WO3/MoO3 Nanocomposite Thin Films and Heterostructures: Interfacial Synergy for Smart and Sustainable Technologies

Tungsten trioxide (WO3) and molybdenum trioxide (MoO3) are redox-active Group VI transition-metal oxides widely used in functional thin-film technologies. Many studies have examined single-phase WO3 and MoO3 films. However, WO3/MoO3 nanocomposite thin films and heterostructures have not yet been comprehensively reviewed as interface-engineered platforms for smart and sustainable technologies. This review addresses this gap by analyzing WO3/MoO3 thin-film nanocomposites through the concept of interfacial synergy. Particular attention is paid to the structural complementarity of WO3 and α-MoO3, oxygen nonstoichiometry, mixed W6+/W5+/W4+ and Mo6+/Mo5+/Mo4+ valence states, crystallographic-shear suboxides, W–O–Mo interfaces, and fabrication routes for mixed, graded, and multilayer films. The functional advantages of these systems do not arise from simply combining the two oxides. Instead, they result from charge and oxygen-vacancy redistribution, shortened ion–electron transport pathways, phase stabilization, and the formation of new active sites at interface boundaries. The review also emphasizes the need to distinguish genuine interfacial synergy from apparent improvements caused by surface area, film thickness, porosity, hydration, or measurement conditions. Finally, the review links structural features, defect chemistry, and interface-controlled properties to device-level functionality. This framework highlights promising directions for WO3/MoO3 nanocomposite films in a wide range of smart and sustainable technologies.