Role of Heterojunction Engineering in Sputtered WO₃/CuWO₄ and WO₃/CuWO₄/TiO₂ Thin Films for Enhanced Photocatalysis

Heterojunction engineering is a promising approach to overcoming the intrinsic limitations of individual semiconductor photocatalysts. In this context, WO₃/CuWO₄ and WO₃/CuWO₄/TiO₂ heterojunctions were deposited by reactive magnetron sputtering. The resulting heterojunctions were subjected to annealing in air at 600 °C and systematically investigated, with respect to their structural, optical, and photocatalytic properties. The characterization of the developed samples, including X-ray diffraction, Rietveld refinement, Raman spectroscopy, scanning electron and atomic force microscopies (SEM and AFM), and optical spectroscopy, revealed the possibility of controlling the WO₃/CuWO₄ ratio, directly influencing the heterojunction band gap and surface morphology. Photocatalytic degradation of methylene blue under 405 nm LED illumination demonstrated that intermediate WO₃/CuWO₄ compositions achieved the highest efficiencies, which may be attributed to enhanced charge separation at the type-II heterojunction interface. Besides, the introduction of an ultrathin TiO₂ overlayer (∼2 nm) further improved the dye degradation activity. This effect is attributed to defect passivation and selective hole transport. In contrast, with thicker TiO₂ overlayers, the heterojunction contribution is suppressed, and the activity behavior is similar to that observed for bulk TiO₂. These results highlight the critical role of precise stoichiometric control and surface overlayer engineering in optimizing the performance of oxide-based heterostructures, establishing sputtering deposition as a scalable strategy for photocatalytic applications in environmental remediation and solar-driven energy conversion.