Bifunctional NaGdF4:Yb3+/Er3+ Upconversion Nanoparticles for Efficient Overall Water Splitting

Abstract

The development of efficient and robust catalysts for overall water splitting is vital for the advancement of hydrogen-based renewable energy systems. This work reports the synthesis of β-NaGdF4:Yb3+/Er3+ upconversion nanoparticles (UCNPs) via a solvothermal method and demonstrates their bifunctional catalytic activities toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), as well as photoelectrochemical (PEC) water splitting. Electrochemical studies reveal that the UCNPs exhibit low overpotentials and high exchange current density with favorable Tafel slopes for both HER and OER, combined with excellent charge-transfer properties and abundant active sites. Especially, under irradiation, the UCNPs exhibit enhanced PEC activities, benefiting from their strong light-matter interactions and photon upconversion capability, which facilitates the absorption of near-infrared photons and converts them into visible emission, thereby enhancing charge carrier dynamics. The bifunctional NaGdF4:Yb3+/Er3+ electrodes also attain efficient overall water splitting, requiring ~1.52 V to reach 10 mA cm-2 under simulated solar irradiation with remarkable stability over 24 h. To the best of our knowledge, this is the first demonstration of NaGdF4:Yb3+/Er3+ UCNPs acting as a bifunctional catalyst for electrochemical and photoelectrochemical overall water splitting, offering new avenues toward full-spectrum solar energy harvesting and sustainable hydrogen production.