Characterization of Hexagonal Tungsten Bronze Cs_xWO3 Nanoparticles and Their Thin Films Prepared by Chemical Coprecipitation and Wet-Coating Methods

摘要

The hexagonal tungsten bronze Cs_xWO3 nanoparticle was synthesized by a chemical coprecipitation method of ammonium tungstate and CS2CO3 in acidic condition. This synthetic method for cesium tungsten bronze is reported for the first time as far as we know. The synthesized Cs_xWO3 as precipitated showed a weak crystallinity of hexagonal unit cell with a crystallite size of about 4 nm without annealing. When the synthesized Cs_xWO3 was annealed in N2 atmosphere, the crystallinity and crystallite size systematically increased maintaining the typical hexagonal tungsten bronze structure as the annealing temperature increased. The analyzed Cs content in the bronze was about 0.3 vs W, which is very close to the theoretical maximum value, 1/3 in cesium tungsten bronze. According to XPS analysis, the reduced tungsten ions existed as both the forms of W~(5+) and W~(4+) and the contents systematically increased as the annealing temperature increased up to 800 °C. The Cs_xWO3 thin films on PET substrate were also prepared by a wet-coating method using the ball-milled solution of the annealed Cs-xWO3 nanoparticles at various temperatures. The near-infrared shielding property of these thin films increased systematically as the annealing temperature increased up to 800 C as expected with the increased contents of reduced tungsten ions.