Na7[SbW6O24]: a new type of turn-off luminescence humidity sensor based on a lanthanide-free polyoxometalate

Abstract

A new type of turn-off photoluminescence (PL) humidity sensor based on a water-responsive anhydrous alkali lanthanide-free [SbW₆O₂₄]⁷⁻ polyoxometalate (POM) is reported. Na₇[SbW₆O₂₄] (NaSbW₆) was recently obtained by heating its parent hydrate Na₇[SbW₆O₂₄]·16H₂O (NaSbW₆·16H₂O) at 200 °C. Upon exposure to a moisture-laden atmosphere at room temperature, the anhydrous phase converted back to the hydrated one, leading to a complete quenching of the intense PL intensity. The present work thoroughly investigates the dehydration/rehydration processes of these materials and their precise impact on their PL responses. First, the complete thermal dehydration of NaSbW₆·16H₂O involves the progressive formation of two intermediary hydrates, Na₇[SbW₆O₂₄]·10H₂O (NaSbW₆·10H₂O) and Na₇[SbW₆O₂₄]·4H₂O (NaSbW₆·4H₂O) which has been characterised by a combination of powder X-ray diffraction (PXRD) analysis, optical microscopy and FT-Raman spectroscopy at variable temperatures. The crystal structure of NaSbW₆·10H₂O has been solved from ab initio powder X-ray diffraction determination, revealing structural similiarities with NaSbW₆·16H₂O. The humidity-dependent stability of the (an)hydrated phases has been also investigated by dynamic vapor sorption (DVS) and PXRD analyses under variable relative humidity (RH). The room-temperature PL properties of NaSbW₆·10H₂O and NaSbW₆·4H₂O have been characterised, and their photophysical parameters determined and compared with those of NaSbW₆·16H₂O and NaSbW₆ recently reported. The PL efficiency of the four materials gradually increases with decreasing their degree of hydration, due to the progressive deletion of non-radiative deactivation pathways caused by O–H vibrators connecting the emitting POM unit. NaSbW₆ exhibits humidity-mediated transformations into the three hydrated phases, and its PL response to RH is linear in three distinct ranges. The best sensitivity is observed within the RH range of 0–23%, with a limit of detection of 2.2% RH.