One‐Step‐Assembled CNC‐Li/rGO Janus Photonic Films With Solvent‐Responsive Structural Color and Directional Actuation

ABSTRACT Cellulose nanocrystal (CNC)‐based photonic films offer a sustainable platform for visual sensing and actuation, but their performance is constrained by limited sensitivity at low water contents and weak coupling between optical response and macroscopic deformation. Here, a bioinspired Janus photonic film composed of lithium‐functionalized CNCs (CNC‐Li) and reduced graphene oxide (rGO) is developed through a one‐step sedimentation‐coupled evaporation‐induced self‐assembly. During drying, CNC‐Li and rGO organize into an asymmetric structure comprising a CNC‐Li‐rich chiral nematic photonic region, an intermediate transition region, and an rGO‐rich bottom region, as supported by cross‐sectional and depth‐resolved analyses. The Li + ‐associated ionic environment enhances solvent affinity and promotes solvent uptake, whereas the through‐thickness asymmetric architecture contributes to asymmetric deformation and directional bending. The film converts ethanol/water composition changes, including low water fractions, into structural‐color and mechanical responses. Single‐sided droplet tests reveal a time‐dependent response consistent with different timescales of solvent uptake and asymmetric deformation across the asymmetric structure. Maximum curvature (κ max ) correlates strongly with reflection‐peak shift (Δλ max ) (R 2 = 0.962), supporting coordinated optical and mechanical responses. The film enables visual and semiquantitative estimation of water content in ethanol/water mixtures. This work establishes a cellulose‐based route to photonic actuators integrating composition‐responsive sensing, visual readout, and adaptive deformation.