Highly transparent and thermally stable cellulose nanofibril films functionalized with colored metal ions for ultraviolet blocking activities

•Highly transparent UV shielding TOCN films were prepared via ions exchanging.•The introduction of Fe3+ led to an excellent UV blocking property.•The UV shielding was persistent when exposed to thermal treatment or UV irradiation.•The metal-carboxylate complexes are advantageous for improving thermal stability. Although many preparation methods have been reported so far, it is still a great challenge for Ultraviolet (UV) protection films that simultaneously have extremely high transparency and excellent UV shielding properties. Herein, we reported a simple, eco-friendly process for preparing high transparency TOCN-COOM (M: metal ions) films with diverse UV-shielding performances via adsorption of colored metal ions (Fe3+, Co2+, Ni2+, Cu2+). The introduction of Fe3+ lead to an excellent UV blocking, especially almost 89% absorption of Ultraviolet A (UVA) and full absorption of Ultraviolet B (UVB). Simultaneously an extremely high transmittance in the visible light region (around 88%) was retained. In addition, the UV shielding films performed stable UV-blocking when exposed to UV irradiation or thermal treatment at 120 °C. The formation of metal-carboxylate complexes is also advantageous for improving thermal stability. Hence, the obtained TOCN-COOM films have wide application prospect in the field of UV shielding.