Low-temperature Atmosphere-free Molten Salt Synthesis of NIR-shielding CsxWO3

With LiCl-KCl molten salt as reaction medium, caesium tungsten bronze (CsxWO3) with high visible transparence and decent NIR (near infrared) shielding ability was synthesized from CsOH·H2O, H2WO4, and W. Experimental results indicated that the molten salt not only enhanced mass transport of the CsxWO3 formation reaction, but also protected CsxWO3 from air oxidization. These merits enabled us to prepare CsxWO3 at a relatively low temperature of 400 °C in air. The as-obtained CsxWO3 delivered both high visible transmittance (54%) and high NIR shielding ability (94%). On the contrary, the solid state reaction product adopted a (Cs2O)0.44W2O6 phase with no detectable NIR shielding ability. This method may also be applicable to synthesize other low-valence materials that are unstable in air at high temperatures.