Effect of doping concentration on particle growth and luminescence properties of monodispersed Dy3+: Y2O3

Accepted Manuscript Effect of doping concentration on particle growth and luminescence properties of 3+ monodispersed Dy :Y2O3 Song Hu, Xianpeng Qin, Guohong Zhou, Xiaoxia Liu, Chunhua Lu, Zhongzi Xu, Shiwei Wang PII:

S0925-8388(15)31998-8

DOI:

10.1016/j.jallcom.2015.12.207

Reference:

JALCOM 36296

To appear in:

Journal of Alloys and Compounds

Received Date: 14 September 2015 Revised Date:

23 December 2015

Accepted Date: 24 December 2015

Please cite this article as: S. Hu, X. Qin, G. Zhou, X. Liu, C. Lu, Z. Xu, S. Wang, Effect of doping 3+ concentration on particle growth and luminescence properties of monodispersed Dy :Y2O3, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2015.12.207. 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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Effect of doping concentration on particle growth and luminescence properties of monodispersed Dy3+:Y2O3 Song Hu, a, b Xianpeng Qin,a,* Guohong Zhou,a Xiaoxia Liu,b Chunhua Lu,b,* Zhongzi Xu,b Shiwei

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Wang a a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

State Key Laboratory of Materials-Orient Chemical Engineering, College of Materials Science

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b

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and Engineering, Nanjing Tech University, Nanjing 210009, People’s Republic of China

Author to whom correspondence should be addressed. a,

* Tel.: +86 2152415219; fax: +86 2152415263; Shanghai 200050, P. R. China.

e-mail: [email protected](X. Qin); b,

* Tel: +86 2583587252; Fax: +86 2583587220; Nanjing 210009, P.R. China.

e-mail: [email protected](C. Lu)

ACCEPTED MANUSCRIPT ABSTRACT Submicron-sized Dy3+: Y2O3 particles were successfully prepared via an urea homogeneous precipitation method, followed by a calcination at 800℃. TG-DSC, FT-IR, X-ray diffraction

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(XRD), Field emission scanning electron microscope (FE-SEM), scanning electron microscope (SEM), photoluminescence (PL), and photoluminescence excitation (PLE) spectra were used to characterize the prepared samples. The particles were spherical shape and monodispersed. More

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importantly, the spherical Y2O3 particles were found to have significant changes in size with

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varying dopant concentration of the Dy3+ ions, ranging approximately from 550 to 840 nm. The possible growth mechanism of the particles was proposed. Under 349nm excitation, the crystalline powders exhibited blue and yellow emissions due to the 4F9/2→ 6H15/2 and 4F9/2→ 6H13/2 transitions of Dy3+ ions, respectively. It was further found that with proper Dy3+ doping concentration, the

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luminescence color hue was tuned close to the ideal white light with color coordinates of (0.33, 0.33). The submicron-sized Dy3+: Y2O3 phosphor is a promising candidate for the white light emitting diodes (WLEDs).

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Keywords: Doping concentration/ Particle size/ Growth mechanism/ Luminescence/ WLEDs.

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1. Introduction

Doping, which involves the intentional incorporation of suitable atoms/ions into the host

lattice, is one of the effective routes to endow functional materials with optical, luminescence, electrical, and magnetic properties [1-5]. Rare earths (RE) are far more attractive as doping elements for their abundant energy levels as well as luxuriant spectral properties [6-9]. To be mentioned, among the REs, Dy3+ was extensively studied due to the wide applications in the fields of thermal neutron dosimetry, white LEDs, photocatalysis, and dye-sensitized solar cells [10-13].

ACCEPTED MANUSCRIPT Especially, the luminescent properties of the Dy3+ doped materials are attracting increasing attentions. When excited by UV light, two intense bands are generated in the visible spectral range which correspond to the hypersensitive transition 4F9/2→ 6H13/2 (∆L=2, ∆J=2, yellow emission)

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and the 4F9/2→ 6H15/2 transition (blue emission), respectively. By tuning the ratio of yellow to blue emission intensities (Y/B), white light can be realized [14]. At present, 19% of the global electric energy is consumed for illumination, therefore, more efficient, energy saving and environmentally

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friendly lighting technologies to reduce power consumption become pressing. In this aspect, Dy3+

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doped luminescent materials for WLEDs devices are becoming attractive [15-17]. Luminescent properties of a material are strongly dependent on not only the phase structure, particle size, the morphology, and the shape [18-22], but also the intrinsic properties of host materials. Therefore, finding an appropriate host material is of vital importance. Among the metal

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oxides, Y2O3 crystals are considered to be one of the best candidates for RE doping, because of low cut-off phonon energy (380 cm-1), high melting point (2400℃), broad transparency, and wide band gap of 5.6 eV [23, 24]. Additionally, the Y3+ ion has similarities in ionic radius and chemical

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properties compared to other REs, making Y2O3 an excellent host material for RE induced

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luminescent applications. To optimize the luminescent properties, many researchers are focusing on doping/co-doping in Y2O3 crystal lattice with different REs, tailoring pH of the solutions, or controlling the morphology and shape of the Y2O3 [25-30]. However, few attentions have been paid to the impact of doping concentration on particle growth of the phosphor and its luminescent property. The dopants are supposed to play important roles in crystal growth, since the ions locating at different lattice sites have their own reaction processes with corresponding surrounding environments. Hence, experiments should be carried out to seek the relationship between the

ACCEPTED MANUSCRIPT dopants and the particle growth. In the present work, submicron-sized Dy3+: Y2O3 spheres were prepared via a facial urea homogenous precipitation method, followed by a calcination at 800 ℃. Different concentrations of Dy3+ ions were incorporated into the Y2O3 particles to investigate the

growth mechanism of Dy3+:Y2O3 has also been proposed.

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2. Experimental procedures

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possible mean that the dopants influence the final particle size and luminescence. The possible

2.1 Preparation

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The Dy3+:Y2O3 submicron-sized spheres were synthesized via a homogeneous precipitation method. Y(NO3)3·9H2O (99.99%), Dy(NO3)3·9H2O (99.99%) and urea (analytic grade) were used as the starting materials. Firstly, the nitrates with a stoichiometric ratio were dissolved in deionized water to make a nitrate solution, according to the chemical formula (Y1-xDyx)2O3,

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(Abbreviated as xDy3+: Y2O3, herein after) (0.5% ≤ x ≤ 2%). The total concentration of Y3+ and Dy3+ ions was kept at 0.015 M. Then the precipitant urea was added with a molar concentration

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ratio of 35:1 to the total metal ions, and the mixed solution was stirred at room temperature for homogenization until a transparent solution was obtained. The solution was then heated in a water

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bath at 90 ± 2℃ for different time durations to help investigate the growth mechanism. A typical time duration was 5 h. After naturally cooled down to room temperature without any disturbance, the resultant suspension was filtered and washed repeatedly with deionized water and alcohol, respectively, to remove by-products during the reaction. The washed precipitate was then dried at 60℃ for 24 h. The resultant Y2O3: Dy3+ precursors were calcined at 800℃ for 2 h in a muffle furnace in air to obtain Y2O3: Dy3+ crystal powders. 2.2 Characterization

ACCEPTED MANUSCRIPT Phase identification of the prepared Y2O3: Dy3+ crystal powders was carried out using X-ray diffraction (XRD) on a Japan Rigaku D/MAX 2200PC diffractometer with Cu Kα radiation (λ=1.54056

Å).

Thermal

analysis

of

the

precursors

was

measured

by

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thermo-gravimetric/differential scanning calorimetry (TG/DSC, STA449C, NETZSCH, Germany) at a heating rate of 10 ℃/min in air. Both the precursors and the powders were mixed with KBr for FT-IR measurement at room temperature using the Fourier transform infrared spectroscopy

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(Nexus 670, Nicolet). The morphologies of the precursors and the powders were observed using a

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scanning electron microscope (SEM, JSM-6390, JEOL, Tokyo, Japan), and a field emission scanning electron microscope (FE-SEM, HITACHI SU8010, Japan). Photoluminescence excitation and emission spectra were measured on a JobinYvon FL3-221 fluorescence spectrophotometer with the resolution of 1 nm using a 450 W Xenon lamp as the excitation source.

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All emission spectra are corrected for the spectral response of the measuring system.

3. Results and discussion

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Fig.1 shows TG/DSC curves of the Y2O3 precursors kept for 5 h in a water bath. The total weight loss is about 40.4%, which can be mainly divided into three periods. Below 180℃, the first

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period experiences a weight loss of about 10 wt%, and the endothermic peak at around 159.2 ℃ is due to the evaporation of absorbed water and the release of bound water. The second period of weight loss occurs in the temperature range between 180 and 625℃, with a total weight loss of 14.06%, which corresponds to the decomposition and oxidation of NO3- ions. A weight loss of 16.34% happens during the third reaction stage. As depicted in the DSC curve, there is an endothermic peak centered at 636.2℃ and an exothermic peak centered at 784.2℃, respectively. They are attributed to the decomposition of hydroxides and carbonates, and the crystallization of

ACCEPTED MANUSCRIPT the Y2O3 crystals, respectively. The TG-DSC results indicate that the Y2O3 crystals are supposed to

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be obtained below 800℃.

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Fig. 1 TG-DSC curves of the Y2O3 precursor aging for 5 h.

FT-IR analysis is crucial for the understanding the composition of the products and reaction mechanism. The FT-IR spectra of both the Y2O3 precursors and the powders calcined at 800℃ are

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presented in Fig.2. It can be seen from the curve of the precursors, the broad-band centered at 3434 cm-1 can be attributed to the couple effects of water molecules. The strong absorption peaks at 1530 cm-1 and 1412 cm-1 can be assigned to the split asymmetrical stretching vibration of

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carbonate and nitrate groups, respectively, indicating that abound of CO32- and NO3- groups are

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adsorbed on the precursor particles. After calcined at 800℃ for 2 h, the curve shows some new bands locating at 562, 465, and 431 cm-1, which are characteristic of Y-O vibrations, indicating the forming of Y2O3 crystals. This result is consistent with that of the TG-DSC analysis. According to the TG-DSC and FT-IR results, as well as an early study [31], the possible reaction formula is proposed: Y3 + NO3- + CO32- + OH- + H2O→ Y2(OH)x·(CO3)y·(NO3)6-x-2y·n H2O

(1)

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Fig.2 FT-IR spectra of the precursor and the powder calcined at 800℃ for 2 h.

In order to obtain xDy3+: Y2O3, Dy3+ ions were added to replace the Y3+ sites. Fig.3 shows the X-ray diffraction patterns of xDy3+: Y2O3 (0.5% ≤ x ≤ 2%) crystal powders. Though the Dy3+ doping concentrations are different, these particles show the similar diffraction peaks that can be

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assigned to the cubic Y2O3 structure (JCPDS No. 05-0574). The similarity in radius of the Dy3+ and Y3+ (rDy3+ = 0.908 Å, rY3+ = 0.880 Å) would lead to a successful replacement of Y3+ with Dy3+ ions. No secondary phases can be found in Fig.3, indicating the formation of single phase and

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incorporation of Dy3+ ions into the Y2O3 crystal lattice.

Fig.3 XRD patterns of the xDy3+:Y2O3 (0.5% ≤ x ≤ 2%) crystal particles.

ACCEPTED MANUSCRIPT Fig.4 shows the FE-SEM morphologies of the 0.8% Dy3+: Y2O3 precursors and crystal powders. It can be clearly seen from Fig.4 (a) that the shape of the precursor is spherical with a mean size of 670 nm. After calcined, the powders remain uniform and spherical shape, as shown

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in Fig.4 (b). However, there is a decrease of about 140 nm in size compared to the precursors. This is attributed to the decomposition of hydroxides, nitrates, and carbonates during the calcinating

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process, which is coincident with the TG-DSC results.

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Fig.4 FE-SEM photographs of the 0.8% Dy3+: Y2O3 (a) precursor and (b) crystal particle calcined at 800℃.

The morphologies of the synthesized precursors with different Dy3+ concentrations are

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presented in Fig.5 (Left). It can be clearly observed in the SEM graphs that the precursors are all spherical in shape and monodispersed, in spite of the variation of Dy3+ concentration. What’s more,

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the sizes of the precursors are changing with increasing Dy3+ concentration. The size distributions of each precursors have been determined by measuring the dimensions of more than 100 particles and are presented in Fig.5 (Right), and the corresponding mean particle sizes are collected in Table 1. It is concluded that the precursor size rises gradually at first with increasing Dy3+ concentration, then reaches a maximum size of about 840 nm when x = 1.5%, and follows a drop in particle size with further increment of Dy3+ concentration.

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Fig.5 SEM photographs (Left) and particle size distribution (Right) of the synthesized xDy3+:Y2O3 precursors via urea homogenous precipitation. (a) x = 0.5%; (b) x = 0.8%; (c) x = 1%; (d) x = 1.5%; (e) x = 2%.

ACCEPTED MANUSCRIPT Table 1 Mean particle size of the xDy3+: Y2O3 precursors and crystals (0.5% ≤ x ≤ 2%). Size of Precursors (nm)

Size of Crystals (nm)

0.5

550

420

0.8

670

530

1.0

754

610

1.5

840

680

2.0

730

610

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x (%)

To clarify the inherent growth mechanism of the Y2O3 precursor, we use FE-SEM graphs to

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record the growth status relevant to the aging time. Morphology and size evolution of the precursor particles are exhibited in Fig.6. As shown in Fig.6, (a) ~ (d) represents the sample aged for 0.5, 2, 5, and 8 h, respectively. When the aging time was 0.5 h, the obtained particles have a size of 150 nm, and are quasi-spherical in shape, but not smooth enough on the surface. Within

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such a short reaction time, numerous small metal (Y3+, and Dy3+) subcarbonates nucleate from the solution once the solution gets supersaturated, and subsequently grow to bigger particles. A

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holding time of 0.5 h is insufficient, leading to relative small size and abnormal morphologies. After that, the particles grow gradually by adsorbing ultra-small particles and simultaneously

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undergo an Oswald ripening process [32]. As can be seen in Fig.6 (b) and (c), the particle size gets larger as aging time goes on. As the Oswald ripening process happens, OH- and CO32- ions release from

the

urea

solution

and

attack

the

cations

in

solution

to

form

ultra-small

Y2(OH)x·(CO3)y·(NO3)6-x-2y·n H2O particles, and the small particles simultaneously attach to the surface of the large particles via electrostatic adsorption. Meanwhile, the small particles locating on the surface of a big particle dissolve to release cations and anions and are immediately devoured by the big one. Fig.6 (e) and (f) show the magnified graphs of the sample aged for 2 h

ACCEPTED MANUSCRIPT and 5 h, respectively. It is clear that the small particles are still locating on the surface of the big particles before they dissolve. With longer aging time (8 h), the particles experience a integrate Oswald ripening process, and have spherical shape with smooth surface, which are dynamically

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steady since then. Fig.6 (d) reveals that the particles no longer grow up with extending the ageing

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time.

Fig.6 FE-SEM photographs of the synthesized Y2O3precursors via urea homogenous precipitation. (a)~(d) are corresponding to the precursors ripening for different times: (a) 0.5 h; (b) 2 h; (c) 5 h; (d) 8h; (e) and (f) are

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magnifications of sample (b) and (c), respectively.

Based on the discussions above, a possible formation schematic diagram of the xDy3+: Y2O3

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precursors is proposed in Fig.7. Firstly, the precursors nucleate from the solution with the assistance of OH-, and CO32-, corresponding to Fig.7 (a). During the second reaction stage, the particles may grow via an electrostatic adsorption and Oswald ripening process, as depicted in Fig.7 (b) → (c). It is notable in Fig.5 that the doping concentration of Dy3+ ions has a great influence on the size of the monodispersed Y2O3 particles. Doping ions usually occupy the surface sites of the lattice when their contents are limited, hence, the Dy3+ ions are expected to firstly expose to the already formed interfaces of the particles and the solution. Dy3+ ions are positively

ACCEPTED MANUSCRIPT charged, whereas the surfaces of the particles are mainly electronegative with the linkage of OH-, CO32-, as a result, small (Y, Dy) (OH)y ·(CO3)x ·nH2O particles are intended to attach to the surface of the larger particles by electrostatic interaction, and the precursors monotonously grow

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from 550 to 840 nm with the increased Dy3+ concentration from 0.5% to 1.5%, as listed in Table 1. However, when the Dy3+ doping concentration is further increased (exceeding 1.5%), size of the precursors decreases. On one hand, the Dy3+ ions partially migrate from the surface to inner sites

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of the Y2O3 lattice, resulting in slowing down of the particle growth. On the other hand, the

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decrease of particle size may be explained by an electrostatic adsorption-repulsion process, as depicted in Fig.7 (d). Surfaces of the big particles are aggregated with large amount of OH-, CO32groups, where the electrostatic repulsion effect might dominant the process of adsorption. Most small particles become difficult to attach to the big ones. As a result, these two combined effects

size.

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caused the particles to stop further growth. On the contrary, the particles get slightly smaller in

During the aging period, the products grow uniformly to particles with spherical shapes to

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obtain a minimum surface energy, as displayed in Fig.7 (e). In fact, the growth rhythm elaborated

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in the present work may enlighten us in the preparation of various nano or submicron sized materials.

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Fig.7 Schematic illustration of the possible formation processes of the yttria precursor.

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Apart from the impact on the particle size, doping concentration is also an important factor affecting the luminescent performance of the materials [16, 33, 34]. To investigate the down-conversion luminescence properties of the Dy3+: Y2O3 powders, the room-temperature PLE and PL spectra of the sintered Dy3+:Y2O3 samples are presented in Fig.8 and Fig.9, respectively.

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The PLE spectrum for the 0.8% Dy3+: Y2O3 phosphors was measured in the range of 300 ~ 500 nm by monitoring at 571 nm, as shown in Fig.8. It consists of several excitation bands corresponding to transitions from ground state to different crystal field-splitting levels of the 4f state of Dy3+ ions.

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The most intense excitation band centered at 349 nm is assigned to the 6H15/2→ 6P7/2

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hypersensitive transition, and the other PLE bands observed at 324 nm, 363 nm, 383 nm, 423 nm, and 454 nm can be ascribed to the transitions from the ground state to the various excited states of 4f9 electronic configuration of the Dy3+ ions [16]. The PL spectra were recorded under the excitation of 349 nm. It can be clearly seen from Fig.9 that, on one hand, the emission spectra of all the samples are similar. They mainly exhibit two emission bands: the blue one centered at 485 nm, and the yellow one centered at 571 nm, which are assigned to 4F9/2→ 6H15/2, and 4F9/2→ 6H13/2 transitions, respectively. On the other hand, the emission intensities are varying with increasing

ACCEPTED MANUSCRIPT Dy3+ ions. With a low doping concentration (0.5 ~ 0.8%), the emission intensity may enhance with the increase of Dy3+ concentration, and reaches the maximum value when the doping content is 0.8%. However, with further increased Dy3+ concentrations (0.8 ~ 2%), the emission intensities of

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the 4F9/2→ 6H13/2 transition decline monotonously, due to the concentration quenching effect [35],

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as can be seen in inset graph in Fig.9.

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Fig.8 Photoluminescence excitation spectrum for the 0.8% Dy3+: Y2O3 phosphors monitored at 571 nm.

Fig.9 Photoluminescence spectra for the Dy3+: Y2O3 phosphors pumped by 349 nm. Inset shows the effect of Dy3+ concentration on the peak intensity of 4F9/2 → 6H13/2 emission of the Dy3+ ions.

ACCEPTED MANUSCRIPT Table 2 Color coordinates for the Dy3+: Y2O3 crystal particles.

0.5% Dy3+: Y2O3

CIE coordinate (x, y) (0.2923, 0.3153) (0.3089, 0.3353)

1% Dy3+: Y2O3

(0.2784, 0.2967)

1.5% Dy3+: Y2O3

(0.2743, 0.2919)

2% Dy3+: Y2O3

(0.2667, 0.2819)

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0.8% Dy3+: Y2O3

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Sample

In order to study the colorimetric performance of the as prepared xDy3+: Y2O3 crystal

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powders, color coordinates for the samples are calculated using the intensity-corrected emission spectra. The detailed calculation procedure is adopted from the previous study [36], and the dependence of color coordinates on the Dy3+ concentration are depicted in Table 2. As can be seen in Table 2 that the color coordinates are getting close to the blue region when the Dy3+ content is

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relatively high (e.g., x ≥ 1). However, when the doping concentration is low (e.g., x = 0.5 or 0.8%), the color coordinates for the as prepared phosphors are closer to the ideal white light illumination

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(0.33, 0.33). The color coordinates for the 0.8% Dy3+: Y2O3 powders are marked in CIE-1931 chromaticity diagram in Fig.10, which are located in the white domain, indicating that the

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prepared Dy3+: Y2O3 phosphors are promising for WLEDs.

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Fig.10 CIE-1931 chromaticity diagram for the 0.8% Dy3+: Y2O3 phosphor under 349 nm excitation.

4. Conclusions

Submicron-sized xDy3+: Y2O3 (0.5% ≤ x ≤ 2%) precursors were successfully synthesized via

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a urea homogeneous precipitation method. Polycrystalline powders were obtained after calcinating at 800℃ for 2 h. It was inspiring to find that the Dy3+ doping concentration had a great impact on the particle growth. With increasing doping content, the particles were undergoing a course of

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enlargement in size at low doping concentration, and a monotonous decrease in size with further

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increased Dy3+concentration. This was supposed to be the Dy3+ ions which were exposed to the solution that influenced the adsorption/ desorption of small subcarbonate to/ from the large particles during the Ostwalt ripening process. Under the excitation of UV-light of 349 nm, Dy3+: Y2O3 powders emitted blue and yellow light which was ascribed to the 4F9/2 → 6H15/2 and 4F9/2 → 6

H13/2 transition of Dy3+ ions, respectively. Through controlling the Dy3+ doping concentration, the

color hue was tuned to white. The color coordinates for the 0.8% Dy3+: Y2O3 powders were calculated to be (0.3089, 0.3353), very close to that of the idea white light illumination (0.33,

ACCEPTED MANUSCRIPT 0.33), indicating that the as prepared powders were potential candidates for WLEDs.

Acknowledgements

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This work was supported by the funding from the Priority Academic Program Development of the Jiangsu Higher Education Institutions (PAPD), and Shanghai Natural Sciences Fund (13ZR1445900).

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 TG-DSC curves of the Y2O3 precursor ageing for 5 h. Fig.2 FT-IR spectra of the precursor and the powder calcined at 800℃ for 2 h.

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Fig.3 XRD patterns of the xDy3+:Y2O3 (0.5% ≤ x ≤ 2%) crystal particles. Fig.4 FE-SEM photographs of the 0.8% Dy3+: Y2O3 (a) precursor and (b) crystal particle calcined at 800℃.

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Fig.5 SEM photographs (Left) and particle size distribution (Right) of the synthesized xDy3+:Y2O3

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precursors via urea homogenous precipitation. (a) x = 0.5%; (b) x = 0.8%; (c) x = 1%; (d) x = 1.5%; (e) x = 2%.

Fig.6 FE-SEM photographs of the synthesized Y2O3 precursors via urea homogenous precipitation. (a)~(d) are corresponding to the precursors ripening for different times: (a) 0.5 h; (b) 2 h; (c) 5 h;

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(d) 8h; (e) and (f) are magnifications of sample (b) and (c), respectively. Fig.7 Schematic illustration of the possible formation processes of the yttria precursor.

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Fig.8 Photoluminescence excitation spectrum for the 0.8% Dy3+: Y2O3 phosphors monitored at

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Fig.9 Photoluminescence spectra for the Dy3+: Y2O3 phosphors pumped by 349 nm. Inset shows the effect of Dy3+ concentration on the peak intensity of 4F9/2→ 6H13/2 emission of the Dy3+ ions. Fig.10 CIE-1931 chromaticity diagram for the 0.8% Dy3+: Y2O3 phosphor under 349 nm excitation.

Tables Captions Table 1 Mean particle size of the xDy3+: Y2O3 precursors and crystals (0.5% ≤ x ≤ 2%). Table 2 Color coordinates for the Dy3+: Y2O3 crystal particles

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Highlights · Y2O3:Dy3+ particles were prepared by urea homogeneous precipitation. · Doping concentration effect on the particle growth of Y2O3: Dy3+ was discovered. · A possible controlled-growth mechanism of particles induced by Ln3+ was proposed. · The luminescence color hue can be tuned to (0.31, 0.33) under UV-light excitation.