Dual-Mode Humidity Sensing Based on Intensity and Lifetime Modulation of Yb₂(MoO₄)₃:Er³⁺ Upconversion Phosphor

Optical sensors have recently attracted considerable attention because of their remarkable advantages, including fast response, high spatial resolution, noncontact measurement capability, and excellent sensitivity. Harnessing intrinsic lattice features for functional sensing offers a promising pathway toward material design. Herein, we strategically exploit the structural characteristics of orthorhombic Yb₂(MoO₄)₃ (space group Pbcn)─comprising a three-dimensional framework of corner-sharing YbO₆ octahedra and MoO₄ tetrahedra with accessible lattice voids─to achieve dual-mode optical humidity sensing. Er³⁺-doped Yb₂(MoO₄)₃ phosphors were synthesized via a hydrothermally assisted method and exhibited both upconversion (UC) and downshifting (DS) luminescence under a 980 nm excitation. The presence of nanoscale lattice cavities capable of accommodating H₂O molecules enabled humidity-dependent modulation of the optical response through nonradiative coupling between the high-energy O-H vibrations and excited states of Yb³⁺/Er³⁺. Upon switching from dry air to 90% relative humidity (RH), the UC green emission intensity decreased to approximately 30% of its initial value, and the luminescence lifetime shortened from 50 to 40 μs, both recovering reversibly upon dehydration. These variations followed the Langmuir-Hinshelwood-type adsorption kinetics, confirming that surface-adsorbed water molecules govern the luminescence modulation. The combination of reversible lattice hydration and a robust host framework endow Yb₂(MoO₄)₃:Er³⁺ with high sensitivity, repeatability (>95%), and dual-mode functionality (intensity and lifetime) at room temperature. This study demonstrates a structure-guided design strategy for converting the inherent hygroscopicity of molybdate frameworks into a robust optical sensing mechanism, paving the way for multifunctional luminescent materials for environmental monitoring.