Improved photo-luminescence by co-doped lithium in the phosphor system CeO2:Eu3+

Author’s Accepted Manuscript Improved photo-luminescence by co-doped lithium in the phosphor system CeO2:Eu3+ Wei Huang, YongJun Tan, Dewei Li, Hongli Du, Xiaowu Hu, Guizhi Li, Yongqing Kuang, Mei Li, Dongcai Guo www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)30639-2 https://doi.org/10.1016/j.jlumin.2018.10.072 LUMIN16020

To appear in: Journal of Luminescence Received date: 10 April 2018 Revised date: 12 October 2018 Accepted date: 15 October 2018 Cite this article as: Wei Huang, YongJun Tan, Dewei Li, Hongli Du, Xiaowu Hu, Guizhi Li, Yongqing Kuang, Mei Li and Dongcai Guo, Improved photoluminescence by co-doped lithium in the phosphor system CeO2:Eu3+, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.10.072 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improved photo-luminescence by co-doped lithium in the phosphor system CeO2:Eu3+ Wei Huanga,b, YongJun Tana, Dewei Lia, Hongli Dua, Xiaowu Hua, Guizhi Lia, Yongqing Kuanga, Mei Lic, Dongcai Guoa,b,* a School of Chemistry and Chemical Engineering, State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China b Hunan Provincial Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions, Changsha 410082, China c Hunan Houde Environmental Protection Technology Co., Ltd. , Changsha 410082, China * Corresponding Author: Dongcai Guo, E-mail address: [email protected] (D. Guo), Tel./fax: +86073188821449

Abstract: Li+ co-doping is used as a prevalent way to improve the luminescent intensity of CeO2:Eu3+[25]. Here, we investigate the structural and luminescent properties of CeO2:Eu3+ by the different co-doping ways of Li+ ions, including the direct co-doping of Li+ by hydro-thermal co-precipitation and the indirect co-doping of Li+ by a post calcining of Li2CO3. The different co-doping ways of Li+ ions was found to cause different effect on structure and luminescence of CeO2:Eu3+ phosphor. The result showed opposite trend with conventional explanations[25] where the substitution of Ce4+ with doped Li+ ions are the prime reason for luminescence intensity enhancement. Oxygen vacancies should be formed due to the charge imbalance. It is found that the Li+ direct co-doped CeO2: Eu3+ didn’t improve the crystallization of the samples due to the increase of oxygen vacancies, as is supported by X-ray diffraction(XRD) patterns and Raman spectra. Meanwhile, the spectroscopic investigations reveal that the Li+ direct co-doped CeO2: Eu3+ lead to less asymmetry around Eu3+ and lower luminescence. However, results show that the indirect co-doping of Li+ lead to a increase of crystallinity and decrease in oxygen vacancies concentration. The luminescent intensity of CeO2:Eu3+ can be significantly enhanced by indirect co-doping of Li+. Consequently, we reason that the enhanced luminescence mainly results from a increase of crystallinity and decrease in oxygen vacancies concentration due to the flux effect of Li+ ions, instead of the substitution of Ce4+ with

doped Li+ ions. Keywords: CeO2; Li+ co-doping ; oxygen vacancies; luminescence; flux effect;

1. Introduction Cerium oxide, a rare earth metal oxide, have been given much attention in the wide range of applications such as catalysis[1], solid oxide fuel cells[2], oxygen storage capacitors[3] and hybrid solar cells[4]. Studies has shown that cerium oxide nanoparticles possess the radical scavenging properties based on their ability to either donate or receive electrons as they change between the +3 and +4 valence states[5]. A new frontier for cerium oxide in biomedical applications were suggested due to the excellent bio-compatibility of cerium oxide nanoparticles[6], such as in vivo optical-based diagnostic imaging[7] and therapy[8]. However, CeO2 matrix is known to have some percentage of Ce3+ sites and oxygen vacancies in a cubic unit cell [9-10]. Weak emission characteristics were shown due to the presence of such defects, which limit its identification in biological and cellular studies. Various approaches[11-16] have been used to enhance the luminescence efficiency of cerium oxide phosphor, such as double or triple doped matrixes, controlled synthesis[17-18]. For example, the potential for therapy applications of CeO2 co-doped with Yb and Ln ( Er, Ho or Tm) was first investigated by Seal et al. [19]. Mono-dispersed Ce4+-Gd3+-Eu3+ oxide phosphors for enhanced red emission under visible excitation was first investigated by Sorbello et al. [20]. In addition, another approaches is alkali metal ion co-doping[21], and the doped alkali metal ion can be easily inserted into the doped phosphor host substitutionally or interstitially because of its low valence state and smaller ionic radius[22-24]. As a result, the oxygen vacancies may be compensated by co-doping alkali metal ions and increase in luminescence intensity. Furthermore, the presence of alkali metal ions can be to serve as flux materials or an effect through the removal of quenching centers. For example, Junga et al.[25] have investigated the effects of alkali metals doping on CeO2:Eu3+ nanocrystals. The result shown that co-doping Li+ enhanced their luminescence intensity. Meanwhile, they consider that Li+ ions are well substituted into the Ce4+

sites in a CeO2 cubic matrix. The effect of the charge compensation and the improvement of crystallinity was shown. However, Florea et al.[26] shown a less effective substitution of Li+ for Ce4+ into ceria lattice. In another work, Min et al.[27] shown that Na+ ions are well inserted in the oxygen vacancy. In all of these studies, although co-doping of alkali metal ions enhances the luminescence of the phosphors by charge compensation and improvement of crystallinity[28-30], the mechanisms of alkali-metal ions co-doping on CeO2:Eu3+ emission is still ambiguous. Here, we investigate the effect of the different co-doping ways of Li+ ions on CeO2:Eu3+ phosphor, including the direct co-doping of Li+ by hydro-thermal co-precipitation and the indirect co-doping of Li+ by a post calcining of Li2CO3. The defect chemistry, luminescence properties, and structural properties in CeO2:Eu3+, Li+ phosphor has been studied using XRD, Raman spectra, UV-visible absorption spectra and luminescence spectra. In this paper, Raman spectra and XRD on Li+ direct co-doped CeO2: Eu3+ system elucidate the substitution of Ce4+ with Li+ ions and induce a amount of oxygen vacancies. The indirect co-doping of Li+ lead to a increase of crystallinity and decrease in oxygen vacancies concentration, which exhibit the flux effect of Li salts. Therefore, a plausible mechanism for luminescence enhancement on Li+ co-doping is discussed in detail.

2. Experimental 2.1 Preparation of samples The raw materials were Ce(NO3)3·6H2O, Eu(NO3)3·6H2O, LiNO3, polyethylene glycol 2000, NH3·H2O (AR). All of them were used as-received without further purification. The Ce(NO3)3·6H2O, Eu(NO3)3·6H2O and LiNO3 was first dissolved in deionized water to obtain 0.1mol/L Ce(NO3)3, 0.1mol/L Eu(NO3)3 and 0.1mol/L LiNO3 aqueous solution. And the 5mol% Li+ direct co-doped CeO2: 5mol% Eu3+ nanocrystals phosphors were synthesized as follows: 0.8 g Polyethylene glycol (2000) and 30 ml deionized water were added to single-mouth flask with magnetic stirring for 20 min at 40℃. 20ml 0.1mol/L Ce(NO3)3, 1ml 0.1mol/L Eu(NO3)3 and 1ml 0.1mol/L LiNO3 aqueous solution were added, respectively. The mixed solution was

stirred for 30 min and formed the molar ratio of Ce : Eu : Li= 0.9: 0.05: 0.05. Subsequently, the mixed solution pH value was adjusted to 12 by addition of NH3·H2O solution and continuously stirred for 1 h[31]. The 52 ml reaction solution was transferred into a 100 ml hydro-thermal reactor and maintained at 180℃for 12 h without stirring. When the reaction was completed, the precursors were rinsed with ethanol and deionized water, respectively. The obtained precursors were dried at 80℃ for 24 h under vacuum. The obtained precursors sintered at 1000℃ for 2 h in the furnace in air and heating rate is 10 degrees per minute. The 5mol% Li+ indirect co-doped CeO2: 5mol% Eu3+ nanocrystals phosphors were synthesized. 0.8g Polyethylene glycol (2000) and 30ml deionized water were added to single-mouth flask with magnetic stirring for 20 min at 40℃. 20ml 0.1mol/L Ce(NO3)3, 1ml 0.1mol/L Eu(NO3)3 aqueous solution were added, respectively. The mixed solution was stirred for 30 min and formed the molar ratio of Ce : Eu = 0.95: 0.05. Subsequently, the mixed solution pH value was adjusted to 12 by addition of NH3·H2O solution and continuously stirred for 1 h[31]. The 51 ml reaction solution was transferred into a 100 ml hydro-thermal reactor and maintained at 180℃for 12 h without stirring. When the reaction was completed, the precursors were rinsed with ethanol and deionized water, respectively. The obtained precipitate were dried at 80℃ for 24 h under vacuum. Finally, the grind of crude product CeO2: 5mol% Eu3+ nanocrystals and 0.0037g Li2CO3 sintered at 1000℃ for 2 h in the furnace in air and heating rate is 10 degrees per minute. 2.3 Characterization The X-ray Powder Diffraction was conducted on a Bruker ADVANCED8 Advance X-ray diffractometer with CuKα X-rays at a scanning rate of 4 °/min-1, speed 0.5 s, and angle from 10 to 80° as well as tube voltage 40 kV and tube current 30 mA. Raman Spectra was measured by Invia-Reflex Confocal Micro Raman spectroscopy. The preparation of sample was as follows: a trace amount of CeO2:Li+, Eu3+ was dispersed in ethanol and was evenly mixed. The mixture was dropped with a dropper to the prepared copper grid and dried to conduct experiment. The absorption characteristics of the synthesized samples were studied in the wavelength range

200-800 nm using a Shimadzu, UV-3600 UV-vis spectrophotometer using barium sulphate as the reference. The photo-luminescence of the phosphors was measured by HITACHI F-2700 Fluorescence Spectrometer (operation voltage 400 V, with a slit width of 5 nm).

3. Results and discussion 3.1 Li+ direct co-doped CeO2: Eu3+ nanocrystals phosphors 3.1.1 Structural Properties Fig. 1 present the results of the XRD patterns of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+. The standard pattern (JCPDS no. 34-0394) of CeO2 is referenced as an initial structural model. As illustrated in Fig. 1a, all the peaks match well with the peaks (111), (200), (220), (311), (222), (400), (331) and (420) of the standard CeO2, without Eu and Li phase separation, suggesting that the formation of solid solution in the entire composition range[7]. In the Fig. 1b, enlarged XRD patterns for (111) show that the diffraction peak of Li+ direct co-doped CeO2: Eu3+ sample decreases in intensity and the peak angle have a shift, indicating that the Li + direct co-doped CeO2: Eu3+ didn’t improve the crystallization of the samples. The result may be ascribed to the increase of oxygen vacancies. It is established in the literature[26] that, on substitution of monovalent ion for Ce4+, oxygen vacancies is formed due to the charge imbalance on the substitution of Ce4+ with monovalent ion. The charge compensation mechanism described in the Kröger Vink notation as: CeO2 + Li2O→2Li’’’Ce + 3VÖ + OXO. Further, the studies[31] on molecular dynamics established that the size of an oxygen vacancy is smaller than that of an oxygen ion. Therefore, the increase of oxygen vacancy may distort the cubic lattice of the host.

Intensity a.u

JCPDS File No. 34-0394 (111) (220)

(200)

(311) (222)

(420) (400)

(331)

+

3+

Li direct co-doped CeO2: Eu

3+

CeO2: Eu

10

20

30

40

50

2θ(degree)

60

70

80

Fig. 1a XRD patterns of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ annealed at 1000℃. CeO2: Eu

3+

+

Intensity a.u

Li direct co-doped CeO2: Eu

28.0

28.5

29.0

29.5

3+

30.0

2θ(degree) Fig. 1b Enlarged XRD patterns of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ for (111).

The morphology of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ is investigated by SEM and the result shown in Fig. 2. The resulting particles show a hard agglomeration of primary nano-sized grains. Because CeO2 nanomaterials is easy to form hard agglomeration upon high-temperature annealing[33]. However, Li+ direct co-doped CeO2: Eu3+ show a rougher surface. Therefore, the average crystallite size of the samples was calculated by Debye-Scherrer equation based on full-width at half maximums of (111) planes, and then the calculated result are shown in Table 1. It was found that Li+ direct co-doped CeO2: Eu3+ did not provide much growth of the

crystallite size of the crystal and even slightly reduced the crystallite size of the crystal. Earlier reports on ions doped ceria reveal that the incorporation of considerable oxygen vacancies in the lattice can cause a decrease in the crystallite size[26].

Fig. 2 SEM image of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ annealed at 1000℃.

Table 1 crystallite size of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ Sample

Peak FWHM (degrees-1)

Crystallite size (nm) Scherrer

CeO2: Eu3+

0.201

45.9

Li+ direct co-doped CeO2: Eu3+

0.245

44.1

To study the behavior of Li+ direct co-doped CeO2: Eu3+ crystals in detail, Raman spectra were conducted. As shown in Fig. 3, the Raman spectra have main bands at about 464 cm-1 which corresponds to the Raman-active F2g mode of ceria cubic fluorite lattice. A band around 545 cm-1 can be observed, which is related to oxygen vacancies produced in the host CeO2[15]. Meanwhile, Raman modes of Eu2O3 phase and Li2O phase at about 340 cm-1 [32]and 520 cm-1 [26]were neither observed in the spectra. Further, the dependence of the ratio of peak 545 cm-1 and 464 cm-1 (A545/A464) on CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ is inset in Fig. 3. It can be seen that the value of A545/A460 becomes larger with the Li+ direct co-doped CeO2: Eu3+, suggesting the increase of oxygen vacancy content, which is in good agreement with the above XRD analysis.

Intensity a.u

CeO2: Eu

3+

+

Li direct co-doped CeO2: Eu

3+

A545/A464

0.36

0.34

0

450

Li concentration x(mol%)

600 750 -1 wavelength/cm

5

900

Fig. 3 Raman spectra of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ annealed at 1000℃(the inset plots the dependence of A545/A464 on Li doping concentration x).

3.1.2 Luminescent properties UV-visible absorption spectra of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ were recorded in the 200−800 nm region and shown in Fig. 4. The absorption spectra exhibit absorption bands in the UV range. The maxima at 250 nm correspond to O2- to Ce3+ charge transfer transitions, and the absorption maxima at 345 nm can be assigned to O2- to Ce4+ charge transfer[32]. It is observed from Fig.4 that when Li+ was co-doped to the CeO2: Eu3+ sample, a decrease in UV-visible absorption was observed. The result may be ascribed to the increase of oxygen vacancy content. The introduction of oxygen vacancies in cerium oxide samples distorts the fluorite lattice, as is supported by the above XRD. In the presence of impurities or lattice disorder, the number of available OCe4 units reduced, and the phonon function is spatially confined, which lead to UV-visible absorption decreased. And the effect of Li+ on luminescence properties still need to be further discussed by excitation and emission spectra.

3+

absorbance a.u

CeO2: Eu

200

+

3+

Li direct co-doped CeO2: Eu

300

400 500 600 wavelength/nm

700

800

Fig. 4 UV-vis absorption spectra of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ annealed at 1000℃.

The excitation and emission properties of CeO2:Eu3+ and Li+ direct co-doped CeO2:Eu3+ shown in Fig. 5. As illustrated in Fig. 5a, the excitation spectra are composed of a broad band centered at about 360-365 nm and a sharp peak at 467 nm. The 467 nm sharp peak originates from the 7F0-5D2 transition of Eu3+. The broad band at about 360-365 nm originates from the charge transfer (CT) transitions between O2and Ce4+ ions. It can be seen from Fig. 5a that the direct co-doped with Li+ decrease the CT intensity and 467 nm sharp peak, which is in agreement with the decrease in UV-visible absorption. In Fig. 5b, the emission spectra monitored at 365 nm also exhibit the same change: the characteristic transition of 5D0-7F2 and 5D0-7F1 of Eu3+ at 593, 613, and 634 nm obviously decreased with direct co-doping of Li+. By combining the information obtained from luminescence spectra with that of Raman data, it is found that the role of direct co-doping of Li+ lead to the enhancement of oxygen vacancies at which the luminescence quenching occurs[9]. The present studies showed opposite trend with conventional explanations where the substitution of Ce4+ with doped Li+ ions are the prime reason for luminescence enhancements[25]. Further, the branching ratio of 5D0 emission of Eu3+ was used as a probe for the assessment of site symmetry, and the result was shown in table 2. It is clear from table 2 that the value of 5D0-7F2/5D0-7F1 is lower than 1 and increase with direct co-doping of Li+ ions. The fact is that the magnetic dipole transition 5D0-7F1 is dominant in a site

with an inversion symmetry, while the 5D0-7F2 electronic transition becomes the strongest one in a site without inversion symmetry. In table 2, the dominant emission is the 5D0-7F2 transition of Eu3+, suggesting that direct co-doping of Li+ ions decrease the inversion symmetry of Eu3+ due to the increase of oxygen vacancies. a

3+

CeO2: Eu

Intensity a.u

+

3+

Li direct co-doped CeO2: Eu

300

325

350 375 400 425 wavelength/nm

450

475

500

Fig. 5a The excitation spectra of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ annealed at 1000℃. 3+

Intensity a.u

CeO2: Eu +

3+

Li direct co-doped CeO2: Eu

555

570

585 600 615 wavelength/nm

630

645

Fig. 5b The emission spectra of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ annealed at 1000℃.

Table 2 the intensity ratio of the transitions 5D0-7F2 to 5D0-7F1 under 365 nm Sample

CeO2:Eu3+ Li+ direct co-doped CeO2:Eu3+

5

D0-7F2 3338 2553

5

D0-7F1 5748 4308

5

D0-7F2/5D0-7F1 0.5807 0.5926

To further understand the behaviour of the luminescent properties of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+, the samples was annealed at different temperatures. Their luminescence spectra was obtained and shown in Fig. 6. It can be seen from Fig. 6a that the luminescent intensity of CeO2: Eu3+ phosphor was found to increase with direct co-doping of Li+ annealed at 600℃. However, Fig. 6b,c show that the luminescent intensity of CeO2: Eu3+ was decreased with direct co-doping of Li+ annealed at 800℃ and then obviously decreased annealed at 1000℃. It can be seen more clearly in Fig. 6d. From the above results we can know, a decrease of luminescent intensity of Li+ direct co-doped CeO2: Eu3+ annealed at 1000℃ may be ascribed to the increase of oxygen vacancies. It is established that a big difference exist in ionic radius and charge of Li+ and Ce4+. Thus annealed at 600℃, there is not enough energy to obtain the substitution of Ce4+ with Li+. The luminescent intensity of CeO2: Eu3+ phosphor was found to increase with direct co-doping of Li+. Upon high-temperature annealing, the Li+ ions have enough energy to replace Ce4+, which is responsible for the increase of oxygen vacancies. Especially, annealed at 1000℃, the luminescent intensity of Li+ direct co-doped CeO2: Eu3+ is obviously decreased. As is supported by the Raman results and XRD, the Li+ direct co-doped CeO2: Eu3+ induced the considerable oxygen vacancies. Consequently, it may be understood in that the direct co-doping of Li+ at high-temperature annealing promotes substitution of Ce4+

a

3+

CeO2: Eu +

600 ℃

3+

Li direct co-doped CeO2: Eu

915.2 680.9

555

570

585 600 615 wavelength/nm

630

645

Intensity a.u

Intensity a.u

by Li+.

b

3+

CeO2: Eu +

3+

Li direct co-doped CeO2: Eu

800 ℃ 2843 2465

555

570

585 600 615 wavelength/nm

630

645

+

3+

Li direct co-doped CeO2: Eu

1000 ℃ 5748 4308

555

570

585 600 615 wavelength/nm

630

645

Intensity a.u

Intensity a.u

c

3+

CeO2: Eu

3+

d

600

CeO2: Eu +

3+

Li direct co-doped CeO2: Eu

700

800

900

Temperature℃

1000

Fig. 6 The emission spectra of CeO2: Eu3+ and Li+ direct co-doped CeO2: Eu3+ annealed at different temperature.

3.2 Li+ indirect co-doped CeO2: Eu3+ nanocrystals phosphors 3.2.1 Structural properties Fig. 7 present the results of the XRD patterns of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+. As illustrated in Fig. 7, all the peaks match well with the peaks (111), (200), (220), (311), (222), (400), (331) and (420) of the standard CeO2, without Eu and Li phase separation, suggesting that the Eu and Li ions are doped in the lattice of CeO2. In the Fig. 7, The diffraction peaks increase in intensity in comparison to CeO2: Eu3+. The result is opposite with Li+ direct co-doped CeO2: Eu3+. The average crystallite sizes of the samples were also calculated using the Debye-Scherrer equation with full-width at half maximums (fwhms) of (111) planes and are shown in Table 3. It was found that the indirect co-doping of Li+ showed a distinctive increase (almost twice) in the crystalline size.

Intensity a.u

JCPDS File No. 34-0394 (111) (220)

(200)

(420)

(311) (222)

(400) (331)

+

3+

Li indirect co-doped CeO2: Eu

3+

CeO2: Eu

10

20

30

40 50 2θ(degree)

60

70

80

Fig. 7 XRD patterns of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ annealed at 1000℃.

Table 3 crystallite size of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ Sample CeO2: Eu3+ +

Li indirect co-doped CeO2: Eu

3+

Peak FWHM (degrees-1)

Crystallite size (nm) Scherrer

0.201

45.9

0.169

81.5

The morphology of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ is investigated by SEM and the result shown in Fig. 8. It can be observed directly from Fig. 8 that the resulting particles show a hard agglomeration of primary nano-sized grains. Since upon high-temperature annealing, CeO2 nanomaterials is easy to form hard agglomeration. Meanwhile, it appears that the boundaries between the neighboring particles melt together with a fairly rough surface. But the Li+ indirect co-doped CeO2: Eu3+ is observed with a smoother and denser surface. By combining the information obtained from XRD with that of SEM result, it is found that the role of indirect co-doping of Li+ lead to a increase of crystallinity and exhibit the flux effect of Li salts.

Fig. 8 SEM image of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ annealed at 1000℃.

To further study the behavior of the indirect co-doping of Li+ in detail, Raman spectra were conducted. As shown in Fig. 9, the Raman spectra have main bands at about 464 cm-1 which corresponds to the Raman-active F2g mode of ceria cubic fluorite lattice. A band around 545 cm-1 is related to oxygen vacancies produced in the host CeO2. Meanwhile, Raman modes of Eu2O3 phase and Li2O phase at about 340 cm-1 and 520 cm-1 was neither observed in the Raman spectra. The dependence of the ratio of peak 545 cm-1 and 464 cm-1 (A545/A464) on CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ is inset in Fig. 9. It can be seen that the value of A545/A460 becomes smaller with the Li+ indirect co-doped CeO2: Eu3+, which suggests the decrease of oxygen vacancy content. It is established that oxygen vacancy can be in the crystal lattice and at the crystal boundary, but most of it is located at the crystal boundary[33]. By combining the information obtained from XRD result with that of Raman data, it is found that the oxygen vacancy at the crystal boundary of samples prepared by the Li+ indirect doping method is significantly reduced compared to the Li+ direct doping method or without Li+ doping.

CeO2: Eu

3+

+

Intensity a.u

Li indrect co-doped CeO2: Eu

3+

A545/A464

0.34

0.32

0.30

0.28

400

0

Li concentration x(mol%)

500 600 700-1 wavelength/cm

800

5

900

Fig. 9 Raman spectra of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ annealed at 1000℃.

3.2.2 Luminescent properties UV-visible absorption spectra of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ were recorded in the 200−800 nm region and are shown in Fig. 10. The absorption spectra exhibit absorption bands in the UV range. The maxima at 250 nm correspond to O2- to Ce3+ charge transfer transitions, whereas the absorption maxima at 345 nm can be assigned to O2- to Ce4+ charge transfer. It is observed that when Li+ is indirectly co-doped to the CeO2: Eu3+ sample, a increase in UV-visible absorption is observed. The result may be ascribed to the increase of crystallite size. Further, the effect of Li+ on luminescence properties is still further discussed in the excitation and emission spectra of Li+ indirect co-doped CeO2: Eu3+.

3+

CeO2: Eu

absorbance a.u

+

200

3+

Li indirect co-doped CeO2: Eu

300

400 500 600 wavelength/nm

700

800

900

Fig. 10 UV-vis absorption spectra of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ annealed at 1000℃.

The luminescent properties of CeO2:Eu3+ and Li+ indirect co-doped CeO2:Eu3+ are investigated and the result were shown in Fig.11. As illustrated in Fig. 11a, the spectra are composed of a broad band centered at about 360-365 nm and a sharp peak at 467 nm. The 467 nm sharp peak originates from the 7F0-5D2 transition of Eu3+. The broad band at about 360-365 nm originates from the charge transfer (CT) transitions between O2- and Ce4+ ions. It can be seen that Li+ indirect co-doped CeO2:Eu3+ enhances the CT intensity, whereas Li+ direct co-doped CeO2:Eu3+ reduce the CT intensity. In Fig. 11b, the emission spectra monitored at 365 nm also exhibit the same change: the characteristic transition of 5D0-7F1 and 5D0-7F2 of Eu3+ at 593, 613, and 634 nm obviously increased with indirect co-doping of Li+. In the case of indirect co-doping of Li+ ions, the substitution of Ce4+ with doped Li+ ions not only need to overcome a big difference in ionic radius and charge of Li+ and Ce4+ but also the resistance between two phases. By combining the information obtained from XRD result with that of Raman data, the role of indirect co-doping of Li+ ions is likely to be as flux materials generates liquid phase at high temperature, which can promote the diffusion of ions and accelerate the crystallization process. As a result, higher crystallinity and larger crystallites is obtained. Meanwhile, oxygen vacancies is decreased due to higher crystallinity and larger crystallites, as is supported by Raman spectra. Further, the result of 5D0-7F2/5D0-7F1 was shown in table 4. It is clear from

table 4 that the value of 5D0-7F2/5D0-7F1 is lower than 1 and gradually decrease with indirect co-doping of Li+ ions, suggesting that Li+ doping promote the inversion symmetry of Eu3+. Consequently, luminescent intensity of CeO2:Eu3+ is improved based on higher crystallinity and larger crystallites.

a

CeO2: Eu +

Li indirect co-doped CeO2: Eu

Intensity a.u 300

3+

325

350 375 400 425 wavelength/nm

450

3+

475

500

Fig. 11a The excitation spectra of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ annealed at 1000℃.

CeO2: Eu

b

3+

+

3+

Intensity a.u

Li indirect co-doped CeO2: Eu

570

585

600 615 wavelength/nm

630

645

Fig. 11b The emission spectra of CeO2: Eu3+ and Li+ indirect co-doped CeO2: Eu3+ annealed at 1000℃.

Table 4 the intensity ratio of the transitions 5D0-7F2 to 5D0-7F1 under 365 nm Sample

CeO2:Eu3+ Li+ indirect co-doped CeO2:Eu3+

5

D0-7F2 3338 5042

5

D0-7F1 5748 9465

5

D0-7F2/5D0-7F1 0.5807 0.5327

4. Conclusion CeO2:Eu3+ is of great interest as a luminescent material, since it is used as important functional materials in biomedical applications. In this study, we investigate the effect of the different co-doping ways of Li+ ions on CeO2:Eu3+ phosphor, including the direct co-doping of Li+ by hydro-thermal co-precipitation and the indirect co-doping of Li+ by a post calcining of Li2CO3. The structural and optical changes were examined. The result show that the direct co-doping of Li+, where oxygen vacancies concentration is increased due to substituting Ce4+ with Li+ in a cubic unit cell, results in less asymmetry around Eu3+ and lower luminescence. The present studies showed opposite trend with conventional explanations where the substitution of Ce4+ with doped Li+ ions are the prime reason for luminescence intensity enhancements. However, the role of indirect co-doping of Li+ ions is to be as flux materials generates liquid phase at high temperature. As a result, higher crystallinity and larger crystallites is obtained. Consequently, we reason that the enhanced luminescence mainly results from a increase of crystallinity and decrease in oxygen vacancies concentration based on flux effect of indirect co-doping of Li+.

Acknowledgments The authors are grateful for the financial support of the National Natural Science Foundation of China (No.J1103312; No.J1210040; No.21341010), the Natural Science Foundation of Hunan Province (No.11JJ5005), Hunan provincial Science &Technology Department (No.2016SK2064), Changsha Science and Technology Bureau (No.kq1701029; No.kq1701164), the Innovative Research Team in University (No.IRT1238), and China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE-No.2011-40). We also thank Dr. William Hickey, the U.S. professor of HRM, for the English editing on this paper.

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