Flexible, Transparent, and Hazy Cellulose Nanopaper with Efficient Near-Infrared Luminescence Fabricated by 2D Lanthanide (Ln = Nd, Yb, or Er) Metal-Organic-Framework-Grafted Oxidized Cellulose Nanofibrils

Three-dimensional (3D) bulk materials, such as metal-organic frameworks (MOFs) and inorganic phosphors, show the properties of large backscattering and stress concentration, which result in low mechanical and inferior transmittance when these materials are hydridized with a polymer matrix. Inspired by the "reinforcement" effects of two-dimensional (2D) materials, such as grapheme, C3N4, MoS2, and Mxene, it was interesting to examine a 2D lanthanide (Ln)-based MOF-grafted natural polymer (nanocellulose) with the goal of achieving light emission, transparency, and good mechanical properties. A series of near-infrared (NIR) luminescent cellulose nanopapers were prepared via 2D Ln-MOF-grafted (2,2,6,6-tetramethylpiperidin-1-yl)oxyl-oxidized cellulose nanofibrils (tCNFs; Ln = Nd, Yb, or Er). In addition to efficient NIR luminescence, these Ln nanopapers exhibited good flexibility, transparency (>90%), and mechanical properties (>28 MPa). Notably, the haze of these nanopapers was increased by 93-95% from 26% due to compositing with 2D Ln-MOFs, which prevented dense packing among the cellulose and formed air cavities in the nanopaper, inducing internal light scattering and improving optical haze. Moreover, these flexible Ln nanopapers exhibited efficient NIR luminescence, which, together with optical haze and transparency, offered an opportunity for utilization in paper-based anticounterfeiting, NIR-light-emitting diodes, or light softening devices.