Enhanced Red Light Emission of OH added Sr5(PO4)3Cl:Eu

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Enhanced Red Light Emission of OH added Sr(PO)Cl:Eu Nanophosphors Hee-Suk Roh, Seongha Lee, Fen Qin, SALIM CALISKAN, Chulsoo Yoon, and Jung-Kun Lee ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00754 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 5, 2018

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ACS Applied Nano Materials

Enhanced Red Light Emission of OH- added Sr5(PO4)3Cl:Eu3+ Nanophosphors Hee-Suk Roh,1,2 Seongha Lee,1 Fen Qin,1 Salim Caliskan,1 Chulsoo Yoon,2 and Jung-Kun Lee*,1,3 1

Department of Mechanical Engineering & Material Science, University of Pittsburgh, PA

15261, United States 2

LED business, Samsung Electronics Co. LTD., Yongin, Gyeonggi-do, 17113, Republic of

Korea 3

Department of Energy Science, Sungkyunkwan University, Suwon, Gyeonggi-do, 16419,

Republic of Korea * Corresponding author: Jung-Kun Lee (Tel: +1-412-648-3395; E-mail address: [email protected])

ABSTRACT : Oxide red nanophosphors convert high energy photons to red light and have potential applications in photonic devices including fluorescent lamps, light emitting devices, and biosensors. In this study, a series of OH- substituted Sr5(PO4)3Cl:Eu3+ nanophosphors emitting red light are successfully synthesized via a microwave reaction method. The effects of OH- substitution on a change in the crystal structure and a photoluminescence of Eu3+ in Sr5(PO4)3Cl(1-x)OHx are investigated. The ratio of Cl- to OH- ions was controlled by changing Sr/Cl ratio in a precursor solution. Though a part of Cl- ions were substituted by OH- ions, the emission intensity ratio of 5D0→7F2 transition to 5D0→7F1 transition (I618 1 Environment ACS Paragon Plus

nm/I593 nm)

did not

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

change. This result indicates that Eu3+ ions preferentially occupies MI site of an apatite structure in Sr5(PO4)3Cl and Sr5(PO4)3Cl(1-x)OHx. In addition, the intensity of the red emission from Sr5(PO4)3Cl(1-x)OHx:Eu3+ increases four times as OH- replaces 20 % of Cl-. Decay lifetime measurement and its analysis by Judd-Ofelt theory show that the addition of OH- ions does not change the dipole transition strength, but decreases the number of non-radiative trapping sites such as point defects. This leads to the improved emission quantum efficiency (η). However, an excessive amount of OH- randomized the atomic arrangement and produced local strain. This, in turn, causes non-radiative relaxation of excited electrons and decreases the intensity of the red emission.

Keywords: Nanophosphor, Apatite, Anion substitution, Eu3+ activator, Emission enhancement

1. Introduction Apatite type alkaline-earth halo-phosphates with the generic molecular formula of M5(PO4)3X (M=Ca, Sr, Ba; X=F, Cl, Br, OH) are widely investigated because they have a broad range of the compositional variation, good thermal stability and excellent biocompatibility. Also, apatite type alkaline-earth halo-phosphates can be produced from cheap raw materials and through a simple synthesis process.1 In addition, apatites are good host materials for phosphor and laser applications.2 When Eu2+ is doped into alkaline-earth halo-phosphates, they turn to promising blue emitting phosphors with high quantum efficiency. Luminescence of Sr5(PO4)3Cl:Eu2+ can be applied to a blue component of high-efficiency compact fluorescent lamps, white LEDs, and flat panel displays.3 Therefore, extensive studies have been performed to find facile synthesis techniques and improve luminescence properties of Sr5(PO4)3Cl:Eu2+.4,5 In recent studies, codoping of rare-earth elements or replacement of cations are reported to enhance the luminescence

2 Environment ACS Paragon Plus

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intensity or tune the photoluminescence emission and excitation of Sr5(PO4)3Cl:Eu2+ system.5,6 While Eu2+ in alkaline-earth halo-phosphates emits blue light, Eu3+ produces intense red light.7 However, there are a limited number of reports on Eu3+-doped Sr5(PO4)3Cl phosphor. Therefore, a local structure around Eu3+ ions and a relation between the red emission and structural change of Sr5(PO4)3Cl:Eu3+ are yet to be known. When the anion site of M5(PO4)3X is taken by OH-, the compound is called hydroxyapatite. Hydroxyapatites M5(PO4)3OH (M=Ca, Sr, Ba) have excellent bioactivity, biocompatibility, and nontoxic properties.8 Therefore, calcium or strontium hydroxyapatites doped with rare earth element activators (e.g. Eu, Tb and Ce) are very useful for in-vivo disease diagnosis, and phototherapy.9 Niu et al. reported applications of Sr5(PO4)3OH:Ln (Ln=Eu3+, Tb3+) in the fields of luminescence, drug delivery, and disease therapy using their photoluminescence and biocompatibility.10 In their study, a part of OH- is also substituted with F- to enhance the stability of hydroxyapatites in acidic environment. Herein, we report the effect of OH- substitution on the red luminescence of Eu3+-doped Sr5(PO4)3Cl nanophosphors which can convert high energy photons to red light for fluorescent lamps, light emitting devices, and biosensors. Materials are synthesized via the microwave reaction because the microwave reaction is suitable for the synthesis of highly crystalline nanoparticles with a narrow size distribution due to a rapid and uniform heating. A direct agitation of the electric dipoles of precursors by microwave radiation provides a rapid and uniform heating.11-13 The anion composition is controlled by changing Sr/Cl ratio in a precursor solution. Changes in the crystal structure, the chemical composition, and the nanoparticle morphology of Eu3+-doped Sr5(PO4)3Cl(1-x)OHx are examined as a function of Cl/OH ratio. These changes have a strong impact on the luminescence of Sr5(PO4)3Cl(1-x)OHx:Eu3+. Addition of OH-



into Cl- site increases the intensity of red luminescence by 400 %. Our study suggests that a change in the anion composition is a promising way to enhance the luminescence of Sr5(PO4)3Cl:Eu3+ phosphors by decreasing the vacancy defect concentration.

2. Experimental In a typical procedure for the synthesis of Sr4.95(PO4)3Cl(1-x)OHx:Eu3+0.05 nanophosphors, Sr(NO3)2 (99.0 %, Sigma-Aldrich), NH4H2PO4 (99.0 %, Fluka), and Eu(NO3)3·6H2O (99.9 %, Acro Organics) were mixed in 50 mL of deionized water with magnetic stirring. After 30 minutes of stirring, total 1 mL of HCl (36.5~38.0 wt.%, Fisher Chemical) and HNO3 (68.0~70.0 wt.%, Fisher Chemical) were added to dissolve all sources and control the ratio of Cl- and OHions in final products. For pure strontium chlorapatite, Sr4.95(PO4)3Cl:Eu3+0.05, 1 mL of HCl was added. To replace Cl- with OH-, the amount of HCl in the precursor solution was decreased. pH of the precursor solution was maintained as 1.5 by adding HNO3. The volume ratio of HCl and HNO3 was 0.153 mL/0.847 mL, 0.136 mL/0.864 mL, 0.119 mL/0.881 mL, and 0.102 mL/0.898 mL for (Sr2++Eu3+):Cl- = 5:0.9, 5:0.8, 5:0.7, and 5:0.6, respectively. As a result, the molecular ratios of Sr:P:Cl:OH:Eu in the solution were designated as 4.95:3:(1-x):x:0.05 (x=0, 0.1, 0.2, 0.3, and 0.4). After all source materials were dissolved, the final pH of the solution was decreased to 9 by slowly dropping NH4OH (28.0~30.0 wt.%, Sigma-Aldrich). The obtained solution was stirred for 30 minutes and transferred into a 100 mL Teflon jar, and then reacted at 200 oC for 2 hours by applying microwave radiation. Microwave reaction was performed using MARS5 (CEM) under max 800 W of microwave radiation. After the microwave reaction, precipitates were separated by 7900 rpm of centrifugation for 30 min, washed with 25 mL of deionized water 3 times, and dried at 95 oC for 6 hours to obtain the final product. The samples were denoted as SPC1.0 (for Sr4.95(PO4)3Cl:Eu3+0.05), SPC0.9 (for Sr4.95(PO4)3Cl0.9OH0.1:Eu3+0.05), SPC0.8 (for


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Sr4.95(PO4)3Cl0.8OH0.2:Eu3+0.05), SPC0.7 (for Sr4.95(PO4)3Cl0.7OH0.3:Eu3+0.05), and SPC0.6 (for Sr4.95(PO4)3Cl0.6OH0.4:Eu3+0.05) according to the molar concentration of Cl- in the solution. All the chemicals were used without further purification. After the microwave reaction, the crystal structure of nanophosphors was investigated using Xray powder diffraction (XRD; PANalytical Empyrean). Data collection was performed in the 2θ range from 20 ° to 60 ° using Co Kα radiation. A scan rate was 0.11 °/s. Accelerating voltage and emission current was 40 kV and 45 mA. Raman spectra ranging from 500 to 3800 ∆cm-1 were collected by a Renishaw inVia Raman microscope equipped with 17 mW of 633 nm He-Ne laser as a light source. FTIR spectra of the synthesized nanophosphors were measured by using a Bruker Vertex-70 LS. Photoluminescence (PL) and decay time measurements were performed using spectrofluorometer (QuantaMaster, PTI). During PL measurement, the temperature of nanophosphors was controlled using a plate-type heating device. Excitation and Emission spectra were measured ranging from 350-450 nm and 550-750 nm, respectively, using 1 nm/s of scan rate. For decay time measurement, 392 nm of excitation wavelengths were used while emission was monitored at 618 nm. The morphology and composition of the samples were examined using a scanning electron microscope (SEM; FEI Apreo) equipped with an energy-dispersive X-ray spectrum (EDS, AMETEK Octane Elite Silicon Drift Detector).

3. Results and Discussion Figure 1(a) shows the XRD pattern of SPC1.0. Diffraction peaks in Figure 1(a) match well with peaks of Sr5(PO4)3Cl in JCPDS card (No. 70–1007). A crystal structure of Sr5(PO4)3Cl belongs to a hexagonal structure with P63/m (176) space group. The inset of Figure 1(a) shows the unit cell structure viewed along with c-axis. The second phase peaks are not detected, indicating that pure apatite particles are synthesized. Figure 1(b) shows PL excitation and



emission spectra of SPC1.0. PL spectra of SPC1.0 exhibit strong red luminescence between 570 nm and 720 nm by absorbing UV light. An excitation peak is observed at 392 nm assigned to 7

F0→5L6 transition of electrons. Red emission indicates that Eu is ionized as Eu3+ in Sr5(PO4)3Cl

structure during the microwave reaction. The strongest emission peak located at 618 nm is assigned to 5D0→7F2 transition of electrons in Eu3+ ions, and the other peaks at 593, 654, and 700 nm are related to the 5D0→7F1, 5D0→7F3, and 5D0→7F4 transitions of electrons in Eu3+ ions which are excited by UV of wavelength 392 nm.7 The thermal stability of SPC1.0 was shown in Figure S1. In Figure S1, the emission peak intensity of SPC1.0 was decreased by 24 % as the temperature of nanophosphors increased from room temperature to 165 oC.


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Figure 1. (a) XRD pattern (inset shows unit cell structure viewed along c-axis) and (b) PL spectra of Sr4.95(PO4)3Cl:Eu3+0.05 (SPC1.0).



Figure 2 shows Raman and FTIR spectra of phosphate powder as a function of Cl content in the precursor. In Raman spectra, the prominent peaks in the region from 500 to 1100 ∆cm-1 represent vibrations of P-O bond within (PO4)3- ion. The peak at 3590 ∆cm-1 is assigned as O–H stretching mode.14 This indicates that a decrease in Cl content causes the addition of OH- into phosphate powder during the microwave reaction. In FTIR spectra, the 950 cm-1 band results from the ν1 symmetric P-O stretching vibration. The strong bands at 1018 and 1072 cm-1 are due to the triply degenerate ν3 antisymmetric P-O stretching vibration of the (PO4)3- groups.15 The broad band centered at 3260 cm-1 is assigned to the O-H stretching mode of H bonded OH-. It is considered that the broad peak centered at 3260 cm-1 is due to H bond between H2O and OH- at the surface of synthesized particle. Therefore, the increase of peak intensity at 3260 cm-1 is related to the increase of OH- in the synthesized particles. The peak at 3590 ∆cm-1 of Raman spectra and the peak at 3260 cm-1 of FTIR confirm that OH- is incorporated into the phosphate powder which are synthesized in Cl- deficient environment.


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Intensity (arb. unit)

(a) SPC1.0 SPC0.8 SPC0.6

ν1 (PO43-)

ν3 (PO43-)

ν4 (PO43-)

ν (OH-)

600

800

1000 3400

3600

3800

Raman shift (∆cm-1)

(b)

Transmittance (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ν (OH-) ν1 (PO4-3)

SPC1.0 SPC0.8 SPC0.6

4000

3500

3000

ν3 (PO4-3)

1500 1200

900

600

Wavenumbers (cm-1) Figure 2. (a) Raman and (b) FTIR spectra of Sr4.95(PO4)3Cl(1-x)OHx:Eu3+0.05 phosphors which were synthesized in precursor solutions containing different Cl- and OH- content.



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To examine the long-range crystal structure of OH- added phosphate particles, XRD analysis was conducted. XRD patterns of synthesized powers which were obtained adding 1 mL of HCl/HNO3 mixture are shown in Figure 3. As Cl- content in the precursor solution decreases, diffraction peaks of synthesized powder become broad and peak intensities decreases. Figure 3(b) shows a change of (211), (112) and (300) peaks. In SPC1.0, (211) and (112) peaks are clearly separated and they are found at 2θ = 35.33 ° and 35.84 °. In SPC0.6, (211) and (112) peaks get closer and they are located at 35.43 ° and 35.74 °. In addition, (300) peak shifts from 36.58 ° for SPC1.0 to 36.78 ° for SPC0.6. In the hexagonal structure, lattice plane spacing and lattice parameters have following relations:

= ℎ + ℎ + +

(1)

From this relation, a- and c-axis lattice parameter is a=9.874 Å and c=7.196 Å for Sr5(PO4)3Cl and a=9.822 Å and c=7.249 Å for Sr5(PO4)3Cl0.6OH0.4. This shows that OH- in Sr4.95(PO4)3Cl0.6OH0.4:Eu3+0.05 shrinks the unit cell along a-axis and expands the unit cell along caxis. Since OH- ion is smaller than Cl- ion, a decrease in a-axis lattice parameter of SPC0.6 is explained by substitution of Cl- with OH-. However, a difference in the ion size does not account for an increase in c-axis lattice parameter of SPC0.6. An answer to this question is found in studies on the crystal structure of the apatite with different anions.16-19 Hughes et al. compared Ca5(PO4)3Cl, Ca5(PO4)3OH and Ca5(PO4)3F and found that an increase in the anion size caused more displacement of anion position along c-axis.19 While the displacement of OH- is only 0.35 Å above or below a mirror plane that is normal to c-axis, larger Cl- ions is displaced 1.2 Å. To release vast stress originating from significant Cl- displacement, anion vacancies are formed periodically along c-axis and c-axis lattice parameter is decreased in Ca5(PO4)3Cl. In contrast, in a case of Ca5(PO4)3OH, smaller OH- decreases not only a-axis lattice parameter but also anion


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displacement and internal stress along c-axis. Therefore, Ca5(PO4)3OH has shorter a-axis lattice parameter, less anion vacancy concentration, and longer c-axis lattice parameter than Ca5(PO4)3Cl. A similar anisotropic change of lattice parameters is observed in SPC1.0 and SPC0.6. Controlling Cl- concentration in precursor solutions of SPC1.0 and SPC0.6 leads to shrinkage of a-axis lattice parameter and elongation of c-axis lattice parameter in SPC0.6. Results of XRD, Raman and FTIR indicate that OH- replaces a significant amount of Cl- in SPC0.6 and the vacancy concentration of SPC0.6 is much smaller than that of SPC1.0.



(222) (302) (213) (321) (140) (402) (004)

(113)

(130)

(211) (112) (300)

(002)

SPC0.6

(102) (210)


(200) (111)

(a)

SPC0.7

SPC0.8

SPC0.9

SPC1.0

JCPDs # 70-1007

20

30

40

50

60

2θ (CoKα)

(300)

(112)

(211)

(b) Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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SPC0.6

SPC0.7

SPC0.8

SPC0.9

SPC1.0

JCPDs # 70-1007

35.0

35.5

36.0

36.5

37.0

2θ (CoKα) Figure 3. XRD patterns of Sr4.95(PO4)3Cl(1-x)OHx:Eu3+0.05 which were synthesized in precursor solutions containing different Cl- and OH- content: (a) 2θ range from 20 ° to 60 ° and (b) 2θ range from 35 ° to 37 °.


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Figure 4 shows PL spectra of Sr5(PO4)3Cl(1-x)OHx:Eu3+. It is noteworthy that the intensity ratio of 5D0→7F2 transition and 5D0→7F1 transition peaks (I618 nm/I593 nm) is almost same regardless of OH- addition. 5D0→7F2 transition of Eu3+ ions is due to electric-dipole radiation and is affected by the electric dipoles of host material. Since the magnitude of the electric dipole is sensitive to the local structure around Eu3+, 5D0→7F1 transition is an indicator of the local structural change. A change in the symmetry around Eu3+ ions changes luminescence intensity at 618 nm. In contrast, 5D0→7F1 transition is due to magnetic dipole radiation and affected only by the magnetic dipoles. Since a change in the local structure does not influence the strength of magnetic dipoles, 5D0→7F1 transition is insensitive to the local crystal structure. Therefore, the intensity ratio of 5D0→7F2 emission over 5D0→7F1 emission (I618 nm/I593 nm) strongly depends on the local symmetry around Eu3+ ions and is called an asymmetric factor.20 In Figure 4, I618 nm/I593 nm

of all SPC samples is maintained at 2, though they have different OH- content. Also, the

intensity ratio of other electric dipole transition, 5D0→7F4 transition to the 5D0→7F1 transition is same as 0.6 through all SPC samples. As a result, CIE 1931 coordinate of all samples is same as shown in the inset of Figure 4. This suggests that OH- addition into Sr5(PO4)3Cl:Eu3+ does not change the local structure around Eu3+ ions.



Excitation @ 618 nm

Emission @ 392 nm 5

7

D0 - F2


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SPC1.0 SPC0.9 SPC0.8 SPC0.7 SPC0.6 5

D0 - 7F1

5

D0 - 7F4

5

D0 - 7F3

350

400

550 600 650 700 750

Wavelength (nm) Figure 4. PL emission and excitation spectra of Sr4.95(PO4)3Cl(1-x)OHx:Eu3+0.05 which were synthesized in precursor solutions containing different Cl- and OH- content: inset exhibits the CIE 1931 coordinate of all samples.


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In the apatite structure, metal cations occupy two crystallographically nonequivalent sites (MI and MII). The first cation site (MI) has trigonal symmetry (C3), since MI sits at the center of a tricapped trigonal prism consisting of nine oxygen atoms. The second cation site (MII) has a coordination number of seven and is surrounded by six oxygen atoms and one X- ion such as OH- ion. Local symmetry of MII belongs to C1h (Cs) point group.21 Hence, addition of OH- ions changes the local crystal structure of MII site more than that of MI site. If Eu3+ ions substitute Sr2+ ions in MII site, an increase in OH- content of Sr5(PO4)3Cl(1-x)OHx decreases the local structural symmetry of Eu3+ at MII and results in change of I618 nm/I593 nm ratio. In contrast, Eu3+ ions at MI of Sr5(PO4)3Cl(1-x)OHx is not expected to change I618 nm/I593 nm ratio, because only oxygen ions surround cations in MI site. A constant I618 nm/I593 nm ratio in Figure 4 suggests that Eu3+ ions are preferentially incorporated into SrI site in Sr5(PO4)3Cl(1-x)OHx. Similar site preference of rare earth element ions in +3 state is reported in Ca10(PO4)6(OH)2. Eu3+ ions preferentially occupy MI site of Ca10(PO4)6(OH)2 and charge balance is maintained by codoping of NH4+ and Eu3+.22 Werts et al. presented the quantitative analysis of Eu3+ emissive properties in matrix by applying the Judd-Ofelt theory which explains emission from forbidden transitions in f-orbitals of trivalent lanthanide by dipole transition.23 Judd and Ofelt independently reported that large dipole strength increased the radiative process constant of 5D0→7FJ transition (AR) and calculated the emission probability using strengths of dielectric and magnetic dipoles:

=

!"# + $ !%# &

(2)

where ν is the average transition energy (in cm-1), h is the Planck constant (6.63×10-27 erg s), (2J+1) is the degeneracy of the initial state (1 for 5D0), n is the refractive index of medium, DED and DMD are the electric and magnetic dipole strengths (in esu2 cm2). In equation (2), external



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electromagnetic field is converted to an effective field which optically active centers in the dielectric medium experience. Among several 5D0→7FJ transitions of Eu3+, only 5D0→7F1 transition is due to magnetic dipole transition that is less sensitive to the ion’s surrounding as described above. For 5D0→7F1 transition of Eu3+, magnetic dipole strength (DMD) is calculated to be 9.6×10-42 esu2 cm2.23 Another 5D0→7FJ transitions such as 5D0→7F2 and 5D0→7F4 is enabled by intervention of electric dipoles. An increase in the electric dipole strength increases the spontaneous emission probability of 5D0→7F2 and 5D0→7F4 transitions of Eu3+. Electric dipole strength (DED) for 5D0→7Fj transition is given by: !"# = ' ∑*4, , )* +〈-.‖0 * 1-′.′〉+

(3)

where e is the elementary charge and +〈-.‖0 * 1-′.′〉+ are the squared reduced matrix

elements. Since +〈-.‖0 * 1-′.′〉+ is not influenced by the chemical environment of the ion, it is treated as a constant.24 This means that a phenomenological parameter (Ωλ) in equation (3) controls the electric dipole strength which determines the emission intensity of 5D0→7Fλ (λ=2, 4 and 6). Ωλ can be calculated from the intensity ratio of 5D0→7Fλ (λ=2, 4 and 6) and 5D0→7F1 transitions, as follows: )* =

#67 8 9 :

> ?: +〈;‖< : 1;==〉+ > ?8

(4)

νλ are the average transition energies obtained from emission spectra as an average over the selected transition region as follows: @* =

>: ? >: ?

A = 1, 2, 3,4

(5)

In this study, the refractive index (1.655) of Sr5(PO4)3Cl was used and the effect of wavelength and OH- addition on the refractive index was ignored. This is because the refractive index is almost constant in the wavelength region of interest (600-700 nm) and the refractive index of


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hydroxyapatite and chlorapatite are almost same.25 Table 1 shows that Ω2 for 5D0→7F2 transition is almost same in SPC1.0 and SPC0.8. Ω2 in our study is comparable to Ω2 in literature.23,24 Quantitative analysis of 5D0→7F1 and 5D0→7F2 transitions confirms that the addition of OH- ions does not change the site symmetry, DED and AR.

Table 1. Ω2, experimental decay lifetimes (τ), radiative process constant (AR) and emission quantum efficiency (η) ratio of SPC1.0 and SPC0.8. Samples

Ω2 (×10-20 cm2)

τ (ms)

AR (s-1)

SPC1.0

3.19

2.06

240

SPC0.8

3.13

4.34

228

ηSPC0.8⁄ηSPC1.0 2

In Figure 4, it is also noted that 20 % addition of OH- ions significantly increases PL intensity, though OH- ion is a well-known quencher for rare-earth activators.26,27 A change in PL intensity between SPC1.0 and SPC0.8 is not due to the traditional quenching effect of OH-. In addition, a negligible change in the site symmetry of Eu3+ indicates that 4 time difference in PL intensity between SPC1.0 and SPC0.8 cannot be explained by an increase in the electric dipole strength.28 Other important factors responsible for emission intensity of phosphors are optical activator concentration, amount of defects and morphology of particles.29,30 Results of EDS analysis in Table 2 shows that SPC0.6 has a much smaller ratio of Cl/(Sr+P) than SPC1.0. This suggests that



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OH- is incorporated into Cl- site in SPC0.6 and SPC0.8. The ratio of Eu/Sr is almost same for all samples, regardless of OH- addition into Cl- site. This excludes the possibility that PL intensity change is due to different activator concentrations. To further investigate a physical origin of PL intensity change in Sr4.95(PO4)3Cl(1-x)OHx:Eu3+0.05, the decay time of SPC1.0 and SPC0.8 was measured. Decay curves and calculated lifetimes are presented in Figure 5. PL decay curves of both samples were fitted using a following single exponential equation: J

F = FG + H' IK

(6)

where I is the luminescence intensity at time t and τ and B represent the decay time and constant. Since Eu3+ ions occupy only MI site, a decay curve was fitted by single exponential equation. SPC0.8 has longer decay time (4.34 millisecond) than SPC1.0 (2.06 millisecond). Relations between measured decay time (τ), radiative process rate constant (AR), non-radiative process rate constant (ANR) and emission quantum efficiency (η, the ratio of emitted photons to absorbed photons) are as follows: L = + M I N =

OP

OP OQP

RSTUV.X RSTU8.V

O

= L Y

= OP,STUV.X YSTUV.X P,STU8.V STU8.V

(7) (8) (9)

Equations (7) - (9) predicts that a difference in the decay time would make η of SPC0.8 larger than that of SPC1.0 by more than 2 times. This is consistent with experimentally measured PL of SPC0.8 and SPC1.0. XRD results in Figure 3 also suggest that increases in decay time and η of SPC0.8 are attributed to a decrease in non-radiative trapping site such as anion vacancies. A decrease in Cl- concentration causes anisotropic changes in the unit cell structure (shrinkage of aaxis lattice parameter and elongation of c-axis lattice parameter). When Cl- is partially replaced by OH-, OH- and Cl- align alternately along c-axis due to the size difference.19 This, in turn, 18 Environment ACS Paragon Plus

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suppresses the formation of anion vacancies which act as luminescent quenching sites.31 A higher vacancy concentration of SPC1.0 explains why SPC1.0 exhibits shorter decay lifetime and lower PL intensity than SPC0.8.

Table 2. Results of composition analysis by energy dispersive X-ray spectroscopy (EDS). Samples

Sr (At.%)

P (At.%)

Cl (At.%)

Eu (At.%)

Cl/Sr (%)

Eu/Sr (%)

SPC1.0

61.85

26.48

10.98

0.68

17.75

1.09

SPC0.8

62.39

28.79

8

0.83

12.82

1.31

SPC0.6

68.60

25.83

4.61

0.96

6.72

1.38

1.0

Normalized Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

Experimental Decay Single exponential Decay Fit

0.6

τSPC1.0 = 2.06 ms τSPC0.8 = 4.34 ms

0.4 0.2 0.0 0

5

10

15

20

Time (ms) Figure 5. PL decay time of Sr4.95(PO4)3Cl:Eu3+0.05 and Sr4.95(PO4)3Cl0.8OH0.2:Eu3+0.05 phosphors (SPC1.0 and SPC0.8).



After reaching to the maximum luminescence intensity, a further increase in OH- content decreases the luminescence intensity. Weaker PL of SPC0.6 is due to disordered atomic structure and particle shape change. First, PL intensity decrease in SPC0.7 and SPC0.6 is due to a decrease of crystallinity.32 As shown in XRD patterns of Figure 3, SPC0.7 and SPC0.6 exhibit smaller peak intensity and broader peak width. This indicates that OH- addition more than 20 mol% increases randomness of the atomic arrangement and local strain of the lattice. Both of them contribute to non-radiative relaxation of excited electrons in f-orbitals of Eu3+. Second, a reduced PL intensity is partially attributed to a change in the particle shape. SEM images in Figure 6 show that rod-shaped and nano-sized particles appear in OH- added Sr5(PO4)3Cl. An aspect ratio of SPC0.7 and SPC0.6 is larger than 5. A similar effect of OH- on the morphology was found in calcium phosphates.33 Since adsorption tendency of OH- is not same for all crystallographic planes of the apatite, a plane of less OH- adsorption grow faster than other planes and an anisotropic particle shape appear.33 Compared with rod particles, spherical particles have less internal reflectance and emit stronger light.34 This suggests that a change in the particle shape also contributes to a decrease of PL intensity in SPC0.7 and SPC0.6.


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Figure 6. SEM images and EDS spectra of Sr4.95(PO4)3Cl(1-x)OHx:Eu3+0.05: (a) SEM image of SPC1.0, (b) EDS spectrum of SPC1.0, (c) SEM image of SPC0.8, (d) EDS spectrum of SPC0.8, (e) SPC0.6 and (f) EDS spectrum of SPC0.6.



4. Conclusions A series of Sr4.95(PO4)3Cl(1-x)OHx:Eu3+0.05 nanophosphors are synthesized via the microwave reaction method. The substitution of Cl- ions by OH- ions was confirmed by Raman, FTIR, and XRD. When Eu3+ ions were doped, they occupied SrI sites of the Sr5(PO4)3Cl matrix to satisfy the charge compensation mechanism due to the existence of NH4+ ions, resulting in almost same I618 nm/I593 nm in PL emission. Also, Quantitative analysis of 5D0→7F1 and 5D0→7F2 transitions by Judd-Ofelt theory confirms that the addition of OH- ions does not change the site symmetry and electric dipole strength around Eu3+ activators. OH- addition until 20 % of Cl- ions significantly increased PL intensity 4 times. The decay time of SPC1.0 and SPC0.8 exhibited that addition of OH- ions increased of emission quantum efficiency (η) by decrease in non-radiative trapping site such as point defects. PL intensity was the highest in SPC0.8 and further replacement of Cl- by OH- decreased PL intensity. XRD and SEM analysis showed that an increase in the atomic arrangement randomness and a change in the particle shape reduced PL intensity of SPC0.6. The results show that a way to fabricate nanophosphors for the potential applications of fluorescent lamps, light emitting devices, and biosensors.

ASSOCIATED CONTENT Supporting Information The following files are available free of charge. The relative emission intensity of SPC1.0 as a function of the temperature (DOC).

AUTHOR INFORMATION Corresponding Author * Jung-Kun Lee (Tel: +1-412-648-3395; E-mail: [email protected]).


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2016M1A2A2940138) and National Science Foundation (NSF 1709307). Notes The authors declare no competing ﬁnancial interest.



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Table of Contents

A series of Sr5(PO4)3Cl(1-x)OHx:Eu3+ nanophosphors were synthesized by a microwave reaction. Eu3+ activators were preferentially located at SrI sites of Sr5(PO4)3Cl due to its effective charge compensation. OH- substitution reduced vacancy defects and increased the intensity of red luminescence by 4 times.


Enhanced Red Light Emission of OH added Sr5(PO4)3Cl:Eu

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