The Na_xWO₃ nanoparticles with different x were synthesized by a solvothermal method and the absorption behavior in visible and near-infrared light (NIR) region was studied. Well-crystallized nanoparticles with sizes of several tens of nanometers were confirmed by XRD, SEM and TEM methods. The absorption valley in visible region shifted from 555 nm to 514 nm and the corresponding absorption peak in NIR region shifted from 1733 nm to 1498 nm with the increasing x. In addition, the extinction behavior of Na_xWO₃ nanoparticles with higher x values were simulated by discrete dipole approximation method and results showed that the changing behavior of optical properties was in good agreement with the experimental results. The experimental and theoretical data indicate that the transparency and NIR-shielding properties of Na_xWO₃ nanoparticles in the visible and NIR region can be continuously adjusted by x value in the whole range of 0 < x < 1. These tunable optical properties of nanocrystalline Na_xWO₃ will expand its application in the fields of transparent heat-shielding materials or optical filters.

Nanocrystalline tungsten bronze is an excellent near-infrared absorbing material, which has a good potential application in the field of transparent heat shielding materials on windows of automobiles or buildings, but it exhibits serious instability in the actual environment, which hinders its further application. In this paper, we coated the Cs_xWO₃ nanoparticles with TiO₂ to prepare core-shell structured Cs_xWO₃@TiO₂, and its crystal structure and optical properties were studied. The results show that the surface of Cs_xWO₃ nanoparticles is coated with a layer of TiO₂ particles with the size of several nanometers, and the shell thickness can be adjusted by the amount of Ti source. The measurement of optical properties illustrates that TiO₂-coated Cs_xWO₃ exhibits good stability in actual environment, and its transparent heat shielding performance will decrease with the increase in TiO₂ shell thickness. This work provides a new route to promote the applications of tungsten bronze as heat shielding materials.