Effect of Eu3+ doping on the structural and photoluminescence properties of cubic CaCO3

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Contents lists available at ScienceDirect

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Effect of Eu3+ doping on the structural and photoluminescence properties of cubic CaCO3

1

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3

Q1

4

Yan Gao, Yidi Sun, Haifeng Zou, Ye Sheng, Xiuqing Zhou, Bowen Zhang, Bing Zhou ∗ College of Chemistry, Jilin University, Changchun 130012, PR China

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6 20

a r t i c l e

i n f o

a b s t r a c t

7 8 9 10 11 12

Article history: Received 7 May 2015 Received in revised form 28 August 2015 Accepted 18 September 2015 Available online xxx

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Keywords: Eu3+ Cubic Calcium carbonate Structure Luminescence

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1. Introduction

14 15 16 17 18

22Q2 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

CaCO3 :xEu3+ (x = 0, 0.010, 0.015, 0.020, and 0.025) cubic nanoparticles were synthesized by carbonation method. The powder XRD patterns and SEM images of the CaCO3 :xEu3+ nanoparticles demonstrate that both the crystalline sizes and average particle sizes of synthesized samples decreased with the increase of Eu3+ content until x = 0.020. Kubelka–Munk plots and bandgap energy estimation indicate that the doping of Eu3+ ions changed optical bandgap of CaCO3 . Photoluminescence (PL) spectra show that the PL intensity of the CaCO3 :xEu3+ nanoparticles was enhanced with the increase of Eu3+ content in cubic CaCO3 :xEu3+ , and concentration quenching occurred when Eu3+ concentration exceeded 2.0 mol%. In addition, the doped sites of Eu3+ in CaCO3 crystalline were identified by the site-selective spectroscopy and decay curves. © 2015 Elsevier B.V. All rights reserved.

Doping, the intentional introduction of impurities into a material, is one of effective routes to control the properties of bulk materials. Many materials are intentionally “doped” by introducing appropriate amounts of foreign elements into hosts to impart electronic, magnetic, and optical properties [1]. For several decades, great endeavors have been devoted to investigate the impact of nanocrystalline structure, size, and shape on the behaviors of dopants [2]. On the contrary, few works concerned the impurity doping-induced control over the growth and phase structure of functional nanocrystals (NCs). In fact, impurity doping was recently found to have significant influence on nucleation and growth of many functional nanomaterials, and provide a fundamental approach to modify the crystallographic phase, size, morphology, and electronic configuration of NCs [3–7]. Recently, much attention has been paid to the preparation and characterization of Eu3+ -doped inorganic luminescent materials, such as oxides [8], fluorides [9], oxy-sulfides [10], silicate [11], and aluminosilicate [12], due to their potential applications in optical displays with strong red emission. Hereinto, there has been an ongoing interest in the preparation and luminescent behaviors of

∗ Corresponding author. Tel.: +86 431 85168428. E-mail address: [email protected] (B. Zhou).

the red phosphor of CaCO3 :Eu3+ because they can be prepared at low temperatures with good crystallinity and regular morphology and can exhibit strong red emission without any further treatment [13–18]. However, to the best of our knowledge, most reports are devoted to investigate the impact of CaCO3 crystallinity and morphology on the luminescence properties of Eu3+ ions, no detailed studies on the effect of Eu3+ ions on the formation of CaCO3 :Eu3+ nanoparticles (NPs) and there are few discussions [19] on the sites of Eu3+ ions in CaCO3 crystals. In fact, the luminescence properties of rare-earth (RE) ions in nanocrystals depend critically on their locations in the host. If RE ions have similar chemical properties (viz., the same oxidation state and similar ionic radius) to the host metallic ions, it is easy to incorporate RE ions into the host lattice to replace the cations [20]. Herein, we discussed the effect of Eu3+ ions doping on the formation of cubic CaCO3 and the sites of Eu3+ ions in CaCO3 crystals for the first time. The results show that Eu3+ ions can also retard the crystal growth of CaCO3 , as a result, the sizes of CaCO3 :Eu3+ cube-like NPs decrease with a little increasing of Eu3+ concentration in CaCO3 cubes. The possible reasons were proposed. In addition, cubic CaCO3 :Eu3+ NPs exhibit strong red emission corresponding to the 5 D0 → 7 F2 transition of the Eu3+ ions under UV light excitation without any posttreatment such as sintering. The PL intensity of cubic CaCO3 :Eu3+ NPs increases with the increasing of Eu3+ concentration in cubic CaCO3 , and concentration quenching occurs when Eu3+ concentration exceeds 2.0 mol%. Site-selective spectroscopy

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and decay curves indicate that Eu3+ ions locate multiple sites in the cubic CaCO3 host.

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81 82 83 84 85 86 87 88 89 90 91 92 93 94 95

2.2. Preparation CaCO3 :Eu3+ NPs were prepared by carbonation method as reported previously [18]. Typically, certain amount of calcium oxide (CaO) was digested in distilled water to form calcium hydroxide (Ca(OH)2 ) slurry (1.0 M, pH = 13). After the slurry was allowed to stand for 24 h, 0.10 M Eu(NO3 )3 aqueous solution (pH = 3–4) was added into it and stirred for 0.5 h (The molar ratio of CaCO3 and Eu3+ was 100:1.0, 100:1.5, 100:2.0, and 100:2.5, respectively.). The resulted slurry was then transferred into a bubble glass column with inside diameter (i.d.) of 3 cm and a gas inlet tube with i.d. of ca. 0.8 cm was put just above the bottom of the column. A gas mixture of carbon dioxide (CO2 ) and nitrogen (N2 ) (molar ratio of 1:2) was introduced into the slurry through the tube. The bubbling process was stopped as soon as the pH value of the slurry decreased to 7. Finally, the precipitates were centrifugally separated from their mother liquor and washed for three times with distilled water. The final product was dried in oven at 80 ◦ C for 24 h.

50

60

70

Intensity (counts)

2θ (degree)

(b)

CaCO3

(104)

80

40

3+ CaCO3:0.010Eu 3+ CaCO3:0.015Eu

Intensity (counts)

79

30

e

20

D

pl

E

B

od

78

C

eC

77

0

A

Sa m

75

2000

( 300) ( 0012)

74

4000

( 12-1) ( 122) ( 124)

73

Calcium oxide, absolute ethanol, and Eu2 O3 (99.99%) was purchased from Beijing Chemical Company. All chemicals were of analytical grade and were used directly without further purification. Distilled water was used throughout the experiment, Eu(NO3 )3 aqueous solution (0.10 M, pH = 3–4) was obtained by dissolving Eu2 O3 (99.99%) in dilute HNO3 solution under heating with ceaseless agitation.

( 018) ( 11-6)

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6000

( 11-3)

2.1. Materials

10000 8000

( 202)

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JCPDS Card no: 47-1743

( 110)

2. Experimental

12000

( 104)

70

14000

(a)

( 012)

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( 006)

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3+ CaCO3:0.020Eu 3+ CaCO3:0.025Eu

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29.0

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30.0

30.5

31.0

2 θ (degree) 96

97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125

2.3. Characterization The crystalline phase of the samples was examined on a ˚ The meaXRD-6000 with Cu-K␣ radiation (wavelength: 1.5418 A). surements were carried out with a range of 20–70o , a step width of 0.06o and half second counting time. The field emission scanning electron microscopy (FESEM) images were obtained by a Hitachi S-4800 scanning electron microscope. Based on the SEM images, diameter of NPs was measured using image visualization software Image J. The UV–vis diffuse reflection spectra (DRS) were obtained on U-4100 spectrophotometer (solid) at room temperature, and the reflection spectra were calibrated with the reflection of white alumina (Al2 O3 , reflection ∼100%) in the wavelength region of 200–800 nm. Thermogravi-metric analysis and differential thermal gravity (TGA–DTG) data were recorded with a thermal analysis instrument (TGA/DSC 1600 LF, METTLER TOLEDO, Switzerland) at a heating rate of 10 ◦ C min−1 in a nitrogen flow of 100 ml min−1 . FT–IR spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. The room-temperature photoluminescence (PL) excitation and emission spectra were recorded using a Jobin Yvon FluoroMax4 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Site-selective spectroscopy was done as follows: the samples were mounted in the helium-exchange gas chamber of a closed-cycle refrigeration system, and their temperature was maintained at 40 K. A rodamine 6G dye laser pumped by the second harmonic of an Nd:yttrium–aluminum–garnet pulsed laser was used as the excitation source. The fluorescence spectra were obtained with a Spex 1403 spectrometer. The photoluminescence signals were detected with a photomultiplier, averaged with a boxcar integrator, and processed by a computer.

Fig. 1. (a) Powder X-ray diffraction patterns of (A) CaCO3 , (B) CaCO3 :0.010Eu3+ , (C) CaCO3 :0.015Eu3+ , (D) CaCO3 :0.020Eu3+ , (E) CaCO3 :0.025Eu3+ ; (b) (1 0 4) peak shifting toward higher 2 value.

3. Results and discussion

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3.1. XRD analysis

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Fig. 1a shows the XRD patterns of CaCO3 :Eu3+ samples with different Eu3+ doping concentration as well as the d-spacing of the standard CaCO3 (JCPDS No. 47-1743). The results of XRD indicate that the diffraction peaks of the CaCO3 :xEu3+ samples can be exactly assigned to the standard diffraction data of calcite CaCO3 (JCPDS No. 47-1743). No other secondary phases were detected, indicating that Eu3+ ions have been successfully doped into the CaCO3 host lattices [21]. It is also observed that the intensities of the diffraction peaks decrease with the increasing of Eu3+ -doped concentration, which indicates that the dopant Eu3+ (ionic radius: 0.095 nm) are substituted in the inner lattice of Ca2+ ions (ionic radius: 0.099 nm) [22,23]. Fig. 1b shows that the diffraction peaks of the phosphor CaCO3 :xEu3+ shift to a little larger diffraction angles as a function of the Eu3+ ions concentration until x = 0.020, which is arisen as a result of contraction in unit cell volume owing to the substitution of Ca2+ ions (0.099 nm) by smaller Eu3+ ions (0.095 nm) in the host lattice. In addition, the diffraction peaks become broader with the increasing Eu3+ dopant content until x = 0.020, indicating a reduction of the average crystallite size until x = 0.020. Since the ionic radius of Eu3+ is close to that of Ca2+ , the changes in the FWHM (full width at half-maximum intensity) are

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Table 1 Crystallite size and microstrain of pure and Eu3+ -doped CaCO3 nanoparticles. Concentration of Eu3+ (%)

Position of (1 0 4) peak (2)

FWHM

Crystallite size (nm)

Microstrain ×10−3 (lines−2 /m4 )

0.0 1.0 1.5 2.0 2.5

29.601 29.619 29.639 29.620 29.580

0.229 0.287 0.304 0.418 0.321

35.48 29.31 26.73 19.44 25.31

0.978 1.184 1.298 1.784 1.371

36

1.9

34

1.8

Concentration of Eu3+ (%)

1.7

32

1.6

30

1.4 26

1.0

18

0.9 0.000

0.005

0.010

0.015

0.020

Fig. 2. Crystallite size and microstrain changed with Eu concentration.

151

in accordance with the crystallite size, which is calculated from the Debye–Scherrer’s formula:

152

D=

0.9 ˇ cos 

(1)

166

where D is the size of the particle,  is the wavelength of Cu ˚ ˇ is the full width at half-maximum intenK␣ radiation (1.5418 A), sity, and  is the peak position. The average crystallite size of pure CaCO3 NPs is calculated as 35.48 nm, and it is found that the average crystallite sizes of CaCO3 :xEu3+ NPs decrease with the Eu3+ concentration until x = 0.020 and represented in Table 1. It is evident that the FWHM increases and the crystallite size decreases with increasing the doping of Eu3+ up to the 2.0 mol% content of Eu3+ . The reduction in the crystallite size is mainly due to the distortion in the host CaCO3 lattice by the foreign impurities (i.e., Eu3+ ) that decrease the nucleation and subsequent growth rate of CaCO3 NPs, but the reduction is not unlimited, it reaches minimum when the dosage of Eu3+ ions is 2.0 mol%. The microstrain can be calculated using the formula:

167

ε=

153 154 155 156 157 158 159 160 161 162 163 164 165

168 169 170 171 172 173 174 175 176 177 178 179 180 181 182

ˇ cos  4

c

4.978 4.976 4.976 4.975 4.977

17.007 17.007 17.002 16.998 16.996

364.92 364.66 364.54 364.33 364.59

3.2. SEM micrographs and particle sizes analysis

0.025

concentration (x)

150

a

Cell volume (Å3 )

the diffraction peaks further illustrate the incorporation of Eu3+ ions into the CaCO3 lattice.

1.1

20

0.0 1.0 1.5 2.0 2.5

Lattice parameters (Å)

4

1.2

22

-2

1.3

24

-3

28

Microstrain

1.5

* 10 lines /m

Crystallite size (nm)

Table 2 Lattice parameters and cell volume of pure and Eu3+ -doped CaCO3 nanoparticles.

(2)

Table 1 and Fig. 2 show the comparison of crystallite size and strain with the increasing of dopant concentration. The substitution of Eu3+ in an interstitial position would affect the concentration of the interstitial Ca, oxygen, and Ca vacancies. The observation of small changes of 2 values in diffraction peaks and the peak broadening is due to the increase of microstrain, and the line broadening may be due to the variation of the size and microstrain of CaCO3 :xEu3+ NPs. The lattice parameters and cell volumes of CaCO3 :xEu3+ NPs are calculated by Jade 6 software. As shown in Table 2, the changes in a and c parameters are observed due to the incorporation of Eu dopant. As displayed in Table 2, the volume of the unit cell of the CaCO3 :xEu3+ decreases with the increase of Eu doping level until x = 0.020, and then increases, which is consistent with the changes of crystalline size. The parameters changes obtained from

Fig. 3A–E show the SEM images of CaCO3 :xEu3+ NPs. Based on the SEM images, size distribution histograms of the samples were obtained (the supporting information, Fig. S1.), and the mean size distribution histograms with dopant concentration are presented in Fig. 3F. From Fig. 3, it can be seen that both the undoped and doped samples are cubic NPs but their average sizes are different. When x = 0.000, 0.010, 0.015, 0.020, and 0.025, the average diameter of CaCO3 :xEu3+ NPs is 52, 33, 24, 18, and ∼20 nm, respectively. The average size of the Eu3+ -doped CaCO3 NPs decreases greatly with the increase of the dopant concentration until the 2.0 mol% Eu3+ dosage. Above discussions indicate that the incorporation of Eu3+ ions suppresses the nucleation and growth of CaCO3 NCs to a great extent. The possible mechanism is shown in Fig. 4. After the addition of the Eu(NO3 )3 aqueous solution (pH = 3–4) to the very basic Ca(OH)2 (pH = 13), Eu(OH)3 gelatinous phase was precipitated. These gelatinous particles may further act like nuclei/seeds for CaCO3 growth and make Eu ions sit in the lattice of the CaCO3 . The increase in the nucleation sites may lead to the crystallite size decrease (as proved by XRD analyses). This may be heavily responsible for the reduction in the nanocube dimensions upon the addition of the Eu(NO3 )3 aqueous solution. The other reason is as follows: Each substitution of Ca2+ by Eu3+ in CaCO3 or interstitial Eu3+ requires extra OH− for charge compensation, and the introduction of such OH− ions into the grain surface may induce transient electric dipoles with their negative poles outward. These transient electric dipoles hinder the diffusion of CO3 2− ions (which are needed for crystal growth) from the solution to the grain surface, thus, retarding the growth of CaCO3 . 3.3. Diffuse reflection spectra of undoped and Eu3+ -doped CaCO3 Fig. 5 shows the diffuse reflection spectra of undoped and Eu3+ doped CaCO3 samples. All lines show a strong drop in reflection in the UV range below 220 nm, corresponding to the valence-toconduction band transitions of the CaCO3 host lattice. In addition, reflections between 230 and 360 nm decrease with increasing Eu3+ concentration, which may be attributed to the Eu–O charge transfer bands. The adsorption edges of CaCO3 :xEu3+ samples show a minor blue shift with the increasing of Eu3+ concentration (Fig. 5), which can be ascribed to the decrease of the lattice constants due to the incorporation of the smaller Eu3+ into the CaCO3 host lattice.

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Fig. 3. Typical SEM images of (A) 0.0, (B) 1.0, (C) 1.5, (D) 2.0, (E) 2.5 mol% Eu3+ -doped CaCO3 nanoparticles and the histograms of variation of the average particle size with

Q8 dopant concentration (F).

Fig. 4. Schematic illustration of the possible Eu3+ doping routes in the CaCO3 host lattice (a) undoped, (b) Eu3+ ions occupied the position of Ca2+ , and (c) interstitial occupation by Eu3+ ions and the impact of Eu3+ doping on the size of cubic CaCO3 nanoparticles.

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Fig. 5. Diffuse reflection spectra of CaCO3 :xEu3+ powder samples. Insets are the Kubelka–Munk functions.

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Fig. 7. TG and DTG traces of CaCO3 and CaCO3 : 0.020 Eu3+ nanoparticles.

calcite crystals based on the in-plane bend (1 ) and on the out-plane band (2 ) at 714 and 873 cm−1 , respectively, and an antisymmetry stretch (3 ) at 1417 cm−1 [26,27]. It can be also seen from Fig. 4 that the width of half peak 3 of Eu-containing sample is obviously narrower than the pure CaCO3 . Eu3+ ions in CaCO3 crystals causes the crystal structural defects, which will weaken Q4 and die away the multiple or mixed frequency peaks, then narrow the absorption peak 3 of Eu-containing CaCO3 samples [28]. Q5 3.5. TG–DTG analysis

Fig. 6. FTIR spectra of (A) CaCO3 , (B) CaCO3 :0.020 Eu3+ nanoparticles.

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The absorption spectra of CaCO3 and Eu3+ -doped CaCO3 samples were obtained from the reflection spectra using the Kubelka–Munk function [24] K (1 − R)2 F (R) = = 2R S

(1)

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where R, K, and S is the reflection, the absorption coefficient and the scattering coefficient, respectively. The absorption spectra (K/S) of CaCO3 and Eu3+ -doped CaCO3 samples are shown in the insets of Fig. 5. The value of the optical bandgap can be calculated by extrapolating the Kubelka–Munk function to K/S = 0. There is only one absorption band for all the samples and their optical bandgaps of CaCO3 : xEu3+ are calculated to be about 5.66 eV (i.e., 219 nm), 5.64 eV (i.e., 220 nm), 5.43 eV (i.e., 228 nm), 5.39 eV (i.e., 230 nm), 5.41 eV (i.e., 229 nm) for x = 0.000, 0.010, 0.015, 0.020, and 0.025, respectively. The optical bandgap of pure CaCO3 is consistent with the former report [25], and the doping of Eu3+ ions brought about the change of optical bandgap of CaCO3 .

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3.4. FT–IR spectra analysis

229 230 231 232 233 234 235 236 237 238 239

The FT–IR spectra of CaCO3 and 2.0 mol% Eu3+ -doped CaCO3 NPs are shown in Fig. 6. Both the samples exhibited absorption bands 243 Q3 at 714, 873, and 1417 cm−1 , which is a characteristic spectrum of 244 242

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The TG and DTG traces of CaCO3 and CaCO3 :0.02Eu3+ were recorded in Fig. 7. From Fig. 7, it can be seen that the thermal decomposition temperature of pure CaCO3 is 725.2 ◦ C, while it was slightly dropped to 716.6 ◦ C after being doped with 2.0 mol% Eu3+ . Thermal analysis suggests that the thermal stability of CaCO3 :Eu3+ nanoparticles decreases a little due to Eu3+ doping. In addition, it should be noted that there is significant difference in the 100–600 ◦ C temperature range for the doped sample compared to the undoped sample. A total of 6–7% weight loss can be seen for the doped sample in this temperature range and this may be due to the existence of Eu rich phases in CaCO3 :Eu3+ NPs, for example, EuCO3 OH or Eu2 (CO3 )3 , etc. [29]. As mentioned above, Eu(OH)3 was precipitated after the addition of the Eu(NO3 )3 aqueous solution to Ca(OH)2 , thus, EuCO3 OH or Eu2 (CO3 )3 can be formed after the introduction of CO2 . The reaction equations are as follows:

254 255 256 257 258 259 260 261 262 263 264 265 266 267 268

2Eu(OH)3 + 3CO2 → Eu2 (CO3 )3 ↓ +3H2 O

(1)

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Eu(OH)3 + CO2 → EuCO3 OH ↓ +3H2 O

(2)

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This also proves that the Eu3+ ions have been incorporated into the CaCO3 . 3.6. Energy level scheme for Eu3+ ions in CaCO3 and their PL–PLE spectra studies Energy level scheme for Eu3+ ions in CaCO3 (Fig. 8a) shows the energy-level scheme for PL and PLE processes in Eu3+ -doped CaCO3 phosphor. The fundamental absorption edge of CaCO3 is at Eg > 5.4 eV (< 230 nm) [30], as shown in Fig. 4. The excitation spectrum of CaCO3 :0.02 Eu3+ NPs in the wavelength range of 300–500 nm is shown in Fig. 8b. It can be seen that the excitation spectrum of CaCO3 : 0.020Eu3+ , monitored with 613 nm emission of Eu3+ (5 D0 → 7 F2 ), consists of the characteristic excitation lines of Eu3+ within its 4 f6 configuration. The peaks at 317,

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Fig. 10. 40 K emission spectra of CaCO3 :0.020Eu3+ cubic like nanoparticles with (a) ex = 464.6, (b) ex = 470.7, and (c) ex = 472.1 nm. The nanoparticles were excited with a xenon lamp at 464–473 nm under the same experimental condition. To eliminate the influence of excitation light, a 495 nm long-pass glass filter was used.

3.7. Site-selective spectroscopy

3+

Fig. 8. (a) Electronic energy-level scheme for Eu in CaCO3 , (b) PL and PLE spectra for CaCO3 : 0.020 Eu3+ phosphor measured at room temperature with em = 613 nm (PLE) and ex = 393 nm (PL).

284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301

Q9

361, 380, 393, 415, and 463 nm can be clearly observed, which correspond to 7 F0 → 5 HJ , 7 F0 → 5 D4 , 7 F0 → 5 GJ , 7 F0 → 5 L6 , 7 F0 → 5 D3 , and 7 F0 → 5 D2 transitions of Eu3+ , respectively. An excitation wavelength of 393 nm gave maximum intensity for all Eu3+ -doped CaCO3 NPs. Therefore, in all the measurements, the optical excitation was kept constant at 393 nm. Upon excitation at 393 nm, the emission spectrum of CaCO3 :0.020Eu3+ NPs is composed of a group of lines peaking at about 579, 590, 613, 651, and 699 nm. They come from the 5 D0 → 7 FJ (J = 0, 1, 2, 3, and 4) transitions of the Eu3+ ions. The room temperature PL and PLE spectra of CaCO3 :xEu3+ (x = 0.010, 0.015, 0.020, and 0.025 mol) NPs are shown in Fig. 9. From the spectra, it can be found that the positions of Eu3+ ions luminescence peaks have not changed but the PL intensity increases with the increasing of Eu3+ ions concentration and the intensity reaches the maximum when the concentration of Eu3+ ions is 2.0 mol%, then decreases with more addition of Eu3+ ions, which is regarded as the typical concentration quenching effect [31,32]. Thus, the optimum concentration of Eu3+ ions is 2.0 mol% in CaCO3 :Eu3+ cube-like NPs.

Because of charge imbalance and lattice distortion, multiple sites are possible for trivalent Eu3+ ions incorporated in cubic CaCO3 NPs. To reveal the multiple sites of Eu3+ in CaCO3 NCs, siteselective emission spectra of CaCO3 :0.020Eu3+ under the excitation at 464.6, 470.7, and 472.1 nm (corresponding to the 7 F0 →5 D2 transitions of sites I, II,and III, respectively) were measured at 40 K. The luminescence of rare earth doped inorganic luminescent materials presents fixed lines determined by the electronic structure of the rare earth and is almost independent of the host matrix; however, the width and the relative intensity of those lines frequently depend on the crystal symmetry of their sites. Thus, depending on the sites’ symmetry, some lines are present, while others are inactive. The 5 D0 → 7 F1 transition at 590 nm was the parity-allowed magnetic dipole transition (J = 1), and the 5 D0 → 7 F2 transition at 613 nm was the electric dipole transition (J = 2) [33]. It is wellknown that the relative intensity of the 5 D0 → 7 F1 and 5 D0 → 7 F2 transition is also determined by the symmetry of the crystal sites of the Eu3+ ions. If Eu3+ ions have a site with inversion symmetry, the 5 D → 7 F transition dominates; while if Eu3+ ion holds a site with0 1 out inversion symmetry, the 5 D0 → 7 F2 transition predominates [34]. As shown in Fig. 10a, under the excitation at 464.6 nm, three broad emission peaks owing to intra-4f transitions 5 D0 → 7 FJ (J = 0, 1, 2) were observed, similar to previous reports for Eu3+ -doped anatase NCs [20,35], and they confirmed that Eu3+ at site I is located at a distorted site, which might be close to the surface. In addition, it should be noted that in Fig. 8a the electric-dipole transition (5 D0 → 7 F2 ) is much stronger than the magnetic dipole transition (5 D0 → 7 F1 ), therefore, the Eu3+ ions locate at low-symmetry sites without an inversion center in the CaCO3 :Eu3+ host lattice. It is further confirmed that the Eu3+ at site I is located at a distorted site,

Fig. 9. PLE (a) and PL (b) spectra of Eu-doped CaCO3 nanoparticles.

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reduction in the size is mainly due to the distortion in the host CaCO3 lattice by the foreign impurities (i.e., Eu3+ ) that decrease the nucleation and subsequent growth rate of CaCO3 NPs. The concentration quenching effect of luminescence happened when the concentration of Eu3+ ions is 2.0 mol%. Furthermore, site-selective spectroscopy shows that the Eu3+ ions are located at two lattice sites: one is close to the surface of CaCO3 ; the other one occupies a site with a higher symmetry, which might be located at the crystal lattice of CaCO3 . The PL decay curve well-verified this result. Acknowledgement This work is financially supported by the National Natural Sci- Q6 ence Foundation of China (Grant nos. 21171066 and 51272085). Fig. 11. The PL decay of 5 D0 of the Eu3+ ions in CaCO3 :0.020Eu3+ by monitoring the 5 D0 → 7 F2 emission at room temperature.

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which might be close to the surface. For comparison, the emission spectrum in Fig. 10b and c are very different from that of Fig. 10a and previous reports for Eu3+ -doped anatase NCs [35], the magnetic dipole transition (5 D0 → 7 F1 ) is stronger than the electric-dipole transition (5 D0 → 7 F2 ), which indicate that the Eu3+ ions occupy the sites with a higher symmetry [36], indicating a well-crystalline surrounding around Eu3+ ions at sites II, III. Although very sharp and intense lines haven not been observed due to the low wavelength resolution, it is confirmed that the crystalline surrounding around Eu3+ ions at sites II, III is different from that of Eu3+ ions at site I.

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3.8. PL decay curves

333 334 335 336 337 338 339 340 341

344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366

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To provide further insight into the position of the Eu3+ ions in CaCO3 :xEu3+ NPs, experiments on the excited-state dynamics of the Eu3+ ions were performed. Fig. 11 shows the decay curve of luminescence of the Eu3+ (em = 613 nm) in the CaCO3 :0.02Eu3+ samples. The curve cannot be fitted into the single-exponential function but can be fitted well into a double-exponential function as I = A1 exp(−t/ 1 ) + A2 exp(−t/ 2 ), in which A is a pre-exponential factor obtained from the curve fitting, and  i (i = 1, 2) is the lifetime. As we know, the rare earth ions are divided into two types: one is at the surface of the nanophosphors and the other is in the “bulk” of the nanophosphors [34]. The lifetimes () are 1.1860 and 0.4424 ms, which proved that there are two different luminous centers of Eu3+ ions in the CaCO3 :Eu3+ NPs. The lifetimes are well-consistent with the results of site-selective spectroscopy. The crystal field around the surface Eu3+ ions is more asymmetric compared with that in the “bulk”. The Eu3+ ions with the decay time of 1.1860 ms is located in the “bulk” of the CaCO3 :Eu3+ . For comparisons, the Eu3+ ions with decay time of 0.4424 ms is located at low-symmetry sites, which might be close to the surface. The CaCO3 :Eu3+ NPs possess many internal defects as quenching centers (traps). When the excited luminescent centers are near traps, the excited energy could be easily transferred to the traps from which it lost nonradiatively. As a result, the decay time becomes short. 4. Conclusions In summary, Eu3+ -doped CaCO3 cubic NPs have been synthesized through a simple carbonate route. The effect of Eu3+ doping on the formation and photoluminescence properties of cubic CaCO3 has been studied. XRD analysis show that Eu3+ ions have been doped into the CaCO3 host lattices and the average crystallite sizes of CaCO3 :Eu3+ NPs decreased with the Eu3+ concentration until 2.0 mol% Eu3+ . SEM studies further prove that the average size of the Eu3+ -doped CaCO3 NPs also decreases with the increase of the dopant concentration until the 2.0 mol% Eu3+ dosage. The

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