Doping, defect engineering, and hybridization of metal oxides for multifunctional applications

Metal oxides have attracted sustained interest as versatile functional materials because their structural, optical, electrical, and surface properties can be tailored for a wide range of technological applications. In particular, doping, defect engineering, nanostructuring, and hybridization through heterostructure formation, composite design, and surface decoration provide effective routes for tuning their physicochemical properties and improving their functional performance. This review presents an overview of modified metal oxides and oxide-based hybrid materials, with emphasis on the relationship between their composition, structure, defects, interfaces, and resulting properties. Here, hybridization refers to combining metal oxides with distinct functional components, including carbon materials, metal-organic frameworks, other semiconductors, and metallic nanoparticles. These approaches are distinguished from lattice doping and ion intercalation to clarify their different contributions to material performance. The discussion first considers structural and morphological evolution, optical behavior, and electrical and charge-transport characteristics, providing a foundation for understanding the origin of their enhanced performance. Recent progress in environmental applications is then highlighted, particularly photocatalytic degradation and water treatment, followed by developments in gas and chemical sensing. The review further examines the growing role of modified metal oxides and their hybrids in energy technologies, including rechargeable batteries, supercapacitors, and solar-energy conversion, with particular attention to charge-transfer processes, interfacial effects, defect chemistry, and compositional optimization. Finally, the remaining challenges associated with stability, reproducibility, dopant and defect control, interfacial compatibility, scalability, and practical implementation are discussed, together with emerging opportunities for multifunctional and integrated metal-oxide systems. Overall, this review provides a materials-oriented perspective on how rational control of composition, defects, interfaces, and architecture can advance metal oxides toward more efficient, durable, and scalable technologies.