Multicolor photoluminescence and energy transfer properties of dysprosium and europium-doped Gd2O3 phosphors

Accepted Manuscript Multicolor photoluminescence and energy transfer properties of dysprosium and europium-doped Gd2O3 phosphors Yanxia Liu, Guixia Liu, Jinxian Wang, Xiangting Dong, Wensheng Yu PII:

S0925-8388(15)30309-1

DOI:

10.1016/j.jallcom.2015.06.193

Reference:

JALCOM 34600

To appear in:

Journal of Alloys and Compounds

Received Date: 13 March 2015 Revised Date:

13 May 2015

Accepted Date: 22 June 2015

Please cite this article as: Y. Liu, G. Liu, J. Wang, X. Dong, W. Yu, Multicolor photoluminescence and energy transfer properties of dysprosium and europium-doped Gd2O3 phosphors, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.06.193. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

Tunable multicolor emissions and energy transfer properties of lanthanides (Ln3+, Ln3+ = Dy3+, Eu3+) doped cubic Gd2O3 submicrospheres prepared by hydrothermal

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method and a subsequent calcination.

ACCEPTED MANUSCRIPT Multicolor photoluminescence and energy transfer properties of dysprosium

and europium-doped Gd2O3 phosphors Yanxia Liu, Guixia Liu*, Jinxian Wang, Xiangting Dong and Wensheng Yu Key laboratory of Applied Chemistry and Nanotechnology at University of Jilin Province, Changchun University of Science and Technology, Changchun 130022, China.

Abstract

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In this study, a series of Gd2O3: Ln3+ (Ln = Dy, Eu) submicrospheres were successfully prepared by a hydrothermal method and a subsequent higher temperature pyrolysis. X-ray diffraction

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(XRD), Fourier transform infrared (FT-IR), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectrometer (EDS), photoluminescence (PL) spectra and

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vibrating sample magnetometer (VSM) were utilized to characterize the as-prepared samples. The precursor sample thoroughly decomposed into Gd2O3 submicrospheres with a diameter of about 550 nm after calcination. Under UV excitation, the samples exhibit multicolor emissions including yellow-green, yellow, red as well as white, moreover, the Dy3+ ions acted as donors can

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transfer the energy to Eu3+ served as acceptors in Gd2O3: Dy3+, Eu3+ system. The interaction between Dy3+ ions and Eu3+ ions is verified to be phonon-assisted electric quadrupole-quadrupole

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interaction. Multicolor luminescence including white light emission can be obtained through varying the content of Eu3+ or adopting different excitation wavelengths in Dy3+ and Eu3+

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co-doped Gd2O3 system. The energy transfer efficiency reaches 88.2% when the doped concentration of Eu3+ is 0.035. The CIE chromaticity diagram directly reveals the variability of the hue of the as-prepared samples. Besides, the as-prepared samples exhibit paramagnetic properties at room temperature. This type of color-tunable luminescence phosphors has promising applications in the fields of photoelectronic devices and biomedical science. Keywords: rare earth oxide; multicolor luminescence; energy transfer; luminescence property *Corresponding author. Tel.: +86-431-85582574. Fax: +86-431-85383815 E-mail address: [email protected]

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1 Introduction

Lanthanide ions (Ln3+) doped multicolor luminescence materials have been extensively investigated because of their wide applications in various fields such as cell imaging[1], full-color displays[2], LEDs[3-6] and fluorescent labels[7] based on their unique magnetic, optical,

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electronic characteristics and the lack of toxicity[8-9]. Particularly, multicolor luminescence materials are often applied to monitor different biochemical functions simultaneously or

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distinguish normal tissue from diseased tissue[10-11]. The development of color-tunable materials has been stimulated own to their comprehensive applications in clinical medicine. In addition,

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compared with conventional fluorescent materials such as organic dyes and quantum dots, lanthanide ions (Ln3+) doped multicolor luminescence materials have many advantages while organic dyes are restricted by a single excitation wavelength and quantum dots are confined by their toxicity and rigorous requirement on their size[12].

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In recent years, Gd-based inorganic compounds (GdPO4[13], NaGdF4[14-15], GdF3[16], Gd2O3[17-18] etc.) have been extensively researched as luminescent and other functional

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materials host because of their unique spectral properties including large Stokes shifts, narrow emission bandwidths, high luminescence efficiency, long lifetime and strong paramagnetic

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properties which originate from their 4f sub-shell electrons. Among all kinds of Gd-based inorganic compounds, cubic phase Gd2O3 is considered to be a prominent luminescent host material due to low phonon energy (phonon cutoff = 600 cm-1)[19] and a wide band gap (5.9 eV)[11, 20], indicating that the quenching of the excited state of rare earth ions can be minimized extremely in the luminescent procedure. Cubic phase Gd2O3 has been widely utilized as host material for obtaining tunable-color luminescence because it can be easily doped with lanthanide ions and displays excellent corresponding luminescent characteristics. Additionally, Gd2O3 is 2

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much easier to be extracted from rare earth mineral resources and can be acquired at a low price compared with the other rare earth oxides. However, in comparison to the other rare earth oxides, the utilization of Gd2O3 is not sufficient to a certain extent, which results in the surplus storage of Gd2O3 and thus has disadvantages on the healthy development of the rare earth industry. Recently,

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more attentions have been given to Gd2O3 with cubic phase to balance the development process of rare earth industry. So far, most reports about Gd2O3 have been focused on various complicated preparation methods and rare earth ions singly activated Gd2O3 host in the literature[21-25]. For

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instance, in Selvalakshmi’s reports, multicolor emitting Gd2O3: Dy3+ phosphors were prepared via

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sol-gel method. Multicolor luminescence even though white color emission could be obtained through controlling of the intensity of each color because Dy3+ can exhibit yellow, blue and feeble red emissions in visible region, but due to lacking of a red component, only the cold white-light that is unsuitable for room lighting was gained. Cho et al. synthesized yolk-shell structured Gd2O3:

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Eu3+ phosphor by spray pyrolysis and studied the luminescence properties of the as-prepared products. Petoral et al. prepared Tb3+-doped Gd2O3 nanocrystals by slightly modified version of

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the “polyol” method and investigated their fluorescent labeling and MRI contrast agent properties. Majeed et al. acquired Gd2O3: Eu3+ spherical hierarchical structures through the transformation of

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GdOOH: Eu3+ spherical hierarchical structures synthesized by microwave irradiation route and discussed their corresponding luminescent properties. Up to now, most studies have been concentrated on rare earth ions singly activated Gd2O3 while rare earth ions co-activated Gd2O3 is found to be rarely reported. However, co-incorporating rare earth ions into Gd2O3 can realize multicolor luminescence effectively via energy transfer between two rare earth ions. Furthermore, since Gd3+ ions possess a high magnetic moment and isotropic electronic ground state 8S7/2, rare earth ions doped Gd2O3 phosphors can function as multifunctional materials which have 3

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promising applications in biomedical field. Herein, it is highly valuable to obtain Gd2O3 phosphors via an effortless hydrothermal method followed by a subsequent calcination process and multicolor luminescence can be realized via doping Dy3+ along with Eu3+ into Gd2O3 host. In this study, Gd2O3 submicrospheres were selected as host while Dy3+ and Eu3+ ions as

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activators to obtain multicolor luminescence via the energy transfer phenomenon from Dy3+ to Eu3+ or adopting different excitation wavelengths. The precursor sample completely turned into

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Gd2O3 submicrospheres after calcination, which was proved by the results of XRD, FT-IR and EDS. Magnetic properties of the samples were measured by using a VSM. The advantages of

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multicolor emission of these samples ensure that they can be utilized for multiple fluorescent labels and full-color display. 2 Experimental section 2.1 Materials

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Gadolinium oxide (Gd2O3), dysprosium oxide (Dy2O3), europium oxide (Eu2O3) were purchased from Sinopharm Chemical Reagent Co., Ltd. Urea (CO(NH2)2) and nitric acid (HNO3)

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were purchased from Xilong Chemical Co., Ltd. All chemicals were of analytical grade and utilized as purchased without any further purification.

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2.2 Preparation

The Gd(NO3)3, Dy(NO3)3, Eu(NO3)3 stock solutions were prepared by dissolving appropriate amounts of Gd2O3, Dy2O3, Eu2O3 in dilute HNO3 (15 mol/L) under heating with agitation followed by evaporating the excess solvent. A series of rare earth ions doped Gd2O3 submicrospheres were synthesized via a hydrothermal method followed by a subsequent calcination process. In a typical procedure for the synthesis of Gd2O3: 0.004Dy3+, 0.004Eu3+ submicrospheres, 2 mmol RE(NO3)3 [including 1.984 mmol Gd(NO3)3, 0.008 mmol Dy(NO3)3 4

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and 0.008 mmol Eu(NO3)3] solutions were added into 30 mL de-ionized water containing 0.8 g CO(NH2)2 under vigorous stirring to form a homogeneous solution. After 30 minutes continuous stirring, the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. After naturally cooling to room temperature, the resulting

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white precipitates were collected by centrifugation and washed with de-ionized water and ethanol several times then dried at 80 °C for 12 h. Finally, Gd2O3: 0.004Dy3+, 0.004Eu3+ phosphors were obtained by annealing the precursor samples at 800 °C for 4 h in air with heating and cooling rate

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of 2 °C min-1. All the other samples were synthesized in a similar way except for using the

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required stoichiometric rare earth ions. 2.3 Characterization

The phase structure and purity were identified by Power X-ray diffraction (XRD) using a Rigaku D/max-RA X-ray diffractometer with Cu Kα radiation (λ=0.15406 nm) and Ni filter,

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operating at 30 mA, 40 kV. Scanning speed, step length and diffraction range were 10° min-1, 0.1° and 10°-90° respectively. A FEI-30 field emission scanning electron microscope (FESEM)

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equipped with an energy-dispersive X-ray spectrometer (EDS) was employed for the detection of the morphology and composition of the samples. The excitation and emission spectra were

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performed on a HITACHI F-7000 Fluorescence Spectrophotometer equipped with a 150 W xenon lamp as the excitation source. A vibrating sample magnetometer was utilized to measure the magnetization of as-obtained samples with the applied magnetic field ranging from -20 to 20 kOe. 3 Results and discussion 3.1 Structure and morphology The structure and phase of the as-prepared samples were analyzed using XRD firstly. Fig. 1 shows the XRD patterns of the precursor sample and the calcined samples of Dy3+ or/and Eu3+ 5

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doped Gd2O3. It is noted that there are only two broad peaks near 28° and 45° for the precursor sample, indicating the precursor is amorphous, which can be confirmed to be Gd(OH)CO3 according to previous reports [26-27]. After calcination at 800 °C for 4 h, it can be discovered that all the diffraction peaks of the as-prepared samples can be readily indexed into pure cubic phase

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of Gd2O3 (JCPDS No.88-2165), indicating the precursor sample completely decomposes into Gd2O3 in the process of annealing. No additional peaks and other phases or obvious shifts of

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diffraction peaks can be discovered, indicating that the as-prepared samples crystallized into pure cubic Gd2O3.

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The FT-IR spectra of the precursor sample and the product annealed at 800 °C for 4 h were employed to further confirm that the precursor sample has thoroughly transformed into Gd2O3, as shown in Fig. 2. From Fig. 2a, the characteristic absorption peaks of ν-OH (3417 cm-1) (originated from surface absorbed water as well as structural hydroxyl group) and νas O-C-O

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(1504 and 1398 cm-1), νs C-O (1096 cm-1), π-CO32- (848 cm-1), δ-CO32- (758 and 692 cm-1) can be detected[28], illustrating the presence of the hydroxyl and carbonate groups. This result indicates

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that the precursor sample is composed of Gd(OH)CO3, which is in good accordance with the previous reports[29-32]. Compared with the precursor sample, it is noted that from Fig. 2b, the

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new strong absorption peak at 546 cm-1 appears after calcinating at 800 °C for 4 h, which is ascribed to the vibration peak of Gd-O bond, forcefully demonstrating that the precursor sample has fully turned into Gd2O3, which is consistent with the XRD results mentioned above. Fig. 3 presents the FESEM images and EDS spectra of the as-prepared samples. From Fig. 3(a) and (b), it can be seen that the products before calcination consist of numerous smooth microspheres with an average diameter of 1.25 µm and the microspheres are uniform in size. After being annealed, the obtained Gd2O3 inherits its parents’ morphologies, but its size (a 6

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diameter of about 550 nm) has a large decrease because of the decomposition of the precursor and the crystallization of Gd2O3, which can be simply depicted by: 2Gd(OH)CO3 → Gd2O3+H2O+2CO2. It is the released gas of H2O and CO2 that make the diameter of the sample decrease. The chemical composition of the as-prepared samples was investigated by EDS.

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According to Fig. 3(c) and (d), no elements can be observed except C, O, Gd, Si, Cr, Dy for the precursor and O, Gd, Si, Cr, Dy for Gd2O3: 0.004Dy3+ (silicon and chromium signals stem from

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silicon substance and spraying chromium procedure), powerfully revealing that the precursor totally transforms into Gd2O3 after calcination, which further supports the results of XRD and

3.2 Photoluminescence properties

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FT-IR spectra.

Fig. 4 gives PL excitation and emission spectra of Gd2O3: 0.004Dy3+ submicrospheres. The excitation spectrum was obtained by monitoring the characteristic emission of the Dy3+ at 573 nm.

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It can be found that the excitation spectrum contains a broad band centered at 232 nm, which dues to host absorption, i.e., electronic transitions from the O2p valance band to the Gd (5d6s)

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conduction band. According to Li et al[33], both the electron transition from the O2p valance band to the Gd (5d6s) conduction band and the O2p valance band to the Dy (5d6s) conduction

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band make contributions to the absorption band of the excitation spectrum. But the electron transition from the O2p valance band to the Gd (5d6s) conduction band is primary as compared with the O2p valance band to the Dy (5d6s) conduction band, indicating the excitation band centered 232 nm mainly results from the electron transition from the O2p valence band to Gd (5d6s) conduction and obviously energy transfer from the Gd2O3 host to Dy3+ occurs in Gd2O3: 0.004Dy3+. Furthermore, there are excitation peaks at about 255, 276, 309, 314, 350, 364 and 386 nm, which can be ascribed to the transitions of 8S7/2 - 6D9/2, 8S7/2 - 6I9/2, 8S7/2 - 6P5/2, 8S7/2 7

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P7/2[34-35] of Gd3+and 6H15/2 - 6P7/2, 6H15/2 - 6P5/2, 6H15/2 - 4I13/2 of Dy3+[36] respectively,

suggesting Gd3+ can transfer energy to Dy3+. Upon excitation at 232, 276 and 350 nm, the characteristic emissions of 487 and 573 nm which are assigned to the magnetic dipole transition 4

F9/2 - 6H15/2 and electric dipole transition 4F9/2 - 6H13/2 of Dy3+ can be observed. Furthermore, the

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intensity of electric dipole transition 4F9/2 - 6H13/2 at 573 nm is much stronger than that of magnetic dipole transition 4F9/2 - 6H15/2 at 487 nm, which illustrates that Dy3+ ions occupy sites

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without inversion symmetry in the crystal lattice. No emissions from Gd3+ can be seen when excited by 232 and 276 nm, further demonstrating an efficient energy transfer from the Gd2O3

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host and Gd3+ to Dy3+ in Gd2O3: 0.004Dy3+. In addition, in these Dy3+-doped products, it can be noted that the optimum concentration of Dy3+ is 0.8 at% (Inset of Fig. 4). Fig. 5 shows the PL excitation and emission spectra of Gd2O3: 0.004Eu3+ submicrospheres. The excitation spectrum contains of two broad band absorption at around 233 and 248 nm, which are

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assigned to the Gd2O3 host absorption and the oxygen-to-europium charge transfer band (CTB)[37], suggesting that both Gd2O3 host and CTB make contributions to the emissions of Eu3+

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ions. The other sharp absorption peaks are assigned to intra-configurationally f-f transitions of Gd3+ at 255 nm (8S7/2 - 6D9/2), 276 nm (8S7/2 - 6I9/2), 309 nm (8S7/2 - 6P5/2), 314 nm (8S7/2 - 6P7/2) and

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Eu3+ at 309 nm (7F0-5I6), 314 nm (7F0-5H6), 323 nm (7F0-5H3), 364 nm (7F0-5D4), 394 nm (7F0-5L6), 417 nm (7F0-5D3), 466 nm (7F0-5D2), 534 nm (7F0-5D1) respectively. Upon excitation with 233 nm, the emission spectrum consists of characteristic f-f transitions 5D2-7F3 (510 nm), 5D1-7F1 (534 nm), 5

D1-7F2 (554 nm), 5D1-7F3 (582 nm), 5D0-7F1 (593 nm), 5D0-7F2 (612 and 630 nm), 5D0-7F3 (651 nm)

of Eu3+ without the characteristic emissions of Gd3+, convincingly suggesting Gd3+ transfers energy to Eu3+. Generally, Eu3+ can occupy two nonequivalent crystallographic sites in cubic Gd2O3 host, S6 (centrosymmetric) and C2 (noncentrosymmetric)[38-40]. It is believed that the 8

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D0-7F2 (C2) electric dipole transition is more sensitive to the surrounding environment than

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D0-7F1 (S6) magnetic dipole transition. The emission spectrum is dominated by the red 5D0-7F2

transition, implying that the Eu3+ ions are in the C2 site without inversion symmetry, as shown in the inset of Fig. 5.

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Fig. 6 presents the excitation and emission spectra of Gd2O3: 0.004Dy3+, 0.004Eu3+ submicrospheres. When monitoring by the emission of Eu3+ (612 nm, 5D0-7F2) or Dy3+ (573 nm, 4

F9/2-6H13/2), the PLE spectra are similar to Gd2O3: 0.004Eu3+ and Gd2O3: 0.004Dy3+ respectively.

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In addition, when monitoring by the 5D0-7F2 emission of Eu3+ at 612 nm, a weak transition (6H15/2

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- 6P7/2) at 350 nm of Dy3+ can be observed, which is the same with the liu’s reports[41], illustrating that Dy3+ transfers energy to Eu3+. Under excitation at 233 nm, compared with Gd2O3: 0.004Eu3+, the Gd2O3: 0.004Dy3+, 0.004Eu3+ displays the characteristic emissions both of Eu3+ (612 and 593 nm) and Dy3+ (573 and 487 nm), which intimates tunable color can be acquired in

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Gd2O3 host by co-doping Dy3+ and Eu3+, at the same time, the emission intensity of Eu3+ of Gd2O3: 0.004Dy3+, 0.004Eu3+ sample is apparently stronger than Gd2O3: 0.004Eu3+, which

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distinctly implies the energy transfer from Dy3+ to Eu3+ occurs. From Fig. 7, it is observed that there is an overlap between the excitation spectrum of Eu3+ and

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the emission spectrum of Dy3+, which is in accordance with the S. Som’s reports[42], so we can speculate that Dy3+ can transfer energy to Eu3+ in Gd2O3 host. The result indicates in Dy3+, Eu3+ co-doped Gd2O3, Dy3+ serves as energy donator and Eu3+ serves as energy acceptor. In order to further study the energy transfer process, a series of phosphors with fixed Dy3+ content at 0.004 and a varying Eu3+ content x in the range of 0-0.035 were synthesized. Fig. 8 depicts the PL emission spectra and the variation of emission intensity of Dy3+ and Eu3+ with the increasing of Eu3+ content for Gd2O3: 0.004Dy3+, xEu3+ samples. One can see that the sample 9

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(x=0) shows the feature emissions of Dy3+ at 573 nm (4F9/2-6H13/5 transition, yellow emission) and 478 nm (4F9/2-6H15/2, blue emission). In addition, with increasing Eu3+ content, the emission intensity of Dy3+ monotonically decreases while emission intensity of Eu3+ increases until the Eu3+ concentration is above 0.020 and then decreases because of concentration quenching effect,

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convincingly demonstrating Dy3+ ions transfer energy to Eu3+ ions in Gd2O3 system. One can see that the intensity of Eu3+ emission lager compared to the Dy3+ emission in Gd2O3 host, so it is believed that the intensity of Eu3+ emission may increase with increasing the Eu3+ content even if

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there is no energy transfer, but it is worth noting that the Dy3+ emission intensity dramatically

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decreased although the content of Dy3+ was fixed, it is because that Dy3+ transfer energy to Eu3+ that Dy3+ emission intensity decreased monotonously. The results are in good with Liu’s and Atabaev’s reports[43-44].

Conversely, one can see that the emission intensity of Eu3+ obviously increases with increasing

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Dy3+ content in the samples with fixed Eu3+ at 0.010 and a varying Dy3+ concentration in the range of 0.002-0.008, as shown in Fig. 9, which further demonstrates the occurrence of the energy

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transfer from Dy3+ to Eu3+ in Dy3+, Eu3+ co-doped Gd2O3 system. In Gd2O3 host, it is believed that both Dy3+ and Eu3+ preferentially occupied C2 site (electric-dipole transition), the preferential

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occupation of Eu3+ at C2 site over Dy3+ at C2 site may contribute the increase the emission for Eu3+. Also, Eu3+ will preferentially displace Gd3+ in Dy3+, Eu3+ co-doped Gd2O3. However, we have a strict control of rare earth ions content and proportion, the emission of Eu3+ increase with the concentration of Eu3+ fixed may be due to an energy transfer from Dy3+ to Eu3+. Also, it can be found a weak absorption at 350 nm of Dy3+ monitored by the characteristic emission of Eu3+. In addition, the conclusions are in good accordance with the Atabaev’s reports[44]. The energy transfer efficiency from sensitizer Dy3+ to activator Eu3+ can be calculated by: ηET = 10

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1-I/I0 [45-46] where I and I0 are the emission intensity of sensitizer Dy3+ with and without the presence of activator Eu3+. As illustrated in Fig. 10, the energy transfer efficiency gradually increases with the increasing of Eu3+ content in Gd2O3 host and the maximum value of ηET is 88.2% at x=0.035, suggesting the energy transfer from the Dy3+ to Eu3+ is efficient.

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In order to explore the nature of energy transfer, the Dexter’s energy transfer formula of multipolar interaction can be expressed by[47-48]: I0/I∝Cn/3 where I0 and I are the corresponding intensities of Dy3+ emission without and with the presence of Eu3+ respectively. C is the sum

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content of Dy3+ and Eu3+ and 6, 8 or 10 stands for dipole-dipole, dipole-quadrupole or

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quadrupole-quadrupole interactions, respectively. The I0/I∝Cn/3 plots are given in Fig. 11 and it can be found that I0/I serves as a liner function of Cn/3 when n=10, indicating the energy transfer mechanism from Dy3+ to Eu3+ is a quadrupole-quadrupole interaction. The possible energy transfer schematic diagram for Dy3+ and Eu3+ co-doped Gd2O3 is shown in

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Fig. 12. From the schematic diagram, it can be found that 4F9/2 of Dy3+ is slightly higher than 5D0 and 5D1 of Eu3+, which makes energy migration from Dy3+ to Eu3+ via nonradiative process

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possible. During the excitation process, electronic transitions from the O2p valence to the Gd (5d6s) conduction band take place under 233 nm UV excitation. When the electrons get back to

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the lower energy levels once again through blue emission as well as transfer energy to Dy3+ and Eu3+, the other energy is released through cross relaxation. In addition, the electrons located at ground state of Gd3+ relax to 6P7/2 state after absorbing energies of photons of UV, meanwhile transfer energy to Dy3+ and Eu3+. Simultaneously, part energies of the 4F9/2 energy level of Dy3+ are transferred to

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D1 energy level of Eu3+ via phonon-assisted electric

quadrupole-quadrupole interaction. All of these results indicate that single phase tunable multicolor phosphors can be obtained by adjusting the energy transfer process in Dy3+ and Eu3+ 11

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co-doped Gd2O3.

It is well known that multicolor luminescence can be achieved by modulating the content of rare earth ions when excited by a single excitation wavelength or properly adopting different excitation wavelengths. Fig. 13 depicts the CIE chromaticity diagram and corresponding

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luminescence photographs of Dy3+ or/and Eu3+ doped Gd2O3 as a function of Eu3+ content excited at 276 nm and Gd2O3: 0.004Dy3+, 0.01Eu3+ under different excitation wavelengths. It is clearly observed that the photoluminescence color can be modulated from yellow-green to white to

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yellow and to red finally using a single wavelength as pumping source through adjusting the Eu3+

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content due to different energy transfer efficiency. The corresponding CIE chromaticity coordinates were calculated to be (a, 0.331, 0.383), (b, 0.402, 0.316), (c, 0.486, 0.347), (d, 0.523, 0.351), (e, 0.545, 0.349) and (f, 0.578, 0.353). In addition, one can see that Gd2O3: 0.004Dy3+, 0.01Eu3+ sample displays white luminescence when excited by 314, 350 and 365 nm respectively

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and the corresponding color gradually moves from warm white light towards to cool white light. The relevant CIE chromaticity coordinates were discovered to be (g, 0.375, 0.332), (h, 0.309,

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0.324), (i, 0.283, 0.257). All of these results show that this kind of phosphors have promising applications in the fields of full-color display, biomedicine and optoelectronic devices.

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3.3 Magnetic properties

In addition to the unique optical properties, the as-obtained Gd2O3: Dy3+, Eu3+ submicrospheres exhibit magnetic properties originating from seven unpaired f-electrons of Gd3+ ions. Magnetic properties of the samples were measured by using a VSM. The plot of magnetization versus applied field at room temperature is presented in Fig. 14. One can see that both Gd2O3 and Gd2O3: 0.004Dy3+, 0.01Eu3+ shows paramagnetic properties and the magnetization reaches around 3.14852 and 2.8731 emu/g when the applied field is 20 kOe. The results indicate that this kind of 12

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materials can be used for clinical medicine[49-50]. 4 Conclusions

In summary, we have successfully synthesized a series of color-tunable Gd2O3: Dy3+, Eu3+ submicrospheres with a diameter of around 550 nm via a hydrothermal method followed by a

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subsequent calcination process. The precursor sample completely turned into final product in the annealing process. By adjusting the Dy3+/Eu3+ ratio, the luminescence color could be altered to yellow-green, white, yellow and red because the different energy transfer efficiency between Dy3+

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and Eu3+ while employing appropriate excitation wavelengths the luminescence color could be

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modified from warm white to cool white. The maximum energy transfer efficiency of Dy3+ to Eu3+ is 88.2% and the energy transfer mechanism is discovered to be quadrupole-quadrupole interaction. The magnetization of the as-obtained samples can reach 2.8731 emu/g at 20 kOe. The

and full-color display. Acknowledgements

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results indicate that the as-prepared phosphors can be applied in the fields of biomedical science

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This work was financially supported by the National Natural Science Foundation of P.R. China (NSFC) (Grant No. 51072026, 50972020) and the Development of science and technology plan

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projects of Jilin province (Grant No. 20130206002GX).

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Figure Captions Figure.1 XRD patterns of (a) the as-prepared precursor sample of Gd2O3: 0.004Dy3+ and the 16

3+ ACCEPTED calcined samples (b) Gd2O3: 0.004Dy , (c) MANUSCRIPT Gd2O3: 0.004Eu3+ and (d) Gd2O3: 0.004Dy3+,

0.004Eu3+ as well as the standard data of Gd2O3 (JCPDS no.88-2165). Figure.2 FT-IR spectra of (a) the precursor and (b) the calcined sample of Gd2O3: 0.004Dy3+. Figure.3 FESEM images of the precursor (a) and Gd2O3: 0.004Dy3+ samples (b); EDS spectra of the precursor (c) and Gd2O3: 0.004Dy3+ samples (d).

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Figure.4 PL excitation (left) and emission (right) spectra of Gd2O3: 0.004Dy3+ submicrospheres. Inset shows emission intensity at 573 nm on the concentration of Dy3+.

Figure.5 PL excitation (left) and emission (right) spectra of Gd2O3: 0.004Eu3+ submicrospheres. Inset shows europium surroundings of the rare earth sites in Gd2O3.

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Figure.6 PL excitation (left) and emission (right) spectra of Gd2O3: 0.004Dy3+, 0.004Eu3+ submicrospheres.

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Figure.7 Excitation spectrum of the Gd2O3: 0.004Eu3+ and emission spectrum of the Gd2O3: 0.004Dy3+.

Figure.8 PL emission spectra and relative intensity (Inset) of the Dy3+ and Eu3+ emissions of the Gd2O3: 0.004Dy3+, xEu3+ (x= 0, 0.004, 0.01, 0.015, 0.020, 0.030, 0.035) samples , as a function of the Eu3+ (λex = 233 nm).

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Figure.9 PL emission spectra of Gd2O3: 0.01Eu3+, yDy3+ (y= 0.002-0.008) under the excitation of 233 nm. Inset shows dependence of the emission intensity at different wavelengths on Dy3+/Eu3+ ratio.

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Figure.10 Energy transfer efficiency (ηET) from Dy3+ to Eu3+ in Gd2O3: 0.004Dy3+, xEu3+ (x=0.004, 0.01, 0.015, 0.020, 0.030, 0.035) samples under 233 nm excitation. Figure.11 Dependence of I0/I of Dy3+ on the (a) CDy+Eu6/3 × 104, (b) CDy+Eu8/3 × 105, (c) CDy+Eu10/3 ×

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106 in the Gd2O3: 0.004Dy3+, xEu3+ (x= 0, 0.004, 0.01, 0.015, 0.020, 0.030) phosphors. Figure.12 The proposed scheme of energetic processes occurring in the Gd2O3: Dy3+, Eu3+ samples.

Figure.13 CIE chromaticity diagram and corresponding luminescence photographs of the selected Gd2O3: Dy3+, Eu3+ phosphors. (a) Gd2O3: 0.004Dy3+; (b) Gd2O3: 0.004Dy3+, 0.005Eu3+; (c) Gd2O3: 0.004Dy3+, 0.01Eu3+; (d) Gd2O3: 0.004Dy3+, 0.02Eu3+; (e) Gd2O3: 0.004Dy3+, 0.03Eu3+; (f) Gd2O3: 0.03Eu3+ under 276 nm excited; Gd2O3: 0.004Dy3+, 0.01Eu3+ under 314 nm (g), 350 nm (h) and 365 nm (i) excited. Figure.14 Magnetization of the Gd2O3 and Gd2O3: 0.004Dy3+, 0.01Eu3+ with the increment of 17

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The as-prepared samples can exhibit multicolor emissions




Dy3+ transfer energy to Eu3+ in Dy3+ and Eu3+ co-doped Gd2O3




The as-prepared phosphor has promising applications in the fields of photoelectronic devices and

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biomedical science