DSpace at VNU: Synthesis and optical properties of ZnAl2O4:Cr 3+, Tb3+ powders

The effect of the Cr3+, Tb3+concentration and heat-treating temperature on structural and optical properties of the synthesized samples has been studied.. With a desire to ex-pand the emi

DOI:10.1051/epjap/2013130008 T HE E UROPEAN

APPLIED PHYSICS

Regular Article

Trinh Thi Loana, Nguyen Thi Thuy, and Nguyen Ngoc Long

Faculty of Physics, VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam

Received: 3 January 2013 / Received in final form: 6 May 2013 / Accepted: 22 July 2013

Published Online: 4 October 2013 – c EDP Sciences 2013

Abstract ZnAl2O4:Cr3+,Tb3+ powders with different dopant contents have been synthesized by sol-gel

method using the following precursors: zinc nitrate (Zn(NO3 2), aluminum nitrate (Al(NO3 3), terbium

nitrate (Tb(NO3 3), chrome nitrate (Cr(NO3 3), and citric acid The effect of the Cr3+, Tb3+concentration

and heat-treating temperature on structural and optical properties of the synthesized samples has been

studied

1 Introduction

Zinc aluminate (ZnAl2O4) with spinel structure is

im-portant in applications It is known as an inert material

with high thermal stability, high mechanical resistance,

hydrophobicity and low surface acidity It is used as

ce-ramic, electronic and catalytic material [1] ZnAl2O4 is

a wide band gap semiconductor with an energy gap of

3.8 eV, hence it is usually used as a host material for

optical activators like rare earth ions [2,3] or transition

metal ions [4 7] The optical properties of ZnAl2O4:Cr3+

were studied by Derkosch and Mikenda [5], Mikenda [6]

and Nie et al [7] They found that the

photolumines-cence (PL) spectrum of ZnAl2O4:Cr3+ consists of

emis-sion lines in the 650–750 nm wavelength range, due to the

optical transitions within Cr3+ ions With a desire to

ex-pand the emission band toward the shorter wavelengths,

for preparing the material of white light emission, in this

paper, we have simultaneously doped ZnAl2O4 powders

with Cr3+ and Tb3+ ions and investigated their optical

properties

2 Experimental

The ZnAl2−x−yTbxCryO4 powders with different dopant

contents have been prepared by sol-gel method The

pow-ders were prepared from Zn(NO3 2, Al(NO3 3, Tb(NO3 3,

Cr(NO3 3, and citric acid solutions Zn(NO3 2, Al(NO3 3,

Tb(NO3 3 and Cr(NO3 3 solutions were mixed with the

Al3+:Tb3+:Cr3+ mole ratios of (2-x-y):x:y Citric acid

aqueous solution was added to the above solution and the

mixed solution temperature was kept constant at 80 ◦C

until a highly viscous gel was formed After drying in air

 International Workshop on Advanced Materials and

Nanotechnology 2012 (IWAMN 2012)

a e-mail:loan.trinhthi@gmail.com

at 100◦C for 24 h, the gel was converted to a xerogel more opaque and dense The xerogel was annealed at different temperatures in air for 3 h

The crystal structure of the samples was character-ized by a Siemens D5005 Bruker, Germany X-ray dif-fractometer (XRD) with Cu-Kα1 (λ = 1.54056 ˚A) irra-diation The composition of the samples was determined

by an energy-dispersive X-ray (EDX) spectrometer (EDS, Oxford Isis 300) attached to the JEOL-JSM 5410 LV scan-ning electron microscope Photoluminescence (PL) spec-tra and photoluminescence excitation (PLE) specspec-tra were measured at room temperature using a Fluorolog FL3-22 Jobin Yvon Spex, USA spectrofluorometer with a xenon lamp of 450 W being used as an excitation source

3 Results and discussion 3.1 Structural properties

The XRD patterns of the of the ZnAl2−x−yTbxCryO4 samples with x = y = 0.005 and heat-treated at

differ-ent temperatures are shown in Figure 1 As evident from the figure, the samples are single-phase material with good crystallinity and all the diffraction peaks can be assigned

to the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2), (5 1 1) and (4 4 0) lattice planes of the face-centered cubic spinel ZnAl2O4phase No characteristic peaks of impurity phase have been observed The lattice constants of samples cal-culated from the XRD patterns are 8.079±0.001 ˚A, which

is somewhat small compared with the standard value of 8.085 ˚A (JCPDS no 05-0669) It is noted that with in-creasing the heat-treating temperature, the crystallinity

of the samples become better and the lattice constants almost keep constant

For studying the effect of dopant concentration on the structural properties of the synthesized samples, the XRD patterns of the samples with different mole fractions x

Fig 1 The XRD patterns of the ZnAl2−x−yTbxCryO4

sam-ples with x = y = 0.005, heat-treated at different

tempera-tures: (a) 750, (b) 850, (c) 950 and (d) 1100◦C

(a)

(b)

Fig 2 The XRD patterns of the ZnAl2−x−yTbxCryO4

sam-ples heat-treated at 1100◦C, (a) withx = 0.05 and different

y, (b) with different x and y = 0.05.

andy, undergone a heat-treatment at 1100 ◦C were

inves-tigated and the results are presented in Figure2

Withx = 0.05 and for y = 0.05–0.15, as can be seen

from Figure2a, the samples were ZnAl2O4single-phase, in

Table 1 The dependence of interplanar spacing dhkl (in

˚ A) and lattice constants on the mole fraction y of the

ZnAl2−x−yTbxCryO4 sample withx = 0.05.

y d220 d311 d422 d511 d440 a (˚A) 0.05 2.859 2.438 1.649 1.557 1.429 8.085 ± 0.003

0.10 2.861 2.440 1.653 1.557 1.430 8.092 ± 0.002

0.15 2.862 2.441 1.653 1.558 1.431 8.096 ± 0.001

Table 2 The dependence of interplanar spacing dhkl (in

˚ A) and lattice constants on the mole fraction x of the

ZnAl2−x−yTbxCryO4 withy = 0.05.

x d220 d311 d422 d511 d440 a (˚A) 0.01 2.857 2.436 1.650 1.555 1.429 8.081 ± 0.002

0.05 2.859 2.438 1.649 1.557 1.429 8.085 ± 0.003

0.15 2.859 2.437 1.651 1.556 1.429 8.085 ± 0.002

which Cr3+ions were incorporated into the ZnAl2O4 lat-tice as Al3+-replaced impurities and no other crystalline phase associated with Cr-impurities was observed It can

be noticed that with increasing the mole fraction y, the

position of the diffraction peaks shifted towards the small theta side, which is associated with an increase in the in-terplanar spacing dhkl and the lattice constants This is because the effective ionic radius of Cr3+ in octahedral sites (0.615 ˚A) is larger than that of Al3+ (0.535 ˚A) The values of dhkl and lattice constants of the samples calcu-lated from the XRD patterns are shown in Table1 With y = 0.05 and for x = 0.01–0.05, as seen from

Figure 2b, the samples were ZnAl2O4 single-phase, no other crystalline phase associated with Tb-impurities was observed But for x = 0.15, in addition to the

diffrac-tion peaks of the ZnAl2O4phase, some weak peaks of the

Tb2O3 (denoted by the asterisks) appeared Unlike the

Cr3+ ion in octahedral sites, effective radius of Tb3+ ion (0.923 ˚A) is much larger than that of Al3+ ion (0.535 ˚A), therefore it is expected that the Tb3+ ions substitute for the Al3+ions in the ZnAl2O4lattice harder than the Cr3+ ions do The influence of the mole fraction x on the

val-ues of dhkl and lattice constants of the samples is demon-strated in Table 2

Figure 3 shows the EDS of the ZnAl2−x−yTbxCryO4 spinel with x = y = 0.005 and x = y = 0.100 For the

samples with the dopant concentrations low as x = y =

0.005, the EDX spectrum only detects the main elements

Zn, Al and O The Cr and Tb elements are detected only

in the samples with the higher dopant concentrations (x =

y = 0.100).

3.2 Optical properties

The PL spectrum of the ZnAl2−x−yTbxCryO4sample with

x = y = 0.005, heat-treated at 1100 ◦C, excited by 412 nm

wavelength is shown in Figure 4 The PL spectrum con-sists of the lines at 450, 487, 543, 585, 621, 658, 667, 676,

686, 698, 708, 717 and 723 nm The fluorescence lines at

487, 543, 585 and 621 nm were assigned to the emission

(b)

Fig 3 The EDX spectra of the ZnAl2−x−yTbxCryO4 spinel

with different mole fraction x and y (a) x = y = 0.005 and

(b)x = y = 0.100.

Fig 4 The PL spectrum excited by 412 nm wavelength of the

ZnAl2−x−yTbxCryO4sample withx = y = 0.005, heat-treated

at 1100◦C

transitions within Tb3+ ions [3]; while the lines at 658,

667, 676, 686, 698, 708, 717 and 723 nm were attributed to

the emission transitions within Cr3+ions [5 7] Hereafter

we analyze the PL spectra in more detail We selected two

excitation wavelengths: 352 nm for the spectrum related

to the Tb3+ions and 528 nm for that related to the Cr3+

ions

Figure5a shows the PL spectrum of the sample with

x = y = 0.005, heat-treated at 1100 ◦C, excited by the

wavelength of 352 nm The lines at 410, 435, 488, 543,

587 and 622 nm correspond to the transitions5D3→7F

5,

5D

3 → 7F

4, 5D4 → 7F

6, 5D4 → 7F

5, 5D4 → 7F

4, and

5D

4 →7F

3 within the Tb3+ ions, respectively.

Figure5b shows PLE spectrum of the sample withx =

y = 0.005, heat-treated at 1100 ◦C, recorded at 543 nm

(a)

(b)

Fig 5 (a) The PL spectrum excited by 352 nm wavelength

and (b) the PLE spectrum monitored at 543 nm emission line

of the ZnAl2−x−yTbxCryO4 sample withx = y = 0.005,

heat-treated at 1100◦C

emission line The PLE spectrum consists of the lines at

303, 317, 340, 352, 368, 377 and 486 nm, corresponding to the 7F6 → 5HJ, 7F6 → 5D0,1, 7F6 → 5GJ, 7F6 → 5D2,

7F6→5L10,7F6→5D3, and7F6→5D4transitions within the Tb3+ ions, respectively

The PL spectrum under 528 nm excitation wavelengths

of the samples exhibits the lines at 652, 658, 667, 675, 686,

698, 708, 717 and 723 nm as shown in Figure6a The line

at 686 nm (noted by R-line) is assigned to the2E→4A2 transitions within the Cr3+ ions in the ZnAl2O4 octahe-dral crystal field [5 7] For identification of these emis-sion lines, their PLE spectra were measured Figure 6b shows the PLE spectra of ZnAl2−x−yTbxCryO4 sample with x = y = 0.005, recorded at 652, 658, 667, 675, 686,

698, 708, 717 and 723 nm Results show that the PLE spectra monitored at these emission lines possess the same shape, indicating that these emission lines have the same origin The PLE spectra consist of two strong broad ab-sorption bands 350–450 nm and 475–600 nm, correspond-ing to spin-allowed4A2→4T

1and4A2→4T

2transitions

of the Cr3+ ions in the octahedral sites of ZnAl2O4 [7] This proves that the lines at 652, 658, 667, 675, 698,

708, 717 and 723 nm are phonon-sidebands of the lines R

(b)

Fig 6 (a) The PL spectrum excited by 528 nm wavelength

and (b) the PLE spectrum monitored at different emission

wavelengths of the ZnAl2−x−yTbxCryO4sample withx = y =

0.005, heat-treated at 1100 ◦C.

(R-PSB), among them the lines at 652, 658, 667 and 675

are the anti-Stokes’ one and the lines at 698, 708, 717 and

723 nm are the Stokes’ one

For studying the effect of the Cr3+ and Tb3+

concen-tration on the optical properties, the PL spectra excited by

352 nm wavelength of ZnAl2−x−yTbxCryO4samples with

various x and y, annealed at 1100 ◦C were measured and

presented in Figure 7 The results showed that when the

concentration of the Tb3+ion did not change (x = 0.005),

the emission related to the transitions within the Tb3+

ions significantly reduced in intensity with increasing the

Cr3+ ion concentration (Fig 7a): the emissions for the

samples withx = 0.005 and y = 0.050, 0.100 were almost

completely extinguished The reason for this perhaps is

due to the energy transition occurring between the Tb3+

and Cr3+ ions under condition of the increased Cr3+

con-centration

Figure7b shows the PL spectrum excited by the

wave-length of 352 nm of the ZnAl2−x−yTbxCryO4 samples,

annealed at 1100 ◦C, with y = 0.005 and various x It is

noted that with increasing the Tb3+ion concentration up

tox = 0.100, luminescence quenching was observed This

is well known the concentration quenching phenomenon

(a)

(b)

Fig 7 The PL spectra excited by the wavelength of 352 nm

of the ZnAl2−x−yTbxCryO4 samples, heat-treated at 1100◦C (a) withx = 0.005 and different y, and (b) with different x and

y = 0.005.

Figure 8 presents the PL spectra excited by 528 nm wavelength of ZnAl2−x−yTbxCryO4 samples with various

x and y, annealed at 1100 ◦C It can be seen from

Fig-ure 8a that at the high Cr3+ concentrations (y = 0.050, 0.100) the intensity of the lines at 667, 675, 686, 698,

708 and 717 nm considerably reduces Meanwhile, some new lines at 707, 732 739 and 746 nm appear After ref-erences [5 7], at the high Cr3+ concentrations the Cr3+

-Cr3+ pairs are expected to form, and the line at 707 nm

is originated from the nearest Cr3+-Cr3+pairs By exam-ining PLE spectrum, the lines at the wavelengths of 732,

739 and 746 nm are believed to be the phonon-sidebands

of the line at 707 nm (the PLE spectra not shown here) Figure8b shows the PL spectra of ZnAl2−x−yTbxCry

-O4 samples, annealed at 1100 ◦C, with y = 0.005 and

various x, excited by the wavelength of 528 nm As seen

from the figure, with increasing the Tb3+ ion concentra-tion up tox = 0.050, 0.100, the shape of the spectra does

not change, but their intensity is decreased Therefore by selecting the appropriate mole fractions x and y, we can

obtain the materials strongly emitting in both blue-green and red regions

(b)

Fig 8 The PL spectra excited by the wavelength of 528 nm

of the ZnAl2−x−yTbxCryO4samples, heat-treated at 1100◦C

(a) withx = 0.005 and different y, and (b) with different x and

y = 0.005.

4 Conclusion

The ZnAl2O4 spinel powders co-doped with Tb3+ and

Cr3+ ions have been successfully synthesized by a sol-gel technique The samples undergone heat-treatment at

1100 ◦C exhibited the best crystallinity The PL spec-tra of the samples showed two emission regions: the first (blue-green) one was related to the emission of Tb3+ ions and the second (red) one was related to the emission of

Cr3+ ions By selecting the appropriate mole fractions

of the Tb3+ ions x and Cr3+ ions y, we can obtain the

materials strongly emitting in both blue-green and red regions

This work is financially supported by VNU University of Science (Project No NT-12-10) and Ministry of Science and Technology of Vietnam (Project No 103.02.51.09 from NAFOSTED)

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