Circular Design of ZnO and TiO₂ Nanocomposites for Energy and Healthcare: Processing, Structure and Multifunctional Performance

ZnO- and TiO₂-based nanocomposites combine semiconductor functionality with the structural, transport, and surface properties of other constituents. Their design requires relationships among processing, phase distribution, interfaces, and performance to be evaluated alongside durability and recovery. This narrative review critically examines these relationships in polymer–particle composites, supported oxides, and composite coatings for energy and healthcare applications. Single-phase and doped oxides and layered heterostructures are considered as constituent materials or architectural comparators, rather than classified indiscriminately as nanocomposites. Examples include chitosan/Ag@ZnO films, zeolite-supported oxides, photovoltaic interfaces and optical modifiers, and ZnO-containing implant coatings. Whey-assisted synthesis and starch-based manufacturing redesign illustrate how precursor chemistry and processing conditions influence the production of functional oxide constituents. The analysis distinguishes multifunctional potential from performance demonstrated in the same component, and material-property improvements from validated device operation. Safe and Sustainable by Design principles and life-cycle assessment connect functional requirements to processing burdens, release, repeated use, and end-of-life options. Reported benefits remain formulation- and application-specific; evidence connecting synthesis, multifunctional service, and recovery for one material stream is limited. Circular nanocomposite design therefore requires traceable processing histories, complementary structural characterization, concurrent functional testing, and requalification of recovered constituents.