Synthesis of hexagonal tungsten bronze nanoparticles at a significantly lower temperature and in a shorter time

Hexagonal tungsten bronze nanoparticles (h-M_xWO₃ NPs, M = alkali metal) have generated significant research interest owing to their exceptional photo-responsive characteristics. Although the conventional solvo-/hydro-thermal method has been proven to be a highly effective route for synthesizing h-M_xWO₃ NPs, the required temperature of over 180 °C and the corresponding production equipments limit its industrialization. Thus, in this work, a milder route for synthesizing h-M_xWO₃ NPs (M = Cs, Rb, K, and Na) was developed, which required a temperature of only 90 °C and a time period of only 8-24 h, enabling the systematic study of h-M_xWO₃. XRD and ICP-MS results showed that the products were hexagonal Cs_0.29WO₃, Rb_0.27WO₃, K_0.24WO₃ and Na_0.15WO₃. FTIR measurements confirmed the presence of -OH and H-O-H in the as-synthesized samples. Compared with Na_0.15WO₃ with a relatively higher symmetry of P6/mmm, the lattice constants and W 4f binding energies of Cs_0.29WO₃, Rb_0.27WO₃ and K_0.24WO₃ with the P6₃/mcm space group were significantly lower. In addition, photo-responsive tests showed that all the as-synthesized h-M_xWO₃ NPs retained their intrinsic properties, including near-infrared shielding, visible light transmission and photothermal conversion. This advancement establishes an experimental foundation for the scale-up industrial production of h-M_xWO₃ nanoparticles.