Hydration-Driven Surface Engineering of Nanocellulose Films for Iridescent Structural Colors

Structural colors, arising from periodic architectures in cellulosic materials through photonic interactions, hold great promise for sustainable optical applications. These cellulosic materials offer a low-impact alternative to conventional polymer-based materials, which typically require energy-intensive processing and pose environmental concerns. However, the working principle of current cellulosic materials is based on their intrinsic liquid crystalline phase in dispersed states, and when processed into solid films, they generally exhibit weak or nonvivid coloration, limiting their practical use. Herein, we develop a hydration-driven imprinting strategy to construct highly ordered micro/nano hierarchical structures on cellulose nanofiber (CNF) films for structural color generation. This approach leverages the excellent hydrophilicity of CNF, resulting from its nanoscale structure and surface chemistry. The mechanical properties, together with simulation results, reveal that hydration-induced swelling weakens interfiber bonding and enhances nanofiber mobility under minimal external force, thereby improving CNF film plasticity and facilitating precise replication of template patterns. As a result, the CNF films exhibit vivid, angle-dependent structural colors. This work offers a scalable and cost-effective strategy for using sustainable cellulosic materials in optical applications.