Effects of Li+, Na+, and K+ doping on the microstructure, fluorescence thermometry, and thermochromism of Ho3+,Yb3+:Bi2WO6 materials†

Abstract

Luminescent materials with 1% M, 3% Ho³⁺, and 10% Yb³⁺:Bi₂WO₆ (M = Li⁺, Na⁺, K⁺) upconversion ability were prepared by a high-temperature solid-phase method. The band structure and density of states of the synthesized material were calculated using density functional theory, and the UV visible absorption spectrum of the synthesized material was measured experimentally. Theoretical calculations and experimental results both indicated that, compared with the Bi₂WO₆ substrate, doping with Ho³⁺/Yb³⁺ or continuing to be doped with M⁺ (M = Li⁺, Na⁺, K⁺) gradually reduced the bandgap of the material. The bandgap was reduced, and the material could absorb photons with lower energy, which was beneficial for the absorption of infrared photons. X-ray diffraction experiments revealed that Li⁺, Na⁺, K⁺, Ho³⁺ and Yb³⁺ doping had no effect on the orthorhombic crystal structure of the Bi₂WO₆ matrix. For 1% M, 3% Ho³⁺, and 10% Yb³⁺:Bi₂WO₆ (M = Na⁺, K⁺), scanning electron micrographs revealed that the powder sample particle size ranged from 1–3 μm, and energy-dispersive spectroscopy (EDS) maps revealed that all the elements were relatively uniformly distributed in the samples. The fluorescence intensity ratio of the Ho³⁺ emission peaks (I_756nm/I_538nm) was used for temperature characterization. The maximum relative thermometric sensitivities of 3% Ho³⁺, 10% Yb³⁺:Bi₂WO₆ doped with Li⁺, Na⁺, or K⁺ in the 298–573 K temperature range were 1.69% K⁻¹ (348 K), 2.54% K⁻¹ (298 K), and 2.65% K⁻¹ (298 K), and the minimum temperature resolutions were 0.14 K (323 K), 0.05 K (298 K), and 0.04 K (298 K), respectively. Under 980 nm excitation from 298 K to 573 K, the luminescence of the samples not doped with Li⁺, Na⁺, or K⁺ changed from yellow–green to yellow and then red, whereas the Li⁺, Na⁺, or K⁺-doped sample luminescence colors all changed from green to yellow, orange, and finally red, with the K⁺-doped samples having the slowest rate of change to red. The luminescent colors of all the samples were reversible during the cooling process within the same temperature range, indicating that the synthesized samples have potential applications in thermochromism and optical anticounterfeiting.