Transparent,\nFluorophore-Doped Cellulose Nanocrystal\nFilms Prepared from Crop Residue: Superior Radiative Cooler and Organic\nPhotodetector

Carbon-neutral\nnanomaterials derived from lignocellulosic\nbiomass\nremain the most preferred choice for lowering down the fossil-fuel-based\nenergy consumption for indoor cooling and metal mining purposes. In\nthis work, the synthetic methodologies for yielding a transparent,\nconducting cellulose nanocrystal (CNC) film from a highly abundant\ncrop residue, rice straw (<i>Oryza sp</i>. Stems), have\nbeen presented. The self-assembly of CNC generates a predominant structural\ngreen color. Even the encapsulation of organic fluorophores in CNC\nfilms retains a chiral nematic order and a photonic band gap. The\nhighly crystalline nature of the hydroxyl groups present in CNC regulates\nthe excited-state dynamics of Rhodamine B encapsulated in the film.\nNevertheless, the transreflective property of the CNC film has been\ndemonstrated owing to its low solar light absorption and high mid-infrared\n(MIR) emissivity at the atmospheric transmission window (8–13\nμm). The CNC film can bring down the temperature of the insulated\nbox by ∼6 <b>°</b>C kept under solar simulator illumination,\nsuggesting an effective radiative cooler. Further, the amine functionalization\nof CNC has resulted in a remarkable increase in current and mobility,\nimproving the film’s conductivity by several folds. The photoresponsivity\nof the RhB-encapsulated-amine-functionalized CNC photodetector shows\na peak response of ∼6.3 mA/W at ∼665 nm under zero bias.\nThe external quantum efficiency of fabricated devices is about 1%.\nThe CNC derived from the crop residue has multidimensional applications\nranging from passive cooling to organic electronics.