Upconversion luminescence and temperature sensing ability of Eu3+ and Yb3+-codoped Bi2WO6 powder in aqueous environments

Bi₂WO₆:x mol% Eu³⁺, y mol% Yb³⁺ (x = 1, 3, 6; y = 3, 6, 9) upconversion luminescent materials were prepared using a high-temperature solid-phase method. XRD results revealed that Eu³⁺ and Yb³⁺ doping did not affect the orthorhombic crystal system structure of the Bi₂WO₆ matrix material. SEM results revealed that Bi₂WO₆ powder with 3 mol% Eu³⁺ and 6 mol% Yb³⁺ consisted of 0.5–8 μm particles with some agglomeration. SEM mapping and EDS measurements of the powder revealed that the elements were relatively uniformly distributed. Under 980 nm excitation, the Eu³⁺ emission intensity was the greatest for the 3 mol% Eu³⁺ and 6 mol% Yb³⁺-codoped sample. The relationship between the luminescence intensity of the Bi₂WO₆:3 mol% Eu³⁺, 6 mol% Yb³⁺ sample and the pump power was determined for excitation powers ranging from 221 to 441 mW. The results indicated that the emission peaks of Eu³⁺ at 475 nm, 521 nm, 541 nm and 649 nm originated from two-photon absorption. Under 980 nm laser excitation, the temperature from 298 K to 573 K was characterized by the intensity ratio of the 475 nm and 541 nm fluorescence from Eu³⁺. The maximum relative temperature sensitivity S_r was 0.022 K^-1 (298 K), and the minimum temperature resolution δT was 0.0182 K (298 K). Under the same excitation conditions, for 187 Ntu samples in an aqueous environment, the maximum S_r from 336 K to 298 K was 0.025 K^-1 (336 K), and the minimum δT was 0.0184 K (336 K). The luminescence color of the powder samples changed between green and blue with increasing temperature, which suggested a potential application in thermochromic optical anticounterfeiting.