Influence of Hydrothermal Temperature on Structure and Optical Response of Ammonium Tungsten Bronze toward Advanced...

Ammonium tungsten bronze (AWO) was prepared through a two-step hydrothermal route. In the first stage, tungsten trioxide (WO₃) was obtained hydrothermally from sodium tungstate dihydrate (Na₂WO₄·2H₂O) and sulfuric acid (H₂SO₄). In the second stage, the obtained WO₃ powder was reacted with thiourea (CH₄N₂S) under hydrothermal conditions at 100-150 °C for 24 h. Structural features, crystallinity, and elemental composition were examined using X-ray diffractometer (XRD) and transmission electron microscopy (TEM), while the morphology was evaluated by scanning electron microscope (SEM). X-ray photoelectron spectroscopy (XPS) verified the mixed-valence tungsten states in AWO, and the optical behavior was assessed by UV-vis diffuse reflectance spectroscopy (UV-vis DRS). At 100-120 °C, the diffraction profiles were mainly associated with WO₃. When the synthesis temperature was raised to 130-150 °C, the hexagonal NH₄WO₃ phase became evident, its crystallinity improved, and WO₃ reflection progressively diminished. The resulting AWO showed strong infrared attenuation, supporting its potential use as a broad-rang NIR-shielding material.