Chromogenic Amorphous MoO_3–x Nanosheets and Their Nanostructured Films for Smart Window Applications

Amorphous molybdenum trioxide (a-MoO3–x) nanosheets of approximately 1.5 nm thickness were synthesized via pseudo-topotactic reactions of two-dimensional MoO2 nanosheets obtained soft-chemically from the exfoliation of layered sodium molybdate, Na0.9Mo2O4. The topotactic phase transformation from the as-grown MoO2 nanosheets to crystalline MoO3–x nanosheets was triggered by a facile calcination at 200 °C in air. Amorphization without significant changes in the sheet structure occurred at 250 °C, yielding a-MoO3–x nanosheets. Among the products, a monolayer film of the a-MoO3–x nanosheets had superior photochromic properties in the visible and infrared ranges via band-gap excitation compared to conventional thin films composed of MoO3-related materials in terms of coloration efficiency, that is, degree of coloration per thickness. The coloration efficiency of the a-MoO3–x nanostructured films fabricated in this study remained unchanged despite an increase in the stacking number of the precursor nanosheets because the amorphous structure inhibited the recombination of electron–hole pairs; moreover, the multilayer structure had interlayer galleries between the nanosheets. Consequently, the thick film comprising layers with a similar a-MoO3–x nanostructure exhibited a remarkable degree of coloration and repeatability of coloration and bleaching cycles, indicating its potential for smart window applications.