A new quaternary type IV deep eutectic solvent (DES) was prepared for the synthesis of WO3/Nb2O5 complexes via a facile solvothermal route under ambient conditions. PXRD confirmed the formation of orthorhombic niobium tungsten oxide with semi-crystalline crystal phases. The SEM micrograph displayed a well aggregated and clustered particle distribution with irregular shapes, while TEM revealed a flower-shaped morphology of the materials. Furthermore, the UV-Vis measurements indicated electronic absorption in the range of 206–282 nm before irradiation with UV-light. After 5 s of irradiation, there was an instant color change from white to blue, which gradually deepened after 180 s. This color change was accompanied by a broad electronic absorption at 658 nm. Moreover, the intense photochromic material displayed fast fading within 20 min of exposure to the open air and remarkable recyclability. These observed properties could open new avenues for smart windows and color display devices.
