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JOURNAL OF RARE EARTHS, Vol. 33, No. 1, Jan. 2015, P. 20

Enhanced green emission from La0.4F3:Ce0.45,Tb0.15/TiO2 core/shell structure

T.K. Srinivasan, B.S. Panigrahi, N. Suriyamurthy*, P.K. Parida, B. Venkatraman

(Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102, India)

Received 28 January 2014; revised 26 May 2014

Abstract: Nano sized La0.4F3:Ce0.45,Tb0.15 (core), La0.4F3:Ce0.45,Tb0.15 (TiO2) (core) shell, La0.55F:Ce0.45, and La0.85F3:Tb0.15 particles were synthesized by adopting co-precipitation technique in acidic environment and coated with TiO2 to form a core-shell structure by

adopting a mechanical dispersion method at room temperature. The synthesized materials were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis) absorption, photoluminescence and lifetime spectroscopy. The crystal structure of La0.4F3:Ce0.45,Tb0.15 remained the same as LaF3 after being doped with Ce and Tb ions but with a slight decrease in the lattice parameter. TEM image confirmed the for-

mation of a core-shell structure. The La0.4F3:Ce0.45,Tb0.15/TiO2 exhibited Tb3+ fluorescence enhancement by a factor of 1.76. Scintillation from the synthesized materials was also observed under X-ray excitation.

Keywords: core-shell; cerium; terbium; luminescence; lanthanum fluoride; scintillation; rare earths

Luminescent nanoparticles of lanthanides are being widely studied for their application in radiation detection through nano composite materials[1,2], biological fluores-

cent labels more efficient than conventional organic probes[3–5]. In the field of radiation detection, nano com-

posite scintillating materials are proposed to be a new class of inexpensive scintillators, easy to prepare and can operate at room temperature[6]. It is always advantageous that scintillators for X or γ-ray detection possess fast decay and cerium possesses efficient radiation absorption and fast radiative decay time (20–40 ns). Therefore ce- rium-doped bulk lanthanum halide compounds are attractive for radiation detection applications. Cerium ion (among few lanthanide ions), has the wavelength of its first excited state higher than the 180 nm (55000 cm–1)[7]. Moreover structurally there were no changes observed in LaF3 matrix even at higher Ce3+ doping concentration, as the radius of lanthanum and cerium are slightly different. Moreover, earlier studies indicate that Ce3+ among all the lanthanide ions, owing to its allowed absorption and board emission band can effectively overlap the acceptors like Tb3+ absorption energy bands[8] and facilitate the excitation energy transfer and convert the UV emission to visible green emission to suit the PM tubes.

Light amplification by a shell structure over the surface of nano sized luminescent material is being studied by many groups. The shell material is either a semiconducting oxide or metallic oxide. Unlike bulk energy states, nano material surface states originate from the discontinuous lattice periodicity and broken chemical bonds. The broken surface bonds aid non radiative trans-

fer and thus involve in reducing the PL intensity. The surface modification helps in reducing the non-radiative relaxation processes by passivating the surface traps leading to fluorescence enhancement[9,10]. Moreover, surface modification stabilizes the surface of the particles. During the past decade there have been a significant number of reports on shelling of LaF3-core structures leading to enhanced light output. In their interesting work, Xie et al.[11] have reported a significant enhancement in the PL intensity of LaF3:Ln3+ (Ln3+:Tb3+ & Nd3+) core with LaF3-shell material codoped with various lanthanides. In another work, SiO2 was used as the shell material over LaF3:Ce, which was found to be effective in enhancing the photoluminescence properties of LaF3:Ce[12].

In this context of exploration of enhancement of light output, doping at higher molar ratios of Ce to Tb was also attempted as the LaF3 with higher molar Ce/Tb dopant does not undergo significant structural changes. In this work, LaF3:Ce core structure with a cerium concentration of 0.45 mol.% was chosen as it exhibited significant quantum yield[8] without affecting the LaF3 lattice structure. This was codoped with terbium (0.15 mol) to yield significant green emission. Also earlier studies on semiconductor ZnS[13], ZnSe[14] coated with TiO2 report significant light intensity enhancement. We reported here

the successful synthesis of La0.4F3:Ce0.45,Tb0.15 nano particles in acidic environment for the first time. TiO2 coat-

ing was carried on the core by adopting a simple mechanical dispersion method at room temperature. The effects of the TiO2 shell thickness on the photolumines-

*Corresponding author: N. Suriyamurthy (E-mail: igcarsuri@yahoo.com; suriya6287@gmail.com; Tel.: +914427488243)

DOI: 10.1016/S1002-0721(14)60377-X

T.K. Srinivasan et al., Enhanced green emission from La0.4F3:Ce0.45,Tb0.15/TiO2 core/shell structure

21

cence properties of La0.4F3:Ce0.45,Tb0.15 were reported

for lifetime measurements. Scintillation under X-ray ex-

here.

citation was observed using an Avantes 2010 fiber Optic

 

spectrometer.

 

1 Experimental

AR grade LaCl3·7H2O (99.99%), CeCl3·7H2O (99.99%) and NH4F·6H2O (99.99%) were used without further purification. TbCl3 was prepared by adding Conc. HCl to Tb4O7 powder purchased from Indian Rare Earths and dried in vacuum and stored in a desiccator. TiO2 powder was prepared using the procedure adopted by Antony et al.[15]. Solution co-precipitation technique was adopted to

synthesize La0.4F3:Ce0.45,Tb0.15. Appropriate quantities LaCl3 (4.549 g–0.03063 mol), CeCl3 (3.397 g–0.01378

mol) and TbCl3 (1.7153 g–0.004594 mol) were dissolved in 120 mL DM water (solution-1). Appropriate amount of NH4F (3.403 g–0.09188 mol.) was dissolved in 200 mL of DM water and acidified by adding conc. HCl to a pH value of 4.6 and kept under constant stirring (solu- tion-2). Solution-2 was heated to 80 ºC for half an hour and then solution-1 was added drop by drop using a syringe pump at 50 mL/h flow rate for about 2.5 h under constant stirring. The mixture was maintained at 80 ºC for 1.5 h after addition of the solution-1 for the reaction to get completed. A milky white colloidal liquid product was formed in the reaction mixture. The product was collected by centrifuging at 9000 r/min for 30 min. The precipitate was washed with enough DM water to remove acidity and the un-reacted chemicals. Finally the precipitate was dried in vacuum oven at 85 ºC for 5 h.

About 1 wt.% of the La0.4F3:Ce0.45,Tb0.15 ~0.01 g of TiO2 particles was disbursed in methanol by ultrasoni-

cating for 30 min (solution-3). 0.5 g of the La0.4F3:Ce0.45, Tb0.15 powder was disbursed in methanol separately for

about 30 min to obtain a uniform colloidal mixture (solu- tion-4). Then solution-3 was added drop by drop to solu- tion-4 and the mixture was stirred thoroughly for 24 h under room temperature. In order to vary the coating thickness another sample was synthesized in a similar way by stirring for 52 h.

X-ray diffraction measurements were carried out using a Shimadzu (XRD-6000) diffractometer equipped with a Cu Kα (0.15406 nm) X-ray source. Transmission electron microscope (TEM) micrographs were recorded using FEI CM200 TEM that operates at 200 kV using a pettier cooled TVIPS 2k X 2k CCD camera. FTIR spectra were recorded using a HORIZON MB3000 ABB spectrophotometer in order to check the surface of TiO2 coating. UV-Vis absorption measurements were carried out using SHIMADZU (UV1800). Photoluminescence (PL) measurements on the nano-powder pellets were carried out using a SHIMADZU spectrofluorimeter [RF-5301PC] in the range of 200–700 nm at 1.5 and 3 nm slit widths. Horiba lifetime spectroscopy was used to

2 Results and discussion

2.1 XRD

Fig. 1 shows XRD pattern of (1) La0.4F3:Ce0.45,Tb0.15,

(2) TiO2 surface modified La0.4F3:Ce0.45,Tb0.15 and (3) La0.55F3:Ce0.45 samples matches with the JCPDS data of LaF3 (32-0483). It is clear that La0.4F3:Ce0.45,Tb0.15 sample had single phase and well formed. The fluoride

products such as LaF3 and CeF3 easily crystallize with a hexagonal-phase structure, which is determined by the intrinsic crystalline structure of lanthanide fluoride[16]. It is evident that there was no shift even with 45 mol.% of Ce (curve (3)). However there is a peak shift towards higher angels (curves (1), (2)) which indicate the substitution of La3+ ions by smaller Tb3+ ions in the lattice structure at certain sites which result in changes in the lattice parameter and homogenous dispersion of Ce3+ and Tb3+ ions in the lattice[17]. The crystallite size is estimated from the Scherrer equation, D=0.90λ/β cosθ, where D is the grain size, λ is the X-ray wavelength (0.15405 nm), and θ and β are the diffraction angle and full-width at half-maximum of an observed peak, respectively. The dominant peaks viz [002], [110], [111], [300], [133] and [302] at 2θ=24.49º, 25.07º, 27.84º, 44.06º, 45.23º and 51.01º respectively were used to calculate the average crystallite size (D) of these nano particles. The estimated average crystallite sizes of uncoated, TiO2 coated

La0.4F3:Ce0.45,Tb0.15 and La0.55F3:Ce0.45 were around 20, 21 and 23 nm. The lattice parameters of all the samples

were estimated using “STOE WINXPOW” an XRD pattern analyzing software which takes into account all the peaks in the XRD pattern. The estimated lattice parameters are provided in Table 1. It is evident from Table 1 that the lattice parameter of uncoated as well as TiO2 coated La0.4F3:Ce0.45,Tb0.15 samples are the same.

Fig. 1 XRD spectra of La0.4F3:Ce0.45,Tb0.15 (1), La0.4F3:Ce0.45, Tb0.15/TiO2 (core/shell) (2) and La0.55F3:Ce0.45 (3)

22

Table 1 Lattice parameter

Sample No.

Sample

a/nm

c/nm

1

La0.4F3:Ce0.45,Tb0.15-uncoated

0.7119

0.7327

2

TiO2 coated La0.4F3:Ce0.45,Tb0.15

0.7119

0.7327

3

La0.55F3:Ce 0.45-uncoated

0.7187

0.7334

2.2 TEM

Fig. 2(a), (b) and (c) show the TEM images of the uncoated, thin and thickly TiO2 coated particles of

La0.4F3:Ce0.45,Tb0.15 with inset showing the diffracted pattern. The inset image displaying SAED (selective area

electron diffraction) patterns taken from an individual nano crystal indicates a single crystalline phase. From Fig. 2(a), it is clear that particles are agglomerated; the average size of the particle was 26 nm. Moreover the “d” (d-inter lattice planar distance) spacing of the lattice parameter was obtained by considering the first five rings of all the diffraction pattern using the Image J software, which closely matched with d planes: d=0.3584 nm (110); d=0.3115 nm (111); d=0.2061 nm (300); d= 0.1858 nm (002) and d=0.1751 nm (221) indicated in the JCPDS (32-0843) data of LaF3. The calculated d values of the particular five orientations were compared with the standard TiO2. The d values did not match with the obtained result indicating absence of TiO2 in the uncoated sample. Similarly, the results of the d spacing comparison obtained from the diffraction pattern of Figs. 2(b) and (c) closely matched with the lattice orientations of LaF3 (002) d=0.3891 nm; d=0.2544 nm (112); d=0.2267 nm (211) and TiO2 of anatase structure d=0.352 (101), d=0.189 nm (200) as well. Figs. 2(b) and (c) indicate that the TiO2 coated the particle appears to be spherical in shape with

JOURNAL OF RARE EARTHS, Vol. 33, No. 1, Jan. 2015

average of 16.06 and 16.6±5 nm size as obtained from

the size distribution analysis. The particle sizes obtained are smaller than the sizes reported by Xie et al.[11] and Wang et al.[17], which may be due to the effect of acidic

environment adopted here during synthesis. This demonstrates the effectiveness of acidic environment to synthesize surfactant-free nano materials. In TEM images, contrast depends on the electron scattering power of the object forming the images. The particle size was calculated using Image J software. The EDS spectrum shows that no impurities are present in the sample except Cu, which was used as the fine grid for holding the sample. The contrast region is utilized in estimating the coating thickness. Using the Image J software the thickness of the coating was estimated. The shell thickness was 1.6–2 nm and 4–6 nm for thinly and thickly TiO2 coated La0.4F3:Ce0.45,Tb0.15 nanoparticles.

2.3 Chemical composition

Tables 2–4 show the chemical composition of the un-

coated, thin and thickly TiO2 coated La0.4F3:Ce0.45, Tb0.15 nanoparticles obtained from the EDS chemical composi-

tional analysis. Tables 3 and 4 reveal the presence of TiO2 along with other constituents, indicating that the products

are La0.4F3:Ce0.45,Tb0.15-TiO2 composite nanoparticles. These data clearly confirm that the products are

La0.4F3:Ce0.45,Tb0.15-TiO2 composite nanoparticles. The elemental analysis revealed the entire elements present (as

shown in Tables 2–4). This technique is only qualitative.

2.4 FTIR

Fig. 3 shows the FTIR spectra of (1) uncoated, (2) plain TiO2 and (3) TiO2 coated La0.4F3:Ce0.45,Tb0.15. In

Fig. 2 TEM images of uncoated (a), thinly TiO2 coated (b) and thickly TiO2 coated (c) La0.4F3:Ce0.45,Tb0.15 with insert showing the

diffraction pattern

Table 2 Chemical composition of the uncoated La0.4F3:

Ce0.45,Tb0.15

Element

Peak

Area

K

Abs.

Mass

Mass

Atomic

 

area

sigma

factor

corn.

fraction/%

fraction/%

ratio/%

O k

95

39

1.871

1.000

0.49

0.20

1.82

F k

2981

117

1.752

1.000

14.51

0.54

45.10

Cu K

8904

152

1.393

1.000

34,45

0.67

32.02

La L

4302

167

2.019

1.000

24.13

0.79

10.26

Ce L

3516

166

2.020

1.000

19.73

0.80

8.32

Tb L

1103

111

2.180

1.000

6.68

0.63

2.48

Table 3 Chemical composition of the TiO2 thinly coated

La0.4F3:Ce0.45,Tb0.15

Element

Peak

Area

K

Abs.

Mass

Mass

Atomic

 

area

sigma

factor

corn.

fraction/%

fraction/%

ratio/%

O k

119

22

1.871

1.000

7.29

1.30

25.34

F k

58

30

1.752

1.000

3.31

1.65

9.68

Ti k

5

17

1.069

1.000

0.17

0.59

0.20

Cu K

1341

60

1.393

1.000

61.26

3.01

53.63

La L

204

41

2.019

1.000

13.54

2.43

5.42

Ce L

218

35

2.020

1.000

14.41

2.11

5.73

Tb L

0

25

2.180

1.000

0.01

1.81

0.00

T.K. Srinivasan et al., Enhanced green emission from La0.4F3:Ce0.45,Tb0.15/TiO2 core/shell structure

23

Table 4 Chemical composition of the TiO2 thickly coated

La0.4F3:Ce0.45,Tb0.15

Element

Peak

Area

K

Abs.

Mass

Mass

Atomic

 

area

sigma

factor

corn.

fraction/%

fraction/%

ratio/%

O k

123

23

1.871

1.000

9.58

1.73

28.59

F k

67

18

1.752

1.000

4.88

1.29

12.26

Cu K

1236

56

1.393

1.000

71.68

3.32

53.85

La L

155

41

2.019

1.000

13.05

3.27

4.48

Ti k

18

22

1.069

1.000

0.81

0.98

0.81

Fig. 3 FTIR of the curves of uncoated La0.4F3:Ce0.45,Tb0.15 (1), TiO2 (2) and TiO2 coated La0.4F3:Ce0.45,Tb0.15 (3)

curve (2), the 3340 and 492 cm–1 wavenumbers are attributed to the OH and Ti–O stretching[18,19]. The band at

1618 cm–1 is attributed to the bending vibration of H–O–H bonds[19]. In curve (3), the 3406, 1450 and 492 cm–1 wavenumbers are of OH, Ti–O–Ti[18] and Ti–O

stretching indicate the presence of TiO2 on the surface.

2.5 UV-Vis absorption

Fig. 4 presents the UV-Vis absorption spectra of TiO2

coated and uncoated La0.4F3:Ce0.45,Tb0.15 colloidal solutions obtained by dispersing 10 mg of powder in 5 mL of

DM water and ultrasonicated for 30 min. Curves (1) and

(2)represent the absorption of the uncoated

La0.4F3:Ce0.45,Tb0.15 and La0.4F3:Ce0.45,Tb0.15/TiO2 coated samples respectively. The five absorptions of Ce3+ are

distinctly clear and it is consistent with that reported by Elias et al. and Dorenbos[20,21]. It is observed that the Ce3+ absorption in the TiO2 coated material was less compared to that of the uncoated sample. The presence of TiO2

around La0.4F3:Ce0.45,Tb0.15 probably has decreased the absorption of the light by Ce3+.

2.6 Photoluminescence studies

Fig. 5 shows the PL emission spectra of the as-pre-

pared uncoated La0.4F3:Ce0.45,Tb0.15 powder sample synthesized at pH 4.6. All excitation and emission peaks of

Tb3+ are discernible in this host. The excitation spectrum recorded for the emission at 543 nm emission is composed of two overlapping bands having maxima at 264, 351 and 379 nm. The peaks observed at 351 and 379 nm are due to f-f transitions of Tb3+. They are less intense owing to parity forbidden nature of f-f transitions. Upon exciting at 280 nm, we have observed emission peaks at

490, 544, 584 and 621 nm that are originating from 5D47F6, 5D47F5, 5D37F4, and 5D47F4 energy levels respectively. Furthermore, in this host blue emission

at 490 nm is suppressed whereas green emission at 543 nm is enhanced precisely due to cross relaxation neighboring terbium Tb3+ ions[22].

Fig. 6 presents the excitation and emission spectra of

La0.4F3:Ce0.45,Tb0.15 core and La0.4F3:Ce0.45,Tb0.15 core/ TiO2 shell nanoparticles. On comparing the core and

core/shell luminescence spectra provided in the above figure it is obvious that, TiO2 shell (1.6–2 nm) of lanthanum fluoride phosphor reduces the cerium photoluminescence intensity by the factor of 2.5 times. The cerium emission intensity further decreased with the increase in the shell thickness (4–6 nm). As discussed previously in

 

Fig. 5 PL spectra of uncoated La0.4F3:Ce0.45,Tb0.15 synthesized

Fig. 4 UV-Vis absorption of uncoated La0.4F3:Ce0.45,Tb0.15 (1)

at pH=4.6 with excitation spectrum λem=543 nm (1) and

and La0.4F3:Ce0.45,Tb0.15/TiO2 (core/shell) (2)

emission spectrum λex=283 nm (2) (slit width=1.5 nm)

24

Fig. 6 Uncoated excitation curve for λem=325 nm (1), emission curve for λex=292 nm (2) of TiO2 coated La0.4F3:Ce0.45, Tb0.15 excitation curves (3); for λem=325 nm (5), emission curves (4), (6) with coating thickness 2–4 nm; λex=283 nm

Fig. 4, the presence of TiO2 shell indeed decreased the absorbance. Therefore, the reduction in absorption has been due to coating by TiO2.

Fig. 7 presents the excitation spectra of core and core shell structures with different shell thickness. The PL excitation and emission intensity for shell thickness (1.6–2

nm) has decreased slightly compared to La0.4F3:Ce0.45,Tb0.15 core. However, upon increasing the shell thickness to 4–

6 nm, there is a significant enhancement in the green emission from 5D4 energy level (544 nm). At this shell thickness, the intensity was enhanced by 1.76 times compared to that from core nano particles. The probable process involved in this interesting observation was discussed as follows. Han et al.[23], have studied the SiO2 as the shell around Y2SiO5:Ce3+,Tb3+ which increased light output of this nano phosphor. According to them, the surface defects in nano materials are quenched in presence of shell structures. The surface defects are known to

Fig. 7 PL spectra of La0.4F3:Ce0.45,Tb0.15 excitation curves of uncoated (1), thinly (2) and thickly (3) TiO2 coated La0.4F3:Ce0.45, Tb0.15 for λem=543 nm and emission curves uncoated for λex=283 nm (4); thinly (2–4 nm) (5) and thickly (4–6 nm) (6) TiO2 coated for λex=268 nm

JOURNAL OF RARE EARTHS, Vol. 33, No. 1, Jan. 2015

increase the non-radiative decay of the excitation energy. Therefore, the reduction in the surface defects may result in minimizing the non-radiative decay process and thereby enhancing the luminescence. In the present study, significant enhancement of luminescence is observed for 4–6 nm TiO2 thickness whereas marginal quenching of luminescence was observed for 1.6–2 nm thickness. This indicates that probably an optimum coating thickness is required for observing enhancement in luminescence. However, more studies are required to understand the surface modification process and its effect on luminescence. Moreover there is a blue shift in the excitation spectra for Tb3+ emission which might be due to the surface TiO2 coating as well as the small size of the TiO2 coated particles. Fig. 8 presents excitation and emission

of uncoated and TiO2 coated La0.85F3:Tb0.15 alone. It is evident from the above figure that, 543 nm green emis-

sion of uncoated is higher than TiO2 coated which might be due to the prevention of excitation and UV photon from reaching the terbium centre. Hence at certain coating thickness the green emission from this host is enhanced desirably.

In Fig. 9, curves (1), (3) and (2), (4) represent the Ce3+ excitation and emission of the uncoated and the TiO2

coated La0.55F:Ce0.45 samples. It is evident that the 325 nm emission of the uncoated is higher than the TiO2 coated,

which might be due to prevention of the excitation UV photons from reaching the Ce3+ centers. It was observed that there was about 2.2 times decrease in the 325 nm emission from the TiO2 coated sample. Though it is evident that thin coating reduces the radiationless transfer which results in the increase in the intensity, in our case the TiO2 coating results in a decrease of the cerium emission.

2.7 Scintillation under X-ray excitation

The powder samples were kept in a dark PVC container at 10 cm away from the X-ray tube. The fiber optic

Fig. 8 PL spectra of La0.85F3:Tb0.15, excitation curves uncoated (1), TiO2 coated for λem=543 nm (3), emission curves uncoated (2) and TiO2 coated λex=379 nm (4)

T.K. Srinivasan et al., Enhanced green emission from La0.4F3:Ce0.45,Tb0.15/TiO2 core/shell structure

25

Fig. 9 PL spectra of uncoated La0.55F3:Ce0.45, excitation curves uncoated (1); TiO2 coated La0.4F3:Ce0.45 (3) for λem=325 nm; emission curves of uncoated (2); TiO2 coated (4) for λex=283 nm

Fig. 11 Time-resolved luminescence intensity decay curves of Ce3+ emission in the uncoated (1), TiO2 coated (2)

La0.4F3:Ce0.45,Tb0.15; uncoated (3) and TiO2 coated (4) La0.4F3:Ce0.45,Tb0.15 Tb3+ emission

cable probe for collecting the light emitted was fixed tightly to a stand and kept at 1 cm away from the powder sample a bit inclined so as not to prevent the X-rays, which is connected to a CCD based spectrometer (Avantes 2010). The spectrometer was operated through a laptop connected to it using inbuilt software. Fig. 10 shows the scintillation spectra obtained by irradiating the powder sample containing TiO2 coated and uncoated

La0.4F3: Ce0.45,Tb0.15. The X-ray tube was operated at 110 kV and 12 mA for 30 s. There are many emission lines in

the figure due to the stray background light in the X-ray chamber. The scintillation intensity was a bit less in the uncoated sample compared to the coated sample. This

indicates that La0.4F3:Ce0.45,Tb0.15/TiO2 (core-shell) marginally improves the scintillation output in this study that

agrees well with data reported in Ref. [24].

2.8 PL Lifetime

Fig. 11 shows the PL emission lifetime of Ce3+ and the inset shows the Tb3+ lifetime from uncoated and TiO2

Fig. 10 Scintillation spectra of uncoated (1) and TiO2 coated (2) La0.4F3:Ce0.45,Tb 0.15 under X-ray excitation

coated La0.4F3:Ce0.45,Tb0.15 colloidal samples excited with 280 nm wavelength using light emitting diode (LED)

source, with the HORIBA-FLOURO Cube compact tabletop spectroflourimeter instrument. The obtained data were fitted with third order exponential which shows that the TiO2 coated Ce3+ emission decays faster than the uncoated. The lifetime of the Ce3+ in uncoated and TiO2 coated were 13.6 and 12.9 ns, which is also evident from the decrease in the Ce3+ emission intensity from the TiO2 coated sample (Fig. 6). In the case of the Tb3+ emission, the uncoated and TiO2 coated decay times were 19.17 and 21.05 μs respectively. This clearly demonstrates that shelling increases the luminescence decay time of Tb3+; the result is in consistence with Tb3+ lifetime reported in Refs. [2,16].

3 Conclusions

After being doped with Ce and Tb ions, no significant change in the crystal structure of LaF3 was observed though a slight decrease in the lattice parameter was observed with a heavy dopant concentration. The estimated

average crystallite sizes of La0.4F3:Ce0.45,Tb0.15 and TiO2 coated La0.4F3:Ce0.45,Tb0.15 nanoparticles were around 21 and 22 nm, respectively. TEM images, the associated

diffraction pattern and the EDAX confirmed the forma-

tion of the La0.4F3:Ce0.45,Tb0.15-core and TiO2 shell structure. FTIR spectra also corroborated the presence of TiO2

on the La0.4F3:Ce0.45,Tb0.15 surface. The coating thickness was around 1.6–2 and 4–6 nm in two different samples.

The particles sizes as per the TEM image were 16.1±5 nm. With TiO2 coating/shelling the cerium luminescence was observed to be quenched. However, in case of terbium, the green emission was enhanced 1.76 times. The scintillation under X-ray excitation of the TiO2 coated and uncoated samples was recorded. The scintillation intensity of the TiO2 coated sample was marginally

26

higher than the uncoated sample.

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