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New synthesis parameters of GGG:Nd nanocrystalline powder prepared by sol–gel method: Structural and spectroscopic investigation
Author’s Accepted Manuscript New synthesis parameters of GGG:ND nanocrystalline powder prepared by sol-gel method: Structural and Spectroscopic investigation Yassin Alshikh mohamad, Yomen Atassi, Zafer Moussa www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(15)00189-1 http://dx.doi.org/10.1016/j.jlumin.2015.04.005 LUMIN13287
To appear in: Journal of Luminescence Received date: 2 November 2014 Revised date: 8 April 2015 Accepted date: 9 April 2015 Cite this article as: Yassin Alshikh mohamad, Yomen Atassi and Zafer Moussa, New synthesis parameters of GGG:ND nanocrystalline powder prepared by solgel method: Structural and Spectroscopic investigation, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.04.005 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.
New synthesis parameters of GGG:Nd nanocrystalline powder prepared by sol-gel method: Structural and Spectroscopic Investigation Yassin Alshikh mohamad*, Yomen Atassi and Zafer Moussa, Department of Applied Physics, Higher Institute for Applied Science and Technology, P.O. Box 31983, Damascus, Syria.
Abstract GGG:Nd nanopowder is synthesized by sol-gel method using formic acid and acetic acid as a chelating agents and ethylene glycol as a cross linking agents. TGADSC, XRD, photoluminescence spectroscopy and fluorescence life time analysis (τ) are used to characterize the powder. XRD is used to optimize the synthesis parameters. According to XRD, complete phase of GGG nanopowder is formed at 800 C˚ for 1 min. Fluorescence life time analyses reveal that the optimum crystallization temperature is 1000˚ C. Keywords: nanopowder, Nd:GGG, sol-gel, photoluminescence spectroscopy, fluorescence life time, chelating agents.
1.Introduction Gadolinium gallium garnet Gd3Ga5O12 (GGG) monocrystals have attracted great attention, since 1964, as laser host materials for their lasing properties when doped with neodymium [1,2]. Monocrystal growth technology is too expensive and is limited to several centimeters in diameters and lengths [3,4]. In the last decade, transparent ceramic laser material technology had achieved many successes [5-8]. The main advantages of ceramic technology are low cost, simple manufacturing process, short time production cycle, high productibility (mass production) and high scalability up to 1 meter. 40 cm long slabs have been successfully fabricated [9]. The production of transparent polycrystalline ceramics needs nanosized, well dispersed, nonagglomerated powders [10]. GGG nanocrystal was synthesized by various wet chemical methods, such as solution combustion [11-14], co-precipitation [15-18], sol-gel [19-23] and microwave gel combustion methods [24,25]. Sol-gel process is one of the best methods to produce nanosized powders. The key benefits of this method are: low temperature process, excellent chemical homogeneity of the final product due to the mixing on the molecular level in the solution and low crystallization temperature. GGG was prepared by sol-gel and combustion methods, with various molar ratios of chelating agent/cations and cross-linking agent/cations, using citric acid as a chelating agent and polyethylene glycol (PEG) as a cross linking agent [13,1921,26,27]. Only experimental observations of the sol/gel were mentioned to evaluate the best molar ratio of citrate/cations [12,28]. Other literature [24] used XRD, particles size distribution and TEM photos to optimize the ratio of citrate/cations. Acetic acid and ethylene glycol (EG), as a chelating agent and a cross-linking agent
*
Corresponding Author: +963-11-6721432 E-mail: [email protected]
respectively, were also used to prepare GGG by sol-gel method[22,23], but no molar ratio was mentioned. On the other hand, the effect of the chelating agents on the fluorescence life time (τ) of Nd in GGG nanocrystals was not reported in the literature, as far as we know. Only one value of τ was mentioned [22] at a specific temperature and time. In this work, we present the preparation of GGG:Nd nano-powders, using the sol-gel process. Our own ratios of chelating agent/cations and crosslinking agent/cations are proposed by new methodical approach. For the first time, to the best of our knowledge, GGG:Nd nanopowder is prepared utilizing formic acid as a chelating agent. Moreover, the influence of different chelating agents on the fluorescence life time () will also be investigated. The variations of with sintering temperature and time are widely studied.
2.Experimental details 2.1.General sample preparation Gallium oxide (Ga2O3, 4N) was dissolved in diluted nitric acid with total reflux. An appropriate amount of gadolinium oxide (Gd2O3, 4N) and neodymium oxide (Nd2O3, 4N) were then added to the previous solution. The molar ratio of Gd2O3/Ga2O3 was the stoichiometric ratio (3:5). The amount of dopant (Nd2O3) was 1% atom. The solution became translucent within several hours. Acetic acid (or formic acid) as a chelating agent was added to the solution. The cross linking agent, ethylene glycol (EG) was added as well as described in diagram 1. All used chemicals were analytical grade. The temperature was controlled to have a transparent solution. White gel was obtained by drying the solution at low temperature. Brown xerogel was obtained by drying the gel at 120 ˚C. The brownish color is due to the initial decomposition of nitrates. Xerogel was grinded to fine powder. Heat treatment of xerogel was performed at various temperatures, in the range of 800˚ C- 1200˚ C, at a heat rate of 2˚C/min. Gallium oxide & HNO3
Gadolinium oxide Neodymium oxide
Acetic/Formic acid
Transparent solution EG
Evaporation Gel
Drying Xerogel Calcination Nanopowder
Diagram 1: Sol-gel process.
2.2. Sample preparation for parameters optimization
1. Many samples have been prepared with different quantities of acetic acid and ethylene glycol. Our aim is to evaluate the influence of experimental parameters like environmental pH, molar ratio of chelating agents and cross-linking agents on the crystallinity of final products. We have dResults and discussion efined α and β as the molar ratios of acetic acid and EG to the total numbers of cations, respectively. α and β were varied in the intervals of 1.2 to 3 and 0 to 3 respectively. Samples were heat treated at 850 ˚C for 2 hours. X-ray analysis was performed. Integral intensities were calculated at the characteristic peak of the GGG (2θ≈32.3˚).
2.3.Instrumental details Thermal gravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) were performed on Labsys-Setaram, France, at a heating rate of 10˚ C/min. Alumina crucible was used as a reference. Structure and phase evolution of the samples were characterized by X-ray diffractometer (XRD) using STOE transmission diffraction system with CuKα radiation at 0.1548 nm. FullProf program was utilized to evaluate FWHM and integrated intensity from X-Ray data. Photoluminescence was recorded using S100 spectrometer-Solar laser system, Belarus. Fluorescence life time was measured using a 50Hz, 808 nm laser diode, a 1064 nm interferential filter, a Si sensor and a GW instek oscilloscope.
3.Results and discussion 3.1.Optimization of parameters The aim of this study is to reduce, as possible, the time and temperature needed for heat treatment. X-ray analysis were performed and the results of all samples coincided with JCPDS 13-493 (this will be discussed later, Figure 5). Figure 1 shows that, X-ray integral intensities (at the characteristic peak of GGG (2θ≈32.3˚)) increase with increasing β (for α =1.2) until it reaches its maximum at the value β=2. This means that the crystallinity is the best at this point. Varying α (while fixing β at 2) we have observed, Figure 2, that the integral intensities decrease with the increase of the amount of acetic acid, i.e. less acetic acid leads to better crystallinity.
X-ray integral intensity (a.u)
1.0
0.8
0.6
0.4
0.2
0
1
2
3
Figure 1: X-ray integral intensity (characteristic peak at 2θ≈32.3˚) as a function of β for α=1.2 (T=850˚C for 2h)
X-ray integral intensity (a.u)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
1.5
2.0
2.5
3.0
Figure 2: X-ray integral intensity as a function of α for β=2 (T=850˚C for 2 h) This trend is also confirmed by figure 3 which shows that the integral intensity increases with pH, i.e. with less amount of acetic acid. pH variation was achieved by adding some ammonia or acetic acid to the medium.
x-ray integral intenseties(A.u)
1.0
0.5
0.0 2.4
2.6
2.8
3.0
3.2
pH
Figure 3: X-Ray integral intensity as a function of pH (T=850 ˚C for 2h). For the rest of our work, α and β will be considered as 1.2 and 2 respectively.
3.2.Structural Characterization 3.2.1.TGA-DSC analysis Figure 4 shows the DSC-TGA analysis of the samples prepared using acetic acid and forming acid as chelating agents with the same other experimental parameters. The overall mass losses are of 32% and 23% for samples prepared with formic acid and acetic acid respectively. Most of mass loss which occurred below 500˚ C is due to the evaporation of adsorbed water, decomposition of nitrate [29] and the first decomposition stages of acetate [30]. Thermal decomposition of Gadolinium acetate contains six steps. These steps were proposed in [30] as follows: ( )
→
Gd(CH3COO)3. 3H2O (
Gd(CH3COO)3. 0.5H2O → ( )
→
Gd(CH3COO)3. H2O
) (
Gd2O2(CO3) →
(
Gd(CH3COO)3 → )
)
( )
→
Gd2O(CO3)2
Gd2O3
Final decomposition stages of gadolinium and neodymium acetate are explained by peaks between 600 ˚C and 800 ˚C (decomposition of Neodymium acetate continues up to 720 ˚C [30]). It is clear that the crystallization peak temperature of GGG is 812 ˚C and 817 ˚C for the samples with acetic acid and formic acid respectively. The difference between DSC diagrams of the samples prepared with acetic acid and formic acid is obvious in the IV and V decomposition stages, because formic acid (HCOOH) does not contain the group (CH3—), while acetic acid (CH3COOH) does. In addition, we notice that the final decomposition stage (VI) occurs at a little higher temperature in the samples with acetic acid.
Figure 4: DSC-TGA analysis of GGG:Nd with (a)-formic acid and (b)-acetic acid. 3.2.2.X-ray diffractometry It is clear from the X-ray diffraction patterns (Figure 5) that the crystallization of powder starts at 750 ˚C. Samples with acetic acid and formic acid, showed the characteristic peaks of GGG (JCPDS No. 13-493) only when treated at 800 ˚C and higher temperatures. At 800 ˚C only the phase of GGG has been crystallized, according to the equation: → The samples prepared with formic acid have a higher integrated X-ray intensity. This is because the diffusion distances of the cations in the samples with
formic acid is shorter than that with acetic acid, so it has more time to be rearranged. This is confirmed by smaller volume contraction of the powder (TGA results) which leads to less agglomeration during the heat treatment process and more occupied volume by the final powder.
Figure 5: XRD of GGG:Nd powder produced with formic acid, treated at various temperatures for different periods of time. The crystallites diameter were evaluated using Scherrer formula: &
√
Where D: is the crystallites diameter, λ: X-ray radiation wave length (0.1548 nm), βa is the apparatus broadening, βp: is the broadening produced by the powder. θ: is the characteristic angle for the highest peak for GGG. Table 1, shows that the crystallite diameter increases with increasing temperature. From table 1 it is clear that the increment of crystallite size prepared with formic acid is less than that of crystallite prepared with acetic acid as shown in figure 6. Temperature
acetic acid Da (nm)
formic acid Df (nm)
800/2h 900/2h 1000/2h 1100/2h 1200/2h
29.4 38.0 59.5 74.1 105.1
27.7 35.7 56.4 69.0 91.5
Table 1: Crystallite diameters as a function of temperature. The preparation of nanosized, non-agglomerated crystallites is the key condition to produce transparent ceramics [31]. In ceramic technology production, growth inhibitors [32] are added to control and limit the growth of grain at high temperatures. This fact is compatible with the small increase of the crystallite prepared with formic acid compared to crystallite prepared with acetic acid, which makes the powder prepared with formic acid more favorable for transparent ceramic production.
Figure 6: Da-Df as a function of temperature.
3.3.Spectroscopic characterization 3.3.1.Photoluminescence of the powder Figure 7 shows the perfect coincidence in the photoluminescence lines positions between photoluminescence spectra of the monocrystal and the nanopowder prepared with formic acid. The difference in intensities is due to the different attenuations filters used to eliminate spectrophotometer saturation.
Figure 7: photoluminescence spectra of nanopowder (solid line) and monocrystal (dashed line) 3.3.2.Fluorescence life time Many samples were prepared; each sample was divided into subsamples. Subsamples were calcinated at various temperatures and time in the ranges of 800˚ C to 1200˚ C and from 1 min, to 2 h, respectively. Fluorescence life time (τ), was measured and plotted as a function of temperature and time in Figure 8 (a and b). With increasing temperature, evaporation of organic residue occurs, and the cations rearrange themselves to have the best possible structure. As consequence, τ increases with temperature until it reaches its maximum at 1000 ˚C (better crystallinity is combined with greater τ). Further temperature increase, as in ceramic science, will lead only to crystal growth and neck formation. In this case the value of τ will tend
down toward the value of monocrystal. It is clear that the temperature treatment is not a critical issue in the range of 900 ˚C-1100 ˚C, for powder prepared with formic acid because the variation of τf is very small in this range. And it will be better to get a luminescent nanoparticles with formic acid, than with acetic acid, in the range of 800 ˚C-1200 ˚C.
Figure 8 (a): τ as a function of time at 800˚ C, (τf) formic acid and (τa) acetic acid.
Figure 8 (b): τ as a function of temperature, (τf) formic acid and (τa) acetic acid (for 2h treatment time) .
Our τ values at 1100 ˚C are greater than the corresponding fluorescence life time, τ=316 μs (for 12h heat treatment at 1100 ˚C) reported in [22].
4.Conclusion GGG:Nd nanopowder was prepared using two chelating agents: formic acid and acetic acid. Our own ratios of chelating agent/cations (α) and cross-linking agent/cations (β) were optimized. These ratios were found to be α=1.2 and β=2, utilizing integral X-ray intensities. X-ray diffractograms and τ measurements have confirmed the formation of luminescent nanoparticles of GGG phase at 800˚ C for 1 min using formic acid as a chelating agent. The analyses of the effect of chelating agents on the variation of τ vs temperature revealed that the optimum treatment temperature is at 1000˚ C. Nanopowder prepared with formic acid is better than that formed with acetic acid, for the following reasons: Less volumic contraction (figure 4); Better crystal structure (figure 5); Less crystallite diameter (table 1); Greater fluorescence life time with less temperature dependency, in the range of 900 ˚C-1100 ˚C (figure 8).
References [1] A.A. Kaminskii, V.V. Osiko, S.E. Sarkisov, New stimulated emission data of GGG:Nd, Phys. Stat. Sol. (a) 49 (1978) 305-311. [2] K. Yoshida, H. Yosida, Y. Kato, Characterization of high average power Nd:GGG slab laser, IEEE J. Quant. Electr. 24 (1988) 1188-1192 [3] C.D. Brandle, Growth of 3" diameter GGG crystal, J. Appl. Phys. 49 (1978) 1855-1858. [4] Y. Fujii, Recent progress in high-power slab laser in Japan, CLEO California, USA 20 (1988) 398-400 ISBN:1-55752-033-x. [5] J. Lu, H. Yagi, K.I. Ueda, T. Yanagitani, A.A. Kaminskii, 110 W ceramic Nd:YAG laser, Appl. Phys. B 79 (2004) 25-28. [6] K. Uda, T. Yanagitani, A.A. Kaminskii, Scalable ceramic laser, Laser Phys. 15 (2005) 927-938. [7] T. Yanagitani, H. Yagi, Rare earth garnet sintered compact, US patent No.: 2005/0215419 A1. [8] A. Ikesue, Y.L. Aung, Ceramic laser materials, Nat. Photonics 2 (2008) 721727. [9] K.I. Ueda, Ceramic solid state lasers, CLEOE (2005) DOI: 10.1109/CLEOE.2005.159779. [10] Y. Rabinovitch, C. Bogicevic, F. Karolak, D. Tedared, Freeze-dried nanometric neodymium doped YAG powders for transparent ceramics, J. Mater. Process. Technol. 199 (2008) 314-320. [11] R. Krsmanovic, V.A. Morozov, O.I. Lebedev, S. Polizzi, A. Speghini, M. Bettinelli, Structural and luminescence investigation on gadolinium gallium garnet nanocrystalline powder prepared by solution combustion synthesis, Nanotechnol. 18 (2007) 325604. [12] H. Dexin, Z. Xuejian, L. Jinghe, Nanostructured Yb:GGG polycrystalline powders via gel combustion method, J. Rare Earth. 27 (2009) 402-405. [13] L. Yanhong, L. Haiyan, S. Guoyi, Upconversion Luminescence of Er3+, Tm3+, and Yb3+ co-doped Gd3Ga5O12 nanocrystals, J. Wuhan Univ. of Technol.-Mater. Sci. Ed. 29 (2014) 13-18. [14] A. Zabiliute, S. Butkute, A. Kareiva, Sol-gel synthesized far-red chromium doped garnet phosphors for phosphor-conversion light-emitting diodes that meet the photomorphogenetic needs of plants, Appl. Opt. 53 (2014) 907-914. [15] G. Wang, X. Li, Y. Geng, Preparation of gadolinium gallium garnet polycrystalline powders for transparent ceramics, J. Alloy. Compd. 505 (2010) 213-216. [16] H. Dexin, Z. Xuejian, L. Jinghe, Z. Dan, Synthesis of polycrystalline Yb:Gd3Ga5O12 nanopowders by homogeneous precipitation method, J. Wuhan Univ. Technol.- Mater. Sci. Ed. 15 (2010) 123-126. [17] Y. Dong, J. Sun, W. Yu, W. Li, F. Teng, Preparation and properties of Nd,Yb:GGG polycrystalline nanopowders, J. Adv. Ceram. 1 (2012) 296-300. [18] X.J. Zhange, C. Li, D.X. Huang, Z.F. Lu, Synthesis of Nd:GGG powder by liquid phase co-precipitation or sold state method and preparation of its transparent ceramic, Appl. Mech. Mater. 651-653 (2014) 119-123. [19] M. Pang, J. Lin, Growth and optical properties of nanocrystalline GGG:Ln (Ln= Eu3+, Tb3+, Er3+) powders and thin films via Pechini sol-gel, J. Crys. Growth 284 (2005) 262-269.
[20] Y. Yan-ping, L. Jing-he, Preparation of Nd-doped GGG laser ceramic powder by sol-gel method, Optoelectron. Lett. 2 (2006) 288-291. [21] M. Daldosso, D. Falcomer, A. Speghini, M. Bettinelli, Synthesis, structural investigation and luminescence spectroscopy of nanocrystalline GGG doped with lanthanide ions, J. Alloy. Compd. 451 (2008) 553-556. [22] L. Lipinska, W. Ryba-Romanowski, A. Rzepka, S. Ganschow, Preparation and characterization of Nd doped gadolinium gallium garnet nanopowders and crystals, Cryst. Res. Technol. 44 (2009) 477-483. [23] A. Kozlowska, M. Nakielska, J. Sarnecki, L. Lipinska, Spectroscopic investigations of rear earth materials for luminescent solar concentrators, Opt. Appl. XLI (2011) 359-365. [24] K. Laishram, R. Mann, R. Sharma, Nd:GGG nanopowders by microwave gel combustion rout and sinterability studies, Defence Sci. J. 64 (2014) 490-494. [25] K. Laishram, R. Maan, N. Malhan, Microwave gel combustion synthesis and sinterability of Nd:GGG, J. Rare Earth. 32 (2014) 521-524. [26] M. Daldosso, D. Falcomer, A. Spegini, M. Bettinelli, Synthesis, EXAFS investigation and optical spectroscopy of nanocrystalline GGG doped with Ln3+ ions (Ln=Eu, Pr), Opt. Mater. 30 (2008) 1162-1167. [27] F.P. Yu, D.R. Yuan, X.L. Duan, S.Y. Guo, A simple process to synthesize sphere-shaped gadolinium gallium garnet nanoparticles for transparent ceramic, J. Alloy. Compd. 465 (2008) 567-570. [28] T. Xueyan, Z. Xiyan, L. Yinzhu, Synthesis and spectral properties of 3+ Nd :GGG nanopowder for transparent laser ceramics, J. Rare Earth. 24 (2006) 443-446. [29] V. Berbenni, C. Milanese, G. Bruni, Thermal decomposition of gallium nitrate hydrate Ga(NO3)3 xH2O, J. Therm. Anal. Calorim. 82 (2005) 401-407. [30] I.S. Shaplygin, V.P. Komarov, V.B. Lazarev, A thermogravimetric study of praseodymium (III), neodymium, samarium, gadolinium and holmium, acetates, benzoates and abietates, J. Therm. Anal. 15 (1979) 215223. [31] G.J. Quarles, V.K. Castillo, M. Dubinskii, L.D. Merckle, Comparison of laser, optical and thermal properties of ceramic laser gain materials with single crystal materials, LEOS 2 (2004) 935-936. DOI 10.1109/LEOS.2004.1363552. [32] C. Barry Carter, M. Grant Norton, Sintering and grain growth, in: Ceramic Materials, Springer, New York, 2007, pp. 427-443.
PII: DOI: Reference:
S0022-2313(15)00189-1 http://dx.doi.org/10.1016/j.jlumin.2015.04.005 LUMIN13287
To appear in: Journal of Luminescence Received date: 2 November 2014 Revised date: 8 April 2015 Accepted date: 9 April 2015 Cite this article as: Yassin Alshikh mohamad, Yomen Atassi and Zafer Moussa, New synthesis parameters of GGG:ND nanocrystalline powder prepared by solgel method: Structural and Spectroscopic investigation, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.04.005 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.
New synthesis parameters of GGG:Nd nanocrystalline powder prepared by sol-gel method: Structural and Spectroscopic Investigation Yassin Alshikh mohamad*, Yomen Atassi and Zafer Moussa, Department of Applied Physics, Higher Institute for Applied Science and Technology, P.O. Box 31983, Damascus, Syria.
Abstract GGG:Nd nanopowder is synthesized by sol-gel method using formic acid and acetic acid as a chelating agents and ethylene glycol as a cross linking agents. TGADSC, XRD, photoluminescence spectroscopy and fluorescence life time analysis (τ) are used to characterize the powder. XRD is used to optimize the synthesis parameters. According to XRD, complete phase of GGG nanopowder is formed at 800 C˚ for 1 min. Fluorescence life time analyses reveal that the optimum crystallization temperature is 1000˚ C. Keywords: nanopowder, Nd:GGG, sol-gel, photoluminescence spectroscopy, fluorescence life time, chelating agents.
1.Introduction Gadolinium gallium garnet Gd3Ga5O12 (GGG) monocrystals have attracted great attention, since 1964, as laser host materials for their lasing properties when doped with neodymium [1,2]. Monocrystal growth technology is too expensive and is limited to several centimeters in diameters and lengths [3,4]. In the last decade, transparent ceramic laser material technology had achieved many successes [5-8]. The main advantages of ceramic technology are low cost, simple manufacturing process, short time production cycle, high productibility (mass production) and high scalability up to 1 meter. 40 cm long slabs have been successfully fabricated [9]. The production of transparent polycrystalline ceramics needs nanosized, well dispersed, nonagglomerated powders [10]. GGG nanocrystal was synthesized by various wet chemical methods, such as solution combustion [11-14], co-precipitation [15-18], sol-gel [19-23] and microwave gel combustion methods [24,25]. Sol-gel process is one of the best methods to produce nanosized powders. The key benefits of this method are: low temperature process, excellent chemical homogeneity of the final product due to the mixing on the molecular level in the solution and low crystallization temperature. GGG was prepared by sol-gel and combustion methods, with various molar ratios of chelating agent/cations and cross-linking agent/cations, using citric acid as a chelating agent and polyethylene glycol (PEG) as a cross linking agent [13,1921,26,27]. Only experimental observations of the sol/gel were mentioned to evaluate the best molar ratio of citrate/cations [12,28]. Other literature [24] used XRD, particles size distribution and TEM photos to optimize the ratio of citrate/cations. Acetic acid and ethylene glycol (EG), as a chelating agent and a cross-linking agent
*
Corresponding Author: +963-11-6721432 E-mail: [email protected]
respectively, were also used to prepare GGG by sol-gel method[22,23], but no molar ratio was mentioned. On the other hand, the effect of the chelating agents on the fluorescence life time (τ) of Nd in GGG nanocrystals was not reported in the literature, as far as we know. Only one value of τ was mentioned [22] at a specific temperature and time. In this work, we present the preparation of GGG:Nd nano-powders, using the sol-gel process. Our own ratios of chelating agent/cations and crosslinking agent/cations are proposed by new methodical approach. For the first time, to the best of our knowledge, GGG:Nd nanopowder is prepared utilizing formic acid as a chelating agent. Moreover, the influence of different chelating agents on the fluorescence life time () will also be investigated. The variations of with sintering temperature and time are widely studied.
2.Experimental details 2.1.General sample preparation Gallium oxide (Ga2O3, 4N) was dissolved in diluted nitric acid with total reflux. An appropriate amount of gadolinium oxide (Gd2O3, 4N) and neodymium oxide (Nd2O3, 4N) were then added to the previous solution. The molar ratio of Gd2O3/Ga2O3 was the stoichiometric ratio (3:5). The amount of dopant (Nd2O3) was 1% atom. The solution became translucent within several hours. Acetic acid (or formic acid) as a chelating agent was added to the solution. The cross linking agent, ethylene glycol (EG) was added as well as described in diagram 1. All used chemicals were analytical grade. The temperature was controlled to have a transparent solution. White gel was obtained by drying the solution at low temperature. Brown xerogel was obtained by drying the gel at 120 ˚C. The brownish color is due to the initial decomposition of nitrates. Xerogel was grinded to fine powder. Heat treatment of xerogel was performed at various temperatures, in the range of 800˚ C- 1200˚ C, at a heat rate of 2˚C/min. Gallium oxide & HNO3
Gadolinium oxide Neodymium oxide
Acetic/Formic acid
Transparent solution EG
Evaporation Gel
Drying Xerogel Calcination Nanopowder
Diagram 1: Sol-gel process.
2.2. Sample preparation for parameters optimization
1. Many samples have been prepared with different quantities of acetic acid and ethylene glycol. Our aim is to evaluate the influence of experimental parameters like environmental pH, molar ratio of chelating agents and cross-linking agents on the crystallinity of final products. We have dResults and discussion efined α and β as the molar ratios of acetic acid and EG to the total numbers of cations, respectively. α and β were varied in the intervals of 1.2 to 3 and 0 to 3 respectively. Samples were heat treated at 850 ˚C for 2 hours. X-ray analysis was performed. Integral intensities were calculated at the characteristic peak of the GGG (2θ≈32.3˚).
2.3.Instrumental details Thermal gravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) were performed on Labsys-Setaram, France, at a heating rate of 10˚ C/min. Alumina crucible was used as a reference. Structure and phase evolution of the samples were characterized by X-ray diffractometer (XRD) using STOE transmission diffraction system with CuKα radiation at 0.1548 nm. FullProf program was utilized to evaluate FWHM and integrated intensity from X-Ray data. Photoluminescence was recorded using S100 spectrometer-Solar laser system, Belarus. Fluorescence life time was measured using a 50Hz, 808 nm laser diode, a 1064 nm interferential filter, a Si sensor and a GW instek oscilloscope.
3.Results and discussion 3.1.Optimization of parameters The aim of this study is to reduce, as possible, the time and temperature needed for heat treatment. X-ray analysis were performed and the results of all samples coincided with JCPDS 13-493 (this will be discussed later, Figure 5). Figure 1 shows that, X-ray integral intensities (at the characteristic peak of GGG (2θ≈32.3˚)) increase with increasing β (for α =1.2) until it reaches its maximum at the value β=2. This means that the crystallinity is the best at this point. Varying α (while fixing β at 2) we have observed, Figure 2, that the integral intensities decrease with the increase of the amount of acetic acid, i.e. less acetic acid leads to better crystallinity.
X-ray integral intensity (a.u)
1.0
0.8
0.6
0.4
0.2
0
1
2
3
Figure 1: X-ray integral intensity (characteristic peak at 2θ≈32.3˚) as a function of β for α=1.2 (T=850˚C for 2h)
X-ray integral intensity (a.u)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
1.5
2.0
2.5
3.0
Figure 2: X-ray integral intensity as a function of α for β=2 (T=850˚C for 2 h) This trend is also confirmed by figure 3 which shows that the integral intensity increases with pH, i.e. with less amount of acetic acid. pH variation was achieved by adding some ammonia or acetic acid to the medium.
x-ray integral intenseties(A.u)
1.0
0.5
0.0 2.4
2.6
2.8
3.0
3.2
pH
Figure 3: X-Ray integral intensity as a function of pH (T=850 ˚C for 2h). For the rest of our work, α and β will be considered as 1.2 and 2 respectively.
3.2.Structural Characterization 3.2.1.TGA-DSC analysis Figure 4 shows the DSC-TGA analysis of the samples prepared using acetic acid and forming acid as chelating agents with the same other experimental parameters. The overall mass losses are of 32% and 23% for samples prepared with formic acid and acetic acid respectively. Most of mass loss which occurred below 500˚ C is due to the evaporation of adsorbed water, decomposition of nitrate [29] and the first decomposition stages of acetate [30]. Thermal decomposition of Gadolinium acetate contains six steps. These steps were proposed in [30] as follows: ( )
→
Gd(CH3COO)3. 3H2O (
Gd(CH3COO)3. 0.5H2O → ( )
→
Gd(CH3COO)3. H2O
) (
Gd2O2(CO3) →
(
Gd(CH3COO)3 → )
)
( )
→
Gd2O(CO3)2
Gd2O3
Final decomposition stages of gadolinium and neodymium acetate are explained by peaks between 600 ˚C and 800 ˚C (decomposition of Neodymium acetate continues up to 720 ˚C [30]). It is clear that the crystallization peak temperature of GGG is 812 ˚C and 817 ˚C for the samples with acetic acid and formic acid respectively. The difference between DSC diagrams of the samples prepared with acetic acid and formic acid is obvious in the IV and V decomposition stages, because formic acid (HCOOH) does not contain the group (CH3—), while acetic acid (CH3COOH) does. In addition, we notice that the final decomposition stage (VI) occurs at a little higher temperature in the samples with acetic acid.
Figure 4: DSC-TGA analysis of GGG:Nd with (a)-formic acid and (b)-acetic acid. 3.2.2.X-ray diffractometry It is clear from the X-ray diffraction patterns (Figure 5) that the crystallization of powder starts at 750 ˚C. Samples with acetic acid and formic acid, showed the characteristic peaks of GGG (JCPDS No. 13-493) only when treated at 800 ˚C and higher temperatures. At 800 ˚C only the phase of GGG has been crystallized, according to the equation: → The samples prepared with formic acid have a higher integrated X-ray intensity. This is because the diffusion distances of the cations in the samples with
formic acid is shorter than that with acetic acid, so it has more time to be rearranged. This is confirmed by smaller volume contraction of the powder (TGA results) which leads to less agglomeration during the heat treatment process and more occupied volume by the final powder.
Figure 5: XRD of GGG:Nd powder produced with formic acid, treated at various temperatures for different periods of time. The crystallites diameter were evaluated using Scherrer formula: &
√
Where D: is the crystallites diameter, λ: X-ray radiation wave length (0.1548 nm), βa is the apparatus broadening, βp: is the broadening produced by the powder. θ: is the characteristic angle for the highest peak for GGG. Table 1, shows that the crystallite diameter increases with increasing temperature. From table 1 it is clear that the increment of crystallite size prepared with formic acid is less than that of crystallite prepared with acetic acid as shown in figure 6. Temperature
acetic acid Da (nm)
formic acid Df (nm)
800/2h 900/2h 1000/2h 1100/2h 1200/2h
29.4 38.0 59.5 74.1 105.1
27.7 35.7 56.4 69.0 91.5
Table 1: Crystallite diameters as a function of temperature. The preparation of nanosized, non-agglomerated crystallites is the key condition to produce transparent ceramics [31]. In ceramic technology production, growth inhibitors [32] are added to control and limit the growth of grain at high temperatures. This fact is compatible with the small increase of the crystallite prepared with formic acid compared to crystallite prepared with acetic acid, which makes the powder prepared with formic acid more favorable for transparent ceramic production.
Figure 6: Da-Df as a function of temperature.
3.3.Spectroscopic characterization 3.3.1.Photoluminescence of the powder Figure 7 shows the perfect coincidence in the photoluminescence lines positions between photoluminescence spectra of the monocrystal and the nanopowder prepared with formic acid. The difference in intensities is due to the different attenuations filters used to eliminate spectrophotometer saturation.
Figure 7: photoluminescence spectra of nanopowder (solid line) and monocrystal (dashed line) 3.3.2.Fluorescence life time Many samples were prepared; each sample was divided into subsamples. Subsamples were calcinated at various temperatures and time in the ranges of 800˚ C to 1200˚ C and from 1 min, to 2 h, respectively. Fluorescence life time (τ), was measured and plotted as a function of temperature and time in Figure 8 (a and b). With increasing temperature, evaporation of organic residue occurs, and the cations rearrange themselves to have the best possible structure. As consequence, τ increases with temperature until it reaches its maximum at 1000 ˚C (better crystallinity is combined with greater τ). Further temperature increase, as in ceramic science, will lead only to crystal growth and neck formation. In this case the value of τ will tend
down toward the value of monocrystal. It is clear that the temperature treatment is not a critical issue in the range of 900 ˚C-1100 ˚C, for powder prepared with formic acid because the variation of τf is very small in this range. And it will be better to get a luminescent nanoparticles with formic acid, than with acetic acid, in the range of 800 ˚C-1200 ˚C.
Figure 8 (a): τ as a function of time at 800˚ C, (τf) formic acid and (τa) acetic acid.
Figure 8 (b): τ as a function of temperature, (τf) formic acid and (τa) acetic acid (for 2h treatment time) .
Our τ values at 1100 ˚C are greater than the corresponding fluorescence life time, τ=316 μs (for 12h heat treatment at 1100 ˚C) reported in [22].
4.Conclusion GGG:Nd nanopowder was prepared using two chelating agents: formic acid and acetic acid. Our own ratios of chelating agent/cations (α) and cross-linking agent/cations (β) were optimized. These ratios were found to be α=1.2 and β=2, utilizing integral X-ray intensities. X-ray diffractograms and τ measurements have confirmed the formation of luminescent nanoparticles of GGG phase at 800˚ C for 1 min using formic acid as a chelating agent. The analyses of the effect of chelating agents on the variation of τ vs temperature revealed that the optimum treatment temperature is at 1000˚ C. Nanopowder prepared with formic acid is better than that formed with acetic acid, for the following reasons: Less volumic contraction (figure 4); Better crystal structure (figure 5); Less crystallite diameter (table 1); Greater fluorescence life time with less temperature dependency, in the range of 900 ˚C-1100 ˚C (figure 8).
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