One-step hydrothermal synthesis of monoclinic vanadium dioxide nanoparticles with low phase transition temperature

[Display omitted] •A simple one-step hydrothermal method for synthesis of VO2(M) nanoparticles.•Controlled easily phase transition temperature of VO2(M) nanoparticles to ∼50 oC.•Controllable oxygen vacancies for tuning phase transition temperature (TC).•Applied hydrazine as both a reducing agent and a shape controller.•Developed VO2/PVB composite film for exemplary optical performance. Vanadium dioxide (VO2) is a promising material for energy-saving smart windows because of its reversible metal-to-insulator transition and accompanying large changes in optical properties. However, the relatively high phase transition temperature (TC ≈ 68 °C) restricts largely its widespread application. In addition, one-step hydrothermal synthesis of VO2(M) nanoparticles with high thermochromic performance that meets commercial requests is still lacking. This study represents the synthesis of oxygen vacancies VO2(M) (Ov-VO2) nanoparticles (32–46 nm) with TC ∼ 50 °C by a simple but efficient one-step hydrothermal method. It is found that when the reaction temperature rises to 300 °C and the pressure reaches 8.5 MPa, Ov-VO2 nanoparticles can be obtained within short time of around 3 h. Specially in processing, hydrazine (N2H4) is selected as both a reducing agent for V5+ to V4+ and a structure directing agent for particle formation and growth. To better understand, the above results are demonstrated by Density Functional Theory (DFT) calculations, which also reveal that N2H4 can lead to the formation of oxygen vacancies. To test its optical performance, a composite film composed by dispersing VO2(M) nanoparticles into polyvinyl butyral (PVB) exhibits excellent optical properties (Tlum = 43.4% and ΔTsol = 17.3%), which close to the simulation results (Tlum = 32.4% and ΔTsol = 23.7%). This may open a path for the simple and scale-up synthesis of VO2 nanoparticles with exemplary performance for smart window applications.