Dual-functional ZnO/Ag/ZnO multilayer thin films for energy-efficient low-emissivity and self-cleaning smart glass

Buildings account for approximately 40% of global energy consumption, with a significant fraction attributable to heat transfer through glazing systems. Low-emissivity (low-ε) coatings that selectively reflect near-infrared (NIR) radiation while maintaining visible transparency are therefore critical for energy-efficient fenestration. However, simultaneously achieving high visible transparency, strong infrared rejection, low thermal transmittance, and photocatalytic self-cleaning within a single scalable coating architecture remains a major challenge due to competing optical and surface-functional requirements. Here, ZnO/Ag/ZnO (ZAZ) multilayers were engineered to systematically balance the trade-off between visible transparency, thermal insulation, infrared reflectance, and photocatalytic self-cleaning within a single sputtered architecture. X-ray diffraction (XRD) confirmed preferential c-axis (002) orientation of ZnO and face-centered cubic (fcc) Ag (111) crystallinity, with Ag interlayer thickness governing crystallite size. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray spectroscopy (EDS) analyses revealed compact nanocrystalline morphologies; Ag incorporation increased surface roughness (up to ∼47 nm peak-to-valley), enhancing photocatalytically active surface area. Ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometry confirmed that ZAZ films maintain >90% transmittance in the visible region (400-750 nm) while exhibiting strong and selective NIR reflectance above 800 nm, a spectral signature essential for solar control glazing. Thermal transmittance (U-value) measurements showed a dramatic reduction from 7.8 W m^-2 K^-1 for bare glass to 1.93 W m^-2 K^-1 for the optimized ZAZ-20 film (∼75% reduction), confirming effective suppression of radiative heat transfer. Ag incorporation also narrowed the optical bandgap below 3.1 eV via localized surface plasmon resonance (LSPR) effects, improving UV light harvesting. Photocatalytic degradation of Rhodamine B under UV irradiation reached 96.4% for ZAZ-20 after 10 h, attributable to enhanced electron-hole separation mediated by Ag nanoparticles acting as electron sinks. The optimized ZAZ-20 coating also demonstrated excellent UV stability, retaining its optical, photocatalytic, and low-emissivity properties after 72 h of continuous UV-A exposure. The results demonstrate that Ag interlayer engineering provides a practical route for integrating passive thermal management and active surface self-cleaning in multifunctional smart-glass coatings.