Controlled Morphology, Improved Photoluminescent Properties, and

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

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Controlled Morphology, Improved Photoluminescent Properties, and Application of an Efficient Non-rare Earth Deep Red-Emitting Phosphor Feng Hong, Haiming Cheng, Guixia Liu,* Xiangting Dong,* Wensheng Yu, and Jinxian Wang

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Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Changchun University of Science and Technology, Changchun 130022, P. R. China ABSTRACT: Transition-metal tetravalent manganese ions (Mn4+) as luminescence center of red phosphors have drawn much attention owing to their broad-band absorption extended from UV to blue regions and narrow red-emissive band. In the present work, a series of Mn4+-doped BaGeF6 red phosphors were obtained via hydrothermal method. X-ray powder diffraction, energy-dispersive X-ray spectrometer, scanning electron microscope, and photoluminescence spectra were employed to determine the crystal structure, composition, morphology, and photoluminescence properties of all samples. The prepared BaGeF6:Mn4+ samples demonstrate two dominant broadband absorption at near-UV (∼366 nm) and blue regions (∼470 nm) and intense red emissions (∼635 nm) under 470 nm excitation. In addition, the morphology and the emission intensities were successfully controlled by adjusting doping concentrations, reaction times, reaction temperatures, barium sources, and surfactants. Concentration quenching and thermal quenching mechanisms were studied in detail. When the BaGeF6:Mn4+ red phosphor was introduced into the light-emitting diode, warm white light-emitting diodes (w-LEDs) were successfully fabricated, which have high color rendering index (Ra = 86.3) and low correlated color temperature (4766 K), indicating that the BaGeF6:Mn4+ red phosphor provides a good opportunity for application in w-LEDs.

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Mn4+ activated red phosphor material can be synthesized at moderate environment, and it has attracted more and more attention because of eco-friendly, low cost, and characteristic optical properties. When Mn4+ occupies the octahedral position, the phosphors exhibit narrow red emitting and broadband excitation extending from ultraviolet to blue light,17,18 which is suitable for w-LED applications. In recent years, Mn4+-doped oxide red-emitting phosphors with outstanding physical/chemical stability have turned into a popular research object for practical applications in w-LEDs. Currently, a lot of Mn4+-doped oxides have been synthesized, such as Mg 3 Ga 2 GeO 8 :Mn 4+ , 19 Ca 14 Zn 6 Al 10 O 35 :Mn 4+ , 20 Li3Mg2NbO6:Mn4+,21 Li2MgTiO4:Mn4+,22 and Sr4Al14O25:Mn4+.23 However, the oxide phosphors are obtained via high-temperature solid-phase method, and the sintering temperature is higher than 1200 °C, which has energy consumption and higher risk of experimental process. By contrast, Mn4+ activated fluoride phosphors can be synthesized under mild conditions. For instance, Lv et al. synthesized red phosphor K2SiF6:Mn4+ from KF and SiO2 in HF solution with the 0.08 mol L−1 of KMnO4 based on facile wet method.24 Sekiguchi et al. obtained BaSiF6:Mn4+ red phosphor by chemical reaction method.25 Jin et al. prepared KNaSiF6:

INTRODUCTION Solid-state white light-emitting diodes (w-LEDs) are expected to replace the original fluorescent and incandescent lamps as next-generation lighting sources, in view of the advantages of low power consumption, high energy efficiency, fast response, long fluorescence lifetime, and environment friendliness material properties.1−7 At present, the appropriate strategy for preparing w-LEDs is combining blue light chip with commercial Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphor. Nevertheless, because of the lack of red light emission, the obtained w-LEDs have lower color-rendering index (CRI, Ra < 80) and higher color temperature (CCT > 5000 K), which makes them unsuitable for domestic lighting or display applications.8−11 Therefore, researchers are looking forward to getting a kind of red phosphor with outstanding luminescent properties and adequate chemical durability. Eu2+-doped sulfide and nitrite compounds, such as CaS:Eu2+,12 Sr2Si5N8:Eu2+,13 and SrLiAl3N4:Eu2+,14 are used as the red phosphors for w-LEDs. However, the synthetic procedures of such red phosphors have inherent complex operation and severe conditions involving the scarcity of nitride raw materials, and the whole process must be separated from air. Furthermore, the poor chemical stability of sulfides makes them inappropriate candidates for competitive samples.15,16 To avoid the above disadvantages, an excellent red phosphor with a maximum broadband excitation at the blue region and narrow red-emissive band, low-production costs, and easy synthesis should be investigated. © XXXX American Chemical Society

Received: April 8, 2018

A

DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Mn4+ via a simple coprecipitation method.26 Zhu et al. proposed a novel method by cation exchange strategy to synthesize the excellent K2TiF6:Mn4+ red phosphor.27 Xu et al. directly synthesized the red-emitting Na2SiF6:Mn4+ and Na2GeF6:Mn4+ phosphors via wet chemical etching of Si wafers and Ge shots in NaMnO4 and HF mixed solution.28 All the prepared phosphors have a sharp red emission peak at ∼630 nm and broad excitation band at ∼460 nm, which reveals more promising for blue GaN excited warm w-LEDs applications.29,30 In contrast to these methods, hydrothermal method can bring high-temperature and high-pressure reaction conditions under relatively mild conditions, which is conducive to dope Mn4+ ions into the matrix and control sample morphology. BaGeF6 crystal possesses a hexagonal crystal system with a space group of R3̅m (166), and the Ge4+ ion is surrounded by six F− ions to construct an octahedron. In virtue of the same valency and ionic radius between Ge4+ (r = 0.53 Å) and Mn4+ (r = 0.53 Å), the BaGeF6 crystal is considered as good luminescence host for Mn4+-doped red phosphors. For example, Sekiguchi and Zhou had made use of BaGeF6 as matrix to synthesize red phosphor.31,32 However, Sekiguchi et al. mainly discussed the luminescence properties of BaGeF6 :Mn4+ prepared by the chemical etching method. Although Zhou et al. synthesized red phosphor by hydrothermal method, they did not discuss in detail the relation between the phosphor properties and the synthesis conditions. In this paper, red phosphors, BaGeF6:Mn4+, were successfully synthesized via hydrothermal method. The phase structure, morphologies, and luminescence properties were also successfully studied by changing the reaction conditions. The thermal stability of the BaGeF6:Mn4+ samples was investigated. Furthermore, a w-LED was fabricated using the as-obtained BaGeF6:Mn4+ red phosphor and commercial YAG:Ce3+ yellow phosphor. The results present a substantial advance toward the commercial application.

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dissolved in 30 mL of distilled water at the same time), and quantities of K2MnF6. The whole synthesis process of BaGeF6:Mn4+ red phosphor is depicted in Figure 1.

Figure 1. Experimental process diagram for synthesizing the BaGeF6:Mn4+ products. Characterization. The phase identification of all phosphors was performed by X-ray powder diffraction (XRD) performed on a Rigaku D/max-RA X-ray diffractometer with Cu Kα radiation (λ = 0.154 18 nm). The 2θ data were collected from 10° to 90°. The morphology and elemental composition of the as-obtained sample were investigated using a JEOL JSM-7610F field emission scanning electron microscope (FE-SEM) equipped with Oxford X-MaxN80 energy-dispersive X-ray spectrometer (EDS). Luminescence properties and fluorescence lifetimes of the samples were investigated by a HITACHI F-7000 fluorescence spectrophotometer with the slits of 2.5 nm at room temperature, and the xenon lamp (150 W) was used as the excitation source. The temperature-dependent photoluminescence (PL) spectra were recorded from 148 to 498 K by a Jobin Yvon fluoro Max-4 equipped with a xenon lamp (150 W) as the excitation source. The quantum efficiency (QE) was measured by the barium sulfate-coated integrating sphere attached to the spectrophotometer at room temperature (C9920-02, Hamamatsu Photonics K. K.).

EXPERIMENTAL SECTION

Materials. The BaGeF6:Mn4+ phosphors were synthesized from the raw materials, including potassium permanganate (KMnO4, >99.5%), potassium fluoride (KF, >98%), hydrofluoric acid solution (HF, 40 wt %), barium fluoride (BaF2, >99.0%), barium nitrate (Ba(NO3)2, >99.5%), barium acetate (Ba(CH3COO)2, >99.0%), barium carbonate (BaCO3, >99.0%), germanium oxide (GeO2, 99.99%), hydrogen peroxide aqueous solution (H2O2, 30.0%), and ethylenediamine tetraacetic acid (EDTA, >99.5%). All of these starting reagents were purchased from the sinopharm Chemical Reagent Co. Ltd. and used directly without further purification. Synthesis of BaGeF6:Mn4+. The hydrothermal method was employed to prepare the Mn4+ activated BaGeF6 red phosphor by using K2MnF6 as the Mn4+ source. According to ref 33, the K2MnF6 precursors could be obtained. Typical BaGeF6:2%Mn4+ red phosphor was prepared for the following procedure, 0.3076 g of GeO2 and 0.0148 g of K2MnF6 were dissolved in 10 mL of HF solution (40 wt %) in a 50 mL plastic breaker to obtain the mixed solution by magnetic stirring. BaF2 (0.5260 g), after it was dissolved into the 30 mL of distilled water under magnetic stirring, was added dropwise into the above solution. After it was vigorously stirred for 30 min, the mixed solution was poured into a 50 mL Teflon-lined stainless steel autoclave and kept there for 8 h at 180 °C. Then, the supernatant was poured out, and the precipitation was washed three times with distilled water and ethanol. Finally, the yellow precipitate was ovendried at 60 °C for 6 h. Other samples were obtained through a similar procedure, except for using different hydrothermal temperatures, times, barium sources, amount of EDTA (EDTA and BaF2 were

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RESULTS AND DISCUSSION Structure, Morphology, and Composition. Figure 2a shows the representative XRD pattern of BaGeF6:Mn4+ phosphor synthesized through hydrothermal method at 180 °C for 8 h. All diffraction peak positions can be exactly in agreement with the rhombohedral BaGeF6 structure (PDF No. 74-0924) with R3̅m (166) space group and unit cell a = b = c = 4.830 Å. No obvious impurity phase is found when the Mn4+ ions are introduced. The crystal structure and cation polyhedral arrangements of BaGeF6 are depicted in Figure 2b. The crystal structure of the BaGeF6 unit cell demonstrates an octahedral structure. There are six F− ions around Ge4+ ion, which constitutes a GeF62−octahedron ionic group and six F− connect with Ba2+ through ionic bond. Mn4+ can occupy the Ge4+ site in the octahedron to form a new MnF62− octahedral structure, because Ge4+ (0.53 Å, CN = 6) and Mn4+ (0.53 Å, CN = 6) have the same radius and coordination environment. The microstructure of BaGeF6:2%Mn4+ was characterized by FE-SEM. B

DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. XRD pattern (a), three-dimensional perspective view of crystal structure for BaGeF6 and the coordination environment of octahedra for GeF62− (b), SEM image (c), and EDS spectrum (d) of BaGeF6:2%Mn4+ samples.

Figure 3. PLE (blue line) and PL (red line) spectra (a) and the CIE chromaticity coordinates (b) of the BaGeF6:2%Mn4+ samples. (inset) Digital image of the BaGeF6:Mn4+ phosphor under blue lamp (460 nm) irradiation.

regions (∼470 nm), they belong to the spin-allowed 4A2g → 4T1g and 4T2g d−d transitions of octahedrally coordinated Mn4+ ions, respectively.30 The strongest excitation peak is located at 470 nm, which meets application requirements in w-LEDs. Under 470 nm excitation, the BaGeF6:2%Mn4+ exhibits intense red emission peaks within the range of 550−700 nm; the peak located at 635 nm has the largest emission intensity, which is originated from the spin-forbidden2Eg → 4A2g transition of Mn4+ in BaGeF6 matrix. The quantum efficiency of BaGeF6:Mn4+ red phosphor is 38%. Figure 3b illustrates the CIE chromaticity diagram of the BaGeF6:Mn4+ phosphor. The corresponding color coordinates are (0.6707, 0.3262) under blue light excitation, which is very near to the NTSC ideal red color

It can be clearly observed that the obtained products show rodlike morphology with smooth surfaces (Figure 2c). The average length is ∼5.2 μm, and the mean diameter is ∼1.0 μm. In the corresponding EDS spectrum (Figure 2d), the elements of Ba, Ge, F, and Mn can be easily recognized, which confirms that the sample contains Ba, Ge, F, and Mn elements. The Si element peak is derived from the silicon wafer in the EDS measurement. Luminescence Properties. Figure 3a is the photoluminescence excitation (PLE) and PL spectra of the BaGeF6:2% Mn4+ product at room temperature. With the emission of Mn4+ at 635 nm as monitor wavelength, the two typical broad-band excitation peaks located in the UV (∼366 nm) and blue C

DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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BaGeF6:Mn4+ red phosphors via different reaction temperatures. All diffraction peaks are consistent with the rhombohedral structure of BaGeF6 crystals (PDF No. 74-0924). No other crystalline phases and structural changes appear, which reveals the formation of pure BaGeF6:Mn4+ samples under different reaction temperatures. The morphological changes of the products are presented in Figure 4B. It is noted that the sample exhibits a rodlike structure when the temperature is 25 °C. The average length and the diameter of the rod structure are ∼3.1 and 1.0 μm, respectively. After the hydrothermal treatment at 100 °C, the sample is bundlelike microstructure, which is composed of many rodlike structures with the average length and diameter of ∼8.7 and 2.2 μm, respectively. With continued increase of temperature to 140 °C, the bundlelike structure is further aggregated to a flowerlike structure, which is also assembled by rodlike structure; the average length and diameter are shortened to ∼5.0 and 1.4 μm, respectively. The hydrothermal temperature then rises to 180 °C, the flower structure disappears, and the sample appears as a rodlike structure that has a length of ∼5.2 μm and a diameter of ∼1.0 μm. On the basis of the morphology evolution of sample at different reaction temperatures, it can be inferred that the hydrothermal treatment may promote the self-assembly of the sample, accelerate the formation of the nucleation and crystal growth rate, and then realize the large size samples. As shown in Figure 4C, it was found that, from 25 to 100 °C, the luminescent intensities of the samples are effectively improved after hydrothermal treatment, owing to the optimized morphology and size of sample at higher temperature.34,35 However, when the hydrothermal temperature is further increased to more than 100 °C, the luminescence intensity is gradually weakened, which may be due to Mn4+ ions becoming unstable at high temperature.36 Figure 5A is the XRD patterns of the as-synthesized BaGeF6:2%Mn4+ red phosphors via hydrothermal method at 180 °C with different times. All diffraction peaks correspond to the standard card of BaGeF6 (PDF No. 74-0924), and no impurity peaks are found, which reveals the single-phase BaGeF6:Mn4+ products are synthesized at different reaction times. The influence of reaction time on the luminescence properties of the BaGeF6:Mn4+ sample is described in Figure 5B. Obviously, the emission peaks have the same shape, and each of them shows the characteristic peaks of Mn4+ in the wavelength extended from 550 to 700 nm. The emission intensity of BaGeF6:Mn4+ initially increases, then achieves the maximum at 8 h. The phenomenon is because the doped concentration of Mn4+ increases with the time increasing and optimization of crystallinity. As the prolongation of the reaction time, the emission intensity gradually decreases, because the Mn4+ in the BaGeF6 matrix tends to instability in a long period time with high temperature, and crystallinity is also changed.37 The influences of different barium sources on the morphology and luminescence of the sample are investigated. Figure 6A describes the XRD patterns of the BaGeF6:2%Mn4+ red phosphors synthesized with different barium sources. All the diffraction peaks of the as-synthesized phosphors accord well with the standard cards (PDF No. 74-0924), and no impurity phases can be observed. The crystal structure of the samples shows no obvious influence through different barium sources. From the SEM images (Figure 6B), it can be seen that the barium sources change the size of the sample, but the morphologies are all still rodlike. Furthermore, it can be seen clearly that the BaGeF6:2%Mn4+ samples prepared by BaCO3 and BaF2 as barium sources have smoother surface. Figure 6C

(x = 0.67, y = 0.33). From the luminescence photograph of the BaGeF6:Mn4+, obvious brilliant red light is emitted from the sample under blue light illumination, indicating that BaGeF6:Mn4+ red phosphors can be potential in the field of interior illumination. Influences of Different Preparation Conditions on Structure, Morphology, and Luminescence Properties. Figure 4A shows XRD patterns of the as-obtained

Figure 4. XRD patterns (A), SEM images (B), and emission spectra (C) of BaGeF6:2%Mn4+ synthesized at different reaction temperatures for 8 h. (inset) The corresponding integrated emission intensity of BaGeF6:2%Mn4+ as a function of reaction temperature. 25 (a), 100 (b), 140 (c), and 180 (d) °C. D

DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. XRD patterns (A) and emission spectra (B) of BaGeF6:2%Mn4+ obtained at 180 °C for different reaction times. (inset) The corresponding integrated emission intensity of BaGeF6:2%Mn4+ as a function of reaction times.

reveals the PL spectra of the as-obtained red phosphors using different barium sources. When 470 nm is used as the excitation wavelength, all of the emission spectra are similar. Nevertheless, the luminescence intensity significantly changes. The BaGeF6:Mn4+ sample obtained from Ba(NO3)2 exhibits the strongest emission peak compared with the samples prepared by other barium sources. The reason for the dramatic difference in luminescence intensity may be derived from the difference in particle size and the number of surface defects.38 To investigate the influence of the amount of EDTA on the morphology evolution in our synthesis process, a series of Mn4+-doped BaGeF6 red phosphors BaGeF6 were obtained through adding different molar ratios of BaF2 to EDTA at 180 °C for 8 h. Figure 7A reveals the XRD patterns of BaGeF6:Mn4+ red phosphor through different amounts of EDTA. All the diffraction peaks correspond to the standard cards of BaGeF6 (PDF No. 74-0924), and no other impurity peaks are present. The position of the peaks has not changed, indicating the participation of the surfactant does not change the purity and the structure of BaGeF6:Mn4+. The microstructures of BaGeF6:2%Mn4+ obtained via different BaF2:EDTA molar ratios were examined using SEM; the images are described in Figure 7B. Morphology of all samples is bundlelike structure consisting of a rodlike structure with length and diameter of ∼7.4 and 1.6 μm, respectively, when the molar ratio of BaF2 to EDTA is 1:1. With an increase in the amount of EDTA surfactant, the sample has evolved into a flowerlike structure, which is assembled by rodlike structure with the length and diameter of ∼5.1 and 1.4 μm, respectively. When the amount of EDTA is increased to 1:3, the flower structure disappears, and the morphology shows a short rodlike structure of the average length and diameter ∼3.9 and 1.4 μm, respectively. In this hydrothermal synthetic system, the EDTA inducer significantly influences the morphology of the BaGeF6:Mn4+ powder. Small amount of EDTA can promote agglomeration of rodlike structures. While excess amount of EDTA was added, the excessive adsorption of EDTA over the surface of the product hinders the ordered growth of the crystal.39 The emission spectra of the red phosphors corresponding to different morphologies are shown in Figure 7C. It is found that BaGeF6:Mn4+ red phosphor with a bundlelike structure has the strongest luminescence intensity, and the flower structure has a stronger luminescence intensity than the short rodlike structure, due to the differences of particle sizes and surface defects.34,35

The doping concentration has a great influence on the photoluminescence performance of the phosphors. Figure 8a reveals the XRD patterns of the as-synthesized samples with different Mn4+ doping concentrations (1%, 2%, 4%, 6%, and 8%). One can clearly see that all the diffraction peaks are matched well with the standard card of the BaGeF6 with rhombohedral structure (PDF No. 74-0924). There is no impurity peak, indicating that different Mn4+ doping concentration does not affect the structure of the BaGeF6 matrix or any undesirable Mn-related compounds. Figure 8b shows the emission spectra of the as-synthesized BaGeF6:Mn4+ doped with different amount of Mn4+ under 470 nm as the excitation wavelength. As the Mn4+ doping concentration increases, the peak shape and position of the emission spectra are just as before. When the concentration of Mn4+ ions is 2%, the emission intensity of BaGeF6:Mn4+ phosphor achieves its maximum value, whereas, by further increasing the concentration beyond 2%, the emission intensity gradually weakens, which is ascribed to the occurrence of concentration quenching. The exchange interaction or electric multipole interaction may be the cause of concentration quenching. To understand which mechanism is possible for energy transfer, the nearest Mn4+, the critical distance (Rc) of energy transfer is calculated. According to the relationship defined by Blasse,40 the Rc value can be evaluated. The calculation is as follows: ij 3V yz zz R c ≈ 2 × jjj j 4πxcN zz k {

1/3

where V represents the unit cell volume, xc represents the doping amount of Mn4+ when concentration quenching happens, and N represents the number of Mn4+ ions in the unit cell. In the BaGeF6 matrix lattice, when V = 109.02 Å, xc = 0.02, N = 1, the critical transfer distance for the concentration of quenching is ∼22 Å. So, the concentration quenching is associated with the multipolar energy transfer mechanism. On the basis of Dexter theory, the type of multipolar energy transfer mechanism can be determined using the following equation:41 I = k[1 + β(x)θ /3 ]−1 x

where I is the photoluminescence intensity when the doping concentration is x, k and β are constant in a specific matrix, and θ is the index of electric multipole character. When θ = 6, 8, and 10, they represent dipole−dipole (d−d), dipole−quadrupole E

DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. XRD patterns (A), SEM images (B), and emission spectra (C) of BaGeF6:Mn4+ samples obtained with different molar ratios of BaF2 to EDTA: 1:1 (a), 1:2 (b), and 1:3 (c)

Note that a value of ∼3.7 is the nearest value of 6 among 6, 8, and 10, which indicates that the interaction between Mn4+ belongs to d−d interaction in the matrix of BaGeF6. Figure 8d reveals the concentration-dependent luminescence decay curves of Mn4+ in BaGeF6:xMn4+ at 635 nm upon the 470 nm light excitation at room temperature. All luminescence decay curves are well-fitted with a single exponential function, which can be demonstrated as below: I(t ) = I0 + Ae(−t / τ)

where I(t) and I0 are the photoluminescence intensity of BaGeF6:xMn4+ at time t and initial condition, respectively. The fluorescence lifetime of BaGeF6:xMn4+ is represented by τ. Obviously, the lifetimes of the Mn4+ ions show a continuous decrease with Mn4+ doping concentration increases, which is assigned to the probability for nonradiative transition between Mn4+−Mn4+ pairs increases. This phenomenon indicates that energy transfer happens in the adjacent Mn4+ ions, and then the energy is transferred to the quenching center, resulting in the reduction of the lifetime of Mn4+ ions. The temperature-dependent photoluminescence intensity of BaGeF6:Mn4+ at temperature range of 148−498 K under 470 nm excitation is illustrated in Figure 9a. The location of the emission peak does not change upon different measurement

Figure 6. XRD patterns (A), SEM images (B), and emission spectra (C) of BaGeF6:Mn4+ synthesized by using the different barium source at 180 °C for 8 h. (inset) The corresponding integrated emission intensity of BaGeF6:2%Mn4+ as a function of barium sources. BaCO3 (a), Ba(NO3)2 (b), Ba(CH3COO)2 (c), and BaF2 (d).

(d−q), and quadrupole−quadrupole (q−q) interactions, respectively. The equation is modified as follows:

θ iIy logjjj zzz = − log(x) + A 3 kx{ Figure 8c shows that the relationship of log(I/x) and log(x) is well linearly fitted. The results show that the fitted slope is 1.223. The value of θ is given by 3 times the slope value. F

DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. XRD patterns (a), emission spectra of BaGeF6:x%Mn4+ (x = 1, 2, 4, 6, and 8) upon 470 nm excitation (b), the linear relationship between log(x) and log(I/x) on the basis of the emission intensity (I) and concentration (x) of Mn4+ (c), and decay curves of Mn4+ in the BaGeF6:x%Mn4+ samples excited at 470 nm under the monitoring of 635 nm (d).

Figure 9. Luminescence intensity of the BaGeF6:Mn4+ red phosphor determined by the test temperature (a), and the possible thermal quenching process of the configurational coordinate diagram for Mn4+ ions in the BaGeF6 matrix (b). ① Red light emission process. ② Temperature quenching process as purple dashed lines.

phosphors.27,30 When the temperature of the phosphor reaches 448 K, the luminescence intensity of BaGeF6:Mn4+ red phosphor can be maintained at 42.5% at room temperature. The nonradiative transition causes the occurrence of thermal

temperatures. The luminescence intensity increases with the change of temperature at 148−298 K. However, above 298 K, the emission intensity decreases gradually. This trend is the same as the thermal stability of Mn4+-doped fluoride G

DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 10. Electroluminescence spectra of the blue chip, LED combined with the blue chip and YAG:Ce3+, and YAG:Ce3+-BaGeF6:Mn4+ mixture driven by current of 20 mA, and photographs of the LED lamps (a). The CIE chromaticity coordinates of blue light chip (A), LED blending the blue light chip and YAG:Ce3+ (B), YAG:Ce3+−BaGeF6:Mn4+ (C), and BaGeF6:Mn4+ (D) (b). (insets) Photos of the BaGeF6:Mn4+ phosphor under visible light and 460 nm blue light excitation.

The availability of BaGeF6:Mn4+ for the blue light chipbased white LED application is checked. Figure 10a reveals the emission spectra and photographs of the blue chip, LED combined with blue chip and yellow phosphor YAG:Ce3+, and YAG:Ce3+-BaGeF6:Mn4+ mixture driven by current of 20 mA. The emission peak located at ∼460 nm is emitted by the blue chip, which can be effectively absorbed through the BaGeF6:Mn4+ red phosphor. Cold bright white light is observed by naked eyes when with the existence of YAG:Ce3+ yellow phosphor. The warm w-LED is fabricated via combining the blue chip, commercial yellow phosphor YAG:Ce3+, and as-prepared red phosphor BaGeF6:Mn4+, and it can be seen clearly the red emission in the 600−700 wavelength range. The correlated color temperature of the w-LEDs is dropped from 5130 to 4766 K, and the color rendering index increases from 74.5 to 86.3. Apparently, because of the introduction of the BaGeF6:Mn4+ red phosphor, the properties of the prepared w-LEDs become more suitable for actual application. The CIE chromaticity diagram of the corresponding LEDs is described in Figure 10b. A point (0.1489, 0.0312) and D point (0.6721, 0.3248) are the CIE chromaticity coordinates of the blue chip and BaGeF6:Mn4+ red phosphor, respectively. The CIE coordinates of the LEDs vary from cool white light of B point (0.3426, 0.3647) to the warm white light of C point (0.3519, 0.3562). The inset photographs in Figure 10b represent the BaGeF6:Mn4+ sample under sunlight and blue light (460 nm) irradiation, respectively. This demonstrates that the great potential of BaGeF6:Mn4+ as red phosphors to improve LED performance and apply in the field of interior illumination.

quenching. Furthermore, the activation energy (ΔE) can be expressed using equation42 below: IT =

I0

(

ΔE

1 + C × exp − kT

)

where I0 and IT, respectively, represent the photoluminescence intensity at initial condition and T temperatures. k is the Boltzmann constant (8.629 × 10−5 eV K−1). ΔE is the activation energy needed to produce thermal quenching, and the activation energy of the BaGeF6:Mn4+ red phosphor can be calculated to be 0.54 eV. The result shows that the BaGeF6:Mn4+ red phosphors possess outstanding thermal stability. The configurational coordinate diagram can clearly show the energy converting process between the ground and the excited states, which can well explain the phenomenon of thermal quenching. As described in Figure 9b, under the near-UV light and blue light excitation, the electrons of Mn4+ are excited from the ground state 4A2g to excited states 4T1g and 4T2g. Then, the electrons will jump back to the ground state 4A2g from the 2Eg state by means of radiation transitions, and the phosphors emit red light, which is shown by ① process. It is found that the origin of the 2T1g state cannot be clearly determined, but it is deduced that ∼0.1 eV above the origin of 2Eg in a great deal of fluoride phosphors. Therefore, the excitation transition peak of 4A2g → 2T1g and the emission transition peak of 2Eg → 4A2g are nearly consistent in energy. However, there is a possibility that the 2Eg → 4A2g emission intensity can be strengthened by the 4A2g → 2T1g excitation transitions and subsequent 2 T1g →2Eg relaxation process in actual phosphors. Therefore, we concluded that the main contribution of the increment of PL intensity from 148 to 298 K is attributed to the enhancement for absorption of sample excitation. However, when the environment temperature is high enough, a portion of the electrons will absorb the activation energy from thermal vibrations and return to the ground state through nonradiative transitions, leading to the occurrence of the thermal quenching phenomenon, as shown in ② process by purple dashed lines.

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CONCLUSIONS In summary, a class of red phosphor BaGeF6:Mn4+ was synthesized by hydrothermal method. The phosphors possess excellent red photoluminescence peaking at 635 nm under the 470 nm blue light excitation. The morphology and size of the phosphors were successfully controlled by different reaction temperatures, barium sources, and amounts of surfactant. Dipole−dipole interaction was considered as an effective H

DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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concentration quenching mechanism. The temperature-dependent photoluminescence intensity results indicate that the BaGeF6:Mn4+ red phosphors possess good thermal stability and meet the needs of the LEDs working temperature. The high-performance w-LED with lower color temperature (CCT = 4766 K) and higher color rendering index (Ra = 86.3) has been obtained via a blue GaN chip integrated with a mixture of BaGeF6:Mn4+ red phosphor and commercial YAG:Ce3+ yellow phosphor. The BaGeF6:Mn4+ phosphors are potential for improving luminous properties of conventional w-LEDs.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-0431-85582574. Fax: +86-0431-85383815. E-mail: [email protected]. (G.X. Liu.) *Phone: +86-0431-85582575. Fax: +86-0431-85383815. E-mail: [email protected]. (X.T. Dong) ORCID

Guixia Liu: 0000-0002-4100-5639 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (51072026, 51573023, 50972020), Natural Science Foundation of Jilin Province of China (20170101185JC, 20170101101JC), and Industrial Technology Research and Development Project of Jilin Province Development and Reform Commission (2017C052-4). The authors appreciate the financial support of the Education Department of Jilin province “13th Five-Year” Science and Technology Research project [Grant No. 2016-382].

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DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00944 Inorg. Chem. XXXX, XXX, XXX−XXX