The hydrothermal synthesis and morphology-dependent optical temperature sensing properties of Er³⁺ doped NaGd(WO₄)₂ phosphor

NaGd(WO₄)₂: Er³⁺ phosphors by using sodium citrate (Na₃Cit, Na₃C₆H₅O₇) as chelating agent were synthesized via hydrothermal method. The structure and morphology were characterized and analyzed by the X-ray diffraction and field emission scanning electron microscopy. It was found that Na₃Cit added into the reaction solution considerably influenced the morphologies of samples. The temperature-dependent down-conversion emission spectra of NaGd(WO₄)₂: Er³⁺ phosphor under the excitation of 377.5 nm fluorescence with different ratios of Cit^3-/Re³⁺ have been measured. The temperature sensing performance for the samples was studied based on the change of emission intensities from two thermally coupled ²H_11/2 and ⁴S_3/2 levels of Er³⁺. It was found that the temperature sensing sensitivities increased with the increase of ratios of Cit^3-/Re³⁺, and the temperature sensing sensitivity can reach a relatively high value of 0.0128 K⁻¹, which was obtained for the sample with the ratio of Cit^3-/Re³⁺ was 1.5. Finally, the physical mechanism of morphology-dependent temperature sensing sensitivity change was explained.