Improved optical properties in nanocrystalline Ce:YGG garnets via normal and reverse strike co-precipitation method

Materials Letters 93 (2013) 21–24

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Improved optical properties in nanocrystalline Ce:YGG garnets via normal and reverse strike co-precipitation method Aditya Verma a,1, Masood Nath a, Neelam Malhan b, Ashok K. Ganguli a,n a b

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Laser Science & Technology Centre, DRDO, Delhi, India

a r t i c l e i n f o

abstract

Article history: Received 22 August 2012 Accepted 7 November 2012 Available online 16 November 2012

There is a difference in the growth of nanoparticles based on the environment in which nucleation occurs as is observed by changing the order of reactant being added to the precipitant present in excess called the normal strike and reverse strike method. The effect of particle size on optical properties of Ce:YGG (Ce doped yttrium gallium garnet) obtained via normal and reverse strike synthesis has been investigated. The normal strike route led to smaller particle size, good chemical homogeneity as well as significant improvement in photoluminescence properties compared to the reverse strike route. A comparative study of Ce:YGG nanoparticles, synthesized via the two different methods and their optical properties are discussed. There is an enhancement in the photoluminescence efficiency in nanoparticles obtained by the normal strike method mainly due to homogeneous distribution of active ions in the host which is essential to avoid quenching. & 2012 Elsevier B.V. All rights reserved.

Keywords: Nanocrystalline materials Particles Nanosize X-ray techniques Phosphors

1. Introduction Rare earth based oxides crystallizing in the garnet structure have been of interest as phosphors [1]. The size reduction of these phosphors is an effective tool to improve the efficiency of the optical properties of the garnets. Yttrium Aluminum Garnet (YAG) having formula Y3Al5O12, possesses good optical properties as well as chemical stability. Apte et al. [2] have attempted the synthesis of YAG powder by normal and reverse strike precipitation. However, they were successful in synthesizing YAG only via the reverse strike (RS) route while normal strike (NS) route led to secondary phases. While Chiang et al. [3] have synthesized Ce:YAG via both RS and NS by choosing appropriate and different sources for aluminum. Yttrium gallium garnet (YGG) is also an interesting garnet with appropriate thermal and spectral properties [4]. It is a phosphor host and is normally synthesized via solid state reaction at high temperature ( 41350 1C). The process needs extensive ball milling and lengthy high temperature treatment with flux which generally introduces additional impurities and defects. YGG particles have also been produced by other techniques [5–7]. In this work, we have optimized a coprecipitation route for the synthesis of homogenously dispersed Ce-doped yttrium gallium garnet (YGG:Ce) which is an important

n

Corresponding author. Tel.: þ91 11 265 91 511; fax: þ 91 11 268 54 715. E-mail addresses: [email protected], [email protected] (A.K. Ganguli). 1 Permanent address: Laser Science & Technology Centre, DRDO, New Delhi. 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.11.025

luminescent material [8]. This is the first report of a comparative study of Ce:YGG using the normal strike method and by the reverse strike method. The effect of particle size on photoluminescence of the YGG:Ce phases has been discussed.

2. Material and methods Sample preparation: Yttrium oxide (99.99%), gallium oxide (99.99%) and cerium oxide (99.99%) were dissolved in 2 M HNO3 and stirred for 2 h at 80–90 1C. A solution of ammonium hydrogen carbonate (AHC, 2 M) was used as the precipitant. In normal strike (NS) the precipitant solution was added slowly into a mixed solution of cations to obtain the precipitate, and in reverse strike (RS) co-precipitation process, mixed cation solution was added slowly into the precipitant solution. After aging for 1 h, the suspension was filtered, and was washed several times with distilled water and absolute alcohol (to remove NO3 and NH4þ ions) and dried in oven. The precursor powder was then calcined at 1000 1C for 2 h to get phase pure Ce:YGG. Characterization: X-ray studies were carried out on a Bruker D-8 Advance X-ray diffractometer using Ni filtered CuKa radiation collected in the 2Y range of 20–701. A step size of 0.021 and a step time of 2 s/ step were used. The Rietveld refinement was carried out using the TOPAZ software. The crystallite size was obtained by Scherrer’s method. IR spectra were recorded on a Bruker Vector-22 spectrometer. SEM–EDAX studies were carried out on a Zeiss EVO40 scanning electron microscope. Diffuse reflectance spectra were obtained using a Perkin Elmer LAMBDA

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A. Verma et al. / Materials Letters 93 (2013) 21–24

1050 spectrometer. A Fluoromax 4 Horiba spectrometer was used to obtain the photoluminescence data.

3. Results and discussion In the co-precipitation method solubility product plays a major role, which directly depends on pH and needs to be controlled to maintain stoichiometry and particle size. In normal strike route, initially the pH is highly acidic which becomes basic by the addition of precipitant. Ga hydroxide precipitates at pH 4 while Y precipitates at higher pH. In the normal strike route initial

(420)

50000

(842)

(800)

(640) (642) (444)

(611)

10000

(521)

(332)

20000

(431)

30000

(422)

% (Ce:YGG via Reverse strike) % (Ce:YGG via Normal strike) % (JCPDS no. 83-1036) (400)

Intensity (a.u.)

40000

0 20

30

40

50

60

70

2 theta Fig. 1. (a) XRD of Ce:YGG synthesized via RS and NS, Rietveld refinement of (b) RS (c) NS.

(NS)

pH is low which favors precipitation of Ga, resulting in primary nuclei and remain small as the rise in pH stops further precipitation of Ga. Y starts precipitating over the Ga nuclei till stable stoichiometry is reached. This indirectly leads to control the overall particle size of YGG. X-ray diffraction studies: Fig. 1 shows the XRD patterns of YGG nanopowders synthesized by normal strike (900 1C for 2 h) and reverse strike (1000 1C). Comparing the XRD result of samples obtained by NS and RS routes, the NS route leads to pure garnet phase at 900 1C; however, the RS precursor was converted to YGG phase by calcining at 1000 1C for 2 h. All the reflections were in agreement with those of the reported YGG crystal structure (JCPDS no. 83-1036) corresponding to the cubic Ia3d space group. We observed that line broadening of reflections are comparatively more pronounced in the oxide obtained by the normal than the reverse strike route. This indicates smaller crystallite size which may be ascribed to reasons as mentioned above. Rietveld refinement (Fig. S1) of the powder X-ray diffraction data of Y2.94Ce0.06Ga5O12 shows close match of observed and calculated patterns. Lattice parameters obtained via both routes are a ¼12.3461(8) A˚ (normal Strike) and 12.3096(6) A˚ (reverse Strike). We observed a slight shift in the lattice parameter which is due to the variation of crystallite size, as had already been discussed earlier [9]. With decrease in particle size there is also a shift of absorption bands in the Fourier Transform Infrared Spectroscopy (FTIR) spectra (toward lower wavenumber) (Fig. S2) along with decrease in intensity, which has been observed earlier by Lukowiak et al. [10]. Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM) studies: Fig. 2(a) and (b) shows SEM images of the samples synthesized by NS as well as for RS route. Micrographs show aggregation of particles with spherical morphology and relatively uniform particle size distribution.

(RS) 200 nm

200 nm

0.198nm

0.287nm

(611)

(431)

1nm

1nm

Fig. 2. (a) SEM images of Ce:YGG prepared by (a) RS and (b) NS strike co-precipitation route with EDAX in their insets and HRTEM micrographs (c) NS (d) RS.

A. Verma et al. / Materials Letters 93 (2013) 21–24

Table 1 Table showing Y/Ga ratio in NS-YGG and RS-YGG with Ce at% doped in.

Normal strike Reverse strike

Y/Ga ratio

Nd at%

0.61 0.52

1.77 1.83

120

Intensity

90

Ce:YGGRS

60

30

0 200

400

600 800 Wavelength (nm)

1000

1200

600

Normal Strike

Energ

YGG:Ce nanopowders have been successfully synthesized by normal and reverse strike co-precipitation methods. Rietveld refinement studies on these compounds show larger lattice ˚ when synthesized by the normal strike parameter (12.3461 A) co-precipitation route as compared to nanopowders synthesized ˚ by the reverse strike co-precipitation method (12.3096 A). The normal strike route is an effective method to obtain smaller nanoparticles of Ce doped YGG ( 50 nm) than the reverse strike co-precipitation method (  100 nm). While EDAX results indicate deviation of Y/Ga ratio from 0.6 in case of RS route indicating NS route is also a better option to maintain cation homogeneity as shown by EDAX. The results illustrate that YGG:Ce synthesized by normal strike method shows enhanced emission which can be attributed to smaller particle size and homogeneity.

Acknowledgment

517nm

Intensity

Photoluminescence property: Photoluminescence measurements were carried out at an excitation wavelength of 410 nm. Strong absorption bands Fig. 3(a) (410 nm) is found in the diffuse reflection spectra. The highest emission intensity was observed at 520 nm in both the cases when excited at 410 nm Fig. 3(b). The emission at 520 nm occurs due to the transition from the lowest crystal field component of 5d1 to the two levels of the ground state 2F5/2 and 2F7/2 [11,12]. The photoluminescence intensity of the YGG:Ce powders increase with decreasing particle size. The analysis shows that the preparation method, in turn precursor homogenization (which is better in normal strike) and particle size profoundly influences the emission intensity [13].

4. Conclusions Ce:YGGNS

400

23

520nm

200

AKG thanks DST, Government. of India for the facilities at IIT Delhi (XRD, HRTEM). Aditya Verma and Neelam Malhan gratefully acknowledge Dr. A.K. Maini, Director, Laser Science & Technology Centre, for providing financial and moral support to carry out this work.

Reverse Strike

Appendix A. Supplementary information 0

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2012.11.025. 400

500 Wavelength (nm)

600

Fig. 3. (a) Diffuse reflectance spectra of Ce:YGG synthesized via RS and NS routes. (b) Emission spectra via RS and NS routes inset showing energy level diagram of Ce:YGG.

The particles of the sample synthesized by NS route are fine and smaller (50 nm) than particles synthesized by RS route (100 nm). Energy Dispersive Spectroscopy (EDAX) results show that Ce content is almost equivalent in both the cases, it is 1.83 at% in case of RS and 1.77 at% in case of NS (note: we have added 2 at% in both the cases). Interestingly Y to Ga ratio varied in both the cases, it is 0.61 (as expected) in case of NS while 0.52 in case of product obtained via RS route(less than expected) indicating precursor inhomogeneity, which is one of the probable reason for difference in emission intensity for the product obtained via two routes. Inset in Fig. 2(a) and (b). Table 1 shows Y/Ga ratio in both samples. HRTEM results are shown in Fig. 2(c) and (d) for NS and RS respectively. It was found that these samples are well crystalline with lattice fringes corresponding to (611) and (431) planes respectively.

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