Scaling up photochromic thin films with antimicrobial functionality: Roll-to-Roll deposition of YHO and YHO/Cu coatings

We report the roll-to-roll (R2R) magnetron sputtering deposition of photochromic oxygen-containing yttrium hydride (YHO) and bilayer YHO/Cu thin films on flexible polyethylene terephthalate substrates, enabling their integration as replaceable coverings or laminates. This scalable fabrication route yields multifunctional coatings that combine reversible optical modulation with antimicrobial activity. The YHO films are nanocrystalline (∼13 nm crystallite size) with a fine-grained surface morphology. Single-layer YHO films exhibit ∼58 % average visible transmittance, whereas YHO/Cu bilayers show ∼47 % mainly due to reflectance from the Cu film. The films darken reversibly, achieving a relative photochromic contrast of 15–19 % after 1 h of UV–violet illumination, and bleach with a time constant of ∼10 min after the illumination. Notably, higher deposition pressures yield lower contrast but faster bleaching, indicating an inverse correlation between optical contrast and bleaching speed. The addition of a Cu top film endows potent antimicrobial performance: the best-performing YHO/Cu bilayers achieved a 5.2 log reduction in Escherichia coli, a 2.2 log reduction in Staphylococcus aureus, and complete inactivation of SARS-CoV-2 (>3.8 log reduction) after 24 h of incubation, indicating broad-spectrum efficacy. The R2R production of YHO-based thin films demonstrates the industrial potential of large-area photochromic coatings with integrated antimicrobial functionality.