Realizing bright blue-red color-tunable emissions from Gd2GeO5:Bi3+,Eu3+ phosphors through energy transfer toward light-emitting diodes

Journal of Luminescence 222 (2020) 117127

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Realizing bright blue-red color-tunable emissions from Gd2GeO5:Bi3þ,Eu3þ phosphors through energy transfer toward light-emitting diodes Qi Sun a, Thangavel Sakthivel a, Balaji Devakumar b, Shaoying Wang a, Liangling Sun a, Jia Liang a, Sanjay J. Dhoble c, Xiaoyong Huang a, * a b c

College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan, 030024, PR China Department of Physics, Mohamed Sathak A.J. College of Engineering, Chennai, 603103, Tamilnadu, India Department of Physics, R.T.M. Nagpur University, Nagpur, 440033, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Phosphors Gd2GeO5 Energy transfer Color-tunable emissions Bi3þ ions Eu3þ ions

Novel Bi3þ/Eu3þ ions co-doped Gd2GeO5 (GGO) phosphors were prepared by a high-temperature solid-state reaction method. X-ray diffraction, photoluminescence (PL), CIE chromaticity coordinates, internal quantum efficiency (IQE), and temperature-dependent PL spectra were applied to analyze the as-obtained phosphors. The emission spectra of the GGO:Bi3þ,Eu3þ phosphors showed both a broad blue band of Bi3þ emission centered at 453 nm and the characteristic sharp red emission lines of Eu3þ, corresponding to the 3P1→1S0 allowed transition of the Bi3þ ions and the 5D0→7FJ (J ¼ 0, 1, 2, 3, 4) transitions of the Eu3þ ions, respectively. Notably, the asprepared GGO:Bi3þ,Eu3þ phosphors exhibited color-tunable emissions from blue (Bi3þ) to red (Eu3þ) with increasing the Eu3þ doping concentration via a high-efficiency energy transfer process. Moreover, the mechanism of energy transfer from Bi3þ to Eu3þ ions was determined to be the dipole-quadrupole interaction. Impressively, the optimal GGO:0.05Bi3þ,0.12Eu3þ phosphors had an outstanding IQE as great as 88% and good thermal sta­ bility. All these meaningful results demonstrated that blue-red color-tunable GGO:Bi3þ,Eu3þ phosphors have potential applications in white light-emitting diodes (LEDs) and plant growth LEDs.

1. Introduction As well known, light-emitting diodes (LEDs) show lots of advantages such as small size, high light efficiency, low energy consumption, long lifetime, environmental protection, and long life, and thus they have been widely used as representative lighting sources for plant growth lighting and solid-state lighting [1–9] Photosynthetic action spectra (PAS) of plants chlorophylls consist of blue (400–500 nm) light and red (600–700 nm) light [10–12]. There are two methods to obtain the LED artificial light sources for plant growth lighting, namely, one is combining blue and red individual chips, while the other is using ul­ traviolet (UV) chip excite blue-red dual-emitting phosphors [13,14]. Compared with the former, the latter could match well with the PAS and cost less. Moreover, white LEDs for solid-state white lighting can be fabricated by combining an UV LED chip with tricolor (blue/green/red) phosphors [15,16]. Thus, it is crucial to find blue-red dual-emitting phosphors, which not only could be applied in plant growth LEDs to promote the photosynthesis of plant but also can be used as

color-conversion materials for white LEDs. Previously, Bi3þ and Eu3þ co-doped oxide phosphors have attracted much attention due to their broadband absorption and tunable color emissions [17–22]. The Bi3þ ion can emit broadband blue emissions originating from 3P1→1S0 transition under UV excitation [23–27], whereas the Eu3þ ion as a typical red-emitting activator can exhibit red emissions around 615 nm corresponding to its 5D0→7FJ (J ¼ 0, 1, 2, 3, 4) transitions upon UV excitation [28–37]. In view of the wide spectral overlap between Bi3þ ion emission band and the Eu3þ ion sharp exci­ tation bands, co-doping of Bi3þ and Eu3þ ions into a single compound is viable to get blue-red dual-emissions. Currently, various Bi3þ ions and Eu3þ ions co-doped phosphors have been investigated, such as Lu2GeO5: Bi3þ,Eu3þ, BaGd2O4:Bi3þ,Eu3þ, SrLu2O4:Bi3þ,Eu3þ, Ba3Y4O9:Bi3þ,Eu3þ, Y2GeO5:Bi3þ,Eu3þ and KAlGeO4:Bi3þ,Eu3þ [27,38–42]. However, to the best of our knowledge, Bi3þ and Eu3þ ions co-doped Gd2GeO5 (GGO) phosphors have not been reported so far. Accordingly, in this work, we successfully prepared GGO:Bi3þ,Eu3þ phosphors by using a conven­ tional high-temperature solid-state method, and systematically

* Corresponding author. E-mail address: [email protected] (X. Huang). https://doi.org/10.1016/j.jlumin.2020.117127 Received 3 October 2019; Received in revised form 10 February 2020; Accepted 13 February 2020 Available online 14 February 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.

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valence states of Bi3þ ions (r ¼ 1.07 Å for coordination number (CN ¼ 7), r ¼ 1.17 Å for CN ¼ 8), Eu3þ ions (r ¼ 1.01 Å for CN ¼ 7, r ¼ 1.066 Å for CN ¼ 8) and Gd3þ ions (r ¼ 1.00 Å for CN ¼ 7, r ¼ 1.053 Å for CN ¼ 8), the Bi3þ and Eu3þ dopants were suggested to occupy the sites of Gd3þ ions [23,45]. Fig. 2(b) shows the Rietveld refinement for XRD pattern of the GGO:0.05Bi3þ,0.10Eu3þ phosphors. The values of the reliable factors were Rp ¼ 7.98%, Rwp ¼ 10.35%, and χ 2 ¼ 0.5034, which demonstrated that the refinement results were reliable. The results exhibited that the sample was crystallized in the structure of monoclinic with the P21/c space group and the lattice parameters were obtained to be a ¼ 9.33(22) Å, b ¼ 7.10(17) Å, c ¼ 6.84(20) Å, α ¼ γ ¼ 90� , β ¼ 105.44(20)� , and V ¼ 436.798(12) Å3. Compared with the pure GGO host, the lattice param­ eters of the GGO:0.05Bi3þ,0.10Eu3þ sample became bigger, which can be attributed to the fact that the ionic radii of Bi3þ and Eu3þ dopants were larger than the Gd3þ ions. Fig. 3(a) shows PLE and PL spectra of GGO:0.05Bi3þ sample. The PLE spectrum monitored at 453 nm contained three different excitation parts in the 225–350 nm wavelength range. First, the weak PLE peak at around 250 nm was ascribed to the 1S0→1P1 transition of Bi3þ ions. Second, the relatively weak PLE peak at around 276 nm could be attributed to the 8S7/2→6Ij transition of Gd3þ ions, suggesting that the energy transfer from Gd3þ ions to Bi3þ ions occurred. Third, the stron­ gest PLE peak at 321 nm was assigned to the 1S0→3P1 transition of Bi3þ ions [46–49]. Under the excitation of 321 nm, the GGO:0.05Bi3þ phosphors exhibited bright blue light peaking at 453 nm, which was originated from the 3P1→1S0 allowed transition of Bi3þ ions [50]. The full width at half-maximum (FWHM) and Stokes shift was about 85 nm and 132 nm, respectively. Fig. 3(b) shows PL spectrum of GGO:0.05Bi3þ phosphors under the excitation of 321 nm together with two Gaussian curves. As can be seen, the broad asymmetric emission band of GGO:0.05Bi3þ sample could be decomposed into two Gaussian bands centered at 443 nm and 479 nm. When monitored the emissions at 443 nm, 453 nm, and 479 nm, the obtained three PLE spectra of GGO:0.05Bi3þ phosphors showed the similar profiles except for the in­ tensity, which could be observed from Fig. 3(c). Moreover, the PL spectra of the GGO:0.05Bi3þ phosphors with various excitation wave­ lengths were studied, and the corresponding PL spectra were shown in Fig. 3(d). As can be seen, with the excitation wavelength gradually increased from 250 nm to 370 nm, all the PL spectra of GGO:0.05Bi3þ phosphors showed a similar broad emission band peaking at about 453 nm, while their only difference was the emission intensity. Taken together, all these results indicated that the intense broad blue emission band peaking at 453 nm was caused by Bi3þ ion occupying only one site of Gd3þ ion in the host. However, as mentioned above, there were two Gd3þ sites in GGO host, namely, the Gd3þ(1) site with CN ¼ 8 (eight-­ coordinated) and the Gd3þ(2) site with CN ¼ 7 (seven-coordinated). Since Gd3þ was substituted by Bi3þ, thus the Bi3þ ions might occupy these two cationic sites. Therefore, it was necessary to clarify which site the observed 453 nm blue emission came from. It is well-known that the shorter the bond length, the higher the covalency. Because in GGO host lattice the Gd3þ(2) site had shorter average Gd–O bond distance than that of Gd3þ(1) site [44], so the coordinated bond of Bi3þ(2)-O was stronger than that of Bi3þ(1)-O. Moreover, it was reported that when the Bi3þ ion was asymmetrically coordinated, the Stokes shift of Bi3þ emission would be large [51]. Consequently, we deduced that the broadband blue emission around 453 nm with large Stokes shift of 132 nm could be attributed to the 3P1→1S0 transition of Bi3þ(2) site with seven-coordination [22,47,52]. In other words, the Bi3þ ions preferen­ tially occupied Gd(2) sites in GGO host material. As a result, the Bi3þ emissions from the Gd(1) sites in GGO:Bi3þ phosphors were not observed. For comparison, the PLE and PL spectra of GGO:0.05Bi3þ, GGO:0.12Eu3þ, and GGO:0.05Bi3þ,0.12Eu3þ phosphors were shown in Fig. 4. Fig. 4(a) shows the PLE and PL spectra of GGO:0.05Bi3þ phos­ phors. Fig. 4(b) displays the PLE (λem ¼ 615 nm) and PL (λex ¼ 276 nm)

investigated their photoluminescence (PL) performances, energy trans­ fer properties from Bi3þ to Eu3þ ions, CIE chromaticity coordinates, internal quantum efficiency (IQE), and temperature-dependent PL spectra. The obtained results indicated that the as-synthesized GGO: Bi3þ,Eu3þ blue-red dual-emitting phosphors had great potential in plant growth LEDs and white LEDs. 2. Experimental section A variety of phosphor samples with the compositions of GGO:0.05Bi3þ,xEu3þ (x ¼ 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.15 and 0.20) were synthesized by using a high-temperature solid-state reaction method. Bi2O3 (analytical reagent), GeO2 (99.99%), Eu2O3 (99.99%), and Gd2O3 (99.99%) were used as starting raw materials, and they were weighed accurately in a stoichiometric ratio and ground thoroughly to form uniform mixture in an agate mortar. Then, the mixed blends were put into crucibles and sintered at 1300 � C for 12 h. Finally, the samples were cooled down to room temperature naturally and then reground into fine powders for further characterization. The X-ray diffraction (XRD) patterns of the samples were identified using a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ ¼ 1.5406 Å). The PL and PL excitation (PLE) spectra were measured by using an Edinburgh FS5 spectrometer equipped with a 150 W continuedwavelength Xenon lamp. All the measurements were performed at roomtemperature. CIE chromaticity coordinates could be obtained by analyzing the emission spectra of the GGO:Bi3þ,Eu3þ phosphors via using the software of Commission International de L’Eclairage (CIE) 1931. The IQE value of the GGO:0.05Bi3þ,0.12Eu3þ phosphors was measured by using the Edinburgh FS5 spectrometer equipped with an integrating sphere coated with BaSO4. The temperature-dependent PL spectra of the GGO:0.05Bi3þ,0.12Eu3þ phosphors ranging from 303 to 483 K were recorded on the same spectrometer with a temperature controlling system. 3. Results and discussion Fig. 1 shows the crystal structure of the GGO host. The GGO host belonged to monoclinic structure with a space group of P21/c, and the crystal parameters were a ¼ 9.323 Å, b ¼ 7.090 Å, c ¼ 6.838 Å, α ¼ γ ¼ 90� , β ¼ 105.4� , V ¼ 435.8 Å3, and N ¼ 4 [43]. There were two occupied sites of Gd3þ ions in the structure of GGO, namely, one was Gd3þ atom in coordination with 8 oxygen atoms (denoted as: Gd(1) site), the other was coordinated by 7 oxygen atoms (denoted as: Gd(2) site) [44]. Fig. 2(a) shows the XRD patterns of the five typical samples of GGO:0.05Bi3þ, GGO:0.10Eu3þ, GGO:0.20Eu3þ, GGO:0.05Bi3þ,0.10Eu3þ, and GGO:0.05Bi3þ,0.20Eu3þ. Apparently, all the diffraction peaks matched well with the standard profile of GGO (JCPDS no. 78–0477) and no impurity phase was observed, indicating that introduction of Bi3þ and Eu3þ into GGO host did not induced sig­ nificant change to the crystal structure. Based on the ionic radii and

Fig. 1. Crystal structure of the GGO host lattice. 2

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Fig. 2. (a) XRD patterns of GGO:0.05Bi3þ, GGO:0.10Eu3þ, GGO:0.20Eu3þ, GGO:0.05Bi3þ,0.10Eu3þ, and GGO:0.05Bi3þ,0.20Eu3þ phosphors. The standard data of the GGO compound (JCPDS no.78–0477) were also shown in for comparison. (b) Rietveld refinement for XRD pattern of GGO:0.05Bi3þ,0.10Eu3þ phosphors.

Fig. 3. (a) PLE (λem ¼ 453 nm) and PL (λex ¼ 321 nm) spectra of GGO:0.05Bi3þ phosphors. (b) PL spectrum of GGO:0.05Bi3þ phosphors under the excitation of 321 nm together with the corresponding Gaussian fitting curves. (c) PLE (λem ¼ 443, 453, 479 nm) spectra of GGO:0.05Bi3þ phosphors. (d) PL spectra of GGO:0.05Bi3þ phosphors pumped by different excitation wavelengths.

spectra of the GGO:0.12Eu3þ phosphors. As can be seen in Fig. 4(b), a broad excitation band covering from 225 to 300 nm was presented, owing to the charge transfer band (CTB) between Eu3þ ions and the surrounding O2 ions. Besides, an intense PLE peak appearing at 276 nm could be attributed to the 8S7/2→6Ij transition of Gd3þ, indicating that the energy transfer from Gd3þ to Eu3þ ions happened [48]. In addition, a series of excitation peaks at about 314, 363, 384, 394, 415, 466, 531 nm

in the wavelength range of 300–550 nm were observed, corresponding to the 7F0→5HJ, 7F0→5D4, 7F0→5L7, 7F0→5L6, 7F0→5D3, 7F0→5D2, and 7 F0→5D1 transitions of Eu3þ ions, respectively [53]. Under 276 nm excitation, the obtained PL spectrum of GGO:0.12Eu3þ phosphors con­ sisted of the characteristic red emission lines of Eu3þ in the region from 550 to 750 nm (see Fig. 4(b)). The PL peaks at 587, 605, 615, 643, 705 nm corresponded to the 5D0→7FJ (J ¼ 0, 1, 2, 3, 4) transitions of Eu3þ 3

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that the obtained PLE spectrum of GGO:0.05Bi3þ,0.12Eu3þ phosphors included the O2 →Eu3þ CTB band (225–300 nm range), a broad exci­ tation band of the Bi3þ ions (250–350 nm range), and a group of the characteristic excitation lines of Eu3þ ions (350–550 nm range). More­ over, under the excitation at 321 nm, both the broad emission band of Bi3þ ions (375–550 nm range) and sharp emission peaks of Eu3þ ions (550–750 nm range) were observed in the PL spectrum of GGO:0.05Bi3þ,0.12Eu3þ phosphors. Thus, the energy transfer from Bi3þ to Eu3þ ions existed in GGO:Bi3þ,Eu3þ phosphors. Fig. 5(a) presents the PL spectra of GGO:0.05Bi3þ,xEu3þ (x ¼ 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.15 and 0.20) phosphors excited at 321 nm. For all the Bi3þ/Eu3þ co-doped samples, both Bi3þ blue emission band and Eu3þ red emission peaks were observed in their PL spectra. Fig. 5(b) shows the normalized PL intensities of the Bi3þ and Eu3þ ions as functions of Eu3þ doping concentration. It can be found that the 453 nm blue emission intensity of Bi3þ ions decreased with the increasing of Eu3þ doping concentration. In contrast, the 615 nm red emission in­ tensity of Eu3þ ions first increased and reached a maximum at x ¼ 0.12, then decreased because of the concentration quenching, which was caused by the nonradiative energy transfer between nearby Eu3þ ions. This phenomenon further proved that energy transfer from Bi3þ to Eu3þ ions indeed took place in GGO:Bi3þ,Eu3þ phosphors [55]. The energy transfer efficiency (ηT ) from Bi3þ to Eu3þ ions could be calculated using the following equation [18,56–61]:

Fig. 4. PLE and PL spectra of (a) GGO:0.05Bi3þ, (b) GGO:0.12Eu3þ, and (c) GGO:0.05Bi3þ,0.12Eu3þ phosphors.

ions [54]. Impressively, by comparing Fig. 4(a) with Fig. 4(b), we could see that an obvious spectral overlap between the broad blue emission of GGO:0.05Bi3þ phosphors and sharp excitation peaks of GGO:0.12Eu3þ phosphors in the 370–550 nm wavelength range, which demonstrated that efficient energy transfer from Bi3þ to Eu3þ ions may occur in the GGO:Bi3þ,Eu3þ phosphors. Fig. 4(c) exhibits the PLE and PL spectra of GGO:0.05Bi3þ,0.12Eu3þ phosphors. When monitored at 453 nm, the resulting PLE spectrum was similar to that of Bi3þ singly-doped GGO sample and the peak at 321 nm was assigned to the Bi3þ ions. When monitored at 615 nm, we could see

ηT ¼ 1

IS ; IS0

(1)

where IS0 and IS are the luminescence intensity of the Bi3þ ions in the absence and presence of Eu3þ ions, respectively. Fig. 5(c) displays the

Fig. 5. (a) PL spectra of the GGO:0.05Bi3þ,xEu3þ (x ¼ 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.15 and 0.20) phosphors excited at 321 nm. (b) The normalized PL intensities of the Bi3þ and Eu3þ ions as functions of Eu3þ doping concentration in GGO:0.05Bi3þ,xEu3þ samples (λex ¼ 321 nm). (c) The Bi3þ→Eu3þ energy transfer efficiency in GGO:0.05Bi3þ,xEu3þ phosphors (λex ¼ 321 nm). (d) Dependences of Iso/Is of Bi3þ ions on (i) C6/3 � 101, (ii) C8/3 � 102, and (iii) C10/3 � 103. 4

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Eu3þ doping concentration dependent energy transfer efficiency (ηT ) in GGO:0.05Bi3þ,xEu3þ phosphors under 321 nm excitation. The values of ηT were about 18.4%, 23.7%, 42.7%, 46%, 55.0%, 58.4%, 66.2%, and 80.6% for GGO:0.05Bi3þ,xEu3þ samples with x ¼ 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.15, and 0.20, respectively. It was found that with increasing Eu3þ concentrations the value of ηT gradually increased and the maximum value reached 80.6% at x ¼ 0.20, which indicated that the energy transfer from Bi3þ to Eu3þ in GGO:0.05Bi3þ,xEu3þ was very efficient. In order to investigate the Bi3þ→Eu3þ energy transfer mechanism, the critical distance (Rc) between the Bi3þ and Eu3þ ions was calculated by the following equation [62–64]: �1=3 � 3V Rc ​ � ​ 2 ; 4πXc N

the lowest vibration mode of 3P1 state [41]. After that, some of the excited electrons returned to the ground state, resulting in the broad­ band blue emission centered at 453 nm, while the others could effi­ ciently transfer their energy to the 5L7 energy level of Eu3þ ions because of the excellent overlap of the emission energy of Bi3þ ions and the excitation energy of Eu3þ ions [42]. In this process, the emission of Bi3þ ions gradually decreased. At the same time, the excited electrons at the 5 L7 energy level of Eu3þ ions would relax nonradiatively to the first excited state 5D0. Finally, the excited electrons of Eu3þ ions at 5D0 state returned to the ground states 7FJ (J ¼ 0, 1, 2, 3, 4), producing the characteristic sharp red emissions of Eu3þ ions due to the 5D0→7FJ (J ¼ 0, 1, 2, 3, 4) transitions [50,68]. Consequently, by carefully controlling the Bi3þ→Eu3þ energy transfer, the intensity ratio of Eu3þ red emission to Bi3þ blue emission can be accurately adjusted, thus leading to blue-red color-tunable emissions. The CIE chromaticity coordinates of GGO:0.05Bi3þ,xEu3þ (x ¼ 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.15 and 0.20) phosphors were shown in Fig. 7. And the corresponding digital photographs of GGO:0.05Bi3þ, xEu3þ phosphors under a 365 nm lamp were also shown in Fig. 7. As can be seen, with increasing Eu3þ doping concentration, the CIE chroma­ ticity coordinates varied from (0.1463, 0.1200) to (0.4370, 0.2570) and the emission color changed from blue to red. Furthermore, we measured the IQE value of the GGO:0.05Bi3þ,0.12Eu3þ sample under 321 nm excitation by using integrating sphere method. The IQE value of GGO:0.05Bi3þ,0.12Eu3þ sample could be calculated using the following equation [69]: R LS R η¼ R (4) ER ES

(2)

where N is the number of sites that activators can substitute in per unit cell; V denotes the cell volume of the unit cell; and xc is the total con­ centration of Bi3þ and Eu3þ ions of the GGO:0.05Bi3þ,0.12Eu3þ phos­ phor. Herein, V ¼ 436.798(12) Å3, xc ¼ 0.17, and N ¼ 4 [43], and thus Rc was calculated to be 10.7 Å, which was larger than 5 Å. Therefore, the mechanism of Bi3þ→Eu3þ energy transfer in GGO:Bi3þ,Eu3þ phosphors can be attributed to electric multipolar interaction. Moreover, according to Dexter’s energy transfer formula of multipolar interaction and Reis­ feld’s approximation, we can use the following relationship to further confirm the multipolar interaction [65]: � ηS0 ηS ∝ Cn=3 (3) where ηS0 and ηS are the luminescence quantum efficiencies of the Bi3þ ions in the absence and presence of Eu3þ, respectively; C is the total concentration of the Bi3þ and Eu3þ ions. Because the value of ηS0 = ηS was not easily to be determined, it could be approximately assessed by the value of IS0/IS [59,66]. Besides, n ¼ 6, 8 and 10 correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole in­ teractions, respectively [67]. The liner fitting between IS0/IS and Cn/3 was shown in Fig. 5(d). Clearly, the values of fitting factor R2 were calculated to be 0.9651, 0.9818, and 0.9772 for n ¼ 6, 8, and 10, respectively. The best linear relationship was achieved when n ¼ 8. Therefore, the Bi3þ→Eu3þ energy transfer mechanism was attributed to the dipole-quadrupole interaction. Fig. 6 shows the simplified energy transfer process from Bi3þ to Eu3þ ions in GGO:Bi3þ,Eu3þ phosphors. Upon 321 nm excitation, the elec­ trons at the ground state 1S0 of Bi3þ ions absorbed the excitation energy and jumped to the excited state 3P1. Then they relaxed nonradiatively to

where η is IQE; LS represents the emission spectrum of the sample; ES and ER correspond to the spectra of excitation light with sample and only with BaSO4 reference, respectively. Thus, the IQE value of GGO:0.05Bi3þ,0.12Eu3þ sample was measured to be about 88%, which was much higher than that of many previously reported Bi3þ and Eu3þ co-doped oxide phosphors, such as Mg3Y2Ge3O12:Bi3þ,Eu3þ (IQE: 63.89%), Lu2GeO5:Bi3þ,Eu3þ (IQE: 43%), and Ba3Y(BO3)3:Bi3þ,Eu3þ (IQE: 35%) [18,27,70]. Thermal stability behavior is an indispensable parameter for the practical application of phosphors in illumination field. Fig. 8(a) shows the temperature-dependent PL spectra of GGO:0.05Bi3þ,0.12Eu3þ sam­ ple under the excitation at 321 nm. Obviously, the emission profiles of GGO:0.05Bi3þ,0.12Eu3þ sample at different temperatures almost did not change except for PL intensity, but the emission intensity decreased continuously with increasing temperature as a result of the occurrence of the thermal quenching effect. Fig. 8(b) shows the normalized PL in­ tensity in the 370–750 nm wavelength range of the GGO:0.05Bi3þ,0.12Eu3þ phosphors as a function of temperature. The emission intensity at 423 K (150 oC) remained 40% of that at 303 K (30 o C). Moreover, the activation energy (Ea) was calculated via the following equation [71,72]: � � I0 Ea ln 1 ¼ lnA (5) I kT where I0 and I represent the emission intensity at 303 K and at different given temperatures T, respectively; k is the Boltzmann coefficient; A is the constant; and Ea is activation energy [73]. Fig. 8(c) shows the linear relationship between ln(I0/I-1) and 1/kT. The slope of the fitting line was 0.189, so the value of Ea was obtained to be 0.189 eV. The results demonstrated that the GGO:0.05Bi3þ,0.12Eu3þ phosphors had good thermal stability. 4. Conclusions In summary, we reported novel blue-red dual-emitting GGO:Bi3þ, Eu phosphors via Bi3þ→Eu3þ energy transfer. The dipole-quadrupole

Fig. 6. The schematic diagram of energy transfer from Bi3þ to Eu3þ in GGO: Bi3þ,Eu3þ phosphors.

3þ

5

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Fig. 7. The CIE chromaticity diagram of the GGO:0.05Bi3þ,xEu3þ (x ¼ 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.15 and 0.20) phosphors (λex ¼ 321 nm). Insets are photographs of these samples under a 365 nm UV lamp. Their corresponding CIE chromaticity coordinates were also shown.

Fig. 8. (a) Temperature-dependent PL spectra of GGO:0.05Bi3þ,0.12Eu3þ sample under 321 nm excitation. (b) The normalized PL intensities of the GGO:0.05Bi3þ,0.12Eu3þ sample at different temperatures from 303 to 483 K. (c) The plot of ln(I0/I-1) versus 1/kT, and the calculated activation energy (Ea) for the GGO:0.05Bi3þ,0.12Eu3þ sample.

interaction could be the main mechanism for Bi3þ→Eu3þ energy trans­ fer. Upon excitation by UV light at 321 nm, blue-red tunable color emissions were realized by adjusting Eu3þ doping concentration in GGO:0.05Bi3þ,xEu3þ phosphors. The GGO:0.05Bi3þ,0.12Eu3þ phos­ phors exhibited high IQE of 88% excited at 321 nm. The temperaturedependent spectra showed that the GGO:0.05Bi3þ,0.12Eu3þ phosphors had good thermal stability, and the corresponding activation energy Ea was 0.189 eV. The ability of converting UV light into blue/red emissions makes GGO:Bi3þ,xEu3þ phosphors promising candidates for plant growth LEDs and white LEDs.

editing. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502190). References [1] X. Huang, Solid-state lighting: red phosphor converts white LEDs, Nat. Photon. 8 (2014) 748–749. [2] X.Y. Huang, New red phosphors enable white LEDs to show both high luminous efficacy and color rendering index, Sci. Bull. 64 (2019) 879–880. [3] L. Sun, B. Devakumar, J. Liang, S. Wang, Q. Sun, X. Huang, Highly efficient Ce3 þ →Tb3þ energy transfer induced bright narrowband green emissions from garnettype Ca2YZr2(AlO4)3:Ce3þ,Tb3þ phosphors for white LEDs with high color rendering index, J. Mater. Chem. C 7 (2019) 10471–10480. [4] Z.-w. Zhang, J.-w. Hou, J. Li, X.-y. Wang, X.-y. Zhu, H.-x. Qi, R.-j. Lv, D.-j. Wang, Tunable luminescence and energy transfer properties of LiSrPO4:Ce3þ,Tb3þ,Mn2þ phosphors, J. Alloys Compd. 682 (2016) 557–564.

CRediT authorship contribution statement Qi Sun: Investigation, Data curation, Writing - original draft. Thangavel Sakthivel: Investigation. Balaji Devakumar: Software, Investigation. Shaoying Wang: Investigation. Liangling Sun: Investi­ gation. Jia Liang: Investigation. Sanjay J. Dhoble: Resources. Xiaoyong Huang: Conceptualization, Supervision, Writing - review & 6

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