VO₂ nanoparticles were ultimately obtained and showed excellent thermochromic properties. The phase-transition temperature T_c of VO₂ (M) nanoparticles is approximately 65 °C. Furthermore, VO₂ flexible foils on PET at a VO₂ solid content of 1.5–2.0% show excellent optical properties with luminous transmittance (T_lum) > 50% and solar energy modulation ability (ΔT_sol) > 15%.