The obtained CsWO displays characteristic diffraction peaks of hexagonal cesium tungsten bronze (Cs0.33WO3, PDF # 83–1334) as shown in Fig. 1a, where the Cs+ ions are introduced into the hexagonal tunnels of tungsten oxygen framework composed of corner-shared WO6 octahedrons to produce a large amount of W5+ species in the crystals for keeping charge balance [36]. The Cs+ ions serving as donors increase the free electron density in the crystal, triggering the localized surface plasmon resonance
