Rational Cationic Disorder in Hexagonal Cesium Tungsten Bronze Nanoparticles for Infrared Absorption Materials.

Hexagonal cesium tungsten bronze (Cs0.33WO3) nanoparticles (NPs) have attracted attention for their potential applications in near-infrared (NIR) absorbing materials. However, the insufficient Cs doping in Cs0.33WO3 NPs has limited their NIR absorbing capabilities and practical stability. In this study, we demonstrate the transition pathway from intermediate W-defective Cs0.33WO3 NPs synthesized by flame spray pyrolysis to cationic (Cs, W)-disordered Cs0.33WO3 NPs prepared through appropriate heat treatments. Direct atomic observations reveal the basal shear and prismatic (Cs, W)-defective planes, which contributed to the disorder of full Cs doping in Cs0.33WO3 NPs. The obtained Cs0.33WO3 NPs with cationic disorder exhibited enhanced practical performance compared with conventional Cs0.33WO3 NPs. Therefore, the developed approach that regulates cationic disorder enables the rational design of defective metal oxides for a variety of applications, including NIR absorbing materials.

Cesium polytungstates with blue-tint-tunable near-infrared absorption.

Revisiting Wöhler's method (1824), Cs-doped tungsten bronzes were synthesized by reducing Cs-polytungstate at high temperature, and were pulverized into nanoparticles for determining their optical properties. The high-temperature reduced Cs4W11O35 crystals absorbed strongly in the near-infrared, providing an improved luminous transparency with a less-bluish tint than normal Cs0.32WO3-y synthesized in a reductive atmosphere. The high-temperature reduction caused an orthorhombic-to-hexagonal phase transformation and a nonmetal-metal transition, which was monitored by spectrophotometry, X-ray diffraction, and X-ray photoelectron spectroscopy measurements, assisted by a first-principles analysis using a DFT+U method. The high-temperature reduction of Cs4W11O35 is concluded to decrease the number of W deficiencies and produce oxygen vacancies, releasing both free and trapped electrons into the conduction band and thereby activating the near-infrared absorption. The comparatively narrow bandgap of Cs4W11O35 was identified as the origin of the less-bluish tint of the produced Cs tungsten bronzes.