Ratiometric Optical Thermometry in Lanthanide-Doped Molybdate Phosphors via Construction of Diverse Charge-Transfer Bands

Optical temperature measurement of fluorescence intensity ratio has the advantages of noncontactness and fast response, so it has been widely studied by scholars. It is difficult to obtain high sensitivity in temperature measurement using a traditional thermally coupled energy-level method. Herein, two different charge-transfer bands are constructed to activate phosphors, and two different emission bands in dual-luminescent centers are used to raise the temperature sensitivity. Tb3+ and Eu3+ dual-doped molybdate phosphors were successfully synthesized, and the host material was sensitized by the charge-transfer state. The absorption energy of MoO42– was transferred to Tb3+ and Eu3+ ions by a resonance interaction, and the spectra of both the ions overlapped greatly, which indicates that there is energy transfer between Tb3+ and Eu3+. Multiphonon de-excitation is the thermal quenching mechanism of lanthanides because Tb3+ has smaller thermal quenching activation energy and faster quenching speed. The fluorescent powder has good temperature sensitivity in the temperature range of 303–503 K, and the maximum relative sensitivity and absolute sensitivity are 1.1% K–1 (483 K) and 0.017 K–1 (503 K), respectively, which are generally superior to those of the previous optical materials. In addition, with an increase of temperature, the luminescent color of the developed phosphor changes from yellow-green to white. To sum up, the prepared phosphors have excellent potential to be used as temperature sensors as well as temperature indicators.