Upconversion luminescence and temperature sensing of Dy³⁺ and Yb³⁺ codoped Bi₂WO₆ powder in aqueous environments

<i>x</i>%Dy³⁺,<i>y</i>%Yb³⁺:Bi₂WO₆ (<i>x</i> = 0.5, 1, 3, 5; <i>y</i> = 1, 6, 10, 15) upconversion luminescent powders were prepared using a high-temperature solid-phase method. X-ray diffraction experiments revealed that moderate amounts of Dy³⁺ and Yb³⁺ doping essentially did not affect the orthorhombic crystal system structure of the Bi₂WO₆ matrix materials. SEM photographs and EDS analysis showed that the particle sizes of the synthesized 0.5%Dy³⁺,10%Yb³⁺:Bi₂WO₆ powder were between 0.5 and 6 μm, and SEM mapping and EDS revealed that all the elements were better distributed in the sample. Under 980 nm excitation, the upconversion luminescence of Dy³⁺ in the powder was strongest when the molar fractions of Dy³⁺ and Yb³⁺ were 0.5% and 10%, respectively. From 10 mW to 100 mW, the emission light intensity <i>I</i> and the excitation power <i>P</i>^<i>n</i> of Dy³⁺ in this sample were linear. The absorbed photon numbers <i>n</i> were 2.63 and 2.65, as calculated from the 546 nm and 660 nm emission peaks of Dy³⁺ in this excitation power range, which indicated that the above two emissions originated from three-photon absorption. Under 980 nm excitation (100 mW), the 660 nm/546 nm fluorescence intensity ratio of the 0.5%Dy³⁺,10%Yb³⁺:Bi₂WO₆ powder was used to characterize the temperature, where the relative thermometry sensitivity <i>S</i>_r was obtained with a maximum value of 0.0151 K^-1 (423 K), and the minimum temperature resolution was 0.58 K. Chromaticity coordinates revealed that, with increasing temperature, the luminescence of this powder shifted from green to the yellow first and then progressively towards the red region. This result indicated that the powder sample synthesized in this work has potential applications in thermochromic optical anticounterfeiting. Under 980 nm excitation (380 mW), for the 60.7 NTU sample in an aqueous environment, from 331 K to 298 K, the temperature was characterized by the fluorescence intensity ratio of 660 nm/546 nm. The obtained maximum <i>S</i>_r was 0.0868 K^-1 (323 K), and the minimum temperature resolution was 3.03 K.