Hydrothermal synthesis of monoclinic vanadium dioxide nanocrystals using phase-pure vanadium precursors for high-performance smart windows

Abstract

The hydrothermal synthesis of monoclinic-phase vanadium dioxide (VO2(M)) has been extensively studied for applications in energy-saving smart windows. In the majority of reported hydrothermal reactions, the reaction intermediates are first synthesized using vanadium-containing agents and additives. These vanadium precursors are then added to pressurized reactors for the hydrothermal synthesis of VO2(M). However, no studies have systematically investigated the structures and compositions of vanadium precursors used for hydrothermal syntheses. In this study, we demonstrated the single-step hydrothermal synthesis of VO2(M) nanocrystals (NCs) using phase-pure vanadium precursors obtained by size-selective purification. We observed that the structures and compositions of the vanadium precursors significantly influenced the hydrothermal synthesis products. Specifically, ammonium metavanadate and hydrazine monohydrate were reacted to yield metastable vanadium precursors that were purified via size-selective precipitation and then selectively transformed into phase-pure VO2(M), VO2(B), and (NH4)(2)V4O9 by hydrothermal synthesis. The obtained phase-pure VO2(M) NCs exhibited enhanced luminous transmittance (55%) and solar modulation ability (18%), the value of the latter being one of the highest reported for hydrothermally synthesized VO2(M). W-doped VO2(M) NCs showed superior phase transition behaviors to those of undoped VO2(M) NCs, while the phase transition temperature was systematically reduced depending on the W-doping concentration. In addition, we experimentally demonstrated that inte-grating the W-doped VO2(M) into the window of a model house reduced the in-house temperature under daytime solar radiation, which exhibits the potential of our VO2(M) films for use in energy-saving window applications.