Size-Controllable M-Phase VO₂ Nanocrystals for Flexible Thermochromic Energy-Saving Windows

Abstract

Ultrafine M-phase VO2 nanocrystals with good crystallinity are desirable for highly efficient thermochromic flexible films. Herein, we report an oxygen-controlled hydrothermal method to achieve the one-step preparation of stoichiometric and size-controllable VO2(M) nanoparticles for scaled-up production. The specific ratio of oxygen intake to V ions was first determined to guarantee the acquisition of stoichiometric VO2(M). Then, the hydrothermal conditions were optimized to perform size-controllable growth of VO2(M) nanoparticles. VO2(M) nanoparticles with average particle sizes of 25, 32, 41, and 52 nm were prepared by changing the precursor solution concentration under the determined oxygen intake/V ion ratio. Smaller VO2(M) nanoparticles exhibit better thermochromic performance because a smaller size favors light transmission and promotes blue-shifting of the local surface plasmon resonance. The flexible film fabricated with 25 nm VO2(M) nanoparticles shows an excellent thermochromic performance with a solar modulation efficiency (ΔT sol) of 19.2% and a luminous transmittance (T lum) up to 66.42%. This work proposes a simple and effective hydrothermal process producing size-controllable VO2(M) nanoparticles with low cost and scalable production and demonstrates that the size reduction of VO2(M) nanoparticles is favorable for flexible films to exhibit excellent thermochromic properties.

Fabrication of Epitaxial W-Doped VO2 Nanostructured Films for Terahertz Modulation Using the Solvothermal Process

“…36 However, this method often results in metastable monoclinic polymorph VO 2 (B) 37 or oxygen-deficient VO 2 . 38 Nevertheless, these VO 2 phases can easily be transformed into VO 2 (M 1 ) by subsequent thermal treatment. This process also retains the morphology of the asgrown product.…”

Recent Advances in Fabrication of Flexible, Thermochromic Vanadium Dioxide Films for Smart Windows

“…Therefore, processes to prevent VO 2 (M) from being oxidized, for example, overcoating of VO 2 (M) films or using NPs with protective layers, should be developed to enable long-term usage of the films. [80], (b) [139], (c) [56], (d) [136], (e) [118], (f) [149], (g) [74], (h), [64], (i) [143], (j) [109].…”

“…All authors have read and agreed to the published version of the manuscript. [139], (c) [56], (d) [136], (e) [118], (f) [149], (g) [74], (h), [64], (i) [143], (j) [109].…”

“…However, the fabrication of VO 2 (M) films with high T lum (> 40%) and ∆T sol (>10%) values as well as a sufficiently reduced T c remains challenging, which limits the utilization of VO 2 (M) in practical applications [56,57,62] Recently, the fabrication of flexible VO 2 (M) films has attracted widespread attention [39,56]. Flexible thermochromic films demonstrate significant potential for large-scale fabrication and commercialization [63][64][65][66]. For example, flexible VO 2 (M) films can be instantly applied to the windows of existing buildings and easily integrated onto curved surfaces, such as automobile windows.…”

“…Figure 13. Comparison of luminous transmittance (Tlum) and solar modulation ability (ΔTsol) between flexible VO2 thin films fabricated via hydrothermal reaction: (a)[80], (b)[139], (c)[56], (d)[136], (e)[118], (f)[149], (g)[74], (h),[64], (i)[143], (j)[109].…”