TOF SIMS analysis and enhanced UVB photoluminescence by energy transfer from Pr³⁺ to Gd³⁺ in Ca₃(PO₄) ₂:Gd³⁺,Pr³⁺ phosphor prepared by urea assisted combustion

Ca₃(PO₄)₂:Gd³⁺,Pr³⁺ phosphors with different concentrations of Gd³⁺ and Pr³⁺ were prepared by combustion process using metal nitrates as precursors and urea as fuel. The phosphors were characterized by X-ray diffraction (XRD), Scanning electron microscopy, Time of Flight-Secondary Ion Mass Spectrometry, ultraviolet-visible and photoluminescence (PL) spectroscopies. The XRD patterns were consistent with the standard rhombohedral phase of Ca₃(PO ₄)₂ referenced in JCPDS Card No. 70-2065. The average crystallite size calculated using the Scherrer equation at different urea masses was in the range of ∼60-120 nm. The PL excitation spectra of Ca ₃(PO₄)₂:Gd³⁺ and Ca ₃(PO₄)₂:Pr³⁺ exhibited peaks at 220-280 and 300-490 nm associated with the f → f transitions of Gd ³⁺ and Pr³⁺ respectively. The ultraviolet B (UVB) emission resulting from the ⁶P_7/2 → ⁸S _7/2 transition of Gd³⁺ was observed at 313 nm when the Ca₃(PO₄)₂:Gd³⁺ phosphor was excited at a wavelength of 274 nm using a monochromatized xenon lamp. Upon Pr ³⁺ co-doping, the excitation peaks due to Gd³⁺ and Pr ³⁺ f → f transitions were suppressed and an intense broad excitation peak ascribed to the 4f → 4f5d transitions of Pr³⁺ was observed at 227 nm. The peak intensity of the UVB emission at 313 nm was shown to improve considerably when the Gd³⁺ and Pr³⁺ co-doped systems were excited at the wavelength of 227 nm suggesting that the Pr³⁺ is a good sensitizer of the 313 nm narrow line UVB emission from Gd³⁺.