Effect of different surfactants on structural and optical properties of Ce³⁺ and Tb³⁺ co-doped BiPO₄ nanostructures

In this paper we report on the Ce³⁺ and Tb³⁺ ions co-doped bismuth phosphate (BiPO₄) nanostructures that were synthesized by a simple precipitation method using different surfactants such as glycerol/H₂O, glycerol/ethylene glycol, oleic acid, and ethylene glycol. The structural (X-ray diffraction, scanning electron microscopy, tunneling electron microscopy), functional groups analysis (Fourier transform infrared and Raman spectroscopy), thermal (thermogravimetry and differential thermal analysis), and optical (photoluminescence, photoluminescence-excitation) properties were investigated. The structural and morphological analysis confirms the pure hexagonal crystal structure of the synthesized nanostructures. From the measured Fourier transform infrared (FTIR) and Raman spectra various functional groups such as υ₃ stretching vibration of the PO₄ group, and δ(O-P-O) and υ₄ (PO₄) vibrations including the υ₂ and υ₁ bending modes of the PO₄ units are identified. Based on the thermal analysis, for all the studied samples an exothermic peak between 680 °C and 700 °C was observed due to phase transition from hexagonal to high temperature monoclinic. The Ce³⁺and Tb³⁺ codoped samples show spectrally broad 5d → 4f luminescence in the blue (centered at 459 nm) wavelength region under the direct optical excitation of Ce³⁺ at 417 nm. Similarly, Tb³⁺ has revealed four main emission bands (⁵D₄ → ⁷F_{6, 5, 4 and 3}) at 490 nm, 545 nm, 585 nm and 621 nm with 378 nm (⁷F₆ → ⁵G₆) as the excitation wavelength, including three more weak emission bands at 647 nm, 669 nm, and 681 nm which could be assigned to ⁵D₄ → ⁷F_{2, 1, 0} emission transitions. Among them, 545 nm (⁵D₄ → ⁷F₅) has shown bright green emission. The Ce³⁺ and Tb³⁺ codoped sample synthesized with pure oleic acid have shown relatively high green emission intensity for Tb³⁺, and relatively weak blue emission intensity for Ce³⁺ under their respective optical excitation wavelengths.