Silver–nickel core–shell nanostructure on cellulose fibers as biodegradable wearable paper heater

In this study, the traditional Korean Hanji or Han-paper, comprising cellulose fibers obtained from mulberry bark, was coated with silver nanowires (AgNWs) via supersonic spraying; this was followed by electroplating with nickel to produce a silver–nickel (AgNW–Ni) core–shell nanostructure, thereby protecting the nanostructures of the AgNWs from thermal breakdown. The Hanji cellulose fibers coated with the core–shell AgNW–Ni were used as a paper heater. The nickel electroplating time was increased to thicken the nickel shell enwrapping the AgNWs. The electrical properties of the Hanji heater were investigated to identify the optimal AgNW–Ni core–shell nanostructure. The surface temperature of the AgNW–Ni Hanji heater reached 125 °C at 7 V without thermal breakup of the AgNWs with long-term stability. Under excessive electroplating, the entire Hanji substrate was covered with nickel. In this scenario, a nickel layer was formed over the Hanji, which substantially reduced the electrical resistivity of the layer. This metal–metal core–shell nanostructure can be attached to any type of substrate, including rigid alumina, polyethylene terephthalate, and bulk plastic; this makes the method versatile and widely applicable. Stringent bending radius and cyclic tests confirmed the mechanical durability and thermal stability of the silver–nickel core–shell Hanji heater.