Phase and morphology evolution of VO2 nanoparticles using a novel hydrothermal system for thermochromic applications: the growth mechanism and effect of ammonium (NH4+)

Abstract

To reveal the formation mechanism of VO₂ nanomaterials in a hydrothermal system, an experimental method was designed to study the growth and crystallization of a VO₂ nanomaterial by combining the reduction of V₂O₅ and homogeneous precipitation method. For Route A without the addition of ammonium, VO₂ (B) nanobelts were assembled by (VO₂)_x·yH₂O thin nano-slices, and for Route B in the presence of ammonium, the VO₂ (M) nanoparticles were decomposed from (NH₄)₂V₄O₉ sheets. The ammonium solution played a crucial role in the formation of the (NH₄)₂V₄O₉ intermediate phase and finally the VO₂ (M) nanoparticles. Therefore, by contrasting Routes A and B, our results revealed that the ammonium (NH₄⁺) ion changed the reaction process and significantly influenced the preparation of well-crystallized VO₂ (M) nanoparticles under hydrothermal conditions. The obtained VO₂ (M) nanoparticles exhibited a high phase transition enthalpy (ΔH = 32.4 J g⁻¹). The VO₂-PET composite films that were derived from these VO₂ (M) nanoparticles exhibited excellent optical switching characteristics (T_lum = 33.5%, ΔT_sol = 16.0%). Moreover, W-doped VO₂ nanoparticles with different W doping levels were also prepared. The efficiency of W⁶⁺ dopants to lower the transition phase temperature (T_c) was determined to occur at a rate of 19.8 K per at%.