English

• 1. INTRODUCTION

  In last decades, researchers have paid many efforts to develop highly efficient luminescent materials for green lighting^[1], 3-dimension display^[2], biological probe^[3], in vivo medical imaging^[4] and anti-fake application^[5]. Rare earth phosphates, as an important family of phosphors, have become the subject of extensive researches in the past few years as a result of their interesting physical and chemical properties as well as their excellent luminescence characteristics which are suitable for a high efficient commercial phosphor in accordance with present applications^[6-8]. Cerium phosphate (CePO₄) is an excellent host among various rare earth phosphates for the production of phosphors as Ce³⁺ exhibits intense 4f-5d absorption/emission band ranging from ultraviolet (UV) to visible (Vis) range^[9-12]. In addition, different morphology of CePO₄ shows different performance of luminescence^[13-16]. In particular, one-dimensional (1D) nanomaterials have attracted much attention due to their unique properties and interesting applications in many fields^[17-20].

  As is well known, luminescence efficiency plays a key role in the application of phosphors, such as green lighting, and the sensitivity of labeling and the effectiveness of therapy both deeply depend on the luminescence intensity of nanocrystals^[21]. Many investigations indicated Ln³⁺ ions were particularly sensitive to the crystal lattice surrounding environment^{[22, 23]}. A hypersensitive transition will be produced by changing the asymmetry of Ln³⁺ ions^, coordination environment. According to the formula of Judd-Ofelt, the Ω₂ parameter is specially susceptive to the surroundings caused by the change of local symmetry of rare earth ions. The crystal field effect will also be produced by the variation of Ln³⁺ ions^, coordination environment^[24]. Recently, Hassouna's group have reported the optical and electrical properties of CePO₄ which can be adjusted by partial Ce³⁺ ions substitution with Cr^3+[25]. Hu's group found the photoluminescence intensity of CePO₄: Tb/GdPO₄ was significantly enhanced compared with that of CePO₄: Tb^[26]. Therefore, we have an idea to take one smaller trivalent ion (Gd³⁺) doped in CePO₄ to manipulate the local coordination surrounding symmetry of rare earth ions, and then enhance the photoluminescence performance of samples. And the role of doped Gd³⁺ was systematically studied by X-ray powder diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and photoluminescence spectra methods.

  2. EXPERIMENTAL

  All reagents including Ce(NO₃)₃·6H₂O, Gd(NO₃)₃·6H₂O and Na₃PO₄·12H₂O are of analytic purity, obtained from Tianjin Fengyue Chemical Reagent Ltd. Co. of China without further purification. Surfactant P123 ((EO)₂₀-(PO)₇₀-(EO)₂₀, Mav = 5800, Aldrich) is a triblock copolymer.

  In the typical preparation of Ce_{(1 – x)}Gd_xPO₄ hexagonal one-dimensional nanowires, an appropriate amount of Ce(NO₃)₃·6H₂O and Gd(NO₃)₃·6H₂O (x = 0.01, 0.05, 0.1, 0.2) was first dissolved in 40 mL 30.0 g·L^-1 solution of P123 surfactant under vigorous stirring for 15 min, and then an appropriate amount of Na₃PO₄ was added. After that, the precipitation immediately appeared in the solution. Then the pH value of the solution was adjusted with 5 mol/L HCl under vigorous stirring, and eventually the precipitation started to dissolve until pH = 1, and the solution was further stirred for 15 min. The above solution was transferred into a stainless-steel autoclave with inner Teflon vessel (volume, 70 mL). After sealing, the autoclave was maintained at 140 ℃ for 24 h and then allowed to be naturally cooled to room temperature. Subsequently, the samples were washed three times with distilled water and absolute alcohol respectively and vacuum-dried at 60 ℃ for 12 h, with the final products appearing as white solid powder.

  The X-ray powder diffraction (XRD) pattern of the as-synthesized sample was determined using a Panalytical X´Pert Pro MPD diffractometer equipped with CuKα radiation (λ = 0.1540596 nm) and operated at 40 KV and 40 mA at a scan rate of 0.02 ^o·s^-1 in the 2θ range from 5^o to 90^o. The morphology and particle sizes of the sample were determined by Tecnai G2 F20 and Talos 200s (FEI Company) transmission electron microscopy (TEM) performed at 200 kV. The photoluminescence excitation and emission spectra were obtained with an instrument (JY, FM-4). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermofisher Scientific ESCA Lab 250 spectrometer with monochromatic AlKα as the X-ray source (1486.6 ev).

  3. RESULTS AND DISCUSSION

  The crystallinity and purity of the as-prepared Ce_1−xGd_xPO₄ were examined by XRD. Nanomaterials with different amount of Gd doping are shown in Fig. 1a. All diffraction peaks can be indexed to hexagonal CePO₄ (JCPDS 34-1380) with space group P6₂22 (180), a = 7.055 and c = 6.439 Å. Diffraction peaks of the prepared samples are well defined without any impurity peaks in the XRD pattern, indicating the phases do not change with raising the Gd³⁺ doping concentration. While peaks are shifted obviously to higher theta as the Gd³⁺ doping concentration increases. In order to understand how the crystal structure changes with different co-doping concentrations of Gd³⁺, the Rietveld refinement of XRD patterns were carried out for all samples with Panalytical HighScore software^[27]. The cell parameters of each sample are shown in Fig. 1b and 1c. Parameters a, c and V decrease almost linearly with increasing the Gd³⁺ concentration, which indicates the successful incorporation of Gd³⁺ ions into the CePO₄ host lattice and thus solid solution forms for the radium of Gd³⁺ (r = 0.0938 nm) is smaller than that of Ce³⁺ (r = 0.1034 nm). When Gd³⁺ is doped into the host lattice, the sites of Ce³⁺ will be occupied by Gd³⁺, which causes the cell shrinkage interestingly, while the values of a/c increase at first and reach the maximum at the concentration of 5 mol% Gd doping, and then decrease with further raising the Gd doping concentration. It reveals that the distortion of host lattice is increased with Gd doping rise and arrives its maximum at 5 mol%, and then decreases when further raising the Gd doping concentration.

  Figure 1

  

  Figure 1.  Lattice parameters (a) XRD pattern of samples (b) a-axis, c-axis (c) volume of Ce_1−xGd_xPO₄ (x = 0.01, 0.05, 0.10 and 0.20) samples obtained from XRD Rietveld refinement

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  The morphology and microstructural details of Ce_1−xGd_xPO₄ were further investigated by high resolution TEM (HRTEM), and its typical morphology is shown in Fig. 2. It clearly shows the nanowires are well-crystallized in 10~25 nm width and 500nm~7μm length, indicating high yield 1D nanostructures were successfully synthesized by this method. Fig. 2 shows the HRTEM images of the Ce_0.95Gd_0.05PO₄ nanowires. A clear consecutive lattice fringe is observed, which means the nanowire shows a single crystalline nature. The calculated interplanar distance between the adjacent atomic lattice fringes is 0.345 nm, corresponding to the standard value of d₁₁₀ spacing in hexagonal Ce_0.95Gd_0.05PO₄. It suggests that the orientational growth of hexagonal Ce_0.95Gd_0.05PO₄ nanocrystals is in the [0 0 1] direction. Selected area electron diffraction (SAED) patterns of hexagonal Ce_0.95Gd_0.05PO₄ nanocrystals are shown in Fig. 2 inset, and crystal planes (1 1 2), (2 0 0) and (1 1 0) are identified. The elemental color mapping reveals the existence of Ce, Gd, O and P elements in Ce_0.95Gd_0.05PO₄ (Fig. 3) with even distribution, showing the existence of Gd in the nanowires. Fig. 3 depicts the EDX spectrum used to confirm the presence of Ce, Gd, P and O elements. Meanwhile, the lattice fringe in Fig. 2 shows obvious distortion caused by the small Gd³⁺ ions substituted on the site of Ce³⁺. It further reveals that Gd³⁺ is doped into the lattice of hexagonal CePO₄ and locates on the site of Ce³⁺, and the distortion indicated the asymmetry of Ce³⁺ coordination surrounding was effectively adjusted by Gd³⁺ doping.

  Figure 2

  

  Figure 2.  TEM and HRTEM images of synthesized hexagonal CePO₄, Ce_0.99Gd_0.01PO₄, Ce_0.95Gd_0.05PO₄, Ce_0.90Gd_0.10PO₄ and Ce_0.80Gd_0.20PO₄ nanowires. The inset is the Ce_0.95Gd_0.05PO₄ SAED pattern of the corresponding sample

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  Figure 3

  

  Figure 3.  Image of elemental mapping of Ce_0.95Gd_0.05PO₄ nanowires and energy dispersive X-ray analysis

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  The photoluminescence emission spectra of all samples were recorded in the range of 290~450 nm under excitation at 274 nm at room temperature (Fig. 4a). The spectrum shows a strong ultraviolet PL emission centered at 348 nm with an acromion peak at about 330 nm. The photoluminescence broad emission band conforms to the excited state ⁵d to ²F_7/2 ground state transition. The peak broadening was due to the spin orbital split of Ce. In the spectrum, the intensity of photoluminescence was significantly enhanced with increasing the Gd³⁺ doping concentration at first and reached the top at 5mol% Gd³⁺ doping (Fig. 4b). It is to be noted that the relative intensity of 5mol% Gd³⁺ doped CePO₄ is significantly higher than pure CePO₄, and the fluorescence integral intensity was enhanced more than 15 times. It indicates that Gd³⁺ codoping is an effective method for enhancing the luminescence performance of CePO₄ phosphor.

  Figure 4

  

  Figure 4.  (a) Photoluminescence emission spectra of hexagonal Ce_1-xGd_xPO₄ nanowires, λ_ex = 274 nm. (b) Trend of integrated intensity with Gd³⁺ doping concentration

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  In order to explore the reasons for the enhancement of luminous intensity, we further studied the XPS of the prepared samples, and results are shown in Fig. 5. The full-scale XPS pattern of Ce_0.95Gd_0.05PO₄ revealed the presence of Ce, Gd, O, C and P (Fig. 5a). The binding energies of all peaks are referenced to a C 1s value of 284.8 eV. The XPS pattern is consistent with the reported bulk CePO₄. The spectrum of Ce 3d showed four components at 881.5, 885.5, 900.1, and 904.1 eV, corresponding to the main peaks and satellites of Ce³⁺ state of CePO₄, which was attributed to 3d_5/2 and 3d_3/2 (Fig. 5b), respectively. The binding energies of Ce, Gd, O and P shift obviously with the rising of Gd³⁺ doping concentration. The binding energy of O 1s shows two peaks without Gd³⁺ doping that indicates O has two coordination states. Correspondingly, the binding energy of P 2P also shows a broadened peak that may be the overlap of two different coordination states; while two XPS peaks of O 1s turn to be one located at their middle after doping Gd³⁺, and the peak of P 2p was narrowed obviously. Then with the rising of Gd³⁺ doping concentration, the peak position was shifted to higher binding energy and reached the top at 5% Gd³⁺ doping, while it reversely shifted with further raising the Gd³⁺ doping concentration. The changed trend of binding energy of Ce 3d is the same. The variety of binding energy may be caused by the exchanging location of Gd to Ce in the crystal lattice, for the electronegativity of Gd is lower than Ce, so O, P and Ce become more surplus of electron with the good distribution of Gd and Ce. However, the dispersion of Gd and Ce should be decreased with the doping concentration of Gd larger than 5%, and then the surplus electrons should disappear. The various binding energies of Ce, P and O indicated the coordination state of Ce was changed obviously with Gd doping, especially at 5% concentration.

  Figure 5

  

  Figure 5.  (a) Full-scale XPS pattern of hexagonal Ce_0.95Gd_0.05PO₄ nanowires. The individual high-resolution XPS spectra of (b) Ce 3d. (c) O 1s. (d) P 2p. (e) Gd 4d

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  Based on the above analysis, the enhancement of luminescence intensity with Gd doping may be caused by the following factors. First, the asymmetry of coordination environment of Ce was obviously changed with increasing the doping concentration of Gd for parameters a/c reach the top and the binding energy of Ce O and P also reach the maximum values with 5% Gd doping. The changing trend of luminescence intensity is the same with that of a/c and the binding energy, indicating the enhancement of luminescence intensity obviously relies on the asymmetry of Ce coor-dination environment. According to Judd-Ofelt theory, the supersensitive transitions were coursed by the asymmetry of crystal coordination field, so the enhancement of luminescence intensity should be coursed by the adjustment of asymmetry of Ce coordination environment. Second, according to the investigation of Qinghong Meng and Cen Shao, the energy level of Ce should be split and adjusted after Gd doping^{[28, 29]}. And then the electron population of excited state levels should be improved, leading to the enhancement of luminescence emission intensity. Third, as we know Gd can be taken as Energy storage ion for other active rare earth ions in order to enhance luminescence emission intensity. The split and adjusted energy levels of Ce should be more effectively matched with the Gd ions'. That should be good for energy transfer between Gd and Ce, and then improve the luminescence intensity.

  4. CONCLUSION

  Gd³⁺ doped hexagonal CePO₄ was successfully synthesized using a hydrothermal method. The phase was not changed after Gd doping. Our results clearly illustrate that the local crystal field of Ce_1-xGd_xPO₄ host lattice can be expectedly tailored by substituting the Ce³⁺ lattice sites with Gd³⁺. The asymmetry of crystal field was effectively adjusted by a suitable amount of Gd doping, and the crystal field asymmetry reaches the top with 5% Gd doping, and the luminescence emission intensity was dramatically improved by 15 folds. This material will have potential applications in the field of optical display, photocatalysis, solar energy, etc., and the research results should be a good reference for preparing other kinds of high performance luminescent materials.

  
  ACKNOWLEDGMENTS: The author would like to thank Dr. Huang Qing-Ming from testing center for helping the analysis of XRD result and also want to thank the testing center of Fuzhou university for characterization.
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• 

  Figure 1  Lattice parameters (a) XRD pattern of samples (b) a-axis, c-axis (c) volume of Ce_1−xGd_xPO₄ (x = 0.01, 0.05, 0.10 and 0.20) samples obtained from XRD Rietveld refinement

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  Figure 2  TEM and HRTEM images of synthesized hexagonal CePO₄, Ce_0.99Gd_0.01PO₄, Ce_0.95Gd_0.05PO₄, Ce_0.90Gd_0.10PO₄ and Ce_0.80Gd_0.20PO₄ nanowires. The inset is the Ce_0.95Gd_0.05PO₄ SAED pattern of the corresponding sample

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  Figure 3  Image of elemental mapping of Ce_0.95Gd_0.05PO₄ nanowires and energy dispersive X-ray analysis

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  Figure 4  (a) Photoluminescence emission spectra of hexagonal Ce_1-xGd_xPO₄ nanowires, λ_ex = 274 nm. (b) Trend of integrated intensity with Gd³⁺ doping concentration

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  Figure 5  (a) Full-scale XPS pattern of hexagonal Ce_0.95Gd_0.05PO₄ nanowires. The individual high-resolution XPS spectra of (b) Ce 3d. (c) O 1s. (d) P 2p. (e) Gd 4d

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