Passive Infrared-to-Visible-Light Upconversion Using NaYF₄:Yb,Er Nanoparticle Films for Greenhouse Façades

Tropical urban greenhouses are prone to heat trapping due to higher daily temperatures compared to temperate climates, and a high energy cost is required to maintain optimal growth conditions. Lanthanide-based nanoparticles are known for their capability of performing upconversion, a process where lower energy photons are absorbed and converted to a single photon of higher energy. These upconversion nanoparticles (UCNPs) possess the innate ability to convert infrared energy to visible light, thereby facilitating efficient heat removal while simultaneously providing upconverted visible light. Consequently, the incorporation of such material into greenhouse façades presents an attractive zero-energy alternative to commonly used greenhouse cooling technologies. In this study, NaYF₄:Yb³⁺/Er³⁺ UCNPs were synthesized and coated onto a simulated greenhouse enclosure with Pak Choi plants to observe the effect of such a film coating on actual crop growth. A reduction of the interior temperature, increased sunlight diffusion, and improved crop yield were demonstrated in this work, showing promising results for future urban greenhouse façade applications.