Red-Emitting

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Controllable Eu2+-Doped Orthophosphate Blue-/Red-Emitting Phosphors: Charge Compensation and Lattice-Strain Control Yi Wei,† Junsong Gao,† Gongcheng Xing,† Guogang Li,*,† Peipei Dang,‡ Sisi Liang,‡ Yu Shu Huang,§ Chun Che Lin,*,§ Ting-Shan Chan,Δ and Jun Lin*,‡,#

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/23/19. For personal use only.

†

Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, 388 Lumo Road, Wuhan 430074, P. R. China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China § Graduate Institute of Nanomedicine and Medical Engineering and International Ph.D. Program in Biomedical Engineering, College of Biomedical Engineering, Taipei Medical University, Taipei 110, Taiwan Δ National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan # School of Applied Physics and Materials, Wuyi University, Jiangmen, Guangdong 529020, P. R. China S Supporting Information *

ABSTRACT: Cation-substitution-induced controllable luminescence tuning could efficiently optimize and improve the luminescence performances of novel phosphor materials for realizing high-quality lighting. As important members of the orthophosphate family, ABPO4 (A = alkali metal Li, Na, K, Rb, Cs; B = alkali earth metal Mg, Ca, Sr, Ba) offers an abundant cation lattice environment for rare earth ions. Herein, we successfully prepared a broad-band red-emitting CsMgPO4:Eu2+ phosphor with an emission peak at 628 nm (fwhm = 118 nm). A series of cation-substitution strategies are designed to adjust and enhance its luminescence performances. The corresponding mechanisms are also investigated and proposed reasonably. A charge-compensation strategy of [Eu2+-Si4+] → [Cs+-P5+] could dramatically enhance the quenching concentration from 0.04 to 0.30, which is attributed to the decrease of Eu3+. Two cation-substitution strategies of larger Ba2+ (Sr2+) ions for Mg2+ ions could achieve superior emission adjustment of Eu2+ ions from the red to blue (yellow) region due to local lattice distortion. Interestingly, a consecutive emission adjustment from the red to blue region by simply changing the annealed temperature is reported for the first time, and the possible emission tuning mechanism is revealed based on a local lattice-strain control. This study could serve as a guide in developing Eu2+-activated ABPO4 phosphors with improving luminescence performance and controllable luminescence adjustment based on charge compensation and lattice-strain control through various cation substitutions.

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INTRODUCTION In the past two decades, phosphor-converted white light emitting diodes (pc-WLEDs) have gradually replaced traditional incandescent and fluorescent lamps in people’s lives, which offer many advantages, such as high luminous efficiency, low energy consumption, and environmental friendliness.1−6 Rare earth ions activated inorganic phosphor materials with high chemical stability and convenient synthesis have been a hot issue to achieve high-quality white illumination with superior color uniformity and excellent color rendering index (CRI). To date, many inorganic compounds have been developed as excellent matrix materials, which are usually categorized into oxides, aluminates, silicates, borates, phosphates, and so on.7 Among various phosphate systems, ABPO4 (A = alkali metal Li, Na, K, Rb, Cs; B = alkali earth metal Mg, Ca, Sr, Ba) as one typical orthophosphate has been extensively exploited as a matrix for doping various rare earth ions (Ce3+, Eu2+/3+, Tb3+, Dy3+, Sm3+, Tm3+) to apply in lighting and display areas.8−11 For example, Lin et al. reported the crystal © XXXX American Chemical Society

structure and luminescence properties of a versatile phosphate phosphor ABPO4:RE (A = Li, K; B = Sr, Ba; RE = Eu2+, Tb3+, and Sm3+) with the density functional theory method.12 Zhang et al. presented an insightful investigation for the luminescence properties of LiBaPO4:Eu2+, which has low thermal quenching.13 Nair et al. reported the theoretical analysis of the electron-vibrational interaction in 4f−5d transitions of Eu2+ ions in an ABaPO4 (A = Li, Na, K, and Rb) system.14 In general, most previous works have realized blue and green emission of ABPO4:Eu2+ in response to n-UV excitation, such as KSrPO 4 :Eu 2+ (blue), KCaPO 4 :Eu 2+ (blue-green), KMgPO4:Eu2+ (blue), LiBaPO4:Eu2+ (blue), and LiCaPO4:Eu2+ (blue).13,15−17 However, it is difficult to obtain red-emitting light in ABPO4:Eu2+ due to the weak nephelauxetic effect induced by the low charge density of phosphate matrixes.18 Received: February 26, 2019

A

DOI: 10.1021/acs.inorgchem.9b00577 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

atmosphere. After being cooled down to room temperature, the annealed powders were ground again to achieve the resulting phosphor powders for further measurements. LEDs Fabrication. Blue LEDs were fabricated by combining the representative CsMg0.6Ba0.4PO4:0.04[Eu2+-Si4+] (CsMgPO4:0.04Eu2+1200 °C) phosphors and 370 nm InGaN chips. Red LEDs were fabricated by combining Cs 0 . 7 MgP 0 . 7 O 4 :0.30[Eu 2 + -Si 4 + ] (Cs0.96MgPO4:0.04Eu2+-1100 °C) and 370 nm n-UV InGaN chips. pc-WLEDs were fabricated by combining blue CsMg0.6Ba0.4PO4:0.04[Eu2+-Si4+], green Ba3Si6O12N2:Eu2+, red Cs0.7MgP0.7O4:0.30[Eu2+Si4+], and 370 nm n-UV InGaN chips. The proper amounts of phosphors were added into the epoxy resins and mixed for 20 min. The acquired mixture was coated on 370 nm InGaN chips and dried at 70 °C to produce LEDs. All measurements were carried out at a 200 mA driven current. Characterization. XRD powder patterns were collected using a D8 Advance diffractometer (Bruker Corporation, Germany) at a scanning rate of 10° min−1 in the 2θ range from 10° to 120° with monochromatized Cu Kα radiation (λ = 1.5406 Å). Synchrotron XRD data were recorded with wavelength λ = 0.774907 Å and a Debye−Scherrer camera installed at the BL01C2 beamline of the National Synchrotron Radiation Research Center (NSRRC). Rietveld profile refinements of the structural models and texture analysis were performed by GSAS (General Structure Analysis System) software. Solid state CPMAS NMR spectra were obtained with the use of a Bruker DSX 300 MHz NMR spectrometer. Diffuse reflectance spectra (DRS) were measured by UV−visible diffuse reflectance spectroscopy UV-2550PC (Shimadzu Corporation, Japan). Photoluminescence excitation (PLE) and emission (PL) spectra were performed by Edinburgh FLSP-920 fluorescence spectrometer equipped with a 450 W xenon lamp as the excitation source. Both excitation and emission spectra were recorded with 1.0 nm interval with the width of monochromator slits adjusted to 0.50 nm. The photoluminescence quantum yield (QY) was collected by an absolute PL quantum yield measurement system C9920-02 (Hamamatsu photonics K.K., Japan). XANES of the Eu L3 edge was recorded with a wiggler beamline BL17C at National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. Temperature-dependent PL spectra from 25 to 250 °C with an interval of 25 °C were obtained by Fluoromax-4P spectrometer (Horiba Jobin Yvon, New Jersey, USA) connected to heating equipment (TAP-02). The Commission Internationale de l’Eclairage chromaticity color coordinates, color rendering index (Ra), and CCT of WLED devices were measured by Starspec SSP6612.

When lacking a red component, pc-WLEDs may emit cold white light, which is harmful to people’s eyes. Therefore, a redemitting phosphor is an indispensable component to improve the luminous efficiency and CRI. The most reported redemitting ABPO4 phosphor usually originates from Eu3+ ions. However, the characteristic 4f−4f transitions of Eu3+ ions are spin and parity forbidden, and thus it presents weak absorption in the n-UV region. So Eu3+ could not be successfully utilized in the pursuit of warm white-emitting pc-WLEDs.19,20 A broad-band, red-emitting ABPO4:Eu2+ phosphor is urgently required. Because of the sensitive 4f−5d electron transition in different crystal field environments, Eu2+ has been considered as the most common activator for the tunable emission in the whole visible region.21 Recently, Zhao et al.22 and Ma et al.23 discovered a red-emitting Cs1−xMgPO4:xEu2+ phosphor with the optimal emission peak located at 630 nm under 340 nm nUV excitation. The reported CsMgPO4 matrix is indexed to an orthorhombic structure with space group P1n1, and Eu2+ ions are proved to occupy Cs+ ions sites through Rietveld refinement. However, the relationship of structure and luminescence properties is not revealed deeply. Moreover, to meet more requirements for practical applications, the controllable luminescence adjustment has attracted a lot of attention.24 Cation substitution is a considerably efficient strategy, which could widely tune luminescence by changing the local lattice strain. Inspired by the above works, we successfully synthesized red-emitting Cs1−xMgPO4:xEu2+ phosphors and designed a series of cation-substitution strategies to eliminate charge defects and modify the lattice environment for improving luminescence performances and realizing tunable emission. In this work, the orthorhombic Cs1−xMgPO4:xEu2+ structure with Pnma space group is synthesized for the first time, and its crystal structure is analyzed by Rietveld refinement based on synchronous X-ray diffraction (XRD) patterns. To overcome charge defects caused by the unequal occupation of Eu2+ ions at Cs+ ions sites, the cosubstitution of [Eu2+-Si4+] for [Cs+-P5+] is introduced, and its effect on the luminescence is investigated in detail. Another cation-substitution of Ba2+ (Sr2+) ions for Mg2+ ions is also designed to achieve superior emission spectra and tune the color from red to blue (yellow) in CsMgPO4:Eu2+ systems. Interestingly, a tunable emission from red to blue is achieved by simply changing the annealed temperature, which is possibly attributed to lattice-strain control in the local coordination environment. Generally, improved luminescence performance and controllable luminescence tuning are successfully achieved through various cation substitutions in Eu2+-activated orthophosphate phosphors, which provide guidance in developing novel rare earth ion-doped phosphate phosphors for pc-WLEDs.

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RESULTS AND DISCUSSION

Charge-Compensation-Induced High-Concentration Doping. Previous works have reported a red-emitting CsMgPO4:Eu2+ phosphor, which crystallized in orthorhombic structure with space group P1n1. Herein, it is the first time to report a CsMgPO4:Eu2+ powder, which is indexed to an orthorhombic structure with space group Pnma (No. 62). The crystal structure of CsMgPO4 matrix is presented in Figure 1a,b, and there exist one Cs, Mg, P crystallographic site and three O crystallographic sites, respectively. [PO4] tetrahedra and [MgO4] tetrahedra connect with each other by sharing a vertex O atom, forming a three-dimensional framework. Degree of condensation k = 0.5, indicating that the steric configuration could be easily adjusted by a cation-substitution strategy. From the b-axis direction, two centrally antisymmetric [CsO6] polyhedra share an edge, forming a [Cs2O10] group. Next, the [Cs2O10] group locates in a dreier ring with alternately linking four [PO4] tetrahedra and four [MgO4] tetrahedra. From the a-axis direction, [CsO6] polyhedra locate in the hexagonal tunnel running with vertex-sharing five [PO4] tetrahedra and one [MgO4] tetrahedron. In addition, [CsO6] polyhedron connects with [PO4] tetrahedron by sharing vertex

EXPERIMENTAL SECTION

Materials and Preparation. Cs1−xMgPO4:xEu2+ (x = 0−0.20), Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0−0.50), and CsMg1−yMyPO4:0.04[Eu2+-Si4+] (M = Sr, Ba; y = 0−0.40) samples were prepared by a traditional high-temperature solid state reaction. Stoichiometric amounts Cs2CO3 (A.R., Aladdin), 4MgCO3·Mg(OH)2·5H2O (A.R., Aladdin), (NH4)2HPO4, (A.R., Aladdin), Eu2O3 (A.R. SigmaAldrich), SiO2 (A.R. Sigma-Aldrich), BaCO3 (A.R. Sigma-Aldrich), and SrCO3 (A.R. Sigma-Aldrich) were mixed and ground together for 30−50 min. Then, the obtained precursors were put into an oven for 12 h drying at 80 °C. Next, the dry powders were transferred into aluminum oxide crucibles and annealed in a horizontal tube furnace at 1100−1200 °C for 3 h under N2/H2 (90%/10%) reduced B

DOI: 10.1021/acs.inorgchem.9b00577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Si4+] samples are summarized in Table S1 (Supporting Information). Moreover, the crystallographic data of Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.004, 0.30) samples are presented in Table 1, and the site occupation of Table 1. Crystallographic Data of Cs1−xMgP1−xO4:x[Eu2+Si4+] (x = 0.004, 0.30) Samples Based on Rietveld Refinement from Synchrotron XRD Data atom

x

y

z 2+

Figure 1. Schematic crystal structure of CsMgPO4 (a) from the b-axis direction, and (b) from the a-axis direction. (c) The XRD patterns of Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.005−0.5). (d) Rietveld refinement of synchrotron XRD data for Cs0.96MgP0.96O4:0.04[Eu2+-Si4+].

O atoms, while [CsO6] polyhedron connects with [MgO4] tetrahedron by sharing a vertex as well as an edge. Considering the charge imbalance and lattice size mismatch caused by bivalent Eu2+ (r = 1.17 Å, CN = 6; where r is ions radius, CN represents coordinate number) substituting monovalent Cs+ ions (r = 1.67 Å, CN = 6), some defects would be generated. The possible substitution scenarios can be described as follows: Eu → EuCs + 1/2VO (neighbor, interstitial)

(i)

Eu → EuCs + VCs

(ii)

Eu → EuCs + CsMg + Mg

0.4950 0.3169 0.2014 0.2780 0.2595 0.0323

Cs1 Eu1 Mg1 P1 Si1 O1 O2 O3

0.4952 0.4952 0.3157 0.1955 0.1955 0.2807 0.2622 0.0138

Uiso (Å2)

Cs0.96MgP0.96O4:0.04[Eu -Si ] 0.2500 0.7034 0.2500 0.0824 0.2500 0.4173 0.2500 0.2653 0.0222 0.4887 0.2500 0.4185 Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] 0.2500 0.7033 0.2500 0.7033 0.2500 0.0862 0.2500 0.4257 0.2500 0.4257 0.2500 0.2839 0.0298 0.4924 0.2500 0.4109

1.00 1.00 1.00 1.00 1.00 1.00

0.0309 0.0009 0.0256 0.0761 0.0477 0.0505

0.69 0.29 1.00 0.75 0.32 1.06 1.03 0.99

0.0274 0.0218 0.0048 0.0578 0.0633 0.0443 0.0494 0.0318

Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] sample shows no Eu2+ and Si4+ trace due to the low doping concentration. For Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] sample, Eu2+ ions occupy Cs+ ions sites at x = 0.4952, y = 0.2500, z = 0.7033, and Si4+ ions occupy P5+ ions sites at x = 0.1955, y = 0.2500, z = 0.4257. Moreover, the refined Eu2+ and Si4+ ion-doping level is 0.29 and 0.32, respectively, which is close to the theoretical value (0.30). Consequently, Eu2+ ions and Si4+ ions successfully occupy Cs+ sites and P5+ sites, forming a solid solution phase. The refined bond length information on Cs−O, Mg−O, P−O of Cs0.96MgPO4:0.04Eu2+ and Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] samples are exhibited in Table S2 (Supporting Information), indicating that the local lattice structure slightly changes with the cosubstitution of [Eu2+-Si4+]. To further determine the successful incorporation of Si4+ ions into the P5+ sites, solid state NMR spectra of Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.005−0.5) samples are collected in Figure 2. At low doping concentration (x < 0.10), only one peak locates at around 8.11 ppm (marked with light gray dashed rectangle), which is assigned to [31PO4] tetrahedra. As the substitution content of [Eu2+-Si4+] increases, the characteristic peaks located at −42.2 and 57.0 ppm (marked with gray dashed lines) of [29SiO4] appear.25 The peak of [31PO4] gradually broadens, indicating the local lattice distortion surrounding [PO4] tetrahedra. These results strongly suggest that the [Eu2+-Si4+] ions successfully dope in the CsMgPO4 matrix and form a solid solution phase. Simultaneously, [PO4] tetrahedra were slightly distorted with the incorporation of [SiO4] tetrahedra. Usually, a wide opticalband gap is a prerequisite for efficient luminescence of activator ions in a matrix.26−28 The optical bandgap (Eg) of CsMgPO4 matrix could be determined by linear extrapolation based on the diffuse reflection spectra. According to the previous report, the optical bandgap (Eg) could be obtained by the following equation:29,30

(iii) 2+

Cs1 Mg1 P1 O1 O2 O3

occupy 4+

+

where EuCs (CsMg) denotes the substitution of Eu (Cs ) for Cs+ (Mg2+) site, VO represents the neighbor or interstitial O vacant defect, and VCs is adjacent Cs vacant defect. To solve the possible O or Cs vacant defects, cation cosubstitution of [Eu2+-Si4+] for [Cs+-P5+] is constructed. Fro m Figure 1c, all XRD d iffract ion peaks of Cs1−xMgP1−xO4:x[Eu2+-Si4+] can match well with the orthorhombic CsMgPO4 (ICSD No. 260423) until x = 0.5, indicating that the as-prepared samples are pure phases. The cosubstitution of [Eu2+-Si4+] does not influence the phase purity and the whole crystal structure of CsMgPO4 matrix. To further validate the crystal structure, the high-quality synchrotron XRD data of the representative Cs0.96MgPO4:0.04Eu2+ and Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] samples are collected for structural refinement. The Rietveld refinement results are plotted in Figure 1d and Figure S1 (Supporting Information), and the acceptable R-factors (Rwp = 0.07, Rp = 0.05, χ2 = 2.27 for Cs0.96MgP0.96O4:0.04[Eu2+-Si4+], Rwp = 0.09, Rp = 0.05, χ2 = 3.46 for Cs0.96MgPO4:0.04Eu2+) also confirm the formation of solid solution. The crystallization degree for samples could slightly improve with codoping Si4+ ions for charge compensation. The refined lattice parameters for Cs0.96MgPO4:0.04Eu2+ and Cs0.96MgP0.96O4:0.04[Eu2+C

DOI: 10.1021/acs.inorgchem.9b00577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Cs0.96MgPO4:0.04Eu2+ and Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] samples present typically broad red emission of Eu2+ ions (5d−4f transitions) from 500 to 800 nm with the peak at 622 nm. Moreover, there is no spectral overlap in both samples. The shape and position for Cs0.96MgPO4:0.04Eu2+ and Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] are basically unchanged, but the emission intensity with charge compensation is obviously higher than that of Cs0.96MgPO4:0.04Eu2+ sample (Figure 4a). Figure 4b shows the normalized emission spectra of Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0−0.5) samples. There are no changes in the shape, position, and fwhm (full width at halfmaximum, 118 nm) with the increase of [Eu2+, Si4+]-doping concentration from 0 to 0.5. However, the quenching concentration of Eu2+ ions is evidently enhanced from 0.04 to 0.30 with using Si4+ ions for charge compensation, as shown in Figure 4c and Figure S2 (Supporting Information). Thus, the cation-substitution of [Eu2+-Si4+] for [Cs+-P5+] greatly increases the tolerance of the matrix lattice to Eu2+ ions. A possible reason to depict the above phenomenon is that the [Eu2+-Si4+] codoping strategy could decrease the formation of the charge vacant defect and simultaneously modify the lattice distortion due to the replacement of Cs+ by Eu2+ through the introduction of Si4+ ions. The CIE chromaticity coordinate position of representative Cs 0.96 MgPO 4 :0.04Eu 2+ and Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] in Figure 4d reveals excellent red emission. A slight redshift occurs from (0.574, 0.420) to (0.598, 0.399) after designing charge compensation with Si4+ ions, which is consistent with PL spectra. To further confirm the valence of Eu in CsMgPO4, X-ray absorption near-edge structure (XANES) of Eu L3-edge for the studied samples is performed and shown in Figure 5. For the Cs0.96MgPO4:0.04Eu2+ sample, two evident peaks at 6975.2 and 6983.2 eV are attributed to 2p3/2-5d electron transitions of E u 2 + an d E u 3 + , r e s p e c t i v e l y . 3 3 , 3 4 A l t h o u g h t h e Cs0.96MgPO4:0.04Eu2+ is prepared under a reducing atmosphere and the main emission belongs to Eu2+ ions, there are still many residues of Eu3+ ions here. This is also a reason for the low efficiency of Eu2+ red emission. By employing the [Eu2+-Si4+] → [Cs+-P5+] cation substitution for charge compensation, the content of Eu3+ ions in the host could be markedly reduced (Figure 5). Therefore, the emission intensity of Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] is much higher than that of Cs0.96MgPO4:0.04Eu2+ (Figure 4a). This result indicates that the cosubstitution-induced charge-compensation in the current

Figure 2. NMR spectra of Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.005− 0.5) samples.

[hvF(R )]1/2 = A(hv ‐ Eg )

(1)

where hv represents the photon energy, R is the reflectance (%) coefficient, and F(R) is the Kubelka−Munk absorption function, in which F(R) = (1 − R)2/2R.31,32 As shown in Figure 3a, the calculated Eg is 6.08 eV, elucidating that CsMgPO4 is a superb matrix for Eu2+ ions doping. Figure 3b displays the diffuse reflectance spectra of representative Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.04, 0.20) samples. Obviously, the strong absorption in the n-UV to blue light region from 200 to 500 nm could be observed due to the 4f− 5d transition of Eu2+ ions, which matches well with the n-UV and blue InGaN chip based pc-WLEDs. Charge defects in phosphor materials easily generate luminescence quenching due to the increase of nonradiative transition. Appropriate charge-compensation could efficiently decrease and even eliminate the charge imbalance and enhance the luminescence efficiency of phosphor. Figure 4 discusses the effect of the cation-substitution of [Eu2+-Si4+] for [Cs+-P5+] on the luminescence properties of Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0−0.5). The photoluminescence excitation (PLE) spectra of Cs0.96MgPO4:0.04Eu2+ and Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] samples in Figure 4a locate in the n-UV region from 200 to 500 nm with the peak position at 350 nm due to the 4f−5d electron transition of Eu2+ ions, which could match well with the n-UV InGaN LED chip. The result coincides with that of diffuse reflectance spectra. Under 350 nm n-UV irradiation,

Figure 3. (a) The calculated optical bandgap based on diffuse reflectance spectra of CsMgPO4 matrix. (b) The diffuse reflectance spectra of representative Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.04, 0.20) samples. D

DOI: 10.1021/acs.inorgchem.9b00577 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. (a) The photoluminescence excitation (PLE, λem = 625 nm) and emission spectra (PL, λex = 350 nm) of Cs0.96MgPO4:0.04Eu2+ and Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] samples. (b) The normalized emission spectra of Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0−0.5) samples. (c) The integrated emission intensity as a function of x in Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.005−0.5) samples. (d) The CIE chromaticity coordinates diagram of Cs0.96MgPO4:0.04Eu2+ (marked as none) and the representative Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] (marked as 0.30).

[Eu2+-Si4+] (y = 0.1−0.35) samples. The diffraction peaks of as-prepared CsMg1−ySryPO4:0.04Eu2+, Si4+ (y = 0.1−0.35) samples are well ascribed to the orthorhombic CsMgPO4 phase (ICSD No. 260423), demonstrating the formation of the pure phase. Figure 6 presents the photoluminescence properties of CsMg1−ySryPO4:0.04[Eu2+-Si4+] (y = 0.1−0.35) samples. The PLE spectra of the representative CsMg0.75Sr0.25PO4:0.04[Eu2+-Si4+] sample give a broad-band emission from 250 to 500 nm, which is consistent with the non-Sr2+ doped sample. Its diffuse reflectance spectra also manifest a strong absorption in the n-UV region in Figure S4 (Supporting Information). Upon excitation with 350 nm n-UV light, the PL spectra of CsMg1−ySryPO4:0.04[Eu2+-Si4+] (y = 0.1−0.35) samples are collected in Figure 6b. As Sr2+ ions replace Mg2+ ions, the emission intensities are remarkably improved to arrive at the maximum until y = 0.25. Subsequently, the emission intensities slightly decrease from y = 0.25 to y = 0.35. Except for the variation in emission intensity, the emission peak position shifts from 623 to 628 nm for y = 0.2 and then gradually returns to 622 nm for y = 0.35. Interestingly, the fwhm could be efficiently tuned with Sr2+ ions gradually replacing Mg2+ ions. When Sr2+ ions concentration is beyond 0.25, the emission band abruptly broadens, which increases from 115 nm for y = 0−0.2 to 152 nm for y = 0.35. The details of emission peak and fwhm are plotted in Figure 6c. And the CIE chromaticity coordinates of CsMg1−ySryPO4:0.04[Eu2+-Si4+] (y = 0−0.35) samples reveal that the emission continuously shifts from the red (0.509, 0.369) to orange (0.497, 0.449) region, mainly resulting from the widening of the emission spectra in Figure 6d. The luminescence photographs in the inset of

Figure 5. Eu L3-edge X-ray absorption near-edge structure (XANES) spectra of the representative Cs 0.96 MgPO 4 :0.04 Eu 2+ and Cs0.96MgP0.96O4:0.04[Eu2+-Si4+] samples.

system is efficient and could distinctly improve the luminescence. Cation-Substitution-Induced Luminescence Tuning. As mentioned above, except the charge defects, the adjacent [MgO4] tetrahedra may be distorted due to the mismatch of ion size with Eu2+ doping in the Cs+ site. Conversely, the distortion of neighboring [MgO4] tetrahedra also changes the coordination environment and further influences the luminescence of Eu2+ ions. Therefore, we intentionally altered the distortion of [MgO4] tetrahedra by gradually substituting Mg2+ ions using a larger cation (Ba2+/Sr2+). Figure S3a (Supporting Information) plots the XRD patterns of CsMg1−ySryPO4:0.04E

DOI: 10.1021/acs.inorgchem.9b00577 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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Figure 6. (a) The PLE spectra of the representative CsMg0.75Sr0.25PO4:0.04[Eu2+-Si4+] sample (λem = 630 nm). (b) The PL spectra of CsMg1−ySryPO4:0.04[Eu2+-Si4+] (y = 0−0.35) samples (λex = 350 nm). (c) The fwhm (magenta line) and emission peak (blue line) as functions of Sr2+ ions doping concentration (y) in CsMg1−ySryPO4:0.04[Eu2+-Si4+] (y = 0−0.35) samples. (d) CIE chromaticity coordinates of CsMg1−ySryPO4:0.04[Eu2+-Si4+] (y = 0−0.35) samples, and the insets are the luminescence photographs of the representative y = 0, 0.2, 0.3, 0.35 samples under 365 nm n-UV light.

Figure 6d confirm the bright red emission and subsequent tuning to the orange region. Emission spectra of Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.005−0.5) and CsMg1−ySryPO4:0.04[Eu2+-Si4+] (y = 0−0.2) samples are highly symmetric due to the only one [CsO6] polyhedron for Eu2+ ions occupying. Nevertheless, the emission spectra of CsMg1−ySryPO4:0.04[Eu2+-Si4+] (y = 0.25−0.35) are obviously asymmetric due to the broadened fwhm, implying the appearance of new Eu2+ luminescence center. Figure 7 demonstrates that the emission band of the representative CsMg0.75Sr0.25PO4:0.04[Eu2+-Si4+] could be well fitted into two Gaussian peaks, in which the Eu1 emission peak locates at 1.984 eV (625 nm), and Eu2 emission peak locates at 2.309 eV (537 nm). Thus, we infer that other distorted Cs+ sites are generated as Sr2+ ions doping. To clarity the abrupt presence of Eu2 emission center, the much bigger Ba2+ ions are then designed to replace Mg2+ ions. The XRD patterns of CsMg1−zBazPO4:0.04[Eu2+-Si4+] (z = 0.1−0.4) samples indicate that the as-prepared Ba2+-substituted samples could form pure CsMgPO4 phase (ICSD No. 260423) before z = 0.4 (Figure S3b, Supporting Information). The photoluminescence properties of CsMg1−zBazPO4:0.04[Eu2+-Si4+] (z = 0−0.4) samples are depicted in Figure 8a−c. Similarly, the PLE spectra also show 250−400 nm broad-band emission with the maximum at 350 nm due to the electron transition 4f−5d level of Eu2+ ions. The diffuse reflectance spectra of the representative CsMg1−zBazPO4:0.04[Eu2+-Si4+] (z = 0.2, 0.3) samples confirm the strong absorption in the nUV (200−450 nm) region (Figure S4, Supporting Information). Surprisingly, the excitation peak intensity gradually increases as the Ba2+ ion concentration increases in the whole

Figure 7. Gaussian fitting emission spectra (λex = 350 nm) of the representative CsMg0.75Sr0.25PO4:0.04[Eu2+-Si4+] sample.

Ba2+-doped series. When exciting with 350 nm n-UV light, the emission spectra of CsMg1−zBazPO4:0.04[Eu2+-Si4+] (z = 0.1− 0.4) samples exhibit two symmetric emission bands from 400 to 750 nm, as presented in Figure 8b. One is the previous redemitting band from 500 to 800 nm with the peak at 625 nm, fwhm = 105 nm. Another new emission band locates in the violet-blue region (400−550 nm) with the peak at 419 nm, fwhm = 35 nm. Clearly, there exists only one red-emitting center without Ba2+ ions doping, and its emission intensity is first increased until z = 0.15 and subsequently decrease. As F

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Figure 8. (a) The PLE spectra of CsMg1−zBazPO4:0.04[Eu2+-Si4+] (z = 0.1−0.4) samples under 420 nm excitation. (b) The PL spectra of CsMg1−zBazPO4:0.04[Eu2+-Si4+] (z = 0−0.4) samples (λex = 350 nm). The inset is the emission peak intensity of blue-violet emission (green line) and orange-red emission (orange line) vs Ba2+-doping concentration z. (c) CIE chromaticity coordinates of CsMg1−zBazPO4:0.04[Eu2+-Si4+] (z = 0−0.4) samples (λex = 350 nm), and the corresponding luminescence photographs under natural light and 365 nm irradiation. (d) The schematic mechanism diagram for spectral tuning with Sr2+/Ba2+ ions substituting Mg2+ ions in CsMg1−yMyPO4:0.04[Eu2+-Si4+] (M = Sr2+, Ba2+) phosphors.

Figure 9. (a) XRD patterns and (b) NMR spectra of Cs0.96MgPO4:0.04Eu2+ at different annealed temperatures (1100 °C, 1150 °C, 1200 °C). The standard XRD card of CsMgPO4 (ICSD No. 260423) is shown as a reference. (c) PLE and (d) PL spectra of Cs0.96MgPO4:0.04Eu2+ at different annealed temperatures (1100 °C, 1150 °C, 1200 °C) (λex = 350 nm).

G

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Figure 10. Temperature-dependent emission spectra of representative (a) Cs0.96MgPO4:0.04Eu2+ and (b) Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] samples from 25 to 250 °C, where the red region represents the highest level and the blue region represents the lowest level. (c) The integrated emission intensity as a function of temperature in Cs0.96MgPO4:0.04Eu2+ (marked as none) and Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0.01−0.30) samples. (d) The emission peak position and fwhm as a function of temperature for Cs0.96MgPO4:0.04Eu2+ (marked as none) and Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] (marked as 0.03) samples.

Ba2+ ions are gradually introduced, the violet-blue emission appears and the emission intensity continuously elevates. Furthermore, the internal quantum efficiency (IQE) is remarkably enhanced from 34.3% for z = 0 to 63.9% for z = 0.4, indicating that cation-substitution strategy could efficiently improve lighting quality as well as spectral adjustment. The CIE chromaticity coordinates of Ba2+-doped samples in Figure 8c further confirm the dramatic shift from the red region (0.510, 0.369) to the blue region (0.194, 0.042). The luminescence photographs under 365 nm n-UV light also indicate that the emission colors have been continuously tuned from red emission to highly efficient blue emission. According to the above results, cation substitution of Sr2+/ Ba2+ ions for Mg2+ ions could induce the intriguing spectral adjustment, which should be related to coordination environment variation in the local lattice. Moreover, the incorporation of Ba2+ ions could achieve a more remarkable effect in tuning emission than Sr2+ ions. A possible mechanism is proposed in Figure 8d. As the larger Sr2+ ions (r = 1.18 Å, CN = 6) or Ba2+ ions (r = 1.35 Å, CN = 6) replace Mg2+ ions (r = 0.57 Å, CN = 4, r = 0.72 Å, CN = 6), the expansion of the [MgO4] tetrahedra may shrink the adjacent [CsO6] polyhedra. Because two [CsO6] polyhedra connect with each other by a sharing edge, the distant [CsO6] polyhedron would slightly expand. As a result, there are two distorted [CsO6] polyhedra for Eu2+ ions occupying, leading to a different crystal field environment for Eu2+. On the basis of the following crystal field strength (Dq) equation:35−38

Dq =

Ze 2r 4 1 × 5 6 dav

(2)

where Z represents the anion valence, e represents the electron charge, r represents the radius of the d wave function, and dav represents the average bond length between the central cation and its ligands. Theoretically, the Dq value is proportional to 1/ dav.5 When the Eu2+ ions occupy a contract [CsO6] polyhedron, the larger crystal field spitting energy of 4f65d excited state would be generated, resulting in red emission (marked as Eu1), while the short-wavelength emission of Eu2 is suggested to occupy the larger [CsO6] polyhedron. Ultimately, the larger separation of two emission peaks in the Ba2+ ion-doping system than that in the Sr2+ ion-doping system is attributed to the bigger ion radius difference between Ba2+ ions and Mg2+ ions, producing a more pronounced lattice distortion. Annealed Temperature-Induced Luminescence Tuning. Except for controlling luminescence tuning through cation-substitution strategies, the external synthesis conditions could influence the internal lattice environment of phosphor materials. 39 Unexpectedly, in this current work, the CsMgPO4:Eu2+ samples also exhibit extraordinarily tunable luminescence under different annealed temperatures of 1100− 1200 °C. The XRD patterns of Cs0.96MgPO4:0.04Eu2+ samples obtained at different annealed temperatures are all indexed to the standard CsMgPO4 (ICSD No. 260423) phase without any trivial impurity (Figure 9a). Figure 9b plots the NMR spectra of Cs0.96MgPO4:0.04Eu2+ at different annealed temperatures (1100 °C, 1150 °C, 1200 °C). The main peaks located at 8.01 ppm should be ascribed to the [PO4] tetrahedra. Moreover, H

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excited electron or photon would experience a nonradiative transfer process through the cross of excited level and ground level.49,50 Consequently, the matrix structure plays a pivotal role in thermal quenching behavior. The high structure rigidity and symmetry could effectively resist the thermal quenching. Therefore, the local lattice variation is usually considered as an index for judging thermal quenching of a phosphor, which could be described with a lattice distortion degree as the following equation:51−53

the peak position and fwhm are almost unchanged with changing annealed temperature, indicating that the dimensional framework of the as-prepared samples does not change in various annealed temperatures. In general, the XRD and NMR results demonstrate the formation of the pure CsMgPO4:Eu2+ phase, and at the macroscopic level, the employed annealed temperature does not affect the crystal structure of the studied samples. Interestingly, conspicuous spectral variation could be observed, as shown by their PLE and PL spectra in Figure 9c,d. Although the PLE spectra possess a similar broad-band absorption from 250 to 500 nm for the three samples, the optimal excitation wavelength shifts from 350 to 400 nm when increasing the annealed temperature. Especially, the PL spectra show an amazing adjustment from red emission to blue emission under the irradiation of optimal excitation wavelength. For the 1100 °C-annealed sample, there only exists a broad red emission with the peak at 630 nm. When the annealed temperature increases to 1150 °C, the emission spectra contain another new blue-emitting band from 375 to 525 nm with the maximum at 434 nm except for the previous red emission. After the annealed temperature rises to 1200 °C, the red emission band completely vanishes, and there is only one 434 nm blue emission band. Simultaneously, the excitation spectrum turns broader with the presence of a shoulder peak at 400 nm, and the optimal excitation wavelength locates at 400 nm. The CIE chromaticity coordinates could further confirm the luminescence tuning from red to blue, and color coordinate positions are (0.575, 0.385) for 1100 °C, (0.402, 0.279) for 1150 °C, and (0.170, 0.081) for 1200 °C (Figure S5, Supporting Information), respectively. According to the above results, the annealed temperature could induce luminescence tuning from the red region to the blue region in the Cs0.96MgPO4:0.04Eu2+ sample. We infer a possible reason is that the decrease of annealed temperature induces some anisotropic lattice distortion of [CsO6] polyhedra in the local region, which modifies the lattice distortion and expands the intrinsic lattice environment. Hence, chemical pressure around the Eu2+ ions releases, resulting in the decreasing crystal field splitting energy of the Eu2+ 4f65d excited level. Thermal Quenching Behavior. Thermal quenching performance of phosphor materials is a pivotal index for evaluating their application prospects in pc-WLEDs lighting and display area.40−46 Here, the thermal quenching behavior for the representative Cs 0 . 9 6 MgPO 4 :0.04Eu 2 + and Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0−0.30) samples is investigated in detail. Figure 10a,b illustrates the thermal-dependent emission spectra f or Cs 0 . 9 6 MgPO 4 :0.04Eu 2 + and Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] samples, which is measured from 25 to 250 °C with a 25 °C interval. Emission intensity of the two samples sharply decreases as the temperature increase. The luminescence intensity could remain at 72% at 12 5 °C o f t h e i n it i a l i n t e n s it y a t 2 5 °C f o r Cs0.96MgPO4:0.04Eu2+ sample. Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] even suffers more serious thermal quenching behavior. Luminescence intensity only remains around 55%−66% at 125 °C of the initial intensity at 25 °C (Figure 10). Generally, the thermal quenching behavior of the emission loss usually originates from the synergism of various factors: (1) The absorption intensity may gradually quench with increasing temperature.47,48 (2) High working temperature could induce electron delocalized from the Eu2+ 4f65d excited level to the conduction band of the matrix.40 (3) Thermally

n

D=

|d − d | 1 ∑ av i n i=1 dav

(3)

where D represents the lattice distortion degree, di is the distance from central atom C (C: Cs, Mg, P) to the ith coordinated O atoms, dav is the average C−O bond length, and n is the coordination numbers, which could be obtained by Rietveld refinement. The calculated lattice distortion is shown in Table S2 (Supporting Information). Codoping of [Eu2+Si4+] increases the local distortion degree of matrix. Thus, the thermal quenching behavior becomes more serious. In addition, the emission peaks of the representative Cs0.96MgPO4:0.04Eu2+ and Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] samples show a slightly blue shift from 630 to 600 nm, and the fwhm becomes broader with increasing temperature, indicating that the thermal stabilities for the as-prepared Cs1−xMgP1−xO4:x[Eu2+-Si4+] (x = 0−0.50) are not good and need to be further optimized (Figure 10d). Fabricated LEDs Performance. To demonstrate that asprepared phosphors is a superb candidate for n-UV based on pc-WLEDs, we fabricate the red, blue, and white LEDs devices driving by n-UV InGaN chip (λmax = 370 nm). The profiles in electroluminescent (EL) spectra are very consistent with corresponding photoluminescence spectra, which confirm the stability luminescence of as-prepared CsMgPO4:Eu2+ phosphor. Red LED devices are fabricated by combining red Cs0.7MgP0.7O4:0.30 [Eu2+-Si4+] phosphor (Figure 11a) and Cs0.96MgPO4:0.04Eu2+ (1100 °C) phosphor (Figure 11d) with n-UV chip. The CIE coordinates, luminous efficiency, and CRI are (0.464, 0.405), 1.22 lm/W, and 93.4 for

Figure 11. Electroluminescent (EL) spectra of LEDs fabricated by (a) red Cs 0 . 7 MgP 0 . 7 O 4 :0.30[Eu 2 + -Si 4 + ] phosphor, (b) blue CsMg0.6Ba0.4PO4:0.04[Eu2+-Si4+] phosphor, (c) red Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] phosphor, green Ba3Si6O12N2:Eu2+ phosphor and blue CsMg0.6Ba0.4PO4:0.04[Eu2+-Si4+] phosphor, (d) red Cs 0.96 MgPO 4 :0.04Eu 2+ (1100 °C) phosphor, (e) blue Cs0.96MgPO4:0.04Eu2+ (1200 °C) phosphor, (f) Cs0.96MgPO4:0.04Eu2+ (1150 °C) and green Ba3Si6O12N2:Eu2+ phosphor driven by n-UV chips (λmax = 370 nm). The insets are the photographs under 200 mA current driving. I

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Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] (0.455, 0.405), 1.19 lm/W and 92.9 for Cs0.96MgPO4:0.04Eu2+ (1100 °C), demonstrating that the [Eu2+-Si4+] charge-compensation strategy could improve the luminescence performance. Blue LEDs devices are fabricated by combining blue CsMg0.6Ba0.4PO4:0.04[Eu2+Si4+] phosphor (Figure 11b) and Cs0.96MgPO4:0.04Eu2+ (1200 °C) phosphor (Figure 11e) with the n-UV chip. The CIE coordinates and color purity are (0.194, 0.079) and 80 for CsMg0.6Ba0.4PO4:0.04[Eu2+-Si4+], and (0.164, 0.040) and 93.9 for Cs0.96MgPO4:0.04Eu2+ (1200 °C), indicating that they are a superb blue-emitting component. White LEDs are fabricated by using a mixture of red Cs0.7MgP0.7O4:0.30[Eu2+-Si4+] phosphor, green Ba3Si6O12N2:Eu2+ phosphor, and blue CsMg0.6Ba0.4PO4:0.04 [Eu2+-Si4+] phosphor (Figure 11c) and the mixture of Cs0.96MgPO4:0.04Eu2+ (1150 °C) and green Ba3Si6O12N2:Eu2+ phosphor driven by the n-UV chip (λmax = 370 nm) (Figure 11f) to coat on the n-UV chip. Both the fabricated white LEDs exhibit a warm light with low corrected color temperature (3469−4942 K) and high CRI (82.4−91.3), and the CIE coordinates are at (0.354, 0.422) and (0.410, 0.400) respectively. The insets in Figure 11 present bright red, blue, and warm white luminescence. These results demonstrate that CsMgPO4:Eu2+ could serve as a promising red/blueemitting phosphor in n-UV based pc-WLEDs.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00577.

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Synchrotron XRD pattern, PL spectra, diffuse reflectance spectra, CIE coordination diagram, crystallographic lattice parameters and bond length information including in Figures S1−S5 and Tables S1−S2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(G.L.) E-mail: [email protected]. *(J.L.) E-mail: [email protected]. *(C.C.L.) E-mail: [email protected]. ORCID

Guogang Li: 0000-0002-0523-5621 Jun Lin: 0000-0001-9572-2134 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 51672259, 51720105015, 51672265, 21521092, 51750110511), Key Research Program Of Frontier Sciences of CAS (YZDY-SSW-JSC018), the CASCroucher Funding Scheme for Joint Laboratories (CAS18204), the Scientific and Technological Department of Jilin Province (Grant No. 20170414003GH), Jiangmen Innovative Research Team Program (2017), and Major Program of Basic Research and Applied Research of Guangdong Province (2017KZDXM083) and in part by grants from Taipei Medical University (Contract Nos. TMU107-AE1-B09 and USTP-NTUT-TMU-108-04).

CONCLUSIONS

In summary, a series of tunable blue/red-emitting Eu2+-doped CsMgPO4 orthophosphate phosphors were successfully prepared by a high-temperature solid state reaction. According to the Rietveld refinement from synchronous XRD data, the asprepared samples are indexed to orthorhombic with space group Pnma (No. 62), which contains one distorted [CsO6] polyhedron and one [MgO4] tetrahedron. Under 350 nm nUV light excitation, Eu2+-doped CsMgPO4 phosphors exhibit broad red emission with peak position at 625 nm (fwhm = 118 nm). Through the cosubstitution strategy of [Eu2+-Si4+] for [Cs+-P5+] for charge compensation, the luminescence intensity is remarkably improved, and the quenching concentration is dramatically enhanced from 0.04 to 0.30. In addition, cation substitutions of Sr2+/Ba2+ ions for Mg2+ ions in CsMgPO4:0.04 [Eu2+-Si4+] achieve controllable emission spectra adjustment from red to blue-violet (yellow) for Ba2+ (Sr2+) substitution, respectively. The proposed mechanism is that the incorporation of Sr2+/Ba2+ may distort the adjacent Cs+ sites, forming two types of distorted [CsO6] polyhedra. Unprecedentedly, Cs0.96MgPO4:0.04Eu2+ sample presents an amazingly spectral adjustment from red (625 nm) to blue (434 nm) by changing the annealed temperature, which is possibly attributed to the distortion of the intrinsic lattice environment to expand local lattice sites. According to the thermal stability research, the luminescence intensity could remain at 72%, 66% at 125 °C of the initial intensity at 25 °C for the Cs0.96MgPO4:0.04Eu2+ sample and Cs0.96MgP0.96O4:0.04 [Eu2+-Si4+] samples, respectively. These results demonstrate that charge compensation and local lattice strain based on various cation-substitution strategies could efficiently improve the luminescence performance and tune the optical properties of the CsMgPO4:Eu2+ phosphor. The employed strategies also offer guidance in optimizing and designing other inorganic phosphor materials.

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DOI: 10.1021/acs.inorgchem.9b00577 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00577 Inorg. Chem. XXXX, XXX, XXX−XXX