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

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.