Up-conversion luminescence and temperature sensing properties of Ho³⁺-, Tm³⁺-, and Yb³⁺-codoped Bi₂WO₆ materials in a...

A series of x%Ho³⁺, 5 %Tm³⁺, y%Yb³⁺:Bi₂WO₆ (x = 0, 0.5, 1, 3, 5; y = 0.5, 1, 3) luminescent materials was prepared using a high-temperature solid-phase method. The microstructure, up-conversion luminescence, and temperature sensing properties of the synthesized powders were analyzed. X-ray diffraction patterns revealed that doping with Ho³⁺, Tm³⁺, and Yb³⁺ ions at certain concentrations did not affect the orthorhombic crystal structure of the Bi₂WO₆ host. Scanning electron microscopy revealed that the morphology of the sample consisted of lumpy particles with a particle size range of 1-5 µm and agglomeration. SEM mapping and energy-dispersive X-ray spectroscopy analyses revealed that each element was relatively uniformly distributed on the particle surface. Under 980 nm excitation (380 mW), the strongest luminescence of the sample was obtained when both Ho³⁺ and Yb³⁺ doping concentrations were 1 %. Compared with the luminescence of the 5 %Tm³⁺ and 1 %Yb³⁺:Bi₂WO₆ sample, with increasing Ho³⁺ concentrations, the luminescence intensity of Tm³⁺ was first enhanced and subsequently weakened, whereas the luminescence of Ho³⁺ was significantly weakened, which indicates the positive energy transfer from Ho³⁺ → Tm³⁺. At 980 nm (80-380 mW), for the 1 %Ho³⁺, 5 %Tm³⁺, and 1 %Yb³⁺:Bi₂WO₆ sample, the 538 nm, 545 nm, 660 nm, and 804 nm emission peaks originated from the two-photon absorption. FIR_{660 nm/804 nm}, FIR_{545 nm/804 nm}, and FIR_{538 nm/804 nm} were used to characterize the temperature and corresponded to temperature sensitivities S_r of 0.0046 K^-1, 0.022 K^-1 and 0.024 K^-1 at 573 K, respectively. At 498 K, the minimum temperature resolution δT values were 0.03384 K, 0.03203 K and 0.04373 K. When the temperature increased from 298 K to 573 K, the powder sample luminescence gradually shifted from the yellow-green region to the red region. The results of environmental discoloration and thermochromic performance tests indicate that this sample has potential application in optical anti-counterfeiting. FIR_{804 nm /660 nm} and FIR_{804 nm /538 nm} were obtained for the 40 NTU turbidity suspension under identical excitation conditions. At 298 K, for the 40 NTU turbidity sample, the maximum S_r values were 0.0197 K^-1 and 0.0405 K^-1; at 340 K, the minimum temperature resolutions δT values were 0.54037 K and 0.66237 K. When the temperature decreased from 340 K to 298 K, the luminescence of the 40 NTU suspension samples gradually shifted from the yellow region to the green region.