A series of Eu2+ and Eu3+ coexisting Ca2P2O7 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 Ca2P2O7 phase can be obtained at 900 °C and Ca3(PO4)2 impurity phase occurs at an increasing preparation temperature (1100 °C). The XRD results confirm that the Eu2+or Eu3+ doped samples all exhibit pure tetragonal Ca2P2O7 crystal with space group P41. Under excitation of 330 nm, Eu2+ singly-doped Ca2P2O7 phosphors showed a broad blue emission band at 422 nm, while the Eu3+ 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 Ca2P2O7 crystal are found due to the strong PL emission at 420 nm. Furthermore, Eu2+/Eu3+ co-doped Ca2P2O7 phosphors prepared by this two-step synthesis show the blue emission at 422 nm (Eu2+ ions) and the orange-red emission (Eu2+ 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 Ca1.88P2O7:0.12Eu 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 Eu3+ ions on the surface of Eu2+/Eu3+ co-doped Ca2P2O7 phosphors. Under NUV excitation in the range from 350 to 400 nm, the optimum Commission International de I’Eclairage (CIE) coordinate of Ca2P2O7:0.12Eu2+/Eu3+ 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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This research is financially supported by the Natural Science Foundation of Liaoning Province (20170540065 and 20170540074), the General Project of Liaoning Provincial Education Department (2016J057) and the Start-up Funding for Doctoral Researchers of Dalian Polytechnic University (61020726).