Direct Synthesis of Defective Tungsten Trioxide Microspheres using Ultrasonic Spray Pyrolysis Process for Enhanced Photothermal Conversion Performance

Defective tungsten trioxide (WO_3-x) is promising as a photothermal conversion material. However, WO_3-x prepared by conventional reduction of WO₃ suffers from limited photothermal efficiency due to oxygen vacancies forming mainly on the surface. In this study, we directly synthesized WO_3-x microspheres with a high concentration of oxygen vacancies using a modified ultrasonic spray pyrolysis (USP) method. Unlike conventional processes, this process can increase the concentration of oxygen vacancies in WO_3-x by adding an organic antioxidant to the precursor solution and applying an oxygen-free atmosphere. To systematically investigate the impact of these modifications, WO₃ or WO_3-x powders were synthesized using three USP conditions: a typical process, a process in an anoxic atmosphere, and a process combining both an anoxic atmosphere and an antioxidant. The resulting powder, with a distinct dark navy color indicating a high oxygen vacancy concentration, showed the most excellent light absorption, the lowest reflectivity, and the smallest band gap energy. This powder achieved the highest temperature increase under various light sources, demonstrating its superior photothermal conversion efficiency.