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One-Step Hydrothermal Synthesis of (NH4)2V4O9 Thin Films Composed of Stacked Single-Crystal Nanosheets

(NH4)2V4O9 thin films composed of vertically aligned single-crystal nanosheets were directly fabricated on quartz glass by a one-step hydrothermal method. In order to study the formation mechanism and to explore the potential applications of the thin films, process experiments were carried out and the band gap was analyzed. An oriented-attachment exfoliation–recrystallization–crystal growth mechanism was proposed for the formation of the (NH4)2V4O9 thin films. In addition, the band gap varied from 2.13 to 2.87 eV with variations in the thickness from 40.21 to 78.64 μm. The move of the band gap main due to the decreasing bond lengths and defect densities with increasing reaction time and which is significant for practical applications.