Synthesis and luminescence properties of single-phase Ca2P2O7:Eu2+, Eu3+ phosphor with tunable red/blue emission
A series of Eu 2+ and Eu 3+ coexisting Ca 2 P 2 O 7 phosphors with tunable red/blue (R/B) emission have been successfully synthesized by a two-step method. Phase composition, luminescence properties and surface states are investigated by the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) emission spectra, fluorescence quantum efficiency, fluorescent lifetime and X-ray photoelectron spectroscopy (XPS). It can be concluded from XRD and FTIR results that pure Ca 2 P 2 O 7 phase can be obtained at 900 °C and Ca 3 (PO 4 ) 2 impurity phase occurs at an increasing preparation temperature (1100 °C). The XRD results confirm that the Eu 2+ or Eu 3+ doped samples all exhibit pure tetragonal Ca 2 P 2 O 7 crystal with space group P 4 1 . Under excitation of 330 nm, Eu 2+ singly-doped Ca 2 P 2 O 7 phosphors showed a broad blue emission band at 422 nm, while the Eu 3+ single-doped samples emitted a strong orange-red light with wavelength of 591, 597 and 618 nm excited by 395 nm. In particular, oxygen vacancies in pure Ca 2 P 2 O 7 crystal are found due to the strong PL emission at 420 nm. Furthermore, Eu 2+ /Eu 3+ co-doped Ca 2 P 2 O 7 phosphors prepared by this two-step synthesis show the blue emission at 422 nm (Eu 2+ ions) and the orange-red emission (Eu 2+ ions) at 591 nm, 597 nm and 618 nm under near-ultraviolet (NUV) excitation of 380 nm. It was detected that the quantum efficiency of the Ca 1.88 P 2 O 7 :0.12Eu phosphor was 37.7%. The tunable R/B emission ratio from 0.13 to 1.56 can be achieved by controlling the recalcined temperature in the second step when excited by 380 nm. The XPS demonstrate the existence of Eu 3+ ions on the surface of Eu 2+ /Eu 3+ co-doped Ca 2 P 2 O 7 phosphors. Under NUV excitation in the range from 350 to 400 nm, the optimum Commission International de I’Eclairage (CIE) coordinate of Ca 2 P 2 O 7 :0.12Eu 2+ /Eu 3+ phosphors is (0.3648, 0.215), which can be potentially used as an emitting source to meet the needs of illumination devices.
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