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

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 (Tlum = 33.5%, ΔTsol = 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 (Tc) was determined to occur at a rate of 19.8 K per at%.